Electrochemical Sensing of Bisphenol A on Facet-Tailored TiO2

Article Cite This: Anal. Chem. XXXX, XXX, XXX−XXX

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Electrochemical Sensing of Bisphenol A on Facet-Tailored TiO2 Single Crystals Engineered by Inorganic-Framework Molecular Imprinting Sites Dan-Ni Pei,† Ai-Yong Zhang,*,†,‡ Xiao-Qiang Pan,† Yang Si,† and Han-Qing Yu*,† †

CAS Key Laboratory of Urban Pollutant Conversion, Department of Chemistry, University of Science & Technology of China, Hefei, 230026, China ‡ Department of Municipal Engineering, Hefei University of Technology, Hefei, 230009, China S Supporting Information *

ABSTRACT: Noble metals, nanostructured carbon, and their hybrids are widely used for electrochemical detection of persistent organic pollutants. However, despite of the rapid detection process and high accuracy, these materials generally suffer from high costs, metallic impurity, heterogeneity, irreversible adsorption and poor sensitivity. Herein, the high-energy {001}-exposed TiO2 single crystals with specific inorganic-framework molecular recognition ability was prepared as the electrode material to detect bisphenol A (BPA), a typical and widely present organic pollutant in the environment. The oxidation peak current was linearly correlated to the BPA concentration from 10.0 nM to 20.0 μM (R2 = 0.9987), with a low detection limit of 3.0 nM (S/N = 3). Furthermore, it exhibited excellent discriminating ability, high anti-interference capacity, and good long-term stability. Its good performance for BPA detection in real environmental samples, including tap water, lake and river waters, domestic wastewater, and municipal sludge, was also demonstrated. This work extends the applications of TiO2 semiconductor and suggests that this material could be used as a highly active, stable, low-cost, and environmentally benign electrode material for electrochemical sensing. sensitivity,1 while noble metals have high cost, low stability, and scarce availability. Compared with the nanostructured carbon and noble metals, transition metal oxides are cheap, stable, and can be readily prepared.8 However, such materials have inert redox properties8,9 and are unsuitable for electrochemical detection of POPs. One good example is TiO2. Although it can be used as a catalyst support or a regeneration catalyst for electrochemical detection,16−18 its application for direct POPs detection is limited by its low conductivity and poor reactivity.10−15 Interestingly, recent studies found that the single-crystalline TiO2 possesses a high electric conductivity because of its continuous and ordered bulk crystal structure.19−21 The arrangement and coordination of surface atoms on high-energy

P

ersistent organic pollutants (POPs) are emerging environmental contaminants with great human health risks. Thus, an in situ monitoring of POPs is highly desired. This calls for rapid, accurate, and sensitive detection methods. Electrochemical sensing offers a great potential due to its high accuracy, rapidity, and simplicity.1,2 An electrochemical sensor typically consists of electrochemical transducer coated with a chemical or biochemical film as conducting material.1 However, the direct electrochemical sensing of POPs is difficult, because the electrochemical sensors have weak responses to some pollutants.1,2 So far, several nanostructured carbon materials (e.g., carbon nanotube and graphene), noble metals (e.g., Au, Pt, and Pd) and their hybrids (e.g., Pt/carbon nanotube and Pd/graphene) and quantum dots (e.g., CoTe) have been used as electrochemical detecting materials.3−7 However, carbon nanomaterials suffer from metallic impurity, heterogeneity, agglomeration, irreversible adsorption, weak response, and poor © XXXX American Chemical Society

Received: October 29, 2017 Accepted: February 20, 2018 Published: February 20, 2018 A

DOI: 10.1021/acs.analchem.7b04466 Anal. Chem. XXXX, XXX, XXX−XXX

Article

Analytical Chemistry

Scheme 1. Fabrication of the Facet-Tailored TiO2 Single Crystals Engineered with BPA Molecular Recognition Sites Anchored onto Surface and Subsurface

{001} polar facet (0.90 J/m2) endow its unique chemical stability, adsorptive properties and catalytic reactivity.20,21 The high electrochemical activity of {001} facet is mainly attributed to the higher density of atomic steps, edges, and kinks of lowcoordinate surface atoms with a large number dangling bonds.20−23 However, TiO2 has a poor surface adsorption capacity for the diffusion-controlled catalysis due to its low surface area and deficient reactive sites. Thus, the interfacial adsorption capacity of TiO2 should be improved if it is to be used as an efficient detecting material for electrochemical sensing. Molecular imprinting (MI) technology offers an effective and versatile tool to improve the surface enrichment performance of a sensor by creating specific tailor-made binding sites with efficient memory of the shape, size, and functional groups of template molecules in appropriate solid matrixes.24−28 By using MI, the shape and functionality of the template can be transcribed onto microporous materials, and the configuration of functional groups may be memorized within a solid matrix.29,30 Recently, a direct inorganic-framework MI method has been developed without organic polymers.31−33 Such a simple imprinting technology exhibits a specific MI ability due to the high affinity between substrate and recognition site on the electrode surface. The binding is driven by various noncovalent interactions, such as hydrogen bonding, van der Waals forces, π−π, and electrostatic effects. Similar to organic MI technology, the main chemical recognition components in inorganic MI technology for selective binding are the surface functional groups anchoring onto inorganic matrix, usually oxygen-containing groups like −OH.31−33 Such an inorganic MI technology is characterized as good stability, high binding sites density, efficient regeneration thermodynamics, and free of surface poisoning. These features favor a selective accumulation and rapid decomposition of analyte for efficient detection.31−33 Herein, we designed a novel TiO2-based electrochemical sensor for detecting POPs in environments. Bisphenol A (BPA) is widely present in the environment and was used as a model POP with severe health risks. The electro-active phenolic groups of BPA make it suitable for electrochemical detection.33−35 TiO2 single crystals (SCs) that possessed high-energy {001} exposed facet and BPA MI sites (MI-TiO2 SCs) were synthesized. The materials were characterized and evaluated as a detecting material. The low electric conductivity and catalytic activity of TiO2 were improved by shape- and facet-engineering, while its poor interfacial adsorption capacity was enhanced by the direct inorganic-framework MI technology. The applicability of the MI-TiO2 SCs BPA detection in water and various other real samples was also examined.

(D520, 5.0 wt %) was purchased from Heshen Co., China. P25 was purchased from Degussa Co., Germany (Figure S1). Hydrofluoric acid (HF, 40 wt %), methanol (CH3OH), ethanol (C2H5OH), isopropanol, phenol (HB), 1,2/1,3/1,4-dihydroxybenzene (DHB), 4-nitrophenol (p-NP), 4-nitroaniline (pNA), and other reagents were purchased from Shanghai Reagent Co., China. All reagents were used as received. Solutions were prepared using deionized water (ρ ≥ 18.2 MΩ cm) and kept in the refrigerator at 4 °C, except BPA stock solution in ethanol. Phosphate buffer solution (PBS, 0.1 M) was freshly prepared with NaH2PO4·2H2O and Na2HPO4· 12H2O, pH was adjusted by 0.1 M H3PO4 or NaOH aqueous solution. Synthesis of TiO2 SCs and MI-TiO2 SCs. MI-TiO2 SCs fabrication is shown in Scheme 1. TiO2 SCs were prepared by a hydrothermal method. Briefly, TBOT was added into HF aqueous solution (24 wt %), then kept at 180 °C for 24 h. To prepare MI-TiO2, 25 mL of TBOT, 15 mL of HF (24 wt %), and 1.5−20 mg of BPA were mixed and stirred for 0.5 h in a 100 mL Teflon autoclave. After cooling to ambient temperature, the white powders were washed with ethanol, distilled water, and 0.1 M NaOH, dried at 60 °C for 6 h in vacuum, and finally calcined at 500 °C for 2 h in air. Electrochemical Detection of BPA. Before modification, the raw glassy carbon electrode (GCE, 3 mm in diameter) was mechanically polished to obtain a mirror-like surface using 0.3 and 0.05 μm Al2O3 powders, followed by rinsing and sonication in distilled water. Then, the electrode was treated by electrochemical activation under continuous cyclic voltammetry (CV) from −1.0 to 1.0 V at 100 mV s−1 in 0.5 M H2SO4 until a stable voltammogram was obtained. For the modified GCE, the polished GCE was uniformly dropped and air-dried by 10 μL of dispersion solution, which was composed of 2 mg of catalyst in 2 mL of isopropanol and 4 μL of nafion after 1-h sonication to form a homogeneous ink. CV and electrochemical impedance spectrum (EIS) measurements were carried out in 5 mM K3[Fe(CN)6]−K4[Fe(CN)6] (1:1) with 0.1 M KCl. Chronocoulometry analysis was performed in 0.1 mM K3[Fe(CN)6] with 0.1 M KCl and 20 μM BPA in 0.1 M PBS (pH 7.0). A differential pulse voltammetry (DPV) test was carried out with BPA of 10−20 μM in 0.1 M PBS (pH 3.0−13.0). The limit of detection (LOD) was estimated from the standard deviation for 10 measurements of blank solution. Plastic products underwent washing, drying, cutting, and leaching by 30 mL of 0.1 M PBS (pH 7.0) at 50 °C in a sealed conical flask with shaking at 30 °C for 1−5 days.36 Surface water samples were collected from Chaohu Lake and Nanfeihe River in Hefei City, China. Tap water samples were collected from our laboratory, while municipal wastewater and sludge samples were collected from a local wastewater treatment plant in Hefei City. All water samples were filtered by a 0.45-μm membrane. All analyses were carried out at ambient temperature.

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EXPERIMENTAL SECTION Chemicals. Titanium tetrabutyloxide (TBOT), BPA, 3,3′,5,5′-tetrabromobisphenol A (TBBPA), and humic acid (HA) were purchased from Aladdin Co., China. Dupont nafion B


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RESULTS AND DISCUSSION Morphological and Structural Properties of MI-TiO2 SCs. Both the pristine and imprinted TiO2 SCs were in the

and structural properties of the well-defined TiO2 SCs remained unchanged after template addition and removal (Figure 1 and Figures S2 and S3). MI sites were formed on the TiO2 SCs surface and subsurface, rather than in lattice spacing.31,32 Fourier transform-infrared spectra provide direct evidence for BPA incorporation and removal to construct specific MI sites on the TiO2 surface and subsurface (Figure 2b). The main peaks occurred at 1598 and 1510 cm−1. The stretching mode of C−O−H occurred at 1237, 1218, and 1177 cm−1 on the aromatic ring. The bending modes of C−H occurred at 827 cm−1 and CH3 at 1421 cm−1. These peaks clearly indicate the presence of BPA in TiO2 matrix and its complete disappearance after calcination.31,32 The stretching mode of O−H near 3340 cm−1 and the bending mode of coordinated Ti−OH at 1612 cm−1 indicate the presence of abundant active groups, which bind specifically with BPA via hydrogen bonding interaction.31,32 Abundant hydroxyls were detected on TiO2 surface, which construct imprinted specific binding cavities with multiple interaction sites toward BPA. Also, the shape and configuration of BPA were memorized by hydrogen bonding and electrostatic interaction.24−28 Moreover, the MI-TiO2 exhibited a larger and rougher surface area to further improve its electrochemical recognition ability over the pristine TiO2 (Table S1). Electrochemical Properties of the MI-TiO2 SCsModified GCE and Its BPA Oxidation Performance. The MI-TiO2 exhibited a much reduced peak-to-peak separation of 120 mV (Figure S4a), suggesting that it was able to accelerate electron transfer with the lowest resistance (Figure S4b). Moreover, its electrochemical surface area (Aeff) was the largest among the three TiO2 materials (Figure S4c,d).

Figure 1. Transmission electron microscope images of TiO2 single crystals (a, b) and MI-TiO2 single crystals with 5.0 mg of BPA template (c, d).

shape of nanosheets, with 50−80 nm of length and 5−10 nm of thickness. For both materials the percentage of exposed {001} facet was over 80% (Figure 1 and Figure S2). After dosing less than 5 mg of BPA as the template, the morphology of TiO2 SCs showed no obvious change. All main peaks of the three samples matched with those of the standard anatase phase (PDF No. 78-2486) (Figure 2a). The TiO2 SCs diffraction peaks became stronger and sharper after calcination.31,32 The morphological

Figure 2. X-ray diffraction patterns (a) and Fourier transform-infrared spectra (b) of TiO2, BPA entrapped-TiO2, and MI-TiO2 with BPA dosage of 5.0 mg. C


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Figure 3. Cyclic voltammetrys of the pure, P25-, TiO2-, and MI-TiO2-modified glassy carbon electrodes (a, b) and their plotted relationships of Ip ∼ v (c), and Ep ∼ lgv (d). Measuring conditions: solution = 0.1 M KCl + 20.0 μM BPA + 0.1 M phosphate buffer solution, pH = 7.0, potential range = 0.0−1.0 V, scan rate = 5, 10, 20, 30, 40, 50, and 100 mV s−1, and effective anode area = 0.196 cm2.

The MI-TiO2 exhibited both the highest current and lowest overpotential among the three TiO2 materials (Figure 3a). In comparison, a smaller overpotential and a lower current were simultaneously observed on the pristine TiO2, compared to P25 (Figure 3a), indicating a reduced activation energy for BPA oxidation on the facet-tailored TiO2 SCs. Also, the weak surface adsorption capacity of the pristine TiO2 limited its overall reaction rate. The peak current increased linearly with a raised scan rate (Figure 3b,c), suggesting that adsorption was involved in the electrochemical BPA oxidation. A linear relationship was found between the peak potential (Ep) and the logarithmic function of potential scan rate (lg v) with a slope of 2.303RT/ αnF (Figure 3d).36,37 Thus, the electron transfer number was around 2 for the BPA oxidation reaction on the MI-TiO2/ GCE.37,38 The BPA diffusion coefficient significantly increased with the increasing BPA surface concentration on the MI-TiO2/GCE (Figures S5−S8), indicating the strong binding interaction between the MI-TiO2 and BPA resulting from its specific molecular recognition capacity. The superior BPA oxidation on the MI-TiO2 (Figure S9a,b) might be attributed to its large electrochemical active surface and excellent surface accumulation ability. In addition, the deposited MI-TiO2 dosage did not significantly affect the DPV detection of BPA (Figure S9c,d). The oxidation current increased when the pH was increased from 3.0 to 6.0, then maintained at a high level at neutral pH 7.0; after that, it decreased as pH was further increased from 8.0 to 13.0 (Figures S10a and S11b). The BPA adsorption onto TiO2 declined under harsh acidic solutions due to its strong protonation ability with a tight electrostatic attraction, whereas it became reverse under strong basic conditions.37,38 The pH value with the highest current was lower than the BPA acid dissociation constant (pKa = 9.73),36

Figure 4. Differential pulse voltammetry responses (a) and calibration curves (b) of the MI-TiO2-modified glassy carbon electrode for different BPA concentrations without additional anodic accumulation. Measuring conditions: solution = 0.1 M KCl + 0.01−20.0 μM BPA + 0.1 M phosphate buffer solution, pH = 7.0, potential range = 0.0−1.0 V, scan rate = 100 mV s−1, and effective anode area = 0.196 cm2.

D


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Analytical Chemistry Table 1. Comparison for BPA Detection between This Work and Referencesa electrode

linear range (μM)

LOD (S/N = 3, nM)

method

ref

/ / / / / CNTs/GCE N-GS tyrosinase-G/CPE tyrosinase-SWCNTs/CPE tyrosinase-MWCNTs/CPE thionine/CPE PAMAM-CoTe QDs/GCE PAMAM-Fe3O4NPs/GCE PEDOT/GCE β-CD-SWCNTs/GCE MAM-MWCNT/GCE MWCNTs/GCE NiTTPS-MWCNTs/GCE COOH-MWCNTs/GCE PAM-MWCNTs/GCE PGA-NH2-MWCNTs/GCE ITO CPE CoPc/CPE MCM-41/CPE CNF/CPE N-CNF/CPE G/GCE Pt-G-CNTs/GCE Au-Pd NPs-G/GCE chitosan-Fe3O4 NPs/GCE chitosan-Fe-rGO/GCE chitosan-G/CILE MWCNTs-Au NPs/GCE Mg-Al-CO3 LDH/GCE tyrosinase-MWCNTs-CoPc-SF/GCE pretreated BDD pretreated pencil C rGO-CNTs-Au NPs/SPE ZnO-CNTs/CILE MI-TiO2 SCs

2.19−219.29 0.0004−0.4000 0.0004−0.2193 79−16600 0.8−12.0 0.3−100.0 0.01−1.30 0.1−15.0 0.1−12.0 1.0−16.0 0.15−45.00 0.013−9.890 0.01−3.07 90.0−410.0 0.018−18.500 0.01−40.80 0.05−20.00 0.05−50.00 0.001−10.000 0.005−20.000 0.1−10.0 5.0−120.0 0.025−1.000 0.0875−12.5000 0.22−8.80 0.8−50.0 0.1−60.0 0.05−1.00 0.06−10.00, 10.00−80.00 0.01−5.00 0.05−30.00 0.06−11.00 0.1−800.0 0.02−20.00 0.01−1.05 0.05−3.00 0.44−5.20 0.05−5.00, 5.00−10.00 0.00145−0.02,0.02−1.49 0.002−700.000 0.01−20.00

170 0.08 0.04 70000 310 98 5 100 20 1000 150 1 5 22000 1 5 20 15000 5 1.7 20 290 7.5 10 38 100 50 46.9 42 4 8 17 26.4 7.5 5 30 210 3.1 0.8 9 3.0

LC−UV ESI-LC−MS GC/MS FL CL LC−amperometry amperometry amperometry amperometry amperometry amperometry amperometry amperometry amperometry amperometry amperometry amperometry amperometry LSV LSV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV SWV DPV

39 40 41 42 43 44 45 46 46 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 62 63 64 65 66 67 67 68 69 70 71 72 73 74 this work

a

Abbreviations: LC−UV (high performance liquid chromatography with UV spectroscopy detector); ESI-LC−MS (high performance liquid chromatography with online electrospray ionization mass spectrometry); GC/MS (gas chromatography/mass spectrometry); FL (fluorescence); CL (chemiluminescence); N-GS (N-doped graphene sheets); SWCNT (single wall carbon nanotube); MWCNT (multiwall carbon nanotube); CNF (carbon nanofiber); N-CNF (N-doped carbon nanofiber); PAM (polyacrylamide); PGA (polyglycolic acid); QDs (quantum dots); NPs (nanoparticles); LDH (layered double hydroxide); MCM (mesoporous molecular sieve); CPE (carbon paste electrode); SPE (screen-printed electrode); CILE (carbon ionic liquid electrode); SF (silk fibroin); ITO (indium tin oxide); G (graphene); rGO (reduced graphene oxide); BDD (boron-doped diamond); MAM (melamine); PAMAM (poly(amidoamine)); PEDOT (poly(3,4-ethylenedioxythiophene)); PGA (polyglutamate acid); Pc (phthalocyanine); TTPS (tetrakis (4-sulfonatophenyl) porphyrin).

described as a two-electron process (Figure 3d), the BPA oxidation on the MI-TiO2 could be described by a typical twoelectron two-protons pathway.37,38 In addition, the electroassisted preadsorption did not exhibit any promoting effects on BPA detection at the MI-TiO2/GCE (Figures S10b,c and S11c,d), further demonstrating the specific reorganization ability and strong binding capacity of the MI-TiO2 SCs for BPA.31,32 In this case, there was no dynamic limitation in the heterogeneous BPA oxidation. Electrochemical Detection of BPA with the MI-TiO2Modified GCE. Figure 4a shows DPV responses at different

indicating a better electrostatic adsorption of the protonated BPA on the negatively charged MI-TiO2 with pHpzc of ∼6.5. The potential for BPA oxidation decreased with a raising pH from 3.0 to 8.0 and then became stable until pH 13.0 (Figures S10a and S11a). In an acidic solution, a linear Ep shift toward negative potential with the increased pH was observed, indicating the involvement of protons. The nearly theoretical slope of 0.0561 V pH−1 indicates the electron transfer was accompanied by an equal number of protons in anodic reaction.37,38 No reduction peak was observed, suggesting the irreversibility of BPA oxidation.36 Since this reaction could be E


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Figure 5. Selectivity (a), stability (b), apparent recovery factor (c), and reproducibility (d) of the MI-TiO2-modified glassy carbon electrode for BPA analysis. Measuring conditions: solution = 0.1 M KCl + 20.0 μM BPA + 0.1 M phosphate buffer solution, pH = 7.0, potential range = 0.0−1.0 V, scan rate = 100 mV s−1, and effective anode area = 0.196 cm2. Intereferring organics: phenol (HB) and 4-nitrophenol (p-NP). Interfering intensity: 1.0 and 100.0 molar ratio to the analyte.

Table 2. Electrochemical Detection of Trace BPA in Real Environmental Samples with the MI-TiO2/GCEa environmental samples testing series measured (nM)

EC HPLCc EC

spiked and found (nM)d HPLCc

average RSD (%) apparent recovery factor (%)

EC HPLCc EC HPLCc

5 10 15 20 5 10 15 20

tap water

lake water

river water

sewage

sewage

municipal sludge

/b /b 4.52 ± 0.10 9.05 ± 0.15 14.61 ± 0.19 20.18 ± 0.35 6.84 ± 0.14 9.83 ± 0.24 13.10 ± 0.51 16.32 ± 0.68 1.73 3.13 94.8 ± 1.63 101.0 ± 3.00

/b /b 5.15 ± 0.12 9.46 ± 0.25 15.29 ± 0.41 20.32 ± 0.53 8.05 ± 0.14 13.22 ± 0.26 18.10 ± 0.47 20.97 ± 0.75 2.57 2.47 100.3 ± 2.57 129.7 ± 3.07

/b /b 4.78 ± 0.09 10.74 ± 0.17 15.60 ± 0.27 19.70 ± 0.45 7.47 ± 0.11 12.07 ± 0.19 15.06 ± 0.32 22.35 ± 0.61 1.86 1.97 101.4 ± 1.89 120.6 ± 2.32

/b /b 5.40 ± 0.08 10.87 ± 0.16 14.52 ± 0.324 19.95 ± 0.50 6.55 ± 0.19 10.52 ± 0.38 15.17 ± 0.44 19.71 ± 0.96 1.95 3.57 103.4 ± 1.97 109.0 ± 3.83

/b /b 4.54 ± 0.12 9.83 ± 0.23 15.27 ± 0.27 19.59 ± 0.39 6.26 ± 0.23 9.48 ± 0.44 11.95 ± 0.69 20.80 ± 0.97 2.18 4.69 97.2 ± 2.11 101.0 ± 4.61

/b /b 5.39 ± 0.14 10.86 ± 0.26 15.83 ± 0.32 20.06 ± 0.47 5.63 ± 0.19 9.48 ± 0.37 13.51 ± 0.54 17.13 ± 0.72 2.34 3.87 105.6 ± 2.47 95.8 ± 3.68

The mean value of five parallel measurements (n = 5) with a RSD < 5.0%. bBelow the detection limit. cBPA was determined by high-performance liquid chromatography (HPLC-1100, Agilent Inc.) with a Hypersil-ODS reversed-phase column and spectrographically detected at 254 nm using a VWD detector. The mobile phase was a mixture of water and methanol (30:70) delivered at a flow rate of 1 mL min−1.22 dDiluted from 50.0 μM stock solution at appropriate folds with or without 0.1 M KCl mineral as supporting electrolyte. a

LOD, and expanded linear range for BPA detection (Table 1).39−74 Although some other materials achieve the same or even better detection performance,75−77 the MI-TiO2 had distinct advantages for electrochemical monitoring attributed to its high activity, low cost, no toxicity, good stability, and earth abundance. The compounds with similar molecular structures and chemical properties, even at a dosage of 100 times, had

BPA dosages on the MI-TiO2 under optimized conditions. Figure 4b illustrates the calibration curve between the oxidation peak current (Ip) and BPA concentration. The oxidation peak current (Ip) was proportional to the BPA dosage from 10.0 nM to 20.0 μM, with a correlation coefficient of 0.9987. The LOD was calculated to be 3.0 nM (S/N = 3) and the limit of quantification (LOQ) was 10.0 nM (S/N = 10). Our approach to prepare the MI-TiO2 enhanced analytic signal, lowered F


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optimized testing conditions, DPV curves were recorded from 0.0 to 1.0 V without electro-assisted accumulation. BPA concentrations in the real samples were too low to be directly detected by both MI-TiO2-based sensor and HPLC (Table 2). Thus, the standard addition method was used to examine its detection accuracy and recovery efficiency.36 Each sample underwent five parallel detections, with a relative standard deviation (RSD) below 5% obtained. The calculated apparent recovery factors of all the samples spiked by 5.0−50.0 nM BPA were in a range of 93.2−109.8% on the MI-TiO2-based sensor (Table 2 and Table S3). This result indicates a good detection trueness and weak substrate effect, both of which are highly indispensable for real sample analysis. Moreover, the high consistency between the result obtained by this TiO2-based electrochemical method and the standard HPLC result (Table 2)23 further demonstrates its great potential for practical applications. In summary, the shape- and facet-tailored synthesis of TiO2 effectively overcame the drawbacks of low electric conductivity and weak catalytic activity, and the adoption of inorganicframework MI technology substantially enhanced its adsorptive capacity. The prepared TiO2 SCs exhibited well-defined morphology and highly specific molecular recognition ability for BPA. The MI-TiO2-modified GCE showed excellent electrochemical sensing performance in terms of sensitivity, discriminating ability, anti-interference capacity, and stability. Its excellent BPA detection performance in real environmental samples was also demonstrated. This work suggests that the high-stable and cost-effective TiO2 could be used as a promising electrode material for electrochemical detection of PoPs like BPA. Our findings provide a new opportunity to develop more practical electrode materials for environmental detection.

negligible interference on BPA detection by MI-TiO2 (Figure 5a). Such an excellent discrimination toward BPA might be attributed to the specific and strong binding interactions between the target BPA and its molecular recognition sites on the TiO2 surface and subsurface.31,32 In addition, it exhibited a high detecting stability. The detection current retained 98.7%, 98.0%, and 97.8%, respectively, after stored in a refrigerator at 4 °C for 10, 20, and 30 days (Figure 5b). After adding 0.5 and 1 μM BPA, the detection current increased to 102.6 and 104.3%, respectively, suggesting a good recovery (Figure 5c). Moreover, the low relative standard deviation (0.9%) indicates its excellent reproducibility (Figure 5d). For surface-mediated anodic reactions, the direct electron transfer from substrate to electrode occurs only for the substrate that is preadsorbed onto electrode surface, and mass-transfer is usually the main rate-limiting step of the electrochemical reaction. Thus, BPA surface enrichment played a governing role in its anti-interference capacity (Figure 5 and Figures S12−S19). Linear sweep voltammetry (LSV) tests showed that the initial BPA concentration on the MI-TiO2 surface was kept stable even under severe interfering conditions (Table S2). This could well explain the superior BPA detection performance of the MI-TiO2/GCE with a high anti-interfering capacity. More interfering substances with either larger or smaller molecular sizes at high concentrations were introduced in the BPA electrochemical detection. Under the given conditions, the MI-TiO2-based sensor kept a relatively stable BPA detection signal (less than 10% reduction) (Figures S12 and S13 and Table S2). Additional interfering tests with four true structural analogues were further carried out for BPA analysis (Figures S14−S16). The four selected structural analogues did not exhibit any recognizable interference to the BPA detection on the MI-TiO2 sensor either. The analytical signal exceeded 95% even under the multi-interfering conditions, indicating the specific recognition and selective enrichment of BPA onto the MI-TiO2. Considering the ubiquitous nature of humic acids, chlorine, bicarbonate, iron, and manganese in water, they were also selected to simulate the real-world scenario for BPA analysis. Again, they showed negligible interference to the BPA detection on the MI-TiO2-based sensor (Figures S17−S19). The good BPA detection performance of the MI-TiO2 should be attributed to its improved electric conductivity, superior anodic activity, excellent molecular recognition, and selective enrichment toward BPA from the abundant surface −OH groups anchored onto the TiO2 matrix. The geometrically oriented hydrogen bonds were formed between −OH groups of the TiO2 matrix and BPA templates in the MI-TiO2 preparation process. The BPA molecules were incorporated into the TiO2 matrix. Thermally removing the imprinted BPA yielded TiO2 filled with specific cavities, whose shape, size, and chemical functionality were complementary to BPA. Such an inorganic-framework MI technology was mainly a surface imprint onto solid matrix, on which −OH groups would be formed around the imprinting sites of BPA templates. The −OH groups then served as the main recognizing components due to the formation of multiple hydrogen bonds. This resulted in the improved BPA enrichment with a much higher local concentration on the MI-TiO2 surface. Also, the shape selection and size matching also contributed to the BPA recognition. Electrochemical Sensing of BPA in Real Samples. The MI-TiO2-modified GCE was used to detect BPA in various real samples to evaluate practical potentials.36−38 Under the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.7b04466. Morphology and structure of P25, calculation for electrochemical surface area, anodic peak potential, initial surface concentration and diffusion coefficient (text), structural parameters of TiO2 (Table S1), initial surface concentration and signal retention efficiency (Table S2), analysis of industrial samples (Table S3), XRD of P25 (Figure S1), SEM of MI-TiO2 (Figure S2), XPS, VB, and BET of TiO2 (Figure S3), electrochemical properties of different electrodes (Figure S4), interfacial diffusion coefficient and initial surface concentration of BPA (Figure S5), i−t and calculated diffusion coefficients (Figure S6), LSV and relationships between peak current with scanning rate (Figures S7 and S8), DPV of MI-TiO2 (Figures S9−S11), and BPA determination with interferences (Figures S12−S19) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Fax: +86-551-63602449. E-mail: [email protected]. *Fax: +86-551-63601592. E-mail: [email protected]. ORCID

Han-Qing Yu: 0000-0001-5247-6244 G


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Analytical Chemistry Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (Grants 21261160489, 21590812, and 51538011), the Anhui P r o v i n c i a l N a t u r a l S c i e nc e F o u n d a t io n ( G r a n t s 1708085MB52), the Collaborative Innovation Center of Suzhou Nano Science and Technology of the Ministry of Education of China, the Opening Foundation of Ministry of Education Key Laboratory of Pollution Processes and Environmental Criteria (KF2017-07) and the CAS Key Laboratory of Reservoir Aquatic Environment, Chongqing Institute of Green and Intelligent Technology for supporting this work.

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