Polyelectrolyte polyacrylic acid (PAA), used in the chemical modification of titanium dioxide (TiO(2)) nanoparticles, allows TiO(2) nanoparticles to remain in suspension at neutral pH. The anti-17beta-estradiol (E2) antibody was immobilized on PAA-modified TiO(2) (PAA-TiO(2)) nanoparticles via covalent bonding between the carboxylic acid of PAA and the amino group of the antibody. The anti-E2-antibody-immobilized TiO(2) (E2Ab-PAA-TiO(2)) nanoparticles can form a suspension at neutral pH, with a particle size of less than 100 nm. The E2-PAA-TiO(2) nanoparticles caused the photocatalytic degradation of a typical TiO(2) substrate, methylene blue. The anti-E2 antibody immobilized on the TiO(2) surface recognized and bound E2 in the solution, thereby improving the efficiency of E2 degradation compared with that of PAA-TiO(2) nanoparticles. These results demonstrate that the E2Ab-PAA-TiO(2) nanoparticles developed in this study can be used in water treatment technology. Furthermore, this strategy of immobilizing proteins on nanoscale TiO(2) particles creates new applications not only in the treatment of environmental waste, but also in medical and public sanitation processes.