Ionophore-Based Ion-Selective Nanospheres Based on Monomer-Dimer Conversion in the Near-Infrared Region

Here, we report ion-selective nanospheres with readout in the near-infrared (NIR) region in both fluorescence and absorbance modes. The nanospheres rely on an ionophore-mediated monomer-dimer conversion of an NIR transducer, DTTC. The DTTC monomer in the nanospheres emits fluorescence around 820 nm, while the dimer in the aqueous environment generates strong blue-shifted emission around 660 nm. With a lead ionophore, an unprecedented lower detection limit of 3 pM for Pb²⁺ was achieved, allowing us to determine Pb²⁺ levels in river water without diluting the sample. Also, the Cu²⁺-selective nanospheres showed a detection limit of 5 nM. Taking advantage of the biologically desired NIR window, blood potassium concentrations were also determined without a complicated sample pretreatment. The sensing process was explained with a theoretical model. The detection range was found finely adjustable by the amount of nanospheres used. Therefore, the nanospheres formed a highly selective, sensitive, versatile, and rapid analytical platform for metal-ion sensing.