Department of Chemistry, University of California , Berkeley, California 94720, United States.
Materials Sciences Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.
Nano Lett. 2017 Jan 11;17(1):414-420. doi: 10.1021/acs.nanolett.6b04383. Epub 2016 Dec 28.
One of the key challenges facing liquid-phase transmission electron microscopy (TEM) of biological specimens has been the damaging effects of electron beam irradiation. The strongly ionizing electron beam is known to induce radiolysis of surrounding water molecules, leading to the formation of reactive radical species. In this study, we employ DNA-assembled Au nanoparticle superlattices (DNA-AuNP superlattices) as a model system to demonstrate that graphene and its derivatives can be used to mitigate electron beam-induced damage. We can image DNA-AuNP superlattices in their native saline environment when the liquid cell window material is graphene, but not when it is silicon nitride. In the latter case, initial dissociation of assembled AuNPs was followed by their random aggregation and etching. Using graphene-coated silicon nitride windows, we were able to replicate the observation of stable DNA-AuNP superlattices achieved with graphene liquid cells. We then carried out a correlative Raman spectroscopy and TEM study to compare the effect of electron beam irradiation on graphene with and without the presence of water and found that graphene reacts with the products of water radiolysis. We attribute the protective effect of graphene to its ability to efficiently scavenge reactive radical species, especially the hydroxyl radicals which are known to cause DNA strand breaks. We confirmed this by showing that stable DNA-AuNP assemblies can be imaged in silicon nitride liquid cells when graphene oxide and graphene quantum dots, which have also recently been reported as efficient radical scavengers, are added directly to the solution. We anticipate that our study will open up more opportunities for studying biological specimens using liquid-phase TEM with the use of graphene and its derivatives as biocompatible radical scavengers to alleviate the effects of radiation damage.
液相透射电子显微镜（TEM）在观测生物样本时面临的一个关键挑战是电子束辐照的破坏作用。强电离电子束会导致周围水分子的辐射分解，形成具有反应活性的自由基。在这项研究中，我们采用 DNA 组装的 Au 纳米颗粒超晶格（DNA-AuNP 超晶格）作为模型系统，证明了石墨烯及其衍生物可用于减轻电子束诱导的损伤。当液体池窗材料为石墨烯时，我们可以在 DNA-AuNP 超晶格的天然盐环境中对其进行成像，但当窗材料为氮化硅时则不行。在后一种情况下，组装的 AuNP 会先发生初始解离，然后随机聚集和刻蚀。采用石墨烯涂覆的氮化硅窗，我们能够复制在石墨烯液体池实现的稳定 DNA-AuNP 超晶格的观察结果。然后，我们进行了相关的拉曼光谱和 TEM 研究，以比较有/无水存在时石墨烯对电子束辐照的影响，发现石墨烯与水辐射分解产物发生反应。我们将石墨烯的保护作用归因于其有效清除自由基的能力，特别是已知会导致 DNA 链断裂的羟基自由基。我们通过向溶液中直接添加氧化石墨烯和石墨烯量子点（最近也被报道为有效的自由基清除剂），并在氮化硅液体池中成功对稳定的 DNA-AuNP 组装体进行成像，从而证实了这一点。我们预计，我们的研究将为使用液相 TEM 研究生物样本提供更多机会，使用石墨烯及其衍生物作为生物相容性的自由基清除剂，以减轻辐射损伤的影响。
