Targeted drug delivery in cancer cells with red-light photoactivated mesoporous silica nanoparticles.

Mesoporous nanoparticles for drug delivery would benefit significantly from further improvements in targeting efficiency and endosomal release. We present a system based on colloidal mesoporous silica nanoparticles with targeting-ligands and a red-light photosensitizer. This nanoparticle system provides spatial and temporal control of the release of drugs into the cytosol of cancer cells. Furthermore, the system presents a general platform since it can be loaded with different cargos and adapted for targeting of multiple cell types.

Manifestations of probe presence on probe dynamics in supercooled liquids.

Experimental studies that follow behavior of single probes embedded in heterogeneous systems are increasingly common. The presence of probes may perturb the system, and such perturbations may or may not affect interpretation of host behavior from the probe observables typically measured. In this study, the manifestations of potential probe-induced changes to host dynamics in supercooled liquids are investigated via molecular dynamics simulations. It is found that probe dynamics do not necessarily mirror host dynamics as they exist either in the probe-free or probe-bearing systems. In particular, for a binary supercooled liquid, we find that smooth probes larger than the host particles induce increased translational diffusion in the host system; however, the diffusion is anisotropic and enhances caging of the probe, suppressing probe translational diffusion. This in turn may lead experiments that follow probe diffusion to suggest Stokes-Einstein behavior of the system even while both the probe-free and probe-bearing systems exhibit deviations from that behavior.

When the Heterogeneous Appears Homogeneous: Discrepant Measures of Heterogeneity in Single Molecule Observables.

Supercooled liquids demonstrate stretched exponential relaxations consistent with the presence of spatially heterogeneous dynamics. Many experimental results are consistent with this picture, but differences in experimental approach may lead to different conclusions about the degree of heterogeneity in a given system. Here, we investigate whether observables accessible with single molecule (SM) approaches are consistent with each other and with ensemble measurements. In particular, the distribution of rotational relaxation times, τ(c), obtained from SM measurements is compared with the stretching exponent determined from a quasi-ensemble treatment of the same data. It is shown that the time-limited trajectories typical of SM experiments can lead to a stretching exponent that suggests homogeneous dynamics even in the presence of heterogeneous dynamics. After correction for the time-limited trajectories, additional discrepancy remains between stretching exponents measured via SM experiments and ensemble techniques. The remaining difference is attributed to the limited dynamic range of the SM experiments.

Probe dependence of spatially heterogeneous dynamics in supercooled glycerol as revealed by single molecule microscopy.

Spatially heterogeneous dynamics in supercooled glycerol over the temperature range 198 K (1.04T(g))-212 K (1.12T(g)) is investigated using widefield single molecule (SM) fluorescence microscopy. Measurements are performed using three different perylenedicarboximide probes to investigate whether probe size and probe-host interactions affect breadth of heterogeneity reported in the glassy host by such SM experiments. Rotational relaxation times of single probe molecules are measured, and for all probes, log-normal distributions of relaxation times are found. No significant change in relaxation time distribution as a function of temperature is evident for a given probe. However, across probes, probe rotational relaxation time is correlated with breadth of heterogeneous dynamics reported. Molecules that undergo changes in dynamics are identified using two complementary approaches that interrogate time scales between 10(3) and 10(6) τ(α), with τ(α) the structural relaxation time of glycerol. Exchange is found on the shortest time scales probed (~30 τ(c), with τ(c) the rotational correlation time of the probe) and is relatively temperature and probe independent. No evidence is found for additional exchange occurring on the longest time scales interrogated. Taken together with the fact that probes that rotate the fastest report the greatest breadth of spatially heterogeneous dynamics in the system, this indicates that exchange times reported from analysis of SM linear dichroism trajectories as described here are upper bounds on the average exchange time in the system.

High power light emitting diode based setup for photobleaching fluorescent impurities.

Single molecule fluorescence experiments, with their associated low signals, require very low background fluorescence in the sample. Even high purity liquids will often possess large numbers of fluorescent impurities that are difficult to completely remove through standard purification techniques such as distillation and recrystallization. We have constructed a simple setup in which such impurities can be photobleached before final sample preparation. The instrument consists of high power light emitting diodes, and it delivers almost 10 W of light to the sample without the heating associated with more conventional light sources or the cost and safety concerns associated with a high power laser.

Spatial and temporal heterogeneity in supercooled glycerol: evidence from wide field single molecule imaging.

We quantify spatial and temporal heterogeneity in supercooled glycerol at T=T(g)+14 K employing a widefield detection scheme and using rubrene as the probe molecule. We describe how microscopy configuration affects measured intensity, linear dichroism, and the resulting autocorrelation function. Rotational relaxation times tau(c) of 241 probe molecules are measured, and we find spatial heterogeneity over almost three orders of magnitude present at this temperature. An approach for detecting temporally heterogeneous molecules and quantifying exchange times is introduced. Of molecules that can be assessed, approximately 15% display evidence of temporal heterogeneity-changes of tau(c) during the measurement-that are detected with the analysis technique employed. Exchanges between dynamically disparate environments occur rarely but in the proportion expected given the rarity of very slowly rotating molecules present. Heterogeneous molecules are characterized by persistence and exchange times. Median exchange and persistence times of the molecules identified as heterogeneous relative to glycerol's structural relaxation time tau(alpha) are found to be tau(ex)/tau(alpha)=202 and tau(pers)/tau(alpha)=405, respectively. These results are discussed in the context of values of exchange times that have been determined in other experiments.

Probe particles alter dynamic heterogeneities in simple supercooled systems.

The authors present results from molecular dynamics simulations on the effect of smooth and rough probes on the dynamics of a supercooled Lennard-Jones (LJ) mixture. The probe diameter was systematically varied from one to seven times the diameter of the large particles of the LJ mixture. Mean square displacements show that in the presence of a large smooth probe the supercooled liquid speeds up, while in the presence of a large rough probe, the supercooled liquid slows down. Non-Gaussian parameters indicate that with both smooth and rough probes, the heterogeneity of the supercooled system increases. From the analysis of local Debye-Waller factors, it is evident that the change in the dynamics of the LJ system is heterogeneous, with the largest perturbations close to the probes. Large smooth and rough probes appear to set up heterogeneities in these supercooled systems that would otherwise not occur, and these heterogeneities persist for long times.