Combined Instrument Reveals Relationships Between Chemistry and Surface Physics

David L. Shenkenberg

Researchers from Georgia Institute of Technology in Atlanta have combined two highly sensitive techniques into a single system and have demonstrated its efficacy on a model process. The combination revealed a correspondence between changes in surface mechanics and changes in overall chemistry that might only have been hinted at if the two instruments had been used separately.

Researchers combined atomic force microscopy and infrared-attenuated total reflection (IR-ATR) spectroscopy. The combination synchronously provides chemical and surface physical information with high sensitivity. This is a top-down schematic of the combined system (FTIR = Fourier transform infrared).

They combined an atomic force microscope (AFM) and equipment for infrared-attenuated total reflection
(IR-ATR) spectroscopy, instruments that are used for measuring surface mechanics and overall chemical changes, respectively. They had sufficient space to arrange the spectroscopy equipment around the AFM because they used an Agilent Technologies microscope that scans from the top down, said principal investigator Boris Mizaikoff, who, together with postdoctoral fellow Martin Brucherseifer and scanning probe microscopist Christine Kranz, developed the system.

To combine the techniques, they attached a zinc sulfide crystal waveguide to the bottom of the sample holder. Parabolic gold mirrors directed the beam to and from the crystal, and the optical path was contained inside a plexiglass compartment filled with nitrogen gas.

They focused the beam from the spectrometer onto the crystal at the critical angle. The beam was totally reflected within the crystal, generating at the waveguide surface a standing wave called an evanescent field, which can interact with sample molecules. The beam ultimately exited the crystal and traveled to a detector, carrying with it the information gained from interactions with the molecules.

The researchers used a modular Fourier transform infrared spectrometer from L.O.T.-Oriel GmbH of Darmstadt, Germany, and a liquid nitrogen-cooled photoconductive mercury cadmium telluride detector from Infrared Associates Inc. of Stuart, Fla.

To demonstrate the efficacy of the system, they investigated the well-studied model process of urea dissolving in an organic solution. They recorded infrared spectra at a rate of 60 per minute while using the AFM to scan an area of 20 × 15 μm at a speed of 83 μm/s, for a total time of 20 min. They achieved a spectroscopic signal-to-noise ratio of >50 per scan and obtained spectral peaks that matched the known spectral peaks of urea. The AFM and IR-ATR spectral data correlated well, demonstrating the feasibility of the combined system.

“The single most important aspect is that it’s literally a combined platform,” Mizaikoff said. “You can do AFM and spectroscopy with space and time synchronization.”

The researchers noted that they could improve the resolution of the technique using single-mode mid-IR waveguides frequency-matched to distributed-feedback quantum cascade lasers, which they previously developed. Mizaikoff said that doing so would enable them to change the waveguide geometry to increase the sensitivity by up to two orders of magnitude.

He believes that it would be relatively easy for other researchers to make a combined system, adding that they would have to integrate the crystal with the sample holder and use an AFM that allows room for the optical path.

Analytical Chemistry, Nov. 15, 2007, pp. 8803-8806.