Adsorption of Gold and Silver Nanoparticles on Polyelectrolyte Layers and Growth of Polyelectrolyte Multilayers: An In Situ ATR-IR Study

Attenuated total reflection infrared (ATR-IR) spectroscopy is used to study the adsorption of gold and silver nanoparticles and the layer-by-layer (LBL) growth of polyelectrolyte multilayers on a Ge ATR crystal. The Ge ATR crystal is first functionalized using positively charged polyelectrolyte poly(allylamine hydrochloride) (PAH). Then citrate-stabilized gold or silver nanoparticles are adsorbed onto the modified Ge ATR crystal. When gold or silver nanoparticles are adsorbed, a drastic increase of the water signal is observed which is attributed to an enhanced absorption of IR radiation near the nanoparticles. This enhancement was much larger for the silver nanoparticles (SNP). On top of the nanoparticles multilayers of oppositely charged polyelectrolytes PAH and poly(sodium 4-styrenesulfonate) (PSS) were deposited, which allowed to study the enhancement of the IR signals as a function of the distance from the nanoparticles. Furthermore, adsorption of a thiol, N-acetyl-L-cysteine, on the nanoparticles confirmed the enhancement. In the case of SNP an absorbance signal of about 15% was observed, which is a factor of about 40 times larger compared to typical signals measure without nanoparticles. ■ INTRODUCTION The layer-by-layer (LbL) deposition of polyelectrolytes is a versatile technique to build up multilayers on flat and curved surfaces. The interest in this technique has rapidly grown since its discovery because the preparation of such multilayers is quick, versatile, reliable, and cheap. It is possible to implement the technique on an industrial scale either using a dipping process or by simply spraying the respective solutions onto the substrate. Furthermore, the environmentally friendly, aqueoussolution-based method can be applied to substrates of almost every shape including planar surfaces, colloidal nanoparticles, quantum dots, and porous solids. The technique has found applications in important fields such as biomedicine, solar cells, drug delivery, and light-emitting diodes. The LbL technique is not limited to polyelectrolytes. For example, charged particles can be incorporated into polyelectrolyte multilayer systems, thus enlarging the field of application. Hybrid films containing both organic and inorganic materials are of special interest due to their particular electronic and optical properties. For example, charged semiconductor particles or monolayer-protected gold nanoparticles can be incorporated into polyelectrolyte multilayer films. We recently reported the preparation and optical properties of polyelectrolyte−metal nanoparticle composite films grown on glass slides. The citrate-stabilized gold and silver nanoparticles were assembled in layers, and the polyelectrolyte LbL technique allowed one to tune the distance between particles within different layers with nanometer precision. In this way the coupling between the plasmons could be tuned by simply varying the number of polyelectrolyte layers between the nanoparticle arrays. The technique is again not limited to flat surfaces. By using the LbL technique, gold nanoparticles were also assembled around silica beads, which lead to the emergence of a magnetic resonance. Such systems are appealing in the metamaterials field and for the development of cloaking devices. In the vicinity of the plasmonic particles the electric field can be enhanced. This is used for surface-enhanced Raman scattering (SERS), but even in the infrared enhancement can be observed, leading to surface-enhanced infrared absorption (SEIRA). SEIRA has been used to study chemical reactions on metal surfaces, and in general SEIRA is attractive for sensing applications. In contrast to techniques that rely on mass uptake or the change in refractive index, SEIRA yields detailed chemical information since an IR spectrum is a characteristic property of a molecule. Using the finite element method (FEM), it was shown that SEIRA increases with increasing particle size. In the present study we used attenuated total reflection infrared (ATR-IR) spectroscopy to investigate the selfassembly of polyelectrolyte layers and polyelectrolyte−nanoparticle composites. In ATR-IR spectroscopy an evanescent field probes the volume close to the internal reflection element. The penetration depth of the field is several hundred nanometers, which is considerably larger than the typical thickness of polyelectrolyte multilayers (tens of nanometers). Received: September 16, 2013 Revised: November 29, 2013 Published: December 4, 2013 Article