Colloidal Au Nanoparticles Research Articles

Photosensitization of TiO2 electrodes immobilized with chiral plasmonic Au nanocolloids by circularly polarized light irradiation.

Photosensitization of semiconductors by excitation of chiral plasmonic metallic nanostructures has attracted much attention, not only for the analysis and detection of circularly polarized light but also for its potential applications in chiral photosynthesis. Although there have been reports on the detection of semiconductor-sensitized current in chiral nanostructures precisely fabricated by physical vapor deposition and/or lithography techniques, there have been no studies using plasmonic metal nanocolloids synthesized by chemical processes. In this study, we report the establishment of a fabrication method for large-area chiral photoelectrodes and the semiconductor photosensitization phenomenon realized using chiral plasmonic nanoparticles. Chiral plasmonic Au nanoparticles prepared by previously reported colloidal methods were immobilized onto a TiO2 thin film electrode by electrophoresis. When TiO2 electrodes loaded with chiral Au nanoparticles synthesized using L-cysteine were irradiated with circularly polarized light, left circularly polarized light irradiation at a wavelength of 500-600nm generated a larger anodic photocurrent than right circularly polarized light irradiation at the same wavelength. This trend was reversed for TiO2 electrodes immobilized with colloidal Au nanoparticles synthesized with D-cysteine. From these results, we conclude that the efficiency of photocurrent generation by chiral plasmon excitation can be controlled by the polarization direction of the incident light.

An overview of the unique absorption and scattering characteristics of gold nanoparticles: experimental and theoretical study

Theoretical prediction of the unusual optical properties shown by the gold (Au) nanomaterials remains challenging. Based on this fact, some colloidal Au nanoparticles (AuNPs) were prepared in ethanol and demonized water using the versatile pulse laser ablation in liquid (PLAL) method and characterized. The pulsed laser irradiation energy-dependent optical absorbance and morphology of these AuNPs were determined. Analytical Mie–Gans scattering model was applied to interpret the experimentally observed absorption spectra of these colloidal AuNPs. In addition, the AuNPs size and shape-dependent absorbance were predicted accurately by fitting the recorded spectra with the analytical model simulation. The results revealed that the dielectric function of the AuNPs played a considerable role to realize the best fit of the experimental absorption data. The average sizes of the AuNPs predicted by the proposed analytical model were tallied well with the experimentally measured ones. It is found that an accuracy of more than 8% on the particle size between experimental and predicted data. It is established that the modified Mie–Gans scattering model can provide a better understanding of the distinct absorption attributes of the colloidal AuNPs synthesized by the PLAL method in various liquid media. Highlights Some AuNPs were prepared in ethanol and deionized water using the PLAL method. The morphology and absorbance of the produced AuNPs were determined. Absorption and scattering traits of these NPs were explained via the modified Mie–Gans model. Proposed AuNPs with distinct optical traits may be beneficial for practical applications.

Thermally Reconfigurable, 3D Chiral Optical Metamaterials: Building with Colloidal Nanoparticle Assemblies.

The three-dimensional, geometric handedness of chiral optical metamaterials allows for the rotation of linearly polarized light and creates a differential interaction with right and left circularly polarized light, known as circular dichroism. These three-dimensional metamaterials enable polarization control of optical and spin excitation and detection, and their stimuli-responsive, dynamic switching widens applications in chiral molecular sensing and imaging and spintronics; however, there are few reconfigurable solid-state implementations. Here, we report all-solid-state, thermally reconfigurable chiroptical metamaterials composed of arrays of three-dimensional nanoparticle/metal bilayer heterostructures fabricated from coassemblies of phase change VO2 and metallic Au colloidal nanoparticles and thin films of Ni. These metamaterials show dynamic switching in the mid-infrared as VO2 is thermally cycled through an insulator-metal phase transition. The spectral range of operation is tailored in breadth by controlling the periodicity of the arrays and thus the hybridization of optical modes and in position through the mixing of VO2 and Au nanoparticles.

Faraday rotation enhancement for colloidal spherical Au and Ag nanoparticles and their mixtures

So far, magnetooptical (MO) properties of silver nanoparticles (AgNPs) have been studied only by magnetic circular dichroism (MCD) spectroscopy at the surface plasmon resonance (SPR) band. However, direct knowledge of the Faraday rotation effect (FR) is very important in fundamental research and applications. In the present work, we study an enhancement of FR of individual gold nanoparticles (AuNPs) and AgNPs and their mixture of colloidal aqueous suspension. The Au and Ag spherical NPs were synthesized independently as colloids in an aqueous suspension and then such mixtures were studied (AgNPs were also studied as a suspension in hexane). The noble metal NPs were characterized by TEM, UV–Vis, and refraction methods. The enhancement of FR of individual NPs was observed and it is consistent with available MCD results. For the mixtures, observed changes of magnitude of FR is small, and coupling of both SPR is rather weak. The obtained FR spectra associated with SPR band have been successfully converted to MCD spectra using the Kramers-Kronig transformation. Calculated MCD spectra are consisted with that obtained by direct measurements.

B-358 Application of Surface-Enhanced Raman Spectroscopy to Improve Sensitivity of Clinical Assays

Abstract Background Current efforts to expand the pool of available markers that can be analyzed in clinical settings is hampered by limited volumes of biomaterial-rich fluids and low concentrations of certain analytes, coupled with poor sensitivity of existing detection techniques. Although progress has been made using advanced sample preparation or assay techniques such as miRNeasy or ELISA for genomic and protein assays, respectively, little research has focused on adapting new optical methods as detection modes in the clinic. Amplified Sciences has developed an enzymatic activity assay platform using SERS detection to improve diagnostic sensitivities in multiple disease indications. Along with the experts at Wasatch Photonics, research focused on adaptation of this assay platform has enabled investigation of multiple markers of disease that had previously been undervalued or overlooked, as well as stacking of multiple markers in precious, low-volume samples. Methods Building upon previously published data using gastricsin as a marker to identify benign, non-mucinous pancreatic cysts, Amplified Sciences has added several SERS laser wavelengths, and different sample preparation methods to the existing platform. To examine the sensitivities of laser and dye combinations, solutions representative of assay samples containing rhodamine-dimer dye-labeled peptide were tested using Wasatch Photonics Raman spectrometers at 532, 638, 785, and 830 nm laser wavelengths. Samples at different pH were analyzed after addition of colloidal Au or Ag nanoparticles to determine the impacts of sample preparation on SERS spectral features. After drying in a desiccator overnight, the samples were reanalyzed to assess the stability of the SERS signals. As a pertinent application of this technology, an activity assay was developed for the known COVID protease 3ClPro. Release of rhodamine dimer dye as a result of 3ClPro cleavage of the substrate was quantified using SERS. The data were analyzed using Bruker OPUS Software package, and plotted using Graphpad Prism 9. Results Each laser, buffer, and metal used for SERS provided a unique spectral signature, suggesting that different transitions are available and potentially tunable depending on the application under investigation. Sensitivity was, as expected, best with the 532 nm SERS system purportedly due to enhancement factors, and the resonance impacts of rhodamine dyes at 532 nm. It was found that the Limit of Detection of Rhodamine-dimer dye was 100-fold lower with SERS compared to fluorescence, even after a 3-fold dilution with colloidal Ag. However, the remaining lasers could be more finely tuned, and, thus, provide alternate spectral properties, and different sensitivities which has implications when new dye classes are introduced into the platform. Furthermore, when SERS was applied to the 3ClPro assay, it was determined that activity could be detected below the dimerization concentration of 3ClPro, described in the literature as ∼25 nM. Also, a monomeric version of 3ClPro that is expected to have no reactivity with the substrate was found to display activity above background at concentrations as low as 1 µM. Conclusion SERS is an appropriate method for measuring activity in clinically relevant markers of disease, with tunable characteristics based upon the dye composition and the spectrometer in use.

Gold nanostars loaded fabrics as flexible, low-cost SERS substrates and swab-based detection of thiram on fruits and vegetables

Currently, flexible surface-enhanced Raman spectroscopy (SERS) substrates have gained tremendous interest because they enable sample collection from uneven surfaces in real sensing applications. Herein, two different flexible textiles (cotton and synthetic fabrics) loaded with star-shaped gold (Au) nanoparticles (NPs) were utilized for detailed studies and are demonstrated as SERS substrates. Both fabrics were soaked in Au NPs synthesized via a simple wet chemical method. The colloidal star-shaped Au NPs were characterized by UV-visible absorption and transmission electron microscopy techniques. The C- and S-fabrics loaded with Au NPs were characterized by FESEM, x-ray diffraction, and x-ray photoelectron spectroscopic techniques. Subsequently, the SERS efficiency of C- and S-fabrics was examined with thiram, methylene Blue (MB), and Nile Blue (NB) molecules. Furthermore, pesticide (thiram) residues on the surfaces of a banana, an apple, and a spinach leaf were investigated by a simple swabbing method. The utility of these substrates is demonstrated by performing the SERS measurements using a portable Raman spectrometer, which is expedient for real-time applications.

Methyl orange-modified Au nanoparticles supported on multiwalled carbon nanotubes as an electrocatalyst for methanol oxidation

The most stable metal-gold (Au) is not usually used as a catalyst component. To explore the catalytic activity of Au, Au colloidal nanoparticles were synthesized by a photochemical method and modified by methyl orange (MO) into modified Au colloidal nanoparticles solution (Au-MO). Then, Au-MO/multiwalled carbon nanotubes (Au-MO/MWCNTs) catalysts were obtained by supporting the Au-MO on the MWCNTs. The structure characterization revealed that MO attracts the electrons of Au, and electron transfer occurs from the Au nanoparticles to MO in the Au-MO/MWCNTs catalyst. Electrochemical measurements demonstrated that Au-MO/MWCNTs have a distinctly higher electrocatalytic activity for methanol oxidation than that of the Au/MWCNT catalyst. It was found that the improved electrocatalytic activity of the Au-MO/MWCNTs is derived from the electron transfer from the Au nanoparticles to MO.

In Situ Observations Reveal the Five-fold Twin-Involved Growth of Gold Nanorods by Particle Attachment.

Crystallization plays a critical role in determining crystal size, purity and morphology. Therefore, uncovering the growth dynamics of nanoparticles (NPs) atomically is important for the controllable fabrication of nanocrystals with desired geometry and properties. Herein, we conducted in situ atomic-scale observations on the growth of Au nanorods (NRs) by particle attachment within an aberration-corrected transmission electron microscope (AC-TEM). The results show that the attachment of spherical colloidal Au NPs with a size of about 10 nm involves the formation and growth of neck-like (NL) structures, followed by five-fold twin intermediate states and total atomic rearrangement. The statistical analyses show that the length and diameter of Au NRs can be well regulated by the number of tip-to-tip Au NPs and the size of colloidal Au NPs, respectively. The results highlight five-fold twin-involved particle attachment in spherical Au NPs with a size of 3-14 nm, and provide insights into the fabrication of Au NRs using irradiation chemistry.

Influence of stabilisers on the catalytic activity of supported Au colloidal nanoparticles for the liquid phase oxidation of glucose to glucaric acid: understanding the catalyst performance from NMR relaxation and computational studies

Impact of stabilisers on the catalytic performance of supported Au colloidal nanoparticles for the liquid phase oxidation of glucose to glucaric acid.

Electrochemical Sensors Based on Au Nanoparticles Decorated Pyrene-Reduced Graphene Oxide for Hydrazine, 4-Nitrophenol and Hg2+ Detection in Water.

Monitoring hazardous chemical compounds such as hydrazine (N2H4), 4-nitrophenol (4-NP) and Hg2+ in natural water resources is a crucial issue due to their toxic effects on human health and catastrophic impact on the environment. Electrochemical nanostructured platforms integrating hybrid nanocomposites based on graphene derivatives and inorganic nanoparticles (NPs) are of great interest for such a purpose. In this work, disposable screen-printed carbon electrodes (SPCEs) have been modified with a hybrid nanocomposite formed by reduced graphene oxide (RGO), functionalized by 1-pyrene carboxylic acid (PCA), and decorated by colloidal Au NPs. These hybrid platforms have been tested for the electrocatalytic detection of N2H4 and 4-NP by differential pulse voltammetry and have been modified with an electropolymerized film of Hg2+ ions imprinted polycurcumin for the electroanalytical detection of Hg2+ by DPV. LODs, lower and in line with the lowest ones reported for state-of-the-art electrochemical sensors, integrating similar Au-graphene < nanocomposites, have been estimated. Additionally, good repeatability, reproducibility, and storage stability have been assessed, as well as a high selectivity in the presence of a 100-fold higher concentration of interfering species. The applicability of the proposed platforms for the detection of the compounds in real complex matrices, such as tap and river water samples, has been effectively demonstrated.

Rapid detection and identification of fungi in grain crops using colloidal Au nanoparticles based on surface-enhanced Raman scattering and multivariate statistical analysis.

Grain crops are easily contaminated by fungi due to the existence of various microorganisms in the storage process, especially in humid and warm storage conditions. Compared with conventional methods, surface-enhanced Raman scattering (SERS) has paved the way for the detection of fungi in grain crops as it is a rapid, nondestructive, and sensitive analytical method. In this work, Aspergillus niger, Saccharomyces cerevisiae, Fusarium moniliforme and Trichoderma viride in grain crops were detected using colloidal Au nanoparticles and SERS. The results indicated that different fungi showed different Raman phenotypes, which could be easily characterized by SERS. Combined with multivariate statistical analysis, identification of a variety of fungi could be accomplished rapidly and accurately. This research can be applied for the rapid detection of fungi in the food and biomedical industries.

Enhanced solar light photocatalytic and antimicrobial activity of green noble metal/TiO2 nanorods

Au/TiO2 and Pt/TiO2 nanocomposites have been processed using a green method. Au and Pt colloidal nanoparticles have been primarily synthesized by mixing their corresponding metal ions with an aqueous solution of corn husk extract, followed by anchoring on the as-synthesized TiO2 nanorods. The structural and morphological properties of green-prepared nanomaterials were systematically investigated by various techniques. The UV–VIS absorption measurements confirmed the formation of colloidal Au and Pt with λmax at 550 and 345 nm, respectively. TEM results show anchoring of spherical Au particles (40 nm) on TiO2 nanorods while smaller Pt particles have been observed on Pt/TiO2 composite. It has been shown that the highest visible light harvesting capability was earned for Au/TiO2 composite due to the surface plasmon resonance (SPR) of Au nanoparticles. The photocatalytic and antimicrobial activity of the nanomaterials was investigated for disinfection of Escherichia coli under solar light irradiation. The green synthesized nanocomposites showed enhanced solar light photocatalytic and antimicrobial activity. The best photocatalytic and antimicrobial activity was obtained for Au/TiO2. This evidences the enhanced SPR of Au nanoparticles and hence an enhancement in the solar light accessible by TiO2 so that a higher amount of reactive oxygen species can be generated, which enhances photocatalytic and antibacterial activity.

Rapid Detection of Aspergillus flavus and Quantitative Determination of Aflatoxin B1 in Grain Crops Using a Portable Raman Spectrometer Combined with Colloidal Au Nanoparticles.

Aspergillus flavus and Aflatoxins in grain crops give rise to a serious threat to food security and cause huge economic losses. In particular, aflatoxin B1 has been identified as a Class I carcinogen to humans by the International Agency for Research on Cancer (IARC). Compared with conventional methods, Surface-Enhanced Raman Scattering (SERS) has paved the way for the detection of Aspergillus flavus and Aflatoxins in grain crops as it is a rapid, nondestructive, and sensitive analytical method. In this work, the rapid detection of Aspergillus flavus and quantification of Aflatoxin B1 in grain crops were performed by using a portable Raman spectrometer combined with colloidal Au nanoparticles (AuNPs). With the increase of the concentration of Aspergillus flavus spore suspension in the range of 102–108 CFU/mL, the better the combination of Aspergillus flavus spores and AuNPs, the better the enhancement effect of AuNPs solution on the Aspergillus flavus. A series of different concentrations of aflatoxin B1 methanol solution combined with AuNPs were determined based on SERS and their spectra were similar to that of solid powder. Moreover, the characteristic peak increased gradually with the increase of concentration in the range of 0.0005–0.01 mg/L and the determination limit was 0.0005 mg/L, which was verified by HPLC in ppM concentration. This rapid detection method can greatly shorten the detection time from several hours or even tens of hours to a few minutes, which can help to take effective measures to avoid causing large economic losses.

Dopamine-mediated self-assembled anisotropic Au nanoworms conjugated with MoS2 nanosheets for SERS-based sensing

Surface-enhanced Raman scattering (SERS) utilizing plasmonic metals coupled with transition metal dichalcogenide (TMD) has received increasing attention in bio-applications due to the synergistically enhanced detection sensitivity. However, there have been several issues to be addressed such as strong SERS background signals and low Raman intensities of analytes. In this study, we report a novel approach for high SERS performance based on anisotropic hybrid plasmonic nanostructures of gold nanoworms (Au NWs) conjugated with 2D Molybdenum disulfide (MoS2) nanosheets. To significantly enhance the SERS enhancement factor (EF), we optimized the gap between Au NWs and MoS2 to ~2 nm by controlling the thickness of polydopamine (PDA). Thus, giant localized electric field enhancement and synergistic coupling effect occurred in the hybrid plasmonic nanostructures, generating abundant hot spots. Consequently, SERS EF of the order of ~107 was achieved, which was 104 times higher than that of colloidal Au nanoparticles. Further, we showcase that our AuNW@MoS2 can detect the 10−10 M level thiram, a typical pesticide, under soil-pollutant muddy condition, evidencing the powerful multimodal sensory platform of our hybrid substrates. This work provides new insight into the rational design of plasmon-coupled 2D nanohybrids for high-performance SERS substrates with a premise for the practical and viable biosensing applications.

Self-Assembled Ligand-Capped Plasmonic Au Nanoparticle Films in the Kretschmann Configuration for Sensing of Volatile Organic Compounds

Films of close-packed Au nanoparticles are coupled electrodynamically through their collective plasmon resonances. This collective optical response results in enhanced light-matter interactions, which can be exploited in various applications. Here, we demonstrate their application in sensing volatile organic compounds, using methanol as a test-case. Ordered films over several cm2 were obtained by interfacial self-assembly of colloidal Au nanoparticles (~10 nm diameter) through controlled evaporation of the solvent. Even though isolated nanoparticles of this size are inherently non-scattering, when arranged in a close-packed film the plasmonic coupling results in a strong reflectance and absorbance. The in-situ tracking of vapor phase methanol concentration through UV-Vis transmission measurements of the nanoparticle film is first demonstrated. Next, in-situ ellipsometry of the self-assembled films in the Kretschmann (also known as ATR) configuration is shown to yield enhanced sensitivity, especially with phase difference measurements. Our study shows the excellent agreement between theoretical models of the spectral response of self-assembled films with experimental in-situ sensing experiments. At the same time, the theoretical framework provides the basis for the interpretation of the various observed experimental trends. Combining periodic nanoparticle films with ellipsometry in the Kretschmann configuration is a promising strategy towards highly sensitive and selective plasmonic thin-film devices based on colloidal fabrication methods for volatile organic compound (VOC) sensing applications.

Uncovering strong π-metal interactions on Ag and Au nanosurfaces under ambient conditions via in-situ surface-enhanced Raman spectroscopy

Uncovering strong π-metal interactions on Ag and Au nanosurfaces under ambient conditions via in-situ surface-enhanced Raman spectroscopy

Oxidative condensation/esterification of furfural with ethanol using preformed Au colloidal nanoparticles. Impact of stabilizer and heat treatment protocols on catalytic activity and stability

The oxidative condensation of furfural and ethanol has been studied using supported gold colloidal nanoparticles. The influence of the nature of stabilizer (polyvinylalcohol, polyvinylpyrrolidone), the choice of thermal treatment and washing for removing the stabilizer, on the catalytic performance has been evaluated. Variation of the mean gold particle size and surface coverage of the gold nanoparticles onto the support surface were achieved by modifying the stabilizer to Au weight ratio. Thus, the mean gold particle size was varied in the range of 3–8 nm. The catalytic results showed that the choice of stabilizer, the stabilizer to Au weight ratio, thermal treatment of the catalyst, affect catalytic activity and selectivity/yield to the desired products. When PVA was the chosen stabilizer, the highest yield to furan-2-acrolein (33%) was attained with a PVA to Au weight ratio of 2.4. On the contrary, when PVP was the stabilizer, the best catalytic performance was achieved in its absence, with the yield of furan-2-acrolein reaching 21%. These results showed the strong impact of stabilizer to control catalytic activity and to enhance, in the case of PVA, the yield to the target product, whereas a strong negative effect was observed with PVP. Moreover, a mild thermal treatment of the catalyst and a washing step for the removal of the stabilizer from the catalyst had a positive effect on the catalytic performance.

Determination of 5-Hydroxymethylfurfural (5-HMF) in milk products by surface-enhanced Raman spectroscopy and its simulation analysis

5-Hydroxymethylfurfural (5-HMF) is a useful indicator of thermal damage degree and freshness of milk. It is of great importance to develop simple, rapid and accurate analytical methods for the sensitive detection of 5-HMF in milk and milk-based products. In this work, surface-enhance Raman spectroscopy (SERS) was used for rapid determination of 5-HMF in processed cheese by colloidal Au nanoparticles (AuNPs) substrate synthesized by the classical solvothermal reduction method. Density functional theory (DFT) calculations were carried out to determine the vibration assignments of 5-HMF and the surface enhancement effect of AuNPs substrate. The results found that a good linear response on the AuNPs substrate for 5-HMF in the concentration range of 0.1–75 mM was established with the detection limit of 75 μM (S/N = 3). Furthermore, the present method could be applied to the determination of 5-HMF in a cheese real sample which revealed its promising application in food safety and analysis.

Temperature-Dependent Activity of Gold Nanocatalysts Supported on Activated Carbon in Redox Catalytic Reactions: 5-Hydroxymethylfurfural Oxidation and 4-Nitrophenol Reduction Comparison

In this study, the temperature-dependent activity of Au/AC nanocatalysts in redox catalytic reactions was investigated. To this end, a series of colloidal gold catalysts supported on activated carbon and titania were prepared by the sol immobilization method employing polyvinyl alcohol as a polymeric stabilizer at different hydrolysis degrees. The as-synthesized materials were widely characterized by spectroscopic analysis (XPS, XRD, and ATR-IR) as well as TEM microscopy and DLS/ELS measurements. Furthermore, 5-hydroxymethylfurfural (HMF) oxidation and 4-nitrophenol (4-NP) reduction were chosen to investigate the catalytic activity as a model reaction for biomass valorization and wastewater remediation. In particular, by fitting the hydrolysis degree with the kinetic data, volcano plots were obtained for both reactions, in which the maximum of the curves was represented relative to hydrolysis intermediate values. However, a comparison of the catalytic performance of the sample Au/AC_PVA-99 (hydrolysis degree of the polymer is 99%) in the two reactions showed a different catalytic behavior, probably due to the detachment of polymer derived from the different reaction temperature chosen between the two reactions. For this reason, several tests were carried out to investigate deeper the observed catalytic trend, focusing on studying the effect of the reaction temperature as well as the effect of support (metal–support interaction) by immobilizing Au colloidal nanoparticles on commercial titania. The kinetic data, combined with the characterization carried out on the catalysts, confirmed that changing the reaction conditions, the PVA behavior on the surface of the catalysts, and, therefore, the reaction outcome, is modified.

Simple self-organization-based synthesis of gold nanoparticle-implanted ZnO aerogels with good sensing performance to gaseous ethanol

Simple fabrication of metal-modified oxide aerogels is expected but remains challenging. This work presents a sample one-pot synthesis method for gold nanoparticle (NP) implanted ZnO (Au–ZnO) aerogels just by sequentially adding (CH3COO)2Zn and NaBH4 solutions into a pre-prepared Au colloidal solution. The typically fabricated Au–ZnO aerogels are constituted by ZnO networks implanted with uniform Au NPs. The Au NPs had a size of about 100 nm, and the ZnO nanochains in the networks were about 10 nm in thickness. Further, the proportion of the Au NPs in the final aerogels could be tuned by using different amounts of the Zn precursors. Furthermore, a mechanism based on metal oxidation and oriented connection growth (a self-organization process) has been presented for describing the formation of such Au–ZnO aerogels. In the typical formation, the Zn2+ ions first convert into ZnO beads, and then are self-organized to form networks wrapping the colloidal Au NPs under the effect of linker molecules, and this matches well with the observed experimental results. Most importantly, these Au–ZnO aerogels show great structurally enhanced gas sensing properties to gaseous ethanol compared with a pure ZnO film. They have a fast response (about 30 s), a high selectivity, and quantitative sensing to the target gas. This work has provided a simple preparation method for Au–ZnO aerogels, and also shows their great potential in gas sensing applications.