Optical properties of biocompatible polyaniline nano-composites

Share Embed


Descrição do Produto

Journal of Non-Crystalline Solids 352 (2006) 3835–3840 www.elsevier.com/locate/jnoncrysol

Optical properties of biocompatible polyaniline nano-composites C. Dispenza a

a,b,*

, M. Leone c, C.Lo. Presti a, F. Librizzi c, G. Spadaro

a,b

, V. Vetri

q c

Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universita` degli Studi di Palermo, Viale delle Scienze, 90128 Palermo, Italy Centro Interdipartimentale di Ricerca sui Materiali Compositi, Universita` degli Studi di Palermo, Viale delle Scienze, 90128 Palermo, Italy c Dipartimento di Scienze Fisiche ed Astronomiche, Istituto Nazionale per la Fisica della Materia, Via Archirafi, 90100 Palermo, Italy

b

Received 10 October 2005; received in revised form 31 May 2006 Available online 1 September 2006

Abstract Polyaniline (PANI) is an electro-active polymer of great interest thanks to its outstanding physical and chemical properties which make it suitable for various applications in optics, bioelectronics, biosensors, diagnostics and therapeutic devices. Unfortunately, PANI is infusible and insoluble in most common solvents and, thus, very difficult to process. In the attempt of improving processability, yet preserving its interesting properties, PANI has been synthesized in the form of particles and dispersed into a hydrogel matrix. The synthesis of PANI–hydrogel composites proceeds via c-irradiation of PANI dispersions as obtained by ‘in situ’ polymerization of aniline in the presence of water-soluble, polymeric stabilizers. The chosen stabilizers are able to undergo to chemical cross-linking when exposed to ionizing radiations, so forming the highly hydrophilic network that entrap PANI particles. The presence of a hydrogel matrix induces biocompatibility to the final composite material which, in a typical bottom-up approach, may become suitable for the development of biocompatible, optoelectronic devices. Some morphological and optical features of these novel soft, functional nano-composites are here presented. Ó 2006 Elsevier B.V. All rights reserved. PACS: 82.35.Np; 82.35.Cd; 78.67. n; 68.37.Ps; 68.37.Hk; 78.40. q; 78.55. m Keywords: Biomaterials; Optical spectroscopy; Atomic force and scanning tunneling microscopy; Scanning electron microscopy; Nanoparticles, colloids and quantum structures; Nano-composites; Nanoparticles; Optical properties; Absorption

1. Introduction Organic conjugated polymers have shown great promise in the field of optoelectronics, demonstrating applications over a wide range of technologies including light detection, electrochromic displays, light emitting diodes, optical switching and optical computing. Specifically, the ability for many polymeric materials to undergo coupled alterations in optoelectronic properties, due to chemical and elecq This paper was originally presented at the First Conference on Advances in Optical Materials, Tucson, AZ, USA, October 12–15, 2005. * Corresponding author. Address: Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universita` degli Studi di Palermo, Viale delle Scienze, 90128 Palermo, Italy. E-mail address: [email protected] (C. Dispenza).

0022-3093/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.06.017

trical perturbations of their redox state lend to them unique advantages within the field of functional materials [1]. Polyaniline, among the other organic conjugated polymers, owns its popularity to the high conductivity in the doped state (up to 1 S/cm), its excellent environmental and thermal stability as well as the electrochemical stability [2]. The transition between the most reduced form (leucoemeraldine), which is insulating, to the half-oxidized, semi-conductive form (emeraldine base), or to the fully oxidized insulating form (pernigraniline base), causes also a strong color change (in transmission) between colorless to blue or to violet, respectively. The emeraldine base form can be easily doped by means of protic acid treatments to obtain protonated emeraldine, which is green in color and electrically conductive. However, the intractability of PANI has limited its application, especially in its pure,

3836

C. Dispenza et al. / Journal of Non-Crystalline Solids 352 (2006) 3835–3840

inherently conductive form. Processing difficulties can seriously restrict devices manufacture and function. Apart from electrochromics and optoelectronic device components, also chemical and biological sensors may benefit from the interesting properties of PANI. The function of a sensor is to provide information on the physical, chemical or biological environment through an electrical response, which is function of the environmental property to be measured. Biosensors, in particular, contain biological entities such as enzymes, antibodies, bacteria, etc. as recognition agents, and in some cases they can couple diagnostic functions with therapeutic technologies (such as in ‘smart’ drug delivery systems) [3,4]. An integration of the sensoring function with an optical effect, intrinsically generated by the material, can be achieved by PANI-based materials. But, especially for biomedical application, the quality of the sensor–human interface has to be carefully considered, being biocompatibility, tolerance toward biofouling, chemical and physical stability obvious requirements. These results can be achieved following two different manufacturing approaches: either by coating the sensing device with a suitable biocompatible material or by applying a typical composite material concept, i.e. by producing a composite material where the sensing material is dispersed in a biocompatible support matrix. The great advantage of the latter approach is the possibility of device miniaturization. The main objective of the present work is to incorporate PANI nanoparticles into a soft, wet and biocompatible matrix, such as a polymeric hydrogel, yet preserving all the important optical features of PANI. The choice of hydrogels as support material also resides on the high permeability of these media by gases and liquids and on the optical transparency to visible light. The inherent hydrogel flexibility should also allow the formation of supramolecular structures in the dry state due to self-assembly of PANI particles [5]. The conductive hydrogel matrix can also work as a template for subsequent electrochemical reactions (such as electrochemical polymerization of a second electrochromic polymer). Synthesis of PANI–hydrogel composites proceeded via ‘in situ’ polymerization of aniline in the presence of steric stabilizers, namely poly-vinyl-pyrrolidone (PVP) or polyvinyl-alcohol (PVA), followed by c-irradiation of the so obtained PANI dispersions, in order to induce chemical cross-linking to either PVP or PVA. The preparation of hydrogels by radiation cross-linking is a very simple and, therefore, very attractive process and it has been subject of intensive research over the years. Hydrogels can be obtained by radiation techniques in several ways, including irradiation of pure polymers, monomers or solution of polymers and/or monomers in bulk, solution or emulsion. Advantages and disadvantages of each approach, as well as the mechanism of reactions leading to gel formation are discussed in the literature [6,7]. Poly-vinyl-pyrrolidone and poly-vinyl-alcohol are among the polymers used for the preparation of hydrogels via high energy irradiation,

especially for wound dressing and tissue engineering applications [8,9]. For the purpose of the present investigation, irradiation conditions, such as irradiation dose and polymer concentration in water, have been selected in order to obtain macroscopic gelification of the whole sample. The absence of residual aniline, in both dispersions with PVP and with PVA, has been assessed by HPLC and GC and the water content of both composite hydrogels has been measured gravimetrically (approx 96% wt for both). Results of the electrical characterization of both dispersions and hydrogel composites, via cyclic voltammetry and impedance spectroscopy, as well as the characterization of the swelling behavior of the pure PVP and PANI/ PVP hydrogels, are elsewhere reported [10]. Pure PVA and PANI/PVA hydrogels behave similarly as far as swelling behavior and electrical properties are concerned (unpublished results), whereas PANI/PVA nano-composites show very distinctive features with respect to the PANI/PVP analogs in terms of morphology and optical properties. In particular, here we discuss some experimental evidences from scanning electron microscopy (SEM) and atomic force microscopy (AFM) analysis, carried out on both films obtained from the dispersions and hydrogels upon freeze drying, and preliminary results of the optical characterization of these novel soft composites. 2. Experimental PANI aqueous dispersions have been obtained by chemical oxidation of an acid solution of aniline using ammonium persulfate as redox initiator and either PVP (Mw = 160 000) or PVA (Mw = 47 000, degree of hydrolysis 88) as steric stabilizer. Details of the synthetic procedure are reported elesewhere [10]. During synthesis the conversion of aniline with reaction time was monitored by both high performance liquid chromatography (HPLC) and gas chromatography (GC) and both techniques confirm that aniline is no longer present after the first 15 min of reaction time for the synthesis carried out in the presence of PVP, and after 30 min when in the presence of PVA. Before irradiation, dispersions were diluted 10-fold with the steric stabilizer/water solution (at 4% wt), already used to prevent macroscopic precipitation of PANI during polymerization, so that stabilizer weight concentration was kept constant. Hydrogel composites were obtained by 60Co c-irradiation of the diluted dispersions of synthesized PANI. Irradiation was performed in glass vials under nitrogen at a dose rate of 2 kGy/h and a total absorbed dose of 40 kGy. During irradiation, temperature was maintained at 10 °C. The original pH of the diluted dispersions and hydrogels was equal to 1.6 ± 0.1; pH of dispersions was brought to 4 ± 0.1, 7 ± 0.1 and 9 ± 0.1 by adding aqueous 1 M NaOH, while pH of hydrogels was brought to the above reported three values by equilibrating them in three different phosphate buffers until constant weight of the swollen hydrogel was attained.

C. Dispenza et al. / Journal of Non-Crystalline Solids 352 (2006) 3835–3840

3837

Morphology of PANI particles was assessed by means of a Philips environmental scanning electron microscope (ESEM, XL-30) on cast films obtained from highly diluted dispersions, purposely made by adding water up to 500 times the initial volume in order to reduce the polymeric stabilizer concentration and minimize particle aggregation in the solid state. In order to enhance contrast, films were gold sputtered prior to SEM analysis. Magnifications that best showed the particulate morphologies were selected. AFM was carried out by means of a nanoscope multimode TM SPM apparatus on spin-coated films (at 4000 rpm for 60 s) obtained from the 10-fold diluted dispersions (with the polymeric aqueous solution) also used for producing composites. Absorption spectra were carried out by using a Jasco V570 Spectrophotometer (scan speed 40 nm/min integration time 2 s, bandwidth 1 nm) at room temperature. Volumes of 0.1 ml of the original dispersions were brought to the volume of 100 ml by adding the corresponding stabilizer/ water solution (4% wt) and samples displaced in 1 cm path-

length couvette. Hydrogels were smeared on a 0.2 mm demountable quartz couvette. Fluorescence measurements were carried out with a Jasco FP-6500 spectrofluorimeter, equipped with a Xenon lamp (150 W), on the same dispersions used for UV–vis absorption measurements after being 10-fold diluted with

Fig. 1. Electron microscopy of the air-dried deposits obtained from highly diluted PANI dispersions with PVP (a) and with PVA (b).

Fig. 2. AFM phase angle images of thin films obtained from PANI dispersions with PVP (a) and with PVA (b).

3838

C. Dispenza et al. / Journal of Non-Crystalline Solids 352 (2006) 3835–3840

the stabilizer/water solution. For the diluted dispersions at each pH, the emission spectra, at the required excitation, were obtained with emission and excitation bandwidth of 3 nm, scan-speed of 100 nm/min and integration time of 1 s, and recorded at 0.5 nm intervals. The experimental errors were about 2%. 3. Results and discussion In the attempt of resolving the morphology of the primary PANI particles, PANI dispersions were highly

diluted with pure water, deposited onto metallic supports and air dried. At high dilution levels particles agglomeration, as a result of the drying process, should be minimized and primary PANI particles with their stabilizer-rich shell resolved. Fig. 1 shows SEM images of polyaniline particles as obtained in the presence of either PVP (Fig. 1(a)) or PVA (Fig. 1(b)). Fig. 1(a) presents spherical particles with an average diameter of 30 nm on the background and bigger agglomerates on the foreground, whereas Fig. 1(b) shows globular particles of approx 100 nm average diameter. In both cases, it is very likely that the PANI particles

PANI-PVA disp pH4 PANI-PVA disp pH7 PANI-PVA disp pH9

ABS (arb.un.)

PANI-PVP disp pH4 PANI-PVP disp pH7 PANI-PVP disp pH9

320

440

560

680

800

320

440

560

680

800

PANI-PVA gel pH4 PANI-PVA gel pH7 PANI-PVA gel pH9

ABS (arb.un.)

PANI-PVP gel pH4 PANI-PVP gel pH7 PANI-PVP gel pH9

320

440

560

680

800

320

440

560

680

PVP

800

ABS (arb.un.)

PVA

320

440

560

680

wavelength (nm)

800

320

440

560

680

800

wavelength (nm)

Fig. 3. Optical absorption spectra of PANI/PVP dispersions (a) and hydrogels (b) at pH 4, 7 and 9; PVP/water solution (c); PANI/PVA dispersions (d) and hydrogels (e) both at pH 4, 7 and 9; PVA/water solution (f). Spectra of dispersions and hydrogels were subtracted of the optical spectra of the ‘matrix materials’, i.e. PVP/water and PVA/water solutions.

C. Dispenza et al. / Journal of Non-Crystalline Solids 352 (2006) 3835–3840

Fluorescence (arb. un.)

resolved also include a stabilizer-rich shell, as significant portions of the polymer may remain anchored to the particle surface, and this can occur either due to strong interfacial interaction with PANI or to difficult dissolution into water, as it may be the case of PVA. Morphological analysis was also carried out on thin films (approx 30 nm) deposited onto glass substrates using an atomic force microscope. In this case, the dispersions maintained always the same PANI/stabilizer concentration. Phase angle images [11] are depicted in Fig. 2. Fig. 2(a) depicts the PANI/PVP dispersion and evidences a tendency of PANI particles to organize into needle-like structures, as it has been already observed by SEM analysis of freeze dried hydrogels [10]. From the corresponding topography image, here not shown, PANI needles stand out for a maximum height of 100 nm. Dispersions with PVA present a significantly different morphology (see Fig. 2(b)). The phase angle image, at the same magnifications as for PVP, reveals a large circle pattern, which from closer, appears to be formed by very small particles arranged in rings around zones covered by the same matrix material. From the corresponding topography image, here not shown, in this case PANI particles stand out for a maximum height of 30 nm. The observed differences in the morphology of these thin films, obtained in the same experimental conditions, are very likely to reflect differences in the nature and strength of PANI/stabilizer interactions in the original dispersion. SEM analysis of freeze dried PVA hydrogels present a globular morphology (results here not reported). UV–vis absorption spectra were collected for both PANI/PVA and PANI/PVP dispersions and for the corresponding hydrogels, in correspondence of three different pH values: 4, 7 and 9 and they are reported in Fig. 3 after subtraction of the optical contribution of the corresponding solvents (PVP/water or PVA/water at 4% wt), whose spectra are also reported. Both PVP and PVA aqueous solutions show no significant absorption bands in the 400–980 nm region, as clearly revealed by Fig. 3(c) and (f), respectively. Both dispersions and hydrogels present the distinctive features of the UV–vis spectra of aqueous PANI contain-

pH=4

3839

ing systems: absorption peaks in the (i) 300–340 nm region and in the (ii) 550–800 nm region [12–14]. The doped PANI in the dispersion and hydrogels at pH 4, beside the characteristic absorption bands at 320–340 nm wavelengths in region (i), and at 740–800 nm wavelengths in region (ii), which are due to the p–p* transition of benzoid rings and the formation of polaron, respectively, also shows an additional band at 400–420 nm due to the formation of a doping level owing to ‘exciton’ transition, caused by interband charge transfer from the benzenoid to the quinoid moieties of the protonated PANI (polaron/bipolaron transition) [13]. Deprotonation of PANI by changing pH to 7 and to 9 produces a gradual blue shift of the absorption bands, and especially in the region (ii) from 800 nm as far as the 550 nm, for the excitation of nitrogen of the quinoid segments present in the violet pernigraniline base form (pH = 9). The ‘exciton’ transition band is completely absent in both the dedoped forms of PANI. The comparison between PVP-based and PVA-based dispersions and hydrogels present some differences in the precise positioning of the absorption bands and, above all, differences in their relative intensity, that can be ascribed to different interactions of PANI with the specific polymeric aqueous environment at different pHs, i.e. the aqueous solution of either PVP or PVA. Note that in the blue range of the spectra a large effect of turbidity is present for the dispersions, whereas for hydrogels the bands at about 320 nm are well resolved, probably due to the restrained mobility of the polymer chains. While the absorption spectra of the different systems characterized presented some common features, the fluorescence behavior of the systems that contain PVP drastically differentiates from that of the systems containing PVA. Both PANI–PVP dispersions and hydrogels did not present any emission band in all the pH conditions observed, whereas PANI–PVA dispersions and hydrogels did. Emission spectra of PANI–PVA dispersions at pH = 4 and pH = 7 under excitation at kexc = 290 nm are presented in Fig. 4. The spectra were corrected for spectral response of the detection system and normalized with respect to sample absorption at 290 nm. The inset in

pH=7 PANI-PVA PVA

2000

150

1500 100 1000

320 370 420 470 520

50 500

300

320 340 360 wavelength (nm)

380

400

420 440 460 wavelength (nm)

480

Fig. 4. Emission spectra for the PANI/PVA dispersions at pH 4 and 7 under excitation at kexc = 290 nm. At pH 9 the system did not show emission signals. Spectra were normalized for absorbance values at the corresponding excitation wavelength. In the inset the comparison between emission spectra of PVA/water and PANI/PVA dispersion at pH = 4.

3840

C. Dispenza et al. / Journal of Non-Crystalline Solids 352 (2006) 3835–3840

Fig. 4 shows the comparison of the emission spectra of PVA/water to the PANI–PVA aqueous dispersion at pH = 4 in order to evidence the contribution of PANI nanoparticles. At the same excitation wavelength different emission bands at different pHs were observed; in particular, an intense band in the UV range is visible at pH = 4 and a less intense and structured band in the visible range at pH = 7. No emission band was observed at pH 9. These results indicate different radiative decay mechanisms, probably ascribable to the different conformations of the chromophore in the excited state in dependence on pH. The absence of fluorescence signals for PANI in the presence of PVP can be related to the intrinsic properties of the primary PANI particles, depending on how they have grown in the different water/stabilizer environments (with different particle size, crystallinity, doping level) [15], or to specific interaction between the PANI particles and their environment (such as synchronized establishment of hydrogen bonding segments of PANI and the stabilizer chains) [16]. More systematic investigations are undergoing in order to elucidate the different optical response of these systems. 4. Conclusions Polyaniline is an electro-active polymer, which exists in different, reversible forms having different electrical and optical properties. This feature makes it a very promising material for optoelectronic applications. Unfortunately this polymer is very difficult to process and its possible applications are actually limited by this problem. A new composite material, with a hydrogel as matrix and PANI particles as filler, was prepared by water dispersion polymerization of aniline followed by c-irradiation, which causes cross-linking of either PVP or PVA. PVP and PVA are water-soluble polymers, generally used during the synthesis to provide both nucleation centers for the growth of PANI chains and steric protection to the PANI nanoparticles against further aggregation. From the morphological investigation it appears that PANI primary particles were always spherical, with an average diameter of 30 nm. For samples obtained in the same experimental

conditions, depending on the nature of the polymeric stabilizer, it drastically changes the way how the nanostructured composite material develop a secondary structure, which can be either fibrillar or globular. UV–vis absorption measurements confirmed the ability of these soft composites and relative dispersions to undergo two, pH ‘switched’, optical transitions and, even more interestingly, fluorescence signals that vary in wavelength and intensity with pH. It is worth noticing that fluorescence was exhibited by only PANI/PVA-based systems, thus stressing the crucial role that the ‘environment’ plays on PANI properties. Acknowledgement The financial support of the University di Palermo (60% funds) is gratefully acknowledged. References [1] D. Kumar, R.C. Sharma, Eur. Polym. J. 34 (8) (1998) 1053. [2] C. Chiang, A.Q.G. MacDiarmid, Synth. Met. 13 (1986) 193. [3] S. Brahim, D. Narinesingh, A. Guiseppi-Elie, Biosens. Bioelectron. 12 (2002) 973. [4] B. Adhikari, S. Majumdar, Prog. Polym. Sci. 29 (2004) 699. [5] Z. Zhang, M. Wan, Synth. Met. 128 (2002) 83. [6] A. Chapiro, Radiat. Phys. Chem. 46 (2) (1995) 159. [7] J.M. Rosiak, F. Yoshii, Nucl. Instrum. Methods Phys. Res., Sect. B 151 (1999) 56. [8] J. Rosiak, P. Ulanski, Radiat. Phys. Chem. 55 (1999) 139. [9] L.F. Miranda, A.B. Lugao, L.D.B. Machado, L.V. Ramanathan, Radiat. Phys. Chem. 55 (1999) 709. [10] C. Dispenza, C. Lo Presti, C. Belfiore, G. Spadaro, S. Piazza, Polymer 47 (2006) 961. [11] D. Raghavan, X. Gu, T. Nguyen, M. VanLandingham, A. Karim, Macromolecules 33 (2000) 2573. [12] M.S. Cho, S.Y. Park, J.Y. Hwang, H.J. Choi, Mater. Sci. Eng. C 24 (2004) 15. [13] P. Ghosh, S.K. Siddhanta, A. Chakrabarti, Eur. Polym. J. 35 (1999) 699. [14] S.S. Umare, A.D. Borkar, M.C. Gupta, Bull. Mater. Sci. 25 (3) (2002) 235. [15] J. Stejskal, I. Sapurina, J. Colloid Interface Sci. 274 (2004) 489. [16] R. Murugesan, G. Anitha, E. Subramanian, Mater. Chem. Phys. 85 (2004) 184.

Lihat lebih banyak...

Comentários

Copyright © 2017 DADOSPDF Inc.