Researchers have created high-resolution pH sensors made of heterostructures of graphene and thin carbon nanomembranes. The sensors have higher sensitivity than any conventional pH meter while also boasting high measurement speed. The graphene sensors are made with planar technology, which makes them compatible with miniaturization, allowing new applications of pH sensors. The research was published in Applied Physics Reviews, a high-impact scientific journal.
A team from several research centers in Germany, with participation from Graphenea, has made use of specific structural and electronic properties of graphene, such as high surface area-to-volume ratio, low electronic noise, and high mobility of charge carriers, to measure pH in the range of 2-12. To address the challenge of the well-known chemical inertness of pristine graphene sheets, the team functionalized the graphene with a molecular nanosheet carbon membrane (CNM). The CNM sheets are terminated with amino groups, which makes them sensitive to a pH variation, but the sheets themselves do not impair the electronic properties of graphene field effect transistors (GFET) which form the detection backbone of the devices. The high surface density of the amino groups (~3nm-2) makes for a strong device sensitivity.
The produced devices were electrically characterized to obtain the charge neutrality point (CNP) of graphene. Strong shifts of the CNP were observed upon changing pH of a solution flown across the surface of the device. To understand the results, the researchers made a theoretical model, comparing the effect of a changing pH on bare graphene, graphene with CNM, and the amino-functionalized graphene/CNM heterostructure. It was apparent that the largest variation in graphene doping due to a changing pH occurs in the latter case, when graphene is covered with an amino-functionalized CNM.The research also showed that these devices react to changes in pH within just several seconds, which has highly practical applications, including highly specific and sensitive micro-sized devices for medical diagnostics.