Noble metals on the nanoscale: optical and photothermal properties and some applications in imaging, sensing, biology, and medicine

Noble metal nanostructures attract much interest because of their unique properties, including large optical field enhancements resulting in the strong scattering and absorption of light. The enhancement in the optical and photothermal properties of noble metal nanoparticles arises from resonant oscillation of their free electrons in the presence of light, also known as localized surface plasmon resonance (LSPR). The plasmon resonance can either radiate light (Mie scattering), a process that finds great utility in optical and imaging fields, or be rapidly converted to heat (absorption); the latter mechanism of dissipation has opened up applications in several new areas. The ability to integrate metal nanoparticles into biological systems has had greatest impact in biology and biomedicine. In this Account, we discuss the plasmonic properties of gold and silver nanostructures and present examples of how they are being utilized for biodiagnostics, biophysical studies, and medical therapy. For instance, taking advantage of the strong LSPR scattering of gold nanoparticles conjugated with specific targeting molecules allows the molecule-specific imaging and diagnosis of diseases such as cancer. We emphasize in particular how the unique tunability of the plasmon resonance properties of metal nanoparticles through variation of their size, shape, composition, and medium allows chemists to design nanostructures geared for specific bio-applications. We discuss some interesting nanostructure geometries, including nanorods, nanoshells, and nanoparticle pairs, that exhibit dramatically enhanced and tunable plasmon resonances, making them highly suitable for bio-applications. Tuning the nanostructure shape (e.g., nanoprisms, nanorods, or nanoshells) is another means of enhancing the sensitivity of the LSPR to the nanoparticle environment and, thereby, designing effective biosensing agents. Metal nanoparticle pairs or assemblies display distance-dependent plasmon resonances as a result of field coupling. A universal scaling model, relating the plasmon resonance frequency to the interparticle distance in terms of the particle size, becomes potentially useful for measuring nanoscale distances (and their changes) in biological systems. The strong plasmon absorption and photothermal conversion of gold nanoparticles has been exploited in cancer therapy through the selective localized photothermal heating of cancer cells. For nanorods or nanoshells, the LSPR can be tuned to the near-infrared region, making it possible to perform in vivo imaging and therapy. The examples of the applications of noble metal nanostructures provided herein can be readily generalized to other areas of biology and medicine because plasmonic nanomaterials exhibit great range, versatility, and systematic tunability of their optical attributes.     

Deposition of platinum on the outer surface of carbon-encapsulated silver nanoparticles

Investigations of the morphological and phase characteristics of silver nanoparticles encapsulated in amorphous carbon shell (Ag@C) during heating, acid treatment and galvanic replacement reactions were carried out. It was found that upon heating of Ag@C nanoparticles up to 800 °C the silver sublimes resulting in the formation of hollow carbon nanocapsules. Washing of Ag@C nanoparticles with nitric or hydrochloric acid solutions also leads to dissolution of encapsulated silver core and formation of hollow carbon nanocapsules. Electron microscopic investigations showed that, for short treatment times of Ag@C nanoparticles with a H₂PtCl₆ solution, nanoparticles of metallic platinum, several nanometers in diameter, are deposited on the outer surface of the carbon shells. With increasing treatment time or concentration of H₂PtCl₆ solution, increasing numbers of noble metal nanoparticles on the surface of the carbon shells, and almost complete dissolution of the metal core, were observed. This process opens broad possibilities for the design and direct preparation of modified by noble metal carbon-based nanomaterials of different compositions for a wide variety of practical applications.     

State of arts on the bio-synthesis of noble metal nanoparticles and their biological application

Nanomaterials are materials in which at least one of the dimensions of the particles is 100 nm and below. There are many types of nanomaterials, but noble metal nanoparticles are of interest due to their uniquely large surface-to-volume ratio, high surface area, optical and electronic properties, high stability, easy synthesis, and tunable surface functionalization. More importantly, noble metal nanoparticles are known to have excellent compatibility with bio-materials, which is why they are widely used in biological applications. The synthesis method of noble metal nanoparticles conventionally involves the reduction of the noble metal salt precursor by toxic reaction agents such as NaBH₄, hydrazine, and formaldehyde. This is a major drawback for researchers involved in biological application researches. Hence, the bio-synthesis of noble metal nanoparticles (NPs) by bio-materials via bio-reduction provides an alternative method to synthesize noble metal nanoparticles which are potentially non-toxic and safer for biological application. In this review, the bio-synthesis of noble metal nanoparticle including gold nanoparticle (AuNPs), silver nanoparticle (AgNPs), platinum nanoparticle (PtNPs), and palladium nanoparticle (PdNPs) are first discussed. This is followed by a discussion of these biosynthesized noble metal in biological applications including antimicrobial, wound healing, anticancer drug, and bioimaging. Based on these, it can be concluded that the study on bio-synthesized noble metal nanoparticles will expand further involving bio-reduction by unexplored bio-materials. However, many questions remain on the feasibility of bio-synthesized noble metal nanoparticles to replace existing methods on various biological applications. Nevertheless, the current development of the biological application by bio-synthesized noble metal NPs is still intensively ongoing, and will eventually reach the goal of full commercialization.     

Embedded metal nanoparticles as localized heat sources: An alternative processing approach for complex polymeric materials

Metal nanoparticles were utilized as heating elements within nanofibers to demonstrate an alternative approach to thermally process nanostructured polymeric materials. In the photothermal process, resonant light excites the surface plasmon of the nanoparticle and the absorbed energy is converted into heat due to electron-phonon collisions. This heating is efficient and strongly localized, generated from the nanometer-sized metal particles embedded within the polymer. Composite polyethylene oxide (PEO) nanofibers, containing differing concentrations and types of nanoparticles, were fabricated by electrospinning and irradiated by a low intensity laser tuned specifically to the metal nanoparticle surface plasmon absorbance; aggregation of fibers, loss of fibrous structure, and ultimately, complete melting were observed. The photothermal response to irradiation increased with nanoparticle concentration as long as particle aggregation was avoided. Pure PEO nanofibers, or those containing metal nanoparticles possessing a non-resonant surface plasmon, were also irradiated but no melting occurred, demonstrating the controllable specificity of this approach.     

挥发性有机化合物催化消除前沿技术及研究进展

挥发性有机化合物(VOCs)是可吸入有害物质形成的重要前体,是大气污染物的重要组成部分。催化氧化法作为末端技术是目前处理VOCs最有效的途径之一。本文讨论了VOCs的热催化氧化、光催化氧化和光热协同催化氧化的研究进展,重点研究常用VOCs的催化氧化机理以及相关催化剂的构筑。其中,热催化燃烧主要以贵金属(Pt、Pd、Au、Ag等)、过渡金属(Mn、Co、Cr等氧化物)及复合型催化剂研究展开;光催化氧化以TiO₂和C₃N₄为典型催化剂进行讨论;光热协同催化研究主要包括碳基催化剂、贵金属负载型以及过渡金属负载型催化剂的开发与应用。此外,本文对基于催化剂的热催化、光催化和光热催化去除VOCs的开发和研究提出了进一步的展望。     

Core shell hybrids based on noble metal nanoparticles and conjugated polymers: synthesis and characterization

Noble metal nanoparticles of different sizes and shapes combined with conjugated functional polymers give rise to advanced core shell hybrids with interesting physical characteristics and potential applications in sensors or cancer therapy. In this paper, a versatile and facile synthesis of core shell systems based on noble metal nanoparticles (AuNPs, AgNPs, PtNPs), coated by copolymers belonging to the class of substituted polyacetylenes has been developed. The polymeric shells containing functionalities such as phenyl, ammonium, or thiol pending groups have been chosen in order to tune hydrophilic and hydrophobic properties and solubility of the target core shell hybrids. The Au, Ag, or Pt nanoparticles coated by poly(dimethylpropargylamonium chloride), or poly(phenylacetylene-co-allylmercaptan). The chemical structure of polymeric shell, size and size distribution and optical properties of hybrids have been assessed. The mean diameter of the metal core has been measured (about 10-30 nm) with polymeric shell of about 2 nm.     

Separation of biosynthesized gold nanoparticles by density gradient centrifugation

The electronic, optical, magnetic, and catalytic properties of noble metal nanoparticles (NPs) strongly depend on their size. The size control of the noble metal NPs is one of the significant issues in the field of nanotechnology. We demonstrate both theoretically and experimentally that gold NPs can be separated by density gradient centrifugation (DGC) according to their size. By analyzing the force balance in a centrifuge with a horizontal rotor, a model was developed that relates the sedimentation shift to the centrifugation time and can be used to predict the effect of separation by the shift of particles within a specific period of time. The influences of centrifugation time and centrifugal force on the separation effect of DGC were evaluated.     

Photochemically reduced polyoxometalate assisted generation of silver and gold nanoparticles in composite films: a single step route

A simple method to embed noble metal (Ag, Au) nanoparticles in organic–inorganic nanocomposite films by single step method is described. This is accomplished by the assistance of Keggin ions present in the composite film. The photochemically reduced composite film has served both as a reducing agent and host for the metal nanoparticles in a single process. The embedded metal nanoparticles in composites film have been characterized by UV–Visible, TEM, EDAX, XPS techniques. Particles of less than 20 nm were readily embedded using the described approach, and monodisperse nanoparticles were obtained under optimized conditions. The fluorescence experiments showed that embedded Ag and Au nanoparticles are responsible for fluorescence emissions. The described method is facile and simple, and provides a simple potential route to fabricate self-standing noble metal embedded composite films.     

以纳米球形聚电解质刷为载体原位还原制备贵金属纳米粒子的研究

以具有核壳结构的纳米球形聚电解质刷为载体,通过吸附不同种类的贵金属离子并将其原位还原,得到分布和粒径不同的贵金属纳米粒子。采用动态光散射(DLS)研究了金属离子浓度和pH对壳层聚电解质链长的影响,使用透射电镜(TEM)观察了贵金属粒子在纳米球形聚电解质刷中的分布,并测量了金属粒子的尺寸。结果表明:金属离子浓度增大,聚电解质链长减小;pH>5时,负载金属粒子之后的聚电解质链长度均大于负载前的长度。不同金属粒子的负载情况相差较大:相同温度下,纳米金属粒子的尺寸符合Ni>Ag>Co的顺序;较高温下制得的金属粒子尺寸要高于低温下制得的金属粒子尺寸;三种纳米金属粒子中,Co纳米粒子具有最好的分布和最小的尺寸。     

Interfacial phenomena between conjugated organic molecules and noble metals

Understanding interfacial interaction between conjugated organic molecules and noble metals is important not only for surface science, but also in relation to organic epitaxy, the architecture of intermolecular networks or nanostructures, and organic electronics. Particularly, properties of interfacial geometric and electronic structures and their related phenomena have attracted much interest for their potential in various electronic and optoelectronic applications, and thus extensive efforts have been devoted to understand and control organic/metal interfaces. We provide an overview of interfacial phenomena between conjugated organic molecules and noble metals via various interactions at the organic/metal interfaces such as surface-molecule and intermolecular interactions, as well as recent progress achieved in this area.     

Progress of Nanomaterials-Based Photothermal Therapy for Oral Squamous Cell Carcinoma

Oral squamous cell carcinoma (OSCC) is one of the top 15 most prevalent cancers worldwide. However, the current treatment models for OSCC (e.g., surgery, chemotherapy, radiotherapy, and combination therapy) present several limitations: damage to adjacent healthy tissue, possible recurrence, low efficiency, and severe side effects. In this context, nanomaterial-based photothermal therapy (PTT) has attracted extensive research attention. This paper reviews the latest progress in the application of biological nanomaterials for PTT in OSCC. We divide photothermal nanomaterials into four categories (noble metal nanomaterials, carbon-based nanomaterials, metal compounds, and organic nanomaterials) and introduce each category in detail. We also mention in detail the drug delivery systems for PTT of OSCC and briefly summarize the applications of hydrogels, liposomes, and micelles. Finally, we note the challenges faced by the clinical application of PTT nanomaterials and the possibility of further improvement, providing direction for the future research of PTT in OSCC treatment.     

基于表面等离子体共振效应的Ag(Au)/半导体纳米复合光催化剂的研究进展

由具有表面等离子体共振(surface plasmon resonance,SPR)效应的贵金属(Ag、Au等)纳米粒子和半导体纳米结构组成的纳米复合光催化剂具有优异的可见光光催化活性,成为新型光催化材料的研究热点之一。本文综述了Ag(Au)/半导体纳米复合光催化剂的制备方法、基本性质以及光催化应用方面的一些重要研究进展;重点介绍了Ag(Au)等纳米粒子的表面等离子共振增强可见光催化活性的机理,以及Ag(Au)纳米粒子与不同类型半导体复合的光催化剂的光催化性能,其中所涉及的半导体包括金属氧化物、硫化物和其他一些半导体;本领域未来几年的研究热点将集中于新型高效的Ag(Au)/半导体纳米复合光催化剂的微结构调控及其用于可见光驱动有机反应的机理研究。本文为基于SPR效应构建Ag(Au)/半导体纳米复合光催化剂的研究提供了有力的参考依据,并且指出Ag(Au)/半导体纳米复合光催化剂的研究是发展可见光高效光催化剂的重要方向。     

Nickel nanoparticles in hydrogen transfer reactions

The transfer hydrogenation of organic compounds is a much safer and more environmentally benign process than reduction reactions involving molecular hydrogen, metal hydrides, or dissolving metals. In transfer hydrogenation, 2-propanol is often preferred as the source of hydrogen because it is cheap, easy to remove, and environmentally friendly. This class of transformation has been mostly pursued through the use of expensive noble metals, such as Ru, Pd, and so forth; research involving cheaper catalytically active metals has been relatively neglected. On the other hand, alcohols have recently emerged as desirable alkylating agents, a useful alternative to organic halides, in reactions of hydrogen autotransfer, also known as the "borrowing of hydrogen" methodology. For instance, the α-alkylation of ketones with alcohols is an atom-efficient process that produces water as the only byproduct in the presence of a noble metal catalyst. Hydrogen autotransfer is also successful in the synthesis of amines through a reductive aza-Wittig reaction, which involves an iminophosphorane and primary alcohol under iridium catalysis. The in situ oxidation-Wittig olefination of primary alcohols with stabilized phosphorus ylides is a commonly practiced method in organic synthesis that precludes the necessity of handling aldehydes. These reactions are normally performed in one pot but sequentially; thus the course of the alcohol oxidation needs monitoring before the ylide addition. In this Account, we describe the development of our discovery that nickel(0), in the form of nanoparticles, can replace the more expensive noble metals in both transfer hydrogenation and hydrogen autotransfer reactions. These nanoparticles were found to catalyze the transfer hydrogenation of olefins and carbonyl compounds, as well as the reductive amination of aldehydes, with 2-propanol as the hydrogen donor. All reactions proceeded in the absence of base, and the catalyst could be easily and successfully reutilized in the case of the carbonyl compounds. The catalyst was fully characterized, and the reaction mechanism, kinetics, and heterogeneous nature of the process were established through a variety of experiments. Moreover, the nickel nanoparticles enabled the activation of primary alcohols for the α-alkylation of ketones and reductive aza-Wittig reaction, with the latter leading to secondary amines. For the first time, these two reactions were achieved with a catalyst that was not one of the noble metals. We also observed that nickel nanoparticles can activate alcohols in the presence of phosphorus ylides. In this case, although the autotransfer of hydrogen failed, the reaction could be used as a key tool to construct carbon-carbon double bonds. In this respect, we describe the one-pot synthesis of stilbenes from alcohols through a Wittig-type olefination reaction promoted by nickel nanoparticles. We report a wide range of polymethoxylated and polyhydroxylated stilbenes, including the naturally occurring polyphenol resveratrol. The utility of the nickel nanoparticles was exceptional in all of the aforementioned reactions when compared with other forms of nickel, including Raney nickel.     

Organic Semiconductors for Photothermal Therapy and Photoacoustic Imaging

In recent years, semiconducting polymer nanoparticles (SPNs) have been attracting considerable attention because of their outstanding characteristics such as higher light and thermal stability. They are widely used in fields of biomedicine such as photoacoustic (PA) imaging (PAI), photodynamic therapy (PDT), and photothermal therapy (PTT). PAI, a new imaging modality based on PA effects, shows great promise in biomedical applications. SPNs that display strong optical absorbance in the visible and near-infrared (NIR) regions can be promising candidates for in vivo PTT and PAI. Here we introduce the preparation of organic conjugated polymer fluorescent nanoparticles in the aqueous phase. We then discuss the application of water-dispersible conjugated polymer nanoparticles in PA and PTT. Finally, we discuss the opportunities and challenges for the development of organic conjugated polymer nanoparticles.     

成核剂在多元醇还原法超细磁粉制备中的应用

主要探讨多元醇还原法制备CoNi类超细磁粉过程中 ,使用Pt或Ag等贵金属作成核剂 ,有利于获得粒度均匀、尺寸可控制的颗粒材料。讨论了贵金属的种类、用量以及合金组分对颗粒材料结构和性能的影响。为制备纳米级、亚微米级的优质超细合金颗粒材料提供了可行方法     

Nanosized catalysts as a basis for intensifications of technologies

This investigation of the catalytic properties of noble metal nanoparticles stabilized in hyper crosslinked polystyrene (HPS) matrix shows the prospect for their application in selective hydrogenation, selective oxidation and enantioselective hydrogenation, which represent key stages for the synthesis of the intermediates and final products of pharmaceutical industry. Commercial use of nanosized catalysts allows shortening the synthetic stages, increasing product yields, and improving the environmental safety of the existing industrial processes. In this review, the synthesis, structure and catalytic properties of mono (Pt, Ru, Pd), bi (Pt-Pd, Pt-Ru, Pd-Ru), and trimetallic (Pt-Pd-Ru) nanoparticles stabilized in the pores of a polymeric HPS matrix are discussed. Physicochemical investigations have shown that the formation of metalcontaining nanoparticles depends on the properties of the porous polymeric structure, the nature of the initial metal precursor, and the synthesis conditions. The use of nanosized catalysts is revealed to be effective in the most important field of fine organic synthesis: preparation of materials for medicine, vitamins, and food additives (e.g. in food and pharmaceutical industries).     

Monitoring early stages of silver particle formation in a polymer solution by in situ and time resolved small angle X-ray scattering

Silver particles have been prepared by reduction of silver nitrate with ascorbic acid in acidic aqueous solution containing a low concentration of a commercial polynaphthalene sulfonate polymer (Daxad 19) as dispersant agent. The reduction has been induced and controlled by the slow addition of ascorbic acid at a fixed rate; in this way, we were able to monitor the formation of a silver crystalline colloidal dispersion by in situ and time resolved Small Angle X-ray Scattering measurements. Modeling the scattering intensity with interacting spherical particles in a polymer-Ag like-fractal template allowed us to distinguish different stages involving liquid-like ordered cluster nucleation, cluster growth up to primary particle formation and particle coalescence. Between primary particle formation and particle coalescence, we observed the occurrence of a transient phase of core-shell type structures having primary particles as stable cores in expanding shells built by the organic polymer. We discuss these results in a twofold perspective pertaining both to technology, relative to controlled fabrication of metal nanoparticles and to basic chemical physics, dealing with non standard stepwise crystallization from solutions.     

Synthesis and properties of near infrared-absorbing magnetic-optical nanopins

Novel near infrared-absorbing iron oxide-gold core-shell nanoparticles in pin shapes were synthesized. The nanopins are superparamagnetic, with 35-fold better surface enhanced Raman scattering activities than the conventional core-shell nanospheres and 50-fold greater photothermal properties than solid gold nanorods. The nanoparticles will have important impact on medical imaging, molecular diagnostics and disease treatment.     

Seeding approach to noble metal decorated conducting polymer nanofiber network

Metal displacement reactions between conducting polymers-"synthetic metals"-and noble metals (Pt, Au and Ag) have been demonstrated using a seeding polymerization technique, to produce a synthetic metal nanofiber network decorated with noble metal nanoparticles, in one-step.