NiCl2 pretreatment: 10 mM NiCl2 aqueous solution was deposited onto a freshly cleaved mica surface and incubated for 1 h. An incubation time of 1 h was chosen based on our previous work [45 (link)]. It should be noted, however, that equivalent results as reported here were also obtained with shorter incubation times, i.e., 1 min to 30 min. The mica substrate was then rinsed with HPLC-grade water to remove excess NiCl2.
PLL pretreatment: PLL was dissolved in HPLC-grade water to yield a 0.1% w/v PLL solution. The PLL solution was deposited onto a freshly cleaved mica surface and incubated for 1 h. An incubation time of 1 h was chosen based on literature to ensure maximum surface coverage [64 (link)]. The mica substrate was then rinsed with HPLC-grade water to remove excess PLL.
Spdn pretreatment: Spdn was dissolved in HPLC-grade water to yield a 5 mg/mL Spdn solution and then deposited onto a freshly cleaved mica surface. After incubation for 5 min the mica substrate was rinsed with HPLC-grade water. An incubation time of 5 min was chosen based on literature [58 (link)].