A microfluidic ultra-violet emission source for direct fluorescence of tryptophan

This paper describes a fluidic microchip for observing the direct fluorescence of tryptophan and other amino acids, which are used in studying protein structure and dynamics. Since the excitation wavelengths of these are in the 250-290 nm ultra-violet range, it is a major challenge to find appropriate light sources that can be integrated onto micrototal analysis systems. This effort demonstrates that illumination from a microdischarge can be used to observe the direct fluorescence of tryptophan. The discharge is ignited across an air gap between an on-chip metal anode and a liquid cathode made of a saturated solution of lead nitrate. Atomic transitions in Pb atoms that are consequently sputtered into the discharge provide the wavelengths necessary to excite amino acids. Unwanted wavelengths are rejected by an optical filter that separates the microdischarge from the tryptophan sample. Measured results include the unfiltered and filtered spectral output of the microdischarge, as well as that of the resulting tryptophan emission, which has the characteristic broad peak from 300-450 nm.