**4. Conclusions**

The use of paper as a platform for sensing devices offers considerable advantages in terms of affordability and availability of functionalization processes; in fact, the hydrophilic nature of paper makes it a suitable tool due to the fast adsorption of different chemical solutions through its porous structure. Since only small volumes of reagents are needed to functionalize the paper device, it is very inexpensive. In addition, this technology does not require qualified personnel to collect and analyze the data. We developed an easy and versatile PAD that is able to measure different pollutant agents, namely NH3, C2H4O, Fe<sup>2</sup><sup>+</sup> and Cu2+, in two different mediums, air and water. The device architecture is a "proof of concept" of a new class of colorimetric sensors. In fact, it could be implemented in several environmental detection technologies, such as drones or aquatic sensors as well as individual protection devices or wearable technologies, by an electronic integration. In addition, the PAD can be used by different citizens of particular geographic areas to map the possible contaminations, with the aim to collect the global data and to build a database to monitor the pollution.

**Author Contributions:** Conceptualization and methodology V.D.M.; experimental V.D.M., G.F., M.C., L.M.; validation V.D.M.; formal analysis V.D.M., M.C., G.F.; data curation V.D.M., M.C.; writing—original draft V.D.M.; review and editing V.D.M., M.C., G.F., R.R.; supervision R.R.; project administration R.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by project PAPER (Paper Analyzer for Particulate Exposure Risk), funded within POR Puglia FESR-FSE 2014-2020—Asse prioritario 1—Azione 1.6—Bando Innonetwork—grant number PH3B166 and the project M.In.E.R.V.A funded within POR Puglia FESR-FSE 2014-2020—Asse prioritario 1—Azione 1.4—Bando Innolabs—grant number NG2WPK0-1.

**Acknowledgments:** V.D.M. kindly acknowledge Franco Mazzotta for the beneficial discussion.

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