Fluorescent Probe as Reporter on the Local Structure and Dynamics in Hydrolysis−Condensation Process of Organotrialkoxysilanes

Study on the Assembly Structure Variation of Cetyltrimethylammonium Bromide on the Surface of Gold Nanoparticles

In this work, the self-assembly behavior of cetyltrimethylammonium bromide (CTAB) on the surface of citrate-capped gold nanoparticles (AuNPs) in solution has been studied by UV-vis absorption spectroscopy, fluorescence probe techniques, ζ potentiometric methods, transmission electron microscopy, etc. The UV-vis spectra show that the color with the increase of CTAB for the mixture containing CTAB and a given amount of AuNPs changes from red to blue and then to red. The absolute value of ζ potential corresponding to this color change decreases initially and then increases. Specially, the reversible color change, from red to blue and then to red, could be observed only in the case of a gradual addition of a AuNP solution to a CTAB solution; however, this reversible change is not suitable for the mixture formed in a reverse order of mixing. The results from pyrene used as the fluorescence probe indicate that the features in the fluorescence spectrum (including fluorescence quenching, I ₁/I ₃, and the excimer) well correspond to those from the UV-vis spectrum mentioned above. Based on the experimental results, the mechanism of the assembly structure variation of CTAB on the surface of negatively charged AuNPs was proposed. For a given amount of AuNPs, the assembly structure of CTAB on the surface of AuNPs undergoes the transformation from a monolayer to a bilayer with the increase of CTAB. In the case of the concentration of CTAB far beyond its critical micelle concentration (CMC) and the higher ratio of CTAB and AuNPs, there is a possibility of the formation of an extra micellar structure only after the formation of a double-layer structure.

One-Step Assembly of TiO2-Liposomes Based on Interfacial Sol-Gel Process within Lipid Bilayer

There is a growing interest in the use of hybrid liposomes for various biochemical and biomedical applications. In this study, we report the first preparation and characterization of a class of TiO₂-reinforced liposomes by a one-step assembly approach. The amphiphilic natural structure of lipids is exploited to localize a hydrophobic molecule, namely, precursor tetrabutyl titanate (TBOT), in the mid-plane of the liposomal bilayer assemblies in the aqueous phase. In situ TiO₂ nanoshell formation is driven by subsequent interfacial hydrolysis of TBOT and the ensuing condensation within the hydrophobic interstices of the lipid bilayer. The core-shell structure, like cell and wall, is demonstrated by means of scanning electron microscopy and transmission electron microscopy images, and the formation of the TiO₂ shell is confirmed using energy-dispersive X-ray spectroscopy, thermogravimetric analysis, and differential scanning calorimetry. To study the structural evolution of the hybrid liposomes during titania formation, fluorescence probe technique and surface pressure versus molecular area (π- A) isotherms are designed. The results demonstrate that the incorporation of TBOT into the mid-membrane of the lipid and titania in the core of the membrane strengthened the assembly of the lipid bilayer. We further demonstrate that titania shell improved the stability and release property of liposomes. We expect that the reported new TiO₂-coated liposomes by co-assembly will be valuable in designing hybrid liposomes, exhibiting integrative capacity for drug encapsulation, compartment reaction, and photocatalysis.

Ultraviolet Light Catalyzed Gelation of 3-Methacryloxypropyltrimethoxysilane via Altered Silicate Spatial Structure

The gelation of 3-methacryloxypropyltrimethoxysilane (MAPTMS) is much more difficult to achieve in conventional conditions. This article describes a novel and concise approach to acquire transparent and firm hybrid gel material by one step promptly without photoinitiator or other tetraalkoxysilane. MAPTMS was hydrolyzed in acidified aqueous solution, which became homogeneous sol in 3 min, and then the sol was irradiated with UV light for a few minutes to form gel. The experimental results indicated that MAPTMS sol gelled in the presence of UV-irradiation was mainly attributed to altering Si-O-Si skeleton structure through hydroxyl radicals, and the gelation originated from the hydrolytic polycondensation of MAPTMS rather than the polymerization of methacryloxy substituent groups. The hydroxyl radicals could break the Si-O-Si ring structure to form cross-linker like species, and these cross-linkers chemically joined linear chains together to form the gel network. This investigation offers not only the photoinduced gelation strategy for MAPTMS sol but also the new insight into the effect of UV-irradiation on the sol-gel process of organotrialkoxysilanes.

Terminating effects of organosilane in the formation of silica cross-linked micellar core-shell nanoparticles

One advanced synthesis strategy for monodisperse silica cross-linked micellar core-shell nanoparticles (SCMCSNs) involves the use of organosilane termination agent R(n)Si(OR')(4 - n). In this study, we investigated the effects of the organosilane termination agent in the formation of SCMCSNs. Experimental data (synthesis results, (29)Si MAS NMR, molecule probe fluorescence spectra, etc.) from a synthesis system with Pluronic F127 as the template indicate that organosilane either covers or reacts with the surface Si-OH groups of nanoparticles. The reduction of reactive surface Si-OH groups helps to stabilize nanoparticles by avoiding aggregation. The terminating behavior of organosilane is determined by its molecular structure, including (1) the value of n, (2) the length of hydrocarbon chain R, and (3) the charge of R. Effective organosilane termination agents are also applicable to other synthesis mixtures such as the systems using Si(OC(2)H(4)OH)(4) as the silica source or F108 or Brij 700 as the template. Furthermore, we can obtain monodisperse nanoparticles by using the trisodium salt of triacetic acid N-(trimethoxysilylpropyl)ethylenediamine (TANED), which acts not only as a termination agent for the successful synthesis of SCMCSNs but also as a functional group to improve the performance of SCMCSNs in potential applications.

Substituent-dominated structure evolution during sol-gel synthesis: a comparative study of sol-gel processing of 3-glycidoxypropyltrimethoxysilane and methacryloxypropyltrimethoxysilane

The sol-gel processes of 3-glycidoxypropyltrimethoxysilane (GPTMS) and methacryloxypropyltrimethoxysilane (MAPTMS) have been followed by fluorescence spectroscopy with pyranine as a photophysical probe. The experimental results showed that this probe is sensitive to the structural evolution and microenvironment polarity. The specific comparison of the structural evolution in two substituted organotrialkoxysilanes, namely, MAPTMS and GPTMS, illustrates the ability of the substituents to interact with the microenvironment via electrostatic interactions. Interestingly, these interactions determine the kinds of intermediate supramolecular structures that form during the sol-gel process and hence control the structure of the ensuing sol-gel end product. In particular, the amphiphile-like character of the MAPTMS intermediates contrasts with the biamphiphilic character of their GPTMS counterparts, driving distinctly different transient and local molecular organizations, which in turn modulate the hydrolysis and condensation reactions during the sol-gel process.