**4. Molecularly imprinted polymer**

In the presence of template molecules, molecularly imprinted polymers (MIPs) are synthesized through polymerization reactions between functional monomers and crosslinking monomers. These polymers are widely used for many applications, including artificial antibodies, drug delivery, and chemical sensors catalysis. Due to

its unique ability to recognize target analytes and chemical stability, MIP has become a research hotspot in chemical sensors. According to Yang et al., ascorbic acid can be quantitatively measured with a ternary QD-based MIP ratiometric fluorescence sensor based on ternary QDs. Under a single excitation wavelength of 380 nm, the sensor exhibited well-resolved emission peaks at 530 and 705 nm, respectively, corresponding to ZnCdS QDs@MIP and CdTeS QDs@SiO2. CdTe QDs were embedded in the SiO2 shell to serve as the reference signal, and ZnCdS QDs were encapsulated in the MIP to serve as the response signal [48].

Qi et al. fabricated MIP using L-arginine, which is a part of microcystin, as the segment template for the adsorption and determination of microcystin. After studying a series of structural analogs of MC-LR, we selected L-leucine as the segment template to prepare MIP. Sol-gel polymerization was used to fabricate the MIP@CQDs@SiO2 fluorescence sensor using 4-aminopropyltriethoxysilane as the monomer and tetraethylorthosilicate as the crosslinker. Polymer layers with specific recognition sites were capable of selectively adsorbing MC-LR molecules, causing CQDs to show fluorescence quenching behavior via electron transfer [49].

Chmangui et al. synthesized a composite material based on the molecularly imprinted technique by using DMC as a dummy template. 5,7-dimethoxycoumarin (DMC) contains coumarin moiety which assists with the specific recognition and imprinting of cavities within the prepared MIP layer. This moiety is identical to that found in AFs. A sensitive and selective detection of total AFs in non-dairy beverages is therefore expected through the anchorage with Mn-doped ZnS QDs. The prepared Mn-doped ZnS QDs were first deposited on a DMC dummy template, which was then coated with polyethyleneglycol (PEG). An efficient fluorescent screening method for non-dairy beverage analysis was found to be reliable with MIP-Mn-doped ZnS QDs composites [50].

## **5. Graphene quantum dots and carbon quantum dots**

As a result of their electronic transport properties, chemical, and exceptional physical, graphene quantum dots (GQDs) and carbon quantum dots (CQDs) are being investigated for the development of next-generation biosensors. An explosion of reports have been published in recent years on biotransducer designs and biosensing applications utilizing these nanomaterials for facilitation, improvement, or otherwise developing novel approaches to analyte detection and monitoring.

Using fluorescent ssDNA probes coupled with CQD sensors, Loo et al. reported complementary DNA strand detection. Fluorescence resonance energy transfer (FRET) is quenched in this scheme thanks to the p-p stacking from DNA base pairs and CQD-conjugated p-systems. Due to electrostatic repulsion, the fluorescence of the ssDNA probe was restored when it hybridized with the target [51].

In order to fabricate a polyphenol index biosensor, Vasilescu et al. developed a molybdenum disulfide (MoS2)/GQD biotransducer on which they drop-casted laccase. It was reported that the caffeic acid sensor had a linear range of 380 nanomolar to 100 millimolar with a low level of detection of 320 nanomolar. A notable finding of their voltammetric studies was that laccase's response was similar with each electrode formulation, although the MoS2/GQD electrode contributed to a higher electron transfer efficiency [52].
