Designing biocompatible and multicolor fluorescent hydroxyapatite nanoparticles for cell-imaging applications T.R. Machado a, * , I.S. Leite b , N.M. Inada b , M.S. Li b , J.S. da Silva a , J. Andr  es c , H. Beltr  an-Mir d , E. Cordoncillo d , E. Longo a a Departamento de Química, CDMF, Universidade Federal de S~ ao Carlos (UFSCar), 13565-905 S~ ao Carlos, S~ ao Paulo, Brazil b Instituto de Física de S~ ao Carlos, Universidade de S~ ao Paulo (USP), 13560-970 S~ ao Carlos, S~ ao Paulo, Brazil c Departamento de Química Física I Analítica, Universitat Jaume I (UJI), 12071 Castell o de La Plana, Spain d Departament de Química Inorg anica I Org anica, Universitat Jaume I (UJI), 12071 Castell o de La Plana, Spain article info Article history: Received 26 August 2019 Received in revised form 10 October 2019 Accepted 14 October 2019 Available online xxx Keywords: Hydroxyapatite Bioimaging Defects Fluorescence Chemical precipitation abstract In recent years, there has been a growing effort toward the synthesis, engineering and property tuning of biocompatible nanoparticles (NPs) that can be detected by confocal microscopy and then used as fluo- rescent probes. Defect-related fluorescent hydroxyapatite (HA) is attracting considerable attention as a suitable material for cell-imaging owing to its excellent biocompatibility, biodegradability, easy cell internalization capability, and its stable and intense blue fluorescence. Although the self-activated fluorescence of HA is advantageous, as it avoids the use of lanthanide dopants, organic dyes, or the need to be combined with other fluorescent inorganic nanocrystals, its preparation by simple procedures with fine control of the defects which govern this property remains challenging. In this study, we pro- pose a new, simple, and cost-effective strategy of fluorescence imaging using HA nanorods (HAnrs) obtained by chemical precipitation followed by heat treatment at relative low temperature (350  C) without using any sophisticated equipment or inorganic/organic additives. Structural, compositional, and morphological analysis, as well as a cytotoxicity assay, are described in detail. The fluorescence char- acterization of HAnrs shows an intense bluish-white broad-band emission (l max ¼ 535 nm), and the defects which cause this behavior are studied by temperature-dependent photoluminescence mea- surements (38e300 K). Moreover, the high density of defects in heat-treated HA leads to tunable fluo- rescent property (l max ¼ 399e650 nm) across the entire visible spectrum as a function of the excitation wavelength (l exc ¼ 330e630 nm) with the potential for further multicolor imaging applications. Labeling results by confocal microscopy show that HAnr co-cultured with human dermal fibroblast cell line exhibit fluorescence signals in the cells even after 48 h of incubation with no evident cytotoxic effects. Therefore, heat-treated fluorescent HAnr can be utilized for tracking and monitoring cells and is a safe alternative for the traditional probes used in bioimaging procedures. © 2019 Elsevier Ltd. All rights reserved. 1. Introduction Fluorescence imaging (or simply bioimaging) is one of the most common methods in bioscience for specific and high-resolution imaging of biological matter. This is a versatile, non-invasive, and low-cost technique which provides detailed information on bio- logical structures and intracellular events with enhanced contrast [1]. The principle for imaging is based on making samples or cells fluorescent from the emitted signal of probes composed of (i) common fluorophores, fluorescent proteins, and lanthanide complexes or (ii) fluorescent systems based on biocompatible nanoparticles (NPs) [2]. Fluorescent probes can significantly improve the effectiveness of diagnosis and therapeutic procedures, such as by the association of imaging and chemotherapy of meta- static tumor cells [3]. In special, there is a vast list of fluorescent NPs which can be internalized into cells for specific intracellular im- aging procedures, including those comprised of polymers [4], up- and down-converting inorganic nanocrystals [5e7], carbonaceous materials [8,9], noble metals [10, 11], or their combination in so- phisticated nanoarchitectures [12e14]. Their surface chemistry can * Corresponding author. E-mail address: tmachado.quimica@gmail.com (T.R. Machado). Contents lists available at ScienceDirect Materials Today Chemistry journal homepage: www.journals.elsevier.com/materials-today-chemistry/ https://doi.org/10.1016/j.mtchem.2019.100211 2468-5194/© 2019 Elsevier Ltd. All rights reserved. Materials Today Chemistry xxx (xxxx) xxx Please cite this article as: T.R. Machado et al., Designing biocompatible and multicolor fluorescent hydroxyapatite nanoparticles for cell-imaging applications, Materials Today Chemistry, https://doi.org/10.1016/j.mtchem.2019.100211