Detection of Metal–Molecule–Metal Junction Formation by Surface Enhanced Raman Spectroscopy

Surface segregation of rigid polyarylene ether amidoxime on polyurethane nanofiber into hierarchical membranes as substrate of flexible SERS nanosensor for sulfamethoxazole detection

Electrospun polymeric nanofibrous membranes are emerging as the promising substrates for preparation of flexible SERS nanosensors due to their intrinsic nanoscale surface roughness, easy scalability as well as rich surface reactivity. Although the nanofiber membranes prepared from high performance thermoplastics exhibit good mechanical stability, the SERS nanosensors based on these substrates normally have lower signal-to-noise ratio because of the interference from background Raman signals of aromatic moieties. Herein, we synthesized an optically transparent polyurethane (PU) and rigid polyarylene ether amidoxime (PEA), which were electrospun into core-shell nanofibers membranes with a "beads-on-web" morphology. Furthermore, the PU-PEA membranes were coated with ultra-thin silver layer and thermally annealed to prepare the flexible SERS nanosensor without any background noises. In addition, the Raman enhancement of SERS nanosensor can be readily improved by tuning of PU-PEA composition, silver thickness as well as thermal annealing temperature. Finally, the optimized SERS nanosensor enables label-free detection of sulfamethoxazole as low as 0.1 nM with a good reproducibility and detection performance in real water sample. Meanwhile, the optimized SERS nanosensor shows long term anti-biofouling capacity. Thanks to its facile fabrication, competitive analytical performance and resistance to biofouling, the current work basically open new way for design of flexible SERS nanosensors for biomedical applications.

Design of competition nanoreactor with shell-isolated colloidal plasmonic nanomaterials for quantitative sensor platform

Shell-isolated colloid plasmonic nanomaterials-based nanoreactor is a well-established platform widely applied in catalyst or Surface Enhanced Raman Scattering (SERS) sensors. The potentials versatility of nanoreactor platform is mainly implemented by the well-defined and tailorable structure of colloid plasmonic nanomaterials. Currently, a competitive conjugative-mediated nanoreactor is introduced to determine glucose with SERS. Glucose-conjugating nanoreactor, as convertors of the sensors, are constructed by coordinated deposition colloidal gold nanoparticles with sodium nitroprusside framework (Au@SNF) and covalently bonded 4-mercaptopyridine (4-Mpy) with self-assembly strategy. The nanoreactor contained the signal-amplifier Au@SNF NPs, conjugative-mediated signal receiver 4-Mpy, and signal internal standard molecular CN^-. In addition to well-defined morphology and functionality, conjugative-mediated and internal standards method are also employed to benefit the nanoreactor. The two-parameter strategy significantly improves the signal indication and correction. Using this proposed platform, the competitive-mediated nanoreactor provides a quantitative SERS detection of glucose, and extends the applicability of SERS in more complicated and reproducibility analysis. Meanwhile, the nanoreactor based sensors also exhibited better properties to detect glucose in various food samples and bio-samples which provided strongly appliance for glucose sensors.

Inductive Effect Alone Cannot Explain Lewis Adduct Formation and Dissociation at Electrode Interfaces

Understanding breaking and formation of Lewis bonds at an electrified interface is relevant to a large range of phenomena, including electrocatalysis and electroadsorption. The complexities of interfacial environments and associated reactions often impede a systematic understanding of this type of bond at interfaces. To address this challenge, we report the creation of a main group classic Lewis acid-base adduct on an electrode surface and its behavior under varying electrode potentials. The Lewis base is a self-assembled monolayer of mercaptopyridine and the Lewis acid is BF₃, forming a Lewis bond between nitrogen and boron. The bond is stable at positive potentials but cleaves at potentials more negative of approximately -0.3 V vs Ag/AgCl without an associated current. We also show that if the Lewis acid BF₃ is supplied from a reservoir of Li⁺BF₄^- electrolyte, the cleavage is completely reversible. We propose that the N-B Lewis bond is affected both by the field-induced intramolecular polarization (electroinduction) and by the ionic structures and ionic equilibria near the electrode. Our results indicate that the second effect is responsible for the Lewis bond cleavage at negative potentials. This work is relevant to understanding the fundamentals of electrocatalytic and electroadsorption processes.

A novel portable Raman scattering platform for antibiotic screening in pig urine

Background and Aim

Public health and food safety are gaining attention globally. Consumer health can be protected from chemical residues in meat by early detection or screening for antibiotic residues before selling the meat commercially. However, conventional practices are normally applied after slaughtering, which leads to massive business losses. This study aimed to use portable surface-enhanced Raman spectroscopy (SERS) equipped with multivariate curve resolution-alternation least squares (MCR-ALS) to determine the concentrations of enrofloxacin, oxytetracycline, and neomycin concentrations. This approach can overcome the problems of business loss, costs, and time-consumption, and limit of detection (LOD).

Materials and Methods

Aqueous solutions of three standard antibiotics (enrofloxacin, oxytetracycline, and neomycin) with different concentrations were prepared, and the LOD for each antibiotic solution was determined using SERS. Extracted pig urine was spiked with enrofloxacin at concentrations of 10, 20, 50, 100, and 10,000 ppm. These solutions were investigated using SERS and MCR-ALS analysis. Urine samples from pigs at 1 and 7 days after enrofloxacin administration were collected and investigated using SERS and MCR-ALS to differentiate the urinary enrofloxacin concentrations.

Results

The LOD of enrofloxacin, oxytetracycline, and neomycin in aqueous solutions were 0.5, 2.0, and 100 ppm, respectively. Analysis of enrofloxacin spiking in pig urine samples demonstrated the different concentrations of enrofloxacin at 10, 20, 50, 100, and 10,000 ppm. The LOD of spiking enrofloxacin was 10 ppm, which was 10 times lower than the regulated value. This technique was validated for the first time using urine collected on days 1 and 7 after enrofloxacin administration. The results revealed a higher concentration of enrofloxacin on day 7 than on day 1 due to consecutive administrations. The observed concentration of enrofloxacin was closely correlated with its circulation time and metabolism in pigs.

Conclusion

A combination of SERS sensing platform and MCR-ALS is a promising technique for on-farming screening. This platform can increase the efficiency of antibiotic detection in pig urine at lower costs and time. Expansion and fine adjustments of the Raman dataset may be required for individual farms to achieve higher sensitivity.

Surface-enhanced Raman spectroscopy detection of organic molecules and in situ monitoring of organic reactions by ion-induced silver nanoparticle clusters

Surface-enhanced Raman spectroscopy (SERS) finds wide applications in the field of organic molecule detection. However, reliable SERS detection of organic molecules and in situ monitoring of organic reactions under natural conditions by metal colloids are still challenging due to the formation of unstable nanoparticle clusters in solution and the low solubility of the organic molecules. Here, we approach the problems by introducing calcium ions to aggregate silver nanoparticles to form stable hot spots and acetone to promote uniform distribution of organic molecules on the nanoparticle surface. Significantly, our method exhibits stable SERS detection of up to 6 types of organic molecules in liquid. With acetone signals as an internal standard, we are able to determine molecule concentrations as well as monitor 3 kinds of organic reactions in situ. Our method shows potential for biomedical analysis, environmental analysis, and organic catalysis research.

Blood identification at the single-cell level based on a combination of laser tweezers Raman spectroscopy and machine learning

Laser tweezers Raman spectroscopy (LTRS) combines optical tweezers technology and Raman spectroscopy to obtain biomolecular compositional information from a single cell without invasion or destruction, so it can be used to "fingerprint" substances to characterize numerous types of biological cell samples. In the current study, LTRS was combined with two machine learning algorithms, principal component analysis (PCA)-linear discriminant analysis (LDA) and random forest, to achieve high-precision multi-species blood classification at the single-cell level. The accuracies of the two classification models were 96.60% and 96.84%, respectively. Meanwhile, compared with PCA-LDA and other classification algorithms, the random forest algorithm is proved to have significant advantages, which can directly explain the importance of spectral features at the molecular level.

Single Molecules in Strong Optical Fields: A Variable-Temperature Molecular Junction Spectroscopy Setup

In order to advance the development of molecular electronic devices, it is mandatory to improve the understanding of electron transport and functionalities in single molecules, integrated in a well-defined environment. However, limited information can be obtained by solely analyzing I-V characteristics, whence multiparameter studies are required to obtain more information on such systems including chemical bonds, geometry, and intramolecular strain. Therefore, we developed an analytical method incorporating an optical near-field technique, which allows us to investigate single-molecule junctions at variable temperatures in strong optical fields. An apertureless near-field emitter acts as a counter electrode and a plasmonic waveguide to focus surface plasmon polaritons into the molecular junctions, where a strongly enhanced evanescent field is confined to only a few nanometers around the apex of the tip. The proof of concept, even at low temperatures, is demonstrated by simultaneously investigating electronic and optical features of the molecule p-terphenyl-4,4″-dithiol in dependence of its charge state. This multichannel method can be employed to analyze a variety of nearly unexplored properties in single-molecule junctions such as photoconductance and photocurrent generation and allows a characterization of the molecular junctions by spectroscopic means as well.

Development of affinity between target analytes and substrates in surface enhanced Raman spectroscopy for environmental pollutant detection

Environmental pollution has long been a social concern due to the variety of pollutants and their wide distribution, persistence and being detrimental to health. It is therefore necessary to develop rapid and sensitive strategies to trace and detect these compounds. Among various detection methodologies, surface enhanced Raman spectroscopy (SERS) has become an attractive option as it enables accurate analyte identification, simple sample preparation, rapid detection and ultra-high sensitivity without any interference from water. For SERS detection, an essential yet challenging step is the effective capture of target analytes onto the surface of metal nanostructures with a high intensity of enhanced electromagnetic field. This review has systematically summarized recent advances in developing affinity between targets and the surface of SERS substrates via direct adsorption, hydrophobic functional groups, boronate affinity, metal organic frameworks (MOFs), DNA aptamers and molecularly imprinted polymers (MIPs). At the end of this review, technical limitations and outlook have been provided, with suggestions on optimizing SERS techniques for real-world applications in environmental pollutant detection.

Coordination controlled electrodeposition and patterning of layers of palladium/copper nanoparticles on top of a self-assembled monolayer

A scheme for the generation of bimetallic nanoparticles is presented which combines electrodeposition of one type of metal, coordinated to a self-assembled monolayer (SAM), with another metal deposited from the bulk electrolyte. In this way PdCu nanoparticles are generated by initial complexation of Pd²⁺ to a SAM of 3-(4-(pyridine-4-yl)phenyl)propane-1-thiol (PyP3) on Au/mica and subsequent reduction in an acidic aqueous CuSO₄ electrolyte. Cyclic voltammetry reveals that the onset of Cu deposition is triggered by Pd reduction. Scanning tunneling microscopy (STM) shows that layers of connected particles are formed with an average thickness of less than 3 nm and lateral dimensions of particles in the range of 2 to 5 nm. In X-ray photoelectron spectra a range of binding energies for the Pd 3d signal is observed whereas the Cu 2p signal appears at a single binding energy, even though chemically different Cu species are present: normal and more noble Cu. Up to three components are seen in the N 1s signal, one originating from protonated pyridine moieties, the others reflecting the SAM-metal interaction. It is suggested that the coordination controlled electrodeposition yields layers of particles composed of a Pd core and a Cu shell with a transition region of a PdCu alloy. Deposited on top of the PyP3 SAM, the PdCu particles exhibit weak adhesion which is exploited for patterning by selective removal of particles employing scanning probe techniques. The potential for patterning down to the sub-10 nm scale is demonstrated. Harnessing the deposition contrast between native and PdCu loaded PyP3 SAMs, structures thus created can be developed into patterned continuous layers.