Purity check: The self‐assembly of magnetic nanoparticle sensors in the presence of stereoselective antibodies causes a decrease in the T2 relaxation time of surrounding protons. The introduction of enantiomeric impurities disrupts the antibody/magnetic‐sensor binding and increases the T2 relaxation time, which can be monitored by magnetic resonance imaging (see picture).

The use of microfabricated cantilevers as bioaffinity sensors was investigated. Since many bioaffinity interactions involve proteins as receptors, we conducted studies of the magnitude, kinetics, and reversibility of surface stresses caused when common proteins interact with microcantilevers (MCs) with nanostructured (roughened) gold surfaces on one side. Exposure of nanostructured, unfunctionalized MCs to the proteins immunoglobulin G and bovine serum albumin (BSA) resulted in reversible large tensile stresses, whereas MCs with smooth gold surfaces on one side produced reversible responses that were considerably smaller and compressive. The response magnitude for nanostructured MCs exposed to BSA is shown to be concentration dependent, and linear calibration over the range of 1-200 mg/L is demonstrated. Stable, reusable protein bioaffinity phases based on unique enantioselective antibodies are created by covalently linking monoclonal antibodies to nanostructured MC surfaces. The direct (label-free) stereoselective detection of trace amounts of an important class of chiral analytes, the alpha-amino acids, was achieved based on immunomechanical responses involving nanoscale bending of the cantilever. The temporal response of the cantilever (delta deflection/delta time) is linearly proportional to the analyte concentration and allows the quantitative determination of enantiomeric purity up to an enantiomeric excess of 99.8%. To our knowledge, this is the first demonstration of chiral discrimination using highly scalable microelectromechanical systems.

Based on the stereoselectivity of immunoglobulins, we have developed a new chiral sensor for the detection of low-molecular-weight analytes. Using surface plasmon resonance detection, enantiomers of free, underivatized alpha-amino acids can be monitored in a competitive assay by their interaction with antibodies specific for the chiral center of this class of substances. The sensitivity to the minor enantiomer in nonracemic mixtures exceeds currently available methods; therefore, such immunosensors can readily detect traces of enantiomeric impurities and are attractive for a range of applications in science and industry.