Clear differences were found between the spectra of the dye
at different pH values. At neutral pH the major bands at 872,
2
1
1
170, 1310, 1371, 1413 and 1559 cm were assigned to nitro
bending and stretching, hydrazo and azo vibrations and were
2
6
detected at a concentration of 1 3 10 M. Thus, a mixture of
both tautomers was present under neutral conditions. On
moving to pH 9 the spectral intensity of the bands increased and
the bands altered to reflect the change in tautomerism. The
dominating bands in this spectrum are those from the hydrazo
2
1
21
system at 1310 cm and nitro at 1371 cm . Under these
2
9
conditions the TNT dye could be easily detected at 1 3 10
M
using a flow cell sampling device. This concentration is still
well above the detection limit of SERRS and approaches the
sensitivity needed for vapour detection of TNT.2
Thus, we have selectively derivatised trinitrotoluene and
subsequently produced a SERRS active species in the form of
an azo dye that can be easily detected at low concentration
levels. Control of the chemistry and optimisation of the
spectroscopy has provided a method capable of detecting TNT
at ultra low concentrations which will be of real use in
restricting the movement and deployment of explosives.
The authors wish to thank Dr Richard Lacey of the Home
Office UK for funding to CM and RK and the BBSRC for the
award of a David Phillips Fellowship to DG.
Fig. 1 UV–vis spectra of the TNT dye at three different pH values. Dye
dissolved in methanol/Hydrion buffer (1+1).
1
1
mol cm ). The hydrazo form of the dye was also observed at
3
21
1
pH 6 as a broad shoulder at 515 nm (e = 6060 dm mol cm ).
At pH 8 deprotonation of the phenol resulted in complete
conversion to the keto-hydrazo form of the dye as observed by
3
21
a single strong absorption at 523 nm (e = 15700 dm mol
Notes and references
1
cm )(Fig. 1).
1
J. Yinon and S. Zitrin Modern Methods and Applications in the Analysis
of Explosives, Wiley, New York, 1993.
Based on the absorbance spectra, SERRS was acquired using
a Renishaw microprobe with spectrometer and 3 mW of 514.5
nm radiation at the source, at neutral pH and at pH 9. (pH 9 was
used as this was easier to achieve from a buffering point of view
with respect to the colloid than pH 8 as used in the UV–visible
studies.) The samples were analysed using a flow cell that has
shown to deliver more reproducible signals due to control of the
mixing of analyte, colloid and aggregating agent.24 Addition-
ally, use of the flow cell for mixing the dye with colloid and
aggregating agent is the first step in production of an automated
system for TNT detection by SERRS (Fig. 2).
2
P. Kolla, Angew. Chem., Int. Ed. Engl., 1997, 36, 800.
3 P. Kolla, Anal. Chem., 1995, 67, 184A.
4 R. Speller, Radiation Phys. Chem., 2001, 61, 293.
5 K. G. Furton and L. J. Myers, Talanta, 2001, 54, 487.
6
T. Urbanski Chemistry and Technology of Explosives (III), Pergamon
Press, Oxford, 1965.
7
8
J. Akhaven, Spectrochim. Acta, 1991, 47A, 1247.
C. Cheng, T. E. Kirkbride, D. N. Batcheldor, R. J. Lacey and T. G.
Sheldon, J. Forensic Sciences, 1995, 40, 31.
9
I. P. Hayward, T. E. Kirkbride, D. N. Batcheldor and R. J. Lacey, J.
Forensic Sciences, 1995, 40, 883.
1
0 C. Passingham, P. J Hendra, C. Hodges and H. A. Willis, Spectrochim.
Acta, 1991, 47A, 1235.
1
1 A. M. Stacy and R. P. Van Duyne, Chem. Phys. Lett., 1983, 102,
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65.
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1
2 P. Hildebrandt and M. Stockburger, J. Phys. Chem., 1984, 88, 5935.
3 C. Rodger, W. E. Smith, G. Dent and M. Edmondson, J. Chem. Soc.,
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1
1
1
1
2
4 K. Kneipp, H. Kneipp, I. Itzkan, R. R. Dasari and M. S. Feld, Chem.
Rev., 1999, 99, 2957.
5 D. Graham, C. McLaughlin, G. McAnally, J. C. Jones, P. C. White and
W. E. Smith, Chem. Commun., 1998, 1187.
6 K. Kneipp, Y. Wang, R. R. Dasari, M. S. Feld, B. D. Gilbert, J. Janni and
J. I. Steinfeld, Spectrochim. Acta, 1995, 51A, 2171.
7 J. M. Sylvia, J. A. Janni, J. D. Klein and K. M. Spencer, Anal. Chem.,
2
000, 72, 5834.
8 T. Urbanski Chemistry and Technology of Explosives ( ), Pergamon
I
Press, Oxford, 1964.
9 T. Urbanski Chemistry and Technology of Explosives (IV), Pergamon
Press, Oxford, 1985.
0 M. Hudlicky Reductions in Organic Chemistry, Ellis Horwood Limited,
New York, 1984.
21 M. O. Terpko and R. F. Heck, J. Org. Chem., 1980, 45, 4992.
22 R. L. Atkins and W. S. Wilson, J. Org. Chem., 1986, 51, 2572.
23 A. T. Nielsen, R. A. Henry, W. P. Norris, R. L. Atkins, D. W. Moore and
A. H. Lepie, J. Org. Chem., 1979, 44, 2499.
Fig. 2 SERRS spectra of the TNT dye at 1 3 102 M at pH 9 using 514.5
9
24 J. C. Jones, C. McLaughlin, D. Littlejohn, D. A. Sadler, D. Graham and
nm excitation for 10 s.
W. E. Smith, Anal. Chem., 1999, 71, 596.
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