BODIPY-based fluorescent sensors for detection of explosives

Abstract

The detecting and screening of explosives has become a global consideration in dealing with potential terrorism threats and the misuse of high explosives. Several detection techniques have been developed, but their disadvantages include the requirement of expensive equipment, complicated pre-treatment, and prolonged testing. So far, fluorescence-based sensors have been of great interest as they overcome the limitations of other techniques. BODIPY, which is a fluorophore, shows excellent fluorescence features, and it can be used as a sensor material for explosives detection. However, to date, BODIPY-based explosive sensors have not been explored in detail. This work reviews the recent developments on explosive sensors based on BODIPY and its analogs.

Keywords

aggregation-induced emission-active BODIPYs explosives detection fluorescence sensor photo-induced electron transfer

Introduction

Explosives are reactive materials containing a great amount of potential energy that can cause detonations, accompanied by the formation of more stable materials. Explosives can be classified in different ways, depending on their chemical aspects (nitro compounds, acid salts, peroxides), their applications (for military uses or commercial uses), and their power release (low, primary high, secondary high). There are nearly 16 common explosives (Figure 1). Nitro-aromatic compounds, which are high explosives, such as 2,4,6-trinitrotoluene (TNT), dinitrotoluene (DNT), and triazacyclohexane (RDX), are usually used as military explosives and are major components in unexploded landmines. Mines and solid rockets for industrial use are mainly made of ammonium salts like ammonium nitrate (AN) and ammonium phosphate (AP).^1,2 Besides, improvised explosive devices have also been developed by using peroxide-based explosives such as triacetone triperoxide (TATP) and hexamethylene triperoxide (HMTD), as they are commercially available and inexpensive materials.

Figure 1.

Common explosives and their vapor pressure at 25 °C (Ewing et al.³ and Haz-Map).

The growth of terrorism worldwide has heightened global concern over methods for the sensitive, selective, and rapid detection of explosives. For terrorist attacks, explosives have been the preferred weapons, being used in more than 80% of worldwide attacks related to firebombs and explosives since 2015 (Global Terrorism Index 2022). Besides, there are also environmental and health risks related to explosive residues released from military sites and ammunition plants (GICHD Annual Report 2021).^4,5 Over-exposure to TNT, for example, can cause anemia, liver problems, and even cataracts because this compound can penetrate through the skin and enter the bloodstream directly.⁶ Consequently, the reliable detection of explosives is one of the pressing concerns in anti-terrorism efforts and environmental pollution control. In particular, rapid, selective, and sensitive explosive-detecting techniques have been an important issue of global concern.

Over the decades, several techniques have been developed for the detection of explosives, such as X-ray imaging, gas chromatography coupled with mass spectroscopy,⁷ ion mobility spectroscopy (IMS),^8,9 and surface-enhanced Raman scattering (SERS).^10,11 Such methods, however, need expensive instrumentation and a complicated set-up followed by a long response time. Furthermore, they are sensitive to contaminants, or may not be amenable to stand-off detection. To date, fluorescence-based sensing technology represents one of the most promising approaches for tracing explosives due to the several advantages of this technique, such as short response times, excellent sensitivity, instrumental simplicity, and low cost.^12–14 Different fluorescent sensory materials (probes) have been developed for sensing explosives and have shown high efficiency. Several such examples are conjugated polymers, supramolecular systems, small-molecule fluorophores, bio-inspired materials, aggregation-induced emission-active (AIE) materials, and so on. Such sensory materials, which have shown excellent results in the sensing of explosives, mainly rely on two sensing mechanisms: fluorescence quenching and/or turn-on fluorescence.¹⁵ Modifications of small-molecule fluorophores, such as conjugated fluorophores and self-assembling small fluorophores, have led to even greater potential in explosives sensing since these molecules can enhance binding and/or exciton migration.¹⁶ BODIPY is among the class of small-molecule fluorophores with great versatility, and their photophysical properties could be fine-tuned over wide ranges using straightforward synthetic protocols.¹⁷ They display both turn-on and turn-off fluorescence in sensing explosives, depending on the polarity of the working media and the type of analyte. Thus, using BODIPY as a fluorescent sensory material holds promise in explosives detection tasks. However, there has not been a lot of research done on developing BODIPY as fluorescence probes for the detection of explosives. Therefore, in this short review, we summarize the research on BODIPY as optical sensory probes for the detection of explosives.

Recent fluorescence-based explosives sensing techniques

Explosives can be detected by using fluorescent sensors, as explosive materials are not fluorescent. The interaction between analytes and probes would result in fluorescence changes, including enhanced or quenched fluorescence intensities, lifetimes, wavelengths, and anisotropy. Although these changes can be used to detect explosives, fluorescence quenching and fluorescence turn-on have been preferred over other mechanisms. The subsequent paragraphs give information on quenching-based detection and turn-on fluorescence detection.

Fluorescence-quenching-based detection

Fluorescence-quenching is the result of either diffusive encounters (collisional quenching) or complex formation processes (static quenching). Collisional quenching refers to the radiationless deactivation of an excited state fluorophore in collisions with quenchers. This process is attributed to photo-induced electron transfer (PET), charge-transfer intersystem crossing (ISC), or Förster resonance energy transfer (FRET) processes. Meanwhile, static quenching refers to the formation of a non-fluorescent complex between a fluorescent probe and analytes.¹⁸ The prevalent way to examine whether the quenching is a static or a collision process is based on fluorescence lifetime changes in the absence or presence of explosive quenchers, given by the ratio of lifetime $τ_{0} / τ$ . The $τ_{0} / τ$ ratio is plotted versus the quencher concentrations as follows: in static quenching, $τ_{0} / τ = 1$ ; in collisional quenching, $τ_{0}$ / $τ = F_{0} / F \neq 1$ . $F_{0} / F$ represents the ratio of fluorescence intensity in the absence and presence of quenchers. This ratio is involved in the Stern–Volmer equation, which describes the collisional quenching process¹⁹

F_{0} / F = 1 + k_{q} τ_{0} [Q] = 1 + K_{D} [Q]

(1)

where $k_{q}$ is the biomolecular quenching constant, $K_{D}$ is the Stern–Volmer quenching constant, and $[Q]$ is the quencher concentration. In some cases, the quenching process can be led by both static quenching and collisional quenching with the same quencher. Thus, the combined collisional and static quenching process can be described by the modified Stern–Volmer equation

F_{0} / F = 1 + (K_{D} + K_{s}) [Q] + K_{D} K_{s} {[Q]}^{2}

(2)

where $K_{D}$ and $K_{s}$ are the Stern–Volmer quenching constants for collisional and static quenching processes, respectively. This equation greatly explains an upward curvature at higher quencher concentrations or an exponential fitting of Stern–Volmer plot in some cases.

In terms of explosives detection ability, static quenching-based sensors show a larger Stern–Volmer quenching constant (K_SV) and a higher sensitivity in general. Fluorescent sensors based on the collisional quenching process have a smaller K_SV, showing a lower sensitivity but faster and more reversible detection.²⁰ The two quenching processes sometimes occur at the same time with the same analytes, allowing for a greater sensing range. For example, the conjugated copolymer PFPy can detect TNT at femtomolar (fM) level (4.94 fM) due to the combination of both collisional and static quenching processes.²¹ In addition, the sensitivity toward explosives detection based on fluorescence quenching depends on different types of sensory platforms and the sensing phase (the detection environment) (Table 1). Conjugate polymers show excellent detection sensitivity at pM and fM levels.^21,22 Small-molecule fluorophores also show excellent sensing ability, and their analogs, such as conjugated fluorophores, showed even greater sensitivity.^23,24

Table 1.

Recent efficient explosive-sensing techniques based on fluorescence quenching.

Sensory platform	Sensory type	Explosive	LOD/K_sv	Detection environment	References
2-(pyren-1-yl)pyridine	Small fluorophore	Picric acid	56 nM	MeOH	Kachwal et al.²⁵
Fluorene-based conjugated polymer	Conjugated polymer	Picric acid	3.2 pM/4.27 × 10⁶ M⁻¹	Aqueous solution (H2O/THF)	Batool et al.²²
Pyoverdines	Bioinspired material	NTO	12 nM	Aqueous medium	Kulkarni et al.²⁶
Quinoline-benzimidazole conjugate	Conjugated fluorophore	Picric acid	21.2 nM/3.57 × 10⁴ M⁻¹	In aqueous solution	Jiang et al.²³
Star-shaped 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)tris(N,N-diphenyl aniline)	Assembled small fluorophore	Picric acid	0.37 nM/3.53 × 10⁵ M⁻¹	In aqueous media	Sathiyan et al.²⁴
BaWO4:5Eu–5Tb nanofibers	Nano fiber	2-nitrotouluen	35 nM/1.73 × 10⁶ M⁻¹	In vapor phase of analytes	George et al.²⁷
Thiophene-substituted tetraphenyl ethylene	AIE material	NACs	1 nM	Water fraction	Qayyum et al.²⁸
Self-assembled monolayer functionalized with small-molecule PA−2PhAn	Doped fluorophore platform	TNT	1.4 ppm (6 $μ M$ )	Vapor TNT	Li et al.²⁹

Fluorescence turn-on-based detection

To date, the fluorescence turn-on approach has received more attention, as it is believed that this approach combines the higher relative intrinsic sensitivity with the higher chemical selectivity and represents an emerging frontier for explosive sensing.³⁰ Despite this, there have not been many studies done on explosive sensors based on fluorescence turn-on, especially for the detection of nitro-aromatics. A recent report introduced a series of supramolecular ensembles that acted as turn-on fluorescence sensors for various nitroaromatic explosives. However, the sensitivity, response time, and sensing mechanism have not been proposed.³⁰ Besides, Yu et al. proposed an enhanced-fluorescence sensor based on a blend consisting of perylene-3,4,9,10-tetracarboxylic acid derivative nanofibers and amberlyst-15 particles. This sensing system showed rapid turn-on fluorescence responses (ca. 5 s) and incredible sensitivity (0.1 ppm) for TATP detection (vapor phase).³¹

BODIPY and its application in explosives sensing

Synthesis and design of BODIPY fluorophores

BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) dyes are a class of small-molecule fluorophores.³² They are fascinating compounds with multiple modification sites on their structure. Also, they have remarkable features, including small Stokes shifts, narrow absorbance bands, sharp emissions with high-fluorescence quantum yields, and excellent stabilities.^31,33–35 BODIPY dyes can be readily made from pyrrole-based materials, often in high yields and in multigram quantities. The synthesis of the BODIPY core can be conducted in different ways, including the following:

Dipyrrin chemistry;³⁶

The acylation of pyrrole followed by condensation and complexation;^37,38

One-pot condensation–decarbonylation of pyrrole-2-carbaldehydes.³⁹

Although the chromophoric core of BODIPY is the key to its success and versatility, novel analogs of BODIPY can easily be made by functionalization at the 8-(meso) positions as well as the pyrrole rings ( $α and β$ positions), allowing the creation of the desired spectroscopic and photophysical characteristics.^40,41 The functionalization at those positions can be performed by nucleophilic substitution reactions $(S_{N} A r)$ ^42,43 and/or transition-metal-catalyzed C-C coupling reactions (Scheme 1).^44–46

Scheme 1.

Different methods for the synthesis of the BODIPY core and functionalization approaches at the preferential sites of attack.

BODIPY in explosive sensing

BODIPY fluorophores have shown versatile spectroscopic and photophysical properties, depending on their chemical structure functionalization and the conjugated system.^32,47 Furthermore, as proposed in several research studies, analogs of the fluorophore, such as conjugated fluorophores and AIE-based fluorophores, displayed an excellent detection sensitivity for explosives based on fluorescence sensing.^23,29,48,49 Perhaps, BODIPY fluorophores and their analogs, such as AIE-based BODIPYs and conjugated BODIPYs, could hold promise as they probably show intrinsic sensitivity, simplicity in design and synthesis, easy set-up, and recognition. The use of BODIPY and the design of its analogs as sensory materials for explosives detection, nevertheless, have not appeared often in the literature. To date, BODIPY-based explosive sensors have only been introduced for the tracing of picric acid (PA), RDX, and peroxide. These BODIPY sensors exhibit dynamic fluorescence in various media and interact with analytes based on the modified sites and attached moieties (refer to Table 2). Among these, PA has attracted more research into BODIPY-based sensors over other analytes, with the detection mechanism based on fluorescence turn-on or fluorescence quenching.^50–55 Later, BODIPY-based sensors for peroxide and RDX were also introduced.^54,56

Table 2.

Photo-spectroscopic data of the introduced BODIPY probes.

Sensor^a	In solution	Without explosives			Exposure to explosives
Sensor^a	In solution	λ_abs (nm)	λ_em (nm)	$ϕ_{f}$	Explosive	λ_abs (nm)	λ_em (nm)	Quenching/enhanced degree/ $ϕ_{f}$
1	CH₃CN/H₂O (v/v, 9:1)	508	514	0.08	PA	523	534	$↑ 6 -fold$
1	In solid state	–	612	–	PA	–	612	$↓ 83$ %
2	ethanol	499	513 (weak)	–	PA	499	513	↑300-fold
3	toluene	505	596	0.52	PA	505	596	↓95%
5	DMSO/H₂O (v/v, 1:1)		522	0.621	PA	–	522	↓95.8%
6	80% acetonitrile	–	540	0.241%^b	RDX	500	540	↑
6	80% acetonitrile	–	540	0.241%^b	PA	–	–	↓
7	20% CH₃CN	500	510	0.01	H₂O₂	498	508	$↑ 78 - f o l d$ (0.56)
8	DMF	–	463	0.65	PA	–	463	0.22
9	DMF	–	–	0.64	PA	–	–	0.24

See Figure 2 for the structures of the sensors.

The relative quantum yield in comparison with quinine sulfate.

Figure 2.

BODIPY probes used for explosives detection.

Detection of PA

Based on fluorescence turn-on mechanism

3,5-Bis(acetal) BODIPY (1) was synthesized and it showed a weak emission peak at 514 nm in chloroform solution due to the PET process that occurred from the interaction with acetal oxygen in the BODIPY core (Figure 2).⁵⁰ The fluorescence of 1 was increased dramatically upon the addition of PA. As PA interacted with the acetal oxygen of BODIPY, and transformed the acetal oxygen into an aldehyde, this transformation stopped the PET process. As a result, the presence of PA enhanced the fluorescence of 1, making it viable as a fluorescence turn-on probe for PA detection. In this sense, the limit of detection (LOD) was found to be 163 ppb (0.71 µM), and $Ka = 9.2 \times 10^{6} M^{- 1}$ in solution. Interestingly, in solid form, 1 showed a strong fluorescence emission, but the fluorescence signal was quenched when exposed to PA vapor. Thus, 1 can serve efficiently for the detection of PA in solution as well as in the vapor phase based on the two opposite fluorescence mechanisms.

In other research, fluorescent meso-diaminophenyl-1,3,5,7-tetramethyl BODIPY dye (2) was investigated and employed for tracing PA by regenerating its fluorescence via PET hindrance.⁵¹ Compound 2 showed a weak emission at 513 nm in ethanol because the amino groups on the angled meso-phenyl ring contribute to PET. The addition of PA caused one of the amine groups associated with PA to be protonated, resulting in a decrease of the electron density in the free amine group compared with the unassociated probe, leading to an emission reset of 2. The sensitivity of 2 toward PA detection was as low as 0.65 ppb (2.8 nM).

Based on the fluorescence quenching mechanism

The triphenylamine-substituted BODIPY derivative (3) was designed and synthesized, and showed excellent luminogenic aggregation due to the combining of twisted intramolecular charge transfer (TICT) and AIE processes.⁵² Compound 3 was synthesized starting from triphenylamine (TPA). As proposed, 3 was more emissive than BODIPY since the bulk junction at the 8-meso position exerts steric effects on the rotational motion of the phenyl rings. This effect can be ascribed to the propeller-shaped TPA core and methyl acrylate groups with strong aggregation properties. Compound 3 possibly disfavored the formation of aggregates in the presence of PA due to the large steric hindrance after producing 4. Thus, the AIE process was suppressed, leading to the disappearance of emission. Furthermore, after capturing the more acidic molecules of PA, PET could be enhanced, because the BODIPY core becomes more electron-deficient, meaning that PA or the picrate $(Ο^{-})$ ion promotes PET quenching of the TPA excited state effectively. As for the detection limit, this was found at 30 ppb (0.13 µM) with a K_sv of $2.1 \times 10^{- 6} M^{- 1}$ in aqueous solution.

P-Pyridine BODIPY (5) for PA detection was synthesized based on a basic N atom as the electronic donor.⁵³ Compound 5 displays strong emission in a mixed H₂O/DMSO (1:1) solution, but upon PA addition the fluorescence was strongly quenched. The fluorescence quenching was assumed to be the result of a PET process from the excited state of 5 to the ground state PA. The PA-OH formed a hydrogen bond with the pyridine N atom, promoting the PET process from the BODIPY core to PA. The LOD of the p-PBP probe for PA was as low as 13.06 nM, which is far lower than the concentration stipulated by the Environmental Quality Standards for surface water. The detection testing on a simulated real sample was also conducted, indicating that 5 can be used in actual case analysis.

A hydrazine-substituted BODIPY (6) was synthesized with the possibility of detecting PA via fluorescence quenching with an LOD at 0.44 µM.⁵⁴ The fluorescence of 6 was quenched in the presence of PA, this being revealed by the inner-filter effect of fluorescence, as the emission spectrum of 6 apparently overlaps with the absorption spectrum of PA.

Two macromolecular chemo-sensors based on BODIPY (8, 9) were also developed with the possibility of detecting PA based on the fluorescence quenching mechanism.⁵⁵ Compounds 8 and 9 were synthesized by a Schiff base forming reaction between BODIPY, trans-cyclohexane-1,4-diamine, and 1,4-phenylenediamine. They showed fluorescent emission at 473 nm, but the fluorescence was quenched in the presence of PA. The LOD of 8 and 9 tracing detection of PA were at 0.038 and 0.017 µM, respectively.

Detection of other explosives

The detection of peroxide-based explosives by using a BODIPY-based fluorescence sensor, m-NBBD (7), was revealed by Li et al., relying on the turn-on fluorescence mechanism.⁵⁶ Compound 7 was obtained from 3-iodobenzoyl chloride and 2,4-dimethyl-1H-pyrrole as starting materials. Compound 7 in aqueous solution showed a very weak fluorescence, but the green fluorescence was enhanced significantly in the presence of peroxide. The turn-on fluorescence of 7 toward peroxide is based on a donor-excited photo-induced electron transfer mechanism (d-PET), and the benzil of 7 holds a key role as a recognition group to turn on the fluorescence of BODIPY for peroxide detection. The sensitivity of 7 toward peroxide vapor was 0.94 ppb. The visual detection of H₂O₂ vapor using a TLC plate coated with 7 was investigated, revealing that H₂O₂ can be observed by the naked eye at 10 ppb.

Interestingly, compound 6 showed its potential in PA detection based on the fluorescence quenching mechanism. In addition, 6 showed great possibility in RDX detection based on the opposite mechanism, with an LOD at 85.8 nM, in an 80% acetonitrile aqueous solution.⁵⁴ Sensing RDX using 6 was the result of an electrophilic addition reaction at the hydrazine moiety by HCHO produced by the photolysis of RDX.

Discussion

In the above examples, BODIPY probes for the detection of explosives have been discussed. The detection mechanism of these BODIPY probes refers to two fluorescence processes, either fluorescence quenching (mainly involved in collisional quenching via PET) or fluorescence turn-on. It can be seen from the described BODIPY compounds that designing BODIPY probes for explosives detection consists of three tasks: (1) extending the π-conjugation, (2) tuning the photospectroscopic properties, and (3) designing the explosive recognition site (Figure 3). To achieve the first task, most of the introduced BODIPY probes were prepared by functionalizing the BODIPY core at its meso-site with aromatic substituents. These substitutions extend the π-conjugation of the BODIPY dyes, allowing them to absorb and emit light at longer wavelengths and achieve higher quantum yields. For instance, compound 3, which was introduced with a TPA substituent, a well-known molecule that displays AIE properties, showed strong emission in less polar solvents and versatile luminescent properties in various solvents.⁵² The second task attached an electron-donating group (EDG) or an electron-withdrawing group (EWG) to BODIPY dyes at specific sites. Therefore, the photophysical properties were fine-tuned (decreased or suppression of emission) via PET, d-PET, and ISC processes. For example, compound 2 has two meta-amino groups on the meso phenyl ring. These groups function as EDGs that promote the PET process, resulting in a temporary suppression of the emission of the molecule.⁵¹ In the third task, recognition sites are the key point to the success of the BODIPY dyes in explosives detection. Upon interaction of the recognition sites and explosives, the photospectroscopic properties of the BODIPYs could be fine-tuned or reset. In few cases, recognition sites and photospectrocopic tuning sites are two different functional groups in a sensor’s structure. But in the most cases of introduced BODIPY dyes, these sites are integrated into the same moiety. For instance, they are oxygen acetal (in compound 1), m-phenyl amino (in compound 2), hydrazine (in compound 6), benzil (in compound 7). These sites are typically EDGs that can interact with an electron-withdrawing moiety of other molecules, while the PA molecule has strong EWGs (3 NO₂) on the aromatic ring. This can explain why most BODIPY sensors were developed for PA detection. In the case of developing BODIPY-based sensors for detecting explosives other than PA, this is attainable by conducting more studies and designing specific recognition sites (for specific explosives) that are associated with photospectroscopic tuning sites.

Figure 3.

Schematic approach for designing a BODIPY-based probe for explosives detection.

Although some research has examined the ability of BODIPY probes in real detection, thereby revealing their versatility and potential use in actual case analysis,^53,54,56 additional studies are needed to integrate these probes into a portable device for practical applications.

Conclusion

In this short review, we have summarized the recent approaches in explosive sensing and the possibilities of using BODIPYs. As a class of small fluorophore molecules, BODIPY and its analogs are promising candidates for explosives detection. Although there have not been many research studies focused on this subject, the available data have revealed the versatility and possibility of using BODIPY for explosives detection in both laboratories and actual cases.

Footnotes

Author contributions

All the authors contributed to the literature search as well as the writing of the manuscript. All the authors have read and agreed to the published version of the manuscript.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was funded by the National Research Foundation of Korea (NRF) grant funded by the Korea government (No. 2021R1I1A3060405) and Basic Science Research Program to Research Institute for Basic Sciences (RIBS) of Jeju National University through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2019R1A6A1A10072987) and by the Big Issue Program funded by KITECH, Republic of Korea (Project No. EO23013).

ORCID iDs

Yen Thi Nguyen

Se Won Bae

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