Highly Efficient versus Null Electrochemical Enantioselective Recognition Controlled by Achiral Colinkers in Homochiral Metal–Organic Frameworks

Chiral materials capable of electrochemical enantiomeric
recognition
are highly desirable for many applications, but it is still very challenging
to achieve high recognition efficiency for lack of the knowledge of
structure–property relationships. Here, we report the completely
distinct enantiomeric recognition related to slightly different achiral
colinkers in isomorphic homochiral metal–organic frameworks
with the same chiral linker. Cu-TBPBe, for which the achiral colinker
has two pyridyl rings connected by CHCH, shows
excellent enantioselectivity and sensitivity for electrochemical recognition
of l-tryptophan (Trp) with a detection limit of 3.16 nM.
The l-to-d ratio of differential pulse voltammetric
(DPV) currents reaches 53, which is much higher than the values (2–14)
reported for previous electrochemical sensors. By contrast, Cu-TBPBa,
in which the achiral colinker has −CH2–CH2– between pyridyl rings, is incapable of discrimination
between l-Trp and d-Trp. Structural and spectral
analyses suggest that the achiral conjugated colinker and the chiral
moieties around it cooperate to produce a chiral pocket in favor of
enantioselective adsorption through multiple hydrogen-bonding and
π–π stacking interactions. The work demonstrated
that Cu-TBPBe can be used to fabricate reliable electrochemical sensors
for ultrasensitive quantification of Trp enantiomers in racemic mixtures
and in complex biological systems such as urine. The work also highlights
that an achiral coligand can be of vital importance in determining
enantiomeric discrimination, opening up a new avenue for the design
of chiral sensing materials.