Label-Free Single-Molecule\nElectrical Sensor for Ultrasensitive\nand Selective Detection of Iodide Ions in Human Urine

Herein, a label-free single-molecule electrical sensor\nwas first\nproposed for the ultrasensitive and selective detection of iodide\nions in human urine. Single-molecule conductance measurements in different\nhalogen ion solutions via scanning tunneling microscopy break junction\n(STM-BJ) clearly revealed that I<sup>–</sup> ions strongly\naffect the stability and displacement distance (Δ<i>z</i>) distribution of molecular junctions. Theoretical calculations prove\nthat the specific adsorption of I<sup>–</sup> ions modifies\nthe surface properties and weakens the molecular adsorption. Furthermore,\nthe average conductance peak area versus the logarithm of the I<sup>–</sup> ion concentration has a very good linear relationship\nin the range of 5 × 10<sup>–6</sup> to 5 × 10<sup>–10</sup> M, with a correlation coefficient of 0.99. This\nquantitative analysis remains valid in the presence of interfering\nions of SO<sub>4</sub><sup>2–</sup>, ClO<sub>4</sub><sup>–</sup>, Br<sup>–</sup>, and Cl<sup>–</sup> as well as interfering\nmolecules of ascorbic acid, uric acid, dopamine, and cysteine. A cross-comparison\nof the human urine detection results of this single-molecule electrical\nsensor with those of the clinical method of As<sup>3+</sup>-Ce<sup>4+</sup> catalytic spectrophotometry revealed an average difference\nof 0.9%, which decreased the detection time of 2 h with the traditional\nmethod to approximately 15 min. This work proves the promising practical\npotential of the single-molecule electrical technique for relevant\nclinical analysis.