Absorption Spectroscopy Using Microdroplet Resonance Fluorescence Intensities

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Abstract
Emission spectra from fluorophor-containing microdroplets exhibit a series of sharp peaks due to microcavity effects, superimposed on the usual broad-band dye emission seen in bulk liquid. The amplitudes of these resonance peaks are shown here to be sensitive absorption indicators, and the decrease in apparent resonance intensities may be used as a practical in situ absorption diagnostic procedure in spectroscopy. We illustrate this concept with two examples using a microdroplet stream (with diameters between 13 to 18 μm) of an ethanol/rhodamine dye solution, additionally doped with either bromocresol green or cresyl violet dye. An ability to obtain spectral absorption information, as well as quantitative absorption coefficient estimates in the range of 10 to 10−3 cm−1, and to infer changes in pH in the droplets is demonstrated. Use of this approach in conjunction with suitable indicator dyes should permit measurement of specific chemical changes occurring in microdroplets.