Piperazine n n bis 2 ethanesulfonic acid pipes

Superparamagnetic iron oxide nanoparticles with an amphiphilic polymer coating possessing different terminal groups [carboxyl (negative net charge) and amino (positive net charge)] were purchased from Ocean NanoTech, USA. The size of NPs, measured by bright-field scanning transmission electron microscopy (BF STEM), was 15.3 ± 2.3 nm and 20.2 ± 2.6 nm, respectively. Chemicals used to prepare the buffer solutions, i.e., Na₂HPO₄, NaH₂PO₄, 4-(2-hydroxyethyl) piperazine-1-ethanesulfonic acid (HEPES), piperazine-N,N′-bis(2-ethanesulfonic acid) (PIPES), ammonium carbonate, and ammonium bicarbonate, as well as sodium hydroxide, sodium chloride, and iron(II) chloride, were purchased from Sigma-Aldrich (St. Louis, MO, USA). Vanadium standard solution was purchased from Fluka (Buchs, Switzerland). Iron ICP standard solution was obtained from Merck Millipore (Darmstadt, Germany). Albumin from human serum was purchased from Sigma-Aldrich (USA). Ultrapure Milli-Q water was obtained from a Millipore Elix 3 system (Merck Millipore) and used throughout this study. The following gases of purity ≥99.999% (Messer, Bad Soden, Germany) were used for the collision/reaction cell in ICP-MS/MS: oxygen, helium, hydrogen, and ammonia (10% NH₃ in He).