Enhancement of Enzymatic IDE Biosensor Response Using Gold Nanoparticles. Example of the Detection of Urea

A new conductometric biosensor based on interdigitated electrodes (IDEs) has been developed for the detection of enzymatic substrates using gold nanoparticles (GNPs), synthesized bellowing the citrate process, with an average diameter of 23 nm and functionalized with urease using layer‐by‐layer technique. A detection limit of 100 µM of urea is obtained when cross‐linked urease is directly immobilized on top of the IDEs (interdigitated distance: 20 µm) whereas a detection limit of 2 µM is obtained when urease functionalized gold nanoparticles are deposited on the top of the IDEs. The use of gold nanoparticles allows the increase of the sensitivity of detection (from 10 µS/mM to 107 µS/mM) due to the decrease of the thickness of probed zone.     

A Novel L‐Cysteine Electrochemical Sensor Using Sulfonated Graphene‐poly(3,4‐Ethylenedioxythiophene) Composite Film Decorated with Gold Nanoparticles

By exploiting the electrostatic interaction between positively charged 3,4‐ethylenedioxythiophene cation radicals and negatively charged sulfonated graphene (SG) sheets, we prepared a poly(3,4‐ethylenedioxythiophene)‐sulfonated graphene (SG‐PEDOT) composite film by a one‐step electrochemical process. The composite was further decorated with gold nanoparticles (AuNPs) and employed as an electrode material for the detection of L ‐cysteine (Cys). The SG‐PEDOT composite film is shown to provide a rough surface for the electrodeposition of AuNPs and to improve substrate accessibility and interaction with Cys. Moreover, the AuNPs‐decorated composite exhibits better electrocatalytic performance than that of a SG‐PEDOT composite only. Under optimum experimental conditions, the amperometric current of the sensor is linearly related to the concentration of Cys in the 0.1 to 382 µM range, and the detection limit is 0.02 µM (at S/N=3). The modified electrode displays favorable selectivity, good stability and high reproducibility. The method was successfully applied to the detection of Cys in spiked human urine.     

Gold Nanoparticles‐β‐Cyclodextrin‐Chitosan‐Graphene Modified Glassy Carbon Electrode for Ultrasensitive Detection of Dopamine and Uric Acid

The nanocomposite (denoted as GR‐AuNPs‐CD‐CS) of graphene (GR), gold nanoparticles (AuNPs), chitosan (CS) and β‐cyclodextrin (β‐CD) was prepared to modify a glassy carbon electrode. The as‐modified electrode was explored for the ultrasensitive detection of dopamine (DA) and uric acid (UA). The modified electrode demonstrated linearly increased current response in the concentration range of 0.1–120 µm for DA and 0.05–70 µm for UA, with so far the best detection limit for DA and UA. Good stability and repeatability were further demonstrated for the as‐made sensor.     

Water‐ and Organo‐Dispersible Gold Nanoparticles Supported by Using Ammonium Salts of Hyperbranched Polystyrene: Preparation and Catalysis

Gold nanoparticles (1 nm in size) stabilized by ammonium salts of hyperbranched polystyrene are prepared. Selection of the R groups provides access to both water‐ and organo‐dispersible gold nanoparticles. The resulting gold nanoparticles are subjected to studies on catalysis in solution, which include reduction of 4‐nitrophenol with sodium borohydride, aerobic oxidation of alcohols, and homocoupling of phenylboronic acid. In the reduction of 4‐nitrophenol, the catalytic activity is clearly dependent on the size of the gold nanoparticles. For the aerobic oxidation of alcohols, two types of biphasic oxidation are achieved: one is the catalyst dispersing in the aqueous phase, whereas the other is in the organic phase. The catalysts are reusable more than four times without loss of the catalytic activity. Selective synthesis of biphenyl is achieved by the homocoupling of phenylboronic acid catalyzed by organo‐dispersible gold nanoparticles.     

A Non‐Enzymatic Hydrogen Peroxide Sensor Based on Gold Nanoparticles/Carbon Nanotube/Self‐Doped Polyaniline Hollow Spheres

In this study, a novel non‐enzymatic hydrogen peroxide (H₂O₂) sensor was fabricated based on gold nanoparticles/carbon nanotube/self‐doped polyaniline (AuNPs/CNTs/SPAN) hollow spheres modified glassy carbon electrode (GCE). SPAN was in‐site polymerized on the surface of SiO₂ template, then AuNPs and CNTs were decorated by electrostatic absorption via poly(diallyldimethylammonium chloride). After the SiO₂ cores were removed, hollow AuNPs/CNTs/SPAN spheres were obtained and characterized by transmission electron microscopy (TEM), field‐emission scanning electron microscopy (FESEM) and Fourier transform infrared spectroscopy (FTIR). The electrochemical catalytic performance of the hollow AuNPs/CNTs/SPAN/GCE for H₂O₂ detection was evaluated by cyclic voltammetry (CV) and chronoamperometry. Using chronoamperometric method at a constant potential of ?0.1 V (vs. SCE), the H₂O₂ sensor displays two linear ranges: one from 5 µM to 0.225 mM with a sensitivity of 499.82 µA mM^?1 cm^?2; another from 0.225 mM to 8.825 mM with a sensitivity of 152.29 µA mM^?1 cm^?2. The detection limit was estimated as 0.4 µM (signal‐to‐noise ratio of 3). The hollow AuNPs/CNTs/SPAN/GCE also demonstrated excellent stability and selectivity against interferences from other electroactive species. The sensor was further applied to determine H₂O₂ in disinfectant real samples.     

Reduced Graphene Oxide/Carbon Nanotube/Gold Nanoparticles Nanocomposite Functionalized Screen‐Printed Electrode for Sensitive Electrochemical Detection of Endocrine Disruptor Bisphenol A

We fabricated a highly sensitive electrochemical sensor for the determination of bisphenol A (BPA) in aqueous solution by using reduced graphene oxide (RGO), carbon nanotubes (CNT), and gold nanoparticles (AuNPs)‐modified screen‐printed electrode (SPE). GO/CNT nanocomposite was directly reduced to RGO/CNT on SPE at room temperature. AuNPs were then electrochemically deposited in situ on RGO/CNT‐modified SPE. Under optimized conditions, differential pulse voltammetry (DPV) produced linear current responses for BPA concentrations of 1.45 to 20 and 20 to 1,490 nM, with a calculated detection limit of an ultralow 800 pM. The sensor response was unaffected by the presence of interferents such as phenol, p‐nitrophenol, pyrocatechol, 2,4‐dinitrophenol, and hydroquinone.     

Comparative study of amphiphilic hyperbranched and linear polymer stabilized organo‐soluble gold nanoparticles as efficient recyclable catalysts in the biphasic reduction of 4‐nitrophenol

A series of organo‐soluble spherical gold nanoparticles (AuNPs) were prepared through the reduction of HAuCl₄ by NaBH₄ in the presence of amphiphilic hyperbranched polymers that had a hydrophilic hyperbranched polyethylenimine core and a hydrophobic shell formed by many palmitamide (C16) chains. For comparison, the corresponding linear polymeric analog derived from linear polyethylenimine was also used to prepare the organo‐soluble AuNPs. The obtained AuNPs were characterized by transmission electron microscopy. It was found that higher feed ratio of polymer to HAuCl₄ and utilization of polymers with higher C16 density usually resulted in smaller AuNPs with relatively lower polydispersity. Except of the polymer having the pronounced low molecular weight, the molecular weight and the morphology of the amphiphilic polymers had almost no obvious effect on the size of the formed AuNPs. These organo‐soluble AuNPs could be used as efficient catalysts for the biphasic catalytic reduction of 4‐nitrophenol by NaBH₄. Their apparent rate coefficients had correlation with the molecular weight of the used amphiphilic polymers, but were less relevant to the morphology of these polymers. These organo‐soluble AuNPs could be conveniently recovered and reused many times. The morphology of the capping polymers had obvious effect on the lifetime of the AuNPs catalysts in the catalytic reduction of 4‐nitrophenol. Except of the pronounced low molecular weight hyperbranched polymer, the other hyperbranched ones with relatively high molecular weight rendered the AuNPs to have bigger turnover number values than their linear analog. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Colorimetric aptamer based assay for the determination of fluoroquinolones by triggering the reduction-catalyzing activity of gold nanoparticles

The authors describe a colorimetric assay for the detection of fluoroquinolones (FQs). It is based on the use of gold nanoparticles (AuNPs) modified with complementary DNA strands and analyte-specific FQ-binding aptamers. The modified AuNPs possess enzyme-like activity that can catalyze the reduction of nitrophenol by NaBH₄. In the absence of ciprofloxacin, the flower-shape coating on the AuNPs prevents the reduction of yellow 4-nitrophenol. In the presence of ciprofloxacin, the DNA/aptamer flower leaves on the AuNPs and the AuNPs can exert their catalytic activity. This results in a color change from yellow to colorless. The assay is highly selective for FQs, fast (1 h), and has a limit of detection as low as 1.2 nM in case of ciprofloxacin. It was successfully applied to the determination of ciprofloxacin in spiked water, serum and milk samples to give LODs of 1.3, 2.6 and 3.2 nM, respectively.     

Amperometric Biosensor for Choline Based on Gold Screen‐Printed Electrode Modified with Electrochemically‐Deposited Silica Biocomposite

A mediator‐free choline biosensor was developed using the electrochemically assisted sol‐gel deposition on gold screen‐printed electrodes. The addition of 12 mM of cationic surfactant CTAB in silica sol allowed enhancing the stability of the sensor. The modified electrode demonstrated catalytic activity and stable amperometric response to choline for over 3 weeks of exploitation with the sensitivity of 6 µA mM^?1 and LOD of 6 µM. The interference of ascorbic acid was reduced by pretreating the analyzed solution with MnO₂ powder. The application of the sensor with the purpose of identifying choline in the baby milk demonstrated satisfactory metrological characteristics.     

Arthrobacter sp. JS443‐Based Whole Cell Amperometric Biosensor for p‐Nitrophenol

An amperometric microbial biosensor for highly sensitive and selective determination of p‐nitrophenol (PNP) is reported. The biosensor consisted of PNP‐degrader Arthrobacter sp. JS443 immobilized by entrapment in Nafion polymer deposited on the top of the carbon paste electrode transducer. The biosensor was based on the measurement of the oxidation current of the intermediates 4‐nitrocatechol and 1,2,4‐benzenetriol formed by the highly selective oxidation of PNP by Arthrobacter sp. The sensor signal and response time were optimized with applied potential of +0.4 V (vs. Ag/AgCl reference electrode) and 0.03 mg of cells and operating in pH 7.5, 50 mM citrate‐phosphate buffer at room temperature. When operated at optimized conditions, the Arthrobacter sp.‐based biosensor measured as low as 5 nM (0.7 ppb) of PNP. The biosensor demonstrated excellent selectivity with no interference from phenolic compounds such as 2‐nitrophenol, phenol and 3‐chlorophenol but was interfered by 3‐nitrophenol and 3‐methyl‐4‐nitrophenol. It had good precision and intra‐ and inter‐day reproducibility, accuracy and was stable up to 3 days when stored in buffer at 4 °C. When applied for measurement in water from Lake Elsinore, CA, the results obtained were in excellent agreement with the amounts determined spectrophotometrically.     

“Click” Chemistry Mildly Stabilizes Bifunctional Gold Nanoparticles for Sensing and Catalysis

A large family of bifunctional 1,2,3‐triazole derivatives that contain both a polyethylene glycol (PEG) chain and another functional fragment (e.g., a polymer, dendron, alcohol, carboxylic acid, allyl, fluorescence dye, redox‐robust metal complex, or a β‐cyclodextrin unit) has been synthesized by facile “click” chemistry and mildly coordinated to nanogold particles, thus providing stable water‐soluble gold nanoparticles (AuNPs) in the size range 3.0–11.2 nm with various properties and applications. In particular, the sensing properties of these AuNPs are illustrated through the detection of an analogue of a warfare agent (i.e., sulfur mustard) by means of a fluorescence “turn‐on” assay, and the catalytic activity of the smallest triazole–AuNPs (core of 3.0 nm) is excellent for the reduction of 4‐nitrophenol in water.     

Electrocatalytic Nitrite Determination Using Iron Phthalocyanine Modified Gold Nanoparticles

Electrochemical detection of nitrite was achieved via electrodeposition of gold nanoparticles (AuNPs) onto glassy carbon electrodes, followed by 3‐mercaptopropionic acid (MPA) self‐assembly, enabling attachment of an iron(III) monoamino‐phthalocyanine (FeMAPc) catalyst via amide bond formation. The use of scanning electron microscopy, energy dispersive X‐ray spectroscopy and ultraviolet‐visible spectroscopy realised surface characterisation while cyclic voltammetry and electrochemical impedance spectroscopy techniques were applied for electrochemical interrogation. The electrochemical behaviour of nitrite at the bare (GCE), AuNPs/GCE, FeMAPc/GCE and FeMAPc‐MPA/AuNPs/GCE was further scrutinised using differential pulse voltammetry in phosphate buffer solution (0.1 M PBS, pH 5.8). Overall the FeMAPc‐MPA/AuNPs/GCE resulted in sensitivity 14.5 nA/µM, which was double that of AuNPs/GCE, 2.4 times FeMAPc/GCE and 3.5 times the response at a bare GCE, with linear range 1.9 µM–2.04 mM (PBS, pH 5.8) and LOD 0.21 µM. An interference study revealed that the proposed sensor (FeMAPc‐MPA/AuNPs/GCE) exhibited a selective response in the presence of interfering anions and the analytical capability of the sensor was demonstrated via nitrite ion determination in real water samples.     

Porous Silica‐Coated Gold Nanorods: A Highly Active Catalyst for the Reduction of 4‐Nitrophenol

The successful coating of thin porous silica layers of various thicknesses [(10±1), (12±1), and (14±1) nm] on cetyl trimethylammonium bromide (CTAB) capped gold nanorods was achieved through a modified Stöber procedure. The resulting material was applied as a novel catalyst for the reduction of 4‐nitrophenol. The catalytic activities of the gold nanorods increased up to eight times after coating with a layer of porous silica and the reaction followed a zero‐order kinetics, having a rate constant as high as 2.92×10^?1 mol L^?1 min^?1. The spectral changes during the reduction reaction of 4‐nitrophenol were observed within a very short span of time and a complete conversion to 4‐aminophenol occured within 5–6 mins, including the induction period of ≈2 mins. The reusability of the catalyst was studied by running the catalytic reaction during five consecutive cycles with good efficiency without destroying the nanostructure. The methodology can be effectively applied to the development of composite catalysts with highly enhanced catalytic activity.     

Stripping Analysis of As(III) by Means of Screen‐Printed Electrodes Modified with Gold Nanoparticles and Carbon Black Nanocomposite

A novel sensor based on carbon black‐gold nanoparticle nanocomposite modified screen‐printed electrode (CB‐AuNPs/SPE) for the detection of As(III) has been developed. The sensor was prepared modifying the SPE with CB and AuNPs by a drop casting automatable deposition. The As(III) was detected by CB‐AuNPs/SPE using anodic stripping voltammetry, with a high sensitivity (673±6 µA µM^?1 cm^?2) and reaching a LOD of 0.4 ppb. Finally, CB‐AuNPs/SPE has been applied to As(III) trace analysis in drinking water, obtaining satisfactory recovery values (99±9 %).     

Gold and Silver Micro‐Wire Electrodes for Trace Analysis of Metals

Micro‐wire electrodes were made from gold and silver wires (diameter: 25 μm; length: 3–21 mm) and sealed in a polyethylene holder; micro‐disk electrodes were made from the same wires and polished. The gold electrodes were electrochemically coated with mercury before use; the silver wires were used without coating. Comparative measurements demonstrated that the micro‐wire electrodes had much higher sensitivity, and a much (10–100×) lower limit of detection, than micro‐disk electrodes, and the sensitivity increased linearly with the area and length of the electrodes. Using a gold micro‐wire electrode of 21 mm and a deposition time of 300 s the limit of detection was 0.07 nM Pb in seawater of natural pH, compared to a limit of detection of 10 nM Pb (more than 100×greater) using a gold micro‐disk electrode of the same diameter. Using the silver micro‐wire electrode the limit of detection of lead was improved by a factor of 10 to 0.2 nM in acidified seawater. It is expected that the improved sensitivity of micro‐wire electrodes will lead to successful in situ detection of metals in natural waters.     

Catalytic Reduction of p‐Nitrophenol and Hexacyanoferrate (III) by Borohydride Using Green Synthesized Gold Nanoparticles

A simple and green approach for the synthesis of well‐stabilized gold nanoparticles (AuNPs) using gum Acacia (GA) is presented here. The gum acacia acts as the reductant and stabilizer. The synthesized gold nanoparticles were characterized by using ultraviolet visible (UV‐Vis), fourier transform infrared spectroscopy (FTIR), x‐ray diffraction (XRD), dynamic light scattering (DLS) and transmission electron microscopy (TEM) techniques. The UV‐Vis study revealed a distinct surface plasmon resonance at 520 – 550 nm, due to the formation of AuNPs. FTIR analysis showed the evidence that –OH groups present in the gum matrix were responsible in reducing the tetra chloroauric acid into AuNPs. XRD studies confirmed the formation of well crystalline nanoparticles with fcc structure and the particle size ranges from 4 – 29 nm, as indicated by TEM analysis. The synthesized gold nanoparticles exhibited homogeneous catalytic activity. The two model reactions studied were the reduction of p‐nitro phenol and the reduction of hexacyanoferrate (III) by borohydride ions. Both the reactions were monitored by UV‐Vis spectroscopy. The kinetic investigations were carried out for the AuNPs‐catalyzed reactions at different temperatures and different amount of catalyst.     

Electrocatalytic Determination of Traces of Hydrazine by a Glassy Carbon Electrode Modified with Palladium‐Gold Nanoparticles

A glassy carbon electrode modified with palladium/gold nanoparticles was successfully prepared by an electrodeposition process. It efficiently oxidizes hydrazine at a low overpotential of ?0.26 V versus SCE. The Pd‐AuNPs with an average size of 50–80 nm are uniformly dispersed at the GCE. The Pd‐AuNPs/GCE was used for determination of hydrazine in phosphate buffer solution of pH 7.0. The amperometric current response of the electrode was increased linearly over a hydrazine concentration of 0.1–500 µM with a limit of detection of 0.07 µM .The prepared hydrazine sensor exhibited high sensitivity, good selectivity reproducibility and long term stability.     

Synthesis,characterization, and heterogeneous catalysis of polymer‐supported poly(propyleneimine) dendrimer stabilized gold nanoparticle catalyst

We report here, the synthesis of two types of heterogeneous nanoparticle catalysts viz., polymer‐supported poly(propyleneimine)‐G2 dendrimer stabilized gold nanoparticle catalysts using crosslinked poly(4‐vinylpyridine) matrix (PSP4VP) as support material. The grafting of dendrimer on the surface of P4VP beads was characterized by FTIR spectrophotometer and CHN analyses. The immobilization of AuNPs was characterized by UV‐Vis spectrum, SEM, and HRTEM studies. The resultant polymer‐supported dendrimer stabilized AuNPs were used as a heterogeneous catalyst for the reduction of 4‐nitrophenol. The catalytic activity is found to be excellent and it can also be reused many times by simple filtration and activity remains maintained. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2525–2532, 2010     

One‐Step Synthesis of β‐Cyclodextrin Functionalized Graphene/Ag Nanocomposite and Its Application in Sensitive Determination of 4‐Nitrophenol

β‐Cyclodextrin functionalized graphene/Ag nanocomposite (β‐CD/GN/Ag) was prepared via a one‐step microwave treatment of a mixture of graphene oxide and AgNO₃. β‐CD/GN/Ag was employed as an enhanced element for the sensitive determination of 4‐nitrophenol. A wide linear response to 4‐nitrophenol in the concentration ranges of 1.0×10^?8–1.0×10^?7 mol/L, and 1.0×10^?7–1.5×10^?3 mol/L was achieved, with a low detection limit of 8.9×10^?10 mol/L (S/N=3). The mechanism and the heterogeneous electron transfer kinetics of the 4‐nitrophenol reduction were discussed according to the rotating disk electrode experiments. Furthermore, the sensing platform has been applied to the determination of 4‐nitrophenol in real samples.     

Electrografting of 4‐tert‐Butylcatechol on GC Electrode. Selective Electrochemical Determination of Homocysteine

A new sensor based on the grafting of 4‐tert‐butylcatechol on the surface of a glassy carbon electrode (GC) was developed for the catalytic oxidation of homocysteine ( Hcy ). The GC‐modified electrode exhibited a reversible redox response at neutral pH. Under the optimum conditions cyclic voltammetric results indicated the excellent electrocatalytic activity of modified electrode toward the oxidation of Hcy at reduced over‐potential about 350 mV. A linear dynamic range of 0.01–3.0 mM and a detection limit of 1.0 µM were obtained for Hcy . The modified electrode was used as an electrochemical sensor for selective determination of Hcy in human blood.