Evaluating Formation and Growth Mechanisms of Silica Particles Using Fluorescence Anisotropy Decay Analysis

Abstract

At present, there is no direct experimental evidence that primary silica particles, which exist only transiently for a few seconds during the Sto ¨ber silica synthesis, can be stable in aqueous solutions. In the present work, we show that primary silica particles are formed spontaneously after the dissolution of diglycerylsilane (DGS) in aqueous solutions and remain stable for prolonged periods of time. By using time-resolved fluorescence anisotropy (TRFA), we demonstrate that this unique property of DGS is ascribed to the slow kinetics of silica particle growth in diluted sols at pH ∼ 9.0. The anisotropy decay of the cationic dye rhodamine 6G (R6G), which strongly adsorbs to silica oligomers and nanoparticles in DGS sols, could be fit to three components: a fast (picosecond) scale component associated with free R6G, a slower (nanosecond) rotational component associated with R6G bound to primary silica particles, and a residual (nondecaying) anisotropy component associated with R6G that was bound to secondary or larger particles that were unable to rotate on the time scale of the R6G emission lifetime (4 ns). The data show that, under conditions where fast hydrolysis is obtained, the initial size of the nuclei depends on the silica concentration, with larger nuclei being present in more concentrated sols, while the rate of growth of primary particles depends on both silica concentration and solution pH. At low silica concentrations and high pHs, it was possible to observe the growth of stable, nonaggregating primary silica particles by a mechanism involving rapid nucleation followed by monomer addition. The presence of stable primary particles was confirmed by atomic force microscopy (AFM) imaging. At higher silica concentrations and lower pHs, there was an increase in the initial size of the nuclei formed, which subsequently grew to a larger radius (>4.5 nm) or aggregated with time, and in such cases, nucleation and aggregation occurred simultaneously in the early stage of silica formation. The data clearly show the power of time-resolved fluorescence anisotropy decay measurements for probing the growth of silica colloids and show that this method is useful for elucidating the mechanism of particle formation and growth in situ.

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“…This is due to the presence of methanol in the latter materials, which causes a significant fraction of the R6G to be located in solvent-filled pores rather than being associated with the silica surface. R6G is far less soluble in the glycerol/water pore solvent in our materials [10] , and thus is more closely associated with the silica surface.…”

“…Single-point fluorescence anisotropy measurements for R6G were performed using an SLM 8100 spectrofluorimeter (Spectronic Instruments, Rochester, NY) with λ ex = 495 nm and λ em = 551 nm, as described elsewhere [10] . Each anisotropy value reported represents the average of three independent measurements (5 replicates per sample) performed on three different samples.…”

“…A detailed description of the instrumentation and experimental procedures has been reported [10] . The fluorescence decays collected by exciting samples with vertically polarized light were used to generate the sum S(t) and the difference D(t) functions, which were used to construct the anisotropy decay r(t):…”

“…This probe has been previously utilized to examine the growth [4][5][6][7][8][9][10][11] and degree of surface modifycation [12][13][14] of silica nanoparticles. Assessment of surface modification relies on measuring the ratio of rapidly rotating R6G (ps correlation time), corresponding to free dye in solution, and to slowly rotating (ns correlation time) or non-rotating dye which results in a residual anisotropy value, which is bound to small and large silica particles, respectively.…”

A comparative study on the dispersion stability of aminofunctionalised silica nanoparticles made from sodium silicate

“…There are two main methods for the preparation of surfactant-free spherical, monodisperse silica nanoparticles: The hydrolysis of tetraalkyl orthosilicates, i.e. the method of Stöber [3] and related syntheses [4,5], and the acidic hydrolysis of sodium silicate [6][7][8][9][10][11][12][13][14]. The latter has a number of advantages compared to the Stöber method, such as using only industrial commodities as starting materials and water as solvent.…”