In this thesis, 4 type of new, robust, fluorescent, and functionated carbon quantum dots (CQDs C-dots or CDs) are prepared by hydrothermal method from various raw materials. Owing to their excellent aqueous dispersibility, functionality, biocompatibility and ease of preparation, as-obtained CQDs are as attractive alternatives to more traditional ‘quantum dots’ in applications of metal ion, bioimaging, and solar energy conversion. To prepare the CDs with acid-evoked fluorescence enhancement under acidic conditions, robust and highly fluorescent N-doped CDs are obtained from a hybrid source, alginic acid (AA) and ethanediamine (EDA). During a hydrothermal process, the raw materials are propelled to form nano-size particles; these resultant CDs possess desirable functional groups on the particle surface. We have further investigated their optical performance under various pH conditions as well as their capacity for sensing metal ions. The N-doped CDs especially exhibit remarkable acid-evoked fluorescence enhancement under acidic conditions. Finally, the as-prepared CDs are tested for their ability to detect of Fe3+ in acidic pure water and urban river water media, the fluorescence-quenching mechanism and recovery properties of the CDs/Fe3+ mixture are also investigated. As one of the most promising fluorometric nanosensors for metal ion sensing, C-dots have been developed rapidly in recent years. However, it is still a thorny problem to shield the mutual interference of two metal ions towards C-dots, e.g., Hg2+ and Fe3+ ions. In the thesis, we proposed the rational routes for the unique interaction between C-dots and Hg2+ or Fe3+. The selectivity of the as-obtained label-free C-dots can be effectively switched by P3O105- and SCN-. The resultant C-dots possess superior photo-luminescence intensity stability in a wide pH range or in certain redox existence, which are highly preferred in terms of practical metal ions detection. Moreover, the quenching-recovery-regeneration performances and quenching mechanisms were also investigated. Based on this stable and recoverable sensing platform, this in-depth study for dispelling molestation of Hg2+ and Fe3+ ions may open up a new paradigm for developing the potential detection of multi-metal ions using one C-dots with switchable selectivity.Furthermore, as-obtained a novel, nitrogenous, biocompatible fluorescent C-dots fabricated from citric acid (CA) and melamine are also manifested desirable imaging ability especially nucleus staining property towards HUVECs, the results suggested that the as-obtained C-dots could be expected for various practical applications in biomedical fields.We also report a new and cheaply carrot as biomaterial sources for preparing high quality CDs instead of semi-conductive quantum dots for bioimaging application. The as-derived CDs possessing down and up-conversion photoluminescence feature was obtained from carrot juice by the common used hydrothermal treatment. The corresponded physiochemical and optical properties were investigated by electron microscopy, fluorescent spectrometers and other spectroscopic methods. The surfaces of obtained CDs were highly covered with hydroxyl groups and nitrogen groups without further modification. The quantum yield of as-obtained CDs was higher up to 5.16%. The cell viability of HaCaT cell against purified CDs aqueous solution was higher than 85% even at higher concentration (700 μg mL−1) after 24 h incubation. Finally, CDs cultured cells exhibited distinguished blue, green and red colors respectively during in-vitro imaging when exciting by three wavelength lasers under a confocal microscope. The excellent optical properties, biocompatibility, low cytotoxicity as well as good cellular imaging capability endowed the carrot juice derived CDs with promising candidate in biomedical applications.Owing to a wide range of light absorption tailing into the visible region, CDs are beneficial for their applications in solar energy conversion including solar cells (SCs) and solar fuels. In this thesis, as-prepared CQDs have extraordinary optical and electrical features such as absorption of ultraviolet range and effective interface for charge separation and transport in active layer. This makes them attractive materials for applications in photovoltaic devices (PV). CQDs possessed important roles in charge extraction in the PV structures. C-dots are synthesized by a simple method and used to insert in active layer of polymer solar cells (PSCs). C-dots are demonstrated that improve charge transport properties of inverted polymer solar cells (iPSCs). As a result, iPSCs with C-dots showed enhancement of more than 12% compared with that of the contrast device in PCE.