Competitive magnetic immunoassay for protein detection in thin channels. 
Functional magnetic nanoparticles are prepared and characterized for protein detection in a magnetic separation channel.  This detection method is based on a competitive immunoassay of magnetic separation in thin channels using functional magnetic nanoparticles.  We used protein A-IgG complex to demonstrate the feasibility.  Free IgG and fixed number of IgG-labeled microparticles were used to compete for limited sites of protein A on the magnetic nanoparticles.  Several experimental parameters were investigated for protein detection.  The deposited percentages of IgG-labeled microparticles at various concentrations of free IgG were determined and used as a reference plot.  The IgG concentration in a sample was deduced and determined based on the reference plot using the deposited percentage of IgG-labeled microparticles from the sample.  The linear range of IgG detection was from 5.0 x 10(-8) to 1.0 x 10(-11) M. The detection limit was 3.69 x 10(-12) M. The running time was less than 10 min.  Selectivities were higher than 92% and the relative errors were less than 7%.  The IgG concentration of serum was determined to be 3.6 mg ml(-1).  This measurement differed by 8.3% from the ELISA measurement.  The recoveries of IgG spiked in serum were found to be higher than 94%.  This method can provide simple, fast, and selective analysis for protein detection and other immunoassay-related applications.