Carbon nanodots (CNDs) derived from a green precursor, kidney beans, was synthesized with high yield via a facile pyrolysis technique. The CND material was easily modified through simple oxidative treatment with nitric acid, leading to a high density “self-passivated” water soluble form (wsCNDs). The synthesized wsCNDs have been extensively characterized by using various microscopic and spectroscopic techniques and were crystalline in nature. The highly carboxylated wsCNDs possessed tunable-photoluminescence emission behavior throughout the visible region of the spectrum, demonstrating their application for multicolor cellular imaging of HeLa cells. The tunable-photoluminescence properties of “self-passivated” wsCNDs make them a promising candidate as a probe in biological cell-imaging applications.