Molecular Electronics for Chemical Sensors: Part 3 of My Notes from 2017 MIT Health Sensing & Imaging Conference Sept. 19-20, 2017.

Molecular Electronics for Chemical Sensors

Timothy Swager, John D. MacArthur Professor of Chemistry, MIT Department of Chemistry

To understand both these talks, please check out the Carbon Nanotube section of wikipedia.

Chemiresistors – light weight low power option for chemical analytics.

Why Chemical Sensors?

Eliminate high overhead (space, people etc) of the state of the art analytical lab by transitioning to small chemical sensors to deliver data through the cloud/IoT.

Features:

  • By putting the technology in materials, less expensive
  • Can be information rich and uniquely identify analytes (Mass spectrometer)

and limitations:

  • More expensive, limited portability
  • Devices often have fixed sampling chambers that can foul
  • Often limited by chemical selectivity (eg can tell an acid or base, but not the specific acid…).

CNT Chemiresistors

Measuring resistance changes across carbon nanotubes that have a corona. They can be low power and small, and can be high sensitivity.

Applications

Gases in Food Management

  • Ethylene is given off by produce during ripening, it induces ripening and is an indicator of plant health.
  • Amines are an indicator of meat/fish spoilage.
  • Ammonia is useful for soil nutrient level monitoring

SWCNT-Based Ethylene Chemiresistors –¬†coordinator chemistry on the side of the nanotubes with a special compound that binds ethylene, causing the circuit to respond.

http://onlinelibrary.wiley.com/doi/10.1002/anie.201201042/abstract

This has lead to a spin-out company. Have made fruit ripening rooms – used to synchronise ripening, and also sensors to detect ethylene – currently tomatoes are used as ethylene sensors around combustion engines in greenhouses!

Sensing with Cross Reactive Arrays

Using statistics to organise the raw data from sensor information.

By adding different functional groups to the nanotubes, they maintain their electrical conductivity whilst changing their function. The sensors for different compounds (water, methanol, various hydrocarbons etc) are reproducible and still work after 1 year.

Used a reaction to “functionalize” the nanotubes very densely with the different functional groups. Now there was a greater reduction in conductance from increased nanotube spacing, requiring electrons to “jump” further. From this data they created a response matrix for different compounds, and did a principal component analysis (statistical analysis) in two dimensions to capture the variance and categorise the compounds.

Image result for j am chem soc 2011 principal component analysis CNT chemiresistor
https://www.researchgate.net/publication/282815807_Chemiresistive_Sensor_Arrays_from_Conductive_2D_Metal-Organic_Frameworks

Carbon Monoxide Sensor

Bio-inspired Gate Enhanced Sensor. Took a molecule that looks like haemoglobin and attached it to a carbon nanotube. In air, iron is Fe3+. When exposed to CO, nothing happens because in the body, iron is Fe2+. So they made an iron sensitive Fe2+ porphyrin, and when exposed to CO, it changes colour. The sensitivity was enhanced using Gate Voltage.

MIT pencil ‘draws’ gas sensors onto paper

Screen Shot 2017-09-19 at 14.35.01
BBC article

Wireless CNT Chemiresistor Sensors

Smartphone sensing – ultra-low power wireless sensors.

Take a RFID (radio frequency identification devices – used as anti-theft devices) tag with an integrated circuit. Disrupt the circuit and draw sensor with nanotubes to reconnect circuit. Either 1) in presence of sensate it becomes conductive or 2) in absence of sensate it becomes conductive.

Application for this is in use of food packaging – and the team created an oxygen dosemeter. Eg in lettuce bags, where oxygen is depleted (modified atmosphere packaging). This means making use of the same iron sensor as above for the CO sensor. When you expose the sensor to oxygen, you get a large response in circuit resistance.

The smartphone can then provide a digital readout depending on the gain set.

Screen Shot 2017-09-19 at 14.08.36

The CO sensor has not been commercialised yet. The food freshness sensor is cheap in terms of sensors but not in terms of adding cost to the food production.

Screen Shot 2017-09-19 at 14.09.39

The answer was that several groups are working on tattoo sensor applications, but the barrier for RFID is that they work best in air, not in conductive material (like extracellular fluid).

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