All reagents used in this work were of analytical grade. The deionized water (resistivity ≥18 MΩ cm at 25 °C) was obtained from a Milli-Q Advantage-0.10 purification system (Millipore). ACE2 Fc Chimera, Human was obtained from GenScript. Spike protein was kindly donated by Scott Hensley, University of Pennsylvania, Philadelphia. EDC and NHS with a degree of purity ≥98%, gold(III) chloride trihydrate (HAuCl40.3H2O) (99.99%), sodium borohydride (NaBH4) with ≥98% purity, cysteamine hydrochloride (cys) with 98% purity, phosphate buffer saline solution, pH = 7.4, and glutaraldehyde (25%, vol/vol) were purchased from Sigma-Aldrich. Graphite pencils 0.7-mm and 0.9-mm diameter under the trade name of Pentel were purchased in a local store in Philadelphia. Ag/AgCl conductive ink was acquired from Creative Materials.
Electrochemical measurements were carried out using a MULTI AUTOLAB M101 potentiostat with six channels, controlled by the NOVA 2.1 software. Cryogenic vials were used as the electrochemical cell (2.0-mL volume), each containing three electrodes. Graphite pencils (0.9 mm) were used as reference and counterelectrodes, and graphite pencils (0.7 mm) were used as the WE. The Ag/AgCl ink was painted over one of the graphites to create the reference electrode.
Electrochemical measurements were carried out using a MULTI AUTOLAB M101 potentiostat with six channels, controlled by the NOVA 2.1 software. Cryogenic vials were used as the electrochemical cell (2.0-mL volume), each containing three electrodes. Graphite pencils (0.9 mm) were used as reference and counterelectrodes, and graphite pencils (0.7 mm) were used as the WE. The Ag/AgCl ink was painted over one of the graphites to create the reference electrode.