Studies On The Fluorescence Characteristics Of Porphyrins And Semiconductor Quantum Dots As Photosensitizers

In the resent years, photodynamic therapy (PDT) attracts more attention in the biomedical research field as a new way dealing with tumor. It is a non-invasive physical diagnostic and therapic method using photosensitizers associated with laser irradiation to produce tumor toxicity material which can damage the tumor tissue. Development of photosensitizers is a critical factor affecting the clinical application of PDT. So far, a lot of work has been done on the investigation of new type of photosensitizers and great progress has been made. The first photosensitizer used in PDT is the porphyrin type compound. Porphyrins are typical photosensitizers with high singlet oxygen quantum yields, convenient molecular modification and preferential accumulation in tumor, so that they play the major role in study of the PDT drugs. Whereas semiconductor quantum dot (QD) is a new type of photosensitizer with the unique quantum size effect. QDs possess contiueous absorption and emission wavelength, high photo- and chemical stability, high fluorescence quantum yield and larger two-photon absorption cross section, making them potential photosensitiers in PDT application.With the rapid development of PDT, studies on photosensitizers become faster and deeper. However, photosensitizer is an interdiscipline research topic, and different research fields focus on different problem, resulting in an imbalance in the study of the photosensitizers. Till now, a lot of works have been reported on the biochemical reaction mechanism of the photosensitizers, but studies on the photoluminescence of the sensitizer itself and the correspongding underlying physical mechanism is pretty rare. As typical photosensitizers, the photoluminescent process of porphyrins and QDs is the precondition of the photosensitization and tumor diagnosis and localization. The photoluminescent properties can provide mass of molecular information, whicn can accelerate the decelopment of new sensitizers and improvement of exist sensitizers.So in the present thesis, femtosecon pulsed laser, which is believed to be a potential light source for PDT, is used as the excitation source to build a steady-state and nanosecond time-resolved fluorescence spectral detected system. Several typical hydrophilic and hydrophobic porphyrins and QDs are studied and their spectral characteristics are analyzed. Two-photon excitation fluorescence spectroscopy is performed. Photodamge behavior of the photosensitizers is observed. Porphyrins are successfully activated by the QDs by way of the excited-state energy transfer and the possible energy transfer mechanism and transfer channel is discussed.Firstly, the spectral characteristics meso-tetraphenylporphyrin (TPP) is measured and analyzed. Its photodamage behavior under one- and two-photon excitation is detailed discussed. Results suggest that laser irradiation on TPP mainly causes two simultaneously occurring photoprocesses: photodamage and formation of a porphine-type photoproduct. This product, which is observed to possess superior photostability and two-photon absorbing ability compared with the original TPP sensitizer, is likely to be treated as a secondary photosensitizer in the activation process of photodynamic therapy (PDT). This work might be helpful for the drug evaluation in the practical application of PDT. The damage rate exhibits different power dependence in one-and two-photon excitation, suggesting higher-order photodamage mechanism operated in the two-photon excitation process.Secondly, comparision study has been done between meso-tetra(4-sulfonatophenyl)porphine dihydrochloride (TSPP) monomer and dimmer. TSPP monomer and dimer are achieved by adjusting the pH value of the solution. Results show that TSPP dimer displays a J-aggregation pattern. The aggregation effect influenced the spectral properties of TSPP greatly, and this aggregation effect is stable. The TSPP monomer possess better photostability during the laser irradiation, and the fluorescence intensity loss is less than 15% within a typical PDT time.Thirdly, CdSe and CdTe QDs are studied by steady-state and time-resolved spectroscopy. Results suggest a combination of two components in the luminescence behavior of CdTe QDs. The fast component is the excitonic state emission caused by narrow band-edge effect and the slow component is the trapping state emission caused by surface-related effect, in which the trapping state emits at a relative lower energy level than the excitonic state emission. The mixture of different size of CdTe QDs is studied. Results show that QDs of small size have a fluorescence enhancemet effect on the QDs of large size, and this enhancemet effect is irrelative to the particle distance. Possible enhancemet mechanism is discussed. The stability of the mixture is also observed and Ostwald ripening is found in the sample system.Finally, use QDs as energy donors and porphyrins as energy acceptors to build a hydrophilic CdTe-TSPP and hydrophobic CdSe-TPP model system, and study their steady-state and time-resolved spectra under 800nm two-photon excitation. Energy transfer process is observed in both of the two model systems. The fluorescence intensity of the QDs decreases and that of porphyrins increases, successfully indirectly activated the porphyrin sensitizers and compensates their small two-photon absorption cross section, which could be helpful for taking use of these model systems in the two-photon excitation PDT. In addition, electron exchange is deduced to be the dominant transfer mechanism by analyzing the time-resolved data, and non-radiative energy transfer is supposed to occur through the trapping state of QDs, which presents a way of raising energy transfer efficiency in this type of donor-acceptor pairs.