Label-free detection of heme in electroactive microbes using a probe-typed optical fiber SPR sensor.

Heme, as a key cytochrome ligand in electroactive microbes, enables sensitive label-free detection for studying extracellular electron transfer-mediated corrosion processes. Developing a reliable, label-free, and high-performance sensor for heme detection is crucial to investigate extracellular electron transfer-mediated microbial corrosion. Herein, we present a probe-typed fiber-optic surface plasmon resonance (SPR) heme sensor based on biomolecular recognition between heme and its transport regulator (FhtR) protein. When heme molecule contacts the sensor surface, FhtR can coordinate with the iron ion of heme and stably embed the hydrophobic porphyrin ring of heme into its hydrophobic pocket through van der Waals forces, thereby changing the refractive index of the sensing surface and causing a shift in the SPR spectrum. Therefore, the concentration of heme can be measured by monitoring the wavelength shift. Experimental results demonstrated that the sensitivity of the sensor was 698.1 nm/(mg/mL) in the range of 0-0.06 mg/mL heme, and the wavelength shift reached 48.95 nm in the range of 0-0.24 mg/mL heme, with a detection limit of 0.9 μg/mL and a response time of 18 min. Rapid determination of trace heme concentration was realized. In the real sample detection experiment of Shewanella oneidensis MR-1, it is verified that the sensor still has good recognition ability for samples diluted 1000 times. To explore the universality of the sensor, the real blood environment is used as the detection environment, which proves the unique innovation and applicability of the sensor in complex environments. In addition, the sensor exhibited excellent selectivity against different humoral molecules and metal ions. It also demonstrated excellent stability against the variation in pH (7.5-8.5), temperature (23.25-51.46 °C), and pressure (0.1-3.6 MPa).