Enhancing SPR biosensor performance for creatinine detection via plasma polymerized heptylamine coatings

Why this matters for everyday health

Kidney problems often develop silently, and by the time symptoms appear, significant damage may already be done. Doctors therefore rely on a small waste molecule called creatinine, measured in blood, as an early warning signal. This article describes a new way to make optical biosensors more sensitive to creatinine, potentially paving the way for faster, more precise kidney checks using tiny, reusable chips instead of slow lab tests.

Turning light into a medical signal

The study focuses on a technology called surface plasmon resonance, or SPR, which turns subtle changes at a metal surface into measurable shifts in reflected light. In these sensors, a thin gold film is attached to a glass prism and lit with a laser at a specific angle. When molecules bind to the gold surface, they slightly change how light behaves, causing the angle of minimum reflection to shift. By tracking that angle, the sensor can “see” when and how much of a target substance has landed on the surface, all without adding dyes or labels and in real time.

Making the sensor surface more welcoming

For this light-based signal to be strong and reliable, the gold surface must be carefully prepared so that biomolecules stick where they should and avoid clumping where they shouldn’t. The researchers concentrated on a property called wettability – essentially how easily water spreads on the surface – which they measured through the water contact angle. A low angle means water spreads out (a more welcoming, hydrophilic surface), while a high angle means it beads up (a more water-repelling, hydrophobic surface). Using a process known as plasma polymerization, they coated the gold with an ultra-thin film made from a compound called heptylamine. By tuning the electrical power of the plasma, they were able to dial in how hydrophilic or hydrophobic the coating became.

Finding the sweet spot for sensitivity

The team systematically varied the plasma power from low to high levels and observed how this changed the contact angle, the thickness of the coating and the light response of the sensor. At lower powers, the surface remained more hydrophilic, which encouraged uniform spreading of liquids and better attachment of biomolecules. At higher powers, the surface gradually shifted toward hydrophobic behavior, and the detailed structure of the coating also changed. Atomic force microscopy revealed that the plasma-grown film added some nanoscale roughness but stayed within the range known to be acceptable for precise optical sensing. By comparing the light reflection curves for different coatings, the researchers identified an optimal coating made at moderate plasma power, with a contact angle around 60 degrees and a thin, well-controlled layer.

Building a better creatinine test

With the surface tuned, the authors then built a working creatinine biosensor. They first activated the heptylamine coating with a common crosslinker and then attached enzymes called creatininase, which specifically recognize and process creatinine. When blood-like solutions containing different creatinine levels flowed across the chip, the binding events at the enzyme layer caused measurable shifts in the SPR angle. Within the clinically important range found in blood, from 0.05 to 0.6 millimoles per liter, the coated sensor produced a clear, nearly linear response, with a sensitivity far higher than that of bare gold surfaces tested under similar conditions. A comparison experiment showed that the uncoated gold could detect creatinine only at much higher concentrations and with far weaker signals.

What this means for future diagnostics

In simple terms, the study shows that carefully tuning how “water friendly” a gold sensor surface is can dramatically boost its ability to catch and measure tiny amounts of a key kidney waste product. The plasma-grown heptylamine coating provides just the right balance: it slightly reduces the raw optical sensitivity of the bare metal, but greatly improves how many enzymes can be attached and how efficiently they interact with creatinine. The end result is a chip that can sensitively detect medically relevant creatinine levels in real time without labels, offering a promising route toward compact, high-performance kidney health monitors. Future work will need to test how well the sensor handles other substances in blood and how stable it remains over time, but the underlying surface strategy could be adapted to many other medical targets beyond creatinine.

Citation: Jamil, N.A., Fatah Yasin, M.F.H., Karim, I.M. et al. Enhancing SPR biosensor performance for creatinine detection via plasma polymerized heptylamine coatings. Sci Rep 16, 10658 (2026). https://doi.org/10.1038/s41598-026-46647-y

Keywords: creatinine biosensor, kidney function, surface plasmon resonance, plasma polymer coating, medical diagnostics