SPR of Ultrathin Organic Films

Surfaceplasmonresonance (SPR) methods are surface-sensitive spectroscopic techniques that can be used to characterize a variety of ultrathin organic monolayers and multilayers at gold, silver, and copper surfaces. For example, SPR measurements have been used to study Langmuir-Blodgett films {1-3), selfassembled organic monolayers (4,5), specifically and nonspecifically adsorbed biopolymers (including DNA) (6-13), nonlinear optical thin films (14), and thin organic films at electrochemical interfaces (15,16). SPR methods have been used to enhance fluorescence (17,18), Raman scattering (18-22), and optical second harmonic generation at metal/dielectric interfaces (23,24), but in their simplest form, SPR measurements are used to monitor the changes in thickness or index of refraction of ultrathin organic films at metal surfaces. In this respect, SPR measurements are similar to ellipsometry (25) except that the two methods have different degrees of surface specificity and sensitivity. The SPR index of refraction measurement is the basis of the recently introduced BIACORE SPR adsorption instrument (1,26). In this Report, three different examples of how SPR techniques can be applied to studying ultrathin organic films are discussed (Figure 1). In Figure la, SPR scanning and imaging measurements are used to study protein adsorption onto chemically modified gold surfaces. In Figure lb, SPR imaging measurements are used to follow the sequential hybridization adsorption of DNA molecules. In Figure lc, the electrostatic fields inside a noncentrosymmetric zirconium phosphonate film incorporated into an electrochemical interface are measured with electrochemically modulated SPR methods.