Luminescent lanthanide-functionalized gold nanoparticles: exploiting the interaction with bovine serum albumin for potential sensing applications.

As luminescent surface-functionalized gold nanoparticles emerged as potential powerful analytical tools in the biomedical fields, understanding the interaction of such systems with proteins has become crucial. In the present study, the interaction of luminescent water-soluble gold nanoparticles (AuNP-1·Eu-nta), obtained through the self-assembly of a naphthalene β-diketone antenna with a Eu(III) cyclen complex tethered to the gold surface via a C(12) alkyl thiol spacer, with bovine serum albumin (BSA) was investigated. The changes in the UV-visible absorption and fluorescence spectra of both the antenna and protein, as well as in the time-resolved Eu(III)-centered emission, of the resulting self-assembly were monitored, at physiological pH, as a function of the BSA concentration. We demonstrate that the Eu(III) emission arising from the self-assembly on the AuNP surface is almost completely quenched upon addition of BSA. Binding constant determination clearly showed that the sensitizing antenna was not displaced and that the quenching was the result of the interaction between the antenna and BSA. Detailed spectroscopic studies performed on the nta-BSA system brought a better insight in the strength of such interaction as well as its effect on the protein secondary structure. Finally, the information gathered on each system resulted in applying AuNP-1·Eu-nta-BSA for the luminescent detection of drugs via the perturbation of the nta-BSA interaction. Competitive titrations using ibuprofen and warfarin showed that nta was located in the binding site II of BSA and that the presence of warfarin, a site I drug, did not interfere with the detection of site II ibuprofen.