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Surface Plasmon Resonance: Signal Amplification Using Colloidal Gold Nanoparticles for Enhanced Sensitivity

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Surface Plasmon Resonance: Signal Amplification Using Colloidal Gold Nanoparticles for Enhanced Sensitivity Rev Anal Chem 2014; 33(3): 153–164 Sabine Szunerits*, Jolanda Spadavecchia and Rabah Boukherroub Surface plasmon resonance: signal amplification using colloidal gold nanoparticles for enhanced sensitivity Abstract: Among all the detection methods developed for plasmons. The unique optical, electronic, and catalytic bioassays, optical-based approaches are the most popu- properties of gold nanoparticles put them into the fore- lar techniques. A widely used and accepted bioanalyti- front of interest for signal amplification as will be outlined cal technique for routine characterization of molecular here. recognition events at a solid interface is surface plasmon resonance (SPR) spectroscopy. Compared with other tech- Keywords: biosensing; gold nanostructures; localized niques, SPR offers a series of advantages including rapid surface plasmon resonance (LSPR); signal amplification; and real-time analysis, in situ detection, and the possibil- surface plasmon resonance (SPR). ity to determine kinetic as well as thermodynamic param- eters of interacting partners without the requirement of DOI 10.1515/revac-2014-0011 tagging one of the two partners. This method is nowadays Received April 16, 2014; accepted July 13, 2014 used in academic and industrial research laboratories for drug discovery processes, safety, and quality con- trol applications as well as for understanding bioaffinity reactions. Past studies have shown that SPR can detect Introduction approximately 100 ng/l of proteins. Currently, much effort is still invested to further improve SPR with respect to the It was in 1983, that Liedberg, Nylander, and Lundström sensitivity in the detection of low molecular weight ana- proposed a new optical concept for the sensing of affin- lytes as well as in the detection limit for different analyte/ ity reactions based on the determination of tiny varia- ligand reactions. This review will focus on the use of gold tions of the refractive index in the surrounding medium nanoparticles as an amplification strategy in SPR sensing. of a thin gold film interface, when illuminated with light Compared to other review articles highlighting the several under a certain angle (Liedberg et al. 1983). Made com- synthetic routes and properties of gold nanoparticles and mercially available in 1990 by Biacore® (GE Healthcare), research utilizing gold nanoparticles for various sensing SPR spectroscopy is now an accepted bioanalytical tech- strategies, this review will exclusively look at the impact nique for routine characterization of molecular recogni- of gold nanostructures for ultrasensitive SPR sensing. Sig- tion events at a solid interface. It has, in particular, proven nal enhancement by gold nanoparticles is caused by sev- to be a powerful approach for the determination of kinetic eral effects such as surface mass increase due to enhanced parameters (association and dissociation rate constants) surface area, larger refractive index changes by the par- as well as thermodynamic parameters, including affinity ticle mass, themselves, and electromagnetic field cou- constants of interacting partners without the requirement pling between the plasmonic properties of the particles of tagging one of the two partners. With such SPR-based (localized surface plasmon resonance) and propagating affinity sensors, it is now easy and quite fast to acquire the relevant data that serve as a basis to understand biologi- cal specificity and function. *Corresponding author: Sabine Szunerits, Institut de Recherche Interdisciplinaire (IRI, USR 3078 CNRS), Université Lille 1, Parc de The basis for the ability of SPR to determine binding la Haute Borne, 50 Avenue de Halley, BP 70478, 59658 Villeneuve affinities and kinetic parameters lies in the high sensitivity d’Ascq, France, e-mail: [email protected] of surface plasmons excited at the metal-dielectric interface Jolanda Spadavecchia: Laboratoire de Réactivité de Surfaces, UMR to changes in the refractive index of the adjacent medium. CNRS 7197, Université Pierre and Marie Curie, Paris VI, Site d’Ivry, Le Surface plasmons, also often known in the literature as Raphaël, 94200 Ivry-sur-Seine, France Rabah Boukherroub: Institut de Recherche Interdisciplinaire (IRI, surface plasmon polaritons (SPPs) or surface plasmon USR 3078 CNRS), Université Lille 1, Parc de la Haute Borne, 50 waves (SPWs), are longitudinal charge-density distributions Avenue de Halley, BP 70478, 59658 Villeneuve d’Ascq, France generated at the metal/dielectric interface (typically gold 154 S. Szunerits et al.: Impact of gold nanoparticles for ultrasensitive SPR sensing or silver) when light propagates through the metal. Such AB SPPs are transversal magnetic waves that propagate parallel along the interface between the metal and a dielectric layer (e.g., air, water, etc). According to the Drude model, the dis- n persion relation of an SPP, which essentially correlates the n+∆n Light intensity relationship between the wave vector along the interface and the angular frequency ω, can be described by: SPR Biosensor Wavelength or incident angle Light line SPP C Analyte flow ω kSP Change in SPR signal 1/2 ω εε k = md (1) SP c εε+ Time md Figure 1 (A) Excitation of surface plasmons by the attenuated total where kSP is the component of the wave vector parallel to reflection (ATR) method in the so-called Kretschmann configuration; the interface, c is the speed of light in a vacuum, and εm and (B) illustration of angular modulation as the refractive index of the sensed dielectric changes from n (solid line) to n+Δn (dashed line); εd are the dielectric constants of the metal and dielectric, respectively. To allow optical excitation of surface plas- (C) SPR sensorgram angle change can be monitored as a function of time. mons, as kSP is outside the light cone, the SPP wave cannot be directly excited from the far field by a plane wave, and the wave vector of the incident light must be increased. Physically, this can be achieved by passing the light wave Many approaches have been explored to improve through an optically denser medium in the attenuated the LOD of propagating SPR such as the use of fluores- total reflection (ATR) configuration (Figure 1A). The suffi- cence (Yu et al. 2004), modification of the sensor chip ciently thin metal film allows some of the incident light to with metallic particles (Lyon et al. 1998), or performing be transmitted through the metal-air boundary, where the a labeled assay using large particles (He et al. 2000). surface plasmon is excited. Prism-based angular modula- Among these, the use of gold nanoparticles (Au NPs) has tion with a conventional SPR structure consisting of a high attracted significant attention. Unlike propagating plas- refractive index glass prism and a thin metallic layer (typi- mons supported on a bulk metal surface, nanoparticles’ cally around 50 nm of gold) has become the most com- plasmons are quantized electron oscillations confined to monly used configuration in SPR sensing technology and nanoscale volumes, which provide a means for manipu- is commercialized by several companies. lating light-matter interactions while circumventing the The excitation of surface plasmons is characterized by diffraction limit. Although Mie’s theory of light scatter- a dip in the angular spectrum of the reflected light at the ing and absorption by gold nanospheres was reported angle of resonance (Figure 1B). The promising potential of more than a century ago (Mie 1908), it is only recently SPR sensors lies in the high sensitivity of the position and that improved computational approaches and theoretical shape of the SPR dip to a change in the refractive index of developments have allowed the mapping of plasmon reso- the dielectric layer or formation of a nanoscale-thick film nances in more complex nanoparticles and assemblies. In (Wijaya et al. 2011, Szunerits et al. 2012, 2013). particular, a new theoretical approach known as plasmon While this technique has proven to be a robust tool hybridization theory (Prodan et al. 2003) draws on the and is now used in research as well in industrial laborato- parallels between the behavior of plasmons in assemblies ries, much effort is still invested to improve SPR, especially of metallic nanoparticles and that of electrons in quantum with respect to sensitivity in the detection of low molecu- molecular orbitals. This theory predicts that the plasmons lar weight analytes. Low molecular analytes ( < 1000 Da) on neighboring metallic nanostructures interact, mix, and are more difficult to quantify by SPR than larger ones hybridize just like the electronic wave functions of simple (Mann et al. 1998). atomic and molecular orbitals. A critical prerequisite for S. Szunerits et al.: Impact of gold nanoparticles for ultrasensitive SPR sensing 155 the use of these materials is the production of high-quality in the collective oscillation of electrons, termed localized metallic nanoparticles with tunable size and controllable surface plasmon (LSP) (Figure 2A) (Szunerits and Bouk- shape to tailor their distinct plasmonic signatures. This herroub 2012, Akjouj et al. 2013). The LSP has two major can be achieved through wet chemical synthesis tech- effects. The electrical fields near the particle’s surface are niques, in which careful optimization of synthesis condi- greatly enhanced, and the particle’s optical extinction tions allows rational control over the nanoparticles’ size displays a maximum at the plasmon resonance frequency, and morphology (Pèrez-Juste et al. 2005, Xia et al. 2009). occurring in the visible wavelength for noble metal parti- Before illustrating the examples of Au NP-enhanced SPR cles (Figure 2B). Gold nanoparticles are the most widely sensing, the synthetic aspects of gold nanostructures and used ones as they are chemically stable and resistant to the way they enhance the SPR signal will be discussed in surface oxidation. more details. Numerous preparation methods for Au NPs have been reported, including both top-down and bottom-up approaches (Daniel and Astruc 2009, Mayer and Hafner 2011, Saha et al. 2012). They allow to form gold nanostruc- Interest of gold nanoparticles for tures of different sizes, shapes, and composition.
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