Construction of a Zinc Porphyrin–Fullerene-Derivative Based

Article pubs.acs.org/ac

Construction of a Zinc Porphyrin−Fullerene-Derivative Based Nonenzymatic Electrochemical Sensor for Sensitive Sensing of Hydrogen Peroxide and Nitrite Hai Wu,†,‡ Suhua Fan,† Xiaoyan Jin,† Hong Zhang,† Hong Chen,† Zong Dai,*,‡ and Xiaoyong Zou*,‡ †

College of Chemistry & Chemical Engineering, Fuyang Normal College, Fuyang, Anhui 236037, P.R. China School of Chemistry and Chemical Engineering, Sun Yat−Sen University, Guangzhou 510275, P.R. China

‡

S Supporting Information *

ABSTRACT: Enzymatic sensors possess high selectivity but suffer from some limitations such as instability, complicated modified procedure, and critical environmental factors, which stimulate the development of more sensitive and stable nonenzymatic electrochemical sensors. Herein, a novel nonenzymatic electrochemical sensor is proposed based on a new zinc porphyrin−fullerene (C60) derivative (ZnP−C60), which was designed and synthesized according to the conformational calculations and the electronic structures of two typical ZnP− C60 derivatives of para-ZnP−C60 (ZnPp−C60) and ortho-ZnP−C60 (ZnPo−C60). The two derivatives were first investigated by density functional theory (DFT) and ZnPp−C60 with a bent conformation was verified to possess a smaller energy gap and better electron-transport ability. Then ZnPp−C60 was entrapped in tetraoctylammonium bromide (TOAB) film and modified on glassy carbon electrode (TOAB/ZnPp−C60/GCE). The TOAB/ZnPp−C60/GCE showed four well-defined quasi-reversible redox couples with extremely fast direct electron transfer and excellent nonenzymatic sensing ability. The electrocatalytic reduction of H2O2 showed a wide linear range from 0.035 to 3.40 mM, with a high sensitivity of 215.6 μA mM−1 and a limit of detection (LOD) as low as 0.81 μM. The electrocatalytic oxidation of nitrite showed a linear range from 2.0 μM to 0.164 mM, with a sensitivity of 249.9 μA mM−1 and a LOD down to 1.44 μM. Moreover, the TOAB/ZnPp−C60/GCE showed excellent stability and reproducibility, and good testing recoveries for analysis of the nitrite levels of river water and rainwater. The ZnPp− C60 can be used as a novel material for the fabrication of nonenzymatic electrochemical sensors.

E

have been constructed for the analysis of glucose,5 ascorbic acid,6 uric acid,7 hydrogen peroxide (H2O2),8 and other biomolecules.9 For example, C60 embedded in tetraoctylammonium bromide (TOAB) film exhibited good repeatability and stability for sensing H2O2 with a linear concentration range from 0.033 to 2.05 mM and a sensitivity of 1.65 μA mM−1.8 C60 hollow microspheres have been developed to fabricate the electrocatalytic sensor for selective detection of dopamine.10 Other fullerenes as mediator were also reported to construct amperometric biosensors.4,11 In spite of the wide applications, these sensors always suffer from the disadvantages of narrow potential range, instability, and low sensitivity. It is well-known that C60 is a most popular electron acceptor material used in biomimetic artificial photosynthetic systems.12,13 Porphyrins (P) are excellent electron donors for its extensive π-conjugated systems, which increases their electron-donating ability and makes them easier to combine with the electron acceptor.14 A

lectrochemical biosensors, especially enzyme-based amperometric sensors, have witnessed an increasing interest due to their advantages of low-cost, simplicity, and high selectivity and sensitivity.1 Unfortunately, the performances of enzyme sensors not only suffer extremely from various environmental factors such as temperature, oxygen, pH, and coexisting chemicals but also depend greatly on the modification procedures of enzyme onto transducers.2 Therefore, nonenzymatic electrochemical sensors have been advocated among various biosensors in recent years. Nonenzymatic sensors are ideal systems for direct oxidation or reduction of analytes that does not need to facilitate fragile and relative enzyme. Although they can avoid the intrinsic defects of enzymes, these sensors vary considerably depending on electrode materials because the slow kinetic process can not lead to discernible faradaic current.3 Therefore, many efforts have been centered on the research of new electrocatalytic materials to improve the kinetic process of electron transfer.2 Fullerene (C60), a zero-dimensional carbon, has attracted enormous interest recently due to its excellent electrical conductivity, charge transport, photophysical behavior, and efficient charge separation.4 Several C60-based nonenzymatic electrochemical sensors © 2014 American Chemical Society

Received: January 20, 2014 Accepted: June 11, 2014 Published: June 11, 2014 6285

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scan rate of 100 mV s−1. Prior to all electrochemical experiments, 0.5 M KCl solution was purged using high purity nitrogen for 15 min to remove dissolved oxygen. NaNO2 or H2O2 solution was introduced into a sealed electrochemical cell using microinjector and stirred carefully before electrochemical detection. For sample analysis, the collected river or rain samples were first filtrated with a 0.45 μM membrane filter and then injected in 0.5 M KCl solution by a microinjector. The concentrations of H2O2 or nitrite in the samples were analyzed by the standard addition method using the proposed sensor. Nitrogen atmosphere was maintained throughout the measurements and all experiments were carried out at room temperature. Field emission scanning electron microscope (FE−SEM) images were obtained on a JSM−6700F field emission scanning electron microanalyzer (JEOL, Japan). Computational Details. Molecular geometry optimizations were performed by Gaussian 09 (revision A.02) using density functional theory (DFT).17 B3LYP method was used with a LanL2DZ basis for Zn and 6-31G* basis for the rest of the atoms.18 The images of the frontier orbitals were generated from GaussView 5.0. The optimized structures and the molecular orbital energies including the frontier highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for ZnPp−C60 are shown in Figure 1 and Table S1 of

long-lived charge-separated state with high quantum yield can be generated on electrodes by combination of porphyrin and C60 as the ideal donor−acceptor pair.15 Moreover, the redox properties of porphyrin−C60 dyads are tunable. Wider potential has been realized by coordinating different metal ions on the cavity of the porphyrin, which extends their potential applications in electrochemistry. So far, although porphyrin−C60 dyads possess excellent electrochemical properties, they are seldom used in the development of nonenzymatic electrochemical sensors. Herein, a nonenzymatic electrochemical sensor is presented based on a p-methoxy zinc porphyrin−C60 derivative (ZnPp− C60, the structure is shown in Scheme S1 of the Supporting Information), which was designed and synthesized by the combination of p-methoxy porphyrin and C60 moieties using a flexible methylene chain. The electronic structure, electrochemical behavior, and the application in electrocatalysis for H2O2 and nitrite were investigated. The results showed that the ZnPp−C60 could be a novel material for construction of nonenzymatic electrochemical sensor for H2O2 and nitrite analysis in a relatively wide potential range, high sensitivity, and good stability.

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EXPERIMENTAL SECTION Chemicals and Reagents. ZnPp−C60 was synthesized according to our previous method.16 Briefly, 5-(4′-hydroxyphenyl)-10,15,20-tri-(p-methoxyphenyl)porphyrin first reacted with 1,4-dibromobutane in the presence of K2CO3 in DMF. The resultant was then converted to 5-[4′-(4″-phenoxybutoxy)phenylaldehyde]-10,15,20-tri-(p-methoxyphenyl) porphyrin (H2Pp−CHO). The ligand 5-[4′-(4″-phenoxybutoxy)phenyl]N-(2-methyl) fulleropyrrolidine-10,15,20-tri-(pmethoxyphenyl)porphyrin (H2Pp−C60) was obtained by the reaction among sarcosine, C60, and H2Pp−CHO. The desired ZnPp−C60 was prepared by metalation of H2Pp−C60 and zinc acetate. Tetraoctylammonium bromide (TOAB, 98%) was obtained from Sigma and used without further purification. Toluene, potassium chlorate (KCl), sodium nitrite (NaNO2), and H2O2 (30%) were purchased from Sinopharm Chemical Reagent Company Ltd. (Shanghai). All other chemicals were of analytical reagent grade and used without further purification, unless stated otherwise. Aqueous solutions were prepared with doubly distilled water at ambient temperature. Preparation of TOAB/ZnPp−C60/GCE. Glassy carbon electrode (GCE, 3 mm in diameter) was polished to a mirror finish mechanically by 0.3 and 0.05 μm alumina powders. The well-polished GCE was sequentially cleaned in absolute ethanol and doubly distilled water by sonication for 1 min, separately. ZnPp−C60 (2.1 mg) and TOAB (10.9 mg) were sonicated in 2.0 mL toluene for 30 min. After that, 8.0 μL of the mixture was spread on the surface of cleaned GCE and dried at ambient temperature. For comparison, TOAB/GCE, TOAB/C60/GCE, and TOAB/H2Pp−C60/GCE were prepared under the same conditions. Apparatus and Measurements. Electrochemical experiments including cyclic voltammetry and amperometric current− time (i−t) curve were carried out on a CHI660C electrochemical workstation (Chenhua, Shanghai, China) equipped with a conventional three-electrode system. A platinum wire and a Ag/AgCl electrode (3 M KCl) were used as counter and reference electrodes, respectively. Modified GCE was used as the working electrode. Cyclic voltammetry was performed in 0.5 M KCl solution in the potential range from 1.0 to −1.32 V at the

Figure 1. Optimized conformation of (A) ZnPp−C60 and (B) ZnPo− C60, and the frontier (C) HOMO and (D) LUMO of ZnPp−C60 calculated by B3LYP methods.

the Supporting Information. For comparison, the ortho position of the C60 moiety (ZnPo−C60) was chosen to probe the orientation effects (Figure S1 and Table S1 of the Supporting Information).

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RESULTS AND DISCUSSION Computational Studies. In order to achieve an optimum porphyrin-C60 derivative with good electrochemical properties, computational studies were performed on the geometry and electronic structure of para- and ortho- dyads (ZnPp−C60 and ZnPo−C60, respectively) by DFT at the B3LYP/6-31G* level. Both dyads were fully optimized to their lowest-energy conformations. In the optimized structure of ZnPp−C60, the C60 spheroid resides above the porphyrin ring system (Figure 1A), with an edge-to-edge distance ree of 2.20 Å (from the zinc in the porphyrin ring to the closet carbon of C60 spheroid), which is close to van der Waals contacts.19 The bent conformation of 6286

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ZnPp−C60 stabilized by attractive van der Waals forces shows relative low energy (−6.99 kcal mol−1), which is better for the intramolecular interactions between porphyrin ring and C60 moiety. In comparison, ZnP o −C 60 in its most stable conformation exhibits an extended and warped conformation (Figure 1B), which increases the ree (9.14 Å), limits the intramolecular interactions, and reduces the stability (ca. 1.86 kcal mol−1) than that of ZnPp−C60. Consequently, ZnPp−C60 affords a longer charge-separated (CS) lifetime and an easier electron transfer capability as compared with other fullerene derivatives.20 Molecular orbitals (MOs) were further applied to investigate the electronic properties of ZnPp−C60 and the intramolecular interactions between the porphyrin ring and the C60 moiety. The calculated HOMO and LUMO of ZnPp−C60 are shown in Figure 1 (panels C and D). Through-space orbital overlap in ZnPp−C60 is present due to the intramolecular interaction between the porphyrin ring and C60 moiety. Thus, HOMO coefficients were found not only on zinc porphyrin ring but also on C60.20 However, for ZnPo−C60, the orbital overlap was not observed due to larger separation. That is, HOMO and LUMO are entirely located on zinc porphyrin ring and C60 spheroid, respectively (Figure S1 of the Supporting Information). The LUMO and HOMO energies for ZnPp−C60 were calculated to be −3.29 and −4.68 eV with a HOMO/LUMO energy gap (ΔE) of 1.39 eV (Table S1 of the Supporting Information). The smaller gas phase energy gap than that of ZnPo−C60 (1.64 eV, Table S1 of the Supporting Information) is ascribed to the shorter distance between the porphyrin ring and the C60 moiety in ZnPp−C60. On the basis of the electronic properties, ZnPp−C60 was chosen and synthesized for construction of nonenzymatic electrochemical sensor. Characterization of TOAB/ZnPp−C60/GCE. The morphologies of ZnPp−C60/GCE and TOAB/ZnPp−C60/GCE were characterized by FE−SEM. ZnPp−C60 with nonhomogeneous three-dimensional structure aggregated and stacked compactly on the surface of GCE (Figure 2A). After being incorporated in TOAB film, ZnPp−C60 was well-dispersed and exhibited a uniform and granular morphology with submicrometer-scale particle sizes of 0.2−0.6 μm (Figure 2B), which is helpful for enhancing the electron transfer rate of ZnPp−C60 and facilitating the access of analytes.8 Figure 2C shows the cyclic voltammograms (CVs) of different modified electrodes in 0.5 M KCl solution. No redox peak was found on TOAB/GCE (curve a). However, four pairs of quasi-reversible redox couples in a wide potential range were clearly observed on TOAB/ZnPp−C60/ GCE (curve b). The formal potentials [(Epred+Epox)/2] of the couples I to IV were −0.236, −0.724, −1.165, and 0.758 V (vs. Ag/AgCl), respectively. The four reduction peak currents were proportional to the square root of scan rate from 0.04 to 0.30 V s−1 (Figure 2D and inset), indicating that the electrochemical reaction of ZnPp−C60 is a diffusion-controlled electrochemical process. The results are in agreement with that of C60 in TOAB film.8,21 Moreover, it should be noted that the electrochemical behaviors of the first and the second CVs were obviously different. As shown in Figure 2C, upon the first scan from 1.0 to −1.32 V, a small peak was observed at approximately −0.118 V, whereas no obvious reduction peak appeared at −0.289 V. The reduction peak of couple IV at 0.720 V was weak. However, in the second and subsequent scans, the peak at 0.720 V increased largely, along with complete disappearance of the small peak at −0.118 V, and appearance of a new sharp reduction peak at −0.289 V (peak I). Similar results were obtained by Echegoyen.21

Figure 2. FE-SEM images of (A) ZnPp−C60/GCE and (B) TOAB/ ZnPp−C60/GCE. (C) (a) CVs of TOAB/GCE and (b) TOAB/ZnPp− C60/GCE in 0.5 M KCl solution. (D) CVs of TOAB/ZnPp−C60/GCE in 0.5 M KCl solution at different scan rate of 0.04, 0.06, 0.08, 0.10, 0.12, 0.14, 0.16, 0.18, 0.20, 0.25, 0.30 V s−1. Inset: plots of the four reduction peak currents (from I to IV) vs the square root of scan rate, respectively.

The peak I is ascribed to the evolution of the small peak at −0.118 V, which may be caused by the change in the structure of the ZnPp−C60 film after the first reduction in the subsequent scans.21 In order to investigate the mechanism of electrochemical process, TOAB/C 60 /GCE, TOAB/H 2 P p −C 60 /GCE, and TOAB/ZnPp−C60/GCE were further studied in Figure 3. On TOAB/C60/GCE, three couples of redox peaks (I, II, and V) were found with the formal potentials of −0.141, −0.619, and −1.251 V (vs Ag/AgCl) (Figure 3A, curve a). These redox peaks correspond to C600/−, C60−/2−, and C602−/3−, respectively.21 On TOAB/ZnPp−C60/GCE, four redox peaks were observed (Figure 3A, curve b). The peaks I and II, which show more negative formal potentials, correspond to ZnPp−C600/− and ZnPp−C60−/2− of the C60 moiety in ZnPp−C60, respectively. In accordance with the results of D’Souza et al.,22 the reduction potential of zinc porphyrin ring is more positive than that of ZnPp−C602−/3−. The ZnPp−C602−/3− cannot show in this potential range due to the appearance at more negative potential. Therefore, the peak III is attributed to the reduction of ZnP0/−− C602−. For identification of the peak IV, the behavior of H2Pp− C60, which does not coordinate zinc, was compared with that of ZnPp−C60. As can be seen, TOAB/H2Pp−C60/GCE did not show any peak at positive potential (Figure 3B, curve a). Therefore, the wide couple of redox peaks IV can be ascribed to the overlap of the redox of the zinc porphyrin in ZnPp−C60 (ZnPp2+/+−C60 and ZnPp+/0−C60). It should be noted that no more redox peaks were found, in comparison with the behavior of porphyrin−C60 derivatives in odichlorobenzene.22 It may be caused by the fact that ZnPp−C60 in TOAB film can easily realize electrochemical reactions,21 leading to the decrease in the resolution and the overlap of redox peaks. Besides, the reduction potential of peak III is more positive than that of peak V, suggesting that ZnP0/−−C602− in ZnPp−C60 is easier to be electrochemically reduced than C602−/3− in the C60 monomer. Consequently, ZnPp−C60 is more suitable for the manufacture of electrochemical sensor. The mechanism for the 6287

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Figure 3. (A) (a) CVs of TOAB/C60/GCE and (b) TOAB/ZnPp−C60/GCE, and (B) (a) CVs of TOAB/H2Pp−C60/GCE and (b) TOAB/ZnPp−C60/ GCE in 0.5 M KCl solution. (C) Mechanism of the redox processes of ZnPp−C60 in TOAB film on GCE.

electrochemical processes of ZnPp−C60 in TOAB film is proposed in Figure 3C. Electrocatalytic Activities of TOAB/ZnPp−C60/GCE Toward H2O2 and Nitrite. The electrocatalytic ability of TOAB/ZnPp−C60/GCE was evaluated by CV and amperometric current−time curve techniques using H2O2 or nitrite as analytes. H2O2 and nitrite were selected because they are widely used in medicine, environment, food products, and biosensor developments.23 Upon the addition of 0.615 mM H2O2, a dramatic increase in the reduction of peak III along with obvious decrease in its oxidation peak was found on the TOAB/ZnPp− C60/GCE, (Figure 4A), indicating an evident electrocatalytic

fer), citric acid−sodium citrate buffer, sodium acetate−acetic acid buffer, and KCl aqueous solution. As shown in Figure 5A, the best electrochemical behavior and maximum current cannot be obtained except for KCl aqueous solution. Therefore, KCl aqueous solution is used as electrolyte solution. The influence of the concentration of KCl on electrochemical behaviors was further studied and shown in Figure 5B. When KCl aqueous solution was diluted from 0.5 to 0.05 M, well-identified voltammetric responses could also be found, but the current responses decreased. The highest response was achieved at 0.5 M

Figure 4. CVs of TOAB/ZnPp−C60/GCE in 0.5 M KCl aqueous solution (A) before and after the addition of 0.615 mM H2O2, (B) before and after the addition of nitrite (the concentrations of nitrite from a to e: 0, 1.6, 6.4, 10.4, and 14.4 μM).

reduction of H2O2. Similarly, with successive additions of NaNO2, the current of anodic peak IV increased greatly and the other anodic peaks unchanged obviously (Figure 4B), suggesting an obvious electrocatalytic oxidation of nitrite. In comparison with the C60-based sensor, the conjunction of porphyrin to C60 extends the potential range, which enables the TOAB/ZnPp−C60/GCE as a nonenzymatic electrochemical sensor to dual-functional determinations of H2O2 and nitrite. Optimization of Conditions. In order to obtain the best electrochemical response, the voltammetric behaviors of TOAB/ ZnPp−C60/GCE were first tested in several commonly used media, including phosphate-buffered saline (PBS), tris(hydroxymethyl)aminomethane hydrochloride (Tris-HCl buf-

Figure 5. (A) CVs of TOAB/ZnPp−C60/GCE in different solutions (a: 0.1 M PBS; b: 0.1 M Tris-HCl; c: 0.1 M citric acid-sodium citrate; d: 0.1 M NaAc-HAc; e: 0.1 M KCl; and f: 0.5 M KCl). (B) CVs of TOAB/ ZnPp−C60/GCE in KCl solutions at different concentrations (a: 0.05 M; b: 0.10 M; c: 0.20 M; d: 0.40 M; and e: 0.50 M). (C) CVs of TOAB/ ZnPp−C60/GCE in 0.5 M KCl solution at different potential ranges (a: 1.0−0.4 V; b: 1.0 ∼ −0.5 V; c: 1.0 ∼ −1.0 V; d: 1.0 ∼ −1.32 V; and e: 1.0 ∼ −1.5 V). (D) The optimization of applied potential in amperometric current−time curves of TOAB/ZnPp−C60/GCE with successive injections of 0.028 M H2O2 into 4.0 mL of 0.5 M KCl aqueous solution (a: −1.30 V; b: −1.25 V; c: −1.17 V; and d: −1.13 V). 6288

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KCl. The results are attributed to the fact that anions of Br− and Cl− can diffuse into and out from TOAB film easier than other anions. Moreover, KCl does not interfere with H2O2 and nitrite detection, and 0.5 M KCl solution was chosen as the bulk solution in the further experiments. The scan range for investigation of the voltammetric behavior of TOAB/ZnPp−C60/GCE was optimized by cyclic voltammetry with various potential ranges. As can be seen from Figure 5C, four pairs of quasi-reversible redox couples were clearly found in the potential range from 1.0 to −1.32 V. However, when the potential was cathodically scanned over −1.32 V, the redox processes became irreversible and the oxidation peak of ZnP0/−− C602− completely disappeared. Moreover, the modified film on the surface of the sensor tends to be unstable. Thus, a potential range from 1.0 to −1.32 V was selected for CV investigation. In addition, the applied potentials for amperometric determinations of H2O2 and nitrite were optimized (Figure 5D). The best catalytic current responses toward H2O2 could be obtained only at the applied potential of −1.17 V. Therefore, −1.17 V was selected as the optimum applied potentials for H2O2 detection, and 0.82 V was selected for nitrite detection (data were not shown). Calibrations of H2O2 and Nitrite. After optimization of experimental conditions, H2O2 and nitrite were amperometrically detected on TOAB/ZnPp−C60/GCE. As shown in Figure 6A, upon the successive additions of H2O2, TOAB/ZnPp−C60/

linear ranges are about two or 3 orders of magnitude, and the LODs are at 1 μM level. The linear ranges of the proposed TOAB/ZnPp−C60/GCE are comparable to those of the previously reported H2O2 and nitrite sensors, while the LOD for H2O2 detection is about 2 orders of magnitude lower than those of the most reported H2O2 sensors. Moreover, the sensitivity of the presented sensor is about 2 orders of magnitude higher than those of the reported sensors. The high sensitivity of the TOAB/ZnPp−C60/GCE can be attributed to the special three-dimensional bent conformation, excellent electrochemical properties, and uniform dispersion of ZnPp−C60, which strongly exhibited electrocatalytic activities toward the oxidation of nitrite and reduction of H2O2. Consequently, ZnPp−C60 supplies a potential novel material for the fabrication of nonenzymatic electrochemical sensor. Interference Study. The influences of potential interferents on the determinations of H2O2 and nitrite were evaluated by the amperometric current−time method. As shown in Figure S3A of the Supporting Information, TOAB/ZnPp−C60/GCE exhibited well-defined amperometric response toward 7.0 μM nitrite, and no noteworthy amperometric responses were observed for the additions of 0.2 mM of possible interferents, including Ca2+, Mg2+, Cu2+, NH4+, F−, Br−, IO3−, CO32−, SO42−, NO3−, SO32−, and NH2OH. Besides, no amperometric response was observed with the addition of some potential coexisting interferents and redox active substances, including 0.1 mM tryptophan (Trp), uric acid (UA), ascorbic acid (AA), acetic acid (HOAc), ethanol, glucose (Glu), and H2O2 (Figure S3B of the Supporting Information), which suggests that the presence of these substances including redox active substances and H2O2 does not interfere with nitrite detection. Furthermore, quick and reproducible responses were observed for the addition of nitrite into the 0.5 M KCl solution containing these foreign substances. The results indicate that the TOAB/ZnPp−C60/GCE has high selectivity for nitrite detection. The influences of interferences on the determination of H2O2 were further evaluated. As can be seen in Figure S3C of the Supporting Information, the TOAB/ZnPp−C60/GCE showed obvious amperometric response toward 10.0 μM H2O2. When further injection of 0.2 mM AA, dopamine (DA), UA, or Glu, no obvious current responses were observed from the additions of DA, UA, and Glu, while some response was found upon the addition of AA with 20-fold higher concentration than that of H2O2. The results suggest that the constructed TOAB/ZnPp− C60/GCE exhibits excellent interference resistance to DA, UA, and Glu, while high concentration of AA may interfere with the detection. Stability and Reproducibility. The long-term storage stability, operational stability, and reproducibility of the proposed TOAB/ZnPp−C60/GCE were further investigated by the amperometric current−time technique. As shown in Figures S4A and S4B of the Supporting Information, the current responses of TOAB/ZnPp−C60/GCEto 35.0 μM H2O2 or 7.0 μM nitrite retained 98.9% and 96.7% of its original current responses after 18 days storage at room temperature. The results reveal a better storage stability, in comparison with those of reported sensors (Tables S4 and S5 of the Supporting Information). The operational stability was examined by continuous determination of 20.0 μM nitrite for 60 min. The response kept stable and was still sensitive toward nitrite after 60 min (Figure S4C of the Supporting Information). Moreover, the reduction peak III and oxidation peak IV of cyclic voltammo-

Figure 6. Representative amperometric current−time curves of TOAB/ ZnPp−C60/GCE in 4.0 mL stirring 0.5 M KCl aqueous solution with successive additions of 0.028 M H2O2 at (A) −1.17 V and (B) 4.0 mM nitrite at 0.82 V. Inset: plots of current vs the concentration of H2O2 or nitrite.

GCE responded quickly to the reduction of H2O2 and achieved the steady-state current within 2 s after every addition of H2O2 (Figure S2 of the Supporting Information). The catalytic current increased linearly with the concentration of H2O2 in the range from 0.035 to 3.40 mM (R = 0.9992, inset in Figure 6A). The sensitivity was obtained to be 215.6 μA mM−1, which is 130-fold larger than that of TOAB/C60/GCE (1.65 μA mM−1).8 The limit of detection (LOD) for H2O2 detection was calculated to be 0.81 μM (S/N = 3).24 Figure 6B shows excellent amperometric response with successive additions of nitrite. The catalytic current increased greatly and achieved the steady-state current within 1.2 s upon every addition of nitrite (Figure S2 of the Supporting Information). The linear concentration range was from 2.0 μM to 0.164 mM (R = 0.9973, inset in Figure 6B), with a sensitivity of 249.9 μA mM−1. The LOD was evaluated to be 1.44 μM (S/N = 3). The analytical performances of the previously reported H2O2 and nitrite sensors are summarized in Tables S2 and S3 of the Supporting Information. As can be seen, the linear ranges for H2O2 detection are mostly over 2 orders of magnitude, and the LODs are at about 10 μM level. As for nitrite sensors, the 6289

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21201037, 21303021, 21175158, and 81171666), Natural Science Foundation of Anhui Province (Grant 1408085QB39), Natural Science Foundation of Higher Education Institutions in Anhui Province (Grants KJ2013B202 and KJ2011A210), and the Innovation Training Program for the Anhui College students (Grant AH201310371039). We thank Prof. Xiaojun Yao from Lanzhou University for the supplement of the Gaussian 09 software.

grams with ten successive scans on a same sensor were almost the same (Figure S5 of the Supporting Information). The relative standard deviations (RSD) were 1.3% and 2.6%, respectively, indicating an excellent operational stability. The intrareproducibility was evaluated by analysis of nitrite using three TOAB/ZnPp−C60/GCEs prepared on a same GCE. As can be seen from Figure S6 of the Supporting Information, the three TOAB/ZnPp−C60/GCEs showed identical responses to the nitrite. The maximum variation coefficient was less than 4.2%. Moreover, the inter-reproducibility was studied by analysis of 7.0 μM nitrite on ten TOAB/ZnPp−C60/GCE with different GCEs independently. The RSD was 5.0% (Figure S4D of the Supporting Information), indicating an excellent reproducibility for reliable analysis. Practicality of TOAB/ZnPp−C60/GCE. In order to evaluate the practical application ability of TOAB/ZnPp−C60/GCE, the determination of nitrite in river water and rainwater was investigated. The collected river and rain samples were first filtrated with a 0.45 μM membrane filter and then injected in 0.5 M KCl solution by microinjector. The concentrations of nitrite in the samples were analyzed by standard addition method using the proposed sensor. As shown in Figure S7 of the Supporting Information and Table 1, the good recoveries from 95.8 to 101.8% were obtained, indicating the appreciable practicality of the presented sensor.

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rain water river water

content (μM)

added (μM)

found (μM)

recovery (%)

RSD (%)

2.99 2.99 3.62 3.62

5.00 10.0 5.00 10.0

7.78 12.84 8.71 13.42

95.8 98.5 101.8 98.0

1.8 2.3 2.2 2.9

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CONCLUSIONS ZnPp−C60 was designed and synthesized to fabricate nonenzymatic electrochemical sensor using TOAB film modified GCE. The electronic structure of ZnPp−C60 investigated by DFT shows a bent conformation with a small energy gap to transport electrons easily. The fabricated sensor has excellent electrochemical properties with four couples of redox peaks in a wide potential range and exhibits highly sensitive electrocatalysis for H2O2 and nitrite detections with wide linear ranges, low LODs, and good stability and reproducibility. The presented sensor provides a promising way to develop efficient nonenzymatic electrochemical sensors.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

(1) Privett, B. J.; Shin, J. H.; Schoenfisch, M. H. Anal. Chem. 2010, 82, 4723−4741. (2) Mu, Y.; Jia, D. L.; He, Y. Y.; Miao, Y. Q.; Wu, H. L. Biosens. Bioelectron. 2011, 26, 2948−2952. (3) Toghill, K. E.; Compton, R. G. Int. J. Electrochem. Sci. 2010, 5, 1246−1301. (4) Zhou, M.; Guo, J.; Guo, L. P.; Bai, J. Anal. Chem. 2008, 80, 4642− 4650. (5) Patolsky, F.; Tao, G.; Katz, E.; Willner, I. J. Electroanal. Chem. 1998, 454, 9−13. (6) Wei, M.; Li, M. X.; Li, N. Q.; Gu, Z. N.; Zhou, X. H. Electroanalysis 2002, 14, 135−140. (7) Goyal, R. N.; Gupta, V. K.; Sangal, A.; Bachheti, N. Electroanalysis 2005, 17, 2217−2223. (8) Liu, W.; Gao, X. Electrochem. Commun. 2008, 10, 1377−1380. (9) Li, M. X.; Li, N. Q.; Gu, Z. N.; Zhou, X. H.; Sun, Y. L.; Wu, Y. Q. Anal. Chim. Acta 1997, 356, 225−229. (10) Wei, L.; Lei, Y. L.; Fu, H. B.; Yao, J. N. ACS Appl. Mater. Interfaces 2012, 4, 1594−1600. (11) Sotiropoulou, S.; Gavalas, V.; Vamvakaki, V.; Chaniotakis, N. A. Biosens. Bioelectron. 2003, 18, 211−215. (12) D’Souza, F.; Amin, A. N.; El-Khouly, M. E.; Subbaiyan, N. K.; Zandler, M. E.; Fukuzumi, S. J. Am. Chem. Soc. 2012, 134, 654−664. (13) Trinh, C.; Kirlikovali, K. O.; Bartynski, A. N.; Tassone, C. J.; Toney, M. F.; Burkhard, G. F.; McGehee, M. D.; Djurovich, P. I.; Thompson, M. E. J. Am. Chem. Soc. 2013, 135, 11920−11928. (14) Hasobe, T.; Saito, K.; Kamat, P. V.; Troiani, V.; Qiu, H. J.; Solladié, N.; Kim, K. S.; Park, J. K.; Kim, D.; D’Souza, F.; Fukuzumi, S. J. Mater. Chem. 2007, 17, 4160−4170. (15) Yamada, H.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Ahn, T. K.; Kim, S. K.; Kim, D.; Fukuzumi, S. J. Am. Chem. Soc. 2003, 125, 9129− 9139. (16) Fan, S. H.; Yang, S. G.; Wu, Z. Y.; Chen, X.; Zhan, M. X. Chin. J. Inorg. Chem. 2006, 22, 1299−1302. (17) Frisch, M. J., et al. Gaussian 09, revision A.02; Gaussian, Inc.: Wallingford, CT, 2009. (18) Yang, L.; Ling, Y.; Zhang, Y. J. Am. Chem. Soc. 2011, 133, 13814− 13817. (19) Schuster, D. I.; Cheng, P.; Jarowski, P. D.; Guldi, D. M.; Luo, C.; Echegoyen, L.; Pyo, S.; Holzwarth, A. R.; Braslavsky, S. E.; Williams, R. M.; Klihm, G. J. Am. Chem. Soc. 2004, 126, 7257−7270. (20) de Miguel, G.; Wielopolski, M.; Schuster, D. I.; Fazio, M. A.; Lee, O. P.; Haley, C. K.; Ortiz, A. L.; Echegoyen, L.; Clark, T.; Guldi, D. M. J. Am. Chem. Soc. 2011, 133, 13036−13054. (21) Song, F.; Echegoyen, L. J. Phys. Chem. B 2003, 107, 5844−5850. (22) D’Souza, F.; Gadde, S.; Zandler, M. E.; Arkady, K.; El-Khouly, M. E.; Fujitsuka, M.; Ito, O. J. Phys. Chem. A 2002, 106, 12393−12404. (23) Ding, Y.; Wang, Y.; Lei, Y. Biosens. Bioelectron. 2010, 26, 390−397. (24) Mani, V.; Dinesh, B.; Chen, S. M.; Saraswathi, R. Biosens. Bioelectron. 2014, 53, 420−427.

Table 1. Determination Results of Nitrite in Water Samples (n = 4) samples

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The authors declare no competing financial interest. 6290

dx.doi.org/10.1021/ac500245k | Anal. Chem. 2014, 86, 6285−6290