Tetrapodal framework based two compounds P and R were synthesized bearing two urea and thiourea moieties respectively. The organic nanoparticles (ONPs) were produced from P and R by re-precipitation method and their recognition behavior was studied by investigating the fluorescence emission pattern of ONPs-analyte complexes. The chemosensing studies of P-ONPs and R-ONPs revealed that P-ONPs (0.009 mM, size 20–30 nm) showed selective recognition behavior towards Pb(II) in aqueous medium among other alkali, alkaline earth and transition metal ions with the limit of detection as 1.56 µM via photoinduced electron transfer (PET)-OFF mechanism. In comparison to other metal ions, prominent peaks with higher intensity at 363 nm and 428 nm for Pb(II) were observed. R-ONPs have not shown any kind of selective chemosensing behavior. The judicially placed two urea bridges in P-ONPs are well suited for Pb(II) sensing.