Tear Drops May Hold Early Warnings of Neurological Disorders

Imagine skipping painful blood tests or brain implants and monitoring your neurological health through a simple teardrop instead.

Researchers at the Federal University of Pelotas in Brazil have made this a reality by developing a low-cost graphene biosensor designed to track dopamine, a neurotransmitter linked to motor control. In laboratory trials, the stamp-sized device accurately detected dopamine concentrations in artificial tears, offering a non-invasive pathway for ultra-early disease screening.

Monitoring dopamine in tears could replace invasive brain testing

When dopamine levels drop or spike, it can act as a warning sign for brain conditions such as Parkinson’s disease, Alzheimer's, and schizophrenia. Tracking this change allows doctors to intervene before symptoms appear.

Unfortunately, current testing tools fall short: doctors rely on blood draws, urine tests, or brain implants to track the neurotransmitter, which can be slow, uncomfortable, and carry surgical risks.

These issues have led scientists to start looking elsewhere for novel non-invasive biosensors—including human tears. Tear fluid would offer a quick and painless alternative, and since scientists know the dopamine baselines for healthy individuals, they are ideal for checkups.

However, although portable electrochemical sensors can easily read fluids, tears contain low concentrations of dopamine, and existing sensors lack the sensitivity to read these signals. They are also difficult to manufacture on a commercial scale.

To bypass these limitations, researchers from the new study engineered an enzyme-free sensor using laser-induced graphene, enhanced with nickel and urea.

Designing a laser-induced graphene biosensor to track dopamine levels

The team used a standard carbon dioxide laser to sketch a three-electrode pattern directly onto a sheet of thin plastic film; this process changes the plastic into a conductive, porous form of carbon known as laser-induced graphene. The resulting biosensor was roughly the size of a postage stamp, measuring 10 mm by 20 mm.

The researchers then coated the main electrode with nickel nitrate and urea before running the laser over it a second time. The heat breaks down the urea, releasing gases that puff up the material, which creates microscopic pores that expand the surface area. The remaining nickel microparticles act as electrical signal amplifiers.

Imaging techniques confirmed that the treatment generated a fourfold increase in active surface area, measuring 13.36 cm² compared to plain graphene.

When tested in a standard salt solution, the sensor tracked dopamine across a range from 0.25–16.44 µmol/L. It also achieved a detection limit of 17.86 nmol/L.

The device performed well with artificial human tears, accurately detecting dopamine levels in concentrations that were similar to those reported in people with Parkinson’s disease, even when mixed with other chemical compounds commonly found in natural eye fluid.

“Our sensor can detect dopamine from levels well below the healthy baseline and up to three times higher,” said lead author Lucas Minghini Gonçalves, a PhD student in the Graduate Program in Materials Science and Engineering at the Federal University of Pelotas. “This capability ensures that a person’s initial dopamine drop can be identified early on, which is crucial to enabling timely, proactive therapeutic interventions.”

The clinical future of tear-based biosensors

This technology could aid the shift to early screening. Instead of waiting for physical indicators such as tremors to emerge, doctors could use this tool to spot the chemical warning signs of Parkinson's disease years in advance.

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“We aim to facilitate the ultra-early detection of neurological disorders, creating opportunities for clinical interventions before major symptoms manifest,” said corresponding author Dr. Neftalí Lênin Villarreal Carreño, a full professor at the Federal University of Pelotas Materials Engineering.

As a disposable tool, the sensor stayed stable and operational for up to 7 days; after this point, its performance gradually dropped over a 30-day period.

However, every new technology faces real-world obstacles, and this biosensor is no different. Human tears are complex fluids; other compounds in the eye can cause variations in the sensor's electrical current.

Sensors often also have an issue called electrode fouling, where a film of dopamine builds up and blocks the active surface. However, the porous structure of this new device minimized this buildup at the lower concentrations found in medical testing.

The team plans to address these issues in their next steps, moving away from lab-made tears to evaluate how the device handles real human tear samples. Their goal is to package this technology into a user-friendly diagnostic tool for clinic waiting rooms—you might just need to be prescribed a sad film to watch first.

Reference: Gonçalves LM, Lopes BV, Noremberg BDS, et al. Toward non-invasive neurological biomarker monitoring: dopamine sensing in tears with laser-induced graphene electrochemical sensors. ACS Omega. 2026;11(24):36141-36150. doi: 10.1021/acsomega.6c03287

This article is a rework of a press release issued by the American Chemical Society. Material has been edited for length and content.