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.

Fig. 1.

Adsorbed mass versus time of poly(l-lysine) (PLL) onto ITO at various substrate electric potentials and from Hepes buffer at pH 7.4, [NaCl] 0.1 M, and [PLL] 0.4 g/liter, as measured via OWLS. (Potentials are relative to a standard hydrogen electrode.) OCP, open circuit potential = 0.2 V. The arrows indicate replacement of the polymer solution by pure buffer.

Fig. 2.

Atomic force microscopy images of the bare ITO substrate (Left) and PLL adsorbed onto ITO at open circuit potential (Center) and under an applied potential of 1.5 V (Right), at pH 7.4, [NaCl] 0.1 M, and [PLL] 0.4 g/liter. Adsorbed layers are produced via 30- and 120-min exposures to PLL solutions, respectively.

Fig. 3.

Adsorbed mass versus time of poly(l-lysine) (PLL), poly(l-ornithine) (PLO), poly(l-histidine) (PLH), poly(l-arginine) (PLA), poly(allylamine hydrochloride) (PAH), poly(ethylene imine) (PEI), poly(l-glutamic acid) (PGA), and cytochrome c onto ITO, at an applied potential of 1.5 V from Hepes buffer of pH 7.4, [NaCl] 0.1 M, and [polymer] 0.4 g/liter as measured via OWLS. In the PAH experiment, the results of a buffer rinse and reapplication of the polymer solution are shown.

Fig. 4.

Adsorbed mass versus time of PLL onto ITO at a substrate electric potential of 1.5 V, from Hepes buffer at various polymer concentrations and at pH 7.4 and [NaCl] 0.1 M, as measured via OWLS. A buffer rinse was conducted after 120 min of adsorption.

Fig. 5.

Adsorbed mass versus time of PLL onto ITO at a substrate electric potential of 1.5 V, from Hepes buffer at various pH values and at [NaCl] 0.1 M and [PLL] 0.4 g/liter, as measured via OWLS. A buffer rinse, a return to OCP, and a reapplication of 1.5 V occurred at 120, 150, and 200 min, respectively. (OWLS signals are affected by the potential, so data under OCP are not shown.)

Fig. 6.

Adsorbed mass versus time of PLL onto ITO at a substrate electric potential of 1.5 V, from Hepes buffer at various [NaCl] and at pH 7.4 and [PLL] 0.4 M, as measured via OWLS. A buffer rinse was conducted after 120 min of adsorption.

Fig. 7.

Adsorption rate of PGA onto a previously adsorbed PLL layer, versus mass of adsorbed PGA. The PLL layer is formed under an applied potential of 1.5 V for 120 min, and PGA is introduced (also under 1.5 V) following a buffer rinse of 10, 20, or 60 min. Solution conditions are [NaCl] 0.1 M, pH 7.4, and [polymer] 0.4 g/liter. Straight lines indicate least squares extrapolation of the (approximately) linear region of surface-limited adsorption kinetics. The intercepts of these lines are proportional to the adsorption rate constants (16).