Preparation, Characterization And In Vitro Degradation Of Poly (Lactic Acid)/QDs Nanocomposites

The birth of tissue engineering has greatly promoted the study of the replacement of the human tissue by biodegradable polymer. It could reduce the use of experimental animals, shorten the research period and reduce the cost of research if the polymer implanted in animals. It can also be called fluorescent markers which can reflect the changing process of the properties of biodegradable polymer structure in living and real time detection. In this paper, the preparation, characterization and in vitro degradation of poly(lactic acid)(PLA)/Cd Se-Zn S QDs nanocomposites were investigated, including the following aspects.1. A solution casting method was adopted to fabricate Cd Se-Zn S quantum dots(QDs)/poly(lactic acid)(PLA) nanocomposites films of various QDs concentrations using chloroform as solvent. The optical properties of the composites were investigated by photoluminescence and UV–vis spectrophotometer. Transmission electron microscopy(TEM) and FT-IR spectrophotometer were used to investigate the morphology and structure of the composites. The TEM photograph confirms that the QDs are uniformly dispersed with around 6 nm diameter in the PLA matrix. With the addition of the QDs, the crystallization peaks of PLA obviously shift to higher temperature, the thermal stability of PLA is sharply declined.2. Fourier transform infrared(FT-IR) spectroscopy, polarized optical microscopy(POM), differential scanning calorimeter(DSC), and X-ray diffractometer(XRD) were used to characterize the crystallization behavior of the nanocomposites films that had been thermally treated differently from the as-cast state. FT-IR and POM results showed that nucleation density and thus crystallization ability of the PLA are greatly enhanced with addition of the QDs in the processes of isothermal crystallization. From the results of DSC, the glass transition temperature of PLA has no obvious change on addition of the QDs. For the QDs/PLA samples isothermally annealed at 130 ℃ or 140 ℃ for 30 min, the intensity of cold-crystallization peak decreases in the DSC melting curve, and the degree of(melt) crystalline increases, with increasing the concentration of the QDs in the PLA. Upon annealing at 130 ℃ or 140 ℃ for 30 min, double crystal-melting peaks are observed from the DSC curve of unfilled PLA, which is apt to become a single melting peak on the QDs incorporation into the PLA. And a crystallization-melting-recrystallization process in the isothermal crystallization of PLA was observed in POM images. Only very small amounts of crystal nucleus formation can be observed in pure PLA at temperature over 130 ℃. After adding quantum dots, even if the temperature improved to 140 ℃, there can still be a large number of small spherulite with uniform distribution. All the results from FT-IR spectroscopy, POM, DSC, and XRD are self-consistent, in that they synergistically suggest that the Cd Se-Zn S QDs function as an efficient heterogeneous nucleating agent for the acceleration and enhancement of the melt crystallization of PLA. The size of crystals and the crystallization degrees can be adjusted by controlling the additive amount of QDs and the crystallization time.3. Cd Se-Zn S QDs as a fluorescent label were mixed with PLA to fabricate PLA/QDs scaffolds, with focus on the degradation behaviors of these PLA/QDs scaffolds in vitro in phosphate-buffered saline(PBS) at 37 ℃. Compressive modulus and thermal stability of the PLA/QDs scaffold are higher than those of the unfilled PLA scaffold. Cytotoxicity test results confirmed the non-cytotoxicity of the PLA/QDs scaffold. During the process of in vitro degradation, the degradation rate of the PLA was accelerated by the presence of the QDs, and the molecular weight distributions of the PLA/QDs scaffolds were much broader as compared with the unfilled PLA ones. During the first 84 weeks of the degradation process, the photoluminescence(PL) intensity of the PLA/QDs scaffolds decreased with almost the same ratio.To sum up, poly(lactic acid)(PLA)/Cd Se-Zn S nanocomposites were successfully prepared by incorporation of surface-modified Cd Se-Zn S quantum dots(QDs) into PLA via a simply solution casting method. It can integrate the advantages of both PLA and QDs to make these nanocomposites. At the same time, it also improved the stability of the quantum dots. This research has laid a good foundation for the application of PLA/quantum dots nanocomposites in biomedical field.