SURFACE ENHANCED RAMAN SPECTROSCOPY Robin L. Garrell Department of Chemistry University of Pittsburgh Pittsburgh, PA 15260
The diverse chemical processes of catalysis, adhesion, and lubrication, as well as many electrochemical reactions, are controlled largely by the interactions of adsorbate molecules or ions with surfaces. The strength of the interactions between adsorbates and surfaces determines the stability of the interface, and the adsorption-desorption kinetics play a key role in determining reaction rates at interfaces. Understanding how molecules interact with surfaces provides the fundamental insight needed for designing more efficient catalysts, electrodes, chromatographic supports, biosensors, coatings, and biocompatible devices. For many analytical techniques, in0003-2700/89/0361-401A/$01.50/0 © 1989 American Chemical Society
cluding perhaps the majority of commonly employed chromatographic separations and electrochemical processes, the interfaces of interest are between two condensed phases: liquidliquid, liquid-solid, and solid-solid interfaces. Ideally, the probes we use to characterize these complex interfaces should be in situ techniques—techniques that do not require high-vacuum conditions or dissection of the device or multilayer system. The techniques should allow us to determine how much is adsorbed on a surface, how strongly the adsorbate is bound, whether the adsorbate has a preferred orientation, and whether it reacts at the interface. The techniques should also allow us to determine whether any of these properties depend on conditions such as the charge at the surface, the solution ionic strength, and the presence of adsorbates that can compete for surface sites.
Vibrational spectroscopic techniques are particularly useful for characterizing adsorbate-substrate interactions. At the very least, the IR or Raman spectrum of an adsorbate may be used as a fingerprint to identify the adsorbed species. Yet much more information is embedded in surface vi-
REPORT brational spectra. The overall spectral intensity provides a measure of the relative surface coverage of the adsorbate, and the relative intensities of individual bands can be used to determine the average orientation of adsorbate molecules on the surface. Shifts in vibrational frequencies and relative intensities in the spectra of adsorbed analytes compared with the same analytes in so-
ANALYTICAL CHEMISTRY, VOL. 61, NO. 6, MARCH 15, 1989 · 401 A
REPORT lution provide information on the aver age conformation of adsorbate mole cules, the relative proximity of differ ent parts of the adsorbate molecules to the surface, and the strength of adsorbate-surface interactions. The wealth of information that can be extracted from the vibrational spec tra of adsorbed species has been a cata lyst for tremendous advances in sur face-sensitive IR techniques, including diffuse reflectance IR-Fourier trans form (DRIFT) spectroscopy, IR exter nal reflection spectroscopy (IR-ERS), IR reflection absorption spectroscopy (IRRAS), subtractively normalized in terfacial FT-IR spectroscopy (SNIFTIRS), and single potential-alteration surface IR spectroscopy (SPAIRS). Al though these techniques are proving to be invaluable tools in the analytical chemist's armamentarium, they suffer from a lack of sensitivity to submonolayer coverages and from the relative difficulty of probing solution-solid in terfaces or polymer-modified surfaces when the bulk phase (e.g., solution or polymer) is strongly absorbing in the IR region. Conventional Raman spec troscopy is suitable for the study of aqueous solution-solid interfaces, but the inherent weakness of the Raman scattering process makes probing thin layers or submonolayers on surfaces extremely difficult. The discovery of the surface-en hanced Raman effect in 1974 generated considerable excitement in the physics and chemistry communities (1). In many ways, surface-enhanced Raman spectroscopy, or SERS, has revolution ized the study of condensed phasemetal interfaces. It is now possible to study molecules and ions adsorbed from solution onto metal surfaces at submonolayer coverages, to observe competitive adsorption processes, and to monitor reactions at solution-metal interfaces. The surface can provide up to 107-fold enhancement of the Raman scattered intensity of the adsorbed spe cies while scattering from the solvent and species in solution remains rela tively weak. Hence SERS offers selec tivity for adsorbed analytes. Because of the large surface enhancement effect and the small sample volumes re quired, SERS is a very sensitive tech nique; current detection limits are in the picomole-to-femtomole range. Greater sensitivity is achieved for adsorbates that exhibit resonance en hancement in addition to surface en hancement of their Raman spectra. The richness of the spectroscopic in formation, surface selectivity, and sen sitivity of SERS provides impetus for using it to solve problems in analytical chemistry. An incredibly wide variety
of adsorbates has been studied by SERS. The review by Seki in the Sug gested Reading list gives a comprehen sive list of more than 500 SERS studies up to 1985, and more than 2800 SERS papers have been published to date. This R E P O R T will review the basis for the technique and its experimental requirements, describe a few examples of the analytical problems to which SERS has been and can be applied, and provide a perspective on the current limitations and frontiers in developing SERS as an analytical technique. The Suggested Reading list contains several excellent reviews of current theories explaining the observed enhancement and applications of SERS to the char acterization of macromolecular adsor bates such as proteins and nucleic ac ids. These reviews also document the extensive work done by analytical chemists who are testing possible mechanisms for SERS, developing novel substrates, and characterizing the adsorption of a wide variety of small molecules and ions on electrode surfaces. SERS theory Our theoretical understanding of the mechanisms for surface enhancement is not yet complete. It is generally agreed that in any given adsorbatesubstrate system several enhancement mechanisms may be operating (2). However, the relative importance of the various mechanisms is somewhat controversial. To get a qualitative feel ing for the enhancement mechanisms, it is useful first to examine the Raman scattering process itself. The intensity of Raman scattering is proportional to the square of the elec tric field-induced dipole moment, P, where P =
