Sensor Assay Microbe Systems

Arsenic (As) in drinking water is a non-negligible problem and affects over 137 million people in more than 70 countries [1]. The aim of this project is to use nonpathogenic Escherichia coli that produce a green fluorescent protein (GFP) after exposure with As. 10μg/L is the maximum As concentration in drinking water recommended by the US-EPA environmental protection agency and the WHO.
Challenges are to minimize all lab equipment usually needed around the small microfluidic polydimethylsiloxane (PDMS) block, especially the large fluorescence microscope, which is to be replaced by a small optical arrangement to collect the light from the GFP and detect it with a high SNR.

Goals

The goals of this project therefore are :

1. to design and construct a microfluidics chip, in which bacterial reporter cells are filled in and exposed to arsenic in drinking water, and
2. to construct a ‘proof-of-principle’ portable device, in which the microfluidics chip can be put and which allows direct detection of the fluorescent signal of the bacteria.

System description

The modified E. coli [2] were encapsulated in agarose beads (Agarose D1 low EE0, Conda) to simplify the handling and the microfluidic aspects [3]. The PDMS microfluidic system is bonded to a glass slide of 150μm. The beads with the E. coli are accumulated in a microfluidic mechanical filter (500x500x70μm3). The water with the As flows around the beads. To allow easy changes of the microfluidic system, optics for excitation and detection are located under the glass slide.
Excitation light, provided by a laser diode (445nm, 60mW, Soraa), is coupled into a multimode fiber (200μm NA 0.37, Thorlabs) and guided to the microfluidic filter under an angle of 40°. Part of the emission light is collected by another optical fiber (1200μm NA 0.39, Thorlabs) and collimated by a lens. Two identical 35nm band pass filters block the excitation light, so that only fluorescence light can be detected by a photodiode (S10357-01, Hamamatsu).

Results

First results show that the compact measuring device is able to measure the fluorescent light produced by E.coli surrounded by an As solution of 50µg/L and 10µg/L. The response of this new device is comparable with that of a standard fluorescence microscope [3].

References
[1] Arsenic in drinking water seen as threat, Associated Press. 2007-08-30
 [2] J. Stocker, D. Balluch, M. Gsell, H. Harms, J. S. Feliciano, S. Daunert, K. A. Malik and J. R. van der Meer, Environ. Sci. Technol., 2003, 37, 4743–4750.
 [3] Nina Buffi et al., “Development of a microfluidics biosensor for agarose-bead immobilized Escherichia coli bioreporter cells for arsenite detection in aqueous samples,” Lab on a Chip (May 26, 2011)