Single-walled carbon nanotube based pH sensors on a flexible parylene-c substrate

Abstract

In this paper, we present a suspended Single-Walled Carbon Nanotube (SWNT) based pH sensor utilizing a low temperature Dielectrophoretic (DEP) assembly process on a flexible parylene-C substrate. Parylene-C, a light weight, flexible and inert material, is compatible with many microfabrication processes. Furthermore, utilizing parylene-C as a flexible substrate, one can readily create a suspended microplatform utilizing an O2 plasma etch process. Suspended nanobridges have larger exposed surface areas and may potentially have enhanced sensitivity for sensing applications. Fabricating these structures on a thin (10 microm) parylene-C substrate allows their utilization as flexible devices or in wearable sensor applications. We have successfully assembled suspended SWNT nanobridges across a spacing of 4 microm. The electrical characterization results from the assembled SWNTs yield ohmic behavior with a measured two-terminal resistance of approximately 17Komega. Furthermore, the conductometric measurements of the SWNT sensors have demonstrated that corresponding to an increase in pH value, the resistance of SWNTs has decreased due to the OH- group that attached on to the wall of the SWNTs and changed the electrical properties of the SWNTs. These novel suspended nanostructures can be used as potential candidates in nanosensor applications.

Recent Progress in Electrochemical pH-Sensing Materials and Configurations for Biomedical Applications

“…Yang el al. provided an example of a time vs resistance plot at 4–10 pH range, specifically using SWNTs on parylene as the sensing membrane material . Linear trend between pH and resistance has also been observed for electrochemically functionalized polyaniline reduced graphene oxide as the sensing membrane material …”

“…105 It is important to note that resistance increases as pH increases due to the increase of OH − ions in the solution. 119 The H + and OH − interact with the material in the sensing layer, which contributes to a change in resistance that the semiconductor device analyzer measures. 128 Yang el al.…”

Electro-conductive porous scaffold with single-walled carbon nanotubes in wormlike micellar networks

“…In fact, individual SWCNTs and SWCNT networks have been used to fabricate pH sensors [1][2][3]15,17,[19][20][21]23]. SWCNT based pH sensors such as SWCNT-glass electrodes [19], polymer-SWCNT networks [20], and SWCNT nanobridges electrodes [21] showed increasing conductivity in the sensor as the pH value of the buffer solution increased. The complexity to isolate and interconnect individual SWCNTs in a device makes SWCNT network-based device more attractive.…”

“…SWCNT networks possess large surface areas, are simple to fabricate, and readily scalable for industrial applications. For example, encapsulating SWCNTs in a scaffold carrier (e.g., polymers or gels) facilitates the formation of SWCNT networks for pH and other sensing applications [1,2,15,17,[19][20][21]23]. However, most of the existing fabrication processes for SWCNT based pH sensors are tedious and costly, involve chemical procedures (i.e., chemical vapor deposition) and sophisticated equipment.…”

“…In an aqueous pH buffer solution, the electronic properties of SWCNTs have displayed considerable changes due to the amount of hydroxide ions (OH À ) present in the solution, implying that SWCNTs can be potentially integrated into a pH sensor to detect chemical or environmental changes [17][18][19][20][21][22][23][24]. In fact, individual SWCNTs and SWCNT networks have been used to fabricate pH sensors [1][2][3]15,17,[19][20][21]23].…”