With the remarkable light-matter interaction and in-situ surface plasmons tunability, 2D plasmonic materials are favored in various disciplines. At the current stage, however, most 2D plasmonic practices are restrained intrinsically at mid-infrared range which brings limitations to integration- and miniaturization-oriented practical applications. To address such challenge, we demonstrate here to employ highly doped atomically thin transition metal oxides (TMOs) as an alternative class of 2D plasmonic material. Molybdenum trioxide (MoO3) is a representative TMO of which the layered crystalized structure enables the formation of 2D morphology. We synthesize and characterize few-layer α-MoO3 nanoflakes highly doped with free electrons, which facilitate surface plasmons in near-infrared (NIR) region. After doping of abundant free electrons, the resultant sub-stoichiometric α-MoO3-x nanoflakes possess quasi-metallic plasmonic behaviors. In our biosensing demonstration, we integrate the MoO3-x nanoflakes with a flexible microfiber which is compliant with the commonly used and cost-effective optical system. The proposed highly integrated fiber-optic biosensor provides a detection limit of bovine serum albumin (BSA) as low as 1 pg/mL.
The layered MoO3 nanoflakes are synthesized by liquid phase exfoliation. Observed under TEM and HRTEM, the as-prepared MoO3 samples are flake-like in shape and preserve the same single crystal nature as their bulk counterparts. AFM reveals the average thickness of MoO3 nanoflakes is ~2.8 nm, which is exactly the thickness of two planar units of α-MoO3. Abundant free electrons are introduced into MoO3 nanoflakes via H+ intercalation, and the color of nanoflakes suspension gradually turns from colorless to dark blue. Meanwhile, a strong absorption peak of nanoflakes suspension appears at ~735 nm. From XPS analysis, we verify that the free electron density increases, and two oxidation states Mo6+ and Mo5+ coexist in MoO3-x after H+ intercalation and account for 71.7% and 28.3%, respectively. Since MoO3-x is positively charged, we immobilize the nanoflakes via electrostatic interaction onto a microfiber evenly functionalized with negative charges. Benefited from the compactness of microfiber, 50 µL MoO3-x nanoflakes suspension is adequate for inducing strong plasmon resonance in the transmission spectrum. The positively charged MoO3-x nanoflakes stabilized on microfiber surface show good affinity to negatively charged BSA molecules. The plasmon resonance performs linear response as BSA concentration increases from 1pg/mL to 100 ng/mL. Based on the surface plasmon attenuation band, we deduce the Drude model of MoO3-x and numerically prove the enhanced plasmon-matter interaction induced by MoO3-x. This study reveals unprecedented potentials of employing 2D TMOs in highly sensitive plasmonic devices with access to frequently used optical windows, high degree of integration and flexibility, and simple fabrication procedures.