## 2 Optical and Electrochemical Sensing for Eukaryotic Cells and Bacteria  
The development of a robust sensing system suitable for biomedical screening needs to cross the boundaries of traditional cell labelling approaches based on fluorophores that, often, are phototoxic, not sensitive enough or specific to eukaryotic cell models, and most relevantly, suffer self-quenching or bleaching in the case of prolonged light exposure [2]. [PERSON] et al. in their paper on the _Study of a Miniaturizable System for Optical Sensing Application to Human Cells_[3], combine the cell labelling by Tris(2,2\({}^{\prime}\)bipyridyl))ruthenium(II) (Ru(bpy)\({}_{3}\)\({}^{2+}\)) fluorophore with the detection by a sensitive and miniaturizable laser beam-photomultiplier based system. The uptake of Ru(bpy)\({}_{3}\)\({}^{2+}\) in human dermal fibroblasts (HDF) or in human colon adenocarcinoma (LoVo) cells was successfully detected by fluorescent time-lapse analysis and confirmed by fluorescence microscopy (FM) and confocal laser scanning microscopy (CLSM). Cells survivalfrom the labelling method was checked by MTT, (3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolzolium bromide, commonly used to detect reductive metabolism in cells for viability, proliferation and cytotoxicity assays), of cells exposed to the fluorophore. Nanocomposites of spherical gold nanoparticles (AuNPs) and graphene oxide (GO) nanosheets, fabricated and characterized by [PERSON] and coworkers in their contribution: _A Hybrid Nanoplatform of Graphene Oxide/Nanogold for Plasmonic Sensing and Cellular Applications at the Nanobiointerface_[4], have been developed as highly promising nanoplatform for theranostic applications targeted to cancer cells. The plasmonic sensing capability of spherical gold nanoparticles--roughly 10 nm in the diameter and mostly confined at the GO basal planes, as shown by atomic force microscopy (AFM) analyses--are maintained and enhanced in the hybrid nanoassembly, despite the strong nanoparticle/nanosheet association revealed by nuclear magnetic resonance (NMR) spectroscopy. The GO-AuNP cytotoxicity, as well as their effects on both total and mitochondrial reactive oxygen species (ROS) production in human neuroblastoma cells (SH-SY5Y line), were demonstrated by MTT assays and by experiments with 2,7-dichlorofluorescein (DCF) and MitoSOX, respectively. Laser scanning confocal microscopy (LSM) imaging confirmed the increased uptake of the nanosheet in the GO-AuNP hybrids compared to the GO alone, which lead to mitochondrial dysfunctions, and in turn, the ROS generation. Electrochemical analysis is based on the redox behavior of the analyte. A variety of electrochemical techniques are available for the detection of biomolecules; however, the development of a sensing platform with high sensitivity and selectivity is still challenging. The innovation introduced by multifunctional nanomaterials has contributed to great advancement of electroanalytical technologies for in vivo and in vitro analyses, especially for point-of-care clinical applications [5]. [PERSON] and coworkers in their contribution on _Nanomaterials-Based Electrochemical Sensors for In Vitro and In Vivo Analyses of Neurotransmitters_[6], review the working principles of the most commonly employed electrochemical analysis techniques for the detection of neurotransmitters, with a special focus on the fabrication and development of nanomaterial-based electrochemical sensors. Challenges and future outlook are described for a more efficient detection of neurotransmitters by electrochemical sensors with improved selectivity in real-time analysis and decreased drawbacks in animal models due to biofouling and interfering molecules.