One-pot synthesis of hollow hydrangea Au nanoparticles as a dual catalyst with SERS activity for in situ monitoring of a reduction reaction

Controllable Preparation of Gold Nanocrystals with Different Porous Structures for SERS Sensing

Porous Au nanocrystals (Au NCs) have been widely used in catalysis, sensing, and biomedicine due to their excellent localized surface plasma resonance effect and a large number of active sites exposed by three-dimensional internal channels. Here, we developed a ligand-induced one-step method for the controllable preparation of mesoporous, microporous, and hierarchical porous Au NCs with internal 3D connecting channels. At 25 °C, using glutathione (GTH) as both a ligand and reducing agent combined with the Au precursor to form GTH-Au(I), and under the action of the reducing agent ascorbic acid, the Au precursor is reduced in situ to form a dandelion-like microporous structure assembled by Au rods. When cetyltrimethylammonium bromide (C₁₆TAB) and GTH are used as ligands, mesoporous Au NCs formed. When increasing the reaction temperature to 80 °C, hierarchical porous Au NCs with both microporous and mesoporous structures will be synthesized. We systematically explored the effect of reaction parameters on porous Au NCs and proposed possible reaction mechanisms. Furthermore, we compared the SERS-enhancing effect of Au NCs with three different pore structures. With hierarchical porous Au NCs as the SERS base, the detection limit for rhodamine 6G (R6G) reached 10^-10 M.

Surface-enhanced Raman spectroscopy for detection of fentanyl and its analogs by using Ag-Au nanoparticles

The problem of opioid abuse has become a global problem. Thus, creating an urgent need for highly sensitive detection of opioid substances. In this work, we developed a method for the controllable preparation of Ag@Au nanocrystals (Ag@Au NCs) for highly sensitive SERS detection of fentanyl and its analogs. By regulating the concentration of ligands on the surface of silver seed, we successfully prepared Ag@Au NCs with three different morphologies, including core-satellite, yolk shell and hollow structure. Firstly, we explored the SERS-enhancing effect of Ag@Au NCs with different morphology using rhodamine 6G as the molecule to be tested. The results show that the core-satellite Ag@Au NCs has the best SERS effect, and the lowest detection concentration for R6G reached to 10^-10 M. Furthermore, we used the prepared core-satellite Ag@Au NCs to detect fentanyl and its five analogs, including carfentanyl, furanylfentanyl, thiofentanyl, 4-fluorobutyrfentanyl and N-4-piperidylacetanilide. Trace detection was achieved for the above six substances. For the environmental water samples spiked with fentanyl, the calculated recovery was 89.2% with an RSD value of 7.3%. Moreover, in order to realize the qualitative analysis of the characteristic peaks of different fentanyl analogs, we performed DFT calculations on the Raman spectra of the above-mentioned 6 substances. By analyzing the DFT calculation results, conventional Raman spectroscopy and SERS spectroscopy, we realized the distinction of six fentanyl analogs with similar structures.

Facile Synthesis of Urchin-like Hollow Au Crystals for In Situ SERS Monitoring of Photocatalytic Reaction

Hollow urchin-like Au nanocrystals have been widely studied due to their excellent surface plasmon resonance properties and large specific surface area, but the controllable preparation of hollow urchin-like Au nanocrystals is still a challenge. In this article, we successfully prepared hollow urchin-like Au nanocrystals using HAuCl4·3H2O and AgNO3 as precursors and ascorbic acid as the reducing agent. No surface ligands or polymer stabilizers are required in the preparation process. HAuCl4·3H2O and AgNO3 will first form AgCl cubes, then the reducing agent, ascorbic acid, will reduce the Au3+ in the solution to Au0, and Au0 will be deposited on the pre-formed AgCl cubes to form AgCl@Au nanocrystals. We characterized the morphology of the prepared Au nanocrystals by scanning electron microscopy and found that by increasing the amount of HAuCl4·3H2O in the reaction, the surface morphology of the Au nanocrystals would change from a rough spherical shape to an urchin-like shape. By further increasing the amount of the precursor HAuCl4·3H2O, urchin-like Au will convert into flake-like morphology. The AgCl in the interior was removed with ammonia water, and finally, hollow urchin-like Au crystals were formed. In addition, we used R6G molecule to explore the surface-enhanced Raman spectroscopy (SERS) enhancement effect of prepared Au crystals. The results show that the minimum detectable concentration of R6G reaches 10−8 M. Moreover, we applied hollow urchin-like Au nanocrystals as catalysts and SERS enhancing materials to detect the photocatalytic reaction of 4-NTP. We used a 785 nm laser as both the SERS light source and the catalytic light source to monitor the photocatalytic effect of the laser on 4-NTP in situ by adjusting the laser power.

Facile Synthesis of Porous Ag Crystals as SERS Sensor for Detection of Five Methamphetamine Analogs

Porous noble metal nanomaterials have attracted extensive attention due to their high specific surface area and surface plasmon resonance effect. However, it is difficult to form porous structures due to the high mobility and low reduction potential of noble metal precursors. In this article, we developed a facile method for preparing porous Ag with a controllable structure at room temperature. Two kinds of Ag crystals with different porous structures were successfully prepared by using AgCl cubes as sacrificial templates. Through the galvanic replacement reaction of Zn and AgCl, Ag crystals with a sponge-like porous structure were successfully prepared. Additionally, using NaBH₄ as the reducing agent, we prepared granular porous Ag cubes by optimizing the amount of reducing agent. Both the sponge-like and granular porous Ag cubes have clean and accessible surfaces. In addition, we used the prepared two porous Ag cubes as substrate materials for SERS detection of five kinds of methamphetamine analogs. The experimental results show that the enhancement effect of granular porous Ag is better than that of sponge-like porous Ag. Furthermore, we probed the hot spot distribution of granular porous Ag by Raman mapping. By using granular porous Ag as the substrate material, we have achieved trace detection of 5 kinds of methamphetamine analogs including Ephedrine, Amphetamine, N-Methyl-1-(benzofuran-5-yl)propan-2-amine (5-MAPB), N-Methyl-1-(4-methoxyphenyl)propan-2-amine (PMMA) and N-Methyl-1-(4-fluorophenyl)propan-2-amine (4-FMA). Furthermore, to achieve qualitative differentiation of analogs with similar structures we performed density functional theoretical (DFT) calculations on the Raman spectra of the above analogs. The DFT calculations provided the vibrational frequencies, Raman activities, and normal mode assignment for each analog, enabling the qualitative differentiation of the above analogs.

Seed-assembly-mediated fabrication and application of highly branched gold nanoshells having hollow and porous morphologies

Highly branched gold nanoshells (BAuNSs) having hollow and porous morphologies have been fabricated via a seed-assembly-mediated strategy. Gold seed assemblies can be prepared by removal of SiO₂nanotemplates with help of polyvinylpyrrolidone (PVP) molecules, which weakly link gold nanoparticles together even after SiO₂etching. L-3,4-dihydroxy phenylalanine (L-DOPA) and AgNO₃are employed as shape-directing agents to induce the anisotropic growth of gold. BAuNSs exhibit 7.4 and 4.4 times stronger activities than SiO₂@Au nanoparticles in catalysis and surface-enhanced Raman scattering (SERS) applications, respectively, due to their large surface areas and numerous hot spots. It is necessary to find the optimal amount of gold deposition in fabrication to effectively utilize the hollow and porous morpologies of BAuNSs for catalysis and SERS applications. Overgrown nanobranches can fill the nanopores and nanogaps of BAuNSs, resulting in decrease of activities in applications. Overall, the seed-assembly-mediated fabrciation can be employed to produce plasmonic nanostructures having unique morphologies and high application activities.

Synthesis of AuAg/Ag/Au open nanoshells with optimized magnetic plasmon resonance and broken symmetry for enhancing second-harmonic generation

The cooperation of magnetic and electric plasmon resonances in cup-shaped metallic nanostructures exhibits significant capability for second-harmonic generation (SHG) enhancement. Herein, we report an approach for synthesizing Au open nanoshells with tunable numbers and sizes of openings on a template of six-pointed PbS nanostars. The morphology of Au nanoshells is controlled by adjusting the amount of HAuCl₄, and the characteristic shapes of pointed nanocaps, open nanoshells, and hollow nanostars are obtained. Owing to the collaboration of electric and magnetic plasmon resonance modes, the Au nanoshells exhibit significantly broadened and highly tunable optical responses. Furthermore, the morphology-dependent SHG of the Au nanoshells shows two maximal SHG intensities, corresponding to four-opening and one-opening Au nanoshells with appropriate opening sizes. Ag/Au and AuAg/Ag/Au open nanoshells were further investigated to achieve enhanced SHG. By adjusting the thickness of the Ag shell, the SHG intensity of Ag/Au open nanoshells reaches a maximum due to the gradient field at the AuAg bimetallic interface. After replacing the Ag shells with Au shells, the SHG intensity of AuAg/Ag/Au open nanoshells reaches a maximum due to further symmetry breaking. These findings provide a strategy to prepare colloidal metal nanocrystals with prospective applications ranging from nonlinear photonic nanodevices to biospectroscopy and photocatalysis.

Controllable preparation of sea urchin-like Au NPs as a SERS substrate for highly sensitive detection of the toxic atropine

Branched Au nanoparticles (Au NPs) can significantly enhance the Raman signal of trace chemical substances, and have attracted the interest of researchers. However, there are still challenges to accurately prepare the morphology of branched Au NPs. In this work, we have successfully prepared sea urchin-like Au NPs and Au nanowires by using the seed-mediate growth method, with cetyltrimethylammonium bromide (CTAB) and glutathione as ligands, and ascorbic acid as a reducing agent. Using Au NPs with a tetrahexahedron (THH) morphology as seeds, and by simply changing the concentration of glutathione, we explored the growth process of sea urchin-like Au and Au nanowires. At low concentrations of glutathione, Au NPs will preferentially grow along the edges and corners of the THH Au seed, forming a core/satellite structure. As the concentration of glutathione increases, Au NPs will grow along the direction of glutathione, forming sea urchin-like Au NPs. To further increase the concentration of glutathione, we will prepare Au nanowires. In addition, we use the prepared Au NPs as a substrate material for surface-enhanced Raman (SERS) high-sensitivity detection. By using 4-aminothiophenol (4-ATP) as the test molecule, we evaluated the SERS effect of the prepared Au NPs with different morphologies. The results showed that sea urchin-like Au NPs have the best enhancement effect. The lowest concentrations of Rhodamine 6G and 4-ATP were 10-10 M and 10-12 M, respectively, using sea urchin Au NPs as the base material. Furthermore, we conducted a highly sensitive SERS detection of the poison atropine monohydrate, and the lowest detected concentration was 10-10 M.

Cl−-Induced selective fabrication of 3D AgCl microcrystals by a one-pot synthesis method

Cl⁻ induces the shape evolution of AgCl crystals with different morphologies.

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.

Tunable plasmonics of hollow raspberry-like nanogold for the robust Raman scattering detection of antibiotics on a portable Raman spectrometer

It is important to develop novel sensors for the simple, rapid, and quantitative detection of residual antibiotics in food, considering their potential threats to human health. Herein, we report the successful fabrication of raspberry-like nanogold (R-like Au) structures with rough surfaces and partially hollow structures for the rapid and sensitive detection of antibiotics in duck meats on a portable Raman spectrometer. The R-like Au with tunable plasmonic wavelengths was fabricated by a two-step method. Ag particles as seeds were substituted by Au particles through a replacement reaction; moreover, the surface plasmon resonances (SPRs) of R-like Au red-shifted with the increase in the Au ion concentration. The R-like Au with a diameter of about 120 nm that matched well with the 785 nm laser on the portable Raman spectrometer was used as an excellent surface-enhanced Raman spectroscopy (SERS) active platform for detecting two kinds of antibiotics, namely, nitrofurantoin (NFT) and nitrofurazone (NFZ), in spiked duck meats. Both of them have sufficient sensitivity (0.05-10 mg L-1), good linear relationship (R2 > 0.99) and high recovery in quantitative SERS analysis. The platform is simple without the need for complex pretreatment of the food samples or use of large scale Raman spectrometers, promising easy implementation for the on-site analysis of residuals of antibiotics in food.

Electrochemical biosensor for detection of MON89788 gene fragments with spiny trisoctahedron gold nanocrystal and target DNA recycling amplification

The shape-controlled synthesis of gold nanocrystals via shape induction of hexadecyltrimethylammonium chloride, potassium bromide, and potassium iodide and enantioselective direction of L-cysteine is reported. The resulting gold nanocrystals (STO-Au) offer spiny trisoctahedron nanostructures with good monodispersity and enhanced exposed high-index facets and high catalytic activity. Construction of the electrochemical sensing platform for MON89788 gene involves the modification of STO-Au, thionine (Thi), and labeled bipedal DNA probe 1 or 2 (P1 or P2) for target DNA-induced recycling amplification. In the detection, two surface DNA probes were immobilized on gold electrode via the Au-S bond. Then, hairpin DNA 1 (H1), Thi-STO-Au-P1, and Thi-STO-Au-P2 self-assemble into two-dimensional DNA nanopores (DNPs) on the electrode surface. Target DNA hybridizes with hairpin DNA 2 (H2) to open hairpin structure of H2. The opened H2 binds with H1 in the DNPs to release Thi-STO-Au-P1, Thi-STO-Au-P2, and target DNA by toehold-mediated strand-displacement. The utilization of target DNA-induced recycling allows one target DNA to release 2N STO-Au-labeled DNA strands, promoting significant signal amplification. The detection signal is further enhanced by the catalyzed redox reaction of Thi with STO-Au. The differential pulse voltammetric signal, best measured at - 0.18 V vs. Ag/AgCl, decreases linearly with increasing concentration of MON89788 in the range 0.02-8 × 104 fM, and the detection limit is 0.0048 fM (S/N = 3). The proposed method was successfully applied for electrochemical detection of MON89788 gene fragments in the PCR products from genetically modified soybean. Graphical Abstract We develop l-cysteine controlled synthesis of spiny trisoctahedron gold nanocrystals with good monodispersity and highly exposed high-index facets. The architecture achieves to ultrahigh catalytic activity. The electrochemical biosensor based on gold nanocrystals and target DNA recycling amplification provides advantage of sensitivity, repeatability, and regeneration-free.

Studies on binding of single-stranded DNA with reduced graphene oxide-silver nanocomposites

The binding reaction of reduced graphene oxide-silver nanocomposites (rGO-AgNCs) with calf thymus single-stranded DNA (ssDNA) was studied by ultraviolet-visible absorption, fluorescence spectroscopy and circular dichroism (CD), using berberine hemisulphate (BR) dye as a fluorescence probe. The absorbance of ssDNA increases, but the fluorescence intensity is quenched with the addition of rGO-AgNCs. The binding of rGO-AgNCs with ssDNA was able to increase the quenching effects of BR and ssDNA, and induce the changes in CD spectra. All of the evidence indicated that there was a relatively strong interaction between ssDNA and rGO-AgNCs. The data obtained from fluorescence experiments revealed that the quenching process of ssDNA caused by rGO-AgNCs is primarily due to complex formation, i.e. static quenching. The increasing trend of the binding equilibrium constant (Ka) with rising temperature indicated that the binding process was an endothermic reaction. The calculated thermodynamic parameters showed that the binding process was thermodynamically spontaneous, and hydrophobic association played predominant roles in the binding of ssDNA to the surface of rGO-AgNCs.

Surfactant-free synthesis of porous Au by a urea complex

We report a facile surfactant-free synthetic method to obtain porous and hollow Au nanoparticles using only urea and HAuCl₄·4H₂O as precursors at 200 °C. The formation mechanism was investigated through X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy. Moreover, the prepared Au nanocrystals present superior catalytic performance for the reduction of 4-nitrophenol in comparison with solid Au nanoparticles. The catalytic efficiency of porous and hollow Au was nearly 6 times higher than that of Au nanoparticles. Furthermore, the porous and hollow Au maintained excellent stability even after 10 catalytic cycles. Therefore, the as-synthesized porous Au nanoparticles will have potential applications in organic catalysis, biosensing, drug delivery, water pollutant removal, and so on.

A gold–urea complex was incinerated to transform into porous hollow gold nanoparticles (PHAuNPs), and the PHAuNPs present high catalytic activity.

Controllable synthesis of Au nanocrystals with systematic shape evolution from an octahedron to a truncated ditetragonal prism and rhombic dodecahedron

Stepwise evolution of Au nanocrystals from an octahedron to a truncated ditetragonal prism and rhombic dodecahedron was achieved by the polyol synthesis method.