Figure 2.

The absolute response of an array of probe-coated cantilevers to different SSC buffer NaCl concentrations. Four cantilevers were coated with different DNA probe sequences, namely BioB1 (BioB1′ denotes another cantilever coated with BioB1), BioB2, and BioB3, and one reference cantilever was modified with hexadecane thiol, which has no ionizable or polar groups. The large response of the reference hexadecane thiol cantilever, 380 nm, reflects the significant nonspecific contribution and changes in refractive index of the buffer solutions to the absolute signal of probe-coated cantilevers. However, a differential measurement deconvoluted the signal to reveal the component that was specific to only the probe DNA, namely a differential deflection of 200 nm arising from the electrostatic repulsion between phosphate groups on neighboring probe molecules and charge screening by Na⁺ in solution. The stepwise appearance of the signal after buffer exchange reflects both the efficient exchange of solvent in the liquid cell and the stability of the instrument.