Umerdaraz
Khan
a,
Abdul
Niaz
a,
Afzal
Shah
*b,
Muhammad Iqbal
Zaman
a,
Muhammad Abid
Zia
c,
Faiza Jan
Iftikhar
b,
Jan
Nisar
d,
Muhammad Naeem
Ahmed
e,
Mohammad Salim
Akhter
f and
Aamir Hassan
Shah
*g
aDepartment of Chemistry, University of Science & Technology, Bannu 28100, KPK, Pakistan
bDepartment of Chemistry Quaid-i-Azam University, Islamabad, 45320, Pakistan. E-mail: afzals_qau@yahoo.com; Fax: +92-5190642241; Tel: +92-5190642110
cUniversity of Education, Attock, Punjab 43600, Pakistan
dNational Centre of Excellence in Physical Chemistry, University of Peshawar, Peshawar, 25120, Pakistan
eDepartment of Chemistry, University of Azad Jammu and Kashmir, Muzaffarabad 13100, Pakistan
fCollege of Science, University of Bahrain, 32038, Bahrain
gUniversity of Chinese Academy of Sciences, Beijing 100049, China. E-mail: quaidian.51214@gmail.com
First published on 29th November 2017
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.
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 DNA27,28 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.
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.
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. 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. |
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) groups32,33 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+.
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 methods34–45 for the detection of Hg2+. The sensitivity of some recent colorimetric methods is shown in Table 1.
Colorimetric detection system | Linear range (nM) | LOD (nM) | Ref. |
---|---|---|---|
Mercaptophenyl boronic acid functionalized-AuNPs | 80–1250 | 37 | 34 |
Green synthesized AgNPs | 50–500000 | — | 35 |
Multi-sulfhydryl hyperbranched polyethylenimine functionalized-AuNPs | 8.76–127000 | 8.76 | 36 |
Cytosine triphosphate-capped AgNPs | 625–5000 | 125 | 37 |
Cysteine-modified Au–Ag core–shell nanorods | 1000–60000 | 273 | 38 |
Heteroepitaxially synthesized unmodified-AgNPs | 100–10000000 | 100 | 39 |
Unmodified-AuNPs | 50–300 | 15 | 40 |
AuNPs formed by H2O2 reduction of HAuCl4 | 0.1–10000 | 0.0089 | 41 |
Glutamine and histidine functionalized AgNPs | 1000–500000 | 900 | 42 |
Biosynthesized AuNPs | 1000–20000 | 1440 | 43 |
Carrageenan-functionalized Ag/AgCl NPs | 1000–100000 | 1000 | 44 |
Bimetallic Ag–Cu nanoparticles | 1–10 | 0.51 | 45 |
Thiamine functionalized-AgNPs | 10–5000 | 5 | This work |
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.
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 |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nj03382f |
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018 |