Figure S1 in the Supporting Information plots a linear fit to the standard curve by measuring absorbance at 570 nm, which can be expressed as Y ) 0.008 47 + 0.002 73X. Then, a sample solution of spheres was prepared at a concentration of 0.0002 g/mL. The number of amine groups on the surface of the spheres was determined by substituting the absorbance measured following the reaction of the mixture of the sample solution with the ninhydrin reagent into the formula. Details of the procedure can be found in the manual on the ninhydrin reagent which is available on the Web site of the product.