Fluorescent nanoparticles (FNPs) are obtained in water by self-assembly from a polymeric ionic liquid, fluorescent carboxylate moiety, and a surfactant through two main supramolecular interactions, that is, ionic bonds and hydrophobic/hydrophilic interactions. The hydrophobicity of the surfactant is tunable and a highly hydrophobic surfactant increases the fluorescence intensity and stability of the FNPs. The fluorescence of the FNPs is sensitive to a quenching effect by various ions with high selectivity, and consequently, they may be used as sensors. The self-assembly approach used to generate the FNPs is considerably simpler than other methods based on more challenging synthetic methods and the flexibility of the approach should allow a wide and diverse range of FNPs to be prepared with specific sensor applications.