Tongue depressor (bio)sensors: A fast decentralized self-testing of salivary biomarkers for personalized medicine

The clinical tests usually make use of blood samples, requiring invasive sample collection protocols, which are not appropriate for decentralized, frequent, and point-of-care (POC) applications. Here, we report disposable (bio)sensors combining electrochemical and colorimetric technologies on an accessible and lightweight wooden tongue depressor platform. The array of (bio)sensors on this commonly used medical tool enables rapid, and safe saliva sampling and showed promise to be used for frequent self-testing of four clinically relevant biomarkers (glucose, uric acid, nitrite, and thiocyanate) in saliva. The electrochemical system was designed to detect uric acid and glucose by chronoamperometric measurements in clinically relevant concentrations ranging from 50 to 500 µmol/L and from 10 to 5000 µmol/L, respectively. Meanwhile, the colorimetric paper-based spot tests were optimized to detect nitrite and thiocyanate in concentrations ranging from 5 to 200 µmol/L and 0.5 to 2.1 mmol/L, respectively, using digital image colorimetry. Our affordable tongue depressor (bio)sensors provide results within 5 min and achieve limits of detection in the µmol/L range. Importantly, the results provided by our tests when applied to human saliva samples presented excellent concordance with the traditional spectrophotometric method (at a 95 % confidence level), demonstrating the reliability and robustness of our method. In addition, the toxicity and biocompatibility studies of the tongue depressor (bio)sensors were evaluated using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), Calcein acetoxymethyl ester (Calcein AM), and Calcein Propidium Iodide Hoechst (Calcein PI) Analysis, confirming the safety use of the test. Therefore, our tongue depressor (bio)sensor facilitates frequent testing of several biomarkers and can contribute to personalized medicine.