Mesoporous Silica and Drug Delivery

8/11/2019 Mesoporous Silica and Drug Delivery

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CHEM 362 Binh Nguyen Nov 24th, 2013

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Mesoporous Silica: Synthesis and Drug Delivery

Introduction

Silica is the most common mineral on Earth and one of the most abundant families

of materials. Silica has been studied for thousands of years and most people would not

expect any revolutionary discovery from Silica. Yet with the advance of nanoscience, we

have been able to open a whole new world of materials made up from our well-known

friend Silica. One of the most notable and interesting Silica materials is nano-mesoporous

silica (MS), which was independently synthesized for the first time in 1990 by researchers in

Japan. Since its first synthesis, it has been widely applied in various applications, including

biosensing, catalytic, templating and, especially, drug delivery. In this paper, I will review the

method to synthesize MS, its special characteristics for drug carrier and three mechanisms of

unloading drug from MS.

Synthesis

There are at least three reported methods to synthesize mesoporous silica with high

yield: Synthesis by Solvent Evaporation2, Synthesis from Commercial Copolymers3 and

Synthesis via Controlled Sol-gel chemistry of Silicon Alkoxide4. In the limited scope of this

paper, I will introduce the most traditional and easiest MS synthesis method using Sol-gel

chemistry and several modifications to get a better control of size and accelerate the rate of

reactions.

Sol-gel chemistry is, briefly, the hydrolysis and condensation of alkoxides, M(OR)n

where M is a metal or semi-metal at the oxidation state n and R is an organic group (usually


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ethyl). The sol-gel chemical sequence leads to the formation of M-O-M bonds and the

release of water and alcohol ROH. The process is highly sensitive to humidity (because water

is involved), the nature of the R group and especially the pH of the solution. The pH of the

solution is crucial to this process: it both pushes the reaction forward by catalyzing the

hydrolysis of alkoxides and backward by breaking the desired M-O-M bonds. One of the

limits of sol-gel chemistry is that researchers have to experimentally study the reaction

condition of the interested system in order to make it proceed as desired.

In the case of MS synthesis, the alkoxide used is tetraethoxy orthosilicate (TEOS), Si-

(OC2H5)4. TEOS is dissolved in a water/ethanol mixtures but reacts very slowly with water

at pH = 7. To catalyze the hydrolysis, ammonia is added to the mixture to raise the pH

value. After a few minutes, some condensations happen and create some MS small nuclei,

which are the seeds for the growth of MS. These small seeds, having high negative surface

charge of silica, repel each other minimizing aggregation (Figure 1). It is worth noting that by

controlling the ionic strength of solution, we can control the aggregation of the seeds: high

ionic strength reduces the repulsion between the seeds and leads to more aggregation. Over

the course of reaction, more TEOS are hydrolyzed and condensed into the seeds, leading to

the growth of MS. Due to its highly porous structure, MS has no crystalline facet or any kind

of lattice. Therefore, the growth of MS is not favorable in any direction, which leads to the

formation of a perfectly spherical nanocrystals (Figure 1).The yield of this process is really

high, exceeding 90%5, and the product is uniform and spherical MS nanoparticles. (The

SEM image in the left of Figure 1)


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Figure 1. Overview of the sol-gel synthesis of mesoporous silica (MS). In the top right are general

chemical reactions of the synthesis. The sol-gel sequence is depicted in the middle. At the bottom, there are two

SEM images of MS produced by this method: in the left is normal mesoporous silica formed, the figure in the

right shows how a regrowth strategy can lead to partial aggregation of MS during growth. Figure taken from

Ref. 1.

As chemists, we would like to have a good control of our products. In the case of

nanomaterials, size is one of the most important factors to their properties. For this sol-gel

synthesis, we can tune the size of the formed MS by controlling the number of the seeds in

solution (either by controlling the reaction conditions or providing pre-formed seeds) and


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the amount of TEOS molecules available for seeds growth. The advantage of using pre-

formed seeds is that you already know how many seeds you have in the solution and how

many TEOS molecules are available, thus can pre-determine the diameter of MS with less

than 10nm of uncertainty1. Moreover, the pre-formed seeds can be other nanoparticles, like

gold nanocrystals (for photothermal therapy) or iron oxide nanocrystals (for imaging),

enabling the creation of multifunctional nano therapeutic system (that we have seen in Ozin

case 2). The main drawback of this approach is the potential aggregation during the growth

(See the SEM image in the right of Figure 1). This problem can be solved using

centrifugation, because the aggregated MS is considerably heavier than the uniform MS.

One different approach to control the size of the formed MS particles is to use

additive agents to adjust the pH of the solution, since pH is crucial to Sol-gel chemistry.

Qiao et al.5used different additive agents to adjust the pH (NaOH, HCl, NaH2PO4) and

showed that the initial pH value of the synthesis system is the main factor affecting the

particle size of the product. The TEM images of their synthesized MS showed that the size

of particles increases from 30nm to 85nm with the decrease of pH values from 10 to 6

(Table 1). The explanation for these results goes back to the nature of Silica with Silanol

groups (-Si-OH) on the surface. With high pH values, the Silanol groups get deprotonated

more, creating MS nuclei with greater negative charge. When their net negative charge is

high enough, the MS nuclei refuse to allow new silicate anion to join and thus stopping the

growth. In the case of high pH values, fewer silicate species are needed to create high


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enough net charge density silicate, thus form smaller MS and vice versa.

Table 1. Synthesis, Experimental Parameters and Diameter of MS4

Besides having uniform and controllable products, a synthesis process need to have a

reasonably fast rate, if it were to be applicable in the industry. Qiao et al5 used a surfactant

molecule, cetyltrimethylammonium chloride (CTA+-Cl-) (which we have seen before in

Murphys work on gold nanorods synthesis), to increase the speed of the reaction. Besides

components of classical sol-gel system (TEOS + EtOH + H20 + NH3), Qiao et al. added

CTA+-Cl-to the reaction solution and, remarkably, the reaction rates become significantly

faster. (However, they did not report their results in the rate of reaction) This can be

explained by [CTA+]s exceptional ability to form stable micelles: the cationic surfactant

micelles attract the negative charged silica species, which concentrate silica species around

the micelle and thus accelerating the reaction.

It is worth noting that the synthesized particles from the sol-gel method have a highly

porous structure, as desired. The condensation process does not need to be complete for the


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particle to be solid. Each silane molecule that makes up the nanoparticles has four silanol

groups (-Si-OH). After the first one or two condensation, these silanol groups are attached

into a rigid structure that makes it harder for these group to find other groups to bond with;

therefore, a large number of these groups dont have the chance to condense, leaving a lot of

empty space inside the particles. This hypothesis is proved by calcination, which is essentially

heating the particles up and making it shrink to smaller size (proving the porous structure).

As the particle is calcinated, the internal stress increases (by the silanol groups trying to

find another group to bond with) then eventually cracks the particles extensively (proving

the rigid structure).

Drug Delivery - Overview

Why is mesoporous silica (MS) so special for drug delivery? Before we can answer

this question, we need to understand the concept of drug delivery, which is relatively simple.

To optimize the efficiency of a drug in treatment, its concentration must remain constant

over a certain period of time and it should be preferably released at or near the site of

interest (for example, a tumor or a cancer cell). Unfortunately, traditional drug that we take

every day does not have these two conditions: its concentration usually shows a peak

followed by a sudden decay and it is delivered by blood in a system wide manner, possibly

risking side effects on other tissues. With the advance of nanoscience, researchers have

developed a series of nanoscaled drug carriers, which can fulfill both conditions mentioned

above: controlled release and targeting specificity allowing a constant release during a certain

period of time at a certain site of interest.


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Among these carriers, MS nanoparticles have stood out as one of the best drug

carriers known. First of all, MS has an excellent biocompatibility due to its intrinsically low

toxicity, the required property of a drug carrier. Researchers have shown that over 90% of

injected MS in mice was and found in urine and feces6. That means MS can be safely

eliminated from the biological body. Moreover, mice treated with free MS on a weekly basis

showed no difference in their history and blood analysis compared to normal mice 7,

signifying the biological safety of using MS. Its biocompatibility makes MS a good drug

carrier, but what makes it an exceptional drug carrier is its rigid platform, yet tunable in size,

shape and surface functionalization. The size and shape of the nanostructure are controllable

in the presented synthesis, thus can be optimized to maximize the cell uptake and increase

drug delivery efficiency. Different cores (for example, Fe3O4 nanoparticle) can be easily

incorporated to MS particles during the synthesis, creating a multifunctional therapeutic

system (for example, a drug carrier with imaging) (Ozin case 2). The silanol groups on the (-

Si-OH) surface of MS bring infinite opportunities to functionalize the surface with anygroup

(-R) of interest by allowing the surface to react with R-SiCl3. This enables various surface

functional groups for actively targeting, controllably load and unload drug, with a lot of

different mechanism (which will be reviewed carefully later in this paper). Last but not least,

MS can be easily synthesized and purified by centrifugation, enabling mass production with

low cost.

Having a highly porous structure, MS has an astonishingly high surface energy, which

enables both the effective loading of the pores and the locking of the substrates in the

pores by stimuli-responsive gating caps.5 Basically, the drugs are loaded to the pores by


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physisorption, by soaking the MS in a solution of the drugs. A stimuli-responsive gate is a

system or a molecule that closes the pores, keeping the drug inside, but sensitive to some

stimuli (i.e. pH, light, redox, enzymes, etc.). With the presence of the appropriate stimuli, the

gate will be opened, thus releasing the drug. Therefore, by controlling the stimuli agents, we

can control the release of the drug from the pores. Due to the ease of surface

functionalization discussed above, various types of gating caps stimuli have been successfully

applied, including pH, light, redox and enzymes. In the rest of the paper, I will review this

exciting feature of MS: controlled release of a drug in several mechanisms with different

stimuli: redox with disulfide bond, pH with acetal linker and light irradiation with

Azobenzene.

Unloading drug mechanisms

The first mechanism is based on the redox-dependent disulfide bond (S-S) cleavage

(Figure 2). Basically, the silane groups (light blue background in the figure) help attaching the

drug to the surface, the amine group (in green) acts as the pore cork and the disulfide

group (pink) is a bridge that can be easily broken by reduction. After loading the drug

molecules to the pores, the amine groups are reacted with a cork of carboxylic groups,

creating a gate to hold the drug inside the pores (Figure 2). Once the pores have been closed,

these MS particles can be washed, without losing the drug inside, and injected into blood for

drug delivery. Targeting might be involved in this process, but varied depending on the

specific treatment. For example, due to the high abundance of folate receptors on many

cancer cells compared to healthy cells, acid folic can be used as the targeting ligands. After


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the MS particles get to the site of interest, the disulfide bridge can be reduced to thiol groups

easily in our biological system, breaking the gate and unload the drug locally.

Figure 2. The schematic view of drug releasing using disulfide as the cap. Three are three steps.

Firstly, drug molecules are loaded to the pores by surface functionalization. Secondly, amidation creates the

anchor closing the pores mouth. And finally, disulfide cleavage removes the cap and releases the drug. Figure

taken from Ref. 1.

The second mechanism uses a light-stimuli system, where a DNA-modified

Azobenzene molecule is the cap, reported by Yuan et al8(Figure 3). Basically, they attached

two chunks of DNA to the azobenzene molecule at 3 and 5 (Figure 3), and attach the

complementary DNA chunks to the surface near the pore mouth of MS. When the

azobenzene is in trans-structure, two complementary DNAs (from the surface and the

azobenzene) can hybridize and close the pore (Figure 3). However, when the azobenzene is

in cis-structure, the azobenzene molecule is folded, creating enough constrain to dehybridize

the DNA linkers and open the pore. Azobenzene is known for undergoing a reversible

isomerization easily under irradiation: from trans to cis by UV light from 300-380nm and

from cis to trans by visible light at above 400nm. Taking advantage of this isomerization,

Yuan et al. was able to use UV light at 365nm to convert trans-azobenzene to cis-azobenzene


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and break the linkers and thus open the pores to unload drugs. To test the capping efficiency

of this approach, Yuan et al. did a series of alternating irradiation wavelengths (Figure 4).

This experiment showed that almost no release happened at visible light (trans-structure,

caps on)

Figure 3. Schematic of azobezene-modified DNA-controlled reversible release system. Visible

irradiation at 450 nm (azobenzene trans) leads to hybridization of the linker and the complementary DNA

arm. Irradiation with UV (365 nm) converts azobenzene to the cis form, leading to dehybridization and

pore opening. Figure taken from Ref. 8.

and substantial release happened at UV light (cis-structure, caps off). To further prove the

usability of azobenzene as the capping agent, Yuan et al. investigated the cytotoxicity of azo-

DNA/MS on human cells. The results showed that more than 95% of the cell lived after

incubated with azo-DNA/MS, proving the biocompatibility of this method. Despite its

promising properties, this approach has a major problem, which is the use of short


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wavelength UV/Visible light (300-400nm), if researchers want to use it directly in

human/animals body. These wavelengths are not in the biological windowthe wavelength

range in which light would not damage the biological tissues, which is from 650nm to

900nm. There are two main ongoing directions of research in the field to solve this problem:

finding a similar system that uses the same mechanism and creating new light-triggered drug

carriers. (Cite something) Researchers have been able to make some real progress in both

directions and the complete results should be reported soon.

Figure 4. Release profile from MSN as a function of time, showing the reversible

dehybridization/rehybridization switching by changing the light wavelength. Data have been normalized to

the largest amount of released dye at 600 min in the experiment. Taken from Ref. 8.

The last method reviewed in this paper is to use pH as the stimuli agent for opening

the cap by Liu et al.9Acidic drug carriers are sought after for the treatment of acidic targets,


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such as tumors and inflammable tissues. Liu et al. capped the pores of MS with gold

nanoparticles attached to the pores mouth by apH-sensitive acetal linker. Firstly, carboxylic

groups were attached to the MS via surface functionalization, then were reacted with excess

3,9-bis(3-aminopropyl)-2,4,8,10-tetraoxaspiro[5,5]undecane (Figure 5), creating the acetal

linker. The amine group in the linker was then reacted with a carboxylic acid modified gold

nanoparticle (Au-COOH) to cap the gold nanoparticle to the mouth of the pores. To test

out of capping efficiency, [Ru(bipy)3]Cl2 was loaded (phisisorbed) to MS and then Au-

COOH was added to the solution to create the cap (via linker described above).

Nanoparticles were washed after that to remove excess [Ru(bipy)3]Cl2 on the surface of MS

then dispersed in water at different pH to examine their controlled release ability. The results

were impressive: at pH = 7.0, no free [Ru(bipy)3]Cl2was found showing efficient capping; at

pH = 4.0, 100% release was reached after 13 hours and at pH = 2.0, 90% release was

reached after 30 minutes. To further prove that the [Ru(bipy)3]Cl2was actually capped inside

the mesoporous structure, Liu et al. also carried out an experiment where no cap (Au-

COOH) was added to the solution. As expected, all the [Ru(bipy)3]Cl2was washed off by the

washing phase and nothing was found in water at any pH value. Although Liu et al. did not

test out the biocompatibility of this MS/Au combination in their research, this method is

still very promising because both MS and Au

nanoparticles are well established as non-toxic

in the literature.


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Figure 5. Schematic Illustration of pH-Responsive Nanogated Ensemble Based on Gold-Capped

Mesoporous Silica through Acid-Labile Acetal Linker. Figure taken from Ref. 9.

In conclusion, I have introduced the simplest method of synthesizing MS using sol-

gel chemistry with size tunability and high production yield. Due to its porous structure, high

biocompatibility, high tunability in size and shape and unique surface functionalization,

mesoporous silica is an exceptional drug delivery carrier. I have picked three different

methods of unloading drug, which use redox, light and pH as the stimuli, to represent the

concept behind stimuli capping agents. After only twenty years after MS first synthesis in

1990, researchers have turned MS silica into the multifunctional system for drug delivery,

biosensing, templating, catalytic and many other applications. Currently, MSs drug delivery

is under extensive research on animals, like mice, and showing great promise. However,

whether it will work as desired in the human body remains unanswered at the moment and it

requires a complete research portfolio on animals before we can move into human testing.

That said, with the advance of nanoscience, the cure for cancer, HIV and a lot of other

diseases might be just around the corner.


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Reference

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5. Lu, C., Willner, B., Willner, B.ACS Nano,2013, 7(10), pp 83208332.

6. Lu, J., Liong, Z, Li, Z., Zink, J., Tamanoi, F. Biocompatibility, biodistribution, and

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Mesoporous Silica and Drug Delivery

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