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Jacques Livage

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C. R. Chimie 21 (2018) 969e973Contents lists avaiComptes Rendus Chimie
www.sciencedirect.comFull paper/MemoireBioinspired nanostructured materials
Materiaux nanostructures bio-inspires
Jacques Livage
College de France, Francea r t i c l e i n f o
Article history:
Received 13 June 2018
Accepted 2 August 2018
Available online 14 September 2018
Keywords:
Organiceinorganic hybrids
Solegel processes
Gels
Nanostructure
Materials scienceE-mail address: jacques.livage@upmc.fr.
https://doi.org/10.1016/j.crci.2018.08.001
1631-0748/© 2018 Académie des sciences. Publish
creativecommons.org/licenses/by-nc-nd/4.0/).a b s t r a c t
Inorganic materials, such as glasses and ceramics, are normally prepared by heating
powders at high temperature via the so-called “shake and bake” chemistry. Nature shows
that microorganisms, such as diatoms, are able to build silica shells at room temperature
via the condensation of silicic acid solutions. Following this example, the so-called “solegel
process” was developed to build nanostructured materials via the polycondensation of
molecular precursors in solution. The mild conditions associated with this “chimie douce”
allow the synthesis of hybrid nanocomposites, in which both organic and inorganic phases
are mixed at a molecular level. Biomolecules, such as enzymes, can be trapped within
inorganic silica matrices opening new possibilities in the field of biotechnology and
nanomedicine. Even microorganisms (bacteria, fungi, yeasts, cells, etc.) can be encapsu-
lated within silica glasses. They retain their bioactivity and can be used for many appli-
cations (biocatalysis, biosensors, bioreactors, drug delivery, etc.). These “living materials”
open new opportunities for the development of “green” technologies.
© 2018 Académie des sciences. Published by Elsevier Masson SAS. This is an open access article
under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Mots-cle:
Hybrides organiqueseinorganiques
Procedes solegel
Nanostructure
Science des materiauxr é s u m é
Les verres et ceramiques sont elabores a partir de poudres, a haute temperature.
L'observation de la nature nous montre pourtant que certains micro-organismes, tels que
les diatomees, sont capables d’elaborer des carapaces de verre a temperature ambiante a
partir de la silice dissoute. Suivant cet exemple, nous avons developpe les procedes « sol
egel », qui permettent d’elaborer des materiaux nanostructures par polycondensaion de
precurseurs moleculaires en solution. Cette « chimie douce » conduit a la synthese
d'hybrides organo-mineraux, dans lesquels les phases organique et minerale sont
associees a l’echelle moleculaire. On peut même associer des biomolecules (enzymes,
medicaments, anticorps) offrant de nouvelles possibilites dans les domaines des bio-
technologies et de la nanomedecine. On peut même encapsuler des micro-organismes
(bacterie, levure, cellules…), qui restent viables et permettent d'obtenir des biocapteurs
ou des bioreacteurs performants. Ces materiaux vivants ouvrent de nouvelles possi-
bilites pour le developpement des biotechnologies « vertes ».
© 2018 Académie des sciences. Published by Elsevier Masson SAS. This is an open access article
under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).ed by Elsevier Masson SAS. This is an open access article under the CC BY-NC-ND license (http://
J. Livage / C. R. Chimie 21 (2018) 969e9739701. Introduction
For centuries, glasses and ceramics have been made via
solid-state reactions at temperatures around 1000 C. Such
high temperatures lead to the usual stable and dense
phases. Nowadays, nanostructured materials are becoming
more and more important for modern technologies. They
open new possibilities and extensive researches are
devoted to improve the synthesis of such advanced mate-
rials. However, solid-state chemistry still involves rather
harsh conditions. Looking at nature shows that living or-
ganisms are able to make biomaterials under much softer
conditions. Biomineralization currently occurs in water at
room temperature leading to nanostructuredmaterials that
often exhibit much better properties than our best mate-
rials. Looking at nature therefore offers a real challenge for
solid-state chemists. In their search for new methods to
make silica-based materials, chemists have turned their
attention toward microorganisms such as radiolarians, di-
atoms, or sponges that are able to make biogenic silica
glasses under ambient conditions [1]. Therefore could we
learn from nature how to make new bioinspired nano-
structured materials? This article describes the soft solu-
tion synthesis of silica-based materials via the so-called
“solegel process.”2. From biogenic glasses to solegel chemistry
Silica glasses are very commonmaterials, currently used
for making windows, bottles, or optical fibers. Many pro-
gresses have been made to improve their properties since
their discovery about 5000 years ago, but they are still
made frommolten sand at temperaturesmuch greater than
1000 C. The example of diatoms shows that nano-
structured silica glasses can be made at room temperature
in water. Diatoms are a widespread group of planktonic
microalgaes that can be found in the sea, fresh water, wet
soils, and damp surfaces (Fig. 1). They exhibit photosyn-
thetic properties and are responsible for about 25% of ox-
ygen on earth. These single cell algaes are known to built a
silica shell called “frustule” to protect themselves against
predators. These shells are made via the condensation of
silicic acidmolecules Si(OH)4 arising from the dissolution of
silicate minerals in water as follows [2]:
SiO2 þ 2H2O / SiðOHÞ4
A silica network is then obtained via the poly-
condensation of silicic acid molecules:
SieOH þ HOeSie / eSieOeSie þ H2O
Silica nanoparticles are formed inside the cell in a silica
deposition vesicle and then transported outside the cell to
build the silica shell.
Actually precipitated silica powders are already made in
industry via the acidification of sodium silicate solutions
(water glass). However, such solutions are highly basic andthe precipitation of silica is not easy to control. Therefore
chemists currently use alkoxide molecular precursors
Si(OR)4 (R ¼ CH3, C2H5, etc.). Upon hydrolysis, these silicon
alkoxides lead to the formation of silica:
SiðORÞ4 þ 2H2O 0 SiO2 þ 4ROH
Colloidal solutions (sols) and gels are progressively
formed and this soft solution synthesis of oxide materials is
commonly called the “solegel process”[3]. Silica glasses can
then be made at temperatures much lower than the usual
melting point, and solegel materials are nowwidely used in
industry. Besides energy saving, the main advantage of sol
egel chemistry is that shaped materials can be obtained
directly from the solution. Following this powderless pro-
cessing of glasses and ceramics, nanoparticles can be formed
by spray, fibers by gel extrusion, and films by dip coating.
Moreover as high temperatures are not required, coatings
can be deposited onto various substrates such as silicon,
metals, electronic components, and even polymers [4].
The mild conditions associated with solegel chemistry
allow the formation of hybrid nanocomposites in which
organic and inorganic phases are mixed at the molecular
level. A whole set of new materials, ranging from brittle
glasses to plastic polymers can then be made. They are
transparent and can be easily shaped by molding and pol-
ishing. Organiceinorganic hybrids already found a large
number of applications in various domains: optics, micro-
electronics, transportation, health, energy, and so forth [5].3. Bioinorganic hybrids
The development of biotechnology requires the immobi-
lization of active biomolecules or cells on solid substrates.
Numerous techniques such as physical adsorption, covalent
grafting, entrapment, or encapsulation have been explored.
But no single method or material has emerged as a generic
anduniversal technology. Nowadays and forobvious reasons,
immobilization is mainly restricted to organic or bio-
polymers. Inorganic silica glasses could offer some advan-
tages such as improved mechanical strength, chemical
stability, and thermal stability. They are biologically inert and
do not swell in aqueous or organic solvents. Therefore
combining fragile biomolecules with tough inorganic mate-
rials becomesahighly innovative researchfield.However, the
usual high temperature processing of ceramics and glasses is
not compatible with fragile biomolecules. Therefore, new
“chimie douce” processes have been developed allowing the
formation of a solid network via biocompatible routes [6].
Solegel encapsulation is quite easy to perform. Bio-
molecules such as enzymes are mixed with a solution of
molecular precursors. Condensation reactions lead to the
growth of a silica network around the biomolecules that
remain physically trapped within the porous solid. Wet
silica gels provide a nontoxic and biologically inert aqueous
environment. Enzymes are trapped into silica cages
tailored to their size. They retain their catalytic activity and
appear to be protected against denaturation and leaching.
Moreover, their catalytic activity can be optimized via a
Fig. 1. Diatoms are singe-cell planktonic microalgae protected by a silica shell called frustule that exhibit a large variety of shapes.
J. Livage / C. R. Chimie 21 (2018) 969e973 971chemical control of the porous solegel matrix [7]. Lipases,
for instance, are widely used in industry as a biocatalyst for
hydrolysis and esterification reactions. However esters are
usually not soluble with water giving rise to emulsions
rather than clear solutions. The catalytic reaction then oc-
curs at the interface between both oil and water phases. It
is therefore very sensitive to the hydrophilicehydrophobic
nature of this interface. The catalytic properties of lipases in
hydrophilic solegel silica are very poor, but they become
much better when hybrid organiceinorganic gels are used.
Solegel lipases are now commercially available [8].
4. Living materials
Diatoms are made of a single cell trapped within a silica
shell. They suggest that living cells could be trapped within
an oxide matrix. Actually, the first article was published by
Carturan et al. [9] who showed the bioactivity of yeast cells
(Saccharomyces cerevisiae) immobilized in porous silicawas
preserved after solegel encapsulation. Since then whole
cell encapsulation has been extended to many other pro-
karyotic and eukaryotic species (bacteria, microalgae, plant
cells, and animal cells) [10,11]. Trapped cells remain
accessible to external reagents through the pores of the
silica matrix and can then be used for the realization of
biosensors or bioreactors. Chlorella vulgaris microalgae, for
instance, have been trapped within a thin film deposited at
the end of an optical fiber tomeasurewater pollution. Their
photosynthetic behavior depends on the presence of her-
bicides in thewater leading to a modification of chlorophyllfluorescence that can be easily measured through the op-
tical fiber [12].
One of the main challenges for the solegel encapsula-
tion of living cells would be to preserve their viability as
long as possible. Trapped cells have to fight against the
stress due to the solid matrix. Escherichia coli bacteria, for
instance, have been shown to retain their biocatalytic ac-
tivity after encapsulation but the number of viable bacteria
decreases quite fast [13,14]. Their viability can be improved
by adding some chemicals such as glycerol to protect the
cell (Fig. 2). Then they can be used as a bioreactor to pro-
duce some chemicals. Serratia marcescens bacteria, for
instance, are known to produce a red pigment called pro-
digiosin that exhibits some cytotoxic activity against cancer
cells. We have shown that these bacteria retain their
metabolic activity when trapped in silica gels. They even
produce more pigment than when they are just suspended
in a solution. This should be because of the diffusion of the
hydrophobic prodigiosin pigment from the hydrophilic
silica matrix toward the supernatant organic solution that
contains all nutrients. This can be easily seen with naked
eyes. The silica matrix where bacteria are trapped remains
colorless whereas the solution becomes pink.
However, the viability of trapped bacteria is not very
high. Only 40% of them remain viable after 1 month.
Nevertheless their viability can be improved significantly
by adding quorum sensing molecules. These molecules
(homoserine lactone) are responsible for the chemical
communication between bacteria. In a culture medium,
they become active when the population increases and
Fig. 2. E. coli cell trapped in a silica gel (bar ¼ 0.5 mm).
J. Livage / C. R. Chimie 21 (2018) 969e973972bacteria come close together to form a colony. We have
shown that adding homoserine lactone to the nutrient
solution improves the production of prodigiosin and in-
creases significantly their lifetime. The plate count tech-
nique shows that almost 100% S. marcescens are still viable
after 1 month in the gel. Solegel encapsulation could also
provide useful information about the mechanism of bac-
terial quorum sensing chemical communication [15]. When
trapped in a silica matrix, bacteria remain far apart so that
there is no chemical communication between them. It is
therefore possible to study how they respond to the addi-
tion of foreign molecules [16,17]. Such studies could pro-
vide useful information to fight against bacterial infections.
Quorum quenching molecules that prevent chemical
communication between bacteria could be used instead of
antibiotics to prevent infection [18].5. Multifunctional nanostructured biomaterials
In industry, both the structure and composition of
advanced materials are controlled to optimize theirFig. 3. Both parts of the frustule of Coscinodproperties. Boron, for instance, is added to the silicate
composition of pyrex glasses to improve their thermal
behavior. Lead is used to increase the reflective index of
crystal glasses and Na to decrease their melting tempera-
ture. This is no longer possible with biomaterials made by
living microorganisms. The same materials have to be used
all along the synthesis but should exhibit several properties
at the same time. Nature has to find a compromise, which is
usually obtained via a careful control of their nano-
structure. Diatom shells, for instance, have to be strong to
protect the cell. But they should also be transparent to
allow their photosynthetic activity. Therefore silica glasses
are chosen instead of calcium carbonate used by most
marine organisms. Moreover, trapped cells should be able
to exchange with the outside medium to perform their
metabolic activity. Therefore, diatom frustules are highly
porous (Fig. 3). This is the main use of diatomaceous earths,
widely used in industry for making filters. The well-known
dynamite invented by A. Nobel was actually made with
diatom earth impregnated with nitroglycerin.
The most remarkable property of the porous shell of
diatoms is that pores are not randomly distributed. Cosci-
nodiscus diatoms, for instance, exhibit a highly nano-
structured frustule. It is made of three overlapping silica
layers containing, respectively, small (z50 nm), medium
(z250 nm), and large (z1 mm) pores (Fig. 4a).
Moreover, these pores exhibit a periodic distribution.
They form a hexagonal array with a lattice parameter of
about 0.6 mm leading to a periodic distribution of the
refractive index (Fig. 4b). Diatom shells then behave as
living photonic crystals. Visible light is focused inside the
cell where photosynthesis occurs, whereas UV light is re-
flected. Frustules then behave as an optical filter protecting
the cell against UV irradiation [19,20].
The nanostructured frustules of diatoms can be exploi-
ted for making a large variety of devices [21,22]. The pho-
toluminescence properties of silica, for instance, have been
used for gas detection (NO, NO2). The light focusing prop-
erties of diatoms can be used to focus solar light onto the “p
en” junction of a solar cell. Diatoms could also be describediscus alesia diatoms are highly porous.
Fig. 4. Pores of different sizes are distributed within three different layers (a). Diatoms exhibit photonic crystal properties because of the periodic distribution of
pores within a silica layer (b).
J. Livage / C. R. Chimie 21 (2018) 969e973 973as living bioreactors. The chemical composition of their
silica frustule can be modified by adding foreign elements
in the culture medium. Adding Ge(OH)4, for instance, leads
to Ge doped frustules that exhibit blue electrolumines-
cence [23]. Interesting results have been obtained with dye
sensitized solar cells. Adding titanium at the end of culture
leads to the formation of a TiO2 layer at the surface of the
frustules. A large interface between dyes and the porous
electrodes is thus obtained, improving the energy conver-
sion efficiency [24].
The whole chemical composition of diatom frustules
can be changed without modifying their nanostructure via
their magnesioisothermic reduction at relatively low tem-
peratures, around 600 C [25]. This so-called Bioclastic and
Shape-preserving Inorganic Conversion process can be
used to obtain other nanostructured ceramics such a
BatiO3, ZrO2, and even BN.
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Bioinspired nanostructured materials

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Semi-empirical correlation of solid solute solubility in supercritical carbon dioxide: Comparative study and proposition of a novel density-based model

Editorial Board - French

Mn(III)-porphyrin/graphene oxide nanocomposite as an efficient catalyst for the aerobic oxidation of hydrocarbons

Tris-hydroxymethylaminomethane (THAM): An efficient organocatalyst in diversity-oriented and environmentally benign synthesis of spirochromenes

Editorial Board - French

Sommaire / Contents

Synthesis, characterization, and biological activity of some complex combinations of nickel with α-ketoglutaric acid and 1-(<i>o</i>-tolyl)biguanide

Editorial Board - English

Bioinspired nanostructured materials

Editorial Board - English

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