Photophysical properties of Chlorin-p6 bound to coated gold nanorods

Effect of different coating materials (surfactant and polymers) and surface plasmon resonance of gold nanorods (AuNRs), on the photophysical properties of a porphyrin based photosensitizer Chlorin-p6 (Cp6) has been investigated. AuNRs with two different aspect ratio (AR), one to match the longitudinal surface plasmon resonance (l-SPR) to the Q band (∼660 nm, AR ∼2.5) of Cp6 and other with l-SPR shifted far red (∼800 nm, AR ∼3.8) were prepared. In the presence of Cp6 (Cp6:AuNR ratio 33) the zeta potential of the coated AuNRs reduced considerably. In particular, for PAH (polymer) coated AuNRs the zeta potential became close to zero and the soret (absorption) band of Cp6 broadened considerably. The fluorescence quantum yield of Cp6 decreased significantly in the presence of coated AuNRs, however the fluorescence lifetime of Cp6 decreased significantly only in the presence of CTAB (surfactant) coated AuNRs. In addition the fluorescence characteristics of the drug was also observed to be different for PAH and PDDAC (polymers) coating. The observed differences have been explained on the basis of distance dependent quenching of the drug’s fluorescence by SPR of AuNRs and difference in the chemical structures of the coating materials. The photobleaching of the drug was also found to be dependent on the nature of the coatings and it reduced significantly for the CTAB and PAH coatings. This suggests that the coating material should be chosen accordingly when utilizing these rods for combined hyperthermia and photodynamic therapy applications.