Imaging single molecules and direct observation of single molecule reaction at a solid-liquid interface

A new fluorescent method was developed for single molecule studies. The fluorophores were excited by the evanescent wave field produced either on the core surface of the optical fiber probe or on the flat surface of a quartz prism. The first configuration was used for single molecule detection. Single rhodamine 6G and fluorescein molecules have been detected. The number of rhodamine 6G molecules imaged by the optical fiber probe showed an excellent linear relationship with the concentrations of the fluorophores. It represents a simpler fluorescent method for the detection of single molecules in solution and at an interface. The second configuration was used to monitor single molecule reaction. Direct observation of single molecule generation from a chemical reaction was achieved at a solid-liquid interface. The reaction between fluorescamine and immobilized N'-(3-trimethoxysilylpropyl)diethylenetriamine was studied at the single molecule level. Time-lapse fluorescence images of single molecule products were recorded to follow the chemical reaction to its completion. Analysis of the photoelectron intensity of the fluorescent product and its distribution shows that the reaction kinetics goes through a transition from zeroth-order to first-order as the reaction proceeds. This approach offered a novel means to study single molecule reactions at the solid-liquid interface.