Near-Field Technique Spectrally Maps Nanoparticles and Viruses

Richard Gaughan

Apertureless near-field scanning optical microscopy produces high-resolution optical images by irradiating with a laser an atomic force microscope (AFM) probe in proximity to a sample to create a concentrated near-field spot of illumination at the apex of the probe.

Standard metallized AFM probes have tip diameters of about 30 nm, so the incident electromagnetic field is concentrated into a spot much smaller than the diffraction limit. Researchers in Germany have exploited this effect in an interferometric method that can identify resonant vibrational modes, providing direct information about constituents in the sample.

Electromagnetic field-enhancement effects in the vicinity of a metallized atomic force microscope probe make it possible to collect infrared spectra on the scale of a few tens of nanometers. In this sample, polymethyl methacrylate spheres (red arrow) are both chemically and topographically distinct from the tobacco mosaic viruses (blue arrow) on the same substrate. Reprinted courtesy of the American Chemical Society.

Developed by Markus Brehm, Fritz Keilmann and their colleagues at Max Planck Institut für Biochemie in Martinsried and at the Center for Nanoscience at Ludwig Maximilians Universität in Munich, the approach employs a line-tunable, alcohol-cooled CO laser from Invivo GmbH of Adelzhausen, Germany, as the illumination source. The laser radiates in the 6-μm-wavelength region.

Surface height data and scattered infrared radiation are collected by translating the sample under the AFM probe. The scattered radiation is recombined interferometrically with the source illumination, creating both an intensity map and a phase map of the sample. If the sample within the field concentration at the probe aperture has a resonant line near the frequency of the laser output, the relative phase change will peak.

After one scan is completed, the CO laser is tuned to another line, and the sequence is repeated. In the current configuration, each scan takes 10 minutes, with an additional five minutes required to change the laser line. Because repeated imaging is a source of error, the scientists are introducing an infrared optical frequency comb source that can obtain a complete spectrum at each pixel.

To demonstrate the selectivity of the technique, they acquired images of polymethyl methacrylate nanospheres and 18-nm-diameter tobacco mosaic viruses on silicon substrates. The method identified the distinct resonances of the two types of samples. It also demonstrated its insensitivity to topographic changes, yielding identical phase contrast signals from 37- and 68-nm-diameter nano-spheres.

The method benefits from the signal-enhancement effects of near-field scanning optical microscopy. In the demonstration, the phase contrast of particles just a few tens of nanometers thick was equivalent to that of a 1-μm-thick sample gathered by conventional IR spectroscopy.

Its high resolution makes the technique applicable anywhere that nanoscale chemical recognition is needed, Brehm said. He added that it is promising for other applications in which small samples must be chemically identified, even when high spatial resolution is not necessary.

“The fact that meaningful spectra can be extracted independent of the substrate, independent of the object’s dimensions and in an environment that is chemically and structurally inhomogeneous should make this method applicable to a wide set of problems,” he said.

Nano Letters, July 2006, pp. 1307-1310.