Nanostructured Tin Oxide as a Surface-enhanced Raman ScatteringSubstrate for the Detection of Nitroaromatic Compounds

Kalimuthu Vijayarangamuthu and Shyama Rath*

Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India

Sensing of low concentrations of two nitroaromatic compounds, 1,2-dinitrotoluene and 2-nitrophenol, is presented. Thesensing mechanism is based on surface-enhanced Raman scattering (SERS) using nanostructured tin oxide as the SERS-activesubstrate. The SnOx nanostructures are synthesized by a simple solgel method and doped with Ag and Au. The Raman signalof a low concentration of the analyte, otherwise extremely weak, becomes significant when the analytes are attached to these

substrates. Doping of SnOx nanopowders with Ag and Au leads to a further increase in the Raman intensities. This studydemonstrates the scope of ceramic–metal nanocomposites as convenient solid-state SERS sensors for low-level detection.

Nitroaromatic compounds are constituents of explo-sives, smokeless detonators, dyes, polyurethane foams,herbicides, and insecticides. Their detection is essentialin environmental protection, landmines, homeland secu-rity applications, and forensic investigations. A particu-larly useful technique for the identification of suchcompounds is Raman spectroscopy (RS), due to theunique Raman spectral identity of every molecular spe-cies. This ability to fingerprint makes RS particularlyattractive for detecting and differentiating multiple spe-cies present in a mixture and having very similar chemi-cal compositions. As an analytical technique, it is rapidand simple, requiring only tiny amounts of the analyte,and has the capability for remote measurements. RS hasbeen used to characterize bulk explosives of wide-rangingchemical compositions including organic, inorganic,nitrogenous, and non-nitrogenous types.1,2 However, theinherent limitation of weak Raman intensities restricts itspractical applications when low concentrations are to bedetected. Surface-enhanced Raman scattering (SERS),which entails sizeable amplifications of the Raman inten-sities when the target molecules are attached to appropri-ately structured metal substrates, extends the scope ofnormal RS enormously to include sensitivity to low-leveldetection in addition to the high molecular specificity.3–6

The large enhancements achieved in SERS have beenused for low-level detection of various chemical and bio-logic analytes. The SERS effect is believed to be due totwo enhancement mechanisms, electromagnetic (EME)and chemical (CE), with the former being the dominantprocess. EME arises due to the giant increase in the localelectric field by the excitation of surface plasmons in the

metallic nanostructures when the analyte interacts withthe incident laser excitation. This effect requires the ana-lyte to be in the vicinity of the local fields. CE is due tothe interaction of the adsorbed molecules with the sub-strate, forming a charge transfer state between the two.In this case, the analyte must be directly adsorbed ontothe substrate.

The potential of SERS for low-level detection ofnitroaromatic explosives and for discriminating signalsarising due to the presence of other compounds7–10 hasbeen explored in a few recent studies. This is particularlyuseful because of the ability of RS to distinguish thesmall frequency shifts of the dominant nitro-group scis-soring and symmetric stretching bands modes arising bythe slight difference in the molecular structure of suchnitro compounds.

The choice of the substrate to which the analyte isattached plays a vital role in SERS. Elemental noblemetal substrates of a wide variety including electrochemi-cally roughened films, colloids, island films, and litho-graphically patterned substrates have commonly beenutilized for a range of analytes due to their ability to pro-vide EME. However, more recently, SERS activity fromalternate substrates such as metal/ceramic composites hasbeen explored. These include Ag/TiO2,

11,12 Au (Ag)-coated ZnO nanowires,13 and Au-coated SnO2.

14 Thedevelopment of portable and low-cost solid substrates isdesirable for the detection of explosive compounds pres-ent in trace levels in difficult environments. The metal/nanostructured semiconductor hybrid systems allow alarger volume of the analyte to be sampled due to anincreased analyte adsorption by the greater surface areaof the porous/nanostructured semiconductor. Most SERSstudies, using either elemental metal or metal compositesas the substrates, have attributed the Raman signal

*srath@physics.du.ac.in

© 2014 The American Ceramic Society

Int. J. Appl. Ceram. Technol., 1–5 (2014)DOI:10.1111/ijac.12266

amplification to the EME arising due to the presence of“hot spots” in the vicinity of the metal. In this study,using semiconducting tin oxide as the SERS substrate,the role of the CE in the SERS process is suggested.

Tin oxide, a wide bandgap semiconductor and hav-ing the advantages of high and mechanical stability, is oftechnological interest for a wide range of applications.Specifically, its application in gas sensing has been widelyreported, and the sensing is based on variations in itselectrical properties. There appears to be a close relationbetween the sensitivity and the surface characteristics.15

It has also been proposed that the introduction of metaldopants enhances the gas-sensitivity behavior.

In this study, we demonstrate the potential of nano-structured SnOx (1 < x < 2) nanopowders as an opticalsensor for the detection of nitroaromatic compoundsbased on the SERS effect. We also compare the SERSsignals with those achieved from commercially availableKlarite substrates purchased from Renishaw Diagnostics,UK. These substrates entail submicrometer scale pattern-ing of a gold-coated silicon surface. The SnOx nanopow-ders are prepared by a simple solgel process. Doping ofthe SnOx nanopowders with Au or Ag enables a furtherenhancement of the signal. We also show a correspon-dence between the structural and surface characteristics ofthe nanopowders with the Raman intensity enhancement.

The SnOx substrates were prepared by a solgel pro-cess where 0.15 molar tin tetrachloride pentahydrate(SnCl4.5H2O) was dissolved in a mixture containing2-proponal, 2-methoxy ethanol, and formamide in theratio 2:1:1. Ag (Au) doping was performed by taking0.005 (0.001) molar ratio of silver nitrate (gold chlo-ride). The solution was refluxed with a magneticstirring arrangement at 80°C for 24 h. After the com-pletion of the reaction, the mixture was cooled downfrom 80°C to room temperature, cleaned with distilledwater, and finally centrifuged to separate the powderfrom the solution. The obtained powder was dried in avacuum oven at 100°C for 1 h. The powders were thenannealed in an oxygen atmosphere at 400 and 500°Cto determine the effect of structural changes on theSERS activity. The structure of the nanopowders wasmonitored by X-ray diffraction and transmission elec-tron microscopy, and Ag (Au) incorporation was con-firmed by energy-dispersive X-ray analysis.16 Thenanopowders were pressed into pellets for their use asconvenient SERS substrates. Doping of Ag (Au) inSnOx induces electron localization on the surface of theSnOx particles and forms smaller aggregated clusters ascompared to the undoped SnOx sample. We alsoobserved a similar nanoparticle size reduction by metaldoping in Co-doped SnO2.

16

Raman scattering measurements were performed atroom temperature in the backscattering configurationusing an in-Via Renishaw micro-Raman system, with anexcitation wavelength of 514.5 nm. The analytes studiedare 2-nitrophenol (NP) and 2,4-dinitrotoluene (DNT)purchased from Sigma-Aldrich. A stock solution of vari-ous concentrations of NP and DNT was prepared in anacetonitrile solvent. For the Raman spectroscopic mea-surements, the samples were prepared by adding 2–5 lLof analyte solution on the different substrates: All theSERS spectra of NP and DNT were acquired using (i)commercial Klarite SERS substrates (ii) SnOx, (iii)Ag-doped, and (iv) Au-doped SnOx in the same manner.Each of the SERS spectra was collected for 10-s exposuretime and five accumulations.

Figure 1a shows the X-ray diffraction pattern of theas-prepared undoped and Ag-/Au-doped SnOx. The dif-fraction peaks observed around 26.6°, 33.8°, and 52° inall the samples can be indexed to the (110), (101), and(211) planes of the tetragonal rutile structure of SnO2

in both position and relative intensity (JCPDS card no:77-0451). The absence of Ag-/Au-related phases is dueto their low concentrations. The broad peaks indicate anamorphous nature of the samples. Figure 1b–d showsthe transmission electron microscopy (TEM) studies ofthe as-prepared undoped and Ag-/Au-doped powders.The particles are observed to be agglomerated together,and the doping is seen as dark spots in the image.

Figure 2a shows the Raman spectra of the as-pre-pared undoped and Ag-/Au-doped SnOx bare substrates.A broad structure between 400 and 700 cm�1, which isdue to a combination of the amorphous modes, a surfacemode at 564 cm�1, and a surface Sn(OH)2 mode,16,17 isobserved in all the substrates. Figure 2b shows the UV–vis spectra of the undoped and Ag-/Au-doped powders.The spectrum of undoped SnOx is featureless. The char-acteristic surface plasmon resonance (SPR) peak due tothe presence of the Au nanoparticles is observed ataround 520 nm in Au-SnOx

18. The SPR peak of Agnanoparticles lies in the range of 325–400 nm, but isnot observed in the Ag-SnOx nanopowder due to alimitation in the instrument’s measurement range and apossible low concentration of Ag19.

Figure 3 shows the measured normal Raman spectraof bulk DNT and NP along with the structure andassignment of the various modes. The two prominentfeatures at around 836 (820) and 1347 (1320) cm�1 inNP (DNT)20 correspond to the nitro out-of-plane bend-ing and symmetric stretching vibration modes, respec-tively. The C-N in-plane stretching mode occurs ataround 1136 cm�1. These modes are the fingerprints forthe detection of nitroaromatic compounds. Their relative

2 International Journal of Applied Ceramic Technology—Vijayarangamuthu and Rath 2014

intensities and the frequency positions can be used toidentify and distinguish between various similar nitroaro-matic compounds.

Figure 4 shows the SERS spectra of low concentra-tions of NP from the various substrates. For a 10�4 Mconcentration, the commercial Klarite substrates (Fig. 4a)show a selective enhancement of the N-O stretchingmodes. The SnOx-based SERS substrates detect almostall the Raman modes observed in the bulk NP while

preserving the intensity ratios. When the concentration isfurther reduced to 10�6 M, the SERS spectrum fromthe Klarite substrates shows broad features that aredifficult to resolve and the signal from the undopedSnOx substrate is featureless (Fig. 4b). However, in theAg- and Au-doped SnOx substrates, (Figs. 4c and d,

(a) (b)

Fig. 2. (a) Raman and (b) UV-vis spectra of bare SnOx, Ag-SnOx, and Au- SnOx substrates.

Fig. 3. Raman spectra of bulk (a) 2-nitrophenol and (b)2,4-dinitrotoluene.

(a) (b)

(c) (d)

Fig. 1. (a) X-ray diffraction pattern and TEM image of as-synthesized (b), undoped SnOx, (c) Ag-SnOx, and (d) Au-SnOx surface-enhanced Raman scattering substrates.

www.ceramics.org/ACT Tin Oxide as a SERS Substrate 3

respectively), well-resolvable Raman spectra are obtainedand the intensity ratios agree with those observed frombulk NP. Along with the nitro symmetric stretchingvibration mode, the C-N modes also have sizeable inten-sities and can also be used as signatures for the detectionof low levels of NP.

A similar study of the SERS spectra of the otheranalyte, DNT, is illustrated in Fig. 5. For a 10�4 molarconcentration, the SERS spectra obtained from all thefour substrates are almost identical with all the Ramanmodes being well resolvable. When the concentration isreduced to 10�6 M, the N-O stretching mode is wellresolved for all substrates; however, the weaker modessuch as N-O bending and the C-N modes are onlyresolvable in the Ag-doped SnOx substrate. In the case ofthe Au-doped SnOx substrate, it is likely that the lowerconcentration of Au in SnOx as compared to Ag preventsthe detection of these modes. The N-O peak is also shar-per in the Ag-doped SnOx substrate as compared to theKlarite substrates for a lower concentration of 10�6 M.

We next investigate the correlation between thestructure of tin oxide and its SERS sensing behavior, asshown in Fig. 6. For this purpose, the as-prepared pow-ders were thermally annealed in an oxygen atmosphere.The Raman spectra of the nanopowders as a functionof annealing are shown in Figs. 6a and c for Ag- and

(a) (b)

(c)(d)

Fig. 4. Surface-enhanced Raman scattering spectra of 2-nitrophe-nol on (a) Klarite, (b) bare SnOx, (c) Ag-SnOx, and (d) Au-SnOx

substrates for 10�4 M (black line) and 10�6 M (red line) concen-trations. In figure, * indicates NO2 mode and # indicates C-Nmode.

(a) (b)

(c)(d)

Fig. 5. Surface-enhanced Raman scattering spectra of 2,4-dini-trotoluene on (a) Klarite (b) bare SnOx, (c) Ag-SnOx, and (d)Au-SnOxsubstrates for 10

�4 M (black line) and 10�6 M (redline) concentrations.

(a) (b)

(c) (d)

Fig. 6. Raman spectra of (a) Ag-SnOx and (c) Au-SnOx as afunction of annealing temperature; corresponding surface-enhancedRaman scattering spectra of 2-nitrophenol from (b) Ag-SnOx and(d) Au-SnOx substrates: (a1) as-prepared, (b1) 400 °C, and (c1)500 °C.

4 International Journal of Applied Ceramic Technology—Vijayarangamuthu and Rath 2014

Au-doped SnOx, respectively. A broad structure between400 and 700 cm�1 observed in the as-prepared and400°C annealed samples is due to a combination of theamorphous modes, a surface mode at 564 cm�1 and asurface Sn(OH)2 mode.17,21 The intensity decrease ofthis feature with annealing temperature indicates a grad-ual suppression of surface effects. The removal of thesurface hydroxyl mode is also further confirmed byRaman and FT-IR measurements in the higher frequency1600–3000 cm�1 wavenumber range17. The appearanceof the well-defined A1g Raman mode at around632 cm�1 after annealing confirms the improvement incrystallinity, growth of the nanoparticles, transformationof stoichiometric SnO2, and a subsequent decrease in thesurface–volume ratio. These structural and chemical fea-tures of the SnOx nanopowders can be related to theSERS activity shown in Figs. 6b and d. The annealedAu- and Ag-doped SnOx show a reduced SERS activityas compared to the as-prepared powders. This shows theimportance of the surface effect, presence of a larger sur-face area and surface bonds, which provides favorableconditions for the attachment of the analytes.

Some key features of this study are firstly, the obser-vation of SERS activity from bare substoichiometricSnOx substrate even without any metal additives and sec-ondly, the suppression of SERS activity in stoichiometricSnO2. These features suggest the role of the CE mecha-nism arising due to the interaction between the surfaceof the substrate and analytes in undoped SnOx and is ashort-range effect. CE occurs through the charge transferprocess between the substrate and the adsorbate, and theenhancement may be likened to a resonance Ramaneffect. The effects of metal addition are (i) a restrictionin the nanoparticle size improving thereby the surface–volume ratio, (ii) an increase in further electronic statesfor charge transfer, and (iii) generation of surface plas-mon effects leading to EME. Thus, Au- and Ag-dopedSnOx substrates for both the analytes studied show betterenhancements as compared to the undoped SnOx sub-strates.

A major advantage that we observe with these SnOx

SERS substrates is the absence of the strong fluorescencethat commonly arises from these nitroaromatic analytes.This is relevant in practical applications where the use ofa convenient visible laser excitation is desirable as SERSexperiments are very often limited by the requirement ofan IR excitation to suppress the strong fluorescence sig-nals. The absence of fluorescence, a simple method ofpreparation, large surface area for adhesion of the ana-lyte, channels for charge transfer in substoichiometricSnOx, and good reproducibility of detection all contrib-ute to SnOx as a good candidate for SERS substrates.

In conclusion, this study demonstrates the potentialof ceramic nanostructures as a SERS sensor. Low-leveldetection of two nitroaromatic compounds, namely NPand DNT, has been demonstrated using nanometric sub-stoichiometric SnOx. The solid substrates are made usingsimple cost-effective chemical techniques and have easyportability. Visible laser excitation is used for the mea-surements, and undesirable fluorescence signals areabsent. RS can be used as a single technique for bothcharacterization of the nanoparticles as well as the sens-ing mechanism. Further work will focus on improvementin the SERS signals both from the points of view ofmaterial optimization and from technique by increasingthe Ag(Au) concentration to obtain better electromag-netic enhancements and studying the excitation wave-length dependence, respectively.

This work was supported by grants from R&Dprogramme of Delhi University, CARS Project 05/P-264/09-10 of DRDO, and Nanomission project ofDepartment of Science and Technology (DST), India.One of the authors (KV) acknowledges Council of Sci-entific & Industrial Research (CSIR), India, for a SRFfellowship.

References

1. I. R. Lewis, N. W. Daniel Jr, N. C. Chaffin, P. R. Griffiths, and M. W.Tungol, Spectrochim. Acta A, 51 1985–2000 (1995).

2. F. T. Docherty, P. B. Monaghen, C. J. McHuge, D. Graham, W. E. Smith,and J. M. Cooper, IEEE Sens. J., 5 632–640 (2005).

3. M. Fleishmann, P. J. Hendre, and A. Mcquilla, Chem. Phys. Lett., 26 163–166 (1974).

4. M. Moskovits, Rev. Mod. Phys., 57 783–826 (1985).5. K. Kneipp, M. Moskovits, and H. Kneipp, Surface Enhanced Raman Scatter-

ing: Physics and Application, Springer, Berlin, Germany, 2006.6. A. Otto, J. Raman Spectrosc., 33 [8] 593–598 (2002).7. K. M. Spencer, J. M. Sylvia, P. J. Marren, J. F. Bertone, and S. D. Christe-

sen, Proc. SPIE, 5269 1 (2004). DOI: 10.1117/12.5148458. S. S. R. Dasary, A. K. Singh, D. Senapati, H. Yu, and P. C. Ray, J. Am.

Chem. Soc., 131 [38] 13806–13812 (2009).9. H. Wackerbarth, L. Gundrum, C. Salb, K. Christou, and W. Viol, Appl.

Opt., 49 4362–4366 (2010).10. X. Liu, L. Zhao, H. Shen, H. Xu, and L. Lu, Talanta, 83 [3] 1023–1029

(2011).11. L. B. Yang, et al., J. Phys. Chem. C, 113 16226–16231 (2009).12. A. Roguska, A. Kudelski, M. Pisarek, M. Lewandowska, M. Dolata, and M.

Janik-Czachor, J. Raman Spectrosc., 40 [11] 1652–1656 (2009).13. M. A. Khan, T. P. Hogan, and B. Shanker, J. Raman Spectrosc., 40 1539–

1545 (2009).14. X. Jiang, L. Zang, T. Wang, and Q. Wan, J. Appl. Phys., 106 [10] 104316

(2009).15. U. Diebold and M. Matzill, Prog. Surf. Sci., 79 47–154 (2005).16. K. Vijayarangamuthu, “Optical Properties of low-dimensional semiconduc-

tors,” Ph.D. Thesis, University of Delhi, Delhi, 2012.17. K. Vijayarangamuthu and S. Rath, Appl. Phys. A, 114 1181–1188 (2013).18. M. Lismont and L. Dreesen, Mater. Sci. Eng., C, 32 1437–1442 (2012).19. L. Suna, et al., J. Hazard. Mater., 171 1045–1050 (2009).20. A. Kovacs, V. Izvekov, G. Keresztury, and G. Pongor, Chem. Phys., 238

231–243 (1998).21. A. Dieguez, A. Romano-Rodrıguez, A. Vila, and J. R. Morante, J. Appl.

Phys., 90 1550–1557 (2001).

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