Surface plasmon resonance (SPR) is a phenomenon occuring at metal surfaces (typically gold and silver) when an incident light beam strikes the surface at a particular angle. Depending on the thickness of a molecular layer at the metal surface, the SPR phenomenon results in a graded reduction in intensity of the reflected light. Biomedical applications take advantage of the exquisite sensitivity of SPR to the refractive index of the medium next to the metal surface, which makes it possible to measure accurately the adsorption of molecules on the metal surface and their eventual interactions with specific ligands. The last ten years have seen a tremendous development of SPR use in biomedical applications. The technique is applied not only to the measurement in real-time of the kinetics of ligand–receptor interactions and to the screening of lead compounds in the pharmaceutical industry, but also to the measurement of DNA hybridization, enzyme–substrate interactions, in polyclonal antibody characterization, epitope mapping, protein conformation studies and label-free immunoassays. Conventional SPR is applied in specialized biosensing instruments. These instruments use expensive sensor chips of limited reuse capacity and require complex chemistry for ligand or protein immobilization. Our laboratory has successfully applied SPR with colloidal gold particles in buffered solution. This application offers many advantages over conventional SPR. The support is cheap, easily synthesized, and can be coated with various proteins or protein–ligand complexes by charge adsorption. With colloidal gold, the SPR phenomenon can be monitored in any UV-vis spectrophotometer. For high‒throughput applications, we have adapted the technology in an automated clinical chemistry analyzer. This simple technology finds application in label-free quantitative immunoassay techniques for proteins and small analytes, in conformational studies with proteins as well as in the real-time association-dissociation measurements of receptor–ligand interactions, for high-throughput screening and lead optimization.

We propose a high-throughput screening method which involves colloidal gold nanoparticles sensitized with the binding protein. Upon interaction with a specific ligand (a polypeptide or a small organic molecule), the surface plasmon resonance absorbance peak of the colloidal gold reagent shifts toward longer wavelengths due to the change in refractive index at the particle surface caused by changes in mass. The shift is proportional to the dose of ligand involved for a fixed amount of binding protein and occurs according to the kinetics of interaction. We applied this property to the analysis of association and dissociation of ligand-binding protein interactions in a small random access clinical chemistry analyzer. The instrument measures the changes in A 600 nm over a period of 20 min for each sample. Due to the high degree of automation, the instrument throughput amounts to 144 samples an hour and can be run during 24 h a day in a walk-away mode. When connected to a computer for data handling, a single instrument can consequently handle over 3000 samples a day. Higher throughput instruments are available which can handle as much as ten times more samples. We validated the technique by comparing the affinity constants (range 10 3 210 12 mol 21 ) calculated for 30 pairs of ligand-protein interactions at different ligand doses with those obtained from other methods, including the BIAcore (slope 0.84; coefficient of correlation r = 0.82).

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The post-genomic era is revolutionizing the drug discovery process. The new challenges in the identification of therapeutic targets require efficient technological tools in order to be properly addressed. Label-free detection systems use proteins or ligands coupled to materials of which the physical properties are measurably modified upon specific interactions. Among the label-free systems currently available, the use of metal nanocolloids offers enhanced throughput and flexibility for real-time biomolecular recognition monitoring at a reasonable cost.