Journal of Colloid and Interface Science 421 (2014) 78–84

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Gold nanoparticles decorated on cobalt porphyrin-modified glassy carbon electrode for the sensitive determination of nitrite ion Palanisamy Muthukumar, S. Abraham John ⇑ Centre for Nanoscience and Nanotechnology, Department of Chemistry, Gandhigram Rural Institute, Gandhigram 624 302, Dindigul, India

a r t i c l e

i n f o

Article history: Received 14 November 2013 Accepted 23 January 2014 Available online 31 January 2014 Keywords: Cobalt porphyrin Self-assembly AuNPs decorated glassy carbon electrode Nitrite ion

a b s t r a c t The present study reports the electrochemical determination of nitrite ion using citrate-gold nanoparticles (cit-AuNPs) decorated on meso-tetra(para-aminophenyl)porphyrinatocobalt(II) (Co(II)MTpAP) self-assembled glassy carbon electrode (GCE). The decoration of cit-AuNPs on Co(II)MTpAP was achieved with the aid of amine groups present on the surface of the self-assembled monolayer (SAM) of Co(II)MTpAP. The SEM image shows that the cit-AuNPs were densely packed on Co(II)MTpAP. The AuNPs decorated electrode was successfully used for the determination of nitrite ion. The cit-AuNPs decorated electrode not only shifted nitrite ion oxidation potential towards less positive potential but also greatly enhanced its current when compared to bare and Co(II)MTpAP SAM modified electrodes. The amperometric current increases linearly while increasing the concentration of nitrite ion ranging from 0.5  106 to 4.7  103 M and the detection limit was found to be 60 nM (S/N = 3). Further, the modified electrode was successfully used to determine nitrite ion in the presence of 200-fold excess of common 2 2 þ  + 2+  interferents such as Na+, NO 3 , I , K , CO3 , Ca , SO4 , NH4 , Cl and glucose. The practical application of the cit-AuNPs decorated electrode was demonstrated by determining nitrite ion in water samples. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction Nitrite ions are widely used as corrosion inhibitor [1], food additive [2] and fertilizer [3]. Because of its more usage in domestic life, nitrite ion made negative impact in both environment and biological processes. According to World Health Organization, the maximum permissible amount of nitrite ion in drinking water is 50 mg/l [4]. Nitrite ion can oxidize haemoglobin to metahaemoglobin and hence the blood oxygen carrying capacity markedly reduced [5]. Further, it interacts with dietary compounds in stomach to form carcinogenic nitrosamines [6]. The major health issues caused by this ion are blue baby syndrome and gastric cancer [6]. Therefore, an accurate determination of nitrite ion is very essential. Several techniques have been employed to determine nitrite ion which includes spectrophotometry [7–9], chromatography [10,11] and voltammetry [12–15]. The conventional spectrophotometric determination of nitrite ion is based on the absorbance of azo dye measured at 526 nm which is formed by the reaction of nitrite with sulphonamide and N-(1-napthyl)ethylene diamine [7,8]. However, this method has serious limitations such as controlling ⇑ Corresponding author. Fax: +91 451 245 3031. E-mail address: [email protected] (S. Abraham John). http://dx.doi.org/10.1016/j.jcis.2014.01.030 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.

of acidity in each step, toxicity of reagents used, more time consumption and the possible interferents of strong oxidants in the sample [7,8,16]. Similarly, the chromatographic determination of nitrite ion has several disadvantages which include tedious sample preparation, requires trained personnel and derivatisation [10,11]. On the other hand, the electrochemical determination of nitrite ion is simple, faster, cheaper and safer when compared to the above two methods. The nitrite ion can be determined either by electrochemical oxidation or reduction [12–15]. The determination of nitrite ion by reduction leads to the interference of molecular oxygen and nitrate [14]. However, the determination of nitrite ion by oxidation often free from these two interferences. Although nitrite ion is electroactive at carbon electrodes, its oxidation needs high overvoltage and hence the oxidizable compounds interfere with the sample [16]. Thus, it is necessary to modify the electrode with suitable material to oxidize the nitrite ion at less overpotential. It has been well-documented in the literature that the electrodes modified with metalloporphyrin and metallophthalocyanine complexes can be used as electrocatalysts [17–19]. The electrochemical oxidation of nitrite ion at metalloporphyrin and metallophthalocyanine modified electrodes has been studied using different strategies such as drop casting [20–23], electropolymerization [24–28], adsorbed on SiO2/SnO2/phosphate modified electrode [29] and layer by layer adsorption [22]. It is well known

P. Muthukumar, S. Abraham John / Journal of Colloid and Interface Science 421 (2014) 78–84

that self-assembly is a suitable method for the preparation of ordered layer on electrode surface [30]. The self-assembly of porphyrin molecule on electrode surface can be achieved by functionalizing them with suitable functional groups [31,32]. Recently, Gallardo et al. reported the spontaneous adsorption of aryl and alkyl amines on glassy carbon surface via the nucleophilic addition of amine with olefinic GCE surface [33]. Based on the knowledge grasped from this paper, very recently, we have successfully self-assembled meso-tetra(para-aminophenyl)porphyrinatocobalt(II) (Co(II)MTpAP) on GCE surface [34]. In recent years, the modification of electrode surface with gold nanoparticles (AuNPs) has been received much interest due to their greater electrocatalytic activity when compared to bulk gold [35]. Several strategies have been employed to immobilize the AuNPs including electrodeposition [36], self-assembly [37,38] and seed mediated growth technique [39]. The self-assembly of AuNPs can be achieved on the surface with suitable functional groups including thiol and amine [37,38]. Although the self-assembly method was successfully used for the immobilization of AuNPs, the complete surface coverage was not achieved due to the interparticle repulsion between the attached AuNPs and AuNPs in solution. For example, only 10% coverage of AuNPs was achieved at SAM terminated with alkyl amine functional group and 0.36% and 2.56% of particle coverage of AuNPs were obtained at alkanedithiol modified electrodes [37,38]. In order to achieve higher surface coverage, it is necessary to find out a suitable linker. In the present study, we have used Co(II)MTpAP as a linker for the immobilization of AuNPs on GCE. In Co(II)MTpAP, four amine groups are present in the meso phenyl ring. Based on the XPS and CV studies, we found that free amine groups were available on the surface of the SAM [34]. In the present study, we have attached AuNPs on the Co(II)MTpAP SAM with the aid of free amine groups. The attachment of AuNPs on Co(II)MTpAP was confirmed by UV–vis spectroscopy, SEM and cyclic voltammetry. The SEM image showed that cit-AuNPs were densely packed on the SAM modified electrode. Finally, the cit-AuNPs decorated Co(II)MTpAP SAM modified electrode was used for the determination of nitrite ion with wide range of concentration. 2. Materials and methods 2.1. Chemicals Pyrrole, 4-nitrobenzaldehyde, boron trifluoride etherate complex (BF3-Et2O), HAuCl43H2O, NaBH4 and trisodium citrate were purchased from Aldrich, India. CoCl26H2O, SnCl22H2O and NaNO2 were purchased from Merck, India. All other chemicals used in this investigation were of analytical grade. Indium tin oxide (ITO) plates were purchased from Asahi Beer Optical Ltd., Japan. Millipore water was used for all the experiments. Phosphate buffer solution (PBS) (pH 7.2) was prepared by using NaH2PO4 and Na2HPO4. 2.2. Synthesis of meso-tetra(para-aminophenyl)porphyrinatocobalt(II) (Co(II)MTpAP) Meso-tetra(para-aminophenyl)porphyrin (MTpAP) was synthesized by the reported procedure and purified by column chromatography on silica gel (60–120 mesh) using dichloromethane–methanol mixture as eluent [40]. The product was confirmed by 1H NMR spectroscopy and ESI-Mass (Figs. S1 and S2). The Co(II)MTpAP was obtained by stirring CoCl26H2O with MTpAP in chloroform and methanol mixture at room temperature for 12 h. The metallated product was confirmed by UV–vis spectroscopy (Fig. S3).

79

2.3. Synthesis of citrate capped gold nanoparticles (cit-AuNPs) All glassware was thoroughly cleaned with freshly prepared aqua regia and rinsed with double-distilled water before use. A colloidal solution of cit-AuNPs was prepared by the reported procedure [41]. Twenty-five milligram of HAuCl44H2O in 83 ml of Millipore water (0.83 mM) was boiled with vigorous stirring in a round-bottom flask fitted with reflux condenser and 8.75 ml of 1% (w/v) trisodium citrate solution was then added rapidly to the flask. The solution was boiled for another 15 min, during which the colour of the solution was changed from pale yellow to deep red. The solution was allowed to cool at room temperature with continuous stirring and stored at 4 °C. The formation of cit-AuNPs was confirmed by UV–vis spectroscopy and TEM (Figs. S4 and S5). 2.4. Preparation of the modified electrodes The GC working electrode was polished with alumina slurry (0.5 lm) and sonicated in Millipore water for 10 min and the surface of the polished electrode was checked with 1 mM K3[Fe(CN)6] in 0.1 M KCl. The SAM of Co(II)MTpAP formed by immersing the well cleaned GCE into the vials containing 1 mM of Co(II)MTpAP in dimethylformamide (DMF) for 6 h. The electrode was then removed from the solution and washed with DMF and Millipore water. For the decoration of cit-AuNPs, the Co(II)MTpAP SAM modified electrode was immersed into the colloidal solution of cit-AuNPs for 6 h. Then, the electrode was washed with Millipore water and used for the electrochemical measurements. The Co(II)MTpAP modified electrode is abbreviated as GCE/Co(II)MTpAP and citAuNPs decorated electrode as GCE/Co(II)MTpAP/cit-AuNPs. 2.5. Instrumentation UV–vis spectra were recorded with a Perkin Elmer Lambda 35 spectrophotometer. The 1H NMR was measured using BRUKER 300 MHz instrument. Electrochemical measurements were performed in a conventional two-compartment three-electrode cell with a GCE of 3 mm diameter (area = 0.07 cm2) as working electrode, a platinum wire as auxiliary electrode and a NaCl saturated Ag/AgCl as reference electrode. All the electrochemical measurements were carried out with CHI Model 650B (Austin, TX, USA) Electrochemical workstation. ESI-Mass analyses were recorded using THERMO-FINNIGAN LCQ ADVANTAGE MAX mass spectrometer. High resolution transmission electron microscopy (HR-TEM) images were measured from a JEOL JEM 3010 operating at 200 kV. The samples were prepared by dropping 2 lL of a colloidal solution onto a carbon-coated copper grid. SEM measurements were carried at VEGA3 TESCAN. 3. Results and discussion 3.1. Spectral and SEM characterization of cit-AuNPs decorated electrode Fig. 1 shows the UV–vis spectra obtained for bare ITO, the SAM of Co(II)MTpAP and cit-AuNPs decorated ITO substrates. No characteristic band was observed for bare ITO (curve a) whereas Co(II)MTpAP SAM modified substrate shows a Soret band at 452 nm and two Q-bands at 554 and 598 nm (curve b). On the other hand, the cit-AuNPs decorated substrate shows an absorbance band at 562 nm in addition to a Soret band at 452 nm (curve c). The absorbance band at 562 nm corresponds to the surface plasmon resonance (SPR) band of cit-AuNPs. This confirms that the citAuNPs were successfully attached on the SAM of Co(II)MTpAP modified substrate. In contrary, the Soret band of Co(II)MTpAP in

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P. Muthukumar, S. Abraham John / Journal of Colloid and Interface Science 421 (2014) 78–84

Co(II)MTpAP modified substrates (images a and b), cit-AuNPs decorated on Co(II)MTpAP modified substrate showed the presence of densely packed spherical AuNPs (image c). The size of the citAuNPs decorated on Co(II)MTpAP modified substrate was found to be 40 nm. When compared to the size of the colloidal cit-AuNPs (13 ± 2 nm; Fig. S5), the size of the cit-AuNPs on Co(II)MTpAP modified surface was increased, which is due to the multipoles induced in AuNPs on the ITO surface [43].

c

Absorbance

0.1

3.2. Electrochemical characterization of cit-AuNPs decorated electrode

b a

0 400

600

800

1000

Wavelength (nm) Fig. 1. UV–vis spectra of (a) bare ITO, (b) ITO/Co(II)MTpAP and (c) ITO/Co(II)MTpAP/ cit-AuNPs.

DMF and the SPR band of colloidal cit-AuNPs were observed at 445 and 524 nm, respectively (Figs. S3 and S4) indicating that both Soret and SPR bands were red shifted when compared to their solution spectrum. The red shift of the Soret band is due to the electronic interaction between the adsorbed molecules whereas SPR band is due to the electromagnetic interaction between the assembled AuNPs and the substrate [42,43]. SEM was used to examine the size and morphology of the cit-AuNPs decorated on Co(II)MTpAP modified surface. Fig. 2 shows the SEM images obtained for bare, SAM of Co(II)MTpAP and cit-AuNPs decorated on Co(II)MTpAP modified ITO substrates. When compared to the SEM images of bare and

The decoration of cit-AuNPs on Co(II)MTpAP SAM modified electrode was also confirmed by cyclic voltammetry. The selfassembly of Co(II)MTpAP on GCE surface was due to nucleophilic addition to the olefinic bond of the GCE [33,34]. The Co(II)MTpAP contains four amine groups. The XPS and CV studies confirm that not all the amine groups were involved in SAM formation on GCE [34]. Thus, we have utilized Co(II)MTpAP SAM as a linker for the decoration of cit-AuNPs in the present study. The cyclic voltammograms (CVs) obtained for the Co(II)MTpAP SAM modified electrode at different time intervals immersed into cit-AuNPs at a scan rate of 0.1 V s1 in 0.2 M PBS (pH 7.2) are shown in Fig. S6. The CV obtained for cit-AuNPs decorated on GCE/Co(II)MTpAP after 1 h shows a oxidation peak at 0.88 V and a reduction peak at 0.35 V (curve a). The peak at 0.88 V is due to the oxidation of Au into Au oxide and the peak at 0.35 V is due to the reduction of Au oxide. While increasing the immersion time, the charge under the reduction of Au oxide was increased (curves b and c) up to 6 h and after that the charge under the reduction of Au oxide remains unchanged (curve d). The coverage of cit-AuNPs on GCE/Co(II)MTpAP at different immersion time intervals was calculated (Eq. (1)) using the charge involved in the reduction of electrochemically formed

(b)

(a)

500 nm

500 nm

(c)

500 nm

Fig. 2. SEM images of (a) bare ITO, (b) ITO/Co(II)MTpAP and (c) ITO/Co(II)MTpAP/cit-AuNPs.

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Au oxide by assuming the charge density for the reduction peak of the Au oxide is 723 lC/cm2 [44].

hp ¼

Au oxide reduction charge ðlCÞ=723 ðlC=cm2 Þ  100 Geometric area of GCE ðcm2 Þ

ð1Þ

with increased current. This illustrates that GCE/Co(II)MTpAP/citAuNPs is a better electrocatalyst for nitrite ion oxidation than bare GCE and GCE/Co(II)MTpAP. According to Guidelli, the oxidation of nitrite ion at neutral medium leads to the formation of nitrate ion as shown in Eqs. (2) and (3) [45].

The AuNPs coverage of cit-AuNPs decorated on Co(II)MTpAP modified electrode at different immersion time intervals is given in Table S1. The highest AuNPs coverage was obtained after 6 h immersion. Thus, we have fixed immersion time of 6 h for the decoration of cit-AuNPs on Co(II)MTpAP SAM modified electrode for all the electrochemical measurements.

2NO2 $ 2NO2 þ 2e

ð2Þ

2NO2 þ H2 O ! NO3 þ NO2 þ 2Hþ

ð3Þ

3.3. Electrochemical oxidation of nitrite ion at GCE/Co(II)MTpAP/citAuNPs

In order to find out the influence of cobalt ion towards nitrite ion oxidation at the GCE/Co(II)MTpAP/cit-AuNPs, we have performed the electrochemical oxidation of nitrite ion at cit-AuNPs decorated on free base porphyrin (MTpAP) modified electrode (GCE/MTpAP/cit-AuNPs). Fig. 4 shows LSVs obtained for the electrochemical oxidation of 0.5 mM nitrite ion in 0.2 M PBS (pH 7.2) at GCE/Co(II)MTpAP/cit-AuNPs and GCE/MTpAP/cit-AuNPs at a scan rate of 0.05 V s1. Both GCE/Co(II)MTpAP/cit-AuNPs (curve a) and GCE/MTpAP/cit-AuNPs (curve b) showed the nitrite ion oxidation peak at 0.80 V. Although the oxidation of nitrite ion occurs at the same potential at both the electrodes, the nitrite ion oxidation current was higher at GCE/Co(II)MTpAP/cit-AuNPs than GCE/MTpAP/cit-AuNPs. This confirms that in addition to cit-AuNPs the cobalt ion was also involved in the oxidation of nitrite ion. It has been already reported that the insertion of metallic ions increases the conductivity of porphyrin macrocyclic ring, which results the efficient electron transport through the SAM via tunnelling mechanism [46,47]. Similar efficient electron transport would also be expected at the GCE/Co(II)MTpAP/cit-AuNPs electrode. Thus, we obtained higher electrocatalytic activity for nitrite ion at cit-AuNPs decorated on Co(II)MTpAP in contrast to MTpAP.

The electrochemical oxidation of nitrite ion at cit-AuNPs decorated electrode was investigated by linear sweep voltammetry (LSV). Fig. 3 shows the LSVs obtained for the oxidation of 0.5 mM nitrite ion in 0.2 mM PBS (pH 7.2) at bare GCE, GCE/Co(II)MTpAP and GCE/Co(II)MTpAP/cit-AuNPs at a scan rate of 0.05 V s1. The oxidation of nitrite ion at bare GCE and GCE/Co(II)MTpAP is obtained at 0.96 and 0.92 V, respectively (curves a and b) and the oxidation current was decreased in the subsequent cycles with the positive shift of oxidation potential at both the electrodes. After five cycles, the oxidation potential of nitrite ion was 60 mV shifted towards more positive potential at both the electrodes (curves a0 and b0 ). On the other hand, GCE/Co(II)MTpAP/cit-AuNPs shows a sharp oxidation peak for nitrite ion at 0.80 V (curve c). Further, the oxidation current remains same without shifting of the potential even after five cycles (curve c0 ). The observed broad wave for the oxidation of nitrite ion at bare GCE and GCE/Co(II)MTpAP indicates the sluggish electron transfer of nitrite ion. On the other hand, the sharp oxidation peak at GCE/Co(II)MTpAP/cit-AuNPs infers the fast electron transfer of nitrite ion at this electrode. Further, GCE/Co(II)MTpAP/cit-AuNPs shows an oxidation peak at 0.88 V (curve d) in the absence of nitrite ion in 0.2 M PBS (pH 7.2) with less oxidation current, which is due to Au oxide formation. This confirms that the oxidation peak observed at 0.80 V in GCE/Co(II)MTpAP/cit-AuNPs (curve c) is due to the oxidation of nitrite ion. When compared to bare GCE and GCE/Co(II)MTpAP, GCE/ Co(II)MTpAP/cit-AuNPs shifted the nitrite ion oxidation potential towards less positive potential of 160 and 120 mV, respectively

c

3.4. Effect of central metal ion

3.5. Effect of particle coverage of cit-AuNPs towards the oxidation of nitrite ion To find out the effect of particle coverage of cit-AuNPs towards the oxidation of nitrite ion, we have prepared the cit-AuNPs decorated on Co(II)MTpAP modified electrode at different time intervals. The particle coverage of cit-AuNPs calculated at different immersion time is given in Table S1. Fig. S7 shows the LSVs obtained for the oxidation of 0.5 mM nitrite ion in 0.2 M PBS (pH 7.2) at different particle coverage of cit-AuNPs decorated on

30

a 30

b c' b' a

Ι/μΑ

20

b

a' d

Ι/μΑ

10

20

10 0 0

1

E/V vs. Ag/AgCl (NaCl sat) Fig. 3. LSVs obtained for 0.5 mM NaNO2 in 0.2 M PBS (pH 7.2) at bare GCE (a) 1st and (a0 ) 5th cycles, GCE/Co(II)MTpAP (b) 1st and (b0 ) 5th cycles and GCE/ Co(II)MTpAP/cit-AuNPs (c) 1st and (c0 ) 5th cycles and (d) LSV obtained for GCE/ Co(II)MTpAP/AuNPs in the absence of NaNO2 in 0.2 M PBS (pH 7.2) at a scan rate of 0.05 V s1.

0 0

1

E/V vs. Ag/AgCl (NaCl sat) Fig. 4. LSVs obtained for 0.5 mM NaNO2 in 0.2 M PBS (pH 7.2) at (a) GCE/ Co(II)MTpAP/cit-AuNPs and (b) GCE/MTpAP/cit-AuNPs at a scan rate of 0.05 V s1.

P. Muthukumar, S. Abraham John / Journal of Colloid and Interface Science 421 (2014) 78–84

Co(II)MTpAP SAM modified electrode at a scan rate of 0.05 V s1. When the particle coverage of cit-AuNPs increases the oxidation current of nitrite ion was also increased and attained maximum at particle coverage of 43%. Further, to study the influence of macrocyclic Co(II)MTpAP towards the oxidation of nitrite ion, we have attached cit-AuNPs on GCE using hexadiamine (HDA) as a linker. Fig. S8 shows LSVs obtained for the electrochemical oxidation of 0.5 mM nitrite ion in 0.2 M PBS (pH 7.2) at GCE/HDA/cit-AuNPs and GCE/Co(II)MTpAP/cit-AuNPs at a scan rate of 0.05 V s1. The GCE/HDA/cit-AuNPs modified electrode shows an oxidation peak at 0.87 V (curve a), which is 70 mV more positive than the GCE/Co(II)MTpAP/cit-AuNPs electrode (curve b). Further, the GCE/HDA/cit-AuNPs modified electrode shows less oxidation current for nitrite ion oxidation than the GCE/Co(II)MTpAP/cit-AuNPs electrode. This is due to the less particle coverage of the former than the later. 3.6. Effect of scan rate Fig. S9 shows the LSVs obtained for 0.5 mM nitrite ion in 0.2 M PBS (pH 7.2) at GCE/Co(II)MTpAP/cit-AuNPs at different scan rates. The oxidation peak current of nitrite ion was increased while increasing the scan rate. A good linearity between the anodic current of nitrite ion and square root of scan rate was obtained with a correlation coefficient of 0.992 (Fig. S9, inset) at scan rates from 25 to 150 mV s1. This indicates that the oxidation of nitrite ion was a diffusion controlled process at GCE/Co(II)MTpAP/cit-AuNPs. To understand the fast electron transfer reaction of nitrite ion at GCE/Co(II)MTpAP/cit-AuNPs, we have calculated the standard heterogeneous rate constant (ks) for nitrite ion at bare GCE, GCE/ Co(II)MTpAP and GCE/Co(II)MTpAP/cit-AuNPs using Velasco Eq. (4) [48]. 1=2 1=2 ks ¼ 1:11D1=2 m 0 ðEp  Ep=2 Þ

ð4Þ

where ks is standard heterogeneous constant, D0 is the apparent diffusion coefficient, Ep is oxidation peak potential, Ep/2 is half-wave oxidation peak potential and m is the scan rate. In order to determine the ks it is necessary to find the diffusion coefficient for nitrite ion. The apparent diffusion coefficient (D0) value was determined by using single step chronoamperometric technique on the Cottrell p slope obtained by plotting 1/ time. Chronoamperometric measurements were carried out for nitrite ion at bare GCE, GCE/Co(II)MTpAP and GCE/Co(II)MTpAP/cit-AuNPs. Based on Cottrell, equation the diffusion coefficient of nitrite ion was found to be 1.08  105, 2.24  105 and 1.34  104 cm2 s1 at bare GCE, GCE/Co(II)MTpAP and GCE/Co(II)MTpAP/cit-AuNPs, respectively [49]. The calculated heterogeneous rate constant (ks) values for the irreversible oxidation of nitrite ion at bare GCE, GCE/Co(II)MTpAP and GCE/Co(II)MTpAP/cit-AuNPs were found to be 2.17  103, 3.12  103 and 1.09  102 cm s1, respectively. The obtained higher ks value for nitrite ion at GCE/Co(II)MTpAP/cit-AuNPs infers that the oxidation of nitrite ion was faster at cit-AuNPs modified electrode than bare and Co(II)MTpAP modified electrodes. 3.7. Amperometric determination nitrite ion at GCE/Co(II)MTpAP/citAuNPs Amperometric method was used to find out the detection limit of nitrite ion using GCE/Co(II)MTpAP/cit-AuNPs. Fig. S10 shows the amperometric i–t curve obtained for nitrite ion at GCE/Co(II)MTpAP/cit-AuNPs in a homogeneously stirred 0.2 M PBS (pH 7.2) by applying a constant potential of 0.9 V. The cit-AuNPs decorated electrode showed the initial current response for 0.5 lM nitrite ion. Further addition of 0.5 lM in each step with a sample interval of 50 s, the current response increases and steady state current re-

sponse was obtained within 2 s. The increase of 4 nA current was observed for the addition of 0.5 lM nitrite ion. Further, the amperometric current was increased linearly with increasing the nitrite ion concentration in the range of 0.5  106 to 5.5  106 M (Fig. S10, inset) with a correlation coefficient of 0.998. We have also carried out the amperometric measurements for the nitrite ion with a wide range of concentration at GCE/Co(II)MTpAP/citAuNPs. Fig. 5 shows the amperometric i–t curve obtained for nitrite ion at GCE/Co(II)MTpAP/cit-AuNPs for the concentration ranging from 0.5  106 to 4.7  103 M. The amperometric current response was linearly increased while increasing the concentration with a correlation coefficient of 0.997 (Fig. 5, inset) and the detection limit was found to be 60 nM (S/N = 3). The wide range of detection and lowest detection limit obtained for nitrite ion at GCE/Co(II)MTpAP/cit-AuNPs was compared with the previously reported porphyrin, phthalocyanine and AuNPs modified electrodes [20,24–26,29,39] and are given in Table 1. As can be seen from the Table 1, the present modified electrode showed the lowest detection limit for nitrite ion with a wide range of concentration when compared to the reported papers [20,24–26,29,39]. 3.8. Effect of interferences The determination of nitrite ion in the presence of common 2 2 þ  + 2+ interfering ions such as Na+, NO 3 , I , K , CO3 , Ca , SO4 , NH4  and Cl and common physiological interference such as glucose was studied at GCE/Co(II)MTpAP/cit-AuNPs using amperometry. Fig. 6 shows the amperometric i–t curve obtained for nitrite ion at GCE/Co(II)MTpAP/cit-AuNPs in the presence of several interferences in a homogeneously stirred 0.2 M PBS. The increased initial current response was due to the addition of 0.5 lM nitrite ion (a) and further addition of 0.5 lM nitrite ion in each step with a sample interval of 50 s, the current response increases and a steady state current response was attained within 2 s (b–d). After that, the addition of 100 lM each (e) NaNO3, (f) NaI and (g) K2CO3 separately with a sample interval of 50 s to the same solution no

25 20

I/μA

82

y = 0.005X + 0.27

50 s

2

R = 0.997

m

15

4μΑ

10 5 0 0

1000

2000

3000

4000

5000

-

Concentration of NO 2 (μM )

l

f k

e d j

a b c g h

i

Fig. 5. Amperometric i–t curve for the determination of nitrite ion at GCE/ Co(II)MTpAP/cit-AuNPs in 0.2 M PBS (pH 7.2). Each addition increases the concentrations of nitrite ion (a) 0.5, (b) 1, (c) 2, (d) 5, (e) 10, (f) 20, (g) 40, (h) 80, (i) 200, (j) 500, (k) 1100, (l) 2300 and (m) 4700 lM at a regular interval of 50 s; Eapp = + 0.9 V. Inset: the corresponding calibration plot obtained for current vs. concentration of NaNO2.

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P. Muthukumar, S. Abraham John / Journal of Colloid and Interface Science 421 (2014) 78–84 Table 1 Comparison of different modified electrodes for the determination of nitrite ion with GCE/Co(II)MTpAP/cit-AuNPs. Electrode

pH

Tetraruthenated porphyrin-modified GCE Nanostructured polymeric tetraruthenated porphyrin film modified GCE Polymeric film of Co(II)-[tetra(4-aminophenyl)porphyrin] modified GCE Nickelphthalocyanine polymer modified GCE Co(II) porphyrin adsorbed onSiO2/SnO2/phosphate modified carbon paste electrode Gold nanoparticles modified GCE by seed mediated growth technique Gold nanoparticles decorated Co(II)porphyrin-modified GCE

change in current response was observed. However, addition of 0.5 lM nitrite ion to the same solution, the current response was again increased (h–j). After three steps, the addition of 100 lM each (k) CaSO4, (l) NH4Cl and (m) glucose separately with a sample interval of 50 s to the same solution caused no change in current response. While 0.5 lM of nitrite ion was added to the same PBS (pH 7.2), an increase in current response was observed and it is similar to the current response observed in the earlier steps (n–p). These results indicated that the determination of 0.5 lM nitrite ion is possible even in the presence of 200-fold excess of common interferents. Further, we have studied the anti-interference ability of the present sensor in the presence of electroactive substances including hydrazine, ascorbic acid, dopamine, sulphite, uric acid, xanthine, folic acid and L-methionine by differential pulse voltammetry. We found that xanthine, folic acid, L-methionine and caffeine interfere the determination of nitrite because their oxidation potentials are close to the oxidation potential of nitrite. On the other hand, the presence of 200-fold higher concentrations of hydrazine, ascorbic acid, uric acid and sulphite does not interfere on the determination of nitrite ion. 3.9. Determination of nitrite ion in real samples The practical application of the GCE/Co(II)MTpAP/cit-AuNPs was demonstrated by determining the concentration of nitrite ion present in the water samples collected from nearby wells. The standard addition technique was used for the determination of nitrite ion in water samples. Fig. S11 shows the determination

8 4.5 7.1 2 5.4 4.6 7.2

Linear range 6

Detection limit 6

5  10 –1000  10 M 1  10–4–8  10–3 M 0.1–10  10–3 M 5  107–8  103 M 5  106–2  103 M 1  105–5  103 M 0.5  106–4.7  103 M

6

0.1  10 M – – 1  107 M 4.2  107 M 2.4  106 M 60  109 M

References [20] [24] [25] [26] [29] [39] This work

of nitrite ion in water sample by differential pulse voltammetry (DPV). The DPV of the GCE/Co(II)MTpAP/cit-AuNPs shows the oxidation peak at 0.79 V in 0.2 M PBS (curve a). This is due to the oxidation of Au into Au oxide. When the same electrode was used to run the DPV in water sample diluted with 0.2 M PBS (pH 7.2) again the oxidation peak observed at 0.79 V without increase in current (curve b). This indicates that the water sample is free from nitrite ion. While adding the commercial nitrite ion (100 lM) into the same solution the oxidation peak was observed at 0.72 V with increased current (curve c). The same procedure was followed for all the samples. The recovery results are given in the Table S2. As shown in Table S2, good recoveries were obtained at cit-AuNPs decorated electrode for the spiked nitrite ion.

3.10. Stability and reproducibility of GCE/Co(II)MTpAP/cit-AuNPs In order to investigate the stability of GCE/Co(II)MTpAP/citAuNPs, the LSVs for 0.5 mM nitrite ion in 0.2 M PBS (pH 7.2) were recorded for every 5 min interval. It was found that oxidation peak current was slightly changed with a relative standard deviation of 1.4% for 20 times repetitive measurements indicating that this electrode has a good reproducibility and does not undergo surface fouling. After voltammetric measurements, the modified electrode was kept in pH 7.2 PBS at room temperature. The current response decreased about 2% in three days and 5.6% in seven days. This infers that the present modified electrode was very much stable. To ascertain the reproducibility of the results, the oxidation of 0.5 mM nitrite ion was tested with three different GCE/Co(II)MTpAP/cit-AuNPs. The peak current obtained for three independent electrodes showed a relative standard deviation of 1.2% confirming that the results are reproducible.

p n

o 4. Conclusions

j kl m i h d e f g c b a 4 nA 50 s

Fig. 6. Amperometric i–t curve responses obtained for 0.5 lM NaNO2 (a–d) and addition of 100 lM of each (e) NaNO3, (f) NaI, (g) K2CO3 and additions were made for 0.5 lM NaNO2 (h–j), addition of 100 lM each of (k) CaSO4, (l) NH4Cl, (m) glucose and then final additions were made for 0.5 lM NaNO2 (n–p) at GCE/Co(II)MTpAP/ cit-AuNPs in a homogeneously stirred 0.2 mM PBS (pH 7) at an interval time of 50 s; Eapp = + 0.90 V.

In the present study, we have demonstrated the electrochemical determination of nitrite ion at cit-AuNPs decorated on Co(II)MTpAP SAM modified electrode. The cit-AuNPs decorated electrode shifted the oxidation potential for nitrite ion oxidation towards 160 and 120 mV less positive potential than bare and Co(II)MTpAP SAM modified electrodes, respectively. The higher electrocatalytic activity of cit-AuNPs decorated electrode towards nitrite ion oxidation is due to the higher coverage of cit-AuNPs besides the cobalt present in the inner core of the porphyrin. The current response was increased linearly with increasing the nitrite ion concentration in the range of 0.5  106–4.7  103 M and the detection limit was found to be 60 nM (S/N = 3). Further, the citAuNPs decorated electrode shows excellent selectivity towards the determination of 0.5 lM nitrite ion even in the presence of 200-fold excess of common interfering agents. The present modified electrode shows the lowest detection limit of nitrite ion when compared to the reported porphyrin and phthalocyanine modified electrodes.

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Gold nanoparticles decorated on cobalt porphyrin-modified glassy carbon electrode for the sensitive determination of nitrite ion.

The present study reports the electrochemical determination of nitrite ion using citrate-gold nanoparticles (cit-AuNPs) decorated on meso-tetra(para-a...
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