Electrical Conductivity + OD Sensor (EC+OD)

In Brief

A set of PCB and 3D printed devices holding 14 mL culture tubes and DropSense electrode. It can be used both as a sensor for the Output devices of our Electro-Genetic Framework and as an actuator for the Input Devices of the toolkit.

Quick Links

Used for:
Inputs
Screening
Gitlab

Used with:
High Throughput Electro Actuator (HTEA)
Electro-Micro-Slide
Electro Planner
Used in:
The screening experiment
Electro-Micro-Slide experiments
Twitter Proof of concept


Description - Rationale

The electrical Conductivity + OD Sensor (EC+OD) is a multi-function device that we designed, built and used to perform measurements of electrical conductivity of the media as an electrical proxy for cell density As shown in [1], growing bacteria’s capture Ions present in their media, and, in doing so, decrease the electrical conductivity of the medium.

Electrical conductivity (EC) measure is common to the fields of environmental science where samples from natural environments such as rivers and ponds are extracted and measured for EC as a proxy for salinity and presence of pulutants[2-5]

To measure electrical conductivity, we used a NE555 Timer. This well known and extremely well characterise (as the component has been unchanged since the 1970) oscilator’s period is modulated by a resistance across 2 of its pins. By replacing this resistance by 2 electrode immerged in medium (acting as the resistor), it is theoretically possible to get a proxy for cell density by reading the oscilation period on a microcontroller (here the ESP8266).

The ESP8266 also offer WIFI capabilities and the code available in our gitlab allow for data logging through WIFI (relevant for continuous measurements in long experiemnts ran in inaccessible places such as incubators).

However, electrical conductivity mesurements using the NE555 was not concluant (data not shown). We believe that, with more time and work, this still represent a viable option.

We decided to turned to using a potentiostat for electrical conductivity mesurments as shown in [1]. We used our IO Rodeo potnetiostat in conjunciton with the DropSense Adaptor and the DropSense 101 screen printed electrode to conduct these experiments.

In the context of our experiments, and more generally when thinking of further work in electro biology, it is relevant to compare OD and electrical activity/conductivity. To meet this goal, epquiped the EC+OD device with OD600 sensing capability using a 600nm LED and an Opt101 analogue light sensor as described in [1]

We also took the opportunity to add led of varied wavelength for potential optogenetic actuation

We used it in:

In our experiments, we used the ACD to gate signals generated by an MHS-2300a dual channel function generator and to dispatch this signal either to the HTEA or the Electro-Micro-Slide

Experiments done with it

We used the EC+OD to conduct experiment on the output part of the project where Electrical conductivity was mesured using the IO Rodeo potentiostat in parallel of OD. We showed that EC could be used as an electronic proxy for cell density, hence, allowing for completly electrical measurement.

Possible use case
Responsive image

In our work, we have used the ACD in conjunction with the Electro Planer, the HTEA and the Micro-Electro-Slide. Many other applications remain highly possible.



Materials and methods

We grew E. coli bacterias overnight in LB and in a 250mL Erlenmeyer flask. In the morning, we diluted the cells into 14mL Culture tubes fitting in the EC+OD Device and in fresh medium at different concentration. Optical density was measured on the spectrophotometer for “ground truth“ measurements used in calibration experiement.

We used the same bacterial population and culture tube to conduct electrical conductivity in parallel.

Figure1 shows measured Analogue signal (Analogue Read) on the Arduino compared to ground truth OD .

Figure 2 shows electrical conductivity measured with the IO Rodeo Potentiostat compared to ground truth OD



References

1. Din, M.O. *et al.* (2020) ‘Interfacing gene circuits with microelectronics through engineered population dynamics’, *Science Advances*, 6(21), p. eaaz8344. Available at: [https://doi.org/10.1126/sciadv.aaz8344](https://doi.org/10.1126/sciadv.aaz8344).

2. Cobb, W. E., & Wells, H. J. (1970). The electrical conductivity of oceanic air and its correlation to global atmospheric pollution. *Journal of Atmospheric Sciences* , *27*(5), 814-819.

3. Akoto, O., Bruce, T.N. and Darko, G. (no date) ‘Heavy metals pollution profiles in streams serving the Owabi reservoir’, p. 6.

4. Essouli, O.F. *et al.* (2022) ‘Hydrochemical Approach to Groundwater in the Environment of the Public Discharge of Lake Mbeubeuss (Dakar, Senegal)’, *Journal of Environmental Science and Pollution Research*, 8(1), pp. 458–465. Available at: [https://doi.org/10.30799/jespr.216.22080101](https://doi.org/10.30799/jespr.216.22080101).



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