Simultaneous Optical Photothermal Infrared (O-PTIR) and Raman Spectroscopy of Submicrometer Atmospheric Particles

Abstract

Physicochemical analysis of individual atmospheric aerosols at the most abundant sizes in the atmosphere (<1 μm) is analytically challenging, as hundreds to thousands of species are often present in femtoliter volumes. Vibrational spectroscopies, such as infrared (IR) and Raman, have great potential for probing functional groups in single particles at ambient pressure and temperature. However, the diffraction limit of IR radiation limits traditional IR microscopy to particles > ∼10 μm, which have less relevance to aerosol health and climate impacts. Optical photothermal infrared (O-PTIR) spectroscopy is a contactless method that circumvents diffraction limitations by using changes in the scattering intensity of a continuous wave visible laser (532 nm) to detect the photothermal expansion when a vibrational mode is excited by a tunable IR laser (QCL: 800–1800 cm–1 or OPO: 2600–3600 cm–1). Herein, we simultaneously collect O-PTIR spectra with Raman spectra at a single point for individual particles with aerodynamic diameters <400 nm (prior to impaction and spreading) at ambient temperature and pressure, by also collecting the inelastically scattered visible photons for Raman spectra. O-PTIR and Raman spectra were collected for submicrometer particles with different substrates, particle chemical compositions, and morphologies (i.e., core–shell), as well as IR mapping with submicron spatial resolution. Initial O-PTIR analysis of ambient atmospheric particles identified both inorganic and organic modes in individual sub- and supermicrometer particles. The simultaneous IR and Raman microscopy with submicrometer spatial resolution described herein has considerable potential both in atmospheric chemistry and numerous others fields (e.g., materials and biological research).

Bond-selective imaging by optically sensing the mid-infrared photothermal effect

“…In summary, these two vibrational spectroscopic techniques are not competing but rather provide complementary information. Notably, there is a growing trend of acquiring both Raman and IR spectra for comprehensive characterization of investigated samples ( 29 , 56 , 101 ).…”

“…Through a dark-field geometry and a resonant amplifier, an unprecedented detection limit of C═O bond at 10 M was reached. This platform was quickly converted into a commercial product mIRage by Photothermal Spectroscopy Corporation (Santa Barbara, CA, USA) and has allowed broad applications in biology and material science (53)(54)(55)(56)(57). Since the 2016 paper, the MIP microscopy field has been quickly expanded by a number of innovations made by many groups.…”

Applications of Vibrational Spectroscopy for Analysis of Connective Tissues

“…In a more recent imaging system, a new modality of MIR spectroscopy called optical photothermal infrared (O-PTIR) spectroscopy has been developed to allow sub-micron spectral imaging [94][95][96]. Moreover, this system can combine O-PTIR and Raman spectroscopy to simultaneously image the same sample region, resulting in complementary spectral images using these different modalities [97]. Even further, both infrared and Raman spectrometers have been combined with atomic force microscopes (AFM) to allow obtaining spectral images with spatial resolutions down to~10 nm [74,98].…”

Resolving Nanocomposite Interfaces via Simultaneous Submicrometer Optical‐Photothermal Infrared‐Raman Microspectroscopy

“…Characteristic IR absorbance peaks ( Figure 1d) at 1738 cm −1 (CO stretching), [21] 1470 cm −1 (-CH 2 -scissoring), [22] and 1173 cm −1 (C-O-C valence vibrations, C-O stretching) [2] along with corresponding active Raman bands ( Figure 1e) at 2876 cm −1 (-CH 2 -/-CH 3 stretching) [2] and 970 cm −1 (Si-O stretching) [23] taken at targetspecific locations near peripheral and central regions confirm the presence of PODA and the underlying native oxide layerpassivated Si-wafer substrate, which agrees well (98.1%) with Wiley's KnowItAll IR spectral database ( Figure S2, Supporting Information) and conventional Raman spectra ( Figure S3, Supporting Information) independently collected. O-PTIR+R has very recently been used to characterize depth-resolved molecules in living cells, [7] polymorphic amyloid aggregates in neurons, [24] sub-micrometer atmospheric particulates, [25] pharmaceutical dry-powder aerosols, [26] <10-µm organic contaminants on hard drives, [27] bioplastic composite interfaces, [28] high-explosive trace materials, [29] and Ruddlesden-Popper hybrid perovskite crystals exhibiting peripheral 2D/3D heterostructure formation, [30] which show excellent correlation to conventional FTIR absorbance spectra.…”