Fluorescent Sensor Can Measure Your Stress Level

Researchers from Skoltech have developed a prototype fluorescent sensor that continuously monitors cortisol concentration in the human body. According to scientists, an implantable sensor based on this technology can be used in clinical practice.

The adrenal cortex secretes glucocorticoid hormones during stress and trauma, helping the human body adapt to adverse conditions. The primary human stress hormone, as many people know, is cortisol. Since cortisol is secreted in response to a stressful situation, measuring its levels is used as an indicator of stress. Also, changes in the amount of the hormone can be a sign of various diseases.

For instance:

• Addison’s disease
• Congenital adrenal hyperplasia
• Hyperaldosteronism
• Adrenal gland tumors
• Adrenocortical carcinoma
• Pheochromocytoma
• Pituitary tumors

Until now, there are no sufficiently accurate and reliable methods for continuous monitoring of cortisol levels in the human body. Existing laboratory methods, particularly the enzyme immunoassay, are reliable but require sample preparation beforehand, making them impossible to use in vivo. Moreover, blood sampling is a stressful procedure in both animals and humans, and under stress, cortisol concentrations rise, and the assay results become unreliable. Therefore, it is essential to develop an implantable sensor to monitor cortisol directly in the bloodstream.

Scientists at the Skoltech Center for Engineering, Manufacturing Technology and Materials, led by Sofia M. Safarian, in collaboration with Skoltech biologists led by Yuri V. Kotelevtsev, developed a prototype immunosensor using gold nanoparticles for quantitative cortisol control. The performance of such a sensor is based on the immune reaction of an antigen with an antibody.

Antibodies are produced in the body in response to the penetration of a foreign substance – antigen. However, substances with a small molecular weight do not disturb the immune system. Still, an immune response occurs if they are weighted down by attaching them to larger molecules, such as proteins. Therefore, to obtain antibodies to cortisol, mice were injected with their protein complexes during the experiment, then the produced antibodies were isolated from the animals’ blood.

In making the sensor, an electron-beam evaporator was used to place gold particles on a glass substrate, which formed clusters of gold on the substrate. Then a substance was applied to the gold layer, which was chemically attached to the antibodies. After that, cortisol molecules in complex with protein and fluorescent marker were placed on the antibodies.

The sensor’s work was tested on saline solutions with different cortisol concentrations. The free cortisol contained in the sample displaced the cortisol molecules in the complex with the protein and fluorescent marker and bound itself to the antibodies. When the cortisol complex occupied all the antibodies with the marker, the emission intensity of the fluorescent molecules was high due to the proximity to the gold nanoparticles. And when replaced with free cortisol, the fluorescence decreased as the marker was far away from the signal-enhancing nanoparticles. The amount of stress hormone in the samples was measured by fluorescence intensity using a spectrophotometer.

Tests showed that the sensor could detect cortisol even in tiny amounts, 0.02 micrograms per milliliter, which corresponds to the average level of the hormone in human blood plasma. Furthermore, the reversibility of cortisol binding to antibodies without measurement error is essential for the continuous operation of the sensor, which has also been demonstrated in the research.

In the future, scientists want to create an implantable sensor in the form of an optical fiber, containing a capillary cell at the end with a semipermeable membrane placed in a thin needle, connected to a portable spectrometer via fiber optics.

Scientists need to develop an implantable sensor with a semipermeable membrane that separates small molecules like cortisol from proteins and other components of biological fluid: blood, saliva, and intertissued fluid. Similar devices for measuring glucose already exist, so creating an implantable sensor requires several complex but feasible tasks to be solved.

Cortisol control has long been of great interest to scientists and physicians. A few years ago, a patch-like sensor was developed to measure the level of the stress hormone using sweat drops.