**4. Prospects and Challenges**

This review provides an overview of the recently developed E-apt sensors and the use of different nanomaterials in sensors. These sensors can be applied to the analysis of various food and water contaminants. By using aptamers with high affinity and specificity for the target and one or more effective signal amplification steps, most sensors exhibit good sensor behavior, including high sensitivity and selectivity.

However, the practical application of electrochemical sensors is still in the preliminary stage, and most heavy metal detection is carried out under laboratory conditions. Results obtained in the laboratory are difficult to verify with results from real samples; there is still a long way to make developments in the practical identification of heavy metal ions. Therefore, further research perspectives that must be taken into account in this area are as follows. The efficiency of the sensor largely depends on the affinity of aptamers, so screening new aptamers with high affinity is the first prerequisite to constructing excellent sensors. In addition, adaptation is also highly dependent on nanomaterials. The design of highly active nanomaterials with long-term stability and reproducibility is the main goal of future efforts. Future research can be carried out towards the advancement of E-apt sensors based on advanced technologies for the multi-residue determination of heavy metal ions in various environments.

Electrochemical sensors have bright application prospects, but there are still some problems that need to be solved Firstly, the binding efficiency of the aptamer and heavy metal ions should be considered. The detection ability of the aptamer sensor depends largely on whether the aptamer can detect the existence of the target sensitively. If the aptamer and the target are combined inefficiently, it is difficult to achieve the ideal detection effect. Secondly, the efficiency of the SELEX method and the specificity of the selected aptamer still need to be improved. Because the aptamer selection method is time-consuming, low cost, and with low efficiency, there are not many aptamers on the market for the detection of heavy metal ions which greatly hinders the development of aptamer-based sensors. Third, because E-apt sensors usually need to use nanomaterials to improve their sensitivity, the cost of nanomaterials is affected by the price of nanomaterials. The development of low-cost nanomaterials with significant effects is also crucial to the construction of sensors. Finally, the survey found that most of the aptamer-based sensors are often only for single heavy metal ion detection and usually, more than in the sample, there are multiple kinds of heavy metal ion pollution. The development of multiple target detection sensors then has important practical significance and broad application prospects.

In conclusion, an aptamer-based biosensor design provides a promising method for the fast and on-site monitoring of heavy metal ions in food safety, but further research and development are still needed.

**Author Contributions:** Methodology, Z.C., M.X. and S.H.; investigation, Z.C., M.X., F.Z. and S.H.; writing—original draft, Z.C. and M.X.; resources, S.H.; funding acquisition, S.H.; project administration, S.H.; writing—original review and editing, S.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded by the Key-Area Research and Development Program of Guangdong Province (2019B020211002).

**Acknowledgments:** This work was supported by the Key-Area Research and Development Program of Guangdong Province (2019B020211002).

**Conflicts of Interest:** The authors declare no conflict of interest.
