Michael Reynolds1 Marcos Guimarães1 Hui Gao1 2 Kibum Kang1 2 Jiwoong Park1 2 Paul McEuen1

1, Cornell University, Ithaca, New York, United States
2, The University of Chicago, Chicago, Illinois, United States

Measuring electrochemical potentials in complex environments is crucial for better understanding of biological and chemical systems. Optical schemes for electrostatic and chemical potential detection are non-invasive, scalable and can be applied to many different systems, but usually have a slow response compared to electrical detection methods. Furthermore, methods using organic fluorophores often suffer from bleaching due to exposure to high-intensity light. These issues can be addressed by using non-organic optical sensors. Among such sensors, two-dimensional (2D) materials are attractive because they are thin, flexible, and compatible with photolithographic patterning.
We show that monolayer MoS2 can be used as a 2D screen to optically detect real-time changes in potential in fluid environments. We characterize the photoluminescence response of MoS2 to fluid potential by liquid-gate measurements in an aqueous electrolyte solution, showing the capability to detect sub-mV changes at timescales of a few milliseconds, only limited by our experimental setup sensitivity. By performing cyclic voltammetry near our MoS2 devices in a solution of ferrocene, we demonstrate that the photoluminescence of electrically floating MoS2 responds to the chemical potential of the solution in the presence of redox active molecules. Based on these findings, we use an array of MoS2 “pixels” to image the ionic diffusion of ferrocene ions (ferrocenium) in real time. These results indicate that monolayer MoS2 can be used as an optical detector of potentials of electrogenic cells and of redox-active biomolecules. Furthermore, the flexibility and ease of transfer of MoS2 devices allows them to be placed on relevant structures such as flexible substrates and optical fibers in order to measure systems of interest.