Analysis of Fiber-Enhanced Raman Gas Sensing Based on Raman Chemical Imaging

Fiber enhanced Raman spectroscopy (FERS) is an arising new technique for versatile highly sensitive and selective multigas analysis in various applications, such as environmental monitoring and medical breath diagnosis. In this study, the performance of FERS was thoroughly studied with the help of a specially designed multichannel Raman chemical imaging. Several types of hollow core photonic crystal fibers were thoroughly analyzed in terms of their performance in light confinement and sensitive gas sensing. The optimal fiber length for Raman gas sensing was found to be 15 cm in our spectroscopic system. To separate the Raman scattering of the target gas molecules from the background generated by the silica microstructure of the fiber, the optimal diameter of a spatial filter was analyzed and quantified as Ø3.9 μm, which balances the suppression of the silica background and the attenuation of the gas signal, originating from different regions in the plane of the fiber end-face. To achieve an easy-to-use gas monitoring system with stable performance, an automated coupling-method was developed, to simplify the alignment of the FERS setup. The optimized design of the FERS setup has remarkable potential for highly sensitive, miniaturized, easy-to-use, and versatile gas sensing

Ultrasensitive Fiber Enhanced UV Resonance Raman Sensing of Drugs

Fiber enhanced UV resonance Raman spectroscopy is introduced for chemical selective and ultrasensitive analysis of drugs in aqueous media. The application of hollow-core optical fibers provides a miniaturized sample container for analyte flow and efficient light-guiding, thus leading to strong light–analyte interactions and highly improved analytical sensitivity with the lowest sample demand. The Raman signals of the important antimalaria drugs chloroquine and mefloquine were strongly enhanced utilizing deep UV and electronic resonant excitation augmented by fiber enhancement. An experimental design was developed and realized for reproducible and quantitative Raman fiber sensing, thus the enhanced Raman signals of the pharmaceuticals show excellent linear relationship with sample concentration. A thorough model accounts for the different effects on signal performance in resonance Raman fiber sensing, and conclusions are drawn how to improve fiber enhanced Raman spectroscopy (FERS) for chemical selective analysis with picomolar sensitivity

Fundamental SERS Investigation of Pyridine and Its Derivates as a Function of Functional Groups, Their Substitution Position, and Their Interaction with Silver Nanoparticles

The Raman and surface-enhanced Raman spectroscopy (SERS) spectra of pyridine, 2-, 3-, and 4-pyridinemethanol and 2-, 3-, and 4-picolylamine were comprehensively studied using two different types of silver nanoparticles as SERS substrates, namely citrate-reduced and hydroxylamine-reduced silver nanoparticles, which are the most commonly used silver colloids. For a robust band assignment, Raman spectra of the investigated molecules were simulated, applying Møller–Plesset perturbation theory in conjunction with the 6 311++G­(d, p) basis set. Subsequently, the observed changes in the spectra were interpreted and discussed in detail, taking into account the simulated Raman spectra. Furthermore, differences in the spectral information resulting from the two different analyzed functional groups, the altering of the substitution position and the molecular orientation relative to the metal surface were discussed, and general conclusions for an application of those molecules as Raman reporters were derived

Fast and Highly Sensitive Fiber-Enhanced Raman Spectroscopic Monitoring of Molecular H₂ and CH₄ for Point-of-Care Diagnosis of Malabsorption Disorders in Exhaled Human Breath

Breath gas analysis is a novel powerful technique for noninvasive, early-stage diagnosis of metabolic disorders or diseases. Molecular hydrogen and methane are biomarkers for colonic fermentation, because of malabsorption of oligosaccharides (e.g., lactose or fructose) and for small intestinal bacterial overgrowth. Recently, the presence of these gases in exhaled breath was also correlated with obesity. Here, we report on the highly selective and sensitive detection of molecular hydrogen and methane within a complex gas mixture (consisting of H₂, CH₄, N₂, O₂, and CO₂) by means of fiber-enhanced Raman spectroscopy (FERS). An elaborate FERS setup with a microstructured hollow core photonic crystal fiber (HCPCF) provided a highly improved analytical sensitivity. The simultaneous monitoring of H₂ with all other gases was achieved by a combination of rotational (H₂) and vibrational (other gases) Raman spectroscopy within the limited spectral transmission range of the HCPCF. The HCPCF was combined with an adjustable image-plane aperture pinhole, in order to separate the H₂ rotational Raman bands from the silica background signal and improve the sensitivity down to a limit of detection (LOD) of 4.7 ppm (for only 26 fmol H₂). The ability to monitor the levels of H₂ and CH₄ in a positive hydrogen breath test (HBT) was demonstrated. The FERS sensor possesses a high dynamic range (∼5 orders of magnitude) with a fast response time of few seconds and provides great potential for miniaturization. We foresee that this technique will pave the way for fast, noninvasive, and painless point-of-care diagnosis of metabolic diseases in exhaled human breath

Fiber-Enhanced Raman Multigas Spectroscopy: A Versatile Tool for Environmental Gas Sensing and Breath Analysis

Versatile multigas analysis bears high potential for environmental sensing of climate relevant gases and noninvasive early stage diagnosis of disease states in human breath. In this contribution, a fiber-enhanced Raman spectroscopic (FERS) analysis of a suite of climate relevant atmospheric gases is presented, which allowed for reliable quantification of CH₄, CO₂, and N₂O alongside N₂ and O₂ with just one single measurement. A highly improved analytical sensitivity was achieved, down to a sub-parts per million limit of detection with a high dynamic range of 6 orders of magnitude and within a second measurement time. The high potential of FERS for the detection of disease markers was demonstrated with the analysis of 27 nL of exhaled human breath. The natural isotopes ¹³CO₂ and ¹⁴N¹⁵N were quantified at low levels, simultaneously with the major breath components N₂, O₂, and ¹²CO₂. The natural abundances of ¹³CO₂ and ¹⁴N¹⁵N were experimentally quantified in very good agreement to theoretical values. A fiber adapter assembly and gas filling setup was designed for rapid and automated analysis of multigas compositions and their fluctuations within seconds and without the need for optical readjustment of the sensor arrangement. On the basis of the abilities of such miniaturized FERS system, we expect high potential for the diagnosis of clinically administered ¹³C-labeled CO₂ in human breath and also foresee high impact for disease detection via biologically vital nitrogen compounds

Raman Spectroscopic Characterization of Packaged <i>L. pneumophila</i> Strains Expelled by <i>T. thermophila</i>

The intracellular lifestyle of <i>L. pneumophila</i> within protozoa is considered to be a fundamental process that supports its survival in nature. However, after ingesting the cells of <i>L. pneumophila</i>, some protozoa expel them as compressed live cells in the form of small rounded pellets. The pellets of tightly packaged viable but not culturable forms (VBNCFs) as well as highly infectious mature intracellular forms (MIFs) of <i>L. pneumophila</i> are considered as infectious particles most likely capable to cause human infection. Since <i>L. pneumophila</i> cells are hardly culturable from these pellets, detection methods for packaged live <i>L. pneumophila</i> forms remaining in water should be cultivation free. Hence, we demonstrate the potential of Raman microspectroscopy to directly sort pellets containing <i>L. pneumophila</i> cells, expelled by <i>T. thermophila</i>, and to characterize them on the basis of their Raman spectra

Photophysical Dynamics of a Ruthenium Polypyridine Dye Controlled by Solvent pH

The photophysics of the novel ruthenium dye [Ru(tmBiBzIm)(dppz)(tbbpy)]²⁺ (tmBiBzIm = 5,5′,6,6′-tetramethyl-2,2′-bibenzimidazole, dppz = dipyrido[3,2-<i>a</i>:2′,3,3′-<i>c</i>]phenazine, tbbpy = 4,4′-di-<i>tert</i>-butyl-2,2′-bipyridine) is investigated, which might be suitable as a model compound for intracellular DNA and pH sensors. The combination of three different bidentate ligands allows for controlling the photophysics by two distinct mechanisms: (i) protonation and deprotonation of the tmBiBzIm and (ii) hydrogen bonding to the phenazine nitrogens of the dppz ligand. As will be reported, deprotonation of the tmBiBzIm ligand causes a bathochromic shift of the metal-to-ligand charge-transfer transition, although the tmBiBzIm ligand itself does not directly contribute to the light absorption. Furthermore, tmBiBzIm deprotonation shortens the overall excited-state lifetime of the complex significantly. Although the protonation stage of the tmBiBzIm directly impacts the excited-state properties of the dye, the overall photoinduced dynamics is dominated by the dppz ligand. Consequently, addition of water to the solvent affects the excited-state relaxation pathway as known from, for example, [Ru(phen)₂dppz]²⁺ (phen = 1,10-phenanthroline) complexes

Toward Culture-Free Raman Spectroscopic Identification of Pathogens in Ascitic Fluid

The identification of pathogens in ascitic fluid is standardly performed by ascitic fluid culture, but this standard procedure often needs several days. Additionally, more than half of the ascitic fluid cultures are negative in case of suspected spontaneous bacterial peritonitis (SBP). It is therefore important to identify and characterize the causing pathogens since not all of them are covered by the empirical antimicrobial therapy. The aim of this study is to show that pathogen identification in ascitic fluid is possible by means of Raman microspectroscopy and chemometrical evaluation with the advantage of strongly increased speed. Therefore, a Raman database containing more than 10000 single-cell Raman spectra of 34 bacterial strains out of 13 different species was built up. The performance of the used statistical model was validated with independent bacterial strains, which were grown in ascitic fluid

Ultrasensitive Detection of Antiseptic Antibiotics in Aqueous Media and Human Urine Using Deep UV Resonance Raman Spectroscopy

Deep UV resonance Raman spectroscopy is introduced as an analytical tool for ultrasensitive analysis of antibiotics used for empirical treatment of patients with sepsis and septic shock, that is, moxifloxacin, meropenem, and piperacillin in aqueous solution and human urine. By employing the resonant excitation wavelengths λ_exc = 244 nm and λ_exc = 257 nm, only a small sample volume and short acquisition times are needed. For a better characterization of the matrix urine, the main ingredients were investigated. The capability of detecting the antibiotics in clinically relevant concentrations in aqueous media (LODs: 13.0 ± 1.4 μM for moxifloxacin, 43.6 ± 10.7 μM for meropenem, and 7.1 ± 0.6 μM for piperacillin) and in urine (LODs: 36.6 ± 11.0 μM for moxifloxacin, and 114.8 ± 3.1 μM for piperacillin) points toward the potential of UV Raman spectroscopy as point-of-care method for therapeutic drug monitoring (TDM). This procedure enables physicians to achieve fast adequate dosing of antibiotics to improve the outcome of patients with sepsis

Simple Ciprofloxacin Resistance Test and Determination of Minimal Inhibitory Concentration within 2 h Using Raman Spectroscopy

Resistant bacteria are spreading worldwide, which makes fast antibiotic susceptibility testing and determination of the minimal inhibitory concentration (MIC) urgently necessary to select appropriate antibiotic therapy in time and, by this, improve patient’s outcome and, at the same time, avoid inappropriate treatment as well as the unnecessary use of broad spectrum antibiotics that would foster further spread of resistant bacteria. Here, a simple and fast Raman spectroscopy-based procedure is introduced to identify antimicrobial susceptibilities and determine the MIC within only 2 h total analysis, marking a huge time savings compared to established phenotypic methods nowadays used in diagnostics. Sample preparation is fast and easy as well as comparable to currently established tests. The use of a dielectrophoresis chip allows automated collection of the bacteria in a micron-sized region for high-quality Raman measurement directly from bacterial suspensions. The new Raman spectroscopic MIC test was validated with 13 clinical E. coli isolates that show a broad range of ciprofloxacin resistance levels and were collected from patients with blood-stream infection. Micro-Raman spectroscopy was able to detect ciprofloxacin-induced changes in E. coli after only 90 min interaction time. Principal component analysis as well as a simple computed ratio of the Raman marker bands at 1458 and 1485 cm^–1 show a clear concentration-dependent effect. The MIC values determined with the new Raman method are in good agreement with MICs obtained by reference methods (broth microdilution, Vitek-2, E-test) and can be used to provide a classification as sensitive, intermediate, or resistant using the clinical breakpoints provided by EUCAST