Accepted Manuscript Virgin silver nanoparticles as colorimetric nanoprobe for simultaneous detection of iodide and bromide ion in aqueous medium Shilpa Bothra, Rajender Kumar, Ranjan K. Pati, Anil Kuwar, Heung-Jin Choi, Suban K. Sahoo PII: DOI: Reference:
S1386-1425(15)00531-4 http://dx.doi.org/10.1016/j.saa.2015.04.059 SAA 13610
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
4 March 2015 11 April 2015 14 April 2015
Please cite this article as: S. Bothra, R. Kumar, R.K. Pati, A. Kuwar, H-J. Choi, S.K. Sahoo, Virgin silver nanoparticles as colorimetric nanoprobe for simultaneous detection of iodide and bromide ion in aqueous medium, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), doi: http://dx.doi.org/10.1016/j.saa. 2015.04.059
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 proof before it is published in its final 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.
Virgin silver nanoparticles as colorimetric nanoprobe for simultaneous detection of iodide and bromide ion in aqueous medium Shilpa Bothraa, Rajender Kumara, Ranjan K Patia, Anil Kuwarb, Heung-Jin Choic and Suban K Sahooa,c,* a
Department of Applied Chemistry, SV National Institute of Technology (SVNIT), Surat-395007,
India. b
c
School of Chemical Sciences, North Maharashtra University, Jalgaon (MS), India. Department of Applied Chemistry, Kyungpook National University, Daegu, 702-701, Korea.
Abstract A simple colorimetric nanoprobe based on virgin silver nanoparticles (AgNPs) was developed for the selective detection of iodide and bromide ions via aggregation and antiaggregation mechanism.
With addition of I- ions, virgin AgNPs, in presence of Fe3+, showed
perceptible color change from yellow to colourless along with disappearance of surface plasmon resonance (SPR) band of AgNPs at 400 nm. But in presence of Cr3+, AgNPs turned yellow upon addition of I-and Br- anions. The developed virgin AgNPs probe showed high specificity and selectivity with the detection limits down to 0.32 µM and 1.32 µM for I- ions via two different mechanistic routes. Also, the designed probe detects Br- with a detection limit down to 1.67 µM. Keywords: Colorimetric sensor, AgNPs, iodide, bromide. *Corresponding author (Dr SK Sahoo): E-mail:[email protected]; Mob: +91-2612201814.
1
1. Introduction Inorganic anions such as chloride, bromide and iodide ions are of immense biomedical significance. They play significant role in ensuring the good human health, besides of environmental, industrial and biological significance [1-3]. Iodide ions help to maintain controlled release of thyroid hormone into the bloodstream. The imbalance in thyroid release results in several metabolic malfunctions and disorders such as anxiety and nervous agitation. Its deficiency may cause spontaneous abortion, increased infant mortality and mental defects [4-6]. Endocrinopathy such as hypo and hyper thyrodism are the most common disorders among humans caused due to iodine deficiency. These disorders are one of the leading causes of the large number of mental retardation cases, which can be prevented by ensuring optimal iodide intake [7]. The bromide ions play an important role in environmental, biological and chemical systems [8]. The toxicity caused by Br- may results in bromism and skin eruptions [9]. Therefore, the sensitive and selective detection of halide ions is of utmost clinical significance. Traditional analytical methods used for the analysis of iodide ion involve complicated sample preparation steps or needs sophisticated instrumentation, such as ion chromatography [10], capillary electrophoresis [11] and indirect atomic absorption spectrometry [12]. Iodide ion on account of its large ionic radii, low charge density, and low hydrogen-bonding ability pose a serious challenge for its correct recognition and detection [13, 14]. Considering biomedical significance of the iodide and bromide ions, problems encountered in their selective and sensitive analysis using traditional methods have motivated the development of colorimetric and fluoremetric assays for reliable detection and determination of these ions [15-19]. In the reported colorimetric and fluorescence methods, emphasis has been on the development of organic 2
molecules functionalized nanoprobes for the detection of iodide ions. Among the several nanoprobes, silver nanoparticles (AgNPs), due to their inherent distinct surface physiochemical properties are excellent probes for analysis and detection of molecules/ions of interest [20, 21]. AgNPs possess localized surface plasmon resonance (LSPR) which is one of the most powerful tools for real-time monitoring of various biological and chemical analytes of interest [22, 23]. The strong LSPR shown by AgNPs, allows sensitive colorimetric detection of analytes with minimal material consumption. The size, shape, interparticle distance and the nature of the surrounding media are of vital significance in formation of the SPR band of AgNPs and sensing of target analytes [24-26]. Recently, we have reported a simple colorimetric method by using unmodified/virgin AgNPs for the selective detection of I- with the detection limit down to 0.24 µM [27]. Continuing our research on development of unmodified nanoprobes, herein, we reports two novel methods for the sensitive and selective detection and analysis of iodide and bromide ions using virgin AgNPs. The probe showed high specificity towards iodide ions with the striking color change from yellow to colorless in presence of Fe3+ while in presence of Cr3+ ions probe showed change in color from orangish-red to yellow. Thus, the probe was successful for the simultaneous detection and analysis of Br- and I- ions. 2. Experimental section 2.1 Materials and instrumentations All chemicals used were of analytical grade or of the highest purity available. All glasswares were cleaned with aqua regia and rinsed with Milli-Q water prior to use. The DI water (18.3 MΩ•cm) was produced from the Millipore water purification system. Silver nitrate 3
(AgNO3) and sodium citrate trihydrate were purchased from Finar India Ltd. Sodium borohydride (NaBH4) was purchased from Merck, India. All the metal ions used for the experiments were purchased from Rankem Pvt. Ltd., India. Stock solutions of all the metal ions (1 mM and 0.1 mM) and inorganic anions (K/Na salts, 1 mM and 0.1 mM) were freshly prepared in Milli-Q water. The freshly prepared stock solutions were used for all spectroscopic studies after appropriate dilution with water. UV-Vis absorption spectra were taken to monitor the changes with and without analytes in AgNPs solution at room temperature in DI water using a UV-Vis spectrophotometer (Cary 50 Varian) and a quartz cell of 1 cm path length. Philips CM 200 transmission electron microscope (TEM) operated at 200 kV was used to obtain the morphology of synthesized virgin AgNPs by dropping the AgNPs solution on the carbon copper grid and drying it at room temperature. The dynamic light scattering (DLS) data were obtained by using a Malvern Zetasize Nano (Malvern, UK). 2.2 Synthesis of virgin AgNPs Virgin AgNPs were prepared by the chemical reduction approach and using citrate as stabilizing agent [28]. Briefly, 3.5 mM freshly prepared aqueous solution of NaBH4 (1.0 ml) was dropwise added to the mixture of 1 mM aqueous solution of AgNO3 (20.0 ml) and 0.1 M sodium citrate solution (0.5 ml) under vigorous stirring. After the reaction under vigorous stirring for 2 h, the product was stored in a brown bottle at room temperature for 24 h prior to further use. 2.3 Interaction of analytes with virgin AgNPs The synthesized virgin nanoparticles were diluted with water prior to use. The concentration of virgin AgNPs used throughout all the experiments was 69.4 µM. The 4
colorimetric response of virgin AgNPs towards different metal ions and anions was first tested by adding metal ions solution of 250 µM and 25 µM in water. Then, at optimized concentration of Fe3+ [12.5 µM] and Cr3+ [125 µM], different anions were added at 125 µM/12.5 µM to study the selectivity. For spectroscopic titrations, 2 mL of AgNPs containing Fe3+ [12.5 µM] or Cr3+ [125 µM] was taken directly into the cuvette and then spectra were recorded after each incremental addition of selective anions. 3. Results and discussion 3.1. Characterization of AgNPs and its interaction with metal ions and anions The yellow colored AgNPs with SPR band at 400 nm were obtained by reducing AgNO3 with NaBH4 and citrate ions as stabilizing agent. The AgNPs were transparent and clear with size of ~10-15 nm (Fig. 1a). The presence of negatively charged citrate ions in the nanopaticle surface results in formation of an electrostatic double layer in the presence of counter ions and thus, nanopaticles are easily dispersed. From the DLS measurement of the virgin AgNPs, the average hydrodynamic diameter of ~10 nm was obtained (Fig. 1b). Then, the effects of individual metal ions and anions on AgNPs solution was tested by adding varying concentrations (250 µM and 25 µM) (Fig. 1S). Among the tested metal ions, Fe3+, Cr3+ and Hg2+ showed perceptible changes in the SPR band and color of the AgNPs. At 250 µM, on addition of Fe3+ and Hg2+, there was an instantaneous change in the color of AgNPs solution from yellow to colorless and disappearance of AgNPs absorption band at 400 nm (Fig. 1SIa, b). However, the yellow colored solution of AgNPs turned to orangish-red along with formation of a new redshifted absorption band at 508 nm in the presence of Cr3+ at 250 µM [29]. The aggregation may be caused by the inter-nanoparticles complexation of metal ions with the carboxylic-O atoms of 5
citrate and/or the oxidation of Ag0 to Ag+ especially in the presence of Hg2+ [30]. At lower concentration of 25 µM, no change was observed on addition of Fe3+ ions (Fig. 1S IIa,b). However, AgNPs on interaction with Cr3+/Hg2+ at 25 µM showed both color and spectral changes (Fig. 1S IIa,b). Further, upon addition of anions only [250 µM and 25 µM] to the virgin AgNPs, no characteristic color or spectral change was observed. A slight decrease in absorbance intensity was observed in case of iodide ions which may be attributed due to the adsorption of Ion AgNPs surface [27]. The slight increase in the size of virgin AgNPs as confirmed by DLS data (Fig. 1c) supported the adsorption of iodide on the NPs surface.
Fig. 1. (a) TEM image and (b) DLS data of virgin AgNPs, (c) DLS of AgNPs in presence of only I- , (d) the presence of both I- [125 µM] and Fe3+ [12.5 µM] in aqueous medium. 6
3.2. Interaction of virgin AgNPs and anions in presence of Fe3+ or Cr3+ The interaction of different inorganic anions, such as I-, F-, Cl-, Br-, HSO4- , H2PO4-, NO3-, NO2-, S2O82- and AcO- [125 µM] with the virgin AgNPs in the presence of Fe3+ [12.5 µM] was next investigated to obtain any detectable color/spectral changes. It was observed that the addition of I- to the AgNPs solution in the presence of Fe3+ caused discoloration along with the disappearance of SPR band (Fig. 2I (a and b)). The DLS data confirm that the decolourisation of AgNPs is caused due to the aggregation, as the average size of NPs increased to ~ 142 nm from ~10-15 nm (Fig. 1d). In contrast, in the presence of Cr3+, prior to the addition of different anions [12.5 µM], the color of virgin AgNPs was orangish-red. Addition of Br- and I-, showed back to yellow coloration along with the disappearance of band at 508 nm and enhancement in the peak intensity at 400 nm (Fig. 2 IIa,b). This clearly indicates that in the simultaneous presence of Cr3+ and Br-/I-, the aggregation of AgNPs induced by Cr3+ would be prevented because of the preferred complex formation occurred between Cr3+ and the anions Br-/I- added. Such antiaggregation mechanism based on preferred complexation mode was adopted for the detection of I- using functionalized AuNPs [31]. The anti-aggregation sensing mechanism was supported by DLS results, the increase in the size of AgNPs in the presence of Cr3+ reverted back to the virgin AgNPs upon addition of Br-/I- (Fig. 2S). The above results clearly delineated that the virgin AgNPs was selective to I- ion in the presence of Fe3+ whereas to Br-/I- in the presence of Cr3+.
7
Fig. 2. Colorimetric I(a) and spectral I(b) response of virgin AgNPs in presence of Fe3+ [12.5 µM] and anions [125 µM]. Also, colorimetric II(a) and spectral II(b) response of virgin AgNPs in presence of Cr3+ [125 µM] and anions [12.5 µM]. 3.3. Practical utility of the nanoprobe The colorimetry detection ability of I- in presence of Fe3+ and Cr3+ by AgNPs was investigated under a competitive environment containing equimolar amounts of other anions at optimized concentrations. No obvious interference was observed in presence of other anions indicating the high specificity of the probe for iodide and bromide detection (Fig. 3S). Further, the sensitivity of the virgin AgNPs towards the selective anions was determined by measuring the absorbance changes of the SPR band at 400 nm by varying the concentrations of I- in presence of Fe3+ [12.5 µM] whereas at 508 nm by varying I-/Br- concentrations in presence of Cr3+ [125 µM]. The intensity of the SPR band of AgNPs was decreased with an increase in the Iconcentrations in presence of Fe3+ (Fig. 4Sa). However, incremental addition of I- and Br- ions in presence of Cr3+ showed decrease in the intensity of the secondary band formed on addition of 8
Cr3+ at ~508 nm along with the attainment of SPR band of AgNPs at 400 nm (Fig. 5Sa and 6Sa). From the UV-Vis absorption titrations, the limit of detection (LOD) of the nanoprobe in presence of Fe3+/Cr3+ was determined for selective anions. First, the absorption measurements of ten blank samples were taken to calculate the relative standard deviation (σ). The calibration curves (∆A vs [I-]) were plotted (Fig. 4Sb, 5Sb and 6Sb), and then the obtained slope was used to calculate the LOD. The LOD for iodide ions was calculated to be 0.32 µM in presence of Fe3+ and 1.32 µM in presence of Cr3+, using the standard relationship, i.e. LOD = 3.3σ/slope. Whereas, the LOD calculated for Br- in presence of Cr3+ was 1.67 µM. The obtained LOD was found to be comparable with the reported sensors as summarized in Table S1. 4. Conclusions In conclusion, we have developed a simple, rapid, inexpensive and sensitive nanoprobe using virgin AgNPs for the selective detection of I- and Br- from aqueous medium based on the aggregation and anti-aggregation mechanism driven by the metal ions (Fe3+/Cr3+). In the presence of Fe3+, the recognition of I- was confirmed by the remarkable color change from yellow to colorless along with the disappearance of SPR band at 400 nm with the detection limit down to 0.32 µM. However, the I- and Br- was colorimetrically detected by AgNPs in presence of Cr3+ via anti-aggregation mechanism with the detection limit of 1.32 µM and 1.67 µM, respectively. Acknowledgments This work was made possible by a grant from the DST, New Delhi (SR/S1/IC54/2012).We would also like to thank SAIF, Indian Institute of Technology (IIT), Bombay for providing TEM facility. 9
References [1] Q. Yang, Q. Tan, K. Zhou, K. Xu, X. Hou, Direct detection of mercury in vapor and aerosol from chemical atomization and nebulization at ambient temperature: exploiting the flame atomic absorption spectrometer, Journal of Analytical Atomic Spectrometry, 20 (2005) 760-762. [2] A. Ohashi, H. Ito, C. Kanai, H. Imura, K. Ohashi, Cloud point extraction of iron(III) and vanadium(V) using 8-quinolinol derivatives and Triton X-100 and determination of 10(7)moldm(-3) level iron(III) in riverine water reference by a graphite furnace atomic absorption spectroscopy, Talanta, 65 (2005) 525-530. [3] T. Katsu, Y. Mori, N. Matsuka, Y. Gomita, Potentiometric flow injection determination of serum bromide in patients with epilepsy, Journal of pharmaceutical and biomedical analysis, 15 (1997) 1829-1832. [4] L. Rong, T. Takeuchi, Determination of iodide in seawater and edible salt by microcolumn liquid chromatography with poly(ethylene glycol) stationary phase, Journal of chromatography. A, 1042 (2004) 131-135. [5] J. Jakmunee, K. Grudpan, Flow injection amperometry for the determination of iodate in iodized table salt, Analytica Chimica Acta, 438 (2001) 299-304. [6] F. Delange, B. de Benoist, E. Pretell, J.T. Dunn, Iodine deficiency in the world: where do we stand at the turn of the century?, Thyroid : official journal of the American Thyroid Association, 11 (2001) 437-447. [7] L. Patrick, Iodine: deficiency and therapeutic considerations, Alternative medicine review : a journal of clinical therapeutic, 13 (2008) 116-127. [8] J. Shao, H. Lin, H.-K. Lin, A simple and efficient colorimetric anion sensor based on a 10
thiourea group in DMSO and DMSO–water and its real-life application, Talanta, 75 (2008) 1015-1020. [9] B.R. Taylor, R. Sosa, W.J. Stone, Bromide Toxicity from Consumption of Dead Sea Salt, The American Journal of Medicine, 123 e11-e12. [10] Z. Huang, Z. Zhu, Q. Subhani, W. Yan, W. Guo, Y. Zhu, Simultaneous determination of iodide and iodate in povidone iodine solution by ion chromatography with homemade and exchange capacity controllable columns and column-switching technique, Journal of Chromatography A, 1251 (2012) 154-159. [11] K. Ito, T. Ichihara, H. Zhuo, K. Kumamoto, A.R. Timerbaev, T. Hirokawa, Determination of trace iodide in seawater by capillary electrophoresis following transient isotachophoretic preconcentration: Comparison with ion chromatography, Anal Chim Acta, 497 (2003) 67-74. [12] M.C. Yebra, M.H. Bollaín, A simple indirect automatic method to determine total iodine in milk products by flame atomic absorption spectrometry, Talanta, 82 (2010) 828-833. [13] Z. Rodriguez-Docampo, S.I. Pascu, S. Kubik, S. Otto, Noncovalent interactions within a synthetic receptor can reinforce guest binding, J Am Chem Soc, 128 (2006) 11206-11210. [14] N. Singh, D.O. Jang, Benzimidazole-based tripodal receptor: Highly selective fluorescent chemosensor for iodide in aqueous solution, Organic letters, 9 (2007) 1991-1994. [15] B. Ma, F. Zeng, F. Zheng, S. Wu, A Fluorescence Turn-on Sensor for Iodide Based on a Thymine–HgII–Thymine Complex, Chemistry – A European Journal, 17 (2011) 14844-14850. [16] J. Zhang, X. Xu, C. Yang, F. Yang, X. Yang, Colorimetric Iodide Recognition and Sensing by Citrate-Stabilized Core/Shell Cu@Au Nanoparticles, Analytical Chemistry, 83 (2011) 39113917. 11
[17] H. Li, C. Han, L. Zhang, Synthesis of cadmium selenide quantum dots modified with thiourea type ligands as fluorescent probes for iodide ions, Journal of Materials Chemistry, 18 (2008) 4543-4548. [18] H.A. Ho, M. Leclerc, New Colorimetric and Fluorometric Chemosensor Based on a Cationic Polythiophene Derivative for Iodide-Specific Detection, Journal of the American Chemical Society, 125 (2003) 4412-4413. [19] D.Y. Lee, N. Singh, M.J. Kim, D.O. Jang, Chromogenic and Fluorescent Recognition of Iodide with a Benzimidazole-Based Tripodal Receptor, Organic letters, 13 (2011) 3024-3027. [20] M.K. Rai, P. Asthana, S.K. Singh, V.S. Jaiswal, U. Jaiswal, The encapsulation technology in fruit plants--A review, Biotechnology advances, 27 (2009) 671-679. [21] V.K. Sharma, R.A. Yngard, Y. Lin, Silver nanoparticles: Green synthesis and their antimicrobial activities, Advances in colloid and interface science, 145 (2009) 83-96. [22] X.D. Hoa, A.G. Kirk, M. Tabrizian, Towards integrated and sensitive surface plasmon resonance biosensors: A review of recent progress, Biosensors & bioelectronics, 23 (2007) 151160. [23] A. Shalabney, I. Abdulhalim, Sensitivity-enhancement methods for surface plasmon sensors, Laser & Photonics Reviews, 5 (2011) 571-606. [24] K.L. Kelly, E. Coronado, L.L. Zhao, G.C. Schatz, The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment, The Journal of Physical Chemistry B, 107 (2002) 668-677. [25] C. Noguez, Surface Plasmons on Metal Nanoparticles: The Influence of Shape and Physical Environment, 111 (2007) 3806-3819. 12
[26] S. Bothra, J. Sholanki, S.K. Sahoo, Functionalized silver nanoparticles as chemosensor for pH, Hg2+and Fe3+ in aqueous medium, Sensor Actuat B Chem, 188 (2013) 937-943. [27] S. Bothra, R. Kumar, A. Kuwar, N. Singh, S.K. Sahoo, Cu2+-driven selective colorimetric sensing of iodide ions and AND logic gate using citrate-capped AgNPs, Materials Letters, 145 (2015) 34-36. [28] J. An, B. Tang, X. H. Ning, J. Zhou , S. P. Xu, B. Zhao , W. Q. Xu, C. Corredor, J. R. Lombardi, Photoinduced Shape Evolution: From Triangular to Hexagonal Silver Nanoplates, J. Phys. Chem. C, 111 (2007) 18055. [29] M. Elavarasi, M.L. Paul, A. Rajeshwari, N. Chandrasekaran, A.B. Mandal, A. Mukherjee, Studies on fluorescence determination of nanomolar Cr(III) in aqueous solutions using unmodified silver nanoparticles, Analytical Methods, 4 (2012) 3407-3412. [30] A. Apilux, W. Siangproh, N. Praphairaksit, O. Chailapakul, Simple and rapid colorimetric detection of Hg(II) by a paper-based device using silver nanoplates, Talanta, 97 (2012) 388-394. [31] L. Chen, W. Lu, X. Wang, L. Chen, A highly selective and sensitive colorimetric sensor for iodide detection based on anti-aggregation of gold nanoparticles, Sensor Actuat B Chem, 182 (2013) 482-488.
13
Graphical Abstract
14
Highlights •
Colorimetric sensor driven by Fe3+/Cr3+ for I– and Br– was developed using AgNPs.
•
Sensor exhibited good I– and Br– sensitivity with the detection limit down to micromolar rage.
•
Sensing of I– and Br– was achieved by aggregation or anti-aggregation of AgNPs.
15
S1386-1425(15)00531-4 http://dx.doi.org/10.1016/j.saa.2015.04.059 SAA 13610
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
4 March 2015 11 April 2015 14 April 2015
Please cite this article as: S. Bothra, R. Kumar, R.K. Pati, A. Kuwar, H-J. Choi, S.K. Sahoo, Virgin silver nanoparticles as colorimetric nanoprobe for simultaneous detection of iodide and bromide ion in aqueous medium, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), doi: http://dx.doi.org/10.1016/j.saa. 2015.04.059
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 proof before it is published in its final 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.
Virgin silver nanoparticles as colorimetric nanoprobe for simultaneous detection of iodide and bromide ion in aqueous medium Shilpa Bothraa, Rajender Kumara, Ranjan K Patia, Anil Kuwarb, Heung-Jin Choic and Suban K Sahooa,c,* a
Department of Applied Chemistry, SV National Institute of Technology (SVNIT), Surat-395007,
India. b
c
School of Chemical Sciences, North Maharashtra University, Jalgaon (MS), India. Department of Applied Chemistry, Kyungpook National University, Daegu, 702-701, Korea.
Abstract A simple colorimetric nanoprobe based on virgin silver nanoparticles (AgNPs) was developed for the selective detection of iodide and bromide ions via aggregation and antiaggregation mechanism.
With addition of I- ions, virgin AgNPs, in presence of Fe3+, showed
perceptible color change from yellow to colourless along with disappearance of surface plasmon resonance (SPR) band of AgNPs at 400 nm. But in presence of Cr3+, AgNPs turned yellow upon addition of I-and Br- anions. The developed virgin AgNPs probe showed high specificity and selectivity with the detection limits down to 0.32 µM and 1.32 µM for I- ions via two different mechanistic routes. Also, the designed probe detects Br- with a detection limit down to 1.67 µM. Keywords: Colorimetric sensor, AgNPs, iodide, bromide. *Corresponding author (Dr SK Sahoo): E-mail:[email protected]; Mob: +91-2612201814.
1
1. Introduction Inorganic anions such as chloride, bromide and iodide ions are of immense biomedical significance. They play significant role in ensuring the good human health, besides of environmental, industrial and biological significance [1-3]. Iodide ions help to maintain controlled release of thyroid hormone into the bloodstream. The imbalance in thyroid release results in several metabolic malfunctions and disorders such as anxiety and nervous agitation. Its deficiency may cause spontaneous abortion, increased infant mortality and mental defects [4-6]. Endocrinopathy such as hypo and hyper thyrodism are the most common disorders among humans caused due to iodine deficiency. These disorders are one of the leading causes of the large number of mental retardation cases, which can be prevented by ensuring optimal iodide intake [7]. The bromide ions play an important role in environmental, biological and chemical systems [8]. The toxicity caused by Br- may results in bromism and skin eruptions [9]. Therefore, the sensitive and selective detection of halide ions is of utmost clinical significance. Traditional analytical methods used for the analysis of iodide ion involve complicated sample preparation steps or needs sophisticated instrumentation, such as ion chromatography [10], capillary electrophoresis [11] and indirect atomic absorption spectrometry [12]. Iodide ion on account of its large ionic radii, low charge density, and low hydrogen-bonding ability pose a serious challenge for its correct recognition and detection [13, 14]. Considering biomedical significance of the iodide and bromide ions, problems encountered in their selective and sensitive analysis using traditional methods have motivated the development of colorimetric and fluoremetric assays for reliable detection and determination of these ions [15-19]. In the reported colorimetric and fluorescence methods, emphasis has been on the development of organic 2
molecules functionalized nanoprobes for the detection of iodide ions. Among the several nanoprobes, silver nanoparticles (AgNPs), due to their inherent distinct surface physiochemical properties are excellent probes for analysis and detection of molecules/ions of interest [20, 21]. AgNPs possess localized surface plasmon resonance (LSPR) which is one of the most powerful tools for real-time monitoring of various biological and chemical analytes of interest [22, 23]. The strong LSPR shown by AgNPs, allows sensitive colorimetric detection of analytes with minimal material consumption. The size, shape, interparticle distance and the nature of the surrounding media are of vital significance in formation of the SPR band of AgNPs and sensing of target analytes [24-26]. Recently, we have reported a simple colorimetric method by using unmodified/virgin AgNPs for the selective detection of I- with the detection limit down to 0.24 µM [27]. Continuing our research on development of unmodified nanoprobes, herein, we reports two novel methods for the sensitive and selective detection and analysis of iodide and bromide ions using virgin AgNPs. The probe showed high specificity towards iodide ions with the striking color change from yellow to colorless in presence of Fe3+ while in presence of Cr3+ ions probe showed change in color from orangish-red to yellow. Thus, the probe was successful for the simultaneous detection and analysis of Br- and I- ions. 2. Experimental section 2.1 Materials and instrumentations All chemicals used were of analytical grade or of the highest purity available. All glasswares were cleaned with aqua regia and rinsed with Milli-Q water prior to use. The DI water (18.3 MΩ•cm) was produced from the Millipore water purification system. Silver nitrate 3
(AgNO3) and sodium citrate trihydrate were purchased from Finar India Ltd. Sodium borohydride (NaBH4) was purchased from Merck, India. All the metal ions used for the experiments were purchased from Rankem Pvt. Ltd., India. Stock solutions of all the metal ions (1 mM and 0.1 mM) and inorganic anions (K/Na salts, 1 mM and 0.1 mM) were freshly prepared in Milli-Q water. The freshly prepared stock solutions were used for all spectroscopic studies after appropriate dilution with water. UV-Vis absorption spectra were taken to monitor the changes with and without analytes in AgNPs solution at room temperature in DI water using a UV-Vis spectrophotometer (Cary 50 Varian) and a quartz cell of 1 cm path length. Philips CM 200 transmission electron microscope (TEM) operated at 200 kV was used to obtain the morphology of synthesized virgin AgNPs by dropping the AgNPs solution on the carbon copper grid and drying it at room temperature. The dynamic light scattering (DLS) data were obtained by using a Malvern Zetasize Nano (Malvern, UK). 2.2 Synthesis of virgin AgNPs Virgin AgNPs were prepared by the chemical reduction approach and using citrate as stabilizing agent [28]. Briefly, 3.5 mM freshly prepared aqueous solution of NaBH4 (1.0 ml) was dropwise added to the mixture of 1 mM aqueous solution of AgNO3 (20.0 ml) and 0.1 M sodium citrate solution (0.5 ml) under vigorous stirring. After the reaction under vigorous stirring for 2 h, the product was stored in a brown bottle at room temperature for 24 h prior to further use. 2.3 Interaction of analytes with virgin AgNPs The synthesized virgin nanoparticles were diluted with water prior to use. The concentration of virgin AgNPs used throughout all the experiments was 69.4 µM. The 4
colorimetric response of virgin AgNPs towards different metal ions and anions was first tested by adding metal ions solution of 250 µM and 25 µM in water. Then, at optimized concentration of Fe3+ [12.5 µM] and Cr3+ [125 µM], different anions were added at 125 µM/12.5 µM to study the selectivity. For spectroscopic titrations, 2 mL of AgNPs containing Fe3+ [12.5 µM] or Cr3+ [125 µM] was taken directly into the cuvette and then spectra were recorded after each incremental addition of selective anions. 3. Results and discussion 3.1. Characterization of AgNPs and its interaction with metal ions and anions The yellow colored AgNPs with SPR band at 400 nm were obtained by reducing AgNO3 with NaBH4 and citrate ions as stabilizing agent. The AgNPs were transparent and clear with size of ~10-15 nm (Fig. 1a). The presence of negatively charged citrate ions in the nanopaticle surface results in formation of an electrostatic double layer in the presence of counter ions and thus, nanopaticles are easily dispersed. From the DLS measurement of the virgin AgNPs, the average hydrodynamic diameter of ~10 nm was obtained (Fig. 1b). Then, the effects of individual metal ions and anions on AgNPs solution was tested by adding varying concentrations (250 µM and 25 µM) (Fig. 1S). Among the tested metal ions, Fe3+, Cr3+ and Hg2+ showed perceptible changes in the SPR band and color of the AgNPs. At 250 µM, on addition of Fe3+ and Hg2+, there was an instantaneous change in the color of AgNPs solution from yellow to colorless and disappearance of AgNPs absorption band at 400 nm (Fig. 1SIa, b). However, the yellow colored solution of AgNPs turned to orangish-red along with formation of a new redshifted absorption band at 508 nm in the presence of Cr3+ at 250 µM [29]. The aggregation may be caused by the inter-nanoparticles complexation of metal ions with the carboxylic-O atoms of 5
citrate and/or the oxidation of Ag0 to Ag+ especially in the presence of Hg2+ [30]. At lower concentration of 25 µM, no change was observed on addition of Fe3+ ions (Fig. 1S IIa,b). However, AgNPs on interaction with Cr3+/Hg2+ at 25 µM showed both color and spectral changes (Fig. 1S IIa,b). Further, upon addition of anions only [250 µM and 25 µM] to the virgin AgNPs, no characteristic color or spectral change was observed. A slight decrease in absorbance intensity was observed in case of iodide ions which may be attributed due to the adsorption of Ion AgNPs surface [27]. The slight increase in the size of virgin AgNPs as confirmed by DLS data (Fig. 1c) supported the adsorption of iodide on the NPs surface.
Fig. 1. (a) TEM image and (b) DLS data of virgin AgNPs, (c) DLS of AgNPs in presence of only I- , (d) the presence of both I- [125 µM] and Fe3+ [12.5 µM] in aqueous medium. 6
3.2. Interaction of virgin AgNPs and anions in presence of Fe3+ or Cr3+ The interaction of different inorganic anions, such as I-, F-, Cl-, Br-, HSO4- , H2PO4-, NO3-, NO2-, S2O82- and AcO- [125 µM] with the virgin AgNPs in the presence of Fe3+ [12.5 µM] was next investigated to obtain any detectable color/spectral changes. It was observed that the addition of I- to the AgNPs solution in the presence of Fe3+ caused discoloration along with the disappearance of SPR band (Fig. 2I (a and b)). The DLS data confirm that the decolourisation of AgNPs is caused due to the aggregation, as the average size of NPs increased to ~ 142 nm from ~10-15 nm (Fig. 1d). In contrast, in the presence of Cr3+, prior to the addition of different anions [12.5 µM], the color of virgin AgNPs was orangish-red. Addition of Br- and I-, showed back to yellow coloration along with the disappearance of band at 508 nm and enhancement in the peak intensity at 400 nm (Fig. 2 IIa,b). This clearly indicates that in the simultaneous presence of Cr3+ and Br-/I-, the aggregation of AgNPs induced by Cr3+ would be prevented because of the preferred complex formation occurred between Cr3+ and the anions Br-/I- added. Such antiaggregation mechanism based on preferred complexation mode was adopted for the detection of I- using functionalized AuNPs [31]. The anti-aggregation sensing mechanism was supported by DLS results, the increase in the size of AgNPs in the presence of Cr3+ reverted back to the virgin AgNPs upon addition of Br-/I- (Fig. 2S). The above results clearly delineated that the virgin AgNPs was selective to I- ion in the presence of Fe3+ whereas to Br-/I- in the presence of Cr3+.
7
Fig. 2. Colorimetric I(a) and spectral I(b) response of virgin AgNPs in presence of Fe3+ [12.5 µM] and anions [125 µM]. Also, colorimetric II(a) and spectral II(b) response of virgin AgNPs in presence of Cr3+ [125 µM] and anions [12.5 µM]. 3.3. Practical utility of the nanoprobe The colorimetry detection ability of I- in presence of Fe3+ and Cr3+ by AgNPs was investigated under a competitive environment containing equimolar amounts of other anions at optimized concentrations. No obvious interference was observed in presence of other anions indicating the high specificity of the probe for iodide and bromide detection (Fig. 3S). Further, the sensitivity of the virgin AgNPs towards the selective anions was determined by measuring the absorbance changes of the SPR band at 400 nm by varying the concentrations of I- in presence of Fe3+ [12.5 µM] whereas at 508 nm by varying I-/Br- concentrations in presence of Cr3+ [125 µM]. The intensity of the SPR band of AgNPs was decreased with an increase in the Iconcentrations in presence of Fe3+ (Fig. 4Sa). However, incremental addition of I- and Br- ions in presence of Cr3+ showed decrease in the intensity of the secondary band formed on addition of 8
Cr3+ at ~508 nm along with the attainment of SPR band of AgNPs at 400 nm (Fig. 5Sa and 6Sa). From the UV-Vis absorption titrations, the limit of detection (LOD) of the nanoprobe in presence of Fe3+/Cr3+ was determined for selective anions. First, the absorption measurements of ten blank samples were taken to calculate the relative standard deviation (σ). The calibration curves (∆A vs [I-]) were plotted (Fig. 4Sb, 5Sb and 6Sb), and then the obtained slope was used to calculate the LOD. The LOD for iodide ions was calculated to be 0.32 µM in presence of Fe3+ and 1.32 µM in presence of Cr3+, using the standard relationship, i.e. LOD = 3.3σ/slope. Whereas, the LOD calculated for Br- in presence of Cr3+ was 1.67 µM. The obtained LOD was found to be comparable with the reported sensors as summarized in Table S1. 4. Conclusions In conclusion, we have developed a simple, rapid, inexpensive and sensitive nanoprobe using virgin AgNPs for the selective detection of I- and Br- from aqueous medium based on the aggregation and anti-aggregation mechanism driven by the metal ions (Fe3+/Cr3+). In the presence of Fe3+, the recognition of I- was confirmed by the remarkable color change from yellow to colorless along with the disappearance of SPR band at 400 nm with the detection limit down to 0.32 µM. However, the I- and Br- was colorimetrically detected by AgNPs in presence of Cr3+ via anti-aggregation mechanism with the detection limit of 1.32 µM and 1.67 µM, respectively. Acknowledgments This work was made possible by a grant from the DST, New Delhi (SR/S1/IC54/2012).We would also like to thank SAIF, Indian Institute of Technology (IIT), Bombay for providing TEM facility. 9
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Graphical Abstract
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Highlights •
Colorimetric sensor driven by Fe3+/Cr3+ for I– and Br– was developed using AgNPs.
•
Sensor exhibited good I– and Br– sensitivity with the detection limit down to micromolar rage.
•
Sensing of I– and Br– was achieved by aggregation or anti-aggregation of AgNPs.
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