An Electrochemical Sensor for Disease Diagnosis and Decision Making By Passively Monitoring Chloride Ion Levels in Human Sweat
Authors: Ganguly Antra , Prasad Shalini 
Published: February 2020

The human body has a crucial responsibility of maintaining a homeostatic electrolytic balance in body fluids. The inability to do so, is often reflective of an underlying pathophysiology which can be both acute or chronic. One such relevant biomarker is the sweat electrolyte, chloride ion. Human sweat is an easily accessible body fluid which is particularly useful for capturing a physiological snapshot of the biofluid composition, especially when frequent probing is needed. Abnormality in sweat chloride concentrations can be indicative of lethal genetic disorders like Cystic Fibrosis or other less severe conditions of hypo or hyperchloremia like dehydration. It is known that certain acute pathologies like dehydration induce a transient departure in biomarker values from their standard healthy homeostatic ranges. By gauging the deviation of a reference electrolyte levels, such as chloride, the fluctuation in the target disease biomarker levels due the actual chronic disorder can be decoupled from that due to the transient dehydrated state. This work proposes a universal, impedance based, electrochemical biosensor that is capable of monitoring chloride ion levels in sweat that can have implications for all the above-mentioned scenarios. The proposed biosensor is based on the affinity capture of chloride ion levels by a novel, chloride ionophore based immune-assay like sensor stack. The sensor can monitor sweat chloride levels reliably and accurately over the entire physiologically relevant sweat pH range (2 to 8) in ultra-low volumes (1-3mL) of human sweat. The electrochemical sensor is universal as it can be tailored to cater to a gamut of diseases associated with chloride level fluctuations and is passively addressable meaning that it does not rely on active stimulation of the eccrine sweat glands by electrical, chemical or exercise-induced stimulation. This greatly reduces the challenges of sample contamination and evaporation by eliminating the issue of handling large sample volumes. The sensor functions as a two-electrode system with gold working and counter/ reference electrodes deposited onto a flexible, hydrophilic substrate. The capture probe, consisting of the chloride ionophore, 4,5-Bis-[N′-(butyl)thioureido]-2,7-di-tert-butyl-9,9-dimethylxanthene, is bound to the electrodes by way of the crosslinker, DSP (dithiobis (succinimidyl propionate or Lomant’s reagent). The ingress of the chloride ions in the interfacial region of the electrode and the electrolyte and their subsequent capture by the ionophore induces a variation in the electrical double layer parameter values as a function of the ion levels. The affinity capture of the chloride ions is achieved by way of highly specific and strong multitopic hydrogen bonds between the donor(ionophore) and the acceptors( chloride ions), which is afforded owing to the highly preorganized bis-thiourea receptors based on a xanthene spacer, in the suitable chemical structure of the capture probe. The modulation in the electrical double layer and the associated Randle’s circuit parameters, like charge transfer resistance, double layer capacitance and solution resistance, in response to the sweat chloride concentration were transduced to relative impedance changes that were quantified by two powerful AC and DC based electroanalytical techniques of Non-faradaic Electrochemical Impedance Spectroscopy (EIS) and Chronoamperometry (CA) respectively. The calibration dose responses obtained from both the methods were linear with an R2 value> 0.95 for a linear dynamic range of sweat chloride of 10-100mM with a limit of detection of 10mM for the entire sweat pH range of 2 to 8. The chloride sensing scheme is envisioned to be incorporated as point-of-care or wearable diagnostic device when coupled with IOT or Machine Learning modalities for real time monitoring of sweat electrolytes. As evidenced by its flexibility and the immunity of sensor readout to bending and ambulation, the sensor shows good potential to be used as a dynamic hydration sensor for real time monitoring of chloride ions levels in sportspersons and athletes during a game or while on the field. The accuracy and passive addressability of the sensor make it viable at-home diagnostic tool for tracking chloride levels in CF patients for checking the efficacy of on-going treatments, making medical decisions and planning visits to the Doctor’s clinic, by reducing, to some extent, the emotional, financial and psychological stress of the individuals. When incorporated within a combinatorial sensing system, the proposed sensor can reduce false positives of disease diagnosis by acting as a precise yardstick for benchmarking levels of other relevant disease biomarkers also coexisting in the sweat sample. Thus, the proposed chloride sensor can be suitably tailored to universally cater to a plethora of disease diagnostic needs thereby improving the quality of human health by empowering patients to make better informed personalized medical decisions.