1. National Basic Research Program of China
Preparation of Porous Materials via Non-Surfactant Template Sol-gel Process
and its Biological Applications
Wentao Zhai
917.603.3139 | wentaozhai1988@gmail.com
143 35 Sanford Ave. apt#1D, NY 11355
International Union of Pure and Applied Chemistry (IUPAC) defines three categories
of porous materials: microporous materials (pore diameter larger than 50 nm, such as activated
carbon), mesoporous materials (pore diameter between 2 nm and 50 nm), and macroporous
materials (pore diameter less than 2 nm, such as zeolite). Macroporous and microporous materials
have a wide distribution in nature. However, there are few examples of mesoporous materials to be
found in between of them. In 1992, Beck and Kresge et al. first synthesized ordered mesoporous
silica materials which led a surging wave of mesoporous research in the 1990s. In the aerospace,
chemical industry, building materials, petrochemistry, machinery, medicine, environmental
protection and many other fields, mesoporous materials have countless applications. The
significance of mesoporous materials manifests itself especially in the areas of catalysis, separation

2. and adsorption, and biomolecular technology in recent years. Mesoporous materials can be
further classified into two types: the disordered and the ordered. The ordered mesoporous materials
have uniformed pore shape through the surfactant method. Furthermore, most mesopore forming
mechanisms base on the surfactant assembling, such as the famous Liquid Crystal Templating
Mechanism (LCT mechanism).
In the early 1990s, Professor Yen Wei and his research group were aiming at
asymmetric synthesis and chiral separation. The motivation comes from the puzzle about the origin
of life. The Miller–Urey experiment has shown that amino acids and nucleotides can be generated
from simple molecules within the atmosphere of the early Earth. Although the products from the
Miller–Urey experiment are racemic, the clear majority of biomolecules has an overwhelming
chiral preference (such as natural amino acids are almost L-isomers and ribose are almost D-
isomers). Based on these perplexities, Professor Yen hypothesized that the pores within ores and
clays of the primitive oceans are chiral cavities. Therefore, to form a chiral pore became the first
step solving the puzzle. Then, D-glucose and dibenzoyl-L-tartaric acid (DBTA) as chiral templates
are introduced into the silica sol-gel system with an expect that chiral pores would be formed.
Despite a massive effort, chirality could not be found in the gel after removing templates. However,
there is a big twist at the end of the story: those small organic molecules construct a worm-like,
disordered, interconnected, and mesoporous morphology inside the silica framework. This
zigzag quest gives birth to the non-surfactant method of making mesoporous materials. Such
method is a very distinct way from the surfactant method mentioned previously.

3. A notable theoretical difficulty emerges immediately with the non-surfactant
method. All present mesopore forming mechanisms derived from the surfactant method (such as the
LCT mechanism) cannot explain this mesopore forming process. There is no surfactant
assembling to form liquid crystals and to occupy the mesoporous space. On the contrary, there are
just dispersed small organic molecules introduced via the non-surfactant method. Although studies
on the mesopore formation mechanism by non-surfactant templates fall behind; the non-surfactant
method has been applied to catalysis, nanoreactors, nanomaterials, and especially, bio-
nanomaterials with unique advantages.
The traditional surfactant method usually removes its template on harsh conditions
such as high temperature and high pressure, strong acid or base, and organic solvent medium which
most active biomolecules cannot survive. Although the surfactant method could produce
mesoporous materials wonderfully, there are many setbacks on its way to the bio-nanomaterial
application. In contrast, the non-surfactant method can provide a mesoporous structure on a
marvelous mild condition. The non-surfactant templates can be washed easily by water attributed
to the nature of small organic molecules. An inventory of small biocompatible molecules is available
for the non-surfactant method, such as glucose, maltose, fructose, dibenzoyl-L-tartaric acid (DBTA),
cyclodextrin, glycerol, soluble starch, citric acid, ascorbic acid, oligopeptides and so on. Various
templates vary the nanostructure of the products. The pore size is adjustable from 2 to 12 nm (by
changing the template species and concentration) to meet different needs. Therefore, the non-
surfactant method offers a room temperature and near neutral pH sol-gel process which enables
the in-situ encapsulation of bio-substances.

4. Enzyme immobilization is the first attempt of the non-surfactant method. The
immobilized enzyme has a high resistance to adverse changes from its environment and gains a
reusable character to lower its costs. A conventional method is covalently bonding enzyme to a
matrix. In this scenario, as what Professor Yen said, bonded peptide is like a dog on a leash. The
conformation of the protein is affected by covalent bonds so that the activity diminishes. The
alternative adsorption method will gradually lose the adsorbed enzyme due to the adsorption-
desorption equilibrium. The enzyme encapsulation would be the best choice. A further problem
comes the nanostructure of the enzyme embedded material. The microporous gel suffers an
extremely low diffusion rate which compromises the catalytic activity; while the macroporous gel
quickly leaks the small-size enzyme away. Such the dilemma demonstrates the importance of the
mesoporous structure. Therefore, only the biocompatible non-surfactant method would be the
most suitable way to reconcile both sides. Furthermore, the thermal stability of the enzyme would
also be improved significantly within the mesoporous space.
The next move of the non-surfactant method is the immobilization of living cells as

5. an extension of the enzyme immobilization. Many complex metabolic reactions could only be
achieved by whole cells such as fermentation processes. Immobilized cells isolate themselves from
the products so that costs of separation will be reduced. Other advantages such as reuse and high
resistance are familiar with immobilized enzyme. However, the immobilization of living cells is a
much rougher approach for the non-surfactant method because of the sensitive and fragile nature
of cells. This project is part of the National Basic Research Program of China (973 Program).
Nature has made some precedents of embedding cells into porous silica matrixes.
Diatom deposits porous silicates from seawater on its surface to form a silica cell wall, which is
believed to act against predators. Since 1989, sol-gel processes have been applied to immobilize
living cells into inorganic oxide structures. There are two major concerns of this field. The first
one is the cell survival within a sol-gel system. Nadine Nassif et al. explored the difference between

6. the protein survival and the cell survival. Even a dead cell can serve as a bag to contain some active
enzyme inside such as β-galactosidase. Thus, the metabolic process is the real criterion to prove
the integrity of encapsulated cells. Because the metabolic process involves complex and interactive
enzyme catalysis systems, which dead cells cannot achieve. The second one comes the
nanostructure of the matrix. C. Jeffrey Brinker et al. used the self-assembly of short-chain
phospholipids to construct an ordered mesoporous silica to embed living cells. Furthermore, they
insert bacteriorhodopsin into the three-dimensional interface to achieve a simultaneous and
synergistic immobilization of living cells and enzyme for the first time.
Comparing to past works, the non-surfactant method concentrates on making another
two breakthroughs with yeast as model cells. The former one is promoting the embedding
material from hydrogels to xerogels. Most studies about living cells encapsulation remain on the
stage of hydrogels which contain more than 60% water. A xerogel contains only 10% water which
seems to be a hard condition for living cells, but the durability and mechanical properties of
xerogels can never be matched through hydrogels. The latter one is the metabolic kinetics of
embedded cells under the effect of the nanostructure of their matrixes. As described before, the
pore size determines the diffusion rate which will consequently limit the reactive activity.

7. The first difficulty stems from the byproduct of the sol-gel process. The precursor,
tetraethoxysilane (TEOS), involves two types of reactions: hydrolysis and polymerization. The
hydrolysis of TEOS generates a lot of alcohol as its byproduct which is toxic for living cells.
Therefore, alcohol must be removed by vacuum extraction before the introduction of living cells.
There are two factors can lead to a possible failure. The first factor is the temperature control. The
initial concentration of alcohol is very high so that a bumping would happen under a sudden vacuum
change. Lower the initial temperature will suppress such bumping. Nevertheless, a long operation
time will cause a premature polymerization of the sol before the introduction of living cells.
Heating the sol on the later stage will accelerate the alcohol removal. However, the conventional
method of heating, water bath as an example, leads another failure. The intense heating of the sol
stimulates the local polymerization so that a crust forms on the inner surface of the vessel. This
problem is solved by a design of air heating operation. The second factor is the concentration
control. As the alcohol is removed, the concentration of the hydrolyzed precursor increases
greatly which would also result in a premature polymerization of the sol. Although the dilution
could counteract a premature polymerization, extra water will expand the volume of the hydrogel
to exacerbate the shrinkage during the xerogel transformation. To balance these two issues, a list
of empirical minimal water additions corresponding to different template concentrations is
established through trial and error. Finally, a carefully designed alcohol removal operation is built.
96% alcohol is removed with a silica weight loss less than 6% under an arbitrary template
concentration.
The second challenge is the preparation of the xerogel. A significant shrinkage will
take place when a hydrogel loses solvents becoming a xerogel. Hydroxyethyl cellulose (800 ~ 1500
m · Pas) is introduced to reduce the shrinkage. Another problem is the dehydration. The water
content of a xerogel is only 10% which forces cells to undergo a high osmotic pressure process.
Preliminary trials are glucose and fructose as classic non-surfactant templates. However, metabolic
kinetics tests show abnormal increases of glucose in the culture medium. Those phenomena indicate
that small monosaccharides penetrate the cell membrane and disturb the metabolic kinetics test.
Then, sucrose, maltose, lactose, and sucralose are chosen to be templates for the disaccharide cannot
be absorbed directly by yeast. Other attempts include glycerol and urea, but they all fail metabolic
kinetics tests because of various reasons. Some of them cannot form a mesoporous structure, and
all these templates cannot preserve yeast during the transition from hydrogels to xerogels.

8. Some extreme examples in nature shed enlightenments upon this project.
Cryptobiosis, as a biological term, refers to a state of organisms with a stagnant metabolic rate to
survive extremely harsh environments such as desiccation, freezing, and oxygen deficiency.
Selaginella, known as resurrection plants, can lose 95% of water and pull out its root to travel with
winds. Until it finds a piece of moist land, selaginella will spread its root and revive again. Water
bears can rise from a moss specimen after a dehydration of 120 years. All organisms mentioned
above contain a high trehalose concentration under cryptobiosis. Trehalose provides a non-specific
protective effect for many bioactive substances such as enzymes and vaccines for industrial
applications. Research shows that trehalose can stabilize membrane proteins under a high osmotic
pressure or dehydration, so that prevent proteins from denaturation and inactivation. Therefore,
trehalose and glycerol with a ratio of 30:1 are chosen to be the template. They can form
mesoporous structures via the non-surfactant method and maintain the vitality of yeast in
xerogels.

9. The last puzzle is the mechanical strength of the xerogel. This hard question haunts
this project for three years and remains unsolved in the end. Although yeast survives in xerogels
with trehalose and glycerol as protective agents and templates, the cell proliferation smashes the
silica framework made of the TEOS precursor. Free yeast cells escape from the encapsulation and
interfere the metabolic kinetics test so that the relationship between embedded cells and the
nanostructure of matrixes become unmeasurable. A silver lining to this cloud is that the breaking
of the silica envelope shows the high vitality of yeast in xerogels. However, this obstacle must be
removed on the way to an investigation of nanostructure effects.

10. A mixture-precursors strategy will improve the mechanical strength of the xerogel
without sintering. A higher ratio of MTES (methyltriethoxysilane) to TEOS makes the xerogel
firmer. But an unexpected structure change comes along with the non-surfactant method. The
existence of MTES sets a ceiling of the template concentration. At a given molar concentration
of MTES, there exists a limit concentration of the template concentration. Beyond the limit, the
mesoporous structure will not disappear, but a macroporous and even a granular structure
will occur simultaneously. Higher the molar concentration of MTES will lower the template
concentration ceiling. These results show that even a macroporous structure can be made via the
non-surfactant method with small organic molecules. Therefore, a further reflection on the
confusion of pore-forming process of the non-surfactant method puts a theoretical hypothesis
forward. Obviously, there is no self-assembling of the non-surfactant method, but a nanoscale
phase separation may take place driven by small organic molecules. The nanoscale phase
separation increases gradually as the template concentration increases resulting in a microporous to
the mesoporous structure. As the molar concentration of MTES increases, the hydrophilicity of the
silica skeleton decreases so that a larger scale phase separation happens. Thus, we observe a
massive phase separation to form a macroporous and even a granular structure. Although a
novel material with structural hierarchy through the non-surfactant method has been found, this
fact points out an inherent conflict between the strategy of mixture-precursors and the non-
surfactant method. The macroporous xerogel is fragile due to the big hollows with the silica matrix.
The maximum mechanical strength under the context of MTES and the non-surfactant method is
the ceiling of macroporous mutation, which is still not strong enough to constrain yeast inside
more than 72 hours. Despite the final scene of yeast leakage, the metabolic kinetics of embedded
cells under the effect of the matrix nanostructure are described quantitatively for the first time.
Within 72 hours, only mesoporous samples metabolize glucose in the culture medium rapidly.
Further efforts to solve the yeast leakage problem include dopa coating, cell cycle control, and
auxotrophic mutation. Unfortunately, all these efforts cannot solve leakage problem ultimately.

12. Wentao Zhai
Columbia University
Master in Chemical Engineering
Tsinghua University
Master in Chemistry
Beijing Forestry University
Bachelor in Chemical Processing of Forest Products
 National Basic Research Program of China
Researcher in laboratory | Beijing, China | 2010.09-2013.09
Preparation of Porous Materials via Non-Surfactant Template Sol-gel Process
and its Biological Applications
 Resolved key technical issues on bio-nanomaterials of enzyme and cell immobilization for high
resistance, reuse, and easy separation
 Invented a room temperature, near neutral pH, and near aqueous sol-gel process to in-situ
encapsulate enzyme and living cells within a mesoporous matrix
 Collected, Processed, and Analyzed the effect of the matrix nanostructure to the date of metabolic
kinetics of embedded cells quantitatively for the first time
 Designed and Implemented a vacuum extraction process to remove the alcohol by time-dependent
temperature control and concentration control with a silica weight loss less than 6%
 Screened possible template candidates to preserve the viability of living cells in a xerogel
 Enhanced the mechanical strength of the gel through a mixture-precursor strategy
 Discovered a novel nanostructure material with structural hierarchy