Communication pubs.acs.org/cm
Non-Resonant Large Format Surface Enhanced Raman Scattering Substrates for Selective Detection and Quantification of Xylene Isomers Phuong Hoang and Niveen M. Khashab* Smart Hybrid Materials Laboratory (SHMs), Advanced Membranes and Porous Material Center (AMPMC), King Abdullah University of Science and Technology (KAUST), Thuwal, 23955 Kingdom of Saudi Arabia S Supporting Information *
X
signals including Langmuir−Blodgett assembly of nanoparticles,21 drop-casting,22 and aggregation of particles with sharp edges like nanostars23 and triangular nanoplates.24 These bottom-up approaches are relatively simple but suffer from limited scalability, poor reproducibility, and uniformity of SERS signals. Other top-down methods such as UV and electron beam lithography afford uniform and highly sensitive SERS substrates; however, they can be costly and restrictive in terms of size and foundry nanotechnologies.25−27 Metal deposition on soft templates provided by well-developed semiconductor structures have been considered as an alternative SERS substrate. Hybrid nanostructures such as Ag decorated ZnO nanowires28 or Ag coated silicon nanotips29 demonstrated Raman scattering with an enhancement of 106−108 orders of magnitude. In these reports, the measurements were conducted at resonant Raman conditions with visible lasers (532 and 633 nm) corresponding to the localized plasmon resonance of the metal nanostructure and at a radiation energy above or equal to the semiconductor bandgap. While resonant Raman has been researched extensively for its impressive sensitivity, this technique suffers from strong interference from fluorescence signals and increased risk of sample photodegradation.30 In this work, a nonresonant uniform SERS substrate based on gold−silica core−shell patterning of a large format Au−ZnO forest is reported. This is prepared by a straightforward “wetchemistry” approach without the need for high-end lithography techniques. The substrate showed a high sensitivity and most importantly reproducibility for rapid detection of xylene isomers without preconcentration of the sample. As it depends on nonresonant Raman, an excitation source in the nearinfrared range is used. This makes this technique more advantageous when dealing with aqueous samples due to the low polarizability of water molecules and suppression of fluorescence signals.31,32 Moreover, the quantum energy in the near-infrared region is low enough to avoid heating or degradation of the semiconductor materials. A schematic illustration of the substrate fabrication is described in Scheme 1. First, ZnO seeds are spin coated on a Au-coated glass slide, from which a ZnO forest is fabricated through chemical bath deposition (Scheme 1a). Au nucleation sites are created directly on ZnO rods through direct reduction
ylenes are common solvents in industry, and they are extensively used in paint, leather, rubber, and other household products. Xylenes and other aromatic compounds such as benzene, toluene, and ethylbenzene (BTEX) are usually additives in mock petroleum and fuel to prevent autoignition at high temperature.1 Xylene isomers (o-xylene, m-xylene, and pxylene) are involved in different petrochemical processes such as the production of polyester fibers and resins starting from pxylene and phthalic anhydride starting from o-xylene.2 Air exposure to xylenes can cause respiratory health problems like forced expository volume (FEV) in 1 s at 100 ppm and oxidative stress when exposed to 80 ppm concentration.3 Moreover, each xylene isomer is metabolized by a different pathway in the body, although all isomers have similar physical properties.4 With a projected global petrochemical market of 758.3 billion dollars by 2022, selective detection of xylene isomers is crucial for general safety and environmental protection.5 Analytical detection of xylene isomers is commonly achieved in laboratories by using solid phase microextraction and highresolution gas chromatography.6−8 These techniques provide high sensitivity but require extensive sample handling and preparation. This greatly limits their use for fast on-site characterization and monitoring. Other indirect sensors like chemiresistor arrays based on metal oxide semiconductors have been developed for selective detection of xylene isomers but have modest detection limits (≤400 ppm).9−11 More recently, a highly sensitive sensor based on an ambipolar diketopyrrolopyrrole copolymer field-effect transistor was reported for xylene isomers detection.12 However, scale up and the tedious fabrication of these systems have proved to be a major challenge. Hence, optical remote sensing using direct sensors including infrared attenuated total reflection (IR-ATR) spectroscopy and surface enhanced Raman scattering (SERS) spectroscopy are emerging as promising candidates for on-site detection. A major drawback of IR-ATR sensing is the narrow spectral range and low selectivity regarding similar molecular structure analytes.13,14 SERS is a highly promising platform for trace chemical and biological detection, environmental monitoring, food security, and forensics.15−19 Symmetrical vibrations of molecules give rise to Raman signals; therefore, Raman spectroscopy provides fingerprint information on an analyte. The efficiency of this technique depends on the availability of a highly enhancing SERS substrate from which reproducible spectra are collected.20 Over the past decade, numerous techniques have been employed to enhance SERS © 2017 American Chemical Society
Received: December 24, 2016 Revised: February 22, 2017 Published: February 22, 2017 1994
DOI: 10.1021/acs.chemmater.6b05431 Chem. Mater. 2017, 29, 1994−1998
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SERS application due to the increased number of hot spots. Figure 1b shows the SEM image for ZnO rods treated with gold salt. The rod surface is rough due to the formation of gold nucleation sites (Figure S1). The SEM image in Figure 1c shows a top-view of the substrate after gold shell coating. A high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image of an individual completed gold shell coated ZnO rod and the corresponding energy dispersive spectrometry (EDX) elemental map are shown in Figures 1d,e, respectively. The thickness of the Au shell around the ZnO rods is approximately 10 nm (Figure S2). The XRD pattern of the gold salt treated substrate (Figure S3) clearly indicates the vertical growth of ZnO as well as the attachment of the Au seeds on the surface. The diffraction peaks at 34.49° and 62.91° correspond to Miller indices (002) and (103) of ZnO. The most intense peak at 34.49° confirms that the preferable growth of ZnO is vertically along the [001] direction.36,37 The peaks at 38.16°, 44.49°, 64.71°, and 77.59° corresponding to (111), (200), (220), and (311) confirm the formation of Au fcc seeds on ZnO.38 The intensity of the Au diffraction peaks increases after gold shell formation (Figure S3c). The UV−visible extinction spectra in the range of 400− 800 nm are also recorded before and after silica coating with extinction maxima at 540 nm (Figure S4). Although the substrate is optically opaque, the absorption spectrum of densely packed nanoparticles can be measured using UV− visible diffuse reflectance spectroscopy according to Kubelka− Munk theory.39 The thin silica coating caused increased extinction and slight red shift in plasmon resonance, which agrees with previous reports.40 Complete composition of the substrate was further confirmed by SEM-EDX (Figure S5). To calculate the enhancement factor for the SERS substrate, 4-mercaptobenzoic acid (4-MBA) was chosen as a nonresonant probe molecule with a known molecular footprint. The Raman peaks of 4-MBA at 1077 and 1575 cm−1 are assigned to the ring breathing mode ν(C−S) and ν(C−C).41 The characteristic peak at 1077 cm−1 is used to compare the performance of the designed substrate. First, in order to investigate the role of the first deposited gold layer (Scheme 1a), we spin coated the ZnO seed layer on plain glass (no Au) and repeated the substrate fabrication process. The substrate with gold coated glass shows stronger signal in contrast to plain glass due to the combined electromagnetic field induced by the gold nanofeatures and the gold layer (Figure S6a). Previous studies have confirmed such field enhancement resulting from nanoparticles coupled with planar metal.42,43 Further Raman measurements at the same concentration of analyte before and after silica coating show a slight decrease in intensity due to the increasing distance between the probe and the metallic surface (Figure S6b). A similar effect has been reported when the thickness of the shell increases above a certain limit.44−46 However, coating the metal substrate with silica is necessary to prevent irreversible absorption of molecules with strong binding thiol or amine groups47 and photodecomposition of the analyte when in direct contact with the active metal.35,48 The SERS signal of 4-MBA (10−6 M) was taken from 15 randomly selected spots on the substrate to check substrate uniformity (Figure S7b). The intensity of the peak at 1077 cm−1 was chosen as a reference for this comparison. The obtained results show 9.5 counts per second (cps) with a standard deviation of 9% (Figure S7c). This validates the substrate uniformity and capability to produce a highly reproducible spectrum. The performance of the substrate was
Scheme 1. Fabrication Scheme of the Designed SERS Substrate
of HAuCl4 solution (Scheme 1b). Further reduction of HAuCl4 will form a complete Au shell around ZnO rods (Scheme 1c). Finally, a silica mask is formed through condensation of (3aminopropyl)triethoxysilane (APTES) and NaSiO3 (Scheme 1d). More details on the synthesis and fabrication process are included in the Supporting Information. It is important to note that the Au nucleation procedure was performed without using any additional chemicals which can interfere with the SERS spectrum.33 A silica coating is employed to improve the stability of the metal surface achieving a clean spectrum of the analyte by preventing surface fouling; thus, our measurement technique is shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINER).34,35 Morphology of the substrate was investigated by scanning electron microscope (SEM). Figure 1a shows that the ZnO rods grow intertwined with each other, which is beneficial for a
Figure 1. (a) SEM images show the change of morphology after growing ZnO nanorod forest on gold coated glass, (b) treated with gold salt showing the formation of gold nucleation sites, and (c) gold shell coated ZnO rod. (d) HAADF STEM image of the coated rod and (e) EDX map of Au (red) and Zn (green) X-ray counts. 1995
DOI: 10.1021/acs.chemmater.6b05431 Chem. Mater. 2017, 29, 1994−1998
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Table 1 and compared to GC results from the manufacturer to show the level of accuracy. The component % (v/v) of the
further quantified by substrate enhancement factor (SEF), which was defined as the ratio of SERS signal compared to normal Raman measurement (non-SERS) condition: SEF = (ISERS/NSERS)/(Ibulk/Nbulk).49 In brief, the calculation was done by comparing the intensity of the spectrum of 4-MBA (20 μM) on the substrate and 0.11 M of 4-MBA on plain glass (Figure S8). The substrate has a SERS enhancement factor of 2.4 × 104. Even with higher concentrations and 10 times more laser power, the performance of plain glass was inferior to that of the designed substrate. Detailed SEF calculations are included in the Supporting Information. SERS not only provides sensitive and selective detection but also can be used as a quantitative tool50,51 due to the strong dependence of Raman Stoke signals to the number of analytes in a defined volume.52 Therefore, we used the obtained SERS spectra to quantify xylene isomers. The unknown concentration of each isomer in the sample can be calculated from a standard curve created from known concentrations of isomers. First, the reference spectra for pure xylene isomers and their mixtures were collected separately (Figure S9). Raman peaks were assigned to strong aromatic ring stretches of p-xylene at 829 cm−1, m-xylene at 726 cm−1, and o-xylene at 735 cm−1.53 In order to create the calibration curve, each isomer was diluted in the concentration range of 10−90% (v/v), and their SERS spectra were recorded (Figure 2a,b). The experimentally
Table 1. Comparative Concentration of Xylene Isomer Calculated from Calibration Data and GC Result from the Manufacturer
p-xylene m-xylene o-xylene
calculated from our SERS substrate % (v/v)
GC result of % (v/v) from manufacturer (product 534056, batch BCBQ2922V)
% difference
18.44 42.42 23.61
17.79 40.92 24.32
3.7 3.7 2.9
sample is within 3.7% deviation from the GC results which demonstrates the reliability of the fabricated SERS substrate for accurate quantification. Another important parameter in quantitative analysis is the limit of detection (LOD) or the lowest concentration of analyte that gives a distinguished Raman signal. It is calculated according to the formula LOD = concentration of analyte/ sensor response × baseline variation.54 Calculation details are described in the Supporting Information. The LOD values for o-xylene, m-xylene, and p-xylene were 14, 35, and 14 ppm, respectively. This is, to the best of our knowledge, the lowest LOD reported for xylene isomers using Raman spectroscopy thus far. In conclusion, a large format, highly sensitive and selective SERS substrate was prepared by employing hybrid structures of Au−ZnO/Au/Si (core/shell/shell). The synergistic effect of the bottom gold layer and gold coated ZnO exhibits efficient and reliable nonresonant Raman signal for quantification of analytes at very low detection limit (14 ppm) in the nearinfrared region. This substrate shows a huge potential to be practically employed in different industrial and commercial settings.
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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/acs.chemmater.6b05431. Experimental details and characterization results from SEM, TEM, XRD, UV−vis, and Raman measurements (PDF)
Figure 2. (a) SERS of m-xylene with different % (v/v) with p-xylene. (b) SERS spectrum of o-xylene with different % (v/v) with p-xylene. (c) Calibration data of intensity at peak 726 cm−1 versus % (v/v) mxylene from b. (d) Calibration data of intensity at peak 735 cm−1 versus % (v/v) m-xylene from b. (e) Calibration data of intensity at peak 828 cm−1 versus % (v/v) p-xylene from a and b. Error bars are standard deviations from three independent measurements. Details of the calibration line fitting are provided in Table S1.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Niveen M. Khashab: 0000-0003-2728-0666 Notes
obtained intensity peaks for the assigned characteristic peaks of each xylene isomer together with the standard curve fit (blue line) are presented in Figure 2c−e. The high coefficient Rsquared (R2 > 0.98) of the fitted line confirms a linear relationship between intensity and % (v/v) of each isomer. The SERS spectrum for the histological grade xylene mixture was measured (Figure S10). The intensities of the assigned peaks belonging to o-, m-, and p-xylene are used in the formula of the calibration curves shown in Figure 2c−e to derive the % (v/v) of each isomer in the mixture sample. The result is shown in
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge King Abdullah University of Science and Technology (KAUST) for the support of this work. REFERENCES
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