Controllable Synthesis,Photoluminescence Mechanisms,and Sensing Application Of Carbon Dots

Recently,carbon dots?CDs?have drawn more and more attention in the field of materials due to their outstanding properties and excellent performance in industrial production and biomedical field.As such,finding an efficient method to quickly synthesize a large scale of relatively high purity CDs is of great importance.In addition,to explore the complex and ever-changing mechanism of photoluminescence can theoretically guide our controllable synthesis of CDs for different purposes.Fluorescence is one of the most attractive features of CDs,and has been mainly used in the field of bio-imaging and optical sensors.Compared with the fluorescence properties,the electrical property of CDs is rarely noticed and studied.In this paper,a novel method is designed for synthesis of CDs,and the photoluminescence mechanism of CDs is further explored.The fluorescence characteristics of CDs have been employed to detect metal ions specifically,and used as bioimaging agent.At the same time,CDs are introduced into the emerging two-dimensional nano-materials,like molybdenum disulfide?MoS₂?and molybdenum trioxide?MoO₃?to improve their humidity sensing performance.As well a new type of resistive humidity sensor is obtained.The main research advances that have been made are as follows:?1?Herein,a facile and novel microwave method has been applied to prepare nitrogen doped CDs?N-CDs?within 8 min using L-glutamic acid as the sole reaction precursor in the solid phase condition.The as-prepared N-CDs with an average size of1.64 nm are well dispersed in aqueous solution.The photoluminescence of N-CDs is pH-sensitive and excitation-dependent.The N-CDs also show a strong blue fluorescence with relatively high fluorescent quantum yield of 41.20%,which remains stable even under high ionic strength.Since the surface is rich in oxygen-containing functional groups,N-CDs can be applied to selectively detect Fe³⁺with a detection limit of 10^-55 M.In addition,they are also used for cellular bioimaging because of their high fluorescent intensity and nearly zero cytotoxicity.The solid-phase microwave method seems to be an effective strategy to rapidly obtain high quality N-CDs and expands their applications in ion detection and cellular bioimaging.?2?Herein,we present a systematic study on the correlation between the formation process and photoluminescence mechanisms through the characterization and analysis of three states of N-CDs obtained by microwave irradiation.At low temperature of160°C,the small organic molecule polymer nanodots whose photoluminescence center is molecule state are obtained with superior quantum yield of about 51.61%.Increasing the reaction temperature up to 200°C,the intermediate transition products named carbon nanodots begin to appear.Prolonging the holding time,the typical carbon quantum dots with a special stable optical properties are finally generated,and their most photoluminescence arises from the carbon cores which are gained through the polymerization,dehydration,carbonation oforganic fluorescent molecules.Furthermore,N-CDs obtained at 160°C have been applied in detection of Fe³⁺in tap wateras well as animal and plant cell fluorescence imaging owing to their excellent water solubility and low cytotoxicity.Our exploration provides the theoretical basis for synthesis of CDs with different properties and purposes.In the near future,more high-quality CDs will be developed in order to better benefit the various fields of mankind.?3?Through a simple and convenient hydrothermal method,CDs with rich surface functional groups are modified to MoS₂ nanosheets,which effectively solves the problem of low sensitivity and long response time of the pure MoS₂ based humidity sensor.The charge transfer mechanism for the enhanced humidity response of CDs modified MoS₂ is proposed through the characterization of physical and chemical properties of the as prepared composites.The introduction of CDs is expected to enlarge the adsorption of water molecules by increasing the specific surface area and surface active sites of MoS₂ nanosheets.At the same time,a three-dimensional conductive network is jointly established by the chemisorbed water molecules,CDs and MoS₂ nanosheets,which ensure continuous transmission of charges in the low humidity environment.As a result,the response performance and the repeatability have been significantly improved in CDs-MoS₂ based humidity sensor.The response curve shows an excellent linear property in the range of 15?80%RH.This work gives potential applications for CDs modified two-dimensional nanomaterials to improve their humidity sensing performance.?4?In this paper,the CDs-MoO₃ composites are rapidly synthesized by microwave method.The experimental results confirm that the doping of CDs greatly enhances the humidity sensing performance of MoO₃.The effects of the amount of thiourea and CDs on the composition and properties of the products are systematically studied.The results showe that MoS₂ is obtained in the case of thiourea excess,but MoO₃ is gained on the condition of thiourea deficiency,and within a certain range,the humidity sensing performance increased with the the amount of CDs added.The response and recovery time of the best performance productbased humidity sensor at 70%RH are22 and 6 s.The response sensitivity is up to 3400%.