Screen-printed electroluminescent lamp modified with graphene oxide as a sensing device

A screen-printed electroluminescent display with different sensing capabilities is presented. The sensing principle is based on the direct relationship between the light intensity of the lamp and the conductivity of the external layers. The proposed device is able to detect the ionic concentration of any conductive species. Using both top and bottom emission architectures, for the first time, a humidity sensor based on electroluminescent display functionalized by a graphene oxide nanocomposite is introduced. In this regard, just by coupling the display to a smartphone camera sensor, its potential was expanded for automatically monitoring human respiration in real time. Besides, the research includes a responsive display in which the light is spatially turned on in response to pencil drawing or any other conductive media. The above mentioned features together with the easiness of manufacturing and cost-effectiveness of this electroluminescent display can open up great opportunities to exploit it in sensing applications and point-of-care diagnosis.


INTRODUCTION
The growing and continuous demand for flexible optoelectronics [1][2][3][4][5][6][7][8] together with the evolution of wearable/portable electronics accessories have made possible that devices with good mechanical manipulation as folding, bending or rolling open up new opportunities for diverse applications [9][10][11][12][13][14][15][16][17] . In this perspective, alternating current electroluminescent (ACEL) displays are very promising due to their intrinsic ability of uniform light emission, low heat generation, flexible architecture and low power consumption [18][19][20][21] . Also, this type of devices can be easily fabricated by inexpensive screen-printing methods. So far, ACEL has been extensively used in commercial applications such as black lighting, decorative lighting, panel display or even largescale billboard 19,[22][23][24] . Nevertheless, the expansion of ACEL display technology toward the sensing field is still rather limited and have been barely exploited [25][26] representing, therefore, a big challenge for the scientific community.
Usually, to build a conventional ACEL display, transparent conductive materials on a transparent substrate are strictly required as a rear electrode in which the emitted light goes through the substrate located at the bottom 20,[27][28][29] . The transparent electrode produced from indium tin oxide (ITO) is typically used for this purpose. Alternatively, top-emission structure (TES) has been explored 19 . In TES architecture, the light from the phosphor layer is emitted directly through a top transparent conductive material, while the substrate is at the bottom (see Figure 1a). In contrast to bottom-emission structure (BES), this reversed architecture usually yields a higher brightness (under the same applied voltage) due to the shorter light path 19 .
Besides, various substrates (e.g. paper, plastic or textile) can be chosen since there is no requisite for the substrate to be transparent 30-31 . By taking advantage of the different configurations, we envisioned that an ACEL display would be useful to create a versatile sensing platform. The light emission of an ACEL display mainly relies on the conductivities of the top conductive layer 32 .
Hence, the change in conductance in the top layer of the ACEL display coming from an exposure of ions/charge carriers could alter the intensity of the emitted light. The direct relationship between the luminance and the conductivity could lead to sensing of a variety of conductive species, through the electroluminescence monitoring.
In this study, we engineered a powerful sensor based on ACEL display architecture lacking in the top electrode. This sensor is capable to detect and directly visualize the target analyte. To demonstrate its sensing ability, the ionic concentration of various water samples was monitored 4 as proof of concept. In addition, we took advantages of graphene oxide (GO) and its interesting properties in devices applications [33][34][35] . Using both TES and BES architectures, for the first time, a   humidity sensor based on ACEL display functionalized by GO nanocomposite is introduced. Furthermore, we demonstrated the high-performance of the developed sensor as point-of-care (POC) device to monitor human breath which may be interesting for further applications.

RESULTS AND DISCUSSION
An ACEL lamp was fabricated using a screen-printing technique. The overall schematic of a lamp structure (5 x 5 cm) is shown in Figure 1a. For full fabrication of ACEL lamp with TES configuration, the process began with coating a layer of silver paste ink onto the substrate as the rear electrode, followed by a sequential coating of the dielectric layer. The phosphor was then coated on top of the dielectric layer. Finally, another layer of clear conductive ink was coated as the top electrode. As was mentioned above, in TES structure, the range of substrates is not limited to a transparent substrate; opaque flexible substrates varying from papers to commercial fabric can be used. As illustrated in Figure S1, the screen-printed electroluminescent lamp was fully achieved with the subsequent white light emission on different substrates (paper and plastic). Such a wide range of substrate selection may tune the resulting electroluminescence as well as the elasticity of the device platform. Other than being just a simple light panel, this ACEL display offers promising applications. As proof of concept, various sensing platforms based on flexible ACEL display are developed and proposed ( Figure 1b). Using the dependence between light emission and conductivity of ACEL display as a primary tool, a variety of sensing approaches were designed and applied. For instance, the sense of (i) ionic concentration/conductance allowing water sample analysis, (ii) writable conductance (responsive board), (iii) relative humidity level, and (iv) human breathing using a smartphone will be demonstrated in this work.
Fabrication of a multilayered ACEL display with TES configuration was further corroborated by scanning electron microscopy (SEM) cross-sectional image. As depicted in Figure 2a, the thickness of the silver paste electrode, dielectric layer, and phosphor layer are about 18 µm, 89 µm and 71 µm, respectively. Energy dispersive X-ray (SEM/EDX) analysis in line-scan mode was also performed to analyze the elemental constituent of each layer ( Figure S2). A major component of phosphor layer was substantiated to be ZnS particles, while the subsequence layer of dielectric is made of BaTiO3 particles. The individual layer surface morphology was also characterized by SEM ( Figure S3). Illustrating in figure S3a is an irregular shape of silver particles, which is extensively deposited over the whole substrate surface. As displayed in figure   S3b, the dielectric layer exhibited a very rough surface, consisting of dense BaTIO3 particles. In addition, in Figure S3c, together with the cross-sectional image, the light emitting phosphor layer appeared as a smoother surface compared to the underlying dielectric layer. The large grain phosphor particles distributed evenly over the surface. This result indicates that the device can determine the ionic concentration/conductivity. Furthermore, using the light sensor embedded in a smartphone camera as an optical detection tool, the linear calibration between light intensity (lux) and logarithmic concentration of NaCl was established in the range of 1-100 mg L -1 (Figure 3b-c). It allows the quantitative evaluation of the ionic concentration. As proof of concept, water samples including milli-Q, distilled, and 9 tap water were inquired with the ACEL display. As expected, it was evident in the Figure 3d that tap and distilled water emit higher luminescence than milli-Q water owing to their larger dissolved ions concentration in the water.
As a result of this unique conductance-based light emission property, any conductive material could be visualized on this display. Thus, a simple responsive ACEL display with TES architecture has been exemplified in this work. Tap water or office pencil, for example, can be used as a conductive stylus to draw onto the surface of an ACEL panel. As indicated in Figure 3e and Movie S2, the light turns on instantaneously when this conductive media touched the electrode bar. Even though the electrode bar is inevitable, minimizing the bar size can make it invisible. We further broaden the potential of the electroluminescent platform to humidity sensing application which is of prime importance in various fields. Numerous publications for humidity monitoring have been reported over years. Most of them are based on capacitive and resistive change [36][37][38][39][40][41][42][43] . However, to the best of our knowledge, there is no report for humidity sensor based on electroluminescence. Therefore, a humidity sensor based on ACEL lamp using BES and TES structure functionalized by graphene oxide (GO)/Nafion nanocomposite as a sensitive material was applied here for the first time.
In this application, the sensing mechanism for an ACEL-based humidity sensor could be attributed to the proton conduction of nanocomposite formed between the 2D material (GO) and Nafion under a hydrated environment. In particular, GO derivative with oxygen-containing functional groups (−O−, −OH, and −COOH) possesses super-permeability to water molecules 44 .
At low relative humidity (RH), the physisorbed water molecules on active site of GO (oxygencontaining functional groups) cannot move freely because of the restriction from double H−bond. For this reason, large energy is required for the hopping transfer of protons between adjacent active site, and therefore, results in strong electrical resistance. However, as %RH increases, the physisorbed water layer becomes mobile and behaves like a bulk liquid. Thus, the proton hopping mechanism between water molecules occurs in GO with charge transport taking place via the conductivity generated by a Grotthuss chain reaction, which in turn yields in an increase of conductance [45][46] . Correspondingly, Nafion, a sulfonated fluoropolymer is also used to synergistically enhance ultrahigh proton conductivity in hydrated conditions [47][48][49] . This sulfonic acid hydrophilic group attached to the hydrophobic backbone in Nafion enables proton transport through hydrated ionic clusters and varies influentially with the humidity 48 . Moreover, the microstructures especially the percolated channel formed by these ionic clusters which may be sculptured by phase separation or colloidal packing is the key factor related to the proton conductivity of Nafion membrane 48,50 .
In this study, a larger light emission sensing area with surrounding square electrode was designed for both TES and BES to obtain maximum light intensity for humidity measurement. To obtain the best sensitivity towards humidity, different proportions of GO and Nafion (1:9, 3:7, 5:5, 7:3 and 9:1) were then investigated at different relative humidity (RH), where RH was controlled in a custom-made chamber (figure 4a (i)) using vapor pressure from a set of saturated salt solution. As illustrated in Figure 4b, the light intensity response was inferior when GO was the major component (proportion) in the composite. The coverage of a phosphor layer with a brownish solution of GO can lead to a poor response. In contrast, when Nafion was in major proportion in the nanocomposite, higher sensitivity was achieved owing to its ultrahigh proton conductivity, especially for 3:7 ratio which exhibited maximum response. It is important to remark that the synergistic effect between GO and Nafion also resulted in higher light intensity response regarding humidity compared to the role of Nafion alone. The GO:Nafion composition of 3:7 was finally selected to obtain maximum efficiency for both TES and BES.
The performance of the sensor with TES configuration was firstly studied. It was obvious that no light is emitted at low RH. However, the light is automatically turned on in high humidity conditions (inset of Figure 4c (lower)). The exponential behavior of light intensity (lux) on the humidity is observed within studied range (inset of Figure 4c (upper)). This exponentiallysensitive behavior has been reported in several GO-based humidity sensors elsewhere, which might arise from mass loading effect of water adsorption at high RH [51][52] . Figure 4c shows the dependence of the logarithmic conductance-related light intensity (lux) on different RH values at room temperature. It is obvious that the logarithmic light intensity response increases linearly with the increment of %RH. Additionally, the humidity sensing capability of the ACEL display 13 with BES architecture was further investigated. In this way, an ACEL light panel was fabricated on a transparent electrode of ITO, following BES structure (see Figure 1a). However, a silver top-electrode was replaced by a sensitive layer of GO nanocomposite as is schematically shown in figure 4d. With this modified architecture, the direct feedback from humidity on nanocomposite layer would allow the phosphor layer on the opposite side to emit light. A light intensity response to different RH degrees is shown in Figure S10. A similar trend can be clearly observed, as in case of TES, whereby the light intensity increases exponentially with %RH. An enhanced sensitivity might be caused by the protective phosphor layer which is not hindered by the layer of GO nanocomposite. This humidity related light emission response holds a great promise to develop an innovative ACEL based humidity sensor for practical applications.
The capability of ACEL humidity sensor can be extended to further uses as point-of-care (POC) sensing device. Such a humidity-dependence light emission enables monitoring of human breathing through the moisture content in the exhaled air. The physical characteristics of the breath, such as breath frequency can provide helpful health condition information about pulmonary and cardiac symptoms [53][54][55] .
In this experiment, the air exhaled was introduced using cylindrical mouthpiece connected to the custom-made chamber as illustrated in the experimental setup in the Figure 4a (ii). The response was then monitored through the light emission from ACEL sensor with TES architecture. The breath sensor performance of repeated exhale/inhale cycles is displayed in Figure S11. Once the breath was blown, the substantial response from light emission is rapidly detected. After the breath is off, the light emission was then gradually decreased. Different  The response and recovery times were further evaluated from single exhale/inhale cycles. As is observed in Figure S12, the sensor response to a breath could be as fast as 1s (time interval of the light sensor was limited at 1s), while the recovery time (defined as the time required to recover up to 90% of initial baseline) was varied depending on breathing patterns. For shallow and normal breathing, the recovery time was around 7s. However, in the case of deep breathing, the recovery time may take up to 12.5s (without applying any heat treatment). A huge difference between response and recovery time could be assigned to the generous hydrophilic functional moiety in GO and Nafion which required a longer time to desorb water molecules 56 . This experiment suggests that the developed breath monitoring sensor is capable to track the breathing frequency by a sensitive ACEL display.
To allow portability and simplicity for on-field POC testing, an ACEL humidity sensor Fabrication of ACEL display: A polyethylene terephthalate (PET) film was used as the substrate; unless stated otherwise. Firstly, a flexible silver paste was printed directly onto PET substrate as the rear electrode and front bus-bar. Then, two layers of dielectric ink, a single layer of phosphor and clear conductor were sequentially printed onto the printed electrode. After each screen-printing layer, the screen-printed film was cured at 130℃ in a box oven for 10 minutes.
For the use of ACEL in sensing applications, the prepared film was used without a clear conductor screen-printing. A wax printing method was used to define circular sensing spots (diameter of 8 mm). The device was completed by drawing a straight line to connect the bus-bar and sensing area with a clear conductor. An AC voltage was powered by an EL inverter build in battery model UTF-EB900-4E (for DC 9.0V input).
Fabrication of GO/Nafion based humidity sensor: A 4-rectangular pattern was screen printed through a polyester screen directly on top of the ACEL display using flexible silver paste. To prepare the composite, 2.5 mg mL -1 GO solution (average lateral (x,y) and through-plane (z) dimension range of ≈ 500 nm and 1-1.2 nm, respectively, and C/O ratio about one unit (supplier's data)) was mixed with Nafion and sonicated for 1 hr. The prepared composite was carefully drop-casted onto an ACEL device and allowed to dry in the oven at 60℃ for 5 minutes.
The humidity measurement was conducted in a custom-made chamber consisting of an adaptable closed chamber, light sensor, and mouthpiece (Figure 4a). To achieve a certain level of humidity, RH was controlled by a series of saturated salt solution ranging low to high humidity level (CH 3 CO 2 K, K 2 CO 3 , NaCl, KCl, KNO 3 ) at room temperature. Each humidity level is kept constant for 30 min to obtain steady RH. The solution chamber was removed from the setup when a human breath is monitored. The breath was then blown directly to an ACEL sensor through a mouthpiece while the light emission is recorded.

Web enhanced Objects
Movie of light emission on ACEL display (Movie 1 ACEL)/responsive writable display (Movie 2 ACEL)