Researchers use nanotechnology to develop paper-based sensor that measures vitamin C
December 09, 2015
By Diego Freire | Agência FAPESP – Researchers at Brazil’s National Nanotechnology Laboratory (LNNano) and National Synchrotron Light Laboratory (LNLS) and at the Chemistry Institutes of the University of Campinas (UNICAMP) and the Catholic University of Campinas (PUCC) have developed a quick and simple method to measure the amount of ascorbic acid (vitamin C) in different samples, such as fruit extracts and industrialized drinks.
The method involves a sensor produced at LNNano by depositing silver nanoparticles on chromatography paper, the kind of paper that chemists use to separate components of mixtures or compounds. The silver serves as a colorimetric reagent, changing color when it comes into contact with different concentrations of ascorbic acid.
The researchers set out to simplify the determination of ascorbic acid levels for quality control purposes in the food, pharmaceutical, chemical and cosmetics industries, which use vitamin C as a preservative because of its antioxidative properties.
“The traditional methods for measuring ascorbic acid require a laboratory environment with sophisticated equipment, large amounts of reagent, and plenty of time to perform experiments. All of this restricts their industrial application. The paper sensor simplifies the process and measures the substance with a high degree of precision,” said Mateus Borba Cardoso, a researcher at LNLS, which together with LNNano belongs to the National Energy & Materials Research Center (CNPEM).
Cardoso is the principal investigator for the research project “Functionalization of composite nanoparticles for biomedical applications”, which is funded by FAPESP.
Silver nanoparticles, which were obtained from silver nitrate (AgNO3), were used by the researchers as the colorimetric agent in this case and were deposited on a prepared portion of chromatography paper. The researchers tested the sensor with ascorbic acid solutions of different concentrations and found that the area containing the nanoparticles changed color. Moreover, the intensity of the color change correlated with the concentration.
The average size of the nanoparticles and their distribution before and after exposure to ascorbic acid were studied by small-angle X-ray scattering (SAXS) at one of the 18 experimental stations at the LNLS, the only synchrotron light source in Latin America. Known as beamlines, the stations are used by researchers in different knowledge areas to study organic and inorganic materials using techniques based on electromagnetic radiation, which ranges from infrared to X-rays.
“The powerful radiation generated by particle acceleration is scattered as it passes through the sample,” Cardoso explained. “The scattering pattern is captured in a two-dimensional image, providing a nanometer-scale view of the sample’s structure. It’s similar to microscopy but offers the advantage of enabling measurements to be made on the paper-based sensor itself rather than in a model.”
Using SAXS, the researchers observed an increase in the average size of the nanoparticles owing to the antioxidative properties of ascorbic acid, which converted the silver ions into metallic silver. This increase caused a change in the color of the chromatography paper, enabling the researchers to develop a colorimetric scale for the measurement of ascorbic acid concentration.
A drop from a sample is enough for the paper to change color in accordance with the ascorbic acid content. The sensor kit includes a chart showing previously measured levels of ascorbic acid for a range of color shades.
First, however, the researchers had to coat the nanoparticles with polyvinylpyrrolidone (PVP), a water-soluble polymer used as a stabilizer.
“Otherwise, the particles would stick together in lumps when they came into contact with the sample and convert to different sizes. It would be difficult to control the results in this case because of color variations not correlated with the variable of interest. We need nanoparticles of exactly the right size to guarantee accurate measurement. The sample won’t react uniformly without them,” Cardoso said.
Gabriela Furlan Giordano, a researcher at LNNano, stressed the speed of the analysis as an advantage of the method. “In half an hour, we had the results for eight samples in the colorimeter, which is an inexpensive portable instrument, and moreover, we created less waste thanks to a reduction in the use of reagents,” she said.
In addition to speed and low cost, the sensor’s portability as well as maintenance of high-precision measurement for weeks under refrigeration and in the absence of light opens up possibilities for industrial applications.
The technique is described in the article “Optical paper-based sensor for ascorbic acid quantification using silver nanoparticles,” by Cardoso, Giordano and others, published in the scientific journal Talanta and available at www.sciencedirect.com/science/article/pii/S0039914015002295.
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