Graphene’s mechanical stability, high electrical conductivity, and compatibility with a diverse array of carbonaceous functionalization chemistries makes it an alluring host system for novel gas sensors. Now researchers have overcome some of the fabrication challenges in the way of using graphene in bio-mimicry of one of nature’s greatest sensors, a dog’s nose.
Dogs’ remarkable sense of smell has been well-known for centuries. Within a dog’s nose lies a rich mesoscopic structure of capillaries, packing an enormous olfactory sensing area into a small volume. Graphene nanoscrolls – nanosheets uniformly wound around themselves – have been proposed for use as a man-made analogue of this capillary structure. The challenge to this point has been the fabrication of graphene nanoscrolls that measure up to performance expectations.
Past attempts at making nanoscrolls started with graphene oxide in its un-reduced or partially reduced states. As a consequence, the nanoscrolls formed lacked the stellar electronic and surface chemistry properties of graphene. Another significant challenge was maintaining good uniformity of the scrolls formed.
Yao Wang, Lei Jiang, Guofu Zhou and colleagues found a method to overcome past challenges to nanoscroll fabrication. They used an aromatic polymer, poly(sodium p-styrenesulfonate) (PSS), which acts as a surfactant when dispersed into an aqueous solution of graphene oxide. Noncovalent interactions between π-orbitals on the polymer and the graphene oxide bind the surfactant to its target and reduce aggregation of graphene oxide nanosheets, allowing full reduction of their surface bonds.
The researchers poured the aqueous solution into a glass bottle, flash-froze it by immersing it in liquid nitrogen, and then loaded it into a “lyophilizer”- freeze drier – to sublimate the ice away. Throughout the sublimation process, the nanosheets begin scrolling gradually, becoming fully scrolled once dehydration is complete.
Examining the wrap-up
Characterizing the resultant nanoscrolls, the research team found excellent uniformity in the length, diameter, and straightness of their tubular structures. They also used molecular dynamics simulations and cryo-SEM during the lyophilization process to shed light on the scrolling behaviour. The flash-freezing procedure establishes a vertical temperature gradient within the glass bottle. Ice crystallizes along this gradient, and thus the embedded nanosheets freeze along this vertical orientation as the process rapidly occurs.
As the ice begins to sublimate, a thin strip of the nanosheet is exposed. Molecular dynamics simulations show that a preferential binding of PSS one side of the graphene oxide nanosheets gives rise to the curvature of the nanosheet during scroll formation, in part from the hydrophilic nature of the PSS layer.
The researchers were then able to create gas sensors by dispensing droplets of dispersed nanoscrolls onto interlacing silver-palladium electrodes. The sensors formed linear ohmic devices whose resistance was modulated by the presence of the target analyte, here NO2. Compared with similar graphene-based sensors, those produced by the new lyophilization approach exhibited an improved sensitivity as well as excellent selectivity for NO2.
Full details of the research are reported in ACS Nano 10.1021/acsnano.7b08294.