Abstract
The use of saliva sampling as a minimally-invasive means for drug testing and monitoring physiology is a subject of great interest to researchers and clinicians. This study describes a new optical method based on non-axially symmetric focusing of light using an oblate spheroid sample chamber. The device is simple, lightweight, low cost and is easily attached to several different brands/models of smartphones (Apple, Samsung, HTC and Nokia) for the measurement of sodium ion levels at physiologically-relevant saliva concentrations. The sample and fluorescent reagent solutions are placed in a specially-designed, lightweight device that excludes ambient light and concentrates 470-nm excitation light, from a low-power photodiode, within the sample through non-axially-symmetric refraction. The study found that smartphone cameras and post-image processing quantitated sodium ion concentration in water over the range of 0.5–10 mM, yielding best-fit regressions of the data that agree well with a data regression of microplate luminometer results. The data suggest that fluorescence can be used for the measurement of salivary sodium ion concentrations in low-resource or point-of-care settings. With further fluorescent assay testing, the device may find application in a variety of enzymatic or chemical assays.
Highlights
Saliva sampling via noninvasive qualitative and quantitative techniques is important in drug testing, as well as for monitoring physiological systems [1]
Where F is the relative fluorescence reading, Fmin is the relative fluorescence reading in the absence of sodium and Fmax is the relative fluorescence of the sodium ion-saturated fluorophore
For the quantitation range of interest, the total sodium ion concentration is much greater than the fluorophore concentration, allowing for the assumption that the free sodium ion concentration is equal to the initial sodium ion concentration
Summary
Saliva sampling via noninvasive qualitative and quantitative techniques is important in drug testing, as well as for monitoring physiological systems [1]. While much attention is being paid to detect hormones or infectious diseases via rapid salivary testing, there has been less effort given to quantifying the levels of important, physiological indicators, such as the concentration of sodium ions. Quantitation of sodium ion levels in saliva can potentially be used to monitor patient wellness, track endogenous cycles, as well as to conduct screening for patients with chronic underlying diseases or those undergoing dialysis [4,5]. The possibility of adding to a saliva sample a premixed fluorophore solution specific for sodium ions followed by measuring the fluorescence directly after mixing, without additional sample preparation procedures or chemical separation steps, would seem to be an attractive means for rapid analysis of this important indicator of physiological function. It should be noted that saliva sampling has specific protocols that yield different concentration levels of analytes depending on the way that the sample is collected [1,2]
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