Identification of chemical species of fluorescein isothiocyanate isomer–I (FITC) monolayers on platinum by doubly resonant sum‐frequency generation spectroscopy

Abstract

AbstractDoubly resonant sum‐frequency generation (DR‐SFG) spectroscopy of fluorescein‐4‐isothiocyanate isomer‐I (FITC) monolayers on platinum was performed. Vibrational spectra of the monolayers for the IR wavenumber of 1750–1450 cm−1 were measured with visible probes ranging from 431 to 582 nm. Two vibrational bands at 1643 and 1610 cm−1 were observed, and their DR‐SFG excitation profiles displayed different shapes. By rinsing the monolayers with an alkaline solution, the smaller wavenumber band disappeared and the larger wavenumber band gained intensity. On the basis of the spectral response to the rinsing, we concluded that the FITC molecules existed on platinum as deprotonated and protonated forms; the former corresponds to the 1643 cm−1 band and the latter to the 1610 cm−1 band. The deprotonated form was assigned to an anionic surface species, and the protonated form to a neutral surface species by comparing the DR‐SFG excitation profiles with electronic absorption spectra of the protolytic forms of fluorescein in an aqueous solution (Sjöback R et al., Spectrochimica Acta A 1995; 51: L7–L21). The results demonstrate that the measurement of DR‐SFG excitation profiles is a useful technique to identify chemical species of monolayers on metal surfaces. Copyright © 2008 John Wiley & Sons, Ltd.