Real-time molecular uptake and membrane-specific transport in living cells by optical microscopy and nonlinear light scattering

We show that the comparably simple and widely available technique, optical transmission microscopy (TM), can be used in a time-resolved way to independently measure uptake of any molecule (that absorbs visible light) into living cells. This technique complements second-harmonic light scattering (SHS), which can be used for time-resolved observation of adsorption and transport of small molecules, with non-zero first-order hyper-polarizability, through membranes in living cells. While either technique can be used alone, a combination of the pair allows a fully complementary and quantitative characterization of molecular uptake with membrane specificity in living cells. Specifically, TM permits detection of the changing bulk molecular concentrations within the cell, and SHS, with its inherent interface sensitivity, provides sequential differentiation of molecular density at membrane surfaces. Simultaneous global analysis of the TM and SHS observations yield molecular diffusion rate constants for all traversed cellular barriers.