Refractometry, namely, the measurement of refractive index (RI), provides information about various sample properties such as concentrations and molecular structure. One physical phenomenon which enables precise determination of a sample’s RI in a microscope is the supercritical-angle fluorescence. This effect is observed when the fluorescence from an emitter near a glass-medium interface is captured by an objective lens with a high numerical aperture. The materials’ index mismatch creates a distinguishable transition in the intensity pattern at the back focal plane of the objective that changes proportionally to the RI of the media. Here, we present a refractometry approach in which the fluorophores are preattached to the bottom surface of a microfluidic channel, enabling highly sensitive determination of the RI using tiny amounts of liquid (picoliters). With this method, we attained a standard deviation of 3.1 × 10^–5 and a repeatability of 2.7 × 10^–5 RI units. We first determine the capabilities of the device for glycerol-water solutions and then demonstrate the relevance of our system for monitoring changes in biological systems. As a model system, we show that we can detect single bacteria (Escherichia coli) and measure population growth.

中文翻译：

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通过超临界角荧光进行超灵敏折光法

折光法，即折射率（RI）的测量，提供了有关各种样品特性（例如浓度和分子结构）的信息。一种能在显微镜下精确确定样品RI的物理现象是超临界角荧光。当来自玻璃-介质界面附近的发射器的荧光被具有高数值孔径的物镜捕获时，可以观察到这种效果。材料的折射率不匹配会在物镜的后焦平面处的强度模式中产生明显的过渡，该过渡与介质的RI成比​​例地变化。在这里，我们提出了一种折光法，其中荧光团预先附着在微流体通道的底部表面，可以使用少量液体（微升）对RI进行高度灵敏的测定。使用这种方法，我们获得的标准偏差为3.1×10^–5和2.7×10 ^–5 RI单位的重复性。我们首先确定甘油水溶液的设备功能，然后证明我们的系统对监测生物系统变化的相关性。作为模型系统，我们表明我们可以检测单个细菌（大肠杆菌）并测量种群增长。