A potassium ion sensor with an acupuncture needle inserted into a glass capillary for small-volume sample detection

Accurate determination of potassium ion (K⁺) concentration in fingertip blood, soil pore water, pipette solution, and sweat is crucial for performing biological analysis, evaluating soil nutrients levels, ensuring experimental precision, and monitoring electrolyte balance. However, current electrochemical K⁺ sensors often require large sample volumes and oversized reference electrodes, which limits their applicability for the aforementioned small-volume samples. In this paper, a K⁺ sensor integrated with a glass capillary and a spiral reference electrode was proposed for detecting K⁺ concentrations in small-volume samples. A K⁺-selective membrane (K⁺-ISM)/reduced graphene oxide-coated acupuncture needle (working electrode) was spirally wrapped with a chitosan-graphene/AgCl-modified Ag wire (reference electrode). This assembly was then inserted into a glass capillary, forming an anisotropic diffusion region of an annular cylindrical gap with width 410 µm and height 20 mm. It was found that the capillary action of the glass capillary results in a raised liquid level of the sample inside it compared to that in the container, which promotes efficient contact between the small-volume sample and the K⁺ sensor. Besides, the formed anisotropic diffusion region limits the K⁺ diffusion from the bulk solution to the K⁺-ISM, which leads to a larger potentiometric response of the K⁺-ISM. The glass capillary-assembled K⁺ sensor displays high performance, including a sensitivity 58.3 mV/dec, a linear range 10⁻⁵–10⁻¹ M, and a detection limit 1.26 × 10⁻⁶ M. Moreover, it reliably determines K⁺ concentrations in artificial sweat of microliter volume. These results facilitate accurate detection of K⁺ concentration in fingertip blood, soil pore water, and pipette solution.