Citrate-capped Gold Nanoparticles Research Articles

Studies on interdependent optical properties of Rhodamine 6G dye and gold nanoparticles at different dilutions of aqueous solutions

Concentration dependent interaction between negatively charged citrate capped gold nanoparticles (GNP) and cationic Rhodamine 6G dye was studied from the change in localized surface plasmon resonance (LSPR) absorption due to dye induced nanoparticles aggregation, fluorescence quenching and surface enhanced Raman scattering (SERS) of the dye. The optical properties resulting from their interaction were found to be highly dependent on dilution of the GNP and concentration of the dye. Composites of up to 1μM dye concentration, relatively low for monolayer coverage on nanoparticles surface, did not result any noticeable change in LSPR wavelength and bandwidth with respect to the pure GNP solution whereas significantly high quenching of the dye fluorescence was observed. In contrast the composites with higher dye concentration were found to exhibit an additional inter-particle coupled LSPR excitation band at higher wavelengths and the characteristics of this absorption band were found to be highly influenced by the dilution of the GNP. The fluorescence quenching efficiency was found to decrease with either increasing the dye concentration or diluting the GNP. Stern–Volmer plots were found to be non-linear at low dye concentrations and almost linear at higher concentrations. The observed fluorescence quenching was attributed to the resonance energy transfer process substantiated by significant change in the excited state lifetime of the dye in the presence of the GNP. Also dye interacting nanoparticles clusters formed on a substrate via solvent evaporation were found to be highly effective for the SERS based detection of very low concentrations of the dye.

Synergistic effect of gold nanoparticles and amine functionalized cobalt porphyrin on electrochemical oxidation of hydrazine

This paper describes the immobilization of citrate capped gold nanoparticles (cit-AuNPs) on indium tin oxide (ITO) and glassy carbon electrode (GCE) surfaces using a electroactive linker, meso-tetra(para-aminophenyl)porphyrinatocobalt(II) Co(II)MTpAP. The immobilization of cit-AuNPs on Co(II)MTpAP was achieved with the aid of amine groups present on the self-assembled monolayer (SAM) of Co(II)MTpAP. This was confirmed by UV-vis, atomic force microscopy (AFM), cyclic voltammetry, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques. The particle coverage estimated from AFM was found to be ∼65% which is higher than the previously reported linkers used for immobilization. The AFM image shows that the cit-AuNPs were densely packed on Co(II)MTpAP. Furthermore, the electrocatalytic activity of the AuNP modified electrode was examined by studying the oxidation of hydrazine. Interestingly, the cit-AuNPs immobilized on Co(II)MTpAP showed a greater catalytic effect towards hydrazine oxidation than cit-AuNPs immobilized on free base porphyrin. This was attributed to the synergistic effect arising from both AuNPs and Co(II).

Behavior of gold nanoparticles in an experimental algal–zooplankton food chain

The release of engineered nanomaterials offers a significant concern due to their unexpected behavior in biological systems. In order to establish the level of threat from releasing nanomaterials into ecosystems, simplified food webs are an effective method to determine toxicity and bioassessment. A study is presented examining the behavior of citrate-capped gold nanoparticles (AuNPs) introduced into a model food chain consisting of a phytoplankton food (Ankistrodesmus falcatus) and a zooplankton grazer (Daphnia magna). UV–Vis spectroscopy is used to monitor the behavior of AuNPs in the presence of algae (Ankistrodesmus) and Daphnia over the span of 5 days. Transmission electron microscopy shows the attachment of gold aggregates to the surface of the Ankistrodesmus. Bright field microscopy shows significant accumulation of AuNPs in the gut of Daphnia via uptake of contaminated Ankistrodesmus and directly from water. No toxicity was evident for Daphnia exposed to AuNPs at the concentration used (880 µg L−1).

Analysis of citrate-capped gold and silver nanoparticles by thiol ligand exchange capillary electrophoresis

We report on the capillary electrophoretic behavior of citrate-capped gold and silver nanoparticles in aqueous medium when applying a ligand-exchange surface reaction with thiols. Gold nanoparticles (AuNPs) and silver nanoparticles (AgNPs) of similar size (39 ± 6 and 41 ± 7 nm, respectively) and shape were synthesized, covered with a citrate shell, and characterized by microscopic and spectroscopic techniques. The analysis of these NPs by CE was accomplished by using a buffer solution (pH 9.7; 40 mM SDS, 10 mM CAPS; 0.1 % methanol) containing the anions of thioctic acid or thiomalic acid. These are capable of differently interacting with the surface of the AuNPs and AgNPs and thus introducing additional negative charges. This results in different migration times due to the formation of differently charged nanoparticles.

Direct Analysis of Triacylglycerols from Crude Lipid Mixtures by Gold Nanoparticle-Assisted Laser Desorption/Ionization Mass Spectrometry

Triacylglycerols (TAGs), essential energy storage lipids, are easily detected by conventional MALDI MS when occurring on their own. However, their signals are easily overwhelmed by other lipids, mainly phosphatidylcholines (PCs) and, therefore, require purification. In order to profile TAGs from crude lipid mixtures without prefractionation, we investigated alternative matrixes that can suppress phospholipid ion signals and enhance cationization of TAGs. We found that an aqueous solution of citrate-capped gold nanoparticles (AuNPs) with a diameter of 12nm is a superior matrix for the laser desorption/ionization mass spectrometry (LDI MS) of TAGs in crude lipid mixtures. The AuNP matrix effectively suppressed other lipid signals such as phospholipids and also provided 100 times lower detection limit for TAGs than 2,5-dihydroxybenzoic acid (DHB), the best conventional MALDI matrix for TAGs. The AuNP-assisted LDI MS enabled us to obtain detailed TAG profiles including minor species directly from crude beef lipid extracts without phospholipid interference. In addition, we could detect TAGs at a trace level from a total brain lipid extract.

PVA/Gluten Hybrid Nanofibers for Removal of Nanoparticles from Water

Recent developments in nanotechnology led to the incorporation of many nanomaterials into consumer products. Disposal of such products will lead to potential contamination of the environment. Nanomaterials are emerging contaminants in water and show significant toxicity to living systems. Considering the diversity in structure and properties, removal of nanopollutants from water warrants novel methods and materials. The objective of this study was to prepare PVA/gluten hybrid nanofibers, which are nontoxic and biodegradable adsorbents for the extraction of nanopollutants from water. Surface morphology, elemental composition, and functional groups on the fiber surface were established using microscopic and spectroscopic techniques. Influence of analytical factors such as experimental pH, time, and concentration of the pollutants toward establishing the efficiency of extraction were quantified using UV–vis spectroscopy. Nanofiber mats with 5 wt % gluten exhibited high extraction efficiency of 99% toward citrate-capped silver (Ag) and gold (Au) nanoparticles with a maximum adsorptive capacity of 31.84 mg/g for Ag NPs and 36.54 mg/g for AuNPs. The kinetic and equilibrium adsorption data were interpreted using Freundlich and Langmuir isotherm models, and a potential adsorption mechanism was suggested. The adsorption kinetics showed a pseudo-second-order model for the extraction of nanoparticles. The prepared PVA/gluten hybrid nanofibers can be utilized as an efficient low-cost adsorbents for removal and recovery of metal nanoparticles from the aqueous environment.

Gold nanoparticles assisted characterization of amine functionalized polystyrene multiwell plate and glass slide surfaces

We demonstrated citrate-capped gold nanoparticles assisted characterization of amine functionalized polystyrene plate and glass slide surfaces through AuNPs staining method. The effect of AuNPs concentration on the characterization of amine modified surfaces was also studied with different concentration of AuNPs (ratios 1.0–0.0). 3-Aminopropylyl triethoxy silane has been used as amine group source for the surface modification. The interactions of AuNPs on modified and unmodified surfaces were investigated using atomic force microscopy and the dispersibility, and the aggregation of AuNPs was analyzed using UV–visible spectrophotometer. Water contact angle measurement and X-ray photoelectron spectroscopy (XPS) were used to further confirmation of amine modified surfaces. The aggregation of AuNPs in modified multiwell plate leads to the color change from red to purple and they are found to be adsorped on the modified surfaces. Aggregation and adsorption of AuNPs on the modified surfaces through the electrostatic interactions and the hydrogen bonds were revealed by XPS analysis. Remarkable results were found even in the very low concentration of AuNPs (ratio 0.2). This AuNPs staining method is simple, cost-effective, less time consuming, and required very low concentration of AuNPs. These results can be read out through the naked eye without the help of sophisticated equipments.

Influence of Gold Nanoparticles on the Kinetics of Alpha-Synuclein Aggregation

Alpha-synuclein (AS) is a presynaptic protein lacking a unique secondary structure in solution. AS amyloid aggregates in dopaminergic neurons are the hallmark of Parkinson's disease (PD). The aggregation involves structural transitions from monomeric AS to oligomeric presumably neurotoxic and fibril formation. In spite of its importance for the understanding PD pathobiology and devising rational, targeted therapeutic strategies, details on the aggregation process remain largely unknown. Methodologies and reagents capable of controlling aggregation kinetics are essential tools for the investigation of the molecular amyloid mechanisms. In this work we investigated the influence of citrate capped gold nanoparticles on the aggregation kinetics of AS using a fluorescent probe (MFC) sensitive to the polarity of the molecular microenvironment via an excited state intramolecular proton transfer (ESIPT). The particular effects on the half time, nucleation time and growth rate were ascertained. Gold nanoparticles produced a strong acceleration, with an influence on the nucleation and growth phases of the mechanism. The effects were dependent on the size and concentration of the nanoparticles, being strongest for nanoparticles 10 nm in diameter, with a 3-fold increase in the overall aggregation rate at low concentrations as 20 nM.

A colorimetric assay for d-Penicillamine in urine and plasma samples based on the aggregation of gold nanoparticles

We report herein the development of a highly sensitive colorimetric method for detection of d-Penicillamine using citrate-capped gold nanoparticles (AuNPs). This assay relies upon the distance-dependent of gold nanoparticles surface plasmon resonance band of gold nanoparticles. By replacing the thiol-containing chelator drug, d-Penicillamine, with citrate on the gold nanoparticles surface, a new peak appearing at a longer wavelength intensifies and shifts further to the red from the original peak position due to aggregation of gold nanoparticles which depends on ionic strength, gold nanoparticles and d-Penicillamine concentration. During this process, the plasmon band at 521 nm decreases gradually along with the formation of a new red-shifted band at 630 nm. The calibration curve which is derived from the ratio intensities of absorbance at longer wavelength (630 nm) to original wavelength (521 nm) displays a linear relation in the range of 5.0 × 10−6–3.0 × 10−4 M d-Penicillamine. Lower limit of detection for d-Penicillamine, at the signal-to-noise ratio of 3 (3σ), was 3.8 × 10−6 M. The developed methodology was successfully applied for the determination of d-Penicillamine in human urine and plasma.

The strong catalytic effect of Pb(ii) on the oxygen reduction reaction on 5 nm gold nanoparticles

Citrate-capped gold nanoparticles (AuNPs) of 5 nm in diameter are synthesized via wet chemistry and deposited on a glassy carbon electrode through electrophoresis. The kinetics of the oxygen reduction reaction (ORR) on the modified electrode is determined quantitatively in oxygen-saturated 0.5 M sulphuric acid solution by modelling the cathode as an array of interactive nanoelectrodes. Quantitative analysis of the cyclic voltammetry shows that no apparent ORR electrocatalysis takes place, the kinetics on AuNPs being effectively the same as on bulk gold. Contrasting with the above, a strong ORR catalysis is found when Pb(2+) is added to the oxygen saturated solution or when the modified electrode is cycled in lead alkaline solution such that lead dioxide is repeatedly electrodeposited and stripped off on the nanoparticles. In both cases, the underpotential deposition of lead on the gold nanoparticles is found to be related to the catalysis.

Novel plasmonic field-enhanced nanoassay for trace detection of proteins

Recently discovered effects of plasmonic field on molecular fluorescence offer new insights into the optical interactions at the nanoscale which can help solving problems encountered in widely applied fluorescent labeling of biomolecules for studying life processes in biomedicine and pharmacy. In this work, we have focused on exploring a novel sensitivity-enhancing phenomenon based on protein modulation of plasmon-controlled fluorescence. We have demonstrated that a protein (cytochrome c (Cytc c) or bovine serum albumin (BSA)) can be employed to gate fluorescence resonance energy transfer occurring from a fluorescein isothiocyanate fluorescent dye to plasmonic citrate-capped gold nanoparticles. By applying plasmonic field gated by protein, facilitated by the formation of multi-shell nanoparticles, (AuNP@Cit/Cytc-FITC or AuNP@Cit/BSA-FITC), low limits of detection for Cyt c (LOD=370pM) and for BSA (LOD=1.8nM) have been achieved even for large fluorophore:protein ratios, up to 30:1 (over-labeling), normally plagued with energy migration and background fluorescence problems. Control experiments confirming adsorption of proteins on AuNPs have been performed using light scattering and piezometric techniques. The proposed nanoassay may be applied in microanalysis of trace amounts of proteins, e.g. in microfluidic devices.

Gold nanoparticle mediated designing of non-hydrolytic sol–gel cross-linked metformin imprinted polymer network: A theoretical and experimental study

A sensitive and selective electrochemical sensor based on molecularly imprinted polymers was developed for trace level detection of metformin-an antidiabetic drug. For the first time, we have applied non-hydrolytic sol-gel matrix as a cross-linking agent in the field of molecular imprinting. To create the sol-gel matrix and enhance the electro-conductivity of the proposed sensor citrate-capped gold nanoparticle were used. The morphologies and properties of the sensor were characterized by scanning electron microscopy, cyclic voltammetry, electron impedance spectroscopy, chronocoulometry and differential pulse voltammetry. Energy of the HOMO and LUMO orbitals and Mülliken's atomic charges of template molecule were also calculated using density functional theory utilizing B3LYP with 3-21G-basis set. The theoretical results allied to the diagnostic criteria of the cyclic voltammetry indicate that the metformin redox mechanism is associated to the irreversible oxidation process of metformin-imino-group to N-hydroxyimino-group. The results demonstrated that the prepared sensor had excellent selectivity and high sensitivity for metformin in the linear range from 0.02 to 80 ng ml(-1) with a detection limit of 0.005 ng ml(-1) (S/N=3). The sensor was also successfully employed to detect metformin in pharmaceutical sample.

Probing Interfacial Interactions Using Core–Satellite Plasmon Rulers

Understanding molecular interactions at the interfaces of nanoparticles is fundamentally important because they determine the stability, affinity, functionality, and assembly of nanoparticles. However, probing the governing intermolecular forces at the interfaces, particularly for the nanoparticles dispersed in solution, remains challenging. Here, we demonstrate that the interfacial interactions between citrate-capped gold nanoparticles and various molecular functional groups can be probed using a plasmon ruler, based on a well-defined core-satellite nanoassembly structure. Different nature of the interactions causes a subtle change in the interparticle distance, and the change is sensitively measured as a shift in the plasmon coupling band of the core-satellite nanoassemblies. Molecular interactions including covalent bonding, hydrogen bonding, electrostatic interactions, and van der Waals interactions are explored.

Influence of Gold Nanoparticles on the Kinetics of α-Synuclein Aggregation

α-synuclein (AS) is a small (140 amino acids), abundant presynaptic protein, which lacks a unique secondary structure in aqueous solution. Amyloid aggregates of AS in dopaminergic neurons of the midbrain are the hallmark of Parkinson's disease (PD). The process of aggregation involves a series of complex structural transitions from innocuous monomeric AS to oligomeric, presumably neurotoxic, forms and finally to fibril formation. Despite its potential importance for understanding PD pathobiology and devising rational, targeted therapeutic strategies, the details of the aggregation process remain largely unknown. Methodologies and reagents capable of controlling the aggregation kinetics are essential tools for the investigation of the molecular mechanisms of amyloid diseases. In this work, we investigated the influence of citrate-capped gold nanoparticles on the aggregation kinetics of AS using a fluorescent probe (MFC) sensitive to the polarity of the molecular microenvironment via excited state intramolecular proton transfer (ESIPT). The particular effects on the half time, nucleation time, and growth rate were ascertained. Gold nanoparticles produced a strong acceleration of protein aggregation with an influence on both the nucleation and growth phases of the overall mechanism. The effects were dependent on the size and concentration of the nanoparticles, being strongest for nanoparticles 10 nm in diameter, which produced a 3-fold increase in the overall aggregation rate at concentrations as low as 20 nM.

Combination of UV–vis spectroscopy and chemometrics to understand protein–nanomaterial conjugate: A case study on human serum albumin and gold nanoparticles

Study of the interactions between proteins and nanomaterials is of great importance for understanding of protein nanoconjugate. In this work, we choose human serum albumin (HSA) and citrate-capped gold nanoparticles (AuNPs) as a model of protein and nanomaterial, and combine UV-vis spectroscopy with multivariate curve resolution by an alternating least squares (MCR-ALS) algorithm to present a new and efficient method for comparatively comprehensive study of evolution of protein nanoconjugate. UV-vis spectroscopy coupled with MCR-ALS allows qualitative and quantitative extraction of the distribution diagrams, spectra and kinetic profiles of absorbing pure species (AuNPs and AuNPs-HSA conjugate are herein identified) and undetectable species (HSA) from spectral data. The response profiles recovered are converted into the desired thermodynamic, kinetic and structural parameters describing the protein nanoconjugate evolution. Analysis of these parameters for the system gives evidence that HSA molecules are very likely to be attached to AuNPs surface predominantly as a flat monolayer to form a stable AuNPs-HSA conjugate with a core-shell structure, and the binding process takes place mainly through electrostatic and hydrogen-bond interactions between the positively amino acid residues of HSA and the negatively carboxyl group of citrate on AuNPs surface. The results obtained are verified by transmission electron microscopy, zeta potential, circular dichroism spectroscopy and Fourier transform infrared spectroscopy, showing the potential of UV-vis spectroscopy for study of evolution of protein nanoconjugate. In parallel, concentration evolutions of pure species resolved by MCR-ALS are used to construct a sensitive spectroscopic biosensor for HSA with a linear range from 1.8 nM to 28.1 nM and a detection limit of 0.8 nM.

Ligand-free gold nanoparticles as colorimetric probes for the non-destructive determination of total dithiocarbamate pesticides after solid phase extraction

In this work, we describe a simple and sensitive non-destructive method for the determination of the total concentration of dithiocarbamate fungicides (DTCs) in real samples. The proposed method combines for the first time the benefits of an extraction method for sample clean-up and preconcentration with a sensitive colorimetric assay based on gold nanoparticle probes. In this two-step procedure, the target DTCs are isolated from the matrix and preconcentrated by solid phase extraction onto commercially available C18 sorbents. Following elution, the extract containing the target dithiocarbamates, free from most interferences and matrix components, is delivered into an aqueous dispersion of plain citrate-capped gold nanoparticles (AuNPs) which aggregate in response to DTCs coordination on AuNPs surface through multiple gold thiolate bonds. This aggregation is evidenced by changes in the spectral properties of the solution involving a decrease in the original absorbance of Au nanoparticles at 522 nm and the appearance of a new absorption band above 700 nm. An ensuing chromatic shift of the solution from wine-red to purple-blue is observed which is visual by naked eye at concentrations as low as 50 μg L(-1). Further improvement in the detection limits can be accomplished by scaling-down the method to micro-volume conditions alleviating the need to preconcentrate larger sample volumes. Overall, by combining sample clean-up and preconcentration with the strong affinity of DTC thiol group for the gold surface, the total concentration of dithiocarbamate pesticides was successfully determined in various water samples at the low and ultra-low μg L(-1) levels without resorting to destructive techniques, sophisticated instrumentation or post-synthetic modification of gold nanoparticles. Method application in real samples showed good analytical features in terms of recoveries (81.0-94.0%), precision (5.6-8.9%) and reproducibility (~9%) rendering the method as an attractive alternative to current methodologies for the determination of DTC fungicide residues in samples of environmental interest.

Colorimetric sensor for thiocyanate based on anti-aggregation of citrate-capped gold nanoparticles

A convenient colorimetric sensor based on anti-aggregation of citrate-capped gold nanoparticles (AuNPs) is demonstrated for thiocyanate (SCN−) sensing. The citrate-capped AuNPs aggregated readily in the presence of sulfuric acid, leading to a color change from red to blue. This particle aggregation could be suppressed by SCN−, which had strong protective effect on the gold colloid. The thiocyanate recognition suffers insignificant interference from most of common ions. With this strategy, 1μM of SCN− can be recognized within 5min by naked-eye observation. This simple, selective, and sensitive approach can be potentially applied for SCN− assays in environmental aqueous samples.

Highly sensitive detection of chromium (III) ions by resonance Rayleigh scattering enhanced by gold nanoparticles

Simple and sensitive determination of chromium (III) ions (Cr3+) has potential applications for detecting trace contamination in environment. Here, the assay is based on the enhancement of resonance Rayleigh scattering (RRS) by Cr3+-induced aggregation of citrate-capped gold nanoparticles (AuNPs). Transmission electron microscopy (TEM) and UV–vis absorption spectroscopy were employed to characterize the nanostructures and spectroscopic properties of the Cr3+–AuNP system. The experiment conditions, such as reaction time, pH value, salt concentration and interfering ions, were investigated. The combination of signal amplification of Cr3+–citrate chelation with high sensitivity of RRS technique allow a selective assay of Cr3+ ions with a detection limit of up to 1.0pM. The overall assay can be carried out at room temperature within only twenty minutes, making it suitable for high-throughput routine applications in environment and food samples.

Disruption of biomolecule function by nanoparticles: How do gold nanoparticles affect Phase I biotransformation of persistent organic pollutants?

The potential influence of nanoparticles on cytochrome P-450 (CYP) isozyme mediated Phase I biotransformation of persistent organic pollutants (POPs) in vitro was investigated using citrate-capped gold nanoparticles (AuNPs) and 2,2′,3,5′,6-pentachlorobiphenyl (PCB 95) as the probe nanoparticle and compound, respectively. AuNPs affected the biotransformation activity of rat CYP2B1 and changed the atropisomeric composition of PCB 95, depending on the incubation time and the AuNP concentration. Electrostatic repulsion between citrate-coated AuNPs and rat CYP2B1 may influence the active conformation of the isozyme and consequently affect its activity and stereoselectivity. In addition, the effects of AuNPs on rat CYP2B1 activity also appeared to be through interference with the CYP catalytic cycle’s electron transfer chain. Incubations with AuNPs had a decline in buffer conductance and an absorbance band red shift of AuNPs, from electrostatic interactions of K+ with negatively-charged AuNP aggregates. These ionic strength changes affected the formation rate of nicotinamide adenine dinucleotide phosphate, which provides electrons for the oxidative reaction cycle, and the biotransformation activity and stereoselectivity of CYP. This study suggests that charged nanoparticles may be able to alter the functions of biomolecules directly, by electrostatic interaction, or indirectly, by changes to the surrounding ionic strength. These factors should be taken into account for further understanding and prediction of the environmental behavior and fate of POPs and nanoparticles.

Dissecting Colloidal Stabilization Factors in Crowded Polymer Solutions by Forming Self-Assembled Monolayers on Gold Nanoparticles

An ideal colloidal system should be highly stable in a diverse range of buffer conditions while still retaining its surface accessibility. We recently reported that dispersing citrate-capped gold nanoparticles (AuNPs) in polymers, such as polyethylene glycol (PEG), can achieve such a goal because of contributions from depletion stabilization. Because AuNPs can weakly adsorb PEG to exert steric stabilization and the remaining citrate can impart charge stabilization, the extent of the contribution of depletion stabilization is unclear. In this work, we aim to dissect the contribution of each stabilizing factor. This is achieved by coating AuNPs with a layer of thiolated compound, which inhibits the adsorption of PEG and also allows for the control of surface charge. We found that depletion stabilization alone was insufficient to stabilize AuNPs at room temperature. However, when working together with other stabilization mechanisms, ultrahigh stability can be achieved. The size of both AuNPs and PEG was systematically varied, and the trends were compared to theoretical calculations. Finally, we report the importance of the surface chemistry of commercial AuNPs.