# Fluorescence Masking Based Multifunctional Quantum Dots+
###### Abstract  
HSP90x is one of the most common stress proteins in cells; hence, it is a good target for developing drugs and testing systems for cancer or physical stress levels in humans. Streptavidin conjugated quantum dots (Sav-QDs) are widely used as fluorophores for biosensing to overcome chemical labelling problems. In this work, we have attempted to develop a multifunctional and robust assay for HSP90\(\alpha\). The detection technique was based on the masking of the fluorescence of spotted Sav-QDs on nitrocellulose chips (NC). Biotinylated ligand/antibody attaches to the spotted Sav-QD and then HSP90\(\alpha\) is attached, which causes the masking of fluorescence. The masking of fluorescence was used to detect protein-ligand interactions, the effect of inhibitors, protein-protein interactions, and the presence of protein in the biological sample. The load of detection (LoD) of the assay lies in the nano molar range, making it a sensitive assay. The results from the experiments suggest that the used approach is promising for developing a multifunctional, robust, and sensitive assay for proteins that can be used for point-of-care detection in complex biological samples. Keywords:quantum dots; Sav-QD; biotin-ATP; biotin-HSP90 antibody; HSP90\(\alpha\); fluorescance masking +
Footnote †: _Applied_  
play a role in the pathogenesis, prognosis, and treatment of various diseases. Due to their cytoprotective function, intracellular HSPs are induced as a result of stress. It is suggested that extracellular HSPs are released from damaged cells and their presence has a regulatory function [7]. Various assays have been developed using isothermal titration calorimetry, ligand immobilization, FRET, and fluorescent polarization to monitor the HSP90 concentration and as well as its interaction with inhibitors. These methods require a high protein concentration or a chemical modification of the ligand. Hence, it is very important to develop a convenient, rapid, and robust assay that can detect HSP90 interaction with small molecules, antibodies, and their presence in biological samples [8; 9]. Transferring emerging nanotechnologies to the field of biology and medicine has led to the widespread usage of quantum dots (QDs). They are a novel class of multifunctional inorganic fluorophores [10; 11; 12]. There are many kinds of QDs available in the market for bioimaging, such as carbon (CQD), graphene (GQD), and semiconductor QDs. GQDs are derived from graphene or graphene oxides, and they are subsets of CQDs. They have a lower toxicity and biocompatibility compared to semiconducting QDs. Their major drawbacks are a lower lifetime and low quantum yield, leading to decreased brightness [13; 14]. Semiconducting QDs have a nanocrystal core of groups II-IV or III-V elements, such as CdSe, CdTe, etc., and ZnS shell [15; 16]. These are spherical particles with size-dependable optical properties such as absorbance, photoluminescence, etc., resulting from the quantum confinement effects of electron and hole carriers at a dimension smaller than the Bohr exciton radius (separation distance between the electron and hole in an exciton) [17; 18]. Their bandgap energy decreases with an increase in the particle size. Upon excitation with a light source, electrons absorb the energy and jump from the valence band, creating a positively charged hole in the conduction band. This creates electron-hole pair (exciton). The electron recombines immediately with the hole, releasing energy equivalent to the bandgap. It is more likely that trap states within the QDs' material will trap the electron or hole, causing a non-radiative emission. Trap states exist due to defects in the lattice structure of the material. To decrease the trap states, the surface passivation of the core material is achieved by coating it with a wider bandgap semiconductor shell (e.g., the CdSe core is coated with a ZnS shell). This makes QDs more favorable as fluorescent labels [18].