Synthesis of mercaptothiadiazole-functionalized gold nanoparticles and their self-assembly on Au substrates

Gold nanoparticles (AuNPs) stabilized with mercaptothiadiazole ligands, 2,5-dimercapto-1,3,4-thiadiazole (DMT), 5-amino-2-mercapto-1,3,4-thiadiazole (AMT) and 5-methyl-2-mercapto-1,3,4-thiadiazole (MMT), were prepared by the reaction of the respective ligands with HAuCl(4) and NaBH(4) in an aqueous medium. TEM images show that the average size of AuNPs was 6.5 ± 0.5?nm, irrespective of the capping ligands. The colloidal solution of both DMT-capped AuNPs (DMT-AuNPs) and AMT-capped AuNPs (AMT-AuNPs) were highly stable for several months. However, several changes were noticed for MMT-capped AuNPs (MMT-AuNPs) after 2?h from its formation. The SPR band intensity at 518?nm decreases and the narrow SPR absorption band slowly changes into a flat absorption pattern with a broad peak from 518 to 1000?nm which was accompanied by a colour change of the solution from red to purple and then blue and thereafter unchanged. The TEM image of MMT-AuNPs after 96?h shows that most of the spherical shape of the AuNPs assembled to form a nanowire-like structure. The observed changes may be due to the absence of a strong stabilizing force on the surface of the MMT-AuNPs. The amino and thiolate groups on the surface of the AMT-AuNPs and DMT-AuNPs, respectively, were directly self-assembled on Au electrodes. They exhibit excellent electrocatalytic activity towards the oxidation of AA by enhancing its oxidation current twice in addition to more than 200?mV negative shift in the oxidation potential in contrast to bare Au electrode.     

Monolayer detection on flat metal surface via surface enhanced Raman spectroscopy and sum frequency generation spectroscopy

Monolayer detection on metal surface requires ultra high sensitivity. Sum Frequency Generation Spectroscopy (SFG) and Surface Enhanced Raman Spectroscopy (SERS) are regarded as two powerful techniques with submolecular sensitivity to detect adsorbents on metal surface. However, in some cases it's still challenge to characterize molecules or groups with relatively high intramolecular symmetry, such as 4-Nitrothiophenol (4NTP), on flat metal surface even combining these two techniques. Basically, this is due to that 4NTP with para-substituted phenol groups is SFG insensitive while flat metal surface is unfavorable to yield strong SERS enhancement. In this concern, a simple and efficient method, silver mirror method, was employed to facilitate the detection of 4NTP SAM on flat gold surface. Silver nanopheres with diameters around 300 nm was fabricated through silver mirror reaction and in situ formed milky overlayer on top of 4NTP SAM adsorbed on gold surface. Significant enhancement on SERS signal can be achieved with such special assembly structure of the "metal-molecule-metal" system. Generally, the silver mirror method provided a complementary approach to facilitate the spectroscopic applications of molecule level detection on various metal surfaces in situ.     

Facile fabrication of gold nanoparticle arrays for efficient surface-enhanced Raman scattering

An effective and facile method for the fabrication of a surface-enhanced Raman scattering (SERS)-active film with closely packed gold nanoparticle (AuNP) arrays is proposed by self-assembly of different sizes (16, 25, 40 and 70?nm) of AuNPs at a toluene/water interface with ethanol as the inducer. The as-prepared AuNP arrays exhibit efficient Raman scattering enhancement, and the enhancement factors estimated using p-aminothiophenol as a probe molecule range from 10(5) to 10(7). This is attributed to the coupling electromagnetic SERS enhancement mechanism with additional localization field within closely packed AuNPs, which have greater SERS activity and reproducibility than that on aggregates and on self-assembled monolayers of isolated AuNPs on glass.     

Synthesis, characterization and electrocatalytic activity of fused gold nanoparticles

This paper describes the synthesis of fused spherical gold nanoparticles (AuNPs) and their electrocatalytic activity towards the oxidation of hydroxylamine (HA). Fused AuNPs were prepared by one-pot synthesis using 2-mercapto-4-methyl-5-thiazoleacetic acid (TAA) as a stabilizing agent and sodium borohydride (NaBH4) as a reducing agent. The HR-TEM images showed that two individual AuNP were joined via on its surface with a size range of approximately 7 nm and a length of approximately 15 nm diameter. The pH studies showed that the synthesized fused AuNPs was stable at pH > 8. This indicated that the carboxylate ion present on the TAA molecule stabilized the AuNPs from aggregation. Further, the fused AuNPs were utilized for the electrocatalytic oxidation of hydroxylamine (HA) after immobilized them on (3-mercaptopropyl)-trimethoxysilane (MPTS) sol-gel film modified Au electrode. The AuNPs modified electrode showed an excellent electrocatalytic activity towards the oxidation of HA in 0.2 M phosphate buffer solution (pH 7.2) by shifting its oxidation potential to 100 mV less positive and enhancing its oxidation current for more than three times when compared to bare Au electrode. Further, it was found that the fused AuNPs modified electrode showed greater electrocatalytic activity towards HA than the spherical AuNPs modified electrodes.     

Quantitative analysis of adsorbed proteins by X-ray photoelectron spectroscopy

Protein adsorption at solid surfaces is central to many phenomena of medical and technological interest. The determination of the amount of protein attached to the surface is a critical measurement performed by using a wide range of methods. X-ray photoelectron spectroscopy (XPS) is able to provide a straightforward quantitative analysis of the amount of protein adsorbed as an overlayer on a material surface. While XPS is commonly employed to assess qualitatively the amount of adsorbed protein, this is usually expressed in terms of the elemental fraction (or at. %) of nitrogen calculated using an assumption of depth homogeneity despite the fact that this does not linearly scale with the amount of protein. In this paper, we have shown that thicknesses derived from XPS data linearly correlated with spectroscopic ellipsometry data on the same samples with a scatter of 10%. A straightforward equation to convert the concentration of nitrogen from XPS into an equivalent thickness of a protein film is presented. We highlight some discrepancies in the absolute thicknesses determined by XPS and ellipsometry on dried films and quartz crystal microbalance on wet films, which appear likely to result from the inclusion of a contribution from water in the latter two techniques.     

Chemical alignment of DNA origami to block copolymer patterned arrays of 5 nm gold nanoparticles

We have used block copolymer patterned arrays of 5 nm gold nanoparticles (AuNPs) for chemically aligned surface attachment of DNA origami. Addition of single-stranded DNA-thiol to AuNPs allowed a base paired attachment of sticky end modified DNA origami. Results indicate a stable, selective attachment between the DNA origami and ssDNA modified AuNPs. Yield data showed 74% of AuNP binding sites forming an attachment with a DNA origami rectangle, and control surfaces showed less than 0.5% nonspecific adsorption.     

TGF‐β1 Conjugated to Gold Nanoparticles Results in Protein Conformational Changes and Attenuates the Biological Function

Gold nanoparticles (AuNPs) are widely used as carriers or therapeutic agents due to their great biocompatibility and unique physical properties. Transforming growth factor‐beta 1 (TGF‐β1), a member of the cysteine‐knot structural superfamily, plays a pivotal role in many diseases and is known as an immunosuppressive agent that attenuates immune response resulting in tumor growth. The results reported herein reflect strong interactions between TGF‐β1 and the surface of AuNPs when incubated with serum‐containing medium, and demonstrate a time‐ and dose‐dependent pattern. Compared with other serum proteins that can also bind to the AuNP surface, AuNP–TGFβ1 conjugate is a thermodynamically favored compound. Epithelial cells undergo epithelial–mesenchymal transition (EMT) upon treatment with TGF‐β1; however, treatment with AuNPs reverses this effect, as detected by cell morphology and expression levels of EMT markers. TGF‐β1 is found to bind to AuNPs through S–Au bonds by X‐ray photoelectron spectroscopy. Fourier transform infrared spectroscopy is employed to analyze the conformational changes of TGF‐β1 on the surface of AuNPs. The results indicate that TGF‐β1 undergoes significant conformational changes at both secondary and tertiary structural levels after conjugation to the AuNP surface, which results in the deactivation of TGF‐β1 protein. An in vivo experiment also shows that addition of AuNPs attenuates the growth of TGF‐β1‐secreting murine bladder tumor 2 cells in syngeneic C3H/HeN mice, but not in immunocompromised NOD‐SCID mice, and this is associated with an increase in the number of tumor‐infiltrating CD4⁺ and CD8⁺ T lymphocytes and a decrease in the number of intrasplenic Foxp3(+) lymphocytes. The findings demonstrate that AuNPs may be a promising agent for modulating tumor immunity through inhibiting immunosuppressive TGF‐β1 signaling.     

Gold-nanoparticle-functionalized In₂O₃ nanowires as CO gas sensors with a significant enhancement in response

We present the room-temperature sensing of gold nanoparticle (AuNP)-functionalized In(2)O(3) nanowire field-effect transistor (NW-FET) for low-concentration CO gas. AuNPs were functionalized onto In(2)O(3) nanowires via a self-assembled monolayer of p-aminophenyltrimethoxysilane (APhS-SAM). The nanowires were mounted onto the Au electrodes with both ends in Schottky contacts. High sensor response toward low concentration of CO gas (200 ppb-5 ppm) at room temperature is achieved. The presence of AuNPs on the surface of In(2)O(3) nanowire serves to enhance the CO oxidation due to a higher oxygen ion-chemisorption on the conductive AuNP surfaces. Detailed studies showed that the sensing capabilities were greatly enhanced in comparison to those of bare nanowires or low coverage of Au NP-decorated nanowires. When the sensor is exposed to CO, the CO molecules interact with the preadsorbed oxygen ions on the AuNP surface. The CO oxidation on the AuNPs leads to the transfer of electrons into the semiconducting In(2)O(3) nanowires and this is reflected as the change in conductance of the NW-FET sensor. This work provides a promising approach for fabricating nanowire devices with excellent sensing capabilities at room temperature.     

Optical Detection of Human Papillomavirus Type 16 and Type 18 by Sequence Sandwich Hybridization With Oligonucleotide-Functionalized Au Nanoparticles

The importance of detecting and subtyping human papillomaviruses (HPVs) in clinical and epidemiological studies has been well addressed. In detecting the most common types of HPV, type 16 (HPV-16) and type 18 (HPV-18), in the cervical mucous of patients in a simple and rapid manner, the assay of a label-free colorimetric DNA sensing method based on sequence sandwich hybridization with oligonucleotide-functionalized Au nanoparticles (AuNPs) was fabricated in this study. Specific oligonucleotide probes were designed for the sequence detection within the L1 gene of HPV-16 and HPV-18, and the probes were capped onto AuNPs, as AuNP probes. The target HPV sequences in clinical specimens were obtained by an asymmetric polymerase chain reaction (PCR) with universal primers, which can amplify the target sequences from several HPV serotypes, including HPV-16 and HPV-18. The DNA sandwich hybridization between the target sequences and the specific AuNP probes was performed at a temperature closer to the theoretical melting temperature of the DNA hybridization. Next, the procedure of increasing salt concentration and cooling the hybridizing solution was immediately utilized to discriminate the target sequences of HPV-16 or HPV-18. If the target sequences were not complementary to sequences of AuNP probes, the AuNPs would aggregate because no duplex DNA formation occurred such that the color of the reaction solution changed from red to purple. If the AuNP probes were a perfect match to the target sequences and a full DNA sandwich hybridization occurred, the reaction solution maintained its red color. A total of 70 mucous specimens from patients with cervical intraepithelial neoplasia were tested by the AuNP probes sandwich hybridization.      

Cytotoxicity of Ultrasmall Gold Nanoparticles on Planktonic and Biofilm Encapsulated Gram‐Positive Staphylococci

The emergence of multidrug resistant bacteria, especially biofilm‐associated Staphylococci, urgently requires novel antimicrobial agents. The antibacterial activity of ultrasmall gold nanoparticles (AuNPs) is tested against two gram positive: S. aureus and S. epidermidis and two gram negative: Escherichia coli and Pseudomonas aeruginosa strains. Ultrasmall AuNPs with core diameters of 0.8 and 1.4 nm and a triphenylphosphine‐monosulfonate shell (Au0.8MS and Au1.4MS) both have minimum inhibitory concentration (MIC) and minimum bactericidal concentration of 25 × 10^?6m [Au]. Disc agar diffusion test demonstrates greater bactericidal activity of the Au0.8MS nanoparticles over Au1.4MS. In contrast, thiol‐stabilized AuNPs with a diameter of 1.9 nm (AuroVist) cause no significant toxicity in any of the bacterial strains. Ultrasmall AuNPs cause a near 5 log bacterial growth reduction in the first 5 h of exposure, and incomplete recovery after 21 h. Bacteria show marked membrane blebbing and lysis in biofilm‐associated bacteria treated with ultrasmall AuNP. Importantly, a twofold MIC dosage of Au0.8MS and Au1.4MS each cause around 80%–90% reduction in the viability of Staphylococci enveloped in biofilms. Altogether, this study demonstrates potential therapeutic activity of ultrasmall AuNPs as an effective treatment option against staphylococcal infections.     

In vivo Gold Nanoparticle Delivery of Peptide Vaccine Induces Anti‐Tumor Immune Response in Prophylactic and Therapeutic Tumor Models

Gold nanoparticles (AuNPs) are promising vehicles for cancer immunotherapy, with demonstrated efficacy in immune delivery and innate cell stimulation. Nevertheless, their potential has yet to be assessed in the in vivo application of peptide cancer vaccines. In this study, it is hypothesized that the immune distribution and adjuvant qualities of AuNPs could be leveraged to facilitate delivery of the ovalbumin (OVA) peptide antigen and the CpG adjuvant and enhance their therapeutic effect in a B16‐OVA tumor model. AuNP delivery of OVA (AuNP‐OVA) and of CpG (AuNP‐CpG) enhanced the efficacy of both agents and induced strong antigen‐specific responses. In addition, it is found that AuNP‐OVA delivery alone, without CpG, is sufficient to promote significant antigen‐specific responses, leading to subsequent anti‐tumor activity and prolonged survival in both prophylactic and therapeutic in vivo tumor models. This enhanced therapeutic efficacy is likely due to the adjuvant effect of peptide coated AuNPs, as they induce inflammatory cytokine release when cultured with bone marrow dendritic cells. Overall, AuNP‐mediated OVA peptide delivery can produce significant therapeutic benefits without the need of adjuvant, indicating that AuNPs are effective peptide vaccine carriers with the potential to permit the use of lower and safer adjuvant doses during vaccination.     

Gold nanoparticles reduced in situ and dispersed in polymer thin films: optical and thermal properties

Optical and thermal activity of plasmon-active nanoparticles in transparent dielectric media is of growing interest in thermal therapies, photovoltaics and optoelectronic components in which localized surface plasmon resonance (LSPR) could play a significant role. This work compares a new method to embed gold nanoparticles (AuNPs) in dense, composite films with an extension of a previously introduced method. Microscopic and spectroscopic properties of the two films are related to thermal behavior induced via laser excitation of LSPR at 532?nm in the optically transparent dielectric. Gold nanoparticles were incorporated into effectively nonporous 680?μm thick polydimethylsiloxane (PDMS) films by (1) direct addition of organic-coated 16?nm nanoparticles; and (2) reduction of hydrogen tetrachloroaurate (TCA) into AuNPs. Power loss at LSPR excitation frequency and steady-state temperature maxima at 100?mW continuous laser irradiation showed corresponding increases with respect to the mass of gold introduced into the PDMS films by either method. Measured rates of temperature increase were higher for organic-coated NP, but higher gold content was achieved by reducing TCA, which resulted in larger overall temperature changes in reduced AuNP films.     

PEI-conjugated AuNPs as a sensing platform for arsenic (AS-III)

We have developed a simple method for synthesis of spherical gold nanoparticles (AuNPs) with enhanced surface properties. The polyethyleneimine (PEI) has good potential to minimise the size of the precursor. The UV–vis spectra of synthesised AuNPs with reducing agents (PEI) have been characterised with a peak at 530?nm. The size and shape measurement of AuNPs was confirmed by transmission electron microscopy (TEM) which shows that the mean diameter is 3.9?nm. The optimal concentration of reducing agents was found to be 1% for synthesis of AuNPs. PEI-conjugated AuNP shows binding with arsenic III (0.1?ppm) as confirmed by scanning electron microscope (SEM)/energy dispersive X-rays mapping. TEM revealed the particle shape and size. Zeta potential, zeta deviation, effective particle size, Z-average diameter, polydispersity index and electrophoretic mobility have been observed in order to understand the stability of AuNPs. The image of SEM confirmed that As (III) particles were eventually distributed in PEI-conjugated AuNPs matrix. Further, this study demonstrated that PEI-conjugated AuNPs is a sensing platform of As (III).     

A structural mass spectrometry strategy for the relative quantitation of ligands on mixed monolayer-protected gold nanoparticles

It is becoming increasingly common to use gold nanoparticles (AuNPs) protected by a heterogeneous mixture of thiolate ligands, but many ligand mixtures on AuNPs cannot be properly characterized due to the inherent limitations of commonly used spectroscopic techniques. Using ion mobility-mass spectrometry (IM-MS), we have developed a strategy that allows measurement of the relative quantity of ligands on AuNP surfaces. This strategy is used for the characterization of three samples of mixed-ligand AuNPs: tiopronin:glutathione (av diameter 2.5 nm), octanethiol:decanethiol (av diameter 3.6 nm), and tiopronin:11-mercaptoundecyl(poly ethylene glycol) (av diameter 2.5 nm). For validation purposes, the results obtained for tiopronin:glutathione AuNPs were compared to parallel measurements using nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS) without ion mobility separation. Relative quantitation measurements for NMR and IM-MS were in excellent agreement, with an average difference of less than 1% relative abundance. IM-MS and MS without ion mobility separation were not comparable, due to a lack of ion signals for MS. The other two mixed-ligand AuNPs provide examples of measurements that cannot be performed using NMR spectroscopy.     

Facile synthesis of anisotropic gold nanoparticles and its synergistic effect on breast cancer cell lines

Gold nanoparticles (AuNPs) possess colourful light‐scattering properties due to different composition, size and shape. Their unique physical, optical and chemical properties coupled with advantages, have increased the scope of anisotropic AuNPs in various fields. This study reports a green methodology developed for the synthesis of anisotropic AuNPs. The aqueous extracts of Alternanthera sessilis (PGK), Portulaca oleracea (PAK) and Sterculia foetida (SF) with gold ions produced violet, purple and pink coloured AuNPs, respectively, under sonication and room temperature methods revealing the formation of different shapes of AuNPs. The results of TEM analysis of AuNPs confirmed the formation of triangular plate AuNPs of the size 35 nm for PAK extract. Spherical‐shaped AuNPs (10–20 nm) were obtained using an extract of PGK. SF extract produced rod, hexagon, pentagon‐shaped AuNPs and nanorice gold particles. The cell viability studies of the PGK, PAK and SF‐mediated AuNPs on MCF‐7 cell lines by MTT assay revealed the cytotoxic activity of AuNPs to depend on the size, shape and the nature of capping agents. The synthesised AuNPs significantly inhibited the growth of cancer cells (MCF‐7) in a concentration‐dependent manner. The size and shape of these anisotropic AuNPs also reveal its potency to be used as sensors, catalysis, photothermal and therapeutic agents.Inspec keywords: toxicology, gold, transmission electron microscopy, catalysis, nanofabrication, biomedical materials, nanomedicine, particle size, cellular biophysics, nanoparticles, cancer, biological organsOther keywords: Au, size 10.0 nm to 20.0 nm, temperature 293.0 K to 298.0 K, size 35.0 nm, TEM analysis, Sterculia foetida, Portulaca oleracea, Alternanthera sessilis, chemical properties, colourful light‐scattering properties, anisotropic AuNP, triangular plate AuNP, spherical‐shaped AuNP, SF‐mediated AuNP, cancer cells, MCF‐7 cell lines, cell viability, nanorice gold particles, gold ions, optical properties, breast cancer cell lines, anisotropic gold nanoparticles     

Selective gold-nanoparticle-based "turn-on" fluorescent sensors for detection of mercury(II) in aqueous solution

A new gold-nanoparticle (AuNP)-based sensor for detecting Hg(II) ions in aqueous solution has been developed. Rhodamine B (RB) molecules that are highly fluorescent in bulk solution fluoresce weakly when they are adsorbed onto AuNP surfaces as a result of fluorescence resonance energy transfer and collision with AuNPs. In the presence of metal ions such as Hg(II), RB molecules are released from the AuNP surface and thus restore the florescence of RB. The modulation of the photoluminescence quenching efficiency of RB-AuNPs in the presence of Hg(II) ions can achieve a large turn-on fluorescence enhancement (400-fold) in aqueous solution, and the entire detection takes less than 10 min. We have improved the selectivity of the probe further by modifying the AuNP surfaces with thiol ligands (mercaptopropionic acid, mercaptosuccinic acid, and homocystine) and adding a chelating ligand (2,6-pyridinedicarboxylic acid) to the sample solutions. Under the optimum conditions, the selectivity of this system for Hg(II) over other metal ions in aqueous solutions is remarkably high (50-fold or more), and its LOD for Hg(II) in the matrix pond water is 2.0 ppb. Our approach demonstrated the feasibility of using the developed nanosensor for rapid determination of Hg(II) in aqueous environmental samples and in batteries.     

Structure and magnetic properties of mono- and bi-layer graphene films on ultraprecision figured 4H-SiC(0001) surfaces

Monolayer and bilayer graphene films with a few hundred nm domain size were grown on ultraprecision figured 4H-SiC(0001) on-axis and 8 degrees -off surfaces by annealing in ultra-high vacuum. Using X-ray photoelectron spectroscopy (XPS), atomic force microscopy, reflection high-energy electron diffraction, low-energy electron diffraction (LEED), Raman spectroscopy, and scanning tunneling microscopy, we investigated the structure, number of graphene layers, and chemical bonding of the graphene surfaces. Moreover, the magnetic property of the monolayer graphene was studied using in-situ surface magneto-optic Kerr effect at 40 K. LEED spots intensity distribution and XPS spectra for monolayer and bilayer graphene films could become an obvious and accurate fingerprint for the determination of graphene film thickness on SiC surface.     

Mixed Fluorinated/Hydrogenated Self‐Assembled Monolayer‐Protected Gold Nanoparticles: In Silico and In Vitro Behavior

Gold nanoparticles (AuNPs) covered with mixtures of immiscible ligands present potentially anisotropic surfaces that can modulate their interactions at complex nano–bio interfaces. Mixed, self‐assembled, monolayer (SAM)‐protected AuNPs, prepared with incompatible hydrocarbon and fluorocarbon amphiphilic ligands, are used here to probe the molecular basis of surface phase separation and disclose the role of fluorinated ligands on the interaction with lipid model membranes and cells, by integrating in silico and experimental approaches. These results indicate that the presence of fluorinated amphiphilic ligands enhances the membrane binding ability and cellular uptake of gold nanoparticles with respect to those coated only with hydrogenated amphiphilic ligands. For mixed monolayers, computational results suggest that ligand phase separation occurs on the gold surface, and the resulting anisotropy affects the number of contacts and adhesion energies with a membrane bilayer. This reflects in a diverse membrane interaction for NPs with different surface morphologies, as determined by surface plasmon resonance, as well as differential effects on cells, as observed by flow cytometry and confocal microscopy. Overall, limited changes in monolayer features can significantly affect NP surface interfacial properties, which, in turn, affect the interaction of SAM‐AuNPs with cellular membranes and subsequent effects on cells.     

Electrophoretic Build‐Up of Alternately Multilayered Films and Micropatterns Based on Graphene Sheets and Nanoparticles and their Applications in Flexible Supercapacitors

Graphene nanosheets and metal nanoparticles (NPs) have been used as nano‐building‐blocks for assembly into macroscale hybrid structures with promising performance in electrical devices. However, in most graphene and metal NP hybrid structures, the graphene sheets and metal NPs (e.g., AuNPs) do not enable control of the reaction process, orientation of building blocks, and organization at the nanoscale. Here, an electrophoretic layer‐by‐layer assembly for constructing multilayered reduced graphene oxide (RGO)/AuNP films and lateral micropatterns is presented. This assembly method allows easy control of the nano‐architecture of building blocks along the normal direction of the film, including the number and thickness of RGO and AuNP layers, in addition to control of the lateral orientation of the resultant multilayered structures. Conductivity of multilayered RGO/AuNP hybrid nano‐architecture shows great improvement caused by a bridging effect of the AuNPs along the out‐of‐plane direction between the upper and lower RGO layers. The results clearly show the potential of electrophoretic build‐up in the fabrication of graphene‐based alternately multilayered films and patterns. Finally, flexible supercapacitors based on multilayered RGO/AuNP hybrid films are fabricated, and excellent performance, such as high energy and power densities, are achieved.     

Synthesis and characterization of sol-gel silica films doped with size-selected gold nanoparticles

Homogeneous nanocomposite silica films uniformly doped with size-selected gold nanoparticles (AuNPs) have been prepared by a combined use of colloidal chemistry and the sol-gel process. For this purpose, stable thiol-functionalized AuNPs (DDT-AuNPs) were first synthesized by a two-phase aqueous/organic system and, subsequently, dispersed in an acid-catalysed sol-gel silica solution. The microstructural morphology of the samples was investigated by x-ray diffraction and field emission scanning electron microscopy. X-ray photoelectron spectroscopy (XPS) and UV-vis optical spectrophotometry were instead employed to investigate the elemental chemical behaviour and the evolution of the surface plasmon resonance (SPR) band of the AuNPs from their synthesis up to the formation of the Au-doped silica films. The results show that the size, shape and crystalline domains of the AuNPs remain unchanged during the entire preparation process, indicating that their aggregation or decomposition was prevented. XPS results show that the DDT-AuNPs lose the capping shells and oxidize themselves when dispersed in acid-catalysed sol-gel solutions, and that bare AuNPs are embedded in the SiO(2) films. A large broadening of the SPR band, observed for systems with DDT-AuNPs, suggests the presence of interface effects which cause a surface electron density lowering. Thiol chain detachment from the AuNPs determines an increase of the SPR peak intensity while the oxidation of the Au surfaces causes a red shift of its position. The latter is no longer observed in doped films, suggesting that no interfacial effects between bare AuNPs and the host medium are present.