Optical resonant microcavities with ultra high quality factors are widely used for biosensing. Until now, the primary method of detection has been based upon tracking the resonant wavelength shift as a function of biodetection events. One of the sources of noise in all resonant-wavelength shift measurements is the noise due to intensity fluctuations of the laser source. An alternative approach is to track the change in the quality factor of the optical cavity by using phase shift cavity ring down spectroscopy, a technique which is insensitive to the intensity fluctuations of the laser source. Here, using biotinylated microtoroid resonant cavities, we show simultaneous measurement of the quality factor and the wavelength shift by using phase shift cavity ring down spectroscopy. These measurements were performed for disassociation phase of biotin-streptavidin reaction. We found that the disassociation curves are in good agreement with the previously published results. Hence, we demonstrate not only the application of phase shift cavity ring down spectroscopy to microcavities in the liquid phase but also simultaneous measurement of the quality factor and the wavelength shift for the microcavity biosensors in the application of kinetics measurements.

Progesterone modulates gamma-aminobutyric acid and excitatory amino acid neurotransmitter systems and has neuroprotective properties in models of hypoxia-ischemia. This study examined the in vitro effects of allopregnanolone, the active progesterone metabolite, in models of N-methyl-D-aspartate (NMDA)-induced necrosis and apoptosis. Cultured NT2 neurons were exposed to 1 mM NMDA. Lactate dehydrogenase (LDH) release was measured 24 h later. NMDA at a concentration of 1 mM produced a 39 +/- 19% release of total LDH. Exposure to 10 microM allopregnanolone prior to NMDA exposure reduced LDH release by 51%(P = 0.0028). NMDA stimulated apoptotic cell changes defined by terminal dUTP nick-end labeling (TUNEL) and 5,5', 6,6'-tetrachloro-1,1,3,3'-tetra ethlybenzimidazolycarbocyanide iodide staining were reduced to baseline values by both 10 microM allopregnanolone and 100 microM MK-801. Pretreatment with allopregnanolone (0-10 microM) reduced the percentage of TUNEL-positive cells in a dose-dependent manner (EC(50)= 2.7 +/- 0.1 nM). Physiologic concentrations of allopregnanolone provided protection against both necrotic and apoptotic injury induced by NMDA excitotoxicity.