Despite
its simplicity and specificity, enzyme-linked immunosorbent
assay (ELISA) requires conjugation of the enzyme to an antibody and
preservation of enzymatic activity during storage and assay. In light
of the high porosity and enormous surface area possessed by metal–organic
frameworks (MOFs), we constructed an immunosensor by integrating antibody
and fluorescent probe (coumarin or COU) with zeolitic imidazolate
framework-8 (ZIF-8), in which about 34 COU molecules can be encapsulated
per ZIF-8. After coating of ZIF-8 with the secondary antibody specific
to cardiac troponin I (cTnI), the final MOF@COU/Ab2 composite
renders an enzyme-free assay that amplifies the detection signal via
release of alkaline-hydrolyzed COU from the MOF interior. Owing to
the isolation of the fluorescent probe before the readout step, our
immunosensors are highly stable and reproducible, with a relative
standard deviation of less than 1.5% for repetitive assays in a 30-day
period. Our MOF-based immunoassay is also highly sensitive and possesses
a wide dynamic range (11.1 fM to 35.6 pM). The detection limit was
estimated to be 4.4 fM (0.099 pg·mL–1), which
is ∼180-fold lower than that of ELISA. We quantified cTnI in
clinical serum samples, yielding results highly comparable to those
measured by ELISA. Particularly remarkable is that the low detection
limit enabled us to quantify cTnl in sera of healthy persons, including
levels that are close to the high-risk threshold but not detectable
by ELISA. The generality of this cost-effective and stable fluorescent
immunosensor renders versatilities for sensitive detections of many
other protein biomarkers.