Chem. Mater. 2011, 23, 789–794 789 DOI:10.1021/cm1020355
Explosive Sensing with Fluorescent Dendrimers: The Role of Collisional Quenching† David A. Olley,‡ Ellen J. Wren,‡ George Vamvounis,‡ Mark J. Fernee,§ Xin Wang,‡ Paul L. Burn,‡ Paul Meredith,‡ and Paul E. Shaw*,‡ ‡
Centre for Organic Photonics & Electronics, The University of Queensland, Brisbane, Queensland 4072, Australia, and §Centre for Quantum Computing Technology, School of Mathematics and Physical Sciences, The University of Queensland, Brisbane, Queensland 4072, Australia Received July 22, 2010. Revised Manuscript Received August 23, 2010
We have investigated a series of branched fluorescent sensing compounds with thiophene units in the arms and triphenylamine centers for the detection of nitrated model compounds for 2,4,6trinitrotoluene (TNT) and the plastic explosives taggant 2,3-dimethyl-2,3-dinitrobutane (DMNB). Stern-Volmer measurements in solution show that the fluorescence is more efficiently quenched by nitroaromatic compounds when compared to a non-nitrated quencher, benzophenone. Simple modification of the structure of the sensing compound was found to result in significant changes to the sensitivity and selectivity toward the nitrated analytes. A key result from time-resolved fluorescent measurements showed that the chromophore-analyte interaction was primarily a collisional process. This process is in contrast to conjugated polymers where static quenching dominates, a difference that could offer a potentially more powerful detection mechanism. Introduction Given current security issues and threats across the globe, there is a critical need for in-field, non-contact methods for sensing dangerous materials such as explosives. In particular, there is great demand for chemosensors suitable for detecting nitroaromatic-based chemicals such as 2,4,6-trinitrotoluene (TNT), which is one of the principle explosives in landmines.1 Nitro groups are also found in taggants such as 2,3-dimethyl-2,3-dinitrobutane (DMNB) that are added to plastic explosives to facilitate detection by canines. Although a number of technological solutions such as X-ray scanning and ion mobility spectrometry exist, these are expensive bulky devices and unsuitable for incorporating into a portable system for in-field, remote, or non-contact detection. A sensitive and potentially valuable route to trace chemical sensing is that of fluorescence quenching. In this regard, nitroaromatics have been shown to quench the fluorescence of conjugated polymers,2-4 offering a route toward compact low-cost sensors. Recent reports have shown that fluorescent dendrimers are also suitable as Accepted as part of the “Special Issue on π-Functional Materials”. *Corresponding author. E-mail:
[email protected].
†
(1) Czarnik, A. W. Nature 1998, 394, 417. (2) Thomas, S. W., III; Joly, G. D.; Swager, T. M. Chem. Rev. 2007, 107, 1339. (3) Toal, S. J.; Trogler, W. C. J. Mater. Chem. 2006, 16, 2871. (4) Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 11864. (5) Cavaye, H.; Smith, A. R. G.; James, M.; Nelson, A.; Burn, P. L.; Gentle, I. R.; Lo, S.-C.; Meredith, P. Langmuir 2009, 25, 12800. (6) Richardson, S.; Barcena, H. S.; Turnbull, G. A.; Burn, P. L.; Samuel, I. D. W. Appl. Phys. Lett. 2009, 95, 063305. (7) Guo, M.; Varnavski, O.; Narayanan, A.; Mongin, O.; Majoral, J.-P.; Blanchard-Desce, M.; Goodson, T., III J. Phys. Chem. A 2009, 113, 4763. r 2010 American Chemical Society
chemosensors for explosives.5-7 Dendrimers are particularly attractive because they can have a high fluorescence quantum yield, and their modular monodisperse macromolecular structure enables tailoring of their properties to specific applications.8 Previous reports concerning conjugated polymer sensors have shown that fluorescence quenching by the explosive molecule or taggant (analytes) occurs predominantly via a static process,9,10 whereby the analyte and sensing chromophore bind to form a ground state complex. Upon photoexcitation, this complex decays nonradiatively, resulting in a so-called dark state. The formation and eventual dissociation of a ground state chromophore-analyte complex are both slow processes, slow that is, on the time scale of the radiative lifetime of the native chromophore (nanosecond), and this could ultimately hinder the device response time and/ or its reuse. When the interaction between the analyte and the chromophore occurs via a collisional mechanism the interaction is brief (much shorter than the radiative lifetime), resulting in a loss of fluorescence only if the chromophore is in the photoexcited state. For quenching to occur via a collisional process, rapid electron transfer to the analyte must occur, which requires the lowest unoccupied molecular orbital (LUMO) of the analyte to be lower in energy than that of the donor, in this case, the excited chromophore. A collisional fluorescence quenching mechanism therefore has the potential to lead to faster and more reversible detection than a static (8) Lo, S.-C.; Burn, P. L. Chem. Rev. 2007, 107, 1097. (9) Zhao, D.; Swager, T. M. Macromolecules 2005, 38, 9377. (10) Thomas, S. W., III; Amara, J. P.; Bjork, R. E.; Swager, T. M. Chem. Commun. 2005, 36, 4572.
Published on Web 09/14/2010
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quenching process as the analyte is not as strongly bound to the sensing chromophore. In this paper, we show that in solution a new series of branched fluorescent chromophores interact with nitroaromatics primarily via a collisional process. This offers a different and potentially more powerful detection mechanism than generally observed in conjugated polymers where static quenching dominates. Furthermore, we show that minor changes to the chemical structure of the chromophores can significantly alter the sensitivity and selectivity toward nitroaromatics and the plastic explosives taggant DMNB. Experimental Details All solutions of the materials were prepared in spectroscopic grade tetrahydrofuran with peak absorbances of approximately 0.1. Absorbance spectra were measured in a Varian Cary 5000 absorption spectrometer with a tetrahydrofuran reference. The fluorescence spectra were measured with a Jobin-Yvon Fluorolog Tau-3 system with excitation at the peak of the absorbance for each material. Slit widths were optimized for signal strength within the limits where the detector response was linear. Repeat fluorescence measurements showed no signal decrease from photodegradation. The solution photoluminescence quantum yields (PLQY) were measured using the relative method with rhodamine 6G in ethanol as the reference (PLQY = 0.95). Solutions were not degassed with materials 1 and 3 excited at 400 nm and materials 2 and 4 excited at 425 nm. For the Stern-Volmer measurements the analytes were dissolved in the dendrimer solution so that during the measurements the dendrimer concentration remained constant and only the analyte concentration varied. For the steady-state measurements, 2.5 mL of dendrimer solution were placed in a 1 cm cuvette and the absorption and fluorescence spectra measured as indicated above. Multiple additions of a known volume of the analyte solution were added to the cuvette with the absorption and fluorescence spectra measured immediately after each addition. The maximum absorbance of the analyte dendrimer solution in the cuvette at the excitation wavelength was limited to approximately 0.6 to ensure that the data could be reliably corrected for the absorbance of the excitation by the analyte. For the time-resolved Stern-Volmer measurements 2.5 mL of dendrimer solution were placed in a 1 cm cuvette and excited with ∼70 fs pulses of approximately 400 nm wavelength at an 80 MHz repetition rate using the frequency doubled output of Spectra-Physics Tsunami Ti:sapphire laser. The fluorescence decays of the dendrimer solutions were captured with a Picoquant TCSPC setup with an instrument response function (IRF) FWHM of about 300 ps. As with the steady-state measurements, multiple additions of a known volume of the analyte solution were added to the cuvette and the fluorescence decay was measured after each addition. All fits to the data were performed following convolution with the IRF.
Figure 1. Solution absorption and fluorescence spectra of the materials (1, 2, 3, and 4) in tetrahydrofuran with the structures for each shown in the insets.
Table 1. Summary of the PLQY, PL Lifetimes, Radiative and Non-Radiative Decay Rates for 1-4 Material
PLQY
PL lifetime (ns)
kR (s-1)
kNR (s-1)
1 2 3 4
0.71 0.33 0.70 0.41
1.67 0.51 1.77 0.65
4.3 108 6.5 108 4.0 108 6.3 108
1.7 108 1.3 109 1.7 108 9.0 108
The sensing materials used in this study consisted of a series of four compounds, two “zeroeth” generation (1 and 2) and two first-generation dendrimers (3 and 4) incorporating triphenylamine centers.11 The chemical structures are shown in
Figure 1 alongside their absorbance and fluorescence spectra in solution. Two variations on the core chromophore were used: the first incorporates a single thiophene (1 and 3) and the second bithiophenes (2 and 4) attached to the triphenylamine center. The molecules are completed with either 4-(2ethylhexyloxy)phenyl surface groups (“zeroeth” generation) or with first-generation biphenyl dendrons with 2-ethylhexyloxy surface groups. The central nitrogen atom enables the conjugation to extend across the three branches and thus each macromolecule has one large chromophore.12 The addition of the extra thiophene likewise extends the conjugation within the core and results as expected in a red shift of both the absorbance and fluorescence maxima relative to the compounds with a single thiophene in each arm (1 and 3). The solution PLQY of the four compounds are summarized in Table 1. The single thiophene per arm materials
(11) Wren, E. J.; Mutkins, K.; Aljada, M.; Burn, P. L.; Meredith, P.; Vamvounis, G. Polym. Chem. 2010, 1, 1117.
(12) Lupton, J. M.; Samuel, I. D. W.; Burn, P. L.; Mukamel, S. J. Phys. Chem. B 2002, 106, 7647.
Results and Discussion
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1 and 3 were found to be approximately twice as luminescent as compounds 2 and 4 with bithiophenes in their arms. Measurements of the fluorescence lifetime revealed that the decrease in the luminescence of compounds 2 and 4 was due primarily to an approximate order of magnitude increase in the nonradiative decay rate (extracted using the steady-state PLQY and radiative lifetime). The nitroaromatics used in this study were 2,4-dinitrotoluene (DNT), 1,4-dinitrobenzene (DNB) and p-nitrotoluene (pNT), all of which are chemically similar to TNT and have high electron affinities. DNT is also a byproduct of TNT manufacturing and degradation, and is easier to detect than TNT itself due to its higher volatility.13 DMNB, a nonaromatic nitro compound, is a widely used taggant in commercial plastic explosives and was also investigated as a potential explosive detection signature. Finally, benzophenone (BP) was used as a control for our experiments, as it is a high-electron-affinity aromatic compound but contains no nitro groups. BP therefore provides an indication of how selective the fluorescence quenching is toward analytes containing nitro groups. To determine the quenching efficiencies of the five analytes with the four sensing materials, Stern-Volmer measurements were performed, enabling quantitative and systematic comparisons to be made. Most Stern-Volmer analyses are done in the steady-state and providing there is only one quenching mechanism the Stern-Volmer equation is F0 ¼ 1 þ KSV ½Q ð1Þ F where F0 is the initial fluorescence of the dendrimer solution prior to analyte addition, F is the fluorescence of the solution for any given analyte concentration Q and KSV is the Stern-Volmer constant.14 Providing the plot of F0/F versus Q is linear, KSV can be determined. What cannot be known is the mechanism of the quenching, that is, whether KSV is the result of static or collisional processes as described earlier. It is important to note that the analytes absorb strongly in the UV and to a lesser extent in the blue region of the spectrum and hence absorb a portion of the excitation energy. Similarly, a fraction of the emission from the sensing material will be absorbed by the sensing material analyte mixture. We have corrected the data for both of these effects following the reported method to avoid both an overestimation of the value of KSV, and a deviation from linearity of the resulting data.15,16 The steady-state Stern-Volmer plots for the first generation dendrimer 3 with each of the analytes are shown in Figure 2. In each case, the fluorescence of the dendrimer is increasingly quenched with successive additions of analyte. The best fits to the data with eq 1 are also shown in Figure 2 (13) Jenkins, T. F.; Leggett, D. C.; Miyares, P. H.; Walsh, M. E.; Ranney, T. A.; Cragin, J. H.; George, V. Talanta 2001, 54, 501. (14) Lackowicz, J. Principles of Fluorescence Spectroscopy; Plenum Press: New York, 1983. (15) Cavaye, H.; Barcena, H.; Shaw, P. E.; Burn, P. L.; Lo, S.-C.; Meredith, P. Proc. SPIE 2009, 7418, 741803–01. (16) Zheng, M.; Bai, F.; Li, F.; Li, Y.; Zhu, D. J. Appl. Polym. Sci. 1998, 70, 599.
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Figure 2. Steady-state Stern-Volmer plots for dendrimer 3 with the analytes DNT, DNB, pNT, DMNB, and BP (quenching versus analyte concentration). Table 2. Summary of the Stern-Volmer Constants Obtained from Fitting to the Steady-State Fluorescence Quenching Data for Each Material with All Five Analytes material
DNT (M-1)
DNB (M-1)
pNT (M-1)
DMNB (M-1)
BP (M-1)
1 2 3 4
49 ( 1 23 ( 2 38 ( 2 26 ( 1
57 ( 2 28 ( 1 45 ( 2 26 ( 2
38 ( 1 19 ( 1 31 ( 1 18 ( 1
20 ( 1 6.0 ( 1.0 14 ( 1 6.0 ( 0.5
14 ( 1 0.2 ( 0.2 5.0 ( 0.2 0.2 ( 0.2
with the KSV values obtained for all four materials with each of the analytes summarized in Table 2. The values of the Stern-Volmer constants are mostly in the range of 560 M-1, which is similar to what has been reported for a range of standard small molecule fluorophores.17 Conjugated polymers designed for the detection of nitroaromatic explosives have been reported to have Stern-Volmer constants in the range of 50-150 M-1 for DNT,9 although there are cases with Stern-Volmer constants an order of magnitude greater.18,19 In this work each material was found to be most strongly quenched by the nitroaromatic analytes with DNB resulting in the greatest response, followed by DNT and then pNT. This trend is consistent with differences in the reduction potential, which become more negative from DNB to DNT to pNT.4 The strong response of compound 1 to DMNB is also noteworthy as although this taggant for plastic explosives is used to facilitate detection by canines, its low electron affinity means it is usually difficult to detect by fluorescence quenching.10 As can be seen in Table 2, the “zeroeth” generation 1 was found to have the largest Stern-Volmer constants for each of the analytes. The addition of a thiophene to each of the branches to give 2 has a large impact on the SternVolmer constants, which in the case of the nitroaromatics are halved with respect to 1. The presence of the bithiophene has a greater impact on the quenching efficiency of DMNB, (17) Meaney, M. S.; McGuffin, V. L. Anal. Chim. Acta 2008, 610, 57. (18) Long, Y.; Chen, H.; Yang, Y.; Wang, H.; Yang, Y.; Li, N.; Li, K.; Pei, J.; Liu, F. Macromolecules 2009, 42, 6501. (19) Kim, T. H.; Kim, H. J.; Kwak, C. G.; Park, W. H.; Lee, T. S. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 2059.
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which is reduced to approximately one-third of its value for 1. However, the biggest change is seen of BP, where almost no quenching is observed in spite of it having a suitable electron affinity. Thus, the inability of BP to quench the emission of 2 indicates that this compound is biased toward the nitro group and therefore offers improved selectivity for the detection of explosives over material 1. The addition of a first-generation biphenyl dendrons to the core to give dendrimer 3 results in an approximate 20-30% decrease in the Stern-Volmer constants for all the nitro-based analytes, but has a bigger impact on the response to BP with about a 60% reduction. In the case of the extended chromophore with an extra thiophene unit in each arm, the addition of the first generation biphenyl dendrons to give dendrimer 4 shows no significant change in the response to the various analytes, preserving the selectivity toward the nitro-containing compounds. These results illustrate the importance of relative KSVs for analyte selectivity in fluorescence quenching, whereas many published studies focus only on large values of KSV. In this regard, it is important to note that an effective sensing material need only show a finite and measurable response to the analyte at requisite low concentrations. Steady-state Stern-Volmer experiments provide a measure of the overall quenching efficiency, but offer little insight into the quenching mechanism involved between the sensing materials and the analytes. The apparent linearity of our Stern-Volmer data as shown in Figure 2 suggests the quenching is predominantly occurring via a single mechanism. Thus, either the analyte is binding to the sensing materials and forming a nonemissive species (static quenching) or the two are unbound and quenching occurs when a photoexcited material interacts briefly with a colliding analyte molecule (collisional quenching). The two mechanisms can be distinguished with timeresolved measurements of the fluorescence decays of the sensing materials across a range of analyte concentrations. With static quenching, the fluorescence decay lifetime of a material will remain unchanged as the concentration of analyte is increased. This is because any molecules not bound to an analyte will decay with their native natural lifetime. However, collisional quenching provides an additional relaxation pathway for the excited molecules and therefore results in a decrease in the average fluorescence lifetime. As the interaction between the sensing molecule and the analyte must occur while the former is in the excited state there is a finite time window during which detection can occur, and thus, longer fluorescence lifetimes are beneficial. It should be noted that a fluorescence lifetime measurement probes the ensemble average excited state lifetime of the system. To elucidate the mechanism behind the quenching, the excited-state lifetimes of the materials were measured before and after multiple additions of analyte solution. The fluorescence decays measured for 1 upon additions of each of the analytes are shown in Figure 3. With increasing analyte concentration, there is a decrease in the fluorescence lifetime of the sensing material. This is clear evidence that the materials are quenched at least to some
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Figure 3. Time-resolved fluorescence decays for solutions of 1 prior to and after additions of known concentrations of each of the five analytes. Single exponential fits to the decays are included as solid black lines and in all cases indicate single lifetime decays.
extent through collisional interactions with the analytes. The fluorescence decays were fitted with single exponentials convolved with the instrument response (as shown in Figure 3) to extract lifetimes for each fluorescence decay profile. The fact that all the measured fluorescence decays are exponential means that the fluorescence intensity is proportional to the lifetime and the data can be analyzed in terms of a Stern-Volmer relationship given by τ0 ¼ 1 þ KC ½Q τ
ð2Þ
where τ0 is the fluorescence lifetime of the material in the absence of the analyte and τ is the lifetime after addition of concentration Q of analyte and KC is the Stern-Volmer constant for collisional quenching.14 As only collisional interactions would result in a change in the fluorescence lifetime, Stern-Volmer analysis of the lifetime data gives a Stern-Volmer constant that only applies to collisional quenching. If all the fluorescence quenching of the materials is by collisional interactions the time-resolved and steady-state Stern-Volmer plots would give the same quenching constant. The Stern-Volmer plots obtained from the time-resolved data for 1 are shown alongside the corresponding steadystate results in Figure 4. With the exception of BP, the time-resolved data indicates a slightly lower Stern-Volmer constant than the corresponding steady-state data, which implies there is more than one mechanism involved. For
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Figure 5. Collisional (shaded bars) and static (white bars) Stern-Volmer constants for each of the materials (1, 2, 3, and 4) with all the analytes.
Figure 4. Time-resolved (open squares) and steady-state (open circles) Stern-Volmer plots for 1 with each of the analytes. Fits to the timeresolved data using eq 2 are shown with a solid line and fits to the steadystate data using eq 3 are included as a dashed line.
combined collisional and static quenching the change in fluorescence is expected to be given by F0 ¼ ð1 þ KC ½QÞð1 þ KS ½QÞ F
ð3Þ
where KS is the static constant.14 However, as can be seen from the steady-state Stern-Volmer plots in Figure 2 the data appears to be linear, an apparent discrepancy that can be explained by expanding eq 3 to give F0 ¼ 1 þ KC ½Q þ KS ½Q þ KC KS ½Q2 F
Figure 6. Collisional quenching rates for the four materials (1, 2, 3, and 4) with all analytes.
ð4Þ
For low analyte concentrations and small Stern-Volmer constants, the contribution from the [Q]2 term to the total quenching will be much less than from the two linear terms and eq 4 would therefore appear linear. The range of analyte concentrations that can be used in a SternVolmer measurement may be limited by a number of factors such as strong absorption of the excitation by the analyte, strong fluorescence quenching or poor solubility. Therefore, it is often not possible to perform steady-state measurements at higher concentrations where strong nonlinearity would be observed. This highlights the
importance of fluorescence lifetime data in the interpretation of steady-state Stern-Volmer measurements. Using the value for the collisional constants obtained from the time-resolved data with eq 2, the steady-state data was fitted with eq 3 to yield a value for the static constants KS. The values of KC and KS for all the analytes with each of the dendrimers are summarized in Figure 5. In general the collisional component represents approximately 70% or more of the overall quenching of the dendrimers by all the analytes. In the case of BP, the quenching is all collisional. It is worthwhile noting that the sum of KC and KS for each dendrimer is generally
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equal to the respective steady-state KSV value, which is consistent with the linearization of eq 4 because of a small contribution from the quadratic term. The collisional Stern-Volmer constant can be used to calculate the collisional quenching rate kQ between the dendrimers and the analyte as KC = kQτ0 with the values plotted in Figure 6. Material 2 has consistently higher collision rates for the nitroaromatics compared to the other materials, which are all similar despite differences in the excited-state lifetime. This indicates that 2 has the largest collisional cross-section with the analytes, which is consistent with the fact that out of all four materials a greater proportion of its molecular structure is comprised of the fluorophore. The similarities in the collisional quenching rates of 1, 3, and 4 indicate that the higher Stern-Volmer constants of 1 and 3 are due to the longer excited-state lifetimes of these two compounds. Conclusions In conclusion, we have reported a series of “zeroeth” and first-generation dendrimers that are quenched by
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both nitroaromatics and the plastic explosives taggant DMNB. Crucially, by modifying the core of the materials the response to the control quencher BP is almost eliminated, which indicates these compounds provide selectivity toward nitrated analytes. Time-resolved measurements of fluorescence demonstrate that in contrast to conjugated polymers, quenching is dominated by collisional interactions between the dendrimers and the analytes with a minor static component. This could have significant, positive implications for detection repeatability, reversibility and response time. Furthermore, our results also show how small structural changes can greatly impact the sensitivity and selectivity of a chemosensing material. Acknowledgment. P.L.B. is recipient of an Australian Research Council Federation Fellowship (Project no. FF0668728). P.M. is a Queensland State Government Smart State Senior Fellow. We acknowledge funding from the Australian Research Council (DP09A6838) and the University of Queensland (Strategic Initiative-Centre for Organic Photonics & Electronics).
