Tip‐enhanced optical spectroscopy for surface analysis in biosciences

Optical spectroscopy provides a wealth of information on the electronic and vibronic states of biological materials and surfaces, thus allowing for a detailed analysis of the sample constituents. Furthermore, information on photoprocesses such as excited‐state relaxation, charge transfer and coupling of individual quantum systems, as for example, in light‐harvesting complexes, can also be obtained. Advances in near‐field optics open up new means to overcome the diffraction limit and extend the range of optical measurements to the length scales of most nanosystems. In this paper, we describe a near‐field microscopic technique that relies on the enhanced electric field near a sharp, laser‐irradiated metal tip. This confined light source can be used for highly localized excitation of photoluminescence (PL) and Raman scattering. We study the properties of the enhanced fields and demonstrate PL and Raman imaging of single‐walled carbon nanotubes (SWCNTs) with sub‐15 nm resolution. The existing studies on biomaterials using tip‐enhanced techniques in the literature are reviewed and potential applications together with the requirements on sample materials are discussed.