Recent food crises have increased the need for manufacturers to guarantee safety, quality, and traceability. A low-cost, robust, and portable chemical detection method would greatly aid this effort. To further this goal, researchers have focused on developing optical chemical sensors (OCSs). OCS technology provides a viable alternative to electrodes and other chemical-sensing devices because it has low sensitivity to electromagnetic noise, compatibility with optical fibers, and multiplexing capability. Yet OCSs still suffer from limitations. They are subject to ambient light interference. In addition, their long-term stability is limited by photobleaching, wherein light exposure destroys the fluorescent molecules used for chemical detection. Stability is also compromised by leaching of the fluorescent indicator, and inadequate immobilization methods.
Detecting amines can provide useful information about food spoilage and changes that occur during meat storage. Yet conventional chromatographic methods1 are time-consuming, expensive, and invasive. In addition, they cannot be applied to real-time, on-line packaging processes. Previously developed OCSs used indicators that were immobilized in plasticized polyvinylchloride (PVC) polymers.2–4 But this sensor material is prone to leaching and evaporation of toxic components (especially plasticizers), making it unsuitable for the food industry. To overcome this challenge, we use sol-gel, a plasticizer-free, relatively benign matrix (i.e., a nanometer-scale cagelike structure), as a solid support for immobilizing the indicator.
Sol-gel technology provides a low-temperature method for obtaining porous silicate glass matrices. Organic reagents and molecular receptors can be easily immobilized in the matrices. They offer many advantages, including optical transparency from the UV to IR wavelengths, chemical and thermal stability, and a fast sensor response. The properties of the final network structure, such as hydrophobicity, thickness, porosity, flexibility, reactivity, and stability, can easily be tailored by controlling the process conditions, the type and size of the precursors, and catalysis.5
We developed a sol-gel-based optical chemical sensor for on-line measurement of aqueous propylamine (PA). We previously reported on the preparation and characterization of the sensor.6 For the preparation of the matrix we used a base-catalyzed sol-gel process (pH 13), with tetraethoxysilane and methyltriethoxysilane as precursors. The ratio of precursors to water was 1:15. We doped the matrix with the chromoionophore XV (ETHT 4001) indicator. The acceptor part of the chromophore has the trifluoroacetyl group, which selectively reacts with the analyte. The sensor layers have a protective coating of microporous white polytetrafluoroethylene (PTFE), which prevents interference from ions and ambient light.
Figure 1. The intensity of the reflection spectra decreased at 398nm and increased at 526nm in the presence of rising concentrations of PA in 0.1mol L- 1 sodium hydroxide. PA: Aqueous propylamine.
To test the device's ability to detect PA, we performed measurements in a flow-through cell in reflection mode. We obtained a relative signal change when the sensor layer was exposed to aqueous PA. As the concentration of PA in 0.1mol L−1 sodium hydroxide solution rose, the reflection intensities decreased at 398nm and increased at 526nm (see Figure 1). The response time (t100) of the sensor layer is 20–30s, and the regeneration time is 70s. Regeneration measurements can be repeated with the same layer at least three times (see Figure 2). The limit of detection is 0.1mmol L−1 and the dynamic range is 0.003–0.3mmol L−1.
Figure 2. Response behavior and reversibility of sensor layer on exposure to different concentrations of PA measured in a flow-through cell at 526nm. The signal intensity decreased by only 1% after the second regeneration.
The sensor layer can be used for more than 9 months, if stored in a dark place at room temperature. The key factor affecting stability is the leaching of the indicator from the host matrix. The signal intensity of the layer decreased after the second regeneration by only 1%, compared to the first regeneration (see Figure 2). In this case the hydrophobic PTFE film prevented the leaching of indicator dye into the solution. Nevertheless, a small fraction of the dye migrates from the sensor layer into PTFE and makes it unsuitable for optical measurements.
The pressure on food manufacturers to ensure safety is increasing and with it, the need for cheap, durable, continuous monitoring sensors. Our sol-gel-based optical sensor may further that goal. Unlike current PVC matrix techniques, its response time, detection limit, and operational and shelf life are suitable for food applications. The next step is to develop the technology for sensor ink-jet printing, which would reduce the manufacturing costs and enable mass production.
Špela Mojca Korent, Aleksandra Lobnik
Center of Sensor Technology
Department of Mechanical Engineering
Špela Mojca Korent is currently seeking a PhD in chemistry and chemical technology at the University of Ljubljana. Korent researches sol-gel materials, silica nanoparticles, and their application to optical sensors.
Aleksandra Lobnik heads the Center of Sensor Technology. Her research focuses on the development and application of various polymers, especially sol-gel materials for optical chemical sensors. She has published over 100 scientific papers on this topic.
Gerhard J. Mohr
Institute of Physical Chemistry
Friedrich-Schiller University Jena
Gerhard J. Mohr received his PhD in chemistry from the University of Graz in 1996. He then moved to the Centre for Chemical Sensors at the Swiss Federal Institute of Technology Zurich, where he developed chemosensors based on reversible chemical reactions. Since 2001 he has worked at the Institute of Physical Chemistry in Jena, focusing on fluorescent sensor nanoparticles.