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

Hybrid gold nanoparticles covalently coated with 5,10,15-p-(ω-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-[ω-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.