7.3.2. Sugars

Among the most popular groups of biomolecules that are the object of scientists' interest are sugars. Particularly noteworthy is glucose, the determination of which is important both in the pharmaceutical and food industries. Importantly, glucose monitoring is essential in the treatment of diabetes that is characterized by long-lasting hyperglycemia, making strict blood glucose control so important [77–81]. Table 4 shows an exemplary QDs-based sensors for sugars determination.


**Table 4.** QDs-based sensors for sugars determination.

Monitoring of glucose in human blood and urine is essential for the diagnosis and treatment of diabetes. The Sarana group has developed a biosensor for the detection of glucose based on cadmium quantum dots with a thiol ligand on the surface. This system has been coupled with glucose oxidase-a catalyst for glucose oxidation reaction, which releases hydrogen peroxide [77]. In constructing this biosensor, a capture mechanism was used, involving the charge transfer. The electron released in the process of reducing H2O2 to O2 has been moved towards the exciton of QDs, acting as an acceptor. As a result of this process, a QDs-ion was formed and decreases of fluorescence intensity were observed [82].

Another sensing method of glucose approach is based on FRET between CdTe QDs as an energy donor and gold nanoparticles (AuNPs) as an energy acceptor. The specific combination of concanavalin A(ConA)-conjugated QDs and thiolated-cyclodextrins (b-SH-CDs)-modified AuNPs assembles a hyperefficient FRET nanobiosensor. In the presence of glucose, the AuNPs-b-CDs segmen<sup>t</sup> of the nanobiosensor is displaced by glucose which competes with b-CDs on the binding sites of ConA, resulting in the fluorescence recovery of the quenched QDs. Experimental results show that the increase in fluorescence intensity is proportional to the concentration of glucose in a linear range of 0.10–50 μM under the optimized experimental conditions. In addition, the sensor has high sensitivity with a LOD as low as 50 nM, and has excellent selectivity for glucose over other sugars and most biological species present in serum [78].

Riedel et al. [79] investigated the light-triggered reaction of the redox molecules, hexacyanoferrate, and ferrocenecarboxylic acid, at CdSe/ZnS quantum dot modified gold electrodes for light-driven applications. Here, electron transfer between QDs and redox mediators has been found to be feasible. Additionally, photoluminescence measurements in solution demonstrate the strong interaction between the QDs and the redox species by quenching of QD fluorescence. Subsequently, the established QD–mediator systems have been combined with the enzymes, pyrroloquinoline quinone-dependent glucose dehydrogenase and fructose dehydrogenase, to the feasibility of electrically contacted

enzyme/QD biohybrids. This demonstrates the photoelectrochemical principle displays applicability for sensing and for driving QD electrodes by biocatalytic sugar consumption.
