Polyelectrolyte polyacrylic acid (PAA), used in the chemical modification of titanium dioxide (TiO2) nanoparticles, allows TiO2 nanoparticles to remain in suspension at neutral pH. The anti-17 beta-estradiol (E2) antibody was immobilized on PAA-modified TiO2 (PAA-TiO2) nanoparticles via covalent bonding between the carboxylic acid of PAA and the amino group of the antibody. The anti-E2-antibody-immobilized TiO2 (E2Ab-PAA-TiO2) nanoparticles can form a suspension at neutral pH, with a particle size of less than 100 nm. The E2-PAA-TiO2 nanoparticles caused the photocatalytic degradation of a typical TiO2 substrate, methylene blue. The anti-E2 antibody immobilized on the TiO2 surface recognized and bound E2 in the solution, thereby improving the efficiency of E2 degradation compared with that of PAA-TiO2 nanoparticles. These results demonstrate that the E2Ab-PAA-TiO2 nanoparticles developed in this study can be used in water treatment technology. Furthermore, this strategy of immobilizing proteins on nanoscale TiO2 particles creates new applications not only in the treatment of environmental waste, but also in medical and public sanitation processes.