(a, c) Schematics illustrating SiNW FET sensors modified with antibodies at low and high density, respectively. (b, d) Power spectra recorded from devices modified with PSA antibodies at low and high density, respectively, for PSA concentrations from 0.15, 5 and 150 pM. The data are red circles, while black continuous lines correspond to Lorentzian fits. For comparison, a Lorentzian curve with a characteristic frequency of 3800 Hz is also added into each figure, shown in green dashed lines.