Sensitivity enhancement of optical immunosensors with nanoparticles

In recent years, several optical sensor techniques have been developed for the direct monitoring of biomolecular recognition processes at the surface of a sensor chip. Applications of these immunosensors for the determination of substances in serum could be demonstrated only for a few analytes due to the lack of sensitivity. Beside nonspecific binding of serum components to the sensor surface, the analytical sensitivity of these sensors is limited by the molecular weight of the analyte, so that smaller analyte molecules give only a moderate sensor response. In order to enhance the sensor signal, the use of mass labels, such as latex particles, was proposed in the literature. However, detection limits comparable to those of conventional ELISA techniques could not be realized so far. We demonstrate the optimization of a "nanoparticle enhanced immunosensor assay" for the detection of thyroid stimulating hormone, with respect to the particle coating, size, and nonspecific binding. The developed prototype assay requires a sample volume of 225 microL and has a measuring range up to 35 mIU/L. For the first time, we obtained a detection limit of 0.03 mIU/L (0.1 pm), which is fully competitive to conventional ELISA techniques. The assay allows serum samples to be measured with good precision and dilution linearity. The sensor can be reused several times and shows an excellent correlation to a commercial enzyme immunoassay.