**1. Introduction**

"Smart Cities" or "intelligent cities" are the cities of the future that offer innovative solutions to improve the quality of life of urban communities in a sustainable and equitable manner. The idea of a "Smart City" represents more efficient cities that better manage their resources, services and technologies and, above all, put them at service of the citizens. People-centric planning will allow a better managemen<sup>t</sup> and efficiency of the city through the deployment of smart infrastructure. By 2050, 85% of the world's population is expected

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**Citation:** Ramírez-Moreno, M.A.; Keshtkar, S; Padilla-Reyes, D.A.; Ramos-López, E.; García-Martínez, M.; Hernández-Luna, M.C.; Mogro, A.E.; Mahlknecht, J.; Huertas, J.I; Peimbert-García, R.E.; et al. Sensors for Sustainable Smart Cities: A Review. *Appl. Sci.* **2021**, *11*, 8198. https://doi.org/10.3390/app11178198

Academic Editor: Juan-Carlos Cano

Received: 29 June 2021 Accepted: 31 August 2021 Published: 3 September 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

to live in cities [1–3]. This means that in the following decades, urban centers will face a growing number of problems such as: (1) energy supply, (2) CO2 emissions, (3) mobility systems planning, (4) raw materials and goods provision, and (5) the provision of health and security services to all the residents in these rapidly growing population centers [4].

To better respond to the increasing volatility driven by climate change, pandemics like COVID-19, and connected political and economic fluctuations, cities must be redesigned to increase their adaptive capacity and resilience [5]. Schemes and models of more liveable cities need to be created, where digital technologies are the key elements for the sustainable development and organic growth of cities [6]. Vulnerabilities of the shared economy in smart cities, such as transport services and their enabling technologies are being tested by the global pandemic and cyber security risks. However, new apps and internet businesses flourished, such as food delivery and online shopping [7–9].

Skepticism towards pervasive digitization, sensing, monitoring, and visualization capacities deployment, and other smart communication technologies used by both the private sector and government, arise, among other factors, from the concerns of citizens regarding the processing of their data, and, therefore, their privacy [10]. The use of new technologies such as artificial intelligence, where personal data play a critical role, are facing increasing challenges [11]. Support for the establishment of codes of conduct are being promoted by the industry itself, and use cases will be key so that its development does not suffer [1]. In the years to come, problems such as intentional disinformation, the evolution of so-called digital rights, or the consolidation of digital identity, must be addressed. The solutions oriented to solve these problems require a long-term perspective, which will steadily become more and more evident as Smart City implementations have become a norm across the globe [12].

With the incoming necessity of communities to become smarter, many applications have started to arise on different countries; and a literature revision that discusses such applications will be useful as a reference for future smart city implementations. In this review, the literature covering several applications of sensors for smart cities is summarized and discussed, in order to fulfill three main objectives: (1) To provide a revision of the most important Smart Cities implementations across the world; (2) To describe the main applications of sensors for smart cities across six main topics (health, security, mobility, water and waste management, and energy efficiency); and (3) To identify common challenges and opportunities of smart city deployments in the proposed six topics.
