Selective electrochemical detection of thiol biomarkers in saliva using multiwalled carbon nanotube screen-printed electrodes

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The selective electrochemical determination of homocysteine and glutathione was realized in spiked synthetic saliva using a two-part voltammetric method at a multiwalled carbon nanotube screen-printed electrode. This method involves first the use of the reaction of electro-oxidized catechol with a thiol via 1,4-Michael addition reaction; this results in an analytically useful adduct peak signal. Secondly, the procedure exploits the different rates of reaction of the targeted thiols with the ortho-quinone, coupled with variable voltage scan rates. At fast scan rate, homocysteine is only detected while at slower voltage scan rate, both homocysteine and glutathione detection is reflected in the adduct signal. Thus, the quantification of both homocysteine and glutathione can be achieved with a combination of both sets of data. Calibration curves were determined for both homocysteine and glutathione in synthetic saliva with sensitivities of ca. 13.2nAμM−1 and ca. 2.1nAμM−1 for homocysteine and glutathione respectively with limits of detection at ca. 0.9μM for homocysteine and ca. 2μM for glutathione within their corresponding linear ranges. Lastly, the use of commercially available multiwalled carbon nanotube screen-printed electrode was applied to the system for selective homocysteine and glutathione detection in different spiked synthetic saliva samples. Hitherto the method has been applied in vitro and to diluted blood plasma; the extension to saliva is attractive for diagnostic use because of the non-invasive nature of the sampling.