Synthesis Of Novel Water-soluble C/Si Quantum Dots And The Study Of Their Optical Performance

Semiconductor quantum dots have become attractive nanomaterials due to their excellent optical performance,biocompatibility and stability.However,the toxicity of heavy metal quantum dots limits its wide application.Therefore,the development of novel non-toxic quantum dots has become particularly important.In recent years,non-toxic C/Si quantum dots have come into the public's attention due to the low cost raw materials and excellent biocompatibility.In this study,we reported a novel N-doped graphene quantum dot?N-GQD?and a imidazole-functionalized silicon quantum dot?IF@Si QD?synthesized via hydrothermal method and explored their structural properties and optical properties.N-GQDs were successfully applied to the detection of the concentration of Hg²⁺ in real water.IF@Si QDs were applied to the determination of trace copper in fruits and vegetables.Based on the competitive coordination of Cu²⁺ between L-histidine?His?and IF@Si QDs,we designed a unique fluorescence ?off-on? response to His at low level and high level.Here we report a novel N-GQDs by using fumaric acid as the carbon source and ammonia as the nitrogen source via hydrothermal method.The characterizations revealed that the as-prepared N-GQDs have a uniform size of about 2.5 nm and a topographic height approximately 200-400 pm.The nitrogen atoms were successfully introduced into the graphene quantum dots.The paper reported a facile synthesis of IF@Si QDs via a simple hydrothermal method,in which N-trimethysilylimidazole and sodium citrate were used as the silicon precursor and reducing agent,respectively.Compared with sodium borohydride,ascorbic acid,bovine serum protein,cysteine and citric acid,the as-prepared silicon quantum dots offered the strongest fluorescence intensity when sodium citrate was used as the reducing agent and stabilizer for the synthesis.The reaction could complete within 2 h at 220?.The obtained silicon quantum dots showed good water-solubility with an average particle size of 2.6 nm,and result of infrared spectroscopic analysis verified the existence of free imidazole groups on the surface.These methods expand the choice of carbon source and silicon source which can be used for the designing and synthesis of functional quantum dots.N-GQDs synthesized via hydrothermal method have excellent optical performance and can be designed as Hg²⁺ fluorescence sensor.The as-prepared N-GQDs exhibit good water-solubility,excellent stability and bright luminescence with a high quantum yield of 29.8%.Furthermore,the as-prepared N-GQDs can combine with Hg²⁺ ions.RLS reveled that the formation of the complex structure N-GQDs-Hg²⁺ resulting in the aggregation of the quantum dots sharply quenched the PL of the N-GQDs.The fluorescence intensity linearly decreases with the increase of Hg²⁺ concentration in the range of 2.3×10^-6-1.8×10^-8 M with the detection limit as lower as 5.8×10^-9 M,so that a novel nanosensor of N-GQDs was performed for the detection of Hg²⁺ ions with high selectivity and sensitivity.N-GQDs were successfully applied to the detection of the concentration of Hg²⁺ in real water.Compared with citric acid and glucose,this kind of N-GQDs synthesized by our group has less functional groups,which are promising for application for biosensors with higher selectivity.IF@Si QDs synthesized via hydrothermal method have excellent optical performance and can be designed as Cu²⁺ fluorescence sensor.The resulted IF@Si QDs exhibits good water-solubility,high photo-stability,strong fluorescence with the quantum yield of 30.6%.By means of the investigation of the fluorescence quenching behavior of copper ions towards the silicon quantum dots at different temperatures,we found that the degree of fluorescence quenching increased with the increase of temperature.There results proved that the fluorescence decrease belongs to static quenching.Namely,the interaction of Cu²⁺ with imidazole groups on the surface of silicon quantum dots formed stable complex.In addition,the resonance light scattering analysis also showed that the fluorescence quenching process was accompanied by the agglomeration of particles.Based on the fluorescence quenching behavior of silicon quantum dots,we established a method for the fluorescent detection of Cu²⁺.When the concentration of Cu²⁺ was in the range of 0.04-2400 ?mol/L,the fluorescence intensity would linearly decrease with the increase of Cu²⁺ concentration,and the detection limit?S/N= 3?reached 1.29×10^-8 mol/L.The method provided high sensitivity,selectivity and reproducibility,and was successfully applied to the determination of trace copper in fruits and vegetables.The competitive coordination of Cu²⁺ between L-histidine?His?and IF@Si QD creates the unique fluorescence?off-on? response to His.Firstly,Cu²⁺ was firmly fixed at the surface of IF@Si QD by the coordination bond between Cu²⁺ and imidazole group to form stable Cum?IF@Si QD?n complex?m > 1,n > 1?.A low concentration of His was introduced to partly replace the IF@Si QD in the complex to produce His-Cu-IF@Si QD,leading to an obvious fluorescence quenching.Based on the fluorescence "off" behaviour,IF@Si QD was developed as a novel optical nanosensor for the detection of His at a low level.Its peak fluorescence intensity linearly decreases with the increase the concentration of His in the range of 1.6×10^-6-2×10^-4 M with the detection limit of 5.2 ×10^-7 M.Further,the His-Cu-IF@Si QD can be changed into non-fluorescent Cu?His?2 complex in the presence of a high concentration of His,accompanying the release of free IF@Si QDs and increase in the fluorescence intensity.Based on the fluorescence "on" behaviour,IF@Si QDs was used for the optical detection of His at a high level.Its peak fluorescence intensity linearly increases with the increase the concentration of His in the range of 2.8×10^-5-5×10^-3 M with the detection limit of 2.2×10^-3 M.The as-proposed method provides the advantage of sensitivity,selectivity and linear range,it has been successfully applied in the fluorescent detection of His in human urine and in medical amino acid injection.