Free Radical Biosensors

Selection includes high selectivity and low detection limit sensors

WPI's biosensors are unique, because they offer a high selectivity and low detection limit (down to nM concentration) with a broad dynamic range, covering physiological concentrations of species with different sizes from nanometer to millimeter. The majority of our sensors are the only commercially available sensors in the world. Scientists across a variety of disciplines have relied on our sensors for over 25 years. These scientists use WPI's sensors for research performed in universities, hospitals, biomedical research labs, pharmaceutical companies, food/nutrition research labs, environmental monitoring centers and military labs. Our popular biosensors are listed in thousands of publications.

Free Radical Sensors Carbon Monoxide Sensors
Glucose Sensors
Hydrogen Peroxide Sensors
Hydrogen Sulfide Sensors
Nitric Oxide Sensors

Comparing Sensors

Choosing the right sensor for your application is critical for successful research. The best way to determine compatibility is to test a sensor in your application, however, cost can make that impossible.Therefore, you must rely on the specifications and information provided by the manufacturer. Five performance factors help you make an informed choice for your given application:


Electrochemical electrodes produce changes in current in response to changes in concentration. "Response" is most often specified in terms of the amount of current per concentration unit: nA/micromole or pA/nM, etc. The larger the current per unit the higher the sensitivity of the sensor.

Detection Limit

The response of a given free radical sensor is meaningless without also specifying the detection limit. Detection limit is the minimum change in concentration that can be reliably seen. This specification is directly related to the noise of the sensor. A sensor with a 100nA/μM response but a 3μM detection limit is not as good as a 10nA/μM response sensor with a 1μM detection limit.

Free Radical Detection
The best sensors have low detection limits and high sensitivity. In the graph above, the response to the addition of 50nM of nitric oxide is shown for the WPI flexible NO sensor and the comparable Brand Z electrodes. The measurements were made with the same meter simultaneously. As can be seen from the graphs response is similar but the noise level on the WPI electrode allows for a more precise estimate of the current.


Free Radical Detection 
A sensor can have a low detection limit and a good response, however, to be useful in long term studies it must be stable when temperature and concentration are constant. A drifting baseline, if monotonic, can be corrected, but wandering baselines limit the utility of sensors to short experiments.


It is a rare instance that the ion species of interest is the only ion in the medium to be measured. In a perfect world your sensor would respond ONLY to the ion of interest. In reality there is always some contribution from competing species. The lower the contribution the better. Graphs detailing the impact of competing species on electrode output are shown on the performance page of this brochure.

The first graph below shows the response of WPI's ISO-NOP007 nitric oxide microsensor following additions of: 50μM ascorbic acid (AA), 50μM nitrite, 100μM L-Arginine (L-Arg), 20 μM Dopamine (DA), 100 nM and 200nM NO. The graph indicates no interference to common reactive species, plus an enhanced response to NO. [Zhang, et al., 2000.]

The second graph below shows the response of typical Nafion-coated nitric oxide carbon microelectrode following additions of: 50μM ascorbic acid (AA), 50μM nitrite, 100μML-Arginine (L-Arg), 2μM Dopamine (DA), 100 nM and 200nM NO. [Zhang,et al., 2000.]

Free Radical Detection Free Radical Detection


For an electrode to be useful and easy to calibrate the response must be "Linear" with changes in concentration over the range of interest. Non-linear behavior requires special curve fit software to calibrate the sensors. This approach is more time consuming and can be unreliable. "Good" linearity is expressed by a R² of 0.900 or higher. (1.00 is perfect.) All of the electrochemical sensors made by WPI are have a 0.9 or better.