Experiments for single particle electrostatic trapping were conducted using negatively charged gold nanoparticles (Au NPs) with a diameter of 80 nm (BBI Solutions, EM.GC80). The purchased gold nanoparticles had intrinsic –COOH functional groups present on the particle surface. These –COOH groups are exposed to the environment and dissociate into H+ and COO− in the aqueous solution, providing a net negative surface charge density to the nanoparticles.
Sample solutions for experiments were prepared using 1 ml of negatively charged 80 nm Au NP solution in a 1.5 ml Eppendorf tube (RNase-free Microfuge tubes, Thermo Fisher Scientific). Nanoparticles were washed by exchanging particle buffer solution with DI water in the following steps: (i) centrifuge the solution at a 2000 relative centrifugal force (rcf) for 20 mins; (ii) discard the supernatant, and resuspend the pellet in 1 ml of DI water; and (iii) repeat the previous cleaning steps (i–ii) two times, and in the last step, resuspend the pellet in 50 μl of DI water instead of 1 ml. The prepared solution had a mM salt concentration remaining from the particle buffer solution and ∼1012 particles/ml.
Sample solutions for experiments were prepared using 1 ml of negatively charged 80 nm Au NP solution in a 1.5 ml Eppendorf tube (RNase-free Microfuge tubes, Thermo Fisher Scientific). Nanoparticles were washed by exchanging particle buffer solution with DI water in the following steps: (i) centrifuge the solution at a 2000 relative centrifugal force (rcf) for 20 mins; (ii) discard the supernatant, and resuspend the pellet in 1 ml of DI water; and (iii) repeat the previous cleaning steps (i–ii) two times, and in the last step, resuspend the pellet in 50 μl of DI water instead of 1 ml. The prepared solution had a mM salt concentration remaining from the particle buffer solution and ∼1012 particles/ml.