Blue-LED-excitable NIR-II luminescent lanthanide-doped SrS nanoprobes for ratiometric thermal sensing

ARTICLES SCIENCE CHINA Materials mater.scichina.com link.springer.com Published online 2 November 2021 | https://doi.org/10.1007/s40843-021-1801-8 Blue-LED-excitable NIR-II luminescent lanthanide-doped SrS nanoprobes for ratiometric thermal sensing Jiaojiao Wei1,2, Youyu Liu1,2, Meiran Zhang2, Wei Zheng1,2,3*, Ping Huang1,2,3, Zhongliang Gong2, Renfu Li2,3 and Xueyuan Chen1,2,3* ABSTRACT Lanthanide (Ln3+)-doped near infrared (NIR)-II luminescent nanoprobes have shown great promise in many technological fields, but are currently limited by the low absorption efficiency of Ln3+ due to the forbidden 4f→4f transition. Herein, we report a novel NIR-II luminescent nanoprobe based on efficient energy transfer from Ce3+ to Er3+ and Nd3+ in sub-10 nm SrS nanocrystals (NCs), which are excitable by using a commercial blue light-emitting diode (LED). Through sensitization by the allowed 4f→5d transition of Ce3+, the NCs exhibit strong NIR-II luminescence from Er3+ and Nd3+ with quantum yields of 2.9% and 2.3%, respectively. Furthermore, by utilizing the intense NIR-II luminescence of Er3+ from the thermally coupled Stark sublevels of 4I13/2, we demonstrate the application of SrS:Ce3+/Er3+ NCs as blueLED-excitable NIR-II luminescent nanoprobes for ratiometric thermal sensing. These findings reveal the unique advantages of SrS:Ln3+ NCs in NIR-II luminescence, which may open up a new avenue for exploring novel and versatile luminescent nanoprobes based on Ln3+-doped sulphide NCs. Keywords: SrS, lanthanide, near-infrared II, nanoprobe, thermal sensing INTRODUCTION Lanthanide (Ln3+)-doped luminescent nanocrystals (NCs) emitting in the second near-infrared (NIR-II: 1000–1700 nm) biological window have recently evoked considerable interest, owing to their superior optical properties such as high photochemical stability, sharp emission peaks, long photoluminescence (PL) lifetimes, and large antenna-generated Stokes shift, in parallel with the benefits of minimal background interference and deep tissue penetration of the NIR light [1–9]. These outstanding features make Ln3+-doped NIR-II luminescent NCs ideal candidates as an alternative to traditional fluorescent probes like organic dyes and quantum dots and as a new generation of luminescent nanoprobes in many technological fields, including deep-tissue bioimaging, non-invasive chemical/biological detection, high-seed optical communication, and non-contact thermal sensing [10–22]. Nonetheless, because of the parity-forbidden nature of the 4f→4f electronic transitions, Ln3+-doped NIR-II luminescent NCs normally suffer from low absorption and emission efficiencies, and exhibit low-tomedium brightness when compared with organic dyes and quantum dots [23–29]. Therefore, a high-power laser is generally demanded to realize bright NIR-II luminescence in Ln3+-doped NCs, which may limit their widespread applications. To circumvent the limitation of Ln3+-doped NIR-II luminescent NCs, it is of fundamental importance to introduce an antenna that can effectively harvest the excitation light and sensitize the NIR-II luminescence of Ln3+ emitters [30–33]. In this regard, optical entities of allowed transitions with large absorption cross-sections such as Ce3+, Bi3+, ligand-to-metal charge transfer states, and the host absorption of semiconductors can be effective sensitizers for Ln3+ luminescence [34–37]. The absorption of these species, however, is influenced significantly by the host matrix with respect to the site symmetry, the crystal field (CF) strength, and the covalency [38–41]. Therefore, a judicious screening of host materials for both the sensitizers and Ln3+ emitters is essential to the design of efficient NIR-II luminescent nanoprobes with desired properties. Hitherto, most of the reported Ln3+-doped NIR-II luminescent nanoprobes have been restricted to fluorides, probably due to the well-established synthetic methods for their upconversion analogues [42–45]. Until recently, the new class of NIR-II luminescent nanoprobes based on Ln3+-doped sulphides (namely, CaS and NaGdS2) have been proposed by our group [46,47]. In comparison with fluorides, sulphide NCs exert a stronger CF and a higher covalency on Ln3+ emitters [48–50]. As a result, the absorption and emission of Ce3+ and Eu2+ ions with allowed 4f→5d transitions locate at lower energies in sulphides (in the visible (vis) region) than in fluorides and oxides (in the ultraviolet (UV) region), which promises Ce3+ and Eu2+ activated sulphides as efficient phosphors in white light-emitting diodes (LEDs) [51–53]. Specifically, the absorption of Ce3+ in alkaline-earth sulfides (e.g., CaS and SrS) in the blue region of the spectrum matches well with the emission of the commercial blue-LED chip. This enables the development of blue-LEDexcitable NIR-II luminescent nanoprobes via energy transfer (ET) from Ce3+ to NIR-II Ln3+ emitters [46]. Moreover, owing to the strong CF level splitting, the luminescence of Ln3+ via 4f→4f transitions in the sulphide system is characterized by sharp CF emission peaks even at room temperature (RT), which may facilitate the devise of smart thermal sensors based on NIR-II 1 College of Chemistry, Fuzhou University, Fuzhou 350116, China CAS Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Key Laboratory of Nanomaterials, and State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China 3 Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou 350108, China * Corresponding authors (emails: (Zheng W); (Chen X)) 2 © Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2021 1 ARTICLES SCIENCE CHINA Materials luminescence of Ln3+ from the thermally coupled CF levels [54,55]. In this work, we develop a unique strategy for the controlled synthesis of Ln3+-doped SrS NCs via a high-temperature coprecipitation method. The optical properties, ET processes, and excited-state dynamics of Ln3+ in Ce3+ singly-doped, Ce3+/Er3+ and Ce3+/Nd3+ co-doped SrS NCs are systematically investigated through concentration- and temperature-dependent steady-state and transient PL spectroscopies. Through sensitization by Ce3+, efficient NIR-II luminescence from Er3+ and Nd3+ is achieved for the first time in sub-10 nm SrS NCs. Furthermore, by virtue of the well-resolved CF transition lines from 4I13/2 of Er3+, we show the potential of SrS:Ce3+/Er3+ NCs as blue-LED-excitable NIR-II luminescent nanoprobes for non-contact thermal sensing with a high sensitivity. EXPERIMENTAL SECTION Chemicals and materials The metal acetates of Sr(CH3COO)2·0.5H2O (99.9%), Ce(CH3COO)3·4H2O (99.99%), Er(CH3COO)3·4H2O (99.99%), and Nd(CH3COO)3·4H2O (99.99%) were bought from Aladdin (China). The organic ligands of oleic acid (OA), oleylamine (OAm), 1-octadecene (ODE), and N,N′-diphenylthiourea (DPTU) were purchased from Sigma (...truncated)