Gastrointestinal tract mechanism of nitrite capture modeled on the self assembled monolayer of thioproline for electrochemical nitrite determination

Abstract

Gastrointestinal (GI) tract has a unique mechanism for nitric oxide (NO) capture in the form of N-nitrosamines. N-Nitrosamines are formed by the reaction between thioproline (THP) and NO under the acidic conditions prevalent in the stomach. The nitrosamines thus formed are excreted in the urine and act as an important biomarker for human NO concentrations. Exploiting the GI tract mechanism of nitrosamine formation, we prepared the self-assembled monolayers (SAMs) of THP on gold wafers and gold nanoparticles (GNPs). The SAMs were characterized by contact angle measurements, ATR-FTIR, FE-SEM with EDS, XPS, DLS and TEM. The SAM-covered gold surfaces were used as a mimetic model of the GI tract to capture nitrite (NO₂⁻) ions for their determination using electrochemical and colorimetric methods. The XPS and ATR-FTIR were used to determine the mechanism of NO₂⁻ binding to THP, supported by electrochemical measurements. The rapid NO₂⁻ capture by THP SAM-coated gold surfaces results in the electrovalent binding of NO₂⁻ with THP within fractions of seconds. Since THP is known to react with NO₂⁻ at about a 1000 times faster rate than the reaction with known reactants, the added advantage of the designed THP-SAM-coated gold surface model is a rapid reaction kinetics which minimize the self-decomposition of NO. The electrochemical detection provides an excellent detection limit with good sensitivity. UV-visible spectroscopy, though it efficiently detects the traces of NO₂⁻, is unable to give the competitive limit of detection and sensitivity obtained from electrochemical detection. Thus, the THP-SAM-coated gold model can be considered as a GI tract model capturing the NO₂⁻ using similar reactants and conditions prevalent to those in GI tract. The developed model can be employed for routine screening and the determination of NO₂⁻ or NO in complex samples or biological matrices without any interference from the matrix as most common impurities/species do not react with THP-SAMs under the employed conditions.