Single-molecule detection in exploring nanoenvironments: an overview

Abstract

In the last one decade or so, a variety of optical experiments have been designed and performed that are capable of exploring down to the regime of single-molecule detection and measurements in all different environments, including solids, surfaces, and liquids. Single-molecule detection in condensed phases has many important chemical and biological applications. A few to list are: rapid DNA sequencing, DNA fragment sizing, medical diagnosis, forensic analysis, understanding of chemical dynamics and mechanisms, etc. Single-molecule spectroscopy allows us to observe the individual molecules hidden in a condensed phase sample, by using a tunable laser light. This technique has the ability to detect and monitor systems with an ultimate sensitivity level of ~1.66 × 10^-24 moles (1/N₀). Measurement at the single-molecule level can completely remove the complicacy associated with ensemble-averaged macroscopic measurements. It allows us to construct a frequency histogram of the distribution of values for a parameter of interest following a large number of measurements on many individual molecules. Such a distribution carries much more information than the average value of the parameter obtained from a macroscopic measurement. As there is no ensemble averaging involved, only measurements at the single-molecule level can give an appropriate test for microscopic dynamical theories. Using single-molecule spectroscopy one can, in principle, follow the temporal evolution of any complex reaction path. As the field is still emerging, with newer methodologies of detecting single molecules with improved signal-to-noise ratios, it is expected that many new physical and chemical phenomena will certainly be explored using this technique. In the present article, our endeavor is to give an overview of the different aspects of single-molecule detection, along with some of its important applications in the areas of bioscience and chemical physics.