The analytical capabilities of the department have been greatly enhanced recently by the development and set-up of a state-of-the-art fluorescence lifetime imaging microscope (FLIM). This system enables us to obtain chemical information not only from the intensity of light emitted from molecules of interest but also from the lifetimes of emitted signals which may last for only a billionth of a second or less. Fluorescence lifetimes are strongly affected by the molecular environment of the emitting species. For example, the presence of oxygen or ion species quenches fluorescence and thus strongly reduces the lifetime of most fluorophores. The laser analytics group of Dr. Kaminski is developing this technology for diagnostics of microscale environments such as living cells or microreactors.

An example of such research, conducted in collaboration with Dr. A. Fisher's group at this department and Dr. J. Frank from Sandia National Laboratories, USA, is displayed in the figure, which shows two microchannels merging into a single channel from right to left. The top channel carries Rhodamine dye dissolved in an aqueous solution of KCl, merging with a stream of Rhodamine dye dissolved in KI from the bottom channel. Flows are from right to left, the width of the merged channel is ~300 µm and flow speeds are about 1 mm sec^-1. The circular feature is an air bubble sticking to the side of the channel wall. The top image shows a conventional fluorescence image of the system, but there is very little contrast between the fluid streams, because the fluorophores across the image display similar levels of emission.

The bottom image shows the result using FLIM. Lifetimes in the KI stream are much shorter (around 2x10^-9 s) compared to the KCl stream (4.5x10^-9 s). In the laminar boundary layer between the two flows a gradual change from shorter to longer lifetimes can be clearly observed. A stagnating layer of KI fluid around the bubble leads to locally low lifetimes there.

Dr. Kaminski's and Dr. Fisher's groups will use this technology to study reactions at the interface of immiscible liquids flowing through such devices and were recently awarded an EPSRC grant for this work.