CuInS₂/ZnS Colloidal Quantum Dots as a Luminescent Nanothermometers

In this work, we present the performance of CuInS₂/ZnS (CIS/ZnS) quantum dots (QDs) employed as fluorescent nanothermometers. We find that both the intensity and wavelength of the photoluminescence (PL) peak is sensitive to temperature and optimize the growth time of ZnS for maximum thermometer sensitivity.<br/>CIS/ZnS QDs are particularly suitable for bio-applications because of environmentally friendly synthesis and absence of photobleaching. Moreover, the size-dependent photoluminescence (PL) and absorption, low power excitation, and long PL lifetimes make them ideal fluorescence markers. In this work, we demonstrate a dual functionality of CIS/ZnS QDs: as biolabels and nanothermometers.<br/><br/>We synthesised CuInS₂ QDs, according to the method described in Ref. [1]. As a result, we obtained CIS QDs with a pyramidal shape, chalcopyrite structure, and an average size of about 2.3 nm. As recently reported, CIS QDs exhibit significant cytotoxicity. In order to produce non-toxic QDs, a ZnS layer was grown around the CIS QD cores as reported in Ref. [2]. We investigated 5 samples with ZnS growth time varied from 0 to 180 minutes. Increasing the growth time resulted in a blue-shift of the PL peak from 673 to 606 nm. Concomitantly, the PL quantum yield increased from 0,9% to 13%. Due to dodecanethiol capping ligands, as-synthesized QDs were hydrophobic. The use of QDs as nanothermometers requires aqueous environment. Therefore, QDs were encapsulated by micelles, as reported in Ref. [3]. To test the optical and thermal stability of the hydrophilic QDs, the PL was measured in 2 hours and 3 cycles of temperature changes from 20^oC to 54^oC. After this time, the PL intensity decreased only by up to 15%. The nanothermometers were then calibrated by measurements in steps of 2^oC. Increasing the temperature to 54^oC, resulted in a PL intensity decrease by up to 70%. We interpret this decrease as a result of thermal activation of non-radiative processes. Moreover, the PL peak red-shifted by up to 10 nm. We interpret the red-shift as a result of closing of the band gap. The two effects afford a precise calibration of the nanothermometer. We found the highest sensitivity — reaching 1.3 %/^oC — is at least comparable to other QD systems. Confocal imaging demonstrated successful entering of CIS/ZnS QDs into HeLa cells. Finally, preliminary Presto Blue cell viability assay revealed negligible CIS/ZnS QD cytotoxicity in HeLa cells.<br/><br/><br/>[1] Li L., et al., Am. Chem. Soc. 2011, 133, 5, 1176–1179<br/>[2] Speranskaya E. S., et al., Langmuir 2014, 30, 25, 7567–7575<br/>[3] Zhang H., et al., J. Mater. Chem. B, 2019,7, 2835-2844<br/><br/>Acknowlegments<br/>This work was supported by National Science Centre Poland grant no. 2019/35/B/ST3/04235 and has been done in the NanoFun laboratories co-financed by the European Regional Development Fund within the Innovation Economy Operational Program, the Project No. POIG.02.02.00-00-025/09/. The research was also partially supported by the project no. UMO-2016/22/E/NZ1/00656.