Measuring small changes in refractive index can provide both sensitive and contactless information on molecule concentration or process conditions for a wide range of applications. However, refractive index measurements are easily perturbed by non-specific background signals, such as temperature changes or non-specific binding. Here, we present an optofluidic device for measuring refractive index with direct background subtraction within a single measurement. The device is comprised of two interdigitated arrays of nanofluidic channels designed to form an optical grating. Optical path differences between the two sets of channels can be measured directly via an intensity ratio within the diffraction pattern that forms when the grating is illuminated by a collimated laser beam. Our results show that no calibration or biasing is required if the unit cell of the grating is designed with an appropriate built-in asymmetry. In proof-of-concept experiments we attained a noise level equivalent to ∼10⁻⁵ refractive index units (30 Hz sampling rate, 4 min measurement interval). Furthermore, we show that the accumulation of biomolecules on the surface of the nanochannels can be measured in real-time. Because of its simplicity and robustness, we expect that this inherently differential measurement concept will find many applications in ultra-low volume analytical systems, biosensors, and portable devices.

中文翻译：

---

非对称纳米流体光栅检测器，用于差分折射率测量和生物传感

测量折射率的微小变化可以为广泛的应用提供有关分子浓度或工艺条件的敏感信息和非接触信息。但是，折射率测量很容易受到非特异性背景信号（例如温度变化或非特异性结合）的干扰。在这里，我们提出了一种光流控设备，用于在单次测量中通过直接背景扣除来测量折射率。该设备由两个纳米流体通道的交叉阵列组成，这些纳米流体通道设计为形成光栅。两组信道之间的光程差可以直接测量通过当光栅被准直的激光束照射时形成的衍射图内的强度比。我们的结果表明，如果光栅的晶胞设计为具有适当的内置不对称性，则无需校准或偏置。在概念验证实验中，我们获得的噪声级相当于大约10 ^-5折射率单位（30 Hz采样率，4分钟的测量间隔）。此外，我们表明，可以实时测量纳米通道表面上生物分子的积累。由于其简单性和鲁棒性，我们希望这种固有的差分测量概念将在超小体积分析系统，生物传感器和便携式设备中找到许多应用。