Silica-based Fluorescent Nanoparticles:Preparations And Applications

As the rapid development of nanomaterials, high-performance fluorescent nanoparticles, such as dye-doped nanoparticles, semiconductor quantum dots and defect-related emissive materials, recently have generated much excitement. Here, the research is focused on preparations and characterization of dye-doped silica nanoparticles, CdTe quantum dots and carbon dots, as well as functionalization of these materials by biocompatible silica to improve their fluorescent properties for further bioapplications.1. We have prepared silica nanocomposites containing fluorescent solid silica core and mesoporous silica shell for drug delivery. Firstly, monodispersed fluorescent silica nanoparticles (F-nSiO2) are synthesized by copolycondensation between 3-(aminopropyl) triethoxysilane conjugation of fluorescein isothiocyanate (FITC) and tetraethyloxysilane. F-nSiO2 shows uniform sphere size of about 68 nm, and the function of silica effectively facilitates the stability of fluorescence. Then, nonporous silica transitional approach is employed to create core-shell architectural nanocomposites, which perform particularly well in morphology and controllable synthesis. The silica nanocomposites containing fluorescent solid silica core and mesoporous silica shell (F-nSiO2/mSiO2) present distinct structures of narrow size distribution, stable and shell thickness independent fluorescence, and high specific surface area. Furthermore, the thickness of mesoporous shell could be precisely tailored by the amount of TEOS and solid SiO2 seeds. These characteristics mentioned above imply that core/shell structured F-nSiO2/mSiO2 had a great potential for controlled drug delivery system, and F-nSiO2/mSiO2 with different mesoporous thicknesses is further used for drug delivery studies. F-nSiO2/mSiO2-DOX showed similar cytotoxicity to pure DOX while no obvious cytotoxicity for blank carrier was observed at concentration as high as 100 μg/mL.2. In this section, we present a ratiometric pH nanoprobe based on ultrastable and highly fluorescent colloidal silica composite nanoparticles encapsulating hydrophilic CdTe quantum dots (QDs). Firstly, CdTe QDs are prepared by hydrothermal method. The emissive peak of the QDs is able to be expediently tuned from 543 nm to 711 nm by increasing incubation time. To avoid inherent chemical instability and serious photoluminescence quenching of quantum dots, we then report a facile electrostatic assembly method to prepare sandwich-like silica/CdTe QDs/silica composite nanoparticles stabilized by mercaptopropyl trimethoxysilane (SQMS). This approach is of high efficiency due to rapid potential reversal by decreasing pH, e.g.,98.9% of quantum dots are instantly absorbed on the surface of silica nanospheres. And nearly 80% of original fluorescence of quantum dots is retained for SQMS while traditional silica coating processes cause dramatical quenching. Finally, the bright SQMS with remarkable stability is modified with pH-sensitive fluorescein isothiocyanate to fabricate a high-resolution pH ratiometric nanoprobe. Above all, SQMS shows uniform sandwich-like structure, narrow size distribution, and ultrastable fluorescence in strong acidic and highly salted solutions. The remarkable stability is in favor of quantitative analyses and nanoprobe.3. By employing glucose as a carbon source and ethanediamine (EDA) as a stabilizer, nontoxic and biocompatible carbon dots (CDs, also called as graphene quantum dots) are synthesized by hydrothermal method. The synthetic process has been optimized by detailed researches on reaction conditions, such as amount of EDA, reaction time, temperature and composition of solvent, and the most robust CDs show a average size of 4.6 nm and bright blue fluorescence under irradiation of a UV lamp. The quantum yield is as high as 22%, and CD fluorescence shows a unique excitation dependent behaviour that the emissive peaks shift from 436 nm to 543 nm as the excitation wavelengths are adjusted from 320 nm to 500 nm. Particularly, we carefully gain insight into upconverted fluorescence of CDs, and find that it is, in fact, second diffraction of longer-wavelength excitation that induces upconversion.4. Based on above preparation of CDs from glucose and EDA, we report an in-situ preparation of CDs confined in silica nanoparticles for fabrication of silica-carbon dot (SCD) composite nanoparticles. The creative route is able to reversibly modulate the excitation-dependent behaviour of CD fluorescence. Glucose, the carbon source of CDs, is pre-packaged in silica nanoparticles by copolycondensation with tetraethoxysilane. Then CDs are grown in situ under the restriction of silica framework to form composite nanoparticles of SCDs. It is found that the restriction of silica shell from sol-gel process is able to dramatically modulate the fluorescent properties of embedded CDs. Compared with the common blue emission of CDs illuminated under an ultraviolet lamp, SCDs suspended in water show bright tunable fluorescence from green to yellow. Furthermore, the excitation dependence of fluorescence for CDs is unprecedentedly dispelled by confining growth of CDs in silica nanoparticles and it recovers after silica is removed. These findings pave the way for mechanism studies on the excitation dependent fluorescence and applications of CDs in bioimaging.