  This paper presents an electrolytically gated broadband microwave sensor
where atomically-thin graphene layers are integrated into coplanar waveguides
and coupled with microfluidic channels. The interaction between a solution
under test and the graphene surface causes material and concentration-specific
modifications of graphene's DC and AC conductivity. Moreover, wave propagation
in the waveguide is modified by the dielectric properties of materials in its
close proximity via the fringe field, resulting in a combined sensing mechanism
leading to an enhanced S-parameter response compared to metallic microwave
sensors. The possibility of further controlling the graphene conductivity via
an electrolytic gate enables a new, multi-dimensional approach merging chemical
field-effect sensing and microwave measurement methods. By controlling and
synchronizing frequency sweeps, electrochemical gating and liquid flow in the
microfluidic channel, we generate multidimensional datasets that enable a
thorough investigation of the solution under study. As proof of concept, we
functionalize the graphene surface in order to identify specific
single-stranded DNA sequences dispersed in phosphate buffered saline solution.
We achieve a limit of detection of ~1 attomole per litre for a perfect match
DNA strand and a sensitivity of ~3 dB/decade for sub-pM concentrations. These
results show that our devices represent a new and accurate metrological tool
for chemical and biological sensing.
