Feasibility of SERS-Active Porous Ag Substrates for the Effective Detection of Pyrene in Water

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous pollutants that are typically released into the environment during the incomplete combustion of fossil fuels. Due to their relevant carcinogenicity, mutagenicity, and teratogenicity, it is urgent to develop sensitive and cost-effective strategies for monitoring them, especially in aqueous environments. Surface-enhanced Raman spectroscopy (SERS) can potentially be used as a reliable approach for this purpose, as it constitutes a valid alternative to traditional techniques, such as liquid and gas chromatography. Nevertheless, the development of an SERS-based platform for detection PAHs has so far been hindered by the poor adsorption of PAHs onto silver- and gold-based SERS-active substrates. To overcome this limitation, several research efforts have been directed towards the development of functionalized SERS substrates for the improvement of PAH adsorption. However, these strategies suffer from the interference that functionalizing molecules can produce in SERS detection. Herein, we demonstrate the feasibility of label-free detection of pyrene by using a highly porous 3D-SERS substrate produced by an inductively coupled plasma (ICP). Thanks to the coral-like nanopattern exhibited by our substrate, clear signals ascribable to pyrene molecules can be observed with a limit of detection of 23 nM. The observed performance can be attributed to the nanoporous character of our substrate, which combines a high density of hotspots and a certain capability of trapping molecules and favoring their adhesion to the Ag nanopattern. The obtained results demonstrate the potential of our substrates as a large-area, label-free SERS-based platform for chemical sensing and environmental control applications.

Keywords: Ag nanostructures; PAH detection; environment monitoring; surface-enhanced Raman spectroscopy (SERS).

Figure 1

UV–Vis reflectance spectrum of the SERS substrate.

Figure 2

Scheme of the preparation of the SERS substrate and sample deposition for SERS measurements. (a) Cleaning and drying of the SERS substrate; (b) parafilm mask composed of nine wells glued on the clean SERS platform; (c) dripping of the pyrene solution into the wells and drying in N${}_2$ atmosphere.

Figure 3

SEM image of a coral-like substrate (scale bar = 1 μm). The inset in the upper-right corner shows the magnification of the SEM image (scale bar = 500 nm), whereas the inset in the lower-left corner shows the PSD.

Figure 4

Comparison of (i) the SERS spectrum of the solvent (water) control acquired on a clean SERS substrate after the evaporation of a distilled water droplet and (ii) the SERS spectrum of a 1 μM pyrene solution. The arrows indicate the presence of further peaks in the SERS spectrum. The Raman spectrum of solid pyrene is also shown (iii) and rescaled with respect to the spectra (i) and (ii) to assure a better visualization.

Figure 5

(a) SERS spectra of a 1 μM pyrene solution from a set of five measurements that were randomly carried out over the surface of the well area $A_w$. The shaded blue area shows the variability of SERS signals corresponding to ∼7%; (b) comparison of the average SERS spectra of pyrene acquired at 0.5 μM (i), 0.05 μM (ii), and 5 nM (iii); (c) plot of the SERS peak intensity at 1029 cm${}^{-1}$ (red) and 3060 cm${}^{-1}$ (blue) versus the concentration of the pyrene solution; (d) linear trend of the signal at 1029 cm${}^{-1}$ in the lower concentration range. The best-fit line is also reported, together with the line’s equation.