The robust glow of molecular fluorescence renders even sparse molecules detectable and susceptible to analysis for concentration, mobility, chemistry, and photophys- ics. Correlation spectroscopy, a statistical-physics-based tool, gleans quantitative information from the spontaneously fluctu- ating fluorescence signals obtained from small molecular en- sembles. This analytical power is available for studying molecules present at minuscule concentrations in liquid solutions (less than one nanomolar), or even on the surfaces of living cells at less than one macromolecule per square micrometer. Indeed, rou- tines are becoming common to detect, locate, and examine individual molecules under favorable conditions.
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