Your next medical test could be biodegradable

Throughout history, paper has been used in a wide variety of applications – it’s the perfect substrate for paints and inks, a key ingredient in construction materials, and the basis of many products used at home in hospitals, and in the lab. And according to a group of Portuguese researchers, it could enable the development of cheap, eco-friendly sensors in the detection of important biomarkers.

Recently published in Carbon [10.1016/j.carbon.2022.06.013], their paper reports on sensors – made from common filter paper – that can successfully detect and quantify the concentration of uric acid (UA) in urine samples.

They started with a cellulose filter paper that had been sprayed with a fine layer of fire retardant. Using a carbon dioxide laser, they irradiated a section of the paper (20 × 5 mm2) in two distinct stages. The first helped to convert cellulose into char, while the second graphitized the char into a porous, conductive graphene foam called laser-induced graphene (LIG).

Graphene and graphene-based materials are known to have excellent electrocatalytic activity and electrochemical performance. LIG has the added advantage of not requiring a separate substrate. The process can directly convert carbon-containing materials, including paper, wood, and cork, into graphene. The porosity provides a large surface area too, which can be particularly beneficial for biosensing applications.

In this study, the new paper-LIG was assembled into an electrode, connected to copper wire, and encapsulated using a specialist varnish. The remaining active area measured 5 x 5 mm2. Once a reference and counter electrode were added (Calomel and platinum wire, respectively), the paper electrochemical cell was ready for testing.

To prepare the cell, and stabilise it for electrochemical measurements with urine, a high volume of PBS electrolyte was added, and a sinusoidal signal passed through it. Next, increasing quantities of uric acid were added to PBS, while differential pulse voltammetry (DPV) was carried out. For samples with a pH of 7.4, there was a strong linear correlation (R2 = 0.999) between peak current density and UA concentration. That linearity was progressively lost with decreasing pH. The limit of detection for pH 7.4 was found to be 3.97 μM, which the authors say is “higher compared to some enzymatic biosensors presented in the literature,” and “also slightly higher compared to non-enzymatic LIG sensors scribed on polyimide and modified with Pt nanoparticles.” However, in practical use, an extremely low limit of detection is not required, as reference values usually under consideration, are above 1 mM.

The paper-LIG cell also provided a sensitivity of 0.363 μA cm−2 μM−1 at that same pH, which “surpasses that of some enzymatic UA sensors reported in the literature that are much more complex and costly to produce.” The authors found that the paper-LIG could be safely stored in ambient conditions for a year without any decline in electroanalytical performance. In selectivity tests in synthetic urine with ‘interfering agents’ like ascorbic acid and dopamine, the paper-LIG performed well at physiologically-relevant concentrations – little or no interference occurred in the measurement, even in non-dilute samples. The same was true for analysis of real human urine, with the reproducibility of different paper-LIG electrodes described as “satisfactory.”

The authors conclude that their results “demonstrate that non-enzymatic uric acid detection is possible using considerably cheaper and more widely available materials than …previously reported,” and that paper-LIG biosensors are “extremely attractive for low-cost, single-use, point-of-care analytical applications, particularly in low-income areas.”


Bohdan Kulyk, Sónia O. Pereira, António  J.S. Fernandes, Elvira Fortunato, Florinda M. Costa, Nuno F. Santos. “Laser-induced graphene from paper for non-enzymatic uric acid electrochemical sensing in urine,” Carbon 197 (2022) 253–263. DOI: 10.1016/j.carbon.2022.06.013