What is a biosensor?
Biosensors are an ingenious range of gas detectors that call on the assistance of biological organisms, principally bacteria, to convert a chemical species from a form that is difficult to measure to a form that is easy to measure.
Biosensors consist of an electrochemical component and a biological component (Figure 1). The electrochemical component is usually a Clark-type microsensor that is embedded into the biological component but separated by a silicon membrane (Figure 2). The biological component, or biochamber, contains the chemical transforming bacteria. An ion-permeable membrane separates the biochamber from the external environment or sensor (Figure 2).
A target gas moves into the biochamber via diffusion and is chemically transformed to a specific chemical species by the bacteria. The new species then moves via diffusion to the Clark-type microsensor where an electrochemical reaction takes place. The voltage output of this reaction is recorded by a picoammeter and, via a calibration curve, converted to known partial pressure of the gas of interest.
Biosensors have been developed to measure oxygen, hydrogen peroxide, methane, nitrate/nitrite (NOx) and nitrite (NO2). However, only NOx and NO2 are currently available as measuring the other gases has proved challenging (see below for more details).
detailed information on biosensors
The following section has been adapted from the Unisense NOx Biosensor Manual and describes in detail how a biosensor measures NOx and NO2. The same measurement principle, although with different chemical and bacteria species, is applicable to other biosensors.
Nitrate or nitrite diffuses into the NOx biosensor from the external environment through a selective membrane (Fig. 2). Bacteria situated in a reaction chamber behind this membrane reduce the nitrate or nitrite to nitrous oxide (N2O), which is detected by an electrochemical nitrous oxide transducer. The amount of N20 reduced on the cathode surface is proportional to the concentration of NOx in the external environment. The respiration of the bacteria oxidizes carbon (the energy source) to form CO2 and water and there is a simultaneous reduction of nitrate and nitrite to nitrous oxide.
The NOx biosensor is equipped with a reservoir containing the carbon source. This carbon compound diffuses into the reaction chamber (Fig. 2), where the bacteria are positioned in opposing gradients of the two substrates, carbon and NOx, required for their growth. Excess cells formed by the growth of the bacteria are pushed upwards into the reservoir, where there is no NOx present. As these cells are unable to respire, growth will cease, and the cells degenerate, releasing cell components, which are used as a carbon source for other living cells.
The bacteria are facultative aerobic microorganisms, meaning that they are able to use both oxygen and NOx for respiration. Respiration with oxygen results in the highest energy yield, and oxygen is therefore used preferentially over NOx. However, oxygen respiration by the bacteria does not affect the sensor’s response to NOx.
The N2O transducer is a Clark-type sensor, in which N2O is reduced to nitrogen gas (N2) on the surface of a polarized cathode. As two electrons are used to reduce a N2O molecule, two electrons are taken from a silver/silver iodide reference electrode (the anode). A picoammeter measures the transport of electrons between the anode and cathode, and the measured current is proportional to the amount of N2O reduced on the cathode surface. The latter is again proportional to the concentration of NOx diffusing into the sensor from the external environment.
A silicone membrane in the tip of the N2O transducer electrically separates bacterial production and electrochemical detection of N2O. The silicone membrane allows the passage of gasses and small, uncharged molecules, while ions are unable to pass.
The bacteria performing the process inside the sensor is a pure culture and must not be contaminated with bacteria from an external source. Therefore the bacteria are separated from the external environment by an ion-permeable membrane, which only allows the passage of small ions, including NOx. Larger molecules and other bacterial cells cannot enter the sensor through this membrane.
advantages and limitations
- Chemical Species
Biosensors are currently limited to only a few different types of chemical species. A great advantage of biosensors is they can measure NOx and NO2 in-situ, continuously and online where otherwise these chemicals need to be tediously measured in the lab. Biosensors offer one of the only methods to measure some nitrogen based chemical species in the field that are critical for environmental scientists and wastewater managers. Some LIX based sensors can measure NO3, NO2 and NH4 in freshwater environments. However, high concentrations of interfering ions renders LIX sensors inaccurate in marine or saltwater environments.
As biosensors rely on living organisms of bacteria, temperature can have a significant effect on performance and measurement range. At low temperatures, bacteria are less active and transform less of the chemical species whereas at high temperatures the bacteria can becomes stressed and die. For the NOx/NO2 biosensors the measurement range at 10°C is 0 to 200 μM whereas at 20°C the measurement range is 0 to 1000 μM.
Biosensors only have a limited lifespan of a few months (depending on level of usage). However, the design of the sensors mean that the biochambers can be replaced once the bacteria have expired.
- Continuous Online Monitoring
NOx and NO2 biosensors provide the opportunity for in-situ, continuous and online monitoring for periods up to 10 days. This feature is particularly advantageous for wastewater managers.