Nanoparticle-fibrous membrane composites are provided as tunable interfacial scaffolds for flexible chemical sensors and biosensors by assembling gold nanoparticles (Au NPs) in a fibrous membrane. The gold nanoparticles are functionalized with organic, polymeric and/or biological molecules. The fibrous membranes may include different filter papers, with one example featuring a multilayered fibrous membrane consisting of a cellulose nanofiber (CN) top layer, an electrospun polyacrylonitrile (PAN) nanofibrous midlayer (or alternate material), and a non-woven polyethylene terephthalate (PET) fibrous support layer, with the nanoparticles provided on the fibrous membranes through interparticle molecular/polymeric linkages and nanoparticle-nanofibrous interactions. Molecular linkers may be employed to tune hydrogen bonding and electrostatic and/or hydrophobic/hydrophilic interactions to provide sensor specificity to gases or liquids. The sensors act as chemiresistor-type sensors. A preferred implementation is a sweat sensor.