Skin-attachable devices are essential to the realization of personalized skin health through continuously monitoring individual's skin surface pH. This paper describes an approach to measure the skin surface pH no matter when or where, just holding a NFC enable phone over the pH-sensor device capable of operating only with NFC energy harvesting. Since NFC can generate the power and batteries are replaced, the proposed device becomes smaller, lighter and thinner. Therefore, it could be attached on the skin by using the ultrathin polymer film called nanosheet. Moreover, the low-power circuit is proposed which implements the constant current circuit and the function of wireless communication.