Layered double hydroxides (LDHs) and their composites with various substances represent
an important class of materials suitable for several existing and future applications
in biological, chemical, and environmental processes. The current widespread interest
in these materials is evidenced by recent reviews [1,2,3,4,5] and high-impact journal
articles [6,7,8] published since this Special Issue was launched. The advantageous
properties of these materials, like biocompatibility, lamellar structure, and compositional
diversity, together with the ease of their synthesis even in larger scales, has attracted
the interest of a large number of scientists working in academia and in applied disciplines.
Such an increased research activity led to progressively growing number of publications
in the recent past, seven of which appear in this Special Issue. They include five
original research papers [9,10,11,12,13] and two review articles [14,15] from world-class
research groups. The topics of these papers are in line with the original goal of
the Special Issue, as they range from the synthesis of novel LDHs and LDH-based composites,
through their physico-chemical characterization, to applications as fertilizers, drug
delivery vehicles, and catalysts. In the following paragraphs, a brief overview of
the articles is provided by the guest editor.
In the first paper released in the Special Issue, Ding et al. reported an immobilization
procedure for dextranase enzyme on Mg/Fe-LDH [13]. This system holds great promise
as a catalyst for the hydrolysis of specific bonds in dextran, which can be utilized
in the sugar industry. The work unambiguously pointed out that dextranase adsorption
is feasible on the outer surface of the LDH due to electrostatic and hydrophobic interactions
between the protein and the surfaces. However, the experimental conditions such as
pH and presence of amino acids affect the extent of adsorption significantly. Therefore,
the article provides the optimal experimental conditions for the immobilization process
of dextranase on Mg/Fe-LDH. Because of the widespread interest in enzyme immobilization,
the results should attract significant attention in the scientific and technological
communities.
The discussion continues with the contribution of Zhao et al. [11], which is concerned
with the development of LDH-based catalytic systems for the reduction of nitrogen
oxides. Given the fact that these gases are largely emitted from the industrial combustion
of fossils, their capture or catalytic conversion to non-harmful substances is crucial
to prevent large-scale environmental pollution. The authors reported the synthesis
of an Mn/Al layered double oxide from an Mn/Al-LDH precursor, which was able to act
as a catalyst without further modification of its structure by other compounds of
catalytic activity. The obtained mixed oxide showed excellent performance, and high
yields were achieved even at moderate temperature.
In energy-related research, supercapacitors represent an important class of devices
owing to their large power density and long lifespan. The review by Yan et al. [15]
summarizes the recent progresses in the design and application of novel Ni/Mn-LDHs
as electrode materials for supercapacitors. These types of LDHs are especially promising
due to their low cost and high specific capacity. The review discusses synthetic strategies
of Ni/Mn-LDHs used for such purposes in the recent past, with special emphasis on
LDH-derived films and composites. The development of such supercapacitor electrodes
will solve existing problems in energy storage.
Colloid chemistry studies, i.e., comprehensive investigations on charging and aggregation
processes in LDH dispersions, were rarely reported in the past, although this issue
is critical in all applications taking place in liquid medium. In heterogeneous catalysis
by LDHs, for instance, homogeneously distributed primary catalyst particles are required
during the run, while they can be removed by aggregation and subsequent sedimentation
once the reaction is terminated. The article by Somosi and his co-workers [12] addressed
this important issue. In this study, the colloidal stability of Mg/Al-LDH dispersions
was tuned by the addition of polyelectrolytes. The study sheds light on the importance
of polyelectrolyte adsorption processes on the outer surface of particles and the
subsequent changes in the surface charges, which govern the strength of the major
interparticle forces. The main novelty of the results is that the formation of polyelectrolyte
bilayers using two types of oppositely charged polymers remarkably improved the stability
of the LDH dispersions. Due to the high colloidal stability and advanced surface properties
necessary for the immobilization of guest molecules, the developed LDH–polyelectrolyte
composite is a promising candidate for delivering biomolecules in living systems.
In the next article published in the Special Issue, Qu et al. [14] reviewed the progress
made in the field of mechanochemically prepared LDHs. Although liquid phase approaches
for LDH synthesis are being used more frequently, LDH preparation by mechanochemical
methods often results in the formation of LDH materials of advanced properties, which
are useful in adsorption and catalytic processes. These features include high surface
chemical activity and abundant surface defects. Concerning these issues, the growing
utilization of this synthetic method can be predicted in the future. The authors also
discuss that certain mechanochemical processes lead to the formation of agglomerated
LDHs, while primary particles of nanoscale size distribution can be obtained by using
additives during the top-down synthetic procedures. This review is an excellent collection
of the scientific information released recently in the relevant literature and it
may provide a stimulus for future projects in the field.
An excellent contribution dealing with the design of surface-modified Mg/Al-LDH particles
and their use for nucleic acid delivery was received from Wu and his co-workers [9].
To improve the colloidal stability of the system, the LDH’s outer surface was coated
with bovine serum albumin, a strongly adsorbing natural polyelectrolyte. The delivery
efficiency was assessed and compared to that of a spherical inorganic particle of
different composition. The results unambiguously revealed that both particles are
able to quickly deliver the target molecules into cells, and these systems are promising
candidates for the development of effective immunotherapy methods for cancer treatment.
Such an application of LDH-based materials possesses huge potential in other biomedical
treatments too, as has already been indicated by the growing research activity in
the field of LDH-based delivery systems.
The last paper included in this Special issue, submitted by Borges et al. [10], is
concerned with a different, but also biorelevant topic, namely, the synthesis and
characterization of sustainable fertilizers based on Mg/Al- and Mg/Fe-LDH materials.
The importance of this topic comes from the fact that new products or methods should
be developed to improve nutrient management, which becomes especially important for
environmental resilience. A mechanochemical method was used to prepare LDHs doped
with dipotassium hydrogen phosphate, and the structure of the obtained composites
was studied in detail with well-chosen methods. This approach led to the development
of excellent slow-release compounds for potassium (K) and phosphate (P). The release
properties of P were further improved by adding carboxymethylcellulose during the
mechanochemical synthesis. The results give useful indications for the further design
of P/K fertilizers with advanced releasing features, to be used in the agriculture
sector.
In conclusion, the contributions of this Special Issue of Nanomaterials entitled ‘Layered
Double Hydroxide-Based Nanomaterials—From Fundamentals to Applications’ cover a wide
range of LDH-related topics. One could learn about novel synthetic methods, strategies
for surface modifications, immobilization of biomolecules, catalytic performance of
LDH-based composites, and their energy-related applications as electrode materials.
It is certain that the goal of the Special Issue has been achieved, since the recent
progress in a wide range of LDH-related topics was covered by the articles, which
were received from excellent research groups with activity in the field of LDH chemistry.
The guest editor believes that by reading these contributions, inspiration for future
research will be given to LDH scientists working in any discipline.