Two-photon scanning microphotolysis for three-dimensional data storage and biological transport measurements

Scanning microphotolysis is a method that permits the user to select, within the scanning field of a confocal microscope, areas of arbitrary geometry for photobleaching or photoactivation. Two-photon absorption, by contrast, confers on laser scanning microscopy a true spatial selectivity by restricting excitation to very small focal volumes. In the present study the two methods were combined by complementing a laser scanning microscope with both a fast programmable optical switch and a titan sapphire laser. The efficiency and accuracy of fluorescence photobleaching induced by two-photon absorption were determined using fluorescein-containing polyacrylamide gels. At optimal conditions a single scan was sufficient to reduce the gel fluorescence by approximately 40%. Under these conditions the spatial accuracy of photobleaching was 0.5 +/- 0.1 micron in the lateral (x.y) and 3.5 +/- 0.5 micron in the axial (z) direction, without deconvolution accounting for the optical resolution. Deconvolution improved the accuracy values by approximately 30%. The method was applied to write complex three-dimensional patterns into thick gels by successively scanning many closely spaced layers, each according to an individual image mask. Membrane transport was studied in a model tissue consisting of human erythrocyte ghosts carrying large transmembrane pores and packed into three-dimensional arrays. Upon equilibration with a fluorescent transport substrate single ghosts could be selectively photobleached and the influx of fresh transport substrate be monitored. The results suggest that two-photon scanning microphotolysis provides new possibilities for the optical analysis and manipulation of both technical and biological microsystems.