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Bifunctionalization of silver nanoparticles with 6-mercaptonicotinic acid and melamine for simultaneous colorimetric sensing of Cr3+ and Ba2+ ions
Accepted Manuscript Title: Bifunctionalization of silver nanoparticles with 6-mercaptonicotinic acid and melamine for simultaneous colorimetric sensing of Cr3+ and Ba2+ ions Author: Richita P. Modi Vaibhavkumar N. Mehta Suresh Kumar Kailasa PII: DOI: Reference:
S0925-4005(14)00076-8 http://dx.doi.org/doi:10.1016/j.snb.2014.01.059 SNB 16479
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
24-10-2013 14-1-2014 17-1-2014
Please cite this article as: R.P. Modi, V.N. Mehta, S.K. Kailasa, Bifunctionalization of silver nanoparticles with 6-mercaptonicotinic acid and melamine for simultaneous colorimetric sensing of Cr3+ and Ba2+ ions, Sensors and Actuators B: Chemical (2014), http://dx.doi.org/10.1016/j.snb.2014.01.059 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.
Bifunctionalization of silver nanoparticles with 6-mercaptonicotinic acid and melamine for simultaneous colorimetric sensing of Cr3+ and Ba2+ ions Richita P. Modi, Vaibhavkumar N. Mehta and Suresh Kumar Kailasa* a
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Department of Applied Chemistry, S. V. National Institute of Technology, Surat-395 007, India * Corresponding author, Phone: +91-261-2201730; Fax: +91-261-2227334 E-mail: [email protected]; [email protected]
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Abstract In this study, silver nanoparticles (Ag NPs) were bifunctionalized with 6mercaptonicotinic acid (MNA) and melamine (MA) for simple, rapid and simultaneous
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colorimetric detection of Cr3+ and Ba2+ ions in water samples. The bifunctionalized Ag NPs were confirmed by UV-visible, FT-IR, dynamic light scattering (DLS), transmission
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electron microscopy (TEM), and atomic force microscopic (AFM) techniques. The
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bifunctionalized Ag NPs were induced to aggregate quickly with Cr3+ and Ba2+ ions through color change from yellow- to reddish brown (Cr3+) and -to orange (Ba2+),
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respectively which was readily seen by the naked eye. The recognition mechanism is attributed to the unique structures of MNA and MA on Ag NPs surfaces, which can yield
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strong interactions (cooperative metal−ligand interaction) between Ag NPs and metal
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ions (Cr3+ and Ba2+). It was observed that the addition of metal ions could induce the
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aggregation of Ag NPs solutions at pH 6.0, yielding changes in color and in UV–visible absorption spectra. Using this system, good liner range was obtained in the range of 10 -
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370 µM, with limit of detection 64.51 and 80.21 nM for Cr3+ and Ba2+ ions, respectively. This method was successfully applied to detect metal ions (Cr3+ and Ba2+) in drinking, tap and river water samples.
Keywords: Bifunctionalized Ag NPs, Cr3+, Ba2+, UV-visible spectrometry, DLS, FT-IR and water samples.
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1. Introduction Due to rapid industrialization and excess use of chemicals, the metal contamination is rapidly growing in the environment and it is widely spreading into the
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soils and the aquatic ecosystem [1]. Metals contamination has been recognized as a major environmental concern due to their serious metabolic poisonous actions in various
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biochemical pathways [2]. Chromium (Cr) and barium (Ba) are considered as heavy
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metals and are widely used in various industrial processes, such as electroplating, glass industry, petroleum industry, alloying, fixing pigments and corrosion protection films,
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respectively. It is well known that chromium exists in two different oxidation states (Cr3+ and Cr6+) in the nature. Among these, Cr6+ is one of the most commonly encountered
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occupational hazards [3-4] and Cr3+ is considered as a fundamental trace element for significant glucose tolerance factor and insulin resistance in humans [5-7]. However, at
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high levels, Cr3+ has showed high affinity to bind with DNA, yielding a detrimental effect on cellular structures and damaging the cellular components (lipids, proteins and DNA)
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[8]. To date, the contaminations of water by Cr3+ and Ba2+ ions are still the most common. In this connection, various traditional methods such as UV-visible spectrometry [9-10], atomic absorption/emission spectroscopy [11], ion-chromatography [12], inductively coupled plasma-atomic emission spectrometry [9], voltammetry [13], and inductively coupled plasma-mass spectrometric [14] techniques have been used for the detection of heavy metal ions including Cr3+ and Ba2+ in water and environmental samples. However, some of the techniques are expensive, labor-intensive, timeconsuming, and required tedious sample preparation and preconcentration procedures. Importantly, these are incapable for on-site assays of metals with minimized samples 3 Page 3 of 40
volumes and reagents. Therefore, the development of novel, simple, rapid and simultaneous detection methods has became an urgent need to meet the practical challenges in metal ions assays in environmental samples.
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The unique size- and shape-dependent physico-chemical and optical properties of metal nanoparticles play key role in the development of simple and novel analytical
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techniques and methodologies for biochemical assays [15-18]. Recently, noble metal
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nanoparticles have been widely used as attractive colorimetric probes because of their excellent and distinctive optical properties such as high visible-region extinction
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coefficients and distance-dependent optical properties, which allow to visualize trace level targets simply by the naked eye [15-20]. As a result, intensive efforts have been
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devoted on the development of Ag NPs-based UV-visible spectrometric analytical
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methods for the sensitive detection of wide variety of molecules including metals, drugs
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and biomolecules [21]. Since, dispersed Ag NPs have shown high affinity to interact with trace level target species at minimum volume of samples, which results a color change in
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NPs that can be observed with naked eye [22]. For example, Li’s group explored the utility of triazole- ester and carboxyl-, glutathione-, and calixarene- modified Ag NPs as colorimetric probes for sensing of metal ions (Cd2+, Co2+ and Ni2+) and pesticides in water samples [22-25]. Farhadi’s and Anthony’s groups independently described the green synthetic approaches for preparation of Ag NPs and used as colorimetric probes for sensing of Hg2+ ion in water samples [26-27]. Zhu et al. developed ascorbic acid-capped Ag NPs-based colorimetric method for the sensitive detection of Cr6+ ion [28]. Wu and co-workers functionalized Ag NPs with 8-hydroxyquinoline-5-sulfonate for the detection
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of traces of Al3+ in water through color change and fluorescence enhancement at pH 7.4 [29]. Recently, significant advances have been made in the development of metallic
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(Au and Ag) NPs-based colorimetric sensing approaches for the detection of chromium and barium ions in various samples. Briefly, Au NPs surfaces were functionalized with
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various reagents such as triazole [30], 5-thio-2-nitrobenzoic acid-horseradish peroxide
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(TNBA-HP) [31], 5,5’-dithio-bis-(2-nitrobenzoic acid) (DTNBA) [32], citrate [33], tripolyphosphate [34], and dithiocarbamate-modified N-benzyl-4-(pyridine-4-ylmethl)
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aniline (BP-DTC) [35], and used as colorimetric sensor for the detection of Cr3+ and Cr6+ ions. Similarly, Cr3+ ion was detected by using Au NPs- [36] and Ag NPs [37] -based
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colorimetric and fluorescence methods. Li and co-workers have designed a fast and
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simple colorimetric method for the detection of Ba2+ ion by using aza-crown ether (ACE)
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functionalized Ag NPs as sensor [38]. Zhang and co-workers have functionalized Au NPs with 2-mercaptosuccinic acid (MSA) and used as a colorimetric probe for simultaneous
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reorganization of Ca2+, Sr2+, and Ba2+ ions in water samples [39]. Bai’s group developed tiopronin functionalized Au NPs for visual detection of Ba2+ ion in water samples [40]. These reports illustrated that the functionalization of NPs play key role to their induced aggregation with target species, and yielding color change from yellow to red. These interactions are mainly attributed due to the presence of free electron donating groups (– NH2, -COOH, -OH), which allows to act as an effective binding sites for trace level target species. In these methods, Ag NPs were functionalized with one organic ligand and used as colorimetric probe for detection of single metal ion. In order to detect more metal ions
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simultaneously, the design of highly selective and sensitive multi-metal ions sensors are of important issue. To date, there were no reports on the simultaneous detection of Cr3+ and Ba2+ ions
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by using Ag NPs. To enhance the capability of Ag NPs for the simultaneous detection of multi-metal ions, we propose the rational design on Ag NPs surfaces with MNA and MA,
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which can be acted as multi-metal ions colorimetric sensor for the simultaneous detection
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of one or more metal ions (Cr3+ and Ba2+) based on their interplay with targets (Supporting Information of Figure S1). The surface chemistry on Ag NPs allows effective
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induced aggregation between Ag NPs and metal ions, which allow to selective and sensitive detection of both metal ions (Cr3+ and Ba2+) simultaneously in water samples.
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2. Experimental section
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2.1 Chemicals and materials
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Silver nitrate, sodium borohydride, tris(hydroxymethyl) aminomethane (Tris), 6mercaptonicotinic acid (MNA), Trisodium citrate dihydrate, metal salts (Ba(NO3)2·4H2O, Co(NO3)2·6H2O,
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Cd(NO3)2·4H2O,
Cr(NO3)2.6H2O,
Cu(NO3)2·3H2O,
FeCl2·4H2O,
FeCl3·6H2O, Hg(NO3)2·H2O, Mg(NO3)2·6H2O, Mn(NO3)2·4H2O, NiSO4·6H2O, Pb(NO3)2, Zn(NO3)2·6H2O) were purchased from Sigma-Aldrich, USA. Melamine and hydrochloric acid (HCl) were purchased from Merck Ltd., India. All chemicals were of analytical grade and used without further purification. Milli-Q-purified water was used for sample preparations. 2.2 Synthesis of MNA-MA-Ag NPs We prepared Ag NPs according to the reported Creighton method in the literature [41] with minor modifications. Briefly, 15 mL of 2 mM sodium borohydride ice-chilled 6 Page 6 of 40
solution was mixed with 1 mL of tri-sodium citrate (1%) and stirred for 5 min. Then, 5 mL of silver nitrate (1 mM) solution was added drop wise into the above solution without stirring, yielding a light yellow color solution which confirms the formation of Ag NPs.
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To assemble MNA and MA molecules on the surfaces of Ag NPs, 250 µL of MNA (0.5 mM) and 500 µL of MA (1 mM) were both added into the above Ag NPs under constant
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of the experiments were performed at room temperature.
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stirring for 2 h to ensure self-assembly of MNA and MA onto the surface of Ag NPs. All
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2.3 Detection of Cr3+and Ba2+ions
For colorimetric detection of Cr3+and Ba2+ ions, 700 μL of standard metal ions
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(1.0 mM) solutions including Cr3+ and Ba2+ were added separately into mixture of 1300
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μL MNA-MA-Ag NPs by using Tris-HCl buffer at pH 6.0. As a result, the color of the
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solution was changed instantly from yellow- to reddish brown and -to orange for only Cr3+ and Ba2+, respectively. The assays were carried out by using UV–visible
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spectrophotometer.
To extent the surface modified Ag NPs towards real samples analysis, we applied
this method for detection of Cr3+ and Ba2+ ions in water samples (drinking water, tap water and river water). Briefly, water samples (drinking water, tap water and river water) were collected from SVNIT and Tapi River, Surat. The collected samples were spiked with known concentration of both metal ions (Cr3+ and Ba2+; 0.1, 0.5 and 1 mM) and other metal ions (1.0 mM), mixed thoroughly, filtered by using 0.45 µm membrane and then analyzed by the aforesaid procedure.
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2.4 Instrumentation UV-visible spectra were measured by using Maya Pro 2000 spectrophotometer (Ocean Optics, USA) at room temperature. Fourier transform infrared (FT-IR) spectra
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were recorded on a Perkin Elmer (FT-IR spectrum BX, Germany). Transmission electron microscopy (TEM) images were taken on a JEOL 3010. DLS measurements were
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obtained by using Zetasizer Nano ZS90 (Malvern, UK). The AFM images were obtained
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by using a Multimode Nanoscope VIII AFM (Bruker AXS, Germany).
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3. Results and discussion 3.1. Characterization
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It is well known that the surface modification of Ag NPs should be carried out in
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a controlled manner by tuning organic assembly on NPs surfaces, which allows to reduce
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the surface energy simultaneously and to prevent NPs agglomeration by forming protective layers on NPs surfaces. Therefore, we bifunctionalized Ag NPs surfaces with
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MNA and MA molecules simultaneously thereby acting as effective binding sites for multi-metal ions sensors. For this, first we optimized MA concentration on Ag NPs surfaces, since the passivation of noble metal NPs surfaces can be effectively performed by using mercapto functionality than that of –NH2 group of MA [42]. Supporting Information of Figure S2a shows UV-visible absorption spectra of Ag NPs capped with different concentrations of MA in ranging from 0.1 to 1.5 mM. To confirm the best concentration of MNA and MA molecules on Ag NPs surfaces, we also studied that the UV-visible spectra of Ag NPs upon the addition of different concentrations of both molecules simultaneously in Ag NPs solution (Supporting Information of Figure S2b). 8 Page 8 of 40
After optimizing MNA and MA concentrations on Ag NPs, we found that 0.5 mM of MNA and 1.0 mM of MA concentrations are more suitable to form uniform (one by one) surface layers on Ag NPs, which access to NPs with core diameters less than 5 nm.
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Importantly, Ag NPs acted as colorimetric sensors when their core diameters are mostly less than 5 nm thereby controlling their surface plasmon resonance (SPR) band at around
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405 nm, and allowing their change color from yellow to red upon addition of target
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species. Figure 1 shows the UV-visible spectra of bare Ag NPs (without MNA and MA) and MNA-MA functionalized Ag NPs. It can be noticed that the both bare Ag NPs and
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MNA-MA Ag NPs have shown strong absorption peaks at 390 and 405 nm, which confirms that the characteristic SPR band of Ag NPs. Furthermore, it can also be
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observed that there is a very slight red-shift in SPR band of Ag NPs before and after
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molecules.
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modifications, which confirms the bifunctionalization of Ag NPs with the both
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To ensure the MNA and MA molecules attachment on the surfaces of Ag NPs, we
characterized bifuncationalized Ag NPs by using FT-IR. It is noticed that the FT-IR spectrum of MNA-MA- functionalized Ag NPs (Supporting Information of Figure S3c) shows the characteristic features of benzene ring stretching modes at 1620, 1582, and 1465 cm-1 and –COOH and –NH2 stretching modes at 1696 cm-1 and 3000 to 3500 cm-1 , respectively. Compared with the FT-IR spectra of MNA and MA (Supporting Information of Figure S3a-S3b), the characteristic bands at 2569 cm-1 (–SH group stretching) was completely disappeared and 3139 cm-1 and at 3336 cm-1 (–NH2 stretching) was slightly shifted in FT-IR spectrum of bifunctionalized Ag NPs (Supporting 9 Page 9 of 40
Information of Figure S3c). The results indicated that MNA- and MA- molecules were successfully attached on the surfaces of Ag NPs through Ag-S and Ag-N linkages.
following formula based on the UV-visible spectra [43].
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Meanwhile, we also calculated the average size of bare Ag NPs by using the
D= (9.8127 X 10-7)λ3 – (1.7147 x 10-3)λ2 + (1.0064)λ – 194.84
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Where D (nm) is the size of a given Ag NPs sample, and λ is the wavelength (nm)
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of the SPR band of Ag NPs. By using the above formula, we estimated the average size of bare Ag NPs is ~3.14 nm, which showed the SPR band at 390 nm. Figure 2 shows the
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TEM images of bare Ag NPs and bifunctionalized Ag NPs. These results demonstrated that the both Ag NPs are well dispersed in solution, with average diameters of 2 and 5 nm
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for bare Ag NPs and MNA-MA-Ag NPs, respectively. As shown in Figure 2, bare Ag
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NPs are well-dispersed and bifunctionalized Ag NPs are polydispersed in solution, which
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is due to the assembly of MNA and MA molecules on Ag NPs surfaces. The hydrodynamic diameters of bare and MNA-MA-Ag NPs were confirmed by DLS (Figure
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5a-b). The hydrodynamic diameters were 2.0 and 5.0 nm for bare and MNA-MA- Ag NPs. The TEM images and DLS data are well agreed with the UV-visible calculations for estimation of an average size of Ag NPs. Furthermore, AFM analysis was also carried out for MNA-MA-Ag NPs. Figure 6a shows the AFM image of the MNA-MA-Ag NPs. This result revealed that the MNA-MA-Ag NPs were spherical and very well dispersed in nature. 3.2. Selective recognition of Cr3+and Ba2+ ions
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It is well known that the functionalized Ag NPs exhibit yellow color due to their SPR vibrations at an average size 5 nm. To know the aggregation ability of Ag NPs, we studied induced aggregation of bifunctionalized Ag NPs with the addition of various
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metal ions (Fe3+, Fe2+, Ni2+, Mg2+, Zn2+, Mn2+, Cd2+, Cu2+, Co2+, Cr3+, Ba2+ and Hg2+; 1.0 mM) separately in NPs solutions and the spectral changes were monitored by UV-visible
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spectrometry and digital camera (Figure 3a). Of all the ions tested, only Cr3+ and Ba2+
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ions were significantly caused to change their UV-visible spectral characteristics, which allow to change its SPR band from 405 nm to 535 nm and to 530 nm for Cr3+ and Ba2+,
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respectively. As shown in Figure 3a, the addition of Cr3+ and Ba2+ ions leads to a decrease in the SPR band intensity of Ag NPs at 405 nm and a dramatic increase in the
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absorbance intensity at 535 nm and 530 nm for Cr3+ and Ba2+ ions, which confirms the induced aggregation of Ag NPs with Cr3+ and Ba2+ ions. Figure 3b shows the color
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change of Ag NPs by the addition of Cr3+ and Ba2+ ions. Supporting information of Figure S4 shows the absorbance ratios of MNA-MA-Ag NPs at A535/530nm/A405nm in the
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presence of Cr3+, Ba2+ and other metal ions.
3.2. Effect of pH
To establish effective experimental conditions, we investigated the effect of pH
on the stability and sensitivity of bifunctionalized Ag NPs sensing system. The bifunctionalization was carried out without addition of HCl or NaOH or without optimizing the pH of reaction mixture. After functionalization of Ag NPs, the pH of bifunctionalized Ag NPs is found at 9.0, which is due to the presence of MA molecules on Ag NPs and they are very stable at this pH. Based on the above observations, we 11 Page 11 of 40
believe that –SH group of MNA and –NH2 group of MA are effectively attached on the surfaces of Ag NPs.
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It is well known that the degree of aggregation of bifunctionalized Ag NPs with metal ions is strongly depended on the pH of the solution. In this connection, we
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investigated the colorimetric responses of bifunctionalized Ag NPs towards Cr3+ and Ba2+
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ions at different pH ranging from 2.0 to 10.0. Figure 4 shows the UV-visible absorption spectra of Ag NPs with Cr3+ and Ba2+ ions at different pH of the solutions. It is noticed
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that the bifunctionalized Ag NPs have shown the most obvious color changes at pH 2.0 – 7.0. To evaluate the degree of Ag NPs aggregations with both metal ions, we measured
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the absorbance ratios at 535 nm to 405 nm (A535 nm/A405 nm) for Cr3+ and at 530 nm to 405
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nm (A530nm/A405nm) for Ba2+ ion (Supporting information of Figure S5). It can also be
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observed that the absorbance ratios (A535nm/A405nm and A585nm/A405nm) at pH 2.0 – 4.0 are very high for Cr3+ and Ba2+, which is due to the self aggregation of metallic NPs
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(neutralization of NPs surface charges) at lower pH < 4.0 [44]. To confirm this, we studied the UV-visible spectra of bifunctionalized Ag NPs at lower pH < 4.0 without addition of metal ions (data was not shown). It was noticed that the spectral changes of Ag NPs were observed due to the leaching of Ag NPs. To confirm this, we studied the hydrodynamic diameter of MNA-MA-Ag NPs at pH 2.0 to 4.0 without addition of metal ions (Supporting Information of Figure S6). It was observed that the hydrodynamic diameters of Ag NPs are increased to ~190 nm (pH 2.0), ~139 nm (pH 3.0) and ~15 nm (pH 4.0), and are larger than that at pH 6.0. This is due to a decrease in electrostatic repulsions between NPs at higher H+ ion concentration, and resulting a high degree of 12 Page 12 of 40
NPs aggregation without addition of analytes [45]. Importantly, the absorbance ratios of bifunctionalized Ag NPs at two wavelengths (A535nm/A405nm for Cr3+ and A530nm/A405nm for Ba2+) are much larger than that at pH 5.0 and 7.0 (for Ba2+ ion only), confirming that
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the both ions are easily induced the aggregation of Ag NPs at pH 6.0 (Supporting information of Figure S5). At basic conditions, there are no obvious color changes in Ag
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NPs solutions. Therefore, we selected pH 6.0 as the optimum pH for effective
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simultaneous sensing of both metal ions using bifunctionalized Ag NPs, which avoids the protonation of –NH2 of MA and the detachment of –SH group of MNA on Ag NPs
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surfaces.
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3.2. Estimation of bifunctionalized Ag NPs aggregation with Cr3+and Ba2+ ions We studied the DLS experiments for the estimation of diameters of bare and
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bifunctionalized Ag NPs and with addition of Cr3+ and Ba2+ ions. As shown in Figure 5, the hydrodynamic diameter of bare Ag NPs is slightly increased from 2.0 nm to 5.0 nm,
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which is due to the assembly of MNA and MA molecules on Ag NPs surfaces. Figure 5cd shows the DLS data of bifunctionalized Ag NPs in the presence of Cr3+ and Ba2+ ions (370 µM) at pH 6.0. It can be noticed that there are significant variations in the hydrodynamic diameter of Ag NPs by the addition of both ions, which confirms that the induced aggregation of Ag NPs with Cr3+ and Ba2+ ions. Based on the DLS data, we confirmed that the hydrodynamic diameter of bifunctionalized Ag NPs is notably increased from 5.0 nm to 164 nm and to 122 nm for Cr3+ and Ba2+ ions, respectively, thereby suggesting the effective aggregation of Ag NPs in the presence of Cr3+ and Ba2+ ions. Figure 2c-d shows the TEM images of MNA-MA-Ag NPs in presence of Cr3+ and 13 Page 13 of 40
Ba2+ ions at 370 µM. It can be noticed that the morphology and sizes of MNA-MA-Ag NPs were greatly changed by the addition of Cr3+ and Ba2+ ions. The selected-area electron diffraction (SAED) patterns of Cr3+ and Ba2+ ions-induced aggregations of
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MNA-MA-Ag NPs were recorded from a region that has no contrast within the nanoparticles, yielded in hollow rings (Figure 2e-f). The selected areas of Cr3+ and Ba2+
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ions-induced aggregations of MNA-MA-Ag NPs were measured at ~200 nm. It can be
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noticed that the hollow rings were appeared for both SAED patterns, which confirmed that the aggregated Ag NPs are in amorphous phase. However, it is also observed that in
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some regions, the electron diffraction patterns are consisted very few unsystematic arrangements of spots. These results confirmed that the hollow rings and unsystematic
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arrangement of spots are depicted from amorphous and crystalline regions in Cr3+ and Ba2+ ions-induced aggregations of MNA-MA-Ag NPs. To confirm this, we also studied
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AFM images of Cr3+ and Ba2+ ions-induced aggregations of MNA-MA-Ag NPs (Figure 6). These results revealed that the MNA-MA-Ag NPs are well dispersed with uniform
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heights (Figure 6a). However, the MNA-MA-Ag NPs structural changes (size and height) were greatly changed by the addition of Cr3+ and Ba2+ ions, confirming that their induced aggregation with Cr3+ and Ba2+ ions (Figure 6b-c). The TEM and AFM images clearly demonstrated that Cr3+ and Ba2+ ions are greatly induced the aggregations of MNA-MAAg NPs, resulting in a red shifted change in their UV-visible absorption spectra. 3.3. Selectivity of bifunctionalized Ag NPs towards Cr3+and Ba2+ ions In order to explore the potentiality of bifunctionalized Ag NPs for assays of Cr3+ and Ba2+ ions in the presence of other metal ions, we studied the UV-visible absorption 14 Page 14 of 40
spectra of MNA-MA-Ag NPs in the presence of mixture of metal ions (1 mM) without and with Cr3+ and Ba2+ ions (Figure 7). It can be noticed that the SPR absorption shift was not observed by the addition of only other metal ions mixture (without Cr3+ and Ba2+
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ions). However, the SPR absorption shift was only observed by the addition of Cr3+ and Ba2+ ions separately along with the mixtures of other metal ions (1 mM), just like as
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solely additions of Cr3+ and Ba2+ ions. These results indicate that only Cr3+ and Ba2+ ions
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are induced the aggregation of Ag NPs and showed good resistance from other metal ions.
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3.4. Analytical performance
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Before selecting the metal ions concentration range for calibration graphs, we
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studied the UV-visible spectra of MNA-MA-Ag NPs by the addition of different concentrations of metal ions from 0.1 to 1000 µM for Cr3+ ion and from 100 to 1000 µM
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for Ba2+ ion, respectively (Supporting Information of Figures S7-S8). These results indicated that the color of MNA-MA-Ag NPs is gradually changing from yellow to reddish brown and yellow to orange for Cr3+ and Ba2+, which can be observed by naked eye (Supporting Information of Figures S7b-S8b). Based on these results, we constructed the calibration graph by using metal ions concentration from 10 to 370 µM. Figure 8 shows the the degrees of aggregation of bifunctionalized Ag NPs in terms of the changes in absorbance ratios at two wavelengths A535nm/A405nm and A530nm/A405nm corresponding to the Cr3+ and Ba2+ ions concentrations ranging from 10 to 370 µM, respectively. It can be noticed that a decrease in the absorption peak at 405 nm and an increase in the absorption 15 Page 15 of 40
peaks at 535 nm and 530 nm upon the addition of increasing concentration of Cr3+ and Ba2+ ions separately into Ag NPs solutions (Figure 8a-b). As shown in Supporting Information of Figure S9, the absorbance ratios (A535nm/A405nm for Cr3+ and A530nm/A405nm
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for Ba2+) are sensitive to the concentrations of both ions. The calibration graphs were obtained in the range of 50 to 330 µM and the equation for the resulting calibration plots
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were y = 0.465x – 0.571 and y = 0.374x – 0.011 for Cr3+ and Ba2+ ions (x was the
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concentration of metal ions, and y was the intercept of the slope), with correlation coefficients of 0.992 and 0.981 for Cr3+ and Ba2+ ions, respectively (Supporting
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Information of Figure S9). This method showed detection limits at 64.51 nM and 80.21 nM for Cr3+ and Ba2+ ions, respectively. The analytical data of the method was shown in
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Table 1.
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3.5. Analysis of Cr3+ and Ba2+ ions in real water samples
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In order to evaluate the practical application of this method, we analyzed Cr3+ and
Ba2+ ions in three different water samples (drinking, tap and river water) by using bifunctionalized Ag NPs as colorimetric probes. The collected water samples spiked with Cr3+ and Ba2+ ions separately, with the concentrations of 0.1, 0.5 and 1 mM, and are analyzed by the aforesaid procedure. Using this method, good recoveries were obtained in the range of 97 to 101% and 98 to 102%, with RSD (%) values 1.17 - 2.32% and 1.24 1.66% for Cr3+ and Ba2+ ions in the drinking, tap and river water samples (Table 2). Table 3 summarizes the comparison of present method with the reported NPs-based UV-visible and fluorescence methods [30-40] for the detection of Cr3+ and Ba2+ ions. These results 16 Page 16 of 40
clearly demonstrated that this method exhibited good selectivity and showed superiority for the simultaneous detection of Cr3+ and Ba2+ ions in environmental samples.
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4. Conclusions
We explore the use of bifunctionalized Ag NPs as colorimetric sensors for
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simultaneous detection of Cr3+ and Ba2+ ions in water samples. The rapid, selective and
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sensitive response to Cr3+ and Ba2+ ions suggests that MNA-MA-Ag NPs colorimetric probe may have potential application in the simultaneous detection of Cr3+ and Ba2+ ions
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in water samples. The bifunctionalized Ag NPs exhibited high affinity towards Cr3+ and Ba2+ ions over a variety of potentially interfering metal ions, yielding an instantaneous
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color change of Ag NPs from yellow to reddish brown and to orange, and corresponding
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SPR absorption shift from 405 nm to 535 nm and 530 nm for Cr3+ and Ba2+ ions,
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respectively. These surface modified Ag NPs were successfully acted as a colorimetric sensor for Cr3+ and Ba2+ ions, providing limit of detection of 64.51 nM and 80.21 nM for
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Cr3+ and Ba2+ ions, respectively. The tuning of multi-molecular assembly and their high stability on Ag NPs surfaces allow to act as efficient colorimetric probes for multi-metal ions sensing in various water samples. Acknowledgements
This work was financially supported by the S. V. National Institute of
Technology, Surat under M.Sc., research project. Special thanks are given to Prof. Z. V. P. Murthy, and Mr. Chetan Patel, Chemical Engineering Department, S. V. National Institute of Technology, Surat for providing DLS instrument facility to this work. We also thank Department of Science and Technology for providing Maya Pro 2000 17 Page 17 of 40
spectrophotometer under the Fast-Track Young Scientist Scheme (2011 – 2014). We would like to thank Mr. Vikas Patel, SICART, V. V. Nagar, Anand for his assistance in
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TEM data.
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Biography of the authors Richita P. Modi completed her M.Sc., Chemistry at S. V. National Institute of Technology.
Her current research interest is development of nanomaterials-based
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colorimetric sensors for metal ions analysis.
Vaibhavkumar N. Mehta completed his Master degree in Nanoscience and
cr
Nanotechnology from Sardar Patel University, Gujarat. Currently he is doing his Ph. D. at
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S. V. National Institute of Technology, Surat. His research interest is functionalization of nanoparticles for colorimetric sensing of biomolecules and metal ions.
an
Suresh Kumar Kailasa completed his Ph. D. in Chemistry from S. V. University, Tirupati in 2005. After completing his two postdoctoral fellowships at South Korea and at
M
National Sun Yat-Sen University, Taiwan, he joined as a faculty in S. V. National
d
Institute of Technology, Surat in 2009. He received Young Scientist Award from Taiwan
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Mass Spectrometry Society in 2013. His interest includes nanomaterials integration in MALDI-MS and the development of nanomaterials-based colorimetric sensors for
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biomolecules, drugs and metal ions.
25 Page 25 of 40
Figure captions Figure 1. UV-visible spectra of bare Ag NPs and MNA-MA-Ag NPs. Inset picture show bare Ag NPs and MNA-MA-Ag NPs.
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Figure 2. TEM images of (a) bare Ag NPs, (b) MNA-MA-Ag NPs, and MNA-MA-Ag NPs induced aggregations by the addition of 370 µM (c) Cr3+ and (d) Ba2+ ion. The
cr
SAED pattern of aggregated MNA-MA-Ag NPs in presence of (e) Cr3+ and (f) Ba2+ ions.
us
Figure 3. (I) UV-visible absorption spectra of MNA-MA-Ag NPs in the presence of different metal ions (Fe3+, Fe2+, Ni2+, Mg2+, Zn2+, Mn2+, Cd2+, Cu2+, Co2+, Cr3+, Ba2+ and
an
Hg2+) (II) Photographic image of bare Ag NPs and MNA-MA-Ag NPs in the presence of various metal ions (Fe3+, Fe2+, Ni2+, Mg2+, Zn2+, Mn2+, Cd2+, Cu2+, Co2+, Cr3+, Ba2+ and
M
Hg2+).
Figure 4. UV-visible absorption spectra of MNA-MA-Ag NPs in presence of (a) Cr3+ at
te
d
pH 2 to 10 and (b) Ba2+ at pH 2 to 10.
Figure 5. DLS of (a) bare Ag NPs (b) MNA-MA-Ag NPs and induced aggregation of
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MNA-MA-Ag NPs with (c) Cr3+ and (d) Ba2+ ions. Figure 6. AFM images of (a) MNA-MA-Ag NPs and their induced aggregations with (b) Cr3+ and (b) Ba2+ ions.
Figure 7. Interference study in presence of different metal ions: (1) MNA-MA-Ag NPs (2) MNA-MA-Ag NPs + different metal ions (Fe3+, Fe2+, Ni2+, Mg2+, Zn2+, Mn2+, Cd2+, Cu2+, Co2+ and Hg2+) (3) MNA-MA-Ag NPs + different metal ions + Cr3+ and (4) MNAMA-Ag NPs + different metal ions + Ba2+ ion. Figure 8. (a) UV–visible spectra of MNA-MA-Ag NPs solutions with various concentrations of Cr3+ with the range of 10 to 370 µM. Inset picture show photographic 26 Page 26 of 40
image of MNA-MA-Ag NPs containing various Cr3+ concentration from 10 to 370 µM. (b) UV–visible spectra of MNA-MA-Ag NPs solutions with various concentrations of Ba2+ with the range of 10 to 370 µM. Inset picture show photographic image of MNA-
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te
d
M
an
us
cr
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MA-Ag NPs containing various Ba2+ concentration from 10 to 370 µM.
27 Page 27 of 40
Table 1. Analytical data for detection of Cr3+ and Ba2+ ions by using bifunctionalized Ag NPs as colorimetric sensors. Bare Ag
MNA-MA-Ag
NPs Color
NPs
Light
MNA-MA-Ag
MNA-MA-Ag
NPs+ Cr3+
NPs + Ba2+
Yellow
Reddish brown
530
535
164.0
122.0
10 to 370
10 to 370
0.465
0.374
0.992
0.981
64.51
80.21
405
Size (nm)
2.0
5.0
-
-
Slope
-
-
R2 value
-
LOD (nM)
-
us
390
an
λ max (nm)
M
(µM)
orange
cr
yellow
Linear range
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Analytical data
d
-
Ac ce p
te
-
28 Page 28 of 40
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Table 2. Colorimetric sensing of Cr3+ and Ba2+ in water samples (drinking, tap, and river water) by using bifunctionalized Ag NPs as
Cr3+ Added
Found (mM) Recovery (%)
RSD (%)
Added
Recovery (%)
RSD (%)
(n=3)
(n=3)
(mM)
(n=3)
(n=3)
0.096
97
2.40
0.1
0.102
102
1.35
0.505
101
1.56
0.5
0.505
101
1.45
0.982
98
2.32
1
0.994
99
1.66
0.099
99
1.57
0.1
0.101
101
1.36
0.5
0.503
100
1.41
0.5
0.492
98
1.61
1
0.994
99
1.61
1
1.020
102
1.57
0.1
0.101
101
1.53
0.1
0.098
98
1.45
0.5
0.498
99
1.17
0.5
0.497
99
1.24
1
1.010
101
2.03
1
1.01
101
1.29
Drinking 0.5 water 1
Ac
ce
0.1
River water
pt
0.1
ed
(mM)
Tap water
Ba2+
M
Sample
an
probes.
Found (mM)
29 Page 29 of 40
i cr
Size (nm)
Au NPs
Triazole
14
Au NPs
TNBA-HP
Au NPs
DTNBA
Au NPs
Citrate
Au NPs
Tripolyphosphate
Au NPs
BP-DTC
Au NPs Ag NPs
Detection method
References
Cr3+
1.4×10-6
UV-visible
[30]
15
Cr3+
0.04×10-6
UV-visible
[31]
13
Cr3+
93.6 ppb
UV-visible
[32]
13.8
Cr6+, Cr3+
4.0 - 0.3×10-6
UV-visible
[33]
11.13
Cr3+
1.0×10-7
UV-visible
[34]
-
Cr3+
31 ppb
UV-visible
[35]
Citrate
30 – 35
Cr3+
1.06×10-7
UV-visible
[36]
-
65±2
Cr3+
2×10-9
Fluorescence
[37]
ACE
-
Ba2+
1.0×10-8
UV-visible
[38]
MSA
-
Ca2+, Sr2+, Ba2+
20, 8.0, and 2.5×10-6
UV-visible
[39]
Tiopronin
13
Ba2+
1.5×10-6
UV-visible
[40]
Cr3+
64.51×10-9
Ba2+
80.21×10-9
UV-visible
This study
ed
pt
ce
Ac
Au NPs
Au NPs Ag NPs
MNA and MA
Analytes
an
Capping agent
Ag NPs
LOD (M)
M
Nanoparticles
us
Table 3. Comparison of MNA-MA-Ag NPs as colorimetric sensor for the detection of Cr3+ and Ba2+ with the reported methods.
5
30 Page 30 of 40
i cr us
an
Graphical Abstract
Capped AgNPs
Fe 3+
Fe 2+
Ni 2+
Mg 2+
Zn2+
Mn2+
Cd2+
Cu2+
Co 2+
Pb2+
Cr3+
Ba2+
Hg 2+
ce
pt
Bare AgNPs
ed
M
We describe the applications of bifunctionalized silver nanoparticles (Ag NPs) with 6-mercaptonitotinic acid (MNA) and melamine (MA) for simple, rapid and simultaneous colorimetric detection of Cr3+ and Ba2+ ions in water samples. The bifunctionalized Ag NPs were confirmed by UV-visible, FT-IR, dynamic light scattering (DLS) and transmission electron microscopy (TEM) techniques. The MNA and MA molecules on Ag NPs surfaces allowed to act as selective binding sites for Cr3+ and Ba2+ ions. As a result, these Ag NPs were induced to aggregate quickly with Cr3+ and Ba2+ ions through color change from yellow- to reddish brown (Cr3+) and -to orange (Ba2+), respectively which was readily seen by the naked eye.
Ac
Photographic image of bare Ag NPs and MNA-MA-Ag NPs in the presence of various metal ions (Fe3+, Fe2+, Ni2+, Mg2+, Zn2+, Mn2+, Cd2+, Cu2+, Co2+, Cr3+, Ba2+ and Hg2+).
31 Page 31 of 40
i cr us
an
Research Highlights of “MNA-MA-Ag NPs”
Bifunctionalized Ag NPs acted as colorimetric sensors for Ba2+ and Cr3+.
MNA-MA-Ag NPs were induced to aggregate quickly with Cr3+ and Ba2+ ions.
M
characterization of MNA-MA-Ag NPs.
FT-IR, DLS, AFM and TEM were used for the
ed
method showed good linearity with LOD 64.51 and 80.21 nM for Cr3+ and Ba2+.
Ac
ce
pt
32 Page 32 of 40
This
Figure(s)
1.4 Bare Ag NPs MNA-MA-Ag NPs
Capped Ag NPs
cr
Bare Ag NPs
us
0.8
0.6
an
Absorbance
1.0
ip t
1.2
M
0.4
0.0 300
400
ed
0.2
500
600
700
800
900
pt
Wavelength(nm)
Ac
ce
Figure 1
Page 33 of 40
(b)
20 nm
(d)
20 nm
20 nm
20 nm
(f)
Ac
(e)
ce
pt
ed
M
(c)
an
us
cr
ip t
(a)
200 nm
200 nm
Figure 2 Page 34 of 40
(I) 1.4
ip t
1.2
cr
Cr3+ Ba2+
us
0.6
an
Absorbance
1.0
0.8
M
0.4
Bare Ag NPs Capped Ag NPs 3+ Fe 2+ Fe 2+ Ni 2+ Mg 2+ Zn 2+ Mn 2+ Cd 2+ Cu 2+ Co 3+ Cr 2+ Ba 2+ Hg
0.0 300
400
ed
0.2
500
600
700
800
900
(II) Capped AgNPs
Fe3+
Fe2+
Ni2+
Mg2+
Zn2+
Mn2+
Cd2+
Cu2+
Co2+
Pb2+
Cr3+
Ba2+
Hg2+
Ac
Bare AgNPs
ce
pt
Wavelength(nm)
Figure 3
Page 35 of 40
1.4
(a)
Bare Ag NPs Capped Ag NPs
1.2
pH 2 pH 4 pH 6
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1.0
pH 10
0.8
2
535 nm
4
8
10
us
0.6
6
cr
Absorbance
pH 8
pH
an
0.4
M
0.2
0.0 400
500
600
700
ed
300
800
900
Wavelength(nm)
pt
(b) 1.6
2
ce
1.4
4
5
6
1.0
0.8
pH 2
7
8
9
10
pH 3 pH 4 pH 5
pH
pH 6
530 nm pH 6
Ac
1.2
Absorbance
3
pH 7 pH 8 pH 9 pH 10
0.6
0.4
0.2
0.0 300
400
500
600
Wavelength(nm)
Figure 4
700
800
900
Page 36 of 40
(a)
cr
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Number (%)
Average hydrodynamic diameter: ~ 2 nm
Size (d. nm)
us
(b)
M
an
Number (%)
Average hydrodynamic diameter: ~ 5 nm
ed
Size (d. nm)
ce Ac
Size (d. nm) Average hydrodynamic diameter: ~ 122 nm
Number (%)
(d)
Size (d. nm) Figure 5
Average hydrodynamic diameter: ~ 164 nm
pt
Number (%)
(c)
Page 37 of 40
an
us
cr
ip t
(a)
ce Ac
(c)
pt
ed
M
(b)
Figure 6
Page 38 of 40
1.4
1.2
2
3
4
cr
1
0.8
us
1. MNA-MA-Ag NPs without metal ions 2. MNA-MA-Ag NPs +other metal ions (not Cr3+ and Ba2+) 3. MNA-MA-Ag NPs+ other metal ions + Cr3+ 4. MNA-MA-Ag NPs+ other metal ions + Ba2+
0.6
an
Absorbance
1.0
ip t
Bare AgNPs Capped AgNPs Capped AgNPs+different metal ion solution of 2+ 3+ 2+ 2+ 2+ 2+ 2+ 2+ 2+ 2+ 2+ Fe ,Fe ,Hg ,Mg .Mn ,Cu ,Co ,Pb ,Zn ,Cd ,Ni 3+ Capped AgNPs+different metal ion solution + Cr 2+ Capped AgNPs+different metal ion solution + Ba
M
0.4
0.0 300
400
ed
0.2
500
600
700
800
900
pt
Wavelength(nm)
Ac
ce
Figure 7
Page 39 of 40
1.4
(a) 10
50
90
130
170
210
250
330
290
370
1.2
Concentration of Cr3+ (μM)
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10
cr
0.8
370 µM 0.6
us
Absorbance
1.0
an
0.4
M
0.2
0.0 400
500
600
ed
300
700
800
900
Wavelength(nm)
1.6
(b)
ce
1.0
Ac
Absorbance
1.2
0.8
50
pt
10
1.4
90
130
170
210
250
290
330
370
Concentration of Ba2+ (μM) 10
370 µM
0.6
0.4
0.2
0.0 300
400
500
600
Wavelength(nm)
Figure 8
700
800
900 Page 40 of 40
S0925-4005(14)00076-8 http://dx.doi.org/doi:10.1016/j.snb.2014.01.059 SNB 16479
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
24-10-2013 14-1-2014 17-1-2014
Please cite this article as: R.P. Modi, V.N. Mehta, S.K. Kailasa, Bifunctionalization of silver nanoparticles with 6-mercaptonicotinic acid and melamine for simultaneous colorimetric sensing of Cr3+ and Ba2+ ions, Sensors and Actuators B: Chemical (2014), http://dx.doi.org/10.1016/j.snb.2014.01.059 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.
Bifunctionalization of silver nanoparticles with 6-mercaptonicotinic acid and melamine for simultaneous colorimetric sensing of Cr3+ and Ba2+ ions Richita P. Modi, Vaibhavkumar N. Mehta and Suresh Kumar Kailasa* a
Ac ce p
te
d
M
an
us
cr
ip t
Department of Applied Chemistry, S. V. National Institute of Technology, Surat-395 007, India * Corresponding author, Phone: +91-261-2201730; Fax: +91-261-2227334 E-mail: [email protected]; [email protected]
1 Page 1 of 40
Abstract In this study, silver nanoparticles (Ag NPs) were bifunctionalized with 6mercaptonicotinic acid (MNA) and melamine (MA) for simple, rapid and simultaneous
ip t
colorimetric detection of Cr3+ and Ba2+ ions in water samples. The bifunctionalized Ag NPs were confirmed by UV-visible, FT-IR, dynamic light scattering (DLS), transmission
cr
electron microscopy (TEM), and atomic force microscopic (AFM) techniques. The
us
bifunctionalized Ag NPs were induced to aggregate quickly with Cr3+ and Ba2+ ions through color change from yellow- to reddish brown (Cr3+) and -to orange (Ba2+),
an
respectively which was readily seen by the naked eye. The recognition mechanism is attributed to the unique structures of MNA and MA on Ag NPs surfaces, which can yield
M
strong interactions (cooperative metal−ligand interaction) between Ag NPs and metal
d
ions (Cr3+ and Ba2+). It was observed that the addition of metal ions could induce the
te
aggregation of Ag NPs solutions at pH 6.0, yielding changes in color and in UV–visible absorption spectra. Using this system, good liner range was obtained in the range of 10 -
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370 µM, with limit of detection 64.51 and 80.21 nM for Cr3+ and Ba2+ ions, respectively. This method was successfully applied to detect metal ions (Cr3+ and Ba2+) in drinking, tap and river water samples.
Keywords: Bifunctionalized Ag NPs, Cr3+, Ba2+, UV-visible spectrometry, DLS, FT-IR and water samples.
2 Page 2 of 40
1. Introduction Due to rapid industrialization and excess use of chemicals, the metal contamination is rapidly growing in the environment and it is widely spreading into the
ip t
soils and the aquatic ecosystem [1]. Metals contamination has been recognized as a major environmental concern due to their serious metabolic poisonous actions in various
cr
biochemical pathways [2]. Chromium (Cr) and barium (Ba) are considered as heavy
us
metals and are widely used in various industrial processes, such as electroplating, glass industry, petroleum industry, alloying, fixing pigments and corrosion protection films,
an
respectively. It is well known that chromium exists in two different oxidation states (Cr3+ and Cr6+) in the nature. Among these, Cr6+ is one of the most commonly encountered
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occupational hazards [3-4] and Cr3+ is considered as a fundamental trace element for significant glucose tolerance factor and insulin resistance in humans [5-7]. However, at
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high levels, Cr3+ has showed high affinity to bind with DNA, yielding a detrimental effect on cellular structures and damaging the cellular components (lipids, proteins and DNA)
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[8]. To date, the contaminations of water by Cr3+ and Ba2+ ions are still the most common. In this connection, various traditional methods such as UV-visible spectrometry [9-10], atomic absorption/emission spectroscopy [11], ion-chromatography [12], inductively coupled plasma-atomic emission spectrometry [9], voltammetry [13], and inductively coupled plasma-mass spectrometric [14] techniques have been used for the detection of heavy metal ions including Cr3+ and Ba2+ in water and environmental samples. However, some of the techniques are expensive, labor-intensive, timeconsuming, and required tedious sample preparation and preconcentration procedures. Importantly, these are incapable for on-site assays of metals with minimized samples 3 Page 3 of 40
volumes and reagents. Therefore, the development of novel, simple, rapid and simultaneous detection methods has became an urgent need to meet the practical challenges in metal ions assays in environmental samples.
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The unique size- and shape-dependent physico-chemical and optical properties of metal nanoparticles play key role in the development of simple and novel analytical
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techniques and methodologies for biochemical assays [15-18]. Recently, noble metal
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nanoparticles have been widely used as attractive colorimetric probes because of their excellent and distinctive optical properties such as high visible-region extinction
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coefficients and distance-dependent optical properties, which allow to visualize trace level targets simply by the naked eye [15-20]. As a result, intensive efforts have been
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devoted on the development of Ag NPs-based UV-visible spectrometric analytical
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methods for the sensitive detection of wide variety of molecules including metals, drugs
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and biomolecules [21]. Since, dispersed Ag NPs have shown high affinity to interact with trace level target species at minimum volume of samples, which results a color change in
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NPs that can be observed with naked eye [22]. For example, Li’s group explored the utility of triazole- ester and carboxyl-, glutathione-, and calixarene- modified Ag NPs as colorimetric probes for sensing of metal ions (Cd2+, Co2+ and Ni2+) and pesticides in water samples [22-25]. Farhadi’s and Anthony’s groups independently described the green synthetic approaches for preparation of Ag NPs and used as colorimetric probes for sensing of Hg2+ ion in water samples [26-27]. Zhu et al. developed ascorbic acid-capped Ag NPs-based colorimetric method for the sensitive detection of Cr6+ ion [28]. Wu and co-workers functionalized Ag NPs with 8-hydroxyquinoline-5-sulfonate for the detection
4 Page 4 of 40
of traces of Al3+ in water through color change and fluorescence enhancement at pH 7.4 [29]. Recently, significant advances have been made in the development of metallic
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(Au and Ag) NPs-based colorimetric sensing approaches for the detection of chromium and barium ions in various samples. Briefly, Au NPs surfaces were functionalized with
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various reagents such as triazole [30], 5-thio-2-nitrobenzoic acid-horseradish peroxide
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(TNBA-HP) [31], 5,5’-dithio-bis-(2-nitrobenzoic acid) (DTNBA) [32], citrate [33], tripolyphosphate [34], and dithiocarbamate-modified N-benzyl-4-(pyridine-4-ylmethl)
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aniline (BP-DTC) [35], and used as colorimetric sensor for the detection of Cr3+ and Cr6+ ions. Similarly, Cr3+ ion was detected by using Au NPs- [36] and Ag NPs [37] -based
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colorimetric and fluorescence methods. Li and co-workers have designed a fast and
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simple colorimetric method for the detection of Ba2+ ion by using aza-crown ether (ACE)
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functionalized Ag NPs as sensor [38]. Zhang and co-workers have functionalized Au NPs with 2-mercaptosuccinic acid (MSA) and used as a colorimetric probe for simultaneous
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reorganization of Ca2+, Sr2+, and Ba2+ ions in water samples [39]. Bai’s group developed tiopronin functionalized Au NPs for visual detection of Ba2+ ion in water samples [40]. These reports illustrated that the functionalization of NPs play key role to their induced aggregation with target species, and yielding color change from yellow to red. These interactions are mainly attributed due to the presence of free electron donating groups (– NH2, -COOH, -OH), which allows to act as an effective binding sites for trace level target species. In these methods, Ag NPs were functionalized with one organic ligand and used as colorimetric probe for detection of single metal ion. In order to detect more metal ions
5 Page 5 of 40
simultaneously, the design of highly selective and sensitive multi-metal ions sensors are of important issue. To date, there were no reports on the simultaneous detection of Cr3+ and Ba2+ ions
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by using Ag NPs. To enhance the capability of Ag NPs for the simultaneous detection of multi-metal ions, we propose the rational design on Ag NPs surfaces with MNA and MA,
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which can be acted as multi-metal ions colorimetric sensor for the simultaneous detection
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of one or more metal ions (Cr3+ and Ba2+) based on their interplay with targets (Supporting Information of Figure S1). The surface chemistry on Ag NPs allows effective
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induced aggregation between Ag NPs and metal ions, which allow to selective and sensitive detection of both metal ions (Cr3+ and Ba2+) simultaneously in water samples.
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2. Experimental section
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2.1 Chemicals and materials
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Silver nitrate, sodium borohydride, tris(hydroxymethyl) aminomethane (Tris), 6mercaptonicotinic acid (MNA), Trisodium citrate dihydrate, metal salts (Ba(NO3)2·4H2O, Co(NO3)2·6H2O,
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Cd(NO3)2·4H2O,
Cr(NO3)2.6H2O,
Cu(NO3)2·3H2O,
FeCl2·4H2O,
FeCl3·6H2O, Hg(NO3)2·H2O, Mg(NO3)2·6H2O, Mn(NO3)2·4H2O, NiSO4·6H2O, Pb(NO3)2, Zn(NO3)2·6H2O) were purchased from Sigma-Aldrich, USA. Melamine and hydrochloric acid (HCl) were purchased from Merck Ltd., India. All chemicals were of analytical grade and used without further purification. Milli-Q-purified water was used for sample preparations. 2.2 Synthesis of MNA-MA-Ag NPs We prepared Ag NPs according to the reported Creighton method in the literature [41] with minor modifications. Briefly, 15 mL of 2 mM sodium borohydride ice-chilled 6 Page 6 of 40
solution was mixed with 1 mL of tri-sodium citrate (1%) and stirred for 5 min. Then, 5 mL of silver nitrate (1 mM) solution was added drop wise into the above solution without stirring, yielding a light yellow color solution which confirms the formation of Ag NPs.
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To assemble MNA and MA molecules on the surfaces of Ag NPs, 250 µL of MNA (0.5 mM) and 500 µL of MA (1 mM) were both added into the above Ag NPs under constant
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of the experiments were performed at room temperature.
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stirring for 2 h to ensure self-assembly of MNA and MA onto the surface of Ag NPs. All
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2.3 Detection of Cr3+and Ba2+ions
For colorimetric detection of Cr3+and Ba2+ ions, 700 μL of standard metal ions
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(1.0 mM) solutions including Cr3+ and Ba2+ were added separately into mixture of 1300
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μL MNA-MA-Ag NPs by using Tris-HCl buffer at pH 6.0. As a result, the color of the
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solution was changed instantly from yellow- to reddish brown and -to orange for only Cr3+ and Ba2+, respectively. The assays were carried out by using UV–visible
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spectrophotometer.
To extent the surface modified Ag NPs towards real samples analysis, we applied
this method for detection of Cr3+ and Ba2+ ions in water samples (drinking water, tap water and river water). Briefly, water samples (drinking water, tap water and river water) were collected from SVNIT and Tapi River, Surat. The collected samples were spiked with known concentration of both metal ions (Cr3+ and Ba2+; 0.1, 0.5 and 1 mM) and other metal ions (1.0 mM), mixed thoroughly, filtered by using 0.45 µm membrane and then analyzed by the aforesaid procedure.
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2.4 Instrumentation UV-visible spectra were measured by using Maya Pro 2000 spectrophotometer (Ocean Optics, USA) at room temperature. Fourier transform infrared (FT-IR) spectra
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were recorded on a Perkin Elmer (FT-IR spectrum BX, Germany). Transmission electron microscopy (TEM) images were taken on a JEOL 3010. DLS measurements were
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obtained by using Zetasizer Nano ZS90 (Malvern, UK). The AFM images were obtained
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by using a Multimode Nanoscope VIII AFM (Bruker AXS, Germany).
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3. Results and discussion 3.1. Characterization
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It is well known that the surface modification of Ag NPs should be carried out in
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a controlled manner by tuning organic assembly on NPs surfaces, which allows to reduce
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the surface energy simultaneously and to prevent NPs agglomeration by forming protective layers on NPs surfaces. Therefore, we bifunctionalized Ag NPs surfaces with
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MNA and MA molecules simultaneously thereby acting as effective binding sites for multi-metal ions sensors. For this, first we optimized MA concentration on Ag NPs surfaces, since the passivation of noble metal NPs surfaces can be effectively performed by using mercapto functionality than that of –NH2 group of MA [42]. Supporting Information of Figure S2a shows UV-visible absorption spectra of Ag NPs capped with different concentrations of MA in ranging from 0.1 to 1.5 mM. To confirm the best concentration of MNA and MA molecules on Ag NPs surfaces, we also studied that the UV-visible spectra of Ag NPs upon the addition of different concentrations of both molecules simultaneously in Ag NPs solution (Supporting Information of Figure S2b). 8 Page 8 of 40
After optimizing MNA and MA concentrations on Ag NPs, we found that 0.5 mM of MNA and 1.0 mM of MA concentrations are more suitable to form uniform (one by one) surface layers on Ag NPs, which access to NPs with core diameters less than 5 nm.
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Importantly, Ag NPs acted as colorimetric sensors when their core diameters are mostly less than 5 nm thereby controlling their surface plasmon resonance (SPR) band at around
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405 nm, and allowing their change color from yellow to red upon addition of target
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species. Figure 1 shows the UV-visible spectra of bare Ag NPs (without MNA and MA) and MNA-MA functionalized Ag NPs. It can be noticed that the both bare Ag NPs and
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MNA-MA Ag NPs have shown strong absorption peaks at 390 and 405 nm, which confirms that the characteristic SPR band of Ag NPs. Furthermore, it can also be
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observed that there is a very slight red-shift in SPR band of Ag NPs before and after
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molecules.
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modifications, which confirms the bifunctionalization of Ag NPs with the both
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To ensure the MNA and MA molecules attachment on the surfaces of Ag NPs, we
characterized bifuncationalized Ag NPs by using FT-IR. It is noticed that the FT-IR spectrum of MNA-MA- functionalized Ag NPs (Supporting Information of Figure S3c) shows the characteristic features of benzene ring stretching modes at 1620, 1582, and 1465 cm-1 and –COOH and –NH2 stretching modes at 1696 cm-1 and 3000 to 3500 cm-1 , respectively. Compared with the FT-IR spectra of MNA and MA (Supporting Information of Figure S3a-S3b), the characteristic bands at 2569 cm-1 (–SH group stretching) was completely disappeared and 3139 cm-1 and at 3336 cm-1 (–NH2 stretching) was slightly shifted in FT-IR spectrum of bifunctionalized Ag NPs (Supporting 9 Page 9 of 40
Information of Figure S3c). The results indicated that MNA- and MA- molecules were successfully attached on the surfaces of Ag NPs through Ag-S and Ag-N linkages.
following formula based on the UV-visible spectra [43].
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Meanwhile, we also calculated the average size of bare Ag NPs by using the
D= (9.8127 X 10-7)λ3 – (1.7147 x 10-3)λ2 + (1.0064)λ – 194.84
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Where D (nm) is the size of a given Ag NPs sample, and λ is the wavelength (nm)
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of the SPR band of Ag NPs. By using the above formula, we estimated the average size of bare Ag NPs is ~3.14 nm, which showed the SPR band at 390 nm. Figure 2 shows the
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TEM images of bare Ag NPs and bifunctionalized Ag NPs. These results demonstrated that the both Ag NPs are well dispersed in solution, with average diameters of 2 and 5 nm
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for bare Ag NPs and MNA-MA-Ag NPs, respectively. As shown in Figure 2, bare Ag
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NPs are well-dispersed and bifunctionalized Ag NPs are polydispersed in solution, which
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is due to the assembly of MNA and MA molecules on Ag NPs surfaces. The hydrodynamic diameters of bare and MNA-MA-Ag NPs were confirmed by DLS (Figure
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5a-b). The hydrodynamic diameters were 2.0 and 5.0 nm for bare and MNA-MA- Ag NPs. The TEM images and DLS data are well agreed with the UV-visible calculations for estimation of an average size of Ag NPs. Furthermore, AFM analysis was also carried out for MNA-MA-Ag NPs. Figure 6a shows the AFM image of the MNA-MA-Ag NPs. This result revealed that the MNA-MA-Ag NPs were spherical and very well dispersed in nature.
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It is well known that the functionalized Ag NPs exhibit yellow color due to their SPR vibrations at an average size 5 nm. To know the aggregation ability of Ag NPs, we studied induced aggregation of bifunctionalized Ag NPs with the addition of various
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metal ions (Fe3+, Fe2+, Ni2+, Mg2+, Zn2+, Mn2+, Cd2+, Cu2+, Co2+, Cr3+, Ba2+ and Hg2+; 1.0 mM) separately in NPs solutions and the spectral changes were monitored by UV-visible
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spectrometry and digital camera (Figure 3a). Of all the ions tested, only Cr3+ and Ba2+
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ions were significantly caused to change their UV-visible spectral characteristics, which allow to change its SPR band from 405 nm to 535 nm and to 530 nm for Cr3+ and Ba2+,
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respectively. As shown in Figure 3a, the addition of Cr3+ and Ba2+ ions leads to a decrease in the SPR band intensity of Ag NPs at 405 nm and a dramatic increase in the
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absorbance intensity at 535 nm and 530 nm for Cr3+ and Ba2+ ions, which confirms the induced aggregation of Ag NPs with Cr3+ and Ba2+ ions. Figure 3b shows the color
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change of Ag NPs by the addition of Cr3+ and Ba2+ ions. Supporting information of Figure S4 shows the absorbance ratios of MNA-MA-Ag NPs at A535/530nm/A405nm in the
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presence of Cr3+, Ba2+ and other metal ions.
3.2. Effect of pH
To establish effective experimental conditions, we investigated the effect of pH
on the stability and sensitivity of bifunctionalized Ag NPs sensing system. The bifunctionalization was carried out without addition of HCl or NaOH or without optimizing the pH of reaction mixture. After functionalization of Ag NPs, the pH of bifunctionalized Ag NPs is found at 9.0, which is due to the presence of MA molecules on Ag NPs and they are very stable at this pH. Based on the above observations, we 11 Page 11 of 40
believe that –SH group of MNA and –NH2 group of MA are effectively attached on the surfaces of Ag NPs.
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It is well known that the degree of aggregation of bifunctionalized Ag NPs with metal ions is strongly depended on the pH of the solution. In this connection, we
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investigated the colorimetric responses of bifunctionalized Ag NPs towards Cr3+ and Ba2+
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ions at different pH ranging from 2.0 to 10.0. Figure 4 shows the UV-visible absorption spectra of Ag NPs with Cr3+ and Ba2+ ions at different pH of the solutions. It is noticed
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that the bifunctionalized Ag NPs have shown the most obvious color changes at pH 2.0 – 7.0. To evaluate the degree of Ag NPs aggregations with both metal ions, we measured
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the absorbance ratios at 535 nm to 405 nm (A535 nm/A405 nm) for Cr3+ and at 530 nm to 405
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nm (A530nm/A405nm) for Ba2+ ion (Supporting information of Figure S5). It can also be
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observed that the absorbance ratios (A535nm/A405nm and A585nm/A405nm) at pH 2.0 – 4.0 are very high for Cr3+ and Ba2+, which is due to the self aggregation of metallic NPs
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(neutralization of NPs surface charges) at lower pH < 4.0 [44]. To confirm this, we studied the UV-visible spectra of bifunctionalized Ag NPs at lower pH < 4.0 without addition of metal ions (data was not shown). It was noticed that the spectral changes of Ag NPs were observed due to the leaching of Ag NPs. To confirm this, we studied the hydrodynamic diameter of MNA-MA-Ag NPs at pH 2.0 to 4.0 without addition of metal ions (Supporting Information of Figure S6). It was observed that the hydrodynamic diameters of Ag NPs are increased to ~190 nm (pH 2.0), ~139 nm (pH 3.0) and ~15 nm (pH 4.0), and are larger than that at pH 6.0. This is due to a decrease in electrostatic repulsions between NPs at higher H+ ion concentration, and resulting a high degree of 12 Page 12 of 40
NPs aggregation without addition of analytes [45]. Importantly, the absorbance ratios of bifunctionalized Ag NPs at two wavelengths (A535nm/A405nm for Cr3+ and A530nm/A405nm for Ba2+) are much larger than that at pH 5.0 and 7.0 (for Ba2+ ion only), confirming that
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the both ions are easily induced the aggregation of Ag NPs at pH 6.0 (Supporting information of Figure S5). At basic conditions, there are no obvious color changes in Ag
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NPs solutions. Therefore, we selected pH 6.0 as the optimum pH for effective
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simultaneous sensing of both metal ions using bifunctionalized Ag NPs, which avoids the protonation of –NH2 of MA and the detachment of –SH group of MNA on Ag NPs
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surfaces.
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3.2. Estimation of bifunctionalized Ag NPs aggregation with Cr3+and Ba2+ ions We studied the DLS experiments for the estimation of diameters of bare and
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bifunctionalized Ag NPs and with addition of Cr3+ and Ba2+ ions. As shown in Figure 5, the hydrodynamic diameter of bare Ag NPs is slightly increased from 2.0 nm to 5.0 nm,
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which is due to the assembly of MNA and MA molecules on Ag NPs surfaces. Figure 5cd shows the DLS data of bifunctionalized Ag NPs in the presence of Cr3+ and Ba2+ ions (370 µM) at pH 6.0. It can be noticed that there are significant variations in the hydrodynamic diameter of Ag NPs by the addition of both ions, which confirms that the induced aggregation of Ag NPs with Cr3+ and Ba2+ ions. Based on the DLS data, we confirmed that the hydrodynamic diameter of bifunctionalized Ag NPs is notably increased from 5.0 nm to 164 nm and to 122 nm for Cr3+ and Ba2+ ions, respectively, thereby suggesting the effective aggregation of Ag NPs in the presence of Cr3+ and Ba2+ ions. Figure 2c-d shows the TEM images of MNA-MA-Ag NPs in presence of Cr3+ and 13 Page 13 of 40
Ba2+ ions at 370 µM. It can be noticed that the morphology and sizes of MNA-MA-Ag NPs were greatly changed by the addition of Cr3+ and Ba2+ ions. The selected-area electron diffraction (SAED) patterns of Cr3+ and Ba2+ ions-induced aggregations of
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MNA-MA-Ag NPs were recorded from a region that has no contrast within the nanoparticles, yielded in hollow rings (Figure 2e-f). The selected areas of Cr3+ and Ba2+
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ions-induced aggregations of MNA-MA-Ag NPs were measured at ~200 nm. It can be
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noticed that the hollow rings were appeared for both SAED patterns, which confirmed that the aggregated Ag NPs are in amorphous phase. However, it is also observed that in
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some regions, the electron diffraction patterns are consisted very few unsystematic arrangements of spots. These results confirmed that the hollow rings and unsystematic
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arrangement of spots are depicted from amorphous and crystalline regions in Cr3+ and Ba2+ ions-induced aggregations of MNA-MA-Ag NPs. To confirm this, we also studied
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AFM images of Cr3+ and Ba2+ ions-induced aggregations of MNA-MA-Ag NPs (Figure 6). These results revealed that the MNA-MA-Ag NPs are well dispersed with uniform
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heights (Figure 6a). However, the MNA-MA-Ag NPs structural changes (size and height) were greatly changed by the addition of Cr3+ and Ba2+ ions, confirming that their induced aggregation with Cr3+ and Ba2+ ions (Figure 6b-c). The TEM and AFM images clearly demonstrated that Cr3+ and Ba2+ ions are greatly induced the aggregations of MNA-MAAg NPs, resulting in a red shifted change in their UV-visible absorption spectra.
spectra of MNA-MA-Ag NPs in the presence of mixture of metal ions (1 mM) without and with Cr3+ and Ba2+ ions (Figure 7). It can be noticed that the SPR absorption shift was not observed by the addition of only other metal ions mixture (without Cr3+ and Ba2+
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ions). However, the SPR absorption shift was only observed by the addition of Cr3+ and Ba2+ ions separately along with the mixtures of other metal ions (1 mM), just like as
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solely additions of Cr3+ and Ba2+ ions. These results indicate that only Cr3+ and Ba2+ ions
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are induced the aggregation of Ag NPs and showed good resistance from other metal ions.
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3.4. Analytical performance
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Before selecting the metal ions concentration range for calibration graphs, we
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studied the UV-visible spectra of MNA-MA-Ag NPs by the addition of different concentrations of metal ions from 0.1 to 1000 µM for Cr3+ ion and from 100 to 1000 µM
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for Ba2+ ion, respectively (Supporting Information of Figures S7-S8). These results indicated that the color of MNA-MA-Ag NPs is gradually changing from yellow to reddish brown and yellow to orange for Cr3+ and Ba2+, which can be observed by naked eye (Supporting Information of Figures S7b-S8b). Based on these results, we constructed the calibration graph by using metal ions concentration from 10 to 370 µM. Figure 8 shows the the degrees of aggregation of bifunctionalized Ag NPs in terms of the changes in absorbance ratios at two wavelengths A535nm/A405nm and A530nm/A405nm corresponding to the Cr3+ and Ba2+ ions concentrations ranging from 10 to 370 µM, respectively. It can be noticed that a decrease in the absorption peak at 405 nm and an increase in the absorption 15 Page 15 of 40
peaks at 535 nm and 530 nm upon the addition of increasing concentration of Cr3+ and Ba2+ ions separately into Ag NPs solutions (Figure 8a-b). As shown in Supporting Information of Figure S9, the absorbance ratios (A535nm/A405nm for Cr3+ and A530nm/A405nm
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for Ba2+) are sensitive to the concentrations of both ions. The calibration graphs were obtained in the range of 50 to 330 µM and the equation for the resulting calibration plots
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were y = 0.465x – 0.571 and y = 0.374x – 0.011 for Cr3+ and Ba2+ ions (x was the
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concentration of metal ions, and y was the intercept of the slope), with correlation coefficients of 0.992 and 0.981 for Cr3+ and Ba2+ ions, respectively (Supporting
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Information of Figure S9). This method showed detection limits at 64.51 nM and 80.21 nM for Cr3+ and Ba2+ ions, respectively. The analytical data of the method was shown in
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Table 1.
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3.5. Analysis of Cr3+ and Ba2+ ions in real water samples
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In order to evaluate the practical application of this method, we analyzed Cr3+ and
Ba2+ ions in three different water samples (drinking, tap and river water) by using bifunctionalized Ag NPs as colorimetric probes. The collected water samples spiked with Cr3+ and Ba2+ ions separately, with the concentrations of 0.1, 0.5 and 1 mM, and are analyzed by the aforesaid procedure. Using this method, good recoveries were obtained in the range of 97 to 101% and 98 to 102%, with RSD (%) values 1.17 - 2.32% and 1.24 1.66% for Cr3+ and Ba2+ ions in the drinking, tap and river water samples (Table 2). Table 3 summarizes the comparison of present method with the reported NPs-based UV-visible and fluorescence methods [30-40] for the detection of Cr3+ and Ba2+ ions. These results 16 Page 16 of 40
clearly demonstrated that this method exhibited good selectivity and showed superiority for the simultaneous detection of Cr3+ and Ba2+ ions in environmental samples.
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4. Conclusions
We explore the use of bifunctionalized Ag NPs as colorimetric sensors for
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simultaneous detection of Cr3+ and Ba2+ ions in water samples. The rapid, selective and
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sensitive response to Cr3+ and Ba2+ ions suggests that MNA-MA-Ag NPs colorimetric probe may have potential application in the simultaneous detection of Cr3+ and Ba2+ ions
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in water samples. The bifunctionalized Ag NPs exhibited high affinity towards Cr3+ and Ba2+ ions over a variety of potentially interfering metal ions, yielding an instantaneous
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color change of Ag NPs from yellow to reddish brown and to orange, and corresponding
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SPR absorption shift from 405 nm to 535 nm and 530 nm for Cr3+ and Ba2+ ions,
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respectively. These surface modified Ag NPs were successfully acted as a colorimetric sensor for Cr3+ and Ba2+ ions, providing limit of detection of 64.51 nM and 80.21 nM for
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Cr3+ and Ba2+ ions, respectively. The tuning of multi-molecular assembly and their high stability on Ag NPs surfaces allow to act as efficient colorimetric probes for multi-metal ions sensing in various water samples. Acknowledgements
This work was financially supported by the S. V. National Institute of
Technology, Surat under M.Sc., research project. Special thanks are given to Prof. Z. V. P. Murthy, and Mr. Chetan Patel, Chemical Engineering Department, S. V. National Institute of Technology, Surat for providing DLS instrument facility to this work. We also thank Department of Science and Technology for providing Maya Pro 2000 17 Page 17 of 40
spectrophotometer under the Fast-Track Young Scientist Scheme (2011 – 2014). We would like to thank Mr. Vikas Patel, SICART, V. V. Nagar, Anand for his assistance in
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TEM data.
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S.Y. Lu, H. L. Ren, Z. S. Liu, Enhanced ultrasensitive detection of Cr(III) using 5-
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Biography of the authors Richita P. Modi completed her M.Sc., Chemistry at S. V. National Institute of Technology.
Her current research interest is development of nanomaterials-based
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colorimetric sensors for metal ions analysis.
Vaibhavkumar N. Mehta completed his Master degree in Nanoscience and
cr
Nanotechnology from Sardar Patel University, Gujarat. Currently he is doing his Ph. D. at
us
S. V. National Institute of Technology, Surat. His research interest is functionalization of nanoparticles for colorimetric sensing of biomolecules and metal ions.
an
Suresh Kumar Kailasa completed his Ph. D. in Chemistry from S. V. University, Tirupati in 2005. After completing his two postdoctoral fellowships at South Korea and at
M
National Sun Yat-Sen University, Taiwan, he joined as a faculty in S. V. National
d
Institute of Technology, Surat in 2009. He received Young Scientist Award from Taiwan
te
Mass Spectrometry Society in 2013. His interest includes nanomaterials integration in MALDI-MS and the development of nanomaterials-based colorimetric sensors for
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biomolecules, drugs and metal ions.
25 Page 25 of 40
Figure captions Figure 1. UV-visible spectra of bare Ag NPs and MNA-MA-Ag NPs. Inset picture show bare Ag NPs and MNA-MA-Ag NPs.
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Figure 2. TEM images of (a) bare Ag NPs, (b) MNA-MA-Ag NPs, and MNA-MA-Ag NPs induced aggregations by the addition of 370 µM (c) Cr3+ and (d) Ba2+ ion. The
cr
SAED pattern of aggregated MNA-MA-Ag NPs in presence of (e) Cr3+ and (f) Ba2+ ions.
us
Figure 3. (I) UV-visible absorption spectra of MNA-MA-Ag NPs in the presence of different metal ions (Fe3+, Fe2+, Ni2+, Mg2+, Zn2+, Mn2+, Cd2+, Cu2+, Co2+, Cr3+, Ba2+ and
an
Hg2+) (II) Photographic image of bare Ag NPs and MNA-MA-Ag NPs in the presence of various metal ions (Fe3+, Fe2+, Ni2+, Mg2+, Zn2+, Mn2+, Cd2+, Cu2+, Co2+, Cr3+, Ba2+ and
M
Hg2+).
Figure 4. UV-visible absorption spectra of MNA-MA-Ag NPs in presence of (a) Cr3+ at
te
d
pH 2 to 10 and (b) Ba2+ at pH 2 to 10.
Figure 5. DLS of (a) bare Ag NPs (b) MNA-MA-Ag NPs and induced aggregation of
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MNA-MA-Ag NPs with (c) Cr3+ and (d) Ba2+ ions. Figure 6. AFM images of (a) MNA-MA-Ag NPs and their induced aggregations with (b) Cr3+ and (b) Ba2+ ions.
Figure 7. Interference study in presence of different metal ions: (1) MNA-MA-Ag NPs (2) MNA-MA-Ag NPs + different metal ions (Fe3+, Fe2+, Ni2+, Mg2+, Zn2+, Mn2+, Cd2+, Cu2+, Co2+ and Hg2+) (3) MNA-MA-Ag NPs + different metal ions + Cr3+ and (4) MNAMA-Ag NPs + different metal ions + Ba2+ ion. Figure 8. (a) UV–visible spectra of MNA-MA-Ag NPs solutions with various concentrations of Cr3+ with the range of 10 to 370 µM. Inset picture show photographic 26 Page 26 of 40
image of MNA-MA-Ag NPs containing various Cr3+ concentration from 10 to 370 µM. (b) UV–visible spectra of MNA-MA-Ag NPs solutions with various concentrations of Ba2+ with the range of 10 to 370 µM. Inset picture show photographic image of MNA-
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te
d
M
an
us
cr
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MA-Ag NPs containing various Ba2+ concentration from 10 to 370 µM.
27 Page 27 of 40
Table 1. Analytical data for detection of Cr3+ and Ba2+ ions by using bifunctionalized Ag NPs as colorimetric sensors. Bare Ag
MNA-MA-Ag
NPs Color
NPs
Light
MNA-MA-Ag
MNA-MA-Ag
NPs+ Cr3+
NPs + Ba2+
Yellow
Reddish brown
530
535
164.0
122.0
10 to 370
10 to 370
0.465
0.374
0.992
0.981
64.51
80.21
405
Size (nm)
2.0
5.0
-
-
Slope
-
-
R2 value
-
LOD (nM)
-
us
390
an
λ max (nm)
M
(µM)
orange
cr
yellow
Linear range
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Analytical data
d
-
Ac ce p
te
-
28 Page 28 of 40
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Table 2. Colorimetric sensing of Cr3+ and Ba2+ in water samples (drinking, tap, and river water) by using bifunctionalized Ag NPs as
Cr3+ Added
Found (mM) Recovery (%)
RSD (%)
Added
Recovery (%)
RSD (%)
(n=3)
(n=3)
(mM)
(n=3)
(n=3)
0.096
97
2.40
0.1
0.102
102
1.35
0.505
101
1.56
0.5
0.505
101
1.45
0.982
98
2.32
1
0.994
99
1.66
0.099
99
1.57
0.1
0.101
101
1.36
0.5
0.503
100
1.41
0.5
0.492
98
1.61
1
0.994
99
1.61
1
1.020
102
1.57
0.1
0.101
101
1.53
0.1
0.098
98
1.45
0.5
0.498
99
1.17
0.5
0.497
99
1.24
1
1.010
101
2.03
1
1.01
101
1.29
Drinking 0.5 water 1
Ac
ce
0.1
River water
pt
0.1
ed
(mM)
Tap water
Ba2+
M
Sample
an
probes.
Found (mM)
29 Page 29 of 40
i cr
Size (nm)
Au NPs
Triazole
14
Au NPs
TNBA-HP
Au NPs
DTNBA
Au NPs
Citrate
Au NPs
Tripolyphosphate
Au NPs
BP-DTC
Au NPs Ag NPs
Detection method
References
Cr3+
1.4×10-6
UV-visible
[30]
15
Cr3+
0.04×10-6
UV-visible
[31]
13
Cr3+
93.6 ppb
UV-visible
[32]
13.8
Cr6+, Cr3+
4.0 - 0.3×10-6
UV-visible
[33]
11.13
Cr3+
1.0×10-7
UV-visible
[34]
-
Cr3+
31 ppb
UV-visible
[35]
Citrate
30 – 35
Cr3+
1.06×10-7
UV-visible
[36]
-
65±2
Cr3+
2×10-9
Fluorescence
[37]
ACE
-
Ba2+
1.0×10-8
UV-visible
[38]
MSA
-
Ca2+, Sr2+, Ba2+
20, 8.0, and 2.5×10-6
UV-visible
[39]
Tiopronin
13
Ba2+
1.5×10-6
UV-visible
[40]
Cr3+
64.51×10-9
Ba2+
80.21×10-9
UV-visible
This study
ed
pt
ce
Ac
Au NPs
Au NPs Ag NPs
MNA and MA
Analytes
an
Capping agent
Ag NPs
LOD (M)
M
Nanoparticles
us
Table 3. Comparison of MNA-MA-Ag NPs as colorimetric sensor for the detection of Cr3+ and Ba2+ with the reported methods.
5
30 Page 30 of 40
i cr us
an
Graphical Abstract
Capped AgNPs
Fe 3+
Fe 2+
Ni 2+
Mg 2+
Zn2+
Mn2+
Cd2+
Cu2+
Co 2+
Pb2+
Cr3+
Ba2+
Hg 2+
ce
pt
Bare AgNPs
ed
M
We describe the applications of bifunctionalized silver nanoparticles (Ag NPs) with 6-mercaptonitotinic acid (MNA) and melamine (MA) for simple, rapid and simultaneous colorimetric detection of Cr3+ and Ba2+ ions in water samples. The bifunctionalized Ag NPs were confirmed by UV-visible, FT-IR, dynamic light scattering (DLS) and transmission electron microscopy (TEM) techniques. The MNA and MA molecules on Ag NPs surfaces allowed to act as selective binding sites for Cr3+ and Ba2+ ions. As a result, these Ag NPs were induced to aggregate quickly with Cr3+ and Ba2+ ions through color change from yellow- to reddish brown (Cr3+) and -to orange (Ba2+), respectively which was readily seen by the naked eye.
Ac
Photographic image of bare Ag NPs and MNA-MA-Ag NPs in the presence of various metal ions (Fe3+, Fe2+, Ni2+, Mg2+, Zn2+, Mn2+, Cd2+, Cu2+, Co2+, Cr3+, Ba2+ and Hg2+).
31 Page 31 of 40
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an
Research Highlights of “MNA-MA-Ag NPs”
Bifunctionalized Ag NPs acted as colorimetric sensors for Ba2+ and Cr3+.
MNA-MA-Ag NPs were induced to aggregate quickly with Cr3+ and Ba2+ ions.
M
characterization of MNA-MA-Ag NPs.
FT-IR, DLS, AFM and TEM were used for the
ed
method showed good linearity with LOD 64.51 and 80.21 nM for Cr3+ and Ba2+.
Ac
ce
pt
32 Page 32 of 40
This
Figure(s)
1.4 Bare Ag NPs MNA-MA-Ag NPs
Capped Ag NPs
cr
Bare Ag NPs
us
0.8
0.6
an
Absorbance
1.0
ip t
1.2
M
0.4
0.0 300
400
ed
0.2
500
600
700
800
900
pt
Wavelength(nm)
Ac
ce
Figure 1
Page 33 of 40
(b)
20 nm
(d)
20 nm
20 nm
20 nm
(f)
Ac
(e)
ce
pt
ed
M
(c)
an
us
cr
ip t
(a)
200 nm
200 nm
Figure 2 Page 34 of 40
(I) 1.4
ip t
1.2
cr
Cr3+ Ba2+
us
0.6
an
Absorbance
1.0
0.8
M
0.4
Bare Ag NPs Capped Ag NPs 3+ Fe 2+ Fe 2+ Ni 2+ Mg 2+ Zn 2+ Mn 2+ Cd 2+ Cu 2+ Co 3+ Cr 2+ Ba 2+ Hg
0.0 300
400
ed
0.2
500
600
700
800
900
(II) Capped AgNPs
Fe3+
Fe2+
Ni2+
Mg2+
Zn2+
Mn2+
Cd2+
Cu2+
Co2+
Pb2+
Cr3+
Ba2+
Hg2+
Ac
Bare AgNPs
ce
pt
Wavelength(nm)
Figure 3
Page 35 of 40
1.4
(a)
Bare Ag NPs Capped Ag NPs
1.2
pH 2 pH 4 pH 6
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1.0
pH 10
0.8
2
535 nm
4
8
10
us
0.6
6
cr
Absorbance
pH 8
pH
an
0.4
M
0.2
0.0 400
500
600
700
ed
300
800
900
Wavelength(nm)
pt
(b) 1.6
2
ce
1.4
4
5
6
1.0
0.8
pH 2
7
8
9
10
pH 3 pH 4 pH 5
pH
pH 6
530 nm pH 6
Ac
1.2
Absorbance
3
pH 7 pH 8 pH 9 pH 10
0.6
0.4
0.2
0.0 300
400
500
600
Wavelength(nm)
Figure 4
700
800
900
Page 36 of 40
(a)
cr
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Number (%)
Average hydrodynamic diameter: ~ 2 nm
Size (d. nm)
us
(b)
M
an
Number (%)
Average hydrodynamic diameter: ~ 5 nm
ed
Size (d. nm)
ce Ac
Size (d. nm) Average hydrodynamic diameter: ~ 122 nm
Number (%)
(d)
Size (d. nm) Figure 5
Average hydrodynamic diameter: ~ 164 nm
pt
Number (%)
(c)
Page 37 of 40
an
us
cr
ip t
(a)
ce Ac
(c)
pt
ed
M
(b)
Figure 6
Page 38 of 40
1.4
1.2
2
3
4
cr
1
0.8
us
1. MNA-MA-Ag NPs without metal ions 2. MNA-MA-Ag NPs +other metal ions (not Cr3+ and Ba2+) 3. MNA-MA-Ag NPs+ other metal ions + Cr3+ 4. MNA-MA-Ag NPs+ other metal ions + Ba2+
0.6
an
Absorbance
1.0
ip t
Bare AgNPs Capped AgNPs Capped AgNPs+different metal ion solution of 2+ 3+ 2+ 2+ 2+ 2+ 2+ 2+ 2+ 2+ 2+ Fe ,Fe ,Hg ,Mg .Mn ,Cu ,Co ,Pb ,Zn ,Cd ,Ni 3+ Capped AgNPs+different metal ion solution + Cr 2+ Capped AgNPs+different metal ion solution + Ba
M
0.4
0.0 300
400
ed
0.2
500
600
700
800
900
pt
Wavelength(nm)
Ac
ce
Figure 7
Page 39 of 40
1.4
(a) 10
50
90
130
170
210
250
330
290
370
1.2
Concentration of Cr3+ (μM)
ip t
10
cr
0.8
370 µM 0.6
us
Absorbance
1.0
an
0.4
M
0.2
0.0 400
500
600
ed
300
700
800
900
Wavelength(nm)
1.6
(b)
ce
1.0
Ac
Absorbance
1.2
0.8
50
pt
10
1.4
90
130
170
210
250
290
330
370
Concentration of Ba2+ (μM) 10
370 µM
0.6
0.4
0.2
0.0 300
400
500
600
Wavelength(nm)
Figure 8
700
800
900 Page 40 of 40