Accepted Manuscript Ultrasonic assisted production of starch nanoparticles: Structural characterization and mechanism of disintegration Sami Boufi, Sihem Bel Haaj, Albert Magnin, Frédéric Pignon, Marianne Impéror-Clerc, Gérard Mortha PII: DOI: Reference:
S1350-4177(17)30429-7 http://dx.doi.org/10.1016/j.ultsonch.2017.09.033 ULTSON 3881
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
19 May 2017 16 September 2017 18 September 2017
Please cite this article as: S. Boufi, S. Bel Haaj, A. Magnin, F. Pignon, M. Impéror-Clerc, G. Mortha, Ultrasonic assisted production of starch nanoparticles: Structural characterization and mechanism of disintegration, Ultrasonics Sonochemistry (2017), doi: http://dx.doi.org/10.1016/j.ultsonch.2017.09.033
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Ultrasonic assisted production of starch nanoparticles: Structural characterization and mechanism of disintegration Sami Boufi1, Sihem Bel Haaj1, Albert Magnin4,5, Frédéric Pignon4, Marianne Impéror-Clerc5, Gérard Mortha2,3
1
University of Sfax- Sfax Faculty of Science-LMSE BP 802-3018 Sfax, Tunisia Univ. Grenoble Alpes, LGP2, F-38000 Grenoble, France 3 CNRS, LGP2, F-38000 Grenoble, France 4a Univ. Grenoble Alpes, CNRS, Grenoble INP*, LRP, 38000 Grenoble, France * Institute of Engineering Univ. Grenoble Alpes 5Laboratoire de Physique des Solides, CNRS, Université Paris-Sud, Université Paris-Saclay, 91400 Orsay, France 2
Abstract In this paper, the disintegration of starch (waxy and standard starch) granules into nanosized particles under the sole effect of high power ultrasonication treatment in water/isopropanol is investigated, by using wide methods of analysis. The
present work aims at a fully
characterization of the starch nanoparticles produced by ultrasonication, in terms of size, morphology and structural properties, and the proposition of a possible mechanism explaining the top-down generation of starch nanoparticles (SNPs) via high intensity ultrasonication. Our dynamic light scattering measurements have indicated a leveling of the particle size to about 40 nm after 80 min of ultrasonication. The WAXD, DSC and Raman have revealed the amorphous character of the SNPs. FE-SEM. AFM observations have confirmed the size measured by DLS and suggested that SNPs exhibited 2D morphology of platelet-like shapes. This morphology is further supported by SAXS. On the basis of data collected from the different characterization techniques, a possible mechanism explaining the disintegration process of starch granules into NPs is proposed. Key words: Starch, nanoparticles, ultrasonication, * Corresponding author: Sami Boufi University of Sfax-Faculty of science BP 1171-3000 Sfax-Tunisia-Fax: 216 74 274 437 E-mail
[email protected]
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1. Introduction Nanoparticles derived from natural resources, more specifically polysaccharides, have become a topic of increasing interest in academic research and industry. Many characteristics of these materials as their nanoscale size, biodegradable character, effective cost, inexpensiveness, non-toxicity [1], light weight, sustainability and non-hazardous effect on the health or the environment drive for further interest. Attention to SNP is relatively recent and dates back to 1996 with the pioneering work by Dufresne et al.[2] who prepared starch nanocrystals through a controlled acid hydrolysis of starch [3]. These SNCs, with a plateletlike morphology, were composed of stacks of elongated elements with a width of 5–7 nm and a length of 20–40 nm. Since this time, several papers focusing on less laborious way to prepare SNCs and other starch nanoparticles (SNPs) have been reported in the literature, e.g. [4, 5]. The preparation of SNPs using physical treatments without any chemical reaction arouses great interest. This eco-friendly approach is an alternative solution to reduce the processing time to generate SNPs, increase the yield in NPs production, and avoid multiple purification steps when adopting the hydrolysis approach. The isolation of SNPs using high-pressure homogenisation, leading to crystalline microparticles turning into amorphous nanoparticles with increasing run numbers, has been reported by Liu et al. [6]. Chin et al.[7] have reported on SNPs synthesized by precipitating dissolved starch solution in absolute ethanol under controlled conditions. However, the mean size of the particles remained large (between 200 to 400 nm) and adding a surfactant during the precipitation process was necessary to obtain particles of size around 200 nm. Another environmentally friendly mechanical approach to produce SNPs with a size less than 400 nm has been described in two patents [8, 9]. The process was based on a reactive extrusion process of cross-linked plasticized starch followed by grinding and high-speed dispersion in water. Another approach reported by Song et al.
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[10] has concerned investigation of the mechanism of SNPs formation via reactive extrusion, analyzing the mains parameters governing the particle size. The use of high-intensity ultrasound for the top-down generation and application of nano-sized materials has become increasingly widespread. High-intensity ultrasound is widely adopted for particle size reduction in emulsification, dispersion and deagglomeration, or milling. These primarily derive from cavitational collapse of bubbles generated during the ultrasonic irradiation, which is caused by the generation of high-velocity interparticle collisions and jets of liquid striking at large velocities onto the surface of particles. Although the ultrasound treatment of starch has been reported in many publications, the first work, to our knowledge, related to the possibility of producing SNPs by simply using a physical method based on high-intensity ultrasonication without chemical additives has been presented in our previous paper [11]. In this way, the complete conversion of the starch granules from micron to nano- scale has been shown to be effective after 75 min of ultrasonication at a temperature of 8-10°C. Changes in properties of starch from different origins, either in the granular or gelatinized form, after an ultrasound treatment have been studied in numerous papers. An interesting review [12] summarizes the current knowledge related to the influence of ultrasonication on composition, structure, physicochemical properties and modifications of starch. Although no consensus on how the ultrasonication affects the morphological and supermolecular structure of starch, we can express the following remarks resulting from the literature data: (i)
the ultrasonication has little effect on the polymorph type of starch [13]
(ii)
the ultrasonication does not significantly change the X-ray pattern of starch nor its crystallinity, [14,15]
(iii)
a decrease in enthalpy of starch gelatinization is observed after ultrasonication; this is explained by the disruption of the double-helical order [16,17] 3
(iv)
the ultrasonication reduces the viscosity of starch solutions after gelatinization, the extent of which depends on the intensity and duration of the sonication[18].
In fact, the effect of ultrasonication on starch is shown to involve extremely complex phenomena and also probably affected by the interference of different factors as ultrasonication power and frequency, temperature and treatment time, as well as the properties of starch dispersions (e.g. the concentration and botanical origin of starch and the dispersion solvent). However, it should be pointed out that most of the reported works have been concerned by chemical modifications. The purpose of this study is the pursuit of our investigation by considering the ultrasoundassisted production of SNPs without the aid of any chemical modification and the proposal of a possible mechanism explaining how SNPs are generated when submitting the starch granules to high intensity ultrasonication. A large range of characterization methods including XRD, Raman, DSC, SAXS, AFM and rheology has been employed in order to acquire accurate information on the morphology, the structural and colloidal properties of SNPs generated from ultrasound treatment of starch.
2. Materials and methods 2.1. Materials For our experiments, we have adopted waxy maize (WaxylisTM, >99% amylopectin, denoted by “WM” in the text) and standard maize starch (70% amylopectin, named in text SS) provided by Roquette S.A, Lesterm, France. 2.2. Starch nanoparticle preparation Corn starch granules (2 g) were suspended in 100 ml of a mixture water-isopropanol (50/50 wt%) and the beaker was placed in a jacketed glass thermostatically controlled by a circulating water system at a temperature of 10 ± 1°C. The suspension was subjected to ultrasonication using a 20 kHz Branson digital Sonifier S-450D (Germany) of power 400 W
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coupled with a horn with a tip diameter of 13 mm. The sonication was performed at 100% amplitude and the pulse mode was set as continuous for 2 s and paused for 2 s. Aliquot was sampled at regular interval and the particle size measured without any purification treatment. 2.3. Particle size determination Particle diameters were measured at 25◦C using a Malvern Nano-Zetasizer ZS (Malvern, UK) with a fixed scattering angle of 173°. The dispersions were not diluted before starting the measurements. Dynamic light scattering (DLS) measurements have provided a Z-average size for further comparisons of the different particles. 2.4. Wide-angle X-ray diffraction (WAXD) A Wide-angle X-ray diffraction method was carried out on powders obtained from freezedried SNP suspensions. Cu radiation, generated with a PAN analytical, Expert PRO MPD diffractometer equipped with an accelerator detector. Sweeps of 4-40° 2θ were made with a step size of 0.084° and step time measurement of 10 s. 2.5. Small angle X-ray scattering (SAXS) Firstly, SAXS experiments were performed in LPS-UMR 8502 (Paris) using a rotating anode generator (Cu radiation, 46 kV, 48 mA). The scattered X-rays were detected on a twodimensional CCD camera detector (Princeton Instruments) at a sample-detector distance of 475 mm and measurement of each sample lasted 4 hours and 12 images of 20 min acquisition time were averaged. The radially averaged intensity I(q) was obtained from the average image over a scattering vector range 0.02
