Carbon quantum dots (CQDs) are fluorescent carbon nanomaterials with potential applications in sensing, optoelectronics, displays, anti-counterfeiting, and environmental technologies. Their optical properties can be adjusted by modifying the carbon structure and surface chemistry, particularly through defect states and the incorporation of heteroatoms. However, achieving predictable and continuous tuning of photoluminescence from a single carbon precursor remains difficult. Many previous approaches rely on different starting materials, synthesis routes, or post-synthetic treatments, making it challenging to determine how individual chemical modifications alter the electronic states responsible for light emission.
Waste polyamide offers an attractive starting material for addressing this challenge. Widely used in textiles, packaging, automotive components, fishing gear, and consumer products, polyamide contributes to substantial post-consumer plastic waste. Converting this material into functional carbon nanomaterials could therefore combine waste valorization with the production of high-value optical materials.
A research team comprising Dr. Christian Ebere Enyoh and Professor Emeritus Qingyue Wang of the Graduate School of Science and Engineering at Saitama University investigated whether the photoluminescence of carbon quantum dots could be systematically controlled using waste polyamide as a single carbon precursor. The researchers prepared eight chemically distinct CQD variants through dry pyrolysis and hydrothermal or solvothermal synthesis, progressively modifying their surface chemistry through oxidation and the introduction of boron-, nitrogen-, sulfur-, and phosphorus-containing functionalities. Using fluorescence spectroscopy, UV–visible spectroscopy, Fourier-transform infrared spectroscopy, optical transition-energy analysis, and colorimetric characterization, the team continuously shifted the emission wavelength from 308 nm in the ultraviolet region to 552 nm in the yellow-green region—a total tuning range of 244 nm—while using the same polyamide precursor throughout. In doing so, the researchers demonstrated that controlled defect-state engineering can systematically regulate the optical emission of waste-polyamide-derived CQDs. The study was published online in Journal of Luminescence on September 21, 2026, under the title "Defect state engineering in polyamide-derived carbon quantum dots enables continuous photoluminescence tuning."
The results revealed a progressive change in both surface chemistry and optical behavior across the eight CQD variants. As chemical modification proceeded, the effective optical transition energy decreased from 4.32 to 2.50 eV, in parallel with the shift toward longer-wavelength emission. The B,O co-functionalized CQDs achieved the highest photoluminescence quantum yield of 62.74%, while an S,N-containing variant reached 59.06%. The P,S,N-modified CQDs produced the longest-wavelength emission at 552 nm and achieved a color purity of 95.20%. Together, the spectroscopic and photophysical results support a progressive transition from predominantly carbon-core-associated emission toward increasing contributions from surface-defect and heteroatom-associated emissive states.
The researchers also introduced two empirical descriptors—the Relative Defect-State Depth Index (Dindex) and the Defect-State Engineering Index (DSEI)—to quantitatively compare how emissive states evolved across the CQD series. Dindex describes the relative energetic depth of an emissive state, while DSEI additionally incorporates electron–phonon coupling through the Huang–Rhys factor. Rather than representing direct measurements of atomic-scale defect density or structure, these parameters provide a comparative framework for linking chemical modification with experimentally observed changes in emission behavior.
"One of the important outcomes of this work is that we can follow how the emissive properties evolve step by step while keeping the carbon precursor unchanged," explained Dr. Enyoh. "By combining optical measurements with the Dindex and DSEI descriptors, we have introduced a way to quantitatively compare how surface and defect-state engineering influences emission energy and excited-state behavior. This could help move CQD design from trial-and-error optimization toward a more systematic approach." The work also connects optical-material design with the valorization of plastic waste. Because the emission wavelength, photoluminescence efficiency, bandwidth, and color purity can be influenced by different aspects of surface chemistry, the results suggest that waste-derived CQDs could eventually be tailored for different optical functions rather than treated as a single general-purpose fluorescent material.
Looking toward the next five to ten years, Professor Emeritus Wang anticipates that further advances in synthesis reproducibility, structural characterization, stability, and scale-up could broaden the practical significance of the approach. "Waste plastics are usually viewed as materials that must simply be collected and disposed of or recycled into lower-value products. Our findings point to another possibility: using their chemical structure as a resource for designing functional nanomaterials," noted Professor Emeritus Wang. "If these materials can be produced reproducibly and at larger scale, tunable waste-derived carbon quantum dots could contribute to applications such as optical sensing, luminescent coatings, displays, anti-counterfeiting technologies, and other light-emitting devices, while also creating new value from discarded polymers."
