Artificial lipid nanoparticles have drawn great attention due to their potential in medicine. Linked with targeting ligands, they can be used as probes and/or gene delivery vectors for specific types of target cells. Therefore, they are very promising agents in early detection, diagnosis and treatment of cancers and other genetic diseases. However, there are several barriers blocking the applications. Controlling the cellular uptake of the lipid nanoparticles is an important technical challenge to overcome. Understanding the mechanism of the endocytosis and the following intracellular trafficking is very important for improving the design and therefore the efficiency as a drug delivery system. By using fluorescence microscopy methods, we studied the endocytosis of lipid nanoparticles by live M21 cells. The movements of the nanoparticles inside the cell were quantitatively characterized and classified based on the diffusion behavior. The trajectories of nanoparticles movement over the cell membrane revealed hop-diffusion behavior prior to the endocytosis. Fast movement in large steps is observed in intracellular trafficking and is attributed to active movement along microtubule. These observations help to understand the mechanism of the endocytosis and the pathway of the particles in cells.
Transition metal complexes such as ruthenium complexes, having metal-to-ligand charge transfer (MLCT) states, are extensively used in solar energy conversion and electron transfer in biological systems and at interfaces. The dynamics of metal-to-ligand charge transfer and subsequent intermolecular, intramolecular, and interfacial electron transfer processes can be highly complex and inhomogeneous, especially when molecules are involved in interactions and
perturbations from heterogeneous local environments and gated by conformation fluctuations. We have employed single-molecule
spectroscopy, a powerful approach for studying inhomogeneous systems, to study the electron transfer dynamics of ruthenium complexes. We have applied a range of statistical analysis methods to reveal nonclassical photon emission behaviors of single ruthenium complexes, e.g., photon antibunching and photophysical ground-state recovering dynamics on a microsecond time-scale. The use of photon antibunching to measure phosphorescence lifetimes and single-molecule electron transfer dynamics at room temperature is demonstrated, which is a novel way of probing ground state regeneration in back electron transfer processes.
Here, we report our results on excitation intensity and nanoscale Ag cluster dependent spectral fluctuation dynamics of surface enhanced Raman scattering. We have studied single-Ag-cluster surface enhanced Raman scattering (SERS) intensity fluctuations under low molecule surface coverage of rhodamine 6G (R6G) and cytochrome c. By applying both experimental and theoretical approaches, we observed that spectral fluctuation phenomena are associated with SERS not only from single-molecule loaded nanoclusters but also from submonolayer molecule loaded nanoclusters. The nanoscale confinement of the local electric field enhancement under the laser excitation defines the SERS fluctuation. A new AFM-coupled two-channel photon time-stamping system, enabling <i>in situ</i> correlation of the topographic and spectroscopic information for single nanoparticle clusters, was used to record Raman intensity fluctuation trajectories at sub-μs resolution. Experimentally, we found that SERS fluctuation dynamics are highly inhomogeneous amongst nanocluster interstitial sites. Although the fluctuation above ~50 W/cm<sup>2</sup> excitation is dominated by photoinduced processes, spontaneous fluctuation can be observed at lower excitation intensity. Although a single Raman-active molecule confined within the volume of an electric field excitation gives a significant Raman spectral fluctuation, observation of the fluctuation alone may not be sufficient in identifying a single-molecule origin of a Raman spectrum. The Raman signal comes predominately from the localized electric field enhancement at interstitial sites, occuring in a very small volume at nanoscale (capable of holding only one or a few molecules), as estimated from finite-element methods simulations of an electric field enhancement using a classical electrodynamics approach. Such a small number of molecules, which are presumably under discrete diffusion and exposed to interactions with a locally strong electric field, results in the observed Raman fluctuation. The fluctuation autocorrelation amplitude is proportional to the reverse number of molecules confined at the volume of the electric field enhancement.