Selective detection of As(III) at the Au(111)-like polycrystalline gold electrode

Selective electrochemical detection of As(III) using a highly sensitive platform based on a Au(111)-like surface is described. The Au(111)-like surface was achieved for the first time by the partial reductive desorption of n-butanethiol (n-BT) from polycrystalline gold (poly-Au), on which a self-assembled monolayer (SAM) of n-BT was formed previously, which allows the selective blockage of the Au(100) and Au(110) surface domains by n-BT while the Au(111) domain remains bare. Square wave anodic stripping voltammetry (SWASV) using the Au(111)-like poly-Au electrode confirms the successful detection of As(III) without any interference from Cu(II). The fabricated electrode is stable and highly sensitive even in the presence of Cu(II), and it shows a linear response for As(III) up to 15 μM. The detection limit (S/N = 3) toward As(III) is 0.28 ppb, which is far below the guideline value given by World Health Organization (WHO). The electrode was applicable for the analysis of spiked arsenic in tap water containing a significant amount of various other ion elements. The results indicate that the Au(111)-like poly-Au electrode could be promising for the electrochemical detection of trace level of As(III) in real samples without any interference from Cu(II).     

"Turn-on" chemiluminescence sensor for the highly selective and ultrasensitive detection of Hg2+ ions based on interstrand cooperative coordination and catalytic formation of gold nanoparticles

Monitoring the levels of potentially toxic metal ions such as Hg(2+) in aquatic ecosystems is important because this ion can have severe effects on human health and the environment. Thus, a novel chemiluminescence (CL) sensor is developed for the highly selective and ultrasensitive detection of Hg(2+) ions in aqueous solution, based on thymine-Hg(2+)-thymine (T-Hg(2+)-T) coordination chemistry and subsequent formation of gold nanoparticles (Au NPs) in a HAuCl(4)/NH(2)OH reaction. The thus-formed Au NPs trigger the reaction between luminol and AgNO(3), producing CL emission. This novel CL technique has several advantages including high sensitivity (0.01 ppb) and selectivity over a spectrum of interfering metal ions. In view of these advantages, as well as the cost-effective, minimized working steps and portable features of the CL techniques, we expect that this CL sensor will be a promising candidate for the field detection of toxic Hg(2+) ions in environment, water, and food samples.     

Detection of polycyclic aromatic hydrocarbons using quartz crystal microbalances

A chemically coated piezoelectric sensor has been developed for the determination of PAHs in the liquid phase. An organic monolayer attached to the surface of a gold electrode of a quartz crystal microbalance (QCM) via a covalent thiol-gold link complete with an ionically bound recognition element has been produced. This study has employed the PAH derivative 9-anthracene carboxylic acid which, once bound to the alkane thiol, functions as the recognition element. Binding of anthracene via pi-pi interaction has been observed as a frequency shift in the QCM with a detectability of the target analyte of 2 ppb and a response range of 0-50 ppb. The relative response of the sensor altered for different PAHs despite pi-pi interaction being the sole communication between recognition element and analyte. It is envisaged that such a sensor could be employed in the identification of key marker compounds and, as such, give an indication of total PAH flux in the environment.     

Detection of mercury(II) by quantum dot/DNA/gold nanoparticle ensemble based nanosensor via nanometal surface energy transfer

An ultrasensitive fluorescent sensor based on the quantum dot/DNA/gold nanoparticle ensemble has been developed for detection of mercury(II). DNA hybridization occurs when Hg(II) ions are present in the aqueous solution containing the DNA-conjugated quantum dots (QDs) and Au nanoparticles. As a result, the QDs and the Au nanoparticles are brought into the close proximity, which enables the nanometal surface energy transfer (NSET) from the QDs to the Au nanoparticles, quenching the fluorescence emission of the QDs. This nanosensor exhibits a limit of detection of 0.4 and 1.2 ppb toward Hg(II) in the buffer solution and in the river water, respectively. The sensor also shows high selectivity toward the Hg(II) ions.     

Simultaneous monitoring of protein adsorption kinetics using a quartz crystal microbalance and field-effect transistor integrated device

We developed an integrated device comprising a quartz crystal microbalance (QCM) and a field-effect transistor (FET) with a single common gold electrode in a flow chamber. An alternating current inducing oscillations in the piezoelectric quartz of the QCM sensor is electrically independent of the circuit for the FET output so that the two sensors in different detection mechanisms simultaneously record binding kinetics from a single protein solution on the same electrode. A conjunction of adsorbed mass from QCM with electric nature of bound protein from FET provided deeper understanding on a complex process of nonspecific protein adsorption and subsequent conformational changes at a solid/liquid interface. Lower apparent k(on) values obtained by FET than those obtained by QCM on hydrophobic surfaces are interpreted as preferred binding of protein molecules facing uncharged domains to the electrode surface, whereas higher k(off) values by FET than those by QCM imply active macromolecular rearrangements on the surfaces mainly driven by hydrophobic association in an aqueous medium. The advanced features of the combined sensor including in situ, label-free, and real-time monitoring provide information on structural dynamics, beyond measurements of affinities and kinetics in biological binding reactions.     

Gold nanoelectrode ensembles for the simultaneous electrochemical detection of ultratrace arsenic, mercury, and copper

Simultaneous electrochemical detection of As(III), Hg(II), and Cu(II) using a highly sensitive platform based on gold nanoelectrode ensembles (GNEEs) is described. GNEEs were grown by colloidal chemical approach on thiol-functionalized solgel derived three-dimensional silicate network preassembled on a polycrystalline gold (Au) electrode. GNEEs on the silicate network have been characterized by field emission scanning electron microscopy, X-ray diffraction, diffuse reflectance spectroscopy, and electrochemical measurements. Square wave anodic stripping voltammetry (SWASV) has been used for the detection of As(III) and Hg(II) without any interference from Cu(II) at the potentials of 0.06 and 0.53 V, respectively. The GNEE electrode is highly sensitive, and it shows linear response for As(III) and Hg(II) up to 15 ppb. The detection limit (signal-to-noise ratio = 4) of the GNEE electrode toward As(III) and Hg(II) is 0.02 ppb, which is well below the guideline value given by the World Health Organization (WHO). The potential application of the GNEE electrode for the detection of As(III) in a real sample collected from the arsenic-contaminated water in 24 North Parganas, West Bengal is demonstrated. The GNEE electrode has been successfully used for the simultaneous detection of As(III), Cu(II), and Hg(II) at sub-part-per-billion level without any interference for the first time. The nanostructured electrode shows individual voltammetric peaks for As(III), Cu(II), and Hg(II) at 0.06, 0.35, and 0.53 V, respectively. The analytical performance of the GNEE electrode is superior to the existing electrodes.     

Cyanide detection using a substrate-regenerating, peroxidase-based biosensor

An enzyme-based, dual working electrode system is described for the sensing of cyanide. Horseradish peroxidase (HRP) is incorporated as the sensing element. A continuous monitoring of oxidative activity by the enzyme results through the generation and regeneration of substrates at the electrode surfaces. Thus, HRP is oxidized by hydrogen peroxide generated from dissolved oxygen, at the primary electrode, and then reduced through the secondary electrode by mediated electron transfer using ferrocene as a carrier. Ferrocene regeneration at this electrode is proportional to the intrinsic activity of HRP. The dynamics of the system are investigated by using a rotating ring-disk electrode. The enzyme is immobilized to provide better control over its catalytic activity and to increase the lifetime of the biosensor. Cyanide inhibition of current can be modeled by reversible binding kinetics. Detection of cyanide is possible in submicromolar (ppb) concentrations, with a half maximal response at 2 microM. The response time for detection of introduced cyanide is within 1 s. The sensor can be operated between 5 and 40 degrees C, and cyanide inhibition is unaffected by pH changes between 5 and 8. The sensor is reproducible for cyanide determination and is stable for over 6 months.     

Hydrogen sensor based on Au and YSZ/HgO/Hg electrode for in situ measurement of dissolved H2 in high-temperature and -pressure fluids

Gold as a hydrogen-sensing electrode for in situ measurement of dissolved H2 in aqueous solutions under extreme conditions is reported. The dissolved H2 sensor, constructed with a Au-based sensing element and coupled with a YSZ/HgO/Hg electrode, is well suited for determining dissolved H2 concentrations of aqueous fluids at elevated temperatures and pressures. The Au electrode is made of Au wire mounted in a quartz bar, which can be pressurized and heated in the high-pressure and -temperature conditions. The Au-YSZ sensor has been tested for its potential response to the concentrations of dissolved H2 in fluids by using a flow-through reactor at high temperatures up to 400 degrees C and pressures to 38 MPa. Good sensitivity and linear response between the hydrogen concentrations in the fluids and the H2 sensor potentials are reported for hydrogen gas in the concentration range of 0.1-0.001 M H2 in aqueous fluids at temperatures up to 340 degrees C and 30 MPa. Nernstian response of the cell potential to dissolved H2 in fluids was determined at 340 degrees C and 30 MPa, described as follows: DeltaE = 0.9444 + 0. 0603 log m H2 The experimental results indicate that the Au-YSZ/HgO/Hg cell can be used to measure the solubility of H2 in aqueous fluid at temperatures and pressures near to the critical state of water. Thus, this type of Au hydrogen sensor could be easily used for in situ measurement of H2 in hydrothermal fluids in a high-pressure vessel, or at midocean ridge, due to its structure of compression resistance.     

Highly Selective Cyanide Coated-Wire Electrode Based on a Recently Synthesized Co(II) Complex With the N,N′ -Bis(2-Quinolinecarboxamido)- 1,2-Benzene Applying Batch and Flow Injection Analysis Techniques

A poly(vinyl chloride) (PVC)-based coated wire electrode based on [Co(II)bqb] (bqb = N,N'-bis(2-quinolinecarboxamido)-1,2-benzene) as a novel sensing material for determination of trace amounts of cyanide ions is successfully developed. The effect of electrode substrate, membrane composition and pH of the working solution on the behavior of the sensor was investigated. The electrode was also used in flow injection potentiometry by a home-made flow cell. The electrode revealed Nernstian response towards cyanide anion over the concentration ranges 3.2times10^-7 to 2.0times10^-4 molldrL^-1 and 5.0times10^-6 to 1.0times10^-3 molldrL^-1 applying batch and flow injection analysis (FIA), respectively. The lower detection limits are 3.2times10^-7 molldrL^-1 and 5.0times10^-6 molldrL^-1 for batch and FIA, respectively. The electrode shows a short response time (<5 s) in the whole concentration range. The selectivity of the electrode in comparison with most of cyanide selective electrodes is high. The flow cell is simple to construct and free from memory effect problems over long periods of use. The proposed electrode was successfully applied for the determination of cyanide in commercially available spring water under both batch and flow injection conditions. A comparative study revealed no significant difference between ASTM method and the proposed technique.     

Mercuric ion sensing by a film bulk acoustic resonator

This paper describes a film bulk acoustic resonator (FBAR) mass sensor for detecting Hg2+ ion in water with excellent sensitivity and selectivity. When a thin Au film was deposited on the surface of an FBAR, the resonant frequency shifted to a lower value when the film was exposed to Hg2+ in aqueous solution. The FBAR sensor detected as low as 10(-9) M Hg2+ (0.2 ppb Hg2+) in water. Other ions such as K+, Ca2+, Mg2+, Zn2+, and Ni2+ had little or no effect on the resonant frequency of the FBAR. Coating of the FBAR Au surface with a self-assembled monolayer (SAM) of 4-mercaptobenzoic acid decreased the Hg2+ response.     

Superoxide dismutase-based third-generation biosensor for superoxide anion

A third-generation biosensor for superoxide anion (O2-) was developed by immobilizing superoxide dismutase (SOD) on a self-assembled monolayer of cysteine on gold electrode; i.e., a SOD/cysteine-modified gold electrode (SOD/Cys/Au) was fabricated. A rapid and direct electron transfer of SOD was realized at the gold electrode by using the cysteine molecule as an electron-transfer promoter. The promoted direct electron transfer of SOD and biomolecular recognition by the exploitation of specific and significant enzyme-substrate reactivity of SOD toward O2- combined with the low operating potential enabled a sensitive measurement of O2-. At SOD/Cys/Au, O2- could be specifically oxidized and reduced to O2 and hydrogen peroxide, respectively, through the inherent catalytic reaction of SOD. This allowed us to measure O2- by polarizing the electrode both anodically and cathodically. We could successfully measure O2- by suitably polarizing the electrode, typically at 300 and -200 mV versus Ag/AgCl without the virtual interference from physiological levels of H2O2, ascorbic acid, uric acid, and metabolites of neurotransmitters. The response mechanism of SOD/Cys/Au to O2- and its sensor characteristics are also presented and discussed.     

Detection of gold nanoparticles using an immunoglobulin-coated piezoelectric sensor

Since the existence of nanoparticles in our environment has already attracted considerable attention due to their possible toxic impact on biological systems, the field detection of nanoparticles is becoming a technology that will be much in need. We have constructed a piezoelectric sensor with an antibody-coated electrode. The antiserum can bind gold nanoparticles with a high degree of selectivity and sensitivity. The biosensor thus constructed can detect 4, 5, or 6?nm gold nanoparticles (GNPs) depending on the coated antiserum. The sensitivity for the detection of 5?nm GNPs was 10.3 ± 0.9?ng?Hz(-1), with the low limit of detection at 5.5?ng. A quartz crystal microbalance (QCM) sensor was capable of detecting GNPs and other types of nanoparticle, such as ZnO, or Fe(3)O(4). The current study provides, for the first time, a platform for detecting nanoparticles in a convenient, economical manner.     

A universal sensor for mercury (Hg, Hg(I), Hg(II)) based on silver nanoparticle-embedded polymer thin film

Detection of mercury at concentration levels down to parts-per-billion is a problem of fundamental and practical interest due to the high toxicity of the metal and its role in environmental pollution. The extensive research in this area has been focused primarily on specific sensing of mercuric (Hg(2+)) ion. As mercury exists in the oxidation states, +2, +1 and 0 all of which are highly toxic, a universal sensor covering all the three while ensuring high sensitivity, selectivity, and linearity of response, and facilitating in situ as well as ex situ deployment, would be very valuable. Silver nanoparticle-embedded poly(vinyl alcohol) (Ag-PVA) thin film fabricated through a facile protocol is shown to be a fast, efficient and selective sensor for Hg(2+), Hg(2)(2+) and Hg in aqueous medium with a detection limit of 1 ppb. The sensor response is linear in the 10 ppb to 1 ppm concentration regime. A unique characteristic of the thin film based sensor is the blue shift occurring concomitantly with the decrease in the surface plasmon resonance absorption upon interaction with mercury, making the sensing highly selective. Unlike the majority of known sensors that work only in situ, the thin film sensor can be used ex situ as well. Examination of the thin film using microscopy and spectroscopy through the sensing process provides detailed insight into the sensing event.     

Anodic stripping voltammetry of arsenic(III) using gold nanoparticle-modified electrodes

A novel method for the detection of arsenic(III) in 1 M HCl at a gold nanoparticle-modified glassy carbon electrode has been developed. The gold nanoparticles were electrodeposited onto the glassy carbon electrode via a potential step from +1.055 to -0.045 V vs SCE for 15 s from 0.5 M H2SO4 containing 0.1 mM HAuCl4. The resulting electrode surfaces were characterized with both AFM and cyclic voltammetry. Anodic stripping voltammetry of arsenic(III) on the modified electrode was performed. After optimization, a LOD of 0.0096 ppb was obtained with LSV.     

Highly responsive sensor on a nanostructured surface via the self-assembly of a biomolecule with an evanescent wave technique

A self-assembled biomolecule was used to create a highly sensitive sensor surface for detecting toxic chemical species (polychlorinated biphenyls, PCBs). We fabricated the nanostructured sensor surface via the self-assembly of cytochrome c on a Au thin film. Surface plasmon resonance (SPR), an evanescent wave technique possessing maximum sensitivity on the surface and characterized by an exponential decay of sensitivity with distance from the surface, was utilized as the principle for signal transduction. When this sensor surface was used for the detection of PCB, even trace amounts of PCB (from 0.1 ppb to 8.0 ppb) in an aqueous solution were readily detectable.     

Ultrasensitive flow injection chemiluminescence detection of DNA hybridization using signal DNA probe modified with Au and CuS nanoparticles

A novel and sensitive flow injection chemiluminescence assay for sequence-specific DNA detection based on signal amplification with nanoparticles (NPs) is reported in the present work. The "sandwich-type" DNA biosensor was fabricated with the thiol-functionalized capture DNA first immobilized on an Au electrode and hybridized with one end of target DNA, the other end of which was recognized with a signal DNA probe labeled with CuS NPs and Au NPs on the 3'- and 5'-terminus, respectively. The hybridization events were monitored by the CL intensity of luminol-H2O2-Cu(2+) after the cupric ions were dissolved from the hybrids. We demonstrated that the incorporation of Au NPs in this sensor design significantly enhanced the sensitivity and the selectivity because a single Au NP can be loaded with hundreds of signal DNA probe strands, which were modified with CuS NPs. The ratios of Au NPs, signal DNA probes, and CuS NPs modified on the gold electrode were approximately 1/101/103. A preconcentration process of cupric ions performed by anodic stripping voltammetry technology further increased the sensor performance. As a result of these two combined effects, this DNA sensor could detect as low as femtomolar target DNA and exhibited excellent selectivity against two-base mismatched DNA. Under the optimum conditions, the CL intensity was increased with the increase of the concentration of target DNA in the range of 2.0 x 10(-14)-2.0 x 10(-12) M. A detection limit of 4.8 x 10(-15) M target DNA was achieved.     

Fabrication of a gold nanoparticles decorated carbon nanotubes based novel modified electrode for the electrochemical detection of glucose

A modified electrode based on gold nanoparticles decorated multiwall carbon nanotubes (MWNTs), MWNT-Au(nano)-ME is fabricated. MWNTs are functionalized with 4-aminothiophenol and coated over the glassy carbon electrode. Further, Au nanoparticles are deposited into MWNTs coated GC electrode by electrochemical reduction of HAuCl4. Field emission transmission electron microscope (FETEM) image shows the formation of approximately 5 nm sized Au nanoparticles without any agglomeration on the MWNTs surface. Further, the presence of Au nanoparticles is confirmed through X-ray photoelectron spectroscopic (XPS) studies. The electrocatalytic activity of the MWNT-Au(nano)-ME towards the detection of glucose is investigated. MWNT-Au(nano)-ME shows enhanced current response than pristine MWNT-ME over the entire (+0.05 to +0.80 V) potential range. The modified electrode shows linear response to current with the concentration of glucose between 1 and 20 mM. Larger current responses to glucose oxidation are witnessed at +0.60 V than at +0.05 V. However, a large interference signal, reflecting the accelerated oxidation of electroactive interference is observed at +0.60 V. No overlapping signal from the interferents such as ascorbic acid, acetaminophen, and dopamine are observed at the MWNT-Au(nano)-ME at +0.05 V. Further, the MWNT-Au(nano)-ME shows high resistance to the toxictiy of chloride ions.     

Electrochemical DNAzyme sensor for lead based on amplification of DNA-Au bio-bar codes

An electrochemical DNAzyme sensor for sensitive and selective detection of lead ion (Pb(2+)) has been developed, taking advantage of catalytic reactions of a DNAzyme upon its binding to Pb(2+) and the use of DNA-Au bio-bar codes to achieve signal enhancement. A specific DNAzyme for Pb(2+) is immobilized onto an Au electrode surface via a thiol-Au interaction. The DNAzyme hybridizes to a specially designed complementary substrate strand that has an overhang, which in turn hybridizes to the DNA-Au bio-bar code (short oligonucleotides attached to 13 nm gold nanoparticles). A redox mediator, Ru(NH3)6(3+), which can bind to the anionic phosphate of DNA through electrostatic interactions, serves as the electrochemical signal transducer. Upon binding of Pb(2+) to the DNAzyme, the DNAzyme catalyzes the hydrolytic cleavage of the substrate, resulting in the removal of the substrate strand along with the DNA bio-bar code and the bound Ru(NH3)6(3+) from the Au electrode surface. The release of Ru(NH3)6(3+) results in lower electrochemical signal of Ru(NH3)6(3+) confined on the electrode surface. Differential pulse voltammetry (DPV) signals of Ru(NH3)6(3+) provides quantitative measures of the concentrations of Pb(2+), with a linear calibration ranging from 5 nM to 0.1 microM. Because each nanoparticle carries a large number of DNA strands that bind to the signal transducer molecule Ru(NH3)6(3+), the use of DNA-Au bio-bar codes enhances the detection sensitivity by five times, enabling the detection of Pb(2+) at a very low level (1 nM). The DPV signal response of the DNAzyme sensor is negligible for other divalent metal ions, indicating that the sensor is highly selective for Pb(2+). Although this DNAzyme sensor is demonstrated for the detection of Pb(2+), it has the potential to serve as a general platform for design sensors for other small molecules and heavy metal ions.     

Quinolino-triazole linked gold nanoparticles as sensitive 'turn-on' fluorescent Cd(2+) probes

Quinoline derivatives were brought into the surface of gold nanoparticles (Au NPs) through click chemistry. The fluorescence was quenched by Au NPs because of electron transfer between Au NPs and quinoline. However, upon addition of Cd(2+) to the quinoline-triazole Au NP solution, it exhibited an effective switch-on fluorescence response, owing to the coordination between quinoline and Cd(2+) which can efficiently block the electron transfer. What's more, the fluorescent sensor can effectively detect Cd(2+) in aqueous solution with a detection limit of 1.0 × 10(-5) M.     

Molecular-imprinted, polymer-coated quartz crystal microbalances for the detection of terpenes

A piezoelectric sensor coated with an artificial biomimetic recognition element has been developed for the determination of L-menthol in the liquid phase. A highly specific noncovalently imprinted polymer (MIP) was cast in situ on to the surface of a gold-coated quartz crystal microbalance (QCM) electrode as a thin permeable film. Selective rebinding of the target analyte was observed as a frequency shift quantified by piezoelectric microgravimetry with the QCM. The detectability of L-menthol was 200 ppb with a response range of 0-1.0 ppm. The response of the MIP-QCM to a range of monoterpenes was investigated with the sensor binding menthol in favor of analogous compounds. The sensor was able to distinguish between the D- and L-enantiomers of menthol owing to the enantioselectivity of the imprinted sites. To our knowledge, this is the first report describing enantiomeric resolution within an MIP utilizing a single monomer-functional moiety interaction. It is envisaged that this technique could be employed to determine the concentration of terpenes in the atmosphere.