Passive Solid Chemosensor as Saliva Point of Need Analysis for Ammonium Determination by Using a Smartphone

Abstract

Point-of-need analysis is of great interest nowadays. It refers to the timely analysis or detection of a specific parameter or substance at the location or moment it is needed, often with the aim of providing rapid and on-site results for informed decision-making or immediate interventions. This approach has gained interest in various fields but has not been extensively explored in bioanalytical chemistry. In order to contribute in this way, the analysis of ammonium in saliva as a biological fluid is proposed here. For that purpose, a passive solid sensor of 1,2-naphthoquinone-4-sulfonic acid sodium salt (NQS) embedded in polydimethylsiloxane (PDMS) doped with silica nanoparticles and an ionic liquid was proposed. The assay was developed by delivering ammonia from saliva in a confined atmosphere containing the sensor for 20 to 45 min. Measurements were carried out by absorbance from a benchtop diffuse reflectance spectrophotometer and a fiber optic miniaturized portable spectrometer coupled to a smartphone for point-of-need analysis. Another option for this kind of analysis was the use of the color intensity from digitalized images obtained by a smartphone by isolating the intensity in the color planes R (red), G (green), and B (blue). Good figures of merit were obtained for all three types of instruments, bearing in mind the ammonium content in saliva. Results for 30 samples of male and female volunteers (n = 30) demonstrated the usefulness of the assay, values of mg NH4+/mL saliva between 0.02 and 0.27 were found, and no matrix effect was present. Recoveries for spiked samples were around 100% for all methodologies. Selectivity was demonstrated from spectra obtained from benchtop instruments and the fiber optic mini spectrometer. Two applications were applied for directly determining the ammonium concentration in saliva.