Despite extensive studies on the distinctive properties of water confined in a nanospace, the underlying mechanism and significance of the lengthscale involved in the confinement effects are still subjects of controversy. The dielectric constant and the refractive index in particular are key parameters in modeling and understanding nanoconfined water, yet experimental evidence is lacking. We report the measurement of the refractive indices of water in 10–100 nm spaces by exploiting the confinement of water and localized surface plasmons in a physicochemically well-defined nanocavity. The results revealed significantly low values and the scaling behavior of the out-of-plane refractive index n_⊥ of confined water. They are attributed to the polarization suppression at the interfaces and the long-range correlation in electronic polarization facilitated by the strengthened H-bonding network. Using the refractive index as a sensing probe, we also observed anomalous stability of water structures over a wide range of temperature. Our measurement results provide essential feedback information for benchmarking water models and molecular interactions under nanoconfinement. This study also opens up a new methodology of using plasmon resonance in characterizing nanoconfined molecules and chemical reactions, and thus gives us fundamental insight into confinement effects.

中文翻译：

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纳米约束水的折射率揭示了其异常的物理性质。

尽管对限制在纳米空间中的水的独特性质进行了广泛的研究，但限制作用所涉及的长度尺度的潜在机制和重要性仍然是有争议的主题。介电常数和折射率尤其是建模和理解纳米受限水的关键参数，但缺乏实验证据。我们报告了通过在物理化学上定义明确的纳米腔中利用水和局域表面等离子体的限制来测量10-100 nm空间中水的折射率。结果显示显著较低值和出的面内的折射率的缩放行为ñ _⊥密闭的水。它们归因于界面处的极化抑制和增强的H键网络促进的电子极化中的长距离相关性。使用折射率作为传感探针，我们还观察到了在宽温度范围内水结构的异常稳定性。我们的测量结果为基准水模型和纳米约束下的分子相互作用提供了重要的反馈信息。这项研究还开辟了一种使用等离振子共振表征纳米受限分子和化学反应的新方法，从而使我们对限制效应有了基本的了解。