Published on Web 05/15/2007
Detection of Explosives with a Fluorescent Nanofibril Film
Tammene Naddo,† Yanke Che,† Wei Zhang,‡ Kaushik Balakrishnan,† Xiaomei Yang,† Max Yen,§
Jincai Zhao,| Jeffrey S. Moore,*,‡ and Ling Zang*,†
Department of Chemistry and Biochemistry, Southern Illinois UniVersity, Carbondale, Illinois 62901, Departments
of Chemistry and Materials Science and Engineering, 600 South Mathews AVenue, UniVersity of Illinois at
Urbana-Champaign, Urbana, Illinois 61801, Materials Technology Center (MTC), Southern Illinois UniVersity,
Carbondale, Illinois 62901, and Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of
Sciences, Beijing 100080, China
Received February 1, 2007; E-mail: lzang@chem.siu.edu; jsmoore@uiuc.edu
Scheme 1. Molecular Structure of ACTC. The Planar Geometry
and Shape-Persistent Core Favors Effective π-π Stacking
between Molecules
Fluorescence-quenching-based chemical detection represents one
of the most sensitive and convenient methods that have been widely
employed in explosives identification.1-6 Aromatic molecules and
conjugated polymers (when fabricated as films) are proven effective
in sensing explosives vapor via fluorescence quenching. Although
porous films of conjugated polymers have typically been utilized,
the quenching efficiency of these materials is often limited by the
short exciton diffusion because of the poor molecular organization
and/or weak intermolecular electronic interactions.2,4 Consequently,
very thin films are needed to achieve desirable amplification of
signal transduction, whereas a sufficiently thick film is usually
required to produce a measurable fluorescence intensity and to
minimize the interference of photobleaching. Because of these
limitations, there is a need to develop new sensing materials that
enable long-range exciton migration and thus produce sensing
systems independent of film thickness and with more flexibility
for device fabrication.
Herein we report an efficient sensing film fabricated from the
alkoxycarbonyl-substituted, carbazole-cornered, arylene-ethynylene
tetracycle (ACTC), shown in Scheme 1. The incorporation of
carbazole enhances the electron donating power of the molecule
and thus increases the efficiency of fluorescence quenching by
oxidative explosives. The large-area planar molecular surface of
ACTC enables effective long-range π-π stacking between the
molecules.7 Nanofibers in length of micrometers can be easily
fabricated via surface casting (see Supporting Information). It has
been demonstrated that one-dimensional π-π stacking is highly
favorable for exciton migration via cofacial intermolecular
electronic coupling.8-10 Thereby, long-range exciton diffusion would
be expected within the film cast from ACTC. Moreover, the shape-
persistent molecular structure of ACTC,11,12 in combination with
the networks formed by interdigitated nanofibers, produces mul-
tiscale porosity, making it an ideal sensing material for probing
oxidative gaseous molecules. Indeed, efficient fluorescence quench-
ing is observed when the ACTC film is exposed to explosives vapor,
leading to potential applications in explosives sensing.
yield of ca. 0.19. Upon exposure to saturated vapor of DNT or
TNT, the fluorescence of the ACTC film was dramatically quenched
(Figure 1A). Since the emission wavelength of ACTC is far above
the absorption range of the two explosives (Figures S5-S6,
Supporting Information), and thus there is no possibility for excited-
state energy transfer, the observed fluorescence quenching must
explicitly be due to the photoinduced electron transfer from the
excited ACTC to the quencher. Such a photoinduced electron
transfer is highly favored by the large driving forces (2.4 and 1.9
eV for TNT and DNT, respectively, Figure S7).
As shown in the inset of Figure 1, the quenching response to
DNT is faster than that to TNT, likely because of the higher vapor
concentration of DNT (ca. 100 ppb, compared to ca. 5 ppb of TNT).
The fluorescence quenching eventually saturated for both explosives
upon reaching the adsorption equilibrium. It is remarkable to note
that at adsorption equilibrium (after ca. 60 s of exposure) the
quenching efficiency of TNT (83%) was comparable to that of DNT
(90%), although the latter provides about 20 times higher vapor
concentration. The relatively strong quenching thus observed for
TNT is likely due to its higher partition into the film. Generally,
the partition coefficient (between a solid adsorbent and air) of a
volatile organic compound tends to scale inversely with its saturated
vapor pressure, as indeed observed for polymer materials, therefore
the partition coefficient of TNT is about 1 order of magnitude higher
than that of DNT. The quenching response observed for the ACTC
film is significantly faster than that previously observed for other
organic materials,1,13,14 consistent with the fibril porous structure
of the film, which facilitates both gaseous adsorption and exciton
migration across the film. The quenching efficiency obtained for
ACTC films is higher than those previously reported for other
explosive sensing materials at the same thickness.2,4
This investigation was primarily focused on two explosives
compounds, 2,4-dinitrotoluene (DNT) and 2,4,6-trinitrotoluene
(TNT), which both exist in commercial explosive products and have
been widely exploited for the purpose of evaluating explosive
sensing devices. The ACTC film was fabricated by spin-casting a
THF solution (0.2-1.0 mM) onto a glass substrate, followed by
annealing in vacuum at 60 °C for 3 h to remove the enclathrated
solvent. The film thus fabricated is quite fluorescent, with a quantum
The porous film morphology (inset of Figure 2) and the extended
one-dimensional π-π stacking facilitate the access of quencher
molecules to the excited states (Supporting Information, part X),
thereby resulting in effective fluorescence quenching, which should
be little dependent on the film thickness as is indeed evidenced by
the observations shown in Figures 2 and S11. This behavior is in
† Department of Chemistry and Biochemistry, Southern Illinois University.
‡ University of Illinois at Urbana-Champaign.
§ Materials Technology Center (MTC), Southern Illinois University.
| Chinese Academy of Sciences.
9
6978
J. AM. CHEM. SOC. 2007, 129, 6978-6979
10.1021/ja070747q CCC: $37.00 © 2007 American Chemical Society