Gold nanoring-enhanced generation of singlet oxygen: an intricate correlation with surface plasmon resonance and polyelectrolyte bilayers

We report the dependence of gold nanoring (Au NR)-enhanced singlet oxygen (1O2) generation on localized surface plasmon resonance (LSPR) wavelength and separation distance between Au NR and photosensitizers. Using Au NR of tunable LSPR and poly(styrene sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) bilayers via layer-by-layer (LbL) assembly on Au NR as molecular spacers, we found that 1O2 generation from Al(III) phthalocyanine chloride tetrasulfonic acid (AlPcS4) conjugated on the Au NR can be optimized through proper combination of LSPR and the separation distance. More importantly, 1O2 enhancement follows a different correlation with the separation distance compared to that of AlPcS4 fluorescence enhancement. Our experimental findings are consistent with the prediction by boundary element method (BEM) calculations. Our study paves the way to the design of LSPR-enhanced 1O2 generation for improved efficacy of photodynamic therapy of cancers.