Fig. 2. r ₁ relaxivities of NaGdF₄@bp-peptide NPs (black line, r₁ = 5.8 mM⁻¹ s⁻¹) and NaGdF₄@CPP NPs (red line, r₁ = 7.9 mM⁻¹ s⁻¹) as a function of the molar concentration of Gd³⁺ in the solution.