Pillar[5]arene-Based Supramolecular Plasmonic Thin Films for Label

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Pillar[5]arene-Based Supramolecular Plasmonic Thin Films for Label-free, Quantitative and Multiplex SERS Detection Verónica Montes García, Borja Gómez-González, Diego M. Solís, Jose M Taboada, Norman Jiménez Otero, Jacobo de Uña-Álvarez, Fernando Obelleiro, Luis García-Río, Jorge Perez-Juste, and Isabel Pastoriza-Santos ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08297 • Publication Date (Web): 19 Jul 2017 Downloaded from http://pubs.acs.org on July 19, 2017

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ACS Applied Materials & Interfaces

Pillar[5]arene-Based Supramolecular Plasmonic Thin Films for Labelfree, Quantitative and Multiplex SERS Detection Verónica Montes-García,† Borja Gómez-González,‡ Diego Martínez-Solís,§ José M. Taboada,∥ Norman Jiménez-Otero,⊥ Jacobo de Uña-Álvarez,⊥ Fernando Obelleiro,§ Luis García-Río,‡ Jorge Pérez-Juste,*,† Isabel Pastoriza-Santos*,† †

Departamento de Química Física y Centro Singular de Investigaciones biomédicas (CINBIO), Universidade de Vigo, 36310 Vigo, Spain ‡

Centro Singular de Investigación en Química Biológica y Materiales Moleculares (CIQUS) y Departamento de Química Física y, Universidad de Santiago, 15782 Santiago, Spain.

§

Departamento de Teoría de la Señal y Comunicaciones, Universidade de Vigo, 36310 Vigo, Spain

∥ Departamento Tecnología de los Computadores y de las Comunicaciones, Universidad de Extremadura, 10003 Cáceres, Spain

Departamento de Estadística e Investigación Operativa, Facultad de Ciencias Económicas y Empresariales & Centro Singular de Investigaciones biomédicas (CINBIO), Universidade de Vigo, 36310 Vigo, Spain

⊥

ABSTRACT: Novel plasmonic thin films based on electrostatic layer-by-layer (LbL) deposition of citrate-stabilized Au nanoparticles (NPs) and ammonium pillar[5]arene (AP[5]A) has been developed. The supramolecular-induced LbL assembly of the plasmonic nanoparticles yields to the formation of controlled hot-spots with uniform interparticle distances. At the same time this strategy allows to modulate the density and dimensions of the Au aggregates, and therefore the optical response, on the thin film with the number of AuNP-AP[5]A deposition cycles. Characterization of the AuNPAP[5]A hybrid platforms as a function of the deposition cycles was performed by means of visible-NIR absorption spectroscopy, scanning electron and atomic force microscopies, showing larger aggregates with the number of cycles. Additionally the surface enhanced Raman Scattering efficiency of the resulting AuNP-AP[5]A thin films has been investigated for three different laser excitations (633, 785 and 830 nm) and using pyrene as Raman probe. The best performance was showed by the AuNP-AP[5]A film obtained with two deposition cycles ((AuNP-AP[5]A)2) when excited with 785 laser line. The optical response and SERS efficiency of the thin films were also simulated using the M3 solver and employing computer aided design (CAD) models built based on SEM images of the different films. The use of host molecules as building blocks to fabricate (AuNP-AP[5]A)2) films has enabled the ultradetection, in liquid and gas phase, of low molecular weight polyaromatic hydrocarbons, PAHs, with no affinity for gold but towards the hydrophobic AP[5]A cavity. Besides these plasmonic platforms allowed to achieve quantitative detection within certain concentration regimes. Finally, the multiplex sensing capabilities of the AuNP-AP[5]A)2 were evaluated for their ability to detect in liquid and gas phase three different PAHs.

Keywords: plasmonic thin films, SERS sensing, pillararene, supramolecular assembly, multiplex detection, PAHs. INTRODUCTION Over the past few decades layer-by-layer (LbL) assembly has emerged as a powerful technology for fabricating nanostructured multilayer films and nanocomposites with tailored composition, structure, thickness and function.1,2 LbL assemblies are formed by stepwise alternate adsorption of complementary building blocks of different nature (such as; polyelectrolytes,3 DNA,4 dendrimers,5 nanoparticles (NPs),6 polypeptides,7 macrocycles,8 etc.,) on a substrate of almost any shape and size, occurring either via electrostatic or non-electrostatic interactions (hydrogen bonding, metal-ion coordination, host-guest interaction,

covalent bonds, and so on).1,9 Therefore, this technology can be considered as highly versatile, simple and efficient and as such it has great potential in a myriad of fields including catalysis, energy, bioelectronics, optics and biomedicine.2,10 Among the wide range of building blocks, the incorporation of macrocycles as building blocks or part of them into the multilayer films offer the possibility of combining host−guest chemistry and LbL assembly technology. Several reports employ macrocycles to build up LbL multilayer films by means of supramolecular interaction between host cavities (e.g. cyclodextrins, cucurbiturils, calixarenes, pillararenes, crown ethers, or por-

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phyrins) and guest molecules (e.g. ferrocene, adamantane, or azobenzene) where both host and guest are incorporated as functional groups in other superior units (dendrimers, polyelectrolytes, particles, etc.).8,11 For instance, organic/inorganic multilayer thin film with nanoscale thickness control was fabricated on the bases of supramolecular interaction between adamantlyterminated dendrimers and cyclodextrin-modified gold nanoparticles.8 Li et al. reported the synthesis of pHresponsive degradable microcapsules with controlled drug release behavior fabricated via host-guest interaction using dextran-graft-β-cyclodextrin and asparticgraft-adamantane.11 Other reports take advantage of the host-guest interaction to incorporate host molecules in previously prepared LbL films for creating multifunctional surfaces or for the recognition and inclusion of a guest molecule.12,13 Cao el al. integrated adamantine in the multilayer film as side chains of the building block, providing sites for the incorporation of host β-CD derivatives with tailored biofunctions.13 Nicolas et al. assembled cucurbit[8]uril as a nanocontainer into polyelectrolyte multilayer films for the reversible uptake and release of a guest molecule.12 Finally, macrocyclic host molecules were used as building blocks to build up supramolecular LbL films with either Au NPs through coordination chemistry or polyelectrolytes/macrocycles through electrostatics.14-16 These microporous host-containing thin films show potential applications in molecular recognition, separation and storage. On the other hand, surface-enhanced Raman scattering (SERS) is a powerful vibrational spectroscopic technique that allows for ultra-sensitive detection of molecules. It is based on the enhancement of the characteristic inelastic Raman scattering signals of an analyte when is close to/adsorbed onto a metal surface sustaining localized surface plasmon resonances (Au and Ag mainly).17 It is well-know that to reach single-molecule level detection a sensing plasmonic platform containing hot spots (such as junctions or gaps between metal nanoparticles) is required. Thus, a hot spot can produce SERS enhancement factor for single molecules of up to 109-1010 orders of magnitude.18,19 However there are several limitations in SERS detection that need to be overcome to achieve its practical applicability. The low Raman cross-section of many interesting analytes requires the use of highly efficient, reliable and reproducible SERS platforms. Though some advances in their fabrication have been made it is still a challenge. On the other hand, many analytes, such as polycyclic aromatic hydrocarbons (PAHs), lack metal-affinity functional groups limiting their effective detection. In that case, the SERS detection is performed by capturing the molecules through the proper functionalization of the metal surface. Supramolecular chemistry has recently attracted a lot of attention as tool to induce the contact between a plasmonic substrate and an analyte.20 Among the various reported macrocycles, the family of pillar[n]arenes is particularly interesting because of its large scale synthesis,

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easy functionalization and outstanding selective binding abilities.21-24 Recently we have demonstrated the high performance of ammonium pillar[5]arene-stabilized (AP[5]A) Au NPs for SERS detection of 2-naphtoic acid and a PAH (pyrene) in water achieving detection limits as low as 10-6 and 10-8 M, respectively.25 Nevertheless, the colloidal nature of this type of sensing platforms limits their possibilities, in terms of reusability, quatitative analysis, multiplexing or gas sensing; limitations which can overcome with the nanoparticles immobilization. Herein, we report the fabrication of Au NP-macrocycle hybrid thin films through LbL technology as highly sensitive, reliable and label-free SERS sensing platform with multiplex capabilities. The hybrid thin film was built up by alternate deposition of Au NPs and AP[5]A macrocycles on a glass substrate through electrostatics. The use of AP[5]A as building block guaranteed uniform interparticle gaps (or hot spots) determined by the macrocycle dimensions (1.5 nm).25 First, we study the formation AuNP-AP[5]A hybrid thin films analyzing their structure, hot spots density as well as their plasmonic response, as a function of deposition cycles. Then we investigate, both theoretically and experimentally, the SERS performance for the different AuNP-AP[5]A hybrid platforms with three different laser lines (633, 785 and 830 nm) employing pyrene as Raman reporter. Additionally, we analyze the sensitivity of the plasmonic platform with the highest SERS efficiency for the single and multiplex detection of three different PAHs; pyrene, nitropyrene and anthracene, in both, liquid and gas phase. The recyclability of the substrates is studied in liquid phase.

EXPERIMENTAL SECTION Materials. Tetrachloroauric(III) acid trihydrate (HAuCl4 3H2O), trisodium citrate dihydrate, poly(diallyldimethylammonium chloride) (PDDA, average Mw 100,000-200,000), poly(acrylic acid sodium salt) (PAA, Mw 15,000) hydrogen peroxide (H2O2, 28%), sulfuric acid (H2SO4, 98%), sodium chloride, nitropyrene, carbon tetrabromide, 1,4-bis(2-hydroxyethoxy)benzene, triphenylphosphine, acetonitrile, methanol, 1,2dichloroethane, paraformaldehyde, boron trifluoride diethyl etherate, petroleum ether, dichloromethane, trimethylamine and ethanol were supplied by Aldrich. Pyrene was supplied by Merk, while anthracene by Alfa Aesar. N,N-dimethylformamide was supplied by Fluka. Milli-Q grade water was used in all the preparations. Synthesis of ammonium pillar[5]arene, AP[5]A. The ammonium pillar[5]arene was synthesized according to the literature.26 Briefly, 39.8 g (120 mmol) of carbon tetrabromide was added to 250 mL of dry acetonitrile containing triphenylphosphine (31.5 g, 120 mmol) and 1,4-bis(2hydroxiethoxy)benzene (10.0 g, 50.5 mmol) at 0 ºC with stirring. The mixture was allowed to react for 4 h under Ar at room temperature. Finally, the product was precipitated, as a white solid, by adding cold water to the reac-


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tion mixture. Then the white solid obtained (14.4 g, yield of 88%) was dissolved under Ar in 230 mL dichloromethane and 0.93 g (30.87 mmol) of paraformaldehyde and 4.83 g (33.86 mmol) of boron trifluoride diethyl etherate were added. After stirring the mixture at room temperature for 2 h, it was washed with water, saturated with sodium bicarbonate solution and brine. After removing the solvent a light yellow solid (3.37 g, yield of 73%) was obtained. Then a mixture of 2 g (1.19 mmol) of this compound and 13 mL of trimethylamine (31-35% in ethanol, 48.11 mmol) in 100 mL of ethanol was refluxed overnight. Finally, a white solid (ammonium pillar[5]arene, 2.56 g, yield of 95%) was collected after vacuum filtration, washed with ethanol and dried under high vacuum. 1H NMR (300 MHz, D2O, room temperature) δ (ppm): 6.95 (s, 10H); 4.45 (s, 20H); 3.94 (s, 10H); 3.80 (s, 20H); 3.22 (s, 90H). 13C NMR (75 MHz, D2O, room temperature) δ (ppm): 149.3, 129.9, 116.5, 64.9, 63.4, 54.1, 29.5. Synthesis of Au nanoparticles. Citrate-stabilized Au NPs (∼60 nm in diameter) were prepared following a seeded growth method previously reported.27 Briefly, 150 mL of an aqueous solution of 2.2 mM trisodium citrate was heated to boiling under vigorously stirring. After 15 min, 1 mL of 25 mM HAuCl4 in water was injected into the boiling mixture. After 10 min the reaction mixture was cooled down to 90 ºC and subsequently, another injection of 1 mL of 25 mM HAuCl4 in water was performed. Another addition was repeated 30 min later. The mixture was allowed to react for 30 min and then 55 mL was extracted from the reaction medium and 53 mL of water and 2 mL of 60 mM sodium citrate (in water) were added. The resulting solution was used as seed, and the process was repeated again six times to yield 60 nm Au NPs. To remove the excess of reactants, the colloids were centrifuged at 1520 g for 20 min and redispersed in the same volume of water. LbL fabrication of Au NP-macrocycle hybrid thin films. Glass slides were washed in piranha solution for 30 min and then copiously rinsed with water and stored in water until use. First of all, the activated glass slides were immersed in an aqueous PDDA solution (1 mg/mL, 0.05 M NaCl) for 15 min, rinsed with water and then dried. The process was repeated twice but alternating the polymer solution; PAA solution (1 mg/mL, 0.05 M NaCl) first, and then PDDA solution (1 mg/mL, 0.05 M NaCl). For the Au NPs films formation, the glass slides were immersed in the previously prepared 0.92 mM Au NPs solution for 3 hours followed by rinsing with water. Then, the substrate was immersed in a 0.1 mM AP[5]A aqueous solution for 2 hours followed by rinsing with water. The process was repeated one or two more times to obtain hybrid thin films with different Au loadings. Note that for each addition, new Au NPs or AP[5]A solutions were used. Moreover, the rinsing step with water was performed three-fold by dipping the glass slides in a different Eppendorf tube for 1 min. Characterization. UV-visible-NIR absorption spectra were recorded using an Agilent 8453 spectrophotometer.

Transmission electron microscopy (TEM) analysis was performed in a JEOL JEM 1010 microscope operating at an acceleration voltage of 100 kV. SEM images were obtained using a JEOL JSM-6700F FEG scanning electron microscope operating at an acceleration voltage of 10.0 kV. Atomic force microscopy was carried out in tapping mode with a VEECO RTESP tip of k=20–80 Nm-1 (exact spring constant not calculated) using a Nanoscope V controller on a multimode microscope. ζ Potential was determined through electrophoretic mobility measurements using a Zetasizer Nano S (Malvern Instruments, Malvern UK). NMR spectra were recorded with a Varian Mercury 300 spectrophotometer or a Varian Inova 400 spectrophotometer using the deuterated solvent as lock and the residual solvent as internal reference. Raman and SERS measurements were conducted with a Renishaw InVia Reflex system. The spectrograph used a high-resolution grating (1200 or 1800 grooves cm-1) with additional band-pass filter optics, a confocal microscope and a 2D-CCD camera. Laser excitation was carried out at 633, 785 and 830 nm with a 50x objective (N.A. 0.75), 0.65 mW of maximum power and 10 s acquisition time. The substrates were immersed in a PAH solution of the desired concentration, incubated for 1h at room temperature and then dried before SERS measurements. For the recyclability study, glass slides were immersed in a DMF solution up to 4 hours or in an analyte solution for 1 hour and dried before SERS measurements. SERS images were obtained using a SERS point-mapping method with a 50x objective (N.A. 0.75), which provided a spatial resolution of about 1.3 μm2. It created a spectral image by measuring the SERS spectrum of each pixel of the image, one at a time. The SERS images of each well were decoded using the characteristic peak intensities of the three PAH molecules using WiRE software V 4.3 (Renishaw, UK). Simulations. Extinction spectra and SERS simulations were calculated using the M3 solver, which implements a full-wave frequency-domain methodology based on boundary-element parameterizations (surface integral equation-method of moments - SIE-MOM),28-30 greatly reducing the resulting algebraic problem size if compared with volumetric discretizations, especially in scattering problems. By applying Love’s equivalence theorem, the metallic nanospheres can be substituted with equivalent electric and magnetic surface currents placed at the surface boundaries that radiate in unbounded media according to the Stratton−Chu expressions. This brings about important advantages with respect to volumetric approaches, strongly reducing the computation domain as only the material boundaries and interfaces (i.e., 2-D surfaces) must be parameterized. Additionally, this methodology yields very accurate and stable solutions, particularly when dealing with resonant metallic (plasmonic) response, as the singular behavior of fields is analytically handled by the Green’s function and its derivatives. Through Maxwell’s equations and boundary conditions for the total electric and magnetic fields, a set of SIEs is derived with the equivalent surface currents as un-



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knowns. These SIEs are subsequently discretized by applying Galerkin procedure in terms of a set of divergenceconforming basis and testing functions, rendering an algebraic dense N×N matrix system of linear equations (wherein N is the number of basis functions that expand the currents). For the realistic simulation of the large-scale plasmonic systems considered in this work, involving assemblies of more than 2900 nanoparticles, we resort to the multilevel fast multipole algorithm (MLFMA) spectral acceleration technique,31 combined with the fast Fourier transform (FFT),332 to compress the otherwise unmanageable SIEMoM dense matrix. Convergence is dramatically increased by using a multilevel non-overlapping additive Schwarz domain decomposition (DD) preconditioner.33,34 The above methodology yields a high algorithmic efficiency —computational cost of O(NlogN) both in memory and CPU time along with a highly-scalable parallel performance in multicore computer clusters. Regarding the simulation of SERS enhancement, we calculate it as the product of near electric-field enhancements produced upon normal irradiations with light of wavelengths corresponding to the laser light and the inelastically emitted Raman signal, respectively. Concretely, for the results of this work zero Raman shift was considered, and weighted by a Gaussian 2D profile with 0.1 λ standard deviation (corresponding to N. A. = 0.75) to account for the lens diffraction (see Figure S1 in SI). A skin-type molecule coverage was assumed with a nanoparticle-to-molecule separation of 0.72 nm (corresponding to half of the AP[5]A molecule length), and a surrounding homogeneous medium of permittivity ϵ = 1 (air). The Raman enhancement is simply given by |E/Einc|4,

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where Einc is the incident laser field and E is the resulting near field.35 The simulations were carried out on a workstation with four 16-core Intel Xeon E7-8867v3 45 MB Smart-Cache processors at 2.50GHz. A high surface mesh density was considered, leading to dense matrix systems of up to 8.6 million unknowns for the substrates with two depositions. A relative error norm of 10-5 was prescribed to stop the Krylov iterative solver. Considering the above, the computation times per wavelength ranged from 5495 s (at 400 nm) to 9163 s (at 1000 nm), with a memory consumption ranging from 29 to 104 GB. Chemometrics. The methodology used is based on principal component analysis (PCA).36 All statistic codes were written in the statistic software R v3.3.2 using the packages: stats and base.

RESULTS AND DISCUSSION LbL assembly of Au NPs and AP[5]A macrocycles. We have built up a LbL assembly via electrostatics using as oppositely charged building blocks ca. 60 nm citratestabilized Au nanospheres27 (LSPR band at 539 nm, Figure S2 in SI) and a cationic water-soluble pillar[5]arene with five quaternary ammonium groups at both rims (AP[5]A, positively charged, Figure S3).26 While Au NPs will provide the platform with plasmonic sensing capabilities, the macrocycle will induce the assembly of Au NPs with controlled interparticle distance as well as will allow for sensing, through host-guest interactions, of molecules with no metal affinity.

Scheme 1. Schematic representation of AuNP-AP[5]A assemblies fabricated by layer-by-layer deposition. See text for details.


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The fabrication of the AuNP-AP[5]A assemblies onto glass substrates was performed as schematically depicted in Scheme 1: (I) negatively charged Au NPs were deposited on positively charged PDDA modified glass substrate via its immersion into an aqueous dispersions of 0.92 mM Au NPs, (II) AP[5]A macrocycles are electrostatically attached on surface of Au NPs via immersion in an aqueous 0.1 mM AP[5]A solution and (III and IV) Au NPs and AP[5]A deposition steps were alternatingly repeated to produce controlled nanoparticle assemblies. Three AuNPAP[5]A depositions were performed in total. Visible-NIR spectroscopy, scanning electron microscopy (SEM) and atomic force microscopy (AFM) analysis were carried out to monitor the AP[5]A-induced assembly of Au NPs on PDDA-modified glass. Figure 1A shows the optical properties of the AuNPs-AP[5]A assemblies in air obtained after one, two and three AuNP-AP[5]A depositions (hereafter (AuNP-AP[5]A)1, (AuNP-AP[5]A)2, (AuNP-AP[5]A)3, respectively) on glass. In the case of (AuNP-AP[5]A)1 films the optical response exhibits a main LSPR band centered at 516 nm and a much less intense and broad band at 698 nm (Figure 1A). The former can be attributed to individual Au NPs which is blue-shifted respect to that one from Au colloids in water (539 nm) due to the lower average refractive index of the Au NPs surrounding medium (air versus water) after their deposition on the glass substrate. The band at lower energies indicated the presence of a small amount of Au aggregates. The presence of a small percentage of aggregates was confirmed by SEM and AFM characterization. As shown in Figure 1B and Figures S4A and S5A in SI, the (AuNP-AP[5]A)1 film is mainly constituted by Au NPs randomly distributed and well separated from each other, although a small percentage of dimer, trimers and quatrimers is also observed. In this first Au NPs layer we wanted to minimize interparticle interactions in order to build up thin films with uniform hot-spots resulting of the AP[5]A-induced Au assemblies. When a second and a third Au NP-AP[5]A deposition cycle was performed the optical response showed a redshift and broadening in the LSPR, along with an increase in extinction (Figure 1A), being the main LSPR band centered at higher wavelengths (> 750 nm). The optical response indicates that a larger number of particles are deposited on the substrate as well as an intense plasmon coupling among particles due to formation of AP[5]Ainduced Au NPs assemblies, and therefore hot spots. It was also verified by SEM and AFM analysis (Figure 1C-D and Figures S4B-C and S5B-C in SI). Moreover in the case of (AuNP-AP[5]A)3 film the analysis also revealed the presence of larger aggregates which is in agreement with its optical response. Additionally, the topography analysis reveals an increase in the film surface roughness with the deposition cycle (see Figure S5). Taking into account that the Au NPs are electrostatically attracted by the macrocycles which are specifically located on the Au surface, the size of the Au nanoparticles aggregates should increase with the deposition cycles. Nevertheless, although with

each addition a full monolayer of NPs is not formed, the AFM analysis showed random places where the cross section of the film matches with the height of three NPs stacked vertically (see Figure S5 in the SI). Finally, we simulated the optical extinction for (AuNPAP[5]A)1, (AuNP-AP[5]A)2, (AuNP-AP[5]A)3 thin films using the M3 solver (see Methods). For doing that, computer aided design (CAD) models corresponding to substrates with one, two, and three additions were generated with 366, 1274 and 2926 spheres, respectively. The spheres, with 60 nm in diameter, were arranged into assemblies trying to emulate as closely as possible the SEM images of Figure 1B-D (see models in Figure S6). The minimum interparticle separation was limited to 1.44 nm, according to the size of the interparticle ligand (AP[5]A)). As show in Figure S6D the calculated extinction spectra for (AuNP-AP[5]A)1, (AuNP-AP[5]A)2, (AuNP-AP[5]A)3 thin films are in good agreement with the experimental data (Figure 1A). After the second deposition cycle the formation of random local particle assemblies induced by AP[5]A gives rise to a red-shift and broadening in the optical response, which becomes more remarkable after the third deposition cycle. Optimization of the SERS substrate. So far we have demonstrated that AP[5]A could act as building block for the LbL deposition of citrate-stabilized gold nanoparticles in a controlled manner on flat surfaces. In order to understand the SERS performance of these new type of plasmonic substrates we have studied, both experimentally and theoretically, the SERS efficiency in terms of number of AuNP-AP[5]A depositions and the wavelength of the excitation laser line.



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Figure 2. A) Molecular structure pyrene and its average SERS spectrum obtained with the plasmonic AuNP-AP[5]A sub-1 strate.. B-D) SERS mappings obtained at 594 cm with the optimized plasmonic substrate (AuNP-AP[5]A)2 for three laser lines (633 (B), 785 (C) and 830 nm (D)). In all cases pyrene concentration was 0.1 µM E) Average intensities of the ring -1 breathing mode (594 cm ) as a function of AuNP-AP[5]A depositions with the three excitation laser lines as indicated. All -2 -2 -2 SERS measurements were carried out with a 50 x objective and a maximum power of 43 kWcm , 52 kWcm and 11 kWcm for the 633, 785 and 830 nm laser lines, respectively. The acquisition time was 1 s. All scale bars represent 20 µm. F) Simulated average SERS enhancement as a function of AuNP-AP[5]A depositions with the three excitation laser lines as indicated (see experimental section for details).

For experimental SERS analysis, pyrene was chosen as model analyte (Figure 2A). Pyrene has been classified as one of the 16 priority PAHs by U.S. EPA (Environmental Protection Agency). Although it shows low affinity for the gold surface it can be selectively trapped by AP[5]A macrocycles via host-guest chemistry as demonstrated by fluorescence spectroscopy (see Figure S7 in SI). This hostguest interaction allows its detection using SERS, as recently demonstrated.25 To record the SERS spectrum of pyrene the plasmonic thin films ((AuNP-AP[5]A)1, (AuNPAP[5]A)2 and (AuNP-AP[5]A)3) were immersed in an aqueous solution of 10-7 M pyrene for 1h and then airdried. Figure 2A shows a representative SERS spectrum dominated by the characteristic signals of pyrene which can be assigned to the ring C=C stretching (1407 and 1242 cm-1), ring deformation (409 cm-1), ring breathing (594

cm-1), ring C=C stretching (1594 and 1628 cm-1), and CH bending (1067 and 1143 cm-1).25 To investigate the SERS performance of the three different AuNP-AP[5]A thin films we first analyzed the uniformity of the SERS signal for three different excitation laser lines (633, 785 and 830 nm). Thus SERS mappings, selecting the ring breathing peak at 594 cm-1 of pyrene, were recorded over extended areas (typically, 80 µm x 80 µm with 3 µm step size). As shown in Figures 2B-D for (AuNP-AP[5]A)2 thin films the obtained SERS mappings are highly uniform and homogenous in intensity over the whole area, independently of the laser line. Similar results were obtained for (AuNP-AP[5]A)1 and (AuNP-AP[5]A)3 films (see Figure S8 in the SI).


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Finally, in order to evaluate the SERS efficiency of the different substrates we have plotted the average SERS intensity (784 points in total) at 594 cm-1 for the three different laser lines (see Figure 2E). Besides, to test the reproducibility of the measurements SERS mappings were recorded on different (AuNP-AP[5]A)n plasmonic substrates (area of 104 µm x 104  µm, 676 points (26x26 spectra, step 4 µm)) leading to similar average SERS intensities of 3.97 ± 0.41 Kcts and 4.43 ± 1.19 Kcts (see SERS mapping in Figure S9 in SI). Interestingly, the results showed a clear increase in the average SERS signal between the first and the second Au NPs deposition cycle (AuNP-AP[5]A)1 and (AuNP-AP[5]A)2, respectively). Nevertheless, the average SERS intensity decreases for the third deposition cycle ((AuNP-AP[5]A)3). A similar behavior has been previously reported for self-assembled Au nanostars on polystyrene beads. When the amount of NPs is increased the SERS efficiency achieves a maximum and then started to decrease. Based on these results, it has been postulated that the strong plasmon coupling between NPs with different orientations and geometries lead to a hot-spot deactivation.37 In summary; one Au NPAP[5]A deposition cycle shows the lowest SERS response for all the laser lines due to the lack of nanoparticle coupling, as it can be predicted from optical properties. Whereas (AuNP-AP[5]A)2 thin films exhibit the best SERS efficiency. Interestingly, for the third Au NPs-AP[5]A addition there is an abruptly decrease in the SERS efficiency. In general the 785 nm excitation laser line leads to the best SERS response independently of the number of cycles probably due to its matching with the plasmon coupling band. A similar picture is obtained from the simulations, which confirms the experimental measurements. We used the CAD models showed in Figure S10 which emulate as closely as possible the (AuNP-AP[5]A)1, (AuNP-AP[5]A)2, (AuNP-AP[5]A)3 thin films (see details above). On these substrates, SERS mappings were calculated for normal irradiations with light at the wavelengths of 633, 785 and 830 nm (see Experimental Section). To more accurately describe the distribution of the analyte molecules, the SERS enhancement was calculated on closed surfaces (namely skins) surrounding each nanoparticle, with a nanoparticle-to-molecule separation of 0.72 nm (corresponding to half of the AP[5]A length). This procedure for the SERS calculation is consistent with a situation in which the analyte molecules can easily penetrate all interstitial regions of the assembly.38 The simulated SERS enhancement mappings (see Figures S10C-H in SI) are highly uniform and homogenous in intensity over the whole area, independently of the laser line. Additionally, the plot of the average SERS enhancement simulated for the different laser lines (see Figure 2F) shows that the (AuNPAP[5]A)2 film exhibit the highest SERS enhancement being the 785 nm laser the most effective, in agreement with the experimental results. Recyclability of the plasmonic substrates. An interesting feature of the proposed sensing platform is its recy-

Figure 3. A) Time-resolved SERS spectra of pyrene with the immersion time of (AuNP-AP[5]A)2 thin film in DMF. B) Reversible SERS behavior of the (AuNP-AP[5]A)2 substrate for the detection of 0.1 µM pyrene along five cycles.

clability which overcomes the single-use limitation present in traditional SERS substrates. As the capture of analytes by the AP[5]A cavity is based on non-covalent interactions, such interaction could be reversed under appropriate conditions. In the current case, pyrene, and PAHs in general, are hydrophobic molecules which can bind in aqueous medium to AP[5]A cavities through hydrophobic forces. Therefore, a decrease in the polarity of the solvent should produce a decrease in the binding affinity of PAH molecules towards AP[5]A cavities, and eventually a release of the previously bound analytes. To perform the analysis of recyclability (AuNP-AP[5]A)2 substrate was chosen since it presents the highest SERS efficiency. Thus the plasmonic substrate was immersed in an aqueous solution of 0.1 µM pyrene for 1 h and after drying the SERS spectrum was recorded (see spectrum “0 h” in Figure 3A). Subsequently, the substrate was immersed in fresh DMF at room temperate and the pyrene release was time-monitored by SERS (see Figure 3A). It should be pointed out that although the complete pyrene release was achieved after 4 hours at room temperature, the process could be speed up by 4-fold increasing the temperature to 40ºC (data not shown). After removing all bound pyrene the substrate could be reused several times without observing changes in its performance. As depicted in Figure 3B, the intensity of the SERS signal (594 cm-1) is



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Figure 4. Top: SERS spectra of pyrene (A), nitropyrene (B) and anthracene (C) obtained from aqueous PAH solutions at different concentrations. The insets show the molecular structure of pyrene (A), nitropyrene (B) and anthracene (C). Bot-1 -1 tom: (D) SERS intensity at 594 cm as a function of pyrene concentration, (E) SERS intensity at 633 cm as a function of -1 nitropyrene concentration, and (F) SERS intensity at 395 cm as a function of anthracene concentration. The dashed lines are linear fits in the quantitative detection regions (ii). All measurements were performed using 785 nm as excitation laser line. Error bars indicate standard deviation.

almost similar in the 5 detection cycle tested, indicating recyclability of the (AuNP-AP[5]A)2 platforms. Ultrasensitive SERS-based detection of PAHs. To demonstrate the sensing and versatile capabilities of the plasmonic substrate we extended the SERS analysis to other PAHs (nitropyrene and anthracene, apart from pyrene). Thus we investigated the limit of detection (LOD) and quantitative SERS detection region for each PAH separately. It was performed by immersing the (AuNP-AP[5]A)2 substrate in PAHs aqueous solutions at different concentrations and recording their SERS spectra after air-drying (Figures 4A-C). Table S1 shows a detailed vibrational band assignment for pyrene, nitropyrene and anthracene. Besides, SERS intensity mappings were also recorded on (AuNP-AP[5]A)2 substrates (typically 45 × 45 μm, 5 μm step size) at 594 cm−1, 633 cm−1 and 395 cm−1 for pyrene, nitropyrene and anthracene, respectively. SERS mappings allowed us to carry out a statistical SERS analysis from at least 81 spectra (see Figures S11-13 in SI). As depicted in Figures 4D-F, the SERS intensity vs PAH concentration reveals three different regions: i) At high PAH concentrations (≥10-6 M for pyrene and ≥10 M for nitropyrene) a saturated region is distinguished where SERS intensity is independent of analyte concentration. This behavior could be attributed to the fact that all AP[5]A cavities are occupied by PAH molecules which are in excess. Interestingly, this saturated region could not be reached for anthracene (Figure 4F) most probably due to its low solubility in water. -5

ii) The second region corresponds to the quantification region, where a linear variation of the SERS signal with the analyte concentration is observed. The SERS measurements showed that a quantitative detection of pyrene could be achieved in a concentration range from 1 µM down to 5 nM (Figure 4D). Within this range the pyrene concentration could be quantitatively expressed by the empirical formula: Log I = 0.63 Log C + 8.11 (R2=0.97) where I is the SERS intensity (in counts/mW s) and C is the pyrene molar concentration. In the case of nitropyrene the quantitative detection was achieved from 5 µM down to 0.05 µM (Figure 4E) being expressed quantitatively by the empirical formula: Log I = 0.51 Log C + 6.61 (R2=0.99). Finally, the quantification detection range for anthracene was from 10 µM down to 50 nM (Figure 4F), and expressed by the empirical formula: Log I = 0.87 Log C + 8.67 (R2=0.99). It should be pointed out that within these concentration ranges (quantification region) the spatial distribution of the PAHs over the substrate was homogenous, since the SERS mappings showed a probability to obtain SERS signals higher than 90% in all cases (see Figure 2C and Figures S8 in SI). iii) At concentrations lower than the quantification region threshold (see Figure 4D-F) the SERS signals could only be detected at random points in the mapping area. Moreover the probability of obtaining SERS signals diminished with the decrease in the analyte concentration. Therefore systematic SERS mapping measurements are necessary in order to determine the limit of detection (see


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Figure 5. (A) SERS spectra of (AuNP–AP[5]A)2 substrate (black), nitropyrene (red), anthracene (blue), pyrene (green) and a mixture of the three PAH (pink). All spectra were recorded using (AuNP–AP[5]A)2.thin film as plasmonic platform. The excitation laser line was 785 nm in all cases. (B) PCA analysis of score plot modeled by the SERS -7 spectra of 5 10 M pyrene, anthracene and nitropyrene (black dots) and its binary mixtures in a 1:1 molar ratio

(pyrene:anthracene in blue, anthracene:nitropyrene in green and nitropyrene:pyrene in red). The lines are only for guide the eye.

Figures S11-13 in the SI). Thus, for pyrene the probability to obtain SERS signal decreases from >90% for 10-8 M down to 37 % and to 3.7 % for 10-10 M and 10-11 M, respectively (see Figure S11 in SI). A similar trend was observed for the other two PAHs; for nitropyrene the SERS probability decreases to 45.6 % for 10-8 M and to 7.4 % for 10-9 M, while for anthracene decrease to 64 % and 4.9 % for 108 M and 10-9M, respectively (see Figures S12 and S13 in SI, respectively). Overall the proposed (AuNP–AP[5]A)2 substrates allowed the ultrasensitive detection of three different PAHs with limits of detection (10-11 M for pyrene, 10-9 for nitropyrene and 10-9 M for anthracene) that are, at least, between one and two orders of magnitude lower than that reported in the literature (see Table S2 in SI). SERS-based multiplex detection. Usually PAHs appear in nature as a mixture so we highlight the importance not only in sensing them separately, but also in mixtures. The

three chosen PAHs (pyrene, nitropyrene and anthracene) have similar structure but exhibit different SERS signatures (Figure 5A, Table S1). This enables to distinguish at least one characteristic SERS peak for each analyte in their mixture so we can assert the validity of this substrate as a multiple PAH sensor (see Figure 5A). Since multiplex spectra are multivariate in nature, it is difficult to differentiate the spectra of the different analytes by eye alone. Alternatively, a multivariate analysis based on principal component analysis (PCA) was performed on the multiplex SERS spectrum. The PCA method, a statistical tool to for dimension reduction which can be used to detect the similarities and differences between data, was applied to the SERS spectra of the three analytes. Up to ten spectra of each PAH, measured at different places of the (AuNP-AP[5]A)2 substrate, were used for the PCA analysis. The Raman spectra were initially summarized by 30 wavenumbers, selected as those attached to the maximum F-statistics in the one-way ANOVA for the three pure analytes (pyrene, anthracene and nitropyrene). This analysis reported 30 Raman shifts around the following four positions: 273, 431, 747 and 1363. These four positions are informative on the differences among Raman spectra (Figure S14 in SI), because they correspond to the wavenumbers for which the largest differences among the three pure analytes were detected by the one-way ANOVA. The other 26 positions suggested by the ANOVA technique were discarded since they provide redundant information. Further dimension reduction from 4 to 2 (so two-dimensional plots can be displayed) was performed through (standardized) Principal Component Analysis applied to the n=30 pure samples. PCA proceeds by computing the linear combinations of the standardized wavenumbers (the loading matrix) which maximize the variance through orthogonal directions. The variance of these principal components is decreasing. In order to reduce dimension to 2, the first two components are retained. As showed in Figure 5B, the three analytes can be clearly identified by the first two components; PC1 and PC2, summarizing 51.88% and 47.44% of the variability, respectively (cumulative variance: 99.33%, see Figure 5B). Additionally, we have also measured the SERS spectra of binary mixtures of the analytes in a 1:1 ratio. Subsequently, we performed PCA analysis for the different mixtures using the loading matrix previously obtained during the PCA of the pure analytes.39 The obtained values of PC1 and PC2 of the different mixtures fall on the line connecting their components. Therefore, this type of PCA analysis could be potentially useful to detect the presence of specific analytes in mixtures. Gas-Phase Detection of PAHs. PAHs are environmental pollutants generated mainly by from anthropogenic activities, such as the incomplete combustion of organic materials or the natural gas extraction, often referred to as "fracking".40 Particularly, low molecular weight PAHs are mainly in the atmosphere in gas-phase. On the other hand, gas-phase SERS detection remains extremely challenging and it is mainly restricted to molecules with high affinity for metal, such as thiols.41 Recently Mueller et al.



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obtained from the PAHs mixture, where characteristic vibrational bands of pyrene (594 cm−1), nitropyrene (633 cm−1) and anthracene (395 cm−1) can be easily distinguished. Interestingly, similar SERS experiments performed using a AuNP substrate with no AP[5]A molecules only shows very weak signals of pyrene but not nitropyrene or anthracene signal. The results corroborate the effectiveness of AP[5]A macrocycles as molecular traps for hydrophobic analytes. Finally, spatial mappings of SERS intensity recording in the very same plasmonic substrate (300 µm x 230 µm) at 594 cm−1 (pyrene), 633 cm−1 (nitropyrene), and 395 cm−1 (anthracene), demonstrated a uniform and multiplexed PAH detection along the plasmonic substrate (Figure 6). CONCLUSIONS

Figure 6. (A) SERS spectra of the three PAHs obtained from a gas-phase mixture using as plasmonic platform (AuNP– AP[5]A)2 thin film (red) and AuNP substrate with no AP[5]A molecules (black). The characteristic vibrational bands of −1 −1 pyrene (594 cm ), nitropyrene (633 cm ) and anthracene (395 −1 cm ) are marked with shadow region. (B-D) SERS intensity -1 mappings of pyrene (B) at 594 cm , nitropyrene (C) at 633 cm 1 -1 , and anthracene (D) at 395 cm recorded in the very same area of (AuNP-AP[5]A)2 thin film. SERS measurements were -2 carried out with a 20 x objective a power of 0.11 kWcm , acquisition 1 s, excitation laser line 785 nm.

demonstrated the SERS detection of pyrene in gas-phase employing poly-N-isopropylacrylamide (pNIPAM)-coated Au nanostars.42 The hydrophobic nature of the pNIPAM coating acted as a trap for pyrene molecules. In the present case the ammonium pillar[5]arene molecules adsorbed on the Au nanoparticle surface could also act as host for gas-phase PAH molecules. First, we tested the detection of each PAH molecule separately in gas-phase. The procedure followed was the same in all cases; typically 10 μL of 10 mM PAH in DMF was placed over a glass slide in a closed container containing the (AuNP-AP[5]A)2 substrate. Subsequently, the container was heated in an oven at 150 °C for 10 minutes to promote the evaporation of the solution. Finally, the substrate was allowed to cool down and SERS analysis was performed. Figure S15 in the SI shows the gas phase SERS spectra of the three single PAHs, as well as, the SERS mappings (310 µm x 210 µm with 10 µm step size) recorded at 594 cm−1, 633 cm−1 and 395 cm−1 for pyrene, nitropyrene and anthracene, respectively. The spatial mappings show a homogenous distribution of the analytes over the substrate. Next, we investigate the gas-phase detection of the three PAHs in a mixture. We proceeded in a similar way as explained above but adding 10 μL of a DMF solution containing pyrene (3.3 mM), nitropyrene (3.3 mM) and anthracene (3.3 mM). Figure 6 shows the SERS spectrum

In summary, we have developed a reliable and label free SERS detection platform based on the electrostatic LbL assembly of Au nanoparticles and pillar[5]arene. This approach allows to control the optical response and therefore the sizes and density of aggregates as function of depositions cycles. Besides the use of pillar[5]arene as building block ensures the formation of aggregates with uniform interparticle distances (that is hot-spots). Analysis of the SERS performance of the Au-AP[5]A thin films as a function of Au- AP[5]A deposition cycles and laser lines revealed that: i) the presence of macrocycles allows to detect molecules without affinity, such as low molecular weight PAHs, through hydrophobic interactions with the macrocycle cavity; ii) all platforms give rise to uniform SERS across large areas; iii) (Au-AP[5]A)2 thin films exhibit the highest SERS efficiency independently of the excitation laser line; iv) 785 nm excitation laser line leads to the best SERS response independently of the number of cycles; v) in liquid phase (AuAP[5]A)2 thin films show lower limits of the detection for pyrene, nitropyrene and anthracene than other SERS platforms reported in the literature and these platforms exhibit quantitative detection region for each PAH; vi) recyclability of the (AuNP-AP[5]A)2 platforms is demonstrated for at least 5 detection cycle in liquid phase; vii) PCA analysis shows the capabilities of this system for the multiplex detection of three PAHs in a liquid mixture; viii) these plasmonic thin films also allow the single and multiplexed detection of PAH in gas-phase.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Visible-NIR extinction spectrum and TEM analysis of citrate-stabilized Au nanoparticles; chemical structure of AP[5]A; SEM and AFM analysis and theoretical optical response of plasmonic substrates with different AuNP-AP[5]A deposition cycles; analysis of the hostguest interaction through fluorescence spectroscopy; CAD models of the different AuNP-AP[5]A assemblies; experimental and simulated SERS mappings obtained for (AuNP-AP[5]A)1 and (AuNP-AP[5]A)3 with 633 nm, 785 nm and 830 nm excitation laser lines; SERS map-


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ACS Applied Materials & Interfaces ping analysis of pyrene, nitropyrene and anthracene at different concentrations; SERS spectra of pyrene, nitropyrene and anthracene obtained from gas-phase experiments; vibrational band assignments for the three PAH molecules; detection limits reported for the three PAH studied.

AUTHOR INFORMATION Corresponding Author * I.P.-S. E-mail: [email protected] * J.P.-J. E-mail: [email protected]

Notes The authors declare no competing financial interest

ACKNOWLEDGMENT This work was supported by the Ministerio de Economía y Competitividad (MINECO, Spain), under Grants MAT201677809-R, MAT2014-58201-C2-1-R, MAT2014-58201-C2-2-R, and Xunta de Galicia/FEDER Grupos de Referencia Competitiva (R2014/030) and AtlantTIC (Agrupación Estratéxica Consolidada de Galicia accreditation 2016-2019). Funding from The European Union (Regional Development Fund ERDF) is also acknowledge. V.M.-G. acknowledges FPU scholarship from the Spanish MINECO. The authors also thank the important support of CACTI (Center for Scientific and Technological support) at the University of Vigo their valuable assistance with SEM, TEM and AFM analysis.

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Pillar[5]arene-Based Supramolecular Plasmonic Thin Films for Label

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