Nitrogen-doped carbon dots in Na₂B₄O₇ matrix: Excitation-dependent phosphorescence for multi-level encryption

To achieve room-temperature phosphorescence (RTP) emission, two effective approaches are constructing rigid microenvironments and enhancing intersystem crossing (ISC) efficiency. However, designing highly efficient RTP carbon dots (CDs) remains challenging. Herein, the RTP CDs were fabricated through a synergistic combination of heteroatom doping and matrix-assisted synthesis strategies. The obtained B, N-doped composite materials N-CDs@Na₂B₄O₇ exhibit long-lived and color-tunable RTP emission. N-doping with lone-pair electrons facilitates the ISC process, thereby enhancing RTP emission. The boron‑oxygen network in sodium tetraborate (Na₂B₄O₇) effectively shields RTP emission from environmental quenching. Characterizations including X-ray Photoelectron Spectroscopy (XPS) demonstrated CB and BN bonds, confirming successful encapsulation of N-CDs within the Na₂B₄O₇ matrix. The RTP lifetime was extended from 32.79 ms to 122.00 ms with the incorporation of the matrix. The wavelength-dependent RTP color variation under UV excitation enables its application in multi-level information encryption. The detection of berberine hydrochloride (BRH) was based on the inner filter effect (IFE) of the N-CDs@Na₂B₄O₇ composite. The combined strategy provides experimental evidence for RTP color regulation. The resulting CDs exhibit excitation-dependent RTP. The resulting CDs demonstrate excellent performance in multi-level information encryption applications. With tunable colors and long lifetimes, making them highly suitable for multi-level information encryption.