Ultrasensitive Colorimetric Chromium Chemosensor Based on Dye

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An Ultrasensitive Colorimetric Chromium Chemosensor Based on Dye Color Switching under the Cr(VI)-Stimulated Au NPs Catalytic Activity Yu-Ting Zhuang, Shuai Chen, Rui Jiang, Yong-Liang Yu, and Jian-Hua Wang Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 26, 2019

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An Ultrasensitive Colorimetric Chromium Chemosensor Based on Dye Color Switching under the Cr(VI)-Stimulated Au NPs Catalytic Activity Yu-Ting Zhuang†, Shuai Chen‡, Rui Jiang†, Yong-Liang Yu†,* and Jian-Hua Wang† †Research

Center for Analytical Sciences, Department of Chemistry, College of

Sciences, Northeastern University, Box 332, Shenyang 110819, P. R., China ‡College

of Life and Health Science, Northeastern University, Shenyang 110169, P. R.,

China

Corresponding Author *E-mail: [email protected] (Y.-L. Yu) Tel: +86 24 83688944; Fax: +86 24 83676698

ABSTRACT: In view of the high toxicity of Cr(VI), simple and rapid on-site analytical approaches are in high demand for environment monitoring. Herein, an innovative chemosensor is developed for on-site sensitive detection of Cr(VI) in minutes by the naked eye. The chemosensor consists of gallic acid-capped gold nanoparticles (GA-Au NPs), methylene blue (MB) and NaBH4. In the presence of Cr(VI) ions, the reduction of MB by NaBH4 is able to greatly accelerate due to the Cr(VI)-stimulated catalytic activity of GA-Au NPs, resulting in a color switching of MB from blue to colorless for the quantitative detection of Cr(VI). The chemosensor

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in solution exhibits excellent selectivity and ultrahigh sensitivity to Cr(VI), with the detection limits of 0.05 nM by UV-vis spectroscopy and 0.1 nM by the naked eye. Similarly, a paper-based chemosensor is obtained by immobilization of GA-Au NPs and MB onto a piece of filter paper, offering a more convenient approach for rapid on-site detection of Cr(VI). In addition, H2O2 as an oxidizing agent is employed to convert Cr(III) into Cr(VI), thus achieving speciation analysis of Cr. The applicability of chemosensor is also validated by the detection of Cr speciation in water samples with satisfactory recoveries.

KEYWORDS: chemosensor, gallic acid-capped gold nanoparticles, methylene blue, colorimetric detection, Cr(VI)

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INTRODUCTION With the increasing concern for human health and ecosystem protection, the toxicity of hexavalent chromium Cr(VI) has been widely recognized around the world.1 Back in 1982 International Agency for Research on Cancer (IARC) considered Cr(VI) to be a powerful carcinogenic agent that modifies the DNA transcription process causing important chromosomic aberration.2,3 Even at a low level of concentration, Cr(VI) presents a serious and persistent threat to public health due to its migration and transform in water, soil, biosphere as well as bioaccumulation.4,5 Therefore, Cr(VI) emission and its content in drinking water and industrial wastewater are strictly restricted by most countries and regions. Conventional analysis of Cr(VI) in water requires highly skilled staff and sophisticated equipment, such as atomic absorption spectrometry,6,7 inductively coupled plasma mass spectrometry (ICP-MS),8,9 surface-enhanced Raman scattering spectroscopy,10 fluorescence and UV-vis spectroscopy.11,12 These instrumentations can be employed for sensitive and accurate detection of Cr(VI), but these approaches are costly along with complex operation process, thus limiting their application for rapid on-site analysis. Chemosensor provides a powerful tool for Cr(VI) analysis. It responds to the particular analyte by converting the chemical stimuli into a signal that can be measured or recorded.13 In chemosensors, nanoparticles capped by different agents (e.g., citrate, 1,4-dithiothreitol, and thymine derivative) or molecules would present the aggregation and disaggregation or the chelation effect in the presence of particular analyte, resulting in the color or fluorescence or UV-vis change.4,14-18

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Currently, chemosensors based on various innovative nanomaterials provide an alternative for Cr(VI) detection, e.g., a Zr4+ MOF was applied to selective sensing of Cr(VI) with a detection limit of 0.004 mg L-1.19 In a recent study, it was found that gallic acid capped gold nanoparticles could be applied for colorimetric detection of Cr(VI) via the aggregation induced color change,12 with the limits of detection (LODs) of 2 μM by eye vision and 0.1 μM by UV-vis spectroscopy. In Canada Cr(VI) guideline is set at 0.001 mg L-1 for marine life and 0.008 mg L-1 for irrigation, respectively.20 Apparently, poor sensitivity of the common chemosensors is far from the expected goal for Cr(VI) detection. In addition, these chemosensors suffer from the sophisticated synthetic process, limited selectivity, and high susceptibility to pH value, making them infeasible for large-scale use.21-23 In this context, a novel chemosensor is developed for sensitive colorimetric detection of chromium speciation (Cr(VI) and Cr(III)). In the presence of Cr(VI) ions, the reduction of MB by the free radicals derived from NaBH4 is facilitated due to the Cr(VI)-stimulated catalytic activity of GA-Au NPs, resulting in MB color switching. The present chemosensor in solution exhibits excellent selectivity and ultrahigh sensitivity to Cr(VI), with a detection limit of 0.1 nM by the naked eye. Similarly, a paper-based chemosensor is also obtained by immobilization of GA-Au NPs and MB onto a piece of filter paper, which is more convenient for rapid on-site detection of Cr(VI). When H2O2 as an oxidizing agent is employed for the oxidization of Cr(III) to Cr(VI), speciation analysis can be carried out by measuring Cr(VI) with and without this preoxidation step. In addition, the GA-Au NPs immobilized filter membrane can

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be applied for the high efficient removal of Cr(VI) in water.

EXPERIMENTAL SECTION Apparatus. UV-vis spectra were obtained by a Hitachi UH5300 spectrophotometer. The morphology was characterized by an Ultra Plus scanning electronic microscope (SEM, Zeiss, Germany) with an energy dispersive X-ray spectroscopy. The details of the morphology were observed by a transmission electron microscopy (TEM, FEI Tecnai G2 F20, USA). High-resolution transmission electron microscope (HRTEM) was operated at 200 kV. High resolution X-ray photoelectron spectroscopy (HR-XPS) scanning curves were obtained on an ESCALAB 250 surface analysis system (Thermo Electron, USA) with an Al Kα 280.00 eV excitation source. Fourier-transform infrared (FT-IR) spectra were characterized by a Nicolet 6700 spectrometer (Thermo Electron, USA) within 400–4000 cm-1. The concentrations of metal ions in solution were measured by an Agilent 7500a ICP-MS (Agilent Technologies) furnished with a Babington nebulizer and double-pass Scott-type spray chamber. Preparation of Gallic Acid-Capped Gold Nanoparticles (GA-Au NPs). The 15-nm Au NPs were prepared by reducing HAuCl4 with sodium citrate.24 Briefly, 700 μL of 1% (m/v) sodium citrate was rapidly added into 20 mL of a boiling 0.24 mM HAuCl4 solution, and consistently stirred and heated for 15 min. The color of the mixture turned from pale yellow to red. The stirring was continued for several minutes after the stop of heating. The concentration of the Au NPs in solution was measured by ICP-MS.

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The GA-Au NPs were prepared by ligand exchange. 90 μL of 27 mM GA was added into 10 mL of citrate-Au NPs solution, and vigorously stirred for 4 h. The mixture was centrifuged at 9300 rpm for 15 min, and the supernatant was removed. All the GA-Au NPs obtained were stored at 4°C before use. Preparation of Paper-Based Chemosensor and Filter Membrane. 20 μL of 40 nM GA-Au NPs solution was dropped on a piece of square filter paper with a side of length 1 cm. After 1 h, the filter paper was immersed into 2 mL of 20 μM MB solution for 3 h. Then the GA-Au NPs and MB immobilized filter paper was taken out from the mixture solution and dried at room temperature for 2 h. This kind of filter paper could be served as a chemosensor for colorimetric detection of Cr(VI). A piece of filter paper with a diameter of 4 cm was immersed into 10 mL of 120 nM GA-Au NPs solution for 24 h. Then the GA-Au NPs immobilized filter paper was taken out from the mixture solution and dried at room temperature for 2 h. This kind of filter paper could be served as a filter membrane for the high efficient removal of Cr(VI) in water. Oxidization of Cr(III) to Cr(VI). 5 mL of Cr(III) solution was adjusted with 1 M sodium hydroxide to pH 11. Then 10 μL of 30% (m/v) H2O2 was added into Cr(III) solution, followed by stirring for 1 h at 50°C. Protocols for Cr(VI) Detection and Removal. For Cr(VI) detection, 10 μL of Cr(VI) solution at various concentrations was added into 1 mL of 20 μM MB solution plus 9 μL of 40 nM GA-Au NPs solution. Then 40 μL of 0.04 M NaBH4 solution prepared freshly was added into the above mixture. Absorption spectra of MB dye

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within the range of 500-750 nm were recorded with a characteristic absorption peak at 664 nm. On the addition of 10 μL of Cr(VI) solution and 20 μL of 0.04 M NaBH4 onto the paper-based chemosensor, the color switching was observed by eye vision, and the solid UV-vis spectrum of MB dye within the range of 500-750 nm was recorded in 2 min with a characteristic absorption peak at 664 nm. A piece of dry GA-Au NP filter paper (Au, 3%) was used to decontaminate 20 mL of waste water containing 50 μg L-1 Cr(VI). Almost 99% Cr(VI) can be fast removed from the aqueous solution in 2 min. Leaching Experiments. The US-EPA methods of TCLP and SPLP were used to evaluate the leaching risk of the paper-based chemosensor. With TCLP model, the leaching behavior of a particular waste material was obtained under municipal landfill conditions. Briefly, the paper-based chemosensor was cut into small pieces with a diameter of PCA> 3, 5-DHBA, p-HBA> 3-HBA. The dihydroxy compounds undergo two-electron oxidation to the corresponding quinone form (Figure S9), and the formed nanoparticles are stabilized by interactions with both the trihydroxy and carboxylic acid groups of the GA. Combined with XPS characterization, it reveals that the presence of the GA drives the reduction of Cr(VI) to Cr(III) (Cr2O3) and the generation of Cr2O3 on the surface of GA-Au NPs.34,38 These results strongly imply that the surface bound molecules of Au NPs are crucial for the formation of Au@Cr2O3 thin layer. Paper-Based Chemosensor for Visual Detection of Cr(VI) and Application

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in Cr(VI) Removal. In order to detect Cr(VI) conveniently in the field, a paper-based chemosensor is constructed by immobilization of GA-Au NPs and MB on the filter paper. Briefly, a piece of filter paper is immersed into the mixture solution of 20 μM MB and 40 nM GA-Au NPs for 2 h, then the GA-Au NPs and MB supported on the filter paper are dried at room temperature for 2 h. Solid UV-vis spectra of the filter paper, GA-Au NPs filter paper, MB filter paper and MB/GA-Au NPs filter paper are shown in Figure S10. After adding different amounts of Cr(VI) onto the paper-based chemosensor, the color change of the chemosensor could be obviously distinguished with the naked eye (Figure 1E). Compared to the filter paper (Figure S11A), the GA-Au NPs are densely distributed on the surface of microscale paper fibers, forming nanoscale roughness on the three dimensional net-like topography in Figure 4A. The SEM corresponding element mapping images further confirm the presence of Au element in the filter paper (Figure S12). The LOD of the paper-based chemosensor for Cr(VI) with the naked eye is found to be 0.1 μM. Although the sensitivity of paper-based chemosensor is not higher than those of some highly sensitive methods, the major advantage lies in visual and on-site monitoring of Cr(VI) without any additional kits. In addition, the LOD obtained by this chemosensor is much lower than the guidelines limit of 50 μg L-1 for Cr(VI) regulated by World Health Organization (WHO) in groundwater. The complicated environment in the practical Cr(VI) detection, e.g., high acid or alkali and high saline, makes the chemosensor withstand more challenges.4,11,14,15 In our study, the paper-based chemosensor is employed for the detection of Cr(VI) within pH 1-12 as shown in Figure S13. The results

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demonstrate that the paper-based chemosensor is hardly affected by pH even in a high acid or alkali environment. Furthermore, a series of Cr(VI) detection experiments are comprehensively carried out in high saline environment (Figure S14). The high anti-interference ability in these environments implies a high stability of the paper-based chemosensor to resist high acid, alkali, and saline environment.

B 1.2

A

Absorbance (a.u.)

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

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1 μm

0.8

0.4

0.0

0

C

1

2

3

4

1

2

5

3 4

5

6

7

8

Cycle number 6

9 10

7

Before

After Cycle number

Figure 4. (A) SEM images of MB/GA-Au NPs filter paper; (B) Recycling of GA-Au NPs as a catalyst for the reduction of MB by excess NaBH4. Conditions: MB concentration: 0.020 mM, 1 mL; Cr(VI) concentration: 100 nM, 10 μL; GA-Au NPs concentration: 40 nM, 9 μL; NaBH4 concentration: 0.04 M, 40 μL; temperature: 25°C; (C) Photo images of cycle number of MB/GA-Au NPs filter paper. Conditions: Cr(VI) concentration: 2 µM, 10 μL; NaBH4 concentration: 0.04 M, 20 μL. What is more, the GA-Au NPs filter paper can serve as point-of-use to remove Cr(VI) from waste water. A filter paper was immersed into 10 mL of 120 nM GA-Au

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NPs solution for 24 h. When the dry filter paper containing 3% GA-Au NP was used to remove 20 mL of 50 μg L-1 Cr(VI) ions in waste water, almost 99% Cr(VI) was removed from the aqueous solution. In contrast, only 20% Cr(VI) was removed by blank filter paper. The gold element in the filtered water is not detected by ICP-MS, but the GA-Au NPs on the filter paper are observed in Figure S11B. Application to Real Samples. To validate the applicability of the present chemosensor to real samples, the quantification of Cr(VI) is carried out in GBW08608, GBW(E)080082, tap water, lake water and waste water, respectively. The recoveries of Cr(VI) spiked with various concentrations for five samples are statistically close to 100% (ranging from 98% to 102%), as listed in Table 1. The results obtained by the present colorimetric method are reasonable agreement with those obtained by the classic ICP-MS method, demonstrating the accuracy of the present chemosensor. When H2O2 as an oxidizing agent is employed for the oxidization of Cr(III) to Cr(VI), speciation analysis can be carried out by measuring Cr(VI) with and without this preoxidation step. Table S2 lists the determination results for total Cr in mixture solution. Furthermore, the paper-based chemosensor is also applied for visual detection of Cr in GBW08608. It can be seen in Figure S15 that the concentration of Cr in GBW08608 is ca. 0.5 μM. Sensor Regeneration, Leaching and Disposal. Reversibility is an important aspect for a chemosensor. For this purpose, 10 μL of 40 μM NaBH4 solution prepared freshly was added into the solution of GA-Au NPs + MB containing 100 nM Cr(VI) and MB/GA-Au NPs filter paper containing 2 µM Cr(VI), respectively. An obvious

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color change from blue to colorless can be observed with the naked eye. When the above colorless solution and filter paper were exposed to air, blue color of MB was recovered. 5% (v/v) HCl was used to elute Cr(VI) ions for regenerating the GA-Au NPs. When the solution of GA-Au NPs + MB and paper-based chemosensor were reused to detect Cr(VI) using the same procedure abovementioned, the reversible colorimetric assay could be repeated 10 and 7 times, respectively (Figure 4B/C). Table 1. Determination results for the certified reference materials and real water samples Chemosensor Sample

GBW08608 GBW(E) 080082

Tap water

Lake water

Spring water

Spiked Cr(VI) Found -1 (nmol L ) (nmol L-1)

ICP-MS Recovery (%)

Found (nmol L-1)

Recovery (%)

0

556.3±19.9

98

566.696±10.949

99

0.50

0.50±0.01

100

0.498±0.017

100

0.80

0.80±0.02

100

0.806±0.012

101

30.0

29.8±0.9

99

30.600±0.663

102

0.50

0.51±0.02

102

0.502±0.018

100

0.80

0.81±0.03

101

0.808±0.018

101

30.0

29.6±1.1

99

30.613±1.270

102

0.50

0.51±0.03

102

0.496±0.028

99

0.80

0.79±0.02

99

0.794±0.015

99

30.0

30.4±1.3

101

30.538±0.879

102

0.50

0.51±0.01

102

0.506±0.016

101

0.80

0.79±0.02

99

0.806±0.022

101

30.0

30.2±1.1

101

30.329±0.778

101

*GBW08608 total Cr: 577.00 nM

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To evaluate the nonhazardous nature of the filter paper after use and the suitability for waste disposal, the leaching risk of the filter paper waste was assessed by using toxicity characteristic leaching procedure (TCLP) and the synthetic precipitation leaching procedure (SPLP). The extraction of chromium and gold from filter paper waste was performed to simulate sanitary landfill conditions and evaluate the leaching potential of land-disposed wastes under acid rainfall (Table S3). The results show that a filter paper after adsorbing 50 mg L-1 chromium would release only 0.0015 and 0.002 mg L-1 from filter paper with TCLP and SPLP, respectively, much below the US-EPA regulatory limit of 5 mg L-1 for waste disposal. Although the filter paper was also loaded with 7.1 ng gold, the result of the leaching experiment showed no leakage of gold. It indicates that the filter paper is nonhazardous and can be disposed by landfilling.

CONCLUSIONS In summary, a novel chemosensor is developed for the colorimetric detection of Cr(VI) in a simple manner. The chemosensor takes advantage of the presence of Cr(VI) to accelerate the reduction of MB by NaBH4 along with a color switching of MB, due to the Cr(VI)-stimulated Au NPs catalytic activity. The coating agent GA on Au NPs promotes the conversion of Cr(VI) into Cr(III) (Cr2O3) depositing on the surface of Au NPs. This chemosensor presents excellent sensitivity for Cr(VI) detection, and provides a simple, convenient, recycled and rapid detection method. Furthermore, it is expected that this work would open up new insights into the structure design of chemosensor for field analysis.

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ACKNOWLEDGEMENTS This work is financially supported by the National Natural Science Foundation of China (21874017, 21727811 and 21605014).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y.-L. Yu) Tel: +86 24 83688944; Fax: +86 24 83676698 Notes The authors declare no competing financial interest.

ASSOCIATED CONTENT Supporting Information Materials; optimization of experimental parameters; UV-vis spectra of Au NPs with different particle sizes; color changes of the detection solution in the presence of 10, 15, 30, and 50 nm Au NPs; UV-vis spectra of Au NPs capped with different amounts of GA; FT-IR spectra of Au NPs, GA, GA-Au NPs and GA-Au NPs + Cr(VI); effect of reaction temperature on catalytic reduction of MB; time-dependent UV-vis absorption spectra for the catalytic reduction of MB by NaBH4 in the presence of GA-Au NPs; smartphone-based colorimeter for the determination of Cr(VI); TEM and HRTEM images of GA-Au NPs after Cr(VI) detection; XPS survey spectrum of GA-Au NPs in the presence of Cr(VI); Cr 2p spectrum of GA-Au NPs in the presence of Cr(VI); two-electron oxidation of gallic acid to corresponding quinone form; solid

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UV-vis spectra of filter paper, GA-Au NPs filter paper, MB filter paper and MB/GA-Au NPs filter paper; SEM images of filter paper and GA-Au NPs filter paper; SEM images of GA-Au NPs filter paper and corresponding element mapping images of C, O, and Au; the colorimetric images of MB/GA-Au NPs filter paper in the presence of Cr(VI) at different pH values; the colorimetric images of MB/GA-Au NPs filter paper in the presence of different metal ions; photo image of the colorimetric filter paper on the addition of different concentrations of Cr(VI) and GBW08608; comparisons of analytical performances of the MB/GA-Au NPs chemosensor to other approaches for the detection of Cr(VI); determination results for total Cr in solution by MB/GA-Au NPs chemosensor; leachability testing of filter paper using the Toxicity Characteristic Leaching Procedure and Synthetic Precipitation Leaching Procedure.

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For TOC only

MB NaBH4

MB paper

7

Cr(VI) accelerating GA-Au NPs MB Cr(VI) Au@Cr2O3

NaBH4

LMB paper

Recycle 7 times

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