Metal–organic
framework (MOF) thin films are promising materials
for multiple technological applications, such as chemical sensing.
However, one potential limitation for their widespread use in different
settings is their stability in aqueous environments. In the case of
ZIF-8 (zeolitic imidazolate framework) thin films, their stability
in aqueous media is currently a matter of debate. Here, we show that
optical waveguide spectroscopy (OWS), in combination with surface
plasmon resonance (SPR) spectroscopy, offers a convenient way for
answering intriguing questions related to the stability of MOF thin
films in aqueous solutions and, eventually provide a tool for assessing
changes in MOF layers under different environmental conditions. Our
experiments relied on the use of ZIF-8 thin films grown on surface-modified
gold substrates, as optical waveguides. We have found a linear thickness
increase after each growing cycle and observed that the growing characteristics
are strongly influenced by the nature of the primer layer. One of
our findings is that substrate surface modification with a 3-mercapto-1-propanesulfonate
(MPSA) primer layer is critical to achieve ZIF-8 layers that can effectively
act as optical waveguides. We observed that ZIF-8 films are structurally
stable upon exposure to pure water and 50 mM NaCl solutions but they
exhibit a slight swelling and an increase in porosity probably due
to the permeation of the solvent in the intergrain mesoporous cavities.
However, OWS revealed that exposure of ZIF-8 thin films to phosphate-buffered
saline solutions (pH 8) promotes significant film degradation. This
poses an important question as to the prospective use of ZIF-8 materials
in biologically relevant applications. In addition, it was demonstrated
that postsynthetic polyelectrolyte modification of ZIF-8 films has
no detrimental effects on the structural stability of the films
The biofunctionalization of graphene field‐effect transistors (GFETs) through vinylsulfonated‐polyethyleneimine nanoscaffold is presented for enhanced biosensing of severe acute respiratory‐related coronavirus 2 (SARS‐CoV‐2) spike protein and human ferritin, two targets of great importance for the rapid diagnostic and monitoring of individuals with COVID‐19. The heterobifunctional nanoscaffold enables covalent immobilization of binding proteins and antifouling polymers while the whole architecture is attached to graphene by multivalent π–π interactions. First, to optimize the sensing platform, concanavalin A is employed for glycoprotein detection. Then, monoclonal antibodies specific against SARS‐CoV‐2 spike protein and human ferritin are anchored, yielding biosensors with limit of detections of 0.74 and 0.23 nm, and apparent affinity constants () of 6.7 and 8.8 nm, respectively. Both biosensing platforms show good specificity, fast time response, and wide dynamic range (0.1–100 nm). Moreover, SARS‐CoV‐2 spike protein is also detected in spiked nasopharyngeal swab samples. To rigorously validate this biosensing technology, the GFET response is matched with surface plasmon resonance measurements, exhibiting linear correlations (from 2 to 100 ng cm−2) and good agreement in terms of KD values. Finally, the performance of the biosensors fabricated through the nanoscaffold strategy is compared with those obtained through the widely employed monopyrene approach, showing enhanced sensitivity.
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