Optical Imaging of the Nanoscale Structure and Dynamics of Biological Membranes

Biological membranes serve as the fundamental unit of life, allowing the compartmentalization of cellular contents into subunits with specific functions. The bilayer structure, consisting of lipids, proteins, small molecules, and sugars, also serves many other complex functions in addition to maintaining the relative stability of the inner compartments. Signal transduction, regulation of solute exchange, active transport, and energy transduction through ion gradients all take place at biological membranes, primarily with the assistance of membrane proteins. For these functions, membrane structure is often critical. The fluid-mosaic model introduced by Singer and Nicolson in 1972 evokes the dynamic and fluid nature of biological membranes.(1) According to this model, integral and peripheral proteins are oriented in a viscous phospholipid bilayer. Both proteins and lipids can diffuse laterally through the two-dimensional structure. Modern experimental evidence has shown, however, that the structure of the membrane is considerably more complex; various domains in the biological membranes, such as lipid rafts and confinement regions, form a more complicated molecular organization. The proper organization and dynamics of the membrane components are critical for the function of the entire cell. For example, cell signaling is often initiated at biological membranes and requires receptors to diffuse and assemble into complexes and clusters, and the resulting downstream events have consequences throughout the cell. Revealing the molecular level details of these signaling events is the foundation to understanding numerous unsolved questions regarding cellular life

Inorganic Semiconductor Quantum Dots as a Saturated Excitation (SAX) Probe for Sub‐Diffraction Imaging

The photoluminescence (PL) saturation of CdSe/ZnS core/shell inorganic semiconductor quantum dots (QDs) and its utility as a probe for saturated excitation (SAX) microscopy are reported. Under saturating excitation power densities, the PL signal was demodulated and recorded at harmonics of the fundamental frequency. For commercially available Qdot® 655 ITK™ QDs, the power density required to achieve saturation was dependent upon the local environment of the QDs. For QDs deposited and dried on a glass substrate, the excitation power density required for PL saturation was less than 1 kW/cm2. Compared to this, saturation of PL for QDs dispersed in water required an excitation power density greater than 200 kW/cm2. This observation is manifested as a limitation in the imaging of hydrated samples, as demonstrated for HeLa cells labelled with biotinylated‐phalloidin followed by labelling with streptavidin‐coated QDs. As saturation affects the obtained spatial resolution in several imaging formats, including confocal imaging, the provided data will aid in obtaining the optimal spatial resolution when using QD probes to image biological samples