Chemical Strategy to Stepwise Amplification of Signals in Surface Plasmon Resonance Imaging Detection of Saccharides and Glycoconjugates.

A stepwise strategy was proposed to controllably amplify surface plasmon resonance imaging (SPRi) signals and used to establish a method for sensitive detection of small saccharides and large glycoconjugates. The key is to enlarge the target analytes step by step through a cyclic recognition reaction of concanavalin A (ConA) with dextran, which can easily be integrated into SPRi detection. The reaction is theoretically expected to proceed with infinite cycles by addition of ConA and dextran one after another, and in practice, it allows for performing signal amplification for up to 20 steps, which nearly reaches the limit of the propagation depth of surface plasmon waves. Because of the nonspecific adsorption effect, the maximum signals of small sugars were measured at steps 5-7 by the use of small molecules like ethanol amine to block the nonspecific adsorption. The real applicability of the method was validated by SPRi determination of either immobilized saccharides and glycoproteins or captured carcinoembryonic antigen (CEA) from patient samples with a limit of detection down to 2.5 μM glucose or 50 pg/mL of CEA. A wide extendibility of the method was confirmed by detecting other ConA-recognizable analytes like human IgG, ovalbumin, α-fetoprotein, and analytes convertible to ConA- or dextran-recognizable forms. Notably, other cyclic reactions specific to saccharides or other analytes are in theory exploitable, which will further widen the applicability of the method and strategy.