Thermally treated bare gold nanoparticles for colorimetric sensing of copper ions

We demonstrate a sensitive and rapid colorimetric assay for selective detection of copper ions based on the strong coordination between Cu(II) ions and the tetrahydroxyaurate anions [Au(OH)₄]^? on the surface of thermally treated bare gold nanoparticles (GNPs). The method for making the unmodified GNPs is simple and results in a nanomaterial with a highly specific response to Cu(II). The thermal treatment of the bare GNPs and the recognition of Cu(II) ions is accomplished in a single step within 5 min. The presence of Cu(II) causes the color to change from red to purple-blue. The limit of detection (LOD) is 0.04 μM of Cu(II) when using UV–vis spectrometry and ratioing the absorbances at 650 and 515 nm, respectively. The method also is amenable to bare eye (visual) inspection and in this case has an LOD of 2.0 μM of Cu(II).

Bimetallic nano-structured glucose sensing electrode composed of copper atoms deposited on gold nanoparticles

We describe the preparation and sensing capabilities of a bimetallic electrode consisting of copper atoms deposited on gold nanoparticles (GNPs). The electrode was obtained by first constructing a GNP template on the surface of a glassy carbon electrode by exploiting the hydrogen-bonding interactions between pyridine groups on the surface of the GNPs and the carboxy groups of poly(acrylic acid). GNPs (60?nm in diameter) were homogeneously and densely deposited in the template (as revealed by scanning electron microscopy). The electro-deposition of copper ad-atoms on GNPs occurred at an underpotential and was proven by electrochemical techniques. The presence of GNPs in the template accelerated the deposition at low potential due to its beneficial effect on the rate of electron transfer. The new electrode was studied for its response to glucose. Highly stable and reproducible catalytic activity towards glucose oxidation is observed and attributed to the synergistic catalytic effect of the copper atoms on the surface of the GNPs. The detection limit is as low as 50?nM (at a signal-to-noise ratio of 3), and the response is between 200?nM and 10?mM of glucose.

Visual detection of silver(I) ions by a chromogenic reaction catalyzed by gold nanoparticles

We report on a novel method for visual detection silver(I) ion. It is based on the finding that Ag(I) ions are rapidly reduced by hydroquinone to form a shell of silver on the surface of gold nanoparticles (AuNPs) which act as catalysts for this reaction. This leads to a color change from red to yellow which can be seen with bare eyes. This scheme is sensitive and highly specific for Ag(I) ions. The detection limits are 5 μM for visual inspection and 1 μM for photometric readout, respectively. The method was successfully applied to the determination of Ag(I) ions in spiked lake water and soil.

In-situ decorated gold nanoparticles on polyaniline with enhanced electrocatalysis toward dopamine

Gold nanoparticles were in-situ decorated on top of a polyaniline film (GNPs–PANI) via the direct electroreduction of the adsorbed AuCl ₄ ^- ions on a glassy carbon electrode that previously was coated with PANI by electropolymerization. The GNPs–PANI composite and the performance of the resultant sensors were investigated in some detail. The sensor was applied to the oxidation of dopamine (DA) with improved catalytic activity. Its catalytic current showed wide linear response toward dopamine ranging from 3 to 115 μM, with a low detection limit of 0.8 μM (S/N=3). In addition, the sensor exhibits easy-operation, fast response to dopamine, as well as excellent reproducibility and stability.

Amperometric hydrogen peroxide biosensor based on a glassy carbon electrode modified with polythionine and gold nanoparticles

We report on a novel hydrogen peroxide biosensor that was fabricated by the layer-by-layer deposition method. Thionine was first deposited on a glassy carbon electrode by two-step electropolymerization to form a positively charged surface. The negatively charged gold nanoparticles and positively charged horseradish peroxidase were then immobilized onto the electrode via electrostatic adsorption. The sequential deposition process was characterized using electrochemical impedance spectroscopy by monitoring the impedance change of the electrode surface during the construction process. The electrochemical behaviour of the modified electrode and its response to hydrogen peroxide were studied by cyclic voltammetry. The effects of the experimental variables on the amperometric determination of H₂O₂ such as solution pH and applied potential were investigated for optimum analytical performance. Under the optimized conditions, the biosensor exhibited linear response to H₂O₂ in the concentration ranges from 0.20 to 1.6?mM and 1.6 to 4.0?mM, with a detection limit of 0.067?mM (at an S/N of 3). In addition, the stability and reproducibility of this biosensor was also evaluated and gave satisfactory results.

Fluorescence correlation spectroscopy of gold nanoparticles,and its application to an aptamer-based homogeneous thrombin assay

We have studied the fluorescence properties and diffusion behaviors of gold nanoparticles (GNPs) in solution by using fluorescence correlation spectroscopy (FCS) at single molecule level. The GNPs display a high photo-saturation feature. Under illumination with strong laser light, they display higher brightness per particle (BPP) despite their low quantum yields. Based on the unique fluorescence properties and diffusion behaviors of GNPs, we have developed a sensitive and homogenous thrombin assay. It is based on a sandwich strategy and is making use of GNPs to which two different aptamers are conjugated. When the differently aptamer-labeled GNPs are mixed with solutions containing thrombin, the affinity reaction causes the GNPs to form dimers or oligomers. This leads to an increase in the diffusion time of the GNPs in the detection volume that is seen in FCS. The FCS method enables sensitive detection of the change in the characteristic diffusion time of the GNPs before and after the affinity reaction. Quantitative analysis of thrombin is based on the measurement of the change in the diffusion time. Under optimal conditions, the calibration plot is linear in the 0.5 nM to 110 nM thrombin concentration range, and the detection limit is 0.5 nM. The method was successfully applied to the direct determination of thrombin in human plasma.

Direct electron transfer and biocatalytic activity of iron storage protein molecules immobilized on electrodeposited cobalt oxide nanoparticles

Ferritin was immobilized on a glassy carbon electrode with electrodeposited cobalt oxide nanoparticles, and its direct electron transfer behavior was studied. It exhibits a pair of redox peaks due to direct electron transfer between ferritin and the nanoparticles. Electrochemical parameters including the formal potential (E^0??), the charge transfer coefficient (??), and the apparent heterogeneous electron transfer rate constant (k_s) were determined. The sensor displays excellent biocatalytic activity in terms of reduction of hydrogen peroxide, and this was applied to electrochemical sensing of hydrogen peroxide.

Sandwich immunoassay for alpha-fetoprotein in human sera using gold nanoparticle and magnetic bead labels along with resonance Rayleigh scattering readout

We describe a sensitive sandwich immunoassay for alpha-fetoprotein (AFP). It is making use of gold nanoparticles (GNPs) and magnetic beads (MBs) as labels, and of resonance Rayleigh scattering for detection. Two antibodies were labeled with GNPs and MBs, respectively, and MB-antigen-GNP complexes were formed in the presence of antigens. The MB labels also serve as solid phase carriers that can be used to magnetically separate the immuno complex. The GNP labels are used as optical probes, and Rayleigh scattering was used to determine the concentration of free GNPs-antibody after separation of the MB-antigen-GNP complexes. The concentration of AFP is related to the intensity of light scattered by free GNPs in the 13.6 pM to 436 pM concentration range, and the limit of detection is 13.6 pM. The method was applied to the determination of AFP in sera of cancer patients, and the results agree well with those obtained by conventional ELISA.

Dendrimer-based biosensor for chemiluminescent detection of DNA hybridization

We report on a highly sensitive chemiluminescent (CL) biosensor for the sequenc-specific detection of DNA using a novel bio barcode DNA probe modified with gold nanoparticles that were covered with a dendrimer. The modified probe is composed of gold nanoparticles, a dendrimer, the CL reagent, and the DNA. The capture probe DNA was immobilized on magnetic beads covered with gold. It first hybridizes with the target DNA and then with one terminal end of the signal DNA on the barcoded DNA probe. CL was generated by adding H₂O₂ and Co(II) ions as the catalyst. The immobilization of dendrimer onto the gold nanoparticles can significantly enhance sensitivity and gives a detection limit of 6 fmol L^-1 of target DNA.

A sensitive sensor for determination of l-tryptophan based on gold nanoparticles/poly(alizarin red S)-modified glassy carbon electrode

This work described a novel sensor for detection of l -tryptophan (Trp) by electrodeposition of gold nanoparticles (AuNPs) onto the poly(alizarin red S) film pre-cast on a glassy carbon electrode (GCE). Alizarin red S (ARS) was deposited on the surface of the GCE by electropolymerization, and gold nanoparticles (AuNPs) were attached onto the poly(ARS) film by electrodeposition, forming an AuNPs–PARS nanocomposite film-modified GCE (AuNPs–PARS/GCE). Then electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM) were used to characterize modified electrodes. The Nyquist diagrams of EIS indicated that the PARS film and AuNPs were successfully immobilized on the surface of GCE, and the electron transfer resistance value of electrode changed efficiently. The SEM image showed that the immobilized AuNPs were spherical in shape. The AuNPs–PARS/GEC displayed excellent amperometric response for Trp. The amperometric responses have two linear ranges from 0.02 to 0.5 μM and 0.5 to 20.0 μM, with sensitivities of 1.63(±0.08) and 0.21(±0.01)?μAμM^?1, respectively. Its detection limit was 6.7 nM at a signal-to-noise ratio of 3. The proposed method was applied to determine Trp.

Sensitive simultaneous determination of catechol and hydroquinone using a gold electrode modified with carbon nanofibers and gold nanoparticles

A highly sensitive electrochemical sensor for the simultaneous determination of catechol (CC) and hydroquinone (HQ) was fabricated by electrodeposition of gold nanoparticles onto carbon nanofiber film pre-cast on an Au electrode. Both CC and HQ cause a pair of quasi-reversible and well-defined redox peaks at the modified electrode in pH?7.0 solution. Simultaneously, the oxidation peak potentials of CC and HQ become separated by 112?mV. When simultaneously changing the concentrations of both CC and HQ, the response is linear between 9.0???M and 1.50?mM. In the presence of 0.15?mM of the respective isomer, the electrode gives a linear response in the range from 5.0 to 350???M, and from 9.0 to 500???M for CC and HQ, respectively, and detection limits are 0.36 and 0.86???M. The method was successfully examined for real sample analysis with high selectivity and sensitivity.

Preparation of a nanocomposite film from poly(diallydimethyl ammonium chloride) and gold nanoparticles by in-situ electrochemical reduction, and its application to SERS spectroscopy and sensing of ascorbic acid

A nanocomposite film is described that is composed of alternating layers of poly(diallydimethyl ammonium chloride) and gold nanoparticles that interact through electrostatic forces. The films of varying thickness were prepared by the layer-by-layer technique, and Au-NPs were generated by electrochemical reduction of hexachloroauric acid. The composite films were characterized by UV?Cvis spectroscopy, X-ray photoelectron spectroscopy, and cyclic voltammetry. Most nanocomposite films exhibit linear, uniform, and regular layer-by-layer growth during the process of formation. The films exhibit unique performance in terms of surface enhanced Raman scattering and electrocatalytic activitiy towards the oxidation of ascorbic acid.

Colorimetric and bare-eye detection of alkaline earth metal ions based on the aggregation of silver nanoparticles functionalized with thioglycolic acid

We describe a simple and rapid method for colorimetric and bare-eye detection of the alkaline earth metal ions Mg(II), Ca(II), Sr(II) and Ba(II) based on the use of silver nanoparticles (AgNPs) functionalized with thioglycolic acid (TGA). The TGA ligand was self-assembled onto the AgNPs to form a probe that undergoes a color change from yellow to orange or red on exposure to the alkaline earth ions. It is presumed that the color change is a result of the aggregation of the AgNPs caused by the interaction of the bivalent ions with the carboxy groups on the AgNPs. The color change can be used for bare-eye and colorimetric determination of the alkaline earth metal ions, for example to rapidly determine water hardness.

Electrochemical synthesis of a graphene sheet and gold nanoparticle-based nanocomposite, and its application to amperometric sensing of dopamine

We describe a simple, green and controllable approach for electrochemical synthesis of a nanocomposite made up from electrochemically reduced graphene oxide (ERGO) and gold nanoparticles. This material possesses the specific features of both gold nanoparticles and graphene. Its morphology was characterized by scanning electron microscopy which reveals a homogeneous distribution of gold nanoparticles on the graphene sheets. Cyclic voltammetry was used to evaluate the electrochemical properties of this nanocomposite towards dopamine by modification of it on surface of glassy carbon electrode (GCE). Compared to the bare GCE, the electrode modified with gold nanoparticles, and the electrode modified with ERGO, the one modified with the nanocomposite displays better electrocatalytic activity. Its oxidation peak current is linearly proportional to the concentration of dopamine (DA) in the range from 0.1 to 10?μM, with a detection limit of 0.04?μM (at S/N?=?3). The modified electrode also displays good storage stability, reproducibility, and selectivity.

A glassy carbon electrode modified with graphene and tyrosinase immobilized on platinum nanoparticles for sensing organophosphorus pesticides

An amperometric biosensor is described for the detection of organophosphorus pesticides. It is based on the enzyme tyrosinase immobilized on platinum nanoparticles and the use of a glassy carbon electrode modified with graphene. Tyrosinase was immobilized on the electrode surface via electrostatic interaction between a monolayer of cysteamine and the enzyme. In the presence of catechol as a substrate, the pesticides chlorpyrifos, profenofos and malathion can be determined as a result of their inhibition of the enzyme which catalyzes the oxidation of catechol to o-quinone. Platinum nanoparticles and graphene effectively enhance the efficiency of the electrochemical reduction of o-quinone, thus improving sensitivity. Under optimum experimental conditions, the inhibition effect of the pesticides investigated is proportional to their concentrations in the lower ppb-range. The detection limits are 0.2, 0.8 and 3?ppb for chlorpyrifos, profenofos and malathion, respectively. The biosensor displays good repeatability and acceptable stability.

Sensitive detection of tuberculosis using nanoparticle-enhanced surface plasmon resonance

We show that the antigen CFP-10 (found in tissue fluids of tuberculosis patients) can be used as a marker protein in a surface-plasmon resonance (SPR) based method for early and simplified diagnosis of tuberculosis. A sandwich SPR immunosensor was constructed by immobilizing the CFP-10 antibody on a self-assembled monolayer on a gold surface, this followed by blocking it with bovine serum albumin. Following exposure of the sensor surface to a sample containing CFP-10, secondary antibody immobilized on nickel oxide nanoparticles are injected which causes a large SPR signal change. The method has a dynamic range from 0.1 to around 150 ng per mL of CFP-10, and a detection limit as low as 0.1 ng per mL. This is assumed to be due to the high amplification power of the NiO nanoparticles.

Surface chemical approach to single-step measurement of antibody in human serum using localized surface plasmon resonance biosensor on microtiter plate system

In clinical settings, serum antibody levels serve as markers of pathology. For example, antibodies related to autoimmune diseases are among the conventional targets in laboratory tests. Simple clinical tests can improve the efficacy of laboratory practice. This study describes a single-step, wash-free technique for optically detecting antibodies in human serum through the localized surface plasmon resonance (LSPR) of gold nanoparticles. As a proof-of-concept experiment, the amount of antibiotin dissolved in human serum was measured with a LSPR-based biosensor in a wash-free manner using a conventional 96-well microtiter plate and a plate reader. For an efficient surface modification of biosensors, zwitterionic copolymer was used as a scaffold on the gold nanoparticle surface to immobilize antigen and blocking reagent. Single-step, wash-free measurement of antibiotin in human serum was successfully achieved. In addition, nonspecific responses from serum contents were significantly reduced because both the copolymer and hydrophilic antigen reagent that we employed were composed of poly(ethylene oxide) spacer. Comparative experiments of the antigen-antibody reaction in serum to that in buffered solution revealed that serum is a favorable environment for the biological reaction. In conclusion, our gold-nanoparticle-based LSPR method may provide a rapid and simple way to measure the amount of antibody in serum quantitatively in clinical practice.

A surface plasmon resonance study on the optical properties of gold nanoparticles on thin gold films

We report on an investigation of the optical properties of gold nanoparticles assembled as thin films of different thickness. The nanoparticles were linked to the surface of a gold chip by dithiol reagents and studied by surface plasmon resonance (SPR) spectroscopy and atomic force microscopy. There is good correlation between the experimental findings and theoretical simulation, and the respective data reveal the presence of ordered nanostructures in the assemblies. The shift in the SPR angle is linearly dependent on the particle size and the ratio of the different particles. SPR spectroscopy also reveals important information in terms of the optical constants of such films. This shall be further applied to in-situ quality control in the fabrication of optoelectronic, solar cell and semiconductor devices.

Protein-mediated synthesis of antibacterial silver nanoparticles deposited on titanium dioxide nanotube arrays

We report on an effective route to decorate titanium nanotube arrays (TiNT) with silver nanoparticles (AgNPs). In this method, surface-adsorbed antibody molecules serve as templates to bind silver ions by electrostatic interaction. The photocatalytic activity of the TiNT under UV irradiation causes the photoreduction of AgNPs to occur, and the biological template is decomposed simultaneously. This route also was successfuly applied to gold nanoparticles (starting from negatively charged metallic precursor ions). Compared to undecorated samples, the AgNPs/TiNT samples under visible light display a much higher antibacterial activity against Escherichia coli.

Biosensor for aluminium(III) based on its inhibition of α-chymotrypsin immobilized on a screen-printed carbon electrode modified with gold nanoparticles

We report on an amperometric assay for Al(III) ions that is based on the inhibition of the enzyme α-chymotrypsin. Screen-printed carbon electrodes modified with gold nanoparticles were used as solid supports for the immobilization of the enzyme. The amperometric response of the synthetic enzyme substrate substrate N-benzoyl-L-tyrosine ethyl ester is affected by Al(III) ions, and this leads to a decrease in the amperometric oxidation current. The assay has a detection limit of 3.3?μM of Al(III). The repeatability and reproducibility of the method are 6.9% (n?=?3) and 6.4% (n?=?5), respectively. Main interferents include Mo(VI), W(VI) and Fe(III) ions. The method was successfully applied to the determination of Al(III) in tap water.