Hyper-branch sensing polymer batch self-assembled on resonant micro-cantilevers with a coupling-reaction route

Abstract

In order to develop a volume fabrication route for chemical sensors, hydrogen-bond acidic hyper-branched polymer is synthesized and directly self-assembled in-batch onto MEMS resonant micro-cantilevers as sensing layer. The batch self-assembly is via a facile coupling reaction between the SH group in the self-assembled monolayer (SAM) and one OH group in the hyper-branched polymer. During the self-assembly process of the hyper-branched polymer, the polyurethane-industry used isophorone diisocyanate (IPDI) is employed as the key coupling reagent, since the two NCO coupling groups in IPDI feature different reactivity. The NCO group with higher reactivity is firstly reacted with the SH group pre-grown at the cantilever surface. Then, another NCO group with lower reactivity reacts with one OH group in the hyper-branched polymer. In this way the hydrogen-bond acidic hyper-branch polymer can be batch grafted onto a run of micro-cantilevers for uniform sensing to targeted gas. Through this route, the resonant cantilever gas sensors can be batch produced for uniform detection of trace organophosphates (OPs). Based on the specific interaction between the OH sensing group and the PO group in OP, the targeted molecules can be captured by the hyper-branched sensing polymer, and the mass addition induced frequency shift of the resonant cantilever is output as sensing signal. Sensing experiment is implemented for the sensors taken from one fabrication batch, resulting in satisfactory consistency in sensing performance. Taking 5 sensors as example, the deviation of the sensing response to 2ppm dimethyl methylphosphonate (DMMP) is less than 1%. Besides, the sensor shows good repeatability/selectivity and good linear response to various concentrations (300ppb to 1.5ppm) of DMMP.