(372b) A Micro-Fabricated Electrochemical Gas Sensor for VOCs Detection

Chemical gas detection is a crucial technology to be developed in our modern world, and in particular,
the ability to quickly identify and quantify molecules present in the atmosphere to trigger appropriate re-
sponses. Several detection technologies are currently commercialized for a variety of application such as
detection of CO, NO_x , formaldehyde, hydrocarbons, explosives ect¹ . However, most of these technologies
are in the form of single and macroscopic devices hard to integrate and miniaturize. Also, these technologies
are often expensive to manufacture and require frequent calibration and maintenance. In this perspective,
recent developments in micro-fabrication of such devices have proven the high potential and large market
for micro-chemical gas sensors² .

Our interest in the development of a micro gas sensor is driven by our vision of a sensor network capable of
continuous monitoring of atmospheric composition at a large scale. In order to achieve such goal, we think
that a sensor should have the following specs:

• Be sensitive to a large number of chemical species
• Be selective to the target molecules such as CH₄ , C₂H₆ , CO, NO_x , H₂ and H₂S
• Be able to differentiate the detected molecules to avoid false positives
• Be inexpensive to manufacture, low power and low maintenance in order to be mass deployable

We present a micro-fabricated chemical gas sensor capable of identifying the detected molecules. The working
principle is based on the electrochemical cell where molecules are oxidized or reduced as they are adsorbed
on the surface of an electrode. The identification capabilities of the sensor is based on the characteristic
redox potential of each molecule. The redox potential corresponds to the electrical potential that has to be
applied to a molecule to oxidize or reduce it. This electrochemical reaction generates an electrical current
that, when measured, gives information on the nature of the molecule as well as the quantity adsorbed on
the electrode. Examples of electrochemical reactions with the previously cited molecules are:

• CO + H₂O → CO₂ + 2H⁺ + 2e^−
• CH₄ + 2H₂O → CO₂ + 8H⁺ + 8e^−
• 2H⁺ + 2e^− → H₂

The sensor has a simple design composed of the three typical electrodes found in electrochemical setup,
namely a working (WE), a counter (CE) and a reference electrode (Ref). The ability of this systems to
operate in the gas phase rather than in liquid environment, as would a conventional electrochemical cell,
comes from the usage of an Ionic Conductive Membrane that transports ions from one electrode to the other.
The total area of the current version of the sensor is 1.5 cm² , but is easily miniturizable to be integrated in
a device.

Currently, the sensor is characterized and tested with cyclic voltammetry. The operation consists of a sweep-
ing the electrical potential between the WE and the CE, while measuring the current flowing between them.
The presence of the reference electrode allows the identification of the detected current peaks on the I-V
curves. The signal for each detected molecules is isolated by using a signal obtained under inert (N₂) gas
flow. We have made the electrodes of the sensor out of Pt because it is known to be sensitive to molecules
such as CO and methane³ . Nevertheless, other materials with different affinity can be used as well for divers
1application, which makes our technology adaptable and versatile.
We have published results for CO detection with an early version of the sensor requiring an
external reference electrode⁴ , and we have now successfully detected CO and methane (CH₄) with a more
advanced version of the sensor embarking a Ag/Ag⁺ reference electrode layer. In these
experiments, it is possible to differentiate the two signals (CO and CH₄). These results open up the path
for the development of sensing solutions for indoor air monitoring as well as Natural Gas pipelines leaks
monitoring.

Our current research and development effort is focused on the detection limit of the sensor by making
measurements in diluted gas flows, exploring its durability, integrate it in a device and developing a signal
analysis software.

References
[1] Xiao Liu, et al. Sensors, 12(7):9635–9665, July 2012.
[2] Jouko Malinen, et al. Advances in miniature spectrometer and sensor development. volume 9101, pages 91010C–91010C–15, 2014.
[3] Tanja Vidaković, et al., Electrochimica Acta, 52(18):5606–5613, May 2007.
[4] Pierre-Alexandre Gross, et al. Micro & Nano Letter, Aug. 2016