Construction of simple gold nanoparticle aggregates with controlled plasmon–plasmon interactions

We have developed a colloidal assembly for the study of plasmon–plasmon interactions between gold nanoparticles. Colloidal aggregates of controlled size and interparticle spacing were synthesized on silica nanoparticle substrates. Following the immobilization of isolated gold nanoparticles onto silica nanoparticles, the surfaces of the adsorbed gold nanoparticles were functionalized with 4-aminobenzenethiol. This molecular linker attached additional gold nanoparticles to the ‘parent’ gold nanoparticle, forming small nanoparticle aggregates. The optical absorption spectrum of these clusters differed from that of gold colloid in a manner consistent with plasmon–plasmon interactions between the gold nanoparticles. q 1999 Elsevier Science B.V. All rights reserved. The self-assembly of nanoparticles into dense arrays, crystals, and Langmuir–Blodgett films yields materials with novel physical properties arising from w x interactions between nanoparticles 1–3 . Particle– particle interactions also regulate the linear and nonlinear optical properties of nanoparticles embedded in matrices or assembled into thin films at a high w x volume fraction 4,5 . Currently, there is great motivation to exploit the unique properties of these materials in electronic and optical device applications. Whereas attention is focussed primarily on the assembly of macroscopic crystals and films of dense cluster matter, there is also interest in developing ) Corresponding author. E-mail: [email protected] methods for assembling small controlled aggregates of nanoparticles, or ‘artificial molecules’. Due to the spherical symmetry and uniform reactivity of individual nanoparticle surfaces, the synthesis of small controlled nanoparticle assemblies is a significant challenge. Recently, however, DNA was utilized to provide the necessary symmetry-breaking mechaw x nism to synthesize nanoparticle dimers 6 . In this Letter, we report the stepwise molecular assembly of small, controlled aggregates of gold nanoparticles. Silica nanoparticles were used as substrates to provide the symmetry-breaking mechanism necessary for controlled assembly of these small gold nanoparticle aggregates. This approach provided hundreds of cm of silica surface area per milliliter of solution, facilitating the observation of 0009-2614r99r$ see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2614 98 01410-9 ( ) S.L. Westcott et al.rChemical Physics Letters 30