Environmental-friendly and biocompatible fluorescent carbon quantum dots (CQDs) show great potential for various applications. The functional groups on the surface of CQDs could dramatically influence the light absorption and emission characteristics of CQDs. A systematic investigation has been carried out by quantifying the oxygen based function groups present on the surface of activated CQDs (AC), dehydrated CQDS, and CQDs prepared by microwave synthesis to understand their role in the fluorescent emission characteristics. The samples were subjected to further treatments to achieve different levels of surface functionality with oxygen-containing functional groups such as carbonyl, carboxyl, phenol, and lactone. Various emissive pathways were identified by comparing the results of surface functionality and the deconvoluted emission spectra. Our results suggest that the energy gap of isolated sp 2 clusters on the surface of CQDs is the highest among the gaps related to various functional groups. The non-radiative transitions from this level can populate the mid level emissive states. As proof for this model, the integrated intensity of the high energy peak was found to decrease, and corresponding enhancements of low-energy peaks were observed as the surface functionality of carboxyl groups increased. Our results suggest that selective surface functionalization could control various emissive pathways in CQDs. These results are quite substantial for understanding the emission mechanism of CQDs. We have also studied the adsorption mechanism of methylene blue (MB) dye on the surface of CQDs at different contact times, pH, concentrations, and temperatures. In this work, we used Langmuir, Freundlich, pseudo-Ξrst, and second-order adsorption models for the analysis. Our results suggest the physisorption of MB on CQDs. Various kinetic and thermal parameters associated with MB adsorption were compared between CQDs prepared by different methods to identify the best CQDs for applications. The results are essential for applying CQDs, such as wastewater treatment. Further, a preliminary investigation of the electrical conductivity of the CQD-PANI polymer composite was done. Our results suggest that the increase in conductivity in the CQD-PANI composite is associated with electron transfer from PANI to CQDs. Our studies give important information to increase CQD's potential for applications.