Facile Fabrication of Self-Healable and Antibacterial Soy Protein

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Applications of Polymer, Composite, and Coating Materials

Facile Fabrication of Self-Healable and Antibacterial Soy Protein Based Films with High Mechanical Strength Feng Li, Qianqian Ye, Qiang Gao, Hui Chen, Sheldon Q. Shi, Wenrui Zhou, Xiaona Li, Changlei Xia, and Jianzhang Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03725 • Publication Date (Web): 08 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

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ACS Applied Materials & Interfaces

Facile Fabrication of Self-Healable and Antibacterial Soy Protein Based Films with High Mechanical Strength Feng Li,a Qianqian Ye,a Qiang Gao,a Hui Chen,a Sheldon Q. Shi,b Wenrui Zhou,a Xiaona Li,c Changlei Xia,b Jianzhang Li*a a

MOE Key Laboratory of Wooden Material Science and Application & Beijing Key Laboratory of

Wood Science and Engineering, Beijing Forestry University, Beijing 100083, China b

Department of Mechanical and Energy Engineering, University of North Texas, Denton, TX 76203,

USA c College

of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China

KEYWORDS: soy protein isolate, polyethyleneimine, self-healing materials, high mechanical strength, antibacterial

ABSTRACT: Soy protein isolate (SPI), a ubiquitous and readily available biopolymer, has drawn increasing attention because of its sustainability, abundance and low price. However, the poor mechanical properties, tedious performance adjustments, irreversible damage and weak microorganism resistance have limited its applications. In this study, a facile but delicate strategy is proposed to fabricate an excellently self-healable and remarkably antibacterial SPI-based material with high mechanical strength by integrating polyethyleneimine (PEI) and metal ions (Cu(II) or 1

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Zn(II)). The tensile strengths of the SPI/PEI-Cu-0.750 and SPI/PEI-Zn-0.750 films reach up to 10.46 ± 0.50 MPa and 9.06 ± 0.62 MPa, which are 367.06% and 306.28% strength increases compared to neat SPI film, respectively. Due to the abundant noncovalent bonds and low glass transition temperature of the network, both SPI/PEI-Cu and SPI/PEI-Zn films exhibit satisfactory self-healing behavior even at room temperature. Furthermore, SPI/PEI-Cu and SPI/PEI-Zn films demonstrate high bacterial resistance against E. coli and S. aureus. This facile strategy of establishing dynamic networks in a biomaterial with numerous excellent properties will enormously expand the scope of its applications, especially in the field of recyclable and durable materials. 1. INTRODUCTION

polymers have captured extensive attention and have been widely exploited in wearable electronics, actuators, tissue

In recent years, interest in exploiting biopolymer materials

engineering and so on.9-11 The healing mechanism is mainly

has substantially increased due to their sustainability,

divided into extrinsic and intrinsic healing. The extrinsic

abundance and low cost, which can not only resolve the

healing strategy is to encapsulate healing agents into the

problem of environmental pollution but also abate the

polymeric matrix, such as microspheres, hollow fibers and

overdependence on petroleum resources.1 Among a variety

microvascular networks.12 Thus, the matrix can be repaired

of biopolymers, soy protein isolate (SPI), the most abundant

at the damaged sites via the release of the healing agents but

plant protein in nature, possesses higher strength than polysaccharide-

and

lipid-based

materials1

and

is limited by healing times and areas.13 The intrinsic healing

has

strategy is to establish noncovalent interactions or dynamic

widespread applications, such as tissue regeneration, gene

covalent bonds so that the failure can be recovered through

delivery, packaging, adhesives, food and so on.2-3 However,

the reformation of reversible bonds.12 Because of the high

there exist numerous problems associated with its use. For

reliability and the ability of multiple healing cycles, the

example, prior research attempted to establish a chemically

intrinsic healing method is an intriguing choice for the future

cross-linked network of SPI-based composites to endow

of self-healing materials.14 Unfortunately, although the

outstanding mechanical performance, but the network was irreversibly Additionally,

damaged SPI-based

when

subjected

materials,

to

similar

development of self-healing polymers has made significant

stress.4-5 to

progress

most

in

synthetic

polysaccharide,17-18

biopolymers, are susceptible to microbes due to their intrinsic

polymers15-16

and

natural

there has been little work on self-healing

soy protein materials.19 Moreover, it is challenging to

structure, resulting in the loss of mechanical performance.6-7

fabricate self-healing polymers with improved mechanical

To overcome the issue of irreversible damage, a self-

properties because the weak dynamic interactions and high

healing polymer, which is an intelligent material inspired by

viscoelastic properties for the polymeric matrix can lower

living organisms, can be used. For such polymers, detriment

these properties.10

triggers the self-recovery response; as such, these polymers

On the other hand, the poor resistance of SPI-based

can be an ideal alternative to prolong service life, improve

materials to microbes severely limits their applications; SPI-

safety and reduce maintenance costs.8 Thus, self-healing

based materials need to be endowed with antibacterial 2


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properties to improve their stability and service life. At

in 57 ml deionized water under stirring for 30 min. After the

present, even though there have been several reports about

SPI/PEI solution was uniformly dispersed, the mixture was

antibacterial SPI-based materials,6-7 this is still a fascinating

heated at 85 °C and stirred continuously for another 30 min.

area to explore.

Then, CuSO4 or ZnCl2 in varying amounts were added dropwise under stirring for 8 hours at ambient temperature.

Considering the abovementioned characteristics, we fabricated

SPI-based

materials

by

To form a more homogeneous composite, the SPI/PEI-M

incorporating

solutions were sonicated for 15 min. For comparison, a neat

polyethyleneimine (PEI) and metal ions (Cu(II) or Zn(II)),

SPI film (control-1) was fabricated in accordance with the

which are named as SPI/PEI-M-x, where M stands for the

procedure described in a previous report.5 Briefly, 1.5 g SPI

metal ion used and x represents the mmol of metal ions added.

and 0.75 g glycerol were dissolved in 57 ml deionized water.

PEI is a water-soluble polymer comprised of a raft of amines,

After the pH of solution was adjusted to 9, the solution was

including primary, secondary, and tertiary amino groups,

heated at 85 °C for 30 min with continuous stirring. To

which allow the PEI to form hydrogen bonds and

fabricate the SPI/glycerol-Cu film (control-2), CuSO4 was

coordination bonds. Meanwhile, Cu(II) and Zn(II) were

added and stirred for 8 hours under ambient temperature. The

selected as central atoms in coordination compound due to

compositions and concentrations of all control and SPI/PEI-

their proper coordination interactions with PEI, low cost and

M samples are summarized in Table S1. The resulting

abundance. The benefits of this system are as follows: (1)

mixtures were then poured into Teflon plates and dried at

compared to the traditional SPI-based composites, it is easy

45 °C. Finally, the films were placed in a saturated-K2CO3-

to establish a tunable system of strong metal-ligand

regulated desiccator (approximately 50% relative humidity).

interactions and weak hydrogen bonds; (2) it possesses a self-

2.3. Characterization.

healing ability due to the reversible nonvalent interactions and the high flowability of PEI; and (3) the coexistence of

The Scanning electron microscopy (SEM) was applied by

PEI and metal ions can increase the antibacterial ability of

an SEM analyzer (FEI Quanta FEG650, USA) with an

SPI-based materials. To the best of our knowledge, it is the

accelerating voltage of 10 kV to observe the surface

first time to explore a readily synthesized, self-healable and

morphology of the samples. The X-ray diffraction (XRD)

antibacterial SPI-based material with high mechanical

patterns were recorded by an X-ray diffractometer (D8

strength. It is believed that this work will enormously expand

Advance, Bruker, Germany) using Cu Ka radiation (40 kV,

the scope of its applications.

40 mA) with a 2θ range between 10 ° and 60 °. The differential scanning calorimetry (DSC) analysis was carried

2. EXPERIMENTAL

out on a DSC apparatus (Q2000, TA Instruments, USA),

2.1. Materials. SPI (95% protein) was provided by Yu

which was calibrated using indium and sapphire standards.

Wang Ecological Food Industry Co., Ltd. (Shandong, China).

Heating and cooling rates of 20 °C/min were used over the

Branched PEI (Mw = ca. 30 000) was purchased from Xiya

studied temperature range of -80 °C to 180 °C. Attenuated

Reagent Co., Ltd. (Sichuan, China). CuSO4 and ZnCl2 were

total reflectance-Fourier transform infrared (ATR-FTIR)

obtained from Tianjin Jinke Fine Chemical Research

(Nicolet 6700, Thermo Scientific, USA) spectra were

Institute (Tianjin, China) and Xilong Chemical Co., Ltd.

obtained within the wavenumber range from 800 to 3550 cm-

(Guangdong, China), respectively. Glycerol (99% pure) and

1

sodium hydroxide were obtained from Beijing Chemical

with 32 scans. Ultraviolet–visible (UV–vis) absorption

spectra were collected using a UV-vis spectrophotometer

Reagents Co., Ltd. (Beijing, China) and used without further

(TU-1901, Beijing Purkinje General, China). The X-ray

purification.

photoelectron spectra (XPS) were recorded on an XPS SPI/PEI-M-x,

spectrometer (ESCALAB 250XI, Thermo, England) with a

SPI/glycerol (control-1) and SPI/glycerol-Cu (control-2)

binding energy range of 0–1200 eV. The SPI/PEI-M samples

films. First, 1.5 g SPI and 3 g PEI (30% wt) were dissolved

examined

2.2.

Preparation

of

SPI/PEI,

were

3


SPI/PEI-M-0.750

composites

unless


otherwise noted. 2.4.

Mechanical

Page 4 of 17

functionalities, including high mechanical strength, selfPerformance.

The

healing and antibacterial capabilities, into one material, a

mechanical

facile but delicate strategy and material system were used.

properties of the films were measured at room temperature

The composite synthesis process and reaction mechanism are

using a universal testing machine (INSTRON 3365,

illustrated in Figure 1. At first, SPI and PEI were mixed at 85

Norwood, MA, USA) equipped with a 100 N load cell. The

°C for 30 min to denature the SPI structure and to obtain

specimen dimensions were 10 mm × 80 mm with a 50 mm

better dispersion between SPI and PEI. As reported in

gauge length. A strain rate of 20 mm min−1 was used. Three

previous works, the incorporation of two contrasting

replicates were tested for each measurement.

structural polymers can balance the mechanical properties

2.5. Self-healing Ability. Following protocols from the

between stiffness and toughness.22 With its long chain and

literature,20-21 damage to the film was applied by cutting the

relatively better crystalline structure, SPI was regarded as the

samples to a depth of 50–70% with a razor blade. Samples

hard component that endows mechanical properties. On the

were healed at 25, 50 and 70 °C, keeping the humidity at 60–

other hand, PEI, with its short chains and highly branched

70%. The healing process was observed by optical

structure, exhibited favorable ductility and acted as a soft

microscope (DP70, OLYMPUS, Japan), and photographs

component to gain stretchability. It is worth mentioning that

were taken with a camera. The self-healing efficiency was

the SPI’s structure in the SPI/PEI composite is quite different

measured by tensile experiments and defined as:

from that in the traditional SPI/glycerol-based composite; the XRD patterns of the SPI/glycerol-based composite exhibit a

η = (𝐸𝐵𝑖 ― 𝐸𝐵1) ∕ (𝐸𝐵0 ― 𝐸𝐵1) × 100%

decrease of the α-helix (2θ = 9°) and an increase of the βsheet (2θ = 20°) structures in the SPI secondary conformation

where EBi is the elongation at break of SPI/PEI-M films at

compared to those of the raw SPI powder (Figures S1a and

different healing times, EB1 is the initial elongation at break

1b) due to the change of conformation induced by heating.

of the damaged SPI/PEI-M films and EB0 is the original

The conformation of the α-helix was destroyed, and the

elongation at break of the SPI/PEI-M films. The average

random coil was unfolded. Additionally, the external heat

value was calculated from at least three independent

energy increased the SPI chain movement, which could

experiments.

rearrange the molecular chain structure and transform it into 2.6. Antibacterial Assessment. The antimicrobial effects

the β-sheet structure.23 In the SPI/PEI-based composite, both

of the pure and modified films were evaluated through a disc

the α-helix and β-sheet conformations were obviously

diffusion test using Escherichia coli (E. coli) and

decreased, indicating that the highly branched structure of

Staphylococcus aureus (S. aureus) bacterial strains. First, the

PEI destroyed the crystal structure of SPI and interrupted the

specimens were cut into a disc with a diameter of 6 mm and

SPI chains, which created an amorphous character (Figure

sterilized under an ultraviolet radiation lamp for 5 hours. The

S1c).24-25 The differences of structure among raw SPI powder,

bacterial dispersion, with a concentration of

108

colony-

SPI/glycerol-based composite and SPI/PEI-based composite

forming units (CFU)/mL, was then applied uniformly on the

were further indicated by DSC analysis (Figure S2). The raw

surface of a nutrient agar plate, and the specimens were

SPI powder showed two endothermic transitions (Tg1 = 65 °C,

placed on the plate. Finally, the plates were incubated at

Tg2 = 121 °C) that were caused by the denaturation of 7S and

37 °C for 24 hours and the diameters of the bacterial

11S proteins and were identified as movements of the SPI

inhibition zones surrounding the discs were measured by a

chains.26 The SPI/glycerol-based composite exhibited two

micrometer. Each group of SPI-based materials was

glass transitions on the DSC curve (Tg1 = -46 °C, Tg2 = 56 °C),

measured with five replicates.

which corresponded to the glycerol-rich domain and the SPIrich domain, respectively, indicating two obviously different

3. RESULTS AND DISCUSSION

phases in the SPI/glycerol-based composite (Figure S3). While there only existed a single glass transition (Tg = 19 °C)

3.1. Design, Synthesis and Characterization of SPI/PEI-M

Based

Films.

To

integrate

on the DSC curve of the SPI/PEI-based composite, the glass

multiple 4


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transition of PEI (Tg = -46 °C) disappeared, indicating that

Zn(II) fails to generate the d-d jump due to the d 10-type of its

there was good miscibility between SPI and PEI and,

d orbits.27 The XRD pattern of SPI/PEI-M showed that the

ultimately, pointing towards a relatively uniform phase.26

broad diffraction peak shifted to a lower degree (2θ = 19°) compared to SPI/PEI-based composite (2θ = 21°) and

Additionally, in order to further improve the mechanical

displayed a sharper peak and enhanced intensity, which

strength and tunability of structures and mechanics, metal

confirmed the interactions between metal ions and the

ions were introduced to form metal-ligand interactions. It was

SPI/PEI composite (Figure 2a). Moreover, the elevation of

observed that the surface morphologies of SPI/PEI-Cu and

the glass transition temperatures in SPI/PEI-M (Tg = 25 °C)

SPI/PEI-Zn-based films had become rougher than that of

compared to SPI/PEI (Tg = 19 °C) and the appearance of a

SPI/PEI-based film (Figure S4). Additionally, the SPI/PEI-

new melting peak (T = 64 °C) further demonstrated the

based mixture turned deep blue after the introduction of

formation of metal-ligand interactions between SPI/PEI and

CuSO4, but there was no color change observed for ZnCl2.

metal ions and, ultimately, enhanced the crystalline nature of

This phenomenon can be explained by the d orbit of Cu(II)

SPI/PEI-M (Figure 2b).

being d 9-type, which can allow d-d jumps under the influence of crystal fields formed by ligands. In contrast,

Figure 1. Fabrication process and schematic of the SPI/PEI-M-based films. To further reveal and verify the interaction among SPI, PEI

characterize the SPI/PEI-based composites. The FTIR

and metal ions, FTIR, UV−vis and XPS were used to

spectra of SPI/PEI (Figure S5) showed a broad band at 31005



3700 cm-1, attributed to the stretching vibrations of -OH and

Page 6 of 17

Cu(II) or Zn(II).31-32

-NH groups in SPI and PEI. The strong absorptions at 1628

The differences of the SPI/PEI-based composite compared

and 1538 cm-1 corresponded to the amide Ⅰ (C=O stretching)

to raw SPI material and the metal-ligand reaction in SPI/PEI-

and amide Ⅱ (N-H bending) bands of SPI, respectively. The

M-based composite were also examined through UV-vis

peaks at 1306, 2848 and 2924 cm-1 referred to the twisting

absorbance spectra (Figure 2d). The raw SPI material had a

motion and symmetric and asymmetric vibrations of -CH2

broad absorbance at approximately 280 nm, whereas this

groups in PEI.28 When the metal ions of Cu(II) or Zn(II) were

absorption peak shifted to 325 nm in the presence of PEI; this

added to the SPI/PEI composite (Figure 2c), the peak at 2848

suggested the formation of hydrogen bonds in the SPI/PEI

cm-1 was slightly shifted to 2851 cm-1, indicating that an

composite. When Cu(II) or Zn(II) were introduced into the

interaction occurred between the PEI and the Cu(II) or

SPI/PEI composite, the absorption peak at 325 nm further

Zn(II).29 In addition, the increase of the CH νs/νas ratio,

shifted to 366 nm due to the ligand-to-metal-charge-transfer

namely, the ratio of intensities of the 2924 cm-1/2848 cm-1

transition, confirming that the metal ions coordinated with

peaks, indicated that the trans/gauche conformer ratio in the

PEI and SPI.33-34 Additionally, a new absorption peak at 630

aliphatic chain increased. It is likely that more trans

nm was observed in SPI/PEI-Cu that corresponded to the

conformers were formed between PEI and Cu(II) or Zn(II),

Cu(II) d-d transition, which further demonstrated that the

as a more ordered conformation.28 Moreover, the adsorption

coordination formed between the SPI/PEI composite and

peaks at 1100 and 1033 cm-1 decreased, which were assigned

Cu(II) and the major chelate is N/Cu(II).35

to the stretching vibrations of C-N bonds.30 This might be due to the formation of interactions between amino groups and

Figure 2. (a) XRD patterns of SPI/PEI, SPI/PEI-Cu and SPI/PEI-Zn composites; (b) DSC curves of SPI/PEI, SPI/PEI-Cu and SPI/PEI-Zn composites; (c) ATR-FTIR spectra of SPI/PEI, SPI/PEI-Cu and SPI/PEI-Zn composites; (d) UV-vis spectra of SPI, SPI/PEI, SPI/PEI-Cu and SPI/PEI-Zn composites. 6


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Figure 3. XPS detailed spectra of (a) SPI/PEI; (b) SPI/PEI-Cu; (c) SPI/PEI-Zn. To better understand the changes of SPI/PEI-based

groups shared a lone pair of electrons with the electron-

composites before and after the introduction of metal ions

withdrawing Cu(II) ion, causing the coordinate interaction

(Cu(II) or Zn(II)), the XPS of different elements at a high-

between N atom and Cu(II) ion. Consequently, the electron

resolution level were deconvoluted to evaluate the

density of N atom was reduced, resulting in an increase of N

contribution of each component (Figure 3). For SPI/PEI, the

1s binding energy.37-38 The O 1s peak of SPI/PEI-Cu

N 1s peak was deconvoluted into two components at binding

composite was deconvoluted into three peaks at 530.7, 531.8

energies of 398.9 and 399.8 eV, which was ascribed to the

and 532.5 eV. The new peak at 532.5 eV was ascribed to the

nitrogen-containing groups of -NH- and -NH2 in SPI and PEI.

chelate interaction between O atom and Cu(II) ion, which

Meanwhile, the O 1s peak was deconvoluted into two peaks

was similar to N atom and Cu(II) ion.37 The higher position

at 530.5 eV and 531.4 eV, owing to the presence of the

shift of O 1s peak further demonstrated the formation of

oxygen-containing groups C=O and

C-O.36-37

However, it has

O…Cu coordination bonds due to the coordination ability of

shown a significant difference on the N 1s and O 1s peaks

oxygen-containing groups in SPI, such as -COOH and -OH.36

after the introduction of Cu(II) or Zn(II) ions into the SPI/PEI

Additionally, the peaks at approximately 932-934 and 952-

based composite. For the SPI/PEI-Cu composite, both N 1s

954 eV corresponded to the signals of Cu 2p3/2 and Cu 2p1/2,

and O 1s peaks shifted to a higher position, which were about

respectively, indicating coordination between the SPI/PEI

0.6 eV and 0.7 eV, respectively. The N 1s spectra comprised

composite and Cu(II) ions.36 After introducing Zn(II) ions,

three peaks with binding energies of 399.2, 399.9 and 402.2

the changes of the N 1s and O 1s peaks in SPI/PEI-Zn

eV, identified via deconvolution. The appearance of new

composite are similar to that in SPI/PEI-Cu composite. The

binding energy at 402.2 eV confirmed that a N…Cu complex

N 1s and O 1s peaks shifted to a higher position as well.

was formed. It can be explained that the N atom in the amine

Meanwhile, a new peak at 532.5 eV appeared in the O 1s 7



Page 8 of 17

spectrum, which was assigned to the O…Zn coordinate

Zn 2p1/2, respectively.40 It can be concluded from these results

bonds.39 In addition, peaks appeared at binding energies of

that the metal ions of Zn(II) can form N…Zn and O…Zn

1,021.4 eV and 1,044.6 eV that corresponded to Zn 2p3/2 and

interactions with SPI/PEI composites.

Figure 4. Mechanical properties of (a),(b) SPI/glycerol (control 1), SPI/PEI and SPI/PEI-Cu-x based films; (c),(d) SPI/glycerol (control 1), SPI/PEI and SPI/PEI-Zn-x based films; (e) SPI/PEI-Cu-0.750 and SPI/PEI-Zn-0.750-based films; (f) Plot of tensile strength, elongation at break and increment of tensile strength of SPI/PEI-M-based films and other reported SPI-based films.4-5, 41-51

3.2. Mechanical Properties of SPI/PEI-M Based Films.

compared to the neat SPI film; it maintained good

To investigate the effects of PEI, the type and content of

stretchability (118.96 ± 15.71%), and exhibited an obvious

metal ion on mechanical properties of the SPI/PEI-M based

increase of toughness (8.18 ± 1.05 MJ/m3). This may be due

films, a series of SPI based composite films were fabricated

to the highly branched structure and multitude of amine

(Figure 4a-f and Table S2). Neat SPI films possessed high

groups in PEI, which can easily destroy the ordered structure

flexibility but poor tensile strength due to the added glycerol,

of SPI chains and form abundant strong hydrogen bonds with

which forms weak hydrogen bonds with SPI. On the other

the SPI, indicating a better miscibility and a relatively more

hand, the SPI/PEI composite showed an obvious increase in

uniform network in the SPI/PEI-based composite (as

tensile strength, from 2.79 ± 0.03 MPa to 8.29 ± 0.12 MPa

opposed to the weak hydrogen bond interactions between SPI 8


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and glycerol and the existence of two obvious phases in the

film, it was still much better than that of SPI-based film

SPI/glycerol

worth

(Figure 4b and Figure 4d). In order to further evaluate the

mentioning that mechanical performance can not only be

contribution of PEI, which plays a major role in forming

tuned by adjusting the content of metal ions but can also be

coordination with metal ions, to SPI/PEI-M based films, we

affected by the type of metal ion. After integrating metal ions

make comparisons with SPI/glycerol-Cu and SPI/PEI-Cu-

(Cu(II) or Zn(II)) into the SPI/PEI mixture, it could be seen

0.375-based films. Neither the tensile strength of the

that the tensile strength was further enhanced by an increase

SPI/glycerol-Cu film (2.79 ± 0.03 MPa) nor was the

in the content of metal ions; the relative strengths of the

stretchability (64.41 ± 0.72%) increased compared to the neat

SPI/PEI-Cu-0.750 and SPI/PEI-Zn-0.750 films reached as

SPI-based film (2.23 ± 0.01 MPa, 166.53 ± 3.02%).

high

as

composite).52

and

is

The

Conversely, the SPI/PEI-Cu film exhibited outstanding tensile strength (9.38 ± 0.74 MPa) and stretchability (70.41 ±

interactions, which were formed among SPI, PEI and metal

13.39%) as well as a remarkably enhanced mechanical

ions, that gradually replaced hydrogen bonds; thus, the

performance compared to the SPI/glycerol-Cu films (Figure

tensile strength was improved as the content of metal ions

S6). The results confirmed that a number of amine groups

increased. Further, increases in the content of metal ions

from PEI, which is the key to the better mechanical

caused the stretchability of SPI/PEI-Cu and SPI/PEI-Zn to

performance, can form strong coordination bonds with Cu(II).

obviously decrease, which is probably attributed to too many

These dynamic reversible bonds were served as “sacrificial

strong coordination bonds restricting the mobility of

bonds” and provided recoverable energy dissipation

chains.9, 21, 39

306.28%,

it

phenomenon can be explained by the stronger metal-ligand

molecular

367.06%

Furthermore,

respectively.

In addition, the SPI/PEI-Cu film

mechanisms via the disruption–reconstruction behavior in

possessed a better tensile strength but lower stretchability

SPI/PEI-based composites.22 In contrast, the SPI/glycerol-

compared to that of SPI/PEI-Zn, indicating that Cu(II)

Cu-based film failed to form strong interactions between SPI

formed a stronger metal-ligand interaction with SPI and PEI

and Cu(II) because of the lack of effective groups, which

than Zn(II). The reason may be due to that though both Cu

result in its poor mechanical performance. Moreover, it could

(Ⅱ) and Zn (II) can chelate with four ligands in the SPI/PEI

be seen that the SPI/PEI-M films have excellent tensile

composite, the second ionization energy of Cu is far greater

strength, good stretchability and the highest increment of

than that of Zn, meaning Cu (Ⅱ) accepts the electron pair

tensile strength compared to other SPI-based films published

from the N atoms on SPI/PEI composite more easily than Zn

at this point (Figure 4f). This might be due to the

(II), so the degree of the crosslinking of SPI/PEI-Cu is more

insufficiency of molecular interactions and/or excessively

than that of SPI/PEI-Zn, which result in the SPI/PEI-Cu film

fragile macromolecular structures in the previously reported

possessing higher tensile strength but lower stretchability

SPI-based modified films, which ultimately resulted in an

than that of SPI/PEI-Zn film.27 Therefore, the properties can

unsatisfactory modification effect. Conversely, the SPI/PEI-

be tuned by changing the type of metal ion. The toughness of

M-based films have introduced abundant dynamic reversible

SPI/PEI-Zn-0.075 was further improved to 10.44 ± 0.58

noncovalent interactions and possess a balance between

MJ/m3,

rigid/long SPI chains and soft/short PEI chains, which give

which was higher than that of SPI/PEI-Cu-0.075

(7.18 ± 0.58

MJ/m3).

Though the toughness of SPI/PEI-Cu

the SPI/PEI-M composites good mechanical properties and

appeared to slightly decrease compared to the SPI/PEI-based

outstanding modification effects.

9



Page 10 of 17

Figure 5. Optical microscopy images of (a) scratched SPI/PEI-Cu films before and after self-healing at 50 °C and humidity of 60 –70% for 30 min, scale bar: 500 μm and (b) scratched SPI/PEI-Zn films before and after self-healing at 50 °C and humidity of 60 –70% for 30 min, scale bar: 500 μm; (c) Tensile curves of SPI/PEI-M films at various healing times at 25 °C, 50 °C and 70 °C; (d) Healing efficiencies of SPI/PEI-M films after different times at different temperatures; (e) Schematic illustration of proposed mechanism for self-healing process. 3.3. Self-healing Capability. SPI/PEI-M films not only

SPI/PEI-M films are summarized in Figure 5c-d. It can be

possess superior mechanical performance but also exhibit a

observed that the tensile strength and elongation at break of

self-healing capability. To this end, the self-healing behavior

SPI/PEI-Cu-0.750 and SPI/PEI-Zn-0.750 films gradually

of the SPI/PEI-Cu-0.750 and SPI/PEI-Zn-0.750 films were

recovered as the healing time was extended. In contrast, the

evaluated. The scratch restoration ability was first observed

self-healing ability of SPI/PEI film is inferior to that of

using an optical microscope (Figure 5a and 5b). It was

SPI/PEI-M films (Figure S7), which indicates that metal-

obvious that damage on the SPI/PEI-Cu and SPI/PEI-Zn

ligand interactions could facilitate the healing of mechanical

films disappeared after healing, which indicated that the

damages of SPI/PEI-M films. Additionally, the required

SPI/PEI-M films possessed favorable self-healable behavior.

restoration time was significantly shortened with the

To further quantitatively evaluate this property, the recovery

elevation of healing temperature. The healing process is

of mechanical properties of the scratched films was

depicted in Figure 5e; it benefited from the high chain

subsequently studied using a tensile testing machine. The

mobility of PEI and the sufficient concentration of reversible

exact tensile properties and self-healing efficiencies of the

bonds,

including

10


hydrogen

bonds

and

coordination

Page 11 of 17 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


interactions. As mentioned above (Figure 2b), both SPI/PEI-

Zn-0.750-based films. This phenomenon might be explained

Cu-0.750 and SPI/PEI-Zn-0.750 composites exhibited low Tg

by the second ionization energy of Cu being far greater than

values (approximately 25 °C), which are attributed to the

that of Zn, meaning Cu(II) accepts the electron pair from the

highly branched structure of PEI and indicate that the

N atoms on PEI more easily than Zn(II).27 Therefore, more

polymer chain could easily flow and be reattached onto the

coordination bonds were reformed in the SPI/PEI-Cu-0.750

fractured surfaces. Meanwhile, owing to the mobility of

films by comparison, resulting in a better healing efficiency.

polymer chains and the abundant amine groups in PEI,

To better estimate the comprehensive performance of self-

dissociated metal ions could reestablish coordination

healable SPI/PEI-M materials, we compared SPI-based

interactions with ligands while hydrogen bonds gradually

materials with some other self-healing polymers from the

reform in the SPI/PEI-based composite. Hence, the dynamic

literature in terms of substrate, mechanical properties and

reversible network is the origin of the self-healing behavior

self-healing performance. Most of the self-healing polymers

in the SPI/PEI-M films. Higher healing temperatures and

were composed of nonrenewable raw materials or exhibited

longer healing times, with more sufficient dynamic

poor mechanical properties or unsatisfactory healing

reversibility, led to better healing efficiency. Moreover, it

performances. However, the SPI-based material displayed an

was surprising to find that SPI/PEI-Cu-0.750-based films

excellent comprehensive performance (Table 1).

generally have a higher healing efficiency than did SPI/PEITable 1. Tensile strength, elongation at break, healing conditions, and healing efficiency of SPI based self-healing material and other self-healing polymers.15, 17-18, 53-64 Substances

Tensile strength (MPa)

Elongation at break (%)

Healing conditions

Healing efficiency (%)

Ref.

50-100

This work

25°C/12 h 10.46 (SPI/PEI-Cu)

75.98

Soy protein

50 °C/90 min + 60-70% humidity 9.06 (SPI/PEI-Zn)

137.35 70 °C/30 min

Boronic esters

4.4

58

25 °C/72 h + 85% humidity

≈90

53

Polybutadiene

3.25

380

110 °C /12 h

75

54

DFTPA-PI-MA

3

118

110 °C/3 min + 60 °C/24 h

90.7

55

Polybenzoxazine

0.12

100

25 °C /12 h + pressure

96

56

Poly(urethane-urea)s

0.93

301

37 °C/12 h

87

57

PDMS

0.7

115

25 °C/4 h

75

58

PDMS-Boroxine

9.46

9.72

70 °C/5 h

95

59

Polycyclooctene

1.85

345

50 °C/16 h

≈100

60

Polyurethane

6.76

923

25 °C/2 h

≈80

16

Polyacrylate

17.89

38.92

60 °C/24 h

95.81

61

Polysaccharide

0.12

600

25 °C/24 h

87.7

18

PAA/CS

3.7

1200

70 °C/24 h

58.33

19

Chitosan

0.026

76.1

25 °C/2 h

100

62

1NDI/CNC

0.5-3.25

10-20

85 °C/5-20 min

≈100

63

PAA/Agar/PVA

0.45

497

25 °C/24 h

84

64

11



Page 12 of 17

Figure 6. Antibacterial activities of SPI-based films against bacteria (a) E. coli; (b) S. aureus after 24 h of contact; (c) the diameters of the inhibition zones against E. coli and S. aureus. 3.4. Antibacterial Activity. To assess the antibacterial

bacteria.65 In addition, when Cu(II) ions were added into the

activity of pure SPI/glycerol and SPI-based modified

SPI/PEI-based composite, the inhibition zone diameters for

materials, a disk diffusion test using Gram-positive

the SPI/PEI-Cu material against E. coli and S. aureus bacteria

(Staphylococcus aureus) and Gram-negative (Escherichia

were further increased to 15.39 ± 0.48 and 15.20 ± 0.64 mm,

coli) bacteria was conducted at 37 °C for 24 hours. The SPI-

respectively. The results indicated that Cu(II) could enhance

based modified materials exhibited obvious antibacterial

the bacterial resistance due to the interaction between Cu(II)

properties against E. coli and S. aureus bacteria, whereas the

and the negatively charged bacterial cell wall, which would

pure SPI/glycerol material failed to inhibit the growth of

lead to protein denaturation and the death of the bacteria.66

bacteria (Figure 6). The inhibition zone diameters for

However, the integration of Zn(II) did not evidently improve

SPI/PEI against E. coli and S. aureus bacteria were 13.85 ±

the antibacterial activity of the SPI/PEI-Zn material, as the

0.90 and 14.49 ± 0.56 mm, respectively, confirming that PEI

observed diameters of the inhibition zones against E. coli and

played a vital role in the antibacterial ability (Figure 6c). The

S. aureus were 14.05 ± 0.61 and 14.28 ± 0.76 mm,

reason may be that PEI is a polycation consisting of massive

respectively. The results were similar to those of the SPI/PEI

positive charges, which can combine with bacteria

material and demonstrated that Zn(II) was inferior to Cu(II)

membranes by ion exchange and, ultimately, kill the

in terms of antibacterial capability.

5. CONCLUSION

endow the SPI/PEI-Cu-0.750 and SPI/PEI-Zn-0.750 films with an outstanding mechanical strength, while tunability

A multifunctional SPI-based material integrating high

was imparted by varying the content and kind of metal ion.

mechanical strength, self-healing properties and antibacterial

Moreover, the ability of PEI and metal ions to interact with

capabilities was successfully fabricated via a facile approach

microorganisms and result in the death of the bacteria

by incorporating PEI and metal ions. Both the high chain

commendably solve the drawback of SPI being vulnerable to

mobility of PEI and the noncovalent bonding resulted in the

microorganisms. This novel construction strategy might

dynamic reversibility of SPI/PEI-M composites, and these

broaden the potential applications of biopolymer-based

composites exhibit excellent self-healing capabilities, even at

materials with recyclable and durable properties.

room temperature. Satisfactorily, this dynamic network has

ASSOCIATED CONTENT

an enhanced mechanical performance. The abundant hydrogen bonds and coordination interactions in the network 12


Page 13 of 17 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


(6) Zhao, S.; Yao, J.; Fei, X.; Shao, Z.; Chen, X. An

Supporting Information

Antimicrobial Film by Embedding In Situ Synthesized

XRD patterrns and ATR-FTIR spectra of different SPI-based

Silver Nanoparticles in Soy Protein Isolate. Mater. Lett.

composites; the glass transition temperatures of different

2013, 95, 142-144.

compontents; schematic illustration of traditional SPI-based

(7) Koshy, R. R.; Mary, S. K.; Thomas, S.; Pothan, L. A.

film; the surface morphology of SPI/PEI-based film; self-

Environment Friendly Green Composites Based on Soy

healing capability of SPI/PEI-based film; mechanical

Protein Isolate – A Review. Food Hydrocolloid. 2015,

properties of SPI-based films; formulations of different SPI-

50, 174-192.

based composites (PDF)

(8) Zwaag, S. Self-Healing Materials: An Alternative Approach to 20 Centuries of Materials Science. Springer

AUTHOR INFORMATION

Series in Materials Science: AA Dordrecht,The

Corresponding Author

Netherlands 2007.

* E-mail: [email protected]

(9) Li, C. H.; Wang, C.; Keplinger, C.; Zuo, J. L.; Jin, L.;

Notes

Sun, Y.; Zheng, P.; Cao, Y.; Lissel, F.; Linder, C.; You,

The authors declare no competing financial interest.

X. Z.; Bao, Z. A Highly Stretchable Autonomous SelfHealing Elastomer. Nat. Chem. 2016, 8, 618-624.

ACKNOWLEDGMENT

(10) Kang, J.; Son, D.; Wang, G. N.; Liu, Y.; Lopez, J.;

This work was supported by the National Natural Science

Kim, Y.; Oh, J. Y.; Katsumata, T.; Mun, J.; Lee, Y.; Jin,

Foundation of China (31722011) and the Nation Key Research

L.; Tok, J. B.; Bao, Z. Tough and Water-Insensitive Self-

and Development Program of China (2017YFD0601205).

Healing Elastomer for Robust Electronic Skin. Adv.

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Facile Fabrication of Self-Healable and Antibacterial Soy Protein

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