Carbon dots (CDs), as a rising star among fluorescent nanomaterials with excellent optical properties and fascinating dual-emission characteristics, have attracted increasing attention in sensing, bio-imaging, drug delivery, and so on. The synthesis of dual-emission CDs (DE-CDs) and the establishment of ratiometric fluorescence sensors can effectively diminish background interference and provide more accurate results than single-emission CDs. Although DE-CDs have generated increased attention in many fields, the review articles about DE-CDs are still insufficient. Therefore, we summarized the latest results and prepared this review. This review first provides an overview of the primary synthesis route and commonly used precursors in DE-CDs synthesis. Then, the photoluminescence mechanism behind the dual-emission phenomenon was discussed. Thirdly, the application of DE-CDs in metal cation detection, food safety analysis, biosensing, cell imaging, and optoelectronic devices has been extensively discussed. Finally, the main challenges and prospects for further development are presented. This review presents the latest research progress of DE-CDs synthesis and its application in ratiometric sensing; hopefully, it can help and encourage researchers to overcome existing challenges and broaden the area of DE-CDs research.

Apurinic/apyrimidinic endonuclease 1 (APE1) selectively cleaves the apurinic/apyrimidinic site (AP site) in DNA, playing a critical role in base excision repair and genomic stability maintenance. Aberrant APE1 expression has been linked to numerous diseases, including cardiovascular disorders, neurological conditions, and various cancers. However, existing methods for detecting trace levels of APE1 remain suboptimal for certain applications. To address this limitation, we developed an innovative biosensing platform for ultrasensitive APE1 detection by integrating APE1-specific site recognition with hybridization chain reaction (HCR)-based signal amplification, enabling enzyme- and label-free bioassays. In this system, APE1 recognizes and cleaves the AP site-containing hairpin (HP) probe, releasing a single-stranded HCR initiator that triggers cascaded HCR amplification. Owing to the high efficiency of HCR, this method achieves ultrahigh sensitivity, with a calculated detection limit of 1.0 × 10⁻⁸ U/mL. Furthermore, the biosensor demonstrates robust performance in cell lysates and is applicable for screening and evaluating APE1 inhibitors.