Aptamer-switching optical bioassay for citrulline detection at the point-of-care

Researchers have found that decreased levels of circulating citrulline could be an indicator of intestinal failure. Typically, this amino acid, which is produced by the intestinal mucosa cells, circulates in the blood at a physiological level of ∼40  μM. The current methodology for measuring this level involves the use of bulky equipment, such as mass spectroscopy and analysis at a central laboratory, which can delay diagnosis. Therefore, the current detection method is unsuited for routine monitoring at a doctor's office. Our research group proposes the development of a point-of-care (POC) device to overcome this issue. The proposed device utilizes surface-enhanced Raman spectroscopy (SERS) coupled with a specifically designed aptamer, capable of binding to citrulline, conjugated to colloidal gold nanoparticles. The assay is then embedded within a vertical flow paper-fluidic platform as a deliverable at the POC, and a handheld Raman spectrometer (638-nm excitation) was used to interrogate the sample. Results showed good dynamic range and specificity with an average 73% decrease in SERS signal intensity with increasing concentrations of citrulline (0 to 50  μM) in phosphate-buffered saline compared to its controls: glycine, glutamine, histidine, and valine, which showed less than 10% average decrease in the presence of 200  μM of each analyte. Further, the limit of detection (LOD) within a chip was determined to be 0.56  μM, whereas the LOD across chips was below 10  μM.