Amorphous calcium phosphate nanoparticles allow fingerprint detection via self-activated luminescence

Herein, stable activator-free photoluminescent nanoparticles based on amorphous calcium phosphate (ACP) were obtained for the first time, and their performance for latent fingerprint imaging was investigated. ACP nano particles with irregular rounded shape and diameters in 10-40 nm range were prepared by a rapid and simple chemical precipitation followed by heat treatment at 400 C for 4 h. Notably, diffuse reflectance spectroscopy, photoluminescence excitation and emission measurements reveal a high density of localized energy states within the wide optical band gap of heat-treated ACP (E-g = 5.25-5.42 eV). This behavior allowed the excitation of ACP in near-ultraviolet region (lambda(exc) = 450 nm, 2.75 eV), leading to an intense defect-related broadband (490-890 nm) photoluminescence emission centered at 540 nm (2.30 eV), 50 times more intense than untreated nanoparticles. The effect of lattice shrinkage due to structural water elimination, the presence of ionic vacancies (V(Ca )and V-O in PO43-) and carbonate groups (CO32-), on the luminescent properties of ACP were discussed in detail, as well as investigated after aqueous-mediated ACP crystallization into hydroxyapatite (HA). ACP nanoparticles were noncytotoxic, as determined by MTT assay in Human Dermal Fibroblast neonatal (HDFn) cell line, with cell viabilities superior to 95% in all tested concentrations (20-320 mu g/mL), after incubation for 24 and 48 h. Latent fingerprint images were obtained using the ACP nanoparticles under near-ultraviolet irradiation (lambda(exc) = 450 nm) in tweezers and LCD smartphone and successfully validated by the Integrated Automated Fingerprint Identification System used by Scientific Police in Spain. The present results evidenced that the new luminescent ACP nanoparticles are safe to be used and agree with the forensic requirements for future legal actions.