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In this paper, we report the demonstration of highly sensitive flexible strain sensors formed by a network of metallic nanoparticles (NPs) grown under vacuum on top of a cracked thin alumina film which has been deposited by atomic layer deposition. It is shown that the sensor sensitivity depends on the surface density of NPs as well as on the thickness of alumina thin films that can both be well controlled via the deposition techniques. This method allows reaching a record strain sensitivity value of 2.6 × 108 at 7.2% strain, while exhibiting high sensitivity in a large strain range from 0.1% to 7.2%. The demonstration is followed by a discussion enlightening the physical understanding of sensor operation, which enables the tuning of its performance according to the above process parameters.

This article aims to provide a comprehensive review of recent advances in the use of gas-phase synthesized nanoparticles in the field of sensing technology. Since there are numerous and diverse reviews that already cover the subject extensively, this review focuses predominantly but not exclusively on gas-phase synthesized metallic nanoparticles and their most prominent sensing-applications. After a brief overview on the main uses of nanoparticles in science and technology, as well as a description of the dominant fabrication methods, the review discusses their incorporation in strain-sensing, chemical sensing and bio-sensing as well as a few other sensing-applications. The review highlights the inherent advantages of nanoparticles, as well as how they combine with flexible gas-phase synthesis processes.

The development of high-performance sensors emulating the response of the human skin to external mechanical stimuli is of increasing importance in the era of artificial intelligence and robotics. To realize artificial skin, various parameters must be met, including flexibility, biocompatibility and low power consumption of the employed sensors. In parallel, a multisensory platform for both detection and storage is required for emulating the physical properties of human skin. With this in mind, in this work we demonstrate an extremely sensitive resistive stretchable sensor that can achieve a gauge factor of ∼107 based on the employment of a polydimethylsiloxane (PDMS) substrate decorated with Pt nanoparticles as the stretch-sensitive medium placed in between two Ag electrodes. A critical step to achieve such performance is the formation of a rippled surface of the PDMS substrate through the combined use of pre-stretch and the deposition of a thin Al2O3 film by atomic layer deposition that enables the fabrication of highly stretchable Ag electrodes. The wavelength of the ripples, as well as the peak-to-valley height between them, can be directly controlled by tuning the applied pre-stretch load on the PDMS. By taking advantage of the extreme sensor sensitivity achieved, emulation of the functionality of a biological mechanoreceptor was further demonstrated by connecting the sensor in a parallel circuit configuration with a SiO2-based conductive-bridge memory. Various synaptic properties of the above sensory system are demonstrated, including paired-pulse facilitation and long-term plasticity, indicating the capabilities of our system to perform neuromorphic computations at the edge.

In this work a Monte-Carlo tool simulating platinum nanoparticle (NP) based strain-sensors, on flexible substrates, is presented. The tool begins by randomly placing the NPs on the simulation area, with the ability to tune the NP surface coverage. After the calculation of the conductive paths that were generated in the previous step, the whole system is represented with an equivalent circuit; the NPs and the NP clusters act as nodes and the inter-particle gaps as resistances. The effective resistance is then calculated with the use of a Laplacian Matrix, which has proven extremely effective in significantly reducing the overall computational time. The simulation results are then benchmarked with experimental measurements from actual strain-sensing devices. The software is capable of predicting the strain-sensitivity for different NP sizes as well as surface coverages, emerging as a powerful computational tool for design-optimization of NP based devices in polymeric substrates, while it could well be extended to other nanocomposite materials used in flexible or stretchable electronic applications.

The present paper discusses the development of a pesticide gas sensor based on self-assembled nanoparticle networks and four distinctive polymer coatings; the sensor is used as a sensing system for chlorpyrifos detection. For the fabrication of the sensing system the sputtering technique is employed for the simultaneous fabrication and deposition of the nanoparticle film while 4 different polymer coatings are then spin-coated on top. The sensors are ultimately able to distinguish between chlorpyrifos (a commonly used pesticide) and its solvent, detecting chlorpyrifos concentrations as low as 0.7 ppb. Each of the 4 polymer coatings produces a unique sensor response when in the presence of chlorpyrifos vapours; this has served as the basis for the development of a multi-sensor array for chlorpyrifos detection. The capability of the system in distinguishing between pesticide and pesticide solvent has been supported by results obtained by means of Principal Component Analysis (PCA).

An impedimetric label-free aptasensor for the simultaneous and highly selective detection of two pesticides is presented. The sensor is based on the combination of self-assembled platinum nanoparticle two-dimensional films (produced in room temperature via the magnetron sputtering technique) and target specific nucleic acids (a.k.a. aptamers). The platinum nanoparticle films have been deposited on top of SiO 2 substrates previously patterned with interdigitated electrodes. The surface of the device has been chemically modified in order to enhance the subsequent aptamer deposition and immobilization. Two distinctive aptamer configurations have been employed in order to facilitate capturing and detection of two pesticides, namely acetamiprid and atrazine. Successful binding events between aptamer and target induce a shift in the impedance of the aptasensor which can be attributed in charge transport hindrance through the nanoparticle film. The incorporation of the nanoparticle film in the fabrication of the aptasensor significantly improves the performance of the device if compared to “naked” interdigitated based aptasensors, allowing the highly sensitive and selective detection of acetamiprid and atrazine down to 0.6 × 10 − 11 M and 0.4 × 10 − 10 M respectively.

A novel impedimetric biosensor was developed for the detection of the two extensively used pesticides, acetamiprid and atrazine. By employing the sputtering and e-beam lithography techniques, platinum nanoparticles (Pt NPs) were deposited in a bridge-like arrangement, in between interdigitated electrodes (IDEs). The resulting Pt NP microwires were chemically functionalized to allow the covalent immobilization of aptamers against the two target analytes onto the sensor surfaces. The biosensing platform facilitated charge transfer through the microwire-bridged IDEs, while upon analyte binding to the immobilized aptamers electron transfer was hindered, resulting in an increase of the electrochemical cell's impedance. The combination of Pt NPs microwires and aptamers allowed the sensitive and highly selective detection of acetamiprid with a linear range of response in the range of 10pM to 100nM with a limit of detection (LoD) at 1pM, and of atrazine with a linear range of responses from 100pM to 1μM and a LoD at 10pM respectively. Its performance was tested against a number of other commonly used pesticides as well as in real water samples.

Molybdenum disulfide (MoS2) has attracted a lot of attention due to its semiconducting behavior. Chemical vapor deposition of MoS2 is considered a relatively simple and user-friendly technique but there are several obstacles that may hinder the growth, and might result in rather small and non-uniformly distributed triangular flakes over the substrate surface. To tackle this issue, we developed an approach for large flakes (>300 μm) grown homogeneously over two substrates at once. This is based on spin-coating one of the two substrates with an aqueous solution of Mo-based salt particles, and covering it with the other. During the growth, the particles will act as nucleation sites for both substrates. Raman and atomic force microscopy measurements show that the grown flakes are monolayer. Finally, electrical measurements demonstrate the semiconducting behavior of the material and further analysis reveal two conduction mechanisms (nearest-neighbor and variable-range hopping) that dominate according to the temperature.

The insertion of an HfO2– y layer within TiO2– x /HfO2– y /TiO2– x resistive random access memory (RRAM) yields in low set power of 50 nW (10 nA at 5 V), low reset power of 3 nW (1 nA at −3 V), and good cycling variability ( $\sigma /\mu ). In addition, under pulse experiments, fast switching time of $1~\mu \text{s}$ , good $10^{7}$ cycling endurance and retention performance at 150 °C, was demonstrated. The confinement of the switching effect into the HfO2– y film, which has the highest oxygen content and deeper oxygen vacancy energy levels compared with the adjusting two layers of TiO2– x which act as two series resistances, can explain the low switching energy. The gradual modulation of the resistance permits also the manifestation of long-term potentiation synaptic plasticity, induced by the application of a train of pulses with different repetition intervals. A quantitative model was applied in order to reproduce the analog SET/RESET responses of the trilayer configuration and highlight the role of the local distribution of oxygen vacancies. These effects in conjunction with the room temperature fabrication process used and the forming-free nature of the thin films are considered as an optimization route toward high-density RRAM design.

Utilizing the sputtering technique, platinum nanoparticles of 5 nm in diameter have been deposited on top of oxidized silicon substrates. The nanoparticle layer has been subsequently functionalized with DNAzymes that are known to self-cleave in the presence of lead (Pb 2+ ) ions. The resulting device has proven to operate as an environmental monitoring sensor for the detection of lead ions down to concentrations of 10 nM, having good reproducibility and stability. The conductive bridging of the DNAzymes that facilitates charge transport between discreet nanoparticles “collapses” due to their self-cleavage upon exposure to lead ions, leading to a shift in the measured resistance of the sensor.

Highlights • A hybrid nanomaterial based gas-sensing array has been used for pesticide detection. • The pesticide is the commercially available organophosphate based Chloract 48 EC. • The array has successfully distinguished between relative humidity and pesticide. • The successful operation of the array has been validated via the PCA method. • This study expands the limited available results related to pesticide gas-sensors., The increased use of pesticides across the globe has a major impact on public health. Advanced sensing methods are considered of significant importance to ensure that pesticide use on agricultural products remains within safety limits. This study presents the experimental testing of a hybrid, nanomaterial based gas-sensing array, for the detection of a commercial organophosphate pesticide, towards its integration in a holistic smart-farming tool such as the “gaiasense” system. The sensing array utilizes nanoparticles (NPs) as the conductive layer of the device while four distinctive polymeric layers (superimposed on top of the NP layer) act as the gas-sensitive layer. The sensing array is ultimately called to discern between two gas-analytes: Chloract 48 EC (a chlorpyrifos based insecticide) and Relative Humidity (R.H.) which acts as a reference analyte since is anticipated to be present in real-field conditions. The unique response patterns generated after the exposure of the sensing-array to the two gas-analytes were analysed using a common statistical analysis tool, namely Principal Component Analysis (PCA). PCA has validated the ability of the array to detect, quantify as well as to differentiate between R.H. and Chloract. The sensing array being compact, low-cost and highly sensitive (LOD in the order of ppb for chlorpyrifos) can be effectively integrated with pre-existing crop-monitoring solutions such as the gaiasense.

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Platinum nanoparticles without any prior chemical modification and poly(2-hydroxyethyl methacrylate) (PHEMA) have been employed as the transducing and gas sensitive layer respectively for this flexible chemical sensing application. Using a modified magnetron sputtering technique, gas phase platinum nanoparticles have been deposited on top of flexible polyimide substrates. The surface density of such nanoparticle films can be controlled via the overall deposition time, producing devices of varying conductivity. A layer of PHEMA polymer (500–700 nm thick) has been ink-jet printed on top of the nanoparticle film finalizing the sensor's fabrication. By exposing the sensor in ethanol or humidity vapors a change in the film's resistance or capacitance is observed facilitating the detection of such gases. The performance of these sensors has been connected with the nanoparticle film resistance.

Chemical gas sensors have been fabricated from platinum and poly(2-hydroxyethylmethacrylate) (PHEMA) polymer. Sputtered platinum nanoparticles have been deposited on flat silicon substrates and a second layer of PHEMA polymer has been deposited on top using ink-jet printing. Density of the platinum nanoparticles and resistance of the device, which can both be tuned by controlling the deposition time, directly affect the response of the sensors as measured in the presence of ethanol and humidity. Chemi-resistive and chemi-capacitive sensors capable of detecting low gas concentrations (1000 ppm of humidity and 500 ppm of ethanol) have been made possible.

A chemical sensor fabricated from gold nanoparticles and poly(2-hydroxyethyl methacrylate) (PHEMA) is presented and its chemi-resistive, chemi-capacitive and chemi-impedance performance is investigated. Detection of humidity and ethanol in the range of 2000–20,000 ppm has been made possible in all the above cases. The sensor was fabricated following a two step process utilizing ink-jet printing to deliver both nanoparticles and PHEMA on oxidized silicon substrates: first a layer of nanoparticles has been delivered on top of gold interdigitated electrodes followed by the successive deposition of PHEMA polymer. By controlling several key design parameters such as nanoparticle size, number of ink-jet printed gold nanoparticles and electrode spacing, the fabrication of chemical sensors with different sensing response and linearity is possible.

A gas sensor formed by a Platinum (Pt) nanoparticle film and a poly(2-hydroxyethyl methacrylate) (PHEMA) polymer layer is discussed in this paper. The Platinum nanoparticles which have been produced by a modified magnetron sputtering technique are deposited on flexible polyimide substrate. The substrate has been initially patterned with gold interdigitated electrodes (IDEs). A second layer of PHEMA polymer has been delivered on top of the Pt nanoparticles using an ink-jet micro printer. By taking advantage of PHEMA's known susceptibility in alcohol or humidity vapors, chemical gas sensors of varying sensitivity can be produced by controlling the nanoparticle deposition time. As a result an inexpensive alternative to gas sensors using conventional silicon technology can be realized.

A novel nanoparticle based biosensor for the fast and simple detection of DNA hybridization events is presented. The sensor utilizes hybridized DNA's charge transport properties, combining them with metallic nanoparticle networks that act as nano-gapped electrodes. The DNA hybridization events can be detected by a significant reduction in the sensor's resistance due to the conductive bridging offered by hybridized DNA. By modifying the nanoparticle surface coverage, which can be controlled experimentally being a function of deposition time, and the structural properties of the electrodes, an optimized biosensor for the in situ detection of DNA hybridization events is ultimately fabricated. The fabricated biosensor exhibits a wide response range, covering four orders of magnitude, a limit of detection of 1nM and can detect a single base pair mismatch between probe and complementary DNA.

The design and implementation of a chemi-resistor sensor utilizing platinum nanoparticles and a poly(2-hydroxyethyl methacrylate) (PHEMA) polymer layer is presented. The sensor was fabricated utilizing a magnetron sputtering system for the gas phase synthesis of metallic nanoparticles and an ink-jet printer for the PHEMA polymer delivery. The platinum nanoparticles have been deposited on oxidized silicon substrates which have been previously patterned with gold interdigitated electrodes. Following the nanoparticles deposition a PHEMA polymer layer has been delivered on top, using the ink-jet printing technique. By controlling the density of the metal nanoparticle film, sensors of different sensitivity can be fabricated.

A technique for the fabrication of chemiresistive sensors using ink-jet printing is reported. Gold nanoparticles stabilised in water were ink-jet printed on gold interdigitated electrodes patterned on an oxidized silicon wafer. As a first step the influence of substrate functionalization, electrode spacing, and nanoparticle density were investigated by electrical measurements and microscopy imaging. Finally a second layer of PHEMA polymer was printed on top of the gold nanoparticle films. The device was exposed to vapours of humidity and ethanol and the obtained results are presented.

Al2O3 thin films deposited by atomic layer deposition on Pt nanoparticle-based strain sensors were studied as humidity barrier coatings for sensor protection. The effect of two deposition parameters-film thickness and growth temperature-is discussed in relation to an Al2O3 coating's ability to isolate the nanoparticle surface and protect strain sensitivity from humidity variations. It is shown that transmission electron microscopy images cannot confirm the effective protection of a nanoparticle surface, thus x-ray photoelectron spectroscopy and electrical measurements have been employed. The existence of a critical thickness of the Al2O3 protective film above which resistance and gauge factor variations are suppressed during humidity change was observed at different deposition temperatures. This ability is linked with the existence of incorporated pinholes and intrinsic hydroxyl groups in the Al2O3 thin film, which are responsible for humidity transport through the oxide.

We report a controllable strain gauge factor obtained using a two-dimensional nanoparticle layer formed from platinum nanoparticles. A vacuum technique is used for room temperature nanoparticle deposition that allows control of the electrical resistance of the film, exhibiting semiconducting-like behavior when nanoparticle arrays cover the surface below a threshold value while above it a metallic behavior is prevalent. The highest sensitivity is obtained for intermediate density values of the nanoparticle assemblies, which could be explained using a tunneling and hopping current expression. The device, which exhibits more than one order of magnitude higher strain sensitivity than continuous metallic films, is fabricated at room temperature through standard lithographic processing allowing for miniaturization and easy integration in silicon technology or flexible substrates.

The fabrication of a chemical sensor based on the technique of gold nanoparticle ink-jet printing and the subsequent functionalization of the nanoparticle film by Polyhydroxyethylmethacrylate (PHEMA) polymer is presented while its chemiresistive, chemi-capacitive and chemi-impedance response are investigated. By controlling the Gold nanoparticle density and selecting different interdigitated electrode distances and nanoparticle sizes, chemical sensors of different linearity and sensitivity can be obtained.