Theranostic nanocomplex of gold-decorated upconversion nanoparticles for optical imaging and temperature-controlled photothermal therapy

• UCNPs were covalently decorated with AuNPs via carbodiimide crosslinking reaction. • The UCNPs-AuNPs complex allows imaging and temperature-controlled photothermal therapy. • The NIR-induced hyperthermia was able to reduce the breast cancer cell viability. The multifunctional hybrid nanomaterials could bring alternative solutions to current public health problems like cancer diagnosis and treatment. In this work, the near-infrared-activated NaYF 4 :Yb,Er up-conversion nanoparticles (UCNPs) were synthesized and covalently decorated with sub-10 nm gold nanoparticles (AuNPs) in a 10 min reaction. The UCNPs in the UCNPs-AuNPs complex converted the deep-penetrating 975 nm near-infrared photons into visible emissions, which were simultaneously used for multiple applications: i) plasmon-induced photothermal therapy, ii) in situ sensing and control of the temperature (nano-thermometer), and iii) contrast agent for fluorescence imaging and cell tracking at the tissues transparency window. The effective energy transfer of green emissions and consequent temperature increment was enhanced by the short separation between donor (UCNPs) and acceptor nanoparticles (AuNPs). The induced hyperthermia locally triggered irreversible cancer cell damage, considerably reducing the cell viability upon 5 min of NIR irradiation but being practically inert in the absence of infrared light exposure. In addition, the non-transferred fractions of the 525 and 545 nm green emission bands were suitable for the ratiometric temperature sensing in a physiologically relevant range (25–50 °C), which allowed to accurately monitor and control the heat generation during the photothermal therapy application (with effective temperature increase from 37 up to 45 °C). Since the 659 nm red emission was only slightly affected by the attached-gold nanospheres, the UCNPs-AuNPs nanocomplex were used to track the MCF-7 breast cancer cells. The simplicity and functionality of this nanocomplex make it an attractive alternative for simultaneous breast cancer detection and temperature-controlled photothermal therapy.