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This paper discusses the use of optical photothermal infrared (O-PTIR) spectroscopy combined with Raman analysis. The new technique overcomes many of the limitations of conventional FTIR and Raman spectroscopy when used alone. It is based on an infrared-visible pump-probe system that incorporates a wavelength-tunable IR laser that emits a pulsed beam that is combined colinearly with the output of a 532-nm green laser. As the paper explains, infrared radiation is partially absorbed by the test target when the wavelength of the laser resonates with the vibrational mode of the material. This excitation process causes the area under the infrared spot to heat up, in turn, causing local expansion along with changes in the refractive indices. These photothermal effects cycle on and off in synch with the pulsed IR beam and the amplitudes of the on-off states are captured by the co-located visible beam and plotted as a function of wavelength over the tunable range of the IR laser. The diffraction limited spot size of the visible beam is approximately 416 nm, corresponding to a spatial resolution of about 1 μm, which is 30 times more precise than conventional FTIR. In addition, by measuring photothermal effects in localized regions, it is possible to identify chemicals in quantities of matter as small as 0.4 pg. By comparison, the sensitivity of transmission mode FTIR is significantly less at around 100 pg.

Optical photothermal infrared (O-PTIR) and Raman spectroscopy and imaging was used to explore the spatial distributions of molecular constituents of a laminate sample consisting of the bioplastics, polyhydroxyalkanoate (PHA) and polylactic acid (PLA), near the interfacial boundary. Highly spatially resolved simultaneous IR and Raman spectra were sequentially collected at 100 nm increments along a line traversing the interface. The set of spectra were subjected to 2D-COS analysis to extract the detailed nature of the spatial distribution of the laminate constituents. It was revealed that the laminate is not a simple binary system of two non-interacting polymers, but consists of different constituents with more complex spatial distributions. Some portion of PLA seems to penetrate into the PHA layer. The crystallinity of PHA near the interface is reduced compared to the rest of the PHA layer. The result suggests the existence of some partial molecular mixing even for these seemingly immiscible polymer pairs. The mixing probably occurs at the segmental level confined to only several hundred nanometers of space at the interface. Such partial mixing may explain the high compatibility between the two bioplastics.

Optical Photothermal Infrared (O-PTIR) spectroscopy is a new technique for measuring submicron spatial resolution IR spectra with little or no sample preparation. This speeds up analysis times benefiting high-volume manufacturers through gaining insight into process contamination that occurs during development and on production lines. The ability to rapidly obtain far-field non-contact IR spectra at high spatial resolution facilitates the chemical identification of small organic contaminants that are not possible to measure with conventional Fourier transform infrared (FT-IR) microspectroscopy. The unique pump-probe system architecture also facilitates submicron simultaneous IR + Raman microscopy from the same spot with the same spatial resolution. With these unique capabilities, O-PTIR is finding utilization in the high-volume and high-value industries of high-tech componentry (memory storage, electronics, displays, etc.).

Rapid identification of organic contamination in the semi and semi related industry is a major concern for research and manufacturing. Organic contamination can affect a system or subsystem’s performance and cause premature failure of the product. As an example, in February 2019 the Taiwan Semiconductor Manufacturing Company (TMSC), a major semiconductor manufacturer, reported that a photoresist it used included a specific element which was abnormally treated, creating a foreign polymer in the photoresist resulting in an estimated loss of $550M [1].

We demonstrate sub-micron, non-contact measurement of infrared spectra from failed interconnects in microelectronics and from a dark contamination particle in a polymer-based computer screen. This unique capability has been achieved with the advent of Optical PhotoThermal InfraRed (O-PTIR) spectroscopy, which has only been recently commercially available in the simultaneous IR and Raman platform. The defective regions of the exhibited examples have been exposed to the surface and have been analyzed as received to highlight the relaxed sample preparation requirement. The presentation illustrates the versatility and high spatial resolution of the O-PTIR technique for contamination identification and failure analysis applications.

We have observed, for the first time, a heterogeneous spatial distribution of crystalline polymorphs in a single electrospun polymer nanofiber. Two crystalline polymorphs of PHBHx, the thermodynamically stable α-form consisting of chains with a 21 helical conformation and the metastable β-form consisting of chains with a planar zigzag conformation, are spatially distributed as a core–shell structure composed of an α-form-rich core and a β-form-rich shell. In addition, it was found that the thickness of the shell is independent of the fiber diameter. The characterization of crystalline polymorphic distribution in individual nanofibers has been made possible by utilizing a technique combining atomic force microscopy (AFM) and infrared spectroscopy (IR), which simultaneously provides the nanoscale spatial resolution and crystalline phase specificity. Based on the experimental results, a possible generation mechanism of this polymorphic heterogeneous core–shell structure is proposed. The implications of this ...

Failure analysis of organics at the microscopic scale is an increasingly important requirement, with traditional analytical tools such as FTIR and Raman microscopy, having significant limitations in either spatial resolution or data quality. We introduce here a new method of obtaining Infrared microspectroscopic information, at the submicron level in reflection (far-field) mode, called Optical-Photothermal Infrared (O-PTIR) spectroscopy, that can also generate simultaneous Raman spectra, from the same spot, at the same time and with the same spatial resolution. This novel combination of these two correlative techniques can be considered to be complimentary and confirmatory, in which the IR confirms the Raman result and vice-versa, to yield more accurate and therefore more confident organic unknowns analysis.

Atomic force microscopy (AFM) and infrared (IR) spectroscopy have been combined in a single instrument (AFM-IR) capable of producing IR spectra and absorption images at sub-micrometer spatial resolution. This new device enables cross sections of multilayer films to be spectroscopically characterized at levels not previously possible. In particular, it was possible to observe nanoscale IR spectroscopic differences, as well as thermal and mechanical property differences, in the tie layers located between the individual polyethylene and polyamide layers of a multilayer cling film of initially unknown structure. It also appears that a two-μm-thick barrier layer between two polyamide layers near the center of the multilayer cling film consists of an ethylene-vinyl alcohol copolymer. Mechanical stiffness and thermal property differences are also observed between the various layers in the film. This powerful capability should prove generally useful for reverse engineering complex unknown multilayer film materials, as well as in aiding the intelligent design and coextrusion of superior multilayer film materials.

Introduction Composite materials are becoming increasingly important in today’s world, where lighter materials with enhanced properties are in high demand. Carbon fibers, carbon black, graphite, graphine, carbon nanotubes, quartz particles, nanocrystalline cellulose, and clays are among the materials being added to bulk polymers in an effort to achieve better properties and performance. It is important not only to determine the size and locations of nanoparticle inclusions in bulk polymers, but also to characterize the important interphase region where the components interact. This article describes how an atomic force microscope (AFM) combined with infrared (IR) spectroscopy and mechanical spectroscopy can be used to not only locate and determine the size of inclusions, but also to characterize them chemically and mechanically. After introducing AFM-IR spectroscopy and Lorentz contact resonance (LCR) methodology for obtaining nanoscale mechanical spectra and images, results from three specific applications will be discussed. These applications include an isotactic poly(propylene) film with added SiO2 particles, a polymer with carbon black particles incorporated under different processing conditions, and a carbon-fiber/epoxy composite material. The first example uses AFM-IR spectroscopy and IR absorbance imaging. The second example employs LCR mechanical property spectroscopy and imaging. The final example includes a combination of AFM-IR and LCR to obtain corroborating information about the important interphase region between carbon fiber and epoxy domains.

The nanoscale distribution of distinctly different water molecules within a perfluorinated sulfonic acid polymer Nafion film has been chemically imaged for the first time. The visualization was achieved using the newly developed AFM-IR microspectroscopy method, which enabled the chemical imaging of water domains to be separated based upon their characteristic water molecular OH-stretching vibration. Chemical imaging results have revealed ionic bound water molecules, iH2O, (3211 cm−1) that were clustered in domains, while the hydrated free bulk-like water molecules, bH2O, (3482 cm−1) were found within transportable channels that linked these clusters. These results give invaluable experimental chemical evidence required to test the numerous cluster-network models that have been proposed so far. A direct visual insight of the Nafion water structure given by this method will enlighten the understanding of the present proton exchange mechanism that is essential for the development of more efficient fuel cells. Keywords: AFM-IR, Atomic force microscopy, Infrared spectroscopy, Nafion, Water channels, Fuel cells

Poly[(R)-3-hydroxyalkanoate]s or PHAs are aliphatic polyesters produced by numerous microorganisms. They are accumulated as energy and carbon reserve in the form of small intracellular vesicles. Poly[(R)-3-hydroxybutyrate] (PHB) is the most ubiquitous and simplest PHA. An atomic force microscope coupled with a tunable infrared laser (AFM-IR) was used to record highly spatially resolved infrared spectra of commercial purified PHB and native PHB within bacteria. For the first time, the crystallinity degree of native PHB within vesicle has been directly evaluated in situ without alteration due to the measure or extraction and purification steps of the polymer: native PHB is in crystalline state at 15% whereas crystallinity degree reaches 57% in commercial PHB. Chloroform addition on native PHB induces crystallization of the polymer within bacteria up to 60%. This possibility of probing and changing the physical state of polymer in situ could open alternative ways of production for PHB and others biopolymers. Graphical abstract An atomic force microscope coupled with a tunable infrared laser (AFM-IR) has been used to record local infrared spectra of biopolymer PHB within bacteria. Deconvolution of those spectra has allowed to determine in situ the crystallinity degree of native PHB.

Polymer and life science applications of a technique that combines atomic force microscopy (AFM) and infrared (IR) spectroscopy to obtain nanoscale IR spectra and images are reviewed. The AFM–IR spectra generated from this technique contain the same information with respect to molecular structure as conventional IR spectroscopy measurements, allowing significant leverage of existing expertise in IR spectroscopy. The AFM–IR technique can be used to acquire IR absorption spectra and absorption images with spatial resolution on the 50 to 100 nm scale, versus the scale of many micrometers or more for conventional IR spectroscopy. In the life sciences, experiments have demonstrated the capacity to perform chemical spectroscopy at the sub-cellular level. Specifically, the AFM–IR technique provides a label-free method for mapping IR-absorbing species in biological materials. On the polymer side, AFM–IR was used to map the IR absorption properties of polymer blends, multilayer films, thin films for active devices such as organic photovoltaics, microdomains in a semicrystalline polyhydroxyalkanoate copolymer, as well as model pharmaceutical blend systems. The ability to obtain spatially resolved IR spectra as well as high-resolution chemical images collected at specific IR wavenumbers was demonstrated. Complementary measurements mapping variations in sample stiffness were also obtained by tracking changes in the cantilever contact resonance frequency. Finally, it was shown that by taking advantage of the ability to arbitrarily control the polarization direction of the IR excitation laser, it is possible to obtain important information regarding molecular orientation in electrospun nanofibers.

The combination of infrared (IR) spectroscopy and atomic force microscopy (AFM) has produced a technique, called AFM-IR, which is becoming one of the most important recent developments in the field of IR spectroscopy and chemical imaging. Conventional Fourier transform infrared (FT-IR) microspectroscopy is well established as a technique for chemical characterization of small samples down to the 3–10 mm size range. This diffraction-imposed size limit has prevented the application of FT-IR microspectroscopy to smaller analysis regions that are relevant to analysis problems in polymer materials and the life sciences. The nanoIR™ instrument (Anasys Instruments, Santa Barbara, CA) described here uses an AFM probe as the IR absorbance sensor and hence breaks through the diffraction limit to attain spatial resolution improvements of between one and two orders of magnitude beyond previous techniques. Thus, the AFM-IR concept provides chemical information from nanoscale regions of polymers and other organic materials. This article describes the physics behind the technique, followed by results from several applications.

Determining the extent of miscibility of amorphous components is of great importance for certain pharmaceutical systems, in particular for polymer-polymer and polymer-small molecule blends. In this study, the application of standard atomic force microscopy (AFM) measurements combined with nanoscale mid-infrared (mid-IR) spectroscopy was explored to evaluate miscibility in binary polymer blends. The miscibility characteristics of a set of 50/50 (w/w) polymer blends comprising of poly(vinylpyrrolidone) (PVP) with dextran or maltodextrin (DEX) of varying molecular weights (MWs) were investigated. Standard AFM characterization results show good agreement with inferences drawn from differential scanning calorimetry (DSC) analysis in terms of forming either single or two phase systems. AFM analysis also provided insight into the microstructure of the two phase systems and how domain sizes varied as a function of polymer MWs. Nanoscale mid-IR evaluation of the blends, performed by collecting local mid-IR spectra or spectral maps, provided an extra dimension of information about the dependence of polymer MWs on chemical composition of the different phases. AFM, combined with nanoscale mid-infrared analysis, thus appears to be a promising technique for the evaluation of miscibility in certain pharmaceutical blends.

Atomic force microscopy (AFM) and infrared (IR) spectroscopy have been combined in a single instrument (AFM-IR) capable of producing sub-micrometer spatial resolution IR spectra and absorption images. This new capability enables the spectroscopic characterization of micro-domain-forming polymers at levels not previously possible. Films of poly(3-hydroxybutyrate- co-3-hydroxyhexanoate) were solution cast on ZnSe prisms, followed by melting and annealing to generate crystalline microdomains of different sizes. A tunable IR laser generating pulses of the order of 10 ns was used for excitation of the sample films. Short duration thermomechanical waves, due to infrared absorption and resulting thermal expansion, were studied by monitoring the resulting excitation of the contact resonance modes of the AFM cantilever. Dramatic differences in the room-temperature IR spectra are observed in the 1200–1300 cm−1 range as a function of position on a spatial scale of less than one micrometer. This spectral region is particularly sensitive to the polymer backbone conformation. Such dramatic spectral differences have also been observed previously in bulk IR measurements, but only by comparing room-temperature spectra with ones collected at higher temperatures. Less dramatic, but significant, AFM-IR spectral differences are observed in the carbonyl stretching region around 1720 cm−1 as a function of location on the sample. Two overlapping, but relatively sharp, carbonyl bands are observed near 1720 cm−1 in more crystalline regions of the polymer, while a broader carbonyl stretching band appears centered at 1740 cm−1 in the more amorphous regions. Using this spectral region, it is possible to monitor the development of polymer crystalline structures at varying distances from a nucleation site, where the site was generated by bringing a heated AFM tip close to a specific location to locally anneal the sample.

Dynamic infrared linear dichroism (DIRLD) spectroscopy is a rheo-optical characterization technique developed specifically to probe the submolecular dynamics of polymer segments. The technique combines the measurement of submolecular orientation based on the directionally selective absorption of polarized IR light with a small-amplitude oscillatory tensile deformation used in dynamic mechanical analysis. A DIRLD spectroscopic study of atactic polystyrene reveals that a dramatic change in the reorientation behavior of aromatic side groups is observed around the glass transition temperature of 100 °C. The transition point for the main chain backbone, on the other hand, is observed at a much higher temperature around 125 °C. Thus, the macroscopically observable glass transition of polystyrene seems to be dominated by the dynamics of side groups rather than that of the coordinated motions of polymer segments along the backbone. This result suggests a fundamental similarity between the glass transition phenomena of polymers and those of small-molecule inorganic glasses.

An atomic force microscope (AFM) and a tunable infrared (IR) laser source have been combined in a single instrument (AFM-IR) capable of producing ~200-nm spatial resolution IR spectra and absorption images. This new capability enables IR spectroscopic characterization of human stratum corneum at unprecendented levels. Samples of normal and delipidized stratum corneum were embedded, cross-sectioned and mounted on ZnSe prisms. A pulsed tunable IR laser source produces thermomechanical expansion upon absorption, which is detected through excitation of contact resonance modes in the AFM cantilever. In addition to reducing the total lipid content, the delipidization process damages the stratum corneum morphological structure. The delipidized stratum corneum shows substantially less long-chain CH2 -stretching IR absorption band intensity than normal skin. AFM-IR images that compare absorbances at 2930/cm (lipid) and 3290/cm (keratin) suggest that regions of higher lipid concentration are located at the perimeter of corneocytes in the normal stratum corneum.

Infrared (IR) spectra were collected on a series of blends of poly(lactic acid) (PLA) and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (P(3HB-co-3HHx) (3HHx=13.4 mol%)) using a single-bounce temperature-controlled attenuated-total-reflection (ATR) accessory with a diamond internal reflection element (IRE). Different approaches were used to analyze the raw spectral data, including peak positions, widths and intensities, principal components, and 2D correlation spectra. Differences in the interactions between the functional groups of the components become apparent as a function of temperature and composition. This study provides insights into the submolecular-level interactions and miscibility of this polymer blend system. In particular for the 40/60 PLA/P(3HB-co-3HHx) blend, two types of crystalline P(3HB-co-3HHx) CO bands were observed. The band representing a more-ordered local CO environment had a peak absorbance at a lower wavenumber than the second, less-ordered CO species. The 2D IR correlation analysis also revealed that the more-ordered material disappeared first upon heating of the sample and the amorphous P(3HB-co-3HHx) CO band formed before the less-ordered P(3HB-co-3HHx) CO band disappeared. This observation is likely the result of the less-ordered crystalline form being replenished by thermally induced disordering of the more-ordered crystalline form. Thus, the less-order crystalline material may be viewed as an intermediate form during the melting process. In the 80/20 PLA/P(3HB-co-3HHx) blend, it was observed that the lower wavenumber peak in the PLA CO doublet preferentially interacts with amorphous P(3HB-co-3HHx). When 10–20% P(3HB-co-3HHx) is added to PLA, the crystallinity of P(3HB-co-3HHx) is restricted, probably due to domain size restriction and inherent low nucleation density. These IR results suggest that P(3HB-co-3HHx) has the potential to increase the toughness of PLA dramatically, thereby enlarging the design space of sustainable materials achievable by blending.

Photoinduced decomposition reaction of all-trans-β-carotene in the absence of oxygen was studied by 2D Raman correlation spectroscopy. Generalized two-dimensional correlation analysis, coupled with NIR-excited Raman scattering measurements, reveals the presence of both simultaneous and sequential changes of Raman intensities associated with the decomposing β-carotene and the creation of photoreaction products. The largest intensity increase occurs for the Raman band at 1537 cm-1, which is likely due to the formation of a photoisomerization product containing one or more cis CC groups. The formation of this product occurs at an earlier stage of the reaction than the steady decrease of band intensities associated with all-trans-β-carotene at 1522 and 1520 cm-1. Smaller intensity increases at 1133, 1569, and 1284 cm-1 occur synchronously with the growth of the 1537 cm-1 band and prior to the overall decrease in the all-trans-β-carotene bands. Thus, the decomposition product associated with the Raman bands at...

Atomic force microscopy (AFM) and infrared (IR) spectroscopy have been combined in a single instrument (AFM-IR) capable of producing IR spectra and absorption images at a sub-micrometer spatial resolution. This new device enables human hair to be spectroscopically characterized at levels not previously possible. In particular, it was possible to determine the location of structural lipids in the cuticle and cortex of hair. Samples of human hair were embedded, cross-sectioned, and mounted on ZnSe prisms. A tunable IR laser generating pulses of the order of 10 ns was used to excite sample films. Short duration thermomechanical waves, due to infrared absorption and resulting thermal expansion, were studied by monitoring the resulting excitation of the contact resonance modes of the AFM cantilever. Differences are observed in the IR absorbance intensity of long-chain methylene-containing functional groups between the outer cuticle, middle cortex, and inner medulla of the hair. An accumulation of structural lipids is clearly observed at the individual cuticle layer boundaries. This method should prove useful in the future for understanding the penetration mechanism of substances into hair as well as elucidating the chemical nature of alteration or possible damage according to depth and hair morphology.

Attenuated total internal reflection (ATR) infrared mapping microspectroscopy using an infrared microscope and a focal plane array is investigated and reported. The study demonstrates the advantages of conducting ATR microspectroscopy using a focal plane array detector. These benefits include the rapid acquisition of molecular specific images, ease of sample preparation, and increased spatial resolution. An experimental determination of the spatial resolution found that the combined system operates very close to the diffraction limit, and a 4 magnification factor associated with the germanium internal reflection element was realized. Experiments conducted on several polymer samples and a biological sample demonstrate the future viability of the method.

The Diffusive Interfacial Transport (DIT) method for performing phase studies (in which all the phases that exist along an isotherm are produced by isothermal swelling and their compositions determined by using refractive index data) was introduced in 1987.1 While this method represented a significant qualitative advance in phase studies methodology, its quantitative accuracy has since been found to be unsatisfactory. We describe herein the DIT-NIR method in which execution of the study is the same as before, but peak-area data on the bend−stretch combination band of water are used to determine composition. Peak-area data are obtained by using Fourier transform near-infrared (FT-NIR) microspectroscopy. Calibration studies with octyldimethylphosphine oxide−water mixtures demonstrated that peak areas vary linearly with composition over the entire composition range. The slopes of calibration lines follow a power-law dependence upon temperature. Phase studies of the C12E3−water system, using both DIT-NIR and ...

Publisher Summary Generalized two-dimensional (2D) correlation spectroscopy is described to show how this technique can be applied to a very broad range of spectral analysis problems. In this spectroscopy, the underlying similarity or dissimilarity among systematic variations in spectroscopic signal intensities is examined. It develops parallel to multiple pulse-based 2D spectroscopy techniques and employs a somewhat different approach to constructing 2D spectra. The signal variations are induced by an external perturbation or physical stimulus applied to the sample, and the pattern of the intensity variation is systematically analyzed by a simple cross-correlation technique. The correlation intensities thus obtained are displayed in the form of a 2D map defined by two independent spectral axes for further analysis. Such a 2D map is referred to as a 2D correlation spectrum, even though many of today's 2D correlation studies include applications in the field outside of spectroscopy, such as chromatography and microscopy. An illustrative example is given for the analysis of attenuated total reflection (ATR) IR spectra collected during the crystallization of biodegradable polymer poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) to demonstrate the merits of this technique, such as the enhancement of spectral resolution, detection of coordinated changes among spectral signals, and determination of relative directions and sequential order of intensity variations.

Two-dimensional infrared (2D IR) correlation spectroscopy was applied to a semicrystalline linear low density polyethylene (LLDPE) sample under varying pressure. A substantial level of spectral changes are observed when even a moderate amount of pressure is applied to the polyethylene sample. Fine features of the spectral changes are readily analyzed by the 2D correlation method. The pressure-induced 2D IR spectra indicate the observed changes in IR spectra may be attributed to the morphological reorganizations of semicrystalline superstructure of polyethylene, e.g., reorientation of lamellae and partial melting under compression.

A 64×64 Mercury–Cadmium–Telluride (MCT) focal-plane array detector attached to a Fourier transform infrared (FT-IR) microscope was used to spectroscopically image 8-μm-thick cross-sections of wheat kernels in the fingerprint region of the infrared spectrum. After fast-Fourier transformation of the raw image interferograms, the data can be displayed as either a series of spectroscopic images collected at individual wavelengths, or as a collection of IR spectra obtained at each pixel position in the image. Image contrast is achieved due to the intrinsic chemical nature of the sample at each pixel location in the image. Individual cell layers near the outer portion of the wheat kernel, as well as the primary root within the germ, can be clearly differentiated in the IR images as a result of this enhanced chemical contrast.

A 64 X 64 Mercury-Cadmium-Telluride (MCT) focal-plane array detector attached to a Fourier transform infrared (FT-IR) microscope was used to spectroscopically image 5-μm sections of human bone tissue in the fingerprint region of the infrared spectrum. Infrared band contours in the 1200–900 cm−1 spectral region, due to phosphorous-containing mineral, change shape as a function of radial distance from centers of bone growth, called osteons. This technique is providing new insights into the biomineralization process.

Conventional infrared spectroscopy is one of the most widely used tools in science and industry to identify materials via vibrational resonances of chemical bonds, but optical diffraction limits its spatial resolution to the scale of many microns. Atomic force microscopy (AFM) enjoys excellent spatial resolution and can measure mechanical, electrical, magnetic and thermal properties of materials, but has historically lacked the ability to perform robust chemical analysis. Two techniques, (1) AFM-based infrared spectroscopy (AFM-IR) and (2) scattering scanning near field optical microscopy (s-SNOM) have been developed which couple AFM with an IR source allowing the chemical identification capabilities of IR spectroscopy to extend to the nanoscale. As complementary techniques, AFM-IR and s-SNOM together provide an unrivaled capability to perform nanoscale chemical analysis on a diverse range of organic, inorganic, photonic and electronic materials. The AFM-IR technique achieves nanoscale spatial resolution by using the AFM probe as a local sensor of the IR absorption [1]. A sample is irradiated with light from a pulsed, tunable infrared laser. When the IR laser is tuned to a wavelength where the sample has an absorption peak, a portion of the incident IR light is absorbed and converted into heat resulting in a rapid thermal expansion of the absorbing region. This generates an impulse force on the AFM tip, inducing resonant oscillations of the AFM cantilever. IR absorption spectra can then be obtained by measuring the cantilever oscillation amplitude as a function of laser wavenumber. Because the amplitude of induced cantilever oscillation is directly proportional to the IR absorption of the sample [2], absorption spectra obtained by AFM-IR compare very well to conventional FTIR without the use of modelling or reference samples. More recent extensions of this technique have allowed chemical measurements to be performed on samples as thin as individual monolayers [3]. Figure 1 shows an example application of the AFM-IR measurement, nanoscale chemical measurements of the environmental degradation of a biomedical material. One challenge to implantable devices is the possible degradation of the material due to exposure to the in vivo environment. Polyurethane was exposed to this simulated environment for sufficient time to allow the initiation of surface degradation. The chemical mechanism of this degradation was then analyzed and mapped using the AFM-IR technique. The AFM-IR spectra clearly show the chain scission which occurs in the ether bonds of the polyurethane by the reduction in the peak at 1108 cm -1 . It also shows the formation of alcohol groups by the increase in the 1176 cm -1 absorption band and carboxylate groups (COO-) shown by the increase in the broad bands at 1652 and around 1400 cm -1 . These absorption bands can be used to map the degradation and determine how the degradation initiates and extends into the sample, information that was previously not achievable at this length scale. The s-SNOM technique has been used in the SPM field for more than two decades [4]. It uses a metallized AFM tip to enhance and scatter radiation from a nanometer scale region of the sample. The scattered radiation is detected in the far field and carries information about the complex optical properties of the nanoscale region of the sample under the metallized tip. Both the optical amplitude

Two-dimensional infrared (2D IR) correlation spectroscopy has found diverse applications in characterizing polymers. The versatility of this powerful technique is demonstrated in the study of a polymeric laminate film by using depth-profiling photoacoustic spectroscopy, mid- and near-IR dynamic rheo-optical measurements, and spectroscopic imaging microscopy. Spatial and temporal variations of near-IR spectra are effectively analyzed by the 2D correlation technique. Step-scanning FT-IR spectrometers provide a unique opportunity to obtain desired spectral information often difficult to access by the conventional rapid-scanning approach.

We demonstrate a new mid-infrared and near-infrared imaging approach which is ideally suited to microscopic applications. The method employs an indium antimonide (InSb) focal-plane array detector and a commercially available step-scan Fourier transform infrared spectrometer (FT-IR). With either a KBr or a CaF2beamsplitter, images from 1 to 5.5 μm (10,000-1818 cm−1) can be rapidly acquired with the use of all the available pixels on the detector. The spectral resolution for each image is easily varied by changing the number of acquired images during the interferometer scan. We apply this technique to noninvasively generate image contrast in sections of monkey brain tissue and to relate these data to specific lipid and protein fractions. In addition, we describe several computational methods to highlight the spatial distributions of components within a sample.

A powerful new mid-infrared spectroscopic chemical imaging technique combining step-scan Fourier transform Michelson interferometry with indium antimonide focal-plane array (FPA) image detection is described. The coupling of an infrared focal-plane array detector to an interferometer provides an instrumental multiplex/multichannel advantage. Specifically, the multiple detector elements enable spectra at all pixels to be collected simultaneously, while the interferometer portion of the system allows all the spectral frequencies to be measured concurrently. With this method of mid-infrared spectroscopic imaging, the fidelity of the generated spectral images is limited only by the number of pixels on the FPA detector, and only several seconds of starting time is required for spectral image acquisition. This novel, high-definition technique represents the future of infrared chemical imaging analysis, a new discipline within the chemical and material sciences, which combines the capability of spectroscopy for molecular analysis with the power of visualization. In particular, chemical imaging is broadly applicable for noninvasive, molecular characterization of heterogeneous materials, since all solid-state materials exhibit chemical nonuniformity that exists either by design or by development during the course of material preparation or fabrication. Imaging, employing Raman and infrared spectroscopy, allows the precise characterization of the chemical composition, domain structure, and chemical architecture of a variety of substances. This information is often crucial to a wide range of activities, extending from the fabrication of new materials to a basic understanding of biological samples. In this study, step-scan imaging principles, instrument design details, and infrared chemical imaging results are presented. Since the prospect of performing high-resolution and high-definition mid-infrared chemical imaging very rapidly has been achieved with the step-scan approach, the implications for the chemical analysis of materials are many and varied.

Several prism-based spectrographs employing a mercury cadmium telluride (MCT) focal plane array detector have been interfaced to an infrared microscope. In the combined system, the area-defining aperture of the microscope also served as the entrance slit to the spectrograph. This investigation considered the fundamental limits of diffraction for both the spectrograph and microscope in order to determine both the spatial and spectral resolution of the system as a whole. Experimental results for spectral resolution, spectral range, and peak-to-peak noise have been presented. Finally, the dynamic capabilities of one spectrograph/microscope combination were investigated.

The applicability of nanoscale mid-infrared (mid-IR) spectroscopy for the study of the micro- and nanostructure of pharmaceutical drug–polymer systems was explored. Felodipine–poly(acrylic acid) (PAA) blends were used as model systems. Standard atomic force microscopy evaluation as a function of drug–polymer composition suggested limited miscibility, in line with previous findings. Localized spectra on a 50:50 (w/w) felodipine–PAA dispersion revealed that the discrete submicrometer domains formed corresponded to an amorphous felodipine-rich phase while the continuous phase tended to be rich in PAA. Further, spectroscopic imaging at selected wavenumbers, enabling discrimination between both constituents, confirmed this finding and made it possible to chemically image differences in composition between each phase with submicrometer resolution. © 2012 Wiley Periodicals, Inc. and the American Pharmacists Association J Pharm Sci 101:2066–2073, 2012

Dynamic 2d IR spectroscopy can provide insights into plymer deformation mechanisms, identify interactions in polymer blends, and enhance the resolution of overlapping spectral features

Advantages and limitations of analyzing authentic and counterfeit pharmaceutical tablets with both macro (nonimaging) attenuated total internal reflection Fourier transform infrared (ATR-FT-IR) spectroscopy and micro ATR-FT-IR spectroscopic imaging have been evaluated. The results of this study demonstrated that micro ATR imaging was more effective for extracting formulation information (sourcing), whereas a macro ATR approach was better suited for counterfeit detection (screening). More importantly, this study demonstrated that a thorough analysis of the counterfeit core can be achieved by combining the results of both techniques.

Dynamic mechanical analysis coupled with polarized step-scan FTIR transmission and two-dimensional correlation analysis (2D FTIR) has been used to monitor the submolecular orientational responses of the components of a semicrystalline 50 : 50 blend of low-density polyethylene (LDPE) and perdeuterated high-density polyethylene (d*-HDPE) to a small amplitude uniaxial 23.47 Hz sinusoidal mechanical strain. Perdeuteration of the HDPE component allowed the distinction of its response from that of the LDPE in the blend samples. The experiments were carried out at room temperature. Analysis of the data indicates that the crystalline parts of the two components reorient at different rates, with the functional groups of the high-density portion reorienting faster, in general, than those of the LDPE in response to the mechanical strain. © 1993 John Wiley & Sons, Inc.

Dynamic two-dimensional infrared (2D IR) correlation maps are a convenient means of examining the information contained in time-resolved IR spectra. Dynamic 2D IR spectra can be collected with the use of either dispersive or Fourier transform (FT) IR spectrometers. Use of a step-scanning FT-IR spectrometer has advantages over conventional rapid-scan FT-IR spectrometry when one is acquiring time-resolved IR data on time scales faster than about 0.1 s, because the spectral multiplexing is removed from the time domain. Dynamic IR spectra of atactic polystyrene (undergoing a small-amplitude oscillatory strain) collected on both dispersive and FT instrumentation are compared. Although the dispersive approach produces higher signal-to-noise ratios over small spectral regions, the multiplex advantage makes the FT approach attractive when broader spectral coverages are required. The first vibrational circular dichroism (VCD) spectrum [of (–)- α;-pinene] collected on a step-scanning interferometer is also presented.

Recent developments in two-dimensional infrared (2D IR) correlation spectroscopy are reviewed. Since the initial introduction of the basic concept seven years ago, the field of 2D IR spectroscopy has evolved considerably. The method for generating 2D IR spectra from perturbation-induced time-dependent fluctuations of IR intensities and the properties of such 2D spectra are summarized first. Applications of 2D IR spectroscopy are then surveyed, and improvements in the instrumentation are reviewed. Different types of external perturbation schemes capable of inducing dynamic fluctuations of IR spectra are listed. Finally, a new 2D correlation method for dynamic spectral data with arbitrary time-dependence is discussed.

Two-dimensional infrared (2D IR) spectroscopy was used to probe the submolecular dynamics of atactic polystyrene. 2D IR is a powerful analytical technique especially suited for the elucidation of localized motions of polymer segments. In 2D IR, a polymer sample is excited by a small-amplitude oscillatory strain at a frequency in the acoustic range. The fluctuation of IR dichroism signals resulting from the strain-induced reorientation of electric dipole-transition moments is monitored with a time-resolved spectrometer. Spectra defined by two independent wavenumber axes are constructed by applying a correlation analysis to such signals. The 2D spectra provide detailed information about the local dynamics of submolecular constituents of the system. From the sign of cross peaks in the synchronous 2D IR spectrum of glassy polystyrene, it is shown that the main-chain backbone of polystyrene reorients in the direction of applied strain. Cross peaks in an asynchronous 2D IR spectrum reveal highly localized reorientational motions of phenyl side groups occurring more or less independently of the main-chain realignment. In the glassy state, the phenyl ring tends to fold back along the main-chain, indicating that there exists a highly constrained local distortion of side groups during deformation.

Hyperspectral reflectance imaging in the visible and NIR spectral ranges has considerable utility for revealing spatial and chemical complexity in both biological systems and manufactured products. Conventional imaging systems are based on broad-band illumination in tandem with a spectrometer or tunable filter placed between the sample and the detector. These systems are typically slow (require seconds of integration per wavelength step), and the CW broad-band source can cause significant heating of the sample. An alternative method is to use a tunable, pulsed, high-peak-power (low average power) source coupled with a broad-band detector. This approach offers a reduction in data acquisition time, the inherent ability to stop motion, and data collection at ambient temperature. An integrated system based on a 5- ns pulsed laser tunable from 430 nm to 2150 nm has been used to obtain hyperspectral images in both the visible and NIR spectral ranges. A number of camera/lens options allow for varied spectral bandwidths and the FOV, ranging from 11 × 15 mm2 to 15 × 20 cm2. An entire hyperspectral image stack can be collected in as little as 20 s. This method, allowing fast, room-temperature data acquisition, has sufficient sensitivity to produce data that can be successfully processed using spectral derivatives and multivariate analysis. We discuss several applications, both in vivo and otherwise, of this alternative approach to visible/NIR hyperspectral imaging.

Infrared and Raman band frequencies, intensities and line shapes are often sensitive to the local molecular environment determined by molecular conformation, surrounding matrix, temperature, pressure, etc. The variety of local environments experienced by a condensed-phase molecule can lead to vibrational spectra with broad bands containing many overlapped spectral features. The spectral resolution of these overlapped features can be enhanced by making perturbations to the sample environment. Examples of perturbations which can be applied to the sample to enhance the information content of infrared spectra are changes in temperature, concentration and mechanical strain. In each instance, the spectra obtained as a function of the perturbation can be cross-correlated to produce a two-dimensional correlation map defined by two independent wavenumber axes. in this representation, infrared bands which respond to the perturbation in a similar or different manner can be clearly identified. This information can be used to help resolve overlapped bands and make unambiguous band assignments.