🔬 applied physics

Comparison of two laser wavelengths for LIBS bioimaging of plants grown in lunar regolith

This study demonstrates that a 2090 nm laser wavelength outperforms the conventional 1064 nm wavelength in Laser-Induced Breakdown Spectroscopy (LIBS) bioimaging of plants grown in lunar regolith simulant by generating hotter, more ionized plasma with higher signal-to-noise ratios, thereby validating LIBS as a viable tool for monitoring nutrient uptake and plant growth in extraterrestrial environments.

Original authors: T. Vozár, L. Čechová, J. Buday, M. Füleky, K. Molnárová, L. Lenža, J. Mužík, Y. Koshiba, M. Smrž, P. Pořízka, J. Kaiser

Published 2026-05-01

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CC BY 4.0

Original authors: T. Vozár, L. Čechová, J. Buday, M. Füleky, K. Molnárová, L. Lenža, J. Mužík, Y. Koshiba, M. Smrž, P. Pořízka, J. Kaiser

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine humanity trying to set up a permanent home on the Moon or Mars. One of the biggest hurdles isn't just the cold or the radiation; it's the food. We can't keep shipping sandwiches from Earth forever—it's too expensive and too complicated. So, the dream is to grow our own salad and broccoli right there in the alien dirt.

But there's a catch: that "dirt" (called regolith) isn't like the rich soil in your garden. It's basically crushed rock, full of sharp edges and strange chemicals, and it's missing the nitrogen plants need to survive.

This paper is about two things: how to check if the plants are eating well in this alien dirt, and finding the best "flashlight" to take a picture of their insides.

The Problem: How do we see what's inside a plant?

Usually, to see what nutrients a plant has absorbed, scientists have to chop it up, boil it in acid, and run it through a machine. It's like taking apart a car engine just to see if the oil is clean. It takes a long time, destroys the plant, and you can't see where the nutrients are located inside the leaves.

The scientists used a technique called LIBS (Laser-Induced Breakdown Spectroscopy). Think of this as a super-fast, super-precise laser "zap."

You point a laser at a tiny spot on a leaf.
The laser zaps a microscopic piece of the leaf, turning it into a tiny, glowing ball of hot gas (plasma).
As that gas cools down, it flashes with light.
The color of that light tells you exactly which elements (like Magnesium or Calcium) were in that spot.

By moving the laser across the leaf, they can build a colorful "heat map" showing exactly where the nutrients are hiding.

The Experiment: Two Different Lasers

The researchers wanted to know: Which laser makes the best "flashlight" for this job?

They tested two different types of laser beams:

The Classic (1064 nm): This is the standard, workhorse laser used in most labs. It's like a bright, reliable flashlight.
The Newcomer (2090 nm): This is a longer-wavelength laser. Think of this like a specialized infrared flashlight that interacts differently with water and organic stuff.

They grew Salad and Broccoli in two types of soil:

Control Soil: A nice, normal potting mix with fertilizer.
Moon Dirt: A special "Lunar Regolith Simulant" (fake Moon dirt) that mimics the actual soil found on the Moon.

What They Found

1. The "Newcomer" Laser Was Better
When they compared the two lasers, the 2090 nm laser (the newcomer) won.

The Analogy: Imagine trying to melt an ice cube. The 1064 nm laser is like a warm breeze; it works, but slowly. The 2090 nm laser is like a blowtorch; it hits the water-rich plant tissue harder and faster.
The Result: The 2090 nm laser created a hotter, brighter, and more efficient "plasma ball." This meant the signals were clearer, the noise was lower, and the scientists could see the elements more distinctly. It's like switching from a grainy, black-and-white photo to a crisp, high-definition color image.

2. The Plants Ate the Moon Dirt (and Liked It)
When they looked at the heat maps of the plants grown in the fake Moon dirt, they found something interesting.

The Analogy: Think of the Moon dirt as a buffet that is very heavy on Calcium and Magnesium (like a plate piled high with cheese and nuts) but light on other things.
The Result: The Salad and Broccoli plants that grew in the Moon dirt absorbed more Magnesium and Calcium than the plants grown in normal soil. The heat maps showed bright, glowing spots of these elements in the Moon-grown plants.
Why? The fake Moon dirt was naturally rich in these specific minerals (oxides of calcium and magnesium). The plants didn't just survive; they actively soaked up these abundant nutrients, proving that they can interact with this alien soil.

The Bottom Line

This study didn't just prove that plants can grow in Moon dirt; it proved that we have a better tool to monitor them.

By using the 2090 nm laser, scientists can get a clearer, more detailed "X-ray" of plant health without destroying the plant. This is a crucial step toward building a future where astronauts can grow their own fresh food on the Moon, using the local soil as a resource rather than a barrier. The paper suggests that if we ever set up a lunar farm, this specific laser wavelength would be the best choice for keeping an eye on our space crops.

1. Problem Statement

The colonization of extraterrestrial bodies (specifically the Moon and Mars) requires sustainable, in-situ food production to avoid the prohibitive costs and logistical challenges of Earth-based supply chains. Growing plants directly in lunar regolith is a promising strategy, but regolith presents significant challenges:

Chemical Limitations: It lacks essential nitrogen and contains toxic elements (e.g., aluminum, chromium) or excessive concentrations of others (e.g., iron, manganese).
Physical Limitations: Its compact structure and sharp particles hinder root development.
Monitoring Needs: There is a critical need for rapid, non-destructive diagnostic tools to monitor plant health, nutrient uptake, and the distribution of toxic elements within plant tissues in space environments.

Current analytical methods (like ICP-OES) require time-consuming sample preparation and destroy spatial information. While Laser-Induced Breakdown Spectroscopy (LIBS) offers rapid, in-situ elemental analysis with spatial resolution, most plant bioimaging relies on the conventional 1064 nm laser wavelength. The efficiency of this wavelength for biological matrices (rich in water and organics) compared to longer wavelengths remains under-explored.

2. Methodology

The study employed a comparative approach using two laser wavelengths for LIBS bioimaging of plant tissues:

Plant Cultivation:
- Species: Salad (Lactuca sativa) and Broccoli (Brassica oleracea).
- Substrates:
  1. Lunar Regolith Simulant (LMS-1, Exolith Lab, USA).
  2. Control substrate (Plagron Growmix, organic peat/worm castings).
  3. Control substrate + Fertilizer (TriPart).
- Conditions: Grown for one month under controlled light (16h/8h), temperature (23°C), and humidity.
- Sample Prep: Leaves were dried, flattened, and fixed on microscope slides with epoxy resin.
LIBS Instrumentation:
- System 1 (Conventional): Nd:YAG laser at 1064 nm (8.8 mJ, 20 Hz, 5.5 ns). Spot size: 15 µm.
- System 2 (Novel): Ho:YAG laser at 2090 nm (2.9 mJ, 100 Hz, 4.1 ns). Spot size: 10 µm.
- Optimization: Laser energies were adjusted to ensure equivalent irradiance ( $I$ ) for both systems.
- Detection: A Czerny–Turner spectrometer (247–410 nm range) with a CMOS detector.
- Data Processing: 2D elemental maps were generated using a step size of 40 µm (1064 nm) and 20 µm (2090 nm). Background correction was applied, and signals from epoxy resin were masked out.
Key Metrics Analyzed:
- Signal-to-Noise Ratio (SNR): For Mg I (285.20 nm) and Ca II (393.36 nm).
- Total Emissivity ( $\varepsilon_{tot}$ ): Integrated spectral emission (247–410 nm).
- Ionization Degree: Ratio of Mg II (279.50 nm) to Mg I (285.31 nm) to estimate plasma temperature.

3. Key Contributions

Wavelength Comparison: This is one of the first studies to systematically compare the efficacy of a 2090 nm laser against the standard 1064 nm laser for plant tissue bioimaging.
Plasma Physics Insight: It demonstrates that the 2090 nm wavelength, due to higher absorption by water and organic compounds in the Short-Wave-Infrared (SWIR) region, generates a hotter and more efficiently ionized plasma.
Nutrient Uptake in Regolith: It provides new empirical data on how specific crops (salad and broccoli) uptake nutrients (Mg and Ca) from Lunar Regolith Simulant (LMS-1) compared to organic soil.

4. Results

A. Laser Wavelength Performance (2090 nm vs. 1064 nm)

Signal Quality: The 2090 nm laser yielded significantly higher performance metrics:
- SNR: Mg I SNR was 35.12 (2090 nm) vs. 23.22 (1064 nm); Ca II SNR was 166.09 vs. 120.67.
- Total Emissivity: The 2090 nm plasma exhibited higher total emissivity, indicating stronger light emission.
- Ionization: The Mg II/Mg I ratio was 1.96 for 2090 nm compared to 1.76 for 1064 nm.
Interpretation: The higher ionization ratio and emissivity suggest the 2090 nm laser creates a hotter plasma. This is attributed to the higher absorption coefficient of plant tissue (water/organics) at 2090 nm, leading to more efficient laser-matter interaction and ablation.

B. Plant Bioimaging (Lunar Regolith vs. Control)

Elemental Distribution: Both broccoli and salad grown in LMS-1 showed visibly higher accumulation of Magnesium (Mg) and Calcium (Ca) compared to plants grown in the control substrate.
Spatial Localization:
- Salad: Nutrients were diffusely distributed throughout the leaf.
- Broccoli: Nutrients were tissue-specific, preferentially localized in veins and midribs, with lower levels in the lamina.
Correlation with Substrate: The increased uptake correlates with the high concentration of MgO and CaO in the LMS-1 simulant, which are less abundant in the organic control substrate.

5. Significance and Conclusion

Diagnostic Tool for Space Agriculture: The study validates LIBS as a viable, rapid diagnostic tool for monitoring plant health and nutrient status in extraterrestrial environments.
Optimization of LIBS for Biology: The findings strongly suggest that 2090 nm lasers are superior to 1064 nm lasers for bioimaging biological tissues. The improved signal quality and plasma temperature could enhance the detection limits for trace elements and toxic metals in future space missions.
Feasibility of Regolith Farming: The results confirm that plants can successfully grow in lunar regolith simulants and actively uptake essential nutrients (Mg, Ca) from the substrate, supporting the concept of sustainable in-situ food production.
Future Outlook: While the 2090 nm wavelength shows promise, further research is needed to fully understand long-term plant growth dynamics and nutrient bioavailability in actual lunar regolith conditions.

In summary, this paper establishes that switching to a 2090 nm laser significantly improves the quality of LIBS bioimaging for plants and confirms that lunar regolith simulants can serve as a source of essential nutrients for crops like salad and broccoli.