This work demonstrated the synthesis and design of ultra-bright and ultra-small fluorescent nanoparticles, which were prepared by encapsulating 4-(diphenylamino)benzaldehyde (DPB) in silica cross-linked micellar nanoparticles (SCMNPs). The DPB-doped SCMNPs (DPB-SCMNPs) exhibited ultra-bright fluorescence in an aqueous medium that was 22 times brighter than that of free DPB molecules in an organic solvent. For the first time, density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations were used to confirm that the enhanced brightness of the DPB-SCMNPs was due to a hydrogen bond-induced mechanism. In addition, the 3D fluorescence spectra and the Commission Internationale de L'Eclairage (CIE) diagram were employed to determine the optical properties and emission colour of the DPB-SCMNPs. Moreover, the DPB-SCMNPs were water-soluble, monodisperse and ultra-small (similar to 12 nm) and should be robust and stable in aqueous media and biological systems. (C) 2018 Elsevier Inc. All rights reserved.