Article pubs.acs.org/Langmuir
Electroactive Layer-by-Layer Plasmonic Architectures Based on Au Nanorods Tiziana Placido,†,‡ Elisabetta Fanizza,†,‡ Pinalysa Cosma,†,‡ Marinella Striccoli,† M. Lucia Curri,*,† Roberto Comparelli,*,† and Angela Agostiano†,‡ †
CNR-IPCF Istituto per i Processi Chimici e Fisici, Sez. Bari, c/o Dip. Chimica Via Orabona 4, 70126 Bari, Italy Dip. Chimica, Università degli Studi di Bari, Via Orabona 4, 70126 Bari, Italy
‡
S Supporting Information *
ABSTRACT: Nanostructured films based on Au nanorods (NRs) have been obtained by layer-by-layer (LbL) assembly driven by electrostatic interaction between metal nanoparticles and polyelectrolytes. Multilayer films have been fabricated by using LbL assembly of poly(sodium styrenesulfonate) (PSS) and positively charged Au NRs on a polyelectrolyte-modified substrate. The effect of fabrication parameters, including the nature of the substrate, the polyelectrolyte initial anchoring layer, and the number of layers has been investigated by means of UV−vis absorbance spectroscopy and atomic force microscopy (AFM). The results demonstrated the dependence of morphology and plasmonic features in the multilayered nanostructured architectures from the nature of the anchoring polyelectrolyte on the substrate, the number of layers, and the kind of NR mutual assembly. In addition, a study of the electrochemical activity at the solid/liquid interface has been carried out in order to assess charge transport through the NR multilayer by using two molecular probes in solution, namely, potassium ferricyanide, a common and well-established redox mediator with reversible behavior, and cytochrome C, a robust model redox protein. The presented systematic study of the immobilization of Au NRs opens the venue to several application areas, such as (bio)chemical sensing.
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components to fabricate structures.2 Such a step is extremely relevant for achieving functional nanostructures, such as films, to be integrated as electrodes and transistors in devices. Various strategies for thin film preparation have been widely employed, such as Langmuir−Blodgett (LB)8 and self-assembled monolayer (SAM) depositions.9 Whereas the former has intrinsic limitation and cannot be used for any material, the latter is not practical for multilayer fabrication. Layer-by-layer (LbL) assembly is carried out by exploiting electrostatic interactions among the components, which result in a stacked structure of alternating positively and negatively charged materials.10 Such a cost-effective and versatile tool allows a large amount of freedom in terms of the chemical nature of the components, the number of layers, and the layering sequence. Diverse types of materials, including organic components (organic dyes and dendrimers),11 inorganic components (nanoparticles, nanowires, nanoplates, clay, and nanosheets),12−14 and biomolecules (DNA, enzymes, proteins, and polysaccharides)15−17 can be firmly incorporated in a multilayered structure. Indeed, the thickness, chemical composition, and surface morphology of an LbL-assembled film can be
INTRODUCTION In the last few years, the use of nanomaterials for the fabrication of biosensing devices has represented one of the most exciting approaches, being able to provide enhanced analytical characteristics in terms of sensitivity, selectivity, reliability, cost effectiveness, and simplicity of fabrication and use. Gold nanoparticles (Au NPs) are nanosized functional materials particularly valuable for the development of highly sensitive devices for application in optics,1 electronics,2 catalysis,3 sensing,4 and plasmonics.5 Among the advantages that make Au NPs interesting candidates for biotechnological application are (i) the retention of biomolecule activity once adsorbed on Au NPs, which is crucial to achieving high biosensor performance; (ii) access to electrochemical transduction without any additional electron-transfer mediator because Au NPs permit the direct electron transfer between redox proteins and bulk electrode materials;6 and (iii) effective exploitation of the Au NP surface as a functional interface for the electrocatalysis of redox processes of H2O2, O2, or NADH molecules, which are highly involved in many significant biochemical reactions.7 The signal transduction mechanism and the general performance of electrochemical sensors are often determined by the surface architectures that connect, on the nanometer scale, the sensing element to the biological sample. Bottom-up nanofabrication approaches are typically driven by assembling © 2014 American Chemical Society
Received: August 13, 2013 Revised: February 11, 2014 Published: February 24, 2014 2608
dx.doi.org/10.1021/la402873c | Langmuir 2014, 30, 2608−2618
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water (Millipore Milli-Q gradient A-10 system). Horse heart cytochrome c (Cyt c) was purchased from Sigma. NaH2PO4 was purchased from J. T. Baker. General Protocol for the Seed-Mediated Synthesis of Au NRs. Surfactant-capped Au NRs with a plasmon coupling band at 726 nm were prepared using a suitable modified literature procedure commonly known a seed-mediated growth method.26 First, a seed solution was prepared by mixing two solutions of CTAB (5 mL, 0.2 M) and HAuCl4·3H2O (5 mL, 5 × 10−4 M) at room temperature right after their preparation, and 0.6 mL of a 0.01 M ice-cold aqueous solution of NaBH4 was added under vigorous stirring. The solution color changed from greenish-yellow to brown, and the mixture was stirred for 2 h. The stirring was stopped, and the seed solution containing CTAB-stabilized Au NPs (
