Gold Flake-Enabled Miniature Capacitive Picobalances

Measurement of masses of microscale objects or weak force with ultrahigh sensitivity (down to nanogram/piconewton level) and compact configuration is highly desired for fundamental research and applications in various disciplines. Here, by using freestanding gold flakes with high reflectivity (≈98% at 980 nm) as the sample tray and silica microfibers with extremely low spring constant (≈0.05 mN m^-1) as the cantilever beams, miniature capacitive balances are reported with piconewton-level detection limit (picobalances) and reliable radiation force-based calibration. In the design, the gold flake is suspended by two silica microfibers, which also functions as an electrode to form a capacitor with an underneath gold electrode. Benefitting from the high reflectivity of the gold flake, the performance of picobalances can be precisely calibrated by exerting piconewton-level radiation pressure on the gold flake (working as a mirror) with a laser, showing a detection limit as low as 6.9 pN. Finally, using a fiber taper-assisted micromanipulation technique, masses of various types of pollens (with weights ranging from 4.6 to 96.3 ng) are readily measured by a picobalance at single-particle level. The miniature picobalances should find applications in precise measurement of masses of micro or nanoscale objects and various types of weak forces.