Perishable Food Detectors using Gas Sensors – CNTs (Carbon Nanotubes)

With rising pressure on the food industry to minimize waste while meeting the need for high food quality and safety, there is a solid inspiration for research into sensor units for checking the quality and general food quality. Deterioration of fresh meat and produce is connected to metabolic activities of bacteria which generally cause the formation of gaseous substances such as CO2. Tracking the freshness as well as the quality of the food could be achieved by keeping track of the release of these gases in the food package.

Today, many perishable food products such as produce and meat are preserved in packages which help secure them from environmental impacts such as light, oxygen, moisture, microorganisms as well as contaminants. The food packaging technology has transformed the food industry, the consumer has to count on the expiry date label to determine the quality or the freshness of the product. Still, the food inside a package could be degraded, leading to the development of byproducts collected within the package. Carbon dioxide (CO2) is one of the most common by-products for food to perish which is the outcome of raising microorganism activity.

A CO2 sensor integrated within the food package could effectively monitor the food freshness and quality of the item up until it gets to the end consumer. As a result of the small amount of CO2 released by derogatory food inside the package, such a gas sensor would need to demonstrate the level of sensitivity in the order of just a few parts per million. The cost per sensor system requires being low, in order to make the quality tracking commercially feasible. In 2012, Esser et al demonstrated using the gas sensor for monitoring the fruit ripeness. They succeeded in making an economical gas sensor, with the sensitivity of much less compared to one component per million, for monitoring ethylene (C2H4) gas released by fruit which can suggest the ripening process.

The nanomaterials for gas sensors have exposed that carbon nanotubes are excellent prospects for developing sensors with the high level of sensitivity, low power consumption, as well as inexpensive. Because the usual concept for gas sensing is absorption/desorption of gas molecules, CNTs with their high contact interface permit developing gas sensors with an exceptional improvement of sensitivity. CNT-based gas sensors are battling to find their method towards commercialization. The primary challenge is the fairly high synthesis temperature level of carbon nanotubes, higher than 600°C.

That temperature goes beyond the maximum temperature permitted for MEMS/CMOS devices, therefore, making the mix of CNTs with CMOS technology challenging. Our strategy provides one technique to conquer this particular challenge by utilizing local synthesis of carbon nanotubes. The local synthesis technique likewise enables the direct combination of CNTs into MEMS/CMOS devices, potentially at wafer-level, to develop a complete sensing system as conceptually defined with signal processing circuits as well as RF communication on a single chip.

At Faststream Technologies, a set of engineers went on to explore this problem and to create sensors for detecting gasses released by this perished food. Our engineers performed sensor tests with CO2 which is a usual gas pertaining to the degradation process of food. The sensor is based on Carbon Nanotubes (CNTs) as sensing aspects. A local synthesis technique is used to directly integrate the CNTs into silicon-based circuits at room temperature. Fabricated CNT-based frameworks have efficiently shown the capability to detect CO2. The level of sensitivity, as well as selectivity of gas sensors to various gas substances, were additionally researched while doing so. The outcomes show that this strategy is guaranteeing for the construction of CNT-based gas sensor at inexpensive for determining of food freshness and quality, implementing complete gas sensor consisting of CNT sensing components, CMOS signal processing as well as RF communication on the same chip, produced as well as integrated at the wafer-level scale.

How this CNTs based chip works

Local synthesis of Carbon Nanotubes is sought proactively making use of the concept of creating locally high temperature (approximately 900°C) utilizing a microheater on the chip. The microheaters were fabricated in the PolyMUMPs commercial process given by MEMSCAP. The geometry of the microheaters, containing the suspended silicon bridges. The range between 2 silicon structures is about 10 µm. 3 nm and 5 nm thick iron films, as the stimulant, were deposited by thermal dissipation.

The deposition thickness was regulated in situ by a quartz-crystal-based thickness monitor. Local thermal annealing was executed before the growth of CNTs for transforming the iron film into nanoparticles at a temperature level of 700°C in the inert atmosphere at atmospheric pressure. The iron nanoparticles work as catalytic sites for CNTs to grow. Following this step, the carbon nanotubes were in locally synthesized at 900°C at 0.4 bar (C2H2 + Ar atmosphere). The temperature of the microheater was monitored by determining the resistance of the silicon bridge, adhering to the procedure.

The silicon bridges were aligned perpendicular to the precursor flow direction. An external electric field (E-field) between 2 silicon bridges was produced by using external bias voltage between them (Vbias). The objective of utilizing external E-field is to align the growth direction of CNTs. In our experiments, E-field aligned with the precursor flow direction. By monitoring the electrical current going through the Si bridges (Ibias), the connection made by CNTs could be identified. Bias resistor (Rbias) is made use of to restrict the current passing through CNTs. The formed Si-CNTs-Si structure is evaluated as CNT-based gas sensor where the sensing aspect is carbon nanotubes.

Two silicon bridges work as supporting structures for the cost-free standing CNTs and are connected to gold pads. Sensitivity, as well as selectivity of CNT-based sensors to Argon (Ar), Carbon Dioxide (CO2) and air (78% N2, 21% O2), are identified. The schematic representation for sensor test experiments for characterized the CNT-based gas sensor, a Wheatstone bridge configuration was used to read out the modification in I-V characteristics of carbon nanotubes when being subjected to exposure to various gas combinations (air/CO2 and also Ar/CO2).

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