  Electrical detection of messenger ribonucleic acid (mRNA) is a promising
approach to enhancing transcriptomics and disease diagnostics because of its
sensitivity, rapidity, and modularity. Reported here is a fast SARS-CoV-2 mRNA
biosensor (<1 minute) with a limit of detection of 1 aM, and dynamic range of 4
orders of magnitude and a linear sensitivity of 22 mV per molar decade. These
figures of merit were obtained on photoresistlessly patterned monolayer
graphene field-effect transistors (FETs) derived from commercial four-inch
graphene on 90 nm of silicon dioxide on p-type silicon. Then, to facilitate
mRNA hybridization, graphene sensing mesa were coated with an ultrathin
sub-percolation threshold gold film for bonding 3'-thiolated single-stranded
deoxyribonucleic acid (ssDNA) probes complementary to SARS-CoV-2 nucleocapsid
phosphoprotein (N) gene. Sub-percolated gold was used to minimize the distance
between the graphene material and surface hybridization events. The
liquid-transfer characteristics of the graphene FETs repeatedly shows
correlation between the Dirac voltage and the copy number of polynucleotide.
Ultrathin percolated gold films on graphene FETs facilitate two-dimensional
electron gas (2DEG) mRNA biosensors for transcriptomic profiling.
