# From NFT 1.0 to NFT 2.0: A Review of the Evolution of Non-Fungible Tokens
## 3 Blockchain and Solutions for NFT
### Blockchain for NFTs  
In this Section, we provide an overview of the main used blockchain for NFT projects to highlight their characteristics, by taking into account the required criteria listed above. Table 4 reports the comparison of the platforms by also showing the NFT standards used by each blockchain. The comparison proposed takes into account transaction speed (transactions per second, tps), confirmation time (time required to confirm a new block), transaction cost fee (free for a standard transaction that transfers an NFT), smart contract programming language, consensus protocol, NFT standards available on the blockchain, and the application field. To determine the application field, we evaluated the top trending collections (by number of NFT transfers and volume) in the largest marketplace for each blockchain, and reported the most common ones. Most NFT projects are proposed on _Ethereum_, which has the two popular token standards ERC-721 and ERC-1155, described above. NFTs are compatible with anything built using Ethereum. To implement smart contracts, Ethereum uses Solidity [17], an object-oriented programming language compiled by the Ethereum Virtual Machine (EVM). The blockchain also features its crypto, called ether (or ETH). The Ethereum transaction fees are based on the network congestion [47] and are usually between USD 20 and USD 60 per transaction (the highest average transaction fee of USD 200 was registered on 1 May 2022) ([[https://etherscan.io/chart/avg-txfe-usd](https://etherscan.io/chart/avg-txfe-usd)]([https://etherscan.io/chart/avg-txfe-usd](https://etherscan.io/chart/avg-txfe-usd)), accessed on 23 May 2023), which is higher compared to other blockchains. Moreover, Ethereum suffers from low throughput. Indeed, it permits 13-15 transactions per second. Nowadays, the top NFT marketplaces have been created on Ethereum, such as OpenSea, Rarible, and SuperRare [48], which explains why the blockchain supports a wide range of application fields. The _Flow_ blockchain ([[https://www.onflow.org/](https://www.onflow.org/)]([https://www.onflow.org/](https://www.onflow.org/)), accessed on 23 May 2023) is referred to as a good alternative to Ethereum. It is a new blockchain built to support arts- and sports-related NFTs. Flow started because of the popular game CryptoKitties. Indeed, the creators of CryptoKitties went to work on fixing issues, and in the process, they created Flow. The FLOW blockchain improves Ethereum's ERC-721 and ERC-1155 standards to provide an NFT implementation that can offer more support to applications. It was proposed in 2020, and it uses a PoS consensus algorithm capable of powering several application categories, such as games. Flow blockchain provides smart contracts programmed in Cadence, the programming language developed by Flow developers, and it ensures greater scalability. Indeed, it can execute more than 10,000 transactions per second. Flow has two fees: the first one is for creating an account, which starts at FLOW 0.001 (the native token), and the other is a transaction fee that is about FLOW 0.000001. _Cardano_ is a PoS blockchain [49] to create a convenient environment for developers. Cardano has two layers: the Cardano Settlement Layer (CSL), used for transferring its crypto value ADA between accounts and recording transactions; and the Cardano Computation Layer (CCL), which contains the data on how values are transferred [50]. Plutus is the smart contract platform of Cardano, and it is based on Haskell [51]. Haskell is also used for Marlowe, the domain-specific language used for creating financial smart contracts [52]. Cardano is capable of processing more than 250 transactions per second. As far as fees are concerned, it reaches about ADA 0.16-0.17 ([[https://www.statista.com/statistics/1278720/gas-price-cardano/](https://www.statista.com/statistics/1278720/gas-price-cardano/)]([https://www.statista.com/statistics/1278720/gas-price-cardano/](https://www.statista.com/statistics/1278720/gas-price-cardano/)), accessed on 23 May 2023).