(100a) Triplet-Triplet Annihilation Upconversion-Based Nanosensors for Fluorescent Detection of Potassium

Ionophore-based optical
nanosensors offer many advantages over traditional organic indicators for
biological monitoring including response tuning, multi-analyte detection, and
ease of development. Typical ionophore-based nanosensors use nile blue-derived
indicators called chromoionophores which must contend with strong background
absorption, autofluorescence, and scattering in biological samples that limit
their usefulness. Here, we demonstrate potassium-selective nanosensors that
utilize triplet-triplet annihilation upconversion to minimize potential optical
interference in biological media and a pH-sensitive quencher molecule to
modulate upconversion intensity in response to changes in analyte
concentration. A triplet-triplet annihilation dye pair (platinum (II)
octaethylporphyrin and 9,10-diphenylanthracene) was integrated into nanosensors
containing an analyte binding ligand (ionophore), charge-balancing additive,
and a pH indicator quencher.

Figure 1. Schematic of nanosensor components co-loaded into the
nanosensor matrix and fluorescence quenching mechanism in response to
decreasing potassium (K⁺) concentrations.

In comparison to
lanthanide systems, TTA-UC requires relatively low power excitation and can be
wavelength tuned by choice of sensitizer and annihilator. The
sensitizer-annihilator dye pairs are organic dyes that are often commercially
available, soluble in the organic phase, and able to be implemented into IBOS.
TTA-UC has been utilized in several forms as a biological imaging agent. TTA-UC
has found limited application in sensing platforms. Thus far, it has been
limited to Borisov et al. creating a polymer optode film to directly sense
oxygen. However, there are already numerous optical oxygen sensing platforms
available. To our knowledge, there have been no reports of TTA-UC used in a
more general sensing platform such as those based on ionophores. Nanosensor
response to potassium was shown to be reversible and stable for three days. In
addition, the nanosensors are selective against sodium and calcium (selectivity
coefficient of -0.23 for calcium), the two main interfering ions in biological
samples.