# Fluorescence Masking Based Multifunctional Quantum Dots+
###### Abstract
QDs are inherently hydrophobic; they are made water-compatible by surface capping with bifunctional ligands that mediate solubility and attachment sites for the bioconjugation of ligands with a terminal carboxylic acid. Positively charged avidin can interact with negatively charged carboxylic acid, forming a bridge for attaching any biotinylated proteins or ligands [11; 19]. Avidin interacts stoichiometrically with biotin, and one biotin binds with one avidin subunit. This high-affinity interaction (K\({}_{\text{d}}\) = 10\({}^{-15}\) M) is exploited for bioimaging and biosensors using biotinylated nucleic acids and proteins [20]. This technique can be used for any type of nanoparticle regardless of the materials' properties [21]. Common organic dyes such as cyanine (Cy5 or Cy3) that have a resonant emission are called resonant dyes. The emission in them originates from the optical transitions of electrons delocalized over whole chromophore. These dyes have narrow absorption and emission bands that often mirror each other. This poor separation often causes crosstalk between different dye molecules. They also have a small stroke shift and moderate-to-high quantum yield [22]. In contrast to these dyes, QDs have a broad excitation/absorption band ranging from UV to visible light so that a single wavelength can excite different-sized QDs. They have a narrow emission band, which is related to the QDs' nanocrystal radius and core material; it is highly suitable for multiplexing applications. QDs have a high fluorescence quantum yield in the visible range and high molar coefficients. In addition, CdSe-ZnS QDs have an exceptional resistance to photodegradation, and heat compared to organic fluorophores [18]. The binding of organic dyes to biomolecules can disrupt the binding affinity of proteins and ligands. Additionally, the binding of proteins to QDs suffers from site-specific and steric hinderance problems. The conjugation of Sav-QDs with biotinylated antibodies or antigens with nanoparticles will increase the physiochemical stability and provide a target-specific probe character to the QD-antibody/antigen complex [18; 23]. Conventional laboratory assays such as an ELISA, RT-PCR, and nucleic acid sequence-based amplification (NASBA) require highly skilled operators, complex procedures, and fancy equipment [24]. A contactless inkjet printer is a low-cost and high-performance method for the printing of biomaterials, based on the ejection of drops from the nozzle in a picolitre volume controlled by piezoelectric micro pumps. This is used for making highly miniaturized, rapid, and robust assays [24; 25]. The current work aims to develop a miniaturized and multifunctional assay system based on the masking of Sav-QDs' fluorescence. The assay is used for detecting the presence of the stress protein (HSP90\(\alpha\)), a protein-ligand interaction, and the effect of the inhibitor [26]. This is achieved by the interaction of a biotinylated antibody or ligand with HSP90\(\alpha\) (Scheme 1). The fluorescence masking results are quantitatively analyzed by a dose-response curve and Stern-Volmer plot. The method is fast, simple, and convenient, with a potential point-of-care application.