Division of Materials Science and Engineering, Boston University, Boston, Massachusetts 02446, USA.
Nanoscale. 2017 Nov 2;9(42):16446-16458. doi: 10.1039/c7nr04296e.
Heterostructured core/shell quantum dots (QDs) are prized in biomedical imaging and biosensing applications because of their bright, photostable emission and effectiveness as Förster resonance energy transfer (FRET) donors. However, as nanomaterials chemistry has progressed beyond traditional QDs to incorporate new compositions, ultra-thick shells, and alloyed structures, few of these materials have had their optical properties systematically characterized for effective application. For example, thick-shelled QDs, also known as 'giant' QDs (gQDs) are useful in single-particle tracking microscopy because of their reduced blinking, but we know only that CdSe/CdS gQDs are qualitatively brighter than thin-shelled CdSe/CdS in aqueous media. In this study, we quantify the impact of shell thickness on the nanoparticle molar extinction coefficient, quantum yield, brightness, and effectiveness as a FRET donor for CdSe/xCdS core/shell and CdSe/xCdS/ZnS core/shell/shell QDs, with variable thicknesses of the CdS shell (x). Molar extinction coefficients up to three orders of magnitude higher than conventional dyes and forty-fold greater than traditional QDs are reported. When thick CdS shells are combined with ZnS capping, quantum yields following thiol ligand exchange reach nearly 40%-5-10× higher than either the commercially available QDs or gQDs without ZnS caps treated the same way. These results clearly show that thick CdS shells and ZnS capping shells work in concert to provide the brightest possible CdSe-based QDs for bioimaging applications. We demonstrate that thicker shelled gQDs are over 50-fold brighter than their thin-shelled counterparts because of significant increases in their absorption cross-sections and higher quantum yield in aqueous milieu. Consistent with the point-dipole approximation commonly used for QD-FRET, these data show that thick shells contribute to the donor-acceptor distance, reducing FRET efficiency. Despite the reduction in FRET efficiency, even the thickest-shell gQDs exhibited energy transfer. Through this systematic study, we elucidate the tradeoffs between signal output, which is much higher for the gQDs, and FRET efficiency, which decreases with shell thickness. This study serves as a guide to nanobiotechnologists striving to use gQDs in imaging and sensing devices.
核壳结构的量子点（QDs）因其明亮、光稳定性的发射以及作为Förster 共振能量转移（FRET）供体的有效性，而在生物医学成像和生物传感应用中备受青睐。然而，随着纳米材料化学的发展超越了传统的量子点，涉及到新的组成、超厚的壳层和合金结构，其中很少有材料的光学性质得到系统的表征，以实现有效的应用。例如，厚壳量子点，也称为“巨型”量子点（gQDs），由于其闪烁减少，在单粒子跟踪显微镜中很有用，但我们只知道 CdSe/CdS gQDs 在水介质中的亮度比薄壳 CdSe/CdS 定性地高。在这项研究中，我们定量地研究了壳层厚度对纳米颗粒摩尔消光系数、量子产率、亮度以及作为 CdSe/xCdS 核/壳和 CdSe/xCdS/ZnS 核/壳/壳 QDs 的 FRET 供体的有效性的影响，其中 CdS 壳层（x）的厚度可变。报道了高达三个数量级高于传统染料和四十倍于传统量子点的摩尔消光系数。当厚的 CdS 壳与 ZnS 帽结合时，通过巯基配体交换后的量子产率达到近 40%-5-10 倍，高于商业上可用的 QDs 或用相同方法处理的没有 ZnS 帽的 gQDs。这些结果清楚地表明，厚的 CdS 壳和 ZnS 帽层协同作用，为生物成像应用提供了尽可能亮的 CdSe 基 QDs。我们证明，由于它们的吸收截面显著增加和在水介质中的量子产率更高，较厚壳层的 gQDs 比它们的薄壳层对应物亮 50 多倍。与通常用于 QD-FRET 的点偶极近似一致，这些数据表明，厚壳层有助于供体-受体距离，降低 FRET 效率。尽管 FRET 效率降低，但即使是最厚壳层的 gQDs 也表现出能量转移。通过这项系统的研究，我们阐明了信号输出（gQDs 的信号输出要高得多）和 FRET 效率（随着壳层厚度的增加而降低）之间的权衡。这项研究为努力在成像和传感设备中使用 gQDs 的纳米生物技术人员提供了指导。
