**5. Surface chemistry of QDs: role of capping agent in aqueous synthesis**

The surface chemistry of the CNCs is very interesting, as well as challenging one to achieve. Due to the presence of high surface to volume ratio, the surface of the QDs generates dangling bonds of both cationic and anionic substituents. Noteworthy, the dangling bonds in a QD are generated due to the unsaturated surface level atoms of the QD, which may alter the electronic transition by introducing new electronic states. The primary strategy to satisfy these surface defects is by using more cationic precursor to satisfy the anionic part, while the stabilizing agent, has to satisfy the dangling bonds formed from the cationic part. In most of the reactions, the presence of a stabilizing agent is very necessary to overcome the instability factor in the colloidal solution. In colloidal synthesis method, it is almost

### *Aqueous-Mediated Synthesis of Group IIB-VIA Semiconductor Quantum Dots: Challenges… DOI: http://dx.doi.org/10.5772/intechopen.82891*

mandatory to use a capping agent [2, 37, 84] as a capping agent does not allow the QDs to grow, after reaching a certain size depending upon the capacity of the agent. On the other hand, in an appropriate solvent, a stabilizing agent, not only caps the QD, but also offers stability to it to retain its size in the solution. The terminology of stabilizing agents is slightly different from surface capping agents. But efforts have been made by researchers to put both the surface modifying agents within a single umbrella by trying to use the same molecule for both the purposes. These molecules attach with the surface of the NCs by dative, covalent or ionic bond and provide them electrostatic stabilization as well as surface passivation. These agents are often called as surface ligands, due to their capacity to act as ligands in a coordination complex, which attach to the metal ion by exchanging electrons. The molecular structure of a stabilizing agent/surface capping agent is very much different in aqueous medium than non-aqueous medium. In non-aqueous solvents, these molecules are normally water insoluble long chain hydrocarbons, whereas in aqueous media the capping agents have short hydrocarbon chain and a polar group at one end. The polar group can provide stabilization to the QD as well as passivation to the surface. But, importantly the ratio of capping agent with the precursor also plays an important role in the nature of the solubility and formation of the QDs. It is because, higher concentration of these molecules may not allow the metal atom to form the semiconducting NCs and lower concentration may lead to the instability of the QD in the medium due to weak interaction between the capping molecule and the metal ion. In this context, till date thiol or carboxyl or phosphate group holding polymers and macromolecules have been tried to achieve better surface stability. However, at the same time, ligands capable of binding with more than one binding site are also drawing tremendous attention of the researchers due to the probability of fetching aspired product. Selecting effective surface modifying agents and optimization of the added amount of them are at utmost importance to develop better synthesis route to achieve desired QDs. Another aspect of choosing an effective surface capping agent is the biocompatibility and thus, their application in biomedical sectors [85–87]. The advantageous positions of these inorganic artificial atoms as fluorophores over organic dyes is due to their bright (10–20 times more than organic dyes) and narrow fluorescent and less photo bleaching (up to 30–40 ns) nature than the traditional dyes. The primary issues regarding the use of these QDs in biological application are non-biocompatibility, ineffective coating and the use of hazardous capping agent. It is well understood that although the synthesis of QDs are performed in water medium, it may not offer the biocompatible nature to the dots and hence they can undergo oxidative degradation inside the cell and harm the same. Moreover, non-functionalization may lead to unspecified exit route of these drugs from the treated organism, hence the treated organism may become intoxicated. Therefore, surface functionalization of these dots is the most important part if we want to cast them inside the body of an organism. Secondly, capping agent has to be a perfect surface passivator, which will act as inhibitor to the surface trap states in the QDs. Thirdly, it is also observed that many a times the capping agent/stabilizer/surface passivating agent itself act as a harmful material toward to treated organism. Hence, selection has to be made on this scenario too. Overall, the research of biologically active and applicative QDs demands the production of such engineered QDs, which can answer all the three major problems. At the same time, the role of capping agent/stabilizer/surface passivator has to be restricted to only one molecule, which can act as capping agent, stabilizer and surface passivator in parallel. Therefore, the quest of potential multiuse QDs with multiuse surface modifying agents is still going on around the globe and warrants a thorough investigation.
