Nanocrystal-molecule energy transfer conjugates for chemical and biological sensing

Abstract

New tools and probes are constantly being developed for chemical and biological sensing. As novel materials emerge, growing demand for sensing in specific applications can be addressed. One such class of materials is fluorescent inorganic semiconductor nanocrystals (NCs), popularly known as quantum dots. The unique, size-dependent properties of NCs are promising for biological microscopy applications in cancer research; however, obstacles such as biocompatibility and sensitivity must be overcome (Chapter I). This Thesis work addresses the challenge of converting the chemically inert NCs into a dynamic equilibrium-based sensor. A strategy of implementing fluorescence resonance energy transfer (FRET) as the signal transduction mechanism of a CdSe/ZnS NC-molecule donor-acceptor pair with a rhodamine-based acceptor dye is investigated. Energy transfer in NC-dye pairs is found to be efficient, with kFRET rates approaching 108 s-1 (Chapter II). A reversible and ratiometric NC pH sensor is synthesized by tethering NCs to a squaraine-based pH dye. The presence of an isosbestic point between the two emission maxima from the NC and the dye allows the sensor to be self-calibrating (Chapter III). The ratiometric nature of the NC-based sensor signifies potential for emission-based sensing in biological environments. Various NC surface modifications and coupling strategies using a physiologically relevant pH dye are compared to determine the characteristics needed to introduce NC based sensors into a biological environment (Chapter IV). NCs functionalized with poly(ethylene glycol) ligands (PEG) were deemed best suited to impart biocompatibility, and first generation PEGylated bio-applicable NC pH sensors were photophysically characterized under single and two-photon excitation and its stability evaluated (Chapter V). The PEGylated NC pH sensors were introduced into an in vivo tumor environment, and using multiphoton laser scanning microscopy (MPLSM), the sensors are able to ratiometrically report a change in pH induced by an external stimulus. Challenges such as calibration in in vivo experiments are currently being addressed (Chapter VI).

(cont.) New conjugation techniques with NCs are further explored with Click Chemistry (Chapter VII). The NC-molecule sensing developed during this Thesis work is general and may be applied towards sensing of other analytes in other applications, using a variety of NC materials (Chapter VIII).