Oriented Self-Assembled Monolayer of Zn(II)-Tetraphenylporphyrin on TiO ₂ Electrode for Photoelectrochemical Analysis

The photoelectrochemical performance of a zinc porphyrin sensitized TiO ₂ photoelectrode fabricated through the axial-coordination strategy has been studied. Zinc(II) tetraphenylporphyrin (TPPZn) was immobilized on the TiO ₂ electrode through the axial coordination reaction with surface premodified methyl 4-(1H-imidazol-1-yl)benzoate (PhImMe). Spectral characterizations were carried out to confirm the robust interaction between TPPZn and PhImMe, while the chemical bonding of PhImMe on TiO ₂ surface has been evidenced by XPS measurement. The modified TiO ₂ electrode produces a stable and enhanced photocurrent signal under 410 nm irradiation at a bias of 0.2 V in phosphate buffer solution, compared with pristine TiO ₂ . A sensitive increment of the photocurrent is observed with the addition of l-glutathione. A possible mechanism for this photoelectrochemical process includes the photoexcitation of the porphyrin, electron injection from porphyrin toward the TiO ₂ , and the hole-scavenging process by the reductants. The "lying-low" surface conformation of the porphyrin plane might facilitate the PEC process owing to the enhanced electronic coupling between the porphyrin plane and the TiO ₂ substrate. A PEC analysis on l-glutathione has been carried out to evaluate the photoelectrochemical performance of the modified TiO ₂ electrode, which presents the wide linear ranges of 5-80 μM and 80-5000 μM, with a lower detection limit of 3.21 μM at an S/N ratio of 3. This work reports for the first time on the PEC performance of metalloporphyrin axially coordinated on a semiconductor surface and could lead to the designing of more efficient photoelectrochemical sensors and devices based on similar electrode structures.