Sensors were expressed in BL21(DE3) competent Escherichia coli. Cultures (500 ml) were grown overnight at 30°C with ampicillin (100 μg/ml) and induced with 1 mM isopropyl-β-d-thiogalactopyranoside at an optical density of 0.6. The bacteria were then spun down at 6000g for 30 min and resuspended in 10 ml of lysis buffer [50 mM sodium phosphate, 300 mM NaCl, and 10 mM imidazole, (pH 8)] with a protease inhibitor cocktail (11873580001, Roche) and 10 μM lysozyme. The resuspended cells were rocked for 30 min at 4°C, lysed with a tip sonicator, and spun at 14,000g for 30 min. The supernatant was incubated with 2 ml of nickel–nitrilotriacetic acid HisPur Resin (Thermo Fisher Scientific) and rocked at 4°C for 2 hours. The solution was then packed into a gravity column, washed three times with 5 ml of wash buffer [50 mM sodium phosphate, 300 mM NaCl, and 20 mM imidazole (pH 7.4)], and incubated with 4 ml of elution buffer [50 mM sodium phosphate, 300 mM sodium chloride, and 250 mM imidazole (pH 7.4)] for 5 min. The eluate was collected and dialyzed overnight into storage buffer [1× phosphate-buffered saline (PBS), 1 mM EDTA, and 2 mM β-mercaptoethanol], flash-frozen, and stored at −80°C. Fractions were characterized by SDS–polyacrylamide gel electrophoresis (SDS-PAGE), and the concentration was determined by ultraviolet–visible (UV-Vis) spectroscopy (fig. S1).

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