Mapping single macromolecule chains using the colloid deposition method: PDADMAC on mica
Graphical abstract
Introduction
Cationic (positively charged) polyelectrolyte molecules exhibit a strong tendency to adsorb on various solid surfaces that are usually negatively charged in aqueous environments. Therefore, they are applied in many industrial processes, such as water treatment, papermaking, as antifouling agents preventing protein adsorption, and bacteria fouling [1], [2], [3].
In other processes, polyelectrolytes are used to produce adhesive substrates for protein and enzyme immobilization, separation [4], and bio-sensing [5]. Consecutive adsorption of cationic and anionic (negatively charged) polyelectrolytes according to the layer by layer (LbL) process is widely exploited in nanocapsule formulations used for controlled drug delivery [6].
Poly(diallyldimethylammonium chloride) (PDADMAC) is a typical representative of a water-soluble polyelectrolyte possessing hydrophilic, positively charged quaternary ammonium groups. It is often exploited as an “anchor layer” [7] for producing multilayers (films) of a desired coverage and structure [8], [9] having application as anion-exchange membranes for fuel cells [10].
PDADMAC is also used for the preparation of magnetic core–shell particles [11], [12], capsules [13] for the preparation of multilayer films on solid substrate such as: silicon wafers, quartz, gold [14], [15], or on porous and soft microgels [16].
Properties of aqueous PDADMAC solutions were determined in Refs. [17], [18]. The dependences of intrinsic viscosity, the second virial coefficient, the radius of gyration on electrolyte composition, and ionic strength were experimentally evaluated [17], [18], [19]. Also, a theoretical modeling based on the combination of molecular dynamics, rotational isomeric states and Monte Carlo procedure was applied to describe the conformation properties of the PDADMAC molecules [20]. Chain conformations in vacuum, pure water and aqueous salt solutions (NaCl, NaBr, and LiCl) were determined. It was postulated in works [19], [20] that the trans conformation of the three rotatable skeletal CH–CH bonds of PDADMAC units is favored, leading to formation of the extended polyelectrolyte chains.
Conformations of PDADMAC molecules in electrolyte solutions were also determined in Ref. [21] by the dynamic light scattering (DLS), micro–electrophoretic and viscosity measurements combined with molecular dynamics modeling. In contrast to previous works [17], [18] the low ionic strength range was also systematically studied. It was confirmed that PDADMAC molecules remain in an expanded state for the lower ionic strength range. Thus, at ionic strength of 10−2 M the effective chain length and the diameter of the molecule were 118 and 1.21 nm, respectively [21]. However, for much higher ionic strength the random coil limit is attained as previously shown in Ref. [17]. It was also experimentally proven in Ref. [21] that the electrokinetic (effective) charge on the PDADMAC molecule was equal to 84 e (where e is the elementary charge) at 0.01 M ionic strength. This suggests that PDADMAC molecules should strongly adsorb at negatively charged surfaces in the side-on orientation. This was experimentally confirmed by Porus et al. [22], [23] who studied adsorption of PDADMAC on gold sensors using the quartz micro balance (QCM) method. They reported the PDADMAC layer thickness to be 1–2 nm for ionic strength of 0.01–0.15 M. Silva et al. [24] studied adsorption of PDADMAC and other polycations at silicon wafer using ellipsometry. It was shown that the mean thickness of the PDADMAC layer at pH 6 and ionic strength of 0.1 M was 2.7 nm. A similar value was also predicted from direct AFM measurements of interaction force between silicon tip and silica covered by PDADMAC [25]. Additionally, in these works, negligible desorption of PDADMAC was confirmed.
However, despite a vital significance for basic science, the low coverage macromolecule adsorption regime has not been considered before in the literature. This is due to the fact that the classical experimental techniques such as ellipsometry, reflectometry or QCM exhibit a limited sensitivity and precision for monolayer coverage below 10%. This also concerns the streaming potential technique used before for studying PDADMAC adsorption on mica [26] that only gives reliable results for polyelectrolyte coverage above 1%. However, using this in situ method one could precisely determine the adsorption/ desorption kinetics of PDADMAC and in consequence the equilibrium adsorption constant and binding energy for various ionic strengths [26].
Therefore, in order to elucidate the PDADMAC adsorption mechanism for the dilute regime, that is the main goal of this work, we have used the colloid deposition method developed in Refs. [27], [28]. Later on it was used for studying protein monolayers on mica [29]. In this technique, the deposition kinetics of negatively charged colloid particles driven by electrostatic attraction on pre-adsorbed macromolecules is quantitatively evaluated. The advantage of this method is that the colloid particle coverage can be directly determined under in situ conditions by optical microscope imaging. Because of a considerable surface area ratio between the macromolecule and the colloid particle (latex microsphere) an enhancement factor of 103 can be attained. This allows one for a nearly one-to one mapping of polyelectrolyte chains for the coverage below 0.1%.
Additionally, the measurements of latex deposition kinetics can be exploited for assessing the range of validity of the random site adsorption models [30], [31], [32] that has an essential significance for basic science.
As far as practical aspects are concerned, the results obtained in this work may have a significance for preparation of polyelectrolyte supporting layers aimed for colloid particle, and bacteria removal by filtration or for protein immobilization and immunosensing.
Section snippets
Materials and methods
Poly(diallyldimethylammonium chloride) hereafter referred to as PDADMAC, the cationic polyelectrolyte having a molar mass of 101 kg mol−1 (number averaged) and 160 kg mol−1 (weight averaged) was purchased from PSS Polymer Standards Service GmbH, Germany. Knowing that the molar mass of the monomer Mwm is 162 g mol−1 one can calculate that the average number of monomers Nm in the PDADMAC molecule is 625.
Natural ruby mica sheets were purchased from Continental Trade, Poland and used as a solid substrate
Characteristics of latex and PDADMAC in the bulk and mica surface
The diffusion coefficient of latex particles was determined as a function of the ionic strength using the DLS method. From these data, the hydrodynamic diameter of particles dl was calculated. At pH 5.8 and the NaCl concentration range 10−2 to 0.15 M, the hydrodynamic diameter of the L800 latex was 820 ± 10 nm. The zeta potential calculated from the electrophoretic mobility measurements, varied between −98 and −46 mV for the NaCl concentration of 10−2 and 0.15 M, respectively (pH 5.8).
The basic
Conclusions
By applying the streaming potential and colloid enhancement methods supplemented by theoretical modeling, the structure of PDADMAC monolayers on mica was uniquely evaluated.
It was confirmed that for the high ionic strength of 0.15 M, a one to one mapping of polyelectrolyte chains by colloid particles can be achieved for the PDADMAC coverage below 0.1%. In this way an enhancement factor of 103 can be attained. This result was quantitatively interpreted in terms of the random sequential adsorption
Acknowledgments
This work was financially supported by the NCN GRANT Umo-2012/07/B/ST4/00559 and Polish Ministry of Science and Higher Education (MNiSW) under Iuventus Plus No. IP2011 0353 71 grant.
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