APPENDIX
A mathematical method for extracting cell secretion rate from
affinity biosensors continuously monitoring cell activity
Yandong Gao*, Qing Zhou, Zimple Matharu, Ying Liu, Timothy Kwa, Alexander
Revzin*
Department of Biomedical Engineering, University of California, Davis, CA 95616, USA.
* Corresponding authors.
Address:
Yandong Gao and Alexander Revzin, Department of Biomedical Engineering, University
of California, Davis, CA 95616, USA
E-mail : ydgao@ucdavis.edu; arevzin@ucdavis.edu
Principle of Aptamer-based electrochemical sensor
The sensing strategy of aptamer-based electrochemical sensor for IFN-γ is shown in
Figure A1. It is based on binding of cytokine to the electrode surface which results in the
displacement of the redox reporter and the decrease in signal measured by square wave
voltammetry1-3. The hairpin aptamer molecules are thiolated at the 3’ end on gold
electrode. A redox label (MB) is linked at the 5’-end of each aptamer. In the absence of
target, the aptamer is thought to fold and the MB molecules are close to the electrode.
Upon binding of target molecule, the hairpin changes conformation and the MB molecule
moves further away from the electrode, altering electron transfer and decreasing the
observed reduction peak.
FIG A1. Schematic of aptamer-based electrochemical detection principle for IFN-γ. (a) The hairpin
aptamer molecules were immobilized on gold electrode. (b) Upon binding of IFN-γ, the hairpin changes
conformation and the redox label (MB) moves away from the electrode. (c) Voltammograms of the IFN-γ
electrochemical sensor. A signal decrease is observed upon binding of IFN-γ to aptamer on gold electrode.
The measured signal was defined as ‘signal suppression’: the ratio of SWV peak
current loss to the initial SWV peak current in the absence of target4,5. We assumed that
the immobilization of aptamer molecules is homogeneous and the binding and report
ability of each molecule are equal.
Therefore, the initial SWV peak has a linear
relationship to the total immobilized aptamer molecules and the ‘current loss’ reflects the
total amount of IFN-γ bound on the electrode. As the result, the ‘signal suppression’ was
written as Eq. (5) in the paper.
S=
P𝑐𝑢𝑟𝑟 ∫ 𝐵(𝑡)𝑑Ω
=
P𝑖𝑛𝑖𝑡
∫ 𝐴0 𝑑Ω
where Pcurr is the reduction peak measured at current time point and Pinit is reduction peak
measured at initial.
Reference
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Y. Liu, N. Tuleouva, E. Ramanculov, and A. Revzin, Analytical Chemistry 82, 8131 (2010).
5
Y. Liu, J. Yan, M. C. Howland, T. Kwa, and A. Revzin, Analytical Chemistry 83, 8286 (2011).