Supramolecular Graphene Quantum Dots/Porphyrin Complex as Fluorescence Probe for Metal Ion Sensing.

Graphene quantum dots (GQDs) obtained by microwave-induced pyrolysis of glutamic acid and triethylenetetramine (trien) are fairly stable, emissive, water-soluble, and positively charged nano-systems able to interact with negatively charged -tetrakis(4-sulfonatophenyl) porphyrin (TPPS). The stoichiometric control during the preparation affords a supramolecular adduct, GQDs@TPPS, that exhibits a double fluorescence emission from both the GQDs and the TPPS fluorophores. These supramolecular aggregates have an overall negative charge that is responsible for the condensation of cations in the nearby aqueous layer, and a three-fold acceleration of the metalation rates of Cu ions has been observed with respect to the parent porphyrin. Addition of various metal ions leads to some changes in the UV/Vis spectra and has a different impact on the fluorescence emission of GQDs and TPPS. The quenching efficiency of the TPPS emission follows the order Cu > Hg > Cd > Pb ~ Zn ~ Co ~ Ni > Mn ~ Cr >> Mg ~ Ca ~ Ba, and it has been related to literature data and to the sitting-atop mechanism that large transition metal ions (e.g., Hg and Cd) exhibit in their interaction with the macrocyclic nitrogen atoms of the porphyrin, inducing distortion and accelerating the insertion of smaller metal ions, such as Zn. For the most relevant metal ions, emission quenching of the porphyrin evidences a linear behavior in the micromolar range, with the emission of the GQDs being moderately affected through a filter effect. Deliberate pollution of the samples with Zn reveals the ability of the GQDs@TPPS adduct to detect sensitively Cu, Hg and Cd ions.