Colorimetric assay conversion to highly sensitive electrochemical

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Colorimetric assay conversion to highly sensitive electrochemical assay for bimodal detection of arsenate based on cobalt oxyhydroxide nanozyme via arsenate absorption Shao-Hua Wen, Xiao-Li Zhong, Yi-Di Wu, Ru-Ping Liang, Li Zhang, and Jian-Ding Qiu Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 30 Apr 2019 Downloaded from http://pubs.acs.org on April 30, 2019

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Analytical Chemistry

Colorimetric assay conversion to highly sensitive electrochemical assay for bimodal detection of arsenate based on cobalt oxyhydroxide nanozyme via arsenate absorption Shao-Hua Wen,1 Xiao-Li Zhong,1 Yi-Di Wu,1 Ru-Ping Liang,1 Li Zhang,1 Jian-Ding Qiu1,2*

1College

of Chemistry, Nanchang University, Nanchang 330031, China

2Engineering

Technology Research Center for Environmental Protection Materials and

Equipment of Jiangxi Province, Pingxiang University, Pingxiang 337055, China *Corresponding authors. Tel/Fax: +86-791-83969518. E-mail: [email protected].

1

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ABSTRACT: This study reports a novel and convenient bimodal method for label-free and signal-off detection of arsenate in environmental samples. Cobalt oxyhydroxide (CoOOH) nanoflakes with facile preparation and intrinsic peroxidase-like activity as nanozyme can efficiently catalyze the conversion of chromogenic substrate like 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) with the presence of H2O2 into the green colored oxidation products. CoOOH nanoflakes can specifically bind with arsenate via electrostatic attraction and As-O bond interaction, which gives rise to inhibit the peroxidase-like activity of CoOOH. Thus, through arsenate specific inhibition of CoOOH nanozyme towards ABTS catalysis, a simple colorimetric method was developed for arsenate detection with a detection limit of 3.7 ppb. Based on the system of CoOOH nanozyme and ABTS substrate, this colorimetric method can be converted into an electrochemical sensor for arsenate assay by the utilization of CoOOH nanoflake modified

electrode.

The

electrochemical

measurement

can

be

realized

by

chronoamperometry, which showed more sensitive and a lower limit of detection as low as 56.1 ppt. The applicability of this bimodal method was demonstrated by measuring arsenate and total arsenic in different real samples like natural waters and soil extracted solutions, and the results are satisfactory accuracy as confirmed by inductively coupled plasma mass spectrometry (ICP-MS) analysis. The bimodal strategy offers obvious advantages including label-free step, convenient operation, on-site assay, low cost and high sensitivity, which is promising for reliable detection of arsenate and total arsenic in environmental samples. 2


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INTRODUCTION Of various toxic metals found in water, arsenic contamination is one of the greatest threats to both ecosystem and mankind.1 Arsenic poisoning stemming from long-term exposure to water containing high level of arsenic is referred to as arsenicosis. The World Health Organization (WHO) classifies arsenic as a carcinogen and has stipulated a guideline of 10 ppb as the maximum permissible level for arsenic in drinking water.2,3 Arsenic in natural water exists mainly as oxyanions having two oxidation states: arsenate (H3AsO4, As(V)) and arsenite (H3AsO3, As(III)). As(V) is more predominant under oxygen-rich aerobic conditions, while As(III) is more predominant under anaerobic conditions with uncharged H3AsO3 form in natural pH.4 As(III) is generally considered to be more toxic than As(V). However, As(V) shows higher toxicity than organic arsenic species. Arsenate is one of the main forms of arsenic, which mainly exists as negatively charged forms like H2AsO4− and HAsO42− in natural waters.3,4 More shocking, arsenate and phosphate have some high similarities of nearly identical size, pKa values, and identical charged oxygen atoms.5 It has been demonstrated that As(V) can directly substitute for phosphate in certain biological process due to their extensive similarity. The uptake of As(V) replacement of phosphate can greatly impact the regular growth of organism6 or plants.7,8 Moreover, As(V) in the soil can participate in the ecological cycle and accumulate in plants like rice, showing high hazard to ecosystem and human health.9 So it is of great necessity and impendency to develop reliable and sensitive methods for detection of arsenate and total arsenic in natural samples. 3



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To date, many tools for arsenate/arsenic quantification are mainly performed by laboratory techniques like atomic absorption10 or emission11 or atomic fluorescence spectrometry,12 inductively coupled plasma mass spectrometry (ICP-MS),13 and high performance liquid chromatography (HPLC).14 However, these methods are neither capable of on-site assay nor readily available in many developing regions.15 Recently, enormous efforts have been exerted to develop cost-effective and sensitive methods for detection of As(V) and As(III) through various techniques. For example, enzyme inhibition biosensors16 and genetically engineered microbial biosensors17 were developed to determinate arsenic in real samples. These techniques often need the professional technical operation to maintain the activity of the biosensors and the rigorous experimental conditions to improve sensor performance. In addition, electrochemical18 and optical19-21 sensors for arsenate assay were reported. Optical sensors like colorimetric methods22,23 have received intensive attention due to their simple operation and on-site assay. Alternatively, the electrochemical methods have achieved more sensitive and low-cost detection of trace heavy metal ions due to their unique stripping voltammetry.24 Huang’s group developed an electrochemical sensing interface based on the combination of Au nanoparticles with Fe3O4 adsorbents for the detection of As(III) and As(V).25 Wang et

al.

separately

utilized

laccase

as

sensing

element

and

flavin

adenine

dinucleotide-dependent glucose dehydrogenase (FAD-GDH) to fabricate bioelectrodes through multiple chemical modification steps.16

The prepared two bioelectrodes can be

used as a self-powered biosensor for As(V) and As(III) detection through the reversible 4


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inhibition of laccase by As(V) and As(III). Ray’s group reported a glutathione, dithiothreitol, and cysteine modified gold-nanoparticle-based colorimetric and dynamic light scattering assay for the detection of As(III) and As(V).22 Chanda with his colleagues reported a colorimetric sensor for the detection of As(III) and As(V) based on poly(ethylene glycol) methyl ether thiol and meso-2,3-dimercaptosuccinic acid co-modified gold nanorods.26 However, most of the previous methods require complicated operation processes, high-cost sensing materials, or suffering from interferential stripping potentials of some coexisted metal cations (e.g. Cu2+ and Hg2+) in electrochemical assay,24,27 which may raise the cost and restrict the actual application of environmental sample detection. Therefore, it is of great challenge and significance to seek a reliable, sensitive, easy-operation, low-cost, on-site, and environmentally friendly method for arsenate detection in real natural samples. Various nanozymes, such as metal oxides (Fe3O4 and Co3O4) nanoparticles, noble metal nanoparticles and carbon-based nanomaterials, refers to nanomaterials featured with natural enzyme-mimicking activity.28-31 The nanozymes have attracted enormous interests of researchers due to their low cost, ease of mass production, robustness to harsh environments, high stability, and size/composition dependent activity.30 The popular testing targets of most studies nanozyme-based often focused on H2O2 and glucose, and received extensive investigation.30,32 Recently, several groups have reported a few of nanozymes as novel colorimetric probes for convenient detection of heavy metal ions in real water.33-36 However, most studies focused on noble metals (platinum,33,34 gold,35 and 5



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palladium36) nanomaterial. The synthesis of noble metal nanomaterials is often required the complicated preparation process like high temperature and long reaction time. Moreover, the stability of these noble metal nanomaterials are easily influenced by the external environment like illumination and storage temperature, and they greatly increase the costs of the analytical methods. Nowadays, novel two-dimension nanomaterials without noble metal (e.g. VOx nanoflakes,37 WSe2 nanosheets,38 Gd(OH)3 nanorods39 and MoS2 nanosheets40) have received increasing attention due to their excellent enzyme-mimic activity, which greatly broadens the application of nanozyme in the fields of sensor, therapeutic, and environmental remediation.30 CoOOH nanoflakes, as a promising nanomaterial, have been used in electrode and catalysis material due to their superior conductivity and electrochemical performance.41,42 Tang et al. developed a CoOOH-modified luminescence nanoparticles probe for assay and imaging of ascorbic acid in cells due to its excellent oxidation property.43 Based on the good

catalysis

ability of CoOOH towards H2O2 and 3,3',5,5'-tetramethylbenzidine (TMB), Ji et al. reported a colorimetric method for ascorbic acid assay.44 Moreover, CoOOH shows strong adsorption ability towards single-stranded DNA via electrostatic interactions between its positively charged surface and polyphosphate of ssDNA.45 Enlighted by the reported studies, the sensing element that taking full advantages of novel nanomaterial CoOOH with the features of simple preparation process, excellent oxidizability, adsorption ability and electrochemical performance may be designed as bimodal analytical methods. To our knowledge, there is no report on suitable nanozyme-based 6


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bimodal and convenient method for arsenate and total arsenic assay. Herein, we report the utilization of CoOOH nanoflakes with peroxidase-like activity for dual mode assay of arsenate (Scheme 1). In the presence of H2O2, CoOOH nanoflakes with excellent peroxidase-like activity can efficiently catalyze the conversion of chromogenic substrate ABTS into distinctly green oxidized products (ABTSox). We found that As(V) can specifically interact with CoOOH nanoflakes via the electrostatic attraction and covalent-type interaction, its presence can significantly attenuate the peroxidase-like activity of CoOOH nanozyme, leading to decrease of the catalytic efficiency and green oxidized resultants. Thus, CoOOH nanoflakes can be used as the colorimetric probe for handy, on-site and signal-off assay of As(V). Taking advantage of the redox conversion between ABTS and ABTSox on the CoOOH-modified electrode

Scheme 1. Schematic illustration of the dual mode assay of arsenate based on CoOOH nanoflakes with peroxidase-like activity. (A) Illustration of the colorimetric method of CoOOH nanoflakes as probe for arsenate detection. (B) Illustration of the electrochemical strategy of CoOOH nanoflakes modified glassy carbon electrode for arsenate detection. 7



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surface, an electrochemical assay was further developed for the detection of trace As(V) with obviously improved sensitivity and detection limit. The practicability of this dual modal method based on CoOOH nanozyme was also demonstrated by measuring As(V) in different environmental samples. Moreover, the level of total arsenic can be detected in the form of As(V), it will be more friendly to the environment than As(III) due to the less toxicity of As(V).

EXPERIMENTAL SECTION Chemicals and Reagents. 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) was provided by Sigma-Aldrich (St. Louis, USA). Cobalt(II) chloride hexahydrate (CoCl2·6H2O, ≥99%), sodium hydroxide (NaOH), sodium acetate (CH3COONa),

acetic

acid

(CH3COOH),

N-(2-Hydroxyethyl)piperazine-N'-(2-ethanesulfonic acid) (HEPES), K4[Fe(CN)6] and K3[Fe(CN)6] were purchased from Sinopharm Chemical Reagents Co., Ltd (Shanghai, China). Sodium arsenate dibasic heptahydrate (Na2HAsO4·7H2O, As(V)), sodium (meta) arsenite (NaAsO2, As(III)) standard solution and NaClO were provided by J&K Scientific Ltd (Shanghai, China). All metal ions, anionic salts and other chemicals were used as received from suppliers without any purification. Ultrapure water (>18.2 MΩ cm) was obtained from a Milli-Q purification system for preparing solutions throughout the whole experiment. Instrumentation and Characterization. UV-vis absorption spectra were measured on 8


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a spectrophotometer (UV-2450, Shimadzu). Fourier transform infrared (FT-IR) spectra of CoOOH nanoflakes were obtained using a FT-IR spectrometer (TENSOR II, Bruker). The morphology of CoOOH nanoflakes was investigated using JEOL JEM-2010 transmission electron microscope (TEM) with an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) characterization was measured on a VG Multilab 2000X instrument (Thermal Electron). A glassy carbon electrode (GCE, Φ = 3 mm), an Ag/AgCl electrode, and a platinum (Pt) wire were used as the working electrode, the reference electrode, and the counter electrode, respectively. All electrochemical assays were performed using a three-electrode system. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) characterizations were conducted on an IviumStat electrochemical workstation (Ivium Technologies) in 2 mL HEPES solution (10 mM, pH 7.2) containing 1 mM K3[Fe(CN)6]/K4[Fe(CN)6] (molar ratio 1:1) and 0.1 M KCl. CV was scanned from -0.3 V to 0.6 V with scan rate of 100 mV/s. EIS was carried out in a frequency range from 0.01 Hz to 1 x 105 Hz. The amperometric current–time (i– t) responses for the electrochemical measurements were recorded by a CHI 630C electrochemical workstation (Shanghai Chenhua instrument Co. Ltd.). The i–t response curves were taken at 150 mV over 180 s. The concentrations of As(V) in real water and soil samples were detected by ICP-MS (Varian 820-MS). Each sample was measured at least for three replicates. All measurements were conducted at room temperature. Preparation of CoOOH Nanoflakes and Other Nanomaterials. The CoOOH nanoflakes were prepared according to the previous study with slight modification.46 In 9



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brief, 1 mL of NaOH (1.0 M) solution was mixed with 4 mL of CoCl2 (10 mM) under violently stirring. After sonication for 1 min, 200 μL volume of NaClO (0.9 M) was added to the above mixture. After sufficient stirring, the mixed solution was sonicated for 20 min. The whole synthetic process was performed at room temperature. Then, the CoOOH nanoflakes were collected and washed with ultrapure water by centrifugation until its pH near neutralization. Finally, the brownish black CoOOH nanoflakes were dispersed in 1 mL of ultrapure water and stored at 4 °C for further experiments. Fe3O4 nanoparticles with a core average diameter ~6-7 nm were prepared according to our previous report.47 Co3O4 nanoparticles with a core average diameter ~7-8 nm were prepared according to Dong's work.48 Co(OH)2 and CoO were also prepared for a comparison. Colorimetry and UV-vis Spectra for As(V) Detection. The colorimetric assay was performed using CoOOH nanoflakes as probe. First, the peroxidase-mimetic activity of CoOOH nanoflakes was investigated at room temperature using CoOOH nanoflakes (28 μg/mL) of 500 μL acetate (50 mM, pH 4.0) buffer solution in the presence of 50 μM freshly prepared ABTS and 1.8 mM H2O2. The formation of green-colored solution was monitored and measured using a spectrophotometer. Then, As(V) solutions with different concentrations were prepared with HEPES (50 mM, pH 7.5) solution. The above different concentrations of As(V) solutions were added into 100 μL of HEPES (50 mM, pH 7.5) containing 5 μL CoOOH nanoflake solutions (2.8 mg/mL). After sufficiently mixing, the obtained solutions were incubated for 120 min on shaking bed. Subsequently, 10


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the reaction solutions were mixed with 200 μL of acetate (250 mM, pH 4.0) buffer, 5 μL of ABTS (5 mM) and 5 μL of H2O2 (0.18 M). The total volume of the reaction systems was kept 500 μL with ultrapure water and further incubated for another 30 min at ambient temperature. The obtained solutions with changed color were measured on the UV-vis spectrophotometer. The spectral intensities at 418 nm (A418) were used to quantify the level of As(V). Selective Investigation. The specificity of CoOOH nanoflakes towards As(V) was investigated by other common ions in environmental water, and the UV-vis absorbance intensities at 418 nm (A418) of the solutions were recorded. Briefly, the solutions of CoOOH nanoflakes were incubated with 1000 ppb of different metal ions as follows. In a 2 mL vial, 5 μL of CoOOH nanoflake solution (2.8 mg/mL) was first added into 100 μL of HEPES buffer (50 mM, pH 7.5) containing 1000 ppb of different interfering ions, such as Na+, K+, Ag+, Ca2+, Mg2+, Zn2, Ba2+, Ni2+, Co2+, Cd2+, Pb2+, Hg2+, Fe2+, Fe3+, Cr3+ and Al3+, SO42-, CO32-, SiO32-, NO3-, Cl-, As(III), PO43-. After thoroughly mixing and incubation for 100 min at ambient temperature, the obtained solutions were mixed with 200 μL of acetate (250 mM, pH 4.0) buffer, 5 μL of ABTS (5 mM) and 5 μL of H2O2 (0.18 M). The total volume of the reaction solutions were kept 500 μL with ultrapure water and further incubated for another 30 min at ambient temperature. The UV-vis spectrum was recorded by spectrophotometer. Fabrication of the Electrochemical Sensor for As(V) Detection. Prior to the modification, the glassy carbon electrode (GCE) was first successively polished by 0.3, 11



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and 0.05 μm alumina slurry on a polishing pad to a mirror-like surface. The polished GCE was ultrasonically cleaned in ethanol and ultrapure water for 2 min, respectively, and dried with high-pure nitrogen gas. 10 μL of CoOOH solution (50 μg/mL) was dropped on the cleaned GCE, and it was dried in the air for overnight. The CoOOH nanoflakes modified GCE (CoOOH-GCE) was used as a working electrode for As(V) detection. Specifically, the CoOOH-GCE was first immersed into 50 μL of HEPES (50 mM, pH 7.6) solutions containing different concentrations of target As(V) at ambient temperature for 100 min. After rinsing with ultrapure water, 2.00 mL acetate (25 mM, pH 4.0, 0.1 M KCl) solution containing 50 μM ABTS and 1.8 mM H2O2 was added. The chronoamperometric response (i−t) curve was measured in the above solutions at 150 mV over 180 s. The selective tests were performed by the electrochemical method similar to the electrochemical sensing process of As(V) except for using various interfering ions with 100-fold higher concentration than As(V). Real Samples Assay. (i) Water Samples. The real water samples were collected from three different natural basins − Ganjiang River, Runxi Lake and well water. The Ganjiang River, located in the southeast (mainly in Jiangxi Province) of mainland China, is situated in the middle and lower regions of the Yangtze River. The Runxi Lake is a one located on the campus of Nanchang University, China. The well water was collected from a well in local village (Xinjian County of Jiangxi Province, China). Briefly, all natural water samples were first centrifuged at 12 000 rpm for 5 min to detach solid impurities, and then filtered twice by a 0.22 μm microfiltration membrane. After the pretreatment with 12


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the solution of CaCl2, the original water samples were first analyzed by the proposed dual mode methods. Finally, these water samples were spiked with As(V) at different concentrations (10 and 50 ppb), which were then detected with the above-mentioned two methods. The detection results were compared with those obtained by ICP-MS method. (ii) Soil Samples. One soil sample was collected from the local paddy field soils in the countryside of Xinjian, Nanchang of China. Another sample was collected from the pond located in the southwest corner of Nanchang University. Soils were oven dried (80 °C) before analysis. Soil samples were then finely ground in a ball mill. Subsamples were weighed (~2.0 g) into quartz glass digestion tubes and then digested using a mixture of HNO3-H2O2 procedure according to the previous method.49 Five milliliters of nitric acid was added to each tube and allowed to stand overnight. The tubes were then placed on a heating block, and the temperature was raised to 60 °C. After adding 3 mL of 30% H2O2, the temperature was gradually raised to 120 °C, and the samples were allowed to digest for 2 h. The digests were then cooled, diluted in deionized water and made up to 20 mL and then filtered through a 0.22 μm microfiltration membrane. All samples were neutralized with NaOH just before the assay in order to work under optimal conditions for detection. To avoid the interference of phosphate, the samples were treated with the solution of CaCl2. The total As in the form of As(V) in soil digests was measured by the proposed methods and ICP-MS method.

RESULTS AND DISCUSSION Characterization of CoOOH Nanoflakes. CoOOH nanoflakes can be easily 13



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synthesized by one-pot ultraphonic process according to the reported method.46 Compared to the absorption band at ~512 nm of the solution of CoCl2 (Figure 1A), the absorption band of CoOOH blue-shifts to ~395 nm, which is similar to the previous literature.46 Figure 1B is the powder X-ray diffraction (XRD) pattern of CoOOH, which shows three peaks at 20.0°, 39.2°, and 50.6° corresponding to the (003), (012), and (018) peaks based on the standard JCPDS card (No. 07-0169), indicating the successful preparation of crystalline CoOOH. As shown in Figure 1C and 1D, the obtained CoOOH is hexagonal ultrathin nanosheets with an average size of ∼50−105 nm and an average thickness of ∼9.2 nm. Besides, the absorption spectra of CoOOH nanoflake solutions show no obvious change after one month stored at 4 °C (Figure S1), indicating its good stability.

14


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Figure 1. (A) UV-vis absorption spectra of aqueous solutions of (a) CoOOH nanoflakes and (b) CoCl2. (B) XRD patterns of CoOOH nanoflakes. (C) and (D) TEM images of CoOOH nanoflakes under different magnifications. The red arrows indicate the thickness of a representative nanoflake. Colorimetric Feasibility of As(V). Several studies have reported that a few of Co-based nanomaterials like Co3O4 nanoparticles,48,50 CoOOH nanoflakes51 and Co4N nanowires52 show intrinsic peroxidase-like activity. Jiang et al. reported that CoOOH nanoflakes possessed intrinsic peroxidase-like activity, which can be used for detection of serum glucose.51 Herein, with the presence of H2O2, CoOOH nanoflakes can catalyze the conversion of chromogenic substrate ABTS into oxidized green products ABTSox as the following equation (Inset tube 8 in Figure 2A and Figure S2): ABTSred + H2O2 CoOOH

ABTSox + H2O

From the absorption spectra and photos of Figure 2A, only the case of solutions including CoOOH, ABTS and H2O2 can show the obvious green change, indicating the intrinsic peroxidase-like activity of the prepared CoOOH nanoflakes. However, when CoOOH nanoflakes are incubated with As(V), a significant decrease of peroxidase-like activity is observed accompanied by a attenuated catalytic efficiency of CoOOH towards ABTS (Figure 2B). The color of solutions is changed from strongly green (vial a) to very pale green (vial b), suggesting the feasibility of colorimetric assay of As(V) based on CoOOH nanozyme as sensing probe. Moreover, Fe3O4 nanoparticles and three Co-based nanomaterials (Co3O4, CoO and Co(OH)2) were used for a comparison experiment. 15



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Among the selected five nanomaterials, CoOOH nanoflakes show the best catalytic activity toward ABTS under the same conditions (Figure 2C). The big differences of peroxidase mimics among these nanomaterials were mainly caused by metal ions, surface properties, morphologies, compositions and electron transfer abilities.48,50 With the same shapes like one-dimensional nanoparticle and size, the catalytic activity of Co3O4 was higher than analogous Fe3O4, which was similar to the previous report.48 As Wang's study reported, Co3O4 nanoplates show higher catalytic activities than those with nanorods and nanocubes morphologies.50 When As(V) with the same concentration was first treated the above five nanomaterials, the absorption intensity change of CoOOH nanoflakes exhibits the most significant decrease (Figure 2C). Therefore, CoOOH nanoflakes were selected as nanozyme probe for arsenate assay.

Figure 2. (A) Absorption spectral observation of CoOOH peroxidase-like activity. Inset is the solutions of different component (from 1 to 8: CoOOH, acetate buffer, H2O2, ABTS, CoOOH + ABTS, buffer + ABTS, H2O2 + ABTS, CoOOH + H2O2 + ABTS) under white light. (B) Absorption spectral of the solutions of CoOOH nanoflakes in the (a) absence and (b) presence of 600 ppb As(V) after reaction with H2O2 and ABTS. Inset 16


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is the corresponding photos. (C) Comparison of absorption intensities at 418 nm of Fe3O4 NPs and several Co-based nanomaterials including Co3O4 NPs, CoO NPs, Co(OH)2 and CoOOH nanoflakes without As(V) (red histogram) and with 1000 ppb As(V) (blue histogram) after reaction with H2O2 and ABTS. The concentrations of the nanomaterials were used as 28 μg/mL. The concentration of As(V) was 1000 ppb. Sensing mechanism of As(V). To investigate the sensing mechanism of CoOOH nanoflakes towards As(V), the ζ potential, FT-IR, elemental mapping distribution and XPS characterizations of the samples were used to illustrate the interaction between CoOOH nanoflakes and As(V). As shown in Figure 3A, the ζ-potential of CoOOH nanoflakes is measured to be ~30.2 mV (column a). With the addition of 200 ppb As(V) at pH 7.5, the amount of the surface positively charge of CoOOH nanoflakes decreases to 12.6 (column b). After incubation with 500 ppb As(V), the surface charge even turns into negative (column c). These changes could be explained by electrostatic adsorption of positive CoOOH nanoflakes towards negative As(V) under pH 7.5. In addition, the binding of As(V) onto CoOOH nanoflakes was investigated by FT-IR spectroscopy. The infrared spectra of the primal CoOOH nanoflakes are shown in curve a of Figure 3B with four major signature bands: 3430, 2920, 1625 and 595 cm-1, which are attributed to the stretching vibration of the hydrogen bonded hydroxyl group (–OH), the stretching vibration band between O–H, the Co–O double bonds, and Co–O2- vibration, respectively.45,46 After incubation with As(V), the band around 2920 cm-1 decreases 17



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obviously and a new band around 860 cm-1 appears due to the symmetry stretching vibration of As-O.53,54 Thus, the adsorption interaction between CoOOH nanoflakes and As(V) includes electrostatic attraction and surface complexation (As-O bond), which is greatly similar to that the previous nanomaterials like β-FeOOH nanorods and Ce–Fe mixed oxide decorated multi-walled carbon nanotubes towards arsenic adsorption.54,55

Figure 3. (A) Histogram of the ζ potential of (a) CoOOH nanoflakes, CoOOH nanoflakes incubated with (b) 200 ppb As(V) and (c) 500 ppb As(V). (B) FT-IR spectra of CoOOH nanoflakes (a) before and (b) after As(V) incubation. (C) Full-range XPS spectra of CoOOH nanoflakes and (D) O1s XPS spectra of CoOOH nanoflakes (a) before and (b) after As(V) absorption. 18


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The samples were then subjected to XPS and elemental analysis. The full-range XPS spectra of CoOOH nanoflakes before and after interaction with As(V) are shown in Figure 3C. The 3d peak (~46.01 eV) of As appears after As(V) adsorption on the surface of CoOOH nanoflakes. The covalent interaction of As-O bond is verified by XPS (Figure 3D). The binding energy of O 1s core level is shifted to higher binding energy about 531.24 eV upon As(V) loading compared with the as-synthesis sample (530.09 eV), indicating that the valence of O in CoOOH is changed because of the interaction with the guest As(V). Elemental mapping distribution of the original CoOOH nanoflakes shows that oxygen disperses uniformly within the CoOOH samples (Figure S3A). However, after As(V) adsorption on the surface of CoOOH nanoflakes, it can be observed that As(V) uniformly disperses on the surface of CoOOH nanoflakes (Figure S3B). These results indicate that CoOOH nanoflakes can effectively adsorb As(V) via electrostatic interaction and As-O linkage, which provides a firm foundation for the colorimetric and spectral detection of As(V). Colorimetric sensing of As(V). The sensing performance of CoOOH nanoflakes as the colorimetric probe was investigated by various concentrations of As(V) under the optimized conditions (Figure S4). The initial reaction rates (V0) were further acquired by calculating the slopes of the initial parts of the kinetic curves obtained with different amounts of As(V) (Figure S5). When the concentration of As(V) gradually increases to 500 ppb, the kinetical reaction is less significant, likely due to surface saturation and inter-As(V) repulsive interaction. So 2 hours was used to insure the highest adsorption of 19



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As(V) onto CoOOH surfaces. The solutions of CoOOH nanoflakes change gradually from obviously green to close colorless with increasing of the concentrations of As(V) (Inset of Figure 4A). The absorbance spectra intensity at 418 nm gradually decreases as the concentration of As(V) is gradually increased from 0 to 1000 ppb (Figure 4A and Figure S6). The intensity of A418 decreases linearly with the concentrations of As(V) ranging from 4 ppb to 500 ppb (Figure 4B), and the limitation of detection (LOD) is calculated to be 3.7 ppb (S/N = 3). The LOD is comparable to most reported different nanomaterial-based colorimetric or even better than some fluorescent/electrochemical methods for As(V) detection (Table 1). The stability tests were conducted by absorption spectrometry, and the detailed results are presented in Figure S7. When CoOOH nanoflakes were stored at 4 °C after 30 days, the absorption spectra and the absorption intensities at 418 nm have no obvious change. So CoOOH nanoflakes can be used as a stable nanozyme probe for As(V) assay (Figure S7). Under the selected conditions, the ΔA418 was used as the index for the selective investigation of CoOOH nanoflakes towards As(V) (Figure 4C). Other interfering ions including Na+, K+, Ag+, Ca2+, Mg2+, Cu2+, Zn2+, Ba2+, Ni2+, Co2+, Cd2+, Pb2+, Hg2+, Fe2+, Fe3+, Cr3+, Al3+, NO3-, Cl-, SO42-, CO32-, and SiO32- with a 100-fold concentration of As(V) do not show obvious absorption change due to their negligible influence on the peroxidase-like activity of CoOOH towards ABTS. It should be noted that As(III) also shows a much smaller absorption change. Owing to the dual interaction of CoOOH nanoflakes towards As(V) via electrostatic attraction and As-O bond, CoOOH nanoflakes 20


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show a high selectivity with As(V) except for PO43-. However, PO43- shows an obvious response of absorption intensity at 418 nm, since PO43- is highly similar to As(V) in terms of structure, pKa, size and charged oxygen atoms.20,21 To effectively improve the selectivity, a pretreatment process for precipitation of PO43- was performed using Ca2+ as precipitant.21 With the help of Ca2+, CoOOH nanoflakes show a specific response towards the target As(V) (Figure 4C).

Figure 4. (A) UV-vis absorption spectra of CoOOH nanoflakes solutions with different concentrations of As(V) (From top to down: 0, 1, 2, 4, 5, 10, 20, 40, 50, 80, 140, 200, 300, 400, 500, 600, 700, 800, 1000 ppb). Inset is the photo of the solutions of CoOOH nanoflakes with various concentrations of As(V) (From a to b: 0, 5, 10, 40, 80, 140, 200, 400, 600, 800, 1000 ppb). (B) The linear relationship between A418 with the logarithm different concentrations of As(V) in the range from 4 ppb to 500 ppb. A418 refers to the absorption intensity of the solutions at 418 nm. (C) Selective comparison of CoOOH nanoflakes towards As(V) (50 ppb) and other 24 interfering ions (5000 ppb) by absorption spectral method. From left to right: (from a to b) As(V), Na+, K+, Ag+, Ca2+, Mg2+, Cu2+, Zn2+, Ba2+, Ni2+, Co2+, Cd2+, Pb2+, Hg2+, Fe2+, Fe3+, Cr3+, Al3+, NO3-, Cl-, 21



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SO42-, CO32-, SiO32-, As(III); (c) PO43-;(d) (PO43- + Ca2+); (e) (cations mixture + As(V)); (f) (anions mixture + Ca2+ +As(V)) and (g) (all other ions mixture + Ca2+ +As(V)). ΔA418 is the difference of the absorption intensity at 418 nm of the blank solution and the sample with one ion. Electrochemical Feasibility of As(V) Sensing. Electrochemical techniques have been widely exploited in the analytical fields of heavy metal ions detection, which show the advantages of low cost, high sensitivity, quick response and even real-time monitoring as compared to other spectroscopic and optical techniques.64 The electrochemically active molecules (e.g. TMB or Ag nanoparticles) can be effectively applied to the electrochemical designs for toxic targets assay, which shows greatly noteworthy potential for highly sensitive detection of toxic targets in various fields.65,66 More importantly, some nanomaterials have been used as highly efficient non-enzymatic electrocatalysts for fabrication of the electrochemical biosensors, which have achieved ultrasensitive electrochemical assay.67,68 So we tried to convert colorimetric assay into electrochemical assay based on CoOOH nanozyme and the substrate ABTS system. Before electrochemical detection, the stepwise modification processes of the GCE surface were investigated by EIS and CV (Figure S9). The peroxidase-like activity of CoOOH nanoflakes was observed through the typical colorimetric oxidation of ABTS by H2O2. Then, the electrochemical sensing feasibility of As(V) was further investigated on CoOOH-modified GCE by electrochemical techniques (Figure 5). Without the presence 22


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of As(V), the CV response on CoOOH-GCE shows an obvious redox peak currents (curve a, Figure 5A), indicating a typical electrocatalytic process, which can be attributed to the mimetic peroxidase catalysis of CoOOH nanoflakes to the oxidation of ABTS by H2O2. When CoOOH-modified GCE was first treated with As(V), the CV responses show the decrease of the peak currents caused by As(V)-inhibited mimetic peroxidase activity of CoOOH (curve b, Figure 5A). The chronoamperometry was further applied to illustrate the electrochemical sensing feasibility of As(V). In the absence of As(V), the CoOOH modified GCE shows a decay curve for the relation of current vs time in pH 4.0 HAc-NaAc buffer containing 0.85 mM ABTS and 0.45 mM H2O2 at +150 mV (vs SCE), which reaches the steady-state reduction current about 80 s. The electrochemical reduction of the oxidized products is on the electrode surface after the catalytic oxidation of ABTS with H2O2 by CoOOH nanoflakes (Scheme 1B and curve a in Figure 5B). While in the presence of As(V), the reduction current decreases distinctly (curve b, Figure 5B). This phenomenon could be ascribed to the fact that the absorption of As(V) onto CoOOH nanoflakes inhibits its peroxidase-like activity and further to resist the reduction of ABTSox on the electrode surface, which could be utilized for the sensitive electrochemical assay of As(V) as shown in Scheme 1B.

23



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Figure 5. (A) Cyclic voltammograms and (B) Chronoamperometric responses of (a) CoOOH modified GCE and (b) CoOOH modified GCE incubation with 200 ppb As(V) in 25 mM pH 4.0 HAc-NaAc buffer containing 50 μM ABTS and 1.8 mM H2O2. Electrochemical Performance of As(V) Detection. The assay performance of CoOOH modified GCE towards As(V) was studied under the optimized conditions. Figure 6A shows the chronoamperometric curves towards different concentrations of As(V). The chronoamperometric responses decrease gradually with increasing the concentrations of As(V) (Figure 6A and Figure S10), indicating that the electrochemical sensor is highly dependent on the concentration of As(V). The current intensity at 160 s is plotted as a function of the logarithm value of As(V) concentration until a plateau is reached. A good linear relationship between the chronoamperometric intensity and the logarithm value of As(V) concentration over the range of 0.4 – 200 ppb (Figure 6B) is obtained, with a detection limit of 56.1 ppt (S/N = 3). The current response is more sensitive than the spectral analysis. The detection limit is much lower than the WHO’s arsenic threshold and even better than most methods in Table 1. 24


Page 25 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. (A) Chronoamperometric responses of As(V) at 0, 0.025, 0.05, 0.1, 0.2, 0.4, 0.8, 1.5, 2, 4, 8, 10, 20, 50, 80, 100, 150 and 200 ppb (from top to down) at +150 mV. (B) The linear relationship of currents at 160 s vs. logarithmic values of As(V) concentrations from 0.1 to 200 ppb. Error bars show the standard deviation for three independent measurements. Selectivity and Stability Tests. To apply the electrochemical method to detect As(V) in environmental samples, the effects of 24 interfering species with 100 times higher content than As(V) were investigated. Compared with blank buffer solution, there is almost no change in the current intensity except for PO43- (Figure 7A). PO43- has an obvious electrochemical response on CoOOH-modified electrode. To effectively improve the selectivity, a pretreatment process for precipitation of PO43- was performed using Ca2+ as precipitant.19 With the help of Ca2+, CoOOH-GCE shows a specific response towards As(V) (Figure 7A). The reproducibility of the electrochemical sensor was investigated by utilizing five 25



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electrodes incubated with 10 ppb As(V). The relative standard deviation (RSD) values were less 3.18%. Therefore, the proposed method possessed acceptable reproducibility. Besides, the stability of the electrochemical sensor was evaluated by measuring relative standard deviations for every five days. It was performed by analyzing As(V)-spiked samples at seven CoOOH-modified electrodes over 30 days with the same concentration of 10 ppb. The current responses values are ranged from 95.8% to 100.7% of the initial current response within 30 days (Figure 7B). One month later, the current signal decreased 4.2% compared with the initial current response signal, indicating an acceptable storage stability of the electrochemical sensor. The above results were demonstrated that the proposed method showed good reproducibility and stability for the determination of As(V).

Figure 7. (A) Specificity tests of the CoOOH-modified GCE towards As(V) (10 ppb). The concentration of other interfering ions was 1000 ppb. From left to right: (from a to b) As(V), Na+, K+, Ag+, Ca2+, Mg2+, Cu2+, Zn2+, Ba2+, Ni2+, Co2+, Cd2+, Pb2+, Hg2+, Fe2+, Fe3+, Cr3+, Al3+, NO3-, Cl-, SO42-, CO32-, SiO32-, As(III); (c) PO43-; (d) (PO43- + Ca2+); (e) 26


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(cations mixture + As(V)); (f) (anions mixture + Ca2+ +As(V)) and (g) (all other ions mixture + Ca2+ +As(V)). ΔCurrent is the difference of the steady-state current intensity at 160 s of the blank solution and the sample with one ion. (B) Stability of the electrochemical sensor stored at 4 °C over 30 days. The concentration of As(V) was 100 ppb. Table 1. Comparison of different methods for As(V) detection. Methods

Materials

Electrochemistry

ACP/PPO

5−592 ppb

2 ppb

56

Electrochemistry

ACP/2-Phospho-l-AA

7.5−98 ppb

8.3 ppb

57

Electrochemistry

Laccase Bioelectrodes

/

10 ppm

16

Fluorescence

FAM-C5 DNA/CeO2

37.6−150 ppb

2.2 ppb

20

Fluorescence

FAM-C15 DNA/Fe3O4

37.6−376 ppb

22.5 ppb

21

Fluorescence

Organic probe APSAL

376−37600 ppb

225 ppb

58

Fluorescence

Organic probe NAPSAL

0.75−67680 ppb

0.38 ppb

59

Fluorescence

DFC-DO

/

17 ppb

60

Fluorescence

Two dyes Acf and RhB

40−90 ppb

10 ppb

61

UV-vis

GNP-MMT@Eu

1−1000 ppb

1.0 ppb

23

UV-vis

AuNPs/ACP/AMP

7.5−7520 ppb

7.5 ppb

62

UV-vis

Ag nanoplates/SiO2-Fh

0.5−30 ppm

0.5 ppb

63

UV-vis

GNR-PEG-DMSA

0.001−10 ppm

1.0 ppb

26

4−500 ppb

3.72 ppb

0.1−200 ppb

56.1 ppt

UV-vis Electrochemistry

Linear range

CoOOH nanoflakes

Detection limit

Ref.

This work

Real sample analysis. Finally, we have tested As(V) in real samples by using the proposed methodology. First, the original real samples were detected by both our methods and ICP-MS. Furthermore, with the aim to avoid any interference from the sample matrix, a standard addition method is implemented by spiking each sample with a 27



Page 28 of 33

known concentration of As(V). The average value of a triplicate of each sample is presented in Table S1 and Table 2, while its standard deviation represents the error bars. The recoveries of As(V) in real water samples are ranging from 96.4% to 106.0% for spectral analysis and from 95.8% to 105.6% for electrochemical analysis (Table S1), which are in good agreement with the values measured by ICP-MS. Generally, inorganic arsenic species exist as both As(III)) and As(V) in water and soil. As(V) is thermodynamically favored in oxic waters and As(III) in anoxic waters. As(III) can be transformed into As(V) by the oxidation of oxidants dissolved in waters, and the negatively charged As(V) can be easily absorbed by mineral substances.4 This is the reason why the concentration of As(V) is much low in above three kinds of original natural waters. To further investigate the proposed method for the detection of total As in the form of As(V), two soil samples were collected from the local field and pond and treated with a mixture of HNO3-H2O2 procedure. After oxidation treatment, the total As was detected in the form of As(V). The results were measured for the soil samples as summarized in Table 2. The recoveries of total As in As(V) form in soil extract samples are calculated as 95.9%−106.5% by UV-vis spectral method, and as 96.1%−106.2% by electrochemical method. These results are in good agreement with the measurements by ICP-MS method. The above results have been demonstrated that the present approaches are feasible strategy for the quantification of arsenate in real samples. More importantly, when the level of total As is detected in the form of As(V), it will be more friendly to the environment than As(III) due to As(III) with higher toxicity. 28


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Table 2. Detection of total arsenic in As(V) forms in two soil samples. As(V) / ppb

Samples

Spiked As(V)/ppb

ICP-MS

RSD (%)

UV-vi s

RSD (%)

Recoverya (%)

Electrochemistry

RSD (%)

Recoverya (%)

Paddy soil 1

0

−

−

−

−

−

−

−

−

Paddy soil 2

10

9.71

2.89

9.59

2.06

95.9

9.63

2.73

96.3

Paddy soil 3

50

49.2

3.06

48.7

1.95

97.4

53.1

3.51

106.2

Pond silt 1

0

4.65

3.17

4.29

3.05

−

4.37

3.36

−

Pond silt 2

10

14.2

2.93

15.3

2.97

106.5

54.1

2.87

96.1

Pond silt 3

50

53.9

3.25

56.8

3.12

104.3

56.5

3.29

103.7

(aRecovery (%) = [(cFound - cBlank)/cAdded] × 100%)

CONCLUSIONS In summary, a dual-mode method based on CoOOH nanozyme had been developed for sensitive and selective detection of As(V). The arsenic-inhibited peroxidase-like activity of CoOOH nanoflakes due to the electrostatic and As-O interactions can be used to establish simple colorimetric and sensitive electrochemical methods for arsenate detection. It is shown that the electrochemical method shows a great improvement of the detection limit down to 56.1 ppt. Besides, the practicability using the bimodal sensing platforms has been verified by analysis of different real samples. The bimodal method could be on-site, low-cost, convenient and environmentally friendly for detection of arsenate, but also promising for reliable assessment of the actual bioavailability of arsenate and total arsenic in complex environmental samples.

ASSOCIATED CONTENT Supporting Information 29



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The supporting information as noted in text. Stability of CoOOH nanoflakes; equation of oxidation of ABTS with the presence of CoOOH and H2O2; comparison of the catalytic performances of several nanomaterials; elemental mapping distribution; optimization of assay conditions; plot of initial rate of nanozyme reaction; absorption spectra and intensity with different amounts of As(V); selectivity of the colorimetric method to As(V); stability of CoOOH nanozyme and the colorimetric method; optimization amount of CoOOH dropped on GCE; electrochemical characterization; chronoamperometric intensity curve and detection of As(V) in three real water samples.

ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21675078), the Jiangxi Province Natural Science Foundation (20165BCB18022), and the Research Innovation Program for College Graduates of Jiangxi Province (YC2016-B002). Notes The authors declare no competing financial interest.

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Colorimetric assay conversion to highly sensitive electrochemical

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