Anion-driven selective colorimetric detection of Hg²⁺ and Fe³⁺ using functionalized silver nanoparticles

Abstract

A novel nanosensing system was developed by the surface functionalization of silver nanoparticles (AgNPs) with β-alanine dithiocarbamate (ADTC) for the selective recognition and monitoring of Hg²⁺ and Fe³⁺ ions in aqueous medium. This system showed a visually detectable colour change from yellow to colourless and the surface plasmon resonance (SPR) band of AgNPs at 402 nm disappeared with the addition of both Hg²⁺ and Fe³⁺ ions due to the aggregation of nanoparticles. Interestingly, the functionalized AgNPs can be applied for the discrimination and selective detection of Hg²⁺ and Fe³⁺ in the presence of Br⁻ and Cl⁻, respectively.

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Introduction

Mercuric ion (Hg²⁺) is an extremely hazardous pollutant and highly carcinogenic transition metal ion that exists mostly in three forms: elemental or metallic mercury, ionic mercury salts (e.g., HgCl₂) and organometallic compounds (e.g., methyl-, phenyl-mercury). Mercuric ions have a strong affinity for sulphur containing ligands and accumulation of this element in the human body results in cognitive and motor disorders as well as Minamata disease.^1,2 Also, interaction of Hg²⁺ with microorganisms in streams and oceans results in the formation of methyl mercury causing serious health problems such as sensory, mental, neurological disorders.^3,4 Similarly, although iron is essential for the proper functioning of all living cells, its deficiency (hypoferremia) and overload (hyperferremia) can be detrimental for proper physiological functioning.^5,6 Therefore, significant attention has been paid not only in the measurement of ferric ion in biological systems but also environmentally, particularly for the evaluation of water quality.^5–8 Hence, it becomes critically important to detect and measure the level of Hg²⁺ and Fe³⁺ in aqueous solution with high sensitivity and selectivity using facile and reasonably fast methods.

Various methods that have been adopted for the detection of these ions (and others) are spectrophotometry,⁹ atomic absorption spectrometry,¹⁰ stripping voltametry¹¹ and inductively coupled plasma atomic emission spectrometry.¹² In order to avoid the use of expensive and sophisticated instrumentation, as well as complicated sample pre-treatment protocols, chemosensors based on colorimetric and fluorescence responses have gained significant attention due to their ability for simple, rapid and sensitive on-site monitoring of target analytes in various biological and environmental samples.¹³ Also, in comparison to organic dyes, functionalized inorganic nanoparticles have gained extensive applications for the colorimetric detection of transition metal ions like Hg²⁺ and Fe³⁺ due to their high extinction coefficient that results in improved visibility on the basis of change in optical brightness and increased sensitivity of UV-visible spectroscopic detection.^14,15 In addition, stable nanoparticles functionalized with suitable receptors have been successfully applied as sensitive and selective colorimetric probes for the detection of DNA,¹⁶ amino acids,¹⁷ metal ions¹⁸ and pesticides.¹⁹

Herein, as a part of our ongoing research on analyte recognition and sensing,²⁰ we have developed a novel nanosensor by the surface functionalization of AgNPs with β-alanine dithiocarbamate (ADTC). The complexation of Fe³⁺ and Hg²⁺ with the capped ADTC resulted in the destabilization followed by aggregation of AgNPs that induced a naked-eye detectable colour change from yellow to colourless and the disappearance of the SPR band at 402 nm in aqueous medium. Furthermore, this nanosensor can also be applied for the selective colorimetric detection of Fe³⁺ and Hg²⁺ in the presence of anions.

Experimental

Materials and reagents

All the starting reagents used for the experiments were purchased commercially in the purest form and were used without further purification. Silver nitrate (AgNO₃) and potassium hydroxide (KOH) were purchased from Finar Ltd., India. Sodium borohydride (NaBH₄), β-alanine and CS₂ were purchased from Merck Pvt. Ltd. All the metal salts used for the experiments were purchased from Rankem Pvt. Ltd., India. All the anions were used in the form of sodium or potassium salts (NaF, NaAcO, NaH₂PO₄·2H₂O, KCl, KBr).

All glassware was cleaned with a diluted HNO₃ solution and rinsed with Milli-Q water prior to use. Stock solutions of the metal ions and anions (1.0 × 10⁻³ M) were prepared freshly in Milli-Q water. These solutions were used for all colorimetric and spectroscopic experiments after appropriate dilution. Hydrochloric acid (0.1 M) and sodium hydroxide (0.1 M) were used to adjust the pH. All UV-vis measurements were recorded on a Cary 50 Varian UV-vis spectrophotometer at room temperature using quartz cells with 1.0 cm path length in aqueous medium. The observed pH was measured as −log[H⁺] using a HANNA HI 2223 pH meter equipped with a calibrated combined glass electrode with standard buffer solutions. Fourier transform infrared (FT-IR) spectra were recorded on a DRS (8400-S-Shimadzu) FTIR spectrophotometer using KBr pellet. The sample for the FT-IR measurement was prepared by adding microvolumes of the synthesized capped AgNPs into KBr followed by drying to make the pellet. The dynamic light scattering (DLS) data were recorded to detect change in the average size of AgNPs as expressed by calculating the mean hydrodynamic diameter (z-average mean) from the autocorrelation function of the intensity of light scattered from the particles undergoing Brownian motion. All the DLS data were obtained using Malvern Zeta size Nano (Malvern, UK).

Synthesis of functionalized AgNPs

The dithiocarbamate derivative of β-alanine (ADTC) was synthesized first by the reaction of β-alanine (1.0 gm, 0.011 moles) with CS₂ (0.66 ml, 0.011 moles) in ethanolic medium in the presence of aqueous KOH (0.61 gm, 0.011 moles). The reaction was refluxed for 4 hours producing yellow thick viscous solution of β-alanine dithiocarbamate giving distinguishable odour. In parallel, silver nanoparticles were also prepared using the chemical reduction process. Briefly, 2 ml solution of 0.01 M AgNO₃ in 98 ml water was reduced by mild reducing agent NaBH₄ (8.8 mg) in an ice cold bath leading to the formation of dispersed, clear yellow solution of bare AgNPs after 30 min of stirring. The ADTC functionalized AgNPs were prepared with the addition of 2 ml of 0.001 M ADTC to the prepared bare nanoparticles, producing brownish-yellow solution. The reaction was stirred for approximately 2 hours to ascertain the self-assembly of ADTC onto the surface of silver nanoparticles. After characterizing the sample from the FT-IR spectrum, the functionalized AgNPs formed were kept in dark for further sensing applications.

Colorimetric detection of analytes

The prepared 100 ml ADTC functionalized AgNPs was diluted further with 50 ml water producing nanoparticles of concentration 1.33 × 10⁻⁴ M with respect to [AgNO₃] and then, the required amount of AgNPs solution was taken for the different experiments. For colorimetric detection of metal ions, many tests were carried out to optimize the sensing conditions for metal ions by adding different concentrations of metal ions to the prepared functionalized AgNPs and to check the instant colorimetric changes. During the experiments, it was found that all the sensing conditions were very stable and no precipitates or flocs were observed.

For spectrophotometric titrations, the required amount of the AgNPs was taken directly into quartz cuvette and then the spectra were recorded after each aliquot addition of metal ions ([Hg²⁺] = [Fe³⁺] = 1.0 × 10⁻³ M) using micropipette. The change in the absorbance at 402 nm was plotted against metal concentration. Similarly, the optical responses of the functionalized AgNPs were tested towards different anions and also in the presence of both anions and cations.

Results and discussion

The functionalized AgNPs was prepared using the chemical reduction method producing intense yellow-brown and transparent aqueous solution resulting in the strong SPR absorption band at 402 nm (Fig. 1S†) and stability due to a secure coating of ADTC on the surface of nanoparticles via zero-length coupling. The FT-IR spectra of β-alanine dithiocarbamate shows the presence of C–S and CS–NH group at 1013 cm⁻¹ and 1200 cm⁻¹ respectively (Fig. 2S†). On functionalization, the slight shifts of the characteristic FT-IR peaks of ADTC were observed which indicate the formation of Ag–S non-covalent interaction on the surface of AgNPs (Fig. 2S†). The DLS measurement of the functionalized AgNPs inferred that the average hydrodynamic diameter of AgNPs obtained after surface modification was ∼6 nm (Fig. 3Sa†). The structure of the ADTC and its ADTC–silver complex was optimized by applying B3LYP/LANL2DZ using the computer program Gaussian 09W²¹ and the plots of frontier molecular orbitals were analysed. As shown in Fig. 1, the HOMO and LUMO of ADTC and the ADTC–silver complex indicates the formation of a charge–transfer complex between ADTC and silver.

Fig. 1 Plots of the frontier molecular orbitals (HOMO and LUMO) of ADTC and ADTC–silver complex.

The absorption spectra of the AgNPs were next recorded by adding various concentrations of NaCl and at different pH values. The peak intensities of the SPR band of AgNPs at different concentrations of NaCl were almost the same as the one without NaCl, which indicate that the ADTC functionalized AgNPs were highly stable under conditions of high ionic strength. The pH of the synthesized AgNPs solution was found to be 6.71. On lowering the pH from 6.71 to 2.50, the intensity of SPR band absorbance drastically decreased between pH 3.43 to 2.50 along with a colour change from yellow to colourless (Fig. 4S†). The change of colour inferred that the silver nanoparticles become unstable in acidic pH due to the aggregation of nanoparticles. However, on increasing the pH from 6.71 to 10.20, there was neither a change in the absorbance intensity nor in the colour of the AgNPs solution. This system can therefore be applicable at wide pH range from 3.43 to 10.20 and also under high ionic strength.

The colorimetric and spectral responses of the functionalized AgNPs was investigated in the presence of different metal ions such as Cu²⁺, Ni²⁺, Co²⁺, Ca²⁺, Cd²⁺, Mn²⁺, Mg²⁺, Fe²⁺, Fe³⁺, Al³⁺, Zn²⁺ and Hg²⁺ (Fig. 2). Addition of Hg²⁺ and Fe³⁺ to AgNPs solution resulted in the instantaneous decolourisation accompanying the disappearance of the SPR absorption maxima at 402 nm. Also, the metal ions Al³⁺ and Fe²⁺ resulted in a detectable colour change and slight red shift in the SPR band of AgNPs. However, the addition of other metals showed no obvious colour or spectral changes. Under competitive environment, the effect of the functionalized AgNPs to various metal ions that often coexisted with Hg²⁺ and Fe³⁺ environment was investigated. As shown in Fig. 5S and 6S,† no obvious interference was seen for the detection of Hg²⁺ and Fe³⁺ in the presence of equimolar amounts of other metal ions, including Al³⁺ and Fe²⁺. These results demonstrate that this nanosensor can be used to specifically detect Hg²⁺ and Fe³⁺ in aqueous medium.

Fig. 2 (a) Colorimetric and (b) UV-vis spectral responses of ADTC functionalized AgNPs in the absence and presence of different metal ions [Mⁿ⁺ = 5 × 10⁻⁴ M].

The DLS analyses showed that the addition of Hg²⁺ and Fe³⁺ to the functionalized AgNPs solution causes a drastic increase in the average hydrodynamic diameter of the AgNPs (Table 1S and Fig. 3S†). These results suggest the aggregation of AgNPs upon addition of Hg²⁺ and Fe³⁺. The ADTC stabilizes the particles via a complexation between the dithiocarbamate groups and the AgNPs, and the carboxylic group of ADTC makes the nanosystem water soluble. As proposed in Fig. 3, the aggregation caused by the inter-nanoparticles complexation of Hg²⁺ and Fe³⁺ with the carboxylic-O atoms of capped ADTC that increases the average diameter of the whole nanoparticles population.^22,23 According to the Mie theory,^20d when the distance between two nanoparticles becomes smaller than the sum of their radii, the SPR band becomes broaden and decreased. Further study of the functionalized AgNPs in the presence of Hg²⁺ and Fe³⁺ showed no characteristic colour and spectral responses on addition of EDTA (5.0 × 10⁻⁴ M) indicating the irreversibility of the AgNP-ion complex.

Fig. 3 Schematic representation of the sensing mechanism of the functionalized AgNPs with Hg²⁺ and Fe³⁺ in the absence and presence of KCl or KBr.

The sensitivity and minimum detectable concentration of the nanosensor towards Hg²⁺ and Fe³⁺ was investigated in aqueous solution by performing UV-vis spectral titrations by adding different concentration of Hg²⁺ and Fe³⁺ to the fixed concentration of AgNPs solution. The successive addition of Hg²⁺ and Fe³⁺ to the yellow coloured AgNPs solution led to a decrease in the SPR band that finally disappeared when the [Hg²⁺] = 7.4 × 10⁻⁵ M (Fig. 7Sa†) and [Fe³⁺] = 5.66 × 10⁻⁵ M (Fig. 7Sb†). The spectral data were used to calculate the binding constant between the AgNPs and the metal ions (Hg²⁺ and Fe³⁺) by applying the Benesi–Hildebrand equation:²⁴

where A is the absorbance measured with different concentration of the metal ions, A₀ is the absorbance of the free AgNPs, A_max is the maximum absorbance of AgNPs and metal ions, [Mⁿ⁺] is the concentration of the metal ions added and K is the binding constant. By plotting 1/(A − A₀) against 1/[Mⁿ⁺] the binding constant (K) was determined from the ratio of intercept/slope and calculated to be 7.77 × 10³ M⁻¹ for Hg²⁺ and 1.44 × 10⁴ M⁻¹ for Fe³⁺ (Fig. 8S†). The BH plots also indicate the formation of donor–acceptor complexes in a 1 : 1 stoichiometry. The higher binding towards Fe³⁺ is presumably due to the preferable complexation process of the hard Fe³⁺ than the Hg²⁺. The detection limit obtained from the calibration curves (Fig. 9S†) was 4.89 μM for Hg²⁺ and 6.18 μM for Fe³⁺ using the equation (3α/s), where ‘α’ is the standard deviation of the blank and ‘s’ is the slope.

The effect of different inorganic anions on the optical properties of AgNPs solution was also tested but the addition of F⁻, Cl⁻, Br⁻, I⁻, H₂PO₄⁻ and AcO⁻ anions did not result in any detectable colour/spectral changes even in abundance. Then, the system AgNPs-anion solutions were added to the optimized concentration of Fe³⁺ [5.6 × 10⁻⁵ M] and Hg²⁺ [6.98 × 10⁻⁵ M] (Fig. 4) i.e. the minimum concentration at which the Fe³⁺ and Hg²⁺ induced complete decolourization of AgNPs by naked-eye observation. It was observed that the addition of Fe³⁺ to the AgNPs–Cl⁻ and AgNPs–I⁻ solutions while Hg²⁺ to the AgNPs–Br⁻ solution only caused decolourisation and disappearance of SPR band presumably due to the competitive interaction that occurred between the ADTC NP surface and the anions added (Fig. 3). These results suggest that the functionalized AgNPs solution can be used to detect and discriminate the presence of Fe³⁺ and Hg²⁺ in any aqueous samples by the judicious choice of anion. In addition, recent reports²⁵ on anion sensors suggest that this nanosensor can also be applied for the detection of anions based on the aggregation or de-aggregation of nanoparticles in the presence of metal ions.

Fig. 4 UV-vis spectra of the functionalized AgNPs in the presence of different anions and (a) Fe³⁺ [5.6 × 10⁻⁵ M] and (b) Hg²⁺ [6.98 × 10⁻⁵ M]. Inset of (a) and (b) shows the change in colour of the functionalized AgNPs on addition of Fe³⁺ and Hg²⁺ in the presence of anions.

The nanosensor was tested further for the selective detection and discrimination of Fe³⁺ and Hg²⁺ in the presence of Cl⁻ and Br⁻ ions. In two separate experiments, the functionalized AgNPs was first diluted with KBr and KCl solution, and then the different metal ion solutions were added. As shown in Fig. 5a, the nanosensor showed the selective decolourization with Hg²⁺ in the presence of KBr but Fe³⁺ did not affect the characteristic AgNPs colour, whereas the reverse optical response was observed in the presence of KCl (Fig. 5b). These results suggest that the AgNPs solution in the presence of anions like Br⁻ and Cl⁻ showed different optical responses for the discrimination of Hg²⁺ and Fe³⁺ in aqueous medium.

Fig. 5 Colorimetric responses of the functionalized AgNPs with different metal ions in the presence of (a) Br⁻ and (b) Cl⁻.

The potential of the proposed functionalized silver nanoparticles was also investigated for the quantification of Hg²⁺ and Fe³⁺ in the presence of Br⁻ and Cl⁻ respectively by continuous addition of Hg²⁺ from 2.49 × 10⁻⁶ M to 1.72 × 10⁻⁵ M (Fig. 6a) and Fe³⁺ from 2.49 × 10⁻⁶ M to 6.98 × 10⁻⁵ M (Fig. 6b). As observed in the absence of anions (Fig. 7S†), with the gradual addition of Hg²⁺ and Fe³⁺, the yellow coloured AgNPs changed in to colourless along with a decrease in the absorbance intensity resulting in the disappearance of SPR band at 402 nm. The association constant obtained from the BH plots (Fig. 10S†) for Hg²⁺ in the presence of Br⁻ is 3.14 × 10⁴ M⁻¹ and for Fe³⁺ in the presence Cl⁻ is 1.97 × 10⁵ M⁻¹. The higher value of binding constant towards the metal ions (Hg²⁺ and Fe³⁺) in the presence of anions may be due to the participation of anions in the complexation process that increase the co-ordination number of the metal ions and hence the stability. It is also observed from the calibration curves (Fig. 11S†) that the aggregation of the functionalized AgNPs is directly related to the concentration of Hg²⁺ and Fe³⁺ in anionic medium with the detection limit down to 2.54 μM for Hg²⁺ in the presence of Br⁻ and 6.08 μM for Fe³⁺ in the presence of Cl⁻. Further, the performance of the present system for the detection of Hg²⁺ and Fe³⁺ in the absence and presence of anion was compared with the reported methods (Table 2S†). Table 2S† inferred that this system showed relatively lower detection limit in the presence of anions, and also the detection limits were comparable with some of the reported AgNPs systems.

Fig. 6 UV-vis spectra of the functionalized AgNPs at various concentrations of (a) Hg²⁺ from 2.49 × 10⁻⁶ M to 1.72 × 10⁻⁵ M and (b) Fe³⁺ from 2.49 × 10⁻⁶ M to 6.98 × 10⁻⁵ M.

Conclusions

We have developed a rapid, facile and highly sensitive colorimetric nanosensor for the detection of Hg²⁺ and Fe³⁺ using a dithiocarbamate functionalized AgNPs based on the mechanism of nanoparticle aggregation. This nanosensor showed a colour change from yellow to colourless for the naked-eye detection and the disappearance of SPR band for the spectrophotometric detection in the presence of Hg²⁺ and Fe³⁺ in aqueous medium. However, in the presence of Br⁻, the AgNPs solution becomes decolourised with Hg²⁺ but not with Fe³⁺ where as the reverse optical response was observed in the presence of Cl⁻. Therefore, this nanosensor was able to detect and discriminate Hg²⁺ and Fe³⁺ in the presence of anions. Also, surprisingly the detection limits in the presence of anions were better as compared to those obtained in the absence of anions. Furthermore, the optical responses of AgNPs indicate the possible molecular logic gate type application through the simultaneous inputs of Fe³⁺ and anion or Hg²⁺ and anion.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra44945a

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