Core-size-dependent catalytic properties of bimetallic Au/Ag core-shell nanoparticles

Bimetallic nanoparticles with sulfated galactan eliminate Vibrio parahaemolyticus in shrimp Penaeus vannamei

Bimetallic (Au/Ag) nanoparticles (BNPs) have shown enhanced antibacterial activity compared to their monometallic counterparts. Sulfated galactans (SG) are a naturally occurring polymer commonly found in red seaweed Gracilaria fisheri. They are biocompatible and biodegradable and environmentally friendly. In this study, we utilized SG in combination with BNPs to develop composite materials that potentially enhance antibacterial activity against shrimp pathogens Vibrio parahaemolyticus and Vibrio harveyi, compared to BNPs or SG alone. BNPs were coated with sulfated galactan (SGBNPs) and characterized using UV-vis spectroscopy, Fourier transform infrared (FTIR) spectroscopy, zeta potential, and transmission electron microscopy (TEM). UV-vis spectroscopy analysis revealed that the surface plasmon peaks of BNPs and SGBNPs appeared at 530 nm and 532 nm, respectively. Zeta potential measurements showed that SGBNPs had a negative charge of -32.4 mV, while the BNPs solution had a positive charge of 38.7 mV. TEM images demonstrated the spherical morphology of both BNPs and SGBNPs with narrow size distributions (3-10 nm). Analysis of the FTIR spectra indicated that SG maintained its backbone structure in SGBNPs, but some functional groups were altered. Notably, SGBNPs showed superior antimicrobial and antibiofilm activities against V. parahaemolyticus and V. harveyi compared to SG and BNPs. Furthermore, treatment with SGBNPs significantly down-regulated the expression of virulence-related genes (toxR, cpsQ, and mfpA) for V. parahaemolyticus 3HP compared to the respective control, bacteria treated with BNPs or SG. Diets supplemented with SGBNPs, BNPs, or SG showed no detrimental impact on the growth of shrimp Penaeus vannamei. Shrimp fed with SGBNPs-supplemented feed showed significantly higher survival rates than those fed with BNPs-supplemented feed when infected with 3HP after being on the supplemented feed for seven days and a subsequent number of fifteen days. These findings collectively demonstrate the benefit of using SG capped Au-Ag BNPs as an antibacterial agent for the prevention and control of Vibrio sp. Infection in shrimp while reducing the risk of environmental contamination.

Synthesis of Au@Ag core-shell nanocubes with finely tuned shell thicknesses for surface-enhanced Raman spectroscopic detection

In this work, we describe a facile method for generating monodisperse Au@Ag core-shell nanocubes with well-controlled size and fine-tuned Ag shell thicknesses. In this synthesis method, Au nanocubes were prepared via the seed-mediated growth method. Then, Au@Ag nanocubes with the core-shell structure were prepared separately by reducing AgNO3 with AA using as-prepared Au nanocubes as seeds. The thickness of Ag shells could be finely tuned from 3.6 nm to 10.0 nm by varying the concentration of the AgNO3 precursor. By investigating the localized surface plasmon resonance (LSPR) properties of Au@Ag nanocubes in relation to the thickness of the Ag shell, we found that the intensity of the characteristic peak of Ag gradually increases and that of Au gradually decreases as the thickness of the Ag shell increases. Additionally, surface-enhanced Raman scattering (SERS) properties of Au@Ag core-shell nanocubes were evaluated using rhodamine 6G (R6G) as the probe molecule. Interestingly, Au@Ag nanocubes exhibit efficient SERS intensities compared to the Au nanocubes, and Ag shell with a thickness of about 8.4 nm exhibits the optimal SERS activity. In addition, our results also demonstrated that Au@Ag nanocubes with an Ag shell thickness of 8.4 nm exhibited high SERS sensitivity and are capable of probing the analyte down to 10-12 M. The results obtained here suggest that Au@Ag core-shell nanocubes might serve as a nanoprobe for SERS-based analytical and biosensing applications.

Unveiling Core-Shell Structure Formation in a Ni3Fe Nanoparticle with In Situ Multi-Bragg Coherent Diffraction Imaging

Solid-state reactions play a key role in materials science. The evolution of the structure of a single 350 nm Ni3Fe nanoparticle, i.e., its morphology (facets) as well as its deformation field, has been followed by applying multireflection Bragg coherent diffraction imaging. Through this approach, we unveiled a demixing process that occurs at high temperatures (600 °C) under an Ar atmosphere. This process leads to the gradual emergence of a highly strained core-shell structure, distinguished by two distinct lattice parameters with a difference of 0.4%. Concurrently, this transformation causes the facets to vanish, ultimately yielding a rounded core-shell nanoparticle. This final structure comprises a Ni3Fe core surrounded by a 40 nm Ni-rich outer shell due to preferential iron oxidation. Providing in situ 3D imaging of the lattice parameters at the nanometer scale while varying the temperature, this study─with the support of atomistic simulations─not only showcases the power of in situ multireflection BCDI but also provides valuable insights into the mechanisms at work during a solid-state reaction characterized by a core-shell transition.

Synergistic reduction of nitrophenols by Au-CDs nanoconjugates with NaBH4

Developing sustainable and innovative approaches for the efficient reduction of nitrophenols is crucial for environmental remediation, for managing health concerns posed by their widespread presence as hazardous pollutants in industrial effluents and contaminated water. We report the use of 12.9 ± 1 nm (TEM data) sized gold carbon dot nanoconjugates (Au@CDs) for catalytic conversion of o, m, p-nitrophenols to aminophenols by sodium borohydride. A simple approach was followed to synthesize ultra-small and highly stable Au@CDs, using citric acid and PEG as reducing and stabilizing agents. X-ray diffraction analysis verified the formation of nano-crystalline nanoconjugates. These nanoconjugates showed a remarkable catalytic activity in the range of 0.22-0.33 s-1(varying with nanoconjugate concentration) which was much higher compared to conventional chemical methods of reduction. All the catalytic reaction experiments were performed at room temperature (27 ± 2 °C). Furthermore, an increase in rate constant was observed with increasing concentration of nanoconjugates. The catalytic activity of Au@CDs nanoconjugates was observed to be in order of m-nitrophenol > o-nitrophenol > p-nitrophenol with apparent rate constant (kaap) values of 0.068, 0.043 and 0.031, respectively. Comparative analysis with GNPs, CDs and Au@CDs nanoconjugates stated that the nanoconjugates had superior catalytic activity. The research can have significant implications in the development of new strategies for environmental remediation and biomedical applications.

'When is a hotspot a good nanospot' - review of analytical and hotspot-dominated surface enhanced Raman spectroscopy nanoplatforms

Substrate development in surface-enhanced Raman spectroscopy (SERS) continues to attract research interest. In order to determine performance metrics, researchers in foundational SERS studies use a variety of experimental means to characterize the nature of substrates. However, often this process would appear to be performed indiscriminately without consideration for the physical scale of the enhancement phenomena. Herein, we differentiate between SERS substrates whose primary enhancing structures are on the hundreds of nanometer scale (analytical SERS nanosubstrates) and those whose main mechanism derives from nanometric-sized gaps (hot-spot dominated SERS substrates), assessing the utility of various characterization methods for each substrate class. In this context, characterization approaches in white-light spectroscopy, electron beam methods, and scanning probe spectroscopies are reviewed. Tip-enhanced Raman spectroscopy, wavelength-scanned SERS studies, and the impact of surface hydrophobicity are also discussed. Conclusions are thus drawn on the applicability of each characterization technique regarding amenability for SERS experiments that have features at different length scales. For instance, while white light spectroscopy can provide an indication of the plasmon resonances associated with 10 s-100 s nm-scale structures, it may not reveal information about finer surface texturing on the true nm-scale, critical for SERS' sensitivity, and in need of investigation via scanning probe techniques.

Efficient Removal and Recovery of Ag from Wastewater Using Charged Polystyrene-Polydopamine Nanocoatings and Their Sustainable Catalytic Application in 4-Nitrophenol Reduction

This study addresses the long-standing challenges of removing and recovering trace silver (Ag) ions from wastewater while promoting their sustainable catalysis utilization. We innovatively developed a composite material by combining charged sulfonated polystyrene (PS) with a PDA coating. This composite serves a dual purpose: effectively removing and recovering trace Ag+ from wastewater and enabling reused Ag for sustainable applications, particularly in the catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). The PS-PDA demonstrated exceptional selectivity to trace Ag+ recycling, which is equal to 14 times greater than the commercial ion exchanger. We emphasize the distinct roles of different charged functional groups in Ag+ removal and catalytic reduction performance. The negatively charged SO3H groups exhibited the remarkable ability to rapidly enrich trace Ag ions from wastewater, with a capacity 2-3 times higher than that of positively-N+(CH3)3Cl and netural-CH2Cl-modified composites; this resulted in an impressive 96% conversion of 4-NP to 4-AP within just 25 min. The fixed-bed application further confirmed the effective treatment capacity of approximately 4400 L of water per kilogram of adsorbent, while maintaining an extremely low effluent Ag+ concentration of less than 0.1 mg/L. XPS investigations provided valuable insights into the conversion of Ag+ ions into metallic Ag through the enticement of negatively charged SO3H groups and the in situ reduction facilitated by PDA. This breakthrough not only facilitates the efficient extraction of Ag from wastewater but also paves the way for its environmentally responsible utilization in catalytic reactions.

Enhanced Fatty Acid Photodecarboxylation over Bimetallic Au-Pd Core-Shell Nanoparticles Deposited on TiO2

Photodecarboxylation of biomass-derived fatty acids to alkanes offers significant potential to obtain hydrocarbons and economic benefits due to the mild conditions and high activity. Herein, the photodecarboxylation of hexanoic acid into alkanes using TiO2-supported monometallic Au or Pd and bimetallic Au-Pd catalysts is reported. It was found that bimetallic Au-Pd catalysts, featuring a core-shell structure evidenced by EDX-mapping and element line profile, show better photocatalytic performance, achieving 94.7% conversion of hexanoic acid and nearly 100% selectivity to pentane under UV-vis irradiation in the absence of H2 than the monometallic Au analogue. This remarkable enhancement in activity compared to its TiO2 supported monometallic Au or Pd analogues can be attributed to the synergistic effect between Au and Pd within the nanostructured Au(core)-Pd(shell) alloy for achieving more efficient charge-separation efficiency upon visible light excitation. This photocatalyst exhibits a wide scope converting multiple fatty acids into hydrocarbons. Moreover, it can even photocatalyze the conversion of raw bio-oils into alkanes directly. No obvious activity loss was observed during the reusability tests, demonstrating the good stability of the present catalyst. Density functional theory (DFT) calculations indicate that oxidation of carboxylates on TiO2 leads to alkyl radicals that become bound to metal nanoparticles. The superior catalytic performance of Au(core)-Pd(shell)/TiO2 is derived from the weaker adsorption for H on the alloy and the lower hydrogen evolution reaction overpotential. Our research can result in an efficient bio-oil upgrading, resulting in the synthesis of biofuels from biomass under mild conditions.

Highly sensitive visual colorimetric sensor for xanthine oxidase detection by using MnO2-nanosheet-modified gold nanoparticles

In this study, a highly sensitive colorimetric assay has been constructed for the determination of xanthine oxidase (XOD) activity by the GNP@MnO₂ core-shell nanoparticles as probe. In the presence of XOD, xanthine can be oxidized to produce H₂O₂, which makes the MnO₂ shell fallen off. With the single particle detection (SPD) based dark field microscopy (DFM), the scattering color of GNP@MnO₂ NP probe shows obvious change before and after etching process. At the single particle level, noticeable color change of the single probe can be easily detected in the existence of trace XOD. This SPD-based colorimetric strategy displays broad linear dynamic range (0.02-4 mU/mL) and low detection limit of 7.82 μU/mL, which is more sensitive than the results from ensemble sample measurement. In addition, we tested the inhibitory effect of quercetin on the activity of XOD and obtained good inhibition effect. As a consequence, this SPD-based colorimetric strategy provides new perception for the ultrasensitive detection of molecules in complex system.

In-Situ Monitoring the SERS Spectra of para-Aminothiophenol Adsorbed on Plasmon-Tunable Au@Ag Core–Shell Nanostars

Plasmon-induced photocatalysis on noble metal surfaces has attracted broad attention due to its application in sunlight energy conversion, while the selectivity of plasmonic platforms remains unclear. Herein, we present the controlled plasmon-mediated oxidation of para-aminothiophenol (p-ATP) by employing Au@Ag core–shell nanostars with tunable tip plasmons in visible–near-infrared range as reactors. In-situ Raman measurements indicate that Au@Ag core–shell nanostars essentially promote the conversion of p-ATP to 4,4′-dimercaptoazobenzene (DMAB) due to hot carriers excited by localized surface plasmon resonance. Au@Ag nanostars with plasmon modes under resonant light excitation suggested higher catalytic efficiency, as evidenced by the larger intensity ratios between 1440 cm⁻¹ (N=N stretching of DMAB) and 1080 cm⁻¹ shifts (C–S stretching of p-ATP). Importantly, the time-dependent surface-enhanced Raman scattering spectra showed that the conversion efficiency of p-ATP was mainly dictated by the resonance condition between the tip plasmon mode of Au@Ag core–shell nanostars and the excitation light, as well as the choice of excitation wavelength. These results show that plasmon bands of metal nanostructures play an important role in the efficiency of plasmon-driven photocatalysis.

Two-Dimensional Self-Assembly of Au@Ag Core-Shell Nanocubes with Different Permutations for Ultrasensitive SERS Measurements

Different self-assembly methods not only directly change the arrangement of noble metal particles on the substrate but also indirectly affect the local electromagnetic field distribution and intensity of the substrate under specific optical excitation conditions, which leads to distinguished different enhancement effects of the structure on molecular Raman signals. In this paper, first, the gold species growth method was used to prepare the silver-coated gold nanocubes (Au@Ag NCs) with regular morphology and uniform size, and then the two-phase and three-phase liquid-liquid self-assembly and evaporation-induced self-assembly methods were used to obtain the substrate structure with different NC arrangement patterns. The optimal arrangement of NCs was found by transverse comparison of Raman signal detection of probe molecules with the same concentration. Subsequently, surface-enhanced Raman scattering (SERS) measurements of Rhodamine (Rh6G) and aspartame (APM) were carried out. Furthermore, the finite element method (FEM) was employed to calculate the local electromagnetic fields of the substrates with different Au@Ag NC arrangements, and the calculated results were in agreement with the experimental results. The experimental results show that the SERS-active substrate was largely associated with the different arrangements of Au@Ag NCs, and the island membrane Au@Ag NCs array substrate obtained by evaporation-induced self-assembly can generate a strong local electromagnetic field due to the edge and corner bonding gap between the tightly arranged NCs; this endows the substrate with benign sensitivity and reproducibility and has great potential in molecular detection, biosensing, and food safety monitoring.

Use of single particle inductively coupled plasma mass spectrometry for understanding the formation of bimetallic nanoparticles

Bimetallic nanoparticles (NPs), including core-shell structure and bimetallic alloy nanoparticles, were synthesized and characterized using flow field-flow fractionation (FlFFF), single particle inductively coupled plasma mass spectrometry (SP-ICP-MS), and transmission electron microscope (TEM) with energy-dispersive x-ray spectroscopy (EDS). For the core-shell particles, a nominal 80 nm commercial core-shell AuAg bimetallic nanoparticle was used to examine the applicability of SP-ICP-MS to determine the core size of Au and shell thickness of Ag. Then, the method was applied to estimate the core size of Au and shell thickness of Ag for the laboratory synthesized particles. The results were compared with those obtained from TEM-EDS. For the alloy nanoparticles, two synthesis protocols, based on the galvanic replacement of Ag seed particles with Au, were used. One was to prepare a hollow AgAu particle by varying the volume of dissolved Au in basic solution (K-gold) to etch some parts of AgNPs to dissolved ionic silver with the formation of AuNPs covering the remaining AgNPs, producing a hole inside the core nanoparticles. Another protocol was to prepare AgAu alloy nanoparticles. SP-ICP-MS was used in combination with FlFFF to provide information on the changes of particle size with varying volume of K-gold reagent. Hydrodynamic diameter increased with increasing K-gold, as observed by FlFFF. With SP-ICP-MS without prior FlFFF, bimodal distributions were observed in the size distribution of Au and Ag. With prior FlFFF, monomodal distributions were observed by SP-ICP-MS, which allow the use of particle concentration and size to estimate the mass concentration of elements on the fractionated bimetallic nanoparticles. This study illustrates the potential use of SP-ICP-MS for gaining information about particle transformation during the synthesis of bimetallic nanoparticles.

Broadband SERS Enhancement by DNA Origami Assembled Bimetallic Nanoantennas with Label-Free Single Protein Sensing

Engineering hotspots in surface-enhanced Raman spectroscopy (SERS) through precisely controlled assembly of plasmonic nanostructures capable of expanding intense field enhancement are highly desirable to enhance the potentiality of SERS as a label-free optical tool for single molecule detection. Inspired by DNA origami technique, we constructed plasmonic dimer nanoantennas with a tunable gap decorated with Ag-coated Au nanostars on origami. Herein, we demonstrate the single-molecule SERS enhancements of three dyes with emission in different spectral regions after incorporation of single dye molecules in between two nanostars. The enhancement factors (EFs) achieved in the range of 10⁹-10¹⁰ for all the single dye molecules, under both resonant and nonresonant excitation conditions, would enable enhanced photostability during time-series measurement. We further successfully explored the potential of our designed nanoantennas to accommodate and detect a single thrombin protein molecule after selective placement in the wide nanogap of 10 nm. Our results suggest that such nanoantennas can serve as a broadband SERS enhancer and enable specific detection of target biological molecules with single-molecule sensitivity.

Size-Dependent Catalytic Activity of PVA-Stabilized Palladium Nanoparticles in p-Nitrophenol Reduction: Using a Thermoresponsive Nanoreactor

Palladium nanoparticles (Pd NPs) of various average global diameters (2.1-7.1 nm) encapsulated with hydrophilic polymer polyvinyl alcohol (PVA) have been synthesized and used as catalysts for sodium borohydride assisted reduction of p-nitrophenol to p-aminophenol. The synthesized catalysts exhibit excellent and typical size-dependent catalytic activity in the green protocol. UV-visible absorption spectroscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy were employed to characterize the prepared Pd NPs. The kinetics of this reaction was easily monitored by a UV-visible absorption spectrophotometer. The mechanism of the reaction is explained by the Langmuir-Hinshelwood model. The catalytic performance increases with decreasing size of the synthesized nanoparticles. The apparent rate constants (k app × 103/s-1) of the catalytic reduction in the presence of Pd NPs of average diameters of 2.1, 3.35, 6.2, and 7.1 nm are determined as 8.57, 7.67, 6.16, and 5.04, respectively, at 298 K by using 2.91 mol % palladium nanocatalyst in each case. Moreover, the estimated activation energy of 22.2 kJ mol-1 obtained for Pd NPs with the smallest average diameter of 2.1 nm is very low as reported in the literature for the reduction. The influences of catalyst dose and concentration of p-nitrophenol on catalytic reduction are fully investigated. The catalyst with the largest diameter shows a temperature-sensitive property that might be due to the presence of a very low amount of rapped PVA used as stabilizer during the fabrication process. Thus, the synthetic protocol provides a unique fabrication process of a catalytically active thermoresponsive nanoreactor consisting of Pd NPs encapsulated into a PVA stabilizing agent.

A Perspective on Modelling Metallic Magnetic Nanoparticles in Biomedicine: From Monometals to Nanoalloys and Ligand-Protected Particles

The focus of this review is on the physical and magnetic properties that are related to the efficiency of monometallic magnetic nanoparticles used in biomedical applications, such as magnetic resonance imaging (MRI) or magnetic nanoparticle hyperthermia, and how to model these by theoretical methods, where the discussion is based on the example of cobalt nanoparticles. Different simulation systems (cluster, extended slab, and nanoparticle models) are critically appraised for their efficacy in the determination of reactivity, magnetic behaviour, and ligand-induced modifications of relevant properties. Simulations of the effects of nanoscale alloying with other metallic phases are also briefly reviewed.

Poly(L-DOPA)-mediated bimetallic core-shell nanostructures of gold and silver and their employment in SERS, catalytic activity, and cell viability

Core-shell gold nanorod (AuNR)@silver (Ag) nanostructures with their unique properties have gained enormous interest and are widely utilized in various applications including sensor systems, catalytic reactions, diagnosis, and therapy. Despite the recent progress, simple, effective, low-cost, and easy-to-tune strategies are heavily required to fabricate these nanoparticles (NP) systems. For this, we propose the employment of the polymer of 3,4-dihydroxyphenyl-L-alanine (L-DOPA) as a ligand molecule. A conformal thin layer of polymer of L-DOPA (PLDOPA) with its various functional groups enabled the reduction of silver ions onto the AuNRs and stabilization of the resultant NPs without using any surfactant, reducing agent, and seed material. The shape and growth model of the AuNR@Ag nanostructures was manipulated by simply tuning the amount of silver ions. This procedure created different NP morphologies ranging from concentric to acentric/island shape core-shell nanostructures. Also, even at the highest Ag deposition, the PLDOPA layer is still conformally present onto the Au@Ag core-shell NRs. The unique properties of NP systems provided remarkable characteristics in surface-enhanced Raman spectroscopy, catalytic activity, and cell viability tests.

Advances and challenges in metallic nanomaterial synthesis and antibacterial applications

Multi-drug resistant bacterial infection has become one of the most serious threats to global public health. The preparation and application of new antibacterial materials are of great significance for solving the infection problem of bacteria, especially multi-drug resistant bacteria. The exceptional antibacterial effects of metal nanoparticles based on their unique physical and chemical properties make such systems ideal for application as antibacterial drug carriers or self-modified therapeutic agents both in vitro and in vivo. Metal nanoparticles also have admirable clinical application prospects due to their broad antibacterial spectrum, various antibacterial mechanisms and excellent biocompatibility. Nevertheless, the in vivo structural stability, long-term safety and cytotoxicity of the surface modification of metal nanoparticles have yet to be further explored and improved in subsequent studies. Herein, we summarized the research progress concerning the mechanism of metal nanomaterials in terms of antibacterial activity together with the preparation of metal nanostructures. Based on these observations, we also give a brief discussion on the current problems and future developments of metal nanoparticles for antibacterial applications.

Light-Scattering Simulations from Spherical Bimetallic Core-Shell Nanoparticles

Bimetallic nanoparticles show novel electronic, optical, catalytic or photocatalytic properties different from those of monometallic nanoparticles and arising from the combination of the properties related to the presence of two individual metals but also from the synergy between the two metals. In this regard, bimetallic nanoparticles find applications in several technological areas ranging from energy production and storage to sensing. Often, these applications are based on optical properties of the bimetallic nanoparticles, for example, in plasmonic solar cells or in surface-enhanced Raman spectroscopy-based sensors. Hence, in these applications, the specific interaction between the bimetallic nanoparticles and the electromagnetic radiation plays the dominant role: properties as localized surface plasmon resonances and light-scattering efficiency are determined by the structure and shape of the bimetallic nanoparticles. In particular, for example, concerning core-shell bimetallic nanoparticles, the optical properties are strongly affected by the core/shell sizes ratio. On the basis of these considerations, in the present work, the Mie theory is used to analyze the light-scattering properties of bimetallic core–shell spherical nanoparticles (Au/Ag, AuPd, AuPt, CuAg, PdPt). By changing the core and shell sizes, calculations of the intensity of scattered light from these nanoparticles are reported in polar diagrams, and a comparison between the resulting scattering efficiencies is carried out so as to set a general framework useful to design light-scattering-based devices for desired applications.

Green fabrication of hydrogel-immobilized Au@Ag nanoparticles using tannic acid and their application in catalysis

A catalytic hydrogel was obtained by immobilizing tannic acid reduced and stabilized Au@AgNPs on a PVA/TA hydrogel, used as the good solid catalyst for the degradation of environmental pollutants such as Congo red, 4-nitrophenol, -etc.

Core-satellite assemblies of Au@polydopamine@Ag nanoparticles for photothermal-mediated catalytic reaction

Engineering plasmonic nanoparticles (NPs) into superstructures comprising two or more distinctive materials is highly desirable because these assemblies can unfold new properties that differ from those exhibited by their individual counterparts. In addition, metal NPs such as Au NPs and Ag NPs have played a major role in environmental remediation. In this study, we designed a heterogeneous NP assembly composed of an Au core and Ag satellite by utilizing a mussel-inspired polydopamine (PDA) strategy. This approach afforded substantial enhancement in the catalytic activity because of the synergistic effect between the Au core and Ag satellite. Specifically, the heat from the localized surface plasmon resonance excitation of the Au NPs can accelerate the reduction reaction of 4-nitrophenol, while the Ag NPs act as a catalyst for reducing the activation energy. Overall, we prepared a facile route to produce heterogeneous metal NP assemblies, which offers promise in scalable synthesis and application in heterogeneous catalysis.

Reversing the Thermodynamics of Galvanic Replacement Reactions by Decreasing the Size of Gold Nanoparticles

Here, we describe the surprising reactivity between surface-attached (a) 0.9, 1.6, and 4.1 nm diameter weakly stabilized Au nanoparticles (NPs) and aqueous 1.0 × 10^-4 M Ag⁺ solution, and (b) 1.6 and 4.1 nm diameter weakly stabilized Au NPs and aqueous 1.0 × 10^-5 M PtCl₄^2-, which are considered to be antigalvanic replacement (AGR) reactions because they are not thermodynamically favorable for bulk-sized Au under these conditions. Anodic Stripping Voltammetry (ASV) and Scanning Transmission Electron Microscopy with Energy-Dispersive X-ray Spectroscopy (STEM-EDS) mapping provide quantitation of the extent of Ag and Pt replacement as a function of Au NP diameter. The extent of the reaction increases as the Au NP size decreases. The percentage of Ag in the AuAg alloy following AGR based on ASV is 17.8 ± 0.6% for 4.1 nm diameter Au NPs, 87.2 ± 2.9% for 1.6 nm Au NPs, and an unprecedented full 100% Ag for 0.9 nm diameter Au NPs. STEM-EDS mapping shows very close agreement with the ASV-determined compositions. In the case of PtCl₄^2-, STEM-EDS mapping shows AuPt alloy NPs with 3.9 ± 1.3% and 41.1 ± 8.7% Pt following replacement with 4.1 and 1.6 nm diameter Au NPs, respectively, consistent with qualitative changes to the ASV. The size-dependent AGR correlates well with the negative shift in the standard potential (E⁰) for Au oxidation with decreasing NP size.

(Gold triangular nanoplate core)@(silver shell) nanostructures as highly sensitive and selective plasmonic nanoprobes for hydrogen sulfide detection

Hydrogen sulfide plays a significant role in living beings, while its abnormal concentration is related to many diseases. Besides, H2S gas is harmful to human beings and the environment. The detection of H2S has therefore attracted much attention in the past several decades. Herein, highly sensitive and selective H2S plasmonic nanoprobes (gold triangular nanoplate core)@(silver shell) (AuTNP@Ag) are reported. By virtue of the high refractive index sensitivity of Au TNPs to the surrounding medium and facile sulfurization of silver by sulfur ions, AuTNP@Ag exhibits great sensitivity to both sulfur ions and H2S gas. The shifts of the plasmon peak are as large as 16 nm for the ventilation of 1 ppm hydrogen sulfide. AuTNP@Ag nanoprobes also exhibit very good sensing linearity at low concentrations of sulfur ions. Moreover, excellent sensing selectivity for sulfur ions is obtained. A type of test gel, which can produce a naked-eye observable color change when exposed to 1-100 ppm hydrogen sulfide gas, is developed using AuTNP@Ag nanoprobes. Owing to the high sensitivity, linearity, and selectivity of the Au TNP@Ag nanoprobes for hydrogen sulfide sensing, this work paves the way for the plasmonic detection of hydrogen sulfide in both biological and environmental applications.

Core-shell Au@AuAg nano-peanuts for the catalytic reduction of 4-nitrophenol: critical role of hollow interior and broken shell structure

Bimetallic hollow core-shell nanoparticles have gained immense attention, especially as a high-performance catalyst due to their large surface area and increased number of uncoordinated atoms. However, the synthesis of an anisotropic hollow structure with large number of uncoordinated atoms and tailored hole size remains elusive. Herein, we report the synthesis of peanut-like core-shell nanostructures consisting of Au nanorods as the core covered by the AuAg alloy shell. The AuAg shell was formed on the Au nanorod core via co-deposition of Ag and Au atoms without disturbing the Au nanorod core. Then, we controllably and selectively removed Ag atoms from the shell to create "Broken Shell Peanuts" with variable hole size between 8 ± 4 nm and 26 ± 7 nm. Further, we utilized these nanostructures with different hole size as catalysts to reduce 4-nitrophenol to 4-aminophenol where the broken shell peanut nanostructures with a hole size of 26 ± 7 nm were found to be 12 times more efficient than the solid shell peanut structures.

Iodine activation: a general method for catalytic enhancement of thiolate monolayer-protected metal clusters

To enhance catalytic activity, the present study details a general approach for partial thiolate ligand removal from monolayer-protected clusters (MPCs) by straightforward in situ addition of iodine. Two model reactions are examined to illustrate the effects on the catalytic activity of glutathione (SG)-capped Au MPCs serving as a catalyst for the NaBH4 reduction of 4-nitrophenol to 4-aminophenol and SG-capped Pd MPCs serving as a catalyst for the hydrogenation/isomerization of allyl alcohol. Iodine addition promoted partial thiolate ligand removal from both MPCs and improved the catalytic properties, presumably due to greater surface exposure of the metal cores as a result of ligand dissociation. The rate of 4-nitrophenol reduction increased from 0.066 min-1 in the absence of I2 to 0.505 min-1 in the presence of 2.0 equivalents I2 (equivalents based on total ligated glutathione). The reaction of allyl alcohol to produce 1-propanol and propanal was similarly accelerated as indicated by the increase in turnover frequency from 131 to 230 moles products per moles catalyst per h by addition of 0.2 equivalents I2. In both reactions, as the amount of I2 added increases the catalyst recyclability decreases due to catalyst instability. Low equivalents of I2 are optimal when considering both reaction rate and catalyst recyclability.

Vapor-phase Synthesis of Bimetallic Plasmonic Nanoparticles

Regulating the nanostructure of composite nanoparticles is essential for making them suitable for various applications. In general, the chemical reduction method is employed for preparing metal nanoparticles in the liquid phase. However, complicated techniques are required to control the nanostructure during particle synthesis. The evaporation/condensation method is used for synthesizing nanoparticles in the vapor-phase. Although this method produces impurity-free particles without aggregation, very few studies have been carried out on the synthesis of composite particles in the vapor-phase. In this study, we synthesized composite nanoparticles in the vapor-phase by using the evaporation/condensation method. The results showed that bimetallic nanoparticles are produced by this method. Moreover, it was indicated that the nanostructure of the synthesized nanoparticles is influenced by the order of the electric furnace with different temperatures.

Ultra-small iridium nanoparticles as active catalysts for the selective and efficient reduction of nitroarenes

The ultra-small noble metal iridium nanoparticles (IrNPs) possessing super catalytic activity can be applied in the efficient and selective catalytic reduction of nitroarenes under mild reaction conditions for the first time.

Tailored surface composition of Au/Pt nanocatalysts synthesized in microemulsions: a simulation study

Au/Pt nanoparticles show an optimized catalytic activity when compared with Pt nanoparticles because Pt activity is improved by the presence of Au on the surface. It was checked whether a controllable surface composition can be achieved by the simple strategy of varying the Au : Pt ratio. We present an in-depth kinetic simulation study on the influence of Au : Pt ratio on the formation of Au/Pt nanoparticles synthesized in microemulsions. This study is able to explain the resulting nanoarrangement as a function of kinetic parameters such as Au : Pt ratio and intermicellar exchange rate. The role of the micelles as a dosing pump of the Au precursor explains that a higher Au amount results in a Au reduction which takes place over a longer period of time. It implies that Au is deposited until longer stages of the synthesis, so Au is present at the nanoparticle surface. Micelles as reaction media produce a minor impact on Pt due to its slower reduction. These different kinetic behaviours of Au and Pt give rise to a surface composition which can be tailored by tuning the Au : Pt ratio. Numerical results on surface composition successfully reproduce experimental data and further support the outcomes of the degree of atomic mixing under different Au : Pt ratios.

Pure Pt surface at low Au content and mixed surface at high Au content.

Asymmetrical Molecular Decoration of Gold Nanorods for Engineering of Shape-Controlled AuNR@Ag Core-Shell Nanostructures

Gold-silver (Au@Ag) core-shell nanostructures have a stronger surface plasma response, wider absorption and scattering in the UV-vis-NIR region, and distinctive optical properties, which are widely explored in biosensors, information processing, photothermal therapy, and catalysis. Core-shell nanostructures are usually formed by the deposition of the second metal atoms onto the first core metal particles via the chemical wet method. The conventional approaches for the manipulation of the shape usually were done by homogeneous growth or etching of isotropic nanoparticles. Through in situ modification of the first metal core at the different locations, the different growth model of the second metal can be regulated to control the shapes of core-shell structures. Herein, we modified the gold nanorods (AuNRs) asymmetrically at the end and side parts using thiolated molecules to regulate the morphology of gold nanorod@silver (AuNR@Ag) core-shell nanoparticles. Interestingly, the obvious eccentric nanostructures of AuNR@Ag core-shell nanoparticles were obtained with the increase of the molecular weight of macromolecules modified at the end of AuNRs. Therefore the growth mode was adjusted from Frank-van der Merwe mode to Stranski-Krastanow mode. By changing the length of the hydrocarbon chain and functional groups of the small mercaptan molecules at the side of AuNRs, the silver shell exhibits selective growth at the side of the AuNRs, resulting in heterogeneous core-shell nanoparticles and various shapes of the AuNR@Ag core-shell. Our method opens up a new avenue toward preparing core-shell nanostructures with controlled shapes, and the obtained structures are promising in various applications.

Tailored Engineering of Bimetallic Plasmonic Au@Ag Core@Shell Nanoparticles

A distinctive synthetic method for the efficient synthesis of multifunctional bimetallic plasmonic Au@Ag core@shell nanoparticles (NPs) with tunable size, morphology, and localized surface plasmon resonance (LSPR) using Triton X-100/hexanol-1/deionized water/cyclohexane-based water-in-oil (W/O) microemulsion (ME) is described. The W/O ME acted as a "true nanoreactor" for the synthesis of Au@Ag core@shell NPs by providing a confined and controlled environment and suppressing the nucleation, growth, agglomeration, and aggregation of the NPs. High-resolution transmission electron microscopic analysis of the synthesized Au@Ag core@shell NPs revealed an "unusual core@shell" contrast, and the selected area electron diffraction and Moiré patterns showed that Au layers are paralleled to Ag layers, thus indicating the formation of Au@Ag core@shell NPs. Interestingly, the UV-visible spectrum of the Au@Ag core@shell NPs exhibited enthralling plasmonic properties by introducing a high-frequency quadrupolar LSPR mode originated from the isolated Au@Ag NPs along with a low-frequency dipolar LSPR mode originated from the coupled Au@Ag NPs. The effective plasmonic enhancement of the Au@Ag core@shell NPs is attributed to the extreme enhancement of the localized electromagnetic field by coupling of the localized surface plasmons of the Au core and Ag shell. The mechanisms for the nucleation and growth of Au@Ag core@shell NPs in W/O ME have been proposed. A unique electron transfer phenomenon between the Au core and Ag shell is elucidated for better understanding and manipulation of the electronic properties, which evinced the development of Au@Ag core@shell NPs through suppression of the galvanic replacement reaction.

Understanding metal-enhanced fluorescence and structural properties in Au@Ag core-shell nanocubes

Au@Ag core–shell structures have received particular interest due to their localized surface plasmon resonance properties and great potential as oxygen reduction reaction catalysts and building blocks for self-assembly. In this study, Au@Ag core–shell nanocubes (Au@AgNCs) were fabricated in a facile manner via stepwise Ag reduction on Au nanoparticles (AuNPs). The size of the Au@AgNCs and their optical properties can be simply modulated by changing the Ag shell thickness. Structural characterization has been carried out by TEM, SAED, and XRD. The metal-induced fluorescence properties of probe molecules near the Au@AgNCs were measured during sedimentation of the Au@AgNCs. The unique ring-like building block of Au@AgNCs has dual optical functions as a fluorescence quencher or fluorescence enhancement medium depending on the assembled regions.

The unique ring-like building block of Au@AgNCs has dual optical functions as a fluorescence quencher and fluorescence enhancement medium.

A nitrile-mediated aptasensor for optical anti-interference detection of acetamiprid in apple juice by surface-enhanced Raman scattering

Currently, the detection of pesticide is critical for food safety assurance, but it is still challenging due to the presence of biological interferents from complex food matrix. In this study, we developed an optical anti-interference surface-enhanced Raman scattering (SERS) aptasensor system for trace detection of acetamiprid. 4-(Mercaptomethyl) benzonitrile (MMBN) containing CN bond was used as Raman tag to provide a sharp peak (2227 cm^-1) in the Raman-silent spectral window (1800-2800 cm^-1) where no Raman signal existed for most of molecules. Gold nanoparticles (AuNPs) bonded with polyadenine (polyA)-mediated aptamer and Raman tag (MMBN-AuNPs-aptamer) was synthesized as Raman probe, while the complementary DNA (cDNA) conjugated with AgNPs-decorated silicon wafer (AgNPs@Si) was used as SERS substrate. As acetamiprid molecule could specifically combine with aptamer, preventing the formation of MMBN-AuNPs-aptamer-cDNA-AgNPs@Si (expressed as "Au-AgNPs@Si") hybrid through DNA sequence linking, Raman signal intensities of MMBN in Au-AgNPs@Si decreased when the concentration of acetamiprid increased. Under the optimum assay condition, the proposed method displayed a linear response for acetamiprid detection in the range of 25-250 nM with a low detection limit of 6.8 nM. Finally, the developed aptasensor was successfully used to determine acetamiprid content in apple juice. Accordingly, this novel anti-interference SERS aptasensor could be a promising acetamiprid sensor for food safety assurance.

Facile Peptide Bond Formation: Effective Interplay between Isothiazolone Rings and Silanol Groups at Silver/Iron Oxide Nanocomposite Surfaces

Proportional to considerable progress in protein-drug conjugations, attention to the efficient peptide coupling reagents is being increased. Hence, in this study, a versatile heterogeneous nanoscale reagent is presented for chemical, biological, and medical purposes. A combination of silver and silica-coated iron oxide nanoparticles (Ag/Fe₃O₄) has been well functionalized with isothiazolone rings via a silver-modified Heck mechanism. An appropriate condition is provided for peptide bond formation through the surface interplay between silanol groups and the loaded isothiazolone rings. A logical mechanism including a series of successive covalent bonds onto the surface of Ag/Fe₃O₄ nanocomposites is suggested for this catalyzed peptide bond formation. Accurate comparisons have been made to obtain the optimum value of the nanocatalyst and suitable conditions. As an additional application, the biological activity of the desired product has also been investigated through antibacterial assay tests. The results showed that our desired product could also be used as an effective heterogeneous nanoscale antibacterial agent for different purposes. In this regard, all of the essential structural and practical analyses have been carried out and precisely interpreted.

Engineering Lignin Stabilized Bimetallic Nanocomplexes: Structure, Mechanistic Elucidation, Antioxidant, and Antimicrobial Potential

Lignin, being a natural antioxidant and antimicrobial underutilized biopolymer derived mainly from agro-waste, is a material of great interest. In this study, lignin was chosen as a matrix to synthesize silver-gold bimetallic and monometallic nanocomplexes to explore the synergistic antioxidant and antimicrobial properties of the lignin stabilized nanoagents. The synthesis of the nanocomplexes was carried out using a one pot method, utilizing lignin as the sole source for reducing, capping, and stabilizing the nanoagents. Further, characterization studies were performed to determine the exact structure of the nanocomplexes. The developed nanocomplexes were found to possess substantial phenolic and flavonoid contents, which contributed to their high antioxidant activity. Further, the antioxidant and antimicrobial activity of the lignin-bimetallic and monometallic nanocomplexes was evaluated and compared with pristine lignin. Moreover, the mechanism behind the antimicrobial activity of the nanocomplexes was elucidated through various methods, namely, reactive oxygen generation, nucleic acid leakage, and DNA cleavage studies. The obtained results were greatly supported by scanning electron microscopy, transmission electron microscopy, and live-dead cell imaging techniques. This study is a contribution in converting waste to value added functional nanomaterials for potential antioxidant and antimicrobial applications.

Ultrasonic Influence on Plasmonic Effects Exhibited by Photoactive Bimetallic Au-Pt Nanoparticles Suspended in Ethanol

The optical behavior exhibited by bimetallic nanoparticles was analyzed by the influence of ultrasonic and nonlinear optical waves in propagation through the samples contained in an ethanol suspension. The Au-Pt nanoparticles were prepared by a sol-gel method. Optical characterization recorded by UV-vis spectrophotometer shows two absorption peaks correlated to the synergistic effects of the bimetallic alloy. The structure and nanocrystalline nature of the samples were confirmed by Scanning Transmission Electron Microscopy with X-ray energy dispersive spectroscopy evaluations. The absorption of light associated with Surface Plasmon Resonance phenomena in the samples was modified by the dynamic influence of ultrasonic effects during the propagation of optical signals promoting nonlinear absorption and nonlinear refraction. The third-order nonlinear optical response of the nanoparticles dispersed in the ethanol-based fluid was explored by nanosecond pulses at 532 nm. The propagation of high-frequency sound waves through a nanofluid generates a destabilization in the distribution of the nanoparticles, avoiding possible agglomerations. Besides, the influence of mechanical perturbation, the container plays a major role in the resonance and attenuation effects. Ultrasound interactions together to nonlinear optical phenomena in nanofluids is a promising alternative field for a wide of applications for modulating quantum signals, sensors and acousto-optic devices.