Distance-based analytical device integrated with carbon nanomaterials for sarcosine quantification in human samples

A straightforward distance-based paper analytical device (dPAD) was developed for monitoring sarcosine levels in human samples for the rapid diagnosis and prognosis of prostate cancer and related symptoms. This assay eliminates the need for the expensive horseradish peroxidase (HRP) enzyme by utilizing carbon nanodots (CDs) as a peroxidase-like nanozyme. The proposed dPAD sensor consists of a sample zone pre-deposited with sarcosine oxidase (SOx) and CDs, and a detection zone containing 3,3′,5,5′-tetramethylbenzidine (TMB). When a solution containing sarcosine is added to the sample zone, hydroxyl radicals (^•OH) are produced through SOx oxidation and subsequent peroxidase catalysis by the CDs. The formed ^•OH radicals immediately flow to the detection zone via capillary force, where they oxidize TMB, resulting in a visible colour change from colourless to blue. Sarcosine quantification is effortlessly accomplished by measuring the distance of the blue colour in the detection zone. The developed dPAD offers a linear working range between 12.5 and 35.0 nmol L⁻¹ (R² = 0.9959) and a detection limit (LOD) of 10.0 nmol L⁻¹. This covers the clinical range for urinary sarcosine determination, thereby suggesting no additional sample preparation or dilution is needed. The sensor shows high precision with the highest relative standard deviation (RSD) of 4.58% and demonstrates excellent selectivity with no observed interferences. Furthermore, recovery studies in human control urine samples ranged from 98.67 to 101.50%, with the highest RSD of 2.03%. Correspondingly, our dPAD method showed a great match with the performance of a commercial ELISA method for detecting sarcosine in human control serum. The sensor is more cost-effective, user-friendly, and accessible than previous methods. Overall, the proposed method represents a promising analytical tool for sarcosine quantification. The concept is also applicable for broader analytical applications in detecting other biomolecules.

Graphical Abstract