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Multi-analyte Biochip (MAB) Based on All-solid-state Ion-selective Electrodes (ASSISE) for Physiological Research

Published: April 18th, 2013



1Department of Agricultural and Biological Engineering, Birck-Bindley Physiological Sensing Facility, Purdue University, 2NASA Ames Research Center, 3Department of Chemistry, Pennsylvania State University Hazleton, 4Cooley LLP, 5NASA Life and Physical Sciences, Human Exploration and Operations Mission Directorate, NASA Headquarters

All-solid-state ion-selective electrodes (ASSISEs) constructed from a conductive polymer (CP) transducer provide several months of functional lifetime in liquid media. Here, we describe the fabrication and calibration process of ASSISEs in a lab-on-a-chip format. The ASSISE is demonstrated to have maintained a near-Nernstian slope profile after prolonged storage in complex biological media.

Lab-on-a-chip (LOC) applications in environmental, biomedical, agricultural, biological, and spaceflight research require an ion-selective electrode (ISE) that can withstand prolonged storage in complex biological media 1-4. An all-solid-state ion-selective-electrode (ASSISE) is especially attractive for the aforementioned applications. The electrode should have the following favorable characteristics: easy construction, low maintenance, and (potential for) miniaturization, allowing for batch processing. A microfabricated ASSISE intended for quantifying H+, Ca2+, and CO32- ions was constructed. It consists of a noble-metal electrode layer (i.e. Pt), a transduction layer, and an ion-selective membrane (ISM) layer. The transduction layer functions to transduce the concentration-dependent chemical potential of the ion-selective membrane into a measurable electrical signal.

The lifetime of an ASSISE is found to depend on maintaining the potential at the conductive layer/membrane interface 5-7. To extend the ASSISE working lifetime and thereby maintain stable potentials at the interfacial layers, we utilized the conductive polymer (CP) poly(3,4-ethylenedioxythiophene) (PEDOT) 7-9 in place of silver/silver chloride (Ag/AgCl) as the transducer layer. We constructed the ASSISE in a lab-on-a-chip format, which we called the multi-analyte biochip (MAB) (Figure 1).

Calibrations in test solutions demonstrated that the MAB can monitor pH (operational range pH 4-9), CO32- (measured range 0.01 mM - 1 mM), and Ca2+ (log-linear range 0.01 mM to 1 mM). The MAB for pH provides a near-Nernstian slope response after almost one month storage in algal medium. The carbonate biochips show a potentiometric profile similar to that of a conventional ion-selective electrode. Physiological measurements were employed to monitor biological activity of the model system, the microalga Chlorella vulgaris.

The MAB conveys an advantage in size, versatility, and multiplexed analyte sensing capability, making it applicable to many confined monitoring situations, on Earth or in space.

Biochip Design and Experimental Methods

The biochip is 10 x 11 mm in dimension and has 9 ASSISEs designated as working electrodes (WEs) and 5 Ag/AgCl reference electrodes (REs). Each working electrode (WE) is 240 μm in diameter and is equally spaced at 1.4 mm from the REs, which are 480 μm in diameter. These electrodes are connected to electrical contact pads with a dimension of 0.5 mm x 0.5 mm. The schematic is shown in Figure 2.

Cyclic voltammetry (CV) and galvanostatic deposition methods are used to electropolymerize the PEDOT films using a Bioanalytical Systems Inc. (BASI) C3 cell stand (Figure 3). The counter-ion for the PEDOT film is tailored to suit the analyte ion of interest. A PEDOT with poly(styrenesulfonate) counter ion (PEDOT/PSS) is utilized for H+ and CO32-, while one with sulphate (added to the solution as CaSO4) is utilized for Ca2+. The electrochemical properties of the PEDOT-coated WE is analyzed using CVs in redox-active solution (i.e. 2 mM potassium ferricyanide (K3Fe(CN)6)). Based on the CV profile, Randles-Sevcik analysis was used to determine the effective surface area 10. Spin-coating at 1,500 rpm is used to cast ~2 μm thick ion-selective membranes (ISMs) on the MAB working electrodes (WEs).

The MAB is contained in a microfluidic flow-cell chamber filled with a 150 μl volume of algal medium; the contact pads are electrically connected to the BASI system (Figure 4). The photosynthetic activity of Chlorella vulgaris is monitored in ambient light and dark conditions.

1. Preparation of Poly(3,4-ethylenedioxythiophene):Poly(sodium 4-styrenesulfonate) (PEDOT:PSS) Electropolymerization Solution for H+ and CO32- Ions

  1. Add 70 mg poly(sodium 4-styrenesulfonate) (Na+PSS-) to 10 ml deionized (DI) water and vortex until completely dispersed (approx. 10 sec).
  2. Add 10.7 μl 3,4-ethlyenedioxythiophene (EDOT) to the solution in 1.1 and vortex until solution is completely mixed.

2. Preparat.......

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An example of a cyclic voltammogram (CV) result of PEDOT:PSS and its corresponding cathodic peak current (ip) vs. the scan rate (v1/2) are shown in Figures 5a and 5b respectively. PEDOT:CaSO4 at various scan rates and its cathodic peak current are not shown. Using Randles-Sevcik analysis 10, the effective surface areas of the solid contact PEDOT:PSS and PEDOT:CaSO4 without ion-selective membrane were found to be 4.4.......

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The MAB biochip consists of ASSISEs that are constructed from an ISM atop a PEDOT-based CP conjugate transduction layer on a Pt electrode, the combination of which transduces the ionic concentration of interest to a measurable electrical signal. A stable electrode potential is defined by both the CP layer and the ISM layer. Both layers also determine the working lifetime of the MAB and the quality (noise, drift) of the measured electrical signal.

PEDOT is especially attractive as a transductio.......

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We would like to thank NASA Astrobiology Science and Technology Instrument Development (ASTID) Program for funding support (grant numbers 103498 and 103692), Gale Lockwood of the Birck Nantechnology Center at Purdue University for wirebonding of the MAB devices, and Joon Hyeong Park for the CAD drawing of the flow-cell chamber.


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Name Company Catalog Number Comments
Name of the items Company Catalog number Comments
3,4-Ethylenedioxythiophene Sigma-Aldrich 483028
Poly(sodium 4-styrenesulfonate) Sigma-Aldrich 243051
EC epsilon galvanostat/potentiostat Bioanalytical Systems Inc. e2P
Saturated Ag/AgCl reference electrode Bioanalytical Systems Inc. MF-2052
Pt gauze Alfa Aesar 10283
Potassium ferricyanide Sigma-Aldrich P-8131
Potassium nitrate J.T. Baker 3190-01
Sodium bicarbonate Mallinckrodt/ Macron 7412-12
Sodium carbonate Sigma-Aldrich S-7127
Calcium chloride J.T. Baker 1311-01
Potassium chloride Sigma-Aldrich P9541
Calcium sulphate Sigma-Aldrich 237132
C3 cell stand Bioanalytical Systems Inc. EF-1085
Flow-cell chip holder Custom, courtesy of NASA Ames
Flow-cell electrical fixture Custom, courtesy of NASA Ames
Table 2. Specific reagents and equipment.

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