Facile fabrication of an electrochemical aptasensor based on magnetic electrode by using streptavidin modified magnetic beads for sensitive and specific detection of Hg²⁺

Highlights

• A novel electrochemical aptasensor was developed for sensitively detecting Hg²⁺.

• The magnetic property of Fe₃O₄-SA was applied to connect well with MGCE.

• Thionine was employed as signal indication for quantitative detection of Hg²⁺.

• The aptasensor displayed low detection limit of 0.33 nmol/L for Hg²⁺.

Abstract

In this work, a novel electrochemical aptasensor was developed for sensitive and specific detection of Hg²⁺ based on thymine-Hg²⁺-thymine (T-Hg²⁺-T) structure via application of thionine (Th) as indicator signal. For the fabrication of the aptasensor, streptavidin modified magnetic beads (Fe₃O₄-SA) was firmly immobilized onto the magnetic glassy carbon electrode (MGCE) benefited from its magnetic character. Then biotin labeled T-riched single stranded DNA (Bio-ssDNA) connected with Fe₃O₄-SA specifically and steadily because of the specific binding capacity between streptavidin and biotin. The stable structure of T-Hg²⁺-T formed in the present of Hg²⁺ provided convenience for the intercalation of Th. The detection of Hg²⁺ was achieved by recording the differential pulse voltammetry (DPV) signal of Th. Under optimal experimental conditions, the linear range of the fabricated electrochemical aptasensor was 1–200 nmol/L, with a detection limit of 0.33 nmol/L. Furthermore, the proposed aptasensor may find a potential application for the detection of Hg²⁺ in real water sample analysis.

Introduction

Heavy metal ions which possess the characteristics of high toxicity, strong biological enrichment, non-natural degradation and complex sources are seriously harmful to the ecological environment (Ye et al., 2012). Mercury ion (Hg²⁺) as one of the most toxic heavy metal ions has gained enormous attention from all walks of life. It is a kind of highly toxic and quite common heavy metal ion that may seriously pollute the environment. The exposure to mercury can lead to a series of harmful effects to health, such as brain damage, kidney failure, nervous system damage and immune system damage (Li et al., 2015). According to the stipulation of United States Environmental Protection Agency, the maximum allowable concentration of Hg²⁺ in drinking water is 10 nmol/L. Therefore, it is indeed crucial to develop a highly sensitive and selective method for the detection of Hg²⁺ in the environment.

For the detection of Hg²⁺, several methods have been promoted, such as fluorescence spectrometry (Deng et al., 2011, Zheng et al., 2013), inductively coupled plasma mass spectrometry (ICP-MS) (Chen et al., 2009, Long and Kelly, 2002), surface-enhanced Raman scattering (SERS) (Duan et al., 2012, Yi et al., 2013), colorimetry (Chai et al., 2010, Lou et al., 2011), and ultraviolet-visible spectrometry (Li et al., 2014, Zhang et al., 2015a). Although these methods can undoubtedly achieve the highly sensitive detection of Hg²⁺, there still existed several problems such as high costs, complex instruments, complicated sample preparation procedures, and time-consuming. Therefore, it is essential to establish a selective and sensitive method for the detection of trace Hg²⁺.

Electrochemical aptasensor is a kind of biosensor which uses aptamer as sensitive recognition element. Aptamer is a kind of DNA or RNA sequence, which can be selected as a kind of material with a specific sequence in vitro. It is a new type of recognition molecules and has many advantages such as chemical synthesis, good stability and no toxicity. Previous reports have demonstrated that Hg²⁺ can specifically link with thymine-thymine (T-T) pairs to form stable T-Hg²⁺-T base pairs, which is even more stable than the Watson-Crick A-T pair (Tanaka et al., 2007, Zhu et al., 2009). Taking advantage of this special structure, researchers have designed large numbers of detection systems like fluorescence aptasensor (Zhang et al., 2015a), electrochemiluminescence aptasensor (Wang et al., 2014), colorimetric aptasensor (Wang et al., 2010) and electrochemical aptasensor (Zhang et al., 2015b) for the detection of Hg²⁺. Among them, electrochemical aptasensor has gained extensive attention due to its advantages of simplicity, sensitivity, low cost, fast response and nontoxicity.

Herein, we developed a novel electrochemical aptasensor for the detection of Hg²⁺ based on magnetic electrode by using streptavidin modified magnetic beads (Fe₃O₄-SA) and thionine (Th). To fabricate the aptasensor, magnetic glassy carbon electrode (MGCE) was modified by Fe₃O₄-SA for the first time, which undoubtedly accelerated the analysis rate. Owing to the magnetic property of electrode, the following treatment would be much easier, which saved a lot of time than the traditional electrode modification methods. Subsequently, the biotin labeled T-riched single stranded DNA (Bio-ssDNA) was modified onto the electrode. Due to the strong specific binding capacity between streptavidin and biotin, the Bio-ssDNA can be immobilized on the electrode firmly, which greatly enhanced the stability of the aptasensor. Afterwards, Hg²⁺was introduced onto the electrode. Based on the strong interaction of T-Hg²⁺-T base pairs, the Bio-ssDNA could fold itself. Eventually, the Th as electron mediator was dropped onto the electrode, which could be detected by differential pulse voltammetry (DPV). The more amount of Hg²⁺ was immobilized onto the electrode, the more Th could be absorbed. According to the different DPV signals of Th, the amount of Hg²⁺ was finally quantified. This fine fabricated electrochemical aptasensor exhibited high sensitivity, good specificity and excellent reproducibility. The proposed strategy may find potential applications in fabricating electrochemical aptasensor for the detection of other heavy metal ions.