NOVELTY - A sensor includes sensing material that changes volume when exposed to target particle(s); and a transducing platform (525) comprising a piezoresistive component to sense change in volume of the sensing material. The sensing material is positioned over the piezoresistive component.    USE - As chemical sensor, e.g. hydrogen sensors for detecting hydrogen gas leaks and for monitoring and controlling hydrogen-based processes for fuel cells; carbon monoxide sensors for detecting unsafe levels of carbon monoxide in a home or garage; propane sensors used in conjunction with gas grills; or industrial sensors for detecting unsafe levels of chemicals or toxins at chemical plants, coal mines, or semiconductor fabrication facilities.    ADVANTAGE - The transducing platform has a supporting structure (521-524) that helps support the piezoresistive component and to yield to placement of strain on the piezoresistive component, thus helping to enhance sensitivity of piezoresistive component to change in volume of sensing material. The heater of the transducing platform helps control temperature of sensing material to help control sensitivity of sensing material to target particle(s) optionally in the presence of non-target particles.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) a sensing device comprising a sensor comprising a piezoresistive layer and sensing material over the piezoresistive layer, and a controller to sense a resistance of the piezoresistive layer; and    (2) a method comprising forming over a substrate (510) a first layer comprising a piezoresistive material to sense change in volume of one or more layers over the first layer, and forming over the first layer a second layer comprising a material that changes volume when exposed to a target particle.    DESCRIPTION OF DRAWING(S) - The figure is a plan view of micro-hotplate structure for piezoresistive chemical sensor.    Micro-hotplate structure (500)    Substrate (510)    Supporting structure (521-524)    Platform (525)    Heater (530)    Lead (531, 533)