Physicochemical properties of in situ microwave-assisted germanium oxide modified nitrogen-doped carbon dots

Preparation of carbon dots (CDs) and nitrogen-doped carbon dots (NCDs) with germanium oxide (GeO2/GeO) using a facile one-step microwave-assisted synthesis procedure is presented. Iso-ascorbic acid, urea and Ge-132 were used as carbon, nitrogen and germanium sources, respectively. Studies have shown that the CDs, NCDs and GeO2/GeO nanoparticles individually have excellent qualities for bio-imaging applications including photoluminescence, biocompatibility, good stability and low toxicity. While CDs have been thoroughly explored in literature, CDs with germanium oxide on their surface are an avenue that has scarce information. Hence the synthesis and physicochemical characterizations of CDs- /NCDs-GeO2/GeO composites were studied using different microscopic and spectroscopic techniques to explore their compatibility for bio-imaging applications. In the presence of Ge, the approximate particle sizes of both the CDs and NCDs increased from 2.5 nm to 5.8 nm and the spherical or quasi-spherical shape was maintained. The X-ray photoelectron spectroscopy survey spectrum showed that the surface of Iso-N-Ge-CDs contained nitrogen and germanium, indicating the successful synthesis of nitrogen-doped carbon dots (NCDs) with germanium oxide (GeO2/GeO). Photoluminescence emissions of longer wavelengths suitable for cell imaging were observed for Iso-N-Ge-CDs (350 nm) and Iso-Ge-CDs (430 nm) and the Iso-N-Ge-CDs are able to fluoresce under the red and blue wavelengths. The Ge-NCDs demonstrated excitation-dependent emission wavelength behaviour, pH sensitivity and Fe3+ sensitivity which may be used as a probe for Fe3+ metal ions and intracellular pH measurements.