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One of the most exciting stages of sensor development is the moment when a technology moves beyond standard laboratory testing and starts measuring real-world samples.
Recently, we discussed a sodium sensing application involving the measurement of sodium in urine using an ion-selective electrode (ISE) operating in open-circuit potentiometry (OCP) mode. The developers had successfully demonstrated that their sensor responded exactly as expected when tested using standard sodium calibration solutions. However, when they moved to urine samples, they encountered some unexpected behaviour.
The good news? What they are observing is actually a very normal part of the journey from technology validation to application development.
Successfully Demonstrating the Sensor
To begin with, the team had established that the sodium sensor was functioning correctly.
Using sodium standards supplied with their kit, they observed the expected increase in potential as sodium concentration was increased. For example, moving through concentrations such as:
- 40 mM sodium
- 80 mM sodium
- 120 mM sodium
- 160 mM sodium
produced the anticipated sensor response.
This is exactly the type of work associated with Technology Readiness Levels (TRL) 3 and 4.
At this stage, the goal is straightforward:
Does the sensor respond correctly to known concentrations under controlled conditions?
If the answer is yes, then the fundamental sensing technology has been demonstrated.
The Challenge of Real Samples
The next step was to begin measuring urine samples.
Rather than simply measuring the urine, the developers attempted a standard addition experiment by adding increasing amounts of sodium calibration solution to the sample.
Intuitively, you might expect the sodium signal to increase.
Instead, they observed the opposite effect.
The signal decreased.
At first glance, this might appear to indicate a problem with the sensor. However, the reality is often much more complicated.
Why Adding Sodium Can Sometimes Reduce the Signal
When working with biological samples, it is important to remember that the sample itself already contains sodium.
If the urine already contains a higher sodium concentration than the calibration solution being added, then the addition can actually dilute the sodium concentration rather than increase it.
Imagine a urine sample containing 320 mM sodium.
If that sample is mixed with a 40 mM sodium calibration solution, the resulting concentration may decrease depending on the volumes used. In this case, the sodium ion-selective electrode would correctly report a lower sodium concentration and therefore a lower potential.
The sensor may not be malfunctioning at all. It may simply be reporting the chemistry accurately.
A Simple Troubleshooting Experiment
Rather than attempting to diagnose every possible cause immediately, a useful approach is to perform a quick sanity-check experiment.
Step 1: Measure the Urine Sample
Place a drop of urine onto the sensor and record the baseline OCP value.
Step 2: Prepare a Concentrated Sodium Sulfate Solution
Prepare a highly concentrated sodium sulfate solution so that only a very small addition volume is required.
Step 3: Add a Small Aliquot
Add a tiny drop of the concentrated sodium sulfate solution to the urine sample.
The objective is to introduce a significant amount of sodium while minimally changing the sample volume.
Step 4: Observe the Response
If the sensor is performing correctly, the signal should increase.
This simple experiment helps confirm:
✅ The sensor is responding to sodium
✅ The sample matrix is not completely suppressing the response
✅ Previous results may have been caused by dilution effects rather than sensor failure
Moving from Sensor Validation to Application Development
This is where projects move beyond TRL3 and TRL4 and begin entering the application development phase.
Questions now become:
- How does the sample matrix affect performance?
- What calibration strategy should be used?
- How should standards be added?
- What concentration ranges are expected in real samples?
- How can measurements be made robust and repeatable?
These are application-development questions rather than sensor-validation questions.
While they can be challenging, they are also a sign that meaningful progress has been made.
Proven Technology for Biological Samples
Sodium ion-selective electrode technology is already widely used in commercial diagnostic systems.
Products such as:
- i-STAT
- epoc
demonstrate that ion-selective electrodes can successfully measure electrolytes in complex biological fluids.
While these systems are primarily designed for blood analysis, the underlying principle remains the same: sodium measurement using ion-selective electrodes is both practical and commercially viable.
For this reason, measuring sodium in urine is a realistic and achievable application.
Recommended Sodium Sensing Development Kit
If you're looking to develop your own sodium sensing application, Zimmer & Peacock offers a complete workflow for sodium sensor development, validation and application testing.
📟 SenseItAll (SIA) Generation 4
The SenseItAll Generation 4 enables potentiometric measurements and provides everything needed to begin evaluating sodium ion-selective electrodes.
🔗 Product Link:
https://shop.zimmerpeacock.com/en-gb/products/senseitall-sia-generation-4-device-only
🧪 Sodium Sensor
Pair the SenseItAll with the Zimmer & Peacock Sodium Sensor to measure sodium concentrations using ion-selective electrode technology.
🔗 Product Link:
https://shop.zimmerpeacock.com/en-gb/products/sodium-sensor
📈 Sodium Calibration Solution Kit
Validate sensor performance, generate calibration curves and verify expected sodium responses using the Sodium Calibration Solution Kit.
🔗 Product Link:
https://shop.zimmerpeacock.com/en-gb/products/sodium-calibration-solution-kit
💧 Biosensor Rinse Solution
Maintain good laboratory practice by rinsing sensors between measurements to minimise carryover and improve repeatability.
🔗 Product Link:
https://shop.zimmerpeacock.com/en-gb/products/biosensor-rinse-solution
Suggested Workflow
A straightforward workflow for sodium sensing development is:
- Obtain the SenseItAll Generation 4.
- Connect a Sodium Sensor.
- Verify sensor performance with the Sodium Calibration Solution Kit.
- Clean between measurements using the Biosensor Rinse Solution.
- Progress from calibration solutions to urine samples.
- Perform spike-and-recovery experiments using concentrated sodium sulfate additions.
- Optimise the method for your specific application and sample type.
This process helps developers rapidly move from proof-of-concept experiments to application-focused testing and eventual commercialization.
Why This Matters
Many researchers become concerned when a sensor that performs perfectly in standards begins producing unexpected responses in biological samples.
In reality, this often indicates progress rather than failure.
You have already demonstrated that the sensor functions correctly. The next challenge is understanding the sample matrix and developing a reliable measurement strategy that works under real-world conditions.
This is the journey from TRL3 and TRL4 validation towards TRL5 application development and beyond.
Importantly, commercial electrolyte analyzers already demonstrate that ion-selective electrode technology can succeed in challenging biological environments. Consequently, sodium measurement in urine should be viewed as a practical and achievable goal.
🚀 Ready to Start Measuring Sodium?
SenseItAll (SIA) Generation 4
https://shop.zimmerpeacock.com/en-gb/products/senseitall-sia-generation-4-device-only
Sodium Sensor
https://shop.zimmerpeacock.com/en-gb/products/sodium-sensor
Sodium Calibration Solution Kit
https://shop.zimmerpeacock.com/en-gb/products/sodium-calibration-solution-kit
Biosensor Rinse Solution
https://shop.zimmerpeacock.com/en-gb/products/biosensor-rinse-solution
Contact Zimmer & Peacock
Have questions about sodium sensing, potentiometry, urine analysis or biosensor commercialization?
📩 https://www.zimmerpeacock.com/contact
Keywords
Sodium Sensor, Sodium Measurement, Sodium in Urine, Ion Selective Electrode, ISE, Open Circuit Potentiometry, OCP, Biosensors, Electrochemistry, Potentiometric Sensors, Urine Analysis, Point of Care Diagnostics, Sensor Development, Technology Readiness Levels, TRL3, TRL4, TRL5, Commercialization, Diagnostic Sensors, Zimmer Peacock.
