Zinc oxide (ZnO) is a common ingredient in sunscreen lotions. Less well-known is the fact that it’s also a wide band gap (WBG) semiconductor (3.4 eV). As a nanomaterial, ZnO is being used to develop a wide variety of sensor technologies. Sensor applications for ZnO include health monitoring, chemical sensing, biomedical, and environmental sensors.
Some of the properties that make ZnO suited for sensor applications include low price, low dielectric constant (8.5), high luminous transmittance, good physicochemical stability, high excitation binding energy (60 meV), high electron mobility (200 cm2/(Vs), non-toxicity and biocompatibility, good piezoelectric properties, and large surface area to volume ratio.
ZnO nanomaterials can enable a variety of UV light sensors for applications like biomedical and environmental monitoring, optical communications, and defense systems.
Physical reservoir computing sensor
ZnO nanoparticles (NPs) have been combined with cellulose nanofibers (CNFs) to produce disposable intelligent optical sensors. The sensors include physical reservoir computing (PRC), which consumes little power and can mimic the human brain. PRC is derived from recurrent neural network theory and blends sensing and AI processing to implement classification and prediction algorithms. PRC is also suited for handling time series data like those common in biological signals.
The ZnO NP / CNF sensor has a gradual change in photocurrent during exposure to UV light with a sub-second time constant. The PRC function exhibited paired-pulse facilitation (PPF), a form of short-term, activity-dependent synaptic response. This smart sensor used sub-second light pulses to perform a short-term memory (STM) task, a parity-check (PC) task, and handwritten character recognition.
A disposable and flexible sensor with integrated PRC capability was developed (Figure 1). The device recognized handwritten digits with 88% accuracy. The accuracy remained unchanged after the device was subjected to 1,000 bending cycles. The device’s disposability was evident in its rapid burn rate like standard office paper.
Flexible and printable ZnO NP photosensors
Directly printing ZnO NPs is challenging. The particles tend to clog the print head and can require high-temperature annealing to become useful. A new technique has been developed that replaces the ZnO NPs with an amine-hydroxide zinc complex precursor for printing. Using an annealing temperature of 250 °C, a ZnO nano-film can be formed on a flexible polyimide substrate. The addition of silver NPs in the ink enabled the creation of a printed UV flexible sensor with adjustable resistance.
The sensor responds linearly to UV light over the intensity range of 50 to 2000 µW/cm2. The completed sensor assembly is a passive wearable device for detecting the intensity of incident UL light and includes near-field communication (NFC) to provide wireless connectivity (Figure 2).
More ZnO nanomaterial categories
ZnO nanomaterials can also be used in the fabrication of highly sensitive and selective electrochemical sensors and biosensors due to their high specific surface area, high catalytic efficiency, and strong absorption ability. ZnO nanomaterials are available in a wide range of sizes and morphologies that can be optimized for specific sensor applications. The four general categories of ZnO nanomaterials include:
- Zero-dimensional, including quantum dots and nanoparticles.
- One-dimensional, including nanowires, nanobelts, and nanoneedles.
- Two-dimensional, including nanofilms and nanosheets.
- Three-dimensional, including porous materials and nanoclusters.
In addition to their use in UV sensing described above, ZnO nanomaterials are suited for a variety of sensing applications. A few examples include:
- Pressure sensors with high sensitivities rely on the piezoresistive and piezoelectric characteristics of ZnO.
- Gas sensors use the high surface area and piezoresistivity of ZnO nanomaterials, which enable the measurement of gas absorption and a corresponding change in resistivity.
- Biosensors and environmental monitoring devices can use tailored zero-dimensional ZnO nanomaterials to achieve high selectivity and responsiveness.
Summary
ZnO nanomaterials have a number of properties that make them highly suitable for sensors, and they are available in a wide range of sizes and morphologies. With the proper additional materials, ZnO NPs can be used to make printed sensors and flexible smart sensors.
References
Disposable and Flexible Paper-Based Optoelectronic Synaptic Devices for Physical Reservoir Computing, Advanced Electronic Materials
Flexible Artificial Intelligence Optoelectronic Sensors Towards Health Monitoring, Tokyo University of Science
Fully Printed Flexible Zinc Oxide Patch for Wearable UV Light Sensing, Advanced Electronic Materials
Preparation of Electrochemical Sensor Based on Zinc Oxide Nanoparticles for Simultaneous Determination of AA, DA, and UA, Frontiers in Chemistry
Progress in ZnO Nanosensors, MDPI sensors
ZnO nano-structured based devices for chemical and optical sensing applications, ScienceDirect
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