tailor the properties of the printed device for each specific application.Currently, printing approaches such as inkjet printing have been limited to a small number of metals because of the inks that are composed of either nano particles or metal-organics. Metal nano particlebased inks are characterized by high metal loading which lead to low contact resistances. However, the inks are prone to agglomeration because of the high particle densities and the high sur face free energy of nanoparticles. For this reason, a number of steps are required to synthesize dispersed and printable inks, including the addition of organic capping groups such as poly(Nvinyl 2pyrrolidone) to sterically stabilize the particles. [9] The interactions between the organics and the metal nanoparticle surface are material specific and most studies have focused on Ag, [10] Au, [11] and Cu. [12] Alternatively, metalloorganic decomposition (MOD) inks composed of molecular metal-organic compounds are particlefree, reducing nozzle clogging, and free of the organic capping groups. [13] A drawback, though, is that these compounds are not readily available and must be carefully designed and syn thesized so that they are stable before and during printing near ambient temperatures, while still capable of being decomposed to produce metallic structures at slightly elevated temperatures. [14] Overall, MOD has focused on Ag, with an increasing number of studies only recently emerging on other metals such as Cu. [12] In addition to the limited number of metals, printing has also been restricted to a few substrates that can withstand relatively high temperatures because of a heating step, referred to gener ally as sintering, that is carried out after printing. In the case of metal nanoparticlebased inks, heating is required to remove the organic capping groups and fuse the metal nanoparticles to form percolating electrically conductive structures. [15] For MOD inks, the heating activates decomposition of the metal-organic compound which leads to release of the organic groups and metal nanoparticle nucleation and growth. [16] Recently, alter native approaches to heating such as chemical, [17] electrical, [18] photonic, [10,19] laser, [20] plasma, [21] and microwave [15,22] have been reported to lower the thermal loading; however, the effec tiveness of these methods varies considerably. [23] Inkjet printing is rapidly emerging as a means to fabricate low-cost electronic devices; however, its widespread adoption is hindered by the complexity of the inks and the relatively high processing temperatures, limiting it to only a few metals and substrates. A new approach for inkjet printing is described, based on commercially available, particle-free inks formulated from inorganic metal salts and their subsequent low-temperature conversion to metallic structures by a non-equilibrium, inert gas plasma. This single, general method is demonstrated for a library of metals including gold (Au), silver (Ag), copper (Cu), palladium (Pd), platinum (Pt), lead (Pb), bismut...

Inkjet printing has emerged as a leading technology for additive manufacturing of electronic devices. It is particularly attractive in applications that require low waste, low-cost fabrication techniques. Most printing processes for electronic device applications involve the fabrication of metal structures owing to the availability of metal-based inks and the high quality structures that can be produced using these inks. As a result of rapid advances in the development of sensor-dependent technology areas like IoT and flexible/wearable electronics, inkjet printing has recently expanded into the sensor area due, in part to its inherent compatibility with a wide variety of polymer substrates and roll-to-roll processing. This review focuses on the development of inkjet-printed elemental metal structures for electrochemical applications. Included in this paper is a review of commonly used and newly emerging ink technologies, post printing sintering processes, functionalization of printed metal surfaces to enhance sensing capabilities and a variety of inkjet-printed electrochemical sensors including gas sensors, ion sensors, pH sensors, glucose sensors, and biomolecule sensors.

Printed electronic devices fabricated using additive manufacturing (AM) processes such as inkjet printing, [1] screen printing, [2] and aerosol printing [3] typically utilize silver or copper for conductors because of their high electrical conductivity and chemical inertness. These two metals can be readily printed using a large variety of commercially available inks including nanoparticle, [4] metallo-organic decomposition (MOD), [5] and inorganic metal salt-based inks. [6] These materials can be processed at low temperature using chemical, [7] photonic, [8] plasma, [9] electrical, [10] microwave, [11] and thermal sintering techniques, enabling the use of low-cost plastics (such as polyesters) as substrates. This combination of print-based manufacturing methods and low-cost materials makes printed electronic devices well suited for large-area or large-number distributed applications, such as item tracking, [12] or environmental monitoring, [13] which could potentially lead to significant amounts of electronic waste if widely implemented. [14] Recently, biodegradable materials have been developed to fabricate electronic devices that can dissolve in biological fluids or decompose in the natural environment. [15] This technology could potentially revolutionize printed electronics applications by reducing electronic waste and facilitating rapid recycling. [16] Instead of silver or copper, printed biodegradable devices use water-soluble, biologically benign metallic elements for electrical conductors such as magnesium, [17] zinc, [18] molybdenum, [19] and iron, [20] which pose minimal threat to life and the environment. These metals can be oxidized and subsequently dissolved in water to form minerals. [21] As only small amounts of zinc are present in the devices, the concentrations of the minerals added to soil by biodegradable conductors are typically lower than the amount already present in the soil and required by the crop. [22] Compared with biocompatible and bioinert metals such as gold and platinum, biodegradable metals are more widely available and less cost prohibitive and potentially well suited for disposable or temporary printed electronics applications. [23] The degradation rate of printed biodegradable devices can be tuned by passivating the readily degradable metals with more slowly degrading polymer encapsulation materials so that the devices remain functional for a tunable period ranging from several days to several months. [24] A challenge in biodegradable device fabrication is formulating and printing conductive metallic inks with high conductivity. [25] Unlike inert metals, biodegradable metals have high oxidation potentials; therefore, instead of solution-based chemical synthesis, nanoparticles are typically obtained through high-energy ball milling of bulk metals. [26] Without the presence of chemical stabilizing and reducing agents, mechanical synthesis normally yields metal-metal oxide core-shell nanoparticles. [27] Noninert metal particles such as copper have inherent tendency to oxidize in a...

Printable metal inks are typically composed of premade nanoparticles that require postdeposition thermal sintering to produce crystalline, electrically conductive features. In this paper, it is shown that particle-free Ag inks made from simple, water-soluble metal salts such as silver nitrate can be ink-jet printed and converted into electrical features with tunable resistivity at low temperature (<100 °C) by exposure to a pure argon plasma. X-ray diffraction confirms that the converted inks are crystalline, and four-point probe electrical measurements show that the sheet resistances are a function of the pressure and power in the plasma. From cross-sectional scanning electron microscopy analysis, it is found that the morphology of the converted silver layer becomes increasingly dense with increasing plasma treatment time, which explains the measured changes in sheet resistance, and that the thickness of the layer is ∼1.5 μm, which yields a minimum resistivity of ∼6 × 10−8 Ω m, approximately 3.8 times higher than bulk resistivity of silver. Interestingly, the resistivity can be varied over a span of 6 orders of magnitude which allows resistor–capacitor filter devices to be fabricated exhibiting varying cut-off frequencies from a single material and geometry.

Inkjet printing has been identified as a cost-effective method to fabricate sensors on polymeric substrates. However, substrate materials suitable for printing are limited by the annealing temperature required by conventional inks. In this article, we describe the fabrication of an inkjetprinted thermistor on polyethylene and cellophane substrates that are not thermally compatible with the conventional inkjet printing processes. Fabrication on these substrates is made possible by a novel plasma-based postprint treatment step that limits the substrate temperature to <50 °C. The sensors exhibited a temperature sensitivity of 0.25 Ω°C −1 that was independent of substrate material. The utility of the fabrication process was demonstrated by fabricating thermistors for common indoor and outdoor applications.

A simple and selective photoelectrochemical (PEC) biosensor was constructed for Hg2+ detection based on zinc phthalocyanine (ZnPc) dye-sensitized CdS using alginate not only as a carrier but also as a binder. First, CdS as a photoactive material was in situ modified on the electrode surface using a rapid and simple electrodeposition to obtain an initial photocurrent signal. Second, ZnPc was loaded in the amphiphilic alginate micelle and then was coated onto the CdS film surface via alginate as the binder. The photocurrent was subsequently enhanced due to the favorable dye sensitization effect of ZnPc to CdS. Finally, the thymine-rich probe DNA was immobilized on the modified ITO surface via coupling reaction between the carbonyl groups of the amphiphilic polymer and the amino groups of the probe DNA. In the presence of Hg2+, the thymine–Hg2+–thymine (T–Hg2+–T) structure was formed due to the specific bond of Hg2+ with thymine, resulting in the decrease of photocurrent due to the increase of steric hindrance on the modified electrode surface. The proposed PEC biosensor for Hg2+ detection possessed a wide linear range from 10 pM to 1.0 μM with a detection limit of 5.7 pM. This biosensor provides a promising platform for detecting other biomolecules of interest.

Additive methods for manufacturing materials have recently emerged, particularly for the fabrication of three-dimensional architectures. Because of their long history in thin-film etching and deposition, plasmas offer unique advantages for many of the materials and surface processes associated with additive manufacturing. Here, we review recent efforts that have been primarily focused on the direct writing of patterned structures and the post-treatment of printed materials. Different configurations, materials, and applications are presented. Current challenges and a future outlook are also provided. 3D printing, additive manufacturing, microdischarges, nanotechnology, roll-to-roll

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