Engineering nitrogen-doped carbon quantum dots: investigating the role of dopant isomer configuration

Nitrogen-doped carbon quantum dots (N-CQDs) have emerged as versatile nanomaterials whose optical and interfacial properties are highly sensitive to dopant chemistry. However, the role of dopant regiochemistry in governing nitrogen incorporation pathways and subsequent structure-property relationships is not well understood. In this work, N-CQDs were synthesized via a one-step microwave-assisted bottom-up approach using citric acid as the carbon source and ortho-, meta, and para-phenylenediamine (o-PD, m-PD, and p-PD) as nitrogen dopants to systematically investigate the influence of dopant configuration. Electron microscopy revealed that o- and m-PD promote the formation of spherical quantum dots with average sizes of 7.63 ± 1.65 and 9.44 ± 2.31 nm, respectively, whereas p-PD favors the growth of micrometer-scale carbon sheets. X-ray photoelectron spectroscopy demonstrates that dopant configuration strongly affects nitrogen bonding states, with o-PD yielding predominantly graphitic nitrogen, m-PD and p-PD incorporating nitrogen mainly as amide. These structural differences lead to pronounced variations in optical behavior: o-PD-derived N-CQDs exhibit excitation-independent blue photoluminescence with a quantum yield of 24%, while m-PD - derived N-CQDs show reduced emission efficiency (16%), and p-PD - derived products are largely non-emissive. To assess the solid-state functionality of the synthesized N-CQDs, they were incorporated into poly(vinyl alcohol) (PVA) matrices. The results reveal that dopant-dependent morphology and surface chemistry significantly influence the crystallization behavior, mechanical performance, and UV-shielding efficiency of the resulting nanocomposites. In particular, N-CQDs derived from o-PD enable effective UV blocking while maintaining high transparency in the visible region and enhancing the mechanical properties of the PVA matrix. The PVA nanocomposite film containing o-PD - derived CQDs demonstrates the ability to block around 90% of the UV radiation at 400 nm. Moreover, incorporation of o-PD - derived CQDs modestly increases the elastic modulus (2286 MPa) and tensile strength (50.3 MPa) compared with pure PVA (1965 MPa and 37.6 MPa, respectively), while substantially enhancing the elongation at break (26.3% versus 19.6%). Overall, this study elucidates how dopant regiochemistry governs early-stage nucleation, nitrogen incorporation, and interfacial interactions, providing insights to rationally design functional CQD-based nanocomposites.