Comprehensive Optical, Electrical and Humidity Sensing Properties of Bifidobacterium infantis 35624 Thin Films

In this study, we present a comprehensive investigation of the structural, optical, and electrical properties of Bifidobacterium longum subsp. longum 35624 (BB35) thin films, and demonstrate their application as a novel relative humidity sensor. UV-Visible spectroscopy revealed that BB35 exhibits two distinct optical absorption regions, corresponding to direct band gaps of 2.1 \pm 0.05 eV and 2.8 \pm 0.05 eV, as confirmed by Tauc plot analysis, establishing BB35 as a genuine wide-bandgap semiconductor material. Photoluminescence measurements under 280 nm excitation exhibited a broad emission spectrum, which was deconvoluted into four Gaussian peaks centered at 434 nm (2.86 eV), 499 nm (2.48 eV), 543 nm (2.3 eV), and 620 nm (2.0 eV), indicating the presence of multiple radiative recombination centers characteristic of semiconducting materials. Electrical characterization revealed dispersive charge transport with current decay following a power-law I \propto t^{-α} (α\approx 0.3), suggesting Poole-Frenkel conduction mechanism typically observed in disordered organic semiconductors. The relative humidity (RH) sensing performance of BB35 films was evaluated using gold interdigital electrodes across 15-90% RH range. The sensor exhibited reversible response with sensitivity increasing linearly from 0.85 to 4.80 as RH increased from 15% to 90%. The devices demonstrated excellent stability over two months with less than 5% degradation in baseline current. These results establish BB35 thin films as a promising eco-friendly semiconducting material for humidity sensing applications and open new avenues for integrating biological materials into electronic and optoelectronic devices.

💡 Research Summary

This paper presents a comprehensive study of thin‑film devices fabricated from the probiotic bacterium Bifidobacterium longum subsp. longum 35624 (referred to as BB35). The authors first prepared uniform ~500 nm films by spin‑coating a bacterial suspension onto gold interdigital electrodes patterned on a plexiglass substrate. Structural characterization confirmed a continuous, defect‑free coating suitable for electronic measurements.

Optical investigations employed UV‑Visible absorption spectroscopy and Tauc‑plot analysis. Two distinct absorption edges were identified, corresponding to direct band gaps of 2.1 ± 0.05 eV and 2.8 ± 0.05 eV. These values place BB35 in the category of wide‑bandgap semiconductors, comparable to many organic‑inorganic hybrid materials. Photoluminescence (PL) measurements under 280 nm excitation revealed a broad emission spectrum that could be deconvoluted into four Gaussian components centered at 434 nm (2.86 eV), 499 nm (2.48 eV), 543 nm (2.30 eV), and 620 nm (2.00 eV). The authors associate these peaks with known bacterial fluorophores such as flavins, tryptophan, and tyrosine, indicating multiple radiative recombination centers within the film.

Electrical characterization showed a highly dispersive charge transport behavior. Current–voltage (I‑V) sweeps displayed non‑Ohmic characteristics, and time‑dependent current decay followed a power‑law I ∝ t⁻ᵅ with α ≈ 0.3 over several orders of magnitude in time. This decay, together with frequency‑dependent admittance measurements (5 Hz–13 MHz), was modeled successfully using a Poole‑Frenkel hopping framework. Fitting yielded α = 0.28 ± 0.02, a dispersion constant M = 0.45, and a characteristic relaxation time τₜ = 2.3 × 10⁻⁴ s, values that are consistent with other disordered organic semiconductors.

The core application explored was relative humidity (RH) sensing. The BB35‑coated interdigital devices were exposed to controlled RH levels ranging from 15 % to 90 % by bubbling nitrogen through de‑ionized water. Upon exposure, the sensor current increased rapidly and reversibly, stabilizing within a few minutes. Sensitivity, defined as the normalized change in current per %RH, rose linearly from 0.85 at 15 % RH to 4.80 at 90 % RH. The devices demonstrated excellent repeatability over multiple humidity cycles and maintained baseline stability within 5 % over a two‑month aging test, indicating robust long‑term performance.

In discussion, the authors link the dual band‑gap structure and multiple PL emission centers to the complex polysaccharide and protein matrix of the bacterial cell wall, suggesting that water adsorption modulates trap occupancy and thus the Poole‑Frenkel conduction pathway. The observed linear humidity response is attributed to the hydrophilic exopolysaccharide (EPS) layer, which facilitates rapid water uptake and protonic conduction.

Overall, the study establishes BB35 thin films as a novel “bio‑semiconductor” that combines wide direct band gaps, intrinsic fluorescence, and dispersive charge transport. The material offers an eco‑friendly, low‑cost alternative to conventional inorganic or polymeric humidity sensors, with competitive sensitivity, fast response, and long‑term stability. The authors propose future work on electrode geometry optimization, temperature dependence, cross‑sensitivity to other gases, and integration into Internet‑of‑Things (IoT) platforms to fully exploit the potential of probiotic‑based electronic devices.