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Cellulose Nanofibril-based HybridMaterials Eco-friendly design towards separation and packaging applications

Luis Valencia

Luis Valencia Cellu

lose Nan

ofibril-based Hybrid M

aterials

Doctoral Thesis in Materials Chemistry at Stockholm University, Sweden 2019

Department of Materials andEnvironmental Chemistry

ISBN 978-91-7797-927-2

Cellulose Nanofibril-based Hybrid MaterialsEco-friendly design towards separation and packaging applicationsLuis Valencia

Academic dissertation for the Degree of Doctor of Philosophy in Materials Chemistry atStockholm University to be publicly defended on Monday 9 December 2019 at 13.00 inMagnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B.

AbstractNanocellulose has been lately considered as the “Holy-Grail” in the design of sustainable materials due to its bio-origin and an unprecedented combination of prominent features, including good mechanical properties, anisotropy andversatile surface chemistry. In addition, nanocellulose in the form of cellulose nanofibrils, can adopt variable structuresand morphologies depending on the processing technique, such as aerogels, films and monoliths.

However, there are limitations that hinder the implementation of cellulose nanofibrils in “real-life applications”, such asinherent interaction with bacteria and proteins, thus leading to surface-fouling; and loss of integrity due to water-inducedswelling. A way to overcome these challenges, and provide further functionality, is through hybridization strategies, atwhich the multiple components act synergistically towards specific properties and applications. In this thesis, the aim is topresent multiple strategies for the synthesis of novel cellulose nanofibril-based hybrid materials, in the form of 2D-filmsand 3D-foams, towards their employment for separation applications or active food packaging.

A novel strategy to surface-functionalize cellulose nanofibril-membranes is proposed via grafting zwitterionic polymerbrushes of poly (cysteine methacrylate). The modification can suppress the absorption of proteins in an 85%, as well asdecreasing the adhesion of bacteria in an 87%, while introducing antimicrobial properties, as demonstrated against S.aureus.

The spontaneous formation of functional metal oxide nanoparticles occurring in situ on cellulose nanofibrils-films duringthe adsorption of metal ions from water is investigated, which occurs without the additional use of chemicals or temperature.Notably, this process not only enables the upcycling of materials through multi-stage applications, but also provides a cost-effective method to prepare multifunctional hybrid materials with enhanced dye-removal/antimicrobial activity.

The processing of functional composite films from cellulose nanofibril-stabilized Pickering emulsions and theirsuitability to be used as active edible barriers was demonstrated. The presence of oil in the films fine-tuned the properties ofthe films, as well as acted as the medium to encapsulate bio-active hydrophobic compounds, providing further functionalitysuch as antioxidant and antimicrobial properties.

Anisotropic porous hybrid foams with ultra-high loading capacity of sorbents (e.g., zeolites and metal-organicframeworks) were produced via unidirectional freeze-casting method using cellulose nanofibrils/gelatin as templatematerial. The foams indeed exhibited ultra-high loading capacity of sorbent nanomaterials, a linear relationship betweensorbent content and CO2 adsorption capacity, and high CO2/N2 selectivity.

Keywords: Cellulose nanofibrils, hybrid materials, membranes, aerogels, separation, food packaging.

Stockholm 2019http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-175622

ISBN 978-91-7797-927-2ISBN 978-91-7797-928-9

Department of Materials and EnvironmentalChemistry (MMK)

Stockholm University, 106 91 Stockholm

CELLULOSE NANOFIBRIL-BASED HYBRID MATERIALS

Luis Valencia

Cellulose Nanofibril-basedHybrid Materials

Eco-friendly design towards separation and packaging applications

Luis Valencia

©Luis Valencia, Stockholm University 2019 ISBN print 978-91-7797-927-2ISBN PDF 978-91-7797-928-9 Printed in Sweden by Universitetsservice US-AB, Stockholm 2019

This thesis is dedicatedto those who never stopchallenging themselvesbeyond what theyalready know

Luis Valencia – PhD Thesis Stockholm University

Abstract

Nanocellulose has been lately considered as the “Holy-Grail” in the design of sustainable materials due

to its bio-origin and an unprecedented combination of prominent features, including good mechanical

properties, anisotropy and versatile surface chemistry. In addition, nanocellulose in the form of cellulose

nanofibrils, can adopt variable structures and morphologies depending on the processing technique, such

as aerogels, films and monoliths.

However, there are limitations that hinder the implementation of cellulose nanofibrils in “real-life

applications”, such as inherent interaction with bacteria and proteins, thus leading to surface-fouling;

and loss of integrity due to water-induced swelling. A way to overcome these challenges, and provide

further functionality, is through hybridization strategies, at which the multiple components act

synergistically towards specific properties and applications. In this thesis, the aim is to present multiple

strategies for the synthesis of novel cellulose nanofibril-based hybrid materials, in the form of 2D-films

and 3D-foams, towards their employment for separation applications or active food packaging.

A novel strategy to surface-functionalize cellulose nanofibril-membranes is proposed via grafting

zwitterionic polymer brushes of poly (cysteine methacrylate). The modification can suppress the

absorption of proteins in an 85%, as well as decreasing the adhesion of bacteria in an 87%, while

introducing antimicrobial properties, as demonstrated against S. aureus.

The spontaneous formation of functional metal oxide nanoparticles occurring in situ on cellulose

nanofibrils-films during the adsorption of metal ions from water is investigated, which occurs without

the additional use of chemicals or temperature. Notably, this process not only enables the upcycling of

materials through multi-stage applications, but also provides a cost-effective method to prepare

multifunctional hybrid materials with enhanced dye-removal/antimicrobial activity.

The processing of functional composite films from cellulose nanofibril-stabilized Pickering

emulsions and their suitability to be used as active edible barriers was demonstrated. The presence of

oil in the films fine-tuned the properties of the films, as well as acted as the medium to encapsulate bio-

active hydrophobic compounds, providing further functionality such as antioxidant and antimicrobial

properties.

Anisotropic porous hybrid foams with ultra-high loading capacity of sorbents (e.g., zeolites and

metal-organic frameworks) were produced via unidirectional freeze-casting method using cellulose

nanofibrils/gelatin as template material. The foams indeed exhibited ultra-high loading capacity of

sorbent nanomaterials, a linear relationship between sorbent content and CO2 adsorption capacity, and

high CO2/N2 selectivity.


List of publications

This thesis is written based on the following publications:

I. Fully bio-based zwitterionic membranes with superior antifouling and antibacterial

properties prepared via surface-initiated free-radical polymerization of poly (cysteine

methacrylate)

Luis Valencia, Sugam Kumar, Blanca Jalvo, Andreas Mautner, German Salazar-Alvarez and

Aji P. Mathew

J. Mat. Chem. A 2018, 6 (34), 16361-16370

I participated in the planning of the project, performed most of experiments and played a leading

role in the writing of the manuscript.

II. Unravelling the spontaneous formation of metal oxide nanoparticles on cellulose

nanofibrils films during adsorption of metal ions: A green alternative towards

multifunctional hybrid materials

Luis Valencia, Sugam Kumar, Emma Nomena, German Salazar-Alvarez and Aji P. Mathew

Submitted, J. Mat. Chem. A



III. Biobased cellulose nanofibril-oil composite films for active edible barriers

Luis Valencia, Emma Nomena, Aji P. Mathew and Krassimir P. Velikov

ACS Applied Materials and Interfaces 2019, 11 (17) 16040-16047

I participated in the planning of the project, performed most of experiments (together with

Emma) and played a leading role in the writing of the manuscript.

IV. Biobased micro/meso/macro-porous hybrid foams with ultra-high zeolite loadings for

selective capture of carbon dioxide

Luis Valencia, Walter Rosas, Andrea Sanchez, Aji P. Mathew and Anders E. C. Palmqvist

ACS Applied Materials and Interfaces 2019, 11, 43, 40424-40431

I developed the research idea, performed most of the experiments and wrote most of the

manuscript.

V. Nanocellulose leaf-like zeolitic imidazolate framework (ZIF-L) foams for selective capture

of carbon dioxide

Luis Valencia and Hani Abdelhamid

Carbohydrate Polymers 2019, 213, 338-345

I developed the research idea, performed most of the experiments and wrote the manuscript.


Publications not included in this thesis

I. Nanocellulose/Graphene oxide layered membranes: Elucidating their behavior during

filtration of water and metal ions in real time

Luis Valencia, Susanna Monti, Sugam Kumar, Chuantao Zhu, Peng Liu, Shun Yu and Aji P.

Mathew

Nanoscale 2019, 10.1039/C9NR07116D

I participated in the experimental part and the writing of the manuscript.

II. Plasma-surface modification of cellulose nanocrystals: a green alternative towards

mechanical reinforcement of ABS

Andres Alanis, Josue Hernandez Valdes, Neira-Velazquez Maria Guadalupe, Ricardo, Lopez,

Aji P. Mathew, Ramon Diaz de Leon and Luis Valencia

RSC Advances 2019, 9 (30), 17417-17424

I developed the research idea, participated in the experimental part and played a leading role in

the writing of the manuscript.

III. Nanolignocellulose extracted from environmentally undesired Prosopis Juliflora

Luis Valencia, Vishnu Arumughan, Blanca Jalvo, Hanna Maria, Sabu Thomas and Aji P.

Mathew

ACS Omega 2019, 4 (2), 4330-4338

I participated in the experimental part and took a leading role in the writing of the manuscript.

IV. Multivalent ion-induced re-entrant transition of cellulose nanofibrils and its influence on

nanomaterials properties

Luis Valencia, Emma Nomena, Susanna Monti, Andrea Sanchez, Walter Rosas, Aji P. Mathew,

Sugam Kumar and Krassimir P. Velikov

Manuscript



V. Recent progress on nanostructured three-dimensional composite materials for CO2

capture

Luis Valencia, Walter Rosas, Aji P. Mathew, Niklas Hedin and Anders E. C. Palmqvist

Manuscript

Review article. I play a leading role in the writing of the manuscript.


Contents

1. Introduction 1

1.1. Nanocellulose: What and why? 1

1.2. Why hybridizing cellulose nanofibrils? 3

1.3. Scope of this thesis 3

2. Preparation of materials and characterization 4

2.1.Preparation of materials 4

2.1.1. Extraction of cellulose nanofibrils

2.1.2. TEMPO-oxidation of cellulose nanofibrils

2.1.3. Manufacturing of membranes

2.1.4. Grafting of poly (cysteine methacrylate)

2.1.5. Preparation of CNF-stabilized Pickering emulsions

2.1.6. Preparation of CNF-based oleofilms

2.1.7. In-situ growth of ZIF-L onto CNF

2.1.8. Fabrication of CNF-based foams

2.2.Characterization methods 5

2.2.1. Atomic force microscopy (AFM)

2.2.2. Scanning electron microscopy (SEM)

2.2.3. Small angle X-ray scattering (SAXS)

2.2.4. X-ray diffraction (XRD)

2.2.5. Surface ζ-potential

2.2.6. Fourier transform infrared spectroscopy (FTIR)

2.2.7. Ultraviolet-visible spectroscopy (UV-vis)

2.2.8. X-ray photoelectron spectroscopy (XPS)

2.2.9. Tensile/compression testing

2.2.10. CO2 adsorption studies

2.2.11. Quartz-crystal microbalance and dissipation (QCMD)

2.2.12. Antibacterial and antifouling activity

2.2.13. Dye-removal

2.2.14. Contact angle measurements

2.2.15. BET porosimetry

3. Cellulose nanofibril-based hybrid films for water purification 8


3.1. Paper I: Zwitterionic cellulose nanofibril-based membranes with superior

antifouling and antibacterial properties 8

3.1.1. Surface properties of the membranes 8

3.1.2. Antifouling performance of PCysMA-grafted membranes 10

3.2. Paper II: Hybrid films based on cellulose nanofibrils/metal oxide nanoparticles

formed spontaneously during adsorption of metal ions 11

3.2.1. Mechanism of formation of in situ growth Cu2O nanoparticles 12

3.2.2. Functionality of the resultant Cu2O@TOCNF hybrid films 13

4. Cellulose nanofibril-based films for active food packaging 15

4.1. Paper III: Cellulose nanofibrils-based oleofilms as active edible barriers 15

4.1.1. Physical properties of the CNF-based oleofilms 15

4.1.2. Encapsulation of bioactive compounds 18

5. Cellulose nanofibril-based hybrid foams for selective CO2 capture 20

5.1. Paper IV: Biobased foams with ultrahigh zeolite loadings for selective CO2

capture 20

5.1.1. Physical properties of the hybrid foams 21

5.1.2. CO2 adsorption capacity of the hybrid foams 22

5.2. Paper V: Biobased hybrid foams prepared by In-situ growth of ZIF-L towards

selective CO2 capture 24

5.2.1. Microstructure and performance of the hybrid foams 25

5.2.2. Capture capacity of CO2 and selectivity 26

6. Conclusions 27

7. Outlook 28

8. Sammanfattning 29

9. Acknowledgements 30

10. References 31


Abbreviations

AFM Atomic force microscopy

BC

BET

Bacterial cellulose

Brunauer-Emmett-Teller

BJH Barret-Joyner-Halenda

CIEL Center of International Environmental Law

CLSM Confocal laser scanning microscopy

CNCs Cellulose nanocrystals

CNFs Cellulose nanofibrils

CO2 Carbon dioxide

CysMA Cysteine methacrylate

DLS Dynamic light scattering

DMSO Dimethyl sulfoxide

DPPH

ECNF

2,2-diphenyl-1-picrylhydrazyl

Electron spun cellulose nanofibers

EDS Energy-dispersive X-ray spectroscopy

FDA Fluorescein diacetate

FTIR Fourier transform infrared spectroscopy

Hmim 2-methylimidazole

KPS Potassium persulfate

MB Methylene blue

MOFs Metal organic frameworks

MPS 3-Methacryloxypropyltrimethoxysilane

NaBr Sodium bromide

NaClO Sodium hypochlorite

NC Nanocellulose

PBS Phosphate-buffered saline

PI Propidium iodide

PCysMA Poly (cysteine methacrylate)

QCMD Quartz-crystal microbalance with dissipation monitoring

RSA Reduction in scavenging activity

SAXS Small angle X-ray scattering

SEM Scanning electron microscopy

SI-FRP Surface-initiated free-radical polymerization

STEM Scanning transmission electron microscopy

TEA Triethylamine

TEMPO 2, 2, 6, 6-tetramethylpiperidine-1-oxyl

TGA Thermogravimetric analysis

TOCNF Tempo-oxidized cellulose nanofibrils

UV-VIS Ultraviolet-visible spectroscopy

XPS X-Ray photoelectron spectroscopy

XRD X-ray diffraction

ZIF-L Zeolitic imadazolate framework L

𝜟D Dissipation change

𝜟f Frequency change


1

1. Introduction

Nowadays, we are at the critical stage at which the environmental damage, occasioned by human

industrialization, is at the edge of being irreversible (at least during the human-age), being fossil-based

plastic industry one of the main contributors to this environmental catastrophe. The topic of fossil-based

plastics is indeed very controversial, considering that these ubiquitous products of the global economy

have actually contributed significantly to human development, making possible the evolution of

computers and most of lifesaving advances in medicine. In fact, both industry and people daily-life

strongly depend on petroleum-based plastics, in a way that annually around 400 million tons of plastics

are produced1.

However, why are petroleum-based plastics so dangerous for our environment? Basically the

combination of their non-renewability and almost non-biodegradability, together with the poor recycling

culture of our society is pretty much filling our environment with massive amount of plastic-waste, as

observed from the 8 million metric tons of plastic that wound up in the ocean every year2. In fact,

nowadays we can find plastics everywhere, not only in waterways and ecosystems, but also in animals

and even in our food (in the form of micro- and nanoplastics)3. Moreover, plastic industry is the fastest-

growing source of industrial green-house emissions (according the latest report from the Center of

International Environmental Law (CIEL)), and if the expansion of plastics continues as predicted, by

2050 the emissions due to plastic production alone will be the responsible for 10-13% of the total

“carbon budget” – which is indeed the amount of carbon dioxide that we can globally emit to remain

below a 1.5 ˚C rise in temperature.

For this reason, it is nowadays crucial to replace petroleum-based plastics as much and as fast as possible

with materials that are renewable and follow the principles of sustainability. In this context, bio-based

polymers, from different biomass sources, represent one of the most prominent alternatives. Among

them, nanocellulose, derived from the most abundant organic polymer on earth (cellulose), have

emerged during the last decades as a key sustainable alternative4.

.

1.1. Nanocellulose: what and why?

Cellulose is the most abundant renewable polymer in the biosphere with an estimated annual production

of 7.5 x 1010 tons5. The use of this “almost unlimited” polysaccharide goes back in time to the beginning

of the human civilization, as it has been used in the form of lumber, textile or cordage. Cellulose has

also been extensively used as raw material for paper, combustible, explosives and cellophane films.

In the early 50´s, Rånby et al. discovered that cellulose could be disintegrated via a top-down approach

into its elemental building-block units6, which would later be identified as “nanocellulose”. This

discovery would bring the possibility to use cellulose as feedstock in way more sophisticated

applications, as it would be realized that within the realm of nanomaterials, nanocellulose is an intriguing

material with great impact in several important applications due to its unique combination of features

such as outstanding mechanical properties, biodegradability, low cost, and tunable surface chemistry

that allows the tailoring of specific properties/applications5. Nanocellulose is already being used to

produce high-value products with low environmental impact such as food rheology modifiers or drug

excipients. It’s also noteworthy that nowadays nanocellulose can be already produced industrially at the

level of tons per day scale.


2

Figure 1.1. Schematic representation of the extraction of nanocellulose, in the form of cellulose

nanofibrils (CNFs) and cellulose nanocrystals (CNCs) from biomass.

Nanocellulose is indeed a general term referring to both cellulose nanocrystals (CNCs) and cellulose

nanofibrils (CNFs), which vary in their structural properties, and the method used for their extraction

(see Figure 1.1) (Bacterial cellulose (BC) and electrospun cellulose nanofibers (ECNF) are also

considered nanocellulose but are much less common and therefore not discussed in this thesis). CNCs

are rigid (highly-crystalline) rod-like particles extracted by acid hydrolysis from cellulose, which exhibit

important properties with potential application in optoelectronics (as they form chiral nematic phases)

and as reinforcement additive of polymer composites7. CNFs, on the other hand, are soft and long chains

with high aspect ratios, ranging from five to fifty nanometers in diameter and micrometer-sized long.

CNFs consist of a bundle of individual nanofibrils entangled together and comprised of alternating

amorphous and crystalline cellulose domains. Different isolating methods can be used to extract CNF.

The most common technique is through mechanical treatments, i.e. microfluidization and grinding,

which breaks the interfibrillar cohesions without disintegrating or damaging the fibrils.8 Some

representative micrographs of CNFs are shown in Figure 1.2.

Cellulose nanofibrils offer several advantages for the preparation of nanomaterials. For instance, their

high aspect ratio allows easy preparation of nanomaterials (due to the robust network formed by vast

entanglement and short-range interactions), such as hydrogels with high gel-strength, and strong

materials upon drying. In this work we have exclusively used cellulose nanofibrils as building block for

the design of nanomaterials. Several applications of CNF have been proposed in the literature during

the last decades, for instance as paper coating9, in energy storage devices (as electrode binder or

electrolyte material)10 and substrate for supercapacitors11. Moreover, CNF is an excellent material to

prepare porous materials (2D-membranes/ 3D-foams) for separation applications (Paper I, II, IV and V)

and films for packaging applications (Paper III).

Figure 1.2. STEM (left) and AFM (right) micrographs of cellulose nanofibrils

Plant cells

Macrofibrers

Biomass

Cellulose

Nanofibrils

Cellulose

Nanocrystals


3

1.1.2. TEMPO-oxidized cellulose nanofibrils (TOCNF)

A major impediment for commercializing CNFs is the elevated energy consumption during their

production, as they require several passes through high-pressure homogenizers12. As an alternative to

overcome this obstacle, Saito et al., proposed a pre-treatment process to oxidize cellulose nanofibrils

via TEMPO (2, 2, 6,6-tetramethylpiperdiine-1-oxyl) radical oxidation using sodium hypochlorite

(NaClO) and sodium bromide (NaBr)13. This pre-treatment allows the ease of disintegration of CNF at

lower energy cost, while yielding quasi-individual nanofibers (width 5-10 nm). TEMPO-mediated

oxidation occurs only at the surface level of the fibrils, introducing a combination of carboxyl/aldehyde

groups, which is the responsible for negatively charged CNF surface that result in repulsion of the

nanofibers (thus ease of defibrillation). Furthermore, the presence of these functional groups can also

provide other interesting properties, as shown in Paper II, IV and V.

1.2. Why hybridizing cellulose nanofibrils?

Even though cellulose nanofibrils exhibit amazing properties and capabilities, there are several

challenges that actually limit their implementation in several applications for instance their inherent

interaction with bacteria and proteins (leading to surface fouling), loss of structural integrity due to

water-induced swelling, high flammability and poor compatibility with polymer matrixes. An efficient

strategy to overcome this, besides of providing further functionality, is through incorporation of other

sub-components such as poly(amino acids) (Paper I14), metal oxide nanoparticles (Paper II), oil (Paper

III), zeolites (Paper IV15) or metal organic frameworks (Paper V16).

An accepted definition of a hybrid (from the materials chemistry point of view), is a material that

comprises two or more components blended on the molecular scale involving chemical bonds among

them17. In this manner, the components act synergistically complementing each other, thus providing

new functionalities that the components by separated do not possess. The physical properties of the

hybrids essentially depend on i. The chemical composition of the phases (nanocellulose, metal organic

frameworks, zeolites, etc.), and ii. The type of interaction between the components that forms the hybrid

(chemically or physically crosslinked). Different hybridization strategies can be carried out depending

on the desired end-use, for instance to prevent cell-adhesion (Paper I), provide antimicrobial/antioxidant

properties (Paper II & III) , or to tailor specific functionalities, such as CO2 capture16 (paper IV & V).

1.3. Scope of this thesis

This thesis is motivated by the urgent demand to develop innovative and functional nanomaterials based

on natural resources, utilizing the advantageous properties of renewable materials. My overall aim is to

develop a platform of sustainable and highly-functional hybrid materials based on cellulose nanofibrils

towards functional applications, such as water purification, CO2 capture and food packaging. All

strategies were designed to be as eco-friendly as possible.

Several strategies are proposed to hybridize cellulose nanofibril materials (in the form of 2D-films and

3D-foams) via different techniques such as: i) surface-initiated free-radical polymerization (Paper I); ii)

in situ growth of metal-oxide nanoparticles or metal organic frameworks (Paper II & IV), or iii) inclusion

of subcomponents such as zeolites or oil (Paper III & V). The research work in this thesis covers the

whole spectra of preparing novel functional materials: processing of CNF-based structures, surface

modification, unravelling the intermediate pathways during hybridization by in situ techniques, and

evaluate the performance of the resultant materials in separation or packaging applications.


4

2. Preparation of materials and characterization

The material preparation and characterization are summarized in this chapter, and detailed information

is available in the corresponding articles in Paper I to V hereby attached.

2.1. Preparation of materials

Extraction of cellulose nanofibrils. The cellulose nanofibrils were extracted from

softwood fibers (Norwegian spruce, 95% cellulose, 4.5% hemicellulose and 0.1% lignin) as

provided by Domsjö Fabriker AB, Sweden. The cellulose fibers were mechanically

defibrillated via an ultrafine friction grinder (Masuko Supermasscolloider, model MKZA

10-15J Corp., Japan) at 1500 rpm, passing 7 times a 2 L suspension with 2 wt% fibers.

TEMPO-mediated oxidation of cellulose nanofibrils. TEMPO-oxidized cellulose

nanofibrils (TOCNF) were produced by subjecting the cellulose pulp to TEMPO-mediated

oxidation following the procedures reported from Isogai et al.,18. After the oxidation, the

pulp was subjected to mechanical defibrillation (5 passes) using a homogenizer (500 bar),

yielding TOCNF with an average diameter of 5-10 nm.

Preparation of CNF-stabilized Pickering emulsions. The Pickering emulsions were

prepared by suspending the CNFs (previously dispersed using a LM5-A Silverson

laboratory mixer (Silver, USA) with a 1 mm screen hole at 3500 rpm for 5 min), and after

oil addition they were emulsified using a microfluidizer MS110S (MF), Microfluidics Corp,

USA, operating at a pressure of 1200 bar.

Manufacturing of membranes. Membranes were prepared by vacuum filtration of a

mixture containing cellulose nanofibrils (1 wt%) and cellulose fibers (2 wt%) in a 1:0.12

ratio using a Buchner funnel with an area of 143 cm2. The membranes were then dried at

room temperature for 2 days under a load of 5 kg.

Preparation of CNF-based oleofilms. Certain amount of CNF-stabilized Pickering

emulsions was degassed to remove any bubbles, poured into Petri dishes and left to dry at

35 ˚C for 24 h.

Fabrication of CNF-based foams. Foams were prepared using the unidirectional freeze-

casting method: a suspension of (15g, 0.5wt%) TOCNF (or ZIF-L@TOCNF) was mixed

with gelatin via a high-speed disperser (5g, 4wt%), and the corresponding amount of sorbent

nanomaterial. Then, the suspensions were cooled at 4 ˚C for 1 h, degassed and freeze-casted

using Teflon molds sitting on a copper plate in contact with dry ice. Followed by freeze-

drying.

Grafting of poly (cysteine methacrylate). (1) Initiator immobilization: Three pre-

manufactured membranes (around 0.15g each) were submerged in a 20 mL solution of

cyclohexane contained in a glass vessel covered by a rubber septum under nitrogen

atmosphere. Then, 3-methacryloxypropyltrimethoxysilane (4 µmol) was injected into the

reaction system, together with n-propylamine (3.1 µmol). The mixture was then stirred for

24 h at 60 ˚C, and washed repeatedly with cycles of cyclohexane, ethanol and water, and

finally vacuum-dried. (2) Surface-initiated polymerization: Pre-synthesized cysteine

methacrylate was submerged in 50 mL water together with the modified membranes, and

0.11 µmol of KPS under nitrogen atmosphere. The mixture was stirred at 70 ˚C for 2 h, and

washed with ethanol and water repeatedly.

In-situ growth of ZIF-L onto cellulose nanofibrils. TOCNF (15g) was added into a 50

mL beaker, followed by the addition of zinc hexahydrate (0.49 mmol), TEA (0.8 mmol) and

2-methylimidazole (7.8 mmol). The mixture was stirred for 2 minutes and left for 1 h for

reaction and the resultant material was purified by washing repeatedly with deionized water

and filtering with filter paper to yield ZIF-L@TOCNF.


5

2.2. Characterization methods

Atomic force microscopy (AFM). AFM micrographs were acquired using a Multimode V,

Veeco Instruments (Santa Barbara, CA, USA) in tapping mode. During the measurements,

the height, amplitude and phase images were recorded using the probe (Model: ScanAsyst-

air, Bruker) under Peak Force tapping mode. The collected data were processed via the

NanoScope Analysis 1.5 (Bruker) software.

Scanning electron microscopy (SEM). The morphology of the samples was analyzed

using a scanning electron microscope Jeol (JSM-7000, Japan) or TEM3000 (Hitachi, Japan)

at accelerating voltages from 5 to 15 kV. Prior imaging, the samples were coated by sputter

coating (JEOL, JFC-1200) with gold for 30 seconds.

Surface ζ-potential. The surface charge of the films was investigated as a function of pH

using a SurPASS electrokinetic analyzer (Anton Paar, Graz, Austria). The films were put in

an adjustable gap cell with a gap width of 100 𝝁m. An electrolyte solution of KCl (1

mmol/L) was pumped through the measuring cell and the pressure steadily increased to 300

mbar. The pH was controlled by titrating HCl and KOH (0.05 mol/L).

Fourier transform infrared spectroscopy (FTIR). Fourier-transform infrared spectra

were measured using attenuated total reflectance Fourier-transform infrared (ATR-FTIR,

Varian 610-IR, UK) in the range of 4000-390 cm-1.

FTIR mapping imaging. The measurements were performed using a Bruker FTIR

microscope HYPERION 3000, coupled to a research spectrometer, VERTEX 80. The

microscope was equipped with two types of detectors: a single element MCT-detector

(Mercury Cadmium Telluride) and a multi-element FPA-detector (Focal Plane Array). The

multi-element FPA-detector consists of 64 × 64 elements covering a sample area of 32 × 32

μm.

X-ray diffraction (XRD). Powder X-ray diffraction (PXRD) patterns were recorded using

a PANalytical X´Pert PRO X-ray system with current, tension, temperature and step size of

40 mA, 45 kV, 25 ˚C and 0.05 respectively.

Ultraviolet-visible spectroscopy (UV-VIS). The Ultraviolet-visible spectra were recorded

using a Thermo Scientific GENESYS UV-vis Spectrophotometer and using quartz cuvettes

for the measurements.

X-ray photoelectron spectroscopy (XPS). XPS analyses were carried out in an Axis Ultra

DLD electron spectrometer (Kratos Analytical Ltd., UK.) using a monochromatized A1 Kα

radiation source operating at 150 W and an energy of 20 eV for individual photoelectron

lines. The binding energies were referenced according to the C 1s peak at 284.6 eV and the

spectra were peak-fitted using a Shirley background subtraction and multiple Gaussian

distributions.

Tensile/compression testing. Tensile and compressive strength of samples were measured

using an Instron mechanical testing machine at a crosshead speed of 1 mm/min with a gauge

length of 20 mm. The measurements were carried out at 50% RH and 25 ˚C.

Contact angle. The wettability of the films was measured in static mode with a KSV

instrument, CAM 200, equipped with a Basler A602f camera. All measurements were

performed at a temperature of 23 ˚C and relative humidity of 40%.

BET porosimetry. The specific surface area and average pore diameter of the materials

were determined from N2 adsorption measurements using a Micromeritics ASAP 2000

instrument at 77 ˚K using BET and BJH models respectively.

Quartz-crystal microbalance with dissipation monitoring (QCMD). QCMD

measurements coupled with Nanoplasmonic Spectroscopy (NPS) were carried out using the

Insplorion Acoulyte (Insplorion AB, Göteborg, Sweden) attached to the QCMD E1 system,

Q-Sense AB. The measurements were done in sensors (provided by Insplorion and


6

containing gold nanodiscs of about 100 nm coated by a SiO2 layer on top) coated with a

monolayer of TOCNF. The flow rate of water during the measurements was 0.200 mL/min.

The real-time kinetics of adsorption was calculated considering that the rate of adsorption

is proportional to the difference in the saturation concentration (M0) and the instant value

of the adsorbed ions (M) at a time t, so that the exponential relation between the adsorbed

mass and time is as follows (Equation 1), where k (sec-1) is the growth constant. Further

details are given in Paper II.

𝑀(𝑡) = 𝑀0[1 − exp(−𝑘𝑡)] (1)

Small angle X-ray scattering. In-situ SAXS experiments were carried out at micro- and

nano-focus X-ray scattering (MiNaXS) beamline P03 at synchrotron source PETRA III,

DESY, Germany. The X-rays of wavelength (λ) 0.09 nm with a beam size of about 20 x 20

𝝁m2 were shot to the samples and the scattering patterns were recorded via a 2D-pixel

detector (Pilatus 300k). The sample-to-detector distance was of 1.5m. The measurements

were carried out in a custom-made vacuum filtration cell where a membrane is hermetically

sealed to filtrate aqueous solutions while perpendicularly shoot the X-ray beam. A

schematic representation of the experimental setup is shown in Figure 2.1. Specific details

regarding the SAXS data analysis can be found in Paper I & II.

Figure 2.1. Schematic representation of experimental setup used for the synchrotron-

based small angle X-ray scattering measurements

Antibacterial and antifouling activity. The antibacterial properties were assessed against

Staphylococcus aureus or E.coli. The antifouling capability was studied via biofilm

formation or by elucidating the protein adsorption using a solution of bovine serum albumin

(BSA). Fluorescein diacetate (FDA) was used to quantify the biofilm formation. The

fluorescence was measured with a fluorometer Fluoroskan Ascent FL. 200 mL of FDA

(0.02% (w/w) in DMSO) were spread over the entire surface of the samples. After 15 min

of pre-incubation at 25 ˚C, FDA was excited at 485 nm and its emission recorded at 538

nm. The visualization of the bacterial cells and biofilms was performed by CLSM after 24

h inoculation using a Leica Microsystems Confocal SP5 fluorescence microscope. Viable

and non-viable bacteria were tracked using a Live/Dead BacLight Bacterial Viability Kit.

For the staining of the membrane, the surface of the samples was covered with 30 mL of

stain (0.5:1 mixture of SYTO 9 and PI in DMSO). For green fluorescence (SYTO 9, intact

cells), excitation was performed at 488 nm and emission recorded at 500– 575 nm. For red


7

fluorescence (PI, dead cells), the excitation/ emission wavelengths were 561 nm and 570–

620 nm, respectively. For matrix visualization, the biofilms were stained with 200 mL of

FilmTracer SYPRO Ruby per film, incubated in the dark for 30 min at room temperature,

and rinsed with distilled water. For matrix staining, the excitation/emission wavelengths

were 450 nm and 610 nm, respectively. Biofilm formation was also visualized by SEM.

BSA adsorption was performed using PBS solution as a buffer. The proteins were dissolved

in 0.01 M PBS solution at a concentration of 1 mg/mL and were quantified and visualized

using a Qubit Protein Assay Kit in a fluorometer/luminometer, Fluoroskan Ascent FL, and

with a Leica Microsystems Confocal SP5 fluorescence microscope, respectively. For BSA

characterization, the excitation/emission wavelengths were 485 nm and 592 nm,

respectively. Further details are given in Paper I, II & III.

Dye-removal. The capacity of the materials to remove dyes from aqueous solutions was

determined using methylene blue (MB) as reference system. For the measurements, a

known amount of dye was incorporated into a glass vial containing the sample to evaluate.

The dye solution was measured using UV-vis spectroscopy at determined times of

irradiation. The efficiency of dye removal was calculated using equation 2. Further details

are given in Paper II.

%𝑑𝑦𝑒𝑟𝑒𝑚𝑜𝑣𝑎𝑙 =𝐴0 − 𝐴𝑡

𝐴0× 100 (2)

CO2 adsorption studies. The CO2 adsorption studies were carried out by two methods: i)

Volumetric gas sorption analysis using a Micromeritics ASAP 2020 adsorption analyzer.

For the measurements all the samples were evacuated at 100 ˚C for 10 h before the analyses.

ii) Thermogravimetric gas adsorption analyses were carried out in a TA instruments

Discovery thermobalance using LABLINE 5.5 CO2, 5.0 N2 gases. The CO2 adsorption was

estimated by the weight difference when switching from N2 to CO2 atmosphere, using a

flow of 100 ml/min. Further details are given in Paper IV & V.


8

3. Cellulose nanofibrils-based hybrid films for water

purification

A prominent application of cellulose nanofibrils is in the fabrication of 2D porous films for purification

of water19–23. CNFs are excellent building blocks for membrane technology due to the ease of

processability by simple methods such as vacuum filtration where the properties can easily be tuned by

calibrating the aspect ratio of the fibrils, thickness of the film or carefully controlling the processing

method, as different pressure/drying stage yield different morphology and architecture24. However,

some challenges need to be overcome in order to implement CNF-based membranes in applications.

One of the main drawbacks of CNF-based membranes is the adsorption of proteins and adhesion of cells

to the cellulose surface, which happens inherently and leads to an exponential flux loss and subsequent

membrane failure. Moreover, neat CNF-based membranes also lack functionality to efficiently remove

pollutants from water streams. A way to improve the performance of CNF-based membranes and

overcome the aforementioned issues is through hybridization with other functional sub-components. In

this chapter we introduce two different strategies to improve CNF-based membranes for water

purification: i. a novel strategy to graft zwitterionic aminoacid-based polymer brushes via surface-

initiated polymer grafting of poly (cysteine methacrylate) and ii. The in situ growth of metal oxide

nanoparticles from CNF films, which we demonstrate to happen spontaneously during the adsorption of

metal ions from aqueous solutions.

3.1. Paper I: Zwitterionic cellulose nanofibril-based membranes with

superior antibacterial and antifouling properties

One of the main impediments for cellulose nanofibrils to be used in membrane technology applications

is the non-specific adsorption of proteins and unwanted adhesion of cells (referred to as fouling) which

leads to dramatic flux-loss and subsequent membrane failure. To overcome this, we proposed a cost-

effective strategy that consists in the grafting of amino-acid based zwitterionic polymer brushes from

cellulosic membranes. The method consists of two stages: 1) the immobilization of a free-radical

initiator via a sylilation reaction, and 2) the surface-initiated free-radical polymerization (SI-FRP) of

cysteine methacrylate (CysMA). Our proposed method allows to homogenously insert brushes of

poly(aminoacids) on surface of membranes, and without the need of the typically required (toxic) metal-

based catalysts. Two different modified membranes with different grafting density were synthesized and

denoted as ZM-1 and ZM-2 (ZM-2 with the highest grafting yield). The method is schematically

represented in Figure 3.1.

Figure 3.1. Conceptual schematic representation of the surface-initiated polymer grafting of P(CysMA) on CNF-

based membranes. (Reprinted with permission14)

3.1.1. Surface properties of the membranes

Upon grafting of PCysMA, the modified-membranes exhibit zwitterionic behavior, which signifies that

at acidic pH the amine groups on the surface are protonated and ionized yielding positive zeta potential


9

values, while a negatively charged surface is shown at basic pH. On the other hand, both functional

groups were ionized at pH 5.3, at which the zeta potential is zero, which is very close to the isoelectric

point of cysteine (5.07). The pH dependence of the surface charge of the modified- and unmodified

membranes is shown in Figure 3.2. It’s worth to mention that this isoelectric point is very close to the

pH of human skin, around 5.5, which suggests a prominent potential to use this approach in wound

dressing applications.

Figure 3.2. Streaming potential of the PCysMA-grafted zwitterionic membranes.

The successful grafting of PCysMA on the membranes was demonstrated by FTIR spectroscopy and

mapping. The resultant spectra are shown in Figure 3.3a. PCysMA grafted-membranes exhibit an

additional IR signal between 1700 and 1760 cm−1 which correspond to the characteristic stretching

vibration of C O from PCysMA. ZM-2 display a higher peak intensity confirming the expected higher

grafting yield. Moreover, the homogenous distribution of the polymer brushes on the surface was

demonstrated via FTIR mapping of ZM-2 (Figure 3.3b). The results display three different regions on

the membrane, shown in different colors, which indeed solely indicate the different surface roughness

of the membranes, as in each of the different spots the peak corresponding to the carboxyl group of

PCysMA at 1700 cm−1 is present. These observations demonstrate the efficiency of the proposed grafting

technique that yields a rather homogeneous grafting density throughout the surface of the membrane.

The modification was further proved by X-ray photoelectron spectroscopy and shown in Paper I.

Figure 3.3. FTIR spectra and mapping of PCysMA-grafted membranes (mapped area = 32 x 32 𝝁m). (Reprinted

with permission14)

b)

a)


10

3.1.2. Antifouling performance of PCysMA-grafted membranes

Zwitterionic polymers have repeatedly demonstrated the capacity to inhibit fouling formation on

surfaces in an efficient manner25–27. The mechanism of zwitterionic polymer brushes to prevent protein

adsorption arise from the combination of different factors, including solvation effect, conformational

entropy and their electrical neutrality (pH dependent)26,28. We evaluated the antifouling performance of

the PCysMA-grafted membranes, and the results are shown in Figure 3.4. From the CLSM images

(Figure 3.4j-l), it can be observed that upon grafting of PCysMA, the modified membranes exhibit a

significant improvement in suppressing the protein adsorption. More specifically the relative

fluorescence decreased from 70.3 (unmodified membrane) up to 11.2, so that the protein adsorption

reduced in 84% (for ZM-2).

In addition, we studied the inhibition of biofouling formation on the surface of the PCysMA-grafted

membranes. The SEM and CLSM micrographs (Figure 3.4a-i) show the growth, adhesion and viability

of S. aureus on the surface of the membranes before and after modification. The SEM micrographs were

completely in agreement with the ones obtained with FilmTracer SYPRO Ruby biofilm matrix staining,

where in both cases the biofilm formation was significantly suppressed after modification. The relative

quantification of S. aureus biofilm formation, based on its metabolic activity, was reduced by 86.92%

in ZM-2, compared to the unmodified one. In addition, our zwitterionic membranes also exhibit

antimicrobial properties, as suggested by the significant decrement in viability of the cells that adhere

to the surface (Figure 3.4g–i). This is shown by the reduction of cells and by the fact that the few

remaining ones were clearly PI-marked as non-viable ones (in red). The ZM-2 membrane display the

better performance in terms of preventing abiotic fouling and biofouling, which suggests the significant

influence of grafting density over antifouling performance.

Figure 3.4. Biofilm formation of S. aureus after 24 h of bacterial incubation shown by SEM (a-c) and CLSM

micrographs (d-i); Qubit Protein Assay Kit showing the BSA adsorption (j-l) on the surface of the unmodified

and PCysMA-grafted membranes using: (d-f) FilmTracer SYPRO Ruby biofilm matrix staining. (g-i) live/dead

double staining cell viability staining. (Reprinted with permission14)


11

3.2. Paper II: Hybrid films based on cellulose nanofibrils/metal oxide

nanoparticles formed spontaneously during adsorption of metal ions

Nanocellulose is known to promote the in situ growth of different nanoparticles, acting as a universal

substraste16,29,30. The mineralization of metal oxide nanoparticles is of special interest, as they are an

important class of semiconductors with a wide range of applications, such as adsorption, electronics,

sensors and catalysis. Even though the hybridization of cellulose nanofibrils with metal oxide

nanoparticles has been achieved before31–36, the current processes require hazardous chemical reduction

methods or hydrothermal procedures34,37. In this work, we demonstrate the ability of TEMPO-oxidized

cellulose nanofibrils (TOCNF) to spontaneously grow metal oxide nanoparticles in situ during the

adsorption of transition metal ions. Although this phenomenon seem to be universal for different metal

ions (Cu2+, Ag+ and Fe3+), most part of this work was dedicated to study the formation of copper oxide

nanoparticles (Cu2O) after its interaction with Cu(NO3)2 aqueous solutions at different concentrations.

The existence of the formed nanoparticles on TOCNF was demonstrated using microscopy, X-ray

diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), and the results are shown in Figure 3.5.

Figure 3.5. In situ growth Cu2O@TOCNF nanoparticles. (a-b) AFM micrographs revealing the surface

topography of TOCNF film after adsorption of metal ions (phase images) (c) Crystalline phase of Cu2O

nanoparticles revealed by XRD. (d) High-resolution Cu2p3 XPS spectra of Cu2O@TOCNF hybrid film.

The presence of the nanoparticles embedded on the surface of the TOCNF films, denominated upon

hybridization as Cu2O@TOCNF was demonstrated by AFM (See Figure 3.4a-b). The nanoparticles

exhibit a diameter of about 26 nm (bigger aggregates of size >100 nm can also be observed at lower

magnifications). The presence of the nanoparticles was further corroborated by XRD (Figure 3.5c)

where the resultant pattern display the characteristic diffraction peaks at 35 and 60˚ corresponding to

the (1 1 1) and (2 2 0) planes respectively of the standard cubic cuprite structure of copper38. However,

the Cu2O diffraction peaks were greatly overshadowed by the predominant peaks of cellulose I

polymorph39. The oxidation state of the Cu2O nanoparticles was also confirmed via high-resolution

Cu2p3 XPS spectra (Figure 3.5d), where two sharp peaks could be observed at 934.5 and 954.5 eV,

attributed to Cu 2p3/2 and 2p1/2 respectively40. The almost undistinguishable satellite features of Cu2p (at

943 eV, marked in fig 3.5d with black arrows) demonstrate the oxidation state of the nanoparticles to

correspond to Cu(I) oxide, considering that Cu (II) shows very remarked signals.

930 940 950 9600

360

720

1080

1440

1800

Inte

nsi

ty (

cps)

B.E. (eV)

Cu2p1/2 and Cu23/2

Cu2p1/2

Cu2p3/2

20 30 40 50 60 700

3800

7600

11400

15200

19000

34,5 36,0

3250

3900

59 60 611200

1800

2400

Inte

nsi

ty (

a.u

.)

2q

JCPDS 78-2076

(111) (220)

200 nm

200 µm

a b

c d


12

3.2.1. Mechanism of formation of in situ growth Cu2O nanoparticles

Through combining a series of cutting-edge in situ techniques (i.e., Quartz-crystal microbalance with

dissipation monitoring, UV-vis spectroscopy, and synchrotron-based small angle X-ray scattering) we

unraveled the structural evolution of the Cu2O@TOCNF nanoparticles during formation. These methods

allow us to propose a plausible mechanism of the intermediate pathways of assembly and the results are

shown in Figure 3.6. A schematic representation of the in situ growth of Cu2O nanoparticles is presented

in the Figures 3.6a-c.

Figure 3.6. (a-c) Conceptual schematic illustration of adsorption of metal ions on TOCNF (a), reduction of

copper ions to Cu(I) by aldehyde groups on TOCNF (b) and growth of copper oxide nanoclusters (c). (d) QCMD

analysis during copper ions adsorption (Cu[NO3]2, 800 mg/L). (e) Adsorbed mass as a function of time time at

different metal ion concentrations (determined by Sauerbrey equation, employing third overtone). (f) UV-vis

spectra of TOCNF at different times (using 400 mg/L of copper nitrate solution. (g) 3-dimensional plots [I(Q)

vs. Q vs. time] and (g) 2-dimensional [I(Q) vs. Q] plots during drying of TOCNF films after copper adsorption.

(h) Size evolution of the Cu2O nanoparticles during drying.

i) Adsorption of Cu2+ ions. The adsorption of metal ions on TOCNF first occurs due to the interaction

between the negatively charged carboxyl groups of TOCNF and the positively charged metal ions

(Figure 3.6a). The carboxyl groups act as anchor groups via electrostatic interactions and covalent

bonding41. To demonstrate the adsorption of copper ions on TOCNF in real time, we carried out QCMD

measurements, where the frequency shift is directly proportional to the adsorbed mass onto

nanocellulose (calculated using the Sauerbrey equation). In this way, the adsorbed Cu2+ ions at different

metal salt concentration can be estimated, as well as gather information regarding the real-time kinetics

of adsorption (Figure 3.6d-e). The results in Figure 3.6e show the efficient and rapid adsorption of the

0,00 33,33 66,67 100,00 133,33 166,67-1680

-1400

-1120

-840

-560

-280

0

-100

0

100

200

300

400

500

600

700

Df

(Hz)

Time (min)

F3

F5

F7

D3

D5

D7

DD

(1

0-6

)

0 500 1000 1500 2000 25000

700

1400

2100

2800

3500

Adso

rbed

mas

s (n

g/c

m2)

Time (sec)

100 mg/L

200 mg/L

400 mg/L

800 mg/L

a

c d e

b

f g

Ab

sorb

ance

330300270240210

Wavelength (nm)

4

5

3

2

1

0

h

0 100 200 300 400 500

8

12

16

20

Pa

rtic

le S

ize

(nm

)

Time (sec)


13

copper ions onto TOCNF showing a similar growth constant (k) at all low metal salt concentrations,

however an abrupt increase is observed for 800 mg∙L-1, at which a significantly faster rate of ion

adsorption is observed compared to the other concentrations. Nevertheless, the saturation value linearly

increases as a function of metal salt concentration, attributable to the shifting equilibrium between the

carboxyl groups of TOCNF and metal ions in solution.

ii) Growth of nanoclusters. The adsorbed Cu2+ ions are converted into nanoclusters presumably by

reduction from the aldehyde groups present on TOCNF, to form Cu+. It’s worth to mention that this

reaction between aldehyde groups and Cu2+ typically requires elevated temperatures, and the fact that

the phenomena reported herein happen at standard room conditions indeed suggest that the anchoring

of the Cu2+ ions by the carboxyl groups of TOCNF can significantly promote the reduction reaction,

probably by converging the ions with the aldehydes, therefore increasing the collision probability. It’s

worth to mention that nanocellulose have previously shown the potential to catalyze other chemical

reactions42,43. Moreover, after reduction, the high concentration of metal monomers end-up in

coalescence leading to nanoclusters (metal monomers tend to bind to other metal atoms forming higher-

mers due to their instability). This phenomenon was verified using UV-vis spectroscopy (Figure 3.6f)

of a TOCNF film during adsorption of Cu2+ ions. The results exhibit the gradual formation of a spectral

feature in the wavelength range of 240 nm. Furthermore, the UV-absorbance increases exponentially as

a function of time, together with the appearance of a shoulder at longer wavelengths that indeed suggests

the growth of the nanoclusters and their increase in population. The size of the nanoclusters were found

to be in the range between 2-5 nm, as estimated by the QCMD measurements.

iii) Coalescence and growth of nanoparticles. The nanoclusters, formed upon reduction of copper

ions, rapidly oxidize and self-aggregate mainly via van der Waals forces 44, to then crystallize and form

primarily Cu2O nanoparticles. This suggests the capacity of TOCNF to stabilize the nanoparticles from

subsequent oxidation and to partially prevent coagulation.

We tracked the in situ growth of the nanoparticles in real-time via synchrotron-based SAXS

experiments, elucidating the structural transformations of the films during adsorption of copper ions

from Cu(NO3)2 solution (100 mg∙L-1) using a custom-built low vacuum filtration cell where the copper

salt solution is vacuum filtered through a cellulosic membrane, while shooting the X-rays

perpendicularly. The growth of the nanoparticles is mainly observed during the drying of the already

wet film (after filtration of the metal salt solution) and the SAXS data is presented in Figure 3.6g-h. The

[I(Q) vs. Q vs. time] SAXS spectra show an evident change in scattering that evolves as a function of

time, extended up to the low Q region, with typical features of the form factor governed scattering. The

data were also fitted by combining the contributions from wet cellulose film and nanoparticles. From

this, it was possible to estimate an approximate of the time-variation in nanoparticle diameters, as shown

in Figure 3.6h. The nanoparticles grow gradually as a function of drying time, and the sizes are in the

range of 10-20 nm after drying, which are consistent the AFM micrographs (Figure 3.5a).

3.2.2. Functionality of the resultant hybrid Cu2O@TOCNF hybrid films

The functionality of the formed Cu2O@TOCNF hybrids, resulting from adsorption of Cu2+ ions and

subsequent formation of nanoparticles, was studied in terms of dye-removal capacity from aqueous

solutions and antimicrobial properties, elucidating their performance upon mineralization. The results

are shown in Figure 3.7.


14

Figure 3.7. (a) UV-vis spectra of MB in the presence of hybrid film under UV-light irradiation. (b) Removal (%)

of MB (20 ppm) during UV-light irradiation. (c) TOCNF film inoculated with E. coli and (d) L. innocua. (e)

Cu2O@TOCNF hybrid film inoculated with E. coli and (f) L. innocua. Scale bar 100 μm.

The ability of Cu2O@TOCNF to remove dyes from aqueous solutions was studied under UV-light

irradiation (MB was chosen as model system), considering that Cu2O nanoparticles can adsorb dyes as

well as photo-catalyze their degradation45–47. The results, shown in Figure 3.7a-b, illustrate the dye-

removal (%) as a function of time, which proves that the presence of Cu2O@TOCNF led to almost

complete MB disappearance from the aqueous solution after 8 min. The presence of Cu2O nanoparticles

increase the dye removal capacity of TOCNF in nearly 60%.

On the other hand, the antibacterial properties of Cu2O@TOCNF films were studied against Gram-

negative (E. coli) and Gram-positive (L. innocua) bacteria. Cu2O nanoparticles exhibit excellent

antibacterial properties due to the possible disruption of biochemical processes inside the bacterial cells 48–50. The antibacterial properties were determined using the Live/Dead Cell Double Staining Kit, and

visualized in the confocal microscope, the results are shown in Figure 3.7c-f. It can be clearly observed

that the number of dead cells (in red) is much higher than the number of live cells (in green) for

Cu2O@TOCNF hybrid membranes. No significant difference was observed between Gram-positive and

Gram-negative bacteria. This clearly demonstrates that the growth of Cu2O nanoparticles provides

significant antibacterial properties against both classes of bacteria. Our insights demonstrate the ease of

preparation of multi-functional hybrid materials upon simply adsorption of metal ions, and introduce a

multi-stage use of nanocellulose-based materials via upcycling the materials after use.

0 3 6 9 12

0

20

40

60

80

100

dy

e re

mo

va

l -

%

Time (min)

Cu2O@TOCNF

TOCNF

MB

400 500 600 7000,0

0,5

1,0

1,5

2,0

2,5

3,0

Ab

sorb

an

ceWavelength (nm)

t = 0

t = 3 min

t = 4 min

t = 5 min

t = 6 min

t = 7 min

t = 8 min

t = 13 min

c

e f

d

a b


15

4. Cellulose nanofibril-based oleofilms for active food

packaging

Owing to the urgent impetus to replace petroleum-based packaging materials, during the last decades,

extensive research effort has been destined to reduce packaging-waste by using bio-based materials from

renewable sources.51 Nanocellulose, especially in the form of nanofibrils, has emerged as a key

alternative for packaging purposes due to its outstanding mechanical properties, biodegradability,

transparency, low cost and renewable nature.52 Several approaches for nanocellulose-based packaging

materials have been developed during the last years, nevertheless, developing a functional packaging

material with no need of chemical modifications or additives that increment the environmental impact,

remains still a challenge.52

4.1. Paper III: Cellulose nanofibrils-based oleofilms as active edible

barriers

An emerging method to prepare novel biomaterials is the use of nanocellulose-stabilized Pickering-

emulsions as building block for different materials (e.g. foams and microcapsules). Pickering emulsions

are emulsions stabilized by (nano) particles that adsorb in the interface between the two phases.

Nanocellulose, due to its amphiphilicity, shows the capacity to stabilize emulsions, and taking advantage

of this, herein we report for the first time the production of CNF-based oleofilms. We studied the

properties of the resultant oleofilms and highlight their potential as active food packaging materials. The

preparation of the oleofilms was carried out by simply solvent evaporation of the emulsions (with

variable oil-content) in a Petri dish. The conceptual schematics of the preparation process of the

oleofilms is presented in Figure 4.1. The stability of the emulsions is explained by depletion effects of

the non-adsorbing CNF due to their amphiphilic nature. Soybean oil was selected as oil phase, as it is

food-grade and it contains well-known nutritional and antioxidant properties, owing to the high

concentrations of genistein, vitamin E, essential fatty acids and lecithin which are typically present in

this vegetable oil. 53,54

Figure 4.1. Conceptual schematic illustration of the preparation of CNF-based oleofilms with low or high oil-

content. (Reprinted with permission55)

4.1.1. Physical properties of the CNF-based oleofilms


16

The influence of the oil-content over the microstructure of the oleofilms was studied by scanning

electron microscopy and the results are shown in Figure 4.2. From the micrographs, it is observable that

a rigid layer of nanofibrils appears on the surface of the oleofilms (Figure 4.2a-d), while the oil phase

remain mainly present between layers of nanofibrils, which forms what it appears as a multi-layered

“sandwich-like” structure. This layered structure can be observed at the highest oil concentrations,

where multiple layers of oil and nanocellulose can be appreciated (Figure 4.2e-h). The micrographs also

demonstrate the homogeneity and smoothness of the oleofilms surface, which does not display any

apparent pores or cracks (Figure 4.2a-d).

Figure 4.2. SEM micrographs of the cellulose nanofibril-based oleofilms with variable oil-content (a-d). CNF

amount was constant in all samples (0.5 wt. %). (Reprinted with permission55)

The resultant oleofilms are indeed transparent (as illustrated in Figure 4.3a), and flexible, which are

properties required for food packaging application. The macroscopic properties of the oleofilms were

analyzed to elucidate the influence of oil in the performance of the films. The transparency of the films

was studied by UV-vis spectroscopy (Figure 4.3b) and the results demonstrate that the transparency

decreases upon incorporation of oil, which is because of the higher refractive index of oil compared to

air and water which surround the nanofibrils in pristine CNF films. The transparency further decreases

as a function of oil-content, which is explained merely by the increase in thickness of the oleofilms, as

proved by the linear increase in mass attenuation coefficient in Figure 4.3b (inlet). The oleofilms also

display enhanced absorptivity at wavelengths below 300 nm (see Figure 4.3c) at the highest oil

concentrations, which arises from the UVB and UVC-blocking potential of soybean oil. This property

is important in food-packaging related applications considering that UV light may generate fat auto-

oxidation, degradation of vitamins and off-flavors development.


17

Figure 4.3. Physical properties of CNF-based oleofilms: (a) photograph of oleofilms (87 wt % oil); (b) optical

transmission at the visible-wavelength range and mass attenuation coefficient of at λ = 700 nm; (c) optical

transmission in the UV-spectral range; (d) experimental and fitted dynamic contact angle of oleofilms; (f–e)

uniaxial tensile properties and (g–i) thermal properties of oleofilms with different oil contents. (Reprinted with

permission55)

The hydrophobicity of the oleofilms increases as a function of oil-content (Figure 4.3d), demonstrating

the presence of oil on the surface of the oleofilms which suggest the improvement in water-resistance

of the materials. The mechanical properties of the films (measured from uniaxial tensile testing, Figure

4.3e) also display direct dependence on the oil-content, as the material become more and more ductile

upon oil incorporation. This behavior suggest that the film can adsorb higher amount of energy before

rupture, which is an important property for an adequate design of packaging-destined films. The thermal

degradation profile, measured by thermogravimetric analysis (TGA) (Figure 4.3g-i), evidence that the

thermal stability of the oleofilms enhances gradually in the presence of oil, owing to the higher thermal

stability of the soybean oil phase, compared to nanocellulose. The TGA results also illustrate the lower

moisture uptake upon incorporation of oil (seen in the %-weight loss at 100 ˚C), which is essential for

food packaging applications, as adsorbed water promotes bacterial-growth.

Moreover, as shown in Figure 4.4, the oleofilms are furthermore easily re-dispersable. By re-submerging

them in water and only 2 minutes of being submitted to a sonication bath, the films are return back to

the original emulsion form, which indeed suggests the edibility of the films, as they should rapidly

disintegrate once ingested, besides of also proving their high-degree of environmental friendliness.

Reference

16 wt% oil

33 wt% oil

50 wt% oil

66 wt% oil

<

83 wt% oil

87 wt% oil

a) b) c)

d) e)216 243 270 297 324

0

12

24

36

48

60

%-T

ransm

issio

n

Wavelenght (nm)

SB3

SB2

SB1

CNF

SB7

SB4

f)

0 20 40 60 80 100

3,36

3,78

4,20

4,62

5,04

Str

ain

at

failu

re (

%)

Oil content (wt.-%)

48 72 96 120 14488

90

92

94

96

98

100

Temperature (oC)

Weig

ht -%

288 312 336 360 38440

50

60

70

80

90

Temperature (oC)

Weig

ht -%

288 312 336 360 38440

50

60

70

80

90

Temperature (oC)

We

igh

t -%

288 312 336 360 38440

50

60

70

80

90

Temperature (oC)

Weig

ht -%

g) h) i)

Wavelength (nm)

0 1 2 3 4 50

20

40

60

80

100

120

140

160S

tress (

MP

a)

Strain (%)

Reference

33 wt.-% oil

66 wt.-% oil

83 wt.-% oil

87 wt.-% oil

100 200 300 400 5000

20

40

60

80

100

Temperature (oC)

We

igh

t -%

Oil content (wt%)0 13 26 39 52

30

45

60

75

90

Time (sec)

Conta

ct a

ngle

()

0 wt% oil

16 wt% oil

50 wt% oil

66 wt% oil

87 wt% oil

0 20 40 60 80 10040

45

50

55

60

65

10 20 30 40 50 60 70 80 9052

54

56

58

60

62

Att

enu

atio

n c

oef

. (c

m2/g

)

Oil content (wt%)

μ (

cm2

/g)

Oil wt-%


18

Figure 4.4. Re-dispersability of the oleofilms in water after 2 min of sonication. (Reprinted with permission55)

4.1.2. Encapsulation of bioactive compounds

Taking advantage of the presence of oil in the oleofilms, we encapsulated a hydrophobic bioactive

compound in order to introduce additional functionalities to the materials. For this, we selected

Curcumin (diferuoyl methane), which is a major component of Curcuma longa, also known as turmeric,

as a model system. Curcumin possess efficient antioxidant properties, as well as in vitro antimicrobial

potential against a wide range of microorganisms including fungi56, as well as several Gram-positive

and Gram-negative bacteria57.

Figure 4.5. Curcumin-containing oleofilms: a) Photograph of curcumin-loaded oleofilm (left side); (b-e) Radical

scavenging activity evaluated by DPPH method. (b-d) UV-vis spectra of DPPH in the presence of oleofilms: b)

Reference, c) oleofilm with 50 wt% oil content and d) oleofilm with 86 wt% oil content. (e) RSA as a function of

oil-content of curcumin-loaded oleofilms. (Rendered with permission55)


19

After curcumin incorporation, the oleofilms exhibit a homogenous yellow tone which suggests an

efficient distribution of curcumin within the oil-phase of the materials (Figure 4.5a). The improvement

in antioxidant activity of the oleofilms after incorporation of curcumin was assessed using the method

based on the scavenging of the DPPH radical molecule, at which the antioxidant activity is calculated

by following the decrease in the absorbance band at 517 nm (see Figure 4.5b-d), which is the maximum

absorbance band of DPPH ethanolic solution. The results clearly show a significant increase in

antioxidant properties of the curcumin-containing oleofilms, which increments as a function of oil

content, as observed in the Figure 4.5e.

Curcumin is an ideal candidate for inhibition of pathogen growth in food packaging due to its

antibacterial and antifungal properties58. We assessed the effect of the presence of curcumin on the

antimicrobial properties of the films against E. Coli, a common food born pathogen (using Live/Dead

double staining method and CLSM). The results (Figure 4.6) suggest that the bacteria growth and

viability is significantly inhibited upon addition of curcumin, even at low concentrations.

Figure 4.6. CSLM overlay images of showing the viability of E. coli 24 hours after inoculation. Live cells are in

green and dead ones in red. Oleofilm without curcumin (a) was used as reference sample. Oleofilms with

curcumin, which oil concentrations are respectively: 16 wt.-% (b), 25 wt.-% (c), 50 wt.-% (d), 66 wt.-% and 87

wt.-% (f). Scale bar 250 µm. (Reprinted with permission55)


20

5. Cellulose nanofibril-based hybrid foams for selective

CO2 capture

Due to the alarming rate at which the concentration of CO2 is increasing in the atmosphere and its

influence over the undergoing global climate change59, there is a critical need to develop new

technologies to reduce industrial CO2 emissions. These technologies must be inexpensive while

following the principles of sustainability. Porous sorbent materials, such as zeolites60 and metal organic

frameworks (MOFs)61, have are prominent alternatives to solve this problem. Nevertheless, there are

still several challenges with common sorbents that need to be overcome prior to their large-scale

utilization, such as their poor processability into desired shapes considering that they are commonly in

powdered crystalline state and the use of binders normally leads to pore blockage.

The use of bio-based foams, which can act as support sorbent materials, appears as a key alternative

towards the industrial implementation of sorbents for CO2 capture; considering their super-porous

structure, biodegradability, low cost and extremely low densities 20. In this chapter, we introduce two

different strategies to prepare cellulose nanofibrils/gelatin-based hybrid foams loaded with sorbent

materials for selective capture of CO2: i. a hybrid foam which exhibits the highest loading capacity of

zeolite (silicalite-1) ever reported in the literature and ii. A hybrid foam prepared by in situ growth of

leaf-like ZIF-L nanoparticles.

5.1. Paper IV: Biobased foams with ultra-high zeolite loadings for

selective CO2 capture We developed hybrid foams that can support ultra-high loadings of sorbent nanomaterials; in this case,

silicalite-1 was used as model system. The matrix of the foams consist of gelatin and TOCNF. For the

preparation, a gelatin suspension is first mixed with TOCNF (previously mixed with silicalite-1), to then

be homogenized and cooled down for 1 hour, as it is known that gelatin undergoes a first order thermo-

reversible gelation transition at low temperatures, recovering partially the collagen triple-helix structure

by disorder-order rearrangements62,63. TOCNF are embedded in the gelatin matrix, forming a strong

network, which is further reinforced by electrostatic interactions (see Figure 5.1). The resultant gel, after

the cooling stage, is then submitted to unidirectional freezing-casting, to obtain the resultant hybrid

foams. A schematic representation of the hybrid foams´ composition is presented in Figure 5.1. EDX

analysis also demonstrate the homogenous distribution of the silicalite-1 embedded in the

TOCNF/gelatin matrix.

Figure 5.1. Conceptual schematic illustration of the network between gelatin/TOCNF and silicalite-1 and the

EDS mapping showing the components distribution in the hybrid foam. (Reprinted with permission15)

TOCNF

C O SiN

Gelatin

Silicalite-1


21

5.1.1. Physical properties of the hybrid foams The hybrid foams display the conventional honeycomb pore structure, which is composed of macropores

(pore diameter > 50 nm) of about 40 µm (see Figure 5.2) resulting from the unidirectional freeze-casting

process at which an anisotropic tubular pore structure is formed. The cell walls of the foams are

interconnected with each other through mesopores64 (not observable in the SEM images), which is

believed to improve the gas diffusion through the foam. The presence of the zeolite in the foams appear

as aggregates at high concentrations, however they did not apparently alter the anisotropic structure of

the foams.

Figure 5.2. SEM micrographs of hybrid foams with different mass fractions of silicalite-1. Right column

corresponds to cross-sectional micrographs. (Reprinted with permission15)

The influence of silicalite-1 content on the pore structure of hybrid foams was studied by BET, and the

corresponding specific surface areas, pore volumes and size distributions are presented in Figure 5.3.

The presence of silicalite-1 lead to significant increase of the porosity and specific surface area of the

foams (see Figure 5.3a-c), which exhibited a quasi-linear relationship between the specific surface area

and the zeolite content. The results confirm that the zeolite particles can be accommodated within the

foam walls without significant pore blockage and therefore the specific surface area increases by

incrementing the amount of silicalite-1 in the foams. The presence of the silicalite-1 introduced

Ref

eren

ce3

3 w

t%5

0 w

t%8

0 w

t%90

wt%

100 µm 50 µm

50 µm

50 µm

50 µm

50 µm

100 µm

100 µm

100 µm

100 µm

150 µm

150 µm

150 µm

150 µm

150 µm


22

microporosity (pore diameter < 2 nm), as it can be observed in the pore size distributions (see Figure

5.3d) where a steep increase and a peak are found below 10 Å and at 25 Å, respectively.

Figure 5.3. (a) Specific pore volume; (b) specific microporous surface area; (c) specific surface area; and (d)

pore size distribution in the hybrid foams. (Reprinted with permission15)

5.1.2. CO2 adsorption capacity of the hybrid foams

The application of the hybrid foams for the selective capture of CO2 was investigated and the results

are shown in Figure 5.4a. The presence of silicalite-1 significantly increases the specific adsorption

capacity of CO2, increasing linearly as a function of zeolite content (see Figure 5.4b). The introduction

of zeolite lead to a significant increase of the porosity and specific surface area of the foams (see Figure

5.4c), which has a strong effect on the number of CO2 molecules that can be adsorbed within the

framework as shown in Figure 5.4a.

The silicalite-1 content and the CO2 adsorption capacity exhibit a linear relationship (see Figure 5.4b),

which confirms that the porous support composed of gelatin/cellulose nanofibrils does not significantly

block the pores of the zeolites or limit the diffusion of the gas molecules to the zeolite particles. The

accessibility of gas molecules to the pores of the zeolites is attributed to the foam porosity and pore

interconnectivity.

The adsorption of N2, also measured by volumetric gas adsorption (see Figure 5.4c-d), was

significantly lower than CO2 adsorption. This suggests a potential application of the hybrid foams for

gas separation of methane-rich gases and hydrogen fuels. The low degree of N2 adsorption is likely a

result of the low polarizability of the N2 molecule65.

10 20 30 40 50 60 70 800,000

0,002

0,004

0,006

0,008

Reference

50 wt% zeolite

75 wt% zeolite

85 wt% zeolite

Po

re V

olu

me

(cm

³/g

·Å)

Pore Width (Å)

33 wt% zeolite

Silicalite-1

0 20 40 60 80 100

0

50

100

150

200

250

Mic

rop

oro

us

are

a (

m2/g

)

Zeolite content (wt%)

a)

c) d)

b)

8.0

6.0

4.0

2.0

0.0

x 10-3

0 20 40 60 80 100

0

50

100

150

200

250

300

350

400

450

Su

rfa

ce a

rea

(m

2/g

)


0 20 40 60 80 1000,00

0,05

0,10

0,15

0,20

0,25

Po

re v

olu

me

(mL

/g)


0.25

0.20

0.15

0.10

0.05

0.00


23

Figure 5.4. a) CO2 adsorption isotherm and b) CO2 capture capacity of hybrid foams as a function of zeolite

content. C) CO2 and N2 adsorption of the foam consisting of 85wt-% silicalite-1; d) amount of CO2 and N2

adsorbed at 1 bar absolute pressure. (Reprinted with permission15)

The reusability and long-term usage of the hybrid foams for CO2 capture was studied by means of

gravimetric adsorption measurement, by subjecting them to repeated CO2/N2 adsorption/desorption

cycles in a TGA instrument at room temperature monitoring the weight-% variation (Figure 5.5a). The

apparent CO2/N2 selectivity was studied by comparing the gravimetric response of CO2 and N2

adsorption, respectively (Figure 5.5b). Overall, the reusability of the foams is extremely efficient, as

only a minor decrease in adsorption capacity could be observed even after 6 cycles. Regarding the

selectivity, it linearly increases as a function of zeolite content, attributed to the higher affinity of

silicalite-1 for CO2 molecules compared to N2.

0 100 200 300 400 500 600 700 8000,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

Am

ou

nt

ad

sorb

ed (

mm

ol/

g)

Absolute Pressure (mmHg)

CO2

N2

0 33 50 66 75 85

0,0

0,5

1,0

1,5

Am

ou

nt

ad

sorb

ed (

mm

ol/

g)

CO2

N2

Materials

0 20 40 60 80 1000,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

CO

2 c

ap

ture

(m

mol/

g)

Zeolite content (%)

0 150 300 450 600 7500,0

0,3

0,6

0,9

1,2

1,5

Absolute Pressure (mmHg)

66 wt% zeolite

75 wt% zeolite

Silicalite-1

CO

2 A

dso

rbed

(m

mo

l/g

)

85 wt% zeolite

Reference

50 wt% zeolite

b)

c) d)

Ref.

33 wt%50 wt%

66 wt%75 wt%

85 wt%

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

a)1.5

1.2

0.9

0.6

0.3

0.014 28 42 56 70

0,0000

0,0016

0,0032

0,0048

0,0064

0,0080

Reference

50 wt% zeolite

75 wt% zeolite

85 wt% zeolite

Po

re V

olu

me

(cm

³/g·Å

)

Pore Width Range (Å)

33 wt% zeolite

Silicalite-1

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

1.5

1.0

0.5

0.0

Zeolite content (wt-%)

Zeolite content (wt-%)


24

Figure 5.5. Gravimetric gas adsorption measurements of the hybrid foams: (a) CO2/N2 adsorption cycles

measured by TGA. (b) Adsorption selectivity of foams with variable zeolite content. (Reprinted with

permission15)

5.2. Paper V: Biobased hybrid foams prepared by in situ growth of ZIF-

L towards selective CO2 capture

Metal organic frameworks are highly-functional porous nanomaterials with high-porosity degree and

high surface area. Some MOFs have the suitable pore size/shape to selectively accommodate CO2

molecules, such as ZIF-L, which make them an attractive alternative towards the development of high-

performance adsorbent materials. In this work, we utilize the versatility of cellulose nanofibrils to be

surface modified by in situ growth of ZIF-L. Then, the modified nanofibrils were mixed with gelatin,

with which they form a strong network (as shown in the previous sub-chapter), which upon freeze-

drying can be translated into robust porous foams with the capacity to selectively capture of CO2.

Figure 5.6. Schematic representation of the preparation of ZIF-L@ CNF/gelatin hybrid foams. (Reprinted with

permission16)

The schematic representation of the method used for the synthesis of the hybrid foams is presented in

Figure 5.6. Three different foams with different ZIF-L content were prepared (MF-1 < MF-2 < MF-3).

0 33 50 66 85 100

0

5

10

15

20

25

30

35

CO

2/N

2


a)

b)0 20 40 60 80 100 120

0

1

2

3

4

5

6

Wei

gh

t-%

Time (min)

Reference

33 wt% zeolite

66 wt% zeolite

75 wt% zeolite

85 wt% zeolite

Silicalite-1


25

The successful modification was confirmed by X-ray diffraction (XRD) which shows the appearance of

the diffraction peaks corresponding to the highly crystalline ZIF-L material (Figure 5.7)

Figure 5.7. XRD pattern for TOCNF, MF-1, MF-2 and MF-3. (Reprinted with permission16)

5.2.1. Microstructure and performance of the hybrid foams Scanning electron microscopy confirmed the formation of anisotropic foams containing macropores,

which arise due to the ice crystal formation during the unidirectional freeze casting process used to

manufacture the foams (Figure 5.8a), besides of the mesopores (pore diameter between 2-50 nm)

interconnecting the cell walls. The SEM images also reveal the presence of white particles that refer to

ZIF-L (as confirmed by EDS mapping, Figure 5.8c-d). The foams were ultralight but mechanically

robust, as they were able to withstand the pressure of 500g (Figure 5.8b).

Figure 5.8. (a) SEM micrographs showing the microstructure of the anisotropic hybrid foams. (b) Photographs

of the foam on a stigma of a flower and 500g object on the top of the hybrid foam. (c-d) EDS mapping of zinc at

the top and edge of the foams. (Reprinted with permission16)


26

5.2.2. Capture capacity of CO2 and selectivity ZIF-L crystals have a large zero-dimensional pore or cavity with dimensions of 9.4 Å × 7.0 Å × 5.3 Å,

where the cushion-shaped cavities of ZIF-L ensure higher selectivity for CO2 adsorption. The CO2

sorption take place via a gate opening mechanism66. These cavities offer flexible adsorption compared

to the tetrahedral structure of other ZIFs.

The influence of in situ growth ZIF-L crystals over the capture of CO2 and its selectivity was studied.

The results are shown in summarized in Figure 5.9. It can be observed that upon growth of ZIF-L, an

important increase in CO2 sorption capacity, reaching almost the value for pure ZIF-L crystals in the

case of the highest loading (MF-3). The N2-adsorption-desorption capacity of the foams was studied

where it was observed that the adsorption capacity of the foams was reduced significantly upon

incorporation of ZIF-L (Figure 5.9a). This behavior is due to the fact that, ZIF-L, in comparison to ZIF-

8, present far lower porosity, although exhibit superior CO2 adsorption capacity due to the cushion-

shaped cavities that ideally accommodate CO2 molecules, which makes it a promising alternative for

gas separation (Figure 5.9b).

Figure 5.9. (a) CO2 adsorption and desorption cycles at 100 mL/min flow rate. (b) CO2/N2 apparent selectivity

of hybrid foams. (Reprinted with permission16)


27

6. Conclusions

This thesis contributes to the development of functional nanomaterials based on natural resources, taking

advantage of the outstanding properties of cellulose nanofibrils and hybridizing with other

subcomponents to provide further functionality. The methods here presented introduce innovative eco-

friendly approaches to tackle current environmental problems to ensure sustainability via water

purification, gas separation and creating biodegradable active food packaging.

A novel surface modification technique of cellulosic films was introduced by surface-

grafting an amino acid based polymer (PCysMA). The insertion of the dense polymer brushes led to an

important enhancement of the antifouling performance of the films, as well as introducing important

antimicrobial properties. Additionally, we mechanistically demonstrated the capacity of carboxylated

cellulose nanofibrils films to spontaneously form metal oxide nanoparticles during the adsorption of

ions from water, and how important properties such as dye-removal from water and antimicrobial

properties are introduced. Moreover, a novel method to examine the structure evolution of films in

operando was developed, by means of synchrotron-based SAXS.

A straightforward strategy to develop highly-mechanically stable hybrid foams

consisting of gelatin and cellulose nanofibrils was introduced. The robust structure of the foams allows

the ultra-high loading of sorbent nanomaterials, such as zeolites or metal-organic frameworks, which

provide high and selective CO2 adsorption capacity.

Lastly, the development of nanocellulose-based oleofilms was first introduced by using

oil-in-water CNF-stabilized Pickering emulsions as building blocks. Moreover, this approach allow us

to encapsulate bioactive hydrophobic compounds which can potentially increase the storage life of food

by introducing antioxidant and antimicrobial properties.


28

7. Outlook

The proposed surface-initiated polymer grafting technique of cellulosic films here introduced could be

exploited in several applications. There are a wide-range of polymers that could be grafted, each of

which would yield different properties. Furthermore, the method here developed to understand the

structural transformations and formation of nanoparticles in operando could be used to study

mineralization of different inorganic materials in organic matrixes. Nevertheless, there is still a lack of

theoretical understanding (e.g., how the reduction of the metal monomers occur) and therefore in situ

spectroscopic studies and computational simulations could be a great contribution.

The hybrid foams with ultra-high loading capacity could be exploited for diverse applications via the

uptake of different nanoparticles. For instance catalysis or separation of gases other than CO2. Moreover,

the potential of these foams at high gas flux should be tested prior their implementation in large-scale

applications.

Different types of oil-phases could be used to prepare CNF-based oleofilms. UV-curable oil, for

instance, could produce ultra-strong materials. Additionally, multiple hydrophobic compounds could be

encapsulated towards different applications such as drug-delivery.


29

8. Sammanfattning

Nanocellulosa har under senare tid setts som en universiell källa för utvecklandet av hållbara material

på grund av dess bio-ursprung och en kombination av framträdande särdrag, bl.a. goda mekaniska

egenskaper, anisotropi och mångsidig ytkemi. Därtill kan nanocellulosa, i form av nanofibriller, anta

varierande strukturer och morfologier beroende på framställningsteknik, såsom aerogeler, filmer eller

monoliter.

Begränsningar finns dock som hindrar införandet av nanofibriller av cellulosa i

tillämpningar i ”verkliga livet”, som växelverkan med bakterier och proteiner, vilket kan leda till

nedsmutsade ytor, eller vattenorsakat svällande. Ett sätt att bemästra dessa utmaningar, och införa

ytterliggare funktionalitet, är genom hybridiseringsstrategier, där ett flertal komponenter samverkar för

att få särskilda egenskaper och tillämpningar. Denna avhandling har som mål att lägga fram flera olika

strategierer för syntes av nya nanofibrillbaserade hybridmaterial av cellulosa, i form av 2D filmer eller

fast 3D skum, riktade mot deras användning i uthålliga tillämpningar som vattenrening, adsorption av

CO2 och aktiva ätbara barriärer.

En ny metodik för funktionalisering av ytor hos nanofibrillmembran av cellulosa föreslås,

innefattande en beläggning med zwitterjoniska polymerborstar av polycystinmetaakrylat.

Modifikationen kan undertrycka absorption av protein upp till 85%, såväl som minska vidhäftningen av

bakterier upp till 87%, samtidigt som antibakteriella egenskaper införs, vilket har visats mot S. Aureus.

Den spontana bildningen in situ av funktionella nanopartiklar av metalloxider på

nanofibrillfilmer av cellulose, utan ytterliggare användning av kemikalier eller temperatur, har

undersökts. Noterbart är att denna process inte bara möjliggör återbruk genom flerstegstillämpningar,

utan ger också en kostnadseffektiv metod för framställning av multifunktionella hybridmaterial med

förbättrade färgborttagnings/antimikrobiella aktiviteter.

Framställning av funktionella kompositfilmer från nanofibrillstabiliserade

Pickeringemulsioner av cellulosa, och deras lämplighet för användning som aktiva ätbara barriärer, har

demonstrerats. Olja i filmerna finjusterade deras egenskaper och verkade också som ett medium för

inkapsling av bioaktiva hydrofoba föreningar, vilket gav ytterliggare funktionalitet som antioxidantiva

och antibakteriella egenskaper.

Anisotropa porösa fasta hybridskum med ultrahög upptagningskapacitet av adsorbenter

(t. ex. zeoliter och metallorganiska ramverk) tillverkades genom en enkelriktad frysgjutningmetod och

användning av cellulosananofibriller/gelatin som templatmaterial. De fasta skummen uppvisade

verkligen ultrahöga upptagningskapaciteter för nanomaterialadsorbenter, med ett linjärt samband mellan

adsorbentinnehåll och adsorptionskapacitet för CO2 och en hög CO2/N2 selektivitet.


30

9. Acknowledgements

I first want to thank my supervisor Prof. Aji Mathew for all the support and trust during the last 3 years.

It was a great decision to have joined your group, I have learnt enormously during this time! I also want

to thank Prof. Krassimir Velikov, who not only mentored me during my industrial internship, but also

taught me a lot (perhaps without realizing) about good leadership and project management.

I want to thank my co-supervisor Prof. Lennart Bergström for the mentoring during the last years. I have

learnt a lot from you. Also thank Prof. German Salazar for your support and trust during the last years.

It´s nice to see that is possible to be an academic person and remain cheerful and simple.

More importantly, I want to thank my great friends and colleagues Dimitrios, Andrea, Blanca, Pierre

and Jinqin. I am very lucky to have met you guys, you are fantastic. Also, I want to thank my good

friend, mentor, collaborator etc., Sugam Kumar, nothing in this thesis would have been the same if I

hadn’t met you.

I also thank Walter Rosas, Susana Monti and Emma Nomena, I have really enjoyed working with you,

and I consider you my friends. You´re awesome.

I would like to thank Prof. Niklas Hedin, Prof. Jiayin, Prof. Mats, Jekabs, Kjell, Zoltan, Cheuk-Wai and

Mirva thanks a lot for all the support during this time. Also my colleagues in MMK, Sahar, Chuantao,

Natalia, Martin, Vera, Atefe, Konstantin, Yingxin, Vahid, Xia, Valentina, and everyone else in the

department. And my Unilever friends Roland and Braulio, it´s been fantastic to spend some time with

you.

I would to thank my MULTIMAT friends: Deniz, Peter, Hanglong, Paula, Remco, Pallahbi and Gerardo.

It has been fantastic to meet you and have a lot of fun together.

Finally, I want to thank my lovely family and dear friends Biyu, Armando and Rishab for all your

support, love and trust. Also I want to thank Denis, nothing would be the same without you.

The European Comission project MULTIMAT (H2020-MSCA-ITN-2014, Grant No. 676045) is

acknowledged for the financial support provided during my doctoral studies.


31

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(6) Rånby, B. G. The Colloidal Properties of Cellulose Micelles. Discuss. Faraday Soc. 1951, 11

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