Silver nanoparticles for printable electronics and biological applications
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An environmentally friendly route to prepare stable concentrated aqueous dispersions of silver nanoparticles is described. It was found that Arabic gum, a well known stabilizing agent, can also rapidly and completely reduce Ag2O to metallic silver in alkaline solutions (pH > 12.0) and elevated temperature (65 C). The average size of the silver nanoparticles could be tailored from 10 to 30 nm by varying the experimental conditions. By hydrolyzing either enzymatically or chemically the polysaccharide, it was possible to isolate dispersed silver nanoparticles suitable for both biological and printable electronics applications. For the latter purpose, concentrated dispersions of silver particles were prepared and used for depositing thin uniform layers, which could be sintered into conductive films at low temperatures.
I. INTRODUCTION
Due to their unique properties, metal nanoparticles are widely used in catalysis,1,2 chemical sensing and biosensing,3,4 photonics,5 electronics,6,7 optics,8 DNA sequencing,9 surface-enhanced Raman spectroscopy,10,11 and pharmaceuticals.12,13 The preferred route for their preparation has been, and still is, the reduction of metal salts in solutions.14 Although dedicated reducing agents,15 solvents,16 and dispersants17–19 are typically used in this approach, in some cases the same additive may play multiple roles. Polyols, for example, can act simultaneously as solvents and reductants,20,21 and it was shown that polyvinyl phenols and polyvinyl alcohols display both reducing and stabilizing properties.22–24 Such dual function polymers are particularly interesting alternatives because they offer the possibility of replacing the undesirable reductants often used in conventional precipitation methods. Thus, cellulose,25 large biomolecules,14 amino acids,26 and even microorganisms27 have been tested recently as reducing and/or dispersing agents in the preparation of metal nanoparticles. In most cases high concentrations of polymeric dispersants are required to obtain stable metal colloids. Consequently, the particles are embedded in the organic matrix, which cannot be removed without affecting the particles’ properties and/ or the dispersion stability and can have a negative effect in many applications. The electronics industry, for example, takes advantage of the low melting point of silver nanoparticles28 to generate conductive films at low temperatures. To sinter in such conditions, the particles cannot be covered with thick layers of organic matter that a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0336
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http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 9, Sep 2009 Downloaded: 14 May 2014
decomposes at elevated temperatures. Although methods capable of preparing dispersant-free particles suitable are available,29–31 they result in well dispersed systems only at low concentrations where the electrostatic stabilization is effective. To obtain the concentrated dispersions needed in most applications, subsequent processin
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