Low-cost Flexible Substrates Research Articles

MXene-based kirigami designs: showcasing reconfigurable frequency selectivity in microwave regime

Today’s wireless environments, soft robotics, and space applications demand delicate design of devices with tunable performances and simple fabrication processes. Here we show strain-based adjustability of RF/microwave performance by applying frequency-selective patterns of conductive Ti3C2Tx MXene coatings on low-cost acetate substrates under ambient conditions. The tailored performances were achieved by applying frequency-selective patterns of thin Ti3C2Tx MXene coatings with high electrical conductivity as a replacement to metal on low-cost flexible acetate substrates under ambient conditions. Under quasi-axial stress, the Kirigami design enables displacements of individual resonant cells, changing the overall electromagnetic performance of a surface (i.e., array) within a simulated wireless channel. Two flexible Kirigami-inspired prototypes were implemented and tested within the S, C, and X (2-4 GHz, 4-8 GHz, and 8-12 GHz) microwave frequency bands. The resonant surface, having ~1/4 of the size of a standard A4 paper, was able to steer a beam of scattered waves from each resonator by ~25°. Under a strain of 22%, the resonant frequency of the wired co-planar resonator was shifted by 400 MHz, while the reflection coefficient changed by 158%. Deforming the geometry impacted the spectral response of the components across three arbitrary frequencies in the 4-10 GHz frequency range. With this proof of concept, we anticipate implementing thin films of MXenes on technologically relevant substrates, achieving multi-functionality through cost-effective and straightforward manufacturing.

Polymeric integration of structure-switching aptamers on transistors for histamine sensing.

Aptamers that undergo large conformational rearrangements at the surface of electrolyte-gated field-effect transistor (EG-FETs)-based biosensors can overcome the Debye length limitation in physiological high ionic strength environments. For the sensitive detection of small molecules, carbon nanotubes (CNTs) that approach the dimensions of analytes of interest are promising channel materials for EG-FETs. However, functionalization of CNTs with bioreceptors using frequently reported surface modification strategies (e.g., π-π stacking), requires highly pristine CNTs deposited through methods that are incompatible with low-cost fabrication methods and flexible substrates. In this work, we explore alternative non-covalent surface chemistry to functionalize CNTs with aptamers. We harnessed the adhesive properties of poly-D-lysine (PDL), to coat the surface of CNTs and then grafted histamine-specific DNA aptamers electrostatically in close proximity to the CNT semiconducting channel. The layer-by-layer assembly was monitored by complementary techniques such as X-ray photoelectron spectroscopy, optical waveguide lightmode spectroscopy, and fluorescence microscopy. Surface characterization confirmed histamine aptamer integration into PDL-coated CNTs and revealed ∼5-fold higher aptamer surface coverage when using CNT networks with high surface areas. Specific aptamers assembled on EG-CNTFETs enabled histamine detection in undiluted high ionic strength solutions in the concentration range of 10 nM to 100 μM. Sequence specificity was demonstrated via parallel measurements with control EG-CNTFETs functionalized with scrambled DNA. Histamine aptamer-modified EG-CNTFETs showed high selectivity vs. histidine, the closest structural analog and precursor to histamine. Taken together, these results implied that target-specific aptamer conformational changes on CNTs facilitate signal transduction, which was corroborated by circular dichroism spectroscopy. Our work suggests that layer-by-layer polymer chemistry enables integration of structure-switching aptamers into flexible EG-CNTFETs for small-molecule biosensing.

Selective flexible sensor for monitoring volatile organic compounds in museum display cases

Aim of this work is the development of a new kind of corrosion sensors, based on low-cost flexible substrates, specifically designed to detect Volatile Organic Compounds (VOCs) producing significant degradation of artifacts in museum display cases. Sensors have been fabricated and tested in relevant environment with three of the most relevant VOCs (acetic acid, formic acid and formaldehyde) to the artifacts preservation. Sensitivity has been measured to be lower than 1 ppm, which is several times better than what reported in the literature on corrosion sensors for the same VOCs. In addition, different sensors, arranged with different active materials, have been implemented for the first time into a so-called multi-sensor architecture, aimed at demonstrating VOC specificity: in particular, we report here on a preliminary test in which the multi-sensor has been able to be selective between acetic acid and formaldehyde.

Malodour classification with low-cost flexible electronics

Understanding body malodour in a measurable manner is essential for developing personal care products. Body malodour is the result of bodily secretion of a highly complex mixture of volatile organic compounds. Current body malodour measurement methods are manual, time consuming and costly, requiring an expert panel of assessors to assign a malodour score to each human test subject. This article proposes a technology-based solution to automate this task by developing a custom-designed malodour score classification system comprising an electronic nose sensor array, a sensor readout interface and a machine learning hardware fabricated on low-cost flexible substrates. The proposed flexible integrated smart system is to augment the expert panel by acting like a panel assessor but could ultimately replace the panel to reduce the test and measurement costs. We demonstrate that it can classify malodour scores as good as or even better than half of the assessors on the expert panel.

In-situ SERS detection of aromatic amine pollutants in fire-fighting wastewater using low-cost flexible substrates

Surface-enhanced Raman scattering (SERS) technology has been widely used in the field of environmental pollutant detection due to its fast and intuitive characteristics and low water interference. Herein, a low-cost and flexible SERS substrate was fabricated by transferring gold nanoparticles (NPs) onto polydimethylsiloxane (AuNPs@PDMS) membranes, which enabled highly sensitive detection of aromatic amine pollutants in wastewater after fire suppression. Finite time domain-difference calculations confirmed that the uniformly distributed NP gap significantly enhanced the localized electromagnetic field. The AuNPs@PDMS substrate performed satisfactorily in testing for rhodamine 6G (R6G) with a detection limit (LOD) of 5.5 × 10−10 M. This substrate also demonstrated high reproducibility for detection of different concentrations of R6G and the relative standard deviation (RSD) was <7 %. These excellent SERS properties made AuNPs@PDMS a promising substrate in the detection of aniline and benzidine. Quantitative analysis of aniline and benzidine was achieved with LODs of 4.5 × 10−9 and 7.8 × 10−9 M, respectively. In addition, this substrate could be applied in rapid analysis of mixtures in wastewater after fire suppression using SERS without any complex pretreatments, displaying a great potential for rapid, high-sensitivity, and on-site detection of contaminants in fire-fighting sites.

Gold nanospheres assembly via corona discharge technique for flexible SERS substrate.

Noble metal nanoparticles (NMNPs) assembly substrates with strongly enhanced local electromagnetic fields provide new possibilities for surface-enhanced Raman spectroscopy (SERS) sensing. Although the external-electric-field-based self-assembly (EEFSA) strategy for decreasing NMNP gap in liquid phase is relatively developed, it is rarely described in solid phase. Here, by combining corona discharge technique (CDT) as a simple EEFSA approach on flexible substrate surface modification, a flexible SERS substrate medicated with gold nanospheres (AuNSs) is produced. Because of the CDT's peculiar discharge event, makes AuNSs aggregation simply achieved. The modified flexible SERS substrate is sensitive to the detection limit of ∼10-5 mM for Rhodamine 6G (R6G), with a maximum enhancement factor of 2.79×106. Furthermore, finite-difference time-domain (FDTD) simulation confirms the SERS enhancement impact of AuNSs-based substrate. This study not only provides a low-cost, simple-to-process, high-yield, high sensitivity, and activity flexible SERS substrate, but also suggests a more practical and adaptable NMNPs self-assembly approach.

Printed zinc tin oxide diodes: from combustion synthesis to large-scale manufacturing

Printed metal oxide devices have been widely desired in flexible electronic applications to allow direct integration on foils and to reduce electronic waste and associated costs. Especially, semiconductor devices made from non-critical raw materials, such as Zn, Sn (and not, for example, In), have gained significant interest. Despite considerable progress in the field, the upscale requirements from lab to fab scale to produce these materials and devices remain a challenge. In this work, we report the importance of solution combustion synthesis (SCS) when compared with sol–gel in the production of zinc tin oxide (ZTO) thin films using a solvent (1-methoxypropanol) that has lower environmental impact than the widely used and toxic 2-methoxyethanol. To assure the compatibility with low-cost flexible substrates in high-throughput printing techniques, a low annealing temperature of 140 °C was achieved for these thin films by combining SCS and infrared annealing in a short processing time. These conditions allowed the transition from spin-coating (lab scale) to flexographic printing (fab scale) at a printing speed of 10 m min−1 in a roll-to-roll pilot line. The ZTO (1:1 Zn:Sn-ratio) diodes show a rectification ratio of 103, a low operation voltage (⩽3 V), promising reproducibility and low variability. The results provide the basis for further optimisation (device size, encapsulation) to meet the requirements of diodes in flexible electronics applications such as passive-matrix addressing, energy harvesting and rectification.

Magnetic, dielectric and structural properties of CoxZn(0.90-x)Al0.10Fe2O4 synthesized by sol–gel method with application as flexible microwave substrates for microstrip patch antenna

In this study, novel flexible microwave substrates based on CoxZn(0.90-x)Al0.10Fe2O4 nanopowder having an average grain size of 20 nm–26 nm were prepared using the sol–gel synthesis technique for microstrip patch antenna applications. At first there are three different molecular composition were chosen having the chemical formula of CoxZn(0.90-x)Al0.10Fe2O4 and specified as Co20, Co40 and Co60 for x = 20%, x = 40% and x = 60% respectively. Then the synthesized ferrites materials were characterized with XRD, FESEM, and VSM analysis, and single-phase spinel with high crystallinity were revealed. Besides, the dielectric and magnetic properties of the proposed Co–Zn ferrites samples were investigated for the affirmations of microwave applications. The values of dielectric constant εr and loss tangent (Tδ) are determined using dielectric assessment kit, and the obtained values are ranges from 3.5 to 4.5 for εr and 0.002 to 0.006 for Tδ for Co20, Co40, and Co60 respectively. Also, the magnetic permeability μr and magnetic loss tangent (Tδm) values are determined to form the predetermined scattering parameters S11 and S21 obtained from rectangular substrate pellets and the obtained values of μr and Tδm are ranges from 0.90 to 1.00 and 0.003 to 0.007 respectively. Finally, a modified diamond-shaped microstrip patch antenna is designed and fabricated on the proposed flexible substrates obtained from the synthesized Co–Zn ferrites nanopowders by mixing with PVA binder to validate their application in the S-band of microwave regime. The finite element method based computer simulation technology software is used to design the proposed microstrip patch antenna; furthermore, the simulated prototype is fabricated, and their reflections coefficients are measured for verification. The simulated and measured result of the proposed flexible antennas exhibits a maximum of 3 GHz bandwidth in between 2.5 and 5.5 GHz. Thus, the proposed materials offers tunable dielectric and magnetic properties with soft magnetic behaviors and easy to use where arbitrarily electromagnetic values are required. Moreover, these materilas also can be played significant role in flexible microwave field as low cost flexible substrates for compact flexible antennas, metamaterials, absorbers, and sensors applications etc.

Nitrogen-Doped Titanium Monoxide Flexible Membrane for a Low-Cost, Biocompatible, and Durable Raman Scattering Substrate.

The development of low-cost, biocompatible, and durable high-performance substrates is an urgent issue in the field of surface-enhanced Raman scattering (SERS). Herein, by reducing and exfoliating the TiO2-layered nanoplates in the gas phase, nitrogen-doped titanium monoxide (N-TiO) ultrathin nanosheets composed of 2-3 single layers with a thickness of only ∼1.2 nm are synthesized. Compared with pure TiO, the oxidation resistance of N-TiO is greatly improved, in which the oxidation threshold is significantly increased from 187.5 to 415.6 °C. The N-TiO ultrathin nanosheets are found to have strong surface plasmon resonance in the visible region. These ultrathin N-TiO nanosheets can be easily assembled into a large-scale flexible membrane and exhibit remarkable SERS effects. Moreover, this low-cost flexible SERS substrate combines the high durability of noble-metal substrates and the high biocompatibility of semiconductor substrates.

Development of Eco-Efficient Smart Electronics for Anticounterfeiting and Shock Detection Based on Printable Inks

Printed electronics are expected to meet an increasing demand for improved functionality and autonomy of products in the context of Internet-of-Things. With this trend, the environmental performance of novel technologies is of growing importance. The current study presents the life cycle assessment of two novel devices: an anticounterfeit label based on the electrochromic display and a shock-detection tag based on the piezoelectric sensor, designed for the use in packaging of pharmaceuticals and luxury items to improve the safety and accountability in the supply chain. The devices are manufactured by means of energy-efficient printing techniques on a low-cost flexible and recyclable paper substrate. Comprehensive cradle-to-grave analysis contributes to industrial-scale energy and material life cycle inventories and identifies the main impact hotspots evaluated for a broad range of categories of the ReCiPe midpoint (H) impact assessment method. Results show that major impact burdens are associated with the near-field communication chip and radio-frequency identification antenna, while the impacts of solvents, process energy, electrochromic display/piezoelectric sensor, Li-ion battery, and substrate are comparatively small. In terms of their global warming potential, both the anticounterfeit label and shock-detection tag embody around 0.23 kg of CO2-equiv. Several material-use reduction and material-substitution strategies are quantified and discussed for their potential to reduce high impacts of the antenna.

Mobility enhancement of indium-gallium oxide via oxygen diffusion induced by a metal catalytic layer

In this study, the effects of a metal capping layer on indium-gallium oxide (IGO) films and associated TFTs were demonstrated. By introducing a Ta metal capping layer and a subsequent annealing process, IGO films are crystallized at the relatively low temperature of 250 °C which is suitable for low-cost flexible substrate such as PI. The Ta-induced IGO TFT shows significantly improved field-effect mobility from 29.3 (device without Ta layer) to 76.9 cm2/Vs. This improvement is due to the reduced number of defects which is consumed preferentially during the crystallization process by injected electron from the metal capping layer. Molecular dynamics calculations were performed in order to obtain theoretical insight into the inner-diffusions of atoms. Oxygen atoms near the interface region break their bonds with the metal cations of IGO and are attracted to Ta capping layer due to difference in the Gibbs free energy of formation, which should constitute the reason for its superior performance.

2D-MoS2/BMN Ceramic Hybrid Structure Flexible TFTs with Tunable Device Properties.

Two-dimensional (2D) layered semiconductor materials have emerged as prospective channel materials in flexible thin-film field effect transistors (TFTs) recently because of their unique electrical and mechanical characteristics. Meanwhile, high-quality ceramics, with outstanding dielectric property and fabrication process compatible with low-cost flexible substrates, have become one of the best candidates of gate dielectric layers in flexible TFTs. In this work, 2D MoS2 and dielectric ceramic Bi2MgNb2O9 (BMN) were utilized to fabricate flexible TFTs on low-cost polyethylene terephthalate substrates. The MoS2/BMN hybrid structure exhibited good quality by Raman, X-ray photoelectron spectroscopy, and atomic force microscopy characterizations. In addition, the flexible MoS2/BMN TFTs indicated good performances with a small gate voltage. More importantly, with the modulation of gate voltage, the flexible TFTs surprisingly exhibited three different device types, that is, multilayer MoS2/BMN n-type TFT (device type 1), homojunction MoS2/BMN TFT (device type 2), and thick MoS2/BMN p-type TFT (device type 3). In particular, with different bias conditions, the homojunction TFT showed bipolarity of transfer characteristics and forward/backward rectifications of output characteristics similar to p-n/n-n junctions. The high dielectric constant and high quality of the BMN ceramic layer enabled the gate to effectively modulate these different structures of MoS2 channels. The operation mechanisms of these three types of flexible TFTs were investigated. Additionally, the flexible MoS2/BMN TFTs showed good flexibility and performance stability with external strains. The results prove the great potential of integration of 2D materials, high-quality dielectric ceramics, and low-cost plastic substrates for high-performance flexible TFTs and further applications of flexible electronics.

Tunable electronic properties of TiO2 nanocrystals by in situ dopamine functionalization for planar perovskite solar cells

A high-quality TiO2 electron transport layer (ETL) should possess a good energy-level alignment, perfect electron mobility, and excellent charge extraction efficiency. Nevertheless, the poor conductivity of amorphous TiO2 film from low-temperature preparation is an inherent problem while the high-crystallined TiO2 after high-temperature sintering (>450 °C) limits its application in the low-cost fabrication and flexible substrates. Here, we develop a low-temperature approach to in situ functionalize TiO2 nanocrystals with dopamine molecules (DA-TiO2 NCs) in the synthesis of the nanocrystals through a non-aqueous sol-gel method. We systematically investigate the tunable electronic properties of TiO2 NCs with the variable DA modification and confirm that DA-TiO2 can tailor the energy level alignment as well as increase the charge-extraction capacity, substantially reducing charge accumulation between the TiO2 and perovskite layer. The in situ anchored amino groups (-NH3+) not only preferentially boost the in-plane growth of perovskite grains but also induce the red-shift absorption of perovskite via the strong electrostatic interaction with Br− ions. Based on the DA-TiO2 NCs ETLs, the power conversion efficiency of the fully ambient-processed planar FAxMA1-xPbBryI3-y PSCs is optimized to 19.45%.

Gentle plasma process for embedded silver-nanowire flexible transparent electrodes on temperature-sensitive polymer substrates

The present study investigates processing routes to obtain highly conductive and transparent electrodes of silver nanowires (AgNWs) on flexible polyethylene terephthalate (PET) substrate. The AgNWs are embedded into a UV-curable polymer to reduce the electrode roughness and enhance its stability. For the purpose of device integration, the AgNWs must partially protrude from the polymer, which demands that their embedding is followed by a transfer step from a host substrate to the final substrate. Since the AgNWs require some sort of curing (thermal or plasma) to reduce the electrode sheet resistance, a thermally stable host substrate is generally used. This study shows that both thermally stable polyimide, as well as temperature-sensitive PET can be used as flexible host substrates, combined with a gentle, AgNW plasma curing. This is possible by adjusting the fabrication sequence to accommodate the plasma curing step, depending on the host substrate. As a result, embedded AgNW electrodes, transferred from polyimide-to-PET and from PET-to-PET are obtained, with optical transmittance of ∼80% (including the substrate) and sheet resistance of ∼13 Ω/sq., similar to electrodes transferred from glass-to-glass substrates. The embedded AgNW electrodes on PET show superior performance in bending tests, as compared to indium-tin-oxide electrodes. The introduced approach, involving low-cost flexible substrates, AgNW spray-coating and plasma curing, is compatible with high-throughput, roll-to-roll processing.

Blue laser-induced photochemical synthesis of CuAg nanoalloys on h-BN supports with enhanced SERS activity for trace-detection of residual pesticides on tomatoes

The copper (Cu)-based nanoalloys loaded on graphene-like nanoplates have a great promise for boosting surface-enhanced Raman scattering spectroscopy (SERS), due to low-cost Cu species in comparison with noble metals and unique additional Raman enhancement originated from two-dimensional (2D) supports. Herein, we report an ingenious strategy to load highly dense and monodisperse CuAg nanoalloys (NAs) on hexagonal boron nitride (h-BN) by blue laser (405 nm) -induced photochemical reaction. Without the aid of any complex auxiliary reagents, the co-reduction of Cu and Ag ions is driven by photon-excited electron on semiconductor h-BN, resulting in the coalescence and overgrowth of “clean” CuAg NAs. As expected, the proposed h-BN/CuAg NAs provide ultrahigh SERS activity with the limit of detections (LODs) of crystal violet (CV) at femtomole level (∼10−16 M), thiram and tricyclazole at picomolar (10−12 M) and nanomolar levels (10−9 M), respectively. Most importantly, the as-prepared h-BN/CuAg NAs were also directly combined with polyethylene terephthalate (PET) by simple dripping and drying processes to form uniform flexible substrates. The low-cost Cu-based flexible substrates can be used to realize the ultrasensitive SERS detection of residual tricyclazole on tomato surfaces, thus facilitating the widespread precise assessment of pesticide residues in various practical applications.

Meshed High-Impedance Matching Network-Free Rectenna Optimized for Additive Manufacturing

Additive manufacturing using direct-write or screen printing represent low-waste methods for fabricating antennas on low-cost flexible substrates. To realize rectennas using low-resolution printing methods, high-impedance antennas with simple printable geometries are required. This article proposes an electrically-small (0.212 × 0.212λ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) folded dipole antenna design with a scalable impedance for directly matching energy harvesting rectifiers. The antenna is demonstrated in a high-efficiency sub-1 GHz rectenna, with varying mesh fill-factors for optical transparency. The proposed solid (non-transparent) and meshed (70%-transparent) rectennas achieve a Power Conversion Efficiency (PCE) of over 70% and 60% from sub-1μW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> power densities, at 940 and 920 MHz, respectively. This represents a 37% improvement in the PCE over state-of-the-art flexible rectennas while maintaining the smallest electrical size and simplest design by not requiring a matching network. The 70%-transparent rectenna's performance is investigated in real-life use-cases showing its suitability for ambient RF energy harvesting with over 500 mV DC output from a phone-call.

A dual-carbon-anchoring strategy to fabricate flexible LiMn2O4 cathode for advanced lithium-ion batteries with high areal capacity

Lithium transition metal oxides (LTMOs) are important cathode materials in lithium-ion batteries (LIBs). Constructing the robust hybrid of LTMO-flexible substrate is of great significance for developing advanced flexible LIBs. However, currently reported flat noble metal-based flexible cathodes are cost-expensive and show quite low areal capacities. Developing low-cost and nanostructured flexible substrates for LTMO cathodes is highly desirable but still rarely reported. Particularly challenging is preventing flexible substrate corrosion and mitigating/eliminating the severe ion migration at interface during necessary high-temperature annealing process. Herein, carbon nanofibers (CNFs) with truncated conical graphene layers are carefully chosen as flexible substrates for the growth of ultrasmall LiMn2O4 nanocrystals. The highly graphitic structure enables good high-temperature oxidation resistance. The plenty of exposed graphitic edge planes afford unexpected strong anchoring of LiMn2O4, evidenced by both experimental results and theoretical calculations. Moreover, an amorphous carbon layer is simultaneously introduced and coated on LiMn2O4 nanocrystals, which provides another strong outer anchoring like a ‘cargo net’. Such dual-carbon-anchoring strategy help produce a 1D LiMn2O4-nanocarbon hybrid with robust interface. As LIB cathode, it owns fast electron conduction, smooth Li+ transportation, good electrochemical stability and especially superior mechanical flexibility, thus enabling a high areal mass loading of 17.7 mg cm-2. The corresponding fabricated flexible LiMn2O4/CNF@C//CNF full cell exhibits a high areal capacity of 2.01 mAh cm-2 as well as good rate capability and cycling stability.

Implementation of Radiating Elements for Radiofrequency Front-Ends by Screen-Printing Techniques for Internet of Things Applications.

The advent of the Internet of Things (IoT) has led to embedding wireless transceivers into a wide range of devices, in order to implement context-aware scenarios, in which a massive amount of transceivers is foreseen. In this framework, cost-effective electronic and Radio Frequency (RF) front-end integration is desirable, in order to enable straightforward inclusion of communication capabilities within objects and devices in general. In this work, flexible antenna prototypes, based on screen-printing techniques, with conductive inks on flexible low-cost plastic substrates is proposed. Different parameters such as substrate/ink characteristics are considered, as well as variations in fabrication process or substrate angular deflection in device performance. Simulation and measurement results are presented, as well as system validation results in a real test environment in wireless sensor network communications. The results show the feasibility of using screen-printing antenna elements on flexible low-cost substrates, which can be embedded in a wide array of IoT scenarios.

Investigation of integrated solid state nano‐ionic metal–insulator–metal switches for electronically reconfigurable band‐stop filter applications

A proof of concept of application of Nano-Ionic Conductive-Bridging Metal-Insulator-Metal (MIM) switches in electronically reconfigurable band stop filters, through design, and experimental results are presented in this article. Two similar designs of band stop filters with different mechanism of operation are proposed herewith. One is a microstrip, resonant open stub based band stop filter and the other is a microstrip tuned shorted stub based band stop filter. Electrical length of the stubs in both these devices are tuned by an integrated MIM switch forcing them to resonate at different frequencies and thus achieving electronically switchable band stop filtering characteristics. Working mechanism of the devices is validated with the help of developed electrical equivalent circuit models. Analysis on time stability of switch states for 1000 minutes with affirmative results is also presented herewith. These devices are fabricated without any soldered electrical components and without the use of any clean room technologies, using proven low cost method, and are promising to work at very-low power and have the potential to be directly printed on low cost flexible substrates like paper, for potential disposable applications.

Enhanced Performance Stability of Iridium Oxide-Based pH Sensors Fabricated on Rough Inkjet-Printed Platinum.

Today, electrochemical sensors are used for a broad range of applications. A fundamental challenge is still the achievement of long-term sensor stability by ensuring good adhesion between the deposited sensing layer and the substrate material, e.g., a metal electrode. Until now, the most applied strategy to overcome this problem is to increase the surface roughness of the metal layer by mechanical etching or by electroplating of additional material layers, which both imply an increase in manufacturing steps and thus the final cost of the overall device. Alternatively, to overcome these adhesion problems, we propose the direct printing of a novel platinum nanoparticle ink, which is compatible with low-cost additive digital inkjet and with flexible low-cost substrates. This water-based platinum ink has two unique features: it leads to highly rough surfaces, which promotes the adhesion of deposited sensing material, and it is a highly low-temperature curing ink, compatible with polymeric substrates that cannot withstand high temperatures. Based on this concept, we report about a long-term stable and highly sensitive solid-state pH sensor functionalized by anodic electrodeposited iridium oxide on a rough nanostructured platinum printed layer. The sensors showed an excellent reproducibility with a linear super-Nernstian response (71.3 ± 0.3 mV/pH unit) in a wide pH range (pH 2-11). Long-term stability tests for over 1 year of application demonstrate an excellent mechanical sensor layer stability, which is correlated to the distinct roughness of the printed platinum layer. This novel approach is useful to simplify the fabrication process and with that the sensor costs.