Diffusion-controlled electron transfer processes and power-law statistics of fluorescence intermittency of nanoparticles

A mechanism involving diffusion-controlled electron transfer processes in Debye and non-Debye dielectric media is proposed to elucidate the power-law distribution for the lifetime of a blinking quantum dot. This model leads to two complementary regimes of power law with a sum of the exponents equal to 2, and to a specific value for the exponent in terms of a distribution of the diffusion correlation times. It also links the exponential bending tail with energetic and kinetic parameters.     

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.     

Photochemical synthesis of polygonal gold nanoparticles

In this paper, we propose a method to generate gold nanoparticles capable of absorbing near infrared light (NIR) radiation through a photochemical reaction. This approach does not require the use of either surfactants or polymers, reducing the difficulties that may arise in further chemical modifications for the gold nanoparticles. The gold nanoparticles with either triangular or hexagonal shapes were generated using the photo-reduction method, mixing hydrogen tetrachloroaurate with sodium oxalate, a reducing agent, in aqueous solution under illumination of a mercury lamp (λ_max = 306 nm) for more than 10 min. The size of the gold nanoparticles varies from 25 to 200 nm, which mainly depends on the duration of light illumination and the concentration of sodium oxalate. Furthermore, we demonstrate that the presence of the gold nanoparticles in aqueous solutions can effectively elevate the temperature of the solutions under irradiation of NIR light (808 nm) within a few minutes. The gold nanoparticles can be potentially used as suitable photothermal agents for hyperthermia.     

Plasma desorption mass spectroscopy of thiol-passivated gold nanoparticles

Plasma desorption mass spectrometry has been applied to characterization of dodecanthiol-passivated gold nanoparticles. An overview of the experimental set-up and mass analyses for the nanoparticles prepared in different conditions are shown. Mass distributions were found to shift to higher mass regions with increasing reaction temperature and reaction period. The results are consistent with those of transmission electron microscopy observations, UV-visible absorption spectra and also with a reported laser desorption mass spectrometry.     

One step alkaline synthesis of biocompatible gold nanoparticles using dextrin as capping agent

Gold nanoparticles (AuNPs) are used in sensing methods as tracers and transducers. The most common AuNP synthesis techniques utilize citrate under acidic reaction conditions. The synthesis described in this article generates glyco-AuNPs under mild alkaline conditions providing a “greener” alternative to Brust and Turkevich methodologies. This biologically compatible one-step technique uses dextrin as a capping agent and sodium carbonate as the reducing agent for chloroauric acid. The generated particles were relatively mono-dispersed and water soluble with a range of controllable mean diameters from 5.9 to 16.8 ± 1.6 nm. The produced AuNPs were stable in water for more than 6 months stored at room temperature (21 °C) in generation solution without protection from light. This article shows the effect of temperature, pH, and dextrin concentration on the synthesis procedure and AuNP diameter. These factors were found to control the reaction speed. The produced glyco-AuNPs were successfully functionalized with DNA oligonucleotides, and the functionalization efficiency was similar to citrate-generated AuNPs. The alkaline synthesis allows future exploration of simultaneous synthesis and functionalization procedures, which could significantly reduce the time of current ligand exchange methodologies.     

Pseudo-template synthesis of gold nanoparticles based on polyhydrosilanes

Highly stable colloidal gold nanoparticles are obtained in a pseudo-template system using a specific polyhydrosilane copolymeric structure. This process takes place in situ by microwaves activation of the polymer solution in a non-polar solvent followed by stirring with solid HAuCl₄ in natural light. The experimental procedure is very simple and the resulted colloidal gold solution is indefinitely stable. The specific surface plasmon resonance absorption band of the gold nanoparticles is strongly red shifted and is strictly related to their size. AFM correlated with DLS analysis showed flattened round shaped colloidal polymer-gold nanoparticles with large diameters. SEM-EDX combined analysis reveals that the polysilane-gold nanoparticles show a natural tendency to auto-assemble in close packed structures which form large areas over the polymer film surface.     

One-pot synthesis of gold nanoparticles using tetradentate porphyrins

In this study, the meso-tetra (p-hydroxyphenyl) porphyrin and meso-tetra (m-hydroxyphenyl) porphyrin were coated on to gold nanoparticles (AuNPs) via thioacetate anchors which easily dissociate to form S–Au bonds. 4-tert-butyl phenyl thioacetate-AuNPs were prepared and used as a monodentate passivant to control the size of the tetradentate porphyrin-AuNPs. The porphyrin-coated AuNPs were characterized by UV–Vis, TEM, XRD, and XPS analyses. The tetradentate porphyrin-AuNPs size is within a range of 5–15 nm in diameter with exotic shapes. The plausible network formation for AuNP@p-TPP-SAc and the capping structure of the AuNP@m-TPP-SAc have been suggested.     

Ultrasonic-assisted spark plasma discharge for gold nanoparticles synthesis

We examine the ultrasonic-assisted spark discharge in gold nanoparticles production. A 1 kHz pulsed power supply is used to generate spark discharge plasma. The appropriate range of peak power (pulse peak current) for nanoparticles production was determined by the Dynamic Light Scattering (DLS) method. Colloidal gold nanoparticles were synthesized in 100 mL deionized water and pure ethanol as liquid dielectrics. Scanning Electron Microscopy (SEM) micrographs show that the nanoparticles diameter in the deionized water is larger than in pure ethanol. Transmission Electron Microscopy (TEM) micrographs of the gold nanoparticles also confirm the SEM results with more accuracy. The present study revealed that the ultrasonic wave increases the shape uniformity of the nanoparticles and decreases their size.     

Controlled synthesis and biomolecular probe application of gold nanoparticles

In addition to their optical properties, the ability of gold nanoparticles (Au NPs) to generate table immobilization of biomolecules, whilst retaining their bioactivities is a major advantage to apply them as biosensors. Optical biosensors using Au NPs are simple, fast and reliable and, recently, they have been moving from laboratory study to the point of practical use. The optical properties of Au NPs strongly depend on their size, shape, degree of aggregation and the functional groups on their surface. Rapid advances in the field of nanotechnology offer us a great opportunity to develop the controllable synthesis and modification of Au NPs as well as to study on their properties and applications. The size-controlled growth of Au NPs requires the isotropic growth on the surface of Au nuclei whereas anisotropic growth will induce the formation of Au NPs of varying shape. Functionalized Au NPs provide sensitive and selective biosensors for the detection of many targets, including metal ions, small organic compounds, protein, DNA, RNA and cell based on their optical, electrical or electrochemical signals. In this review, we will discuss the size- and shape-controlled growth and functionalization of Au NPs to obtain Au nanoprobes. The basis of the optical detection of Au nanoprobes and their applications in nucleic acid, protein detection and cell imaging are also introduced.     

Fluorescence of polystyrene microspheres during photochemical synthesis of gold nanoparticles

The results of investigation of the fluorescence of carboxylated polystyrene microspheres in the process of photochemical synthesis of gold nanoparticles are presented. The introduction of an aqueous solution of hydrogen tetrachloroaurate (III) into an unirradiated polymer dispersion causes a decay of the excimer fluorescence in polystyrene at 335 nm. UV irradiation (λ_exc = 254 nm) of polystyrene dispersions in the presence of AuCl ₄ ^? ions leads to the formation of metal particles, which occurs after an induction period and affects the polystyrene fluorescence.     

Synthetic antibacterial agent assisted synthesis of gold nanoparticles: Characterization and application studies

In this study, we report synthesis of water-soluble gold nanoparticles (Au-NPs), having an average diameter of ca. ∼20 nm, using ciprofloxacin (CF) as a reducing/stabilizing agent. The synthesized Au-NPs have been characterized by scanning electron microscopy (SEM), EDX, TEM, UV-visible spectroscopy (UV-vis), X-ray diffraction and cyclic voltammetry. TEM and SEM combined with EDX analysis confirmed that spherical-shaped Au-NPs were formed. UV-vis spectra of the Au-NPs showed two absorption bands corresponding to the capping agent ciprofloxacin and surface plasmon absorption bands at 274 and 527 nm, respectively. The synthesized Au-NPs are used to modify a glassy carbon electrode (GCE) and its electrochemical and electrocatalytic properties are investigated. The Au-NPs modified electrode showed excellent electrocatalytic activity towards the oxidation of methanol at +0.33 V in alkaline solution, which was not observed on the unmodified GCE. Further, electrocatalytic reduction of oxygen was also studied using the Au-NPs modified electrode at lower potential. Here, CF was used as a reducing agent for the preparation of Au-NPs dispersion. This Au-NPs dispersion is highly stable, and can be stored for more than three months in air at room temperature.     

Pulsed spark-discharge assisted synthesis of colloidal gold nanoparticles in ethanol

A green method, using pulsed spark-discharge (PSD) to synthesize gold nanoparticles (AuNPs) in ethanol, is studied in this article. Unlike conventional methods for metal nanoparticles synthesis, the PSD method does not require the addition of chemical surfactants and stabilizers. The size of PSD–AuNPs is examined by transmission electron microscopy, with a range 5–50 nm. The chemical compounds, crystal structure, and surface plasmon resonance of PSD–AuNPs are studied using energy dispersive X-ray spectroscopy, X-ray diffraction, and UV–Visible spectroscopy, respectively. Zeta potential analysis shows that a negative charge (−40 mV) on the surface of the PSD–AuNPs may be contributing to the stability of the suspension. During the gold electrodes discharge in the ethanol, under an intensive electric field and thermal energy, bulk metallic gold and ethanol may produce AuNPs and varieties of chemical derivatives, which are also studied by GC/MS and FTIR to investigate the suspension mechanism. The analysis results show that there is an oxidation reaction of ethanol occurring during the PSD process to produce ethanol derivatives, such as acetaldehyde, acetic acid, and ethyl acetate, which may modify the surface of AuNPs by coordination of oxygen atoms. However, only acetic acid can form a negative charge by the deprotonation of the carboxylic group of surface in ethanol, resulting in the creation of a repulsion force between the particles to form the stable colloid system. The experimental results indicate that PSD is an alternative green process to synthesize gold nanoparticles suspension in ethanol. Moreover, with a gold rod consumption rate of 15 mg/L, concentrations of gold nanoparticles ~9 ppm have been observed; therefore, the net production rate is around 60%.     

金纳米棒核／二氧化硅壳纳米复合结构的可控制备及细胞成像

采用模板合成以及溶胶凝胶方法制备了金纳米棒核／二氧化硅壳（GNR@SiO2）纳米复合粒子,探讨了这种新型纳米复合结构的可控制备、光谱性质、细胞毒性和细胞成像。通过紫外可见分光光度计、透射电镜、共聚焦显微镜对样品进行表征,结果表明：通过对反应时间的调控,获得的纳米复合粒子的二氧化硅壳层厚度可以控制在20～30nm。由于二氧化硅壳层的存在,大大提高了金纳米棒的稳定性,同时降低了金纳米棒的细胞毒性;此外,由于二氧化硅壳层具有良好的化学修饰作用,因此可以将荧光探针分子标记在二氧化硅壳层表面,修饰后的纳米复合粒子可以通过细胞内吞作用进入细胞,从而实现细胞内的光学成像。因此,该纳米粒子复合材料在生物传感、细胞成像以及光热治疗等方面有着良好的应用前景。     

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.     

Effect of ultrasonic irradiation power on sonochemical synthesis of gold nanoparticles

In this work, optimized size distribution and optical properties in the colloidal synthesis of gold nanoparticles (GNPs) were obtained using a proposed ultrasonic irradiation assisted Turkevich-Frens method. The effect of three nominal ultrasound (20 kHz) irradiation powers: 60, 150, and 210 W have been analyzed as size and shape control parameters. The GNPs colloidal solutions were obtained from chloroauric acid (HAuCl₄) and trisodium citrate (C₆H₅Na₃O₇·2H₂O) under continuous irradiation for 1 h without any additional heat or stirring. The surface plasmon resonance (SPR) was monitored in the UV–Vis spectra every 10 min to found the optimal time for localized SPR wavelength (${\lambda }_{\mathrm{L}\mathrm{S}\mathrm{P}\mathrm{R}}$), and the 210 sample procedure has reduced the ${\lambda }_{\mathrm{L}\mathrm{S}\mathrm{P}\mathrm{R}}$ localization at 20 min, while 150 and 60 samples have showed ${\lambda }_{\mathrm{L}\mathrm{S}\mathrm{P}\mathrm{R}}$ at 60 min. The nucleation and growth of GNPs showed changes in shape and size distribution associated with physical (cavitation, temperature) and chemical (radical generation, pH) conditions in the aqueous solution. The results showed quasi-spherical GNPs as pentakis dodecahedron (${\lambda }_{\mathrm{L}\mathrm{S}\mathrm{P}\mathrm{R}}$ = 560 nm), triakis icosahedron (${\lambda }_{\mathrm{L}\mathrm{S}\mathrm{P}\mathrm{R}}$ = 535 nm), and tetrakis hexahedron (${\lambda }_{\mathrm{L}\mathrm{S}\mathrm{P}\mathrm{R}}$ = 525 nm) in a size range from 12 to 16 nm. Chemical effects of ultrasound irradiation were suggested in the disproportionation process, electrons of AuCl₂⁻ are rapidly exchanged through the gold surface. After AuCl₄⁻ and Cl⁻ were desorbed, a tetrachloroaurate complex was recycled for the two-electron reduction by citrate, aurophilic interaction between complexes AuCl₂⁻, electrons exchange, and gold seeds, the deposition of new gold atoms on the surface promoting the growth of GNPs. These mechanisms are enhanced by the effects of ultrasound, such as cavitation and transmitted energy into the solution. These results show that the plasmonic response from the reported GNPs can be tuned using a simple methodology with minimum infrastructure requirements. Moreover, the production method could be easily scalable to meet industrial manufacturing needs.     

Optimization of components in high-yield synthesis of block copolymer-mediated gold nanoparticles

The optimization to achieve stable and high-yield gold nanoparticles in block copolymer-mediated synthesis has been examined. Gold nanoparticles are synthesized using block copolymer P85 in gold salt HAuCl₄·3H₂O solution. This method usually has a very limited yield which does not simply increase with the increase in the gold salt concentration. We show that the yield can be enhanced by increasing the block copolymer concentration but is limited to the factor by which the concentration is increased. On the other hand, the presence of an additional reductant (trisodium citrate) in 1:1 molar ratio with gold salt enhances the yield by manyfold. In this case (with additional reductant), the stable and high-yield nanoparticles having size about 14 nm can be synthesized at very low block copolymer concentrations. These nanoparticles thus can be efficiently used for their application such as for adsorption of proteins.     

Molecular histology analysis by matrix-assisted laser desorption/ionization imaging mass spectrometry using gold nanoparticles as matrix

Gold nanoparticles (AuNPs) were applied and optimized as matrix for matrix-assisted laser desorption/ionization mass spectrometry analysis of animal tissues, and enabled histological analysis of animal tissues at molecular level by imaging mass spectrometry (IMS). AuNPs were coated on animal tissue in a solvent-free manner via argon ion sputtering. Metabolites, including neurotransmitters, fatty acids and nucleobases, were directly detected from mouse brain tissue. Based on region-specific chemical profiles, fine histological features of mouse brain tissue and heterogeneous regions of tumor tissue were both revealed.     

Surface property modification of well-dispersed amphiphilic gold nanoparticles as individuals

Amphiphilic gold nanoparticles (AuNPs) functionalized with mixed monolayers consisting of hydrophobic and hydrophilic ligands find widespread applications in biosensing, drug delivery, and bioimaging. One important aspect of amphiphilic AuNPs in such applications is the tuning of the surface properties of these AuNPs by modifying the composition of the ligands. In this study, well-dispersed AuNPs as individuals with mixed monolayers of hydrophobic and hydrophilic ligands were synthesized and the ratios of hydrophilic and hydrophobic ligands on the AuNP surfaces with varying ligand lengths were investigated by electrostatic titration. We demonstrated that longer hydrophobic ligands have higher affinity for the AuNP surface, and that the relative ligand length plays an important role in determining the maximum hydrophobic coverage on the AuNP surface at which the ratio of the amount of hydrophobic to that of hydrophilic ligands on the AuNP surface is the largest, for AuNPs to remain as individuals. We expect that the AuNPs synthesized with diverse ratios of hydrophobic and hydrophilic ligands on the surface can be useful in biological applications.

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Room-temperature synthesis of gold nanoparticles and nanoplates using Shewanella algae cell extract

Biosynthesis of spherical gold nanoparticles and gold nanoplates was achieved at room temperature and pH 2.8 when cell extract from the metal-reducing bacterium Shewanella algae was used as both a reducing and shape-controlling agent. Cell extract, prepared by sonicating a suspension of S. algae cells, was capable of reducing 1 mol/m³ aqueous AuCl₄ ⁻ ions into elemental gold within 10 min when H₂ gas was provided as an electron donor. The time interval lapsed since the beginning of the bioreductive reaction was found to be an important factor in controlling the morphology of biogenic gold nanoparticles. After 1 h, there was a large population of well-dispersed, spherical gold nanoparticles with a mean size of 9.6 nm. Gold nanoplates with an edge length of 100 nm appeared after 6 h, and 60% of the total nanoparticle population was due to gold nanoplates with an edge length of 100–200 nm after 24 h. The yield of gold nanoplates prepared with S. algae extract was four times higher than that prepared with resting cells of S. algae. The resulting biogenic gold nanoparticle suspensions showed a large variation in color, ranging from pale pink to purple due to changes in nanoparticle morphology.