Applied Spectroscopy

Significant advancements in waveguide technology in the mid-infrared (MIR) regime during recent decades have assisted in establishing MIR spectroscopic and sensing technologies as a routine tool among nondestructive analytical methods. In this review, the evolution of MIR waveguides along with state-of-the-art technologies facilitating next-generation MIR chem/bio sensors will be discussed introducing a classification scheme defining three “generations” of MIR waveguides: (1) conventional internal reflection elements as “first generation” waveguides; (2) MIR-transparent optical fibers as “second generation” waveguides; and most recently introduced(3) thin-film structures as “third generation” waveguides. Selected application examples for these each waveguide category along with future trends will highlight utility and perspectives for waveguide-based MIR spectroscopy and sensing systems.

Haematococcus pluvialis has promising applications owing to its ability to accumulate astaxanthin under stress conditions. In order to acquire higher astaxanthin productivity from H. pluvialis, it is critical not only to develop efficient mutagenesis techniques, but also to establish rapid and effective screening methods which are highly demanded in current research and application practice. In this work, we therefore attempted to develop a new approach to screening the astaxanthin-hyperproducing strains based on spectroscopic tools. Using Fourier transform infrared (FT-IR) and Raman microspectroscopy, we have achieved rapid and quantitative analysis of the algal cells in terms of astaxanthin, β-carotene, proteins, lipids, and carbohydrates. In particular, we have found that the ratio of the IR absorption band at 1740 cm⁻¹ to the band at 1156 cm⁻¹ can be utilized for identifying astaxanthin-hyperproducing strains. This work may therefore open a new avenue for developing high-throughput screening methods necessary for the microbial mutant breeding industry.

Two chiral ionic liquids (ILs), namely 1-ethyl-3-methylimidazole l-tartrate (EMIML-Tar) and 1-ethyl-3-methylimidazole l-lactate (EMIML-Lac), were used to modify gold nanoparticles (AuNPs) for chiral recognition of amino acid enantiomers. Transmission electron microscopy, infrared spectroscopy, ultraviolet-visible spectroscopy, and capillary electrophoresis were used for the characterization of chiral IL-modified AuNPs. Meanwhile, the performance of l-tartaric acid and l-lactic acid as modifiers was investigated to make a comparison. The chiral recognition mechanism is further discussed.

Photothermal behavior of binary liquid mixtures has been studied using a high repetition rate (HRR) Z-scan technique with femtosecond laser pulses. Changes in the peak–valley difference in transmittance (ΔT_P–V) for closed aperture Z-scan experiments are indicative of thermal effects induced by HRR femtosecond laser pulses. We show such indicative results can have a far-reaching impact on molecular properties and intermolecular interactions in binary liquid mixtures. Spectroscopic parameters derived from this experimental technique show that the combined effect of physical and molecular properties of the constituent binary liquids can be related to the components of the binary liquid.

We explore the size and spatial microheterogeneity of contact pin-printed spots formed on porous silicon (pSi). Glycerol was contact printed at room temperature onto as-prepared, hydrogen-passivated pSi (ap-pSi) using 50 or 200 µm diameter solid pins. The pSi was then subjected to a strong oxidizing environment (gaseous O₃) and washed to remove the glycerol masks. The glycerol-free regions were converted to oxidized pSi (ox-pSi); the glycerol-coated regions were protected from O₃, but not entirely. The final array is described as circularly shaped “ap-pSi” regions on a field of ox-pSi. When comparing the areas outside and inside the glycerol-masked pSi spots, one finds dramatic differences in the Si–O–Si, SiH _x (x = 1–3) and O _y SiH _x (y, x = 1–3) levels with a spatially dependent continuum of compositions across the spot diameter. Experimental conditions could be adjusted to tune the final ap-pSi spot diameter and edge widths from 90 µm to 520 µm and 20 µm to 130 µm, respectively. The resulting ap-pSi spot diameter is explained by using molecular kinetic theory and time-dependent glycerol imbibement into the pSi within a one-dimensional Darcy’s law model.

This study seeks to estimate the mechanical properties of thermally modified wood (TMW) using near-infrared (NIR) spectroscopy to measure 80 samples in three-point bending tests. Near-infrared spectra collected from the transverse, radial, and tangential sections of wood, coupled with chemometric techniques, were used to predict the mechanical properties of southern pine wood, from which NIR models were constructed based on partial least squares and corresponding cross-validation. The coefficient of determination between NIR transverse section spectra, as well as two mechanical properties of wood samples, modulus of rupture (MOR) and modulus of elasticity (MOE), were above 0.92 and greater than values for other sections. Spectral data from the transverse sections were richer than data from other sections, and thus, a model based on transverse sections better predicts the mechanical properties of wood. A close relationship between the values for mechanical properties (MOE and MOR) and the NIR spectra of thermally modified southern pine wood can be demonstrated, which provides the potential to predict the mechanical properties of untreated and thermally modified wood.

In a recent report we demonstrated a miniature static Fourier transform spectrometer (FTS) that was implemented with a LiNbO₃ (LN) waveguide electro-optic modulator (EOM) combined with the dispersion relation between its half-wave voltage and wavelength. The FTS was verified to be able to measure laser wavelength and for low-resolution spectroscopy. In this report, we successfully applied the resolution enhancement algorithm to the FTS, resulting in at least a three-fold increase in its spectral resolution without causing obvious distortion of the measured spectra. The algorithm method used is based on an autoregressive (AR) model, singular value decomposition (SVD), and forward–backward linear prediction (FBLP). The combination of these methods allows the FTS to remain a small size but to possess good spectral resolution, effectively mitigating the conflict between the small size and high resolution of the device. This study opens the way to development of high-resolution miniature FTS.

Water-dispersed Cu@Ag core–shell nanoparticles (NPs) with 15 nm-diameter Cu core and 5 nm-thick Ag shell can be synthesized by a facile one-step chemical reduction at room temperature without any protective atmosphere. To obtain a homogeneous Ag coating on Cu, the influence of [Cu/Ag] molar ratio was investigated. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) confirmed that Ag formed a dense coating on the surface of Cu, and that phase-pure spherical Cu@Ag core–shell bimetallic NPs were prepared when the [Cu/Ag] molar ratio was between 1/0.5 and 1/0.75. The time dependence of ultraviolet-visible (UV-Vis) spectra and XRD patterns of six-month stored Cu@Ag NPs showed that the as-prepared Cu@Ag NPs have a long-term antioxidant activity. Also, the surface-enhanced Raman scattering (SERS) signals had a high stability and reproducibility for the substrates. Hence, the as-prepared Cu@Ag nanostructures can be used as an efficient substrate for SERS signals.

Laser-induced breakdown spectroscopy (LIBS) was used to detect and measure the concentrations of Ce and Gd in aqueous aerosol solutions. A total of 36 standards, with concentrations of Ce and Gd ranging from 100 parts per million (ppm) to 10 000 ppm, were made to explore the relationship between them. In this study, a Collison nebulizer with an argon carrier gas was used to generate the aerosol droplets. For each liquid sample, ten repetitions of 200 laser shots each were recorded. The percent relative standard deviations (%RSD) were on an average of 7.5% between the ten different sample repetitions. Due to the close proximity of the Ce and Gd lines, it was challenging to identify peaks with low interferences. However, several lines were identified, calibration curves were constructed, and the best curves were generated using the 457.228 nm line for Ce and the 409.861 nm line for Gd. The LODs for these curves were calculated to be 209.7 ppm and 216.4 ppm for the Ce line and Gd line, respectively.

The parathyroid glands are small and often similar to lymph nodes, fat, and thyroid tissue. These glands are difficult to identify during surgery and a biopsy of the parathyroid for identification can lead to damage of the gland. The use of static and time-resolved fluorescence techniques to detect biochemical composition and tissue structure alterations could help to develop a portable, minimally invasive, and nondestructive method to assist medical evaluation of parathyroid tissues. In this study, we investigated 10 human parathyroid samples using absorbance, fluorescence, excitation, and time-resolved fluorescence measurements. Moreover, we compared the results of time-resolved fluorescence measurements with 59 samples of thyroid tissues. The fluorescence lifetimes with emission at 340 nm were 1.09 ± 0.10 and 4.46 ± 0.06 ns for healthy tissue, 1.01 ± 0.25 and 4.39 ± 0.36 ns for benign lesions, and 0.67 ± 0.36 and 3.92 ± 0.72 ns for malignant lesions. The lifetimes for benign and malignant lesions were significantly different, as attested by the analysis of variance with confidence levels higher than 87%. For each class of samples (healthy, benign, and malignant) we perceived statistical differences between the thyroid and parathyroid tissue, independently. After further investigations, fluorescence methods could become a tool to identify normal and pathological parathyroid tissues and distinguish thyroid from parathyroid tissues.

The normal infrared (IR) spectral emissivity of pure titanium TA1 is experimentally investigated using a self-designed emissivity measurement apparatus. The apparatus and the measurement method are described in detail. Seven samples are treated with abrasive paper to obtain the roughness needed. The emissivity of one sample is obtained between 473 and 1035 K with wavelength range of 3–27 µm in an argon environment. The other six samples are oxidized at a high temperature (873 K) for various times. The surface roughness and composition of the samples are analyzed using roughness tester and X-ray diffraction before and after the emissivity measurement. The results show that the spectral emissivity of titanium sample increases with the increase of temperature and decreases with the increase of wavelength from 3 to 27 µm. For the oxidized samples, the spectral emissivity increases with increasing oxidization time, and after being heated for 12 h, the emissivity values increase slightly. The influence of the oxide film thickness on the spectral emissivity is discussed based on interference theory, and the thickness of oxide film with different oxidation time is accurately measured using scanning electron microscope.

A dispersive nanosecond transient absorption instrument was developed to enable rapid time-resolved and steady-state measurements in the mid-infrared (mid-IR) region for thin films without the need for gated integrators or lock-in amplifiers. Two detectors are used depending on the experimental needs (100 MHz and 16 MHz) with time resolution from nano-millisecond and spectral coverage from 1000–5000 cm⁻¹ (2000–10 000 nm). The instrument utilizes flexible digitization resolution (8 bit to 14 bit) to enable high sensitivity (10⁻⁵) measurements on thin films under low excitation (<50 µJ/cm²). We highlight the instrument’s improvement over prior state-of-the-art time-resolved capabilities by measuring transient species (e.g., polarons) under extremely low energy densities (<5 µJ/cm²) in less than 10 minutes to achieve high fidelity signals. Additionally, to highlight the spectral capabilities we study two optoelectronic materials for which we resolve vibrational features as small as 10 µOD.

In this paper, we report a pyridinium salt “turn-on” fluorescent probe, 4-[2-(4-Dimethylamino-phenyl)-vinyl]-1-methylpyridinium iodide (p-DASPMI), and applied its time-resolved fluorescence (TRF) to monitor the protein conformational changes. Both the fluorescence lifetime and quantum yield (QY) of p-DASPMI were increased about two orders of magnitude after binding to the protein bovine serum albumin (BSA). The free p-DASPMI in solution presents an ultrashort fluorescence lifetime (12.4 ps), thus it does not interfere the detection of bound p-DASPMI which has nanosecond fluorescence lifetime. Decay-associated spectra (DAS) show that p-DASPMI molecules bind to subdomains IIA and IIIA of BSA. The TRF decay profiles of p-DASPMI can be described by the multi-exponential decay function ( ∑ α n exp ( - t / τ n ) ), and the obtained parameters, such as lifetimes ( τ n ), fractional amplitudes ( α n ), and fractional intensities ( α n τ n ), may be used to deduce the conformational changes of BSA. The pH and Cu²⁺ induced conformational changes of BSA were investigated through the TRF of p-DASPMI. The results show that the p-DASPMI is a candidate fluorescent probe in studying the conformational changes of proteins through TRF spectroscopy and microscopy in the visible range.

Fluorescence spectroscopy is a sensitive and selective technique, which can be of great value in bioprocesses to provide online, real-time measures of chemical compounds. Although fluorescence spectroscopy is a widely studied method, not much attention has been given to issues concerning intensity variations in the fluorescence landscapes due to pH fluctuations. This study elucidates how pH fluctuations cause intensity changes in fluorescence measurements and thereby decreases the quality of the subsequent quantification. A photo-degradation process of riboflavin was investigated using fluorescence spectroscopy and used as a model system. A two-step modeling approach, combining weighted PARAllel FACtor analysis (PARAFAC) with weighted nonlinear regression of the known reaction kinetics, is suggested as a way of handling the fluorescence intensity shifts caused by the pH changes. The suggested strategy makes it possible to compensate for uncertainties in the shifted data and thereby obtain more reliable concentration profiles for the chemical compounds and kinetic parameters of the reaction.

In this paper, ultrathin carbon film protected silver substrate (Ag/C) was prepared via a plasma-enhanced chemical vapor deposition (PECVD) method. The morphological evolution of silver nanostructures underneath, as well as the surface-enhanced Raman scattering (SERS) activity of Ag/C hybrid can be tuned by controlling the deposition time. The stability and reproducibility of the as-prepared hybrid were also studied.

Archeological ceramic paste material typically consists of a mix of a clay matrix and various millimeter and sub-millimeter sized mineral inclusions. Micro X-ray fluorescence (XRF) is a standard compositional classification tool and in this work we propose and demonstrate an improved fluorescence map processing protocol where the mineral inclusions are automatically separated from the clay matrix to allow independent statistical analysis of the two parts. Application of this protocol allowed us to enhance the discrimination between different ceramic shards compared with the standard procedure of working with only the spatially averaged elemental concentrations. Using the new protocol, we performed an initial compositional classification of a set of 83 ceramic shards from the western slopes of the south central Andean region in the Arica y Parinacota region (Chile). Comparing the classifications obtained using the new versus the old (average concentrations only) protocols, we found that some samples were erroneously classified with the old protocol. From an archaeological perspective, a broad and heterogeneous regional sample set was used in this experimental study due to the fact that this was the first such analysis to be performed on ceramics from this region. This allowed a general overview to be obtained, however further work on more specific sample sets will be necessary to extract concrete archaeological conclusions.

In the present work, an improved model of the Raman gas analyzer (RGA) of natural gas (NG) developed by us is described together with its operating principle. The sensitivity has been improved and the number of measurable gases has been expanded. Results of its approbation on a real NG sample are presented for different measurement times. A comparison of the data obtained with the results of chromatographic analysis demonstrates their good agreement. The time stability of the results obtained using this model is analyzed. It is experimentally established that the given RGA can reliably determine the content of all molecular NG components whose content exceeds 0.005% for 100 s; moreover, in this case the limiting sensitivity for some NG components is equal to 0.002%.