Fully Transparent Gas Sensor Based on Carbon Nanotubes.

In this paper, we demonstrate the feasibility of realization of transparent gas sensors based on carbon nanotubes (CNTs). Both sensing layer and electrodes consist of CNTs deposited by spray deposition. The transparent sensor-with a transmittance higher than 60% in both sensing layer and electrodes-is characterized towards NH3 and CO2 and compared with a reference sensor with the same active layer but evaporated Au electrodes. In particular, the sensitivity towards NH3 is virtually identical for both reference and transparent sensors, whereas the transparent device exhibits higher sensitivity to CO2 than the reference electrode. The effect of the spacing among consecutive electrodes is also studied, demonstrating that a wider spacing in fully CNT based sensors results in a higher sensitivity because of the higher sensing resistance, whereas this effect was not observed in gold electrodes, as their resistance can be neglected with respect to the resistance of the CNT sensing layer. Overall, the transparent sensors show performance comparable-if not superior-to the traditionally realized ones, opening the way for seamlessly integrated sensors, which do not compromise on quality.

Development of Flexible Dispense-Printed Electrochemical Immunosensor for Aflatoxin M1 Detection in Milk.

Detection of mycotoxins, especially aflatoxin M1 (AFM1), in milk is crucial to be able to guarantee food quality and safety. In recent years, biosensors have been emerging as a fast, reliable and low-cost technique for the detection of this toxin. In this work, flexible biosensors were fabricated using dispense-printed electrodes, which were functionalized with single-walled carbon nanotubes (SWCNTs) and subsequently coated with specific antibodies to improve their sensitivity. Next, the immunosensor was tested for the detection of AFM1 in buffer solution and a spiked milk sample using a chronoamperometric technique. Results showed that the working range of the sensors was 0.01 µg/L at minimum and 1 µg/L at maximum in both buffer and spiked milk. The lower limit of detection of the SWCNT-functionalized sensor was 0.02 µg/L, which indicates an improved sensitivity compared to the sensors reported so far. The sensitivity and detection range were in accordance with the limitation values imposed by regulations on milk and its products. Therefore, considering the low fabrication cost, the ease of operation, and the rapid read-out, the use of this sensor could contribute to safeguarding consumers' health.

Fully Printed Flexible Single-Chip RFID Tag with Light Detection Capabilities.

A printed passive radiofrequency identification (RFID) tag in the ultra-high frequency band for light and temperature monitoring is presented. The whole tag has been manufactured by printing techniques on a flexible substrate. Antenna and interconnects are realized with silver nanoparticles via inkjet printing. A sprayed photodetector performs the light monitoring, whereas temperature measurement comes from an in-built sensor in the silicon RFID chip. One of the advantages of this system is the digital read-out and transmission of the sensors information on the RFID tag that ensures reliability. Furthermore, the use of printing techniques allows large-scale manufacturing and the direct fabrication of the tag on the desired surface. This work proves for the first time the feasibility of the embedment of large-scale organic photodetectors onto inkjet printed RFID tags. Here, we solve the problem of integration of different manufacturing techniques to develop an optimal final sensor system.

Printed and Flexible Microheaters Based on Carbon Nanotubes.

This work demonstrates a cost-effective manufacturing method of flexible and fully printed microheaters, using carbon nanotubes (CNTs) as the heating element. Two different structures with different number of CNT layers have been characterized in detail. The benchmarking has been carried out in terms of maximum operating temperature, as well as nominal resistance and input power for different applied voltages. Their performances have been compared with previous reports for similar devices, fabricated with other technologies. The results have shown that the heaters presented can achieve high temperatures in a small area at lower voltages and lower input power. In particular, the fully printed heaters fabricated on a flexible substrate covering an area of 3.2 mm2 and operating at 9.5 V exhibit a maximum temperature point above 70 °C with a power consumption below 200 mW. Therefore, we have demonstrated that this technology paves the way for a cost-effective large-scale fabrication of flexible microheaters aimed to be integrated in flexible sensors.

Low-Cost Gas Sensing: Dynamic Self-Compensation of Humidity in CNT-Based Devices.

Solid-state gas sensors are a cost-effective and well-spread alternative to conventional gas sensing, employable in most environments, ranging from homes and offices to harsh industrial scenarios. The emergence of carbon nanotubes (CNTs) as sensing material in solid-state gas sensors paved the way to a new class of devices, which are semitransparent, flexible, and with a remarkably low environmental impact. These devices, however, lack selectivity to other gases and concurring physical phenomena, such as temperature and pressure changes. In this contribution, we show how by measuring the impedance of CNT-based gas sensors at different frequencies, it is possible to evaluate sensitivity coefficient for the immediate compensation of moisture content in the air, while still preserving in the considered ranges of average sensitivities as high as 0.045, 0.112, 7.842 × 10-5, and 0.041 % Z/ppm for ammonia, carbon dioxide, carbon monoxide and ethanol gas, respectively. With this simple approach, it will be possible to develop simple sensor read-out systems, with reduced external electronic, simplifying the route to low-cost and low-power sensor nodes for the internet of things.

Cost-effective PEDOT:PSS Temperature Sensors Inkjetted on a Bendable Substrate by a Consumer Printer.

In this work, we report on a fabrication protocol to produce fully inkjet-printed temperature sensors on a bendable polyethylene terephthalate (PET) substrate. The sensing layer is made of polymer-based Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) ink that is electrically contacted by an underlying interdigitated electrode (IDE) structure based on a silver nanoparticle (AgNP) ink. Both inks are available commercially, and no further ink processing is needed to print them using a cost-effective consumer printer with standard cartridges. The fabricated sensor modules are tested for different IDE dimensions and post-deposition treatments of the AgNP film for their response to a temperature range of 20 to 70 °C and moisture range of 20 to 90% RH (relative humidity). Attributed to the higher initial resistance, sensor modules with a larger electrode spacing of 200 µm show a higher thermal sensitivity that is increased by a factor of 1.8 to 2.2 when compared to sensor modules with a 150 µm-spacing. In all cases, the sensors exhibit high linearity towards temperature and a response comparable to state of the art.

A Handwriting Method for Low-Cost Gas Sensors.

In this study, we report on an automated method based on a handwritten technique for the fabrication of low-cost gas sensors based on carbon nanotube (CNT) networks. Taking advantage of the inherent low-cost, flexible, and uncomplicated characteristics of pen-based techniques and combining them with an automated robotic system allows for high-resolution patterns, high reproducibility, and relatively high throughput considering the limitations of parallel processing. To showcase this, gas sensors capable of sensing NH3, CO2, CO, and ethanol, as well as temperature and relative humidity, are fabricated and characterized displaying competitive performance in relation to previously reported devices. The presented process is compatible with a variety of solutions and inks and, as such, allows for an easy integration into existing printing and coating frameworks with the greatest advantage being the ease of creating prototypes because of the nonstringent material requirements.

Use of SU8 as a stable and biocompatible adhesion layer for gold bioelectrodes.

Gold is the most widely used electrode material for bioelectronic applications due to its high electrical conductivity, good chemical stability and proven biocompatibility. However, it adheres only weakly to widely used substrate materials such as glass and silicon oxide, typically requiring the use of a thin layer of chromium between the substrate and the metal to achieve adequate adhesion. Unfortunately, this approach can reduce biocompatibility relative to pure gold films due to the risk of the underlying layer of chromium becoming exposed. Here we report on an alternative adhesion layer for gold and other metals formed from a thin layer of the negative-tone photoresist SU-8, which we find to be significantly less cytotoxic than chromium, being broadly comparable to bare glass in terms of its biocompatibility. Various treatment protocols for SU-8 were investigated, with a view to attaining high transparency and good mechanical and biochemical stability. Thermal annealing to induce partial cross-linking of the SU-8 film prior to gold deposition, with further annealing after deposition to complete cross-linking, was found to yield the best electrode properties. The optimized glass/SU8-Au electrodes were highly transparent, resilient to delamination, stable in biological culture medium, and exhibited similar biocompatibility to glass.

Transfer Printed P3HT/PCBM Photoactive Layers: From Material Intermixing to Device Characteristics.

The fabrication of organic electronic devices involving complex stacks of solution-processable functional materials has proven challenging. Significant material intermixing often occurs as a result of cross-solubility and postdeposition treatments, rendering the realization of even the simplest bilayer architectures rather cumbersome. In this study we investigate the feasibility of a dry transfer printing process for producing abrupt bilayer organic photodiodes (OPDs) and the effect of thermal annealing on the integrity of the bilayer. The process involves the transfer of readily deposited thin films of poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) using a polydimethylsiloxane (PDMS) stamp. Fabricated structures are characterized by means of cross-sectional scanning electron microscopy (SEM), UV/vis absorption spectroscopy, and time-of-flight secondary ion mass spectrometry (TOF-SIMS). Joint consideration of all results unveils abrupt interfaces with no thermal treatment applied and significant material intermixing for samples annealed above 100 °C. The role of the thermally assisted intermixing in determining the performance of complete devices is evaluated through the comparison of J-V characteristics and external quantum efficiencies (EQEs) of identical photodiodes subject to different annealing conditions. It is shown that the performance of such devices approaches the one of bulk heterojunction photodiodes upon thermal annealing at 140 °C for 5 min. Our results demonstrate that transfer printing is a reliable and simple process for the realization of functional multilayers, paving the way for organic electronic devices incorporating complex stacks. It further contributes to a fundamental understanding of material composition within photoactive layers by elucidating the process of thermally assisted intermixing.

Fully-sprayed and flexible organic photodiodes with transparent carbon nanotube electrodes.

In this study, we demonstrate the feasibility of TCO-free, fully sprayed organic photodiodes on flexible polyethylene terephthalate (PET) substrates. Transparent conducting films of single-wall carbon nanotubes are spray deposited from aqueous solutions. Low roughness is achieved, and films with sheet resistance values of 160 Ω/sq at 84% in transmittance are fabricated. Process issues related to the wetting of CNTs are then examined and solved, enabling successive spray depositions of a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer and a blend of regioregular poly(3-hexylthiophene-2,5-diyl) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). The active layer is then optimized, achieving a process yield above 90% and dark currents as low as 10(-4) mA/cm(2). An external quantum efficiency of 65% and high reproducibility in the performance of the devices are obtained. Finally, the impact of the characteristics of the transparent electrode (transmittance and sheet resistance) on the performances of the device are investigated and validated through a theoretical model and experimental data.