Sensing. In sensing applications, self-assembly can be used to obtain the actual sensing interface. For example, for metal oxide (MOS) based chemo-resistive gas sensing, metal oxide nanoparticles can be self-assembled into thin films or clusters. In self-assembled films, individual metal oxide nanoparticles can at first be subjected to pre-modification or functionalization for sensitivity enhancement . In general, colloidally synthesized high surface area nanoparticles provide a higher contact interface compared to planar thin films, resulting in higher sensitivity given that the other sources of resistance are eliminated. Miao et al. showed that CuO nanosheets obtained by air-water interfacial self-assembly result in excellent sensitivity as such films provide large surface area but small thickness (~30 nm), Figure . Indeed, with increasing thickness of the films by multiple layer assembly, the sensitivity decreases. Borah et al. showed that different metal oxide nanoparticle films can be obtained by self-assembly after the hydrophobization by appropriate ligand functionalization. Similarly, the decoration of Au nanoparticles by self-assembly or other methods can enhance light assisted gas sensing. Another way self-assembly can enhance sensing is by making highly porous nanostructures by the clustering of nanoparticles in three dimensions. As shown by Suematsu et al., SnO2 nanoparticle clusters exhibit considerably higher sensitivity in H2 and CO sensing as compared to isolated nanoparticles, Figure . The sensitivity is then further enhanced by the incorporation of Pd. Plasmonic nanostructures are well-known for their application in SERS (Surface Enhanced Raman Spectroscopy) based sensing, where the inelastic Raman scattering signal of probe molecules is enhanced by the near-field enhancement or the light concentration effect of the nanostructures. It has been shown that in self-assembled periodic plasmonic nanoparticles/nanostructures, the near-field enhancement is stronger than for isolated nanoparticles due to the inter-particle coupling. As shown by Shin et al., interparticle gaps of desired dimensions can be obtained by subsequent etching steps to remove the SiO2 shell over the Au nanoparticles in the self-assembled film, Figure ). Importantly, for Raman enhancement, the etching step is important as the probe molecules are exposed to the near-field enhancement only if the silica shells are removed. For certain cases, to shift the SERS enhancement to longer wavelengths, the plasmon resonance must be red shifted. For this, supercrystals of Au nanoparticle clusters can be obtained by combining self-assembly and lithography resulting in resonances in the infrared region, Figure . On the other hand, with magnetic-plasmonic bifunctional nanoparticles, self-assembled clusters of nanoparticles for SERS enhancement can be obtained by the application of magnetic fields. While SERS application of self-assembled plasmonic nanoparticles has been studied quite extensively, some recent developments have shown new applications in biosensing. Self-assembled plasmonic Au nanoparticle clusters combined with metal organic frameworks are excellent platforms for SERS based detection of VOCs (volatile organic compounds) in human breath that are indicators of certain diseases like cancer. For example, among different VOCs, certain aldehydes in breath have been identified as biomarkers of lung cancer The combination of MOFs with self-assembled plasmonic nanoparticles facilitates high absorption of the probe molecules in the porous MOF, where their SERS intensity is amplified by the strong nearfield enhancement of coupled plasmonic response of the nanoparticles, Figure . Recently, Borah et al. have shown an alternative strategy for the detection of VOCs by self-assembled Au nanoparticle films in the Kretschmann (i.e. Attenuated Total Reflection, ATR) configuration, where the adsorbed VOCs on the nanoparticles can be detected by the change in the optical intensities due to the change in refractive index, Figure . In the ATR configuration, the plasmon resonance is excited by the evanescent field due to the total internally reflected incident beam. The evanescent field is strongly dependent on the direction of polarization of the incoming light. It is shown that further improvement in sensitivity can be achieved by the use of ellipsometry instead of intensity based optical spectroscopy. Refractive index based sensing by plasmonic nanostructures is based on the change in the optical intensities or plasmon resonance positions (usually a red shift) due to the change in the refractive index in the surrounding medium of the nanoparticles after interaction with the analyte. Naturally, the adsorption of biomolecules (in liquid phase), gases, VOCs, etc., on the plasmonic nanostructure leads to this change in the refractive index. Plasmonic nanoparticle films have a very short-range evanescent field (or near-field enhancement) that senses the changing refractive index. Thus, self-assembled plasmonic nanoparticle films can selectively probe only the molecules that are in the immediate vicinity of the film. This can facilitate higher selectivity as compared to planar thin films which have long range evanescent fields that can interact with distant molecules (or resulting refractive index changes). Naturally, refractive index-based sensing itself is not selective as the adsorption of any molecule can lead to the change in the refractive index. Thus, the selectivity in this case can be achieved by selective adsorption of the probe molecules on the sensing platform. In this regard, combining metal organic frameworks (MOFs) with plasmonic nanoparticles reappears as a promising approach as the adsorption of molecules in MOFs can be highly selective. For example, CHCl3 shows preference in adsorption to CH2Cl2 from a mixture, in the adsorption to a Cu(II)-metal organic framework, which is linked to the hydrophobic nature of the pores. Such trends have also been demonstrated for aromatic molecules. (a) Self-assembled CuO nanosheets for H2S gas sensing applications. A monolayer film yields higher sensitivity than multilayer due to low thickness with high surface area. Reprinted with permission © 2019, American Chemical Society (b) Self-assembled SnO2 nanoparticle clusters have stronger sensitivity than isolated nanoparticles in H2 and CO gas sensing due to higher porosity resulting from clustering. Reprinted with permission © 2014, American Chemical Society (c) Self-assembled Au@SiO2 core-shell nanoparticle films with controlled interparticle gap for SERS applications. Reprinted with permission © 2015 John Wiley & Sons. (d) Self-assembled supercrystals of Au nanoparticle clusters can shift SERS enhancement to infrared wavelengths due to the red-shifted lattice plasmon resonance. Reprinted with permission © 2018, American Chemical Society (e) Demonstration of a SERS based biosensing device: self-assembled Au nanoparticle clusters combined with MOF facilitates SERS signal enhancement of cancer biomarker VOCs from breath that adsorb onto the MOF in high concentration. Reprinted with permission © 2022 John Wiley & Sons. (f) Schematic of ellipsometric VOC sensing by self-assembled Au nanoparticle films in the Kretschmann (or ATR) configuration: the optical response is driven by the evanescent field (varies with the polarization) that interacts with the nanoparticles and the surrounding environment (not to scale).Reprinted with permission © 2022, American Chemical Society.