Graphene Quantum Dots: Preparation And The Interactions With Dna

The lateral dimensions of graphene sheets affect their properties dramatically. In general, the graphene sheets with micrometer lateral sizes have zero band gaps, and exhibit semimetal properties. For the graphene sheets with lateral dimensions less than 100 nm, usually called graphene quantum dots(GQDs), show typical semiconducting properties due to the quantum confinement and edge effects. Besides the potential application in optoelectronic devices, the unique photoluminescence, chemical inertia, good dispersion in water and nontoxicity make GQDs more suitable for biological applications. So far, several protocols have been proposed for the fabrication of GQDs, but they are not suitable for mass scale preparation owing to the harsh consitions, time consumption, and low yield. Therefore, the controllable preparation of GQDs in bulk scale is still challenging, which also limits the practical application of GQDs. In this work, two new approaches we explored to cut micrometer-sized GO into GQDs rapidly in water through the Photo-Fenton reaction of GO and the reaction of GO with sodium hypochlorite(NaClO). Meanwhile, the interaction of GQDs with DNA also has been studied. The main results of the work are as following:1. Using hydroxylamine as reductant, chemically reduced graphene oxide(CRGO) with different reduction extents were prepared successfully by controlling the reduction time(< 60 min) under a mild condition in water(~90℃). In comparison with hydrazine and L-ascorbic acid which were used often as reductants for GO reduction, the reduction of GO by hydroxylamine could take place quickly under a mild condition. The as-obtained CRGO could be suspended in water stably without using any of the stabilizing and dispersing reagents by simply adjusting the pH of reaction solution system. The as-generated CRGO sheets show also high electrical conductivity and fair structural integrity. These results illustrated that hydroxylamine should be a proper reductant to reduce the GO in aqueous solution and should be potentially useful for bulk scale or even industrial productions of CRGO.2. Photo-Fenton reaction of GO were studied systematically and GQDs were prepared successfully. It was demonstrated that under the ultraviolet(UV) irradiation, the micrometer-sized GO sheets could be subsequently converted into porous GO, GQDs, small organic molecules, and finally CO2 with the time increasing. For 0.1 mg m L-1 GO aqueous solution, by controlling the reaction time from 15 to 60 min, GQDs with different sizes were obtained. For instance, when the reaction time was 15 min, the as-generated GQDs assumed average lateral size of ~ 40 nm, and the yield was about 45%. The high resolution microscopic and spectroscopic data showed that the GQDs has defect-free hexagonal lattice structure and contains periphery carboxylic groups, which afford GQDs good dispersing capability in water. The reaction mechanism was also explored and it was believed that the reaction is initiated by ·OH radicals attacking the carbon atoms connected with the oxygen-containing groups. With reaction processing, some carbon atoms were removed through the oxidization, meanwhile, more novel reaction sites, the carbon atoms connected with oxygen-containing groups, could be generated. Thus, the GO sheets were cut into the porous GO, GQDs, and finally CO2. Nevertheless, the work illustrated that the Photo-Fenton reaction of GO should be a simple and efficient strategy to prepare GQDs in mass scale.3. It was illustrated that under the UV irradiation, NaClO releases lots of oxygen radicals [O] which can react with GO and cut GO into GQDs quickly. In comparison with the Photo-Fenton reaction of GO, which can only take place under acidic condition, the reaction of GO with NaClO can be conducted under either acidic or basic condition. The yield of as-obtained GQDs can reach up to 78%, which is much higher than that of the GQDs obtained though Photo-Fenton reaction. The results illustrate that the reaction of GO with Na ClO should be potentially useful for the preparation of GQDs with high quality.4. The interactions of GQD with DNA molecules were studied preliminarily. It was found that, in comparison with GO, GQDs showed good performance in DNA molecule cleavage under certain conditions. More pronouncedly, it was demonstrated that GQDs can induce and stabilize the DNA i-motif structure. Specifically, under acidic condition, the DNA i-motif structure can be stabilized by GQDs, and under alkaline or neutral conditions, GQDs can induce the formation of i-motif structure. It was proved that the interaction of GQDs with i-motif structure is based on end-stacking of the bases at its loop regions. The results should be useful for further exploring the potential biological and medical applications of GQD and its derivatives.