Water-Based Assembly and Purification of Plasmon-Coupled Gold Nanoparticle Dimers and Trimers

We describe a simple one-pot water-based scheme to produce gold nanoparticle groupings with short interparticle spacings. This approach combines a cross-linking molecule and a hydrophilic passivation layer to control the level of induced aggregation. Suspensions of dimers and trimers are readily obtained using a single electrophoretic purification step. The final interparticle spacings allow efficient coupling of the particle plasmon modes as verified in extinction spectroscopy. 1. Introduction The reproducible synthesis of well-defined metal nanoparticle groupings has been the subject of intense research during the last few years [1–6], driven largely by their use as substrates for surface-enhanced Raman scattering (SERS) [7–10]. The electromagnetic field impinging on pairs of silver or gold particles separated by a few nanometers is enhanced in the gap region by several orders of magnitude [11], allowing for single-molecule sensitivity of the SERS signal [12, 13]. For a 1？nm gap, the SERS enhancement factor is larger than 108 for gold particles larger than 30？nm in diameter [7], becoming sufficient to achieve single-molecule SERS sensitivity. Several approaches have been described using organic linkers [1–4, 6] or salt-induced aggregation [5, 9] to produce shortly spaced nanoparticle assemblies. In general, particle groupings with controlled geometries and valencies are obtained after several purification and reaction steps. Furthermore, in order to separate dimers and trimers from single particles and larger aggregates, the particles need to be sufficiently stable to sustain electrophoresis [2] or differential centrifugation [5]. Since the colloidal stability of metal nanoparticles decreases for larger diameters, it is difficult to combine efficient purification and large enough particles for SERS. Electrophoresis was used successfully to purify symmetric or asymmetric gold nanoparticle (AuNP) dimers and trimers linked by DNA strands [2]. Using this technique, we recently demonstrated the controlled assembly of gold nanoparticles smaller than 20？nm with a 1？nm gap using a DNA template [7]. Using DNA as a linker offers a large flexibility since a single oligonucleotide can be grafted on the surface of one particle [14]. However, controlled DNA functionalization is difficult to adapt to particles larger than 20？nm in diameter because of the intrinsic limitations of electrophoresis [15]. In particular, for particles larger than 30？nm in diameter, the gold surface needs to be passivated before electrophoresis using ethylene glycol oligomers [16].