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J. Phys. Chem. B 2001, 105, 8468-8473
Signal Amplification in Multichromophore Luminescence-Based Sensors Igor A. Levitsky* and Sergei G. Krivoshlykov ALTAIR Center, LLC, 1 Chartwell Circle, Shrewsbury, Massachusetts 01545
Jay W. Grate EnVironmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352 ReceiVed: April 12, 2001; In Final Form: June 16, 2001
A method for signal amplification in the detection of vapors with luminescence-based sensors is described. Amplification involves energy transfer between two or more fluorescent chromophores in a carefully selected polymer matrix. A quantitative model has been derived that can be applied to any luminescence sensor comprising donor-acceptor pairs, and it can be generalized to multichromophore systems with n chromophores leading to n-fold signal amplification. Signal amplification has been demonstrated experimentally in the fluorescent sensing of dimethyl methylphosphonate (DMMP) using two dyes, 3-aminofluoranten (AM) and Nile Red (NR), in a hydrogen-bond acidic polymer matrix. The selected polymer matrix quenches the fluorescence of both dyes and shifts dye emission and absorption spectra relative to those of more inert matrixes. Upon DMMP sorption, the AM fluorescence shifts to the red at the same time that the NR absorption shifts to the blue, resulting in more band overlap and increased energy transfer between chromophores. In addition, the emission of both chromophores is enhanced. Using an excitation wavelength tuned to the AM dye, we found that the absolute signal magnitude observed upon DMMP exposure in the two-dye film was an order of magnitude greater than that observed when using a single-dye NR-containing film. The ratio of the response signal under vapor exposure to the signal prior to exposure was 250 for the two-dye film compared to 15 for single-dye films. The two-dye approach to signal amplification also significantly increases the selectivity relative to the potentially interfering vapors. Experimental results to date favor a reabsorption mechanism over a Fo¨rster radiationless direct energy-transfer mechanism.
Introduction Today, optical chemical sensors are used for a variety of applications, demonstrating high selectivity, sensitivity, fast response, and robustness.1-3 A special place among optochemical indicators (optodes) belongs to luminescence-based chemosensors2,4-11 because luminescence is more sensitive to change in the chromophore microenvironment than to absorption or reflection.6 Hence, such methods exhibiting the highest selectivity to the specific analytes should be optimal for the detection of low concentrations of target molecules. Most of the luminescence sensory materials known today consist of chromophores isolated in an inert matrix and cannot use the amplification effect resulting from energy migration or transfer in the excited states. Meanwhile, the application of the “molecular wire approach”8 based on the energy migration12 in conjugated polymers to the amplification of the response emission has received recent attention. Pioneering studies by the Swager group9,10 demonstrated that fluorescence quenching in polymers was considerably more effective than that of the monomer analogues. Recently, the same effect was obtained for watersoluble polymers.13 Also, signal amplification has been observed for emissive dendrimers14 (in which one metal ion quenched a great number of dendritic units) and for fluorescent dye adsorbed on polymer nanobeads.15 * Corresponding author. E-mail:
[email protected]. Fax: (508)-8455349.
Another way to improve the sensitivity is through the use of radiationless direct energy transfer (RDET) or emissive energy transfer (EET) between donor and acceptor chromophores isolated in an inert matrix. RDET is a distance-dependent transfer of electronic excitation from donor to acceptor due to dipole-dipole interaction (Fo¨rster mechanism16,17), and EET is a result of the acceptor reabsorption of the donor emission.17 Several recent studies have been related to the fluorescence response of a bichromophore system involving the RDET and EET effects.18-28 Physiological fiber-optic pH sensors were studied for eosin (donor) and phenol red (acceptor) covalently attached optical fibers by copolymerization,18,19 and watersoluble polymers contained these dyes.20,21 Also, the pH sensor was fabricated for donor-acceptor pairs isolated in sol-gel films22 employing RDET between Texas Red Hydrazine (donor) and bromothymol blue (acceptor). The luminescent ruthenium complex-thymol blue donor-acceptor pairs were used for carbon dioxide and chloride sensing.23,24 In these studies, analyte binding or pH changes affect the absorbance of acceptor species that alters RDET and, consequently, changes the donor quantum yield and lifetime. Here, only one chromophore (energy acceptor) was sensitive, while the second chromophore (donor) remained insensitive to analyte or pH changes. In this case, as it follows from the results presented here, it is not possible to increase the sensor sensitivity due to signal amplification; as a result, these studies23,24 were focused on other benefits coming from the energy-transfer effect. For example, it was shown that
10.1021/jp011376h CCC: $20.00 © 2001 American Chemical Society Published on Web 08/11/2001
Multichromophore Luminescence-Based Sensors
Figure 1. Structure of NR, AM, and BSP3 repeat units.
sensor performance can be improved if a luminescent ruthenium complex with a long decay time is used as a donor chromophore.24 The first study of sensitivity enhancement for fluorescence sensors was reported by Walt and co-workers.25-27 Here, the inner-filter effect, a result of the EET between specially chosen pH-sensitive dyes, was employed to increase the pH sensitivity. In the study reported by Yang et al.,28 the amplification of the sensitivity has been observed when both donor and acceptor chromophores (RDET mechanism) were affected by analyte binding. However, the explanation of this effect was not correct. According to ref 28, the main reason for the amplification was the quenching of simultaneously excited donor and acceptor (excited pair), which is inconsistent with the energy-transfer principle: only one chromophore (donor or acceptor) can be excited when energy transfer occurs.16 We now present a quantitative model describing the amplification effect in a luminescence-based sensor as a result of the energy transfer (RDET and EET) between donor and acceptor chromophores. This approach has universal character and can be applied to any luminescence sensors comprising donoracceptor pairs. Moreover, it can be generalized to a multichromophore system consisting of n chromophores, which consequently transfer energy from the first donor to the last acceptor, leading to n-fold signal amplification. We have found experimental confirmation of the proposed model using a sensory film comprising two dyes (Nile Red as an acceptor and 3-aminofluoranten as a donor) isolated in a hydrogen-bond acidic polymer. The subject of interest is the sensing of basic vapors that exhibit strong binding to acidic polymer groups,29,30 affecting photophysical properties of both dyes and energy transfer between them. In our previous study,31 we found that Nile Red isolated in hydrogen-bond acidic polymer films exhibits strong fluorescence enhancement and spectral shift under exposure to basic vapors. In the present paper, we demonstrate also that an amplification effect not only increases the sensor sensitivity but also considerably improves the selectivity to the target vapors. Experimental Section Nile Red (NR), 3-aminofluoranten (AM), spectroscopic-grade chloroform, ethanol, and other organic solvents used in our experiments were commercial products purchased from Aldrich. They were used without further purification. The BSP3 polymer was prepared at Pacific Northwest National Laboratory (PNNL) according to published procedures.32 Structures of BSP3 polymer, NR, and AM are shown in Figure 1.
J. Phys. Chem. B, Vol. 105, No. 35, 2001 8469 Solutions for spin casting were prepared in spectroscopicgrade chloroform. The polymer concentration was 5 × 10-2 M in repeat units for all polymers, and the concentrations of AM and NR were 2.5 × 10-2 and 2.5 × 10-4 M, respectively, for bichromophore films. For monochromophore films, the dye concentration was 2.5 × 10-4 M. Spin cast films were prepared on glass substrates at a spinning rate of 1000 rpm. Thickness measurements were carried out by ellipsometry, giving measured thicknesses (error (20%) of 550 and 420 Å for BSP3 and PMMA polymers, respectively. UV-vis spectra were measured using a Perkin-Elmer Lambda spectrometer. Uncorrected fluorescence spectra were recorded with a SLM 8001 fluorimeter. All solution measurements were made in spectroscopic-grade chloroform. All films were positioned into a quartz cuvette at an angle of 23° to the excitation beam. Fluorescence measurements under exposure to saturated vapors of dimethyl methylphosphonate (DMMP) were carried out by adding a small amount (50 µL) of the analyte solvent directly in the bottom of the cuvette. DMMP is a strong basic vapor that mimics well Sarin, Soman, and other organophosphorus vapors belonging to chemical warfare agents. After the cuvette was capped, the fluorescence signal was recorded as a function of time. Generally, final fluorescence spectra were recorded under equilibrium conditions 10-15 min after the solvent was added. Model In this section we will develop a quantitative model describing the amplification of the fluorescent response signal in the bichromophore sensor that results from energy transfer. In particular, this model gives an explanation for the experimental observation of the higher sensitivity of AM-NR donoracceptor chromophores in the BSP3 matrix with respect to that of a monochromophore sensor. Let us consider the fluorescence enhancement as a mechanism of the signal transduction; however, the same approach can be applied to the fluorescence quenching. The sensitivity and intensity of a sensor signal can be introduced as S ) I/I0, where I0 and I are fluorescence intensities prior to and after analyte exposure, respectively. Then, monochromophore system populations of excited states prior to (n0) and after (n) analyte exposure are given by
0 ) -k0n0 + JN0 0 ) -kn + JN
(1)
where k0 and k are deactivation rates (inverse lifetimes) of excited chromophores prior to and after analyte exposure, respectively, N is the number of chromophores transferred to the new excited state after analyte exposure, N0 is the total number of chromophores, and J is the intensity of excited light. The general expression for signal intensity is
I)
kr J kr + kd
(2)
where kr and kd are radiation and radiationless deactivation rates, respectively, and k ) kr + kd. Usually, the radiationless processes (kd parameter) is more sensitive to a change of the chromophore environment. Therefore, in the following report we will consider only kd changes, keeping kr as a constant. Then, the monochromophore sensor sensitivity, SM, is given by
8470 J. Phys. Chem. B, Vol. 105, No. 35, 2001
SM )
Levitsky et al.
I koN k0 ) 0 ) KB × 0 k I kN
(3)
Here, KB ) N/N0 is the binding constant for analytechromophore interaction. We omit the remaining part of the chromophore molecules (N0 - N) at an unchanged state under analyte exposure due to the condition where k0 . k (strong fluorescence enhancement). If there is initially no fluorescence at all (I0 ) 0), then the sensitivity S can be defined as SM ) I. The bichromophore scheme proposes to selectively excite only donor chromophores and detect the response fluorescence signal from acceptor chromophores. Acceptor fluorescence, in this case, is a result of the direct energy transfer in accordance with RDET (Fo¨rster mechanism) or EET (reabsorption). We begin consideration with RDET, and then demonstrate that substitution to the reabsorption does not change the final formula. Prior to analyte exposure, the balanced equations for donor and acceptor populations are given by
0 ) -(k0D + k0TR)n0D + JN0D 0 ) -k0An0A + k0TRn0DN0A
bichromophore sensitivity will be 4 times higher than the sensitivity of the monochromophore system (SB ) 4SM). If reabsorption of the donor emission by acceptor chromophores (EET) is the main mechanism of the energy transfer, then eqs 4 and 5 should be modified to
0 ) -k0Dn0D + JN0D 0 ) -k0An0A + J0RBN0A and
I0A )
JN0AN0D
(5)
k0A(k0TR + k0D)
Here, k0D and k0A are donor and acceptor deactivation rates, respectively, and k0TR is the RDET rate between them. Also, 0 k0r A is the radiation deactivation rate of the acceptor, and NDand 0 NA are the total numbers of donor and acceptor chromophores, respectively. In the most general case, the analyte exposure can affect both the donor and the acceptor deactivation rates and also the energy transfer between them. Then, the fluorescence intensity after the exposure can be presented in the same way as eq 5:
IA )
k0r A kTR
JNAND
(6)
kA(kTR + kD)
where NA ) KBAN0A and ND ) KBDN0D are the numbers of acceptor and donor chromophores, respectively, transferred to the new excited state; other parameters have the same meaning as those in eq 5 (the parameters indicated without a superscript 0 correspond to values that were changed after exposure). The relation kTR < kD is a reasonable condition since the usual concentration of chromophores in a solid matrix cannot provide an average distance between the donor and acceptor less than typical a Fo¨rster radius, which is 20-60 Å for most organic compounds.12,17 Finally, we can obtain the sensitivity SB of the bichromophore sensor by
SB )
IA I0A
) KBD × KBA ×
(9)
(4)
and 0 k0r A kTR
k0r A 0 JRBN0AN0D k0A
The reabsorption intensity is J0RB ∼ I0OV × Q0D, where I0OV is the integral characterizing the overlapping between acceptor absorption and donor fluorescence spectra and Q0D is the donor quantum yield. Following the above procedure for eq 7 we can derive the expression for bichromophore sensitivity SB in the reabsorption case as
SB ) I0A )
(8)
k0D k0A kTR × × kD kA k0
(7)
TR
A comparison of eq 7 with eq 3 demonstrates the sensitivity amplification for a bichromophore system with respect to that of a monochromophore system. For example, let us assume that for strong analyte-chromophore binding (KBA ) KBD j 1), analyte exposure doubles the fluorescence enhancement of the donor and acceptor fluorescence intensities and the RDET between them (k0D ) 2kD, k0A ) 2kA, k0TR ) kTR/2). Then, the
IA I0A
) KBD × KBA ×
k0D k0A IOV × × kD kA I0
(10)
OV
Equation 10 is similar to eq 7, where the RDET rate (kTR) is substituted by the overlapping integral (IOV). The stronger the overlapping between donor and acceptor spectra, the higher the IOV and kTR values. However, there is a difference between eqs 10 and 7. In accordance with the Fo¨rster model,16 kTR ∼ (RF),3 and the Fo¨rster radius RF ∼ IOV × QD ∼ IOV/kD. After substitution of kTR in eq 7 we found that RDET sensitivity will depend nonlinearly upon parameters k0D/kD and IOV/I0OV. Thus, despite the different nature of the energy transfer between donor and acceptor chromophores, both RDET and EET provide amplification of the sensitivity for bichromophore sensory films. A bichromophore sensor can be generalized to an nchromophore sensor by
Sn ) KBn ×
k0n kn
∏
n-1 i)1 KBi
×
k0i Si × 0 ki S
(11)
i
where KBi is the binding constant of the analyte to the i chromophore, k0i and ki are deactivation constants of the i chromophore prior to and after analyte exposure, respectively, and S0i and Si are the RDET rates or the overlapping integral between i and i + 1 chromophores prior to and after analyte exposure, respectively. In such a sensing scheme, only the first chromophore should be excited (energy donor, i ) 1), and its excited energy is consequently transferred through other chromophores to the final acceptor chromophore (i ) n) that emits the fluorescence. If the binding constants are rather high (KBi e 1), and the chromophore-analyte interaction leads to fluorescence enhancement (k0i /ki > 1), then the n-fold amplification of the sensitivity occurs with respect to monochromophore sensors. The amplification can be even greater if analyte binding induces an increase in the energy transfer between chromophores (Si/S0i > 1). Results and Discussion The responses of NR fluorescence in single-dye NR/BSP3 films under DMMP exposure were investigated in detail in our previous study.31 Pronounced NR fluorescence enhancement and
Multichromophore Luminescence-Based Sensors
J. Phys. Chem. B, Vol. 105, No. 35, 2001 8471
Figure 3. Spectral shifts of AM fluorescence and NR absorbance in BSP3 films upon DMMP exposure. AM fluorescence spectra in AM/ BSP3 films are shown before (1) and after (3) DMMP exposure, showing the fluorescent enhancement and red shift. Spectrum 2 is spectrum 1 normalized to spectrum 3. NR absorbance in NR/BSP3 films is shown before (5) and after (4) DMMP exposure normalized to spectrum 3, showing the blue shift in absorbance. Figure 2. Nile Red fluorescence spectra before (1) and after (2) DMMP exposure in BSP3 (a) and PMMA (b) spin cast films (λex ) 530 nm). Spectrum 3 is spectrum 1 × 10.
spectral blue shift were found upon exposure to DMMPsaturated vapor when NR was in the BSP3 matrix; rather small changes were observed with NR in the PMMA matrix (Figure 2). The strong enhancement of NR fluorescence (Figure 2a) observed was interpreted as a competition between NR and DMMP for interaction with hydrogen-bonding sites in the BSP3 polymer.31 NR most likely forms hydrogen bonds with this polymer, resulting in the low quantum yield and the red shift. NR has four basic sites (two nitrogen and two oxygen atoms) with the potential to hydrogen bond to the BSP3 polymer. The solute DMMP is a strong hydrogen-bond basic vapor that also seeks to hydrogen bond with polymeric hydroxyl groups. In general, phosphonates are stronger hydrogen-bond bases than amines and much stronger hydrogen-bond bases than ketones or ethers. Thus, DMMP molecules having a higher binding affinity for BSP3 sites than NR can break NR hydrogen bonds, “releasing” the dye to a less hydrogen-bonded state. The fluorescence spectrum then more closely resembles the spectrum of NR in a PMMA matrix (Figure 2b). The dye has a more intensive emission, and the spectrum is blue-shifted relative to the spectrum of the dye in BSP3 in the absence of DMMP. The fluorescence of AM isolated in BSP3 films under DMMP exposure also shows an enhancement; however, in contrast to NR, it exhibits a red spectral shift (Figure 3). NR absorption in BSP3 films is blue shifted under exposure to DMMP-saturated vapors. As a result, there is a strong overlap of the AM fluorescent band and the NR absorption band. Thus, the analyte exposure induces an increase in the energy transfer between AM (donor) and NR (acceptor) and the fluorescence enhancement for both chromophores. Such a situation is ideal for the demonstration of the amplification effect in the bichromophore system according to the proposed model (eqs 7 and 8). Indeed, a strong NR fluorescence was observed in bichromophore AMNR/BSP3 sensory films that were exposed to DMMP vapors. Figure 4 shows the fluorescence spectra of AM-NR/BSP3 films before and after DMMP exposure (spectra 3 and 4, respectively). The samples were excited at λex ) 400 nm in the absorption band of AM, where NR practically does not absorb light. Therefore, prior to DMMP exposure, only the AM fluorescence
Figure 4. Fluorescence of AM-NR/BSP3 films before (3) and after (4) DMMP exposure (λex ) 400 nm). For comparison, the fluorescence (λex ) 400 nm) of AM/BSP3 is shown before (1) and after (2) DMMP exposure, where 2 has been normalized to the height of 4 (spectrum 1 was multiplied by the same factor as spectrum 2). Then, the difference between 4 and 2 gives 5, the fluorescent peak from NR in the AMNR/BSP3 film, which is in agreement with results from NR/BSP3 films under DMMP exposure as shown in Figure 2.
band was observed without any sign of NR fluorescence (the condition of selective donor excitation is fulfilled here). DMMP exposure leads to the appearance of an intensive NR fluorescence band (590 nm) that is comparable to the AM emission as a result of the signal amplification (strong spectral overlapping and fluorescence enhancement of both chromophores). To estimate the bichromophore sensitivity (SB value), we should extract the NR fluorescence band from the spectrum composed of NR and AM emissions (Figure 4, spectrum 4) and take the ratio of its intensity to that of NR emission before exposure. The fluorescence spectrum of the AM/BSP3 film after DMMP exposure was used as a reference (spectrum 2 in Figure 4). Finally, spectrum 5 is a result of the subtraction of spectrum 2 from spectrum 4 (Figures 4 and 5). The same procedure was applied to extract the NR fluorescence band before exposure in the AM-NR/BSP3 films; however, the resulting spectrum had almost a background-level intensity. Therefore, we used the NR fluorescence obtained from NR/BSP3 films at an excitation of λex ) 400 nm (Figure 5, spectrum 7). The ratio of the peak intensity of spectrum 5 (IA value, after exposure) to that of
8472 J. Phys. Chem. B, Vol. 105, No. 35, 2001
Figure 5. Fluorescence spectra of AN-NR/BSP3 films (5) and NR/ BSP3 films (6, 7) after DMMP exposure. Spectrum 5 is the same spectrum 5 shown in Figure 4.
Figure 6. Normalized NR absorption (4, 5) and AM fluorescence (2, 3) spectra in spin cast NR/BSP3 and AM/BSP3 films prior to (2, 5) and after (3, 4) ethanol exposure. Spectrum 2 is the normalized spectrum 1 that demonstrates the AM fluorescence enhancement.
spectrum 7 (I0A value, prior to exposure) gives SB ∼ 250, which is considerably higher than SM ) 15 (monochromophore sensitivity) obtained for the NR/BSP3 film at λex ) 400 and 530 nm (Figure 2a). Also, an absolute magnitude of the response signal for AM-NR/BSP3 films (Figure 5) after DMMP exposure exceeds that for the NR/BSP3 film by more than 1 order of magnitude at the same excitation wavelength (λex ) 400 nm) and by more than 5 orders of magnitude at λex ) 530 nm. This is another advantage of bichromophore sensing over the monochromophore detection scheme. In the latter case, the 5-fold enhancement over the values obtained with the monochromophore sensor can be explained by the fact that the concentration of the donor species (AM) is 2 orders of magnitude greater than that of the acceptor species (NR). In such conditions, the energy transfer from the donor can provide a higher intensity of the acceptor than that provided by direct donor excitation, even in the maximum wavelength of the donor absorption (530 nm). We investigated also the selectivity of the bichromophore AM-NR/BSP3 films. Note that the strong overlapping between donor and acceptor spectra under exposure of the specific analyte is a rather unique property of the proposed film composition. It means that such a system should be very selective to a given analyte, diminishing the false response from other interfering vapors. Indeed, the response signal is the result of the acceptor fluorescence harvesting light only from donor emission due to energy transfer. If overlapping between donor emission and
Levitsky et al.
Figure 7. Fluorescence spectra of AM-NR/BSP3 films prior to (1) and after DMMP (2) and ethanol (3) exposures. Spectrum 3 is normalized to the maximum of spectrum 2.
acceptor absorption cannot provide an efficient energy transfer, the response signal will be dramatically low. The experimental results presented in Figures 6 and 7 confirm this idea. We chose an ethanol-saturated vapor as an interfering gas, which can induce the false response in the monochromophore NR/BSP3 film.31 As one can see from Figure 7, the ethanol exposure does not result in any NR fluorescence in AM-NR/BSP3 films. This is consistent with the lower degree of spectral overlapping produced under ethanol exposure (Figure 6) in comparison to that produced under DMMP exposure (Figure 3). Hence, the AM-NR/BSP3 bichromophore films are completely insensitive to the interfering ethanol vapors, in contrast to the monochromophore NR/BSP3 films. Note that the concentration of the ethanol-saturated vapor exceeded the concentration of the DMMP-saturated vapor by ∼25 times.31 Also, we tested other organic vapors such as benzene and chloroform and did not find any sizeable NR fluorescence. The interesting aspect is related to the nature of the energytransfer effect inducing NR fluorescence enhancement in the AN-NR/BSP3 films. The main question here is what kind of mechanism (RDET or reabsorption) prevails in this case? The Fo¨rster RDET process should exhibit a strong dependence on the donor and acceptor concentration in contrast to the process of reabsorption. The AM, NR, and BSP3 concentrations in the chloroform solution prior to spin cast were CAM ) 2.5 × 10-2 M, CNR ) 2.5 × 10-4 M, and CBSP3 ) 5 × 10-2 M, respectively. An average distance between the donor (AM) and acceptor (NR) species in solution should be equal to the average distance between donor molecules for CAM . CNR. Its magnitude was estimated to be approximately 500 Å. The average distance in the film (after solvent evaporation) is less than 500 Å, and it can become comparable to the Fo¨rster radius (20-60 Å). However, it is difficult to get the real value of this distance due to the unknown amount of chromophores remaining in the film, possible phase separation, and partial dye aggregation. Therefore, such an estimation cannot give the real contribution of the Fo¨rster mechanism to the energy transfer. On the other hand, decreasing the magnitude of CAM should increase the average distance between donor and acceptor species and sharply (nonlinearly) decrease the RDET between them,16 leading to a disappearance (or decrease in the intensity) of the NR fluorescence. On the contrary, for reabsorption17 the ratio between intensities of the NR and AM band should remain unchanged. We found that decreasing the AM concentration in two times does not decrease the ratio of NR/AM intensities and even increases it by approximately 1.3 times. Hence, in our case,
Multichromophore Luminescence-Based Sensors the reabsorption mechanism prevails on the Fo¨rster RDET. An increase of the NR/AM ratio can be associated with the decrease of reabsorption between AM molecules for the lower AM concentration. Then, NR molecules absorb more AM emission. Conclusion A successful amplification performed according to the approach described above requires a four-parameter system involving two dyes, a polymer matrix, and an interacting analyte vapor. Amplification can occur when the vapor interaction results in an improved overlap between the donor emission spectrum and the acceptor absorbance spectrum. In our case, the vapor interaction also led to a significant enhancement of the fluorescent intensities of both dyes. The polymer matrix played a significant role in this. The selection of the polymer to quench fluorescence while shifting absorbance and fluorescence spectra prior to vapor sorption created a composite film whose fluorescence could change significantly upon vapor sorption, provided that the sorbed vapor significantly alters the local environments of the dye molecules. Thus, the films will be selective only for those vapors whose sorption can cause these spectral changes. In the example given in this paper, a strong hydrogen-bond acidic polymer that was originally designed to promote sorption of basic vapors such as organophosphorus compounds was used as the dye matrix. Since these and many other dyes are bases, this type of polymer can induce significant spectral shifts relative to those of more inert matrixes. Detection of basic vapors is then promoted because such vapors are strongly sorbed and because they compete with the dyes for interactions with the polymer matrix. Using the two-dye approach, we found that fluorescence was greatly amplified relative to single-dye fluorescence sensing and that the selectivity was enhanced. Acknowledgment. The authors acknowledge funding from the U.S. Department of Energy (DOE). I.A.L. and S.G.K. are grateful for funding under the SBIR program, Grant DE-FG0299ER82737. The DOE support does not constitute an endorsement by DOE of the views expressed in the paper. J.W.G. is grateful for funding from the Office of Nonproliferation and National Security, NN-20. The authors acknowledge David A. Nelson of PNNL for the synthesis of the BSP3 sample. The Pacific Northwest National Laboratory is a multiprogram national laboratory operated for the Department of Energy by Battelle Memorial Institute.
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