Thiamine-functionalized silver nanoparticles for the highly selective and sensitive colorimetric detection of Hg²⁺ ions

Abstract

Hg²⁺ contamination is a serious threat to the environment; hence, the development of methods for its trace level detection is urgently required. To contribute in this domain, we report for the first time the application of a highly sensitive and selective colorimetric sensing platform for Hg²⁺ ions using thiamine-functionalized silver nanoparticles (Th-AgNPs). Upon the addition of Hg²⁺ ions to the solution, the AgNPs exhibited a noticeable color change from yellow to violet-blue. This color change was monitored by the UV-vis spectrophotometric observation of a significant decrease in the absorption band at 395 nm along with the appearance of a new peak at 550 nm. The designed sensor demonstrated good sensitivity in the concentration range of 1 × 10⁻⁸ to 5 × 10⁻⁶ M with a detection limit of 5 nM. The sensor also showed high selectivity for Hg²⁺ when tested in the presence of several competing metal ions. Moreover, the method was found to be applicable to river water samples with satisfactory percentage recoveries.

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1. Introduction

The unintended agricultural and industrial growth in the last century has triggered widespread water contamination by heavy metal ions. Among the heavy metal ions, mercury is very toxic in all its oxidation states as well as in its elemental form and organometallic complex form such as methyl-mercury.^1–4 Owing to high risks to human health and water-dwelling species, the approximate permissible level of Hg²⁺ in water bodies is suggested to be 2 ppb (10 nM) by the United States Environmental Protection Agency (USEPA).⁵ Being non-biodegradable, Hg²⁺ accumulates in human bodies and causes dysfunction of body cells, which ultimately results in kidney, brain, liver and central nervous system related diseases.⁶ These serious threats related to mercury-contaminated water systems have compelled the scientific community to search for sensitive analytical tools which could detect this water toxin below the threshold limit suggested by the USEPA.

The much higher toxicity of Hg²⁺ among other heavy metal ions has been reported to be related to its stronger affinity for the thiol groups present in specific amino acids of proteins and enzymes.^7,8 This high degree of toxicity, posing a serious health hazard, demands immense attention and effort for the development of a highly selective and sensitive sensing system for the routine analysis of Hg²⁺ ions in food and water bodies. In this context, a number of techniques such as electrochemistry,⁹ reversed-phase high-performance liquid chromatography,¹⁰ fluorometry,¹¹ atomic spectrometry,^12,13 and inductively coupled plasma-mass spectrometry¹⁴ have been developed for the trace level determination of Hg²⁺. Although these methods are sensitive towards Hg²⁺ detection, they generally require cumbersome and time-consuming sample preparation procedures, specialized laboratories, and expensive and bulky instrumentation which render them difficult to be used particularly in field monitoring. In contrast, colorimetric sensors based on noble metal nanoparticles, especially gold nanoparticles (AuNPs) and silver nanoparticles (AgNPs), have gained a great deal of attention for the detection of various organic and inorganic ions.^15–17 These nanoparticles confer unique properties to the sensors depending on their size, shape and interparticle distance. These sensing systems are simple, low cost, rapid and suitable for real time and on site monitoring with limited resources. Moreover, these nanoparticle-based sensors provide high sensitivity and selectivity which are mainly attributed to their distinct strong absorption and high extinction properties in the UV-vis region. Moreover, in such kinds of colorimetric sensors, the dispersion and aggregation state of the nanoparticles essentially leads to a change in color that can be observed by the naked eye.^16,17 Higher extinction coefficients and lower costs prompted us to prefer AgNPs over AuNPs in designing nanoparticle-based colorimetric sensors.¹⁸

The colorimetric detection of ions greatly depends on the ligand attached to the surfaces of the nanoparticles. For this purpose, a number of ligands have been proposed to modify AuNP and AgNP surfaces for the sensitive and selective recognition of Hg²⁺ ions. Due to the strong affinity of Hg²⁺ towards sulfur and nitrogen atoms, many Hg²⁺ sensors have been designed with ligands which contain these kinds of donor atoms. Examples of such ligands include dithioerythritol,¹⁹ mercaptoundecanoic acid,²⁰ mercaptopropionic acid,²¹ thioctic acid,²²L-cysteine,²³ peptides,²⁴ proteins,²⁵ quaternary ammonium group-terminated thiols,²⁶ DNA^27,28 and N-1-(2-mercaptoethyl) thymine.²⁹ The presence of these groups on the surfaces of the nanoparticles can easily and selectively recognize Hg²⁺ ions through metal–ligand coordination, resulting in the aggregation of nanoparticles. Conversely, Duan et al. reported a colorimetric method based on the anti-aggregation of AgNPs, in which Hg²⁺ ions prevented the aggregation of AgNPs induced by 6-thioguanine.³⁰ DNA and N-1-(2-mercaptoethyl)thymine have been found to be more suitable sensing ligands for the detection of Hg²⁺ ions;³¹ however, their synthetic methods are complex and tedious. Moreover, expensive chemicals are required for the modification of surfaces using these ligands. Therefore, cost-effective, stable and easily available ligands are highly desired for the selective and sensitive detection of Hg²⁺ ions. Therefore, we used thiamine for the first time as a suitable ligand to modify AgNPs for the development of a highly selective and sensitive sensor for Hg²⁺ detection. Thiamine, also known as vitamin B₁, is a water soluble vitamin and an easily available ligand. Its structure consists of a thiazole ring which contains –S functionality and an aminopyrimidine ring which contains –N functionality. AgNPs could be easily modified with thiamine molecules by the formation of Ag–S bonds. Upon the addition of Hg²⁺ ions to the solution, the thiamine-modified AgNPs are expected to readily aggregate due to specific interactions between the –N groups on the surface of the Th-AgNPs and Hg²⁺ to form an N–Hg²⁺–N complex, in a similar fashion as thymine–Hg²⁺–thymine coordination (T–Hg²⁺–T).²⁹ This induced aggregation by Hg²⁺ ions can result in a color change from yellow to red or violet-blue, which could be readily observed by the naked eye or using a UV-vis spectrophotometer. Our designed sensing system provides high selectivity and sensitivity, a broad linear range and trace level detection of Hg²⁺ in aqueous solutions.

2. Materials and methods

2.1. Materials

All chemicals utilized in this work were of analytical grade and used without further purification. Thiamine hydrochloride and mercuric chloride were obtained from Merck, Germany. Sodium borohydride (NaBH₄), silver nitrate (AgNO₃) and other heavy metal salts in the form of either chlorides or nitrates were acquired from Sigma Aldrich, Germany. Deionized water was used for the preparation of all stock solutions and the final washing of glassware. Before starting any experiment, all glassware including cuvettes were first thoroughly washed with 3 : 1 HCl/HNO₃ (aqua regia) and then rinsed with deionized water. All measurements and experimental work including the synthesis of AgNPs were performed at room temperature.

2.2. Instrumentation

UV-vis absorption spectra were recorded on a UV-1800 double beam spectrophotometer (Shimadzu, Japan), using quartz cuvettes of 1.0 cm path length. The absorption spectra were recorded in the UV-vis range from 200 to 800 nm.

For HR-TEM measurements, a Hitachi H-8100 (Tokyo, Japan) electron microscope, was used with an accelerating voltage of 200 kV. The HR-TEM analysis was carried out after placing a drop of the colloidal solution of AgNPs onto a carbon-coated copper grid and allowing it to dry at room temperature.

2.3. Procedure for the synthesis of AgNPs

AgNPs were prepared according to the method reported by Wang et al.³² 20 mL of 1.0 mM AgNO₃ was added dropwise into a 500 mL conical flask containing 70 mL of 0.28 mM trisodium citrate solution and 7.6 mM NaBH₄. The mixture was stirred at room temperature for 1 h. The color of the mixture changed to pale yellow which indicated the formation of colloidal AgNPs. To complete the reaction time, the mixture was kept at room temperature for 48 h before its use.

2.4. Synthesis of thiamine-functionalized AgNPs (Th-AgNPs)

To prepare Th-AgNPs, 5 mL of the as-prepared AgNPs was diluted up to 10 mL with deionized water, and then 40 μL of 0.01 M thiamine was added to this solution. The mixture was stirred for 2 h and then left undisturbed for 30 h to achieve complete functionalization of the AgNPs with thiamine molecules. The as-prepared Th-AgNPs were used for the colorimetric detection of Hg²⁺.

2.5. Colorimetric detection procedure for Hg²⁺

For the detection of Hg²⁺, different concentrations of Hg²⁺ were added into a beaker/test tubes containing 5 mL of Th-AgNPs solution and stirred to mix them uniformly. Then the mixture was allowed to equilibrate for 5 min. During this time the color of the solution completely turned from yellow to red or violet-blue. The solution was then transferred to quartz cuvettes to monitor the absorption spectra. For real sample analysis, the Hg²⁺ concentration was determined in river water samples collected from the Kurram river, Pakistan, following the standard addition method. Different known concentrations of Hg²⁺ were added into a solution containing 3.5 mL of Th-AgNPs plus 1.5 mL of river water and then the absorption spectra were measured. All absorption measurements were performed in triplicate.

3. Results and discussion

3.1. The colorimetric detection ability of the Th-AgNPs

Thiamine has –S and –N functionalities in its structure. Owing to the strong affinity of sulfur atoms with Ag, the surface of the AgNPs could be easily modified with thiamine molecules through Ag–S bonds. FT-IR analysis was carried out to verify the possible interaction between thiamine molecules and AgNPs. As problems were faced in isolating the functionalized AgNPs from the solution, to record the IR data we added drop by drop solutions of thiamine and AgNPs directly into the KBr salt and then dried the samples in a desiccator. The FT-IR spectra of thiamine and thiamine-functionalized AgNPs are presented in Fig. S1 (ESI†). The absorption bands of thiamine at about 3670 cm⁻¹, 1603 cm⁻¹ and 1390 cm⁻¹ can be related to the –NH (–NH₂) stretching, –NH bending and C–N stretching of the amine groups in the pyrimidine ring, respectively. Observation of Fig. S1 (ESI†) further reveals the appearance of strong absorption peaks at about 3245 cm⁻¹ and 1065 cm⁻¹ which correspond to the –OH and C–O stretching vibrations of the hydroxyl group, respectively, attached to the thiazole ring via a hydrocarbon chain. A small peak around 740 cm⁻¹ corresponding to the stretching vibration of C–S–C also appeared in the spectrum of thiamine. While recording the IR spectrum of thiamine-functionalized AgNPs, typical bands of –OH at 3245 cm⁻¹ and C–S–C at 740 cm⁻¹ almost disappeared, while the signals of the amino groups still remained. These peaks offer evidence of the attachment of thiamine molecules onto the surface of the AgNPs through the –OH and C–S–C groups of the thiazole ring.

It is well known that the amino groups present in the DNA molecules can bind to Hg²⁺ ions to form a T–Hg²⁺–T complex.^27,28 Similar to the thymine structure, thiamine also offers –N functionalities to the surface of AgNPs that can easily interact with Hg²⁺ to form a N–Hg²⁺–N complex. Thus the Th-AgNPs have the ability to sense Hg²⁺ in solution. The proposed sensing mechanism for Hg²⁺ is illustrated in Fig. 1. Initially, the color of the dispersed Th-AgNPs was yellow which showed a strong surface plasmon resonance (SPR) band at 395 nm, as shown in Fig. 2.

Fig. 1 A proposed scheme for the Hg²⁺ induced colorimetric sensing response of Th-AgNPs.

Fig. 2 The UV-vis absorption spectra of Th-AgNPs in the absence and presence of 5 × 10⁻⁶ M Hg²⁺; inset: a photo of the corresponding color change.

Upon the addition of a small amount of Hg²⁺ to the solution containing Th-AgNPs, the color of the solution immediately turned from yellow to red or violet-blue, as shown in the inset of Fig. 2. This color change can be related to the aggregation of Th-AgNPs from their dispersed state due to the formation of N–Hg²⁺–N complexes between thiamine and Hg²⁺. The color change and aggregation of Th-AgNPs in the presence of Hg²⁺ were monitored using a UV-vis spectrophotometer. Upon the addition of Hg²⁺ ions to a solution of Th-AgNPs, the intensity of the SPR band at 395 nm decreased along with the appearance of a new peak at about 550 nm, as demonstrated in Fig. 2. This result suggests that Hg²⁺ ions could be readily detected using Th-AgNPs. The Hg²⁺-induced aggregation of Th-AgNPs was further confirmed by the HR-TEM analysis both in the absence and in the presence of Hg²⁺, as represented by HR-TEM images in Fig. 3. The observation of Fig. 3(A) reveals that in the absence of Hg²⁺, Th-AgNPs with a particle size of about 13.0 nm were well dispersed in the solution, but in the presence of 1 × 10⁻⁵ M Hg²⁺, aggregation of Th-AgNPs occurs as shown in Fig. 3(B). This aggregation behavior can be attributed to the interactions between Th-AgNPs and Hg²⁺ through N–Hg²⁺–N linkages which are expected to reduce the inter-particle distance with a concomitant increase in the size of the AgNPs. Moreover, a change in the particle size of the AgNPs before and after the addition of Hg²⁺ to the Th-AgNP solution was observed by dynamic light scattering (DLS) analysis. The observation of Fig. 4(A) manifests that the size of the Th-AgNPs in the absence of Hg²⁺ is around 23.51 nm. However, in the presence of Hg²⁺ the dynamic size of the Th-AgNPs significantly increased to a diameter of around 234.9 nm, as shown in Fig. 4(B). Furthermore, the size measured by DLS analysis is somewhat larger than that measured by HR-TEM analysis, as DLS measures the hydrodynamic diameter of the AgNPs dispersed in the aqueous system. These results clearly indicate that Hg²⁺ triggers the aggregation of the Th-AgNPs.

Fig. 3 HR-TEM images of (A) dispersed Th-AgNPs in the absence of Hg²⁺ and (B) aggregated Th-AgNPs in the presence of 1 × 10⁻⁵ M Hg²⁺ ions.

Fig. 4 DLS analysis of Th-AgNPs, (A) before and (B) after Hg²⁺ addition.

3.2. Effect of pH on the sensing system

The effect of pH on the colorimetric detection of Hg²⁺ was examined in the range of 2–10 using UV-vis absorption spectrophotometry. Initially the pH of the functionalized nanoparticles was 8. For investigating the influence of pH on the sensing matrix, the pH was decreased by the addition of hydrochloric acid and increased by the addition of sodium hydroxide. In the absence of Hg²⁺, decreasing the pH from 8 to 2 led to a gradual increase in the absorption ratio (A₅₅₀/A₃₉₅) of the Th-AgNPs with the maximum value of this ratio at pH 4, as shown in Fig. 5.

Fig. 5 The effect of pH on the absorption ratio (A₅₅₀/A₃₉₃) change of Th-AgNPs in the absence (a) and presence of 1 × 10⁻⁵ M Hg²⁺ ions (b). The A₅₅₀/A₃₉₅ values in the presence of Hg²⁺ were subtracted from the blank values (only Th-AgNPs) at each pH.

This maximum value indicates that the Th-AgNPs have aggregated themselves, which could be attributed to the strong inter-particle interaction via H-bonding between the protonated (–NH) and (–N) groups^32,33 at the heterocyclic free end of the thiamine molecule on the AgNP surfaces. Additionally, with the increase of pH from 8 to 10, no obvious changes in A₅₅₀/A₃₉₅ can be observed. This could be mainly due to the de-protonation of (–NH) groups in the heterocyclic ring that may not cause aggregation of the nanoparticles via H-bonding. This result suggests that the colorimetric detection of Hg²⁺ could not be performed below pH 8, because the Th-AgNPs prefer self-aggregation. Subsequently, the A₅₅₀/A₃₉₅ change was evaluated in the presence of Hg²⁺ in the pH range from 8 to 2. As shown in Fig. 5, the highest A₅₅₀/A₃₉₅ ratio was obtained at pH 8, as no interparticle H-bonding could occur at this pH value. Upon further increase in the pH of the solution, the A₅₅₀/A₃₉₅ ratio gradually decreased possibly due to the formation of mercuric hydroxide. From this study, it can be concluded that there is no need to adjust the pH conditions below or above 8 for the colorimetric detection of Hg²⁺.

The reaction time of the assay was examined by measuring the A₅₅₀/A₃₉₅ ratio of the Th-AgNP solution after the addition of Hg²⁺. Fig. S2 (ESI†) shows that the A₅₅₀/A₃₉₅ ratio increases quickly up to 5 min and then attains a steady state after further increasing the reaction time from 5 to 15 min. This behavior suggests that the Th-AgNPs get aggregated immediately within 5 min after the addition of Hg²⁺, thus demonstrating a fast colorimetric response for the detection of Hg²⁺.

3.3. Sensitivity of the proposed system

To test the colorimetric detection of Hg²⁺, different amounts of Hg²⁺ were added into the solution containing Th-AgNPs and then examined after 5 min of mixing. Fig. 6 shows that with the increase of Hg²⁺ concentrations ranging from 10 to 10 000 nM, the absorption band at 395 nm decreases gradually while at the same time the bands at a longer wavelength (550 nm) undergo red shift.

Fig. 6 (A) Photo images representing color variations of Th-AgNP solutions induced by the addition of different concentrations of Hg²⁺ ions. (B) The UV-vis absorption spectra of Th-AgNP solutions upon the addition of 0, 1 × 10⁻⁸, 2.0 × 10⁻⁸, 5 × 10⁻⁸, 1 × 10⁻⁷, 2.5 × 10⁻⁷, 5 × 10⁻⁷, 1 × 10⁻⁶, 2.5 × 10⁻⁶ and 5 × 10⁻⁶ M Hg²⁺.

Similarly, along with the spectral changes, progressive color variations from yellow to red and to violet-blue were also observed, as shown in Fig. 6. The color changes could be easily recognized by the naked eye when the concentration is over 100 nm. Based on the spectral changes (A₅₅₀/A₃₉₅), a calibration plot was constructed between the absorption ratio (A₅₅₀/A₃₉₅) and concentration of Hg²⁺, as shown in the inset of Fig. 6. The plot is nicely linear in the concentration range from 10 to 5000 nM, with a good linear correlation coefficient (R²) of 0.9962. The limit of detection (LOD) was found to be 5 nM using the equation 3σ/slope. Thus, the proposed sensor exhibits good sensitivity with a broad linear range and a lower detection limit as compared to most of the previously reported colorimetric methods^34–45 for the detection of Hg²⁺. The sensitivity of some recent colorimetric methods is shown in Table 1.

Table 1 Comparison of the sensitivity of the proposed sensing platform with the reported methods for the colorimetric detection of Hg²⁺

Colorimetric detection system	Linear range (nM)	LOD (nM)	Ref.
Mercaptophenyl boronic acid functionalized-AuNPs	80–1250	37	34
Green synthesized AgNPs	50–500 000	—	35
Multi-sulfhydryl hyperbranched polyethylenimine functionalized-AuNPs	8.76–127 000	8.76	36
Cytosine triphosphate-capped AgNPs	625–5000	125	37
Cysteine-modified Au–Ag core–shell nanorods	1000–60 000	273	38
Heteroepitaxially synthesized unmodified-AgNPs	100–10 000 000	100	39
Unmodified-AuNPs	50–300	15	40
AuNPs formed by H2O2 reduction of HAuCl4	0.1–10 000	0.0089	41
Glutamine and histidine functionalized AgNPs	1000–5 00 000	900	42
Biosynthesized AuNPs	1000–20 000	1440	43
Carrageenan-functionalized Ag/AgCl NPs	1000–1 00 000	1000	44
Bimetallic Ag–Cu nanoparticles	1–10	0.51	45
Thiamine functionalized-AgNPs	10–5000	5	This work

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3.4. Selectivity of the method

To explore the selectivity of Th-AgNPs towards Hg²⁺, we tested their colorimetric response in the presence of different potential interfering metal ions including Zn²⁺, Pb²⁺, Ni²⁺, Fe²⁺, Fe³⁺, Cr³⁺, Cu²⁺, Mn²⁺, Co²⁺, Al³⁺, Cd²⁺, Ca²⁺ and Mg²⁺. The color and the spectral changes of the Th-AgNPs upon the addition of various interfering metal ions are presented in Fig. S3 (ESI†).

No obvious color change was observed after the addition of different metal ions except Hg²⁺, as can be seen from the photo images shown in Fig. S3(A and C) (ESI†). Similarly, no noticeable change can be observed in the UV-vis absorption spectra of the Th-AgNPs in the presence of various metal ions (see the inset of Fig. S3(B and C), ESI†). Based on the UV-vis spectral changes obtained for various metal ions, the A₅₅₀/A₃₉₅ ratio was calculated to be: 0.823, 0.023, 0.0392, 0.01, 0.017, 0.017, −0.0104, 0.1, 0.02, −0.063, 0.065, 0.08, 0.05, and 0.069 for Hg²⁺, Zn²⁺, Pb²⁺, Ni²⁺, Fe²⁺, Fe³⁺, Cr³⁺, Cu²⁺, Mn²⁺, Co²⁺, Al³⁺, Ca²⁺, Cd²⁺ and Mg²⁺, respectively. This study reveals that the developed sensor has excellent recognition ability towards Hg²⁺ ions even in the presence of other metal ions. Thus, the high selectivity of the sensor is apparent. Furthermore, we probed the effects of NaCl on the stability of the Th-AgNPs. As the observation of Fig. S4 (ESI†) reveals, there is no significant effect on the absorption signal of the Th-AgNPs in the presence of 0.005 M and 0.025 M concentrations of NaCl. This result indicates that the Th-AgNPs are highly stable in saline conditions, and thus, the sensor is suitable for the detection of Hg²⁺, even at relatively high concentrations of NaCl.

3.5. Application to real water samples

The reliability of the method was tested by spiking different known concentrations of Hg²⁺ into solutions containing Th-AgNPs and river water. The absorption spectrum of each spiked sample was recorded after 5 min, as represented in Fig. S5 (ESI†). The results obtained are shown in Table 2. The percentage recovery in the range from 96.2 to 107.5% suggests that the Th-AgNP probe has the potential to practically determine Hg²⁺ in real water samples.

Table 2 Colorimetric determination of Hg²⁺ in real water samples

Sample	Added (μM)	Found (n = 3) (μM)	Recovery (%)
River water	0.00	—	Not detected
	0.02	0.021	105.0
	3.30	3.500	107.5
	5.00	4.810	96.2

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4. Conclusions

A highly selective and sensitive colorimetric method was successfully developed for the detection of Hg²⁺ ions using thiamine-functionalized AgNPs. The Th-AgNPs displayed a visible color change from yellow to red or violet-blue accompanied by a distinct spectral change. The presence of thiamine molecules on the surface of the AgNPs provided suitable functionalities that recognized Hg²⁺ ions using N–Hg²⁺–N coordination chemistry. The proposed sensing system demonstrated high selectivity for Hg²⁺ ions in the presence of various competing metal ions. The designed sensor utilizes a cost-effective, greener and easily available ligand compared to DNA or other ligands which could be obtained from difficult synthetic processes. Moreover, the sensor provides high sensitivity and selectivity requirements. Good percentage recoveries obtained from the practical analysis of Hg²⁺ ions in river water confirmed that the designed sensing method could be easily implemented for the routine analysis of Hg²⁺ ions.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support from the Quaid-i-Azam University and Higher Education Commission of Pakistan is gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nj03382f

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