Optimization of Ionic Liquid-Mediated Red-Emission Carbon Dots and Their Imaging Application in Living Cells

Recently, fluorescent carbon dots (CDs) have emerged as novel carbon nanomaterials in terms of their unique optical properties, robust chemical inertness, and excellent biocompatibility. However, synthesis of efficient red-emission carbon dots (R-CDs) remains highly desirable for sensing applications, especially in the field of bioimaging, on account of their deeper tissue penetration and greater bioimaging capability than CDs with low-wavelength emission. Herein, novel R-CDs are synthesized by one-step solvothermal treatment of o-phenylenediamine and the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate. The as-prepared R-CDs exhibit remarkable acidophilic ability; the results show that the fluorescence intensity of R-CDs at 620 nm decreases with increasing pH from 1.5 to 6.0 and with a linear response to the extreme acidity range of 1.5–4.0. Importantly, it is nearly 4.7-fold more sensitive to pH response than phosphate-mediated R-CDs. The proposed pH sensor is further applied to monitor extreme pH fluctuations in Escherichia coli (E. coli). Compared to those CDs with low-wavelength emission, ionic liquid-mediated R-CDs have lower energy, stronger penetrability, and greater bioimaging capabilities, making them more preferable for biological and medical applications. To our knowledge, no R-CDs mediated by ionic liquids are reported until now, which would open a new chapter in the synthesis of R-CDs with ionic liquids.