Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 (Singapore), Fax: (+65) 6791-1961.
Chemistry. 2014 Jan 3;20(1):217-22. doi: 10.1002/chem.201303582. Epub 2013 Dec 5.
Graphene oxide (GO)-based materials offer great potential for biofunctionalization with applications ranging from biosensing to drug delivery. Such biofunctionalization utilizes specific functional groups, typically a carboxyl moiety, as anchoring points for biomolecule. However, due to the fact that the exact chemical structure of GO is still largely unknown and poorly defined (it was postulated to consist of various oxygen-containing groups, such as epoxy, hydroxyl, carboxyl, carbonyl, and peroxy in varying ratios), it is challenging to fabricate highly biofunctionalized GO surfaces. The predominant anchoring sites (i.e., carboxyl groups) are mainly present as terminal groups on the edges of GO sheets and thus account for only a fraction of the oxygen-containing groups on GO. Herein, we suggest a direct solution to the long-standing problem of limited abundance of carboxyl groups on GO; GO was first reduced to graphene and consequently modified with only carboxyl groups grafted perpendicularly to its surface by a rational synthesis using free-radical addition of isobutyronitrile with subsequent hydrolysis. Such grafted graphene oxide can contain a high amount of carboxyl groups for consequent biofunctionalization, at which the extent of grafting is limited only by the number of carbon atoms in the graphene plane; in contrast, the abundance of carboxyl groups on "classical" GO is limited by the amount of terminal carbon atoms. Such a graphene platform embedded with perpendicularly grafted carboxyl groups was characterized in detail by X-ray photoelectron spectroscopy, cyclic voltammetry, and electrochemical impedance spectroscopy, and its application was exemplified with single-nucleotide polymorphism detection. It was found that the removal of oxygen functionalities after the chemical reduction enhanced the electron-transfer rate of the graphene. More importantly, the introduction of carboxyl groups promoted a more efficient immobilization of DNA probes on the electrode surface and improved the performance of graphene as a biosensor in comparison to GO. The proposed material can be used as a universal platform for biomolecule immobilization to facilitate rapid and sensitive detection of DNA or proteins for point-of-care investigations. Such reactive carboxyl groups grafted perpendicularly on GO holds promise for a highly efficient tailored biofunctionalization for applications in biosensing or drug delivery.
基于氧化石墨烯（GO）的材料具有很大的生物功能化潜力，可应用于从生物传感到药物输送等领域。这种生物功能化利用特定的官能团，通常是羧基部分，作为生物分子的锚固点。然而，由于 GO 的确切化学结构仍然在很大程度上未知且定义不明确（据推测，GO 由各种含氧基团组成，如环氧、羟基、羧基、羰基和过氧基，比例不同），因此制造高度生物功能化的 GO 表面具有挑战性。主要的锚固位点（即羧基）主要存在于 GO 片的边缘作为端基，因此仅占 GO 中含氧基团的一部分。在这里，我们提出了一种直接解决 GO 上羧基数量有限的长期问题的方法；首先通过自由基加成反应将 GO 还原为石墨烯，然后使用异丁腈进行合理合成，使羧基垂直接枝到其表面，随后进行水解。这种接枝氧化石墨烯可以含有大量的羧基，用于随后的生物功能化，其中接枝的程度仅受石墨烯平面中碳原子数量的限制；相比之下，“经典”GO 上羧基的丰度受末端碳原子数量的限制。这种嵌入垂直接枝羧基的石墨烯平台通过 X 射线光电子能谱、循环伏安法和电化学阻抗谱进行了详细表征，并通过单核苷酸多态性检测实例说明了其应用。研究发现，化学还原后氧官能团的去除增强了石墨烯的电子转移速率。更重要的是，与 GO 相比，羧基的引入促进了 DNA 探针更有效地固定在电极表面上，并提高了石墨烯作为生物传感器的性能。所提出的材料可用作生物分子固定的通用平台，以促进 DNA 或蛋白质的快速和灵敏检测，用于即时检测研究。GO 上垂直接枝的反应性羧基有望实现高效的定制生物功能化，应用于生物传感或药物输送等领域。
