Nanoporous Thin Films Formed by Salt-Induced Structural Changes in Multilayers of Poly(acrylic acid) and Poly(allylamine)

Abstract

We report here on the influence of changes in the solution salt concentration on the structure of multilayers of weak polyelectrolytes. For poly(acrylic acid) (PAA) and poly(allylamine) (PAH) multilayers assembled by the layer-by-layer process in the presence of sodium chloride, washing with pure water after deposition of each layer produces films with considerable surface roughness (root-mean-squared (rms) roughness ∼17 nm for a 10 layer film), as assessed by scanning force microscopy. In contrast, relatively smooth (rms roughness ∼1 nm) and homogeneous PAH/PAA multilayer films are formed when the salt concentration is kept constant both during the assembly process and in the washing steps. For such smooth films, subsequent exposure to pure water leads to the introduction of regular, discrete, nanometer-sized pores, thus providing a means of introducing lateral structure into the PAH/PAA multilayer films. Electrochemical measurements revealed that the pores formed in less than 10 min. The sensitivity of the multilayer films to salt as well as the subsequent creation of nanopores potentially makes them attractive candidates for use in controlled-release applications where defined permeability characteristics are desired.

Polyelectrolyte Complex Membranes for Immobilizing Biomolecules, and Their Applications to Bio-analysis

“…Moreover, the pore size can be controlled to 17 nm based on sodium chloride addition while preparing the membrane. 8 A modification of the nanoporous characteristics of the membranes can be achieved by changing the components of the membranes. We found that molecular-size nanopores can be achieved in a PIC membrane composed of PSS and PLL, where the penetration of the molecular species is strongly influenced by the molecular size; i.e., species with molecular weights less than that of 110 g/mol can penetrate through the PIC membrane, whereas larger species are excluded by the membrane (Fig.…”

Mass Transfer in Amperometric Biosensors Based on Nanocomposite Thin Films of Redox Polymers and Oxidoreductases

“…[31] Others have observed that polyelectrolyte nanocomposite thin films are highly compact, have a nanoporous structure and can limit the diffusion of small molecules such as ferricyanide. [15,16,22] Given that the nanocomposite films described here do undergo significant structural changes upon oxidation and reduction, these films are likely very compact and may posses nanoporosity as seen in other polyelectrolyte thin films. Actual electrode response Anticipated electrode response in the absence of mass transfer limitations in the nanocomposite film Figure 5.…”

“…For all the enzymes, one would expected that the enzyme is highly intercalated with the redox polymer layer, given the highly charged nature of each macromolecule in the multilayer structure. [7,13,15,16,21,22] The SPR signal can be directly related to the surface concentration of proteins with a shift of θ SPR by 1° degree corresponding to the deposition of 10 ng/mm 2 of protein. [23] Because the SPR signal can be alternatively recorded in RI units or angle θ SPR , the surface concentration of enzyme for each deposition step was easily found using the conversion factor of 7x10 Assuming the density of protein to be 1.3 g/cm 3 and a densely packed layer, [12] the thickness of the first deposited enzyme layer was approximately 19 Å.…”

“…Because nanocomposite films created from layers of polyelectrolytes have been shown to be nanoporous, the expansion and collapse of the thin film with potential may also represent a change in pore structure and volume at the nanometer level. [16] Fery and colleagues observed changes in nanopore structure based on changes in salt concentration of layerby-layer polyelectrolyte nanocomposites. One may hypothesize that changes in salt concentration as produced by changes in potential may produce a similar effect.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] A promising application of this assembly approach is in the area of biosensor fabrication. Layer-by-layer assembly represents a potentially reproducible method to control the architecture of the nanometer-scale biosensing films.…”