Patterning of Poly(acrylic acid) by Ionic Exchange Reactions in Microfluidic Channels

Here, we describe the in-plane patterning of thin films of poly(acrylic acid) (PAA) within microfluidic channels by crosslinking with metal cations. The patterned polymer and embedded metal ions (Pb, Ba, Zn, Pd, Cu, La, or Ho) are a platform for the rapid “on-chip” growth of patterned metallic and semiconductor nanoparticles; these particles, in turn, serve as catalysts for electroless deposition of a metal film. The patterned, crosslinked polymer serves as both a reactant and a matrix for subsequent chemistry; nanoparticles grow in patterned regions defined by the crosslinked PAA in seconds, rather than self-assembling over hours on a preformed patterned surface. We patterned the PAA film with a solution of metal cations in methanol/water flowing through a poly(dimethylsiloxane) (PDMS) microfluidic channel that was in contact with the film of PAA. This solution crosslinked the polymer and thus reduced its solubility in water. After removing the PDMS stamp and rinsing the substrate with methanol, water was used to dissolve those regions of the PAA film that were not crosslinked; this process generated a topologically patterned surface comprising features of PAA with lateral dimensions as small as 500 nm and vertical dimensions of ca. 350–500 nm. Reductants (e.g., BDC: borane dimethylamine complex) and other reagents (e.g., Na2S) converted appropriate crosslinking cations (e.g, Pd or Zn) to metallic or semiconducting nanoparticles, respectively, throughout the PAA matrix. All cation conversion was completed within a microfluidic channel, permitting several types of nanoparticles to be patterned simultaneously on the same “chip”. Nanoparticles patterned on the sub-micrometer scale are useful as nucleation sites for the growth of patterned nanostructures. Our approach for crosslinking the polymer within microfluidic channels enables further reactions to be done using a combinatorial strategy: we can in principle simultaneously introduce a set of crosslinking solutions of different multivalent metal cations into the channels in the PDMS array and thereby form a pattern of lines (or any desired shape), each with a different functionality. In addition, soft lithography allows rapid fabrication of a wide array of channel geometries. Thin polymer films patterned on glass, semiconductor, and plastic substrates are useful in microelectronics, optics, catalysis, and medical diagnostics. Conventional and more specialized (e.g., topographically directed and near-field contact mode) photolithography techniques for patterning polymers are limited because they are not compatible with a large number of heatand light-sensitive polymers and substrates. Electron-beam writing and laser ablation are capable of patterning a wider range of materials than photolithography but are inherently nonparallel and slow. Several successful techniques, such as solvent-assisted micromolding (SAMIM) and micromolding in capillaries (MIMIC), both based on soft lithography, can pattern a diverse set of organic films and polymers into arbitrary geometries. Nanoimprint lithography, step and flash lithography, and capillary force lithography fabricate features in a polymer substrate defined from a topographically patterned stamp (typically PDMS). PAA is an organic polymer with carboxylate functionalities that can be crosslinked by binding metallic cations. PAA has several desirable features: i) Crosslinked PAA structures are insoluble in both organic solvents and water (at pH 7), so it is a robust polymer for biological applications, such as the controlled release of drugs in vivo and cell patterning on hydrophobic surfaces. ii) The crosslinking of PAA is a reversible process; the crosslinked polymer again becomes soluble when it is exposed to metal chelating agents (e.g., ethylenediaminetetraacetic acid: EDTA) or non-crosslinking ionic solutions (e.g., 1 M NaCl). This reversibility makes PAA an ideal sacrificial layer for applications such as surface micromachining. iii) The metallic cations bound to PAA can either be reduced to form metallic colloids or precipitate as semiconducting colloids. We patterned PAA within microfluidic channels using metal-cation solutions. Following the procedure established by Linder et al. (Fig. 1), we spin-coated a 3.5 % (w/v) aqueous C O M M U N IC A TI O N