The authors gratefully acknowledge financial support from the Belgian Federal government (Belspo through the IAP VI/27 and IAP-7/05 programs), the European Union's Seventh Framework Programme (FP7/2007-2013 under grant agreement no. 310651 SACS), the Flemish government in the form of long-term structural funding "Methusalem" grant METH/08/04 CASAS, the "Strategisch Initiatief Materialen" SoPPoM program, and the Fund for Scientific Research Flanders (FWO) grant G.0349.12. W.B. gratefully acknowledge the chemistry department of the KU Leuven for a FLOF-scholarship. The authors thank UOP Antwerp for the kind donation of zeolite samples and the mechanical workshop of the KU Leuven for helping with the design and construction of the heating/vacuum cell used in this study.

Caution: The chemicals and reagents used in this report were handled with care using the appropriate safety protections (lab coats, gloves, safety goggles, fume hoods). This study deals with the manipulation of microporous inorganic materials (zeolites with sizes ranging from 1 to 5 microns), therefore special attention was directed to the use of adequate dust protection (dust masks). We recommend the consultation of the relevant material safety data (MSDS) of the chemicals and reagents employed in this work before use for proper manipulation and/or waste disposal.
1. Zeolite Pre-treatment
<ol>
	<li>Heat pre-treatment 
	NOTE: Pre-treat the zeolite materials before use to remove impurities, such as organic impurities, that could hinder the silver cluster formation and luminescence.
	<ol>
		<li>Weigh 10 g of commercial LTA(Na) zeolites (commercial LTA zeolites contain sodium as counter-balancing ions in their frameworks) and deposit it homogeneously on a porcelain tray.</li>
		<li>Heat the zeolite powder overnight in a muffle oven at 450 &#176;C using a temperature ramp of 5 &#176;C/min with intervals of 1 hr at 80 &#176;C and 110 &#176;C to avoid zeolite structure damage.</li>
		<li>Remove the zeolite material from the oven and let it cool down to room temperature under ambient conditions.</li>
	</ol>
	</li>
	<li>Size selection of zeolite particles 
	NOTE: This will generate a more uniform grain size distribution of the starting zeolite materials, necessary for the creation of a homogenous film. This step also removes large amorphous particles, which are often present in industrially produced zeolites.
	<ol>
		<li>Weigh 10 g of commercial LTA(Na) and suspend it into 1 L of deionized water.</li>
		<li>Sonicate the suspension for 1 hr, vigorously shaking the suspension by hand every 10 min.</li>
		<li>Pour the suspension into an Atterberg cylinder (1 L) for 30 min. Particles smaller than 10 &#181;m in size remain in suspension, but larger particles precipitate.</li>
		<li>Decant the suspension and recover the powder by filtration using a B&#252;chner funnel. Wash the recovered powder three times with deionized water.</li>
		<li>Heat treat the powder as described in step 1.1.2.</li>
	</ol>
	</li>
</ol>
2. Preparation of Luminescent LTA(Na)-Ag Zeolite Composites
<ol>
	<li>Synthesis of luminescent silver exchanged LTA zeolite [LTA(Na)-Ag] as reference material
	<ol>
		<li>Dissolve 74.8 mg of silver nitrate in 200 ml deionized water in a 250 ml high density polypropylene (HDPE) bottle.</li>
		<li>Weigh 1 g of the pre-treated LTA(Na) sample and suspend it in the silver nitrate solution.</li>
		<li>Leave the HDPE flask agitating overnight into an end-over-end shaker oven at room temperature.</li>
		<li>Filter the suspension using a B&#252;chner funnel and wash the zeolite powder 3 times with deionized water.</li>
		<li>Heat the recovered powder in a muffle oven at 450 &#176;C, using the same procedure as described in step 1.1.2.</li>
		<li>Cool down the sample and place it in a desiccator with controlled humidity (98% relative humidity). Control the relative humidity by placing a saturated potassium sulfate solution inside the desiccator.15</li>
		<li>Measure the excitation and emission spectra of the samples (at different wavelengths) using a spectrofluorometer as well as their external quantum efficiencies.
		<ol>
			<li>Measure two dimensional excitation-emission plots by placing the sample in a 1 mm path quartz cuvette. Collect emission spectra starting 30 nm above the excitation wavelength up to 800 nm using 5 nm steps and a dwell time of 0.1 sec.</li>
			<li>Apply corrections using the instrument software for the lamp intensity and the wavelength dependent detection of the emission path to the raw data. Additionally, use a long pass filter to avoid second order peaks in the two dimensional plots.</li>
			<li>Perform quantum efficiency measurements by using an integrating sphere attached to the spectrofluorometer.16 Record the emission scan from 240 nm to 600 nm for both the zeolite sample and BaSO4 reference using 260 nm as excitation wavelength, and then calculate the quantum efficiency using the instrument software.</li>
		</ol>
		</li>
	</ol>
	</li>
</ol>
3. Preparation of Luminescent [LTA(Li)-Ag] Zeolite Composites
<ol>
	<li>Synthesis of partially exchanged lithium LTA zeolite [LTA(Li)] 
	Note: The procedure followed for the fabrication of partially exchanged LTA(Li) zeolites was adapted from the report by Yahiro and collaborators.17
	<ol>
		<li>Dissolve 17.2 g lithium nitrate in 2.5 L deionized water.</li>
		<li>Pour 0.5 L of the lithium nitrate solution into a 1 L HDPE flask.</li>
		<li>Weigh 3 g of pre-treated LTA(Na) zeolite and suspend it in the HDPE flask containing the lithium nitrate solution.</li>
		<li>Agitate the flask using an end-over-end shaker oven at room temperature overnight.</li>
		<li>Filter the suspension using a B&#252;chner funnel and wash the recovered powder 3 times with deionized water.</li>
		<li>Perform lithium exchange
		<ol>
			<li>Add 0.5 L of fresh lithium nitrate solution (3.1.1) to a 1 L HDPE flask containing the recovered powder from the filtration step (3.1.5).</li>
			<li>Repeat steps 3.1.4 and 3.1.5.</li>
			<li>Repeat steps 3.1.6.1 and 3.1.6.2 another 4 times.</li>
		</ol>
		</li>
		<li>Recover the zeolite powder and heat it in a muffle oven at 450 &#176;C overnight using a temperature ramp of 5 &#176;C/min with intervals of 1 hr at 80 &#176;C and 110 &#176;C.</li>
	</ol>
	</li>
	<li>Synthesis of luminescent [LTA(Li)-Ag] zeolites
	<ol>
		<li>Dissolve 74.8 mg silver nitrate in 200 ml deionized water using a 250 ml HDPE bottle.</li>
		<li>Weigh 1 g of the partially exchanged lithium LTA zeolite [LTA(Li)] and suspend it in the silver nitrate solution (3.2.1).</li>
		<li>Agitate the HDPE flask using an end-over-end shaker oven at room temperature overnight.</li>
		<li>Filter the suspension using a B&#252;chner funnel and wash the recovered zeolite powder 3 times with deionized water.</li>
		<li>Heat-treat the powder in a muffle oven at 450 &#176;C overnight using a temperature ramp of 5 &#176;C/min with intervals of 1 hr at 80 &#176;C and 110 &#176;C.</li>
		<li>Cool down the sample under controlled humidity conditions using a desiccator containing a saturated potassium sulfate solution inside (98% relative humidity).15</li>
		<li>Measure the excitation and emission spectra of the samples as well as their external quantum efficiencies following the procedure described in step 2.1.7.</li>
		<li>Perform thermogravimetric analysis (TGA) to determine the water content in the sample at different temperatures.1 Briefly, place 30 to 50 mg of the as-prepared sample on a platinum sample holder and load it into the TGA device. Measure the weight loss from 50 &#176;C until 600 &#176;C using a heat rate of 5 &#176;C/min under a nitrogen flow (90 ml/min).</li>
	</ol>
	</li>
</ol>
4. Fabrication of a LTA(Li)-Ag / Polyethylenimine (PEI) Composite Deposited Film for Humidity Sensing Applications
Note: The deposition procedure used in this report was modified and adapted from reference 18.
<ol>
	<li>LTA(Li)-Ag colloidal suspension preparation.
	<ol>
		<li>Dilute 1 ml of the commercial 50 wt% PEI solution to 100 ml with deionized water.</li>
		<li>Weigh 250 mg of the luminescent LTA(Li)-Ag material.</li>
		<li>Mix the zeolite and PEI solution together in a 125 ml HDPE bottle and shake the suspension vigorously.</li>
		<li>Place the bottle in a 40 kHz sonicator bath at room temperature overnight, to obtain a homogeneous suspension.</li>
		<li>Pour the LTA(Li)-Ag / PEI suspension into a spray bottle.</li>
	</ol>
	</li>
	<li>Deposition of a LTA(Li)-Ag / PEI film onto a quartz plate for sensor prototype production.
	<ol>
		<li>Clean a quartz plate by rinsing it with deionized water and acetone consecutively, prior to the film deposition. Dry the clean plates in an oven at 80 &#176;C for 1 hr.</li>
		<li>Spray coat the quartz plate on one side, by placing the quartz plate horizontally on a clean sheet of aluminum foil and spraying three times (3 sec each time) from a distance of about 20 cm. Place the coated plate inside a drying oven at 50 &#176;C for 30 min.</li>
		<li>Repeat step 4.2.2 another 4 times until the film is uniform.</li>
	</ol>
	</li>
	<li>Hydration/dehydration of the sensor prototype.
	<ol>
		<li>Place the coated quartz plate into the sample compartment of an in-house heating/vacuum cell.5 </li>
		<li>Close the sample chamber of the cell by placing a clean quartz plate in combination with a rubber ring on top of the coated plate and seal the cell using a Teflon stopper and screws as depicted in Figure 2.</li>
		<li>Apply high vacuum, using a pressure below 10-3 mbar, to the cell overnight in order to dehydrate the sample.</li>
		<li>Visually monitor the emission color changes (in the visible region) of the deposited film by using a UV lamp.</li>
		<li>Open the sample chamber to monitor the emission color changes, in the visible region, upon rehydration of the film using a UV lamp.</li>
		<li>Repeat the cycle multiple times starting from step 4.3.2 to 4.3.5 to test the reversibility of the LTA(Li)-Ag/PEI film.</li>
	</ol>
	</li>
</ol>

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Mater. 22, 957-960 (2010).</li><li>Coutino-Gonzalez, E., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=X-ray+Irradiation-Induced+Formation+of+Luminescent+Silver+Clusters+in+Nanoporous+Matrices.">X-ray Irradiation-Induced Formation of Luminescent Silver Clusters in Nanoporous Matrices.</a> Chem. Commun. 50, 1350-1352 (2014).</li><li>De Cremer, G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=In+Situ+Observation+of+the+Emission+Characteristics+of+Zeolite-Hosted+Silver+Species+During+Heat+Treatment.">In Situ Observation of the Emission Characteristics of Zeolite-Hosted Silver Species During Heat Treatment.</a> ChemPhysChem. 11, 1627-1631 (2010).</li><li>Coutino-Gonzalez, E., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Thermally+Activated+LTA(Li)-Ag+Zeolites+with+Water-Responsive+Photoluminescence+Properties.">Thermally Activated LTA(Li)-Ag Zeolites with Water-Responsive Photoluminescence Properties.</a> J. Mater. Chem. 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Comm. 14, 7157-7160 (2012).</li><li>Qi, H., Mader, E., Liu, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Unique+Water+Sensors+Based+on+.">Unique Water Sensors Based on .</a> Sensor. Actuat. B-Chem. 185, 225-230 (2013).</li><li>Basabe-Desmonts, L., Reinhoudt, D. N., Crego-Calama, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Design+of+Fluorescent+Materials+for+Chemical+Sensing.">Design of Fluorescent Materials for Chemical Sensing.</a> Chem. Soc. Rev. 36, 993-1017 (2007).</li><li>Yamazoe, N., Shimzu, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Humidity+Sensors+-+Principles+and+Applications.">Humidity Sensors - Principles and Applications.</a> Sensor. 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The authors have nothing to disclose.

A simple device to demonstrate the proof of concept of using LTA(Li)-Ag as a luminescence based humidity sensor was produced by spray coating the LTA(Li)-Ag powder suspended in a PEI solution onto a quartz plate. The PEI solution produces a polymer layer with homogenous thickness when the water is evaporated. The polymer-zeolite composite layer displays similar luminescent properties as that of the zeolite in powder form. The PEI/LTA(Li)-Ag zeolite composite displays the expected water-responsive luminescent properties, whose emission color changes upon variations in the water content present in the composite at relatively low humidity scale. 
Replacing Na with Li ions in LTA zeolites (calculated exchange rate 33%) has a notable impact on the self-assembly and stabilization of luminescent silver clusters in the LTA(Li) scaffolds leading to unique optical properties. The EQE of LTA(Li)-Ag as compared to LTA(Na)-Ag samples is enhanced by more than one order of magnitude. Moreover, the emission colors displayed by the LTA(Li)-Ag samples have a water-dependence, providing a potential application of the samples as luminescence based humidity sensors. 
We have thus demonstrated an easy method to fabricate a luminescent film-like humidity sensor through which changes in hydration levels can be visually monitored simply by using a UV lamp. The availability of the raw materials, the direct visualization of the color changes correlated with humidity content, the photo-stability of the films, and the relative ease of fabricating cost-effective devices make these luminescent materials potential candidates to compete with state-of-the-art humidity sensors based upon electrical responses. The procedure described in this report could also be applied and extended to different substrates, at different micro and macro scales, to make the sensor more flexible. Additionally, several critical steps during the fabrication of Ag-zeolites, which play an important role in determining the final optical properties of such materials, were discussed in this protocol. For instance, the pre-cleaning of the raw zeolite material leads to the removal of optical and chemical impurities, as well as to homogenous zeolite crystal size distribution. This is crucial for the incorporation of zeolites into functional devices. One limitation of the present methodology is the restriction on the use of thin film sensors beyond 75 &#176;C. This is mainly due to the decomposition of the PEI polymer, rather than to the degradation of the LTA(Li)-Ag zeolites, which can withstand up to 500 &#176;C. The use of heat-resistant polymers, such as polyvinyl alcohol, could expand the temperature range up to 200 &#176;C. We expect that further investigations will be directed to the development of methodologies for the synthesis of nanostructured Ag-zeolite composites with (multi)functional properties and finally to the design of advanced sensor prototypes.

Small sub-nanometer oligoatomic silver clusters formed by self-assembly in confined zeolite matrices display unique optical properties.1-5 Such silver-zeolite composites have high chemical and photo-stability. However, their photoluminescence properties are highly dependent on the local environment of the silver clusters. The environmental conditions that influence the optical features in silver-zeolite composites can be divided into intrinsic and extrinsic properties. Intrinsic properties are related to the zeolite topology, the type of counter-balancing ions, and the silver loading.1 On the other hand, extrinsic properties are associated to the post-synthetic changes, such as the presence of adsorbates or water molecules in the zeolite cavities.3,4 The latter properties confer to silver-zeolite composites the ability to optically respond to external stimuli, such as variations of moisture within the zeolite scaffold6-8 or the presence of determined gases; therefore their use as water vapor and gas sensors has been suggested.9,10
In a recent study we have demonstrated that the optical response of Ag-zeolites to moisture is not only correlated to changes in the absorption or quenching of their emission but also to the appearance of different emission colors with respect to their water content.5 The stabilization of silver clusters in partially Li exchanged LTA zeolites led to the formation of a moisture-responsive material in which changes in the relative low humidity scale were reflected in a dynamic color change from a blue to green/yellow emission in dehydrated and hydrated samples, respectively. Therefore the use of these materials as luminescence-based humidity sensors was proposed. To date, different types of materials such as electrolytes, ceramics, polymers, and nanostructured composites have been proposed for monitoring changes in humidity based on electrical and optical responses.11,12 In this detailed protocol we aim to demonstrate a proof-of-concept for the application of LTA(Li)-Ag zeolites as humidity sensors and for further prototype developments. Due to the versatility of LTA(Li)-Ag zeolites to be incorporated into different substrates, their potential scalability and cost-effective fabrication, the prototype design might be facilitated.13 Such sensors could have potential applicability in different industrial sectors, such as in agriculture, as well as the automobile and paper industry.14

<table border="0" cellpadding="0" cellspacing="0" width="833">
	<tbody>
		<tr height="20">
			<td height="20" style="height:20px;width:268px;">LTA(Na) zeolite</td>
			<td style="width:169px;">UOP</td>
			<td style="width:167px;">Molsiv adsorbent 4A</td>
			<td style="width:229px;"></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Silver nitrate</td>
			<td>Sigma Aldrich</td>
			<td>209139</td>
			<td>&#8805;99,0%</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Lithium nitrate</td>
			<td>Sigma Aldrich</td>
			<td>62574</td>
			<td>&#8805;99,0%, calc. on dry substances</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Polyethyleneimine solution</td>
			<td>Sigma Aldrich</td>
			<td>3880</td>
			<td>~50% H2O</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Scanning electron microscope (SEM)</td>
			<td>JEOL</td>
			<td>JSM-6010LV</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Thermogravimetric analyzer</td>
			<td>TA instruments</td>
			<td>Q500</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Spectrofluorimeter</td>
			<td>Edinburgh instruments</td>
			<td>FLS980-s</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Integrating sphere</td>
			<td>Labsphere</td>
			<td>4P-GPS-033-SL</td>
			<td></td>
		</tr>
	</tbody>
</table>

nanostructured ag-zeolite composites as luminescence-based humidity sensors

Small silver clusters confined inside zeolite matrices have recently emerged as a novel type of highly luminescent materials. Their emission has high external quantum efficiencies (EQE) and spans the whole visible spectrum. It has been recently reported that the UV excited luminescence of partially Li-exchanged sodium Linde type A zeolites [LTA(Na)] containing luminescent silver clusters can be controlled by adjusting the water content of the zeolite. These samples showed a dynamic change in their emission color from blue to green and yellow upon an increase of the hydration level of the zeolite, showing the great potential that these materials can have as luminescence-based humidity sensors at the macro and micro scale. Here, we describe the detailed procedure to fabricate a humidity sensor prototype using silver-exchanged zeolite composites. The sensor is produced by suspending the luminescent Ag-zeolites in an aqueous solution of polyethylenimine (PEI) to subsequently deposit a film of the material onto a quartz plate. The coated plate is subjected to several hydration/dehydration cycles to show the functionality of the sensing film.

SEM micrographs of the LTA-Ag zeolite were recorded after the cation exchange and heat-treatment step. Subsequently the photoluminescence two-dimensional (2D) excitation/emission plots were measured for both the hydrated LTA(Na)-Ag and LTA(Li)-Ag zeolites (Figure 1). Elemental analysis was performed by XPS on the Ag exchanged zeolites to determine their chemical composition. The analysis shows that silver exchange on LTA(Na) and LTA(Li) zeolites is very close with a silver weight percentage of 19.6 wt% and 21.5 wt%, respectively. The difference in weight percentage could be ascribed to the lower atomic weight of Li atoms. Furthermore the elemental analysis also showed that after Li exchange 33% of Na is replaced. The cation exchange and subsequent heat-treatment step performed on the samples do not seem to affect the structure of the LTA crystals, as demonstrated by SEM. Moreover, the formation of larger silver nanoparticles on the surface of the zeolite crystals was not visualized. The luminescent properties largely differ between both the LTA(Li)-Ag and LTA(Na)-Ag samples in their hydrated state. By incorporating lithium into the zeolite framework as a counter-balancing cation, a blue shift in the excitation maximum occurs from 370 nm to 260 nm, for LTA(Na)-Ag and LTA(Li)-Ag, respectively. In contrast the emission maximum undergoes a small red shift from 550 to 565 nm by adding Li into the system. The largest difference between these samples is observed in their external quantum efficiencies (EQE). LTA(Na)-Ag zeolites possess an EQE of about 4% at its excitation maximum (370 nm), whereas the EQE for LTA(Li)-Ag zeolites reaches 62% (when excited at 260 nm). This results in a bright yellow emitting powder under 254 nm UV- illumination.
The luminescent properties of the LTA(Li)-Ag sample are also dependent on the water content of the system. This was shown by a combination of TGA and temperature dependent luminescence experiments, TGA correlates temperature to hydration level of the zeolite. Additionally, the temperature was indirectly related to the emission color displayed by the LTA(Li)-Ag sample by using an in-house heating cell (Figure 2). The emission color shifts from yellow over green to blue, when removing water from the LTA(Li)-Ag system. The EQE steadily drops from 62% (hydrated state) to 21% (dehydrated state).
Because of the water-responsive behavior of LTA(Li)-Ag, this material was used to fabricate a luminescence based humidity sensor prototype by suspending the powder into a PEI solution and subsequently spray coating the composite onto a quartz plate. Pictures (under daylight and UV illumination) and SEM micrographs of the spray-coated LTA(Li)-Ag/PEI film are displayed in Figure 3. We observed that by using this coating procedure, a relatively homogenous layer of the polymer-zeolite composite in terms of luminescence was obtained. The SEM micrograph shows that the zeolite crystals are not altered by the coating procedure. By using an in-house heating/vacuum cell it was also demonstrated that the polymer-zeolite film retains the water-responsive properties which were observed in the zeolite in powder form.
<img alt="Figure 1" src="/files/ftp_upload/54674/54674fig1.jpg" /> 
Figure 1: SEM pictures and luminescent properties of silver exchanged LTA zeolites. SEM micrographs and 2D excitation-emission plots of LTA(Na)-Ag (a,b) and LTA(Li)-Ag (c,d). The insets in the 2D excitation-emission plots display the simulated emission colors of the samples under different excitation wavelengths (254, 300, 366 and 450 nm). <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54674/54674fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/54674/54674fig2.jpg" /> 
Figure 2: Effect of hydration level on the luminescent properties of LTA(Li)-Ag. a) Schematic representation of the in-house heating/vacuum cell employed in this study. b) TGA plot for the LTA(Li)-Ag sample. c) Normalized emission spectra (upon 260 nm excitation) of the LTA(Li)-Ag sample measured at different temperatures. d) Scheme displaying the emission color change of real samples with respect to the water content. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54674/54674fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/54674/54674fig3.jpg" /> 
Figure 3: Luminescent humidity sensor based on a LTA(Li)-Ag/PEI composite. a) Photographs of the coated plate under daylight illumination. (b,c) Pictures of the dehydrated and hydrated coated plate under 254 nm UV-light irradiation, respectively. d) SEM micrograph of the deposited film showing the distribution of LTA(Li)-Ag crystals on the quartz surface. The inset displays an enlargement of a selected area of the original SEM micrograph. e) Emission maxima of the hydrated and dehydrated PEI/LTA(Li)-Ag zeolite composite, during 10 hydration/dehydration cycles using 260 nm as excitation wavelength. f) Plot displaying the emission maxima profiles behavior of the PEI/LTA(Li)-Ag zeolite composite after 10 hydration/dehydration cycles. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54674/54674fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Nanostructured Ag-zeolite Composites as Luminescence-based Humidity Sensors at JoVE.com

Nanostructured Ag-zeolite Composites as Luminescence-based Humidity Sensors

A protocol for the synthesis of moisture-responsive luminescent Ag-zeolite composites is described in this report.

Small silver clusters confined inside zeolite matrices have recently emerged as a novel type of highly luminescent materials. Their emission has high ...

nanostructured-ag-zeolite-composites-as-luminescence-based-humidity

Nanyang Technological University

Research

JoVE Journal

Engineering

10.1K Views.  KU Leuven. A protocol for the synthesis of moisture-responsive luminescent Ag-zeolite composites is described in this report.

Video: Nanostructured Ag-zeolite Composites as Luminescence-based Humidity Sensors

Development of Whispering Gallery Mode Polymeric Micro-optical Electric Field Sensors

Optical modes of dielectric micro-cavities have received significant attention in recent years for their potential in a broad range of applications. The optical modes are frequently referred to as "whispering gallery modes" (WGM) or "morphology dependent resonances" (MDR) and exhibit high optical quality factors. Some proposed applications of micro-cavity optical resonators are in spectroscopy1, micro-cavity laser technology2, optical communications3-6 as well as sensor technology. The WGM-based sensor applications include those in biology7, trace gas detection8, and impurity detection in liquids9. Mechanical sensors based on microsphere resonators have also been proposed, including those for force10,11, pressure12, acceleration13 and wall shear stress14. In the present, we demonstrate a WGM-based electric field sensor, which builds on our previous studies15,16. A candidate application of this sensor is in the detection of neuronal action potential.
The electric field sensor is based on polymeric multi-layered dielectric microspheres. The external electric field induces surface and body forces on the spheres (electrostriction effect) leading to elastic deformation. This change in the morphology of the spheres, leads to shifts in the WGM. The electric field-induced WGM shifts are interrogated by exciting the optical modes of the spheres by laser light. Light from a distributed feedback (DFB) laser (nominal wavelength of ~ 1.3 &mu;m) is side-coupled into the microspheres using a tapered section of a single mode optical fiber. The base material of the spheres is polydimethylsiloxane (PDMS). Three microsphere geometries are used: (1) PDMS sphere with a 60:1 volumetric ratio of base-to-curing agent mixture, (2) multi layer sphere with 60:1 PDMS core, in order to increase the dielectric constant of the sphere, a middle layer of 60:1 PDMS that is mixed with varying amounts (2% to 10% by volume) of barium titanate and an outer layer of 60:1 PDMS and (3) solid silica sphere coated with a thin layer of uncured PDMS base. In each type of sensor, laser light from the tapered fiber is coupled into the outermost layer that provides high optical quality factor WGM (Q ~ 106). The microspheres are poled for several hours at electric fields of ~ 1 MV/m to increase their sensitivity to electric field.

A high-sensitivity photonic micro sensor was developed for electric field detection. The sensor exploits the optical modes of a dielectric sphere. Changes in the external electric field perturb the sphere morphology leading to shifts in its optical modes. The electric field strength is measured by monitoring these optical shifts.

Optical modes of dielectric micro-cavities have received significant attention in recent years for their potential in a broad range of applications. The ...

A Testing Platform for Durability Studies of Polymers and Fiber-reinforced Polymer Composites under Concurrent Hygrothermo-mechanical Stimuli

The durability of polymers and fiber-reinforced polymer composites under service condition is a critical aspect to be addressed for their robust designs and condition-based maintenance. These materials are adopted in a wide range of engineering applications, from aircraft and ship structures, to bridges, wind turbine blades, biomaterials and biomedical implants. Polymers are viscoelastic materials, and their response may be highly nonlinear and thus make it challenging to predict and monitor their in-service performance. The laboratory-scale testing platform presented herein assists the investigation of the influence of concurrent mechanical loadings and environmental conditions on these materials. The platform was designed to be low-cost and user-friendly. Its chemically resistant materials make the platform adaptable to studies of chemical degradation due to in-service exposure to fluids. An example of experiment was conducted at RT on closed-cell polyurethane foam samples loaded with a weight corresponding to ~50% of their ultimate static and dry load. Results show that the testing apparatus is appropriate for these studies. Results also highlight the larger vulnerability of the polymer under concurrent loading, based on the higher mid-point displacements and lower residual failure loads. Recommendations are made for additional improvements to the testing apparatus.

The durability of polymers and fiber-reinforced polymer composites in service is a critical aspect for their designs and condition-based maintenance. We present a novel low-cost laboratory testing platform for the investigation of the influence of concurrent mechanical and environmental loadings, and may help design more efficient yet safer composite structures.

The durability of polymers and fiber-reinforced polymer composites under service condition is a critical aspect to be addressed for their robust designs ...

Preparation and Reactivity of Gasless Nanostructured Energetic Materials

High-Energy Ball Milling (HEBM) is a ball milling process where a powder mixture placed in the ball mill is subjected to high-energy collisions from the balls. Among other applications, it is a versatile technique that allows for effective preparation of gasless reactive nanostructured materials with high energy density per volume (Ni+Al, Ta+C, Ti+C). The structural transformations of reactive media, which take place during HEBM, define the reaction mechanism in the produced energetic&#160;composites. Varying the processing conditions permits fine tuning of the milling-induced microstructures of the fabricated composite particles. In turn, the reactivity, i.e., self-ignition temperature, ignition delay time, as well as reaction kinetics, of high energy density materials depends on its microstructure. Analysis of the milling-induced microstructures suggests that the formation of fresh oxygen-free intimate high surface area contacts between the reagents is responsible for the enhancement of their reactivity. This manifests itself in a reduction of ignition temperature and delay time, an increased rate of chemical reaction, and an overall decrease of the effective activation energy of the reaction. The protocol provides a detailed description for the preparation of reactive nanocomposites with tailored microstructure using short-term HEBM method. It also describes a high-speed thermal imaging technique to determine the ignition/combustion characteristics of the energetic materials. The protocol can be adapted to preparation and characterization of a variety of nanostructured energetic composites.

This protocol describes the preparation of gasless nanostructured energetic materials (Ni+Al, Ta+C, Ti+C) using the short-term high-energy ball milling (HEBM) technique. It also describes a high-speed thermal imaging method to study the reactivity of mechanically fabricated nanocomposites. These protocols can be extended to other reactive nanostructured energetic materials.

High-Energy Ball Milling (HEBM) is a ball milling process where a powder mixture placed in the ball mill is subjected to high-energy collisions from the ...

Surface Enhanced Raman Spectroscopy Detection of Biomolecules Using EBL Fabricated Nanostructured Substrates

Fabrication and characterization of conjugate nano-biological systems interfacing metallic nanostructures on solid supports with immobilized biomolecules is reported. The entire sequence of relevant experimental steps is described, involving the fabrication of nanostructured substrates using electron beam lithography, immobilization of biomolecules on the substrates, and their characterization utilizing surface-enhanced Raman spectroscopy (SERS). Three different designs of nano-biological systems are employed, including protein A, glucose binding protein, and a dopamine binding DNA aptamer. In the latter two cases, the binding of respective ligands, D-glucose and dopamine, is also included. The three kinds of biomolecules are immobilized on nanostructured substrates by different methods, and the results of SERS imaging are reported. The capabilities of SERS to detect vibrational modes from surface-immobilized proteins, as well as to capture the protein-ligand and aptamer-ligand binding are demonstrated. The results also illustrate the influence of the surface nanostructure geometry, biomolecules immobilization strategy, Raman activity of the molecules and presence or absence of the ligand binding on the SERS spectra acquired.

We describe the fabrication and characterization of nano-biological systems interfacing nanostructured substrates with immobilized proteins and aptamers. The relevant experimental steps involving lithographic fabrication of nanostructured substrates, bio-functionalization, and surface-enhanced Raman spectroscopy (SERS) characterization, are reported. SERS detection of surface-immobilized proteins, and probing of protein-ligand and aptamer-ligand binding is demonstrated.

Fabrication and characterization of conjugate nano-biological systems interfacing metallic nanostructures on solid supports with immobilized biomolecules ...

Preparation and Evaluation of Hybrid Composites of Chemical Fuel and Multi-walled Carbon Nanotubes in the Study of Thermopower Waves

When a chemical fuel at a certain position in a hybrid composite of the fuel and a micro/nanostructured material is ignited, chemical combustion occurs along the interface between the fuel and core materials. Simultaneously, dynamic changes in thermal and chemical potentials across the micro/nanostructured materials result in concomitant electrical energy generation induced by charge transfer in the form of a high-output voltage pulse. We demonstrate the entire procedure of a thermopower wave experiment, from synthesis to evaluation. Thermal chemical vapor deposition and the wet impregnation process are respectively employed for the synthesis of a multi-walled carbon nanotube array and a hybrid composite of picric acid/sodium azide/multi-walled carbon nanotubes. The prepared hybrid composites are used to fabricate a thermopower wave generator with connecting electrodes. The combustion of the hybrid composite is initiated by laser heating or Joule-heating, and the corresponding combustion propagation, direct electrical energy generation, and real-time temperature changes are measured using a high-speed microscopy system, an oscilloscope, and an optical pyrometer, respectively. Furthermore, the crucial strategies to be adopted in the synthesis of hybrid composite and initiation of their combustion that enhance the overall thermopower wave energy transfer are proposed.

A protocol for conducting thermopower wave experiments is presented. The synthesis of hybrid composites of a chemical fuel and micro/nanostructured material, manufacturing of a thermopower wave generator, and methods for measuring the corresponding physical phenomena are described.

When a chemical fuel at a certain position in a hybrid composite of the fuel and a micro/nanostructured material is ignited, chemical combustion occurs ...

Iridium Oxide-reduced Graphene Oxide Nanohybrid Thin Film Modified Screen-printed Electrodes as Disposable Electrochemical Paper Microfluidic pH Sensors

A facile, controllable, inexpensive and green electrochemical synthesis of IrO2-graphene nanohybrid thin films is developed to fabricate an easy-to-use integrated paper microfluidic electrochemical pH sensor for resource-limited settings. Taking advantages from both pH meters and strips, the pH sensing platform is composed of hydrophobic barrier-patterned paper micropad (&#181;PAD) using polydimethylsiloxane (PDMS), screen-printed electrode (SPE) modified with IrO2-graphene films and molded acrylonitrile butadiene styrene (ABS) plastic holder. Repetitive cathodic potential cycling was employed for graphene oxide (GO) reduction which can completely remove electrochemically unstable oxygenated groups and generate a 2D defect-free homogeneous graphene thin film with excellent stability and electronic properties. A uniform and smooth IrO2 film in nanoscale grain size is anodically electrodeposited onto the graphene film, without any observable cracks. The resulting IrO2-RGO electrode showed slightly super-Nernstian responses from pH 2-12 in Britton-Robinson (B-R) buffers with good linearity, small hysteresis, low response time and reproducibility in different buffers, as well as low sensitivities to different interfering ionic species and dissolved oxygen. A simple portable digital pH meter is fabricated, whose signal is measured with a multimeter, using high input-impedance operational amplifier and consumer batteries. The pH values measured with the portable electrochemical paper-microfluidic pH sensors were consistent with those measured using a commercial laboratory pH meter with a glass electrode.

The study demonstrates the growth of iridium oxide-reduced graphene oxide (IrO2-RGO) nanohybrid thin films on irregular and rough screen-printed carbon substrate through a green electrochemical synthesis, and their implementation as a pH sensor with a patterned paper-fluidic platform.

A facile, controllable, inexpensive and green electrochemical synthesis of IrO2-graphene nanohybrid thin films is developed to fabricate an easy-to-use ...

Ultrasonic Welding of Thermoplastic Composite Coupons for Mechanical Characterization of Welded Joints through Single Lap Shear Testing

This paper presents a novel straightforward method for ultrasonic welding of thermoplastic-composite coupons in optimum processing conditions. The ultrasonic welding process described in this paper is based on three main pillars. Firstly, flat energy directors are used for preferential heat generation at the joining interface during the welding process. A flat energy director is a neat thermoplastic resin film that is placed between the parts to be joined prior to the welding process and heats up preferentially owing to its lower compressive stiffness relative to the composite substrates. Consequently, flat energy directors provide a simple solution that does not require molding of resin protrusions on the surfaces of the composite substrates, as opposed to ultrasonic welding of unreinforced plastics. Secondly, the process data provided by the ultrasonic welder is used to rapidly define the optimum welding parameters for any thermoplastic composite material combination. Thirdly, displacement control is used in the welding process to ensure consistent quality of the welded joints. According to this method, thermoplastic-composite flat coupons are individually welded in a single lap configuration. Mechanical testing of the welded coupons allows determining the apparent lap shear strength of the joints, which is one of the properties most commonly used to quantify the strength of thermoplastic composite welded joints.

A straightforward procedure for ultrasonic welding of thermoplastic composite coupons for basic mechanical testing is described. Key characteristics of this ultrasonic welding process are the use of flat energy directors for simplified process preparation and the use of process data for the fast definition of optimum processing conditions.

This paper presents a novel straightforward method for ultrasonic welding of thermoplastic-composite coupons in optimum processing conditions. The ...

Fiber Optic Distributed Sensors for High-resolution Temperature Field Mapping

The reliability of computational fluid dynamics (CFD) codes is checked by comparing simulations with experimental data. A typical data set consists chiefly of velocity and temperature readings, both ideally having high spatial and temporal resolution to facilitate rigorous code validation. While high resolution velocity data is readily obtained through optical measurement techniques such as particle image velocimetry, it has proven difficult to obtain temperature data with similar resolution. Traditional sensors such as thermocouples cannot fill this role, but the recent development of distributed sensing based on Rayleigh scattering and swept-wave interferometry offers resolution suitable for CFD code validation work. Thousands of temperature measurements can be generated along a single thin optical fiber at hundreds of Hertz. Sensors function over large temperature ranges and within opaque fluids where optical techniques are unsuitable. But this type of sensor is sensitive to strain and humidity as well as temperature and so accuracy is affected by handling, vibration, and shifts in relative humidity. Such behavior is quite unlike traditional sensors and so unconventional installation and operating procedures are necessary to ensure accurate measurements. This paper demonstrates implementation of a Rayleigh scattering-type distributed temperature sensor in a thermal mixing experiment involving two air jets at 25 and 45 &#176;C. We present criteria to guide selection of optical fiber for the sensor and describe installation setup for a jet mixing experiment. We illustrate sensor baselining, which links readings to an absolute temperature standard, and discuss practical issues such as errors due to flow-induced vibration. This material can aid those interested in temperature measurements having high data density and bandwidth for fluid dynamics experiments and similar applications. We highlight pitfalls specific to these sensors for consideration in experiment design and operation.

We demonstrate use of a fiber optic distributed sensor for mapping the temperature field of mixing air jets. The Rayleigh scattering-based sensor generates thousands of data points along a single fiber to provide exceptional spatial resolution that is unattainable with traditional sensors such as thermocouples.

The reliability of computational fluid dynamics (CFD) codes is checked by comparing simulations with experimental data. A typical data set consists ...

Advanced Compositional Analysis of Nanoparticle-polymer Composites Using Direct Fluorescence Imaging

The fabrication of polymer-nanoparticle composites is extremely important in the development of many functional materials. Identifying the precise composition of these materials is essential, especially in the design of surface catalysts, where the surface concentration of the active component determines the activity of the material. Antimicrobial materials which utilize nanoparticles are a particular focus of this technology. Recently swell encapsulation has emerged as a technique for inserting antimicrobial nanoparticles into a host polymer matrix. Swell encapsulation provides the advantage of localizing the incorporation to the external surfaces of materials, which act as the active sites of these materials. However, quantification of this nanoparticle uptake is challenging. Previous studies explore the link between antimicrobial activity and surface concentration of the active component, but this is not directly visualized. Here we show a reliable method to monitor the incorporation of nanoparticles into a polymer host matrix via swell encapsulation. We show that the surface concentration of CdSe/ZnS nanoparticles can be accurately visualized through cross-sectional fluorescence imaging. Using this method, we can quantify the uptake of nanoparticles via swell encapsulation and measure the surface concentration of encapsulated particles, which is key in optimizing the activity of functional materials.

Here we present a reliable method to monitor the incorporation of nanoparticles into a polymer host matrix via swell encapsulation. We show that the surface concentration of cadmium selenide quantum dots can be accurately visualized through cross-sectional fluorescence imaging.

The fabrication of polymer-nanoparticle composites is extremely important in the development of many functional materials. Identifying the precise ...

High Temperature Fabrication of Nanostructured Yttria-Stabilized-Zirconia (YSZ) Scaffolds by In Situ Carbon Templating Xerogels

We demonstrate a method for the high temperature fabrication of porous, nanostructured yttria-stabilized-zirconia (YSZ, 8 mol% yttria - 92 mol% zirconia) scaffolds with tunable specific surface areas up to 80 m2&#183;g-1. An aqueous solution of a zirconium salt, yttrium salt, and glucose is mixed with propylene oxide (PO) to form a gel. The gel is dried under ambient conditions to form a xerogel. The xerogel is pressed into pellets and then sintered in an argon atmosphere. During sintering, a YSZ ceramic phase forms and the organic components decompose, leaving behind amorphous carbon. The carbon formed in situ serves as a hard template, preserving a high surface area YSZ nanomorphology at sintering temperature. The carbon is subsequently removed by oxidation in air at low temperature, resulting in a porous, nanostructured YSZ scaffold. The concentration of the carbon template and the final scaffold surface area can be systematically tuned by varying the glucose concentration in the gel synthesis. The carbon template concentration was quantified using thermogravimetric analysis (TGA), the surface area and pore size distribution was determined by physical adsorption measurements, and the morphology was characterized using scanning electron microscopy (SEM). Phase purity and crystallite size was determined using X-ray diffraction (XRD). This fabrication approach provides a novel, flexible platform for realizing unprecedented scaffold surface areas and nanomorphologies for ceramic-based electrochemical energy conversion applications, e.g. solid oxide fuel cell (SOFC) electrodes.

High Temperature Fabrication of Nanostructured Yttria-Stabilized-Zirconia YSZ Scaffolds by In Situ Carbon Templating Xerogels

A protocol for fabricating porous, nanostructured yttria-stabilized-zirconia (YSZ) scaffolds at temperatures between 1,000 &#176;C and 1,400 &#176;C is presented.

We demonstrate a method for the high temperature fabrication of porous, nanostructured yttria-stabilized-zirconia (YSZ, 8 mol% yttria - 92 mol% zirconia) ...