Towards a high-throughput real-time confocal microfluidic system for monitoring absorbance spectra in mixed-phase chemical reactions

In this work, we present a compact, real-time absorbance spectroscopy instrument, designed with a particular focus on taking spectroscopic readings in a microfluidic channel environment and selectively analysing an active volume in this channel by adding confocality. There are time-saving advantages to industry in carrying out and monitoring chemical reactions in a high-throughput microfluidic environment as opposed to manually mixing and then analysing chemicals. In this paper, we use absorbance spectroscopy to investigate a particular complex mixed-phase reaction (specifically, the reaction of colloidal diluted hair dye with an oxidising agent and catalyst), which has an additional complication of oxygen gas being released then trapped in the mixture as the reaction progresses. We find that the results obtained using our instrument are comparable to those obtained in a standard spectrometer. Oxygen bubbles formed in the reaction, however, present a significant obstacle to obtaining the correct sample depth in narrow flow channels. This is mitigated through the use of a camera which views the reaction between the glass and liquid through the use of remote reimaging, allowing the bubbles to be detected via histogram analysis while spectroscopy takes place enabling rogue readings to be removed, and also serves to monitor the overall density of the ‘gassy colloidal’ mixture produced.