Low cost reversible sensors for volatile organic compounds (VOC) are of increasing interest for various applications like environmental monitoring or in medical applications ever since. Here we want to present a novel method to achieve a fast and reversible response by employing a quasi 2-Dimensional system of polymer-metal nanocomposites. The method operates on the principle of swelling of polymers in the presence of organic vapors and the advantages of nano-sized metal clusters. The difference to common sensing devices is, that we do not use a wet-chemical process here in order to incorporate metal nanoparticles in the polymer. Our sensors are made out of a spin-coated polymer substrate on which we deposit gold clusters with a cluster density near the percolation threshold via thermal evaporation. The benefit of this method is that it is very fast, easy and precisely to control and that only a minimum amount of waste (like organic solvents) is produced. In order to create a homogeneous and reproducible pattern of metal clusters a defined concentration of point defects was created by ion bombardment [1] of the polymer surfaces. In order to stabilize the configuration of metal clusters grown on top of the polymer, the samples were annealed up to the glass transition temperature. At this temperature the clusters are embedded in the polymer matrix [2] and the change in the electrical tunnel conductivity of these polymer-metal nanocomposites was controlled. The polymer swells in the presence of VOC leading to a change in the interparticle distance and conductivity of the composites. The degree of swelling primarily depends on the type of polymer, the degree of cross linking and the organic solvent i.e. solubility parameter of the polymer and the vapor. The conductivity changes until the saturation limit of around 1ppm of the VOC is reached. The amount of change in the measured conductivity is a unique quantity and is different for different combination of polymers and organic vapors. Therefore, the pattern leads to the fingerprint of a particular polymer towards different vapors. These fingerprints can be employed as a parameter for detecting VOCs.
[1] V. Zaporojtchenko et al., NIMB 236, 95 (2005).
[2] J. Erichsen et al., Macromolecules 37, 8813 (2004) |