Comparative Investigation of Graphene Quantum Dots and Graphitic-phase C3N4 Nanosheets in Terms of Photoluminescence Properties and Biomedical Imaging

Abstract

In recent years, fluorescent nanomaterials have attracted great attention due to their broad applications. Extensive efforts have been made on the development of novel metal-free fluorescent nanomaterials. Among these nanomaterials, graphene quantum dots (GQDs) have demonstrated chemical inertness, photostability, low cytotoxicity and favorable biocompatibility. Due to their excellent quantum confinement effect and edge effect, GQDs are usually characterized with distinct photoluminescence (PL) behaviors for biomedical imaging. Similar to GQDs, graphitic-phase C3N4 (g-C3N4) nanosheets have also exhibited high biocompatibility, low toxicity, and unique optical properties. Both GQDs and g-C3N4 nanosheets have been extensively investigated as potential biomedical imaging candidates. However, there are very few systematic comparative studies on GQDs and g-C3N4 nanosheets in terms of photoluminescence properties and biological imaging characteristics. Therefore, in this study, we mainly focus on investigating these differences. Highly water-dispersible GQDs and g-C3N4 nanosheets were prepared by a similar top-down process, chemically oxidizing thermally exfoliated graphite oxide and bulk g-C3N4 with concentrated nitric acid, respectively. UV-Vis absorption and photoluminescence spectral analyses were used to compare their optical properties. Compared to GQDs, g-C3N4 nanosheets demonstrate higher photoluminescence quantum yield. Cytotoxicity measurements indicate that both GQDs and g-C3N4 nanosheets are of low cytotoxicity with favorable biocompatibility. The remarkable performances of GQDs and g-C3N4 nanosheets in biomedical imaging are demonstrated by a simple incubation study with HepG2 cells. All these studies suggest that both GQDs and g-C3N4 nanosheets prepared by the above method have low cytotoxicity and are greatly promising for biomedical applications, such as in vitro and in vivo imaging studies.