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。芯片规模的集成光子传感系统结合了光学、电学和流体功能，由于光学传感器的高灵敏度、芯片规模系统的小尺寸以及采用成熟的大规模生产技术制造的系统可能的低成本加工，因此对传感应用特别有吸引力。虽然人们已经通过各种方法实现了对单个纳米粒子的光学传感，如散射干涉显微镜、光热显微镜和纳米光纤传感器，但微腔传感由于其高品质因数(Q因数，物理上表示能量损失相对于总存储能量的比率)和小的模式体积使光-物质相互作用得到显著增强而引起人们的关注。微腔传感已经取得了巨大的进展，并通过对单个纳米颗粒和单个生物分子的检测展示了其传感性能。然而，只有在极少数情况下，才能使用在封闭环境中浸泡或充满液体的介质微谐振器来实现对具有回声走廊模式(WGM，即由外部腔界面的全部内反射支撑的闭合圆形光束)腔的液体的检测。到目前为止，还没有关于非固体光学谐振腔真实稳定的高Q因子传感实验的报道。在这个项目中，弗吉尼亚理工大学成立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。生物启发的纳米科学和机械工程将与所有级别的K-12教育相结合。参与拟议的研究将激励代表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帮助实现具有超高灵敏度和Q因子的芯片上检测和分级系统，为研究单粒子分析物和单生物分子的性质和动力学奠定基础，其精度是集合测量无法达到的。研究任务如下：(1)具有工程微/纳米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，并且润滑剂的覆盖有助于防止水滴的快速蒸发；(2)新型WGM系统配置能够使液体微腔与衬底上的内置波导进行弹性耦合；(3)理想的探测系统不仅需要具有痕量分析物响应或单粒子响应的能力，还需要能够快速检测目标。将采用电润湿技术来驱动液滴在可编程路径上的运动，并通过内置波导将其传输到靶上进行WGM传感；(4)首次使用超灵敏WGM来检测液滴上是否存在耗尽壳；(5)总体目标是开发集光、电和流体功能于一体的芯片级集成光子传感系统。这一建议的高Q因数方法将为传感器对环境中的微小变化具有前所未有的敏感性铺平道路。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.