In this work, we demonstrate the performance of CuInS2/ZnS core/shell quantum dots as spectroscopic nanothermometers.
Temperature detection is one of the key issues in intracellular research. Monitoring changes in the temperature allow us to track processes taking place in the cell. Therefore, it is important to find a non-invasive, non-toxic material that will enable temperature detection in individual parts of the cell. For spectroscopic temperature detection, semiconductor quantum dots(QDs) offer stable optical signals and broadband absorption–two characteristics that other systems such as fluorescent polymers or upconverting nanoparticles, lack.
In this work, we investigate the structural and optical properties of CuInS2/ZnS (CIS/ZnS) core/shell QDs and demonstrate their performance as nanothermometer. The advantage of CIS/ZnS in comparison to other QD materials is that they do not contain toxic heavy metal ions.
CIS/ZnS QDs were synthesized according to the procedure described in Ref.[1], which resulted in suspensions of QDs in toluene. X-ray diffraction studies reveal that the QDs exhibit chalcopyrite structure. Transmission electron micrographs show QDs with pyramidal shapes. Absorption measurements show featureless spectra as reported before [2]. The analysis of these spectra nevertheless allow to determine the average size of the QDs and we find that core-only CIS QDs reveal sizes about 2.3 nm. The photoluminescence (PL) spectra appear strongly red-shifted with respect to the absorption onset. The growth of the ZnS shell was performed using the procedure from Ref [3]. Increasing the growth time leads to a blue-shift of both the PL peak and the absorption onset. To transfer the QDs to water (i.e. to a biocompatible environment), we applied a procedure described in Ref. [4], whereby the CIS/ZnS QDs were encapsulated into micelles. PL spectra measured on water suspensions show a slight red-shift of the spectral features.
In general, increasing increased temperature activates non-radiative recombination channels due to surface states and leads to a decrease of the PL intensity. We exploit this mechanism to detect the temperature. Indeed, we find that the PL intensity decreases by almost 50% as the temperature is increased from 20^oC to 54^oC. We also monitor the optical stability of the samples and find that the PL intensity changes less than 5% over the course of about 2 hours and three temperature cycles. Furthermore, we determine the absolute sensitivity of 1.3% ^oC¹. This work proves that CIS/ZnS QDs can be employed as spectroscopic nanothermomenters in biological applications.