Multilayer thin films based on polyelectrolyte-complex nanoparticles

https://doi.org/10.1016/S0927-7757(02)00133-4Get rights and content

Abstract

Polyelectrolyte complex nanoparticles, obtained by mixing two solutions of oppositely charged polyelectrolytes in non-stoichiometric ratios, were assembled into multilayer films on flat substrates by their alternate adsorption with oppositely charged polyelectrolyte. The incorporation of these nanoparticles in thin films was examined by UV-vis spectroscopy, quartz crystal microgravimetry and atomic force microscopy. The observed film thickness is up to 45% larger than films conventionally prepared from uncomplexed polyelectrolytes (for the same number of adsorption cycles), yielding an average thickness of approximately 40 nm for a ten layer film. The system presented here provides a viable approach to prepare thick polyelectrolyte-based films of defined composition, with potentially different permeability and mechanical characteristics than their layer-by-layer assembled polycation/polycation film counterparts.

Introduction

The layer-by-layer (LbL) self-assembly of polyelectrolytes represents a general and powerful method to build tailored ultrathin films of defined thickness, composition and structure. Following the pioneering work of Decher on pure polyelectrolyte multilayers [1], [2], [3], the LbL approach has been used extensively to prepare thin films for a variety of applications, including biosensing [4], [5], catalysis [6], [7], separations [8], [9], [10] and optics [11]. In the conventional LbL process, substrates are alternately dipped in solutions containing anionic and cationic species, which adsorb onto the solid support primarily via electrostatic interactions (for highly charged polymers). One advantage of the LbL method is that it allows remarkable nanometer-level control over the thickness of the resulting layers and, within the limit of interpenetration of the polyelectrolyte layers, vertical structuring [12]. The thickness of the films can be tuned either through the number of layers that are deposited or, in the case of polyelectrolytes, by increasing the ionic strength of the solutions from which the films are formed [13], [14], [15].

Similar to the association of oppositely charged polyelectrolytes on surfaces, when polyelectrolytes bearing opposite charges are mixed directly in water, aggregates form mainly due to the strong coulombic interactions. In mixtures of two weak polyelectrolytes with large differences in molecular weight, these complexes consist of a long host molecule with shorter ones sequentially attached (single- and double-stranded regions). These systems are water-soluble and have been characterized by Tsuchida et al. [16] and Kabanov and co-workers [17]. In contrast, for most strong polyelectrolyte systems, larger aggregates form, which then phase separate from solution and precipitate. These aggregates are therefore used as flocculants to remove dyes and colloid particles from solution [18]. The ratio of the monomeric units in insoluble aggregates of strong polyelectrolytes was found to be close to the stoichiometric ratio of 1:1 [19], [20]. However, in some systems soluble, non-stoichiometric polyelectrolyte complexes (nPECs) can be found at certain, low polyelectrolyte concentrations and at a range of mixing ratios of the polyelectrolytes [21], [22]. As these aggregates contain an excess amount of one polyelectrolyte, they are charged either positively or negatively (depending on the charge of the polyelectrolyte in excess), and are thus electrostatically stabilized. Such complexes formed by mixing polyelectrolyte solutions are readily accessible and variable model systems for polymer nanoparticles, as the charge of nPECs is determined by the mixing ratio of the polyelectrolytes in solution, as well as the medium conditions and the molecular parameters of the polyelectrolytes.

A model for the structure of nPECs was proposed by Karibyants et al. [23]. The nPECs were treated as consisting of a core of closely coiled polymer where the charges are entirely compensated. The nPEC core is surrounded by a corona of more loosely attached, charged polyions of the excess compound stabilizing the nanoparticles. However, studies show that on changing the conditions in the solution (salt, polyion ratio, temperature, etc.) the nPEC structure changes with variation of the environment [22], [24]. nPECs formed from the polyelectrolyte pair poly(styrenesulfonate) (PSS)/poly(diallyldimethylammonium chloride) (PDDA) have been well characterized [25], [26]: the resulting complexes are polydisperse and are stable for weeks. For cationic and anionic nPECs formed from a range of mixing ratios from 1:0.6 to 1:0.9, no free polyelectrolytes exist in the solution, as determined by analytical ultracentrifugation [23]. The average nPEC size is controlled mainly by the ionic strength of the solution and the component concentration, and varies between 60 and 150 nm [23]. The size additionally depends on the degree of swelling. The possibility to modify powders by adsorbing nPECs was demonstrated by Kramer et al. [21]: In that study it was shown that the adsorbed aggregates have basically the same stoichiometry as the performed nPECs in solution. It has also been reported that such aggregates, when adsorbed, reverse the surface charge of negatively charged silicate powder [27].

There are numerous reports on the inclusion of nanoparticles into LbL assembled polyelectrolyte-based multilayer films. Various inorganic (e.g. silica, magnetite, gold and semiconductor) nanoparticles [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38] as well as polystyrene beads [39] have been LbL assembled with oppositely charged polyelectrolyte. In this paper, we report the multilayer buildup of thin films comprising of nPECs, or ‘polymer-nanoparticles’, assisted by the alternate adsorption of oppositely charged linear polyelectrolytes. Assembling multilayers from these nPEC nanoparticles may provide an alternative route to alter the ratio of the total amount of polyanion and polycation in the films. In the past, this has often been realized by varying the salt concentrations of the solutions or the total number of layers deposited. In a similar approach to that demonstrated for proteins [7], [40], this method of precomplexing could also provide a means to incorporate polyelectrolytes in thin films that are not readily amenable to deposition via the LbL method; for example, low molecular weight and/or lowly charged polyelectrolytes [41]. There is also the possibility to use these systems, when tuned for maximum deposition amounts and composition, to build thick(er) films of polyelectrolytes with tailored properties (e.g. permeability, elasticity and structure). Furthermore, the nanoparticle structures could afterwards be converted to more uniform and entangled layers using a change in environmental conditions, e.g. ionic strength, pH or temperature.

Section snippets

Materials

Branched polyethyleneimine (PEI) (MW 25 000), poly(styrenesulfonate) (PSS) (MW 70 000) and poly(diallyldimethylammonium chloride) (PDDA) (MW 100 000–250 000) were purchased from Sigma-Aldrich. PSS was dialysed prior to use. The quartz crystal microbalance (QCM) gold electrodes with a resonance frequency of 9 MHz were obtained from Kyushu Dentsu (Nagasaki, Japan). The quartz substrates for UV-vis measurements were purchased from Hellma (Jena, Germany). Water used in all experiments was taken

Results and discussion

The positively charged complexes (nPEC+) used were prepared according to the method of Dautzenberg et al. with a monomer ratio of PDDA:PSS of 1:0.7, and the negative complexes (nPEC) with a ratio of PSS:PDDA of 1:0.5 [24], [25]. nPECs formed under these conditions are strongly charged, as confirmed by electrophoresis measurements: ζ-potential of +48 mV for the cationic nPECs with an excess of PDDA, and −53 mV for anionic nPECs with PSS as excess polyelectrolyte.

Multilayers of positive nPECs

Conclusion

In this work we demonstrated the successful incorporation of nPEC nanoparticles into polyelectrolyte thin films by utilizing the LbL method. Both the QCM and UV-vis data revealed film growth (per bilayer), showing that nPEC nanoparticles can be used as charged species in thin film formation by alternation with oppositely charged polyelectrolyte. The QCM frequency, and hence, mass changes for the nPEC/polyelectrolyte films were up to 45% higher than those for polyelectrolyte multilayer

Acknowledgements

Björn Schöler is thanked for the AFM measurements and Christine Pilz for technical assistance. H. Dautzenberg is acknowledged for helpful discussions and H. Möhwald for supporting the work within the MPI-Interface department. This work was funded by the BMBF.

References (51)

  • G. Decher et al.

    Thin Solid Films

    (1992)
  • M. Onda et al.

    J. Biosci. Bioeng.

    (1999)
  • F. van Ackern et al.

    Thin Solid Films

    (1998)
  • J. Koetz et al.

    Polymer

    (1996)
  • G. Kramer et al.

    Colloids Surf. A

    (1997)
  • B. Philipp et al.

    Prog. Poly. Sci.

    (1989)
  • G. Kramer et al.

    Colloids Surf. A

    (1998)
  • A. Krozer et al.

    J. Colloid Interface Sci.

    (1995)
  • Y. Liu et al.

    Chem. Phys. Lett.

    (1998)
  • T. Serizawa et al.

    Colloids Surf. A

    (2000)
  • M. Onda et al.

    J. Biosci. Bioeng.

    (1999)
  • J.J. Ramsden et al.

    Thin Solid Films

    (1995)
  • G. Decher et al.

    Thin Solid Films

    (1994)
  • G. Decher et al.

    Ber. Bunsen-Ges. Phys. Chem.

    (1991)
  • G. Decher

    Science

    (1997)
  • P. Bertrand et al.

    Macromol. Rapid Commun.

    (2000)
  • F. Caruso et al.

    Langmuir

    (1998)
  • F. Caruso et al.

    Langmuir

    (2000)
  • J.J. Harris et al.

    Langmuir

    (2000)
  • T.D. Dubas et al.

    J. Am. Chem. Soc.

    (2001)
  • A. Wu et al.

    J. Am. Chem. Soc.

    (1999)
  • X. Arys et al.

    Macromolecules

    (2001)
  • G. Decher et al.

    J. Prog. Colloid Poly. Sci.

    (1992)
  • J.B. Schlenoff et al.

    J. Am. Chem. Soc.

    (1998)
  • J.B. Schlenoff et al.

    Macromolecules

    (2001)
  • Cited by (47)

    • Food packaging applications of biopolymer-based (nano)materials

      2021, Biopolymer-Based Metal Nanoparticle Chemistry for Sustainable Applications: Volume 2: Applications
    • pH stimuli drug loading/release platforms from LbL single/blend films: QCM-D and in-vitro studies

      2020, Colloids and Surfaces A: Physicochemical and Engineering Aspects
    • Salt effects on the structural tailoring of layer-by-layer assembled polyelectrolyte complexes and salt-containing polyelectrolyte films

      2018, Thin Solid Films
      Citation Excerpt :

      Generally, building blocks that can adjust their structures and properties with external environment are ideal for LbL assembly as it enables more possibilities to achieve the structural tailoring of the films and therefore broaden the application range of LbL assembled films [5,14,25,42]. Recently, polyelectrolyte complexes (PECs), which are a class of supramolecular assemblies intertwined together mainly by electrostatic interaction between polycations and polyanions [5,43–45], have been widely used to fabricate LbL assembled films with new structures as well as functions [5,46–55]. Compared with uncomplexed polyelectrolyte, the PECs have abundance of composition and relatively large dimension, which facilitates the functional integration and rapid fabrication of LbL assembled film materials [5,48–54].

    • From polyelectrolyte complexes to polyelectrolyte multilayers: Electrostatic assembly, nanostructure, dynamics, and functional properties

      2017, Advances in Colloid and Interface Science
      Citation Excerpt :

      The authors reported that at pH 3 a dense coacervate was formed upon complexation, at pH 5 a precipitate was formed in solution, whereas at pH 7 the bulk behavior shifted away from precipitation. Polyelectrolyte complexes, especially those that deviate from 1:1 stoichiometry and exhibit a net charge, can be used as building blocks of PEMs, offering the advantage of rapid growth [93–99]. LbL films incorporating PECs are found to exhibit porous structure [93].

    • Structure and properties of layer-by-layer self-assembled chitosan/lignosulfonate multilayer film

      2012, Materials Science and Engineering C
      Citation Excerpt :

      All these steps were performed under air stream flow condition. The self-assembly process was in situ determined by UV spectroscopy (Hitachi U-4000 spectrophotometer) equipped with an integrating sphere detection system due to the UV absorption intensity proportional to the layer number as seen in literature [31–33]. The FTIR spectra of multilayer films were recorded using a NEXUS-670 (Nicolet Co., Ltd) spectrometer in transmission mode by aligning the film on a silicon wafer substrate (1–2 cm2) at a Brewster's angle of 75° with respect to the incident beam.

    • Controllable disintegration of temperature-responsive self-assembled multilayer film based on polybetaine

      2011, Colloids and Surfaces A: Physicochemical and Engineering Aspects
    View all citing articles on Scopus
    View full text