Conductive Gold Films Assembled on Electrospun Poly(methyl methacrylate) Fibrous Mats

Abstract

The preparation and design of confinement structures on a sub-micrometer scale have attracted great attention because of the potential for new materials properties and applications in a broad range of areas. Many materials with sub-micrometer dimensions have been fabricated, including inorganic and organic materials. Self-assembly of colloidal metal particles has generated great interest as a powerful fabrication method of macroscopic surfaces with well-defined and controllable nanostructures. Gold is of great interest because of its chemical and electrical properties. Electrical conductivities of assembled materials that are close to that of bulk gold have been achieved by selfassembly of colloidal gold particles using linker molecules between the particles or by increasing the gold coverage by a reduction process. The applications of assembled gold films include as surface plasmon resonance (SPR) substrates, substrates for surface-enhanced Raman spectroscopy (SERS), catalytic surfaces, electrodes, and production of patterns in microscale and nanoscale structures. However, most of the prior work has centered around fabrication of films on substrates such as glass or other oxide surfaces, silicon, and polymers. Electrospinning is a simple and convenient method of generating polymer fibers, inorganic fibers, and hybrid fibers with diameters ranging from tens of nanometers to several micrometers. The spinning process uses a high static voltage to create a strong electric field and to impart some charge to a viscous polymer solution. Once the field strength has surpassed a threshold to overcome the surface tension of the polymer, fine jets of the polymer solution are ejected from the capillary tip and undergo a whipping process, which leads to the formation of ultrathin fibers as the solvent evaporates. The fibers deposit onto a grounded conductor as a nonwoven mat and can act as a template or substrate for organic and inorganic materials. For example, poly(p-xylylene) and aluminum sub-micrometer tubes have been fabricated using chemical vapor deposition (CVD) to coat electrospun fibers. Titanium dioxide tubes have been prepared from polymer fibers coated with titanium dioxide by the sol-gel technique, and continuous thin coatings of titanium dioxide or tin dioxide on electrospun polymer fibers have also been prepared by solution deposition. In this communication, we report on using electrospun fibrous mats as substrates for a solution deposition of continuous thin gold coatings on fibers. The procedure used for coating gold consists of three steps. First, poly(methyl methacrylate) (PMMA) fibers containing a gold salt are produced by electrospinning; second, the gold salt is reduced by a dilute solution of NaBH4 and then washed with water and dilute HCl solution thoroughly; finally, gold is plated on the surface of the fibers by using gold particles embedded on the fiber surface to catalyze reduction of Au by hydroxylamine so that the size of the gold particles increases until a continuous coating is formed. Figure 1A shows two scanning electron microscopy (SEM) images of hybrid PMMA/gold-salt fibers electrospun from a chloroform solution of PMMA and a gold salt. It can be seen that the fibers exhibit random orientation because of the bending instability associated with the spinning jet. The surfaces of the fibers are smooth and the average diameter of the fibers is about 500 nm, suggesting that the gold salt is dispersed in the PMMA fibers uniformly. After the gold salt embedded in fibers is reduced with NaBH4, the yellow fibrous mat becomes brownish. Its SEM image is shown in Figure 1B. In contrast to a gold colloid assembly on a silane-coated polymer surface where the spherical gold particles are clearly visible, but in accordance with previous research by Kim et al. on electrospun poly(ethylene oxide) (PEO)/goldnanoparticle hybrid fibers, we presume that the gold nanoparticles may be dispersed both on the fibers’ surfaces and interiors. A transmission electron microscopy (TEM) image (Fig. 1C) shows that the gold nanoparticles are roughly spherical in shape and separated from each other, each having a diameter of approximately 5–10 nm, although some particles aggregate into clusters. Because the compact polymer network can prevent the gold particles from growing further after nucleation, gold nanoparticles have a small size and roughly uniform dispersion in ultrafine PMMA fibers. Figure 1D shows the UV-vis spectra of the gold salt (spectrum A) and gold nanoparticles (spectrum B) that came from the hybrid fibers dissolved in chloroform. The gold salt has an absorption C O M M U N IC A IO N S