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(a) Sensor electrical scheme. Four piezoresistors are arranged in a ¼ active wheatstone bridge configuration: two are embedded into the cantilevers and two into the chip substrate. The bridge can be balanced by two 1 kΩ potentiomenters in series with the substrate resistors. (b) SEM image of the cantilevers and close up of the cantilever end. (c) SEM image of the substrate. Each substrate chip hosts 4 rows of 100 μm high pillars. On the top of each pillar, a 10 μm high tip is defined. The radius of the tip varies between 10 nm and 300 nm.
(a) Sensitivity measurement. An AFM probe performs an F-z curve on the cantilever under test. The approach and retract curves are represented in black and red, respectively: from the slope of the retract curves, we calculate a sensitivity of 394 μV/μm. (b) Sensor noise power spectral density (PSD) in air. Considering the 1 Hz-10 kHz bandwidth, we have a noise of 3.7 μV in air (black) and 3.8 μV in water (blue). The Hooge and thermal noise fittings are reported in gray.
(a) Force sensor chip onto the nose cone. The die is bonded onto the PCB which is held by a magnet placed into the nose cone. (b) F-z measurement in air (black and red curves) and water (blue and light blue). Approaching the sample in air, a jump-on contact event is detected (black line). Retracting the cantilever for 1 μm is not sufficient to overcome the adhesion force, therefore there is no jump-off-contact (red curve). In water, instead, there are no jump phenomena and the sensitivity has no significant variation. From the slopes, we calculated sensitivities of 354 μV/μm and 359 μV/μm for air and water environments, respectively. (c) The peak to peak noise has a value of 18 μV in air (between 0 and 6 s) and in grounded PBS solution (between 12 and 18 s). When the solution is not grounded (between 6 and 12 s), the noise is more than 2 times higher. There is no significant noise variation between air and grounded PBS.
AFS curves obtained functionalizing the cantilever by b-BSA and the substrate by neutravidin. (a) No event is detected. (b) Non-specific event. The slope of the retract curve is constant before the jump-off. (c) Specific unbinding event between biotin and neutravidin. The slope of the retract curve varies before the jump off. The unbinding force is around 70 pN.
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