It is reported that a mixture of WS2 nanosheets (WS2 NS) and silver nanoclusters (AgNCs) displays strongly enhanced peroxidase-mimicking activity. The catalytic effect of the mixture was studied by colorimetry, fluorometry, chemiluminescence (CL) and electrochemistry. The effect is interpreted in terms of a difference between the Fermi energy level of the two nanomaterials. This leads to the formation of charge separation regions which act as active sites for enzyme mimetic interaction with the substrates. The mixture of WS2 NS and AgNCs was exploited for the non-enzymatic determination of H2O2 and glucose. A stopped-flow method was applied as a sensitive CL detection system using the bicarbonate-H2O2 reaction. The mixture has a powerful peroxidase mimicking activity on the bicarbonate-H2O2 CL reaction, and this effect is much larger than that of any single constituent. In addition, the CL emission is improved several times by using the stopped-flow technique. Under optimum condition, H2O2 can be determined in the 2.5–1500 nM concentration range. Moreover, glucose levels in human serum can be quantified via glucose oxidase based oxidation which leads to the generation of H2O2. Using this CL assay, a linear relationship was obtained between the intensity of the CL emission and glucose concentration in the range of 0.03–20 μM, with a limit of detection (3S) of 13 nM.
Graphical abstract An enhanced peroxidase-like catalytic activity for WS2 nanosheets (WS2 NS) was revealed in the presence of silver nanoclusters (AgNCs), and was exploited for the non-enzymatic determination of H2O2, and of glucose (via glucose oxidase; GOx) using a stopped-flow CL method.
A photoelectrochemical wire microelectrode was constructed based on the use of a TiO2 nanotube array with electrochemically deposited CdSe semiconductor. A strongly amplified photocurrent is generated on the sensor surface. The microsensor has a response in the 0.05–20 μM dopamine (DA) concentration range and a 16.7 μM detection limit at a signal-to-noise ratio of 3. Sensitivity, recovery and reproducibility of the sensor were validated by detecting DA in spiked human urine, and satisfactory results were obtained.
Graphical abstract Schematic of a sensitive photoelectrochemical microsensor based on CdSe modified TiO2 nanotube array. The photoelectrochemical microsensor was successfully applied to the determination of dopamine in urine samples.
The authors describe a dye-sensitized photoelectrochemical immunoassay for the tumor marker carcinoembryonic antigen (CEA). The method employs the rhodamine dye Rh123 with red color and absorption maximum at 500 nm for spectral sensitization, and a 3D nanocomposite prepared from graphene oxide and MoS2 acting as the photoelectric conversion layer. The nanocomposite with flower-like 3D architectures was characterized by transmission electron microscopy, scanning electron microscopy, X-ray powder diffraction, and UV-vis diffuse reflectometry. A photoelectrochemical sandwich immunoassay was developed that is based on the use of the nanocomposite and based on the specific binding of antibody and antigen, and by using a secondary antibody labeled with Rh123 and CdS (Ab2-Rh123@CdS). Under optimal conditions and at a typical working voltage of 0 V (vs. Hg/HgCl2), the photocurrent increases linearly 10 pg mL?1 to 80 ng mL?1 CEA concentration range, with a 3.2 pg mL?1 detection limit.
Graphical abstract Flower-like GO-MoS2 complex with high efficiency of electron transport was synthesized to construct photoelectrochemical platform. The sandwich-type immunoassay was built on this platform based on specific binding of antigen and antibody. Carcinoembryonic antigen in sample was detected sensitively by using sensitization of rhodamine dye Rh123 as signal amplification strategy.
We describe a highly sensitive glucose probe based on carbon dots modified with MnO2. A strong reduction of the green fluorescence of the carbon dots (CDs) happened due to the surface energy transfer (SET) from CDs to the deposited MnO2. In the presence of H2O2 (formed via enzymatic oxidation of glucose), fluorescence is restored because the MnO2 nanosheets are reduced to form colorless Mn(II) ions. These findings were used to design a fluorometric glucose assay that has a detection limit as low as 44 nM (at an S/N ratio of 3).
Graphical Abstract A strong reduction of the green fluorescence of the carbon dots (CDs) occurs due to surface energy transfer (SET) from CDs to the deposited MnO2. In the presence of H2O2 (formed by enzymatic action of glucose oxidase) the MnO2 nanosheets are reduced to form colorless Mn(II) ions, and glucose can be quantified by the fluorescence restored.
The authors describe a highly efficient photoelectrochemical (PEC) scheme for the determination of hydrogen peroxide (H2O2). BiVO4 microrods were hydrothermally synthesized and deposited on fluorine - doped tin oxide (FTO) glass which acts as the working electrode. Scanning electron microscopy, X-ray powder diffraction and Raman spectroscopy were utilized for the characterization of the microrods. On irradiation with visible light, the holes generated in the microrods are capturing electrons from H2O2 to produce a photocurrent at an operating potential of 0 V vs. Ag/AgCl. Under optimal conditions, the photocurrent increases with the concentration of H2O2 in the range from 50 μmol·L?1 to 1.5 mmol·L?1, and the limit of detection is 8.5 μmol·L?1 (at 3σ). A repeatability and intermediate precision of ≤6.6% was accomplished at H2O2 levels of 0.1, 0.5 and 1.0 mmol·L?1. The method was applied to the determination of H2O2 in spiked sterilized milk samples and gave satisfactory results. As the method works at zero potential, the photocurrent can be measured with simple instrumentation such as digital multimeters, and this will enable expensive electrochemical workstations to be replaced in future.
Graphical abstract An enzyme-free photoelectrochemical sensing strategy is described for sensitive determination of hydrogen peroxide in foodstuff using fluorine-doped tin oxide electrode modified with BiVO4 microrods.
The authors describe a photoelectrochemical (PEC) immunoassay for determination of aflatoxin B1 (AFB1) in foodstuff. The competitive immunoreaction is carried out on a microplate coated with a capture antibody against AFB1 using AFB1-bovine serum albumin (BSA)-liposome-coated mesoporous silica nanoparticles (MSN) loaded with L-cysteine as a support. The photocurrent is produced by a photoactive material consisting of cerium-doped Bi2MoO6. Initially, L-cysteine acting as the electron donor is gated in the pores by interaction between mesoporous silica and liposome. Thereafter, AFB1-BSA conjugates are covalently bound to the liposomes. Upon introduction of the analyte (AFB1), the labeled AFB1-BSA complex competes with the analyte for the antibody deposited on the microplate. Accompanying with the immunocomplex, the liposomes on the MSNs are lysed upon addition of Triton X-100. This results in the opening of the pores and in a release of L-cysteine. Free cysteine then induces the electron-hole scavenger of the photoactive nanosheets to increase the photocurrent. The photocurrent (relative to background signal) increases with increasing AFB1 concentration. Under optimum conditions, the photoactive nanosheets display good photoelectrochemical responses, and allow the detection of AFB1 at a concentration as low as 0.1 pg·mL?1 within a linear response in the 0.3 pg·mL?1 to 10 ng·mL?1 concentration range. Accuracy was evaluated by analyzing naturally contaminated and spiked peanut samples by using a commercial AFB1 ELISA kit as the reference, and well-matching results were obtained.
Graphical abstract Schematic presentation of a photoelectrochemical immunoassay for AFB1. It is based on the use of Ce-doped Bi2MoO6 nanosheets and of liposome-coated mesoporous silica nanoparticles loaded with L-cysteine.
A novel photoelectrochemical (PEC) aptasensor with graphitic-phase carbon nitride quantum dots (g-C3N4; QDs) and reduced graphene oxide (rGO) was fabricated. The g-C3N4 QDs possess enhanced emission quantum yield (with an emission peak at 450 nm), improved charge separation ability and effective optical absorption, while rGO has excellent electron transfer capability. Altogether, this results in improved PEC performance. The method is making use of an aptamer against sulfadimethoxine (SDM) that was immobilized on electrode through π stacking interaction. Changes of the photocurrent occur because SDM as a photogenerated hole acceptor can further accelerate the separation of photoexcited carriers. Under optimized conditions and at an applied potential of +0.2 V, the aptasensor has a linear response in the 0.5 nM to 80 nM SDM concentration range, with a 0.1 nM detection limit (at S/N =?3). The method was successfully applied to the analysis of SDM in tap, lake and waste water samples.
Graphical abstract Graphitic-phase carbon nitride (g-C3N4) quantum dots (QDs) and reduced graphene oxide (rGO) were used to modify fluorine-doped SnO2 (FTO) electrodes for use in a photoelectrochemical (PEC) aptasensor. SDM oxidized by the hole on valance band (VB) of g-C3N4 QDs promote the separation of electron in the conductive band (CB), which made the changes of photocurrent signal.
A method is described for the determination of the activity of alkaline phosphatase (ALP). It is based on the reversible modulation of the fluorescence of WS2 quantum dots (QDs). The fluorescence of the QDs is quenched by Cr(VI) but restored by free ascorbic acid (AA). The detection scheme relies on the fact that ALP hydrolyzes the substrate ascorbic acid 2-phosphate to produce AA, and that enzymatically generated AA can restore the fluorescence of the QDs. The signal (best measured at excitation/emission peak wavelengths of 365/440 nm) increases linearly in the 0.5 to 10 U·L?1 ALP activity range, with a detection limit of 0.2 U·L?1. The method was applied to the determination of ALP activity in human serum samples and demonstrated satisfactory results.
Graphical abstract The fluorescence of chromate-loaded tungsten disulfide quantum dots (QDs) is quenched but restored after reaction with ascorbic acid that is formed by the catalytic action of alkaline phosphatase (ALP) on ascorbic acid 2-phosphate (AAP). The increase in fluorescence can be related to the activity of ALP.
The authors describe a fluorometric glucose assay that is based on the use of MnO2 nanosheets and copper nanoclusters (CuNCs) acting as nanoprobes. The CuNCs were synthesized by using bovine serum albumin as a template by chemical reduction of copper(II) sulfate. On addition of MnO2 nanosheets to a colloidal solution of CuNCs, the fluorescence of CuNCs (measured at excitation/emission wavelengths of 335/410 nm) is quenched. However, in the presence of enzymatically generated H2O2, the MnO2 nanosheets are reduced to form Mn(II) ions. As a result, fluorescence intensity recovers. The glucose assay is based on the enzymatic conversion of glucose by glucose oxidase to generate H2O2 and glucuronic acid. The calibration plot is linear in the 1 μM to 200 μM glucose concentration range, and the detection limit is 100 nM. The method was successfully applied to the determination of glucose in spiked human serum samples.
Graphical abstract A sensitive fluorescent bioassay is reported for the detection of glucose based on the hydrogen peroxide-induced decomposition of a quencher system composed of MnO2 nanosheets and copper nanoclusters (CuNCs).
The paper describes a sensitive method for simultaneous sensing of morphine (MOR) and diclofenac (DCF). The surface of a MgFe2O4/graphite paste electrode was modified with multi-walled carbon nanotubes, and the resulting sensor was characterized by cyclic voltammetry, differential pulse voltammetry, chronoamperometry, and electrochemical impedance spectroscopy. The electrode showed an efficient synergistic effect in term of oxidation of DCF and MOR, with sharp oxidation peaks occurring at +0.370 and 0.540 V (vs Ag/AgCl) at pH 7.0. The calibration plot for MOR is linear in the 50 nM to 920 μM concentration range, and the detection limit is 10 nM (at a signal-to-noise ratio of 3). The respective data for DCF are 100 nM to 580 μM, with a 60 nM LOD. The sensor was applied to the determination of MOR and DCF in spiked serum and urine samples, with recoveries ranging between 91.4 and 100.7 %.
Graphical abstract A sensitive method for simultaneous sensing of morphine (MOR) and diclofenac (DCF) is described. The surface of MgFe2O4/graphite paste electrode was modified with multi-walled carbon nanotubes, and the resulting sensor showed an efficient synergistic effect in terms of oxidation of DCF and MOR. The calibration plot for MOR is linear in the 50 nM to 920 μM concentration range, and the detection limit is 10 nM. The respective data for DCF are 100 nM to 580 μM, with a 60 nM LOD.
The authors report that carbon nitride quantum dots (CN QDs) exert a strong enhancing effect on the Cu(II)/H2O2 chemiluminescent system. Chemiluminescence (CL) intensity is enhanced by CN QDs by a factor of ~75, while other carbon nanomaterials have a much weaker effect. The possible mechanism of the effect was evaluated by recording fluorescence and CL spectra and by examining the effect of various radical scavengers. Emitting species was found to be excited-state CN QDs that produce green CL peaking at 515 nm. The new CL system was applied to the sensitive detection of H2O2 and glucose (via glucose oxidase-catalyzed formation of H2O2) with detection limits (3σ) of 10 nM for H2O2 and 100 nM for glucose. The probe was employed for glucose determination in human plasma samples with satisfactory results.
Graphical abstract The effect of carbon nitride quantum dots (CN QDs) on Cu(II)-H2O2 chemiluminescence reaction was studied and the new CL system was applied for sensitive detection of glucose based on the glucose oxidase (GOx)-catalyzed formation of H2O2.
We describe a chemical exfoliation method for the preparation of MoS2 nanosheets. The nanosheets were incorporated into poly(3,4-ethylenedioxythiophene) (PEDOT) by electrodeposition on a glassy carbon electrode (GCE) to form a nanocomposite. The modified GCE is shown to enable simultaneous determination of ascorbic acid (AA), dopamine (DA) and uric acid (UA). Due to the synergistic effect of MoS2 and PEDOT, this electrode displays better properties in terms of electrocatalytic oxidation of AA, DA and UA than pure PEDOT, which is illustrated by cyclic voltammetry and differential pulse voltammetry (DPV). Under optimum conditions and at pH 7.4, the respective sensitivities and best working potentials are as follows: AA: 1.20 A?mM?1?m?2, 30 mV; DA: 36.40 A?mM?1?m?2, 210 mV; UA: 105.17 A?mM?1?m?2, 350 mV. The calculated detection limits for AA, DA and UA are 5.83 μM, 0.52 μM and 0.95 μM, respectively. The modified electrode was applied to the detection of the three species in human urine samples and gave satisfactory results.
Graphical abstract MoS2 nanosheets were prepared by a facile chemical exfoliation method. MoS2 and poly(3,4-ethylenedioxythiophene) nanocomposite modified glassy carbon electrodes were fabricated, which are shown to enable simultaneous determination of ascorbic acid, dopamine and uric acid with high sensitivity and selectivity.
Cuprous oxide (Cu2O) thin films have been deposited onto fluorine doped tin oxide (FTO) glass substrates by using electrochemical route. The structural, morphological, and chemical composition of the deposited films have been studied by using X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Energy dispersive x-ray spectroscopy (EDAX) techniques respectively. The optical studies have been carried out by using UV-Vis spectroscopy. The effect of potential, pH and bath temperature onto absorption and band gap of Cu2O thin films have been studied. The highest sensitivity 6.25 mA·mM·cm-2 is observed for the thin films which shows glucose concentration 7 mM in 0.1 M NaOH solution. The results indicates Cu2O is promising material for glucose sensor with high sensitivity, high stability, and repeatability.
Graphical abstract The surface morphology of Cu2O thin films was found to be tip-truncated octahedral. The films were prepared by electrodeposition. The Cu2O thin films were used to construct low cost, highly sensitive and stable glucose sensor.
Thin films of La2O3 were deposited onto glass substrates by ultrasonic spray pyrolysis. Their structural and morphological properties were characterized by X-ray diffraction, Fourier transform Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray photo-electron spectroscopy, Brunauer-Emmett-Teller and optical absorption techniques. The sensor displays superior CO2 gas sensing performance at a low operating temperature of 498 K. The signal change on exposure to 300 ppm of CO2 is about 75%, and the signal only drops to 91% after 30 days of operation.
Graphical abstract Schematic diagram of the CO2 gas sensing mechanism of an interconnected web-like La2O3 nanostructure in presence of 300 ppm of CO2 gas and at an operating temperature of 498 K.
An Al-doped ZnO@Fe3O4 nanocomposite was synthesized and used as a magnetic sorbent for solid-phase extraction of Cd(II) prior to its determination by flame atomic absorption spectrometry (FAAS). The size and morphology of the nano-sorbent were characterized via X-ray diffraction analysis, scanning electron microscopy and FTIR. Following its desorption with acetic acid, cadmium was quantified by FAAS. Factors affecting the extraction of the Cd(II) were optimized. Under optimized experimental conditions, the calibration graph is linear in the 0.6 to 60 ng mL?1 concentration range. The limit of detection is 0.17 ng mL?1 and the pre-concentration factor is 50. The inter- and intra-day relative standard deviations for six replicate determinations at a Cd(II) level of 40 ng mL?1 are 3.8% and 2.5%, respectively. The method was successfully applied to the trace determination of Cd(II) in spiked water samples. The accuracy of the method was confirmed by analyzing the certified reference material NIST SRM 1643e.
Graphical abstract Schematic of the synthesis of an Al-doped ZnO@Fe3O4 nanocomposite and its application as a magnetic sorbent for solid-phase extraction of Cd(II) prior to its determination by flame atomic absorption spectrometry (FAAS).
The authors report on the preparation of a hollow-structured cobalt ferrite (CoFe2O4) nanocomposite for use in a non-enzymatic sensor for hydrogen peroxide (H2O2). Silica (SiO2) nanoparticles were exploited as template for the deposition of Fe3O4/CoFe2O4 nanosheets, which was followed by the removal of SiO2 template under mild conditions. This leads to the formation of hollow-structured Fe3O4/CoFe2O4 interconnected nanosheets with cubic spinel structure of high crystallinity. The material was placed on a glassy carbon electrode where it acts as a viable sensor for non-enzymatic determination of H2O2. Operated at a potential of ?0.45 V vs. Ag/AgCl in 0.1 M NaOH solution, the modified GCE has a sensitivity of 17 nA μM?1 cm?2, a linear response in the range of 10 to 1200 μM H2O2 concentration range, and a 2.5 μM detection limit. The sensor is reproducible and stable and was applied to the analysis of spiked urine samples, where it provided excellent recoveries.
Graphical abstract Schematic of a cobalt ferrite (CoFe2O4) hollow structure for use in electrochemical determination of H2O2. The sensor shows a low detection limit, a wide linear range, and excellent selectivity for H2O2.
The article describes a method for determination of tannic acid in extracts of medicinal plants. Tannic acid (TA) is an antioxidant and has anticancer and antimicrobial properties. Titanium dioxide nanoparticles (TiO2) were co-sensitized with 5-methylphenazinium methosulfate (PMS) and carboxy-functionalized cadmium telluride quantum dots (CdTe QDs), and immobilized on a fluorine-doped tin oxide electrode. The surface morphology and electrochemical properties of the modified electrode were investigated by scanning electron microscopy and amperometry, respectively. A composite consisting of TiO2, PMS and CdTe QDs in a nafion film has a response to TA under LED light higher than that observed for each separate component. Under optimized experimental conditions and at an applied voltage of +0.4 V vs Ag/AgCl, the photoelectrochemical sensor has a linear response in the 0.2 to 200 μmol L?1 TA concentration range and a detection limit of 60 nmol L?1. The sensor was successfully applied to the determination of TA in spiked extracts from three medicinal plants, with recovery values between 98.3 and 103.9 %.
Graphical abstract Schematic diagram for photoelectrochemical detection of tannic acid based on a fluorine doped tin oxide electrode modified with titanium oxide, 5-methylphenazinium methosulfate and carboxy-functionalized cadmium telluride quantum dots
Platinum nanoparticles (PtNPs) were uniformly grown on the surface of gold nanorods (AuNRs) by a laser irradiation procedure. Transmission electron microscopy confirmed that the PtNPs are uniformly grown on the surface of the AuNRs. The formation of PtNPs on the AuNRs leads to a red-shift of the absorption maximum from 734 nm to 766 nm. In addition, the efficiency of surface enhanced Raman scattering (SERS) is increased, but the photothermal conversion efficiency is decreased compared to pure AuNRs. The result indicates that electron transfer occurs between gold and platinum. The peroxidase mimicking effect of PtNPs, AuNRs and Au/Pt NRs by catalyzing the oxidation of colorless 3,3’,5,5’-tetramethylbenzidine (TMB) to blue oxidized 3,3’,5,5’-tetramethylbenzidine (oxTMB; a quinone) in the presence of H2O2. The catalytic activity of Au/Pt NRs is higher than that of sole AuNRs or PtNPs by factors of 4.2 and 2.1, respectively. Thus, Au/Pt NRs have been used for the detection of peroxide and the limit of detection is 0.04 μM. This work provides an approach to integrate the peroxidase mimicking effect with SERS enhancement for potential application in detection.
Graphical abstract A schematic diagram for the laser-induced growth of Au/Pt NRs and the colorimetric determination of hydrogen peroxide concentration with their peroxidase mimicking properties. The limit of detection is 0.04 μM based on the use of Au/Pt NRs as a catalyst.
The authors report on a composite based electrocatalyst for methanol oxidation and H2O2 sensing. The composite consists of Pt nanoparticles (NPs), Pd nanoflakes, and MoS2. It was synthesized by chemical reduction followed by template-free electro-deposition of Pt NPs. FESEM images of the Pd nanoflakes on the MoS2 reveal nanorod-like morphology of the Pd NPs on the MoS2 support, whilst FESEM images of the Pt-Pd/MoS2 composite show Pt NPs in high density and with the average size of ~15 nm, all homogeneously electrodeposited on the Pd-MoS2 composite. A glassy carbon electrode (GCE) was modified with the composite to obtain an electrode for methanol oxidation and H2O2 detection. The modified GCE exhibits excellent durability with good catalytic efficiency (the ratio of forward and backward peak current density, If/Ib, is 3.23) for methanol oxidation in acidic medium. It was also used to sense H2O2 at an applied potential of ?0.35 V vs. Ag|AgCl which can be detected with a 3.4 μM lower limit of detection. The sensitivity is 7.64 μA μM?1 cm?2 and the dynamic range extends from 10 to 80 μM. This enhanced performance can be explained in terms of the presence of higher percentage of metallic 1T phase rather than a semiconducting 2H phase in MoS2. In addition, this is a result of the high surface area of MoS2 with interwoven nanosheets, the uniform distribution of the Pt NPs without any agglomeration on MoS2 support, and the synergistic effect of Pt NPs, Pd nanoflakes and MoS2 nanosheets. In our perception, this binder-free nano-composite has promising applications in next generation energy conversion and in chemical sensing.
Graphical abstract A composite consisting of palladium nanoflakes and molybdenum disulfide was decorated with platinum nanoparticles and then placed on a glassy carbon electrode which is shown to be a viable electrocatalyst for both methanol oxidation and detection of hydrogen peroxide.
A photoelectrochemical (PEC) aptasensor for the highly sensitive and specific detection of thrombin is described. This aptasensor is based on an indium tin oxide (ITO) support that is covered with carbon quantum dot (CQD)-sensitized TiO2 and acts as a photoactive matrix. The ITO/TiO2/CQD electrode was prepared by impregnation assembly. It displays an enhanced and steady photocurrent response under irradiation by visible light. A carboxyl-functionalized thrombin-binding aptamer was covalently immobilized on the modified ITO to obtain a PEC aptasensor whose photocurrent decreases with increasing concentration of thrombin. Under 420 nm irradiation at a bias voltage of 0 V, the aptasensor has a linear response in the 1.0 to 250 pM thrombin concentration range, with a 0.83 pM detection limit. Conceivably, this approach can be extended to numerous other PEC aptasensors for the detection of targets for which appropriate aptamers are available.
Graphical abstract Schematic of a PEC aptasensor for thrombin. It is based on the use of CQD as the sensitizer, TiO2/CQDs as the photoactive matrix, and the thrombin aptamer as the recognition element.
