Multifunctional human serum albumin in the surface-enhanced Raman spectroscopy of porphyrin: demetalation promoter, molecular spacer and stabilizer

Human serum albumin (HSA), a model protein, was introduced to the surface-enhanced Raman spectroscopy (SERS) of cationic porphyrin 5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21H,23H-porphine (H(2)TMPyP4). HSA was found to have a great influence not only on Ag nanoparticle aggregation state but also on the interaction between Ag nanoparticle and H(2)TMPyP4 molecules. In the (H(2)TMPyP4-Ag colloid)/HSA system, addition of H(2)TMPyP4 to Ag colloid led to a quick Ag colloid aggregation, and subsequent HSA addition could stabilize this system. The SERS spectrum was dominated by a combination of Ag(II)TMPyP4 and free base H(2)TMPyP4. More interestingly, a photoinduced demetalation of Ag(II)TMPyP4 to free base H(2)TMPyP4 was observed in the (H(2)TMPyP4-Ag colloid)/HSA system. This demetalation process was partially reversible when the laser was turned off or the laser power was reduced. In this case, HSA acts as both a stabilizer and a demetalation promoter. In the (HSA-H(2)TMPyP4)/Ag colloid system, when H(2)TMPyP4 was premixed with HSA prior to the Ag colloid addition, no obvious Ag colloid aggregation appeared, and the SERS spectrum was just characteristic of free base H(2)TMPyP4. In this case, HSA is proposed to function as both a stabilizer and a molecular spacer. Copyright (C) 2010 John Wiley & Sons, Ltd.