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Nanoscience and Nanotechnology Applied to Art Conservation: Improved Oddy Test Using Silver Nanoparticle Sensor

Honors Undergraduate Thesis Submitted to:

The Mellon College of Science Chemistry Honors Committee Mellon College of Science

5000 Forbes Avenue The Carnegie Mellon University

By:

Laura Moussa Department of Chemistry

Spring 2007

Approved by:

Dr. Paul Whitmore, Advisor Art Conservation Research Center

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Acknowledgements

I would like to thank Dr. Paul Whitmore and Dr. Hannah Morris for providing me the

opportunity to work in the Art Conservation Research Center located at the Pittsburgh

Technology Center. I also greatly appreciate the help I received from my mentor Dr. Rui Chen

with her support and help all throughout my project, Dr. Rongchao Jin and Dr. Alexander

Ryabov for their teachings and answering any questions I had, and Joseph Suhan for obtaining

the TEM images. I would also like to thank Karen Stump for her support and guidance through

my years as a student at Carnegie Mellon University and finally the Art Conservation Research

Group for providing such a wonderful work environment. Without the aforementioned people,

this project would not have been possible.

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Abstract

Damage to metal artifacts from the materials used for display and storage cases is of great

concern to art museums. The current technology used, the “three in one” Oddy Test, tests for the

suitability of these construction materials by placing silver, copper, and lead metal coupons in

one container along with the material at 60°C and 100% relative humidity. After a 28-day period

the metal coupons are assessed for any visual changes. There are many shortcomings to this

simple test. It is time consuming, irreproducible, slow, hard to evaluate and most importantly

gives no quantitation. A new way to test for the suitability of these materials using a silver

nanoparticle sensor is described here. Two types of silver nanoparticle shapes, spherical and

triangular, were self-assembled using polyethylenimine (PEI) onto a glass coverslip to produce

two different sensors. The spherical nanoparticle sensor gave a yellow color and when evaluated

for its sensitivity to hydrogen sulfide gas changed colors from yellow to colorless. The

triangular nanoparticle sensor gave a blue color and when evaluated for its sensitivity to

hydrogen sulfide gas changed colors from blue to colorless. Color changes were followed

through UV-Vis spectrophotometry, which showed a decrease in absorption of the initial

characteristic peak after exposure to hydrogen sulfide. Kinetic studies were performed on the

spherical and triangular nanoparticles, and the reaction rates were determined to be first-order

with k = 0.0002 for the triangular nanoparticles and k = 0.0001 for the spherical nanoparticles.

When compared to the Oddy Test, both the spherical and triangular silver nanoparticle sensors

reacted fully by showing the characteristic color change before the 28-day period of the test. The

nanoparticle sensor will allow for high sensitivity, easy evaluation, and quantitative analysis for

corrosive gases that would react with silver.

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Contents

Chapter 1-Introduction 6

1.1 Current Methodology………………………………………………………...………6

1.1.1 Oddy Test ……………………………………………………………….....6

1.1.2 Other Tests……………………………………………………………..…..7

1.2 Principles of Operations……………………………………………………………..8

1.2.1 Color Theory…………………………………………………………..…..8

1.2.2 Reaction with hydrogen sulfide gas………………………………………11

Chapter 2- Sensor Fabrication 14

2.1 Synthesis of Metal Nanoparticles- A colloidal system………………………...……14

2.1.1 Spherical Ag nanoparticles from aqueous solution………………………16

2.1.2 Ag spherical nanoparticle synthesis in organic solvent……………………….…18

2.1.3 Triangular Ag nanoparticles from aqueous solution………………….…19

2.2 Self-Assembly…………………………………………………………………...…20

2.2.1 Silane coupling agents……………………………………………………21

2.2.1.1 Acid Washing Procedure………………………………….……21

2.2.1.2 Monolayers assembly…………………………………………..21

2.2.1.3 Multilayer assembly …………………………………………...22

2.2.1.4 Results and Discussion…………………………………………23

2.2.2 PEI-Coupling……………………………………………………………..25

2.2.2.1 Base Washing Procedure………………………………………25

2.2.2.2 Monolayer………………………………………………………25

2.2.2.3 Multilayer………………………………………………………26

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2.2.2.4 Results & Discussion…………………………………………26

2.3 Summary………………………………………………………………………. …29

Chapter 3-Sensitivity of Nanoparticles to Hydrogen Sulfide 30

3.1 Products and Kinetics……………………………………………………….………33

3.1.1 Verification of products…………………………………………….………33

3.1.2 Kinetic Studies…………………………………………...…………………37

Chapter 4-Performance evaluation of Ag nanoparticle sensors

4.1 Stability………………………………………………………………..……………42

4.1.1 Silane-Coupled Sensors……………………………………………………43

4.1.2 PEI-coupled sensors………………………………………………….…….46

4.1.3 Summary…………………………………………………..………………..53

4.2 Comparison to conventional Oddy Test…………………………………...………..54

Chapter 5- Conclusion and future work 59

References 61

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Chapter 1-Introduction

1.1 Current Methodology

Damage to metal artifacts from the environment is of great concern to art museums.

Everything from the carpeting on the floor to the paint on the wall can possibly produce

corrosive vapors that will destroy these artifacts. For this reason, all materials with which the

artwork may be in contact must be tested.

1.1.1 Oddy Test

Currently, the “three in one” Oddy Test is the method most commonly used in museums

to assess the effects of materials on metal artifacts. Andrew Oddy, Keeper of Conservation at

the British Museum, developed the original Oddy Test in 19731. The original Oddy Test tested

for gases hazardous to copper, silver, and lead using three different set ups for each type of metal

coupon. Each set up enclosed the construction material to be tested along with a coupon of the

metal in a sealed glass vessel. Moistened cotton placed in a small vial would be added to the

vessel to produce 100% relative humidity conditions. The whole set up would then be sealed

and placed in an oven at 60ºC for four weeks. At the end of the time period the metal coupons

are checked for evidence of corrosion based on any observed changes in color, texture, or luster.

With these observations and a comparison to control vessels, the material tested would be

grouped into one of three categories: Permanent use, Temporary use, or Unsuitable use.

To further improve the Oddy Test, the “three in one” Oddy Test2, 3 was introduced and is

the current technology in use. The procedure simplified the original Oddy Test by placing all

three metal coupons in the same container and using a different sealing method to keep the vials

airtight, preventing loss of water vapor or other gasses that might be released (Figure 1).

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Metal Coupons (Silver, Lead, Copper)

Figure 1: Variant Oddy Test Set-Up with all three metal coupons and the material to be tested

Both the original and variant Oddy tests are very easy to use and the set up is not

complicated. The 28-day time period also gives a good indication of what can be expected to

occur to the artifacts from several months to years. However, along with the advantages to this

simple test there are also many shortcomings. One disadvantage is how time consuming and

slow the test is; sometimes museums need answers within a few weeks and cannot wait a month

to find out whether or not a material is suitable. The Oddy Test is also irreproducible and hard to

evaluate; glassware variations and any slight variation in the set up itself can cause different test

results. The main disadvantage is that the Oddy test provides only qualitative information based

on a subjective visual analysis leading to discrepancies in the results.

1.1.2 Other Tests

Material to be tested

Water

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Many other visual and chemical methods to test for corrosive gas emission have been

mentioned in conservation literature, but are not widely used due to their irreproducibility. The

Iodide-Azide Test4 is a spot-test that detects the presence of reducible sulfides in materials.

Measurements of pH are also used to give an idea about acid content of materials5. A

Chromotropic Acid Test tests for the presence of aldehydes (formaldehyde) and the Iodide-

Iodate sensor tests for the presence of volatile acids6. These two tests are known as timesaving

options to the Oddy Test. Many others are also used but they all have disadvantages due to their

limitation in only detecting a specific type corrosive agent.

1.2 Principles of Operations

1.2.1 Color Theory

To improve upon the disadvantages of the current methodology, the use of metal

nanoparticles developed as sensors to detect off-gassing of display materials was explored.

These sensors were compared against current methodology to determine if they had improved

sensitivity, allowed for quantitative analysis and allowed easy visual analysis. The use of metal

nanoparticles, specifically silver nanoparticles, will be explored here in developing a metal-based

nanoparticle sensor that will test for the suitability of materials in art museums. Silver is

tarnished by sulfur containing gases and much research has already been performed behind both

the synthesis and assembly of silver nanoparticles. Metal nanoparticles are extremely interesting

in that they do not show exactly the same characteristics as bulk materials. For one, they show

characteristic color changes depending on the size and shape of the metal nanoparticle7. These

optical properties of nanoparticles offer the prospects for easy, quantitative evaluation, and thus

as a basis for a gas sensor.

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The color of these nanoparticles is determined by the surface plasmon resonance8. The

surface plasmon resonance is the frequency at which conduction electrons are displaced relative

to the nuclei in response to the alternating electric field. The electric field induces a dipole in the

nanoparticle and in effect a unique resonance wavelength is seen. Silver is an example of a

metal that contains plasmon resonances in the visible spectrum. The visible spectrum can

roughly be taken as a sum of blue, green and red. For spherical Ag nanoparticles, they absorb at

~400 nm, which is blue light. Therefore, when one looks at the colloids or a glass slide with Ag

nanoparticles, only the complementary light (Green + Red) is seen by the eye. In the spherical

silver nanoparticle case the color is yellow (Green + R = yellow). Similarly, triangular Ag

nanoparticles mainly absorb in the red, ~700 nm, so Green + Blue = Cyan, which is close to blue.

The Drude model9 of free electrons describes the mechanics of the electrons inside a

metal, which are considered free and independent. It states that the motion of a whole cloud

equals the sum of the motion of the individual electrons, which is most perfect embodiment of

plasmon absorption (Figure 2). The coupling between the electrons is considered as a maximum

with the electrons all acting in phase.

Figure29: The displacement of the conduction electron cloud relative to the nuclei due to the electromagnetic wave.

Electric Field Metal Spheres

Force of electron

Force of electron

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The Mie Theory is the simplest theoretical explanation for predicting the optical

properties of nanoparticles. It is a solution of Maxwell’s equations for the scattering of light by

spherical particles. The Mie theory takes on the assumptions that the material is linear,

homogeneous, and isotropic. In the equation below10:

!

E(") =24# 2

a3$m

32

" ln(10)

$i(")

($r

+ %$m)2

+ $i

2

&

' (

)

* +

Eqn 1: Mie Theory equation

E(λ) is the extinction (viz., sum of absorption and scattering) magnitude, a is the radius of the

metallic nanosphere, εm is the dielectric constant of the medium surrounding the metallic

nanosphere (assumed to be a positive, wavelength independent, real number), λ is the

wavelength of the absorbing radiation, εi is the imaginary portion of the metallic nanosphere’s -

dielectric function, εr is the real portion of the metallic nanosphere’s dielectric function, and χ is

the term that describes the aspect ratio of the polycrystalline domains. For spherical

nanoparticles χ = 2, but χ increases as the aspect ratio of the nanoparticle increases. The surface

plasmon resonance occurs when the denominator of the term in the brackets approaches zero.

When εI is small and εr = -2εm, the long λ absorption by the bulk metal is condensed into a single

surface plasmon band. For nonspherical nanoparticles the wavelength occurs when εr = -χεm.

The different values inserted into equation 1 will change the surface plasmon resonance

frequency; hence, different sizes and shapes should result in different colors. The Mie theory

exhibits a size dependence of the intensity of the surface plasmon band with changes to the

radius of the metallic nanosphere. The radius has an intrinsic effect on the dielectric constant of

the metal, which in effect causes a position and bandwidth shift. The affect of different shapes

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on the surface plasmon band has yet to be fully explained due to its intrinsic complexity.

Thin films of nanoparticles also show optical properties. The dipole-dipole interactions

between neighboring nanoparticles affect the frequency of a central particle and therefore the

Mie theory cannot be used to explain the optical properties. Effective medium theories11 by

Maxwell-Garnett can account for the optical properties of multi-layered or thin film

nanoparticles. The theory equation of the optical effect does not depend on the size or shape of

the nanoparticles but rather the volume of the nanoparticles and the surrounding medium they are

in. The effective medium theory gives an expression of the dielectric constant within a matrix

containing small metallic spheres. However, these assemblies can become very complex

resulting in multiple colors due to strong nanoparticle-to-nanoparticle interaction when they are

close to each other, resulting in additional absorption peak(s).

Metal nanoparticles and assembled layers will have intrinsic color according to the

theories above. Reaction with gases should change these intrinsic optical properties, and perhaps

size and shape, which would alter the color. These color changes should provide sensitive

quantitative measurements of the extent of reaction.

1.2.2 Reaction with hydrogen sulfide gas

Hydrogen sulfide gas is known to tarnish silver, and for this reason it was used to

examine the effects a corrosive gas would have on assembled silver nanoparticle sensors. When

the corrosion of silver nanoparticles takes place in a closed system, the sulfidation mechanism

reaction would be:

2Ag + H2S Ag2S + H2

Eqn 3: Sulfidation mechanism in a closed system

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In the presence of oxygen, the reaction of the sulfidation mechanism is:

2Ag + H2S + ½ O2 Ag2S + H2O

Eqn 4: Sulfidation mechanism in the presence of oxygen

Wool is composed of many amino acids including cystine, which contains one of the

largest sulfur-containing side groups, and was therefore chosen as the construction material in

the Oddy Tests performed. In the protein chain of wool, cystine linked with tyrosine undergoes

free radical processes (Figure 3). An oxidation reaction occurs between the two, which produces

sulfur radicals. In the presences of moisture in the air, the sulfur radicals form sulphenic acid

side groups and thiol side groups. Upon further exposure to atmospheric moisture, the sulphenic

acid side groups leave, resulting in a cysteine side group and aldehyde side group. During this

process hydrogen sulfide is produced.

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Figure 3: Formation of Hydrogen Sulfide by wool under atmospheric moisture12

It is important to note that the sulfidation reaction is also dependent on the amount of

water on the silver surface since the moisture provides the proper medium for the reaction to

occur with silver13. Therefore, at high humidity the reaction is accelerated.

Figure 4: Schematic diagram of atmospheric corrosion process of silver exposed to H2S environments14

Experiments on the kinetics of hydrogen sulfide with triangular and spherical silver nanoparticles

and on their reaction to wool in the Oddy Test will give light to their sensitivities.

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Chapter 2- Sensor Fabrication

2.1 Synthesis of Metal Nanoparticles- A colloidal system

The current focus is on replacing the silver metal coupon with a silver nanoparticle

sensor. A variety of synthesis routes for silver nanoparticles exist to give various shapes, which

may provide differences in reactivity and ease of detecting color changes. Only spherical and

triangular shapes were evaluated in this study.

Two methods exist for the synthesis of metal nanoparticles, a top down and a bottom up

approach. The top down method is a physical method that utilizes thermal evaporation, laser

ablation, or mechanical grounding. The bottom up approach is a chemical method and is the

method of choice in our synthesis. The reaction to produce colloidal silver is a reduction of the

metal salt silver nitrate, AgNO3, using reducing agent sodium borohydride, NaBH4 (eqn 5).

Another form of preparation of nanoparticles is to use a seeded-growth method,15 which involves

using metal seeds that have already been reduced to grow larger particles. This method allows

for better control of particle size, which could affect sensitivity of the nanoparticles. We have

used the sodium borohydride reduction for making the silver nanoparticles in this study.

Ag+ + e- → Ag0

BH4 - e- → ½H2 + ½B2H6

AgNO3 + NaBH4 → Ag0 + ½H2 + ½B2H6 + NaNO3

Eqn(5): The two half reactions for the reduction of Ag+ using sodium borohydride to give the overall equation

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There are two important stages in colloid synthesis of nanoparticles, which are described

by the LaMer diagram16: Nucleation and Growth (Figure 5). These two stages are responsible

for the size of the metal nanoparticles produced. After reduction of the metal salt, nucleation and

growth begin to produce the nanoparticle. Several factors control the size of nanoparticles

during their growth such as the reducing agent, reaction temperature, glassware, etc. To produce

smaller metal nanoparticles a strong reducing agent is utilized, which allows for a larger number

of nuclei and more stabilizing agent to prevent aggregation.

Figure 5: LaMer Diagram where part I is reduction of metal salt part II is nucleation and part III is growth

The main idea in synthesizing metal nanoparticles is to have a reducing agent along with

a stabilizer and a metal salt. In solution phase nanoparticles, the stabilizers are used to prevent

the particles from aggregating. Three types of stabilizers exist: ions, large molecules, and

ligands. The most commonly used ion stabilizer is trisodium citrate, giving citrate ions. Large

molecules are those such as polymers or surfactants and can be natural or synthetic. Natural

large molecules in synthesis include starch or gelatin. Synthetic large molecules include

polyvinylpyrrolidone (PVP), polyvinyl alcohol and copolymers. An example of a surfactant

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stabalizer is sulfonated triphenylphosphine. Ligand stabilizers include alkanethiol, phosphines,

and amines. These systems provide an electrostatic stabilization, preventing aggregation by

charge repulsion, or steric stabilization, preventing close approach of the nanoparticle with bulky

coating layers.

Controlling the shape of the nanoparticles is very important because different shapes

create different colors. Silver metals are face centered cubic structures whose important crystal

planes are shown in the figure 6 below. The shape of the nanocrystal is mostly dependent on the

rate of growth along <100> and that of <111> plane, the two most stable planes. Capping

molecules such as stabilizers play an important role in determining the direction in which growth

of the nanocrystal occurs. For example PVP will have a more distinct effect on the <100> facets

than on <111> facets17 whereas trisodium citrate can promote growth along the <111> facet18.

Therefore, the combination of stabilizers determines the shape of the nanoparticle. Other factors

that control shape include the reaction temperature or the pH of the solution and the types of seed

used if synthesis is followed using the seeded growth method.

Figure 6: Important crystal planes of fcc nanocrystal shaded in red.

2.1.1 Spherical Ag nanoparticles from aqueous solution

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Spherical Ag nanoparticles were synthesized19 using trisodium citrate as a stabilizer and

sodium borohydride as a strong reducing agent. Using a 250 mL 3-neck round bottom flask,

HPLC grade water (47.5 ml) was deoxygenated by bubbling with nitrogen gas for 30 min. Then,

an aqueous solution of trisodium citrate (0.5 mL, 30mM) and an aqueous solution of AgNO3 (1

mL, 5 mM) were added. Freshly prepared NaBH4 (0.5 mL, 50 mM) was quickly added and the

suspension immediately turned a light yellow color. After 30 seconds, an aqueous solution of

PVP (0.5 mL, 5 mg/mL, Mw= 55,000) was added. The suspension changed to a darker yellow

color after reaction had proceeded for another 30 min. Vigorous stirring took place at ~1200

rpm during the entire process. The as-synthesized dispersion of Ag nanoparticles suspension was

characterized by UV-Vis spectrophotometry in transmittance mode. The absorption band at

~388 nm was assigned to the surface plasmon resonance of the Ag nanoparticles20. A high-

resolution transmission electron microscopy (HRTEM) image of the spherical particles was also

taken and revealed the particle size to be 10 ± 4 nm.

a b c

d

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Figure 7: (a) UV-Vis spectrum of (b) synthesized spherical silver nanoparticles and (c) TEM image. Black bar in TEM image is 10 nm. (d) Histogram data of spherical silver nanoparticles, x-axis is size (nm). 2.1.2 Ag spherical nanoparticle synthesis in organic solvent

To make more concentrated Ag nanoparticle suspensions syntheses were performed in

organic solvents21. Stirring occurred at ~1200 rpm into a 300 mL three-neck round bottom flask.

HPLC grade methanol (25 mL) was injected into the flask immersed in an ice bath, in which

nitrogen gas slowly flowed during the synthesis. PVP (1 mL, 66mg/ml, Mw= 55,000) was added

quickly to the solution. The solution mixed for a couple of minutes before AgNO3 solution in

methanol (2mL, 6.8 mg, 20 mM) was added dropwise into the flask. After 10 minutes NaBH4 in

ice-cold methanol (20.2 mg, 2.5 mL) was added. The suspension was allowed to react for about

15 minutes in the dark. The suspension turned from colorless to a dark red/brown. The

suspension was then aged overnight while stirring in the dark. After the reaction was complete

the suspension was centrifuged twice at max speed for 20 minutes to isolate the nanoparticles.

UV-Vis transmittance spectra of the nanoparticle suspension was measured, and the absorption

band at ~408 nm was assigned to the surface plasmon resonance of the Ag nanoparticles. The

size distribution of the nanoparticles was found through analysis of HRTEM images of particles

distributed on carbon coated copper grids. Histogram data revealed the particle size to be about

28 ± 9 nm.

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Figure 8: (a) UV-Vis spectrum of (b) concentrated spherical Ag nanoparticles in methanol and (c) TEM image along with (d) histogram data, x-axis is size (nm)

2.1.3 Triangular Ag nanoparticles from aqueous solution

For comparison to sensitivity of the spherical nanoparticles, triangular Ag nanoparticles

were also synthesized22. HPLC grade water (50 mL) was added to a 300 mL three-neck round

bottom flask. Stirring at 1200 rpm, trisodium citrate (3 mL, 30 mM) was added and allowed to

mix for about 10 minutes. AgNO3 (1 mL, 5 mM) was added dropwise followed by PVP (3 mL,

16 mg/ 1.5 mL, Mw= 55,000), which was added quickly. H2O2 (120 µL) was then added

followed by freshly prepared NaBH4 (0.5 mL, 100 mM). The reaction took place in the dark.

The suspension changed from a faint yellow color to a light blue indigo color after 20 minutes.

The reaction was allowed to continue for 5 hours. The absorption band at ~626 nm was assigned

to the in-plane dipole surface plasmon resonance and the band at ~330nm was assigned to the

a

b

c

d

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out-of-plane quadrupole resonance of the Ag nanoparticles. The size distribution of the

nanoparticles was found through analysis of HRTEM images of particles distributed on carbon

coated copper grids. A histogram of the data revealed the triangles to be 36 ± 9 nm in size.

Figure 9: (a) UV-Vis spectrum of (b) concentrated triangular Ag nanoparticles synthesized and (c) TEM along with (d) histogram data, x-axis of histogram is size (nm)

2.2 Self-Assembly

The silver nanoparticles were immobilized in layers on glass cover slips using chemical

self-assembly23 methods. Self-assembly is a spontaneous formation of ordered aggregates or

networks of nanoparticles. Electrostatic and chemical self-assembly are the most commonly

a

b

c

c

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used techniques to form both monolayer and multilayer assemblies. Ionic forces govern

electrostatic self-assemblies whereas chemical self-assemblies use covalent bonds between

molecules and nanoparticles. Because chemical self-assembly utilizes stronger interactions

between the glass substrate and the nanoparticles, this technique leads to a more stable structure,

and more robust sensor. Different coupling agents were evaluated to make single and multiple

layers.

2.2.1 Silane coupling agents

2.2.1.1 Acid Washing Procedure

Microscope cover glass slides (No. 2, 18 mm square, Corning) were first cleaned using a

piranha solution (3:1 sulfuric acid: hydrogen peroxide) for 20-30 minutes. They were then

washed three times using methanol and ultrasonicating for 15-20 minutes. The pH of the

methanol solution was then tested to make sure no more acid was present on the glass slides.

The clean glass cover slides could then be stored in the methanol until used.

2.2.1.2 Monolayers assembly

The assembly was accomplished by activating the glass cover slides by ultrasonicating in

methanol for 10 minutes. Then the clean glass slide was placed in a 3-

aminopropyltrimethylsilane (APTMS) (1% v/v in methanol)24 for 24 hours. After the overnight

conditioning of the glass slides, they were removed from the APTMS, rinsed with HPLC grade

water and placed in a water bath for 2 hrs. After the two hours the glass slide was rinsed again

with fresh HPLC grade water. The layer of nanoparticles was then deposited by immersing the

functionalized cover glass slide in the silver nanoparticle suspension overnight. A monolayer of

the spherical silver nanoparticles gave a very slight tint of yellow on the glass substrate and a

very slight tint of blue for the triangular silver nanoparticles.

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2.2.1.3 Multilayer assembly

Multilayers were also analyzed to see if a deeper color on the glass coverslips could be

achieved to allow for an easier visual analysis. The covalent layer-by-layer assembly of the

nanoparticles to produce a multilayer consisted of repetitive dips of the functionalized cover

glass into the nanoparticle suspension and a bifunctional linker, β-mercaptoethanol25. The glass

slide was conditioned with APTMS (10% v/v in methanol) for 1.5 hours and then washed with

methanol for about two minutes followed by water for another two minutes. The slide was then

allowed to soak in a water bath for 2 hours. The slide was removed and rinsed with fresh water

and placed in the silver nanoparticle suspension overnight. Once the first layer of nanoparticles

was assembled, the slide was immersed in solution of 2-mercaptoethanol (BME) for 10 minutes.

BME acts as a coupling agnet and allows for the attachment of a second layer of nanoparticles

onto the first layer. The glass slide was then washed with water for two minutes and then soaked

in a water bath for one hour and placed in the silver nanoparticle suspension again. When the

BME process was repeated along with copious rinsing between immersions, multilayers of the

nanoparticles were achieved giving rise to different colors on the glass slides depending on the

number of layers of nanoparticles assembled.

Figure 10: 13-layers spherical Ag nanoparticles suspended in water self-assembled onto a cover glass slide giving blue-green color

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2.2.1.4 Results and Discussion

The cleaning process using the piranha solution leaves the coverslip surface hydrophilic

and hydroxylised and therefore appropriate for organosilane deposition. During surface

attachment, APTMS reacts with the glass substrate allowing to the alkoxy groups of APTMS to

leave and allowing the amino terminus of APTMS to interact with the silver nanoparticles

leading to the formation of a monolayer. To form additional covalent layer-by-layer assembly,

BME is used as the bilinker between silver nanoparticles.

Figure: Chemical structure of (a) 3-aminopropyltrimethoxysilane (APTMS) and (b) 2-Mercaptoethanol (BME)

An effective sensor would easily allow for determination of a gas reaction occurring

between the sensor and the display material based on an intense color change of the film. A

monolayer assembly provides a minimal amount of color by visual inspection and was difficult

to use. Absorbance in the UV-Vis was very low, and peaked around 0.025 absorbance. Such low

absorbance is accounted for by the fact that organosilane itself is very sensitive to moisture

which causes silane to polymerize and thus lose its useful functional groups for the assembly26

leading to just a few nanoparticles attaching to the glass coverslip. Also it was found that piranha

cleaning method for glass slips was not efficient and reproducible films were hard to make with

this method.

a b

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Figure 11: UV-VIS of monolayer assembly of (a) spherical nanoparticles and (b) triangular nanoparticles

The multilayer on the other hand gives a deep color but only achieved this deep color after

multiple assemblies, which is very time consuming and difficult to keep a uniform assembly

each time. Since the color on the glass coverslip is so dark, a different method of the UV-Vis

must be utilized in which % reflectance is measured. The UV-Vis spectrum below shows a %R

of the multilayer to be around 11.5 %R.

Figure 12: UV-Vis spectrum in reflectance mode of 13-layer assembly of Ag spherical

nanoparticles synthesized in water

a

b a

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2.2.2 PEI-Coupling

Another assembly method using a polymer, polyethylenimine (PEI), was tried next to see

if any improvement could be done on the challenges of using the silane-assembly method. For

example silane assembly required coupling of silver nanoparticles by sulfur-containing

molecules, which could cause problems in determining whether or not the sensors were reacting

to the coupling agent or the sulfur gases in the surroundings. Also, PEI does not polymerize in

the presence of water or lose its functional groups, so its efficiency in coupling silver

nanoparticles to class coverslips would not be compromised.

2.2.2.1 Base Washing Procedure

Glass coverslips were first cleaned using a cotton swab, sparkleen soapy water and

distilled water. Then the glass coverslips were placed in a staining jar (peolco international, 17

mm x 23 mm x 30 mm height) filled with NaOH/ethanol solution (7 g NaOH, 28 mL HPLC

water, 42 mL ethanol). The jar was agitated for two hours in a wrist-action shaker. The

NaOH/ethanol solution was poured out and the glass coverslips were rinsed once with HPLC

grade water. The coverslips continued to be ultrasonicated with HPLC water for four times with

20 minutes each time. The coverslips were then stored in HPLC water until use but sonicated

with fresh HPLC water for 20 minutes before using.

2.2.2.2 Monolayer

To assemble the monolayer of silver nanoparticles on the glass coverslips, the coverslips

were soaked in 1% polyethylenimine (PEI)27 aqueous solution for an allotted time and then

washed with HPLC water bath. The coverslips were then immersed into the Ag nanoparticles

after thoroughly rinsing with water and then methanol only if the nanoparticles were dispersed in

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methanol. The film was then taken out of the nanoparticle suspension and washed again and

allowed to air dry.

2.2.2.3 Multilayer

To obtain a mutilayer, the same procedure was used as the monolayer except the

coverslips were immersed in PEI and the silver nanoparticle suspension alternately with

thorough washing with water for the triangular nanoparticles and both water and methanol for

the spherical nanoparticles.

Figure 13: Multilayer assembly with the polyanion being silver nanoparticles and the polycation being PEI in part B. Part A, 1 is PEI, 2 is HPLC water wash, 3 is Ag nanoparticle suspension, 4 is HPLC water wash. 2.2.2.4 Results & Discussion

The structure of polyethylenimine is shown below. Hydrogen bonding interactions

between the amino groups and the hydroxylised surface of the glass coverslip allow for

attachment of PEI. The amino groups then react with the negatively charged Ag nanoparticles

through electrostatic or ligand donation stabilization to achieve assembly.

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Figure 14: Chemical structure of polyethylenimine (PEI)

Monolayer assemblies using PEI were very efficient and allowed absorbances measured

with UV-Vis as high as 0.36 absorbance. One layer of spherical nanoparticles synthesized in

methanol gave a yellow color when first taken out of the silver suspension but after washing and

drying the color turned to a yellow color with a pink hue. Two absorbance peaks are seen in the

UV-Vis spectrum, one at ~400 nm and one broad one at ~550 nm (Figure 15a). One layer of

triangular nanoparticles created using PEI gave a blue color and an absorbance maximum in the

near IR at ~750 nm (Figure 15b).

Figure 15: UV-Vis spectrum in transmittance mode of a single layer silver nanoparticle assembly using PEI of (a) spherical nanoparticles and (b) of triangular nanoparticles

a

b a

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A two-layer assembly of spherical nanoparticles synthesized in methanol using PEI gives

a yellow color when taken out of the Ag nanoparticle suspension but after washing and drying

turns to a purple color. In addition to the absorption peak at 400 nm, the second peak at ~550 nm

is more prominent in the UV-Vis spectrum (Figure 16a). This colloidal silver suspension was

very concentrated, and stacking of layers could exist on parts of the film. The small broad peak

seen in the monolayer could therefore be due to stacking of nanoparticles, which is why the same

peak is at higher intensities in the 2-layer film. This phenomenon could be improved by diluting

the suspension. A two-layer assembly of triangular nanoparticles gives the same blue color as

the one-layer assembly but a little deeper. The UV-Vis spectrum shows a similar absorbance

peak to the one-layer assembly, however it is a little wider (Figure 16b). After the second

exposure of the glass coverslip to PEI, the glass coverslip looked a fainter color after washing.

This trend was seen in both the spherical and triangular assemblies, but the color of the glass

coverslip was refreshed after the second dipping in the silver suspension to achieve the next

layer.

Figure 16: UV-Vis spectrum of a (a) 2-layer Ag nanoparticle assembly using PEI of (a) spherical nanoparticles and of (b) triangular nanoparticles

a b

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2.3 Summary

The self-assembly of Ag nanoparticles onto glass coverslips to give a deep color was very

successful with the PEI. PEI coupling allowed an easier visual of the assembled silver

nanoparticles with only a monolayer, unlike the silane coupling which required a multilayer to

get a better visual analysis. PEI also did not contain any sulfur functional groups that might have

reacted with the silver nanoparticles after assembly and the functional groups do not polymerize

in the presence of water preventing them from efficient coupling. In fact, PEI-coupling only

took a few hours, whereas silane-coupling required over night treatments. The color of the

triangular Ag nanoparticles gave a blue colored sensor in both the silane and PEI coupling. The

spherical Ag nanoparticles gave a yellow/pink sensor for a monolayer and a purple sensor for

two –layer coupling using PEI and a yellow sensor for silane coupling.

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Chapter 3-Sensitivity of Nanoparticles to Hydrogen Sulfide

Once the self-assembled silver nanoparticle sensors had been made, their performance

was tested in exposures to hydrogen sulfide to determine how the reaction affected the film

color. Both static and dynamic exposures of H2S were utilized. The set-up scheme is shown in

Figure 17. A static exposure consists of a steady exposure of H2S for a certain amount of time

where the valve to the bubbler was be closed. The dynamic exposure consisted of the same gas

handling system with the H2S constantly flowing through the set-up below and escaping through

the bubbler.

Figure 17: Scheme for exposure to H2S gas

For exposure, the silver sensor produced through self–assembly was placed into an

exposure chamber, which was then flushed with nitrogen and evacuated all under a closed

system. Once this was completed, either pure or 100 ppm H2S gas was released into the system

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with the bubbler closed. The monolayer was exposed to a 100 ppm concentration of H2S and the

multilayer was exposed to pure H2S for a prescribed amount of time (Table 1). The 1-layer

silane-coupled sensors were exposed to both static and dynamic conditions of 100 ppm hydrogen

sulfide at various times. The multi-layer silane-assembled sensor and PEI-assembled sensor was

exposed to a static condition of pure hydrogen sulfide gas.

H2S Exposure

Spherical Ag NPs/H2O Silane-coupled

Triangular Ag NPs/H2O PEI- Coupled

Spherical Ag NPs/MeOH PEI-coupled

Monolayer Multilayer Multi Mono Dynamic-100 ppm

1) 35 min

Static-100 ppm

1) 30 min 2) 1.75 hr

Static-pure

1) 40 min 1) 45 min 1) 30 min

Table 1: Exposure times of sensors to pure or 100 ppm H2S gas at static or dynamic conditions

After exposure to hydrogen sulfide, the sensor was analyzed for any changes due to the

reaction. The UV-Vis absorption spectrum was recorded to measure a before and after

absorption of the sensor. As seen in the figure 18, the original peak validating that the

nanoparticles are assembled onto the glass slide is reduced in intensity after exposure to H2S.

This demonstrates that the sensors are reacting with the H2S gas.

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Figure 18: UV-Vis spectra in transmittance mode of silane-coupled monolayers spherical Ag NPs synthesized in water sensors exposed to hydrogen sulfide gas for (a) 30 minutes static (b) 35 min dynamic and (c) 1.75 hrs static. UV-Vis spectra in reflectance mode for a (d) ~13 layer multilayer spherical Ag nanoparticle sensor exposed to pure hydrogen sulfide gas for 40 min.

Figure 19: (a) UV-Vis spectrum in transmittance mode of PEI-coupled monolayer of spherical Ag nanoparticles synthesized in methanol sensor exposed to hydrogen sulfide gas for 30 min static. (b) UV-Vis spectrum in transmittance mode of PEI-coupled monolayer triangular Ag nanoparticle sensor exposed to hydrogen sulfide gas for 45 min static.

b

c

b a

d

a

a

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Reduction of the characteristic peak shows sensors are sensitive to corrosive gas. The

resulting UV-Vis spectrum of the sensor after being exposed to hydrogen sulfide gas and fully

reacting (red color in spectra of Figures 18c -19a) showed a characteristic rise in the spectrum

near the blue region, which is consistent with the optical absorption spectra of silver sulfide28. A

color change in the sensors was also seen, in which both the one-layer yellow spherical Ag

nanoparticle sensor and the one-layer blue triangular Ag nanoparticle sensor turned colorless

after exposure. The multi-layer spherical Ag in water sensor turned from a blue-green to a

grayish color after exposure.

Figure 20: Multi-layer spherical silane assembly (a) Pre-exposure to pure hydrogen sulfide and (b-c) post-exposure to pure hydrogen sulfide for 40 minutes at different angles. 3.1 Products and Kinetics 3.1.1 Verification of products To better understand the reaction of silver nanoparticles with hydrogen sulfide, further

analysis was completed. Energy Dispersive X-ray spectroscopy analysis (EDX) was run to

verify the sulfur reaction and kinetic studies were performed to examine the nature of the surface

reaction. Samples for transmission electron microscopy (TEM) were prepared on formvar/carbon

copper grids with the same PEI-assembly procedure as making films on glass coverclips. The

samples on copper grids were exposed to H2S gas for the same time as the films treated.

Morphological details and was performed on a high-resolution transmission electron

microscope (HRTEM), operating at 200 kV. In the STEM mode, a focused electron beam is

a b c

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scanned over a selected sample area. STEM images were recorded with a high-angle annular

dark field (HAADF) detector. An energy-dispersive X-ray (EDX) spectrometer attached to the

TEM performs elemental analyses at spots selected in the HADDF dark field STEM images,

with a spot size of 1 nm.

HRTEM images showed a blurring of the edges of the nanoparticles indicating a layer of

reaction product had formed (Figure 21 1a vs. 1b). This blurring of the edges could be due to

small clustering of Ag2S on the nanoparticle surface indicating that the silver nanoparticles did

not react uniformly, but rather in areas of low PVP coverage29. This could be due to variable

concentrations of PVP covering the areas of the nanoparticle. Alternatively, the Ag-Ag2S

attractive forces are weak, and the surface energy causes the Ag2S to form small clumps rather

than a thin uniform film30.

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1. 2. Figure 21: (1a) High resolution TEM image of spherical Ag NPs, showing sharp outlines of nanoparticles. (1b) High resolution scanning TEM image of spherical Ag NPs. (2a) High resolution TEM image of reacted Ag NPs with H2S, showing blurred edges. (2b) High resolution dark field scanning TEM image of reacted Ag NPs with H2S, showing blurred edges.

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Figure 22: (a) EDX spectrum of spherical nanoparticles. The C signal comes from both the TEM grid and PVP surrounding the nanoparticles; O comes from the PVP. (b) EDX spectrum of reacted Ag NPs with H2S showing sulfur at about half the intensity of silver.

Silver with no signs of Sulfur

Silver and Sulfur peaks seen after exposure

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Looking at the initial EDX data (Figure 22a), the Cu comes form the TEM grid and the

silver is the spherical silver nanoparticles. In figure 22b, the Ag peak has decreased and a sulfur

peaked has appeared indicating the reaction with hydrogen sulfide had occurred. The intensity

of the sulfur seems to be about half of the intensity of the silver, which is the correct atomic

proportion of silver compared to sulfur in Ag2S. Because sulfur is present in the Ag particles

after reaction, it indicates the reaction occurred and color changes in the sensor were due to the

reaction with hydrogen sulfide and not impurities.

3.1.2 Kinetic Studies

To determine the reactivity of the triangular and spherical silver nanoparticle sensors to

hydrogen sulfide gas, kinetic studies were performed. One-layer of the nanoparticles was self-

assembled onto the inner surfaces of a glass cuvette using the same PEI method used on the glass

coverslips. The cuvette was allowed to dry overnight. The cuvette was then attached to the

hydrogen sulfide system, flushed with nitrogen gas and vacuumed. Then, pure hydrogen sulfide

gas was flushed into the cuvette at a pressure of 2psi, in which the cuvette was then closed off

and removed for further analysis using the UV-Vis, taking consecutive spectrums over time. The

formation of silver sulfide was monitored by measuring the change in absorbance of the

triangular silver nanoparticles at 750 nm and the spherical silver nanoparticles at 400 nm.

To analyze how the concentration of silver nanoparticles changed as the reaction

progressed, characteristic kinetic plots were made simulating zero, first and second order

reactions. A zero order is the plot of the concentration of Ag (absorbance) against time with the

differential rate law being –d[Ag] / dt = k and therefore the integrated rate law is [Ag] = -kt.

First order plots ln ([Ag]/ [Ag]0) against time and so –d[Ag] / dt = k[Ag] resulting in [Ag] =

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[Ag]0 e-kt. Second order plots 1/[Ag] against time and so –d[Ag] / dt = k[Ag]2 and the integrated

rate law is [Ag] = ([Ag]0 / (1 + kt[Ag]0)).

A linear characteristic kinetic plot determined a one-layer assembly of the Ag

nanoparticles to give a first order reaction rate for both the spherical and the triangular with

respect to silver. Plotting ln [Ag] or ln [Ag] / [Ag]0 against time gives a straight line with slope

of –k. The plot should be linear up to a 80-90% conversion, that is up to the point at which 80-

90% of the concentration of the limiting reactant is consumed. The rate constant, k, of the

spherical Ag nanoparticle reaction was 0.0001 sec-1 and for the triangular Ag nanoparticle

reaction was 0.0002 sec-1. Since triangular Ag nanoparticles have a higher rate constant, they are

more sensitive to the reaction with hydrogen sulfide and will react quicker than the spherical

nanoparticles.

a

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Figure 23: 1/2 hr PEI-assembly of triangular Ag nanoparticles on a cuvette exposed to pure hydrogen sulfide gas (a) Zero-Order simulation (b) First-Order simulation (c) Second-Order Simulation

b

c

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a

b

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Figure 24: 1/2 hr PEI-assembly of spherical Ag nanoparticles in methanol exposed to pure hydrogen sulfide gas (a) Zero-Order simulation (b) First-Order simulation (c) Second-Order Simulation

The sensitivity of the triangular Ag nanoparticles over the spherical Ag nanoparticles

could be due to the chemical stability of spherical nanoparticles, which is greater than the

triangular. It could also be the fact that the spherical nanoparticles were synthesized in methanol

which reduces the amount of water on the silver surface and does not allow the gas to react as

quickly. To further investigate the kinetics, dose response experiments should be performed

where the pressure of the H2S gas is varied and one can evaluate the if the reaction is still linear

with first order.

c

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Chapter 4-Performance evaluation of Ag nanoparticle sensors

4.1 Stability

Once the Ag nanoparticle sensors had been demonstrated to react to the target gas (H2S),

they were then tested for their stability under Oddy Test conditions of 100% relative humidity

and 60°C, without harmful material as a control. The sensors were also kept in a jar with 10 mL

of DI water at room temperature to determine their stability when not exposed to high

temperatures.

The jars used to perform the stability testing were glass, with stainless steel caps having a

teflon face liners. The sensors were placed in a holder and then in a 20 mL beaker, which was

placed inside the jar. Different holders were used to determine the best one. The first holder

tried was made of glass slides and the sensor was glued on using a glue gun. The second holder

used was two pieces of Teflon with a square cut inside. The Teflon pieces were held together by

aluminum foil when placed inside the jar. The last holder consisted of a small, square Teflon

piece with three slits cut across so the sensor could stand up vertically.

Figure 25: (a) glass slide holder (b) two-piece Teflon holder (c) square-piece Teflon holder

a b c

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4.1.1 Silane-Coupled Sensors

To prepare for the Oddy test set up, all containers were cleaned using piranha solution for

20-30 minutes, then rinsed with deionized water twice and washed again with water two more

times while ultrasonicating, then checked for a neutral pH. A fresh silane-assembled, one-layer

silver nanoparticle film was glued onto the glass slide holder and set inside the small beaker.

About 10 mL of deionized water were added to the jar and the beaker was placed inside the jar,

which was then capped, tightened and placed in the 60°C oven. After one hour, the cap to the jar

was retightened to ensure gases would not escape.

UV-Vis spectra of the sensor was taken pre-exposure to determine the absorbance at

initial time. Maximum absorbance using silane-assembly of spherical nanoparticles synthesized

in water only reached around 0.025, which is very low. After exposing the spherical

nanoparticle sensor to control Oddy conditions (inside jar with only water placed in 60°C oven),

a big decrease in absorbance of the spectra occurred after only a few days and continued to

decrease throughout the 28 days period. The exponential time dependence for the decrease was

consistent with the first-order reaction kinetics for this process, indicating a gas reaction of some

sort.

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Figure 26: (a) UV-Vis spectrum of a 1-Layer silane assembly of spherical nanoparticles synthesized in water exposed to Oddy Test conditions without harmful material over a 28-day period in a 60° oven (b) Peak absorbance at 410 nm versus the number of days the sensor was exposed to controlled Oddy Test conditions in a 60° oven

Another monolayer silane-coupled assembly of spherical nanoparticles synthesized in

water was placed in controlled Oddy Test conditions, however this time the jar was placed at

room temperature instead of in the oven. The sensor was placed in the two-piece Teflon holder

and UV-Vis spectrums were taken over a period of 26 days. The same trend in decrease of

absorbance was also observed after only five days.

Figure 27: (a) UV-Vis spectrum of a 1-Layer silane assembly of spherical nanoparticles synthesized in water exposed to Oddy Test conditions without harmful material over a 28-day period at room temperature (b) Peak absorbance at 410 nm versus the number of days the sensor was exposed to controlled Oddy Test conditions at room temperature

a

a b

a b

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A monolayer silane-coupled assembly of triangular nanoparticle synthesized in water was

tested at room temperature. The UV-Vis max absorbance of the sensor was only .0045 A to start

with and a decrease was also seen within three days. Another interesting observation of the

triangle one-layer assembly included a blue shift of the spectra when placed in controlled Oddy

conditions. This phenomenon will later be explained.

Figure 28: (a) UV-Vis spectrum of a 1-Layer silane assembly of triangular nanoparticles synthesized in water exposed to Oddy Test conditions without harmful material over a 28-day period at room temperature (b) Peak absorbance versus the number of days the sensor was exposed to controlled Oddy Test conditions at room temperature

One-layer silane-coupled sensors came with a lot of challenges to be resolved. The

stability of the nanoparticles was poor, both in Oddy Conditions in the oven andin ambient room

temperature conditions. Also, once the silver nanoparticles were assembled, the color of the

sensors was very faint and hard to see. The faint color was shown in the low absorbance of UV-

Vis spectra. These factors made the silane-coupled sensors unsuitable for replacing the silver

coupon in the Oddy Test, and therefore no further studies were conducted utilizing the silane-

coupled nanoparticle sensors. Out of the three types of holders used, the square Teflon holder

was the best and was the method of choice for the rest of the studies conducted.

a

b a

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4.1.2 PEI-coupled sensors

PEI-coupled nanoparticle sensors were also first tested for their stability in Oddy

conditions. To prepare for the Oddy test set up, all containers were cleaned using

NOCHROMIX, then rinsed with deionized water twice and allowed to sit in fresh water

overnight. The glass was then removed and rinsed with fresh water along with scrubbing using a

cotton swab. The glass was again left to sit in fresh water overnight.

A fresh PEI-coupled monolayer assembly of spherical Ag nanoparticles synthesized in

methanol was placed in the square Teflon holder and placed inside the small beaker. About 10

mL of deionized water were added to the jar and the beaker was placed inside the jar, which was

then capped, and placed in the 60°C oven. After one hour, the cap to the jar was retightened to

ensure gases would not escape.

A UV-Vis spectrum of the sensor was taken before exposure. Absorbance of the PEI-

coupled spherical nanoparticles can be adjusted as high as 0.3, much higher than the silane-

coupled assembly. In the Oddy test conditions, a one-layer spherical assembly starting with 0.12

absorbance showed two peaks in the spectrum turning into one peak at 410 nm with a higher

absorbance than the initial spectrum indicating a thermal-induced rearrangement of the silver

nanoparticles. The one-layer spherical assembly was measured for 77 days and showed to be

stable over the time period with only slight deviations in the max absorbance of the spectrum.

a

Moussa 47

Figure 29: (a) UV-Vis spectrum of a 1-Layer PEI-assembly of spherical nanoparticles

synthesized in methanol exposed to Oddy Test conditions without harmful material over a 77-day period in the 60°C oven (b) Peak absorbance at 410 nm versus the number of days the sensor was exposed to controlled Oddy Test conditions in the 60°C oven (initial peak absorbance at Day 0 omitted)

A 2-layer PEI-coupled assembly of spherical nanoparticles synthesized in methanol was

tested under controlled Oddy conditions over a 28-day period in a 60°C oven, and only slight

deviations were observed in the max absorbance of the spectra, with the peaks still absorbing

above 0.3 at 420 nm. The two absorbance peaks turned into one peak, again indicating thermal-

induced rearrangement of the silver nanoparticles.

b a

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Figure 30: (a) UV-Vis spectrum of a 2-Layer PEI-coupled assembly of spherical nanoparticles synthesized in methanol exposed to Oddy Test conditions without harmful material over a 28-day period in the 60°C oven (b) Peak absorbance at 420 nm versus the number of days the sensor was exposed to controlled Oddy Test conditions in the 60°C oven

PEI-coupled triangular two-layer and one-layer sensors were also tested for their stability

in controlled Oddy conditions. The one-layer PEI-assembly of triangular silver nanoparticles

started with 0.26 absorbance of the peak at ~715 nm and then started to decrease and blue shift

and another peak ~ 410 nm started to form. Once the peak at 700 nm was completely fully react

around day 14, the peak at ~410 nm was very stable and showed slight deviations throughout the

test period. These changes in the UV-Vis spectra were also observed visually as the blue sensor

started to form deep yellow spots until eventually the whole sensor was yellow. This yellow

color stayed stable even after 77 days.

a

b

b

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Figure 31: (a) UV-Vis spectrum of a 1-layer PEI-assembly of triangular nanoparticles synthesized in water exposed to Oddy Test conditions without harmful material over a 77-day period in the 60°C oven (b) Peak absorbance versus the number of days the sensor was exposed to controlled Oddy Test conditions in the 60°C oven

The one-layer PEI-assembly of triangular silver nanoparticles was also tested in

controlled Oddy conditions at room temperature. Similar observations were seen in this test as

the one done at 60° C. The peak at ~735 nm started to decrease but stabilized in absorbance

from day 7 to 35. The blue shift phenomena was observed again and the peak ~410 nm did not

appear until after 67 days.

Figure 32: (a) UV-Vis spectrum of a monolayer PEI-assembly of triangular nanoparticles synthesized in water exposed to Oddy Test conditions without harmful material over a 67-day period at room temperature (b) Peak absorbance versus the number of days the sensor was exposed to controlled Oddy Test conditions at room temperature

a

b

a b

a

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These changes in the UV-Vis spectra were also observed visually as the blue sensor

started to form deep yellow spots. After 67 days, blue color was still visible on the sensor along

with the deep yellow color.

Figure 33: The monolayer PEI-assembly of triangular silver nanoparticles after 67 days in controlled Oddy conditions at room temperature. Originally the sensor was all just the blue color seen in the picture.

You may do an experiment to verify that, take a solution of Ag nanoprisms and heat up

while stirring, the blue color should change to yellow; but if you first bubble the colloids with N2

for ~half an hour (to get rid of O2), then seal the flask and heat up, I believe the blue color will

sustain, at least for a much longer time.

The blue to yellow color change of the triangular particle sensor was examined in more

detail. It was postulated that the triangular Ag nanoparticles on the sensor were turning into the

spherical Ag nanoparticles due to the humidity and O2 in the jars. O2 will etch Ag nanoprisms,

and in the presence of a water layer (or moisture) on the Ag nanoparticle surface, mass transfer is

easier to occur, thus, Ag nanoprisms will easily convert to spherical particles, which would

explain the yellow color of the sensors. It was later determined through HRTEM analysis that

this was true. Three carbon coated grids were prepared onto which the triangular Ag

nanoparticles were self-assembled onto using the PEI method. One carbon grid was placed in

controlled Oddy test conditions in a 60°C oven (in a jar with only water and no harmful

material), one grid was placed in controlled Oddy test conditions at room temperature (in a jar

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with just water and no harmful material), and one grid was placed in a sealed container away

from moisture as a control. TEM images of the copper grid placed in the oven show spherical

nanoparticles whereas the control is mostly triangular nanoparticles, confirming the change from

triangular to spherical due to humidity in the air.

Figure 34: TEM images of triangular Ag nanoparticles assembled on carbon grid using PEI method (a) control (b) placed in Oddy conditions at room temperature (c) placed in Oddy conditions in 60° C oven.

For the two-layer PEI-coupled assembly of triangular Ag nanoparticles, a slight decrease

in the absorbance spectrum was observed along with a remarkable blue shift from 780 nm to ~

410 nm. The two-layer assembly started with an absorption of 0.32 and took ~14 days to reach

~410 nm. The sensor showed only slight deviations in absorbance for the first 17 days. Then, a

slight decrease in absorbance occurred and from day 17 to 21, but from day 21 to 77 the

absorbance did not vary much. Visually, color could still be seen on the sensor for up to 77 days,

with absorbance near 0.19.

a b c

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Figure 35: (a) UV-Vis spectrum of a 2-Layer PEI-assembly of triangular nanoparticles synthesized in water exposed to Oddy Test conditions without harmful material over a 77-day period in the 60°C oven (b) Peak absorbance versus the number of days the sensor was exposed to controlled Oddy Test conditions in the 60°C oven

The two-layer PEI-coupled assembly of triangular silver nanoparticles was also tested in

controlled Oddy conditions at room temperature. The peak at ~710 nm decreased and blue

shifted as a peak at ~410 nm appeared, which started to increase in intensity. The peak at ~410

nm did not appear until after 16 days, and the peak at ~710 nm disappeared after 35 days.

Figure 36: (a) UV-Vis spectrum of a 2-Layer PEI-assembly of triangular nanoparticles synthesized in water exposed to Oddy Test conditions without harmful material over a 67-day period at room temperature (b) Peak absorbance versus the number of days the sensor was exposed to controlled Oddy Test conditions at room temperature

a b

a

a b

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These changes in the UV-Vis spectra were also observed visually as the blue sensor

started to form deep yellow spots. After 67 days, blue color was not visible on the sensor, which

had turned to a yellow color.

Figure 37: 2-Layer PEI-assembly of triangular silver nanoparticles after 67 days of being exposed to controlled Oddy conditions at room temperature.

The sensors also showed stability when placed inside a sealed plastic bag. No changes

were seen in the UV-Vis spectrum of these sensors even after 12 days, which showed that the

sensors could be mass-produced and used at a later time. Since both the monolayer and 2-layer

assemblies gave good visual color on the glass coverslip, it was decided to use the monolayers

for Oddy testing with harmful material. This would allow one surface to interact with the

corrosive gas instead of having to permeate through two layers. Originally, the only reason why

2-layers was looked into was for visual analysis reasons.

4.1.3 Summary

Overall, the monolayer PEI-coupled sensors were most stable. The monolayer PEI-

coupled spherical Ag nanoparticle sensors showed stability for over one month and were the

most stable in visual color. The monolayer triangular PEI-coupled Ag nanoparticle sensors

showed a transition from nanoprisms to spherical nanoparticles under the presence of moisture

and oxygen. When the nanoprisms did however make the final transition over to spherical

shape, they showed stability with little changes in the UV-Vis absorption spectra at ~410 nm,

showing deep color for over one month.

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4.2 Comparison to conventional Oddy Test

Since the PEI-coupled sensors showed stability for over one month in Oddy test

conditions, the next tests incorporated wool to a jar having a silver coupon to monitor the effects

of the harmful gasses emitted from the wool. The wool was placed in a shell vial and the silver

coupon was hung from the side of the beaker (Figure 38). The wool used in the Oddy Test was

N/500 wool galoon fabric from Testfabrics, Inc.

Figure 38: Jar setup for Oddy test with wool in shell vial and square Teflon holder

When the Oddy Test has been completed, the silver coupon starts to show signs of tarnish after 3

days in the oven with wool and makes it impossible to quantify partial reaction.

Figure 39: Silver coupons exposed to Oddy Conditions with 1 cm x 1 cm wool after (a) 1 day (b) 2 days (c) 3 days (d) 4 days (e) 5 days. Picture taken at two different angles.

a b c d e

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When the PEI-coupled one-layer triangular Ag nanoparticle sensor was exposed to Oddy

Test conditions with wool, the number of days it took for the sensor to fully react depended on

the concentration of nanoparticles on the sensor and the amount of the wool in the test.

Figure 40: Analysis of the number of days it took various concentrations of triangular Ag nanoparticles exposed to Oddy Test conditions with wool to react fully. For the one-layer triangular nanoparticle PEI-assembly, an assembly time of 10 minutes

allowing for UV-Vis absorption of the sensor around 0.18 was the optimal condition.

This sensor reacted within one day showing a dissapearance of the characteristic peak at 780 nm

before the silver coupon showed signs of tarnish from a 1 cm x 1 cm piece of wool. It took the

silver coupon about 3 days to show signs of tarnish. The visual analysis of the sensor showed a

change from the blue color to a colorless glass coverslip.

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Figure 41: (a) UV-Vis spectrum of optimal one-layer triangular Ag nanoparticle sensor pre and

post exposure to Oddy Test conditions with wool. One-layer triangular nanoparticle PEI-

assembled sensor (b) before and (c) after exposure to Oddy Test conditions with wool.

When the one-layer spherical/methanol nanoparticle PEI- assembly was exposed to Oddy

Test conditions with wool, the number of days it took for the sensor to fully react also depended

on the concentration of nanoparticles. However, when compared to the triangular nanoparticle

sensor, the spherical/methanol sensor was not as sensitive, taking three days to react fully to

Oddy Test conditions with wool at a very low absorbance of 0.0638. These results are consistent

with the kinetic studies performed showing the triangular nanoparticles are more sensitive than

the spherical shape. Visual analysis at this absorbance of spherical nanoparticles is very hard,

since only a light tint of yellow is produced on the glass slide.

a b c

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Figure 42: (a) Analysis of the number of days it took various concentrations of spherical/methanol Ag nanoparticles exposed to Oddy Test conditions with wool to react fully.

The resulting UV-Vis spectrum of the sensor after fully reacting with the wool in the

Oddy test conditions did show a characteristic rise in the spectrum near the blue region which is

consistent with the optical absorption spectra of silver sulfide and similar to the results seen in

the triangular nanoparticle sensor (Figure 43, red line).

Figure 43: UV-Vis of optimal one-layer spherical/methanol Ag nanoparticle sensor pre and post exposure to Oddy Test conditions with wool.

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Pieces of wool that were already used in the Oddy Test were reused again in other tests to

act as less harsh off-gassing materials. This slowed down the reaction of the sensor during the

Oddy Test and the reaction progress was followed through UV-Vis. This allowed a quantitative

way of determining the effect of the material on the silver nanoparticle sensors as opposed to just

a visual analysis with the tarnishing of the metal coupon. Therefore, along with following the

changes in color of the sensors as a visual analysis, decrease in UV-Vis spectra absorbance

(Figure 44) showed how far along the reaction was, and one could tell at what point the reaction

was half way done.

Figure 44: UV-Vis spectrum showing reaction progress a reused piece of wool using a (a) monolayer PEI-coupled spherical Ag NPs in methanol sensor and (b) a 2-layer PEI-coupled triangular Ag NPs in water sensor. The spherical silver nanoparticle sensor showed color changes in a decrease in intensity of the

yellow color until it reached colorless. The triangular silver nanoparticle sensor showed color

changes from blue to purple to colorless, purple being the midpoint of the reaction.

a a

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Chapter 5- Conclusion and future work

The research shows that the replacement of the silver coupon in the original Oddy Test

with a silver nanoparticle sensor is very promising. Both triangular and spherical sensors allow

for analysis visually through color changes and quantitatively through UV-Vis spectroscopy

when reacted with corrosive gas. Both sensors showed stability in controlled Oddy conditions

without any harmful material when placed in the oven. The blue triangular sensor turned to a

deep yellow color indicating shape transformation and the spherical sensor retained its yellow

color. The shape transformation of the triangular nanoparticles could be accounted for by the O2

in the air in the presence of humidity oxidizing the triangles and turning them into spheres. This

process was accelerated at high temperatures.

Kinetic studies revealed that the triangular nanoparticle sensors are more sensitive than

the spherical sensors, which was seen in the Oddy Test when the triangular sensors gave results

before the silver coupon. This could be due to the chemical stability of spherical nanoparticles,

which is greater than the triangular. It could also be the fact that the spherical nanoparticles were

synthesized in methanol which reduces the amount of water on the silver surface and does not

allow the gas to react as quickly. To further investigate the kinetics, dose response experiments

should be performed where the pressure of the H2S gas is varied and one can evaluate the if the

reaction is still linear with first order.

The PEI-coupled triangular Ag nanoparticle sensor reacted within one day with a starting

UV-Vis absorbance peak around 0.18. Visual analysis was dramatic in which the sensor turned

from blue to colorless. However, the color shift during the reaction due to the oxidation process

by O2, can make it hard to evaluate intermediate reactions. Until the triangular nanoparticles can

be made stable on the glass coverslips, their use in Oddy conditions would only be suitable

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before the shape transformation to prevent misinterpretations. However, visual analysis can still

be followed as the sensor changes from blue to purple and finally to colorless to indicate reaction

with corrosive gas, or from blue to yellow to indicate transformation of shape and no signs of

corrosion.

The PEI-coupled spherical Ag nanoparticle sensor also showed color changes to give a

visual analysis from yellow to colorless, which is not as easy to see as the blue. The starting

absorbance of the UV-Vis peak at 0.064 of the spherical sensor took 3 days to react fully

showing the spherical Ag nanoparticle sensor was not as reactive as the triangular Ag

nanoparticle sensor. UV-Vis analysis by following the decrease in the absorbance of the

characteristic peak at ~400 nm was easier to evaluate in the spherical sensor than in the

triangular sensor because the spherical sensor’s peak did not shift during the reaction.

Spectroscopic analysis allows for quantitative evaluations of partially reacted sensors.

The extent of the corrosive gas reaction on metal coupons is hard to follow and almost

impossible to quantify how much the silver coupon has tarnished. By following the decrease in

absorption of the characteristic peak of the sensors through UV-Vis, one can know exactly how

far along in the process is.

In the future, there will be hopes to try to replace the copper and lead metal coupons with

respective nanoparticles. This will however take time since not much research has been put into

synthesizing copper and lead nanoparticles. Copper does have surface plasmon resonances in the

visible field (~580 nm), but they are not prominent for spherical shapes so it would be hard to

develop a sensor allowing UV-Vis spectroscopy for quantification. If one can make Cu nanorods

or other nonspherical nanoparticles, a similar colorimetric detection scheme could be developed.

Pb has unfortunately no surface plasmon resonance, so it would be difficult to develop a sensor.

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