Carbon Dots Based Metal Enhanced Fluorescence Mechanism And Fe(Ⅲ) Detection

Fluorescence analysis technology,as a technique for qualitative/quantitative analysis of substances by using fluorescence spectrum,is widely used in biochemical detection fields due to its high sensitivity and selectivity.Nowadays,the technical requirements for fluorescence analysis technology increased with the increasing complexity of the problems.The traditional fluorescent probes have low quantum yield,weak luminous intensity and poor biocompatibility,which limit their further applications in detection field.Therefore,the development of new fluorescent probes with high brightness,low light drift,and good biocompatibility needs to be studied urgently.The metal enhanced fluorescence（MEF）effect near metal nanostructure can effectively improve the detection sensitivity and reliability of the fluorescent probe by increasing their fluorescence intensity and reduce their fluorescence lifetime.In this thesis,silver nanoparticles（Ag NPs）with controllable particle sizes were synthesized using chemical reduction method.Meanwhile,fluorescent material carbon dots（CDs）with good luminous performance and low biological toxicity were prepared through wet oxidation method.Based on this,a glass slide-silver nanoparticles-carbon dots（GS-Ag NPs-CDs）composite structure was designed and prepared to study the effects of size of Ag NPs and the distance between Ag NPs and CDs on MEF.Moreover,based on the excellent optical properties of CDs,it was designed as a fluorescent probe for the Fe³⁺detection.The main work of this thesis includes the following aspects:（1）The CD aqueous solutions with excellent water solubility and luminescence properties were prepared by wet oxidation methods and a series of characterizations were performed.In TEM,the CDs were observed to be in a spherical shape with an average particle size of 3.28±0.57 nm and distributed uniformly.In the spectral characterization,the CDs exhibit strong absorption in the short-wave region in ultraviolet spectral,and the emission spectrum of the CDs varies with the wavelength of the excitation light,which is consistent with the optical characteristics of common CDs.The fluorescence quantum yield of CDs measured by the reference method was 10.68%,and the fluorescence lifetime of CDs measured by time-correlated single photo counting method was 3.46627 ns,which can further meet the luminescence requirements of subsequent experiments.（2）Five kinds of Ag NPs with increasing particle size were prepared by chemical reduction method and a series of characterizations were performed.In TEM,the Ag NPs were observed to be in the form of spheres and their average particle sizes were 9.82±2.08 nm,30.81±7.01 nm,35.41±6.88 nm,40.2±6.16 nm,and 52.07±5.69 nm respectively.In UV spectrum,the five kinds of Ag NPs showed the maximum absorption peaks which were red-shifted in order of particle size from small to large,and were named with the maximum absorption peaks of 391 nm,404 nm,422 nm,428 nm,and436 nm.（3）The GS-Ag NPs-CDs composite structure in a planar system was designed and prepared,and the effects of Ag NPs size on MEF effect was studied using this structure.Meanwhile,the Ag NPs were coated with polyvinylpyrrolidone（PVP）which was used as a spacer between the metal and the fluorophore to study the effect of distance on MEF effect.The fluorescence characterization of the composite structure was performed by fluorescence inverted microscope and the experimental results showed that as the distance between Ag NPs and CDs increases,the enhancement factor（EF）decreases.And as the particle size of Ag NPs increases,the EF increases first and then decreases.On the surface of the non-PVP-coated Ag NPs 428 nm,CDs can achieve the fluorescence enhancement with a maximum EF of 31.72.The MEF mechanism was analyzed by using the radiation rate interaction model of metal and fluorophore.It was found that the fluorescence lifetime of CDs on the surface of GS-Ag NPs-CDs structure has been attenuated to varying degrees which proves that the MEF under this experimental condition is caused by coupling of the fluorophore and metal particles to increase its radiation rate.（4）Based on the quenching effect of Fe³⁺on CDs fluorescence,CDs were designed as fluorescent probes for Fe³⁺detection.The probe can work effectively when the Fe³⁺concentration was in the range of 10-1000μM and the LOD was 1.3394μM,which has good sensitivity and reliability.Furthermore,the mechanism of quenching CDs fluorescence by Fe³⁺was analyzed using the Stern-Volmer equation.This quenching process is considered to be the result of the combined action of energy transfer caused by ion collision and coordination binding between fluorophore and quencher.