The fibres are made out of gold nanoparticles that are coated with compounds called hexanethiol and alkanethiol, and bind to charged particles, also called ions, of heavy metal elements.
The more pollutants that are captured, the more conductive the fibres become, which means that a sample can be easily measured by passing an electrical current through it.
The technique has been tested on levels of methyl mercurcy in the waters of Lake Michigan, off Chicago, and in mosquitofish caught in the Everglades National Park in Florida.
In both cases, the samples had ultra-low levels of mercury and measurements tallied with analyses made separately by US environmental watchdogs.
Mercury, a heavy metal that affects the brain and nervous system, occurs naturally but is a bigger risk as a byproduct of industrialisation. It accumulates up the food chain.
Each nano-velcro testing tab costs €10 (12 franc, $12) at most, and the measuring equipment would cost several thousand euros, the scientists said in a press release.
Samples can be tested in situ, which helps inspectors who need an on-the-spot, instant assessment. Levels of industrial pollution can vary hugely according to output and environmental conditions.
"With conventional methods, you have to send samples to a lab, where the equipment costs millions of euros," said Francesco Stellacci of the Federal Polytechnic School of Lausanne, Switzerland.
The study appears in the journal Nature Materials.