Pulsed Laser Assisted Synthesis Of Carbon Quantum Dots And Gadolinium Carbide/Carbon Shell Dots,Crystalstructure And Enhanced Magnetic Resonance Properties

Nanotechnology has been integrated in many aspect of living cultures,including physics,energy,environment and biology.As for luminescent quantum dots?QDs?,photoluminescent carbon quantum dots?CDs?is environmental materials.Hence there is great desire for the synthesis of photoluminescent CDs via chemical and physical processes,which would explode extreme endeavor after fullerene,carbon nanotube and graphene.The luminescent QDs also have potential applications in biology and biomedicine.To avoid toxicity of semiconductor QDs,core-shell structures have been widely designed.However,the crystal shell is hard to be extended over the whole QDs due to small sizes and the lipoid packaging may be vitiated in acid/alkali solution due to week bonding.Thus stable shells over QDs are still challenges.Nowadays,cancer and tumor are two major public diseases due to its high death rate over the world.In clinical trials,enhancement agents have been recommended for the diagnosis of these diseases under magnetic resonance imaging?MRI?.The gadolinium chelates are a kind of such enhancement agents,on the other hand,may bring potential toxicities due to gadolinium accumulation.Compared with these chelates,the nontoxicity magnetic QDs would achieve ultimate progress in MRI enhancement,inject dose and biocompatibility.With these aims,we start our work based on the synthesis of carbon related QDs under pulsed laser irradiation.The main results are as follows.1.The KrF excimer pulsed laser,operated at 248 nm with energy density of 170mJ/pulse and duration of 20 ns at 10 Hz,had been employed to irradiate the toluene solution directly.After irradiation,light yellow toluene solution emits blue-green luminescence under 365 nm UV-light.The sizes of these photoluminescent CDs are about 1.3-4.0 nm.Raman spectra,with typical D peak and G peak,have pointed out that these CDs are graphite structure dots.However,in some samples,a peak around 1330cm^-1 gives some evidence that there may be diamond QDs in these CDs.In addition,the photodissociation processes have been discussed by First-Principle calculations,demonstrating that electrons can be transited from Highest Occupied Molecular Orbital?HOMO?to Lowest Unoccupied Molecular Orbital?LUMO?by 248 nm single/multi-photon excitation including Stark effect,which furthermore achieve the photo-dissociation of toluene molecule and directly synthesize the CDs.2.Since the Raman spectra with a peak around 1330 cm^-1 presents the existence of diamond QDs.In this part,we made careful analysis of high resolution transmission electron microscopy?HRTEM?images of CDs.Some cubic structure diamond and hexagonal structure lonsdaleite QDs have been found.The results are verified by the HRTEM simulations,where these HRTEM images must originate from these two kinds of diamond structure.Besides,two kinds of fullerenes,C₆₀ and C₁₈₀,have been discovered also in these CDs.Furthermore,direct photodissociation of aniline solution was studied and carbon nitride QDs were synthesized.We have present some HRTEM images with two dimensional spots.However,the structure of carbon nitride QDs still needs to be further identified due to the differences compared to the HRTEM simulations of C₃N₄.3.A Q-Switched Nd:YAG laser system was employed and the pulsed laser operated at 355 nm,tripled from the Q-switched 1064 nm laser with pulse duration of10 ns.After laser ablation of gadolinium target at the bottom of the toluene liquid under neutral gas argon protection,gadolinium carbide/carbon shell?GC/CS?dots with face-centered cubic GdC_0.4 core have been successfully synthesized and two kinds of GC/CS dots with diameters 1-3 nm and 4-10 nm were formed.Furthermore,some HRTEM images exhibit novel two dimensional spots have been cautiously studied comparing to gadolinium,gadolinium carbide,and gadolinium oxide.Benefit from the simulations of HRTEM and selected area electron diffraction?SAED?pattern,we finally assume a new cubic structure GdC₃ with space group of Pm-3m?a=b=c=4.05?,?=?=?=90o?.4.As a kind of paramagnetic species,the GC/CS dots revealed an excellent enhancement in MRI at high magnetic field 14.1T with ultralow concentration.In terms of the relaxivity values of the 1-3 nm GC/CS sample,both longitudinal and transverse relaxivity values r₁ and r₂ have been dramatically increased to 107.3 mM^-1s^-1 and86.5 m M^-1s^-1 respectively,which are about twenty times higher than that of Magnevist.Compared to Feridex,a kind of hematite with diameter about 4-5 nm,the transverse relaxivity value r₁ of 1-3 nm GC/CS dots is a bit higher than Feridex and the longitudinal relaxivity value r₂ is about eight times higher than Feridex.Besides,the MTT assay of HepG₂ cells had revealed the immunoconjugates with non-/low-cytotoxicities,thereby bringing the great potential of the GC/CS dots to be utilized as advanced magnetic resonance agent for the diagnosis of cancers with high relaxivity value,ultralow dose and non-cytotoxicity.