Nanocelluloses
The term nanocellulose generally refers to long, thread-like cellulose nanofibers (CNF), ribbon-like bacterial
nanocellulose (BNC) or short, rigid rods called cellulose nanocrystals (CNC). A common figurative distinction
is to describe CNF as spaghetti and CNC as rice, which gives an idea of their relative aspect ratios and
flexibility. All nanocellulose materials combine the high strength of cellulose with high surface area,
rendering them promising candidates in, for example, bio-based composite materials.
Cellulose nanofibers
1 µm
Bacterial nanocellulose
a)
Cellulose nanocrystals
b)
1 µm
Cellulose nanofibers (CNF)
Synonyms in literature: cellulose nanofibrils, nanofibrillar cellulose, nanofibrillated cellulose, microfibrillar
cellulose, microfibrillated cellulose
Dimensions: width 3-50 nm, length 0.5-5.0 µm
CNFs are essentially isolated microfibrils from the plant cell wall and their preparation entails breaking up
the hierarchical fibre matrix. If true individualization results, the width of CNF depends on the botanical
source and it corresponds to the width of the microfibril in the original plant. In the case of wood-based
sources, for example, the diameter of truly individualized CNF is 3-4 nm. Such narrow widths can be gained
with the aid of catalytic oxidation, namely the so-called TEMPO-mediated. Often, however, the CNFs are
not individual, isolated microfibrils but rather bundles of microfibrils. The width of bundle-like CNFs is
usually polydisperse, ranging from ~5 nm to few tens of nanometres. Such CNFs are prepared by harsh
mechanical grinding (sometimes in combination with enzymatic or chemical pretreatments) that breaks up
the cell wall matrix and liberates the microfibrils. Because of the coarser dimensions, these CNFs can
sometimes go under the name microfibrillar cellulose.
Research example 1: Films and composites
Because of the high water uptake of CNF, preparation of
CNF films is fairly demanding. The picture on the right
presents an effortless method to prepare films by
simple hot pressing of filtrated NFC gel. These films
possess high strength and good solvent resistance, and
their gas barrier properties are not significantly
hampered by moisture at moderate humidity values.
Films and composites are among the most popular applications of nanocellulosic materials. The
department has based its high profile work with such applications on its traditionally solid knowledge on
the behaviour of wood-based components.
Research example 2: Model film research with CNF
The department is one of the leading research units with
model films of cellulose. Model films are ultrathin layers
whose chemistry and morphology are well-defined and they
pH 5
pH 5
are used to interpret physico-chemical phenomena that are
pH 8
otherwise difficult to interpret with heterogeneous,
pH 8
macroscopic fibres. A host of sophisticated instruments
Increased swelling
designed for surface and colloid chemistry are utilized in such
pH 10
experiments. The figure on the left exposes a simple but
pH 10
illustrative series of swelling of model CNF films are different
pH conditions. The swelling has been followed with quartz
crystal microbalance with dissipation monitoring (QCM-D)
which is a very sensitive balance, capable of following changes in mass and viscoelastic properties of very
thin films. The fundamental knowledge gained from model film experiments can be utilised further in the
applications of, for example, CNF composites.
Change in frequency
pH 3.5
Bacterial nanocellulose (BNC)
Dimensions: width 20-100 nm length 1.0-5.0 µm
Cellulose is also synthesized extracellularly by several bacterial species such as Gluconacetobacter,
Agrobacterium, Pseudomonas, Rhizobium, and Sarcin. In general, cellulose in BNC has the same chemical
composition as that from plants but it is produced in the absence of other polymers (such as hemicelluloses
or lignin) which makes it chemically pure.
BNC is produced by bacterial cultivation in aqueous culture media containing glucose, phosphate, and
oxygen. It has a ribbon-like shape (less than 100 nm wide) and high crystallinity index. Due to the
replication of bacteria, nanocellulose fibrils in the BNC pellicle form a randomly assembled tangled
structure, which has good mechanical properties even in the wet state.
Research example: Cellulose-based biointerfaces
Since BNC growth takes place only in the presence
of oxygen, their assembly can be directed to form
different shapes, depending on the air-water
interface used, for example in closed vessels and
tubes (BNC-tubes). Moreover, the non-toxicity and
high stability of BNC makes it an ideal material in
medical and diagnostic applications. An important
aspect is that the BNC tubes have shown the
potential to significantly resist the internal pressure
which is a prerequisite in biofiltration. The scheme
on the left illustrates the synthesis of CMCmodified BNC-tubes and their subsequent functionalization with affibodies. The incorporation of
antibodies, peptide ligands, protein A, etc. onto the inner walls of BNC tubes is expected to open new
possibilities in detection and separation of specific target proteins.
Cellulose nanocrystals (CNC)
Synonyms in literature: nanocrystalline cellulose, cellulose whiskers, cellulose nanowhiskers, cellulose
microcrystals (ambiguous)
Dimensions: width 3-20 nm, length 50-1000 nm
CNCs are generally prepared by subjecting native fibres to controlled acid hydrolysis that cleaves the
glycosidic bonds of cellulose in the disordered (or amorphous) regions of microfibrils, leaving the crystalline
segments intact. The typical source material consists of cotton fibres, resulting in CNCs with a width of 6-7
nm and a length of 50-300 nm (average ~130 nm).
The most common route to CNCs involves the use of concentrated sulphuric acid which simultaneously
with the hydrolysis introduces sulphate half-esters on the surface of CNCs. The charged sulphate groups
result in stable suspensions of CNCs because of electrostatic repulsion. Interestingly, the sulphate-stabilized
CNCs spontaneously form chiral nematic liquid crystal phases in aqueous suspensions.
Research example 1: Nanoforest from end-functionalized CNCs
Nanocellulosic materials are often difficult to modify
chemically. The schematics on the right reveal how the
specific direction of cellulose chains can be utilised to
end-functionalize the CNCs with a thiol moiety. After
modification, it is feasible to attach the CNCs on a gold
surface, forming a cellulosic nanoforest which is
essentially an artificial cilia structure. Such structures
can be exploited in fluid manipulation and as a template for controlled adsorption/desorption.
Research example 2: Assembly of CNCs on surfaces
Charged CNCs are stable in aqueous
suspensions, but it can be excessively
difficult to prepare controlled surface
structures and films out of CNC because it
always involves the removal of water. The
images on the left demonstrate how it is
feasible to prepare continuous ultrathin
2 µm
films with even CNC distribution or tunable
cellular networks consisting of CNCs. The
tuning of the surface assemblies is made feasible by simple tuning of the chemical conditions upon the film
deposition. Thin films and various defined surface patterns are useful in applications like sensors,
transducers, or photonic devices to name a few.