The researchers developed a highly sensitive and eco-friendly uric acid sensor using carbon quantum dots (CQDs) synthesized from lemon juice and integrated it into a gold-based SPR platform.
The sensor detects uric acid at ultra-low concentrations (0.002 μM), outperforming traditional methods in speed, sensitivity, and sustainability. Their findings were published in Scientific Reports.
Uric acid (UA) is a key biological marker that plays roles both as a metabolic waste product and as an antioxidant. Its concentrations in blood and urine are closely linked to various health conditions, including gout, cardiovascular disease, and renal dysfunction.
Conventional detection methods, such as enzymatic assays and chromatography, are accurate but often require either expensive reagents, lengthy processing times, or specialized instruments.
SPR sensing is a compelling alternative with its high sensitivity, real-time response, and label-free detection capabilities.
Green Nanomaterial Synthesis
The researchers synthesized CQDs from pulp-free lemon juice using a hydrothermal approach. Photoluminescence (PL) analysis revealed that samples treated at 180 °C had the strongest emission at around 450 nm under 350 nm excitation.
FTIR spectroscopy confirmed the presence of functional groups, including amino, carboxyl, and hydroxyl, which contribute to the CQDs’ water solubility and biocompatibility.
Transmission electron microscopy revealed that the CQDs were predominantly spherical, with an average diameter of 4.7 nm.
These CQDs were spin-coated onto a 50 nm thick gold layer deposited on a glass substrate, forming a 12 nm sensing layer. This gold/CQDs composite functioned as the active interface in the SPR sensor.
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Optical Modeling and Layered Nanomaterial Design
The sensor used a p-polarized helium-neon laser at 632.8 nm directed through a prism onto the gold/CQDs film.
A flow cell introduced various UA concentrations over the sensor surface, and a photodiode captured the reflected light. The resulting data was processed using a lock-in amplifier and modeled using Winspall software based on Fresnel equations.
Modeling revealed the complex refractive index of the gold/CQDs layer to be approximately 1.6014 + 3.7i. The CQD layer itself had a refractive index (n) of 1.343 and an extinction coefficient (k) of 0.1137.
As UA concentrations increased, both the refractive index and thickness of the CQD layer showed slight increases, indicating adsorption of UA onto the sensing surface, a key factor in the system’s sensitivity.
Sensitivity, Detection Limits, and Binding Analysis
The SPR sensor demonstrated a clear and linear response to UA concentrations ranging from 0.05 to 1 µM.
The limit of detection was found to be 0.002 µM, which is significantly lower than many existing uric acid sensors. The response curve showed two distinct sensitivity regions: From 0 to 0.1 µM, the sensitivity was 3.968° per µM, while between 0.1 and 1.0 µM, it dropped to 0.4098° per µM.
This reduction was attributed to surface saturation effects at higher analyte concentrations.
The binding interaction between UA and the CQDs was modeled using the Langmuir isotherm, yielding a binding affinity constant of 6.95 MM-1 for the CQD-enhanced film, far higher than the 0.94 MM-1 measured for the gold-only surface.
These results confirmed that the lemon CQDs significantly enhanced the film’s affinity for UA molecules.
Selectivity and What Lemon-Derived Quantum Dots Could Mean for Uric Acid Detection
While direct selectivity tests against potential interfering substances were not performed, the authors suggest that functional groups on the CQD surface, particularly amine and carboxyl moieties, likely contribute to specific binding through electrostatic interactions and hydrogen bonding.
These surface properties, combined with the sensor’s sensitivity, indicate promising potential for clinical and environmental applications.
The authors note that future work will explore the sensor’s selectivity under more complex conditions and validate its performance using real biological samples.
Journal Reference
Afandi M.A.S.M.N., et al. (2025). Eco-friendly synthesis of carbon quantum dots from lemons for enhanced uric acid detection using gold-integrated surface plasmon resonance. Scientific Reports 15, 38520 (2025). DOI: 10.1038/s41598-025-16423-5