Surface Enhanced Raman Spectroscopy of 4-Mercaptopyridine Molecules on Pb3 O4 Nanoparticles1 1 Supported by the National Natural Science Foundation of China (Nos. 20273022, 20375014 and 20473029), Program for Changjiang Scholars and Innovative Research Team in University, Program for New Century Excellent Talents in University and the Innovative Scholars of Jilin University (No. 2004CX035).