Hydrogen peroxide solution

Analytical-grade L-arginine (ARG), L-histidine (HIS), L-aspartic acid (ASP), and L-glycine (GLY) were purchased from Acros Organics. Other amino acids including L-cysteine (CYS), L-proline (PRO), L-methionine (MET), L-tyrosine (TYR), and L-glutamic acid (GLU) were obtained from MP Biomedicals at >98% purity. The structures of the selected amino acids are shown in Fig. S1. PAA solution (∼39% PAA, 6% H₂O₂ and ∼45% acetic acid, w/w), sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium thiosulfate, and sodium borohydride were purchased from Sigma Aldrich (St. Louis, MO, USA). Hydrogen peroxide solution (∼30% H₂O₂ in water, w/w) was purchased from Fisher Scientific (Waltham, MA, USA). Reagent-grade deionized water (≥18 mΩ-cm) was generated from a Milli-Q integral water purification system (Millipore, Billerica, MA, USA) and used to prepare working solutions. The working solution of each amino acid at 10 mM was freshly prepared and used within 24 h. Fresh PAA working solutions at 100 mM were prepared every week by diluting the PAA stock. The H₂O₂ working solution was prepared at 11 mM. All stock and working solutions were stored at 4–5 °C before use.

Following incubation of cells with Gd-EOB-DTPA for 90 min, the media was aspirated and cells were washed with sterile PBS. The cells were then treated with 0.25 % trypsin-EDTA to detach from the surface, and centrifuged at 350 × g for 5 min. The cells were washed with sterile PBS an additional 5 times, and then approximately 5 × 10⁶ cells (determined by cell counting using a C-chip^™ disposable hemocytometer, Fisher Scientific) were lysed using 0.5 ml RIPA lysis buffer (Santa Cruz Biotech, Dallas, TX). Subsequently, 2 mL of 70 % (v/v) concentrated nitric acid (Fisher Scientific) was added to each sample, stirred, and heated carefully on a hot plate for approximately 24 h, inside a fume hood. After this, 1 mL of 30 % hydrogen peroxide solution (Fisher Scientific) was added to each sample, stirred, and further heated on hot plate for ~ 24 h in a fume hood until the sample solution appeared clear and colorless. Each sample was then diluted using deionized water to reach a final concentration of 10% (v/v) nitric acid. The gadolinium concentrations in the ensuing samples were determined by Thermo iCAP 6300 Inductively coupled plasma atomic emission spectrometer (ICP-AES), using serial dilutions of a gadolinium ICP standard (Inorganic Ventures, Avantor^™) for calibration.