The chemistry
How microplastic test kits work.
Every legitimate microplastic test kit is a packaged version of the same five-step lab protocol: sample, digest, stain, filter, image. The chemistry was developed in the 1980s for biology (Greenspan & Fowler, 1985, used Nile Red to image lipid droplets in cells), adapted for microplastics in 2017 by Maes and colleagues at the Centre for Environment, Fisheries and Aquaculture Science, and standardized for consumer surveys by Mason and colleagues at SUNY Fredonia in the 2018 Orb Media bottled-water study. The in-home microplastics test kit is the same protocol with the dilutions pre-measured — minus the peroxide digest, which we don't ship because digestion-strength peroxide is hazmat. In the home it's three steps: stain, filter, look.
The five steps
- 1
Sample
100 mL of water (tap, bottled, filtered — anything pourable) goes into a graduated cup. The volume matters because particle count is reported per 100 mL.
- 2
Digest (lab step — not in our kit)
In the published lab protocol, hydrogen peroxide oxidizes organic matter that would otherwise stain non-specifically and clog the filter. Our kit does not include this step: digestion-strength peroxide is regulated as a hazardous material for shipping, so we ship a rapid, safe screen without it. The consequence is that organic-rich samples read high — see our limitations page.
- 3
Stain
Drop in the Nile Red vial. The hydrophobic, solvatochromic dye partitions out of the water and binds to hydrophobic surfaces in the sample — preferentially plastic, though some organic matter can take it up too. Wait 15 minutes minimum; longer is fine, and better.
- 4
Filter
Push the full 100 mL through a 25mm, 1 µm membrane filter. Particles ≥1 µm are trapped on the disc, stained ones included.
- 5
Image
Place the filter under the digital microscope. Stained particles — likely microplastics — fluoresce bright pink against the dark filter. The included microscope sees particles down to about 5 µm. You count them yourself, live, in minutes.
Those five steps are the whole method. For the practical walkthrough — what to buy, how long each step takes, and how to read your filter — see the full guide on how to test for microplastics in the home.
Why Nile Red binds to plastic
Nile Red (9-diethylamino-5H-benzo[α]phenoxazine-5-one) is a small, planar, lipophilic fluorophore — chemistry speak for “tiny molecule that loves oily things and glows under UV-to-blue light.” In water, it doesn't dissolve; it partitions onto any hydrophobic surface it can find. The most common consumer plastics — polyethylene, polypropylene, polystyrene, PET, PVC — are all hydrophobic. So in a water sample containing plastic, Nile Red preferentially binds the plastic.
The fluorescence itself comes from solvatochromism — the dye changes emission color based on the polarity of its local environment. In water, Nile Red barely fluoresces. On a hydrophobic plastic surface, it fluoresces brightly in the pink-to-red range, 580–630 nm. The digital microscope excites the stain and passes only that pink-to-red emission, so the bound dye is the only thing left to see.
Nile Red stains hydrophilic materials poorly — most minerals, dissolved salts, calcium carbonate, sand, glass. But some natural organic matter (lipids, biofilm, plant tannins) can also take up the stain and fluoresce. A hydrogen peroxide digestion step is the standard lab control for that: it breaks organics down before staining so they don't produce a false-positive glow. The current kit does not include that step (peroxide is hazmat to ship), so particles that glow are counted as likely microplastics — a screening estimate, not a lab-confirmed identification — and on organic-rich water the count runs high. See limitations for what that means for your count.
Two more things to know when you read the filter. Black plastics (e.g. tire wear, black packaging) often don't take up the dye, so they can appear dark rather than glowing. Sand and sediment show up as non-glowing, 3D-textured grains — they're not counted.
Why the kit uses a 1 µm filter
Filter pore size is the capture floor. Anything smaller than the pore passes through; anything larger gets trapped. The Maes et al. and Mason et al. methods both settled on ~1 µm because:
- It catches the size class actually called "microplastic" in published surveys (1 µm to 5 mm). Sub-1 µm particles are "nanoplastic" and require different methods to detect.
- Smaller pores (0.45 µm, 0.22 µm) clog fast on real-world samples and produce noisy, hard-to-count filters.
- Most consumer-relevant plastic in water — fragments from PET bottles, fibers from synthetic textiles, flakes from PEX piping — falls in the 1–100 µm range, above the filter's capture floor.
The trade-off: at 1 µm the filter captures the microplastic fraction, and the included digital microscope sees particles down to about 5 µm (a more powerful microscope can see down to the filter's ~1 µm capture size). Nanoplastics — particles under 1 µm, small enough to cross cell membranes, the gut lining, and the blood-brain barrier — are below the floor. The 2024 PNAS bottled-water paper used stimulated Raman scattering to count ~240,000 particles per liter, ~90% of them nanoplastics, far past anything an optical method can see. The micron count the kit gives you is the visible proxy for that larger invisible nano load from the same source. We say so on the nanoplastics explainer and on the accuracy page.
Where the method came from
The Nile Red molecule was synthesized in the 19th century but its modern fluorescent use traces to Greenspan & Fowler, 1985 — a Journal of Lipid Research paper that used it to image lipid droplets in cultured cells. For the next 30 years it was mostly a tool in cell biology and biochemistry.
Maes, Lyons, Devriese, & Vlimant, 2017 inScientific Reports repurposed it for microplastic detection. They demonstrated that Nile Red stained common plastic polymers cleanly and could be paired with fluorescence imaging on filter membranes to produce quantifiable counts.
Mason, Welch, & Neratko, 2018 in Frontiers in Chemistry applied the method at scale to the Orb Media bottled-water survey — 259 samples across 11 brands. That paper is still the most-cited single source on bottled microplastic contamination, and it's the methodological backbone of every consumer Nile Red kit (including ours).
Full citations and the validation roadmap on the methodology page and the Nile Red microplastics test deep dive.
FAQ
Why does Nile Red bind to plastic specifically?
Two molecular properties. Nile Red is hydrophobic — it doesn't dissolve in water — so it partitions out of solution and onto any hydrophobic surface it can find. Plastic polymers (polyethylene, polypropylene, PET, PVC) are also hydrophobic, so they're the preferred binding target in a water-dominant sample. It isn't plastic-exclusive — some organic matter can also take up the stain and fluoresce, which is why the result is a screening count. Nile Red is also solvatochromic, meaning its fluorescence emission shifts depending on the polarity of its environment. When bound to a hydrophobic plastic surface, it emits in the pink-to-red range (~580–630 nm) — bright and easy to see against a dark filter.
Why does the digital microscope excite the stain with ~450nm light?
Because that's the excitation wavelength where Nile Red bound to a hydrophobic surface fluoresces most strongly. Nile Red has a broad excitation profile peaking in the blue-green range, but the published microplastic-staining protocols (Maes 2017, Mason 2018) settled on ~450nm because it produces clean pink emission with minimal background fluorescence from natural organics. The microscope's built-in illumination is tuned to that band.
Why does the kit use a 1 µm filter and not something smaller?
Filter pore size sets the smallest particle captured. 1 µm catches the size class typically called "microplastic" in published surveys (1 µm to 5 mm) and matches the pore size used in the Orb Media bottled-water study. Smaller pore sizes (0.45 µm, 0.22 µm) catch more particles but clog quickly with organic matter and ambient dust, producing a noisy filter that's hard to count. 1 µm is the published compromise between sensitivity and clean filters. Note the filter isn't the only limit: the included digital microscope sees particles down to about 5 µm, which is the kit's practical detection limit as shipped.
What does the hydrogen peroxide actually do?
In the lab protocol, wet chemical oxidation: peroxide attacks organic carbon — sugars, proteins, biofilm, tannins, milk fats — breaking it into smaller, water-soluble fragments that pass through the 1 µm filter instead of getting trapped on it. Without a digestion step, that organic material stays on the filter alongside any plastic and stains non-specifically with Nile Red (organic compounds are partially hydrophobic). Note: the current kit does not include the peroxide digest, because digestion-strength peroxide is regulated as a hazardous material for shipping. So on organic-rich samples (tea, formula, old plumbing), treat the count as an upper bound — see thewatertest.com/limitations.
How does the microscope make the pink particles stand out?
The excitation light is much brighter than the pink fluorescence it triggers, so the digital microscope carries a built-in long-pass filter — it blocks the shorter excitation wavelengths (below ~550nm) and passes the longer ones (orange through red). What's left on screen is only the pink/red fluorescence emitted by stained particles, against a black filter background. High contrast, easy to count.
Can I see the particles with the naked eye, or do I need the microscope?
Only the largest stained particles are visible by eye, and counting needs the microscope. The included digital microscope sees particles down to about 5 µm. Particles between 1 and 5 µm are captured on the filter but need a more powerful microscope to see.