Citrate-stabilized AuNPs Research Articles

Sensitive fluorescent detection of melamine in raw milk based on the inner filter effect of Au nanoparticles on the fluorescence of CdTe quantum dots

A simple, rapid and sensitive fluorescent assay for determination of melamine has been developed based on inner filter effect (IFE) of gold nanoparticles (AuNPs) on the fluorescence of CdTe quantum dots (QDs). When thioglycolic acid-capped CdTe QDs was mixed with citrate-stabilized AuNPs, the fluorescence of CdTe QDs was significantly quenched via the IFE of AuNPs. With the presence of melamine, melamine could induce the aggregation and corresponding absorbance change of AuNPs, which then resulted in the recovery of IFE-decreased emission of CdTe QDs. Under the optimum conditions, the detection limit for melamine in raw milk was 0.02mgL−1. The application of this method in samples of melamine-spiked raw milk suggested a recovery between 103% and 104%. Therefore, the obvious merits provided by the present assay, such as simplicity, rapidity, low cost, and high sensitivity, would make it promising for on-site screening of melamine adulterant in raw milk.

Site-Selective Assembly and Reorganization of Gold Nanoparticles along Aminosilane-Covered Nanolines Prepared on Indium–Tin Oxide

We have fabricated gold nanoparticle (AuNP) arrays on indium-tin oxide (ITO) substrates in a nearly one-dimensional fashion. AuNPs were site-selectively immobilized on ITO of which the surface had been patterned by a nanolithography process based on scanning probe microscopy. The fabricated nanoscale lines covered with aminosilane self-assembled monolayer served as chemisorption sites for citrate-stabilized AuNPs of 20 nm in diameter, accordingly, AuNP nanolines with a thickness of single nanoparticle diameter were spontaneously assembled on the lines. In this 1D array, the AuNPs were almost separated from each other due to the electrostatic repulsion between their negatively charged surface layers. Furthermore, a reorganization process of the immobilized AuNP arrays has been successfully demonstrated by replacing each AuNP's surface layer from citric acid to dodecanethiol. By this process, the AuNPs lost their electrostatic repulsion and became hydrophobic so as to be attracted to each other through hydrophobic interaction, resulting in reorganization of the AuNP array. By repeating the deposition and reorganization cycle, AuNPs were more densely packed. The optical absorption peak of the arrays due to their plasmonic resonance was found to shift from 526 to 590 nm in wavelength with repeating cycles, indicating that the resonance manner was changed from the single nanoparticle mode to the multiple particle mode with interparticle coupling.

Ultrasensitive detection of Cu2+ with the naked eye and application in immunoassays

In this study, a novel method for the fast, sensitive and selective detection of Cu2+ using gold nanoparticles (AuNPs) was developed and used in immunoassays. In the presence of L-cysteine, L-cysteine can bind to the surface of citrate-stabilized AuNPs through Au-S bonds. As a result, aggregation of AuNPs occurs through electrostatic interactions between the cysteine-bound AuNPs. In contrast, in the presence of Cu2+, Cu2+ can catalyze O2 oxidation of cysteine, leading to the quick formation of disulfide cystine. An increase in the concentration of Cu2+ decreased L-cysteine-induced AuNPs aggregation by decreasing the number of free cysteine thiol groups, and the solution color changed from purple to red. Therefore, the concentration of Cu2+ can be detected with the naked eye or with ultraviolet–visible spectroscopy, and the detection limits of Cu2+ were 20 nM and 10 nM, respectively. This sensitivity was approximately three orders of magnitude higher than that of traditional AuNPs-based colorimetric Cu2+ detection methods. Because of the high sensitivity of the proposed method, we further used it with a labeled antibody in colorimetric immunoassays. The detection limit of the cancer biomarker α-fetoprotein was 2 ng ml−1, which is comparable to the detection limit of the enzyme-linked immunosorbent assay method. Huang-Hao Yang and co-workers at Fuzhou University have devised an ultra-sensitive method to detect copper ions (Cu2+), whose accumulation in the body has been linked to several diseases. The researchers' method relies on gold nanoparticles, popular in colourimetric sensing because they turn a solution from red to purple on aggregation. Here, however, instead of triggering the aggregation directly, the copper ions catalyse a ‘supporting reaction’ – the oxidation of a monomeric amino acid (cysteine) into a dimeric one (cystine). Only cysteine triggers the aggregation of the nanoparticles, which means the presence or absence of copper results in a red or purple solution, respectively. The method's sensitivity is very high because only catalytic quantities of copper are needed. The researchers further used this rapid, convenient, sensitive method in an immunoassay for a human protein. A novel method based on the AuNPs for fast, sensitive and selective detection of Cu2+ was developed and applied in immunoassays.

Gold Nanoparticles Aggregation: Drastic Effect of Cooperative Functionalities in a Single Molecular Conjugate

Aggregation of gold nanoparticles (AuNPs) can be utilized in chemical and biomolecular sensing as a sensitive and easy-to-visualize process. However, interpretation of experimental results requires a clear understanding of physicochemical processes that take place upon multiple interactions between an analyte and AuNPs. In this article, interactions between citrate-stabilized AuNPs and organic compounds bearing various functional groups in an aqueous medium were experimentally and theoretically studied using spectrophotometry of the localized surface plasmon resonance (LSPR), transmission electron microscopy (TEM), conductometry, zeta potential measurements, and finite-difference time-domain (FDTD) modeling. As a result, it has been found that organic compounds containing both thiol and amine groups strongly promote the aggregation of AuNPs due to their cooperative functionalities. FDTD modeling has enabled consideration of the light extinction (i.e., LSPR response) properties of nanoparticle aggregates involving single, chain-like, and globular structures. Taking one billion distributions of differently structured aggregates into account, the theoretical light extinction was fitted to that of the experimental result with a root-mean-square deviation of 7%.

Tumor necrosis factor interaction with gold nanoparticles

We report on a systematic investigation of molecular conjugation of tumor necrosis factor-α (TNF) protein onto gold nanoparticles (AuNPs) and the subsequent binding behavior to its antibody (anti-TNF). We employ a combination of physical and spectroscopic characterization methods, including electrospray-differential mobility analysis, dynamic light scattering, polyacrylamide gel electrophoresis, attenuated total reflectance-Fourier transform infrared spectroscopy, fluorescence assay, and enzyme-linked immunosorbent assay. The native TNF used in this study exists in the active homotrimer configuration prior to conjugation. After binding to AuNPs, the maximum surface density of TNF is (0.09 ± 0.02) nm(-2) with a binding constant of 3 × 10(6) (mol L(-1))(-1). Dodecyl sulfate ions induce desorption of monomeric TNF from the AuNP surface, indicating a relatively weak intermolecular binding within the AuNP-bound TNF trimers. Anti-TNF binds to both TNF-conjugated and citrate-stabilized AuNPs, showing that non-specific binding is significant. Based on the number of anti-TNF molecules adsorbed, a substantially higher binding affinity was observed for the TNF-conjugated surface. The inclusion of thiolated polyethylene glycol (SH-PEG) on the AuNPs inhibits the binding of anti-TNF, and the amount of inhibition is related to the number ratio of surface bound SH-PEG to TNF and the way in which the ligands are introduced. This study highlights the challenges in quantitatively characterizing complex hybrid nanoscale conjugates, and provides insight on TNF-AuNP formation and activity.

G4-DNA-Coated Gold Nanoparticles: Synthesis and Assembly

Here, we describe the preparation of stable 15 nm gold nanoparticles (Au-NPs) coated with parallel-stranded G-quadruplexes (G4-DNA), comprising phosphorothioate residues on both sides of the DNA. Phosphorothioate residues located on the surface of the coated particles can anchor them to noncoated ones. Their incubation with more than 20-fold excess of 15 nm citrate-stabilized Au-NPs leads to the formation of flower-shaped structures comprising a central noncoated particle and five to six G-quadruplex-coated ones at the periphery, as revealed by TEM imaging analysis. The absorption band of the structures is shifted toward long wavelengths compared to individual particles not connected to each other. We show a strong dependence of plasmon coupling strength on the length of the DNA connecting Au-NPs.

NaBH4-Induced Assembly of Immobilized Au Nanoparticles into Chainlike Structures on a Chemically Modified Glass Surface

A facile method of obtaining chainlike assemblies of gold nanoparticles (AuNPs) on a chemically modified glass surface based on NaBH(4) treatment is developed. Citrate-stabilized AuNPs (17 nm) are immobilized on a glutaraldehyde-functionalized glass surface and assembled into chainlike structures after treatment with aqueous sodium borohydride (NaBH(4)) solution. The production and morphology of the AuNP chainlike assemblies are controlled by the density of the immobilized NPs, the concentration of NaBH(4) solution, and the treatment time. The AuNP assemblies are stable in water and can undergo drying. X-ray photoelectron spectroscopic data show that the number of citrate ions on the AuNPs decreased by 43% after treatment with 5 mg/mL NaBH(4) solution. The NaBH(4)-induced partial removal of the citrate ions and the roughness of the glass surface greatly affect the binding force of AuNPs on the substrate. The immobilized AuNPs begin to move at the solid-liquid interface without desorbing when the strength of the binding force was decreased. These mobile NPs form chainlike assemblies under the driving force of van der Waals interaction and diffusion. This interface-based formation of chainlike assemblies of AuNPs may provide a simple protocol for the 1D assembly of other Au-coated colloidal nanoparticles.

Colorimetric detection of melamine in complex matrices based on cysteamine-modified gold nanoparticles

A sensitive assay for melamine in complex matrices is built using cysteamine-modified gold nanoparticles (AuNPs) and an effective sample pretreatment protocol. Citrate-stabilized AuNPs were modified by cysteamine in order to weaken the electrostatic repulsion force between the gold nanoparticles. Detection sensitivity gained through this modification increased about 100 fold compared with the result using the unmodified AuNPs. Direct colorimetric visualizations of melamine in milk products, eggs and feeds was successfully demonstrated within the linear ranges of 1-200 mg L(-1) and detection limits below 1 mg L(-1). The proposed scheme could be an alternative means for onsite detection of melamine without costly instruments.

Single-step generation of fluorophore-encapsulated gold nanoparticle core–shellmaterials

We report a simple route to produce fluorophore-encapsulated gold nanoparticles (AuNPs)in a single step under aqueous conditions using the fluorophore 1-pyrenemethylamine(PMA). Different amounts of PMA were used and the resulting core–shell goldnanoparticles were analyzed using UV–visible absorption spectroscopy, fluorescencespectroscopy, and transmission and scanning electron microscopy. Electron microscopyanalysis shows nanoparticles consisting of a gold nanoparticle core which is encapsulatedwith a lower contrast shell. In the UV–visible spectra, we observed a significant red shift(37 nm) of the localized surface plasmon resonance (LSPR) absorption maximum (λmax) compared to citrate-stabilized AuNPs of a similar size. We attribute the prominent LSPRwavelength shift for PMA–AuNP conjugates to the increase in the local dielectricenvironment near the gold nanoparticles due to the shell formation. This simple,aqueous-based synthesis is a new approach to the production of fluorophore-encapsulatedAuNPs that could be applicable in biological sensing systems and photonic devicefabrication.

Estimating conformation content of a protein using citrate-stabilized Au nanoparticles

Herein we report the use of the optical properties of citrate-stabilized gold nanoparticles (Au NPs) for estimation of native or denatured conformation content in a mixture of a protein in solution. The UV-vis extinction spectrum of citrate-stabilized Au NPs is known to broaden differently in the presence of native and denatured states of alpha-amylase, bovine serum albumin (BSA) or amyloglucosidase (AMG). On the other hand, herein we show that when a mixture of native and denatured protein was present in the medium, the broadening of the spectrum differed for different fractional content of the conformations. Also, the total area under the extinction spectrum varied linearly with the change in the mole fraction content of a state and for a constant total protein concentration. Transmission electron microscopy (TEM) measurements revealed different levels of agglomeration for different fractional contents of the native or denatured state of a protein. In addition, time-dependent denaturation of a protein could be followed using the present method. The rate constants calculated for denaturation indicated a possible fast change in conformation of a protein before complete thermal denaturation. The observations have been explained based on the changes in extinction coefficient (thereby oscillator strength) upon interaction of citrate-stabilized NPs with proteins being in different states and levels of agglomeration.

The Effect of Aniline Concentration in the Ligand Exchange Reaction with Citrate-Stabilized Gold Nanoparticles

Sodium citrate reduction of hydrogen tetrachloroaurate is one of the most efficient routes in the synthesis of gold nanoparticles (AuNPs). However, a major limitation of this method is that it results only in citrate-stabilized AuNPs, and exchange reactions of the citrate for another stabilizer must be undertaken in cases where other surface functionalities are desired. These exchange reactions can be studied using a variety of techniques. Two techniques we have used for analysis are ultraviolet-visible spectroscopy, which takes advantage of the plasmon resonance (PR) of AuNPs, and liquid chromatography (LC) to examine residual exchanger in the supernatant at equilibrium. The exchange of the citrate on the surface of AuNPs with aniline was studied where different concentrations of aniline are added. While the equilibrium PR band spectra were similar for high and low concentrations of added aniline, the spectra as a function of time differed markedly. Surprisingly, increasing the concentration of added aniline slowed the evolution of a second red-shifted band. Transmission electron microscopy (TEM) imaging showed that addition of neither high nor low concentrations of aniline caused a significant difference in particle size, and the appearance of the large second band was most likely due to the presence of an oligomeric aniline species surrounding the particles. As aniline is known to undergo polymerization under a variety of conditions, it is postulated that aniline polymerization is a competitive event to the ligand exchange process, and increasing the aniline concentration increases the likelihood of polymerization kinetically competing with association of the aniline species interacting with the AuNP surface. This was further supported by LC data, which demonstrated that the aniline exchanged at approximately 4% of the initial amount of citrate present in the synthetic mixture.

Short-time preparation and electrochemical properties of a single layer of tetraoctylammonium bromide capped Au nanoparticles on dithiol self-assembled monolayer-modified Au electrode

Short time immobilization of densely packed tetraoctylammonium bromide (TOAB) stabilized gold nanoparticles (AuNPs) were established on a Au electrode modified with a self-assembled monolayer (SAM) of 1,6-hexanedithiol (HDT) or 1,4-benzenedimethanethiol (BDMT). The quartz crystal microbalance experiment showed densely packed TOAB–AuNPs single layer formation on both SAMs was achieved within 20 min. AFM images demonstrated that the immobilized TOAB–AuNPs on the SAMs were densely packed and the AuNPs film thickness was 6–7 nm. The electronic communication between the immobilized AuNPs and the underlying bulk electrode was confirmed by cyclic voltammetry and electroreflectance spectroscopy. A reversible electron transfer reaction was observed for both [Fe(CN) 6] 4−/3− and [Ru(NH 3) 6] 2+/3+ at TOAB–AuNPs immobilized on HDT (Au/HDT/AuNPs) and BDMT (Au/BDMT/AuNPs) modified electrodes. The electroreflectance spectra show a red-shifted strong positive-going plasmon resonance bands at 551 nm and 584 nm, respectively, for the Au/BDMT/AuNPs and Au/HDT/AuNPs electrodes. The observed reversible redox response for the solution redox species and red-shifted plasmon resonance bands for the immobilized AuNPs again indicated that the AuNPs were immobilized on the SAMs in a densely packed manner. An advantage of TOAB–AuNPs modified electrode prepared by short time immersion over citrate-stabilized AuNPs modified electrode was demonstrated by the enhanced oxidation of ascorbic acid (AA) at these electrodes. The oxidation of AA was shifted to 90 mV less positive potential with higher oxidation current at TOAB–AuNPs modified electrode when compared to citrate-stabilized AuNPs modified electrode.

Rapid fabrication of Au nanoparticle films with the aid of centrifugal force

In this work, rapid fabrication of Au nanoparticle (Au NP) films has been simply achievedby alternate adsorption of citrate-stabilized Au NPs and poly(diallyldimethylammoniumchloride) with the aid of centrifugal force. In contrast to conventional electrostaticassembly, we carried out the assembly process in a centrifuge with a rotating speed of4000 rpm, where centrifugal force can be imposed on Au NPs. Scanning electron microscopyand cyclic voltammetry were employed to characterize the assembly procedure and thethus-prepared thin solid films. Our results demonstrate that centrifugal force can promotethe assembly of Au NPs and therefore enable the rapid fabrication of functional Au NPfilms. The thus-prepared Au NP films can serve as surface enhanced Raman scatting(SERS) substrates with tunable SERS signal intensity. This method is simple, rapid, andcan be used as a general method to rapidly assemble other charged nanoparticles.

Using plasmonic heating of gold nanoparticles to generate local SER(R)S-active TiO2 spots

The plasmonic heating generated by laser irradiation of gold nanoparticles was exploited for fabricating SER(R)S-active anatase spots.

Self-Assembly of Ionic Liquid (BMI-PF6)-Stabilized Gold Nanoparticles on a Silicon Surface: Chemical and Structural Aspects

Ultrafine monodisperse gold nanoparticles (AuNPs) were synthesized by an elegant sputtering of gold onto 1- n-butyl-3-methylimidazolium hexafluorophosphate (BMI-PF(6)) ionic liquid. It was found that the BMI-PF(6) supramolecular aggregates were loosely coordinated to the gold nanoparticles and were replaceable with thiol molecules. The self-assembly of BMI-PF(6)-stabilized AuNPs onto a (3-mercaptopropyl)trimethoxysilane (MPS)-functionalized silicon surface in 2D arrays, followed by dodecanethiol (DDT) treatment, have been demonstrated using X-ray photoelectron spectroscopy, field emission scanning electron microscopy, and contact angle measurements. DDT treatment of tethered AuNPs revealed two types of interactions between AuNPs and the MPS-functionalized surface: (a) AuNPs anchor through Au-S chemisorption linkage resulting in strong immobilization and (b) some of the AuNPs are supported by physisorption, driven by BMI-PF(6). The attachment of these particles remains unchanged with sonication. The replacement of BMI-PF(6) aggregates from physisorbed AuNPs with DDT molecules advances the dilution of their interaction with the MPS-functionalized surface, and they subsequently detach from the silicon surface. The present finding is promising for the immobilization of ionic liquid-stabilized nanoparticles, which is very desirable for electronic and catalytic device fabrication. Additionally, these environmentally friendly AuNPs are expected to replace conventional citrate-stabilized AuNPs.

Spatially Controlled Functionalization and Chemical Manipulation to Fabricate Two-Dimensional Arrays of Gold Nanoparticles onto Indium Tin Oxide

We demonstrate the formation of two-dimensional (2D) arrays of gold nanoparticles (AuNPs) by controlled functionalization in a regular linear fashion, on the basis of the self-assembly and vacuum ultraviolet (VUV) lithography approaches. An octadecyltrimethoxysilane (ODS) monolayer, self-assembled on an indium tin oxide (ITO) surface, was exposed to VUV light (172 nm) through a photomask to construct the linear pattern by photodegradation of the ODS monolayer. Subsequently, an aminopropyltriethoxysilane (APS) monolayer was deposited on VUV-exposed sites by chemical vapor deposition. The immobilization of AuNPs on amino-terminated sites is due to the electrostatic attraction between citrate-stabilized AuNPs and amino groups of the APS monolayer, while the methyl-terminated sites remain unexposed. The structural organization of AuNPs in an alternating linear fashion was demonstrated by scanning electron microscope (SEM) analysis. The X-ray photoelectron microscopy (XPS) measurements were performed to probe the self-assembly of ODS, the APS monolayer, photopatterning and AuNPs immobilization.

Optical sensing of biomedically important polyionic drugs using nano-sized gold particles

A simple optical method for the sensing of biomedically important polyionic drugs, protamine and heparin based on the reversible aggregation and de-aggregation of gold nanoparticles (AuNPs) is described. The polycationic protamine induces the aggregation of negatively charged citrate-stabilized AuNPs, resulting in a shift in the surface plasmon (SP) band and a consequent color change of the AuNPs from red to blue. Addition of polyanionic heparin dissipates the aggregated AuNPs due to its strong affinity to protamine and the blue color changes to the native color. The color change was monitored using UV–vis spectrophotometry. The aggregation and de-aggregation was confirmed by transmission electron microscopic (TEM) measurements. The degree of aggregation and de-aggregation is proportional to the concentration of added protamine and heparin, allowing their quantitative detection. The change in the absorbance and SP band position has been used to monitor the concentration of protamine and heparin. This optical method can quantify protamine and heparin as low as 0.1 μg/ml and 0.6 μg/ml, respectively and the calibration is linear for a wide range of concentration.

Immobilization of glucose oxidase onto gold nanoparticles with enhanced thermostability

Immobilized proteins and enzymes were widely investigated in medical field as well as in food and environmental fields. In this paper, glucose oxidase (GOD) monolayer was covalently immobilized on the surface of gold nanoparticles (AuNPs) to fabricate bioconjugate complex. The citrate-stabilized AuNPs were first functionalized by a carboxyl-terminated alkanethiol and the terminal carboxyl groups were subsequently bonded with side-chain amino groups of protein surface through EDC/NHS coupling reaction. The enzyme activity assays of the obtained bioconjugates display an enhanced thermostability and similar pH-dependence behavior in contrast with that of free enzyme. Such GOD/AuNPs bioconjugates can be considered as a catalytic nanodevice to construct nanoreactor based on glucose oxidation reaction for biotechnological purpose.