Evanescent cavity ring-down spectroscopy (e-CRDS) of hemoglobin absorption at the silica-water interface

The absorption of hemoglobin (Hb) from controlled urine samples was observed using the technique of evanescent cavity ring-down spectroscopy (e-CRDS). A room temperature, alexandrite laser pumped LiF:F₂+ color-center pulsed laser was used to excite Hb at 425 nm. A minimum absorbance level of 2.57 × 10^-4 was achieved corresponding to a minimum detectable concentration of Hb in urine of 5.8 nM. These levels could have advantages in the diagnosis of hemoglobinurea. The formation of layers of Hb upon the silica surface allowed for an increased sensitivity for smaller concentrations of Hb than would be expected for only a free floating solution. The formation of the layers also suggested a higher binding constant of Hb to the silica surface than between other layers of Hb molecules. Future studies are underway to understand the effects of salinity on the observed absorption due to the competitive binding of Na⁺ to the surface. Absorption isotherm modeling will also be used to better understand the development of layers upon the surface.