Characterization of particle shape, zeta potential, loading efficiency and outdoor stability for chitosan-ricinoleic acid loaded with rotenone

Bohua Feng; Muhammad Aqeel Ashraf; Liufen Peng

doi:10.1515/biol-2016-0050

Article Open Access

Characterization of particle shape, zeta potential, loading efficiency and outdoor stability for chitosan-ricinoleic acid loaded with rotenone

Bohua Feng , Muhammad Aqeel Ashraf and Liufen Peng

Published/Copyright: December 2, 2016

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From the journal Open Life Sciences Volume 11 Issue 1

Abstract

Carboxymethyl chitosan grafted with ricinoleic acid (CMC-g-RA), an amphiphilic drug carrier, was synthetized, loaded with rotenone (Rot), and characterized for particles shape, zeta potential, loading efficiency and outdoor stability. Results show that as the ratio of carrier to drug increased, the formulation exhibited monodisperse nanoparticles negative surface charge. The loading efficiency of the formulation was up to 68%. The outdoor test also indicated that the formulation with the higher loading efficiency prevented Rot degradation in natural environments.

Keywords: nanocarrier; chitosan derivative; ricinoleic acid; drug carrier

1 Introduction

Water-dispersion pesticides are known to be environmentally friendly. Most lipid-soluble drugs can be solubilized in water using various surfactants, and polymers surfactants are superior to micro molecule surfactants in many respects. Surfactants with hydrophobic groups on polymer side chains could have much better loading capacity, which would improve water dispersion stability and prolong release time. Furthermore, the biodegradability of polymers surfactants would be beneficial to environmental protection. Chitosanis known as a biodegradable alkaline polysaccharide, having pharmacological activity and growth regulation for crops [1]. The self-aggregation of the chitosan derivative, modified with a hydrophobic group, may indicate that could be an effective carrier for lipid-soluble drugs [2].

It our early research, we synthetized a new chitosan derivative by grafting ricinoleic acid onto carboxymethyl chitosan (CMC-g-RA, Fig. 1) [3]. Both hydrophobic long carbon chain (ricinoleic acid, RA) and hydrophilic carboxymethyl were grafted onto a chitosan chain. RA is well known as a natural green pesticide, which suggested that CMC-g-RA could form self-assembling polymer micelles, allowing lipid-soluble drugs to be solubilized in neutral water. This is significant because lipid soluble pesticides have been conventionally dispersed in highly toxic solvents with additives required for emulsion, concentration, and suspension, usually with particle sizes greater than 1 μm It was considered that minimizing particles size to nanoscales would be beneficial to improving the bioavailability of drug, owing to the large surface area to volume ratio [4]. In this study, the well-known botanical and lipid-soluble pesticide rotenone (Rot) [5] would be carried by CMC-g-RA. The Rot degradation could be restrained with the protection of a carrier, which would be evaluated in an outdoor test. Micro molecule surfactants RA-Na were selected as control group. The methods of the water dispersion formulation are demonstrated in detail.

Fig. 1

Preparation of CMC-g-RA [3].

2 Material and Methods

2.1 Materials

CMC-g-RA and RA-Na were prepared the Department of Materials Science and Engineering at Jinan University (Guangzhou, China), rotenone standard was purchased from Sigma (USA), and rotenone was obtained from Jiaxing fuli chemistry factory (98%, Fengshun of Guangdong, China).

2.2 Devices

EM (XL-30E, Philips, Netherland), FTIR (Equi-nox55, Bruk, Germany), ZetaPALS (Brookhaven, USA), Elemental Analyzer (EA2400II, Perkin-El-mer, USA), Sprayer (QH-003, Qihua Plastic Fact-ory), HPLC (1100, Agilent, USA), Supercentrifuge (3K30, Sartorius-Sigma Laboratory Centrifuges, Germany), Freeze drier(FD1.0-60E, Heto, Denmark).

2.3 Preparation of Rot/CMC-g-RA water dispersion

The samples of CMC-g-RA were dissolved in deionized water to obtain a carrier solution (10 mg/ml). The Rot solution was then mixed with the carrier solution to form Rot/CMC-g-RA water dispersions, with individual Rot concentrations ranging from 0.20 to 0.20 mg/ml.

2.4 Characterization of Rot/CMC-g-RA water dispersion formulation

After ultracentrifuging Rot/CMC-g-RA (25,000 rpm, 4°C, 30 min), the precipitation was collected for freeze drying. Then the molecular structure of this precipitation was identified using ATR-FTIR (IR) spectrograms. Particles sizes, polydispersity indexes (PDI) and zeta potentials were measured with the ZetaPALS. Morphological structures were characterized by SEM after dropping and drying the solutions on base chips. The concentration of unloaded drug in supernatant (1.5 ml) was measured by HPLC, and the loading efficiency (LE) was calculated. HPLC details: Chromatographic column 250 mm × 4.0 mm, C18 reversed-phase column, column temperature 30°C, mobile phase volume ratio methanol: wate r= 75: 15, flow velocity 1.0 ml/min, wavelength 281 nm and sampling volume 15 μl were followed.

2.5 Outdoor stability

Rot/CMC-g-RA water dispersion was first prepared by Rot (0.10 mg/ml) and CMC-g-RA (0.50 mg/ml). Then control groups prepared by small molecule surfactant RA-Na and water/acetone solution were obtained with the same Rot concentration. After nylon sheets (size 20 cm × 20 cm per sheet) were immersed in the water dispersion and control groups respectively. All sheets were dried outdoor under sun from 1 to 11 days, which were then submerged in methanol to remove unbound the Rot every 2 days. Rot concentrations were measured by HPLC.

3 Results and Discussion

3.1 Rot/CMC-g-RA FTIR spectroscopy analysis

FTIR spectrograms of CMC-g-RA particles, Rot/CMC-g-RA and Rot were separately shown in Fig. 2.

Fig. 2

FTIR spectrums of CMC-g-RA(a), Rot/CMC-g-RA(b) and Rot(c).

In the CMC-g-RA spectrogram plot (Fig. 2a), a substitution reaction occurred on -NH₂ groups, shifting the adsorption peak of from 1597 cm^-1 to 1560 cm^-1. The peak at 1735 cm^-1 identifies the amide carbonyl produced by the reaction. The characteristic absorption peak of the C=C appeared at 3018 cm^-1, and C-H stretching vibration peak at 2920 cm^-1 and 2852 cm^-1 became sharp and strong. A peak at 720 cm^-1 was observed, corresponding to the -(CH₂)_n adsorption peak (n > 4) of the RA. In the Rot/CMC-g-RA spectrogram (Fig. 2b), the main adsorption peaks shown in Fig. 2a were present, although with weaker intensity.

The clear peak at 1151 cm^-1 corresponds to the C-O-C group of the Rot molecule. The above changes indicate that Rot was loaded on carrier CMC-g-RA.

3.2 Nanoparticle morphological structure

The SEM photos of Rot/CMC-g-RA particles are shown in Fig. 3. They appear as smooth spherical particles less than 500 nm in diameter. The hydrophilic carboxymethyl groups oriented toward the water phase and the hydrophobic RA chain toward the oil phase, resulting in a shell formation which reduced surface tension. Rot was effectively loaded on the carrier, and the hexagonal plate crystalline form of Rot was not observed

Fig. 3

SEM photos of Rot / CMC-g-RA nanoparticles, drying at ambient temperature (a); drying at 60 C (b) & (c) where [CMC-g-RA] = 0.50 mg/ml, [Rot] = 0.10 mg/ml. A simulation graph of polymers cage-shaped micelle (d), where green and gray represent hydrophobic and hydrophilic segments, respectively [6].

Rot/CMC-g-RA particles with porous structures were also identified (Fig. 3b) by drying the solution at 60C before SEM observation instead at room temperature. The details in Fig. 3c clearly demonstrated the coreshell structure of Rot/CMC-g-RA particle. A model graph according to self-consistent mean field theory (SCMFT) could be drawn like Fig. 3d, which showed amphiphilic polymers could form porous cage-shaped particle. It was inferred that the higher drying temperature accelerated the solvent evaporation, thus the shape of Rot/CMC-g-RA particles in water dispersion was reserved.

3.3 Effects of concentration on nanoparticles size, PDI and Zeta potential

To explore the characteristics of particle size, PDI and zeta potential in formulation Rot/CMC-g-RA under varying concentrations of carriers and drugs were measured using dynamic light scattering (DLS). Results listed in table 1 show particles sizes between 200 and 400 nm, where the PDIs were less than 0.25nm. The smaller PDI suggests a more uniformity organization [7]. Since carboxymethyl groups oriented toward the water phase as expected, zeta potential data in table 1 indicates a negative surface charge.

Table 1

Particles size, PDI and zeta potential under varying concentrations of CMC-g-RA and Rot.

CMC-g-RA [mg/ml]	Rot [mg/ml]	Particles size [nm]	PDI	Zeta potential [mV]
0.10	0.02	190.2 ± 12.3	0.433 ± 0.002	-58.50 ± 0.91
0.10	0.03	289.7 ± 20.5	0.344 ± 0.002	-55.62 ± 1.68
0.10	0.04	271.5 ± 19.9	0.284 ± 0.002	-40.73 ± 5.49
0.10	0.05	256.4 ± 20.0	0.157 ± 0.002	-50.50 ± 1.52
0.10	0.06	376.3 ± 27.3	0.185 ± 0.003	-45.55 ± 5.52
0.30	0.02	316.2 ± 17.5	0.206 ± 0.002	-36.98 ± 0.95
0.30	0.03	402.3 ± 31.0	0.171 ± 0.001	-63.27 ± 0.68
0.30	0.04	345.3 ± 27.1	0.153 ± 0.001	-68.30 ± 2.04
0.30	0.05	475.2 ± 37.2	0.138 ± 0.004	-60.33 ± 2.15
0.30	0.06	562.2 ± 28.2	0.154 ± 0.001	-58.47 ± 1.29
0.50	0.02	218.4 ± 18.1	0.075 ± 0.001	-28.00 ± 4.21
0.50	0.03	352.2 ± 24.8	0.081 ± 0.001	-60.14 ± 6.85
0.50	0.04	341.2 ± 18.6	0.112 ± 0.001	-56.81 ± 1.75
0.50	0.05	306.9 ± 17.5	0.140 ± 0.001	-69.64 ± 2.91
0.50	0.06	511.4 ± 26.7	0.137 ± 0.001	-52.28 ± 0.95
0.70	0.02	251.5 ± 20.0	0.104 ± 0.001	-45.84 ± 13.66
0.70	0.03	253.6 ± 16.0	0.126 ± 0.001	-54.69 ± 5.28
0.70	0.04	309.6 ± 16.1	0.009 ± 0.000	-65.13 ± 0.91
0.70	0.05	348.9 ± 22.3	0.003 ± 0.001	-74.69 ± 2.21
0.70	0.06	355.8 ± 21.1	0.010 ± 0.001	-70.35 ± 6.62

In the water dispersion formulation, the particles size could be regulated by altering carriers or drugs concentrations. The minimum particles size was 190.2 nm when lower CMC-g-RA and Rot concentrations were present (Fig. 4a). CMC-g-RA concentration was increased from 0.1 to 0.3 mg/ml coupled with greater particles sizes, while Rot concentrations were kept steadily as shown in Fig. 4e. Particles size increased to562.2 nm, and appeared after Rot concentrations were increased to 0.06 mg/ml, while concentrations of CMC-g-RA were invariant.

Fig. 4

Effect of CMC-g-RA concentrations on particles sizes, a-e: [Rot] = 0.2-0.6 mg/ml.

The linear relationship between the solubilizing capability and micelle size is known to increase with the concentration of polymer carrier [8]. Higher carrier concentrations mean that adding additional hydrophobic RA groups result in better compatibility between CMCg-RA and Rot when the Rot concentrations are constant. Accordingly, the Rot molecules tend to enter hydrophobic domains in CMC-g-RA micelles. Furthermore, according to the steric stabilization theory, coverage of CMC-g-RA would reduce interfacial energy among the Rot and water, and this may help form a stable water dispersion status. The interfacial energy mentioned could also be lower after increasing Rot concentrations while maintaining constant CMC-g-RA concentrations. When the ratio of carriers to drugs was less than 10 (Fig. 5), particles size increased along with the growing ratio, but if CMC-g-RA concentration increased as well ([CMC-g-RA] > 0.3 mg/ml in Fig. 5), the force among micelles, surface micelle molecules, and solution molecules dramatically increased as well. Thus, bigger micelles would disaggregate to form smaller ones due to the decreasing free energy from mixing. Therefore, particles sizes were reduced correspondingly, shown in Fig. 5, where ratio of carriers to drugs were more than 10.

Fig. 5

Effect of [CMC-g-RA]/[Rot] on particles size.

By adjusting the concentration ratios of CMC-g-RA to Rot, particle sizes in nanoscale could be controlled. As the PDI should be less than 0.25 to unify particles and improve the bioavailability of drugs, the Rot concentrations remained constant while the PDIs became much smaller with CMC-g-RA increasing concentrations (Fig. 6). For example, CMC-g-RA concentrations increased from 0.1mg/ml to 0.5 mg/ml while PDIs decreased from 0.344 to 0.081 (Fig. 6b). When the ratios of carriers to drugs were less than 10, PDIs decreased rapidly along with the increasing ratios, as shown in Fig. 7a.

Fig. 6

Effect of CMC-g-RA concentrations on PDIs, a-e: [Rot] = 0.2–0.6 mg/ml.

Fig. 7

Effect of [CMC-g-RA]/[Rot] on PDIs (a) and Zeta potentials (b).

Zeta potentials were also influenced by the concentration ratios of CMC-g-RA to Rot. As, shown in Fig. 7b. higher zeta potentials (usually > 30 mV) handled stronger electrostatic repulsive forces, which was beneficial to maintain steady water dispersion formulation [9]. Zeta potential increased rapidly with increasing ratios up to 1:15, reaching a maximum -74.69mV as concentrations of CMC-g-RA and Rot were 0. 70 mg/ml and 0.05 mg/ml respectively (Table 1). Because more carboxymethyl groups with negative charges were added to the formulation along with increasing carrier concentrations, the stronger affinity among hydrophobic groups in carriers and drugs resulted in more carboxymethyl groups specifically entering the water phase. In brief, stable and uniform nanoparticles could be obtained when concentration ratios of CMCg-RA to Rot were regulated between 5 and 10.

3.4 LE of Rot/CMC-g-RA nanoparticles

According to HPLC data, the Rot standard’s retention time was 5.718 min [10], and LE of Rot/CMC-g-RA samples could be calculated by the equation below:

LE of Rot/CMC−g - RA=A−BA

where A is total Rot concentrations and B is free Rot concentration in formulation supernatant.

Firstly, the LEs increased to 68% and then decreased to 22% along with increasing of CMC-g-RA concentrations (Fig. 8a), when the Rot concentration of Rot/CMCg-RA samples was 0.05 mg/ml. During this process, concentration ratios of CMC-g-RA to Rot ranged from 2 to 18. This trend is similar to that observed in Fig. 5, where the peak value of LE corresponded to [CMC-g-RA]/[Rot] = 10. When Rot concentration was 0.10 mg/ml (Fig. 8b), the LE increased from 22% to 65% along with increasing CMC-g-RA concentration, which coincided with the trends shown in Fig. 5 when the ratios of [CMC-g-RA]/[Rot] < 10.

Fig. 8

LE of Rot/CMC-g-RA, [Rot] = 0.05 mg/ml (a) and [Rot] = 0.10mg/ml (b).

3.5 Outdoor stability of water dispersion

The stability of Rot/CMC-g-RA was evaluated by measuring residual Rot concentrations under sampling times in leaching liquor (Fig. 9). Samples ([CMC-g-RA]/ [Rot] = 5, [CMC-g-RA] = 0.50 mg/ml, [Rot] = 0.10 mg/ml) with the LE of 68 % (Fig. 9b) were chosen, with Rot/RA-Na - Rot samples as control groups. During 11 days of outdoor testing, the residual Rot concentration of the samples was higher than control groups. In the first 3 days, Rot concentration decreased from 5.8 ppm to 3.7 ppm, while control groups curves dropped more rapidly. After 4 days the residual Rot concentration remained at 2.3-1.1 ppm. Polymer carrier CMC-g-RA efficiently restrained the Rot degradation when compared to control groups. High LE implied the unloading Rot molecules were seldom outside the micelle, which would not reduce the Rot concentration rapidly. The polymer CMC-g-RA also provided effective protection for Rot compared with small molecule carrier RA-Na or no carrier in water. Thus, the carrier CMC-g-RA was beneficial for stabilizing, releasing, and maintaining higher concentrations of Rot.

Fig. 9

Rot concentrations under sampling times in leaching liquor of Rot/CMC-g-RA (a), Rot/RA-Na (b) and Rot water dispersion (c).

4 Conclusion

The amphiphilic CMC-g-RA self-assembled into polymeric micelles in water, acting as an efficient drug carrier for the lipid-soluble pesticide Rot. The Rot/ CMC-g-RA water dispersion were prepared successfully by blending carrier and drug together. Rot/CMC-g-RA particles were smooth and spherical in nanoscale, and this small-size effect could correspondingly improve the infiltration and bioavailability of drugs. Adjusting the concentration ratios of CMC-g-RA to Rot, especially [CMC-g-RA]/[Rot] = 5 to 10, resulted in nanoparticles with smaller and more uniform particles size, as well as and higher zeta potential. Rot was protected effectively by carriers in micelles, with the highest LE of Rot/CMC-g-RA at 68% when [CMC-g-RA]/[Rot] = 5. Therefore, Rot was stably released over the 11 days in an open environment. The results of this may be applied to the production of non-toxic drug carriers that can be used solubilize, stabilize, and control the release of lipid-soluble Rot in water. This will contribute to the development of green and efficient nano-botanical pesticide preparing methods for water-based formulations. Future studies will characterize the insecticidal effects of Rot/CMC-g-RA water dispersion and its performance on improving bioavailability.

Acknowledgments

The authors would like to thank professor Zi-Yong Zhang for his guidance. This project was supported by Medical Scientific Research Foundation of Guangdong Province, P.R. China (B2014203) and Science & Technology Planning Project of Guangdong Province, P.R. China (2016A020215162). The authors also appreciate the Polymers Laboratory in the Department of Materials Science and Engineering at Jinan University for supporting this work.

Conflict of Interests: The authors declare no conflict of interest regarding the publication of this paper.

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Received: 2016-3-5

Accepted: 2016-8-21

Published Online: 2016-12-2

Published in Print: 2016-1-1

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