A Supersensitive Probe for Rapid Colorimetric Detection of Nickel Ion

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A supersensitive probe for rapid colorimetric detection of nickel ion based on a sensing mechanism of anti-etching Ningyi Chen, Yujie Zhang, Hongyu Liu, Huimin Ruan, Chen Dong, Zheyu Shen, and Aiguo Wu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01326 • Publication Date (Web): 20 Sep 2016 Downloaded from http://pubs.acs.org on September 22, 2016

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ACS Sustainable Chemistry & Engineering

A supersensitive probe for rapid colorimetric detection of nickel ion based on a sensing mechanism of anti-etching

Ningyi Chena,b, Yujie Zhanga, Hongyu Liua,b, Huimin Ruana, Chen Donga, Zheyu Shena,*, Aiguo Wua,*

a

Key Laboratory of Magnetic Materials and Devices & Key Laboratory of Additive Manufacturing

Materials of Zhejiang Province & Division of Functional Materials and Nanodevices, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, 1219 Zhong-guan West Road, Zhen-hai District, Ning-bo, Zhe-jiang 315201, P. R. China. b

Nano Science and Technology Institute, University of Science and Technology of China, 166

Ren-ai Road, Du-shu-hu-gao-jiao District, Su-zhou, Jiang-su 215123, China.

*Corresponding authors E-mail: [email protected]; or [email protected] Tel: +86 574 86685039; or +86 574 87617278; Fax: +86 574 86685163

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ACS Paragon Plus Environment


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ABSTRACT Redundant nickel is harmful to human health and can result in skin diseases, allergies, or cancer formation. Although lots of probes based on noble metal nanoparticles have been established for rapid heavy metal ion detection by the naked eyes or UV-vis spectroscopy, few noble metal nanomaterials have been developed for Ni2+ detection. In this study, we propose novel triangular silver nanoprisms (AgNPRs) stabilized with glutathione (GSH) for rapid colorimetric detection of Ni2+ based on a sensing mechanism of anti-etching, which has been affirmed by Raman spectra, UV-vis spectra, transmission electron microscopy (TEM) and dynamic light scattering (DLS). At the optimal experimental parameters, our GSH-AgNPRs-based Ni2+ probe has an excellent selectivity compared with 26 other ions because Ni2+ can inhibit the AgNPR etching by iodide ion (I–) (i.e. anti-etching), but other ions cannot. The limit of detection (LOD) of our Ni2+ probe is 50 nM via the naked eyes and 5 nM via UV-vis spectroscopy. They are both negligible compared with the permissible limit of Ni2+ in drinking water (0.34 µM) prescribed by the World Health Organization (WHO). Especially, the latter is far less than the LOD values of other reported Ni2+ probes based on noble metal nanomaterials. A satisfying linear relationship reinforces that our probe can be utilized for the quantitative analysis of Ni2+. The detection of real water samples indicates that our probe could be used for rapid Ni2+ colorimetric detection with supersensitivity and excellent selectivity in real environmental water samples.

KEYWORDS: functionalized triangular silver nanoprisms, rapid colorimetric detection, nickel ion, supersensitivity, excellent selectivity

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INTRODUCTION Nickel is one of the essential elements for aquatic organisms because it is able to interact with phosphates, amino acids, proteins and nucleic acids forming complexes. It is also very important for some enzyme activities and catalytic processes.1 But the nickel dosage required for organisms is quite low, and redundant nickel is harmful to human health and can result in skin diseases, allergies, or cancer formation.2 Thus the permissible limit of nickel ion (Ni2+) in drinking water is 0.02 mg/L (0.34 µM) prescribed by the World Health Organization (WHO).3 The conventional detection methods for Ni2+ include atomic absorption spectrometry (AAS),4 electrochemical methods,5 inductively coupled plasma-atomic emission spectrometry (ICP-AES)6 and flow injection analysis.7 The advantages of these methods are high sensitivity and good selectivity. However, these methods need big expensive instruments. However, the measurement costs are expensive, the measurement processes are complicated and the measurement time is long, which make them non-applicable for on-site detection.8 In recent years, lots of probes based on noble metal nanoparticles have been established for rapid heavy metal ion detection by the naked eyes or UV-vis spectroscopy.9,10 Most of them are designed and developed for the rapid colorimetric detection of Hg2+, Pb2+, Cr3+, Mn2+, Cu2+ and so on.11-16 However, few noble metal nanomaterials have been developed for Ni2+ rapid detection via eye-vision or UV-vis spectroscopy. Li et al. synthesized the silver nanoparticles (AgNPs) stabilized by glutathione (GSH) in aqueous solution and developed a rapid detection system of Ni2+ using the GSH-AgNPs.17 Unfortunately, the sensitivity is very low and the limit of detection (LOD) is 75 µM, which is very high compared with the permissible Ni2+ limit in drinking water prescribed by the WHO (0.34 µM). Annadhasan et al. and

Zhang

et

al.

respectively

explored

sunlight

irradiation

AuNPs

and

N-acetyl-L-cysteine-functionalized AgNPs for rapid colorimetric detection of Ni2+ based on the mechanism of aggregation.18,19 However, as for the former, the accuracy is not good and the linear regression coefficient (R2) of Ni2+ detection is found to be 0.9870. As for the latter, the sensitivity is not high and the LOD is 0.30 µM, which is close to the permissible limit in drinking water (0.34 3



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µM). Feng et al. established a colorimetric filtration method for the highly selective and sensitive determination of Ni2+ and Pb2+ ions.20 Although the LOD of Ni2+ is 94 nM, there are several ions interfering the detection, which limited its application. Therefore, new probes based on noble metal nanomaterials are sorely demanded for rapid colorimetric detection of Ni2+.

Scheme 1. The detection mechanism of GSH-AgNPRs-based probe for rapid colorimetric detection of Ni2+. When Ni2+ is absent, the steric hindrance of GSH is not big and I– can be associated with the AgNPR corners and edges. Formation of AgI leads to etching of the AgNPRs, whose morphology changes from nanoprism to nanodisk. However, Ni2+ could catalyze the GSH oxidation by oxygen leading to fast forming of GSSG, whose high steric hindrance could inhibit the I- etching to frozen the shape of AgNPRs.

Herein, we developed novel improved triangular silver nanoprisms (AgNPRs) for rapid colorimetric detection of Ni2+ with supersensitivity and excellent selectivity. GSH, a tripeptide with the sequence γ-Glu-Cys-Gly, is used to stabilize the AgNPRs through formation of Ag-N or/and Ag-S bonds.21,22 The mechanism of the GSH-AgNPRs-based system for rapid colorimetric sensing of Ni2+ is shown in Scheme 1. In the absence of Ni2+, the steric hindrance of GSH is not big and iodide ion (I–) can be associated with the AgNPR corners and edges. Forming of AgI leads to etching of the AgNPRs resulting in shape change from triangular nanoprism to round nanodisk, clear blue shift of the SPR band, and evident change of the AgNPR solution color from blue to red.23 But Ni2+ could catalyze the GSH oxidation by oxygen leading to fast forming of GSSG (i.e. oxidized glutathione),24-30 whose high steric hindrance can inhibit the etching of AgNPRs by I– to frozen the shape of AgNPRs. On that 4


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point, the solution color will always be blue. Therefore, in view of the above-mentioned mechanism of anti-etching, Ni2+ can be rapidly visualized via eye-vision or measured by analysis of the UV-vis absorption spectra. Our developed GSH-AgNPRs-based Ni2+ probe has a super sensitivity and an excellent selectivity because the extinction coefficient of AgNPRs is extremely high and the catalysis of GSH oxidation by Ni2+ is specific.

EXPERIMENTAL SECTION Materials. L-glutathione (GSH), trisodium citrate dehydrate (C6H5Na3O7·2H2O), AgNO3, NaCl, KCl, CaCl2, ZnCl2, MnCl2·4H2O, Pb(NO3)2, HgSO4, BaCl2·2H2O, CoCl2·6H2O, MgCl2·6H2O, CuCl2·2H2O, AlCl3, FeCl3, CrCl3·6H2O, K2Cr2O7, Na2SO3, Na2SO4NaNO3, Na2CO3, Na3PO4, Na4P2O7, KBr, NaF, NaAc and KClO4 were purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). NiCl2.6H2O, 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid buffer (HEPES), sodium borohydride (NaBH4), Poly(vinylpyrrolidone) (PVP) (MW 58,000 daltons), hydrogen peroxide(H2O2), and KI were obtained from Aldrich. The glassware was thoroughly washed with aqua-regia (the volume ratio of HNO3 to HCl is 1/3) and Milli-Q water before using. Preparation of AgNPRs. AgNRPs was synthesized by the reported approach with minor modifications.9,31 In a typical procedure, 112 mL of aqueous solution was prepared by mixing Milli-Q water (99.5 mL), PVP solution (6.0 mL, 0.7 mM), C6H5Na3O7 solution (6.0 mL, 30 mM), AgNO3 solution (0.5 mL, 20 mM) and H2O2 (240 µL, 30 wt%). The mixture was vigorously stirred at room temperature. Then, a NaBH4 aqueous solution (0.1 M, 1.0 mL) was injected into the above-mentioned solution under stirring for reducing AgNO3. After that, the solution was kept in 20 o

C of water bath for 80 minutes. The final obtained AgNRP dispersion was kept in the fridge. Sensitivity of Ni2+ detection using the GSH-AgNPRs-based probe. The colorimetric detection

of Ni2+ using the GSH-AgNPRs-based probe was carried out at room temperature. Typically, GSH solution in the range of 5.0 ~ 30 µM (0.1 mL) was mixed with 800 µL of the AgNPR solution to obtain the GSH-AgNPRs-based probe solution. Ni2+ solutions (50 µL) at different concentrations 5



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were respectively mixed with the GSH-AgNPRs-based probe solutions. Next, KI aqueous solutions (50 µL, 0.5 ~ 5.0 mM) were respectively charged to the above-mentioned mixtures. The solutions were vortexed for 20 s and then kept standing at room temperature for different time ranging from 1.0 to 30 min. In the end, pictures of the solutions were taken by a camera to observe the changes of the solution color via eye-vision, and the corresponding UV-vis spectra were obtained and analyzed. Selectivity of Ni2+ detection using the GSH-AgNPRs-based probe. The selectivity of our probe to detect Ni2+ was investigated via comparison with detection of other ions including K+, Na+, Ca2+, Mn2+, Mg2+, Cu2+, Ba2+, Pb2+, Co2+, Hg2+, Zn2+, Al3+, Cr3+, Fe3+, F-, Cl-, Br-, NO3-, Ac-, ClO4-, SO42-, CO32-, SO32- , Cr2O72-, PO43- and P2O74-. The detection procedure was same to that proposed above. The ion concentration was 50 µM for K+, Na+, Ca2+, Ba2+, Mg2+, Pb2+, Al3+, Fe3+, F-, Cl-, Br-, NO3-, Ac-, ClO4-, SO42-, CO32-, SO32- , Cr2O72-, PO43- and P2O74-, and 5.0 µM for Mn2+, Cu2+, Co2+, Hg2+, Zn2+ and Cr3+. Application of our probe in environmental water samples. In order to verify the feasibility in real water samples, we applied this GSH-AgNPRs-based probe for rapid colorimetric detection of Ni2+ in the tap water and lake water collected from our institute. The real water samples were purified by 200 nm of syringe filters prior to use. Ni2+ solutions with various concentrations were then added into the purified real water samples to prepare stock solutions. Finally, the samples were detected utilizing the GSH-AgNPRs-based Ni2+ system or ICP-MS.

RESULTS AND DISCUSSION Sensing Mechanism of our Ni2+ probe. The construction of the GSH-AgNPRs-based Ni2+ probe and its sensing mechanism for Ni2+colorimetric detection are shown in Scheme 1. When Ni2+ is absent, I– can be associated with the AgNPR corners and edges leading to etching of the AgNPRs, whose shape changes from triangular nanoprism to nanodisk. The I− is causing etching because Ksp of AgI is very small (8.49 × 10−17), and the silver atoms at the corners and edges of AgNPRs are active and easy to coordinate with I−.9 When Ni2+ is present, it could catalyze the GSH oxidation by 6


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oxygen leading to fast forming of GSSG, whose high steric hindrance can prevent the I- etching and fix the shape of AgNPRs. This sensing mechanism has been verified by Raman spectra (Figure S1), UV-vis spectra (Figure 1), TEM (Figure 2) and DLS (Figure S2).

Figure 1. UV-vis spectra (a) and color change (b) of the AgNPR-based solutions with I−, and/or GSH, and/or Ni2+. The blank AgNPR solution is used as a control. Ⅰ: AgNPRs (blank); Ⅱ: AgNPRs incubated with I− (5.0 mM); Ⅲ: AgNPRs stabilized with GSH (20 µM); Ⅳ: AgNPRs stabilized with GSH (20 µM) and incubated with I− (5.0 mM); Ⅴ: AgNPRs stabilized with GSH (20 µM) with addition of Ni2+ (5.0 µM); Ⅵ: AgNPRs stabilized with GSH (20 µM) and incubated with 5.0 mM of I− with addition of Ni2+ (5.0 µM).

GSH is a tripeptide with sequence of γ-Glu-Cys-Gly, exists in biological fluids, and serves a variety of fundamental physiological functions. It contains functional groups of –SH, –NH2 and –COOH. Therefore, it is easy for GSH to be attached on AgNPR surfaces as a stabilizer via formation of Ag-S bonds, which has been demonstrated by our previously reported XPS spectra.9 The XPS spectra showing the binding energies of Ag 3d and S 2p indicated the interaction through the –SH groups and Ag atoms. The presence of Ni2+ can inhibit the etching of AgNPRs by I− because GSH can complex with Ni2+ and the complexation of GSH-Ni2+ can catalyze the GSH oxidation by oxygen leading to fast 7



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forming of GSSG.24-30 We also verified this point by Raman spectra (Figure S1). The stretching vibration peak of the disulfide bond (-S-S-) at 540 cm-1 is obvious in the Raman spectrum of “GSH+Ni2+” sample, but not in that of the “GSH” sample. The UV-vis spectra and color change of the AgNPR-based solutions with I−, and/or GSH, and/or Ni2+ are shown in Figure 1. It is found that the solutions of blank AgNPRs (Ⅰ), AgNPRs stabilized with GSH (Ⅲ) and AgNPRs stabilized with GSH in the presence of Ni2+ (Ⅴ) are all blue and their UV-vis spectra with three absorption peaks are all similar. The three absorption peaks are respectively situated at around 660, 450 and 330 nm, and the strongest absorption locates at around 660 nm. Both GSH and Ni2+ have no influence on the color and UV-vis absorption of the AgNPRs solution, which indicates that both GSH and Ni2+ cannot etch AgNPRs. However, the solution of AgNPRs incubated with I− (Ⅱ) changes to red and its UV-vis spectrum shifts toward blue (the strongest peak wavelength at around 660 nm shifts to 493 nm). That’s because I– can be associated with the AgNPR corners and edges and the formation of AgI (Ksp[AgI] = 8.49 × 10-17)9,32 leads to etching of the AgNPRs, which further causes shape change from nanoprism to nanodisk.23,32 In addition, the solution of AgNPRs stabilized with GSH and incubated with I− (Ⅳ) also changes to red and its UV-vis spectrum also shifts towards blue. The reason should be that the steric hindrance of the stabilizer GSH is not big enough to prevent the etching of AgNPR corners and edges by I−. While the GSH-stabilized AgNPRs are incubated with Ni2+ before introducing of I− (Ⅵ), the solution color remains blue and the corresponding shift of UV-vis absorption is negligible. That’s because Ni2+ could catalyze the GSH oxidation by oxygen leading to fast forming of GSSG,24-30 whose high steric hindrance can inhibit the etching of AgNPR corners and edges by I– and frozen the shape of AgNPRs (Scheme 1).

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Figure 2. TEM pictures of AgNPRs with I−, and/or GSH, and/or Ni2+. The blank AgNPR solution is used as a control. (a): AgNPRs (blank); (b): AgNPRs after incubation with 5.0 mM of I−; (c): AgNPRs stabilized with 20 µM of GSH; (d): AgNPRs stabilized with 20 µM of GSH after incubation of 5.0 mM of I−; (e): AgNPRs stabilized with 20 µM of GSH with addition of Ni2+ (5.0 µM); (f): AgNPRs stabilized with GSH (20 µM) and incubated with I− (5.0 mM) with addition of Ni2+ (5.0 µM).

Figure 2 shows the corresponding TEM pictures of the AgNPRs with I−, and/or GSH, and/or Ni2+. It can be seen that the morphologies of the blank AgNPRs (Figure 2a), the AgNPRs stabilized with GSH (Figure 2c) and the AgNPRs stabilized with GSH with addition of Ni2+ (Figure 2e) are all triangular. This result demonstrates that the GSH and Ni2+ cannot etch the AgNPRs. Moreover, the shapes of the AgNPRs incubated with I− (Figure 2b) and the AgNPRs stabilized with GSH and incubated with I− (Figure 2d) both show nanodisks, but not triangular nanoprisms. This result indicates that I– can etch AgNPR corners and edges resulting in shape change and the GSH is not able to inhibit the etching. However, if the GSH-stabilized AgNPRs are mixed with Ni2+ before introducing of I− (Figure 2f), the shape of the AgNPRs remains triangular nanoprisms because Ni2+ catalyzes the oxygen oxidation of GSH resulting in rapid formation of GSSG and its high steric hindrance prevents the I– etching. All the results of TEM are consistant with the UV-vis spectra and the AgNPR solution color change as shown in Figure 1. 9



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The morphology change observed by the TEM images is reconfirmed by size distributions of the GSH-stabilized AgNPRs mixed with or without Ni2+ before introducing of I− (Figure S2). We can see the size of the GSH-stabilized AgNPRs mixed with Ni2+ before introducing of I− is bigger than that without Ni2+, which reconfirmed that the Ni2+ can inhibit the AgNPR etching by I–.

Optimization of the probe parameters. It has been reported that the type of the reducing agent, stabilizer agent, reaction temperature, pH of solution, and the concentration of the reactants can influence the formation of AgNPRs, and the uniform and stable AgNPRs with different size and color can be synthesized by controlling the various influencing factors.9,31 Because the solution color and the UV-vis absorbance are dependent on the particle size of the AgNPRs, in this study, the AgNPRs with same synthesis conditions and same particle size are used for the detection of nickel ion compared with other ions. Our Ni2+ probe’s selectivity and sensitivity based on GSH-AgNPRs can be influenced by four experimental conditions, which are the concentration of GSH used to stabilize the AgNPRs, the concentration of I− used as an etching agent of the AgNPRs, the pH value of the GSH-AgNPRs-based Ni2+ probe, and the incubation time of the GSH-AgNPRs-based probe and Ni2+. They were systematically investigated and confirmed in accordance with our GSH-AgNPRs-based probe’s detection efficiency for Ni2+ (Figure S3). Figure S3a shows influence of our probe’s GSH concentration on Ni2+ detection efficiency. The wavelength shift is calculated between the peak wavelengths (in UV-vis absorption spectra) of the GSH-AgNPRs incubated with I− in the presence of Ni2+ and those in the absence of Ni2+. When the GSH concentration is lower than 20µM, the wavelength shift increases with increasing of the GSH concentration. That’s because higher GSH concentration accelerates the formation of GSSG induced by Ni2+. However, when the GSH concentration is higher than 20µM, the wavelength shift ceases to increase with increasing of the GSH concentration because the GSH should be saturated. In accordance with the detection efficiency of our Ni2+ probe, 20 µM is chosen as the maximum 10


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GSH concentration for the subsequent experiments. The peak wavelengths in UV-vis spectra of the GSH-AgNPRs incubated with I− (1.0-10 mM) with addition of Ni2+ (5.0 µM) are compared with those without addition of Ni2+, and the wavelength shift is plotted versus the I− concentration (Figure S3b). We can see the higher I− concentration from 0.5 to 5.0 mM, the stronger wavelength shift. That’s because more and more GSH-AgNPRs in the absence of Ni2+ can be etched in 15 min with increasing of I− concentration. In addition, the wavelength shift decreases at higher I− concentrations (> 5.0 mM) because some of the GSH-AgNPRs in the presence of Ni2+ can also be etched in 15 min when the I− concentration is too high. Therefore, the I− concentration is fixed at 5.0 mM as one of the probe’s optimal parameters. The mechanism of accelerating air oxidation of GSH by Ni2+ forming GSSG in alkaline solutions has been systematically investigated by Wojciech Bal et al. and Artur Krezel et al.24,25 Ni2+ and GSH can form complexes when the GSH is excess and the pH value is in the range of 6-12 as given below: NiHL, Ni2L22-, NiHL23-, NiL24-, and NiH-1L25-. Herein, the NiHL is an octahedral species, coordinated through the donors of the Glu moiety of GSH, while the remaining ones are largely squareplanar, with participation of the thiol in Ni2+ coordination. Among the molecular forms of GSH, HL2- is the one most susceptible to air oxidation, due to a presence of ionic interactions between its protonated amine and deprotonated thiol moieties. The complexation of Ni2+ accelerates air oxidation of GSH in alkaline solutions by a factor of 4, but this effect is absent at neutral pH. Therefore, influence of the pH value in the range of 7.5-9.3 on the UV-vis absorption of the GSH-AgNPRs is investigated (Figure S3c). We can see the UV-vis spectrum is blue shifted with decreasing of the pH values in the range of 7.5-9.3. In order to get a bigger wavelength shift at same conditions, the pH value of the GSH-AgNPRs is optimized to be 9.3 as one of the probe’s optimal parameters. Influence of the detection time (i.e. the incubation time for Ni2+ and GSH-AgNPRs (with 20 µM of GSH and 5.0 mM of I−, pH=9.3)) on Ni2+ (5.0 µM) sensing effect is studied by analyzing the UV-vis spectra (Figure S3d). With increasing of the incubation time, the wavelength shift first 11



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increases continuously, arrives at a maximum value at 15 min, and then starts to decrease after 15 min. That’s because more and more GSH-AgNPRs in the absence of Ni2+ can be etched with increasing of the incubation time within 15 min, and some of the GSH-AgNPRs in the presence of Ni2+ can also be etched with an extremely long incubation time. Therefore, the detection time is optimized to be 15 min as one of the probe’s optimal parameters.

Selectivity of our Ni2+ probe based on GSH-AgNPRs. For evaluating the selectivity of our Ni2+ probe based on GSH-AgNPRs at the optimized conditions, 26 kinds of ions were selected as the comparison investigation including K+, Na+, Co2+, Ca2+, Ba2+, Cu2+, Al3+, Mg2+, Pb2+, Mn2+, Hg2+, Zn2+, Cr3+, Fe3+, F−, Cl−, Br−, Ac−, NO3−, CO32−, C2O42−, SO32−, SO42−, Cr2O72−, PO43− and P2O74−. Figure 3a presents the wavelength shift between the peak wavelengths (in UV-vis absorption spectra) of the probe with addition of single metal ion and that without addition of metal ions. Figure 3b shows the corresponding data of Ni2+ compared with those of the anions. The inset images show the corresponding color change of the solutions. It is found that the wavelength shift for Ni2+ is far stronger compared with other ions. In addition, the solution color changes to red with addition of other ions, but remains blue in the presence of Ni2+. That’s because Ni2+ is able to specifically catalyze the GSH oxidation by oxygen on the AgNPR corners and edges forming GSSG to prevent the I− etching, but other metal ions and anions cannot. These results demonstrate our Ni2+ probe has a good selectivity to be distinguished from other ions. The corresponding UV-vis spectra are shown in Figure S4. The absorbance spectra of the sample containing Ni2+ showed an obvious wavelength shift compared with the blank sample, and the other metal ion and anion samples under the same conditions. Influence of other ions on the Ni2+ detection efficiency is further studied to confirm the selectivity of our Ni2+ detection system (Figure S5). Figure S5a presents the photos of our probe with addition of Ni2+ plus other metal ions. Figure S5b shows that in the presence of Ni2+ plus anions. We can see the solution color is blue in the presence of Ni2+ and other competing ions. 12


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These results indicate the Ni2+ probe has an excellent selectivity without significant interference from other metal ions and anions.

Figure 3. Selectivity of our Ni2+ probe based on GSH-AgNPRs. (a): Wavelength shift between the peak wavelengths (in UV-vis spectra) of the probe with addition of single metal ion (5.0 µM for Ni2+, Mn2+, Cu2+, Co2+, Hg2+, Zn2+, Cr3+ and 50 µM for other metal ions) and that without addition of metal ions. (b): Wavelength shift between the peak wavelengths of the probe with addition of single ion (5.0 µM for Ni2+ and 50 µM for other anions) and that without addition of ions. The inset images show the corresponding color change of the solutions. The pH value is 9.3 and the incubation time is 15 min as optimized conditions.

Figure 4. Photographs of the probes stabilized with GSH (20 µM) and incubated with I− (5.0 mM) with addition of Ni2+ at different concentrations (0-3.0 µM). The colorimetric probe stabilized with GSH (20 µM) and incubated with I− (5.0 mM) without addition of Ni2+ is the control. The pH value is 9.3 and the detection time is 15.0 min as optimized conditions.

Sensitivity of our Ni2+ probe based on GSH-AgNPRs. The UV-vis spectra and color change of our GSH-AgNPRs-based Ni2+ probe are both used to estimate the sensitivity. Photographs of the 13



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probes stabilized with GSH (20 µM) and incubated with I− (5.0 mM) with addition of Ni2+ at different concentrations (0-3000 nM) are shown in Figure 4. The colorimetric probe stabilized with GSH (20 µM) and incubated with I− (5mM) without addition of Ni2+ is the control. It is easy to distinguish the solution color at Ni2+ concentration of 50 nM or higher with that of the control. However, the solution color at Ni2+ concentration of 30 nM is very similar with that of the control. Therefore, our probe’s limit of detection (LOD) by eye-vision is 50 nM, which is negligible compared with the Ni2+ permissible limit in drinking water prescribed by the WHO (0.34 µM).3

Figure 5. Sensitivity of our probe based on GSH-AgNPRs for rapid measurement of Ni2+. (a): UV-vis absorption spectra of the colorimetric probe stabilized with GSH (20 µM) and incubated with I− (5.0 mM) with addition of Ni2+ at different concentrations (0-5000 nM). (b) Plot of the wavelength shift versus Ni2+ concentrations ranging from 5 to 5.0 µM. The inset plot shows that in the range of 5 ~ 300 nM. The pH value is 9.3 and the incubation time is 15 min as optimized conditions. (Mean ± SD, n = 3) 14


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The sensitivity of our Ni2+ probe based on GSH-AgNPRs is also determined by UV-Vis spectra. UV-vis spectra of the colorimetric probe stabilized by GSH (20 µM) and incubated with I− (5mM) with addition of Ni2+ at different concentrations (0-5.0 µM) are shown in Figure 5a. We can see the red shift increases with increasing of Ni2+ concentration because the Ni2+ is able to catalyze GSH oxidation by oxygen on the AgNPR corners and edges forming GSSG to prevent the I−etching. In addition, the wavelength shift between the peak wavelengths (in UV-vis absorption spectra) of the GSH-AgNPRs-based colorimetric probes with addition of Ni2+ and that in the absence of Ni2+ is utilized to obtain a standard curve for Ni2+ quantitative analysis. Figure 5b shows the plot of the wavelength shift versus Ni2+ concentrations ranging from 5 to 5000 nM, and the inset plot shows that in the range of 5 ~ 300 nM. A satisfying linear relationship (i.e., Y = 7.14309 + 0.15893 X, R2 = 0.992) in the range of 5 ~ 300 nM. Thus, we can draw the conclusion that the Ni2+ probe based on GSH-AgNPRs can be utilized for Ni2+ quantitative analysis in aqueous solutions. In addition, from Figure 5b, we can see the LOD of our probe by the UV-vis spectroscopy is 5 nM. It’s not only much lower than the permissible limit prescribed by the WHO (0.34 µM)3, but also much lower than the LOD values of other reported Ni2+ probes based on noble metal nanomaterials (Table S1).17-20,33-35 Therefore, we can say our GSH-AgNPRs-based Ni2+ probe has a supersensitivity according to the drinking water criteria by the WHO and the reported corresponding probes.

Figure 6. Photographs of our Ni2+ probes based on GSH-AgNPRs stabilized with GSH (20 µM) and incubated with I− (5.0 mM) (the pH value is 9.3 and the detection time is 15 min as optimized conditions) in the presence of various Ni2+ concentrations in environmental water samples.

Detection of environmental water samples. The Ni2+ probe based on GSH-AgNPRs is 15



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subsequently utilized for the real-time and on-site analysis of Ni2+. In this study, the real water samples were first purified by 200 nm of syringe filters prior to use, and then 0.1 or 0.3 µM of Ni2+ was added into the real water samples adopting standard addition method. Therefore, although the speciation of metal ions in aqueous systems (collected from natural resources) varies, the free Ni2+ is the only speciation of nickel ion for our real water samples. Figure 6 shows the photographs of our Ni2+ probes based on GSH-AgNPRs at optimized conditions with different concentrations of Ni2+ in the environmental water samples. The solution color at Ni2+ concentration of 0.1 or 0.3 µM is quite different with the control. Moreover, the detection results of Ni2+ in the environmental water samples by the Ni2+ probe based on GSH-AgNPRs or ICP-MS are summarized in Table 1. It is found that the results detected by both methods are similar and the recoveries are between 85.7 to 90.0 %, which is a satisfying range. Although these real water samples contain some organic matters, it seems that they do not interfere the Ni2+ detection by our GSH-AgNPRs-based probe. Therefore, our GSH-AgNPRs-based probe can be used for rapid colorimetric detection of Ni2+ with supersensitivity and excellent selectivity in real water samples. The UV-vis spectra and photographic images of our Ni2+ probes (with 20 µM of GSH, 5.0 µM of Ni2+ and 5.0 mM of I−) with various concentration of NaBH4 (Figure S6) demonstrate that the GSSG could be reduce back under a strong reductive environment inducing the AgNPR etching by I-. Therefore, the application of our probe Ni2+ should avoid a strong reductive environment.

Table 1. Ni2+ detection in environmental water samples by the Ni2+ probe or ICP-MS. ICP-MS AgNPRs-based probe Recoverya Sample Added (µM) (µM) (µM) (Mean±SE, n=3) (%) Tap water Lake water

0.100

0.11

0.09±0.03

88.0

0.300

0.34

0.26±0.01

88.0

0.100

0.11

0.09±0.01

90.0

0.300

0.32

0.26±0.02

85.7

a

Calculated from an equation given below: (the observed value with addition of Ni2+ – the observed value without addition of Ni2+) / the added value. 16


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CONCLUSIONS In summary, we explored novel AgNPRs for fast Ni2+ colorimetric detection with supersensitivity and excellent selectivity. In the absence of Ni2+, the steric hindrance of the stabilizer GSH is not big enough to prevent etching of the AgNPR corners and edges by I–, which leads to shape change from triangular nanoprism to round nanodisk, obvious blue shift of the SPR band, and clear change of the AgNPR solution color. But Ni2+ could catalyze GSH oxidation by oxygen causing fast GSSG formation, whose high steric hindrance can prevent the I– etching. This sensing mechanism has been verified by Raman spectra, UV-vis spectra, TEM and DLS. In accordance with the detection efficiency of our Ni2+ probe based on GSH-AgNPRs, the GSH concentration used to stabilize the AgNPRs, the concentration of I− used as an etching agent of the AgNPRs, the pH value of the GSH-AgNPRs-based Ni2+ probe, and the incubation time of the GSH-AgNPRs-based probe and Ni2+ are respectively optimized to be 20 µM, 5.0 mM, 9.3 and 15 min. Under the optimal parameters, our probe has an excellent selectivity for Ni2+ compared with 26 kinds of other ions because Ni2+ is able to inhibit the AgNPRs etching by I–, but other ions cannot. The LOD of our GSH-AgNPRs-based Ni2+ probe is 50 nM by naked eyes and is 5 nM by UV-vis spectroscopy. They are both negligible compared with the permissible limit of Ni2+ in drinking water prescribed by the WHO (0.34 µM). The LOD by UV-vis spectroscopy is also far lower than the LOD values of other reported Ni2+ probes based on noble metal nanomaterials. A satisfying linear relationship (R2 = 0.992) reinforces that our probe can be utilized for the quantitative analysis of Ni2+ concentration in aqueous solutions. The detection of real water samples demonstrates the developed Ni2+ probe based on GSH-AgNPRs could be utilized for Ni2+ rapid colorimetric detection with supersensitivity and excellent selectivity in real water samples.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Figure S1: 17



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Raman spectra; Figure S2: size distributions by DLS; Figure S3: optimization of the probe parameters; Figure S4: selectivity of the colorimetric probe; Figure S5: influence of other metal ions or anions; Figure S6: UV-vis spectra of the probe with or without NaBH4; Table S1: comparison of Ni2+ detection probes (PDF).

AUTHOR INFORMATION Corresponding authors E-mail: [email protected]; Tel: +86 574 86685039; Fax: +86 574 86685163. E-mail: [email protected]; Tel: +86 574 87617278 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS Authors acknowledge the financial support by the Project for Science and Technology Service of Chinese Academy of Sciences (KFJ-SW-STS-172 & KFJ-EW-STS-016), Hundred Talent Program of Chinese Academy of Sciences (2010735), Youth Innovation Promotion Association of Chinese Academy of Sciences (2016269), the aided program for Science and Technology Innovative Research Team of Ningbo Municipality (Grant No. 2014B82010, and 2015B11002), Natural Science Foundation of China (Grants Nos. 31128007), Natural Science Foundation of Ningbo (2016A610266) and Zhejiang Provincial Natural Science Foundation of China (Grant No. R5110230).

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For Table of Contents Use Only

A novel supersensitive probe for rapid colorimetric detection of nickel ion based on a sensing mechanism of anti-etching

Ningyi Chena,b, Yujie Zhanga, Hongyu Liua,b, Huimin Ruana, Chen Donga, Zheyu Shena,*, Aiguo Wua,*

Synopsis Glutathione (GSH)-stabilized triangular silver nanoprisms (AgNPRs) are developed for rapid colorimetric detection of Ni2+ with supersensitivity and excellent selectivity based on a sensing mechanism of anti-etching.

23



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Graphical Abstract Novel glutathione (GSH)-stabilized triangular silver nanoprisms (AgNPRs) are developed for rapid colorimetric detection of Ni2+ with supersensitivity and excellent selectivity based on a sensing mechanism of anti-etching because Ni2+ can protect the AgNPRs from I− etching via rapid formation of oxidized glutathione (GSSG), but other ions cannot.


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A Supersensitive Probe for Rapid Colorimetric Detection of Nickel Ion

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