Facile colorimetric detection of Hg²⁺ based on anti-aggregation of silver nanoparticles

Highlights

• A colorimetric sensor for Hg²⁺ was developed by mixing 6-Thioguanine and AgNPs.

• Colorimetric sensing was obtained based on the Hg²⁺-inhibited aggregation of AgNPs.

• The method showed high sensitivity and selectivity.

• Determination of Hg²⁺ in water samples was studied.

Abstract

This paper describes an investigation of a facile colorimetric sensor for Hg²⁺ in aqueous solution based on the anti-aggregation of silver nanoparticles (AgNPs). In the absence of Hg²⁺, the addition of 6-Thioguanine to AgNPs solution led to the aggregation of AgNPs, resulting in a color change from yellow to brown with a red shift of absorption spectra. However, the presence of Hg²⁺ inhibited the 6-Thioguanine-induced aggregation of AgNPs accompanying with a color change from brown to yellow. Under the optimal conditions, the ratio between the absorbance at 530 nm and 394 nm (A₅₃₀/A₃₉₄) was linearly proportional to the Hg²⁺ concentration in a range from 0 to 333 nM with a detection limit of 4 nM. Other environmentally relevant metal ions did not interfere with the detection of Hg²⁺. The proposed method was simple, cost-effective and rapid without any complicated modifying step. It was successfully applied to detect Hg²⁺ in environmental water samples.

Introduction

Heavy metal ions, released from both natural and industrial sources, have severe adverse effects on human health and environment even at low concentrations. Among them, mercury ions (Hg²⁺) is one of the most hazardous metal ions which widely exists in water and soil, it can cause damage to the brain, kidneys, nervous system and immune system (Fan et al., 2009a, Wang et al., 2010a). Thus, the rapid, economical, sensitive and selective detection of Hg²⁺ is urgently needed. Up to date, various analytical methods have been developed to detect Hg²⁺, including electrochemical methods (Liu et al., 2009), chromatography (Fitzgerald and Gill, 1979, Krishna et al., 2007), atomic absorption/emission spectroscopy (Madden and Fitzgerald, 2009, Zhu and Alexandratos, 2007), cold vapor atomic fluorescence spectrometry (Yu and Yan, 2004) and inductively coupled plasma mass spectrometry (Moreton and Delves, 1998). Although many of these methods can provide low detection limits, they generally require sophisticated sample preparation procedures or expensive equipment.

In comparison with the above methods, colorimetric methods are especially promising in the analysis of Hg²⁺, due to their naked-eye applications free of expensive equipment (Hirayama et al., 2008, Lee et al., 2007a, Lin et al., 2010). Functionalized metal nanoparticles (NPs) including gold nanoparticles (AuNPs) and silver nanoparticles (AgNPs) have been developed as effective colorimetric sensors for detecting Hg²⁺ at low concentrations, due to their high extinction coefficients and the distance-dependent optical properties (Li, L. et al., 2009). If AuNPs or AgNPs aggregate, a visual color change as well as UV–vis spectral change would be observed, due to the decrease of interparticle distance (Zhou et al., 2013). Up to now, most of the developed Hg²⁺ colorimetric sensors were based on AuNPs. In which, the interaction between the surface modifiers of AuNPs and Hg²⁺ induced the aggregation of AuNPs with a red-to-blue color change (Chai et al., 2010, Chen et al., 2011, Darbha et al., 2008, Liu et al., 2008, Si et al., 2007, Wang et al., 2011, Xu et al., 2009) or the anti-aggregation of initially aggregated AuNPs with a blue-to-red color change (Du et al., 2011, Hung et al., 2010, Li et al., 2011, Lou et al., 2011, Wu et al., 2011, Yang et al., 2011). Although the above methods are promising tools for Hg²⁺ detection, they usually need complicated procedures to modify DNA or thiol-containing organic molecules on AuNPs surface, which makes them time-consuming and relative highly costly. In fact, compared with AuNPs, AgNPs are more cost-effective and have higher extinction coefficients relative to AuNPs of the same size (Bae et al., 2010, Lee et al., 2007b, Li et al., 2009a, Ling et al., 2008). So, AgNPs are also good candidates for Hg²⁺ colorimetric sensor design. Until now, only very few AgNPs-based colorimetric sensors for Hg²⁺ have been reported, and most of them were based on the redox reaction between AgNPs and Hg²⁺ (Apilux et al., 2012, Bera et al., 2010, Fan et al., 2009b, Farhadi et al., 2012, Ramesh and Radhakrishnan, 2011). Recently, few colorimetric sensing methods of Hg²⁺ based on the aggregation of AgNPs were also developed (Bothra et al., 2013, Tan et al., 2013, Wang et al., 2010b, Wang et al., 2012). It should be noted that the aggregation-based colorimetric assays often exhibit poor selectivity because many other external factors can also induce the aggregation of metal NPs (Li et al., 2011). Conversely, the anti-aggregation-based colorimetric sensing strategy may improve the selectivity owing to the lower possibility of false positive results. To the best of our knowledge, no colorimetric sensor for Hg²⁺ based on the anti-aggregation of AgNPs has been reported.

Herein, for the first time, 6-Thioguanine-capped AgNPs were synthesized and used as sensitive and selective colorimetric sensor for Hg²⁺ based on the Hg²⁺-induced anti-aggregation of AgNPs. The addition of 6-Thioguanine to AgNPs resulted in the aggregation of AgNPs with a yellow-to-brown color change. While in the presence of Hg²⁺, the 6-Thioguanine-induced aggregation of AgNPs could be inhibited by Hg²⁺, leading to a reverse color change from brown to yellow. This colorimetric method for the detection of Hg²⁺ showed high sensitivity (4 nM) and selectivity. It could be applied to the determination of Hg²⁺ in water samples.