To measure the loading efficiency of DNA, 16 μl of the chitosan–DNA NP was spun down at 15,000 rpm for 30 min (Avanti J-25 centrifuge, Beckman Coulter Inc., Fullerton, CA, USA). The supernatant was mixed with Hoechst 33258 (0.1 μg/ml) at a ratio of 1:1 (v/v) and measured for DNA content in the fluorimeter at excitation and emission wavelengths of 350 and 455 nm, respectively. Hoechst 33258 is a bisbenzimidazole derivative that exhibits enhanced fluorescence upon binding to the A–T rich regions of double stranded DNA [17], thereby allowing for DNA quantification in solutions and cells by fluorometry [18]. DNA loading efficiency (%) for each batch of NP was the amount of condensed DNA (difference between the total amount of DNA used for the NP preparation and the amount of non-entrapped DNA remaining in the supernatant) calculated as a percent of the total amount of DNA used.