Detection of unlabeled oligonucleotide targets using whispering gallery modes in single, fluorescent microspheres

The development of an inexpensive, highly sensitive, whispering gallery mode (WGM) based biosensing system is demonstrated. The system comprises a silica microsphere functionalized with a fluorophore and a dense monolayer of single-strand oligonucleotides. The adsorption of the complementary strand causes spectral shifts in the emission spectrum of the microsphere that can be registered using a conventional optical microscope and CCD detector. The system is capable of detecting low-abundance, unlabeled-oligonucleotide targets. The spectral shifts can be used to monitor both the hybridization kinetics and the denaturation of the double-strand DNA at elevated temperatures. The hybridization appears to be completely reversible. Complementary oligonucleotide probes more than 30 bases in length are most readily detected while a complete assay takes only a few minutes.