Unprecedented Spatially Resolved Chemical Analysis Via Nanoscale Spectroscopy

Kevin Kjoller, Craig Prater and Roshan Shetty, !%Anasys Instruments Corp.%!

The phrase “lab on a tip,” referring to nanoscale property measurements made by the probe of an atomic force microscope (AFM), has been a dream for the developers of analytical instrumentation. There also has been a drive for “hyphenated techniques” that make multiple types of complementary measurements simultaneously. This has been driven with investment in nanotechnology in many fields, notably in the materials and life sciences, especially for studies that correlate structure and property or function. Looking at a polymer company, one sees that many analytical techniques are used, with infrared spectroscopy and thermal analysis being two of the most common. However, the resolution of such “macro” techniques will not explain behavior on length scales less than 1 µm. For example, how do polymers behave at the domain level; e.g., on the scale of 100 nm?

The solution has been provided with nanoIR technology from Anasys Instruments Corp., which overcomes the major barriers in AFM and conventional IR spectroscopy (Figure 1). AFM has outstanding resolution on the nanoscale but cannot perform chemical spectroscopy. IR spectroscopy has been the benchmark tool for chemical characterization but has lacked the spatial resolution to address nanoscale problems.

Figure 1. Anasys Instruments Corp.’s nanoIR system.

In addition to revealing chemical composition, the nanoIR system provides high-resolution characterization of local topographic, mechanical and thermal properties. This new tool will help facilitate materials and life sciences research at the nanoscale.

The nanoIR system combines the nano-scale spatial resolution capabilities of AFM with IR spectroscopy’s ability to characterize and identify chemical species. Users of nanoIR technology can quickly survey regions of a sample via AFM and then rapidly acquire high-resolution chemical spectra at the selected regions. The system also can be programmed to automatically acquire spectra from an array of points across the sample. Mechanical and thermal properties, such as local thermal transitions, also may be mapped with nanoscale resolution.

How does nanoIR work?

The science behind the system applies the patent-pending technology of photo-thermal induced resonance, a technique pioneered by Dr. Alexandre Dazzi of the Laboratoire de Chimie Physique at the Université de Paris-Sud in Orsay, France. The nanoIR system uses a pulsed, tunable IR source to excite molecular absorption in a sample that has been mounted on a ZnSe prism. Samples are prepared in one of two ways. For many samples, ultramicrotomy is used to cut sections with thicknesses between 100 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 onto the prism.

The IR beam illuminates the sample by total internal reflection similar to conventional attenuated total reflection spectroscopy (Figure 2). As the sample absorbs radiation, it heats up, leading to rapid thermal expansion that excites resonant oscillations of the cantilever, which are 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, by measuring the amplitudes of the cantilever oscillation as a function of the source wavelength, local absorption spectra are created.

Figure 2. The basic principles of the nanoIR technology.

The oscillation frequencies of the ringdown also are 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.

At work with nanoIR

Potential application areas for nanoIR are broad. They include polymer blends, multilayer films and laminates, organic defect analysis, tissue morphology and histology, subcellular spectroscopy and organic photovoltaics. Polymer spectra acquired with the nanoIR system are rich and interpretable, and they have demonstrated good correlation with bulk Fourier transform infrared spectra. The nanoIR software allows researchers to export nanoscale IR absorption spectra to standard analysis packages. With this interface, nanoIR spectra can be used to rapidly analyze samples and identify specific chemical components.

Integrated measurements that permit the user to see multiple results from a single sample from one instrument platform are very beneficial. This power is well illustrated in this landmark example of a nylon-ethylene acrylic acid composite.

Figure 3. (a) A high-resolution AFM image shows the topography of a composite sample comprising nylon and ethylene acrylic acid (EAA). (b) The composite of chemicals characterized by nanoscale IR spectroscopy. (c) The mechanical properties of the composite are mapped at the nanoscale. (d) Local transition temperatures are reported through nanothermal analysis.

In Figure 3, four data sets are shown. Figure 3a shows a familiar high-resolution AFM image that provides the topography of the surface of the material. This contrast image shows stripes of the component polymers. Figure 3b shows the chemical characterization by nanoscale IR spectroscopy – measured by photothermal induced resonance. Figure 3c illustrates nanoscale mechanical property mapping measured by contact resonance, and Figure 3d shows local transition temperatures reported through nanothermal (nanoTA) analysis, which is analogous to the macro thermomechanical analysis.

Figure 4. A nanospectroscopic examination of a polyethylene terephthalate-nylon composite.

A further example is of a polyethylene terephthalate-nylon composite (Figure 4). Here, by highlighting individual components, the spectroscopy data is examined more closely. Biodegradable polymers are important materials in a variety of applications ranging from tissue engineering, drug delivery and food packaging to textiles. Such materials are increasingly complex blends of base materials and performance-enhancing additives. The nanoIR system has been used to map, characterize and even identify specific polymer additives.

The data in Figure 5 shows the spectral mapping of a biodegradable polymer blend. AFM measurements allow spatial mapping of polymer matrix and additives. The nanoIR can then spatially map variations in chemical components. In the spectral line 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.

Figure 5. Spectral mapping of a biodegradable polymer blend.

More power for analysis

Leading scientists have offered their thoughts on the use and potential of nanoIR. President-elect of the Society of Applied Spectroscopy and a retired research fellow from Procter & Gamble, Dr. Curtis Marcott, said that IR micro-spectroscopy has already proved itself extremely valuable for addressing a wide range of problems in science and industry. He said he is excited over the new nanoIR technology because it will allow scientists to break through the submicron spatial resolution barrier and apply IR spectroscopy to new classes of problems beyond current capabilities.

Dr. Katherine (Kallie) Willets, an expert in nanomaterials spectroscopy and microscopy at the University of Texas at Austin, was quoted in Chemical & Engineering News following Pittcon (March 29, 2010, Vol. 88, No. 13). She said, “By far, the product that excited me most in molecular spectroscopy was the nanoIR from Anasys Instruments. This system combines the rich vibrational information of IR spectroscopy with the high-resolution imaging offered by atomic force microscopy. The instrument’s ability to measure IR absorption spectra from nanoscale regions of a sample is extremely exciting and opens up new avenues for nanoscale characterization using vibrational spectroscopy.”

In the literature, several new papers have shown how the developers continue to identify new and exciting applications. A selection is provided below. This all goes to underscore the value of this exciting new technique, nanoTA.

Acknowledgments

The nanoIR system is the result of several million dollars of government and private investment. Anasys Instruments was awarded $2.6 million in research grants from the NIST Advanced Technology Program and the National Science Foundation. US and foreign patents are pending.

Further reading

C. Mayet et al (2010) In situ identification and imaging of bacterial polymer nanogranules by infrared nanospectroscopy. Analyst, pp. 2540-2545.

A. Dazzi et al (2010). Theory of infrared nano-spectroscopy by photothermal induced resonance. J Appl Phys, p. 124519.

K. Kjoller et al (2010). High-sensitivity nanometer-scale infrared spectroscopy using a contact mode microcantilever with an internal resonator. Nanotechnology, p. 185705.

J. Houel et al (2009). Mid-IR absorption measured at lambda/400 resolution with AFM. Opt Exp, pp. 10887-10894.

Meet the author

Roshan Shetty is the chief executive officer of Anasys Instruments; e-mail: roshan@anasysinstruments.com