Electrochemical Sensor for Lead Cation Sensitized with a DNA

Article pubs.acs.org/ac

Electrochemical Sensor for Lead Cation Sensitized with a DNA Functionalized Porphyrinic Metal−Organic Framework Lin Cui, Jie Wu, Jie Li, and Huangxian Ju* State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, P. R. China S Supporting Information *

ABSTRACT: An efficient electrochemical sensor was presented for lead cation detection using a DNA functionalized iron−porphyrinic metal− organic framework (GR−5/(Fe−P)n-MOF) as a probe. The newly designed probe showed both the recognition behavior of GR−5 to Pb2+ with high selectivity and the excellent mimic peroxidase performance of (Fe−P)nMOF. In the presence of Pb2+, GR−5 could be specifically cleaved at the ribonucleotide (rA) site, which produced the short (Fe−P)n-MOF-linked oligonucleotide fragment to hybridize with hairpin DNA immobilized on the surface of screen-printed carbon electrode (SPCE). Because of the mimic peroxidase property of (Fe−P)n-MOF, enzymatically amplified electrochemical signal was obtained to offer the sensitive detection of Pb2+ ranging from 0.05 to 200 nM with a detection limit of 0.034 nM. In addition, benefiting from the Pb2+-dependent GR−5, the proposed assay could selectively detect Pb2+ in the presence of other metal ions. The SPCE based electrochemical sensor along with the GR−5/ (Fe−P)n-MOF probe exhibited the advantages of low-cost, simple fabrication, high sensitivity and selectivity, providing potential application of on-site and real-time Pb2+ detection in complex media.

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derivatives have been used to catalyze organic reactions,26−30 including olefin epoxidation29 and CO2/propylene oxide coupling reactions.30 Fe(III)-based MOFs have been reported to exhibit peroxidase-like catalytic activity and used to construct colorimetric biosensing methods for detection of H2O2 and ascorbic acid.24 Recently, a porous iron-porphyrinic MOF has been synthesized to show peroxidase activity similar to heme protein myoglobin in an organic solvent.26 Our previous works also demonstrated the good performance of a series of ironporphyrinic MOFs as mimetic catalysts and employed them to construct electrochemical DNA sensing platforms.31,32 In the present work, a highly sensitive and selective electrochemical sensor for Pb2+ was proposed with a newly designed DNA functionalized iron−porphyrinic MOF (GR−5/ (Fe−P)n-MOF). The stable (Fe−P)n-MOF could produce multisignal species to achieve the high sensitivity, while the GR−5, which was linked to (Fe−P)n-MOF via the Au nanoparticles (AuNPs), achieved the high selectivity of this method (Scheme 1). Different from the fluorescence detection,5,6 after the GR−5 substrate strand was catalytically cleaved by Pb2+, the short (Fe−P)n-MOF-linked oligonucleotide fragment further hybridized with hairpin probe (HP) immobilized on the surface of screen-printed carbon electrode (SPCE) to act as an enzyme mimic. Because of the affinity

s a well-known and nondegradable environmental pollutant, lead cation (Pb2+) can accumulate in human body through the food chain and lead to adverse effects on the immune, central nervous, and reproductive systems, particularly in children.1−4 Therefore, it is essential to develop the analytical techniques for routine and effective monitoring of Pb2+. Different Pb2+ sensors have been proposed for sensitive detection of Pb2+ by using cost-effective Pb2+-dependent DNAzyme.5−9 This enzyme is quite stable and can remain its activity or binding ability after repeated denaturation and renaturation.10,11 The detection sensitivity of electrochemical sensors for Pb2+ can be improved by replacing small redox molecules (e.g., ferrocene12) with signal amplification strategies by employing inorganic nanoparticles combined with additional amplification processes, such as surface preconcentration13,14 or enzymatic recycling.15,16 However, these amplification ways usually need long detection time, and the native enzymes suffer from the certain disadvantages of time-consuming separation and purification, high cost, easy denaturation, and being susceptible to environmental conditions.17−19 Thus, a new detection strategy to produce more signal products with costefficient and stable enzyme mimic has been considered as another opportunity for electrochemical detection of Pb2+.20,21 Metal−organic frameworks (MOFs), which consist of metal ions or clusters connected by organic linker groups, have recently emerged as one kind of promising materials to develop novel artificial enzymes, because their uniform cavities can generate a high density of biomimic active centers.22−25 For example, MOFs created with porphyrin or porphyrin © XXXX American Chemical Society

Received: August 27, 2015 Accepted: October 2, 2015

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DOI: 10.1021/acs.analchem.5b03287 Anal. Chem. XXXX, XXX, XXX−XXX

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mV s−1. Chronoamperometric response was recorded at +100 mV. The scanning electron microscopic (SEM) morphology was examined at Hitachi S-4800 scanning electron microscope (Japan). Transmission electron microscopic (TEM) images were obtained with a JEM 2100 high-resolution (JEOL, Japan). Powder X-ray diffraction (XRD) patterns were measured on RigakuDmax 2200 X-ray diffractometer with Cu Kα radiation (λ = 1.5416 Å). The ultraviolet−visible (UV−vis) absorption spectra were recorded with a Nanodrop-2000C nanophotometer (Thermo Scientific). Fourier transform-infrared (FT-IR) spectra were recorded on a Nicolet NEXUS870 Fourier transform-infrared spectrometer (Madison, WI). Nitrogen adsorption−desorption isotherms and pore size distributions were measured at 77 K using a micrometeritics ASAP 2020 system. Preparation of (Fe−P)n-MOF. The (Fe−P)n-MOF was prepared according to the literature with slight modification:33 TCPP (39.54 mg), FeCl3·6H2O (40.55 mg) and HCl (1.2 mL, 0.1 M in ethanol) were homogeneously dissolved in a mixture of 4 mL of DMF and 8 mL of ethanol with assistance of sonication treatment. The final mixture was placed into a 50 mL Teflon stainless vessel and was then thermally treated at 150 °C in an oven for 48 h, followed by slow cooling to room temperature. The crystals were collected via filtration and washed with DMF and ethanol, then dried at 60 °C in a vacuum oven. Preparation of GR−5/(Fe−P)n-MOF Probe. First, (Fe− P)n-MOF (6.0 mg) was dispersed in 1 mL of ethanol. This dispersion was added to a mixture of 0.2 mL of ammonium hydroxide (1.48 M) and 13.8 mL ethanol, and then TEOS (1.8 mL, 8.12 mmol) was added. After the mixture was stirred for reaction at room temperature for 2 h, the products were harvested by centrifuging and washing three times with ethanol and dried under vacuum overnight to get the SiO2 coated (Fe− P)n-MOF composite. Then, 4 mg of SiO2 coated (Fe−P)nMOF was added into 4 mL of 20 mM MPTS ethanol solution and refluxed at 65 °C for 3 h. After centrifugation and washing several times with ultrapure water, the (Fe−P)n-MOF modified with MPTS was dried at 50 °C for 12 h. The MPTS modified (Fe−P)n-MOF was further modified with AuNPs by mixing 4 mL of AuNPs (13 nm diameter) solution34 with 2 mL of MPTS modified (Fe−P)n-MOF (1 mg mL−1) and shaking vigorously for 2 h. The formed AuNPs(Fe−P)n-MOF composite was collected by centrifugation, washed with ultrapure water three times, and dispersed in 6 mL of 50 mM Tris-acetate buffer (pH 7.0). A volume of 70 μL of GR−5 (10 μM, in 50 mM pH 8.2 Trisacetate buffer containing 300 mM NaCl) was heated at 95 °C for 10 min and then cooled to room temperature. After the treated GR−5 was activated with 5 μL of 10 mM TCEP to reduce disulfide bond, it was mixed with 500 μL of AuNPs(Fe−P)n-MOF suspension to incubated for 24 h at room temperature to obtain GR−5/(Fe−P)n-MOF probe, which was blocked with 1% BSA (w/v) for 1 h and dispersed in 500 μL of 50 mM Tris-acetate buffer (containing 300 mM NaCl, pH 7.0) to store at 4 °C before use. Preparation of Electrochemical Sensor. The SPCE was first electrochemical activated by performing the CV scan (10 cycles) in 0.01 M phosphate buffer (pH 7.2) with a potential range from −0.3 to +0.6 V at a scan rate of 500 mV s−1. After washing with ultrapure water and drying with nitrogen, 3 μL of 0.05 mg mL−1 chitosan and AuNPs solution were sequentially dropped on the working electrode. After drying at room

Scheme 1. Schematic Representation of (A) Preparation of GR−5/(Fe−P)n-MOF Probe and (B) Electrochemical Detection of Pb2+

recognition property of GR−5 to Pb2+ and the mimic peroxidase performance of (Fe−P)n-MOF, the assay method could selectively detect Pb2+ down to 0.034 nM. The good performance showed its potential application for the detection of Pb2+ in different environments.

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EXPERIMENTAL SECTION Materials and Reagents. Chloroauric acid (HAuCl4· 4H2O) and trisodium citrate were obtained from Shanghai Reagent Company (Shanghai, China). Tris(2-carboxyethyl)phosphine hydrochloride (TCEP), 5,10,15,20-tetrakis(4-carboxyl)-21H,23H-porphyrin (TCPP), tetramethoxysilane (TEOS), 3-mercaptopropyltrimethoxysilane (MPTS), chitosan (≥85% deacetylation), bovine serum albumin (BSA), 3,3′,5,5′tetramethylbenzidine (TMB, 99%), hemin, and horseradish peroxidase (HRP) (EC 1.11.1.7, 261 Units mg−1, powder) were used as received from Sigma-Aldrich. Iron(III) chloride hexahydrate (FeCl3·6H2O), sodium acetate (NaAc), and acetic acid (HAc) were purchased from Nanjing Chemical Reagent Co., Ltd. Lead nitrate (Pb(NO3)2·3H2O) was purchased from Shanghai Sinpeuo Fine Chemical Co., Ltd. (China). Other reagents were of analytical grade and used as received. Ultrapure water obtained from a Millipore water purification system (≥18 MΩ, Milli-Q, Millipore) was used in all assay. The synthetic oligonucleotides were purchased from Sangon Biological Engineering Technology & Services Co., Ltd. (Shanghai, China) and purified using high-performance liquid chromatography. Their sequences were as following: HP: 5′-ACAGACATCATCTCTTCCTCTGT-(CH2)6-SH3′ GR−5: 5′-ACAGACATCATCTCTGAAGTAGCGCCGCCGTATAGTGAGAAACTCACTATrAGGAAGAGATGATGTCTGTTTTTT-(CH2)6-SH3′ Apparatus. All electrochemical experiments were performed on CHI 660D electrochemical workstation (CH Instruments Inc.) with a portable homemade SPCE, which consisted of a carbon working electrode (2 mm diameter), a carbon auxiliary electrode, and an Ag/AgCl reference electrode. Cyclic voltammetry (CV) was carried out at a scan rate of 50 B

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Figure 1. (A) XRD pattern and (B) N2 adsorption (●) and desorption (◀) isotherms of (Fe−P)n-MOF, (C) FT-IR spectra of (a) TCPP and (b) (Fe−P)n-MOF, and (D) UV−vis absorption spectra of (a) AuNPs, (b) (Fe−P)n-MOF, and (c) AuNPs-(Fe−P)n-MOF.

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RESULTS AND DISCUSSION Characterization of (Fe−P)n-MOF. XRD method was used to examine the phase and structure of the (Fe−P)n-MOF. As shown in Figure 1A, the diffraction peaks around 9.18°, 18.43°, 23.72° and 31.11° were observed, which corresponded to the (100), (200), (111), and (121) planes, respectively. This result was coincident with that of the previously reported MOF,33 indicating the synthesized (Fe−P)n-MOF exhibited well-recognized crystalline diffraction patterns. The N 2 adsorption−desorption cueves of (Fe−P)n-MOF showed the typical type I isotherm (Figure 1B), which sharply increased at low relative pressure, suggesting that the micropores were dominant. The surface area of (Fe−P)n-MOF was determined to be 279 m2 g−1 by the Brunanuer−Emmer−Teller (BET) method, which was beneficial to the formation of the catalytic interface. The structure of (Fe−P)n-MOF was characterized with FT-IR (Figure 1C). Compared to TCPP (curve a), (Fe− P)n-MOF showed a downshift of the CO stretch from 1701 to 1675 cm−1, an intense fingerprint of Fe−N stretching at 1008 cm−1 and the disappearance of N−H in-plane bending vibration at 967 cm−1 (curve b), reflecting the metalation of porphyrin ring with iron.35,36 Meanwhile, (Fe−P)n-MOF, similar to TCPP, exhibited the characteristic peaks for big ring skeleton absorption at 1606, 1562, and 1401 cm−1, and benzene ring at 866 and 796 cm−1. This result indicated the (Fe−P)n-MOF maintained the pyrrole ring structure of TCPP.32 The modification of (Fe−P)n-MOF with AuNPs was characterized with UV−vis absorption spectra (Figure 1D). The absorption spectrum of (Fe−P)n-MOF showed an intense Soret band at 426 nm (curve b), which was attributed to the coordination reaction between iron atoms and porphyrin. After further modification with AuNPs, an absorption peak at 520 nm appeared (curve c), suggesting the successful modification of AuNPs on (Fe−P)n-MOF. The morphologies of (Fe−P)n-MOF played a critical role in the preparation of mimic enzyme. The (Fe−P)n-MOF

temperature, the modified electrode was washed with ultrapure water and dried with nitrogen. On the other hand, the thiolated HP (0.1 μM) was activated by 10 μM TCEP for 1 h to reduce disulfide bonded oligos. Then, 3 μL of the HP was pipetted onto the AuNPs modified working electrode and incubated overnight at room temperature in 100% humidity. After rinsing with 50 mM Tris-acetate buffer (pH 7.0), the working electrode was passivated with 1% BSA for 2 h to remove nonspecific adsorption sites. The obtained HP/AuNPs modified working electrode was used as the electrochemical sensor for Pb2+. Detection of Pb2+. A volume of 10 μL of Pb2+ at various concentrations or samples and 90 μL of GR−5/(Fe−P)n-MOF probe were mixed in 50 mM Tris-acetate buffer (300 mM NaCl, pH 7.0). Then, 3 μL of the mixture was dropped onto the HP modified electrode and incubated at 37 °C for 90 min. After washing with 50 mM Tris-acetate buffer (pH 7.0), 100 μL of 0.1 M pH 5.5 HAc-NaAc buffer containing 0.89 mM TMB and 0.61 mM H2O2 was added on the sensor surface. Measurements were immediately made with a chronoamperometric method at +100 mV. Sample Preparation. The 1 g of soil sample was mixed with 10 mL of HNO3 (99.7%)/H2O [1:1 (v/v)] and heated at ∼95 °C (without boiling) for 15 min. After cooling to below 70 °C, 5 mL of concentrated HNO3 was added and heated under reflux for 30 min at ∼95 °C. The sample was evaporated to ∼5 mL without boiling and cooled to below 70 °C. The 2 mL of ultrapure water and 10 mL of H2O2 (30%) were slowly added, respectively. The solution was heated until effervescence subsided. Concentrated HCl (5 mL) and ultrapure water (10 mL) were then added and cooled to below 70 °C. The obtained sample was heated to reflux (without boiling) for 15 min. After cooling to room temperature, the sample was diluted to 100 mL with ultrapure water and filtered. The resulting sample solution was spiked with standard Pb2+ solutions for Pb2+ determination using the proposed procedure. C

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and the blue solution changed to be yellow (c). These changes could be expressed as follows:

consisted of a rod shape-like crystal with the diameter of 80− 100 nm (Figure 2A,B). The AuNPs with an average diameter of

The catalytic performance of (Fe−P)n-MOF for oxidation of TMB by H2O2 was further confirmed by electrochemical measurements (Figure 3B). Compared with the CV response of TMB on bare SPCE (curve a), the CV response on (Fe−P)nMOF modified SPCE showed the increase of the reduction peak current (curve b), indicating a typical electrocatalytic process, which was attributed to the enzymatic catalysis of the immobilized (Fe−P)n-MOF to the oxidation reaction of TMB by H2O2. The mechanism of enzymatic catalysis was similar to that with natural or the mimetic peroxidase reported previously.37,38 Accordingly, a significant reduction current was obtained on (Fe−P)n-MOF modified SPCE by chronoamperometric detection at +100 mV (Figure 3C), which was used for the sensitive detection of Pb2+ in the present work. The steady-state kinetic assay of the (Fe−P)n-MOF in the catalytic system of TMB and H2O2 showed a Michaelis constant (Km) value of 1.27 mM with H2O2 as the substrate (Figure S1 in the Supporting Information), which was smaller than 3.70 mM of HRP and 2.74 mM of hemin, suggesting that the (Fe−P)n-MOF had higher affinity to H2O2 than HRP and hemin. Moreover, the Km value of (Fe−P)n-MOF with TMB as the substrate was calculated to be 0.63 mM, which was close to 0.434 mM of HRP and much smaller than 4.84 mM of hemin, suggesting that the (Fe−P)n-MOF had also a high affinity to TMB mediator.39 Stability of (Fe−P)n-MOF. A favorable mimic enzyme should process good catalytic ability in broad conditions. After the natural enzyme (HRP), common mimic (hemin) and (Fe− P)n-MOF modified SPCEs were exposed to different pHs ranging from 3 to 10 (Figure 4A) and temperatures ranging from 4 to 60 °C (Figure 4B) for 2 h, their relative activities were measured in the detection solution. After exposure to pH 3.0 and 10.0 solution for 2 h, the catalytic activity of (Fe−P)nMOF could remain of 62.3% and 41.9%, respectively (curve a), while the HRP and hemin remained only 1.3% and 18.6% of their activity at pH 3.0, and 11.6% and 20.8% of their activity at pH 10.0, respectively (curves b and c). The effect of the temperature on its stability showed similar phenomena. In the temperature range of 4−60 °C, the (Fe−P)n-MOF was stable

Figure 2. (A) SEM image of (Fe−P)n-MOF and TEM images of (B) (Fe−P)n-MOF, (C) AuNPs, and (D) AuNPs-(Fe−P)n-MOF.

13 nm (Figure 2C) were well dispersed on the surface of (Fe− P)n-MOF matrix (Figure 2D), conforming the successful modification of AuNPs on (Fe−P)n-MOF using MPTS linker. Mimetic Catalysis of (Fe−P)n-MOF. The peroxidase-like catalytic performance of (Fe−P)n-MOF was evaluated with an typical colorimetric oxidation reaction of TMB by H2O2 (Figure 3A). The (Fe−P)n-MOF could be well dispersed in water to form a homogeneous suspension with black color (a). Upon the addition of 5 μL of 1 mg mL−1 (Fe−P)n-MOF suspension to 500 μL of the mixture of TMB and H2O2, TMB molecule lost one electron and transformed into the status of cation radical, then the colorless solution turned to blue color immediately (b), indicating (Fe−P)n-MOF possessed the peroxidase-like activity to catalyze the oxidation of TMB in the presence of H2O2. Like the oxidation reaction catalyzed by peroxidase, the catalytic activity of (Fe−P)n-MOF could be stopped by H2SO4, leading to the cation radical of TMB molecule, which further lost another electron to form diamine,

Figure 3. (A) Photograph of (a) 1 mg mL−1 (Fe−P)n-MOF, (b, c) the mixtures of 0.89 mM TMB, 0.61 mM H2O2, and 0.01 mg mL−1 (Fe−P)nMOF before (b) and after (c) adding H2SO4 to quench the catalytic reaction of (Fe−P)n-MOF. (B) Cyclic voltammograms and (C) chronoamperometric curves of 0.1 M pH 5.5 HAc-NaAc buffer containing 0.89 mM TMB and 0.61 mM H2O2 at SPCE (a) and (Fe−P)n-MOF/ SPCE (b). D

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Figure 4. Relative electrocatalytic activity of (a) (Fe−P)n-MOF, (b) HRP, and (c) hemin modified SPCEs in 0.1 M HAc-NaAc buffer containing 0.89 mM TMB and 0.61 mM H2O2 after exposure to different (A) pHs at 37 °C and (B) temperatures at pH 7.0 for 2 h.

Figure 5. Effects of (A) HP concentration for sensor preparation, (B) pH, (C) reaction time and (D) reaction temperature for Pb2+ detection on chronoamperometric response to 100 nM Pb2+ at +100 mV. Error bars represent standard deviations (S.D., n = 3).

and remained at 87% of its activity at 60 °C (curve a), but HRP and hemin showed narrow temperature ranges for their remaining activity and remained at 22% and 47% of their activity at 60 °C, respectively (curves b and c). Obviously, (Fe− P)n-MOF was more favorable in a wider range of pH and temperature than HRP and hemin that were very sensitive to pH and temperature, suggesting that (Fe−P)n-MOF had a great potential application in bioanalysis. Feasibility of Sensing Method. In the presence of 100 nM Pb2+, the electrochemical sensor showed a decay curve for the plot of current (I) vs time (t) in 0.1 M pH 5.5 HAc-NaAc buffer containing 0.89 mM TMB and 0.61 mM H2O2 at +100 mV (vs Ag/AgCl reference electrode), which rapidly reached the steady-state current (∼672 nA) within 20 s, and the steadystate current was ∼24 times higher than that in the absence of Pb2+ (∼28 nA) (Figure S2 in the Supporting Information). This result could be contributed to the fact that the Pb2+ activated GR−5 to cleave the substrate strand into two DNA fragments at the ribonucleotide (rA) site, and the short (Fe− P)n-MOF-linked oligonucleotide fragment hybridized with the immobilized HP at the sensor surface, which led to the linkage of the (Fe−P)n-MOF to the sensor surface to catalyze the oxidation of TMB by H2O2 and the electrochemical reduction of the oxidation product on the sensor surface, as shown in Scheme 1. The catalytic current increased greatly the sensitivity for detection of Pb2+. Optimization of Sensing Conditions. In order to obtain high sensitivity for Pb2+ detection, the influences of HP concentration for sensor preparation, and reaction conditions such as pH of reaction solution, reaction time, and reaction temperature on the chronoamperometric response of the resulting sensor in 0.1 M pH 5.5 HAc-NaAc buffer containing 0.89 mM TMB and 0.61 mM H2O2 were examined (Figure 5). The chronoamperometric response increased with the increasing concentration of the used HP up to 0.1 μM, afterward the response decreased due to the fact that the dense HP on sensor surface was unfavorable to the hybridization of the short (Fe−P)n-MOF-linked oligonucleotide fragment (Figure 5A). Thus, 0.1 μM HP was selected for sensor preparation. The effect of pH on the cleavage of GR−5 and the hybridization of short (Fe−P)n-MOF-linked oligonucleotide fragment with HP was shown in Figure 5B. In the examined pH range, the maximum chronoamperometric response occurred at approximately pH 7.0, which was considered as the optimum pH. Fast detection was an important character of the simple Pb2+ sensor. The chronoamperometric response increased with

the increasing reaction time and tended to a steady value at 90 min (Figure 5C). Thus, 90 min was enough for the reaction. In addition, the chronoamperometric response increased gradually along with the temperature up to 37 °C and then rapidly declined at higher temperature (Figure 5D). Thus, the optimal reaction temperature was chosen as 37 °C. Detection of Pb2+. Under optimal sensing conditions, the chronoamperometric response of the obtained sensor in 0.1 M pH 5.5 HAc-NaAc buffer containing 0.89 mM TMB and 0.61 mM H2O2 at +100 mV increased with the increasing concentration of Pb2+ (Figure 6A). The calibration plot showed

Figure 6. (A) Chronoamperometric detection of Pb2+ at 0, 0.05, 0.1, 0.5, 1, 5, 10, 50, 100, and 200 nM (from a to j) at +100 mV and (B) plot of current at 100 s vs logarithmic value of Pb2+ concentration. Error bars show the SD for three independent experiments.

a good linear relationship between the chronoamperometric response and the logarithm (log) value of Pb2+ concentration ranging from 0.05 to 200 nM with a correlation coefficient of 0.9994. The detection limit for Pb2+ was experimentally found to be 0.034 nM (3δ), which was at least 350 times lower than that of 10 μg L−1 in drinking water permitted by World Health Organization.40 The detection limits of the proposed and previously reported sensors were compared in Table S1. The detection limit of the proposed sensor was much lower than those of other electrochemical methods and much superior to the fluorescent and colorimetric sensors for Pb2+. This result indicated that the artificial enzyme was able to replace the redox label to amplify the signal through the catalytic reaction. Selectivity of the Sensor. To investigate the selectivity, the sensor was determined by exposing it to several metal ions E

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Analytical Chemistry including Ca2+, Fe3+, Cd2+, Co2+, Zn2+, Mn2+, Ni2+, Cu2+, Hg2+, Mg2+, and Ag+ (Figure 7). A negligible response was observed

linear range for Pb2+ detection and good accuracy. The proposed electrochemical sensing method could be extended to detect other analytes with corresponding (Fe−P)n-MOF-based probes and thus possessed great potential for real-time testing.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.5b03287. Catalytical reaction rate plots, Lineweaver-Burk plots, chronoamperometric curves, and comparison of the detection performance among different Pb2+ sensors (PDF)

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Figure 7. Chronoamperometric responses of 5 μM Ca2+, Fe3+, Cd2+, Co2+, Zn2+, Mn2+, Ni2+, Cu2+, Hg2+, Mg2+, Ag+, and 0.1 μM Pb2+ at +100 mV. Mixture represents a solution containing 0.1 μM Pb2+ and 5 μM of these interfering ions.

*Phone/fax: +86-25-83593593. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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when the sensor was challenged with above possible interference ions even at a 50-fold higher concentration than Pb2+ (5 μM vs 0.1 μM). In addition, the current responses of 0.1 μM Pb2+ in the presence of these interfering ions were similar to those without interfering ion. Therefore, the proposed sensor has demonstrated an excellent selectivity for the detection of Pb2+, indicating its potential application for analysis of complex samples. Determination of Pb2+ in Real Samples. To evaluate the analytical reliability and application potential of the proposed sensor in environmental analysis, it was used to detect Pb2+ in soil samples. It was found that the content of free Pb2+ in the tested samples was too low to be detected by the proposed method. However, there was an obvious increase in readout signal after different concentrations of Pb2+ were spiked into the samples. The analytical results were summarized in Table 1.

ACKNOWLEDGMENTS We gratefully acknowledge the National Special Project for Key Scientific Apparatus Development (Grant 2012YQ170000302) and the National Natural Science Foundation of China (Grants 21135002 and 21361162002).

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added (nM)

found (nM)

RSD (%) (n = 3)

recovery (%)

soil 1

1.0 5.0 1.0 5.0

0.95 5.08 1.04 4.86

3.5 3.4 3.7 3.5

95.0 101.6 104.0 97.2

soil 2

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Table 1. Determination of Pb2+ in Soil Samples Using the Proposed Method samples

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The recoveries of 95.0% ∼ 104.0%, as well as the acceptable relative standard deviation (RSD), indicated good accuracy of the proposed method for the detection of Pb2+ in environmental samples.

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CONCLUSIONS A novel GR−5/(Fe−P)n-MOF probe is designed to realize specific recognition and signal amplification for highly sensitive and selective detection of Pb2+. The label-free GR−5 ensures the excellent selectivity, and the (Fe−P)n-MOF as artificial enzyme possesses high affinity to TMB mediator and H2O2 substrate, which leads to high sensitivity of the suggested method. The mimic peroxidase shows good stability in wide ranges of pH and temperature. Moreover, this method based on the low-cost homemade SPCE with the probe shows a wide F

DOI: 10.1021/acs.analchem.5b03287 Anal. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.analchem.5b03287 Anal. Chem. XXXX, XXX, XXX−XXX