Luminescent nanomaterials are encouraging scaffolds for diverse applications such as chemical sensors and biosensors, imaging, drug delivery, diagnostics, catalysis, energy, photonics, medicine, and so on. Carbon dots (CDs) are a new class of luminescent carbonaceous nanomaterial that have appeared recently and reaped tremendous scientific interest. Herein, we have exploited a simple approach to prepare tuneable and highly fluorescent CDs via surface functionalization. The successful synthesis of CDs is manifested from several investigations like high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The CDs exhibit excellent water solubility and with increasing nitrogen content fluorescence quantum yield increases whereas cell toxicity decreases. The CD synthesized at high temperature (180 degrees C) shows very high quantum yield (more than 56%). The tuneable optical properties of CDs are systematically studied using UV-vis and fluorescence spectroscopy. The cell viability evaluation and in vitro imaging study reveals that the synthesized CDs can be employed as a potential fluorescent probe for bio-imaging without further modification. (C) 2017 Elsevier Inc. All rights reserved.