## 1 Introduction
Moreover, although this information is somehow overlooked, recent and current studies are progressively showing the complexity of this visible fluorescence, as selective emissions in different wavelength ranges can be stimulated. Indeed, it has been demonstrated that some specific amyloids may produce multicolor fluorescence with the maximum of the emission located in the blue, green, red, and even in the infrared region of the electromagnetic spectrum (see Figure 2). Figure 1: Lateral (**A**) and top (**B**) view of amyloid fibrils formed by the low-complexity domain of TDP-43 ( TDP-43 LCD) in the cryo-EM structure (PDB code 7 KWZ). In Panel (**A**), ribbons are represented with a color code (blue to red) from the N- to the C-terminus. The model has a similar orientation in Panel (**B**). The H-bonding pattern of the parallel \(\beta\)-sheet formed by residues 371-378 is shown in Panel (**C**), in which the N-terminal side of the chain is on the left. It is important to note, however, that, despite the progressive interest in this peculiar phenomenon and its many implications in both basic and applied science, its underlying structural and electronic basis is yet to be unraveled. Although different hypotheses have been proposed, the topic is still highly debated and no consensus has been hitherto achieved. In recent years, several insightful reviews have focused on different aspects of fluorescence emission by amyloids. In particular, these studies have addressed both basic aspects of this puzzling phenomenon as well as promising applications [9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26]. Nevertheless, systematic surveys of amyloid systems that can emit visible fluorescence and their spectroscopic properties have not yet been provided. An exhaustive analysis of the literature reporting the fluorescence emission of proteins and peptides in the amyloid state could represent a useful source of information for studies aimed at identifying the elusive structural basis of this intriguing phenomenon and stimulating new applications. We here report a systematic analysis of the papers reporting original data on this topic. Interestingly, as the discovery of the visible fluorescence by amyloids has stimulated similar studies on self-assembling systems with a smaller size, such as individual amino acids, here, we also survey the visible fluorescence of these compounds, as they could share a common underlying mechanism. To provide quantitative information on this puzzling phenomenon, in vitro studies represent the main focus of the manuscript, although in vivo applications will also be mentioned. Exhaustive Search of Literature Articles Describing the Visible Fluorescence Emission by Amyloid-like Systems  
Papers describing intrinsic visible fluorescence emission by amyloid-like systems were identified using multiple PUBMED and Google Scholar searches (February 2023). The papers identified in these searches were individually inspected to check their relevance to the topic of the review. In particular, considering that intrinsic fluorescence has been identified in the literature using different notations (label-free fluorescence, intrinsic fluorescence, photoluminescence, etc. ), several distinct PUBMED searches were performed (Table S1). From this initial survey, we selected 87 manuscripts of potential interest. Due to the heterogeneity in the designation of this fluorescence, we integrated these PUBMED searches with Google Scholar surveys looking at the 185 documents citing one of the pioneering and seminal papers on this subject [27]. This led to the identification of 213 novel manuscripts of interest. All of the 213 selected papers were individually inspected selecting only original reports of fluorescent emission by amyloid-like systems. This further selection meant that we ended up with 92 papers, and from which the information described in the present manuscript was retrieved.