Zeta potential values measured for uncoated SPIONs in Ro-3306 different suspension vehicles demonstrated a dramatic impact of charged buffer ions on the diffuse layer capacitance. This effect is further augmented by an increased particle concentration facilitating significant aggregation. The results from these experiments imply that surface adsorption of trivalent citrate ions most effectively protect SPIONs from aggregation. Even at a concentration of 1.0 mg/mL, the mean particle size was significantly smaller
than that measured for the same colloid at a 50-fold lower concentration in Hanks’ balanced salt solution (HBSS) or phosphate Tucidinostat buffered saline (PBS). Zeta potential values in excess of -32.4 mV implied strong electrostatic repulsion due to surface-associated, fully ionized citrate ions [23]. To fabricate lipid-coated Fe3O4 nanoparticles at the desired target size range of <200 nm, the avidin coating step was performed at 0.02 mg/mL in citrate buffer, which afforded particle populations with a mean hydrodynamic diameter of approximately 80 nm. The lipid composition was selected with the objective to fabricate find more thermoresponsive colloids that exhibit a transition temperature consistent with clinical hyperthermia applications (40°C to 45°C) [11]. Table 1 Physicochemical properties
of uncoated and lipid-coated SPIONs in different buffer solutions at pH 7.4 Buffer system Particle concentration (mg/mL) Particle size (nm) Zeta potential (mV) Uncoated SPIONs Lipid-coated SPIONs Uncoated SPIONs Lipid-coated SPIONs 1.0 520.0 ± 45.4 651.6 ± 25.3 -32.4 ± 1.0 -11.9 ± 1.4 Citrate, pH 7.4 0.24 286.6 ± 25.4 460.3 ± 15.4 -40.7 ± 1.4 -15.6 ± 1.4 0.02 80.0 ± 1.7* 179.3 ± 35.0** -47.1 ± 2.6* -19.1 ± 1.3** 1.0 1860.0 ± 180.9a 2422.0 ± 223.5a -11.2 ± 1.0 -4.5 ± 0.9 HBSS, pH 7.4 0.24 1255.0 ± 35.2a 1560.0 ± 135.2a -12.3 ± 1.1 -5.5 ± 1.0 0.02 580.0 ± 8.5 193.5 ± 32.6**
-23.3 ± 0.8 -7.4 ± 1.4 1.0 2800.0 ± 320.4a 2990.0 ± 412.5a -10.3 ± 0.5 -2.2 ± 0.6 PBS, pH 7.4 0.24 2214.0 ± 45.3a 2500.0 ± 245.3a mafosfamide -10.8 ± 1.0 -3.4 ± 1.1 0.02 931.0 ± 4.5 229.9 ± 12.42** -22.5 ± 0.8 -5.2 ± 1.6 Data are represented as mean ± SD (n = 4). aValue outside qualification range of Zetasizer Nano-ZS. *Significantly different from uncoated control SPIONs (p < 0.05). **Significantly different from lipid-coated SPIONs (p < 0.05). Earlier experiments performed in our laboratory with DPPC-coated SPIONs revealed limited colloidal stability in physiological buffer systems due to low surface charge (zeta potential -5.0 mV) [12]. DPPG is a negatively charged phosphatidyl glycerol with the same transition temperature as DPPC (i.e., 41°C). Stability of liposomes prepared with mixtures of these two phospholipids has been studied previously, and an equimolar lipid ratio was demonstrated to enhance colloidal stability [24].