Self-assembling siloxane bilayer directly onSiO2 surface of micro-cantilevers for long-term highly repeatable sensing to trace explosives

Abstract

This paper presents a novel sensing layer modification technique for static micro-cantileversensors that detect trace explosives by measuring specific adsorption-induced surface stress.For the first time, a method of directly modifying a siloxane sensing bilayer on anSiO2 surface is proposed to replace the conventional self-assembled monolayers (SAMs)of thiols on Au to avoid the trouble from long-term unstable Au–S bonds. Formodifying the long-term reliable sensing bilayer on the piezoresistor-integratedmicro-cantilevers, a siloxane-head bottom layer is self-assembled directly on theSiO2 cantilever surface, which is followed by grafting another explosive-sensing-groupfunctionalized molecule layer on top of the siloxane layer. The siloxane-modified sensorhas experimentally exhibited a highly resoluble response to 0.1 ppb TNT vapor.More importantly, the repeated detection results after 140 days show no obviousattenuation in sensing signal. Also observed experimentally, the specific adsorptionof the siloxane sensing bilayer to TNT molecules causes a tensile surface stresson the cantilever. Herein the measured tensile surface stress is in contrast tothe compressive surface stress normally measured from conventional cantileversensors where the sensitive thiol-SAMs are modified on an Au surface. The reasonfor this newly observed phenomenon is discussed and preliminarily analyzed.