Miniaturised holographic pH sensors for nanobioreactors

Microfluidics enables small volume, multiplexed experiments to be performed with minimal amount of reagents and high throughput testing and monitoring of cell metabolites. In order to monitor cell growth accurately in real-time, integrated sensors would require a non-consumptive and non-contact method to minimise analyte utilisation and cross-contamination. The holographic sensor works by an equilibrative mechanism, and relies on an optical method to observe changes in the cell environment. The sensor consists of a smart hydrogel embedded with analyte-sensitive ligands and Ag⁰ fringes. Based on Bragg's law, the periodic fringes reflect incident light of different wavelength at a particular angle depending on the Donnan potential of the gel. A change in analyte concentration shifts the Donnan potential and consequently expands or contracts the gel. This results in a corresponding colourimetric shift which makes the holographic sensor an integrated system of a sensitive recognition element, transducer and display. In this talk, the use of a miniaturised holographic pH sensor embedded in a microfluidic chip will be discussed. The growth of a lactic acid bacterium, Lactobacillus casei, which is commonly found in probiotic beverages, was monitored with the sensor by examining the corresponding fall in pH resulting from its homolactic fermentation.