Photopolymers are used as photosensitive holo-graphic recording materials for holographic opticalmemories [1] and holographic sensing [2]. Comparedto other types of holographic recording materials,photopolymers present many advantages, such asself development, wide spectral sensitivity, and rela-tively low cost [3].
In most cases, the photopolymers used as record-ing materials contain one or more monomers and aphotoinitiator typically consisting of two compo-
nents, a sensitizing agent and an electron donor.The photopolymer components can be spatially redis-tributed when illuminated by an optical interferencepattern created by two or more mutually coherent la-ser beams, resulting in a holographic diffraction grat-ing. The layer regions exposed to bright fringes willbe composed mainly of polymer (in the case of poly-acrylamide with a refractive index, n ¼ 1:5) whilethose exposed to the dark fringes will be depletedof monomer through concentration-gradient-drivendiffusion and, thus, have a lower refractive index.
Different classes of volume holographic recordingmaterials, such as organically modified silica glass[4], sol-gel materials containing zirconium isopropox-ide [5], and photopolymerizable nanocomposites
containing solid nanoparticles, such as SiO2 [6], TiO2[7], and ZrO2 [8,9] as dopants, have been developedin the past decade. These recording materials showimproved holographic properties, such as higher dy-namic range and lower level of shrinkage during ho-lographic recording, which is why they have beenstudied mainly for applications such as holographicmemories. These materials are usually sandwichedbetween two glass slides, which limit their area ofexposure to the surrounding environment conditions,which can be a disadvantage if the intended use isfor sensing chemical compounds present in theatmosphere.
The dark diffusionmechanisms in photopolymeriz-able materials have been studied both experimen-tally [10] and theoretically by several authors[11,12]. In the particular case of photopolymerizablenanocomposites, Tomita [13] proposed that photo-insensitive nanoparticles, which are assumed to bean inactive component, undergo diffusion from thebright to the dark regions. Meanwhile, as monomeris polymerized in bright regions, unused monomerdiffuses from dark to bright regions driven by theresulting concentration gradient. These polymeriza-tion-driven mutual-diffusion processes essentiallycontinue until the photopolymerization is completed.It was assumed that the inorganic nanoparticles areinert and take part only in the mass transport me-chanism during holographic exposure. Recently,Goldenberg et al. [14] observed that inorganic goldnanoparticles are not inert and promote monomerspatial segregation in addition to mass transporteffects observed in the nanocomposite.
In our group, two types of nanocomposites contain-ing colloidal zeolite Beta (BEA-type structure) andzeolite A (LTA-type structure) [15] were prepared.Both types of microporous nanoparticles are zeolites:a class of crystalline aluminosilicalites having athree-dimensional porous structure arising fromcorner-sharing SiO4 and AlO4 tetrahedra [16]. It wasshown that zeolite Beta nanoparticles were redis-tributed as a result of holographic recording [17].The redistribution effect, in combination with thenanoparticles ability to adsorb targeted chemicalsubstances,makes this nanocomposite a suitable can-didate for the design of holographic sensors [18].Moreover, thin films based on zeolites A and Betahave proved to be effective materials for sensing pur-poses [19,20]. High sensitivity, good reversibility, andlong life of zeoliteA-based sensorswere demonstratedfor detection of water at low concentrations. The Betafilms showed a higher sorption capacity towardwatervapor than the zeolite A films [20].
Zeolite Beta is also a promising material for theadsorption of toluene and propene [21,22]. Using to-luene as a probe test, our aim is to detect volatileorganic compounds by the change of the diffractionefficiency of holographic sensors.
This paper reports on development of nanocompo-site holographic recording materials in the form of anacrylamide-based photopolymer containing zeolite
nanoparticles exhibiting properties suitable for sen-sing applications. The interactions between the hostphotopolymer components and zeolite nanoparticlesare studied by UV–visible spectroscopy and carbonnuclear magnetic resonance (13CNMR) spectroscopy.The advantage in the development of this class ofvolume holographic materials is the fact that thepermeable polymer can be coated either in glass orplastic substrates and exposed directly to the envir-onmental conditions.
2. Experimental Section
A. Materials
The photosensitive nanocomposite material consistsof a solution of acrylamide-based photopolymer andzeolite A and Beta nanoparticles. Zeolite Beta is alarge-pore microporous material characterized bythree sets of mutually perpendicular channels with12-membered ring apertures [23] (pore sizes of 6:4 ×7:6 Å and 5:5 × 5:5 Å), while zeolite A is a small-porematerial with a pore diameter size of 4:2 Å [15].
The photopolymer composition for holographicrecording was described previously [24]. In thisstudy, 2:0 ml of triethanolamine (TEA) (Sigma) wasadded to 9 ml of a polyvinyl alcohol (PVA) solution(20 wt: %) (Sigma), then the monomers were added,i.e., 0:6 g of acrylamide (AA) (Aldrich) and 0:2 g ofN;N0-methylenebisacrylamide (BA) (Sigma). The fi-nal solution was stirred for 20 min, 4 ml of erythro-sine B solution (0:11 wt: %) (Aldrich) were added,and the resulting mixture was stirred for an addi-tional 10 min.
The nanosized zeolite crystals were synthesizedfrom clear precursor suspensions containing organictemplates: tetramethylammonium hydroxide (TMA·5H2O) in the case of zeolite LTA and tetraethylam-monium hydroxide (TEAOH) in the case of zeoliteBEA. The synthesis procedures for zeolite A andBeta are described in detail in [25,26], respectively.The crystalline nanoparticles were extracted fromthe reaction mixture by a three-step centrifugation(20; 000 rpm for 180 min) and, subsequently, redis-persed in distilled water to obtain stabilized zeolitesuspensions in water with a solid concentration of1 wt: %. Dynamic light scattering (DLS) measure-ments using a Malvern Zetasizer Nano ZS werecarried out to determine the mean hydrodynamicdiameter of the crystalline zeolite particles. The mor-phology of the crystals and film thickness were char-acterized using scanning electronic microscopy(SEM, Philips XL 40).
Before use, the aqueous nanoparticle suspensionswere sonicated for 20 min to obtain a homogeneousparticle size distribution and to redisperse the ag-glomerates. These suspensions were added to thephotopolymer solution and themixturewas sonicatedfor an additional 10 min. Different concentrations ofnanoparticles were introduced in the dry layers(Table 1). To obtain similar viscosities of the solutionscontaining different concentrations of nanoparticles,
deionized water was added to the photopolymer solu-tions. DLS measurements of the suspensions wereperformed immediately and again after 24 h to checktheir stability.
The photopolymerizable nanocomposite films wereprepared by spreading 0:4 ml of the suspensionson glass plates with dimensions of 26 mm × 38 mm,followed by drying on an optical table for 24 h.
B. Measurements
Unslanted volume holographic transmission grat-ings were recorded by exposure of the holographicrecording material to two mutually coherent s-polarized beams of wavelength 532 nm. The totalexposure energy was 600 mJcm−2 at a spatial fre-quency of 1000 l mm−1. The grating growth wasmon-itored in real time by probing with an unexpandedHe─Ne laser of 633 nm wavelength, incident atthe Bragg angle. The sample was held on a rotationstage, allowing the grating angular selectivity to bemeasured, at the completion of exposure, by adjust-ing the incident angle of the probe beam. The angularselectivity curve was used to calculate the effectivethickness of the grating. This parameter and themaximum diffraction efficiency, η (here defined bythe ratio of the intensity of the first diffraction orderand the incident intensity of the probe beam) wereused to calculate the refractive indexmodulation am-plitude, n1, using Kogelnik’s coupled-wave theory[27]:
n1 ¼ λ cos θ arcsin ffiffiffiηpπd ; ð1Þ
where λ is the reconstructing beam wavelength, θ isthe reading beam incidence angle, and d is the thick-ness of the photosensitive layer.
The absorption spectra of solutions were recordedusing a Perkin-Elmer Lambda 900 UV/VIS/NIRspectrometer. Solutions of erythrosine B in water,photopolymer, and water solutions containing onlyone of the matrix components (PVA 10%, acrylamide,or TEA) were spectrally characterized.
The solid-state 13Cmagic angle spinning (MAS) H-decoupled NMR measurements of the samples wereperformed using a Bruker Biospin 400 spectrometer,
operating at a resonance frequency of 100:62 MHz.All spectrawere recordedat a spinning rate of12 kHz,recycle delay of 60 s, and pulse length of 3:8 μs.
To evaluate the sensing properties of thematerials,recorded volume gratings of acrylamide-based photo-polymer and Beta nanocomposite were exposed totoluene vapors (and their efficiency was measuredagain by means of the angular selectivity curve. Thegratingswere placed ina container intowhich toluenewasadded and the container sealed for theduration ofthe exposure (10 min). This experiment was carriedout in a fume hood, and contact from ignition sourceswas prevented. The concentration of toluene insidethe volume of the container was estimated to be19 ppm.
3. Results and Discussion
A. Characterization of PhotopolymerizableNanocomposites
The morphology of the zeolite nanocrystals (A andBeta) prior tomixingwith the photopolymer solutionswas characterized by DLS and scanning electron mi-croscopy (SEM) (Figs. 1 and2). As can be seen, the sizeof the crystals is below 100 nm for both types of zeo-lite, and both materials exhibit monomodal particledistribution. Zeolite Beta and A have spherical andalmost cubic shape, respectively (Fig. 1).
Table 1. Compositions Used for Preparations of PhotopolymerizableNanocomposite (NC) Layers
The size of the zeolite particles and their stabilityin the photopolymer suspensions were characterizedby measuring the DLS curves for the photopolymer-izable nanocomposites. No aggregation of the zeolitenanoparticles directly after mixing and after 24 haging was observed (Fig. 2). These results demon-strate that the zeolite nanoparticles are compatiblewith the photopolymer; thus, homogeneous coatingsuspensions and dry layers are obtained.
Furthermore, the surface morphology of the filmswas studied using SEM. The SEM picture taken fromthe film with zeolite Beta loading of 5 wt:% is shownin Fig. 3(a). It can be seen that the nanoparticles arerandomly and homogeneously distributed within andat the surface of the film. The thickness of the film isabout 50 μm as estimated from the scratch (whiteportion shows the glass substrate) in the SEM pic-ture [Fig. 3(a)]. Moreover, the zeolite photopolymer-izable nanocomposite containing both types of zeoliteparticles yield good optically transparent films,which are thus suitable for optical applications. Thisis further supported by Fig. 3(b), which shows an ac-rylamide-based photopolymer containing zeolite Ananoparticles (5 wt:%) assembled in film on glasssupport.
B. Holographic Properties of PhotopolymerizableNanocomposites
Holographic properties, i.e., diffraction efficiency,thickness, and refractive index modulation, of thephotopolymerizable nanocomposites assembled inthe films with different concentrations of zeolitesBeta and A were studied. As can be seen fromFig. 4(b), the diffraction efficiency (η) of the gratingsfor Beta-loaded nanocomposites increases with theconcentration of the particles up to 5 wt:%. The max-imum value of η is about 35% for the film with athickness of 40 μm [Fig. 4(a) (squares)]. The thick-ness of the films increases from 20 to 50 μm as theconcentration of zeolite Beta increases from 0 to5 wt:%. However, the thickness of the films dopedwith zeolite A nanocrystals is constant at 30 μm.There is a slight improvement of the refractive index
modulation for the films with small additions ofzeolite A nanoparticles [Fig. 4(c) (circles)], and theoverall the refractive index is higher than in the caseof the addition of the same concentration of Betananoparticles [Fig. 4(c) (circles)].
C. Influence of the Matrix on Grating Formation inPhotopolymerizable Nanocomposites
To determine how the presence of zeolite nanoparti-cles affects the holographic properties of acrylamide-based photopolymer, we compared the recordingcharacteristics of layers having different concentra-tions of polymer components. Layers were preparedusing stock solutions with 5, 10, and 20 wt:% polyvi-nyl alcohol (PVA). The compositions of the differentsuspensions are presented in Table 2. The refractiveindex modulation determined for all samples areshown in Fig. 5.
The layers showed lower refractive index modula-tion with increasing PVA concentration, which isexpected because monomer concentration in thedry layer is 20� 0:8 wt: % in the case of the 5%
Fig. 2. DLS curves (number weighted) of pure zeolite suspensions(black), acrylamide-based photopolymer and zeolite nanoparticlesfreshly mixed (gray), and after 24 h at ambient conditions (graywith stars).
Fig. 3. (Color online) (a) SEM of the acrylamide-based photopo-lymer layer (50 μm thickness) doped with Beta nanoparticles(5 wt:%); the scale bar is 50 μm. (b) Optically transparent filmof acrylamide-based photopolymer doped with zeolite A nanopar-ticles (5 wt:%).
PVA stock solution, 16� 0:6 wt: % for 10% PVA stocksolution, and 12� 0:3 wt: % for 20% PVA stock solu-tion. These standard deviations account for similarmonomer concentrations when using the concentra-tion of the PVA stock solution. Despite this fact, wehave observed different rates of decrease of therefractive index modulation with the increasingconcentration of nanoparticles for the two nano-composites. For the nanocomposite containing Beta[Fig. 5(a)], the decrease is greater than the one ob-served in layers containing the same percentage ofzeolite A [Fig. 5(b)].
Because the observed difference in the decrease inthe refractive index modulation for the doped andundoped layers cannot be accounted for by differ-ences in the monomer concentration, one can suspectthat it could be caused by the TEA presence. There isaround a 10% TEA concentration difference in thenanocomposites concentration prepared with 20%PVA stock solution and with 5% PVA stock solution.This hypothesis is investigated Subsection 3.D.
D. Interactions Between Photopolymer Components andZeolite Nanoparticles Studied by Visible Spectroscopy
The interactions between zeolite nanoparticles (Aand Beta) and the photopolymer components werestudied by means of visible spectroscopy. Typical ab-sorption spectra are shown in Fig. 6(a) for aqueous
solutions of pure erythrosine B (sensing dye) and amixture of erythrosine B and zeolites. No shift inthe absorption spectra of the dye was observed afterthe addition of the zeolite nanoparticles, indicatingthe absence of interaction between the dye and thetwo types of nanoparticles.
Spectra were also taken from photopolymer solu-tions. The shapes of the spectra remained the samebut a redshift of the maximum of the absorption peakis observed, by 9 and 10 nm in the case of solutionswith Beta and A, respectively. This is expected be-cause the absorption maxima of dyes are dependenton solvent polarity. In non-hydrogen-bond donatingsolvents, solvation of dye molecules probably occursvia dipole–dipole interactions, whereas, in hydro-gen-bond donating solvents, the phenomenon is morehydrogen bonding in nature [28,29]. Studies inxanthenes dyes, such as eosin and erythrosin, showedthat the maximum absorption peak in aqueous solu-tions shifts to higher wavelengths in ethanol, and thesame type of solvent dependency of the absorptionspectra was observed for hidroxyxanthenes, in whichthe shift in the absorption maximum is due to hydro-gen bonding [30].
Different components of the photopolymer (one ata time) were added to the solution of zeolite anderythrosine B [Fig. 6(b)]. It was observed that the ad-dition of acrylamide to the water dispersion of zeolite
Table 2. Composition of Photopolymerizable Layersa Prepared with Different Concentrations of Matrix Componentsb
Matrixcomponent
Matrix with 20% PVA Matrix with 10% PVA Matrix with 5% PVA
aDry content wt:%.bConcentrations of matrix components: (1) undoped layer; (2) nanocomposite containing concentration 1 of zeolite; (3) nanocomposite
containing concentration 2 of zeolite. Zeolite is either Beta or A.
Fig. 4. Holographic properties of zeolite nanocomposites: (a) thickness of layers, (b) diffraction efficiency, and (c) refractive indexmodulation. Gratings recorded at 1000 l mm−1, exposure energy of 600 mJ cm−2, and using 20% PVA solution.
nanoparticles and erythrosine B dye leads to a 2 nmredshift in the erythrosine B absorption peak in bothsolutions containing zeolite Beta and A nanoparti-cles. This indicates that a similar change of the sol-vent polarity after acrylamide additions occurs inboth solutions. A redshift of 5 nm in the absorptionpeak for zeolite Beta was detected in the presence ofTEA, but there was no change in the case of zeolite Asolution. This indicates that there is a difference inthe interaction between the two types of nanoparti-cles and the TEA molecules. Both solutions (waterdispersions of Beta and zeolite A) showed a largerredshift in the absorption peak of erythrosine B inpresence of PVA (16 nm in the case of zeolite Betaand 12 nm for zeolite A).
E. Interactions Between Photopolymer Components andZeolite Nanoparticles Studied by NMR
To further clarify the nature of the difference be-tween the two nanocomposite layers of acrylamide-based photopolymer with 5 wt:% of Beta and zeoliteA nanoparticles, the layers were studied by 13CNMRand compared with undoped layers (Fig. 7). In thisspectroscopic technique, the interaction of the nucleiof carbon isotopes with a static magnetic field can bestudied and one should expect to distinguish organic
molecules incorporated or adsorbed in the zeolite fra-mework on the basis of their chemical shifts in the13C NMR spectra.
In Beta containing nanocomposite, the two peakscorresponding to TEA are shifted to a lower field ashighlighted by the arrows in Fig. 7(a), indicating aninteraction (possibly hydrogen bonding) between theTEA and the nanoparticles. However, no shift in the13CNMR peaks is observed in the zeolite A nanocom-posite. The spectra were collected outside and insidethe grating area. As can be seen, a more pronouncedshift in the C peak is obtained for Beta–photopoly-mer inside the grating, thus confirming the stronginteractions between the particles and the photo-polymer.
F. Photopolymerizable Nanocomposites for Sensing ofToluene
Unslanted volume holographic transmission diffrac-tion gratings in photopolymerizable materials re-corded as described in Subsection 2.B were testedas sensors [11]. The principle of operation of this typeof sensor is based on a change of the diffractionefficiency (η).
Fig. 5. Refractive indexmodulation of acrylamide-based photopo-lymer nanocomposites containing nanoparticles of (a) zeolite Betaand (b) zeolite A. The layers were prepared with PVA stock solu-tions with a concentration of 5, 10, and 20 wt: % PVA.
Fig. 6. (a) Visible absorption spectra of erythrosine B in water(black), aqueous solution of zeolite A (light gray), and aqueous so-lution of zeolite Beta (dark gray). (b) Change in the position of theabsorption maximum in aqueous solutions containing erythrosineB and zeolite Beta or A when compared to aqueous solution in thepresence of acrylamide—AA, TEA, and PVA.
Diffraction efficiency can be obtained from Eq. (1):
η ¼ sin2
�n1πdλ cos θ
�: ð2Þ
Deriving Eq. (2), we can estimate the change in dif-fraction efficiency:
Δη ¼ sin�2n1πd
λ
� πcos θλ
�dΔn1 þ n1Δd −
�n1dλ
�Δλ
�;
ð3Þ
where n1 is the refractive index modulation, d is thethickness of the layer, λ is the wavelength of light,and θ is the angle between the two recording beams.
A change in the diffraction efficiency is caused by achange of a thickness (Δd), which is due to shrink-age/swelling of the layer or is caused by the redistri-bution of the nanoparticles during the recording ofthe holographic diffraction gratings.
Chemical vapors, such as acetone (n ¼ 1:36),chloroform (n ¼ 1:45), and toluene (n ¼ 1:50) havebeen reported [31] to change the overall average re-fractive index of a reflection grating recorded in un-doped acrylate polymer. Most importantly, this isachieved in the case of transmission gratings, where
the diffraction efficiency does not depend on thisparameter.
Both undoped acrylamide and Beta photopolymer-izable nanocomposite were exposed to toluene, andtheir angular selectivity curves were obtained beforeand after exposure (Fig. 8). The results are summar-ized in Fig. 9. As can be seen, the nanocompositegrating exhibits a greater change in diffraction effi-ciency than the undoped photopolymer grating. Forgratings made in undoped photopolymer, with an in-itial diffraction efficiency ranging from 20% to 80%, achange in the diffraction efficiency of less than 10%occurred after exposure to toluene (the maximum ob-served was Δη ¼ 9:8% for a grating with initial dif-fraction efficiency of 58%). As discussed previously,the change in the diffraction efficiency in undopedlayers is possibly due to a toluene-induced dimen-sional change of the layer since one should not expecta change in the refractive index modulation. Moredetailed studies are in progress to identify the exactmechanism of diffraction efficiency change of trans-
Fig. 7. 13C NMR spectra of acrylamide-based photopolymer con-taining (a) zeolite Beta and (b) zeolite A nanoparticles: 1, undopedacrylamide-based photopolymer; 2, zeolite-doped photopolymer(outside grating); 3, zeolite-doped photopolymer (inside grating).Spatial frequency 1000 l mm−1 and recording energy of600 mJ cm−2.
Fig. 8. Angular selectivity curves measured before and after ex-posure to toluene (19 ppm) for the highest change in the diffractionefficiency observed: (a) undoped photopolymer; (b) photopolymercontaining Beta nanoparticles.
mission gratings in undoped layers exposed to to-luene. The gratings recorded in the composite layerscontaining 5% Beta nanoparticles, exhibit a maxi-mum diffraction efficiency change of 22% (double thevalue observed for undoped layers) in a range of in-itial diffraction efficiencies ranging from 20% to 60%.An improvement in the sensitivity toward toluene forBeta–photopolymer composite (Fig. 9) is observed. Itis also worth noting that the higher the initial dif-fraction efficiency of the doped layers is, the greaterchange caused by the exposure to toluene. This canbe explained by the fact that, in higher diffraction ef-ficiency gratings, the redistribution of nanoparticlesis more effective—that is, there is a greater differ-ence between the concentrations of nanoparticles inthe bright and dark regions. It is also expected thatthe refractive index in the bright regions is higherthan the refractive index in the dark regions rich inzeolite nanoparticles. Because of adsorption of to-luene in the nanocomposite grating, the refractiveindex of the dark regions is increased. Thus, the re-sulting refractive index modulation, which is definedas the difference between the refractive index in thebright and in the dark regions, decreases. This ulti-mately is observed as a decrease of the measured dif-fraction efficiency. The observed decrease is expectedto be higher when nanoparticle redistribution ismore effective, which is confirmed by the experimen-tal data shown in Fig. 9.
4. Conclusions
Zeolite–photopolymer nanocomposites with good op-tical quality were prepared by combining differentconcentrations of zeolite nanoparticles (A and Beta)with various concentrations of a photopolymer bin-der. The addition of zeolite Beta to an acrylamide-based photopolymer leads to an interaction between
the nanoparticles and the TEA photopolymer compo-nents. However, this interaction does not occur in thecase of zeolite A nanoparticles, which can be ex-plained by the high hydrophilicity and small poresof this zeolite. The Beta nanoparticles act as noninertcomponents during holographic recording, bondingto the TEA electron donor molecules, while no evi-dence of these interactions was found regarding zeo-lite A. These interactions could be responsible for thedifferent holographic characteristics observed for thetwo types of nanocomposites. First, the layer thick-ness increases and, second, the diffraction efficiencyis almost doubled (5 wt:% BEA concentration) incomparison with the undoped photopolymer. This ef-fect is exploited for the fabrication of holographicsensors. The addition of 5 wt:% Beta to the polymerlayers yields to an increase of the transmission grat-ing sensitivity toward toluene in comparison to theundoped photopolymer.
This work was supported by the Technological Sec-tor Research: Strand I—Post-Graduate R&D SkillsProgramme. The authors acknowledge the Facilityfor Optical Characterization and Spectroscopy at Du-blin Institute of Technology for technical support andS. Martin for constructive suggestions.
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