Applications | Micro-Raman Drug Residue Screening in Seafood

Before fish reaches the market, are any drug residues slipping through? Micro-Raman is changing the rules of aquatic product residue testing.
 

The sea bass you pick at the market, the shrimp you buy—from the aquaculture pond to the dining table, they must pass a critical checkpoint: veterinary drug residue testing. Malachite green, crystal violet, enrofloxacin, ciprofloxacin, nitrofuran metabolites, chloramphenicol… These names are unfamiliar to ordinary consumers, but they are frequent entries on aquatic product inspection lists. They were once used to prevent and treat saprolegniasis, septicemia, and intestinal infections in aquatic animals. Because they were cheap and effective, they were once abused.
The problem is: traditional testing is too slow, on-site rapid testing is too coarse, and laboratory precision is too expensive. The window from pond to table is often only tens of hours, while methods such as HPLC and LC-MS/MS require hours of sample preparation and another half-day or more for reports.
Is there a technology that can both see where drug molecules are located under a microscope and identify what they are like a fingerprint check? This is precisely why micro-confocal Raman spectroscopy has been reconsidered in food rapid testing in recent years. This article uses Optosky’s ATR8100 micro-confocal Raman spectroscopic imaging instrument as an example to explain clearly: what it is, what it can actually do in aquatic product residue detection, and where its boundaries lie.

01  Why Is Fish Testing Always a Step Behind?

First, let us clarify the reality of veterinary drug residue testing in aquatic products. The two mainstream systems each have their own difficulties.

The first is laboratory precision methods. Liquid chromatography (HPLC) and liquid chromatography-tandem mass spectrometry (LC-MS/MS) are accurate and quantitative. But the cost is: samples must undergo homogenization, extraction, purification, derivatization, and multiple pretreatment steps. A batch often takes 4–8 hours. The equipment is expensive and requires high environmental and personnel standards, so it can only be completed in central laboratories.

The second is on-site rapid testing. Enzyme-linked immunosorbent assay (ELISA) and colloidal gold immunochromatography are common methods at aquaculture ponds and wholesale markets. They produce results in about ten minutes and the equipment is inexpensive. But they are essentially antigen–antibody reactions: one test can only detect one target or one class of targets, the false positive rate is relatively high, and positive results must still be sent to a laboratory for confirmation.

In other words, equipment that can truly provide molecular-level evidence on site has long been absent. Regulators, aquaculture enterprise quality control teams, and distribution channels all lack an instrument that can see clearly, identify accurately, and be taken into the field.

02  Raman Spectroscopy: Taking a Fingerprint Photo of Molecules

To understand why Raman is suitable for this task, start with a basic physical phenomenon. When a monochromatic laser beam hits a sample, the vast majority of photons are bounced back unchanged (Rayleigh scattering). However, a very small number of photons transfer a little energy to the vibrational modes of molecular chemical bonds, causing a slight frequency shift in the scattered light—this is Raman scattering. Different molecules have different chemical bond structures, so the position and intensity of this frequency shift differ. Plotted as a spectrum, it becomes the molecule’s unique fingerprint. Malachite green, enrofloxacin, crystal violet, and chloramphenicol each have a set of characteristic peaks in their Raman spectra, identifiable like human fingerprints.

The advantages are obvious:

• Almost no sample pretreatment. Common matrices such as water, glass, and plastic have weak Raman signals. Water-containing aquatic samples can be measured directly, eliminating cumbersome extraction.

• A single scan captures the full spectrum. Unlike immunoassays, which require one reaction per molecule, one spectrum can contain fingerprints of multiple targets simultaneously.

• Non-destructive and fast. A laser exposure takes seconds to tens of seconds to obtain a spectrum, and the sample is not destroyed.

• Compatible with microscopy. The laser spot can be focused to the micrometer scale and precisely aimed at a tiny suspicious particle or crystal in fish tissue—this is micro-Raman.
 

03  But Raman Has an Inherent Weakness: The Signal Is Too Weak

The photon efficiency of Raman scattering is extremely low—approximately one Raman photon is generated for every million to hundreds of millions of incident photons. Direct measurement of pure substances is fine, but veterinary drug residues in aquatic products are usually at the μg/kg or even ng/kg level, which native Raman cannot see. This requires a key technology: surface-enhanced Raman scattering (SERS).

When a sample is placed on a gold or silver nano-sol or nano-array substrate, the metal nanostructures generate localized surface plasmon resonance under laser excitation, amplifying the local electric field by several orders of magnitude. The Raman signal of drug molecules adsorbed on the substrate can be enhanced by millions to trillions of times. This is why a large number of recent studies have applied SERS to trace residue detection of malachite green, enrofloxacin, chloramphenicol, and similar compounds. Published research shows that using silver nano-sol as an enhancement substrate, SERS methods can achieve detection limits for malachite green, enrofloxacin, and ciprofloxacin in aquatic products at the μg/L level or even lower. Combined with thin-layer pretreatment (acetonitrile extraction, purification, and reduction/oxidation), the entire process can be compressed from several hours in traditional methods to the minute level. This is also why instrument manufacturers working on rapid aquatic product residue testing have been focusing simultaneously on three directions: high sensitivity, microscopic localization, and SERS compatibility.
 

04  ATR8100: Putting “Seeing” and “Identifying” into One Machine

Returning to the ATR8100. Its core positioning is a micro-confocal Raman spectroscopic imaging instrument that integrates an optical microscope and a Raman spectrometer into one device. Against the actual needs of aquatic product residue detection, several key parameters deserve separate mention.

First, the spot reaches the micrometer level—truly “what you see is what you measure.” The ATR8100 laser spot diameter is less than 1 μm. Equipped with a 5-megapixel industrial camera and a conjugate focusing system, the operator can see suspicious crystals, particles, or stained patches in fish tissue on the screen and directly aim the laser at them to measure their Raman spectra. This is very practical in inspection: malachite green often remains on fish gills and scales as trace crystals or dye stains. Color changes may be visible to the naked eye, but molecular-level evidence is needed for confirmation.

Second, signal-to-noise ratio greater than 2000:1, up to 11000:1 with cooled models. Aquatic product matrices are complex. Fluorescence backgrounds from fat, protein, and collagen can severely interfere with Raman signals. High signal-to-noise ratio combined with long integration time (up to 1.2 hours for DC-cooled models) is the physical basis for stable SERS trace detection peaks.

Third, multiple excitation wavelengths are available: 532/638/785/830/1064 nm. This is often overlooked by non-specialists but is extremely critical for biological samples. Short wavelengths (532 nm) have a larger Raman scattering cross-section and stronger signals, but aquatic samples have heavy fluorescence backgrounds. 785 nm and 1064 nm can significantly suppress fluorescence. Choosing the appropriate wavelength for different drugs and different matrices is standard practice in real testing scenarios.

Fourth, the MP model supports mapping scanning imaging. The automatic platform movement range is 50 × 50 mm, with a movement resolution of 0.1 μm. For a fish tissue section, area scanning can be performed, stitching together the Raman spectrum of each pixel to generate a distribution map of drug molecules in the tissue. This is very useful for studying drug enrichment locations, conducting mechanism analysis, or tracing contamination sources.

Fifth, the instrument has a built-in Windows system, touchscreen, and lithium battery with approximately 4 hours of battery life. This means it does not have to be fixed in a central laboratory. It can be placed in aquaculture enterprise quality control rooms, aquatic product wholesale market testing points, and customs on-site rapid testing rooms. In terms of temperature stability, wavenumber drift is no more than 1 cm⁻¹ in the 5–40°C environment, and peak intensity variation is less than 10%. This is crucial for repeatability in field environments.

05  Putting It into Real Aquatic Product Residue Testing Scenarios

Translating parameters into scenarios, the ATR8100 can broadly cover three types of work in the aquatic product testing chain.

Scenario 1: Rapid qualitative screening in aquaculture and distribution. When fish arrive at port or are sampled at wholesale markets, for visually suspicious stained gills or abnormal spots, suspicious particles can be directly located under the microscope. A Raman fingerprint spectrum can be obtained within tens of seconds and compared with standard spectral libraries for malachite green, crystal violet, enrofloxacin, and others, quickly yielding a conclusion of suspected positive or no characteristic peaks. Positive samples are then sent for LC-MS/MS confirmation, greatly reducing the number of samples requiring laboratory testing.

Scenario 2: Laboratory SERS trace method development. Universities, research institutes, and third-party testing agencies can use the ATR8100 with homemade gold/silver nano-substrates to develop SERS detection methods for specific veterinary drugs, such as quinolone antibiotics, nitrofuran metabolites, and chloramphenicol. The MP scanning imaging mode can simultaneously obtain information on drug distribution uniformity on the substrate, helping optimize extraction and spike recovery procedures.

Scenario 3: Contamination tracing and mechanism research. Mapping area scanning can output distribution maps of drug molecules on fish tissue sections, gill tissue, and scale surfaces, answering questions that traditional point testing cannot answer, such as where drugs are enriched and what the residue gradient is. This is valuable for research on fishery drug use standards and environmental exposure assessment..

06  Several Common Misconceptions Need to Be Clarified First

Before discussing capabilities, it is more important to discuss boundaries. Regarding the application of Raman in aquatic product testing, at least four statements need to be viewed calmly.

Misconception 1: Raman can directly replace LC-MS/MS for legally valid reports. A more prudent statement is: Raman is suitable for rapid on-site qualitative screening. At present, the national standard methods for veterinary drug residues in aquatic products are still based on chromatography and mass spectrometry. Raman/SERS methods in China are still in the stage of methodological validation and accumulation of group standards. Their real value is to screen out suspicious samples at the earliest opportunity, reducing the number of samples sent and waiting time—not to directly issue legally binding quantitative conclusions.

Misconception 2: As long as the instrument is highly sensitive, trace residues can be measured. Instrument sensitivity is only one condition. The core variable in SERS detection is actually the substrate: the particle size, uniformity, repeatability, and activity of gold/silver nanoparticles, as well as the extraction efficiency of pretreatment, often determine the actual detection limit. No matter how good the instrument is, if the substrate is poor, the signal will still fluctuate. This is why most papers in this field focus on substrate optimization rather than instrument parameters.

Misconception 3: Micro-Raman can only measure pure substances; biological matrices are too dirty to use. On the contrary, the significance of micro-confocal Raman is precisely that it isolates the target particle within the microscope field of view and measures it separately, avoiding interference from the surrounding matrix. Combined with 785 nm or 1064 nm excitation wavelengths to suppress fluorescence, it has quite good adaptability to water-containing and fat-containing samples such as fish meat and gills. The multiple SERS experiments and cell metabolite tests already completed with the ATR8100 in the PDF materials also validate this path.

Misconception 4: Portable means reduced performance. The ATR8100 has a built-in Windows system, touchscreen, and battery, and a compact body, but its optical path uses solid-state assembly (no moving parts for optical path switching; patent applied). Its 8-hour spectral stability σ/μ is less than 0.5%, and wavenumber drift at 5–40°C is ≤1 cm⁻¹. It is not a stripped-down laboratory instrument; rather, it brings the core specifications of confocal Raman to a mobile platform.

Conclusion

The real difficulty in veterinary drug residue testing of aquatic products has never been whether it can be measured at all. It is whether a molecular-level preliminary answer can be obtained before the fish spoils, before customs clearance, and before a dispute arises. The value of micro-confocal Raman spectroscopy is not that it suddenly replaces something else, but that it puts “seeing” and “identifying” into the same instrument—one that can be moved and can produce results quickly. With the continuous accumulation of SERS substrates and standard spectral libraries, it is moving from the laboratory to ponds, markets, and customs rapid testing rooms. For regulators, it means faster on-site judgment. For aquaculture enterprises, it means shorter self-inspection cycles. For consumers, it means one more molecular-level safeguard for the fish on the table. Technology will not make risks disappear, but it will make risks visible earlier.

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