PEGylate porphyrin-gold nanoparticles conjugates as removable pH-sensor nano-probes for acidic environments

Hybrid gold nanoparticles covalently coated with 5,10,15-p-(?-methoxy polyethyleneoxyphenyl)-20-p-(hydroxyphenyl) porphyrin molecules, having about 16 repetitive units in each of its poly(oxyethylene) branches, were synthesized and characterized by UV-vis, fluorescence, dynamic light scattering (DLS), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) measurements. Their [H3O+] optical sensing behavior was also studied and compared to that of the similar water-soluble star polymer 5,10,15,20- tetrakis p-[?-methoxypoly(ethyleneoxy)]phenylporphyrin. Changing the pH of a solution, the UV-vis spectra of the PEGylate porphyrin gold nanoparticles conjugates showed the gradual disappearance of the Soret band at 424 nm and the progressive increase of a new band at 451 nm. Upon acidic titration, also the fluorescence spectra showed a gradual disappearing of the fluorescence at 654 nm. Moreover, an unusual B-Band red-shift and a decreasing of porphyrin core basicity were observed for this porphyrin bonded to the gold nanoparticle surfaces. To rationalize these phenomena an in silico TD-DFT calculation study was then conducted. For its properties, this new system: can be used in both aqueous and organic media; by a simple treatment of the solution at 80 °C under N2 gas stream the starting Soret signals can be recovered; the nanoparticles can be eventually removed from the solution by centrifugation.