Surface Ligand Controlled Photophysics and Photopatterning with InP/ZnS Quantum Dots

Abstract

In nature, interactions play an important role in dictating functions to systems - for example, H-bonding in DNA hybridization for carrying genetic information, and substrate-enzyme interactions resulting in efficient biochemical transformations. So, how to control these interactions for an inorganic nanoparticle (NP)? Most of the interactions and forces in nature originate from atoms and molecules. Thus, one of the most efficient strategies to control the interactions/forces in NPs is to play with their surface chemistry. Out of different inorganic NPs known, several protocols are available in the literature for tuning the surface chemistry of metal NPs. Whereas, for semiconductor NPs or Quantum Dots (QDs) the availability of similar strategies is limited. For instance, there are only a few reports available to generate permanent positive charge on the surface of QDs, and that too on highly toxic metal ion based QDs such as CdSe and CdTe. In this direction, my thesis starts with developing a place exchange protocol to impart a permanent positive charge on the surface of eco-friendly and less toxic QDs like indium phosphide over-coated with zinc sulfide (InP/ZnS).

Next, the two important properties of QDs, namely bioimaging and light induced resonance energy transfer, were successfully demonstrated in cationic InP/ZnS QD. A proof of concept for future nano-bio studies with cationic InP/ZnS QD was presented via an electrostatically driven Förster resonance energy transfer (FRET) with oppositely charged Merocyanine-540 dye (MC), both in solution and solid states. This was followed by developing a robust technique for the creation of reusable multicolor luminescent photopatterns from a single QD-nanohybrid system, as opposed to the common practice of using different colored QDs. FRET process in InP/ZnS QD – MC dye film was regulated through the selective and controlled photodegradation of organic dye molecules. Consequently the FRET efficiency was regulated between completely ON and OFF states through a moderately efficient state to generate three distinctly different colors.

Further, the potency of cationic InP/ZnS QD as a donor in light-induced electron transfer process with oppositely charged Indocyanine green (ICG) dye was demonstrated. A strong electrostatic attraction resulted in a highly efficient electron transfer (~95 %) from cationic InP/ZnS QD to anionic ICG dye, leading to a completely non-fluorescent QD nanohybrid system. Finally, the processes of FRET and electron transfer were coupled in InP/ZnS QD nanohybrid system comprising of both MC and ICG dyes for the generation of non-luminescent (black) regions, which helped in enhancing the color contrast of multicolor photopatterns.

In summary, our efforts helped in introducing environmentally friendly InP/ZnS QDs to the family of cationic nanoparticles as a practical alternative to toxic metal ion based QDs, for future interaction driven light-harvesting and biological applications.