Tuning the photophysical properties of conjugated polyelectrolytes using silver nanoparticles and amphiphilic macromolecules -

Abstract

The optoelectronic, semiconducting, and light amplifying properties of conjugated polyelectrolyte (CPEs) make them promising class of materials for applications in light emitting diodes, solar cells, biosensing and bioimaging. A major drawback, however, is their large extent of reversible and irreversible photobleaching leading to their short survival time and low quantum yield. The photophysical properties of CPEs are highly dependent on their microenvironment and backbone conformation. When extended, the polymer quantum yield increases substantially and its photostability improves. In this thesis work, we report on the enhancement of the photophysical properties of an anionic conjugated polyelectrolyte poly[5-methoxy-2-(3-sulfopropoxy)-1,4phenylenevinylene] (MPS-PPV) when positioned at precise distance from the core of metallic nanoparticles and upon its complexation with an amphiphilic polymer. Using silver nanoparticles coated with a nano-layer of silica modified with 3-(Aminopropyl)trimethoxysilane, we were able to achieve 3.5 folds fluorescence enhancement on the emission intensity of MPS-PPV. Parameters such as silica shell thickness, particle concentration and surface charge are discussed in details on how to get the highest fluorescence enhancement. Upon complexation with an amphiphilic polymer (polyvinylpyrrolidone), the anionic conjugated polyelectrolyte MPS-PPV was prepared as small nanoparticles with exceptional photostability and brightness. The polymer fluorescence was enhanced by 23 folds. Single molecule experiments revealed a complete suppression of blinking. In addition, a remarkable amount of photons were emitted per nanohybrid particle (~10⁹, on average) after only losing 18percent of the original intensity. This number is many folds greater than most of the popular organic fluorescent dyes. We believe that an intimate contact between the two polymers is shielding the conjugated polymer from the destructive photo-oxidation effects. In the last section, we discuss the use of antioxidants to