Facile Synthesis of Chitosan-Coated Silica Nanocapsules via

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Facile synthesis of chitosan-coated silica nanocapsules via interfacial condensation approach for sustained release of vanillin Qianqian Fan, Jianzhong Ma, Qunna Xu, John Wang, and Yanxiao Ma Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00217 • Publication Date (Web): 12 Apr 2018 Downloaded from http://pubs.acs.org on April 13, 2018

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Industrial & Engineering Chemistry Research

1

Facile synthesis of chitosan-coated silica nanocapsules via interfacial condensation

2

approach for sustained release of vanillin

3

Qianqian Fan a, c, Jianzhong Ma a, c∗, Qunna Xu a, c*, John Wang b*, Yanxiao Ma d

4

a

5

China

6

b

Department of Materials Science & Engineering, National University of Singapore (NUS), 117456, Singapore

7

c

National Demonstration Center for Experimental Light Chemistry Engineering Education (Shaanxi University of Science &

8

Technology), Xi’an 710021, China

9

d

10

College of Bioresources Chemical and Materials Engineering, Shaanxi University of Science & Technology, Xi'an 710021,

Department of Chemistry, The University of Alabama, Tuscaloosa, AL 35487, United States

Abstract

11

Chitosan-coated silica nanocapsules with a double-shelled structure were crafted using Pluronic

12

F127 as the template via interfacial condensation approach. The shell of these nanocapsules was

13

composed of an inner layer of silica and an outer layer of chitosan, which was designed for sustained

14

release of vanillin. Average size of nanocapsules was proved to be approximately 37 nm, which could

15

be tunable by varying the usage of chitosan. Significantly, these double-shelled nanocapsules possessed

16

enhanced encapsulation efficiency of 95.5% for vanillin and showed sustained release behaviors.

17

Thermal gravimetric results indicated that the double-shelled structure could endow capsules with

18

improved thermal stability. All of these make them promising candidates for sustained release of small

19

volatile molecules in industrial field.

20

Keywords: Chitosan; Silica; Double-shelled nanocapsule; Vanillin sustained release

21

1. INTRODUCTION

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Nanocapsules have been widely investigated as they are capable of encapsulating active

2

ingredients in drugs, cosmetics and foods 1-5. One of the most important characteristics for nanocapsules

3

is their greater effective protection for small volatile cargoes, which offers long-lasting release behavior

4

6

5

for controllable permeability, whereas inorganic capsules exhibit high permeability to encapsulated

6

compounds, but it would have outstanding mechanical properties and thermal stability 7. Among

7

inorganic nanocapsules, silica nanocapsules have attracted much attention in industrial and engineering

8

fields due to their large surface areas, tunable pore sizes and excellent thermal stability

9

they both lack of expected sustained release property. Therefore, organic/inorganic hybrid capsules

10

could be more promising in practical application 10-12. Lee et al. 13 prepared polymer microcapsules with

11

a silica reinforced polyelectrolyte thin shell layer based on the layer-by-layer technique. They found that

12

the deposition of silica nanoparticles with polyelectrolytes can not only remarkably reinforce the shell

13

layer, but also enhance cargoes retention against leakage. In our previous work, casein-based silica

14

nanoparticles have been designed for ibuprofen’s delivery. The results confirmed that hybrid

15

nanoparticles showed a lasting release behavior and enhanced drug load efficiency 14,15.

. Generally, capsules can be made from either inorganic or organic compounds. Organic capsules allow

8,9

. However,

16

Chitosan is a heteropolysaccharide sourced from insect or crustacean shells. This polymer is

17

derived from N-deacetylation of chitin, which is the second most abundant biomass after cellulose. It

18

has also been recognized as a suitable material for capsule shell due to its biocompatibility,

19

biodegradability and nontoxicity. Recently, increasing attention and extensive efforts have been focused

20

on chitosan-based hybrid nanocapsules in release systems

21

chitosan/SiO2 hybrid hollow capsule by the layer by-layer approach and they observed that drugs in a

22

wide range of molecular weight up to 70 kDa Mw could be loaded into the hollow hybrid microcapsules

23

for their sustained release.

16-18

. Rajamanickam et al.

19

prepared the

24

Vanillin, which is extracted from the vanilla plant, is an important flavour and aroma molecule in

25

the world. It is extensively used in food, textile and home-care industries due to its pleasant smell.

26

However, highly volatile nature of vanillin often causes undesired loss during storage or application.

27

Encapsulation is an effective method to mitigate the volatility of small molecule, which can introduce a 2 ACS Paragon Plus Environment

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protective shell around vanillin as a diffusion barrier, thus enhancing the retention of vanillin. For

2

traditional capsules, they always contain a single shell layer, and exhibit relatively high permeability of

3

small cargoes inside incurring relatively rapid release. Differently, multi-layered capsules, for example,

4

double-shelled ones may show sustained release behaviors

5

sometimes complicated even crucial since maybe high temperature, strong acidic or alkaline solution

6

are required, thus restricting their industrial applications

7

extremely necessary. In Wang’s group, a facile and highly benign approach was reported for fabricating

8

silica nanocapsules via interfacial templating condensation using non-ionic block copolymer (F127,

9

PEO106-PPO70-PEO106) micelles as the template

10

24-27

22,23

20,21

. However, their synthesis routes are

. Therefore, a facile synthesis procedure is

. However, the obtained capsules demonstrated a

single shell structure, which may be not good for long-lasting release of volatile flavor.

11

Based on the above work, in our recent cooperative research, a facile synthesis strategy for the

12

construction of chitosan-coated silica nanocapsules with a double-shelled structure was developed

13

(Scheme 1) to offer nanocapsules sustained release property. In this study, silica precursor

14

(tetramethoxysilane, TMOS) and chitosan were employed to construct the inner and outer shells of the

15

capsule, respectively. Scheme 1 depicts the specific route for preparing this double-shelled

16

chitosan-coated

17

γ-methacryloxypropyltrimethoxysilane (KH570, a silane coupling agent) were dissolved in

18

tetrahydrofuran (THF) solution. Upon injecting the mixed solution into an aqueous solution of chitosan,

19

rapid changes in polarity of the solvent helped accelerate the formation of F127 micelles, each of which

20

consists of a PPO core and PEO corona. During this micellization process, vanillin, TMOS and KH570

21

due to their hydrophobic characters were simultaneously encapsulated inside the core of F127 micelles.

22

Subsequently, hydrolysis of TMOS occurred at the interface region between the core and corona of

23

F127 micelles, which would result in the generation of a thin silica inner shell. Meanwhile, the outer

24

shell of chitosan formed via the H-bond interactions between silica and chitosan in the presence of

25

hydrolyzed KH570. Accordingly, double-shelled chitosan-coated silica nanocapsules for vanillin

26

sustained release were obtained. These nanocapsules may exhibit higher vanillin encapsulation capacity

silica

nanocapsules.

Initially,

F127,


TMOS,

vanillin,

and


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1

and more sustained release behaviors, which hold much promise in sustained release of volatile small

2

molecules for industrial use.

3 4

Scheme 1. Synthesis strategy for chitosan-coated silica nanocapsules via interfacial condensation approach.

5

2. EXPERIMENTAL SECTION

6

2.1 Materials

7

Chitosan oligosaccharide (MW=5 kDa, 95% deacetylation) was obtained from Qingdao Yunzhou

8

Biochemistry Co., Ltd. Pluronic F127 (PEO106PPO70PEO106), tetramethoxysilane (TMOS, 98%), and

9

vanillin were all procured from Sigma Aldrich. Tetrahydrofuran (THF) and acetic acid (99%) were

10

purchased from Tianjin Kemiou Co., Ltd. KH570 (γ-methacryloxypropyltrimethoxysilane) was

11

provided from Tianjin Fuyu Co., Ltd. All the chemicals were of analytical grade and used without

12

further treatment.

13

2.2 Preparation of Chitosan-Coated Silica Nanocapsule

14

Chitosan-coated silica nanocapsules were prepared by using F127 as the template via interfacial

15

condensation method. Briefly, 75 mg of F127 was dissolved into 900 µL of THF under magnetic

16

stirring in a 4 mL vial to form a clear solution. Meanwhile, an aqueous solution of chitosan (0.4 wt%) 4 ACS Paragon Plus Environment

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1

was prepared by dissolving chitosan in acetic acid solution (1 wt%) at 50 °C for 24 h. Then, 55 µL of

2

TMOS and 25 µL of KH570 were fed into F127 solution to obtain a mixed solution A, which was then

3

ultra-sonicated for 5 min. After the mixed solution A was slowly injected into 10 g of the chitosan

4

solution and immersed in water bath, the system was stirred for an additional 4 days to evaporate off

5

THF and ensure complete hydrolysis and condensation of TMOS. Finally, chitosan-coated silica

6

nanocapsule aqueous dispersion was obtained.

7

2.3 Vanillin Encapsulation

8

Vanillin encapsulation could be accomplished during the capsule’s formation, as shown in Scheme

9

1. Firstly, 50 mg of vanillin and 75 mg of F127 were co-dissolved into 900 µL THF. In the micellization

10

process, vanillin was encapsulated into the hydrophobic core of F127 micelles. The following steps

11

were the same as the preparation of chitosan-coated silica nanocapsule mentioned above. Eventually,

12

vanillin-loaded chitosan-coated silica nanocapsules were acquired.

13

In the encapsulation process, loss of vanillin due to evaporation is an inevitable problem, and it

14

may affect the calculation of encapsulation efficiency from mass balance. Herein, test of vanillin loss

15

during handing was performed. In this test, chitosan, F127, TMOS and KH570 were not added while

16

other experimental procedures were the same as the preparation of capsules. Loss of vanillin during

17

handing was monitored from the mass loss of vanillin in the mixture before and after the test, which was

18

detected by a TU-1901 UV-Vis spectrometer (Beijing Puxi Co. Ltd.) at 310 nm (λmax for vanillin) 3.

19

Standard curve of vanillin solution (Figure S1) and equation of relationships between absorbance and

20

concentration were acquired (see supporting information). The data represents an average value from 3

21

measurements.

22 23 24

Mass loss of vanillin can be calculated by the following equation 1, where w0* represents the total mass of vanillin added in the mixture; wf* represents the vanillin mass measured after 4 days’ stirring. Mass loss of vanillin (%)=100%× (w0*- wf* )/ w0*

(1)

25

After testing, it was found that only 1.057 mg vanillin was evaporated when 50 mg vanillin was

26

added in the mixture, and the mass loss ratio of vanillin was 0.21% which is within a margin of error, so

27

the loss of vanillin due to evaporation during the handling can be ignored. On this basis, encapsulation 5 ACS Paragon Plus Environment


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efficiency could be determined from the mass loss of vanillin before and after the encapsulation.

2

Equation 2 can be used to calculate encapsulation efficiency of vanillin, where w0 represents the total

3

mass of vanillin added in the initial mixture; wf represents the vanillin mass measured after the

4

formation of nanocapsule. Furthermore, each encapsulation efficiency result represents the average

5

value of 3 measurements to make the result more accurate. Encapsulation efficiency (%)=100%× (w0- wf )/ w0

6 7

(2)

2.4 Vanillin Release

8

Vanillin release properties were investigated by the USP basket method at a rotational speed of 100

9

rpm using the water bath thermostat oscillator (Shanghai Puguang Physical and Chemical Reagent Co.

10

Ltd.). Vanillin release properties of the nanocapsule were determined by a TU-1901 UV-Vis

11

spectrometer at 310 nm 3,14. In a typical release test, 50 µL of the as-prepared nanocapsule solution was

12

diluted 100 times with THF/water mixture (volume ratio 1:9). A vial containing the above suspension

13

was kept at room temperature, and the aliquots of supernatant were taken at time intervals. Meanwhile,

14

fresh THF/water mixture was supplemented after taking each sample to keep the total volume of the

15

suspension constant. Total release of vanillin can be calculated by the equation 3, where w0 represents

16

the total mass of vanillin encapsulated in the capsule; C(n) represents the vanillin concentration when

17

measured for n times (mg/mL); C(n-1) represents the vanillin concentration when measured for (n-1)

18

times (mg/mL).

Vanillin release (%) = 100 × [40C ( n ) + 4∑ C( n - 1 ) ] w0

19 20

(3)

2.5 Characterization

21

Morphology and structure of the as-obtained samples were acquired by a FEI transmission electron

22

microscope (TEM, FEI Tecnai G2 F20 S-TWIN), scanning electron microscopy (SEM, FEI

23

Q45+EDAX, an acceleration voltage of 20 kV) and atomic force microscope (AFM, SPA-400, Seiko

24

Instruments Inc., Japan), separately. Before testing, samples were diluted with acetic acid solution (1

25

wt%). One drop (2 µL) of the aqueous nanocapsule solution was dried on a copper TEM grid for 10 min.

26

For TEM measurement, the dried nanocapsule specimen was then stained with 1% phosphotungstic acid 6 ACS Paragon Plus Environment

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1

solution followed by drying out. The average diameter of the diluted nanocapsules was assessed by a

2

laser particle-measuring instrument (DLS, BT9300Z, Malvern Co., Ltd., UK). FTIR spectrometer (IR

3

Prestige-21, Shimadzu, Japan) was used in the spectra range from 4000 cm-1 to 400 cm-1 via the KBr

4

pellet method to confirm the composition of the prepared nanocapsules. X-ray diffractrometer (XRD,

5

D/max-2200PC, Rigaku, Japan) was used to confirm the vanillin distribution in capsule, with Cu Kα

6

radiation at a scanning rate of 8 deg/min in the 2θ range from 5º to 70º. Thermal gravimetric (TG)

7

experiments were carried out on a thermogravimetric analyzer (Q500 TG, TA, USA) with a heating rate

8

of 10 °C min-1 from 25 to 800 °C in nitrogen atmosphere. In TG experiments, all samples were kept in

9

the dryer before testing to avoid moisture absorption.

10

3.RESULTS AND DISCUSSION

11

3.1 Characterization of Chitosan-Coated Silica Nanocapsule

12

To visualize double-shelled structure of the resultant nanocapsules, TEM images of nanocapsule

13

with 0.4 wt% chitosan usage were acquired. Moreover, nanocapsules templated by different

14

concentrations of F127 were also checked by TEM to see if the template works (Figure S2). However,

15

capsule-like particles only can be observed at presence of 7.5 g/L F127 while concentrations of F127

16

above or below 7.5 g/L induced the formation of random complexes and cube-like particles,

17

respectively, due the self-assemble behaviors of F127 micelle at different concentrations 28, 29. Therefore,

18

capsule templated by 7.5 g/L F127 was applied on TEM and stained by phosphotungstic acid to confirm

19

its double-shelled structure.

20

Figure 1 shows the TEM images of chitosan-coated silica nanocapsule (0.4 wt% chitosan) with

21

(Figure 1a) and without (Figure 1b) staining treatment. The black and white regions in these capsules

22

correspond to the inorganic and organic phases, respectively, due to different electron penetrability. In

23

Figure 1, spherical and regularly distributed hollow capsules with average size of 37 nm could be

24

observed. The white region observed in Figure 1a may correspond to the chitosan shell and PEO chains

25

of F127 in capsule, while the black circle in Figure 1b should correspond to the silica shell. After

26

staining treatment, only organic phases appeared in Figure 1a due to enhanced contrast by 7 ACS Paragon Plus Environment


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phosphotungstic acid. In Figure 1a, white particles with bright white shell were found. It was worth

2

mentioning that white circle can be observed both in the nanocapsules with and without staining

3

treatment, and the average diameter of the white circle is nearly the same (37 nm). Furthermore, the

4

diameter of black circle in Figure 1b is smaller than the size of particles within the white circle in Figure

5

1a. All of these suggested that white circle corresponds to PEO corona on the surface of nanocapsules

6

and chitosan was coated on the silica shell of capsule. Additionally, SEM image of the obtained

7

nanocapsule (0.4 wt% chitosan) affords similar results on the size of these capsules (Figure S3).

8

Although there are some aggregates of nanocapsules observed in TEM and SEM image, the colloidal

9

stability of the reaction system is good, no gel, precipitates or other unstable phenomena appeared

10

during the reaction. Meanwhile, stability of the capsule aqueous dispersion was still good after being

11

placed for 6 months (Figure S4) .

12 13 14

Figure 1. TEM images of chitosan-coated silica nanocapsules (a) with and (b) without staining.

According to the literature, AFM is a very useful modern analysing technology for investigating 30,31

15

topography and size distribution of latex particles

. Hence, AFM images were used to further

16

demonstrate the morphology and size distribution of the same samples as that used in TEM test. As

17

shown in Figure 2, the resultant nanocapsules showed spherical shapes with an approximate size of 38

18

nm, which corresponded to the TEM observations, while larger particles were discovered due to slight

19

aggregations of particles. It was important to highlight that the height of these nanocapsules measured

20

by AFM correlated well with the diameter determined by TEM, suggesting that there was a robust shell

21

in the nanocapsule, which may prevent the collapse of nanocapsule on substrates 8 ACS Paragon Plus Environment

32,33

. All of this

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1

indicated that the obtained chitosan-coated silica nanocapsules were stable in structure once formed and

2

robust even when the solution was evaporated out.

3 4 5

Figure 2. AFM images of chitosan-coated silica nanocapsules and their height proﬁles (a, b) obtained from the corresponding AFM height images.

6

Since the as-synthesized nanocapsules are well dispersed in water without any further surface

7

modification, their hydrodynamic size and size distribution in water were estimated by DLS

8

measurement. As is shown in Figure 3a, the number-average Dh of nanocapsules with 0.4 wt% chitosan

9

usage was about 40 nm. Compared to the particle size determined by TEM, the larger Dh was possibly

10

due to swelling of nanoparticles in solution and their surface property 34. Significantly, in Figure 3a, the

11

hydrodynamic size of these nanocapsules increased slightly as the amount of chitosan increasing.

12

Accordingly, TEM images of nanocapsules with different chitosan contents were provided in Figure

13

3b-d. It was found that particle size of nanocapsules increased with an increase of chitosan usage, which

14

corresponded with the results of DLS measurement, indicating that particle diameter of these

15

nanocapsules could be tunable by varying the usage of chitosan. When the usage of chitosan was

16

increased, the thicker outer shell would form and then larger-sized capsule observed. It

17

was noteworthy that when the chitosan usage was increased more than 0.4%, particle size of

18

nanocapsules increased obviously, indicating that PEO corona may be immersed by chitosan shell as

19

chitosan usage increased to a certain extent.



Page 10 of 22

1 2 3

Figure 3. Size distributions (a) and TEM images (b, 0.2%; c, 0.4%; d, 0.8%) of these nanocapsules as function of chitosan (CS) fraction.

4

To better understand the chemical structure of chitosan-coated silica nanocapsule, FTIR

5

spectroscopy was utilized to examine their functional groups. Figure 4 represents the FTIR spectra of

6

chitosan, chitosan-coated silica nanocapsule (0.4 wt% chitosan) and F127, separately. FTIR spectrum of

7

chitosan shows two peaks at 1637 and 1560 cm-1 because of the strong N-H bending (1560 cm-1) in the

8

primary amine (-NH2) groups and the amide I (C=O stretching, 1637 cm-1) and amide II (weaker N-H

9

bending than that in the primary amine, 1560 cm-1) in the residual (5%) acetylated amine

10

(-NH-(C=O)-CH3) groups of chitosan 35. For chitosan-coated silica nanocapsule, peaks at 465 cm-1 and

11

1100 cm-1 are attributed to the bending and asymmetric stretching vibration of Si-O-Si. A peak at 956

12

cm-1 belongs to the stretching vibration of Si-OH group. All of these indicate the occurring of

13

condensation reaction of Si-OH derived from the hydrolysis of TMOS or KH570. It was noted that N-H

14

bending of chitosan at 1560 cm-1 (curve a) transferred to 1720 cm-1(curve b), which indicated the

15

presence of H-bond between silica (-OH) and chitosan (-NH2) in the capsules. Furthermore, the 10 ACS Paragon Plus Environment

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1

characteristic absorption bands of chitosan and F127 can be found in the FTIR spectrum of

2

chitosan-coated silica nanocapsule, thus confirming the hybrid structure of nanocapsules. a - Chitosan b - Chitosan-coated silica nanocapsule c - F127

a

Amide I Amide II

Si-OH

b

Si-O-Si

c

4000

5

3000

2500

2000

1500

1000

500

Wavenumber(cm-1)

3 4

3500

Figure 4. FTIR spectra of chitosan (a), chitosan-coated silica nanocapsules (b) and F127 (c).

3.2 Vanillin Encapsulation and Thermal Stability

6

As described in Scheme 1, vanillin encapsulation could be accomplished during the capsule

7

formation process. Before determination of encapsulation efficiency, it was imperative to confirm

8

whether vanillin had been successfully encapsulated into the capsules.

9

XRD is a technique that not only can provide the characterization of crystal substances, but also 36

10

can identify the compounds

. According to the previous investigation, vanillin is a crystalline

11

substance and its most intense XRD signal appears at 13◦ of 2θ (Figure S5) 37. In Peña’s research, XRD

12

was used with the purpose to determine the distribution of vanillin crystals in capsules 38. If the capsule

13

presents vanillin crystals over its surface, it was expected to ﬁnd out a sharp and intense peak of vanillin

14

at 13◦ of 2θ. Interestingly, in Figure 5, chitosan-coated silica nanocapsules (0.4 wt% chitosan) with and

15

without vanillin loaded presented similar spectra, and both nanocapsules had an amorphous component.

16

In addition, there was no sharp peak appeared at 13◦ of 2θ in the spectra of vanillin-loaded nanocapsules,

17

which indicated that vanillin might be successfully loaded in the nanocapsules either inside the core or

18

pores of shell 38. 11 ACS Paragon Plus Environment


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Without vanillin With vanillin

10

30

40

50

60

70

Diffraction angle (2θ )

1 2

20

Figure 5. XRD analysis of chitosan-coated silica nanocapsules with and without vanillin loaded.

3

Encapsulation efficiency of double-shelled nanocapsules obtained at different initial concentration

4

of vanillin was studied and showed in Figure S6. It can be observed that encapsulation efficiency of

5

vanillin gradually decreased as the increase of initial concentration of vanillin, which is ascribed to the

6

limited loading capacity of capsule. To verify the advantages of these double-shelled nanocapsules in

7

vanillin encapsulation, encapsulation efficiency of chitosan-coated silica nanocapsule (0.4 wt% chitosan)

8

and pristine silica nanocapsule were both studied. The initial concentration of vanillin, F127 and TMOS

9

usage, reaction condition are kept in the same in the preparation of these two nanocapsules to make this

10

comparison fair. Results showed that vanillin encapsulation efficiency of chitosan-coated silica

11

nanocapsule was around 95.5%, while that for pristine silica nanocapsules was only 80.0%. This result

12

confirmed that the double-shelled structure facilitates improved encapsulation efficiency, which was

13

possibly due to the stronger barrier ability of double-shelled nanocapsules during the encapsulation

14

process of vanillin.

15

Thermal stability of vanillin is very important for industrial use 39-41. In this study, thermal stability

16

of vanillin (Figure 6a), vanillin-loaded silica nanocapsules (Figure 6b) and vanillin-loaded

17

chitosan-coated silica nanocapsules (0.4 wt% chitosan, Figure 6c) was tested by TG technique. As is

18

displayed in Figure 6, a trivial weight loss of 2.0%, 8.0% and 3.0% observed in curve a, b and c,

19

respectively, can be explained by the fact of vaporization of residual moisture in the samples. Weight 12 ACS Paragon Plus Environment

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1

loss at 400 °C appeared in the two kinds of nanocapsules (curve b and c) could be ascribed to the

2

decomposition of F127 structure, while thermal decomposition of 160-185 °C was observed for vanillin

3

itself 26. When temperature reached at 185 °C, the weight of pristine vanillin was reduced by 99.8%. For

4

vanillin-loaded silica nanocapsules (curve b), complete decomposition of vanillin was found until

5

temperature reached at 230 °C and the weight-loss rate decreased compared with that of pristine vanillin

6

(curve a), presumably resulted from the barrier of inner silica shell 42. In sharp contrast, a relatively slow

7

weight loss occurring at 160-280 °C caused by vanillin’s decomposition was detected for

8

vanillin-loaded chitosan-coated silica nanocapsules (curve c). This phenomenon further confirmed the

9

existence of double shells in the nanocapsules

10

43

. Therefore, it is proved that double-shelled structure

could endow nanocapsules with improved thermal stability.

11 12

Figure 6. TG curves of vanillin (a), vanillin-loaded silica nanocapsules (b) and vanillin-loaded chitosan-coated

13

silica nanocapsules (c).

14

3.3 Vanillin Release Behavior

15

Figure 7 illustrates vanillin release behaviors of the nanocapsules in THF/water mixture medium at

16

room temperature. Pristine silica nanocapsules were also taken as the control sample. It could be

17

observed that more than 96% of vanillin in pristine silica nanocaspules was released after 120 h, while

18

only 65% of vanillin was released in the double-shelled nanocapsules (in the case of 0.2wt% chitosan 13 ACS Paragon Plus Environment


1

used). The enhanced sustained release behaviors could be attributed to the double-shelled structure,

2

which formed double-layered barriers to vanillin molecules. Significantly, when the usage of chitosan

3

increased, more sustained release behavior was observed, which indicated that the addition of chitosan

4

was beneficial for giving long-lasting release of vanillin. 0 wt% Chitosan 0.2wt% Chitosan 0.4wt% Chitosan 0.6wt% Chitosan 0.8wt% Chitosan

100

Vanillin release (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80 60 40 20 0 0

20

40

60

80

100

120

Time (h)

5 6

Figure 7. Vanillin release of chitosan-coated silica nanocapsule as function of chitosan fraction.

7

To understand the kinetic mode of vanillin release from these capsules, the data was analyzed

8

according to the Ritger-Peppas equation, and fitted to the following power law equation 44:

9

Mt/M∞=ktn×100%

10

where (Mt/M∞) represents the fraction amount of vanillin released at time t (Mt) with respect to the

11

maximum amount of vanillin (M∞) that would be released at time t =∞ in all cases;

12

related to structural and geometric characteristics of the capsules; n is the diffusion exponent indicating

13

the kinetics and release mechanism involved. In case of spheres, n is usually between 0.43 and 0.85. If n

14

is close to 0.43, Fickian diffusion is the major driving force; when n is close to 0.85, release mechanism

15

mainly tends to the case-II transport; If n is 0.43

Facile Synthesis of Chitosan-Coated Silica Nanocapsules via

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