Abstract: The Light-Addressable Potentiometric Sensors (LAPS) and Scanning Photo-induced Impedance Microscopy (SPIM) was developed for the image detection of multilayer yeast Saccharomyces cerevisiae. Low gelling temperature agarose gel was shown to be an effective immobilization reagent for yeast cells in LAPS measurements without background signal. The images were produced by the photocurrent differences between the area where yeast cells were attached and the 1, 8-nonadiyne modified silicon on sapphire (SOS) surface, which were induced by the surface negative charge and local impedance of yeast cells.

Keywords: Light-Addressable Potentiometric Sensors, cell image, Saccharomyces cerevisiae

Introduction

LAPS has been used successfully for the chemical imaging of specific ions, DNA detection, monitoring of enzymatic reactions, microorganisms, activities and impedance of cells.^[1] The resolution was about 10-100 µm generally using LED arrays as the light source. We optimized the setup by scanning a focused laser beam across the sample thereby exciting local photocurrents and achieved submicrometer resolution^[2], which we are now proposing to use for imaging living cells with high resolution. Here we report the LAPS image detection of multilayer yeast Saccharomyces cerevisiae.

Results and Discussion

Multiple yeast cells (4 × 10⁷cells/mL, 0.2 µL) were immobilized onto the 1, 8-nonadiyne modified SOS surface by low melting agarose gel. Pure agarose gel was also dropped onto the same sample surface as the control experiment (Figure 1(A)).

Figure 1 (B) showed the LAPS image of yeast-agarose gel on a 1, 8-nonadiyne modified SOS surface at 0.7 V using a 405 nm laser. As control experiment, the pure agarose gel did not show a difference in the LAPS signal with the 1, 8-nonadiyne modified SOS. The maximum photocurrents on the yeast attached area were lower than that on the blank monolayer surface and the agarose gel attached area, indicating an increase of the local impedance. After normalizing, the I-V curves on the yeast attached area shifted by +90 mV compared to the blank monolayer surface and the agarose gel attached area, corresponding to the negative charge of the yeast cells in pH 7.4 PBS solution.

Figure captions

Figure 1. Yeast-agarose gel on a 1,8-nonadiyne modified SOS surface; (A) Optical microscope image of (a) pure agarose gel and (b) yeast-agarose gel; (B) LAPS photocurrent image measured at 0.7 V; (C) I-V curves on yeast-agarose gel, blank surface, and agarose gel; (D) normalised I-V curves and potential shift.

Conclusions

We report the LAPS image of multilayers yeast cells. The signals of yeast cells came from the negative surface charge and the local impedance. In the future we hope to achieve single cell LAPS images.

References

[1] T. Yoshinobu, K. Miyamoto, T. Wagner, M. J. Schöning, Sensors and Actuators B: Chemical 2015, 207, 926-932.

[2] L. Chen, Y. Zhou, S. Jiang, J. Kunze, P. Schmuki, S. Krause, Electrochemistry Communications 2010, 12, 758-760.

Acknowledgements

The authors thank EU for the providing a Marie Skłodowska-Curie Individual Fellowship (H2020-MSCA-IF-2014-660489) for this work.