Preparation and characterization of pH-responsive Pickering emulsion stabilized by grafted carboxymethyl starch nanoparticles

Journal Pre-proofs Preparation and Characterization of pH-Responsive Pickering Emulsion Stabilized by Grafted Carboxymethyl Starch Nanoparticles Zhigang Xiao, Lishuang Wang, Chunyue Lv, Shilong Guo, Xuanxuan Lu, Liwei Tao, Qingsong Duan, Qingyu Yang, Zhigang Luo PII: DOI: Reference:

S0141-8130(19)37469-0 https://doi.org/10.1016/j.ijbiomac.2019.10.261 BIOMAC 13762

To appear in:

International Journal of Biological Macromolecules

Received Date: Revised Date: Accepted Date:

16 September 2019 28 October 2019 28 October 2019

Please cite this article as: Z. Xiao, L. Wang, C. Lv, S. Guo, X. Lu, L. Tao, Q. Duan, Q. Yang, Z. Luo, Preparation and Characterization of pH-Responsive Pickering Emulsion Stabilized by Grafted Carboxymethyl Starch Nanoparticles, International Journal of Biological Macromolecules (2019), doi: https://doi.org/10.1016/ j.ijbiomac.2019.10.261

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

© 2019 Published by Elsevier B.V.

Preparation

and

Characterization

of

pH-Responsive

Pickering Emulsion Stabilized by Grafted Carboxymethyl Starch Nanoparticles

Zhigang Xiaoa1, Lishuang Wanga1, Chunyue Lva, Shilong Guoa, Xuanxuan Luc, Liwei Taoa, Qingsong Duana, Qingyu Yanga*, Zhigang Luob*

a. College of Grain Science and Technology, Shenyang Normal University, Shenyang 110034, China; b. School of Food Sciences and Engineering, South China University of Technology, Guangzhou 510640, China; c. Department of Food Science, Rutgers, The State University of New Jersey, 65 Dudley Road, New Brunswick, New Jersey 08901, USA.

Abstract: Copolymer nanoparticles with pH-responsive properties were prepared by grafting different amounts of 2-(dimethylamine) ethyl methacrylate (DMAEMA) on carboxymethyl maize starch (CMS). The formation of CMS-g- DMAEMA was verified by different techniques and was then used for stabilization of Pickering emulsion at different pH values. The CMS-g-DMAEMA nanoparticles showed pH-responsive properties in Pickering emulsion and the pH-responsivity increased as the amount of DMAEMA increased in the CMS-g-DMAEMA copolymer. The prepared Pickering emulsion underwent emulsion/demulsification transitions at different pH

1

values. The microscopy analysis verified the attachment of CMS-g-DMAEMA on oil-water interface of Pickering emulsion. Pickering emulsion showed different droplets size, contact angle, and Zeta-potential depend on pH value and CMS/DMAEMA ratio in the CMS-g-DMAEMA copolymer. The obtained results revealed that CMS-g-DMAEMA copolymer can be successfully used for preparing and stabilizing of Pickering emulsion at pH 10. Keywords: Carboxymethyl corn starch; Pickering emulsion; pH-responsive; Microstructure; Rheological property

2

1. Introduction Pickering emulsions are emulsion systems stabilized by solid particles at the oil/water interface instead of surfactants [1,2]. Pickering emulsions are highly stable, biologically compatible, and environment-friendly compared with traditional surfactant stabilized emulsions [3,4,5]. Different particles such as carbohydrate, protein, and lipid-based particles have been used to stabilize Pickering emulsions [6,7,8]. However, phase separation is also necessary in the fields of functional food ingredient delivery, biocatalysis, drug delivery, food storage, pharmaceutical, material coating, cosmetics, and emulsion polymerization, where stimuli-responsive Pickering emulsions could be switched by emulsion/demulsification under different conditions [3,6,7,8]. Nowadays, there is increasing interest in developing of stimuli responsive materials for controlled formation of responsive Pickering emulsions that can undergo reversible transition because of internal or external conditions such as temperature, pH, light, electrolytes, CO2, and redox [3,6,7]. Nanoparticles with good switchable interfacial properties are desirable in stabilized responsive Pickering emulsion. The surface activity of Pickering emulsion stabilization particles can be well controlled, resulting in on/off switch ability in response to stimuli such as CO2 [9], pH [10], temperature [11], magnetic fields, metal ions [12], or redox changes [13]. The chemical properties of pH responsive molecules and their interfacial characteristics can be controlled by manipulating pH of a solution. The molecular assembly and disassembly of responsive particles can be adjusted by external stimuli because of the controllable behavior and reversibility of noncovalent interaction. The solid particles 3

undergo a fast reversible phase transition from a soluble configuration to an insoluble configuration between acids and bases aqueous solutions. Responsive polymers, including lipids, starch, protein, chitosan, and other macromolecules have attracted increasing interest because of the increasing demand for ‘‘intelligent’’ materials [14]. Starch based polymers, which exhibit response to the external stimuli changes, such as temperature, pH, light, magnetic fields, and ionic strength have attracted increasing interest [15,16]. 2-(dimethylamine) ethyl methacrylate (DMAEMA) is a pH- and thermo- sensitive monomer and the copolymerization of DMAEMA with hydrophilic and hydrophobic sections can be controlled. Due to non-toxicity of the DMAEMA graft copolymers, they were used for biological applications such as gene and drug delivery [17]. Polymers of hydrophilic and hydrophobic transition ability attracted great interest because of their advantages to promote transformation, formation, or deformation of aggregates in response to pH. The amino and carboxylic groups in starch-based polymers help to achieve emulsion/emulsion of pH-responsive emulsion [6,8,18]. The pH-responsive DMAEMA grafting carboxymethyl starch (CMS) nanoparticle was synthesized via free

radical

polymerization

in

the

presence

of

the

crosslinker

N,N′-methylenebisacrylamide (BIS) and the initiator potassium persulfate [6]. The introduction of carboxyl groups into native starch could provide carboxymethyl starch (CMS) derivatives with good pH-swichability [19,20]. Also, DMAEMA is considered as a superior candidate to reversibly starch particle for stabilization of Pickering emulsions because of its good pH-sensitivity. Amphiphiles can be fabricated without 4

purification, and the phase transition is simple and rapid. However, although switching between emulsification and

demulsification using pH-responsive

amphiphiles in a reversible transition has many potential industrial applications, few studies have been performed on the formation of stimuli-responsive amphiphiles. In order to explore novel pH responsive nanoparticles for stabilizing Pickering emulsion, it is necessary to understand the mechanism of pH responsive Pickering emulsion and the pH responsive nanoparticles in oil/water or water/oil emulsion system. The objective of this work was to synthesize a pH-responsive copolymer by grafting DMAEMA to surface of carboxymethyl maize starch (CMS). The resulting responsive grafted starch nanoparticles (CMS-g- DMAEMA) were used as stabilizer in oil/water or water/oil emulsion system and the effect of different pH values on the stability of the prepared Pickering emulsions was investigated.

2. Materials and methods 2.1 Materials Carboxymethyl maize starch (CMS) was purchased from Yan Xing Chemical Co., Ltd (China). The degree of substitution of CMS was 0.12. Grape seed oil was provided by Liaoning Jiashi Nutritional Plant Oil Development Co., Ltd. (Shenyang, China).2-(Dimethylamine)

ethyl

methacrylate

(DMAEMA),

and

N,

N′-methylenebisacrylamide (BIS), and potassium persulfate (KPS) were purchased from Aladdin Chemical Reagent Co., Ltd. (Shanghai, China). All other chemicals and reagents used in the study were of analytical grade. 5

2.2 Preparation of CMS-g-DMAEMA Nanoparticles The paste of carboxymethyl maize starch (CMS) was prepared by suspension of 5 g of carboxymethyl maize starch in 95 mL of distilled water in a 250 mL three-necked flask with a magnetic stirrer. Starch gelatinization was initiated by rising the temperature of starch suspension to 100 °C and maintained for 30 min. The gelatinized CMS paste was cooled to 60 °C and 3 mL of (NH4)2S2O8 (KPS) aqueous solution (1.0 wt.%) was added. Then, a pH-responsive monomer (DMAEMA) grafted starch nanoparticles were prepared through polymerization reaction in the presence of N, N-methylenebisacrylamide (BIS, 0.05 g) at 65 °C for 3 h. For complete activation of CMS, the system was stirred gently and bubbled with a slow stream of nitrogen for 30 min to obtain carboxymethyl starch copolymer. Then, 0.3 g of the cross-linking agent (BIS) and certain amounts of DMAEMA monomer (1, 2, or 3 g, equal to 20, 40, and 60 g / 100 g CMS) were dissolved in 15 mL of distilled water and then added to the CMS paste to initiate the free radical polymerization. Based on the DMAEMA/CMS ratio (g/ 100 g), samples were marked as CMS-20, CMS-40, and CMS-60. The obtained starch copolymer was washed and centrifuged for 5 times to destroy long-chain DMAEMA homopolymers and remove impurities. Then, dialysis (MW cutoff of 12 000 Da) of the residues was conducted for 36 h in ethanol to remove un-grafted monomers and short chain polymers to obtain grafted copolymers nanoparticles. The nanosized granulation process was performed using an ethanol precipitation method. Starch paste (10 mL) was sucked with a disposable syringe, 6

then injected dropwise in ethanol at 1 mL/min rate. The obtained suspensions were stirred at 600 rpm for 15 min, centrifuged, and freeze dried. The grafting efficiency ˄GE%˅was calculated by the following equation [21]:

GE (%)

mass of grafted polymer u 100 mass of grafted polymer+mass of homopolymer

(1)

2.3 Preparation and stabilization of Pickering emulsions CMS-g-DMAEMA nanoparticles of varying grafting efficiency were used for preparing and stabilization of Pickering emulsions. The grape oil/water emulsions were prepared at a constant oil/water ratio of 1:2 (v/v) and CMS-g-DMAEMA nanoparticles concentration of 1% (w/v). Samples were mixed using a homogenizer (Ultraturrax T25, IKA, Germany) at 15, 000 rpm and 25 °C for 2 min.

2.4 FT-IR analysis 7KH )RXULHU 7UDQVIRUP ,QIUDUHG 6SHFWURVFRS\ )7,5  VSHFWUD IRU carboxymethyl maize starch and CMS-g-DMAEMA nanoparticlesZHUHUHFRUGHG EHWZHHQDQGFP-XVLQJD9HFWRU0,5)7,5VSHFWURSKRWRPHWHU %UXNHU*HUPDQ\ ZLWKVFDQVDWDUHVROXWLRQRIFP-6DPSOHVPJ HDFK ZHUHJURXQGZLWKVSHFWURVFRSLFJUDGH.%USRZGHUPJ DQGSUHVVHG LQWRPPSHOOHWV$EODQNGLVFZDVXVHGDVEDFNJURXQG>@  2.5 Nuclear Magnetic Resonance (1H NMR) The 1H NMR spectra were obtained by an AVANCE digital 400 spectrometer 7

1

1

(Bruker, Germany) at 400 MHz for H NMR spectroscopy. The H spectra were

recorded in 128 individual scans with a sweep width of 16 ppm and a delay time of 1 s. Prior to 1H NMR analysis, starch-based nanoparticles were dissolved in deuterium dimethyl sulfoxide (DMSO-d6) at 40 °C for 20-40 min to obtain completely dissolved samples [23]. The carboxymethyl starch and CSM-g-DMAEMA nanoparticles analysis was carried out at 25 °C.

2.6 X-ray Diffraction The X-ray diffraction patterns of carboxymethyl starch and CSM-g-DMAEMA nanoparticles were recorded with an X-ray diffractometer equipped with a detector (D8-ADVANCE, Bruker, Germany). Diffractograms were collected at 40 kV and 30 mA with nickel-filtered Cu Kα radiation (λ = 1.5405 Å). Samples were measured using Bragg angle (2θ) at 5°-40° with a scan rate of 5° min-1. Before XRD test, samples were sealed in a vessel at 75% relative humidity using saturated sodium chloride [6]. The relative crystallinity (RC) was calculated using MIDI-Jade 6.0 software.

2.7 Contact Angle Measurements Contact angles of carboxymethyl starch and CMS-g-DMAEMA in air were measured at room temperature using a DCA40 optical contact angle measuring device (Data Physics Instrument GmbH. Germany) equipped with a CDD camera and the WINDROP software. Prior to measurement, the samples were pressed into film. A 8

droplet of water (constant 4 μL) was deposited on the granule surface using a precision syringe and image was captured immediately by a video camera, and the profile of the droplet was numerically solved and fitted to the Laplace-Young equation. The average contact angle value was obtained at ten different positions for each sample. The three-phase contact angle of particles at the oil-water interface was measured by the compressed disk method as described by [24].

2.8 Zeta Potential Measurements Zeta potential of samples was measured using a Zetasizer Nano ZS90 (Malvern, UK) equipped with a dip cell at room temperature. The concentration of CMS and CMS-g-DMAEMA nanoparticles was 0.1 wt% [25].

2.9 Particle Size Distribution Starch-based nanoparticles size distribution was determined by a Nano ZS instrument (Malvern Instrument Ltd, UK) at room temperature. The light source wavelength was 633 nm and the scattering angle was set at 90° [26].

2.10 Droplet Size of Pickering Emulsion Droplet sizes of CMS and CMS-g-DMAEMA nanoparticles stabilized Pickering emulsion were determined by a 2000 Malvern Mastersizer analyzer. The measured particle sizes are expressed as volume-weighted mean diameter: D43 = Σnidi4/Σnidi3, where ni is the number of droplets with diameter di. The refractive values of 1.53 and 9

1.33 were employed as proposed for oil phase and water phase, respectively. The volume-mean diameter (D43) values were recorded [27].

2.11 Microscopy analysis The microstructures of CMS-g-DMAEMA stabilized Pickering emulsion samples were evaluated using a BH2 light microscope equipped with a digital recorder (Tokyo, Japan). A small amount of Pickering emulsion was put on a glass slide with a glass cover. The prepared samples were observed at room temperature with cedar wood oil at a magnification of 500x [8]. The microstructure of Pickering emulsion was also analyzed by a TCS SP5 confocal laser scanning microscope (Leica Microsystems Inc., Heidelberg, Germany), using an argon krypton laser with an excitation line of 488 nm and a helium neon laser (He/Ne) with excitation at 633 nm. Pickering emulsions were pre-stained with Nile red (oil phase) and Nile Blue (aqueous phase) during Pickering emulsion fabrication process. In the confocal images, the red and green color represented the oil phase and starch phase, respectively. Combined pictures under two channels were obtained through FV10-ASW 4.2 Viewer software.

2.12 Measurement of pH Sensitivity The CMS-g-DMAEMA nanoparticles (0.1%, w/v) were suspended in aqueous solutions with different pH values and then transferred to glass tubes. Images for solution appearance were captured by a digital camera (Canon, Shanghai, China) [6]. 10

2.13 Switching On/Off Cycling Emulsions stabilized by 1 wt % of CMS-g-DMAEMA nanoparticles at pH =10 were switched off (destabilized) by the addition of 0.1 M HCl solution reducing the pH to below 7.0, followed by stirring with a glass rod or magnetic rotator. Emulsion samples were switched on by the addition of 0.1 M HCl solution, followed by homogenization at 11, 000 rpm for 2 min. All experiments were carried out at room temperature (20 to 25°C).

2.13 Rheological measurements The flow sweep experiments of the emulsion were performed by a HAAKE RheoStress 600 rheometer (Thermo electron GmbH, Karlsruhe, Germany) equipped with a parallel plate geometry. The apparent viscosity was at across shear rates in range 0.01- 1000 s-1 at 25 °C. The flow sweep experiments were performed as follows: frequency was 1 Hz; shear rate from 0.01 to 1000 s-1 [28].

2.16 Statistical analysis Data obtained from triplicate measurements were statistically analyzed and expressed as the mean ± standard deviation. Statistical significance (p < 0.05) was evaluated by one-way analysis of variance (ANOVA). Statistical computations and analyses were conducted using SPSS 21.0.

11

3. Results and Discussion 3.1 FT-IR spectra FT-IR spectra of CMS and CMS-g-DMAEMA nanoparticle samples are shown in Fig. 1. A new peak appeared at 1251 cm-1 for CMS-20, CMS-40, and CMS-60 samples, which may correspond to C-N bond. This peak indicates that DMAEMA is introduced on CMS backbone and C-N bond was grafted on CMS skeleton. However, bond of amide groups was not observed for CMS. Other three peaks at 2926 cm -1ˈ 1450cm-1, and 1370 cm-1 are attributed to the C-H stretching and bending vibration of the methylene in CMS-DMAEMA [29]. However, the peak at 1750 cm-1 for carboxymethyl starch sample is attributed to -C=O stretching vibration of ester group [22,28,30]. On the other hand, the peak bands at 1570 and 1251 cm−1 are corresponding to -N-H and C-N bonds, which are presented in CMS-g-DMAEMA molecules. The differences in peaks at 1570 cm-1, 1251 cm-1, and 1750 cm-1 between CMS and CMS-g-DMAEMA are attributed to the grafting of DMAEMA on CMS surface.

12

Fig. 1. FT-IR spectra of CMS and CMS-g-DMAEMA copolymer with different DMAEMA/CMS grafting ratios. CMS-20, CMS-40, CMS-60 represented the grafting ratio of 14.72%, 22.21%, and 33.21%, respectively.

3.2 1H NMR Analysis Typical 1H NMR spectra of CMS and CMS-g-DMAEMA nanoparticle samples are shown in Fig. 2. The residual DMSO-d6 signal at 2.549 ppm was used to calibrate chemical shift scale. The signals of starch backbone proton are observed at 5.3 ppm for H-1, at 3.5 ppm (overlapped by H-4) for H-2, at 1.9 ppm for H-4, and at 3.8 ppm for H-3, H-5, and H-6. Signals at the region of 4.5-5.5 ppm were assigned to protons at 2, 3, 6 positions of the anhydroglucose unit [23]. The area of OH-2,3 protons for anhydroglucose is corresponding to 5.4-5.5 ppm. Also, the area of OH-6 and H-1 protons for anhydroglucose were corresponding to 4.57 ppm, and 5.1 ppm, respectively. The grafting efficiency of CMS-20, CMS-40, and CMS-60 was found to be 14.72%, 22.21%, and 33.21%, respectively. 13

The proton signal of methylene (-CH2-) at 3.5 ppm is observed for CMS, indicating the occurrence of methylene in CMS. However, no peak could be observed at 2.3 ppm, 5.9 ppm, and 5.4 ppm for CMS, indicating absence of tertiary amine group (-CH2-N(CH3)2) in CMS. However, 2.3 ppm, 5.9 ppm, and 5.4 ppm are seen for CMS-g-DMAEMA,

indicating

occurrence

of

tertiary

amine

group

in

CMS-g-DMAEMA. These results are consistent with findings of previous studies [31,32,33]. As shown in Fig. 2 and Table 1, the area of protons for OH-2,3 in CMS-20, CMA-40, and CMS-60 samples slightly decreased (from 2.01 to 1.90), indicating that protons of OH-2,3 were partially replaced by functional groups. The chemical shifts of OH-2, 3 and OH -6 were possible to assign peaks between 5.4-5.5 ppm and 4.58 ppm, respectively [6]. The high OH-2,3 reactivity was verified by esterification and etherification processes [6]. In contrast, the area of protons for OH-6 is more stable than that of OH-2,3. The mechanism for the reaction between CMS and DMAEMA is shown in Fig. 3. Both FT-IR and 1H NMR spectra confirmed the grafting of DMAEMA to carboxymethyl starch.

Fig. 2. 1H NMR spectra of CMS and CMS-g-DMAEMA copolymer with different DMAEMA/CMS grafting ratios. (a); CMS-20(b); CMS-40 (c), and CMS-60 (d). 14

Fig. 3. Scheme for the preparation of CMS-g-DMAEMA copolymer nanoparticles.

Table 1. The grafting rate, relative crystallinity contact angle and the area of

characteristic protons from 1H NMR spectrum and calculated composition information of CMS and CMS-g-DMAEMA nanoparticles

Samples

CMS CMS-20 CMS-40 CMS-60

Grafting

Relative

efficiency

crystallinity

(%)

(%)

— 14.72c 22.21b 33.21a

a

27.8 8.4b 4.6c 1.6d

Area of protons

Contact angle (Ʊ)

5.4-5.5ppm

4.58ppm

5.1ppm

(OH-2,3)

(OH-6)

(H-1)

— 119.3a 89.1b 81.2c

2.01 1.96 1.93 1.9

1.09 1.08 1.06 1.05

1.00 1.00 1.00 1.00

3.3 X-Ray diffraction patterns Fig. 4 presents the X-ray diffraction patterns of CMS and CMS-g-DMAEMA nanoparticles. The XRD profiles of the CMS and CMS-g-DMAEMA exhibited distinct diffraction peaks and crystallinity, compared with that of carboxymethyl 15

starch (Fig. 4). The diffractogram of CMS was A type crystalline structure with diffraction peaks at 2θ of 15.01◦, 17.04◦, 18.09◦ and 22.96◦. No significant diffraction peaks were observed for CMS-g-DMAEMA polymer, indicating that the crystalline region was melted. The loss of crystallinity can be attributed to the completely gelatinization of starch under heating. The starch gelatinization caused a destruction of CMS, which could significantly increase the low molecular weight starch chains [6]. The completely gelatinized CMS-DMAEMA showed a relative crystallinity close to zero. During cooling, gel networks are formed because the entanglement of amylose molecules through hydrogen bonding [34]. As a result, there are some crystalline residues still occur in samples. Moreover, starch functionality inhibited the rearrangement of molecular chains as indicated by the low crystallinity of the grafted starch nanoparticles [6]. However, as DMAEMA grafting ratio increased, the relative crystallinity sharply decreased to 1.6%. This may be attributed to variation in the hydroxyl groups replacement and the steric effects of chain substitutions, which prevented the hydrogen bonding among the amylose molecules [6].

16

Fig. 4. XRD spectra of CMS and CMS-g-DMAEMA copolymer with different DMAEMA/CMS grafting ratios.

3.4 Particle Size Distribution The particle size distribution of carboxymethyl starch and CMS-g-DMAEMA nanoparticles suspension at different pH values is shown in Fig. 5. No significant difference was found in particle size of CMS at different pH values. However, as the pH increased from 2.0 to 10.0, the particle size of CMS-20, CMS-40, and CMS-60 decreased. This may be attributed to the electrostatic repelling force among amino groups for a subtle stretching of outer chains [35]. These results are consistent with findings of Miao et al [28]. A possible mechanism for CMS-g-DMAEMA formation was proposed (Figure 3). The DMAEMA amine groups were protonated and positively charged at low pH values [6,11]. Also, DMAEMA chains were stretched because of the electrostatic repulsion among the protonated amine groups. The DMAEMA layer can serve as a pH-responsive sponge for transition at different pHs. 17

This pH-responsive behavior was reversible and driven by ionization of a single terminal amino group and carboxylic acid on DMAEMA (Fig. 1). pH-switchable Pickering

emulsion

can

induce

a

reversible

phase

transformation

of

CMS-g-DMAEMA in a solution.

Fig. 5. Particle size distribution of CMS-20, CMS-40, and CMS-60 at different pH values.

3.5 Zeta-potential Zeta potentials and responsive property of CMS-g-DMAEMA nanoparticles suspensions at different pH are shown in Fig. 6 and Fig. 7. For CMS-20 solutions at pH 2.0 to 6.0, the zeta potential decreased, which can be attributed to aggregation induced by intermolecular attractive forces including hydrogen bonding and van der Waals forces. When the pH of CMS-g-DMAEMA suspension increased from 6.0 to 10.0, the zeta potential increased from -31.5 to -27.0. However, for CMS-40 and 18

CMS-60 solutions, the zeta potential decreased as the pH increased from 2.0 to 8.0. Then, the zeta potential of CMS-g-DMAEMA nanoparticles dispersion slightly increased as the pH increased from 8.0 to 10.0. These results are consistent with zeta potentials of starch nanocrystals suspension, the carboxyl and sulfate esters were in protonated state under acidic conditions [25]. The zeta potential changed with the increase of DMAEMA content, producing more negative charges on the surface of CMS-g-DMAEMA nanoparticles. Similar results were found by Han et al [11]. However, the negative electricity of CMS-40 and CMS-60 solution slightly increased to 3.5 mV and 0.4 mV when the dispersion pH changed from 8.0 to 10.0. The zeta potentials of CMS-g-DMAEMA decreased as the ionic strength increased, which could be attributed to their surface charges by salt ions at different pH values. At low pH, the tertiary amine protonation induced surface hydrophilic of CMS-g-DMAEMA because of its bearing charges [6]. The dispersion properties of CMS-g-DMAEMA at different pH conditions at ambient temperature (25 °C) are shown in Fig. 7. CMS-g-DMAEMA solution was turbid at pH 2.0 at ambient temperature (25 °C). An obvious flocculation was observed at pH 2.0 and most of the CMS-g-DMAEMA was deposited at the bottom of the bottle at pH 6.0 However, the solution of CMS-g-DMAEMA was clear and transparent at pH 10. This could be explained by that a phase transformation from precipitate to solution occurred at pH higher than 6. The pH-switchable behavior can be attributed to the protonation and deprotonation among amino groups and carboxyl groups in CMS-g-DMAEMA solution. Solution properties were found to increase 19

with increase in the pH, which may be attributed to the hydrogen bonds and ionic interactions of the functional groups of polymer chains [36,37]. In addition, the polymer’s electric charge can be decreased by several approaches, including decreasing of pH, neutralizing the electric charge and reducing the hydrophilicity of the polymeric macromolecules [37,38].

Fig. 6. Zeta Potential of pH profiles of CMS-20 (filled square), CMS-40 (filled circle), and CMS-60 (filled triangle).

Fig. 7. Solution observations of CMS and CMS-g-DMAEMA copolymer suspensions at different pH values, respectively (a, CMS; b, CMS-20; c, CMS-40; d, CMS-60). 20

3.6 Surface Wettability of CMS-g-DMAEMA Nanoparticles The contact angle of CMS and CMS-g-DMAEMA nanoparticles are shown in Fig. 8. No contact angle was observed for CMS, indicating that the CMS particles are not able to stabilize Pickering emulsion. The contact angle of water on CMS-20, CMS-40, and CMS-60 were 119.3°, 89.1°, and 81.2°, respectively. These results suggest that the hydrophilic starch granules became hydrophobic when CMS imparted with an amphipathic group of DMAEMA molecular. Obviously, the carboxymethyl starch was hydrophilic but the CMS-g-DMAEMA nanoparticles showed different hydrophobicity at different pH values. CMS-20 showed the largest contact angle; while, CMS-60 showed the smallest contact angle. Different amount of DMAEMA were grafted to CMS showed variable. Different amounts of DMAEMA grafted to carboxymethyl starch skeleton, CMS-g-DMAEMA nanoparticles showed different wetting characteristics. When the contact angle is too large(θ>160°) or too small(θ<20°), the adsorption energy E reduced significantly, it is hard to form a stable emulsion. The presence of DMAEMA molecules in carboxymethyl starch may have significant effect on the hydrophobic and responsive property. After atom transfer radical

polymerization

reaction,

prominent

hydrophobicity

feature

of

demonstrate

that

CMS-g-DMAEMA nanoparticles was improved. The

measurements

of

contact

angle

(θw/o)

clearly

CMS-g-DAMAEMA is a very suitable material for interfacial adsorption. Hence, CMS-g-DAMAEMA nanoparticles could serve well as emulsifier particle for 21

Pickering emulsion. The contact angle of CMS-20 is 119.3°; therefore, it can also be used t The contact angle results indicated that CMS-40 and CMS-60 can be used to stabilize oil-in-water Pickering emulsion. o stabilize water-in-oil Pickering emulsion. It was reported that the wetting property of particles by water and oil is crucial to their effectiveness in stabilizing of Pickering emulsions [39]. By homogenization, the hydrophobicity of starch could be improved due to the grafted DMAEMA on CMS skeleton (see Fig. 3). Upon free radical polymerization reaction, the wettability of nanoparticles is difference among CMS-g-DMAEMA nanoparticles with different grafting ratios.

119.3e

CMS-20

CMS 89.1e e

81.2e e

89 1e

CMS-40 Fig.

8.

Contact

angle

CMS-60 of

CMS-g-DMAEMA

copolymer

with

different

DMAEMA/CMS grafting ratios.

3.7 Microstructure of Pickering emulsions Micrographs for microstructure of Pickering emulsions stabilized by 22

CMS-g-DMAEMA nanoparticles at different grafted ratios as obtained by confocal laser scanning microscope are shown in Fig. 9. An obvious starch layer surrounding the oil droplets could be seen by staining the oil with Nile red and aqueous starch phase with Nile blue. The emulsions were stabilized by adsorption of CMS-g-DMAEMA at the surface of emulsion droplet and networking among the CMS-g-DMAEMA nanoparticles, preventing the accumulation of emulsion droplet. The emulsions showed lower droplet sizes at pH 10. Similar nanoparticle clustering phenomena were observed for CMS-g-DMAEMA nanoparticles at pH higher than 6.0. When the electrostatic repulsions among nanoparticles increased with the increase in pH value, particle aggregation increased. CMS-g-DMAEMA nanoparticles exhibited a good interfacial property at alkali condition. Furthermore, as shown in Fig. 10, the oil interface was covered by CMS-g-DMAEMA nanoparticles (red color). The close packing and connection of Pickering emulsion droplets could allow them to interact strongly with each other, thus contributes to the increase in the interface strength and stability of Pickering emulsion network [7,40]. It is known that the textural properties of Pickering emulsion are highly depend on the emulsion droplet size, and reducing the droplet size could obviously increase the rigidity of emulsion stability; while, the oil droplets function as particle emulsifier to adjust Pickering emulsion stability (Fig. 10). Micrographs of Pickering emulsion stabilized by different CMS-g-DMAEMA nanoparticles at different pHs as obtained by light microscope are shown in Fig.11. The DMAEMA was grafted onto the CMS backbone to improve interfacial activity of 23

carboxymethyl starch. When the acid aqueous phase was turned to alkaline by the addition of NaOH solution, obvious change was observed in Pickering emulsions stabilized by 1.0 wt % CMS-g-DMAEMA nanoparticles. The Pickering emulsions presented good interface stability at either neutral or alkaline aqueous medium. These samples were chosen because the difference in droplet size between low and high ionic strength systems were observed at different pH values. Under acidic conditions (pH 2.0), Pickering emulsion stabilized by CMS-g-DMAEMA nanoparticles showed smaller particle size and more hydrophilic particle surface. This led to a separation of nanoparticles from emulsion interface to the oil phase, resulting in emulsion destabilization and separation. The protonated tertiary amine groups decreased as pH decreased to 2.0, resulting in a significant decrease of electrostatic interactions, hydrogen bonding, covalent bonding and van der Waals forces [12]. The amino-amide reacted with hydrochloric acid to reassemble supramphiphile as H+ increased, which increased reversible emulsified and demulsified transitions. $YHUDJHGURSOHWVL]H W\SH DQGOD\HUWKLFNQHVVRIWKHUHJHQHUDWHG3LFNHULQJHPXOVLRQDUHVLPLODUVXJJHVWLQJ WKDWWKHHPXOVLILFDWLRQSHUIRUPDQFHRI&06J'0$(0$QDQRSDUWLFOHVZDVQRWFKDQJHG GXULQJWKHILQLWHHPXOVLILFDWLRQGHPXOVLILFDWLRQVZLWFKDEOH

A series of dispersions at pHs in the range from 2.0 to 10.0 was obtained by the addition of NaOH solutions to the original neutral solution (Fig. 11). The droplets size of Pickering emulsion at pH 10 stabilized by 1.0 wt % CMS-g-DMAEMA nanoparticles showed more uniform droplets. This can be attributed to that with the 24

addition of strong base, the CMS-g-DMAEMA become more hydrophobicity at pH higher than 6.0 and the nanoparticles migrated to the emulsion interface, which improved the stability of Pickering emulsion. Therefore, amino-amide/hydrochloric acid amphiphile dissociated to interfacial inactive amino-amide because of the break of electrostatic interaction and the hydrolysis of the amphiphile. This weakened the interactions among anionic (COO-) electrostatic and quaternary ammonium groups (NH3+) [6]. This may be attributed to the shorter alkyl chain in CMS-60 than in carboxymethyl starch molecules, which increased the hydrophobicity of CMS at similar adsorption levels. The quaternary ammonium (-N+) and the anionic (-COO-) group electrostatic interaction could stabilize the Pickering emulsions. It can also be explained by that the adsorption of amphiphile molecules at the oil/water interface resulted in interactions with H+ and OH- ions, which provide possibility for pH stimuli. Pickering emulsion stabilization was deteriorated by the high concentration of salt, resulted from the excess addition of HCl and NaOH. Adjusting the pH to more neutral and basic conditions, i.e. pH > 6, caused deprotonation of the CMS-g-DMAEMA, which resulted in more lipophilic particle surface and caused emulsion phase inversion.

25

Fig.

9.

CLSM

analysis of

Pickering emulsions

stabilized

by

1%

(w/v)

CMS-g-DMAEMA copolymer nanoparticles. a, b, and c represented CMS-20, CMS-40,

CMS-60

stabilized

Pickering

emulsion

at

pH

10,

respectively.

Fig. 10. Schematic illustration of the synthesis of CMS-g-DMAEMA stabilized Pickering emulsion and their pH-responsive behavior at different pH values.

26

Fig. 11. Optical microscopic image of Pickering emulsion stabilized by 1.0 wt% CMS-g-DMAEMA copolymer nanoparticles with different grafting ratio. a, b, and c represented CMS-20, CMS-40, CMS-60 stabilized Pickering emulsion at pH 2.0,6.0, 10.0, respectively.

3.9 Droplet size distribution Droplet size of Pickering emulsions stabilized by CMS-g-DMAEMA nanoparticles at different pH values is shown in Fig. 12. Droplet size distribution decreased with the increase in pH of Pickering emulsion. The droplets size of Pickering emulsion stabilized by CSM-60 was decreased from 31.2 μm to 6.71 μm as pH increased from 2.0 to 10.0. The average droplet sizes in emulsions formed by CSM-20 and CSM-40 decreased from 12.1μm, 15.3μm to 8.82μm and 7.57 μm as pH increased from 2.0 to 10.0, respectively. These results indicate more particles sediments in the aqueous phase at pH of 2.0, which reduced the particles adsorption at droplet interfaces with a consequent increase in droplet size. The polydispersity index (PDI) of CMS-20, CMS-40, and CMS-60 were 0.07, 0.10 and 0.18, respectively. The 27

polydispersity index of emulsion droplet at pH 2.0 was also larger, indicating a significant emulsion droplet aggregation. The droplet size of emulsion is positively correlated with the particle size of CMS-g-DMAEMA nanoparticles. This may be attributed to that -OH groups of CMS and DMAEMA were deprotonated to anionic -O- at pH above 7.0. The surface charge of these particles influenced their state in complex immiscible system, which could be influenced by the deprotonation of the carboxyl groups and protonation of amino groups [27]. The increase in particle adsorption increased the total interface area, and correspondingly a smaller droplet size was formed at high pH. The repulsion overcame attractive forces that resulted from hydrogen bond or hydrophobic interactions led to particle dispersion. When the pH increased to 10.0, Na+ showed a shielding effect, which decreased the zeta potential and caused particle aggregation.

28

Fig. 12. Average Pickering emulsion droplet size of CMS-g-DMAEMA nanoparticles-stabilized Pickering emulsion at different pH values. a, b, and c represented CMS-20, CMS-40, CMS-60 stabilized Pickering emulsion at pH 2.0 and 10.0, respectively.

3.10 ζ -potential of Pickering emulsion The zeta potential of Pickering emulsion at different pH values is shown in Fig. 13. The ζ-Potential of CMS-20, CMS-40, CMS-60 stabilized Pickering emulsions changed from 20.8 mV, 10.7 mV, and 5.44 mV, to -10.3 mV, -12.3 mV, and -18.9 mV when pH increased from 2.0 to 10.0, respectively. The hydrophobicity of CMS-g-DMAEMA nanoparticles, which is weakened in acidic media and enhanced in alkaline media, is also supported by the zeta potential measurements. At low pH, the protonation of tertiary amine increased the hydrophobicity of CMS-g-DMAEMA surface because of its bearing charges. Also, surfactant molecules become cationic at at low pH and the negative charges on particle surfaces neutralized by adsorption. Larger number of carboxylic groups is protonated at low pH, which decreased the zeta potential and internal electrostatic repulsion of particles [41,42]. Many hydrophobic groups from DMAEMA were embedded in carboxymethyl starch; therefore, hydrophobicity of particles increased. These results indicate that there was enough repulsive force among Pickering emulsion droplets, which kept droplets away from each other at high pH. A possible explanation for this phenomenon is that -OH of CMS-g-DMAEMA was deprotonated to anionic -O-. Therefore, repulsion overcame 29

attractive forces that resulted from hydrogen bonding or hydrophobic interactions, leading to particle dispersion. All Pickering emulsions samples displayed good stability at pH higher than 6.0, indicating that emulsions are more stable at neutral or alkaline pH. The carboxyl groups in CMS-g-DMAEMA nanoparticles could be ionized in neutral or alkaline system, and the nanoparticles were mutual repulsed for more steric hindrance.

Fig. 13. Zeta potential of CMS-g-DMAEMA nanoparticles-stabilized Pickering emulsion at different pH values.

3.11 Flow behavior of Pickering emulsions Flow curves of Pickering emulsion stabilized by CMS-g-DMAEMA nanoparticles at different pH values are shown in Fig. 14. The rheological properties revealed the stability and functionality of Pickering emulsions. As shear rates increased, emulsion viscosity decreased and reached a steady value, suggesting that 30

Pickering emulsion stabilized by CMS-g-DMAEMA presented a shear-thinning behavior. The Pickering emulsion stabilized by CMS-g-DMAEMA nanoparticles showed a shear thinning (pseudoplastic) behavior at shear rates of 1-1000 s-1; while, Pickering emulsion stabilized by CMS-20, CMS-40 or CMS-60 showed a similar Newtonian fluid over the whole shear rate range. Similar results were reported for Pickering emulsions stabilized by hydrophobic maize starch particles [28]. All CMS-g-DMAEMA nanoparticles stabilized Pickering emulsion showed pseudoplastic and shear thinning behaviors. These results indicate a formation of weak droplet network, which is consistent with findings of Song et al. [43]. CMS-g-DMAEMA with hydrophobic and hydrophilic moieties adsorbed at O/W interface and formed a stable system by steric repulsion. The shear thinning behavior of the Pickering emulsion was a typical of weak associative interactions, which indicate a formation of a weak droplet network structure. As can also be seen from Fig. 14, the Pickering emulsion showed a prominent shear thinning behavior (n < 1). At pH 6.0 and 10.0, Pickering emulsion exhibited highly viscous and strong shear-thinning behavior, especially the CMS-60 nanoparticles stabilized Pickering emulsion. This may be attributed to that the droplets in these emulsions were highly flocculated, and that the flocculation became progressively deformed and disrupted as the shear rate increased. The droplet flocculation at low pH value can be attributed to the weak charge of starch because the pH is near its isoelectric point; therefore, promoting its aggregation. However, CMS-g-DMAEMA might have appreciable regions of both positive and negative 31

charge on their surfaces at low pH values, which could promote their aggregation through electrostatic attraction. At pH 10.0, the apparent viscosity of the samples remained relatively constant across a wide range of shear rates, suggesting that the droplets in these Pickering emulsions were not strongly flocculated. These results are consistent with findings for emulsions contained highly flocculated protein-coated oil droplets [44,45]. Moreover, the apparent viscosity of Pickering emulsion improved as pH value increased from 2.0 to 10.0. The good pH response property of CMS-g-DMAEMA stabilized emulsion can be attributed to that the pH is higher than isoelectric point of CMS-g-DMAEMA; therefore, there would be a strong electrostatic repulsion between the positively charged CMS-g-DMAEMA molecules.

Fig. 14. Shear-rate dependence of viscosity for O/W emulsions stabilized by CMS-g-DMAEMA nanoparticles. (a) CMS-20; (b) CMS-40; (c) CMS-60 at different pH values. 32

Conclusion In this work, pH-responsive Pickering emulsifiers were synthesized by grafting DMAEMA on CMS through polymerization reaction. The CMS-g-DMAEMA nanoparticles exhibited dissolution and precipitation at different pH values. The synthesized nanoparticles showed good interfacial and surface properties when the amino groups on DMAEMA were highly deprotonated. An efficient approach was developed for preparing a pH-switchable Pickering emulsion stabilized by CMS-g-DMAEMA nanoparticles. Furthermore, the CMS-g-DMAEMA nanoparticles stabilized Pickering emulsion presented good pH responsive property. The pH-responsive Pickering emulsion, stabilized by starch-based nanoparticles, are interesting because of their switchable emulsion/demulsification functions. Further studies involving the controlled DMAEMA to elucidate the effect of emulsion and demulsification on the stability of nutritional element in Pickering emulsion are in progress.

Corresponding Author *(Q.-Y. Y), Phone: 86-24-86506208, fax: 86-24-86506860. E-mail: [email protected]. *(Z.-G. L). E-mail: [email protected]. 1

Both authors contributed equally to this work.

Funding This work is supported by grants from Liaoning Province Xingliao Talents Plan 33

Project (XLYC1807144); Sponsored by the Special Fund of Liaoning Provincial Universities' Fundamental Scientific Research Projects (LQN201708); Liaoning Province Natural science foundation of Doctoral Scientific Research Foundation (20180540119); Key Project of Scientific and Technological Achievements Conversion Project of Shenyang city “Double Hundred project” (Z18-5-019); Shenyang Young and Middle aged Science and Technology Innovation Talents Project (RC170367);

Heilongjiang

Province

Key laboratory of

grain

by-products

(HGW2017004); The project by Ningbo University of Technology Fenghua Research Institute (FHI-018119).

Notes The authors declare no competing financial interest.

34

REFERENCES [1] T. Saigal, H. Dong, K. Matyjaszewski, R.D. Tilton, Pickering emulsions stabilized by nanoparticles with thermally responsive grafted polymer brushes, Langmuir. 26 (19) (2010) 15200-15209. http://doi.org/10.1021/la1027898. [2] J.T. Tang, M.F.X. Lee, W. Zhang, B.X. Zhao, R.M. Berry, K.C. Tam, Dual responsive Pickering emulsion stabilized by poly [2-(dimethylamino) ethyl methacrylate] grafted cellulose nanocrystals, Biomacromolecules. 15 (8) (2014) 3052-3060. http://doi.org/10.1021/bm500663w. [3] L. Hao, C. Yegin, I-C. Chen, J.K. Oh, S.H. Liu, E. Scholar, B. Jiang, pH-responsive emulsions with supramolecularly assembled shells, Ind. Eng. Chem. Res. 57 (28) (2018) 9231-9239. http://doi.org/10.1021/acs.iecr.8b00984. [4] F.L. Román, M. Schmidt, H. Löwen, Colloidal particles in emulsions, Phys. Rev. E. 61 (5) (2000) 5445-5451. http://doi.org/10.1103/PhysRevE.61.5445. [5] U. Lesmes, D.J. McClements, Structure-function relationships to guide rational design and fabrication of particulate food delivery systems, Trends Food Sci. Technol. 20 (10) (2009) 448-457. http://doi.org/10.1016/j.tifs.2009.05.006. [6] L. Qi, Z.G. Luo, X.X. Lu, Facile synthesis of starch-based nanoparticle stabilized Pickering emulsion: its pH-responsive behavior and application for recyclable catalysis,

Green

Chem.

20

(7)

(2018)

1538-1550.

http://doi.org/10.1039/C8GC00143J. [7] Z.L. Wan, Y.G. Sun, L.L. Ma, X.Q. Yang, J. Guo, S.W. Yin, Responsive emulsion gels with tunable properties formed by self-assembled nanofibrils of natural 35

saponin Glycyrrhizic acid for oil structuring, J. Agric. Food Chem. 65 (11) (2017) 2394-2405. http://doi.org/10.1021/acs.jafc.6b05242. [8] J.F. Li, F.Y. Ye, L. Lei, Y. Zhou, G.H. Zhao, Joint effects of granule size and degree of substitution on octenylsuccinated sweet potato starch granules as Pickering emulsion stabilizers, J. Agric. Food Chem. 66 (17) (2018) 4541-4550. http://doi.org/10.1021/acs.jafc.7b05507. [9] Y.M. Zhang, S. Guo, X.F. Ren, X.F. Liu, Y. Fang, CO2 and redox dual responsive Pickering

emulsion,

Langmuir.

33

(45)

(2017)

12973-12981.

http://doi.org/10.1021/acs.langmuir.7b02976. [10] Y. Zhu, J.Z. Jiang, K.H. Liu, Z.G. Cui, B.P. Binks, Switchable Pickering emulsions stabilized by silica nanoparticles hydrophobized in situ with a conventional cationic surfactant, Langmuir. 31 (11) (2015) 3301-3307. http://doi.org/10.1021/acs.langmuir.5b00295. [11] X. Han, X.X. Zhang, H.F. Zhu, Q.Y. Yin, H.L. Liu, Y. Hu, Effect of composition of

PDMAEMA-b-PAA

block

copolymers

on

their

pH-and

temperature-responsive behaviors, Langmuir. 29 (4) (2013) 1024-1034. http://doi.org/10.1021/la3036874. [12] J.T. Tang, P.J. Quinlan, K.C. Tam, Stimuli-responsive Pickering emulsions: recent advances and potential applications, Soft Matter. 11 (18) (2015) 3512-3529. http://doi.org/10.1039/C5SM00247H. [13] J. Li, H.D. Stöver, Doubly pH-responsive Pickering emulsion, Langmuir. 24 (23) (2008) 13237-13240. http://doi.org/10.1021/la802619m. 36

[14] J.M. Zhuang, M.R. Gordon, J. Ventura, L.Y. Li, S. Thayumanavan, Multi-stimuli responsive macromolecules and their assemblies, Chem. Soc. Rev. 42 (17) (2013) 7421-7435. http://doi.org/10.1039/c3cs60094g. [15] Y. Kotsuchibashi, M. Ebara, T. Aoyagi, R. Narain, Recent advances in dual temperature responsive block copolymers and their potential as biomedical applications,

Polymers.

8

(11)

(2016)

380-405.

http://doi.org/10.3390/polym8110380. [16] M. Elsabahy, K.L. Wooley, Design of polymeric nanoparticles for biomedical delivery

applications,

Chem.

Soc.

Rev.

41

(7)

(2012)

2545-2561.

http://doi.org/10.1039/c2cs15327k. [17] F. Seidi, A. Zarei, ATRP grafting of poly (N, N-dimethylamino-2-ethyl methacrylate) onto the fatty-acid-modified “grafting-from”

technique,

Starch-Stärke.

agarose backbone via 68

(7-8)

(2016)

the

644-650.

http://doi.org/10.1002/star.201500352. [18] H. Saari, K. Heravifar, M. Rayner, M. Wahlgren, M. Sjöö, Preparation and characterization of starch particles for use in pickering emulsions, Cereal Chem. 93 (2) (2016) 116-124. http://doi.org/10.1094/CCHEM-05-15-0107-R. [19] O.S. Lawal, M.D. Lechner, B. Hartmann, W.M. Kulicke, Carboxymethyl cocoyam starch: synthesis, characterisation and influence of reaction parameters, Starch-Stärke. 59 (5) (2007) 224-233. http://doi.org/10.1002/star.200600594. [20] L.F. Wang, S.Y. Pan, H. Hu, W.H. Miao, X.Y. Xu, Synthesis and properties of carboxymethyl kudzu root starch, Carbohydr. Polym. 80 (1) (2010) 174-179. 37

http://doi.org/10.1016/j.carbpol.2009.11.008. [21] J.Y. Xu, E.F. Krietemeyer, V.L. Finkenstadt, D. Solaiman, R.D. Ashby, R.A. Garcia, Preparation of starch-poly-glutamic acid graft copolymers by microwave irradiation and the characterization of their properties, Carbohydr. Polym. 140 (2016) 233-237. http://doi.org/10.1016/j.carbpol.2015.12.034. [22] H.J. Zhu, L. Tian, L. Zhang, J.X. Bi, Q.Q. Song, H. Yang, J.J. Qiao, Preparation, characterization and antioxidant activity of polysaccharide from spent Lentinus edodes

substrate,

Biomacromolecules.

112

(2018)

976–984.

http://doi.org/10.1016/j.ijbiomac.2018.01.196. [23] D. Binh, P.T.T. Hong, N.N. Duy, N.T. Duoc, N.N. Dieu, A study on size effect of carboxymethyl starch nanogel crosslinked by electron beam radiation, Radiat. Phys.

Chem.

81

(7)

(2012)

906-912.

http://doi.org/10.1016/j.radphyschem.2011.12.016. [24] C.Y. Xie, S.X. Meng, L.H. Xue, R.X. Bai, X. Yang, Y.L. Wang, Z.P. Qiu, B.P. Binks, T. Guo, T. Meng, Light and magnetic dual-responsive Pickering emulsion microreactors,

Langmuir.

33

(49)

(2017)

14139-14148.

http://doi.org/10.1021/acs.langmuir.7b03642. [25] B.X. Wei, X.T. Hu, H.Y. Li, C.S. Wu, X.M. Xu, Z.Y. Jin, Y.Q. Tian, Effect of pHs on dispersity of maize starch nanocrystals in aqueous medium, Food Hydrocolloids.

36

(2014)

369-373.

http://doi.org/10.1016/j.foodhyd.2013.08.015. [26] Q.Y. Yang, X.X. Lu, Y.Z. Chen, Z.G. Luo, Z.G. Xiao, Fine structure, crystalline 38

and physicochemical properties of waxy corn starch treated by ultrasound irradiation,

Ultrason

sonochem.

51

(2018)

350-358.

http://doi.org/10.1016/j.ultsonch.2018.09.001. [27] Y.H. Sun, S.A. Zhong, Molecularly imprinted polymers fabricated via Pickering emulsions stabilized solely by food-grade casein colloidal nanoparticles for selective protein recognition, Anal. Bioanal. Chem. 410 (13) (2018) 3133-3143. http://doi.org/10.1007/s00216-018-1006-x. [28] F. Ye, M. Miao, B. Jiang, B.R. Hamaker, Z.Y. Jin, T. Zhang, Characterizations of oil-in-water emulsion stabilized by different hydrophobic maize starches, Carbohydr.

Polym.

166

(2017)

195-201.

http://doi.org/10.1016/j.carbpol.2017.02.079. [29] J.P. Wang, S.J. Yuan, Y. Wang, H.Q. Yu, Synthesis, characterization and application of a novel starch-based flocculant with high flocculation and dewatering

properties,

Water

Res.

47

(8)

(2013)

2643-2648.

http://doi.org/10.1016/j.watres.2013.01.050. [30] C. Qiu, J. Yang, S.J. Ge, R.R. Chang, L. Xiong, Q.J. Sun, Preparation and characterization of size-controlled starch nanoparticles based on short linear chains from debranched waxy corn starch, LWT-Food Sci. Technol. 74 (2016) 303-310. http://doi.org/10.1016/j.lwt.2016.07.062. [31] C. Boyer, G. Boutevin, J.J. Robin, B. Boutevin, Study of the telomerization of dimethylaminoethyl

methacrylate

(DMAEMA)

with

mercaptoethanol,

Application to the synthesis of a new macromonomer, Polymer. 45 (23) (2004) 39

7863-7876. http://doi.org/10.1016/j.polymer.2004.09.020. [32] B.Y. Zhang, W.D. He, W.T. Li, L.Y. Li, K.R. Zhang, H. Zhang, Preparation of block-brush PEG-b-P (NIPAM-g-DMAEMA) and its dual stimulus-response, Polymer.

51

(14)

(2010)

3039-3046.

http://doi.org/10.1016/j.polymer.2010.05.012. [33] J.J. Nie, W.Y. Zhao, H. Hu, B.R. Yu, F.J. Xu, Controllable heparin-based comb copolymers and their self-assembled nanoparticles for gene delivery, ACS Appl. Mater.

Interfaces.

8

(13)

(2016)

8376-8385.

http://doi.org/10.1021/acsami.6b00649. [34] C.Y. Lii, M.L. Tsai, K.H. Tseng, Effect of amylose content on the rheological property

of

rice

starch,

Cereal

chem.

73

(4)

(1996)

415-420.

http://doi.org/10.1021/bp960041s. [35] J.R. Lovett, N.J. Warren, L.P.D. Ratcliffe, M.K. Kocik, S.P. Armes, pH-responsive non-ionic diblock copolymers: ionization of carboxylic acid end-groups induces an order-order morphological transition, Angew. Chem.-Int. Edit. 54 (4) (2015) 1279-1283. http://doi.org/10.1002/ange.201409799. [36] Y.G. Shi, M.Y. Liu, K. Wang, F.J. Deng, Q. Wan, Q. Huang, L.H. Fu, X.Y. Zhang, Y. Wei, Bioinspired preparation of thermo-responsive graphene oxide nanocomposites in an aqueous solution, Polym. Chem. 6 (32) (2015) 5876-5883. http://doi.org/10.1039/c5py00844a. [37] W.K. Wang, A.H. Milani, Z.X. Cui, M.N. Zhu, B.R. Saunders, Pickering emulsions

stabilized

by

pH-responsive 40

microgels

and

their

scalable

transformation to robust sub-micrometer colloidoisomes with selective permeability,

Langmuir.

33

(33)

(2017)

8192-8200.

http://doi.org/10.1021/acs.langmuir.7b01618. [38] R.P. Shaikh, V. Pillay, Y.E. Choonara, L.C. du Toit, V.M.K. Ndesendo, P. Bawa, S. Cooppan, A review of multi-responsive membranous systems for rate-modulated drug

delivery,

AAPS

PharmSciTech.

11

(1)

(2010)

441-459.

http://doi.org/10.1208/s12249-010-9403-2. [39] B.P. Binks, D.Z. Yin, Pickering emulsions stabilized by hydrophilic nanoparticles: in situ surface modification by oil, Soft matter. 12 (32) (2016) 6858-6867. http://doi.org/10.1039/c6sm01214k. [40] E. Dickinson, Emulsion gels: the structuring of soft solids with protein-stabilized oil

droplets,

Food

Hydrocolloids.

28

(1)

(2012)

224-241.

http://doi.org/10.1016/j.foodhyd.2011.12.017. [41] K. Chen, G.B. Yu, F.R. He, Q.F. Zhou, D.C. Xiao, J.C. Li, Y.H. Feng, A pH-responsive emulsion stabilized by alginate-grafted anisotropic silica and its application in the controlled release of λ-cyhalothrin, Carbohydr polym. 176 (2017) 203-213. http://doi.org/10.1016/j.carbpol.2017.07.046. [42] Y. Tan, K. Xu, C. Niu, C. Liu, Y.L. Li, P.X. Wang, B.P. Binks, Triglyceride-water emulsions stabilised by starch-based nanoparticles, Food Hydrocolloids. 36 (2014) 70-75. http://doi.org/10.1016/j.foodhyd.2013.08.032. [43] X.Y. Song, Y.Q. Pei, M.W. Qiao, F.L. Ma, H.T. Ren, Q.Z. Zhao, Preparation and characterizations of Pickering emulsions stabilized by hydrophobic starch particles,

Food

Hydrocolloids. 41

45

(2015)

256-263.

http://doi.org/10.1016/j.foodhyd.2014.12.007. [44] X.M. Li, Q.T. Xie, J. Zhu, Y. Pan, R. Meng, B. Zhang, H.Q. Chen, Z.Y. Jin, Chitosan Hydrochloride/Carboxymethyl Starch Complex Nanogels as Novel Pickering Stabilizers: Physical Stability and Rheological Properties, Food Hydrocolloids.

93

(2019)

215-225.

http://doi.org/10.1016/j.foodhyd.2019.02.021. [45] K.H. Liu, J.Z. Jiang, Z.G. Cui, B.P. Binks, pH-Responsive Pickering emulsions stabilized by silica nanoparticles in combination with a conventional zwitterionic surfactant,

Langmuir.

33

http://doi.org/10.1021/acs.langmuir.6b04459.

42

(9)

(2017)

2296-2305.

Highlights: 1)

Carboxymethyl

starch-grafted-2-(dimethylamine)

ethyl

methacrylate

(CMS-g-DMAEMA) nanoparticles was prepared. 2) CMS-g-DMAEMA nanoparticles exhibited pH responsivity in Pickering emulsion. 3) The Pickering emulsion stability was enhanced by the addition of CMS-g-DMAEMA at pH 10. 4) The transformation mechanism of Pickering emulsion at different pH is explained.

43