Biological imaging using light-addressable potentiometric sensors and scanning photo-induced impedance microscopy

Abstract

Light-addressable potentiometric sensors (LAPS) and scanning photo-induced impedance microscopy (SPIM) use photocurrent measurements at electrolyte-insulator-semiconductor substrates for spatio-temporal imaging of electrical potentials and impedance. The techniques have been used for the interrogation of sensor arrays and the imaging of biological systems. Sensor applications range from the detection of different types of ions and the label-free detection of charged molecules such as DNA and proteins to enzyme-based biosensors. Imaging applications include the temporal imaging of extracellular potentials and dynamic concentration changes in microfluidic channels and the lateral imaging of cell surface charges and cell metabolism. This paper will investigate the current state of the art of the measurement technology with a focus on spatial and temporal resolution and review the biological applications, these techniques have been used for. An outlook on future developments in the field will be given.

Keywords: biosensors; electrochemical imaging; impedance; ion concentrations; potentiometric sensor.

Figure 1.

(a) Basic set-up for LAPS and SPIM measurements; (b) equivalent circuit for photocurrent measurements: i_P is the photocurrent generated in the space charge layer, C_SC is the DC voltage-dependent capacitance of the space charge layer and i_Photo the current measured in the external circuit; (c) LAPS measure changes in the surface potential (shift from curve A to B) by monitoring the shift of the I–V curve at a constant current or the change in the current at a constant voltage. SPIM monitors changes in the maximum photocurrent (shift from curve B to C), which are directly related to the local impedance of the illuminated part of the sensor chip. (Online version in colour.)

Figure 2.

(a) Photocurrent image of an SU-8 pattern on an SOS substrate with a 0.5 µm thick silicon layer and an anodic oxide and (b) photocurrent line scans across the edge of the photoresist at different wavelengths. Reprinted with permission from Chen et al. [11] (Copyright © 2010 Elsevier). (Online version in colour.)

Figure 3.

Experimental set-up for high-resolution LAPS and SPIM measurements. V_AC, AC voltage; RE, reference electrode; CE, counter electrode; WE, working electrode; SAM, self-assembled monolayer. Reprinted with permission from Wang et al. [9] (Copyright © 2016 Elsevier). (Online version in colour.)

Figure 4.

High-speed laser scanning set-up for LAPS using a two-axis analogue micromirror [31]. PDMS, polydimethylsiloxane; DAQ, data acquisition. (Online version in colour.)

Figure 5.

Optical fibres were mounted above the LAPS substrate in a microfluidic channel for dynamic monitoring of pH changes. Reprinted with permission from Miyamoto et al. [42] (Copyright © 2014 Elsevier). (Online version in colour.)

Figure 6.

The sandwich ELISA model used in LAPS. F, fluorescein; b, biotin. (Online version in colour.)

Figure 7.

A pH image of colonies of yeast. Each dark region corresponds to a colony. Reprinted with permission from Nakao et al. [1] (Copyright © 1994 Elsevier).

Figure 8.

Microcapsules attached on a COOH-terminated SOS substrate. (a) SPIM image measured at 0.9 V, (b) corresponding optical image. Reprinted with permission from Wang et al. [9] (Copyright © 2016 Elsevier). (Online version in colour.)

Figure 9.

Visualization of buffering action at 100 fps. (a) Injection of 10 mM NaOH solution into phosphate buffer and (b) injection of 10 mM HCl solution into phosphate buffer. Reprinted with permission from Miyamoto et al. [42] (Copyright © 2014 Elsevier). (Online version in colour.)