Without a doubt the Internet of Things (IoT) paradigm has been gaining attention over the last years. With the deployment of low cost sensors, a plethora of information is made available, giving rise to intelligent systems that combine data from multiple sources. Multiple IoT platforms have been developed to support different application domains such as smart cities, health, industry 4.0, etc. However, the vertical focus and usually isolated development of such platforms pose difficulties in offering cross-domain smart applications to meet society's needs. To this end, interoperability frameworks have emerged to allow the interplay of different platforms as well as the formation of federations among the entities contributing IoT devices through such platforms. The decentralized nature of IoT data federations presents challenges when it comes to their management and governance, particularly in ensuring fairness, security, trustworthiness and transparency among its members. However, permissioned blockchain-based solutions hold significant potential in addressing these issues due to their inherent characteristics. In this work, we propose a blockchain solution based on the Hyperledger Fabric for the decentralized management of IoT federations and their gover-nance by utilizing voting-based configurable rules that abide to the concept of Decentralized Autonomous Organizations (DAOs). The solution includes smart contracts for the secure creation and management of federations exposed through APIs, as well as a voting application for enabling the DAO dynamic. Our solution demonstrates a secure, trustworthy and transparent way for the formation and membership control of IoT federations, that holds potential for extension with smart contracts related to data marketplace, as well as reputation and tokenization mechanisms.

In recent years, on the one hand, we have witnessed the rise of blockchain technology, which has led to better transparency, traceability, and therefore, trustworthy exchange of digital assets among different actors. On the other hand, achieving trustworthy content exchange has been one of the primary objectives of the Semantic Web, part of the World Wide Web Consortium. Semantic Web and blockchain technologies are the fundamental building blocks of Web3 (the third version of the Internet), which aims to link data through a decentralized approach. Blockchain provides a decentralized and secure framework for users to safeguard their data and take control over their data and Web3 experiences. However, developing trustworthy decentralized applications (Dapps) is a challenge because many blockchain-based functionalities must be developed from scratch, and combined with data semantics to open new innovative opportunities. In this survey paper, we explore the cross-cutting domain of the Semantic Web and blockchain and identify the critical building blocks required to achieve trust in the Next-Generation Internet. The application domains that could benefit from these technologies are also investigated. We developed a deep analysis of the published literature between 2015 and 2023. We performed our analysis in different digital libraries (e.g., Elsevier, IEEE, ACM), and as a result of our research, we retrieved 137 papers, of which 97 were retrieved as relevant to include in the paper. Furthermore, we studied several aspects (e.g., network type, transactions per second) of existing blockchain platforms. Semantic Web and blockchain technologies can be used to realize a verification and certification process for data quality. Examples of mechanisms to achieve this are the Decentralized Identities of the Semantic Web or the various blockchain consensus protocols that help achieve decentralization and realize democratic principles. Therefore, Semantic Web and blockchain technologies should be combined to achieve trust in the highly decentralized, semantically complex, and dynamic environments needed to build smart applications of the future.