Dependence of electrochemical charging of gold nanoparticle monolayer films on counterion proximity

• We report the dependence of nanoparticle film charging on the counterion size.
• The monolayer film charging occurs with small counterions but not big ones.
• The dependence on the counterion size is related to the counterion proximity.
• Monolayer film charging is coupled by formation of ion pairs with counterion.

Herein we report the nontrivial effect of counterion size on the electrochemical charging of gold nanoparticle (or called MPC) monolayer films in aqueous electrolyte solutions. The electrochemical charging response was observed with relatively small counterions (e.g., BF4−, ClO4− and PF6−) but not large ones like bis(trifluoromethylsulfonyl)amide (Tf2N−), tetrakis(pentafluorophenyl)borate (TB−) or bis(triphenylphosphoranylidene) (BA+). This effect was explained by the proximity of counterions to the MPC surface, which determines whether the electronic charges on MPCs can be effectively compensated by formation of ion pairs. Only small counterions are permitted to approach the MPC surface by permeating the alkanethiolate layer to achieve a sufficient charge compensation. In contrast, big ions are sterically excluded and thus disfavor the electrochemical charging. This finding helps in further understanding the ion-coupled electrochemical charging mechanism of nanoparticle films.

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