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24 Mar 2025
Near-IR technique spots microplastics and monitors water uptake of plants.
Near-infrared spectroscopy can offer insights into several environmental parameters of interest, including the presence of contamination in water and the health of growing plants.
A project group including Ruhr University Bochum (RUB) and Fraunhofer Institute for Microelectric Circuits and Systems (Fraunhofer IMS) has now developed an improved spectroscopic technique for this environmental monitoring.
Described in Advanced Science, the group's findings could point towards real-time imaging via simpler and less expensive optical platforms than currently necessary.
The project's HyperNIR device employs a "phasor acquisition" approach, a known numerical method for simplifying complex fluorescence lifetime data.
This involves parameters like spectral width and emission lifetime being Fourier transformed and represented as 2D plots, so that a physical position on the plot indicates some of the spectral data.
A direct optical version of the same approach is possible, but "the potential of direct phasor-acquisition has not been leveraged yet for hyperspectral imaging, and the NIR range of the spectrum is especially challenging," noted the project in its paper, not least since commercially available spectral filters are not best suited to the operation.
The RUB project built an alternative architecture incorporating two parallel linear polarizers, sandwiched around a tunable liquid crystal retarder able to further modify the polarization state of light traveling through it. The combination transforms optical signals into a 2D spectral (phasor) space, and changing the degree of retardance allows different NIR fluorophores to be identified in the spectral data.
Molecules vital to plant health
In trials, as few as three images taken at different retardances were sufficient to isolate the presence of polyethylene and polyamide microplastics. The same approach also showed the influence of water uptake in pepper plants through changing NIR reflectivity of the plant leaves, without contact and without using dyes.
"The ability to analyze different materials and their properties in real time can considerably increase the efficiency of processes in environmental monitoring," commented Sebastian Kruss from RUB. "While conventional methods require time-consuming scanning of a sample, the HyperNIR camera is significantly faster."
The HyperNIR approach could also potentially be applied to detection of other molecules vital to plant health, allowing similar hyperspectral monitoring to be used in monitoring the nutrient content in a plant, or to detect pest infestation and plant stress at an early stage.
It may also be possible to combine HyperNIR method with conventional fluorescence microscopy, using it to differentiate between various fluorescent molecules being used as markers. This means that the system is potentially of interest for biomedical research as well as plant studies.
"Integrating the process into drones could help to solve pressing environmental issues in the field of agriculture by opening up a new dimension in data collection and analysis," said Sebastian Kruss.
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