History of FCS
The theory of FCS is based on molecular diffusion, and the origin of the
technique can be traced back to the discovery of the Brownian motion of molecules.
The first person to describe the mathematics behind Brownian motion was Thorvald N.
Thiele in 1880 in a paper92 on the method of least squares. Then it was followed
independently by Louis Bachelier in his PhD thesis "The theory of speculation" in
1900,93 in which he proposed a stochastic analysis of the stock and option markets.
However, it was Albert Einstein94 (1905) and Marian Smoluchowski95 (1906) who
independently brought the solution of the problem to the attention of physicists, and
presented it as a way to indirectly confirm the existence of atoms and molecules. Albert
Einstein explained that the movement of particles suspended in liquids, which is called
Brownian motion, is due to the random thermal motion of the solvent molecules. He also
predicted that the displacement of a particle should be, on average, proportional to the
square root of its diffusion time. This prediction was later been proved experimentally by
Jean Perrin in 1909, whose studies provided the most compelling evidence for
molecular diffusion.96 This was also the first time that scientists analyzed a fluctuating
signal to study molecular-scaled properties, the first example of "molecular fluctuation
analysis".
Between the late 1960's and early 1970's, the invention of lasers led to the
development of an important technique, named dynamic light scattering (DLS), for
molecular-size characterization. This advance opened the door for molecular-scaled
analysis.