Fiber Optic-Based Sensor Measures pH Levels in Cell-Growth Environments

Scientists at the National Institute of Standards and Technology (NIST) are developing a photonic sensor to study tissue growth in the lab. The proof-of-concept sensor uses a light-based signal to measure pH, an important property in cell-growth studies. Unlike conventional sensors, the photonic sensor could be used to monitor the environment in a cell culture for weeks at a time without the need to disturb the cell-growth environment.

As cells grow, their environment becomes more acidic. If the environment becomes too acidic or too basic, the cells will die. “An increment of 0.1 pH is significant,” researcher Zeeshan Ahmed said. If researchers disturb the growing cells every time they have to measure the cell culture’s pH, they introduce another kind of uncertainty to their measurements, since they are altering the cells’ environment. For tissue engineering research, Ahmed said, a measurement system that can stay inside an incubator without needing to be removed or calibrated for weeks at a time is needed.

For years, Ahmed and his team have developed photonic sensors that use optical fibers etched with a fiber Bragg grating (FBG). Changes in temperature or pressure alter the wavelengths that can pass through the grating.

The researchers demonstrated that sensors with Bragg gratings could be adapted to a pH measurement. Using a fiber optic-based platform featuring FBGs, they developed a sensor that could measure the heat released by pH-sensitive chromophores upon absorption of light. They were able to correlate visible light absorption by the chromophores to heat released and changes in the FBG signal over a pH range of 2.5 to 10.

An empty petri dish with two optical fibers, illustrating one version of the experiment. The fiber on the left (usually shining infrared light, but depicted here as visible red light) is a temperature sensor. The top fiber shines green, red, or blue light into the petri dish to adjust the signal that the temperature sensor measures. Courtesy of J.L. Lee/NIST.

For their initial experiment, the researchers filled a petri dish with a solution made with red cabbage juice powder, which changes color in response to changes in pH. They positioned one optical fiber above the dish, connected the fiber to a laser pointer, and shined light into the sample. They embedded a second optical fiber in the cabbage juice solution. This fiber contained the Bragg grating and acted as the temperature sensor. The pH levels in the solution were controlled manually.

To measure pH, the researchers shined one color of light into the solution from above. Depending on its pH level, the cabbage juice solution absorbed the light to varying degrees. The photonic thermometer fiber picked up these slight changes in the juice’s heat. The researchers shined a second color of light into the solution and repeated the process.

By comparing how much heat was generated by each wavelength, the researchers could determine the color of the cabbage juice, and thus its pH level at time of measurement. “Literally we said, ‘Can we turn two laser pointers on and off for a few minutes and see if we can turn that into a pH meter?’ And we were able to show that it works over a wide range,” Ahmed said. Further research indicated that the pH measurements taken with the photonic sensor are accurate to plus or minus 0.13 pH units and are stable for at least three weeks.

For their next round of experiments, the researchers are using phenol red, a pH-sensitive dye, and plan to encapsulate the dye in a plastic coating around the fiber so that it does not interact with the cell medium. The team is also conducting its first test of the system in a real cell culture, with help from NIST colleagues who specialize in tissue engineering. Future plans include measuring quantities beyond pH, which would require swapping out phenol red for a different dye sensitive to whatever property the researchers want to measure.

In this version of the experiment, a petri dish was filled with a solution of phenol red. The lower pH of the solution on the left gives the substance a deep yellow color. The higher pH of the solution on the right gives the substance a reddish color. Depending on its color, the liquid absorbs more or less of the green versus the blue laser light. Note that the green light is much more visible inside the yellow liquid (top left) than it is in the red liquid (top right). In contrast, the blue light is much more visible in the red liquid (bottom right) than it is in the yellow liquid (bottom left). The more light the liquid absorbs, the more its temperature increases. By measuring these temperature changes with the optical fiber (shining red light in this illustration), researchers can assess the exact color of the liquid, which tells them its pH. Courtesy of J.L. Lee/NIST.

NIST researchers will also be testing how cell cultures are affected by the slight, temporary temperature changes (about 1 to 2 kelvins) in localized areas of the sample that occur as a result of this measurement method. Ahmed says that so far, potential collaborators are not overly concerned about the issue of localized heating, and that his team will be working to reduce the temperature changes as much as possible.

One day, Ahmed said, the measurement scheme could potentially be used to monitor the growth of tissue in the human body. “The long-term goal is being able to put implantable devices into people where you’re trying to grow bones and muscles, and then hopefully over time the sensors could be designed to dissolve away and you wouldn’t even have to go back in and remove them,” he said.

The research was published in Sensors and Actuators B: Chemical (www.doi.org/10.1016/j.snb.2019.127076).