The traditional pyrolysis of vegetable ghee leads to the fabrication of graphitic photoluminescent, water soluble carbon nano-onions (wsCNO) with tunable photoluminescence without using any metal catalyst. Simple oxidative treatment by nitric acid fabricated a high density “self-passivated” water soluble version. As-synthesized wsCNO possessed tunable photoluminescence behavior from the visible-to-near infrared region. Further small sized wsCNO separated from the bulk as-synthesized wsCNO via gel filtration achieved a highly fluorescent colored fraction, used for cell imaging (Escherichia coli and Pseudomonas putida) and selective, immediate sensing of glucose molecules based upon a simple fluorescence “turn-off”/“turn-on” technique.