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Colorimetric detection of Cr (VI) based on the leaching of gold nanoparticles using a paper-based sensor
Author’s Accepted Manuscript Colorimetric detection of Cr (VI) based on the leaching of gold nanoparticles using a paper-based sensor Jian-feng Guo, Dan-qun Huo, Yang Mei, Changjun Hou, Jun-jie Li, Huan-bao Fa, Hui-bo Luo, Ping Yang www.elsevier.com/locate/talanta
PII: DOI: Reference:
S0039-9140(16)30704-4 http://dx.doi.org/10.1016/j.talanta.2016.09.032 TAL16882
To appear in: Talanta Received date: 25 May 2016 Revised date: 7 September 2016 Accepted date: 11 September 2016 Cite this article as: Jian-feng Guo, Dan-qun Huo, Yang Mei, Chang-jun Hou, Jun-jie Li, Huan-bao Fa, Hui-bo Luo and Ping Yang, Colorimetric detection of Cr (VI) based on the leaching of gold nanoparticles using a paper-based sensor, Talanta, http://dx.doi.org/10.1016/j.talanta.2016.09.032 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Colorimetric detection of Cr (VI) based on the leaching of gold nanoparticles using a paper-based sensor Guo Jian-feng1, Huo Dan-qun1*, Yang Mei1*, Hou Chang-jun1,2, Li Jun-jie1, Fa Huan-bao3, Luo Hui-bo4, Yang Ping5 1
Key Laboratory of Biorheology Science and Technology, Ministry of Education, College of Bioengineering, Chongqing
University, Chongqing, 400044, China 2
National Key Laboratory of Fundamental Science of Micro/Nano-Device and System Technology, Chongqing University,
Chongqing, 400044, China 3
College of Chemistry and Chemical Engineering, Chongqing University, Chongqing, 400044, China
4
Liquor Making Bio-Technology & Application of Key Laboratory of Sichuan Province, Zigong 643000, China
5
National Engineering Research Center of Solid-State Brewing, Luzhou Laojiao Group Co. Ltd., Luzhou, Sichuan, 646000,
China
[email protected]
[email protected] *
Corresponding author. Tel.: +862365112673; fax: +862365102507.
Abstract Herein, we have developed a simple, sensitive and paper-based colorimetric sensor for the selective detection of Chromium (Ⅵ) ions (Cr (VI)). Silanization- titanium dioxide modified filter paper (STCP) was used to trap bovine serum albumin capped gold nanoparticles (BSA-Au NPs), leading to the fabrication of BSA-Au NPs decorated membrane (BSA-Au NPs/STCP). The BSA-Au NPs/STCP operated on the principle that BSA-Au NPs anchored on the STCP were gradually etched by Cr (VI) as the leaching process of gold in the presence of hydrobromic acid (HBr) and hence induced a visible color change. Under optimum conditions, the paper-based colorimetric sensor showed clear color change after reaction with Cr (VI) as well as with favorable selectivity to a variety of possible interfering counterparts. The amount-dependent colorimetric response was linearly correlated with the Cr (VI) concentrations ranging from 0.5 µM to 50.0 µM with a detection limit down to 280 nM. Moreover, the developed cost-effective colorimetric sensor has been successfully applied to real environmental samples which demonstrated the potential for field applications.
Keywords: paper-based; gold nanoparticles; non-aggregation; colorimetric; Cr (VI)
1. Introduction Cr (III) and Cr (VI) are the most common valence states observed in nature environment.[1] Nevertheless, compared to Cr (III), Cr (VI) is much more notorious for its strong oxidability, hypertoxicity, carcinogenicity, non-biodegradability which posing serious threats to both environment and human health.[2] Generally, exposure to even a low concentration of Cr (VI) might cause hemolysis, renal, liver failure and even various types of cancer.[3] With the global expansion of industrial factories, extensive
utilization of chromium in industrial processes such as stainless steel, chrome plating, leather tanning and pigment production always causing chromium-containing effluents or slags discharged into the environment immoderately.[4] With increasing attention on chromium pollution, the United States Environmental Protection Agency (US-EPA) suggests that the concentration of Cr (VI) in drinking water should under 100 ppb and the World Health Organization (WHO) also proposes a guideline value for total chromium and Cr (VI) in drinking water which is 50 ppb and 16 ppb, respectively. [5, 6] Therefore, it is of significant importance to develop a simple and practical method for the determination of Cr (VI). Previously, a number of strategies have been devised to detect Cr (VI), those include methods based on large equipments such as inductively coupled plasma mass spectrometry (ICP-MS),[7] atomic absorption spectroscopy(AAS),[8] X-ray fluorescence (XRF) and solid phase extraction.[9] [10] On one hand, those large equipment-based methods usually have superior accuracy and stability. But on the other hand, they usually suffer from disadvantages such as sophisticated pretreatment, requirement of professional operation, and also high-cost, which render them unsuitable for real-time field monitoring. In the meantime, colorimetric assay based on metal nanoparticles have attracted increasing attentions in recent years because of requiring no complex instrumentations and signal recognition can be achieved by just naked eye in the form of color change.[11-13] Of particular interest is those based on the gold nanoparticles (Au NPs), as they have strong surface plasmon resonance (SPR) absorption with extremely high extinction coefficients (108-1010 M-1•cm-1) in the visible region.[14] The past few years have witnessed Au NPs widely employed in colorimetric detection for various analytes including detection of cells,[15] viruses,[16] proteins,[17] nucleic acids,[18] small molecules[19]. Especially, Au NPs have been frequently used in the detection of metal ions, such as Pb (II),[20] Hg (II),[21] Mn (II),[22] Fe (Ⅲ) and As (Ⅲ)[23], Cr(Ⅲ) and Cr (VI)[24]. Majority of these probes based on target-inducing particles aggregation in the solution-phase, have proven to be sensitive as well as rapid response. Nevertheless, Au NPs in solution are vulnerable to aggregate and may fail in detecting metal ions in sophisticated matrixes.[25] Besides, there also have difficulty in discerning color hue or intensity changes when detecting colored samples or complex mixtures, such as blood and waste water samples. While transforming the detecting process from solution-phase to solid-phase may alleviate these problems effectively, as membrane-based colorimetric sensor have better stability properties as well as visual detection which pose a good expectation for on-site sensing. Herein, we have developed a practical and sensitive paper-based colorimetric sensor for the rapidly detection of Cr (VI). After functional modification of a titanium dioxide (TiO2) intermediate on commercial
cellulose filter papers, sulfhydryl and amino groups’ decorations were successfully realized to endow the interface with appropriate hydrophilicity/hydrophobicity as well as strong binding sites for BSA-Au NPs. In the presence of HBr, the BSA-Au NPs anchored on the surface of paper were gradually dissolved by Cr (VI) as the etching process of gold. The leaching of gold results in rapid and remarkable damping of the SPR and hence induces a visible color change. The designed paper-based colorimetric sensor, as compared to other solution-based sensors, is more economical, facile and practical for the detection of Cr (VI) without sophisticated equipment. Moreover, we also demonstrated the practicality of using this approach for the detection of Cr (VI) in real river water sample.
2. Experimental sections 2.1. Apparatus Images of dispersed Au NPs were achieved by transmission electron microscopy (TEM, JEOL Ltd., Japan). Inductively coupled plasma mass spectroscopy (ICP-MS) was performed with ICP-MS instrument (Agilent 7500, USA). Ultraviolet spectrum measurement was performed using a UV-2700 spectrophotometer (Tsushima, Japan). Zeta potential measurements were performed on Malvern Zetasizer Nano-ZS90 (Malvern Instruments Ltd., U.K.). X Ray Fluorescence (XRF) images were achieved by super ED-XRF 1050 spectrometer(Skyray Ltd., China). 2.2. Chemicals Gold
acid
chloride
trihydrate
(HAuCl4•3H2O)
(99.0%),
chitosan
(80%-95%
deacetylated),
(3-Mercaptopropyl)-trimethoxysilane (MPTMOS) and (3-Aminopropyl)-trimethoxysilane (APTEOS) were purchased from Aladdin Industrial Inc. (Shanghai, China). Bovine serum albumin (BSA) was purchased from Sigma-Aldrich (St Louis, MO, USA). Hydrobromic acid (HBr) (47% aqueous solution), potassium dichromate (K2CrO4) (99.8%), Tetrabutyltitanate (TBT), ethyl alcohol (EtOH), acetic acid (HAc) and all other fundamental reagents (analytical reagent grade) were purchased from Titan (Shanghai, China) without further purification. Commercial cellulose filter paper (diameter=7 cm) was purchased from Shuangquan Filter Paper Ltd.Co. (Hangzhou, China). Ultra-pure water was generated by a Millipore Direct-Q Water system (Mosheim, France). 2.3. Synthesis of BSA-stabilized gold nanoparticles The Au NPs was synthesized by a green method according to a former report with slightly modifications.[26] Briefly, chitosan solution of 3 mg/mL was prepared with 1.0% HAc by stirring at 70°C.
After obtaining a clear solution, addition of 2 mL 1wt.% HAuCl4•3H2O solution (25.0 mM) into 50 mL as-prepared chitosan solution, the mixture was heated to 100°C and kept stirring until the color turned into deep red and then cooled at room temperature with continuous stirring. To obtain BSA-Au NPs, an aliquot of the BSA aqueous solution (50.0 μM, 3 mL) was added to the above 50 ml Au NPs solution, equilibrated for 2h at room temperature. Finally, the BSA-Au NPs solution was stored at 4°C under darkness. 2.4. Fabrication of the BSA-Au NPs/STCP probe The fabrication of nano-TiO2 modified filter papers (TCP) was prepared according our previously reported.[27] Briefly, 5%(v/v) hydrochloric acid (HCl) aqueous solution was first used to remove surface contaminations on cellulose filter paper and washed with deionized water. Then immersed 5 pieces of filter paper in 20 mL EtOH for 2h and drying at 60°C for 1h. Then the papers were added into 100 mL mixed solution of ethanol, TBT, and HAc in a ratio of 10:3:1 in a culture dish, which was allowed to hydrolyze for 30 minutes under shaking (100npr) at ambient conditions. The uneven residual solution on the surface of filter paper was removed by issue paper and then dried at 60°C for 10 minutes. After that, papers were kept in 300 mL deionized water to finally hydrolyze for 4h at 140°C and dried for 2h to obtain TCP. To further obtain STCP, the as-prepared TCP were immersed into 100 mL APTEOS of 3.0% concentration (v% in EtOH) and allowed to react for 30 minutes at 100°C. The papers were washed successively with deionized water to remove unreacted reagents and dried for 10 minutes at 50°C. After that 100 mL MPTMOS (3%/v%) ethanol solution was applied to repeat the procedures similar to that of APTEOS. Then, a piece of STCP was cut to a size 0.9 cm (length)•0.9 cm (width) and dipped with BSA-Au NPs solution (12.6 nM, 0.6μL) for several times. After drying with a blower for 5 minutes, BSA-Au NPs/STCP were gently washed with deionized water to remove any weakly bonded or impurities and then dried in air at room temperature for 2h prior to use. 2.5. Detecting Cr (VI) with the BSA-Au NPs/STCP For the determination of Cr (VI), the as-prepared BSA-Au NPs/STCP was immersed in 1.0 M HBr solution premixed with target or standard aqueous samples ranged from 0-80µM. After incubating at 60°C for 10 minutes, the test papers were gently washed with deionized water and dried at 50°C for 5 minutes. Finally, the images of BSA-Au NPs/STCP were scanned by a scanner and color analysis was performed using an Image J computer program that measured the red, green, and blue (RGB) color intensities. In the present study, we recorded the green component absorbance (Gabs) of the BSA-Au NPs/STCP after it reacted with the leaching agent (1.0 M HBr) in the absence and presence of Cr (VI).
2.6. Analysis of real river water sample The real water sample was collected from Jialing River (Chongqing, China) and filtered through a 0.2-μm membrane before use. Firstly, the pretreated river water (0.99 mL) and HBr (1.0 ml, 2 M) were spiked with standard Cr (VI) solutions (10 µL) at concentrations over the range of 0-50 µM. Then, BSA-Au NPs/STCP were immersed in the above solution and incubated at 60°C for 10 minutes. After that, the strips were gently washed with deionized water and dried at 50°C for 5 minutes. The accurate concentration of Cr (VI) in river water was detected using ICP-MS and subsequently used to calculate recovery values.
3. Results and discussion 3.1. Characterization of Au NPs and BSA-Au NPs/STCP Here, the Au NPs were synthesized by a green biological method, using chitosan as both the reducing and protecting agent. Compared to general methods for the preparation of Au NPs using organic compounds such as citrate and NaBH4,[28-31] the present strategy will not introduce any environmental toxicity or biological hazards as the excellent biodegradable and biocompatible characteristics of chitosan.[32, 33]. TEM images was employed to characterize the morphology of Au NPs and shown in Fig. 1A, Au NPs were in globose appearance with multi-dispersed sizes that corresponding well with previous reports.[26, 34] The average particles size of as-prepared Au NPs was calculated to be 14.06±3.14 nm by counting the diameter of 200 particles (Fig. 1B). We also monitored UV-visible absorption spectroscopy of Au NPs and found the absorption peak was located at 520 nm that matched with former studies.[33, 35] Concentration of the as-prepared Au NPs was estimated to be 12.6 nM according to Lambert-Beer's law using an extinction coefficient of 2.43×108 M-1•cm-1 at 520 nm for the 14.2 nm Au NPs.[14] According to previous researches,[36] Au NPs can be stabilized by interaction of protein side chains or domains, resulting in a reduction in entropy and a loss of solvation enthalpy, both of which cause an colloid repulsion. As proven in Fig. S1, the mean zeta potential value of Au NPs in the absence and presence of BSA is about -3.55 and 34.6 mV, respectively. As the isoelectric point of BSA is 4.6, and therefore BSA is positively charged at strong acid environment (e.g. pH=0). Thus the significant potential change might attribute to the binding of positively charged BSA to negatively charged surfaces of Au NPs and induced colloid repulsion among BSA-Au NPs. Therefore, the BSA could greatly reduce the aggregation of Au NPs and improve their stability in aqueous solution. In order to illuminate the fabrication processes of BSA-Au NPs/STCP, SEM was applied to observe the
morphologic properties in different procedures. As Fig. 2A shows, the surface of cellulose is smooth and there are barely particles adsorbed on it before impregnation with TiO2. Generally, cellulose was rich in −C−O−O− and −OH groups, which can easily enable Ti ion to firmly anchor onto the cellulose surface. [37] After a TiO2 film was uniformly deposited onto the nanofiber surfaces using the sol–gel method[39], the nanofiber was endowed with a rough surface morphology and larger specific surface area.(Fig. 2B) It is beneficial for providing an intermediate layer with broad surface area chemically active for subsequent modifications. After loading BSA-Au NPs solution on the TiO2 film, spherical gold nanoparticles anchored on the TiO2 film can easily observed (Fig. 2C) which demonstrated the successfully fabrication of BSA-Au NPs/STCP. Moreover, X Ray Fluorescence (XRF) was employed to monitor the metallic elements during the fabrication processes. As shown in Fig. S2, the XRF spectra of untreated filter paper showed no metallic element initially. With the deposition of TiO2 film on the nanofiber, there began to arise obvious peaks of Ti, and the peaks of Au appear subsequently after loading BSA-Au NPs. The successive appearance of Ti, Au in the XRF spectra also indicated successful fabrication of BSA-Au NPs/STCP. 3.2. Sensing mechanism Previously, the nitrocellulose membrane (NCM) has been employed as blotting matrix for protein capped gold nanoparticles immobilizations.[25, 35] However, hydrophobic interactions as the main binding forces between proteins and nitrocellulose that might lead the immobilized BSA-Au NPs easily leakage from NCM that make false-positive conclusion in detecting process. In the present study, as shown in Scheme 1, a functional cellulose filter paper was used to immobilize BSA-Au NPs through strong Au-S bond as well as hydrophobic interactions and fabricated the BSA-Au NPs/STCP for detecting Cr (VI). The mechanism of this paper-based sensor for sensing Cr (VI) is investigated in our study. Generally, Cr (VI) does not have enough ability to oxidize gold as the standard electron potential of Au (I)/Au (0) and Cr (VI)/Cr (III) is 1.69 eV and 1.33 eV, respectively.[38] Nevertheless, in the presence of Br- the potential of Au (I)/ Au (0) decreased as the formation of AuBr2- complexes, that resulting Au NPs redox etching by Cr (VI). (Equation 1) The leaching process of gold induced by Cr (VI) caused a dramatic diminishment of particle amounts as well as the particles size of Au NPs, leading to observable color change on the surface of BSA-Au NPs/STCP. Actually, several studies have also reported similar mechanism that based on the etching of gold in sensing metal ions in aqueous solution, such as Fe3+, Cu2+, and Pb2+, with the satisfactory results demonstrating its simplify and selectivity as well as practicality in detecting metal ions.[25, 39, 40] 𝟑𝐀𝐮 + 𝟔𝐁𝐫 − + 𝐂𝐫 𝟔+ → 𝟑𝐀𝐮𝐁𝐫𝟐− + 𝐂𝐫 𝟑+ (Equation 1)
3.3. Optimization of the detecting conditions for Cr (VI) For optimizing the detecting conditions, relevant experimental parameters, including pH value, concentration of Br-, incubation temperature and response time were carefully evaluated in the present study. The descriptors Gabs and G○abs represent the green component absorbance (Gabs) of the BSA-Au NPs/STCP probe in the presence and absence and of Cr (VI), respectively. △Gabs represents the value change between Gabs and G○abs. Firstly, we examined the effect of dipping frequency of BSA-Au NPs solution. As Fig. 3A shows, the Gabs intensity responded linear correlation to the dipping frequencies with a good correlation coefficient (R2=0.9847). On one hand, the stronger Gabs intensity could extend the detection range of Cr (VI) as well as do a favor to the visual detection process. But on the other hand, excessive loading BSA-Au NPs on the surface of STCP may reduce the interaction forces between BSA-Au NPs and the surface of paper, which might decrease the sensitivity and selectivity toward Cr (VI). As proved in Fig. 3B, successive increasing dipping frequency of BSA-Au NPs solution from 3 to 8 times caused the corresponding colorimetric responses and reached maximum at 6. Taking both the intensity of Gabs and stability into consideration, dipping 6 times of BSA-Au NPs solution was chosen for further investigation. As a critical factor for the detection process, the influence of pH was also investigated in our study. The oxidizability of Cr (VI) is greatly relying on the environment pH. For example, the electron potentials of Cr (VI)/Cr (III) are 1.271 eV and 0.995 eV when the pH values are 1.0 and 3.0, respectively.[41] The low pH circumstance can greatly accelerate the etching process as the increasing oxidation of Cr (VI). Nevertheless, the excessive increasing of acidity might also reduce the selectivity and stability of the paper-based sensor. As expected, the colorimetric response of the sensor keep increasing with the pH value decreasing from 7.0 to 1.0 and achieved maximum response when the pH was lower than 1.0 (Fig. 4A). Considering the sensitivity and selectivity, the pH=0 was chosen for further investigation. Ligand reagents such as 2-mercaptoethanol, thiocyanate, and thiourea are important parameter influencing the dissolution of gold in the etching process. [14, 39, 42]. Here, Br- acted as the ligands of gold and formed AuBr2- to accelerate the leaching process by reducing the electron potential of Au (I)/Au (0). As showed in Fig. 4B, colorimetric responses enhanced with the increasing of Br- concentration ranged from 0 to 1.0 M. However, the continuously increasing concentration of Br- might decrease the selectivity and the excessive Br- might also react with Cr (VI) as the electron potentials of Br/Br- is 1.09 eV. Therefore, we chose 1.0 M Br- as the optimized concentration and applied in our further study.
Finally, the influence of incubation temperature and response time was taken into consideration. Effect of temperature was investigated over the range from 30°C to 80°C. As shown in Fig. 5A, the etching rate of gold induced by Cr (VI) response very strongly dependent on the incubation temperature. With temperature increasing, the oxidative corrosion of Au NPs enhanced and the response reached maximum at 60°C. Besides, as a significant parameter for any sensor, the response time of as-developed sensor was investigated at different temperature (Fig. 5B). The colorimetric response was graded enhanced with prolonging incubation time. With the increasing incubation temperature, the reaction rate became faster and the response time for etching process decreased obviously. However, there is no further reduction of response time when the incubation temperature is over 60°C and that might due to the reaching of maximum reaction rate at 60°C. Thus, the response time of the paper-based sensor was investigated to be 10 minutes by setting incubation temperature at 60°C. 3.5. Sensitivity for detecting of Cr (VI) with BSA-Au NPs/STCP We investigated the performance of BSA-Au NPs/STCP with various concentrations of Cr (VI) under optimized conditions. The rate of leaching of Au NPs increased upon increasing the concentration of Cr (VI) (0-80µM), resulting in various degrees of colorimetric response on the surface of STCP. We obtained a good linear relationship (R2=0.9928) between the colorimetric responses (△Gabs) and the concentrations of Cr (VI) over the range from 0.5 μM to 50 μM. (Fig. 6) The detection limit was 0.28 μM calculated with 3δ/k, where δ is the standard deviation for 11 blank samples and k is the slope of calibration line. Moreover, this facile paper-based sensor enabled visual monitoring of Cr (VI) as low as 1.0 μM which demonstrated the practical detection of Cr (VI) on-site. 3.6. Selectivity for detecting of Cr (VI) with BSA-Au NPs/STCP To further realize the selectivity of proposed paper-based sensor, other common metal ions and anions , including Na+, K+, Zn2+, Al3+, Fe3+, Mn2+, Pb2+, Ni2+, Cr3+, Ag+ , Ca2+, Ge4+, Mn2+, Ni2+, Sn2+, Hg2+, Cd2+, As3+, Cl-, NO3-, SO42- were examined under optimized conditions. As indicated in Fig. 7, the presence of 500 μM other interfering ions induced a negligible color change while it was observed dramatic color change after addition of 40 μM Cr (VI). The striking contrast indicated the BSA-Au NPs/STCP has strong resistance to interference ions and the satisfactory selectivity might attribute to the excellent stability of paper-based sensor and the unique oxidation of Cr (VI).[25, 35, 41] 3.7. The repeatability and reproducibility of BSA-Au NPs/STCP in detecting Cr (VI) Favorable repeatability and reproducibility usually be mandatory for sensors especially for those
paper-based sensors. Thus, we also tested the repeatability and reproducibility of as-developed sensor in detecting Cr (VI) by evaluating the colorimetric response in different test batches, each of which was measured with 7 repeated tests. As shown in Figure S3, despite slight fluctuations of response for different parallel tests in different batches, the average signal remains consistent after reaction with 40 μM Cr (VI). According to the statistical analysis, the relative standard deviation (RSD) of 7 repeat tests in different batches were ranged 1.66 % to 4.01% and the RSD of the 10 different batches test was 1.78%, which indicates that this proposed sensor performed satisfactory repeatability and reproducibility. 3.8. Detection of Cr (VI) in real water samples To test the practicality of proposed paper-based sensor, we applied this detecting method to determine the concentrations of Cr (VI) in samples of Jialing River (Chongqing, China) water. As shown in Table 1, the recoveries ranged from 99.05% to 103.03% in detecting real river water samples. Besides, the Student’s t-test values for the correlations between the as-developed method and ICP-MS were 1.75 and 1.26 respectively (The t-test value is 2.77 at a 95% confidence level), suggesting that the two methods did not provide significantly different results. The satisfactory results indicated the high potential of this paper-based sensor for Cr (VI) quantification in real water samples.
4. Conclusion In summary, we have developed a paper-based sensor for the sensitive detection of Cr (VI) based on the selectively redox etching of gold in the nanomolar range. The leaching of BSA-Au NPs anchored on the surface of STCP results in rapid and remarkable damping of the SPR and hence induced a visible color change. Compared with aggregation-based or solution-based colorimetric sensors depend on electrostatic absorption or cross-linking, the proposed sensor shows high resistance to the interferences from substrates in environmental samples. Additionally, the strategy for monitoring Cr (VI) was achieved with simple visual inspection as low as 1.0 μM in less than 10 minutes without sophisticated equipment or complicated chemosensors, making it a promising approach for on-line detection of Cr (VI) in water sample.
ACKNOWLEDGMENTS The authors would like to acknowledge the financial support from National Natural Science Foundation of China (No. 31171684), Chongqing Graduate Student Research Innovation Project (CYS15010), Key Technologies R&D Program of China (No.2014BAD07B02), the workstation in Sichuan Province
GY2015-01, the Fundamental Research Funds for the Central Universities (No.106112016CDJXY230005), and sharing fund of Chongqing University’s large equipment.
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label-free visual assay for the detection and quantification of viral RNA using peptide nucleic acid (PNA) and gold nanoparticles (AuNPs), Analytica chimica acta 795 (2013) 1-7. [17] G. Goebel, R. Lange, J.-M. Hollidt, F. Lisdat, Development of a fast and simple test system for the semiquantitative protein detection in cerebrospinal liquids based on gold nanoparticles, Talanta 146 (2016) 49-54. [18] A. Jyoti, P. Pandey, S.P. Singh, S.K. Jain, R. Shanker, Colorimetric Detection of Nucleic Acid Signature of Shiga Toxin Producing Escherichia coli Using Gold Nanoparticles, Journal of Nanoscience and Nanotechnology 10(7) (2010) 4154-4158. [19] P. Pannopard, C. Boonyuen, C. Warakulwit, Y. Hoshikawa, T. Kyotani, J. Limtrakul, Size-tailored synthesis of gold nanoparticles and their facile deposition on AAO-templated carbon nanotubes via electrostatic self-assembly: Application to H2O2 detection, Carbon 94 (2015) 836-844. [20] W. Chu, Y. Zhang, D. Li, C.J. Barrow, H. Wang, W. Yang, A biomimetic sensor for the detection of lead in water, Biosensors & bioelectronics 67 (2015) 621-624. [21] D.F. Moyano, M. Goldsmith, D.J. Solfiell, D. Landesman-Milo, O.R. Miranda, D. Peer, V.M. Rotello, Nanoparticle hydrophobicity dictates immune response, J Am Chem Soc 134(9) (2012) 3965-7. [22] L. Xun, J. Qiao, L. Qi, J. Huang, H. Cai, Polyacrylamide-protected gold nanoparticles for the determination of manganese ions, Analytical Methods 7(23) (2015) 9906-9911. [23] S. Kaviya, E. Prasad, Sequential detection of Fe3+ and As3+ ions by naked eye through aggregation and disaggregation of biogenic gold nanoparticles, Analytical Methods 7(1) (2015) 168-174. [24] M. Elavarasi, S.A. Alex, N. Chandrasekaran, A. Mukherjee, Simple fluorescence-based detection of Cr(III) and Cr(VI) using unmodified gold nanoparticles, Analytical Methods 6(24) (2014) 9554-9560. [25] Y.-F. Lee, C.-C. Huang, Colorimetric Assay of Lead Ions in Biological Samples Using a Nanogold-Based Membrane, Acs Applied Materials & Interfaces 3(7) (2011) 2747-2754. [26] H.Z. Huang, X.R. Yang, Synthesis of polysaccharide-stabilized gold and silver nanoparticles: a green method, Carbohydrate Research 339(15) (2004) 2627-2631. [27] S.S. Chou, M. De, J. Luo, V.M. Rotello, J. Huang, V.P. Dravid, Nanoscale graphene oxide (nGO) as artificial receptors: implications for biomolecular interactions and sensing, J Am Chem Soc 134(40) (2012) 16725-33. [28] Y. Guo, Z. Wang, W. Qu, H. Shao, X. Jiang, Colorimetric detection of mercury, lead and copper ions simultaneously using protein-functionalized gold nanoparticles, Biosensors & bioelectronics 26(10) (2011) 4064-4069. [29] L. Beqa, A.K. Singh, S.A. Khan, D. Senapati, S.R. Arumugam, P.C. Ray, Gold Nanoparticle-Based Simple Colorimetric and Ultrasensitive Dynamic Light Scattering Assay for the Selective Detection of Pb(II) from Paints, Plastics, and Water Samples, Acs Applied Materials & Interfaces 3(3) (2011) 668-673. [30] X. Fu, T. Lou, Z. Chen, M. Lin, W. Feng, L. Chen, "Turn-on" Fluorescence Detection of Lead Ions Based on Accelerated Leaching of Gold Nanoparticles on the Surface of Graphene, Acs Applied Materials & Interfaces 4(2) (2012) 1080-1086. [31] L. Zhao, Y. Jin, Z. Yan, Y. Liu, H. Zhu, Novel, highly selective detection of Cr(III) in aqueous solution based on a gold nanoparticles colorimetric assay and its application for determining Cr(VI), Analytica chimica acta 731 (2012) 75-81. [32] H.Z. Huang, X.R. Yang, Synthesis of chitosan-stabilized gold nanoparticles in the absence/presence of tripolyphosphate, Biomacromolecules 5(6) (2004) 2340-2346. [33] H. Guan, J. Yu, D. Chi, Label-free colorimetric sensing of melamine based on chitosan-stabilized gold nanoparticles probes, Food Control 32(1) (2013) 35-41. [34] Y. Wu, U. Wollenberger, M. Hofrichter, R. Ullrich, K. Scheibner, F.W. Scheller, Direct electron transfer of
Agrocybe aegerita peroxygenase at electrodes modified with chitosan-capped Au nanoparticles and its bioelectrocatalysis to aniline, Sensors and Actuators B-Chemical 160(1) (2011) 1419-1426. [35] Y.-F. Lee, T.-W. Deng, W.-J. Chiu, T.-Y. Wei, P. Roy, C.-C. Huang, Visual detection of copper(II) ions in blood samples by controlling the leaching of protein-capped gold nanoparticles, Analyst 137(8) (2012) 1800-1806. [36] S.H. Brewer, W.R. Glomm, M.C. Johnson, M.K. Knag, S. Franzen, Probing BSA binding to citrate-coated gold nanoparticles and surfaces, Langmuir 21(20) (2005) 9303-9307. [37] S.X. Li, X. Lin, F.Y. Zheng, W. Liang, Y. Zhong, J. Cai, Constituting fully integrated visual analysis system for Cu(II) on TiO(2)/cellulose paper, Anal Chem 86(14) (2014) 7079-83. [38] J. Xin, F. Zhang, Y. Gao, Y. Feng, S. Chen, A. Wu, A rapid colorimetric detection method of trace Cr (VI) based on the redox etching of Ag-core-Au-shell nanoparticles at room temperature, Talanta 101 (2012) 122-127. [39] S.K. Tripathy, J.Y. Woo, C.-S. Han, Colorimetric detection of Fe(III) ions using label-free gold nanoparticles and acidic thiourea mixture, Sensors and Actuators B-Chemical 181 (2013) 114-118. [40] Y.-M. Fang, J. Song, J.-S. Chen, S.-B. Li, L. Zhang, G.-N. Chen, J.-J. Sun, Gold nanoparticles for highly sensitive and selective copper ions sensing-old materials with new tricks, Journal of Materials Chemistry 21(22) (2011) 7898-7900. [41] F.-M. Li, J.-M. Liu, X.-X. Wang, L.-P. Lin, W.-L. Cai, X. Lin, Y.-N. Zeng, Z.-M. Li, S.-Q. Lin, Non-aggregation based label free colorimetric sensor for the detection of Cr (VI) based on selective etching of gold nanorods, Sensors and Actuators B-Chemical 155(2) (2011) 817-822. [42] S. Wang, Z. Chen, L. Chen, R. Liu, L. Chen, Label-free colorimetric sensing of copper(II) ions based on accelerating decomposition of H2O2 using gold nanorods as an indicator, Analyst 138(7) (2013) 2080-2084.
Scheme 1. Schematic representation of the preparation of BSA-Au NPs/STCP for colorimetric sensing Cr (VI) based on the leaching of gold.
Fig. 1. UV-vis spectra of as-prepared Au NPs and the inset shows corresponding TEM image (A), and particle size distribution of the Au NPs (B).
Fig. 2. SEM images of (A) filter paper after immersion in EtOH, (B) TiO2 modified filter paper, (C) TiO2 modified filter paper after loading AuNPs; a, b, and c are the corresponding images at high magnification.
Fig. 3. (A) Colorimetric responses with repetitive dipping of BSA-Au NPs solution and related real images. (B) The influence of repetitive dipping BSA-Au NPs solution on the colorimetric response in the presence of 30 μM Cr (VI).
Fig. 4. Influence of the pH (A) and concentration of bromide ions (B) in the presence of 40 μM Cr (VI).
Fig. 5. Influence of the incubation temperature (A) and response time at different temperature (B) in the presence of 40 μM Cr (VI).
Fig. 6 Responses of BSA-Au NPs/STCP in the presence of different concentrations of Cr (VI) and related real images. Other condition: (1.0 M of HBr, 10 minutes at 60℃)
Fig. 7 Responses of BSA-Au NPs/STCP in the presence of Cr (VI) and other interfering ions. Concentration of Cr (VI) was 40 μM and concentration of each of the other interfering ions was 500 μM. Other condition: (1.0 M of HBr, 10 minutes at 60℃).
Table 1 Determination of Cr (VI) in river samples by the BSA-Au NPs/STCP Samples
Spiked [Cr (VI)]
Certified value
Found by BSA-Au NPs/STCP
(μM)
(μM)
Mean ± SD (μM, n=5)
Recovery (%)
1#
0
0.024
/
/
2#
10.00
10.384
10.531±0.168
101.42±1.61
3#
20.00
20.542
20.740±0.349
100.96±1.91
Highlights
A functional paper-based sensor was designed to detect Cr (VI) based on the leaching of gold nanoparticles. Detection limit for Cr (VI) was as low as 280 nM with a wide linear range (0.5 nM to 50µM) and the concentration of visual inspection was as low as 1.0 μM. The proposed non-aggregation and paper-based colorimetric sensor showed high resistance to the interference ions and also demonstrated the potential for field applications.
PII: DOI: Reference:
S0039-9140(16)30704-4 http://dx.doi.org/10.1016/j.talanta.2016.09.032 TAL16882
To appear in: Talanta Received date: 25 May 2016 Revised date: 7 September 2016 Accepted date: 11 September 2016 Cite this article as: Jian-feng Guo, Dan-qun Huo, Yang Mei, Chang-jun Hou, Jun-jie Li, Huan-bao Fa, Hui-bo Luo and Ping Yang, Colorimetric detection of Cr (VI) based on the leaching of gold nanoparticles using a paper-based sensor, Talanta, http://dx.doi.org/10.1016/j.talanta.2016.09.032 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Colorimetric detection of Cr (VI) based on the leaching of gold nanoparticles using a paper-based sensor Guo Jian-feng1, Huo Dan-qun1*, Yang Mei1*, Hou Chang-jun1,2, Li Jun-jie1, Fa Huan-bao3, Luo Hui-bo4, Yang Ping5 1
Key Laboratory of Biorheology Science and Technology, Ministry of Education, College of Bioengineering, Chongqing
University, Chongqing, 400044, China 2
National Key Laboratory of Fundamental Science of Micro/Nano-Device and System Technology, Chongqing University,
Chongqing, 400044, China 3
College of Chemistry and Chemical Engineering, Chongqing University, Chongqing, 400044, China
4
Liquor Making Bio-Technology & Application of Key Laboratory of Sichuan Province, Zigong 643000, China
5
National Engineering Research Center of Solid-State Brewing, Luzhou Laojiao Group Co. Ltd., Luzhou, Sichuan, 646000,
China
[email protected]
[email protected] *
Corresponding author. Tel.: +862365112673; fax: +862365102507.
Abstract Herein, we have developed a simple, sensitive and paper-based colorimetric sensor for the selective detection of Chromium (Ⅵ) ions (Cr (VI)). Silanization- titanium dioxide modified filter paper (STCP) was used to trap bovine serum albumin capped gold nanoparticles (BSA-Au NPs), leading to the fabrication of BSA-Au NPs decorated membrane (BSA-Au NPs/STCP). The BSA-Au NPs/STCP operated on the principle that BSA-Au NPs anchored on the STCP were gradually etched by Cr (VI) as the leaching process of gold in the presence of hydrobromic acid (HBr) and hence induced a visible color change. Under optimum conditions, the paper-based colorimetric sensor showed clear color change after reaction with Cr (VI) as well as with favorable selectivity to a variety of possible interfering counterparts. The amount-dependent colorimetric response was linearly correlated with the Cr (VI) concentrations ranging from 0.5 µM to 50.0 µM with a detection limit down to 280 nM. Moreover, the developed cost-effective colorimetric sensor has been successfully applied to real environmental samples which demonstrated the potential for field applications.
Keywords: paper-based; gold nanoparticles; non-aggregation; colorimetric; Cr (VI)
1. Introduction Cr (III) and Cr (VI) are the most common valence states observed in nature environment.[1] Nevertheless, compared to Cr (III), Cr (VI) is much more notorious for its strong oxidability, hypertoxicity, carcinogenicity, non-biodegradability which posing serious threats to both environment and human health.[2] Generally, exposure to even a low concentration of Cr (VI) might cause hemolysis, renal, liver failure and even various types of cancer.[3] With the global expansion of industrial factories, extensive
utilization of chromium in industrial processes such as stainless steel, chrome plating, leather tanning and pigment production always causing chromium-containing effluents or slags discharged into the environment immoderately.[4] With increasing attention on chromium pollution, the United States Environmental Protection Agency (US-EPA) suggests that the concentration of Cr (VI) in drinking water should under 100 ppb and the World Health Organization (WHO) also proposes a guideline value for total chromium and Cr (VI) in drinking water which is 50 ppb and 16 ppb, respectively. [5, 6] Therefore, it is of significant importance to develop a simple and practical method for the determination of Cr (VI). Previously, a number of strategies have been devised to detect Cr (VI), those include methods based on large equipments such as inductively coupled plasma mass spectrometry (ICP-MS),[7] atomic absorption spectroscopy(AAS),[8] X-ray fluorescence (XRF) and solid phase extraction.[9] [10] On one hand, those large equipment-based methods usually have superior accuracy and stability. But on the other hand, they usually suffer from disadvantages such as sophisticated pretreatment, requirement of professional operation, and also high-cost, which render them unsuitable for real-time field monitoring. In the meantime, colorimetric assay based on metal nanoparticles have attracted increasing attentions in recent years because of requiring no complex instrumentations and signal recognition can be achieved by just naked eye in the form of color change.[11-13] Of particular interest is those based on the gold nanoparticles (Au NPs), as they have strong surface plasmon resonance (SPR) absorption with extremely high extinction coefficients (108-1010 M-1•cm-1) in the visible region.[14] The past few years have witnessed Au NPs widely employed in colorimetric detection for various analytes including detection of cells,[15] viruses,[16] proteins,[17] nucleic acids,[18] small molecules[19]. Especially, Au NPs have been frequently used in the detection of metal ions, such as Pb (II),[20] Hg (II),[21] Mn (II),[22] Fe (Ⅲ) and As (Ⅲ)[23], Cr(Ⅲ) and Cr (VI)[24]. Majority of these probes based on target-inducing particles aggregation in the solution-phase, have proven to be sensitive as well as rapid response. Nevertheless, Au NPs in solution are vulnerable to aggregate and may fail in detecting metal ions in sophisticated matrixes.[25] Besides, there also have difficulty in discerning color hue or intensity changes when detecting colored samples or complex mixtures, such as blood and waste water samples. While transforming the detecting process from solution-phase to solid-phase may alleviate these problems effectively, as membrane-based colorimetric sensor have better stability properties as well as visual detection which pose a good expectation for on-site sensing. Herein, we have developed a practical and sensitive paper-based colorimetric sensor for the rapidly detection of Cr (VI). After functional modification of a titanium dioxide (TiO2) intermediate on commercial
cellulose filter papers, sulfhydryl and amino groups’ decorations were successfully realized to endow the interface with appropriate hydrophilicity/hydrophobicity as well as strong binding sites for BSA-Au NPs. In the presence of HBr, the BSA-Au NPs anchored on the surface of paper were gradually dissolved by Cr (VI) as the etching process of gold. The leaching of gold results in rapid and remarkable damping of the SPR and hence induces a visible color change. The designed paper-based colorimetric sensor, as compared to other solution-based sensors, is more economical, facile and practical for the detection of Cr (VI) without sophisticated equipment. Moreover, we also demonstrated the practicality of using this approach for the detection of Cr (VI) in real river water sample.
2. Experimental sections 2.1. Apparatus Images of dispersed Au NPs were achieved by transmission electron microscopy (TEM, JEOL Ltd., Japan). Inductively coupled plasma mass spectroscopy (ICP-MS) was performed with ICP-MS instrument (Agilent 7500, USA). Ultraviolet spectrum measurement was performed using a UV-2700 spectrophotometer (Tsushima, Japan). Zeta potential measurements were performed on Malvern Zetasizer Nano-ZS90 (Malvern Instruments Ltd., U.K.). X Ray Fluorescence (XRF) images were achieved by super ED-XRF 1050 spectrometer(Skyray Ltd., China). 2.2. Chemicals Gold
acid
chloride
trihydrate
(HAuCl4•3H2O)
(99.0%),
chitosan
(80%-95%
deacetylated),
(3-Mercaptopropyl)-trimethoxysilane (MPTMOS) and (3-Aminopropyl)-trimethoxysilane (APTEOS) were purchased from Aladdin Industrial Inc. (Shanghai, China). Bovine serum albumin (BSA) was purchased from Sigma-Aldrich (St Louis, MO, USA). Hydrobromic acid (HBr) (47% aqueous solution), potassium dichromate (K2CrO4) (99.8%), Tetrabutyltitanate (TBT), ethyl alcohol (EtOH), acetic acid (HAc) and all other fundamental reagents (analytical reagent grade) were purchased from Titan (Shanghai, China) without further purification. Commercial cellulose filter paper (diameter=7 cm) was purchased from Shuangquan Filter Paper Ltd.Co. (Hangzhou, China). Ultra-pure water was generated by a Millipore Direct-Q Water system (Mosheim, France). 2.3. Synthesis of BSA-stabilized gold nanoparticles The Au NPs was synthesized by a green method according to a former report with slightly modifications.[26] Briefly, chitosan solution of 3 mg/mL was prepared with 1.0% HAc by stirring at 70°C.
After obtaining a clear solution, addition of 2 mL 1wt.% HAuCl4•3H2O solution (25.0 mM) into 50 mL as-prepared chitosan solution, the mixture was heated to 100°C and kept stirring until the color turned into deep red and then cooled at room temperature with continuous stirring. To obtain BSA-Au NPs, an aliquot of the BSA aqueous solution (50.0 μM, 3 mL) was added to the above 50 ml Au NPs solution, equilibrated for 2h at room temperature. Finally, the BSA-Au NPs solution was stored at 4°C under darkness. 2.4. Fabrication of the BSA-Au NPs/STCP probe The fabrication of nano-TiO2 modified filter papers (TCP) was prepared according our previously reported.[27] Briefly, 5%(v/v) hydrochloric acid (HCl) aqueous solution was first used to remove surface contaminations on cellulose filter paper and washed with deionized water. Then immersed 5 pieces of filter paper in 20 mL EtOH for 2h and drying at 60°C for 1h. Then the papers were added into 100 mL mixed solution of ethanol, TBT, and HAc in a ratio of 10:3:1 in a culture dish, which was allowed to hydrolyze for 30 minutes under shaking (100npr) at ambient conditions. The uneven residual solution on the surface of filter paper was removed by issue paper and then dried at 60°C for 10 minutes. After that, papers were kept in 300 mL deionized water to finally hydrolyze for 4h at 140°C and dried for 2h to obtain TCP. To further obtain STCP, the as-prepared TCP were immersed into 100 mL APTEOS of 3.0% concentration (v% in EtOH) and allowed to react for 30 minutes at 100°C. The papers were washed successively with deionized water to remove unreacted reagents and dried for 10 minutes at 50°C. After that 100 mL MPTMOS (3%/v%) ethanol solution was applied to repeat the procedures similar to that of APTEOS. Then, a piece of STCP was cut to a size 0.9 cm (length)•0.9 cm (width) and dipped with BSA-Au NPs solution (12.6 nM, 0.6μL) for several times. After drying with a blower for 5 minutes, BSA-Au NPs/STCP were gently washed with deionized water to remove any weakly bonded or impurities and then dried in air at room temperature for 2h prior to use. 2.5. Detecting Cr (VI) with the BSA-Au NPs/STCP For the determination of Cr (VI), the as-prepared BSA-Au NPs/STCP was immersed in 1.0 M HBr solution premixed with target or standard aqueous samples ranged from 0-80µM. After incubating at 60°C for 10 minutes, the test papers were gently washed with deionized water and dried at 50°C for 5 minutes. Finally, the images of BSA-Au NPs/STCP were scanned by a scanner and color analysis was performed using an Image J computer program that measured the red, green, and blue (RGB) color intensities. In the present study, we recorded the green component absorbance (Gabs) of the BSA-Au NPs/STCP after it reacted with the leaching agent (1.0 M HBr) in the absence and presence of Cr (VI).
2.6. Analysis of real river water sample The real water sample was collected from Jialing River (Chongqing, China) and filtered through a 0.2-μm membrane before use. Firstly, the pretreated river water (0.99 mL) and HBr (1.0 ml, 2 M) were spiked with standard Cr (VI) solutions (10 µL) at concentrations over the range of 0-50 µM. Then, BSA-Au NPs/STCP were immersed in the above solution and incubated at 60°C for 10 minutes. After that, the strips were gently washed with deionized water and dried at 50°C for 5 minutes. The accurate concentration of Cr (VI) in river water was detected using ICP-MS and subsequently used to calculate recovery values.
3. Results and discussion 3.1. Characterization of Au NPs and BSA-Au NPs/STCP Here, the Au NPs were synthesized by a green biological method, using chitosan as both the reducing and protecting agent. Compared to general methods for the preparation of Au NPs using organic compounds such as citrate and NaBH4,[28-31] the present strategy will not introduce any environmental toxicity or biological hazards as the excellent biodegradable and biocompatible characteristics of chitosan.[32, 33]. TEM images was employed to characterize the morphology of Au NPs and shown in Fig. 1A, Au NPs were in globose appearance with multi-dispersed sizes that corresponding well with previous reports.[26, 34] The average particles size of as-prepared Au NPs was calculated to be 14.06±3.14 nm by counting the diameter of 200 particles (Fig. 1B). We also monitored UV-visible absorption spectroscopy of Au NPs and found the absorption peak was located at 520 nm that matched with former studies.[33, 35] Concentration of the as-prepared Au NPs was estimated to be 12.6 nM according to Lambert-Beer's law using an extinction coefficient of 2.43×108 M-1•cm-1 at 520 nm for the 14.2 nm Au NPs.[14] According to previous researches,[36] Au NPs can be stabilized by interaction of protein side chains or domains, resulting in a reduction in entropy and a loss of solvation enthalpy, both of which cause an colloid repulsion. As proven in Fig. S1, the mean zeta potential value of Au NPs in the absence and presence of BSA is about -3.55 and 34.6 mV, respectively. As the isoelectric point of BSA is 4.6, and therefore BSA is positively charged at strong acid environment (e.g. pH=0). Thus the significant potential change might attribute to the binding of positively charged BSA to negatively charged surfaces of Au NPs and induced colloid repulsion among BSA-Au NPs. Therefore, the BSA could greatly reduce the aggregation of Au NPs and improve their stability in aqueous solution. In order to illuminate the fabrication processes of BSA-Au NPs/STCP, SEM was applied to observe the
morphologic properties in different procedures. As Fig. 2A shows, the surface of cellulose is smooth and there are barely particles adsorbed on it before impregnation with TiO2. Generally, cellulose was rich in −C−O−O− and −OH groups, which can easily enable Ti ion to firmly anchor onto the cellulose surface. [37] After a TiO2 film was uniformly deposited onto the nanofiber surfaces using the sol–gel method[39], the nanofiber was endowed with a rough surface morphology and larger specific surface area.(Fig. 2B) It is beneficial for providing an intermediate layer with broad surface area chemically active for subsequent modifications. After loading BSA-Au NPs solution on the TiO2 film, spherical gold nanoparticles anchored on the TiO2 film can easily observed (Fig. 2C) which demonstrated the successfully fabrication of BSA-Au NPs/STCP. Moreover, X Ray Fluorescence (XRF) was employed to monitor the metallic elements during the fabrication processes. As shown in Fig. S2, the XRF spectra of untreated filter paper showed no metallic element initially. With the deposition of TiO2 film on the nanofiber, there began to arise obvious peaks of Ti, and the peaks of Au appear subsequently after loading BSA-Au NPs. The successive appearance of Ti, Au in the XRF spectra also indicated successful fabrication of BSA-Au NPs/STCP. 3.2. Sensing mechanism Previously, the nitrocellulose membrane (NCM) has been employed as blotting matrix for protein capped gold nanoparticles immobilizations.[25, 35] However, hydrophobic interactions as the main binding forces between proteins and nitrocellulose that might lead the immobilized BSA-Au NPs easily leakage from NCM that make false-positive conclusion in detecting process. In the present study, as shown in Scheme 1, a functional cellulose filter paper was used to immobilize BSA-Au NPs through strong Au-S bond as well as hydrophobic interactions and fabricated the BSA-Au NPs/STCP for detecting Cr (VI). The mechanism of this paper-based sensor for sensing Cr (VI) is investigated in our study. Generally, Cr (VI) does not have enough ability to oxidize gold as the standard electron potential of Au (I)/Au (0) and Cr (VI)/Cr (III) is 1.69 eV and 1.33 eV, respectively.[38] Nevertheless, in the presence of Br- the potential of Au (I)/ Au (0) decreased as the formation of AuBr2- complexes, that resulting Au NPs redox etching by Cr (VI). (Equation 1) The leaching process of gold induced by Cr (VI) caused a dramatic diminishment of particle amounts as well as the particles size of Au NPs, leading to observable color change on the surface of BSA-Au NPs/STCP. Actually, several studies have also reported similar mechanism that based on the etching of gold in sensing metal ions in aqueous solution, such as Fe3+, Cu2+, and Pb2+, with the satisfactory results demonstrating its simplify and selectivity as well as practicality in detecting metal ions.[25, 39, 40] 𝟑𝐀𝐮 + 𝟔𝐁𝐫 − + 𝐂𝐫 𝟔+ → 𝟑𝐀𝐮𝐁𝐫𝟐− + 𝐂𝐫 𝟑+ (Equation 1)
3.3. Optimization of the detecting conditions for Cr (VI) For optimizing the detecting conditions, relevant experimental parameters, including pH value, concentration of Br-, incubation temperature and response time were carefully evaluated in the present study. The descriptors Gabs and G○abs represent the green component absorbance (Gabs) of the BSA-Au NPs/STCP probe in the presence and absence and of Cr (VI), respectively. △Gabs represents the value change between Gabs and G○abs. Firstly, we examined the effect of dipping frequency of BSA-Au NPs solution. As Fig. 3A shows, the Gabs intensity responded linear correlation to the dipping frequencies with a good correlation coefficient (R2=0.9847). On one hand, the stronger Gabs intensity could extend the detection range of Cr (VI) as well as do a favor to the visual detection process. But on the other hand, excessive loading BSA-Au NPs on the surface of STCP may reduce the interaction forces between BSA-Au NPs and the surface of paper, which might decrease the sensitivity and selectivity toward Cr (VI). As proved in Fig. 3B, successive increasing dipping frequency of BSA-Au NPs solution from 3 to 8 times caused the corresponding colorimetric responses and reached maximum at 6. Taking both the intensity of Gabs and stability into consideration, dipping 6 times of BSA-Au NPs solution was chosen for further investigation. As a critical factor for the detection process, the influence of pH was also investigated in our study. The oxidizability of Cr (VI) is greatly relying on the environment pH. For example, the electron potentials of Cr (VI)/Cr (III) are 1.271 eV and 0.995 eV when the pH values are 1.0 and 3.0, respectively.[41] The low pH circumstance can greatly accelerate the etching process as the increasing oxidation of Cr (VI). Nevertheless, the excessive increasing of acidity might also reduce the selectivity and stability of the paper-based sensor. As expected, the colorimetric response of the sensor keep increasing with the pH value decreasing from 7.0 to 1.0 and achieved maximum response when the pH was lower than 1.0 (Fig. 4A). Considering the sensitivity and selectivity, the pH=0 was chosen for further investigation. Ligand reagents such as 2-mercaptoethanol, thiocyanate, and thiourea are important parameter influencing the dissolution of gold in the etching process. [14, 39, 42]. Here, Br- acted as the ligands of gold and formed AuBr2- to accelerate the leaching process by reducing the electron potential of Au (I)/Au (0). As showed in Fig. 4B, colorimetric responses enhanced with the increasing of Br- concentration ranged from 0 to 1.0 M. However, the continuously increasing concentration of Br- might decrease the selectivity and the excessive Br- might also react with Cr (VI) as the electron potentials of Br/Br- is 1.09 eV. Therefore, we chose 1.0 M Br- as the optimized concentration and applied in our further study.
Finally, the influence of incubation temperature and response time was taken into consideration. Effect of temperature was investigated over the range from 30°C to 80°C. As shown in Fig. 5A, the etching rate of gold induced by Cr (VI) response very strongly dependent on the incubation temperature. With temperature increasing, the oxidative corrosion of Au NPs enhanced and the response reached maximum at 60°C. Besides, as a significant parameter for any sensor, the response time of as-developed sensor was investigated at different temperature (Fig. 5B). The colorimetric response was graded enhanced with prolonging incubation time. With the increasing incubation temperature, the reaction rate became faster and the response time for etching process decreased obviously. However, there is no further reduction of response time when the incubation temperature is over 60°C and that might due to the reaching of maximum reaction rate at 60°C. Thus, the response time of the paper-based sensor was investigated to be 10 minutes by setting incubation temperature at 60°C. 3.5. Sensitivity for detecting of Cr (VI) with BSA-Au NPs/STCP We investigated the performance of BSA-Au NPs/STCP with various concentrations of Cr (VI) under optimized conditions. The rate of leaching of Au NPs increased upon increasing the concentration of Cr (VI) (0-80µM), resulting in various degrees of colorimetric response on the surface of STCP. We obtained a good linear relationship (R2=0.9928) between the colorimetric responses (△Gabs) and the concentrations of Cr (VI) over the range from 0.5 μM to 50 μM. (Fig. 6) The detection limit was 0.28 μM calculated with 3δ/k, where δ is the standard deviation for 11 blank samples and k is the slope of calibration line. Moreover, this facile paper-based sensor enabled visual monitoring of Cr (VI) as low as 1.0 μM which demonstrated the practical detection of Cr (VI) on-site. 3.6. Selectivity for detecting of Cr (VI) with BSA-Au NPs/STCP To further realize the selectivity of proposed paper-based sensor, other common metal ions and anions , including Na+, K+, Zn2+, Al3+, Fe3+, Mn2+, Pb2+, Ni2+, Cr3+, Ag+ , Ca2+, Ge4+, Mn2+, Ni2+, Sn2+, Hg2+, Cd2+, As3+, Cl-, NO3-, SO42- were examined under optimized conditions. As indicated in Fig. 7, the presence of 500 μM other interfering ions induced a negligible color change while it was observed dramatic color change after addition of 40 μM Cr (VI). The striking contrast indicated the BSA-Au NPs/STCP has strong resistance to interference ions and the satisfactory selectivity might attribute to the excellent stability of paper-based sensor and the unique oxidation of Cr (VI).[25, 35, 41] 3.7. The repeatability and reproducibility of BSA-Au NPs/STCP in detecting Cr (VI) Favorable repeatability and reproducibility usually be mandatory for sensors especially for those
paper-based sensors. Thus, we also tested the repeatability and reproducibility of as-developed sensor in detecting Cr (VI) by evaluating the colorimetric response in different test batches, each of which was measured with 7 repeated tests. As shown in Figure S3, despite slight fluctuations of response for different parallel tests in different batches, the average signal remains consistent after reaction with 40 μM Cr (VI). According to the statistical analysis, the relative standard deviation (RSD) of 7 repeat tests in different batches were ranged 1.66 % to 4.01% and the RSD of the 10 different batches test was 1.78%, which indicates that this proposed sensor performed satisfactory repeatability and reproducibility. 3.8. Detection of Cr (VI) in real water samples To test the practicality of proposed paper-based sensor, we applied this detecting method to determine the concentrations of Cr (VI) in samples of Jialing River (Chongqing, China) water. As shown in Table 1, the recoveries ranged from 99.05% to 103.03% in detecting real river water samples. Besides, the Student’s t-test values for the correlations between the as-developed method and ICP-MS were 1.75 and 1.26 respectively (The t-test value is 2.77 at a 95% confidence level), suggesting that the two methods did not provide significantly different results. The satisfactory results indicated the high potential of this paper-based sensor for Cr (VI) quantification in real water samples.
4. Conclusion In summary, we have developed a paper-based sensor for the sensitive detection of Cr (VI) based on the selectively redox etching of gold in the nanomolar range. The leaching of BSA-Au NPs anchored on the surface of STCP results in rapid and remarkable damping of the SPR and hence induced a visible color change. Compared with aggregation-based or solution-based colorimetric sensors depend on electrostatic absorption or cross-linking, the proposed sensor shows high resistance to the interferences from substrates in environmental samples. Additionally, the strategy for monitoring Cr (VI) was achieved with simple visual inspection as low as 1.0 μM in less than 10 minutes without sophisticated equipment or complicated chemosensors, making it a promising approach for on-line detection of Cr (VI) in water sample.
ACKNOWLEDGMENTS The authors would like to acknowledge the financial support from National Natural Science Foundation of China (No. 31171684), Chongqing Graduate Student Research Innovation Project (CYS15010), Key Technologies R&D Program of China (No.2014BAD07B02), the workstation in Sichuan Province
GY2015-01, the Fundamental Research Funds for the Central Universities (No.106112016CDJXY230005), and sharing fund of Chongqing University’s large equipment.
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Scheme 1. Schematic representation of the preparation of BSA-Au NPs/STCP for colorimetric sensing Cr (VI) based on the leaching of gold.
Fig. 1. UV-vis spectra of as-prepared Au NPs and the inset shows corresponding TEM image (A), and particle size distribution of the Au NPs (B).
Fig. 2. SEM images of (A) filter paper after immersion in EtOH, (B) TiO2 modified filter paper, (C) TiO2 modified filter paper after loading AuNPs; a, b, and c are the corresponding images at high magnification.
Fig. 3. (A) Colorimetric responses with repetitive dipping of BSA-Au NPs solution and related real images. (B) The influence of repetitive dipping BSA-Au NPs solution on the colorimetric response in the presence of 30 μM Cr (VI).
Fig. 4. Influence of the pH (A) and concentration of bromide ions (B) in the presence of 40 μM Cr (VI).
Fig. 5. Influence of the incubation temperature (A) and response time at different temperature (B) in the presence of 40 μM Cr (VI).
Fig. 6 Responses of BSA-Au NPs/STCP in the presence of different concentrations of Cr (VI) and related real images. Other condition: (1.0 M of HBr, 10 minutes at 60℃)
Fig. 7 Responses of BSA-Au NPs/STCP in the presence of Cr (VI) and other interfering ions. Concentration of Cr (VI) was 40 μM and concentration of each of the other interfering ions was 500 μM. Other condition: (1.0 M of HBr, 10 minutes at 60℃).
Table 1 Determination of Cr (VI) in river samples by the BSA-Au NPs/STCP Samples
Spiked [Cr (VI)]
Certified value
Found by BSA-Au NPs/STCP
(μM)
(μM)
Mean ± SD (μM, n=5)
Recovery (%)
1#
0
0.024
/
/
2#
10.00
10.384
10.531±0.168
101.42±1.61
3#
20.00
20.542
20.740±0.349
100.96±1.91
Highlights
A functional paper-based sensor was designed to detect Cr (VI) based on the leaching of gold nanoparticles. Detection limit for Cr (VI) was as low as 280 nM with a wide linear range (0.5 nM to 50µM) and the concentration of visual inspection was as low as 1.0 μM. The proposed non-aggregation and paper-based colorimetric sensor showed high resistance to the interference ions and also demonstrated the potential for field applications.