Diffusion of a multi-species component and its role in oxygen and water transport in silicates

Kinetic and spectroscopic responses of pH-sensitive nanoparticles: influence of the silica matrix

Intracellular pH sensing with fluorescent nanoparticles is an emerging topic as pH plays several roles in physiology and pathologic processes. Here, nanoparticle-sized pH sensors (diameter far below 50 nm) for fluorescence imaging have been described. Consequently, a fluorescent derivative of pH-sensitive hydroxypyrene with pK a = 6.1 was synthesized and subsequently embedded in core and core-shell silica nanoparticles via a modified Stöber process. The detailed fluorescence spectroscopic characterization of the produced nanoparticles was carried out for retrieving information about the environment within the nanoparticle core. Several steady-state and time-resolved fluorescence spectroscopic methods hint to the screening of the probe molecule from the solvent, but it sustained interactions with hydrogen bonds similar to that of water. The incorporation of the indicator dye in the water-rich silica matrix neither changes the acidity constant nor dramatically slows down the protonation kinetics. However, cladding by another SiO2 shell leads to the partial substitution of water and decelerating the response of the probe molecule toward pH. The sensor is capable of monitoring pH changes in a physiological range by using ratiometric fluorescence excitation with λ ex = 405 nm and λ ex = 488 nm, as confirmed by the confocal fluorescence imaging of intracellular nanoparticle uptake.

Silica nanoparticle stability in biological media revisited

The stability of silica nanostructure in the core-silica shell nanomaterials is critical to understanding the activity of these nanomaterials since the exposure of core materials due to the poor stability of silica may cause misinterpretation of experiments, but unfortunately reports on the stability of silica have been inconsistent. Here, we show that luminescent silver nanodots (AgNDs) can be used to monitor the stability of silica nanostructures. Though relatively stable in water and phosphate buffered saline, silica nanoparticles are eroded by biological media, leading to the exposure of AgNDs from AgND@SiO₂ nanoparticles and the quenching of nanodot luminescence. Our results reveal that a synergistic effect of organic compounds, particularly the amino groups, accelerates the erosion. Our work indicates that silica nanostructures are vulnerable to cellular medium and it may be possible to tune the release of drug molecules from silica-based drug delivery vehicles through controlled erosion.

Internal friction of hydrated soda-lime-silicate glasses

The internal friction of hydrated soda-lime-silica glasses with total water content (C(W)) up to 1.9 wt. % was studied by dynamic mechanical analysis (DMA) using temperature-frequency sweeps from 723 K to 273 K and from 1 s(-1) to 50 s(-1). Total water content and concentrations of H2O molecules (C(H2O)) and OH groups (C(OH)) in the DMA specimens were determined by infrared spectroscopy. For low water contents (C(W) ≈ C(OH) < 0.25 wt. %) two discrete internal friction peaks below the glass transition (α relaxation) were assigned to the low-temperature motion of alkali ions (γ relaxation) and cooperative movements of dissimilar mobile species under participation of OH at higher temperature (β(OH) relaxation). For large water contents (C(W) > 1 wt. %), where significant amounts of molecular water are evident (C(H2O) > 0.15 wt. %), however, internal friction spectra change unexpectedly: the β(OH) peak heights saturate and a low temperature shoulder appears on the β-relaxation peak. This emerging relaxation mode (β(H2O) relaxation) was assigned to the motions of H2O molecules. β(H2O) relaxation was found to be faster than β(OH) but slower than γ relaxation. Activation energy of the different relaxation modes increased in the order γ < β(H2O) < β(OH) < α.