The detection of single bacterial cells and novel absorbing labels has been demonstrated through optical resonances in microdroplets. The setup enables high throughput detection of single Escherichia coli (E. Coli) cells without any direct labeling although Rhodamine 6G (R6G) was used as the signal transduction mechanism. A micro droplet acts as an optical cavity that supports Morphology Dependent Resonances (MDRs) at wavelengths where the droplet circumference is an integer multiple of the emission wavelength. The cells inside the droplet have a direct effect on the fluorescence lasing spectrum of R6G fluorescence by means of scattering and local refractive index change. The change in the lasing spectrum can be observed at the concentrations where each droplet has as little as one cell. C60 fluorescence quenching has also been demonstrated in microdroplets. R6G in ethanol (10μM) was used for the fluorescence spectrum measurements. Quenching of the optical resonances was observed when C₆₀ dissolved in ethanol was mixed with the R6G-Ethanol solution. Quenching can be observed at C₆₀ concentrations of 1μM in the final solution. The background signal was also checked by repeating the experiment with only R6G and only C₆₀ in the solvent, assuring that the signal reduction was due to the addition of C₆₀ in to the solution. This quenching mechanism may have many applications in multiplexing in bioassays.

Date Published: 29 March 2004
PDF: 8 pages
Proc. SPIE 5275, BioMEMS and Nanotechnology, (29 March 2004); doi: 10.1117/12.522806

Published in SPIE Proceedings Vol. 5275:
BioMEMS and Nanotechnology
Dan V. Nicolau; Uwe R. Muller; John M. Dell, Editor(s)