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 at-home microplastic test kit is the same protocol with the dilutions pre-measured.
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
Add hydrogen peroxide. Oxidation breaks down organic matter that would otherwise interfere with staining and clog the filter. Wait 30 minutes.
- 3
Stain
Add Nile Red dye. The hydrophobic, solvatochromic dye partitions out of the water and binds to any hydrophobic surface in the sample — primarily plastic. Wait 30 minutes for binding to saturate.
- 4
Filter
Push the full 100 mL through a 25mm membrane filter (1 µm pore, PTFE). All particles ≥1 µm trapped on the disc; stained plastic comes with them. Speed matters — slow pushes let dye soak into the filter background.
- 5
Image
Filter face up on a dark surface. Lights off. 450nm blue LED on. Orange long-pass filter clipped to phone camera. Stained plastic fluoresces pink. Photograph, count.
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. Excite with 450nm blue light, filter out the blue with an orange long-pass, and the bound dye is the only thing left to see.
Crucially, Nile Red does not bind to hydrophilic materials — most minerals, dissolved salts, calcium carbonate, sand, glass. It binds weakly to some natural organic matter (lipids, biofilm, plant tannins), which is why the hydrogen peroxide digest step exists: to break those down before staining so they don't produce a false-positive glow.
Why the kit uses a 1 µm filter
Filter pore size is the detection 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, well above the floor.
The trade-off is honest: at 1 µm the kit catches the microplastic fraction. Nanoplastics — the 100,000+ particles per liter the 2024 PNAS bottled-water paper counted using stimulated Raman scattering — are below the floor. We say so on the pillar page 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 validation data 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. 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 kit use a 450nm blue LED specifically?
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. Generic "blue" LEDs (470nm) work but produce weaker signal.
Why does the kit use a 1 µm filter and not something smaller?
Filter pore size sets the lower detection limit. 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.
What does the hydrogen peroxide actually do?
Wet chemical oxidation. Peroxide attacks organic carbon — sugars, proteins, biofilm, tannins, milk fats — breaking them into smaller, water-soluble fragments that pass through the 1 µm filter instead of getting trapped on it. Without this digestion step, organic material gets trapped on the filter alongside any plastic and stains non-specifically with Nile Red (organic compounds are partially hydrophobic). The peroxide step is the difference between a clean signal and a falsely dense filter.
Why do you need an orange filter on the phone?
Because the blue LED's light is much brighter than the pink fluorescence it excites. Without filtering, the phone camera sensor saturates on the blue and you see a blue-purple field with no detail. The orange filter is a long-pass — it blocks wavelengths below ~550nm (blue) and passes wavelengths above (orange through red). With the blue blocked, the camera only sees the pink/red fluorescence emitted by the stained plastic, against a black filter background. High contrast, easy to count.
Could I see the particles without a phone camera?
Some of them, yes — but counting is much easier with the camera. With a strong 450nm LED and a dark room, the largest stained particles (5+ µm) are visible to the dark-adapted eye through the orange filter held in front of your face. Smaller particles (1–5 µm) are at the edge of what the human eye can resolve and benefit from the camera's longer exposure and pinch-to-zoom. The kit ships the orange filter as a phone clip because that's the easiest way to image and document the result.