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Colorimetric detection toward halide ions by a silver nanocluster hydrogel
Talanta 211 (2020) 120717
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Colorimetric detection toward halide ions by a silver nanocluster hydrogel Yun Ma, Xiao-Fang Shen, Fei Liu, Yue-Hong Pang
T
∗
State Key Laboratory for Food Science and Technology, School of Food Science and Technology, Jiangnan University, Wuxi, 214122, China
ARTICLE INFO
ABSTRACT
Keywords: Ag nanoclusters hydrogel Colorimetric sensor Halides Real water samples
We reported a novel colorimetric method for highly selective halide ions (Cl−, Br−and I−) recognition by Ag nanoclusters hydrogel (Ag–NCs hydrogel). The Ag–NCs hydrogel could discriminate Cl−, Br−and I− ions from a wide range of environmentally important anions, identified by the distinct UV–vis absorption band changes or the change in the color of Ag–NCs hydrogel. On the basis of this strategy, 20 μM and 200 μM of Cl−, 5 μM and 100 μM of Br−, 5 μM and 100 μM of I− could be recognized within 5 min by UV–vis spectrum and naked eye observation, respectively. The surface color of hydrogel changed from yellow to dark green for Cl−, to brown for Br−, and to deep brown for I−. In addition, this sensing method had been applied successfully to detect chloride anion in real water samples such as tap water, pond water and pure water. Therefore, this rapid, facile, and costeffective colorimetric assay based on Ag–NCs hydrogel was attractive and promising.
1. Introduction Inorganic anions are prevalent in biological system, which play important roles in industrial, medical, environmental processes, and so on. Meanwhile, over two thirds of cofactors and substrates in nature involved in biological transformations are anionic [1]. Inorganic anions such as halide ions, nitrite, and phosphate can be both beneficial and harmful to the environment and human health, up to their concentrations. Chloride ion, one kind of halide ions, plays pivotal roles in drinking water, biological systems and it would be very beneficial for establishing the diagnosis and curing methods of related diseases (e.g., cystic fibrosis, renal function and acid-base disorders) [2,3]. Iodide ion, one of the indispensable elements for the synthesis of thyroid hormone, regulates cell metabolism and supports for the development and growth of the neuromuscular tissue (especially in the fetus brain). Either of iodine deficiency or iodine excess will cause great harm to human health [4]. The determination of inorganic anions can not only afford a way for environment monitoring and water quality characterization, but also benefit clinical diagnosis and related diseases cure. Given the pressing demand for inorganic anions detection, considerable progress in analytical methodology development for inorganic anions has been made in the past decade, such as surface enhanced raman scattering (SERS) [5], electrochemical characterisation [3,6], and traditional ion chromatography (IC) [7–9]. Among them, the IC is the most widely used technique for the separation and simultaneous determination of anion ions. Recently, chemosensors capable of recognizing and detecting anions, namely fluorogenic or chromogenic
∗
anion receptors based on supramolecular, have become one of the most attractive research areas [10]. Although these chemosensors show remarkable selectivity, most of them have several drawbacks, like complicated organic synthesis process, only working in water-incompatible media, and high detection limits, preventing their practical application in real water samples [11–14]. Therefore, there is in sore need of developing simple, sensitive, and water-compatible approaches for detecting anions. Colorimetric sensors, based on noble metal (e.g. Ag and Au) nanomaterials, provide attractive candidates for detecting anions because of their visual signal feedback without the aid of advanced instruments [1,15]. Ag nanoclusters (< 2 nm) have gained much attention in recent years for their unique size and shape dependent optoelectronic properties and potential applications in biosensing and bioimaging [16–19]. However, the applications of Ag nanoclusters in colorimetric detection of halide ions are still few. Herein, we report a simple, rapid and sensitive colorimetric method based on our newly synthesized Ag nanocluster hydrogel (Ag–NCs hydrogel) for the recognization of halide ions (Cl−, Br−and I−) in aqueous environments. 2. Materials and methods 2.1. Ag–NCs hydrogel synthesis Ag–NCs hydrogel was prepared according to our previously reported method [20]. Namely 0.1 M AgNO3 was mixed with 0.01 M L-
Corresponding author. E-mail address: [email protected] (Y.-H. Pang).
https://doi.org/10.1016/j.talanta.2020.120717 Received 28 August 2019; Received in revised form 29 December 2019; Accepted 3 January 2020 Available online 07 January 2020 0039-9140/ © 2020 Elsevier B.V. All rights reserved.
Talanta 211 (2020) 120717
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cysteine solution in a molar ratio of 1.5: 1 and photo-reduction was carried out with a high-pressure mercury lamp for 4.5 h. Then the solution underwent hydrogelation at room temperature for 0.5 h. The detailed procedure can be seen in our previously study [20]. The asprepared Ag–NCs hydrogel was directly applied without any treatment as the probe to study the colorimetric detection of halide ions in this work. 2.2. Halides recognition 2.2.1. Selectivity of the Ag–NCs hydrogel The 400 μL of other different anions (5 mM) such as NO3−、CH3COO−、SO42−、SO32−、PO43−、NO2− and 0.1 mM of CO32−、OH− were added into 50 μL of the as-prepared Ag–NCs hydrogel. After reaction for 5 min at room temperature, the system was diluted to 2 mL by water. Then UV–vis spectra of the diluted solutions after shaking by hand for about 30 s were recorded by the UV-1800 spectrophotometer (Shimadzu, Japan).
Fig. 1. a) UV/Vis spectra of Ag–NCs hydrogel. Inset: photograph of Ag–NCs hydrogel. b) The structure of Ag–NCs hydrogel.
3. Results and discussion 3.1. Synthesis of Ag–NCs hydrogel The Ag–NCs hydrogel was prepared according to our previously method [20]. Fig. 1a shows the UV–vis spectrum of the as-prepared Ag–NCs hydrogel. The Ag–NCs hydrogel demonstrated a characteristic absorption peak at 408 nm, suggesting that Ag was formed in the hydrogel and Fig. 1b shows the structure of Ag–NCs hydrogel, Ag was surrounded with the L-cysteine by the Ag–S and hydrogel was formed due to the presence of hydrogen bonding, the specific information can be seen in our previously work.
2.2.2. Colorimetric detection of halide ions The 400 μL of halide ions (Cl−, Br−and I−) at different concentrations were added into 50 μL Ag–NCs hydrogel. After reaction for 5 min at room temperature, the system was diluted to 2 mL by water. The UV–vis spectra of diluted solutions after shaking by hand for about 30 s were then recorded by the UV-1800 spectrophotometer (Shimadzu, Japan).
3.2. Selectivity of Ag–NCs hydrogel toward halide ions
2.2.3. Detection of halides in real water samples Firstly, known concentration of Cl−solutions (0.2 mM and 7 mM) were prepared as reference, respectively, according to the hygienic standard of bottled purified water for drinking in China (GB173242003) and the national standards for drinking water in China (GB57492006), where the concentrations of Cl− should be not more than 6 mg L−1 (about 0.2 mM) and 250.0 mg L−1 (about 7 mM). And one tapping water, two kinds of pure water and four kinds of mineral water as real water samples were selected. Then 400 μL of the above mentioned solutions were added into 50 μL Ag–NCs hydrogel. After reaction for 5 min at room temperature, the system was diluted to 2 mL by water. The UV–vis spectra of diluted solutions after shaking by hand for about 30 s were then recorded by the UV-1800 spectrophotometer (Shimadzu, Japan). The ion chromatography (IC) was conducted to validate our proposed method using ICS2100 (Dionex，USA) in the isocratic elution mode. The eluent of KOH was delivered at a flow rate of 1 mL/min, and the injected sample volume was 500 μL. The column of AS19 temperature was kept at 45 °C throughout the analysis and the suppressor current was 90 mA. All of the water samples were filtered through 0.45 μm filter membrane (Millipore, USA) before use.
It is reported that the absorption band of silver nanostructures are sensitive to the presence of adsorbed substances, thus resulting in appreciable changes in color and optical properties, particularly for anions [21]. To evaluate the selectivity of the proposed colorimetric assay, we studied the values of individual anions such as NO3−, NO2−, CH3COO−, SO32−, SO42−, PO43−, S2−, CO32−, OH−, F−, Cl−, Br−and I−. Fig. 2 shows that most of the tested ions could not lead to similar absorbance or color change as Cl−, Br−and I− did, even if the concentration of each type of anions were increased to 5 mM, CO32－and OH− were 0.1 mM, due to their larger solubility of silver salts (the Ksp is given in Table S1) [19,22], except for S2− (see Fig. S2). Thus, the detection of halide ions could be interfered by S2− because of its low solubility. However, such interference could be negligible under the condition that its concentration is very low, for instance, in mineral water or tap water. So in all, such selective experiment result highlights the remarkably specific identification of halide ions by our Ag–NCs hydrogel. 3.3. Sensitivity of Ag–NCs hydrogel toward halide ions The sensitivity of Ag–NCs hydrogel toward halides (Cl−, Br−and I−) was hence recognized by the UV–vis spectra. Firstly the optimizations about the sensing conditions such as Ag–NCs hydrogel volume and its volume ratio to halide ions (Cl−, Br−and I−) were conducted (Fig. S1), and the colorimetric determination was performed at room temperature. Two different volumes of 50 μL and 100 μL Ag–NCs hydrogel were used and the volume ratio of Ag–NCs hydrogel to 5 mM Cl− was kept at 2:1, 1:1, 1:2, 1:4, 1:6, 1:8 and 1:10. The intensity of the absorption peak in two systems at 408 nm decreased with increasing the volume ratio of Ag–NCs hydrogel to Cl−, together with the appearance of a new absorption peak at high wavelength. Eventually, the absorbance peak at 408 nm of the Ag–NCs hydrogel achieved a minimum and kept steady when the volume ratio was 1:8. Therefore, we chose a volume ratio of 1:8 of 50 μL Ag–NCs hydrogel to 5 mM Cl− in the following tests. To further investigate the sensitivity of our Ag–NCs hydrogel towards halide ions and to obtain the corresponding minimum detectable
2.3. Transmission electron microscopy (TEM) TEM images were taken on a JEOL TEM-2100 microscope. The Ag–NCs hydrogel after reaction with halides (Cl−, Br−and I−) were diluted to 2 mL using water, then the solutions were spotted on a carbon coated copper grid and dried in ambience. 2.4. Zeta (ζ)-potential Zeta (ζ)-potential measurements were performed by laser Doppler electrophoresis using a ZetaPALS (Brookhaven Instruments Corporation, U.S.A). The Ag–NCs hydrogel after reaction with halides (Cl−, Br−and I−) were diluted to 2 mL, then the Zeta (ζ)-potential of the solutions were measured. 2
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Fig. 2. a) UV–vis spectra and (b) selectivity of Ag–NCs hydrogel after interaction with different anions. The insets in a) are the photographs showing respective colorimetric responses.
Fig. 3. UV–vis spectra of Ag–NCs hydrogel upon the addition of different concentrations of halide ions.
concentration, we monitored the UV–vis spectra and color changes by varying the concentrations of Cl−, Br−and I− (Fig. 3). The results based on the UV–vis absorption can be classified into two stages, the absorption peak at 408 nm first slightly enhanced with increasing the concentration of halide ions (Cl−: 0–200 μM, Br−and I−: 0–50 μM), and then gradually damped together with a new absorption peak appeared at higher wavelength when reaching a certain concentration. This new peak might be ascribed to the aggregation of Ag–NCs in the presence of adsorbed ions and the adsorption of ions on the surfaces of Ag–NCs. Besides, Br−and I−were much more sensitive, in comparison to Cl−. The minimum detectable concentration was estimated to be 20 μM, 5 μM and 5 μM for Cl−, Br−and I−, respectively. These obtained minimum detectable concentration values were much lower than those of comparable anion sensors based on other nanomaterials reported
recently (Table S2) [19,21,23–26]. Furthermore, the UV–vis spectra responses were replenished by the observation of visible color changes of the Ag–NCs hydrogel. The surface color of hydrogel changed from yellow to dark green, to brown and to deep brown for Cl−, Br−, I−, respectively, which darken with increasing the concentration of Cl−, Br−and I− (Fig. 4). The minimum identifiable concentrations of halides by the colorimetric method as a cue were 200 μM for Cl−, 100 μM both for Br− and I−. Furthermore, the interaction of halide ions with Ag–NCs hydrogel was found to be time-dependent and UV–vis spectra were measured to evaluate the kinetic process of Ag–NCs hydrogel upon the addition of different halides, as displayed in Fig. S3. The change of peak intensity with reaction time showed that the reaction almost terminated within 10 min. Therefore, to ensure reproducibility and achieve rapid reionization, 5 min was 3
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Table 1 The zeta (ζ)-potential of Ag–NCs hydrogel before and after interaction with 200 μmol/L Cl−, 20 μmol/L Br− and 20 μmol/L for 5 min. Halides added
Cl− Br− I−
Zeta (ζ)-potential of Ag– NCs hydrogel (mV) Original
After addition of halides
34.49 34.49 34.49
28.55 29.20 31.22
tap water, two kinds of pure water and four kinds of mineral water without any pretreatment. Typical detection results are given in Fig. 5. We used the hygienic standard of bottled purified water for drinking in China (GB17324-2003) and the national standards for drinking water in China (GB5749-2006) as reference standards, where the concentration of Cl− should be not more than 6 mg L−1 (0.2 mM) and 250.0 mg L−1 (7 mM), respectively. Firstly, known concentration of Cl−solutions prepared based on the requirement of the two standards (0.2 mM and 7 mM) were measured by colorimetric method and UV–vis spectra. When 7 mM Cl− was added, the surface color of Ag–NCs hydrogel changed obviously from yellow to dark green and the absorption peak at 408 nm declined significantly together with a new absorption peak emerged at higher wavelength. Unfortunately, for the tap water, we could not obtain the exact UV–vis spectra result because there existed floccules in the diluted solution. However, we could still estimate the Cl− level by the colorimetric method that the color change of Ag–NCs hydrogel was less obvious than that of 7 mM Cl−. Therefore, we can reach the conclusion that the Cl− level in tap water was below the requirement of GB5749-2006. Similarly, when 0.2 mM Cl− was added, the color around the surface of Ag–NCs hydrogel changed to white visually and the intensity of the absorption peak slightly enhanced with increasing the concentration of halide ions when the concentration of Cl− was range from 0 to 200 μM (as displayed in Fig. 3). For the two kinds of pure water, no color change was observed and the intensity of the UV–vis absorption peak was lower than that of 0.2 mM Cl−. Thus, the Cl− level in two kinds of pure water met the requirement of GB17324-2003. Based on the above results, we directly applied the
Fig. 4. Photographs of colorimetric response of Ag–NCs hydrogels toward different concentrations of halide ions (Cl−, Br−and I−).
selected for detecting Cl−, Br−and I− in this work. 3.4. Application of Ag–NCs hydrogel in real water samples The measurement of Cl− would suffer from the disturbance by higher concentrations of Br−, I− and S2− owing to their superior affinity to silver, thus some pretreatments might be desired for real sample detection. However, fortunately, such interference was negligible in some cases if these anions concentrations were very low, such as tap water or pure water. Thus to verify the feasibility of our proposed sensing strategy for the analysis of halides in real water samples, the Ag–NCs hydrogel sensor was applied to measure the levels of Cl− in one
Fig. 5. UV–vis spectra and photographs of colorimetric response of Ag–NCs hydrogel after detection of Cl− in water samples. 4
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Fig. 6. TEM images of Ag–NCs hydrogels upon the addition of different concentrations of Cl−, Br−and I− for 5 min, a) 200 μM Cl−, b) 20 μM Br− and c) 20 μM I−.
Scheme 1. Scheme for the recognition mechanism of AgNCs hydrogel toward halide ions.
colorimetric method to estimate the Cl− level in four kinds of mineral water, as shown in Fig. 5c, there were no color change visually compared to that of 0.2 mM Cl−, this suggested that the Cl− levels in four kinds of mineral water met the requirement of GB17324-2003. To verify the reliability of our method, we used ion chromatography to measure those real water samples mentioned above. It suggested that the detection results by our method were consistent with that of ion chromatography (Table S3), suggesting that our system was practical and reliable.
than 1 nm appeared in the TEM images. According to those previous profound researches the above mentioned together with our experimental results, we speculated the following mechanism of halide-induced aggregation resulting in the changes of UV–vis absorption intensity and color of Ag–NCs hydrogel, as illustrated in Scheme 1. Since two L-cysteine molecules might be crosslinked together by the S–S bond [32], more Ag–NCs exposed into the solution were activated when Cl−, Br−and I− ions were added into the Ag–NCs hydrogel, leading to the increase of UV absorption intensity. On the other hand, the three ions would extremely easy to adsorb onto the surface of Ag–NCs to form a monolayer of AgX (AgCl, AgBr, and AgI) on account of the higher affinity of Ag-X bond and the lower Ksp of AgX compounds. Meanwhile, the corrosiveness of halides would also etch a small number of Ag–NCs into smaller Ag–NCs. It was known that the stability of Ag–NCs depended on their surface electric double layer and coulomb repulsion. The aggregation of Ag–NCs appeared with the increase of the halides concentration. This is due to the chemisorption of halides to the surface Ag–NCs and the surface charge of Ag–NCs was neutralized, as a result, the van der Waals attractive forces increased between Ag–NCs, thus triggering the initiation of aggregation, which was accompanied by the changes of UV–vis spectra and the color responses [19].
3.5. Recognition mechanism of Ag–NCs hydrogel toward halide ions The sensitive identification of halide ions by Ag–NCs hydrogel rests with the unique reactions between halides and Ag atoms, as well as the oxidative etching and aggregation of Ag–NCs induced by halides. To date, there have been several reports about the mechanism of halides recognition by the Ag nanoparticles. For example, Espinoza et al. [27] elucidated comprehensively the kinetics of halide-induced disintegration and aggregation of Ag nanoparticles. Besides, Baalousha [28], Li [29,30] and Huynh [31] reported the chloride-induced surface oxidation, dissolution and aggregation process of Ag nanoparticles. Herein, more analysis were performed by TEM and Zeta (ζ)-potential measurements to clearly understand the recognition mechanism. The original Ag–NCs hydrogel possessed positive charges and after reaction with 200 μM Cl−, 20 μM Br−and 20 μM I− for 5 min, the zeta (ζ)potential of AgNCs hydrogel decreased from 34.49 mV to 28.55 mV, 29.20 mV and 31.22 mV, respectively (Table 1). Meanwhile, the Ag–NCs size increased from original 1.9 nm [20] to more than a dozen nanometers (Fig. 6). Besides, there were some new particles smaller
4. Conclusion To summarize, we have developed a convenient colorimetric method for halide ions identification based on the Ag–NCs hydrogel. This protocol highly exhibited selectivity toward halides over other tested anions and it was applied successfully to the recognition of 5
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chloride in real water samples. The recognition mechanism was also proposed in this paper on the basis of our optical, TEM and zeta potential results, which was mainly according to the special reactions between halides and Ag atoms. Considering the important roles of halide ions in the areas of the environment, industrial processes, human health and so on, such method can be likely to have potential applications, such as the immediate analysis of chloride ions in water, the distinguishment of edible salt from industrial salt and so forth.
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Notes The authors declare no competing financial interest. Acknowledgements This work was supported by the National Key R&D Program of China (2018YFC1604202) and the Fundamental Research Funds for the Central Universities (JUSRP51714B). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.talanta.2020.120717. References [1] G.-L. Wang, X.-Y. Zhu, Y.-M. Dong, H.-J. Jiao, X.-M. Wu, Z.-J. Li, The pH-dependent interaction of silver nanoparticles and hydrogen peroxide: a new platform for visual detection of iodide with ultra-sensitivity, Talanta 107 (2013) 146–153. [2] A. Graefe, S.E. Stanca, S. Nietzsche, L. Kubicova, R. Beckert, C. Biskup, G.J. Mohr, Development and critical evaluation of fluorescent chloride nanosensors, Anal. Chem. 80 (17) (2008) 6526–6531. [3] J. Jin, X. Ouyang, J. Li, J. Jiang, H. Wang, Y. Wang, R. Yang, Nucleic acid-modulated silver nanoparticles: a new electrochemical platform for sensing chloride ion, Analyst 136 (18) (2011) 3629–3634. [4] B. Hetzel, Iodine deficiency disorders and their eradication, The Lancet 322 (8359) (1983) 1126–1129. [5] P. Pienpinijtham, X.X. Han, S. Ekgasit, Y. Ozaki, Highly sensitive and selective determination of iodide and thiocyanate concentrations using surface-enhanced Raman scattering of starch-reduced gold nanoparticles, Anal. Chem. 83 (10) (2011) 3655–3662. [6] X. Qin, H. Wang, Z. Miao, X. Wang, Y. Fang, Q. Chen, X. Shao, Synthesis of silver nanowires and their applications in the electrochemical detection of halide, Talanta 84 (3) (2011) 673–678. [7] C. Villagrán, M. Deetlefs, W.R. Pitner, C. Hardacre, Quantification of halide in ionic liquids using ion chromatography, Anal. Chem. 76 (7) (2004) 2118–2123. [8] Y. Noguchi, L. Zhang, T. Maruta, T. Yamane, N. Kiba, Simultaneous determination of fluorine, chlorine and bromine in cement with ion chromatography after pyrolysis, Anal. Chim. Acta 640 (1) (2009) 106–109. [9] R. Michalski, M. Jabłonska, S. Szopa, A. Łyko, Application of ion chromatography with ICP-MS or MS detection to the determination of selected halides and metal/ metalloids species, Crit. Rev. Anal. Chem. 41 (2) (2011) 133–150. [10] J. Zhang, X. Xu, Y. Yuan, C. Yang, X. Yang, A Cu@ Au nanoparticle-based colorimetric competition assay for the detection of sulfide anion and cysteine, ACS Appl.
6
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Talanta journal homepage: www.elsevier.com/locate/talanta
Colorimetric detection toward halide ions by a silver nanocluster hydrogel Yun Ma, Xiao-Fang Shen, Fei Liu, Yue-Hong Pang
T
∗
State Key Laboratory for Food Science and Technology, School of Food Science and Technology, Jiangnan University, Wuxi, 214122, China
ARTICLE INFO
ABSTRACT
Keywords: Ag nanoclusters hydrogel Colorimetric sensor Halides Real water samples
We reported a novel colorimetric method for highly selective halide ions (Cl−, Br−and I−) recognition by Ag nanoclusters hydrogel (Ag–NCs hydrogel). The Ag–NCs hydrogel could discriminate Cl−, Br−and I− ions from a wide range of environmentally important anions, identified by the distinct UV–vis absorption band changes or the change in the color of Ag–NCs hydrogel. On the basis of this strategy, 20 μM and 200 μM of Cl−, 5 μM and 100 μM of Br−, 5 μM and 100 μM of I− could be recognized within 5 min by UV–vis spectrum and naked eye observation, respectively. The surface color of hydrogel changed from yellow to dark green for Cl−, to brown for Br−, and to deep brown for I−. In addition, this sensing method had been applied successfully to detect chloride anion in real water samples such as tap water, pond water and pure water. Therefore, this rapid, facile, and costeffective colorimetric assay based on Ag–NCs hydrogel was attractive and promising.
1. Introduction Inorganic anions are prevalent in biological system, which play important roles in industrial, medical, environmental processes, and so on. Meanwhile, over two thirds of cofactors and substrates in nature involved in biological transformations are anionic [1]. Inorganic anions such as halide ions, nitrite, and phosphate can be both beneficial and harmful to the environment and human health, up to their concentrations. Chloride ion, one kind of halide ions, plays pivotal roles in drinking water, biological systems and it would be very beneficial for establishing the diagnosis and curing methods of related diseases (e.g., cystic fibrosis, renal function and acid-base disorders) [2,3]. Iodide ion, one of the indispensable elements for the synthesis of thyroid hormone, regulates cell metabolism and supports for the development and growth of the neuromuscular tissue (especially in the fetus brain). Either of iodine deficiency or iodine excess will cause great harm to human health [4]. The determination of inorganic anions can not only afford a way for environment monitoring and water quality characterization, but also benefit clinical diagnosis and related diseases cure. Given the pressing demand for inorganic anions detection, considerable progress in analytical methodology development for inorganic anions has been made in the past decade, such as surface enhanced raman scattering (SERS) [5], electrochemical characterisation [3,6], and traditional ion chromatography (IC) [7–9]. Among them, the IC is the most widely used technique for the separation and simultaneous determination of anion ions. Recently, chemosensors capable of recognizing and detecting anions, namely fluorogenic or chromogenic
∗
anion receptors based on supramolecular, have become one of the most attractive research areas [10]. Although these chemosensors show remarkable selectivity, most of them have several drawbacks, like complicated organic synthesis process, only working in water-incompatible media, and high detection limits, preventing their practical application in real water samples [11–14]. Therefore, there is in sore need of developing simple, sensitive, and water-compatible approaches for detecting anions. Colorimetric sensors, based on noble metal (e.g. Ag and Au) nanomaterials, provide attractive candidates for detecting anions because of their visual signal feedback without the aid of advanced instruments [1,15]. Ag nanoclusters (< 2 nm) have gained much attention in recent years for their unique size and shape dependent optoelectronic properties and potential applications in biosensing and bioimaging [16–19]. However, the applications of Ag nanoclusters in colorimetric detection of halide ions are still few. Herein, we report a simple, rapid and sensitive colorimetric method based on our newly synthesized Ag nanocluster hydrogel (Ag–NCs hydrogel) for the recognization of halide ions (Cl−, Br−and I−) in aqueous environments. 2. Materials and methods 2.1. Ag–NCs hydrogel synthesis Ag–NCs hydrogel was prepared according to our previously reported method [20]. Namely 0.1 M AgNO3 was mixed with 0.01 M L-
Corresponding author. E-mail address: [email protected] (Y.-H. Pang).
https://doi.org/10.1016/j.talanta.2020.120717 Received 28 August 2019; Received in revised form 29 December 2019; Accepted 3 January 2020 Available online 07 January 2020 0039-9140/ © 2020 Elsevier B.V. All rights reserved.
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cysteine solution in a molar ratio of 1.5: 1 and photo-reduction was carried out with a high-pressure mercury lamp for 4.5 h. Then the solution underwent hydrogelation at room temperature for 0.5 h. The detailed procedure can be seen in our previously study [20]. The asprepared Ag–NCs hydrogel was directly applied without any treatment as the probe to study the colorimetric detection of halide ions in this work. 2.2. Halides recognition 2.2.1. Selectivity of the Ag–NCs hydrogel The 400 μL of other different anions (5 mM) such as NO3−、CH3COO−、SO42−、SO32−、PO43−、NO2− and 0.1 mM of CO32−、OH− were added into 50 μL of the as-prepared Ag–NCs hydrogel. After reaction for 5 min at room temperature, the system was diluted to 2 mL by water. Then UV–vis spectra of the diluted solutions after shaking by hand for about 30 s were recorded by the UV-1800 spectrophotometer (Shimadzu, Japan).
Fig. 1. a) UV/Vis spectra of Ag–NCs hydrogel. Inset: photograph of Ag–NCs hydrogel. b) The structure of Ag–NCs hydrogel.
3. Results and discussion 3.1. Synthesis of Ag–NCs hydrogel The Ag–NCs hydrogel was prepared according to our previously method [20]. Fig. 1a shows the UV–vis spectrum of the as-prepared Ag–NCs hydrogel. The Ag–NCs hydrogel demonstrated a characteristic absorption peak at 408 nm, suggesting that Ag was formed in the hydrogel and Fig. 1b shows the structure of Ag–NCs hydrogel, Ag was surrounded with the L-cysteine by the Ag–S and hydrogel was formed due to the presence of hydrogen bonding, the specific information can be seen in our previously work.
2.2.2. Colorimetric detection of halide ions The 400 μL of halide ions (Cl−, Br−and I−) at different concentrations were added into 50 μL Ag–NCs hydrogel. After reaction for 5 min at room temperature, the system was diluted to 2 mL by water. The UV–vis spectra of diluted solutions after shaking by hand for about 30 s were then recorded by the UV-1800 spectrophotometer (Shimadzu, Japan).
3.2. Selectivity of Ag–NCs hydrogel toward halide ions
2.2.3. Detection of halides in real water samples Firstly, known concentration of Cl−solutions (0.2 mM and 7 mM) were prepared as reference, respectively, according to the hygienic standard of bottled purified water for drinking in China (GB173242003) and the national standards for drinking water in China (GB57492006), where the concentrations of Cl− should be not more than 6 mg L−1 (about 0.2 mM) and 250.0 mg L−1 (about 7 mM). And one tapping water, two kinds of pure water and four kinds of mineral water as real water samples were selected. Then 400 μL of the above mentioned solutions were added into 50 μL Ag–NCs hydrogel. After reaction for 5 min at room temperature, the system was diluted to 2 mL by water. The UV–vis spectra of diluted solutions after shaking by hand for about 30 s were then recorded by the UV-1800 spectrophotometer (Shimadzu, Japan). The ion chromatography (IC) was conducted to validate our proposed method using ICS2100 (Dionex，USA) in the isocratic elution mode. The eluent of KOH was delivered at a flow rate of 1 mL/min, and the injected sample volume was 500 μL. The column of AS19 temperature was kept at 45 °C throughout the analysis and the suppressor current was 90 mA. All of the water samples were filtered through 0.45 μm filter membrane (Millipore, USA) before use.
It is reported that the absorption band of silver nanostructures are sensitive to the presence of adsorbed substances, thus resulting in appreciable changes in color and optical properties, particularly for anions [21]. To evaluate the selectivity of the proposed colorimetric assay, we studied the values of individual anions such as NO3−, NO2−, CH3COO−, SO32−, SO42−, PO43−, S2−, CO32−, OH−, F−, Cl−, Br−and I−. Fig. 2 shows that most of the tested ions could not lead to similar absorbance or color change as Cl−, Br−and I− did, even if the concentration of each type of anions were increased to 5 mM, CO32－and OH− were 0.1 mM, due to their larger solubility of silver salts (the Ksp is given in Table S1) [19,22], except for S2− (see Fig. S2). Thus, the detection of halide ions could be interfered by S2− because of its low solubility. However, such interference could be negligible under the condition that its concentration is very low, for instance, in mineral water or tap water. So in all, such selective experiment result highlights the remarkably specific identification of halide ions by our Ag–NCs hydrogel. 3.3. Sensitivity of Ag–NCs hydrogel toward halide ions The sensitivity of Ag–NCs hydrogel toward halides (Cl−, Br−and I−) was hence recognized by the UV–vis spectra. Firstly the optimizations about the sensing conditions such as Ag–NCs hydrogel volume and its volume ratio to halide ions (Cl−, Br−and I−) were conducted (Fig. S1), and the colorimetric determination was performed at room temperature. Two different volumes of 50 μL and 100 μL Ag–NCs hydrogel were used and the volume ratio of Ag–NCs hydrogel to 5 mM Cl− was kept at 2:1, 1:1, 1:2, 1:4, 1:6, 1:8 and 1:10. The intensity of the absorption peak in two systems at 408 nm decreased with increasing the volume ratio of Ag–NCs hydrogel to Cl−, together with the appearance of a new absorption peak at high wavelength. Eventually, the absorbance peak at 408 nm of the Ag–NCs hydrogel achieved a minimum and kept steady when the volume ratio was 1:8. Therefore, we chose a volume ratio of 1:8 of 50 μL Ag–NCs hydrogel to 5 mM Cl− in the following tests. To further investigate the sensitivity of our Ag–NCs hydrogel towards halide ions and to obtain the corresponding minimum detectable
2.3. Transmission electron microscopy (TEM) TEM images were taken on a JEOL TEM-2100 microscope. The Ag–NCs hydrogel after reaction with halides (Cl−, Br−and I−) were diluted to 2 mL using water, then the solutions were spotted on a carbon coated copper grid and dried in ambience. 2.4. Zeta (ζ)-potential Zeta (ζ)-potential measurements were performed by laser Doppler electrophoresis using a ZetaPALS (Brookhaven Instruments Corporation, U.S.A). The Ag–NCs hydrogel after reaction with halides (Cl−, Br−and I−) were diluted to 2 mL, then the Zeta (ζ)-potential of the solutions were measured. 2
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Fig. 2. a) UV–vis spectra and (b) selectivity of Ag–NCs hydrogel after interaction with different anions. The insets in a) are the photographs showing respective colorimetric responses.
Fig. 3. UV–vis spectra of Ag–NCs hydrogel upon the addition of different concentrations of halide ions.
concentration, we monitored the UV–vis spectra and color changes by varying the concentrations of Cl−, Br−and I− (Fig. 3). The results based on the UV–vis absorption can be classified into two stages, the absorption peak at 408 nm first slightly enhanced with increasing the concentration of halide ions (Cl−: 0–200 μM, Br−and I−: 0–50 μM), and then gradually damped together with a new absorption peak appeared at higher wavelength when reaching a certain concentration. This new peak might be ascribed to the aggregation of Ag–NCs in the presence of adsorbed ions and the adsorption of ions on the surfaces of Ag–NCs. Besides, Br−and I−were much more sensitive, in comparison to Cl−. The minimum detectable concentration was estimated to be 20 μM, 5 μM and 5 μM for Cl−, Br−and I−, respectively. These obtained minimum detectable concentration values were much lower than those of comparable anion sensors based on other nanomaterials reported
recently (Table S2) [19,21,23–26]. Furthermore, the UV–vis spectra responses were replenished by the observation of visible color changes of the Ag–NCs hydrogel. The surface color of hydrogel changed from yellow to dark green, to brown and to deep brown for Cl−, Br−, I−, respectively, which darken with increasing the concentration of Cl−, Br−and I− (Fig. 4). The minimum identifiable concentrations of halides by the colorimetric method as a cue were 200 μM for Cl−, 100 μM both for Br− and I−. Furthermore, the interaction of halide ions with Ag–NCs hydrogel was found to be time-dependent and UV–vis spectra were measured to evaluate the kinetic process of Ag–NCs hydrogel upon the addition of different halides, as displayed in Fig. S3. The change of peak intensity with reaction time showed that the reaction almost terminated within 10 min. Therefore, to ensure reproducibility and achieve rapid reionization, 5 min was 3
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Table 1 The zeta (ζ)-potential of Ag–NCs hydrogel before and after interaction with 200 μmol/L Cl−, 20 μmol/L Br− and 20 μmol/L for 5 min. Halides added
Cl− Br− I−
Zeta (ζ)-potential of Ag– NCs hydrogel (mV) Original
After addition of halides
34.49 34.49 34.49
28.55 29.20 31.22
tap water, two kinds of pure water and four kinds of mineral water without any pretreatment. Typical detection results are given in Fig. 5. We used the hygienic standard of bottled purified water for drinking in China (GB17324-2003) and the national standards for drinking water in China (GB5749-2006) as reference standards, where the concentration of Cl− should be not more than 6 mg L−1 (0.2 mM) and 250.0 mg L−1 (7 mM), respectively. Firstly, known concentration of Cl−solutions prepared based on the requirement of the two standards (0.2 mM and 7 mM) were measured by colorimetric method and UV–vis spectra. When 7 mM Cl− was added, the surface color of Ag–NCs hydrogel changed obviously from yellow to dark green and the absorption peak at 408 nm declined significantly together with a new absorption peak emerged at higher wavelength. Unfortunately, for the tap water, we could not obtain the exact UV–vis spectra result because there existed floccules in the diluted solution. However, we could still estimate the Cl− level by the colorimetric method that the color change of Ag–NCs hydrogel was less obvious than that of 7 mM Cl−. Therefore, we can reach the conclusion that the Cl− level in tap water was below the requirement of GB5749-2006. Similarly, when 0.2 mM Cl− was added, the color around the surface of Ag–NCs hydrogel changed to white visually and the intensity of the absorption peak slightly enhanced with increasing the concentration of halide ions when the concentration of Cl− was range from 0 to 200 μM (as displayed in Fig. 3). For the two kinds of pure water, no color change was observed and the intensity of the UV–vis absorption peak was lower than that of 0.2 mM Cl−. Thus, the Cl− level in two kinds of pure water met the requirement of GB17324-2003. Based on the above results, we directly applied the
Fig. 4. Photographs of colorimetric response of Ag–NCs hydrogels toward different concentrations of halide ions (Cl−, Br−and I−).
selected for detecting Cl−, Br−and I− in this work. 3.4. Application of Ag–NCs hydrogel in real water samples The measurement of Cl− would suffer from the disturbance by higher concentrations of Br−, I− and S2− owing to their superior affinity to silver, thus some pretreatments might be desired for real sample detection. However, fortunately, such interference was negligible in some cases if these anions concentrations were very low, such as tap water or pure water. Thus to verify the feasibility of our proposed sensing strategy for the analysis of halides in real water samples, the Ag–NCs hydrogel sensor was applied to measure the levels of Cl− in one
Fig. 5. UV–vis spectra and photographs of colorimetric response of Ag–NCs hydrogel after detection of Cl− in water samples. 4
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Fig. 6. TEM images of Ag–NCs hydrogels upon the addition of different concentrations of Cl−, Br−and I− for 5 min, a) 200 μM Cl−, b) 20 μM Br− and c) 20 μM I−.
Scheme 1. Scheme for the recognition mechanism of AgNCs hydrogel toward halide ions.
colorimetric method to estimate the Cl− level in four kinds of mineral water, as shown in Fig. 5c, there were no color change visually compared to that of 0.2 mM Cl−, this suggested that the Cl− levels in four kinds of mineral water met the requirement of GB17324-2003. To verify the reliability of our method, we used ion chromatography to measure those real water samples mentioned above. It suggested that the detection results by our method were consistent with that of ion chromatography (Table S3), suggesting that our system was practical and reliable.
than 1 nm appeared in the TEM images. According to those previous profound researches the above mentioned together with our experimental results, we speculated the following mechanism of halide-induced aggregation resulting in the changes of UV–vis absorption intensity and color of Ag–NCs hydrogel, as illustrated in Scheme 1. Since two L-cysteine molecules might be crosslinked together by the S–S bond [32], more Ag–NCs exposed into the solution were activated when Cl−, Br−and I− ions were added into the Ag–NCs hydrogel, leading to the increase of UV absorption intensity. On the other hand, the three ions would extremely easy to adsorb onto the surface of Ag–NCs to form a monolayer of AgX (AgCl, AgBr, and AgI) on account of the higher affinity of Ag-X bond and the lower Ksp of AgX compounds. Meanwhile, the corrosiveness of halides would also etch a small number of Ag–NCs into smaller Ag–NCs. It was known that the stability of Ag–NCs depended on their surface electric double layer and coulomb repulsion. The aggregation of Ag–NCs appeared with the increase of the halides concentration. This is due to the chemisorption of halides to the surface Ag–NCs and the surface charge of Ag–NCs was neutralized, as a result, the van der Waals attractive forces increased between Ag–NCs, thus triggering the initiation of aggregation, which was accompanied by the changes of UV–vis spectra and the color responses [19].
3.5. Recognition mechanism of Ag–NCs hydrogel toward halide ions The sensitive identification of halide ions by Ag–NCs hydrogel rests with the unique reactions between halides and Ag atoms, as well as the oxidative etching and aggregation of Ag–NCs induced by halides. To date, there have been several reports about the mechanism of halides recognition by the Ag nanoparticles. For example, Espinoza et al. [27] elucidated comprehensively the kinetics of halide-induced disintegration and aggregation of Ag nanoparticles. Besides, Baalousha [28], Li [29,30] and Huynh [31] reported the chloride-induced surface oxidation, dissolution and aggregation process of Ag nanoparticles. Herein, more analysis were performed by TEM and Zeta (ζ)-potential measurements to clearly understand the recognition mechanism. The original Ag–NCs hydrogel possessed positive charges and after reaction with 200 μM Cl−, 20 μM Br−and 20 μM I− for 5 min, the zeta (ζ)potential of AgNCs hydrogel decreased from 34.49 mV to 28.55 mV, 29.20 mV and 31.22 mV, respectively (Table 1). Meanwhile, the Ag–NCs size increased from original 1.9 nm [20] to more than a dozen nanometers (Fig. 6). Besides, there were some new particles smaller
4. Conclusion To summarize, we have developed a convenient colorimetric method for halide ions identification based on the Ag–NCs hydrogel. This protocol highly exhibited selectivity toward halides over other tested anions and it was applied successfully to the recognition of 5
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chloride in real water samples. The recognition mechanism was also proposed in this paper on the basis of our optical, TEM and zeta potential results, which was mainly according to the special reactions between halides and Ag atoms. Considering the important roles of halide ions in the areas of the environment, industrial processes, human health and so on, such method can be likely to have potential applications, such as the immediate analysis of chloride ions in water, the distinguishment of edible salt from industrial salt and so forth.
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