Phi 5600 spectrometer

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).

Successful PEGylation of pQCT NPs was verified by XPS. Briefly, TiO₂-coated silicon substrates were cleaned by consecutive sonication in Milli-Q water, acetone, and isopropanol for 10 min each, followed by drying under a stream of N₂ and then plasma discharge (Harrick Plasma Cleaner, Ithaca, NY, USA) at 60 W for 5 min. A drop of each NP suspension was then placed onto the surface of the substrates and left to dry overnight. Substrates were completely dried under vacuum prior to analysis using a PHI 5600 spectrometer (PerkinElmer, Waltham, MA, USA) equipped with an Al monochromated 2 mm filament and a built-in charge neutralizer. The X-ray source operated at 350 W power and 15.0 V voltage. Survey scans were performed between 0 and 1200 eV electron binding energies. High resolution spectra of the C 1s, N 1s, and O 1s regions were obtained between 280 – 305, 395 – 410, and 525 – 535 eV, respectively. Charge correction was performed by setting the C 1s peak at 285 eV. Data analysis was conducted using MultiPak software version 9.6.015.