Department of Bioscience and Biotechnology, National Taiwan Ocean University, Keelung, 20224, Taiwan.
Nanoscale. 2017 Nov 30;9(46):18359-18367. doi: 10.1039/c7nr07639h.
Carbon quantum dots (CQDs) have attracted enormous interest in recent years owing to their low cytotoxicity, excellent biocompatibility and strong fluorescence. They have been successfully employed in sensor, bio-imaging, and drug carrier applications. A complete understanding of their core-surface structure is essential for tuning their physical and chemical properties for various applications. Conventional characterizations of CQDs are conducted with electron microscopy or spectroscopy, such as transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and Raman spectroscopy. However, these techniques cannot fully resolve the core-surface structure of CQDs. In this study, we attempt to analyze the structures of CQDs by laser desorption/ionization mass spectrometry (LDI-MS) using three model CQDs synthesized from citric acid (CA-CQDs), diammonium citrate (AC-CQDs) and spermidine trihydrochloride (Spd-CQDs). Both CA-CQDs and AC-CQDs produced anionic carbon cluster ions ([C], n = 4-9) during the laser desorption/ionization process. Additionally, AC-CQDs produced fragments containing C, N, and O that appeared at m/z values of 41.999, 91.015, and 107.008, which were identified by N isotopes as [CNO], [CHNO], and [CHNO], respectively. By contrast, subjecting Spd-CQDs to the same analysis did not yield carbon cluster ions ([C]); instead, strong chlorine-associated ions with a unique isotopic pattern were observed, strongly implying that Spd-CQDs contain chlorine. The lack of carbon cluster ion formation in nitrogen- and chlorine-doped Spd-CQDs indicates that nitrogen and chlorine are abundantly and homogenously doped in the CQDs. We also found a shot-dependent fragmentation behavior for AC-CQDs that produces nitrogen- and oxygen-containing ions and carbon cluster ions ([C]) during initial fragmentation of the surface, with a gradual destruction of the nanocrystalline carbon core after additional shots. These results suggest that LDI-MS can be used as a tool for analyzing the core-surface structure of CQDs, particularly when it contains a heteroatom doped carbon core with various surface functional groups containing nitrogen, oxygen and halogens.
碳量子点（CQDs）由于其低细胞毒性、优异的生物相容性和强荧光性，近年来引起了极大的兴趣。它们已成功应用于传感器、生物成像和药物载体等领域。为了调整其物理和化学性质以满足各种应用的需求，全面了解其核-壳结构至关重要。传统的 CQDs 特性分析采用电子显微镜或光谱学技术，如透射电子显微镜、X 射线光电子能谱、傅里叶变换红外光谱和拉曼光谱。然而，这些技术无法完全解析 CQDs 的核-壳结构。在这项研究中，我们尝试使用激光解吸/电离质谱（LDI-MS）分析三种由柠檬酸（CA-CQDs）、二铵柠檬酸盐（AC-CQDs）和 spermidine trihydrochloride（Spd-CQDs）合成的模型 CQDs 的结构。在激光解吸/电离过程中，CA-CQDs 和 AC-CQDs 均产生带负电荷的碳团簇离子([C]，n=4-9)。此外，AC-CQDs 产生了含有 C、N 和 O 的碎片，在 m/z 值为 41.999、91.015 和 107.008 处出现，这些碎片通过 N 同位素鉴定为[CNO]、[CHNO]和[CHNO]。相比之下，对 Spd-CQDs 进行相同的分析并未产生碳团簇离子([C])；相反，观察到具有独特同位素模式的强氯关联离子，强烈表明 Spd-CQDs 含有氯。氮掺杂和氯掺杂的 Spd-CQDs 中未形成碳团簇离子表明，氮和氯在 CQDs 中丰富且均匀掺杂。我们还发现 AC-CQDs 存在与发射次数相关的碎片化行为，即在初始表面碎片化时会产生含氮和含氧离子以及碳团簇离子([C])，在经过额外的发射次数后，纳米晶碳核逐渐被破坏。这些结果表明，LDI-MS 可用于分析 CQDs 的核-壳结构，特别是当 CQDs 含有含有各种表面官能团的掺杂有氮、氧和卤素的杂原子的富碳核时。
