Journal of Luminescence 223 (2020) 117237 Available online 28 March 2020 0022-2313/© 2020 Elsevier B.V. All rights reserved. Temperature induced emission enhancement and investigation of Nd 3þ →Yb 3þ energy transfer efficiency in NaGdF 4 :Nd 3þ , Yb 3þ , Er 3þ upconverting nanoparticles I. Mikalauskaite , G. Pleckaityte , L. Sinusaite , V. Plausinaitiene , A. Katelnikovas * , A. Beganskiene Institute of Chemistry, Vilnius University, Naugarduko 24, LT-03225, Vilnius, Lithuania A R T I C L E INFO Keywords: Downconversion Upconversion Temperature induced luminescence NaGdF 4 Nd 3þ co-doping ABSTRACT Inorganic upconverting nanoparticles that can be excited with 808 nm laser radiation are gaining much attention recently. Living tissues, as well as water do not heat up as intensively when embedded upconverting nano- particles are excited under such wavelength if compared to commonly used 980 nm laser radiation for excitation. To this end, a series of NaGd 0.995-x Yb x Nd 0.005 F 4 nanoparticles were synthesized in order to investigate energy transfer efficiency between Nd 3þ and Yb 3þ dopants under 808 nm laser excitation to better suit biological ap- plications. The impact of Yb 3þ concentration increase on crystal phase formation as well as downconversion (DC) properties was investigated. Temperature dependent luminescence measurements in NIR region revealed that no thermal quenching of Nd 3þ  Yb 3þ pair in NaGdF 4 system was observed in the entire 77–500 K temperature range. Finally, Er 3þ ions were introduced in the investigated system in order to obtain multiple excitation and visualisation tool with performance in both VIS and NIR regions. The effective energy difference (ΔE) was also calculated from the thermally coupled levels (TCLs) of Er 3þ . The promising results and observations of syn- thesized compounds will be discussed, as NaGd 0.475 Yb 0.5 Er 0.02 Nd 0.005 F 4 has a potential to be used as a lumi- nescent temperature sensor. 1. Introduction Upconverting (UC) nanoparticles (NPs) have been considered as a beneficial luminescent tool in tissue and cell imaging fields. Despite the promising future perspectives, the performance improvement of these nanomaterials is still necessary in order to meet the requirements for practical application. The major challenges that limit the successful application of such nanomaterials in biomedical field are overheating of biological tissues when 980 nm laser is used for excitation, and low luminescence efficiency of upconversion process [1]. The conventional bio-imaging techniques utilize fluorescent dyes or quantum dots. In this sense, the upconverting NPs have lots of advantages, because the near infrared (NIR) radiation used to excite these NPs, can penetrate deeper into the tissue (several centimetres [1]) if compared to short wavelength radiation used to excite fluorescent dyes or quantum dots. Moreover, UC NPs provide narrower emission bands (lines) and weaker auto- fluorescence [2,3]. Recently, a 915 nm excitation wavelength was pro- posed for NaYbF 4 system yielding lower temperature rise of biological tissue sample over the prolonged irradiation [4]. Another way of shifting excitation to even shorter wavelengths is adding Nd 3þ ions into the inorganic matrices. Neodymium ions are frequently considered as an alternative sensitizer in UC systems since they possess larger absorption cross-section [5] than Yb 3þ ions, and several absorption bands in the NIR region where absorption of water is relatively weak. Thus, a deep tissue imaging using NIR emission of Nd 3þ ions is also considered as an advantageous approach, which eliminates the negative impact of the biological components from the outcoming signal [6,7]. To avoid any overheating and suppression of signal due to the interaction with bio- logical tissue, an emission in the second near infrared (NIR-II) biological window has been proposed as a subject for investigation. There are several lanthanide ions (i.e., Tm 3þ , Nd 3þ , Yb 3þ , Pr 3þ , Er 3þ , and Ho 3þ ) possessing emission bands in the NIR-II region. These bands can be employed in bio-imaging through downconversion (DC) approach [8–10]. The advantage of Nd 3þ doping is the shift of excitation wave- length to ca. 808 nm, what significantly reduces the overheating effect. Besides, an additional combination of DC and UC emission in single * Corresponding author. E-mail address: arturas.katelnikovas@chf.vu.lt (A. Katelnikovas). Contents lists available at ScienceDirect Journal of Luminescence journal homepage: http://www.elsevier.com/locate/jlumin https://doi.org/10.1016/j.jlumin.2020.117237 Received 15 November 2019; Received in revised form 19 March 2020; Accepted 21 March 2020