ZnO Nanostructure-Based Intracellular Sensor

Abstract

Recently ZnO has attracted much interest because of its usefulness for intracellular measurements of biochemical species by using its semiconducting, electrochemical, catalytic properties and for being biosafe and biocompatible. ZnO thus has a wide range of applications in optoelectronics, intracellular nanosensors, transducers, energy conversion and medical sciences. This review relates specifically to intracellular electrochemical (glucose and free metal ion) biosensors based on functionalized zinc oxide nanowires/nanorods. For intracellular measurements, the ZnO nanowires/nanorods were grown on the tip of a borosilicate glass capillary (0.7 µm in diameter) and functionalized with membranes or enzymes to produce intracellular selective metal ion or glucose sensors. Successful intracellular measurements were carried out using ZnO nanowires/nanorods grown on small tips for glucose and free metal ions using two types of cells, human fat cells and frog oocytes. The sensors in this study were used to detect real-time changes of metal ions and glucose across human fat cells and frog cells using changes in the electrochemical potential at the interface of the intracellular micro-environment. Such devices are helpful in explaining various intracellular processes involving ions and glucose.

Keywords: ZnO nanowire/nanorods, functionalization, intracellular measurement, glucose, metal ions, human fat cells, frog oocytes, electrochemical sensor.

Figure 1

Field emission scanning electron microscope images at different magnifications of the Ag-coated glass tip without (A) and with (B,C) grown ZnO nanorods [3,5].

Figure 2

Schematic experimental setup for the intracellular potentiometric measurements.

Figure 3

A calibration curve showing the electrochemical potential difference vs. the Ag/AgCl reference electrode in response to the glucose concentration using the functionalized ZnO nanorods as working electrode [3].

Figure 4

Shows potentiometric response vs. time as the concentrations of Ca²⁺ concentration is changed in the buffer surrounding the cell for the case where for partial insertion of the functionalized ZnO nanorods. The insert shows a typical the calibration curve of the present working electrode [5].

Figure 5

Schematic diagram showing the principle of measurements with different penetration depths during the experiment (a) the case with partial insertion of the functionalized ZnO nanorods and in (b) when all the functionalized ZnO nanorods are inserted inside the cell [5].

Figure 6

Scanning electron microscopy images showing the working electrode after intracellular measurements at two different magnifications.

Figure 7

A calibration curve showing the electrochemical potential difference between the Mg²⁺-selective ZnO nanorod-covered and the Ag/AgCl reference microelectrode vs. the Mg²⁺ concentration. Insets show images of human adipocytes and frog oocytes with arrows pointing at measured intracellular levels of Mg²⁺ for the respective cells [17].

Figure 8

The output response with (black and green) and without (red) interfering ions [17].

Figure 9

A calibration curve showing the electrochemical potential difference between the Na⁺-selective ZnO nanorod and the Ag/AgCl reference microelectrodes vs. the Na⁺ concentration [18].

Figure 10

Experimental setup for simultaneous test solution injection and potentiometric measurements, (A) Schematic illustration of the setup and (B) Photography of Xenopus oocyte penetrated by the reference electrode (left), measurement electrode (right), and injector (middle).

Figure 11

Representative K⁺ current recordings and the corresponding I(V) curves measured electrophysiological in Kv channel expressing Xenopus oocytes. (A) Shows data for control oocytes and (B–D) for oocytes injected with indicated test solution. The holding potential was set to −80 mV and test pulses ranging from −80 to + 50 mV. The current generated by stepping to 0 mV is marked in red in each recording [26].

Figure 12

Intracellular K⁺ concentrations in Kv channel-expressing Xenopusoocytes measured with electrophysiological and K⁺-selective microelectrode methods. The Field emission scanning electron microscopy images of the K⁺-selective microelectrode before (a,b) and after intracellular measurements (c). Data points are expressed as mean values for control oocytes and oocytes injected with 50 nL of indicated test solution (d). Error bars show SE. n = 3–5) [26].