Development of a New Generation of Ammonia Sensors on Printed

Subscriber access provided by Aston University Library & Information Services

Article

Development of a New Generation of Ammonia Sensors on Printed Polymeric Hotplates Ehsan Danesh, Francisco Molina-Lopez, Malick CAMARA, Alexia Bontempi, Andrés Vásquez Quintero, Damien Teyssieux, Laurent Thiery, Danick Briand, Nico F. de Rooij, and Krishna C. Persaud Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac501908c • Publication Date (Web): 21 Aug 2014 Downloaded from http://pubs.acs.org on August 26, 2014

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Analytical Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Analytical Chemistry

Development of a New Generation of Ammonia Sensors on Printed Polymeric Hotplates Ehsan Danesh1, Francisco Molina-Lopez2, Malick Camara2, Alexia Bontempi3, Andrés Vásquez Quintero2, Damien Teyssieux3, Laurent Thiery3, Danick Briand2, Nico F. de Rooij2, Krishna C. Persaud1*. 1. School of Chemical Engineering & Analytical Science, the University of Manchester, Manchester, M13 9PL, UK 2. Ecole Polytechnique Fédérale de Lausanne (EPFL), Institute of Microengineering (IMT), Sensors, Actuators and Microsystems Laboratory (SAMLAB), Neuchâtel, Switzerland 3. FEMTO-ST, Department MN2S, CNRS, Besançon, France

* Corresponding author: [email protected]; Tel. +44 (0) 161 3064892

1 Environment ACS Paragon Plus


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Conducting polyaniline-based chemiresistors on printed polymeric micro-hotplates were developed, showing sensitive and selective detection of ammonia vapor in air. The devices consist of a fully inkjet-printed silver heater and interdigitated electrodes on a polyethylene naphthalate substrate, separated by a thin dielectric film. The integrated heater allowed operation at elevated temperatures, enhancing the ammonia sensing performance. The printed sensor designs were optimized over two different generations, to improve the thermal performance through careful design of the shape and dimension of the heater element. A vapor-phase deposition polymerization technique was adapted to produce polyaniline sensing layers doped with poly(4-styrenesulfonic acid). The resulting sensor had better thermal stability and sensing performance when compared with conventional polyaniline-based sensors, and this was attributed to the polymeric dopant used in this study. Improved long-term stability of the sensors was achieved by electrodeposition of gold on the silver electrodes. Response to sub-ppm concentrations of ammonia even under humid conditions was observed.

KEYWORDS: polyaniline, chemiresistor, ammonia, inkjet printing, micro-hotplate, vapor phase deposition polymerization


Page 2 of 26

Page 3 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.

INTRODUCTION

Low-cost integrated-on-plastic chemiresistor devices for chemical sensing require the development of novel materials and technologies compatible with large scale manufacturing such as inkjet printing 1,2. For fabrication of inexpensive chemiresistors, intrinsically conducting polymers (ICP) such as polypyrrole (PPy) and polyaniline (PANI) as well as their derivatives are attractive due to their high sensitivity, ease of synthesis and ability to act as chemical vapor sensors under ambient conditions 3,4. Poor processability of ICPs including PANI 5 restricts the mass production of ICPbased sensors exploiting common solution-based thin film fabrication methods such as spin coating and inkjet printing. Hence, we investigated the deposition of thin films of conjugated polymers by in-situ oxidative polymerization directly onto a substrate. One way to achieve this is to apply the oxidant (precursor) by spin coating and subsequently expose the coated surface to monomer vapor, a process known as vapor-phase deposition polymerization (VDP) 6. VDP has previously been adapted to produce high quality thin films of poly(3,4-ethylenedioxythiophene) (PEDOT), PPy and PANI 7-12, as well as ultra-sensitive ICP-based gas sensors

13,14

. Since many

appropriate oxidizing agents such as ammonium peroxydisulfate (APS) are water-soluble, solution-based deposition techniques are easily adapted to coat or pattern the substrate with the precursor layer. This makes the VDP technique compatible with printing technologies 15,16. Since there is no restriction on the type of substrate that can be used in VDP process as opposed to electrochemical polymerization, inexpensive plastic substrates and high throughput fabrication technologies can be used to manufacture chemical sensors. Our target analyte was ammonia, since a great need exists for robust and cheap systems for real-time measurement of ammonia at ppb-ppm levels in environmental monitoring, food processing and medical applications

17,18

. PANI has previously been well-characterized for its



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

sensitivity to ammonia gas 19-21. However, the high affinity of PANI for ammonia has drawbacks due to slow desorption, so that recovery after exposure to ammonia may take a significant time. This problem, together with baseline drift and irreversible reactions in PANI-based sensors, has hitherto impeded their practical use for continuous monitoring applications. A strategy adapted by some researchers is to maintain the sensing layer at a higher temperature using external heaters

22,23

, in order to enhance recovery and reversibility of sensors based on PANI. This

increase in operating temperature affects the kinetics of binding between ammonia and the sensing layer and facilitates desorption of ammonia from PANI. However, conventional dopedPANI suffers from low thermal stability 24. Moreover, the use of external heaters is cumbersome and unfeasible for low-cost and efficient devices. Integrated heaters on foil using clean room technologies have already been demonstrated

25,26

, but cost-effective printed heaters on foil are

not yet available. Here, conducting polymer chemiresistors integrated on polymeric micro-hotplates are developed using techniques compatible with large area manufacturing. These devices consist of fully inkjet-printed silver heater and comb electrodes separated by a thin patternable dielectric film, and a polymeric acid-doped polyaniline sensing layer on top (Figure 1 and S1). The detection of very low concentrations of ammonia in sub-ppm range is demonstrated.


Page 4 of 26

Page 5 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.

EXPERIMENTAL SECTION

The sensor consisted of a PEN substrate, meander-shaped metallic heater, dielectric dry foil, interdigitated metallic electrodes and conducting polymer sensing layer. Two different generations of µ-hotplates were designed and fabricated by inkjet printing silver nanoparticlebased ink on PEN foil aiming to minimize the power consumption as well as the thermal gradient over the sensing area. PEN was selected as a substrate material because it presented the best trade-off between price and thermal stability, with a glass transition of 155 °C and a maximum process temperature higher than 180 °C 27. An initial development of a 24 mm2 sensor allowed exploring the heater design, the manufacturing process and the sensor materials. This preliminary study along with an amelioration of the printing resolution led to an improved 2nd generation sensor that was 24 times smaller (1 mm2), used thinner PEN substrate (50 µm instead of 125 µm) and also had a thinner dielectric dry foil film between heater and combed electrodes (14 µm instead of 50 µm). This reduction in device size and dielectric layer thickness reduced the power consumption and the thermal inertia, and minimized heat dissipation.

2.1.

Heaters

The heaters from the 1st generation were designed as large meanders to simplify printing, whereas the heaters belonging to the improved second generation were smaller in area and presented a symmetrical square double meander shape to ameliorate thermal homogeneity. The design of the two generations of the heaters is depicted in Figure S2. The thermal design of the heaters was supported by finite element method (FEM) modelling (see Figure S3). These heaters were fabricated by inkjet printing (Dimatix DMP 2800 printer) of a silver nanoparticle-based ink (SuntTronic Jet EMD506 from SunChemical) on 50 or 125 µm-thick



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

PEN (Teonex®Q65FA from Dupont Teijim Films). Figure 1 documents the printed electrodes produced. Two layers were printed selecting a drop-to-drop distance of 25 µm for the first generation of heaters, and three layers at a drop-to-drop distance of 40 µm were printed to fabricate the second generation. The patterns were sintered in a convection oven for 3 hours at 180 °C. Electrodeposition of a thin layer of gold on the silver heater and electrodes was also carried out to improve robustness 27,28.

2.2.

Dielectric layer and interdigitated electrodes (IDE)

Once the heater was fabricated, a photo-patternable dry film photoresist (PerMX™ 3000 from DuPont™) was laminated onto the heater as a dielectric film (Figure S1b). The dry photoresist was patterned via photolithography to open contact windows on the pads of the device. The film must be as thin as possible to optimize the heat transfer from the heater to the sensing layer onto the film. Interdigitated electrodes were then inkjet-printed on the dielectric film to complete the fabrication of the transducer (Figure S1c). Two layers of the same silver ink used for the heater were deposited with a drop-to-drop space of 40 µm, and thermally annealed for 2 hours at 180 °C. Silver is known to oxidize which limits chemical compatibility with many sensing layers. Therefore, electrodeposition of 400 ± 270 nm of gold on top of the printed electrodes was adapted to enhance compatibility and stability. Figure 1 shows optical images of µ-hotplate examples from both generations, before deposition of the conducting PANI sensing layer. A device with an interdigitated electrode plated with gold is also depicted. Using inkjet technology enabled us to fix almost all printing defects on demand, resulting in a good fabrication yield.


Page 6 of 26

Page 7 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.3.

Preparation of the PANI sensing layer

The sensing layer was deposited onto the IDE area of the substrates (see Figure S1d). The VDP method adapted to make a thin polymeric acid-doped PANI sensing layer on the polymeric hotplates. A 3 wt.% aqueous solution of poly(4-styrenesulfonic acid) (PSSA) was prepared by dilution from the commercial solution (PSSA, Mw ~75,000, 18 wt.% solution in water). 300 mg of the oxidizing agent, APS, was then added into 5 ml of the solution to make homogenous mixture of dopant and oxidant. The substrate was spin coated with the precursor solution and then exposed to monomer (freshly distilled aniline) vapor in a custom-built VDP chamber for 1 hr. The monomer container was heated to 75 ºC during the VDP process in order to facilitate its evaporation. The resulting PSSA-doped PANI film on the µ-hotplate was removed from the VDP chamber, rinsed with distilled water and dried at 65 °C. The process was the same for both generations of the µ-hotplates. All the chemicals were purchased from Sigma-Aldrich. The aqueous solutions were made using ultra-high purity water purified using a Milli-Q 50 system (Millipore Co.).

2.4.

Characterization

The power consumption and the thermal distribution over the active area of the µ-hotplates were characterized using type S micro-thermocouples (1.3 µm diameter) mapping surface temperature over the device area

29,30

, while different DC currents were supplied to the heater. The results

were compared to FEM simulations carried out using COMSOL (version 4.2) software (see Supporting Information section S2 and Figure S3). UV-visible spectroscopy was used to study the doping behavior of PANI films deposited on cleaned glass substrates using the VDP method. The transmittance spectrum was recorded using a Perkin-Elmer lambda 35 spectrometer with a scanning velocity of 120 nm min−1. A homemade



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

automatic ammonia vapor generation system was designed and utilized for characterizing the sensing performance of the sensors. The electrical response of the fabricated sensors towards different NH3 vapor concentrations in dry air was measured as the change in the resistance. Ammonia vapor over the range of 250 ppb to 3.65 ppm was generated using a permeation tube with calibrated permeation rate of 1042 ± 14 ng min-1 at 25 ºC. A custom-built humidifier provided moist air whenever needed.

3.

RESULTS AND DISCUSSION

3.1.

Thermal characteristics of the µ-hotplates

Figure 2 shows the relationship between temperature and dissipated power for the two generations of the fabricated heaters. The power ranges from 0 to 350 mW for the first generation and up to 80 mW for the second generation, enough in both cases to reach the maximum temperature of 100 °C targeted for the operation of the sensor. The reported temperature is that at the center of the device, on the surface of the dielectric laminated above the heater. The data for both generations of heaters showed linear relationship with a gradient of 0.248 ± 0.008 mW °C-1 and of 1.93 ± 0.02 mW °C-1 for the 1st and the 2nd generations, respectively. The second generation of heaters exhibited an efficiency of 7.8 times greater than the first. The generated temperature for a given power value matched the FEM simulation (Figure S3). 3.2.

PSSA-doped PANI thin films

Usually electrically conducting polyaniline is made by the protonation of emeraldine base with low molecular weight acids such as hydrochloric acid. However, the electrical properties of these materials change abruptly with increasing temperature, due to evaporation or segregation of the dopant

24

. Using macromolecular polymeric acids to protonate PANI can enhance the heat 8 Environment ACS Paragon Plus

Page 8 of 26

Page 9 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


stability and life-time of the layer

31

. Here, PSSA is used in the VDP precursor solution to

produce polymeric-doped conducing polymer thin films with good uniformity and high stability. We observed that the presence of water vapor was essential in the VDP process. Without H2O, the synthesis did not result in conducting PANI. The role of water has been already highlighted in several papers

7,8,32-34

, as an effective proton scavenger in the polymerization process of

PEDOT and PPy. We believe the same mechanism is operative for PANI, where water deprotonates dimers formed in the early stages of polymerization. The stabilized dimers then participate in polymer chain growth. UV-Vis transmittance spectrum of the deposited PANI layer on glass (Figure S4) showed characteristic peaks at 435 and 810 nm which are entirely consistent with reports on the doped PANI structure

35,36

: the peak at 435 nm is due to the

presence of localized semiquinone population or the polaron absorption (equivalent to: polaron band to π* transition). The peak at 810 nm is due to the trapped excitons centered on quinoid (imine) (equivalent to: π- to polaron band transition). 3.3.

Ammonia sensing properties

3.3.1. The first generation sensors Emeraldine salt exhibits p-type semiconductor characteristics; consequently, electron-supplying gases such as NH3 reduce the charge-carrier (polaron) concentration and decrease the conductivity, in a reversible chemisorption process: ⋯ + ⇌ +

Equation 1

The 1st generation sensors were fabricated by depositing thin films of PSSA-doped PANI on the sensing area of the first generation µ-hotplates using VDP process. Figure 3 compares the response of the sensor obtained for consecutive ammonia vapor concentrations over the range of 250 ppb to 3.65 ppm in dry air at different operating temperatures. The sensor was then purged



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 26

with air for recovery. The response is defined by the relative change in the resistance of the sensor: ∆ − = × 100%

Equation 2

where R and Rb are the sensor’s transient and baseline resistance, respectively. Increase in the operating temperature from ~20 to ~80 ºC significantly enhanced recovery of the response to initial value after exposure to ammonia. This can be explained by the equilibrium interaction between doped PANI and ammonia molecules as stated in Equation 1: heating increases the dissociation rate of ammonium ion into ammonia and proton, shifting the equilibrium to the left. Hence, the sensor resistance approaches faster to its Rb due to reprotonation of PANI. The enhanced recovery also diminishes the baseline drift over time due to faster regeneration of the sensor at elevated temperatures. Nevertheless, less ammonia is absorbed by PANI layer at elevated temperatures, so the response magnitude is reduced compared to room temperature (RT ~20 °C). To ascertain the effect of temperature on the response performance of the 1st generation sensor, the sensitivity (% ppb-1) and recovery (%) values were defined at each temperature for the concentration range and time intervals studied here (Figure 4a and b), and the results are shown in Figure 4c. As the temperature increases, sensitivity decreases exponentially according to the equation obtained from curve fitting (Figure 4c, solid line): Sensitivity = 0.0107 + 0.0830 × exp$−0.0983 × T', where T is the temperature in degrees Celsius. Such behavior is in agreement with Kukla and co-workers’ report 37 in which ammonia adsorption onto poly(methyl methacrylate)-PANI composite was considered as a reversible chemisorption process. On the other hand, recovery increases linearly from ~45 % at ambient temperature to ~90 % at 80 °C (Recovery = 0.776 × $T + 33.045') (Figure 4c, dashed line). Thus, heating to 80 ºC results in


Page 11 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


desorption of the most of the tightly bound NH ions, followed by sensor regeneration. Sensors operated at temperatures higher than 80 ºC showed no measurable response up to 750 ppb. 3.3.2. The second generation sensors The same procedure was used to deposit a thin PSSA-doped PANI on the 2nd generation µhotplates’ sensing area. The sensor was exposed sequentially to ammonia vapor and dry air for 5 min and 15 min, respectively, and the response magnitude was compared at RT and 95 ºC (Figure 5a). Although the heated sensor showed a small drop in sensitivity compared to that of RT, it remained operative in detection of ammonia at very low concentrations. The average recovery of the heated sensor was improved by a factor greater than 2 when compared to recovery of the sensor operated at RT (Figure 5b). The gold plating of the IDEs did not result in any significant change in response of the 2nd generation sensor over the original silver electrodes (Figure S6). The theoretical limit of detection (LOD) was improved from 129 ppb for the 1st generation to 92.2 ppb for the 2nd generation devices (see section S4 of the Supporting Information). Both sensor designs showed acceptable device-to-device reproducibility (Figure S6). 3.3.3. Effect of humidity Humidity often affects many conducting polymer sensors, so the effect on electrical conductivity and ammonia sensing of the PSSA-doped PANI was investigated. The electrical resistance of the sensor decreased with increase in the humidity level, both at RT and when heated (Supporting Information section S6, Figure S7). The effect of water vapor on the ammonia sensing of the sensor is summarized in Table 1. The 2nd generation sensor was exposed to four different concentrations of ammonia vapor in dry air and absolute humidity of 5000 mg m-3. At RT, the response magnitude is greater for the sensor



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 26

working in humid conditions. When operating in humid air, water molecules sorbed in the sensing layer resulted in an increase in conductivity. Following exposure to NH3 molecules, these not only interact with polyaniline, but also react with water in the layer and produce ammonium hydroxide: $1' ⇌ $23' + 4 5 ⇌ + 5 6

Equation 3

hence, hydroxide ions produced further deprotonate PANI and increase its resistance. Moreover, the higher Rb in dry air may attribute to its lower relative response. Due to the opposing effects of water vapor and ammonia on the conductivity, the overall sensitivity remains lower in humid air compared to dry air, in spite of higher response magnitudes. When the heater is operated at 95 ºC, the moisture content is reduced in the sensing layer. The response behavior at this condition can be described based on competitive sorption of NH3 and H2O molecules. During exposure to ammonia, the NH3 molecules are able to displace weakly sorbed H2O molecules, directly interact with the PANI and deprotonate it. However, the Equation 1 is shifted to the left at elevated temperatures and both the response magnitude and sensitivity are lower compared to RT conditions. 3.3.4. Cross-sensitivity To assess the cross-sensitivity to other chemical vapors, the 2nd generation sensor was characterized (Figure 6). The sensor showed only a slight increase in conductivity in exposure to high concentrations of methanol, ethanol, chloroform, acetone and n-butylacetate (Figure 6inset), while the exposure to ammonia in comparable concentrations (2.83 ppth) resulted in a drastic increase in resistance. This results show the high selectivity of the sensor to ammonia. Although the sensor response to these chemicals were fast and reversible, the regeneration of the sensor after exposure to ammonia at such a high concentration was slow and it took 18 hr for the


Page 13 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


sensor to recover to ~90 % of its original baseline resistance, even when heated. The partial recovery indicates irreversible chemical reactions may have occurred in the sensing layer at these high concentrations. Sensor poisoning was not observed for the sensors exposed to low concentrations of ammonia (up to 5 ppm) even over extended periods. 3.3.5. Sensor stability The long term stability of the sensors was examined by monitoring their baseline drift over a three-week period. The 1st generation sensors as well as the 2nd generation sensors (with and without gold electroplating) were heated to around 95 °C and a flow of dry air passed over the sensors at a constant rate. The data were recorded after an initial 24 hr induction period and the results are compared in Figure 7. All the sensors showed a continuous decrease in conductance over time. This is a common phenomenon for conducting polymer-based sensors 22,38 and can be attributed to the gradual oxidation of PANI in the presence of oxygen molecules in air which results in degradation of electrical conductivity. Besides, dry air flow in combination with the elevated temperature accelerates the evaporation of water molecules trapped in the bulk of sensing layer due to the hygroscopic nature of the polymeric dopant, and causes a further drop in conductivity. The baseline resistance values remained comparatively stable up to 7 days for all three samples; however the baseline resistance of the devices with silver IDE started to increase monotonically after one week. Surface oxidation of the silver electrodes may cause this issue. Moreover, the poly(4-styrenesulfonic acid) may slowly react with the silver at the interface of the sensing layer and the electrodes, and convert it to silver sulfonate moieties (Figure 7-inset a). Heating speeds up these processes which eventually increase the contact resistance, and the measured conductance of the sensor drops. In contrast, the chemically inert gold-plated IDE device showed no change in its surface (Figure 7-inset b), and baseline resistance was



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 26

significantly more stable with R/R0=1.928 after 21 days. However, in spite of the baseline drift, even the silver IDE sensors were operational for at least 2 months. CONCLUSIONS PSSA-doped polyaniline chemiresistors have been realized on novel inkjet-printed polymeric µhotplates using vapor-phase deposition polymerization method for robust detection of ammonia vapor in sub-ppm concentration range in air. Two generations of printed micro-hotplates with different dimensions were developed on PEN substrate. The hotplates were processed using a combination of silver ink-jet printing and lamination processes compatible with large area manufacturing. The second generation of devices outperformed the first due to their smaller size, thinner substrate and thinner dielectric film used, demonstrating the crucial importance of good printing resolution and structure design. Both generations of micro-hotplates displayed improved sensing characteristics at elevated temperatures. However, the devices based on the second generation of heaters showed the best performance, achieving sensitive and selective ammonia detection with theoretical detection limit of less than 100 ppb when operated at 95 °C. The sensors were operational even at a very high humidity. The electrodeposition of gold on the silver seed patterns has been demonstrated for improved long-term stability. Such devices may be incorporated in smart RFID tags for food freshness and quality traceability control through packaging. ASSOCIATED CONTENT Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENTS


Page 15 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The authors are grateful to the FP7 EU Marie Curie Initial Training Network, FlexSmell (grant n°238454), for partially funding the work presented, as well as to Dr. Bill A. MacDonald from Dupont Teijim Films for supplying the plastic foils used as the substrate.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 26

REFERENCES (1) Briand, D.; Oprea, A.; Courbat, J.; Bârsan, N. Materials Today 2011, 14, 416-423. (2) Claramunt, S.; Monereo, O.; Boix, M.; Leghrib, R.; Prades, J. D.; Cornet, A.; Merino, P.; Merino, C.; Cirera, A. Sensors and Actuators B: Chemical 2013, 187, 401-406. (3) Janata, J.; Josowicz, M. Nat Mater 2003, 2, 19-24. (4) Persaud, K. C. Materials Today 2005, 8, 38-44. (5) Wessling, B. In Handbook of Nanostructured Materials and Nanotechnology, Vol.5: Organics, Polymers and Biological materials, Nalwa, H. S., Ed.; Academic Press, 1999, pp 501-576. (6) Winther-Jensen, B.; Chen, J.; West, K.; Wallace, G. Macromolecules 2004, 37, 5930-5935. (7) Mohammadi, A.; Hasan, M. A.; Liedberg, B.; Lundstrom, I.; Salaneck, W. R. Synthetic Metals 1986, 14, 189-197. (8) Fabretto, M.; Zuber, K.; Hall, C.; Murphy, P. Macromolecular Rapid Communications 2008, 29, 1403-1409. (9) Winther-Jensen, B.; West, K. Macromolecules 2004, 37, 4538-4543. (10) Bhattacharyya, D.; Howden, R. M.; Borrelli, D. C.; Gleason, K. K. Journal of Polymer Science Part B: Polymer Physics 2012, 50, 1329-1351. (11) Kim, J.; Kim, E.; Won, Y.; Lee, H.; Suh, K. Synthetic Metals 2003, 139, 485-489. (12) Kim, J.-Y.; Lee, J.-H.; Kwon, S.-J. Synthetic Metals 2007, 157, 336-342. (13) Chen, Y.-s.; Li, Y.; Wang, H.-c.; Yang, M.-j. Carbon 2007, 45, 357-363. (14) Kwon, O. S.; Park, S. J.; Yoon, H.; Jang, J. Chemical Communications 2012, 48, 1052610528. (15) Cho, J.; Shin, K.-H.; Jang, J. Thin Solid Films 2010, 518, 5066-5070. (16) Cho, J.; Shin, K.-H.; Jang, J. Synthetic Metals 2010, 160, 1119-1125. (17) Abel, T.; Ungerbock, B.; Klimant, I.; Mayr, T. Chemistry Central Journal 2012, 6, 124. (18) Timmer, B.; al., e. Sensors and Actuators B: Chemical 2005, 107, 666-677. (19) Hu, H.; Trejo, M.; Nicho, M. E.; Saniger, J. M.; Garcı́a-Valenzuela, A. Sensors and Actuators B: Chemical 2002, 82, 14-23. (20) Wojkiewicz, J. L.; Bliznyuk, V. N.; Carquigny, S.; Elkamchi, N.; Redon, N.; Lasri, T.; Pud, A. A.; Reynaud, S. Sensors and Actuators B: Chemical 2011, 160, 1394-1403. (21) Liu, H.; Kameoka, J.; Czaplewski, D. A.; Craighead, H. G. Nano Letters 2004, 4, 671-675. (22) Crowley, K.; Morrin, A.; Hernandez, A.; O'Malley, E.; Whitten, P. G.; Wallace, G. G.; Smyth, M. R.; Killard, A. J. Talanta 2008, 77, 710-717. (23) Danesh, E. and Persaud, K. C., In 14th International Meeting on Chemical Sensors - IMCS 2012; AMA Science: Nürnberg/Nuremberg, Germany, 2012, pp 1134 - 1136. (24) Nicolas-Debarnot, D.; Poncin-Epaillard, F. Analytica Chimica Acta 2003, 475, 1-15. (25) Briand, D.; Colin, S.; Courbat, J.; Raible, S.; Kappler, J.; de Rooij, N. F. Sensors and Actuators B: Chemical 2008, 130, 430-435. (26) Courbat, J.; Briand, D.; Yue, L.; Raible, S.; de Rooij, N. F. Sensors and Actuators B: Chemical 2012, 161, 862-868. (27) Camara, M.; Molina-Lopez, F.; Danesh, E.; Mattana, G.; Bontempi, A.; Teyssieux, D.; Thiery, L.; Breuil, P.; Pijolat, C.; Persaud, K. C.; Briand, D.; de Rooij, N. F. In Solid-State Sensors, Actuators and Microsystems (TRANSDUCERS & EUROSENSORS XXVII), 2013 Transducers & Eurosensors XXVII: The 17th International Conference on, 2013, pp 10591062. 16 Environment ACS Paragon Plus

Page 17 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(28) De Koninck, D. A.; Lopez, F. M.; Briand, D.; de Rooij, N. F. In Micro Electro Mechanical Systems (MEMS), 2012 IEEE 25th International Conference on, 2012, pp 64-67. (29) Thiery, L.; Briand, D.; Toullier, S.; Teyssieux, D. Journal of Heat Transfer 2008, 130, 091601-091601. (30) Bontempi, A.; Thiery, L.; Teyssieux, D.; Briand, D.; Vairac, P. Review of Scientific Instruments 2013, 84, 103703. (31) Lu, X.; Tan, C. Y.; Xu, J.; He, C. Synthetic Metals 2003, 138, 429-440. (32) Fabretto, M.; Zuber, K.; Hall, C.; Murphy, P.; Griesser, H. J. Journal of Materials Chemistry 2009, 19, 7871-7878. (33) Mueller, M.; Fabretto, M.; Evans, D.; Hojati-Talemi, P.; Gruber, C.; Murphy, P. Polymer 2012, 53, 2146-2151. (34) Ha, Y. H.; Nikolov, N.; Pollack, S. K.; Mastrangelo, J.; Martin, B. D.; Shashidhar, R. Advanced Functional Materials 2004, 14, 615-622. (35) Tzou, K.; Gregory, R. V. Synthetic Metals 1993, 53, 365-377. (36) Xia, Y.; Wiesinger, J. M.; MacDiarmid, A. G.; Epstein, A. J. Chemistry of Materials 1995, 7, 443-445. (37) Kukla, A. L.; Shirshov, Y. M.; Piletsky, S. A. Sensors and Actuators B: Chemical 1996, 37, 135-140. (38) Yoon, H. Nanomaterials 2013, 3, 524-549.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 26

TABLES: Table 1. Effect of temperature on the 2nd generation sensor response to ammonia vapor in dry and humid air. (AH = absolute humidity; mg m-3). Sensor Response† (∆Rmax/Rb, %) NH3 concentration (ppb)

Room Temperature

95 ºC (Heater Power=35 mW)

AH=0

AH=5000

AH=0

AH=5000

350

1.87 (±0.42)

3.76 (±0.59)

1.59 (±0.56)

1.61 (±0.47)

500

2.89 (±0.56)

4.33 (±0.72)

2.26 (±1.20)

2.05 (±0.61)

1000

5.97 (±0.88)

7.51 (±0.45)

4.86 (±1.40)

3.14 (±0.64)

2000

10.94 (±2.65)

11.49 (±1.31)

8.89 (±1.65)

6.20 (±1.14)

Sensitivity‡ (% ppb-1)

0.0053 (R2=0.9963)

0.0047 (R2=0.9885)

0.0044 (R2=0.9964)

0.0028 (R2=0.9952)

†: Average maximum response of the sensor over a 5-min exposure. The figures in parenthesis are the standard deviation of 5 measurements using the same sensor. ‡: Sensitivity is calculated based on the slope of the linear regression fit to the quasi linear regime of the calibration curve.


Page 19 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FIGURES:

Figure 1. Optical images of µ-hotplates depicting examples of both generations, including flexible arrays, close views of a single device and a device with a gold-plated interdigitated electrode. The IDE consisted of two interdigitated combs. The pitch of the electrodes was 120 µm, corresponding to a width of 68 ± 8 µm and an inter-finger spacing of 52 ± 8 µm. The electrodes’ total thickness was 260 ± 50 nm.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 26

Figure 2. Temperature measured at the center of the micro-hotplate as a function of the heating power: Comparison between the first and second generations.


Page 21 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Response of the 1st generation sensor towards various concentrations of ammonia vapor from 250 ppb to 3.65 ppm in dry air, at four temperatures: RT, 40, 60 and 80 °C with exposure and recovery time of 10 min and 30 min, respectively.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4. a) Sensitivity is defined as the slope of the quasi-linear regime of the calibration curve (response magnitude vs. concentration). b) Recovery is defined as the ratio of the recovered 7

response to the maximum response ( 789:

67;

'; where Rb, Rmax and Rrec are baseline resistance,

maximum resistance reached during exposure to the analyte and the resistance at the end of the specified recovery period. c) Effect of µ-hotplate temperature on sensitivity (open square) and recovery (closed circle) of the 1st generation sensor. The sensor was exposed 10 min to ammonia, followed by 30 min recovery. The solid curve and dashed line show curve fits for sensitivity and recovery data, respectively. The error bars represent the standard deviation of 5 measurements using the same sensor.


Page 22 of 26

Page 23 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. a) The 2nd generation sensor response magnitude (∆Rmax/Rb) to ammonia during a 5 min exposure at RT (closed square) and 95 ºC (open circle; the heater power consumption was 35 mW). b) Enhancement of the recovery of the sensor at 95 ºC compared to that of RT. The sensor was purged with clean dry air for 15 min after each ammonia exposure. The error bars represent the standard deviation of 5 measurement cycles using the same sensor.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 26

Figure 6. Selectivity test of the 2nd generation sensor: resistance change in exposure to: 1) acetic acid (3.19 ppth), 2) acetone (1.16 ppth), 3) chloroform (2.26 ppth), 4) n-butyl acetate (0.58 ppth), 5) ethanol (3.12 ppth), 6) methanol (4.51 ppth) and 7) ammonia (2.83 ppth). The ordinate on the inset was multiplied by a factor of 50 from the ordinate on the main plot of the figure. The value at the low end and high end of the ordinate of the inset are 3.12 and 4.12 MΩ, respectively. The baseline value in the inset is 3.87 MΩ.


Page 25 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. Baseline drift of the sensors over an aging period of 21 days. The sensors were heated to about 95 °C and dry air passed over them. R0 is the initial resistance on day 1. The inset shows the optical image of the electrodes’ surface on the 2nd generation sensor after 2 months: (a) an oxidized silver electrode and (b) an intact gold-electroplated electrode.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

FOR TOC ONLY:


Page 26 of 26

Development of a New Generation of Ammonia Sensors on Printed

Recommend Documents