Abstract
The promise and success of current biomedical research is highly dependent on understanding the mechanisms of fundamental biological processes such as protein folding, the function of molecular machines, or the interaction of proteins with molecular complexes. In particular, on the cellular level the heterogeneous and fluctuating environment as well as the intrinsic asynchronicity of protein function requires observation of individual copies rather than ensembles if a detailed understanding of their function is to be obtained. As a consequence, real‐time in vivo imaging is one of the most desirable tools for investigating biological systems at microscopic and nanoscopic levels. In this chapter, we summarize some of the recent advances in optical microscopy with a particular focus on the capabilities and limitations of studies at the single‐molecule and nanoparticle level. We present a detailed account of the intricate interplay between single‐molecule photophysical properties and the attainable signal‐to‐noise ratio in the ability to see, localize and trace single molecules. Furthermore, we discuss the possibility of single‐molecule labels being replaced by tiny light scatterers, such as gold nanoparticles, which offer an inherent photostability and lack of saturation. In particular, we introduce a novel interferometric technique for detecting nanoscopic objects and demonstrate its potential use both for the study of gold nanoparticle labels in scattering media and the label‐free detection of single biological nanoparticles in the absence of additional scatterers.
