IR Spectroscopy Maps Linker Films

Anne L. Fischer

In fields as diverse as genomics and defense, surfaces must be scanned for traces of chemicals and other substances. The use of self-assembled monolayers is an effective method of modifying the properties of these surfaces, acting as linker films that adsorb biomolecules for sensing applications.

To improve the characterization of such films, researchers at Technische Universität Dresden in Germany and Vilnius University in Lithuania have used polarization modulation infrared reflection absorption spectroscopy in conjunction with a Fourier transform infrared (FTIR) spectrometer to reveal inhomogeneities within monolayers of phosphonic acid.

The polarization modulation infrared reflection absorption spectroscopy system was constructed as an accessory to a Fourier transform IR spectrometer.
Scanning tunneling and atomic force microscopy techniques have been used to image self-assembled monolayers. To probe the quality of a microstructured film across a surface a few millimeters in size, however, it is necessary to image the whole film, said Gerald Steiner of Institut für Analytische Chemie at Technische Universität Dresden. Both microscopy techniques can image a surface at a resolution of less than 1 nm, but they are not suitable for imaging a surface or microstructure larger than 100 μm.

The polarization modulation IR reflection absorption spectroscopy mapping accessory (yellow ray) is shown with a surface plasmon resonance imaging (red ray) unit.

The scientists thus turned to polarization modulation infrared reflection absorption spectroscopy to obtain a spatial resolution down to 50 μm. Their setup was constructed as an accessory to an FTIR spectrometer from Bruker Optik GmbH of Ettlingen, Germany.

The IR excitation beam from the spectrometer is focused by a spherical gold mirror, linearly polarized by a wire grid polarizer and modulated by a ZnSe photoelastic modulator from Hinds Instruments Inc. of Hillsboro, Ore. A pinhole in the focal point of the beam shields marginal rays and forms a small point source for an IR objective. A second IR objective collects the radiation reflected from the sample and focuses it onto a cooled HgCdTe detector. Spectra are collected by a point-by-point rastering of the sample.

This bright-field image taken with the IR spectroscopy system shows chemical information in the spectral region of 1500 to 1000 cm^–1. The sample comprises self-assembled monolayers of phosphonic acid attached to a microstructure of aluminum oxide on a gold-coated glass substrate. The red and yellow pixels indicate a high amount of phosphonic acid, corresponding to the aluminum-oxide microstructure. The blue and green pixels indicate a low amount of phosphonic acid, corresponding to gold.

Steiner said that the mapping setup has commercialization potential but that the researchers must figure out how to overcome the energy-limited spatial resolution. Many applications, such as the characterization of biosensor arrays and microstructures, require a high spatial resolution and signal-to-noise ratio. To solve this problem, a high-intensity IR source is needed.

Analytical Chemistry, online March 14, 2006, doi:10.1021/ac050481a.