Trakoolwilaiwan, Thithawat; (2023) Development of Thermochromic Lateral Flow Assay for Sensitive Detection. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Lateral flow assay (LFA) is a cost-effective and simple detection method providing a prompt colourimetric result to users. However, the low sensitivity of detection is one of the main disadvantages of the LFA. This project aims to develop a new technique based on thermal sensing strategy to enhance the sensitivity of LFA detection. Thermochromic sheet, along with photothermal nanoparticles and a light-emitting diode (LED), was utilised in the LFA. In the thermal sensing LFA, photothermal nanoparticles were optically stimulated to generate heat energy whereas the temperature change was monitored and converted into a visual signal by a thermo-responsive material. The novel thermal sensing platforms were developed to separately detect two antigens: Salmonella typhimurium and dengue virus serotype 2 NS1 (DENV2-NS1). Various types and shapes of photothermal nanoparticles were fabricated, of which the photothermal response was investigated. At the same mass, gold nanospheres (AuNSPs) with a diameter of 12 nm could raise to a relatively higher temperature than other materials. Hence, they were chosen to be utilised in the thermal sensing platform. The analytical performances of the typical and the thermochromic LFA were compared in the thesis. The colour intensities of the typical visual signal and the thermochromic signal was quantified using MATLAB. The power and duration of LED illumination were optimised to maximise the performance of the thermochromic assay. The thermochromic LFA was deployed to identify Salmonella typhimurium. The result showed that the typical visual measurement had a detection limit of 0.25 x 107 colonies forming unit (CFU) per mL, while using the thermochromic technique reduced the detection limit to 0.5x106 CFU/mL. For the detection of DENV2-NS1, the typical LFA yielded the detection limit of 6.125 ng/mL, while the thermochromic LFA provided the limit of detection (LOD) of 1.5 ng/mL. The linear relationship between the colour intensity ratio and the concentration of the analytes was found in both detection platforms. Furthermore, the thermochromic assay offered a semi-quantificative analysis to estimate the range of analyte concentration without the requirement of additional equipment. In conclusion, various types and shapes of nanoparticles were prepared and examined for their photothermal response. AuNSPs were chosen to be the visual signal provider in the conventional LFA and the heat generator in the thermochromic LFA. Two analytes were tested in this thesis to assess the performance of the thermochromic LFA. It was found that the analytical performance was significantly improved. Therefore, the thermochromic-based method is an effective colourimetric detection method for sensitive detection and early diagnosis.

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