A photoelectrochemical aptasensor for thrombin based on the use of carbon quantum dot-sensitized TiO<Subscript>2</Subscript> and visible-light photoelectrochemical activity

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.

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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.

     

A cathodic photoelectrochemical sensor for chromium(VI) based on the use of PbS quantum dot semiconductors on an ITO electrode

Under visible-light irradiation, a cathodic photoelectrochemical (PEC) sensor is presented for highly sensitive determination of Cr(VI) at a potential of ?0.25 V (vs SCE). PbS quantum dots (QDs) were capped with mercaptoacetic acid and assembled on the surface of an indium tin oxide (ITO) electrode via the linker poly(diallyl dimethyl ammonium chloride) providing a photoactive sensor. Cr(VI) accepts the photoelectrons generated by the PbS QDs. This promotes the separation of electron holes and enhances the cathodic photocurrent generated by a 470-nm LED. The sensor has 10 pM detection limit and a linear working range from 0.02 nM to 2 μM of chromate. The method was successfully applied to the determination of Cr(VI) and total chromium in spiked environmental water samples.

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Graphical abstract Schematic illustration of the photocurrent enhancement response of ITO/PbS toward chromium(VI). In the presence of Cr(VI) (red line), Cr(VI) accepts the photoelectrons generated by the PbS QDs under 470-nm LED irradiation, resulting in improved photocurrent of ITO/PbS.

     

Sensitive simultaneous determination of catechol and hydroquinone using a gold electrode modified with carbon nanofibers and gold nanoparticles

A highly sensitive electrochemical sensor for the simultaneous determination of catechol (CC) and hydroquinone (HQ) was fabricated by electrodeposition of gold nanoparticles onto carbon nanofiber film pre-cast on an Au electrode. Both CC and HQ cause a pair of quasi-reversible and well-defined redox peaks at the modified electrode in pH?7.0 solution. Simultaneously, the oxidation peak potentials of CC and HQ become separated by 112?mV. When simultaneously changing the concentrations of both CC and HQ, the response is linear between 9.0???M and 1.50?mM. In the presence of 0.15?mM of the respective isomer, the electrode gives a linear response in the range from 5.0 to 350???M, and from 9.0 to 500???M for CC and HQ, respectively, and detection limits are 0.36 and 0.86???M. The method was successfully examined for real sample analysis with high selectivity and sensitivity.

Artificial photosynthesis by using chloroplasts from spinach adsorbed on a nanocrystalline TiO2 electrode for photovoltaic conversion

Photovoltaic conversion has been achieved by use of chloroplasts (photosynthetic organs) from spinach adsorbed on a nanocrystalline TiO₂ film on an indium tin oxide (ITO) glass electrode (chloroplast/TiO₂ electrode). The shape of the absorption spectrum of the chloroplast/TiO₂ electrode is almost the same that of a dispersion of the chloroplasts. Absorption maxima of the chloroplast/TiO₂ electrode observed at 430, 475, and 670 nm were attributed to carotenoid and chlorophyll molecules, suggesting that chloroplasts have been adsorbed by the nanocrystalline TiO₂ film on the ITO electrode. The photocurrent responses of chloroplast/TiO₂ electrodes were measured by using a solution of 0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile as redox electrolyte in the presence or absence of water and 100 mW cm^?2 irradiation. The photocurrent of the chloroplast/TiO₂ electrode was increased by adding water to the redox electrolyte. The photocurrent responses of chloroplast/TiO₂ electrodes irradiated with monochromatic light (680 nm, the absorption band of photosystem II complexed with evolved oxygen) were measured by use of a solution of 0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile as redox electrolyte in the presence or absence of water. A chloroplast/TiO₂ electrode photocurrent was observed only when the redox electrolyte containing water was used, indicating that the oxygen evolved from water by photosystem II in chloroplasts adsorbed by a nanocrystalline TiO₂ film on an ITO electrode irradiated at 680 nm is reduced to water by the catalytic activity of the platinum electrode. The maximum incident photon-to-current conversion efficiency (IPCE) was 0.8 % on irradiation at 670 nm.     

Voltammetric determination of catechol and hydroquinone using nitrogen-doped multiwalled carbon nanotubes modified with nickel nanoparticles

Nitrogen-doped multiwalled carbon nanotubes modified with nickel nanoparticles (Ni/N-MWCNT) were prepared by a thermal reduction process starting from urea and Ni(II) salt in an inert atmosphere. The nanocomposite was deposited on a screen printed electrode and characterized by X-ray diffraction, scanning and transmission electron microscopy, nitrogen adsorption, X-ray photoelectron spectroscopy, and thermogravimetric analyses. The performance of the composite was investigated by cyclic voltammetry, differential pulse voltammetry and chronoamperometry. The numerous active metal sites with fast electron transfer properties result in enhanced electrocatalytic activity towards the individual and simultaneous detection of catechol (CC) and hydroquinone (HQ), best at 0.21 V for CC and 0.11 V for HQ (vs. Ag/AgCl). For both targets the detection limit (S/N of 3) was 9 nM (CC) and 11 nM (HQ), and the Ni/N-MWCNT-electrode showed linear response from 0.1–300 μM CC, and 0.3–300 μM HQ. The electrode is selective over many potentially interfering ions. It was applied to the analysis of spiked water samples and gave satisfactory recoveries. It also is sensitive for CC (5.396 μA·μM^?1 cm^?2) and HQ (5.1577 μA·μM^?1 cm^?2), highly active, durable, acceptably repeatable and highly reproducible.

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Graphical abstract Voltammetric determination of catechol and hydroquinone using nitrogen-doped multiwalled carbon nanotubes modified with nickel nanoparticles.

     

有机钌螯合物/TiO_2杂化膜修饰电极上Pt纳米团簇的光电流增强效应

利用LB膜技术可控制备了纳米单层和多层的二氧化钛-有机钌螯合物杂化膜,并研究了上述无机-有机杂化膜修饰电极在Pt纳米团簇敏化后的光电流增强效应.实验结果表明:(1)纳米单层TiO2/[Ru(phen)2(dC18bpy)]2+(简称为TiO2-Ru)杂化膜的平均厚度为(3.6±0.5)nm;(2)在光照条件下TiO2-Ru杂化膜能有效催化还原[Pt(NH3)6]4+形成粒径位于20～160nm之间的Pt纳米团簇;(3)Pt纳米团簇的引入消除了金属钌螯合物中配体对电子传递的阻碍作用,改变了电子传递途径,从而有效减少了电子空穴对的复合,提高了Pt纳米团簇敏化的n层杂化膜修饰电极(ITO/(TiO2-Ru)n/Pt)在支持电解质中的光电流.与纳米单层TiO2-Ru杂化膜修饰的ITO电极(ITO/TiO2-Ru)相比,当工作电压为900mV时,ITO/TiO2-Ru/Pt在0.1mol·L-1的NaClO4电解质溶液中和光照(λ360nm)条件下,单位面积的光电流提高了约5倍;(4)ITO/(TiO2-Ru)n/Pt电极光电流的大小与杂化膜的层数密切相关,当TiO2-Ru杂化膜的层数从一层、二层增加到四层时,光电流呈现先升高后下降行为,这表明ITO/(TiO2-Ru)n/Pt电极的电子传递过程直接通过非电活性的二氧化钛纳米单层进行.     

Simultaneous determination of hydroquinone and catechol using a glassy carbon electrode modified with gold nanoparticles,ZnS/NiS@ZnS quantum dots and L-cysteine

We describe an electrochemical sensor for simultaneous determination of hydroquinone (HQ) and catechol (CC). A glassy carbon electrode (GCE) was modified with gold nanoparticles, L-cysteine, and ZnS/NiS@ZnS quantum dots using a layer-by-layer technique. The materials were characterized by X-ray diffractometry, field emission scanning electron microscopy, and electrochemical impedance and Fourier transform infrared spectroscopy. Cyclic voltammetry and differential pulse voltammetry revealed this modified GCE to represent a highly sensitive sensor for the simultaneous determination of HQ and CC. The anodic peak current for HQ at a working voltage of 80 mV (vs. Ag/AgCl) is related to its concentration in the 0.1 to 300 μM range (even in the presence of 0.1 mM of CC). The anodic peak current for CC at a working voltage of 184 mV is related to its concentration in the 0.5 to 400 μM range (even in the presence of 0.1 mM of HQ). The detection limits (at an S/N ratio of 3) are 24 and 71 nM for HQ and CC, respectively. The modified GCE was successfully applied to the determination of HQ and CC in aqueous solutions and gave satisfactory results.

A glassy carbon electrode was modified with gold nanoparticles, ZnS/NiS@ZnS quantum dots and L-cysteine and used for simultaneous determination of hydroquinone and catechol.

     

Aptamer based photoelectrochemical determination of tetracycline using a spindle-like ZnO-CdS@Au nanocomposite

The authors have synthesized spindle-like ZnO nanorods closely anchored to CdS nanoparticles (NPs) placed on gold NPs (ZnO-CdS@Au). It is shown that the ZnO-CdS@Au nanocomposite can serve as a photoactive material for use in photoelectrochemical (PEC) detection by efficiently absorbing light and then promoting electron transfer. A visible light-driven PEC detection platform for tetracycline (TET) was fabricated by placing the nanocomposite on an ITO and immobilizing the TET-binding aptamer as biorecognition element. PEC can be quantified by applying a bias potential of +?0.4 V (vs. SCE) and visible light irradiation. The aptamer on the electrode specifically captures the TET present in the solution to produce a restored photocurrent signal through the reaction between the captured TET and the photogenerated holes. The electrode has a linear response in the 50 to 200 nM TET concentration range, with a 4.5 nM detection limit (at an S/N ratio of 3). In our perception, this novel PEC detection strategy based on ZnO-CdS@Au nanocomposite demonstrated an ultrasensitive method for TET detection with high selectivity and good stability.

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Graphical abstract Gold nanoparticles and CdS nanoparticles were deposited on the spindle-like ZnO nanorod surface (ZnO-CdS@Au). Photoelectrochemical detection of tetracycline (TET) was realized with high selectivity and good stability utilizing ZnO-CdS@Au as transducer and TET-binding aptamer as biorecognition element.

     

Ultrafine α-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> nanocrystals anchored on N-doped graphene: a nanomaterial with long hole diffusion length and efficient visible light-excited charge separation for use in photoelectrochemical sensing

The authors have coupled ultrafine α-Fe₂O₃ nanocrystals to N-doped graphene (NG) to obtain a novel material for use in a photoelectrode. The presence of NG is shown to strongly affect the morphology and size of the α-Fe₂O₃ nanocrystals formed on the NG sheets, and to improve their photoelectrochemical (PEC) activity. Interestingly, the PEC performance of the nanocomposite is closely correlated to the size of the α-Fe₂O₃ nanocrystals in that small nanocrystals display better PEC properties. The photocurrent of α-Fe₂O₃-NG is nearly 3.3-fold stronger than that of α-Fe₂O₃ nanocrystals. Based on the remarkable PEC performance of this nanocomposite, a PEC sensor was constructed for the sensitive determination of 1,4-dihydroxybenzene (HQ). Its photocurrent increases with the HQ concentration in the range from 3.0 nM to 3.3 μM, and the detection limit is 1.0 nM (at an S/N ratio of 3). In our perception, the study presented here can serve as a basis for a better understanding of the relationship between morphologies and PEC performance of such nanomaterials. Conceivably, it may be extended to other PEC sensing system and to other fields associated with nanotechnology.

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Graphical abstract Ultrafine α-Fe₂O₃ nanocrystals were prepared via coupling with N-doped graphene (NG) substances (α-Fe₂O₃-NG). They exhibit superior photoelectrochemical (PEC) performance and have been successfully utilized for PEC-based sensing.

     

Highly sensitive lable-free electrochemical aptasensor for thrombin detection with cobalt hexacyanoferrate as the electrochemical probe

A highly sensitive label-free electrochemical aptasensor has been constructed for the electrochemical detection of thrombin (TB), where two layers of cobalt hexacyanoferrate (CoHCF) redox probes sandwiched with carbon nanotubes–Nafion were directly immobilized on the electrode surface by electrodeposition. Through the strong interaction between CN^? (CoHCF) and gold nanoparticles (GNPs), GNPs were assembled on the CoHCF-modified electrode for the immobilization of thiolated thrombin aptamers (TBA). In the presence of target TB, TBA on the electrode surface could catch TB to form TBA–TB complex, which made a barrier for the electron transfer, resulting in a greater decrease in CoHCF redox probe signals. Thus, the proposed aptasensor showed a high sensitivity and a much wider linearity to TB in the range of 1.0 pg/mL?～?1.0 μg/mL with a detection limit of 0.28 pg/mL.     

A Photoelectrochemical Sensor for Firstly the Detection of Amlodipine Besylate Based on an MnC4Pc Coated ZnO Composite Materials

In this study, tetracarboxylic manganese phthalocyanine coated nano-zinc oxide (MnC₄Pc-ZnO) composite material was prepared by in-situ growth method and modified with indium tin oxide (ITO) glass electrode to construct a photoelectrochemical (PEC) sensor. A PEC sensor for the determination of amlodipine besylate (AB) was developed for the first time based on the principle of precipitation reaction between heavy metal ions and dihydropyridine and the recombination suppression effect of the material. The morphology and optical properties of the MnC₄Pc-ZnO composites were characterized by scanning electron microscopy (SEM) and ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS). Chronoamperometry (i-t) and electrochemical impedance spectroscopy (EIS) were used to study the PEC behavior of ITO electrodes modified by MnC₄Pc-ZnO composite material. The study found that the MnC₄Pc-ZnO composite material has a good photocurrent response to AB, and there was a good linear relationship between the concentration range of 75 nM-250 μM, the linear equation was I(μA)=−5.2×10⁻⁸×lgC+3.2×10⁻⁸ (r=0.9947), a limit of detection (LOD) of 20 nM. In addition, MnC₄Pc-ZnO/ITO also has good selectivity and stability. The PEC sensor detects amlodipine besylate tablets, amlodipine besylate dispersible tablets, and biological samples, with standard addition recovery rates of 96.11 % and 103.96 %, respectively. The determination result has good accuracy, and the PEC sensor provides a new method for detecting AB.     

Design of poly-l-methionine–gold nanocomposit/multi-walled carbon nanotube modified glassy carbon electrode for determination of amlodipine in human biological fluids

The present paper describes a sensitive electrochemical detection of amlodipine (AMLO) at the poly-l-methionine–gold nanoparticles/multi-walled carbon nanotube modified glassy carbon electrode (PLM–GNPs/MWCNTs/GCE) by differential pulse voltammetry (DPV) technique at physiological pH 7.12. Cyclic voltammetry results demonstrate that the proposed electrode shows excellent electrocatalytic activity toward oxidation of AMLO. Kinetic parameters of the electrochemical reaction are calculated, and analytical variables such as MWCNT volumes, drug accumulation time, electropolymerization cycles and pH values are also optimized. Under optimal conditions, the linear range covering from 5 nM to 2.5 μM along with detection limit of 1 nM is obtained. Moreover, this method is successfully used to detect AMLO in pharmaceutical samples and biological fluids of a dosage received by the volunteer.     

Electrochemical behavior of dihydroxybenzene isomers at MWCNTs modified electrode and simultaneous determination in neutral condition

Simultaneous determination of dihydroxybenzene isomers in neutral condition was successfully realized by a simple and easy prepared modified electrode without previous chemical or physical separations. The multi-walled carbon nanotubes modified glassy carbon electrode (MWCNTs/GCE), which was prepared by the drop-coating method, was characterized by FE-SEM and TEM. Then, the electrochemical behavior of dihydroxybenzene isomers at MWCNTs/GCE was systematically studied at different temperature and pH conditions. The oxidation peak potentials were separated in neutral condition with 105 mV to hydroquinone (HQ) and catechol (CC) and 390 mV to CC and resorcinol (RS). And in neutral condition, the amperometric current were found to be linear with concentration of HQ, CC, and RS (20–140 μM) with the presence of 100 μM other isomers. Furthermore, excellent anti-interference, stability, and reproducibility were also presented by this modified electrode.     

三苯胺功能染料的合成及在谷胱甘肽超低电位光电检测中的应用

设计合成了一种具有D-π-A结构的三苯胺功能染料(TCA),并通过分子结构中的羧基将其配位于TiO_2纳米粒子修饰的光电极表面,发展了一种可在超低电位下高灵敏检测谷胱甘肽(GSH)的光电传感方法.该TCA分子以三苯胺为电子给体,噻吩为桥连基团,氰基乙酸为电子受体.在可见光的照射下,TCA通过分子内电子转移将光电子由三苯胺经噻吩和羧基注入到TiO_2的导带能级,进而注入基底光电极,产生阳极光电流;同时,TCA被氧化到氧化态.由于氧化态TCA的稳定性好,可循环被生理活性小分子GSH还原,并产生放大的阳极光电流.TCA功能化的TiO_2纳米粒子修饰电极对GSH表现出了极高的催化活性,在波长为480 nm的可见光照射下,在0 V的超低电位下即可实现对GSH的催化氧化.基于这一性质,发展了一种可用于GSH检测的光电传感方法.在最优条件下,该传感器对浓度为2~100μmol/L和0.1~2.4 mmol/L的GSH具有良好的线性响应,检出限低达1μmol/L.此外,该光电传感器具有较好的选择性,可排除13种氨基酸和生理活性物质多巴胺及氢醌的干扰,因此具有一定的实际应用前景.     

Simultaneous determination of hydroquinone and catechol based on glassy carbon electrode modified with gold-graphene nanocomposite

We have synthesized a virtually monodisperse gold-graphene (Au-G) nanocomposite by a single-step chemical reduction method in aqueous dimethylformamide solution. The nanoparticles are homogenously distributed over graphene nanosheets. A glassy carbon electrode was modified with this nanocomposite and displayed high electrocatalytic activity and extraordinary electronic transport properties due to its large surface area. It enabled the simultaneous determination of hydroquinone (HQ) and catechol (CC) in acetate buffer solution of pH?4.5. Two pairs of well-defined, quasi-reversible redox peaks are obtained, one for HQ and its oxidized form, with a 43 mV separation of peak potentials (ΔE_p), the other for CC and its oxidized form, with a ΔE_p of 39 mV. Due to the large separation of oxidation peak potentials (102 mV), the concentrations of HQ and CC can be easily determined simultaneously. The oxidation peak currents for both HQ and CC increase linearly with the respective concentrations in the 1.0 μM to 0.1 mM concentration range, with the detection limits of 0.2 and 0.15 μM (S/N?=?3), respectively. The modified electrode was successfully applied to the simultaneous determination of HQ and CC in spiked tap water, demonstrating that the Au-G nanocomposite may act as a high-performance sensing material in the selective detection of some environmental pollutants.

Ni0.1Co0.9Fe2O4-based electrochemical sensor for the detection of paracetamol

A novel electrochemical sensor based on nickel-doped cobalt ferrite nanoparticles (Ni_0.1Co_0.9Fe₂O₄)-modified glassy carbon electrode (NCF/GCE) was presented for the sensitive detection of paracetamol. Experimental conditions such as pH, applied potentials and concentration were investigated using cyclic voltammetric and chronoamperometric techniques. The modified electrode exhibited excellent catalytic response towards the oxidation of paracetamol with good reproducibility. The overpotential for oxidation of paracetamol is decreased, and the current response enhanced significantly on the modified electrode in comparison with that of bare electrode. Linear calibration curve is obtained over the range 2 μM to 8,000 μM having a detection limit of 11 nM. The modified electrode facilitated the simultaneous detection of paracetamol, ascorbic acid, and dopamine with good reproducibility.     

Amperometric Determination of Cholesterol-Reducing Drug,Ezetimibe, Using Glassy Carbon Electrode Modified with Multiwalled Carbon Nanotubes and Sodium Dodecylsulfate

A simple and sensitive electrochemical method is described for the determination of the cholesterol-reducing drug ezetimibe in aqueous solution. A glassy carbon electrode, modified with multiwalled carbon nanotubes and sodium dodecylsulphate was used as the working electrode. Ezetimibe yields a well-defined anodic peak at the surface of the electrode in an aqueous solution of pH 13. A linear amperometric calibration curve was obtained in the range of 1.2–78 μM (0.5–32.0 μg/mL) of ezetimibe, with a sensitivity of 88.6 nA/μM and a detection limit of 300 nM (0.12 μg/mL). The sensor was applied successfully to the determination of ezetimibe in pharmaceutical preparations.     

A novel photoelectrochemical hydrogen peroxide sensor based on nickel(II)-potassium hexacyanoferrate-graphene hybrid materials modified n-silicon electrode

A platinum (Pt) film coated n-silicon (Pt/n-n⁺-Si) was modified with nickel(II)-potassium hexacyanoferrate (NiHCF)-graphene sheets (GS) hybrid and used as a photo-electrochemical (PEC) sensor for non-enzyme hydrogen peroxide (H₂O₂) detection. A NiHCF film was deposited on the surface of GS/Pt/n-n⁺-Si electrode by chemical method. The structure and composition of the NiHCF film was characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). PEC behavior of the NiHCF-GS/Pt/n-n⁺-Si electrode was investigated using cyclic voltammetry (CV) under illumination. The modified electrode has been used as PEC sensor for H₂O₂ detection with a linear range of 2.0 × 10^?6–2.9 × 10^?3 M and a detection limit of 1.0 × 10^?6 M at a signal-to-noise ratio of 3 in a two-electrode cell with a Pt plate as counter electrode. The characteristics of GS layer have been discussed in both the improvement of sensibility and selectivity.     

Enhanced electron transfer by bovine serum albumin covalently attached to glassy carbon electrode and its application to determination of hydroquinone

Bovine serum albumin (BSA) was covalently attached to glassy carbon electrode (GCE) surface by the electrochemical method. An enhancement for the redox of hydroquinone (HQ) on BSA/GCE was confirmed by cyclic voltammetry and electrochemical impedance spectroscopy measurement. The electron transfer rate constant (k _s) on the BSA/GCE electrode is almost three orders of magnitude higher than that on bare GCE. The enhancing effect can be attributed to the electrostatic force between the positively charged HQ and negatively charged BSA. It is found that the enhanced redox process of HQ can be used to determine HQ sensitively. The oxidation current can reach 95% of its steady-state value within 30 s. The linear range for HQ determination is from 2.5 × 10^?8 M to 1.325 × 10^?6 M with a detection limit of 8.6 × 10^?9 M at a signal-to-noise ratio of 3. The study may provide a simple, rapid and sensitive method for determination of HQ which is present in the natural environment and in chemical industry effluent.