Dept. of Applied Physics, Chalmers University of Techology, Gothenburg, Sweden.
Analyst. 2016 Jun 21;141(12):3803-10. doi: 10.1039/c6an00046k. Epub 2016 Feb 12.
Plasmonic nanohole arrays are widely used for optical label-free molecular detection. An important factor for many applications is the diameter of the apertures. So far nanohole arrays with controllable diameters below 100 nm have not been demonstrated and it has not been systematically investigated how the diameter influences the optical properties. In this work we fine-tune the diameter in short range ordered nanohole arrays down to 50 nm. The experimental far field spectra show how the wavelength of maximum extinction remains unaffected while the transmission maximum blue shifts with smaller diameters. The near field is visualized by numerical simulations, showing a homogenous enhancement throughout the cylindrical void at the transmission maximum for diameters between 50 and 100 nm. For diameters below 50 nm plasmon excitation is no longer possible experimentally or by simulations. Further, we investigate the refractive index sensing capabilities of the smaller holes. As the diameter was reduced, the sensitivity in terms of resonance shift with bulk liquid refractive index was found to be unaltered. However, for the transmission maximum the sensitivity becomes more strongly localized to the hole interior. By directing molecular binding to the bottom of the holes we demonstrate how smaller holes enhance the sensitivity in terms of signal per molecule. A real-time detection limit well below one protein per nanohole is demonstrated. The smaller plasmonic nanoholes should be suitable for studies of molecules confined in small volumes and as mimics of biological nanopores.
等离子体纳米孔阵列被广泛用于光学无标记分子检测。对于许多应用来说，一个重要的因素是孔径的大小。到目前为止，还没有展示出具有可控制直径小于 100nm 的纳米孔阵列，也没有系统地研究过直径如何影响光学性质。在这项工作中，我们将短程有序纳米孔阵列的直径精细调整到 50nm 以下。实验远场光谱表明，最大消光波长如何保持不变，而随着直径的减小，透射最大值蓝移。通过数值模拟可视化近场，显示在 50nm 到 100nm 之间的直径下，在透射最大值处圆柱形空隙中存在均匀的增强。对于直径小于 50nm 的孔，实验上或通过模拟都无法激发等离子体。此外，我们研究了较小孔的折射率传感能力。随着直径的减小，以体相液体折射率的共振位移为单位的灵敏度被发现保持不变。然而，对于透射最大值，灵敏度变得更强烈地局限于孔内部。通过将分子结合到孔的底部，我们证明了较小的孔如何通过每个分子的信号来提高灵敏度。证明了实时检测下限低于每个纳米孔一个蛋白质。较小的等离子体纳米孔应该适用于研究限制在小体积中的分子，以及作为生物纳米孔的模拟物。
