Fabrication of periodical Ag–Au compound nanostructure films with controllable Ag nanoparticle aggregate patterns: a study on surface‐enhanced Raman scattering

The Ag–Au compound nanostructure films with controllable patterns of Ag nanoparticle (NP) aggregates were fabricated. A strategy of two‐step synthesis was employed toward the target products. Firstly, the precursor Au NP (17 nm) films were synthesized as templates. Secondly, the Ag NPs (45 nm) were deposited on the precursor films. Three types of Ag NP aggregates were obtained including discrete Ag NPs (discrete type), necklace‐like Ag NP aggregates (necklace type), and huddle‐like Ag NP aggregates (huddle type). The surface‐enhanced Raman scattering (SERS) property was studied on these nanostructures by using the probing molecule of rhodamine 6G under the excitation laser of 514.5 nm. Interestingly, the different types of samples showed different enhancement abilities. A statistical method was employed to assess the enhancement. The relative enhancement factor for each Ag NP was estimated quantitatively under the ratio of 1 : 25 : 18 for the discrete‐type, necklace‐type, and huddle‐type samples at the given concentration of 10⁻⁸ mol/l. This research shows that the enhancement ability of each Ag NP is dependent on the aggregate morphology. Moreover, the different enhancement abilities displayed different limit detection concentrations up to 10⁻⁸, 10⁻¹¹, and 10⁻⁹ mol/l, separately. The understanding of the relationship between the defined nanostructures and the SERS enhancement is very meaningful for the design of new SERS substrates with better performance. Copyright © 2015 John Wiley & Sons, Ltd.     

Enhancement in SERS intensity with hierarchical nanostructures by bimetallic deposition approach

We investigate the plasmonic enhancement arising from bimetallic (Au/Ag) hierarchical structure and address the fundamental issues relating to the design of multilayered nanostructures for surface‐enhanced Raman scattering (SERS) spectroscopy. SERS‐active nanosphere arrays with Ag underlayer and Au overlayer were systematically constructed, with the thickness of each layer altered from 40 to 320 nm. The SERS responses of the resultant bimetallic structures were measured with 2‐naphthalenethiol dye as the test sample. The results confirm the dependency of SERS enhancement on the thickness ratio (Au : Ag). Compared with Au‐arrays, our optimized bimetallic structures, which exhibit nanoprotrusions on the nanospheres, were found to be 2.5 times more SERS enhancing, approaching the enhancement factor of an Ag‐array. The elevated SERS is attributed to the formation of effective hot‐spots associated with increased roughness of the outer Au film, resulting from subsequent sputtering of Au granules on a roughened Ag surface. The morphology and reflectance studies suggest that the SERS hot‐spots are distributed at the junctions of interconnected nanospheres and over the nanosphere surface, depending on the thickness ratio between the Au and Ag layers. We show that, by varying the thickness ratio, it is possible to optimize the SERS enhancement factor without significantly altering the operating plasmon resonance wavelength, which is dictated solely by the size of the underlying nanospheres template. In addition, our bimetallic substrates show long‐term stability compared with previously reported Ag‐arrays, whose SERS efficiency drops by 60% within a week because of oxidation. These findings demonstrate the potential of using such a bimetallic configuration to morphologically optimize any SERS substrate for sensing applications that demand huge SERS enhancement and adequate chemical stability. Copyright © 2011 John Wiley & Sons, Ltd.     

Surface characteristics of Ag‐doped Au nanoparticles probed by Raman scattering spectroscopy

We have examined the surface characteristics of Ag‐doped Au nanoparticles (below 5 mol% of Ag) by means of the surface‐enhanced Raman scattering (SERS) of 2,6‐dimethylphenylisocyanide (2,6‐DMPI) and 4‐nitrobenzenethiol (4‐NBT). When Ag was added to Au to form ∼35‐nm‐sized alloy nanoparticles, the surface plasmon resonance band was blue‐shifted linearly from 523 to 517 nm in proportion to the content of Ag up to 5%. In the SERS spectra of 2,6‐DMPI, the N‐C stretching peak also shifted almost linearly from 2184 to 2174 cm⁻¹ when the Ag content was 5 mol% or less; the peak then remained the same as that of the pure Ag film. The potential variation of the SERS spectrum of 2,6‐DMPI in an electrochemical environment, as well as the effect of organic vapor, also showed a similar tendency. From the SERS of 4‐NBT, we confirmed the occurrence of a surface‐induced photoreaction converting 4‐NBT to 4‐aminobenzenethiol, when Ag was added to Au to form alloy nanoparticles. The photoreaction induction ability also increased linearly with the Ag content, reaching a plateau level at 5 mol% of Ag. All these observations suggest that the surface content of Ag should increase almost linearly as a function of the overall mole fraction of Ag and, once the Au/Ag nanoparticles reach 5 mol% of Ag, their surfaces are fully covered with Ag, showing the same surface characteristics of pure Ag nanoparticles. Copyright © 2011 John Wiley & Sons, Ltd.     

应用球形和海胆状金混合SERS基底检测高环多环芳烃

合成了海胆状金银复合纳米材料,并与球形金纳米材料混合作为表面增强拉曼活性基底实现了对水中高环多环芳烃的痕量检测。对海胆状材料进行表征,粒径大小约为300～400 nm,表面有40～100 nm明显的刺状凸起。与球形金溶胶混合后并优化pH值及混合比例等参数,产生了优于球形金溶胶2～3倍的增强效果。利用此增强基底检测了危害严重的高环多环芳烃污染物——芘(四环)、苯并蒽(四环)、苯并芘(五环),得到的光谱数据反映出混合SERS基底有良好的重复性和稳定性,对测得光谱进行特征峰归属分析,固体拉曼光谱与水溶液SERS光谱有确定的对应关系,并且在低浓度范围多环芳烃特征峰峰强与其水溶液浓度有良好的线性关系。经计算,芘(四环)、苯并蒽(四环)、苯并芘(五环)的检测限分别为0.44,2.92和1.64 nmol·L-1。该研究的创新点为合成了海胆金纳米颗粒,与球形金溶胶混合后制成新型高效SERS检测基底;选用自制高效SERS基底,实现了高环PAHs痕量检测。结果表明,利用该方法制备的活性基底,可实现对水中高环多环芳烃的痕量检测,为检测水中高环多环芳烃提供了实验室依据。     

A facile method for the synthesis of large‐size Ag nanoparticles as efficient SERS substrates

Silver nanoparticles (Ag NPs) enjoy a reputation as an ultrasensitive substrate for surface‐enhanced Raman spectroscopy (SERS). However, large‐scale synthesis of Ag NPs in a controlled manner is a challenging task for a long period of time. Here, we reported a simple seed‐mediated method to synthesize Ag NPs with controllable sizes from 50 to 300 nm, which were characterized by scanning electron microscopy (SEM) and UV–Vis spectroscopy. SERS spectra of Rhodamine 6G (R6G) from the as‐prepared Ag NPs substrates indicate that the enhancement capability of Ag NPs varies with different excitation wavelengths. The Ag NPs with average sizes of ~150, ~175, and ~225 nm show the highest SERS activities for 532, 633, and 785‐nm excitation, respectively. Significantly, 150‐nm Ag NPs exhibit an enhancement factor exceeding 10⁸ for pyridine (Py) molecules in electrochemical SERS (EC‐SERS) measurements. Furthermore, finite‐difference time‐domain (FDTD) calculation is employed to explain the size‐dependent SERS activity. Finally, the potential of the as‐prepared SERS substrates is demonstrated with the detection of malachite green. Copyright © 2016 John Wiley & Sons, Ltd.     

The SERS response of semiordered Ag nanorod arrays fabricated by template oblique angle deposition

Semiordered Ag nanorod arrays are fabricated by template oblique angle deposition (OAD) using regular Au nano‐post arrays with different diameters as seed patterns. The Au nano‐post arrays do not give an observable surface‐enhanced Raman scattering (SERS) activity under our detection configuration, whereas the patterned Ag nanorod arrays can produce a very strong SERS signal. These SERS intensities increase monotonically with the decrease in the diameter and separation of the Ag nanorods, which demonstrates that one can improve the SERS detection by tuning the diameter and separation of the Ag nanorods, and the template OAD method can help produce more uniform, reproducible, and sensitive Ag nanorod SERS substrates. Copyright © 2010 John Wiley & Sons, Ltd.     

Enhancements in intensity and thermal stability of Raman spectra based on roughened gold substrates modified by underpotential deposition of silver

In this study, electrochemically roughened gold is modified with underpotential deposition (UPD) silver to investigate the effects on enhancements in the intensity and the thermal stability of surface‐enhanced Raman scattering (SERS). The SERS of Rhodamine 6G (R6G) adsorbed on the UPD Ag‐modified Au substrate exhibits a higher intensity by six‐fold of magnitude, as compared with that of R6G adsorbedon the unmodified Au substrate. Moreover, the SERS enhancement capabilities of UPD Ag‐modified Au and unmodified Au substrates are seriously depressed at temperatures higher than 200 and 150 °C, respectively. It indicates that the modification of UPD Ag can significantly depress the thermal destruction of SERS‐active substrates. 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.     

Surface‐enhanced Raman scattering‐based approach for DNA detection at low concentrations via polyvinyl alcohol‐protected silver grasslike patterns

A simple method is demonstrated to detect DNA at low concentrations on the basis of surface‐enhanced Raman scattering (SERS) via polyvinyl alcohol‐protected silver grasslike patterns (PVA‐Ag GPs) grown on the surface of the common Al substrate. By the SERS measurements of sodium citrate and thymine, the PVA‐Ag GPs are shown to be an excellent SERS substrate with good activity, stability and reproducibility. With the use of the tested molecule of thymine, the enhancement factor of the PVA‐Ag GPs is up to ~1.4 × 10⁸. The PVA‐Ag GPs are also shown to be an excellent SERS substrate with good biocompatibility for DNA detection, and the detection limit is down to ~10⁻⁵ mg/g. Meanwhile, the assignations of the Raman bands and the adsorption behaviors of the DNA molecules are also analyzed. In this work, the geometry optimization and the wavenumber analysis of adenine–Ag and guanine–Ag complexes for the ground states are performed using density functional theory, B3LYP functional and the LanL2DZ basis set. The transition energies and the oscillator strengths of adenine–Ag and guanine–Ag for the lowest six singlet excited states were calculated by using the time‐dependent density functional theory method with the same functional and basis set. The results show that the charge transfer in the adenine–Ag and guanine–Ag complexes should be the chemical factor for the SERS of the DNA molecules. Lastly, this method may be employed in large‐scale preparation of substrates that have been widely applied in the Raman analysis of DNA because the fabrication process is simple and inexpensive. Copyright © 2011 John Wiley & Sons, Ltd.     

Experimental parameters for the SERS of nitrate ion for label‐free semi‐quantitative detection of proteins and mechanism for proteins to form SERS hot sites: a SERS study

We have explored the effects of the experimental parameters on the surface‐enhanced Raman scattering (SERS) intensities of NO₃⁻ and proteins observed by a heat‐induced SERS method developed by our group. The results have shown that a strong SERS signal can be obtained at pH 4.0, using an Ag colloid prepared with the reduction time of 15 min (the average size of Ag nanoparticle is 56.5 nm) dilution prepared Ag colloid by a factor of 2 by use of a 5 mM citrate buffer, using 6 mM NaNO₃ and drying the sample at 100 °C, respectively. Based on the results, two possible mechanisms for proteins to form SERS hot sites during the sample preparations are proposed. A semi‐quantitative SERS detection of ribonuclease B has been investigated. Also, NaNO₂, Mg (NO₃)₂, MgSO₄ and Na₂SO₄ have been found to be suitable for the heat‐induced SERS method. Importantly, samples prepared by the heat‐induced SERS method are so stable that these samples can be used as a standard and transferred to different laboratories for direct comparison. Namely, it can overcome uncontrollable aggregation of Ag colloids in a solution sample. All these advantages and the simplicity of experimental setup have demonstrated that the heat‐induced SERS method using NaNO₃ as an electrolyte is very promising for label‐free routine and quantitative detection of proteins. Copyright © 2011 John Wiley & Sons, Ltd.     

Biocompatible gold/silver nanostars for surface‐enhanced Raman scattering

Raman‐enhancing properties of chitosan (CS)‐coated gold/silver nanostars (Au/AgNSs) were demonstrated by using them as a surface‐enhanced Raman scattering (SERS) probe. Based on the energy‐dispersive X‐ray spectroscopy element distribution maps and highly enhanced SERS spectra, we suggest that the incorporation of silver into the NS tips leads to a stronger SERS behavior. The SERS spectra of the proteins adsorbed on the NS surface greatly differ from their respective Raman spectra in both the band positions and relative intensities, indicating that the protein molecules penetrate through the CS coating layer and interact closely with the NS surface. Raman and SERS spectra of Chlamydia trachomatis protease/proteasomelike activity factor are reported for the first time, demonstrating the potential of these NSs for the development of a diagnosis method for Chlamydia based on SERS. The results showed a good SERS performance of the Au/AgNSs and their potential for SERS detection of biomolecules. Copyright © 2016 John Wiley & Sons, Ltd.     

Detection and quantitative analysis of carbendazim herbicide on Ag nanoparticles via surface‐enhanced Raman scattering

Carbendazim (MBC) is a fungicide widely used in agriculture, and there are serious concerns regarding the health risks that could be caused by this fungicide. Here, we explore its ultrasensitive detection by surface‐enhanced Raman scattering (SERS). First, to obtain maximum SERS signal, the adsorption of the target molecule onto metallic surface is essential. Therefore, we study the adsorption of the MBC onto the nanoparticle surface by SERS under different experimental conditions, such as different synthesis methods of nanoparticle, variable excitation wavelength, and fungicide concentration with the aim to detect MBC at low concentrations. Experiments are carried out with three kinds of colloidal nanoparticles: Ag and Au reduced by citrate and Ag reduced by hydroxylamine. However, mainly Ag colloids are highly efficient in the SERS detection of MBC. In addition, theoretical calculations of MBC Raman spectrum and that of the surface complex are used to help with the understanding the mechanisms responsible for the interaction between MBC and Ag. Ultraviolet–visible absorption spectroscopy showed displacement to the red of the plasmon resonance of Ag colloid in the presence of MBC. Copyright © 2015 John Wiley & Sons, Ltd.     

Facile preparation of Au/Ag bimetallic hollow nanospheres and its application in surface-enhanced Raman scattering

In this paper, an Au/Ag bimetallic hollow nanostructure was obtained by using SiO₂ nanospheres as sacrificial templates. The nanostructure was fabricated via a three steps method. SiO₂@Au nanospheres were first synthesized by the layer-by-layer technique, and then they were coated with a layer of Ag particles, finally, the Au/Ag bimetallic hollow nanospheres were obtained by dissolution of the SiO₂ core by exposure in HF solution. Several characterizations, such as transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and UV visible absorption spectroscopy were used to investigate the prepared nanostructures. The effectiveness of these Au/Ag bimetallic hollow nanospheres as substrates toward surface-enhanced Raman scattering (SERS) detection was evaluated by using rhodamine 6G (R6G) as a probe molecule. We show that such Au/Ag bimetallic hollow nanospheres structure films which consisting of larger interconnected aggregates are highly desirable as SERS substrates in terms of high Raman intensity enhancement. The Au/Ag bimetallic hollow nanostructured aggregate, interconnected nanostructured aggregate and nanoscale roughness are important factors responsible for this large SERS enhancement ability.     

Ag核Au壳复合纳米粒子为标记溶胶免疫检测的SERS研究

以种子生长法合成Ag核Au壳复合纳米粒子,苯硫酚分子(TP)在其表面的SERS增强随Au摩尔比例的增加呈现先增强后减弱的趋势,其最大增强为相应Ag纳米粒子的10倍。将标记分子TP,羊抗小鼠抗体固定在Ag核Au壳复合纳米粒子表面形成标记免疫溶胶,其与被基底捕获抗原分子发生免疫识别,通过TP分子的SERS信号进行免疫检测。     

Surface‐enhanced Raman scattering sensing on black silicon

Reactive ion etching was used to fabricate black‐Si over the entire surface area of 4‐inch Si wafers. After 20 min of the plasma treatment, surface reflection well below 2% was achieved over the 300–1000 nm spectral range. The spikes of the black‐Si substrates were coated by gold, resulting in an island film for surface‐enhanced Raman scattering (SERS) sensing. A detection limit of 1 × 10^?6 M (at count rate > 10² s^?1 . mW^?1) was achieved for rhodamine 6G in aqueous solution when drop cast onto a ～ 100‐nm‐thick Au coating. The sensitivity increases for thicker coatings. A mixed mobile‐on‐immobile platform for SERS sensing is introduced by using dog‐bone Au nanoparticles on the Au/black‐Si substrate. The SERS intensity shows a non‐linear dependence on the solid angle (numerical aperture of excitation/collection optics) for a thick gold coating that exhibits a 10 times higher enhancement. This shows promise for augmented sensitivity in SERS applications.     

Silica‐overcoated substrates for detection of proteins by surface‐enhanced Raman spectroscopy

Ag film over nanosphere (AgFON) substrates for surface‐enhanced Raman spectroscopy (SERS) are shown to be ineffective for the detection of proteins in phosphate buffer solution (PBS) because of the decomposition of the substrate resulting in a total loss of SERS activity. However, modification of these substrates with SiO₂ overlayers overcomes this problem. The SiO₂ overlayers are produced by filtered arc deposition (FAD) and are characterised by atomic force microscopy (AFM). Their porosity is examined using Raman spectroscopy and the detection of cytochrome c and bovine serum albumin in PBS is successfully demonstrated. These findings show promise for the detection of proteins in biologically relevant conditions using Ag‐based SERS substrates. Copyright © 2008 John Wiley & Sons, Ltd.     

Near‐infrared surface‐enhanced Raman spectroscopy (NIR‐SERS) studies on oxyheamoglobin (OxyHb) of liver cancer based on PVA‐Ag nanofilm

A near‐infrared surface‐enhanced Raman spectroscopy (NIR‐SERS) method was employed for oxyheamoglobin (OxyHb) detection to develop a simple blood test for liver cancer detection. Polyvinyl alcohol protected silver nanofilm (PVA‐Ag nanofilm) used as the NIR‐SERS active substrate to enhance the Raman scattering signals of OxyHb. High quality NIR‐SERS spectrum from OxyHb adsorbed on PVA‐Ag nanofilm can be obtained within 16 s using a portable Raman spectrometer. NIR‐SERS measurements were performed on OxyHb samples of healthy volunteers (control subjects, n = 30), patients (n = 40) with confirmed liver cancer (stage I, II and III) and the liver cancer patients after surgery (n = 30). Meanwhile, the tentative assignments of the Raman bands in the measured NIR‐SERS spectra were performed, and the results suggested cancer specific changes on molecule level, including a decrease in the relative concentrations and the percentage of aromatic amino acids of OxyHb, changes of the vibration modes of the C_aH_m group and pyrrole ring of OxyHb of liver cancer patients. In this paper, principal component analysis (PCA) combined with independent sample T test analysis of the measured NIR‐SERS spectra separated the spectral features of the two groups into two distinct clusters with the sensitivity of 95.0% and the specificity of 85.7%. Meanwhile, the recovery situations of the liver cancer patients after surgery were also assessed using the method of discriminant analysis‐predicting group membership based on PCA. The results show that 26.7% surgeried liver cancer patients were distinguished as the normal subjects and 63.3% were distinguished into the cancer. Our study demonstrated great potentials for developing NIR‐SERS OxyHb analysis into a novel clinical tool for non‐invasive detection of liver cancers. Copyright © 2013 John Wiley & Sons, Ltd.     

SERS microRaman spectral probing of adsorbate‐containing,liquid‐overlayed nanosponge Ag aggregates assembled from fractal aggregates

Adsorbate‐containing, nanosponge Ag aggregates overlayed by a thin (~1.5 mm) liquid layer are reported as a new type of sample for Surface‐enhanced Raman scattering (SERS) microRaman spectral measurements and adsorbate (analyte) detection. Macroscopic Ag aggregates (of about 1.5 × 1.0 × 0.025 mm size) with the nanosponge internal morphology (revealed by Scanning electron microscopy (SEM)) were prepared by 3D assembling of fused fractal aggregates (D = 1.84 ± 0.04) formed in Ag nanoparticle hydrosol/HCl/adsorbate systems with 2,2’‐bipyridine (bpy) and/or a cationic free‐base tetrakis(2‐methyl‐4‐pyridiniumyl) porphine (H₂TMPyP) as the testing adsorbates. For SERS microRaman measurements, the macroscopic aggregate was overlayed by a thin (~1.5 mm) layer of the residual liquid. Preparation procedure, nanoscale imaging, and SERS spectral probing including the determination of the detection limits of the adsorbates revealed the following advantages of the adsorbate‐containing, liquid‐overlayed 3D nanosponge aggregate as a sample for SERS microRaman spectral measurements: (1) localization of adsorbate (analyte) into hot spots and, simultaneously, prevention of the analyte decomposition during the spectral measurement (carried out without an immersion objective), (2) fast and simple sample preparation, and (3) minimization of sample volume and an efficient concentration of hot spots into the focus of the laser beam. The advantages of the nanosponge Ag aggregates are further demonstrated by the 40 fmol limit of detection of bpy as Ag(0)‐bpy surface complex, as well as by preservation of the native structure of the cationic free‐base porphyrin H₂TMPyP. Copyright © 2015 John Wiley & Sons, Ltd.     

Comparison of SERS effectiveness of copper substrates prepared by different methods: what are the values of enhancement factors?

Surface‐enhanced Raman scattering (SERS) spectroscopy is an analytical method for the detection of low amounts of analytes adsorbed on an appropriate coinage metal (Au, Ag, Cu) surface. Generally, the values of the enhancement factor are the highest on silver, lower on gold and relatively very low on copper. In this study, we have focused on the estimation of the enhancement factors of copper surface/substrates formed by different preparation procedures. The SERS activity of large electrochemically prepared substrates and colloidal systems is compared. The surface morphology of the large substrates was studied using scanning electron microscopy and atomic force microscopy. The size distribution of colloidal nanoparticles was monitored by dynamic light scattering. The values of enhancement factor are in both cases more than 10⁵ for the FT‐SERS spectra, demonstrating the fundamental role of nanostructured copper as a substrate material at the excitation wavelength (1064 nm) used. Copyright © 2011 John Wiley & Sons, Ltd.     

On‐line surface enhanced Raman spectroscopic detection in a recyclable Au@SiO2 modified glass capillary

A facile method was developed to fabricate a high sensitive, reproducible and recyclable surface enhanced Raman spectroscopy (SERS) active glass capillary. The Au nanoparticles were synthesized through a seed‐mediated growth approach and then self‐assembled onto the inner wall of glass capillaries. The attached Au nanoparticles were homogeneously coated with thin silica shell by using the silane coupling agent to functionalize the Au surface. By using thiophenol (TP) as SERS probe molecules, the substrate exhibited robust SERS effects. The adsorbed SERS probe molecules could be rapidly and completely removed away by flowing sodium borohydride solution and thus to obtain a refresh Au@SiO₂ film‐coated substrate for the cyclic detection on different species. The on‐line detection of TP and malachite green (MG) with different concentrations was performed in the flowing system. The intensities of SERS signals were dependent on concentrations of the detected molecules. The results indicated that the SERS‐active substrate has potential applications on the on‐line qualitative and quasi‐quantitative analysis. Copyright © 2014 John Wiley & Sons, Ltd.