Facile detection of polycyclic aromatic hydrocarbons by a surface-enhanced Raman scattering sensor based on the Au coffee ring effect.

Surface-enhanced Raman scattering (SERS) analysis of environmental hydrophobic pollutants without chemical functionalization of a bare nanoparticle (NP) substrate presents a challenge. The motivation for our study is to develop a highly reproducible and robust portable SERS sensor for detection and identification of polycyclic aromatic hydrocarbons (PAHs) using bare Au NPs. Our hypothesis is that the coffee ring effect could separate PAHs from the bulk solution and concentrate them on the closely packed Au NP ring, consequently enhancing their Raman scattering. This premise was confirmed with the commonly used citrate-reduced Au NPs in 20 nm, having no structural uniqueness. Because of the coffee ring effect, however, closely packed but not aggregated Au NP arrays were formed and, consequently, facilitated the separation and concentration of hydrophobic PAHs. As a result, a prominent SERS enhancement can be obtained on the ring because of the electromagnetic mechanism. A mixture of six PAHs with different numbers of benzene rings, namely, naphthalene, anthracene, pyrene, benzo[a]pyrene, benzo[g,h,i]perylene, and indeno[1,2,3-cd]pyrene, could be readily identified in river water. This portable SERS sensor based on the coffee ring effect provides a robust and versatile approach in PAH detection without the need for stringent structural requirements for Au NPs.