ARTICLE pubs.acs.org/ac
Multicomponent Direct Detection of Polycyclic Aromatic Hydrocarbons by Surface-Enhanced Raman Spectroscopy Using Silver Nanoparticles Functionalized with the Viologen Host Lucigenin I. Lopez-Tocon,*,† J. C. Otero,† J. F. Arenas,† J. V. Garcia-Ramos,‡ and S. Sanchez-Cortes*,‡ † ‡
Department of Physical Chemistry, Faculty of Science, University of Malaga, Unidad Asociada CSIC, E-29071-Malaga, Spain Instituto de Estructura de la Materia, CSIC, Serrano 121, E-28006-Madrid, Spain
bS Supporting Information ABSTRACT:
Silver nanoparticles (NPs) functionalized with the molecular assembler bis-acridinium dication lucigenin (LG) have been used as a chemical sensor system to detect a group of polycyclic aromatic hydrocarbon (PAH) pollutants in a multicomponent mixture by means of surface-enhanced raman scattering (SERS). The effectiveness of this system was checked for a group of PAHs with different numbers of fused benzene rings, namely anthracene, pyrene, triphenylene, benzo[c]phenanthrene, chrysene, and coronene. In order to determine the host capacity of this sensor system, the self-assembly of the LG viologen on a metallic surface has been checked by analyzing SERS intensities of PAH bands at different LG concentrations. The NP-LG-analyte affinity is derived from the analysis of PAH band intensities at different concentrations of pollutants, the adsorption isotherm of each PAH on NP-LG cavities has been studied, and the corresponding adsorption constants have been evaluated. The limit of detection at tracelevel concentration is confirmed by the presence of their characteristic fingerprint vibrational bands. The SERS spectra of PAH mixtures confirm that LG viologen dication shows a higher analytical selectivity to PAHs constituted by four fused benzene rings, mainly pyrene and benzo[c]phenanthrene, in agreement with their higher affinity which is also related to their better fit into the intermolecular LG cavities. As a conclusion, SERS spectra recorded on modified NP-LG surfaces are a powerful chemical tool to detect organic pollutants.
S
urface-enhanced raman scattering (SERS) can be used as an analytical technique because of its high sensitivity and selectivity in molecular identification.1-3 This fact is mainly based on the giant electromagnetic (EM) enhancement induced by nanostructured noble metal surfaces and associated with their localized plasmon resonances.4,5 Moreover, this EM intensification occurs in very localized regions of the metallic surface called hot spots6 (HS), such as interparticle junctions or gaps between metal nanoparticles (NPs).7 In this way, single molecule detection8,9 (SMD) is related to the existence of these interparticle HS. Previous works10,11 show that many molecules adsorbed on metal surfaces are not only able to induce the formation of the HS r 2011 American Chemical Society
but can also act as molecular hosts of specific analytes.12-15 The molecules that fulfill this double function show two main features: the ability to interact with the metal surface, and affinity to bind the analyte by acting as electron acceptors, which makes them able to form CT complexes with electron-donor species. It has been shown that viologen dications characterized by an aromatic moiety and by the presence of two charged quaternary nitrogens are good candidates in the formation of highly sensitive Received: October 21, 2010 Accepted: February 10, 2011 Published: March 10, 2011 2518
dx.doi.org/10.1021/ac102771w | Anal. Chem. 2011, 83, 2518–2525
Analytical Chemistry HS localized in junctions between Ag NPs as well as in detecting pollutant compounds such as polycyclic aromatic hydrocarbons (PAHs).12,15,16 We have employed SERS-active sensing devices in which the viologen dication lucigenin (N,N0 -dimethyl-9,90 -biacridinium dication, LG) adsorbed on hydroxylamine Ag NPs is used as a molecular tether to create interparticle HS, capable to enhance the SERS signal as well as to allocate analyte molecules. In our previous works,12,15,17 this ability was optimized for the individual detection of several PAHs. In the present work, the detection ability of such substrates is tested in the analysis of a group of six PAHs with different number of fused benzene rings, namely anthracene (ANT), pyrene (PYR), triphenylene (TP), benzo[c]phenanthrene (BcP), chrysene (CHR), and coronene (COR). These PAHs are hydrophobic and dangerous pollutants, and many of them are considered strong carcinogens.18,19 Therefore, it is of great importance to find a routine control analysis for their trace detection. In this work, the experimental conditions are changed with respect to the previous papers12,15 in order to improve the analytical procedure as well as the preparation of silver NPs at room temperature by reduction of silver nitrate with hydroxylamine instead of citrate,15 which needs instead a more complex preparation under reflux at controlled temperature, to avoid the possible interference of citrate (or subproducts) SERS bands. Moreover, SERS macro detection is used instead of micro conditions, which requires specific Raman instrumentation and the averaging of several spectra recorded on different sites as well as much more complex manipulations to immobilize the silver substrate.12,15 Another goal of this work is to monitor the effectiveness of LG-functionalized Ag NPs in the detection of PAHs to investigate the influence of their molecular structure, i.e., the number of benzene rings, geometry, and aromaticity, on the guest-host interaction. As a first approach of this work, the self-assembly of LG viologen on metallic surface has been checked by analyzing SERS intensities of PAH bands at different LG concentrations. The SERS detection of each PAH was allowed by the presence of their characteristic fingerprint vibrational bands at trace-level concentrations. Thus, the NP-LG-analyte affinity of each PAH is derived from the analysis of PAH band intensities at different concentrations of pollutant. From this analysis, the limit of detection (LOD) for each PAH was determined, and the corresponding hosting constants on NP-LG cavities have been estimated from the respective adsorption isotherms. This is a quite interesting issue given that PAHs do not form complexes with LG in water and do not show any affinity toward Ag surfaces lacking functionalization, making it impossible to record SERS spectra. Under the analytical point of view, the main interest of this work consists of the fact that it is a multicomponent direct detection of a family of PAHs. Very scarce SERS works on multicomponent analysis have been published up to date, including a set of recent studies.20-23 However, the adsorption of an analyte on the silver surface may be influenced by the presence of other pollutants. Accordingly, LG-functionalized Ag NPs were finally applied to the direct detection of multicomponent PAH mixtures at different concentrations of each pollutant and different laser exciting lines. From this assay, it is possible to estimate the LG/PAH affinity modification by the presence of other species, which is another key point to be considered in the determination of the SERS viability in chemical analysis.
ARTICLE
’ EXPERIMENTAL SECTION PAHs and LG of the highest available purity were purchased from Aldrich and used as received. Solutions of each PAH and LG were prepared using acetone (99%) and water as solvents, respectively. Pure water was obtained from a Milli-Q instrument. Preparation of Silver Sols. The procedure to prepare Ag NP suspensions has been explained elsewhere.13,24 Briefly, it consists of reducing an aqueous solution of 10-2 M AgNO3 with hydroxylamine hydrochloride in basic medium under vigorous stirring. Other reducing agents such as citrate and NaBH4 can be used to obtain colloidal suspensions, but it has been shown that the NPs obtained from hydroxylamine show a more uniform distribution of size and shape and no interference from the remaining oxidation products are detected.25 The Ag NP suspension was characterized by the resonance spectra of metallic plasmons, showing a maximum at 410 nm with an average full width at halfheight (fwhm) of 80 nm. Samples for SERS experiences were prepared by adding 20 μL of a 0.5 M aqueous solution of KCl to 1000 μL of the silver colloid. The chloride solution is employed to activate the surface of the silver nanoparticles, inducing their aggregation and then the adsorption of the linker (LG) via either the strong interaction through an ionic pair or the formation of a CT complex.13 The aggregation leads to a slight change of the color as well as an enhancement of the plasmon adsorption at higher wavelengths. Then 10 μL of a solution of the assembler LG was added in order to functionalize the surface and, afterward, 10 μL of the PAH solution to reach the desired final concentration. SERS spectra at different concentrations of LG but constant concentration of PAH have been recorded in order to optimize the concentration of LG. Then, SERS spectra at different concentrations of PAH but a constant concentration of LG have been recorded in order to establish the detection limit. Instrumentation. SERS spectra obtained with the 785 nm exciting line have been recorded on a Renishaw Raman Microscope System RM2000 fitted with a diode laser and an electrically cooled CCD camera. The laser power reaching the sample was about 2.0 mW, and the spectral resolution was set to 2 cm-1. The SERS spectra obtained with the 1064 nm exciting line have also been recorded on an FT-Raman spectrometer (Bruker RFS 100/S instrument) provided with an Nd:YAG laser and a Ge detector cooled with liquid nitrogen. The laser power reaching the sample was about 200 mW, and the spectral resolution was set to 4 cm-1. All the SERS spectra were recorded under macro conditions by using standard 1 cm path length quartz cells. Theoretical Calculations. Optimized structure and force field of PAHs were calculated at B3LYP/6-31G* level of theory by using GAUSSIAN03 program package.26 Geometry optimizations of PAHs have been constrained to their respective planar structures, and their corresponding Cartesian coordinates have been submitted as Supporting Information (Table S-1). Figure 1 shows the B3LYP/6-31G* optimized geometries of LG and PAHs. The calculated harmonic frequencies are scaled as usual by a factor of 0.985 to account for anharmonicity and electron correlation effects.27 All the calculated wavenumbers are real, indicating that the structures correspond to equilibrium geometries, except for CHR and BcP, for which one and three imaginary frequencies were calculated, respectively. These imaginary frequencies correspond to out-of-plane deformations of the aromatic rings, indicating that CHR and BcP are nonplanar due to steric hindrance between adjacent CH bonds. 2519
dx.doi.org/10.1021/ac102771w |Anal. Chem. 2011, 83, 2518–2525
Analytical Chemistry
ARTICLE
Figure 2. (a) Raman spectrum of aggregated colloid with KCl. (b) Typical SERS spectra of any PAH without LG. (c) LG (10-4 M) SERS spectrum. (d, e, f, g, and h) LG/PAH (10-4/10-6 M) SERS spectrum of ANT, PYR, TP, CHR, and BcP, respectively. (i) LG/COR (10-4/10-8 M) SERS spectrum. Exciting line 785 nm. Asterisks mark characteristic PAH bands.
Figure 1. B3LYP/6-31G* optimized geometries of lucigenin (LG) and PAHs: anthracene (ANT), pyrene (PYR), triphenylene (TP), chrysene (CHR), benzo[c]phenanthrene (BcP), and coronene (COR).
coverage, on the size of the cavity, and on the structure of the LG molecule, which shows a more or less titled structure (Figure 1).17 Once the NP surface is functionalized, PAH molecules may be hosted into the NP-LGhost cavities: NP - LGhost þ PAH / NP - LGhost - PAH
’ RESULTS AND DISCUSSION Adsorption of PAHs on NP-LG Cavities. PAHs do not
adsorb directly on silver NPs, and therefore functionalization of the metal surface is required to record their SERS spectra:
LG is the organic molecule selected to create hydrophobic cavities on the surface able to host PAHs. LG is adsorbed on NPs according to the equilibrium: NP þ LG / NP - LG However, not any adsorbed LG molecules on the NP surface (NP-LGTotal) are suitable to host PAH molecules (NP-LGhost). The total concentration can be expressed as:
This equilibrium refers to the hosting process, so that the affinity of the PAHs toward the NP-LGhost cavities can be quantified by means of the respective hosting constant Khost. Consequently, the SERS signal of PAHs depends on the number of NP-LGhost-PAH systems, which depends in turn on the molar concentration of both LG and PAH through the respective adsorption isotherms. Thus, we have studied first the effect of the LG surface coverage on the SERS signal at a constant concentration of PAHs. Thereafter, the PAH concentration is changed, keeping constant the optimum LG concentration. The SERS intensity of each PAH band (ISERS) reaches a maximum (ISERS max ) once the available NP-LGhost cavities are filled, which makes the hosting process resemble a Langmuir isotherm. The hosting constant Khost can be estimated from the well-known equation: ½PAH ½PAH 1 ¼ SERS þ SERS SERS I Imax Khost Imax
NP - LGtotal ¼ NP - LGNO - host þ NP - LGhost, where NP-LGhost fraction increases at higher LG concentrations, but it could decrease because of either excessive compact packing of the LG monolayer or excessive self-aggregation in the multilayers, which can destroy the suitable NP-LGhost cavities. Therefore, the hosting capacity depends on the LG surface
ð1Þ
Raman and SERS Spectra of PAHs. Figure 2a shows the Raman spectra of the Ag colloid aggregated with KCl using the 785 nm exciting line. No SERS spectra of any PAH can be obtained in the absence of LG (Figure 2b). However, characteristic 2520
dx.doi.org/10.1021/ac102771w |Anal. Chem. 2011, 83, 2518–2525
Analytical Chemistry
Figure 3. Raman spectra of solid PAHs: (a) ANT, (c) PYR, (e) TP, (g) CHR, (i) BcP, and (k) COR and their respective SERS spectra (LG/ PAH: 10-4/10-6 M) after subtraction of LG (10-4 M) SERS bands. All spectra are recorded using the 785 nm exciting line.
bands corresponding to each analyte are observed when SERS spectra of LG/PAHs are recorded (Figure 2d-i). In order to assign the bands belonging to LG and PAHs, SERS spectra of LG (Figure 2c) and Raman spectra of PAHs in the solid state (Figure 3) have been recorded. Figure 3 shows also the SERS spectra of each particular PAH obtained after subtraction of the LG SERS spectrum. Quite similar spectra were recorded by using the 1064 nm exciting line. Tables 1 and 2 show the proposed assignment of strong and medium Raman and SERS bands of PAHs on the basis of the B3LYP/6-31G* force field results. It can be seen that no significant
ARTICLE
wavenumber shifts are observed between SERS and Raman spectra, which suggests that negligible structural modifications of the PAHs occur when adsorbed on the LG-functionalized NPs. All the SERS spectra display strong bands in the 1400-1350, 590-750, and 500-400 cm-1 regions that can be assigned to the totally symmetric modes 14, 1, and 12/6a, respectively. These bands are characteristic of fused benzene rings and are highlighted in bold in the tables. Taking into account the selection rules of the EM mechanism of SERS, one could conclude that the PAHs adsorb on the metal surface in a more or less perpendicular orientation given that only in-plane SERS bands are observed. All this suggests that LG adsorbs on the metal surface, forming hydrophobic cavities able to host the PAHs. Previous analysis of the Raman and SERS spectra16,17 showed that LG stands slightly tilted with respect to the metal surface provided that very weak SERS enhancement of the out-of-plane vibrations was observed. The B3LYP/6-31G* optimized geometry of LG yields a conformation in which each acridinium plane is twisted 90.10° with respect to the other (Figure 1). This conformation allows for the existence of intermolecular cavities of LG across the self-assembled monolayer where PAHs can be hosted and become close to the Ag surface. It has been shown that the interaction between LG and the metal surface takes place through the formation of either an ionic pair or, more probably, a charge transfer (CT) complex with the Cl- ions.28,29 This interaction mechanism implies that the guest-host binding is due to a π-π stacking interaction.13,14 SERS wavenumbers of isolated LG correlate quite well with those of the LG/PAHs spectra, very slight shifts being observed between them (Table S-2 of Supporting Information). These could be related to the adsorption of LG on the metal surface and to the twisting of the acridinium planes as shown in previous work.15 Therefore, the structural conformation of LG should only be slightly changed when PAHs molecules are hosted in the intermolecular cavities formed between each pair of LG dications.15 Effect of the LG Surface Coverage on the Raman Signal of PAHs. Surface functionalization and self-assembly of the LG viologen on the metallic surface in order to build adequate NPLGhost cavities have been checked by analyzing SERS intensity of isolated LG (Figure S-2 in Supporting Information) and LG/PAHs mixtures recorded at different LG concentrations ranging from 10-4 down to 5 10-10 M, while keeping constant the PAH concentration at 10-6 M. This allows us to determine the optimum LG concentration for the detection of PAHs. We have only monitored those PAHs whose characteristic bands are clearly resolved, namely LG/PYR, LG/TP, and LG/BcP systems (see Figure 2). Concerning the reproducibility of the results, SERS intensities have been averaged over at least three different spectra recorded under the same experimental conditions. Standard deviations for the LG bands are shown in Figure S-2, and Table S-3 (Supporting Information) collects typical values of the relative errors for LG and LG/PYR, LG/TP, and LG/BcP mixtures at the representative concentration of 10-6 M. The uncertainty in the measurements of the intensities for these systems is
