Ultrathin silver‐coated gold nanoparticles as suitable substrate for surface‐enhanced Raman scattering

Surface‐enhanced Raman scattering (SERS) is an extremely powerful tool for the analysis of the composition of bimetallic nanoparticle (BNP) surfaces because of the different adsorption schemes adopted by several molecules on different metals, such as Au and Ag. The preparation of BNPs normally implies a change in the plasmonic properties of the core metal. However, for technological applications it could be interesting to synthesize core–shell structures preserving these original plasmonic properties. In this work, we present a facile method for coating colloidal gold nanoparticles (NPs) in solution with a very thin shell of silver. The resulting bimetallic Au@Ag system maintains the optical properties of gold but shows the chemical surface affinity of silver. The effectiveness of the coating method, as well as the progressive silver enrichment of the outermost part of the Au NPs, has been monitored through the SERS spectra of several species (chloride, luteolin, thiophenol and lucigenin), which show different behaviors on gold and silver surfaces. A growth mechanism of the Ag shell is proposed on the basis of the spectroscopic and microscopic data consisting in the formation and deposit of Ag clusters on the Au NP surface. Copyright © 2009 John Wiley & Sons, Ltd.     

Highly Enhanced Emission of Visible Light from Core–Dual‐Shell‐Type Hybridized Nanoparticles

Core–dual‐shell‐type hybridized nanoparticles (NPs) having Au‐core/dye‐doped silica inner shell/Au outer shell are successfully fabricated by developing a biphasic process that is a kind of so‐called “one‐pot” method. The resulting hybridized NPs exhibit evidently about 20‐fold enhancement of fluorescence intensity, increase in fluorescence quantum yield, and decrease in fluorescence lifetime. These effects depend on the metal nanostructure being optimized, compared with the reference hybridized NPs with neither a Au‐core nor a Au outer shell, due to the gap‐mode effect induced by localized surface plasmon resonance in the core–dual‐shell‐type MIM‐like nanostructure. More detailed elucidation concerning the enhancement mechanism will provide the possibility of photonic device application, for example as a high‐performance point light source, nanolaser, or sensor for bioimaging in the visible region in the near future.     

SHINERS and plasmonic properties of Au Core SiO2 shell nanoparticles with optimal core size and shell thickness

Shell‐isolated nanoparticles (NPs)‐enhanced Raman spectroscopy (SHINERS) can be potentially applied to virtually any substrate type and morphology. How to take a step forward to prepare SHINERS NPs (SHINs) with superior performance is critical for the practical applications of surface‐enhanced Raman scattering (SERS) in the breadth and depth. Here, we present a method to obtain 120 nm diameter gold NPs coated with ultrathin silica shells (1–4 nm). The silica shell can be controlled growth through carefully tuning a series of parameters, such as amount of 3‐aminopropyl triethoxysilane used, pH, reaction time, and reaction temperature. We compare the enhancement factor of the obtained 120 nm Au with a 4 nm silica shell NPs to the 55 nm Au with a 4 nm silica shell NPs, and the activity of a 120 nm SHINs is nearly 24 times that the 55 nm SHIN from a single particle view. We also compare the enhancement factor of 1 nm silica shell Au@SiO₂ NPs with the bare Au NPs. The enhancement factor of 1 nm silica shell Au@SiO₂ NPs was found to be about twice that of the bare particles. For a deeper understanding of the source of the giant enhanced electrical field of the 1 nm silica shell Au@SiO₂ NPs, we study the plasmonic property of single 1 nm silica shell Au@SiO₂ NP on a gold film substrate through correlation of the structure of single NP using SEM with its SPR spectroscopy. We find that the multipolar interaction between the single Au@SiO₂ NP and gold film substrate is important for the SERS. Our studies on the performance of 120 nm SHINs and the plasmonic property of these particles can significantly expand the applications of SHINERS technique and improve the understanding of physical nature of SHINs. Copyright © 2013 John Wiley & Sons, Ltd.     

High–Yield Production of Uniform Gold Nanoparticles with Sizes from 31 to 577 nm via One‐Pot Seeded Growth and Size‐Dependent SERS Property

In this work, uniform, quasi‐spherical gold nanoparticles (Au NPs) with sizes of 31–577 nm are prepared via one‐pot seeded growth with the aid of tris‐base (TB). Distinct from the seeded growth methods available in literature, the present method can be simply implemented by subsequently adding the aqueous dispersion of the 17 nm Au‐NP seeds and the aqueous solution of HAuCl₄ into the boiling aqueous TB solution. It is found that at the optimal pH range, the sizes of the final Au NPs and their concentrations are simply controlled by either the particle number of the Au seed dispersion or the concentration of the HAuCl₄ solution, while the latter enables us to produce large Au NPs at very high concentration. Moreover, as‐prepared Au NPs of various sizes are coated on glass substrates to test their surface‐enhanced Raman scattering (SERS) activities by using 4‐aminothiophenol (4‐ATP) molecules as probes, which exhibit “volcano type” dependence on the Au NP sizes at fixed excitation wavelength. Furthermore, the Au NPs with sizes of ≈97 and 408 nm exhibit the largest SERS enhancement at the excitation wavelength of 633 and 785 nm, respectively.     

Surface‐enhanced Raman spectroscopy in living plant using triplex Au?Ag?C core–shell nanoparticles

This paper presents, for the first time, noninvasive imaging of a livingplant using biocompatible carbon‐encapsulated Au Ag nanoparticles (NPs) using micro‐Raman spectroscopy (MRS). A convenient and controllable hydrothermal synthetic route was developed to synthesize the layer‐by‐layer triplex Au Ag C core–shell NPs, which can incorporate the reporter molecule 4‐mercapto benzoic acid (4‐MBA). A unique approach was devised to deliver the carbon‐encapsulated surface‐enhanced Raman scattering (SERS) tags into the leaf of Nicotiana benthamiana. In vivo SERS mapping was subsequently performed to monitor the distribution of tags inside the leaf, which successfully avoided interference of autofluorescence from plant tissue. The imaging modality reported here and further the bio‐functionalized carbon‐encapsulated SERS NPshold significant potential as a strategy forbiochemical imaging in living plantsin a noninvasive and nontoxic manner, whichmight open up exciting opportunities for plant sciences. Copyright © 2010 John Wiley & Sons, Ltd.     

Au@SiO2 core/shell nanoparticle assemblage used for highly sensitive SERS‐based determination of glucose and uric acid

The use of Au@SiO₂ core/shell nanoparticle (NP) assemblage with highly sensitive surface‐enhanced Raman scattering (SERS) was investigated for the determination of glucose and uric acid in this study. Rhodamine 6G dye molecules were used to evaluate the SERS enhancement factor for the synthesized Au@SiO₂ core/shell NPs with various silica shell thicknesses. The enhancement of SERS signal from Rhodamine 6G was found to increase with a decrease in the shell thickness. The core/shell assemblage with silica layer of 1–2 nm over a Au NP of ~36 nm showed the highest SERS signal. Our results show that the SERS technique is able to detect glucose and uric acid within wide concentration ranges, i.e. 20 ng/dL to 20 mg/dL (10⁻¹²–10⁻³ M) and 16.8 ng/dL to 2.9 mg/dL (10⁻¹¹–1.72 × 10⁻⁴ M), respectively, with associated lower detection limits of ~20 ng/dL (~1.0 × 10⁻¹² M) and ~16.8 ng/dL (~1.0 × 10⁻¹¹ M). Our work offers a low‐cost route to the fabrication of agile sensing devices applicable to the monitoring of disease progression. Copyright © 2013 John Wiley & Sons, Ltd.     

Design of Enhanced Catalysts by Coupling of Noble Metals (Au,Ag) with Semiconductor SnO2 for Catalytic Reduction of 4‐Nitrophenol

The reduction of 4‐nitrophenol (Nip) into 4‐aminophenol (Amp) by NaBH₄, which is catalyzed by both binary and ternary yolk–shell noble‐metal/SnO₂ heterostructures, is reported. The binary heterostructures contain individual Au or Ag nanoparticles (NPs) and the ternary heterostructures contain both Au and Ag NPs. The Au@SnO₂ yolk–shell NPs are synthesized via a silica seeds‐mediated hydrothermal method. Subsequently, the Au@SnO₂@Ag and Au@SnO₂@Au yolk–shell–shell (YSS) NPs are synthesized, whereby SnO₂ is located between the Au and Ag NPs. The morphology, composition, and optical properties of the as‐prepared samples are analyzed. For the binary heterostructures, the rate of the reduction reaction increases with decreasing particle size. The catalytic results demonstrate the synergistic effect of Au and Ag in the ternary metal–semiconductor heterostructures, which is beneficial to the catalytic reduction of Nip into Amp. Both the binary and ternary heterostructures exhibit significantly better catalytic performances than the corresponding bare Au and Ag NPs. It is envisaged that the current synthesized strategy will promote further interest in the field of bimetal NP‐based catalysis.     

Temperature–Light Dual‐Responsive Au@PNIPAm Core‐Shell Microgel‐Based Optical Devices

Au nanoparticle (AuNP) core particles coated with a poly(N‐isopropylacrylamide) (pNIPAm) shell (Au@pNIPAm) are synthesized by seed mediated free radical polymerization. Subsequently, a temperature–light‐responsive photonic device is fabricated by sandwiching the Au@pNIPAm particles between two thin layers of Au. The optical device exhibits visual color and characteristic multipeak reflectance spectra, where peak position is primarily determined by the distance between two Au layers. Dual responsivities of the photonic device are achieved by combining the photothermal effect of AuNPs core (localized surface plasmon resonance (LSPR) effect) and the temperature responsivity of the pNIPAm shell. That is, the pNIPAm shell collapses as the temperature is increased above pNIPAm's lower critical solution temperature, either by direct heat input or heat generated by AuNPs' LSPR effect. To investigate the effect of AuNPs distribution in the microgels on the devices' photothermal responsivity, the Au@pNIPAm microgel‐based etalon devices are compared with that fabricated by AuNP‐doped pNIPAm‐based microgels; in terms of response kinetics and optical spectrum homogeneity. The uniform Au@pNIPAm microgel‐based devices show a fast response and exhibit a comparatively homogeneous spectrum over the whole slide. These materials can potentially find use in drug delivery systems, active optics, and soft robotics.     

Individual SERS substrate with core–satellite structure decorated in shrinkable hydrogel template for pesticide detection

Individual Au@PNIPAM/Ag composite has been designed and fabricated as surface‐enhanced Raman scattering (SERS) substrate in this paper. Because of the high porosity of the polymer shell and the driving force of the Au core to Ag⁺(H₂O)_n (n = 1–4) in aqueous solution, chemical reactions can be carried out while aggregation is completely avoided. Also, this makes the formation of vast and monodisperse Ag nanoparticles within PNIPAM and increases the colloidal stability. The Au cores with different sizes and the vast Ag nanoparticles then form core–satellite structures that can generate plasmon resonance. Moreover, this kind of individual Au@PNIPAM/Ag composite can be seen directly through Raman optical microscope, and uncertain effects on SERS signals resulting from variability of the configurations are minimized because these individual composite particles are relatively uniform. Importantly, the gaps between the Au and Ag nanoparticles can decrease because the PNIPAM shrinks from swollen to collapse state, so the substrate can also be used for inspecting pesticide residues accurately and rapidly. Copyright © 2014 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering studies of carbon nanotubes using Ag‐core Au‐shell nanoparticles

Surface‐enhanced Raman scattering from carbon nanotube bundles adsorbed with plasmon‐tunable Ag‐core Au‐shell nanoparticles (Ag@Au nps) was carried out for the first time. By utilizing nanoparticles whose plasmon resonance peak (541, 642 nm) closely matches the commonly used Raman excitation sources (532, 632.81 nm), we can observe a large enhancement in the Raman signatures of carbon nanotubes. We obtain greater enhancement in the Raman signal for the above case when compared to nanotubes adsorbed with conventional Ag, Au or other ‘off resonant’ Ag@Au nps. The power‐dependent SERS experiment on single‐walled nanotubes (SWNTs) with resonant Ag@Au nps reveals a linear behavior between the G‐band intensity and the photon flux density, which is in agreement with the vibrational pumping model of SERS. The observed enhancement by resonance matching is pronounced for carbon nanotubes and may lead to insights into understanding nanotube–nanoparticle interaction. Copyright © 2009 John Wiley & Sons, Ltd.     

High performance Au/Ag core/shell bipyramids for determination of thiram based on surface‐enhanced Raman scattering

Au/Ag core/shell bipyramids were used as surface‐enhanced Raman scattering (SERS) substrates to determine the thiram. The metallic substrates showed high SERS performance and are very suitable for the analytical sensors. The fabrication and characterization of the Au/Ag core/shell bipyramids were described. The influence of experimental parameters, such as the thickness of Ag shell of the bipyramids, sodium chloride concentration, and pH value on SERS of thiram was examined and optimized. Under the optimum conditions, thiram molecules were effectively adsorbed onto bipyramids and the SERS intensity is proportional to the concentration of thiram in the range of 3.3 to 400.0 ng mL^–1. The corresponding correlation coefficient of the linear equation is 0.997, which indicates that there is a good linear relationship between SERS intensity and thiram concentration. The limit of detection for thiram is 2.0 ng mL^–1. The experimental results indicate that the proposed method is a viable method for determination of thiram. Some environmental water samples were analyzed and the analytical results were satisfactory. Copyright © 2012 John Wiley & Sons, Ltd.     

Preparation of monodisperse bimetallic nanorods with gold nanorod core and silver shell and their plasmonic property and SERS efficiency

In this study, monodisperse bimetallic nanorods with gold (Au) nanorod core and silver (Ag) shell (Au@AgNRs) were synthesized through seed‐mediated growth process by reduction of AgNO₃ using Au nanorods with narrow size and shape distribution as seeds. With increasing the used amount of AgNO₃, the Ag shell thickness of their lateral facets is raised faster than that of their two tips, leading to a decrease of their aspect ratios. Four plasmon bands are observable on the extinction spectra of Au@AgNRs, which are attributed to the longitudinal dipolar plasmon mode, transverse dipolar plasmon mode, and octupolar plasmon mode of the core‐shell structured bimetallic nanorods, respectively. As their Ag shell thickness increases, their longitudinal plasmon band blue‐shifts notably with the transverse plasmon band blue‐shifting and the two octupolar plasmon bands red‐shifting slightly, due to the decrease of their aspect ratios and enhancement of Ag plasmon resonance contribution. When used as surface‐enhanced Raman scattering (SERS) substrate for probing minute amounts of 4‐mercaptobenzoic acid in aqueous solution, Au@AgNRs have much stronger SERS activity than Au nanorods, and the obtained Raman signals are highly reproducible arising from their excellent monodispersity. Their SERS activity is remarkably increased with their Ag shell thickness thanks to the enhancing surface electric field and the chemical enhancement associated with electronic ligand effect. Copyright © 2014 John Wiley & Sons, Ltd.     

Charge‐dependent adsorption of rhodamine 6G on gold nanoparticle surfaces: fluorescence and Raman study

We study the adsorption behaviors of rhodamine dyes on gold nanoparticles (Au NPs) depending on their surface charges. Rhodamine 6G (Rh6G) dye is tested comparatively for positively and negatively charged Au NPs prepared by the reduction of chitosan and citric acid, respectively. The adsorption of Rh6G is found to be weaker on the positively charged Au NPs, whereas more substantial aggregation is found on negatively charged Au NPs. An increase in the concentration of Au NPs enhances the surface‐enhanced Raman scattering (SERS) intensities only for the Au(−) NPs, whereas the Au(+) NPs do not exhibit any strong SERS signals. Our findings suggest that SERS and reciprocal fluorescence measurements of Rh6G can be used to estimate the surface charges and atomic percentages of Au NPs less than ∼5 ppm. Copyright © 2011 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering spectroscopy via gold nanostars

Anisotropic metallic nanoparticles (NPs) have unique optical properties, which lend them to applications such as surface‐enhanced Raman scattering (SERS) spectroscopy. Star‐shaped gold (Au) NPs were prepared in aqueous solutions by the seed‐mediated growth method and tested for Raman enhancement using 2‐mercaptopyridine (2‐MPy) and crystal violet (CV) probing molecules. For both molecules, the SERS activity of the nanostars was notably stronger than that of the spherical Au NPs of similar size. The Raman enhancement factors (EFs) for 2‐MPy on Au nanostars and nanorods are comparable and estimated as greater than 5 orders of magnitude. However, the enhancement for CV on nanostars was significantly higher than for nanorods, in particular at CV concentrations of 100 nM or lower. This article is a US Government work and is in the public domain in the USA. Published in 2008 by John Wiley & Sons, Ltd.     

A Sensitive “Optical Nose” for Detection of Volatile Organic Molecules Based on Au@MOFs Nanoparticle Arrays through Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) is an effective technique for detecting toxic gas and volatile organic molecules (VOMs); however, recent SERS-based gas sensors have disadvantages and lack an effective approach to capture toxic gas and insufficient reproducibility of SERS substrates. Herein, a facile strategy is developed to integrate metal-organic frameworks with Au nanoparticle (NP) arrays to form Au@ZIF-8 NP arrays, which can be used as an “optical nose” based on SERS to detect toxic VOMs with good reproducibility and sensitivity. Toluene as a target molecule is recognized at ppm levels by the Au@ZIF-8 NP arrays in situ. And the analytical enhancement factor of Au@ZIF-8 NP arrays for toluene is about 1.2 × 10⁵. Importantly, this SERS substrate can also detect the 1-butanol molecule, which provides an idea for designing a universal VOM sensor. In addition, the coating method of the ZIF-8 shell can be extended to synthetize various NPs@ZIF-8 core–shell composites, such as Au nanospheres@ZIF-8, Au@Ag nanorods@ZIF-8, PS microspheres@ZIF-8, and Fe₂O₃ microellipsoids@ZIF-8 composites.     

Improved surface‐enhanced Raman scattering properties of TiO2 nanoparticles by Zn dopant

In this paper, pure and Zn‐doped TiO₂ nanoparticles (NPs) with various content of Zn were prepared by a sol–hydrothermal method and were employed as active substrates for surface‐enhanced Raman scattering (SERS). On the 3% Zn‐doped TiO₂ substrate, 4‐mercaptobenzoic acid(4‐MBA) molecules exhibit a higher SERS intensity by a factor of 6, as compared with the native enhancement of 4‐MBA adsorbed on undoped TiO₂ NPs. Moreover, the higher SERS activity was still observed on the 3% Zn‐doped TiO₂ NPs at temperature even up to 125 °C. These results indicate that an appropriate amount of Zn doping can improve the SERS performances of TiO₂ SERS‐active substrates. The introduction of Zn dopant can enrich the surface states (defects) of TiO₂ and improve the separation efficiency of photo‐generated charge carriers (electrons and holes) in TiO₂, according to measurements of X‐ray diffraction, UV‐visible diffuse reflectance spectroscopy, and photoluminescence, which are responsible for the influence of Zn dopant on the improved SERS performances of TiO₂ NPs. Copyright © 2009 John Wiley & Sons, Ltd.     

Green synthesis and characterization of Au@Pt core–shell bimetallic nanoparticles using gallic acid

In this study, we developed a facile and benign green synthesis approach for the successful fabrication of well-dispersed urchin-like Au@Pt core–shell nanoparticles (NPs) using gallic acid (GA) as both a reducing and protecting agent. The proposed one-step synthesis exploits the differences in the reduction potentials of AuCl₄⁻ and PtCl₆²⁻, where the AuCl₄⁻ ions are preferentially reduced to Au cores and the PtCl₆²⁻ ions are then deposited continuously onto the Au core surface as a Pt shell. The as-prepared Au@Pt NPs were characterized by transmission electron microscope (TEM); high-resolution transmission electron microscope (HR-TEM); scanning electron microscope (SEM); UV-vis absorption spectra (UV-vis); X-ray diffraction (XRD); Fourier transmission infrared spectra (FT-IR). We systematically investigated the effects of some experimental parameters on the formation of the Au@Pt NPs, i.e., the reaction temperature, the molar ratios of HAuCl₄/H₂PtCl₆, and the amount of GA. When polyvinylpyrrolidone K-30 (PVP) was used as a protecting agent, the Au@Pt core–shell NPs obtained using this green synthesis method were better dispersed and smaller in size. The as-prepared Au@Pt NPs exhibited better catalytic activity in the reaction where NaBH₄ reduced p-nitrophenol to p-aminophenol. However, the results showed that the Au@Pt bimetallic NPs had a lower catalytic activity than the pure Au NPs obtained by the same method, which confirmed the formation of Au@Pt core–shell nanostructures because the active sites on the surfaces of the Au NPs were covered with a Pt shell.     

Optimization of extinction efficiency of gold‐coated polystyrene bead substrates improves surface‐enhanced Raman scattering effects by post‐growth microwave heating treatment

We report a novel post‐growth microwave treatment approach to selectively modify the surface morphologies of gold (Au) films coated on the polystyrene (PS) bead substrates for effectively improving the surface‐enhanced Raman scattering (SERS) effect on the analytes. The discrete dipole approximation (DDA) model was introduced to evaluate the enhancement effects by calculating the localized electromagnetic field distribution and extinction efficiency based on the sizes of the trenches and voids, and the surface roughness of the modified Au–PS bead substrates. The SERS performance of microwave‐modified Au–PS substrates on rhodamine 6G (Rh 6G) and saliva yields at least 10‐fold improvements in SERS intensities compared to the as‐grown substrates, which is also in agreement with theoretical predictions by DDA modeling. This work demonstrates both experimentally and theoretically the efficacy of the microwave heating treatment on modifying the Au–PS bead substrates for the realization of high SERS performance in biomedical applications. Copyright © 2009 John Wiley & Sons, Ltd.     

Identification of two Ag‐2,2′:6′,2″‐terpyridine surface species on Ag nanoparticle surfaces by excitation wavelength dependence of SERS spectra and factor analysis: evidence for chemical mechanism contribution to SERS of Ag(0)–tpy

Measurement and interpretation of the excitation wavelength dependence of surface‐enhanced Raman scattering (SERS) spectra of molecules chemisorbed on plasmonic, e.g. Ag nanoparticle (NP) surfaces, are of principal importance for revealing the charge transfer (CT) mechanism contribution to the overall SERS enhancement. SERS spectra, their excitation wavelength dependence in the 445–780‐nm range and factor analysis (FA) were used for the identification of two Ag‐2,2′:6′,2″‐terpyridine (tpy) surface species, denoted Ag⁺–tpy and Ag(0)–tpy, on Ag NPs in systems with unmodified and/or purposefully modified Ag NPs originating from hydroxylamine hydrochloride‐reduced hydrosols. Ag⁺–tpy is a spectral analogue of [Ag(tpy)]⁺ complex cation, and its SERS shows virtually no excitation wavelength dependence. By contrast, SERS of Ag(0)–tpy surface complex generated upon chloride‐induced compact aggregate formation and/or in strongly reducing ambient shows a pronounced excitation wavelength dependence attributed to a CT resonance (the chemical mechanism) contribution to the overall SERS enhancement. Both the resonance (λ_exc = 532 nm) and off‐resonance (λ_exc = 780 nm) pure‐component spectra of Ag(0)–tpy obtained by FA are largely similar to surface‐enhanced resonance Raman scattering (λ_exc = 532 nm in resonance with singlet metal to ligand CT (¹ MLCT) transition) and SERS (λ_exc = 780 nm) spectra of [Fe(tpy)₂]²⁺ complex dication. Copyright © 2014 John Wiley & Sons, Ltd.     

Analysis of self‐repair mechanisms of Phaseolus vulgaris var. saxa using near‐infrared surface‐enhanced Raman spectroscopy

We used surface‐enhanced Raman spectroscopy (SERS) to investigate ultrastructural changes in cell‐wall composition during the self‐repair of lacerated hypocotyls of Phaseolus vulgaris var. saxa. A detailed study of self‐repair mechanisms requires localized information about cell‐wall structure and morphology in addition to the chemical cell‐wall composition. Characteristic Raman and SER spectra yielded two‐dimensional maps of cross sections of P. vulgaris var. saxa visualizing chemical compositions in the walls of different cell types and during various repair phases. SERS substrate particles were produced by the reduction of gold chloride on the plant tissue surface and characterized with absorption spectroscopy, scanning electron microscopy and energy‐dispersive X‐ray spectroscopy. The SERS results were compared with stained cross sections of the same plant using dark‐field microscopy with focus on lignin and suberin contents in repairing cells. In addition, SERS measurements revealed Au cyanide compounds on the cell surface, indicating the formation of hydrogen cyanide during the self‐repair phase. Copyright © 2009 John Wiley & Sons, Ltd.