Highly fluorescent sterically stabilized silver nanoparticles (Cf-AgNPs) have been prepared by a facile thermochemical method in a single step and with excellent long-term stability. The method involves reduction of silver nitrate to Ag via carbonyl groups of the β-lactam ring and the amide of cefaclor. The materials have been characterized by fluorescence and absorption spectroscopy, atomic force microscopy, FT-IR spectroscopy, ¹H NMR and MALDI-TOF-MS. The luminescence arises from small metal clusters (i.e., Ag₂, Ag₃ and Ag₄), which is efficiently quenched by the paramagnetic quencher Cu(II). The Cf-AgNPs allowed us to detect Cu(II) in the range of 0.8 to 3.3 µM with a detection limit of 0.282 µM. Moreover, Cu(II) was selectively recognized in human blood plasma by Cf-AgNPs. We have also evaluated the cytotoxicity of Cf-AgNPs and explored their scavenging potential against Cu(II) in B and T lymphocytes with the help of AFM.