Flow cells

A method, comprising:

• introducing a first fluid to a flow channel of a flow cell including a working electrode having an unmodified surface that is at least partially exposed to the flow channel, wherein a complex present in the first fluid includes a linking moiety and an orthogonal functional group that does not attach to the unmodified surface, wherein the linking moiety chemically attaches the complex to the surface to form a temporarily modified surface of the working electrode and wherein the orthogonal functional group is biotin;
• introducing, into the flow channel, a second fluid including a hydrogel having i) a plurality of primers attached thereto and ii) a reactive functional group attached thereto that is reactive with the orthogonal functional group, wherein the reactive functional group is streptavidin;
• and
• performing a sensing operation involving the complex of the temporarily modified surface, wherein the sensing operation involves:
• amplifying a template nucleic acid strand using the plurality of primers to generate a cluster of template nucleic acid strands;
• introducing, into the flow channel, a third fluid including a plurality of optically labeled nucleotides;
• and
• optically detecting an incorporation event of a respective one of the plurality of optically labeled nucleotides into a nascent strand along at least some of the template nucleic acid strands;
• and
• applying a desorption voltage of the linking moiety to the working electrode, thereby detaching the linking moiety and regenerating the surface.