Silicone-Rubber Based Multiplex Ion-Sensitive Light-Addressable Potentiometric Sensor (ISLAPS) System for Physiological Ions Detection

Introduction

Light-addressable potentiometric sensor (LAPS) is a field-effect chemical sensor with the ability of spatial resolution proposed by Hafeman [1] and has an electrolyte-insulator-semiconductor (EIS) structure. With the illumination of focused light, photocurrent can be generated locally, providing the information of surface potential. As a member of potentiometric sensors, technologies for ion-selective electrodes (ISEs) and ion-sensitive field-effect transistor (ISFET) are also applicable to LAPS. For example, by depositing the ion-sensitive membrane (ISM) on the surface of the LAPS, an ion-sensitive LAPS (ISLAPS) can be obtained. Conventional ISEs which contains liquid contacts have some limitations such as requiring maintenance, proper handling, leakage of inner filling solution and primary ion diffusion, which will deteriorate limit of detection. In this work, a multiplex ISLAPS detection system has been proposed combined with Na⁺, K⁺, and Ca²⁺ all-solid-state ISM and a conventional pH-LAPS. The matrix of the plasticizer free ISM is the silicone-rubber, which has not only the same sensitivity of PVC-based ISM, but also better adhesion and longer lifetime. With the help of a program-controlled two-axis translation stage, the detection sites of the sensor were sequentially illuminated by modulated light from the backside. Different from the conventional multi-channel potential sensors, LAPS system essentially uses a single-channel instrument to realize multi-parameter detection, and can be expanded easily by adding other ISMs only. The multiplex ISLAPS can meet the ion detection requirements, and it is a promising physiology detection platform.

Method

Figure 1a shows the structure of LAPS chip, the fabrication is similar to the previous report [2], except the thinned illumination area in the center of backside. The sensing material on the oxide layer is 30 nm Al₂O₃ layer deposited by atomic layer deposition (ALD) [3] for pH detection, and silicone-rubber ISMs for Na⁺, K⁺, and Ca²⁺ detection. The chip was cut into 1 cm×1 cm with a 4 mm×4 mm thinned area, in turn washed by acetone, ethanol and deionized water to store and use.

Preparation of the ISM is as follows: Firstly, about 300mg silicone-rubber (RTV 730) was evenly dissolved in 1.5mL THF and centrifuged. Then the supernatant was mixed evenly with the ionophores and ion additives in 30min ultrasonic bath. Before deposition of ISM, a P3OT layer was applied on the surface of SiO₂-LAPS chip. After that the ISM mixture was spin-coated on the P3OT layer and dried overnight at room temperature. The three ISM-coated LAPS chips and a Al₂O₃-LAPS chip were placed in a PMMA chamber, pasted with conductive silver glue on one bonding pad of PCB, with four holes to expose the thinned area of each site. The edges of chips were encapsulated by epoxy adhesive.

The schematic of the multiplex ISLAPS system is shown in Figure 1b. The modulated light with collimator was fixed on the translation stage, illuminating the thinned area of sensor. The data acquisition (DAQ) device performed bias voltage output, signal acquisition, and was controlled by LabVIEW software together with the translation stage. The ISMs are sequentially illuminated and the response of four ions can be obtained in one measurement.

Results and Conclusions

Sensitivities of the multiplex ISLAPS were calibrated with series of concentration gradient solutions. The background electrolyte for Na⁺, K⁺ and Ca²⁺ is 0.1M CH₃COOLi, and PBS solutions adjusted with HCl/NaCl were used in pH sensitivity determination. The ISMs were conditioned in the solutions of 10^-2 M corresponding ions for 1h. The I-V curves and the bias-concentration fit lines are shown in Figure 1c. The sensitivity was obtained from the maximum slope points of I-V curves. The limits of detection (LODs) of the Na⁺, K⁺ and Ca²⁺ were about 10^-6M, and the sensitivities were 57.37mV/pH, 56.9mV/pNa, 58.4mV/pK, and 25.3mV/pCa respectively, close to the Nernst theoretical value. The LOD and linear ranges can meet the requirements of physiological ions detection.

The real DMEM samples were tested with standard addition method. The light spot moved with the stage and stayed at the center of each site for 10s. The responses of standard and spiked samples are shown in Figure 1d and results are listed in Table 1, indicating that the multiplex ISLAPS can be applied in physiological ions detection.

Table 1 The results of DMEM samples detection

In summary, we have firstly combined silicone-rubber ISM with LAPS and proposed a multiplex ISLAPS system for Na⁺, K⁺, Ca²⁺ and H⁺ with good performance. The ISM-coated chips are packaged in one detection channel, and the response of multiple ions can be recorded sequentially. It seems to be a promising physiology detection platform.

References

[1] Hafeman D G, Parce J W, McConnell H M. Light-addressable potentiometric sensor for biochemical systems[J]. Science, 1988, 240(4856): 1182-1185.

[2] Liang T, Gu C, Gan Y, et al. Microfluidic chip system integrated with light addressable potentiometric sensor (LAPS) for real-time extracellular acidification detection[J]. Sensors and Actuators B: Chemical, 2019, 301: 127004.

[3] Ismail A B M, Harada T, Yoshinobu T, et al. Investigation of pulsed laser-deposited Al2O3 as a high pH-sensitive layer for LAPS-based biosensing applications[J]. Sensors and Actuators B: Chemical, 2000, 71(3): 169-172.

Figure 1