  Flowing water can be used as an energy source for generators, providing a
major part of the energy for daily life. However, water is rarely used for
information or electronic devices. Herein, we present the feasibility of a
polarized liquid-triggered photodetector in which polarized water is sandwiched
between graphene and semiconductor. Due to the polarization and depolarization
processes of water molecular under the driven of photogenerated carriers, a
photo-sensitive current can be repeatably produced, resulting in a
high-performance photodetector. The response wavelength of the photodetector
can be finely tuned as a result of the free choice of semiconductors as there
is no requirement of lattice match between graphene and the semiconductors.
Under zero voltage bias, the responsivity and specific detectivity of Gr/NaCl
(0.5 M)/N-GaN reaches values of 130.7 mA/W and 2.8*10^12 Jones under 350 nm
illumination. Meanwhile, using a polar liquid photodetector can successfully
read the photoplethysmography signals to produce accurate oxygen blood
saturation and heart rate. Compared with the commercial pulse oximetry sensor,
the average errors of oxygen saturation and heart rate in the designed
photoplethysmography sensor are~1.9% and ~2.1%, respectively. This study
transfers the message that water can be used as high-performance photodetector
in informative industries.
