**1. Introduction**

With the rapid development of industry and the improvement of urbanization, more and more chemical substances are used in daily life and agricultural production. Increasingly frequent industrial activities such as mining, metallurgy, and oil extraction produces many toxic and harmful substances. These toxic and harmful substances, even after purification treatment, will still leave some residues in the natural water system, including heavy metals, inorganic salts, and agricultural veterinary drugs, which cause pollution and damage the water environment [1–3]. Unlike organic pollutants, heavy metals cannot be biodegraded under natural conditions [4] and will be passively ingested by plants through drinking and irrigation, and eventually, will enter the human body through continuous accumulation in the food chain. Mercury, cadmium, lead, Chromium, Thallium, Antimony, and arsenic are the most common heavy metal pollutants. According to WHO standards, they usually do not exceed 2 ppb. The heavy metals ingested into the human body are likely to form complexes with biological substances such as proteins, enzymes, and nucleic acids. The formation of such complexes alters the molecular composition and mechanism of biological matter, causing it to fail to perform its original physiological function or causing distortion [5]. The accumulation of these elements can cause serious damage to the gut, bones, central nervous system, liver, kidneys, and reproductive system. Since these elements cannot be removed by normal removal methods, even trace amounts of heavy metals can pose a serious threat to living things [6].

**Citation:** Chen, Z.; Xie, M.; Zhao, F.; Han, S. Application of Nanomaterial Modified Aptamer-Based Electrochemical Sensor in Detection of Heavy Metal Ions. *Foods* **2022**, *11*, 1404. https://doi.org/10.3390/ foods11101404

Academic Editor: Haiying Cui

Received: 25 April 2022 Accepted: 10 May 2022 Published: 12 May 2022

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In these cases, detection of heavy metal ions in environmental and water systems to prevent heavy metal pollution from the source of the food chain is a vital need. In recent years, many detection methods for heavy metal ions have been developed. Traditional detection methods mainly calculate the concentration of an atom based on its characteristic spectral intensity, including atomic absorption spectroscopy (AAS), inductively coupled plasma mass spectroscopy (ICP-MS), X-ray fluorescence spectrometry (XRF), neutron activation analysis (NAA), and inductively coupled plasma-atomic emission spectrometry (ICP-AES) [7]. These methods can perform accurate qualitative and quantitative analyses of heavy metal ions with high sensitivity, but they are also expensive and require laborious pre-processing [8]. Therefore, a cost-effective, fast, and efficient detection method for heavy metal ions needs to be developed.

Biosensors play an indispensable role in the development of biotechnology and are a fast analytical tool for detection at the molecular level. Usually, the recognition factor used in this technology is an antibody, but the emergence of the aptamer brings new prospects and possibilities to the field of biosensor analysis [9]. An aptamer is an artificial, single-stranded oligomer probe of DNA and RNA consisting of 10–100 bases, obtained by the SELEX index enrichment method. In Figure 1 the general process of SELEX is shown in detail. The initial ssDNA library was exposed in a container filled with target molecules, and the sequences with specific binding ability to the target were separated from the library. Then, a PCR reaction was used for bulk amplification and the next round of screening was carried out until ssDNA with the highest affinity to the target was obtained. Aptamers can fold into complex structures, selectively binding to the target with high affinity and specificity, known as artificial antibodies [10–13]. Compared with antibodies, aptamers have obvious advantages such as high chemical stability, low cost, easy operation, and they are easy to obtain [14]. Aptamers have a wide range of recognition targets, such as proteins, small molecules, agricultural veterinary drugs, bacteria, and heavy metal ions [15–19], which have broad application prospects in the field of food detection. A variety of biosensors using aptamers as recognition factors have been developed and applied to the detection of heavy metal ions, providing a new, efficient, and fast platform for the detection of heavy metal ions.

**Figure 1.** SELEX flowchart.

The electrochemical aptamer sensor (E-apt sensor), which is composed of biometric elements and signal sensors, has attracted more and more attention for this purpose. The signal sensor usually consists of an electrode substrate, modified layer, and electrochemical signal detection system. The most widely used electrode substrates include a gold electrode (AuE), glassy carbon electrode (GCE), indium tin oxide electrode (ITO), reduced graphene electrode (ERGO), and screen-printed electrode (SPE). Different electrode materials can have different degrees of signal enhancement after being modified by an appropriately modified layer and a variety of nanomaterials can be selected for the modified layer. The aptamer is fixed on the surface of the electrode by intermolecular force with the modified layer. This operation will change the impedance of the electrode and cause a current change. Therefore, nanomaterial modification in electrodes becomes an important part of electrochemical sensor construction. The electrochemical signal detection system includes a signal amplifier, a processor, and a display screen. The electrochemical sensor has been widely used in recent years because of its advantages of simplicity, high efficiency, strong specificity, and high sensitivity. As shown in Figure 2, the number of papers and patents published with the keywords "Electrochemical" and "Aptamer" has increased year by year since 2012 (data for 2022 is up to March).

**Figure 2.** Web of Science report for the number of indexed papers and patents about the application of the E-apt sensor (keyword: "Electrochemical" and "Aptamer" accessed on 27 March 2022).

This paper reviews the application of the E-apt sensor for the detection of heavy metal ions in recent years and systematically introduces the working principles of electrochemical sensors with different configurations of various nanomaterials. A Schematic diagram of nanomaterial modified aptamer-based electrochemical sensor are briefly illustrated in Figure 3. The strengths and limitations of this technology are summarized and its future challenges and application trends are envisioned.

**Figure 3.** Schematic diagram of nanomaterial modified aptamer-based electrochemical sensor.
