Surface enhanced raman scattering from Au and Ag nanoparticle coated magnetic microspheres

Abstract

A novel SERS substrate was prepared by coating Au or Ag nanoparticles onto magnetic microspheres prepared by a modified suspension polymerization method. The micron-sized magnetic microspheres were prepared in two steps: In the first step, inorganic core which consisted of oleic acid coated magnetic magnetite nanoparticles were prepared by co-precipitation method. The second step was the encapsulation of oleic acid coated magnetite nanoparticles by a modified suspension polymerization method. Magnetic microspheres were modified with amine functional groups in order to immobilize Au or Ag nanoparticles onto magnetic microspheres via amine groups of magnetic microspheres, however, a high background signal was obtained in Raman measurements due to the amine groups. Alternatively, Au or Ag nanoparticles were coated directly onto magnetic microspheres by hydroxylamine and sodium borohydrate reduction methods for Au nanoparticle coating and sodium borohydrate for Ag nanoparticle coating. For the first time, Au and Ag nanoparticle coated magnetic microspheres were prepared and used as SERS substrate successfully. The magnetic microspheres were characterized by Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), Energy dispersive X-Ray spectroscopy (EDX) attached to SEM, Raman spectroscopy and X-Ray Diffraction (XRD). The average size of magnetic microspheres is measured to be 22 µm from their SEM images. EDX analysis demonstrated that magnetic microspheres were coated with Au or Ag nanoparticles. Moreover, commercially available amine functionalized magnetic microspheres were immobilized with Au nanoparticles and its SERS activity was significantly than the Au nanoparticle coated magnetic microspheres prepared in this study. Enhancement factors for Au and Ag nanoparticle coated magnetic microspheres were calculated to be ca. 105 and 107 , respectively, however, in case of Au nanoparticle immobilized Spherotech magnetic microspheres, enhancement factor was only 2x102 using Rhodamine 6G as SERS probe. Interactions of aspartic acid with Ag and Au nanoparticles were followed by Raman spectroscopy at various pH values. pH dependent interactions of aspartic acid with Au and Ag metals were followed depending on pH for the first time. Protonation or deprotonation of amine or carboxyl groups on aspartic acid depending on pH of the solution affects the interacting functional groups with metal nanoparticles and increase in the signal of the corresponding group was measured. It is found that aspartic acid interacts through amine and carboxyl groups with Ag surface at low pH values and via only carboxyl groups at higher pH values. However, aspartic acid interacts with Au surface through amine and carboxyl groups at all pH values under investigation.