Another common sensing technique that is ubiquitous nowadays is colorimetric sensing. The lateral flow assay (LFA) test strips became quite common during the COVID19 pandemic. The colors of the control line and the test line in LFA tests originate from plasmonic (commonly Au nanoparticles) nanoparticles that show a strong color contrast against the cellulosic background. Since it has been shown that in plasmonic nanoparticle clusters, the plasmonic coupling increases the optical cross section of the individual nanoparticles, self-assembled nanoparticle clusters can be interesting for LFA tests. Chen et al. showed that Au nanoparticle clusters obtained by emulsion directed self-assembly provide ~40 times higher sensitivities than isolated 40 nm Au nanoparticles. With increasing cluster size, the sensitivity increases up to a maximum before dropping again, Figure (a) . Similarly, Oh et al. reported higher sensitivities with even smaller assemblies of several Au nanoparticles for SARS-CoV-2 sensing with LFA tests, Figure (b) . It is important to note that while increasing size leads to the increase in the optical intensities, the surface area also increases. For a fixed amount of analyte or biomarkers, more analyte per nanoparticle may be required with increasing size. Thus, the ideal situation is where high optical cross sections (or intensities) are combined with small nanostructures. Recently, there have been several studies that have shown significant sensitivity enhancement in LFA tests by self-assembled nanoparticles. Apart from LFA, other colorimetric sensing methods can also make use of self-assembled nanoparticle clusters or the process of clustering itself. As clustering comes with a change in color compared to individual nanoparticles, the process of selfassembly itself can be directly useful in colorimetric sensing in general. For example, Mirkin and co-workers showed back in 1997 the possibility of colorimetric detection of DNA in liquid phase by self-assembly or aggregation of Au nanoparticles, Figure ). Such plasmonic coupling mediated colorimetric sensing has since been studied extensively and further enhancement by the incorporation of modern optical sensors still holds great promise. Similar to how clustering of plasmonic nanoparticles affects the overall optical properties, the clustering of magnetic nanoparticle also leads to changes in the magnetic resonance (MR) signal. It has been shown that during the self-assembly of magnetic nanoparticles, the nanoparticles act as magnetic relaxation switches by causing extensive spinspin relaxation time changes (δT2) of surrounding water molecules. Thus, clustering of superparamagnetic iron oxide nanoparticles labelled with antibodies specific to antigens presented on specific viral capsids result in detectable perturbations of the T2 magnetic relaxation times. This strategy by self-assembly of magnetic nanoparticles can be used to detect viruses up to concentrations as few as five viral particles in 10 μL of 25% protein solution without the need of PCR amplification, Figure . Another recent yet fast developing field is the sensing with chiral plasmonic nanostructures. These chiral structures can also be obtained by self-assembly of nanoparticles, especially by DNA directed techniques to achieve high control over the structure. A number of other review articles have specifically addressed chiral sensing in great detail already. Figure . (a) Sensitivity enhancement in lateral flow assay (LFA) device by the use of self-assembled Au nanoparticle clusters © CC-BY 4.0. (b) Sensitivity in LFA can be further enhanced by the use of larger Au nanoparticles for SARS-CoV-2 sensing. The comparison between 15 and 40 nm nanoparticles shows that 40 nm nanoparticles have a stronger color due to stronger plasmonic coupling. © CC-BY 4.0. (c) In solution phase, colorimetric detection of DNA in liquid phase is possible by self-assembly or aggregation of Au nanoparticles that changes the color of the colloid. Reprinted with permission © 1997 AAAS, Reprinted with permission © 2005, American Chemical Society (d) Superparamagnetic iron oxide nanoparticles (brown spheres) labelled with antibodies (green) specific to antigens (blue) presented on viral capsids (red) will form aggregates in the presence of target viruses, which result in detectable perturbations of the magnetic relaxation times of protons in the surrounding media. Reprinted with permission © 2003, American Chemical Society.