Engineered Phages for Fast, Antibody-Free SARS-CoV-2 Test

The platform is designed for rapid, label-free detection under low-voltage conditions and was evaluated in buffer and in complex matrices, including fetal bovine serum, pasteurized milk, and wastewater. The system is positioned as a laboratory-scale proof-of-concept and was tested using synthetic S1 protein rather than intact virus or clinical samples.

Get all the details: Download your PDF here! Antibody-based biosensors remain a gold standard in diagnostics, but they come with logistical constraints. Production can be costly and time-intensive, storage often requires cold-chain control, and batch-to-batch variability can complicate deployment, particularly in decentralized or resource-limited settings.

Aptamers offer a synthetic alternative but require extensive selection procedures and can lose structural stability in protein-rich environments. M13 bacteriophages – filamentous viruses that infect E. coli – provide a different strategy. Using phage display, researchers can genetically program these particles to present target-binding peptides on their coat proteins.

The resulting constructs are inexpensive to produce in bacterial hosts, genetically tunable, and robust to environmental fluctuations. In this study, the team engineered M13 phages to display a previously identified SARS-CoV-2 spike-binding peptide on the pIII coat protein. A scrambled peptide variant served as a specificity control.

Building The Phage-Graphene Sensor The sensing platform relies on reduced graphene oxide, a conductive nanomaterial known for its high surface area and electrical sensitivity. Graphene oxide was deposited onto glass substrates and thermally reduced to form rGO. The surface was then functionalized with a pyrene-based linker (PBASE), which binds to graphene via π-π stacking while presenting reactive NHS ester groups.

These esters form covalent bonds with primary amines on the phage coat proteins, immobilizing whole M13 particles onto the rGO surface. Because M13 presents multiple amine groups along its filamentous body (pVIII proteins), immobilization is non-directional, forming a dense phage layer across the electrode.

In this proof-of-concept configuration, residual NHS esters were not quenched, and no additional blocking step was introduced. The authors note that this may permit minor nonspecific adsorption and could be optimized in future iterations. Surface characterization using SEM, AFM, XRD, and EDS confirmed graphene reduction and stepwise functionalization.

Low-Voltage, Sub-Second Detection Rather than relying on traditional redox electrochemistry, the device operates through a chemiresistive mechanism. When the target S1 protein binds to the displayed peptide, it perturbs charge distribution at the graphene interface, producing a transient change in electrical current.

Measurements were performed at a fixed low bias of -0.8 mV, a voltage identified as optimal for maximizing signal-to-noise while minimizing nonspecific activation, joule heating, and faradaic side reactions. Each analyte addition generated a rapid current spike that peaked within approximately 300 milliseconds before returning to baseline.

The peak amplitude (ΔIpeak) served as the analytical readout. Detection Limit And Statistical Framework In buffer, the biosensor achieved an operational limit of detection of 10-4 pg/mL for the S1 protein. Detection was defined using a statistically conservative threshold calculated as the mean blank signal plus three standard deviations.