5.2
Impact Factor
Generic selectors
Exact matches only
Search in title
Search in content
Post Type Selectors
Search in posts
Search in pages
Filter by Categories
Corrigendum
Current Issue
Editorial
Erratum
Full Length Article
Full lenth article
Letter to Editor
Original Article
Research article
Retraction
Retraction notice
Review
Review Article
SPECIAL ISSUE: ENVIRONMENTAL CHEMISTRY
5.3
Impact Factor
Generic selectors
Exact matches only
Search in title
Search in content
Post Type Selectors
Search in posts
Search in pages
Filter by Categories
Corrigendum
Current Issue
Editorial
Erratum
Full Length Article
Full lenth article
Letter to Editor
Original Article
Research article
Retraction
Retraction notice
Review
Review Article
SPECIAL ISSUE: ENVIRONMENTAL CHEMISTRY
View/Download PDF

Translate this page into:

Original article
13 (
1
); 2955-2963
doi:
10.1016/j.arabjc.2018.08.005

Time-biased square wave differential electrolytic potentiometry for determination of ascorbic acid in a complex matrix at multi-walled carbon nanotubes modified silver electrodes

, ,
a Chemistry Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
b College of Science & Engineering, HBKU, Qatar

⁎Corresponding author. abdalghaffar@kfupm.edu.sa (Abdalghaffar Mohammad Osman)

Received: , Accepted: ,
© The Author(s) 2018
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Carbon nanotubes modified silver electrodes (CNTs/Ag) have been prepared by the catalytic thermal decomposition of ethylene in a chemical vapor deposition reactor. CNTs growth parameters such as ratio of ethylene to hydrogen, temperature and time were optimized, to enhance the electrode functionality for application. The electrode surface was characterized by scanning electron microscopy, transition electron microscopy and Raman spectroscopy. The optimum ethylene to hydrogen (C2H2:H2) ratio, temperature and time were found to be 75:90 sccm, 800 °C, and 20 min respectively. The CNTs/Ag electrodes prepared by this method exhibited well adhesion of the CNTs to the metal surface enabling their use for multiple times. The CNTs/Ag electrodes were successfully applied as indicating system in biased square wave differential electrolytic potentiometry (DEP) for the determination of ascorbic acid in a drug formulation and complex Baobab fruit matrix. CNTs/Ag electrodes showed high performance and durability, and the biased square wave led to enhanced DEP signal and lowered the detection limit for ascorbic acid to less than 25 µM. These promising results open the way for the use of CNTs/Ag electrodes as indicating setup for automated flowing systems like flow injection analysis.

Keywords

CNTs modified silver electrode
Chemical vapor deposition
Differential electrolytic potentiometry
Time biased square wave
1

1 Introduction

DEP is a technique in which two similar working electrodes are used. Working electrodes may be polarized by passing a minute direct current (d.c.) between them (Bishop, 1956) or by bias-free square, sine or triangle waveform (Bishop, 1973), while the potential across them is measured. Time-biased square wave DEP technique utilizes unsymmetrical, biased square waveform to polarize electrodes. During the course of the titration, the difference in potential (ΔE) is measured. The position of the end point in DEP titration is determined from the maximum of the obtained first differential curve. It was found that periodic reversing of the electrode signal prevents buildup of films on the electrode surfaces and keeps them active during the titration. Therefore, electrodes reach the equilibrium in a short period of time resulting in sharper peaks and enhanced response (Bishop, 1973). Moreover, the difficulties caused by salt bridges are eliminated since no reference electrode is required. DEP titrations using platinum indicator electrodes polarized with direct current (Al-Ghannam and Al-Olyan, 2005; Bishop, 1958, 1956) and bias-free waveforms (Bishop, 1973) have been applied to different oxidation-reduction reactions. No time biased-square wave DEP has been reported for redox reactions.

Carbon nanotubes (CNTs) possess high surface area and unique electronic properties (Iijima, 1991). Due to which they showed enhanced electrocatalytic activity during electrochemical analyses. CNTs offer current densities exceeding 109 A/cm2 (Yao et al., 2000). Therefore, they have been extensively used as electrode material for electrochemical studies and electroanalytical applications. For instance, CNTs modified electrodes have been reported to promote electron transfer reactions (Musameh et al., 2002; Mx, 2004) and have been applied in different electrochemical techniques such as amperometry (Ong et al., 2005; Trojanowicz, 2006), potentiometry (Crespo et al., 2008; Darroudi et al., 2015; Ganjali et al., 2010; Ghaedi et al., 2015; Khani et al., 2010; Mohammadabadi et al., 2015; Shehab and Mansour, 2015; Siqueira et al., 2009; Zhu et al., 2009) and voltammetry (Liang and Zhuobin, 2003; Wei et al., 2005; Zeng and Huang, 2004; Zhao et al., 2005). CNTs have not been much explored as electrode modifiers/materials for DEP redox reactions. They have been reported only for DEP determination of cyanide (Amayreh and Abulkibash, 2017; Amro et al., 2014). Silver metal has a high electric conductivity, high melting point and has been used successfully as electrode material in some previously reported DEP applications (Abulkibash et al., 2003; Amayreh and Abulkibash, 2017; Amro et al., 2014).

Ascorbic acid, also known as vitamin C, is one of the safest and most effective nutrients found in many fruits and vegetables. For instance, Baobab fruit pulp contains vitamin C seven to ten times more than the oranges (Gebauer et al., 2002; Kamatou et al., 2011; Rahul et al., 2015). Ascorbic acid plays an important role in protecting humans against various diseases through maintaining the immune system (Chambial et al., 2013; Edefonti et al., 2015; Forastiere et al., 2000; Johnston et al., 2014; Padayatty et al., 2003). Furthermore, the growing demands for ascorbic acid have increased significantly in various sectors such as food industries. This necessitates the development of simple, low-cost, and effective methods for determination of ascorbic acid in different fruits and products. Different methods including direct titrations (Nweze et al., 2015; Suntornsuk et al., 2002; “USP-NF25,” 2007), liquid chromatography (Bansal et al., 2015; Klimczak and Gliszczyńska-Świgło, 2015; Routoir and Analytique, 1988) and electrochemical methods (Pisoschi et al., 2014) have been reported for quantitation of ascorbic acid. Electrodes modified with nanomaterials (Baghizadeh et al., 2014) particularly CNTs have been applied for the determination of ascorbic acid in different matrices (Cui et al., 2012; Gheibi et al., 2015; Huang et al., 2010; Manjunatha et al., 2010; Ragupathy et al., 2010; Wang et al., 2002). These techniques achieve low detection limits but some of them require longer sample preparation. Direct current DEP method has been applied for determination of ascorbic acid in pharmaceutical formulation (Al-Ghannam and Al-Olyan, 2005).

This study combines for the first time the advantages of both periodic square wave DEP and CNTs modified electrodes to determine ascorbic acid in complex matrix Baobab fruits and vitamin C tablets.

2

2 Experimental

2.1

2.1 Reagents

All solutions were prepared using deionized water. A stock standard solution of 0.02 mol/L ascorbic acid (99.5%, Fluka) was freshly prepared prior to use and then series of standard solutions was prepared by dilution with deionized water. A stock solution of 0.02 mol/L of potassium iodate (min 99.5%, Fisher scientific) was prepared and then used to prepare other diluted solutions as needed. 0.1 mol/L sulfuric acid (Sigma Aldrich) was also prepared as supporting electrolyte.

2.2

2.2 Electrode preparation

The growth of carbon nanotubes on the silver metal surface was achieved in a quartz tube (125 cm × 2.5 cm ID) placed in two separate tube furnaces (Lindburg/Blue M TF55035A-1, USA and OTF 1200X, MTI Corporation, USA). 60 mg of ferrocene (98%, Aldrich) was placed in a ceramic boat located in the first heating zone while silver wire (2 cm × 0.1 cm) was held on another boat positioned in the second heating zone.

The operation of the chemical vapor deposition (CVD) system started with argon gas being released into the system at a flow rate of 200 sccm until the second furnace reached the desired temperature. Ferrocene was then vaporized at 150 °C and swept into the second furnace, where carbon nanotubes grew, for 5 min. Thereafter, the argon flow was stopped followed by passing a mixture of ethylene and hydrogen at specified flow rates for different times (10, 15, 20, 25 and 30 min). Finally, the reactor was allowed to cool under argon environment, and silver coated with CNTs was then collected.

2.3

2.3 Characterization

The surface of the prepared electrodes was characterized using FE-SEM (TESCAN VELA3), FE-TEM (JOEL-2100F) and (LabRAM HR Evolution, HORIBA Scientific) Raman spectrometer.

2.4

2.4 Square-wave DEP titration method

For the purpose of measurements, the indicating system was formed by soldering CNTs/Ag electrodes to a copper wire placed into a glass jacket and attached to a square wave polarizing source.

A 2.0 mL volume of 0.02 mol/L freshly prepared ascorbic solution acid was added to 20 mL of 0.1 mol/L sulfuric acid in the titration cell. Two CNTs/Ag electrodes, which are part of the square wave DEP circuit, were placed in the mixture and polarized by a bias ranging from 0 to 20%. A standard potassium iodate solution was dispensed from a micropipette while stirring the mixture, and the potential difference (ΔE) between the two electrodes was recorded. The optimum bias was determined and fixed for the measurements of other standards of ascorbic acid; each of them was prepared and analyzed three times. The same titration was repeated by applying direct current. The optimum bias was then applied for the rest of experiments. Standard solutions containing 4.00 × 10−2, 1.00 × 10−2, 2.00 × 10−3, 1.00 × 10−3 and 5.00 × 10−4 mmole of ascorbic acid were used to establish a calibration curve in order to determine ascorbic acid in vitamin C tablets and in a fresh Baobab fruit. 2 g of the fruit powder was stirred with 20 mL of 0.1 mol/L sulfuric acid for a few minutes. The mixture was spiked with 0.002, 0.005, 0.05 and 0.1 mmole of ascorbic acid and titrated against 0.01 mol/L potassium iodate solution. The sample preparation and titration has been repeated three times.

3

3 Results and discussion

3.1

3.1 Characterization and optimization of electrodes

Preparation of CNTs/Ag electrodes by CVD method requires optimization of some important parameters that influence the quantity and quality of the grown nanotubes. Such parameters include hydrogen gas concentration, temperature and time.

In this study, C2H2 flow rate was held constant (75 sccm) while H2 flow rate was varied. At a temperature of 700 °C with low H2 flow rate (25 sccm), only amorphous carbon was formed while the CNTs growth was enhanced by increasing the H2 flow rate from 25 to 90 sccm as shown in Fig. 1(a and b). However, beyond a flow rate of 100 sccm amorphous carbon were observed. This result was in agreement with previous results for CVD production of MWCNTs (Iyuke et al., 2006).

SEM micrographs showing the surface of CNTs/Ag electrode prepared at H2 flow rate (a) 25 sccm (b) 90 sccm. Temperature is 700 °C, growth time 15 min and C2H2 flow rate is 75 sccm.
Fig. 1 SEM micrographs showing the surface of CNTs/Ag electrode prepared at H2 flow rate (a) 25 sccm (b) 90 sccm. Temperature is 700 °C, growth time 15 min and C2H2 flow rate is 75 sccm.

At a temperature of 600 °C, only amorphous carbon particles were formed, not shown here, without the formation of any CNTs. Upon increasing the growth temperature to 700 °C, the formation of CNTs was noticed, and which was further enhanced at 750 °C. The maximum amount of CNTs was achieved at 800 °C (Fig. 2a). Moreover, at this temperature, CNTs were strongly adhered to the metal surface. This may enhance the properties of the fabricated electrodes and keep them stable for longer and multiple periods of applications. Obviously, as the growth temperature increases, the CNTs/Ag electrodes get better in terms of both quality and surface coverage. This could be attributed to the increase in the degree of crystalline perfection at higher temperature due to the increase of the diffusion rate of carbon atoms in the iron nanoparticles layer formed on the silver metal because of the thermal decomposition of the ferrocene. This increase in the diffusion rate enhances the graphitic sheets to build with a less defect (Kim et al., 2005), which in turn leads to the increase of the metallic conductivity of the MWCNTs (Jang et al., 2002). However, some distortion in the graphitic structure was represented by the D′-band as a shoulder peak around 1600 cm−1 (Veríssimo et al., 2006).

(a) SEM and (b) TEM micrographs showing the surface of CNTs/Ag electrode prepared at a temperature of 800 °C, growth time 20 min. C2H2:H2 flow rate ratio is 75:90 sccm.
Fig. 2 (a) SEM and (b) TEM micrographs showing the surface of CNTs/Ag electrode prepared at a temperature of 800 °C, growth time 20 min. C2H2:H2 flow rate ratio is 75:90 sccm.

Another parameter that is important for the fabrication of an efficient electrode is the growth time. It was observed that the coverage of the silver metal surface with CNTs increased with growth time up to 30 min. However, beyond 20 min, amorphous carbon particles start to form on CNTs due to the exhaustion of the catalyst with time (Brukh and Mitra, 2006). These results agree with other previously reported ones (Harutyunyan et al., 2005; Kong et al., 1998). Therefore, 20 min was selected as an optimal growth time.

Raman spectrometry is considered as one of the most important analysis techniques for the study of graphitic carbon. Raman analysis further confirmed the formation of well graphitized CNTs with least amount of other carbon species at higher temperatures. G′-band peak which appears around 2700 cm−1 may represent a more accurate measurement of MWCNTs quality or purity, since its intensity was dramatically enhanced by carbon nanotubes (DiLeo et al., 2007). D-band at 1350 cm−1 is due to the distorted sp2 carbon of non-nanotube graphitic components and indicates the presence of impurities (Dillon et al., 1999). In this study as shown in Fig. 3, at 700 °C, the D-band peak at approximately 1325 cm−1 was of high intensity compared to the G-band one at ∼1580 cm−1 while there is no significant peak was observed around 2655 cm−1. Increasing the growth temperature to 750 °C and 800 °C revealed an enhancement of G′-band at ∼2655 cm−1 and decrease in the D-peak intensity indicating the formation of CNTs with good graphitization. Transition electron microscopy also confirmed the formation of multiwall carbon nanotube on silver metal as shown in Fig. 2(b).

Raman spectra for CNTs/Ag electrode prepared by CVD at different temperatures. C2H2:H2 flow rate ratio is 75:90 sccm and growth time is 20 min.
Fig. 3 Raman spectra for CNTs/Ag electrode prepared by CVD at different temperatures. C2H2:H2 flow rate ratio is 75:90 sccm and growth time is 20 min.

3.2

3.2 Determination of ascorbic acid

0.002 mol/L of freshly prepared ascorbic acid solution in 0.1 mol/L sulfuric acid was titrated with potassium iodate at different square wave biases in the range of 0–20%. Changing the percent bias applied to electrodes from 0% to 10% showed an enhancement in the peak potential difference (Fig. 4a). Beyond a bias of 10%, no significant change was observed. However, indicator electrodes polarized by square wave -provided larger ΔE and sharper peaks than electrodes polarized with direct current, shown also in Fig. 4(a). This is because the indicator electrodes retain full activity for very long periods, their response is greatly accelerated, and warning is given of the approach of the endpoint. Therefore, periodic mode DEP has an advantage over the direct current mode in giving error-free end points (Bishop, 1973). An optimum bias of 10% was chosen for the square waveform and applied for the rest of the experiments.

(a) Effect of bias change on the peak periodic potential compared to d.c. DEP peak, for (0.02 mol/L) ascorbic acid in 0.1 mol/L H2SO4 titrated with potassium iodate solution. (b) Biased square wave DEP peaks for different concentrations of ascorbic acid titrated with potassium iodate solutions.
Fig. 4 (a) Effect of bias change on the peak periodic potential compared to d.c. DEP peak, for (0.02 mol/L) ascorbic acid in 0.1 mol/L H2SO4 titrated with potassium iodate solution. (b) Biased square wave DEP peaks for different concentrations of ascorbic acid titrated with potassium iodate solutions.

3.2.1

3.2.1 Method validation

3.2.1.1
3.2.1.1 Linearity

A calibration curve was established with standard solutions containing 4.00 × 10−2, 1.00 × 10−2, 2.00 × 10−3, 1.00 × 10−3 and 5.00 × 10−4 mmole of ascorbic acid. Triplicate titrations were performed for each calibration point. A good linear regression (Fig. 5) was obtained between the amounts of analyzed ascorbic acid and those measured from the DEP peaks using Microsoft Excel. Origin Pro 8 software was employed to locate end points from peaks maxima shown in Fig. 4(b). The linear range covers the concentration of ascorbic acid normally found in food and drug samples. Analytical data are provided in Table 1.

Standard calibration curve of a biased square wave DEP titration of ascorbic acid with potassium iodate.
Fig. 5 Standard calibration curve of a biased square wave DEP titration of ascorbic acid with potassium iodate.
Table 1 Analytical parameters for DEP determination of ascorbic acid at CNTs/Ag electrodes with comparison against reference iodimetric titration. The number of measurements is 3.
Ascorbic acid spiked (mmole) DEP titration method Iodimetric titration method
Ascorbic acid detected (mmole) Recovery (%) CV (%) N = 3 Calibration parameters Ascorbic acid detected (mmole) Relative error (%)
Slope y-intercept R2
4.00 × 10−2 4.06 × 10−2 101.4 2.6 0.996 5.00 × 10−5 0.996 3.93 × 10−2 3.2
1.00 × 10−2 1.03 × 10−2 103.1 2.4 9.73 × 10−3 4.4
2.00 × 10−3 2.04 × 10−3 102.0 2.9 1.98 × 10−3 3.0
1.00 × 10−3 1.04 × 10−3 103.9 4.2 1.01 × 10−3 2.8
5.00 × 10−4 4.83 × 10−4 96.7 3.7 5.05 × 10−4 −4.4

3.2.1.2
3.2.1.2 Precision and accuracy

Intra-assay and instrument precision were evaluated by analyzing aliquots of standard ascorbic acid solutions for three times. The method showed good reproducibility expressed in terms of percent coefficient of variation, (CV%). It also showed high accuracy and recovered good percentage of the spiked standard analyte. The same standard ascorbic acid solutions were analyzed by a normal titration against standardized 0.0466 mol/L iodine solution using starch indicator, according to the USP (“USP-NF25,” 2007), and compared with DEP titration for validation. The obtained results (Table 1) agreed fairly well with those obtained by DEP titrations using the CNTs/Ag electrodes.

3.2.2

3.2.2 Determination of ascorbic acid in real samples

Due to complex chemical composition of the Baobab fruit (Gebauer et al., 2002; Kamatou et al., 2011), and in order to perform the determination of ascorbic acid with minimum interference effect from the matrix, the standard addition calibration method was applied. The amounts of ascorbic acid spiked into the sample solutions were 2.00 × 10−3, 1.00 × 10−2, 5.00 × 10−2 and 1.00 × 10−1 mmol. DEP titration peaks, Fig. 6, were obtained. The sample preparation and titration has been repeated three times from which an excellent linear relation shown in Fig. 7 was established. Other analytical data are shown in Table 2. Ascorbic acid content in Redoxon® tablets was also determined by this method and the resulting differential peak is shown in Fig. 8. Other analytical data are also provided in Table 2. Analytical performance for some electrochemical determinations of ascorbic acid are shown in Table 3 for comparative purposes.

DEP titration peaks for 0.0, 0.002, 0.005, 0.05 and 0.1 mmole of ascorbic acid spiked in Baobab fruits powder in 0.1 mol/L sulfuric acid and titrated with 0.01 mol/L potassium iodate.
Fig. 6 DEP titration peaks for 0.0, 0.002, 0.005, 0.05 and 0.1 mmole of ascorbic acid spiked in Baobab fruits powder in 0.1 mol/L sulfuric acid and titrated with 0.01 mol/L potassium iodate.
Standard addition calibration for DEP titration of ascorbic acid in Baobab fruits.
Fig. 7 Standard addition calibration for DEP titration of ascorbic acid in Baobab fruits.
Table 2 Analytical parameters for DEP determination of ascorbic acid in Baobab fruit pulp and in Redoxon tablets at CNTs/Ag electrodes.
Sample Sample mass (g) Ascorbic acid spiked (mmole) Expected amount of ascorbic acid Detected amount of ascorbic acid CV(%)N = 3 Calibration parameters
Slope y-intercept R2
Baobab fruit powder 2.05 0.00 300 mg/100 g 279 mg/100 g 7.7 33.93 1.09 0.999
2.00 × 10−3
1.00 × 10−2
5.00 × 10−2
1.00 × 10−1
Redoxon® tablets 0.032 0.00 1000 mg/tablet 1040 mg/tablet 6.4 –
Biased square wave DEP peak for ascorbic acid in Redoxon, titrated with 0.01 mol/L potassium iodate.
Fig. 8 Biased square wave DEP peak for ascorbic acid in Redoxon, titrated with 0.01 mol/L potassium iodate.
Table 3 Some analytical performances attained in electrochemical determination of vitamin C.
Type of electrochemical detection Transducer Linear response (mol L−1) Detection limit (mol L−1) Relative standard deviation Reference
Potentiometric Graphite-epoxy composite electrode 8.0 × 10−6−4.5 × 10−4 4.5 × 10−6 4% Fernandes et al. (1999)
Voltammetric (CV) Poly caffeic acid film modified glassy carbon electrode 2.0 × 10−5−1.2 × 10−3 9.0 × 10−6 – Li et al. (2008)
Voltammetric (CV, DPV) Cu-zeolite A/graphene-modified glassy carbon electrode 2.0 × 10−5–2.0 × 10−4 1.1 × 10−5 – He et al. (2012)
Voltammetric (DPV) Single-walled CNT-modified carbon–ceramic electrode 5.0 × 10−6–7.0 × 10−4 3.0 × 10−3 3.50% Habibi et al. (2011)
Voltammetric (DPV) Poly(diallyl dimethylammonium chloride) modified helical CNTs coated glassy carbon electrode 2.5 × 10−6–1.05 × 10−3 1.2 × 10−7 – Zhang et al. (2013)
Amperometric and voltammetry Glassy carbon electrode modified by deposition of MWCNTs and poly (NileblueA) 5.0 × 10−5–1.0 × 10−3 1.6 × 10−6 Less than 5% Kul et al. (2013)
Voltammetric (DPV) Gold nanoparticles modified gold glassy carbon electrode 0.3–1.4 × 10−3 9.0 × 10−5 – Raoof et al. (2010)
Direct current DEP platinum electrodes 1.14 × 10−4–5.7 × 10−2 1.14 × 10−4 – Al-Ghannam and Al-Olyan (2005)
Square wave DEP CNTs/Ag electrodes 2.5 × 10−5–4.02.5 × 10−2 2.5 × 10−5 3.5% Current work

4

4 Conclusion

In this study, the modification of silver metal surface with good quality multiwalled CNTs through catalytic chemical vapor deposition method is reported. The prepared electrodes were used as indicating system in square wave differential electrolytic potentiometry for determination of ascorbic acid in a drug formulation and in a complex matrix of Baobab fruit. These electrodes showed high efficiency in terms of sensitivity, reproducibility and long-term stability. The obtained CNTs provided high surface area for the reaction while polarizing electrodes with a biased square wave offered clean electrode surface during the reaction. Ascorbic acid was determined at concentrations less than 25 µM. The promising results obtained with square wave DEP technique using these types of electrodes have encouraged us to develop a flowing system for the determination of ascorbic acid.

Acknowledgment

Abdalghaffar M. Acknowledges the support of Chemistry and Chemical Engineering Departments at King Fahd University of Petroleum and Minerals.

References

  1. , , , , . Differential electrolytic potentiometric titration method for the determination of ciprofloxacin in drug formulations. Talanta. 2003;61:239-244.
    [CrossRef] [Google Scholar]
  2. , , . Differential electrolytic potentiometric titration of vitamin C in pharmaceutical preparations. J. Food Drug Anal.. 2005;13:295-300.
    [Google Scholar]
  3. , , . Differential electrolytic potentiometry: a detector in the flow injection analysis of cyanide using silver electrodes modified with carbon nanotubes. Arab. J. Sci. Eng.. 2017;42:4445-4451.
    [CrossRef] [Google Scholar]
  4. , , , . Carbon nanotubes as nanosensor for differential electrolytic micropotentiometry. Am. J. Anal. Chem.. 2014;5:879-890.
    [Google Scholar]
  5. , , , , , . A voltammetric sensor for simultaneous determination of vitamin C and vitamin B6 in food samples using ZrO2 nanoparticle/ionic liquids carbon paste electrode. Food Anal. Methods. 2014;549–557
    [CrossRef] [Google Scholar]
  6. , , , , . Rapid HPLC method for determination of vitamin C, phenolic acids, hydroxycinnamic acid, and flavonoids in seasonal samples of emblica officinalis juice. J. Liq. Chromatogr. Relat. Technol.. 2015;38:619-624.
    [CrossRef] [Google Scholar]
  7. , . Differential electrolytic potentiometry with periodic polarisation part XXII. Symmetrical periodic current differential electrolytic potentiometry in oxidation - reduction titrimetry. Analyst. 1973;98:712-724.
    [CrossRef] [Google Scholar]
  8. , . Differential electrolytic potentiometry. Part II. Precision and accuracy of applications to redox titrimetry. Analyst. 1958;83:212-222.
    [CrossRef] [Google Scholar]
  9. , . Ultramicro potentiometric titrimetric analysis. differential electrolytic potentiometry in redox systems. Microchim. Acta. 1956;44:619-629.
    [Google Scholar]
  10. , , . Mechanism of carbon nanotube growth by CVD. Chem. Phys. Lett.. 2006;424:126-132.
    [CrossRef] [Google Scholar]
  11. , , , , , . Vitamin C in disease prevention and cure: an overview. Indian J. Clin. Biochem.. 2013;28:314-328.
    [CrossRef] [Google Scholar]
  12. , , , . Ion-selective electrodes using carbon nanotubes as ion-to-electron transducers. Anal. Chem.. 2008;80:1316-1322.
    [CrossRef] [Google Scholar]
  13. , , , , . Simultaneous determination of dopamine, ascorbic acid, and uric acid using helical carbon nanotubes modified electrode. Sensors Actuat. B Chem.. 2012;161:1139-1143.
    [CrossRef] [Google Scholar]
  14. , , , , , , , . A novel carbon paste electrode for potentiometric determination of vanadyl ion. Iran. J. Chem. Chem. Eng. 2015;34:89-96.
    [Google Scholar]
  15. , , , . Purity assessment of multiwalled carbon nanotubes by Raman spectroscopy. J. Appl. Phys.. 2007;101:64307.
    [CrossRef] [Google Scholar]
  16. , , , , , , . A simple and complete purification of single-walled carbon nanotube materials. Adv. Mater.. 1999;11:1354-1358.
    [CrossRef] [Google Scholar]
  17. , , , , , , , , , , , , , , , , , , , , , , , , . Natural vitamin C intake and the risk of head and neck cancer: a pooled analysis in the international head and neck cancer epidemiology consortium. Int. J. Cancer. 2015;137:448-462.
    [CrossRef] [Google Scholar]
  18. , , , . Potentiometric biosensor for l-ascorbic acid based on ascorbate oxidase of natural source immobilized on ethylene–vinylacetate membrane. Anal. Chim. Acta. 1999;385:3-12.
    [CrossRef] [Google Scholar]
  19. , , , , , , , , , . Consumption of fresh fruit rich in vitamin C and wheezing symptoms in children. SIDRIA collaborative group, Italy (Italian studies on respiratory disorders in children and the environment) Thorax. 2000;55:283-288.
    [CrossRef] [Google Scholar]
  20. , , , , , . Determination of Pb2+ions by a modified carbon paste electrode based on multi-walled carbon nanotubes (MWCNTS) and nanosilica. J. Hazard. Mater.. 2010;173:415-419.
    [CrossRef] [Google Scholar]
  21. , , , . Baobab (Adansonia digitata L.): a review on a multipurpose tree with promising future in the Sudan. Gartenbauwissenschaft. 2002;67:155-160.
    [Google Scholar]
  22. , , , , , , , , . Preparation of iodide selective carbon paste electrode with modified carbon nanotubes by potentiometric method and effect of CuS-NPs on its response. Electroanalysis. 2015;27:1516-1522.
    [CrossRef] [Google Scholar]
  23. , , , , . A new voltammetric sensor for electrocatalytic determination of vitamin C in fruit juices and fresh vegetable juice using modified multi-wall carbon nanotubes paste electrode. J. Food Sci. Technol.. 2015;52:276-284.
    [CrossRef] [Google Scholar]
  24. , , , . Differential pulse voltammetric simultaneous determination of acetaminophen and ascorbic acid using single-walled carbon nanotube-modified carbon–ceramic electrode. Anal. Biochem.. 2011;411:167-175.
    [CrossRef] [Google Scholar]
  25. , , , . Liquid as a required catalyst phase for carbon single-walled nanotube growth. Appl. Phys. Lett.. 2005;87:51919.
    [CrossRef] [Google Scholar]
  26. , , , , , , . Zeolite A functionalized with copper nanoparticles and graphene oxide for simultaneous electrochemical determination of dopamine and ascorbic acid. Anal. Chim. Acta. 2012;739:25-30.
    [CrossRef] [Google Scholar]
  27. , , , . Simultaneous determination of norepinephrine, uric acid, and ascorbic acid at a screen printed carbon electrode modified with polyacrylic acid-coated multi-wall carbon nanotubes. Biosens. Bioelectron.. 2010;25:2351-2355.
    [CrossRef] [Google Scholar]
  28. , . Helical microtubules of graphitic carbon. Nature. 1991;354:56-58.
    [Google Scholar]
  29. Iyuke, S.E., vZ Pienaar, H.C., Abdulkareem, A.S., Afolabi, A.S., 2006. Optimising Carbon Nanotube Continuous Production in a Swirled Floating Catalyst Chemical Vapour Deposition Reactor, in: South African Chemical Engineering Congress. pp. 1–8.
  30. , , , , , , . Metallic conductivity in bamboo-shaped multiwalled carbon nanotubes. Solid State Commun.. 2002;122:619-622.
    [CrossRef] [Google Scholar]
  31. , , , . Vitamin C supplementation slightly improves physical activity levels and reduces cold incidence in men with marginal vitamin C status: a randomized controlled trial. Nutrients. 2014;6:2572-2583.
    [CrossRef] [Google Scholar]
  32. , , , . An updated review of adansonia digitata: a commercially important African tree. South Afr. J. Bot.. 2011;77:908-919.
    [CrossRef] [Google Scholar]
  33. , , , , , . Multi-walled carbon nanotubes-ionic liquid-carbon paste electrode as a super selectivity sensor: application to potentiometric monitoring of mercury ion (II) J. Hazard. Mater.. 2010;183:402-409.
    [CrossRef] [Google Scholar]
  34. , , , , , , , . Investigation on the temperature-dependent growth rate of carbon nanotubes using chemical vapor deposition of ferrocene and acetylene. Chem. Phys. Lett.. 2005;401:459-464.
    [CrossRef] [Google Scholar]
  35. , , . Comparison of UPLC and HPLC methods for determination of vitamin C. Food Chem.. 2015;175:100-105.
    [CrossRef] [Google Scholar]
  36. , , , . Chemical vapor deposition of methane for single-walled carbon nanotubes. Chem. Phys. Lett.. 1998;292:567-574.
    [CrossRef] [Google Scholar]
  37. , , , , . A novel amperometric sensor for ascorbic acid based on poly(Nile blue A) and functionalised multi-walled carbon nanotube modified electrodes. Talanta. 2013;111:76-84.
    [CrossRef] [Google Scholar]
  38. , , , . Simultaneous voltammetric measurement of ascorbic acid and dopamine on poly(caffeic acid)-modified glassy carbon electrode. J. Solid State Electrochem.. 2008;12:693-699.
    [CrossRef] [Google Scholar]
  39. , , . Direct electrochemistry of glucose oxidase at a gold electrode modified with single-wall carbon nanotubes. Sensors. 2003;3:544-554.
    [Google Scholar]
  40. , , , , , . Simultaneous determination of ascorbic acid, dopamine and uric acid using polystyrene sulfonate wrapped multiwalled carbon nanotubes bound to graphite electrode through layer-by-layer technique. Sensors Actuat. B Chem.. 2010;145:643-650.
    [CrossRef] [Google Scholar]
  41. , , , . Fabrication of a Tb3+ carbon paste ion selective electrode by using nanosilica and multi-walled carbon nanotubes (MWCNTs) Int. J. Electrochem. Sci.. 2015;10:2791-2800.
    [Google Scholar]
  42. Musameh, M., Wang, J., Merkoci, A., Lin, Y., 2002. Low-potential stable NADH detection at carbon nanotube modified glassy carbon electrodes 4, 743–746.
  43. Mx, J., 2004. Direct electrochemical oxidation of NADPH at a low potential on the carbon nanotube modified glassy carbon electrode, pp. 167–171.
  44. , , , . Comparative analysis of vitamin C in fresh fruits juice of malus domestica, Citrus sinensi, Ananas comosus and Citrullus lanatus by iodometric titration. Int. J. Sci. Environ. Technol.. 2015;4:17-22.
    [Google Scholar]
  45. , , , , , , . Electrochemistry and electroanalytical applications of carbon nanotubes: a review. Anal. Sci.. 2005;21:1383-1393.
    [Google Scholar]
  46. , , , , , , , , , , , . Vitamin C as an antioxidant: evaluation of its role in disease prevention. J. Am. Coll. Nutr.. 2003;22:18-35.
    [CrossRef] [Google Scholar]
  47. , , , , . Electrochemical methods for ascorbic acid determination. Electrochim. Acta. 2014;121:443-460.
    [CrossRef] [Google Scholar]
  48. , , , . Electrocatalytic oxidation and determination of ascorbic acid in the presence of dopamine at multiwalled carbon nanotube-silica network–gold nanoparticles based nanohybrid modified electrode. Sensors Actuat. B Chem.. 2010;143:696-703.
    [CrossRef] [Google Scholar]
  49. , , , , , , , , . Adansonia digitata L. (baobab): a review of traditional information and taxonomic description. Asian Pac. J. Trop. Biomed.. 2015;5:79-84.
    [CrossRef] [Google Scholar]
  50. , , , , , . Simultaneous voltammetric determination of ascorbic acid and dopamine at the surface of electrodes modified with self-assembled gold nanoparticle films. J. Solid State Electrochem.. 2010;14:1171-1176.
    [CrossRef] [Google Scholar]
  51. , , . High-performance liquid chromatographic determination vitamin C in fresh fruits from west Africa. J. Food Compos. Drug Anal.. 1988;269:265-269.
    [Google Scholar]
  52. , , . Potentiometric multi-walled carbon nanotube Zn-sensor based on a naphthalocyanine neutral carrier: experimental and theoretical studies. RSC Adv.. 2015;5:58416-58427.
    [CrossRef] [Google Scholar]
  53. , , , , , , , . Layer-by-layer assembly of carbon nanotubes incorporated in light-addressable potentiometric sensors. J. Phys. Chem. C. 2009;113:14765-14770.
    [CrossRef] [Google Scholar]
  54. , , , , . Quantitation of vitamin C content in Herbal Juice using direct titration. J. Pharm. Biomed. Anal.. 2002;28:849-855.
    [CrossRef] [Google Scholar]
  55. , . Analytical applications of carbon nanotubes: a review. Trends Anal. Chem.. 2006;25:480-489.
    [Google Scholar]
  56. USP-NF25, 2007. USP-NF25.
  57. , , , , , , . Different carbon nanostructured materials obtained in catalytic chemical vapor deposition. J. Braz. Chem. Soc.. 2006;17:1124-1132.
    [CrossRef] [Google Scholar]
  58. , , , , , . Carbon nanotube-modified electrodes for the simultaneous determination of dopamine and ascorbic acid. Analyst. 2002;127:653-658.
    [CrossRef] [Google Scholar]
  59. , , , . Electrochemical behavior and determination of uric acid at single-walled carbon nanotube modified gold electrodes. Microchim. Acta. 2005;150:219-224.
    [CrossRef] [Google Scholar]
  60. , , , . High-field electrical transport in single-wall carbon nanotubes. Phys. Rev. Lett.. 2000;84:2941-2944.
    [Google Scholar]
  61. , , . Electrochemical behavior and determination of fluphenazine at multi-walled carbon nanotubes/(3-mercaptopropyl) trimethoxysilane bilayer modified gold electrodes. Talanta. 2004;64:380-386.
    [Google Scholar]
  62. , , , , , , , , . Simultaneous electrochemical determination of ascorbic acid, dopamine and uric acid with helical carbon nanotubes. Electrochim. Acta. 2013;91:261-266.
    [CrossRef] [Google Scholar]
  63. , , , , . Characterization of dodecanethiol SAM and multi-walled carbon nanotube modified gold electrodes, and voltammetric determination of prochlorperazine. Microchim. Acta. 2005;150:179-185.
    [CrossRef] [Google Scholar]
  64. , , , . Preparation of all solid-state potentiometric ion sensors with polymer-CNT composites. Electrochem. Commun.. 2009;11:1684-1687.
    [CrossRef] [Google Scholar]

Fulltext Views
975

PDF downloads
669
View/Download PDF
Download Citations
BibTeX
RIS
Show Sections

Suggested read for related articles: