Effect of Humidity on Electrical Conductivity of Pristine and Nanoparticle-Loaded Hydrogel Nanomembranes

Abstract

We studied here the effect of humidity on the electrical conductivity of pristine and nanoparticle-loaded hydrogel nanomembranes. The membranes were fabricated by the thermally activated cross-linking of amine-and epoxy-terminated, starbranched poly(ethylene glycol) (PEG) oligomers. The resistance of the pristine membrane changed by ∼5.5 orders of magnitude upon relative humidity (RH) variation from 0% to 100%, which is an unprecedented response for homogeneous materials. The dependence of the resistance on the moisture uptake into the membrane could be coarsely described by an exponential function. The loading of the membranes with gold and silver nanoparticles (NPs) resulted in a noticeable improvement of their conductance at low RH but in a small improvement or even a negative effect on the conductance at high RH. Both pristine and NP-loaded PEG hydrogel membranes have significant potential as highly sensitive elements in humidity sensors and moistureresponsive nanoelectronic devices. ■ INTRODUCTIONThere is a continuous demand for light, small, and flexible assemblies based on advanced inorganic and organic materials for different applications as sensors and stimuli-responsive systems. A particularly important area is humidity-sensitive systems, which are of significance in various fields, such as medicine, agriculture, industry, goods storage, and environmental monitoring. In this context, different materials including but not limited to metal oxides, ceramics, and polymers have been used, relying on different kinds of transduction techniques, such as capacitive, resistive, hygrometric, and gravimetric ones. 1,2 Since recently, nanostructured materials and organic/inorganic hybrid systems are utilized because of their superior performance in comparison to macroscopic onecomponent systems. Among nanostructured materials, monoclinic VO 2 nanostructures 3 and VS 2 ultrathin nanosheets 4 can be mentioned, showing particularly high sensitivity at a relative humidity (RH) above 50%. 4 As representative organic/ inorganic hybrid systems, tin oxide nanoparticle (NP) loaded cellulose, 5 Li-loaded nanoporous polymers, 6 multilayer graphene oxide/polyelectrolyte nanocomposite films, 7 multiwalled carbon nanotubes dispersed in cross-linked polyelectrolyte, 8 and LiCl−polymer composite nanofibers 9 can be listed, showing quite promising performance at high humidity 5,7 but also in the entire RH range. 7−9Among organic materials used in humidity-responsive hybrid assemblies, hydrogels are in particular attractive because of their intrinsic ability to absorb moisture, affecting their mechanical, optical, and electrical properties. 10−12 Some of the recently developed humidity-sensitive systems include superabsorbent poly(acrylamide)−montmorillonite composite hydrogels, 13 fluophore-loaded acrylamide, 12 hydrogel-actuated nanorod assembly, 11 and conductive polymer hydrogel cross-linked by phytic acid in poly(N-isopropylacrylamide) matrix. 14 Most of the above assemblies are, however, complex, requiring in some cases also a ...

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