Polyethylene-glycol-Stabilized Ag Nanoparticles for Surface-Enhanced Raman Scattering Spectroscopy: Ag Surface Accessibility Studied Using Metalation of Free-Base Porphyrins

Silver nanoparticles (Ag NPs) stabilized with a permeable layer of long-chain polyethylene glycol (PEG) represent a new generation of surface-enhanced Raman scattering (SERS) substrates for bioanalytical applications. This paper will shed light on their efficiency in the SERS detection of biomolecules. Two types of thiol-terminated PEG (average molecular weight 5000 g/mol) were used. Metalation of free-base porphyrins (10 –9 –10 –6 M) of different size and charge was employed as the probe of accessibility of the Ag-PEG NP surface. The influence of NaCl (0.01 M) on the system was examined. The metalation was significantly decreased by the interaction of porphyrins with propionylamino groups of PEG1, while no interaction with the neutral chain of PEG2 was observed. Sterical hindrance of the porphyrin side groups seemed to cause a rather perpendicular porphyrin orientation on both types of Ag-PEG NP surfaces, inhibiting the metalation. Moreover, chloride anions influenced the structure of the polymer coating and improved the porphyrin accessibility on the Ag-PEG1 NPs. Finally, the studied Ag-PEG NPs remained isolated even after addition of porphyrins and chlorides, therefore being favorable for SERS applications inside the living cells.