The electrochemical sensor based on electrochemical oxidation of nitrite on metalloporphyrin–graphene modified glassy carbon electrode

Abstract

In this study, 5-(4-aminophenyl)-10,15,20-triphenylporphyrin]Mn(III) (MnNH₂TPP) and graphene oxide (GO) composite materials (GO–MnNH₂TPP) were successfully used to modify a glassy carbon electrode (GC) by the drop casting method. The GO–MnNH₂TPP/GC composite electrode was used to investigate the electrocatalytic oxidation features of nitrite ion by cyclic voltammetry (CV) and amperometric I–T curve techniques. These experimental results show the GO–MnNH₂TPP/GC composite electrode has excellent electrocatalytic performance for the detection of nitrite. The oxidation peak current of nitrite ion at the GO–MnNH₂TPP/GC composite electrode has shifted negatively and the intensity of the oxidation peak current increased greatly compared with that at the GC, GO/GC and MnNH₂TPP/GC electrodes. A linear relationship has been established between the oxidation current and the nitrite ion concentration. The detection limit of the GO–MnNH₂TPP/GC composite electrode for the detection of nitrite ion was found to be 1.1 μM and 2.5 μM (S/N = 3) using CV and amperometric I–T curve techniques, respectively. The GO–MnNH₂TPP/GC electrode possesses excellent electrocatalytic activity, rapid response time, low detection limit, high selectivity for nitrite and was applied to the detection of nitrite in real water samples.

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Introduction

Metalloporphyrin and porphyrin free-base derivatives with many excellent properties play an important role in biological process.^1,2 Metalloporphyrin derivatives serving as electron transfer agents have been extensively explored in many important industrial reactions. They can greatly enhance the electron transfer kinetics of many electroactive species. In general, metalloporphyrin derivatives have higher electrocatalytic activity than metal phthalocyanines towards π-acceptor species such as nitric oxide and carbon oxides. Furthermore, the hydrogen atom in the meso-position of the metalloporphyrin could be replaced by many other electron-acceptor or electron-donor groups, which could effectively regulate the structural properties of the porphyrin compound. Because metalloporphyrin derivatives have been widely used in mimicking enzymatic systems, the electrocatalytic capabilities of metalloporphyrin derivatives as various analysts have been put forward using electrocatalytic reactions. Metalloporphyrin-modified glassy carbon electrodes have been employed in determination of all types of small molecule compounds.^3–5 The electrocatalytic mechanism of the interactions of metalloporphyrin with various small molecules has been extensively investigated by several authors.^6–9 However, the direct utilization of metalloporphyrins serving as electrode modifying materials is not very convenient because of the solubility in water or organic solvents and poor stability and reproducibility in electrochemical measurements. Therefore, the insoluble catalyst was preferred because it can be maintained on the surface of the electrode for a long time when it is immersed in solution. Therefore, the intercalation of metalloporphyrins into two-dimensional matrices has attracted much scientific interest, for example, metal oxide semiconductors,^10,11 intercalation of porphyrin into hydrated vanadium(V) oxide,¹² layered niobates,^13,14 and silica¹⁵ were utilized in catalysis and electrochemistry. Many other methods have also been proposed to improve the abovementioned inconvenience such as self-assembled monoplayers (SAM)¹⁶ or electropolymerization¹⁷ of metalloporphyrins on electrode surfaces. Recently, we have reported that a metalloporphyrin and graphene covalent composite material modified glassy carbon electrode exhibited excellent electrocatalytic oxidation of ascorbic acid.¹⁸ Graphene contains sp²-hybridized carbon atoms packed into a hexagonal structure forming a two-dimensional (2D) network carbon nanomaterial. Graphene has been extensively used as an electrode modification material because it has the following merits:^19–24 (1) graphene as an electrode modifying material has a large surface area that can effectively increase the contact area between the electrode and the substrates, which enhances the detection sensitivity of substrates; (2) graphene could effectively avoid encapsulating the residual metals in other carbon-based electrode materials, which can effectively improve the electrochemical stability and reproducibility; (3) graphene as an electrode modifying material has excellent electron transfer properties, which improve the electron interflow between the electrode and the substrates; (4) graphene could be easily modified by covalence or non-covalence with various other functional molecules or nanomaterials. Therefore, we selected the graphene and MnNH₂TPP covalent composite material to modify a glassy carbon electrode, which can play a synergistic effect when the composite electrode is used to detect biological small molecules. Nitrite (NO₂⁻) originating from anthropogenic fertilizer wastes exists in soils and waters;²⁵ nitrite (NO₂⁻), as a typical inorganic pollutant, has seriously threatened human health and the living environment. Nitrite can reduce the blood capacity to transport oxygen by the irreversible oxidization of haemoglobin.²⁶ In addition, nitrite can lead to cancer^27,28 by forming N-nitrosamines when reacting with amines in the stomach. Therefore, it is important that a convenient and sensitive method should be selected to detect the nitrite ion. Many analytical technologies have been developed for nitrite detection, for example, capillary electrophoresis chemiluminescence,²⁹ chromatography,³⁰ spectrophotometry³¹ and electrochemistry.^32–35 In the present methods, electrochemical techniques were one of the most ideal ways to undertake the determination of nitrite. Most nitrite electrochemical sensors are based on the catalysis of proteins for the reduction of nitrite. However, these types of nitrite electrochemical sensors are complicated and the products are complex. The nitrogen in nitrite is in an intermediate valence state, nitrite can be reduced into the lower valence state nitrogen, mainly including the nitrous oxide (N₂O), nitric oxide (NO), and hydroxylamine, until its lowest valence state ammonia; therefore, there are a lot of difficulties in accurately detecting nitrite due to the diverse reduction products that bring out serious environmental pollution. However, the oxidation of nitrite is simple because the only oxidation product of nitrite is the nitrate ion that brings about only light environmental pollution and we could accurately detect nitrite. To date, there are few reports in the literature about the detection of nitrite based on electrocatalytic oxidation of nitrite ion. In this study, a GO–MnNH₂TPP composite material was used to modify a glassy carbon (GC) electrode, which was successfully used for the electrocatalytic oxidation of nitrite ion. It significantly increases the stability and improves the electrochemical performance of the electrochemical sensor. The oxidation peak current for nitrite ion at the GO–MnNH₂TPP/GC composite electrode increased significantly compared with that at the bare GC, GO/GC composite and MnNH₂TPP/GC composite electrodes. Moreover, a negative shift in the oxidation potential of nitrite indicates that GO–MnNH₂TPP has a quite good catalytic activity for oxidation of nitrite; the relationship has been established between the oxidation peak current and the nitrite ion concentration using cyclic voltammetry and amperometric I–T curve techniques. The detection limit of the GO–MnNH₂TPP/GC composite electrode for the nitrite ion was found as 1.1 μM and 2.5 μM using CV and amperometric I–T curve techniques, respectively, based on the principle that the signal-to-noise ratio is 3 (S/N = 3). The GO–MnNH₂TPP/GC composite electrode possesses not only a lower detection limit but also good stability and repeatability when it was used to detect nitrite. Therefore, the GO–MnNH₂TPP/GC composite electrode has potential application in the detection of nitrite ion.

Results and discussion

The UV-Vis absorption spectra of GO, MnNH₂TPP, GO–NH₂TPP and GO–MnNH₂TPP in DMF are displayed in Fig. 1; the maximum absorption peak of GO dispersion in DMF displays at 270 nm, which could be attributed to the π–π* transition of aromatic C=C bonds, and a weak shoulder peak at 305 nm, which corresponds to the n–π* transition of the C=O bonds.³⁶ UV-Vis absorption spectra of GO–NH₂TPP exhibited an absorption peak at 268 nm, which should be the corresponding graphene moiety and a blue shift of 2 nm relative to that of GO. The absorption peak at about 418 nm should be the corresponding Soret absorption band of the NH₂TPP moiety.^37–40 These results illustrate that the conjugated structure of GO was lightly affected because of the covalent connectivity combination of NH₂TPP. GO–MnNH₂TPP and MnNH₂TPP were also measured in DMF. The UV-Vis absorption spectra of MnNH₂TPP showed the typical absorption band of a metalloporphyrin, a strong absorption band appeared at 478 nm, which was attributed to Soret absorption band of MnNH₂TPP and one weak Q absorption band at 553 nm. In the UV-Vis absorption spectra of GO–MnNH₂TPP, a strong absorption band at 268 nm was attributed to the absorption band of the graphene moiety and the absorption band at 468 nm should be the corresponding Soret absorption band of the MnNH₂TPP moiety. By comparing the UV-Vis absorption spectra of GO–MnNH₂TPP with that of MnNH₂TPP, the data showed the absorption band of the graphene moiety in GO–MnNH₂TPP did not change relative to that of GO–NH₂TPP; however, the Soret absorption band of the MnNH₂TPP moiety in GO–MnNH₂TPP blue shifts about 10 nm relative to that of MnNH₂TPP, which could be attributed to the effect on axial coordination of the manganese porphyrin when GO and MnNH₂TPP were connected by covalent bonding. Furthermore, the spectral and surface morphological properties of GO–MnNH₂TPP composite materials have been reported by us elsewhere.¹⁸

Fig. 1 UV-Vis spectra of GO, GO–NH₂TPP, GO–MnNH₂TPP and MnNH₂TPP in DMF.

The effect of pH on the electrochemical behavior of the GO–MnNH₂TPP/GC electrode was examined in 10 mM nitrite in phosphate-buffered saline (PBS) solution with different pH values by the cyclic voltammetry. The electrochemical window was selected from 0.2 to 1.2 V (Fig. 2(a)). Comparing the oxidation peak position and the oxidation peak current under different pH values on GO–MnNH₂TPP/GC electrodes, the oxidation peak position was negatively shifted from 0.92 to 0.76 V and negatively shifted about 160 mV with pH in the range of 1.0–4.0, whereas the oxidation peak position positively shifted from 0.76 to 0.84 V with pH in the range 4.0–7.0. Therefore, when pH is about 4.0, the oxidation position attained the most negative value in the present experimental conditions; the oxidation peak current increased from 71.32 to 100.72 μA with pH in the range from 1.0 to 4.0, whereas in the pH range from 4.0 to 7.0, the peak current decreased from 100.72 to 77.22 μA. The variation trends of oxidation peak position and the oxidation peak current of nitrite with the change of pH are displayed in Fig. 2(b). Based on the abovementioned changes of the oxidation peak position and the oxidation peak current, when the pH was 4, the oxidation peak position was the most negative and the oxidation peak current was a maximum. In addition, in strong acidic media, nitrite is not stable and can undergo the following disproportionation reaction when pH is less than 4.^41–44

3HONO → H⁺ + 2NO + NO₃⁻ + H₂O

Fig. 2 (a) pH dependence of CV at GO–MnNH₂TPP/GC composite electrode in 10 mM NaNO₂ PBS (b) the potential positions and the intensity of current versus different pH.

The disproportionation reaction of nitrite could generate the lower valence state nitrogen compounds and decrease the concentration of nitrite on the surface of the GO–MnNH₂TPP/GC electrode, which would cause more difficulties to nitrite detection due to slowing down electron transfer between the electrode and nitrite ion. The surface of GO–MnNH₂TPP/GC electrode has many –OH groups, which can act as an electron transport medium between nitrite and the electrode. When pH is greater than 4.0, the electrocatalytic oxidation of nitrite becomes more difficult because of the shortage of protons. According to the abovementioned experimental results, when pH is 4, not only could the disproportionation reaction be effectively avoided, but also the electron transfer between nitrite and the electrode could be greatly accelerated; therefore, the optimal experimental condition was selected as pH = 4 when the GO–MnNH₂TPP/GC electrode was used to detect nitrite. The schematic for the electrochemical sensing of nitrite with GO–MnNH₂TPP/GC was displayed in Scheme 1.

Scheme 1 Mechanism for the electrochemical sensing of nitrite with GO–MnNH₂TPP/GC.

The sensing mechanism of oxidation processes of nitrite could be deduced according to previous reports.^45–49 The oxidation of nitrite undergoes two successive one electron oxidation steps on the GO–MnNH₂TPP/GC composite electrode. The ambidentate nitrite ion can coordinate to metalloporphyrins along the axial direction, which leads to formation of the 5-coordinate monodentate nitrato O-nitrito complex Mn^IIINH₂TPP(η¹-ONO)^45,46 and nitrite ions were active. The nitrite ions undergo one electron transfer when the unstable 5-coordinate monodentate nitrato O-nitrito complex Mn^IIINH₂TPP(η¹-ONO) was rapidly transformed into 6-coordinate O-nitrito Mn^IVNH₂TPP(H₂O)(η¹-ONO).⁴⁶ As followed, nitrite undergoes a second electronic transfer during the process of transformation from 6-coordinate O-nitrito Mn^IVNH₂TPP(H₂O)(η¹-ONO)^48,49 to more stable monodentate nitrato Mn^V(TPP)(OH)(η¹-ONO₂) complexes⁴⁵ by the intramolecular electron rearrangement in solution. Nitrite was oxidized into the nitrate.

CV was recorded in 10 mM nitrite ion in PBS (0.1 M, pH = 4.0) under N₂-saturated conditions using a bare GC electrode, a MnNH₂TPP/GC composite electrode, and GO/GC and GO–MnNH₂TPP/GC composite electrodes (Fig. 3). The results showed that GC electrode and GO/GC composite electrode did not exhibit effective electrocatalytic oxidation response towards nitrite ion within the electrochemical window from 0.2 V to 1.2 V, which illustrated that the GC electrode and GO/GC composite electrode had no electrocatalytic activity toward nitrite ion in nature. However, the CV curves' intensity on the GO/GC composite electrode greatly increased relative to that of the bare GC electrode. When GO was modified onto the surface of the GC electrode, GO with a huge surface area and excellent electron transfer capacity can effectively enhance the contact area between the electrode and the electrolyte solution and effectively promote the electron transfer rate between the GC electrode and GO. The electron transfer kinetics of the nitrite ion oxidation are dependent on the surface properties of the electrode, which might be attributed to the enhancement of the effective surface area of the modified electrode in the presence of GO. The surface of GO with many –OH and C=O groups modified onto the surface of the GC electrode can act as an electron transport medium between the electrolytic solution and the electrode. Many –OH and C=O groups on the surface of GO were helpful to promote the interfacial charge transfer and the surface charges of the GO increase access of the ions to the electrode surface for electron communication, which will greatly improve charge transfer between the ions and the electrode. The other main reason can be attributed to significantly enhance electrical conductivity due to the sp² hybrid network of the GO. In addition, compared with the MnNH₂TPP/GC composite electrode and the GO–MnNH₂TPP electrode, a weak oxidation peak current of nitrite ion observed at 1.08 V and no reduction peak current observed on the MnNH₂TPP/GC composite electrode within the electrochemical window from 0.2 V to 1.2 V reveal that nitrite ion is an irreversible oxidation process, which can be ascribed to the strong interaction between nitrite ion and the axial direction of the MnNH₂TPP; the corresponding reaction equation of the electrode was as follows:

Mn^IIINH₂TPPCl + NO₂⁻ → Mn^IIINH₂TPPNO₂ + Cl⁻

Mn^IIINH₂TPPNO₂ + H₂O → Mn^IVNH₂TPP(H₂O)NO₂ + e

Mn^IVNH₂TPP(H₂O)NO₂ + Cl⁻ → Mn^VNH₂TPP(Cl)NO₃ + H⁺ + e

Mn^VNH₂TPP(Cl)NO₃ + 2e → Mn^IIINH₂TPPCl + NO₃⁻

Fig. 3 CVs of 5 mM NaNO₂ PBS (0.1 M, pH 4.0) at different electrodes. Scan rate: 100 mV s⁻¹.

The overall reaction is:

NO₂⁻ + H₂O → NO₃⁻ + 2H⁺ + 2e

On the GO–MnNH₂TPP/GC composite electrode, a strong irreversible oxidation peak current of nitrite ion was observed at 0.76 V. The oxidation peak current on the GO–MnNH₂TPP/GC composite electrode was negatively shifted about 320 mV and the intensity of the oxidation peak current was enhanced 10 times at the GO–MnNH₂TPP/GC composite electrode relative to that at the MnNH₂TPP/GC composite electrode, which could be attributed to enhancing the concentration of nitrite ion on the surface of the GO–MnNH₂TPP/GC composite electrode. This will supply clear evidence of excellent conductivity and improve electrocatalytic activity of the GO–MnNH₂TPP/GC electrode. The GO–MnNH₂TPP/GC electrode has not only a large effective surface area of GO, but also the strong interaction between the nitrite ion and the axial direction of the MnNH₂TPP; the electron transfer between the electrode and nitrite ion was accelerated due to providing many active sites to the surface of the GO–MnNH₂TPP/GC composite electrode.

To obtain an acceptable reproducibility and long-term stability of the GO–MnNH₂TPP/GC composite electrode during the detection of nitrite ion, the cyclic voltammetry was repeatedly performed in 10.0 mM nitrite PBS solution at room temperature; the relative standard deviation (RSD) was evaluated as 3.02% for sequential measurements (n = 10) at 10.0 mM nitrite (Fig. 4). The intensity of the nitrite oxidation peak at the GO–MnNH₂TPP/GC composite electrode was slightly enhanced after 10 complete cycles, which indicates the GO–MnNH₂TPP/GC composite electrode was very well stable and reproducible in 10.0 mM nitrite PBS solution. According to the abovementioned fact, a relative accuracy rate should be higher than 95% when the GO–MnNH₂TPP/GC composite electrode was used to detect nitrite in PBS. In addition, the slight increase of the oxidation peak currents of nitrite at the GO–MnNH₂TPP/GC composite electrode could be attributed to a typical diffusion-controlled process, when the scan rate attained 100 mV s⁻¹, the nitrite ion concentration close to the GO–MnNH₂TPP composite electrode was enhanced with the increase of the reaction time due to the diffusion speed of nitrite ion. The excess nitrite ions accumulate on the surface of the composite electrode, which leads to the increase of the oxidation peak currents.

Fig. 4 The reproducibility and stability of GO–MnNH₂TPP/GC composite electrode in 10 mM NaNO₂ in PBS (0.1 M, pH 4.0).

To further confirm the abovementioned cases, the scan rate effect on the oxidation current of nitrite was performed on the GO–MnNH₂TPP composite electrode in 10 mM nitrite PBS solution, the results illustrated the oxidation peak currents of nitrite enhanced with the increase of the scan rate in the range of the scan range from 50 to 300 mV s⁻¹ (Fig. 5(a)). The relationship between the oxidation peak currents of nitrite and the scan rate was nonlinear. However, the oxidation peak currents of nitrite obtained a good linear relationship to the square root of the scan rate (ν^1/2). The linear regression equation was I_p,a (μA) = 4.069ν^1/2 + 6.24, R² = 0.999 (Fig. 5(b)). This fact suggests that the electrode reaction of nitrite on the surface of the GO–MnNH₂TPP/GC composite electrode is a typical diffusion-controlled process.⁵⁰ The oxidation of nitrite on the GO–MnNH₂TPP/GC composite electrode is a surface-confined oxidation couple; the strong axial interaction of MnNH₂TPP with nitrite played an important role in the oxidation of nitrite due to the surface accumulation process of nitrite being slow on the surface of the GO–MnNH₂TPP electrode. In addition, in the electrochemical system, the oxidation of nitrite at the GO–MnNH₂TPP electrode was an irreversible course, which can be confirmed by the change in oxidation potential of nitrite with the changes in the scan rates. We investigated the changes of the oxidation peak currents of nitrite ion with the change of the scan rates on the GO–MnNH₂TPP/GC composite electrode by cyclic voltammetry. The oxidation potentials moved gradually to the positive orientation when the scan rate changed from 50 to 300 mV s⁻¹. The experimental results can be reasonably explained from the limitation arising from charge transfer kinetics. Based on the abovementioned discussion, the reaction at the GO–MnNH₂TPP electrode was a typical diffusion-controlled process; when the scan rate was increased, the nitrite ion concentration close to the GO–MnNH₂TPP electrode decreased comparatively due to the diffusion speed of nitrite ion. In addition, the oxidation peak currents increased at higher scan rates, and the oxidation peak currents were linearly dependent on the scan rate in the range of 50–300 mV s⁻¹. The linear regression equation was I_p,a (μA) = 1.407ν^1/2 + 11.907, (R² = 0.993). These results showed that the oxidation process of nitrite ion at the GO–MnNH₂TPP electrode is a surface-confined oxidation couple because the contribution of the axial coordination effect of MnNH₂TPP played an important role in the electrode reaction due to the surface accumulation process of nitrite ion being slow on the surface of the GO–MnNH₂TPP electrode.

Fig. 5 (a) Scan rate dependence of CV at the GO–MnNH₂TPP/GC composite electrode in 5 mM NaNO₂ in PBS (0.1 M, pH 4.0); (b) plot of the peak currents versus scan rate (50, 100, 150, 200, 250, 300 mV s⁻¹).

The GO–MnNH₂TPP/GC composite electrode as a working electrode was used to detect nitrite ion by cyclic voltammetry. Fig. 6(a) displays the oxidation peak currents of nitrite ion at GO–MnNH₂TPP electrode change with changes in the nitrite ion concentration at 0.76 V. The intensity of oxidation peak current increased with the increase of the nitrite concentration when the nitrite concentration changed from 0 mM to 10 mM. The linear relationship has been established between the intensity of the oxidation peak current of nitrite ion and the nitrite concentration on the GO–MnNH₂TPP/GC composite electrode and linear fitting curves are displayed at Fig. 6(b); the corresponding linear regression equation was I (μA) = 10.082C (μM) − 0.0352, R² = 0.999. The GO–MnNH₂TPP/GC composite electrode has a good linear dynamic range for detection of nitrite ion. To further confirm the lowest detection limit of the linear equations for nitrite ion concentration and according to other authors' reported method,⁵¹ we determined the lowest detection limit for nitrite to be 1.1 μM on the GO–MnNH₂TPP/GC composite electrode based on the principle that the signal-to-noise ratio is 3 (S/N = 3).

Fig. 6 (a) Concentration dependence of CVs of NaNO₂ in PBS (0.1 M, pH 4.0) at the GO–MnNH₂TPP/GC composite electrode; (b) plot of the peak currents versus concentration of nitrite ion.

The GO–MnNH₂TPP/GC electrode was employed as an electrochemical sensor material for the detection of nitrite ions by amperometric I–T curve techniques. The current responses were measured for successive additions of nitrite at a regular time interval of 60 s to uniformly stirred 0.1 M PBS (pH 4.0) with an applied potential of 0.76 V (Fig. 7(a)). The sensor displayed a rapid response and unmistakable signals were gained for every addition of nitrite. The current response increased in proportion to the nitrite concentration in the range from 10 μM to 160 μM. The linear relationship is established between the intensity of current and the nitrite concentration on the GO–MnNH₂TPP/GC electrode. The linear fitting curve was displayed at Fig. 7(b). The corresponding linear regression equation was I (μA) = 0.2241C (μM) + 0.0250, R² = 0.994. The lowest detection limit (LOD) was 2.5 μM (S/N = 3), which was far below the guidance level of nitrite ions based on the World Health Organization.⁵²

Fig. 7 (a) Amperometric response at the GO–MnNH₂TPP/GC electrode in 0.1 M PBS (pH 4) with an applied potential of 0.76 V; (b) plot of the peak currents versus concentration of nitrite ion.

The property of the GO–MnNH₂TPP/GC electrode for detection of nitrite was compared with other previous reports (Table S1†).⁵³ It can be observed from Table S1† that the GO–MnNH₂TPP/GC electrode showed the lower detection limit and a wider linear range than some previous reports.⁵⁴ It may be attributed to the GO–MnNH₂TPP/GC electrode having a large effective surface area, which could effectively enhance the contact area between the GO–MnNH₂TPP/GC electrode and the electrolytic solution. Therefore, the GO–MnNH₂TPP/GC electrode is suitable for the determination of nitrite in real samples.⁴³

The selectivity of the GO–MnNH₂TPP/GC modified electrode for nitrite was investigated by adding 100 μM of several common interferents such as NaCl, Cu(NO₃)₂, KCl, Na₂SO₄, Na₂HPO₄, hydrogen peroxide, and ascorbic acid. The amperometric curve was recorded for the successive addition of nitrite and the interferents in a uniformly stirred 0.1 M PBS (pH 4.0) solution (Fig. 8). After a few additions of nitrite (10 μM), the oxidation peak current produces a ladder-shape change. When the interferents, one after another, were injected into the continuously stirred nitrite PBS solutions, the intensity of the oxidation peak current did not change. However, the injection of 10 μM NO₂⁻ to the same solution again lead to a clear and quick response. It obviously indicated that the current enhanced only when the nitrite was added. These results demonstrated that even in the presence of interferents the GO–MnNH₂TPP/GC electrode has high selectivity towards NO₂⁻,⁵⁵ which may be ascribed to the strong interaction between nitrite ion and the axial direction of the MnNH₂TPP.

Fig. 8 Amperometric I–T curve of the GO–MnNH₂TPP/GC electrode for the addition of 10 μM nitrite and each 100 μM addition of other interferents, such as NaCl, KCl, Cu(NO₃)₂, Na₂SO₄, Na₂HPO₄, ascorbic acid, and hydrogen peroxide, in 0.1 M PBS (pH 4) at a regular time interval of 60 s (applied potential was 0.76 V).

To further investigate the possibility of the GO–MnNH₂TPP/GC electrode applied to practical samples, the detection of nitrite in various water samples (river water, tap water and pickled foods) was carried out. After the water samples were filtered, the water samples were examined at the GO–MnNH₂TPP/GC electrode by the abovementioned addition method and the amperometric I–T curve technique. The river water was collected from the campus, the tap water was from the Harbin Institute of Technology and pickled foods were bought from the supermarket. The results of the real sample analysis are listed in Table 1.

Table 1 Results of the determination of nitrite in different samples

Sample	Analyte	Added (μM)	Found (μM)	Recovery (%)
River water	NO2^−	50.00	44.68	89.36
Tap water	NO2^−	50.00	40.30	80.60
Pickled food	NO2^−	50.00	50.23	100.46

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As can be observed from Table 1, 50 μM of nitrite was added. The examined results were: river water 44.68 μM (89.36%), tap water 40.30 μM (80.60%), pickled foods 50.23 μM (100.46%). Although potential interferents were contained in the water samples, the recoveries were still high, which was basically consistent with the practical results.⁵⁶ This suggested that the GO–MnNH₂TPP/GC electrode was reliable and sensitive enough for detection of nitrite in practical applications. The high recovery and sensitivity may owe to the stronger axial coordination effect between the nitrite and MnTPP relative to the unknown interferents.

Experimental

Materials and methods

All chemical reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. in Shanghai without any further purification. Phosphate buffer solutions (0.1 M, pH 4.0) acted as the supporting electrolyte and were prepared using 0.1 M Na₂HPO₄ and 0.1 M NaH₂PO₄ (PBS). A typical synthetic procedure and characterization of graphene oxide (GO), 5-(4-aminophenyl)-10,15,20-triphenylporphyrin (NH₂TPP), MnNH₂TPP, GO–MnNH₂TPP and the preparation of all composite electrodes have been reported by ourselves elsewhere.¹⁸

Synthesis of covalently attached MnNH₂TPP and graphene oxide hybrids

50 mg GO was refluxed in 30 mL SOCl₂ and 2 mL DMF at 70 °C for 24 h under a nitrogen atmosphere. After removing the excess SOCl₂, 3 mL triethylamine and 70 mL DMF were injected into the reaction container and continuously reacted for 72 h at 80 °C under a nitrogen atmosphere and then the solution was intermittently sonicated for 8 h in an ultrasonic bath. This was followed by adding 100 mg MnNH₂TPP into the reaction solution and reaction for 24 h at 80 °C under a nitrogen atmosphere; 300 mL ether was poured into the solution after the solution was cooled to room temperature and filtered with a 0.45 μm Millipore filter. The precipitate was successively washed with CH₂Cl₂ and H₂O. The precipitate was sonicated for 30 min in 50 mL DMF; GO–MnNH₂TPP was collected by centrifuging at 10 000 rpm.

Apparatus

Spectra were obtained on a Varian 4000 UV-Vis spectrometer (Varian Company, USA). Electrochemical measurements were performed using a Bio-Logic VSP-300-6 electrochemical workstation. A conventional three electrode system was used for all electrochemical experiments, which comprised a high purity platinum rod (φ = 3 mm) as auxiliary electrode, a saturated calomel electrode (SCE) as reference electrode, and GC electrode (φ = 3 mm), GO/GC composite electrode, MnNH₂TPP/GC composite electrode and GO–MnNH₂TPP/GC composite electrode as the working electrode; all the potential values were measured in pH = 4.0 PBS solution.

Preparation of graphene oxide–MnNH₂TPP modified GC electrode

For the GO–MnNH₂TPP modified GC electrode preparation, the GC electrode was polished with 0.03 and 0.05 mm alumina slurry and washed with doubly-distilled water and ethanol to discard the physically absorbed substances under an ultrasonic bath. Then, the GC electrode was dried at room temperature. The CVs were performed in 10.0 mM K₃[Fe(CN)₆] aqueous solution and the GC electrode acted as the working electrode. The pre-treated GC electrode could meet our experimental requirements until the potential difference (ΔE) between the oxidation potential and the reduction potential was less than or equal to 80 mV. The ΔE of the pre-treated GC electrode is 72 mV; therefore, the pre-treated GC electrode was used to fabricate the GO–MnNH₂TPP/GC composite electrode; in all our electrochemical performances, 10 mg GO–MnNH₂TPP was added into 20 mL DMF to form a 0.5 mg mL⁻¹ of GO–MnNH₂TPP homogeneous dispersion with mild ultrasonication. A volume of 15 μL of the resulting GO–MnNH₂TPP homogeneous dispersion solution was dropped onto the surface of the GC electrode and dried at room temperature for 24 h. Then, the GO–MnNH₂TPP modified GC electrode was stocked in 0.1 M pH = 7.4 PBS solution at 4 °C. The other modified electrodes were prepared by same procedures.

Conclusions

The GO–MnNH₂TPP/GC composite electrode was successfully fabricated and used to detect nitrite based on the electrocatalytic oxidation of nitrite by cyclic voltammetry and amperometric I–T curve techniques. When the GO–MnNH₂TPP/GC composite electrode was used for the detection of nitrite ion, the linear relationship was established between the oxidation peak current and the nitrite concentration. The corresponding linear equation was I (μA) = 10.082C_nitrite (μM) − 0.0352, R² = 0.999 with a linear range of 0–10 mM by cyclic voltammetry (CV) and the linear regression equation was I (μA) = 0.2241C (μM) + 0.0250, R² = 0.994 with a linear range of 10–160 μM by amperometric I–T curve techniques. The detection limits were 1.1 μM and 2.5 μM (S/N = 3) using CV and amperometric I–T curve techniques, respectively. Compared with the other nitrite detection methods, which have been previously reported, the preparation method of the GO–MnNH₂TPP/GC composite electrode and the detection method for nitrite have been greatly simplified. In addition, the GO–MnNH₂TPP/GC composite electrode has good stability, repeatability, excellent electrocatalytic activity, rapid response time, low detection limit and high selectivity, which gives it an enormous potential for practical detection of nitrite ion.

Acknowledgements

Support for this study by the Natural Scientific Research Innovation Foundation in Harbin Institute of Technology (HIT IBRSEM.2009.003, HIT.ICRST. 2010012) and The Department of International Cooperation of Science and Technology (2012DFR30220).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra08269f

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