Exploring the unusual SERS and fluorescence behavior of silver ranelate nanoparticles and their interaction with copper( ) and Tb(iii) ions

Abstract

The chemistry of silver nanoparticles incorporating a covalently bound complexing agent is dealt with in this paper. The particles have been obtained by the direct reaction of silver ions with the ranelate species obtained from an old antiosteoporotic drug. The ranelate ion exhibits a thiophene structure encompassing a cyanonitrile and two carboxylic groups, in addition to a chelating nitrilebis(diacetate) moiety. Through metal substitution and redox processes involving decarboxylation at the C5-thiophene position, chocolate-brown silver-ranelate nanoparticles (AgNP@Ran) have been generated. As corroborated by the XPS and Raman data, the nanoparticles are strongly stabilized by Ag–C bonds and are inert to substitution. This allows the exploration of their coordination chemistry in the presence of transition and rare earth metal ions, under the influence of plasmonics and the SERS effect. Due to the small nanoparticle size and the high energy spectral band (415 nm), although no significant SERS response has been detected in the excitation range of 488–785 nm, a gradual enhancement has been observed over time, at 488 nm. This observation was ascribed to the laser-induced aggregation of the silver nanoparticles, leading to the SERS effect. The silver ranelate nanoparticles exhibited dramatic changes in their optical and SERS spectra in the presence of transition metal ions such as Cu(II), forming stable complexes in aqueous solution. In the case of Tb³⁺ ions, instead of the SERS effect, an enhanced fluorescence emission was observed, involving energy transfer from the ranelate ligand to the terbium 4f levels. In this way, silver ranelate nanoparticles make interesting optical sensors for metal ions and can be useful as fluorescent/plasmonic markers for chemical and biological systems.