Laser writing creates sensors on fallen leaves

14 May 2024

Federal University of São Carlos converts leaf cellulose into graphite to fabricate detectors.

A project at Brazil's Federal University of São Carlos (UFSCar) has manufactured sensors for potential use in medical and industrial applications by laser writing onto fallen leaves.

Published in ACS Sustainable Chemistry & Engineering, the findings could indicate a route to rapid creation of sensors for a number of uses, while also adding value to a low-cost natural raw material that is usually discarded.

Paper-based biosensors, often exploiting the electrochemical behavior of a cellulose material before and after contact with an analyte, have been a topic of research for some time. In addition to a widely available and recyclable base material, these devices should be versatile, with various molecular technologies and nanomaterials available to enhance the measured signal changes.

Methods used to fabricate these devices have included a number of printing, electrode deposition and lithography techniques, but the UFSCar project wanted to investigate whether direct laser writing could also be used to create workable sensors.

"The pursuit of novel substrates derived from renewable sources, characterized by their accessibility and wide availability, is important in the advancement of cutting-edge electrochemical sensors," commented the team in its paper.

"In this study the utilization of fallen tree leaves is presented for the first time, capitalizing on the pyrolytic transformation induced by a carbon dioxide laser to fabricate electrochemical sensors."

The project employed its carbon dioxide laser to irradiate the leaf surface, where careful control of laser power and scan rate can convert leaf cellulose to graphite by pyrolysis, and form desired patterns on the leaf surface.

After the graphite markings had been created on the leaves, they were characterized using electrochemical, morphological, and physicochemical methods, including Raman spectral analysis. Their behavior as sensors was established by depositing suitable test chemicals across the treated areas and determining how the electrical conductivity was affected.

Lower cost and increased sustainability

"The sensors were characterized by morphological and physicochemical methods, permitting exhaustive exploration of the novel carbonized surface generated on the leaves," commented Bruno Janegitz, head of the Laboratory for Sensors, Nanomedicines, and Nanostructured Materials at UFSCar.

"Furthermore, the applicability of the sensors was confirmed by tests involving the detection of dopamine and paracetamol in biological and pharmaceutical samples."

In those trials the leaf detector showed a linear range of 10 to 1,200 micromoles per liter of dopamine, with a detection limit of 1.1 micromole per liter. For paracetamol, the device operated across a linear range of 5 to 100 micromoles per liter, with a detection limit of 0.76 micromole per liter, according to the project team.

This proof-of-concept testing showed that electrochemical sensors derived from fallen tree leaves "attained a satisfactory analytical performance and noteworthy reproducibility," said the team, highlighting the potential of such devices to be used as alternatives to conventional substrates. This substitution could reduce cost and increase environmental sustainability.

"The leaves would have been incinerated, or at best composted," said Bruno Janegitz. "Instead, they were used as a substrate for high value-added devices in a major advancement for the fabrication of next-generation electrochemical sensors."