Experimental and density functional theory study of Raman and SERS spectra of 5-amino-2-mercaptobenzimidazole

Synthesis and Raman Detection of 5-Amino-2-mercaptobenzimidazole Self-Assembled Monolayers in Nanoparticle-on-a-Mirror Plasmonic Cavity Driven by Dielectric Waveguides

Functionalization of metallic surfaces by molecular monolayers is a key process in fields such as nanophotonics or biotechnology. To strongly enhance light-matter interaction in such monolayers, nanoparticle-on-a-mirror (NPoM) cavities can be formed by placing metal nanoparticles on such chemically functionalized metallic monolayers. In this work, we present a novel functionalization process of gold surfaces using 5-amino-2-mercaptobenzimidazole (5-A-2MBI) molecules, which can be used for upconversion from THz to visible frequencies. The synthesized surfaces and NPoM cavities are characterized by Raman spectroscopy, atomic force microscopy (AFM), and advancing-receding contact angle measurements. Moreover, we show that NPoM cavities can be efficiently integrated on a silicon-based photonic chip performing pump injection and Raman-signal extraction via silicon nitride waveguides. Our results open the way for the use of 5-A-2MBI monolayers in different applications, showing that NPoM cavities can be effectively integrated with photonic waveguides, enabling on-chip enhanced Raman spectroscopy or detection of infrared and THz radiation.

Engineering the Au-Cu2 O Crystalline Interfaces for Structural and Catalytic Integration

Precise structural control has attracted tremendous interest in pursuit of the tailoring of physical properties. Here, this work shows that through strong ligand-mediated interfacial energy control, Au-Cu₂ O dumbbell structures where both the Au nanorod (AuNR) and the partially encapsulating Cu₂ O domains are highly crystalline. The synthetic advance allows physical separation of the Au and Cu₂ O domains, in addition to the use of long nanorods with tunable absorption wavelength, and the crystalline Cu₂ O domain with well-defined facets. The interplay of plasmon and Schottky effects boosts the photocatalytic performance in the model photodegradation of methyl orange, showing superior catalytic efficiency than the AuNR@Cu₂ O core-shell structures. In addition, compared to the typical core-shell structures, the AuNR-Cu₂ O dumbbells can effectively electrochemically catalyze the CO₂ to C₂₊ products (ethanol and ethylene) via a cascade reaction pathway. The excellent dual function of both photo- and electrocatalysis can be attributed to the fine physical separation of the crystalline Au and Cu₂ O domains.

Ultrasensitive Sandwich-Type SERS-Biosensor-Based Dual Plasmonic Superstructure for Detection of Tacrolimus in Patients

Tacrolimus (FK506) is widely used in the prevention of organ transplant rejection and the treatment of autoimmune diseases, but it is difficult to detect within the low and narrow concentration range in practical clinical fields. A magnetic plasmonic superstructure-targets-plasmonic superstructure-based sandwich-type SERS biosensor is presented here to ultrasensitively detect FK506 in the blood of organ transplant patients. The spiky Fe₃O₄@SiO₂@Ag flower magnetic superstructure and hollow Ag@Au superstructure enhanced the SERS signals by providing rich sharp tips, cavities, and abundant hot spot gaps. And the magnetic feature makes it easy to concentrate and separate the biological target. Using the designed sandwich-type SERS biosensor, FK506 could be detected within a range of 0.5-20 ng/mL with a detection limit of 0.33 ng/mL. All results indicated that the sandwich-type SERS biosensor has good stability, sensitivity, and anti-interference properties. It is noteworthy that this allowed us to successfully analyze FK506 in the blood of transplant patients, which is in strong agreement with the clinical results. Consequently, the attractive sandwich-type SERS biosensor can be used for the detection of FK506 in real samples, which is promising for clinical diagnosis.

The characteristics of Raman spectroscopy of isomer CBD- and THC-Au nanoparticles using the density functional theory

The isomers cannabidiol (CBD) and Δ9-tetrahydrocannabinol (THC) can both be extracted from cannabis. We use density functional theory to study the Raman activity spectra, frontier molecular orbitals, and molecular electrostatic potentials of CBD, THC, and their respective gold complexes. A "selectivity enhancement" phenomenon for the spectral peaks at frequencies of 1144 cm^-1 and 1553 cm^-1 in the Raman spectrum of the CBD-Au_n complex, and at frequencies of 865 cm^-1, 1335 cm^-1, and 1553 cm^-1 in the Raman spectrum of the THC-Au_n complex, was observed and explained. The frontier molecular orbital energy gaps of CBD and THC are 5.4085 eV and 5.4461 eV, respectively, indicating that CBD is more likely to react than THC. The CBD/THC-Au complexes had the strongest chemical activities and greater charge transfer effects with an Au₃ cluster. The most electronegative sites of CBD and THC were found from molecular electrostatic potential (MEP) mapping. It is assumed that these sites are the adsorption sites of the CBD/THC molecules and gold surface. The MEP of the CBD/THC complexes also demonstrates the charge transfer effect between CBD/THC and Au. Both the "selectivity" phenomenon in the Raman activity spectra of the complex and the above assumption are explained by a surface selection rule. The conformation of the CBD/THC molecules on the gold surface are determined, showing that CBD is adsorbed vertically through the resorcinol structure while THC is adsorbed vertically through the tetrahydropyran and benzene ring.

Ultrasensitive detection of plant hormone abscisic acid-based surface-enhanced Raman spectroscopy aptamer sensor

Abscisic acid (ABA), as the most common plant hormone in the growth of wheat, can greatly affect the yield when its levels deviate from normal. Therefore, highly sensitive and selective detection of this hormone is greatly needed. In this work, we developed an aptamer sensor based on surface-enhanced Raman spectroscopy (SERS) and applied it for the high sensitivity detection of ABA. Biotin-modified ABA aptamer complement chains were modified on ferrosoferric oxide magnetic nanoparticles (Fe₃O₄MNPs) and acted as capture probes, and sulfhydryl aptamer (SH-Apt)-modified silver-coated gold nanospheres (Au@Ag NPs) were used as signal probes. Through the recognition of the ABA aptamer and its complementary chains, an aptamer sensor based on SERS was constructed. As SERS internal standard molecules of 4-mercaptobenzoic acid (4-MBA) were encapsulated between the gold core and silver shell of the signal probes; the constructed aptamer sensor generated a strong SERS signal of 4-MBA after magnetic separation. When there were ABA molecules in the detection system, with the preferential binding of ABA aptamer and ABA molecule, the signal probes were released from the capture probes, after magnetic separation, leading to a linear decrease in SERS intensity of 4-MBA. Thus, the detection response was linear over a logarithmic concentration range, with an ultra-low detection limit of 0.67 fM. In addition, the practical use of this assay method was demonstrated in ABA detection from fresh wheat leaves, with a relative error (RE) of 5.43-8.94% when compared with results from enzyme-linked immunosorbent assay (ELISA). The low RE value proves that the aptamer sensor will be a promising method for ABA detection.

The Raman, SERS and computational studies of 3,5-dimethoxy-4-hydroxycinnamic acid and its silver complex

Optimized chemical structure, Raman and SERS spectra of 3,5-dimethoxy-4-hydroxycinnamic acid (35D4HCA) molecule and its silver (Ag) complex were calculated using time-dependent density functional theory in conjunction with B3LYP functional and LANL2DZ/6-311 + G(d,p) basis sets. Moreover, excitation and HOMO-LUMO energies were computed by the same level of theory. Because of the fact that energy of excitation electronic transition depends on infinite lifetime approximation, Raman activities were calculated under this approximation. Normal Raman spectra and SERS spectra of 35D4HCA and its silver complex were obtained with different excitation laser frequencies such as 532 nm and 785 nm and so interaction between metal surface and 35D4HCA was examined using SERS spectra. Moreover, theoretical and experimental UV-VIS spectra in the water of 35D4HCA and its silver complex were obtained and transitions, wavelengths and energy values of samples were shown. ¹H NMR experiment of 35D4HCA and its silver complex were performed and it was determined interaction between Ag atoms and 35D4HCA. In order to determine thermal properties of 35D4HCA and its silver complex, TG and DTA analysis were carried out. HOMO and LUMO energy levels corresponding to these energy values were determined and transitions between these levels were determined.

Raman and SERS spectra of thiamethoxam and the Ag3-thiamethoxam complex: an experimental and theoretical investigation

The insecticide thiamethoxam (TMX) is one of the most important neonicotinoid pesticides. The chromatographic methods currently employed to detect TMX require multiple operational steps. This study proposes a simple method that detects TMX via surface-enhanced Raman scattering (SERS) spectroscopy with Ag nanoparticles (NPs) as the SERS active substrate. Density functional theory (DFT) was used to calculate the structures and vibrational modes of the Ag- and Ag₃-TMX complexes at the B3LYP/6-311++G(d,p)(C,H,N)/LanL2DZ(Ag) level of theory. The results reveal that the atoms in the thiazole ring all lie in the same plane, while the six-membered ring is perpendicular to the thiazole ring. Data from both Ultraviolet-visible and Raman spectroscopy indicated that TMX bonds to Ag through its nitro group, vertically. A weak intramolecular (N22-O23…H26) hydrogen bonding and Ag-O bands shift N-O symmetrical vibration to down to lower wavenumber. This was supported by the appearance of a strong 866 cm^-1 band in the SERS spectrum assigned to the N-O symmetrical vibration coupled with the N-N stretching vibrational mode of different excitation wavelength. Notably, a good linear relationship was observed in the TMX concentration range 1.0 × 10^-6-1.0 × 10^-4mol·L^-1 (R² = 0.9667). SERS is an extremely simple and rapid technique that requires little sample for analysis.

Indirect glyphosate detection based on ninhydrin reaction and surface-enhanced Raman scattering spectroscopy

Glyphosate is one of the most commonly-used and non-selective herbicides in agriculture, which may directly pollute the environment and threaten human health. A simple and effective approach to assessment of its damage to the natural environment is thus quite necessary. However, traditional chromatography-based detection methods usually suffer from complex pretreatment procedures. Herein, we propose a simple and sensitive method for the determination of glyphosate by combining ninhydrin reaction and surface-enhanced Raman scattering (SERS) spectroscopy. The product (purple color dye, PD) of the ninhydrin reaction is found to SERS-active and directly correlate with the glyphosate concentration. The limit of detection of the proposed method for glyphosate is as low as 1.43×10^-8mol·L^-1 with a relatively wider linear concentration range (1.0×10^-7-1.0×10^-4mol·L^-1), which demonstrates its great potential in rapid, highly sensitive concentration determination of glyphosate in practical applications for safety assessment of food and environment.

Modular assembly of proteins on nanoparticles

Generally, the high diversity of protein properties necessitates the development of unique nanoparticle bio-conjugation methods, optimized for each different protein. Here we describe a universal bio-conjugation approach which makes use of a new recombinant fusion protein combining two distinct domains. The N-terminal part is Glutathione S-Transferase (GST) from Schistosoma japonicum, for which we identify and characterize the remarkable ability to bind gold nanoparticles (GNPs) by forming gold-sulfur bonds (Au-S). The C-terminal part of this multi-domain construct is the SpyCatcher from Streptococcus pyogenes, which provides the ability to capture recombinant proteins encoding a SpyTag. Here we show that SpyCatcher can be immobilized covalently on GNPs through GST without the loss of its full functionality. We then show that GST-SpyCatcher activated particles are able to covalently bind a SpyTag modified protein by simple mixing, through the spontaneous formation of an unusual isopeptide bond.

Detection of Pesticide Residues in Food Using Surface-Enhanced Raman Spectroscopy: A Review

Pesticides directly pollute the environment and contaminate foods ultimately being absorbed by the human body. Their residues contain highly toxic substances that have been found to cause serious problems to human health even at very low concentrations. The gold standard method, gas/liquid chromatography combined with mass spectroscopy, has been widely used for the detection of pesticide residues. However, these methods have some drawbacks such as complicated pretreatment and cleanup steps. Recent technological advancements of surface-enhanced Raman spectroscopy (SERS) have promoted the creation of alternative detection techniques. SERS is a useful detection tool with ultrasensitivity and simpler protocols. Present SERS-based pesticide residue detection often uses standard solutions of target analytes in conjunction with theoretical Raman spectra calculated by density functional theory (DFT) and actual Raman spectra detected by SERS. SERS is quite a promising technique for the direct detection of pesticides at trace levels in liquid samples or on the surface of solid samples following simple extraction to increase the concentration of analytes. In this review, we highlight recent studies on SERS-based pesticide detection, including SERS for pesticide standard solution detection and for pesticides in/on food samples. Moreover, in-depth analysis of pesticide chemical structures, structural alteration during food processing, interaction with SERS substrates, and selection of SERS-active substrates is involved.