Au nanoparticle monolayers covered with sol-gel oxide thin films: optical and morphological study

In this work, we provide a detailed study of the influence of thermal annealing on submonolayer Au nanoparticle deposited on functionalized surfaces as standalone films and those that are coated with sol-gel NiO and TiO(2) thin films. The systems are characterized through the use of UV-vis absorption, X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and spectroscopic ellipsometry. The surface plasmon resonance peak of the Au nanoparticles was found to red-shift and increase in intensity with increasing surface coverage, an observation that is directly correlated to the complex refractive index properties of Au nanoparticle layers. The standalone Au nanoparticles sinter at 200 °C, and a relationship between the optical properties and the annealing temperature is presented. When overcoated with sol-gel metal oxide films (NiO, TiO(2)), the optical properties of the Au nanoparticles are strongly affected by the metal oxide, resulting in an intense red shift and broadening of the plasmon band; moreover, the temperature-driven sintering is strongly limited by the metal oxide layer. Optical sensing tests for ethanol vapor are presented as one possible application, showing reversible sensing dynamics and confirming the effect of Au nanoparticles in increasing the sensitivity and in providing a wavelength dependent response, thus confirming the potential use of such materials as optical probes.     

AuNPs/PNIPAM复合颗粒的制备及其温敏性质

将金纳米颗粒(AuNPs)组装到聚N-异丙基丙烯酰胺(PNIPAM)水凝胶微球表面制备出AuNPs/PNIPAM复合颗粒. 将PNIPAM 凝胶的温敏特性与AuNPs的光学性质结合, 通过改变温度调节AuNPs的局部表面等离子共振(LSPR)吸收峰位置. 研究结果表明, 温度升高使AuNPs的LSPR吸收峰发生红移, 并且这种效应是可逆的. 同时发现, AuNPs的光学性质还可以作为表征PNIPAM水凝胶微球温敏行为的一种手段. 利用透射电镜、紫外-可见光谱仪及动态光散射仪对AuNPs/PNIPAM复合颗粒的形貌、光学性质、粒径变化等进行了分析.     

Interfacing colloidal graphene oxide sheets with gold nanoparticles

Aqueous solutions of graphene oxide (GO) and citrate-stabilised gold nanoparticles (AuNPs) are two classic, negatively charged colloids. Using the surface plasmon resonance spectra of AuNPs as a probe, we illustrate how the two like-charged colloids interact with each other and in so doing, reveal the unique solution behaviour of GO. We demonstrate that the electrical double layer of the GO sheets in water plays a key role in controlling the interaction between GO and AuNPs, which displays a one-way gate effect. It is shown that GO can capture and disperse AuNPs in water in a controllable fashion, without the need for additional chemical linkers. This discovery allows the successful synthesis of uncapped, yet solution-dispersible metal-nanoparticle assemblies. Such metal nanostructures have long been pursued for nano-plasmonics and sensing applications, but have remained difficult to prepare using conventional polymer dispersants. This work also makes clear that the combination of the two-dimensional conformation of GO along with its large molecular size and self-contained functional groups allows it to act as a unique soluble nanocarrier/substrate (the thinnest, functionalised flat substrate possible in nature) for the synthesis of new, soluble functional materials.     

Metal Oxide/Graphene Composites for Supercapacitive Electrode Materials

Graphene composites with metal or metal oxide nanoparticles have been extensively investigated owing to their potential applications in the fields of fuel cells, batteries, sensing, solar cells, and catalysis. Among them, much research has focused on supercapacitor applications and have come close to realization. Composites include monometal oxides of cobalt, nickel, manganese, and iron, as well as their binary and ternary oxides. In addition, their morphological control and hybrid systems of carbon nanotubes have also been investigated. This review presents the current trends in research on metal oxide/graphene composites for supercapacitors. Furthermore, methods are suggested to improve the properties of electrochemical capacitor electrodes.     

Antimicrobial Treatment of Different Metal Oxide Nanoparticles: A Critical Review

Many nanomaterials can be used as metal oxides (Ti, Ag, Zn, Cu, Mg, Ca, Ce, Yt, Al). Metal oxide nanoparticles have strong antimicrobial properties. The oxides that play a large role as antimicrobial agents can be divided into two major groups based on their mechanism of action i.e., those that involve oxidation and those that inhibit the production of Reactive Oxygen Species (ROS). Previous studies have shown that, toxic metals like silver and titanium, and their metals oxides, employ the ROS‐mediated mechanism that leads to oxidative stress‐related cytotoxicity, cancer, and heart diseases. Oxidative stress further leads to increased ROS production and also delays the cellular processes involved in wound heal‐ ing. Other metal oxide nanoparticles, like Y₂O₃, CeO₂ and Al₂O₃ act as free radical scavengers. Out of these, aluminium oxide nanoparticles are more effective antimicrobial agents, than the other metal oxide nanoparticles. A combination of Al₂O₃ and other antimicrobial agents such as TiO₂ may act as ideal antimicrobial agents, along with possessing free radical scavenging activity. This critical review aims to study the antimicrobial properties of different metal oxide nanoparticles and the mechanism of action in‐ volved, besides comparing their efficacy to eliminate bacteria.     

Contribution of charge-transfer mechanisms to surface-enhanced Raman scattering with near-IR excitation.

Surface-enhanced Raman scattering under near-IR excitation is investigated for p-aminothiophenol (PATP) molecules that are either adsorbed on an electrochemically roughened silver electrode or embedded in an Au/PATP/Ag molecular junction assembled on an indium-doped tin oxide electrode. The contribution from chemical enhancement can be amplified relative to the contribution from electromagnetic enhancement, because the energy of the near-IR excitation is far from the surface plasmon resonance of the nanosized metal particles. The energy required for the charge-transfer process for the Au/PATP/Ag molecular junction is much lower than that of the PATP molecules adsorbed on the electrochemically roughened silver electrode. Coadsorption of chloride ions on the metal nanoparticles may result in an alteration of the local Fermi level of the metal nanoparticles, thus leading to better energy matching between the energy level of the interconnecting PATP molecules and the Fermi level of the metal nanoparticles.     

A novel route to obtain metal and oxide nanoparticles co-existing on a substrate

In this work we present a novel route to cover large surfaces with metal and oxide nanoparticles (NPs) by growing and annealing of metallic bilayers. We have used this method to fabricate ensembles of Au and α-Fe₂O₃ NPs on silica substrates from Fe/Au bilayers. The morphology of the hybrid nanostructures and the presence of defects and disorder can be tuned through the processing parameters as the initial film thickness and the annealing conditions. The proximity effects between both types of NPs alter their physical properties. In particular, we observe that the presence of α-Fe₂O₃ NPs modifies the surface plasmon resonance of Au NPs.     

Enhancement of Nonlinear Optical Scattering by Gold Nanoparticles through Aggregation-Induced Plasmon Coupling in the Near-Infrared

Gold nanoparticles (AuNPs) are regarded as promising building blocks in functional nanomaterials for sensing, drug delivery and catalysis. One remarkable property of these particles is the localized surface plasmon resonance (LSPR), which gives rise to augmented optical properties through local field enhancement. LSPR also influences the nonlinear optical properties of metal NPs (MNPs) making them potentially interesting candidates for fast, high resolution nonlinear optical imaging. In this work we characterize and discuss the wavelength dependence of the hyper-Rayleigh scattering (HRS) behavior of spherical gold nanoparticles (GNP) and gold nanorods (GNR) in solution, from 850 nm up to 1300 nm, covering the near-infrared (NIR) window relevant for deep tissue imaging. The high-resolution spectral data allows discriminating between HRS and two photon photoluminescence contributions. Upon particle aggregation, we measured very large enhancements (ca. 10⁴) of the HRS intensity in the NIR, which is explained by considering aggregation-induced plasmon coupling effects and local field enhancement. These results indicate that purposely designed coupled nanostructures could prove advantageous for nonlinear optical imaging and biosensing applications.     

纳米金的制备及其在重金属离子检测中的应用——一个分析化学综合性研究型实验

具有可控粒径、形貌纳米金的制备是近年来的一大研究热点。本实验以铯盐硼簇(Cs₂B₁₂H₁₂)为还原剂和包覆剂,成功在水相中将氯金酸还原制备了纳米级别的单分散纳米金颗粒。并将制备得到的纳米金胶体应用于重金属离子(Cr₃₊和Pb₂₊)的比色法检测中,进一步利用紫外可见光谱可以对重金属离子实现定量检测。通过该实验,可以让学生学习制备纳米金的方法,掌握利用紫外等多种分析手段对纳米粒子结构及性能进行表征的技能。     

The fabrication of photosensitive self-assembly Au nanoparticles embedded in silica nanofibers by electrospinning

In this study, we demonstrated a simple, efficient, and low-cost method to fabricate large-area self-assembly Au nanoparticles (AuNPs) encapsulated within silica nanofibers (Au/SiO₂). The method is based on electrospinning and thermal decomposition of hybrid nanofibers prepared from the solution of tetraethylorthosilicate (TEOS), polyvinylpyrrolidone (PVP) and AuNPs. This study employed the electrospinning technique for the first time as a successful method for preparing a self-organized AuNP peapod chain in a silica nanofiber matrix, under mild conditions. It has the advantage of easily controlling the diameters of the silica nanofibers as well as the concentration of the AuNPs in the spinning solution. The Au/SiO₂ hybrid nanofibers fabricated by this method exhibited an obvious photoelectric response under the illumination wavelength around the Au/SiO₂ nanofibers surface plasmon resonance (SPR) absorption band, whereas no photoelectric response was observed for the pure silica fibers. The excellent characteristics of photoelectric response suggest that the electrospinning technique has a great potential for large-scale fabrication of functional nanofiber devices. The ability of coupling light responses into the nanosystems dependent on metallic nanoparticle SPR opens up new prospects for the construction of nanoscale waveguiding devices, sensors and optoelectronics.     

Gold Nanoparticles Decorated Graphene as a High Performance Sensor for Determination of Trace Hydrazine Levels in Water

Electrochemical sensors provide a selective, sensitive and an easy approach to detect hazardous substances such as hydrazine. Herein, we investigate a facile route for the fabrication of a nanostructured composite based on Au nanoparticles (AuNPs) decorated graphene and present its sensing performance towards hydrazine. Our strategy involves electrophoretic deposition (EPD) of graphene oxide (GO) on Au substrate to obtain a uniform layer EPD‐GO, followed by electrochemical reduction of GO to yield high quality graphene ERGO and electrodeposition of monodispersed AuNPs on ERGO (AuNPs/ERGO/Au). The modified AuNPs/ERGO/Au electrode was characterized using scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT‐IR) techniques. The sensor exhibited an improved catalytic activity with a peak potential of +87 mV (vs. Ag/AgCl) for hydrazine oxidation. The high performance of this hybrid electrode is due to the presence of a synergistic effect between AuNPs and ERGO at their interface. Insights into the mechanism and kinetics of hydrazine oxidation are withdrawn from varying the voltage scan rate as the reaction is fully irreversible and diffusion‐controlled. The proposed hydrazine sensor showed suitability for nanomolar detection (detection limit of 74 nM), high selectivity in the presence of common ions and efficiency for application in water samples.     

Synthesis and applications of mesoporous nanocomposites containing metal nanoparticles

Metal nanoparticles (NP) and mesoporous (MP) oxides are complementary materials, since the size scale of pores in MP oxides matches that of NP and both systems have potential applications in similar fields. Besides, nanocomposites obtained through their combination possess not only the intrinsic properties of each component, but also new features derived from the synergy between them, mainly due to the high interfacial area between the metal and the oxide. Thus, new optical, catalytic and sensing properties can be achieved that are not easily available from the individual components. In this review, we focus our attention on such NP@MP composites, not only from the point of view of the most common synthesis pathways but also briefly describing their applications in fields as diverse as (photo)catalysis, sensing, photochromism and other optical properties, as well as patterning.     

组装在贵金属基底上的金纳米粒子对CO的电化学催化氧化

研究了组装在Au, Pt电极表面的金纳米粒子对CO的电化学催化氧化行为, 首次在实验上观察到较大粒径金纳米粒子(粒径＞10 nm)对CO的电催化氧化活性. 考察了金粒子表面金氧化物对粒子电催化活性的影响, 发现表面金氧化物的形成是金纳米粒子对CO具有电催化氧化活性的前提. 对于相同粒径的金纳米粒子, 随着粒子表面金氧化物量的增加,催化活性增大.     

Gold and silver nanoparticles in sensing and imaging: sensitivity of plasmon response to size, shape, and metal composition

Plasmonic metal nanoparticles have great potential for chemical and biological sensor applications, due to their sensitive spectral response to the local environment of the nanoparticle surface and ease of monitoring the light signal due to their strong scattering or absorption. In this work, we investigated the dependence of the sensitivity of the surface plasmon resonance (frequency and bandwidth) response to changes in their surrounding environment and the relative contribution of optical scattering to the total extinction, on the size and shape of nanorods and the type of metal, that is, Au vs Ag. Theoretical consideration on the surface plasmon resonance condition revealed that the spectral sensitivity, defined as the relative shift in resonance wavelength with respect to the refractive index change of surrounding materials, has two controlling factors: first the bulk plasma wavelength, a property dependent on the metal type, and second on the aspect ratio of the nanorods which is a geometrical parameter. It is found that the sensitivity is linearly proportional to both these factors. To quantitatively examine the dependence of the spectral sensitivity on the nanorod metal composition and the aspect ratio, the discrete dipole approximation method was used for the calculation of optical spectra of Ag-Au alloy metal nanorods as a function of Ag concentration. It is observed that the sensitivity does not depend on the type of the metal but depends largely on the aspect ratio of nanorods. The direct dependence of the sensitivity on the aspect ratio becomes more prominent as the size of nanorods becomes larger. However, the use of larger nanoparticles may induce an excessive broadening of the resonance spectrum due to an increase in the contribution of multipolar excitations. This restricts the sensing resolution. The insensitivity of the plasmon response to the metal composition is attributable to the fact that the bulk plasma frequency of the metal, which determines the spectral dispersion of the real dielectric function of metals and the surface plasmon resonance condition, has a similar value for the noble metals. On the other hand, nanorods with higher Ag concentration show a great enhancement in magnitude and sharpness of the plasmon resonance band, which gives better sensing resolution despite similar plasmon response. Furthermore, Ag nanorods have an additional advantage as better scatterers compared with Au nanorods of the same size.     

Use of thermally annealed multilayer gold nanoparticle films in combination analysis of localized surface plasmon resonance sensing and MALDI mass spectrometry

A self-assembled film of gold nanoparticles (AuNPs) with a raspberry-like morphology was prepared on a glass plate by the layer-by-layer thermal annealing of multilayer films of AuNPs. It was possible to control the morphology of the obtained films of AuNPs by changing the annealing temperature, duration of annealing, and number of layers. On investigating the plasmonic properties of these films, we found that AuNP films with a raspberry-like morphology yielded the highest refractive index unit, which is a critical parameter in localized surface plasmon resonance (LSPR) sensing, as compared to other types of AuNP films. Self-assembled AuNP films with a raspberry-like morphology were subsequently functionalized with 11-mercaptoundecanoic acid (MUA) to enable the binding of lysozyme to the MUA-modified Au surface. The superior limit of detection for the LSPR sensing of lysozyme in a buffer solution was found to be in the picomolar range (～10(-12) M). The high sensitivity observed in the region was attributed to the raspberry-like morphology, where the AuNPs were packed closely together, and the electromagnetic field confinement was most intense (i.e., at hot spots). The MUA-modified, self-assembled AuNP films with a raspberry-like morphology were finally used in the combination analysis of LSPR sensing and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) for the selective detection and identification of lysozyme in human serum.     

One‐step Synthesis of AuAg Alloy Nanodots and its Electrochemical Studies towards Nitrobenzene Reduction and Sensing

Efficient, stable, and low‐cost electrocatalysts for the degradation and sensing of environment pollutants are essential components of clean environment monitoring. Here we report, one‐step synthesis and characterization of 1–3 nm diameter sized bi‐metallic AuAg nanodots (NDs) embedded in amine functionalized silicate sol‐gel matrix (SSG) and its electrochemical studies toward nitrobenzene. The SSG was used as a reducing agent as well as stabilizer for the prepared mono‐ and bi‐metallic nanoparticles (NPs). From the HRTEM, STEM‐EDS and XPS analyses, the bi‐metallic AuAg NDs were identified as an alloy and not the mixtures of Au and Ag NPs. Characteristic surface plasmon resonance (SPR) band between the Au and Ag NPs SPR absorption region was noticed for the prepared AuAg NDs. The AuAg alloy NDs with different concentrations of Au and Ag (Au₂₅Ag₇₅, Au₅₀Ag₅₀ and Au₇₅Ag₂₅ NDs) modified electrodes exhibited synergistic electrocatalytic effect than did the Au and Ag NPs towards nitrobenzene reduction and detection. Together with ultra‐small size and exceptional colloidal stability features within these SSG‐AuAg NDs pave convenient way for nanotechnology‐based catalysts development and sensor applications.     

Enhancement of ethanol electrooxidation on plasmonic Au/TiO2 nanotube arrays

Novel electrocatalysts Au/TiO₂ nanotube arrays (Au/TiO₂NTs) were prepared by loading low-content(1.9 at.%) of Au nanoparticles (AuNPs) onto highly ordered TiO₂ nanotube arrays (TiO₂NTs). Ethanol electrooxidation indicates that visible-light (λ > 400 nm) irradiation can significantly enhance the activity as well as resistpoisoning of Au/TiO₂NTs electrocatalysts that are activated by plasmon resonance. Au/TiO₂NTs catalysts calcinated at 300 °C display the highest performance due to the strong synergistic interactions between TiO₂ and Au NPs. The combination of visible-light irradiation with a controllable potential offers a new strategyfor enhancing the performance of anodes in direct ethanol fuel cell (DEFC).     

A facile synthesis and characterization of Ag, Au and Pt nanoparticles using a natural hydrocolloid gum kondagogu (Cochlospermum gossypium)

An environmentally benign method for the synthesis of noble metal nanoparticles has been reported using aqueous solution of gum kondagogu (Cochlospermum gossypium). Both the synthesis, as well as stabilization of colloidal Ag, Au and Pt nanoparticles has been accomplished in an aqueous medium containing gum kondagogu. The colloidal suspensions so obtained were found to be highly stable for prolonged period, without undergoing any oxidation. SEM-EDXA, UV-vis spectroscopy, XRD, FTIR and TEM techniques were used to characterize the Ag, Au and Pt nanoparticles. FTIR analysis indicates that -OH groups present in the gum matrix were responsible for the reduction of metal cations into nanoparticles. UV-vis studies showed a distinct surface plasmon resonance at 412 and 525 nm due to the formation of Au and Ag nanoparticles, respectively, within the gum network. XRD studies indicated that the nanoparticles were crystalline in nature with face centered cubic geometry. The noble metal nanoparticles prepared in the present study appears to be homogeneous with the particle size ranging between 2 and 10 nm, as evidenced by TEM analysis. The Ag and Au nanoparticles formed were in the average size range of 5.5±2.5 nm and 7.8±2.3 nm; while Pt nanoparticles were in the size range of 2.4±0.7 nm, which were considerably smaller than Ag and Au nanoparticles. The present approach exemplifies a totally green synthesis using the plant derived natural product (gum kondagogu) for the production of noble metal nanoparticles and the process can also be extended to the synthesis of other metal oxide nanoparticles.     

Tunable localized surface plasmon resonances in tungsten oxide nanocrystals

Transition-metal oxide nanocrystals are interesting candidates for localized surface plasmon resonance hosts because they exhibit fascinating properties arising from the unique character of their outer-d valence electrons. WO(3-δ) nanoparticles are known to have intense visible and near-IR absorption, but the origin of the optical absorption has remained unclear. Here we demonstrate that metallic phases of WO(3-δ) nanoparticles exhibit a strong and tunable localized surface plasmon resonance, which opens up the possibility of rationally designing plasmonic tungsten oxide nanoparticles for light harvesting, bioimaging, and sensing.