Angstrom-scale-porous plasmonic molybdenum oxide for ultrasensitive optical chemical sensing

Abstract Nanoporous plasmonic nanostructures based on noble metals have received extensive attention on the high-performance detection of chemical analytes. However, the size of such pores is rarely at the angstrom scale given the limitation of current fabrication methods. This leads to a relatively poor performance in the detection of ions. Here, we demonstrate the formation of angstrom-scale pores in ultra-thin plasmonic ammonium doped molybdenum oxide through crystal nucleation. The molybdenum oxide octahedra assemble into hexagonal rings, in which dopants fill in parts of the rings to stabilize the crystal structure and simultaneously generate a broad plasmon resonance across the visible to near-infrared regions. As a result, the unfilled centers of the rings effectively become angstrom-scale pores. Na+ ion sensing capability is investigated by integrating plasmonic ammonium doped molybdenum oxide onto D -shaped optical fibers. The ions are facilely accommodated within the pores and induce a charge re-distribution in the host. This alters the plasmon resonance behavior and modulates the optical output of the fiber transducing platform, through a strong light-matter interaction. The structure is sensitive to a wide concentration range of Na+ ions from subnanomolar (sub-nM) to submolar (sub-M) with the limit of detection (LOD) of ~5 fM, and high selectivity in both the aqueous solution and simulated serum conditions, which is a superior sensitivity over other reported optical ion sensors. This work demonstrates the strong potential of angstrom-scale porous plasmonic materials for chemical detection, and the possibility of being integrated with popular optical transducing platforms for practical high-performance sensing.