On the Limit of Detection in Infrared Spectroscopic Imaging

In situ quantification of fungicide residue on wheat leaf surfaces using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry imaging technology

To overcome the time-consuming off-site limitations in conventional pesticide detection, this contribution presents an in situ quantitative analysis detection strategy for pesticides on leaf surfaces using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry imaging technology. Taking fungicide metrafenone as a representative, we initially screened seven commonly used matrices, and identifying α-cyano-4-hydroxycinnamic acid as the most effective one in positive mode. Subsequently, coating the matrix using sublimation spraying method resulted in the highest mass intensity. The optimal combination of operation parameters were established with imaging step size of 30 μm × 30 μm, laser diameter of 75 μm, and laser energy of 4.01 μJ/pulse. Under these conditions, a lowest detection limit of 0.6 ng/mm2 was reached for metrafenone, demonstrating extremely high sensitivity and strong repeatability. This method exhibits great potential for the analysis of various pesticides, including triadimefon and tebuconazole, thereby offering innovative approaches to the analysis of pesticide residues.

Benchtop IR Imaging of Live Cells: Monitoring the Total Mass of Biomolecules in Single Cells

Absolute quantity imaging of biomolecules on a single cell level is critical for measurement assurance in biosciences and bioindustries. While infrared (IR) transmission microscopy is a powerful label-free imaging modality capable of chemical quantification, its applicability to hydrated biological samples remains challenging due to the strong IR absorption by water. Traditional IR imaging of hydrated cells relies on powerful light sources, such as synchrotrons, to mitigate the light absorption by water. However, we overcome this challenge by applying a solvent absorption compensation (SAC) technique to a home-built benchtop IR microscope based on an external-cavity quantum cascade laser. SAC-IR microscopy adjusts the incident light using a pair of polarizers to precompensate the IR absorption by water while retaining the full dynamic range. Integrating the IR absorbance over a cell yields the total mass of biomolecules per cell. We monitor the total mass of the biomolecules of live fibroblast cells over 12 h, demonstrating promise for advancing our understanding of the biomolecular processes occurring in live cells on the single-cell level.

Label-Free Monitoring of Coculture System Dynamics: Probing Probiotic and Cancer Cell Interactions via Infrared Spectroscopic Imaging

Evaluating the dynamic interaction of microorganisms and mammalian cells is challenging due to the lack of suitable platforms for examining interspecies interactions in biologically relevant coculture conditions. In this work, we demonstrate the interaction between probiotic bacteria (Lactococcus lactis and Escherichia coli) and A498 human cancer cells in vitro, utilizing a hydrogel-based platform in a label-free manner by infrared spectroscopy. The L. lactis strain recapitulated in the compartment system secretes polypeptide molecules such as nisin, which has been reported to trigger cell apoptosis. We propose a mid-infrared (IR) spectroscopic imaging approach to monitor the variation of biological components utilizing kidney cells (A498) as a model system cocultured with bacteria. We characterized the biochemical composition (i.e., nucleic acids, protein secondary structures, and lipid conformations) label-free using an unbiased measurement. Several IR spectral features, including unsaturated fatty acids, β-turns in protein, and nucleic acids, were utilized to predict cellular response. These features were then applied to establish a quantitative relationship through a multivariate regression model to predict cellular dynamics in the coculture system to assess the effect of nisin on A498 kidney cancer cells cocultured with bacteria. Overall, our study sheds light on the potential of using IR spectroscopic imaging as a label-free tool to monitor complex microbe-host cell interactions in biological systems. This integration will enable mechanistic studies of interspecies interactions with insights into their underlying physiological processes.

Phasor Representation Approach for Rapid Exploratory Analysis of Large Infrared Spectroscopic Imaging Data Sets

Infrared (IR) spectroscopic imaging is potentially useful for digital histopathology as it provides spatially resolved molecular absorption spectra, which can subsequently yield useful information by powerful artificial intelligence methods. A typical analysis pipeline in using IR imaging data for chemical pathology often involves iterative processes of segmentation, evaluation, and analysis that necessitate rapid data exploration. Here, we present a fast, reliable, and intuitive method based on a phasor representation of spectra and discuss its unique applicability for IR imaging data. We simulate different features extant in IR spectra and discuss their influence on the phasor waveforms; similarly, we undertake IR image analysis in the transform space to understand spectral similarity and variance. We demonstrate the potential of phasor analysis for biomedical tissue imaging using a variety of samples, using fresh frozen surgical prostate resections and formalin-fixed paraffin-embedded breast cancer tissue microarray samples as model systems that span common histopathology practice. To demonstrate further generalizability of this approach, we apply the method to data from different experimental conditions─including standard (5.5 μm × 5.5 μm pixel size) and high-definition (1.1 μm × 1.1 μm pixel size) Fourier transform IR (FTIR) spectroscopic imaging using transmission and transflection modes. Quantitative segmentation results from our approach are compared to previous studies, showing good agreement and quick visualization. The presented method is rapid, easy to use, and highly capable of deciphering compositional differences, presenting a convenient tool for exploratory analysis of IR imaging data.

Amino-Termination of Silicon Carbide Nanoparticles

Silicon carbide nanoparticles (SiC NPs) are promising inorganic molecular-sized fluorescent biomarkers. It is imperative to develop methods to functionalize SiC NPs for certain biological applications. One possible route is to form amino groups on the surface, which can be readily used to attach target biomolecules. Here, we report direct amino-termination of aqueous SiC NPs. We demonstrate the applicability of the amino-terminated SiC NPs by attaching bovine serum albumin as a model for functionalization. We monitor the optical properties of the SiC NPs in this process and find that the fluorescence intensity is very sensitive to surface termination. Our finding may have implications for a few nanometers sized SiC NPs containing paramagnetic color centers with optically read electron spins.

Sampling strategies and analytical techniques for assessment of airborne micro and nano plastics

The atmosphere is pervasively polluted by microplastics and nano plastics (M/NPs) released into indoor and outdoor areas. However, various methodologies and their limitations along with non-standardization make the comparison of information concerning their prevalence difficult. Such diversity in techniques greatly limits the interpretation of results. Herein, We extracted data from publications on PubMed and Embase database up to the year 2022 regarding sampling strategies, identification methods, and reporting data for M/NPs quantification. In this review, 5 major areas for measuring airborne M/NPs have been identified including pre-sampling/ sampling/ post-sampling/ analysis/ and contamination avoidance. There are many challenges specific to each of those sections that need to be resolved through further method development and harmonization. This review mainly focuses on the different methods for collecting atmospheric M/NPs and also the analytical tools which have been used for their identification. While passive sampling is the most user-friendly method, the most precise and reproducible approach for collecting plastic particles is an active method which is directly followed by visual counting as the most common physical analysis technique. Polymers collected using visual sorting are most frequently identified by spectroscopy (FTIR; Raman). However, destructive analytical techniques (thermal degradation) also provide precise chemical information. In all cases, the methods were screened for advantages, limitations, and fieldwork abilities. This review outlines and critiques knowledge gaps, and recommendations to support standardized and comparable future research.

Resolution Limit in Infrared Chemical Imaging

Chemical imaging combines the spatial specificity of optical microscopy with the spectral selectivity of vibrational spectroscopy. Mid-infrared (IR) absorption imaging instruments are now able to capture high-quality spectra with microscopic spatial detail, but the limits of their ability to resolve spatial and spectral objects remain less understood. In particular, the sensitivity of measurements to chemical and spatial changes and rules for optical design have been presented, but the influence of spectral information on spatial sensitivity is as yet relatively unexplored. We report an information theory-based approach to quantify the spatial localization capability of spectral data in chemical imaging. We explicitly consider the joint effects of the signal-to-noise ratio and spectral separation that have significance in experimental settings to derive resolution limits in IR spectroscopic imaging.