pH-Responsive, TiO2-Attached Porphyrin for Singlet Oxygen Production in an Aqueous Solution

A pH-responsive, TiO2-attached sensitizer was prepared based on the adsorption of 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (TCPP) onto TiO2 nanoparticles. This colloidally dispersed TiO2-attached TCPP behaves as a single-phase colloidal sensitizer at pH 1.0−3.3 with quantum yields of singlet oxygen production (ΦΔ) between 0.20 and 0.25, as a heterogeneous particle sensitizer at pH 3.5−6.0 with ΦΔ between 0.25 and 0.50, and as homogeneous free TCPP molecules in alkaline solutions with ΦΔ = 0.53. The changes in ΦΔ are fully consistent with pH-dependent adsorption of TCPP onto the TiO2 surface. Recovery yields of 99.8% for TCPP and 98.8% for TiO2 were obtained from 1.4 mM TiO2-attached TCPP. We attribute its photosensitization ability to retaining TCPP solubility on the TiO2 surface and, hence, activity. This novel system shows a potential to bridge the gap between easily recoverable and highly efficient sensitizers.