First Steps Towards a Joint Measurement System for Light-Addressable Potentiometric Sensors and Light-Addressable Electrodes
Authors: Welden Rene , Schneider Benno , Breuer Lars , Iken Heiko , Schubert Jürgen , Schoening Michael J. , Wagner Patrick , Wagner Torsten 
Published: May 2020

Introduction

A rapid diagnostic of the patients’ disease is essential for the success of the therapy. To support the doctor, lab-on-a-chip devices could be used. These devices contain different components, which one would primarily find in an analytical laboratory. Typically, sensors, liquid-handling devices or actuators are integrated in a microfluidic channel to allow a medical analysis on a single chip. Through the miniaturization of the single components, only a small volume e.g., of the patients’ blood, is needed and a first diagnosis can be done on-site.
In the past research, mostly rigid sensor and actuator structures such as electrodes are integrated in lab-on-a-chip devices. For their implementation, photolithographic fabrication processes are necessary. On the contrary, devices based on semiconductor structures can be spatially and temporally resolved by light, whereby the sophisticated structuring process is no longer required. For sensing, light-addressable potentiometric sensors (LAPS) can detect different target species in the analyte [1,2], while with light-addressable electrodes (LAE), photoelectrochemical reactions can be triggered [3,4].
In this work, the joint implementation of these two technologies integrated in a microfluidic channel will be presented for the first time, as depicted in figure 1.

Methods and Materials

LAPS are based on a semiconductor-insulator-transducer sandwich structure. An applied d.c. bias voltage will induce a space-charge region at the semiconductor-insulator interface. By illumination with an intensity-modulated light source from the rear-side, electron-hole pairs are generated leading to a detectable a.c. photocurrent. Local variations of the analyte concentration at the transducer surface will influence the width of the space-charge region and therewith the photocurrent. Therefore, moving the light source in a scanning like-manner, a spatially resolved mapping of the photocurrent, which is proportional to the analyte concentration, can be recorded.
To fabricate the Al/p-Si/SiO2/Ta2O5 field-effect structure of the LAPS chip, SiO2 was grown by a thermal dry oxidation step on a p-doped silicon wafer. Subsequently, Ta was deposited by electron-beam evaporation followed by an oxidation step to achieve the Ta2O5 layer. In a final step, an Al ohmic rear-side contact was prepared and an illumination window was etched inside the Al layer for rear-side illumination.
The LAE can be used for the spatially resolved manipulation inside the microfluidic channel. Semiconductors can be applied as electrode materials where charge carriers are generated upon illumination. In contrast to LAPS, LAEs have no insulating layer, which enables a direct charge transfer with the analyte and triggering of surface reactions. By spatially resolved illumination, a localized photocurrent will occur at the illuminated spot, while the remaining electrode is still isolated.
A titanium dioxide layer was fabricated by means of pulsed laser deposition on a SnO2:F (FTO) glass substrate. The TiO2 target was vaporized by a KrF excimer laser (wavelength = 248 nm) with a repetition rate of 10 Hz at a pressure of 2x102 hPa O2. During deposition, the FTO glass substrate has been heated up to 400 °C.

Results and Conclusions

A possible strategy for the implementation of both technologies into a microfluidic channel was introduced. Furthermore, as a proof-of-concept, by local illumination, the pH value inside a microchannel was changed photoelectrocatalytically by the LAE. Simultaneously, a chemical image of the localized pH shift has been recorded by the LAPS. Since both systems can be addressed by light, dynamic manipulation and sensing is possible, which enables an easy adjustment of both technologies at run time. A detailed description of the experiment and the obtained results will be presented and discussed.
Figure 1: Schematic illustration of the combination of a light-addressable electrode and a light-addressable potentiometric sensor. The pH gradient, generated by the LAE can be measured by the LAPS.

Acknowledgements

This work was supported by the German Federal Ministry of Education and Research (BMBF) within “NanoMatFutur“ (13N12585).

References

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[3] R. Welden, S. Scheja, M.J. Schöning, P. Wagner, T. Wagner, Electrochemical evaluation of light‐addressable electrodes based on TiO2 for the integration in lab‐on‐chip systems, physica status solidi (a). 215(15) (2018) 1800150. doi.org/10.1002/pssa.201800150.
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Figure 1