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Title: Miniaturisation of pH holographic sensors for nano-bioreactors
Author: Chan, Leon Cong Zhi
ISNI:       0000 0004 7224 6026
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2017
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Monitoring and controlling pH is of utmost importance in bioprocessing as it directly affects product yield and quality. Multiplexed experiments can be performed in nanobioreactors for optimisation of yield and cell heterogeneity in a relatively quick and inexpensive manner. In this thesis, a pH holographic sensor (holosensor) is miniaturised to 3.11 nL in volume and integrated into a PDMS-glass microfluidic chip for monitoring the growth of Lactobacillus casei Shirota. Although other established methods for monitoring cell cultures can be utilised, miniaturised holosensors enable real-time and non-consumptive monitoring of the bacterial cell culture growth medium. The 2-hydroxyethylmethacrylate (HEMA)-co-2-(trifluoromethyl) propenoic acid (TFMPA) holosensor was fabricated using an adapted technique from photolithography, coupled with the use of a polymerisation inhibitor to control the gel polymerisation with diameters not exceeding a standard deviation of 0.067. The hologram brightness was optimised to 1.05 ms integration time with 36X magnification using a low power (0.290 mW) 532 nm green continuous wave (CW) laser with a devised beam-offset technique. The holosensor was characterised with ionic strength balanced (9.50 mS/cm) McIIvaine pH buffers and a calibration curve plotted together with measured ionic strength, optical density at 600 nm (OD600) and pH. Correspondingly, RGB-xyY transformed values were plotted in the CIE 1931 chromaticity diagram. Later, a miniaturised 0.4φ HEMA-co-TFMPA holosensor and array was also demonstrated. Together with the 3.0φ holosensor, an accuracy parameter for the 0.4φ spot and array holosensors were calculated to be 99.08%, 99.38% and 97.77% respectively. Further work involved studying the issues associated with fabricating gels with unusually flat gel profiles. Other preliminary results suggested the alternative of utilising polymers as a holosensor substrate, together with a dye-free method for hologram fabrication, outlined the prospective possibility of a miniaturised holosensor integrated into a polymer microfluidic chip with the flexibility of hologram colour customisation for cell culture monitoring.
Supervisor: Lowe, Christopher R. Sponsor: Agency for Science ; Technology and Research (A*STAR)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Holographic sensor ; biosensor ; pH monitoring ; microfluidic chip ; bacterial growth ; microbioreactor