The two-dimensional quantum dots (2D-QDs) have been developed significantly in the past decades. The 2D-QDs could be used in bioimaging, biosensing, drug/gene delivery, and photodynamic/photothermal therapy. The potential applications in biology receive increasing attention, which makes them the novel and emerging candidates in biomaterial research fields. In this context, we discuss a variety of 2D-QDs with different physical and chemical properties. We focuse on the latest synthesis progress and recent applications in biotechnological, and biomedical applications of the 2D-QDs and we also evaluate the challenges and prospects in this field.

The dual-emissive N, S co-doped carbon dots (N, S-CDs) with a long emission wavelength were synthesized via solvothermal method. The N, S-CDs possess relatively high photoluminescence (PL) quantum yield (QY) (35.7%) towards near-infrared fluorescent peak up to 648 nm. With the advanced characterization techniques including X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), etc. It is found that the doped N, S elements play an important role in the formation of high QY CDs. The N, S-CDs exist distinct pH-sensitive feature with reversible fluorescence in a good linear relationship with pH values in the range of 1.0–13.0. What is more, N, S-CDs can be used as an ultrasensitive Ag⁺ probe sensor with the resolution up to 0.4 μM. This finding will expand the application of as prepared N, S-CDs in sensing and environmental fields.

Heteroatom-doped carbon dots (CDs) with a high photoluminescent quantum yield (PLQY) have recently attracted attention due to their applications in chemical sensors, photocatalysis, bioimaging, and drug delivery. Nitrogen and phosphorus are in close proximity to carbon in the periodic table and are key tracking elements in the field of biomedical imaging. These two elements alter the optical and electronic properties of CDs and help improve the fundamental understanding of their PLQY. This can also lead to multifunctional usage in photoimaging and photothermal therapy. However, most PLQYs resulting from the synthesis of P-doped CDs are currently below 50%. These CDs have limited usefulness in the fields of bioimaging and drug delivery. In this study, a single-step, high-efficiency hydrothermal method was applied to synthesize nitrogen and phosphorous-doped carbon dots ((N, P)-CDs) with a PLQY of up to 53.8% with independent emission behavior. Moreover, the CDs presented high monodispersity, robust excitation-independent luminescence, and stability over a large pH range. Spectroscopic investigations indicated that the PLQY of the (N, P)-CDs was primarily due to the addition of P and the passivation effect of the oxidized surface. The excellent fluorescence properties of (N, P)-CDs can be effectively and selectively quenched by Hg²⁺ ions. Such systems show a linear response in the 0–900 nM concentration range with a short response time, indicating their potential for applications in the fields of chemistry and biology.