Nanoparticles in explosives detection – the state-of-the-art and future directions
Forensic Sci Med Pathol
Nanoparticles in explosives detection - the state-of-the-art and future directions
William J. Peveler 0
Sultan Ben Jaber 0
Ivan P. Parkin 0
0 Department of Chemistry, University College London , 20 Gordon St, WC1H 0AJ, London , UK
1 Ivan P. Parkin
Nanoparticle-ligand systems can be targeted to specific analytes
to effect a change in the properties of the nanoparticles. We will
examine two examples in which the electromagnetic properties
of the nanoparticles (arising from their small size) are altered by
analyte binding, and can be applied as a transducer in a chemical
sensing system for explosive analytes. The first property
reviewed is the surface plasmon resonance (SPR) band of
colloidal gold nanoparticles (AuNPs), and the second, the
fluorescence of colloidal semiconductors (quantum dots – QDs).
Sensors based on these nanoparticle properties have the
potential to detect picomolar or lower concentrations of explosive
analytes, and can operate for both solution phase and gas phase
]. In addition, measurement of the signal produced
by the nanoparticle transducer uses standard scientific
instrumentation, making it easier to build complete detection systems from
standard components - an important consideration [
Example 1 - gold nanoparticles
In AuNPs the free electrons of the metal surface interact
strongly with light causing large oscillations in the surface
electromagnetic field. The particles therefore absorb light
strongly at the particular resonant frequencies of these
electrons, giving rise to SPR bands.
One method to exploit the plasmons of AuNPs for sensing
is to use them in surface enhanced Raman spectroscopy
(SERS). A Raman spectrum is a powerful way to fingerprint
a molecule, using incident light to excite Raman active
vibrational modes (Fig. 1a), causing inelastic scattering of the
photons, and giving rise to a unique spectrum that provides
information on molecular shape and connectivity. The spectrum
obtained from an unknown analyte can be compared to a
library of known spectra and used to identify a threat.
However, Raman scattering is very weak, and so detection
of low levels of analyte, enhancement is required.
If a molecule is bound (chemi- or physisorbed) to a metal
surface, incident light (usually a monochromated laser pulse)
excites the surface plasmons, inducing polarization in the
bound molecules, increasing the amount of inelastic scattered
light from the Raman vibrational modes (Fig. 1b). This leads
to a signal enhancement of up to E4, where E is the
electricfield magnitude. The intensity of the SERS effect is largely
attributed to the monolayer of molecules absorbed to the
nanoparticles, and is highly dependent on the form (and hence
plasmonic field) of the nanoparticles [
]. This adsorption
onto the substrate also creates new vibrational selection rules,
and surface-complex formation can lead to altered electronic
properties of the absorbed molecule .
The best SERS enhancement is achieved by having strong
localized plasmons, that fall within the wavelength of the
Raman laser excitation. For this reason gold and silver are often
chosen, as their SPR bands are typically within 400–800 nm,
which is easy to access with a visible laser [
]. In addition they
are chemically inert and thus stable against air, and strong
oxidizing or reducing agents. Other materials have been
successfully applied as SERS substrates, such as other noble metals (Pt
and Pd) and even transition metals and semiconductors [
Strong plasmon hotspots created in between individual
particles can improve the SERS enhancement effect. This can be
21 } vibrational states
0 - ground state
achieved through aggregation of the particles, either in
solution by trapping at an interface or chemical aggregation with
linking molecules, or by solvent removal [
In recent years SERS on nanoparticulate colloids has been
heavily applied to the detection of illicit materials, such as
]. The technique has been shown to be
capable of detecting a range of high explosives, with good
detection limits (into the nanomolar region or below), even with
raw colloidal solutions of Au and AgNPs, such as those
applied by the group of Hernández-Rivera [
aggregating AuNPs at an interface, Edel et al. created a regular
monolayer array, which showed enhanced sensitivity to a
range of compounds, including some explosives [
A more targeted approach has been taken by utilizing NPs
functionalized with cysteine to form Mesienheimer complexes
with nitroaromatics [
]. Xu et al. demonstrated enhanced
detection of DNT by using cyclodextrin coated triangular
nanoprisms of gold . SERS also extends beyond the
nitroaromatics, for example it has been shown that RDX can
be detected at concentrations as low as 0.15 mg/L in ground
water samples [
]. This illustrates a key benefit of SERS, that
it is label free, requiring no special binding groups to target
particular analytes, but that it can become more targeted with
SERS is a very useful technique for detection of explosives as
it can can detect solution or vapor phase materials at very low
concentrations. The individual Raman fingerprint of each
different molecule makes specificity high, however in complex
mixtures, deconvolution can be a challenge [
]. A second
problem is the dependence of the Raman signal on the SERS
substrate. Particular vibrational modes in the Raman spectrum can
be enhanced or suppressed depending on the binding mode of
the analyte, and if the substrate (e.g. AuNP concentration and
aggregation amount) is not identical in every instance then the
fingerprints may differ slightly, causing loss of specificity.
Therefore, a key requirement is the development of cheap,
identical SERS substrates, that exhibit powerful enhancement,
ensuring strong and repeatable spectral fingerprints can be obtained on
Recently we reported a new method of designing such
substrates based on a new enhanced Raman technique –
photoinduced enhanced Raman spectroscopy – PIERS. It was shown
that by combining semiconducting TiO2 substrates with
plasmonic AuNPs, and pre-irradiating the material, a PIERS
substrate was created that displayed an order of magnitude
enhancement over conventional SERS techniques [
]. We performed
sensing of DNT, TNT, RDX and PETN explosives in solution,
with high quality spectral finger prints obtained even at
subnanomolar concentrations (Fig. 2). In particular nanomolar
concentrations of DNT and TNT were detected both in solution and
in the vapor phase, demonstrating that this PIERS technique
might have interest for stand-off detection of explosives. We
also showed that via the pre-irradiation step the substrates could
be fully cleaned over several cycles, meaning the same substrate
can be used multiple times, unlike many commercial SERS
substrates on the market today.
We undertook a thorough investigation into the mechanism
of this enhancement and suggest that it arises from interaction
between the irradiated TiO2 and gold nanoparticles, causing
improved charge transfer and electromagnetic enhancement at
the surface of the substrate (Fig. 2). Further investigations into
the underlying mechanism and optimization for field detection
of explosives, and other threats, are ongoing.
Example 2 - quantum dots
Quantum dots (QDs) are semiconducting nanoparticles which
are small enough to confine a generated hole-electron pair
(exciton) within all 3 spatial dimensions, leading to
quantization of the energy levels. This causes the electronic structure
of the material to sit between a classical semiconductor, and a
molecular material (Fig. 3a).
The result of this quantization of states is that the
nanoparticles display sharp photon absorption and emission
bands, and the band gap is closely related to the size of
the nanoparticle. The fluorescence arises from
photoexcitation of the nanoparticles, causing exciton formation.
Recombination of this exciton will then occur through
radiative (fluorescence) or non-radiative (trap-states,
oxidation, energy transfer) pathways. The fluorescence from
Pre-irradiation with UV Incident laser
QDs is easily tuned to the visible or near-IR region of the
spectrum, by choice of semiconductor material and
particle size, making QDs useful fluorophores (Fig. 3b).
Introducing molecules around the surface of the QD will
affect the rate of recombination of the exciton, and may also
disrupt its recombination. In particular, the conduction band
electron may be lost to a local species in a process termed
photoinduced electron transfer (PET). In PET the loss of the excited
electron to the acceptor results in the prevention of
recombination, and thus the loss of fluorescence. The more efficient this
process, the larger the fluorescence quenching of the system.
This electron transfer mechanism has been used
extensively to transduce the presence of analytes in proximity to a QD,
and thus form the sensor element in a chemical sensor
]. The surface of the QD can be targeted to certain
analytes by the placement of receptors on the surface that
preferentially bind the analyte and bring it into close proximity
with the QD, facilitating the PET mechanism, and quenching
the observed fluorescence [
QDs, therefore, have many properties of interest to
chemical sensing - they exhibit high fluorescence quantum yields,
are resistant to photobleaching, and have broad absorption
giving rise to narrow emission bands. This means they lend
themselves well to a multiplexed or multichannel fluorophore
system, with a single excitation wavelength causing emission
from many different species of varying color. The surface of
the particles is easily modified with targeting ligands to allow
specific and sensitive fluorescence enhancement or quenching
on association with an analyte via a PET mechanism.
In the security domain, QDs have been used for
approximately 10 years as a fledgling sensor for explosives. Due
to explosives’ (particularly nitro/conventional explosives’)
electron deficient nature, they make excellent PET
]. Initially antibody targeting of explosives
was used, but this is complicated by the procedures to
obtain, purify and conjugate the antibodies [
cheaper and simpler targeting approach has been the
formation of Meisenheimer complexes, using surface amines
to bind nitroaromatics, such as TNT or picric acid [
This system can be highly specific towards nitroaromatic
materials, but is very difficult to apply more widely to
other explosives (even DNT). A novel donor/acceptor
system for the sensing of explosives beyond TNT has been
pioneered by Willner et al. They have utilized both a redox
couple based on NAD+/NADH and a donor/acceptor
scheme, based on surface bound dopamine derivatives, to
sense a range of explosives, including RDX [
To further this work we have recently exploited the attractive
optical properties of QDs to build an explosive sensing array of
quantum dots. By combining multiplexed fluorophores with
variable response do different explosives, with array statistics, it was
possible to ‘fingerprint’ 5 different explosives and identify them
at low concentration [
]. This has potential applications in
wastewater management and testing, as well as drinking water
evaluation in areas where explosives contamination is a health issue,
such as ordinance ranges and manufacturing sites, as well as for
In this system, the surfaces of the QDs were modified with
a range of supramolecular functionalities to control their
selective interactions with different explosives, and DNT, TNT,
RDX, PETN and tetryl were successfully detected and
discriminated using the array of just three QDs. It was also
shown that the QDs in the array could operate in the format of
a paper-based test, in addition to the solution-based assay.
Limits of detection down to ppb levels were obtained, and
most importantly the array could read out information on what
type of explosive was present, rather than just if there was a
particular single explosive or not (Fig. 4).
Nanomaterials have shown great promise in the
explosives detection field and it is likely that future commercial
developments in this area will make some use of these
types of matter. Two key areas of interest are the Raman
enhancing properties of plasmonic gold nanoparticles, and
the fluorescent nanomaterials such as quantum dots that
can be used in complex photonic systems for sensing
different types of explosive at low levels. Each of these has
shown their worth in the laboratory and efforts must now
focus on more rigorous device design and field testing to
move towards end-user applications.
Acknowledgements WJP is supported by an EPSRC Doctoral Prize
Fellowship (EP/M506448/1). SBJ acknowledges the support of the
government of Saudi Arabia, Ministry of Interior, King Fahd Security
Open Access This article is distributed under the terms of the Creative
C o m m o n s A t t r i b u t i o n 4 . 0 I n t e r n a t i o n a l L i c e n s e ( h t t p : / /
creativecommons.org/licenses/by/4.0/), which permits unrestricted use,
distribution, and reproduction in any medium, provided you give
appropriate credit to the original author(s) and the source, provide a link to the
Creative Commons license, and indicate if changes were made.
1. Peveler WJ , Roldan A , Hollingsworth N , Porter MJ , Parkin IP . Multichannel detection and differentiation of explosives with a quantum dot array . ACS Nano . 2016 ; 10 : 1139 - 46 .
2. Ben-Jaber S , Peveler WJ , Quesada-Cabrera R , Cortés E , SoteloVazquez C , Abdul-Karim N , et al. Photo-induced enhanced Raman spectroscopy for universal ultra-trace detection of explosives, pollutants and biomolecules . Nat Commun . 2016 ; 7 : 12189 .
3. Smith RG , D'Souza N , Nicklin S. A review of biosensors and biologically-inspired systems for explosives detection . Analyst . 2008 ; 133 : 571 - 84 .
4. Wolfbeis OS . Probes, sensors, and labels: why is real progress slow ? Angew Chem Int Ed. 2013 ; 52 : 9864 - 5 .
5. Liu Z , Zhang F , Yang Z , You H , Tian C , Li Z , et al. Gold mesoparticles with precisely controlled surface topographies for single-particle surface-enhanced Raman spectroscopy . J Mater Chem C. 2013 ; 1 : 5567 - 76 .
6. Tiwari VS , Oleg T , Darbha GK , Hardy W , Singh JP , Ray PC . Nonresonance SERS effects of silver colloids with different shapes . Chem Phys Lett . 2007 ; 446 : 77 - 82 .
7. Kedia A , Kumar PS . Controlled reshaping and plasmon tuning mechanism of gold nanostars . J Mater Chem C. 2013 ; 1 : 4540 .
8. Peveler WJ , Parkin IP . Rapid synthesis of gold nanostructures with cyclic and linear ketones . RSC Adv . 2013 ; 3 : 21919 - 27 .
9. Guerrini L , Graham D . Molecularly-mediated assemblies of plasmonic nanoparticles for surface-enhanced Raman spectroscopy applications . Chem Soc Rev . 2012 ; 41 : 7085 - 107 .
10. Ghosh SK , Pal T. Interparticle coupling effect on the surface plasmon resonance of gold nanoparticles: from theory to applications . Chem Rev . 2007 ; 107 : 4797 - 862 .
11. Abdelsalam ME , Mahajan S , Bartlett PN , Baumberg JJ , Russell AE . SERS at structured palladium and platinum surfaces . J Am Chem Soc . 2007 ; 129 : 7399 - 406 .
12. Wang X , Shi W , She G , Mu L . Surface-enhanced Raman scattering (SERS) on transition metal and semiconductor nanostructures . Phys Chem Chem Phys . 2012 ; 14 : 5891 - 901 .
13. Kasera S , Biedermann F , Baumberg JJ , Scherman OA , Mahajan S. Quantitative SERS using the sequestration of small molecules inside precise plasmonic nanoconstructs . Nano Lett . 2012 ; 12 : 5924 - 8 .
14. Taylor RW , Lee T-C , Scherman OA , Esteban R , Aizpurua J , Huang FM , et al. Precise subnanometer plasmonic junctions for SERS within Gold nanoparticle assemblies using cucurbit[n]uril “Glue.” . ACS Nano . 2011 ; 5 : 3878 - 87 .
15. Kundu S. A new route for the formation of au nanowires and application of shape-selective au nanoparticles in SERS studies . J Mater Chem C. 2013 ; 1 : 831 - 42 .
16. Dasary SSR , Senapati D , Singh AK , Anjaneyulu Y , Yu H , Ray PC . Highly sensitive and selective dynamic light-scattering assay for TNT detection using p-ATP attached gold nanoparticle . ACS Appl Mater Interfaces . 2010 ; 2 : 3455 - 60 .
17. Dasary SSR , Rai US , Yu H , Anjaneyulu Y , Dubey M , Ray PC . Gold nanoparticle based surface enhanced fluorescence for detection of organophosphorus agents . Chem Phys Lett . 2008 ; 460 : 187 - 90 .
18. Jerez-Rozo JI , Primera-Pedrozo OM , Barreto-Caban MA , Hernandez-Rivera SP . Enhanced Raman scattering of 2 , 4 ,6 -TNT using metallic colloids . IEEE Sensors J . 2008 ; 8 : 974 - 82 .
19. Primera-Pedrozo OM , Jerez-Rozo JI , La Cruz-Montoya DE , LunaPineda T , Pacheco-Londono LC , Hernandez-Rivera SP . Nanotechnology-based detection of explosives and biological agents simulants . IEEE Sensors J . 2008 ; 8 : 963 - 73 .
20. Cecchini MP , Turek VA , Paget J , Kornyshev AA , Edel JB . Selfassembled nanoparticle arrays for multiphase trace analyte detection . Nat Mater . 2013 ; 12 : 165 - 71 .
21. Xu Z , Hao J , Braida W , Strickland D , Li F , Meng X . Surface-enhanced Raman scattering spectroscopy of explosive 2,4- dinitroanisole using modified silver nanoparticles . Langmuir . 2011 ; 27 : 13773 - 9 .
22. Dasary SSR , Singh AK , Senapati D , Yu H , Ray PC . Gold nanoparticle based label-free SERS probe for ultrasensitive and selective detection of trinitrotoluene . J Am Chem Soc . 2009 ; 131 : 13806 - 12 .
23. Xu JY , Wang J , Kong LT , Zheng GC , Guo Z , Liu JH . SERS detection of explosive agent by macrocyclic compound functionalized triangular gold nanoprisms . J Raman Spectrosc . 2011 ; 42 : 1728 - 35 .
24. Hatab NA , Eres G , Hatzinger PB , Gu B . Detection and analysis of cyclotrimethylenetrinitramine (RDX) in environmental samples by surface-enhanced Raman spectroscopy . J Raman Spectrosc . 2010 ; 41 : 1131 - 6 .
25. Kasera S , Herrmann LO , Barrio JD , Baumberg JJ , Scherman OA . Quantitative multiplexing with nano-self-assemblies in SERS . Sci Rep . 2014 ; 4 : 6785 .
26. Freeman R , Willner I . Optical molecular sensing with semiconductor quantum dots (QDs) . Chem Soc Rev . 2012 ; 41 : 4067 - 85 .
27. Algar WR , Stewart MH , Scott AM , Moon WJ , Medintz IL . Quantum dots as platforms for charge transfer-based biosensing: challenges and opportunities . J Mater Chem B . 2014 ; 2 : 7816 - 27 .
28. Sun X , Wang Y , Lei Y. Fluorescence based explosive detection: from mechanisms to sensory materials . Chem Soc Rev . 2015 ; 44 : 8019 - 61 .
29. Hezinger AFE , Teßmar J , Göpferich A . Polymer coating of quantum dots - a powerful tool toward diagnostics and sensorics . Eur J Pharm Biopharm . 2008 ; 68 : 138 - 52 .
30. Shi GH , Shang ZB , Wang Y , Jin WJ , Zhang TC . Fluorescence quenching of CdSe quantum dots by nitroaromatic explosives and their relative compounds . Spectrochim Acta Mol Biomol Spectrosc . 2008 ; 70 : 247 - 52 .
31. Goldman ER , Medintz IL , Whitley JL , Hayhurst A , Clapp AR , Uyeda HT , et al. A hybrid quantum dot-antibody fragment fluorescence resonance energy transfer-based TNT sensor . J Am Chem Soc . 2005 ; 127 : 6744 - 51 .
32. Wilson R , Spiller DG , Prior IA , Bhatt R , Hutchinson A . Magnetic microspheres encoded with photoluminescent quantum dots for multiplexed detection . J Mater Chem . 2007 ; 17 : 4400 - 6 .
33. Bai M , Huang S , Xu S , Hu G , Wang L . Fluorescent nanosensors via photoinduced polymerization of hydrophobic inorganic quantum dots for the sensitive and selective detection of nitroaromatics . Anal Chem . 2015 ; 87 : 2383 - 8 .
34. Carrillo-Carrión C , Simonet BM , Valcárcel M. Determination of TNT explosive based on its selectively interaction with creatinine-capped CdSe/ZnS quantum dots . Anal Chim Acta . 2013 ; 792 : 93 - 100 .
35. Pazhanivel T , Nataraj D , Devarajan VP , Mageshwari V , Senthil K , Soundararajan D. Improved sensing performance from methionine capped CdTe and CdTe/ZnS quantum dots for the detection of trace amounts of explosive chemicals in liquid media . Anal Methods . 2013 ; 5 : 910 - 6 .
36. Singh K , Chaudhary GR , Singh S , Mehta SK . Synthesis of highly luminescent water stable ZnO quantum dots as photoluminescent sensor for picric acid . J Luminescence . 2014 ; 154 : 148 - 54 .
37. Tian X , Chen L , Qing X , Yu K , Wang X , Wang X . Hybrid cadmium tellurium quantum dots for rapid visualization of trace-level nitroaromatic explosives . Anal Lett . 2014 ; 47 : 2035 - 47 .
38. Wang YQ , Zou WS . 3 -Aminopropyltriethoxysilane-functionalized manganese doped ZnS quantum dots for room-temperature phosphorescence sensing ultratrace 2,4,6-trinitrotoluene in aqueous solution . Talanta . 2011 ; 85 : 469 - 75 .
39. Freeman R , Willner I. NAD + /NADH-sensitive quantum dots: applications to probe NAD +−dependent enzymes and to sense the RDX explosive . Nano Lett . 2009 ; 9 : 322 - 6 .
40. Freeman R , Finder T , Bahshi L , Gill R , Willner I. Functionalized CdSe/ZnS QDs for the detection of nitroaromatic or RDX explosives . Adv Mater . 2012 ; 24 : 6416 - 21 .