Large-Area Organization of pNIPAM-Coated Nanostars as SERSPlatforms for Polycyclic Aromatic Hydrocarbons Sensing in GasPhaseMareen 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 onthe 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 pollutantsfrom the gas phase. pNIPAM-coated gold nanostars wereorganized into parallel linear arrays. The optical properties ofthe fabricated substrates are investigated, and applicability foradvanced sensing is demonstrated through the detection in thegas phase of pyrene traces, a well-known polyaromatic hydrocarbon.

■ INTRODUCTIONSurface enhanced Raman spectroscopy (SERS) is a spectro-scopic technique with the ability to yield ultrasensitivedetection of molecules under environmental conditions withoutspecial sample preparation. The requirements to obtain strongSERS include an optical enhancer, typically a metallicnanostructure, that produces an intense electromagnetic fieldat its surface upon excitation with the appropriate light (i.e.,localized surface plasmon resonance, LSPR) and the proximityof analyte molecules to the metallic nanostructure. So far, avariety of nanostructures of gold and silver have beenintroduced in the literature as optical enhancers. For example,single-molecule SERS was reached for the first time using silvernanoparticle aggregates.1,2 The formation of aggregates wasnecessary to obtain highly active regions where the electro-magnetic field is gigantic due to the interparticle plasmoninteraction (i.e., hot spots).3 Unfortunately, aggregation yieldsheterogeneous surfaces where the hot spots are randomlydistributed in intensity, shape, and density. This makesquantitative detection impossible, as all the spots for a givensurface are intrinsically different in their optical properties.4 Inother words, quantitativity is sacrificed in order to gainsensitivity. To solve this issue, much effort has been dedicatedto produce metallic structures, which are well-defined on thenanoscale and thus give rise to hot-spots of uniform intensity,shape, and density: Evaporated films have been lithographicallyetched with electron or ion beams providing homogeneousoptical platforms.5 Unfortunately, lithographic etching is time-consuming, expensive, and cannot be carried out over largeareas. Alternatives to these methods have been proposed. For

example, the Van Duynes group developed a surfacemodification by combining nanosphere lithography withmetal films over the nanospheres (NSL-FON), which resultsin a homogeneous high efficient optical platform that can beproduced over large areas.6,7 On the other hand, anotherapproach recently developed by our groups consists of theconfinement of preformed colloidal nanoparticles aided bycontrolled polymer wrinkling.8 This alternative yields inex-pensive, highly homogeneous, and efficient SERS platforms thatcan be patterned over large areas with minimum technicalrequirements.9,10

As mentioned before, the other requirement to obtain strongSERS relies on the molecule to be analyzed. SERS is eminentlya first-layer effect. In general, molecules that have strong affinitytoward the metallic nanostructured surface lead to strong SERSsignals. However, molecules with low or no affinity usually donot yield SERS. To solve this problem, nanostructures havebeen functionalized with a variety of materials that may attract/increase the concentration of the analyte close to the plasmonicsurface.11 As an example, polycyclic aromatic hydrocarbons(PAHs), a class of naturally formed persisting air pollutants thatcan induce severe cancer and poisoning, have been detected insolution by adding functionalities to the plasmonic surfacessuch as thiolated alkyl chains,12 viologen,13 humic acids,14 orcyclodextrins.15 Poly-N-isopropylacrylamide (pNIPAM)-coated

Special Issue: Colloidal Nanoplasmonics

Received: January 31, 2012Revised: March 1, 2012Published: March 2, 2012

Article

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© 2012 American Chemical Society 9168 dx.doi.org/10.1021/la300454q | Langmuir 2012, 28, 9168−9173

nanoparticles have been demonstrated as one of the mosteffective molecular traps, especially for hydrophobic analytes.16

However, a homogeneous coating of the nanoparticles bypolymers such as pNIPAM completely inhibits the plasmoncoupling of the metal cores if the polymer shell has a certainthickness. In other words, the polymer shell, acting as a stericalspacer between the metal cores, prevents the formation of hotspots. Therefore, the achieved sensitivity is quite modest. Onealternative relies on the generation of aggregates, which willincrease the SERS intensity17 but, as previously commented,will lead to intensity fluctuations due to the random generationof hot spots. Another alternative exploits the coating of star-shaped nanoparticles.18−22 In these highly branched nano-particles, the core acts as a plasmonic nanoantenna, while thetips concentrate the electromagnetic field at their apex.23,24 Infact, these nanostars are one of the very little number of SERSplatforms able to sustain an electric field large enough to beused as single-particle SERS substrates.25−27 Notably, adistinctive property of these particles is that, upon aggregation,their optical efficiency decreases. This phenomenon occursbecause the interaction of different particles with differentsymmetries leads to field deactivation rather than plasmon

coupling.28 Thus, in this scenario the polymer coating not onlyadds a trapping functionality to the structure but will preserveits unique optical properties.Herein we show the preparation of optically active gold

nanostars coated with pNIPAM and their controlled assemblyinto highly ordered linear structures. The optical properties ofthe imprinted particle assemblies were characterized, and theSERS efficiency was compared with films of pNIPAM-coatedgold stars and coated gold spheres, respectively. Finally,demonstration of their capabilities for gas sensing wasillustrated by the detection of the PAH pyrene in the gas phase.

■ MATERIALS AND EXPERIMENTAL METHODSChemicals. Gold(III) chloride trihydrate (HAuCl4 × 3H2O),

trisodium citrate dihydrate, N,N-dimethylformamide, N-isopropylacry-lamide (NIPAM), butenylamine hydrochloride (BA), ethanol,benzenethiol (BT), pyrene and poly(vinylpyrrolidone) (PVP, Mw =10 000) were purchased from Aldrich. Polydimethylsiloxane (PDMS)Sylgard (184) silicon elastomer, curing agent, and precursor werepurchased from Dow Corning, USA. N,N-Methylenebisacrylamide(BIS) and potassium peroxodisulfate (PPS) were acquired from Fluka.Sodium dodecyl sulfate (SDS) was from Ajax Laboratory Chemicals.All chemicals were used as received. Water was purified using a

Scheme 1. (1) Synthesis and Funtionalization of Au Nanoparticles. (2) pNIPAM Coating of Au Nanoparticles. (3) PVPFuntionalization and Nanostar Overgrowth. (4) Fabrication of a PDMS Wrinkled Surface. (5) Nanoparticle Organization

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Millipore Milli-Q system (resistivity higher than 18 MΩ cm−1). Allexperiments were carried out at room temperature.Synthesis of pNIPAM-coated Gold Nanoparticles. Au@

pNIPAM core−shell particles were prepared by precipitation polymer-ization of the monomer NIPAM in the presence of functionalized,spherical gold nanoparticles as reported elsewhere.29 First, goldnanoparticles approximately 15 nm in diameter were synthesizedaccording to the well-known method by Turkevich and Enustun.30

Briefly, 2.5 mL of an aqueous solution of HAuCl4 (0.1 M) was addedto 497.5 mL of water in a conical flask of 1 L volume. The light yellow,clear solution was heated to heavy boiling under continuous stirringwith a magnetic stirrer, and 25 mL of water containing 255 mgtrisodium citrate was added quickly. The solution turned colorlessimmediately after the addition of citrate and turned deep red withinthe next 5 min. The solution was kept boiling for 20 min upon theaddition of citrate and was then cooled to room temperature. Then,the gold particles were surface functionalized by the addition of 3 mLof an aqueous SDS solution (0.62 mM) and, after 20 min, the additionof 980 μL of a solution of BA in ethanol (0.12 mM). Twenty minutesafter the addition of BA, the particle dispersion was cleaned andconcentrated by repeated centrifugation. Three centrifugation stepswere performed (each for 90 min at 3500 rpm). Finally, the residueswere collected, resulting in a deep red nanoparticle dispersion with aconcentration [Au] = 0.01 M (see Scheme 1.1).The growth of the pNIPAM shell around the gold nanoparticles was

performed by precipitation polymerization using 1.188 g of themonomer NIPAM and 243 mg of the cross-linker BIS, dissolved in350 mL of water. The solution was stirred in a three-neck round-bottom flask of 500 mL volume and heated to 45 °C while degassingwith nitrogen. After 20 min of degassing, 5.32 g of the concentrateddispersion of functionalized gold nanoparticles ([Au] = 0.01 M) wereinjected dropwise with a syringe. In the following, the red and clearreaction medium was heated to 70 °C and after an equilibration timeof 10−15 min, 5 mg PPS dissolved in 1 mL of water were added inorder to initiate the polymerization. The reaction was allowed to occurfor 2 h. Then, the turbid, red dispersion was cooled to roomtemperature. Cleaning of the core−shell particles was done byrepeated centrifugation and redispersion in water (3 h at 9000 rpm,two repetitions). Finally, the residues were collected and freeze-dried(see Scheme 1.2).PVP Functionalization and Nanostar Growth. The previously

described pNIPAM coated gold nanoparticles (0.023 g) weredispersed in 5 mL ethanol via sonication during 20 min. This solution(350 μL) was added to a solution containing 4.88 g of PVP in 35 mLof N,N-dimethylformamide. The solution was sonicated for 30 minand was left standing overnight to allow diffusion of PVP to the Ausurface. Next, 105 μL of an aqueous solution of 0.105 M HAuCl4 wereadded rapidly under vigorous stirring. Within 20 min, the color of thesolution changed from pink to colorless, and finally turned blue,indicating the formation of gold nanostars. The dispersion was keptovernight with continuous stirring (see Scheme 1.3). After that, thesolution was centrifuged at 7000 rpm for 20 min, the clear supernatantwas discarded, and the precipitate was redispersed in 40 mL of ethanol.This process was repeated three times. Before organizing the particles,the solution was concentrated to a final volume of 2 mL viacentrifugation steps at 7000 rpm for 20 min. Finally, 1 mL of thepreviously concentrated solution was centrifuged again at 7000 rpmfor 20 min, the supernatant was discarded, and the precipitateredispersed in a total volume of 0.5 mL of milli-Q water.Particle Organization. In order to organize the particles on

substrates, the fabrication of a PDMS wrinkled surface is required as atemplate.8−10 The PDMS elastomer was prepared by mixing Sylgard184 with a 10:1 ratio by mass of prepolymer to curing agent. Themixture was stirred and filled in a clean, flat Petri dish. After 24 h atroom temperature and baking at 80 °C for 5 h, the cross-linked PDMSwas cut into 30 mm × 8 mm stripes. One of these stripes was stretcheduniaxially in a custom-made apparatus to a strain of 125% of the initiallength. The stretched substrate was oxidized during 900 s in oxygenplasma at 0.2 mbar using a plasma etcher operating at 0.1 kW(flecto10, Plasma Technology, Germany). After that, the sample was

relaxed, and wrinkles with a periodicity of 1700 nm were obtained (seeScheme 1.4). The hydrophilic wrinkled substrates were used directlyafter plasma treatment. Briefly, a drop of the nanoparticles solution(20 μL) was spin coated onto the wrinkled PDMS at 1500 rpm for 1min.31 Next, a drop of water (10 μL) was placed on a clean glassslide,9,32 and the wrinkled PDMS containing the particles was placedon top of the water droplet. The system was left undisturbed for 2 h inorder to allow the water to evaporate. After that, the PDMS wasremoved, and the particles remained organized into lines on the glassslide (see Scheme 1.5).

Spin coating at 1500 rpm for 1 min of 15 nm spherical gold corescoated with pNIPAM and 130 nm gold stars coated with pNIPAM wasalso performed for control experiments.

Characterization. Ultraviolet−visible (UV−vis) spectroscopy(PerkinElmer, Lambda 19), transmission (TEM) and scanning(SEM) electron microscopies (LEO 922 EFTEM operating at 200kV and LEO 1530 FE-SEM, Zeiss, respectively) were applied tocharacterize the optical response, structure, and size of thenanoparticles and their arrays.

SERS. Optical enhancing activity was characterized using BT in thegas phase. BT was adsorbed on the whole surface of the metallicsamples by casting a drop of BT (0.1 M in ethanol) in a Petri dishwhere the substrate was also contained. SERS spectra were collected inbackscattering geometry with a Renishaw Invia Reflex systemequipped with two-dimensional charge-coupled device (2D-CCD)detector and a Leica confocal Microscope. The spectrograph uses ahigh-resolution grating (1200 g cm−1) with additional bandpass filteroptics. Excitation of the sample was carried out with a 830 nm diodelaser line, with acquisition times of 200 ms and power at the sample ofabout 1 mW, using the Renishaw’s StreamLine accessory. The laserwas focused onto the sample with a 50× objective. For pyrenedetection, the analyte was adsorbed in the gas phase by placing thesamples and a drop of pyrene (10 mM in ethanol) (organized lines ofpNIPAM-coated nanostars and a film with uncoated nanostars forreference) in an oven at 150 °C with vacuum for 10 min.

■ RESULTS AND DISCUSSIONIn order to obtain gold nanostars coated with homogeneouspolymer shells based on pNIPAM, core−shell particles withspherical gold cores of 15 nm in diameter and pNIPAM shellswith a total diameter of 273 nm as determined by dynamic lightscattering (DLS) at 20 °C (AuNP@pNIPAM, Figure 1A) were

prepared according to a recently reported protocol.29 Althoughthe direct pNIPAM coating of gold nanostars (AuNS) has beenrecently reported,18 this approach requires a rather time-consuming procedure and the design of specific chemicals.Hence, to fabricate the final AuNS@pNIPAM, pNIPAM-coatedgold spheres were used as seed particles for the furtherovergrowth of the metal core. In this synthetic approach, it isvery important to previously functionalize the seed particleswith PVP before adding the growth solution, thus allowing the

Figure 1. TEM images of (A) AuNP@pNIPAM and (B) AuNS@pNIPAM core−shell particles.

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epitaxial growth of the gold onto the spheres and the formationof spikes, as can be seen in Figure 1B. During this process, thecolor of the solution gradually turns from red to dark blue. Thischange of color indicates the formation of stars. The obtainedparticles have a nanostar core of 130 nm in diameter. Theoverall diameter of the core−shell particle is 250 nm, againdetermined using DLS at 20 °C.The UV−vis−near-infrared (UV−vis−NIR) spectra of the

prepared nanoparticles (Figure 2) show how the nanospheres

are characterized by the typical LSPR at 520 nm, while, for thenanostars, this band remains as a shoulder of a much strongerabsorption peak located at 810 nm. This last signal is attributedto the tip growth and is broad because of the overlapping of theplasmonic bands of tips with different lengths and apertureangles as previously reported.25

In order to organize the core−shell particles in a linearfashion on planar substrates, we employed PDMS wrinkledsubstrates with a periodicity of 1700 nm as an assemblytemplate, by spinning the particles onto the wrinkled PDMS31

and transferring them on a clean glass slide.9,32 Notably, asobserved by SEM (Figure 3) this method leads to a highlyhomogeneous patterning that can be extended over cm2 (see,for example, the iridescent colors of the digital camera pictureof the stamped colloidal lines, inset in Figure 3). The upper

insets of Figure 3 show the internal geometry of the structure.This is characterized by double-particle zigzag lines with anoverall width of 340 nm and a periodicity between them of1700 nm. Within the line, the particles remain separated due tothe pNIPAM shell. This spacing is also confirmed in the UV−vis−NIR spectrum of the patterned film (Figure 2, blue). It canbe seen that the plasmon peak position remains very similar tothat of the sample in solution with only a small blue-shift (10nm). This shift is attributed to the change of the dielectricmedium (air in the film, ethanol in the solution). Thus, thesimilarity in the plasmonic response indicates that there is noplasmonic interaction between neighbor particles.To test the optical enhancing properties, BT was retained in

the gas phase over all the film. Figure 4A shows the SERS

spectrum on BT recorded on the AuNS@pNIPAM pattern.The spectrum is characterized by the BT typical vibrationalpattern including the ring stretching (1573 cm−1), C−Hbending (1022 cm−1), ring breathing (1073 and 999 cm−1), andlow frequency mode with contribution of C−S stretching (417cm−1) modes. Figure 4B,C shows the optical images and theSERS mappings over an extended area (40 × 40 μm, step size 1μm) for the AuNS@pNIPAM patterned lines and spin-coatedfilms. Notably, while the line patterned film shows slightlylower intensity than the corresponding spin-coated one, thesignal remains constant spot to spot in contrast to the strongfluctuations found in the spin-coated film. For comparison, aspin-coated AuNP@pNIPAM film was also mapped (Figure3D) without any SERS signal. This illustrates the opticalefficiency of the stars as previously described elsewhere.28

Besides the previously mentioned role of the pNIPAM shellto act as a separation layer between the individual Au nanostarsand thus avoiding the deactivation of the field enhancement attheir tips,33 the pNIPAM shell could be used as an effectivemolecular trap for hydrophobic analytes. In order to do that,

Figure 2. UV−vis−NIR spectra of the AuNP@pNIPAM (red) andAuNS@pNIPAM (yellow) in solution and after organization into lineson a substrate (blue).

Figure 3. SEM images of the linear assembled AuNS@pNIPAMparticles at different magnifications and a digital camera photographshowing the characteristic iridescence provided by the colloidal gratingover ca. 1 cm.

Figure 4. (A) SERS spectrum of BT on the AuNS@pNIPAM linepatterned film. Optical and Raman maps (1073 cm−1) over extendedareas (40 × 40 μm, step size 1 μm) on (B) AuNS@pNIPAM linepatterned, (C) AuNS@pNIPAM monolayered, and (D) AuNP@pNIPAM monolayered films.

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the ability of the composite AuNS@pNIPAM particles to trapand analyze potential pollutants in in the air was demonstratedby exposing the organized lines to an atmosphere containingtraces of pyrene. Pyrene is formed during combustion oforganic matter; thus to promote its dispersion into the gasphase, the patterned film together with a film of bare nanostarsand an ethanolic solution containing pyrene were placed in avacuum oven at 150 °C for 10 min. After exposure, bothsamples, the pNIPAM-coated and the uncoated one, wereanalyzed by SERS. Figure 5 shows the obtained experimental

spectra for both samples, pNIPAM-coated stars (red) and bareAu stars (black). This experiment reveals the vibrational SERSpattern of pyrene when pNIPAM was used (red). Thisspectrum correlates well with the Raman spectrum of thesolid,14 which is characterized by vibrational modes at 1627,1601, 1557, 1440, and 1398 cm−1 due to the ring stretchings,1241 cm−1 CCH in plane deformation and the 601 and 413cm−1 CCH out of plane deformations. Notably, when the barenanostars were used (black), the spectrum only shows veryweak signals of pyrene because it is not retained on the goldsurface, thus corroborating the effectiveness of pNIPAM shellsas molecular traps for hydrophobic analytes.In summary, here we demonstrate the possibility of

generating 2D structured arrays of pNIPAM-coated nanostarsextend over square centimeters via the wrinkle assistedassembly in order to create a new, versatile SERS platformsuitable for gas phase sensing. Also the optical properties andenhancing activity are characterized. In addition, trappingefficient for gas phase pollutants is demonstrated with thedetection of pyrene, a well-known polyaromatic hydrocarbon.We anticipate that these materials will be a key technology inthe development of highly efficient advanced air- or gas-sensingdevices with environmental, industrial, and homeland securityapplications.

■ AUTHOR INFORMATION

Corresponding Author*(N.P.P.) Phone: (+49) 921-55-2750; Fax: (+49) 921-55-2059; E-mail: nicolas.pazos@uni-bayreuth.de. (A.F.) (+49)921-55-2753; Fax: (+49) 921-55-2059; E-mail: andreas.fery@uni-bayreuth.de.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors would like to thank Carmen Kunert for helpingduring TEM characterization. This work was funded by theSpanish Ministerio de Ciencia e Innovacion (CTQ2011-23167)and by the German Science Foundation within SFB 840, TPB5.

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Figure 5. SERS spectra of pyrene from the gas phase on the AuNS@pNIPAM line patterned film (red) and on a film of bare AuNS (black).

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