Novel Multilayer Nanostructured Materials for Recognition of

Research Article www.acsami.org

Novel Multilayer Nanostructured Materials for Recognition of Polycyclic Aromatic Sulfur Pollutants and Express Analysis of Fuel Quality and Environmental Health by Surface Enhanced Raman Spectroscopy Olga E. Eremina,† Alexander V. Sidorov,‡ Tatyana N. Shekhovtsova,† Eugene A. Goodilin,*,†,‡,§ and Irina A. Veselova† †

Faculty of Chemistry, Moscow State University, Lenin Hills, 1-3, Moscow 119991, Russia Faculty of Materials Science, Moscow State University, Lenin Hills, 1-73, Moscow 119991, Russia § Institute of General and Inorganic Chemistry, Leninskii prosp., 31, Moscow 119071, Russia ‡

S Supporting Information *

ABSTRACT: A novel concept of advanced SERS (surface enhanced Raman spectroscopy) planar sensors is suggested for fast analysis of sulfur-containing hazardous oil components and persistent pollutants. The main advantage of the proposed sensors is the utilization of an additional preconcentrating layer of optically transparent chitosan gel, which is chemically modified with appropriate π-acceptor compounds to selectively form charge-transfer complexes (CTCs) at the interface with nanostructured silver coatings. The CTCs shift absorption bands of polycyclic aromatic sulfur heterocycles (PASHs) and other important analytes in a controllable way and thus provide a surplus enhancement of vibration modes due to resonant Raman scattering. This novel indicator system provides multiplex determination of PASHs in different forms in a small volume of oil without any tedious sample pretreatment steps. This approach opens new possibilities of increasing either spectral and concentration sensitivity or specificity of SERS-based sensors, allowing for new developments in the fields of ecology, advanced fuel analysis, and other prospective applications. KEYWORDS: surface enhanced Raman scattering, charge-transfer complexes, oil quality markers, polycyclic aromatic sulfur heterocycles, dibenzothiophene

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INTRODUCTION Some of the most challenging and pressing problems today are related to environmental issues caused by pollutants originating from oil and oil-derived fuels as the traditional sources of energy.1 The most common undesirable element in crude oil and its distillates is sulfur, usually ranging in its content from 0.05% to 5% and up to 14% in heavier oil.2 A mature industrial technology of hydrodesulfurization is effective only for removal of aliphatic S-compounds, such as thiols, thioethers, and disulfides; however, heterocyclic S-compounds, such as thiophene, benzothiophene, dibenzothiophene (DBT), and their alkylated derivatives, cannot be completely removed in this way.3 Among these, one of the most important markers of fuel quality is 4,6-dimethyldibenzothiophene (4,6-DMDBT), which survives after hydrodesulfurization due to the alkyl groups attached to the ring structure near sulfur because of huge steric hindrance after adsorption on a catalyst surface.1,4,5 DBT and its methylated derivatives in fossil fuels are believed to cause acid rains and air pollution.6 Moreover, polycyclic aromatic sulfur heterocyclic (PASH) compounds are widely © 2017 American Chemical Society

known to possess biocancerogenic, carcinogenic, mutagenic, and teratogenic properties.7 Additionally, due to the photodegradation of DBT in water, its oxidized forms such as dibenzothiophene-5,5-dioxide (DBTO2), which is a patented herbicide, and different alkylated aromatic sulfonic acids, which serve as surfactants, are formed unavoidably leading to the increased toxicity.8 Thus, DBT with its high toxicity may perniciously affect human health and ecosystems.9 Futhermore, sulfur in fuel causes poisoning of the catalysts in catalytic converters of vehicles,1 while the stronger environmental standards such as Euro-5 (2011)10 and ASTM (2015)11 require stricter control over pollutants: the maximum permissible level of sulfur content in diesel fuels in Europe and USA is currently 10 and 1 ppm, respectively, with targets of their zero emissions. New environmental regulations on sulfur content have promoted studies of new alternative technologies Received: February 11, 2017 Accepted: April 12, 2017 Published: April 12, 2017 15058

DOI: 10.1021/acsami.7b02018 ACS Appl. Mater. Interfaces 2017, 9, 15058−15067

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ACS Applied Materials & Interfaces

special linker molecules resulting in greatly enhanced SERS sensitivity, specificity, and reproducibility of such multilayer sensors. We propose a quite simple and effective solution of key problems of important analytes detection usually associated with either improper positions of analyte absorption regions, often found in the UV-range, far above the energy of silver and gold plasmonic bands,27,28 or a low binding ability of analyte molecules to the surface of SERS-substrates. These restrictions usually do not allow SERS to be effectively applied as a highly sensitive tool for the detection of the above-mentioned analytes. Thus, in this Article, we present a new concept of preconcentration, recognition, binding, and optical properties adjustment of analytes using planar optical sensors based on nanostructured silver substrates coated with a thin layer of optically transparent chitosan gel. The unique features of the proposed approach consist of registration of highly enhanced signal of charge-transfer complexes (CTCs) formed in a thin layer of chitosan film, chemically modified by a π-acceptor compound, that is capable of recognizing important analytes by binding with them in CTC, thus excluding tedious sample pretreatment and providing the detection of individual PASH compounds as new electronic-vibrational structures that can be seen in Raman scattering spectra.

to purify fossil fuels like catalytic oxidative desulfurization (ODS). Dibenxothiophene-5-oxide (DBTO) and dibenzothiophene-5,5-dioxide (DBTO 2) are thought to be model compounds for understanding of this process.12 Practically, these regulations mean that concentrations of DBTs and DBTO/DBTO2 to be measured are low, and therefore preconcentration techniques normally have to be employed to increase analytical signals, especially in the case of oxidized forms of DBTs.13 Gas chromatography with flame-ionization detectors (GC−FID), mass selective detectors (GC−MS), sulfur chemiluminescence detectors (GC−SCD), and high performance liquid chromatography with UV-detectors (HPLC−UV) are commonly used for the determination of DBTs;14 however, it is not always possible to confirm, identify, and quantify all of the products individually.15 GC−FID or GC−MS can detect about 280 aliphatic, aromatic, and biomarker compounds, but a complex separation scheme is needed to fractionate a fuel for such an analysis.16 GC−SCD has been applied widely for specification of sulfur compounds and determination of total sulfur content only.17 Therefore, the search for new advanced analytical techniques to identify and quantify these degradation products is of urgent interest. One of the most interesting and promising techniques toward direct ultrasensitive detection of various analytes in complex samples is surface enhanced Raman scattering (SERS) spectroscopy. SERS is a promising diagnostic tool due to its ultrahigh sensitivity, label-free detection of molecules at extremely low concentrations, and high specificity by their vibrational fingerprint. For these reasons, SERS is widely known to be an outstanding technique used in a broad range of applications, especially biosensorics.18−21 Few successful attempts are also known to apply SERS for nonpolar chemical pollutants such as polycyclic aromatic hydrocarbons (PAHs) at low concentration.22 Huge Raman enhancement factors, up to 1010−1014, can be observed, particularly if molecules are adsorbed within “hot spots” of rough nanostructured surface of noble metals like silver and gold.23−25 Silver nanoparticles (AgNPs) are a common choice for SERS measurements due to their broad plasmon resonance, high stability, facile fabrication methods, etc.18−22,26 An optimum tuning of chemical properties of the nanostructured substrates may lead to their higher affinities toward certain classes of analytes. As a result, a huge increase of sensitivity and selectivity toward quantification can be achieved in the developed systems.29 For example, modifying of AgNPs with dithiocarbamate calix[4]arene host molecules provides detection of several PAHs at relatively low concentrations (ca. 1 mM).30 A host−guest mechanism takes place through a π−π stacking interaction leading to a charge transfer between the arene complex and the metallic surface, which induces a notable influence on the surface charge of the metallic nanoparticle.31,32 However, this approach still allows neither quantitative nor multiplex determination of PASHs in their complex mixtures. Polymer thin layers can be used to tune SERS efficiency and obtain optimal conditions for maximizing the enhancement factor (EF) in terms of both fundamental and application aspects.33 This innovative idea to detect and quantify sulfurorganic molecules in oil and environment by SERS can be readily implemented using easy-to-prepare active elements of optical sensors with no need of any tedious and complex sample preparation steps.22 In this Article, we discuss a novel simple and effective approach to construct the interface of plasmonic nanostructures with polymer layers that contain

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EXPERIMENTAL SECTION

Materials. Silver nitrate (AgNO3), sodium hydroxide (NaOH), ammonium hydroxide (NH4OH, 30%), dibenzothiophene (C12H8S, 98%), 4,6-dimethyldibenzothiophen (C14H14S, 98%), dibenzothiophene sulf-5,5-dioxide (C12H8SO2, 97%), pyrene (C16H10, 98%), 1,4benzoquinone (C6H4O2), 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (C8O2N2Cl2, 98%), 7,7,8,8-tetracyanoquinodimethane (C12H4N4, 98%), acetic acid (CH3COOH), polyvinylpyrrolidone ((C6H9NO)n, Mw 40 000), poly(vinyl alcohol) (Mw 25 000, 88% degree of deacetylation), and chitosan (50−190 kDa, 75−85% degree of deacetylation) were purchased from Sigma-Aldrich. Hydroxyethylcellulose (C6H8O5R3, R = H or C2H4OH) was purchased from Natrosol. Dibenzothiophene sulf-5-oxide (C12H8SO, 97%) was purchased from Santa Cruz Biotechnology. In all of the experiments, ultrapure 18 MΩ cm water (Milli-Q, Millipore) was used. Substrate Preparation. SERS-active substrates containing nanostructured silver were prepared in two simple steps. First, spray pyrolysis deposition of aqueous diaminsilver hydroxide was used18 because we have previously found that this is a universal approach to design highly effective SERS-substrates.18−21,34 Aqueous sodium hydroxide solution (0.1 M) was added dropwise to freshly prepared aqueous silver nitrate solution (10 mM) until complete precipitation of a black-brown silver(I) oxide. Prepared oxide then was thoroughly washed with deionized water and dissolved in a 2-fold molar excess of a 10% aqueous ammonia solution (prepared from 30% ammonium hydroxide) to have 0.01 M solution of a silver(I) complex; a higher concentration deteriorates the silver ring structure formation, while superstoichimetric ammonia is needed to prevent Ag2O precipitation at the aerosol production stage. The obtained transparent silver complex solution was filtered through Millex-LCR syringe driven filter units (Millipore, 0.45 μm pores). In the ultrasonic silver rain deposition process, this initial ammonia solution of silver(I) oxide was nebulized into mist, and 1−5 μm droplets were streamed onto “warm” (200−270 °C) glasses.18−21,43 Solution Preparation. Samples of π-donor compounds, dibenzothiophene, 4,6-dimethildibenzothiophen, dibenzothiophene sulf-5oxide, dibenzothiophene sulf-5,5-dioxide, and pyrene, were dissolved in isooctane. Samples of π-acceptor compounds, 1,4-benzoquinone, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone, and 7,7,8,8-tetracyanoquinodimethane, were dissolved in chloroform. The prepared SERS-active substrates were dipped first into a solution of DDQ/TCNQ in CHCl3 with a concentration of 5 × 10−3/5 × 10−4 M. Next, 10 μL of DBT/ 15059


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Figure 1. Molecular orbital (MO) energy levels estimated using DFT: (a) the preferable interaction is between LUMO (DDQ)−HOMO (DBT) and LUMO (TCNQ)−HOMO (DBTO); and (b) energy HOMO levels of π-donors and LUMO levels of π-acceptors. The calculations of the molecular orbital levels were performed with the B3LYP method (Spartan ’16).44

Figure 2. Absorption spectra of PASH analytes: (a) (1) DBT, 4,6-DMDBT, DBTO, or DBTO2 (in isooctane) and (2) DDQ (in chloroform), and concentrated (5 mM) solutions of their charge-transfer complexes (3) DBT:DDQ, (4) 4,6-DMDBT:DDQ, (5) DBTO:DDQ, and (6) DBTO2:DDQ; (b) (1) TCNQ (in chloroform) and (2) DBT, 4,6-DMDBT, DBTO, or DBTO2 (in isooctane), and concentrated (5 mM) solutions of their charge-transfer complexes (3) DBT:TCNQ or 4,6-DMDBT:TCNQ, (4) DBTO:TCNQ, and (5) DBTO2:TCNQ. microscopy (TEM) combined with electron diffraction (ED) on a LEO 912 AB OMEGA, Carl Zeiss setup at 100 kV accelerating voltage. The assumptions of complexes formation were confirmed by density functional theory (DFT) estimations of molecular orbitals of ether donors and acceptors. All of the theoretical calculations were carried out using the Spartan ’16 software.44 B3LYP method with 6311+G** basis set was used in all cases. This package provides both visualization of 3D-structures and quantum-chemical calculations.44 To perform Raman measurements, an InVia Raman confocal microscope (Renishaw, UK) was used. All SERS spectra were acquired using a 20 mW 633 nm argon laser with power neutral density filter of 10%. The spectra were collected using a 20× objective lens and 10 s of acquisition time. A silicon wafer was used for calibration.

4,6-DMDBT/DBTO/DBTO2 solution in CHCl3 was deposited using different concentrations of 10−4−10−7 M. To obtain the most suitable porous polymer film on the rough metal surface, the solution of chitosan was prepared. A primary solution of acidic acid (1 vol %) in Milli-Q water was made up by mixing glacial acidic acid (500 μL) with water (49.50 mL). Chitosan (2.000 g) was dissolved in 40.00 mL of this acidic acid solution (1 vol %) and stirred for at least 5 h. A drop of the resulting chitosan solution (0.5 wt %) was deposited onto SERS-active nanostructure surface and dried under ambient conditions, thus coating the active layer with a protective film of chitosan. Characterization. The obtained substrates were examined by XRD measurements using a Rigaku D/MAX 2500 machine (Japan) with a rotating copper anode (Cu Kα irradiation, 5−90° 2θ range, 0.02° step). Diffraction maxima were indexed using the PDF-2 database. UV−vis absorption spectra were recorded using the Lambda 950 (PerkinElmer) UV−vis spectrophotometer with an attached diffuse reflectance accessory. Measurements were performed in the spectral range of 250−850 nm with a scan step of 1 nm. The obtained nanostructured substrates were characterized by scanning electron microscopy (SEM) Carl Zeiss NVision 40 and transmission electron

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RESULTS AND DISCUSSION Formation of Colored Charge-Transfer Complexes. All sulfur organic polycyclic pollutants, DBT, 4,6-DMDBT, DBTO, and DBTO2, absorb UV at 230−290 nm but have no strong absorption modes in a visual light range, as they alone demonstrate weak enhancement in SERS spectra (Figure S1). DBT can act both as an n-type donor by the lone pair of the 15060


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Figure 3. Microstructural features of SERS-active samples with a chemical preparation history: (a−c) top views of a hierarchic structure of silver coated substrates under different magnifications, (a) transmission optical images of the ring structure of the samples with 5 min of deposition time, (b) SEM details of the rims and interiors of silver rings after deposition of a layer of nanostructured silver, (c) silver clusters of different sizes inside the silver rings, (d) cross-section view of the substrates (1) coated with a nanostructured silver layer (2), (e) the same with an additional coating by a polymer (3), (f) typical XRD data on the chemically coated silver substrates, and (g) the multilayered structure of the final SERS-active element, optical image of a sample cross section with an interface between silver coating (2) deposed on a glass (1) and a polymer layer (3) above. 15061


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Figure 4. Optical features of SERS-active substrates including properties of individual layers of the multilayer optical sensor and the interface between nanostructured silver coating and the polymer layer. Reflectivity data (a−c) of the bare silver coating (a), the polymer coatings themselves (b), and the sandwich structure after polymer deposition onto the silver nanostructures (c): (1) Denotes reference data of pure glass substrate in all of the graphs, (2) for chitosan (CS, ε = 3.3−3.7), (3) hydroxiethylcellulose (HEC, ε = 3.2−7.5), (4) polyvinylpyrrolidone (PVP, ε = 1.8−2.2), (5) poly(vinyl alcohol) (PVA, ε = 1.9−2.1), (6) no polymer coating (meaning bare nanostructured silver, 5 min of deposition time); the other numbers (7)−(15) denote bare silver nanostructures possessing a ring-like morphology obtained by chemical deposition of different duration: (7) 30 s, (8) 1.5 min, (9) 2.5 min, (10) 3.5 min, (11) 5 min, (12) 10 min, (13) 15 min, (14) 20 min, (15) 25 min in (a); the same notations are used in the description of the Raman scattering spectra (d) showing a low noise background for the substrates.

sulfur atom and as a π-type donor by the delocalized electrons of the aromatic rings (Figure 1b). Considering the electron-rich structure of PASHs, the idea to form stable charge-transfer complexes (CTCs) with an appropriate π-acceptor seems to be natural. Formation of CTC is generally accompanied by the appearance of a new absorption Benesi−Hildebrand band.35−37 In general, such CTC can be successfully involved within a universal approach of the detection of other important analytes like nonpolar organic molecules, thus expanding the suggested approach toward environmental control of water, air, soil, and quality control of fuel. The suggested strategy of express diagnostics of fuels and environmental health with the use of commercially available portable equipment opens new frontiers toward prospective applications of SERS.38−42 Formation of CTCs originates with the HOMO (highest occupied molecular orbitals) of the donor compounds interacting with the LUMO (lowest unoccupied molecular orbital) of the acceptor molecules. Neglecting geometric and steric effects, calculation of HOMO/LUMO may provide a rough classification of the donor/acceptor abilities (Figure 1b; Tables S1 and S2). According to the MO calculations, the most stable CTC should be formed between 4,6-DMDBT/DBT and DDQ, while TCNQ turned out to be not such an effective acceptor except when interacting with DBTO/DBTO2 donors. Experimentally, as shown in Figure 2 (spectra a, 1 and b, 2), DBT, 4,6-DMDBT, DBTO, and DBTO2 themselves absorb in the UV-region (at 230−290 nm). Consequently, it is possible to selectively “visualize” such π-donors if an appropriate πacceptor is intentionally chosen, because it gives a bathochromic shift typical of donor−acceptor complexes making

them easily detectable via UV−vis spectroscopy (Figure 2).38−42 It is also important (Figure 2, spectra a, 3−5 and b, 3−6) that the formed CTCs provide light absorption maxima in the 500−650 nm range, thus giving the possibility to agitate their electronic structure by standard 514 and/or 633 nm lasers typically used for Raman spectrometers. In accordance with the above preliminary data (Figures 1 and 2), 7,7,8,8-tetracyanoquinodimethane (TCNQ) and 2,3dichloro-5,6-dicyanobenzoquinone (DDQ) are suggested as suitable π-acceptors for further experiments. Nonoxidized forms, as predicted, can be effectively trapped with either DDQ or TCNQ; however, the situation is quite different for TCNQ complexes: despite the relatively high difference in the HOMO and the LUMO values (Figure 1b), these were the most stable in the series. To explain this fact, the presence of an oxygen atom in the structure of DBTO and related geometric redistribution of the electron density of the HOMO can be considered, which is as important as the energy difference between the LUMO and the HOMO.45 So, the revealed preferred interactions are shown with solid arrows and possible nonpreferred with the dashed arrows (Figure 1a). However, quantitative and selective determination of main PASHs impurities by the UV−vis spectroscopy is impossible as their content in fuel is less than 0.1 mM, and this technique is poorly selective. Consequently, a promising approach would consist of colored CTCs formation combined with their application as molecular traps in chemically modified planar optical sensors based on the SERS effect. The enhancement of Raman scattering and, ultimately, the low detection limits potentially attainable from SERS depend strongly upon the nanoscale 15062


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Figure 5. RS and SERS spectra of (a) polymer layers on the nanostructured silver ring layer: 1, pure glass (for comparison); 2, HEC; 3, PVP; 4, PVA; 5, CS coatings; and 6, the same with 5-fold increased thickness (RS spectra were measured using power neutral density filter 50%). SERS/ resonant SERS of charge-transfer complex DBT-DDQ on the silver rings with (a) 7, PVP; 8, PVA; 9, CS; 10, HEC; (b) with various CS coatings of 1, 1 wt %; 2, 0.1 wt %; 3, 0.005 wt %; and 4, 0.5 wt %. (The spectra were measured for identical analyte concentrations of 0.1 mM of DBT.) SERS/ resonant SERS spectra of different analytes on CS-coated SERS-substrate: (c) with DDQ (5 mM), 1, no analyte; 2, DBTO2; 3, DBTO; 4, CTC with 4,6-DMDBT; 5, CTC with DBT; 6, CTC with a model mixture of DBT:4,6-DMDBT:DBTO (1:10:9); and (d) with TCNQ (0.5 mM), 1, no analyte; 2, DBT; 3, 4,6-DMDBT; 4, CTC with DBTO2; 5, CTC with model mixture DBT:4,6-DMDBT:DBTO (1:10:9); 6, CTC with DBTO; (e) diesel oil matrix with 1, DDQ; 2, TCNQ; (f) 1, pyrene, and DDQ with 2, pyrene; 3, pyrene in diesel oil (Euro-5); and (g) TCNQ with 1, pyrene; 2, pyrene in diesel oil (Euro-5). All of the spectra were registered for identical analyte concentrations of 0.01 mM using a 20 mW 633 nm argon laser and power neutral density filter (10%), 20× objective lens, and 10 s of acquisition time.

Ag + + OH− = Ag 0 + 1/4O2 + 1/2H 2O

surface morphology of the enhancing material, making it difficult to achieve reproducible and quantitative measurements. Thus, a further important step consists of the preparation of a stable and reproducible nanostructured sensor with a wide plasmonic band. Bare Silver Substrate Structure. SERS-active planar substrates with pure nanostructured silver were prepared by decomposition of an aqueous silver(I) oxide ammonia complex under mild conditions without special reducing agents, salts, stabilizers, or anionic pollutants:18

3Ag + + NH3 + 3OH− = 3Ag 0 + 1/2N2 + 3H 2O

These transformations are utilized using thermal decomposition of ultrasonic mist of the complex. As a result, pure nanostructured silver coatings are deposited on a glass surface. The silver complex decomposition occurs in droplets acting as microreactors; the latter also provides constrained growth of AgNPs diminishing their size. During the aerosol deposition procedure, each droplet suffers a thermal shock followed by a 15063


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observed from the pristine layers of the polymers placed onto the silver layer of the substrates even if the polymer layer thickness is essentially increased as compared to the routine procedure used for substrate coating (Figure 5). Chitosan films on the glass were transparent, very flexible, and had smooth surfaces. As CS is a linear polysaccharide composed of randomly distributed deacetylated and acetylated D-glucosamine units, it may hold in its structure a solvent and the dissolved molecules, thus preconcentrating them. The presence of both amino and hydroxyl groups in CS monomers causes excellent chelating and film-forming properties. Because of the effect of molecular sieve and hydrophobic interactions, CS could capture various compounds from nonpolar solvents such as saturated hydrocarbons (e.g., isooctane, a model solvent of oil). At the same time, the CS layer did not affect the silver nanostructures. In particular, to find an optimum thickness of the CS gel applied on the nanostructured silver layers, aqueous solutions of chitosan were used with the following concentrations: 0.05, 0.1, 0.5, and 1 wt %. The sample with a concentration of chitosan gel of 0.5 wt % is found to be optimal for the formation of the maximal signal (Figure 5b). This polymer coating seems to be superior as compared to other tested variants, and we used such a CS layer in the consequent experiments. Chitosan films are known to have a great absorption effect, so they can hold linker molecules, acceptors, in its structure for latter trapping of PASHs as colored CTC complexes. Chemical Modification of SERS-Substrate by πAcceptors. SERS spectra in the region of aromatic ring stretching of PASHs in corresponding CTCs are shown in Figure 5. The spectra reveal typical vibrational modes of PASH bands; detailed assignments of the peaks are given in Table S2. Also, the intensities of the obtained SERS signals were reproducible for the samples with the same analyte concentrations (Figures S10−S12). Good signal-to-noise ratios were obtained from millimolar solutions of PASHs at 10 s signal accumulation time with laser power of 20 mW (Figure 5) and demonstrated a low level of photodegradation (Figure S13). The bands that appeared at 701 cm−1 can be indicated as the C−C−C in-plane bending modes α(CCC) or the C−H out-of-plane bending modes γ(HCCC). The signal at 1134 cm−1 corresponds to the vibration of thiophene ring, 1350 cm−1 to stretching vibrations of thiophene, and 1650−1670 cm−1 to oscillations of CC bonds of the aromatic ring.46−48 SERS spectra of the DBT−DDQ complex show the presence of DBT as the analyte immobilized on the SERS-sensor (Figure 5a, spectrum 10). In addition, the proposed method allows one to distinguish some alkyl derivatives of DBT: several additional bands in the experimental spectra (929 and 1567 cm−1) can be attributed to the characteristic peaks of 4,6-DMDBT; see the Supporting Information for details. We investigated the possibility of multiplex PASHs determination in the model complex mixtures of DBT:4,6DMDBT:DBTO in the concentration ratio of 1:10:9. In Figure 5c, a SERS spectrum of the model solution DBT:4,6DMDBT:DBTO for the sensor with DDQ is shown. Some visible characteristic signals related to DBT and 4,6-DMDBT structural fragments vibrations at 1134 cm−1 are found, along with the vibration of thiophene ring at 1584−1627 cm−1, and stretching modes of aromatic CC bonds are also present. Contrary to that, the spectrum recorded with a TCNQmodified sensor shows (Figure 5d) that the only set of peaks is visible that corresponds well to oxidized products, DBTO.

decrease of ammonia concentration and partial water evaporation, then followed by decomposition of droplets into films of interlacing silver rings (Figure 3a and b) with a highly hierarchic structure of their rims (Figure 3a−e).18 This surprisingly simple and robust approach was applied to produce the bare nanostructured silver substrates (Figure 3f) that are rough enough (Figure 3c and d) and uniform in its crosssection (Figure 3g), taking into consideration (Figure 3d) possible natural variations of silver layer thickness between the silver ring rims (1.4−1.5 μm) and the interior regions (300− 400 nm) consisting of 30−3000 nm silver clusters (Figure 3c). This variation is diminished when the rings interlace heavily at longer deposition time (more than 5 min). There is no special adhesion layer found at the interface between the substrate and the silver layer; however, the layer is attached strongly to the substrate with no delamination problems found, and also it is interconnected laterally keeping its unity even after breaking the substrate (Figure 3d). The polymer layer has typically a thickness of 1 μm and strengthens mechanically the silver layer (Figure 3e), demonstrating also good adhesion to silver. The polymer layer is not glassy (Figure 3e) and would have submicrometer porosity, while it is able to transfer analytes to the silver−polymer interface by swelling. The properties of such SERS-substrates can be optimally tuned by variation of simple experimental parameters18−21 like overall silver complex concentration and the process temperature and/or duration (Figure 4, Figure S5). It is also important that these bare nanostructured silver substrates give a very low and smooth background signal (Figure 4d) evidencing that the surface of silver is pure and contains no impurities or adsorbates. The applied silver deposition procedure is shown to be effective in enhancement of Raman scattering signals with an optimal deposition time of 15−20 min (Figure 4a, Figure S5). At the same time, such rough, hierarchically structured layers can be easily coated with a covering micrometer-thick polymer layer, which do not significantly interfere with optical properties of silver nanostructures themselves (Figure 4b). Polymer Layers. Despite the key importance of the silver nanostructured layer, it should be coated by a microporous polymer film with thickness of 5−10 μm, which is chemically modified with acceptor compounds ensuring the generation of bright SERS signals from the targeted analytes (Figure 5a). Such a newly proposed microporous optically transparent polymer layer acts as a protective coating and, simultaneously, as an effective absorbent of PASHs. Another important role of the polymer consists of trapping of the analytes. Polymers with different chemical motifs were investigated to find an optimum coating: chitosan (CS), hydroxyethylcellulose (HEC), poly(vinyl alcohol) (PVA), and polyvinylpyrrolidone (PVP). Polymer coatings do not destroy the unique silver-ring morphology of the prepared silver nanostructured substrates (Figure 3e and g, Figure 4). It was found that only chitosan and hydroxyethylcellolose coatings form continuous layers, while poly(vinyl alcohol) and polyvinylpyrolidone demonstrate dewetting and polymer film disruption. Otherwise, all of the investigated polymers are transparent, demonstrate no luminescence in the working region, and do not suppress or shift much the plasmonic band position typical for bare silver substrates (Figure 4b). The observed broadening of the plasmonic peaks at the interface of silver and polymers correlates well with the increase of dielectric constants of the multilayer structure when compared to a single layer of silver nanoparticles. Also, there is no pronounced Raman scattering 15064


Research Article

ACS Applied Materials & Interfaces Therefore, the proposed approach is highly selective and works well even for complex mixtures, allowing for a multiplex analysis because of the selectivity of the formation of relevant CTC complexes (Figure S15). We reconsidered a possibility of real sample analysis of diesel fuel (Euro-5) using our scheme (Figure S14). Obviously (Figure 5e), the concentration of PASHs in a given diesel fuel sample does not exceed 5 ppm, indicating the claimed quality of the product (Euro-5). However, in the case of TCNQ, the diesel fuel sample formed a charge-transfer complex with DBTO, which proved the presence of desulfurization products in the analyzed sample of oil (compare Figure 5e, spectrum 2 with Figure 5d, spectrum 6). The same results were demonstrated by a standard GC−MS analysis (Figure S9)49 for DBT determination, but the selectivity across different oxidized forms of DBT could be achieved only using an additional column. The influence of polyaromatic hydrocarbons (PAHs) in PASHs determination was examined because such products might interfere with the correct results on PASHs impurities level (Figure S16), as they might also serve as π-donors in CTC. Pyrene (C16H10) was chosen as a model PAH because it was present typically in oil. Pyrene gives itself no SERS signal in the system with TCNQ (Figure 5f). This discrepancy is explained by the absorption maxima of PAHs and metal plasmon bands: there is no charge transfer from the surface of the metal particles and the formation of new excited states of complex “molecule−metal”. However, pyrene revealed characteristic signals of its structural fragments, 600, 1246, 1404, and 1586 cm−1, if DDQ is immobilized on the silver surface using the CS layer. These characteristic signals of pyrene do not coincide with the characteristic signals of DBT/4,6-DMDBT in the system with DDQ. Pyrene in concentrations of about 1 × 10−5 M in the sample of diesel is “invisible” by SERS (Figure 5f). Therefore, pyrene (and other PAHs) has a negligible interfering effect on the selective determination of DBT and 4,6-DMDBT in petroleum products by SERS spectroscopy when using DDQ as a π-acceptor. At the same time, pyrene admixed to diesel fuel, if using TCNQ as immobilized acceptor, is clearly visible in the SERS spectrum. However, pyrene also does not interfere with DBTO determination (Figure 5g). Because charge-transfer complexes of DBT or 4,6-DMDBT with DDQ are the most stable and the same is observed for the DBTO−TCNQ combination (Tables S2 and S3), the concentration dependences on enhanced Raman intensity were then analyzed. Importantly, the intensity of SERS signals of DBT aromatic moiety (1596 cm−1) linearly depends on the concentration of DBT in the range 0.5−500 μM for our sensors (Figure 6). Analytical signals of DBT solutions with a concentration higher than 0.5 mM were almost constant. This fact can be attributed to a “saturability” of the interface between the nanostructured silver coating and the chitosan gel layer. Thus, the proposed approach allows one to determine the DBT in the concentration range from 5 × 10−7 to 5 × 10−4 M (Table 1). The specified range includes the maximum quantity of DBT corresponding to modern quality standards of diesel fuel.10,11 Also, determining DBTO with TCNQ on the same silver coating using SERS spectroscopy is possible. A signal from the (−CH2−CH−S(O)−CHCH2−) fragment at 1386−1390 cm−1 can be taken as a characteristic vibration (see the Supporting Information for details).

Figure 6. Dependence of SERS intensities (1596−1598 cm−1) on concentration of DBT (1) in CTC with DDQ (5 mM) and of SERS intensities (1386−1390 cm−1) on concentration of DBTO (2) in CTC with TCNQ (0.5 mM) with confidence and prediction bands (95%). The inset shows the dependence of SERS intensities for DBT (0.01− 10 mM).

Table 1. Metrological Parameters of SERS Determination of DBT/DBTO on Nanostructured Silver Surface Modified by Chitosan Gel (0.5 wt %) with Immobilized DDQ (5 × 10−3 M) at 1596 cm−1 for DBT and 956 cm−1 for 4,6-DMDBT, and Immobilized TCNQ (5 × 10−4 M) at 1390 cm−1 for DBTO (n = 6; P = 0.95) I = (a ± Δa)C + (b ± Δb) analyte

(a ± Δa) × 10−7

(b ± Δb)

DBT 4,6-DMDBT DBTO

20 ± 1 15 ± 1 100 ± 3

1530 ± 30 985 ± 20 2860 ± 30

linear range, μM 0.5−500 0.5−500 5.0−500

LOD, μM 0.1 0.1 1.0

Thus, the developed method makes it possible to control quantitative yields of catalytic oil desulfurization processes with necessary accuracy and precision as well as to determine the purity of industrial fuel and concentration of environmental pollutants. The obtained experimental data show that our novel indicator system based on trapping analytes within colored charge-transfer complexes allows a multiplex analysis of PASHs due to significant differences in the Raman spectra of the analytes with different molecular structures. Selectivity and stability of the analysis are provided by the correct choice of appropriate π-acceptor compounds, which are immobilized within the polymer layer of the novel SERS-sensor.

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CONCLUSIONS A novel approach is developed to build up advanced SERS planar sensors with a nanostructured silver layer coated with a thin layer of optically transparent chitosan gel. A chemical modification of the polymer with appropriate π-acceptor compounds revealed that such a sensor benefits from absorbing and preconcentrating of the analytes, followed by their selective binding into charge-transfer complexes (CTCs). These novel phenomena provide 2 or 3 orders of magnitude higher enhancement of vibrational modes intensities in the region measured by resonant SERS, due to a controllable shift of CTC absorption bands into the region of both laser agitation and the plasmonic peak of the silver nanostructures. In particular, 2,315065


Research Article

ACS Applied Materials & Interfaces dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and 7,7,8,8tetracyanoquinodimethane (TCNQ) were found to be most suited to selectively trap and quantitatively determine levels of DBT even in the presence of its derivatives. DDQ and TCNQ are thus shown to act as a readily available acceptors, which facilitate rapid formation of colored donor−acceptor CTCs. This novel indicator system allows for a multiplex determination of different forms of PASHs without any tedious sample pretreatment steps in small volumes of oil. This approach opens new possibilities of either increasing spectral and concentration sensitivity or increasing specificity of SERS-based sensors with foreseeable applications in the fields of ecology and advanced fuel analysis, among many others.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02018. Estimations of association constants and the formation Gibbs energy characterizing the new complexes; details of SERS experiments in comparison with a standard GC−MS analysis for model systems and real diesel fuel samples; typical transmission electron microscopy images; optimization of acceptor concentration on the nanostructured silver surface; and reproducibility of the SERS analysis for detecting DBT in the presence of oxidized forms and photodegradation of CTC (PDF)

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HEC, hydroxyethylcellulose HPLC-UV, high performance liquid chromatography with UV-detector PAH, polycyclic aromatic hydrocarbons PVA, poly(vinyl alcohol) PVP, polyvinylpyrrolidone SERS, surface enhanced Raman scattering TCNQ, 7,7,8,8-tetracyanoquinodimethane

AUTHOR INFORMATION

Corresponding Author

*Fax: +7-(495)-939-09-98. E-mail: [email protected]. ORCID

Olga E. Eremina: 0000-0002-2776-4743 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is governed and supported by Innopraktika (MSU). O.E.E., E.A.G., and I.A.V. acknowledge the support of the Russian Science Foundation (grant no. 14-13-00871). We thank A. A. Semenova, A. S. Baranchikov, V. K. Ivanov, and S. V. Savilov for their fruitful discussions, N. A. Belich for SEM images, and D. B. Eremin for GC−MS analysis. Mass spectra were recorded using facilities of the Department of Structural Studies, N.D. Zelinsky Institute of Organic Chemistry RAS; and Raman spectra measurements were performed within the ongoing Program of Development of Moscow State University.

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ABBREVIATIONS AgNP, silver nanoparticle ASP, aerosol silver pyrolysis CTC, charge-transfer complex CS, chitosan DBT, dibenzothiophene DBTO, dibenzothiophene sulf-5-oxide DBTO2, dibenzothiophene sulf-5,5-dioxide 4,6-DMDBT, 4,6-dimethyldibenzothiophene DDQ, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone GC-FID, gas chromatography with flame-ionization detector GC-MS, mass selective detector GC-SCD, sulfur chemiluminescence detector 15066


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