Zeptomolar detection of Hg2 + based on label-free electrochemical aptasensor: One step closer to the dream of single atom detection

An ultra-sensitive and highly selective electrochemical label-free aptasensor is proposed for the quantitation of Hg^2 + based on the hybridization/dehybridization of double-stranded DNA (dsDNA) on a gold electrode. Thiol-substituted single-stranded DNA (ssDNA) is self-assembled on the gold electrode surface through the SAu interaction. The hybridization of ssDNA with complementary DNA (cDNA) and the consequences of dehybridization in the presence of mercury ions are followed through differential pulse voltammetry (DPV) responses using a [Fe(CN)₆]^{3 −/4 −} redox probe. The formation of a thymine–Hg^2 +–thymine (T–Hg^2 +–T) complex is the key to producing a highly selective and sensitive aptasensor for Hg^2 + determination. Specifically, the present electrochemical aptasensor is able to quantify Hg^2 + ions in concentrations from 5 zeptomolar (zM) to 55 picomolar (pM) with a limit of detection of 0.6 zM, close to the dream of single atom detection, without requiring a complicated procedure or expensive materials.     

Nucleic acid immobilized polypyrrole–polyvinylsulphonate film for Mycobacterium tuberculosis detection

21-mer peptide acid nucleic acid (PNA) probe specific to 16s–23s rRNA spacer region of Mycobacterium tuberculosis has been covalently immobilized on polypyrrole–polyvinylsulphonate film electro-chemically deposited onto indium-tin-oxide (ITO) glass for detection of complementary target by monitoring guanine oxidation and redox indicators (methylene blue and ruthenium complex) up to 0.1 fmole, 0.1 attomole and 1.0 pmole, respectively within 30 s of hybridization time. The peptide nucleic acid immobilized polypyrrole–polyvinylsulphonate electrode can be used for hybridization detection with complementary sequence in heat-shocked genomic DNA and in serum samples containing genomic M. tuberculosis DNA up to 2.5 pg/μl within about 60 min at 30 °C and can be used 8–9 times.     

Direct electrochemistry of single-stranded DNA on an ionic liquid modified carbon paste electrode

Electrochemical oxidation of thermally denatured single-stranded DNA (ssDNA) was studied on a room temperature ionic liquid N-butylpyridinium hexafluorophosphate (BPPF₆) modified carbon paste electrode (IL-CPE). A distinct oxidation peak appeared at +0.772 V (vs. SCE) on the IL-CPE after preconcentration of ssDNA at +0.35 V for 160 s in pH 7.0 phosphate buffer solution (PBS), which was attributed to the oxidation of guanine residue on the ssDNA molecular structure. The results showed an apparent negative shift of the oxidation peak potential and a great enhancement of the oxidation peak current on the IL-CPE compared with that of CPE. The electrochemical parameters of ssDNA on the IL-CPE were further calculated. Under the selected conditions, a linear calibration curve for ssDNA detection was obtained in the concentration range from 10.0 to 110.0 μg mL⁻¹ with the detection limit of 1.5 μg mL⁻¹(3σ).     

Impedance‐Based DNA Biosensor Employing Molecular Beacon DNA as Probe and Thionine as Charge Neutralizer

In this paper, we present an impedance‐based DNA biosensor using thionine intercalation to amplify DNA hybridization signal. Beacon single‐stranded DNA (ssDNA) probe and mercaptoacetic acid were self‐assembled onto a Au electrode by forming Au? S bonds. These beacon ssDNAs were hybridized with the complementary sequences around the loop structure. Then thionine was intercalated into the double‐stranded DNA (dsDNA) immobilized on the Au electrode surface. Due to the neutralization of the negative charges of dsDNA by the intercalated thionine, the electronic transfer resistance (R_et) of the DNA modified Au electrode was significantly diminished. Herein, the decreased value of R_et resulted from the thionine intercalating into dsDNA was employed as the hybridization signal. SDS was used to reduce the unspecific adsorption between ssDNA and thionine. Several experimental conditions, including the surface coverage of ssDNA probe on Au electrode, the hybridization temperature and time were all optimized. Moreover, the hybridization reactions of the unstructured linear ssDNA probe and the structured beacon ssDNA probe with their complementary sequences were compared in this work. The sensitivity of the presented DNA biosensor highlighted that the intercalation of thionine into dsDNA was an efficient approach to amplify the hybridization signal using impedance detection technique. Additionally, in this DNA biosensing protocol, beacon ssDNA has a good ability to distinguish target DNA sequences. This results in a higher specificity than using traditional unstructured DNA probe.     

DNA surface nanopatterning by selective reductive desorption from polycrystalline gold electrode

Selective electrochemical desorption was employed to pattern polycrystalline gold electrodes with thiolated DNA. The sacrificial thiol 3-mercaptopropionic acid (3-MPA) was selectively desorbed from the crystallographic plane Au(1 1 1) to revealed bare gold domains, surrounded by SAMs of 3-MPA present on the adjacent low index planes Au(1 1 0) and Au(1 0 0). Thiolated DNA sequences were further immobilised on the revealed Au(1 1 1) and the hybridisation efficiency towards complementary and non-complementary sequences evaluated. All derivatisation steps were followed by cyclic voltammetry and faradaic electrochemical impedance spectroscopy. Successful hybridisation resulted in large drops in resistance to charge transfer, attributed to the extension of the DNA surface duplex into solution resulting in an increased diffusion of electrochemical probes to the electrode surface. The results demonstrated the feasibility of the method to generate a DNA sensor able to efficiently discriminate between complementary and non-complementary sequences with good reproducibility.     

Nanostructured zirconium oxide based genosensor for Escherichia coli detection

A deoxyribonucleic acid (DNA) biosensor has been fabricated via immobilization of 17 base terminal single stranded DNA (ssDNA) identified from the 16s rRNA coding region of Escherichia coli onto sol–gel derived nanostructured zirconium oxide (NanoZrO₂) film. An oligonucleotide probe with a terminal 5′-phosphate group has been attached to the surface of the electrode via affinity of NanoZrO₂ for phosphate. The results of hybridization studies carried out with the complementary, non-complementary and genomic DNA reveal that ssDNA/NanoZrO₂/ITO bioelectrode has a high selectivity and sensitivity towards hybridization detection with limits of 10^?6–10⁶ pM of complementary DNA.     

Electrochemical biosensor based on hairpin DNA probe using 2-nitroacridone as electrochemical indicator for detection of DNA species related to Chronic Myelogenous Leukemia

In this article, a new kind of hairpin DNA Electrochemical biosensor using nitroacridone as electrochemical indicator was first designed, and used to detect BCR/ABL fusion gene in Chronic Myelogenous Leukemia (CML). The results indicated that in pH 7.0 Tris–HCl buffer solution, the oxidation peak current was linear with the concentration of complementary strand in the range of 6.2 × 10⁻⁸–3.1 × 10⁻⁷ mol/l with a detection limit of 5.3 × 10⁻⁹ mol/l. This new hairpin DNA electrochemical biosensor demonstrates its excellent specificity for single-base mismatch and complementary (dsDNA) after hybridization, and this probe has been used for assay of PCR product of a real sample with satisfactory result.     

Anodic stripping voltammetric analysis of trace arsenic(III) enhanced by mild hydrogen-evolution at a bimetallic Au–Pt nanoparticle modified glassy carbon electrode

A glassy carbon electrode (GCE) modified with electrodeposited bimetallic Au–Pt nanoparticles (Au–PtNPs) was applied to sensitively detect As(III) by linear sweep anodic stripping voltammetry (LSASV). In 0.5 M aqueous H₂SO₄, atomic hydrogen and molecular hydrogen were easily electrogenerated at the Pt sites on Au–PtNPs/GCE, which can chemically reduce As(III) to As(0) and enhance the cathodic preconcentration of As(0) at both the Pt sites and the neighboring Au sites. Since the As(0)–Au affinity is weaker than the As(0)–Pt affinity, the preconcentrated As(0) can be rapidly oxidized on/near the surface Au sites of Au–PtNPs/GCE, yielding sharper and higher LSASV current peaks. Under optimum conditions (700 s preconcentration at − 0.1 V, 5 V s^− 1), the LSASV peak current for the As(0)–As(III) oxidation responded linearly to As(III) concentration from 0.005 to 3.0 μM with a limit of detection (LOD) of 3.7 nM (0.28 ppb) (S/N = 3), while that for the As(III)–As(V) oxidation was linear with As(III) concentration from 0.01 to 3.0 μM with a LOD of 6.0 nM (0.45 ppb) (S/N = 3). This method was applied for analysis of As(III) in real water samples.     

Influence of chemical composition of electrode material on the differential capacitance characteristics of the ionic liquid | electrode interface

The electrical double layer structure at polycrystalline metal | ionic liquid interface has been studied using cyclic voltammetry, electrochemical impedance spectroscopy and in situ infrared methods. Polycrystalline Bi(PC), Pb(PC), Au(PC) and Pt(PC) electrodes have been prepared using ultra-high vacuum magnetron sputtering method. Noticeable dependence of differential capacitance on the electrode potential has been observed. For all electrodes, a wide well-expressed minimum in capacitance, potential (C, E) curve has been shown. For graphene, C(0001), carbide-derived carbon and Bi(PC) U-shaped curves and for Pb(PC), Au(PC) and Pt(PC) M-shaped C, E curves have been measured. Dependence of the C, E curve shape on the electrode chemical composition has been explained by the different position of the image plane of surface charge, dependent on the electronic characteristics of the electrodes under study.     

Label-free electrochemical detection of Avian Influenza Virus genotype utilizing multi-walled carbon nanotubes–cobalt phthalocyanine–PAMAM nanocomposite modified glassy carbon electrode

A novel DNA electrochemical biosensor for label-free determination of DNA sequence related to the Avian Influenza Virus (AIV) genotype was demonstrated in this paper. First, the multi-walled carbon nanotubes–cobalt phthalocyanine (MWNTs–CoPc) nanocomposite and poly (amidoamine) (PAMAM) dendrimer (generation 4.0) were modified on the glassy carbon electrode (GCE) sequentially. Then, DNA probes were successfully immobilized on the modified electrode with G4 PAMAM dendrimer acting as the coupling agent. The hybridization events were monitored by differential pulse voltammetry (DPV) measurement based on the oxidation signals of guanine without any external labels. Under the optimal conditions, the difference in guanine oxidation signal of the probe modified GCE in the absence and presence of complementary target (ΔI_p) was linear with the logarithmic value of the complementary target concentration from 0.01 to 500 ng/ml with a correlation coefficient of 0.998 and a detection limit of 1.0 pg/ml.     

Microwave-radiated synthesis of gold nanoparticles/carbon nanotubes composites and its application to voltammetric detection of trace mercury(II)

Gold nanoparticles/carbon nanotubes (Au-NPs/CNTs) composites were rapidly synthesized by microwave radiation, and firstly applied for the determination of trace mercury(II) by anodic stripping voltammetry (ASV). The structure and composition of the synthesized Au-NPs/CNTs nanocomposites were characterized by transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), UV–vis absorption spectroscopy and cyclic voltammetry. Au-NPs/CNTs nanocomposites modified glassy carbon electrode (Au-NPs/CNTs/GCE) exhibited excellent performance for Hg(II) analysis. A wide linear range (5 × 10⁻¹⁰–1.25 × 10⁻⁶ mol/L) and good repeatability (relative standard deviation of 1.84%) were obtained for Hg(II) detection. The limit of detection was found to be 3 × 10⁻¹⁰ mol/L (0.06 μg/L) at 2 min accumulation, while the World Health Organization’s guideline value of mercury for drinking water is 1 μg/L, suggesting the proposed method may have practical utility.     

Quasi-reversible two-electron reduction of oxygen at gold electrodes modified with a self-assembled submonolayer of cysteine

The electrochemical reduction of molecular oxygen (O₂) has been performed at gold electrodes modified with a submonolayer of a self-assembly (sub-SAM/Au) of a thiol compound (typically cysteine (CYST)) in O₂-saturated 0.5 M KOH. At bare gold electrode O₂ reduction reaction proceeds irreversibly, while this reaction is totally hindered at gold electrodes with a compact structure of CYST over its surface. The partial reductive desorption of the compact CYST monolayer was achieved by controlling the potential of the CYST/Au electrode, leading to the formation of a submonolayer coverage of the thiol compound over the Au electrode surface (sub-SAM/Au), at which the CYST molecules selectively block the Au(1 0 0) and Au(1 1 0) fractions (the so-called rough domains) of the polycrystalline Au while the Au(1 1 1) component (the so-called smooth domains) remains bare (i.e., uncovered with CYST). This sub-SAM/Au electrode extraordinarily exhibits a quasi-reversible two-electron reduction of molecular oxygen (O₂) in alkaline medium with a peak separation (ΔE_p) between the cathodic and anodic peak potentials (E_p^c,E_p^a) of about 60 mV. The ratio of the anodic current to the cathodic one is close to unity. The formal potential (E^o′) of this reaction is found to equal −150 mV vs. Ag/AgCl/KCl(sat.).     

Electrochemical synthesis of Au/polyaniline–poly(4-styrenesulfonate) hybrid nanoarray for sensitive biosensor design

Au/polyaniline (PANI)–poly(4-styrenesulfonate) (PSS) hybrid nanoarray is fabricated for biomolecular sensing in neutral aqueous solutions. Firstly, an array of one-dimensional Au nanorods (diameter = ca. 200 nm, length = ca. 3 μm) is formed by a template-electrodeposition method using a porous anodic alumina membrane, and then a thin PANI–PSS composite layer is electropolymerized on the surface of the Au nanorods. The resulting Au/PANI–PSS hybrid nanoarray exhibits a quasi-reversible redox electrochemical process at ca. +0.11 V and electrocatalytic oxidation of reduced β-nicotinamide adenine dinucleotide (NADH) is attained with a detection limit of 0.3 μM in a neutral solution.     

Amperometric biosensing of glutamate using carbon nanotube based electrode

Amperometric biosensing of glutamate using nanobiocomposite derived from multiwall carbon nanotube (CNT), biopolymer chitosan (CHIT), redox mediator meldola’s blue (MDB) and glutamate dehydrogenase (GlDH) is described. The CNT composite electrode shows a reversible voltammetric response for the redox reaction of MDB at −0.15 V; the composite electrode efficiently mediates the oxidation of NADH at −0.07 V, which is 630 mV less positive than that on an unmodified glassy carbon (GC) electrode. The CNTs in the composite electrode facilitates the mediated electron transfer for the oxidation of NADH. The CNT composite electrode is highly sensitive (5.9 ± 1.52 nA/μM) towards NADH and it could detect as low as 0.5 μM of NADH in neutral pH. The CNT composite electrode is highly stable and does not undergo deactivation by the oxidation products. The electrode does not suffer from the interference due to other anionic electroactive compounds such as ascorbate (AA) and urate (UA). Separate voltammetric peaks have been observed for NADH, AA and UA, allowing the individual or simultaneous determination of these bioanalytes. The glutamate biosensor was developed by combining the electrocatalytic activity of the composite film and GlDH. The enzymatically generated NADH was electrocatalytically detected using the biocomposite electrode. Glutamate has been successfully detected at −0.1 V without any interference. The biosensor is highly sensitive, stable and shows linear response. The sensitivity and the limit of detection of the biosensor was 0.71 ± 0.08 nA/μM and 2 μM, respectively.     

Electrochemical properties of niosomes modified Au electrode and DNA recognition

Non-ionic surfactant vesicles (NSVs), also referred to as niosomes, have been studied as an alternative to conventional liposomes. In this paper, electrochemical inspection of the interaction between Herring sperm DNA and niosomes has been investigated after a simple and novel method for the formation of niosomes on Au electrode. Each step of electrode modification has been confirmed with cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The assembly of octadecanethiol (ODT) layer on the electrode surface generates a packed film that introduces a barrier to the interfacial electron transfer (R_et), and the subsequent immobilization of niosomes onto the self-assembled monolayer (SAM) layer results in a further increase of R_et, due to the formed bilayer almost blocked the redox probe to the electrode surface. When Herring sperm DNA was added, the R_et value decreased, indicating that the barrier of the redox probe to the surface was disrupted. The addition of DNA caused the formation of some transmembrane channels for the redox probe across the niosomes. A good linear relationship between R_et value and DNA concentration was found over the 0–0.05 mg mL⁻¹ concentration range.     

Photoelectrochemical determination of inorganic mercury ions based on energy transfer between CdS quantum dots and Au nanoparticles

A novel photoelectrochemical (PEC) sensor for mercury ions (Hg^2 +) was fabricated based on the energy transfer (ET) between CdS quantum dots (QDs) and Au nanoparticles (NPs) with the formation of T–Hg^2 +–T pairs. In the presence of Hg^2 + ions, a T-rich single-strand (ss) DNA labeled with Au NPs could hybridize with another T-rich ssDNA anchored on the CdS QDs modified electrode, through T–Hg^2 +–T interactions, rendering the Au NPs in close proximity with the CdS QDs and hence the photocurrent decrease due to the ET between the CdS QDs and the Au NPs. Under the optimal condition, the photocurrent decrease was proportional to the Hg^2 + concentration, ranging from 3.0 × 10^− 9 to 1.0 × 10^− 7 M, with the detection limit of 6.0 × 10^− 10 M.     

Bismuth nano-powder electrode for trace analysis of heavy metals using anodic stripping voltammetry

In the present work, a more sensitive and conveniently usable electrode sensor for a trace analysis of heavy metal was developed by using Bi nanopowder synthesized by levitational gas condensation (LGC) method. It was observed from the TEM image that the Bi nanopowder is spherical in shape with a size of nearly 50 nm. The XRD pattern revealed intense peaks which can be indexed as a rhombohedral structure of Bi without any other diffraction peaks corresponding to an oxide or an impurity. This indicates that the resulting nanopowder synthesized by the LGC method is a highly crystallized Bi with a high purity. The square wave anodic stripping voltammograms (SWASV), experimentally measured for the Bi nanopowder electrode, showed well-defined and highly reproducible electrochemical responses relating to the stripping of Cd and Pb. The detection limit of the electrode was estimated to be 0.15 μg/l and 0.07 μg/l for Cd and Zn, respectively, on the basis of the signal-to-noise characteristics (S/N = 3) of the response for the 1.0 μg/l solution under a 10 min accumulation.     

Synthesis of Au/C and Au/Pani for anode electrodes in glucose microfluidic fuel cell

Gold nanoparticles have been prepared by two methods: chemical (ex-situ, Au/C) by two phase protocol, and electrochemical (in-situ, Au/Pani) by electroreduction of gold ions on a polyaniline film and compared as anode catalysts in a glucose microfluidic fuel cell. In this paper the structural characteristics and electrocatalytic properties were investigated by X-ray diffraction and electrochemical measurements. The catalytic behavior of both anodes was tested in a microfluidic fuel cell with a reference electrode incorporated, by means of linear sweep voltammetry (LSV), showing a cathodic shift in the glucose oxidation peak for Au/Pani. Results show a higher power density (0.5 mW cm^? 2) for Au/C anode compared with an already reported value, where a glucose microfluidic fuel cell was used in similar conditions.     

Hydrogen bubble dynamic template synthesis of porous gold for nonenzymatic electrochemical detection of glucose

Gold (Au) films with open interconnected macroporous walls and nanoparticles have been successfully sculptured using the hydrogen bubble dynamic template synthesis followed by a galvanic replacement reaction. Copper (Cu) films with open interconnected macroporous walls and nanoparticles were synthesized using the electrochemically generated hydrogen bubbles as a dynamic template. Then through a galvanic replacement reaction between the porous Cu sacrificial templates and KAu(CN)₂ in solution, the porous Cu films were converted to porous Au films with the similar morphologies. Additional electrochemical dealloying process was introduced to remove the remaining Cu from the porous Au films. X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), Energy-dispersive X-ray (EDX), X-ray diffraction (XRD) and electrochemical methods were adopted to characterize the porous Au films. The resulted porous Au films show excellent catalytic activity toward the electrooxidation of glucose. A nonenzymatic glucose sensor based on those Au film electrodes shows a linear range from 2 to 10 mM with a sensitivity of 11.8 μA cm⁻² mM⁻¹, and a detection limit of 5 μM.     

Amperometric ascorbic acid biosensor based on carbon nanoplatelets derived from ground cherry husks

In this work, a novel amperometric biosensor based on carbon nanoplatelets derived from ground cherry (Physalis peruviana) husks (GCHs-CNPTs) is reported for the sensitive and selective detection of ascorbic acid (AA). The structure of the nanoplatelets, the oxygen-containing groups and edge-plane-like defective sites (EPDSs) on the GCHs-CNPTs were characterized by scanning electron microscopy, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. The presence of GCHs-CNPTs with a high density of EPDSs effectively enhances the electron transfer between AA and the glassy carbon electrode (GCE), and thus induces a substantial decrease in the overvoltage for AA oxidation compared with both a bare GCE and a GCE modified with carbon nanotubes (CNTs/GCE). In particular, an amperometric biosensor based on GCHs-CNPTs exhibited a wider linear range (0.01–3.57 mM), higher sensitivity (208.63 μA mM^− 1 cm^− 2), a lower detection limit (1.09 μM, S/N = 3) and better resistance to fouling for AA determination compared to a CNTs/GCE. The great potential of the GCHs-CNPTs/GCE for practical and reliable AA analysis was demonstrated by the successful determination of AA in samples taken from a medical injection dose and a soft drink.