Electrowetting-Tuned Liquid Droplets on Lubricated Superhydrophobic Surfaces for Whispering-Gallery-Mode Sensing

小型化、便携式、灵敏和低成本的传感系统对于医疗和环境诊断和监测应用是重要的。由于光学传感器的高灵敏度、芯片级系统的小形状因数以及对于利用成熟的大规模生产技术制造的系统可能的低成本处理，所以组合了光学、电学和流体功能的联合收割机集成光子传感系统对于传感应用特别有吸引力。虽然已经通过各种方法实现了检测限低至单个纳米颗粒的光学传感，例如散射干涉和光热显微镜以及MEMS传感器，但微腔传感吸引了很多关注，因为它们的高品质因子（Q因子，其物理上表示相对于总存储能量的能量损失率）和小模式体积能够显著增强光-物质相互作用。微腔传感技术已经取得了巨大的进展，并通过检测单个纳米颗粒和单个生物分子来证明其传感性能。然而，利用回音壁模式（WGM，即，由外部腔界面处的全内反射支持的闭合圆形光束）腔仅在极少情况下使用在封闭环境中浸没或填充有液体的介电微谐振器来实现。迄今为止，还没有真实的和稳定的高Q值非固体光学谐振腔传感实验的报道。在这个项目中，弗吉尼亚理工大学的团队旨在用直接由液滴制成的微谐振器进行光学传感。该项目的新研究成果将与教育工作相结合。生物启发的纳米科学和机械工程将与各级K-12教育相结合。参与拟议的研究将激励代表性不足的群体攻读高级学位。除研究生外，本科生和女学生将通过论文、项目型课程或多学科高级设计项目参与研究，本项目的研究目标是协助实现具有超高灵敏度和Q因子的片上检测和尺寸测量系统，为研究单个粒子分析物和单个生物分子的性质和动力学奠定基础，其精度无法达到使用集合测量。研究任务如下：（1）具有工程微/纳米结构的润滑的超疏水表面可以使液滴以足够大的接触角站立以用于WGM感测，并且润滑剂遮盖可以帮助防止水滴的快速蒸发;（2）新颖的WGM系统配置使得液体微腔与基板上的内置波导能够弹性耦合;（3）理想的检测系统不仅要求具有痕量分析物响应或单粒子水平响应的能力，而且要求能够快速检测目标。电润湿将被应用于驱动液滴在一个可编程的路径和运输到目标与内置波导WGM传感;（4）第一次，超灵敏度WGM将被用来检测液滴上的耗尽壳的存在;（5）首要目标是开发芯片级集成的光子传感系统，联合收割机结合光学，电学和流体功能。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Miniaturized, portable, sensitive, and low cost sensing systems are important for medical and environmental diagnostic and monitoring applications. Chip scale integrated photonic sensing systems that combine optical, electrical, and fluidic functions are especially attractive for sensing applications due to the high sensitivity of optical sensors, the small form-factor of chip scale systems, and the low-cost processing possible for systems fabricated with well-developed mass production techniques. While optical sensing with a detection limit down to single nanoparticles has been achieved by various methods, such as scattering interferometric and photothermal microscopy and nanofiber sensors, microcavity sensing attracts much attention because their high quality factors (Q factor, which physically represents the rate of energy loss relative to the total stored energy) and small mode volumes enable significant enhancement of light-matter interactions. Microcavity sensing has seen tremendous progress and the sensing performance has been demonstrated by detecting single nanoparticles and single biological molecules. However, detection in liquids with whispering gallery mode (WGM, i.e., closed circular beams supported by total internal reflections at the external cavity interface) cavities was achieved only in rare cases using dielectric micro-resonators that were immersed or filled with liquids in a closed environment. No real and stable high Q-factor sensing experiment with non-solid optical resonators has been reported to date. In this program, the team formed at Virginia Tech aims to perform optical sensing with micro-resonators made directly of liquid droplets. New research outcomes from this project will be integrated with educational endeavors. Bio-inspired nanoscience and mechanical engineering will be integrated with all levels of K-12 education. Participation in the proposed researches will motivate the underrepresented groups for advanced degrees. In addition to graduate students, undergraduate and female students will participate in the research through thesis, project-based courses, or multidisciplinary senior design projects.The research objective of this project is to assist in realizing on-chip detection and sizing systems with ultra-high sensitivity and Q-factor, laying the groundwork to investigate the properties and dynamics of single particle analyte and single biological molecules with an accuracy that cannot be achieved using ensemble measurements. The research tasks are below: (1) The lubricated superhydrophobic surfaces with engineered micro/nanostructures can make the liquid droplet stand with a sufficiently large contact angle for WGM sensing and the lubricant cloaking can help prevent fast evaporation of water droplet; (2) The novel WGM system configuration enables resilient coupling of the liquid microcavity with the built-in waveguide on the substrate; (3) An ideal detection system requires not only the ability of trace analyte response or single-particle-level response, but also the rapid detection of targets. Electrowetting will be applied to actuate liquid droplets in a programmable path and transport to the target with built-in waveguide for WGM sensing; (4) For the first time, ultra-sensitivity WGM will be employed to detect the existence of depletion shell on a liquid droplet; (5) The overarching goal is to develop chip-scale integrated photonic sensing systems that combine optical, electrical, and fluidic functions. This proposed high Q-factor approach will pave the way for sensors that have unprecedented sensitivity to tiny changes in their environments.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.