Abstract
Choline is an officially established essential nutrient and precursor of the neurotransmitter acetylcholine. It is employed as a cholinergic activity marker in the early diagnosis of brain disorders such as Alzheimer’s and Parkinson’s disease. Low levels of choline in diets and biological fluids, such as blood plasma, urine, cerebrospinal and amniotic fluid, could be an indication of neurological disorder, fatty liver disease, neural tube defects and hemorrhagic kidney necrosis. Meanwhile, it is known that choline metabolism involves oxidation, which frees its methyl groups for entrance into single-C metabolism occurring in three phases: choline oxidase, betaine synthesis and transfer of methyl groups to homocysteine. Electrocatalytic detection of choline is of physiological and pathological significance because choline is involved in the physiological processes in the mammalian central and peripheral nervous systems and thus requires a more reliable assay for its determination in biological, food and pharmaceutical samples. Despite the use of several methods for choline determination, the superior sensitivity, high selectivity and fast analysis response time of bioanalytical-based sensors invariably have a comparative advantage over conventional analytical techniques. This review focuses on the electrocatalytic activity of nanomaterials, specifically carbon nanotubes (CNTs), CNT nanocomposites and metal/metal oxide-modified electrodes, towards choline detection using electrochemical sensors (enzyme and non-enzyme based), and various electrochemical techniques. From the survey, the electrochemical performance of the choline sensors investigated, in terms of sensitivity, selectivity and stability, is ascribed to the presence of these nanomaterials.
Highlights
This review describes electrochemical sensors for choline detection with fast response times based on the electrocatalytic activity of MWCNT, MWCNT composite and metal oxide nanomaterials as electrode modifiers
Highly selective recognition matrices such as polymers, nanomaterials and biological recognition elements such as enzymes have been incorporated into choline sensors to improve their selectivity in the presence of possible interferents
The selectivity of the fabricated choline sensors reviewed in the presence of the aforementioned possible interfering species was evaluated by various authors
Summary
Choline can be oxidized to betaine through an enzymatic process involving choline dehydrogenase and betaine dehydrogenase; hydrolyzed to trimethylamine by the bacterial deaminase enzyme; acetylated to acetylcholine by a cytosolic enzyme called choline acetyltransferase; and phosphorylated to phosphocholine by choline kinase [4,5]. Choline metabolism is closely related to that of different B vitamins and methionine. The pathways of choline and one-carbon metabolism intersect at the formation of methionine from homocysteine [6,7]. Methionine is regenerated (re-methylated) from homocysteine in a reaction catalyzed by betaine homocysteine methyl transferase, in which betaine, a metabolite of choline, serves as the methyl donor [5,8,9,10]. Choline is an essential nutrient officially estab-
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