**6. Aptamers**

In nucleic acids, aptamers are short, single-stranded sequences that bind specific molecules selectively and strongly. To create them, ligands undergo systematic transformations and exponential evolutions. Several applications rely on aptamers, including cancer cell imaging, biomedicine, therapeutics, gene therapy, biosensing, or targeted drug delivery, despite their high binding affinity, long-term stability, ease of modification, low immunogenicity, and low cost.

An ATmega328P prototype biosensor for the detection of E. coli in water samples using aptamer I and II-conjugated SPIONs and CdTe QDs has been developed by Pandit et al. We then conjugated E. coli-specific aptamers I and II to both types of nanoparticles. In terms of microbial detection, conjugated SPIONs and CdTe-MPA QDs display high quantum yields and significant magnetic and fluorescent properties. It is an effective tool to detect microbes using a conjugated SPION followed by a CdTe-MPA QD that has a high quantum yield, as well as significant magnetic and fluorescent properties. Bioconjugation of *E. coli*-specific aptamer I and II with SPIONs as well as QDs studied using various methods demonstrated selective separation and subsequently, detection of *E. coli*. The ATmega 328P prototype biosensor was further used to capture and detect E.coli using CdTe MPA QDs conjugated with aptamer II. Up to 100 cfu can be detected by the biosensor, demonstrating its sensitivity. We can detect on the spot, it is easy to use and less time-consuming. The ATmega328P prototype biosensor is capable of detecting E. coli in water if it is combined with cadmium-based QDs. As a result of the technology's rational extension and exploration, other pathogenic microorganisms can be detected in a wide variety of food samples. Additionally, the system meets the need for a portable, miniaturized pathogen detection device that can be deployed in the field [53].

Using SPCE/C60/MWCNTs-PEI/PQdot/GLA/APT aptasensors, Jamei et al. analyzed thrombin protein. By modifying SPCE electrodes with C60/MWCNTs-PEI/PQ dot nanocomposites, sensitive electrochemical aptasensors could be manufactured, as shown in **Figure 3**. In addition to their suitable stability, their high number of surface amine groups, their surface-to-volume ratio, their fast electron transfer kinetics, and their high electrical conductivity, these C60/MWCNTs-PEI/PQ dot nanocomposites possess unique characteristics. This nanocomposite's high surface-to-volume ratio and high number of surface amine groups increased the number of thrombin binding sites. A sensitive aptasensor was first developed using polymer quantum dots, which exhibit unique characteristics and are easy to synthesize [55].

A QDs-based biosensor was reported by Liu et al. using Pr3+–rutin complexes as both quenchers and receptors. Using the Pr3+–rutin complex, we were able to detect QDs in a favorable "off" state by efficiently quenching their fluorescence. Our approach relies on the electrostatic association of the cationic Pr3+–rutin complex on the surface of QDs. The static association reduces the fluorescence intensity of QDs due to ultrafast photoinduced electron transfer. In addition, Pr3 + −rutin complexes also serve as receptors for dsDNA. As a result of adding herring sperm DNA, Pr3+–rutin complexes were removed from QD surfaces and the QDs' fluorescence was restored [54].

The FRET method was employed by RezanejadeBardajee et al. in order to detect complementary DNA (as a positive control) and RNA from the COVID-19 virus by high-sensitivity detection of CdTe-ZnS QDs bioconjugates. A ligand exchange process replaces thiolated DNA (capture DNA) with surface-bound TGA molecules to form

**Figure 3.**

*Schematic of stepwise preparation electrochemical aptasensor and C60/MWCNTsPEI/PQdot [54].*

DNA-conjugated quantum dots (QDs-DNA). Besides designing the BHQ2-DNA oligonucleotide, an oligonucleotide from the viral genome was also designed for the FRET experiment. As a final application, these elements can be used for detecting viral RNAs (via FRET experiment) and for sensing target DNA sequences (via the QDs-DNA bioconjugate nanoprobe) [56].
