Laser-Fabricated APTES-Functionalized Interdigitated Electrodes with a Portable Readout: A Proof-of-Concept Impedimetric Response to Aqueous Nosema Spore Suspensions

Nosemosis, caused by the microsporidian parasites Nosema apis and Nosema ceranae, is a major contributor to honeybee (Apis mellifera) colony losses, yet its diagnosis still relies on destructive sampling followed by light microscopy or nucleic-acid amplification in a laboratory. We describe an impedimetric transducer intended as a first step toward a non-destructive alternative. Interdigitated gold electrodes were patterned on standard microscope slides by nanosecond laser ablation, a route that requires neither photolithography nor cleanroom processing, and were functionalized with (3-aminopropyl)triethoxysilane (APTES) deposited from either ethanol or toluene. Because the 0.01–50 MHz measurement window lies far above the range in which faradaic charge transfer contributes measurably, the response is non-faradaic and is governed by the permittivity and the ionic conductivity of the deposited droplet rather than by interfacial charge transfer. Films deposited from ethanol gave the largest and most reproducible contrast between an aqueous spore suspension (1500 spores/µL) and a deionized-water control. At the structural resonance of the electrode (≈16 MHz) the contrast in impedance magnitude is approximately 10% and is not resolved above the chip-to-chip scatter, which is expected because the magnitude at resonance is set by the series resistance of the electrode path rather than by the droplet. The two liquids are instead separated by the reactive component: the imaginary part of the impedance differs by 15.7 ± 1.8 MΩ near 6.8 MHz, about nine times the replicate uncertainty, and the phase by 33 ± 7° at 4.5 MHz. A self-contained readout built around an ESP32 microcontroller reproduced the direction of this change, its output voltage rising by approximately 33% after the suspension was applied. The scope of these observations is deliberately narrow: a single spore concentration in deionized water, no spore-free filtrate control, no calibration series, no selectivity panel and no measurement in a fecal matrix. The contribution is accordingly methodological—a cleanroom-free fabrication route, a quantified solvent dependence of the APTES film, and the transfer of the measurement from a benchtop spectrometer to a portable unit—rather than a validated diagnostic assay.