SERS microRaman spectral probing of adsorbate-containing, liquid-overlayed nanosponge Ag aggregates assembled from fractal aggregates

Abstract

Adsorbate-containing, nanosponge Ag aggregates overlayed by a thin (~1.5 mm) liquid layer are reported as a new type of sample for Surface-enhanced Raman scattering (SERS) microRaman spectral measurements and adsorbate (analyte) detection. Macroscopic Ag aggregates (of about 1.5 × 1.0 × 0.025 mm size) with the nanosponge internal morphology (revealed by Scanning electron microscopy (SEM)) were prepared by 3D assembling of fused fractal aggregates (D = 1.84 ± 0.04) formed in Ag nanoparticle hydrosol/HCl/adsorbate systems with 2,2’-bipyridine (bpy) and/or a cationic free-base tetrakis(2-methyl-4-pyridiniumyl) porphine (H2TMPyP) as the testing adsorbates. For SERS microRaman measurements, the macroscopic aggregate was overlayed by a thin (~1.5 mm) layer of the residual liquid. Preparation procedure, nanoscale imaging, and SERS spectral probing including the determination of the detection limits of the adsorbates revealed the following advantages of the adsorbate-containing, liquid-overlayed 3D nanosponge aggregate as a sample for SERS microRaman spectral measurements: (1) localization of adsorbate (analyte) into hot spots and, simultaneously, prevention of the analyte decomposition during the spectral measurement (carried out without an immersion objective), (2) fast and simple sample preparation, and (3) minimization of sample volume and an efficient concentration of hot spots into the focus of the laser beam. The advantages of the nanosponge Ag aggregates are further demonstrated by the 40 fmol limit of detection of bpy as Ag(0)-bpy surface complex, as well as by preservation of the native structure of the cationic free-base porphyrin H2TMPyP. Copyright © 2015 John Wiley & Sons, Ltd.