Nano-porous electrode systems by colloidal lithography for sensitive electrochemical detection: fabrication technology and properties

Abstract

A porous metal-insulator-metal sensor system was developed with the ultimate goal of enhancing the sensitivity of electrochemical sensors by taking advantage of redox cycling of electro active molecules between closely spaced electrodes. The novel fabrication technology is based on thin film deposition in combination with colloidal self-assembly and reactive ion etching to create micro-or nanopores. This cost effective approach is advantageous compared to common interdigitated electrode arrays (IDA) since it does not require high definition lithography technology. Spin-coating and random particle deposition, combined with a new sublimation process are discussed as competing strategies to generate monolayers of colloidal spheres. Metal-insulator-metal layer systems with low leakage currents < 10 pA and an insulator thickness as low as 100 nm were obtained at high yield (typically > 90%). We also discuss possible causes of sensor failure with respect to critical fabrication processes. Short circuits which could occur during or as a result of the pore etching process were investigated in detail. Infrared microscopy in combination with focused ion beam etching/SEM were used to reveal a defect mechanism creating interconnects and increased leakage current between the top and bottom electrodes. Redox cycling provides for amplification factors of >100. A general applicability for electrochemical diagnostic assays is therefore anticipated.

Micro- and Nanopatterning of Surfaces Employing Self Assembly of Nanoparticles and Its Application in Biotechnology and Biomedical Engineering

“…On the other hand, well separated pores are for example required to provide for sufficient mechanical stability of a filter membrane. To this end, a homogeneous distribution of separated particles has been achieved by freeze drying (Lohmüller, Müller et al 2008). After preparation of a monolayer of homogeneously distributed particles, while the thickness of the solvent layer is still much larger than the particle diameter so that electrostatic interactions between particles and substrate are predominant, the substrate is cooled below freezing point of the solvent, followed by sublimation of the frozen solvent in a vacuum chamber.…”

“…In order to induce particle adhesion, particles and substrate surface have to bear opposite charges. In case of the substrate this has been achieved by choice of an appropriate material such as an oxide or mica (Semmler, Mann et al 1998;Antelmi & Spalla 1999) or by deposition of synthetic or biological polyelectrolyte mono-or multilayer coatings (Serizawa, Takeshita et al 1998;Serizawa, Kamimura et al 2000;Lohmüller, Müller et al 2008), which can be deposited under well controlled conditions yielding reproducible results. The charge density of such a polymer layer can be tuned within a certain range by the choice of the salt concentration of the polymer solution during deposition.…”

“…As has been shown by Serizawa et al, this will affect final particle density as well as adhesion kinetics (Serizawa, Takeshita et al 1998;Serizawa, Kamimura et al 2000). Other synthetic and biological polymers such as polyethylene-imine and bovine serum albumin, respectively, which provide for a positive charge density at neutral pH have been used to coat substrates in order to promote electrostatic interaction with negatively charged polystyrene particles (Lohmüller, Müller et al 2008). Polyimide membranes were successfully modified using diethylene triamine (DETA) to introduce positive charges and provide for favourable wetting properties (Santoso, Albrecht et al 2003).…”

“…As a cost effective alternative, nanoporous metal -insulator -metal electrode systems (Fig.3) have been developed and were shown to exhibit superior redox-cycling properties (Neugebauer, Müller et al 2006;Lohmüller, Müller et al 2008). Amplification ratios of up to www.intechopen.com…”

“…Once arranged on the surface, particles have been used in a variety of ways to produce nanostructures. Firstly, particles may serve as shadow masks during the deposition of an etching mask (Musil, Jeggle et al 1995;Haginoya, Ishibashi et al 1997;Burmeister, Schäfle et al 1998;Choi, Yu et al 2003;Choi, Jang et al 2004;Lohmüller, Helgert et al 2008;Lohmüller, Müller et al 2008). After removal of the particle layers a porous etching mask is obtained having micro-or nanopores at the prior location of the particles.…”