Fig 1.
(a) Microelectrode configuration. (b) SEM image of fabricated planar microelectrode with a SU8 passivation layer. Inset in (b) shows an SEM image of the planar microelectrode overview without the SU8 passivation layer. The SEM images were taken at 70° tilted-view.
Fig 2.
The fabrication process of the planar microelectrode, pristine ZnO NW microelectrode and metal encapsulated ZnO NW microelectrodes.
(a) Si/SiO2 substrate preparation; (b) photolithography; (c) deposition of Cr/Au layer by thermal evaporation followed by lift-off, resulting in a planar microelectrode; (d) photolithography; (e) Sputter deposition of a ZnO seed layer followed by lift-off; (f) photolithography; (g) hydrothermal growth of ZnO nanowires; (h) lift-off to remove ZnO nanowire debris, resulting in a ZnO NW microelectrode; (i) photolithography; (j) deposition of a metallic encapsulation layer by thermal or e-beam evaporation followed by lift-off, resulting in a metal encapsulated ZnO NW microelectrode. The fabrication of all microelectrodes was completed by SU8 photolithography, as shown from the top-view in (k), which deposited a SU8 passivation layer to cover the entire substrate surface except the regions above the working microelectrodes and outer electrodes for electrical measurements.
Fig 3.
Electrochemical impedance spectroscopy (EIS) measurement configuration.
Two-point measurement configuration was applied for measuing EIS of each individual microelectrode. Hpot and Hcur represent the high potential and current terminals of the impedance analyser instrument connected to a tungsten electrode probe placed in the phosphate-buffered saline solution (PBS-1x). Lpot and Lcur represent the low potential and current terminals of the instrument connected to another tungsten electrode probe attached to the outer electrode contact. The polydimethylsiloxane (PDMS) frame kept the PBS solution on the central working microelectrodes, excluding the outer electrode contacts.
Fig 4.
(a, b) SEM images of a ZnO NW microelectrode before and after photolithography of a SU8 passivation layer, respectively. Inset in (a) shows an SEM image of the ZnO NWs grown from the seed layer on the microelectrode. The SEM images were taken at 70° tilted-view.
Fig 5.
SEM image and energy-dispersive X-ray spectroscopy (EDS) mapping of a ZnO NW microelectrode.
(a) SEM image of the ZnO NW microelectrode enclosed with a SU8 passivation layer. (b)–(f) EDS mapping of the ZnO NW microelectrode shown in (a). The EDS mapping shows (b) silicon (Si) of the Si/SiO2 substrate; (c) gold (Au) of the underlying Cr/Au microelectrode; (d) zinc (Zn) of ZnO nanowires; (e) oxygen (O) of ZnO nanowires and the Si/SiO2 substrate; and (f) carbon (C) of the SU8 passivation layer.
Fig 6.
Electrochemical impedance spectroscopy (EIS) of planar microelectrodes, microelectrodes with ZnO seed layers and with pristine ZnO NWs.
(a, b) Impedance magnitude and (c, d) impedance phase are plotted vs frequency with dashed lines indicating the frequency of 1 kHz. (b) and (d) show the electrochemical impedance magnitude and phase, respectively, for the frequency range of 40 Hz to 2 kHz in linear scale. Error shades represent one standard deviation (N ≥ 20 electrodes).
Fig 7.
SEM images of (a) pristine ZnO NWs, (b) Cr/Au-ZnO NWs, (c) Pt-ZnO NWs, and (d) Ti-ZnO NWs. The SEM images were taken from the top-view.
Fig 8.
Electrochemical impedance spectroscopy (EIS) of microelectrodes with metal encapsulated ZnO NWs.
(a, b) Impedance magnitude and (c, d) impedance phase are plotted vs frequency with dashed lines indicating the frequency of 1 kHz. (b) and (d) show the electrochemical impedance magnitude and phase, respectively for the frequency range of 40 Hz to 2 kHz in linear scale. Error shades represent one standard deviation (N ≥ 20 electrodes).
Fig 9.
Modified Randles equivalent circuit model of microelectrodes for electrochemical impedance modelling.
The equivalent circuit comprises a series resistor (Rs), charge transfer resistor (Rct), adsorption capacitor (Cad), and constant phase element (CPE). Zf represents the Faradaic impedance of the circuit as a series combination of Rct and Cad impedance.
Table 1.
Impedance parameters of the microelectrodes.
Parameters are measured by fitting the measured impedance (after the open/short circuit compensation) to the modified Randles equivalent circuit.