Gold and Silver Nanomaterial‐Based Optical Sensing Systems

Gold and silver nanomaterials (NMs) such as nanoparticles (NPs) and nanoclusters (NCs) possessing interesting optical properties have become popular sensing materials. With strong surface plasmon resonance (SPR) absorption, extraordinary stability, ease in preparation, conjugation, and biocompatibility, Au NPs are employed to develop sensitive and selective sensing systems for a variety of analytes. However, small sizes of Au and Ag NCs with interesting photoluminescence (PL) properties are used in many PL‐based sensing systems for the detection of important analytes. In addition, many bimetallic AuM NMs possessing strong catalytic activity are used to develop highly sensitive fluorescent sensors. This review article is categorized in four sections based on the NMs used in the sensing systems, including Au NPs, bimetallic AuM NMs, Au NCs, and DNA–Ag NCs. In each section, synthetic strategies and optical properties of the NMs are provided briefly, followed by emphasis on their analytical applications in the detection of small molecules, metal ions, DNA, proteins, and cells. Current challenges and future prospects of these NMs‐based sensing systems will be addressed.     

不同Ag壳厚度Au@Ag纳米粒子的调控制备表征及表面增强拉曼光谱的效应

采用化学还原法制备了以Au为核、包覆Ag的双金属核壳Au@Ag纳米粒子,并成功地用于表面增强拉曼光谱(SERS)分析测试。通过改变制备液中Ag/Au的量比来调控Ag壳包覆厚度。采用透射电子显微镜(TEM)和紫外-可见光谱仪(UV-Vis)对Au@Ag纳米粒子的构貌进行表征。TEM显示明显存在核壳结构,且Ag壳层随Ag/Au的量比的增加而逐渐变厚;UV-Vis表明随着Ag/Au的量比的增加,Au@Ag纳米粒子出现了Au核与Ag壳吸收峰的2个等离子体共振峰,同时伴随着Au峰的蓝移和Ag峰的红移。以双甲脒为分析物,考察了不同Ag/Au的量比时的Au@Ag纳米粒子的SERS活性。结果表明,SERS活性随Ag/Au的量比的增加先增大后减小,在6∶5时其SERS增强效应最佳,此时Ag壳厚度约为6 nm。以对巯基苯胺(4-ATP)、结晶紫(CV)和双甲脒为分析测试对象,对比了Au@Ag、Ag、Au 3种基底的SERS活性。结果表明,所制备的Au@Ag纳米粒子的SERS活性要明显优于单纯的Au、Ag纳米粒子。     

Strain distributions of confined Au/Ag and Ag/Au nanoparticles

The strain distributions of Au/Ag and Ag/Au nanoparticles confined in the Al2O3 matrix with different core sizes are investigated by using the finite element method, respectively. The simulation results clearly indicate that the compressive strains exerted on the Au/Ag and Ag/Au nanoparticles can be induced by the Al2O3 matrix. Moreover, it can be found that the strain gradient existing in a Au/Ag nanoparticle is much larger than that in a Ag/Au nanoparticle, which could be due to the larger Young's modulus of Au than that of Ag. With the core size increasing, the strain gradient existing in the Au/Ag nanoparticle becomes larger, while the strain gradient existing in the Ag/Au nanoparticle keeps constant. These different strain distributions may have significant influences on the structures and morphologies of the Au/Ag and Ag/Au nanoparticles, leading to the different physical properties for potential applications.     

Monofunctional gold nanoparticles: synthesis and applications

The ability to control the assembly of nanoparticle building blocks is critically important for the development of new materials and devices. The properties and functions of nanomaterials are not only dependent on the size and properties of individual particles, but also the interparticle distance and interactions. In order to control the structures of nanoassemblies, it is important to first achieve a precise control on the chemical functionality of nanoparticle building blocks. This review discusses three methods that have been reported recently for the preparation of monofunctional gold nanoparticles, i.e., nanoparticles with a single chemical functional group attached to each particle. The advantages and disadvantages of the three methods are discussed and compared. With a single functional group attached to the surface, one can treat such nanoparticles as molecular building blocks to react with other molecules or nanoparticles. In other words, by using appropriate chemical reactions, nanoparticles can be linked together into nanoassemblies and materials by covalent bonds, similar to the total chemical synthesis of complicated organic compounds from smaller molecular units. An example of using this approach for the synthesis of nanoparticle/polymer hybrid materials with optical limiting properties is presented. Other potential applications and advantages of covalent bond-based nanoarchitectures vs. non-covalent interaction-based supramolecular self-assemblies are also discussed briefly in this review.     

Silver,gold and the corresponding core shell nanoparticles: synthesis and characterization

Simple strategies for producing silver and gold nanoparticles (AgNP and AuNP) along with the corresponding core shell nanoparticles (Au–Ag and Ag–Au) by reduction of the metal salts AgBF₄ and HAuCl₄ by NaBH₄ in water will be presented. The morphologies of the obtained nanoparticles are determined by the order of addition of reactants. The obtained NPs, with sizes in the range 3–40 nm, are characterized by transmission electronic microscopy (TEM) and UV–Vis absorption spectroscopy, so as to evaluate their qualities. Moreover, a direct electrochemical detection protocol based on a cyclic voltammetry in water solution that involves the use of glassy carbon electrode is also applied to characterize the prepared NPs. The developed NPs and the related electroanalytical method seem to be with interest for future sensing and biosensing applications including DNA sensors and immunosensors.     

Chiral Fluorescence Recognition by Anthracene Fluorescent Dyes ⊂ Water-Soluble Pillar[5] arene containing Phosphonic Acid Group (PWP[5])

Chirality plays a pivotal role in drugs, agrochemicals and food additives et al. The enantiomers of a chiral molecule often show huge difference in bioactivity, metabolism, and toxicity et al. thereby, the recognition of chiral molecules shows an increasingly important priority. In this paper, a novel method for chiral fluorescence recognition based on anthracene fluorescent dyes (AD)???water-soluble pillar[5] arene containing phosphonic acid group (PWP[5]) is developed. The AD as guest molecule can complex with PWP [5] to form 1:1 AD???PWP[5] assembly, and this assembly can be further used as a fluorescent probe to identify D/L-phenylalanine and D/L-phenylalaninol by fluorescent titration. The fluorescence intensity of the assembly was significantly reduced for D-phenylalanine and D-phenylalaninol, while L-phenylalanine or L-phenylalaninol was added to AD???PWP[5] assembly, the fluorescence intensity of the assembly almost unchanged. Hence, the chiral recognition based on assembly between the achiral fused ring fluorescent dye and achiral PWP[5] was developed.

     

Study of thermal diffusivity of nanofluids with bimetallic nanoparticles with Au(core)/Ag(shell) structure

The thermal diffusivity of Au/Ag nanoparticles with core/shell structure, at different compositions (Au/Ag = 3/1, 1/1, 1/3, 1/6), was measured by using the mismatched mode of the dual-beam thermal lens (TL) technique. This study determines the effect of the bimetallic composition on the thermal diffusivity of the nanofluids. In these results we find a lineal increment of the nanofluid it thermal diffusivity when the Ag shell thickness is increased. Our results show that the nanoparticle structure is an important parameter to improve the heat transport in composites and nanofluids. These results could have importance for applications in therapies and photothermal deliberation of drugs. Complementary measurements with UV-vis spectroscopy and TEM, were used to characterize the Au(core)/Ag(shell) nanoparticles.     

没食子酸还原法制备金银合金纳米粒子及其吸收光谱研究

用没食子酸一步液相还原制备粒度均匀和性质稳定的Au-Ag合金纳米粒子,研究在不同反应温度和Au／Ag摩尔比下Au-Ag合金纳米粒子的吸收光谱特性。实验结果表明Au-Ag合金纳米粒子仅有1个吸收峰,介于纯Ag和纯Au纳米粒子的吸收峰之间,且随着反应温度的提高,合金纳米粒子的最大吸收峰位逐渐蓝移,吸光度增加,半峰宽变窄。随着Au/Ag摩尔比的增加,合金纳米粒子的最大吸收峰位红移,且与其所含Au摩尔百分数呈线性关系。HRTEM表征结果说明制备的Au/Ag合金纳米粒子近球形、粒度均匀、无包覆现象。EDX分析结果证实单个纳米粒子由Au和Ag组成,其元素组成比与参加反应的Au/Ag摩尔比接近。     

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.     

An insight into the effect of composition for enhance catalytic performance of biogenic Au/Ag bimetallic nanoparticles

A comprehensive knowledge of composition‐activity property relationship of nanoparticulate materials is highly desirable for applications in various catalysis reactions. We have addressed a facile green aqueous approach for preparation of Au, Ag monometallic, Au/Ag alloy as well as core‐shell bimetallic nanoparticles. The phytochemicals present in lemon grass leaves extract were employed both as natural reducing and capping agents at room temperature. X‐ray diffraction pattern, UV‐Vis spectroscopy, and energy dispersive X‐ray studies confirmed the formation of bimetallic system. The ensuing Au core/Ag shell and Au/Ag alloy bimetallic nanoparticles were crystalline and spherical in nature with identical average diameter of ~ 18 nm as measured via transmission electron microscopy. The bimetallic systems incredibly display higher catalytic potential than their monometallic counterparts which were vividly reckoned on structural effect, lattice compression, and synergistic electronic effect.     

Photocatalytic properties of porous C-doped TiO2 and Ag/C-doped TiO2 nanomaterials by eggshell membrane templating

Porous organic carbon-doped titania (C-TiO₂) nanomaterials and their composites with Ag nanoparticles (Ag/C-TiO₂) were synthesized by an eggshell membrane templating method, and their structural and photocatalytic properties were systematically characterized. These nanomaterials, exhibiting a macroscopic morphology of a thin film, are composed of interwoven tubes, and the tube wall consists of nanocrystals. The doped organic carbon was composed of the active carbon and carbonate species, which could form a layer around the surface of TiO₂ nanoparticles, while the silver was incorporated into Ag/C-TiO₂ composites as separated Ag nanoparticles. The degradation of methylene blue under visible light irradiation was employed to evaluate the photocatalytic activity of these as-prepared TiO₂-based materials. Both C-TiO₂ and Ag/C-TiO₂ nanomaterials showed higher photocatalytic activity than pure TiO₂ material–commercial Degussa P25. These results can be accounted for the coupling effect of the incorporation of carbon species and Ag nanoparticles.     

Superchiral fields generated by nanostructures and their applications for chiral sensing

Chirality is ubiquitous in natural world. Although with similar physical and chemical properties, chiral enantiomers could play different roles in biochemical processes. Discrimination of chiral enantiomers is extremely important in biochemical, analytical chemistry, and pharmaceutical industries. Conventional chiroptical spectroscopic methods are disadvantageous at a limited detection sensitivity because of the weak signals of natural chiral molecules. Recently, superchiral fields were proposed to effectively enhance the interaction between light and molecules, allowing for ultrasensitive chiral detection. Intensive theoretical and experimental works have been devoted to generation of superchiral fields based on artificial nanostructures and their application in ultrasensitive chiral sensing. In this review, we present a survey on these works. We begin with the introduction of chiral properties of electromagnetic fields. Then, the optical chirality enhancement and ultrasensitive chiral detection based on chiral and achiral nanostructures are discussed respectively. Finally, we give a short summary and a perspective for the future ultrasensitive chiral sensing.     

硫氰根在金银复合基底上的表面增强拉曼光谱研究

采用自组装技术在硅基底卜进行金银纳米粒子的复合组装,通过控制组装溶液中金溶胶和银溶胶的体积比进而控制基底上金银纳米粒子的密度.采用紫外可见漫反射和SEM对复合基底进行了表征,结果显示硅基底上金银同时存在且呈亚单层均匀分布.以硫氰根为探针分子研究了纯金、纯银以及混合组装时复合基底的SERS效应.通过一系列的校正表明,在金银同时存在的复合基底上,硫氰根的三键伸缩振动频率和特征峰的形状相对于单组分基底而言都接近于在银基底上的特征谱峰.对实验结果进行分析后认为,当金银同时组装在基底上时,金和银之间有一定的耦合作用,这种耦合作用最终导致金的SERS效应逐渐趋向于银.     

Influence of solvent on the chiral resolution of organic molecules on Au(111): EC-STM study of biphenyl dicarboxylic acid on Au(111) in an aqueous environment

Adsorption-induced chiral resolution of organic molecules is important due to its potential applications in stereo-selective catalysis. We studied the adsorption-induced chiral resolution using a model achiral molecule of 4,4′ biphenyl dicarboxylic acid (BPDA) on Au(111) in 0.1 M perchloric acid (HClO₄) by electrochemical scanning tunneling microscopy (EC-STM). Our experimental data showed that the BPDA molecules formed island structures with distinctive preferred orientations at the length scale of the molecular size. The molecules did not show any orientational ordering above the length scale, indicating that chiral resolution was absent in the aqueous environment. Previously, the molecules were found to have chiral resolution on Au(111) in ultra-high vacuum conditions (UHV). We calculated angle-dependent binding energy between the substrate and a BPDA molecule, the intermolecular interactions between the BPDA molecules, and their interactions with water molecules. The calculations suggest that the absence of chiral resolution in the aqueous environment originated from the decrease in the intermolecular energy of the BPDA molecules due to their hydrogen bonds with the surrounding water molecules. The strength of the hydrogen bonding between BPDA molecules was sufficient to overcome the energy barrier for chiral resolution through rotational motion in UHV, but not in an aqueous environment.     

Silver-Mediated Growth of Chiral Ag/Au-Cysteine Hybrid Nanospheres with Giant Chiroptical Response

The chiral Ag/Au-cysteine hybrid nanospheres (HNSs) have been prepared by using cysteine (Cys) as an inductive agent via wet chemistry methods. Unlike the case of the dipole approximation, the circular dichroism spectra can be divided into two components: 1) one region associated with the interband absorption enhanced optical activity of structural arrangement of Cys molecules at 200–320 nm; 2) another region corresponding to a ligand-to-metal charge transfer mixed with ligand-to-metal-metal charge transfer excitation due to the synergetic interplay of the electrostatic interaction between Cys side chains and the Au(I)⋅⋅⋅Au(I) aurophilic attraction in the Au(I)-Cys complexes, located at 350–400 nm. The chiroptical response increases dramatically with increasing the concentration of Au from 0.1 to 1.56 × 10⁻³ m , and subsequently decreases with further increasing the concentration of Au from 3.12 to 12.5 × 10⁻³ m , which is similar to the sergeants and soldiers effect of chiral polymer materials. Furthermore, the circinate-like morphology of the alloyed nanospheres is strongly dependent on the presence of Ag and the chiral bimetal HNSs present excellent enantioselective recognition for amino acids in catalytic electrochemical reactions due to the specific activity and chiral active spots.     

消光式核壳二聚体传感器的设计与光谱特性分析

根据ITO/Au纳米核壳二聚体粒子在生物医学领域的应用合理性,设计了一种实时检测生物液体的核壳二聚体探针消光式传感器;由偶极子理论推导出输出波长与外界环境折射率关系;利用MATLAB设计ITO/Au纳米核壳二聚体粒子结构;采用软件DDSCAT7.3结合离散偶极近似法,利用二聚体有效半径模拟计算了300~950nm可见光到红外光波段不同核壳比、二聚体间距、以及不同介质折射率的消光光谱;根据传感芯片折射率与偶极共振、耦合八级共振的响应关系得出ITO/Au二聚体的折射率灵敏特性。与传统Ag/Au核壳纳米粒子相比,ITO/Au纳米核壳二聚体结构引入了可作为传感芯片灵敏性自参考参数的耦合八级共振峰,同时ITO/Au二聚体结构的折射率灵敏度可达到419nm/RIU。这些工作及其结果对制作消光式传感器具有重要的意义。     

利用激光烧蚀法制备表面等离子体可调谐的金核银壳层纳米颗粒胶体

利用激光烧蚀方法在水中制备了金核银壳层纳米颗粒胶体,发现这种复合胶体的等离子体振动吸收峰频率会随着激光烧蚀时间的不同而发生改变。利用等离子杂化理论定性解释了共振吸收峰可调谐的物理机制.     

Influence of Silver and Gold Nanoparticles and Thin Layers on Charge Carrier Generation in InGaN/GaN Multiple Quantum Well Structures and Crystalline Zinc Oxide Films

It has been shown that Ag and Au nanoparticles and thin layers influence charge carrier generation in InGaN/GaN multiple quantum well structures and crystalline ZnO films owing to the surface morphology heterogeneity of the semiconductors. When nanoparticles 10 < d < 20 nm in size are applied on InGaN/GaN multiple quantum well structures with surface morphology less nonuniform than that of ZnO films, the radiation intensity has turned out to grow considerably because of a plasmon resonance with the participation of localized plasmons. The application of Ag or Au layers on the surface of the structures strongly attenuates the radiation. When Ag and Au nanoparticles are applied on crystalline ZnO films obtained by rf magnetron sputtering, the radiation intensity in the short-wavelength part of the spectrum increases insignificantly because of their highly heterogeneous surface morphology.     

Characterization of exposure to silver nanoparticles in a manufacturing facility

An assessment of the extent of exposure to nanomaterials in the workplace will be helpful in improving the occupational safety of workers. It is essential that the exposure data in the workplace are concerned with risk management to evaluate and reduce worker exposure. In a manufacturing facility dealing with nanomaterials, some exposure data for gas-phase reactions are available, but much less information is available regarding liquid-phase reactions. Although the potential for inhaling nanomaterials in a liquid-phase process is less than that for gas-phase, the risks of exposure during wet-chemistry processes are not negligible. In this study, we monitored and analyzed the exposure characteristics of silver nanoparticles during a liquid-phase process in a commercial production facility. Based on the measured exposure data, the source of Ag nanoparticles emitted during the production processes was indentified and a mechanism for the growth of Ag nanoparticle released is proposed. The data reported in this study could be used to establish occupational safety guidelines in the nanotechnology workplace, especially in a liquid-phase production facility.     

Recent Advances of Light Propagation in Surface Plasmon Enhanced Quantum Dot Devices

Surface plasmons are of particular interest recently as their performance is approaching the enhancement of light emission efficiencies, after synthesized close to the vicinity of solid state materials, i.e., semiconductor structure. As other scientific works have been proposed to improve the light-emitting efficiency, such as the use of resonant cavities, photon recycling, and thin-light emitting layers with periodic surface texturing, surface plasmon possesses a promising way to the light enhancement, due to the energy coupling effect between the emitted photons from the semiconductor and the metallic nanoparticles fabricated by nanotechnology. The usual pathway of plasmon enhanced light emitting devices is the use of Ag/Au nanoparticles coating the surface of semiconductor quantum dot (QD) or quantum well (QW) structures. However, apart from efforts to extract as much light as possible from single-driven surface plasmon-QD/QW, it is possible to enhance the light emission rate with double optical-excitations. This approach is based on the quantum interference between the external lasers and the localized quantum light, and promised to stimulate the development of plasmon-enhanced optical sensors. In this review, we describe the quantum properties of light propagation in hybrid nanoparticle and semiconductor materials, i.e., quantum dot or nanomechanical resonator coupled to Ag/Au nanoparticles, driven by two optical fields. Distinct with single excitation, plasmon-assisted complex driven by two optical fields, exhibit specific quantum interference characteristics that can be used as sensitive all-optical devices, such as the slow light switch, nonlinear optical Kerr modulator, and ultra-sensitive mass sensing. We summarize the recent advances of light propagation in surface plasmon-enhanced quantum dot devices, driven by two optical fields, which would stimulate the development of novel optical materials, deeper theoretical insights, innovative new devices, and plasmonic applications with potential for significant technological and societal impact.