Transformation of Ag nanocubes into Ag-Au hollow nanostructures with enriched Ag contents to improve SERS activity and chemical stability

Quantitative and specific detection of cancer-related microRNAs in living cells using surface-enhanced Raman scattering imaging based on hairpin DNA-functionalized gold nanocages

A highly sensitive surface-enhanced Raman scattering (SERS) strategy based on hairpin DNA-functionalized gold nanocages for the detection of intracellular miR-125a-5p.

Three-Dimensional Hierarchical Reticular Nanostructure of Fulfora candelaria Wing Decorated by Ag Nanoislands as Practical SERS-Active Substrates

Although surface-enhanced Raman scattering (SERS) technology has been widely explored nowadays in various fields, the fabrication of practical SERS-active substrates with prominent recognition ability for various analyte molecules is still defective. Natural Fulfora candelaria wing (FCW) with three-dimensional (3D) hierarchical reticular nanostructure was selected as a new bioscaffold for rough silver (Ag) nanoislands to be assembled on to prepare a practical SERS substrate (Ag/FCW substrate). By adjusting the sputtering time of metal Ag, the morphology of the substrates could be easily tuned to control the formation and distribution of “hot spots”. Three-dimensional finite-difference time-domain (3D-FDTD) simulation indicated that the excellent SERS performance under optimal morphology was ascribed to the local enhanced electric field in rough Ag surface and effective “hot spot” areas. The SERS measurement results show that the optimal Ag/FCW substrates had high SERS performance in terms of Raman signal sensitivity, reproducibility, uniformity and recognition ability for various analyte molecules. Coupled with flexibility of the biological substrates and the cost effectiveness, the sensitive SERS detection of varied analytes based on Ag/FCW substrates offered great potential for practical applications.

Facile One-Pot Synthesis of Nanodot-Decorated Gold-Silver Alloy Nanoboxes for Single-Particle Surface-Enhanced Raman Scattering Activity

Surface-enhanced Raman scattering (SERS) is an important, highly sensitive technique for chemical and biological analysis, which is critically dependent upon high-performance metallic substrates. Anisotropic gold (Au)-silver (Ag) alloy nanoboxes are attractive SERS substrates because of the greatly enhanced Raman signals from the strong electromagnetic fields on the sharp corners. Yet, the routine approach of Au-Ag alloy nanobox synthesis is still challenging because of the complicated procedures and use of biologically/environmentally unfriendly reagents. To facilitate the usage of Au-Ag alloy nanoboxes for broad SERS applications, we propose a facile green strategy to synthesize Au-Ag alloy nanoboxes with superior single-particle SERS sensitivity. Our novel straightforward strategy involves HAuCl₄ and AgNO₃ reduction by ascorbic acid, which is achieved in an aqueous one-pot reaction at ambient temperature. Significantly, the surfaces of the prepared Au-Ag alloy nanoboxes are judiciously designed to introduce nanodots, generating numerous "hot spots" for high Raman signal enhancement as indicated by rigorous numerical simulations. By combining scanning electron microscopy and Raman mapping images, we demonstrate the application of Au-Ag alloy nanoboxes for single-particle sensing SERS activity. The as-prepared Au-Ag alloy nanoboxes are expected to facilitate their further applications in quantitative and ultrasensitive SERS detection.

Composition-Tunable Hollow Au/Ag SERS Nanoprobes Coupled with Target-Catalyzed Hairpin Assembly for Triple-Amplification Detection of miRNA

Detecting disease-related biomarkers is of great significance for disease diagnosis and therapy. In this work, we develop an ultrasensitive surface-enhanced Raman scattering (SERS) biosensor for the detection of an acute myocardial infarction-related miRNA (miR-133a) using composition-adjustable hollow Ag/Au nanosphere-based SERS probes coupled with the target-catalyzed hairpin assembly (CHA) strategy. Bimetallic probes displaying high stability and a strong surface plasmon resonance effect were synthesized with a controllable ratio of silver and gold by a galvanic replacement method and then captured by a duplex linker produced in the CHA process to accomplish signal amplification. In this way, the target miR-133a can be detected in a wide linear range with a detection limit of 0.306 fM and high selectivity over other miRNAs expressed in human hearts. Practical applications in human blood samples reveal the strong anti-interference ability and ideal sensitivity of our developed sensing platform in physiological environments, benefiting its potential biomedical applications.

Simple, Low-Cost Fabrication of Highly Uniform and Reproducible SERS Substrates Composed of Ag⁻Pt Nanoparticles

Ag⁻Pt nanoparticles, grafted on Ge wafer, were synthesized by the galvanic replacement reaction based on their different potentials. Detailed characterization through scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDS) and X-ray photo-elelctron spectroscopy (XPS) proved that Ag⁻Pt nanoparticles are composed of large Ag nanoparticles and many small Pt nanoparticles instead of an Ag⁻Pt alloy. When applied as surface-enhanced Raman scattering (SERS) substrates to detect Rhodamine 6G (1 × 10-8 M) or Crystal violet (1 × 10-7 M) aqueous solution in the line mapping mode, all of the obtained relative standard deviation (RSD) values of the major characteristic peak intensities, calculated from the SERS spectra of 100 serial spots, were less than 10%. The fabrication process of the SERS substrate has excellent uniformity and reproducibility and is simple, low-cost and time-saving, which will benefit studies on the platinum-catalyzed reaction mechanisms in situ and widen the practical application of SERS.

Overgrowth Versus Galvanic Replacement: Mechanistic Roles of Pd Seeds during the Deposition of Pd-Pt

Here, a systematic study of the roles played by Pd seeds during seed-mediated coreduction of Pd-Pt is presented. Either nanoparticles with porous, hollow architectures or concave nanocubes were achieved, depending on whether the synthesis conditions favored galvanic replacement or overgrowth. Prior works have shown that the galvanic replacement reaction between seeds and a precursor can be suppressed by introducing a faster, parallel reaction that removes one of the reagents (e.g., adatom generation in solution rather than surface-catalyzed precursor reduction). Here, we show that the galvanic replacement reaction depends on the size and concentration of the Pd seeds; the former of which can be manipulated during the course of the reaction through the use of a secondary reducing agent. This insight will guide future syntheses of multimetallic nanostructures by seeded methods, allowing for a range of nanocrystals to be precisely engineered for a variety of applications.

Galvanic Replacement-Driven Transformations of Atomically Intermixed Bimetallic Colloidal Nanocrystals: Effects of Compositional Stoichiometry and Structural Ordering

Galvanic replacement reactions dictated by deliberately designed nanoparticulate templates have emerged as a robust and versatile approach that controllably transforms solid monometallic nanocrystals into a diverse set of architecturally more sophisticated multimetallic hollow nanostructures. The galvanic atomic exchange at the nanoparticle/liquid interfaces induces a series of intriguing structure-transforming processes that interplay over multiple time and length scales. Using colloidal Au-Cu alloy and intermetallic nanoparticles as structurally and compositionally fine-tunable bimetallic sacrificial templates, we show that atomically intermixed bimetallic nanocrystals undergo galvanic replacement-driven structural transformations remarkably more complicated than those of their monometallic counterparts. We interpret the versatile structure-transforming behaviors of the bimetallic nanocrystals in the context of a unified mechanistic picture that rigorously interprets the interplay of three key structure-evolutionary pathways: dealloying, Kirkendall diffusion, and Ostwald ripening. By deliberately tuning the compositional stoichiometry and atomic-level structural ordering of the Au-Cu bimetallic nanocrystals, we have been able to fine-maneuver the relative rates of dealloying and Kirkendall diffusion with respect to that of Ostwald ripening through which an entire family of architecturally distinct complex nanostructures are created in a selective and controllable manner upon galvanic replacement reactions. The insights gained from our systematic comparative studies form a central knowledge framework that allows us to fully understand how multiple classic effects and processes interplay within the confinement by a colloidal nanocrystal to synergistically guide the structural transformations of complex nanostructures at both the atomic and nanoparticulate levels.

Functionalized Au@Ag-Au nanoparticles as an optical and SERS dual probe for lateral flow sensing

Lateral flow assay strips (LFASs) with Au nanoparticles (NPs) have been widely used as a probe for biomarkers in point-of-care testing; however, there still remain challenges in detection sensitivity and quantitative analysis. In this study, we developed a surface-enhanced Raman scattering (SERS)-based LFAS for quantitative analysis of a biomarker in the low concentration range. Moreover, apart from conventional Au NPs, three other types of citrate-capped Au-Ag bimetallic NPs: Au core with Ag shell NPs (Au@Ag NPs), rattle-like Au core in Ag-Au shell NPs (Au@Ag-Au NPs) and Ag-Au NPs were prepared and functionalized, and their solution-based SERS activities were comprehensively studied by experimental measurement and theoretical analysis. The results clearly indicated that the citrate-capped Au@Ag-Au NPs exhibited the highest SERS activity among the probes tested. Au@Ag-Au NPs were used as both optical and SERS probes in a SERS-based LFAS. In the presence of the analyte at high concentrations, a purple color appeared in the test zone. Highly sensitive and quantitative analysis was realized by measurement of SERS signals from the test lines. One of the most specific markers for cardiac injury, cardiac troponin I (cTnI), was chosen as the detection model. The detection limit of the SERS-based LFAS for cardiac troponin I was 0.09 ng/mL, lowered by nearly 50 times compared with visual results, and could be further lowered by optimization. These results demonstrated that the SERS-based LFAS using citrate-capped Au@Ag-Au NPs as probes can be a powerful tool for highly sensitive and quantitative detection of biomarkers. Graphical abstract A surface-enhanced Raman scattering (SERS)-based lateral flow assay strip using rattle-like Au core in Ag-Au shell (Au@Ag-Au) nanoparticles as probes was developed for quantitative analysis of a biomarker, with a detection limit nearly 50 times lower than that of visual assessment. C control line, T test line.

Site-Selective Carving and Co-Deposition: Transformation of Ag Nanocubes into Concave Nanocrystals Encased by Au-Ag Alloy Frames

We report a facile synthesis of Ag nanocubes with concave side faces and Au-Ag alloy frames, namely Ag@Au-Ag concave nanocrystals, by titrating HAuCl₄ solution into an aqueous mixture of Ag nanocubes, ascorbic acid (H₂Asc), NaOH, and cetyltrimethylammonium chloride (CTAC) at an initial pH of 11.6 under ambient conditions. Different from all previous studies involving poly(vinylpyrrolidine), the use of CTAC at a sufficiently high concentration plays an essential role in carving away Ag atoms from the side faces through galvanic replacement. Concurrent co-deposition of Au and Ag atoms via chemical reduction at orthogonal sites on the surface of Ag nanocubes leads to the generation of Ag@Au-Ag concave nanocrystals with well-defined and controllable structures. Specifically, in the presence of CTAC-derived Cl^- ions, the titrated HAuCl₄ is maintained in the AuCl₄^- species, enabling its galvanic replacement with the Ag atoms located on the side faces of nanocubes. The released Ag⁺ ions can be retained in the soluble form of AgCl₂^- by complexing with the Cl^- ions. Both the AuCl₄^- and AgCl₂^- in the solution are then reduced by ascorbate monoanion, a product of the neutralization reaction between H₂Asc and NaOH, to Au and Ag atoms for their preferential co-deposition onto the edges and corners of the Ag nanocubes. Compared with Ag nanocubes, the Ag@Au-Ag concave nanocrystals exhibit much stronger SERS activity at an excitation of 785 nm, making it feasible to monitor the Au-catalyzed reduction of 4-nitrothiophenol by NaBH₄ in situ. When the Ag cores are removed, the concave nanocrystals evolve into Au-Ag nanoframes with controllable ridge thicknesses.

Highly effective and chemically stable surface enhanced Raman scattering substrates with flower-like 3D Ag-Au hetero-nanostructures

We demonstrated flower-like 3D Ag-Au hetero-nanostructures on an indium tin oxide glass (ITO glass) for surface enhanced Raman scattering (SERS) applications. The flower-like 3D Ag nanostructures were obtained through electrodeposition with liquid crystalline soft template which is simple, controllable and cost effective. The flower-like 3D Ag-Au hetero-nanostructures were further fabricated by galvanic replacement reaction of gold (III) chloride trihydrate (HAuCl4·3H2O) solution and flower-like Ag. The flower-like Ag-Au hetero-nanostructure exhibited stronger SERS effects and better chemical stability compared with flower-like Ag nanostructure. The localized surface plasmon resonance (LSPR) spectra, field emission scanning electron microscope (FESEM) photos and Ag-Au ratios were studied which show that the surface morphology and shape of the flower-like Ag-Au hetero-nanostructure play significant roles in enhancing SERS. The flower-like 3D Ag-Au hetero-nanostructures fabricated by electrodeposition in liquid crystalline template and galvanic replacement reaction are simple, cheap, controllable and chemical stable. It is a good candidate for applications in SERS detection and imaging.

Thermal degradation mechanism of triangular Ag@SiO2 nanoparticles

Triangular silver nanoparticles are promising materials for light harvesting applications because of their strong plasmon bands; these absorption bands are highly tunable, and can be varied over the entire visible range based on the particle size. A general concern with these materials is that they are unstable at elevated temperatures. When thermally annealed, they suffer from changes to the particle morphology, which in turn affects their optical properties. Because of this stability issue, these materials cannot be used in applications requiring elevated temperatures. In order to address this problem, it is important to first understand the degradation mechanism. Here, we measure the changes in particle morphology, oxidation state, and coordination environment of Ag@SiO2 nanotriangles caused by thermal annealing. UV-vis spectroscopy and TEM reveal that upon annealing the Ag@SiO2 nanotriangles in air, the triangular cores are truncated and smaller nanoparticles are formed. Ag K-edge X-ray absorption spectroscopy (XANES and EXAFS) shows that the small particles consist of Ag(0), and that there is a decrease in the Ag-Ag coordination number with an increase in the annealing temperature. We hypothesize that upon annealing Ag in air, it is first oxidized to AgxO, after which it subsequently decomposes back to well-dispersed Ag(0) nanoparticles. In contrast, when the Ag@SiO2 nanotriangles are annealed in N2, since there is no possibility of oxidation, no small particles are formed. Instead, the triangular core rearranges to form a disc-like shape.

Molecular Tilting Alignment on Ag@C Nanocubes Monitored by Temperature-Dependent Surface Enhanced Raman Scattering

Core@shell Ag@C nanocubes (NCs) with a cubic silver core (~60 nm of side length) and a coating of ultrathin amorphous carbon (~4 nm) have been synthesized on a large scale by a one-pot hydrothermal method. The carbon layer not only protects the Ag@C nanocubes from oxidation under hydrothermal condition, but also stabilizes the structure of Ag cores. Considering that optical properties of nanostructured metals strongly depend on the temperature for SERS measurement, in this work we systemically investigate the relationship between the orientation of molecules adsorbed on Ag@C NCs and temperature by SERS spectra. Results suggest that the adsorbed 4-MBA molecules prefer a flat orientation on the NC surface with temperature decreasing. In addition, Ag@C NCs after one-year storage in water still maintain high SERS-active capability. Our synthesized Ag@C NCs with excellent and stable optical properties can be potentially applied in the field of sensor and ultrasensitive spectral analysis.

Biocompatible 3D SERS substrate for trace detection of amino acids and melamine

A novel, low-cost and biocompatible three-dimensional (3D) substrate for surface-enhanced Raman spectroscopy (SERS) is fabricated using gold nanoparticles (AuNPs) loaded on cellulose paper for detection of amino acids and melamine. Dysosma pleiantha rhizome (Dp-Rhi) capped AuNPs (Dp-Rhi_AuNPs) were prepared by in situ using aqueous extract of Dp-Rhi and in situ functionalized Dp-Rhi on AuNPs surface was verified by Fourier transform infrared spectroscopy and zeta potentials analysis shows a negative (-18.4mV) surface charges, which confirm that presence of Dp-Rhi on AuNPs. The biocompatibility of Dp-Rhi_AuNPs is also examined by cell viability of FaDu cells using MTS assay and compared to control group. In conclusion, the SERS performance of AuNPs@cellulose paper substrates were systematically demonstrated and examined with different excitation wavelengths (i.e. 532, 632.8 and 785nm lasers) and the as-prepared 3D substrates provided an enhancement factor approaching 7 orders of magnitude compared with conventional Raman intensity using para-nitrothiophenol (p-NTP), para-aminothiophenol (p-ATP) and para-mercaptobenzoic acid (p-MBA) as probe molecules. The strong electromagnetic effect was generated at the interface of AuNPs and pre-treated roughened cellulose paper is also investigated by simulation in which the formation of possible Raman hot-spot zone in fiber-like microstructure of cellulose paper decorated with AuNPs. Notably, with optimized condition of as-prepared 3D AuNPs@cellulose paper is highly sensitive in the SERS detection of aqueous tyrosine (10^-10M) and melamine (10^-9M).

Imaging Laser-Triggered Drug Release from Gold Nanocages with Transient Absorption Lifetime Microscopy

Nanoparticles have shown promise in loading and delivering drugs for targeted therapy. Many progresses have been made in the design, synthesis, and modification of nanoparticles to fulfill such goals. However, realizing targeted intracellular delivery and controlled release of drugs remains challenging, partly because of the lack of reliable tools to detect the drug-releasing process. In this paper, we applied femtosecond laser pulses to trigger the explosion of gold nanocages (AuNCs) and control the intracellular release of loaded aluminum phthalocyanine (AlPcS) molecules for photodynamic therapy (PDT). AuNCs were found to enhance the encapsulation efficiency and suppress the PDT effect of AlPcS molecules until they were released. More importantly, we discovered that the excited-state lifetimes of the AlPcS-AuNC conjugate (∼3 ps) and free AlPcS (∼11 ps) differ significantly, which was utilized to image the released drug molecules using transient absorption lifetime microscopy with the same laser source. This technique extracts information similar to fluorescence lifetime imaging microscopy but is superior in imaging the molecules that hardly fluoresce or are prone to photobleaching. We further combined a dual-phase lock-in detection technique to show the potential of real-time imaging based on the change in transient optical behaviors. Our method may provide a new tool for investigating nanoparticle-assisted drug delivery and release.

Galvanic replacement reaction: recent developments for engineering metal nanostructures towards catalytic applications

Recent developments to achieve further physicochemical control in metallic nanomaterials by galvanic replacement are discussed towards applications in catalysis.

Plasmonic and photo-electrochemical enhancements of the AuAg@Au/RGO–C3N4 nanocomposite for the detection of DA

Plasmonic photocatalyst has attracted significant attention due to its valuable theoretical study and promising practical applications in solar cells, functional composites, and sensors.

m-Cresol purple functionalized surface enhanced Raman scattering paper chips for highly sensitive detection of pH in the neutral pH range

m-Cresol purple functionalized SERS chips for sensitive detection of pH in the neutral pH range relying on the SERS to SERRS mechanism.

Engineering high refractive index sensitivity through the internal and external composition of bimetallic nanocrystals

High refractive index sensitivity (RIS) of branched Au-Pd nanocrystals (NCs) is engineered through lowering the dielectric dispersion at the NC resonant wavelength with internal or external atomic % Pd. To our knowledge, these NCs display the highest ensemble RIS measurement for colloids with LSPR maximum band positions ≤900 nm, and these results are corroborated with FDTD computations.

Use of Single-Layer g-C3N4/Ag Hybrids for Surface-Enhanced Raman Scattering (SERS)

Surface-enhanced Raman scattering (SERS) substrates with high activity and stability are desirable for SERS sensing. Here, we report a new single atomic layer graphitic-C3N4 (S-g-C3N4) and Ag nanoparticles (NPs) hybrid as high-performance SERS substrates. The SERS mechanism of the highly stable S-g-C3N4/Ag substrates was systematically investigated by a combination of experiments and theoretical calculations. From the results of XPS and Raman spectroscopies, it was found that there was a strong interaction between S-g-C3N4 and Ag NPs, which facilitates the uniform distribution of Ag NPs over the edges and surfaces of S-g-C3N4 nanosheets, and induces a charge transfer from S-g-C3N4 to the oxidizing agent through the silver surface, ultimately protecting Ag NPs from oxidation. Based on the theoretical calculations, we found that the net surface charge of the Ag atoms on the S-g-C3N4/Ag substrates was positive and the Ag NPs presented high dispersibility, suggesting that the Ag atoms on the S-g-C3N4/Ag substrates were not likely to be oxidized, thereby ensuring the high stability of the S-g-C3N4/Ag substrate. An understanding of the stability mechanism in this system can be helpful for developing other effective SERS substrates with long-term stability.

Direct Cross-Linking of Au/Ag Alloy Nanoparticles into Monolithic Aerogels for Application in Surface-Enhanced Raman Scattering

The direct cross-linking of Au/Ag alloy nanoparticles (NPs) into high surface area, mesoporous Au/Ag aerogels via chemical oxidation of the surface ligands is reported. The precursor alloy NPs with composition-tunable morphologies were produced by galvanic replacement of the preformed Ag hollow NPs. The effect of Au:Ag molar ratio on the NP morphology and surface plasmon resonance has been thoroughly investigated and resulted in smaller Au/Ag alloy NPs (4-8 nm), larger Au/Ag alloy hollow NPs (40-45 nm), and Au/Ag alloy hollow particles decorated with smaller Au NPs (2-5 nm). The oxidative removal of surfactant ligands, followed by supercritical drying, is utilized to construct large (centimeter to millimeter) self-supported Au/Ag alloy aerogels. The resultant assemblies exhibit high surface areas (67-73 m(2)/g), extremely low densities (0.051-0.055 g/cm(3)), and interconnected mesoporous (2-50 nm) networks, making them of great interest for a number of new technologies. The influence of mesoporous gel morphology on surface-enhanced Raman scattering (SERS) has been studied using Rhodamine 101 (Rd 101) as the probe molecule. The alloy aerogels exhibit SERS signal intensities that are 10-42 times higher than those achieved from the precursor Au/Ag alloy NPs. The Au/Ag alloy aerogel III exhibits SERS sensing capability down to 1 nM level. The increased signal intensities attained for alloy aerogels are attributed to highly porous gel morphology and enhanced surface roughness that can potentially generate a large number of plasmonic hot spots, creating efficient SERS substrates for future applications.

Ag@Au Concave Cuboctahedra: A Unique Probe for Monitoring Au-Catalyzed Reduction and Oxidation Reactions by Surface-Enhanced Raman Spectroscopy

We report a facile synthesis of Ag@Au concave cuboctahedra by titrating aqueous HAuCl4 into a suspension of Ag cuboctahedra in the presence of ascorbic acid (AA), NaOH, and poly(vinylpyrrolidone) (PVP) at room temperature. Initially, the Au atoms derived from the reduction of Au(3+) by AA are conformally deposited on the entire surface of a Ag cuboctahedron. Upon the formation of a complete Au shell, however, the subsequently formed Au atoms are preferentially deposited onto the Au{100} facets, resulting in the formation of a Ag@Au cuboctahedron with concave structures at the sites of {111} facets. The concave cuboctahedra embrace excellent SERS activity that is more than 70-fold stronger than that of the original Ag cuboctahedra at an excitation wavelength of 785 nm. The concave cuboctahedra also exhibit remarkable stability in the presence of an oxidant such as H2O2 because of the protection by a complete Au shell. These two unique attributes enable in situ SERS monitoring of the reduction of 4-nitrothiophenol (4-NTP) to 4-aminothiophenol (4-ATP) by NaBH4 through a 4,4'-dimercaptoazobenzene (trans-DMAB) intermediate and the subsequent oxidation of 4-ATP back to trans-DMAB upon the introduction of H2O2.

Label-free SERS study of galvanic replacement reaction on silver nanorod surface and its application to detect trace mercury ion

It is significant to explore a rapid and highly sensitive galvanic replacement reaction (GRR) surface enhanced Raman scattering (SERS) method for detection of trace mercury ions. This article was reported a new GRR SERS analytical platform for detecting Hg(II) with label-free molecular probe Victoria blue B (VBB). In HAc-NaCl-silver nanorod (AgNR) substrate, the molecular probe VBB exhibited a strong SERS peak at 1609 cm(-1). Upon addition of Hg(II), the GRR occurred between the AgNR and Hg(II), and formed a weak SERS activity of Hg2Cl2 that deposited on the AgNR surfaces to decrease the SERS intensity at I1609 cm(-1). The decreased SERS intensity was linear to Hg(II) concentration in the range of 1.25-125 nmol/L, with a detection limit of 0.2 nmol/L. The GRR was studied by SERS, transmission electron microscopy and other techniques, and the GRR mechanism was discussed.

Seed-Mediated Growth of Silver Nanocubes in Aqueous Solution with Tunable Size and Their Conversion to Au Nanocages with Efficient Photothermal Property

Two seed-mediated approaches for the growth of silver nanocubes in aqueous solution have been developed. Addition of a silver-seed solution to a mixture of cetyltrimethylammonium chloride (CTAC), silver trifluoroacetate, and ascorbic acid and heating the solution at 60 °C for 1.5 h produces uniform Ag nanocubes with tunable sizes from 23 to 60 nm by simply adjusting the volume of silver-seed solution introduced. Alternatively, the silver-seed solution can be injected into a mixture of cetyltrimethylammonium bromide (CTAB), silver nitrate, copper sulfate, and ascorbic acid and heated to 80 °C for 2 h to generate 46 nm silver nanocubes. Plate-like Ag nanocrystals exposing {111} surfaces can be synthesized by reducing Ag(NH3 )2 (+) with ascorbic acid in a CTAC solution. Relatively large Ag nanocubes were converted to cuboctahedral Au/Ag and Au nanocages and nanoframes with empty {111} faces through a galvanic replacement reaction. The nanocages showed a progressive plasmonic band red-shift with increasing Au content. The nanocages exhibited high and stable photothermal efficiency with solution temperatures quickly reaching beyond 100 °C when irradiated with an 808 nm laser for large heat and water vapor generation.

Synthesis of Small Au-Ag Core-Shell Cubes, Cuboctahedra, and Octahedra with Size Tunability and Their Optical and Photothermal Properties

Aqueous phase synthesis of small Au-Ag core-shell nanocubes, cuboctahedra, and octahedra is achieved through the deposition of Ag shells on small octahedral Au cores. These nanocrystals show efficient photothermal activity and can assemble into supercrystals.

Influencing Factor Investigation on Dynamic Hydrothermal Growth of Gapped Hollow BaTiO3 Nanospheres

Gapped hollow BaTiO3 nanospheres with an apparent diameter of 93 ± 19 nm (shell thickness of 10-20 nm) were synthesized via a dynamic hydrothermal process using TiO2 sols and Ba(2+) ions as the Ti and Ba sources in alkaline aqueous solutions. The phases and morphologies of the BaTiO3 samples were characterized by X-ray diffraction (XRD), SEM, TEM, and Raman spectra. The effects of the hydrothermal temperatures and durations, NaOH concentrations, and Ba/Ti ratios on the formation of gapped hollow BaTiO3 nanospheres were systematically investigated. The optimum conditions for forming gapped hollow BaTiO3 nanospheres are hydrothermal treatment at 180 °C for 10-20 h under a continuous magnetic stirring with NaOH concentrations of about 1 mol/L and molar Ba/Ti ratios of 1.2-1.5. The formation mechanism of the gapped hollow BaTiO3 nanospheres is understood as the combination of the orientated attachment and reversed crystal growth.

Seeding a New Kind of Garden: Synthesis of Architecturally Defined Multimetallic Nanostructures by Seed-Mediated Co-Reduction

Bimetallic nanoparticles display unique optical and catalytic properties that depend on crystallite size and shape, composition, and overall architecture. They may serve as multifunctional platforms as well. Unfortunately, many routes toward shape and architecturally controlled bimetallic nanocrystals yield polydisperse samples on account of the challenges associated with homogeneously nucleating a defined bimetallic phase by co-reduction methods. Developed by the Skrabalak laboratory, seed-mediated co-reduction (SMCR) involves the simultaneous co-reduction of two metal precursors to deposit metal onto shape-controlled metal nanocrystalline seeds. The central premise is that seeds will serve as preferential and structurally defined platforms for bimetallic deposition, where the shape of the seeds can be transferred to the shells. With Au-Pd as a model system, a set of design principles has been established for the bottom-up synthesis of shape-controlled bimetallic nanocrystals by SMCR. This strategy is successful at synthesizing symmetrically stellated Au-Pd nanocrystals with a variety of symmetries and core@shell Au@Au-Pd nanocrystals. Achieving nanocrystals with high morphological control via SMCR is governed by the following parameters: seed size, shape, and composition as well as the kinetics of seeded growth (through manipulation of synthetic parameters such as pH and metal precursor ratios). For example, larger seeds yield larger nanocrystals as does increasing the amount of metal deposited relative to the number of seeds. This increase in nanocrystal size leads to red-shifts in their localized surface plasmon resonance. Additionally, seed shape directs the overgrowth process during SMCR so the resultant nanocrystals adopt related symmetries. The ability to tune structure is important due to the size-, shape- and composition-dependent optical properties of bimetallic nanocrystals. Using this toolkit, the light scattering and absorption properties of Au-Pd octopods, 8-branched nanocrystals, could be tuned and were shown to be highly sensitive to changes in refractive index. The refractive index sensitivity displayed a linear correlation to the localized surface plasmon resonance initial position, where the sensitivity is greater than that of monometallic Au structures. Due to their bimetallic composition and unique architecture enabled by SMCR, Au-Pd octopods are promising refractive index based sensors. This Account summarizes the underlying principles for synthesis of bimetallic nanocrystals by SMCR, which have been established by systematic manipulation of synthetic parameters in a model Au-Pd system. These principles are anticipated to be general to other bimetallic systems, allowing for the design and synthesis of new nanocrystals with fascinating optical and catalytic properties.

Intense Raman scattering on hybrid Au/Ag nanoplatforms for the distinction of MMP-9-digested collagen type-I fiber detection

Well-ordered Au-nanorod arrays were fabricated using the focused ion beam method (denoted as fibAu_NR). Au or Ag nanoclusters (NCs) of various sizes and dimensions were then deposited on the fibAu_NR arrays using electron beam deposition to improve the surface-enhanced Raman scattering (SERS) effect, which was verified using a low concentration of crystal violet (10(-)(5)M) as the probe molecule. An enhancement factor of 6.92 × 10(8) was obtained for NCsfibAu_NR, which is attributed to the combination of intra-NC and NR localized surface plasmon resonance. When 4-aminobenzenethiol (4-ABT)-coated Au or Ag nanoparticles (NPs) were attached to NCsfibAu_NR, the small gaps between 4-ABT-coated NPs and intra-NCs allowed detection at the single-molecule level. Hotspots formed at the interfaces of NCs/NRs and NPs/NCs at a high density, producing a strong local electromagnetic effect. Raman spectra from as-prepared type I collagen (Col-I) and Ag-NP-coated Col-I fibers on NCsfibAu_NR were compared to determine the quantity of amino acids in their triple helix structure. Various concentrations of matrix-metalloproteinase-9-digested Col-I fibers on NCsfibAu_NR were qualitatively examined at a Raman laser wavelength of 785nm to determine the changes of amino acids in the Col-I fiber structure. The results can be used to monitor the growth of healing Col-I fibers in a micro-environment.

Au-Ag@Au Hollow Nanostructure with Enhanced Chemical Stability and Improved Photothermal Transduction Efficiency for Cancer Treatment

Despite the fact that Au-Ag hollow nanoparticles (HNPs) have gained much attention as ablation agents for photothermal therapy, the instability of the Ag element limits their applications. Herein, excess Au atoms were deposited on the surface of a Au-Ag HNP by improving the reduction power of l-ascorbic acid (AA) and thereby preventing the reaction between HAuCl4 and the Ag element in the Au-Ag alloy nanostructure. Significantly, the obtained Au-Ag@Au HNPs show excellent chemical stability in an oxidative environment, together with remarkable increase in extinction peak intensity and obvious narrowing in peak width. Moreover, finite-difference time-domain (FDTD) was used to simulate the optical properties and electric field distribution of HNPs. The calculated results show that the proportion of absorption cross section in total extinction cross section increases with the improvement of Au content in HNP. As predicted by the theoretical calculation results, Au-Ag@Au nanocages (NCs) exhibit a photothermal transduction efficiency (η) as high as 36.5% at 808 nm, which is higher than that of Au-Ag NCs (31.2%). Irradiated by 808 nm laser at power densities of 1 W/cm(2), MCF-7 breast cancer cells incubated with PEGylated Au-Ag@Au NCs were seriously destroyed. Combined together, Au-Ag@Au HNPs with enhanced chemical stability and improved photothermal transduction efficiency show superior competitiveness as photothermal agents.

Recent advances in noble metal based composite nanocatalysts: colloidal synthesis, properties, and catalytic applications

This Review article provides a report on progress in the synthesis, properties and catalytic applications of noble metal based composite nanomaterials. We begin with a brief discussion on the categories of various composite materials. We then present some important colloidal synthetic approaches to the composite nanostructures; here, major attention has been paid to bimetallic nanoparticles. We also introduce some important physiochemical properties that are beneficial from composite nanomaterials. Finally, we highlight the catalytic applications of such composite nanoparticles and conclude with remarks on prospective future directions.

Controlled synthesis of bimetallic Pd-Rh nanoframes and nanoboxes with high catalytic performances

Bimetallic nanoframes and nanoboxes of Pd-Rh are synthesized by selective removal of Pd cores from different Pd-Rh nanocubes prepared by a hydrothermal reaction of PdCl2, RhCl3 and HCHO. HCHO in the procedure alters the reaction kinetics and the growth behavior of Pd and Rh, resulting in different nanocubes that determine the following hollow nanostructures, nanoframes or nanoboxes. The catalytic properties of the hollow nanostructures are investigated using the oxidation of o-phenylenediamine (OPDA) to 2,3-diaminophenazine (DAP) as a model reaction. The resulting bimetallic nanoframes and nanoboxes show enhanced conversion efficiencies compared to their solid counterparts. This method offers a convenient way for mass production of bimetallic hollow nanomaterials.

Synthesis of hollow and trimetallic nanostructures by seed-mediated co-reduction

Seed-mediated co-reduction coupled with galvanic replacement is a new route to structurally defined trimetallic nanoparticles.

Kinetically controlled nucleation of silver on surfactant-free gold seeds

We report on the heterogeneous nucleation of Ag on Au seeds using a surfactant-free synthesis where nanoparticle aggregation is nullified through the immobilization of bare Au seeds on the surface of a substrate. Requiring only silver nitrate, ascorbic acid, and Au seeds, the synthesis is facile and, from a mechanistic standpoint, far less convoluted than conventional protocols. The results reveal that, even in the absence of surfactants, highly anisotropic growth modes are achieved which result in a lone Ag structure emanating from a single (100) Au facet. Consistent with surfactant-based protocols is the ability to vary the product of the reaction by varying the reaction rate. It allows for kinetic control which is able to direct the reaction toward either a bimetallic heterodimer or a core-shell configuration. The observed growth modes cannot be explained in terms of those proposed for surfactant-based growth modes where surfactants, surface diffusion, and/or collision patterns are used to rationalize the reaction product. We, instead, propose a growth mode reliant on the formation of a space charge region around each seed consisting of a double layer of ions, where the integrity of the layer is dependent upon the facets expressed by the seed, the rate at which the reduced ions are being deposited, and the pH of the solution. Our work reveals the rich nature of surfactant-free heteroepitaxial growth modes as well as the utility of the substrate-based platform in defining growth pathways.

Diffusion and seed shape: intertwined parameters in the synthesis of branched metal nanostructures

Branched nanocrystals display interesting optical and catalytic properties on account of their high surface areas and tips with small radii of curvatures. However, many synthetic routes toward branched nanocrystals result in inhomogeneous samples on account of asymmetric branching. Seed-mediated coreduction is a recently developed route to symmetrically branched nanocrystals where the symmetry of the seeds is transferred to the final stellated morphologies. Here, general guidelines to stellated nanocrystals are outlined by surveying coreduction of Au and Pd precursors in the presence of a variety of shape-controlled Au seeds to achieve Au/Pd nanostructures. Single-crystalline, twinned, and anisotropic seeds were analyzed to expand the classes of stellated nanostructures synthetically accessible. Significantly, single-crystalline Au seeds adopt {100}-terminated intermediates prior to branching, regardless of initial seed shape. We compared these results with those obtained with shape-controlled Pd seeds, and seed composition was identified as an important synthetic parameter, with Pd seeds being more resistant to shape changes during overgrowth. This difference is attributed to the greater diffusion rate of Au atoms on Au seeds compared to Au atoms on Pd seeds. These results provide guidelines for the seeded synthesis of symmetrically branched nanocrystals and architecturally defined bimetallic nanostructures in general.

Galvanic replacement-free deposition of Au on Ag for core-shell nanocubes with enhanced chemical stability and SERS activity

We report a robust synthesis of Ag@Au core-shell nanocubes by directly depositing Au atoms on the surfaces of Ag nanocubes as conformal, ultrathin shells. Our success relies on the introduction of a strong reducing agent to compete with and thereby block the galvanic replacement between Ag and HAuCl4. An ultrathin Au shell of 0.6 nm thick was able to protect the Ag in the core in an oxidative environment. Significantly, the core-shell nanocubes exhibited surface plasmonic properties essentially identical to those of the original Ag nanocubes, while the SERS activity showed a 5.4-fold further enhancement owing to an improvement in chemical enhancement. The combination of excellent SERS activity and chemical stability may enable a variety of new applications.