The authors report pH-dependent conductance behaviors of single-walled carbon nanotube (SWCNT) multilayer thin-film sensors. SWCNTs are functionalized with carboxylic groups, and layer-by-layer assembled alternatively with polycation, polydiallyldimethyammonium chloride on the microfabricated metal electrodes. Current-voltage (I-V) characteristics show that the conductance of SWCNT multilayer thin-film sensors decreases with increase of pH values. On the other hand, the multilayer resistors with polymethylmethacrylate (PMMA) as a passivation layer demonstrate the increasing conductance with pH, opposite to the case of the absence of PMMA layer. The conductance change in the absence of PMMA attributes to doping/undoping of holes as charge carriers by molecular protonation/deprotonation of p-type semiconducting SWCNTs. With PMMA layer, concentration gradient of hydrogen ions on the dielectric forms the gate bias voltage, which changes the conductance of underlying semiconducting SWCNT layer. Each pH-dependent behavior has versatile applications for chemical and biological detections.

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