Article pubs.acs.org/ac
Sensitive Surface-Enhanced Raman Spectroscopy (SERS) Detection of Organochlorine Pesticides by Alkyl Dithiol-Functionalized Metal Nanoparticles-Induced Plasmonic Hot Spots Jana Kubackova,†,‡ Gabriela Fabriciova,† Pavol Miskovsky,†,§ Daniel Jancura,†,§ and Santiago Sanchez-Cortes*,‡ †
Department of Biophysics, Faculty of Science, P.J. Safarik University, Jesenna 5, Kosice 04 154, Slovakia Instituto de Estructura de la Materia, IEM-CSIC, Serrano 121, 28006 Madrid, Spain § Center for Interdisciplinary Biosciences, Faculty of Science, P.J. Safarik University, Jesenna 5, Kosice 04 154, Slovakia ‡
S Supporting Information *
ABSTRACT: In this work, we report the detection of the organochlorine pesticides aldrin, dieldrin, lindane, and α-endosulfan by using surface-enhanced Raman spectroscopy (SERS) and optimization of the SERS-sensing substrate. In order to overcome the inherent problem of the low affinity of the above pesticides, we have developed a strategy consisting of functionalization of the metal surface with alkyl dithiols in order to achieve two different goals: (i) to induce the nanoparticle linkage and create interparticle junctions where sensitive hot spots needed for SERS enhancement are present, and (ii) to create a specific environment in the nanogaps between silver and gold nanoparticles, making them suitable for the assembly and SERS detection of the analyzed pesticides. Afterward, an optimization of the sensing substrate was performed by varying the experimental conditions: type of metal nanoparticles, molecular linker (aromatic versus aliphatic dithiols and the length of the intermediate chain), surface coverage, laser excitation wavelength. From the adsorption isotherms, it was possible to deduce the corresponding adsorption constant and the limit of detection. The present results confirm the high sensitivity of SERS for the detection of the organochlorine pesticides with a limit of detection reaching 10−8 M, thus providing a solid basis for the construction of suitable nanosensors for the identification and quantitative analysis of this type of chemical.
S
phenomenon is of great importance in SERS, where metal colloids are commonly used to obtain signal intensification even for single molecule detection experiments.11,12 An ideal situation for building interparticle HS is the use of bifunctional molecules, which act as NP linkers.13 Many molecules adsorbed on metal surfaces are not only able to induce the formation of the HS, but can also act as molecular hosts of specific analytes.14 In recent works, we have employed bifunctional molecules for the detection of organic pollutants and doping substances, even at very low concentrations,6,15 which otherwise are unable to link the metal surface and approach the HS on the NP surface. In the present work, we have employed dithiols for the functionalization of metal NPs to obtain SERS-active substrates for the detection of the organochlorine pesticides (OPs) aldrin, α-endosulfan, dieldrin, and lindane. The characterization of the adsorption mechanisms of aliphatic dithiols with different chain lengths on metal (silver and gold) NPs surface and morphology of the obtained NPs assemblies have been thoroughly
urface-enhanced Raman scattering (SERS) is an extremely sensitive method that gives an enhancement of ∼106 in scattering efficiency over normal Raman scattering1 (in some extreme cases, the enhancement can be up to ∼1014−1015); thus, it is a powerful analytical technique. Such great enhancement is, under certain conditions, sufficient for the detection of a single molecule or a few molecules, making SERS an extremely useful technique for very sensitive chemical and biomolecular detection.2,3 The enhancement of the Raman intensity requires localization of the analyzed molecules near or at nanostructured noble metal surfaces and is associated with localized surface plasmon resonance (LSPR) induced on the metal surface.1,4−6 Currently, the most employed SERS substrates are metal nanoparticles (NPs) in suspensionthat is to say, colloids.7 The sensitivity and selectivity of metal NPs to analytes can be remarkably enhanced by changing the interfacial properties of metal surfaces. One of the possible modifications can be functionalization of the metal surface by molecules that are able to activate the formation of intramolecular or intermolecular cavities, thus acting as hosts to attach analytes.8,9 Nowadays, it is widely accepted that the main part of the field intensification occurs in special regions of the metallic surface called hot spots (HS), which are mainly localized in interparticle gaps.10 This © 2014 American Chemical Society
Received: September 11, 2014 Accepted: November 23, 2014 Published: December 15, 2014 663
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most frequently methods used to analyze water samples containing the studied OPs are gas chromatography equipped for electron capture detection (QC/ECD) and gas chromatography−tandem mass spectrometry (GC-MS).27,28 GC/ECD provides a higher degree of sensitivity than GC/MS; however, ECD does not provide the molecular structure information that can be obtained with an MS detector. The structural information increases the level of confidence that the compound being measured has been correctly identified.29 However, the development of low-cost, easy-going, and reliable approaches for the detection of pesticides is still needed. In this regard, SERS technique can be employed to develop a detection system capable of analyzing low concentrations of pesticides. In order to accomplish this task, we have employed aliphatic and aromatic dithiols for two different purposes: (i) to link metallic nanoparticles creating the appropriated physical conditions to induce plasmonic hot spots, and (ii) to modify the chemical properties of these hot spots, making them more suitable for the molecular assembly of liposoluble pesticides. In the future, this approach should be applicable for the practical analysis of the samples from different areas of the environment (water, soil), because of the advantages provided by the SERS method: univocal identification of the chemicals, fast and in situ analysis, and last but not least, low financial cost, in comparison with the currently utilized techniques.
investigated by us and the results have been recently published.16,17 Aldrin, α-endosulfan, dieldrin, and lindane are pesticides that belong to the organochlorine family (structures are shown as insets in Figure 1). All these pesticides are poisons and pose
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EXPERIMENTAL SECTION Preparation of Silver and Gold Colloids. Ag NPs in suspension were prepared by two different methods. Citrate Ag NPs (AgC) were obtained by the following procedure:30 1 mL of a 1% (w/v) trisodium citrate aqueous solution was added to 50 mL of a boiling 10−3 M silver nitrate aqueous solution, and boiling was continued for 1 h. The obtained colloid showed a turbid gray aspect and had a final pH of 6.5. Hydroxylamine Ag NPs (AgH) were obtained by the method described previously by Cañamares et al.31 Briefly, 10 mL of a 10−2 M silver nitrate solution are added dropwise to 90 mL of a 1.6 × 10−3 M solution of hydroxylamine hydrochloride adjusted to pH 9 under vigorous stirring. The resulting spherical Ag NPs have an average size (diameter) of 35 nm.31 Au NPs in colloidal suspension (AuC) were prepared using the method described by Frens.32 Briefly, 0.1 mL of an aqueous solution of HAuCl4 (0.118 M) was diluted in 40 mL of water under intense stirring. Then, 1 mL of 1% sodium citrate solution in volume was added dropwise. The yellow solution was refluxed for 5 min, resulting in a red colloidal suspension. The average diameter of Au nanoparticles prepared by the above method is 15−20 nm.33 All aqueous solutions needed for preparations of the metal colloids were prepared using Milli-Q water. Preparation of Samples for SERS Experiments. Samples for SERS measurements were prepared as follows. A total of 1 mL of the colloid (silver as well as gold) was activated by the addition of 40 μL of 0.5 M KNO3. The activation of the citrate colloids by nitrate is also related to the removal of the citrate excess existing on the fabricated nanoparticles after their preparation. Then, 10 μL of a solution of aliphatic dithiol, the concentration of which was adjusted according to the final dithiol concentration, were added to 1 mL of the colloidal suspension. After the aggregation of NPs by the dithiol, 10 μL of the organochlorine pesticide, also at an appropriate concentration, were added to the above mixture. In all SERS
Figure 1. Raman spectra of the solid state of the studied pesticides: aldrin (spectrum (a)), endosulfan (spectrum (b)), lindane (spectrum (c)), and dieldrin (spectrum (d)). Insets show the molecular structure of the associated analyzed pesticide. Excitation line at 785 nm.
long-term danger to the environment and humans through their persistence in nature and adipose tissue.18 Aldrin, dieldrin, and α-endosulfan were extensively used mainly in agriculture as a seed dressing for food or soil insecticides on commodity crops, such as tea, coffee, grain, fruits, and vegetables, as well as on cereals and also on nonfood crops such as tobacco and cotton.19,20 Endosulfan and lindane have been also used as a wood preservative.21,22 In addition, lindane has been used as an insecticide for several fruit and vegetable crops, in baits and seed treatments for rodent control, and for the treatment of scabies (mites) and lice.23,24 Because aldrin has harmful effects on human health and the health of wildlife, this chemical is no longer produced or used.19 The threat posed by endosulfan to the environment evocated a global ban of its use, and its manufacture was considered under the Stockholm Convention on persistent organic pollutants.25 The production of lindane is also limited, because of the relationship between an exposure to lindane or its isomers and the appearance of cancer in human beings.9 Even if some of these pesticides are currently banned in many countries, they are still spread throughout the world in many environments and represent a serious danger to human health, because of the persistent character of these compounds, their long-term accumulation in adipose tissues, and their demonstrated carcinogenic effect.26 Therefore, the detection and identification of these chemicals is very important. The 664
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Figure 2. (a) Scheme displaying the pesticide hosting (in this case, for aldrin) in the dithiol layer organized in interparticle gaps. SERS spectra of aldrin (10−5 M) (b) on AgC in the absence of dithiols and (d) on DT8-functionalized AgC. The latter spectrum shows the bands corresponding to DT8 that are seen in panel (c) for AgC functionalized with DT8 (10−5 M). The Raman of the solid aldrin is presented in spectrum (e). All spectra were obtained at 785 nm excitation. (f) SERS spectra of the analyzed pesticides (10−5 M) on DT8-functionalized AgC NPs exciting at 785 nm, showing the C−Cl stretching bands of the fingerprint region (300−400 cm−1) and the C−S stretching bands of DT8 in the deduced multilayer highly ordered conformation employed as reference band for quantitative analysis.
weak CC stretching bands in the case of the bicyclic pesticides (aldrin, α-endosulfan, and dieldrin), appearing at 1599−1605 cm−1 and attributed to the stretching vibration of the Cl−CC−Cl moiety. Aldrin also exhibits an extra CC band appearing at 1560 cm−1, corresponding to the CC localized in the adjacent cycle of the molecule (Figure 1a). Finally, the bands observed between the two above-mentioned regions correspond to CC and CH stretching and CH2 bending from the aliphatic cyclic structure of the analyzed molecules. SERS Spectra of Organochlorine Pesticides. The OPs studied in this work do not display any Raman band in the presence of metal NPs, using either Ag or Au colloids, as can be seen in Figure 2b for aldrin (10−5 M). This observation can be explained by the extremely low affinity of the analyzed pesticides to link the metal surface and, consequently, to benefit from the EM field enhancement induced on the surface. To change this situation, we have modified the chemical properties of the metal surface by functionalizing the metal nanoparticles with molecular linkers, alkyl dithiols (AD) and aromatic dithiols (ArD). AD adsorbed on the Ag and Au NPs
measurements, the sample was placed in a 1-cm-optical-path quartz cuvette, and these measurements were performed 10 min after the addition of the pesticide to the mixture.
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RESULTS AND DISCUSSION Raman Spectra of Organochlorine Pesticides. The Raman spectra of the solid state of aldrin, α-endosulfane, lindane, and dieldrin are shown in Figure 1. The tentative assignment of the vibrational bands of the studied pesticides was made on the basis of the data found in the literature.34,35 Briefly, the Raman spectra are dominated by a group of bands appearing in the 300−400 cm−1 region, attributed to C−Cl stretching vibrations (ν(CCl)) (Figure 1). These bands are the most intense in the Raman spectra, since the C−Cl stretching induces a large variation of the polarizability of the analyzed molecules. Therefore, these bands can be employed as an actual fingerprint region to carry out the SERS detection of the studied OPs. The bands appearing below 300 cm−1 are attributed to bending vibrations (δ(CCCl), δ(ClCCl), and δ(CCC)). The Raman spectra of these pesticides also include 665
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fulfill two different roles (see Figure 2a for a schematic explanation): (i) linking of NPs and forming hot spots, as demonstrated in previous works;36 and (ii) functionalization of the metal surface to attach the pollutant just to the gap between NPs, where the EM field is much more enhanced than in other parts of the metal surface. The NPs linking induces large changes in the extinction spectra due to the strong NP assembly induced by AD. These effects are summarized in Figure S1 in the Supporting Information for the special case of the addition of ADs to Ag NPs at a final concentration of 10−5 M. The main effect is the intensity decrease of the single particle band (at 405 nm) and the appearing of a second, wider band centered in the 770−850 nm range. The functionalization of metallic NPs with AD leads to a change of the chemical properties of the interface that allows the observation of the intense Raman bands of the pesticides in the 300−400 cm−1 region corresponding to the ν(CCl) bands. In Figure 2d, we show this effect for the particular case of aldrin. These bands correlate very well with the bands seen in the classical Raman spectrum of the solid aldrin (Figure 2e), and are clearly distinguished from the dithiol bands in the 600− 750 cm−1 region (Figure 2c), which were employed as reference to calculate the relative SERS intensities of the analyzed pesticides. A detailed analysis of the observed SERS spectra reveals that only slight changes are produced in the position of the main bands of either the dithiol or the pesticide molecule, indicating that the interaction between these molecules is not very strong. However, this interaction is sufficient to induce the inclusion of the pesticide into the nonpolar aliphatic environment by van der Waals and/or hydrophobic forces. These interactions also occur in the adipose tissue of mammals and in the cells plasmatic membrane where the OPs can be accumulated. The small relative change in the intensities of the SERS bands of the pesticides are attributed to the specific orientation of these molecules inside the dithiol layer, formed on the metal surface, as deduced from the propensity rules predicted by the electromagnetic mechanism of SERS.37 Optimization of the Detection of Organochlorine Pesticides by SERS. To optimize the functionalization of NPs by the adsorption of AD, we have carried out a study consisting of the variation of several key factors essential for the ability of these systems to detect the OPs by SERS. These factors are (i) the structure of dithiols, (ii) the chemical properties of the metal surface, (iii) the aliphatic layer organization on the surface, and (iv) the excitation wavelength. The molecular structure of dithiols varied by using aliphatic or aromatic dithiols and by changing the length of the chain in aliphatic dithiols. For this purpose, we have used 1,6-hexanedithiol (DT6), 1,8-octanedithiol (DT8), and 1,10-decanedithiol (DT10). The chemical properties of the metal surface were modified by using plasmonic NPs prepared in different ways: hydroxylamine and citrate Ag NPs and citrate Au NPs. The layer organization around NPs was modified by varying the surface coverage of NPs at different dithiol concentrations. An important step in the optimization process of the SERS detection of the pesticides was to determine the most suitable type of metal substrate. In order to carry out this study, we have investigated three types of plasmonic NPs: AgC, AgH, and AuC. Figure 3b (red bars) shows the relative intensity of the most intense SERS band of aldrin at 10−5 M, which is the band appearing at 350 cm−1 (Figure 2d). The SERS spectra of aldrin
Figure 3. Optimization of the linker (AD) and the plasmonic substrate in the detection of aldrin: (a) Schematic showing the interparticle gaps, and (b) the intensity of the 350 cm−1 band of aldrin is displayed as a function of different dithiol-functionalized metal NPs. DT8 provided the most intense spectrum on AgC substrate due to the best plasmonic conditions of the interparticle gaps.
were obtained in the presence of all of the ADs used, but we only show the results obtained on all the studied NPs systems covered by DT8. It is evident that the most intense bands appear in the SERS spectrum in the presence of AgC colloid, while utilization of Au NPs leads to a spectrum with low SERS intensity (Figure 3). In the presence of the AgH colloid, the SERS spectrum of aldrin is also intense; however, this is not the case for AgC. The relative intensities of the aldrin bands in the 300−400 cm−1 region are identical in both spectra obtained using AgC and AgH colloids. This observation suggests that the method of preparation of silver colloids does not influence the orientation of the pesticide inside the dithiol layer. The AgC was also determined to be the best SERS substrate for the other analyzed pesticides (results not shown), although a worse matching with the plasmon resonance of aggregates is observed (see Figure S2 in the Supporting Information). The reason to obtain a higher efficiency in the pesticide detection on AgC NPs, in comparison with other analyzed SERS substrates, seems to be the differences in the chemical properties of the exposed interfaces. In fact, the easiest removal of the citrate ions existing in AgC by the added dithiols, in comparison to Cl− ions existing in AgH, can be the reason for the better properties of this type of colloid, since, in the case of AgH NPs, the Cl− ions adsorbed on the surface of this substrate31 are more strongly attached on the surface avoiding the adsorption of dithiols. Au NPs are less effective for SERS detection of the pesticides, because the structure of dithiols adsorbed on this metal is more disordered, as reported in our previous work.17 This fact prevents the formation of suitable sites for the incorporation of the pesticides inside nanoparticle gaps. The employment of aromatic dithiols (1,4-benzenedithiol (BDT) and biphenyl-4,4′-dithiol (BPDT)) in the detection of 666
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pesticide hosting is expected. At submonolayer concentrations ( aldrin > endosulfan > lindane
The different values of the affinity constants are probably associated with the variable solubility of the analyzed substances in the aliphatic multilayer created by the adsorption of DT8 on Ag NPs. 668
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(8) Guerrini, L.; Garcia-Ramos, J. V.; Domingo, C.; Sanchez-Cortes, S. Anal. Chem. 2009, 81 (4), 1418−1425. (9) Lopez-Tocon, I.; Otero, J. C.; Arenas, J. F.; Garcia-Ramos, J. V.; Sanchez-Cortes, S. Anal. Chem. 2011, 83, 2518−2525. (10) Le Ru, E. C.; Etchegoin, P. G.; Meyer, M. J. Chem. Phys. 2006, 125, 204701−204714. (11) Futamata, M. Faraday Discuss. 2006, 132, 45−61. (12) Vlckova, B.; Moskovits, M.; Pavel, I.; Siskova, K.; Sladkova, M.; Slouf, M. Chem. Phys. Lett. 2008, 455, 131−134. (13) Guerrini, L.; Garcia-Ramos, J. V.; Domingo, C.; Sanchez-Cortes, S. J. Phys. Chem. C 2008, 112, 7527−7530. (14) Lopez-Tocon, I.; Otero, J. C.; Arenas, J. F.; Garcia-Ramos, J. V.; Sanchez-Cortes, S. Langmuir 2010, 26, 6977−6981. (15) Izquierdo-Lorenzo, I.; Sanchez-Cortes, S.; Garcia-Ramos, J. V. Langmuir 2010, 26, 14663−14670. (16) Izquierdo-Lorenzo, I.; Kubackova, J.; Manchon, D.; Mosset, A.; Cottancin, E.; Sanchez-Cortes, S. J. Phys. Chem. C 2013, 117, 16203− 16212. (17) Kubackova, J.; Izquierdo-Lorenzo, I.; Jancura, D.; Miskovsky, P.; Sanchez-Cortes, S. Phys. Chem. Chem. Phys. 2014, 16, 11461−11470. (18) Botella, B.; Crespo, J.; Rivas, A.; Cerrillo, I.; Olea-Serrano, M. F.; Olea, N. Environ. Res. 2004, 96 (1), 34−40. (19) Jorgenson, J. L. Environ. Health Perspect. 2001, 109, 113−139. (20) Toxicological Profile for Endosulfan; Agency for Toxic Substances and Disease Registry (ATSDR), 2013; pp 10−128 and 209−212. (21) Oliveira, E. C.; Grisolia, C. K.; Paumgartten, F. J. R. Chemosphere 2009, 75, 398−404. (22) Peper, M.; Ertl, M.; Gerhard, I. Am. J. Ind. Med. 1999, 35, 632− 641. (23) Swackhamer, D. L.; Hites, R. A. Environ. Sci. Technol. 1988, 22, 543−548. (24) Nolan, K.; Kamrath, J.; Levitt, J. Pediat. Dermatol. 2012, 29, 141−146. (25) Gilden, R. C.; Huffling, K.; Sattler, B. J. Obstet. Gynaecol. Neonat. Nurs. 2010, 39, 103−110. (26) Peluso, F.; Dubny, S.; Othax, N.; Castelain, J. G. Hum. Ecol. Risk Assess. 2014, 20, 1177−1199. (27) Xu, X. Q.; Yang, H. G.; Li, Q. L.; Yang, B. J.; Wang, X. R.; Lee, F. S. C. Chemosphere 2007, 68, 126−139. (28) Ali, M.; Kazmi, A. A.; Ahmed, N. Chemosphere 2014, 102, 68− 75. (29) Li, X.; Dai, X.; Yin, X.; Li, M.; Zhao, Y.; Zhou, J.; Huang, T.; Li, H. J. Chromatogr. A 2013, 1277, 69−75. (30) Cañamares, M. V.; Garcia-Ramos, J. V.; Sanchez-Cortes, S.; Castillejo, M.; Oujja, M. J. Colloid Interface Sci. 2008, 326, 103−109. (31) Cañamares, M. V.; Garcia-Ramos, J. V.; Gomez-Varga, J. D.; Domingo, C.; Sanchez-Cortes, S. Langmuir 2005, 21, 8546−8553. (32) Frens, G. Nat. Phys. Sci. 1973, 241, 20−22. (33) Rivas, L.; Sanchez-Cortes, S.; Garcia-Ramos, J. V.; Morcillo, G. Langmuir 2000, 16, 9722−9728. (34) Mishra, S.; Vallet, V.; Poluyanov, L. V.; Domcke, W. J. Chem. Phys. 2006, 125. (35) Guerrini, L.; Aliaga, A. E.; Carcamo, J.; Gomez-Jeria, J. S.; Sanchez-Cortes, S.; Campos-Vallette, M. M.; Garcia-Ramos, J. V. Anal. Chim. Acta 2008, 624, 286−293. (36) Guerrini, L.; Izquierdo-Lorenzo, I.; Rodriguez-Oliveros, R.; Sanchez-Gil, J. A.; Sanchez-Cortes, S.; Garcia-Ramos, J. V.; Domingo, C. Plasmonics 2010, 5, 273−286. (37) Creighton, J. A. In Spectroscopy of Surfaces; Clark, R. J. H. and Hetser, R. E., Eds.; John Wiley & Sons: New York, 1988; p 37. (38) Analytical Toxicology for Clinical, Forensic and Pharmaceutical Chemists, WHO/SDE/WSH/03.04/73, World Health Organization Report, 2003. (39) Opinion of the Scientific Panel on Contaminants in the Food Chain on a Request from the Commision Related to Aldrin and Dieldrin as Undesirable Substance in Animal Feed. EFSA J. 2005, 285, 1−43.
substrates using AgC nanoparticles linked with DT8 at a concentration of 10−5 M. Under these conditions, the alkyl chains of dithiols are organized in multilayers, providing a maximum number of binding sites just at the formed hot spots localized in the gaps between nanoparticles. The adsorption isotherms deduced from SERS data fit a Langmuir curve, indicating that the analyzed pesticides are inserted in the dithiol multilayer with a negligible pesticide−pesticide intermolecular interaction. The limits of detection reported here for the analyzed organochlorine pesticides are comparable to those determined by other techniques. The SERS detection of pesticides offers the advantage of a direct analysis of samples which does not imply the sample pretreatment required by other techniques. Others advantages of this technique are the low cost and the possibility of an in situ analysis of samples. For these reasons, SERS is proposed here as a reliable technique to design, optimize, and implement novel and robust optical sensing devices based on the plasmonic technology in the detection of the organochlorine pesticides.
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ASSOCIATED CONTENT
S Supporting Information *
Experiment section, including materials, methods and instrumentation and LSPR extinction spectra. This material is available free of charge via the Internet at http://pubs.acs. org/.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work has been supported by the Agency of the Ministry of Education of Slovak Republic for the Structural funds of the European Union, Operational Program Education (Doctorand, ITMS code: 26110230013 and KVARK, ITMS code: 26110230084) and Operational Program Research and Development (NanoBioSens (ITMS code: 26220220107) and CEVA II (ITMS code: 26220120040)), by the Slovak Research and Development Agency under Contract No. APVV-0242-11, by the project CELIM (316310) funded by7FP EU, by the Spanish Ministerio de Economiá y Competitividad (MINECO, Grant No. FIS2010-15405).
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REFERENCES
(1) Aroca, R. Surface-Enhanced Vibrational Spectroscopy; John Wiley & Sons: Chichester, U.K., 2006. (2) Kneipp, K.; Kneipp, H.; Manoharan, R.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Bioimaging 1998, 6, 104−110. (3) Nie, S. M.; Emery, S. R. Science 1997, 275, 1102−1106. (4) Moskovits, M. Rev. Mod. Phys. 1985, 57, 783−826. (5) Wokaun, A. Mol. Phys. 1985, 56, 1−33. (6) Guerrini, L.; Peyton, L.; Campos-Vallette, M.; Domingo, C.; Garcia-Ramos, J. V.; Sanchez-Cortes, S. In Surface Enhanced Raman SpectroscopyAnalytical and Life Science Applications; Schlücker, S., Ed. Wiley-VCH Verlag GmbH & Co: Weinheim, Germany, 2010; pp 103−124. (7) Aroca, R. F.; Alvarez-Puebla, R. A.; Pieczonka, N.; SanchezCortez, S.; Garcia-Ramos, J. V. Adv. Colloid Interface Sci. 2005, 116, 45−61. 669
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Sensitive Surface-Enhanced Raman Spectroscopy (SERS) Detection of Organochlorine Pesticides by Alkyl Dithiol-Functionalized Metal Nanoparticles-Induced Plasmonic Hot Spots Jana Kubackova,†,‡ Gabriela Fabriciova,† Pavol Miskovsky,†,§ Daniel Jancura,†,§ and Santiago Sanchez-Cortes*,‡ †
Department of Biophysics, Faculty of Science, P.J. Safarik University, Jesenna 5, Kosice 04 154, Slovakia Instituto de Estructura de la Materia, IEM-CSIC, Serrano 121, 28006 Madrid, Spain § Center for Interdisciplinary Biosciences, Faculty of Science, P.J. Safarik University, Jesenna 5, Kosice 04 154, Slovakia ‡
S Supporting Information *
ABSTRACT: In this work, we report the detection of the organochlorine pesticides aldrin, dieldrin, lindane, and α-endosulfan by using surface-enhanced Raman spectroscopy (SERS) and optimization of the SERS-sensing substrate. In order to overcome the inherent problem of the low affinity of the above pesticides, we have developed a strategy consisting of functionalization of the metal surface with alkyl dithiols in order to achieve two different goals: (i) to induce the nanoparticle linkage and create interparticle junctions where sensitive hot spots needed for SERS enhancement are present, and (ii) to create a specific environment in the nanogaps between silver and gold nanoparticles, making them suitable for the assembly and SERS detection of the analyzed pesticides. Afterward, an optimization of the sensing substrate was performed by varying the experimental conditions: type of metal nanoparticles, molecular linker (aromatic versus aliphatic dithiols and the length of the intermediate chain), surface coverage, laser excitation wavelength. From the adsorption isotherms, it was possible to deduce the corresponding adsorption constant and the limit of detection. The present results confirm the high sensitivity of SERS for the detection of the organochlorine pesticides with a limit of detection reaching 10−8 M, thus providing a solid basis for the construction of suitable nanosensors for the identification and quantitative analysis of this type of chemical.
S
phenomenon is of great importance in SERS, where metal colloids are commonly used to obtain signal intensification even for single molecule detection experiments.11,12 An ideal situation for building interparticle HS is the use of bifunctional molecules, which act as NP linkers.13 Many molecules adsorbed on metal surfaces are not only able to induce the formation of the HS, but can also act as molecular hosts of specific analytes.14 In recent works, we have employed bifunctional molecules for the detection of organic pollutants and doping substances, even at very low concentrations,6,15 which otherwise are unable to link the metal surface and approach the HS on the NP surface. In the present work, we have employed dithiols for the functionalization of metal NPs to obtain SERS-active substrates for the detection of the organochlorine pesticides (OPs) aldrin, α-endosulfan, dieldrin, and lindane. The characterization of the adsorption mechanisms of aliphatic dithiols with different chain lengths on metal (silver and gold) NPs surface and morphology of the obtained NPs assemblies have been thoroughly
urface-enhanced Raman scattering (SERS) is an extremely sensitive method that gives an enhancement of ∼106 in scattering efficiency over normal Raman scattering1 (in some extreme cases, the enhancement can be up to ∼1014−1015); thus, it is a powerful analytical technique. Such great enhancement is, under certain conditions, sufficient for the detection of a single molecule or a few molecules, making SERS an extremely useful technique for very sensitive chemical and biomolecular detection.2,3 The enhancement of the Raman intensity requires localization of the analyzed molecules near or at nanostructured noble metal surfaces and is associated with localized surface plasmon resonance (LSPR) induced on the metal surface.1,4−6 Currently, the most employed SERS substrates are metal nanoparticles (NPs) in suspensionthat is to say, colloids.7 The sensitivity and selectivity of metal NPs to analytes can be remarkably enhanced by changing the interfacial properties of metal surfaces. One of the possible modifications can be functionalization of the metal surface by molecules that are able to activate the formation of intramolecular or intermolecular cavities, thus acting as hosts to attach analytes.8,9 Nowadays, it is widely accepted that the main part of the field intensification occurs in special regions of the metallic surface called hot spots (HS), which are mainly localized in interparticle gaps.10 This © 2014 American Chemical Society
Received: September 11, 2014 Accepted: November 23, 2014 Published: December 15, 2014 663
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most frequently methods used to analyze water samples containing the studied OPs are gas chromatography equipped for electron capture detection (QC/ECD) and gas chromatography−tandem mass spectrometry (GC-MS).27,28 GC/ECD provides a higher degree of sensitivity than GC/MS; however, ECD does not provide the molecular structure information that can be obtained with an MS detector. The structural information increases the level of confidence that the compound being measured has been correctly identified.29 However, the development of low-cost, easy-going, and reliable approaches for the detection of pesticides is still needed. In this regard, SERS technique can be employed to develop a detection system capable of analyzing low concentrations of pesticides. In order to accomplish this task, we have employed aliphatic and aromatic dithiols for two different purposes: (i) to link metallic nanoparticles creating the appropriated physical conditions to induce plasmonic hot spots, and (ii) to modify the chemical properties of these hot spots, making them more suitable for the molecular assembly of liposoluble pesticides. In the future, this approach should be applicable for the practical analysis of the samples from different areas of the environment (water, soil), because of the advantages provided by the SERS method: univocal identification of the chemicals, fast and in situ analysis, and last but not least, low financial cost, in comparison with the currently utilized techniques.
investigated by us and the results have been recently published.16,17 Aldrin, α-endosulfan, dieldrin, and lindane are pesticides that belong to the organochlorine family (structures are shown as insets in Figure 1). All these pesticides are poisons and pose
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EXPERIMENTAL SECTION Preparation of Silver and Gold Colloids. Ag NPs in suspension were prepared by two different methods. Citrate Ag NPs (AgC) were obtained by the following procedure:30 1 mL of a 1% (w/v) trisodium citrate aqueous solution was added to 50 mL of a boiling 10−3 M silver nitrate aqueous solution, and boiling was continued for 1 h. The obtained colloid showed a turbid gray aspect and had a final pH of 6.5. Hydroxylamine Ag NPs (AgH) were obtained by the method described previously by Cañamares et al.31 Briefly, 10 mL of a 10−2 M silver nitrate solution are added dropwise to 90 mL of a 1.6 × 10−3 M solution of hydroxylamine hydrochloride adjusted to pH 9 under vigorous stirring. The resulting spherical Ag NPs have an average size (diameter) of 35 nm.31 Au NPs in colloidal suspension (AuC) were prepared using the method described by Frens.32 Briefly, 0.1 mL of an aqueous solution of HAuCl4 (0.118 M) was diluted in 40 mL of water under intense stirring. Then, 1 mL of 1% sodium citrate solution in volume was added dropwise. The yellow solution was refluxed for 5 min, resulting in a red colloidal suspension. The average diameter of Au nanoparticles prepared by the above method is 15−20 nm.33 All aqueous solutions needed for preparations of the metal colloids were prepared using Milli-Q water. Preparation of Samples for SERS Experiments. Samples for SERS measurements were prepared as follows. A total of 1 mL of the colloid (silver as well as gold) was activated by the addition of 40 μL of 0.5 M KNO3. The activation of the citrate colloids by nitrate is also related to the removal of the citrate excess existing on the fabricated nanoparticles after their preparation. Then, 10 μL of a solution of aliphatic dithiol, the concentration of which was adjusted according to the final dithiol concentration, were added to 1 mL of the colloidal suspension. After the aggregation of NPs by the dithiol, 10 μL of the organochlorine pesticide, also at an appropriate concentration, were added to the above mixture. In all SERS
Figure 1. Raman spectra of the solid state of the studied pesticides: aldrin (spectrum (a)), endosulfan (spectrum (b)), lindane (spectrum (c)), and dieldrin (spectrum (d)). Insets show the molecular structure of the associated analyzed pesticide. Excitation line at 785 nm.
long-term danger to the environment and humans through their persistence in nature and adipose tissue.18 Aldrin, dieldrin, and α-endosulfan were extensively used mainly in agriculture as a seed dressing for food or soil insecticides on commodity crops, such as tea, coffee, grain, fruits, and vegetables, as well as on cereals and also on nonfood crops such as tobacco and cotton.19,20 Endosulfan and lindane have been also used as a wood preservative.21,22 In addition, lindane has been used as an insecticide for several fruit and vegetable crops, in baits and seed treatments for rodent control, and for the treatment of scabies (mites) and lice.23,24 Because aldrin has harmful effects on human health and the health of wildlife, this chemical is no longer produced or used.19 The threat posed by endosulfan to the environment evocated a global ban of its use, and its manufacture was considered under the Stockholm Convention on persistent organic pollutants.25 The production of lindane is also limited, because of the relationship between an exposure to lindane or its isomers and the appearance of cancer in human beings.9 Even if some of these pesticides are currently banned in many countries, they are still spread throughout the world in many environments and represent a serious danger to human health, because of the persistent character of these compounds, their long-term accumulation in adipose tissues, and their demonstrated carcinogenic effect.26 Therefore, the detection and identification of these chemicals is very important. The 664
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Figure 2. (a) Scheme displaying the pesticide hosting (in this case, for aldrin) in the dithiol layer organized in interparticle gaps. SERS spectra of aldrin (10−5 M) (b) on AgC in the absence of dithiols and (d) on DT8-functionalized AgC. The latter spectrum shows the bands corresponding to DT8 that are seen in panel (c) for AgC functionalized with DT8 (10−5 M). The Raman of the solid aldrin is presented in spectrum (e). All spectra were obtained at 785 nm excitation. (f) SERS spectra of the analyzed pesticides (10−5 M) on DT8-functionalized AgC NPs exciting at 785 nm, showing the C−Cl stretching bands of the fingerprint region (300−400 cm−1) and the C−S stretching bands of DT8 in the deduced multilayer highly ordered conformation employed as reference band for quantitative analysis.
weak CC stretching bands in the case of the bicyclic pesticides (aldrin, α-endosulfan, and dieldrin), appearing at 1599−1605 cm−1 and attributed to the stretching vibration of the Cl−CC−Cl moiety. Aldrin also exhibits an extra CC band appearing at 1560 cm−1, corresponding to the CC localized in the adjacent cycle of the molecule (Figure 1a). Finally, the bands observed between the two above-mentioned regions correspond to CC and CH stretching and CH2 bending from the aliphatic cyclic structure of the analyzed molecules. SERS Spectra of Organochlorine Pesticides. The OPs studied in this work do not display any Raman band in the presence of metal NPs, using either Ag or Au colloids, as can be seen in Figure 2b for aldrin (10−5 M). This observation can be explained by the extremely low affinity of the analyzed pesticides to link the metal surface and, consequently, to benefit from the EM field enhancement induced on the surface. To change this situation, we have modified the chemical properties of the metal surface by functionalizing the metal nanoparticles with molecular linkers, alkyl dithiols (AD) and aromatic dithiols (ArD). AD adsorbed on the Ag and Au NPs
measurements, the sample was placed in a 1-cm-optical-path quartz cuvette, and these measurements were performed 10 min after the addition of the pesticide to the mixture.
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RESULTS AND DISCUSSION Raman Spectra of Organochlorine Pesticides. The Raman spectra of the solid state of aldrin, α-endosulfane, lindane, and dieldrin are shown in Figure 1. The tentative assignment of the vibrational bands of the studied pesticides was made on the basis of the data found in the literature.34,35 Briefly, the Raman spectra are dominated by a group of bands appearing in the 300−400 cm−1 region, attributed to C−Cl stretching vibrations (ν(CCl)) (Figure 1). These bands are the most intense in the Raman spectra, since the C−Cl stretching induces a large variation of the polarizability of the analyzed molecules. Therefore, these bands can be employed as an actual fingerprint region to carry out the SERS detection of the studied OPs. The bands appearing below 300 cm−1 are attributed to bending vibrations (δ(CCCl), δ(ClCCl), and δ(CCC)). The Raman spectra of these pesticides also include 665
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fulfill two different roles (see Figure 2a for a schematic explanation): (i) linking of NPs and forming hot spots, as demonstrated in previous works;36 and (ii) functionalization of the metal surface to attach the pollutant just to the gap between NPs, where the EM field is much more enhanced than in other parts of the metal surface. The NPs linking induces large changes in the extinction spectra due to the strong NP assembly induced by AD. These effects are summarized in Figure S1 in the Supporting Information for the special case of the addition of ADs to Ag NPs at a final concentration of 10−5 M. The main effect is the intensity decrease of the single particle band (at 405 nm) and the appearing of a second, wider band centered in the 770−850 nm range. The functionalization of metallic NPs with AD leads to a change of the chemical properties of the interface that allows the observation of the intense Raman bands of the pesticides in the 300−400 cm−1 region corresponding to the ν(CCl) bands. In Figure 2d, we show this effect for the particular case of aldrin. These bands correlate very well with the bands seen in the classical Raman spectrum of the solid aldrin (Figure 2e), and are clearly distinguished from the dithiol bands in the 600− 750 cm−1 region (Figure 2c), which were employed as reference to calculate the relative SERS intensities of the analyzed pesticides. A detailed analysis of the observed SERS spectra reveals that only slight changes are produced in the position of the main bands of either the dithiol or the pesticide molecule, indicating that the interaction between these molecules is not very strong. However, this interaction is sufficient to induce the inclusion of the pesticide into the nonpolar aliphatic environment by van der Waals and/or hydrophobic forces. These interactions also occur in the adipose tissue of mammals and in the cells plasmatic membrane where the OPs can be accumulated. The small relative change in the intensities of the SERS bands of the pesticides are attributed to the specific orientation of these molecules inside the dithiol layer, formed on the metal surface, as deduced from the propensity rules predicted by the electromagnetic mechanism of SERS.37 Optimization of the Detection of Organochlorine Pesticides by SERS. To optimize the functionalization of NPs by the adsorption of AD, we have carried out a study consisting of the variation of several key factors essential for the ability of these systems to detect the OPs by SERS. These factors are (i) the structure of dithiols, (ii) the chemical properties of the metal surface, (iii) the aliphatic layer organization on the surface, and (iv) the excitation wavelength. The molecular structure of dithiols varied by using aliphatic or aromatic dithiols and by changing the length of the chain in aliphatic dithiols. For this purpose, we have used 1,6-hexanedithiol (DT6), 1,8-octanedithiol (DT8), and 1,10-decanedithiol (DT10). The chemical properties of the metal surface were modified by using plasmonic NPs prepared in different ways: hydroxylamine and citrate Ag NPs and citrate Au NPs. The layer organization around NPs was modified by varying the surface coverage of NPs at different dithiol concentrations. An important step in the optimization process of the SERS detection of the pesticides was to determine the most suitable type of metal substrate. In order to carry out this study, we have investigated three types of plasmonic NPs: AgC, AgH, and AuC. Figure 3b (red bars) shows the relative intensity of the most intense SERS band of aldrin at 10−5 M, which is the band appearing at 350 cm−1 (Figure 2d). The SERS spectra of aldrin
Figure 3. Optimization of the linker (AD) and the plasmonic substrate in the detection of aldrin: (a) Schematic showing the interparticle gaps, and (b) the intensity of the 350 cm−1 band of aldrin is displayed as a function of different dithiol-functionalized metal NPs. DT8 provided the most intense spectrum on AgC substrate due to the best plasmonic conditions of the interparticle gaps.
were obtained in the presence of all of the ADs used, but we only show the results obtained on all the studied NPs systems covered by DT8. It is evident that the most intense bands appear in the SERS spectrum in the presence of AgC colloid, while utilization of Au NPs leads to a spectrum with low SERS intensity (Figure 3). In the presence of the AgH colloid, the SERS spectrum of aldrin is also intense; however, this is not the case for AgC. The relative intensities of the aldrin bands in the 300−400 cm−1 region are identical in both spectra obtained using AgC and AgH colloids. This observation suggests that the method of preparation of silver colloids does not influence the orientation of the pesticide inside the dithiol layer. The AgC was also determined to be the best SERS substrate for the other analyzed pesticides (results not shown), although a worse matching with the plasmon resonance of aggregates is observed (see Figure S2 in the Supporting Information). The reason to obtain a higher efficiency in the pesticide detection on AgC NPs, in comparison with other analyzed SERS substrates, seems to be the differences in the chemical properties of the exposed interfaces. In fact, the easiest removal of the citrate ions existing in AgC by the added dithiols, in comparison to Cl− ions existing in AgH, can be the reason for the better properties of this type of colloid, since, in the case of AgH NPs, the Cl− ions adsorbed on the surface of this substrate31 are more strongly attached on the surface avoiding the adsorption of dithiols. Au NPs are less effective for SERS detection of the pesticides, because the structure of dithiols adsorbed on this metal is more disordered, as reported in our previous work.17 This fact prevents the formation of suitable sites for the incorporation of the pesticides inside nanoparticle gaps. The employment of aromatic dithiols (1,4-benzenedithiol (BDT) and biphenyl-4,4′-dithiol (BPDT)) in the detection of 666
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pesticide hosting is expected. At submonolayer concentrations ( aldrin > endosulfan > lindane
The different values of the affinity constants are probably associated with the variable solubility of the analyzed substances in the aliphatic multilayer created by the adsorption of DT8 on Ag NPs. 668
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(8) Guerrini, L.; Garcia-Ramos, J. V.; Domingo, C.; Sanchez-Cortes, S. Anal. Chem. 2009, 81 (4), 1418−1425. (9) Lopez-Tocon, I.; Otero, J. C.; Arenas, J. F.; Garcia-Ramos, J. V.; Sanchez-Cortes, S. Anal. Chem. 2011, 83, 2518−2525. (10) Le Ru, E. C.; Etchegoin, P. G.; Meyer, M. J. Chem. Phys. 2006, 125, 204701−204714. (11) Futamata, M. Faraday Discuss. 2006, 132, 45−61. (12) Vlckova, B.; Moskovits, M.; Pavel, I.; Siskova, K.; Sladkova, M.; Slouf, M. Chem. Phys. Lett. 2008, 455, 131−134. (13) Guerrini, L.; Garcia-Ramos, J. V.; Domingo, C.; Sanchez-Cortes, S. J. Phys. Chem. C 2008, 112, 7527−7530. (14) Lopez-Tocon, I.; Otero, J. C.; Arenas, J. F.; Garcia-Ramos, J. V.; Sanchez-Cortes, S. Langmuir 2010, 26, 6977−6981. (15) Izquierdo-Lorenzo, I.; Sanchez-Cortes, S.; Garcia-Ramos, J. V. Langmuir 2010, 26, 14663−14670. (16) Izquierdo-Lorenzo, I.; Kubackova, J.; Manchon, D.; Mosset, A.; Cottancin, E.; Sanchez-Cortes, S. J. Phys. Chem. C 2013, 117, 16203− 16212. (17) Kubackova, J.; Izquierdo-Lorenzo, I.; Jancura, D.; Miskovsky, P.; Sanchez-Cortes, S. Phys. Chem. Chem. Phys. 2014, 16, 11461−11470. (18) Botella, B.; Crespo, J.; Rivas, A.; Cerrillo, I.; Olea-Serrano, M. F.; Olea, N. Environ. Res. 2004, 96 (1), 34−40. (19) Jorgenson, J. L. Environ. Health Perspect. 2001, 109, 113−139. (20) Toxicological Profile for Endosulfan; Agency for Toxic Substances and Disease Registry (ATSDR), 2013; pp 10−128 and 209−212. (21) Oliveira, E. C.; Grisolia, C. K.; Paumgartten, F. J. R. Chemosphere 2009, 75, 398−404. (22) Peper, M.; Ertl, M.; Gerhard, I. Am. J. Ind. Med. 1999, 35, 632− 641. (23) Swackhamer, D. L.; Hites, R. A. Environ. Sci. Technol. 1988, 22, 543−548. (24) Nolan, K.; Kamrath, J.; Levitt, J. Pediat. Dermatol. 2012, 29, 141−146. (25) Gilden, R. C.; Huffling, K.; Sattler, B. J. Obstet. Gynaecol. Neonat. Nurs. 2010, 39, 103−110. (26) Peluso, F.; Dubny, S.; Othax, N.; Castelain, J. G. Hum. Ecol. Risk Assess. 2014, 20, 1177−1199. (27) Xu, X. Q.; Yang, H. G.; Li, Q. L.; Yang, B. J.; Wang, X. R.; Lee, F. S. C. Chemosphere 2007, 68, 126−139. (28) Ali, M.; Kazmi, A. A.; Ahmed, N. Chemosphere 2014, 102, 68− 75. (29) Li, X.; Dai, X.; Yin, X.; Li, M.; Zhao, Y.; Zhou, J.; Huang, T.; Li, H. J. Chromatogr. A 2013, 1277, 69−75. (30) Cañamares, M. V.; Garcia-Ramos, J. V.; Sanchez-Cortes, S.; Castillejo, M.; Oujja, M. J. Colloid Interface Sci. 2008, 326, 103−109. (31) Cañamares, M. V.; Garcia-Ramos, J. V.; Gomez-Varga, J. D.; Domingo, C.; Sanchez-Cortes, S. Langmuir 2005, 21, 8546−8553. (32) Frens, G. Nat. Phys. Sci. 1973, 241, 20−22. (33) Rivas, L.; Sanchez-Cortes, S.; Garcia-Ramos, J. V.; Morcillo, G. Langmuir 2000, 16, 9722−9728. (34) Mishra, S.; Vallet, V.; Poluyanov, L. V.; Domcke, W. J. Chem. Phys. 2006, 125. (35) Guerrini, L.; Aliaga, A. E.; Carcamo, J.; Gomez-Jeria, J. S.; Sanchez-Cortes, S.; Campos-Vallette, M. M.; Garcia-Ramos, J. V. Anal. Chim. Acta 2008, 624, 286−293. (36) Guerrini, L.; Izquierdo-Lorenzo, I.; Rodriguez-Oliveros, R.; Sanchez-Gil, J. A.; Sanchez-Cortes, S.; Garcia-Ramos, J. V.; Domingo, C. Plasmonics 2010, 5, 273−286. (37) Creighton, J. A. In Spectroscopy of Surfaces; Clark, R. J. H. and Hetser, R. E., Eds.; John Wiley & Sons: New York, 1988; p 37. (38) Analytical Toxicology for Clinical, Forensic and Pharmaceutical Chemists, WHO/SDE/WSH/03.04/73, World Health Organization Report, 2003. (39) Opinion of the Scientific Panel on Contaminants in the Food Chain on a Request from the Commision Related to Aldrin and Dieldrin as Undesirable Substance in Animal Feed. EFSA J. 2005, 285, 1−43.
substrates using AgC nanoparticles linked with DT8 at a concentration of 10−5 M. Under these conditions, the alkyl chains of dithiols are organized in multilayers, providing a maximum number of binding sites just at the formed hot spots localized in the gaps between nanoparticles. The adsorption isotherms deduced from SERS data fit a Langmuir curve, indicating that the analyzed pesticides are inserted in the dithiol multilayer with a negligible pesticide−pesticide intermolecular interaction. The limits of detection reported here for the analyzed organochlorine pesticides are comparable to those determined by other techniques. The SERS detection of pesticides offers the advantage of a direct analysis of samples which does not imply the sample pretreatment required by other techniques. Others advantages of this technique are the low cost and the possibility of an in situ analysis of samples. For these reasons, SERS is proposed here as a reliable technique to design, optimize, and implement novel and robust optical sensing devices based on the plasmonic technology in the detection of the organochlorine pesticides.
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ASSOCIATED CONTENT
S Supporting Information *
Experiment section, including materials, methods and instrumentation and LSPR extinction spectra. This material is available free of charge via the Internet at http://pubs.acs. org/.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work has been supported by the Agency of the Ministry of Education of Slovak Republic for the Structural funds of the European Union, Operational Program Education (Doctorand, ITMS code: 26110230013 and KVARK, ITMS code: 26110230084) and Operational Program Research and Development (NanoBioSens (ITMS code: 26220220107) and CEVA II (ITMS code: 26220120040)), by the Slovak Research and Development Agency under Contract No. APVV-0242-11, by the project CELIM (316310) funded by7FP EU, by the Spanish Ministerio de Economiá y Competitividad (MINECO, Grant No. FIS2010-15405).
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REFERENCES
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