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

小型化、便携、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非常重要。结合光学、电学和流体功能的芯片级集成光子传感系统对传感应用特别有吸引力，因为光学传感器的高灵敏度，芯片级系统的小尺寸，以及用成熟的批量生产技术制造的系统的低成本加工。虽然通过散射干涉法、光热显微镜和纳米纤维传感器等各种方法已经实现了对单个纳米颗粒的检测限制，但微腔传感由于其高质量因子（Q因子，物理上表示相对于总存储能量的能量损耗率）和小模式体积能够显著增强光-物质相互作用而受到广泛关注。微腔传感技术取得了巨大的进步，其传感性能已被证明可以检测单个纳米颗粒和单个生物分子。然而，在窃窃私语通道模式（WGM，即由外部腔界面的全内反射支撑的封闭圆光束）的液体中，只有在极少数情况下才能使用浸入或充满液体的介电微谐振器在封闭环境中进行检测。迄今为止，还没有报道过用非固体光学谐振腔进行真实、稳定的高q因子传感实验。在这个项目中，弗吉尼亚理工大学组建的团队旨在利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。仿生纳米科学和机械工程将与K-12教育的各个层次相结合。参与拟议的研究将激励代表性不足的群体获得更高的学位。除研究生外，本科生和女生将通过论文、项目课程或多学科高级设计项目参与研究。该项目的研究目标是协助实现具有超高灵敏度和q因子的片上检测和分级系统，为研究单颗粒分析物和单个生物分子的特性和动力学奠定基础，其精度无法通过系综测量实现。研究任务如下：(1)工程微纳结构的润滑超疏水表面可以使液滴保持足够大的接触角以进行WGM传感，润滑剂的隐形可以防止液滴的快速蒸发；(2)新型WGM系统配置使液体微腔与衬底上的内置波导具有弹性耦合；(3)理想的检测系统不仅要求具有痕量分析物响应或单颗粒级响应的能力，而且要求对目标的快速检测。电润湿将应用于驱动液滴在可编程路径上，并通过内置波导传输到目标，用于WGM传感；(4)首次利用超灵敏WGM检测液滴上是否存在耗尽壳；(5)总体目标是开发集光学、电学和流体功能于一体的芯片级集成光子传感系统。这种提出的高q因子方法将为对环境微小变化具有前所未有灵敏度的传感器铺平道路。该奖项反映了美国国家科学基金会的法定使命，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.