7.3.1. Proteins

Proteins are among the most important biomolecules found in the body. In addition to the basic building, transporting and regulating functions, proteins also act as biological catalysts–enzymes. The function of proteins is also invaluable in the immune system-acting as immunoglobulins. Due to the extremely important functions of proteins, it is necessary to monitor their concentration and the processes in which they take part [19,71–75]. QDs-based FRET nanosensors have been developed to monitor a variety of enzymes including alkaline phosphatase, ATPase renin, protein kinase, DNA methyltransferase, DNA glycosylase, and telomerase [76]. A different strategies for determination of proteins are presented in Table 3.


**Table 3.** QDs-based sensors for proteins determination.

Xu et al. have presented a novel label-free fluorescent assay for monitoring the activity and inhibition of protein kinases based on the aggregation behavior of unmodified CdTe QDs with very high LOD 5.0 fM. In this assay, cationic substrate peptides induce the selective aggregation of unmodified QDs with an anionic surface charge, whereas phosphorylated peptides do not. Phosphorylation by kinase alters the net charge of peptides and subsequently inhibits the aggregation of unmodified QDs, causing an enhanced QDs fluorescence [19].

Lv et al. [71] have proposed detection of C-reactive protein (CRP) based on fluorescence changes by CdSe/ZnS QDs, where QDs surfaces were modified with monoclonal antibodies. The fluorescence intensity has increased with the increasing of antigens concentration. The assay for the detection of CRP can provide a wide analytical range of 1.56–400 ng/mL with the LOD 0.46 ng/mL and the limit of quantification = 1.53 ng/mL.

Another example of a protein biosensor was developed by Zhang group. Using DNA-ZnS:Mn2+ QDs as the energy donor and WS2 as the energy acceptor. DNA-ZnS:Mn2+ QDs were hybridized with biotin-DNA to obtain dsDNA. When Exonuclease III was added into the system, the biotin-DNA was hydrolyzed for the stepwise removal of mononucleotides from the 30-hydroxyl termini of dsDNA, releasing DNA-ZnS:Mn2+ QDs into solution. After incubation with WS2 nanosheets, DNA-ZnS:Mn2+ QDs were absorbed on the surface of WS2 due to their stronger affinity towards ssDNA than that of dsDNA. As the results, the fluorescence intensity was reduced with the increasing concentration of Exonuclease III. There is a good linear relationship between the fluorescence intensities and the concentration of SA (biotin-streptavidin) in the range of 5–150 ng/mL. The LOD was calculated as 2.8 ng/mL [72].
