Overview of Non-invasive Sampling Techniques for Biomarkers and In Vitro and In Vivo Monitoring Strategies for Neurotransmitter Detection

The brain is a complex network accounting for ∼5% of human body mass but it consumes nearly 20% of the energy. Uncovering the mysteries of the brain's functions for motion, memory, mental health, etc. remains an exciting but challenging hotspot for research. It is difficult to diagnose disorders of the brain chemically. Neurochemicals, especially neurotransmitters, play vital roles in mediating and modulating normal brain function. Neurotransmitters are molecules that transfer chemical signals between neurons to convey messages for any action conducted by the nervous system. Variations in neurotransmitter levels, i.e. their abnormal release or imbalances in concentration, are a characteristic manifestation of several neurological diseases, such as epilepsy, Alzheimer's disease, and Parkinson's disease. It is estimated that approximately 800 million people will be living with a neurological disorder by 2050. Great interest in designing and optimizing tools for directly detecting chemical biomarkers implicated in neurological disorders to improve diagnosis is therefore a pressing current need. Tools capable of monitoring brain chemicals, i.e. neurotransmitters, must be biocompatible, with high spatiotemporal resolution, high selectivity, and high sensitivity. Recent advances in detection methods, device fabrication, and new materials have resulted in the development of neurochemical sensors with improved performance. The development of cutting-edge technologies with improved detection efficacy and sensitivity, and less invasive measurements, has been widely reported for the detection of neurotransmitters. In this chapter, we highlight significant technical advances that have helped facilitate the development of diagnostic tools for neurochemical sensor development. Finally, we address the challenges and future outlook for developing biosensors for biomarker detection.