All-electronic
DNA biosensors based on graphene field-effect transistors
(GFETs) offer the prospect of simple and cost-effective diagnostics.
For GFET sensors based on complementary probe DNA, the sensitivity
is limited by the binding affinity of the target oligonucleotide,
in the nM range for 20 mer targets. We report a ∼20 000×
improvement in sensitivity through the use of engineered hairpin probe
DNA that allows for target recycling and hybridization chain reaction.
This enables detection of 21 mer target DNA at sub-fM concentration
and provides superior specificity against single-base mismatched oligomers.
The work is based on a scalable fabrication process for biosensor
arrays that is suitable for multiplexed detection. This approach overcomes
the binding-affinity-dependent sensitivity of nucleic acid biosensors
and offers a pathway toward multiplexed and label-free nucleic acid
testing with high accuracy and selectivity.
We developed a high-yield synthesis of highly crystalline bilayer graphene (BLG) with two preferential stacking modes using a Ni-Cu gradient alloy growth substrate. Previously reported approaches for BLG growth include flat growth substrates of Cu or Ni-Cu uniform alloys and "copper pocket" structures. Use of flat substrates has the advantage of being scalable, but the growth mechanism is either "surface limited" (for Cu) or carbon precipitation (for uniform Ni-Cu), which results in multicrystalline BLG grains. For copper pockets, growth proceeds through a carbon back-diffusion mechanism, which leads to the formation of highly crystalline BLG, but scaling of the copper pocket structure is expected to be difficult. Here we demonstrate a Ni-Cu gradient alloy that combines the advantages of these earlier methods: the substrate is flat, so easy to scale, while growth proceeds by a carbon back-diffusion mechanism leading to high-yield growth of BLG with high crystallinity. The BLG layer stacking was almost exclusively Bernal or twisted with an angle of 30°, consistent with first-principles calculations we conducted. Furthermore, we demonstrated scalable production of transistor arrays based crystalline Bernal-stacked BLG with a band gap that was tunable at room temperature.
We have developed a scalable fabrication process for the production of DNA biosensors based on gold nanoparticle-decorated graphene field effect transistors (AuNP-Gr-FETs), where monodisperse AuNPs are created through physical vapor deposition followed by thermal annealing. The FETs are created in a four-probe configuration, using an optimized bilayer photolithography process that yields chemically clean devices, as confirmed by XPS and AFM, with high carrier mobility (3590 ± 710 cm/V·s) and low unintended doping (Dirac voltages of 9.4 ± 2.7 V). The AuNP-Gr-FETs were readily functionalized with thiolated probe DNA to yield DNA biosensors with a detection limit of 1 nM and high specificity against noncomplementary DNA. Our work provides a pathway toward the scalable fabrication of high-performance AuNP-Gr-FET devices for label-free nucleic acid testing in a realistic clinical setting.
Lyme disease is an infectious disease caused by the Borrelia burgdorferi bacterium. Early diagnosis of Lyme disease could prevent patients from developing serious side effects such as chronic arthritis and permanent neurological disorders. Lyme disease diagnosis is currently held back by a lack of reliable tools that are sufficiently sensitive and specific to allow early stage detection. Here, we demonstrate all electronic nano-biosensors for multiplexed detection of antigens of B. burgdorferi at concentrations as low as 2 pg ml−1. The sensors are based on graphene field-effect transistors (GFETs) coupled with genetically engineered antibody fragments. Single-chain variable fragment (scFv) antibodies are used to obtain a closer proximity of the target-binding event to the graphene sensor surface and for higher immobilization density. When compared to GFET nano-biosensors that use the parental immunoglobulin G (IgG) antibodies, scFv GFET nano-biosensors achieve approximately a 4000 × improvement to the limit of detection. We also demonstrate multiplexed detection of B. burgdorferi antigens through site-specific immobilization of scFvs on GFET arrays, which can potentially reduce the false-positive diagnosis ratio of Lyme disease. This work offers a pathway towards point-of-care detection of Lyme disease at an early stage.
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