High molecular-weight silk peptide (SP) was used to functionalize the surface of nanosheets of reduced graphene oxide (rGO). The SP-rGO nanocomposite was then mixed with mouse anti-human prostate specific antigen monoclonal antibody (anti-PSA) and coated onto a glassy carbon electrode to fabricate an immunosensor. By using the hexacyanoferrate redox system as electroactive probe, the immunosensor was characterized by voltammetry and electrochemical impedance spectroscopy. The peak current, measured at the potential of 0.24 V (vs. SCE), is distinctly reduced after binding prostate specific antigen (PSA). Response (measured by differential pulse voltammetry) is linearly related to PSA concentration in the range from 0.1 to 5.0 ng · mL−1 and from 5.0 to 80.0 ng∙mL−1, and the detection limit is 53 pg∙mL−1 (at an SNR of 3). The immunosensor was successfully applied to the determination of PSA in clinical serum samples, and the results were found to agree well with those obtained with an enzyme-linked immunosorbent assay.
Nanosheets of reduced graphene oxide were functionalized with silk peptide and used to immobilize anti-PSA to fabricate an immunosensor for PSA.
We describe an electrochemical sensor for nitric oxide that was obtained by modifying the surface of a nanofiber carbon paste microelectrode with a film composed of hexadecyl trimethylammonium bromide and nafion. The modified microelectrode displays excellent catalytic activity in the electrochemical oxidation of nitric oxide. The mechanism was studied by scanning electron microscopy and cyclic voltammetry. Under optimal conditions, the oxidation peak current at a working voltage of 0.75 V (vs. SCE) is related to the concentration of nitric oxide in the 2 nM to 0.2 mM range, and the detection limit is as low as 2 nM (at an S/N ratio of 3). The sensor was successfully applied to the determination of nitric oxide released from mouse hepatocytes.
NO electrochemical sensor based on CTAB-Nafion/CNFPME was fabricated through a simple method and applied to detect NO released from mouse hepatocytes successfully.
A label-free and single-step method is reported for rapid and highly sensitive detection of bisphenol A (BPA) in aqueous samples. It utilizes an aptamer acting as a probe molecule immobilized on a commercially available array of interdigitated aluminum microelectrodes. BPA was quantified by measuring the interfacial capacitance change rate caused by the specific binding between bisphenol A and the immobilized aptamer. The AC signal also induces an AC electrokinetic effect to generate microfluidic motion for enhanced binding. The capacitive aptasensor achieves a limit of detection as low as 10 fM(2.8 fg ⋅ mL − 1) with a 20 s response time. The method is inexpensive, highly sensitive, rapid and therefore provides a promising technology for on-site detection of BPA in food and water samples.
A. AC electrokinetics effect plays a vital role in BPA detection by introducing microfluidic movement to accelerate the molecular transport to the electrode surface.
B. The ACEK capacitive aptasensor has a limit of detection as low as 10 fM (2.8 fg ⋅ mL − 1) with a 20-s response time.
This article describes an electrochemical immunosensor for rapid determination of Salmonella pullorum and Salmonella gallinarum. The first step in the preparation of the immunosensor involves the electrodeposition of gold nanoparticles used for capturing antibody and enhancing signals. In order to generate a benign microenvironment for the antibody, the ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate was used to modify the surface of a screen-printed carbon electrode (SPCE). The single steps of modification were monitored via cyclic voltammetry and electrochemical impedance spectroscopy. Based on these findings, a sandwich immunoassay was worked out for the two Salmonella species by immobilizing the respective unlabeled antibodies on the SPCE. Following exposure to the analytes, secondary antibody (labeled with HRP) is added to form the sandwich. After adding hydrogen peroxide and thionine, the latter is oxidized and its signal measured via CV. A linear response to the Salmonella species is obtained in the 104 to 109 cfu · mL−1 concentration range, and the detection limits are 3.0 × 103 cfu · mL−1 for both species (at an SNR of 3). This assay is sensitive, highly specific, acceptably accurate and reproducible. Given its low detection limit, it represents a promising tool for the detection of S. pullorum, S. gallinarum, and - conceivably - of other food-borne pathogens by exchanging the antibody.
We describe an electrochemical sandwich assay based on a screen-printed carbon electrode, gold nanoparticles and ILs and capable of detecting Salmonella pullorum and Salmonella gallinarum. The preparation is outlined in the Schematic.
We are presenting an electrochemical immunosensor for the determination of the β-agonist and food additive ractopamine. A glassy carbon electrode (GCE) was modified with gold nanoparticles and a film of a composite made from poly(arginine) and multi-walled carbon nanotubes. Antibody against ractopamine was immobilized on the surface of the modified GCE which then was blocked with bovine serum albumin. The assembly of the immunosensor was followed by electrochemical impedance spectroscopy. Results demonstrated that the semicircle diameter increases, indicating that the film formed on the surface hinders electron transfer due to formation of the antibody-antigen complex on the modified electrode. Under optimal conditions, the peak current obtained by differential pulse voltammetry decreases linearly with increasing ractopamine concentrations in the 0.1 nmol•L−1 to 1 μmol•L−1 concentration range. The lower detection limit is 0.1 nmol•L−1. The sensor displays good stability and reproducibility. The method was applied to the analysis of spiked swine feed samples and gave satisfactory results.
Immunoassay for ractopamine based on glassy carbon electrode modified with gold nanoparticles and a film of a composite made from poly (arginine) and multi-walled carbon nanotubes was proposed. Under optimal conditions, the peak currents obtained by differential pulse voltammetry decreases linearly with increasing ractopamine concentrations in the 0.1 nmol•L−1 to 1 μmol•L−1 concentration range. The detection limit is 0.1 nmol•L−1.
Alloy nanoparticles of the type PtxFe (where x is 1, 2 or 3) were synthesized by coreduction with sodium borohydride in the presence of carbon acting as a chemical support. The resulting nanocomposites were characterized by scanning electron microscopy and X-ray diffraction. The nanocomposite was placed on a glassy carbon electrode, and electrochemical measurements indicated an excellent catalytic activity for the oxidation of glucose even a near-neutral pH values and at a working voltage as low as 50 mV (vs. SCE). Under optimized conditions, the sensor responds to glucose in the 10.0 μM to 18.9 mM concentration range and with a 3.0 μM detection limit (at an S/N ratio of 3). Interferences by ascorbic acid, uric acid, fructose, acetamidophenol and chloride ions are negligible.
Nonenzymatic sensing of glucose is demonstrated at neutral pH values and low working potential using a glassy carbon electrode modified with platinum-iron alloy nanoparticles on a carbon support.
Titanium dioxide nanorods (TNR) were grown on a titanium electrode by a hydrothermal route and further employed as a supporting matrix for the immobilization of nafion-coated horseradish peroxidase (HRP). The strong electrostatic interaction between HRP and TNR favors the adsorption of HRP and facilitates direct electron transfer on the electrode. The electrocatalytic activity towards hydrogen peroxide (H2O2) was investigated via cyclic voltammetry and amperometry. The biosensor exhibits fast response, a high sensitivity (416.9 μA·mM−1), a wide linear response range (2.5 nM to 0.46 mM), a detection limit as low as 12 nM, and a small apparent Michaelis-Menten constant (33.6 μM). The results indicate that this method is a promising technique for enzyme immobilization and for the fabrication of electrochemical biosensors.
A TiO2 nanorod film was directly grown on Ti substrate by a hydrothermal route, and was further employed for a supporting matrix to immobilize horseradish peroxidase as a biosensor electrode. The as-prepared hydrogen peroxide biosensor based on Nafion/HRP/TNR/Ti electrode exhibited fast response and excellent electrocatalytic activity toward H2O2, i.e., a high sensitivity (416.9 μA mM−1), a wide linear range (2.5 × 10−8 to 4.6 × 10−4 M) with a low detection limit (0.012 μM) and a small apparent Michaelis-Menten constant (33.6 μM).
This article describes an electrochemical sensor for the dye additive Sunset Yellow (SY). It consists of a carbon paste electrode modified with nanostructured resorcinol-formaldehyde (RF) resin. The RF resin warrants strong signal enhancement and a strongly increased oxidation peak currents of SY at 0.66 V (vs. SCE). The effects of pH value, amount of RF polymer, accumulation potential and time were optimized. The sensor has a linear response to SY in the 0.3 to 125 nM concentration range, and the limit of detection is 0.09 nM after a 2-min accumulation time. The electrode was applied to the analysis of samples of wastewater and drinks, and the results are consistent with those obtained by HPLC.
Nanostructured resorcinol-formaldehyde (RF) resin was prepared and used as a material for electrochemical determination of Sunset Yellow.
Thin-layered molybdenum disulfide (MoS2) was intercalated, via ultrasonic exfoliation, into self-doped polyaniline (SPAN). This material, when placed on a glassy carbon electrode (GCE), exhibits excellent electrical conductivity and synergistic catalytic activity with respect to the detection of bisphenol A (BPA). The electrochemical response of the modified GCE to BPA was investigated by cyclic voltammetry and differential pulse voltammetry. Under optimal conditions, the oxidation peak current (measured best at 446 mV vs. SCE) is related to the concentration of BPA in the range from 1.0 nM to 1.0 μM, and the detection limit is 0.6 nM.
Thin-layered molybdenum disulfide (MoS2) was intercalated into self-doped polyaniline (SPAN) via ultrasonic exfoliation. The special conjugated structure and functional groups of MoS2-SPAN composite help to adsorb BPA easily. MoS2-SPAN has a synergistic effect for catalyzing the oxidation of BPA. The BPA electrochemical sensor based on MoS2-SPAN has a high sensitivity and low detection limit.
We report on an electrochemical method for the determination of the activity of trypsin. A multi-functional substrate peptide (HHHAKSSATGGC-HS) is designed and immobilized on a gold electrode. The three His residues in the N-terminal are able to recruit thionine-loaded graphene oxide (GO/thionine), a nanocover adopted for signal amplification. Once the peptide is cleaved under enzymatic catalysis by trypsin (cleavage site: Lys residue), the His residues leave the electrode, and the GO/thionine cannot cover the peptide-modified electrode anymore. Thus, the changes of the electrochemical signal of thionine, typically acquired at a voltage of -0.35 V, can be used to determine the activity of trypsin. A detection range of 1 × 10−4 to 1 U, with a detection limit of 3.3 × 10−5 U, can be achieved, which is better than some currently available methods. In addition, the method is highly specific, facile, and has the potential for the detection of trypsin-like proteases.
Graphene oxide was adopted as a nanocover for the development of a sensitive electrochemical method to detect the activity of trypsin.
Magnetic Fe3O4@SiO2 core shell nanoparticles containing diphenylcarbazide in the shell were utilized for solid phase extraction of Hg(II) from aqueous solutions. The Hg(II) loaded nanoparticles were then separated by applying an external magnetic field. Adsorbed Hg(II) was desorbed and its concentration determined with a rhodamine-based fluorescent probe. The calibration graph for Hg(II) is linear in the 60 nM to 7.0 μM concentration range, and the detection limit is at 23 nM. The method was applied, with satisfying results, to the determination of Hg(II) in industrial waste water.
We are presenting an electrochemical method for the determination of pyrophosphate ions (PPi) that is based on the competitive coordination of Cu(II) ion to a nanofilm of cysteine (Cys) and dissolved PPi. Cys was immobilized on the surface of a gold electrode by self-assembly. The Cys-modified gold electrode was loaded with Cu(II) ion which is released from the surface on addition of a sample containing PPi. The sensor shows an unprecedented electrochemical response to PPi, and the reduction peak currents is linearly related to the logarithm of the concentration of PPi in the 100 nM to 10 mM range (with an R2 or 0.982). The limit of detection is ~10 nM which is lower than the detection limits hitherto reported for PPi. Adenosine triphosphate (ATP), adenosine diphosphate (ADP), adenosine monophosphate (AMP) and common anions give a much weaker response. The method demonstrated here is simple, effective, highly sensitive, hardly interfered, and does not require the addition of a reagent. The method was applied to the determination of PPi in (spiked) serum samples.
Schematic illustration of the pyrophosphate sensing process.
We describe an electrochemical immunoassay for the Cry1Ab toxin that is produced by Bacillus thuringiensis. It is making use of a nanobody (a heavy-chain only antibody) that was selected from an immune phage displayed library. A biotinylated primary nanobody and a HRP-conjugated secondary nanobody were applied in a sandwich immunoassay where horseradish peroxidase (HRP) is used to produce polyaniline (PANI) from aniline. PANI can be easily detected by differential pulse voltammetry at a working voltage as low as 40 mV (vs. Ag/AgCl) which makes the assay fairly selective. This immunoassay for Cry1Ab has an analytical range from 0.1 to 1000 ng∙mL-1 and a 0.07 ng∙mL-1 lower limit of detection. The average recoveries of the toxin from spiked samples are in the range from 102 to 114 %, with a relative standard deviation of <7.5 %. The results demonstrated that the assay represented an attractive alternative to existing immunoassays in enabling affordable, sensitive, robust and specific determination of this toxin.
Nanobodies specific to Cry1Ab toxin were isolated from an immunized camel. A biotinylated primary nanobody and a HRP-conjugated secondary nanobody were applied in a sandwich immunoassay with horseradish peroxidase being used to produce polyaniline, which can be easily detected by differential pulse voltammetry.
We have investigated the gas sensing properties of ZnO thin films (100 to 200 nm thickness) deposited by room-temperature radio frequency magnetron sputtering. The sensitivity of the films to ethanol vapor was measured in the 10 to 50 ppm concentration range at operating temperatures between 200 and 400 °C. A synergetic effect of decreasing grain size and increasing operating temperature was observed towards the improvement of the sensitivity, reaching a value of 54 and a limit of detection as low as 0.61 ppm. The decrease in the grain size resulted in prolonged response time but faster recovery. In any case, both response time and recovery time are < 400 s. The results demonstrate that room-temperature magnetron sputtering is a viable approach to enhance the performances of ZnO films in sensors for ethanol vapor.
Sensor response for ZnO films in presence of 50 ppm ethanol as a function grain size and temperature
Co@Pt core-shell nanoparticles (NPs) were synthetized by a two-step reductive method using carbon (Vulcan XC-72) as a solid support. The NPs were characterized by X-ray diffraction, field emission gun scanning electron microscopy, energy dispersive X-ray spectroscopy, and transmission electron microscopy. Their electrochemical performance was evaluated by cyclic voltammetry, electrochemical impedance spectroscopy and chronoamperometry, and these showed that the Co@Pt NPs display an electrocatalytic activity towards the oxidation of glucose that is much better than that of plain Pt NPs. Under optimized conditions and at pH 7.0, the oxidation current of glucose at a working potential of −50 mV (vs. SCE) is linearly related to its concentration in the 1.0 to 30 mM range, and the detection limit is 0.3 mM (S/N = 3). It therefore covers the clinical range. The sensor also exhibits excellent stability and repeatability.
Co@Pt core-shell nanoparticles (NPs) display an electrocatalytic activity towards the oxidation of glucose that is much better than that of plain Pt NPs. The oxidation current for glucose is linearly related to its concentration in the 1.0 to 30 mM range, and the detection limit is 0.3 mM (S/N =3).
We describe a nanostructured immunosensor for the cardiovascular biomarker netrin 1. A glassy carbon electrode was consecutively modified with multi-walled carbon nanotubes (MWCNTs), nafion (to retain the MWCNTs), thionine-coated gold nanoparticles (Thi@AuNPs), and monoclonal antibodies against netrin 1. The modified electrode was characterized by transmission electron microscopy, cyclic voltammetry, differential pulse voltammetry, UV-visible spectrophotometry and X-ray diffraction. The presence of Thi@AuNPs warrants direct and convenient immobilization of the antibody. This immunoelectrode enables netrin 1 to be determined, best at a voltage of −300 mV (vs. SCE), with a limit of detection of 30 fg mL−1 (at an S/N ratio of 3) after a 50 min incubation time. The detection range extends from 0.09 to 1800 pg∙mL−1. The method is simple, sensitive, specific and reproducible. We presume this stable and reproducible biosensor to be useful for the early detection of cardiovascular diseases.
A high sensitivity immunoassay was developed for the detection of netrin 1 based on multi-walled carbon nanotubes, thionine and gold nanoparticles. Its excellent performance is ascribed to the good conductivity of MWCNTs and the combination of materials.
A simple, rapid and sensitive fluorescence resonance energy transfer (FRET) method is presented for the determination of thiols. It is based on the thiol-induced enhancement effect of the surfactant sodium dodecyl sulfate (SDS) on the efficiency of fluorescence resonance energy transfer (FRET) in nanospheres consisting of a magnetic (Fe3O4) core and a phenol-formaldehyde resin (PFR) shell containing gold nanoparticles (AuNPs). The luminescence of the core-shell nanospheres at excitation/emission wavelengths of 390/445 nm, respectively, is quenched by the AuNPs which act as energy acceptors. The interaction of AuNPs with thiol compounds in the presence of SDS suppresses FRET and gives rise to a fluorescent signal whose intensity is proportional to the thiol concentration. The analytical features of seven thiols (homocysteine, thioglycolic acid, glutathione, dodecanethiol, cysteamine, cysteine and N-acetylcysteine) were studied. Detection limits are in the range from 0.14 to 0.49 μmol L−1. The precision of the method, expressed as the relative standard deviation, ranges from 0.4 to 4.9 %. The method was applied to the determination of total thiols in water samples with recovery values between 88.7 and 104.6 %.
The fluorescence resonance energy transfer in magnetic-resin core-shell nanospheres coated with gold nanoparticles is inhibited by thiol compounds in the presence of sodium dodecyl sulfate. This gives rise to a fluorescent signal whose intensity is proportional to the thiol concentration.
Platinum nanoparticles (Pt-NPs) with sizes in the range from 10 to 30 nm were synthesized using protein-directed one-pot reduction. The model globular protein bovine serum albumin (BSA) was exploited as the template, and the resulting BSA/Pt-NPs were studied by transmission electron microscopy, energy dispersive X-ray spectroscopy, and resonance Rayleigh scattering spectroscopy. The modified nanoparticles display a peroxidase-like activity that was exploited in a rapid method for the colorimetric determination of hydrogen peroxide which can be detected in the 50 μM to 3 mM concentration range. The limit of detection is 7.9 μM, and the lowest concentration that can be visually detected is 200 μM.
Pt-NPs were synthesized using BSA-directed one-pot reduction and BSA/Pt-NPs composite can effectively catalyze the oxidation of TMB producing blue solution in the presence of H2O2.
We describe an electrochemical immunosensor for the simultaneous determination of alpha-fetoprotein (AFP) and prostate specific antigen (PSA) via a modified glassy carbon electrode. Silica nanoparticles (200–300 nm i.d.) with good monodispersity and uniform shape were synthesized, and the following species were then consecutively immobilized on their surface: gold nanoparticles (AuNPs; 5–15 nm i.d.), secondary antibody (Ab2) and the redox-probes Azure A or ferrocenecarboxy acid (Fc). In parallel, two types of primary antibodies (Ab1) were co-immobilized on the surface of the dissolved reduced graphene oxide sheets (rGO) that were also decorated with AuNPs. In the presence of antigens (AFP or PSA), the Ab2/Si@AuNPs carrying Azure A and Fc are attached to the AuNP/rGO conjugate via a sandwich type immunoreaction. Differential pulse voltammetry (DPV) was employed to measure the resulting changes in the signal of Fc or Azure A. Two well-resolved oxidation peaks, one at −0.48 V (corresponding to Azure A) and other at + 0.12 V (corresponding to Fc; both vs. SCE) can be observed in the DPV curves. Under optimal conditions, AFP and PSA can be simultaneously determined in the range from 0.01 to 25 ng mL‾1 for AFP, and from 0.012 to 25 ng mL‾1 for PSA. The detection limits are 3.3 pg mL‾1 for AFP and 4.0 pg mL‾1 for PSA (at a signal-to-noise ratio of 3). The method was applied to (spiked) real sample analysis, and the recoveries are within 96.0 and 107.2 % for PSA, and within 100.9 and 105.8 % for AFP, indicating that this dual immunosensor matches the requirements of clinical analysis.
(A) Two types of signal labels preparation process. (B) The immunosensor preparation and detection process.