Low potential detection of NADH based on Fe₃O₄ nanoparticles/multiwalled carbon nanotubes composite: fabrication of integrated dehydrogenase-based lactate biosensor

Fe(3)O(4) magnetic nanoparticles were in situ loaded on the surface of multiwalled carbon nanotubes (MWCNTs) by a simple coprecipitation procedure. The resulting Fe(3)O(4)/MWCNTs nanocomposite brings new capabilities for electrochemical sensing by combining the advantages of Fe(3)O(4) magnetic nanoparticles and MWCNTs. It was found that Fe(3)O(4) has redox properties similar to those of frequently used mediators used for electron transfer between NADH and electrode. The cyclic voltammetric results indicated the ability of Fe(3)O(4)/MWCNTs modified GC electrode to catalyze the oxidation of NADH at a very low potential (0.0 mV vs. Ag/AgCl) and subsequently, a substantial decrease in the overpotential by about 650 mV compared with the bare GC electrode. The catalytic oxidation current allows the stable and selective amperometric detection of NADH at an applied potential of 0.0 mV (Ag/AgCl) with a detection limit of 0.3 μM and linear response up to 300 μM. This modified electrode can be used as an efficient transducer in the design of biosensors based on coupled dehydrogenase enzymes. Lactate dehydrogenase (LDH) and NAD(+) were subsequently immobilized onto the Fe(3)O(4)/MWCNTs nanocomposite film by covalent bond formation between the amine groups of enzyme or NAD(+) and the carboxylic acid groups of the Fe(3)O(4)/MWCNT film. Differential pulse voltammetric detection of lactate on Fe(3)O(4)/MWCNT/LDH/NAD(+) modified GC electrode gives linear responses over the concentration range of 50-500 μM with the detection limit of 5 μM and sensitivity of 7.67 μA mM(-1). Furthermore, the applicability of the sensor for the analysis of lactate concentration in human serum samples has been successfully demonstrated.     

A cyclodextrin host-guest recognition approach to an electrochemical sensor for simultaneous quantification of serotonin and dopamine

An electrochemical sensor for simultaneous quantification of serotonin (5-hydroxytryptamine, 5-HT) and dopamine (DA) using a β-cyclodextrin/poly(N-acetylaniline)/carbon nanotube composite modified carbon paste electrode has been developed. Synergistic effect of multi-walled carbon nanotube (MWCNT) in addition to the pre-concentrating effect of β-cyclodextrin (β-CD) as well as its different inclusion complex stability with 5-HT and DA was used to construct an electrochemical sensor for quantification of these important neurotransmitters. The overlapping anodic peaks of 5-HT and DA at 428 mV on bare electrode resolved in two well-defined voltammetric peaks at 202 and 363 mV vs. Ag/AgCl respectively. The oxidation mechanism of 5-HT and DA on the surface of the electrode was investigated by cyclic voltammetry and it was found that the electrode processes are pH dependent and electrochemical oxidation of 5-HT is totally irreversible while the electrode gave a more reversible process to DA. Under optimized conditions, linear calibration curves were obtained in the range of about 4-200 μM with a detection limits down to sub-μM levels (S/N=3) after 20-s accumulation, for both. The proposed sensor was shown to be remarkably selective for 5-HT and DA in matrices containing different species including ascorbic acid and uric acid. The suitability of the developed method was tested for the determination of 5-HT and DA in the Randox Synthetic Plasma samples and acceptable recoveries were obtained for a set of spiked samples.     

Electrochemical monitoring of aflatoxin M1 in milk samples using silver nanoparticles dispersed on α‐cyclodextrin‐GQDs nanocomposite

Aflatoxins are potential food pollutants produced by fungi. One of important toxins is aflatoxin M1 (AFM1). A great deal of concern is associated with AFM1 toxicity. In the present study, an innovative electrochemical interface for quantitation of AFM1 based on ternary signal amplification strategy was fabricated. In this work, silver nanoparticles was electrodeposited onto green and biocompatible nanocomposite containing α‐cyclodextrin as conductive matrix and graphene quantum dots as amplification element. Therefore, a multilayer film based on α‐cyclodextrin, graphene quantum dots, and silver nanoparticles was exploited to develop a highly sensitive electrochemical sensor for detection of AFM1. Fully electrochemical methodology was used to prepare a transducer on a glassy carbon electrode, which provided a high surface area toward sensitive detection of AFM1. The surface morphology of electrode surface was characterized by high‐resolution field emission scanning electron microscope. The proposed sensing platform provides a simple tool for AFM1 detection. Under optimized condition, the calibration curve for AFM1 concentration was linear in 0.015mM to 25mM with low limit of quantification of 2μM. The practical analytical utility of the modified electrode was illustrated by determination of AFM1 in unprocessed milk samples.     

Graphene nanosheets modified glassy carbon electrode for simultaneous detection of heroine, morphine and noscapine

In the present study, the graphene nanosheets (GNSs) modified glassy carbon (GC) electrode is employed for simultaneous determination of morphine, noscapine and heroin. To the best of our knowledge this is the first report of the simultaneous determination of these three important opiate drugs based on their direct electrochemical oxidation. Field emission scanning electron microscopy (FESEM) technique is utilized in order to study the surface morphology of the modified electrode. The modified electrode shows excellent electrocatalytic activity toward oxidation of morphine, noscapine and heroin at reduced overpotentials in wide pH range. In the performed experiments, differential pulse voltammetric determination of morphine, noscapine and heroin yields calibration curves with the following characteristics; linear dynamic range up to 65, 40 and 100 μM, sensitivity of 275, 500 and 217 nA μM(-1) cm(-2), and detection limits of 0.4, 0.2 and 0.5 μM at 3S(B), respectively. Fast response time, signal stability, high sensitivity, low cost and ease of preparation method without using any specific electron-transfer mediator or specific reagent are the advantageous of the proposed sensor. The modified electrode can be used for simultaneous or individual detection of three major narcotic components, heroin, noscapine and morphine at micromolar concentration without any separation or pretreatment steps.     

Application of a Cu–chitosan/multiwalled carbon nanotube film-modified electrode for the sensitive determination of rutin

A new sensitive electrochemical sensor, a glassy carbon electrode modified with chemically cross-linked copper-complexed chitosan/multiwalled carbon nanotubes (Cu–CS/MWCNT/GCE), for rutin analysis was constructed. Experimental investigations of the influence of several parameters showed that the rutin can effectively accumulate on the surface of the Cu–CS/MWCNT/GCE, which accumulation caused a pair of well-defined redox peaks in the electrochemical signal when measurements were carried out in Britton–Robinson buffer solution (pH 3, 0.04 M). The surface of the Cu–CS/MWCNT/GCE was characterized by field-emission scanning electron microscopy, transmission electron microscopy, and X-ray diffractometry analysis. In a rutin concentration range of 0.05–100 μM and under optimized conditions, a linear relationship between the oxidation peak current of rutin and its concentration was obtained with a detection limit of 0.01 μM. The Cu–CS/MWCNT/GCE showed good selectivity, stability, and reproducibility. Moreover, the sensor was used to determine the presence of rutin in fruits with satisfactory results.     

Voltammetric detection of anti-HIV replication drug based on novel nanocomposite gold-nanoparticle–CaCO<Subscript>3</Subscript> hybrid material

A novel bionanocomposite, horse radish peroxidase- gold-nanoparticle–Calcium carbonate (HRP-AuNPs-CaCO₃), hybrid material was encapsulated by silica sol on a glassy carbon electrode (GCE). The fabricated modified electrode was used as a novel voltammetric sensor for electrochemical sensing of anti-HIV replication drug i.e. deferiprone. The surface morphology of the modified electrode was characterized by scanning electron microscopy (SEM). Results obtained from the voltammetric measurements show that HRP-AuNPs-CaCO₃ modified GCE offers a selective and sensitive electrochemical sensor for the determination of deferiprone. Under experimental conditions, the proposed voltammetric sensor has a linear response range from 0.01 to 10,000 μM with a detection limit of 0.01 μM. Furthermore, the fabricated sensor was successfully applied to determine deferiprone level in spiked urine and serum samples.     

Selective electrochemical detection of cysteine in complex serum by graphene nanoribbon

Selective detection of cysteine in serum samples was achieved on a graphene nanoribbon (GNR) and Nafion nanocomposite modified electrode with high precision. The superior conductivity and abundant amount of active chemical oxygen groups on the edge of GNR led to extremely highly electrocatalytic activity of GNR towards the electrochemical oxidation of cysteine at +0.025 V. The electrocatalytic behavior was further used for sensitive detection of cysteine by differential pulse voltammetry. Under optimized conditions, the calibration curve was linear in the range from 25 nM to 500 μM. The electrochemical sensor showed strong antifouling ability, good stability and selectivity. It could effectively exclude the interferences from other kinds of biothiols and the biological relevant species, thus had great perspective for in vivo analysis of biological samples.     

Sensitive monitoring of riboflavin in commercial multivitamins using poly (chitosan)‐based nanocomposite

The rapid and sensitive determination of riboflavin (RF) is important for the treatment of seborrheic and glossitis dermatitis, sunlight sensitivity, mucosal, and skin disorders. In this work, an electrochemical sensor was developed by electrodes modification using poly (chitosan) to sensitive detection of RF in commercial multivitamin. Electrodeposition of chitosan on the surface of glass carbon electrode was performed using cyclic voltammetry technique in the range of ?1 to +1 V. The modified electrode surface morphology was characterized using a high‐resolution field emission scanning electron microscope. The modified electrode was used as an effective electrical interface for the detection of RF using cyclic, differential pulse, and square wave voltammetry techniques. Finally, the sensor was applied to determine RF in commercial multivitamins. In optimum conditions, the linear range for the standard sample of RF and commercial multivitamins 94 to 333μM and 24.6 to 176μM were obtained, respectively. Low limit of quantification (LLOQ) were obtained as 24.6μM.     

Cobalt(II) salophen-modified carbon-paste electrode for potentiometric and voltammetric determination of cysteine

A chemically modified electrode constructed by incorporating N,N(')-bis(salicylidene)-1,2-phenylenediaminocobalt(II) into carbon-paste matrix was used as a sensitive electrochemical sensor for detection of cysteine. The resulting electrode exhibits catalytic properties for the electrooxidation of cysteine and lowers the overpotential for the oxidation of this compound. The faster rate of electron transfer results in a near-Nernstian behavior of the modified electrode and makes it a suitable potentiometric and voltammetric sensor for the fast and easy determination of cysteine. A linear response in concentration range from approximately 2 microM to 0.01 M was obtained with a detection limit of 1 microM for the potentiometric detection of cysteine. The modified electrode was also used for the amperometric and differential pulse voltammetric determination of cysteine and the results were compared with those of the potentiometric method.     

In situ synthesis of palladium nanoparticle-graphene nanohybrids and their application in nonenzymatic glucose biosensors

A nonenzymatic electrochemical biosensor was developed for the detection of glucose based on an electrode modified with palladium nanoparticles (PdNPs)-functioned graphene (nafion-graphene). The palladium nanoparticle-graphene nanohybrids were synthesized using an in situ reduction process. Nafion-graphene was first assembled onto an electrode to chemically adsorb Pd(2+). And Pd(2+) was subsequently reduced by hydrazine hydrate to form PdNPs in situ. Such a PdNPs-graphene nanohybrids-based electrode shows a very high electrochemical activity for electrocatalytic oxidation of glucose in alkaline medium. The proposed biosensor can be applied to the quantification of glucose with a wide linear range covering from 10 μM to 5mM (R=0.998) with a low detection limit of 1 μM. The experiment results also showed that the sensor exhibits good reproducibility and long-term stability, as well as high selectivity with no interference from other potential competing species.     

Electrochemical imprinted sensor for determination of oleanic acid based on poly (sodium 4-styrenesulfonate-co-acrylic acid)-grafted multi-walled carbon nanotubes-chitosan and cobalt hexacyanoferrate nanoparticles

A novel sensitive and selective imprinted electrochemical sensor for the determination of oleanic acid was constructed on a carbon electrode by stepwise modification of functional multi-walled carbon nanotubes, cobalt hexacyanoferrate nanoparticles and a thin imprinted sol-gel film. The fabrication of a homogeneous porous poly (sodium 4-styrenesulfonate-co-acrylic acid)-grafted multi-walled carbon nanotubes/SiO(2)-chitosan nanocomposite film was conducted by controllable electrodeposition technology. The surface morphologies of the modified electrodes were characterized by scanning electron microscope. The performance of the imprinted sensor was investigated by cyclic voltammetry, square wave voltammetry and electrochemical impedance spectroscopy in detail. The imprinted sensor displayed high sensitivity and selectivity towards oleanic acid. A linear relationship between the sensor response signal and the logarithm of oleanic acid concentrations ranging from 1.0×10(-8) to 1.0×10(-3) mol L(-1) was obtained with a detection limit of 2.0×10(-9) mol L(-1). It was applied to the determination of oleanic acid in real capsule samples successfully.     

Xanthine biosensor based on XO/AuNP/PtNP/MWCNT hybrid nanocomposite modified GCPE

A new xanthine (X) biosensors based on a hybrid nanocomposite containing multi-walled carbon nanotubes (MWCNT), Pt nanoparticles (PtNP) and gold nanoparticle (AuNP) was presented. X biosensor was fabricated by dropping AuNP/PtNP/MWCNT onto xanthine oxidase (XO) modified glassy carbon paste electrode (GCPE). Resulted XO/AuNP/PtNP/MWCNT/GCPE biosensor showed two linearity between 2.0 and 50 µM and 0.25 and 6.0 mM for X. RSD value was calculated as 2.46 (n = 5). Finally, the biosensor was applied to the X detection in synthetic serum samples and good recovery value was obtained.     

A highly sensitive nanostructure-based electrochemical sensor for electrocatalytic determination of norepinephrine in the presence of acetaminophen and tryptophan

A new electrochemical sensor for the determination of norepinephrine (NE), acetaminophen (AC) and tryptophan (Trp) is described. The sensor is based on carbon paste electrode (CPE) modified with 3,4-dihydroxybenzaldehyde-2,4-dinitrophenylhydrazone (DDP) and takes the advantages of carbon nanotubes (CNTs), which makes the modified electrode highly sensitive for the electrochemical detection of these compounds. Cyclic voltammetry (CV) at various scan rates was used to investigate the redox properties of the modified electrode. The apparent charge transfer rate constant, k(s), and transfer coefficient, α, for electron transfer between DDP and CNT paste electrode were calculated. The mediated oxidation of NE at the modified electrode was investigated by CV and the values of k, α and diffusion coefficient (D) were calculated. Under the optimum pH of 7.0, the oxidation of NE occurs at a potential about 215 mV less positive than that of the unmodified CPE. Differential pulse voltammetry (DPV) of NE at the modified electrode exhibited two linear dynamic ranges with a detection limit (3σ) of 77±2 nM. DPV was used for simultaneous determination of NE, AC and Trp at the modified electrode, and quantitation of NE in some real samples by the standard addition method.     

DNA electrochemical biosensor based on thionine-graphene nanocomposite

A novel protocol for development of DNA electrochemical biosensor based on thionine-graphene nanocomposite modified gold electrode was presented. The thionine-graphene nanocomposite layer with highly conductive property was characterized by scanning electron microscopy, transmission electron microscopy, cyclic voltammetry and electrochemical impedance spectroscopy. An amino-substituted oligonucleotide probe was covalently grafted onto the surface of the thionine-graphene nanocomposite by the cross-linker glutaraldehyde. The hybridization reaction on the modified electrode was monitored by differential pulse voltammetry analysis using an electroactive intercalator daunomycin as the indicator. Under optimum conditions, the proposed biosensor exhibited high sensitivity and low detection limit for detecting complementary oligonucleotide. The complementary oligonucleotide could be quantified in a wide range of 1.0 × 10(-12) to 1.0 × 10(-7)M with a good linearity (R(2)=0.9976) and a low detection limit of 1.26 × 10(-13)M (S/N=3). In addition, the biosensor was highly selective to discriminate one-base or two-base mismatched sequences.     

Bimetallic PtM (M=Pd, Ir) nanoparticle decorated multi-walled carbon nanotube enzyme-free, mediator-less amperometric sensor for H₂O₂

A new highly catalytic and intensely sensitive amperometric sensor based on PtM (where M=Pd, Ir) bimetallic nanoparticles (NPs) for the rapid and accurate estimation of hydrogen peroxide (H(2)O(2)) by electrooxidation in physiological conditions is reported. PtPd and PtIr NPs-decorated multiwalled carbon nanotube nanocatalysts (PtM/MWCNTs) were prepared by a modified Watanabe method, and were characterized by XRD, TEM, ICP, and XAS. The sensors were constructed by immobilizing PtM/MWCNTs nanocatalysts in a Nafion film on a glassy carbon electrode. Both PtPd/MWCNTs and PtIr/MWCNTs assemblies catalyzed the electrochemical oxidation of H(2)O(2). Cyclic voltammetry characterization measurements revealed that both the PtM (M=Pd, Ir)/MWCNTs/GCE possessed similar electrochemical surface areas (～0.55 cm(2)), and electron transfer rate constants (～1.23 × 10(-3)cms(-1)); however, the PtPd sensor showed a better performance in H(2)O(2) sensing than did the PtIr counterpart. Explanations were sought from XAS measurements to explain the reasons for differences in sensor activity. When applied to the electrochemical detection of H(2)O(2), the PtPd/MWCNTs/GC electrode exhibited a low detection limit of 1.2 μM with a wide linear range of 2.5-125 μM (R(2)=0.9996). A low working potential (0V (SCE)), fast amperometric response (<5s), and high sensitivity (414.8 μA mM(-1)cm(-2)) were achieved at the PtPd/MWCNTs/GC electrode. In addition, the PtPd/MWCNTs nanocatalyst sensor electrode also exhibited excellent reproducibility and stability. Along with these attractive features, the sensor electrode also displayed very high specificity to H(2)O(2) with complete elimination of interference from UA, AA, AAP and glucose.     

羧基化石墨烯/半胱胺修饰金电极对多巴胺的电化学行为研究

为了基于羧基化石墨烯/半胱胺修饰金电极建立更为先进的多巴胺生物传感器，以定量检测儿茶酚胺类神经递质多巴胺，利用自组装技术将半胱胺修饰于金电极上，再利用1-乙基-\[3-二甲基氨基丙基\]碳酰二亚胺盐酸化物/N-羟基琥珀酰亚胺（EDC/NHS）交联剂将羧基化石墨烯固定在修饰后的金电极上制成多巴胺电化学传感器。先对修饰电极进行表征以检验其灵敏度，再利用循环伏安法研究该电极在多巴胺溶液中的电化学行为，包括检测条件的优化和传感器性能的测定。修饰电极表征结果表明，羧基化石墨烯/半胱胺修饰金电极提高了电极传递电子的能力，具有较高的灵敏度。经单因素实验得出，最佳检测条件为利用pH 6.00的0.30 mol·L-1磷酸盐缓冲溶液（PBS）配制多巴胺溶液，扫描速率设定为200 mV·s-1。在最佳检测条件下，制备的多巴胺电化学传感器电流的大小随着多巴胺浓度的增大而增大，在1.0×10-3~3.5×10-3 mol·L-1范围内呈现良好的线性关系，线性回归方程为I=8.120 6C+7.017，相关系数R2为0.999 5。且该传感器精密度好，稳定性强，具有一定的抗干扰能力。研究结果为药物盐酸多巴胺注射液中多巴胺含量测定提供了支撑。     

Multienzymatic amperometric biosensor based on gold and nanocomposite planar electrodes for glycerol determination in wine

Amperometric biosensors based on gold planar or nanocomposite electrode containing multiwalled carbon nanotubes for determination of glycerol were developed. The biosensors were constructed by immobilization of a novel multienzyme cascade consisting of glycerol kinase/creatine kinase/creatinase/sarcosine oxidase/peroxidase between a chitosan "sandwich." A measuring buffer contained adenosine 5'-triphosphate (ATP), creatine phosphate, and an artificial electrochemical mediator ferrocyanide. The currents proportional to glycerol concentration were measured at working potential of -50 mV against Ag/AgCl reference electrode. The biosensors showed linearity over the ranges of 5-640 μM and 5-566 μM with detection limits of 1.96 and 2.24 μM and sensitivities of 0.80 and 0.81 nA μM(-1), respectively. Both types of biosensors had a response time of 70s. The biosensors demonstrated satisfactory operational stability (no loss of sensitivity after 90 consecutive measurements) and excellent storage stability (90% of the initial sensitivity after 15 months of storage at room temperature). The results obtained from measurements of wines correlated well with those obtained with an enzymatic-spectrophotometric assay. The presented multienzyme cascade can be used also for determination of triglycerides or various kinase substrates when glycerol kinase is replaced by other kinases.     

Reduced graphene oxide/PAMAM-silver nanoparticles nanocomposite modified electrode for direct electrochemistry of glucose oxidase and glucose sensing

Reduced graphene oxide/PAMAM-silver nanoparticles nanocomposite (RGO-PAMAM-Ag) was synthesized by self-assembly of carboxyl-terminated PAMAM dendrimer (PAMAM-G3.5) on graphene oxide (GO) as growing template, and in-situ reduction of both AgNO(3) and GO under microwave irradiation. The RGO-PAMAM-Ag nanocomposite was used as a novel immobilization matrix for glucose oxidase (GOD) and exhibited excellent direct electron transfer properties for GOD with the rate constant (K(s)) of 8.59 s(-1). The fabricated glucose biosensor based on GOD electrode modified with RGO-PAMAM-Ag nanocomposite displayed satisfactory analytical performance including high sensitivity (75.72 μA mM(-1) cm(-2)), low detection limit (4.5 μM), an acceptable linear range from 0.032 mM to 1.89 mM, and also preventing the interference of some interfering species usually coexisting with glucose in human blood at the work potential of -0.25 V. These results indicated that RGO-PAMAM-Ag nanocomposite is a promising candidate material for high-performance glucose biosensors.     

Ultra-fine Pt nanoparticles supported on ionic liquid polymer-functionalized ordered mesoporous carbons for nonenzymatic hydrogen peroxide detection

Poly(ionic liquid) (PIL) coated ordered mesoporous carbons (OMCs) were prepared by in situ polymerization of 3-ethyl-1-vinylimidazolium tetrafluoroborate ([VEIM]BF(4)) monomer on OMCs matrix. PIL on the surface of OMCs can provide sufficient binding sites to anchor the precursors of metal ion. PIL/OMCs were employed as support material for the deposition and formation of ultra-fine Pt nanoparticles, via the self-assembly between the negative Pt precursor and positively charged functional groups of PIL-functionalized OMCs. The combination of the unique properties of each component endows Pt/PIL/OMCs as a good electrode material. Compared with the Pt/OMCs nanocomposite, the Pt/PIL/OMCs modified electrode displays high electrocatalytic activity towards hydrogen peroxide (H(2)O(2)) and gives linear range from 1.0 × 10(-7) to 3.2 × 10(-3) M (R=0.999). The Pt/PIL/OMCs responds very rapidly to the changes in the level of H(2)O(2), producing steady-state signals within 4-5s. A high sensitivity of 24.43 μA mM(-1) and low detection limit of 0.08 μM was obtained at Pt/PIL/OMCs modified electrode towards the reduction of H(2)O(2). The improved activity makes Pt/PIL/OMCs nanocomposite promising for being developed as an attractive robust and new electrode material for electrochemical sensors and biosensors design.     

Sensitive label-free electrochemical immunoassay based on a redox matrix of gold nanoparticles/Azure І/multi-wall carbon nanotubes composite

A sensitive label-free electrochemical immunosensing platform was designed by a redox matrix of gold nanoparticles (GNPs), Azure І and multi-wall carbon nanotubes (MWCNT) self-assemblying nanocomposite. To construct the immunosensor, MWCNT was first dispersed in Nafion (Nf) to obtain a homogeneous solution and then it was dropped on the surface of the gold electrode (Au). Then the positively-charged redox molecule, Azure І, was entrapped into MWCNT–Nf film to form a redox nanostructural membrane. Next, the negatively charged gold nanoparticles (GNPs) were assembled to the interface through the electrostatic force. Finally, carcinoembryonic antibody molecules could be absorbed into the GNPs/Azure І/MWCNT–Nf immobilization matrix. Using carcinoembryonic antigen (CEA) as a model protein, the electrochemical immunosensor exhibited good stability and reproducibility, as well as good selectivity and storage stability. This strategy presented a promising platform for sensitive and facile monitoring of CEA.