Preparation Of Fluorescence Carbon Quantum Dots As Sensor For Hg²⁺ And Co²⁺

Carbon dots（CDs）are fluorescent carbon nanomaterials with small particle size,excellent photostability,and adjustable fluorescence emission wavelength.CDs-based fluorescence sensors with features such as easy operation,high efficiency,and rapid detection,has shown their unique application potential.Hg²⁺,one of the most toxic heavy metal pollutants,is widely present in the form of stable inorganic salts and exists in the natural environment.It is highly toxic and difficult to degrade and poses a great threat to the environment and human health.Cobalt is well-known as a trace element in nature,and is benefical to the organism.But excessive intake of this element can lead to asthma,rhinitis,allergic dermatitis vasodilation and cardiomyopathy in human and animals.Therefore,finding efficient,rapid,and simple methods is significant for Hg²⁺and Co²⁺determination.In this paper,three kinds of CDs were prepared by hydrothermal method and used for the detection of Hg²⁺and Co²⁺.The details are summarized as follows:（1）A green hydrothermal synthetic method for producing nitrogen-doped carbon dots（NCDs-1）by using ammonium citrate and urea is introduced.The obtained NCDs-1 were characterized by transmission electron microscopy（TEM）,fourier transform infrared spectra（FTIR）,UV-vis absorption and fluorescence spectra.These results showed that the prepared NCDs-1 were spherical with a size of about 3.5nm,emitting strong and stable blue fluorescence when excited at 352 nm.It was noting that the NCDs enable sensitive and selective determination of Hg²⁺with a linear range of 0.05μM-25μM based on static quenching.The limit of detection（LOD）was 0.047μM.（2）Fluorescent nitrogen-doped carbon dots（NCDs-2）with an average size of about 4.28 nm were hydrothermally synthesized using arcylic acid and ethylenediamine.When excited at 350 nm,the NCDs-2showed a strong emission peak centered at 425 nm,corresponding to blue fluorescence.Interestingly,for NCDs-2,significant decrease in fluorescence was observed upon addition of Co²⁺.On the basis of this particular phenomenon,the synthesized NCDs-2 were demonstrated as an effective fluorescence probe for the selective and quantitative analysis of low concentration Co²⁺in a linear range of 1μM-60μM.LOD is 0.25μM.Further detailed investigations on the interactions of NCDs-2 with Co²⁺as quencher by using various techniques（UV-vis absorption and time-resolved fluorescence techniques）confirmed that the fluorescence quenching is mainly induced by the interaction of between Co²⁺and some functional groups of NCDs-2.（3）Aqueous CDs were prepared using natural onion as a new carbon source by one-pot hydrothermal method.Through hydrothermal carbonization of onion fragment,fluorescence CDs were prepared and then modified with polyethylenimine（PEI）,thus obtaining the PEI-CDs.The as-prepared PEI-CDs exhibited bright and stable blue fluorescence with a maximum fluorescent intensity at 462 nm under an excitation wavelength of 340 nm.Upon addition of Co²⁺into the solution of PEI-CDs,the fluorescence of PEI-CDs decreased sharply.On the basis of this particular phenonmenon,a rapid,effective fluorescence Co²⁺sensor with a low LOD of 0.042μM was constructed.