Development of polymeric photonic crystals as colorimetric sensors for chemical sensing
Creators: name: Evans, Ashley Lewis
Publisher: name: University of Surrey
Year: 2024
Subjects: chemical sensing
photonic crystals
nanomaterials

description: There are millions of avoidable injuries and deaths across the globe due to chemicalexposure in industries such as transportation, utilities, farming and manufacturingevery year. We must strive to find ways of alleviating this plight on members of ourglobal community. Sensing harmful chemicals is often costly, complicated, andcumbersome, though it is paramount that we have effective monitoring, so to achievethis, we explore the possibility of a portable, easy-to-use, colorimetric device, thatallows clear and obvious detection of harmful substances.Within the introduction to this thesis, colorimetric sensors were examined, specificallyphotonic crystals (PCs), and their physical principles. Exploration was undertaken ofhow the interesting and novel functionalities of a 2D nanomaterial, molybdenumdisulphide (MoS2), could enhance PCs and explore possible exfoliation route, liquid-phase exfoliation (LPE), as a low-cost and scalable approach.Within this work, I use spectroscopic ellipsometry (SE) to probe the optical constantsof thin films of LPE MoS2. After developing a modelling protocol to fit spectroscopicdata, I successfully compared our methods to data reported in literature usingmechanically exfoliated samples. From this, I pushed the capabilities of SE anddescribed the optical constants of vacuum filtered and spray deposited films of LPEMoS2, something which has not been achieved in the literature so far. I then wentfurther, exposing spray deposited LPE MoS2 films to chemicals of industrial relevance.Through SE, I measured increases and decreases in MoS2 optical constants,depending on whether the analyte was of an electron accepting and donating nature,respectively. Alongside these experiments, I successfully adapted a method of PCfabrication, evaporation driven self-assembly, of polymeric nanospheres to producerobust, free-standing crystals. The adaptation described here allowed us to embedLPE MoS2, locking the nanomaterial within interstitial sites of our PCs, to producephenomenal increases in the structural colour of the crystals. After refining thismethod, through changing our bespoke latex to a commercially available one with asuitable particle size, I solved issues related to the creation of thin film crystals, whichare better suited to spectrophotometric analysis. This set the stage to be able tomonitor the PC stopband (SB) through quantitative spectrophotometric analysis of thincrystals and, in parallel, carry out qualitative observation of structural colour changeswith the inclusion of MoS2 or the exposure to chemical analytes.This platform was then used to perform sensing experiments, whereby thin and thickcrystals containing different loadings of MoS2 were exposed to various concentrationsof electron accepting/donating analytes. Observed within this thesis is the red- andblue-shifting of the SB and visual colour of PCs, due to these respective chemicals.Possible mechanisms which lead to this interesting interaction were then described.Within the final experimental chapter, the ability for PC crystals to be used in real-world scenarios was showcased, by undertaking cell culture experiments upon theirsurface which showed total biocompatibility with mammalian cells. This could act as amodel for exposure to skin cells in a wearable device or give a foundation for futureworks which push the technology into the biomedical field as a biosensor.This work showcases several groundbreaking achievements in the realm of chemicalsensing via polymeric PCs and nanomaterials. It pushes the envelope for possibilitiesof SE through the use of this apparatus in optical characterisation and sensingcapabilities of LPE MoS2. Altogether, this thesis shows the ability of a novel system tohelp combat a global issue, inherent to many sectors, and potentially allows us tofurther this technology to ultimately save lives and prevent tragedy by allowing robust,simple, and clear, colorimetric chemical detection.