Nanophotonic resonators can confine light to deep-subwavelength volumes with highly enhanced near-field intensity and therefore are widely used for surface-enhanced infrared absorption spectroscopy in various molecular sensing applications. The enhanced signal is mainly contributed by molecules in photonic hot spots, which are regions of a nanophotonic structure with high-field intensity. Therefore, delivery of the majority of, if not all, analyte molecules to hot spots is crucial for fully utilizing the sensing capability of an optical sensor. However, for most optical sensors, simple and straightforward methods of introducing an aqueous analyte to the device, such as applying droplets or spin-coating, cannot achieve targeted delivery of analyte molecules to hot spots. Instead, analyte molecules are usually distributed across the entire device surface, so the majority of the molecules do not experience enhanced field intensity. Here, we present a nanophotonic sensor design with passive molecule trapping functionality. When an analyte solution droplet is introduced to the sensor surface and gradually evaporates, the device structure can effectively trap most precipitated analyte molecules in its hot spots, significantly enhancing the sensor spectral response and sensitivity performance. Specifically, our sensors produce a reflection change of a few percentage points in response to trace amounts of the amino-acid proline or glucose precipitate with a picogram-level mass, which is significantly less than the mass of a molecular monolayer covering the same measurement area. The demonstrated strategy for designing optical sensor structures may also be applied to sensing nano-particles such as exosomes, viruses, and quantum dots.

纳米光子谐振器可以将光限制在具有高度增强的近场强度的深亚波长体积中，因此广泛用于各种分子传感应用中的表面增强红外吸收光谱。增强的信号主要由光子热点中的分子贡献，光子热点是具有高场强的纳米光子结构区域。因此，将大部分（如果不是全部）分析物分子输送到热点对于充分利用光学传感器的传感能力至关重要。然而，对于大多数光学传感器来说，将水性分析物引入设备的简单直接的方法（例如应用液滴或旋涂）无法实现将分析物分子靶向传递到热点。反而，分析物分子通常分布在整个设备表面，因此大多数分子不会经历增强的场强。在这里，我们提出了一种具有无源分子捕获功能的纳米光子传感器设计。当分析物溶液液滴被引入传感器表面并逐渐蒸发时，该器件结构可以有效地将大部分沉淀的分析物分子捕获在其热点中，从而显着提高传感器的光谱响应和灵敏度性能。具体来说，我们的传感器会产生几个百分点的反射变化，以响应具有皮克级质量的痕量氨基酸脯氨酸或葡萄糖沉淀物，这明显小于覆盖相同测量区域的分子单层的质量.