Abstract
Here, we report a label-free optical relative refractive index (RI) sensing technique based on the image contrast of surface plasmon resonance microscopy (SPRM). The positive or negative contrast of a SPRM pattern reflects the relative RI of a nanosized object to the medium. Through directly imaging the relative RI, we classified two kinds of surface nanobubbles generated by varied pathways. The former type of nanobubbles nucleated from local oversaturation of gas molecules had a RI lower than that of water and a transient lifetime. The second type of nanobubbles formed by a methanol-water exchange process had a RI higher than that of water and a long lifetime. The relative RI of surface nanobubbles indicated that the gas state is sparse for transient nanobubbles and compressed inside the stable nanobubbles.