Hybrid Sensors Fabricated by Inkjet Printing and Holographic

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Hybrid sensors fabricated by inkjet printing and holographic patterning Izabela Naydenova, Julien Grand, Tatsiana Mikulchyk, Suzanne Martin, Vincent Toal, Veselina GEORGIEVA, Sebastien Thomas, and Svetlana Mintova Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b02629 • Publication Date (Web): 10 Aug 2015 Downloaded from http://pubs.acs.org on August 15, 2015

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Hybrid sensors fabricated by inkjet printing and holographic patterning Izabela Naydenova1 *, Julien Grand2, Tatsiana Mikulchyk1, Suzanne Martin1, Vincent Toal1, Veselina Georgieva2, Sebastien Thomas2, Svetlana Mintova2 * 1

Centre for Industrial and Engineering Optics, School of Physics, Dublin Institute of Technology, Kevin Street, Dublin 8, Ireland. 2 Laboratoire Catalyse & Spectrochimie, ENSICAEN, Université de Caen Basse-Normandie, CNRS, 6, boulevard du Maréchal Juin, 14050 Caen Cedex, France

ABSTRACT: Inkjet printing and patterning strategies have been developed for fabrication of hybrid holographic sensors using zeolite nanocrystals on glass-supported photopolymers. The flexibility of the proposed techniques was demonstrated by fabrication and characterization of two types of holographic sensors. The first type, which is a reversible sensor, is based on a transmission hologram recorded in a hydrophobic MFI-type zeolite doped layer with high sensitivity toward alcohols. In this type of sensor the patterning of the zeolite nanocrystals in the volume of the polymer layer is achieved by holographic recording; the pattern periodicity is in the sub-micrometer range. The second type of sensor is based on a reflection hologram and it is produced by inkjet printing of zeolite nanocrystals on photopolymer layer before holographic recording. The resulting localized presence of zeolite nanocrystals in the layer is key for the performance of the sensor. Irreversible humidity sensors based on photopolymer layers doped with hydrophilic EMT-type zeolite are fabricated using the second approach and characterized in a controlled humidity environment. We demonstrate that the inkjet printing approach enables fabrication of a variety of patterns with high precision and uniformity, using zeolite nanocrystals (10-50 nm sizes). Limitations and future directions of this fabrication technique are discussed.

The development of photopolymer-based holography has received great attention due to the high sensitivity, selfprocessing nature and relatively low cost of production of photopolymer materials, which make them a material of choice for applications such as holographic sensors, diffractive optics and data storage.1-6 Some recent holographic sensors rely on the development of hybrid devices combining organic and inorganic nanomaterials, which enhance the holographic properties of the recording material.7,8 The use of nanodopants in the photopolymers enables change of the refractive index modulation of the photopolymer layer in the presence of analytes.1 The introduction of zeolites as nanodopants in the holographic recording medium was previously reported; the formation of zeolite-based photonic structures allows for a change in their optical properties in the presence of the analyte.10–12 This kind of application requires the use of zeolite nanocrystals with uniform diameter, controlled size and high stability in water or ethanol suspensions. A decrease in the particle size from micro-size to nano-size leads to crucial changes of their properties: larger surface areas and lower diffusion rate limitations are among the numerous properties of nanosized zeolites.13-15 The zeolite nanoparticle size is also very important for achieving high optical quality layers with minimal scattering. Zeolite crystals are defined as crystalline materials with 1D or 3D framework structures that form regular and uniform pores.16,17 Zeolite crystals consist of tetrahedral units (T = Si, Al) bonded by oxygen atoms resulting in variable frameworks consisting of cages and channels of distinct sizes and shapes.

Thanks to this well-defined porous structure and their hydrophilic/hydrophobic nature, they are selective sorbents for organic molecules that are stable in gas and liquid phases. In addition, their rigid crystalline nature (uniform channels and pores) permits their use as molecular discriminant, a property useful to improve the selectivity of a sensing material.18, 19 The aim of this work is to reveal the potential and the current limitations of fabrication of holographic sensors based on zeolite nanocrystals. For the first time we demonstrate inkjet printing strategy for deposition of a range of nanosized zeolites for holographic sensors application. The focus is on the preparation of low cost sensors for environmental and humidity monitoring. The use of an inkjet system will enable printing zeolite nanoparticles (diameter of 10-50 nm) in a variety of patterns with high precision and uniformity. When applied to holographic sensors the zeolite nanocrystals can be added to photosensitive materials such as photopolymers. The zeolite nanocrystals can be included as dopants in photopolymer layers using two techniques; either by direct addition to the photopolymer solution before layer fabrication or deposition on the dry photopolymer layer by ink jet printing. Deposition of the nanoparticles by ink jet printing is particularly useful for fabrication of sensors based on reflection holograms. For sensors based on transmission holograms due to the current limitations of the resolution of the printers, ink jet printing does not bring significant advantage in the sensor fabrication. For this reason currently for the preparation of transmission holographic sensors, the nanoparticles are added directly to the photopolymer solution. Initially they are distributed homoge-

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nanocomposite on a glass substrate and drying for 24 hours at room temperature. Both types of layers were photosensitive and the drying process was carried out in the dark. Subsequently, the zeolite suspension was inkjet printed on the glass supported photopolymer layer (Figure 1, Step 2). After diffusion and evaporation of the solvent (water), the photopolymer was exposed to a specific light pattern. Transmission holographic gratings were recorded by exposure to laser light of wavelength 532 nm and intensity 5 mWcm-2 for 60 s. Reflection gratings were recorded by exposure to light of 633 nm wavelength and intensity 1.5 mWcm-2 for 100 s. This choice of recording conditions has been made after carrying out initial optimization experiments. The principle of operation of zeolite containing holograms can be explained as follow. In the presence of analytes (water or hydrocarbons), the zeolite rich areas will change their refractive index differently than the areas with no zeolite nanocrystals and this will lead to change of the properties of the hologram either change of diffraction efficiency or change of spectral characteristics of the diffracted light. The sensitive devices will be used as transmission or reflection holograms utilizing the main advantage of each of the two types. In transmission mode, the analyte absorption can be monitored in real time while the reflection mode permits visual detection as shown in Figure 2.

neously in the layer, but after holographic recording the nanoparticles are patterned as demonstrated before.7 A holographic structure consisting of periodically redistributed zeolite nanocrystals is capable of changing its optical properties under adsorption of a target analyte.9 The principle of operation of holographic sensors is described in details in Ref. 1. Briefly, the factors that can influence the spectral characteristics and the diffraction efficiency of the holographic gratings are change in the refractive index modulation, effective refractive index and the layer’s thickness. Depending on the geometry of recording and the type of holographic recording material one of these factors will usually dominate the response.1 The nanoparticles used in this study were MFI- and EMT- type zeolites. MFI- doped photopolymer layers for holographic recording were studied previously and it was observed that the MFIzeolite pores remain empty.7,20 This is beneficial for fabrication of sensors, as the pores are available to the studied analyte. Reversible sensors made using transmission holograms containing pure silica hydrophobic MFI-zeolite doped layers are expected to have high sensitivity toward hydrocarbons (alcohols) and not be influenced by water while the EMT- type zeolite crystals are well known for their large water capacity and therefore were used for fabrication of irreversible humidity sensors. In addition, both nanosized zeolites are highly crystalline (SI, Figure S1) and thermally stable (Figure S2). They have high micropore volume and high external surface area (Figures S3, Table S1). The stability of zeolite nanocrystals with monomodal particle size distribution in the colloidal suspension allows their long-term storage and deposition in films with controlled thickness and variable shapes (Figure S4).

Figure 2. Principle of operations of zeolite containing holograms in (left) transmission and (right) reflection modes. In the case of the transmission holographic grating, the diffraction efficiency before exposure to the analyte (η1), defined as the ratio of Id and It, is a function of the thickness of the layer (d1), the refractive index modulation (∆n1) and the

Figure 1. Fabrication of the holographic sensors: (left) direct addition of zeolite nanocrystals to the photopolymer solution for preparation of transmission holographic sensors, and (right) inkjet printing of zeolite nanocrystals for preparation of reflection holographic sensors. I0, Id and It correspond to the intensity of the incident light, diffracted light and transmitted light, respectively.

fringe spacing (Λ1). The direction of the diffracted light is determined by the Bragg angle (θ1). The interaction with the analyte leads to alterations in the direction of the diffracted light and the diffraction efficiency. Under exposure, the diffraction efficiency (η2) is determined by d2, ∆n, Λ2 and the

The different steps for the preparation of the holographic sensor are schematically shown in Figure 1.

maximum intensity of the diffracted light is observed at θ2.

A photopolymer solution containing two monomers (acrylamide and N, N’-methylenebisacrylamide), a free radical generator (triethanolamine), a binder (polyvinyl alcohol) and a dye sensitizer (Erythrosine B) was prepared following the procedure described in Ref. 7. The layers used for recording of transmission holograms were prepared by addition of zeolite nanocrystals to the photopolymer solution, deposition of the

When the reflection holographic grating is illuminated with white light, the light with the wavelength λ1 is diffracted in a particular direction. The wavelength of the observed light depends on the effective refractive index n1, Λ1 and θ1. Under exposure to the analyte, the sensitive medium shrinks or swells

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leading to alterations of the fringe spacing, the Bragg angle and the effective refractive index. Thus, the wavelength of the diffracted light (λ2) is determined by n2, Λ2 and θ2. The obtained MFI-zeolite holograms were used for the detection of four alcohols including methanol, ethanol, butanol and iso-propanol. The experimental set-up used to test the samples when exposed to the gas analytes is shown in Figure S5. In a gas chamber, the sample was exposed to each alcohol vapor as a single analyte by controlling a constant atmosphere of 32 Torr. The adsorption of each analyte on the hologram was monitored by recording the diffraction efficiency response of the MFI-zeolite doped transmission grating. Obtained normalized diffraction efficiency spectra at different exposure times to different analytes are shown in Figure 3.

Figure 4. Diffraction energy vs. time recorded in MFI- zeolite doped photopolymer layer for methanol, ethanol, iso-propanol, and butanol.

The fastest responses, less than 1 minute, for the sensors were obtained by their exposure to methanol and ethanol, while more than 2 minutes were required for iso-propanol. This difference could be explained by a higher diffusion rate of methanol and ethanol in the zeolite nanocrystals compared to the slower diffusion of iso-propanol. Concerning the butanol, the response and the saturation of the diffracted energy appeared almost at the same time (2 minutes) with a smalldiffracted energy decrease of 0.25. In addition, it was demonstrated that this holographic sensor could selectively detect alcohols with negligible interference from aromatic hydrocarbons such as xylene. Indeed, the exposure to xylene has also been monitored and no change in diffraction efficiency has been observed (Figure S6). In summary, reversible selective sensing toward alcohols using holographic transmission grating containing MFI-zeolite nanocrystals has been demonstrated. The observed change is fully reversible as the initial diffraction efficiency was restored after the analyte vapour pressure was set at 0 Torr (Figure S7). In addition, the size selectivity of the holographic transmission sensor is demonstrated, i.e., higher and faster detection of methanol in comparison to the other alcohols with longer alkyl chains is observed. The ink-jet printing approach permits deposition of zeolite nanocrystals (different shapes and sizes) in various patterns by controlling the amount of zeolites deposited and even the deposition of more than one zeolite type on the same photopolymer sensor is possible. Several types of zeolite nanocrystals including EMT-, LTL-, AEI- and MFI- were successfully deposited on the photopolymer layers by inkjet printing. Herein, the EMT- type zeolite nanocrystals will be presented due to their high selective sorption properties and small crystalline sizes (10-50 nm) that allow homogeneous deposition via ink jet printing. The EMT-type zeolite nanocrystals were dispersed in water by sonication followed by filtration to obtain a homogenous coating suspension. The resulting suspension of zeolite nanoparticles was ink-jet printed on the glass supported photopolymer layer. Concentric circles were used as pattern for the EMT- zeolite nanocrystals. In all cases, homogeneous

Figure 3. Alcohols response of a transmission grating recorded in MFI- zeolite doped photopolymer layer: (a) methanol, (b) ethanol, (c) iso-propanol, and (d) butanol.

The diffracted light was recorded in the visible spectrum around 600 nm, and initial unexposed spectra were normalized to a diffraction efficiency of 1. As shown, the diffraction efficiency responses of the sensors are significant under exposure to the analytes. It was noticed that only methanol resulted in an increase of the diffraction efficiency up to 3.2, while the other analytes produced a decrease in the diffraction efficiency (Figure 4). Indeed, the diffraction efficiency decreased down to 0.75, 0.2 and 0.2 for butanol, ethanol and iso-propanol, respectively. In order to explain the sensor response two main contributors to the diffraction efficiency change have to be considered: (i) adsorption of the analyte on the zeolite nanoparticles and (ii) swelling of the layer causing change in the thickness of the hologram. The adsorption of the analyte in the zeolite doped photopolymer layer is expected to decrease the diffraction efficiency of the grating (as observed in the case of ethanol, butanol and iso-propanol) while the swelling of the layer will cause increase of the diffraction efficiency. The response of the sensor to methanol implies that significant swelling of the layer takes place, which can be explained by the smaller size and larger polarity of the methanol molecule. Such significant swelling is not observed under exposure to other alcohol vapors with longer alkyl chains. This property of the photopolymer containing zeolite nanocrystals is considered as a sign of size selectivity.

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for 10 min the relative humidity in the chamber was brought back to 20%. This time the sensor was visible in green color (where the zeolite nanoparticles were deposited), Figure 5c. The reason for this is the absorption of water by the zeolite nanoparticles and the increase of their effective refractive index. The refractive index of the areas without nanoparticles remains the same, thus they appear relatively colorless. The appearance of the sensor at different RH demonstrates clearly its irreversible water sensing properties. In conclusion, suspensions of nanosized EMT- and MFItype zeolite nanocrystals with particle size ranging from 10 to 50 nm have been used for the preparation of holographic sensors by inkjet printing and holographic patterning strategies. The zeolite nanocrystals of different shapes and sizes were deposited on a photopolymer layer supported on glass in accurate patterns and in controlled amounts. The MFI-zeolite holograms were used for reversible detection of alcohols. The response to four alcohols (methanol, ethanol, butanol and isopropanol) demonstrated that these transmission gratings were selective towards alcohols. Methanol produced the most pronounced diffraction response, while towards ethanol the fastest response was measured. In addition, the EMT- zeolite doped hologram device was prepared by inkjet printing and used for irreversible detection of water. After exposure to 80% RH, dehydrated EMT-type zeolite films irreversibility absorbed water resulting in an irreversible color change of the sensor from colorless to green. Moreover, this color change is visually detected. Holographic sensors still are in development. However, the introduction of zeolite nanocrystals on photopolymer layers offers great advantages to control the performance and properties of the holographic sensors. At the same time, the inkjet printing strategy ensured controlled, precise and reproducible deposits of the nanosized zeolites. The holographic devices based on nanosized zeolites presented herein open new routes for nanosized zeolites in applications concerning optical quality thin films and hybrid organic/inorganic nanomaterial based holographic devices.

deposits with a controlled thickness of 50-350 nm were obtained, as shown in the SEM images (Supporting information, Figure S8). Ink jet printing of highly hydrophilic EMT-type zeolite nanocrystals on holographic sensors with efficient irreversible absorption of water is presented in Figure 5. The high sorption capacity and fast response for pure EMT- zeolite was studied initially (see the adsorption isotherm in Figure S9). Then the holographic sensors incorporating EMT-type zeolite were exposed to a change in relative humidity, and the sensor gave a clear visual response as shown in Figure 5.

ASSOCIATED CONTENT Supporting Information. Experimental section included the synthesis procedures and characterization for nanosized zeolites assembled in films by ink jet printing. This material is available free of charge via the Internet at http://pubs.acs.org. Figure 5. Holographic sensors based on EMT-zeolite doped layers exposed to relative humidity (RH) (a) 60% (as prepared), (b) 20% (sensor annealed at 120 °C for 20 minutes), and (c) 20% (relative humidity in the chamber was brought back to 20% after exposure to 80% RH for 10 min).

AUTHOR INFORMATION

The as prepared holographic sensor containing EMT zeolite nanocrystals exposed to RH of 60% exhibits red color (Figure 5a). In order to activate the holographic sensor, it was kept at 120 °C for 20 minutes to remove the water. Once the water was completely evacuated from the chamber, the water free EMT-zeolite holographic sensor shows no color, and only the white zeolite nanocrystals assembled in circles can be seen (Figure 5b). The reasons for this observation are (1) the shrinkage of the layer and (2) the decrease of the fringe spacing of the holographic structure. After exposure to 80% RH

Author Contributions

Corresponding Author * [email protected] * [email protected] The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT The authors gratefully acknowledge funding from the Ulysses 2014 Programme.

REFERENCES

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