Arrays and cascades of fluorescent liquid-liquid waveguides: broadband light sources for spectroscopy in microchannels.

This paper describes the fabrication and operation of fluidic broadband light sources for use "on-chip" in integrated microanalytical systems. These light sources consist of liquid-core, liquid-cladding (L2) microchannel waveguides with liquid cores containing fluorescent dyes, excited by incident light from an external halogen bulb. Simultaneous use of multiple fluorophores in a common solution, in a single L2 light source, is not possible, because energy transfer from fluorophores emitting at shorter wavelength to fluorophores emitting at longer wavelength is essentially complete. Two approaches circumvent this problem of energy transfer; both use spatial separation of the fluorophores in different streams. The first setup uses a cascade (series) of single-core, single-dye light sources of increasing absorption energy to generate a combined broadband output. The second approach uses a parallel array of single-core, single-dye light sources. The spectral content of the light output for both cascade and array light sources can be controlled through choice of flow rates and dyes. Output intensity from these light sources is comparable to standard fiber-optic spectrophotometer light sources. The paper also discusses the efficiency of energy transfer between parallel liquid cores as a function of the fluid medium (index of refraction, path length, and rate of flow).