2D-titanium carbide (MXene) based selective electrochemical sensor for simultaneous detection of ascorbic acid,dopamine and uric acid

Two-dimensional (2D) titanium carbide (MXene) nanosheets exhibited excellent conductivity,flexibility,high volumetric capacity,hydrophilic surface,thermal stability,etc.So,it has been exploited in various applications.Herein,we report synthesis of mixed phase 2D MXene as a catalytic material for simultaneous detection of important biomolecules such as ascorbic acid (AA),dopamine (DA) and uric acid (UA),Crystalline structure,surface morphology and elemental composition of mixed phase titanium carbide (Ti-C-Tx) MXene (Tx =-F,-OH,or-O) nanosheets were confirmed by X-ray diffraction (XRD),Raman spectroscopy,high-resolution transmission electron microscopy (HR-TEM),high-resolution scanning electron microscopy (HR-SEM) and Energy-dispersive X-ray spectroscopy (EDS) mapping analysis.Furthermore,Ti-C-Tx modified glassy carbon electrode (GCE) was prepared and its electrochemical properties are studied by cyclic voltammetry (CV) and differential pulse voltammetry (DPV).It was found that Ti-C-Tx modified GCE (Ti-C-Tx/GCE) showed excellent electrocatalytic activity and separated oxidation peaks of important biomolecules such as AA (at 0.01 V),DA (at 0.21 V) and UA (at 0.33 V).Also,Ti-C-Tx/GCE sensor is enabled their simultaneous detection in physiological pH from 100 to 1000 μM for AA,0.5-50 μM for DA and 0.5-4 μM & 100-1500 μM for UA.The limit ofdetection's (LOD) was estimated as 4.6 μM,0.06 μM and 0.075 μM for AA,DA and UA,respectively.Moreover,real sample analysis indicated that spiked AA,DA and UA can be determined accurately by Ti-C-Tx/GCE with the recovery ratio in the range between 100.5％-103％ in human urine samples.The proposed Ti-C-Tx modified electrode exhibited good stability,selectivity and reproducibility as an electrochemical sensor for the detection ofAA,DA and UA molecules.     

Synthesis of composite material of cobalt oxide (Co3O4) with hydroxide functionalized multi-walled carbon nanotubes (MWCNTs) for electrochemical determination of uric acid

The gout is mainly found due to accumulation of uric acid crystals into the joints which produces the inflammatory symptoms. Thus, it is highly demanded to detect uric acid from our body. Herein, we prepare a composite material of cobalt oxide (Co₃O₄) with hydroxide functionalized multi-walled carbon nanotubes (MWCNTs) by hydrothermal method. The composite material is used for the modification of glassy carbon electrode (GCE) and investigated for the electrochemical determination of uric acid (UA). The analytical techniques such as scanning electron microscopy (SEM), powder X-ray diffraction (XRD), energy dispersive spectroscopy (EDS) and Fourier Infra-red spectroscopy (FTIR) are used to characterize the composite material. The Co₃O₄ exhibits a dendrite morphology and very well chemically coupled with MWCNTs. The elemental analysis confirms the presence of cobalt (Co), oxygen (O) and carbon (C) as main constituent of the composite material. The Co₃O₄ exhibitsa cubic unit cell crystallography in the composite system. The FTIR study reveals the characteristic bands of Co–O bands in the composite material. The cyclic voltammetry isused to study the electrochemical properties of prepared materials. The composite sample with highest percentage of MWCNTs shows an excellent electrochemical activity towards the oxidation of uric acid in phosphate buffer solution pH 7.3. The enzyme free uric acid sensor possesses a linear range of 0.1 mM to 3 mM with a quantified limit of detection of 0.005?±?0.0023 mM. The modified electrode is stable, selective, and very sensitive towards uric acid, therefore it may be used for the monitoring of uric acid from clinical samples. The proposed composite material can be of great interest for energy and biomedical fields.

     

Facile deposition of polyvinylpyrrolidone-grafted multi-walled carbon nanotube on glassy carbon electrode and its application in the sensing of biomolecules

Polyvinylpyrrolidone-grafted multi-walled carbon nanotube (PVP/MWCNT) was deposited on carboxylated glassy carbon electrode (GCE) from acidic water via the electrostatic force between the positively charged PVP/MWCNTs and negatively charged GCE. It was found out that PVP/MWCNT-modified GCE (PVP/MWCNT/GCE) exhibited great catalytic capability for the oxidation of dopamine (DA), uric acid (UA), tyrosine (Tyr) and nitrite) by enhancing their oxidation currents and lowering their overpotentials. For PVP/MWCNT/GCE, the linear calibration plots for DA, UA, Tyr and nitrite were obtained over the range of 0.1–50 μM, 1–500 μM, 1–200 μM and 1–500 μM with detection limits of 0.04 μM, 0.17 μM, 0.26 μM and 0.30 μM, respectively. In addition, satisfactory results were obtained by applying PVP/MWCNT/GCE in the determination of DA, UA, Tyr and nitrite in human serum samples with standard addition method.     

Hydrothermal synthesis of ZnO flower-reduced graphene oxide composite for electrochemical determination of ascorbic acid

In this work, reduced graphene oxide coated zinc oxide flower (ZnO–RGO) nanocomposite has been prepared via a simple one-pot hydrothermal synthesis method. The morphology and properties of the proposed ZnO–RGO were characterized using SEM, Raman and UV–VIS spectroscopy. Then, an ascorbic acid (AA) electrochemical sensor was prepared based on a glassy carbon electrode (GCE) modified with the ZnO–RGO nanocomposite. An advanced performance was recorded on the ZnO–RGO/GCE compared with that of the bare GCE, ZnO/GCE, and RGO/GCE. The proposed electrochemical sensor exhibited a wide linear detection from 5 μM to 2 mM with a low detection limit of 1.2 μM. Further, the proposed AA electrochemical sensor showed a good repeatability, reproducibility and stability.     

Development of an amperometric l-lactate biosensor based on l-lactate oxidase immobilized through silica sol–gel film on multi-walled carbon nanotubes/platinum nanoparticle modified glassy carbon electrode

Platinum nanoparticles (Ptnano) were used in combination with multi-walled carbon nanotubes (MWCNTs) for fabricating sensitivity-enhanced electrochemical l-lactate biosensor. The composite film of MWCNTs and Ptnano was dispersed on the surface of the glassy carbon electrode (GCE). l-lactate oxidase (LOD) was immobilized on MWCNTs/Ptnano/GCE surface by adsorption. The resulting LOD/MWCNTs/Ptnano electrode was covered by a thin layer of sol–gel to avoid the loss of LOD in determination and to improve the anti-interferent ability. Moreover, the sol–gel microenviroment contributes to both intensified stability and permselectivity. The cyclic voltammetry results indicated that MWCNTs/Ptnano catalyst displayed a higher performance than MWCNTs. Under the optimized conditions of applied potential 0.5 V, pH 6.4, room temperature, the proposed biosensor showed a large determination range (0.2–2.0 mM), a short response time (within 5 s), a high sensitivity (6.36 μA mM^{− 1}) and good stability (90% remains after 4 weeks). The fabricated biosensor had practically good selectivity against interferences. The results for whole blood samples measured by the present biosensor showed a good agreement with those measured by spectrophotometric method.     

Sensitive determination of uric acid by using graphene quantum dots as a new substrate for immobilisation of uric oxidase

A novel strategy for highly sensitive electrochemical detection of uric acid (UA) was proposed based on graphene quantum dots (GQDs), GQDs were introduced as a suitable substrate for enzyme immobilisation. Uric oxidase (UOx) was immobilised on GQDs modified glassy carbon electrode (GCE). Transmission electron microscope, scanning electron microscopy, cyclic voltammetry and electrochemical impedance spectroscopy techniques were used for characterising the electrochemical biosensor. The developed biosensor responds efficiently to UA presence over the concentration linear range 1–800 μM with the detection limit 0.3 μM. This novel biosensing platform based on UOx/GQDs electrode responded even more sensitively than that based on GCE modified by UOx alone. The inexpensive, reliable and sensitive sensing platform based on UOx/GQDs electrode provides wide potential applications in clinical.Inspec keywords: organic compounds, graphene devices, quantum dots, enzymes, biosensors, biochemistry, electrochemical electrodes, electrochemical sensors, transmission electron microscopy, scanning electron microscopy, voltammetry (chemical analysis), electrochemical impedance spectroscopy, nanomedicine, molecular biophysicsOther keywords: sensitive uric acid determination, graphene quantum dots, uric oxidase immobilisation, electrochemical detection, GQD, enzyme immobilisation, glassy carbon electrode, GCE, transmission electron microscope, scanning electron microscopy, cyclic voltammetry, electrochemical impedance spectroscopy, electrochemical biosensor, C     

Poly-cobalt[tetrakis(o-aminophenyl)porphyrin] nanowire and single-walled carbon nanotube for the analysis of hydrogen peroxide

Nanowires of poly-cobalt[tetrakis(o-aminophenyl)porphyrin] (PCoTAPPNW) were fabricated by electrochemical polymerization by the cyclic voltammetric method in anodic aluminum oxide membranes. A glassy carbon electrode (GCE) modified by PCoTAPPNW and single-walled carbon nanotubes (SWNT) without any binder was investigated with voltammetric methods in phosphate buffer saline (PBS) at pH 7.4. The PCoTAPPNW + SWNT/GCE exhibited strongly enhanced voltammetric and amperometric sensitivity towards hydrogen peroxide (H2O2), which shortened the response time (< 5 seconds), showed detection limit of 1.0 microM and enhanced the sensitivity for H2O2 detection with 194 microA mM(-1) cm(-2). The PCoTAPPNW + SWNT/GCE can be used to monitor H2O2 at very low concentration in physiological pH as an efficient electrochemical H2O2 sensor.     

Direct electrochemical determination of ascorbic acid by a cobalt(II) tetra-neopentyloxy phthalocyanine-multi-walled carbon nanotubes glassy carbon electrode

A cobalt(II) tetra-neopentyloxy phthalocyanine-multi-walled carbon nanotubes (CoTNPPc–MWNTs) composite was synthesized and characterized by UV–Vis spectra and transmission electron microscopy. The CoTNPPc–MWNTs glassy carbon electrode (CoTNPPc–MWNTs/GCE) was prepared by drop coating. The electrocatalytic performance of the chemically modified electrode was investigated for oxidation of ascorbic acid (AA). It was found that in phosphate buffer solution at pH = 6.60, the chemically modified electrode exhibited excellent electrocatalytic activity toward the oxidation of AA. The oxidation peak current increased linearly with the concentration of AA in the range of 10 μM–1.6 mM within the detection limit of 5 μM and low response time of 4 s.     

Conducting polymer‐coated, palladium‐functionalized multi‐walled carbon nanotubes for the electrochemical sensing of hydroxylamine

Electrochemical sensors of hydroxylamine were fabricated on glassy carbon electrodes (GCEs) by the electropolymerization of 3,4‐ethylenedioxypyrrole (EDOP) and 3,4‐ethylenedioxythiophene (EDOT) on palladium (Pd) nanoparticles attached to thiolated multi‐walled carbon nanotubes (MWCNTs), denoted as PEDOP/MWCNT‐Pd/GCE and PEDOT/MWCNT‐Pd/GCE. The sensors were characterized by field emission scanning electron microscopy and electrochemical impedance spectroscopy. They showed strong catalytic activity toward the oxidation of hydroxylamine. Cyclic voltammetry and amperometry were used to characterize the sensors' performances. The detection limits of hydroxylamine by PEDOP/MWCNT‐Pd/GCE and PEDOT/MWCNT‐Pd/GCE were 0.22 and 0.24 μM (S/N = 3), respectively. The sensors' sensitivity, selectivity, and stability were also investigated.     

Multicenter uric acid biosensor based on tetrapod-shaped ZnO nanostructures

In this work, we developed a tetrapod-shaped ZnO nanostructure (T-ZnO) biosensor to determine uric acid (UA), which is the primary end product of purine metabolism. The as-fabricated UA sensor presents a higher performance than that of the reported biosensors based on ZnO nanorods and ZnS quantum dots, etc. High-quality ZnO nanotetrapods were characterized by field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectra (EDX), X-ray diffraction (XRD) and Raman spectroscopy, respectively. A high affinity of uricase/ZnO to UA was revealed by cyclic voltammograms. The biosensor performance has been systematically investigated by amperometric response measurements. A fast current response time is within 9 s. It was also found that the uricase/T-ZnO biosensor presented a high and reproducible sensitivity of 80.0 microA cm(-2) mM(-1) and an experiment limit of detection of 0.8 microM. This study provides an insight utilizing the unique ZnO nanostructure to develop the highly sensitive and rapidly responsive nano-bio devices.     

Selective voltammetric and amperometric detection of uric acid with oxidized diamond film electrodes

Electrochemically anodized diamond film electrodes were used for selective detection of uric acid (UA) in the presence of high concentrations of ascorbic acid (AA) by differential pulse voltammetry and chronoamperometry. Because the oxidation peak potential for AA is approximately 450 mV more positive than that for UA at anodized diamond electrodes, UA can be determined with very good selectivity. By use of chronoamperometry, linear calibration curves were obtained for UA over the concentration range up to 1 x 10(-6) M in 0.1 M HClO4 solution, with the lowest experimental value measured being 5 x 10(-8) M. This is consistent with the fact that a statistical analysis of the calibration curve yielded a detection limit of 1.5 x 10(-8) M (S/N = 3). AA in less than 20-fold excess does not interfere. The practical analytical utility of the method is demonstrated by the measurement of UA in human urine and serum without any preliminary treatment.     

Highly selective and sensitive voltammetric sensor based on modified multiwall carbon nanotube paste electrode for simultaneous determination of ascorbic acid,acetaminophen and tryptophan

A carbon-paste electrode modified with multiwall carbon nanotubes (MWCNTs) was used for the sensitive and selective voltammetric determination of ascorbic acid (AA) in the presence of 3,4-dihydroxycinnamic acid (3,4-DHCA) as mediator. The mediated oxidation of AA at the modified electrode was investigated by cyclic voltammetry (CV), chronoamperommetry and electrochemical impedance spectroscopy (EIS). Also, the values of catalytic rate constant (k), and diffusion coefficient (D) for AA were calculated. Using square wave voltammetry (SWV), a highly selective and simultaneous determination of AA, acetaminophen (AC) and tryptophan (Trp) has been explored at the modified electrode. The modified electrode displayed strong function for resolving the overlapping voltammetric responses of AA, AC and Trp into three well-defined voltammetric peaks. In the mixture containing AA, AC and Trp, the three compounds can well separate from each other with potential differences of 200, 330 and 530 mV between AA and AC, AC and Trp and AA and Trp, respectively, which was large enough to determine AA, AC and Trp individually and simultaneously.     

Determination of Cu2+ using poly(2-aminothiazole)/ multi-walled carbon nanotubes composite film modified glassy carbon electrodes

2-Aminothiazole was electropolymerized by cyclic voltammetry (CV) on the multi-walled carbon nanotubes (MWCNTs) modified glassy carbon electrode (GCE) surface. Poly(2-aminothiazole)/MWCNTs/GCE was used for determination of copper ions. The anodic peak currents of copper ions evaluated by differential pulse stripping voltammetry (DPSV) are linear with the concentrations in the range from 1.0 x 10(-7) M to 2.0 x 10(-5) M with a linear coefficiency of 0.9985. The detection limit is 2.0 x 10(-9) M calculated for a signal-to-noise ratio of 3 (S/N = 3). The proposed method was applied successfully to the determination of copper ions in drinking water, and the recovery was 96%.     

Simultaneous measurement of dopamine and ascorbate at their physiological levels using voltammetric microprobe based on overoxidized poly(1,2-phenylenediamine)-coated carbon fiber

Overoxidized poly-(1,2-phenylenediamine) (OPPD)-coated carbon fiber microelectrodes (CFMEs) exhibit, in combination with square-wave voltammetry (SWV) detection mode, the attractive ability to simultaneously measure low nM dopamine (DA) and mM ascorbate (AA) in a pH 7.4 medium. The PPD polymer film is electrodeposited onto a carbon fiber at a constant potential of 0.8 V versus Ag/AgCl using a solution containing sodium dodecylsulfate as the dopant. After overoxidation using cyclic voltammetry (CV) in the potential range from 0 to 2.2 V at a scan rate of 10 V/s, the resulting OPPD-CFME displays a high SWV current response to cationic DA at approximately 0.2 V and has a favorably low response to anionic AA at approximately 0.0 V vs Ag/AgCl. The preparation of the new OPPD-sensing film has been carefully studied and optimized. The OPPD properties and behavior were characterized using CV and SWV under various conditions and are discussed with respect to DA and AA detection. The linear calibration range for DA in the presence of 0.3 mM AA is 50 nM to 10 microM, with a correlation coefficient of 0.998 and a detection limit of 10 nM using 45-s accumulation. The detection limit for DA in the absence of AA was estimated to be 2 nM (S/N = 3). The linear range for AA in the presence of 100 nM DA is 0.2-2 mM, with a correlation coefficient of 0.999 and a detection limit of 80 microM. The reproducibilities of SWV measurements at OPPD-CFCMEs are 1.6% and 2.5% for 100 nM DA and 0.3 mM AA, respectively. Potential interfering agents, such as 3,4-dihydroxyphenylacetic acid, uric acid, oxalate, human serum proteins, and glucose, at their physiologically relevant or higher concentrations did not have any effect. These favorable features offer great promise for in vitro and in vivo application of the proposed OPPD-coated microprobe.     

Electrochemical determination of nitrate with nitrate reductase-immobilized electrodes under ambient air

Nitrate monitoring biosensors were prepared by immobilizing nitrate reductase derived from yeast on a glassy carbon electrode (GCE, d = 3 mm) or screen-printed carbon paste electrode (SPCE, d = 3 mm) using a polymer (poly(vinyl alcohol)) entrapment method. The sensor could directly determine the nitrate in an unpurged aqueous solution with the aid of an appropriate oxygen scavenger: the nitrate reduction reaction driven by the enzyme and an electron-transfer mediator, methyl viologen, at -0.85 V (GCE vs Ag/AgCl) or at -0.90 V (SPCE vs Ag/AgCl) exhibited no oxygen interference in a sulfite-added solution. The electroanalytical properties of optimized biosensors were measured: the sensitivity, linear response range, and detection limit of the sensors based on GCE were 7.3 nA/microM, 15-300 microM (r2 = 0.995), and 4.1 microM (S/N = 3), respectively, and those of SPCE were 5.5 nA/microM, 15-250 microM (r2 = 0.996), and 5.5 microM (S/N = 3), respectively. The disposable SPCE-based biosensor with a built-in well- or capillary-type sample cell provided high sensor-to-sensor reproducibility (RSD < 3.4% below 250 microM) and could be used more than one month in normal room-temperature storage condition. The utility of the proposed sensor system was demonstrated by determining nitrate in real samples.     

Electron microscopic and voltammetric analysis of carbon fibre electrode pretreatments

The comparison between the differential pulse voltammetric capability of untreated or treated 12 µm or 30 µm diameter carbon fibre electrodes (CFE) to detect electroactive compoundsin vitro has been analysed. Their affinity for ascorbic acid (AA), catechols (dopamine (DA); 3,4-dihydroxyphenylacetic acid (DOPAC)), indoles (serotonin (5HT); 5-hydroxyindoleacetic acid (5HIAA)) and uric acid (UA) has been quantified. It appeared that only the 12 µm CFE, when electrically pretreated, can selectively and simultaneously detect the oxidation of AA, catechols and indoles, while the electrically pretreated 30 µm CFE are specifically sensitive to the indoles. However, both electrodes displayed highest sensitivity for serotonin.To avoid the detection of acids and selectively monitor the oxidation of DA and 5HT the 12 µm CFE and the 30 µm CFE were chemically (chromic acid) and electrically pretreated then electrically coated with Nafion. However, the 30 µm CFE alone selectively measured these compoundsin vitro as the 12 µm CFE did not accept the coat of Nafion.Using transmission electron microscopy (TEM) and scanning electron microscopy (SEM), we have investigated the effect of electrical and/or chemical pretreatments on the structural characteristics of the CFE. It appeared that both pretreatments increased the surface area of the carbon fibres by pitting, roughening and fracturing the carbon. Holes were also observed on the surface of the 30 µm dia. carbon fibres following the electrical pretreatment. The possibility of a correlation between these modifications and the pretreatment-induced voltammetric high sensitivity of the CFE is discussed.     

A highly-sensitive l-lactate biosensor based on sol-gel film combined with multi-walled carbon nanotubes (MWCNTs) modified electrode

A new type of amperometric l-lactate biosensor based on silica sol-gel and multi-walled carbon nanotubes (MWCNTs) organic–inorganic hybrid composite material was developed. The sol-gel film was used to immobilize l-lactate oxidase on the surface of glassy carbon electrode (GCE). MWCNTs were used to increase the current response and improve the performance of biosensor. The sol-gel film fabrication process parameters such as H₂O : TEOS and pH were optimized, Effects of some experimental variables such as applied potential, temperature, and pH on the current response of the biosensor were investigated. Analytical characteristics and dynamic parameters of the biosensors with and without MWCNTs in the hybrid film were compared, and the results showed that analytical performance of the biosensor could be improved greatly after introduction of the MWCNTs. Sensitivity, linear range, limit of detection (S / N = 3) were 2.097 μA mM^− 1, 0.3 to 1.5 mM, 0.8 × 10^− 3 mM for the biosensor without MWCNTs and 6.031 μA mM^− 1, 0.2 to 2.0 mM, 0.3 × 10^− 3 mM for the biosensor with MWCNTs, respectively. This method has been used to determine the l-lactate concentration in real human blood samples.     

Multi-walled carbon nanotube modified carbon paste electrode as an electrochemical sensor for the determination of epinephrine in the presence of ascorbic acid and uric acid

A biocompatible electrochemical sensor for selective detection of epinephrine (EP) in the presence of 1000-fold excess of ascorbic acid (AA) and uric acid (UA) was fabricated by modifying the carbon paste electrode (CPE) with multi-walled carbon nanotubes (MWCNTs) using a casting method. The electro-catalytic activity of the modified electrode for the oxidation of EP was investigated. The current sensitivity of EP was enhanced to about five times upon modification. A very minimum amount of modifier was used for modification. The voltammetric response of EP was well resolved from the responses of AA and UA. The electrochemical impedance spectroscopic (EIS) studies reveal the least charge transfer resistance for the modified electrode. The AA peak that is completely resolved from that of EP at higher concentrations of AA and the inability of the sensor to give an electrochemical response for AA below a concentration of 3.0 × 10^? 4 M makes it a unique electrochemical sensor for the detection of EP which is 100% free from the interference of AA. Two linear dynamic ranges of 1.0 × 10^? 4–1.0 × 10^? 5 and 1.0 × 10^? 5–5.0 × 10^? 7 M with a detection limit of 2.9 × 10^? 8 M were observed for EP at modified electrode. The practical utility of this modified electrode was demonstrated by detecting EP in spiked human blood serum and EP injection. The modified electrode is highly reproducible and stable with anti fouling effects.     

Morphological and compositional engineering of Ni/carbon nanotube composite film via a novel cyclic voltammetric route

Ni/multi-walled carbon nanotubes (MWCNTs) composite films were deposited on the glassy carbon electrode (GCE) by a Ni plating bath containing homogeneously dispersed MWCNTs using polyvinylpyrrolidone (PVP) as dispersion additive. Incorporation of MWCNTs into Ni matrix was greatly enhanced by the application of cyclic voltammetric (CV) deposition technique. The structure and nature of the Ni/MWCNT were characterized by field emission scanning electron microscope (FE-SEM) and X-ray diffraction (XRD). The results show that the content of MWCNT and the morphology of the deposited Ni/MWCNT composite film can be controlled by selecting the appropriate electroplating conditions. Further study indicates that the obtained Ni/MWCNT showed excellent electro-catalytic activity for the oxidation of ethanol in alkaline solution.     

Copper germanate nanowire electrode for the electrochemical detection of glyoxalic acid using cyclic voltammetry method

Glyoxalic acid was determined by electrochemical cyclic voltammetry method using copper germanate (CuGeO₃) nanowires as the modified glassy carbon electrode (GCE) materials. The CuGeO₃ nanowire modified GCE exhibited good detection performance for glyoxalic acid in neutral solution. The intensities of two anodic peaks vary linearly with the increase of the scan rate from 25 to 200 mVs^?1 and glyoxalic acid concentration from 0.001 to 2 mM. The detection limit was 8.5 μM and 0.78 μM for cvp1 and cvp2 with the correlation coefficient of 0.991 and 0.998, respectively. The CuGeO₃ nanowire modified GCE exhibited good reproducibility and stability.