Theoretical Study On The Effect Of Metal Ions On The Photoluminescence Of Graphene Quantum Dots

Graphene quantum dots are a new type of fluorescent nano-carbon material.Because of their low toxicity,controllable photoluminescence,good light stability,low preparation cost,and good biocompatibility,graphene quantum dots have become a new generation of sensing platforms for metal ion detection.However,due to the complexity of graphene quantum dot structure and the diversity of its interaction with metal ions,there are still some problems such as unclear mechanism,which seriously limit the application of graphene quantum dots in metal ion fluorescence detection.Based on this,this paper uses the quantum chemical calculation method to study the effect of metal ions on the photoluminescence of graphene quantum dots,and compares it with the experimental results to study the microscopic interaction mechanism between metal ions and graphene quantum dots from the molecular level,reveal the influence of metal ions on the photoluminescence of graphene quantum dots,and provide better guidance for the optical application of graphene quantum dots.The main research results include the following aspects:（1）The investigation of the effects of 12 metal cations such as Li⁺,Na⁺,K⁺,Mg²⁺,Ca²⁺,Zn²⁺,Cu²⁺,Al³⁺,Fe³⁺,Cr³⁺,In³⁺and Ga³⁺on graphene quantum photoluminescence.The experimental results show that among the 12 metal ions,only three kinds of Cu²⁺,Fe³⁺,and Cr³⁺ions quench the fluorescence of graphene quantum dots,and it is found that Al³⁺,In³⁺,Ga³⁺can redshift the fluorescence of quenched graphene quantum dots.Through the quantum chemical calculation of the corresponding system,we found that there is strong adsorption and obvious electron conversion between Cu²⁺,Fe³⁺,Cr³⁺,and the graphene quantum dot model(C₂₄H₁₂).The corresponding adsorption energies are-289.67 kcal/mol,-693.52 kcal/mol and-636.35 kcal/mol,respectively.The electron transfer values are Cu²⁺0.55e,Fe³⁺2.62e,and Cr³⁺2.91e,respectively,and all the three systems transfer electrons from graphene quantum dots to metal ions.For the oxygen-containing graphene quantum dot model(C₂₄H₁₁O₁₂),Cu²⁺,Fe³⁺,and Cr³⁺can interact with the oxygen-containing functional groups on the graphene quantum dot to form a stable chemical bond,thus destroying the conjugation effect on the surface of the aromatic ring.This is the main reason for the fluorescence quenching of graphene quantum dots caused by three metal ions.After the interaction of Al³⁺,In³⁺,Ga³⁺ions with quenched graphene quantum dots,Cu²⁺,Fe³⁺,Cr³⁺ions were removed from the surface of graphene quantum dots,and the conjugation ofπon the surface of aromatic rings was restored,thus the fluorescence of graphene quantum dots was restored.（2）The exploration of the effects of metal ions with different valence states of the same element(Fe³⁺/Fe²⁺,Cr³⁺/Cr²⁺,and Cu²⁺/Cu⁺)on the photoluminescence of graphene quantum dots.The experimental results show that the high-valence and low-valence metal ions have different fluorescence responses to graphene quantum dots.The theoretical calculation results show that the adsorption energies of Fe³⁺,Cr³⁺,Cu²⁺,and graphene quantum dots are-705.66 kcal/mol,-717.10 kcal/mol,and-317.53 kcal/mol,respectively.The adsorption energies of low valence Fe²⁺,Cr²⁺,Cu⁺and graphene quantum dots are-309.20 kcal/mol,-234.13 kcal/mol and-58.35 kcal/mol,respectively.The results of charge transfer analysis show that electrons are transferred from graphene quantum dots to metal ions.The electron transfer of Fe³⁺,Cr³⁺,Cu²⁺,and graphene quantum dots is 2.12 e,2.15 e,and 1.25 e,respectively,and the charge transfer of high valence metal ions is 1 e higher than that of low valence metal ions.The results of natural orbit analysis of adsorption energy show that the difference in adsorption energy between high and low valence systems is mainly charge transfer.The orbital contribution analysis shows that the high valence ions make a great contribution to the graphene quantum dot complexes.The difference in the interaction between these different valence metal ions and graphene quantum dots results in their different fluorescence responses to graphene quantum dots.（3）To investigation of the effect of Fe³⁺/Pb²⁺/Ag⁺on photoluminescence of graphene quantum dots with defects.C₉₆H₂₄(C₉₆)is used as the template of graphene quantum dots to design the defect structure,and its structure,energy,and optical properties are calculated.Four types of defects are considered,namely,topology defects(55-77-C₉₆),single vacancy defects(5-9-C₉₆),multi-vacancy defects(5-8-5-C₉₆,555-777-C₉₆,5555-6-7777-C₉₆),and doped external atoms(B-C₉₆,N-C₉₆,O-C₉₆).The theoretical results show that the maximum absorption wavelength of C₉₆is 593.54 nm,5-9-C₉₆,and the maximum absorption wavelength of O-C₉₆defects is 591.05 nm and 590.94 nm,respectively.The maximum absorption wavelength of 5-8-5-C₉₆,555-777-C₉₆defect is blue-shifted,which is 554.59 nm and 504.83 nm,respectively.The maximum absorption wavelengths of 55-77-C₉₆,5555-6-7777-C₉₆,B-C₉₆,and N-C₉₆defects are red-shifted,which are 606.35 nm,620.01 nm,615.93 nm,and 650.40 nm,respectively.The electron density diagram shows that when heteroatoms are introduced into graphene quantum dots,electrons transfer from aromatic rings to heteroatoms.The maximum absorption wavelengths of B-Ag⁺-C₉₆,B-Pb²⁺-C₉₆,and B-Fe³⁺-C₉₆are 615.64 nm,669.43 nm,and964.45 nm,respectively.With the increase of charge of Fe³⁺/Pb²⁺/Ag⁺,the absorption wavelength of graphene quantum dots are red-shifted.The above research results show that it is feasible to design the quantum optical properties of graphene by adjusting metal ions and defects.