Oriented Self-Assembled Monolayer of Zn(II)-Tetraphenylporphyrin on TiO2 Electrode for Photoelectrochemical Analysis.

The photoelectrochemical performance of a zinc porphyrin sensitized TiO2 photoelectrode fabricated through the axial-coordination strategy has been studied. Zinc(II) tetraphenylporphyrin (TPPZn) was immobilized on the TiO2 electrode through the axial coordination reaction with surface pre-modified 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 TiO2 surface has been evidenced by XPS measurement. The modified TiO2 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 TiO2. Sensitive increment of photocurrent is observed with the addition of L-glutathione. Possible mechanism for this photoelectrochemical process includes the photoexcitation of the porphyrin, electron injection from porphyrin toward the TiO2 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 TiO2 substrate. A PEC analysis on L-glutathione has been carried out to evaluate the photoelectrochemical performance of the modified TiO2 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 semiconductor surface and could lead to the designing of more efficient photoelectrochemical sensors and devices based on similar electrode structures.