Ratiometric multisensing with heteroaggregates of aqueous carbon quantum dots and rare earth doped nanocrystals

Combinations of two different colloidal nanostructures can inherit properties of both and give access to new, emerging properties. Here, we create optically downconverting heteroaggregates of carbon quantum dots and lanthanide doped nanocrystals with high quantum yields and stability against blinking and bleaching. These heteroaggregates are heavy metal-free and display visible emissions that are suitable for multimodal sensing. Red emitting europium Eu³⁺ doped CaF₂ nanoparticles were combined with orange emitting carbon quantum dots prepared by solvothermal reactions. Simple mixing led to the formation of heteroaggregates as confirmed by dynamic light scattering and zeta potential analysis. The assembly caused electromagnetic coupling, changed the particles' response to temperature and pH in aqueous dispersion, and increased the sensing performance. Emission dropped by 39% at 550 nm (of carbon quantum dots) and 8% at 613 nm (of Eu³⁺ doped CaF₂ nanocrystals) from 17% and 33% of the pure nanoparticle dispersions. Changing pH from 1.3 to 12.5 quenched emissions by 92% and 74% at the same wavelengths, increasing from 62% and 47% of the pure dispersions. Ratiometric calibrations based on the intensity ratio of the 550 nm and 613 nm emissions were constructed to use the heteroaggregates as sensors. They performed as optical thermometers in the range from 293 K to 333 K with a temperature resolution of 0.35 K, and pH values within a range from 1.3 to 12.5 with a resolution of 0.15 pH units. Our results show that heteroaggregates of coupled nanostructures provide improved performance in temperature and pH sensing.