Analytical tools for the nano world

Abstract

Nanotechnology and nanoscience are very important areas of modern research. They are supposed to give us judicious control over the properties of matter at the nanoscale (i.e., lengths of well below 1 μm in at least two dimensions). Nano objects include structures such as subcellular compartments, “molecular machines” and small biological objects, nanocomposites, and components of molecular electronics. However, there is currently a lack of chemical diagnostics and characterization methods for these nano objects and nanoscale structures, particularly methods for characterizing their molecular compositions. Established methods for studying matter with nanoscale lateral resolution, such as atomic force microscopy (AFM), scanning tunneling microscopy (STM), and scanning electron microscopy (SEM), typically give very little or no chemical information. Some exceptions exist—for example derivatized AFM tips that can be used for chemical recognition. In general, however, only the shape of a nanostructure, its surface topography, and its local electric properties can be measured, albeit in many cases with excellent spatial resolution, down to the atomic scale. However, with the advent of nanotechnology and nanoscience, the need for chemical and in particular molecular analysis has become urgent. Several new approaches to spectroscopic observation and imaging with nanometer-scale resolution have appeared recently, and will continue to set trends in the fields of nanoscience and nanotechnology. Scanning near-field optical microscopy (SNOM), the “optical cousin” of STM and AFM, emerged in the 1980s [1]. Standard aperture SNOM works by guiding light from a laser into an optical fiber whose tip is sharpened and metallized on the outside such that only a very small aperture (diameter ≈50 nm) passes laser light. If brought close to the object under investigation (within a few nm, i.e., into the optical near-field), this nanometer light source can be used to illuminate a spot on a sample that is considerably smaller than the optical diffraction limit. In order to obtain a high-resolution optical image of the sample, the SNOM tip is scanned over the sample surface with nanometer accuracy, using piezo elements for motion control. SNOM observation was initially restricted to fluorescence [2], but was later extended to Raman spectroscopy [3] and even to laser ablation mass spectrometry [4, 5]. One of the main limitations of SNOM is its optical throughput when using very small apertures. This limits both the achievable spatial resolution and the optical brightness of SNOM nano light sources. A way to circumvent this limitation is to use apertureless near-field methods, such as tip-enhanced Raman scattering (TERS). In this case, Raman scattering is amplified by the presence of a metallic nanoparticle brought close to the sample, using an AFM cantilever for example. Conceptually, this is an inversion of the geometry used in surface-enhanced Raman scattering (SERS). The advantage of this approach is that the sample can be illuminated using conventional optics, avoiding the much lower transmission of an aperture SNOM probe. Such apertureless or “tip-enhanced” nearfield spectroscopy experiments have been realized using coated AFM tips or sharpened metal tips, and are used for Anal Bioanal Chem (2008) 390:215–221 DOI 10.1007/s00216-007-1640-1