A research team led by the University of California, San Diego, has demonstrated the first direct integration of an insect olfactory receptor with a graphene field-effect transistor (gFET), creating a label-free biosensor capable of selectively detecting small organic compounds (SOCs) with high sensitivity.
The work centers on MhOR5, an odorant receptor from Machilis hrabei, which was produced at scale for the first time. The researchers obtained the protein in a stable tetrameric form with approximately 80% purity, maintaining structural integrity for up to 3 months under frozen storage. This stability enabled consistent device functionalization and testing. To translate biological recognition into an electrical signal, the team fabricated wafer-scale graphene transistors and subjected 6,655 devices to electrical screening, with more than 80% passing quality control in each batch. Each chip contained an array of fifteen graphene channels (100 × 10 µm), along with coplanar gate and sensing electrodes to ensure stable liquid-gated operation.
The sensing mechanism relies on graphene’s extreme sensitivity to local charge perturbations. MhOR5 receptors were immobilized onto the graphene surface using a pyrene-based linker, positioning them close enough to the conducting channel that ligand binding events could modulate the local electrostatic environment. These interactions shift the Dirac voltage of the transistor, providing a measurable electrical readout.
The MhOR5-functionalized gFETs were evaluated against a panel of sixteen chemically diverse SOCs, including eugenol, DEET, hexanol, octanol, and sugars, across concentrations ranging from 200 nM to 10 mM. All tested analytes produced concentration-dependent responses under controlled buffer conditions. Binding analysis revealed a wide affinity range, with acetophenone showing the strongest interaction (logKD = 0.28) and sulcatone the weakest (logKD = 4.4).
Control experiments confirmed that the observed signals depended on the functional receptor. Devices with intact MhOR5 generated more than twice the response to eugenol compared to sensors with denatured protein, while protein-free graphene devices exhibited small shifts in the opposite direction. These results indicate that preserving the receptor’s folded structure plays a key role in signal generation.
Importantly, the platform retained some degree of molecular selectivity. The sensor was able to distinguish between eugenol and its structural isomer, isoeugenol, at 10 mM in buffer, despite the two compounds differing only by the position of a double bond. This level of discrimination is difficult to achieve with conventional chemical sensors.
However, the electrical response does not directly replicate biological receptor activity. For example, acetophenone produced strong responses both in the graphene device and in prior electrophysiological measurements, while eugenol - previously classified as a strong activator - yielded a more modest signal on graphene. This discrepancy reflects the different physical mechanisms involved: ion flow through membrane channels versus charge redistribution near a transistor surface.
The study also highlights practical limitations. While the sensors performed well in controlled buffer solutions, performance degraded in more complex environments. In tap water, DEET concentrations above 10 µM still produced measurable signals, but the response lost its clear concentration dependence due to interference from other dissolved species affecting the graphene surface.
Overall, the results establish a modular biosensing platform that combines the molecular recognition capabilities of insect olfactory receptors with the electrical sensitivity of graphene. At the same time, they underscore the challenges of interpreting signals in real-world conditions, where environmental factors can obscure the contribution of receptor-ligand interactions.
