Nanomaterial-based optical sensors for mercury ions

Highlights

• The unique optical properties of nanomaterials made them candidates in the design of optical sensors.

• The review summarized the design principles of nanomaterials-based optical sensors for Hg²⁺ detection.

• Three kinds of optical nanosensors were included colorimetric assay, fluorometric analysis and SERS detection.

Abstract

As one of the most toxic heavy metals, mercury ion (Hg²⁺) has become a concern focus for its severe threats to environment and human health. As a result, it is of great importance to develop novel methods to realize the recognition and quantification of Hg²⁺. The past decades witness the development of nanomaterial-based optical sensors for Hg²⁺ detection, showing the benefits of simplicity, rapidity, high sensitivity and selectivity, and cost-effectiveness. The reported methods have allowed the detectability down to nanomolar concentrations or much lower levels, and proved their practical applications for detecting and quantifying Hg²⁺ in synthetic solutions or natural water samples. In this review, we summarize the published innovations in nanomaterial-based optical sensors for the detection of Hg²⁺ according to different sensing strategies, including colorimetric, fluorescent and surface enhanced Raman scattering detection. Moreover, some challenges and significant attempts related to these methods are also discussed.

Introduction

Mercury ion (Hg²⁺) is one of the extremely hazardous metal ions which are widely presented in the ambient air, water, soil and biota [1], [2], [3]. Due to its high toxicity and bioconcentration effect, Hg²⁺ can cause numerous severe negative impacts on human health, such as kidney failure, brain damage, and various cognitive and motion disorders [4]. Therefore, it is of great importance to research and develop sensitive sensing strategies for the recognition and detection of Hg²⁺ in biological and environmental systems. Traditionally reported approaches available to monitor Hg²⁺ in aqueous samples include atomic absorption spectrometry [5], [6], atomic fluorescence spectrometry [7], [8], [9], inductively coupled plasma-mass spectrometry [10], [11], fluorometry [12], [13], [14], [15], [16] and electrochemical analysis [17], [18], [19], [20], [21], [22]. Recently, liquid crystal based sensor system and resonance Rayleigh scattering aptasensor are also be used to detect Hg²⁺ [23], [24]. However, most of these methods are limited in their extensive applications because of the use of expensive and bulky instruments, complex sample pretreatment processes or low sensitivity and selectivity. As a result, it is very necessary to explore new approaches for determination of Hg²⁺ to overcome the restrictions of above mentioned methods.

With the flourish development of nanoscience and nanotechnology, various nanomaterials with different morphologies have been successfully synthesized, such as gold/silver nanoparticles (Au/Ag NPs) [25], [26], gold nanorods (Au NRs) [27], semiconductor quantum dots (QDs) [28], graphene [29], carbon dots (CDs) [30], etc. On account of their unique optical properties, nanomaterial becomes an important optical-translating unit in the construction of optical sensors. Typically, the optical properties of nanomaterials can be varied via target-induced aggregation/anti-aggregation or their surface change [31], [32]. By combining with the recognition unit, such as small molecular ligands, biological macromolecules and specific chemical/biological reactions, the nanomaterial-based optical sensors show specific and selective response to the target molecule. According to the reported design strategies for optical nanosensors in recent years, the most popular sensing techniques include colorimetric detection, fluorometry and surface enhanced Raman scattering (SERS) [33]. Except the function of translating sensing behavior of target into optical signals, the use of these novel nanomaterials can also endow high sensitivity to the proposed nanosensors.

Among a variety of target molecules, optical nanosensors for the determination of Hg²⁺ have attracted increasing interest, and numerous related sensors have been constructed. Recently, some excellent reviews on this topic have appeared in special journal issues based on the importance of the usage of nanomaterials [34], [35], [36], [37]. Xu et al. [34] reviewed the development of different nanomaterial-based approaches for the detection of mercury according to the kinds of nanomaterials, and the speciation analysis of mercury using nanomaterials was also be discussed. Botasini et al. [35] critically discussed the applicability of the nanotechnology-based sensors to the analysis of real samples for Hg²⁺ determination. Chansuvarn et al. [36] summarized colorimetric and visual assays for determination of Hg²⁺ based on gold-based nanomaterials. The above mentioned reviews offer different perspectives to overview the nanomaterial-based optical sensors, however, a comprehensive summary in the design strategies of optical nanosensors for Hg²⁺ has not been given, especially the fluorometric and SERS detection strategies.

In this review, the prominent optical nanosensors for Hg²⁺ are summarized based on different detection techniques. Firstly, we discuss nanomaterial-based colorimetric detection of Hg²⁺ based on the aggregation and morphology transition of nanomaterials; in the following, fluorescent nanosensors for Hg²⁺ are overviewed including Hg²⁺-induced aggregation of fluorescent nanomaterials and fluorescence energy transfer system; moreover, nanomaterial-based SERS detection is focused on for the detection of Hg²⁺ via direct aggregation of nanoparticles and Hg²⁺ induced signal alteration on SERS active and support-assisted nanonmaterials. Finally, the review ends with the conclusions of the latest advances and outlook in the development of nanosensors for Hg²⁺.