<title>Surface plasmon resonance imaging instrumentation and data handling for biochips: review and perspectives</title>

Abstract

ABSTRACT Sensors based on surface plasmon resonance have the potential to provide information on the binding of biologicalmolecules on adequate substrates over typically thousand channels in parallel, without the need for any marker and inreal time compared to the scale of biochemical reactions. The need to optimize selectivity and sensitivity has triggeredcontinued research efforts. We review those related with optics and image processing, at the same time identifying someaspects that deserve further investigation before the potential ofthe technique is fully utilized.Keywords: surface plasmon resonance imaging, biosensors, biochips, image processing 1. INTRODUCTION Since first proposed by Chen and Chen (1), the sensitivity of surface plasmon resonances (2-8) occurring around theinterface between a metal layer and a dielectric to changes in the dielectric refractive index has been used for sensingminute changes in the latter, in particular for applications to biochips, the topic of this communication.Biochips are devices designed to detect and/or quantify biological interactions occurring at the surface of a suitablyprepared substrate. Applications include research in molecular biology, medical tests, agriculture, water and airpollution, and safety. That broad list suggests the rather large variety of molecular interactions that may come into play.Typically, the biochemical reaction of interest involves two molecules interacting through identified sites in a fluidmedium, one of which is in fact bound to a solid substrate. A major aspect of the biochip technology is therefore thedesign and optimization of biochemical processes that can bind one of the two reagents to the surface while minimallyperturbing the reaction of interest. At the interface between the substrate and the fluid, some physical transductionprocess is used to monitor the result through some physical quantity.In optical biochips, our focus, the transduction process is an optical phenomenon, in most cases fluorescence or surfaceplasmon resonances. The present review concentrates on surface plasmon resonances but considers fluorescence as wellfor comparison purposes. The chemistry for binding reagents to the glass surface typically used for fluorescencebiochips is very different from that appropriate for the metal layers that can support surface plasmons, and thatdifference in chemistry is obviously one major aspect of the comparison. However, in this work we concentrate on theoptical instrumentation and data processing aspects rather than on chemistry. Let it suffice to mention that our recentwork on surface plasmon resonance biochips has relied on the avidine/biotine system (9), on electropolymerizationbased on pyrroles (10), and on thiol bounding on gold (11, 12, 13). In the following, after briefly reviewing thephenomenon of surface plasmon resonance, we address the peculiarities of surface plasmon resonance imaging (SPRI),i.e. the fact that the phenomenon can be used on at least several hundreds of spots per squared centimeter in parallel onthe same biochip. Appropriate optical setups are mentioned. We then list the main comparison parameters betweenSPRI biochips and fluorescence biochips, stressing the real time capacity of SPRI and the fact that it does not requireany marker. Finally, we consider the issue of processing the image data from the photodetector array used to acquire theSPRI image and point out that the ultimate sensitivity of the method may not have been reached yet.