Solvent-controlled plasmon-assisted surface catalysis reaction of 4-aminothiophenol dimerizing to p,p'-dimercaptoazobenzene on Ag nanoparticles

Distinction of Plasmonic Intrananoparticle and Internanoparticle Molecular Reaction Rates at the Three-Phase Contact Line of an Evaporating Sessile Droplet

Molecular reactions on the surface of a plasmonic nanoparticle (intrananoparticle) and between nanoparticles (internanoparticle) may differ in kinetics, dynamics, and product selectivity. We report that the difficulty in distinguishing the kinetics in a dispersion medium could be overcome by probing the reactions at the three-phase contact line (TPCL) of an evaporating sessile droplet using surface-enhanced Raman spectroscopy (SERS). Thus, when an evaporating aqueous droplet on glass containing 4-aminothiophenol-stabilized Ag nanoparticles was monitored by SERS at the TPCL, dimerization into 4,4'-dimercaptoazobenzene followed two steps, each preceded by the loss of H-bonded water accordingly. On the basis of the results, we assigned the first step with a higher relative kinetic rate (∼3 times) to be an intrananoparticle reaction and the second one as an internanoparticle reaction. In D2O medium, the ratio of the rates was ∼1.8. The observed vibrational signatures of the losses of water molecules before reactions and product formations were accounted for by using density functional theoretical calculations.

Machine-learning-assisted SERS nanosensor platform toward chemical fingerprinting of Baijiu flavors

A novel fingerprinting platform for multiplex detection of flavor molecules in Baijiu was developed by using a surface-enhanced Raman scattering (SERS) nanosensor array in combination with machine learning. The SERS sensors were constructed by core-shell Fe₃O₄@Ag nanoparticles modified with molecules carrying end-groups of hydroxyl, pyridyl, methyl, and amino, respectively, which interacted with flavors and led to changes in the sensors' spectra. All the Raman spectra acquired from the nanosensor array contacting with the sample were concatenated into a single SERS super-spectrum, representing the flavor fingerprint which was recognized through machine learning. Principal component analysis, support vector machine, and partial least squares were utilized to build classification and quantitation models for predictive analyses. The SERS nanosensor array was successfully used for fingerprinting ten typical flavors in Baijiu including four esters, three alcohols, and three acids, with an accuracy of 100%, linear detection ranges over two orders of magnitude, and limits of detection ranging from 3.45 × 10^-3 mg/L of phenylethyl acetate to 1.21 × 10^-2 mg/L of ethyl hexanoate. It was also demonstrated that satisfactory accuracies (recoveries) ranging from 96.2 to 104% and relative standard deviations ranging from 0.65 to 2.78% were obtained for the simultaneous quantification of 3-methylbutyl acetate and phenylethyl acetate in eighteen Baijiu samples of three flavor types including sauce flavor, strong flavor, and light flavor. Compared with the existing detection techniques, this chemical fingerprinting platform is easy to use, highly sensitive, and can perform multiplex detection, which has great potential for practical applications.

Noninvasive and Point-of-Care Surface-Enhanced Raman Scattering (SERS)-Based Breathalyzer for Mass Screening of Coronavirus Disease 2019 (COVID-19) under 5 min

Population-wide surveillance of COVID-19 requires tests to be quick and accurate to minimize community transmissions. The detection of breath volatile organic compounds presents a promising option for COVID-19 surveillance but is currently limited by bulky instrumentation and inflexible analysis protocol. Here, we design a hand-held surface-enhanced Raman scattering-based breathalyzer to identify COVID-19 infected individuals in under 5 min, achieving >95% sensitivity and specificity across 501 participants regardless of their displayed symptoms. Our SERS-based breathalyzer harnesses key variations in vibrational fingerprints arising from interactions between breath metabolites and multiple molecular receptors to establish a robust partial least-squares discriminant analysis model for high throughput classifications. Crucially, spectral regions influencing classification show strong corroboration with reported potential COVID-19 breath biomarkers, both through experiment and in silico. Our strategy strives to spur the development of next-generation, noninvasive human breath diagnostic toolkits tailored for mass screening purposes.

Applied surface enhanced Raman Spectroscopy in plant hormones detection, annexation of advanced technologies: A review

Plant hormones are the molecules that control the vigorous development of plants and help to cope with the stress conditions efficiently due to vital and mechanized physiochemical regulations. Biologists and analytical chemists, both endorsed the extreme problems to quantify plant hormones due to their low level existence in plants and the technological support is devastatingly required to established reliable and efficient detection methods of plant hormones. Surface Enhanced Raman Spectroscopy (SERS) technology is becoming vigorously favored and can be used to accurately and specifically identify biological and chemical molecules. Subsistence molecular properties with varying excitation wavelength require the pertinent substrate to detect SERS signals from plant hormones. Three typical mechanisms of Raman signal enhancement have been discovered, electromagnetic, chemical and Tip-enhanced Raman spectroscopy (TERS). Though, complex detection samples hinder in consistent and reproducible results of SERS-based technology. However, different algorithmic models applied on preprocessed data enhanced the prediction performances of Raman spectra by many folds and decreased the fluorescence value. By incorporating SERS measurements into the microfluidic platform, further highly repeatable SERS results can be obtained. This review paper tends to study the fundamental working principles, methods, applications of SERS systems and their execution in experiments of rapid determination of plant hormones as well as several ways of integrated SERS substrates. The challenges to develop an SERS-microfluidic framework with reproducible and accurate results for plant hormone detection are discussed comprehensively and highlighted the key areas for future investigation briefly.

Enhanced solid-state plasmon catalyzed oxidation and SERS signal in the presence of transition metal cations at the surface of gold nanostructures

The effect of several metal cations (Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺) on the photochemical conversion of 4-aminothiophenol (4-ATP) into 4,4'-dimercaptoazobenzene (DMAB) is probed using surface enhanced Raman scattering (SERS). The coupling reaction is carried out on the surface of Au nanoparticles and Au nanorods using 532 nm and 632.8 nm laser excitations, respectively, in the absence and presence of metal cations. Here, we report that DMAB formation on the surface of Au nanostructures - when carried out in the solid state - is augmented significantly (by a factor of 1.98 to 4.07 and 3.34 to 5.74 for Au nanoparticle and Au nanorod substrates, respectively, and depending on the metal). Furthermore, the SERS signal is also markedly enhanced. Thus, the results underpin a new way of carrying out a photochemical reaction with a higher yield along with a higher SERS signal.

Revealing the effects of molecular orientations on the azo-coupling reaction of nitro compounds driven by surface plasmonic resonances

A recent report on the azo coupling of 4-nitrobenzo-15-crown-ether (4NB15C) and 4-nitrothiophenol (4NTP) indicated that the reaction barrier could be reduced greatly with surface plasmonic effects on silver dendritic nanostructures in aqueous solution. Accordingly, an azo coupling reaction mechanism was proposed based on one or two SERS peaks. Toward a profound understanding of this azo coupling reaction mechanism, it is crucial to scrutinize the origin of the full SERS spectrum. Here, we construct a molecular model consisting of 4NTP and 4NB15C on an Ag₇ cluster that simulates a silver dendritic nanostructure, and investigate the SERS spectra of the azo coupling of these two molecules. We propose five different adsorption sites and 13 different orientations of 4NTP on the Ag₇ cluster and optimize the geometries of the five configurations. With each optimized configuration of 4NTP adsorbed on Ag₇, we further consider the azo coupling product with a 4NB15C molecule and simulate the corresponding Raman spectra. Comparing the measured Raman spectra and model analysis, we conclude that the azo coupling reaction depends decisively on a parallel molecular orientation of the adsorbed 4NTP relative to the facets of Ag₇, the orientation of which further directs the subsequent reaction for the product of 4NB15C-4NTP.

Plasmon catalytic PATP coupling reaction on Ag-NPs/graphite studied via in situ electrochemical surface-enhanced Raman spectroscopy

Plasmon-induced hot holes and electrons play different roles in the PATP coupling reaction, resulting in two different catalytic reaction paths.