Acid-mediated synthesis coupled with liquid–liquid extraction separation for obtaining red and orange double-color carbon dots: Application for pH, water sensing and cell-imaging

Abstract

Owing to their various advantages, such as low photodamage and high tissue penetration, carbon dots (CDs) with long emission wavelength have gained much attention, and are used to compensate for blue to green fluorescent CDs. However, their synthesis has been challenging. In the present study, an acid-mediated synthesis method was developed for the preparation of long-wavelength CDs by using N-phenyl-o-phenylenediamine as a precursor, phosphoric acid as an acidity regulator. According to the principle of similarity compatibility, liquid–liquid extraction was further adopted to separate red CDs (R-CDs) and orange double-color CDs (O-CDs) from long-wavelength CDs mixture for the first time. By optimizing the factors that affect quantum yield (QY), R-CDs with a QY of 4.16% and O-CDs with a QY of 60.80% were obtained. Structural analysis revealed that both types of CDs had a graphite-like structure with an abundance of various groups on their surface. Their fluorescence colors were quite different because of differences in the nature of these functional groups (e.g. contents of N and P). To explore the application value of two kinds of CDs, the potential use of R-CDs, for pH sensing and O-CDs for water content detection in organic solvents was studied systematically. The results revealed that R-CDs was acid sensitive and can be used to detect different pH values. While O-CDs can be used for the detection of various water content in different organic solvents including ethyl acetate, ethanol, methanol, DMSO, DMF and acetone. Finally, due to the low toxicity and good biocompatibility, the potential of R-CDs as probe for cell imaging was explored preliminarily. Co-localization experiments with commercial dye Lyso-tracker Red showed that R-CDs was localized in lysozyme and can be used for lysosomal imaging, further proving that R-CDs is a potential lysosomal probe.