Label-Free Single-Molecule Electrical Sensor for Ultrasensitive and Selective Detection of Iodide Ions in Human Urine

Herein, a label-free single-molecule electrical sensor was first proposed for the ultrasensitive and selective detection of iodide ions in human urine. Single-molecule conductance measurements in different halogen ion solutions via scanning tunneling microscopy break junction (STM-BJ) clearly revealed that I^- ions strongly affect the stability and displacement distance (Δz) distribution of molecular junctions. Theoretical calculations prove that the specific adsorption of I^- ions modifies the surface properties and weakens the molecular adsorption. Furthermore, the average conductance peak area versus the logarithm of the I^- ion concentration has a very good linear relationship in the range of 5 × 10^-6 to 5 × 10^-10 M, with a correlation coefficient of 0.99. This quantitative analysis remains valid in the presence of interfering ions of SO₄^2-, ClO₄^-, Br^-, and Cl^- as well as interfering molecules of ascorbic acid, uric acid, dopamine, and cysteine. A cross-comparison of the human urine detection results of this single-molecule electrical sensor with those of the clinical method of As³⁺-Ce⁴⁺ catalytic spectrophotometry revealed an average difference of 0.9%, which decreased the detection time of 2 h with the traditional method to approximately 15 min. This work proves the promising practical potential of the single-molecule electrical technique for relevant clinical analysis.