Intelligent materials, the latest revolution in the field of materials science, can adapt
their properties depending on changes in their surroundings.
They can be used in everything from self-healing mobile phone screens, to shape-shifting aeroplane
wings, and targeted drug delivery.
Delivering drugs to a specific target inside the body using intelligent materials is particularly
important for diseases like cancer, as the smart material only releases the drug payload
when it detects the presence of a cancer cell, leaving the healthy cells unharmed.
Now, researchers from the Centre for Advanced 2D Materials (CA2DM) at the National University
of Singapore (NUS) have created a new class of intelligent materials.
It has the structure of a two-dimensional (2D) material, but behaves like an electrolyte
– and could be a new way to deliver drugs within the body.
Just like traditional electrolytes, these new “2D-electrolytes” dissociate their
atoms in different solvents, and become electrically charged.
Furthermore, the arrangement of these materials can be controlled by external factors, such
as pH and temperature, which is ideal for targeted drug delivery.
The 2D-electrolytes also show promise for other applications that require a material
to be responsive to environmental changes, such as artificial muscles and energy storage.
Their trailblazing results were published in the journal Advanced Materials.
In materials science, a 2D material is a solid material that exists in a single layer of
atoms.
It can be thought of as an atomically-thin sheet that has a specific height and width,
but effectively no depth, hence, it is essentially two-dimensional.
On the other hand, an electrolyte is a substance that produces an electrically conducting suspension
when dissolved in a solvent, such as water.
There are numerous 2D materials in existence today, and electrolytic behaviour has been
well-established in countless other compounds.
However, the results from the NUS researchers show the first instance of materials that
have both 2D structure and properties of electrolytes, with a particular trend to shapeshift their
form reversibly in liquid medium.
The NUS team achieved this feat by using organic molecules as reactive species to add different
functionalities to 2D materials such as graphene and molybdenum disulfide (MoS2).
By adding different chemical groups that become positively or negatively electrically charged
in solvents, the researchers altered traditional 2D materials and came up with a novel class
of smart materials that have their electronic properties controlled by morphological conformation.
The methods used by the researchers to create 2D-electrolytes are only a few possible examples
among many potential options, making this discovery an exciting new research area to
explore.
A major breakthrough of this research was that the orientation of the 2D-electrolytes
could reversibly change by tweaking the external conditions.
Currently, the electrical repulsion between the surface charge in a 2D material leads
to it be laid out in a flat sheet.
By altering the pH, the temperature, or the ionic concentration of the suspensions, the
NUS researchers demonstrated the ability of the 2D-electrolyte sheet to shapeshift and
form scroll-like arrangements.
These experimental results are supported by detailed theoretical analysis in which they
explain the physical mechanism behind the scroll formation and stability.
These scroll orientations have such a small diameter that they could be described as one-dimensional
(1D), leading to different physical and chemical properties.
Moreover, this transition from 2D to 1D is reversible by altering the external conditions
back to their original values.
One can think of 2D-electrolytes as the higher dimensional analogues of 1D electrolytes,
commonly known as polyelectrolytes.
Important examples of polyelectrolytes include many biologically relevant materials, such
as DNA and RNA.
Discovering this class of materials has opened up new areas of exploration for materials
scientists, since it brings together two fields of research that have been traditionally unlinked,
namely, 2D materials in the field of Physics, and electrolytes (in the area of Electrochemistry).
There is an uncountable number of ways to functionalise graphene and other 2D materials
to transform them into 2D-electrolytes.
The researchers hope that their work will inspire scientists from different fields to
further explore the properties and possible applications of 2D-electrolytes.
The researchers anticipate that as 2D-electrolytes have similarities with biological or natural
systems, they are capable of spontaneously self-assemble and cross-link to form nanofibers
that are promising for applications in filtration membranes, drug delivery, and smart e-textiles.