Detection of chemical alterations at internal walls of microchannel flow cells by nondestructive fluorescence depolarization

Abstract

A recent trend is the production of workable microchannel flow cells (MF cells). The nondestructive methods used to assess their reliability are based mainly on output monitoring and do not evaluate internal chemical interactions. We investigate a nondestructive method for evaluating changes in the chemical composition of the inner walls based on evaluation of the extent of alignment of a fluorescent probe in a liquid flowing within MF cells. Two MF cells were built with a 10-μm inner spacing. Their inner walls had four parallel SnO2 strips, 2.00 mm wide, separated by 0.50-mm-wide glass strips. One cell had strips parallel to the flow and the other perpendicular. Flow-induced intermolecular alignment of rhodamine B in monoethylene glycol was scanned with 28-μm precision by fluorescence depolarization, using polarized-laser-induced fluorescence within induced flows (PLF-FI).No changes of polarization were seen when the flow was stopped. Under flowing conditions, polarization was always 4% lower in the glass region as compared to SnO2. Glass had a higher solid–liquid interfacial tension (determined by contact angle measurements), thus being more wettable and increasing the drag, which propagates into the liquid flow, decreasing polarization. PLF-FI can thus identify regions with different chemical constitutions.