Green Synthesis of Multicolor Emissive Nitrogen-Doped Carbon Dots for Bioimaging of Human Cancer Cells

In this work, banana peel (BP) was utilized as a green carbon precursor to synthesize carbon dots (CDs) through the single-step hydrothermal-carbonization method. The structural and optical properties of the resulting BP-CDs were investigated by various techniques. The transmission electron microscopy measurement of BP-CDs displayed uniform morphology with a quasi-spherical shape of 5 nm in size. The optical studies of BP-CDs revealed that BP-CDs emit excitation-dependent fluorescence emission behaviors (redshift) without any capping or passivation agent. The maximum emission was observed at an excitation wavelength of 340 nm, showing an acceptable quantum yield of 19%. The abundant functional groups such as nitrogen- (amine and amide) and oxygen-containing (carbonyl and hydroxyl) groups on the surface of the BP-CDs were confirmed from X-ray photoelectron spectroscopy, and Attenuated total reflection-Fourier transform infrared studies. These functional groups in BP-CDs are responsible for the negative zeta potential. Since the BP-CDs showed excellent long-term stability (120 days) and photostability (120 min). The biocompatibility of BP-CDs was examined by cytotoxicity studies on cancer cells and utilized as a multi-colored nano-probe for imaging human cancer cells. The aforesaid properties demonstrate that the BP-CDs can be applied to imaging human cells without further modifications.