Non-invasive and user-friendly visual detection of salivary phosphate using enzymatic biosensor test strip for chronic kidney disease screening.

Salivary phosphate is a potential biomarker for chronic kidney disease (CKD) screening. This study offers a practical, non-invasive, and patient-centric alternative to traditional blood-based assays, by leveraging saliva for phosphate screening. As a pioneering effort, we developed a paper-based colorimetric platform for the detection of phosphate in saliva. The platform comprises a saliva collection strip and a hydrogel-based sensing module integrated onto cellulosic filter paper. The sensing module utilizes a sodium alginate (SA) hydrogel that encapsulates the alkaline phosphatase (ALP) enzyme generating a colorimetric response through the reaction between phosphate and ALP. The extent of ALP inhibition in the presence of phosphate serves as the basis for quantifying phosphate levels. The method is straightforward and allows quantification of phosphate concentration by a visual color detection when the images are processed through RGB analysis and values are measured by color intensity. The RGB analysis showed a good correlation with spectrophotometric results, confirming the reliability of the proposed method. The method permits detection of phosphate in a linear range of 0.15-10 mM, with an LOD of 0.12 mM. The saliva test strips were tested on spiked artificial saliva samples and recovery values ranged from 92 to 99% (RSD < 5%), signifying its on-spot applicability. The method was also found to be highly selective, sensitive, and reproducible with considerable stability of saliva strips. The proposed biosensor can provide a rapid, non-invasive monitoring strategy as a viable alternative for CKD screening in clinical settings and for continuous therapeutic monitoring in personalized therapy.