Superhydrophobic Surfaces as a Source of Airborne Singlet Oxygen through Free Space for Photodynamic Therapy

A superhydrophobic (SH) sandwich system has been developed to enable "contact-free" airborne singlet oxygen (¹O₂) delivery to a water droplet. The contact-free feature means that the sensitizer is physically separated from the droplet, which presents opportunities for photodynamic therapy (PDT). Trapping of airborne ¹O₂ in a H₂O droplet residing on a lower SH surface was monitored with 9,10-anthracene dipropionate dianion by varying distances to an upper ¹O₂-generating surface. Short distances of 20 μm efficiently delivered airborne ¹O₂ to the droplet in single-digit picomolar steady-state concentrations. Delivery decreases linearly with distance, but 50% of the ¹O₂ steady-state concentration is trapped at a distance of 300 μm from the generating surface. The 1270 nm luminescence intensity was measured within the SH sandwich system, confirming the presence of airborne ¹O₂. Physical quenching of ¹O₂ to ground-state ³O₂ by the water droplet itself and both physical and chemical quenching of ¹O₂ by the water droplet containing the trap 9,10-anthracene dipropionate dianion are observed. Unlike a majority of work in the field of PDT with dissolved sensitizers, where ¹O₂ diffuses short (hundreds of nanometers) distances, we show the delivery of airborne ¹O₂ via a superhydrophobic surface is effective through air in tenths of millimeters distances to oxidize an organic compound in water. Our results provide not only potential relevance to PDT but also surface bacterial inactivation processes.