Chemical analysis breakthrough: AFM-IR nanospectroscopy
Figure 1: The Anasys nanoIR system displaying a topographic image of a PET/polyamide composite.
For many years, infrared spectroscopy has provided the ability to characterize and identify chemical species. However, it has always been restricted to spatial resolution in the order of 5 – 10 microns and then only when applying attenuated total reflection spectroscopy. Now, when combined with the nanoscale spatial resolution of an Atomic Force Microscope (AFM) tip, it is possible to measure and map local chemical composition below the diffraction limit of light. The technology described here as nanoIR will also perform nanoscale topographic, mechanical and thermal analyses.
This exciting and unique technology is provided through a new platform called nanoIR, a new product from Anasys Instruments, Santa Barbara, Calif. The nanoIR is a probe-based measurement tool that reveals the chemical composition of samples at the nanoscale (Figure 1). This laboratory solution combines key elements of both infrared spectroscopy and atomic force microscopy (AFM) to enable the acquisition of infrared spectra at spatial resolutions of 50 - 200 nm, well beyond the optical diffraction limit. Potential application areas span polymer science, materials science, and life science, including detailed studies of structure-property correlations.
How it works
The science behind the system applies the patent-pending technology of photothermal induced resonance (PTIR), a technique pioneered by award-winning researcher Dr. Alexandre Dazzi of the Laboratoire de Chimie Physique ,CLIO, Université Paris-Sud, Orsay, France.
Figure 2: Schematics to illustrate how the nanoIR works
The nanoIR system uses a pulsed, tunable IR source to excite molecular absorption in a sample that has been mounted on a ZnSe prism (figures 2 & 3). Samples are prepared in one of two ways. For many samples, ultramicrotomy is used to cut sections with thicknesses between 100 nm and 1000 nm. These are then transferred to the prism surface. In other sample preparations, it is possible to cast thin films from solvent directly on the prism.
The IR beam illuminates the sample by total internal reflection similar to conventional ATR spectroscopy. As the sample absorbs radiation, it heats up, leading to rapid thermal expansion that excites resonant oscillations of the cantilever which is detected using the standard AFM photodiode measurement system. These induced oscillations decay in a characteristic ringdown which can be analyzed via Fourier techniques to extract the amplitudes and frequencies of the oscillations. Then, measuring the amplitudes of the cantilever oscillation as a function of the source wavelength, local absorption spectra are created.
The oscillation frequencies of the ringdown are also related to the mechanical stiffness of the sample. With maximum flexibility, the IR source can also be tuned to a single wavelength to simultaneously map surface topography, mechanical properties, and IR absorption in selected absorption bands.
The potential application areas are found in the polymer, materials and life sciences offering notable potential in producing detailed studies of structure-property correlations.
Figure 3: Sample mounted on ZnSe prism
The ability to combine nanoscale spatial resolution with chemical spectroscopy provides users of nanoIR technology with the tools to quickly survey regions of a sample via AFM and then rapidly acquire high-resolution chemical spectra at selected regions on the sample. Making additional mechanical and thermal properties measurements with nanoscale resolution adds significantly to the power of this technique.
The nanoIR system enables researchers to harness the full power of mid-IR spectroscopy. The system’s IR source, designed using proprietary technology, is continuously tunable from 3600 to 1200 cm-1. This range covers a major portion of the mid-IR, including important CH, NH, and CO bands, as well as carbonyl and amide I/II bands. Polymer spectra acquired with the nanoIR system have demonstrated good correlation with bulk FTIR spectra. The nanoIR software also allows researchers to export nanoscale IR absorption spectra to standard analysis packages (e.g., Bio-Rad’s KnowItAll) to rapidly analyze samples and identify chemical components.
Figure 4: The multifunctional measurement capability of the nanoIR
The nanoIR software also allows integrated thermal and mechanical property mapping in addition to its ability to provide high-resolution infrared spectra. Mechanical properties of a sample can be collected using a contact resonance method to map stiffness variations simultaneously with the topography while the nanoIR platform can also perform nanoscale thermal analysis providing researchers to work beyond bulk thermal analysis measurement to obtain information not available with any other technique.
To give a very clear example of the technique is to look at the results of figure 4. This illustrates topographic (A), spectroscopic (B), mechanical (C) and thermal (D) data sets from a composite sample of nylon and ethylene acrylic acetate.
The mechanical and spectroscopic data was obtained simultaneously, thus allowing direct correlation of mechanical stiffness information with chemical composition data. Note that the transitions in contact stiffness correlate extremely well with the strength of the CH absorption. Nanothermal analysis was also performed on the same sample clearly identifying softening at different temperatures for the Nylon and EAA layers.
Figure 5: Spectral mapping of a degradable polymer blend
Figure 5 illustrates the topographic and point spectroscopic mapping of a degradable polymer blend. It shows that AFM measurements allow spatial mapping of polymer matrix and additives. The nanoIR can then spatially map variations in chemical components. In the line spectral map (right), note the spatially varying concentration of the C=O carbonyl band (1740 cm-1) and the single bond C-O peak at around 1100 cm-1.
Looking forward – applications developments
Applying AFM-IR in the nanoIR system combines infrared spectroscopy and atomic force microscopy to provide high resolution topographic, chemical, mechanical and thermal mapping. This combination provides spatial resolution at length scales well below the diffraction limit of conventional IR spectroscopy and adds chemical spectroscopy to the field of atomic force microscopy. The potential for future applications is very broad and includes polymer blends, multilayer films and laminates. In the life sciences, sub cellular spectroscopy and the study of tissue morphology/histology will be eagerly awaited while in materials science, prospects are good for the study of organic photovoltaics and defect analysis.