Crystal Structure and Property of Proton Acid and Polypeptide Co

Nov 28, 2016 - An approach was presented in this article for organizing the polyaniline (PANI) nanoparticles using polypeptide as the template. PANIs ...
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Crystal Structure and Property of Proton Acid and Polypeptide Codoped Polyaniline Qiaozhen Yu* Department of Materials and Textile Engineering, Zhejiang Experimental Center of Materials and Textile Engineering, Jiaxing University, Jiaxing, Zhejiang 314001, P.R. China ABSTRACT: An approach was presented in this article for organizing the polyaniline (PANI) nanoparticles using polypeptide as the template. PANIs were chemically synthesized in different weight ratios of poly( D -Glu- D -Lys) (PDGDL)/aniline (Ani) (mg/g) in the feed, doped acid, and polymerization time. Scanning electron microscopy and X-ray diffraction analysis showed that the PDGDL dictated the organization of the PANI nanoparticles. The formation and shape of PANI crystal structures were strongly affected by the parameters including weight ratio of PDGDL/Ani, doped acid, and polymerization time. Fourier transform infrared and UV− vis near-infrared spectrometry confirmed the strong interaction between PDGDLs and PANIs. Semiconductor parameter analyzer test showed that the conductivity of proton acid and polypeptide co-doped PANIs increased with the increase of the weight ratio of PDGDL/Ani and increased with the polymerization time extended from 12 to 60 h, but decreased from 60 to 72 h. Solubility test indicated that the solubility of these PANIs was also strongly affected by the parameters as mentioned above. My investigation suggested that there was an optimum weight ratio and polymerization time to obtain certain regular structure and properties of the proton acid and PDGDL co-doped PANIs. The utilization of biomolecules such as DNA15 and proteinbased materials16,17 to dictate the organization of nanoparticles has been shown to be an effective and robust paradigm. Peptides have molecular recognition function.18 This smart recognition function can also address the biological nanomaterials to exact locations on substrate where their complementary recognition groups are marked.19−21 Organization of nanoparticles using polypeptide templates represents a robust and flexible bottom-up nanofabrication method. However, it has been reported4 that the doping of protonic acid is a prerequisite process for transforming PANI emeraldine into the form of conductive emeraldine salt by the formation of radical ions at PANI backbone. PANI is not electroactive at pH > 4. Wang et al.22 found that the pH value of 4.0 was the most suitable for obtaining both a relative high conductivity and a high doping ratio of co-polypeptide. In this study, polypeptide was used to dictate the organization of PANI nanoparticles, and the protonic acids were mainly used to adjust the pH value of the reactant to obtain the PANIs with a relative high conductivity, a high doping ratio of polypeptide, and a high solubility in common solvents. The relationship between the structure and property of proton acid and polypeptide co-doped PANIs was

1. INTRODUCTION Electroactive materials have been developed for actuators, organic sensors, and artificial muscles. Electroactive biomaterials could also be advantageous since many types of cells, including neurons and muscle cells, respond to electrical stimulations.1 Electrical stimulation increases neurite and axon extension in vitro and nerve regeneration in vivo. PANI is a biocompatible organic conducting polymer that has been used in the stimulation of neural growth and regeneration both in vitro and in vivo.2,3 It has been shown1,4 that PANI has the ability to control electrical signals and influence cell behavior. PANI has the potential in biomedical applications, such as implantable electrochemical biosensors, electro-stimulated drug release devices, and neural prosthetics.5 However, the inherently poor solubility in common solvents, which originates from the strong inter and intrachain interactions, has limited PANI’s practical applications in many areas.6 The properties of nanoscaled materials are influenced by their sizes and shapes.7 For synthesis of nanostructured PANI, many approaches, including template and nontemplate, have been used and developed.8−10 Self-assembling systems offer convenient and powerful bottom-up strategies for the creation of nanostructures that can be deployed in the realization of functional nanoscale devices.11,12 For example, Jayakannan et al.13,14 reported a unique soft templating approach based on an in-built amphiphilic surfactant for tuning various types of polyaniline nanomaterials such as fibers, rods, spheres, and tubes. © XXXX American Chemical Society

Received: August 7, 2016 Revised: November 14, 2016

A

DOI: 10.1021/acs.jpcc.6b07969 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

1. From Figure 1a−c, it could be seen that the morphology of lysine-doped PANI was strongly affected by the weight ratio of

researched systematically. The related mechanisms were also discussed.

2. EXPERIMENTAL SECTION 2.1. Materials. Aniline (Ani) monomer was distilled under reduced pressure prior to use. Poly(D-Glu-D-Lys) (PDGDL) (Glu/Lys = 6:4 mol/mol, Mw = 20,000−50,000) and hexafluoroisopropanol (HFIP, 99.5% purity) were purchased from Beijing Hua Wei Rui Ke Chemical Industry Co., China. Ammonium persulfate (APS) was supplied by Shanghai Qiangsheng Chemical Industry Co., China. Proton acid (lysine, HCl, and H2SO4) and all the other reagents were obtained from Shanghai Guoyao Chemical Industry Co., China and were used without further purification. 2.2. Synthesis of Proton Acid and Polypeptide Codoped PANI Nanostructures. Proton acid and polypeptide co-doped PANI was prepared by the following procedures. According to the weight ratio of PDGDL/Ani (mg/g), Ani monomer and PDGDL were dissolved in deionized water and cooled down to 0 °C in an ice bath. Then the reactant was purged with nitrogen for 30 min. APS aqueous solution (0 °C) was slowly added into Ani aqueous solution under vigorous magnetic stirring. After the APS aqueous solution was dropped up, the lysine, HCl, or H2SO4 (1.0 mol/L) was used to adjust the pH value to about 4.0. Then the polymerization proceeded for at least 12 h with continuously stirring, and the reaction temperature was maintained at about 5 °C in a circulation bath. The filtered precipitates were sequentially washed with distilled water and acetone in order to remove all residues until the filtrate became colorless, and then dried in a vacuum oven at room temperature for 24 h. 2.3. Characterization. The morphologies of proton acid and PDGDL co-doped PANI particles were observed using S570 scanning electron microscopy (SEM). X-ray diffraction (XRD) patterns were performed on a Rigaku D/max-2600 model instrument operating at 40 kV and 120 mA, using Cu Kα radiation (λ = 1.5406 nm). UV−vis near-infrared (NIR) absorption spectra of proton acid and PDGDL co-doped PANI solutions were obtained with Cary 5000 spectrophotometer. Fourier transform infrared (FT−IR) spectroscopy was done with a 470 FT−IR at transmission mode in the resolution of 4 cm−1. The conductivities of samples were determined by fourprobe method (ST512-SZT-2A). The doped PANI pellets were compressed from its powders at 300 MPa with a manual hydraulic press, using a current source SMU Keithley 237 and a Multimeter Keithley 2010 voltmeter with a 2000 SCAN 10channel scanner card. The properties of the viscosity of proton acid and PDGDL co-doped PANI solutions were investigated using rotation viscometer (NDJ-5S). A differential scanning calorimeter (DSC, PerkinElmer DSC7, USA) was used to determine the glass transition temperature (Tg) and the melting temperature (Tm) from endothermic peaks. Each sample (5−10 mg) was hermetically sealed in an aluminum crucible, and then the DSC cell was kept in an atmosphere equilibrated with liquid nitrogen. Each heating and cooling cycle was performed at a 10 °C/min scan rate covering the range from 30 to 300 °C.

Figure 1. SEM images of lysine and PDGDL co-doped PANIs synthesized with different weight ratios of PDGDL/Ani (mg/g): (a) 3:1 (scale bar = 250 nm); (b) 6:1 (scale bar = 333 nm); (c) 8:1 (scale bar = 333 nm); (d) 10:1 (scale bar = 333 nm).

Figure 2. SEM images of lysine and PDGDL co-doped PANI with different weight ratios of PDGDL/Ani in high magnification: (a) 3:1 (scale bar = 67 nm); (b) 8:1 (scale bar = 200 nm); (c) 10:1 (scale bar = 133 nm).

PDGDL/Ani, the PANIs piled closely (Figures 1a,b and 2a), and no regular structure was found when the weight ratio of PDGDL/Ani was 3:1 or 6:1. As the weight ratio of PDGDL/ Ani was 8:1, many nanobelts (rectangular crystals) with a dimension of about 2160 × 160 × 70 nm appeared (Figure 1c). However, to our surprise, a large number of nanotubes with a diameter of 30 nm were observed as the weight ratio of PDGDL/Ani increased to 10:1 (Figure 1d). Moreover, as the weight ratio was 8:1, in high magnification (Figure 2b), some rectangular nanoblocks were also found in the dimension of about 400 × 200 × 100 nm. While the weight ratio was 10:1 (Figure 2c), it appeared that a kind of multiporous structure piled with square crystals in the dimension of about 100 × 100 × 100 nm. These indicate that the interaction between lysine or PDGDL and PANI increased with the increase of the weight

3. RESULTS AND DISCUSSION 3.1. Morphology and Structure Characterization. As the polymerization time was 12 h and the concentration of aniline was 0.6%, the SEM images of lysine and polypeptide codoped PANIs with different weight ratios were shown in Figure B

DOI: 10.1021/acs.jpcc.6b07969 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C ratio of PDGDL/Ani. Lysine or PDGDL may dictate the organization of the PANI nanoparticles. As the polymerization time was 12 h and the concentration of Ani was 0.6%, the shape and size of HCl and PDGDL codoped PANI also vary with weight ratio of PDGDL/Ani. When the weight ratio was 8:1, a multiporous net structure composed of PANI nanorods was obtained. The nanorods had a diameter of about 25 nm and a length of about 100 nm, but no lamellar crystal was found, as shown in Figure 3a. While the weight ratio

Figure 4. SEM images of lysine and PDGDL co-doped PANIs with different concentrations of Ani: (a) 0.3% (scale bar = 150 nm); (b) 0.6% (scale bar = 500 nm).

In addition, when the weight ratio of PDGDL/Ani was 8:1 and concentration of Ani was 0.3%, the morphology of H2SO4 and PDGDL co-doped PANI crystal varied with the polymerization time, as shown in Figure 5. A multiporous net structure

Figure 3. SEM images of proton acid and PDGDL co-doped PANI synthesized with different type of proton acid and weight ratios of PDGDL/Ani: (a) HCl, 8:1 (scale bar = 111 nm); (b) HCl, 10:1 (scale bar = 167 nm); (c) HCl, 16:1 (scale bar = 133 nm); (d) H2SO4, 8:1 (scale bar = 200 nm) (the concentration of Ani was 0.6%, and polymerization time was 12 h).

increased to about 10:1, a lot of lamellar crystals appeared in the dimension about 1000 × 480 × 40 nm, although the size of these lamellar crystals was little smaller than those of lysine and PDGDL co-doped PANI (Figure 3b), but no lamellar crystal was found as the weight ratio increased to 16:1 (Figure 3c), further indicating the formation of lamellar crystal was affected by the weight ratio of PDGDL/Ani. There was an optimal weight ratio to obtain the lamellar crystal with certain regular shape. It may be due to the following two aspects. On the one hand, lysine or PDGDL may dictate the organization of PANI nanoparticles. On the other hand, the doping of lysine or PDGDL, increased the interaction between molecular chains due to the formation of a hydrogen bond between the carboxy group and the amino group,23 resulting in the twist of PANI molecular chain and the decrease of ordering degree of PANI molecular chains, as confirmed by XRD analysis. The morphologies of both lysine, PDGDL co-doped PANI and HCl, PDGDL co-doped PANI were different from that of HCl-doped PANI, which was floc with a lot of nanofibers about 10 nm in diameter,24 indicating all of lysine and PDGDL acted as the template in the formation of lamellar crystal of PANI, and the lysine or PDGDL dictated the organization of the PANI nanoparticles. Meanwhile, we also found the morphology of PANI lamellar crystal was affected by the concentration of aniline. As the concentration of Ani was about 0.3%, the obtained crystals were mainly lamellar crystals with some nanorods (Figure 4a); while the concentration of Ani was about 0.6%, the obtained crystals were mainly rectangular crystals (Figure 4b).

Figure 5. SEM images of H2SO4 and PDGDL co-doped PANI with different polymerization times: (a) 12 h (scale bar = 200 nm); (b) 36 h (scale bar = 1 μm); (c) 48 h (scale bar = 1.7 μm); (d) 48 h (scale bar = 250 nm); (e) 48 h (scale bar = 400 nm); (f) 72 h (scale bar = 1.3 μm) (the weight ratio of PDGDL/Ani was 8:1, and concentration of aniline was 0.3%).

composed of PANI nanorods was obtained, and no obvious crystals were observed when the polymerization time was 12 h (Figure 5a). As the polymerization time extended to 36 h, many petal-like crystals of flos rosae sinensis were observed (Figure 5b). Further extended the polymerization time to 48 h, we found many rectangular single crystals with the dimension about 10.5 μm × 400 nm × 40 nm (Figure 5c), lamellar polycrystals (Figure 5d), and snowflake-like crystals (Figure 5e). While the polymerization time was 72 h, long belts with the dimension about 25 μm × 75 nm × 40 nm were formed except the snowflake-like crystals and lamellar polycrystals (Figure 5f). The long belt crossed over several amorphous regions and crystal regions, indicating the polymerization time is also an important fact on the formation of crystals with certain regular shape. C

DOI: 10.1021/acs.jpcc.6b07969 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 6. FT−IR spectra of (A) lysine and PDGDL and (B) HCl and PDGDL co-doped PANI synthesized with different weight ratios of PDGDL/ Ani: (a) 3:1; (b) 6:1; (c) 8:1; (d) 10:1; (e) 16:1.

Figure 7. UV−vis−NIR absorption spectra of (A) lysine and PDGDL and (B) HCl and PDGDL co-doped PANIs synthesized with different weight ratio of PDGDL/Ani: (a) 3:1; (b) 6:1; (c) 8:1; (d) 10:1.

and PDGDL co-operated PANI, the main characteristic bands are nearly similar to the characteristic bands of HCl-doped PANI, indicating the strong interaction between PDGDL and PANI was weakened by lysine. It may result from the hydrogen bond interaction between the lysine and PDGDL. These results confirmed that the lysine and PDGDL are indeed doped in PANIs. From Figure 6A,B it could be found that the variation of weight ratio of PDGDL/Ani had little effect on the intensity and location of the absorption peaks of PANI. 3.2. Optical Properties of Lysine or HCl and PDGDL Co-doped PANI. UV−vis−NIR absorption spectra of lysine or HCl-doped PDGDL modified PANIs synthesized with different weight ratios of PDGDL/Ani were shown in Figure 7. The absorption peak attributed to the π−π* transition of the benzenoid ring was at around 293 nm, the absorption corresponding to the benzenoid−quinoid excitonic transition was located around 849 nm, and the absorption peaks around 270 and 496 nm were due to the n−π* transition and polaron−π transition for pure PANI, respectively. As shown in Figure 7A,B, the absorption peak around 270 nm due to the n−π* transition shifted to higher wavelengths and the peak around 496 nm due to polaron−π transition shifted to lower wavelengths in the lysine and PDGDL co-doped PANI relative to pure PANI, suggesting that the lysine and PDGDL had strong impact on the optical property of PANI. The polaron band around 800−2526 nm in the UV−vis−NIR spectrum, characteristic of the emeraldine salt form of PANI, is responsible for the high conductivity in PANI.27,28 However, we found that the absorption peak around 800−2526 nm only

When the weight ratio of PDGDL/Ani was 8:1, concentration of Ani was 0.6% and polymerization time was 12 h, the morphology of lysine and PDGDL co-doped PANIs was different from that of HCl or H2SO4 and PDGDL co-doped PANIs. The shape of lysine and PDGDL co-doped PANIs was nanobelt (Figure 1c) or rectangular nanoblock (Figure 2b), while the shape of HCl or H2SO4 and PDGDL co-doped PANI was nanorods, and no lamellar crystal was found (Figure 3a,d). The results show that the lysine acted with the same function as PDGDL in organizing the PANI particles. FT−IR spectra of lysine or HCl and PDGDL co-doped PANIs obtained with different weight ratio of PDGDL/Ani are shown in Figure 6. The main characteristic bands of HCl-doped PANIs were assigned as follows:25 the band at 3440 cm−1 was attributed to N−H stretching vibration, CN and CC stretching vibration for the quinoid and benzenoid rings occurred at 1560 and 1481 cm−1, and the bands at about 1299 and 1240 cm−1 were attributed to C−N stretching mode for benzenoid ring, while the peak at 1127 cm−1 was assigned to a plane bending vibration of C−H (mode of NQN, QN+ H−B, and B−N+ H−B), which was formed during protonation.26 While in the curves of HCl and PDGDL co-doped PANI: the band attributed to N−H stretching vibration shifted to lower wavenumber, CN and CC stretching vibration for the quinoid and benzenoid rings, the peak assigned to a plane bending vibration of C−H (mode of NQN, QN+ H−B, and B−N+ H−B), all shifted to higher wavenumbers (except to the curve c), indicating the strong interaction between PDGDL and PANI. However, in the curve of lysine D

DOI: 10.1021/acs.jpcc.6b07969 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

weight ratio increased from 6:1 to 8:1, the intensities of the peaks around 2θ = 20.6° and 25.5° increased in HCl and PDGDL co-doped PANIs, indicating the increase of crystallinity; while the intensities of these peaks decreased as the weight ratio increased from 8:1 to 16:1, reflecting the decrease of crystallinity and ordered degree of PANI chains. In addition, the intensities of the peaks around 2θ = 25.5° increased in H2SO4 and PDGDL co-doped PANIs with the extension of polymerization time from 12 to 36 h, indicating the increase of crystallinity. It was reported that15,16 the peak centered at about 19.9° may be ascribed to the periodicity parallel to the polymer chain, which was a characteristic peak of amorphous emeraldine base form of PANI; while the peak at about 25.1° might be caused by the periodicity perpendicular to the polymer chain, which was the mark of the highly ordered crystalline structure. Thus, the amorphous and highly ordered crystalline structure coexisted in HCl and PDGDL co-doped PANIs. Moreover, the morphology and crystallinity were the ratio of PDGDL/Ani and polymerization time dependence. The differential scanning calorimetry (DSC) of the HCl and PDGDL co-doped PANIs was shown in Figure 9A. The DSC of HCl and PDGDL co-doped PANI showed two endothermic transitions. The first transition, which occurred at a temperature of about 100 °C, was attributed to the loss of lattice water.30 The second transition that occurred at temperature of about 273 °C represented the glass transition temperature (Tg) of polyaniline.30 Moreover, the transition temperature varied with weight ratio and polymerization time. When the weight ratio was 10:1, Tg and crystallinity increased from 269 to 276 °C with the extension of polymerization time from 12 to 36 h, indicating the increase of molecular weight and crystallinity. Figure 9B showed the relationship between viscosity and weight ratios of PDGDL/Ani of HCl and PDGDL co-doped PANIs. With the increase of the weight ratio, the viscosity of HCl and PDGDL co-doped PANIs increased, indicating the increase of molecular weight. 3.4. Solubility of Proton Acid and PDGDL Co-doped PANI. The solubility of proton acid and PDGDL co-doped PANI in formic acid, HFIP, and their mixture were listed in Table 3. It could be seen that the lysine and PDGDL co-doped PANIs were soluble in HFIP, mostly soluble in the mixture of formic acid/HFIP, and partially soluble in formic acid. Differently, HCl and PDGDL co-doped PANIs were partially soluble in HFIP, formic acid, and the mixture of formic acid/ HFIP. At the same weight ratio of PDGDL/Ani, the solubility of lysine and PDGDL co-doped PANIs was better than that of HCl or H2SO4 and PDGDL co-doped PANIs in these three solvents. Moreover, the solubility of HCl or lysine and PDGDL co-doped PANIs decreased with the increase of weight ratio of PDGDL/Ani. This was related to the crystallinity and molecular weight. As mentioned above, the crystallinity of HCl and PDGDL co-doped PANIs was better than that of lysine and PDGDL co-doped PANIs, and the molecular weight was increased with the increase of weight ratio of PDGDL/Ani.

appeared in the curve d in the UV−vis−NIR absorption spectra of lysine and PDGDL co-doped PANIs, while it appeared in all the curves in the UV−vis−NIR absorption spectra of HCl and PDGDL co-doped PANIs, as shown in the sets of Figure 7. 3.3. Conductivity of Proton Acid and PDGDL Codoped PANIs. As the polymerization time was 12 h, it could be found from Table 1 that the conductivity of the HCl and Table 1. Conductivity of HCl and PDGDL Co-doped PANIs weight ratio conductivity (10−2 × S/cm)

6:1 0.14

8:1 0.27

10:1 1.59

16:1 33.33

PDGDL co-doped PANIs increased from 13.6 × 10−2 to 33.33 × 10−2 S/cm with the increase of weight ratio of PDGDL/Ani from 6:1 to 16:1. It was two times the high electrical conductivity of the PANI nanospheres doped with 100 ppm of HCl, their optimum doping state, reported by Neelgund et al.,29 which was 6 × 10−2 S/cm. The possible reason for this may be that the degree of conjugation of the π electron increased with the increase of weight ratio of PDGDL/Ani, which resulted from the interaction between the PDGDL and PANI chains as analyzed above, and the increase of molecular weight and crystallinity, as confirmed by following DSC analysis. While the conductivity of lysine and PDGDL co-doped PANIs was too low to detect by four-probe method except for the one synthesized with a weight ratio of PDGDL/Ani of 10:1. The conductivity of this sample was 2.1 × 10−4 S/cm. This was in agreement with the analysis in UV−vis−NIR absorption spectra. The reason may be mainly due to the pH value of reactant with lower than 4 and lower crystallinity as shown in XRD analysis. Wang et al.24 reported the pH value of 4.0 was the most suitable for obtaining both a relative high conductivity and a high doping ratio of co-poly(Glu−Lys). Moreover, when the weight ratio of PDGDL/Ani was 8:1, the conductivity of H2SO4 and PDGDL co-doped PANI increased from 3.26 × 10−2 to 4.48 × 10−2 S/cm with the extension of polymerization time from 12 to 60 h, then decreased as the polymerization time further extended to 72 h, indicating the existence of optimal polymerization time to obtain the high conductivity. It may be the combined action of the increased crystallinity and peroxidation reaction. When the weight ratio of PDGDL/Ani was 8:1, concentration of Ani was 0.6% and polymerization time was 12 h, the conductivity of PANIs also varied with the type of proton acid, as listed in Table 2. The conductivity of H2SO4 and PDGDL Table 2. Conductivity of Different Proton Acid and PDGDL Co-doped PANIs proton acid conductivity (10−2 × S/cm)

HCl 0.27

lysine