THE DEVELOPMENT OF DYE-DOPED SEMICONDUCTING POLYMER DOTS FOR FLUORESCENCE IMAGING AND SENSING APPLICATIONS

Abstract

Fluorescent semiconducting polymer dots are a new type of fluorescent nanoparticle that has shown great potential for fluorescence imaging due to their exceptional chemical and photophysical properties. Pdots are composed of semiconducting polymers that offer several advantages over inorganic QDs and organic dyes, including facile synthesis, high photostability, tunable absorption and emission, and exceptionally high absorptivity and brightness. On a per-particle basis, fluorescent Pdots are several times brighter than similar emitting QDs and organic dyes. This brightness is the result of fast emissive rates, large extinction coefficients, and high emission quantum yields. Additionally, Pdots are resistant to photobleaching, making them highly applicable to fluorescence imaging. The one major limitation of Pdots compared to organic dyes and quantum dots is their broad emission that can span several hundred nanometers. Theresearch presented in this dissertation aims to solve this problem and expand the fluorescence imaging applications of this promising nanoparticle. First, we employ a series of porphyrin dyes to tune the optical properties of Pdots. When doped into the polymer matrix, the dyes act as energy acceptors, quenching polymer emission and causing the Pdot to take on the emission properties of the dye. This significantly narrows and red-shifts Pdot fluorescence, while maintain the strong absorption properties and aqueous solubility of the Pdots. We utilized these dye-doped Pdots for NIR cellular imaging and demonstrated their potential for super-resolution fluorescence imaging. The introduction of dyes to the Pdots did produce a new problem: photobleaching. Intense energy transfer from the polymer to the dye results in rapid photobleaching of the dye and the return of polymer fluorescence. Designing dyes to be more resistant to photobleaching has not yet yielded desired results and requires further study. Second, we investigated new series of dyes to impart additional properties onto our Pdots. Using phthalocyanine dyes produced extremely bright NIR emission, though over a small range of wavelengths. BBTD allowed us to shift the wavelength of emission much deeper into the NIR, with one dye-doped Pdot reaching the NIRII window. This capability is highly desired because the NIRII window has even lower autofluorescence and scatter than NIRI, producing higher signal-to-noise ratios. Finally, we used a ruthenium dye to impart oxygen sensing properties to our Pdots, demonstrating that doped dyes maintain their sensing properties when doped into Pdots. We also found that Pdots possess excellent two-photon absorption capability, which make them interesting candidates for two-photon fluorescence microscopy. Finally. We investigated the relationship between Pdots and lipids. Amphiphilic polymers were substituted with lipids to produce Pdots with altered surface functionalization and optical properties. The interactions between Pdots and liposomes were also carried out to study their potential harmful interactions with cell membranes. We found that Pdot-lipid interactions were controlled by ligand. The distributed negative charge on PSMA prevented all interactions between the liposome and the semiconducting polymer, keeping the Pdots form disrupting the membrane. This result bodes well for PdotsÕ potential use as in vivo imaging probes, though more extensive study is needed. Our development of the dye-doped Pdot approach has led to the creation of a highly modular fluorophore system. The absorption characteristics of the nanoparticle can be altered by simply changing out the semiconducting polymer, while fluorescence properties can be tailored as desired using different organic dyes. Surface functionalization can be easily tuned by switching out ligands or with post-synthesis modification. Our Pdots have also been shown to be highly biocompatible, which makes them excellent candidates for in vivo fluorescence imaging. Once we solve photostability issues, these Pdots will be highly applicable as fluorescence sensors or detectors.