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Hou et al. Microstructures 2023;3:2023039 https://dx.doi.org/10.20517/microstructures.2023.37 Page 9 of 17

the MOF-derived carbon materials can be tuned to meet specific application requirements; (3) Low cost:
MOF-derived carbon materials can be synthesized from abundant and inexpensive precursors, making
them cost-effective alternatives to other carbon materials and (4) Versatility: MOF-derived carbon materials
can be synthesized in various forms, including powders, fibers, films, and monoliths, which makes them
suitable for a wide range of applications.

In summary, the prospects for the multifunctionality of MOFs are very promising. Their unique properties
make them highly versatile materials that can be tailored for a wide range of applications. We expect that
future MOF-based flame retardants can show their functions in more aspects while ensuring fire safety. As
research in this field continues, we can expect to see even more exciting applications of MOFs in the years
to come.

LIGAND DESIGN OF MOF-BASED FLAME RETARDANTS
Traditional MOFs typically lack flame retardant properties due to the flammability of their organic
components and the absence of other inherent flame-retardant elements, except for transition metals in
metal frameworks. However, researchers have started to try to improve the flame-retardant properties of
MOFs by different methods [Figure 4]. A commonly employed strategy involves the functionalization of
ligand precursors with flame retardant moieties, such as phosphorus or nitrogen-containing groups, prior to
their utilization in MOF synthesis [67,68] . Phosphorus is an effective flame retardant because it can capture
reactive radicals upon heating, which can dilute and quench the flame. Phosphorus-containing ligands, such
as phosphonic acid and phosphonate derivatives, can act as flame retardant additives by releasing
phosphoric acid and other phosphorus-containing species upon thermal decomposition, which promotes
the formation of an expanded carbon layer. To achieve this, phosphonate or phosphonate groups can be
incorporated into the organic linkers used to construct the MOF. In addition, transition metal ions that can
coordinate with phosphorus-containing ligands can be used to enhance the flame-retardant properties of
the MOF. Lu et al. synthesized an organic ligand containing P/N and combined it with Co to form a novel
P-MOF . The researchers found that adding the P/N-treated cobalt MOFs to the lignin-based epoxy resins
[69]
(EP) significantly improved their flame retardancy, as demonstrated by reduced peak heat release rates and
decreased total heat release.

Bio-functional MOFs (Bio-MOFs) are biocompatible and biodegradable porous materials made from
organic and inorganic building blocks with tunable porosity and high surface area. They can be
functionalized with biological molecules to enhance selectivity and specificity, making them useful in
[70]
biotechnology and environmental science . Nabipour et al. used adenine as an efficient adsorbent for in
situ removal of zinc ions, resulting in the formation of a bio-MOF. The as-prepared bio-MOF was then
added to the EP matrix to investigate its impact on the thermal stability, smoke suppression, and flame
retardancy of the EP composites. The results showed that the addition of a boom improved the flame
[71]
retardancy and smoke suppression properties of the EP composites . Zhou et al. present a study on a
sustainable approach for simultaneously achieving flame retardancy, UV protection, and reinforcement in
polylactic acid (PLA) composites using fully bio-based complexing couples. The authors used two bio-based
complexing couples, tannin acid/ferric salt (TAFe) and chitosan/phytic acid (CTSPA), to fabricate the
composites. The thermal and burning properties of the composites were evaluated by various tests,
including TGA, limiting oxygen index (LOI), UL-94 test, and cone calorimetry. The results showed that the
composites with TAFe and CTSPA had an earlier mass loss and higher char residue than pristine PLA.
CTSPA also enhanced the LOI value of PLA from 19.6% to 30.5%. The study demonstrates a promising
approach for developing sustainable composite materials with multiple desirable properties .
[72]