A team of nanoscientists from Spain and Germany has developed a new technique for chemically identifying materials at the nanoscale, with both high sensitivity to chemical composition and ultra-high spatial resolution. The approach can chemically identify practically any material by its molecular vibrational infrared fingerprint, as long as it is infrared active. The new optical technique, called nano-FTIR, could offer a range of research applications in polymer chemistry, biomedicine, pharmaceutical and organic electronics, as well as in the identification of nanoscale sample contaminations.
A variety of high-resolution imaging techniques already exist, such as electron microscopy and scanning probe microscopy, but their chemical sensitivity cannot meet the demands of modern chemical nano-analytics. Optical spectroscopy, on the other hand, does offer high chemical sensitivity, but its spatial resolution is limited by diffraction to around half the incident light wavelength, thus preventing nanoscale resolved chemical mapping.
However, this new study, published in Nano Letters [Huth et al, Nano Lett (2012) doi: 10.1021/nl301159v], showed how nano-FTIR, combining scattering-type scanning near-field optical microscopy (s-SNOM) with Fourier transform infrared (FTIR) spectroscopy, allows for the chemical identification and mapping of nanomaterials. The technique involves scattering infrared laser light at the metalized tip of an atomic force microscope (AFM). The backscattered light is analyzed with a Fourier transform infrared spectrometer, yielding the material's local infrared response. This method provides a spatial resolution of about 10 nm – up to 1000 times better than conventional FTIR microscopy.
As researcher Rainer Hillenbrand pointed out,“Our asymmetric spectrometer allows for recording the real and imaginary part of the scattered field as a function of the wavelength. We could demonstrate that the imaginary part yields infrared spectra that correspond to conventional FTIR absorption spectra. “This is an aspect of enormous practical relevance – because the nano-FITR spectra match extremely well with conventional FTIR spectra, standard FTIR databases can be directly used for analyzing nano-FTIR spectra. This is expected to significantly boostthe acceptance of nano-FTIR.
The technology could also provide benefits in the fields of mineralogy and geology, especially as crystal vibrations display strong infrared resonances that are perfectly suitable for chemical – as well as structural – identification. The research team will now attempt to strengthen the core technology by offering even broader infrared light sources, and look to increasethe key factors of spatial resolution, sensitivity and speed.