A low-cost and simple-to-use disposable sensor, made of cardboard and containing gold nanoparticles, could prove to be a useful tool for monitoring the quality of water consumed by the population. In the final phase of development, the analytical device was designed by the team of chemist Thiago Regis Longo Cesar da Paixão, from the Chemistry Institute of the University of São Paulo (IQ-USP) and coordinator of the Electronic Languages and Chemical Sensors Laboratory at the same institution. The research, supported by FAPESP, resulted in an article published earlier this year in the scientific journal
Sensors & Diagnostics . A patent application for the manufacturing process is being prepared by the group.
“Producing cheap sensors that can be spread across Brazil makes it possible to monitor the water served to the population in real time. The data collected can guide the creation of public policies by government agents and help water treatment companies make decisions”, highlights Paixão. The estimated cost of the sensor, subject to confirmation, is R$0.50.
The manufacturing process of the device, a small rectangle of cardboard measuring 15 millimeters (mm) wide by 20 mm long and 1 mm thick, is practically free of chemical reagents, commonly used in the manufacture of sensors, and almost completely automated. In addition to cardboard, which may come from a recycling process, the researchers used adhesive glue, waterproofing spray and a small volume of gold solution (30 micrograms). A carbon dioxide (CO) laser ) applied to cardboard is responsible for creating the conductive tracks, the basis of the detection electrodes. The gold solution is added to the tracks and then a further laser application synthesizes the gold nanoparticles ( see infographic
“The nanoparticles are responsible for improving the device's performance”, explains the researcher. “The sensor measures electrical current arising from an electrochemical reaction that occurs on the conductive surface when an electrical potential is applied. The greater the concentration of the chemical substance you want to identify in the water sample placed on the sensor, the greater the current generated will be.” The electrical potential that must be applied to the central electrode to make the device work is -0.2 volts (V), lower than that of a small AAA battery (1.5 V).
The fact that it is produced without human manipulation gives it advantages. “We often have a series of manual steps in the laboratory to create sensors. They mean that the devices do not have much reproducibility. When we use machines, such as the one that emits the laser, we avoid this artisanal intervention in the device manufacturing process”, says Paixão.
Rodrigo Cunha
Chemist Wendell Karlos Tomazelli Coltro, director of the Chemistry Institute at the Federal University of Goiás (IQ-UFG), who did not participate in the study, agrees. “The technology based on the use of laser is very attractive as it allows production to be scaled up with high reproducibility”, he assesses. For Coltro, the device presents a high level of innovation. “The USP team was a pioneer in proposing the use of lasers to produce sensors in cardboard. The use of this material makes the device sustainable and allows it to be manufactured anywhere in the world,” he says.
In laboratory tests to evaluate the sensor's performance, the researchers used sodium hypochlorite as a proof of concept. Popularly known as chlorine, the substance is used as a disinfectant in swimming pool water. In high concentrations, it can be harmful to health. The maximum level of free chlorine allowed by the World Health Organization (WHO) in swimming pools is 3 to 5 parts per million (ppm). In the study carried out at the Chemistry Institute, according to the article published in Sensors & Diagnostics it was possible to detect up to 0.50 ppm of sodium hypochlorite in the water.
To identify other chemical species in water samples, the platform would have to be adapted – the term chemical species refers to the different forms in which chemical substances are found in nature, such as atoms, molecules, ions. “We have already designed sensors to measure toxic metals, pesticides and pharmaceuticals, as well as other species of environmental interest, such as nitrite and nitrate”, says the USP researcher. For each substance it would be necessary to design a specific sensor, but there is the possibility of assembling an array of sensors to carry out the simultaneous detection of several substances.
For now, only the sensor has been created, but the USP researchers say they have the capacity to design the complete system, which includes the device that reads the data, with no estimated cost yet. It is also possible to use a portable reader model available on the market – something similar to what is done today with strips to measure blood glucose levels, which are inserted into instruments called glucometers.
The next step in the research is to create a pilot plan for testing the sensor on a large scale, in people's homes, by untrained users. Testing with the population will help improve the device. In a later stage, the group intends to find a company that is interested in commercially producing the sensor.
“There are already conversations underway”, reveals Paixão, highlighting that the search for cardboard sensors or arrangements of these devices to monitor water quality in real time does not only occur in Brazil. The startups LAIIER, in London, England, iFlux, in Niel, Belgium, and OmniVis, in San Francisco, in the United States, informs the researcher, are also working on the design of devices of this type.
Project Miniaturized and integrated chemical sensors: New manufacturing platforms for biological, clinical and environmental applications (nº 18/08782-1); Modality Thematic Project; Responsible researcher Mauro Bertotti (USP); Investment R$ 4,756,859.76.
Scientific article ARANTES, IVS et al . Laser-induced fabrication of gold nanoparticles onto paper substrates and their application on paper-based electroanalytical devices. Sensors & Diagnostics . v. 2, p. 111-21. 2023.