We present the construction of layer-by-layer (LbL) assemblies of polyethylenimine and urease onto reduced-graphene-oxide based field-effect transistors (rGO FETs) for the detection of urea. This versatile biosensor platform simultaneously exploits the pH dependency of liquid-gated graphene-based transistors and the change in the local pH produced by the catalyzed hydrolysis of urea. The use of an interdigitated microchannel resulted in transistors displaying low noise, high pH sensitivity (20.3µA/pH) and transconductance values up to 800 µS. The modification of rGO FETs with a weak polyelectrolyte improved the pH response because of its transducing properties by electrostatic gating effects. In the presence of urea, the urease-modified rGO FETs showed a shift in the Dirac point due to the change in the local pH close to the graphene surface. Markedly, these devices operated at very low voltages (less than 500mV) and were able to monitor urea in the range of 1-1000µm, with a limit of detection (LOD) down to 1µm, fast response and good long-term stability. The urea-response of the transistors was enhanced by increasing the number of bilayers due to the increment of the enzyme surface coverage onto the channel. Moreover, quantification of the heavy metal Cu(with a LOD down to 10nM) was performed in aqueous solution by taking advantage of the urease specific inhibition.
Nanomaterial-based
FET sensors represent an attractive platform
for ultrasensitive, real-time, and label-free detection of chemical
and biological species. Nevertheless, because their response is screened
by mobile ions, it remains a challenge to use them to sense in physiological
ionic strength solutions. In this work, it is demonstrated, both experimentally
and theoretically, that polyelectrolyte multilayers are capable of
increasing the sensing range of graphene-based FETs. Potential shifts
at graphene surfaces and film thickness are recorded upon the construction
of PDADMAC/PSS polyelectrolyte multilayer (PEM) films. By correlation
of the potential shift with the film thickness, the electrostatic
screening length and the concentration of mobile ion inside the films
have been deduced. Across the polymer interface the Debye length is
increased more than 1 order of magnitude. The fundamentals of this
strategy are described by a conceptually simple thermodynamic model,
which accounts for the entropy loss of ion confinement and incorporates
the effect of ions finite volume. Interestingly, the electrostatic
screening inside the film strongly depends on the polymer density
and the ionic strength of the solution. Of particular interest in
physiological condition sensing, the PEM interfaces can extend the
Debye length from 0.8 to 10 nm.
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