Poly(lactic--glycolic acid) (PLGA)-based nanoparticles featuring covalently incorporated 1,8-naphthalimide fluorophores were developed through DFT/TD-DFT-guided molecular engineering. Systematic variation of 4-position substituents and carbon spacer lengths established clear structure-property relationships governing spectral-luminescent characteristics and conjugate stability. Computational modeling accurately predicted experimental absorption/emission features. The optimized PLGA-fluorophore conjugates yielded nanoparticles with high aqueous fluorescence, excellent colloidal stability, and approximately two-fold higher photostability than PLGA-Cy5 under the tested imaging conditions. Live-cell confocal microscopy (405 nm excitation) demonstrated strong emission and homogeneous distribution in 4T1/HeLa cells, confirming suitability for cellular imaging with high cell viability. This covalent PLGA labeling platform establishes a quantifiable foundation for advanced fluorescent nanomaterials in cellular imaging applications. While these results establish a robust platform for cellular imaging applications, extended validation remains a goal for future work.