Second harmonic spectroscopy of aqueous nano- and microparticle interfaces.

Applications of the interface-selective methods of second harmonic (SHG) and sum frequency (SFG) spectroscopy to the investigation of equilibrium and time-dependent properties of interfaces are increasing at an explosive rate.1-53 Among the interfaces studied with these methods are liquid/ gas, solid/gas, and the buried liquid/liquid, liquid/solid, and solid/solid interfaces.35,36,39,41,51,53 In 1996, I was able to write a Chemical ReViews article that was a reasonably comprehensive review of the field. However today, this would require a book-length review if only second harmonic spectroscopy was considered. Though restricting the review to aqueous interfaces for the purposes of this Chemical ReViews issue is helpful, it does not reduce the extensive research topics to a manageable length. On the basis of these considerations, I have limited the review to a rapidly growing area of research, which uses SHG as the primary method. This new area addresses the aqueous interfaces of nanoand microsize particles suspended in aqueous solutions. These small particles in aqueous solution are commonly referred to as colloids. Although colloidal chemistry is a wellestablished field, important in basic science, technology, and medicine,54,55 it is only in the past decade that SHG and SFG spectroscopies have been used specifically to probe aqueous colloid surfaces. One can anticipate that SHG and SFG will have a major impact on the future development of interfacial colloid chemistry.