Minimizing the photobleaching of self-assembled multilayers for sensor applications

Layer-by-layer electrostatic self-assembled (ESA) multilayer nanostructures containing 1-hydroxypyrene-3,6,8-trisulfonate (HPTS), a fluorophore widely used for pH sensing, were built-up onto optical fiber substrates. The self-assembled structures are based on a basic matrix of alternating polyelectrolytes; poly(allylamine hydrochloride) (PAH) as polycation and poly(acrylic acid) (PAA) as polyanion. Doping the polyanionic solution with HPTS, the resultant (PAH/PAA + HTPS)_n multilayer coatings exhibited a pH-dependent fluorescencent behavior appropriate for optical sensing applications. Unfortunately, these sensitive coatings show short lifetimes due to their high rate of photobleaching. In order to improve the performance of the devices, the fabrication atmosphere and some post-treatments, like thermal curing and the addition of an antifading agent, 1,4-diazabicyclo[2.2.2]octane, have been experimentally studied. After these modifications were introduced in the fabrication routine, the photobleaching rate was drastically reduced: the initial sensitive coatings show a decrease in fluorescence intensity due to photobleaching of 58% after 90 min of continuous illumination and the optimized sensitive coatings show a decrease of only 4.7% after 3 days of continuous illumination.