Self‐Assembled Au Nanoparticles as Substrates for Surface‐Enhanced Vibrational Spectroscopy: Optimization and Electrochemical Stability

Three‐dimensional nanostructured metallic substrates for enhanced vibrational spectroscopy are fabricated by self‐assembly. Nanostructures consisting of one to 20 depositions of 13 nm‐diameter Au nanoparticles (NPs) on Au films are prepared and characterized by means of AFM and UV/Vis reflection–absorption spectroscopy. Surface‐enhanced polarization modulation infrared reflection–absorption spectroscopy (PM‐IRRAS) is observed from Au NPs modified by the probe molecule 4‐hydroxythiophenol. The limitation of this kind of substrate for surface‐enhanced PM‐IRRAS is discussed. The surface‐enhanced Raman scattering (SERS) from the same probe molecule is also observed and the effect of the number of Au‐NP depositions on the SERS efficiency is studied. The SERS signal from the probe molecule maximizes after 11 Au‐NP depositions, and the absolute SERS intensities from different batches are reproducible within 20 %. In situ electrochemical SERS measurements show that these substrates are stable within the potential window between ?800 and +200 mV (vs. Ag/AgCl/sat. Cl^?).     

Gas-Phase Deposited Plasmonic Nanoparticles Supported on 3D-Graphene/Nickel Foam for Highly SERS Detection

In this work, we proposed a novel three-dimensional (3D) plasmonic nanostructure based on porous graphene/nickel foam (GNF) and gas-phase deposited Ag nanoparticles (NPs).Ag NPs with high density were directly deposited on the surface of 3D GNF by performing a novel cluster beam deposition approach. In comparison with traditional Ag substrate(SiO₂/Ag), such hot-spots enriched 3D nanostructure showed extremely high electromag-netic field enhancement under incident light irradiation which could be used as a sensitive chemical sensor based on surface enhanced Raman scattering (SERS). The experimental results demonstrated that the proposed nanostructure showed superior SERS performance in terms of Raman signal reproducibility and sensitivity for the probe molecules. 3D full-wave simulation showed that the enhanced SERS performance in this 3D hierarchical plasmonic nanostructure was mainly obtained from the hot-spots between Ag NPs and the near-field coupling between Ag NPs and GNF sca olds. This work can provide a novel assembled SERS substrate as a SERS-based chemical sensor in practical applications.     

内部具有可移动金核的中空银纳米颗粒的制备、表征及其表面增强拉曼效应

采用振荡法和种子生长技术制备出核壳结构的Au@SiO2纳米颗粒及夹层结构的Au@SiO2@Ag纳米颗粒, 用HF将Au@SiO2@Ag NPs夹层的SiO2溶解, 得到内部带有粒径为30 nm的可移动金核、壳层厚度约为30 nm的中空银纳米颗粒(Au@air@Ag NPs). 用扫描电子显微镜和透射电子显微镜对所得到的纳米微球的形貌进行了表征, 并以罗丹明B为探针分子研究了Au@air@Ag 纳米颗粒的表面增强拉曼(SERS)效应, 发现Au@air@Ag 纳米颗粒是一种可应用于SERS的理想材料.     

小尺寸金石墨纳米颗粒的合成与表征

在表面增强拉曼光谱(SERS)的研究领域中,基于局域表面等离子体共振效应的等离子体SERS基底的制备成为过去几十年的研究热点。然而,通常开发的等离子体金属基底具有较差的稳定性和重现性。对于SERS而言,石墨烯类材料具有拉曼化学增强效应,除此之外,还具有分子富集、强的稳定性与荧光猝灭能力等优点,因此基于石墨金属复合纳米材料的SERS基底受到了研究人员的重视。我们利用化学气相沉积(CVD)法制备了小尺寸的金石墨核壳纳米颗粒(Au@G),其粒径约为17 nm。我们通过在Au NP上包覆介孔二氧化硅来控制Au@G的尺寸,同时还研究了包覆二氧化硅过程中,正硅酸乙酯(TEOS)的浓度对于石墨壳层形成的影响。结果表明当TEOS在一定浓度范围内,其浓度的降低有利于得到石墨化程度高的Au@G。进一步利用Au@G对结晶紫分子进行拉曼检测,也表明了Au@G具有较好的拉曼增强效果。这种小尺寸的Au@G在分子检测与细胞成像分析领域中具有广泛的应用潜力。     

Silver overlayer-modified surface-enhanced Raman scattering-active gold substrates for potential applications in trace detection of biochemical species

Because Ag and Au nanoparticles (NPs) possess well-defined localized surface plasmon resonance (LSPR) they are popularly employed in the studies of surface-enhanced Raman scattering (SERS). As shown in the literature and in our previous studies, the advantage of SERS-active Ag NPs is their higher SERS enhancement over Au NPs. On the other hand, the disadvantage of SERS-active Ag NPs compared to Au NPs is their serious decay of SERS enhancement in ambient laboratory air. In this work, we develop a new strategy for preparing highly SERS-active Ag NPs deposited on a roughened Au substrate. This strategy is derived from the modification of electrochemical underpotential deposition (UPD) of metals. The coverage of Ag NPs on the roughened Au substrate can be as high as 0.95. Experimental results indicate that the SERS of Rhodamine 6G (R6G) observed on this developed substrate exhibits a higher intensity by ca. 50-fold of magnitude, as compared with that of R6G observed on the substrate without the deposition of Ag NPs. The limit of detection (LOD) for R6G measured on this substrate is markedly reduced to 2 × 10⁻¹⁵ M. Moreover, aging of SERS effect observed on this developed substrate is significantly depressed, as compared with that observed on a generally prepared SERS-active Ag substrate. These aging tests were performed in an atmosphere of 50% relative humidity (RH) and 20% (v/v) O₂ at 30 °C for 60 day. Also, the developed SERS-active substrate enables it practically applicable in the trace detection of monosodium urate (MSU)-containing solution in gouty arthritis without a further purification process.     

银/金纳米线阵列表面增强拉曼基底的制备及对孔雀石绿的高灵敏度检测

利用种子介导的软模板生长方法制备了金纳米线(Au NWs)阵列, 通过调节生长温度控制Au NWs阵列的形貌, 最后在经硼氢化钠(NaBH₄)清洗过的Au NWs阵列上化学沉积银纳米颗粒(Ag NPs), 制得银/金纳米线(Ag/Au NWs)阵列作为表面增强拉曼散射(SERS)基底. 选用罗丹明6G(R6G)作为拉曼探针分子测定了Ag/Au NWs阵列的SERS性能. 结果表明, Ag/Au NWs阵列作为SERS基底具有高灵敏度、 优异的信号均匀性和良好的稳定性. 使用Ag/Au NWs阵列对孔雀石绿(MG)检测的检出限可低至1×10^-8 mol/L, 线性范围为 1×10^-8~1×10^-4 mol/L. NaBH₄可以在不影响SERS性能的情况下去除Ag/Au NWs阵列上吸附的分子, 使得 SERS基底可以重复使用. 使用Ag/Au NWs阵列对湖水中的MG进行检测, 得到了可靠的回收率, 证明Ag/Au NWs 阵列在检测环境水体中的孔雀石绿上具有应用潜力.     

Au/TiO<Subscript>2</Subscript>/Graphene Composite with Enhanced Photocatalytic Activity Under Both UV and Visible Light Irradiation

Au/TiO₂/graphene composite was synthesized by the combination of electrostatic attraction and photo-reduction method. In the composite, graphene sheets act as an adsorption site for dye molecules to provide a high concentration of dye near to the TiO₂ and Au nanoparticles (NPs), and work as an excellent electron transporter to separate photoinduced e ^?/h ⁺ pairs. Under UV irradiation, photogenerated electrons of TiO₂ are transferred effectively to Au NPs and graphene sheets, respectively, retarding the recombination of electron–hole pairs. Under visible light irradiation, the Au NPs are photo-excited due to the surface plasmon resonance effect, and charge separation is accomplished by the interfacial electron injection from the Au NPs to the conduction band of TiO₂ and then transfer further to graphene sheets. As a result, compared with pure TiO₂, Au/TiO₂/graphene composite exhibited much higher photocatalytic activity for degradation of methylene blue under both UV and visible light irradiation, based on the synergistic effect of Au, graphene in contact with TiO₂, allowing response to the visible light, effective separation of photoinduced charges, and better adsorption of the dye molecules.     

Focused-ion-beam-fabricated Au nanorods coupled with Ag nanoparticles used as surface-enhanced Raman scattering-active substrate for analyzing trace melamine constituents in solution

A well-ordered Au-nanorod array with a controlled tip ring diameter (Au_NRs_d) was fabricated using the focused ion beam method. Au_NRs_d was then coupled with Ag nanoparticles (Ag NPs) to bridge the gaps among Au nanorods. The effect of surface-enhanced Raman scattering (SERS) on Au_NRs_d and Ag NPs/Au_NRs_d was particularly verified using crystal violet (CV) as the molecular probe. Raman intensity obtained from a characteristic peak of CV on Au_NRs_d was estimated by an enhancement factor of ≈10⁷ in magnitude, which increased ≈10¹² in magnitude for that on Ag NPs/Au_NRs_d. A highly SERS-active Ag NPs/Au_NRs_d was furthermore applied for the detection of melamine (MEL) at very low concentrations. Raman-active peaks of MEL (10⁻³ to 10⁻¹² M) in water or milk solution upon Au_NRs_d or Ag NPs/Au_NRs_d were well distinguished. The peaks at 680 and 702 cm⁻¹ for MEL molecules were found suitable to be used as the index for sensing low-concentration MEL in a varied solution, while that at 1051 cm⁻¹ was practical to interpret MEL molecules in water or milk solution bonded with Au (i.e., Au_NRs_d) or Ag (i.e., Ag NPs/Au_NRs_d) surface. At the interface of Ag NPs/Au_NRs_d and MEL molecules in milk solution, a laser-induced electromagnetic field or hotspot effect was produced and competent to sense low-concentration MEL molecules interacting with Ag and Au surfaces. Accordingly, Ag NPs/Au_NRs_d is very promising to be used as a fast and sensitive tool for screening MEL in complex matrices such as adulteration in e.g., food and pharmaceutical products.     

Fabrication of small-sized silver NPs/graphene sheets for high-quality surface-enhanced Raman scattering

In this paper, small-sized and highly dispersed Ag nanoparticles (NPs) supported on graphene nanosheets are fabricated via a strategy for etching a copper template with Ag(+). Firstly, big-sized Cu NPs are supported on graphene, and then the small-sized and highly dispersed Ag NPs are supported on graphene by replacement reaction, mainly making use of graphene passing electrons between Cu and Ag(+). The graphene used in the experiment is prepared by in situ self-generating template and has good dispersion, excellent crystallinity and little defects. Thus, in the process of Ag/graphene synthesis, there is no any intervention of surfactant, which ensures that SERS activity sites have not been passivated. And, the little defects of graphene benefit the excellent conductivity of graphene and ensured the replacement reaction between Cu and Ag(+). The obtained material exhibits significant high-quality and distinctive SERS activity. Especially, a serial new peak of p-aminothiophenol (PATP) is observed, this is suggested two reasons: one is "surface geometry" of the PATP on small-sized Ag NPs and another is the charge-transfer between Ag and graphene.     

Surface-enhanced Raman spectroscopic detection of Bacillus subtilis spores using gold nanoparticle based substrates

The detection of bacterial spores requires the capability of highly sensitive and biocompatible probes. This report describes the findings of an investigation of surface-enhanced Raman spectroscopic (SERS) detection of Bacillus subtilis spores using gold-nanoparticle (Au NP) based substrates as the spectroscopic probe. The SERS substrates are shown to be highly sensitive for the detection of B. subtilis spores, which release calcium dipicolinate (CaDPA) as a biomarker. The SERS bands of CaDPA released from the spores by extraction using nitric acid provide the diagnostic signal for the detection, exhibiting a limit of detection (LOD) of 1.5×10(9) spores L(-1) (or 2.5×10(-14) M). The LOD for the Au NP based substrates is quite comparable with that reported for Ag nanoparticle based substrates for the detection of spores, though the surface adsorption equilibrium constant is found to be smaller by a factor of 1-2 orders of magnitude than the Ag nanoparticle based substrates. The results have also revealed the viability of SERS detection of CaDPA released from the spores under ambient conditions without extraction using any reagents, showing a significant reduction of the diagnostic peak width for the detection. These findings have demonstrated the viability of Au NP based SERS substrates for direct use with high resolution and sensitivity as a biocompatible probe for the detection of bacterial spores.     

Rapid Synthesis of Hollow Ag–Au Nanodendrites in 15 Seconds by Combining Galvanic Replacement and Precursor Reduction Reactions

Metallic nanomaterials displaying hollow interiors as well as sharp tips/branches at their surface (such as hollow nanodendrites) are attractive, because these features enable higher surface‐to‐volume ratios than their solid and/or rounded counterparts. This paper describes a simple strategy for the synthesis of Ag–Au nanodendrites in 15 s using Ag nanospheres prepared in a previous synthetic step as seeds. Our approach was based on the utilization of Ag nanospheres as seeds for Au deposition by a combination of galvanic replacement reaction between Ag and AuCl₄^?(aq) and AuCl₄^?(aq) reduction using hydroquinone in the presence of polyvinylpyrrolidone (PVP) as a stabilizer and water as the solvent. The produced Ag–Au nanodendrites presented monodisperse sizes, and their surface morphologies could be tuned as a function of growth time. Owing to their hollow interiors and sharp tips, the Ag–Au nanodendrites performed as effective substrates for surface‐enhanced Raman scattering (SERS) detection of 4‐MPy (4‐mercaptopyridine) and R6G (rhodamine 6G) as probe molecules. We believe that the approach described herein can serve as a protocol for the fast and one‐step synthesis of Ag–Au hollow nanondendrites with a wide range of sizes, compositions, and surface morphologies for applications in SERS and catalysis.     

Temperature-assisted ionic liquid dispersive liquid-liquid microextraction combined with high performance liquid chromatography for the determination of PCBs and PBDEs in water and urine samples

We report on silver–gold core-shell nanostructures that contain Methylene Blue (MB) at the gold–silver interface. They can be used as reporter molecules in surface-enhanced Raman scattering (SERS) labels. The labels are stable and have strong SERS activity. TEM imaging revealed that these nanoparticles display bright and dark stripe structures. In addition, these labels can act as probes that can be detected and imaged through the specific Raman signatures of the reporters. We show that such SERS probes can identify cellular structures due to enhanced Raman spectra of intrinsic cellular molecules measured in the local optical fields of the core-shell nanostructures. They also provide structural information on the cellular environment as demonstrated for these nanoparticles as new SERS-active and biocompatible substrates for imaging of live cells.

Size-controllable synthesis of surface-enhanced Raman scattering-active gold nanoparticles coated on TiO2

As shown in the literature, gold nanoparticles (NPs) were popularly used in the fields of catalyst and surface-enhanced Raman scattering (SERS). In this work, size-controllable Au NPs coated on TiO(2) are synthesized by adjusting the pH of solutions based on sonoelectrochemical methods. The size-controlled Au NPs on TiO(2), ranging from 2 to 80 nm in diameter, can be obtained by varying the pH of solutions from 3 to 7 and placing the sample for 3 h before sonoelectrochemical reductions. The optimal particle sizes of Au NPs on TiO(2) to obtain the strongest SERS effects under an irradiation of 785 nm for probe molecules of adsorbed Rhodamine 6G (R6G) and deposited polypyrrole (PPy) are all ca. 60 nm.     

Surface enhanced Raman scattering of p-aminothiophenol self-assembled monolayers in sandwich structure fabricated on glass

A sandwich structure consisting of Ag nanoparticles (NPs), p-aminothiophenol (p-ATP) self-assembled monolayers (SAMs), and Ag NPs was fabricated on glass and characterized by surface enhanced Raman scattering (SERS). The SERS spectrum of a p-ATP SAM in such sandwich structure shows that the electromagnetic enhancement is greater than that on Ag NPs assembled on glass. The obtained enhancement factors (EF) on solely one sandwich structure were as large as 6.0 +/- 0.62 x 10(4) and 1.2 +/- 0.62 x 10(7) for the 7a and 3b(b(2)) vibration modes, respectively. The large enhancement effect of p-ATP SAMs is likely a result of plasmon coupling between the two layers of Ag NP (localized surface plasmon) resonance, creating a large localized electromagnetic field at their interface, where p-ATP resides. Moreover, the fact that large EF values (approximately 1.9 +/- 0.7 x 10(4) and 9.4 +/- 0.7 x 10(6) for the 7a- and b(2)-type vibration modes, respectively) were also obtained on a single sandwich structure of Au NPsp-ATP SAMsAg NPs in the visible demonstrates that the electromagnetic coupling does not exist only between Ag NPs but also between Au and Ag NPs. The lower EF values on Au-to-Ag NPs compared to those on Ag-to-Ag NPs demonstrate that the Au-to-Ag coupling must be less effective than the Ag-to-Ag coupling for the induction of SERS in the visible.     

Assembly of Multilayered Films on Core Substrate:Preparation and Characterization

Certain colloidal metals such as.silver (Ag), gold (Au) and copper, (Cu), when properly or assembled, display remarkable enhancement effect to the Raman scattering cross section of adsorbed molecules. This surface-enhanced Raman scattering (SERS) phenomenon has found wide applications in the study of interfacial chemical processes and is a potentially non-invasive technique in molecule-specific analysis. However, the SERS activity of metal colloids depends sensitively on both the synthetic method and the aggregation and assembly procedure, making it difficult to develop SERS into a reliable and quantitative analytical technique. To solve this problem, one needs to develop a substrate with a well-defined adsorption area and SERS activity. One approach to achieve this goal is to assemble a monolayer of uniform colloidal metals onto a well-defined secondary substrate. Here we report our effort in assembling monolayers of uniform Au nanoparticles on the well-defined optical-inactive microparticles in a layer-by-layer (LbL) manner and the use of such assembly as SERS-active substrate.     

Trace molecules detectable by surface-enhanced Raman scattering based on newly developed Ag and Au nanoparticles-containing substrates

In this work, Ag and Au nanoparticles-containing substrates were first developed for obtaining a stronger surface-enhanced Raman scattering (SERS) intensity of Rhodamine 6G (R6G) and reducing the limit of detection (LOD) of trace molecules. First, the optimum electrochemically roughening conditions employed on Ag substrates for obtaining strongest SERS of R6G were investigated. Then the optimally roughened Ag substrates were incubated in the prepared Cl- and Au-containing solutions for different couples of minutes to undergo the galvanic replacement reactions. Encouragingly, the SERS of R6G adsorbed on this roughened Ag substrate modified by the replacement of Ag with Au for 5 min exhibits a higher intensity by 8-fold of magnitude, as compared with the SERS of R6G adsorbed on an unmodified roughened Ag substrate. Moreover, the practical LOD of R6G can be reduced by one order of magnitude from 1 ppq to 0.1 ppq. Further investigations indicate that the compositions of complexes formed on the substrates demonstrate decided effects on the improved SERS.     

Fabrication of bimetallic microfluidic surface-enhanced Raman scattering sensors on paper by screen printing

Au–Ag bimetallic microfluidic, dumbbell-shaped, surface enhanced Raman scattering (SERS) sensors were fabricated on cellulose paper by screen printing. These printed sensors rely on a sample droplet injection zone, and a SERS detection zone at either end of the dumbbell motif, fabricated by printing silver nanoparticles (Ag NPs) and gold nanoparticles (Au NPs) successively with microscale precision. The microfluidic channel was patterned using an insulating ink to connect these two zones and form a hydrophobic circuit. Owing to capillary action of paper in the millimeter-sized channels, the sensor could enable self-filtering of fluids to remove suspended particles within wastewater without pumping. This sensor also allows sensitive SERS detection, due to advantageous combination of the strong surface enhancement of Ag NPs and excellent chemical stability of Au NPs. The SERS performance of the sensors was investigated by employing the probe rhodamine 6G, a limit of detection (LOD) of 1.1 × 10⁻¹³ M and an enhancement factor of 8.6 × 10⁶ could be achieved. Moreover, the dumbbell-shaped bimetallic sensors exhibited good stability with SERS performance being maintained over 14 weeks in air, and high reproducibility with less than 15% variation in spot-to-spot SERS intensity. Using these dumbbell-shaped bimetallic sensors, substituted aromatic pollutants in wastewater samples could be quantitatively analyzed, which demonstrated their excellent capability for rapid trace pollutant detection in wastewater samples in the field without pre-separation.     

Au-Ag合金纳米粒子制备及其表面增强拉曼光谱研究

首先采用柠檬酸钠法制得Au-Ag合金纳米种子, 然后采用盐酸羟胺生长法得到不同组成的Au-Ag合金纳米粒子. 在其UV-Vis光谱中只观察到一个位于单金属银和金之间的等离子体共振峰, 表明Au-Ag合金纳米粒子已经形成. TEM结果表明, 合金纳米粒子的粒径约为60 nm, 且颜色均一, 没有明显的核壳结构. 用苯硫酚(TP)作为探针分子研究了合金纳米粒子的表面增强拉曼光谱(SERS). 结果表明, SERS强度与合金纳米粒子的组成和尺寸有关. 当纳米粒子粒径一定时, 除Au25Ag75外, 随着金的增加SERS强度增强. Au25Ag75的粒径比Ag小, 导致SERS强度比Ag低. Au50Ag50和Au75Ag25加入TP分子后, 其聚集方式与Au相似, 等离子体共振峰逐渐靠近1064 nm, 金含量较高时, TP的SERS归于聚集体的等离子体共振增强的贡献.     

Designed fabrication of ordered porous au/ag nanostructured films for surface-enhanced Raman scattering substrates

This article reports the designed preparation of two different kinds of novel porous metal nanostructured films, namely, an ordered macroporous Au/Ag nanostructured film and an ordered hollow Au/Ag nanostructured film. Different from previous reports, the presently proposed method can be conveniently used to control film structures by simply varying the experimental conditions. The morphology of these films has been characterized by scanning electron microscopy (SEM), and their performance as surface-enhanced Raman scattering (SERS) substrates has been evaluated by using rhodamine 6G (R6G) as a probe molecule. We show that such porous nanostructured films consisting of larger interconnected aggregates are highly desirable as SERS substrates in terms of high Raman intensity enhancement, excellent stability, and reproducibility. The interconnected nanostructured aggregate, long-range ordering porosity, and nanoscale roughness are important factors responsible for this large SERS enhancement ability.     

Synthesis, characterization and SERS activity of Au-Ag nanorods

The formation mechanism and morphology of Au-Ag bimetallic colloidal nanoparticles depend on the composition. Ag coated Au colloidal nanoparticles have been prepared by deposition of Ag through chemical reduction on performed Au colloid. The composition of the Au(100-x)-Ag(x) particles was varied from x=0 to 50. The obtained colloids were characterized by UV-vis spectroscopy and transmission electron microscopy (TEM). The Au(80)-Ag(20) colloid consists of alloy nanorods with dimension of 25nmx100nm. The activity of these nanorods in surface enhanced Raman spectroscopy (SERS) was checked by using sodium salicylate as an adsorbate probe. Intense SERS bands are observed indicating its usefulness as a SERS substrate in near infrared (NIR) laser excitation.