Immobilization of CoII-(N,N′-bis(salicylidene)-2-aminobenzylamine) on Poly(pyrrole-co-o-anisidine)/Chitosan Composite Films: Application to Electrocatalytic Oxidation of Catechol

In this study, poly (pyrrole-co-o-anisidine)/chitosan composite (Cs) films were prepared by cyclic voltammetry technique on platinum electrode using different pyrrole and o-anisidine mole ratios. Immobilization process was accomplished in Co^II-(N,N′-bis(salicylidene)-2-aminobenzylamine)(CoL) dissolved 0.15 M acetonitrile-LiClO₄ solution by cyclic voltammetry technique at 0.2–2.0 V potential range. Three electrode methods were applied in all electrochemical studies. After immobilization process, the characterizations of the electro catalytic surfaces (Cs−CoL−Pt) were carried out by cyclic voltammetry and SEM images. The SEM images clearly indicated that the [CoL] complex is immobilized onto composite films. The electrocatalytic activity of the modified electrodes on the catechol was investigated using buffer solutions of different pH values. The results of catalytic studies revealed that, pH=10 buffer solution was the optimal solution and 1 : 1 Cs−CoL−Pt electrode was the best electrode for catechol oxidation. In square wave voltammetry measurements using this electrode, two linear working ranges were determined. The linear response ranges for catechol determination were found as 3.0 μM–6.0 μM and 16 μM–80 μM for the first and the second linear working ranges, respectively, with 1.1 μM detection limit.     

Sensitive and Facile Determination of Catechol and Hydroquinone Simultaneously Under Coexistence of Resorcinol with a Zn/Al Layered Double Hydroxide Film Modified Glassy Carbon Electrode

A stable dihydroxybenzene sensor was fabricated by electrochemical deposition of Zn/Al layered double hydroxide film on glassy carbon electrode (LDHf/GCE). The sensitive and facile electrochemical method for the simultaneous determination of catechol (CA) and hydroquinone (HQ) under coexistence of resorcinol (RE) has been achieved at the LDHf/GCE in phosphate buffer solution (pH 6.5). Under the optimized conditions, the differential pulse voltammetry response of the modified electrode to CA (or HQ) shows a linear concentration range of 0.6 μM to 6.0 mM (or 3.2 μM to 2.4 mM) with a correlation coefficient of 0.9987 (or 0.9992) and the calculated limit of detection is 0.1 μM (or 1.0 μM) at a signal‐to‐noise ratio of 3. In the presence of 50 μM isomer, the linear concentration ranges for CA and HQ are 3.0 μM to 1.5 mM and 12.0 μM to 0.8 mM, respectively. The detection limits are 1.2 μM and 9.0 μM. Further, the proposed method has been performed to successfully detect dihydroxybenzene isomers in analysis of real samples, such as water and tea.     

A novel palygorskite-modified carbon paste amperometric sensor for catechol determination

A palygorskite-modified carbon paste electrode (CPE) was constructed using graphite powder mixed with palygorskite particles. Compared with the unmodified CPE, the resulting palygorskite-modified CPE remarkably increases the peak currents of catechol, and greatly lowers the peak potential separation. Therefore, the palygorskite exhibits catalytic activity to catechol and significantly improves the determining sensitivity. The electrocatalytic activity of palygorskite is attributed to its high adsorption capability and the –OH groups on its surface, which plays an important role in the electron transfer between the modified CPE and the catechol in the solution. The sensor shows a linear response range between 5 and 100 μM catechol with a correlation coefficient of 0.998. The detection limit was calculated as 0.57 μM (s/n = 3).     

Platinum Nanoparticles/Poly(isoleucine) Modified Glassy Carbon Electrode for the Simultaneous Determination of Hydroquinone and Catechol

A new electrochemical sensor based on Poly(Isoleucine) modified glassy carbon electrode decorated with platinum nanoparticles (Pt/Poly(Isoleucine)/GCE) was developed for sensitive individual and simultaneous determination of hydroquinone (HQ) and catechol (CC). Scanning electron microscopy (SEM), Electrochemical impedance spectroscopy (EIS), Cyclic voltammetry (CV) and Differential pulse voltammetry (DPV) were performed in order to characterize the Pt/Poly(Isoleucine)/GCE nanocomposite. For simultaneous determination of HQ and CC, Pt/Poly(Isoleucine)/GCE showed wide linear range between the 0.01–100.0 μM. The detection limits were 0.006 μM for HQ and 0.005 μM for CC. The Pt/Poly(Isoleucine)/GC electrode exhibited good sensitivity and reliability in the simultaneous electroanalysis of two isomers in PBS of pH 7.5. The modified electrode was used to detect the isomers in naturel samples.     

Construction of amperometric biosensor modified with conducting polymer/carbon dots for the analysis of catechol

Phenolic compounds used in food industries and pesticide industry, are environmentally toxic and pollute the rivers and ground water. For that reason, detection of phenolic compounds such as catechol by using simple, efficient and cost-effective devices have been becoming increasingly popular. In this study, a suitable and a novel matrix was composed using a novel conjugated polymer, namely poly[1-(5-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophen-2-yl)furan-2-yl)-5-(2-ethylhexyl)-3-(furan-2-yl)-4H thieno[3,4-c]pyrrole-4,6(5H)-dione] (PFTBDT) and carbon dots (CDs) to detect catechol. PFTBDT and CDs were synthesized and the optoelectronic properties of PFTBDT were investigated via electrochemical and spectroelectrochemical studies. Laccase enzyme was immobilized onto the constructed film matrix on the graphite electrode. The proposed biosensor was found to have a low detection limit (1.23 μM) and a high sensitivity (737.44 μA/mM.cm⁻²) with a linear range of 1.25–175 μM. Finally, the applicability of the proposed enzymatic biosensor was evaluated in a tap water sample and a satisfactory recovery (96–104%) was obtained for catechol determination.     

Simultaneous Determination of Hydroquinone and Catechol at a Glassy Carbon Electrode Modified with Multiwall Carbon Nanotubes

A simply and high selectively electrochemical method for simultaneous determination of hydroquinone and catechol has been developed at a glassy carbon electrode modified with multiwall carbon nanotubes (MWNT). It was found that the oxidation peak separation of hydroquinone and catechol and the oxidation currents of hydroquinone and catechol greatly increase at MWNT modified electrode in 0.20 M acetate buffer solution (pH 4.5). The oxidation peaks of hydroquinone and catechol merge into a large peak of 302 mV (vs. Ag/AgCl, 3 M NaCl) at bare glassy carbon electrode. The two corresponding well‐defined oxidation peaks of hydroquinone in the presence of catechol at MWNT modified electrode occur at 264 mV and 162 mV, respectively. Under the optimized condition, the oxidation peak current of hydroquinone is linear over a range from 1.0×10^?6 M to 1.0×10^?4 M hydroquinone in the presence of 1.0×10^?4 M catechol with the detection limit of 7.5×10^?7 M and the oxidation peak current of catechol is linear over a range from 6.0×10^?7 M to 1.0×10^?4 M catechol in the presence of 1.0×10^?4 M hydroquinone with the detection limit of 2.0×10^?7 M. The proposed method has been applied to simultaneous determination of hydroquinone and catechol in a water sample with simplicity and high selectivity.     

Development and optimization of a solid composite tyrosinase biosensor for phenol detection in flow injection systems

Bulk-modified epoxy-graphite tyrosinase biosensors were fabricated by four different procedures. The influence of these fabrication procedures on the analytical performance of the enzyme electrode in an amperometric wall-jet flow cell has been studied. The bioprobe performance is assessed by cyclic voltammetry. Higher current densities and narrower peaks were obtained when the enzyme was introduced in the dry state into the epoxy-graphite material, instead of introducing it previously dissolved in the buffer. In the F1 system responses of 11.79 μA cm⁻² and 1.43 μA cm⁻² are then obtained for catechol and phenol respectively for 50 μL injections of 20 μM solutions. Moreover, if gold/palladium is introduced into the epoxy-graphite, a further increase in current is achieved resulting in 27.70μA cm⁻² and 4.90μA cm⁻²for catechol and phenol, respectively. This biosensor can operate in aqueous as well as in mixed aqueous-organic environments.     

Amperometric Detection of Catechol and Catecholamines by Immobilized Laccase from DeniLite

DeniLite laccase immobilized Pt electrode was used for detection of catechol and catecholamines. The enzymatically oxidized substrates were measured amperometrically. The sensitivities are 210, 75, 60 and 45 nA/μM with the upper limits of linear ranges of 58, 40, 55 and 55 μM and the detection limits (S/N=3) of 0.07, 0.2, 0.3 and 0.4 μM for catechol, dopamine (DA), norepinephrine (NEPI) and epinephrine (EPI), respectively. The response time (t_90%) is about 2 seconds for each substrate and the long‐term stability is around 40–50 days with retaining 80% of initial activity. The very fast response and the remarkable long‐term stability are the principal advantages of this sensor. In case of catechol, the pH response of the sensor is mainly determined by enzyme's pH profile, however, in case of catecholamines, both enzyme's pH profile and reversibility of the substrate are operated and the optimal pHs for NEPI and EPI shift towards acidic range compared to that for DA. The presence of ascorbic acid (<50 μM) did not interfere with the measurement.     

Amperometric sensors based on tyrosinase-modified screen-printed arrays

This paper describes the design, development and characteristics of a tyrosinase (polyphenol oxidase) modified amperometric screen-printed biosensor array, with the enzyme cross-linked in a redox-hydrogel namely the PVI₁₃-dmeOs polymer. Two types of Au-screen-printed four-channel electrode arrays, differing in design and insulating layer, were compared and investigated. Au-, graphite-coated-Au- and Carbopack C-coated-Au-surfaces, serving as the basis for tyrosinase immobilisation, were investigated and the performances of the different arrays were evaluated and compared in terms of their electrocatalytic characteristics, as well as operational- and storage stability using catechol as model substrate. It was found that the Carbopack C-coated array was the best choice for tyrosinase immobilisation procedure mainly due to a higher mechanical stability of the deposited enzyme layer, combined with good sensitivity and stability for up to 6 months of use. In the batch mode the biosensors responded linearly to catechol up to 30 μM with limits of detection from 0.14 μM. Parameters from cyclic voltammograms indicated that the reversibility of the direct electrochemical reaction for catechol on the three types of electrode surfaces (no tyrosinase modification) was not the limiting factor for the construction and performance of tyrosinase biosensors.     

Highly Sensitive Temperature‐responsive Sensor Based on PS‐PDEA‐PS/C60‐MWCNTs for Reversible Switch Detection of Catechol

A new type of temperature‐responsive electrochemical sensor was constructed based on a glass carbon electrode modified by the composite containing temperature‐responsive polymer polystyrene‐poly N, N‐diethyl acrylamide‐polystyrene (PS‐PDEA‐PS) and fullerenes‐carboxylate multi‐walled carbon nanotubes (C₆₀‐MWCNTs). The sensor was used for the electrochemical detection of catechol (CC). When the temperature is higher than the critical temperature (LCST) of PS‐PDEA‐PS, the electrochemical behavior of CC can be detected, which it is in the “on” state. When the temperature is lower than LCST, the composite modified film is in the “off” state and the electrochemical behavior of CC was not detected. Under the best experimental conditions, the sensor has a good detection range for catechol from 4.0 to 135.0 μM, with a LOD of 1.45 μM. In addition, the proposed sensor has good stability and reproducibility, and was successfully applied to the determination of catechol in real tap water.     

Pyrolyzed Photoresist Carbon Electrodes for Microchip Electrophoresis with Dual‐Electrode Amperometric Detection

The fabrication and evaluation of pyrolyzed photoresist films (PPF) for microchip capillary electrophoresis (CE) with dual‐electrode electrochemical (EC) detection is described. The sensitivity, linearity, and reproducibility were evaluated using catecholamines and related compounds, including dopamine (DA), 5‐hydroxyindole‐3‐acetic acid (5‐HIAA), ascorbic acid (AA), and catechol. Initial studies with DA show the response of the PPF electrodes to be linear between 25 and 500 μM (r²=0.999) with a limit of detection (LOD) of 5 μM (S/N=3) and sensitivity of 5.8 pA/μM. Selectivity was further enhanced by employing dual‐electrode detection in the series configuration for detection of species exhibiting chemically reversible redox reactions.     

Ag Doped Titanium Dioxide Nanocomposite‐modified Glassy Carbon Electrode as Electrochemical Interface for Catechol Sensing

The electrochemical sensing of catechol (CC) on a glassy carbon electrode modified with the ionothermal assisted synthesis of Ag doped TiO₂ a nanoparticle has been successfully demonstrated for the first time.Ag doped TiO₂ nanoparticles composite modified glassy carbon electrode exhibits higher electrocatalytic activity towards oxidation of catechol than glassy carbon electrode itself. The modified electrode also exhibits high selectivity towards this analyte in the presence of some of the metal ions and some of the biological compounds. Linear ranges and the limit of detections with the above electrode are 1–15 µM and 0.0249 µM respectively. The optimized protocol has been utilized for monitoring the catechol in some of the natural samples like apple juice and green tea and in industrial effluents.     

Synthesis of Ferrosoferric Oxide‐graphene Oxide Nanocomposite by Isoelectric Point Method for the Detection of Catechol

Ferrosoferric oxide functionalized graphene oxide nanocomposite with layer by layer structure was synthesized by isoelectric point method in this work. The prepared material was characterized by Fourier transform infrared spectroscopy, X‐ray diffraction, scanning electron microscopy, transmission electron microscopy and Raman spectroscopy. Then the material was used to modify a glassy carbon electrode to fabricate an electrochemical sensor for the detection of catechol. The electrochemical sensor exhibited excellent electrochemical performance towards the detection of catechol with a linear response in the range of 5–205 μM and a detection limit of 2.32 μM. Furthermore, the electrochemical sensor showed excellent selectivity, stability and repeatability. These results revealed ferrosoferric oxide functionalized graphene oxide nanocomposite has potential applications in the fabrication of electrochemical sensors.     

Graphene Paper with Sharp‐edged Nanorods of Fe−CuMOF as an Excellent Electrode for the Simultaneous Detection of Catechol and Resorcinol

A flexible composite paper Fe?Cu‐based metal‐organic framework (MOF)/reduced graphene oxide (rGO) (Fe?CuMOF/rGO) electrode was prepared by using a simple electrochemical method for the simultaneous detection of catechol (CC) and resorcinol (RC). Free‐standing, flexible and double‐sided Fe?CuMOF/rGO composite paper was obtained by applying the electrochemical deposition process on the rGO paper electrode in the solution containing Fe?CuMOF composite. The morphological analysis of Fe?CuMOF/rGO composite paper showed that sea urchin‐like structures formed on the rGO electrode surface consist of numerous sharp‐edged nanorods of Fe?CuMOF. Flexible Fe?CuMOF/rGO paper electrode exhibited high sensitivity, wide linear range and low detection limit for the simultaneous determination of CC and RC. The linear ranges of concentration for CC and RC were 0.1–800 and 0.1–720 μM, respectively, and the corresponding limits of detection (S/N=3) were 0.016 and 0.020 μM. The outstanding performance of this flexible electrode could be attributed to the sharp‐edged urchin‐like Fe?CuMOF structures which provide an increment of the surface area and the electrochemical activity of the composite paper electrode. Stability tests showed that Fe?CuMOF/rGO composite paper electrode has excellent flexibility, high durability, and good reproducibility. Furthermore, this electrode exhibited high sensitivity and selectivity for the determination of CC and RC in real sample analysis.     

Simultaneous Determination of Catechol and Hydroquinone on Nano-Co/L-Cysteine Modified Glassy Carbon Electrode

In this study, a novel and highly sensitive electrochemical method for simultaneous determination of catechol (CC) and hydroquinone (HQ) was developed, which worked at GCE modified with Nano cobalt (Nano-Co) by electrodeposition and L-Cysteine by electrochemical polymerization. The Nano-Co/L-Cysteine GCE was investigated by cyclic voltammetry (CV), SEM and EIS. The excellent conditions have been selected including supporting electrolyte, pH, accumulation time and scan rate. The calibration curves of were obtained that the linear regression equation was I=0.0734c+6×10⁻⁶ in the range of 5.8 μM to 103 μM (R²=0.9942) for CC and the linear regression equation was I=0.0566c+5×10⁻⁶ in the range of 5.8 μM to 100 μM (R²=0.9967) for HQ. The obtained detection limits of CC and HQ both were 6×10⁻⁷ M. The modified electrode was successfully applied to the simultaneous determination of CC and HQ in water samples.     

Enhanced Electrochemical Determination of Catechol and Hydroquinone Based on Pd Nanoparticles/Poly(Taurine) Modified Glassy Carbon Electrode

Here, Pd nanoparticles and poly(taurine) film was prepared on the glassy carbon electrode surface (Pd/Poly(TAU)/GCE) by the rapid electrochemical technique. The proposed composite surface was characterized by scanning electron microscopy(SEM), X‐ray photoelectron spectroscopy(XPS) and electrochemical impedance spectroscopy(EIS). Enhanced electron transfer ability and higher electroactive surface area were achieved at Pd/Poly(TAU)/GCE as compared to the bare GCE and polymer film electrode. The new and highly stable Pd/Poly(TAU)/GCE was employed for the individual and simultaneous determination of hydroquinone and catechol which were environmentally toxic. Under the optimized conditions, HQ and CC were individually determined by using the differantial pulse voltammetry in the linear ranges of 0.008–100 μM and 0.001–100 μM with the detection limits of (LOD) 2.1 nM and 0.68 nM, respectively. In case of simultaneous determination, LODs were found as 10 nM and 0.88 nM for HQ and CC, respectively. The content of both analytes in the real sample analysis was evaluated in the river water and tap water successfully.     

Simultaneous determination of hydroquinone and catechol based on three-dimensionally ordered macroporous polycysteine film modified electrode

A three-dimensionally ordered macroporous (3DOM) polycysteine (PCE) film was electropolymerized on the glassy carbon electrode (GCE) using polystyrene spheres as template. The electrochemical behaviors of hydroquinone (HQ) and catechol (CC) were studied, and two independent oxidation peaks were observed. Compared with the bare GCE and GCE modified with PCE without using template, this electrode displays larger peak currents which may be attributed to the structure of PCE and the large surface area of the nanopore array structure. As a result, a novel electrochemical method was developed for the simultaneous determination of HQ and CC. Under the optimized conditions, the peak currents were linear to concentrations in the wider ranges of 9 to 700 μM for HQ and from 3 to 700 μM for CC. The method was successfully applied to the simultaneous determination of HQ and CC in spiked water samples, and the results are satisfactory.     

Electropolymerized Diphenylamine on Functionalized Multiwalled Carbon Nanotube Composite Film and Its Application to Develop a Multifunctional Biosensor

Diphenylamine (DPA) monomers have been electropolymerized on the amino‐functionalized multiwalled carbon nanotube (AFCNT) composite film modified glassy carbon electrode (GCE) by cyclic voltammetry (CV). The surface morphology of PDPA‐AFCNT was studied using field‐emission scanning electron microscopy (FE‐SEM). The interfacial electron transfer phenomenon at the modified electrode was studied using electrochemical impedance spectroscopy (EIS). The PDPA‐AFCNT/GCE represented a multifunctional sensor and showed good electrocatalytic behavior towards the oxidation of catechol and the reduction of hydrogen peroxide. Rotating‐disk electrode technique was applied to detect catechol with a sensitivity of 1360 µA mM^?1 cm^?2 and a detection limit of 0.01 mM. Amperometric determination of hydrogen peroxide at the PDPA‐AFCNT film modified electrode results in a linear range from 10 to 800 µM, a sensitivity of 487.1 µA mM^?1 cm^?2 and detection limit of 1 µM. These results show that the nano‐composite film modified electrode can be utilized to develop a multifunctional sensor.     

Laccase Biosensor on Monolayer‐Modified Gold Electrode

Laccase enzymes from two different sources, namely, tree laccase from Rhus vernicifera and fungal laccase from Coriolus hirsutus were used for the development of biosensor for catechol. Laccase was immobilized onto the amine terminated thiol monolayers on gold surface by glutaraldehyde coupling. From the different thiol monolayers investigated, cystamine was found to be optimal with respect to sensitivity, stability, reproducibility, and other electrochemical properties of the enzyme electrode. Linear calibration in the range between 1 and 400 μM for catechol was obtained for fungal laccase covalently coupled on the electrode surface. The kinetic parameters determined using the Lineweaver‐Burk and Eadie‐Hofstee plots were K_m=0.65 mM and V_max=24.5 μA for fungal laccase compared to K_m=5.4 mM and V_max=6.6 μA for tree laccase on cystamine monolayer. The electrode showed good stability for 1 month without loosing appreciable activity when stored dry in a refrigerator at ?20 °C.     

Electrochemical Detection of Catechol on Boron‐doped Diamond Electrode Modified with Au/TiO2 Nanorod Composite

Au/TiO₂ nanorod composites with different ratios of [TiO₂]:[Au] have been prepared by chemically reducing AuCl₄^‐ on the positively charged TiO₂ nanorods surface and used to modify boron‐doped diamond (BDD) electrodes. The electrochemical behaviors of catechol on the bare and different Au/TiO₂ nanorod composites‐modified BDD electrodes are studied. The cyclic voltammetric results indicate that these different Au/TiO₂ nanorod composites‐modified BDD electrodes can enhance the electrocatalytic activity toward catechol detection, as compared with the bare BDD electrode. Among these different conditions, the Au/TiO₂‐BDD3 electrode (the ratio of [TiO₂]:[Au] is 27:1) is the most choice for catechol detection. The electrochemical response dependences of the Au/TiO₂‐BDD3 electrode on pH of solution and the applied potential are studied. The detection limit of catechol is found to be about 1.4 × 10^‐6 M in a linear range from 5 × 10^‐6 M to 200 × 10^‐6 M on the Au/TiO₂‐BDD3 electrode.