Dual-Signal Capillary Sensor Based on Viscosity Variation for Bacterial Endotoxin Detection

Endotoxin, a key pathogenic component of Gram-negative bacteria, represents a serious public health concern due to its ubiquity, exceptional thermal stability, and potent biological activity. Therefore, developing a reliable, rapid, and convenient method for endotoxin detection is critical. Herein, we report a distance-color dual-signal capillary sensor (DC-CapSensor) that detects endotoxin via viscosity-induced signal variations. The distance signal, generated through membrane-driven capillary flow, varies proportionally with the solution viscosity. For colorimetric readout, horseradish peroxidase (HRP) is doped into the viscous solution as a tracer, yielding a color signal via the HRP-catalyzed H₂O₂/TMB reaction. Changes in viscosity affect the residual HRP concentration within the capillary, thereby modulating the intensity of the colorimetric response. This dual-signal strategy enables reliable discrimination of viscous solutions with different viscosities or concentrations. The cascade reaction between Tachypleus Amebocyte Lysate (TAL) and endotoxin induces specific viscosity changes, enabling the DC-CapSensor to detect endotoxin effectively. The detection limits were 0.0407 EU/mL (distance signal) and 0.0614 EU/mL (colorimetric signal). Furthermore, the method was successfully applied to quantify endotoxin levels in commercial glucose and sodium chloride injections. This method provides high reliability, minimal reagent consumption, and operational simplicity, representing a promising strategy for practical endotoxin detection in analytical applications, especially for low-viscosity liquid samples with simple matrices.