Sensing of dissolved-gas concentration in water using a rugate-structured porous silicon membrane

A constant pressure difference was applied between opposite sides of a rugate-structured porous silicon (RPS) membrane placed in water, causing water to pass through the nanosized pore structure of the membrane from the high-pressure side to the low-pressure side. During this process, we found that bubbles formed and grew on the membrane surface on the low-pressure side. Further, when reflectivity spectra were measured at the RPS membrane surface with the bubbles, the peak intensity of the stop-band decreased linearly as a function of time. In addition, the declining slopes of the stop-band intensity curves were found to be different depending on the total dissolved-gas concentration in the water. Accordingly, we propose that the experimental setup used in this study can be utilized as an optical sensor to sense the total dissolved-gas concentration in liquids. A mechanism for the bubble formation and growth is also discussed to explain the experimental results.