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Carbon quantum dots (CQDs) are among the most feasible allotropes of carbon-based nanomaterials with unique characteristics of photoluminescence, bio-compatibility, and high stability. Herein, a green and eco-friendly approach has been propagated for the fabrication of CQDs from different biomass waste materials including sugarcane bagasse (SCB), garlic peels (GP), and taro peels (TP) by using ultrasonic-assisted wet-chemical-oxidation method. This top-down approach involves oxidation of the carbonized biomass wastes by H2O2. Another purpose of our work is to make a comparative study on the three CQDs produced from the three different biomass wastes. The properties of the fabricated CQDs were evaluated by using High Resolution-Transmission Electron Microscopy (HR-TEM), Fourier Transform-Infrared (FT-IR) spectroscopy, X-ray Diffraction (XRD), and X-ray Photoelectron spectroscopy (XPS), respectively. The CQDs showed the characteristic photo-physical behaviours as evident from the UV-visible and fluorescence (FL) spectroscopic analyses. The CQDs are found to be highly water soluble possessing strong blue-fluorescence under UV light with excellent quantum yield around 4-27%. The comparative study on the different physico-chemical properties of the three wastes biomass-derived CQDs are also discussed in the paper. The FL properties of CQDs derived from taro peels waste shows the best fluorescence quantum yield among the three and keeping in view of this, an on-off-on fluorescence nanoprobe was designed by using taro peels-derived CQDs (i.e. T-CQDs) and Eu3+ ion. The FL emission of T-CQDs was observed to be significantly quenched by Eu3+ leading to the formation of a CQDs-Eu3+ nanoprobe. The CQDs-Eu3+ nanoprobe was promisingly used for sensing of fluoride ions in water.