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Surfaces, Interfaces, and Applications

Versatile and robust integrated sensors to locally assess humidity changes in fully-enclosed paper-based devices Murilo Santhiago, Priscila Costa, Mariane Peres Pereira, Cátia C. Corrêa, Vitoria B Morais, and Carlos César Bof Bufon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12780 • Publication Date (Web): 24 Sep 2018 Downloaded from http://pubs.acs.org on September 26, 2018

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ACS Applied Materials & Interfaces

Versatile and robust integrated sensors to locally assess humidity changes in fullyenclosed paper-based devices Murilo Santhiago, Priscila G. Costa, Mariane P. Pereira, Cátia C. Corrêa, Vitória B. de Morais, Carlos C.B. Bufon* Brazilian Nanotechnology National Laboratory (LNNano), Brazilian Center for Research in Energy and Materials (CNPEM), Zip Code 13083-970, Campinas, Sao Paulo, Brazil. * E-mail: [email protected] Keywords: polypyrrole, surface modification, paper-microfluidics, three-dimensional devices, flexible devices, foldable devices. Abstract: The synergic combination of materials and interfaces to create novel functional devices is a crucial approach for various applications, including low-cost paper-based point-of-care systems. In this work, we demonstrate the implementation of surfacemodified polypyrrole (PPy) structures, monolithically integrated into a three-dimensional multilayered paper-based microfluidic device, to locally assess humidity changes. The fabrication and integration of the system include the deterministic incorporation of PPy into the paper-based structure by gas-phase polymerization, and the modification of the polymer properties to allow the local humidity monitoring. The functionalization of PPy changes both the wettability and the chemical composition of the interface, what is of fundamental importance for the sensor’s operation. The PPy structure has excellent mechanical stability, enduring at least 600 bending cycles, what is of relevance on flexible electronics. The electrical resistance correlates with the local relative humidity (RH) inside of the sealed

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microfluidic system, and the sensor response is fully reversible. The integrated system capable of locally monitoring the RH allowed us to verify that inside the microfluidic channel water molecules can diffuse across the wax barriers - a possibility disregarded so far. Our results attest that RH variations of 5 – 10% can affect the flow of extended channels (> 5 cm) even when they are fully enclosed.

1. Introduction Since it was first introduced in 2007, paper-based microfluidics has been pushing forward the field of low-cost and portable devices.1 The remarkable advances were mainly driven by the fact that the paper devices’ substrate is an inexpensive material, lightweight, biodegradable, worldwide available, and easily printed, coated, folded, and/or impregnated with reagents.2–4 Moreover, the surface chemistry of cellulose fibers is well-known and allows different functionalization processes.5,6 The capability of paper to store reagents combined with its porous structure, which helps a pump-free delivery of liquid samples, have permitted the construction of a myriad of microfluidic paper-based sensors and biosensors.7–9 One of the critical factors in paper-based microfluidic devices (µPADs) is the possibility to monitor and to control the liquid flow for specific applications.4,10 Since the flow velocity decreases with time, as predicted by the Lucas-Washburn equation,11 one particular aspect of interest is to accelerate the flow to reduce analysis time. Different strategies have been used to accomplish this task, such as hollow structures,12 open channels,13 different channel format,14 and creating two-ply channels.15 On the other hand,


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flow delay may be useful to give more time to reactions take place and/or for sequential delivery of reagents. The flow delay can be obtained by incorporating sugar delays,16 waxprinted pillars,17 triboelectric delays,18 and dissolvable bridges19, to name a few. Another essential aspect in paper-based microfluidic channels is the evaporation rate of solvents, which can be strongly affected by humidity changes.20 In fact, humidity changes play a critical role for extended channels (> 30 mm).15,18,21 Also, if one considers the worldwide commercialization of µPADs, the humidity can have a significant impact due to the variations observed in different countries (11 – 100 %). This particular issue has been addressed by sealing or covering the paper-based devices.22 Toner,21 Polyethylene terephthalate (PET)18 and adhesive layers15,23 are the most used materials to minimize the effect of solvent evaporation. These strategies have successfully been used in many sensors and biosensors.22 However, there is no quantitative analysis of relative humidity (RH), inside of enclosed devices, reported so far. The understanding of local RH changes in such devices may help the development of more reliable, efficient and novel µPADs. Several reports have presented the fabrication of humidity sensors on paper-based substrates. Some of the conductive elements employed used to measure the variations in RH are carbon-based nanomaterials,24 silver nanoparticles,25 interdigitated carbon/metallic electrodes26 and organic semiconductors, such as PEDOT:PSS.27 The incorporation of solution-processed semiconductors is an exciting approach for paper substrates since they can be patterned along the entire thickness of the paper.28 However, the combination of materials of different natures, often observed on flexible hybrid electronics, is not always possible in paper based-devices due to incompatible fabrication and integration processes. For instance, the deposition of metallic thin-films on paper may lead to the degradation of the substrate because of the elevated temperatures involved during the metal deposition. As



an alternative for creating hybrid structures in/on paper, we recently described threedimensional (3D) polypyrrole (PPy) conductive inter-connects as a versatile and straightforward patterning route.29 By considering the technological potential of organic structures in the field of flexible electronics and electrochemical systems, the use of direct patterning and modification of conducting polymers is a unique opportunity to develop sophisticated paper-based devices further. Herein, we report the deterministic incorporation and modification of 3D PPy structures, inside of fully-enclosed paper-based microfluidic channels, as a sensing element. As a proof-of-concept, the 3D PPy structure was modified to monitor the RH. The conductive structures were made by adding copper chloride dissolved in water in hydrophilic channels and exposing the substrate to pyrrole monomers, in the gas-phase, in a second step.30 The chemical modification of the PPy was performed by removing the excess of ions during a controlled washing step in water. The effect of the chemical modification on the physical properties of the PPy was monitored by electrical measurements, X-ray photoelectron spectroscopy, and contact-angle experiments. The removal of the counter ions was accompanied by an increase in the electrical resistance of the structures. Such increasing indicates that the modification procedure adopted here leads to the very same macroscopic effects observed after either a PPy electrochemical reduction or a synthesis in the presence of low-concentration of counterions.31,32 Moreover, the modification process promotes changes in the chemical composition of the interface by switching the surface from hydrophilic to hydrophobic. The PPy structure has excellent mechanical stability, enduring at least 600 bending cycles, what is of relevance on flexible electronics. Considering the RH dependence of the resistance, we conclude that the increase of the cellulose fibers volume is also responsible for the RH response. Finally, we verify for


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the first time that small variations in humidity can significantly affect the liquid flow, even when the paper-based microfluidic channel is fully-enclosed. Such an observation was only possible by the direct monitoring of the RH inside of the microfluidic system.

2. Experimental section 2.1. Materials All chemicals are analytical grade and used as received without further purification. The device structures were created in Software PowerPoint and printed on Chromatography Paper 1CHR. Graphite powder, cellulose acetate, and cyclohexanone were purchased from Sigma-Aldrich, SP, Brazil. Acetone was acquired from Synth, SP, Brazil. Copper (II) chloride dihydrate was from Merck, SP, Brazil, and pyrrole 98+% from Alfa Aesar. Silver ink from SPI supplies, PA, USA was used to construct conductive pads. Single and doublesided tape from 3M, SP, Brazil were used to pattern the electrodes. All the solutions were prepared using purified water (18.2 MΩ.cm) from Elga Veolia model Purelab Option-Q, UK. An analytical balance from Shimadzu AUW220D, SP, Brazil with five decimals places was used to weight all the reagents. All the characterizations were performed on freshly prepared samples.

2.2. Preparation of conducting graphite ink The graphite ink was prepared as follows: 2.85 g of a mixture of cyclohexanone and acetone (1:1)33 were added to 150 mg of cellulose acetate. The mixture was vigorously stirred for 2 hours. Next, 1.0 g of graphite was mixed to 2.5 g of the solution containing cellulose acetate and the mixture was stirred for an additional 2 minutes.



2.3. Patterning graphite ink on paper Chromatography Paper 1CHR was selected for the fabrication of all the devices. In brief, the device contains three main parts: (i) main channel, (ii) reservoir, (iii) humidity sensor (see Figure 1a). The hydrophobic patterns were wax-printed on paper by using a XEROX printer model ColorQube 8570. The wax patterned paper received a thermal treatment on a hot plate (Tecnal, SP, Brazil) at120 °C for 5 minutes. Following the thermal treatment, the paper could cool at room temperature for 5 minutes before the application of the ink. We added adhesive layers from 3M to delimitate the area of the conductive carbon pathways. Additional details are in Figure S1.

2.4. In situ Polymerization of pyrrole After patterning, the device made of the graphite ink, 4.0 µL of CuCl2 4M solution was added to the hydrophilic region between the graphite tracks. Before the polymerization, we kept the device at room temperature for 10 minutes. The polymerization process was very similar to our previous work.29 Briefly, 5.0 mL of pyrrole (Alfa Aesar, 98+%) were added to a one-neck flask and heated at 100 oC. We let the monomer pass through the system for 10 minutes as a pre-conditioning step. Next, the device was taken to the polymerization chamber and exposed to the monomer in the gas-phase for 30 minutes. After the polymerization process, the device was rinsed with deionized water for 1 hour. The PPy resistance was measured after keeping the sample for 2.5 hours in a desiccator.


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2.5. Calibration and construction of the fully-integrated humidity sensor For assessing the RH variations, the device was first placed inside of a sealed calibration chamber under a dry argon atmosphere. The increasing of RH occurs by forcing a flow of dry argon through a reservoir containing DI water. During the calibration procedure, both the temperature and humidity were monitored simultaneously using a digital thermo-hygrometer. The electrical properties were obtained by using a semiconductor parameter analyzer (4200) from Keithley. Copper wires were attached to the carbon tracks by using silver ink and epoxy resin. The calibration curve was done with the unfolded microfluidic devices by varying humidity inside the chamber and monitoring the electrical resistance. Before the folding steps, we cut the regions highlighted by the red lines in Figure 1a. Next, three folding steps are performed, as illustrated in the same Figure 1a. See Figure S2 for more details. Finally, we added an adhesive tape (3M Scotch) to seal the device entirely. Figure 1b shows a picture of the sealed device. We monitored the liquid flow in the main channel and assessed the variations in humidity using the integrated PPy sensor (Figure 1c) previously calibrated. The values of the resistance from the calibration curve were converted to RH and plotted as a function of time (s). All measurements take place at 22oC.



Figure 1. Schematics of the fabrication process. (a) The device is fabricated on a single sheet of paper. By folding steps, the integration and compacting are achieved. (b) Picture of the device during the liquid flow and electrical measurements. (c) The liquid flow (its position inside the channel) and RH were simultaneously monitored using the paper-based device (qualitative scheme).

2.6. Characterization The determination of the contact angle at room temperature was made using a Tensiometer Optical Theta L with software Attension. A thermal hygrometer from Minipa MTH-1362W, SP, Brazil was used to measure temperature and RH simultaneously. The electrical characterization was done using a Keithley digital multimeter model 2000. The investigation of the chemical composition of the modified interface was studied by X-ray photoelectron spectroscopy (XPS), using a K-Alpha system from Thermo Scientific. A 3D Laser Scanning Confocal Microscope (LSCM) from Keyence (model VK-X200 series), Osaka, Japan was used to map the surface roughness of the paper-based microfluidic devices. The surface roughness was outlined in four independent samples using bare and modified surfaces.


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3. Results and discussion 3.1. Polymerization and surface characterization To prepare the PPy conductive structures on paper, we have used an in situ polymerization procedure recently described by us.29 In brief, first copper ions in water are added to hydrophilic paper-based channels. Next, we expose the devices to pyrrole monomers in the gas-phase. This process leads to the formation of PPy conductive pathways within the entire thickness of the paper by coating the surface of the cellulose fibers. Such incorporation is possible because copper ions can freely flow through the complex network of cellulose fibers. Consequently, an electrical current path connects the top to the bottom surface of the paper. Here it is worth to mention that the monolithic integration of conducting tracks in the paper is not always possible since the porous nature of the substrates may suppress the penetration nanomaterials. As a porous material, the paper-based structures allow the diffusion of pyrrole monomers (Py) through the substrate. The time necessary for the pyrrole monomers to flow through the paper can be described by the Equation 1. 34

τ ∼

Equation (1)

where, τ is time (s), L is the distance in one dimension (m), and D is the diffusion constant of molecules of gas (m2 s-1). The changes in the paper color were used to monitor the PPy growth. During the synthesis, the polymerized region undergoes a color transition from green/light blue (copper solution) to black (PPy). Such a change occurs in less than 1 second, which is in good agreement with the theoretical value (16 ms). The polymerization procedure renders 3D conductive tracks with a low sheet resistance of approximately 40 Ω sq-1.



The functionalization process is made immersing the PPy structures in deionized water, to remove the excess of chlorine ions and to create an active layer more sensitive to the humidity changes.

35

The PPy modification is followed by controlling the resistance of

the structures as a function of the washing time. As expected, (see Figure 2a) the electrical resistance of the system increases with the washing time. The removal of chlorine ions is one of the probable causes for such increase. We noticed that for longer washing times the dispersion of the resistance values increases indicating that the PPy undergoes structural changes. Furthermore, both the fiber swelling and the uncontrolled removal of counterions may also contribute to the resistance dispersion. During the washing process, the Cl- ions diffuse out from the array of cellulose fibers promoting a decreasing of induced charges along the PPy backbone. We did not observe any fiber orientation in this type of paper.

Figure 2. (a) Variation or resistance versus washing time. X-ray photoelectron spectra for (b) as-grown and (c) after 60 minutes of washing. (d) Simplified schematic view of the chemical transformations of the PPy backbone during the washing process. Static contactangle measurement for each step of the preparation process, (e) as-grown and (f) after the modification process.


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By setting the washing time to 60 minutes, we investigated the chemical composition of the PPy structure interfaces by X-ray photoelectron spectroscopy (XPS). Figure 2b-c shows the N1s spectra for the bare and modified PPy structures on paper, respectively. Figure 2b presents a significant peak at 399.7 eV, which is ascribed to pyrrolylium nitrogens (−NH−). The second peak at 401.3 eV is attributed to positively charged nitrogen (−N+H−).36 Such peaks refer to the oxidized form of PPy. The chloride ions counterbalance the positive charge on the nitrogen atom. After the washing process, we have observed a shoulder in the XPS spectra at 398.1 eV that was not present in the previous spectra (see Figure 2c). This new shoulder is associated with the presence of imine-like structures (=N−).36,37 We found that the imine-like nitrogen represents approximately 28 % of the total of nitrogen in PPy. This percentage is in good agreement with others deprotonated PPy/anion complexes.36 The chemical composition at the interface obtained by the survey spectra revealed that the Cl/N ratio is 0.31. This result also agrees with other published works.37 Figure 2d shows a simplified scheme that illustrates the chemical changes in the PPy structure caused by the modification process. It is clear from Figures 2b-c that the modification by washing changes the surface chemistry of the material apart from the removal of Cl- ions. The presence of neutral imine-like structures may be responsible for the increasing the resistance of the PPy structure in/on paper. Moreover, the wettability of the interface is modified due to the formation of new chemical groups after modification. We performed contact-angle measurements to obtain the wettability of each interface.

Figure 2e-f shows the static contact-angle measurements of the bare and

modified structures, respectively. By comparing the two images, it is possible to observe



that the presence of imine-like structures affects the wettability of the PPy surface. The removal of Cl- ions renders the surface hydrophobic, with static contact-angles higher than 90o. It is important to highlight that paper have randomly arranged fibers that can work as micrometric protrusions. Also, air pockets localized on the interface between the water droplet and the substrate can have an impact on the wettability of the surface.38 To account for this effect, we performed advancing and receding contact-angles, as demonstrated in Table 1. Table 1. Static (CAst), advancing (CAad) and receding (CArec) contact-angles of PPy on paper. Bare PPy modified PPy (deg)

(deg)

Static

54 (±2)

110 (±10)

Advancing

58 (±4)

101 (±7)

Receding

20 (±3)

14 (±2)

Standard deviation represents 5 measurements.

The advancing and receding contact-angles of the water droplets are related to a different position at the three-phase contact line on the cellulose microfibers.39 The surface roughness before and after the PPy modification was found to be 13.8 (± 2.6) µm and 12.3 (± 1.7) µm, respectively. Figure S3 shows one of the laser scanning confocal images used to obtain the roughness. By considering the standard deviation obtained above in four independent samples, it is worth to point out that the surface roughness remains the same after the PPy modification, indicating that the treatment did not contribute to creating any additional structures that eventually could increase the contact angle.


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3.2. Bending tests The bending capability of conducting polymers have been recently studied on paper to prepare functional devices.40–42 The oxidation of pyrrole using FeCl3 on bare office paper has led to stable responses after 27 bending cycles.41 Supercapacitors prepared by first soaking pyrrole on paper and then adding FeCl3 to form PPy have shown to be stable after 100 bending cycles.40 These two examples of bare PPy on office paper show attractive features, namely low sheet resistance, and moderate folding stability. Filter papers have no additives and are more porous than office papers, which allows the fabrication of conductive structures by merely flowing the conducting polymers, or other precursors, to prepare the conductive patterns.28,29 For instance, Hamedi et al. prepared conductive tracks on paper that supports up to 500 folding cycles by flowing PEDOT:PSS in paper-based microfluidic channels.28 In this work, we tested the bending capability of the modified PPy incorporated in porous filter paper (see Figure 3).

Figure 3. (a) Bending test performed with the PPy sensor. The red baseline is the control experiment. (b) Bending angle versus time. Each cycle is completed in 10 s. The symbol (*) indicates the same curve in both graphs. As can be observed in Figure 3a, the resistance of the structure increases during the bending process and reaches its highest value in the region of maximum mechanical stress.



One cycle is completed when the bending angle reaches ninety degrees and returns to the flat position, as illustrated in Figure 3b. The changes in the electrical resistance are mainly caused by the mechanical stress on the cellulose fibers modified with PPy. These changes are reversible, and several bending cycles were obtained for the modified PPy. The cycling stability of the modified PPy in/on paper is kept over 600 cycles by considering a baseline increasing of 5%.

3.3. Humidity and mechanism of response We proceed to discuss the response of the modified PPy under RH variations. We monitored the changes of the resistance of the structure by switching the RH values between 3.6 and 73 % in ten cycles (see Figure 4a). Considering that the resistance systematically rises with RH, we hypothesize that water molecules are increasing the fiber volume. This swelling process may increase the PPy-coated cellulose fibers’ separation in the conducting network.43 The functionalization of cellulose fibers with vapors of organosilane renders the surface superhydrophobic, but care was taken to avoid exposing uncoated regions.34 A similar experiment was performed in other fiber-like surfaces modified with precursors in the gas-phase.44 Here, we cannot rule out the presence of uncoated cellulose fibers because R-OH groups are present in the structure and are prone to interact with water molecules readily.5 Moreover, the paper has a complex network of pores and fibers where water molecules may penetrate and increase the volume. To check our hypothesis, we created an experimental setup to map the paper surface by confocal laser microscopy during the exposure of water vapor.


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Figure 4. (a) The reversible response of the device by alternating RH between two levels (3.6 and 73 %). (b) Schematic layout of the experimental setup used for the in-situ surface mapping by the 3D laser scanning confocal microscope. Images of the surface before (c) and after (d) adding water underneath the modified PPy structure. The environment RH was stable at 59 %. Figure 4b shows a schematic layout of the paper-based device used in this experiment. The source of water molecules was kept 130 µm apart from the PPy structure. Figure 4c-d shows the 3D images for the control and water-vapor exposed surfaces, respectively. We observed that the blue regions (deeper regions) practically disappear from the map once water is added to the hydrophilic reservoir located underneath the PPy structure. This result suggests that water molecules in the gas-phase diffuse through the PPy porous embedded in the paper. It is important to mention that this image was obtained in a controlled environment and no extra gas flow was used in this experiment. The changes observed in the PPy layer illustrated in Figure 4d are due to the natural evaporation of water from the paper reservoir located underneath the sensor. We measured the volume (µm3) of the two selected regions, as depicted by the small squares positioned in Figure 4c-d. The results show an increase of 51% of the volume in the region 1 and 26% in 2. Thus, we



demonstrate that water vapor changes the volume of the PPy-coated cellulose fibers. Consequently, the paper swelling is also a relevant mechanism that modifies the current pathway in the conductive network. 3.4. Temporal humidity measurements inside of fully-enclosed microfluidic systems The combination of flexible electronics with microfluidic elements has motivated the creation of 3D conductive tracks patterned by hydrophobic walls on paper. Two fabrication approaches are usually adopted to accomplish it: (i) the flow of precursors in the microfluidic channels that will further react with monomers to form in situ the conductive polymer;29,45 and (ii) the direct incorporation of water-based organic conductive polymers in the channels.28 However, both routes share the same limitations when the goal is the fabrication of large-area electronics: the solvent evaporation. Next, we demonstrate that even fully enclosed, multi-layered devices can have their flow properties tailored by locally increasing the humidity inside of the microfluidic device. Such evaluation was possible because the RH sensor based on modified PPy is monolithically integrated inside of the microfluidic system. The flow in microfluidic paper-based analytical devices can be described by the Lucas-Washburn Equation 2:11 =

γ θ , µ

Equation (2)

where, l is the distance of the fluid front, γ the solution surface tension, r the effective pore radius of the paper, θ the solution contact-angle with the paper, µ the viscosity, and t the elapsed time. Equation (2) indicates that the flow velocity decreases with t for paper-based channels. Moreover, low local RH may also delay the flow.


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Figure 5a shows the curves obtained experimentally for l as a function of t under three distinct conditions, namely: open channel, filled-fully-enclosed channel, and emptyfully-enclosed channel. In general, for short distances (l < 2-3 cm) there is a small difference between open and closed devices. However, when the distance traveled by the liquid front is higher than 5.0 cm, the total flow time is significantly affected (see the open device measurements presented in Figure 5a). The average time necessary for the liquid front reaches 7.5 cm can decrease 10 minutes once the device is encapsulated. Such a process helps to minimize the fluid evaporation. It is important to mention that the wax patterns are hydrophobic but still very porous. Consequently, we cannot exclude the effect of evaporation inside of the enclosed device, as water molecules may diffuse through the hydrophobic pores. To minimize this effect, we have added water to the device reservoir and performed the flow experiments, as shown in Figure 5a (curve in blue). This new condition leads to a further decrease of the time required for the solution reaches the top of the channel, probably due to better control of the local humidity inside of the enclosed device.

Figure 5. (a) Distance versus time plots for the three conditions studied, namely open/empty reservoir, fully-enclosed/empty reservoir and fully-enclosed/H2O filled



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reservoir. (b) Calibration curve ∆R versus RH (%). (c) RH (%) versus time during the flow experiments.

The direct measurement of RH, and how it varies during the experiments inside of fully-enclosed devices are connected to both, the changes in the paper volume and the increasing of the electrical charge localization.46 In a bare PPy structure, the amount of counterions present in the film is high enough to guarantee a low electrical resistance, regardless of the presence of water. However, for the modified PPy, there is a lack of counterions and, consequently, the charge carriers are more localized.

35

As the RH

increases, the water molecules may trap an additional amount of counterions, promoting a further rise in the resistance by decreasing the delocalization length. Figure 5b shows the calibration curve of the RH sensor based on the modified PPy structure. The discussed modification route allows the sensing layer to be present on both sides of the substrate, and also on multi-layered structures.

29

Such a property represents a

step forward to detect analytes in 3D paper-based microfluidic devices with complexes architectures. There are RH sensors with a better performance,47–49 however, it is important to highlight that our goal is to directly and locally assess humidity changes in paper-based microfluidic devices using a simple but effective fabrication route. Figure 5c shows the temporal measurements of the local RH as water flows inside the channel. The RH values for the device with an empty reservoir (red trace) barely changes during the first 480 seconds. After that, a systematic increasing of RH is observed. We observe that at t=480 seconds, the position of the fluid front is well aligned with the modified PPy sensor. At this position, we estimate from Equation (1) that the time necessary for the water molecules, in the gas phase, to diffuse through the wax barriers and reaches the sensor, is ~8 seconds (See SI4 for more details). This result indicates that water


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locally increases the RH near the channel. In contrast, we notice that the RH increases continuously during the flow experiments, meaning that the water reservoir establishes a stable environment, by compensating the evaporation from the main channel followed by its diffusion. For instance, the time necessary for water molecules in the gas-phase to diffuse from the inlet channel to the PPy sensor is ~10 minutes, which is inconsistent with our observations. Thus, the continuous increase of RH inside of the device comes mainly from the water evaporation from the reservoir. When the reservoir gets filled, the RH inside of the microfluidic system remains practically constant. A further increase is observed once the liquid front approaches the sensor, as illustrated in Figure S5. By adding water to the reservoir, the RH increases around 5 to 10% at the final part of the channel. This variation has a significant effect on the curves illustrated in Figure 5a. Such differences are responsible for the decreasing of the time needed for the liquid to reach the top of the device.

4. Conclusions In this work, we reported for the first time the fabrication, characterization, and application of modified polypyrrole (PPy) structures directly integrated into a multilayered paper-based microfluidic device to assess local humidity changes. The sensor was fabricated by combining paper-microfluidics, in-situ polymerization, and the modification of the PPy structures. The later process changed the wettability and the surface chemistry of the material under investigation. The binary cooperation process combines surface transitions from hydrophilic to hydrophobic and the tailoring of the number of counterions



embedded in the PPy structure. The mechanism of response was studied by measuring the increase in the volume of cellulose fibers and the relationship between the RH and the structure’s electrical resistance. We propose that the rise in electrical resistance is a combined result of the screening of counterions by water molecules coupled with the rise of the cellulose fiber volume. The modified PPy on paper proved to be an attractive candidate for functional layers in flexible systems since the operation was very stable under mechanical stress tests, enduring more than 600 bending cycles with a response loss of about 5%. We demonstrate that there is an increase in the local RH in the regions patterned with wax, as the solution flows into the hydrophilic channel. By increasing the RH inside of the device, variations of 5-10% have a significant impact on the time necessary for the solution penetrates the channel in enclosed devices. The very same strategy employed for creating the modified PPy structure can be applied to construct novel functional elements to monitor additional physical/chemical properties, such as pressure and chemical composition, inside of the enclosed-device structure.43

Supporting Information: Patterning graphite ink on paper, pictures of the device and RH versus time plot during the process of filling the reservoir with water. Corresponding Author *E-mail: [email protected], phone +55(19)35175098 Author Contributions The manuscript was written through contributions of all authors. All authors have approved the definitive version of the manuscript. Notes


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The authors declare no competing financial interest. Acknowledgment The authors thank CNPq (Project 483550/2013-2) and FAPESP (Project 2013/22127-2 and 2014/25979-2) for the financial support. We also acknowledge Davi H. S. de Camargo and Leirson Daniel Palermo for their valuable help.

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ToC figure


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