Detection Of Heavy Metal Ions By Boron-nitrogen Co-doped Graphene Quantum Dots And Their Composite

Graphene quantum dots?GQDs?,as a quasi-zero-dimensional carbon nanomaterial with a lateral dimension below 100 nm,has become the focus of attention in the large family of carbon materials.Because of its unique photoelectric properties,biological low toxicity,good water solubility,high light stability,environmental protection and other outstanding advantages.It has great application prospects in the fields of establishing chemical sensors,biological imaging,photoelectric devices and so on.We know that there are a large number of defects on the surface of the original GQDs structure,and the fluorescence quantum yield?QY?is very low.In order to change this situation and expand the application of GQDs in the field of sensors.By introducing heteroatoms boron and nitrogen and successfully doping them into the framework of GQDs,we can completely change the electronic properties of the original GQDs,significantly improve the surface defects of GQDs,low fluorescence QY,less active sites and a series of other problems.In this paper,we mainly focus on the synthesis of boron-nitrogen co-doped graphene quantum dots?B,N-GQDs?by"bottom-up"and"top-down"methods.And the synthesis of B,N-GQDs and natural polysaccharide chitosan?CS?composite.Their structural characteristics,optical properties,electrochemical properties and their application in the detection of heavy metal ions were studied in detail.In addition,all the research contents in this paper have strong novelty,which have played a role in attracting other researchers in the establishment of sensors based on B,N-GQDs and their composite for the detection of heavy metal ions.?1?B,N-GQDs was synthesized by"bottom-up"method and designed as a fluorescent sensor which can successfully detect heavy metal Hg²⁺in aqueous media.B,N-GQDs was synthesized by one-pot method using citric acid?CA?,boric acid and urea as carbon source,boron source and nitrogen source,respectively.By testing the structural characterization and optical properties of B,N-GQDs,it was known that it had a high photoluminescence quantum yield?PLQY?with a value of 17.16%and showed bright blue fluorescence.And it had a narrow size distribution and an average diameter of 2.0 nm,which was a highly crystalline structure.Establishing a fluorescent sensor based on B,N-GQDs can detect metal ions by monitoring its fluorescence intensity.The experimental results showed that only the heavy metal Hg²⁺can quench the fluorescence of B,N-GQDs,while other metal ions had no obvious phenomenon.Therefore,B,N-GQDs fluorescent sensor can realize the specific detection of heavy metal Hg²⁺.The B,N-GQDs fluorescent sensor can respond quickly in a wide range of Hg²⁺concentration from 1 to 300?M,the best linear relationship was from 0 to 4?M,and the minimum detection limit was as low as 4.3 nM.In addition,B,N-GQDs can also be successfully applied to the detection of heavy metal Hg²⁺in real water samples.?2?Using the"top-down"method,B,N-GQDs was successfully synthesized by one-pot hydrothermal method using graphene oxide?GO?as a carbon precursor,boric acid,and ammonia as boron and nitrogen sources.It showed blue fluorescence,PLQY was 5.13%,with good dispersion and uniform particle size,and the average diameter was about 3.8 nm.Establishing a fluorescent sensor based on B,N-GQDs can realize the detection of heavy metal Hg²⁺,and also detect the metal ions Fe³⁺and Cu²⁺,which can eliminate their interference after adding chelating agents.During the detection of heavy metal Hg²⁺,it was found that the fluorescence quenching efficiency of B,N-GQDs increased with the increase of Hg²⁺concentration.It had a good linear relationship in the concentration range of 0.2-1?M,and the minimum detection limit can be as low as 6.4 nM.Finally,we also successfully applied it to the detection of heavy metal Hg²⁺in real water samples.?3?Based on the B,N-GQDs/CS composite synthesized from B,N-GQDs and chitosan?CS?,an electrochemical sensor was established to detect heavy metal Cu²⁺.Their bonding can be seen clearly by X-ray photoelectron spectroscopy?XPS?and Fourier transform infrared spectroscopy?FT-IR?.Moreover,the transmission electron microscope?TEM?characterization can more clearly showed the lattice fringes of B,N-GQDs and the morphology of CS,which can further confirmed that the B,N-GQDs/CS composite was successfully synthesized.The electrochemical sensor based on B,N-GQDs/CS composite can be used to detect heavy metal Cu²⁺specifically,rapidly and sensitively.When the Cu²⁺concentration range was between 0.5 and 30?M,the peak current value increased with the increase of Cu²⁺concentration.And it had a good linear relationship,the calculated minimum detection limit was 52 nM.As an electrode material for the establishment of electrochemical sensor,the composite had better performance than the single electrode material of B,N-GQDs or CS.