Solid State Dendrimer Sensors: Effect of Dendrimer Dimensionality

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Solid State Dendrimer Sensors: Effect of Dendrimer Dimensionality on Detection and Sequestration of 2,4-Dinitrotoluene Hamish Cavaye,† Paul E. Shaw,† Arthur R. G. Smith,† Paul L. Burn,*,† Ian R. Gentle,† Michael James,‡,§ Shih-Chun Lo,† and Paul Meredith*,† †

Centre for Organic Photonics & Electronics, The University of Queensland, Brisbane, Queensland, 4072 Australia Bragg Institute, Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia § School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia ‡

ABSTRACT: We compare two dendrimers, which contain the same luminescent chromophores but differ in dimensionality, for the detection of an explosive analyte via PL quenching. Each dendrimer has first generation biphenyl dendrons with 2-ethylhexyloxy surface groups but differ in the core units. One dendrimer has a bifluorene core and hence has a “planar” structure, whereas the second has four bifluorene units tetrahedrally arranged around an adamantyl center and hence has a “three-dimensional” structure. Solution Stern
Volmer measurements have previously been reported to show that the three-dimensional dendrimer has a higher binding constant than that of the more planar compound. Films of the dendrimers rapidly detect 2,4-dinitrotoluene (DNT) with thinner films (∼25 nm) being more responsive than thicker films (∼85 nm). Neutron reflectometry measurements show that the analyte can diffuse completely through the films with the three-dimensional dendrimer absorbing more of the analyte. The rate of recovery of the PL was faster for the planar dendrimer than the three-dimensional material showing that large binding constants are not necessary for reversible detection of analytes.

’ INTRODUCTION In recent years interest in photoluminescent sensors for explosives has grown rapidly.1,2 These sensors rely on the ability of high electron affinity analytes, such as nitroaromatic compounds, to oxidatively quench the photoluminescence (PL) of a sensing material either in solution or the solid state.3,4 Solid state photoluminescent thin films can provide a convenient medium for portable sensing devices.5 Originally, much of the literature was focused on conjugated polymer sensors;3,6
12 however, more recently the field of photoluminescent dendrimer sensors has expanded and dendrimer sensors are now recognized as an important emerging technology.13
19 A dendrimer is a branched macromolecule comprising three components: a core, branching dendrons, and surface groups. The modular design of dendrimers provides a high level of synthetic control over the properties of the final compound, for example, the electronic properties of the core can be modified without affecting the solubilizing properties of the surface groups. Dendrimers are also monodisperse, which means that properties such as film morphology and device performance are often more readily reproduced than with polymers. Materials based upon dendrimers have already been successfully developed for use in a variety of organic electronic devices including light-emitting diodes,20
24 photovoltaics,25
28 and field effect transistors.29
31 It has been shown previously for polymeric sensors that the shape of the photoluminescent sensing material is important. It has been found that by using rigid three-dimensional moieties to structurally shield the polymer backbone it is possible to dramatically r 2011 American Chemical Society

decrease interchromophore quenching as well as improving quenching rates in the solid state.10,32 In this paper we explore the effect of dendrimer dimensionality (“planar” versus three-dimensional core) on the ability of solidstate dendrimer sensors to detect the nitroaromatic analyte 2, 4-dinitrotoluene (DNT). We also investigate how dimensionality affects the capacity of a dendrimer film to absorb DNT vapors, thus gaining important insight into how to improve the design of future sensor materials. DNT was chosen as a target since it is both a degradation product of and impurity found in 2,4,6-trinitrotoluene (TNT). TNT is an inexpensive explosive that is still used as one of the primary energetic materials in landmines throughout the world.33,34 Due to the significantly greater vapor pressure of DNT (180 ppb) compared to TNT (10 ppb),3 the presence of DNT can be used to more easily detect solid samples of TNT,33 making DNT an important real-world analyte. Figure 1 shows the chemical structures of the two dendrimers used in this study. Their syntheses and sensing properties in solution have been reported previously.17,19 The dendrimers consist of diphenylbifluorene chromophores, with D1 being relatively planar and containing one chromophore whereas D2 has four of these chromophores in a three-dimensional arrangement around an adamantane center. It would be expected that the significantly different dimensionalities of these two dendrimers would change Received: June 14, 2011 Revised: August 10, 2011 Published: August 30, 2011 18366

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Figure 1. Chemical structures of the bifluorene-based dendrimers D1 and D2 used in this study.

the packing in the solid state, which in turn would have an effect on the interactions with the analyte vapors.

’ EXPERIMENTAL SECTION Thin films of D1 and D2 on glass substrates (for photoluminescent quenching measurements) and quartz substrates (for PL recovery measurements) were formed by spin-coating from toluene at a concentration of 7 (for thin films) and 20 mg mL
1 (for thick films) at a speed of 2000 rpm. Film thicknesses were measured by a Veeco Dektak 150 surface profilometer giving D1 films with thicknesses of 23 ( 5 and 83 ( 5 nm, and D2 films with thicknesses of 25 ( 5 and 83 ( 5 nm. Uniformity of the films was confirmed by measuring the thickness at multiple points. PL spectra and intensity measurements were recorded on a Horiba Jobin-Yvon Fluorolog Tau-3 system. Excitation wavelengths of 352 (D1) and 354 nm (D2) were used. Quenching response measurements were carried out in the following manner. A quartz cuvette was charged with a small quantity of solid 2,4-dinitrotoluene (DNT) (Sigma-Aldrich, 97%, used as supplied) covered with cotton wool, and the cuvette was sealed and left to saturate for at least 24 h. The cotton wool serves two purposes; the first is to prevent any physical contact of the analyte with the sensor film, and the second is to help keep the vapor pressure of analyte inside the cuvette at a constant value when the lid is removed. The dendrimer films were then placed in the cuvette after it had been mounted in the spectrophotometer, and the peak wavelength PL intensity was monitored once every 0.5 s

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for a period of approximately 600 s. All measurements were repeated four times and performed at 23
25 °C to ensure constant analyte vapor pressure. PL recovery measurements were carried out in the following manner. Approximately 50 mg of DNT was placed in a 20 mL glass vial with cotton wool, and the vial sealed and left to saturate for at least 24 h. The dendrimer films were then placed in the vial for a period of 10 min to allow quenching to occur. Upon removal, the quenched films were placed into the spectrophotometer, and the peak wavelength PL intensity was monitored once every 0.5 s for a period of approximately 600 s. During the recovery a stream of clean, dry nitrogen (5 L min
1) was directed at the film to aid the removal of the analyte vapors. Measurements were repeated twice for each dendrimer and film thickness combination. The thin film PL quantum yields (PLQYs) were measured in an integrating sphere using the method described by Greenham et al.35 The 325 nm emission line from a HeCd laser was used as the excitation source. The laser power was attenuated to be in the range of 200
300 μW with a beam diameter at the sample of approximately 1 mm. To minimize the impact of photodegradation, the interior of the integrating sphere was flushed with nitrogen. Neutron reflectometry measurements were recorded using the Platypus time-of-flight neutron reflectometer36,37 and a cold neutron spectrum (3.0 Å e λ e 18.0 Å) at the OPAL 20 MW research reactor [Australian Nuclear Science and Technology Organisation (ANSTO), Sydney, Australia]. Neutron pulses of 23 Hz were generated using a disk chopper system (EADS Astrium GmbH) in the medium resolution mode (Δλ/λ = 4%) and recorded on a 2-dimensional helium-3 neutron detector (Denex GmbH). Reflected beam spectra were collected at 0.5° for 1 h (0.4 mm slits) and 2.0° for 4 h (1.6 mm slits). Direct beam measurements were collected under the same collimation conditions for 1 and 3 h (with attenuator) for each respective slit size. In situ PL spectra of the films were simultaneously measured with an Ocean Optics USB2000 spectrometer exciting with a 365 nm Nichia UV-LED. For the neutron studies, films of D1 and D2 were spin-coated onto 2 in. silicon wafers (Si-Mat, Germany) from solutions of concentrations 10 and 25 mg mL
1 in toluene at 2000 rpm to give films of thickness (as measured by surface profilometry): D1 36 ( 5 nm and 90 ( 5 nm; D2 38 ( 5 nm and 103 ( 5 nm. Glass jars containing approximately 50 mg of perdeuterated DNT (prepared by dinitration of d8-toluene) covered with cotton wool were left to equilibrate overnight. For saturated film measurements the films of the dendrimers were placed in the jars for approximately 5 h at room temperature, which ensured equilibrium between analyte vapors and analyte in the films was reached. A small amount of solid analyte was also placed in the neutron sample chamber in order to maintain complete quenching throughout the neutron reflectivity measurement. For recovered film measurements a stream of clean, dry air or nitrogen was used to remove the analyte. Nitrogen exposure lasted ∼10
15 min for films of D1 and 4
5 h for films of D2 before no more increase in luminescence was seen. Analysis of the reflectivity profiles was performed using the Motofit reflectometry analysis program.38 Quoted uncertainties are statistical uncertainties based upon the quality of the fit to the measured data. All models include a 0.8
1.5 nm silicon oxide layer on the surface of the substrate and consist of a single organic layer of uniform scattering length density (SLD). Interfacial roughnesses for the organic layers were all between 0.4 and 1.3 nm and showed little change upon exposure to and removal of the analyte. 18367

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Figure 4. NR profiles (symbols) and fits (solid lines) for D1: (a) thin film and (b) thick film. The profiles have been offset for clarity. Figure 2. Normalized absorption and emission spectra of D1 and D2 in the solid state.

Figure 5. NR profiles (symbols) and fits (solid lines) for D2: (a) thin film and (b) thick film. The profiles have been offset for clarity.

Figure 3. Quenching of photoluminescent films of D1 and D2 during exposure to vapors of DNT. Also shown are the baseline curves for photodegradation in air: D1 (solid line) and D2 (dotted line).

’ RESULTS AND DISCUSSION Photoluminescence and Quenching. Normalized film absorption and emission spectra of D1 and D2 can be seen in Figure 2. Analogous with the result previously reported in solution,19 the solid state absorption and emission spectra of D1 and D2 are almost identical. However, although both dendrimers have a solution PLQY of approximately 90%, in the solid state D1 has a PLQY of 49 ( 5% and D2 has a PLQY of 72 ( 6%. The stronger solid state luminescence of D2 compared to D1 is consistent with the more three-dimensional structure of D2 reducing interchromophore quenching. For a photoluminescent sensing film to be useful in practical applications it is necessary for a measurable level of quenching to occur in the first few seconds of analyte exposure. That is, the rate of PL quenching upon exposure to explosive vapors is important. In order to test the effect of dendrimer dimensionality and film thickness on the rate of PL quenching an experiment similar to those performed by Yang and Swager3 was carried out on films of D1 and D2, which had been spin-coated onto glass at two different thicknesses. Figure 3 shows the reduction in PL intensity with exposure to DNT vapors over time for the dendrimer films. As might be expected, thinner films of both dendrimers displayed more rapid quenching of the PL than thicker films. After ∼2 s 10% of the PL

from films of D1 is quenched, with the thinner film being quenched slightly faster. For D2, however, the difference in quenching rate between different film thicknesses is more pronounced. The thinner film of D2 was 10% quenched in just 0.5 s, whereas it took 5 s to reach the same level of quenching for the thicker film of D2. These levels of quenching are comparable to the best reported quenching rates for polymer-based PL explosive sensors,3,12,32,39
41 many of which were obtained with films below 10 nm in thickness. There are a number of benefits of being able to use thicker films in a sensing device, such as greater absolute PL intensity, which in turn can lead to a better signal-to-noise ratio or permit the use of simpler optical components. These results demonstrate that both dendrimer dimensionality and film thickness have an effect on the rate of PL quenching when the films are exposed to vapors of DNT. It is generally accepted that thinner films quench more rapidly than thick films because it takes less time for the analyte to diffuse throughout the film when the film is thinner. However, the link between dimensionality and quenching rate is less obvious. D1 and D2 pack differently in the solid state, and this difference in film structure will change the interactions between each dendrimer and the analyte, potentially leading to the differences observed in the PL quenching rate. In order to investigate these differences we utilized neutron reflectometry (NR) to examine the molecular packing of D1 and D2 in these films; as well as the distribution and amount of analyte that the sensors could absorb Neutron Reflectometry. NR allows us to probe the layered structure (perpendicular to the plane of the substrate) of the dendrimer films, as well as their ability to sequester analyte vapors. Using NR we have previously shown that films of D1 readily absorb the analyte 4-nitrotoluene and that the analyte spreads throughout the entire thickness of the films. 17 The contrast necessary to distinguish dendrimer from analyte in the NR measurements 18368

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Figure 6. SLD profiles for D1: (a) thin film and (b) thick film.

Figure 8. PL spectra recorded during the NR measurements for D1: (a) thin film and (b) thick film using an Ocean Optics USB2000 spectrometer. The small peak at ∼365 nm is scattered excitation light.

Figure 7. SLD profiles for D2: (a) thin film and (b) thick film.

was achieved using per-deuterated DNT. Our custom-built sample chamber allows for simultaneous PL and NR measurements, which allowed levels of quenching and analyte distribution to be directly compared in situ. Three NR profiles were recorded for each dendrimer and film thickness combination; one of the as-cast film, one of the film after being saturated with deuterated DNT vapors, and one after removal of the analyte (the recovered film). PL spectra were also recorded for each NR measurement (see later discussion). Figures 4 and 5 show the NR profiles and fits for the films of D1 and D2, respectively. All models included a silicon oxide layer of 0.8
1.5 nm on the substrate and a single dendrimer or dendrimer + analyte layer. Figures 6 and 7 show the scattering length density (SLD) versus film thickness determined from the NR profiles for D1 and D2 respectively. Before discussing the changes that occur in each of the films, it is important to note the relationship between the SLD and mass density of a film. The mass density of a film is related to its measured SLD by the molecular mass and the neutron scattering length of the material as follows: F¼

M SLD NA bi

∑i

ð1Þ

where F is the mass density of the film, M is the molecular mass of the material in the film, NA is Avogadro’s number, and Σbi is the sum of the bound coherent neutron scattering lengths of the atoms in the material, which are tabulated by Sears.42 The as-cast films of D1 have SLDs of (0.99 ( 0.01)  10
6 and (1.02 ( 0.01)  10
6 Å
2 for the thin and thick films, respectively. These SLDs correspond to a thin film mass density of ∼1.01 g cm
3 for D1. The as-cast films of D2 have SLDs of (1.15 ( 0.01)  10
6 and (1.08 ( 0.01)  10
6 Å
2 for the thin and thick films, respectively; that is, the thinner film appears to have a slightly higher SLD than the thicker film. These

Figure 9. PL spectra recorded during the NR measurements for D2: (a) thin film and (b) thick film using an Ocean Optics USB2000 spectrometer. The small peak at ∼365 nm is scattered excitation light.

SLDs correspond to thin film mass densities of ∼1.03 and ∼0.97 g cm
3 for the thin and thick films of D2, respectively. Importantly, the average thin film mass densities for both D1 and D2 are very similar. After the films are saturated with vapors of the deuterated DNT a number of changes occur. First, for all of the films studied, exposure to the analyte causes the films to swell (they become thicker). For D1 the saturated films have a thickness of 38.2 ( 0.1 and 97.6 ( 0.1 nm having increased from 36.6 ( 0.1 and 94.9 ( 0.1 nm, respectively. These changes correspond to an ∼3
4% increase in film thickness after exposure. For D2 the saturated films swell in thickness from 40.9 ( 0.1 to 43.7 ( 0.1 nm and from 104.4 ( 0.1 to 112.2 ( 0.1 nm, corresponding to an ∼7
8% increase in film thickness upon saturation with analyte vapors. The films of D2 increase in thickness approximately twice as much as films of D1, regardless of the original thickness of the film. This result shows that the shape of the dendrimer has an effect on the ability of the film to reorganize itself to accommodate the analyte molecules. Second, it is clear from the PL spectra (Figures 8 and 9) that the emission of each of the films is completely quenched by the analyte. This result alone suggests that there is at least one analyte molecule within the exciton diffusion range of all the dendrimers in each film. In a bifluorene-cored dendrimer related to D1, the solid-state exciton diffusion length was reported to be 6 nm,43 meaning that for full quenching to occur the analyte must have diffused through the bulk of the organic layer. This was confirmed by the changes in SLD observed in the films after exposure. For all the DNT saturated films measured, the SLD of the organic layer increases compared to that in the as-cast film. Importantly, this increase appears to occur uniformly throughout the thickness of 18369

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The Journal of Physical Chemistry C the films, meaning that the analyte has penetrated through the entirety. A similarly high level of analyte penetration was seen previously with films of D1 and the analyte 4-nitrotoluene.17 However, the increase in SLD upon exposure to the analyte is not the same for both of the dendrimers tested. The changing thickness of the film has an effect on the density of the dendrimer film, which means a simple comparison of the increase in SLD is misleading. To obtain a measure of how much analyte has been absorbed by each film it is necessary to calculate the average number of analyte molecules per unit volume in each of the saturated films. A basic premise of the calculation is that as the film absorbs the analyte the total amount of dendrimer does not change. This is reasonable as there is no solvent to remove the dendrimers and they do not sublime. The SLD of the saturated film is a combination of the SLD of the dendrimer and the SLD of the analyte. The contribution of the analyte to the SLD of the saturated film (SLDA) can be expressed as Z Z SLDsaturated dz
SLDas
cast dz ð2Þ SLDA ¼ lsaturated where SLDsaturated and SLDas-cast are the SLD profiles of the saturated and as-cast films, respectively, and lsaturated is the thickness of the saturated film. The value of SLDA calculated in this way is analogous to the SLD that would be measured by NR if a film of the absorbed analyte could be created without the dendrimer matrix supporting it. As such, SLDA can be used with eq 1 to obtain the mass density of the analyte in the film and consequently the number of analyte molecules per unit volume averaged over the entire film can be calculated. Using this method we have determined that there are 0.19 ( 0.01 and 0.11 ( 0.01 DNT molecules per cubic nanometre for the D1 thin and thick films, respectively, while there are 0.35 ( 0.01 and 0.39 ( 0.01 DNT molecules per cubic nanometre for the D2 thin and thick films, respectively. These data clearly show that the films of D2 are able to sequester 2
4 times more DNT vapor than films of D1, which is attributed to the different film morphologies arising from the different dendrimer dimensionalities. A final NR experiment was undertaken to determine how much analyte could be removed from the films. Each of the saturated films was blown with air or nitrogen until no further changes in PL intensity were observed then NR profiles were measured. As can be seen from Figures 8 and 9 films of D1 recovered more of the PL intensity than those of D2, and in both cases the thin films recovered to a greater degree than the respective thick films. Figures 4 and 5 show the NR profiles measured for these recovered films, with the corresponding SLD profiles again shown in Figures 6 and 7. These figures show that the SLDs have returned to the level of the as-cast films but the films are still slightly thicker. The fact that the PL has not returned to the levels of the as-cast films for all but the thin D1 film shows that there is still analyte present. In order to determine how much analyte still remained in these films the analyte concentration was calculated for the recovered films in the same way as for the saturated films above. In all four cases it was found that, within the uncertainty of the fits from the model, the level of the analyte remaining was below 0.01 molecules per cubic nanometre. Thus the increase in thickness of the films, which is ∼1
2% for the thicker films and ∼3
4% for the thinner films, suggests that a slight morphological (density) change has occurred during

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Figure 10. PL recovery of films of D1 and D2 after quenching with DNT then blowing with nitrogen.

the ingress and removal of the analyte, which is akin to solvent annealing of the organic layer that remains. Importantly, the results show that the analyte absorption is at least partially reversible even after complete saturation of the films with thin D1 films essentially fully recovered. Photoluminescence Recovery. Due to the recovery of the PL occurring to a greater extent in the films of D1, an experiment was performed to compare the rates of analyte desorption in films of D1 and D2. Both dendrimers were spin-coated onto quartz substrates to give films of the same thickness as those used in the PL quenching section above. These films were then prequenched with DNT vapors before the recovery of PL was monitored as nitrogen was blown over the films. Figure 10 shows how the PL intensities of the films changed over time. The thinner films of both D1 and D2 recovered their PL intensity at a greater rate than the respective thicker films. As shown by the NR experiments, the analyte molecules are distributed throughout the film, which means that to reach the surface and be removed the analyte must diffuse further in the thick films. However, a more stark result is the difference between D1 and D2. Films of D1 recover their PL intensity at a much greater rate than those of D2, with both thicknesses of the D1 films recovering faster than those of D2. In fact, after 9 min the thicker film of D2 still shows very little change in PL intensity; that is, there is enough analyte to still fully quench the film. These data suggest that the binding of DNT within a film of D2 is stronger than the binding of DNT to D1. As D1 and D2 are constructed of essentially the same chemical components, this result shows that the higher dimensionality of D2 is likely to be the cause of the stronger interactions. This is consistent with the solution measurements where the Stern
Volmer constant is ∼50% higher for D2 than for D1. However, these results show that a higher solution Stern
Volmer constant may not always be advantageous for a reversible solid-state detection system.

’ CONCLUSIONS Films of two highly photoluminescent dendrimers were spincoated to create rapid and sensitive sensors for the nitroaromatic analyte DNT. The dendrimers comprise almost identical bifluorene chromophores but differ in their dimensionality, with D1 being relatively planar and D2 being more three-dimensional. Photoluminescent films of D1 and D2 were exposed to DNT vapor and the rate of quenching monitored. Thin films of both dendrimers were shown to quench more rapidly than the respective thick films, with the thin films of D2 quenching most 18370

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The Journal of Physical Chemistry C rapidly. NR measurements were performed on films of both dendrimers saturated with DNT, which showed that DNT penetrates evenly throughout the entire thickness of the dendrimer films. Upon exposure to the analyte all the films increased in thickness as the analyte was absorbed, with D2 swelling approximately twice as much as D1 regardless of the original film thickness. DNT concentration in saturated films of D2 was ∼2
4 times higher than in D1. Finally, the rate of analyte removal was significantly greater for films of D1 than D2. These results demonstrate that the higher dimensionality dendrimer D2 has significantly greater interactions with the analyte than D1.

’ ACKNOWLEDGMENT P.L.B. is the recipient of an Australian Research Council Federation Fellowship (Project FF0668728) and P.M. was a Queensland Smart State Senior Fellow during the tenure of this work. H.C. is supported by an Endeavour International Postgraduate Research Scholarship and University of Queensland Living Allowance Scholarship. A.R.G.S. thanks AINSE for a Postgraduate Research Award. The research was supported by the Australian Research Council through the Discovery Program (DP0986838). The Centre for Organic Photonics & Electronics is a strategic initiative of the University of Queensland. ’ REFERENCES (1) Meaney, M. S.; McGuffin, V. L. Anal. Bioanal. Chem. 2008, 391 (7), 2557–2576. (2) Germain, M. E.; Knapp, M. J. Chem. Soc. Rev. 2009, 38 (9), 2543–2555. (3) Yang, J. S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120 (46), 11864–11873. (4) Germain, M. E.; Vargo, T. R.; McClure, B. A.; Rack, J. J.; Van Patten, P. G.; Odoi, M.; Knapp, M. J. Inorg. Chem. 2008, 47 (14), 6203–6211. (5) Fisher, M.; laGrone, M.; Sikes, J. Proc. SPIE 2003, 5089, 991–1000. (6) Czarnik, A. W. Nature 1998, 394 (6692), 417–418. (7) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. Rev. 2000, 100 (7), 2537–2574. (8) Thomas, S. W.; Joly, G. D.; Swager, T. M. Chem. Rev. 2007, 107 (4), 1339–1386. (9) Toal, S. J.; Trogler, W. C. J. Mater. Chem. 2006, 16 (28), 2871–2883. (10) Yang, J. S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120 (21), 5321–5322. (11) Sohn, H.; Sailor, M. J.; Magde, D.; Trogler, W. C. J. Am. Chem. Soc. 2003, 125 (13), 3821–3830. (12) Liu, Y.; Mills, R. C.; Boncella, J. M.; Schanze, K. S. Langmuir 2001, 17 (24), 7452–7455. (13) Li, W.-S.; Aida, T. Chem. Rev. 2009, 109, 6047
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