*2.1. Gold Nanoparticles*

Among the aptamer-based sensors developed, metal nanoparticles such as gold nanoparticles (AuNPs) are the most common advanced materials. They have high stability and oxidation resistance and can be prepared through various physical and chemical methods, among which the most classical preparation method was proposed by Turkevich et al. in 1951 [29]. The high surface-volume ratio and excellent optical properties of AuNPs contribute to the high sensitivity and selectivity of the target detection. The optical properties of AuNPs are largely dependent on the size, shape, and aggregation state of nanoparticles. Colloidal AuNPs are usually red or pink and turn purplish-blue when aggregated, forming the basis of colorimetric detection [30]. AuNPs are also conductive and fluorescent and, thus, have the ability to respond to external electrochemical and optical stimulation.

One of the most common approaches to using AuNPs in electrochemical sensors is to anchor aptamers. The aptamer is usually fixed by the force of the Au-S bond; Yuan et al. [31] used this method to develop a sensor for detecting Pb2+ and Cd2+ at the same time. Due to the specific binding of aptamers to metal ions, methylene blue or ferrocene labeled aptamers were competitively separated from the gold electrode, resulting in a weaker electrochemical signal. The detection limits of Cd2+ and Pb2+ were 89.31 and 16.44 pM. Deng et al. [32] deposited L-cysteine and AuNPs layers on the electrode surface successively providing a large surface area to anchor many sulfur-capped auxiliary probes through the mercaptan-gold interaction. Additionally, to fix the aptamer, Liu et al. used chitosan to attach AuNPs to the electrode to enhance electron transfer, thus improving the sensitivity of the sensor [33].

In addition to being used alone, AuNPs are also used as composite materials coupled with other metal ions. The composite materials tend to enhance the electrochemical signal so that even subtle changes can be detected. Silver and gold alloy nanoparticles (Ag-Au alloy NPs) promise to create inexpensive and stable electrochemical sensors [34]. This is because silver-gold alloy nanoparticles have a large specific surface area and good electrical conductivity which can act as a conductive center and promote electron transfer, thus trapping more heavy metal ions on the electrode. Zhao et al. [35] used DNA enzyme functionalized (Ag-Au alloy) NPs to form core-shell nanoparticles, which can be used as signal tags. Because the core-shell structure increases the specific surface area and catalyzes the reaction, the electrochemical signal can be enhanced. Ag-Au Alloy NPs were also used to modify GCE and the resulting sensor had higher sensitivity and reproducibility. Miao et al. [36] used Fe3O4@AuNPs to carry a DNA probe, and the other two were labeled with independent electrochemical substances which can detect both Hg2+ and Ag2+. Due to their excellent signal amplification, AuNPs have been one of the most used transduction materials to construct E-apt sensors for the analysis of food and water contaminants.
