**5. Conclusions**

In this review we focused on the recent progress, main figures-of-merit and potential of different miniaturized sensing systems based on nanostructured materials. In particular, we reported on the use of organic and hybrid nanostructured materials into optoelectronic, electrochemical and plasmonic elements for highly integrated smart sensors.

The engineering of the functional properties of nanostructured materials through their chemical and structural design, together with the design of the architecture of the single-component devices, is demonstrated to be playing a major role in the realization of e ffective and integrated detection schemes. Indeed, for on-site bio-diagnostics and environmental/food monitoring purposes, the application of most standard and traditional analytical techniques is in contrast with the current need for rapid, cheap, easy-to-use and portable devices.

Above all, electrochemical and optical chemical sensors are promising tools with interesting analytical features. For electrochemical sensing, the major strategy that is implemented for enhancing both sensitivity and selectivity is the surface modification by nanophase-functional materials. Nanophase materials, such as carbon allotropes, metal- and metal oxide NPs, molecularly imprinted polymers, through di fferent physical and chemical mechanisms, or acting as binders for biological recognition elements, can strongly enhance the limit of detection and analyte a ffinity of electrochemical sensors.

Optical sensors generally also show a good sensitivity and selectivity. Also, with respect to electronic devices, optoelectronic devices are less a ffected by external electric or magnetic fields and self-interference e ffects. The massive development of miniaturized and e fficient single optoelectronic components based on nanostructured organic and hybrid materials, which has occurred over the last few decades, has provided enormous potential for the progress of new concepts and applications of optical and optical chemical sensors. In parallel, architecture engineering succeeded in avoiding the need for additional expensive optical components, thus allowing e ffective miniaturization and multiplexing.

On the other hand, plasmonic and photonic components obtained by nanostructuring allow the sensitivity of optical sensors to be increased. The features of the plasmonic and photonic systems are quite complementary and a new interest emerged in combining them into hybrid multicomponent systems to take advantage of the best properties of both. As one of the main advantages, the same plasmonic/photonic detection scheme can be applied to detect a large panel of molecules, often with a label-free approach (i.e., SPR). Furthermore, a specific functionalization step with biological recognition elements enable to obtain optoplasmonic or photonic chemical sensors with a high selectivity for specific target molecules.

Finally, we highlighted that integration is a key-factor to unravel the potentiality of optical-chemical sensors in terms of disposability, reliability, miniaturization and multiplexing while providing laboratory-quality analysis. In view of developing functional sensors for real-setting applications, a smart and e ffective system-engineering approach is necessary for realizing fast-responding, non-invasive, broadly adaptable, potentially highly sensitive and multiplexing sensors. Such systems could be employed to overcome existing limitations in measurements which are currently used in environmental and agri-food fields and advanced biodiagnostics.

**Funding:** This work received funding from the European Union's Horizon 2020 research and innovation program under gran<sup>t</sup> agreemen<sup>t</sup> no. 780839 (MOLOKO project). We kindly acknowledge financial support from Programma Operativo POR-FESR 2014-2020 of Regione Emilia Romagna, Azione 1.2.2, through the project FORTRESS "Flexible, large-area patches for real-time detection of ionizing radiation", CUP I38D18000150009-PG/2018/629121.

**Conflicts of Interest:** The authors declare no conflict of interest.
