Metal-organic framework thin film for enhanced localized surface Plasmon resonance gas sensing

Despite its high refractive index sensitivity, localized surface plasmon resonance (LSPR) spectroscopy has been generally restricted to large biological analytes. Sensing of smaller molecules is a compelling target for this technique; in particular, LSPR spectroscopy could be utilized to detect hazardous or toxic gases and manage industrial processes involving gaseous chemicals. Here, we report sensing of pure gases over Ag nanoparticles using LSPR spectroscopy, where the detected changes in bulk refractive index are <5×10^-4 refractive index units (RIU). We further demonstrate a novel strategy for amplifying the sensing signal by coating the plasmonic substrate with a metal-organic framework (MOF) material. Cu₃(BTC) ₂(H₂O)₃, BTC = benzenetricarboxylate, was grown on Ag nanoparticles using a layer-by-layer method in order to control the MOF thickness, which we show greatly affects the sensor response. Preferential concentration of CO₂ within the MOF pores produces a 14-fold signal enhancement for CO₂ sensing. In principle, MOFs can be tailored for sorbing different analytes, making them ideal materials for this amplification strategy. Because the sensing signal originates in the nanoparticle extinction spectrum and not in the MOF itself, this comprises a generalizable sensing scheme applicable to any porous MOF and any analyte.