Fabrication of anionic sulfate-functionalized nanoparticles as an immunosensor by protein immobilization.

Anionic sulfate (SO(4)(-))-functionalized polystyrene (PS) nanoparticles were prepared by the thermal decomposition of potassium persulfate (KPS) in the presence of sodium tetraborate via emulsion polymerization. The presence of a SO(4)(-) group at a solid/liquid interface of a particle surface was confirmed by a zeta potential value of -40.6 mV as well as the shifting of S 2p spectra toward a lower-binding-energy region around 162.7 eV (2p(3/2)) and 164.4 eV (2p(1/2)) in X-ray photoelectron spectroscopy (XPS) analysis. The electrostatic attraction between positively charged antibodies of human immunoglobulin G (hIgG) and cardiac troponin I (cTnI) and negatively charged particle surfaces was accomplished. The atomic force microscopy (AFM) measurement and bicinchoninic acid (BCA) assay results show binding structure between hIgG and antibodies of hIgG (anti-hIgG) with a gradual increase in particle diameter to 152.6 nm (bare), 170.2 nm (hIgG), and 178.9 nm (hIgG/anti-hIgG). Surface coverage densities of 331.4 ng/cm(2) (hIgG) and 320.3 ng/cm(2) (cTnI) and the binding capacity of hIgG to HyLite-750-labeled Fab-specific anti-hIgG (approximately 81.2%) indicate that the majority of hIgG was immobilized with a Y-shaped orientation. The sandwich immunoassay results provide the evidence that the immunological activity of cTnI on the PS nanoparticle surface was retained because the binding activity of the cTnI-PS nanoparticle/cTnI (antigen)/detection cTnI-antibody reaction showed a 5-fold higher activity than that of the cTnI-PS nanoparticle/human serum albumin (HSA)/detection cTnI antibody used as a negative control.