Microfiber Fluorescence

Your washing machine is a significant source of microplastics, in the form of fabric fibers, entering the environment. Your clothes wear out eventually, right? Part of that wearing out is shedding fibers into the wash water as your clothes jostle around. Some of those fibers stick to other clothes and may end up in the dryer lint collector or going out the dryer exhaust. A lot of the fibers go out the discharge hose when the washing machine empties, going into your septic system or the local sewer system.

Due to their interest in (a) the planet and (b) learning more about fluorescence and microfibers, the researchers at NIGHTSEA purchased a Lint LUV-R filter from Environmental Enhancements. This is one of a number of alternatives for keeping fibers from escaping down the drain, and in one test it was found to remove over 80% of the fibers. The researchers were not out to make quantitative measurements, but rather to see how the collected fibers appeared in white light, and when fluorescing under excitation by two different wavelengths of light - longwave Ultraviolet and Royal Blue. Note that this is not their first foray into looking at the fluorescence of fibers - take a look at their web pages on dryer lint and fiber transfer (they certainly know how to have a good time at NIGHTSEA). Read on as they describe their findings.

The filter was installed on a home washing machine and we (the researchers at NIGHTSEA) then documented the collection through white-light and fluorescence photography after running three loads of laundry. These images show the complete filter after installation, and with the filter bowl removed.

(Click any image for larger view)

Now we move in a bit closer and take images with white light (left), fluorescence excited by ultraviolet light (center), and by blue light (right). The bright glow in the ultraviolet image is from fibers collected on the filter. The metal is not fluorescent so appears black in the fluorescence images. Note that all sets of white light - fluorescence images are of exactly the same area.

We then removed the stainless steel filter from the bracket and set up the camera to take close-ups of portions of the filter surface.

As a reality check of what might be going into the washing machine, the three images below show the sleeve of a polyester fleece jacket. They were taken with white light (left), fluorescence excited by ultraviolet light (center), and by blue light (right).

Not all of the lint that enters the Lint Luv-R assembly sticks to the filter. The picture on the left below is looking down into the filter bowl filled with clean water. On the right is the bowl with the water after those three washloads. This is why the instructions that come with the filter tell you not to just pour that water down the drain when you clean the filter - you should pour it into a coffee filter or something similar that will catch the suspended material.

We wanted to look at what was in the water, so we poured it into a coffee filter and dried the filter in a toaster oven at low heat.

We then looked at the filter under a stereo microscope, again with white light and fluorescing under ultraviolet and blue light using the NIGHTSEA Model SFA Stereo Microscope Fluorescence Adapter with the white light head and the ultraviolet (UV) (~360-380nm) and Royal Blue (RB) (~440 - 460nm) filter sets. It is immediately evident that you see very different things with the two excitation wavelengths. Fibers that are highly fluorescent under blue light may be weakly or even non-fluorescent under ultraviolet, and vice versa. In some cases it can be difficult to convince yourself that you are even looking at the same area of the sample! The black lines that you can see in many of the images are hairs (it was the cat).

The photography can be tricky at times, because some of the fibers fluoresce with very different intensities. The two images on the right below were both taken with the blue excitation but at different exposure times. When the image is properly exposed for the bright green and orange fibers the rest of the field of view is dark. Increasing the exposure time overexposes those bright fibers but enables you to record many more fluorescing features.

An area of the sample that did not have any large features fluorescing under blue light still revealed many very small bright spots of undetermined origin, as seen in the images below. The set in the bottom row were taken at higher magnification. The scale bar shows that many of the small bright spots are on the order of 10 microns or less.

We extracted a few representative fibers and put them on a microscope slide, along with a non-fluorescent hair.

For the technically inclined, we measured excitation and emission spectra for a blue-fluorescent, green-fluorescent, and orange-fluorescent fiber picked somewhat at random. These should not be interpreted as being representative of all particles.

Note that the observations and spectral measurements described here do not tell you exactly what type of fiber you are collecting. Fluorescence is very useful as a detection technology, but less so for specific identification. The fluorescence can arise from a variety of additives, like the optical brighteners commonly used in detergents, or to fiber colorants included in the manufacturing process.

Recommendations

Our recommendations from this small, non-scientific experiment:

• If you want to find small clothing fibers in a sample, fluorescence is more effective than white light for providing contrast.
• Your first choice for an excitation wavelength to find the most fibers should be longwave ultraviolet. It makes the fibers far easier to see than in white light and makes more fibers fluoresce than blue light excitation.
• To find even more fibers the next choice should be Royal Blue excitation.

We tried our Violet (VI) light set with these samples, thinking that since it falls between the Ultraviolet and the Royal Blue we might get the best of both worlds. This turned out NOT to be the case. The fibers that fluoresced brightly under UV fluoresced less brightly under Violet, and the fibers that fluoresced well under Royal Blue did not show up well at all under Violet. Note that if your interest is in microplastic fragments rather than the microfibers described here, our first-choice recommendation is Royal Blue, with Ultraviolet as the recommended backup to find even more. The Royal Blue seems to make more particles fluoresce than ultraviolet, and also works well for particles stained with Nile red.

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