Layer-by-Layer Assembly of Bifunctional Nanofilms ... - ACS Publications

J. Phys. Chem. C 2009, 113, 5087–5095

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Layer-by-Layer Assembly of Bifunctional Nanofilms: Surface-Functionalized Maghemite Hosted in Polyaniline Leonardo G. Paterno,*,† Maria A. G. Soler,‡ Fernando J. Fonseca,† Joa˜o P. Sinnecker,§ Elis H. C. P. Sinnecker,§ Emilia C. D. Lima,| Miguel A. Novak,§ and Paulo C. Morais‡ Depto de Engenharia de Sistemas Eletroˆnicos, EPUSP, Sa˜o Paulo SP 05508-900, Brazil, UniVersidade de Brası´lia, Instituto de Fı´sica, Brası´lia DF 70910-900, Brazil, UniVersidade Federal do Rio de Janeiro, Instituto de Fı´sica, Rio de Janeiro RJ 21945-970, Brazil, UniVersidade Federal de Goia´s, Instituto de Quı´mica, Goiaˆnia GO 74001-970, Brazil ReceiVed: October 19, 2008; ReVised Manuscript ReceiVed: January 18, 2009

This study reports on the pioneering use of the layer-by-layer (LbL) technique to produce multilayered (1 to 50 bilayers) bifunctional nanocomposite films consisting of negatively charged citrate-coated maghemite nanoparticle (cit-MAG) hosted in positively charged conducting polyaniline (doped-PANI). The aim is to use the LbL assembly to fabricate thin nanocomposite films displaying superparamagnetic and conductivity properties and with fine control of the end properties as a function of the preparation condition. Multilayered cit-MAG/PANI bifunctional nanocomposite films were systematically investigated in order to access information regarding the nanofilm structure, electrical conductivity, and magnetic properties. Using the isothermal adsorption of each individual electrolyte (cit-MAG dispersion and dopedPANI solution) onto solid substrates (silicon and glass) the average time for deposition of a single layer (cit-MAG or doped-PANI) was fixed in 3 min. Independent evaluation using UV-vis spectroscopy and atomic force microscopy indicated a linear correlation between the nominal number of adsorbed citMAG/PANI bilayers and the material content (film thickness), even for the smallest number of adsorbed bilayers. Values of electrical conductivity (film thickness) found for the 10-bilayered cit-MAG/PANI nanocomposite films were in the range of 10-2-10-4 Scm-1 (25-63 nm) for γ-Fe2O3 concentration within the employed magnetic fluid suspension in the range of 10-4-10-3 g L-1. Values of the blocking temperature obtained from ZFC/FC curves recorded for the nanofilm produced using the highest γ-Fe2O3 concentrated suspension (2 × 10-3 g L-1) monotonically increase from 30 to 40 K as the number of cit-MAG/PANI bilayers increases from 5 to 50 bilayers. Therefore, we found that the end properties can be easily and precisely modulated by varying the concentration of the magnetic fluid used for film deposition and/or controlling the nominal number of cit-MAG/PANI bilayers in the nanocomposite. Introduction Nanocomposite materials consisting of conducting polymers hosting magnetic nanoparticles have recently attracted a great deal of attention as they exhibit multifunctionalities, combining for instance electrical conductivity and magnetization in a unique material system.1 They can be used in different applications, such as absorbers to shield electronic circuits from electromagnetic interference (EMI shielding),2 in chemical and biological sensors,3 and as magnetic sorbents.3c Conducting polymers are promising materials for EMI shielding due to their relatively high electrical conductivity and dielectric constant.2a Moreover, their association with magnetic nanoparticles may create nanocomposites with a higher shield efficiency because of the contribution of the magnetic loss, tan δm () µ′′/µ′) provided by the magnetic filler.2f Also, conducting polymer-magnetic nanoparticle nanocomposites may be designed to work as signal-triggered materials with properties being modulated by the application of an external stimulus, in * To whom correspondence should be addressed. Tel: +5511 3091 5256. Fax: +5511 3091 5585. E-mail: [email protected]. † Depto de Engenharia de Sistemas Eletroˆnicos, EPUSP. ‡ Universidade de Brası´lia, Instituto de Fı´sica. § Universidade Federal do Rio de Janeiro, Instituto de Fı´sica. | Universidade Federal de Goia´s, Instituto de Quı´mica.

this case electric or magnetic. Control of the charge transport within a polyaniline (PANI) nanocomposite film was recently demonstrated,3a incorporated with maghemite-Au core-shell nanoparticles, by the use of an external magnet. The external magnetic field induced the swelling/shrinking of the polymer matrix, and thus changed the conduction paths through the film. This so-called magnetoswitchable material was used as a mediator on the biolectrocatalytic oxidation of glucose by glucose oxidase.3a In another approach, Pal et al.3b produced a bioconjugated nanocomposite with PANI, maghemite nanoparticles, and specific antibodies for detecting Bacillus anthracis, the causative agent of anthrax. The bioconjugate was used simultaneously for preconcentrating the target antigens under the effect of a magnetic field and for transducing the chemical events into electrical signal. Magnetically separable PANI nanofibers were produced upon polymerization of aniline in the presence of iron oxide nanopowder.3c The resulting nanofibers were used for immobilization of lipases that were then employed in the enantioselective esterification of ibuprofen. The lipases immobilized onto the nanofibers retained more than 80% of their original activity after repeated use and could be easily recovered from the reaction media by using a magnet. Among the hosting conducting polymers, PANI has been used due to several reasons, including easy synthesis route at high

10.1021/jp8092463 CCC: $40.75  2009 American Chemical Society Published on Web 03/09/2009

5088 J. Phys. Chem. C, Vol. 113, No. 13, 2009 yields, appreciable electrical conductivity controlled by simple doping/dedoping chemistry, and enhanced stability.4 Further, the electric functionality of PANI combined with nanoscale processing have enabled its use in several practical applications that include organic light-emitting diodes,5 chemical and biological sensors,6 photovoltaic cells,7 and EMI shielding.2d-f As the magnetic counterpart, superparamagnetic iron oxide (SPIO), as for instance magnetite (Fe3O4) and maghemite (γFe2O3), have been extensively explored. In addition to their unique size-dependent magnetic properties,8 SPIO can be prepared using different methods (physical and chemical), some of them allowing fine control of size, size distribution, shape, crystallinity, and stability.9 Interest has been focused on nanocomposites derived from the grafting or homogeneous dispersion of iron oxide nanoparticles in PANI. First attempts to prepare SPIO/PANI nanocomposites were made by Wan et al.,10 who reported the preparation of iron oxide/PANI nancomposite using a chemical method. In that study, the authors synthesized nanocomposites of Fe3O4/ PANI using a solution of the emeraldine base form dissolved in NMP solvent reacting with aqueous solution of iron(II) sulfate under nitrogen atmosphere. Maghemite/PANI nanocomposite films cast from homogeneous solutions containing camphor sulfonic acid (CSA)-doped PANI and dodecylbenzenesulfonic acid (DBSA)-coated maghemite nanoparticle, both components synthesized separately, were prepared by Tang et al.1e Most of the subsequent investigations involved the synthesis of the conducting polymer in the presence of nanoparticles or viceversa, resulting in powdery materials requiring further processing to obtain nanocomposite films.11 Indeed, strategies of doping acids and solvents for the preparation of SPIO/polymeric-based nanocomposites have led to different nanostructures, such as nanospheres, nanotubes, and nanorods exhibiting electrical conductivity and superparamagnetic behavior at room temperature.12 Apart from the chemical routes developed so far, physical approaches have also been explored for the production of nanocomposite films consisting of polymer templates hosting SPIO particles. The electrostatic layer-by-layer (LbL) technique, under which layers of charged nanoparticle are alternately adsorbed with common polyelectrolytes onto solid substrates, is one key example.13 Actually, LbL has been successfully employed for the preparation of films with several classes of materials, due to its simplicity and low cost. Furthermore, LbL enables one to properly tune the properties of nanostructured thin films at the molecular scale, thus providing a method for the preparation of multifunctional materials. Just to mention a few, LbL has been applied to obtain SPIO/polyectrolytes planar layers, such as magnetite/poly(dimethyldiallylammonium chloride);14 magnetite and exfoliated montmorillonite clay alternated with poly(diallyldimethylammonium bromide);15 magnetite layers alternated with amphiphilic azobenzene compounds;16 magnetite with poly(ethyleneimine), poly(allylamine hydrochloride), and poly(styrene sulfonate), PSS.17 In a previous study, we reported on the preparation of nanocomposites consisting of positively charged maghemite nanoparticles supported in poly(o-ethoxyaniline), POEA and PSS, via the LbL technique. The obtained nanocomposite films presented conductivity similar to the pure template (POEA/PSS film) while exhibiting, in addition, superparamagnetic behavior at room temperature.18 The aim of this work is to obtain thin nanocomposite films containing nanosized maghemite particles hosted on PANI, using the LbL approach. We found this preparation route

Paterno et al. allowing easy and fine control of the magnetic and conductivity properties of the nanocomposite film. Before fabrication of the nanocomposite maghemite nanoparticles were synthesized, surface coated with citrate (cit) and suspended as magnetic fluid (cit-MAG) samples with fine control of maghemite content. The assembly of 3D nanocomposite films consisting of multiple bilayers of cit-MAG/PANI was carried out as a model system to investigate: (i) the influence of the deposition condition on the adsorption of cit-MAG in the nanofilm and (ii) the electrical and magnetic behavior of the as-produced bifunctional nanocomposite films. In order to access information regarding the material’s structure, the composition of the end nanocomposite films was varied by changing both the number of deposited cit-MAG/PANI bilayers from 1 to 50 and the concentration of cit-MAG in the starting magnetic fluid (MF) sample from 1.4 × 10-4 up to 2.9 × 10-3 gL-1 (in grams of γ-Fe2O3 per liter). Adsorption of cit-MAG was investigated by atomic force microscopy (AFM) and UV-vis spectroscopy. The electrical conductivity and magnetic properties of cit-MAG/PANI multilayered nanofilms were evaluated, and the results were interpreted and correlated with the nanocomposite’s structure. Experimental Section Chemicals and Materials. All chemicals used for polymer and nanoparticle synthesis, substrate cleaning and film deposition were analytical grade, used without additional purification. Optical glass slides (25 × 10 × 1 mm) and silicon stripes (10 × 3 × 1 mm) were used as substrates. Ultrapure water (resistivity: 18 MΩ · cm) was provided by the Millipore Milli-Q Plus 185 purification system. Synthesis of Polyaniline. Polyaniline (PANI, Mw 20 000 g/mol) was synthesized via oxidation of aniline (ANI: Aldrich, USA) by ammonium peroxydisulfate (APS: Merck, Germany) in 1.0 molL-1 HCl solution at low temperature (0-5 °C).19 The employed molar ratio ANI:APS was 4:1 and the resulting powder (doped PANI) was collected by filtration and washed with 1.0 molL-1 HCl solution. Once doped, PANI is insoluble in most solvents the obtained polymer was dedoped by dispersion in 0.1 molL-1 NH4OH solution under magnetic stirring, for 18 h. The dedoped PANI was then collected by filtration, washed several times with distilled water, dried in vacuum, and stored in a desiccator for further use in the experiments. The resulting dedoped PANI is soluble in polar organic solvents, as for instance in N′,N′′-dimethylacetamide (DMA) and N-methylpyrrolidone (NMP). Synthesis of Citrate-Coated Maghemite Nanoparticle. Citrate-coated maghemite nanoparticles (cit-MAG) were produced and peptized in aqueous medium according to the procedure previously reported in the literature.20 Shortly, magnetite (Fe3O4) nanoparticle was synthesized by mixing FeCl2 (Merck, Germany) and FeCl3 (Merck, Germany) aqueous solutions (2:1 molar ratio) with concentrated ammonia under vigorous stirring. The resulting black magnetite precipitate was washed several times with water and separated from the supernatant using a permanent magnet. The oxidation of magnetite to maghemite was performed by dispersing the magnetite nanoparticulate sample in boiling solution of 0.35 molL-1 Fe(NO3)3 (Synth, Brazil) under stirring, for 1 h.21 The nanoparticulate sample was then washed with 2.0 molL-1 HNO3 solution and separated from the supernatant using the permanent magnet. The reddish sediment, composed of maghemite nanoparticles, was dispersed in deionized water and the dispersion was shaken for 12 h. The dispersion containing maghemite

Polyaniline Hosting Maghemite Nanoparticles

Figure 1. Particle size histogram of the cit-MAG sample. The inset is a typical TEM micrograph of the sample.

nanoparticle bearing positive charge was further centrifuged at 5000 RPM for 5 min. for separation of insoluble material. This maghemite-based dispersion (20 mL) was then treated with a stock solution of citric acid (Aldrich, USA) in order to obtain the MF sample consisting of cit-MAG particles stably suspended in aqueous medium with citric acid concentration of ∼0.05 molL-1. The as-produced MF sample was shaken for 12 h and then dialyzed for 12 h against deionized water to eliminate the free citric acid from the bulk dispersion. The pH was adjusted to 7.0-7.2 and large aggregates were removed out from the MF sample by centrifugation at 5000 RPM, for 5 min. As expected, the resulting MF sample was very stable, composed by dispersed citric-coated maghemite nanoparticles bearing negative charge due to citrate adsorption. The zeta-potential (ζpotential) of the aqueous-suspended cit-MAG at pH 6.5 was quoted as -32.9 mV. Transmission electron microscopy (TEM) micrographs of the cit-MAG were recorded in order to access the average particle size and size dispersion. Vertical bars in Figure 1 represent the particle size histogram obtained from the TEM micrographs whereas the solid line results from the curve-fitting of the data using the log-normal distribution function.22 Values of the average particle diameter (Dm) and standard diameter deviation (σ) obtained from the fitting of the data presented in Figure 1 were 7.5 ( 0.1 nm and 0.33 ( 0.03, respectively. The inset of Figure 1 shows a typical TEM micrograph of the cit-MAG sample, recorded in a JEOL 1011 transmission electron microscope. X-ray diffraction patterns (data not shown) obtained from the powder cit-MAG sample confirms the maghemite phase, with an average particle diameter of 7.7 nm. Magnetic characteristics of the powder cit-MAG sample (provided in Supporting Information) were evaluated from both magnetic hysteresis loops measurements recorded at 4 and 300 K and zero-field-cooled (ZFC)/field-cooled (FC) curves obtained between 2 and 280 K. Magnetization measurements were carried out in a Cryogenic SQUID S600 magnetometer. At 300 K, the M × H curve presented no hysteresis (zero coercivity), as expected due to the average diameter of the cit-MAG sample. The observed saturation magnetizations of the cit-MAG sample were 55 (4 K) and 46 emug-1 (300 K). The coercive field obtained from the hysteresis loop at 4 K was 180 Oe. ZFC/FC magnetization curves exhibit the typical blocking process of an assembly of superparamagnetic particles, with a blocking temperature at around 80 K.

J. Phys. Chem. C, Vol. 113, No. 13, 2009 5089 Production of Nanocomposite Films. Substrates. Previous to film deposition, glass and silicon substrates were cleaned according to the following protocol: (i) immersion in piranha solution (H2SO4:H2O2, 7:3, v/v) for 15 min; (ii) rinsing in deionized water; (iii) immersion in RCA solution (H2O:H2O2: NH4OH, 5:1:1 v/v) at 70 °C for 15 min; and (iv) rinsing in deionized water. Depositions were conducted immediately after substrate preparation. PANI Solution and cit-MAG Dispersion. PANI solution (0.15gL-1) was prepared by dissolution of dedoped PANI in DMA (Synth, Brazil) under magnetic stirring, for 18 h. The viscous solution was filtered (paper filter at 10 µm porosity) and acidified (for doping) by dropping 0.1 molL-1 HCl until the color of the solution changed from deep blue to dark green. The solution was then diluted with HCl (pH 2.7) solution to reach a solvent composition of 20% DMA and 80% HCl solution. PANI solutions remained stable for at least two weeks before polymer precipitation. Cit-MAG aqueous dispersions were prepared directly from dilution of the stock MF sample with ultrapure water. Three different dilutions of the MF sample containing cit-MAG (2 × 10-3, 2 × 10-4, and 1 × 10-4 gL-1 of γ-Fe2O3) were employed for nanocomposite preparations. The maghemite concentration (in grams per liter) in each dispersion was determined from a calibration curve obtained by UV-vis spectroscopy, provided in the Supporting Information. To build the calibration curve, the concentration of iron in the stock MF was first determined by atomic emission spectrometry (plasma ionization) and then the amount of maghemite was calculated. The MF fluid was then diluted with different amounts of ultrapure water and UV-vis spectra of each solution were recorded. Nanocomposite Deposition. Cit-MAG/PANI nanocomposite films were deposited via the electrostatic LbL technique directly from cit-MAG dispersions (anions) and doped-PANI solutions (polycation). Since the substrates used were negatively charged, they were first immersed in doped-PANI solution (pH 2.7), following rinse in HCl solution (pH 2.7). The as-deposited PANI layer was dried with compressed air and then immersed in the cit-MAG dispersion (pH 6.2). The cit-MAG/PANI “bilayer” was rinsed in ultrapure water (pH 5.5) and dried again with compressed air. The immersion time of the substrate into both starting materials was the same (3 min) and in order to obtain multilayered nancomposites, the immersion steps were repeated until the desired number of bilayers was reached. In the present work, the time for cit-MAG deposition was determined from adsorption isotherms obtained by UV-vis spectroscopy, as discussed later on in this paper. Preparation of nanocomposite films consisting of 1 to 50 cit-MAG/PANI bilayers is herein reported. Sample Characterization. The amount of cit-MAG and PANI in solution and in the as-produced nanocomposite films was evaluated by UV-vis spectroscopy, using a Shimadzu spectrophotometer (model UVPC 1600). The morphological characterization of the as-produced nanofilms was conducted in an atomic force microscope (Digital, Nanoscope II), using silicon nitride tips (V-shaped) attached to a cantilever with a spring constant of 0.09 N m-1. All images were recorded in the tapping mode, and the surface root-mean-square roughness (Rrms) was calculated using the software provided by the instrument. Values measured at three different spots on the sample surface were used to calculate the average Rrms. Nanocomposite film thicknesses were also measured by AFM (see Table 1), following a procedure developed by Lobo et al.23

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Figure 2. Typical tapping-mode AFM images (1 × 1 µm) recorded at different adsorption times of cit-MAG first layer onto the predeposited PANI layer. (a) 0 (pure PANI layer), (b) 10, (c) 60, and (d) 180 s.

TABLE 1: Thickness and Electrical Characteristics of cit-MAG/PANI Multilayered Nanocomposite Samples Prepared from cit-MAG Suspensions Containing Different Maghemite Concentration (expressed in gL-1 of γ-Fe2O3)a

a

cit-MAG concentration (gL-1 of γ-Fe2O3)

thickness (nm)b

2 × 10-3 2 × 10-4 1 × 10-4 controlc

63.3 ( 0.5 30.0 ( 0.5 25.0 ( 0.5 28.5 ( 0.5

sheet resistance (ohm) 4.27 × 108 2.27 × 108 5.16 × 107 8.33 × 106

resistivity (ohm × cm)

conductivity (ohm-1 × cm-1)

2700 682 129 23.7

3.70 × 10-4 1.47 × 10-3 7.75 × 10-3 4.21 × 10-2

The PANI concentration was fixed at 0.15 gL-1. b measured by AFM. c PANI/PSS control film. [PANI] ) 0.15 gL-1.

The electrical conductivity of the three as-prepared nanocomposite films was measured using the van der Pauw method. The electrical conductivity of a PANI/PSS film was also measured for control. Film samples deposited onto optical glass were cut in 1 cm2 squares, doped by immersion in 1 mol L-1 HCl solution for 1 min, and then silver contacts were painted at the corner of the samples through which 1 mm diameter copper wires were glued. Small currents (10-100 nA) were applied to two of the contacts while the drop voltage was acquired by the other two contacts, using a programmable power source/meter model 2420-C from Keithley Instruments Inc. Values of current and potential were used to determine initially the sheet resistance (in Ω), which was used to calculate the electrical resistivity (in Ωcm) and the electrical conductivity (in Ω-1cm-1 or Scm-1), as quoted in Table 1. Magnetization measurements (field and temperature dependent) of cit-MAG concentrated nanocomposite samples were conducted in a Quantum Design PPMS 6000 magnetometer. Magnetization measurements (field and temperature dependent) of less concentrated cit-MAG nanocomposite samples were obtained in the Cryogenic SQUID S600 magnetometer. Zerofield-cooled (ZFC) curves of the as-produced nanocomposite films were obtained while warming up the samples previously

cooled down under zero field condition. Field-cooled (FC) measurements were recorded while warming up the samples after a previous field cooling. Results and Discussion Adsorption of Cit-MAG. Adsorption of cit-MAG onto a predeposited PANI layer was observed via AFM images. Figure 2 shows AFM micrographs recorded from samples obtained at different times (10, 60, and 180 s) of immersion of the precoated PANI substrate into the cit-MAG dispersion. As shown in Figure 2a the surface of the precoated PANI layer presents flattened globules with in-plane average diameter of 35 nm and height of 5 nm. According to previous investigations, these structures are originated by a two-step adsorption mechanism,24 involving the nucleation and growth of PANI globules. First, a fast nucleation process occurs when isolated, positively charged, PANI segments anchor to the negative sites of the substrate creating small nuclei. Later on, in a second stage, the small nuclei increase their size, mostly in the in-plane direction, by incorporating the upcoming PANI chains from the solution. The size of the resulting flattened globules depends on the adsorption conditions, mainly on the pH. When the PANI-coated substrate is immersed into the cit-MAG dispersion its surface becomes

Polyaniline Hosting Maghemite Nanoparticles

J. Phys. Chem. C, Vol. 113, No. 13, 2009 5091 SCHEME 1: (a) Ilustrative Picture of Cit-MAG/PANI Bilayer Formation. (b) Proposed Internal Structure of the Multilayered Nanocomposite

Figure 3. Time dependence of both UV-vis absorbance measured at 480 nm (left-hand vertical axis) and surface roughness (right-hand vertical axis) measured by AFM. The data is related to the nanocomposite film consisting of the first cit-MAG layer adsorbed onto the PANI layer.

coated by a continuous layer of cit-MAG nanoparticle at the very first stage of the adsorption process around 10 s, as shown in Figure 2b. Figure 2, parts c and d, shows that this adsorption process proceeds in the same way as the immersion time increases. AFM images reveal the presence of some aggregates of nanoparticles. Similar features were observed for films assembled using more diluted cit-MAG dispersions. UV-vis spectroscopy was used to monitor the amount of adsorbed cit-MAG onto PANI as the adsorption process proceeded. Figure 3 shows the adsorption isotherm (UV-vis data), indicating a steep increase in the amount of cit-MAG adsorbed at the beginning of the process, followed by a plateau starting at ∼120 s and extending up to 4000 s. Plot of the film surface roughness value (AFM data) versus adsorption time (see Figure 3, right-hand vertical axis) resulted in a similar isotherm, indicating that the surface roughness reaches a maximum around 120 s, stabilizing afterward in the same time window as observed by UV-vis measurements. Such a fast adsorption process observed either from UV-vis measurements or from AFM data reflects the strong interaction between cit-MAG and PANI, more likely electrostatic in nature, once attempts in adsorbing positively charged MAG nanoparticles onto PANI layers were unsuccessful. As first pointed out by Pastoriza-Santos et al.25 and later on by Ostrander et al.26 and Tang et al.,27 the morphology of LbL films deposited with polyelectrolytes and inorganic nanoparticles depends on the interplay between interactions among nanoparticles and polyelectrolytes and between the nanoparticles themselves. When the attraction between polyelectrolytes and nanoparticles and the repulsion between nanoparticles themselves are strong enough, the resulting films are formed by dense-packed nanoparticle monolayers. However, if one of the above interactions dominates over the other, then the film morphology is characterized by nanoparticle clustering or else by low particle density. On the basis of such a model picture, it is possible to hypothesis for the existence of a strong interaction between cit-MAG and PANI, once the observed morphology of the resulting films is characterized by dense-packed layers of nanoparticles. However, a significant interaction among nanoparticles is also occurring, leading to the observation of few aggregates. In fact, since PANI pK is around 3, the pH of cit-MAG dispersion (pH 5.5) is high enough to neutralize some of the positive charges in the adsorbed PANI chains, thus decreasing the electrostatic attraction among the

PANI layer and the cit-MAG. Scheme 1a gives an illustrative picture of the adsorption of cit-MAG onto the PANI layer. In the preparation of flat nanocomposite films containing nanoparticles is always desirable to improve the nanoparticle packing and, consequently, minimize the surface roughness. Although cit-MAG nanoparticles seem to be agglomerated to some extent, the mean roughness of the resulting cit-MAG layer is about 6 nm, which is less than the average nanoparticle diameter (7 nm). This finding can be interpreted as a result of the dense-packing of cit-MAG, thus reducing the number of voids and islands on the film surface and, consequently, decreasing surface roughness. Further, the roughness of the multilayered cit-MAG/PANI nanocomposites is also smaller than the average nanoparticle diameter, indicating that the LbLbased protocol used in the preparation of all nanocomposite samples keeps the same absorption mechanism. While the reduced surface roughness in the as-prepared cit-MAG/PANI nanocomposites is a result of its own adsorption process, external effects may also help to decrease this parameter. The drawback of this extra treatment, however, is the need of an extra step in the nanocomposite film production. In a particular case in which the LbL process was employed the surface roughness of nanocomposites containing magnetite nanoparticles hosted into poly(dimethyldiallylammonium chloride) was decreased by treating the polyelectrolyte layer with microwaves.14 Multilayered Nanocomposite Deposition. We found that the average time required for the deposition of a complete cit-MAG layer onto PANI is about 3 min, as indicated by the adsorption isotherms presented in Figure 3. Therefore, production of multilayered cit-MAG/PANI nanocomposites requires multiple, 3 min cycles, comprising immersion of the substrate into the selected cit-MAG dispersion. Deposition of nanocomposites onto substrates was carried out with a fixed PANI concentration and three different concentrations of cit-MAG dispersions. Monitoring the nanocomposite deposition either at the 480 nm wavelength (Figure 4a) or at 770 nm (data not shown) revealed a linear dependence of the nanocomposite absorbance with the number of deposited layers, as shown in Figure 4b. This finding supports the picture that regular contents of both cit-MAG and PANI were adsorbed in every deposited bilayer at the end of each deposition cycle. In fact, this behavior is expected, once cit-MAG/PANI adsorption is essentially driven by electrostatic interaction, thus making the process self-regulated. Moreover, similar behavior was found during LbL deposition of other charged nanoparticles and polyelectrolytes.14-17 Additionally, varying the concentration of cit-MAG in the dispersion allowed one to control the amount of cit-MAG within the as-produced bifunctional nanocomposite film, while keeping the linear dependence between absorbance and number of layers. The UV-vis spectra (see Figure 4a) of the resulting nanocomposites are similar to those presented by both materials individually,

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Figure 4. Multilayer nanocomposite deposition of cit-MAG and PANI. (a) UV-vis absorption spectra of 10-bilayer cit-MAG/PANI nanocomposite films produced from different cit-MAG concentrations in the aqueous dispersion, as quoted and (b) absorbance dependence of the cit-MAG/PANI nanocomposite film thickness, in units of bilayers, for different cit-MAG concentrations, evaluated at 480 nm.

presenting the same characteristic absorption peaks: peak at 770 nm due to n-π* transition (polaron band) attributed to dopedPANI28 and the shoulder at 480 nm characteristic of charge transfer mechanism involving FeIII-O and FeII-FeIII.29 Estimation of the nanocomposite film thickness was independently performed by AFM evaluation. The AFM tip was used to scratch and peel out a small spot of the nanocomposite film deposited onto the solid substrate in order to create a step (see, for instance, Figure 5, parts a and b), allowing estimation of the film thickness. A linear dependence was found between the nanocomposite film thickness and the number of nominal cit-MAG/PANI bilayers (see Figure 5c), demonstrating that the cycling assembly process provides increasing adsorption of PANI and cit-MAG with fine control of film thickness and structure. After 10 cycles, the average thickness of a single citMAG/PANI bilayer, as obtained from the AFM evaluation shown in Figure 5c, is 5.9 nm. However, this value (5.9 nm) is smaller than the average cit-MAG diameter. At this point, we argue that as the assembly proceeds in building a new layer the upcoming cit-MAG must preferably accommodate into the voids left by the previous cit-MAG layer (top-to-bottom) rather than vertically aligned on top of it (top-to-top), thus explaining the reduction of the effective bilayer thickness to values smaller than the average nanoparticle diameter. PANI chains are adsorbed in between cit-MAG, acting as “glue”, holding the nanoparticles together within the nanocomposite film while minimizing electrostatic repulsions. Additionally, the surface

Figure 5. Thickness evaluation of cit-MAG/PANI nanocomposite film using AFM. (a) Typical AFM image (15 µm2), showing the spot selected by the AFM tip on the sample film containing 3-bilayers of cit-MAG/PANI; (b) topographic profile of the step; and (c) dependence of the film thickness on the number of cit-MAG/PANI bilayers. Deposition conditions: [cit-MAG] ) 2 × 10-3 gL-1, pH 6.2; [PANI] ) 0.15 gL-1 at pH 2.7.

roughness (root-mean-square, Rrms) of the multilayered nanocomposites did not vary with the number of cit-MAG/PANI bilayers corroborating the adsorption model proposed above. Rrms roughness values estimated by using the instrument software were as follows: 5.9 nm (1 bilayer); 5.8 nm (3 bilayers); 5.9 (5 bilayers), and 5.7 (10 bilayers).The close proximity of nanoparticles in adjacent layers increases mutual interaction, which has remarkable influence on the magnetic properties of the resulting nanocomposites, as will be further demonstrated by magnetization measurements. Scheme 1b illustrates the internal structure of the multilayered nanocomposite, emphasizing topto-bottom and top-to-top alignments. Electrical Conductivity. Preparation of nanocomposite films mixing magnetic nanoparticles and conducting polymers is

Polyaniline Hosting Maghemite Nanoparticles

Figure 6. Magnetization versus field curves for the 50-bilayer citMAG/PANI nanocomposite films measured at different temperatures, as quoted therein. The inset shows a magnified window of the M × H curves.

mostly driven by the possibility in obtaining materials with electrical conductivity and magnetization in a bifunctional material system, aiming the opportunity of several technological applications. However, introduction of magnetic materials based on SPIO into conducting polymer matrices usually leads to lower electrical conductivity than that presented by the pure conducting polymer. This is caused by the insulating characteristics of SPIO which partially blocks the conduction path within the polymer matrix. The electrical conductivity (van der Pauw measurements) of a 10-bilayer cit-MAG/PANI nanocomposite film, when varying the cit-MAG concentration in the starting aqueous-based dispersion, is listed in Table 1. Notice from Table 1 that the electrical conductivity of the multilayered nanocomposite films varies in the range between 10-4 and 10-2 Scm-1, depending on the concentration of the cit-MAG in the starting aqueousbased suspension. As expected, the electrical conductivity of the nanocomposite film decreases as the content of cit-MAG increases. However, for the lowest cit-MAG content sample, the resulting nanocomposite film presented conductivity (7.75 × 10-3 Scm-1) and thickness (25 nm) comparable to those presented by the control film (PANI/PSS): respectively equal to 4.21 × 10-2 Scm-1 and 28.5 nm. Nonetheless, even at this low content of cit-MAG, the resulting nanocomposite film still presents magnetization, as will be reported latter on in this study. Magnetic Properties. Magnetic properties of the cit-MAG/ PANI nanocomposite films (5, 10, 25, and 50 bilayers) were accessed via magnetization versus magnetic field curves (M × H), in the ( 20 kOe window, at different temperatures (4, 40, and 300 K). Magnetization data were normalized to the %wt content of maghemite, as determined by UV-vis spectroscopy. Additionally, the maghemite content of the 50-bilayers sample was confirmed by atomic absorption technique. Figure 6 shows typical M × H curves obtained at 4, 40, and 300 K for the 50-bilayer nanocomposite film, which was prepared using the cit-MAG dispersion at 2 × 10-3 gL-1 of γ-Fe2O3. The inset shows details of the M × H curves at small fields. No remanence (Mr) or coercivity (Hc) was observed at room temperature, as expected from samples containing noninteracting nanosized (about 7 nm average diameter) maghemite-based systems. The room-temperature 10 kOe-magnetizations are 56, 80, and 73 emug-1 for cit-MAG/PANI samples with 10-, 25-, and 50bilayers, respectively. With the exception fo the 10-bilayer citMAG/PANI sample, in which an overestimation of the maghemite mass has possibly occurred, the obtained value of the room-

J. Phys. Chem. C, Vol. 113, No. 13, 2009 5093

Figure 7. ZFC-FC curves obtained with a DC field of 100 Oe. The sample is the cit-MAG/PANI nanocomposite film containing 50bilayers.

temperature saturation magnetization (Ms) is in accordance with the value reported for bulk maghemite (76 emug-1).30 Supported, noninteracting nanoparticles, exhibit enhanced surface anisotropy at low temperatures, which is claimed to be responsible for the nonsaturation of the magnetization.31 For all samples investigated, the Ms values increase as the temperature of the M × H measurements decreases, as expected. The 4 K curve displays a coercivity value that indicates the presence of magnetically blocked particles. At 4 K, the nanocomposite film containing 50-bilayers shows Ms ) 83 emug-1, Mr ) 41 emug-1, and Hc ) 212 Oe. These values are slightly smaller than the values reported for bulk maghemite (Ms ) 85 emug-1 and Hc ) 250-400 Oe). Similar features were observed for cit-MAG/ PANI nanocomposite films prepared with more diluted cit-MAG dispersions. In nanoparticulated magnetic systems, the zero-field-cooled (ZFC)/field-cooled (FC) magnetizations are sensitive to the size, morphology, and particle-particle interaction.32 The ZFC/FC curves (DC probing field of 100 Oe) of the cit-MAG/PANI nanocomposite film containing 50-bilayers are presented in Figure 7. The data shown in Figure 7 can be used to estimate the blocking temperature (TB) as ∼40 K, which is in accordance with the observed behavior of the magnetization curves, with almost no coercivity above 40 K (see Figure 6). However, the ZFC curve does not exhibit the typical Curie-Weiss behavior above 40 K. This deviation can be attributed to the particle-particle interaction and is presently under more detailed investigation. Values of TB obtained from the recorded ZFC/FC curves using the cit-MAG/PANI nanocomposite films (prepared with citMAG dispersion containing 2 × 10-3 gL-1) containing 5-, 10-, 25-, and 50-bilayers were 30, 35, 39, and 40 K, respectively. The monotonic increase of TB with the number of deposited bilayers indicates that interactions among nanoparticles increase the more cit-MAG are incorporated within the nanocomposite film. When nanosized magnetic particles are brought close together the magnetic interaction (dipole and/or exchange interaction) will affect the superparamagnetic relaxation. Modeling real systems of nanosized magnetic interacting particles is a complex task, even in the limiting case of weak interaction. Different theoretical and phenomenological studies proposed to account for nanosized magnetic particle-particle interaction predict an increase of the blocking temperature as the single particle anisotropy energy barrier increases with the interparticle interaction.33

5094 J. Phys. Chem. C, Vol. 113, No. 13, 2009 Increase of interactions among nanoparticles may be caused by the decrease on the interparticle distance. With the exception of the cases where rigid and large molecular units (e.g., aluminosilicate platelets) are used for film assembly most of the LbL-based films are absent of interlayer boundaries, leading to close contact among adjacent layers. For the case of cit-MAG/ PANI system, AFM images revealed that immersion of the PANI-coated substrate into the magnetic fluid sample led to a dense layer of cit-MAG and some aggregates. This is the first contribution of the LbL assembly toward the increase of the nanoparticle interaction. As the multilayer deposition proceeds, more nanoparticles are incorporated within the nanocomposite film and nanoparticle densification occurs, with nanoparticles being accommodated into the voids left by previous layers (see Scheme 1b). This is the second contribution of the LbL assembly toward the increase of the particle-particle interaction. This model picture explains the observed monotonic increase of TB as the number of bilayers increase in the cit-MAG/PANI nanocomposite films. Conclusions In summary, this work reports on the pioneering use of the layer-by-layer (LbL) technique to assembly bifunctional nanofilms consisting of citrate-coated maghemite nanoparticle (citMAG) hosted in conducting polyaniline (doped-PANI), exhibiting electrical conductivity and superparamagnetism at room temperature. We found that the cit-MAG/PANI nanocomposites’ properties (structural, electrical, and magnetic) can be successfully and precisely modulated by the amount of magnetic nanoparticle incorporated during the deposition of each citMAG/PANI bilayer. One key difference between the protocol we report here and the current literature is that any single adsorbed bilayer is build while dipping the solid substrate in individual materials suspensions, namely doped-PANI solution first following immersion into the magnetic fluid sample (citMAG). This strategy allows fine control of the linear relationship between the amount of as-deposited material and the number of deposited bilayers (deposition cycles), as corroborated by UV-vis spectroscopic evaluation. Likewise, the fine control of the linear relationship between the nanofilm thickness and the number of deposition cycles is confirmed by atomic force microscopy investigation. In addition to the fine control of the nanofilm morphology in easily controllable time scale (3 min per each deposited layer), the possibility of varying the citMAG concentration in the magnetic fluid sample extends the capability of the protocol reported here far beyond the protocols already reported in the literature. The increase of the net nanofilm thickness, with the corresponding monotonic decrease of the electrical conductivity as the cit-MAG concentration increases is simply a consequence of the fine control of the film morphology and its correlation with the introduction of nonconducting nanoparticles in the hosting conducting template. Similarly, the fine control of the film morphology provided by the protocol reported in the present study leads to the observed monotonic increase of the blocking temperature as the net nanofilm thickness increases. Finally, the hypothesis here is that the adsorption of cit-MAG onto the doped-PANI layer results in a dense-packed layer of nanoparticles, leading to the observed surface roughness smaller than the average nanoparticle diameter. While building a new cit-MAG layer, the model picture used to describe the multilayered film morphology includes the stacking of nanoparticles into the voids left by previous layer, modulating the average particle-particle distance. The fine control of the morphology and macroscopic properties of the

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