*3.4. Cadmium Sulfide Nanoparticles Quantum Dots (CdS QDs)*

In contrast to traditional fluorescent organic dyes and green fluorescent proteins, CdS QDs seem to be superior as they overcame the limitations associated with different factors, such as spectral overlapping, weak signal intensity, and photobleaching [81]. The multiple characteristics of QDs are high photostability, symmetric, slow decay rates, fine emission spectra, wide absorption cross-sections, and broad absorption spectra. The emission color of QDs depends upon their size and surface chemistry; chemical composition used can be altered from the UV to visible or near NIR wavelengths. The increasing interest in the use of CdS QDs is because they act as luminescent probes and labels for biological imaging, disease diagnosis, and molecular histopathology. The studies revealed that the QDs derived from plants did not aggregate [82]. Borovaya et al. synthesized CdS NPs with the help of aqueous extract of roots of *Linaria maroccana*, CdSO4, and Na2S. First, the mixture solution was incubated for 4 days at 28 ◦C resulting in the formation of the clear homogeneous solution with a bright yellow color. This indicates the formation of CdS NPs, which are water-soluble and spherical, with sizes of 5–7 nm [82]. In 2015, again biosynthesis of luminescent CdS NPs using mycelium of *P. ostreatus*, CdSO<sup>4</sup> and Na2S. In brief, CdSO<sup>4</sup> solution was mixed with mycelium followed by the incubation for 10 days at 26 ◦C, followed by the addition of Na2S solution. Obtained NPs were spherically shaped,

having the size in the range of 4 to 7 nm. In particular, cadmium sulfide QDs are highly useful in investigating the biomolecules interaction and cellular signaling pathway with the help of fluorescent microscopy [81].
