Localized and Propagating Surface Plasmon Resonance Sensors: A Study Using Carbohydrate Binding Protein
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Localized and Propagating Surface Plasmon Resonance Sensors: A Study Using Carbohydrate Binding Protein Chanda Yonzon and Richard P. Van Duyne Department of Chemistry, Northwestern University Evanston, IL 60208, U.S.A ABSTRACT This work encompasses a comparative analysis of the properties of two optical biosensor platforms: (1) the propagating surface plasmon resonance (SPR) sensor based on a planar, thin film gold surface and (2) the localized surface plasmon resonance (LSPR) sensor based on surface confined Ag nanoparticles fabricated by nanosphere lithography. The binding of Concanavalin A (ConA) to mannose-functionalized self-assembled monolayers (SAMs) is chosen to illustrate the similarities and the differences of these sensors. A comprehensive set of non-specific binding studies demonstrate that the single transduction mechanism is due to the specific binding of ConA to the mannosefunctionalized surface. Finally, an elementary (2x1) multiplexed version of a LSPR carbohydrate sensing chip to probe the simultaneous binding of ConA to mannose and galactose-functionalized SAMs is also demonstrated. INTRODUCTION The last two decades has seen a tremendous advancement of optical biosensors and their applications in environmental protection,1,2 biotechnology,3 medical diagnostics,4 drug screening,5 food safety,2,6 and security.7 The potential of surface plasmon resonance (SPR) biosensors was realized in early 1980’s by Liedberg and coworkers who were able to sense antibodies by observing the change in the critical angle when the antibodies bound selectively to a Au film.8 Furthermore, in late 1990’s, nanoparticle-based localized surface plasmon resonance (LSPR) sensors have been reported to detect biological9,10 and chemical entities.11 Propagating surface plasmons are evanescent electromagnetic waves bounded by flat smooth metal-dielectric interfaces and arise from oscillations of the conduction electrons in the metal.12 When surface plasmons are confined on either periodic,13 colloidal,11 or other nanosystems,14 localized optical modes are observed. These optical modes lead to highly localized electromagnetic fields outside the particles. Both SPR and LSPR sensors are sensitive to the local refractive index changes that occur when target analyte binds to the metal film or nanoparticles. Surface refractive index sensors have an inherent advantage over optical biosensors that require a chromophoric group or other label to transduce the binding event. Furthermore, they require very little ligand purification due to the specific ligand/receptor binding of these sensors. Also, these sensors provide real-time information on the course of binding and are applicable over a broad range of binding affinities. Additionally, LSPR sensing elements are inherently the size of a single nanoparticle, making the LSPR sensors potentially applicable for in situ detection in biological systems. The sensing capability of LSPR sensors can also be tuned by changing the shape, size and material composition of the nanoparticles.15,1
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