SE(R)RS microspectroscopy of porphyrins on immobilized Au nanoparticles: Testing spectral sensitivity and reproducibility

Although surface-enhanced (resonance) Raman scattering SE(R)RS spectroscopy is currently employed as a (bio)analytical technique, designing and optimizing of cheaply and easily available SERS-active substrates are still in progress. In this paper, we tested SERS substrates prepared by immobilization of Au colloidal nanoparticles via aminosilane on glass from the point of view of their spectral sensitivity and reproducibility. This preparation procedure is easy, cheap and provides uniform SERS substrates on a large scale. We measured SE(R)RS spectra of free-base cationic 5,10,15,20-tetrakis(1-methyl-4-pyridyl)porphyrin (TMPyP) as a testing biomolecule using a confocal Raman microspectrometer and two excitation wavelengths (514.5 and 632.8 nm). SERS microspectroscopy had several advantages in comparison to macro-Raman technique: namely the possibility of spectral mapping over the surface, shorter collection times and the absence of strong Raman bands from the glass support. Analytical enhancement factor (AEF) of Au substrates between 105 and 106 and limits of detection (LOD) of TMPyP ∼5 × 10−8 M were determined. Excellent spectral reproducibility of Au substrates (relative standard deviation of signal ∼15%) was proved. Immobilized Au colloidal nanoparticles can be therefore considered as suitable substrates for SE(R)RS (bio)analytical applications.