Continuous polyelectrolyte adsorption under an applied electric potential

Abstract

Interactions between charged macromolecules (e.g., proteins, nucleic acids, polyelectrolytes) and charged surfaces govern many natural and industrial processes. We investigate here the influence of an applied electric potential on the adsorption of charged polymers, and report the following significant result: the adsorption of certain amine side chain-containing polycations may become continuous, i.e., asymptotically linear (or nearly linear) in time over hours, upon the application of a modest anodic potential. Employing optical waveguide lightmode spectroscopy (OWLS) and an indium tin oxide (ITO) substrate, we show that asymptotic kinetics, and the adsorbed mass at the onset of the asymptotic regime, depend sensitively on polymer chemistry (in particular, side chain volume and charge location), increase with applied potential and ionic strength (conditions favoring a thicker initial layer), and are independent of bulk polymer concentration (suggesting postadsorption events to be rate limiting). X-ray photoelectron spectra reveal a suppressed polymer charge within layers formed via continuous adsorption, but no evidence of electrochemical reactions. We propose a mechanism based on polymer-polymer binding within the adsorbed layer, enabled by suppressed electrostatic repulsion and/or enhanced ionic correlations near the conducting surface, and stabilized by short-range attractive interactions. Continuous adsorption under an applied electric potential offers the possibility of nanoscale films of tailored polymer content realized in a single step.optical waveguide lightmode spectroscopy ͉ poly(L-lysine) ͉ protein adsorption ͉ indium tin oxide I nteractions between charged macromolecules and charged surfaces are ubiquitous in nature (e.g., protein-cell membrane) and are often exploited in technological applications. For example, weakly charged colloidal systems such as paints, inks, and waste water may be stabilized through an adsorbed layer of charged polymer (polyelectrolyte) (1, 2), and polyelectrolyte films containing functional entities (e.g., biomolecules, nanoparticles) may serve as sensors, separation membranes, and electrochemical components (3, 4). Adsorption is usually spontaneous, with electrostatic interactions naturally playing a key role. These interactions, and therefore the adsorption process itself, may be controlled through solution variables such as salt concentration and pH (5-12). However, there generally exists an upper limit to the extent of polyelectrolyte adsorption. The typical situation is for adsorption to be quite rapid (usually at a transport limited rate) and then to saturate, corresponding to the point where interfacial charge accumulation suppresses additional (net) adsorption (1). A clever way to avoid this limit is through the layer-by-layer (LbL) method, where a substrate is alternately exposed to solutions of oppositely charged polyelectrolytes (3, 13). Each exposure results in an increase in adsorbed mass and an overall interfacial charge reversal. The LbL method allows one t...

Electrochemical quartz crystal microbalance study of polyelectrolyte film growth under anodic conditions

“…Furthermore, we appear to remove part of the Pt during the process. Thus, besides verifying observations previously reported by others [7,15], we report methods for identifying, and even dealing with, the challenges associated with gas evolution at and etching of a Pt electrode.…”

“…via a layer-by-layer (LbL) method, is an obvious possible solution [3]. Studies highlighting the absorption of polyelectrolytes at solid-liquid interfaces are numerous [4][5][6][7][8]. While considering variables such as ionic strength and solution pH, most of these studies neglect the substrate's electric potential, which is the focus in this study [9].…”

“…Others have reported the electrochemical desorption of LbL films containing PLL with potential biological applications such as cell release [3,[11][12][13]. In the case of LbL thin film assembly, the construction of films by alternately adsorbing positively-and negatively-charged polyelectrolytes has been investigated using optical waveguide lightmode spectroscopy (OWLS) or ellipsometry with indium tin oxide (ITO) as the substrate [6,11,12,14,15] van Tassel et al have also reported the continuous adsorption of a polyelectrolyte under an applied anodic potential [7] via a similar adsorption process for both strongly-and weaklycharged polymers [8,16], and hypothesized that the film growth mechanism changes during the process [8], resulting in a different orientation of the polymer chains (secondary structure). The assembly of thin films of chitosan by anodic electrodeposition has also recently been reported [17].…”

Electrodes and Electrokinetic Systems for Biotechnological Applications

“…As we were primarily interested in the response from the QCM and how the applied potential influenced the deposition of PLL on Pt and ITO, we did not analyze the current in detail. However, we used the current when comparing the rate of water electrolysis to previous results obtained by van Tassel et al 39,40 This will be further discussed in Chapter 4.…”

“…A discovery made in 2007 by Van Tassel et al was that certain polyelectrolytes continuously adsorb over the time-scale of several hours when a positive electric potential is applied to the underlying substrate. 39 One advantage with this method, compared to, e.g., the LbL method, is that only one polyelectrolyte is needed, leading to single-component films achieved in one step. The scientists behind this study used an optical technique called optical waveguide light-mode spectroscopy (OWLS), and indium tin oxide (ITO) as the conducting substrate upon which the polyelectrolytes were adsorbed.…”