Investigation On The Photoluminescence Properties Of CDs By Measurement Of The Fluorescence Anisotropy

Carbon dots(CDs),which are usually considered to be composed of two parts: a carbon core in the center and functional groups surrounding,have attracted much interest recently.CDs can be modified easily,therefore they always obtain high water-solubility,chemical inertness,stability against photobleaching,biocompatibility and low toxicity.Carbon dots have been widely investigated and extensively used since this kind of nanomaterial was first synthesized and characterized by purification of single-wall nanotube incidentally in 2004.Semiconductor quantum dots(QDs)have been widely investigated in biosensing and bioimaging due to their high intensity and tunable fluorescence emission for many years.However,the high toxicity of quantum dots limited their clinical applications.So,CDs are becoming a promising fluorescence nanomaterial and taking the place of the semiconductor quantum dots.In recent years,by controlling the carbon core size and surface chemical properties,great improvement has been made in the synthesis methods of CDs.The quantum yield(QY)of CDs has been improved by optimization of the precursor,the synthesis methods and the subsequent treating process.Although lots of efforts have been made to uncover the origin and mechanism of the photoluminescence(PL)of CDs,the PL mechanism remains debatable.The carbon core apparently plays an important role in the PL process of CD as the major part of it.But the influence on the carbon cores of the PL properties and the relationship between the cores and the fluorescence moieties in CDs are still not clear.We investigated the influence of carbon cores with different sizes on the rotational motion of fluorescence moieties to determine the relationship between carbon cores and fluorescence moieties.We used a one-step microwave synthesis method to obtain the CDs with citric acid(CA)and urea as precursor.By changing the carbonization time in the synthesis process,we can control the sizes of the carbon cores.The core sizes and the rotational correlation times(RCTs)are measure using transmission electron microscopy(TEM)and fluorescence anisotropy,respectively.The size of the rotating unit can be calculated by the rotational correlation times of the fluorescence moieties in CDs solution.The calculated average rotating unit radius of all the samples is about 0.9 nm,which is much smaller than the smallest carbon cores(about 4 nm),and the rotating unit sizes do not change with the carbon core sizes.It can be inferred that the carbon cores make no influence on the motion of the fluorescence moieties in CDs and the fluorescence moieties possess totally independent motion.Such relationship indicates that the connection between carbon cores and fluorescence moieties may be relatively week.So that the fluorescence moieties are neither tightly connected to the carbon cores nor exist in the solution freely.Our work is helpful for the investigation of the influence on the PL of CDs,and it also provides an useful method to investigate the fluorescence component of CDs.