“Lab on a chip” advances for water quality testing

The YouTube video shows nano-sized droplets being formed in the “lab on a chip” micro-fluids laboratory of engineering professor Sindy Tang at Stanford University. The droplets contain different dyes that emit different wavelengths of light when excited optically. The radius of each droplet is about 20 micrometers (20 micrometers = 0.000787401575 inches = very, very small droplet) and the speed of the video is one hundred-plus times slower than “real-time.” (Reads like a script for a Star Trek episode, doesn’t it?)

In a CA Sea Grant project beginning this month, Tang will seek to demonstrate the utility of droplet generation for water quality testing. Why droplets? Inside one of these super-tiny drops, a single “bad” bacterial cell suddenly looks big and can be detected, under the right conditions. Imagine a long series of droplets either containing a cell or not. The series of 0’s and 1’s could be counted to estimate cell counts in water samples. Droplets are just a high-tech means for detecting pathogens that in the environment exist at low concentrations and can go undetected.

Here is a more technical summary of Tang’s project that appeared in our recent Program Directory:

In-situ Detection of Fecal Indicator Bacteria by Digitization and Concentration in Microfluidic Picoliter Droplets


Sindy Tang, Stanford University,

The researcher has designed but not fully tested an intensely high-tech approach for monitoring low concentrations of pathogenic bacteria in water samples. In the method, water samples are mixed with probes that enzymes in bacteria convert to a fluorescent-colored product. Picoliter (trillionth-of-a-liter) droplets are formed from this mixture and the brightly colored drops are then counted to estimate bacterial concentrations.  The key step in the approach is the ability to form the droplets, within which a single cell will have a very high effective concentration, on the order of 10^9 cfu/mL. Besides amplifying the pathogenic signal, the approach also reduces the assay time for detecting bacteria, which is critical for protecting public health. The main objective of this proof-of-concept project is to demonstrate the ability to form the droplets and count cells for the fecal indicator bacteria Escherichia coli and Enterococcus sp. The method’s accuracy will be verified for samples with known cell counts. The scientist will also characterize the rate at which color intensity builds in “incubating” droplets, as a function of droplet size, to identify an optimal drop size and assay time for the bacteria. The enzyme-substrate probe technology to be employed in this project  has been approved and is expected to become adopted by the EPA. Outcomes from this project will further efforts to quantitatively measure low concentrations of water-borne pathogens through a technique that “packages” the EPA method in picoliter containers.

Written by Christina Johnson


NOAA’s California Sea Grant College Program is a statewide, multi-university program of marine research, extension services, and education activities administered by Scripps Institution of Oceanography at the University of California, San Diego. It is one of 33 Sea Grant programs and is part of the National Oceanic and Atmospheric Administration (NOAA), U.S. Department of Commerce. Visit our website ( to sign up for email news or follow us on Facebook or Twitter.


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