Enhancement of Anion Binding in Lanthanide Optical Sensors

Abstract

In the design of molecular sensors, researchers exploit binding interactions that are usually defined in terms of topology and charge complementarity. The formation of complementary arrays of highly cooperative, noncovalent bonding networks facilitates protein-ligand binding, leading to motifs such as the ‘lock-and-key.’ Synthetic molecular sensors often employ metal complexes as key design elements as a way to construct a binding site with the desired shape and charge to achieve target selectivity. In transition metal complexes, coordination number, structure and ligand dynamics are governed primarily by a combination of inner-sphere covalent and outer-sphere noncovalent interactions. These interactions provide a rich variable space that researchers can use to tune structure, stability and dynamics. In contrast, lanthanide(III)-ligand complex formation and ligand-exchange dynamics are dominated by reversible electrostatic and steric interactions, because the unfilled f shell is shielded by the larger, filled d shell. Luminescent lanthanides such as terbium, europium, dysprosium and samarium display many photophysical properties that make them excellent candidates for molecular sensor applications. Complexes of lanthanide ions act as receptors that exhibit a detectable change in metal-based luminescence upon binding of an anion. In our work on sensors for detection of dipicolinate, the unique biomarker of bacterial spores, we discovered that the incorporation of an ancillary ligand (AL) can enhance binding constants of target anions to lanthanide ions by as much as two orders of magnitude. In this Account, we show that selected ALs in lanthanide/anion systems greatly improve sensor performance for medical, planetary science and biodefense applications. We suggest that the observed anion binding enhancement could result from an AL-induced increase in positive charge at the lanthanide ion binding site. This effect depends on lanthanide polarizability, which can be established from the ionization energy of Ln3+ → Ln4+. These results account for the order Tb3+ > Dy3+ > Eu3+ ≈ Sm3+. As with many lanthanide properties, ranging from hydration enthalpy to vaporization energy, this AL-induced enhancement shows a large discrepancy between Tb3+ and Eu3+ despite their similarity in size, a phenomenon known as the ‘gadolinium break.’ This discrepancy, based on the unusual stabilities of the Eu2+ and Tb4+ oxidation states, results from the half-shell effect, as both of these ions have half-filled 4f-shells. The high polarizability of Tb3+ explains the extraordinarily large increase in the binding affinity of anions for terbium compared to other lanthanides. We recommend that researchers consider this AL-induced enhancement when designing lanthanide-macrocycle optical sensors. Ancillary ligands also can reduce the impact of interfering species such as phosphate) commonly found in environmental and physiological samples.

Interfacial interactions of hard polyelectrolyte-stabilized luminescent colloids with substrates

“…This tendency points to the equilibrium (3) as most probable reason for the DF-induced luminescence response, where DF is coordinated with Tb(III) through its basic nitrogen-containing moiety. This coordination mode is less efficient than the well-known coordination through the anionic carboxylate group, but it can be favored by the presence of the ancillary ligand [16]. The previously reported effect of fluoroquinolones on the luminescence of tris-diketonate Eu(III) complexes in solutions and in the polyelectrolyte coated colloids confirms the reliability of the above mentioned coordination mode of fluoroquinolones [15].…”

“…It is well understood that ancillary ligands bound with lanthanide ion facilitate a sensing of chelating substrates through lanthanide-centered luminescence response [16,17]. Thus lanthanide complexes are preferably applied instead of lanthanide ions in sensing.…”

Designed Synthesis of Multiluminescent Materials Using Lanthanide Metal‐Organic Frameworks and Carbon Dots as Building‐Blocks

“…The carboxylic groups are known to coordinate strongly to the lanthanide ions. [15,16,33] Moreover, the combination of distinct emissive centers, the Eu 3+ ion and the CDs, enable the design of a multiluminescent material. Thus, it is expected that the CDs will bind covalently to the MOF framework via the lanthanide ions which have accessible coordination sites.…”

Effect of silica coating and further silica surface decoration by phospholipid bilayer on quenching of Tb(III) complexes by adrenochrome

“…The sensitization of Tb(III) luminescence by EP and dopamine (DA) exemplifies easy route for their luminescence determination in alkaline conditions [20], although the luminescence is not enough for low detection limit. The impact of ancillary ligands in fluorescent response on catecholamines is also highlighted in literature [24]. Unfortunately this approach is not selective enough to discriminate between EP and DA.…”