XPS samples were prepared as we have previously described.24 (link) Substrates were cleaned by serial sonication in ultrapure water, acetone, and isopropanol for 10 min each followed by plasma cleaning at 60 W for 10 min (Harrick Plasma Cleaner, Ithaca, NY, USA). Then, 20-50 μL droplets of concentrated PDA or RV@PDA suspensions (~ 0.25-1 mg/mL) were placed onto clean gold-coated silicon substrates and dried overnight. Substrates were dried completely under high vacuum prior to analysis in a PHI 5600 spectrometer (PerkinElmer) equipped with an Al monochromated 2 mm filament and a built-in charge neutralizer. The X-ray source operated at 350 W, 14.8 V, and 40° take-off angle. The atomic concentrations (atom %) of nitrogen, oxygen, and carbon of drop-casted nanomaterial was determined relative to total nitrogen, oxygen, and carbon content by performing survey scans between 0 and 1100 eV electron binding energies. Charge correction was performed setting the C1s peak at 285.0 eV. Data analysis was conducted using MultiPak software version 9.6.015 (Physical Electronics, Chanhassen, MN, USA) and OriginPro 2017 software (Student version, OriginLab, Northampton, MA, USA).