Stable and tunable plasmon resonance of molybdenum oxide nanosheets from the ultraviolet to the near-infrared region for ultrasensitive surface-enhanced Raman analysis

Preparation of color-tunable and stable plasmonic MoO3 nanomaterials remains challenging, due to the lack of an effective preparation strategy and surface protection in heavily doped MoO3. Herein, we report a facile and reliable method for synthesis of oxygen-deficient MoO3 (MoO3-x) nanosheets using dopamine as the reducing agent and precursor for the formation of a polydopamine (PDA) surface coating. The PDA-coated MoO3-x nanosheets show stable and tunable localized surface plasmon resonance (LSPR) from the ultraviolet to the near-infrared region (361-809 nm) via altering the pH value of the medium, accompanying the generation of multicolor nanosheet dispersions, such as deep blue, faint bluish, orange, yellow and black. Importantly, the resulting PDA-coated MoO3-x nanosheets are quite stable even in the presence of oxidants, and they can be used as an ultrasensitive surface-enhanced Raman scattering (SERS) substrate. The limit of detection for rhodamine 6G (R6G) dye is down to 0.3 fM concentration, and the corresponding Raman enhancement factor reaches 1 x 10(10). The coupling of charge transfer between R6G and PDA-coated MoO3-x nanosheets and molecular resonances may be responsible for the strong SERS effect.