Journal of Molecular Liquids 221 (2016) 666–672

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Journal of Molecular Liquids

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Fabrication of novel electrochemical sensor for determination of vitaminC in the presence of vitamin B9 in food and pharmaceutical samples

Fatemeh Khaleghi a,⁎, Zahra Arab b, Vinod Kumar Gupta c,⁎, M.R. Ganjali d, Parviz Norouzi d,Necip Atar e, Mehmet L. Yola f

a The Health of Plant and Livestock Products Research Center, Mazandaran University of Medical Sciences, Sari, Iranb Department of Food Science, Sari Branch, Islamic Azad University, Sari, Iranc Department of Applied Chemistry, University of Johannesburg, Johannesburg, South Africad Center of Excellence in Electrochemistry, Faculty of Chemistry University of Tehran, Tehran, Irane Pamukkale University, Department of Chemical Engineering, Denizli, Turkeyf Sinop University, Department of Metallurgical and Materials Engineering, Sinop, Turkey

⁎ Corresponding authors.E-mail addresses: khaleghi.ft@gmail.com (F. Khaleghi)

(V.K. Gupta).

http://dx.doi.org/10.1016/j.molliq.2016.06.0610167-7322/© 2016 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 26 May 2016Received in revised form 14 June 2016Accepted 14 June 2016Available online 16 June 2016

Synthesis and application of NiO-multiwall carbon nanotube nanocomposite (NiO/MWCNTs) and 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF4) in the carbon paste matrix as high sensitive sensors forvoltammetric determination of vitamin C have been reported. The oxidation peak potential of vitamin C at thesurface of the ([Bmim]BF4) NiO/MWCNT carbon paste electrode (NiO/MWCNTs/([Bmim]BF4/CPE) appeared at440 mV, which was about 200 mV lower than the oxidation peak potential at the surface of the simple carbonpaste electrode under a similar condition. Also, the oxidation peak current was increased for about 3.7 times atthe surface of NiO/MWCNTs/([Bmim]BF4/CPE compared to carbon paste electrode. At an optimum condition(pH 7.0), the two peaks are separated ca. 0.44 and 0.85 V for vitamin C and vitamin B9. This point shows that vi-tamin C can be determined in the presence of vitamin B9. The linear response range and detection limit werefound to be 0.1–1000 μM and 0.06 μM, respectively. The proposed sensor was successfully applied for the deter-mination of vitamin C in food and drug samples.

© 2016 Elsevier B.V. All rights reserved.

Keywords:Vitamin CVitamin B9NiO/MWCNTsIonic liquids

1. Introduction

Vitamin C is used to prevent low levels of vitamin C in human bodythat do not get enough of the vitamin from their diets.Most peoplewhoeat a normal diet do not need extra vitamin C. Low levels of it can resultin a condition called scurvy. Scurvy may cause symptoms such as mus-cle weakness, rash, joint pain, tiredness, or tooth loss. Vitamin C is animportant antioxidant, along with vitamin E, beta-carotene, and manyother plant-based nutrients. Antioxidants block some of the damagecaused by free radicals, substances that damage DNA. Many analyticalmethods such as chromatography [1,2], spectrophotometry [3,4], massspectrometry [5], flow injection [6], chemiluminescence [7] and electro-chemicalmethods [8–12] have been proposed for determination of vita-min C in food and pharmaceutical samples and different levels.Electrochemical based methods have attracted in comparison to otheranalytical methods in the recent years due to their high sensitivity,lower cost, good accuracy, high dynamic range and simplicity [13–20].

, vinodfcy@gmail.com

Folic acid is a type of B vitamin that is usually found in foods samplessuch as dried beans, peas, lentils, oranges, asparagus, beets, broccoli,brussels sprouts, and spinach. Vitamin B9 helps human body for main-tain new cells, and also helps prevent changes to DNA that may leadto cancer. Forwomenwhomay get pregnant, vitamin B9 is really impor-tant [21]. So, its determination is very important in human body and bi-ological samples.

Chemically carbon paste modified electrodes (CCPME) are best suit-ed for the electrochemical determination of pharmaceutical, environ-mental or biological samples [22–30]. CCPME reduce the over-potential required for either the oxidation or reduction of theelectroactive compounds [31–37]. As we know the direct electro-oxida-tion of vitamin C is slow at most conventional electrodes surfaces,studying the electrocatalytic reaction of vitamin C is important usingmodified electrodes.

In this work, we describe the synthesis and application of a novelNiO/MWCNTs nanocomposite modified carbon ionic liquid paste elec-trode, which utilizes [Bmim]BF4 as a good conductive binder. The elec-trochemical behavior of vitamin C at NiO/MWCNTs/[Bmim]BF4/CPE, atcarbon paste electrode modified with [Bmim]BF4 ([Bmim]BF4/CPE), atNiO/MWCNTs paste electrode (NiO/MWCNTs/CPE), and at carbonpaste electrode (CPE) was investigated. The results showed the

Fig. 1. SEM images of NiO/MWCNT nanocomposite.

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superiority of NiO/MWCNTs/[Bmim]BF4/CPE to the other electrodes interms of better reversibility and higher sensitivity. The proposed meth-od is selective and sensitive enough for the determination of vitamin Cin food and pharmaceutical samples.Wehave also evaluated the analyt-ical performance of the NiO/MWCNTs/[Bmim]BF4/CPE for quantifica-tion of vitamin C in the presence of vitamin B9 in some real samples.

2. Experimental section

2.1. Apparatus and compounds

All of the voltammetric investigation performed using μ-Autolab,potentiostat/galvanostat connected to a three-electrode cell, Metrohm

Fig. 2. Plot of potential, Epa, vs. pH for the electro-oxidation of 150 μMvitamin C at a surface of Nat a surface of the modified electrode.

Model 663 VA stand, linked with a computer (Pentium IV) and with μ-Autolab software. Three-electrode cell assembly consisting of a plati-num wire as an auxiliary electrode and an Ag/AgCl (KClsat) electrodeas a reference electrode was used.

Samples for scanning electron microscopy (SEM) analysis were pre-pared by evaporating a hexane solution of dispersed particles on amor-phous carbon coated copper grids. NiO/MWCNTs were synthesizedaccording to reported paper [31]. All of the chemicals were of A.R.grade and were used as unless otherwise stated from merck.

2.2. Preparation of the electrode

NiO/MWCNTs/[Bmim]BF4/CPEwas prepared bymixing 0.2 g of ionicliquids, 0.80 g of the liquid paraffin, 0.2 g of NiO/MWCNTs, and 0.8 g ofgraphite powder. Then the mixture was mixed well for 65 min until auniformly wetted paste was obtained. A portion of the paste was filledfirmly into one glass tube as described above to prepare NiO/MWCNTs/[Bmim]BF4/CPE. Unmodified carbon paste past was preparedby hand-mixing of 1.0 g of graphite powder plus paraffin at a ratio of70:30 (w/w) and mixed well for 55 min until a uniformly wettedpaste was obtained.

2.3. Preparation of real samples

Fresh juices were obtained using a mechanical squeezer. The juicesobtained were filtered into a beaker and acidified (pH= 2) using citricacid. A 2.0-mL portion of the filtrate was added to the buffer solution involtammetric cell. Vegetable juices were obtained using a grater (poly-mer material) and a centrifuge respectively, a 2.0-mL portion of vegeta-ble juice was subjected for the voltammetric measurement. In all cases,the amounts of vitamin C in the sampleswere evaluated by the standardaddition method.

For the tablet analysis, an accurately weighed portion of finely pow-dered sample obtained from five tablets, equivalent to about 50 mg ofvitamin C dissolved in 100 mL water with ultrasonication. Then,1.0 mL of the solution plus 9.0 mL of the buffer (pH 7.0) was used forthe analysis with standard addition method.

iO/MWCNTs/[Bmim]BF4/CPE. Inset: influence of pH on cyclic voltammograms of vitamin C

Fig. 3. Cycic voltammograms of a) NiO/MWCNTs/[Bmim]BF4/CPE, b) [Bmim]BF4/CPE, c) NiO/MWCNTs/CPE and d) CPE in presence of 150 μMvitamin C at a pH 7.0, respectively. Inset: thecurrent density derived from cyclic voltammograms responses of 100 μM vitamin C at pH 7.0 at the surface of different electrodes with a scan rate of 100 mV/s.

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3. Results and discussion

3.1. NiO/MWCNT nanocomposite characterization

The morphology of the as-grown NiO/MWCNTs nanostructures wascharacterized by SEM. Typical SEM micrograph of the synthesized NiO/MWCNTs nanostructures is shown in Fig. 1. Results confirm synthesis ofNiO/MWCNTs nanostructures.

3.2. Voltammetric investigation

The effect of pH on electroxidation of vitamin C was investigatedusing cyclic voltammetry technique (Fig. 2 insert). Result shows thatthe electro-oxidation peak current for vitamin C increased slowly from

Fig. 4. Plot of Ipa vs. ν1/2 for the oxidation of vitamin C A at NiO/MWCNTs/[Bmim]BF4/CPE. Insscan rates of a) 5, b) 10, c) 20, d) 35, e) 50, f) 70, g) 100 and h) 130 mV/s in 0.1 M phosphate

pH 5.0 to 7.0, and then the current conversely decreased when the pHvalue increased from 7.0 to 8.0. According to Fig. 2 results, the pH 7.0was chosen as the best optimal experimental condition for other exper-imental. Also, we study the relationship between the oxidation peak po-tential of vitamin C and pH. A linear shift of Epa towards negativepotential with an increasing pH can be obtained and obeyed the regres-sion equation of Epa (V)=−0.052 pH+ 0.768 (R2 = 0.994), which in-dicates that protons are directly involved in the oxidation of vitamin C(Fig. 2). A slope of 52.0 mV/pH suggests that the number of electrontransfer is equal to the proton number involved in the sensor reaction.

The current density derived from the cyclic voltammograms of150 μM vitamin C at different electrodes shows in Fig. 3. The resultsshow that the presence of NiO/MWCNTs and [Bmim]BF4 togethercauses the increase of the electrode. Fig. 3 shows cyclic voltammograms

et shows cyclic voltammograms of vitamin C at NiO/MWCNTs/[Bmim]BF4/CPE at differentbuffer, pH 7.0.

Fig. 5. Tafel plot for NiO/MWCNTs/[Bmim]BF4/CPE in 0.1 M PBS (pH 7.0) with a scan rate of 5 mV/s in the presence of 100 μM vitamin C.

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of 150 μM vitamin C at pH 7.0 at the surface of different electrodes witha scan rate of 100mV/s. NiO/MWCNTs/[Bmim]BF4/CPE exhibited signif-icant oxidation peak current around 440 mV with the peak current of55.7 μA (Fig. 3, curve a). In contrast, low electro-oxidation activitypeak was observed at NiO/MWCNTs/CPE (Fig. 3, curve c) and at CPE(Fig. 3 curve d) over the same condition. The vitamin C oxidation peakpotentials at NiO/MWCNTs/CPE and at carbon paste electrode were ob-served around 600 and 640 mV vs. the reference electrode with the ox-idation peak current of 30.9 and 15.5 μA, respectively. In addition, at thesurface of bare ionic liquid modified carbon paste electrode, the oxida-tion peak appeared at 505 mV with the peak current was 21.6 μA (Fig.3, curve b), which indicated the presence of [Bmim]BF4 in CPE could en-hance the peak currents and decrease the oxidation potential. A signifi-cant negative shift of the currents starting from oxidation potential forvitamin C and dramatic increase of current of vitamin C indicated thecatalytic ability of NiO/MWCNTs/[Bmim]BF4/CPE to vitamin C oxidation.

Fig. 6. A) Chronoamperograms obtained at CdO/NPs/ILs/CPE NiO/MWCNTs/[Bmim]BF4/CPE in(pH 7.0). B) Cottrell's plot for the data from the chronoamperograms.

The results indicated that the presence of NiO/MWCNTs on NiO/MWCNTs/[Bmim]BF4/CPE surface had great improvement with theelectrochemical response, which was partly due to excellent character-istics of NiO/MWCNTs such as good electrical conductivity. The suitableelectronic properties of metal oxide nanocomposite together with the[Bmim]BF4 gave the ability to promote charge transfer reactions, goodanti-fouling properties, especiallywhenmixedwith a higher conductivecompound such as ILs when used as an electrode.

The effect of scan rate (υ) on the oxidation current of vitamin C wasalso examined (Fig. 4 inset). The results of this investigation showedthat the peaks current increased linearly with increasing the squareroot of scan rate that ranged from 5 to 130 mV/s (Fig. 4). The resultshows that the electrode process for oxidation of vitamin C is controlledunder the diffusion step [38,39]. Also, the peaks potential shifts in neg-ative direction when the scan rate increases, meaning that the electro-chemical reaction is irreversible.

the presence of a) 400; b) 600; c) 800 and d) 1000 μM vitamin C in the buffer solution

Table 1Interference study for the determination of 35.0 μM vitamin C under the optimizedconditions.

SpeciesTolerant limits(WSubstance/Wvitamin C)

Glucose, sacarose, lactose, feroctose 800Tryptophan, lucine, glycine, methionine, alanine,valine, histidine

700

Starch Saturation

Fig. 7. Plot of the electrocatalytic peak current as a function of vitamin C concentration. Insert: SWVs of NiO/MWCNTs/[Bmim]BF4/CPE in 0.1 M phosphate buffer solution (pH 7.0)containing vitamin C and vitamin B9 concentration; a) 90.0 + 100; b) 140 + 200.0; c) 190 + 300.0; d) 255.0 + 400.0; e) 300.0 + 470.0 and f) 350.0 + 600.0 μM, respectively.

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To obtain further information on the rate determining step, a Tafelplot was developed for the vitamin C at a surface of NiO/MWCNTs/[Bmim]BF4/CPE using the data derived from the raising part of the cur-rent–voltage curve (Fig. 5). The slope of the Tafel plot is equal to n(1−α)F/2.3RT which comes up to 0.2353 V decade−1. We obtained α as0.87.

Chronoamperometric measurements of vitamin C at NiO/MWCNTs/[Bmim]BF4/CPE investigated using the data derived from the raisingpart of the current–voltage curve (Fig. 6A). The slope of thewere carriedout by setting the electrode potential at 600 mV for the various concen-tration of vitamin C in buffered aqueous solutions (pH 7.0) (Fig. 6A). Forvitamin C with a diffusion coefficient of D, the current observed for theelectrochemical reaction at the mass transport limited condition is de-scribed by the Cottrell equation. Experimental plots of I vs. t−1/2 wereemployed, with the best fits for different concentrations of vitamin C(Fig. 6B). The slopes of the resulting straight lines were then plottedvs. vitamin C concentration. From the resulting slope and Cottrell equa-tion themean value of the Dwas found to be 1.24 × 10−5 cm2/s. Analyt-ical parameters and simultaneous determination of vitamin C andVitamin B9.

Squarewave voltammetry (SWV)were used to determine vitamin Cconcentrations (Not shown). The SWvoltammograms clearly show thatthe plot of peak current versus vitamin C concentration is linear for 0.1–1000 μMof vitamin C. The detection limitwas determined at 0.07 μMvi-tamin C according to the definition of YLOD = YB + 3σ.

Themain object of this study was to detect vitamin C and vitamin B9simultaneously using NiO/MWCNTs/[Bmim]BF4/CPE. This was per-formed by simultaneously changing the concentrations of vitamin Cand vitamin B9, and recording the SWVs. The voltammetric resultsshowedwell defined anodic peaks at potentials of 440 and 850mV, cor-responding to the oxidation of vitamin C and vitamin B9, respectively(Fig. 7). This is indicating that simultaneous determination of thesecompounds is feasible at NiO/MWCNTs/[Bmim]BF4/CPE as shown inFig. 7. The sensitivity of the modified electrode towards the oxidationof vitamin C in the presence of vitamin B9 was found to be 0.0501 μA/μM (Fig. 7). This is very close to the value obtained in the absence of vi-tamin B9 (0.0521 μA/μM) indicating that the oxidation processes ofthese compounds at the NiO/MWCNTs/[Bmim]BF4/CPE are indepen-dent and therefore, simultaneous determination of their mixtures ispossible without significant interferences.

3.3. Stability and reproducibility

The repeatability and stability of modified electrode was investigat-ed by SWVmeasurements of 20.0 μMbisphenol A. The relative standarddeviation (RSD%) for ten successive assays was 1.7%. When using sevendifferent electrodes, the RSD% for nine measurements was 2.7%. Whenthe electrode stored in the laboratory, the modified electrode retains96% of its initial response after 10 days and 92% after 40 days. These re-sults indicate that CdO/NPs/ILs/CPE has good stability and reproducibil-ity, and could be used for bisphenol A.

3.4. Interference study

The influence of various substances as potentially interfering com-poundswith the determination of vitaminCwas studied under the opti-mum conditions 35.0 μMvitamin C at pH 7.0. The potentially interferingsubstances were chosen from the group of substances commonly foundwith this compound in food samples. The tolerance limit was defined asthemaximum concentration of the interfering substance that caused anerror less than ±5% for the determination of vitamin C. The results areshown in Table 1. Those results confirm the suitable selectivity of theproposed method for determination of vitamin C.

3.5. Real sample analysis

In order to evaluate the analytical applicability of the NiO/MWCNTs/[Bmim]BF4/CPE, also it was applied to the determination of vitamin C infood anddrug samples. Standard additionmethodwas used formeasur-ing its concentration in the samples. The proposed method was

Table 2Determination of vitamin C in real samples (n = 3).

SampleFound (AA) proposedmethod (μM)

Found (AA) othermethod (μM) Fex Ftab tex ttab(95%)

Tablet 5.35 ± 0.43 5.22 ± 0.55 8.8 19.0 1.4 3.8Fruitjuices

– – – – – –

Orange ″ 160.35 ± 1.25 159.74 ± 1.85 11.9 19.0 3.1 3.8Apple ″ 24.32 ± 0.85 23.93 ± 0.95 10.2 19.0 2.7 3.8Lemon 68.35 ± 0.95 69.01 ± 1.02 10.8 19.0 2.8 3.8Vegetablejuices

– – – – – –

Pimento 35.25 ± 0.53 34.95 ± 0.75 9.8 19.0 2.0 3.8

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compared with a published method too [8]. The results are given inTable 2, confirm that the modified electrode retained its efficiency forthe determination of vitamin C in real samples.

4. Conclusions

In this study, the 1-butyl-3-methylimidazolium tetrafluoroboratemodified NiO/MWCNTs carbon paste electrode was used to investigatethe electrochemical behaviors of vitamin C. The NiO/MWCNTs/[Bmim]BF4/CPE showed great improvement to the electrode processof vitamin C compared to the carbon paste electrode. Themodified elec-trode successfully resolves the overlapped voltammetric peaks of vita-min C and vitamin B9 by ≈410 mV, so that the modified electrodedisplays high selectivity in the SWV measurement of vitamin C and vi-tamin B9 of in their mixture solutions. The electrode was successfullyused for the determination of vitamin C in food and pharmaceuticalsamples.

Acknowledgments

The authors wish to thank The Health of Plant and Livestock Prod-ucts Research Center, Mazandaran University of Medical Sciences, Sari,Iran for their support.

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