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Abstract
Self assembled conjugated organic nano-structures have been developed as a model to study fluorescence resonance energy transfer (FRET). We have used polymerized polydiacetylenes (PDA) liposomes and multilayered nanostructures. In the first study, we investigated reversing the role of donor and acceptor in conjugated liposome system using PDA and lissamine rhodamine fluorescence dye (LR). The stress-induced electronic absorption transition from blue-phase to red-phase of the PDA was used as a unique system to study this phenomenon. The colorimetric transition from blue-phase PDA to red-phase PDA was achieved through heating the conjugated liposomes. In this work, the FRET mechanism was evaluated without violating the rule of energy flow that energy flows "down hill" and that the roles of donor and acceptor in FRET are fixed. We found that LR is an excellent energy donor and blue-phase PDA is an excellent acceptor. However, after thermo-chromatic transition from blue phase-PDA to red phase-PDA, PDA acted as a donor and LR acted as an acceptor. We have also evaluated FRET efficiency in both cases, and we found that FRET efficiency is higher in the first case (LR donor and PDA acceptor) than in second case. This was rationalized due to energy transfer kinetics and quantum yield differences between LR and red-phase PDA. This research reports the unique phenomena that can be used in sensing of biomolecules and bioparticles.
Self assembled nanotubes from dicarboxylic PDA (DCPDA) were synthesized using layer-by-layer (LBL) techniques inside a template. Alumina templates with 100 nm pore diameter were used for the synthesis of DCPDA nanotubes. The Zr-carboxylate was used for interlayer linking chemistry between the DCPDA layers because Zr-carboxylate is robust and bond formation kinetics is fast. We characterized the multilayer films on flat surfaces (such as gold, quartz and glass) using FTIR, UV-Vis, fluorescence and ellipsometric techniques. The DCPDA nanotubes were characterized using scanning electron microscopy and ionic current measurements. Ionic current measurements provided a simple and convenient method to probe subtle changes (of few nanometers) in nanotube diameter which otherwise is difficult to analyze using conventional characterization techniques. Ellipsometry, electronic absorption and emission studies showed a uniform deposition of DCPDA with deposition cycles. A model is also proposed to explain dis-ordering of -(CH2)8- alkyl position in DCPDA multilayer. It also explains the strong emission from DCPDA films without application of external stress. The self assembled chemistry using the materials may find potential applications in areas such as sensors, actuators, and computational devices at micro and nano scale, as well as an encapsulation drug delivery vehicle. Both these studies offer new insight to the unique properties of polymerized PDA liposomes and films.
