Advances in submicron infrared vibrational band chemical imaging

The technique of infrared near-field microscopy with submicron resolution is an important addition to the chemical sciences arsenal in the last few years. Although related to highly successful scanning optical probe microscopies in the visible, infrared near-field microscopy had to overcome several obstacles, which slowed its development. This review illustrates the history as well as the state of the art of this new field, its limitations and perspectives. At present, two main experimental approaches have been successful: the apertureless metal tip approach and the fibre tip aperture approach. The two variants are compared from the point of view of resolution, ease of implementation in the laboratory and image formation mechanisms. The techniques using chemically specific vibrational absorption contrast are emphasized here, in the general context of chemical microscopy, which includes other methods such as chemical force,]Raman and fluorescence microscopies. The phenomenon of surface-enhanced infrared absorption is also mentioned in relation to near-field infrared microscopy, with regard to important aspects of image formation and possible improvements. The main advantages of spatial resolution, chemical sensitivity; non-intrusiveness, minute amounts of specimen and the possibility of quantitative analytical measurements make infrared near-field microscopy a powerful tool. We also examine here possible future applications that go beyond the limits of classical vibrational microspectroscopy, as well as directions for additional advances.