Singlet oxygen generation on porous superhydrophobic surfaces: effect of gas flow and sensitizer wetting on trapping efficiency

Abstract

We describe physical-organic studies of singlet oxygen generation and transport into an aqueous solution supported on superhydrophobic surfaces on which silicon-phthalocyanine (Pc) particles are immobilized. Singlet oxygen ((1)O2) was trapped by a water-soluble anthracene compound and monitored in situ using a UV-vis spectrometer. When oxygen flows through the porous superhydrophobic surface, singlet oxygen generated in the plastron (i.e., the gas layer beneath the liquid) is transported into the solution within gas bubbles, thereby increasing the liquid-gas surface area over which singlet oxygen can be trapped. Higher photooxidation rates were achieved in flowing oxygen, as compared to when the gas in the plastron was static. Superhydrophobic surfaces were also synthesized so that the Pc particles were located in contact with, or isolated from, the aqueous solution to evaluate the relative effectiveness of singlet oxygen generated in solution and the gas phase, respectively; singlet oxygen generated on particles wetted by the solution was trapped more efficiently than singlet oxygen generated in the plastron, even in the presence of flowing oxygen gas. A mechanism is proposed that explains how Pc particle wetting, plastron gas composition and flow rate as well as gas saturation of the aqueous solution affect singlet oxygen trapping efficiency. These stable superhydrophobic surfaces, which can physically isolate the photosensitizer particles from the solution may be of practical importance for delivering singlet oxygen for water purification and medical devices.

Figure 1

Geometry of the ¹O₂ photoreactor device. A poly(methyl methacrylate) (PMMA) cuvette was modified to incorporate a superhydrophobic surface embedded with Pc particles printed onto a porous membrane. The printed membrane is held on a plastic support plate that defines the top of the plenum. Holes were drilled through the plate enabling gas to flow from the plenum to the plastron. A gas input needle inserted into the bottom of the plenum is used to introduce a controlled flow of gas.

Scheme 1

Figure 2

Schematic images of PDMS posts coated with Pc particles at controlled locations. (A) Surface A has Pc particles coating the PDMS posts. (B) Surface B has Pc particles primarily embedded at the PDMS tips. (C) Surface C has Pc coating the PDMS post base, where tips are capped with a layer of PDMS and SiO₂ nanoparticles.

Figure 3

Schematic of the fabrication of surface B.

Figure 4

Schematic of the fabrication of surface C.

Figure 5

SEM images of PDMS posts coated with Pc particles at controlled locations: (A) surface A with particles coating the entire PDMS surface; (B) surface B with particles adhered only to the top portion of the PDMS posts; and (C) surface C where the surface prepared as in surface A was capped with a layer of silica nanoparticle adhered to a layer of PDMS.

Figure 6

Endoperoxide 2 yield in static experiments where D₂O solutions were presaturated with O₂ or N₂. There was no gas sparging through the plenum of the device. Error bars represent the standard deviation obtained from 3 measurements.

Figure 7

Optical images of surface A, showing (i) plastron with a planar and reflective air–water interface, (ii) gas bubble forming over the surface, and (iii) gas bubble releasing from the surface.

Figure 8

Endoperoxide 2 yield in bubbling experiments where O₂ or N₂ gas was sparged through the plenum into the D₂O solution. Error bars represent the standard deviation were obtained from three measurements.

Figure 9

Mechanism of singlet oxygen generation via O₂ flowing through the plastron of a superhydrophobic sensitizer surface.