Water contamination by moldy odorants such as geosmin and 2-methylisoborneol compromises drinking water safety and acceptability globally. Existing detection methods predominantly rely on advanced laboratory instruments or human panels, which pose challenges for sensitivity, quantification, and operational complexity.
Insect olfactory systems are highly sensitive due to numerous olfactory receptor neurons (ORNs) that express odorant receptors coupled with a co-receptor (Orco) and can detect trace odorant molecules. Unlike mammalian ORs, insect ORs are ligand-gated ion channels, facilitating ion influx upon odorant binding.
This biological mechanism enables novel biosensor designs that use insect ORs to detect odorants via ionic or fluorescent signals. Previous approaches employing biological components faced limitations, including limited mobility, which this study aims to address by developing a portable cell-based sensor system.
Engineering Sensor Cells and Device Assembly
To build the biosensor, the authors engineered Sf21 insect cell lines to co-express the geosmin-specific odorant receptor Or56a, its co-receptor Orco, and the calcium-sensitive fluorescent protein GCaMP6s. Both wild-type and codon-optimized Or56a genes were transfected into Sf21 cells, followed by clone selection based on fluorescence response magnitude to geosmin exposure.
The best-performing clones exhibited robust, homogeneous increases in fluorescence upon odorant binding, indicating successful receptor expression and signaling. For a portable system, sensor cells were immobilized on glass surfaces within cartridges using an oleyl-PEG-NHS adhesion protocol, thereby enabling consistent, stable fluorescent measurements. A handheld Quantus™ Fluorometer measured fluorescence changes upon sample addition, and the performance was validated with pure geosmin solutions and environmental samples.
Geosmin detection specificity was assessed by comparing responses to 2-methylisoborneol (2-MIB) and other control odorants, establishing selectivity. Dose-response curves were generated to determine detection limits and sensitivity. Field applicability was tested by analyzing water samples from a dam reservoir onsite, with sensor responses compared to parallel GC/MS measurements conducted in a water quality laboratory.
Performance and Field Application
The sensor cells expressing codon-optimized Or56a exhibited enhanced fluorescence responses upon geosmin exposure, with selected clones showing increases in fluorescence intensity exceeding 30%. In contrast, controls and unselected cells exhibited minimal responses, verifying the importance of receptor optimization and screening. Immobilized sensor cell cartridges maintained operational stability across batches, with fluorescence changes averaging around 25% in response to geosmin and showing little variation, confirming reproducibility.
Selectivity tests revealed the sensor cells specifically responded to geosmin but not to 2-MIB or other tested moldy odorants, demonstrating high target specificity. Dose-dependent measurements indicated a detection limit of 100 pM (18.2 parts per trillion), comparable to the sensitivity of traditional GC/MS methods but achieved without complex pre-concentration steps. This sensitivity aligns with typical environmental geosmin concentrations in water reservoirs, making the sensor well-suited for practical applications.
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On-site measurements at a dam reservoir confirmed the sensor cartridge’s capability to detect natural geosmin levels in untreated surface water. Fluorescence intensity increases correlated with GC/MS quantification, supporting the system’s reliability in field conditions. Although environmental samples sometimes exhibited slightly reduced fluorescence responses due to ionic composition variations or background geosmin habituation, calibration adjustments effectively corrected these effects.
The portable system, consisting of the sensor cartridge and handheld fluorometer, is compact (weighing approximately 2 kg) and enables rapid detection within minutes. This contrasts starkly with GC/MS systems requiring laboratory infrastructure, specialized staff, and lengthy analysis times.
The sensor platform’s modularity facilitates adaptation to various odorants by exchanging target ORs, given the diversity of insect olfactory receptors. The authors note that further integration of multi-OR sensor arrays and dual-fluorescence channels could enhance performance through pattern-recognition approaches.
While the Sf21 cell line proved effective for receptor expression, exploring alternative cell hosts, such as Drosophila-derived cell lines, may improve expression efficiency. Environmental factors, such as water ion concentrations, were observed to affect fluorescence output, underscoring the need for calibration protocols when analyzing diverse sample types.
Potential and Future Directions
This work successfully developed a novel portable biosensor system employing insect odorant receptor-expressing Sf21 cells immobilized on glass cartridges for selective geosmin detection. When coupled with a handheld fluorometer, the device detects geosmin concentrations as low as 100 pM without requiring pre-concentration.
The sensor demonstrated high specificity, operational stability, and field applicability at a dam reservoir, matching traditional GC/MS sensitivity while enabling rapid, on-site measurements. The platform’s design enables broad adaptability to other target odorants by substituting insect OR genes, offering versatile applications in water quality monitoring, food safety, and potentially disease diagnostics. This study highlights the practical advantages of leveraging insect olfactory mechanisms in cell-based biosensors and paves the way for accessible, sensitive odor-detection technologies outside laboratory settings.
Journal Reference
Mitsuno H., Araki S., et al. (2026). Portable geosmin detection system based on sensor cells expressing insect odorant receptors. Scientific Reports 16, 12577. DOI: 10.1038/s41598-026-41786-8, https://www.nature.com/articles/s41598-026-41786-8