Self-Assembly of Metal-Organic Framework Photonic SensorsNanyang Research Programme

Loi Si XianRaffles Institution

INTRODUCTION

Photonic Crystals

Photonic Crystals

● Photonic crystals consist of periodic arrangements of regularly shaped materials with different dielectric constants

● These crystals exhibit a photonic band gap, where electromagnetic propagation is completely prohibited in the specified frequency band, giving the crystals a structural colour.

Photonic Crystals

● Isotropic spherical colloids have conventionally been used to create ordered photonic superstructures, but anisotropic particles offer more packing geometries and functions.

● Unlike ordered superstructures, amorphous photonic structures (APSs) with short range order and randomly packed scatters, do not require homogeneous shapes and particle sizes, and are easier to fabricate.

Objective

● To create amorphous photonic sensors through the use of zeolitic imidazolate framework-8 (ZIF-8) metal-organic framework (MOF) particles

● HYPOTHESIS: The APS will be able to demonstrate optical properties comparable to ordered superstructures and should show potential for vapour sensing applications

MOF Particles

● Metal-organic framework (MOF) particles are hybrid, crystalline materials that have a dynamic porous framework

● ZIF-8, an MOF particle, has high crystallinity, a large surface area and good chemical and thermal stability

Structure of ZIF-8 (Lee, 2015)

METHODS

Synthesis of MOF Particles

Colloidal rhombic dodecahedral (RD) ZIF-8 Particles:● 2-methylimidazole (MiM), polyvinylpyrrolidone (PVP), and

varying amounts of sodium acetate were dissolved in methanol

● Methanol solution with Zn(NO3)2·6H2O was subsequently added, and the reaction mixture was incubated overnight

● Particles were purified by repeated centrifugation and washing with ethanol.

Synthesis of MOF Particles

Cubical ZIF-8 Particles:● Cetyltrimethylammonium bromide (CTAB)

was used to modulate the growth rates of the ZIF-8 particles

● CTAB selectively attaches to the {100} facets of the nanocrystals to facilitate the synthesis of cubical ZIF-8 particles.

Schematic of the facets of ZIF-8 (Avci, 2017)

Infiltration Driven Colloidal Assembly of MOF Photonic Structures

● Rapid infiltration of a 15μL drop of colloidal solution transports particles to be fixed on photo paper

Illustration of three different infiltration methods (a) Infiltration driven colloidal assembly (b) Infiltration driven coating (c) Radial infiltration driven colloidal assembly

MOF APSs for Vapour Sensing● Vapour was run through a gas chamber where the sensor

was placed in.

Overview of experimental process

RESULTS

Characterisation of MOF Particles

Average particle height: 111.03nm Average particle height: 178.07nm

A B

Average particle height: 232.92nm Average particle height: 268.56nm

C D

SEM images of cubical (A-D) ZIF-8 particles with varying sizes

Characterisation of MOF ParticlesA B

Average particle height: 264.70nmSEM images of cubical (E-H) RD ZIF-8 particles with varying sizes

Average particle height: 199.35nm Average particle height: 219.62nm

Average particle height: 414.37nm

Optical Properties of MOF Photonic Structures

● Patterned MOF APSs with different structural colours were obtained, with clear peaks in optical reflectance spectrum

Rod coating MOF APSs by RD particles with (a) different colours and (b) corresponding reflectance

Optical Properties of MOF Photonic Structures● Linear relationship observed between the particle height and maximum

reflection wavelength (λmax) for RD particles assembled through infiltration driven colloidal assembly

● Peaks in reflectance spectrum indicate photonic band gap functionality

(a) λmax observed in the optical reflectance spectrum and (b) λmax plotted against particle height for RD ZIF-8 particles

MOF APSs for Gas Sensing

● Obvious colour change from green to yellow upon exposure to ethanol (ETOH) vapour (Figure 8a), and a red-shift in reflection peak from 550nm to 577nm (Figure 8b), due to change in refractive index

● Such sensors have higher sensitivity and shorter response times● Clear change in colour allows for colorimetric vapour sensing

(a) Optical images and (b) reflection spectra of the activated MOF APSs (infiltration driven coating) exposed to N2 and saturated ETOH vapour

CONCLUSION AND FUTURE WORK

Conclusion and Future Work

● Morphology and optical properties of ZIF-8 particles can be controlled

● RD ZIF-8 particles can be used to create MOF APSs for colorimetric vapour sensing

● Further extensions include to develop MOF APSs with distinct kinds of MOF materials with improved sensitivity to different vapours and vapour concentrations

● Develop and construct of MOF vapour sensing arrays

“Acknowledgements

I would like to extend my sincere gratitude to my research mentor, Dr Bai Ling, the Nanyang Research Programme, the School of Chemical and

Biomedical Engineering, Nanyang Technological University, my school, Raffles Institution, and my school coordinator, Dr Lena Lui, for their guidance

and support throughout this research journey.

Thank you!