Efficient synthesis of a new bifunctional Cu(I) chelator

Rational Design of a Water-Soluble, Lipid-Compatible Fluorescent Probe for Cu(I) with Sub-Part-Per-Trillion Sensitivity

Knowledge-driven optimization of the ligand and fluorophore architectures yielded an ultrasensitive Cu(i)-selective fluorescent probe featuring a 180-fold fluorescence contrast and 41% quantum yield.

Fluorescence probes represent an attractive solution for the detection of the biologically important Cu(i) cation; however, achieving a bright, high-contrast response has been a challenging goal. Concluding from previous studies on pyrazoline-based fluorescent Cu(i) probes, the maximum attainable fluorescence contrast and quantum yield were limited due to several non-radiative deactivation mechanisms, including ternary complex formation, excited state protonation, and colloidal aggregation in aqueous solution. Through knowledge-driven optimization of the ligand and fluorophore architectures, we overcame these limitations in the design of CTAP-3, a Cu(i)-selective fluorescent probe offering a 180-fold fluorescence enhancement, 41% quantum yield, and a limit of detection in the sub-part-per-trillion concentration range. In contrast to lipophilic Cu(i)-probes, CTAP-3 does not aggregate and interacts only weakly with lipid bilayers, thus maintaining a high contrast ratio even in the presence of liposomes.

Influence of PNIPAm on log K(f) of a copolymerized 2,2'-bipyridine: revised bifunctional ligand design for ratiometric metal-ion sensing

Here we describe the synthesis of a model compound (1) based upon a previously reported bifunctional 2,2'-bipyridine (2). Ligand pKa and thermodynamic stability constants were investigated by potentiometric titrations for 1 in order to assess the metal-binding capabilities of 2 following subsequent incorporation within a temperature-responsive polymer that functions as a fluorescent metal-ion indicator. While the log KCu1 measured here was found to be 8.86 ± 0.05 at 25 °C, this value was previously seen to fall 2.8 orders of magnitude following copolymerization of 2 with poly(N-isopropylacrylamide) (PNIPAm). This drop in affinity was attributed to stabilization of the neutral ligand by the polymer environment and elevated temperatures at which metal-binding experiments were performed. ΔH (-54.4 kJ mol(-1)) and ΔS (-12.8 J K(-1) mol(-1)) were therefore determined through variable temperature titrations in order to establish the temperature dependence of log KCu1. Doing so enabled elucidation of the overall effect that the polymer environment exerts on thermodynamic stability of copolymerized 2. Specifically, the polymer indicator was found to decrease the thermodynamic stability by 2.2 orders of magnitude, whereas elevated temperatures account for the additional 0.6 order of magnitude drop observed. This finding has implications regarding the design of future bifunctional ligands for ratiometric sensing within our temperature-responsive polymer indicator.