AuNP-modified Electrode Research Articles

Visual and Electrochemical Detection of let-7a: A Tumor Suppressor and Biomarker.

Let-7a, a type of low-expressed microRNAs in cancer cells, has been investigated as a promising biomarker and therapeutic target for tumor suppression. Developing simple and sensitive detection methods for let-7a is important for cancer diagnosis and treatment. In this work, the hybridization chain reaction (HCR) was initiated by let-7a via two hairpin primers (H1 and H2). After the HCR, the remaining hairpin H1 was further detected by lateral flow assay (LFA) and electrochemical impedance spectroscopy. For LFA, biotin-modified H1(bio-H1) and free H2 were used for HCR. With the decrease of let-7a concentration, the color of T line gradually increased. As for electrochemical methods, the H1'-AuNP-modified electrode was used for detection of bio-H1 based on the difference of impedance (ΔRct) detected without and with different concentrations of let-7a participating in the HCR. This method could detect let-7a in the range of 10.0 fM and 1.0 nM with detection limits of 4.2 fM.

Enzyme-Free Electrochemical Sensors for in situ Quantification of Reducing Sugars Based on Carboxylated Graphene-Carboxylated Multiwalled Carbon Nanotubes-Gold Nanoparticle-Modified Electrode.

The reducing sugars of plants, including glucose, fructose, arabinose, galactose, xylose, and mannose, are not only the energy source of plants, but also have the messenger function of hormones in signal transduction. Moreover, they also determine the quality and flavor of agricultural products. Therefore, the in situ quantification of reducing sugars in plants or agriculture products is very important in precision agriculture. However, the upper detection limit of the currently developed sugar sensor is not high enough for in situ detection. In this study, an enzyme-free electrochemical sensor for in situ detection of reducing sugars was developed. Three-dimensional composite materials based on carboxylated graphene–carboxylated multi-walled carbon nanotubes attaching with gold nanoparticles (COOH-GR–COOH-MWNT–AuNPs) were formed and applied for the non-enzymatic determination of glucose, fructose, arabinose, mannose, xylose, and galactose. It was demonstrated that the COOH-GR–COOH-MWNT–AuNP-modified electrode exhibited a good catalysis behavior to these reducing sugars due to the synergistic effect of the COOH-GR, COOH-MWNT, and AuNPs. The detection range of the sensor for glucose, fructose, arabinose, mannose, xylose, and galactose is 5–80, 2–20, 2–50, 5–60, 2–40, and 5–40 mM, respectively. To our knowledge, the upper detection limit of our enzyme-free sugar sensor is the highest compared to previous studies, which is more suitable for in situ detection of sugars in agricultural products and plants. In addition, this sensor is simple and portable, with good reproducibility and accuracy; it will have broad practical application value in precision agriculture.

A Reusable Electrochemical Aptasensor for the Sensitive Detection of Pb (II) with an Electrodeposited AuNP-Modified Electrode based on the Formation of a Target-Induced G-Quadruplex

A Reusable Electrochemical Aptasensor for the Sensitive Detection of Pb (II) with an Electrodeposited AuNP-Modified Electrode based on the Formation of a Target-Induced G-Quadruplex

Novel Ce(III)-Metal Organic Framework with a Luminescent Property To Fabricate an Electrochemiluminescence Immunosensor.

We designed a novel luminescent metal-organic framework (MOF) named Ce-TCPP-LMOF (CTM) through a simple one-pot solvothermal method. CTM was synthesized by using the emerging electrochemiluminescent (ECL) material (4,4',4″,4‴-(porphine-5,10,15,20-tetrayl)tetrakis(benzoic acid) as the organic ligand and Ce(III) as the metal node. We found that CTM not only has the remarkable ability to emit light but also has a uniform "sandwich biscuit" shape and suitable nanoscale size, which are promising for further applications. We also applied CTM to construct a novel ECL immunosensor and achieve sensitive detection of the proprotein convertase subtilisin/kexin type 9 (PCSK9), a biomarker related to cardiovascular diseases. To further amplify the ECL signal of CTM, a novel dual-amplified signal strategy was established by inducing a polyamidoamine dendrimer (PAMAM) and gold nanoparticles (AuNPs). Importantly, we first proved that the ECL signal of the CTM/S2O82- system could be enhanced by the PAMAM electric field. As the electron transfer rate was accelerated by the AuNP layer, this ECL signal was further enhanced in AuNP-modified electrodes. The ECL immunosensor showed desirable performance for PCSK9 analysis within a detection range of 50 fg mL-1 to 10 ng mL-1 and a low limit of detection of 19.12 ± 2.69 fg mL-1. Real sample detection suggested that the immunosensor holds great potential for analyzing clinical serum samples.

In Situ Plant Virus Nucleic Acid Isothermal Amplification Detection on Gold Nanoparticle-Modified Electrodes.

Solid-phase isothermal recombinase polymerase amplification (RPA) offers many benefits over the standard RPA in homogeneous phase in terms of sensitivity, portability, and versatility. However, RPA devices reported to date are limited by the need for heating sources to reach sensitive detection. With the aim of overcoming such limitation, we propose here a label-free highly integrated in situ RPA amplification/detection approach at room temperature that takes advantage of the high sensitivity offered by gold nanoparticle (AuNP)-modified sensing substrates and electrochemical impedance spectroscopic (EIS) detection. Plant disease ( Citrus tristeza virus (CTV)) diagnostics was selected as a relevant target for demonstration of the proof-of-concept. RPA assay for amplification of the P20 gene (387-bp) characteristic of CTV was first designed/optimized and tested by standard gel electrophoresis analysis. The optimized RPA conditions were then transferred to the AuNP-modified electrode surface, previously modified with a thiolated forward primer. The in situ-amplified CTV target was investigated by EIS in a Fe(CN6)4-/Fe(CN6)3- red-ox system, being able to quantitatively detect 1000 fg μL-1 of nucleic acid. High selectivity against nonspecific gene sequences characteristic of potential interfering species such as Citrus psorosis virus (CPsV) and Citrus caxicia viroid (CCaV) was demonstrated. Good reproducibility (RSD of 8%) and long-term stability (up to 3 weeks) of the system were also obtained. Overall, with regard to sensitivity, cost, and portability, our approach exhibits better performance than RPA in homogeneous phase, also without the need of heating sources required in other solid-phase approaches.

An impedimetric biosensor for E. coli O157:H7 based on the use ofself-assembled gold nanoparticles and protein G.

Two kinds of electrochemical impedimetric biosensors for the detection of E. coli O157:H7 are described and compared. They were fabricated using self-assembled layers of thiolated protein G (PrG-thiol) on (i) planar gold electrodes and (ii) gold nanoparticles (Au NPs) modified gold electrodes. The fabrications of the biosensors were characterized using cyclic voltammetry, electrochemical impedance spectroscopy, scanning electron microscopy and atomic force microscopy techniques. The modification of the planar gold electrode by Au NPs via self-assembled monolayer of 1,6-hexadithiol as a linker molecule increased the electrochemically active surface area by about 2.2 times. The concentration of PrG-thiol and its incubation time, as well as the concentration of IgG were optimized. The Au NP-based biosensor exhibited a limit of detection of 48 colony forming unit (cfumL-1) which is 3 times lower than that of the planar gold electrode biosensor (140cfumL-1). It also showed a wider dynamic range (up to 107cfumL-1) and sensitivity. The improved analytical performance of the Au NP-modified biosensor is ascribed to the synergistic effect between the Au NPs and the PrG-thiol scaffold. The biosensor exhibited high selectivity for E. coli O157:H7 over other bacteria such as Staphylococcus aureus and Salmonella typhimurium. Graphical abstract Schematic representations of sensor fabrication using Au NP-modified electrode (HKEC = heat- killed E. coli O157:H7).

Gold nanostar electrodes for heavy metal detection

This paper presents a novel application of gold nanostars (AuNSs) for the electrochemical detection of arsenic (As(III)), mercury (Hg(II)), and lead (Pb(II)) in water. The AuNSs were synthesized using the Good’s buffer method, a simple, environmentally friendly procedure. Boiling the AuNS suspension enabled the formation of spherical gold nanoparticles (AuNPs), which were compared to the AuNSs to determine if the shape of the nanoparticle contributed to the functionality of the sensor. When compared to AuNSs, AuNPs exhibited a fourfold increase in specific geometric surface area (per liter) and a threefold increase in electroactive surface area (EASA). The effect of particle loading on charge transfer resistance, RCT, and double layer capacitance, CDL was assessed: as particle loading increased, RCT decreased and CDL increased for both AuNS- and AuNP-modified electrodes; however, RCT was lower for the AuNPs. For the detection of As(III), Hg(II), and Pb(II), square wave stripping voltammetry (SWSV) parameters were individually optimized and peak heights, Ip, were obtained for a range of NP loadings. The Ip values varied with respect to NP loading and displayed a unique trend for each analyte and particle shape. For As(III), the AuNSs produced higher peak heights than AuNPs of the same loading. Ip values for Hg(II) decreased dramatically at high AuNP loadings and decreased minimally at high AuNS loadings. NP loadings did not have a significant influence on Ip values associated with Pb(II). These findings suggest that gold nanostructure shape and loading can influence the efficacy of electrochemical heavy metal detection and should be taken into consideration.

Detection of bisphenol-A using electrochemical immunosensor: Comparison between competition and displacement format assay

Antibodies against bisphenol-A (Ab∝BPA) and BPA-butyric-horseradish peroxidase (HRP) were generated and utilised to fabricate a gold-nanoparticles (AuNPs)-based electrochemical biosensor for the detection of BPA. The glassy carbon electrode surface was modified with AuNPs via chronoamperometry (CA) to increase the surface-to-volume ratio and provide a robust sensing surface. The Abs∝BPA were generated and immobilized on the AuNP-modified electrode using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC)/N-hydroxysuccinimide (NHS) method with thioctic acid as linker. The assay was conducted using competition and displacement formats. Then, the working electrode, counter electrode and reference electrode were placed in a phosphate-buffered saline (PBS) containing ferrocenemethanol (FcOH) and H2O2. Sensitivity data were interpreted using the same sensor surface, enabling a direct comparison between the reduction currents of the oxidized form of ferrocenemethanol in the two assay formats which indicated the concentration of BPA bound to Ab∝BPA. Starting from a pre-blocked sensor surface, the competition assay was completed in 1h and the displacement assay took 2h. The limit of detection (LOD) in competition assay and displacement assay were 12.35μgL−1 and 4.11μgL−1, respectively.

Insect biofuel cell using an electrode with gold nanoparticles deposited by sputtering

A high-stability living battery that uses a metallic catalyst for glucose oxidation instead of enzymes is investigated. In this battery, glucose in insect haemolymph is oxidised by a gold nanoparticle (AuNP)-modified electrode, which is utilised as the anode of the battery. The AuNP-modified electrode was fabricated by sputtering gold onto a carbon cloth in a low-vacuum state. First, the oxidation performance values of glucose were evaluated and the oxidation current of glucose was obtained by the AuNP-modified electrode from cockroach haemolymph (CHL). Then, the electrocatalytic stability of the AuNP-modified electrode was compared with an enzymatic electrode. Furthermore, a glucose biofuel cell consisting of the AuNP-modified electrode and a bilirubin oxidase-modified electrode was constructed; the respective power densities obtained from 100 mM glucose solution and CHL were 22.4 and 15.1 µW/cm2, respectively. It is concluded that AuNPs are a useful catalyst for living batteries.

Large-area plasmonic electrodes and active plasmonic devices generated by electrochemical processes

This work describes a fast electrodeposition method for generating gold plasmonic electrodes and their use as active plasmonic devices. Gold nanoparticles (AuNPs) are easily obtained on large areas (up to several square centimeters) on indium tin oxide (ITO) electrodes using electroreduction of an AuIII salt by chronoamperometry. These AuNP substrates exhibit a strong localized surface plasmon resonance (LSPR) signal in the visible range, and constitute low-cost plasmonic electrodes. The average size, the density and the dispersity of the AuNPs, as seen by scanning electron microscopy (SEM), can be modulated by the potential applied during electroreduction. In this way, the LSPR can be varied from 680nm to 580nm with a concomitant marked evolution of the full-width at half-maximum from 200 to 80nm. An important result is that the LSPR of substrates generated at −0.9V using a charge density of 20mCcm−2 is close to that of AuNP gratings obtained using e-beam lithography. In a second step, the best AuNP-modified electrodes were covered with an ultrathin organic film of bisthienylbenzene (BTB) generated by electroreduction of the corresponding diazonium salt. This film switches between a conducting and an insulating state depending on the applied voltage. The effect of the switch on the plasmonic properties of AuNPs is reported. Despite the small thickness of the film (below 15nm), its conductance switch leads to a reversible modulation of the LSPR intensity by as much as 25%.

Detection of Gliadin in Foods Using a Quartz Crystal Microbalance Biosensor That Incorporates Gold Nanoparticles

This work develops a label-free gliadin immunosensor that is based on changes in the frequency of a quartz crystal microbalance (QCM) chip. A higher sensitivity was obtained by applying 25 nm gold nanoparticles (AuNPs) to the surface of a bare QCM electrode. Subsequently, chicken anti-gliadin antibodies (IgY) were immobilized directly on the AuNP-modified surface by cross-linking amine groups in IgY with glutaraldehyde. Experimental results revealed that the change in frequency exhibited when 2 ppm gliadin was bound to the AuNP-modified electrode was 35 Hz (48%) greater than that of the bare gold electrode. The linear dynamic range in 60% ethanol was from 1 × 10(1) to 2 × 10(5) ppb gliadin, and the calculated limit of detection (LOD) was 8 ppb. The entire detection process was completed in 40 min and was highly repeatable. Additionally, the AuNP-modified QCM system generated results in the detection of gliadin in 10 commercial food products that were consistent with those obtained using an AOAC-approved gliadin kit. In conclusion, the QCM platform provides a potential alternative means of ensuring that people with wheat allergies and celiac patients have access to gliadin-free food.

Mediatorless sugar/oxygen enzymatic fuel cells based on gold nanoparticle-modified electrodes

We report on the fabrication and characterisation of a gold-nanoparticle (AuNP)-based mediatorless sugar/oxygen biofuel cell (BFC) operating in neutral sugar-containing buffers and human physiological fluids, such as blood and plasma. First, Corynascus thermophilus cellobiose dehydrogenase (CtCDH) and Myrothecium verrucaria bilirubin oxidase (MvBOx), used as anodic and cathodic bioelements, respectively, were immobilised on gold electrodes modified with 20nm AuNPs. Detailed characterisation and optimisation of a new CDH/AuNP-based bioanode were performed and the following fundamental parameters were obtained: (i) the redox potential of the haem-containing centre of the enzyme was measured to be 75mV vs. NHE, (ii) the surface coverage of CtCDH was found to be 0.65pmolcm−2 corresponding to a sub-monolayer coverage of the thiol-modified AuNPs by the enzyme, (iii) a turnover number for CtCDH immobilised on thiol-modified AuNPs was calculated to be ca. 0.5s−1, and (iv) the maximal current densities as high as 40μAcm−2 were registered in sugar-containing neutral buffers. Second, both biomodified electrodes, namely the CtCDH/AuNP-based bioanode and the MvBOx/AuNP-based biocathode, were combined into a functional BFC and the designed biodevices were carefully investigated. The following characteristics of the mediator-, separator- and membrane-less, miniature BFC were obtained: in phosphate buffer; an open-circuit voltage of 0.68V, a maximum power density of 15μWcm−2 at a cell voltage of 0.52V and in human blood; an open-circuit voltage of 0.65V, a maximum power density of 3μWcm−2 at a cell voltage of 0.45V, respectively. The estimated half-lives of the biodevices were found to be >12, <8, and <2h in a sugar-containing buffer, human plasma, and blood, respectively. The basic characteristics of mediatorless sugar/oxygen BFCs were significantly improved compared with previously designed biodevices, because of the usage of three-dimensional AuNP-modified electrodes.

Formation and modification of a binary self-assembled monolayer on a nano-structured gold electrode and its structural characterization by electrochemical impedance spectroscopy

Formation and electrochemical characterization of uni- and binary-self-assembled monolayers (SAMs) are presented. One of the purposes of this study is to show the effects of a gold nanoparticle (AuNP)-modified surface on the reductive desorption of SAMs. The binary SAM consisted of 3-mercaptopropionic acid (MPA, HS–(CH 2) 2–COOH) and 1-tetradecanethiol (TDT, HS–(CH 2) 13CH 3) was formed on a bare gold (Au) and AuNP-modified electrode. Then, MPA was removed off the electrode’s surface using selective desorption technique to form a uni-SAM of TDT with defect sites. Comparisons of structures and electrochemical characteristics were performed using electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). Electrochemical parameters investigated for the comparisons were charge transfer resistance ( R p), double layer capacitance ( C dl), electron transfer rate constant ( k et), and the time constant ( τ et). It was demonstrated that the AuNP-modified electrode not only enhanced the electrochemical properties of the electrode, but also it affected the further SAM formation on and desorption from the electrode tailored with a binary SAM. The obtained nano-structures of the gold electrodes provided higher electrocatalytic activity than a planar gold electrode as characterized by EIS. The electron transfer rate constant, k et, of the AuNP electrode (2.741 × 10 −2/s) turned out to be about two times greater than the bare electrode. When both MPA and TDT were layered on the AuNP, k et lowered 10 times from the bare AuNP to 3.070 × 10 −3/s. However, for the bare gold, the lowering effect of the binary SAM on k et was much greater than the AuNP, where k et (1.343 × 10 −5/s) of the MPA + TDT SAM was three orders of magnitude lower than that of the bare gold. When only TDT remained on the surface after selective desorption of MPA, k et raised 27 times to 3.661 × 10 −4/s for the bare gold, and for the AuNP, k et doubled from 3.070 × 10 −3 of the binary SAM. For the each case, the AuNP showed higher k et than the bare gold by an order to three orders of magnitude. The effect of SAMs on k et was much more significant for the bare gold than for the AuNP, and a possible explanation was discussed. The TDT SAM-modified AuNP electrode had medium k et and C dl, which indicates a fast and sensitive electrochemical electrode. Selective desorption for two different SAMs proved to be a valid technique to control the surface coverage of a binary SAM on the gold nano-structure.

Surface Molecular Self-Assembly for Organophosphate Pesticide Imprinting in Electropolymerized Poly(p-aminothiophenol) Membranes on a Gold Nanoparticle Modified Glassy Carbon Electrode

This paper reports a surface molecular self-assembly strategy for molecular imprinting in electropolymerized polyaminothiophenol (PATP) membranes at the surface of gold nanoparticles (AuNPs) modified glassy carbon (gc) electrode for the electrochemical detection of pesticide chlorpyrifos (CPF). Electropolmerizable p-aminothiophenol (ATP) was first assembled on the AuNPs at the gc electrode surface by the formation of Au-S bonds, and subsequently, the CPF template was further assembled onto the monolayer of ATP through the hydrogen-bonding interaction between amino group and CPF. A conductive hybrid membrane was fabricated at the assembled gc electrode by the electropolymerization in a mixing solution containing additional ATP and CPF templates, and meanwhile, the CPF was spontaneously imprinted into the PATP/AuNP film. The amount of imprinted sites at the PATP/AuNP film was significantly increased due to the additional replenishment of CPF templates. The cyclic voltammetric response of the imprinted PATP-AuNP-gc sensor to CPF is about 3.2-fold as compared with the imprinted PATP-Au sensor, and the detection limit for CPF is about 2 orders of magnitude lower than that by the imprinted PATP-Au sensor. An excellent electrochemical selectivity for CPF over other pesticides was also achieved. The combination of surface molecular self-assembly with electropolymerized molecular imprinting on a larger surface area of a AuNP-modified electrode produces a high ratio of imprinted sites and, thus, provides an ultrasensitive electrochemical detection of organophosphate pesticide.

High performance bioanode based on direct electron transfer of fructose dehydrogenase at gold nanoparticle-modified electrodes

The direct electron transfer reaction of fructose dehydrogenase (FDH) from Gluconobacter sp. on alkanethiol-modified gold nanoparticles (AuNPs) was examined. AuNP-modified electrodes were simply fabricated by depositing citrate-reduced gold nanoparticles onto a gold electrode and carbon fiber paper and then covering the surface with a self-assembled monolayer of alkanethiols. The immobilization of AuNPs provided a large effective surface area for the adsorption of FDH. Catalytic oxidation currents based on the direct electron transfer reaction of FDH were observed from a potential about −100 mV (vs. Ag/AgCl, 3 M NaCl) in the presence of d-fructose without a mediator. The current density reached as high as 14.3 ± 0.93 mA/cm 2 (at +500 mV), which was achieved in the presence of 200 mM d-fructose by immobilization of FDH on 2-mercaptoethanol-modified AuNP/carbon fiber paper electrodes.

Synthesis, Characterization, and Electrochemiluminescence of Luminol‐Reduced Gold Nanoparticles and Their Application in a Hydrogen Peroxide Sensor

It was found that chloroauric acid (HAuCl(4)) could be directly reduced by the luminescent reagent luminol in aqueous solution to form gold nanoparticles (AuNPs), the size of which depended on the amount of luminol. The morphology and surface state of as-prepared AuNPs were characterized by transmission electron microscopy, UV/visible spectroscopy, X-ray photoelectron spectroscopy, FTIR spectroscopy, and thermogravimetric analysis. All results indicated that residual luminol and its oxidation product 3-aminophthalate coexisted on the surface of AuNPs through the weak covalent interaction between gold and nitrogen atoms in their amino groups. Subsequently, a luminol-capped AuNP-modified electrode was fabricated by the immobilization of AuNPs on a gold electrode by virtue of cysteine molecules and then immersion in a luminol solution. The modified electrode was characterized by cyclic voltammetry, electrochemical impedance spectroscopy, and scanning electron microscopy. The as-prepared modified electrode exhibited an electrochemiluminescence (ECL) response in alkaline aqueous solution under a double-step potential. H2O2 was found to enhance the ECL. On this basis, an ECL sensor for the detection of H2O2 was developed. The method is simple, fast, and reagent free. It is applicable to the determination of H2O2 in the range of 3x10(-7)-1x10(-3) mol L(-1) with a detection limit of 1x10(-7) mol L(-1) (S/N=3).