CNT–DNA metamaterial microwave biosensor for dual-parameter label-free DNA sensing: A proof-of-concept study

Label-free nucleic acid sensing with high sensitivity and compatibility with electronic compatibility remains a major challenge in biosensor development. This work presents a proof-of-concept microwave biosensing platform based on a complementary split-ring resonator (CSRR)-integrated metamaterial patch antenna functionalized with a single-walled carbon nanotube–single-stranded DNA (SWCNT–ssDNA) nanocomposite. The study investigates the electromagnetic sensing mechanism by correlating changes in the dielectric properties of the CNT–DNA nanocomposite with resonance-frequency and phase variations. A metamaterial patch antenna incorporating three CSRRs into the radiating patch and exhibiting X- and Ku-band resonances was fabricated, with the Ku-band resonance selected for DNA sensing. SWCNT–ssDNA nanocomposite films with different DNA concentrations were selectively deposited over the high-electric-field CSRR region to maximize electromagnetic interaction. DNA sensing was performed by simultaneously monitoring the resonance frequency and phase of the reflection coefficient, enabling dual-parameter interrogation. Compared with the CNT-coated reference device, the CNT–DNA-functionalized biosensor exhibited maximum frequency and phase shifts of approximately 1 GHz and 126°, respectively. Within the experimentally investigated DNA concentration range of 50–66.7 nM (1.98–2.64 ng/mL), frequency and phase variations of approximately 540 MHz and 33° were observed. A theoretical detection limit of 0.6 pM (24 pg/mL) was estimated from the measured sensitivity and standard deviation of repeated Vector Network Analyzer measurements using the 3σ criterion. TEM and SEM analyses confirmed DNA incorporation within the CNT nanocomposite. These results demonstrate the feasibility of CSRR-confined near-field microwave sensing and provide a foundation for future sequence-specific DNA recognition and hybridization assays.