, ATP-driven 29 , although in this case proton translocation cannot 622 be triggered electrochemically, but addition of an initiator (e.g., ATP) is required. In the latter 623 case, there is no need to use a gold-modified cover slip. Moreover, provided that a suitable 624 membrane-impermeable and ion-sensitive fluorescent dye is used, this method can be extended 625 to other ionic pumps, e.g., sodium and potassium. 626 627 ACKNOWLEDGMENTS: 628 The authors acknowledge the BBSRC (BB/P005454/1) for financial support. NH was funded by 629 the VILLUM Foundation Young Investigator Program

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