Facile Detection of Polycyclic Aromatic Hydrocarbons by a Surface- Enhanced Raman Scattering Sensor Based on the Au Coffee Ring Effect Jianwei Xu, †,‡ Jingjing Du, ‡ Chuanyong Jing,* ,‡ Yongli Zhang,* ,† and Jinli Cui ‡ † College of Architecture and Environment, Sichuan University, Chengdu 610065, People’s Republic of China ‡ State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences (RCEES), Chinese Academy of Sciences (CAS), Beijing 100085, People’s Republic of China * S Supporting Information ABSTRACT: 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. KEYWORDS: SERS, coffee ring effect, Au nanoparticles, PAHs ■ INTRODUCTION Surface-enhanced Raman scattering (SERS) is a rapid and ultrasensitive spectroscopic technique in chemical analysis, taking advantage of the explosive growth in nanofabrication. 1-3 Great efforts have been made in the attempt to synthesize SERS-active noble metal nanoparticles (NPs) with tunable size, shape, and functionality. These NPs deposited on solid surfaces can form closely packed but non-aggregated arrays to promote substantial SERS enhancement. 4 Thus, a broad diversity of techniques have been exploited to fabricate periodic and reproducible arrays, including spin-coating, 5 electron beam lithography, 6 vapor deposition, 7 and self-assembly on function- alized surfaces. 8 These techniques result in NPs with exceptional SERS enhancement and high reproducibility. However, stringent and laborious protocols and expensive equipment are the two minimum prerequisites to reliably control the SERS substrate characteristics. 9-11 Therefore, developing a facile and robust SERS sensor for environmental monitoring purposes motivates our research. An intrinsic difficulty in environmental analysis is separating trace amounts of analytes from complex matrices, and this difficulty is also a factor in SERS implementation. For example, rigorous sample pretreatment is generally required before identification and quantification of polycyclic aromatic hydro- carbons (PAHs) using expensive instruments, including high- performance liquid chromatography (HPLC), gas chromatog- Received: February 1, 2014 Accepted: April 10, 2014 Published: April 10, 2014 Research Article www.acsami.org © 2014 American Chemical Society 6891 dx.doi.org/10.1021/am500705a | ACS Appl. Mater. Interfaces 2014, 6, 6891-6897