Total internal reflection fluorescence correlation spectroscopy for counting molecules at solid/liquid interfaces

Fluorescence correlation spectroscopy, using tota internal reflection excitation (TIRFCS), is developed as a method to allow quantitative determination of molecular populations at solid/liquid interfaces. Population fluctuations of fluorescent molecules at the interface are observed as excess low-frequency noise on a fluorescence signal. Since the noise arises from molecular origins, its magnitude can be evaluated by Poisson statistics to determine the number of molecules in the interface volume. This quantitative information is available without sensitivity calibration or the preparation of standards and without fitting the transients to a kinetic model. Unlike single-molecule counting measurements, TIRFCS can produce these quantitative results even when the number of photoelectrons detected per molecule is small. Surface populations of rhodamine 6G dye molecules were measured at C-18-derivatized, flat silica surfaces in contact with aqueous solutions and compared with predicted values derived from chromatographic retention data. In addition, electrostatic and nonpolar contributions to the free energy of adsorption of the dye to C-18-modified silica surfaces were examined.