Some materials have properties that enable scalability more easily. To create a (optical) nanosensor, optically active (e.g. fluorescent) materials are combined with recognition units that permit molecular recognition (Figure ). To link both components, a suitable linker strategy is required. As the sensor response of most nanosensors is caused by a perturbation of the nanoparticle environment (corona) upon binding, the following section will briefly introduce the main "components" as well as the main properties that affect sensing. Nanomaterials. Nanoscale objects often exhibit size-dependent properties. Of those materials, metallic nanoparticles represent, due to the high tunability of size and optical properties, the most frequently used imaging modality. Pure metals (Au, Cu, Ag, …) are electronically conducting and their optical properties depend on surface plasmon resonance (SPR) effects. Consequently, changes in their optical properties are typically due to a change in an oscillating electric field, e.g. coupling effects between materials with different resonances. In contrast, quantum dots are metal chalcogenides (CdSe, PbS, InAs, …) with a size-dependent band gap that determines their optical properties. A smaller version, carbon-based derivative are carbon dots. Carbon dots are spherical nanomaterials with a diameter between 1 and 10 nm. Their π-system absorbs light in the ultra-violet UV range and emits light in the visible. Similarly to carbon dots, single-walled carbon nanotubes (SWCNTs) also possess inherent fluorescent properties. However, semiconducting SWCNTs are fluorescent in the near-infrared (NIR) and can be excited at different wavelengths that correspond to the respective energy transitions (Typically E11: NIR; E22: VIS; E33: UV). Moreover, SWCNTs are photostable and do not bleach. The nature of the fluorescence in SWCNTs is excitonic. Excitons in SWCNTs have a size on the order of nm and diffuse over the length of a few hundred nanometers. This renders the fluorescence of SWCNTs highly sensitive to their chemical surrounding, which is a key advantage to other fluorescent materials that are relatively inert. The introduction of quantum defects (sometimes called organic color centers) to SWCNTs provides an additional opportunity to tune the fluorescent properties of SWCNTs and provides a handle for further functionalization. Linker. To functionalize a nanomaterial for (bio-)sensing, recognition units need to be connected to the nanomaterial. In this context, conjugation methods with high yields are often used to ensure efficient functionalization. Examples include traditional chemical strategies such as reactions between maleimides and thiols, carboxyl groups and amines, the reactions between alkynes and azides (CuAAC, SPAAC) as well as the functionalization of gold nanoparticles with thiols. For an overview of different bioconjugate techniques see. Additionally, the introduction of protein handles (biotin/streptavidin, His-, SNAP-, Halo-, Clip-tags, …) and DNA-based approaches relying on hybridization, or the formation of selective interfaces allow for rapid assembly of nanosensors. While functionalizing a nanomaterial, it is important to remember, that the optical properties of a material can be affected by its functionalization. For example, the introduction of quantum defects with low defect densities leads to novel fluorescent features that can be used to tune the quantum yield and sensing properties of SWCNTs. An extensive functionalization of a nanoparticle can, however, lead to the loss of its fluorescent properties, if the fluorescence is affected by the surface modification. Similarly, the controlled fixation of anchor-structures on the surface of highly sensitive nanoparticles can be used as a tailored passivation strategy to improve selectivity and stability. By constraining a recognition unit to such an anchor, the conformational freedom of a recognition unit can be further influenced. Such approaches furthermore may therefore influence the binding affinities and allow for an optimization of the signal translation and selectivity before the introduction of recognition units thereby providing an interesting opportunity for high throughput approaches.