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Journal of Molecular Catalysis A: Chemical 396 (2015) 245–253

Contents lists available at ScienceDirect

Journal of Molecular Catalysis A: Chemical

jou rn al hom epage: www.elsev ier .com/ locate /molcata

lectrocatalytic oxidation of sulfide and electrochemical behavior ofhloropromazine based on organic–inorganic hybrid nanocomposite

ader Amini, Mojtaba Shamsipur, Mohammad Bagher Gholivand ∗

epartment of Analytical Chemistry, Razi University, Kermanshah, Iran

r t i c l e i n f o

rticle history:eceived 30 January 2014eceived in revised form 17 May 2014ccepted 7 September 2014vailable online 16 September 2014

eywords:hloropromazine (CPZ)ilica nanoparticles (SNPs)

a b s t r a c t

For the first time, this work describes the electrochemical behavior of chloropromazine as a modifieron the surface of electrodes. The electrochemical properties of chloropromazine in the silica nanopar-ticles/chloropromazine/Nafion (SNPs/CPZ/Nf) nanocomposite at pH 2–10 were investigated at a glassycarbon electrode. Well defined reversible redox couples were observed in acidic solutions and irreversiblein alkaline solutions. The (SNPs/CPZ/Nf) nanocomposite modified electrodes were characterized witha transmission electron microscopy (TEM), electrochemical impedance spectroscopy (EIS) and cyclicvoltammetry (CV). The apparent electron transfer rate constant (Ks), transfer coefficient (˛) and the sur-face concentration (� c) were determined by cyclic voltammetry and they were about 0.025 s−1, 0.50

−6 −2

ulfideodified electrodeafionlectrocatalysis

and 1.26 × 10 mol cm , respectively. Moreover, electrocatalytic oxidation of sulfide on the surface ofmodified electrode was investigated with cyclic voltammetry and amperometry methods at pH 7. Theprepared modified electrode showed several advantages, such as a simple preparation method, high sen-sitivity, very low detection limits and excellent reproducibility. Moreover, the proposed sensor can beused for sulfide analysis in water samples.

. Introduction

The discovery of the antipsychotic agent chloropromazine inhe early 1950s and the advent of even more powerful phenoth-azinic psychopharmacological agents represent a landmark in theistory of the medical and psychiatric sciences [1]. Chloropro-azine hydrochloride is the most important compound in the large

roup of phenothiazine derivatives. It is widely used as a thera-eutic agent for treating various mental and personality disorders,

n the prevention of vomit spasms and as an intravenous anti-ypertensive. One of the common properties of phenothiazine and

ts derivatives is that they are easily oxidized and lose a single elec-ron to become cation radicals [2–4] which are very active and caneact with a number of substances [5–9]. It was found that cationadicals are colored and stable in acidic media. It has the abilityor oxidation by many oxidizing agents with the formation of col-red oxidation products. These cation radicals are easily formed

y chemical [10–12], electrochemical [13,14], enzymatic [15] andhotochemical [16] oxidation. The oxidation process involves twoubsequent and distinct one-electron steps. The first is reversible

∗ Corresponding author. Tel.: +98 831 4274557; fax: +98 831 4274559.E-mail addresses: mbgholivand@yahoo.com,

bgholivand2013@gmail.com (M.B. Gholivand).

ttp://dx.doi.org/10.1016/j.molcata.2014.09.011381-1169/© 2014 Elsevier B.V. All rights reserved.

© 2014 Elsevier B.V. All rights reserved.

and results in the formation of a colored cation-radical. The sec-ond is irreversible and gives rise to the colorless sulfoxide [17].One of the redox mediators that was used as a suitable homoge-neous mediator in the electrooxidation of various compounds ischlorpromazine (CPZ) [18,19]. To our best knowledge, there are noreports concerning the use of CPZ in modification of electrodes forfabricated sensor.

Because of their unique electronic, optical and catalytic proper-ties, nanoparticles have become the focus for scientific researchers[20,21]. The integration of nanoparticles and biomolecules into thinfilms is extremely significant, which has opened up a new route forthe fabrication of chemical sensors and biosensors. As a non-metaloxide, silica (SiO2) nanoparticles have extensive applications inchemical mechanical polishing and as additives to drugs, cosmetics,printer toners, varnishes and food. In recent years, the use of SiO2nanoparticles has been extended to biomedical and biotechnolo-gical fields, such as biosensors [22], biomarkers [23], cancer therapy[24], DNA delivery [25,26], drug delivery [27] and enzyme immo-bilization [28]. On the other hand, silica nanoparticles, because oftheir large surface area, good biocompatibility, and suitability formany surface immobilization mechanism, have been effectively

used [29,30] and have proven to be excellent substrates in manyfields ranging from biosensors to interfacial interaction studies[31]. Since the single silica material cannot transfer electron fromanalyte to electrode, as a result, organic–inorganic nanocomposite,

2 Cataly

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uWwbatephspWwpCttmNoons

46 N. Amini et al. / Journal of Molecular

uch as the SiO2 nanoparticles coupled with a redox mediator, haveecome attractive for many new electronic, optical or magneticpplications [32,33]. The resultant “nanocomposition” determinesoth the compatibility and the suitability of the probe towards thenalyte, and thence assays are possible [34].

Sulfides (H2S, HS− and S2−) are found widely in natural wateramples and wastewater and serve as a main pollution index forater [35]; also, it is used as a preservative in the food industry

ecause its addition to several products (vegetables, fruits and sev-ral beverages) prevents oxidation, inhibits bacterial growth andssists in preserving vitamin C [36]. Due to potential toxicity of sul-de, the sulfide content more than the established threshold levelhould be strictly limited and must be adequately labeled. There-ore, the existence of methods allowing an accurate measurementf sulfide is very important. In aqueous solutions hydrogen sulfide isn equilibrium with bisulfide and sulfide ions. Total sulfide concen-rations are usually reported as the sum of all three species. Manynalytical methods for the sulfide assay have been reported, suchs high performance liquid chromatography [37,38], capillary elec-rophoresis [39], chemiluminescence [40], and spectrophotometry41]. Compared to electrochemical detections, these techniques areot convenient. For example, for the determination of traces ofulfide in solution, cathodic stripping voltammetry (CSV) can be

more suitable technique than the commonly used spectropho-ometry from the viewpoints of detection limits and operationalasiness [42].

Therefore, there is still an increasing demand in electrochemi-al detections, as these generally have the advantages of simplicity,hort analysis time, low cost, high reproducibility, and sensitivity.

major problem arises due to the electrochemical oxidation ofulfide at bare electrodes, referred to as electrode fouling or poison-ng, which decreases electrode reactivity and shortens its workingife [43–46]. This problem can be overcome by construction ofhemically modified electrodes (CME). The properties and electro-hemical activities of CME that are constructed by the attachmentf a catalytic species (organic or inorganic) to the surface of aase electrode, are found to be highly influenced by the naturef the adsorbed species and the electrode surface morphology47–52]. Moreover, the sensing capability of these modified elec-rodes as environmental sensing tools for sulfide detection wasnother important area of application due to their high sensitivity,electivity, simplicity and cheap construction [53–57].

A literature survey showed that there were not any reports onsing CPZ as a mediator for construction of modified electrodes.e immobilized CPZ onto a GC electrode. However, the filmsere easily washed from the electrode with buffer solution and

ecame unstable during the potential scan. SiO2NPs/EtOH/water isttractive metrics for the incorporation of phenothiazine deriva-ives due to their excellent physical and chemical stability and thease with which the silica gels can be prepared. The CPZ incor-orated into SiO2NPs/EtOH/water on the surfaces of GC electrodead well-defined electron transfer properties; however, the diffu-ion loss of CPZ into external solutions occurred. To overcome thisroblem, we immobilized CPZ into Nafion–silica composite films.hen CPZ@SiO2NPs were conjugated with a polymer Nafion (Nf)ith favorable compatibility and film-forming ability, a promisinglatform for electrochemical analysis of CPZ and sulfide base onPZ/SiO2/Nf was further developed. The present research describeshe electrochemical behavior of chloropromazine as a modifier onhe surface of electrode and an organic–inorganic nanocomposite

aterial composed of silica nanoparticles, chloropromazine andafion (SNPs/CPZ/Nf nanocomposite) was used for the construction

f a new amperometric sensor for sulfide detection. The advantagesf the sulfide amperometric detector based on the SNPs/CPZ/Nfanocomposite GCE are as follows: a very low detection limit, highensitivity inherent stability at pH 7, excellent catalytic activity

sis A: Chemical 396 (2015) 245–253

for sulfide oxidation and remarkable antifouling property towardsulfide and its oxidation product.

2. Experimental

2.1. Reagents

Chloropromazine and sodium sulfide were purchased fromAldrich and Merck and used as received. Silica nanoparticles wereprocured from Sigma–Aldrich. Nafion (Nf) (v/v 5%) was obtainedfrom Sigma chemical Co. A buffer solution (0.05 M) was preparedfrom di-sodium hydrogen phosphate (Na2HPO4), sodium dihydro-gen phosphate (NaH2PO4), hydrogen chloride (HCl) and sodiumhydroxide (NaOH) and was also purchased from Merck. Doubly dis-tillated water was used to prepare all solutions. All electrochemicalexperiments were carried out at 25 ± 0.10 ◦C.

2.2. Apparatus

Electrochemical experiments were performed with a com-puter controlled �-Autolab modular electrochemical system (EcoChemie, Utrecht, The Netherlands) driven with GPES software(Eco Chemie). A conventional three-electrode cell consisting of aAg/AgCl [KCl (sat)], Pt wire and (unmodified or modified) glassycarbon were used as reference, counter and working electrode,respectively. A personal computer was used for data storage andprocessing.

2.3. Preparation of the modified electrode

Prior to the modification, the bare GCE was polished successivelywith No. 1 to No. 6 emery papers and 0.5 �m alumina slurry to mir-ror – like smoothness. Then, it was sonicated in ethanol and distilledwater for 10 min to remove adsorbed particles. A 1.0% (v/v) Nafionsolution was prepared by diluting the 5.0% (v/v) Nafion solutionwith ethanol. 1 mg silica nanoparticles and 1 mg CPZ were added to5 mL 1.0% (v/v) Nafion solution, and then ultrasonicated for 30 minto form a homogeneous CPZ–SiO2NPs–Nafion solution. Next, 5 �Lof the above solution was dripped onto cleaned GC electrode sur-face and dried at room temperature. Finally, the electrode wasimmersed in 0.05 M phosphate buffer solution (pH 2) and cycledin the potential range of 0.2–0.8 V using scan rate of 100 mV s−1 forstabilization of the CPZ in the nanocomposite.

2.4. General analytical procedure

Cyclic voltammetric and amperometric techniques were usedfor sulfide detection. The cyclic voltammogram (CV) of 10 mLphosphate buffer solution with pH 7 was used for background cor-rection. Then sulfide solution with different concentrations wasadded and their CVs were used for calibration curve construction.The CVs were recorded in the range of 0.3–0.85 V. The ampero-metric response of sulfide on SNPs/CPZ/Nf nanocomposite/GCE wasobtained by addition of successive aliquots of 100 nM sulfide inphosphate buffer solution (pH 7) under the constant potential of0.8 V.

3. Results and discussion

3.1. Morphological and electrochemical characterization ofSNPs/CPZ/Nf/GC electrode

Fig. 1 shows the TEM images of SiO2 nanoparticles (SNPs) andSNPs/CPZ/Nf nanocomposite film on the surface of glassy carbonelectrode (GCE) revealing the attachment of nanocomposite film

N. Amini et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 245–253 247

s/CPZ

dtf

isctfmq

lfpseCistsGGtbta

3m

obricto

Fig. 1. The TEM of SNPs (a) and SNP

irectly on the glassy carbon electrode. As shown, silica nanopar-icles are uniformly distributed with an average diameter rangingrom 20 to 30 nm.

In an attempt to clarify the differences in the electrochem-cal performance of the electrodes, electrochemical impedancepectroscopy (EIS) was employed as a technique for the electro-hemical characterization of each electrode surface. Fig. 2 showshe impedance spectra represented as Nyquist plots (Zim vs. Zre)or bare GCE (Fig. 2a), and GCE/Nf/SiO2NPs (Fig. 2c) composite film

odified electrode in 5 mM Fe(CN)63−/4− at 0.35 V and in the fre-

uency range of 0.01–104 Hz.In the EIS, the semicircle part corresponds to electron transfer

imited process and its diameter is equal to the electron trans-er resistance, Rct, that controls electron transfer kinetics of redoxrobe at the electrode interface. The electrochemical impedancepectroscopy data of the two electrodes were fitted on a simplequal circuit, which is shown in Fig. 2a (inset). In this circuit, Rs,

and Rct represent solution resistance, the double layer capac-tance and the charge transfer resistance, respectively. It can beeen from the Nyquist plots (Fig. 2) that semicircle of bare elec-rode (Rct = 1.859 k� curve a) characteristics of a diffusion limitingtep of the electrochemical process, decreases for GC/MWCNT andCE/CPZ/Nf/SiO2NPs electrodes (Rct = 1.033 k� curve b). When theC electrode was modified with CPZ/Nf/SiO2NPs nanocomposite,

he EIS curve’s diameter (curve b) became smaller than that of theare GC electrode. These results suggest that electron transfer inhe CPZ/Nf/SiO2NPs is easier between solution and the electrode,nd thus CPZ promotes electron transfer.

.2. Electrochemical behavior of the SNPs/CPZ/Nf nanocompositeodified GC electrode

The one electron quasi-reversible mechanism of electro-xidation of CPZ and its use as a homogeneous mediator haveeen reported previously [18,19]. Till now there have been noeports of using the CPZ as an electrode material to develop chem-

cally modified electrode (CME). In the current work the CPZ washosen as a mediator for construction of a novel modified elec-rode for monitoring of sulfide ions. The cyclic voltammogramsf SNPs/CPZ/Nf/GC modified electrode in phosphate buffer

/Nf nanocomposite (b) respectively.

solution (pH 2) were recorded at different scan rates (from 10to 100 mV s−1) (not shown). The experimental results showed awell-defined anodic and cathodic peaks related to CPZ/CPZ•+ redoxcouple with quasireversible behavior. CPZ shows a redox coupleat about 0.45 and 0.57 V versus Ag/AgCl at experimental condi-tions. The value of formal potential [E◦′ = (Epa + Epc)/2] was 0.51 Vat scan rate of 100 mV s−1. Both anodic and cathodic peak currentsincreased along with the increase in scan rate. Moreover, the peakpotential of the waves is practically independent of the scan rate.Thus, the overall redox process confined at the electrode surfacecan be considered to be relatively fast on the voltammetric timescale. At these sweep rates, the peaks current became proportionalto the square root of the scan rate, indicating a diffusion controlledprocess which is related to the relatively slow diffusion of counterions into the electrode surfaces.

The peak to peak potential separation (�Ep) is about 70 mVfor sweep rate below 1000 mV s−1, suggesting facile charge trans-fer kinetics over this range of sweep rate. At higher sweep rates(� > 1000 mV s−1), the peak separation begins to increase indicat-ing limitation due to charge transfer kinetics (not shown). Basedon Laviron’s theory [58], the electron transfer rate constant (Ks)as well as the transfer coefficient (˛) can be determined by mea-suring the variation of peak potential with scan rate. The values of�Ep = (Ep − E◦′) were proportional to the logarithm of the scan rateshigher than 1000 mV s−1 (Fig. 3).

The calculated values of Ks and ̨ for a SNPs/CPZ/Nf nanocom-posite modified GC electrode in buffer solution (pH 2) were about0.025 s−1 and 0.5, respectively. Since the electron transfer rateconstant for the CPZ/CPZ•+ couple is high, the CPZ doped in thenanocomposite is an excellent electron transfer mediator and itcan be used for electrocatalytic process. The surface concentra-tion of electroactive species � c can be approximately calculatedfrom the slop plot of peak current versus scan rate (not shown).For a reversible surface reaction, the peak current is given by thefollowing equation [59]:

Ip = n2F2vA�c (1)

4RT

where v is the sweep rate, A is the effective surface area(0.0314 cm2) of the modified electrode and the other symbols havetheir usual meaning. From the slope of the anodic peak current

248 N. Amini et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 245–253

b) bar

vn

3n

ebsCo1

Fig. 2. Electrochemical impedance spectroscopy(EIS) of (a) equal circuit; (

ersus scan rate plot, the surface concentration (� c) of SNPs/CPZ/Nfanocomposite was found to be 1.26 × 10−6 mol cm−2 for n = 1.

.3. Stability and pH dependence of the SNPs/CPZ/Nfanocomposite modified GC electrode

The stability and reproducibility of the modified electrodes werexamined by repetitive recording of the cyclic voltammograms inuffer solution (pH 2) at a scan rate of 100 mV s−1. After 100 succes-

ive scans, the peak height and peak potential of the immobilizedPZ remained nearly unchanged and the amount of CPZ remainingn the electrode surface were almost 95% of its initial value after00 cycles. In addition, no significant decrease was seen when the

e GC electrode; (c) SNPs/CPZ/Nf/GC electrode in 5 mM probe Fe(CN)64−/3− .

old electrolyte (after 100 cycles) was replaced by a new fresh one.The electrochemical behavior of CPZ/Nf/GC electrode in phosphatebuffer with pH 2 was also investigated by cyclic voltammetry. Noredox peaks were observed in the resulted CV indicating that in theabsence of silica NPs the immobilized film (CPZ/Nf) is not stable.Therefore, to overcome this problem and improve the stability ofthe electrode, silica NPs was added to the composite film.

One of the variables that usually influence the shape of voltam-mograms is solution pH and hence the effect of this parameter was

investigated. Cyclic voltammograms of SNPs/CPZ/Nf/GC electrodein 0.05 M phosphate buffer solution at different pH ranging from 2to 10 were recorded. A pair of well defined and stable redox peakswas obtained for modified electrode in solutions with pHs lower

N. Amini et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 245–253 249

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0 1 2 3 4 5 6 7 8

log(v/mVs-1)

E p(v

)

y = 0.0615x - 0.1325R2 = 0.982 2

y = -0.0615x + 0.132 5R2 = 0.982 2

-0.18

-0.13

-0.08

-0.03

0.02

0.07

0.12

0.17

2.5 3 3.5 4 4.5

Log(v/mVs-1)

E p(v

)

∆

∆

modi

tolamswn

3n

aHmetopscu

Fig. 3. The variation of �Ep(Ep − E◦) versus log v for the

han 9. The shift in both reduction and oxidation peaks potentialf the CPZ/CPZ•+ couple at the SNPs/CPZ/Nf/GC electrode was neg-igible, which means that the voltammetric behavior of CPZ is not

proton transfer process under experimental conditions. Further-ore, no change was observed in peaks current when the pH of

olution was varied from 4 to 8. At pHs greater than 9 no redox peaksere observed that may be due to instability of the SNPs/CPZ/Nfanocomposite in basic media.

.4. Electrocatalytic oxidation of sulfide on SNPs/CPZ/Nfanocomposite modified GC electrode

Since sulfide ion can exist in various forms of H2S, HS− and S2−

t different pH in solution (pKa1 = 6.96 and pKa2 = 11), therefore theS− ion is the predominant species that are actively involved in theechanism in neutral pH [60,61]. As it was mentioned above the

lectrochemical behavior of SNPs/CPZ/Nf/GCE is pH independenthus, the cyclic voltammetric study of sulfide oxidation was carriedut in a solution with pH 7.0. In the presence of 6 �M sulfide and

otential CPZ/CPZ•+ redox couple on the SNPs/CPZ/Nf/GC electrodeurface and are consistent with a strong electrocatalytic effect. Foromparison, a bare GCE was also subjected to the sulfide oxidationnder similar experimental conditions. No peak was observed at

fied GCE. Inset is the same plots at higher sweep rates.

GCE in this potential window which might be due to the existenceof high over potential associated with sulfide oxidation (Fig. 4).

It is not possible to compare the behavior of the CPZ modifiedelectrode in Nafion without silica NPs because immobilization ofCPZ in the absence of silica NPs is not stable and can immediatelybe realized into the solution as in the absence of silica NPs we didnot observed any cathodic or anodic peak during CV recording. Weimmobilized CPZ into Nafion–silica composite films.

In regards the electrostatic interaction (repulsion force)between the sulfonic groups of Nafion and the anionic species ofthe analyte (S2−), CPZ can be easily oxidized and produce a colorcation radical which can cancel the negative charge of sulfonicgroups of Nafion. Therefore, the diffusion of the anionic speciestowards the surface of the electrode is possible. These results indi-cate that the SNPs/CPZ/Nf nanocomposite is a suitable mediator toshuttle electrons between sulfide and working electrode and facil-itate electrochemical regeneration following electrons. The effectof pH on the electrocatalytic response of the modified electrodeto sulfide oxidation was investigated. The cyclic voltammogramsof the modified electrode in 6 �M sulfide solution at different pH

ranging from 2 to 10 were recorded and the results showed that thepeak current was pH independent in pHs lower than 8 and electro-catalytic activity of the electrode was diminished at pHs greaterthan 8. Therefore, neutral pH 7 was selected for subsequent uses.

250 N. Amini et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 245–253

d

c

b

a

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.3 0.5 0.7 0.9Potential(v) vs .Ag/AgCl

Cur

rent

(A

)µ

F er solu(

Fe

mra

F(

ig. 4. Cyclic voltammograms of SNPs/CPZ/Nf nanocomposite modified GCE in buffd), (a) and (b) are same results as (c) and (d) for bare GC electrode.

urthermore, the variation in the solution pH had no significantffect on the peak potential of CPZ.

The effect of increasing amounts of sulfide on the cyclic voltam-

ogram of SNPs/CPZ/Nf modified electrode was also studied. The

esults showed that the catalytic peak current which was centeredt 0.67 V increased linearly with sulfide in the concentration range

ig. 5. (a) Cyclic voltammetry response of a GCE modified with SNPs/CPZ/Nf nanocompofrom inner to outer) 10–100 mV s−1. (b) Plot of Ip versus v1/2. (c) Plot of Ep versus log v.

tion pH 7 at scan rate of 10 mV s−1 in the absence (c) and presence of 6 �M sulfide

of 1.0–6.0 �M with an excellent correlation coefficient (R2 = 0.993)and sensitivity of 0.08 �A �M−1. The detection limit (0.49 �M) andthe determination limit (1.85 �M) were calculated as the concen-

tration that gave a signal to noise of 3 and 10, respectively.

Fig. 5 shows the cyclic voltammograms of 4 �M of sulfide solu-tion at different scan rates. The peak current for the oxidation of

site in a phosphate buffer (pH 7) containing 5 �M of sulfide at different scan rates

N. Amini et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 245–253 251

F E at 0.a

srttr

C

C

ale

E

tacorw7s

TA

ig. 6. Amperommetric responses of the SNPs/CPZ/Nf nanocomposite modified GC shows plot of the peak currents as a function of concentration.

ulfide is proportional to the square root of the scan rate in theange of 10–100 mV s−1 (Fig. 5b) suggesting that the process is con-rolled by diffusion as expected for a catalytic system [66]. Based onhe obtained results, the following catalytic scheme describes theeaction sequence in the oxidation of sulfide with CPZ mediator:

PZ − e− � CPZ•+ (2)

PZ•+ + sulfide → CPZ + sulfurproduct (3)

In order to get information regarding the rate-determining step, Tafel plot was used. The Tafel slope (b) was obtained from theinear relationship between Ep and log v according to the followingquation [62]:

p = b(log v)2

+ const (4)

Based on Fig. 5c the b value was found to be 49.2 mV, indicatinghat one electron process was involved in the rate limiting step byssuming a charge transfer coefficient of ̨ = 0.5. Under the aboveonditions for an EC′ mechanism, the Andrieux and Savent the-retical model [63] was used to calculate the constant catalytic

ate. According to the Andrieux and Savent approach and usingorking curve (Fig. 1 of Ref. [63]), the average value of Kcat was

.6 × 102 M−1 S−1 indicating the potential catalytic activity of theensor for sulfide monitoring.

able 1nalytical parameters of several modified electrodes for determination of sulfide.

Electrode Method

Cobalt pentacyanonitrosylferrate/glassy carbon electrode Cyclic voltammetry

Horseradish peroxidase Amperometry

Nickel powder/sol–gel carbon ceramic electrode Amperometry

Homogeneous mediator Tris(2,2′-bipyridyl)Ru(II) Linear sweep voltammFerrocene sulfonates Cyclic voltammetry

Nickel electrodes Cyclic voltammetry

Carbon nanotube modified electrodes Amperometry

Au nanoclusters/glassy carbon electrode Cyclic voltammetry

Chloropromazine–SiO2NPs–Nafion/glassy carbon electrode Amperometry

8 V upon successive addition of 100 nM sulfide in to phosphate buffer (pH 7). Inset

3.5. Amperometric response to sulfide

Fig. 6 displays a typical amperometric response of sulfide onSNPs/CPZ/Nf nanocomposite/GCE sensor with the addition of suc-cessive aliquots of 100 nM sulfide in phosphate buffer solution (pH7) under the constant potential of 0.8 V. The fabricated sensor yieldsa rapid and sensitive response to each injection of sulfide, a risein current followed by a steady-state value (Iss) within 28 s. Theplot of Iss versus sulfide concentration (inset, Fig. 6) depicted a lin-ear relationship from 100 to 600 nM with regression equation ofI(�A) = 0.0019[sulfide]nM + 0.1536 (R2 = 0.9906). Based on signal tonoise ratio (S/N) of 3, the detection limit was estimated to be 90 nM.

3.6. Comparison of the figures of merit of the proposed sensorwith those of previous electrochemical methods

Table 1 compares, the limit of detection, sensitivity, and opti-mized pH of the proposed sensor for sulfide determination withthose of the other electrodes that have been reported previously[64–71]. As it is obvious from Table 1, the proposed electrode isone the most sensitive electrode which can be used for nanomo-

lar detection of sulfide. On the other hand, the limit of detection ofthe SNPs/CPZ/Nf electrode is ten time lower that the most sensitiveelectrodes [65,70] presented in Table 1. Furthermore, the applica-bility of the sensor in neutral medium is the other advantage of

pH LOD Sensitivity Refs.

5 23 �M – [64]6.5 0.3 �M – [65]13 1.9 �M 7.58 �A/�M [66]

etry 10 0.33 �M – [67]6.8 14 �M 0.005 �A/�M [68]13 19 �M – [69]7.4 0.3 �M 0.115 �A/�M [70]Toluene and methanol 13.2 �M 6.2 �A/�M [71]7 90 nM 0.0019 �A/nM This work

252 N. Amini et al. / Journal of Molecular Cataly

Table 2The analysis of water samples.

Added (�M) Found (�M) Recovery (%) RSD (%)

Tap water2 2.1 105 6.33 3.3 1106 5.8 97

Wastewater2 2.2 110

ts

3

StantSdtrsMItmp

3

Swmrstops%r

3

ctwseftreatm

[[

[

[[[[

[

[

[[[

[

[[

[

[[

[[[[

[

[

[

[[[[

[J. Chromatogr. A 1084 (2005) 101–107.

3 3.1 103 8.36 5.6 93

he proposed electrode. Therefore, the monitoring of sulfide in realamples with neutral pH such as waters can be possible.

.7. Interference effects

Selectivity is an important factor in the performance of aNPs/CPZ/Nf based sensor. The influence of various substances con-aining sulfur atom such as SO3

2−, SO42− and S2O4

2− and somenions and cations that may be presented in real samples such asatural and wastewaters as potential interference compounds onhe determination of sulfide under the optimum conditions withNPs/CPZ/Nf/GCE at pH 7 was studied. The tolerance limit wasefined as the maximum concentration of the interfering substancehat caused an error less than 3% for determination of sulfide. Theesults showed, the peak current of the proposed electrode is notignificantly affected by all conventional cations (Na+, Li+, K+, Ca2+,g2+, Pb2+and Cd2+), anions (SO4

2−, IO3−, PO4

3−, I−, Cl−, Br−, F−,O4

− and BrO3−) when their concentration are 50 times greater than

hat of sulfide ion. But SO32− and S2O4

2− at the same concentrationentioned above show interfering effect and increase the catalytic

eak current of sulfide.

.8. Repeatability and lifetime

In order to examine the stability and lifetime of theNPs/CPZ/Nf/GC electrode towards sulfide, the response currentas monitored every day for one month. After two weeks theodified electrode retained about 91% of sulfide initial sensitivity,

epresenting the modified electrode has good stability towards theulfide detection. After four weeks the electrode responses droppedo 73% of the initial current. The results implied that the stabilityf the proposed sensors was acceptable. The repeatability of theroposed sensor was also evaluated by cyclic voltammetry mea-urements of 5 �M of sulfide. The relative standard deviation (RSD) for five successive assays was 5%, which revealed an excellentepeatability for the designed electrochemical system.

.9. Application

In order to evaluate the performance of SNPs/CPZ/Nf glassyarbon electrode, in practical analytical applications, quantita-ive measurement of sulfide ion in Kurdistan (Saghez) water, andastewater was attempted. Reliability was checked by spiking the

ample and the accuracy of the method was examined by recoveryxperiment. No pretreatment for the wastewater was done exceptor dilution with 0.1 mol L−1, pH 7.0 phosphate buffer. The con-ent of sulfide was determined by using calibration curve and theesults are shown in Table 2. As clearly shown, the obtained recov-ries are in the range of 97–105% with RSD = 6.3% for drinking water

nd 93–110% with RSD = 8.3% for wastewater. It was concluded thathe modified electrode may have applications in selective sulfide

onitoring in water samples.

[[

[

sis A: Chemical 396 (2015) 245–253

4. Conclusion

A new composite material modified electrode asSNPs/CPZ/Nf/GCE was prepared by loading SNPs/CPZ/Nf com-posite on the surface of GC electrode. Compared with GC andSNPs/Nf/GC electrodes, the combination of unique properties ofSiO2 nanoparticle and CPZ resulted in the improvement of both thereversibility and current responses of SNPs/CPZ/Nf/GC electrode.The GC electrode modified with a thin film of SNPs/CPZ/Nf showsstable and reproducible electrochemical behavior, long stabilityand excellent electrochemical reversibility. This modified electrodemakes evident excellent catalytic activity for sulfide oxidation.Some kinetic parameters, such as the electron transfer coefficientand the catalytic rate constant Kcat of the catalytic reaction werecalculated. The proposed sensor was used for determination of lowlevels of sulfide by using the amperometric method.

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