Talanta 85 (2011) 35–42 Contents lists available at ScienceDirect Talanta journal homepage: www.elsevier.com/locate/talanta Biosensors elaborated on gold nanoparticles, a PM-IRRAS characterisation of the IgG binding efficiency Anne-Laure Morel a,b , Souhir Boujday a,b , Christophe Méthivier a,b , Jean-Marc Krafft a,b , Claire-Marie Pradier a,b,∗ a Université Pierre et Marie Curie, Paris 6, UMR CNRS 7197, Laboratoire de Réactivité de Surface, F75005 Paris, France b CNRS, UMR 7197, Laboratoire de Réactivité de Surface, F75005 Paris, France article info Article history: Received 6 October 2010 Received in revised form 11 February 2011 Accepted 20 February 2011 Available online 24 February 2011 Keywords: Gold nanoparticles Immunosensors AFM PM-IRRAS SERS abstract This work is focused on studying the grafting of gold nanoparticles (Np) on a cystamine self-assembled monolayer on gold, in order to build sensitive immunosensors. The synthesis and deposition of gold nanoparticles, 13 and 55 nm sizes, were characterised by combining Polarisation Modulation Infrared Reflection–Absorption Spectroscopy (PM-IRRAS), X-ray Photoelectron Spectroscopy (XPS) Sur- face Enhanced Raman Scattering (SERS), and Atomic Force Microscopy (AFM) which all indicated the formation of a dispersed layer of nanoparticles. This observation is explained by the compromise between the high reactivity of amine-terminated layers towards gold, and interparticle repulsions. Nps were then functionalised with antibody probes, and the recognition by an anti-rIgG was assayed both on planar and Np gold surfaces. The important result is that nanoparticles of 55 nm are preferable for the following reasons: they enable to build a denser and well dispersed layer and they increase both the number of receptors (IgGs) and their accessibility. Beside these geometric improvements, a net enhancement of the Raman signal was observed on the 55 nm nanoparticle layer, making this new platform promising for optical detection based biosensors. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The use of gold nanoparticles (AuNps) is particularly attractive in biological and medical fields for imaging or optical detection enhancement thanks to the unique properties of surface plasmon resonance (SPR) and resonance light scattering, while they are bio- compatible and easily chemically modified [1]. After recalling the principle of bioconjugate chemistry on gold, as well as of AuNp- based techniques like SPR and SERS, Astruc and Boisselier describe, in a recent review, the fantastic interest of AuNps for medical diagnosis and therapeutic applications [2]. As an example, AuNps, conjugated to antibodies, may be used as cancer biomarkers; alter- natively, they may be stabilized by dendrimers thus leading to supramolecular properties applicable to encapsulation or specific substrate-specific interactions [3]. Recent developments in the syn- thesis and optical properties of gold nanoparticles are given in reference [4]. ∗ Corresponding author at: Université Pierre et Marie Curie, Paris 6, UMR CNRS 7197, Laboratoire de Réactivité de Surface, F75005 Paris, France. Tel.: +33 1 44 27 55 33. E-mail address: claire-marie.pradier@upmc.fr (C.-M. Pradier). Gold nanoparticles have been used for improving the sensitiv- ity of biosensors thanks to their optical properties or because they enable easy electrochemical detection of biomolecular recognition phenomena [5]. Various procedures to modify electrodes by gold NPs have been implemented and compared, showing the interest of immobilising AuNp on thiol monolayers [6,7], or using Nps assem- bled on a polymer grafted on an electrode [8]. Another example reports the construction of an amperometric immunosensor on a mixed NH 3 + /SH-terminated SAM layer, modified with gold Nps, taking advantage of the high affinity of gold to either of these two functions [9]. Antibodies are subsequently directly immobilised on the gold Nps, by simple incubation in an antibody solution, resulting in an efficient immunosensor for -fetoprotein; the enhanced num- ber and accessibility of probes were mentioned but not measured in this case [6]. In a very pioneer work, Natan and co-workers demonstrated the possibility to take advantage of the electronic coupling between gold planar films and gold nanoparticles to obtain larger changes in reflectivity and, thus, amplified SPR signals [10]. More recently, gold nanoparticles were immobilised on a dithiol layer or within a protein–polymer mixture, both methods resulting in the enhanced sensitivity of surface plasmon resonance biosensors [11,12]. The interest of building an immunosensor on gold nanoparticles, in particular the increase of the number of molecular receptors 0039-9140/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2011.02.028