Melatonin, the ‘chemical signal of darkness’, is responsible to regulate biological rhythms and different physiological processes. It is mainly produced by the pineal gland as a hormone in a rhythmic daily basis, but it may also be synthesized by other tissues, such as immune cells, under inflammatory conditions. Its abnormal circulating levels have been related to several diseases such as type 2 diabetes, Alzheimer’s disease and some types of cancer. Currently, melatonin is exclusively quantified by ELISA or radioimmunoassays, which although are very sensitive techniques and present low detection limits, usually require specialized personal and equipment, restricting the tests to a limited number of patients. To overcome such limitations, we developed a novel easy-to-use electrochemical immunosensor for rapid melatonin quantification. Anti-melatonin antibodies were immobilized into Indium tin oxide (ITO) platforms using (3- Aminopropyl)triethoxysilane (APTES), 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and N-hydroxysuccinimide (NHS) crosslinkers. The platforms were assayed with synthetic and biologically-present melatonin containing samples. The developed device displayed a linear response in the concentration range from 0.75 to 7.5 µmol/L and a limit of detection of 0.175 µmol/L using Electrochemical Impedance Spectroscopy (EIS) (R2=0.989) and 0.513 µmol/L using Cyclic Voltammetry (CV) (R²=۰٫۹۵۳) for synthetic melatonin. Furthermore, the sensors exhibited a good stability and reproducibility (3.45% and 2.87% for EIS and CV, respectively, n=3), maintaining adequate response even after 30 days of assembly. On biologically-present melatonin-containing samples the device displayed a similar performance when compared to ELISA technique (deviation of 13.31%). We expect that the developed device contributes significantly to the medical area allowing precise and complete diagnosis of the diseases related to abnormal levels of melatonin.

Introduction

Melatonin (N-acetyl-5-methoxy tryptamine) is an indoleamine present in most biological interfaces. In several species, melatonin plays important roles as a hormone and it is believed that its biological signaling is one of the oldest existing in nature.1 In humans, melatonin biosynthesis is rhythmically active in the pineal gland. Some other tissues, which includes the retina and gastrointestinal tract, may also synthesize melatonin in a rhythmic manner. 2 Melatonin may also be synthesized on demand by macrophages, lymphocytes and microglia when activated by pathogens or abiotic injuries.2-4 . Under normal conditions its maximum concentration in human plasma is about 50-200 pg/mL (0.22-0.86 nmol/L) during the night and lower than 2.0 pg/mL (8.6 pmol/L) at daylight, 5-7 which is in accordance with one of its major functions: translating environmental darkness to adjust endogenous physiology. Excretion of melatonin in the urine is also a pivotal marker of melatonin bioavailability. It is noteworthy to mention that melatonin can also be found in human saliva at concentrations ten times lower than in plasma, which highlights the possibility for the development of non-invasive and sensitive methods for detection of the indoleamine in different biological fluids. The idea that melatonin is only linked to the circadian rhythms or seasonal behavior was long challenged by the scientific community. In the last decades, intense efforts have been spent in understanding the influence of this hormone in other actions and physiological systems3,8-10 . For example, studies have shown that melatonin presents a very important role on the organization of the immune response11 . In this case, the hormone derived from the pineal gland is ceased but is locally produced by defense cells and acts locally, as regulator of the immune response. Also, under inflammatory frames, nocturnal pineal melatonin suppression favors the mounting of inflammation by allowing the rolling and adherence of leukocytes as well as its vascular permeability.