Controllable particles sizes of starch nanoparticles were synthesized via a precipitation in water-in-oil microemulsion approach. Microemulsion method offers the advantages of ultralow interfacial tension, large interfacial area, and being thermodynamically stable and affords monodispersed nanoparticles. The synthesis parameters such as stirring rates, ratios of oil/cosurfactant, oil phases, cosurfactants, and ratios of water/oil were found to affect the mean particle size of starch nanoparticles. Starch nanoparticles with mean particles sizes of 109 nm were synthesized by direct nanoprecipitation method, whereas by using precipitation in microemulsion approach, starch nanoparticles with smaller mean particles sizes of 83 nm were obtained.
Starch is one of the most commonly used biopolymers in industries because of nontoxicity, biodegradability, biocompatibility, low cost, and being renewable and abundantly available in nature [
Various synthetic methods for synthesis of starch nanoparticles such as high-pressure homogenization and miniemulsion cross-linking [
Interest in using microemulsion for nanoparticles synthesis arises mainly from the versatile nature of microemulsion system such as mild reaction conditions, simple procedure [
In this work, starch nanoparticles with controllable particle size were prepared by precipitation of locally available native sago starch (
Native sago (
1% (w/v) starch solution was prepared by dissolution of 0.5 g native sago starch powder in 50 mL of 0.5 M NaOH solution. The mixture was heated to 80°C in a water bath for 1 hr with magnetic stirring until all starch powder was completely dissolved and homogeneous starch solution was obtained. This starch solution was then cooled to room temperature.
Starch nanoparticles were formed when 1 mL of starch solution was added dropwise into 20 mL of absolute ethanol solution. The same procedure was repeated by varying the stirring rates (300 rpm, 600 rpm, and 900 rpm) and magnetic stirring for 1 hr in order to investigate the effect of stirring rates on the particle size and morphology of starch nanoparticles.
1 mL of starch solution was added dropwise to an oil phase (15 mL of cyclohexane, 5 mL of ethanol, and certain amount of surfactant) with magnetic stirring at 900 rpm for 1 hr. The same procedure was repeated by varying the surfactants concentrations, ratios of oil/cosurfactant, oil phases (hexane, olein palm oil, sunflower oil, and oleic acid), cosurfactants (methanol, propanol, butanol, and acetone), and ratios of water/oil.
The morphologies of samples were observed by using a scanning electron microscope (SEM) (JOEL JSM-6390 LA). The mean particles sizes of around 100 starch nanoparticles were measured randomly using SmileView software.
0.5 M of sodium hydroxide (NaOH) was used to completely dissolve the native sago starch powder into aqueous solution with constant magnetic stirring. The dissolution of starch was carried out at 80°C in order to ensure complete solubilization of starch granules. Dissolution of starch was achieved by disrupting the starch granule to release the starch molecules into the solution. NaOH has commonly been used for dissolution of starch [
Schematic representation of alkalization reaction between starch and sodium hydroxide.
Figure
Effects of stirring rates on particles size of starch nanoparticles.
As shown in Figure
SEM micrographs of (a) native sago starch; starch nanoparticles prepared (b) without stirring; and (c) with stirring rate at 900 rpm.
The effects of surfactant concentrations on the particles sizes of starch nanoparticles were investigated by using various concentrations of span 60 (3 : 1 ratio of cyclohexane and ethanol) as shown in Figure
Effect of various span 60 concentrations on the mean particles sizes of starch nanoparticles.
In the presence of surfactant, the particle sizes of starch nanoparticles sizes were observed to decrease since surfactant reduced the interfacial tension between oil and water phases and stabilized the dispersed phase against coalescence once it was formed [
As can be observed from Figure
Effect of various ratios of cyclohexane/ethanol on the mean particles sizes of starch nanoparticles.
However, increase of cosurfactant up to 1 : 5 ratio of cyclohexane/ethanol resulted in larger particle size of around 121 nm due to dilution effect of microemulsion as high cosurfactant volume led to destruction of the microemulsion droplets [
Figure
Effect of types of oil phases on the mean particles sizes of starch nanoparticles.
SEM micrographs of starch nanoparticles prepared in various oil phases: (a) cyclohexane, (b) hexane, and (c) oleic acid.
Starch nanoparticles prepared using cyclohexane and hexane produced smaller nanoparticles; however the particles were observed to be more aggregated. When oleic acid was used, more monodispersed and discrete nanoparticles were formed due to the more hydrophobic nature of oleic acid. However, oil with excessive long hydrocarbon chains or high molecular weight such as palm oil and sunflower oil was difficult to microemulsify [
Figure
Effect of types of cosurfactants on the mean particles sizes of starch nanoparticles.
Increase of alcohol chain used for synthesis has resulted in increase of the particles sizes of starch nanoparticles. This could be due to the lipophilicity of the cosurfactant that increased with the carbon chain length from propanol to butanol and the longer the alcohol chain, the less effective the cosurfactant because it is more soluble in the oil phase [
Figure
Effect of various ratios of starch solution/oleic acid on the mean particles sizes of starch nanoparticles.
For synthesis of smaller nanoparticles, it is essential to maintain low water/oil ratio as increase of water fraction in the microemulsion system would increase the particles sizes of starch nanoparticles due to reinforcement of the interfacial tension between water and oil in the microemulsion system [
Controllable particles sizes of starch nanoparticles were successfully synthesized by precipitation in microemulsion system under controlled conditions. All of the starch nanoparticles obtained were spherical in shape and have uniform particles sizes distribution. Direct nanoprecipitation method without microemulsion produced larger particle size in the range of
The authors of this paper have no direct financial relation with the commercial entities mentioned in this paper.
The authors declare that there is no conflict of interests regarding the publication of the paper.
Financial support by Ministry of Higher Education (MOHE) Fundamental Research Grant Scheme (FRGS), Grant no. 01(17)746/2010(32), and MyBrain (My Master) Programme for graduate scholarship were gratefully acknowledged.