© The Electrochemical Society. Resonant sensors for ultimate sensing are developed. By scaling down, the sensitivity of a magnetic resonance force microscopy probe is improved, and three-dimensional imaging of radical density in a poly-10-(4-vinylbenzyl)-10H-phenothiazine particle is demonstrated at room temperature. Also resonant thermal sensor with a microchannel is developed and applied to detect heat from a single brawn fat cell.