On the use of Au@Ag core-shell nanorods for SERS detection of Thiram diluted solutions

Gold nanorod in silver tetrahedron: Cysteamine mediated synthesis of SERS probes with embedded internal markers for AFP detection

BACKGROUND

Plasmonic core-shell nanostructures with embedded internal markers used as Raman probes have attracted great attention in surface-enhanced Raman scattering (SERS) immunoassay for cancer biomarkers due to their excellent uniform enhancement. However, current core-shell nanostructures typically exhibit a spherical shape and are coated with a gold shell, resulting in constrained local field enhancement.

RESULTS

In this work, we prepared a core-shell AuNR@BDT@Ag structure by depositing silver on the surface of Raman reporter-modified gold nanorods (AuNR). With cysteamine-driven specific deposition of Ag atoms, the internal standard nanoprobes with an ortho-tetrahedral morphology were obtained. Owing to its tetrahedral morphology and the effective plasmon coupling at the Ag-Au interface, this internal standard Raman probe exhibited excellent Raman enhancement. Also, with embedded Raman reporter, the probes of Au nanorod in Ag tetrahedron avoided the desorption of Raman reporter and competitive adsorption of interfering molecule, which greatly improved the stability and reproducibility of SERS signal and addressed the drawbacks of low reproducibility existing in SERS immunoassay. The feasibility of the AuNR@BDT@Ag probe was demonstrated by the sensitive detection of the liver cancer biomarker alpha-fetoprotein (AFP) in Eppendorf (EP) tubes and microfluidic chips. The results in EP tubes revealed a linear range of 1 fg/mL-1 ng/mL and a detection limit of 0.631 fg/mL. When the detection was performed in microfluidic chips, the linear range was 10 pg/mL to 0.1 μg/mL, with a limit detection of 8.29 pg/mL.

SIGNIFICANCE

The performance of AFP detection in EP tubes and microfluidic chips demonstrates that the tetrahedral core-shell AuNR@BDT@Ag nanostructures used as internal standard Raman probes have the potential to detect biomarkers in blood samples for cancer screening.

Development of a smartphone-assisted multiple colorimetric detection assay for GSH in food based on the degradation of gold nanorods

Glutathione (GSH) is a tripeptide and natural reducing agent composed of glutamic acid, glycine, and cysteine. Its level in the human body is closely linked to human health, such as diabetes, Alzheimer's disease, and cancer. The supplementation of exogenous GSH could bring health benefits and GSH detection in food is of considerable importance. However, the existing assays for GSH detection such as high-performance liquid chromatography/mass spectrometry, electrochemiluminescence and fluorescent nanoprobe were not satisfactory because of the disadvantages of equipment and site requirements. In this study, a multiple-colorimetric detection assay for GSH was developed based on GSH's reaction with gold nanorods. During the reaction with varying concentrations of GSH, the gold nanorods degraded into spherical nanoparticles with multiple color changes, which could be used to determine GSH concentrations. The transverse surface plasmon resonance absorption peak of gold nanorods (AuNRs) significantly shifted, indicating a novel mechanism distinct from etching or surface coating, which typically altered the longitudinal surface plasmon absorption peak. Under optimized conditions, the assay exhibited commendable specificity and reliability in actual samples. The assay accurately quantified GSH ranging from 1 to 10 µM, with detection limits of 439 nM and 260 nM for spectrophotometry and visual analysis, respectively. It was firstly to use GSH as a reducing agent to react with AuNRs in the presence of AgNO3 and the mechanism was different from etching or surface coating. The study's assay shows potential for detecting GSH in food samples and provides an alternative approach for the development of colorimetric detection assays based on AuNRs.

Advances in Surface-Enhanced Raman Spectroscopy for Urinary Metabolite Analysis: Exploiting Noble Metal Nanohybrids

This review examines recent advances in surface-enhanced Raman spectroscopy (SERS) for urinary metabolite analysis, focusing on the development and application of noble metal nanohybrids. We explore the diverse range of hybrid materials, including carbon-based, metal-organic-framework (MOF), silicon-based, semiconductor, and polymer-based systems, which have significantly improved SERS performance for detecting key urinary biomarkers. The principles underlying SERS enhancement in these nanohybrids are discussed, elucidating both electromagnetic and chemical enhancement mechanisms. We analyze various fabrication methods that enable precise control over nanostructure morphology, composition, and surface chemistry. The review critically evaluates the analytical performance of different hybrid systems for detecting specific urinary metabolites, considering factors such as sensitivity, selectivity, and stability. We address the analytical challenges associated with SERS-based urinary metabolite analysis, including sample preparation, matrix effects, and data interpretation. Innovative solutions, such as the integration of SERS with microfluidic devices and the application of machine learning algorithms for spectral analysis, are highlighted. The potential of these advanced SERS platforms for point-of-care diagnostics and personalized medicine is discussed, along with future perspectives on wearable SERS sensors and multi-modal analysis techniques. This comprehensive overview provides insights into the current state and future directions of SERS technology for urinary metabolite detection, emphasizing its potential to revolutionize non-invasive health monitoring and disease diagnosis.

Plasmonics nanorod biosensor for in situ intracellular detection of gene expression biomarkers in intact plant systems

The intracellular developmental processes in plants, particularly concerning lignin polymer formation and biomass production are regulated by microRNAs (miRNAs). MiRNAs including miR397b are important for developing efficient and cost-effective biofuels. However, traditional methods of monitoring miRNA expression, like PCR, are time-consuming, require sample extraction, and lack spatial and temporal resolution, especially in real-world conditions. We present a novel approach using plasmonics nanosensing to monitor miRNA activity within living plant cells without sample extraction. Plasmonic biosensors using surface-enhanced Raman scattering (SERS) detection offer high sensitivity and precise molecular information. We used the Inverse Molecular Sentinel (iMS) biosensor on unique silver-coated gold nanorods (AuNR@Ag) with a high-aspect ratio to penetrate plant cell walls for detecting miR397b within intact living plant cells. MiR397b overexpression has shown promise in reducing lignin content. Thus, monitoring miR397b is essential for cost-effective biofuel generation. This study demonstrates the infiltration of nanorod iMS biosensors and detection of non-native miRNA 397b within plant cells for the first time. The investigation successfully demonstrates the localization of nanorod iMS biosensors through TEM and XRF-based elemental mapping for miRNA detection within plant cells of Nicotiana benthamiana. The study integrates shifted-excitation Raman difference spectroscopy (SERDS) to decrease background interference and enhance target signal extraction. In vivo SERDS testing confirms the dynamic detection of miR397b in Arabidopsis thaliana leaves after infiltration with iMS nanorods and miR397b target. This proof-of-concept study is an important stepping stone towards spatially resolved, intracellular miRNA mapping to monitor biomarkers and biological pathways for developing efficient renewable biofuel sources.

A core-shell structured AuNPs@ZnCo-MOF SERS substrate for sensitive and selective detection of thiram

Surface-enhanced Raman scattering (SERS) enables pesticide residue monitoring to become facile and efficient. In this study, a core-shell structured gold nanoparticles@ZnCo metal-organic framework (AuNPs@ZnCo-MOF) SERS substrate was designed and successfully synthesized for efficient and selective detection of thiram. The bimetallic ZnCo-MOF shell can not only enrich the targeted molecules in the electromagnetic field because of its excellent absorptive capacity, but also act as a stabilized matrix for protecting the AuNPs from aggregation. The AuNPs@ZnCo-MOFs exhibited a high enhancement factor (EF) of 3.51 × 106 and a low detection limit of 1 × 10-7 mol L-1. Besides, the substrate material showed exceptional stability for up to 28 days at room temperature. The AuNPs@ZnCo-MOFs were used to detect thiram which displayed wide linearity (1 × 10-7 to 1 × 10-4 mol L-1) and high recoveries (83.45-99.61%). Moreover, the AuNPs@ZnCo-MOF SERS substrate exhibited excellent anti-interference ability and size selectivity for the target molecules. These indicate that the AuNPs@ZnCo-MOF substrate has great potential for the detection of thiram residues in practical applications.

Increasing gold nanostars SERS response with silver shells: a surface-based seed-growth approach

A straightforward method to prepare surface enhanced Raman spectroscopy (SERS) chips containing a monolayer of silver coated gold nanostars (GNS@Ag) grafted on a glass surface is introduced. The synthetic approach is based on a seed growth method performed directly on surface, using GNS as seeds, and involving a green pathway, which only uses silver nitate, ascorbic acid and water, to grow the silver shell. The preparation was optimized to maximize signals obtaining a SERS response of one order of magnitude greater than that from the original GNS based chips, offering in the meantime good homogeneity and acceptable reproducibility. The proposed GNS@Ag SERS chips are able to detect pesticide thiram down to 20 ppb.

Surface-enhanced Raman scattering detection of thiram and ciprofloxacin using chitosan-silver coated paper substrates

Fast detection of contaminants of emerging concern (CECs) in water resources is of great environmental interest. Ideally, sustainable materials should be used in water quality monitoring technologies implemented for such purposes. In this regard, the application of bio-based materials aimed at the fabrication of analytical platforms has become of great importance. This research merges both endeavors by exploring the application of chitosan-coated paper, decorated with silver nanoparticles (AgNPs), on surface-enhanced Raman scattering (SERS) spectroscopy studies of two distinct types of CECs dissolved in aqueous samples: an antibiotic (ciprofloxacin) and a pesticide (thiram). Our results indicate the superior SERS performance of biocoated substrates compared to their non-coated paper counterparts. The detection limits achieved for thiram and ciprofloxacin using the biocoated substrates were 0.024 ppm and 7.7 ppm, respectively. The efficient detection of both analytes is interpreted in terms of the role of the biopolymer in promoting AgNPs assemblies that result in local regions of enhanced SERS activity. Taking advantage of these observations, we use confocal Raman microscopy to obtain Raman images of the substrates using ciprofloxacin and thiram as molecular probes. We also demonstrate that these biobased substrates can be promising for on-site analysis when used in conjunction with portable Raman instruments.

Differences between surfactant-free Au@Ag and CTAB-stabilized Au@Ag star-like nanoparticles in the preparation of nanoarrays to improve their surface-enhanced Raman scattering (SERS) performance

In this study, we assessed the controlled synthesis and efficacy of surface-enhanced Raman scattering (SERS) on two distinct types of star-like Au@Ag core-shell nanoarrays. These nanoarrays were designed based on gold nanostars (AuNSs), which were synthesized with and without CTAB surfactant (AuNSs-CTAB and AuNSs-FS, respectively). The AuNS-FS nanoparticles were synthesized via a novel modification process, which helped overcome the previous limitations in the free-surfactant preparation of AuNSs by significantly increasing the number of branches, increasing the sharpness of the branches and minimizing the adsorption of the surfactant on the surface of AuNSs. Furthermore, the differences in the size and morphology of these AuNSs in the created nanoarrays were studied. To create the nanoarrays, a three-step method was employed, which involved the controlled synthesis of gold nanostars, covering them with a silver layer (AuNSs-FS@Ag and AuNSs-CTAB@Ag), and finally self-assembling the AuNS@Ag core-shelled nanoparticles via the liquid/liquid self-assembly method. AuNSs-FS@Ag showed higher ability in forming self-assembled nanoarrays than the nanoparticles prepared using CTAB, which can be attributed to the decrease in the repulsion between the nanoparticles at the interface. The nano-substrates developed with AuNSs-FS@Ag possessed numerous "hot spots" on their surface, resulting in a highly effective SERS performance. AuNSs-FS featured a significantly higher number of sharp branches than AuNSs-CTAB, making it the better choice for creating nanoarrays. It is worth mentioning that AuNSs-CTAB did not exhibit the same benefits as AuNSs-FS. The morphology of AuNSs with numerous branches was formed by controlling the seed boiling temperature and adding a specific amount of silver ions. To compare the SERS activity between the as-prepared nano-substrates, i.e., AuNS-CTAB@Ag and AuNS-FS@Ag self-assembled nanoarrays, low concentrations of crystal violet aqueous solution were characterized. The results showed that the developed AuNSs-FS@Ag could detect CV at trace concentrations ranging from 1.0 ng mL-1 to 10 ng mL-1 with a limit of detection (LOD) of 0.45 ng mL-1 and limit of quantification (LOQ) of 1.38 ng mL-1. The nano-substrates remained stable for 42 days with a decrease in the intensity of the characteristic Raman peaks of CV by less than 7.0% after storage. Furthermore, the spiking method could detect trace amounts of CV in natural water from the Dong Nai River with concentrations as low as 1 to 100 ng mL-1, with an LOD of 6.07 ng mL-1 and LOQ of 18.4 ng mL-1. This method also displayed good reproducibility with an RSD value of 5.71%. To better understand the impact of CTAB stabilization of the Au@Ag star-like nanoparticles on their surface-enhanced Raman scattering (SERS) performance, we conducted density functional theory (DFT) calculations. Our research showed that the preparation of AuNSs-FS@Ag via self-assembly is an efficient, simple, and fast process, which can be easily performed in any laboratory. Furthermore, the research and development results presented herein on nanoarrays have potential application in analyzing and determining trace amounts of organic compounds in textile dyeing wastewater.

Fabrication of a Novel Au Star@AgAu Yolk-Shell Nanostructure for Ovarian Cancer Early Diagnosis and Targeted Therapy

Purpose

A novel CYPA-targeted, SiO₂ encapsulated Au star@AgAu yolk-shell nanostructure (YSNS) was synthesized and used for ovarian cancer early diagnosis and therapy.

Methods

Diverse spectroscopic and microscopic methods were utilized to investigate the pattern of the yolk-shell nanostructure. In addition, in vitro and in vivo experiments were carried out.

Results

It can be found that the ratio of HAuCl₄ and AgNO₃ played a critical role in the constitution of the yolk-shell nanostructure. The as-prepared yolk-shell nanostructure showed excellent SERS performance, which could be utilized as SERS substrate for specific sensitivity analysis of ovarian cancer markers cyclophilin A (CYPA) with detectable limit of 7.76*10^-10 μg/mL. In addition, the as-prepared yolk-shell nanostructure possessed outstanding photothermal performance, which could be used as photothermal agent for ovarian cancer therapy. Experiments in vitro and in vivo proved that the as-prepared yolk-shell nanostructures are ideal candidate for early diagnosis and therapy for ovarian cancer in one platform.

Conclusion

This work holds promise to offer a new method for the detection and therapy of ovarian cancer in the early stage.

Plasmonic nanorod probes' journey inside plant cells for in vivo SERS sensing and multimodal imaging

Nanoparticle-based platforms are gaining strong interest in plant biology and bioenergy research to monitor and control biological processes in whole plants. However, in vivo monitoring of biomolecules using nanoparticles inside plant cells remains challenging due to the impenetrability of the plant cell wall to nanoparticles beyond the exclusion limits (5-20 nm). To overcome this physical barrier, we have designed unique bimetallic silver-coated gold nanorods (AuNR@Ag) capable of entering plant cells, while conserving key plasmonic properties in the near-infrared (NIR). To demonstrate cellular internalization and tracking of the nanorods inside plant tissue, we used a comprehensive multimodal imaging approach that included transmission electron microscopy (TEM), confocal fluorescence microscopy, two-photon luminescence (TPL), X-ray fluorescence microscopy (XRF), and photoacoustics imaging (PAI). We successfully acquired SERS signals of nanorods in vivo inside plant cells of tobacco leaves. On the same leaf samples, we applied orthogonal imaging methods, TPL and PAI techniques for in vivo imaging of the nanorods. This study first demonstrates the intracellular internalization of AuNR@Ag inside whole plant systems for in vivo SERS analysis in tobacco cells. This work demonstrates the potential of this nanoplatform as a new nanotool for intracellular in vivo biosensing for plant biology.

Recent advances of Au@Ag core-shell SERS-based biosensors

The methodological advancements in surface-enhanced Raman scattering (SERS) technique with nanoscale materials based on noble metals, Au, Ag, and their bimetallic alloy Au-Ag, has enabled the highly efficient sensing of chemical and biological molecules at very low concentration values. By employing the innovative various type of Au, Ag nanoparticles and especially, high efficiency Au@Ag alloy nanomaterials as substrate in SERS based biosensors have revolutionized the detection of biological components including; proteins, antigens antibodies complex, circulating tumor cells, DNA, and RNA (miRNA), etc. This review is about SERS-based Au/Ag bimetallic biosensors and their Raman enhanced activity by focusing on different factors related to them. The emphasis of this research is to describe the recent developments in this field and conceptual advancements behind them. Furthermore, in this article we apex the understanding of impact by variation in basic features like effects of size, shape varying lengths, thickness of core-shell and their influence of large-scale magnitude and morphology. Moreover, the detailed information about recent biological applications based on these core-shell noble metals, importantly detection of receptor binding domain (RBD) protein of COVID-19 is provided.

Surface-Enhanced Raman Spectroscopy Chips Based on Silver Coated Gold Nanostars

Surface-enhanced Raman scattering (SERS) is becoming widely used as an analytical tool, and the search for stable and highly responsive SERS substrates able to give ultralow detection of pollutants is a current challenge. In this paper we boosted the SERS response of Gold nanostars (GNS) demonstrating that their coating with a layer of silver having a proper thickness produces a 7-fold increase in SERS signals. Glass supported monolayers of these GNS@Ag were then prepared using simple alcoxyliane chemistry, yielding efficient and reproducible SERS chips, which were tested for the detection of molecules representative of different classes of pollutants. Among them, norfloxacin was detected down to 3 ppb, which is one of the lowest limits of detection obtained with this technique for the analyte.

Wafer-scale Silver Nanodendrites with Homogeneous Distribution of Gold Nanoparticles for Biomolecules Detection

We report the fabrication and demonstrate the superior performance of robust, cost-effective, and biocompatible hierarchical Au nanoparticles (AuNPs) decorated Ag nanodendrites (AgNDs) on a Silicon platform for the trace-level detection of antibiotics (penicillin, kanamycin, and ampicillin) and DNA bases (adenine, cytosine). The hot-spot density dependence studies were explored by varying the AuNPs deposition time. These substrates’ potential and versatility were explored further through the detection of crystal violet, ammonium nitrate, and thiram. The calculated limits of detection for CV, adenine, cytosine, penicillin G, kanamycin, ampicillin, AN, and thiram were 348 pM, 2, 28, 2, 56, 4, 5, and 2 nM, respectively. The analytical enhancement factors were estimated to be ∼10⁷ for CV, ∼10⁶ for the biomolecules, ∼10⁶ for the explosive molecule, and ∼10⁶ for thiram. Furthermore, the stability of these substrates at different time intervals is being reported here with surface-enhanced Raman spectroscopy/scattering (SERS) data obtained over 120 days.

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Wafer-scale surface-enhanced Raman spectroscopy/scattering (SERS) substrate of Ag nanodendrites decorated with Au nanoparticles prepared

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Trace level detection of antibiotics achieved

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Versatility of these substrates demonstrated by detecting explosive, dye molecules

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Typical enhancement factors achieved were 10⁵–10⁷

Silicon Microchannel-Driven Raman Scattering Enhancement to Improve Gold Nanorod Functions as a SERS Substrate toward Single-Molecule Detection

The investigation of enhanced Raman signal effects and the preparation of high-quality, reliable surface-enhanced Raman scattering (SERS) substrates is still a hot topic in the SERS field. Herein, we report an effect based on the shape-induced enhanced Raman scattering (SIERS) to improve the action of gold nanorods (AuNRs) as a SERS substrate. Scattered electric field simulations reveal that bare V-shaped Si substrates exhibit spatially distributed interference patterns from the incident radiation used in the Raman experiment, resulting in constructive interference for an enhanced Raman signal. Experimental data show a 4.29 increase in Raman signal intensity for bare V-shaped Si microchannels when compared with flat Si substrates. The combination of V-shaped microchannels and uniform aggregates of AuNRs is the key feature to achieve detections in ultra-low concentrations, enabling reproducible SERS substrates having high performance and sensitivity. Besides SIERS effects, the geometric design of V-shaped microchannels also enables a "trap" to the molecule confinement and builds up an excellent electromagnetic field distribution by AuNR aggregates. The statistical projection of SERS spectra combined with the SIERS effect displayed a silhouette coefficient of 0.83, indicating attomolar (10^-18 mol L^-1) detection with the V-shaped Si microchannel.

Nylon membranes modified by gold nanoparticles as surface-enhanced Raman spectroscopy substrates for several pesticides detection

Surface enhanced Raman spectroscopy (SERS) is an attractive means for trace compound detection because of its high sensitivity, however, the poor reproducibility is a major challenge. Herein, we propose a facile SERS strategy employing the several developed test processes to improve the repeatability of the SERS analysis based on regular nylon membranes as substrates to detect trace compounds. Various methods, including in situ reduction, immersion adsorption, and filtration, were first compared to prepare composite substrates using nylon membranes and gold nanoparticles. The substrates prepared by filtration showed the best test parallelism (RSD = 7.85%). Its limit of detection (LOD) could reach 10-8 g mL-1 with a good linear relationship in the range 10-8 to 10-7 g mL-1. Finally, three pesticide solutions were tested to verify the substrate applicability. A superior LOD of 10-8 g mL-1 was observed for thiram, whereas the LODs of both phorate and benthiocarb could reach 10-6 g mL-1. Overall, modifying nylon membrane substrates with gold nanoparticles improves the repeatability and economic viability of SERS and favors its wider commercial application for detecting trace compounds.

Quantitative detection of dithiocarbamate pesticides by surface-enhanced Raman spectroscopy combined with an exhaustive peak-seeking method

Surface-enhanced Raman spectroscopy (SERS) based on nanosilver colloid substrates has great potential for rapid detection of pesticide residues because of its advantages of sensitivity, rapidity, simplicity, low cost, etc. However, its poor repeatability and narrow linear quantitative range limit its practical application. In this paper, a silver colloid SERS analysis method combined with an exhaustive peak-seeking method was introduced for quantitative determination of thiram and ziram. This method can establish a linear quantitative relationship in a wide range by use of an own characteristic peak of analysis as an internal standard (IS) which is found via judging the linear correlation between the intensity ratios of two SERS peaks of analytes and the concentrations. Combined with improving the preparation method of silver colloids, adding suitable activators and optimizing the detection process, a liquid detection system with good repeatability and a wide linear quantitation range was obtained. The relative standard deviation (RSD) of the strongest SERS peak is no more than 8.98%, which is better than the general case of the silver colloid SERS substrate. The ratio of I₁₃₈₄/I₁₁₄₈ has a good linear relationship with the concentration of thiram solution, and the 1148 cm^-1 characteristic peak was utilized as the IS to establish the standard curve equation for the determination of thiram concentration. The equation is I₁₃₈₄/I₁₁₄₈ = -1.7930 × lg[c_thiram (ppm)] + 6.0078 with a linear range of 10^-2 to 10² ppm (4.16 × 10^-8 to 4.16 × 10^-4 mol L^-1) and a limit of detection (LOD) of 10^-2 ppm. The peak of IS for the determination of ziram concentration is at 938 cm^-1, and the equation is I₁₃₈₄/I₉₃₈ = 4.5531 × lg[c_ziram (ppm)] + 6.4792 with a linear range of 10^-1 to 10² ppm (3.27 × 10^-7 to 3.27 × 10^-4 mol L^-1) and a LOD of 10^-4 ppm. Thiram or ziram in apple juice was successfully detected by using this liquid detection system. This analysis system effectively solves the problem of poor repeatability and a narrow linear quantification range in SERS analysis based on silver colloid substrates, and the linear quantification range meets the requirements of the national standard (GB-2763-2019).

Unravelling the shell growth pathways of Au-Ag core-shell nanoparticles by in situ liquid cell transmission electron microscopy

Controlling the growth, structure and morphology of core-shell nanoparticles (NPs) is significant for catalytic applications and it can be achieved by adding chemical additives to the synthesis reaction mixture. However, achieving precise control over NP synthesis would require a comprehensive understanding of the mechanisms of NP formation under different chemical conditions, which is quite challenging. Here, using in situ liquid cell transmission electron microscopy (TEM), the overgrowth mechanisms of Ag on Au nanobipyramids (NBPs) are studied in AgNO3 aqueous solution with ascorbic acid as the reducing agent. Au-Ag core-shell NPs are formed via two mechanistic modes: (1) atom deposition during which the Ag atoms are deposited directly onto Au NBPs without the addition of poly(vinyl)pyrrolidone (PVP) and (2) nuclei coalescence during which the Ag nanocrystals (NCs) adsorb onto Au NBPs in the presence of PVP. High-resolution imaging reveals the dynamics of the coalescence process of Ag NCs upon addition of PVP. This study helps us to understand the effect of chemical additives during the evolution of a core seed into core-shell NPs with a well-defined composition and shape. It is useful for synthesizing NPs with greater design flexibility and expanding their various technological applications.

Loading of Au/Ag Bimetallic Nanoparticles within and Outside of the Flexible SiO2 Electrospun Nanofibers as Highly Sensitive, Stable, Repeatable Substrates for Versatile and Trace SERS Detection

In this paper, we propose a facile and cost-effective electrospinning technique to fabricate surface-enhanced Raman scattering (SERS) substrates, which is appropriate for multiple analytes detection. First of all, HAuCl4∙3H2O was added into the TEOS/PVP precursor solution, and flexible SiO2 nanofibers incorporated with gold nanoparticles (SiO2@Au) were prepared by electrospinning and calcination. Subsequently, the nanofibrous membranes were immersed in the tannic acid and 3-aminopropyltriethoxysilane solution for surface modification through Michael addition reaction. Finally, the composite nanofibers (Ag@T-A@SiO2@Au) were obtained by the in-situ growth of Ag nanoparticles on the surfaces of nanofibers with tannic acid as a reducing agent. Due to the synergistic enhancement of Au and Ag nanoparticles, the flexible and self-supporting composite nanofibrous membranes have excellent SERS properties. Serving as SERS substrates, they are extremely sensitive to the detection of 4-mercaptophenol and 4-mercaptobenzoic acid, with an enhancement factor of 108. Moreover, they could be utilized to detect analytes such as pesticide thiram at a low concentration of 10-8 mol/L, and the substrates retain excellent Raman signals stability during the durability test of 60 days. Furthermore, the as-fabricated substrates, as a versatile SERS platform, could be used to detect bacteria of Staphylococcus aureus without a specific and complicated bacteria-aptamer conjugation procedure, and the detection limit is up to 103 colony forming units/mL. Meanwhile, the substrates also show an excellent repeatability of SERS response for S. aureus organelles. Briefly, the prime novelty of this work is the fabrication of Au/Ag bimetallic synergetic enhancement substrates as SERS platform for versatile detection with high sensitivity and stability.

Metal Nanoparticles-Enhanced Biosensors: Synthesis, Design and Applications in Fluorescence Enhancement and Surface-enhanced Raman Scattering

Metal nanoparticles (NP) that exhibit localized surface plasmon resonance play an important role in metal-enhanced fluorescence (MEF) and surface-enhanced Raman scattering (SERS). Among the optical biosensors, MEF and SERS stand out to be the most sensitive techniques to detect a wide range of analytes from ions, biomolecules to macromolecules and microorganisms. Particularly, anisotropic metal NPs with strongly enhanced electric field at their sharp corners/edges under a wide range of excitation wavelengths are highly suitable for developing the ultrasensitive plasmon-enhanced biosensors. In this review, we first highlight the reliable methods for the synthesis of anisotropic gold NPs and silver NPs in high yield, as well as their alloys and composites with good control of size and shape. It is followed by the discussion of different sensing mechanisms and recent advances in the MEF and SERS biosensor designs. This includes the review of surface functionalization, bioconjugation and (directed/self) assembly methods as well as the selection/screening of specific biorecognition elements such as aptamers or antibodies for the highly selective bio-detection. The right combinations of metal nanoparticles, biorecognition element and assay design will lead to the successful development of MEF and SERS biosensors targeting different analytes both in-vitro and in-vivo. Finally, the prospects and challenges of metal-enhanced biosensors for future nanomedicine in achieving ultrasensitive and fast medical diagnostics, high-throughput drug discovery as well as effective and reliable theranostic treatment are discussed.