Embed Size (px)
Citation preview
Journal of Molecular Liquids 223 (2016) 431–435
Contents lists available at ScienceDirect
Journal of Molecular Liquids
j ourna l homepage: www.e lsev ie r .com/ locate /mol l iq
Highly sensitive nanostructure voltammetric sensor employing Pt/CNTsand 1-butyl-3-methylimidazolium hexafluoro phosphate fordetermination of tryptophan in food and pharmaceutical samples
Fatemeh Khaleghi a,⁎, Abolfazl Elyasi Irai b, Vinod Kumar Gupta c,⁎, Shilpi Agarwal c,Majede Bijad d, Maryam Abbasghorbani e
a The Health of Plant and Livestock Products Research Center, Mazandaran University of Medical Sciences, Sari, Iranb Young Researchers and Elite Club, Ayatollah Amoli Branch, Islamic Azad University, Mazandaran 46351-43358, Iranc Department of Applied Chemistry, University of Johannesburg, Johannesburg, South Africad Department of Food Science, Sari Branch, Islamic Azad University, Sari, Irane Gas Division, Research Institute of Petroleum Industry, P.O. Box 14665-137, Tehran, Iran
⁎ Corresponding authors.E-mail addresses: [email protected] (F. Khaleghi)
(V.K. Gupta).
http://dx.doi.org/10.1016/j.molliq.2016.08.0580167-7322/© 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 oArticle history:Received 13 June 2016Received in revised form 14 August 2016Accepted 17 August 2016Available online 18 August 2016
A novel, sensitive, selective and simple method for the direct and quantitative determination of L-tryptophan(Trp) was proposed in this work. Pt/CNTs nanocompoite made by polyol technique, were used to modify ionicliquid carbon paste electrode (CPE) without any treatment to study the electrochemical behaviors of the Trpusing cyclic voltammetry (CV) and chronoamperometry (CA) and square wave voltammetry (SWV) methods.The results demonstrated that the 1-butyl-3-methylimidazolium hexafluoro phosphate (([C4mim]-[PF6])) Pt/CNTs modified carbon paste electrode ([C4mim]-[PF6]/Pt/CNTs/CPE) exhibited high catalytic activity and analyt-ical performance towards the electro-oxidation of Trp. The linear oxidation response range and limit of detectionwere found to be 0.1–400 μMand 0.04 μM, respectively. The [C4mim]-[PF6]/Pt/CNTs/CPEwas successfully appliedfor the voltammetric determination of Trp in food and pharmaceutical samples.
© 2016 Elsevier B.V. All rights reserved.
Keywords:TryptophanPt/CNTs1-Butyl-3-methylimidazolium hexafluorophosphateVoltammetry
1. Introduction
Tryptophan is well known as an essential amino acid in human andherbivores bodies, and the precursors of catecholamine synthesis inhuman body [1]. Trp has to be taken by foods in daily diet to maintainthe nitrogen balance. It also serves as serotonin and melatonin precur-sors which regulate several human functions such as sleep, mood andvarious aspects of mental health [2]. Since, Trp is a usual constituent ofmost protein-based foods or dietary proteins, then rapid and sensitivedetermination of Trp in food and medicine is of concern for scientists.Many analytical methods have been suggested for the determinationof Trp, including high performance liquid chromatography [3–5],chemiluminescence [6–8], spectroscopic [9,10], and electrochemicalmethod [11–14]. Among them, electrochemical sensors have advan-tages such as simple analysis, good selectivity, lower cost, high sensitiv-ity and high speed for electroactive compounds analysis [15–25]. So, weused this strategy for trace analysis of Trp in food and pharmaceuticalsamples.
Chemicallymodified electrodeswith room temperature ionic liquidsand nanomaterials (CMILNME) are high quality tools for the trace anal-ysis of electrochemical determination for biological, environmentaland pharmaceutical samples in different condition [26–34]. Scientificinvestigations show that nanomaterials have a good quality for applica-tion in sensor filed due to high surface area of nanomaterials and goodelectrical conductivity [35–40]. Reducing the over-potential andincreasing the sensitivity, including the substantial benefits of applica-tion of CMILNME as voltammetric electroactive compound analysis[41–51]. Modification of electrodes are necessary for trace analysis ofTrp due to high overvoltage of this compound at a surface of bare elec-trode [51].
In this study, we describe the synthesis and application of Pt/CNTsnanocomposite modified carbon ionic liquid paste electrode, with utili-zation of [C4mim]-[PF6] as a good conductive binder. The electrochemi-cal behavior of Trp at [C4mim]-[PF6]/Pt/CNTs/CPE, at carbon pasteelectrode modified with [C4mim]-[PF6] ([C4mim]-[PF6]/CPE), at Pt/CNTs/CPE paste electrode (Pt/CNTs/CPE), and at carbon paste electrode(CPE) was investigated. The results showed the superiority of [C4mim]-[PF6]/Pt/CNTs/CPE to the other electrodes in terms of higher sensitivity.The [C4mim]-[PF6]/Pt/CNTs/CPE is sensitive enough for the determina-tion of Trp in meat and tablet samples.
Fig. 1. TEM images of Pt/CNTs nanocomposite.
432 F. Khaleghi et al. / Journal of Molecular Liquids 223 (2016) 431–435
2. Experimental section
2.1. Apparatus and compounds
Mineral oil, Tryptophan, NaOH and graphite powder (b50 μm)wereobtained from Merck. All of the voltammetric investigation performedusing Autolab, potentiostat/galvanostat connected to a three-electrodecell, Azar Electrode, linked with a computer (Pentium IV) and withAutolab 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.
2.2. Preparation of the [C4mim]-[PF6]/Pt/CNTs/CPE
[C4mim]-[PF6]/Pt/CNTs/CPE was prepared by mixing of 0.2 g of[C4mim]-[PF6], 0.80 g of the paraffin oil, 0.15 g of Pt/CNTs, and 0.85 gof graphite powder. Then the mixture was mixed well for 40 min untila uniformlywetted pastewas obtained. A portion of the paste was filledfirmly into one glass tube as described above to prepare Pt/CNTs.
2.3. Synthesis of Pt/CNTs nanostructure
Pt/CNTs nanocomposite obtained by mixing stoichiometric of puri-fied carbon nanotubes, Pt (acac)2 (0.5 mmol) and 1,2 Hexadecanediol(5 mmol) in 40 mL phenylether at room temperature under a flow ofN2 atmosphere, and then the solution was heated at 120 °C for45 min. Reduction of Pt atoms in the presence of 1,2 Hexadecanediolhave been obtained until starting the reduction of Pt salt and nucleationof Pt nanoparticles. Afterward the solution was heated to 250 °C for100 min during the reflux process. After completion of reaction, theblack product solution was cool to room temperature under flow of N2
atmosphere. Then the product was rinse with ethanol and hexane solu-tion for 3 times and dried in 200 °C under Ar atmosphere.
Scheme 1. Electro-oxidation mechan
2.4. Preparation of real samples
5mL of meat sample extract was added to a volumetric flask and di-luted to the mark with the supporting electrolyte (PBS pH 8.0).
50 mg of Trp tablet finely powdered and then dissolved in 50 mLwater with ultrasonication. Then, 0.5 mL of the solution plus 9.5 mL ofthe buffer (pH 8.0) was used for the analysis with standard additionmethod.
3. Results and discussion
3.1. Pt/CNTs nanocomposite characterization
The morphology of the as-grown Pt/CNTs nanocomposite was char-acterized by TEM. Typical TEM micrograph of the synthesized Pt/CNTsnanocomposite is shown in Fig. 1. Results confirm synthesis of Pt/CNTs nanocomposite.
3.2. Voltammetric investigation
According to Scheme 1, we found that electro-oxidation of Trp is rel-ative to pH value in aqueous solution. Therefore, the effect of pH onelectro-oxidation of Trp was investigated using SWV technique (Fig. 2inset). Result shows that the electro-oxidation peak current for Trphas a maximum sensitivity at a pH 8.0 and this condition was chosenas the best optimal experimental condition for this work (Fig. 2).
The active surface area of the using sensors was estimated accordingto the Randles–Sevcik equation for a known concentration ofK4Fe(CN)6/K3Fe(CN)6. The results obtained were 0.24, 0.19, 0.13 and0.09 cm2 for [C4mim]-[PF6]/Pt/CNTs/CPE, [C4mim]-[PF6]/PE, Pt/CNTs/CPE and CPE, respectively. The obtained results show that the presenceof nanoparticles and ionic liquids together contributed to an increase inthe active surface area of the using sensor.
Current density derived from the cyclic voltammograms of 100 μMTrp (pH 8.0) at the surface of modified and bare electrodes with ascan rate of 50 mV/s shows in Fig. 3 inset. The results confirm that thepresence of Pt/CNTs and [C4min]-[PF6] together causes the increase ofthe electrode current density. Fig. 3 shows CV of 100 μM Trp atpH 8.0 at the surface of modified and unmodified electrodes with ascan rate of 50 mV/s. [C4mim]-[PF6]/Pt/CNTs/CPE exhibited significantoxidation peak current around 785 mV with the peak current of13.1 μA (Fig. 3, curve a). However, low electro-oxidation activity peakwas observed at Pt/CNTs/CPE (Fig. 3, curve c) and at CPE (Fig. 3, curved) over the same condition. The Trp oxidation peak potential at Pt/CNTs/CPE and at carbon paste electrode observed around 820 mV vs.the reference electrode with the oxidation peak current of 3.52 and7.05 μA, respectively. In addition, at the surface of [C4mim]-[PF6]/CPE,the oxidation peak appeared at 790 mV with the peak current was10.0 μA (Fig. 3, curve b), which indicated the presence of [C4mim]-[PF6] in CPE could enhance the peak currents and decrease the oxidationpotential. A significant negative shift of the currents starting from
ism for Trp in aqueous solution.
Fig. 4. Plot of Ipa vs. ν1/2 for the oxidation of Trp at [C4mim]-[PF6]/Pt/CNTs/CPE. Insetshows cyclic voltammograms of Trp at [C4mim]-[PF6]/Pt/CNTs/CPE at different scan ratesof a) 5, b) 10, c) 15, d) 20, e) 30, f) 45 and g) 60 mV/s in 0.1 M phosphate buffer, pH 8.0.
Fig. 2. Plot of potential, Ipa, vs. pH for the electro-oxidation of 150 μM Trp at a surface of[C4mim]-[PF6]/Pt/CNTs/CPE. Inset: influence of pH on SW voltammograms of Trp at asurface of the modified electrode.
433F. Khaleghi et al. / Journal of Molecular Liquids 223 (2016) 431–435
oxidation potential for Trp and dramatic increase of current of Trpindicated the catalytic ability of [C4mim]-[PF6]/Pt/CNTs/CPE to Trpoxidation. The results indicated that the presence of Pt/CNTs on[C4mim]-[PF6]/Pt/CNTs/CPE surface had great improvement with theelectrochemical response, whichwasmainly owing to excellent charac-teristics of Pt/CNTs such as good electrical conductivity. The suitableelectronic properties of Pt/CNTs nanocomposite together with the[C4mim]-[PF6] gave the ability to promote charge transfer reactions,good anti-fouling properties, especially when mixed with a higher con-ductive compound such as ILs as an electrode particle.
The effect of scan rate (υ) on the electrochemical oxidation peak cur-rent of Trpwas also examined (Fig. 4 inset). The results of this investiga-tion showed that the Trp oxidation peaks current increased linearlywith increasing the square root of scan rate that ranged from 5 to60 mV/s (Fig. 4). The result shows that the electrode process for oxida-tion of Trp is controlled under the diffusion step [52–65]. Also, the peakspotential shifts in negative direction when the scan rate increases,meaning that the electrochemical reaction is irreversible.
To obtain further information on the rate determining step, a Tafelplot was developed for the Trp at a surface of [C4mim]-[PF6]/Pt/CNTs/
Fig. 3. Cyclic voltammograms of a) [C4mim]-[PF6]/Pt/CNTs/CPE, b) [C4mim]-[PF6]/CPE,c) Pt/CNTs/CPE and d) CPE in presence of 100 μM Trp at a pH 8.0, respectively.
CPE using the data derived from the raising part of the current–voltagecurve (Fig. 5). The slope of the Tafel plot is equal to n(1 − α)F/2.3RTwhich comes up to 0.2594 V decade−1. We obtained α as 0.77.
Chronoamperometric measurements of Trp at [C4mim]-[PF6]/Pt/CNTs/CPE using the data derived from the raising part of the current–voltage curve (Fig. 6A). For Trp with a diffusion coefficient of D, the cur-rent observed for the electrochemical reaction at the mass transportlimited condition is described by the Cottrell equation. Experimentalplots of I vs. t−1/2were employed,with the best fits for different concen-trations of Trp (Fig. 6B). The slopes of the resulting straight lines werethen plotted vs. Trp concentration. From the resulting slope and Cottrellequation the mean value of the D was found to be 9.56 × 10−5 cm2/s.
3.3. Figure of merit
Limit of detection and linear dynamic range are two importantanalytical parameters that must be report for new analytical sensors[58-66]. The electrochemical determination of Trp has been studiedat the optimum condition using SWV method. Fig. 7 shows the SWvoltammograms (inset) responses and the calibration curves forTrp. The results showed that the dynamic linear range of Trp was0.1–400.0 μM with the correlation coefficient of 0.9918. The detectionlimit of the Trp was 0.04 μM (S/N = 3) along with the sensitivity of0.0469 μA/μM.
Fig. 5. Tafel plot for [C4mim]-[PF6]/Pt/CNTs/CPE in 0.1 M PBS (pH 8.0) with a scan rate of10 mV/s in the presence of 400 μM Trp.
Table 1Determination of Trp in real samples (n = 3).
Sample Added (Trp) Expected (Trp) Founded (Trp) Recovery (%)
Tableta – 5.00 4.79 ± 0.55 95.810.00 15.00 15.68 ± 0.85 104.53
Meat extractb – – 1.93 –
a The report concentration is according to μM.b The report concentration is according to (mg per 100 g meat).
Fig. 6. A) Chronoamperograms obtained at [C4mim]-[PF6]/Pt/CNTs/CPE in the presence ofa) 300 and b) 400 μMTrp in the buffer solution (pH 8.0). B) Cottrell's plot for the data fromthe chronoamperograms.
434 F. Khaleghi et al. / Journal of Molecular Liquids 223 (2016) 431–435
3.4. Stability and reproducibility
Wehave investigated the stability of the [C4mim]-[PF6]/Pt/CNTs/CPEin this study. It was observed that [C4mim]-[PF6]/Pt/CNTs/CPE con-served their activity for long time durations (i.e. 40 days) when theystored in a laboratory. To investigate the reproducibility of the[C4mim]-[PF6]/Pt/CNTs/CPE, electrochemical oxidation of 20.0 μM Trpat the same conditions were performed by using 5 different [C4mim]-[PF6]/Pt/CNTs/CPE. The results showed that an acceptable reproducibil-ity with a RSD of 3.1% was obtained. The results indicate that the[C4mim]-[PF6]/Pt/CNTs/CPE has good fabrication reproducibility.
3.5. Interference study
The possible interference for Trp determination at [C4mim]-[PF6]/Pt/CNTs/CPEwas investigated. Some amino acids, including valine, alanine,threonine, isoleucine, glutamine, aspartic acid, and histidine, had no in-fluence on the current response for 40 μMTrp.Meanwhile, the [C4mim]-[PF6]/Pt/CNTs/CPE showed an inhibiting effect on the oxidation of otherelectroactive species such as glucose, lactose, fructose and methanol(500 times content).
Fig. 7. Plot of the electro-oxidation peak currents as a function of Trp concentration. Inset:SWVs of [C4mim]-[PF6]/Pt/CNTs/CPE in 0.1 M phosphate buffer solution (pH 8.0)containing different concentrations of Trp.
3.6. Real sample analysis
We study the analytical utility of the [C4mim]-[PF6]/Pt/CNTs/CPE sen-sor was assessed by applying it to the determination of Trp in a food andtablet samples using standard additionmethod by SWVmethod. The re-sults are given in Table 1, confirm that the [C4mim]-[PF6]/Pt/CNTs/CPEretained its efficiency for the determination of Trp in real samples.
4. Conclusions
The carbon paste electrode modified with [C4mim]-[PF6] and Pt/CNTs have excellent electrocatalytic activity towards Trp. Besides thegood analytical performance, [C4mim]-[PF6]/Pt/CNTs/CPE has theadvantages of good reproducibility, stability and selectivity. Under theoptimum conditions, the peak current was proportional to the Trp con-centration in the range of 1.0 to 400.0 μM with the detection limit of0.04 μM. The [C4mim]-[PF6] and Pt/CNTs was successfully used for thedetermination of Trp in food and pharmaceutical samples.
Acknowledgments
The authors wish to thank The Health of Plant and LivestockProducts Research Center, Mazandaran University of Medical Sciences,Sari, Iran for their support.
References
[1] G.-P. Jin, X.-Q. Lin, The electrochemical behavior and amperometric determinationof tyrosine and tryptophan at a glassy carbon electrode modified withbutyrylcholine, Electrochem. Commun. 6 (2004) 454–460.
[2] A. Özcan, Y. Şahin, A novel approach for the selective determination of tryptophan inblood serum in the presence of tyrosine based on the electrochemical reduction ofoxidation product of tryptophan formed in situ on graphite electrode, Biosens.Bioelectron. 31 (2012) 26–31.
[3] M.E. Soto, A.M. Ares, J. Bernal, M.J. Nozal, J.L. Bernal, Simultaneous determination oftryptophan, kynurenine, kynurenic and xanthurenic acids in honey by liquid chro-matography with diode array, fluorescence and tandem mass spectrometry detec-tion, J. Chromatogr. A 1218 (2011) 7592–7600.
[4] I.N. Mefford, J.D. Barchas, Determination of tryptophan and metabolites in rat brainand pineal tissue by reversed-phase high-performance liquid chromatography withelectrochemical detection, J. Chromatogr. B Biomed. Sci. Appl. 181 (1980) 187–193.
[5] J. Landry, S. Delhaye, D. Jones, Determination of tryptophan in feedstuffs: compari-son of two methods of hydrolysis prior to HPLC analysis, J. Sci. Food Agric. 58(1992) 439–441.
[6] Y.-D. Liang, J.-F. Song, Flow-injection chemiluminescence determination of trypto-phan through its peroxidation and epoxidation by peroxynitrous acid, J. Pharm.Biomed. Anal. 38 (2005) 100–106.
[7] H. Chen, L. Li, M. Zhou, Y.J. Ma, Flow-injection chemiluminescence determination oftryptophan using galangin-potassium permanganate–polyphosphoric acid system,Chin. Chem. Lett. 19 (2008) 203–206.
[8] A.A. Alwarthan, Chemiluminescent determination of tryptophan in a flow injectionsystem, Anal. Chim. Acta 317 (1995) 233–237.
[9] H. Edelhoch, Spectroscopic determination of tryptophan and tyrosine in proteins,Biochemistry 6 (1967) 1948–1954.
[10] L. Servillo, G. Colonna, C. Balestrieri, R. Ragone, G. Irace, Simultaneous determinationof tyrosine and tryptophan residues in proteins by second-derivative spectroscopy,Anal. Biochem. 126 (1982) 251–257.
[11] X. Tang, Y. Liu, H. Hou, T. You, Electrochemical determination of L-tryptophan, L-tyrosine and L-cysteine using electrospun carbon nanofibers modified electrode,Talanta 80 (2010) 2182–2186.
[12] S. Shahrokhian, L. Fotouhi, Carbon paste electrode incorporating multi-walled car-bon nanotube/cobalt salophen for sensitive voltammetric determination of trypto-phan, Sensors Actuators B Chem. 123 (2007) 942–949.
[13] K.-J. Huang, C.-X. Xu, W.-Z. Xie, W. Wang, Electrochemical behavior andvoltammetric determination of tryptophan based on 4-aminobenzoic acid polymerfilm modified glassy carbon electrode, Colloids Surf. B: Biointerfaces 74 (2009)167–171.
435F. Khaleghi et al. / Journal of Molecular Liquids 223 (2016) 431–435
[14] Y. Fan, J.-H. Liu, H.-T. Lu, Q. Zhang, Electrochemistry and voltammetric determina-tion of L-tryptophan and L-tyrosine using a glassy carbon electrode modified witha Nafion/TiO2-graphene composite film, Microchim. Acta 173 (2011) 241–247.
[15] N. Atar, T. Eren, M.L. Yola, H. Karimi-Maleh, B. Demirdögen, Magnetic iron oxide andiron oxide@gold nanoparticle anchored nitrogen and sulfur-functionalized reducedgraphene oxide electrocatalyst for methanol oxidation, RSC Adv. 5 (2015)26402–26409.
[16] N. Atar, M.L. Yola, T. Eren, Sensitive determination of citrinin based on molecularimprinted electrochemical sensor, Appl. Surf. Sci. 362 (2016) 315–322.
[17] T. Eren, N. Atar, M.L. Yola, H. Karimi-Maleh, A sensitive molecularly imprinted poly-mer based quartz crystal microbalance nanosensor for selective determination oflovastatin in red yeast rice, Food Chem. 185 (2015) 430–436.
[18] H. Karimi-Maleh, F. Tahernejad-Javazmi, N. Atar, M.L.t. Yola, V.K. Gupta, A.A. Ensafi,A novel DNA biosensor based on a pencil graphite electrode modified with polypyr-role/functionalized multiwalled carbon nanotubes for determination of 6-mercaptopurine anticancer drug, Ind. Eng. Chem. Res. 54 (2015) 3634–3639.
[19] B.J. Sanghavi, G. Hirsch, S.P. Karna, A.K. Srivastava, Potentiometric stripping analysisof methyl and ethyl parathion employing carbon nanoparticles and halloysitenanoclay modified carbon paste electrode, Anal. Chim. Acta 735 (2012) 37–45.
[20] B.J. Sanghavi, J.A. Moore, J.L. Chávez, J.A. Hagen, N. Kelley-Loughnane, C.-F. Chou, N.S.Swami, Aptamer-functionalized nanoparticles for surface immobilization-free elec-trochemical detection of cortisol in a microfluidic device, Biosens. Bioelectron. 78(2016) 244–252.
[21] B.J. Sanghavi, S. Sitaula, M.H. Griep, S.P. Karna, M.F. Ali, N.S. Swami, Real-time elec-trochemical monitoring of adenosine triphosphate in the picomolar to micromolarrange using graphene-modified electrodes, Anal. Chem. 85 (2013) 8158–8165.
[22] N.F. Atta, M.F. El-Kady, A. Galal, Simultaneous determination of catecholamines, uricacid and ascorbic acid at physiological levels using poly (N-methylpyrrole)/Pd-nanoclusters sensor, Anal. Biochem. 400 (2010) 78–88.
[23] N.F. Atta, M.F. El-Kady, A. Galal, Palladium nanoclusters-coated polyfuran as a novelsensor for catecholamine neurotransmitters and paracetamol, Sensors Actuators BChem. 141 (2009) 566–574.
[24] H. Yaghoubian, H. Karimi-Maleh, M.A. Khalilzadeh, F. Karimi, Electrocatalytic oxida-tion of levodopa at a ferrocene modified carbon nanotube paste electrode, Int. J.Electrochem. Sci. 4 (2009) 993–1003.
[25] M.L. Yola, V.K. Gupta, N. Atar, Newmolecular imprinted voltammetric sensor for de-termination of ochratoxin A, Mater. Sci. Eng. C 61 (2016) 368–375.
[26] M. Elyasi, M.A. Khalilzadeh, H. Karimi-Maleh, High sensitive voltammetric sensorbased on Pt/CNTs nanocomposite modified ionic liquid carbon paste electrode fordetermination of Sudan I in food samples, Food Chem. 141 (2013) 4311–4317.
[27] R. Bavandpour, H. Karimi-Maleh, M. Asif, V.K. Gupta, N. Atar, M. Abbasghorbani, Liq-uid phase determination of adrenaline uses a voltammetric sensor employingCuFe2O4 nanoparticles and room temperature ionic liquids, J. Mol. Liq. 213 (2016)369–373.
[28] M. Najafi, M.A. Khalilzadeh, H. Karimi-Maleh, A new strategy for determination ofbisphenol A in the presence of Sudan I using a ZnO/CNTs/ionic liquid paste electrodein food samples, Food Chem. 158 (2014) 125–131.
[29] H. Karimi-Maleh, S. Rostami, V.K. Gupta, M. Fouladgar, Evaluation of ZnO nanoparti-cle ionic liquid composite as a voltammetric sensing of isoprenaline in the presenceof aspirin for liquid phase determination, J. Mol. Liq. 201 (2015) 102–107.
[30] T. Jamali, H. Karimi-Maleh, M.A. Khalilzadeh, A novel nanosensor based on Pt:Conanoalloy ionic liquid carbon paste electrode for voltammetric determination of vi-tamin B9 in food samples, LWT–Food Sci. Technol. 57 (2014) 679–685.
[31] M. Bijad, H. Karimi-Maleh, M.A. Khalilzadeh, Application of ZnO/CNTs nanocompos-ite ionic liquid paste electrode as a sensitive voltammetric sensor for determinationof ascorbic acid in food samples, Food Anal. Methods 6 (2013) 1639–1647.
[32] A. Baghizadeh, H. Karimi-Maleh, Z. Khoshnama, A. Hassankhani, M. Abbasghorbani,A voltammetric sensor for simultaneous determination of vitamin C and vitamin B6in food samples using ZrO2 nanoparticle/ionic liquids carbon paste electrode, FoodAnal. Methods 8 (2015) 549–557.
[33] E. Afsharmanesh, H. Karimi-Maleh, A. Pahlavan, J. Vahedi, Electrochemical behaviorof morphine at ZnO/CNT nanocomposite room temperature ionic liquid modifiedcarbon paste electrode and its determination in real samples, J. Mol. Liq. 181(2013) 8–13.
[34] H. Beitollah, M. Goodarzian, M.A. Khalilzadeh, H. Karimi-Maleh, M. Hassanzadeh, M.Tajbakhsh, Electrochemical behaviors and determination of carbidopa on carbonnanotubes ionic liquid paste electrode, J. Mol. Liq. 173 (2012) 137–143.
[35] H. Karimi-Maleh, P. Biparva, M. Hatami, A novel modified carbon paste electrodebased on NiO/CNTs nanocomposite and (9,10-dihydro-9,10-ethanoanthracene-11,12-dicarboximido)-4-ethylbenzene-1,2-diol as a mediator for simultaneousdetermination of cysteamine, nicotinamide adenine dinucleotide and folic acid,Biosens. Bioelectron. 48 (2013) 270–275.
[36] H. Karimi-Maleh, F. Tahernejad-Javazmi, A.A. Ensafi, R. Moradi, S. Mallakpour, H.Beitollahi, A high sensitive biosensor based on FePt/CNTs nanocomposite/N-(4-hydroxyphenyl)-3,5-dinitrobenzamide modified carbon paste electrode for simulta-neous determination of glutathione and piroxicam, Biosens. Bioelectron. 60 (2014) 1–7.
[37] R. Moradi, S. Sebt, H. Karimi-Maleh, R. Sadeghi, F. Karimi, A. Bahari, H. Arabi, Synthe-sis and application of FePt/CNTs nanocomposite as a sensor and novel amide ligandas a mediator for simultaneous determination of glutathione, nicotinamide adeninedinucleotide and tryptophan, Phys. Chem. Chem. Phys. 15 (2013) 5888–5897.
[38] M.R. Shahmiri, A. Bahari, H. Karimi-Maleh, R. Hosseinzadeh, N. Mirnia,Ethynylferrocene–NiO/MWCNT nanocomposite modified carbon paste electrodeas a novel voltammetric sensor for simultaneous determination of glutathioneand acetaminophen, Sensors Actuators B Chem. 177 (2013) 70–77.
[39] A.A. Ensafi, H. Karimi-Maleh, Modified multiwall carbon nanotubes paste electrodeas a sensor for simultaneous determination of 6-thioguanine and folic acid usingferrocenedicarboxylic acid as a mediator, J. Electroanal. Chem. 640 (2010) 75–83.
[40] A.A. Ensafi, H. Karimi-Maleh, S. Mallakpour, M. Hatami, Simultaneous determinationof N-acetylcysteine and acetaminophen by voltammetric method using N-(3,4-dihydroxyphenethyl)-3,5-dinitrobenzamide modified multiwall carbon nanotubespaste electrode, Sensors Actuators B Chem. 155 (2011) 464–472.
[41] W. Sun, M. Yang, R. Gao, K. Jiao, Electrochemical determination of ascorbic acid inroom temperature ionic liquid BPPF6 modified carbon paste electrode, Electroanal-ysis 19 (2007) 1597–1602.
[42] M. Arvand, M. Dehsaraei, A simple and efficient electrochemical sensor for folic aciddetermination in human blood plasma based on gold nanoparticles-modified car-bon paste electrode, Mater. Sci. Eng. C 33 (2013) 3474–3480.
[43] A. Safavi, N. Maleki, F. Farjami, E. Farjami, Electrocatalytic oxidation of formaldehydeon palladium nanoparticles electrodeposited on carbon ionic liquid composite elec-trode, J. Electroanal. Chem. 626 (2009) 75–79.
[44] T. Tavana, M.A. Khalilzadeh, H. Karimi-Maleh, A.A. Ensafi, H. Beitollahi, D. Zareyee,Sensitive voltammetric determination of epinephrine in the presence of acetamino-phen at a novel ionic liquid modified carbon nanotubes paste electrode, J. Mol. Liq.168 (2012) 69–74.
[45] H. Karimi-Maleh, K. Ahanjan, M. Taghavi, M. Ghaemy, A novel voltammetric sensoremploying zinc oxide nanoparticles and a new ferrocene-derivative modified car-bon paste electrode for determination of captopril in drug samples, Anal. Methods8 (2016) 1780–1788.
[46] H. Karimi-Maleh, M. Moazampour, M. Yoosefian, A.L. Sanati, F. Tahernejad-Javazmi,M. Mahani, An electrochemical nanosensor for simultaneous voltammetric determi-nation of ascorbic acid and Sudan I in food samples, Food Anal. Methods 7 (2014)2169–2176.
[47] M. Shabani-Nooshabadi, M. Roostaee, Modification of carbon paste electrode withNiO/graphene oxide nanocomposite and ionic liquids for fabrication of high sensi-tive voltammetric sensor on sulfamethoxazole analysis, J. Mol. Liq. 220 (2016)329–333.
[48] M. Shabani-Nooshabadi, M. Roostaee, F. Tahernejad-Javazmi, Graphene oxide/NiOnanoparticle composite-ionic liquid modified carbon paste electrode for selectivesensing of 4-chlorophenol in the presence of nitrite, J. Mol. Liq. 219 (2016) 142–148.
[49] T. Alizadeh, M.R. Ganjali, M. Akhoundian, P. Norouzi, Voltammetric determination ofultratrace levels of cerium (III) using a carbon paste electrode modified with nano-sized cerium-imprinted polymer and multiwalled carbon nanotubes, Microsc. Acta183 (2016) 1123–1130.
[50] S. Gheibi, H. Karimi-Maleh, M.A. Khalilzadeh, H. Bagheri, A new voltammetric sensorfor electrocatalytic determination of vitamin C in fruit juices and fresh vegetablejuice using modified multi-wall carbon nanotubes paste electrode, J. Food Sci.Technol. 52 (2015) 276–284.
[51] J.B. Raoof, R. Ojani, H. Karimi-Maleh, Carbon paste electrode incorporating 1-[4-(ferrocenyl ethynyl) phenyl]-1-ethanone for electrocatalytic and voltammetric de-termination of tryptophan, Electroanalysis 20 (2008) 1259–1262.
[52] H. Karimi-Maleh, F. Tahernejad-Javazmi, M. Daryanavard, H. Hadadzadeh, A.A.Ensafi, M. Abbasghorbani, Electrocatalytic and simultaneous determination ofascorbic acid, nicotinamide adenine dinucleotide and folic acid at ruthenium (II)complex-ZnO/CNTs nanocomposite modified carbon paste electrode, Electroanaly-sis 26 (2014) 962–970.
[53] R. Sadeghi, H. Karimi-Maleh, A. Bahari, M. Taghavi, A novel biosensor based on ZnOnanoparticle/1,3-dipropylimidazolium bromide ionic liquid-modified carbon pasteelectrode for square-wave voltammetric determination of epinephrine, Phys.Chem. Liq. 51 (2013) 704–714.
[54] M. Baghayeri, M. Namadchian, H. Karimi-Maleh, H. Beitollahi, Determination of ni-fedipine using nanostructured electrochemical sensor based on simple synthesisof Ag nanoparticles at the surface of glassy carbon electrode: application to the anal-ysis of some real samples, J. Electroanal. Chem. 697 (2013) 53–59.
[55] A.A. Ensafi, M. Izadi, H. Karimi-Maleh, Sensitive voltammetric determination ofdiclofenac using room-temperature ionic liquid-modified carbon nanotubes pasteelectrode, Ionics 19 (2013) 137–144.
[56] A.A. Ensafi, H. Karimi-Maleh, S. Mallakpour, N-(3,4-dihydroxyphenethyl)-3,5-dinitrobenzamide-modified multiwall carbon nanotubes paste electrode as anovel sensor for simultaneous determination of penicillamine, uric acid, and trypto-phan, Electroanalysis 23 (2011) 1478–1487.
[57] A.A. Ensafi, S. Dadkhah-Tehrani, H. Karimi-Maleh, A voltammetric sensor for the si-multaneous determination of L-cysteine and tryptophan using a p-aminophenol-multiwall carbon nanotube paste electrode, Anal. Sci. 27 (2011) 409.
[58] R.N. Goyal, V.K. Gupta, S. Chatterjee, Voltammetric biosensors for the determinationof paracetamol at carbon nanotube modified pyrolytic graphite electrode, SensorsActuators B. Chem. 149 (2010) 252–258.
[59] V.K. Gupta, A.K. Jain, S. Agarwal, G. Maheshwari, An iron (III) ion-selective sensorbased on a m bis (tridentate) ligand, Talanta 71 (2007) 1964–1968.
[60] V.K. Gupta, A.K. Singh, S. Mehtab, B. Gupta, ACobalt (II) selectivePVCmembranebased on a Schiff base complex of N, N’-bis (salicylidene)-3,4-diaminotoluene, Anal. Chim. Acta 566 (2006) 5–10.
[61] V.K. Gupta, A.K. Singh, M. Al Khayat, Barkha Gupta, Neutral carriers based poly431-meric membrane electrodes for selective determination of Mercury (II), Anal. Chim.Acta 590 (2007) 81–90.
[62] V.K. Gupta, R. Prasad, R. Mangla, P. Kumar, New nickel (II) selective poten434tio-metric sensor based on 5,7,12,14-tetramethyldibenzotetraazaannulene in apoly(vinyl chloride) matrix, Anal. Chim. Acta 420 (2000) 19–27.
[63] R.N. Goal, V.K. Gupta, S. Chatterjee, Simultaneous determination of adenosineandinosine using single-wall carbon nanotubes modified pyrolytic graphiteelectrode,Talanta 76 (2008) 662–668.
[64] V.K. Gupta, A.K. Jain, G. Maheshwari, Novel Aluminum (III) selective potentiometricsensor based on morin in poly (vinyl chloride) matrix, Talanta 72 (4) (2007)1469–1473.
[65] V.K. Gupta, M.R. Ganjali, P. Norouzi, H. Khani, A. Nayak, S. Agarwal, ElectrochemicalAnalysis of some Toxic Metals and Drugs by Ion Selective Electrodes, CRC Crit. Rev.Anal. Chem. 41 (2011) 282–313.
[66] H. Karimi-Maleh, A. Fallah Shojaei, K. Tabatabaeian, F. Karimi, S. Shakeri, R. Moradi,Simultaneous determination of 6-mercaptopruine, 6-thioguanine and dasatinib asthree important anticancer drugs using nanostructure voltammetric sensoremploying Pt/MWCNTs and 1-butyl-3-methylimidazolium hexafluorophosphate,Biosens. Bioelect. 86 (2016) 879–884 (313313).
