Large-Area Organization of pNIPAM-Coated Nanostars as SERS Platforms for Polycyclic Aromatic Hydrocarbons Sensing in Gas Phase Mareen Mueller, † Moritz Tebbe, † Daria V. Andreeva, † Matthias Karg, ‡ Ramon A. Alvarez Puebla, § Nicolas Pazos Perez,* ,† and Andreas Fery* ,† † Physical Chemistry II Department, University of Bayreuth, 95440, Bayreuth, Germany ‡ Physical Chemistry I Department, University of Bayreuth, 95440, Bayreuth, Germany § Departamento de Quimica Fisica, Universidade de Vigo, 36310, Vigo, Spain ABSTRACT: Here, a new surface enhanced Raman spectros- copy (SERS) platform suitable for gas phase sensing based on the extended organization of poly-N-isopropylacrylamide (pNIPAM)-coated nanostars over large areas is presented. This system yields high and homogeneous SERS intensities, and simultaneously traps organic chemical agents as pollutants from the gas phase. pNIPAM-coated gold nanostars were organized into parallel linear arrays. The optical properties of the fabricated substrates are investigated, and applicability for advanced sensing is demonstrated through the detection in the gas phase of pyrene traces, a well-known polyaromatic hydrocarbon. ■ INTRODUCTION Surface enhanced Raman spectroscopy (SERS) is a spectro- scopic technique with the ability to yield ultrasensitive detection of molecules under environmental conditions without special sample preparation. The requirements to obtain strong SERS include an optical enhancer, typically a metallic nanostructure, that produces an intense electromagnetic field at its surface upon excitation with the appropriate light (i.e., localized surface plasmon resonance, LSPR) and the proximity of analyte molecules to the metallic nanostructure. So far, a variety of nanostructures of gold and silver have been introduced in the literature as optical enhancers. For example, single-molecule SERS was reached for the first time using silver nanoparticle aggregates. 1,2 The formation of aggregates was necessary to obtain highly active regions where the electro- magnetic field is gigantic due to the interparticle plasmon interaction (i.e., hot spots). 3 Unfortunately, aggregation yields heterogeneous surfaces where the hot spots are randomly distributed in intensity, shape, and density. This makes quantitative detection impossible, as all the spots for a given surface are intrinsically different in their optical properties. 4 In other words, quantitativity is sacrificed in order to gain sensitivity. To solve this issue, much effort has been dedicated to produce metallic structures, which are well-defined on the nanoscale and thus give rise to hot-spots of uniform intensity, shape, and density: Evaporated films have been lithographically etched with electron or ion beams providing homogeneous optical platforms. 5 Unfortunately, lithographic etching is time- consuming, expensive, and cannot be carried out over large areas. Alternatives to these methods have been proposed. For example, the Van Duyne ́ s group developed a surface modification by combining nanosphere lithography with metal films over the nanospheres (NSL-FON), which results in a homogeneous high efficient optical platform that can be produced over large areas. 6,7 On the other hand, another approach recently developed by our groups consists of the confinement of preformed colloidal nanoparticles aided by controlled polymer wrinkling. 8 This alternative yields inex- pensive, highly homogeneous, and efficient SERS platforms that can be patterned over large areas with minimum technical requirements. 9,10 As mentioned before, the other requirement to obtain strong SERS relies on the molecule to be analyzed. SERS is eminently a first-layer effect. In general, molecules that have strong affinity toward the metallic nanostructured surface lead to strong SERS signals. However, molecules with low or no affinity usually do not yield SERS. To solve this problem, nanostructures have been functionalized with a variety of materials that may attract/ increase the concentration of the analyte close to the plasmonic surface. 11 As an example, polycyclic aromatic hydrocarbons (PAHs), a class of naturally formed persisting air pollutants that can induce severe cancer and poisoning, have been detected in solution by adding functionalities to the plasmonic surfaces such as thiolated alkyl chains, 12 viologen, 13 humic acids, 14 or cyclodextrins. 15 Poly-N-isopropylacrylamide (pNIPAM)-coated Special Issue: Colloidal Nanoplasmonics Received: January 31, 2012 Revised: March 1, 2012 Published: March 2, 2012 Article pubs.acs.org/Langmuir © 2012 American Chemical Society 9168 dx.doi.org/10.1021/la300454q | Langmuir 2012, 28, 9168-9173