Piezoresistive Microcantilever with SAM-Modified ZnO-Nanorods@Silicon-Nanopillars for Room-Temperature Parts-per-Billion NO₂ Detection

Abstract

Organic–inorganic hybrids are ideal for gas detection, considering their selectivity and sensitivity to single gas species under moderate working conditions. However, the poor surface-to-volume ratio and low electron density of organic materials hinder their application in high-performance resistive gas sensors. Instead herein, a gravimetric sensor is realized on the basis of an in-plane self-actuating and self-reading piezoresistive microcantilever-chip (PMC), which is patterned with an (inorganic) 3D framework of ZnO nanorods on a Si-nanopillar array (3D ZnO-NRs@Si-NPLs) and functionalized by a thin (organic) self-assembled monolayer (SAM, (3-aminopropyl)trimethoxysilane (APTES)) for interacting with NO2. For stable adsorption/desorption rates of NO2, this SAM-on-3D ZnO-NRs@Si-NPL PMC (S3-PMC) was exposed to constant light illumination by an LED (wavelength: 530 nm, intensity: 10 mW/cm2), realizing a limit of detection (LOD) of about 2 parts per billion by volume (ppbv) for NO2 at room temperature, together with fast response and complete recovery within times of 42.1 ± 6.6 s and 112 ± 17.4 s, respectively, to NO2 concentrations ranging up to 1000 ppbv. Moreover, the sensor shows reliable stability under both short- and long-time (31 days) exposure to NO2, where resonance frequency-shift deviations of merely at most ±5% and ±9%, respectively, are observed. These unprecedented results indicate an enormous potential of the S3-PMC for portable gas sensor arrays in high-resolution real-time-monitoring applications.