Surface Of Carbon Ionic Liquid Electrode Research Articles

Preparation and Application of Electrochemical Horseradish Peroxidase Sensor Based on a Black Phosphorene and Single-Walled Carbon Nanotubes Nanocomposite.

To design a new electrochemical horseradish peroxidase (HRP) biosensor with excellent analytical performance, black phosphorene (BP) nanosheets and single-walled carbon nanotubes (SWCNTs) nanocomposites were used as the modifier, with a carbon ionic liquid electrode (CILE) as the substrate electrode. The SWCNTs-BP nanocomposite was synthesized by a simple in situ mixing procedure and modified on the CILE surface by the direct casting method. Then HRP was immobilized on the modified electrode with Nafion film. The electrocatalysis of this electrochemical HRP biosensor to various targets was further explored. Experimental results indicated that the direct electrochemistry of HRP was realized with a pair of symmetric and quasi-reversible redox peaks appeared, which was due to the presence of SWCNTs-BP on the surface of CILE, exhibiting synergistic effects with high electrical conductivity and good biocompatibility. Excellent electrocatalytic activity to trichloroacetic acid (TCA), sodium nitrite (NaNO2), and hydrogen peroxide (H2O2) were realized, with a wide linear range and a low detection limit. Different real samples, such as a medical facial peel solution, the soak water of pickled vegetables, and a 3% H2O2 disinfectant, were further analyzed, with satisfactory results, further proving the potential practical applications for the electrochemical biosensor.

Electrochemical DNA Biosensor Based on Platinum-gold Bimetal Decorated Graphene Modified Electrode for the Detection of Vibrio parahaemolyticus Specific tlh Gene Sequence

Background: By using bimetal nanocomposite modified electrode, the electrochemical DNA biosensor showed the advantages of high sensitivity, low cost, rapid response and convenient operation, which was applied for disease diagnosis, food safety, and biological monitoring. Objective: A nanocomposite consisting of platinum (Pt)-gold (Au) bimetal and two-dimensional graphene (GR) was synthesized by hydrothermal method, which was modified on the surface of carbon ionic liquid electrode and further used for the immobilization of probe ssDNA related to Vibrio parahaemolyticus tlh gene to construct an electrochemical DNA sensor. Method: Potassium ferricyanide was selected as electrochemical indicator, cyclic voltammetry was used to study the electrochemical behaviours of different modified electrodes and differential pulse voltammetry was employed to test the analytical performance of this biosensor for the detection of target gene sequence. Results: This electrochemical DNA biosensor could detect the Vibrio parahaemolyticus tlh gene sequence as the linear concentration in the range from 1.0×10-13 mol L-1 to 1.0×10-6 mol L-1 with the detection limit as 2.91×10-14 mol L-1 (3σ). Conclusion: This proposed electrochemical DNA biosensor could be used to identify the special gene sequence with good selectivity, low detection limit and wide detection range.

Porous biomass carbon and gold nanoparticles modified electrode for myoglobin direct electrochemistry and electrocatalysis

AbstractIn this paper, a stable porous biomass carbon (BC) and gold (Au) nanoparticles modified carbon ionic liquid electrode (CILE) was prepared for the investigation of direct electrochemistry and electrocatalysis of myoglobin (Mb). BC was synthesized via pyrolysis of Hainan's natural tropical plant Lagerstroemia indica leaves, which was modified on the surface of CILE with Au nanoparticles further electrodeposited on the electrode surface. Then, Mb was cast on the nanocomposite‐modified electrode to obtain an electrochemical sensing platform with Nafion film (Nafion/Mb/Au−BC/CILE). Direct electrochemistry of Mb on the modified electrode was investigated with appearance of a pair of enhanced redox peaks, indicating direct electron transfer rate of Mb was accelerated due to the presence of Au−BC nanocomposite. Au−BC nanocomposite exhibited high conductivity, good biocompatibility, and large surface area, which was beneficial for electron transfer of Mb. The Mb modified electrode showed excellent electrocatalytic activity toward the reduction of trichloroacetic acid (TCA) and sodium nitrite (NaNO2), which was further used to establish a new electroanalytical method with satisfied sensitivity in real samples analysis.

Myoglobin- and Hydroxyapatite-Doped Carbon Nanofiber-Modified Electrodes for Electrochemistry and Electrocatalysis.

In this paper, a hydroxyapatite (HAp)-doped carbon nanofiber (CNF)-modified carbon ionic liquid electrode (CILE) was prepared and used for the investigation on the direct electrochemistry and electrocatalysis of myoglobin (Mb). HAp nanoparticles were mixed within a polyacrylonitrile (PAN) solution, and a HAp@PAN nanofiber was synthesized by electrospinning process, which was further controlled by carbonization at 800 °C for 2 h in a nitrogen atmosphere to get a HAp@CNF nanocomposite. Various techniques were used to check the physicochemical properties of HAp@CNF. Mb was mixed with a HAp@CNF dispersion solution and casted on the surface of CILE to obtain an electrochemical sensing platform. The direct electrochemistry of Mb on the modified electrode was checked when a pair of enhanced redox waves appeared, indicating the direct electron transfer of Mb. HAp@CNF exhibited high conductivity, good biocompatibility, and large surface area, which was beneficial for Mb immobilization. The modified electrode showed excellent electrocatalytic activity toward the reduction of trichloroacetic acid and sodium nitrite, which was further used to establish a new electroanalytical method. Real samples were analyzed by the proposed method with satisfactory results.

An Electrochemical Sensor Based on an Ionic Liquid Covalently Functionalized Graphene Oxide for Simultaneous Determination of Copper (II) and Antimony (III)

AbstractGraphene oxide (GO) was chemically grafted by ionic liquid (IL) containing amino groups for preparation of IL covalently functionalized GO (IL–CFGO). SEM, FTIR and XRD results confirmed that IL had been successfully anchored on GO surface, resulting in more active sites and higher conductivity. The IL–CFGO composite was coated on the surface of carbon IL electrode (CILE) to construct electrochemical sensor for simultaneous detection of Cu(II) and Sb(III). The experimental factors including pH, deposition time and potential were optimized. Differential pulse voltammetry was employed for the simultaneous determination of Cu(II) and Sb(III) under the optimal conditions. The IL–CFGO/CILE exhibited the wide linear ranges of 0.1∼28 μM for Cu(II) and 0.1∼20 μM for Sb(III) with detection limits of 0.04 and 0.06 μM, respectively. The proposed sensor displayed good reproducibility, selectivity and acceptable stability owing to the contributions of the more active sites and the enhanced conductivity.

Gold Nanocage-Based Electrochemical Sensing Platform for Sensitive Detection of Luteolin.

A simple and sensitive electrochemical sensor was developed for the detection of tracelevels of luteolin. The sensoris based on a novel type of chemically modified electrode: gold nanocage (AuNCs)-modified carbon ionic liquid electrode (CILE). To construct this electrochemical sensing platform for luteolin, CILE is initially prepared by using 1-hexylpyridinium hexafluorophosphate as the binder and then AuNCs are coated on the surface of CILE to fabricate AuNCs-modified CILE (AuNCs/CILE). Electrochemical studies have shown that AuNCs/CILE can exhibit enhanced electrocatalytic activity toward the redox reaction of luteolin, therefore, the redox peak current of luteolin can be greatly improved, resulting in the high sensitivity of the developed sensor. Under the optimal conditions, the oxidation peak currents of the sensor increase linearly with an increase in the luteolin concentration in a range from 1 to 1000 nM with a detection limit of 0.4 nM, which is lower than those of most reported electrochemical luteolin sensors. Moreover, the reproducibility, precision, selectivity, and stability of this sensor are excellent. Finally, the sensing system was applied to the analysis of luteolin-spiked drug samples and the recovery in all cases was 95.0–96.7%, indicating the potential application of this simple, facile, and sensitive sensing system in pharmaceutical analysis.

Sensitive electrochemical detection of bisphenol A with chitosan and urchinlike MnO2 microsphere modified carbon ionic liquid electrode

In this article, a carbon ionic liquid electrode (CILE) was fabricated by using ionic liquid N-hexylpyridinium hexafluorophosphate as the binder and the modifier. Then urchinlike MnO2 microsphere and chitosan (CTS) was further casted on the CILE surface step-by-step to get a modified electrode that was denoted as CTS/MnO2/CILE. Cyclic voltammetric studies indicated that bisphenol A (BPA) exhibited a well-defined oxidation peak at 0.486 V in 22.83 g L−1 pH 8.0 Britton‒Robinson buffer solution, which was attributed to the electro-oxidation of BPA on the modified electrode. The presence of urchinlike MnO2 microsphere on the electrode surface could increase the oxidation peak current (Ipa) greatly, which may be due to the larger surface area that could adsorb more BPA on the electrode surface. Electrochemical parameters of BPA on the modified electrode were calculated with the electron transfer coefficient (α) as 0.66 and the apparent heterogeneous electron transfer rate constant (ks) as 0.50 s−1. Under the optimal conditions, a linear relationship between the Ipa of BPA and its concentration was obtained in the range from 1.37 × 10–1 mg L−1 to 182.6 mg L−1 with the detection limit as 7.31 × 10–3 mg L−1 (3σ). The CTS/MnO2/CILE was applied to the detection of BPA content in different kinds of samples with satisfactory results.

Application of Titanium Dioxide Nanowires for the Direct Electrochemistry of Hemoglobin and Electrocatalysis

AbstractTitanium dioxide (TiO2) nanowires were synthesized and used for the realization of direct electrochemistry of hemoglobin (Hb) with carbon ionic liquid electrode (CILE) as the substrate electrode. TiO2‐Hb composite was casted on the surface of CILE with a chitosan film and spectroscopic results confirmed that Hb retained its native structure in the composite. Direct electron transfer of Hb on the modified electrode was realized with a pair of quasi‐reversible redox waves appeared, indicating that the presence of TiO2 nanowires could accelerate the electron transfer rate between the electroactive center of Hb and the substrate electrode. Electrochemical behaviors of Hb on the modified electrode were carefully investigated with the values of the electron transfer coefficient (α), the electron transfer number and the heterogeneous electron transfer rate constant (ks) as 0.58, 0.98 and 1.62 s‐1. The Hb modified electrode showed excellent electrocatalytic activity to the reduction of trichloroacetic acid and NaNO2 with wider linear range and lower detection limit, indicating the successful fabrication of a new third‐generation biosensor.

Application of nanosized gold and graphene modified carbon ionic liquid electrode for the sensitive electrochemical determination of folic acid

In this study a new electrochemical folic acid (FA) sensor was proposed by using gold nanoparticles (AuNPs) and graphene (GR) modified carbon ionic liquid electrode (CILE) as the working electrode. GR and AuNPs were directly electroreduced on the surface of CILE to get the modified electrode, which exhibited excellent electrochemical performances due to the specific characteristics including higher conductivity and larger surface area. The modified electrode showed excellent electrocatalytic ability to the oxidation of FA with an irreversible oxidation peak appeared at +1.05V (vs. SCE) in pH5.5 phosphate buffer solution. Electrochemical behaviors of FA on the modified electrode were carefully investigated with the electrochemical parameters calculated. Under the optimal conditions and by using differential pulse voltammetry, the oxidation peak currents were linear to FA concentration in the range from 0.01μmolL−1 to 50.0μmolL−1 with the detection limit as 2.7nmolL−1 (3σ). The proposed method was successfully applied to the FA tablet sample determination with satisfactory results.

Electrochemical deoxyribonucleic acid biosensor based on electrodeposited graphene and nickel oxide nanoparticle modified electrode for the detection of salmonella enteritidis gene sequence.

In this paper a new electrochemical DNA biosensor was prepared by using graphene (GR) and nickel oxide (NiO) nanocomposite modified carbon ionic liquid electrode (CILE) as the substrate electrode. GR and NiO nanoparticles were electrodeposited on the CILE surface step-by-step to get the nanocomposite. Due to the strong affinity of NiO with phosphate groups of ssDNA, oligonucleotide probe with a terminal 5'-phosphate group could be attached on the surface of NiO/GR/CILE, which could further hybridize with the target ssDNA sequence. Methylene blue (MB) was used as the electrochemical indicator for monitoring the hybridization reaction. Under the optimal conditions the reduction peak current of MB was proportional to the concentration of salmonella enteritidis gene sequence in the range from 1.0×10(-13) to 1.0×10(-6)molL(-1) with a detection limit as 3.12×10(-14)molL(-1). This electrochemical DNA sensor exhibited good discrimination ability to one-base and three-base mismatched ssDNA sequences, and the polymerase chain reaction amplification product of salmonella enteritidis gene sequences were further detected with satisfactory results.

Sensitive electrochemical determination of oxalic acid in spinach samples by a graphene-modified carbon ionic liquid electrode

In this paper, a new electrochemical oxalic acid (OA) sensor based on graphene (GR)-modified carbon ionic liquid electrode (CILE) was proposed. The GR nanosheets were deposited on the surface of CILE by electroreduction with graphene oxide as precursor and electrocatalytic oxidation of OA on GR/CILE was further investigated. As compared with that of bare CILE, the oxidation peak current of OA on GR/CILE were greatly improved with the decrease of the oxidation potential. Electrochemical parameters of the electrooxidation of OA were calculated. Under the selected conditions, the oxidation peak currents increased with OA concentration in the range from 8.0 μM to 6.0 mM with the detection limit as 0.48 μM (3σ). The proposed method exhibited higher sensitivity and wider linear range, which was applied to determination of OA concentration in spinach samples with satisfactory results.

Sensitive electrochemical detection of dopamine with a DNA/graphene bi-layer modified carbon ionic liquid electrode

A DNA and graphene (GR) bi-layer modified carbon ionic liquid electrode (CILE) was fabricated by an electrodeposition method. GR nanosheets were electrodeposited on the surface of CILE at the potential of −1.3V and then DNA was further deposited at the potential of +0.5V on GR modified CILE. Electrochemical performances of the fabricated DNA/GR/CILE were carefully investigated. Then electrochemical behaviors of dopamine (DA) on the modified electrode were studied with the calculated electrochemical parameters. Under the optimized conditions, a linear relationship between the oxidation peak current and the concentration of DA was obtained in the range from 0.1μmol/L to 1.0mmol/L with a detection limit of 0.027μmol/L (3σ). The modified electrode exhibited excellent reproducibility, repeatability, stability, validation and robustness for the electrochemical detection of DA. The proposed method was further applied to the DA injection solution and human urine samples determination with satisfactory results.

A novel electrochemical sensor based on a silver nanoparticle modified carbon ionic liquid electrode for selective and sensitive determination of levetiracetam in pharmaceutical tablets and blood plasma samples

An electrochemical sensor based on a silver nanoparticle (AgNP) modified carbon ionic liquid electrode (CILE) was prepared for the determination of ultra trace levels of levetiracetam (LEV) in human plasma and pharmaceutical tablets. The AgNPs were electrodeposited on the CILE surface using a double pulse potentiostatic technique, which can suitably control the size and morphology of AgNPs electrodeposited on the electrode. The AgNPs deposited on the CILE surface revealed an excellent electrocatalytic activity towards the oxidation of LEV. The sensor exhibited a fast response towards LEV with a linear concentration range of 1–300 ng mL−1 and a limit of detection of 0.7 ng mL−1. The possible interference from several common ions and drugs that usually accompany LEV was tested. The method was successfully applied to the determination of LEV content in real samples such as tablets and human plasma samples with good recovery, and the obtained results were checked by HPLC. The proposed sensor is suitable for routine analysis of LEV in plasma samples to monitor the therapeutic or toxic levels of LEV and for drug pharmacokinetic studies.

Electrochemical myoglobin biosensor based on carbon ionic liquid electrode modified with Fe3O4@SiO2 microsphere

In this paper, a Fe3O4@SiO2 core-shell structure microsphere was synthesized and used to investigate the direct electron transfer of myoglobin (Mb) with a 1-butylpyridinium hexafluorophosphate based carbon ionic liquid electrode (CILE) as the substrate electrode. The mixture of Mb and Fe3O4@SiO2 microsphere could form an organic–inorganic composite, which was immobilized on the surface of CILE with a chitosan (CS) film. Cyclic voltammetric experiments indicated that a pair of well-defined quasi-reversible redox peaks appeared on CS/Mb-Fe3O4@SiO2/CILE with the formal peak potential (E0′) located at −0.31 V (vs. saturated calomel electrode), which was corresponded to the electroactive center of Mb heme Fe(III)/Fe(II) redox couples. Direct electrochemical behaviors of Mb in CS-Fe3O4@SiO2 composite film were carefully investigated with the electrochemical parameters calculated. The CS/Mb-Fe3O4@SiO2/CILE showed good electrocatalytic behaviors to the reduction of trichloroacetic acid in the concentration range from 0.2 to 11.0 mmol L−1 with the detection limit of 0.18 mmol L−1 (3σ). Based on CS/Mb-Fe3O4@SiO2/CILE, a new third-generation reagentless electrochemical biosensor was constructed with higher sensitivity and reproducibility.

Cu3Mo2O9 nanosheet incorporated with hemoglobin on carbon ionic liquid electrode for the direct electrochemistry and electrocatalysis

In this paper Cu3Mo2O9 nanosheet was prepared by a hydrothermal method and further used to investigate the direct electrochemistry of hemoglobin (Hb) with a carbon ionic liquid electrode (CILE) as the substrate electrode. Hb was mixed with Cu3Mo2O9 nanosheet and cast on the CILE surface with chitosan (CTS) as the film-forming material. UV-vis and FT-IR spectroscopic results showed that Hb remained in its native structure in the composite film. Direct electron transfer of Hb on the modified electrode was realized with a pair of well-defined quasi-reversible redox waves that appeared on cyclic voltammograms. The redox peak potential appeared at −0.252 V (Epc) and −0.141 V (Epa), respectively, with the formal peak potential calculated as −0.196 V, which was the characteristic of electroactive center of Hb heme Fe(III)/Fe(II). The result could be attributed to the presence of Cu3Mo2O9 nanosheet on the electrode surface that was of benefit for the protein orientation and promoted direct electron transfer between the redox active center of Hb and the substrate electrode. The CTS/Cu3Mo2O9–Hb/CILE showed good electrocatalytic ability in reducing different substrates such as trichloroacetic acid, H2O2 and O2, with wider linear range and lower detection limit, thus exhibiting the potential application of the Cu3Mo2O9 nanosheet in third-generation electrochemical biosensors.

Electrochemical DNA Biosensor Based on Partially Reduced Graphene Oxide Modified Carbon Ionic Liquid Electrode for the Detection of Transgenic Soybean A2704‐12 Gene Sequence

AbstractIn this work a partially reduced graphene oxide (p‐RGO) modified carbon ionic liquid electrode (CILE) was prepared as the platform to fabricate an electrochemical DNA sensor, which was used for the sensitive detection of target ssDNA sequence related to transgenic soybean A2704‐12 sequence. The CILE was fabricated by using 1‐butylpyridinium hexafluorophosphate as the binder and then p‐RGO was deposited on the surface of CILE by controlling the electroreduction conditions. NH2 modified ssDNA probe sequences were immobilized on the electrode surface via covalent bonds between the unreduced oxygen groups on the p‐RGO surface and the amine group at the 5′‐end of ssDNA, which was denoted as ssDNA/p‐RGO/CILE and further used to hybridize with the target ssDNA sequence. Methylene blue (MB) was used as electrochemical indicator to monitor the DNA hybridization. The reduction peak current of MB after hybridization was proportional to the concentration of target A2704‐12 ssDNA sequences in the range from 1.0×10−12 to 1.0×10−6 mol/L with a detection limit of 2.9×10−13 mol/L (3σ). The electrochemical DNA biosensor was further used for the detection of PCR products of transgenic soybean with satisfactory results.

Poly(methylene blue) functionalized graphene modified carbon ionic liquid electrode for the electrochemical detection of dopamine

An ionic liquid 1-butylpyridinium hexafluorophosphate based carbon ionic liquid electrode (CILE) was used as the substrate electrode and a poly(methylene blue) (PMB) functionalized graphene (GR) composite film was co-electrodeposited on CILE surface by cyclic voltammetry. The PMB–GR/CILE exhibited better electrochemical performances with higher conductivity and lower electron transfer resistance. Electrochemical behavior of dopamine (DA) was further investigated by cyclic voltammetry and a pair of well-defined redox peaks appeared with the peak-to-peak separation (ΔEp) as 0.058V in 0.1molL−1 pH 6.0 phosphate buffer solution, which proved a fast quasi-reversible electron transfer process on the modified electrode. Electrochemical parameters of DA on PMB–GR/CILE were calculated with the electron transfer number as 1.83, the charge transfer coefficients as 0.70, the apparent heterogeneous electron transfer rate constant as 1.72s−1 and the diffusional coefficient (D) as 3.45×10−4cm2s−1, respectively. Under the optimal conditions with differential pulse voltammetric measurement, the linear relationship between the oxidation peak current of DA and its concentration was obtained in the range from 0.02 to 800.0μmolL−1 with the detection limit as 5.6nmolL−1 (3σ). The coexisting substances exhibited no interference and PMB–GR/CILE was applied to the detection of DA injection samples and human urine samples with satisfactory results.

Electrochemical DNA biosensor for the detection of Listeria monocytogenes with dendritic nanogold and electrochemical reduced graphene modified carbon ionic liquid electrode

A new electrochemical DNA biosensor was fabricated by using a dendritic gold nanoparticles and electrochemical reduced graphene (GR) composite modified carbon ionic liquid electrode (CILE) as the platform. Ionic liquid 1-butylpyridinium hexafluorophosphate was used as the binder for the preparation of CILE and GR film was further decorated on the CILE surface by electrochemical reduction. Then the dendritic nanogold was electrodeposited on the surface of GR/CILE to get a modified electrode as Au/GR/CILE, which was further used for the formation of mercaptoacetic acid self-assembling film. The amino modified ssDNA probe sequence was covalently linked with mercaptoacetic acid to get the ssDNA modified electrode for the further hybridization. Methylene blue (MB) was used as the electrochemical indicator for monitoring the hybridization reaction after hybridized with the target ssDNA. Under the optimal conditions the specific Listeria monocytogenes hly ssDNA sequences could be detected by measuring the differential pulse voltammetric responses of the accumulated MB molecules on dsDNA molecules. The linear concentration range was achieved from 1.0×10−12 to 1.0×10−6mol/L with the detection limit as 2.9×10−13mol/L (3σ). This electrochemical DNA sensor exhibited excellent selectivity with the good discrimination ability of one-base and three-base mismatched ssDNA sequences. The polymerase chain reaction product of L. monocytogenes hly gene that extracted from deteriorated fish was successfully detected, which indicated that this electrochemical DNA sensor could be further used for the detection of specific ssDNA sequence in real biological samples.

Direct Electrochemistry and Electrocatalysis of Myoglobin with Ionic Liquid through Multilayers Film on Carbon Ionic Liquid Electrode

AbstractMultilayers of myoglobin (Mb) with ionic liquid 1‐ethyl‐3‐methylimidazolium tetrafluoroborate ([EMIM]BF4) was assembled on carbon ionic liquid electrode (CILE) based on the electrostatic attraction between the negatively charged Mb and the positively charged imidazolium ion of IL. The CILE was fabricated with 1‐ethyl‐3‐methylimidazolium ethylsulfate ([EMIM]EtOSO3) as the modifier, which exhibited imidazolium ion on the electrode surface. Then Mb molecules were assembled on the surface of CILE step‐by‐step to get a {IL/Mb}n multilayer film modified electrode. UV‐Vis adsorption and FT‐IR spectra indicated that Mb remained its native structure in the IL matrix. In deaerated phosphate buffer solution (pH 7.0) a pair of well‐defined quasi‐reversible redox peaks appeared with the apparent formal potential (E0′) as ‐0.212 V (vs. SCE), which was the characteristic of Mb heme Fe(III)/Fe(II) redox couples. The results indicated that the direct electron transfer of Mb was realized on the modified electrode. The {IL/Mb}n/CILE displayed excellent electrocatalytic ability to the trichloroacetic acid reduction in the concentration range from 2.0 to 22.0 mmol/L with the detection limit of 0.6 mmol/L (3σ). The proposed method provides a new platform to fabricate the third generation biosensor based on the self‐assembly of redox protein with ILs.

Square Wave Voltammetry for Selective Detection of Dopamine Using Polyglycine Modified Carbon Ionic Liquid Electrode

AbstractThe electrochemical polymerization of glycine on carbon ionic liquid electrode (CILE) was described. The presence of ionic liquid on the surface of CILE facilitated the electropolymerization of glycine. The polyglycine modified CILE provided a valid and simple approach to selectively detect dopamine in the presence of AA in physiological environment. The proposed sensor not only decreased the voltammetric responses of AA but also dramatically enhanced the oxidation peak current of DA compared to bare CILE. Using square wave voltammetry, the modified CILE showed good electrochemical behavior to DA, a linear range of 1.0×10−7–3.0×10−4 M in the presence of 1 mM ascorbic acid (AA) and a detection limit of 5.0×10−9 M was estimated (S/N=3).