Synthesis, Characterization Of Rhodamine-based PMOs And Their Applications In Metal-ions Detection

Periodic mesoporous organosilicas (PMOs) are a new type of organic-inorganic hybrid materials in which the organic moieties bound to two or more SiO1.5 units are uniformly located in the framework as building blocks. PMOs with different bridging organic groups could be potentially applied in the fields of adsorption, catalysis, drug delivery, metal ion detection and optics. Recently, much attention has been paid to luminescent PMOs due to their utilization potential in biological labeling, fluorescence sensing, and lighting devices. PMOs that display optical properties in the visible-light region can be synthesized using bridged organosilanes with a large pconjugation system. Owing to the protection of the pore walls, bridging chromophore organosilanes can be densely packed in the silica matrix with high stability to avoid photobleaching. Besides, organosilica precursors exist in the form of alkoxy substituents, which will efficiently prevent fluorescence quenching due to aggregation of dyes. Consequently, PMOs will show more intense photoabsorption and a stronger fluorescence intensity than the mesoporous materials synthesized by grafting or adsorption methods.By using the long-chain midazole ionic quid (1-hexadecyl-3-methylimidaolium bromide, C16mimBr) as the template, the rhodamine derivative bridged periodic mesoporous organosilicas (PMOs) with crystal-like pore walls were synthesized through hydrothermal method. The synthesized rhodamine-based PMOs displayed strong fluorescent response after coordination with some specific metal ions, which would have a great application potential in metal-ion detecting. We mainly use X-ray powder diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), scanning electron microscopy (SEM), N2 adsorption/desorption,29Si magic angle spinning NMR (29Si MAS NMR), X-ray absorption fine structure (XAFS) analysis to characterize the material structure, and use UV/Vis diffuse reflection spectra, UV/Vis absorption spectra, steady-state fluorescence spectra, time-resolved fluorescent spectra and fluorescent confocal microscopy to investigate the optical property. We mainly focused on the following content:(1) By using bis-alkoxysilane (RS-Si2) and tetraethoxysilane (TEOS) as the organosilica precursors, mono-rhodamine Schiff-based PMOs (RSPMOs) with different rhodamine contents were synthesized through hydro-thermal method. When the molar fraction of rhodamine-based organosilica in the total precursors was above 1%, PMOs with crystal-like pore walls, i.e. molecular periodicity could be obtained due to the strong face-to-face π-π interaction among rhodamine groups embedded in the framework. The results of small-angle XRD and HR-TEM also demonstrated that RSPMOs possessed both mesoscopic and molecular scale periodicity, which could be confirmed by the lattice fringes observed in the pore walls. Based on the "OFF-ON" spiro-structure in rhodamine moieties, RSPMOs could be used as the fluorescent chemosensors for copper ions. No or little fluorescence could be observed when there were no copper ions, however, the system would display strong fluorescence after chelation with Cu2+ due to the opening of the spiro-structure in rhodamine moieties based on the Forster Resonance Energy Transfer (FRET) mechanism. The detection limit for RSPMOs to Cu2+ was at or even below 6.5 ppb (1.0×10-7 M) in C2H5OH-HEPES (9:1 v/v, pH 6.8). In addition, after being simply treated with ethylene diamine tetraacetic acid (EDTA), RSPMOs would show the potential of recycle and regeneration.(2) We used the benzyl-bridged bis(rhodamine) tetra-siloxane (BRh-Si4) and TEOS as the organosilica precursors and the long-chain imidazole ionic liquid C16mimBr as the surfactant, the BRh bridged PMOs (BRhPMOs) which could be used as the fluorescent chemosensors with ultra-high selectivity towards copper ions were synthesized. The detection limit of BRhPMOs towards Cu2+ was 1.0×10-5M (0.64 ppm). This value was worse than that of RSPMOs, which might be because of the formation of non-fluorescent H-type rhodamine aggregates in the framework during the self-assembly and co-condensation process., Through the Cu k-edge XAFS study of Cu-chelated BRhPMOs, we may draw the conculusin that the high selectivity of BRhPMOs towards Cu2+ could be attributed to the strong chelation of "N" atoms of the Schiff base groups in BRh units to copper ions, which would finally lead to the effective charge transfer between the BRh fluorophores and targeted metal ions.(3) By using the pentyl-linked bis(rhodamine)-derived tetra-siloxane (PRh-Si4) as the organosilica precursor, a new type of bis(rhodamine)-derived PMOs (PRhPMOs) which could be used as the ratiometric fluorescent nanosensors for copper(II) were successfully fabricated. Compared with the conventional fluorescent chemosensors, PRhPMO suspensions showed dual-fluorescent emission at X=550 nm and 623 nm. The fluorescent emission centered at 550 nm was orginated from the ring-opening of the spiro structure in rhodamine moieties, and the peak located at 623 nm was due to the fluorescent aggregates of PRh units formed in the framework. Based on the different intensity ratios of the two fluorescence signals (FI550/623), PRhPMOs could be used as the turn-ON type ratiometric fluorescent chemosensors for Cu2+. Afterwards, through the single-exciton theory and further experiments, the aggregation states of rhodamine fluorophores in these hybrid materials were discussed in detail. It could be deduced that the fluorescent aggregates formed in the silica framework were of the J-type aggregates and with a coplanar configuration. For this reason, PRhPMOs would display excellent fluorescent emission properties and very high optical stability.