JOURNAL OF COSMETIC SCIENCE 54 Table III Content Quantifi cation Results of Formulations Formulation code Drug content (%) ± SD F1 86.077 ± 1.294 F9 113.911 ± 2.520 F19 103.488 ± 1.352 F27 107.928 ± 1.043 F31 89.457 ± 0.373 The particle sizes of the formulations were in the range of 148.9 ± 1,019 and 1.688 ± 223.5 nm. In literature review, this value was generally lower than 400 nm for topical nanogel formulations, and in our study, formulations with similar particle size (F1, F19, and F27) were selected optimally (24,25). The PI describes the unequal degree of size distribution of the particles. The PI value ranging from 0.1 to 0.25 indicates that the particle size distribution is narrow, but if it is greater than 0.5, it shows a very wide dis- tribution. As the PI approaches 1, the particle size distribution increases. When long- term stability is desired, it is important to ensure that the PI parameter is as low as possible. Formulations with a PI of less than 0.5 were selected optimally. DETERMINATION OF ZETA POTENTIAL OF FORMULATIONS The zeta po tential of the formulations without vitamin C ac tive substance loading was performed, as described in the Method section. The zeta potentials of the nanogels are shown in Table II. Zeta potential is a scientifi c term for electrokinetic pote ntial in colloidal systems and a measure of surface charge. It can affect particle stability as well as cellular uptake and intracellular processes. The stability of the colloidal system depends on the balance be- tween the two opposing forces. These are van der Waals forces and electrical double lay- ers. The photon correlation spectroscopy technique was chosen to measure zeta potential. Positive zeta potentials or negative zeta potentials greater than 30 mV can lead to mono- dispersity. On the other hand, values less than 5 mV can lead to agglomeration. The zeta potential is infl uenced not only by the properties of the nanoparticles but also by the nature of the solution, such as pH and ionic strength. The formulations prepared have zeta potential between -16.01 ± 3.05 and 23 ± 1.84. The zeta potential of the formulations is expected to be positive because of the cationic charge of CS. However, only zeta potential of F1 formulation was negative. It suggests that there is a problem during preparation. The zeta potentials of nanogel formulations prepared by Islam et al. (26) were found to be positive because of the cationic charge of CS. They observed a reduction in zeta potential by the addition of alginate. According to the obtained characterization results, F19 formulatio n which is selected optimally based on particle size and zeta potential contains low–molecular weight CS. When medium– and high–molecular weight CS species are used, turbidity and pre- cipitate are formed, and particle size and PDI are higher than those of low–molecular weight CS formations.

PHARMACEUTICALLY OPTIMIZED TOPICAL NANOGEL FORMULATION 55 CONTENT QUANTITY DETERMINATION A validated HPLC method was used to measure the concentration of v itamin C in aque- ous phase. The determination correlation coeffi cient (r2) for the regression line was 0.9998 with the linear regression equation y = 11177.7 + 154703x. The method was validated according to International Conference on Harmonisation guidelines with respect to lin- earity (r2 = 0.9998), specifi city, precision as repeatability and reproducibility (coeffi cient of variation d 2%), accuracy (coeffi cient of variation d 2%), and stability (27). The limit of detection and the limit of quantifi cation of citamin C were found to be 0.061 and 0.123 μg/mL, respectively. Table IV pH Results of Formulations Formulation code PH F1 5.4 ± 0.004 F9 5.95 ± 0.003 F19 5.95 ± 0.007 F27 5.4 ± 0.011 F31 5.4 ± 0.005 Figure 1. Three-dimensional AFM image of formulations.