Photocleavable groups (cages) are small organic molecules that can be attached to a functional group of a biomolecule and used to light-trigger its bioactivity. The presence of the cage changes the original molecular structure, conformation or charge that results in a dramatic alteration of the biological function. Upon light exposure, the photolabile group is cleaved and the bioactivity is restored. Light has important advantages as external trigger. It can be readily available and focused, allowing a precise temporal and spatial definition of the activation process. Light dose can be precisely controlled over the irradiation time and intensity, allowing precise modulation of the concentration of active molecule, a feature that is not easy to achieve with other triggering sources.

By using photoremovable cages with differential response to selected wavelengths, the bioactivity of coexisting molecules can be independently triggered. The independent response relies on the different photosensitivity of the chromophores at the selected wavelength (as given by the quantum yield of the photolytic reaction and extinction coefficient of the chromophore at the selected wavelength). This strategy allows individual and highly controlled activation of multiple processes confined into the same space.

UV-Vis spectra of caging groups showing differences in extinction coefficients

If a caged biomolecule is attached to a surface, a latent surface functionality is generated that can be activated by posterior light exposure. The irradiated surface exposes active groups that can, for example, recognize complementary targets from solution and bind them specifically to the solid support. Proteins, oligonucleotides or cells can be selectively addressed by using appropriate surface caged ligands. Using wavelength-selective cages multiple surface functional states can be generated depending on the applied irradiation sequence. When combined with site-specific irradiation (either using masks or writing with focused lasers), different biomolecules can be attached to specific sites of a caged substrate with micrometric lateral resolution (microarrays). By tuning the exposure dose, the surface density of active groups can be precisely tuned.

Fluorescently labelled protein micropattern. A surface modified with a photocleavable aminoterminated silane was patterned using a scanning laser and different exposure doses. Fluorescently labelled protein was coupled to the exposed fields and. The fluorescent pattern reveals different protein surface densities that correlate with the exposure dose.	Bifunctional pattern obtained after masked irradiation of a surface layer containing two photocleavable groups at two selected wavelengths.

References:

1. Modulating surface density of proteins via caged surfaces and controlled light exposure, M. Álvarez, J. M. Alonso, O. Filevich, M. Bhagawati, R. Etchenique, J. Piehler, A. del Campo*, Langmuir, in press
2. Near field lithography by two-photon induced photocleavage of organic monolayers, M. Álvarez, A. Best, A. Unger, J.M. Alonso, A. del Campo, M. Schmelzeisen, K. Koynov, M. Kreiter, Adv. Funct. Mater. 20(24), 4265-4272 (2010)
3. Photoactive branched and linear surface architectures for functional and patterned immobilization of proteins and cells onto surfaces: a comparative study, P. Stegmaier, A. del Campo*, ChemPhysChem 10, 357-369 (2009)
4. Photoresponsive surfaces with two independent wavelength-selective functional levels, P. Stegmaier, J. M. Alonso, A. del Campo*, Langmuir 24, 11872-11879 (2008)