Trace Detection of Malachite Green in Stream Water

Significance of the Topic

This application describes a rapid, on‐site method for detecting trace levels of malachite green (MG) in stream water using surface‐enhanced Raman spectroscopy (SERS). Malachite green, once widely used in aquaculture as a fungicide, poses significant health risks due to its persistence in fish tissue and acute toxicity to both aquatic life and humans. Regulatory limits for MG in food products are extremely low, and unauthorized use continues to threaten consumer safety.

Objectives and Study Overview

The main goal was to demonstrate a simple, field‐deployable assay that can detect MG at sub‐ng/mL concentrations directly in unprocessed water samples. The study outlines the development of a reference library for MG, evaluates the analytical performance of the Metrohm Instant SERS Analyzer (MISA) coupled with gold nanoparticles, and establishes a straightforward test protocol for aquaculture sites.

Methodology

Stream water samples were spiked with known concentrations of MG (from 10 µg/mL down to 1 ng/mL). For each test, 0.1 mL of sample (pH 7.0) was mixed with 0.8 mL of gold colloid and 0.1 mL of 500 mmol/L NaCl. After gentle mixing, SERS spectra were acquired using the MISA vial attachment.

Used Instrumentation

• MISA Advanced (Metrohm Instant SERS Analyzer) with Orbital‐Raster‐Scan (ORS) technology
• MISA Vial Attachment and P‐SERS Attachment
• ID Kit – Au NP (gold colloidal solution, pipettes, vials)
• Software: Misa Cal V1.0.15; Firmware 0.9.33
• Acquisition settings: Laser power level 5, integration time 1 s, 10 averages, raster scan ON

Main Results and Discussion

SERS spectra showed clear MG signals down to 1 ng/mL after baseline correction. The characteristic vibrational peaks of MG were reproducibly detected across the concentration range, demonstrating high sensitivity and selectivity in a complex water matrix. The detection limit achieved is well below regulatory thresholds for food safety.

Benefits and Practical Applications

• On‐site testing without elaborate sample preparation or laboratory infrastructure
• Rapid results (seconds per measurement) suitable for high‐throughput screening
• Minimal operator training required
• Portable instrumentation ideal for field inspections or mobile laboratories

Future Trends and Opportunities

Advances in nanoparticle substrate design and integration of automated data interpretation could further lower detection limits and reduce analysis time. Expanding the SERS library to include additional aquaculture contaminants will enable multiplexed screening. Integration with cloud‐based reporting systems could streamline regulatory compliance and supply‐chain monitoring.

Conclusion

The Metrohm Instant SERS Analyzer combined with gold nanoparticles provides a sensitive, user‐friendly method for detecting malachite green in stream water at trace levels. This simplified assay meets regulatory requirements and offers a practical solution for on‐site monitoring in aquaculture operations.

References

Application note AN‐RS‐018, Metrohm AG