- Email: [email protected]
Highly selective Hg2+ colorimetric sensor using green synthesized and unmodified silver nanoparticles
Sensors and Actuators B 161 (2012) 880–885
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Highly selective Hg2+ colorimetric sensor using green synthesized and unmodified silver nanoparticles Khalil Farhadi a,∗ , Mehrdad Forough a , Rahim Molaei a , Salahaddin Hajizadeh a , Aysan Rafipour b a b
Department of Chemistry, Faculty of Science, Urmia University, Urmia, Iran Department of Chemistry, Faculty of Science, Payam-e-Noor University, Khoy, Iran
a r t i c l e
i n f o
Article history: Received 3 August 2011 Received in revised form 4 November 2011 Accepted 21 November 2011 Available online 19 December 2011 Keywords: Mercury (II) ion Silver nanoparticles Biological Colorimetry Sensor Detection
a b s t r a c t The reaction between biologically green synthesized silver nanoparticles (Ag NPs) and mercury (II) ions was introduced as a new and high potential colorimetric sensor for the selective recognition and monitoring of mercuric ions in aqueous samples. The green synthesized silver nanoparticles were characterized with surface plasmon resonance (SPR) ultraviolet spectroscopy (UV–vis), SEM and X-ray diffraction analysis (XRD) techniques. The fresh biologically synthesized silver nanoparticles are yellowish-brown in color due to the intense SPR absorption band. In the presence of Hg2+ , the yellow Ag NPs solution was turned to colorless, accompanying the broadening and blue shifting of SPR band. The sensitivity and selectivity of green prepared Ag NPs toward other representative transition-metal ions, alkali metal ions and alkaline earth metal ions were studied. Also the effect of the concentration of Hg2+ to the Ag NPs was considered and the LOD for mercury (II) ion was 2.2 × 10−6 mol L−1 . The proposed method has been successfully used for the determination of mercury (II) ions in various water samples. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Recently, selective and sensitive chemosensors which are broadly used in heavy metal ions determination have been considerably attended because these metals play important roles in living systems and have a severely toxic impact on the environment [1–5]. Among them, mercury is considered as one of the most dangerous metal ions for environment and has most commonly toxic risks for human contacting areas as a result of natural processes, because it is widely distributed in air, water and soil and it is a toxic element that exists in metallic, inorganic, and organic forms [6]. Mercuric ion (Hg2+ ), the most stable form of inorganic mercury, exists mostly in surface water due to its high water solubility and it can cause several developmental delays and health problems that can damage the brain, nervous system, kidneys, and endocrine system [7,8]. Therefore, it is critical to be able to detect and measure the level of Hg2+ in both environmental and biological samples under aqueous conditions with high sensitivity and selectivity and without interference of other metal ions.
Abbreviations: Ag NPs, Ag nanoparticles; SPR, surface plasmon resonance; ICPMS, inductively coupled plasma mass spectrometry; AAS/AES, atomic absorption/emission spectrometry; ISE, ion selective electrode; XRD, X-ray diffraction; SEM, scanning electron microscopy; EDS, energy-dispersive X-ray spectrometer; RSD, relative standard deviation; HPLC, high performance liquid chromatography; LOD, limit of detection. ∗ Corresponding author. Tel.: +98 441 2972050; fax: +98 441 3443442. E-mail addresses: [email protected], [email protected] (K. Farhadi). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.11.052
There are various classical methods described in the literature for mercury detection. These include atomic absorption/emission spectrometry (AAS/AES) [9–11], inductively coupled plasma mass spectrometry (ICPMS) [12,13], atomic fluorescence spectrometry (AFS) [14], high-performance liquid chromatography (HPLC) [15], ion selective electrode (ISE) and flame photometry [16], which are powerful techniques for the determination of Hg2+ , however their excellent performance is achieved at the expenses of expensive instrumentation and time-consuming sample preparation and preconcentration procedures [17]. Therefore, new analytical methods from alternative techniques always could be useful. Hence, the interest in swapping instrumental analytical tools with suitable, selective and sensitive Hg2+ ion sensors has been increasing. To date, colorimetry is generally well known, and commonly used for routine analysis. Recently, colorimetric sensors and several methods providing the immediate optical feedback have been extremely attended due to their simplicity, rapidity, high sensitivity and ease of measurement. Various sensor systems for detection of Hg2+ , based on chromophores or fluorophores [18–27], organic compounds [28–32], polymers [26,33,34], oligonucleotides [35,36], DNA [37], proteins [38] and nanoparticles and nanorods [27,39,40] have been reported. For example, previously, Yang et al. [41] have presented a selective multisignaling optical-electrochemical chemosensor for Hg2+ based on a rhodamine dye. Also, recently Liu et al. [42] reported a selective water-soluble probe for upconversion luminescence sensing of intracellular mercury ions based on chromophoric ruthenium complex-assembled nanophosphor (N719-UCNPs). Optical sensors which require no longer any special
K. Farhadi et al. / Sensors and Actuators B 161 (2012) 880–885
instrumentation are particularly attractive for the visual detection of Hg2+ . Although, such optical sensors are simpler than conventional methods, most of them are either limited with respect to sensitivity, selectivity, need to electronic heating unit, sophisticated synthesis of the probe materials or being not suitable for on-the-spot sample assays. Furthermore, it is highly desirable to develop a sensitive and selective detection colorimetric system besides simple and practical implementation and economical operation. Metal nanoparticles, particularly silver nanoparticles with well-controlled size, have been the focus of great interest because the color changes associate with the surface plasmon absorption band (SPR) which is dependent on a number of parameters such as the size and shape of the particle, the adsorbed species, the dielectric properties of the medium, and the distance between particles [43]. Consequently, metallic nanoparticles especially Ag NPs can be clearly observed by naked eyes, allowing sensitive detection with minimal consumption of materials. Unlike dyes, silver nanoparticles are photostable and do not undergo photobleaching, allowing these nanoparticles to be utilized as ideal color reporting groups for colorimetric sensor design [44]. Among the approaches proposed thus far, mercury’s optical detection generally relied on the complexation of Hg2+ ion to ligands. In this paper, the green and biological synthesis of silver nanoparticles in water media would be discussed and a label free colorimetric assay of Hg2+ based on freshly biological and unmodified synthesized silver nanoparticles which are prepared by the method, as previously described in [45] is reported. The green synthesized Ag NPs are employed as a selective colorimetric sensor to probe Hg2+ in water media without interference of other transition-metal ions, alkali metal ions and alkaline earth metal ions, resulting in appreciable changes in color and optical properties, especially with the assistance of UV radiation. We also employed our proposed colorimetric method, for the practical analysis and detection of the Hg2+ in a complicated lake water matrix. The results confirm that our method is simple, quantitative, cost-effective, sensitive and selective for colorimetric detection of Hg2+ in water media. 2. Experimental 2.1. Instruments UV-vis absorption spectra were recorded using a doublebeam, Lambda 25 UV-vis spectrophotometer (Perkin Elemer, USA) with 1 cm quartz cell. Scanning electron micrographs (SEM) were recorded by a Philips XL-30 electron microscope operating at 30 kV and equipped with an energy-dispersive X-ray spectrometer (EDS). Photographs of the Ag NPs suspension used for visual colorimetric detection were taken by a Canon PowerShot S3 IS digital camera. Freeze drying of the samples before and after addition of mercury (II) ion into Ag NPs was carried out using a ALPHA 1-4 freeze dryer (CHRIST, Germany) under vacuum conditions at −50 ◦ C for 40 h. 2.2. Chemical and materials All chemicals used were of analytical grade or of the highest purity available. All solutions were prepared with double-distilled, deionized water. Silver nitrate (AgNO3 99.8%) was purchased from Merck (Darmstadt, Germany). All different cations, in the form of nitrate or chloride salts including (LiCl, NaNO3 , KNO3 , MgCl2 ·6H2 O, Ca (NO3 )2 ·4H2 O, Sr (NO3 )2 , BaCl2 ·2H2 O, Zn (NO3 )2 ·6H2 O, Co (NO3 )·6H2 O, Ni (NO3 )3 ·6H2 O, Mn (NO3 )2 ·4H2 O, Cd (NO3 )2 ·4H2 O, Hg (NO3 )2 ·H2 O and Cu (NO3 )2 ·3H2 O) were purchased from Merck or Aldrich chemical companies and used as received without further purification. The stock solutions of metal ions were prepared by dissolving a known amount (per mg) of nitrate and/or chloride
881
Fig. 1. Photos (a) and UV–vis spectra (b) of 10 ml green synthesized Ag NPs solution after drop by drop addition of 10−3 mol L−1 Hg2+ .
salts of them in 100 ml deionized water containing a few drops of concentrated HCl, and was further diluted whenever necessary. All glassware were thoroughly cleaned with aqua regia and rinsed thoroughly with deionized water prior to use. 2.3. Preparation of silver nanoparticles The biological and green preparation of Ag NPs was described in our previous study [45]. Briefly, 10 ml of freshly prepared extract of soap-root plant as a stabilizer agent was added to 100 ml of 3 mM aqueous silver nitrate solution and incubated in a rotary shaker for 2 h in dark conditions at 25 ◦ C, and then 15 ml of the aqueous extract of manna of hedysarum plant as a reducing agent was added into the mixture at 86 ◦ C. The mixture obtained, was purified by repeated centrifugation at 12,000 rpm for 20 min to obtain the fresh biologically Ag NPs solution. The characteristics of the synthesized Ag NPs were studied using X-ray diffraction analysis (XRD), energydispersive spectroscopy (EDS), and scanning electron microscopy (SEM). The EDS spectrum of the solution containing silver nanoparticles confirmed the presence of an elemental silver signal without any peaks of impurities [45]. 2.4. General procedure for the colorimetric determination of Hg2+ For colorimetric detection of mercury (II) ion, as-prepared Ag NPs were diluted three times by double distilled deionized water and the resulting concentration calculated to be diluted three-fold. To investigate the metal ions detection ability of biologically synthesized unmodified Ag NPs, representative alkali (Li+ , Na+ , K+ ), alkaline earth (Mg2+ , Ca2+ , Sr2+ , Ba2+ ), and transition-metal ions (Ni2+ , Mn2+ , Cu2+ , Zn2+ , Hg2+ , Co2+ , Cd2+ ) at the same conditions and of the same concentration (0.5 ml, 10−3 mol L−1 ) were added into the three time diluted solution of fresh prepared Ag NPs solution (3 ml), respectively. The assays and the changes in the UV–vis absorption spectra were performed and monitored at room temperature. The photographs were taken with a digital camera after 5 min of mixing.
882
K. Farhadi et al. / Sensors and Actuators B 161 (2012) 880–885
Fig. 2. Photos (a) and UV–vis spectra (b) of 3 ml green synthesized Ag NPs solution with different transition-metal ions.
2.5. Preparation of water samples Water samples were collected from Mahabad Lake (Mahabad, Iran). All the collected samples were spiked with a suitable amount of standard solution (30 M) of Hg2+ and other metal ions, mixed thoroughly, filtered through a 0.2 m membrane, and then centrifuged for 20 min at 9000 rpm. A suitable volume of water samples
Fig. 4. (a) Photo images, UV–vis spectra of (A) biologically synthesized Ag NPs, (B) Ag NPs in the presence of 10 M Hg2+ , and (C) Ag NPs after addition of 100 M Hg2+ . (b) Possible mechanism and schematic illustration of mercury (II) detection principle.
were analyzed according to the proposed general procedure and UV–vis titration study. 3. Results and discussion 3.1. Interaction of silver nanoparticles with metal ions
Fig. 3. Photos (a) and UV–vis spectra (b) of 3 ml green synthesized Ag NPs solution with alkali metal and alkaline metal ions. (c) The colorimetric response of silver nanoparticles to various cations.
It is well known that biologically synthesized Ag NPs exhibit a yellowish-brown color in aqueous solution due to the excitation of surface plasmon resonance vibrations (SPR band) in silver nanoparticles [45]. After addition of Hg2+ solution to freshly prepared Ag NPs, the color of Ag NPs solution changes from yellowish-brown to pale yellow and its color decreases gradually with increasing concentration of Hg2+ and finally changes to colorless after addition of a known concentration of 10−3 mol L−1 mercury (II). Fig. 1 clarifies the photos and UV–vis spectra of 10 ml Ag NPs solution after drop by drop addition of mercury (II) ion. By the way, the photos of Ag NPs solution were taken immediately after drop by drop addition of Hg2+ (Fig. 1a) and related UV–vis spectra (3 ml of sample) monitored and obtained after 5 min of interaction (Fig. 1b). Fig. 1a also confirms that major changes in color of Ag NPs solution are expected when further concentration of 10−3 mol L−1 mercury (2–5 drops) is added. To investigate the response of biologically Ag NPs to other transition metal, alkali and alkaline earth metal ions, the same concentration (0.5 ml, 10−3 mol L−1 ) of the stock solutions of the salts of these metals were added into the 3 ml of Ag NPs solution under the same conditions. As revealed in Figs. 2 and 3, upon interaction of freshly prepared Ag NPs with various metal ions, the solution contacting Hg2+ changes from yellow to colorless, while
K. Farhadi et al. / Sensors and Actuators B 161 (2012) 880–885
883
Fig. 5. Typical SEM micrographs of Ag nanoparticles (a) and product of the reaction between the Ag nanoparticles and mercury (II) ion (b) after freeze drying the samples.
the effect of the other alkali metal (Li+ , Na+ , K+ ), alkaline earth metal (Mg2+ , Ca2+ , Sr2+ , Ba2+ ), and transition-metal ions (Ni2+ , Mn2+ , Cu2+ , Zn2+ , Hg2+ , Co2+ , Cd2+ ) on the color and SPR band of Ag NPs solution is very negligible. The results demonstrate that adding a known concentration of Li+ , Na+ , K+ , Mg2+ , Ca2+ , Sr2+ , Ba2+ , Ni2+ , Mn2+ , Cu2+ , Zn2+ , Co2+ and Cd2+ have no obvious effect on the SPR band and color of Ag NPs as compared to 15 M Hg2+ , which made the color of solution changes from yellow to colorless otherwise the same conditions, indicating that the assay approach has very high selectivity and specificity toward Hg2+ and biologically synthesized Ag NPs are not significantly sensitive to other transition-metal, alkali metal and alkaline metal ions under similar conditions.
3.2. Sensing detection of Hg2+ As-prepared Ag NPs were characterized before and after addition of Hg2+ using UV–vis spectroscopy and SEM techniques. As shown in Fig. 4, characteristic SPR band of the Ag NPs after diluting 3 times was observed at 408 nm (Fig. 4, curve A) and the color of the solution was yellowish-brown in the absence of Hg2+ . Mercury (II) ion with a closed-shell d10 configuration has no optical spectroscopic signature. After addition of 10 M Hg2+ to Ag NPs, the color of solution decreases accompanying with the broadening and blue shifting of the SPR band, and finally turns to colorless after addition of 100 M of Hg2+ . To gain further information about the features of the Ag NPs in the absence and presence of Hg2+ , analysis of the freeze dried samples was performed using SEM. The corresponding SEM images were depicted in Fig. 5. As can be seen, while well defined Ag NPs have been appeared in Fig. 5a, there are no nanoparticles after adding the mercury (II) to Ag NPs solution (Fig. 5b). Based on these images, it may be stated that the observed phenomena is probably related to a redox reaction between zero-valent silver and Hg2+ with the standard potential of 0.8 V (Ag+ /Ag) and 0.85 V (Hg2+ /Hg), respectively [46]. According to proposed mechanism, it is expected that after addition of Hg2+ to the freshly green synthesized unmodified Ag NPs, mercury (II) ion bounds to the Ag NPs surface to move biological stabilizer compounds (from soap-root plant) away from the silver surface; as a result, a redox reaction between silver and mercury ions would be occurred (Fig. 4b).
3.3. Sensitivity and UV–vis titration studies
Fig. 6. (a) UV–vis absorbtion response of 3 ml silver nanoparticles solution upon addition of Hg2+ ions (0–100 M). (b) Plot of absorbance intensity at 408 nm versus Hg2+ concentration.
To evaluate the sensitivity of this method and investigate the minimum detectable concentration of Hg2+ in aqueous solution by color change of the system, and monitoring the UV–vis absorbance values, different concentrations of aqueous solution of mercury ion were added to a solution of the Ag NPs (2 ml) at room temperature. The limit of detection for Hg2+ was determined using the characteristic SPR peak of Ag NPs by UV–vis spectra. Several repeated experiments confirmed that the mercury (II) ion concentration <5 M do not change the color of biologically synthesized Ag NPs. As indicated from Fig. 6a, the absorbance peak of Ag NPs decrease by increasing the concentration of mercury (II) ion and there is a linear relationship (y = 0.0102x + 0.1925, R2 = 0.9968), between the absorbance intensity changes and the concentration of Hg2+ ion over the range from 10 M to 100 M at 408 nm (Fig. 6b). Thus, we suggest that the green synthesized Ag NPs can be used for the colorimetric detection of Hg2+ with the limit of detection (LOD) 2.2 × 10−6 M. Fig. 6a also demonstrates that the increased concentration of Hg2+ ion may cause a slight blue shift on the surface plasmon absorption band. It has been reported that mercury ions could radiolytically be reduced in aqueous silver sol from a mercury layer around silver particles (see Fig. 4.b), accompanied by a broadening and blue shift of the plasmon absorption band [47].
884
K. Farhadi et al. / Sensors and Actuators B 161 (2012) 880–885
In our experiments, Hg2+ ion reacted with silver nanoparticles to form metallic mercury. The freshly generated mercury atoms could strongly be bonded on the silver surface, which could be accounted for the slight blue shift of the SPR band of silver nanoparticles. The results represent the fast performance of this probe for detection of mercury ion. It is well known that different types of mercury including Hg, Hg (OH)2 , HgO, CH3 Hg+ and CH3 HgCl can be transformed into Hg2+ ions by using a digestive method [17]. Thus the proposed probe may offer great promise as a colorimetric detection method for determination of total mercury forms. 3.4. Determination of Hg2+ in water samples The applications of proposed method were evaluated for determination of Hg2+ in a complicated lake water matrix. In such environmental samples, such as lake water, the concentration of other metal ions or some unknown contamination are significantly higher than that of Hg2+ , so potential practical assay is necessary, and it is critical issue to the application of most common sensors. Water sample was found to be free or undetectable values from mercury. So the sample spiked with various concentration range of mercury (30 M) of Hg2+ . The relative standard deviations (RSD) of four measurements are 2.2%. This result suggests that the proposed method has great potential for the sensing of Hg2+ in environmental samples. 4. Conclusion In summary, a simple, label free, cost effective, portable, selective and sensitive detection method has been developed using unmodified and biologically prepared Ag NPs as a colorimetric probe that allows rapid and real time detection of Hg2+ . This approach probably relies on simple redox reaction between Ag NPs and Hg2+ in solution. The presence of Hg2+ can be monitored by naked eye assay and the UV–vis spectrophotometer at room temperature. Compared with previous methods this direct colorimetric technique does not need to use any DNA, complexation of Hg2+ ion to any ligands, fluorescent compounds or dyes and temperature control unit to probe Hg2+ in aqueous solution. Influence of various metal ions has also been investigated. This method shows high selectivity for mercury (II) ion over other transition metal, alkali metal and alkaline earth metal ions with a detection limit of 2.2 × 10−6 mol L−1 . The easy preparation and high stability of the biological synthesized Ag NPs allow this method to be very simple and easy to implement. We believe that using biologically synthesized silver nanoparticles without further modification may offer a new approach for detection of mercury (II) in aqueous environmental samples. Acknowledgements We would like to express our gratitude to the Iranian Nanotechnology Initiative council for the financial support of this research. We also wish to thank Mr. Abdolhamid Rezaie (operator of scanning electron microscopy center) from Tarbiat Modares University for his cooperation and useful advices and with especial thanks to Mrs. Sima Farhadi for her English edition. References [1] A.W. Czarnik, Fluorescent Chemosensors for Ion and Molecule Recognition, American Chemical Society, Washington, DC, 1993. [2] A.P. de Silva, D.B. Fox, A.J.M. Huxley, T.S. Moody, Combining luminescence, coordination and electron transfer for signalling purposes, Coord. Chem. Rev. 205 (2000) 41–57.
[3] A.P. de Silva, H.Q.N. Gunaratne, T. Gunnlaugsson, A.J.M. Huxley, C.P. McCoy, J.T. Rademacher, T.E. Rice, Signaling recognition events with fluorescent sensors and switches, Chem. Rev. 97 (1997) 1515. [4] J.P. Desvergne, A.W. Czarnik, Chemosensors of Ion and Molecule Recognition, Kluwer, Dordrecht, 1997. [5] P.B. Tchounwou, W.K. Ayensu, N. Ninashvili, D. Sutton, Environmental exposure to mercury and its toxicopathologic implications for public health, Environ. Toxicol. 18 (2003) 149–175. [6] F.A. Cotton, G. Wilkinson, C.A. Murillo, M. Bochmann, Advanced Inorganic Chemistry, 6th ed., John Wiley & Sons, New York, 1999. [7] T.W. Clarkson, L. Magos, G.J. Myers, The toxicology of mercury–current exposures and clinical manifestations, N. Engl. J. Med. 349 (2003) 1731–1737. [8] Y. Wang, F. Yang, X. Yang, Colorimetric biosensing of mercury(II) ion using unmodified gold nanoparticle probes and thrombin-binding aptamer, Biosens. Bioelectron. 25 (2010) 1994–1998. [9] Q. Yang, Q. Tan, K. Zhou, K. Xu, X.J. Hou, Direct detection of mercury in vapor and aerosol from chemical atomization and nebulization at ambient temperature: exploiting the flame atomic absorption spectrometer, J. Anal. Atom. Spectrom. 20 (2005) 760–762. [10] C.J.D. Maciel, G.M. Miranda, D.P. de Oliveira, M.E. de Siqueira, J.N. Silveira, E.M.A. Leite, J.B.B. da Silva, Determination of cadmium in human urine by electrothermal atomic absorption spectrometry, Anal. Chim. Acta 491 (2003) 231–237. [11] R.F. Suddendorf, J.O. Watts, K. Boyer, Simplified apparatus for determination of mercury by atomic absorption and inductively coupled plasma emission spectroscopy, J. AOAC Int. 64 (1981) 1105–1110. [12] D. Karunasagar, J. Arunachalam, S. Gangadharan, Development of a ‘collect and punch’ cold vapour inductively coupled plasma mass spectrometric method for the direct determination of mercury at nanograms per litre levels, J. Anal. Atom. Spectrom. 13 (1998) 679–682. [13] B. Fong, W. Mei, T.S. Siu, J. Lee, K. Sai, S. Tam, Determination of mercury in whole blood and urine by inductively coupled plasma mass spectrometry, J. Anal. Toxicol. 31 (2007) 281–287. [14] J.J.B. Nevadoa, R.C.R. Martín-Doimeadiosb, F.J.G. Bernardo, M.J. Moreno, Determination of mercury species in fish reference materials by gas chromatography–atomic fluorescence detection after closed-vessel microwave-assisted extraction, J. Chromatogr. A 1093 (2005) 21–28. [15] S. Ichinoki, N. Kitahata, Y. Fujii, Selective determination of mercury(II) ion in water by solvent extraction followed by reversed-phase HPLC, J. Liq. Chromatogr. Rel. Technol. 27 (2004) 1785–1798. [16] B. Kuswandi, A. Nuriman, H.H. Dam, D.N. Reinhoudt, W. Verboom, Development of a disposable mercury ion-selective optode based on trityl-picolamide as ionophore, Anal. Chim. Acta 591 (2007) 208–213. [17] A. Fan, Y. Ling, C. Lau, J. Lu, Direct colorimetric visualization of mercury (Hg2+ ) based on the formation of gold nanoparticles, Talanta 82 (2010) 687–692. [18] E. Coronado, J.R. Galan-Mascaros, C. Marti-Gastaldo, E. Palomares, J.R. Durrant, R. Vilar, M. Gratzel, M.K. Nazeeruddin, Reversible colorimetric probes for mercury sensing, J. Am. Chem. Soc. 127 (2005) 12351–12356. [19] X.J. Zhu, S.T. Fu, W.K. Wong, J.P. Guo, W.Y. Wong, A near-infrared-fluorescent chemodosimeter for mercuric ion based on an expanded porphyrin, Angew. Chem. Int. Ed. 45 (2006) 3150–3154. ˇ [20] J.V. Ros-Lis, M.D. Marcos, R.M. Mánez, K. Rurack, J. Soto, A regenerative chemodosimeter based on metal-induced dye formation for the highly selective and 2+ sensitive optical determination of Hg ions, Angew. Chem. Int. Ed. 44 (2005) 4405–4407. [21] Y.K. Yang, K.J. Yook, J. Tae, A rhodamine-based fluorescent and colorimetric chemodosimeter for the rapid detection of Hg2+ ions in aqueous media, J. Am. Chem. Soc. 127 (2005) 16760–16761. [22] X.J. Zhu, S.T. Fu, W.-K. Wong, J.P. Guo, W.Y. Wong, A near-infrared-fluorescent chemodosimeter for mercuric ion based on an expanded porphyrin, Angew. Chem. 118 (2006) 3222–3226. [23] X. Guo, X. Qian, L. Jia, A highly selective and sensitive fluorescent chemosensor for Hg2+ in neutral buffer aqueous solution, J. Am. Chem. Soc. 126 (2004) 2272–2273. [24] A. Caballero, R. Martinez, V. Lloveras, I. Ratera, J. Vidal-Gancedo, K. Wurst, A. Tarraga, P. Molina, J. Veciana, Highly selective chromogenic and redox or fluorescent sensors of Hg2+ in aqueous environment based on 1,4-disubstituted azines, J. Am. Chem. Soc. 127 (2005) 15666–15667. [25] S. Yoon, E.W. Miller, Q. He, P.H. Do, C.J. Chang, A. Bright, Specific fluorescent sensor for mercury in water, cells, and tissue, Angew. Chem. Int. Ed. 46 (2007) 6658–6661. [26] Y. Zhao, Z. Zhong, Tuning the sensitivity of a foldamer-based mercury sensor by its folding energy, J. Am. Chem. Soc. 128 (2006) 9988–9989. [27] J.S. Lee, M.S. Han, C.A. Mirkin, Colorimetric detection of mercuric ion (Hg2+ ) in aqueous media using DNA-functionalized gold nanoparticles, Angew. Chem. Int. Ed. 46 (2007) 4093–4096. [28] S. Yoon, A.E. Albers, A.P. Wong, C.J. Chang, Screening mercury levels in fish with a selective fluorescent chemosensor, J. Am. Chem. Soc. 127 (2005) 16030–16031. [29] C.C. Huang, H.T. Chang, Selective gold-nanoparticle-based turn-on fluorescent sensors for detection of mercury(II) in aqueous solution, Anal. Chem. 78 (2006) 8332–8338. [30] H. Lu, Y. Tang, W. Xu, D. Zhang, S. Wang, D. Zhu, Highly selective fluorescence detection for mercury (II) ions in aqueous solution using water soluble conjugated polyelectrolytes, J. Macromol. Rapid Commun. 29 (2008) 1467–1471.
K. Farhadi et al. / Sensors and Actuators B 161 (2012) 880–885 [31] J. Wang, B. Liu, Highly sensitive and selective detection of Hg2+ in aqueous solution with mercury-specific DNA and Sybr Green I, Chem. Commun. 39 (2008) 4759–4761. [32] C.K. Chiang, C.C. Huang, C.W. Liu, H.T. Chang, Oligonucleotide-based fluorescence probe for sensitive and selective detection of mercury(II) in aqueous solution, Anal. Chem. 80 (2008) 3716–3721. [33] X. Liu, Y. Tang, L. Wang, J.Z. Song, C. Fan, S. Wang, Optical detection of mercury(II) in aqueous solutions by using conjugated polymers and label-free oligonucleotides, J. Adv. Mater. 19 (2007) 1471–1474. [34] I.B. Kim, U.H.F. Bunz, Modulating the sensory response of a conjugated polymer by proteins: an agglutination assay for mercury ions in water, J. Am. Chem. Soc. 128 (2006) 2818–2819. [35] S.J. Liu, H.G. Nie, J.H. Jiang, G.L. Shen, R.Q. Yu, Electrochemical sensor for mercury(II) based on conformational switch mediated by interstrand cooperative coordination, Anal. Chem. 81 (2009) 5724–5730. [36] Z.Q. Zhu, Y.Y. Su, J. Li, D. Li, J. Zhang, S.P. Song, Y. Zhao, G.X. Li, C.H. Fan, Highly sensitive electrochemical sensor for mercury(II) ions by using a mercuryspecific oligonucleotide probe and gold nanoparticle-based amplification, Anal. Chem. 81 (2009) 7660–7666. [37] J. Liu, Y. Lu, Rational design of turn-on allosteric DNAzyme catalytic beacons for aqueous mercury ions with ultrahigh sensitivity and selectivity, Angew. Chem. Int. Ed. 46 (2007) 7587–7590. [38] S.V. Wegner, A. Okesli, P. Chen, C. He, Design of an emission ratiometric biosensor from MerR family proteins: a sensitive and selective sensor for Hg2+ , J. Am. Chem. Soc. 129 (2007) 3474–3475. [39] J. Chen, A. Zheng, A. Chen, Y. Gao, C. He, X. Kai, G. Wu, Y. Chen, A functionalized gold nanoparticles and Rhodamine 6G based fluorescent sensor for high sensitive and selective detection of mercury(II) in environmental water samples, Anal. Chim. Acta 599 (2007) 134–142. [40] M. Rex, F.E. Hernandez, A.D. Campiglia, Pushing the limits of mercury sensors with gold nanorods, Anal. Chem. 78 (2006) 445–451. [41] H. Yang, Z. Zhou, K. Huang, M. Yu, F. Li, T. Yi, C. Huang, Multisignaling opticalelectrochemical sensor for Hg2+ based on a rhodamine derivative with a ferrocene unit, Org. Lett. 29 (2007) 4729–4732. [42] Q. Liu, J. Peng, L. Sun, F. Li, High-efficiency upconversion luminescent sensing and bioimaging of Hg(II) by chromophoric ruthenium complex-assembled nanophosphors, ACS Nano. 5 (2011) 8040–8048, doi:10.1021/nn202620u. [43] K. Aslan, J.R. Lakowicz, C.D. Geddes, Nanogold–plasmon-resonance-based glucose sensing, Anal. Biochem. 330 (2004) 145–155.
885
[44] C. Sonnichsen, B.M. Reinhard, J. Liphardt, A.P. Alivisatos, A molecular ruler based on plasmon coupling of single gold and silver nanoparticles, Nat. Biotechnol. 23 (2005) 741–745. [45] M. Forough, K. Farhadi, Biological and green synthesis of silver nanoparticles, Turk. J. Eng. Environ. Sci. 34 (2010) 281–287. [46] A.J. Bard, R. Parsons, J. Jordan, Standard Potentials in Aqueous Solution, International Union of Pure and Applied Chemistry Marcel Dekker Inc., New York, 1985. [47] L. Katsikas, M. Gutiérrez, A. Henglein, Bimetallic colloids: silver and mercury, Phys. Chem. 100 (1996) 11203–11206.
Biographies Khalil Farhadi is a professor of chemistry at the Urmia University, Urmia, Iran. He received a BC in chemistry from Urmia University, Urmia, Iran, in 1987, and MS in analytical chemistry from Tabriz University, Tabriz, Iran, in 1990, and a PhD in analytical chemistry from Razi University, Kermanshah, Iran, in 2000. His research interest is in the fields of electroanalytical chemistry, biosensors and separation methods. Mehrdad Forough received BC in chemistry from Payame-Noor University (PNU), in Urmia, Iran, in 2005, and MS in analytical chemistry from Payame-Noor University (PNU), Khoy, Iran, in 2009. His main area of interest is nanochemistry. He is a PhD student in analytical chemistry at the Urmia University, Urmia, Iran. Rahim Molaei received a BC in chemistry from Urmia University, Urmia, Iran in 2005, and MS in analytical chemistry from same university in 2007. His main area of interest is development of new sample preparation methods. He is a PhD student in analytical chemistry at the Urmia University, Urmia, Iran. Salahaddin Hajizadeh received a BC in chemistry from Payame-Noor University (PNU), Urmia, Iran in 2006, and MS in analytical chemistry from Urmia University, Urmia, Iran, in 2010. His main area of interest is application of nanoparticles in analytical chemistry. Aysan Rafipour received a BC in chemistry from Urmia University, Urmia, Iran in 2006, and MS in analytical chemistry from Khoy Payamour University, Khoy, Iran, in 2010. Her main area of interest is application of nanoparticles in analytical chemistry.
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Highly selective Hg2+ colorimetric sensor using green synthesized and unmodified silver nanoparticles Khalil Farhadi a,∗ , Mehrdad Forough a , Rahim Molaei a , Salahaddin Hajizadeh a , Aysan Rafipour b a b
Department of Chemistry, Faculty of Science, Urmia University, Urmia, Iran Department of Chemistry, Faculty of Science, Payam-e-Noor University, Khoy, Iran
a r t i c l e
i n f o
Article history: Received 3 August 2011 Received in revised form 4 November 2011 Accepted 21 November 2011 Available online 19 December 2011 Keywords: Mercury (II) ion Silver nanoparticles Biological Colorimetry Sensor Detection
a b s t r a c t The reaction between biologically green synthesized silver nanoparticles (Ag NPs) and mercury (II) ions was introduced as a new and high potential colorimetric sensor for the selective recognition and monitoring of mercuric ions in aqueous samples. The green synthesized silver nanoparticles were characterized with surface plasmon resonance (SPR) ultraviolet spectroscopy (UV–vis), SEM and X-ray diffraction analysis (XRD) techniques. The fresh biologically synthesized silver nanoparticles are yellowish-brown in color due to the intense SPR absorption band. In the presence of Hg2+ , the yellow Ag NPs solution was turned to colorless, accompanying the broadening and blue shifting of SPR band. The sensitivity and selectivity of green prepared Ag NPs toward other representative transition-metal ions, alkali metal ions and alkaline earth metal ions were studied. Also the effect of the concentration of Hg2+ to the Ag NPs was considered and the LOD for mercury (II) ion was 2.2 × 10−6 mol L−1 . The proposed method has been successfully used for the determination of mercury (II) ions in various water samples. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Recently, selective and sensitive chemosensors which are broadly used in heavy metal ions determination have been considerably attended because these metals play important roles in living systems and have a severely toxic impact on the environment [1–5]. Among them, mercury is considered as one of the most dangerous metal ions for environment and has most commonly toxic risks for human contacting areas as a result of natural processes, because it is widely distributed in air, water and soil and it is a toxic element that exists in metallic, inorganic, and organic forms [6]. Mercuric ion (Hg2+ ), the most stable form of inorganic mercury, exists mostly in surface water due to its high water solubility and it can cause several developmental delays and health problems that can damage the brain, nervous system, kidneys, and endocrine system [7,8]. Therefore, it is critical to be able to detect and measure the level of Hg2+ in both environmental and biological samples under aqueous conditions with high sensitivity and selectivity and without interference of other metal ions.
Abbreviations: Ag NPs, Ag nanoparticles; SPR, surface plasmon resonance; ICPMS, inductively coupled plasma mass spectrometry; AAS/AES, atomic absorption/emission spectrometry; ISE, ion selective electrode; XRD, X-ray diffraction; SEM, scanning electron microscopy; EDS, energy-dispersive X-ray spectrometer; RSD, relative standard deviation; HPLC, high performance liquid chromatography; LOD, limit of detection. ∗ Corresponding author. Tel.: +98 441 2972050; fax: +98 441 3443442. E-mail addresses: [email protected], [email protected] (K. Farhadi). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.11.052
There are various classical methods described in the literature for mercury detection. These include atomic absorption/emission spectrometry (AAS/AES) [9–11], inductively coupled plasma mass spectrometry (ICPMS) [12,13], atomic fluorescence spectrometry (AFS) [14], high-performance liquid chromatography (HPLC) [15], ion selective electrode (ISE) and flame photometry [16], which are powerful techniques for the determination of Hg2+ , however their excellent performance is achieved at the expenses of expensive instrumentation and time-consuming sample preparation and preconcentration procedures [17]. Therefore, new analytical methods from alternative techniques always could be useful. Hence, the interest in swapping instrumental analytical tools with suitable, selective and sensitive Hg2+ ion sensors has been increasing. To date, colorimetry is generally well known, and commonly used for routine analysis. Recently, colorimetric sensors and several methods providing the immediate optical feedback have been extremely attended due to their simplicity, rapidity, high sensitivity and ease of measurement. Various sensor systems for detection of Hg2+ , based on chromophores or fluorophores [18–27], organic compounds [28–32], polymers [26,33,34], oligonucleotides [35,36], DNA [37], proteins [38] and nanoparticles and nanorods [27,39,40] have been reported. For example, previously, Yang et al. [41] have presented a selective multisignaling optical-electrochemical chemosensor for Hg2+ based on a rhodamine dye. Also, recently Liu et al. [42] reported a selective water-soluble probe for upconversion luminescence sensing of intracellular mercury ions based on chromophoric ruthenium complex-assembled nanophosphor (N719-UCNPs). Optical sensors which require no longer any special
K. Farhadi et al. / Sensors and Actuators B 161 (2012) 880–885
instrumentation are particularly attractive for the visual detection of Hg2+ . Although, such optical sensors are simpler than conventional methods, most of them are either limited with respect to sensitivity, selectivity, need to electronic heating unit, sophisticated synthesis of the probe materials or being not suitable for on-the-spot sample assays. Furthermore, it is highly desirable to develop a sensitive and selective detection colorimetric system besides simple and practical implementation and economical operation. Metal nanoparticles, particularly silver nanoparticles with well-controlled size, have been the focus of great interest because the color changes associate with the surface plasmon absorption band (SPR) which is dependent on a number of parameters such as the size and shape of the particle, the adsorbed species, the dielectric properties of the medium, and the distance between particles [43]. Consequently, metallic nanoparticles especially Ag NPs can be clearly observed by naked eyes, allowing sensitive detection with minimal consumption of materials. Unlike dyes, silver nanoparticles are photostable and do not undergo photobleaching, allowing these nanoparticles to be utilized as ideal color reporting groups for colorimetric sensor design [44]. Among the approaches proposed thus far, mercury’s optical detection generally relied on the complexation of Hg2+ ion to ligands. In this paper, the green and biological synthesis of silver nanoparticles in water media would be discussed and a label free colorimetric assay of Hg2+ based on freshly biological and unmodified synthesized silver nanoparticles which are prepared by the method, as previously described in [45] is reported. The green synthesized Ag NPs are employed as a selective colorimetric sensor to probe Hg2+ in water media without interference of other transition-metal ions, alkali metal ions and alkaline earth metal ions, resulting in appreciable changes in color and optical properties, especially with the assistance of UV radiation. We also employed our proposed colorimetric method, for the practical analysis and detection of the Hg2+ in a complicated lake water matrix. The results confirm that our method is simple, quantitative, cost-effective, sensitive and selective for colorimetric detection of Hg2+ in water media. 2. Experimental 2.1. Instruments UV-vis absorption spectra were recorded using a doublebeam, Lambda 25 UV-vis spectrophotometer (Perkin Elemer, USA) with 1 cm quartz cell. Scanning electron micrographs (SEM) were recorded by a Philips XL-30 electron microscope operating at 30 kV and equipped with an energy-dispersive X-ray spectrometer (EDS). Photographs of the Ag NPs suspension used for visual colorimetric detection were taken by a Canon PowerShot S3 IS digital camera. Freeze drying of the samples before and after addition of mercury (II) ion into Ag NPs was carried out using a ALPHA 1-4 freeze dryer (CHRIST, Germany) under vacuum conditions at −50 ◦ C for 40 h. 2.2. Chemical and materials All chemicals used were of analytical grade or of the highest purity available. All solutions were prepared with double-distilled, deionized water. Silver nitrate (AgNO3 99.8%) was purchased from Merck (Darmstadt, Germany). All different cations, in the form of nitrate or chloride salts including (LiCl, NaNO3 , KNO3 , MgCl2 ·6H2 O, Ca (NO3 )2 ·4H2 O, Sr (NO3 )2 , BaCl2 ·2H2 O, Zn (NO3 )2 ·6H2 O, Co (NO3 )·6H2 O, Ni (NO3 )3 ·6H2 O, Mn (NO3 )2 ·4H2 O, Cd (NO3 )2 ·4H2 O, Hg (NO3 )2 ·H2 O and Cu (NO3 )2 ·3H2 O) were purchased from Merck or Aldrich chemical companies and used as received without further purification. The stock solutions of metal ions were prepared by dissolving a known amount (per mg) of nitrate and/or chloride
881
Fig. 1. Photos (a) and UV–vis spectra (b) of 10 ml green synthesized Ag NPs solution after drop by drop addition of 10−3 mol L−1 Hg2+ .
salts of them in 100 ml deionized water containing a few drops of concentrated HCl, and was further diluted whenever necessary. All glassware were thoroughly cleaned with aqua regia and rinsed thoroughly with deionized water prior to use. 2.3. Preparation of silver nanoparticles The biological and green preparation of Ag NPs was described in our previous study [45]. Briefly, 10 ml of freshly prepared extract of soap-root plant as a stabilizer agent was added to 100 ml of 3 mM aqueous silver nitrate solution and incubated in a rotary shaker for 2 h in dark conditions at 25 ◦ C, and then 15 ml of the aqueous extract of manna of hedysarum plant as a reducing agent was added into the mixture at 86 ◦ C. The mixture obtained, was purified by repeated centrifugation at 12,000 rpm for 20 min to obtain the fresh biologically Ag NPs solution. The characteristics of the synthesized Ag NPs were studied using X-ray diffraction analysis (XRD), energydispersive spectroscopy (EDS), and scanning electron microscopy (SEM). The EDS spectrum of the solution containing silver nanoparticles confirmed the presence of an elemental silver signal without any peaks of impurities [45]. 2.4. General procedure for the colorimetric determination of Hg2+ For colorimetric detection of mercury (II) ion, as-prepared Ag NPs were diluted three times by double distilled deionized water and the resulting concentration calculated to be diluted three-fold. To investigate the metal ions detection ability of biologically synthesized unmodified Ag NPs, representative alkali (Li+ , Na+ , K+ ), alkaline earth (Mg2+ , Ca2+ , Sr2+ , Ba2+ ), and transition-metal ions (Ni2+ , Mn2+ , Cu2+ , Zn2+ , Hg2+ , Co2+ , Cd2+ ) at the same conditions and of the same concentration (0.5 ml, 10−3 mol L−1 ) were added into the three time diluted solution of fresh prepared Ag NPs solution (3 ml), respectively. The assays and the changes in the UV–vis absorption spectra were performed and monitored at room temperature. The photographs were taken with a digital camera after 5 min of mixing.
882
K. Farhadi et al. / Sensors and Actuators B 161 (2012) 880–885
Fig. 2. Photos (a) and UV–vis spectra (b) of 3 ml green synthesized Ag NPs solution with different transition-metal ions.
2.5. Preparation of water samples Water samples were collected from Mahabad Lake (Mahabad, Iran). All the collected samples were spiked with a suitable amount of standard solution (30 M) of Hg2+ and other metal ions, mixed thoroughly, filtered through a 0.2 m membrane, and then centrifuged for 20 min at 9000 rpm. A suitable volume of water samples
Fig. 4. (a) Photo images, UV–vis spectra of (A) biologically synthesized Ag NPs, (B) Ag NPs in the presence of 10 M Hg2+ , and (C) Ag NPs after addition of 100 M Hg2+ . (b) Possible mechanism and schematic illustration of mercury (II) detection principle.
were analyzed according to the proposed general procedure and UV–vis titration study. 3. Results and discussion 3.1. Interaction of silver nanoparticles with metal ions
Fig. 3. Photos (a) and UV–vis spectra (b) of 3 ml green synthesized Ag NPs solution with alkali metal and alkaline metal ions. (c) The colorimetric response of silver nanoparticles to various cations.
It is well known that biologically synthesized Ag NPs exhibit a yellowish-brown color in aqueous solution due to the excitation of surface plasmon resonance vibrations (SPR band) in silver nanoparticles [45]. After addition of Hg2+ solution to freshly prepared Ag NPs, the color of Ag NPs solution changes from yellowish-brown to pale yellow and its color decreases gradually with increasing concentration of Hg2+ and finally changes to colorless after addition of a known concentration of 10−3 mol L−1 mercury (II). Fig. 1 clarifies the photos and UV–vis spectra of 10 ml Ag NPs solution after drop by drop addition of mercury (II) ion. By the way, the photos of Ag NPs solution were taken immediately after drop by drop addition of Hg2+ (Fig. 1a) and related UV–vis spectra (3 ml of sample) monitored and obtained after 5 min of interaction (Fig. 1b). Fig. 1a also confirms that major changes in color of Ag NPs solution are expected when further concentration of 10−3 mol L−1 mercury (2–5 drops) is added. To investigate the response of biologically Ag NPs to other transition metal, alkali and alkaline earth metal ions, the same concentration (0.5 ml, 10−3 mol L−1 ) of the stock solutions of the salts of these metals were added into the 3 ml of Ag NPs solution under the same conditions. As revealed in Figs. 2 and 3, upon interaction of freshly prepared Ag NPs with various metal ions, the solution contacting Hg2+ changes from yellow to colorless, while
K. Farhadi et al. / Sensors and Actuators B 161 (2012) 880–885
883
Fig. 5. Typical SEM micrographs of Ag nanoparticles (a) and product of the reaction between the Ag nanoparticles and mercury (II) ion (b) after freeze drying the samples.
the effect of the other alkali metal (Li+ , Na+ , K+ ), alkaline earth metal (Mg2+ , Ca2+ , Sr2+ , Ba2+ ), and transition-metal ions (Ni2+ , Mn2+ , Cu2+ , Zn2+ , Hg2+ , Co2+ , Cd2+ ) on the color and SPR band of Ag NPs solution is very negligible. The results demonstrate that adding a known concentration of Li+ , Na+ , K+ , Mg2+ , Ca2+ , Sr2+ , Ba2+ , Ni2+ , Mn2+ , Cu2+ , Zn2+ , Co2+ and Cd2+ have no obvious effect on the SPR band and color of Ag NPs as compared to 15 M Hg2+ , which made the color of solution changes from yellow to colorless otherwise the same conditions, indicating that the assay approach has very high selectivity and specificity toward Hg2+ and biologically synthesized Ag NPs are not significantly sensitive to other transition-metal, alkali metal and alkaline metal ions under similar conditions.
3.2. Sensing detection of Hg2+ As-prepared Ag NPs were characterized before and after addition of Hg2+ using UV–vis spectroscopy and SEM techniques. As shown in Fig. 4, characteristic SPR band of the Ag NPs after diluting 3 times was observed at 408 nm (Fig. 4, curve A) and the color of the solution was yellowish-brown in the absence of Hg2+ . Mercury (II) ion with a closed-shell d10 configuration has no optical spectroscopic signature. After addition of 10 M Hg2+ to Ag NPs, the color of solution decreases accompanying with the broadening and blue shifting of the SPR band, and finally turns to colorless after addition of 100 M of Hg2+ . To gain further information about the features of the Ag NPs in the absence and presence of Hg2+ , analysis of the freeze dried samples was performed using SEM. The corresponding SEM images were depicted in Fig. 5. As can be seen, while well defined Ag NPs have been appeared in Fig. 5a, there are no nanoparticles after adding the mercury (II) to Ag NPs solution (Fig. 5b). Based on these images, it may be stated that the observed phenomena is probably related to a redox reaction between zero-valent silver and Hg2+ with the standard potential of 0.8 V (Ag+ /Ag) and 0.85 V (Hg2+ /Hg), respectively [46]. According to proposed mechanism, it is expected that after addition of Hg2+ to the freshly green synthesized unmodified Ag NPs, mercury (II) ion bounds to the Ag NPs surface to move biological stabilizer compounds (from soap-root plant) away from the silver surface; as a result, a redox reaction between silver and mercury ions would be occurred (Fig. 4b).
3.3. Sensitivity and UV–vis titration studies
Fig. 6. (a) UV–vis absorbtion response of 3 ml silver nanoparticles solution upon addition of Hg2+ ions (0–100 M). (b) Plot of absorbance intensity at 408 nm versus Hg2+ concentration.
To evaluate the sensitivity of this method and investigate the minimum detectable concentration of Hg2+ in aqueous solution by color change of the system, and monitoring the UV–vis absorbance values, different concentrations of aqueous solution of mercury ion were added to a solution of the Ag NPs (2 ml) at room temperature. The limit of detection for Hg2+ was determined using the characteristic SPR peak of Ag NPs by UV–vis spectra. Several repeated experiments confirmed that the mercury (II) ion concentration <5 M do not change the color of biologically synthesized Ag NPs. As indicated from Fig. 6a, the absorbance peak of Ag NPs decrease by increasing the concentration of mercury (II) ion and there is a linear relationship (y = 0.0102x + 0.1925, R2 = 0.9968), between the absorbance intensity changes and the concentration of Hg2+ ion over the range from 10 M to 100 M at 408 nm (Fig. 6b). Thus, we suggest that the green synthesized Ag NPs can be used for the colorimetric detection of Hg2+ with the limit of detection (LOD) 2.2 × 10−6 M. Fig. 6a also demonstrates that the increased concentration of Hg2+ ion may cause a slight blue shift on the surface plasmon absorption band. It has been reported that mercury ions could radiolytically be reduced in aqueous silver sol from a mercury layer around silver particles (see Fig. 4.b), accompanied by a broadening and blue shift of the plasmon absorption band [47].
884
K. Farhadi et al. / Sensors and Actuators B 161 (2012) 880–885
In our experiments, Hg2+ ion reacted with silver nanoparticles to form metallic mercury. The freshly generated mercury atoms could strongly be bonded on the silver surface, which could be accounted for the slight blue shift of the SPR band of silver nanoparticles. The results represent the fast performance of this probe for detection of mercury ion. It is well known that different types of mercury including Hg, Hg (OH)2 , HgO, CH3 Hg+ and CH3 HgCl can be transformed into Hg2+ ions by using a digestive method [17]. Thus the proposed probe may offer great promise as a colorimetric detection method for determination of total mercury forms. 3.4. Determination of Hg2+ in water samples The applications of proposed method were evaluated for determination of Hg2+ in a complicated lake water matrix. In such environmental samples, such as lake water, the concentration of other metal ions or some unknown contamination are significantly higher than that of Hg2+ , so potential practical assay is necessary, and it is critical issue to the application of most common sensors. Water sample was found to be free or undetectable values from mercury. So the sample spiked with various concentration range of mercury (30 M) of Hg2+ . The relative standard deviations (RSD) of four measurements are 2.2%. This result suggests that the proposed method has great potential for the sensing of Hg2+ in environmental samples. 4. Conclusion In summary, a simple, label free, cost effective, portable, selective and sensitive detection method has been developed using unmodified and biologically prepared Ag NPs as a colorimetric probe that allows rapid and real time detection of Hg2+ . This approach probably relies on simple redox reaction between Ag NPs and Hg2+ in solution. The presence of Hg2+ can be monitored by naked eye assay and the UV–vis spectrophotometer at room temperature. Compared with previous methods this direct colorimetric technique does not need to use any DNA, complexation of Hg2+ ion to any ligands, fluorescent compounds or dyes and temperature control unit to probe Hg2+ in aqueous solution. Influence of various metal ions has also been investigated. This method shows high selectivity for mercury (II) ion over other transition metal, alkali metal and alkaline earth metal ions with a detection limit of 2.2 × 10−6 mol L−1 . The easy preparation and high stability of the biological synthesized Ag NPs allow this method to be very simple and easy to implement. We believe that using biologically synthesized silver nanoparticles without further modification may offer a new approach for detection of mercury (II) in aqueous environmental samples. Acknowledgements We would like to express our gratitude to the Iranian Nanotechnology Initiative council for the financial support of this research. We also wish to thank Mr. Abdolhamid Rezaie (operator of scanning electron microscopy center) from Tarbiat Modares University for his cooperation and useful advices and with especial thanks to Mrs. Sima Farhadi for her English edition. References [1] A.W. Czarnik, Fluorescent Chemosensors for Ion and Molecule Recognition, American Chemical Society, Washington, DC, 1993. [2] A.P. de Silva, D.B. Fox, A.J.M. Huxley, T.S. Moody, Combining luminescence, coordination and electron transfer for signalling purposes, Coord. Chem. Rev. 205 (2000) 41–57.
[3] A.P. de Silva, H.Q.N. Gunaratne, T. Gunnlaugsson, A.J.M. Huxley, C.P. McCoy, J.T. Rademacher, T.E. Rice, Signaling recognition events with fluorescent sensors and switches, Chem. Rev. 97 (1997) 1515. [4] J.P. Desvergne, A.W. Czarnik, Chemosensors of Ion and Molecule Recognition, Kluwer, Dordrecht, 1997. [5] P.B. Tchounwou, W.K. Ayensu, N. Ninashvili, D. Sutton, Environmental exposure to mercury and its toxicopathologic implications for public health, Environ. Toxicol. 18 (2003) 149–175. [6] F.A. Cotton, G. Wilkinson, C.A. Murillo, M. Bochmann, Advanced Inorganic Chemistry, 6th ed., John Wiley & Sons, New York, 1999. [7] T.W. Clarkson, L. Magos, G.J. Myers, The toxicology of mercury–current exposures and clinical manifestations, N. Engl. J. Med. 349 (2003) 1731–1737. [8] Y. Wang, F. Yang, X. Yang, Colorimetric biosensing of mercury(II) ion using unmodified gold nanoparticle probes and thrombin-binding aptamer, Biosens. Bioelectron. 25 (2010) 1994–1998. [9] Q. Yang, Q. Tan, K. Zhou, K. Xu, X.J. Hou, Direct detection of mercury in vapor and aerosol from chemical atomization and nebulization at ambient temperature: exploiting the flame atomic absorption spectrometer, J. Anal. Atom. Spectrom. 20 (2005) 760–762. [10] C.J.D. Maciel, G.M. Miranda, D.P. de Oliveira, M.E. de Siqueira, J.N. Silveira, E.M.A. Leite, J.B.B. da Silva, Determination of cadmium in human urine by electrothermal atomic absorption spectrometry, Anal. Chim. Acta 491 (2003) 231–237. [11] R.F. Suddendorf, J.O. Watts, K. Boyer, Simplified apparatus for determination of mercury by atomic absorption and inductively coupled plasma emission spectroscopy, J. AOAC Int. 64 (1981) 1105–1110. [12] D. Karunasagar, J. Arunachalam, S. Gangadharan, Development of a ‘collect and punch’ cold vapour inductively coupled plasma mass spectrometric method for the direct determination of mercury at nanograms per litre levels, J. Anal. Atom. Spectrom. 13 (1998) 679–682. [13] B. Fong, W. Mei, T.S. Siu, J. Lee, K. Sai, S. Tam, Determination of mercury in whole blood and urine by inductively coupled plasma mass spectrometry, J. Anal. Toxicol. 31 (2007) 281–287. [14] J.J.B. Nevadoa, R.C.R. Martín-Doimeadiosb, F.J.G. Bernardo, M.J. Moreno, Determination of mercury species in fish reference materials by gas chromatography–atomic fluorescence detection after closed-vessel microwave-assisted extraction, J. Chromatogr. A 1093 (2005) 21–28. [15] S. Ichinoki, N. Kitahata, Y. Fujii, Selective determination of mercury(II) ion in water by solvent extraction followed by reversed-phase HPLC, J. Liq. Chromatogr. Rel. Technol. 27 (2004) 1785–1798. [16] B. Kuswandi, A. Nuriman, H.H. Dam, D.N. Reinhoudt, W. Verboom, Development of a disposable mercury ion-selective optode based on trityl-picolamide as ionophore, Anal. Chim. Acta 591 (2007) 208–213. [17] A. Fan, Y. Ling, C. Lau, J. Lu, Direct colorimetric visualization of mercury (Hg2+ ) based on the formation of gold nanoparticles, Talanta 82 (2010) 687–692. [18] E. Coronado, J.R. Galan-Mascaros, C. Marti-Gastaldo, E. Palomares, J.R. Durrant, R. Vilar, M. Gratzel, M.K. Nazeeruddin, Reversible colorimetric probes for mercury sensing, J. Am. Chem. Soc. 127 (2005) 12351–12356. [19] X.J. Zhu, S.T. Fu, W.K. Wong, J.P. Guo, W.Y. Wong, A near-infrared-fluorescent chemodosimeter for mercuric ion based on an expanded porphyrin, Angew. Chem. Int. Ed. 45 (2006) 3150–3154. ˇ [20] J.V. Ros-Lis, M.D. Marcos, R.M. Mánez, K. Rurack, J. Soto, A regenerative chemodosimeter based on metal-induced dye formation for the highly selective and 2+ sensitive optical determination of Hg ions, Angew. Chem. Int. Ed. 44 (2005) 4405–4407. [21] Y.K. Yang, K.J. Yook, J. Tae, A rhodamine-based fluorescent and colorimetric chemodosimeter for the rapid detection of Hg2+ ions in aqueous media, J. Am. Chem. Soc. 127 (2005) 16760–16761. [22] X.J. Zhu, S.T. Fu, W.-K. Wong, J.P. Guo, W.Y. Wong, A near-infrared-fluorescent chemodosimeter for mercuric ion based on an expanded porphyrin, Angew. Chem. 118 (2006) 3222–3226. [23] X. Guo, X. Qian, L. Jia, A highly selective and sensitive fluorescent chemosensor for Hg2+ in neutral buffer aqueous solution, J. Am. Chem. Soc. 126 (2004) 2272–2273. [24] A. Caballero, R. Martinez, V. Lloveras, I. Ratera, J. Vidal-Gancedo, K. Wurst, A. Tarraga, P. Molina, J. Veciana, Highly selective chromogenic and redox or fluorescent sensors of Hg2+ in aqueous environment based on 1,4-disubstituted azines, J. Am. Chem. Soc. 127 (2005) 15666–15667. [25] S. Yoon, E.W. Miller, Q. He, P.H. Do, C.J. Chang, A. Bright, Specific fluorescent sensor for mercury in water, cells, and tissue, Angew. Chem. Int. Ed. 46 (2007) 6658–6661. [26] Y. Zhao, Z. Zhong, Tuning the sensitivity of a foldamer-based mercury sensor by its folding energy, J. Am. Chem. Soc. 128 (2006) 9988–9989. [27] J.S. Lee, M.S. Han, C.A. Mirkin, Colorimetric detection of mercuric ion (Hg2+ ) in aqueous media using DNA-functionalized gold nanoparticles, Angew. Chem. Int. Ed. 46 (2007) 4093–4096. [28] S. Yoon, A.E. Albers, A.P. Wong, C.J. Chang, Screening mercury levels in fish with a selective fluorescent chemosensor, J. Am. Chem. Soc. 127 (2005) 16030–16031. [29] C.C. Huang, H.T. Chang, Selective gold-nanoparticle-based turn-on fluorescent sensors for detection of mercury(II) in aqueous solution, Anal. Chem. 78 (2006) 8332–8338. [30] H. Lu, Y. Tang, W. Xu, D. Zhang, S. Wang, D. Zhu, Highly selective fluorescence detection for mercury (II) ions in aqueous solution using water soluble conjugated polyelectrolytes, J. Macromol. Rapid Commun. 29 (2008) 1467–1471.
K. Farhadi et al. / Sensors and Actuators B 161 (2012) 880–885 [31] J. Wang, B. Liu, Highly sensitive and selective detection of Hg2+ in aqueous solution with mercury-specific DNA and Sybr Green I, Chem. Commun. 39 (2008) 4759–4761. [32] C.K. Chiang, C.C. Huang, C.W. Liu, H.T. Chang, Oligonucleotide-based fluorescence probe for sensitive and selective detection of mercury(II) in aqueous solution, Anal. Chem. 80 (2008) 3716–3721. [33] X. Liu, Y. Tang, L. Wang, J.Z. Song, C. Fan, S. Wang, Optical detection of mercury(II) in aqueous solutions by using conjugated polymers and label-free oligonucleotides, J. Adv. Mater. 19 (2007) 1471–1474. [34] I.B. Kim, U.H.F. Bunz, Modulating the sensory response of a conjugated polymer by proteins: an agglutination assay for mercury ions in water, J. Am. Chem. Soc. 128 (2006) 2818–2819. [35] S.J. Liu, H.G. Nie, J.H. Jiang, G.L. Shen, R.Q. Yu, Electrochemical sensor for mercury(II) based on conformational switch mediated by interstrand cooperative coordination, Anal. Chem. 81 (2009) 5724–5730. [36] Z.Q. Zhu, Y.Y. Su, J. Li, D. Li, J. Zhang, S.P. Song, Y. Zhao, G.X. Li, C.H. Fan, Highly sensitive electrochemical sensor for mercury(II) ions by using a mercuryspecific oligonucleotide probe and gold nanoparticle-based amplification, Anal. Chem. 81 (2009) 7660–7666. [37] J. Liu, Y. Lu, Rational design of turn-on allosteric DNAzyme catalytic beacons for aqueous mercury ions with ultrahigh sensitivity and selectivity, Angew. Chem. Int. Ed. 46 (2007) 7587–7590. [38] S.V. Wegner, A. Okesli, P. Chen, C. He, Design of an emission ratiometric biosensor from MerR family proteins: a sensitive and selective sensor for Hg2+ , J. Am. Chem. Soc. 129 (2007) 3474–3475. [39] J. Chen, A. Zheng, A. Chen, Y. Gao, C. He, X. Kai, G. Wu, Y. Chen, A functionalized gold nanoparticles and Rhodamine 6G based fluorescent sensor for high sensitive and selective detection of mercury(II) in environmental water samples, Anal. Chim. Acta 599 (2007) 134–142. [40] M. Rex, F.E. Hernandez, A.D. Campiglia, Pushing the limits of mercury sensors with gold nanorods, Anal. Chem. 78 (2006) 445–451. [41] H. Yang, Z. Zhou, K. Huang, M. Yu, F. Li, T. Yi, C. Huang, Multisignaling opticalelectrochemical sensor for Hg2+ based on a rhodamine derivative with a ferrocene unit, Org. Lett. 29 (2007) 4729–4732. [42] Q. Liu, J. Peng, L. Sun, F. Li, High-efficiency upconversion luminescent sensing and bioimaging of Hg(II) by chromophoric ruthenium complex-assembled nanophosphors, ACS Nano. 5 (2011) 8040–8048, doi:10.1021/nn202620u. [43] K. Aslan, J.R. Lakowicz, C.D. Geddes, Nanogold–plasmon-resonance-based glucose sensing, Anal. Biochem. 330 (2004) 145–155.
885
[44] C. Sonnichsen, B.M. Reinhard, J. Liphardt, A.P. Alivisatos, A molecular ruler based on plasmon coupling of single gold and silver nanoparticles, Nat. Biotechnol. 23 (2005) 741–745. [45] M. Forough, K. Farhadi, Biological and green synthesis of silver nanoparticles, Turk. J. Eng. Environ. Sci. 34 (2010) 281–287. [46] A.J. Bard, R. Parsons, J. Jordan, Standard Potentials in Aqueous Solution, International Union of Pure and Applied Chemistry Marcel Dekker Inc., New York, 1985. [47] L. Katsikas, M. Gutiérrez, A. Henglein, Bimetallic colloids: silver and mercury, Phys. Chem. 100 (1996) 11203–11206.
Biographies Khalil Farhadi is a professor of chemistry at the Urmia University, Urmia, Iran. He received a BC in chemistry from Urmia University, Urmia, Iran, in 1987, and MS in analytical chemistry from Tabriz University, Tabriz, Iran, in 1990, and a PhD in analytical chemistry from Razi University, Kermanshah, Iran, in 2000. His research interest is in the fields of electroanalytical chemistry, biosensors and separation methods. Mehrdad Forough received BC in chemistry from Payame-Noor University (PNU), in Urmia, Iran, in 2005, and MS in analytical chemistry from Payame-Noor University (PNU), Khoy, Iran, in 2009. His main area of interest is nanochemistry. He is a PhD student in analytical chemistry at the Urmia University, Urmia, Iran. Rahim Molaei received a BC in chemistry from Urmia University, Urmia, Iran in 2005, and MS in analytical chemistry from same university in 2007. His main area of interest is development of new sample preparation methods. He is a PhD student in analytical chemistry at the Urmia University, Urmia, Iran. Salahaddin Hajizadeh received a BC in chemistry from Payame-Noor University (PNU), Urmia, Iran in 2006, and MS in analytical chemistry from Urmia University, Urmia, Iran, in 2010. His main area of interest is application of nanoparticles in analytical chemistry. Aysan Rafipour received a BC in chemistry from Urmia University, Urmia, Iran in 2006, and MS in analytical chemistry from Khoy Payamour University, Khoy, Iran, in 2010. Her main area of interest is application of nanoparticles in analytical chemistry.