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Sensors and Actuators B 123 (2007) 59–64

Dye-doped inorganic/organic composite films as fluorescencesensors for methanol vapor

Nathan Stevens ∗, Daniel L. AkinsCenter for Analysis of Structures and Interfaces (CASI), Department of Chemistry, The City College of The City University of New York,

138th Street & Convent Avenue, New York, NY 10031, United States

Received 24 April 2006; received in revised form 24 July 2006; accepted 24 July 2006Available online 1 September 2006

bstract

The sol–gel method has been employed in the fabrication of mesoporous composite films consisting of a non-ionic surfactant, Pluronic P123,s the organic component, and silica as the inorganic component. The hybrid nature of these films resulted in them having an internal structureonsisting of nanometer size self-assembled organic mesostructures surrounded by a silica framework. These films served as the host matrix for theaser dye coumarin 481 (C481), and an energy transfer complex formed between C481 and J-aggregated meso-tetra(4-sulfonatophenyl)porphyrinTSPP). Upon exposure to methanol vapor, a rapid and reversible decrease in fluorescence intensity occurs for films containing C481 alone, as

ell as the energy transfer complex. Steady-state and time-resolved spectroscopic studies indicated that the decrease in fluorescence intensity wasrimarily due to an excited state interaction between methanol and C481. Additionally, morphological changes within the film appear to play aole for films containing the C481/TSPP energy transfer complex.

2006 Elsevier B.V. All rights reserved.

eywords: Coumarin dye; Methanol; J-aggregate; Sol–gel; Composite film; Optical sensor

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. Introduction

Over the years, thin-film technology has been widelymployed in the development of artificial olfactory systems forhe detection of a wide range of volatile organic compoundsVOC) [1–4]. The success of these films is in large part due tohe ability of VOC to rapidly diffuse through and interact withhe intercalated sensing agent or agents, inducing a measurableesponse. The ability to readily deposit such films on a vari-ty of substrates is another factor that has contributed to theiruccessful adoption. Depending on the composition of the film,he sensing response may be optical or electrical in nature. Anlectrical response is most often measured as a change in thelectrical current on exposure to VOC vapors [5,6], while an

ptical response is often a change in absorption intensity at par-icular wavelengths [7,8]. Moreover, the flexibility inherent tohin-film technology has led to continued research in the devel-

∗ Corresponding author. Tel.: +1 212 650 6953; fax: +1 212 650 6848.E-mail address: nstevens@sci.ccny.cuny.edu (N. Stevens).

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925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2006.07.021

pment of films with desirable sensing characteristics such asast response, good analyte discrimination, high sensitivity, andeversibility.

We report here on the changes in fluorescence intensityf a C481 and C481/TSPP energy transfer complex thatre intercalated into mesoporous inorganic/organic compositelms, on exposure to methanol vapor. Films were preparedrom sols that utilized silica serving as the inorganic compo-ent and the nonionic surfactant, Pluronic P123, poly(ethylenexide)–poly(propylene oxide)–poly(ethylene oxide) triblockopolymer (structure shown in Fig. 1), serving as the organicomponent. Steady-state fluorescence measurements indicatehat the methanol vapor is able to rapidly diffuse into the filmsnd cause a reduction in the fluorescence intensity of the sensinggents. Moreover, a combination of time-resolved and steady-tate fluorescence measurements have aided in understandinghe change in fluorescence intensity as primarily due to an

xcited state interaction between methanol and C481. The datalso suggests that for films containing the C481/TSPP complex,change in the film’s internal morphology enhances the response

o methanol vapor.

60 N. Stevens, D.L. Akins / Sensors and Actuators B 123 (2007) 59–64

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. Experimental

The synthesis of the silica/organic sol follows a formulationreviously reported to prepare a pure silica sol [9]. In the presenttudy, 40.0 ml of tetraethylorthosilicate (TEOS, Acros 98%),06.6 ml 200 proof ethanol, 11.4 ml distilled water, and 0.2 mlf HCl (2.6 M) were placed into a 250 ml Erlenmeyer flask andigorously stirred for 1 h. Ten grams of Pluronic P123 (BASF)riblock copolymer paste was then added to the resultant partiallyydrolyzed sol and the mixture was stirred for an additional 4 hntil the sol was visually clear. To aliquots of this sol, C481Exciton) alone, as well as in combination with TSPP (Frontiercientific, Inc.) were added, with sonication used to promoteissolution. The concentration (in mg/ml of sol) of C481 andSPP used were 1.5 and 0.04 mg/ml, respectively. Films were

abricated by drop-casting the dye/sol mixture onto glass cap-llary tubes or glass coverslips. Films deposited on coverslipsere used to conduct the right-angle (RA) and front-face (FF)uorescence measurements. The chemical structures of C481,SPP and Pluronic P123 are shown in Fig. 1.

UV–vis absorption measurements of the coated films andolution samples were performed using a Perkin-Elmer Lambda8 spectrometer while fluorescence spectra were obtained usingn Ocean Optics fiber optic spectrometer (Model HR4000).ime-resolved fluorescence spectra were obtained using a streakamera system described in detail elsewhere [10]. Film thick-ess was measured using a Mikropack NanoCalc-2000 spec-roscopic reflectometer. Surface topology and roughness were

easured using a Thermomicroscope ExplorerTM atomic forceicroscope (AFM). The pore structure of the composite was

etermined from adsorption of nitrogen at its boiling point: aAP 2010 analyzer (Micromeritics) was used for the measure-ents of adsorption isotherms. The pore structure determination

f three composite samples was conducted. One was formedrom the pure sol, while a second contained ∼0.2 wt.% anti-ony doped tin oxide nanoparticles, Nanophase Technologies

NanoTek®), with an average particle size quoted as 20 nm. Thehird sample contained ∼0.2 wt.% of an organic dye.

For determination of methanol sensing properties, a simpleapor generation and fluorescence detection system was utilized.

diagram of this system is presented in Fig. 2. The genera-ion of methanol vapor was accomplished by flowing a carrier

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1, TSPP, and Pluronic P123.

as, in this case dry air, through a 20 ml vial containing ∼10 mlethanol. The vapor/air mixture was directed to the sample cellhich was housed within the sample chamber of a fluorescence

pectrometer. Having the sample cell in this location allowedhe 450 W Xenon light source and monochromator of the spec-rometer to be used as the excitation source. The sample cellas constructed from a 30 mm diameter clear plastic petri dishith three small holes drilled into it. Two of the holes were

or the vapor inlet and outlet tubes. The remaining hole accom-odated a capillary tube onto which the dye-containing filmas coated over part of its length. The capillary tube acted aswaveguide, directing the fluorescence from the intercalated

ye to the collection lens of the Ocean Optics spectrometer.he concentration of methanol vapor was obtained analyticallyy measuring the decrease in methanol volume in the sampleial over a known period of time while maintaining a con-tant flow rate of air at constant room temperature (25 ◦C).or example, a loss of 1.8 ml of methanol after 60 min of airow at a rate of 200 ml/min would result in a concentration

ig. 2. Schematic diagram showing the key vapor generation and fluorescenceetection components of the sensor evaluation system.

s and Actuators B 123 (2007) 59–64 61

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The absorption and fluorescence spectra of the C481 andTSPP are presented in Fig. 4. The absorption maximumof C481 is similar to that in solution [18]. The absorbancespectrum of TSPP, on the other hand, shows clear evidence of

N. Stevens, D.L. Akins / Sensor

. Results and discussion

In deciding on the composition of the thin-film a numberf factors needed to be taken into account. First and foremost,as the ability of the film’s matrix to effectively solubilize theigh concentration (∼10−3 M) of C481 and TSPP used. Thelm also needed to be porous to permit the analyte vapor toapidly diffuse through and interact with the dye. Moreover,ood quality films needed to be fabricated with relative ease,sing the simplest of deposition techniques possible (i.e. drop-asting). As a result of these requirements, the materials that weound most suited for fabrication of the thin-film was a sol–gelerived inorganic/organic composite. This type of compositeas the solubilizing properties of a polymer while retaining theorosity of a sol–gel derived xerogel [11].

Although several organic molecules were tried as the organicomponent for the composite film, the one that resulted in theighest quality film was the nonionic surfactant Pluronic P12312]. One advantage of using this surfactant is that films can beeadily fabricated using the simple drop-casting method. AFMopographic images reveal that the resultant films, whose averagehickness were found to be 6 ± 0.5 �m, had an average RMSurface roughness of only 2.4 nm over a 100�m × 100 �m area.his indicates that there were few surface defects which waso doubt due to the films’ inorganic/organic hybrid nature, withluronic P123 functioning as a drying control agent [13].

Given the concentration of Pluronic P123 used, the qualityf the films should also be related to the specific interactionshat take place between the copolymer and forming silica net-ork. Several studies have shown that Pluronic P123 molecules

elf-assemble into rod-shape nanometer size mesostructureshat serve as templates around which the silica network forms14]. Within the resulting films, these supramolecular assem-lies occupy a significant portion of the overall volume, leadingo distinct hydrophobic domains at the core of Pluronic P123ssemblies. It is these hydrophobic domains that serve as a favor-ble microenvironment in which polar organic dyes can reside.upport for the existence of these mesostructures is providedy the observation that when no dye is added to the sol, theesulting composite is nonporous. Upon dye addition, however,he composite becomes mesoporous, with an average pore sizeistribution of 2.7 ± 0.1 nm.

As a reference system, antimony doped tin oxide nanoparti-les were added to the sol at the same weight percent as was usedor the dye. In this case, the resulting composite was found to bef significantly reduced porosity, suggesting that the hydropho-ic interactions between the dye and the Pluronic P123 drive theormation of the rod-shape mesostructures. The porosity differ-nce between the dye and nanoparticle containing compositess shown in Fig. 3.

Since the primary aim of this study was to use changes in anntercalated dye’s fluorescence intensity as a means to detect theresence of methanol vapor, the highly luminescence laser dye

481 was chosen as a model dye. Prior work has already shown

hat the fluorescence intensity of a closely related dye, coumarin60, was dependent on the methanol concentration in solution15]. Additionally, TSPP was co-intercalated along with C481

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ig. 3. Plot of pore volume versus pore diameter of the composite materials con-aining an organic dye (filled squares) or antimony doped tin oxide nanoparticlesfilled circles) at equivalent weight percent.

n order to form a Coulombic-type energy transfer complex.or this latter case, individual TSPP molecules are expected toelf-assemble into J-aggregate structures [16]. Although these-aggregates strongly absorb in the spectral region where C481uoresces, they have very low radiative quantum yields. Asresult of these properties a long-range energy transfer pro-

ess from C481 (donor) to TSPP J-aggregates (acceptor) shouldesult. Furthermore, due to the distance dependence of such annergy transfer process [17], the fluorescence from the complexhould be sensitive to changes in the film’s internal structure.f the film expands when exposed to methanol vapor then thesewo molecular species will move further apart, likely leading todecrease in the energy transfer efficiency and conversely an

ncrease in the fluorescence intensity; if the film contracts, thenhe opposite would occur. Having such a complex should allowhanges in the film’s internal morphology, in the presence ofethanol vapor, to be actively probed.

ig. 4. Normalized absorption (ab) and fluorescence(fl) spectra of films contain-ng C481 and TSPP. Flourescence spectra are obtained at the both the right-angleRA) and front-face (FF) collection geometry using an excitation wavelength of00 nm.

62 N. Stevens, D.L. Akins / Sensors and Actuators B 123 (2007) 59–64

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decrease in fluorescence intensity is likely due to an excited stateinteraction between methanol molecules and C481 moleculesin the S1 state. Upon photoexcitation, C481 forms radiativeintramolecular charge transfer (ICT) state in which electron

ig. 5. Normalized fluorescence spectra of C481 and C481/TSPP films beforend after exposure to methanol vapor at ∼150 ppm.

-aggregate formation: sharp narrow band centered at 490 nm16]. Evidence for the formation of the energy transfer complexan be seen when the fluorescence spectra of the C481 filmsith and without TSPP are compared. When no TSPP isresent, the fluorescence spectrum of C481 is characterize bysingle broad band, but when TSPP is present there is a sharpip in the spectrum at the wavelength corresponding to the-aggregate (490 nm). Moreover, the shape of the fluorescencepectrum is strongly dependent on the collection geometry.s can be seen, the spectrum acquired using the right-angle

RA) configuration differs from that observed for the spectrumcquired in the front-face (FF) configuration. This is most likelyue to the well-known inner filter effect [17]. In the right-angleonfiguration, the photons that are detected have traveled auch greater distance through the film than those detected in

he front-face configuration. As a result of this, the emittedhotons from the C481 molecules are absorbed and scattered byhe TSPP J-aggregates to a greater degree, leading to significanthanges in the fluorescence band shape. In a sense, the TSPP J-ggregates act as miniature spectral filters for the emitted C481hotons.

The spectral changes that occur on exposure to methanolapor are readily apparent in Fig. 5. As can be seen, only

decrease in the intensity occurs for both the C481 and481/TSPP films. The lack of any shifts in the fluorescenceaxima suggest that methanol vapor simply quenches the flu-

rescence and does not induce any structural changes in thelectronically excited C481 molecules [17].

A histogram showing the variation in the fluorescence inten-ity when a composite film containing C481 is exposed to air thenethanol vapor over several cycles is presented in Fig. 6. The

istogram for films containing the C481/TSPP energy transferomplex is shown in Fig. 7. Each cycle consists of dry air flow,t a rate of 500 ml/min for 30 s, followed by dry air/methanolapor (concentration of ∼150 ppm) for 30 s at the same flowate. A 30 s interval is chosen since no further spectral changes

re observed to take place at this point. The fluorescence inten-ity is obtained by spectral integration in the region from 490o 540 nm. Several measurements were made on each type ofample to ensure the reproducibility of the results.

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ig. 6. Histogram of the response to methanol vapor (black bars) at a concen-ration of ∼150 ppm for a film contaning C481.

When films containing C481 is exposed to methanol vapor,he histogram that results (Fig. 6) indicates a clear response inhe fluorescence intensity. After the initial exposure, the inten-ity decreases by ∼40% and recovers to ∼94% when dry air ishen introduced into the sample cell. This trend is more or lessonsistent over the remaining cycles. A noticeable aspect of thisata is that the fluorescence intensity never fully recovers. Oneikely reason for this is that a small fraction of the methanol

olecules become trapped within the film and cannot be com-letely removed. Such a transient effect has been observed forther methanol sensing films [5]. To test this assertion, dry airas flowed through the sample for an extended period of time

30 min), and resulted in an almost complete recovery of theuorescence intensity (98%). The lack of complete recoveryay indicate an irreversible morphological change occurringithin the film or, possibly, a chemical change in the C481olecules.As previously reported by Dadge et al. [15], the observed

ig. 7. Histogram of the response to methanol vapor (black bars) at a concentra-ion of ∼150 ppm for a film contaning the C481/TSPP energy transfer complex.

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ensity from the nitrogen is transfered to the oxygen atom [18].his results in the nitrogen having a partial positive chargend the oxygen a partially negative charge. Interaction withethanol molecules would help to stabilize this highly polar

xcited state, while likely resulting in an additional nonradiativeecay pathway to the ground state, and thus, a decrease in theuorescence intensity. Direct evidence for the formation of thisCT excited state is provided by time-resolved fluorescenceata. In methanol solution, C481 exhibits a single componentifetime (0.34 ns), but within the film a multicomponent lifetimes found with time constants of 5.29 ns (66%), 1.75 ns (15%),nd 4.78 ns (19%) (data not shown). The 1.75 ns components a rise component and indicates the formation of the radia-ive ICT state from the initial excited state species as timerogresses.

The response to methanol vapor of films containing the481/TSPP J-aggregate energy transfer complex exhibited a flu-rescence intensity decrease whose magnitude is dependent onhe number of exposures to methanol (i.e., the sensing cycles).s can be seen (Fig. 7), on initial exposure to methanol vapor theuorescence intensity decreases to ∼50% of the original, versus60% for films containing only C481. However the enhancedagnitude of this response is not retained and comes into parityith the C481 films over several sensing cycles. Furthermore,

he decrease in intensity at cycle 5 for these films is less thanhat for the C481 films and indicates that as the number of cyclesncreases the enhancement effect of TSPP J-aggregates is lost.

oreover, we found that the fluorescence intensity does notecover to the same extent as for C481 films, despite an extendederiod of dry air flow.

The results described above for the C481/TSPP J-aggregatelm can be explained as due to a morphological change occur-ing within the film’s structure. The initial enhancement inethanol detection can be explained in terms of Forster energy

ransfer theory [17]. According to this theory, the overall energyransfer efficiency from a donor (i.e., C481) to an acceptor (i.e.,SPP J-aggregate) is strongly dependent on the average dis-

ance between the two. Since a small variation in the averageistance results in a substantial change in the energy transferfficiency, any morphological changes within the composite filmhat causes the average distance between the C481 and TSPP-aggregate to vary, would result in a change in the fluores-ence intensity. If the distance between the donor and acceptorecreases, then the fluorescence intensity will also decrease,hile if it increases, the intensity will increase. From the ini-

ial enhanced decrease in fluorescence intensity, compared tolms containing only C481, it appears that a morphologicalhange occurs within the film that brings the C481 and TSPP-aggregates into closer proximity. The loss of this enhancedesponse over several sensing indicates that this morphologi-al change is not fully reversible and helps explain why theuorescence intensity cannot be fully recovered to the initialalue. In other words, over several cycles the average distance

etween C481 and TSPP J-aggregates permanently decreases,hereby increasing the energy transfer efficiency which trans-ates into a reduction in the overall fluorescence intensity of481.

Actuators B 123 (2007) 59–64 63

. Conclusion

Silica/Pluronic P123 nanocomposite mesoporous films wererepared using the sol–gel method. The hydrophobic domainsithin the self-assembled mesostructures act as a favorableicroenvironment for the intercalation of the highly lumines-

ent laser dye C481. The incorporate dye is shown to be ableo indicate the presence of methanol vapor at a concentrationf ∼150 ppm via changes in the fluorescence intensity. The co-ntercalation of TSPP J-aggregates along with C481 into thelms led to the formation of an energy transfer complex thatxhibit an enhanced response to methanol vapor, but only forhe first few sensing cycles, at which point the response returnso a constant value close to that for the system in which only481 is present. This enhancement has been ascribed to a mor-hological change within the film that leads to the C481 (donor)nd TSPP J-aggregates (acceptor) coming into closer proximity,nd thereby decreasing the fluorescence efficiency of the C481s a result of energy transfer to the aggregates. The limitations,uch as sensitivity, long term photostability, and specificity toethanol vapor of these films will need to be determined in

rder to ascertain their potential as commercially viable opticalensors. Quantifying these limitations and studying the sensingesponse of other dye containing composite films towards var-ous VOC will be the focus of future work. Also, modificationf the film’s composition will be attempted so that the enhancedensing response of an energy transfer complex can be retainedver a greater number of sensing cycles.

cknowledgments

We thank Dr. Teresa J. Bandosz for pore structure determi-ation of the composite materials. DLA also thanks the NSFnd DoD-ARO for support of this work, in part, through theollowing awards: (1) the NSF-IGERT program under grantGE-9972892; (2) the NSF-MRSEC program under grantMR-0213574; the (3) NSF-NSEC program under grant CHE-117752; and (4) DoD-ARO under Cooperative AgreementAAD19-01-1-0759 and grant W911NF-04-1-0029.

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iographies

athan Stevens received his BA degree in chemistry from Queens College,ew York, in 2001. He obtained his PhD from the The City University of Nework, Graduate Center, New York, for work on nonlinear photonic films forltrafast optical switching applications in 2006. His current research interest areanoscale photonic materials and optical chemical sensors.

aniel L. Akins is a professor of chemistry at the City College of New Yorksince 1981) and director of the CUNY—Center for Analysis of Structures andnterfaces, a center that conducts research focused on nanomaterials and theirses. Prof. Akins was an undergraduate at Howard University (Washington,C) and received his PhD in physical chemistry (in 1968) from the Universityf California, Berkeley. He served as assistant professor and associate professorf chemistry at the University of South Florida, Tampa, FL, from 1970 to 1977.e then served for 2 years as program officer for the Dynamics Program of

he National Science Foundation, followed by 2 years as a senior scientist athe Polaroid Corporation in Waltham, MA. His research focus is on quantumroperties of molecular nanostructures and the exploitation of such properties forormulating new nanomaterials with uses in molecular photonic devices (MPDs)nd/or chemical sensors.