A Paper‐Based Platform for Long‐Term Deposition of Nanoparticles with Exceptional Redispersibility,Stability, and Functionality

Although nanoparticles (NPs) can be carefully engineered to have maximal stability and functionality desirable for use in diverse applications, they are generally not suitable for long‐term storage in solution. It is also difficult to store NPs in a dry state because dried NPs generally become aggregated and cannot easily be redispersed. Thus, a new strategy allowing long‐term storage of NPs with high stability, redispersibility, and functionality is highly demanded. By passivating the 13 nm gold nanoparticle (AuNP) surface with stabilizing agents and treating a paper substrate with both bovine serum albumin and sucrose after coating with a hydrophobic polyvinyl butyral layer, it is possible to fully redisperse (≈100%) dried AuNPs with colloidal stability comparable to that of as‐prepared AuNPs. Furthermore, AuNPs physically stabilized with polyvinylpyrrolidone can react with thiol‐containing compounds, such as 1,4‐dithiothreitol (DTT). Taking advantage of the oxidation reaction of hypochlorous acid with DTT, it is possible to demonstrate a paper‐based colorimetric sensor for detection of residual chlorine in water. Since this strategy is applicable to large‐sized AuNPs (30–90 nm), silver NPs, oleic acid‐capped magnetic NPs, and cetrimonium bromide‐passivated gold nanorods, it can be used for diverse NPs requiring long‐term storage for many applications.     

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.     

Deliberate Introduction of Particle Anisotropy in Helical Gold Nanoparticle Superstructures

Helical nanoparticle (NP) superstructures are an important class of chiral NP assemblies. The nature of the constituent NPs (size and shape) within these assemblies dictates their optical properties. However, the construction of helical NP superstructures consisting of various anisotropic NPs remains challenging. Here, a set of cetyltrimethylammonium bromide derivatives is employed to transform constituent spherical gold NPs (≈3 nm) within a chiral single‐helical assembly into gold nanoprisms (edge length ? 10 nm). Careful optimization of this strategy may lead to designed chiral NP architectures with tunable optical properties.     

Structure‐Making Effects of Metal Nanoparticles in Amyloid Peptide Fibrillation

There is growing concern that nanoparticles (NPs) may accelerate amyloid protein aggregation and thus cause amyloid‐related diseases. Here, the potential of silver and gold NPs is explored (diameter 20 nm) on the aggregation of the amyloid peptide sequences NNFGAIL from human islet amyloid polypeptide and the yeast prion protein sequence GNNQQNY, which are both the sequences of the full systems, which are able to aggregate into characteristic amyloid cross‐beta sheet fibrillar structures. Here, it is shown that silver and gold NPs in physiological aqueous solution at ambient temperatures accelerate the aggregation kinetics of both peptides significantly (in vitro). Scanning electron microscopy and X‐ray diffraction provide solid evidence for a “structure‐making” effect of the NPs. In particular, we are able to image the initial peptide corona and measure its structural reorganization in time‐resolved kinetic experiments. After a conversion time Δt, the coated NPs appear to act as templates or seeds for rapid fibrillation. Interestingly, cross‐fibrillation experiments with different peptide‐coated NPs (pcNPs) reveal that they can efficiently induce aggregation of similar peptides once the pcNPs are structurally converted. It is discussed that these structurally converted pcNPs may display similar kinetic features as toxic and aggregation inducing oligomers/protofibrils in normal amyloid aggregation, without being transient and very low‐concentration species. Finally, we suggest and discuss a simple mechanistic picture with the biomolecule corona of NPs being central to the function of the coated NPs in amyloid fibrillation.     

Self‐Assembled Silver–Gold Nanoisland Films on Glass for SERS Applications

We present a study of resonant optical properties of gold‐protected silver nanoisland films. Silver nanoislands were grown on a glass substrate using out‐diffusion technique, the growth was followed by the deposition of nanometer‐thick gold coatings. Scanning electron microscopy and optical spectroscopy were used to characterize morphology and extinction spectra of the grown combined silver–gold nanostructures. Micro Raman spectroscopy of the combined nanoislands has demonstrated their signal enhancement factor exceeding that one of the initial silver nanoislands.     

The Toxicity of Silver Nanoparticles Depends on Their Uptake by Cells and Thus on Their Surface Chemistry

A set of three types of silver nanoparticles (Ag NPs) are prepared, which have the same Ag cores, but different surface chemistry. Ag cores are stabilized with mercaptoundecanoic acid (MUA) or with a polymer shell [poly(isobutylene‐alt‐maleic anhydride) (PMA)]. In order to reduce cellular uptake, the polymer‐coated Ag NPs are additionally modified with polyethylene glycol (PEG). Corrosion (oxidation) of the NPs is quantified and their colloidal stability is investigated. MUA‐coated NPs have a much lower colloidal stability than PMA‐coated NPs and are largely agglomerated. All Ag NPs corrode faster in an acidic environment and thus more Ag(I) ions are released inside endosomal/lysosomal compartments. PMA coating does not reduce leaching of Ag(I) ions compared with MUA coating. PEGylation reduces NP cellular uptake and also the toxicity. PMA‐coated NPs have reduced toxicity compared with MUA‐coated NPs. All studied Ag NPs were less toxic than free Ag(I) ions. All in all, the cytotoxicity of Ag NPs is correlated on their uptake by cells and agglomeration behavior.     

Optical Chirality from Dark‐Field Illumination of Planar Plasmonic Nanostructures

Dark‐field illumination is shown to make planar chiral nanoparticle arrangements exhibit circular dichroism in extinction, analogous to true chiral scatterers. Single oligomers, consisting rotationally symmetric arrangements of gold nanorods, are experimentally observed to exhibit circular dichrosim at their maximum scattering with strong agreement to numerical simulation. A dipole model is developed to show that this effect is caused by a difference in the projection of a nanorod onto the handed orientation of electric fields created by a circularly polarized dark‐field normally incident on a glass‐air interface. Owing to this geometric origin, the wavelength of the peak chiral response is experimentally shown to shift depending on the separation between nanoparticles. All presented oligomers have physical dimensions less than the operating wavelength, and the applicable extension to closely packed planar arrays of oligomers is demonstrated to amplify the magnitude of circular dichroism. This realization of strong chirality in these oligomers demonstrates a new path to engineer optical chirality from planar devices using dark‐field illumination.     

Metal Nanoparticle Photocatalysts: Synthesis,Characterization, and Application

Metal nanoparticles (NPs) have emerged as a kind of new photocatalyst to drive various chemical reactions by visible‐light irradiation. A distinct advantage of metal NP photocatalysts is that their light absorption is not limited to a certain wavelength but instead they are able to utilize a broad range of wavelengths, constituting a large fraction of the solar spectrum. Metal NPs like gold, silver, and copper NPs can strongly absorb visible light due to the localized surface plasmon resonance (LSPR) effect. Recent developments have shown that the light absorption properties strongly depend on the shape, size, and particle–particle interactions of NPs, which directly influence their photocatalytic activities. In this review, an overview of the preparation of metal NPs photocatalysts with various morphologies is given along with a brief discussion of the relationship between the morphology/composition and optical properties. The latest photocatalytic applications of these morphologies are also presented, and some of the challenges for the development of metal NPs photocatalysts are provided.     

Tunable Aminooxy‐Functionalized Monolayer‐Protected Gold Clusters for Nonpolar and Aqueous Oximation Reactions

Aminooxy (–ONH₂) groups are well known for their chemoselective reactions with carbonyl compounds, specifically aldehydes and ketones. The versatility of aminooxy chemistry has proven to be an attractive feature that continues to stimulate new applications. This work describes application of aminooxy click chemistry on the surface of gold nanoparticles. A trifunctional amine‐containing aminooxy alkane thiol ligand for use in the functionalization of gold monolayer‐protected clusters (Au MPCs) is presented. Diethanolamine is readily transformed into an organic‐soluble aminooxy thiol ( AOT ) ligand using a short synthetic path. The synthesized AOT ligand is coated on ≤2‐nm‐diameter hexanethiolate‐(C₆S)‐capped Au MPCs using a ligand‐exchange protocol to afford organic‐soluble AOT /C₆S (1:1 ratio) Au mixed monolayer‐protected clusters (MMPCs). The synthesis of these Au(C₆S)( AOT ) MMPCs and representative oximation reactions with various types of aldehyde‐containing molecules is described, highlighting the ease and versatility of the chemistry and how amine protonation can be used to switch solubility characteristics.     

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.     

Janus Nanoparticle Emulsions as Chiral Nanoreactors for Enantiomerically Selective Ligand Exchange

Janus nanoparticles capped with a hydrophobic and hydrophilic hemisphere of mercapto ligands can self‐assemble into hollow, emulsion‐like nanostructures in controlled media. As the nanoparticle emulsions are chiroptically active exhibiting a plasmonic circular dichroism absorption in the visible range, they can be exploited as a unique chiral nanoreactor by selective encapsulation of d ‐enantiomer into the water phase of the water‐in‐oil emulsions for directional functionalization of the nanoparticles and endow the resulting nanoparticles with select chirality. This is demonstrated in the present study with gold Janus nanoparticles functionalized with (hydrophobic) hexanethiolates and (hydrophilic) 3‐mercapto‐1,2‐propandiol, and d ,l ‐cysteine is used as the molecular probe. Experimental results demonstrate that d ‐cysteine is the preferred enantiomers entrapped within the nanoparticle emulsions, where the ensuing ligand exchange reaction is initially confined to the hydrophilic face of the Janus nanoparticles. This suggests that with a deliberate control of the reaction time, chiral Janus nanoparticles can be readily prepared by ligand exchange reactions even with a racemic mixture of ligands.     

Chiral plasmonics and enhanced chiral light-matter interactions

Chirality, which describes the broken mirror symmetry in geometric structures, exists macroscopically in our daily life as well as microscopically down to molecular levels. Correspondingly, chiral molecules interact differently with circularly polarized light exhibiting opposite handedness(left-handed and right-handed). However, the interaction between chiral molecules and chiral light is very weak. In contrast, artificial chiral plasmonic structures can generate "super-chiral" plasmonic near-field, leading to enhanced chiral light-matter(or chiroptical) interactions. The "super-chiral" near-field presents different amplitude and phase under opposite handedness incidence, which can be utilized to engineer linear and nonlinear chiroptical interactions. Specifically,in the interaction between quantum emitters and chiral plasmonic structures, the chiral hot spots can favour the emission with a specific handedness. This article reviews the state-of-the-art research on the design, fabrication and chiroptical response of different chiral plasmonic nanostructures or metasurfaces. This review also discusses enhanced chiral light-matter interactions that are essential for applications like chirality sensing, chiral selective light emitting and harvesting. In the final part, the review ends with a perspective on future directions of chiral plasmonics.     

Development of a sensitive,stable and EGFR‐specific molecular imaging agent for surface enhanced Raman spectroscopy

We created and studied a novel nanoprobe for spectroscopic molecular imaging of the epidermal growth factor (EGF) receptor, whose over‐expression is a hallmark of a wide range of cancers. Silver nanoparticles (AgNPs) of 45 nm diameter were synthesized and coupled to EGF by α‐lipoic acid, a short ligand that exhibits excellent silver binding affinity. Time‐of‐flight mass spectroscopy demonstrates formation of the protein complex. Enzyme‐linked immunosorbent assay verifies the protein complex is 100% active for the EGF receptor, alone and, following conjugation to silver nanoparticles. Compared with its monosulfide analog, 6‐mercaptohexanoic acid, α‐lipoic acid is stabilized by binding to silver with a total energy that is lower by 1.38 eV, as found from Density Functional Theory (DFT)/natural bond analysis calculations. A Highest Occupied Molecular Orbital (HOMO)‐Lowest Unoccupied Molecular Orbital (LUMO) gap energy of 5.25 (spin‐up electrons) and 5.74 eV (spin‐down electrons) was obtained for the silver‐α‐lipoic acid complex. This is the first report of silver nanoparticles being attached to EGF, and the first theoretical and experimental report on the surface enhanced Raman spectroscopy spectral interpretation of α‐lipoic acid bound to silver. These nanoprobes exhibit surface enhanced Raman spectroscopy, when aggregated in solution, at picomolar concentrations and have the necessary properties – specificity, sensitivity and stability – to serve as molecular imaging agents. Copyright © 2015 John Wiley & Sons, Ltd.     

Synthesis of highly stable silver colloids stabilized with water soluble sulfonated polyaniline

A simple technique was developed for the synthesis of silver nanoparticles (NPs) in an aqueous medium using water soluble sulfonated polyaniline as a new non-covalent effective stabilizer. The narrow size distribution of the NPs was achieved through the synthesis. In neutral and basic solutions the as-prepared silver NPs demonstrated resistance toward aggregation over several months and at least a few days at pH 1. The versatility of the procedure was demonstrated also for the preparation of gold nanoparticles. Transmission electron microscopy with electron microdiffraction, UV-vis spectroscopy, XRD, XPS and FTIR analyses were used to characterize the structure and chemical composition of as obtained silver NPs.     

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.     

Lock‐In Thermography to Analyze Plasmonic Nanoparticle Dispersions

The physicochemical properties of nanoparticles (NPs) strongly rely on their colloidal stability, and any given dispersion can display remarkably different features, depending on whether it contains single particles or clusters. Thus, developing efficient experimental methods that are able to provide accurate and reproducible measures of the NP properties is a considerable challenge for both research and industrial development. By analyzing different NPs, through size and concentration, it is demonstrated that lock‐in thermography, based on light absorption and heat generation, is able to detect and differentiate the distinct aggregation and re‐dispersion behavior of plasmonic NPs, e.g., gold and silver. Most importantly, the approach is nonintrusive and potentially highly cost‐effective compared to standard analytical techniques.     

Antibacterial activity of silver and zinc oxide nanoparticles produced by spark discharge in deionized water

Inorganic antibacterial agents such as metal nanoparticles (NPs) are very important in biomedical and pharmaceutical areas. There are many methods of synthesizing these NPs, but all of them have their own disadvantages. In this study, ultrasonic‐assisted spark discharge is employed to produce colloidal silver (Ag) and zinc oxide (ZnO) NPs which are stable without using any stabilizers or surfactants. Different tests such as X‐ray diffraction, field emission scanning electron microscopy, and ultraviolet–visible absorption spectroscopy are used for the characterization of the quantity and quality of these NPs, and their antibacterial activity is evaluated by the disk diffusion method and determination of the minimum inhibitory concentrations against Escherichia coli . The results show that the overall antibacterial activity of Ag NPs is higher than that of ZnO NPs.     

Synthesis of Atom‐Precise Chiral Ag14 Clusters Protected by Penicillamine Ligands

A pair of atom‐precise chiral silver(I) nanocluster enantiomers ( Ag₁₄‐d and Ag₁₄‐l ) protected by d ‐ and l ‐penicillamine ligands is reported. Crystallographic structures reveal that the nanoclusters consist of a S^2? template and a chiral Ag₁₄ core stabilized by 12 penicillamine ligands. The penicillamine ligands show two binding fashions: (i) only thiolate coordination, and (ii) thiolate and carboxylate co‐coordination. Meanwhile, the two enantiomers show strong circular dichroism with opposite signals (mirror image relationship) owing to the chiral metallic core induced by chiral ligands, suggesting that the nanoclusters have well‐defined stereostructures as common chiral molecules do. The proton conductivity is also explored due to the existence of both amino groups and carboxylate groups from the penicillamine ligands, which is beneficial to construct H‐bond network for proton transfer.     

What is the origin of chirality in the cholesteric phase of virus suspensions?

We report a study of the cholesteric phase in monodisperse suspensions of the rodlike virus fd sterically stabilized with the polymer polyethylene glycol (PEG). After coating the virus with neutral polymers, the phase diagram and nematic order parameter of the fd-PEG system then become independent of ionic strength. Surprisingly, the fd-PEG suspensions not only continue to exhibit a cholesteric phase, which means that the grafted polymer does not screen all chiral interactions between rods, but paradoxically the cholesteric pitch of this sterically stabilized fd-PEG system varies with ionic strength. Furthermore, we observe that the cholesteric pitch decreases with increasing viral contour length, in contrast to theories which predict the opposite trend. Different models of the origin of chirality in colloidal liquid crystals are discussed.     

基于功能化的金纳米的共振瑞利散射方法手性识别肉碱对映体

一种不经分离而同时测定手性对映体的简便方案是非常有趣和有用的.提出一种基于共振瑞利散射(RRS)光谱技术手性识别新方法,利用功能化的金纳米粒子(Au NPs)同时检测肉碱对映体.Au NPs的RRS强度很弱,但当Cu2+存在时,RRS强度显著增加.更有趣的是,肉碱对映体均可以降低Cu2+-Au NPs体系的RRS强度,但D-肉碱使RRS降低更多.在最优实验条件下,均有良好的线性关系并有很好的相关系数以及较低的检出限.由此,这种方法可以计算出肉碱对映体的对映体比率和对映体分数.并应用于胶囊样品中肉碱对映体混合物手性识别的研究.该方法不需要复杂的手性修饰处理,并具有简捷低消耗、灵敏度高、选择性好等优点.