A photoactivatable light tracer

Abstract

The development of experimental protocols to probe quantitatively electromagnetic radiation propagating through microscaled objects is essential to unravel the fundamental factors controlling the ability of microoptical elements to transmit light and, hence, guide their design and fabrication. The results obtained in our study demonstrate that the photochemical and photophysical properties of photoactivatable fluorophores together with the sectioning capability of confocal laser-scanning microscopy (CLSM) are particularly valuable in this context. Specifically, the introduction of a photoactivatable borondipyrromethene (BODIPY) fluorophore, which can interconvert between two emissive states with resolved fluorescence upon exposure to activating illumination, inside microscaled poly(dimethylsiloxane) (PDMS) pyramids permits the three-dimensional visualization of the light propagating within these objects. The fluorescence of either one of the two states of this compound appeared predominantly within a cylindrical volume along the main axis of each pyramid with a monotonic intensity increase in the base-to-tip direction. The predominant localization of light at the pyramid core and the resulting four-fold enhancement at the pyramid tip enables photochemical transformations with spatial control. The resolved fluorescence of the two interconvertible states allows, once again, the direct visualization of the restricted volumes, in which the photochemical conversions occur. These results suggest that photoactivatable fluorophores can be optimal chemical probes to trace propagating light in microoptics and provide definitive experimental evidence of the light-guiding and light-enhancing capabilities of these particular pyramidal PDMS objects.