Sajo P. Naik

Novel fluorescent silica nanoparticles: towards ultrabright silica nanoparticles.

Fluorescent nanoparticles (FNPs) are of considerable interest nowadays as labels, tracers, sensors, and photon sources. The largest areas of application are nanobiotechnology and bioinformatics. Both areas require biocompatible, nontoxic FNPs. Silica FNPs satisfy both requirements. Brightness of the particles is desired because it improves the signal-tonoise ratio when measuring florescent signals. An well-known example of such bright, commercially available FNPs are quantum dots (QDs), nanometer-scale semiconductor crystals. QDs are brighter and typically more photostable than organic fluorescent dyes even when used in aqueous media. However, use in many applications is still restricted because of their toxicity. Organic fluorescent dyes encapsulated in silica have presented a promising alternative to quantum dots. Because of a large variety of available fluorescent dyes, it is plausible to expect creation of FNPs with more versatile fluorescent properties than the fluorescence of quantum dots. Recently, synthesis of a new type of fluorescent material has been reported. Dyes were noncovalently (physically) encapsulated in self-sealed nanometer-sized channels inside micrometer-sized silica particles in high concentration. The self-sealed topology of the channels manifested itself in virtually no leakage of the dyes out of the particles. Moreover, physical confinement only of the dye molecules resulted in no detectable decrease of the fluorescence quantum yield. Finally, a specific one-dimensional (1D) entrapment of the dye molecules inside silica nanochannels resulted in attaining much higher concentrations of the dye molecules inside the channels without quenching fluorescence. Up to four orders of magnitude higher concentration of the dyes can be reached inside the tubes without dimerization (i.e., quenching the fluorescence) compared to that in water ( 4mM for rhodamine 6G dye). This made those particles 170–260 times brighter than the particles of the same size made of QDs encapsulated in polymeric matrix. It would definitely be a serious breakthrough in labeling technology if one was able to synthesize nanometer-sized silica particles with similar properties.

Mesoporous silica with short-range MFI structure

Abstract A new type of mesoporous silica has been prepared which showed 780 m 2 /g of BET surface area and 0.6 ml/g of primary mesopores narrowly distributed around 4.2 nm. More importantly however, is that it showed short-range zeolite crystallinity as demonstrated by FTIR and XRD analysis, and hydrophobicity as demonstrated by water and n -hexane adsorption. This material was synthesized via a dual-template, three-step hydrothermal–flocculation–steaming synthesis procedure developed by us recently. Briefly, MFI nanoprecursors (NPs) were first prepared by a low-temperature hydrothermal step using TPAOH as template for zeolite structure, and then flocculated using a surfactant that served as the template for the mesopores. The collected NPs are mesoporous silica exhibiting short-range MFI domains when directly calcined. However, the steaming step promoted the crystallization of the NPs and created uniform mesopores. It was found that almost every detail in these procedures affected the properties of the final product. The most important variables, however, were identified as the duration the flocculants were kept in contact with the liquid phase, and the humidity under which the steaming was conducted. By properly adjusting the procedures, the said mesoporous silica, as well as nanocrystals having high external surface area, could be produced at will.

Synthesis of silicalite nanocrystals via the steaming of surfactant protected precursors

A new scheme has been developed for the confined synthesis of silicalite nanocrystals. The scheme involves: (1) Prepare a clear solution that is known to produce colloidal TPA-silicalite upon extended hydrothermal reaction. (2) Subject the solution to hydrothermal condition but stop before the appearance of colloidal silicalite. (3) Protect the TPA-silicalite precursor nanoparticles with cationic surfactant and collect them as flocculated mass. (4) Convert the precursor/surfactant hybrid into nanocrystals via high temperature steaming. It was found that an intermediate dilution during the hydrothermal step helped to delay the appearance of colloidal silicalite, and to produce more precursor nanoparticles. A steaming temperature of 150 °C was also found enough to convert the collected precursor into nanocrystals. The obtained nanocrystals were smaller than 30 nm with good XRD and IR crystallinity, which survived after 550 °C calcination.

Dynamics of molecular diffusion of rhodamine 6G in silica nanochannels

We describe a method to study diffusion of rhodamine 6G dye in single silica nanochannels using arrays of silica nanochannels. Dynamics of the molecules inside single nanochannel is found from the change of the dye concentration in solution with time. A 10(8) decrease in the dye diffusion coefficient relative to water was observed. In comparison to single fluorescent molecule studies, the presented method does not require fluorescence of the diffusing molecules.

Preparation of zeolitic mesoporous aluminosilicate by vapor phase transport method

A vapor phase transport method is applied to the synthesis of composite zeolitic mesoporous aluminosilicate. Analytical data suggest that aluminosilicate in mesopore wall is partially transformed to a zeolitic unit, and the composite shows the zeolitic nature.

Clay Catalyzed Disruption of Quorum Sensing in Vibrio harveyi Bacteria

This work describes the use of clay minerals as catalysts for the degradation of quorum sensing molecule N-(3-oxooctanoyl)-dl-homoserine lactone. Certain clay minerals as a result of their surface properties and porosity can catalytically degrade the quorum sensing molecule into smaller fragments. The disruption of quorum sensing by clay in a growing Gram-negative Vibrio harveyi bacteria culture was also studied by monitoring luminescence and population density of the bacteria, wherein quenching of bacterial quorum sensing activity was observed by means of luminescence reduction. The results of this study show that food-grade clays can be used as biocatalysts in disrupting bacterial activity in various media.