Fig. 4.

Correlation of the extent of hybridization x with the dimensionless group Π. Onset of hybridization was observed at Π = 2.2. Π was calculated using Eq. 1 and C_P = S_PN_P/HN_A, where S_P is the experimentally determined probe coverage, N_P = 20 is the number of phosphate charges per probe, N_A is Avogadro's number, and H is thickness of the probe layer. H was approximated by the probe contour length, H = 10 nm, based on polyelectrolyte brush theory, which predicts the probes to be highly stretched at the hybridization cross-over due to the still significant osmotic pressure of small ions (31).