Detection of High-Explosive Materials within Fingerprints by Means of Optical-Photothermal Infrared Spectromicroscopy

Abstract

As we live under a constant threat of global terrorism, the effective detection of highly energetic materials is one of the critical procedures needed at a variety of locations, including airports, border checkpoints, and entrances to high-security buildings. In this work, the application of optical-photothermal infrared (O-PTIR) spectromicroscopy for the detection of highly explosive materials within fingerprints is described. High-explosive (HE) materials (e.g., PETN, RDX, C-4, or TNT) were used to prepare contaminated fingerprints. These were subsequently deposited on various objects, including microscopic glass slides, a table, a mug, etc. Samples deposited on glass slides were directly sent for analyses; for other samples, adhesive tapes were used to lift off fingermarks. In cases of difficulty in locating fingerprints, additional powders were used to enhance their visibility. Experiments were performed with a mIRage IR microscope working in a noncontact, far-field reflection mode, offering submicron IR spectroscopy and imaging. Fast imaging (several characteristic absorbances were selected for every substance of interest) was used to locate “suspicious” particles among various residues present in fingerprints. Subsequently, spectra were collected for those particles. Reflection mode O-PTIR spectra taken from powdered and nonenhanced fingerprints were of comparable quality to transmission mode FTIR spectra collected for pure HEs. On the basis of the performed experiments, we consider O-PTIR spectromicroscopy to open a new avenue for the nondestructive, efficient, and reliable analysis of exogenous substances deposited within fingerprints. The real significance of O-PTIR is in its ability to deliver high-quality, spatially resolved FTIR transmission-like spectra below the diffraction limit of infrared wavelengths, doing so in an easy-to-use reflection (far-field) mode. Collected spectra are also searchable and interpretable in both commercial and institutional IR databases without mathematical modeling.

Fluorescence-Detected Mid-Infrared Photothermal Microscopy

“…The MIP signal is then obtained by probing these changes using a visible beam which provides a much smaller diffraction limit than the mid-infrared illumination, enabling a spatial resolution down to 300 nm . Following the first demonstration of MIP imaging of living cells, technical innovations have been made to enable MIP detection in wide field using scattering or phase signals. − Meanwhile, MIP microscopy and its commercial product have found various applications in studying living cells, pharmaceuticals, viruses, and bacteria. ,,,− Yet, the photothermal effect induces only a tiny change in the intensity and angular distribution of the scattered probe light due to the weak thermal dependence of the particle size and refractive index. Typical fractional change is on the order of 10 –4 / K , set by the intrinsic thermal properties of most materials.…”

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.…”

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

“…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.…”