A microfluidic device for continuous sensing of systemic acute toxicants in drinking water

Abstract

A bioluminescent-cell-based microfluidic device for sensing toxicants in drinking water was designed and fabricated. The system employed Vibrio fischeri cells as broad-spectrum sensors to monitor potential systemic cell toxicants in water, such as heavy metal ions and phenol. Specifically, the chip was designed for continuous detection. The chip design included two counter-flow micromixers, a T-junction droplet generator and six spiral microchannels. The cell suspension and water sample were introduced into the micromixers and dispersed into droplets in the air flow. This guaranteed sufficient oxygen supply for the cell sensors. Copper (Cu2+), zinc (Zn2+), potassium dichromate and 3,5-dichlorophenol were selected as typical toxicants to validate the sensing system. Preliminary tests verified that the system was an effective screening tool for acute toxicants although it could not recognize or quantify specific toxicants. A distinct non-linear relationship was observed between the zinc ion concentration and the Relative Luminescence Units (RLU) obtained during testing. Thus, the concentration of simple toxic chemicals in water can be roughly estimated by this system. The proposed device shows great promise for an early warning system for water safety.

Figure 1

Construction of the cell-based LOC device. (a) The chip has three layers, but the three major structures are in the middle layer, including two counter-flow micromixers, a T-junction droplet generator and six spiral micro-channels. A photo of the device is shown in the top left corner. The schematic and micrographs of the counter-flow micromixer are illustrated at the top right part, which houses the counter-flow units (on the middle layer) and the inlet channels (on the bottom layer). The tiny inlet port is located in the center of the counter-flow unit. (b) Two counter-flow micromixers are connected in series. The solution flowing in the bottom layer penetrates through the tiny inlet ports in the counter-flow units and then mixes with another solution near the pillar gaps around the tiny inlet port. (c) The map of the chip domains is shown at the bottom. The sample, buffer solution and the cell suspension are mixed and used to form droplet within the air flow.

Figure 2

The process flow of the cell-based LOC device. (a) The fabrication steps of the bottom layer are marked from Step B1 to Step B5 and diagramed in the top left corner. (b) The fabrication processes of the middle layer are illustrated in the top right corner. The steps are labeled from M1 to M5. (c) The bonding of the three layers is shown at the bottom.

Figure 3

Schematic of the detection system. (a) The droplet flow is generated by the cell-based LOC and moved into the observation chamber. The H10723-01 PMT is located on top of the observation chamber to collect the luminescence. The control voltage of the PMT is programed by a DA converter (Analog Devices, Inc.^®). The voltage output of PMT passes through a low-pass filter to improve the signal-to-noise ratio before AD conversion, the activity of which is controlled by a microcontroller unit (Texas Instruments Inc.^®, Dallas, TX, USA). The digital signals are transported into a computer (PC) through the USB interface. (b) Dynamic diagram of the observation chamber. The droplets flow from the cell-based LOC through the observation chamber continuously. After a short period, the contents of the observation chamber are totally replaced. Accordingly, the detection system measures the observation chamber periodically.

Figure 4

Toxicity test results for the V. fischeri cell-based sensing system. (a) The six sample solutions (2% NaCl, deionized water, 0.05 mg/L Cu²⁺ solution, 0.5 mg/L Cu²⁺ solution, 2.5 mg/L Cu²⁺ solution and 60 mg/L cetylpyridinium chloride (CPC)) were tested at several droplet flow rates [18]. (b) Nonlinear regression between [Zn²⁺] and the RLU data. (c) Response time and sampling interval of the sensing device. (d) Observation chambers with volumes of 5 µL, 10 µL and 20 µL were tested. (e) The drinking water sample and five artificial solutions were tested in the cell-based system, including 2% NaCl, deionized water, a drinking water sample taken from Tønsberg, Norway, 50 mg/L potassium dichromate, 4 mg/L 3,5-dichlorophenol and 60 mg/L CPC. (f) Parallel tests for the six spiral microchannels.