Search

Salmonella Typhimurium is the prevalent pathogenic bacterium that causes foodborne-related diseases. Here, we developed a surface-enhanced Raman scattering (SERS) biosensor for S. Typhimurium detection. The biosensor was based on Au@Ag core-shell nanorods (Au@Ag NRs) to enhance Raman signal and aptamer-specific identification. 4-mercaptobenzoic acid (4-MBA) and aptamer-modified Au@Ag NRs were used as Raman signal probes. The aptamer adsorbed on the Au@Ag NRs surface when the reaction system was free of bacteria. Once S. Typhimurium was added, the aptamer then specifically identified and captured the cells. The decrease of free aptamers in solution restoring the SERS "hot spot" between the Au@Ag NRs and enhancing the 4-mercaptobenzoic acid (4-MBA) Raman signal. Quantitative analysis of S. Typhimurium was performed using Raman-enhanced spectroscopy. Under optimised conditions, the 4-MBA SERS intensity at 1072 cm−1 was used to determine S. Typhimurium in the range of 102–107 CFU mL−1 (y = 1660.84x+2340.7, R2 = 0.9912). The limit of detection was 64 CFU mL−1. This method also has high specificity and is not interfered by the other five control bacteria to complete the detection of S. Typhimurium. The developed method is simple, rapid, sensitive and specificity, which can be used to detect actual samples. It can be potentially used for detecting bacteria. • A novel aptamer based SERS assay for rapid detection of S. typhimurium was proposed. • The method is ultrasensitive for the detection of S. typhimurium with the LOD of 64 CFU mL−1. • The developed method showed good recoveries in real food samples contaminated with S. typhimurium. [ABSTRACT FROM AUTHOR]

An indirect aptamer-based SERS assay for insulin-like growth factor 2 receptor (IGF-IIR) protein was developed. The gold substrate and silver nanoparticles (AgNPs) were employed simultaneously to achieve double enhancement for SERS signals. Firstly, the five commercial SERS substrates including Enspectr, Ocean-Au, Ocean-AG, Ocean-SP and Q-SERS substrates were evaluated using 4-mercaptobenzoic acid (4-MBA). The Q-SERS substrate was selected based on low relative standard deviation (RSD, 8.6%) and high enhancement factor (EF, 8.7*105), using a 785 nm laser. The aptamer for IGF-IIR protein was designed to include two sequences: one grafted on gold substrate to specifically capture the IGF-IIR protein and a second one forming a 3′ sticky bridge to capture SERS nanotags. The SERS nanotag was composed by AgNPs (20 nm), 4-MBA and DNA probes that can hybridize with the aptamer. Due to the steric-hindrance effect, when the aptamer doesn't combine with IGF-IIR protein, it only can capture the SERS nanotags. Therefore, there was a negative correlation between the concentration of IGF-IIR protein and the intensity of 4-MBA at 1076 cm−1. The detection limit reached to 141.2 fM and linear range was from 10 pM to 1 μM. The SERS aptasensor also exhibits a high reproducibility with an average RSD of 4.5%. The interference test was conducted with other four proteins to verify the accuracy of measuring. The study provides an approach to quantitative determination of proteins based on specific recognition and nucleic acid hybridization of aptamers, to establish sandwich structure for SERS enhancement. [ABSTRACT FROM AUTHOR]

Semiconductor nanomaterial is a kind of important enhancement substrate in surface-enhanced Raman scattering (SERS), and the charge-transfer (CT) process contributes dominantly when they are used as the enhancement substrate for SERS. Doping has significant effect on the CT process of semiconductor nanomaterials. Yet till now, none attempts have been made to explore how doping affects the CT process between the semiconductor and probe molecules. For the first time, this paper investigates the effect of gallium (Ga) doping on the CT process between ZnO nanoparticles and 4-mercaptobenzoic acid (4-MBA) monolayer. In this paper, a series of Ga-doped ZnO nanoparticles (NPs) with various ratio of Ga and Zn are synthesized and their SERS performances are studied. The study shows that the doped Ga can cause the band gap shrinkage of ZnO NPs and then affect the CT resonance process form the valence band (VB) of ZnO NPs to the LUMO of 4-MBA molecules. The band gap of Ga-doped ZnO NPs is gradually narrowed with the increasing doping concentration, and a minimum value (3.16 eV) is reached with the Ga and Zn ratio of 3.8%, resulting in the maximum degree of CT. This work investigates the effects of doping induced band gap shrinkage on CT using SERS and provides a new insight on improving the SERS performance of semiconductor NPs.

Site-selected growth of L-shaped Janus Au-Ag gold nanodumbbells with appropriate concentration of 4-MBA embedded on the Au NDs and polarization orientation-dependent plasmonic extinction spectra and local electric field enhancement. [Display omitted] • The synergistic surface energy effects between gold nanodumbbells (Au NDs) with a negative surface curvature and 4-mercaptobenzoic acid (4-MBA) can direct the site-selective growth of anisotropic silver domains on gold nanodumbbells. • By adjusting the 4-MBA concentration-dependent interfacial energy, the Au NDs@Ag NPs could be continuously tuned from dumbbell-like core–shell structures, to L-shaped Janus, and then rod-like core–shell structures. • The Au NDs@Ag L-shaped Janus NPs with Ag island domains locaed on one tip-side of Au NDs created polarization orientation-dependent plasmonic extinction spectra. • The Au NDs@Ag L-shaped Janus NPs with Ag island domains created hot spots around the negatively curved waist and Ag domains. • The best SERS enhancement factor of 1.41 × 107 for 4-MBA was achieved. Ligand-mediated interface control has been broadly applied as a powerful tool in constructing asymmetric multicomponent nanoparticles (AMNP), and induces the anisotropic growth with fine-tuning morphology, composition, plasmonic property and functionality. As a new kind of AMNP, the synthesis of Janus Au-Ag nanoparticles with tunable negative surface curvature is still a challenge. Here, we demonstrate that the synergistic surface energy effects between gold nanodumbbells (Au NDs) with a negative surface curvature and 4-mercaptobenzoic acid (4-MBA) can direct the site-selective growth of anisotropic silver domains on gold nanodumbbells (Au NDs@Ag NPs). By adjusting the 4-MBA concentration-dependent interfacial energy, the Au NDs@Ag NPs could be continuously tuned from dumbbell-like core–shell structures, to L-shaped Janus, and then rod-like core–shell structures with directional and asymmetric spatial distributions of resizable Ag domains by site-selective growth. Based on the calculation results of discrete dipole approximation (DDA) method, it has been found that the Au NDs@Ag L-shaped Janus NPs with Ag island domains created polarization orientation-dependent plasmonic extinction spectra and hot spots around the negatively curved waist and Ag domains. The L-shaped Janus Au NDs@Ag NPs exhibited significantly plasmonic spectrum properties with four apparent LSPR peaks that cover from visible to near-infrared range and higher surface-enhanced Raman scattering (SERS) activity compared with the original Au NDs. The best SERS enhancement factor of 1.41 × 107 was achieved. This synergistic surface energy effect-based method involving the asymmetric growth of silver coating on gold nanoparticles with negatively curved surface presents a new method to design and fabricate nanometer optical devices based on asymmetric multicomponent nanoparticles. [ABSTRACT FROM AUTHOR]

Abstract: The differences between electromagnetic enhancement and chemical enhancement in surface‐enhanced Raman scattering (SERS) and their deep insight are very important and valuable for the explanation of spectral behaviors and the practical application of SERS technique. In this work, the typical semiconductor substrate TiO2 nanoparticles and noble‐metal substrate Ag colloid were selected as two representative SERS‐active substrates to investigate the differences between the electromagnetic (EM) mechanism and the charge‐transfer (CT) mechanism using 4‐mercaptobenzoic acid as probe molecule. The 4‐mercaptobenzoic acid molecules on two substrates exhibit the obvious differences in spectral behavior and signal intensity of SERS as well as the detectability, which can be attributed to the essential differences between the EM mechanism and the CT mechanism. The EM enhancement more depends on the behavior of the substrate, which more tends to be nonselective for the adsorbed molecules and their enhancement. The CT enhancement not only depends on the behavior of the substrate but also depends on the behavior of the adsorbed molecule due to the CT between substrate and molecule, which more tends to be selective for the adsorbed molecules and their enhancement. Moreover, it is also proved that the semiconductor (TiO2) SERS‐active substrate is characterized by the high stability and the recyclability. [ABSTRACT FROM AUTHOR]

In this work, a structural and electronic properties of 4-methoxybenzaldehyde (4MBA) have been presented. The density functional theory (DFT) along with B3LYP hybrid functional is employed. The optimized structure was found to be in well consistent with the X-ray diffraction geometry. The 4MBA crystal is stabilized by C-H⋯O intermolecular interactions along with π⋯π interactions. Various intermolecular interactions involved in 4MBA crystal have been analyzed deeply through topological atom-in-molecules (AIM) analysis and noncovalent interactions (NCI) method. Besides, Hirshfeld surface (HS) analysis and fingerprint plots are performed to determine the contribution of intermolecular contacts in 4MBA crystal packing. The electronic properties of the title compound have been investigated. Nonlinear optic (NLO) properties of 4MBA have been interpreted through the calculated first hyperpolarizability value. 4-substituted benzaldehydes, including 4MBA, are known with their competitive inhibitory activity on Tyrosinase, which also known as polyphenol oxidase (PPO). This enzyme is a rate limiting enzyme that controls the production of melamine and brown coloring of foods. Thus, molecular docking behaviors of 4MBA are presented in comparison with that of benzaldehyde (BA), 4-ethylBA, 4-tertbutylBA, 4-isoprophylBA, 4-propoxyBA, 4-butoxyBA, and Hexylresorcinol on four selected PPOs from sweet potato, grape, and mushroom. [ABSTRACT FROM AUTHOR]

4-mercaptobenzoic acid (4-MBA) adsorbed on Ag colloids has been investigated by surface-enhanced Raman scattering (SERS) spectroscopy. From the intensity variation of the symmetric stretching COO − vibration band corresponding to different pH values, it shows that 4-MBA molecules are bonded to the Ag surface only through the S atoms at acidic conditions, but through the S atoms and the COO − groups at neutral or alkaline conditions. The 4-MBA bonding to Ag shows SERS activity with multilayer adsorption (Freundlich isotherm model). Raman spectra calculated by density functional theory (DFT) with emphasis on long-range charge-transfer effect were compared with experimental observation to interpret the adsorption behavior of 4-MBA on the Ag surface. From both DFT simulations and experimental observation, we have verified that the contribution of electromagnetic and chemical enhancements in the SERS spectra of 4-MBA adsorbed on Ag are equally important. [ABSTRACT FROM AUTHOR]

In this paper, anatase and rutile TiO2 nanoparticles as well as their mixed crystal phase structure TiO2 nanoparticles were synthesized by a sol-hydrothermal method, and were served as active substrates for surface-enhanced Raman scattering (SERS) study. The results show that the 4-mercaptobenzoic acid probe molecules exhibit different degree SERS enhancements on the surface of different phase structure TiO2 nanoparticles. The mixed crystal structure TiO2 with an appropriate proportion of anatase and rutile phase is favourable to SERS enhancement of adsorbed molecules. These are mainly attributed to the contributions of the TiO2-to-molecule charge transfer mechanism and the mixed crystal effect. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Surface-enhanced Raman spectroscopy (SERS) with pyramidal gold nanostructures increases the signal of Raman active analytes, since hotspots form at the edges and tip of a nanopyramid under illumination. 2D hexagonal arrays of pyramidal nanostructures with a quadratic base are fabricated through cost-effective nanosphere lithography and transferred onto elastomeric polydimethylsiloxane. By making use of the {111} crystal plane of a silicon (100) wafer, an inverted pyramidal array is etched, which serves as the complementary negative for the gold nanostructures. Either a continuous gold thin-film with protruding pyramids or separate isolated nanopyramids are produced. Three basic fabrication strategies are presented. The SERS enhancement is verified by Raman mapping of 4-mercaptobenzoic acid (4-MBA) molecules. Fabrication on a flexible substrate paves the way for future applications on curved surfaces or in situ tunable resonances., (© 2021 IOP Publishing Ltd.)

A metal-organic semiconductor-molecule model was developed with Ag nanoparticles (NPs), poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and 4-mercaptobenzoic acid (4-MBA) via the layer-by-layer self-assembly method. In the SERS spectrum of the Ag/PEDOT:PSS/4-MBA system, structural changes in the PEDOT chain were discovered, which provides a deeper understanding of the charge transfer (CT) mechanism in SERS and helps in the development of a method to construct metal-organic semiconductor SERS substrates. A quantitative calculation of the degree of charge transfer (ρ CT(κ)) determines the CT contribution of PEDOT:PSS to the SERS intensity of the Ag/PEDOT:PSS/4-MBA system. On this basis, we propose the formation of a resonance complex between Ag NPs and PEDOT:PSS to explore the CT mechanism, which is beneficial for studying interface CT and for understanding the CT mechanism in SERS. The introduction of organic semiconductors in this study not only broadens the research scope of SERS substrates but also contributes to the exploration of SERS mechanisms. A metal/organic semiconductor/molecule model was fabricated with Ag NPs, PEDOT:PSS and 4-MBA via the layer-by-layer self-assembly method. The structural changes of the PEDOT chain in the SERS spectrum of the Ag/PEDOT:PSS/4-MBA system provide a deeper understanding of the CT mechanism in SERS and helps in the development of a method to construct metal-organic semiconductor SERS substrates. The introduction of organic semiconductors in this study not only broadens the research scope of SERS substrates but also contributes to the exploration of SERS mechanisms. Unlabelled Image • A metal/organic semiconductor/molecule model was fabricated via the layer-by-layer self-assembly method. • SERS spectrum of the Ag/PEDOT:PSS/4-MBA system provided a deeper understanding of the CT mechanism. • The introduction of organic semiconductors in this study not only broadens the research scope of SERS substrates. [ABSTRACT FROM AUTHOR]

• SERS has shown a high promise in microenvironmental pH sensing. • Evaluation of response range, resistance ability, accuracy and stability of four pH probes was performed. • 4-Mpy showed the best performance in pH detection with real biological samples. • 2-ABT has been found not suitable for SERS pH application due to the inadequate pH response. Surface-enhanced Raman spectroscopy (SERS) has shown a high promise in microenvironmental pH sensing in recent years, which is significant for understanding various biological processes and multiphase chemical reactions. Generally, the pH sensing capability of the SERS substrate is realized by the surface functionalization of the substrate with a pH sensitive molecule (pH probe). However, to the best of our knowledge, there is no systematic evaluation of mainstream SERS pH probes reported in literatures on the pH response range, resistance to co-present ions, accuracy and stability. Here a systematic analysis of common pH probes, including 4-mercaptobenzoic acid (4-MBA), 4-mercaptopyridine (4-Mpy), 2-aminobenzenethiol (2-ABT), and 4-aminothiophenol (4-ATP) has been carried out with a series of performance assessments. Density functional theory (DFT) calculation was carried out to investigate the vibration mode in SERS spectra. Moreover, the practical application comparison was conducted with urine samples. Overall, 4-MBA showed an excellent sensing performance in the neutral and alkaline area, and meanwhile, the pH response of 4-Mpy and 4-ATP was inclined to the acidic and neutral range. The pH probe 2-ABT is not recommend because of the poor performance. [ABSTRACT FROM AUTHOR]

Semiconductor nanomaterial is a kind of important enhancement substrate in surface-enhanced Raman scattering (SERS), and the charge-transfer (CT) process contributes dominantly when they are used as the enhancement substrate for SERS. Doping has significant effect on the CT process of semiconductor nanomaterials. Yet till now, none attempts have been made to explore how doping affects the CT process between the semiconductor and probe molecules. For the first time, this paper investigates the effect of gallium (Ga) doping on the CT process between ZnO nanoparticles and 4-mercaptobenzoic acid (4-MBA) monolayer. In this paper, a series of Ga-doped ZnO nanoparticles (NPs) with various ratio of Ga and Zn are synthesized and their SERS performances are studied. The study shows that the doped Ga can cause the band gap shrinkage of ZnO NPs and then affect the CT resonance process form the valence band (VB) of ZnO NPs to the LUMO of 4-MBA molecules. The band gap of Ga-doped ZnO NPs is gradually narrowed with the increasing doping concentration, and a minimum value (3.16 eV) is reached with the Ga and Zn ratio of 3.8%, resulting in the maximum degree of CT. This work investigates the effects of doping induced band gap shrinkage on CT using SERS and provides a new insight on improving the SERS performance of semiconductor NPs.