Determining the Subnanometer Thickness of the Water-Depletion Layer at the Interface between Water and the Hydrophobic Substrate

Surface plasmon resonance (SPR) is one of the most popular and powerful techniques for label-free detecting and quantitatively analyzing the interfacial refractive index (RI). So far, most of the SPR measurements are mainly applied to detect the relative change of RI upon biological and chemical events occurring at the interface, while the determinations on the absolute value of RI remains challenging. However, the absolute value of RI has become increasingly urgent in some cases, such as the existence and physical properties of the water depletion layer (WDL). WDL refers to a subnanometer-thick layer with reduced density between water and the hydrophobic substrate. The detailed explanations of how water meets hydrophobic surface have been studied by several kinds of techniques for decades but it remains under debate. In this work, we successfully established a method to measure the absolute RI at a gold–liquid interface by surface plasmon resonance microscopy (SPRM) and 2D Fourier transformation image processing and further applied this method to study the existence and physical nature of WDL. It was found that a 0.6 nm thick WDL existed at the interface of water and the hydrophobic substrate, leading to a reduced refractive index of 1.3295 ± 0.0006 compared with the standard value of 1.3325. Our results further indicated that the WDL consisted of a uniform layer rather than numerous isolated surface nanobubbles that distributed at the interface with high density.