Inorganic Nanoparticles as Carriers of Nucleic Acids into Cells

Abstract

The transfer of nucleic acids (DNA or RNA) into living cells, that is, transfection, is a major technique in current biochemistry and molecular biology. This process permits the selective introduction of genetic material for protein synthesis as well as the selective inhibition of protein synthesis (antisense or gene silencing). As nucleic acids alone are not able to penetrate the cell wall, efficient carriers are needed. Besides viral, polymeric, and liposomal agents, inorganic nanoparticles are especially suitable for this purpose because they can be prepared and surface-functionalized in many different ways. Herein, the current state of the art is discussed from a chemical viewpoint. Advantages and disadvantages of the available methods are compared.

Full text

Nucleic Acid Carriers DOI: 10.1002/anie.200703039
Inorganic Nanoparticles as Carriers of Nucleic Acids into
Cells
Viktoriya Sokolova and Matthias Epple*
Angewandte
Chemie
Keywords:
cell biology · gene therapy ·
nanoparticles · nucleic acids ·
transfection
Dedicated to Professor Hans-Curt Flemming
on the occasion of his 60th birthday
M. Epple and V. Sokolova
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1382 www.angewandte.org  2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2008,47, 1382 – 1395

1. Introduction
The application of nanoparticles in medicine is an
emerging field of nanobiotechnology.[1] As a result of their
small size, nanoparticles can penetrate the cell wall and
deliver drugs or biomolecules into living systems, usually for a
therapeutic purpose.[2–5] Many different kinds of nanoparticles
are known, many have been tested on biosystems, and some
approaches have made it into clinical trials. Herein we are
summarizing the state of the art of inorganic nanoparticles as
carriers for nucleic acids (DNA, RNA, and oligonucleotides)
to influence the gene expression of a cell. Because of the huge
amount of literature on bioorganic nanoparticulate systems
(such as, polycationic and liposomal agents and dendrimers),
we will restrict ourselves to inorganic nanoparticles.
2. Transfection
The introduction of DNA, RNA, or oligonucleotides into
eukaryotic cells is called transfection.[6] This process involves
the uptake of extracellular molecules through the cell
membrane into the cytoplasm and also further into the
nucleus. If DNA is brought into the nucleus, it can be
incorporated into a cells genetic material and induce the
production of specific proteins.[6–8] We distinguish between a
transient transfection, where DNA does not integrate into the
host chromosome, and a stable transfection, where the foreign
DNA is integrated into the chromosome and passed over to
the next generation. In contrast, the introduction of small-
interfering RNA (siRNA) can selectively turn off the
production of specific proteins (“gene silencing” or “antisense
technology”).[9–15] Such a controlled introduction of genetic
sequences into mammalian cells has become an essential tool
for analyses of gene structure, function and regulation; it is
also the conceptual basis for a medical technique called “gene
therapy” that potentially allows the treatment of a wide
variety of diseases of both genetic and acquired origin.
Naked DNA itself cannot successfully enter cells; it
requires the assistance of a suitable vector.[16] There are many
reports about the direct injection of naked DNA into
different tissues, for example, skeletal muscle,[17] liver,[18]
thyroid,[19] heart muscle,[20] brain,[21] and urological organs.[22]
The cellular uptake of plasmid DNA by injection is very
inefficient, for example, in muscle cells less than 1% of the
injected dose is taken up.[17] For instance, a tail-vein injection
of naked DNA into mice did not result in gene expression in
major organs[23] because of its rapid degradation by nucleases
in the blood.[24,25]
The cell membrane is a permeable lipid bilayer which
constitutes the outer border of a cell. The amphiphilic
membrane lipid molecules (mostly phospholipids) have a
polar hydrophilic head and two hydrophobic hydrocarbon
tails.[26,27] In the cell membrane, there are also receptor
proteins, recognition proteins, and transport proteins. The
transport of small molecules across the cell membrane can
occur by diffusion through channels (so-called passive trans-
port) or with the help of transport proteins (so-called active
transport).[28–30] Active transport requires energy, usually in
the form of adenosine triphosphate (ATP). For the uptake of
macromolecules or nanoparticles most cells use endocytosis,
that is, the penetration of the cell membrane and the
incorporation into an intracellular vesicle.[31,32] Vonarbourg
et al. recently reviewed the factors which influence the uptake
of nanoparticles of different nature by the mononuclear
phagocyte system (monocytes and phagocytes). This process
is the typical mechanism by which nanoparticles are elimi-
nated from the blood.[33]
Figure 1 shows the DNA delivery pathway. First, nano-
particles are adsorbed on the cell membrane. Then, by
endocytosis, nanoparticles are taken up by cells.[34,35] Some
intracellular processes can prevent the transport of DNA
across the cell to the nucleus. Endosomal degradation of
DNA can occur during endocytosis inside an endosome if
DNA does not escape from the endosome before the fusion
The transfer of nucleic acids (DNA or RNA) into living cells, that is,
transfection, is a major technique in current biochemistry and
molecular biology. This process permits the selective introduction of
genetic material for protein synthesis as well as the selective inhibition
of protein synthesis (antisense or gene silencing). As nucleic acids
alone are not able to penetrate the cell wall, efficient carriers are
needed. Besides viral, polymeric, and liposomal agents, inorganic
nanoparticles are especially suitable for this purpose because they can
be prepared and surface-functionalized in many different ways.
Herein, the current state of the art is discussed from a chemical
viewpoint. Advantages and disadvantages of the available methods are
compared.
From the Contents
1. Introduction 1383
2. Transfection 1383
3. Methods for Gene Transfer into
Living Cells 1384
4. Chemical Methods Based On
Nanoparticles 1385
5. Summary 1391
[*] Dr. V. Sokolova, Prof. Dr. M. Epple
Institut fr Anorganische Chemie
and
Center for Nanointegration Duisburg-Essen (CeNIDE)
Universitt Duisburg-Essen
Universittsstrasse 5–7, 45117 Essen (Germany)
Fax : (+49) 201-183-2621
E-mail: matthias.epple@uni-due.de
Transfection
Angewandte
Chemie
1383Angew. Chem. Int. Ed. 2008,47, 1382 – 1395  2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

with lysosomes (in which the pH value is under 5).[36] After a
successful release of DNA into the cytoplasm, degradation of
DNA by specific enzymes (nucleases) can occur. For an
efficient introduction of DNA into the nucleus, it has to
overcome such obstacles and must be protected from
nucleases. The next step is the introduction of DNA into
the nucleus. In general, the transfer of molecules into the
nucleus occurs through nuclear pore complexes (NPCs), that
is, large proteins (nucleoporins) that are inserted into the
double nuclear membrane that consists of two lipid bilay-
ers.[37,38] NPCs are highly permeable to small molecules, but
they restrict the movement of larger molecules across the
nuclear envelope. To overcome this barrier, macromolecules
that carry a nuclear localization sequence (NLS) can be
recognized by importins and then be actively transported
through the pore into the nucleus.[39,40]
Despite extensive studies on nuclear targeting,[41–43] it is
still not clear how DNA is transported into the nucleus, that is,
alone or incorporated into nanoparticles. One of the possi-
bilities is that nanoparticles are slowly dissolved by acid in the
endosomal vesicle and/or in the cytoplasm. Another possi-
bility is that DNA-loaded nanoparticles go to the surface of
the nucleus where the import of DNA can occur. In this case,
it is advantageous if the DNA is protected by a nanoparticle
until its uptake into the nucleus.
3. Methods for Gene Transfer into Living Cells
Gene therapy is the treatment of genetically caused
diseases by manipulation of the genetic material of an
organism. For such therapy an efficient method for the
introduction of a therapeutic gene into cells is required.[44]
Gene-delivery systems are generally divided into two cate-
gories: viral and nonviral systems. In Table 1, the current
transfection methods are summarized and their advantages
and disadvantages are shown. Viral carriers (which work by
the same mechanisms as natural viruses that cause infectious
diseases) are a most effective but rather dangerous method
because of the risk of recombination, leading to the gener-
ation of viruses capable of replication. Electroporation is a
safe, easy, and rather efficient method, but it needs a large
amount of DNA and has to be optimized for every cell type.
Microinjection only allows one cell at a time to be transfected
and is therefore not feasible for a whole organism. Using the
gene-gun technique, a shallow penetration of DNA into the
tissue is accomplished. Cationic compounds and recombinant
proteins were used in clinical trials; however, cationic
compounds are usually toxic and recombinant proteins are
expensive to prepare.
3.1. Viral Gene-Delivery Systems
Viral gene-delivery systems are based on the ability of
viruses to infect cells. Part of the original gene segment of the
viral carrier is eliminated and the reporter gene is inserted.
This is the oldest method for gene transfer, first demonstrated
on Salmonella in 1952.[45] Later, for gene transfer into cells,
different viral vectors based on retroviruses,[46,47] adenovi-
ruses,[48] adeno-associated viruses,[49] herpes simplex virus,[50]
and other viruses were introduced. It is a most efficient
method with which to introduce DNA into cells, but it carries
Viktoriya Sokolova graduated in biology at
the V. N. Karazhin Kharkiv National Univer-
sity, Ukraine, in 2003. For doctoral studies,
she joined the group of Prof. Epple and
obtained her Ph.D. in 2006. In 2007, she
received the Young Scientist Award of the
Klee Family of the German Society for
Biomedical Technology, the Award of the
German Society for Biomaterials, and was a
finalist for the DSM Science and Technology
Award 2007. Her PhD research focused on
the synthesis, characterization, and applica-
tion of calcium phosphate nanoparticles for
the transfection of cells.
Matthias Epple studied chemistry at the
University of Braunschweig, Germany, and
received his PhD in 1992 with Prof. Cam-
menga. H was a Postdoc at the University
of Washington (Seattle) and completed his
Habilitation at the University of Hamburg
(Prof. Reller) in 1997. He was appointed
Associate Professor at the Ruhr-University of
Bochum in 2000. In 2003 he became
Professor of Inorganic Chemistry at the Uni-
versity of Duisburg-Essen. His research inter-
ests include the development of biomateri-
als, biomimetic crystallization, the applica-
tion of synchrotron radiation-based methods, the synthesis of nanoparticles,
and the reactivity of solids.
Figure 1. The transfer mechanism of nanoparticles (green circles) into
a cell and into its nucleus. IAdsorption on the cell membrane.
II Uptake by endocytosis. III–IV Escape from endosomes and intra-
cellular release. VNuclear targeting. VI Nuclear entry and gene expres-
sion, see text for details. Red foreign DNA, brown lipids, orange nu-
clear membrane, green cell DNA.
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serious risks, such as the possibility of recombination, strong
immunogenicity, inflammatory response, and carcinogenic-
ity.[48,51–53]
At present, there are no viral-based methods which would
allow a safe and efficient gene delivery for clinical treat-
ment.[54] Therefore, nonviral delivery systems have advan-
tages for gene transfer even though they show a lower
efficiency than viral systems. Helm et al. reviewed their
applicability for spinal fusion through the induction of the
production of bone-growth-stimulating proteins (such as,
bone morphogenic proteins (BMPs)).[55]
3.2. Recombinant Proteins
Recombinant proteins, so-called TAT proteins (TAT=
trans-activating transcriptional activator), are a special type
of DNAvector which contain a nuclear localization sequence.
Like a virus, they have the capability to penetrate a cell
membrane and especially to overcome the nuclear-membrane
barrier to deliver their genetic material. Such proteins may
include polylysine segments,[56] protamine,[57,58] or histones to
bind DNA and to form stable complexes which help to
protect DNA from intracellular degradation by nucleases.[58]
3.3. Electroporation
Electroporation is a popular in vitro technique for intro-
ducing plasmid DNA into living cells. It was introduced in
1982 for the transfection of mammalian cells.[59] The applica-
tion of electric pulses opens pores in the cell membrane
through which DNA can pass and directly enter into the
cytoplasm. Then, the pores close again and the DNA is
trapped within the cell. This technique was applied to
introduce plasmid DNA into tissues, such as muscles,[60]
melanoma,[61] and liver.[62] Its efficiency varies greatly with
cell types.[60,63]
3.4. Microinjection
Conceptually, the microinjection of naked plasmid DNA
into a cell is the easiest method for DNA delivery. Its
drawback is its sequential character, that is, the fact that only
one cell at a time can be treated with DNA. It is, therefore,
not applicable for research with large numbers of cells and for
in vivo DNA delivery.[17–24,64]
3.5. Gene Gun
The gene gun (“biolistic particle delivery”) is the most
novel physical transfection method.[65] This technique is based
on gold nanoparticles which are coated with DNA and then
shot into target tissues or cells.[66] This approach allows DNA
to penetrate directly through cell membranes into the
cytoplasm or even the nucleus, and to bypass the endosomes,
thus avoiding enzymatic degradation. The major limitation is
the shallow penetration of the particles into the tissue. The
depth of the particle penetration in the skeletal muscle of
mouse did not exceed 0.5 mm.[67] Skin, liver, and muscle were
all transfected by the gene-gun technique, but the efficiency
depended on the tissue, for example, 10–20% of skin
epidermal cells were transfected, whereas only 1–5% of
muscle cells.[66–68] In vivo gene-gun application typically
results in short-term and low-level gene expression. Never-
theless, it might be suitable for genetic vaccination.[69]
4. Chemical Methods Based On Nanoparticles
The chemical methods are generally based on nano-
particles, liposomes, or micelles which form a complex with
DNA or incorporate DNA and serve as carriers. These
methods can be divided into three groups: Cationic com-
pounds, recombinant proteins, and inorganic nanoparticles.
The different types of nanoparticles are shown in Figure 2.
4.1. Cationic Organic Molecules and Polymers
This approach uses the electrostatic attraction between
negatively charged nucleic acids and cationic carriers, typi-
cally cationic polyelectrolytes (e.g. polylysine[70,71] or polyeth-
yleneimine[72–75]) or liposomes/micelles from cationic surfac-
tants (usually lipids).[76] These nanoparticle assemblies are
taken up by cells.[77] In 1987, Felgner and co-workers were the
Table 1: Comparison of different gene-delivery systems.
Transfection method Advantages Disadvantages
Viral methods[44,45, 202] highly efficient[46–50] immunogenicity,[48,51, 52] carcinogenicity,[48,51, 52]
inflammation[53]
Physical
methods
electroporation[59–63,203–205] easy to perform; efficient optimization for every cell line required; a large
amount of DNA is necessary
microinjection[17–24,64] exact direction of nucleic acid into a
single cell
one cell after the other, that is, a slow, sequential
method
gene gun[65–69] useful for genetic vaccination shallow penetration of DNA into the tissue
Chemical
methods
cationic compounds[76] easy preparation toxicity[72,75, 82]
recombinant proteins[56–58] high biocompatibility expensive
polymeric nanoparticles, for example,
polylactide[206,207]
easy preparation; size controllable ;
easy functionalization
limited efficiency; some are toxic
inorganic nanoparticles[98,99] easy preparation; size-controllable;
easy functionalization
limited efficiency; some are toxic
Transfection
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first to use the cationic lipid dioleoyltrimethylammonium
chloride (DOTMA) in a 1:1 molar ratio with the neutral lipid
dioleoylphosphatidylethanolamine (DOPE) to condense
DNA for transfection.[78] Since then, a variety of cationic
lipids was developed for gene transfection; liposomes also
play a major role.[79,80]
One of the first polymers in nonviral gene delivery was
poly(l-lysine) (PLL).[70,71] PLL particles with a size around
100 nm were easily taken up by cells, although the trans-
fection efficiency remained low.[70] The reporter-gene expres-
sion was improved by the inclusion of targeting moieties, such
as chloroquine[81] or fusogenic peptides. However, poly(l-
lysine) is toxic and not approved for clinical use.[82] Another
polymer which is widely used for transfection is poly(ethyle-
nimine) (PEI). DNA-loaded PEI particles were delivered into
liver[73] and lung tissue.[74] Again, the major drawback of this
polymer is its toxicity.[72,75] Two frequently used commercial
transfection agents are Polyfect and Lipofectamin. Polyfect
consists of dendrimer molecules that radially branch from a
central core. Amino groups at the end of the branches are
positively charged and therefore strongly interact with the
negatively charged phosphate groups of nucleic acids, forming
compact structures.[83] Lipofectamine is a cationic-lipid trans-
fection agent used for the introduction of DNA into
eukaryotic cells. It was efficiently applied to many cell lines,
for example, NIH 3T3, COS-1, and fibroblasts.[84]
The practical problems which are encountered when a
synthetic compound is brought from the laboratory to a
clinical application were outlined by McNeil and Perrie for
cationic liposomes.[8] There are problems with the toxicity of
cationic polymers[72,75] and liposomes,[82] and in general, the
efficiency of nonviral systems is smaller than that of viral
systems.[7] However, some cationic-lipid–DNA complexes
were used in clinical trials.[85,86] They were successfully applied
to deliver plasmid DNA to the lung,[87] the brain,[88]
tumors,[89,90] and the skin.[91]
4.2. Inorganic Nanoparticles as Carriers of Nucleic Acids
The fact that cells take up nanoparticles can be used to
bring nucleic acids into a living cell.[92] The chemistry of
inorganic nanoparticles is highly advanced,[93–96] therefore
many classes of inorganic nanoparticles have been used as
carriers.[97–99] The inorganic materials used for DNA delivery
comprise calcium phosphate, carbon nanotubes, silica, gold,
magnetite, quantum dots, strontium phosphate, magnesium
phosphate, manganese phosphate, and double hydroxides
(anionic clays).
Although inorganic nanoparticles show only moderate
transfection efficiencies, they possess some advantages over
organic nanoparticles: They are not subject to microbial
attack, they can be easily prepared, they often have a low
toxicity, and they exhibit a good storage stability. It must be
emphasized that DNA must be protected from intracellular
attack by suitable “packaging”. DNA that is only adsorbed on
the surface of a nanoparticle is easily degraded by nucleases
(see Ref. [64] for a review on the requirements for a successful
transfection). Table 2 summarizes some features of inorganic
nanoparticles for biological application.
4.2.1. Metallic Nanoparticles
The chemistry of metallic nanoparticles is well explored,
particularly with respect to nanoparticles of the noble metals,
gold, silver, palladium, platinum.[94] Usually, they are pre-
pared by reduction of the corresponding metal salts in the
presence of a suitable protecting group which prevents further
aggregation (e.g., Au55 clusters[100]).
Gold nanoparticles (typical sizes: 10–20 nm) are easily
taken up by cells.[101–104] It was recently shown by Schmid et al.
that Au55 clusters effectively interact with DNA[105] and can be
used as anticancer agent.[105] This interaction appears to be a
matter of particle size (1.4 nm for Au55 clusters), that is, these
small gold clusters are intercalated into DNA strands. The
surface of gold can be conveniently covalently functionalized
using thiols (as in self-assembled monolayers (SAMs)), and
oligonucleotides can be attached to the particle surface.[106]
Oishi et al. reported polymer nanoparticles which were
assembled with gold nanoparticles and functionalized by
thiol-oligonucleotide conjugates.[107] Oligonucleotide-loaded
gold nanoparticles were also used for gene-silencing experi-
ments by Mirkin et al.[108] Salem et al. reported bimetallic
nanorods consisting of gold and nickel as a nonviral gene-
delivery system.[109] The gold and nickel segments in these
nanorods can selectively bind plasmid DNA and target
ligands. The pathway of gold–peptide nanoparticles inside
cells was studied by Tkachenko et al.[110]
Silver has been used for a long time as bactericide,[111] for
example, to prevent biofilms. This research has now been
extended to silver nanoparticles[112] which can be prepared in
many different sizes and shapes[113] which is important
Figure 2. Different types of nanoparticles which can be used for the
transfer of nucleic acids into living cells.
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because the biocidal action appears to be size-dependent.[114]
However, there are still many open questions, for example,
the dosage dependence and the risk of bacterial resistance.[115]
4.2.2. Iron Oxides
The magnetic properties of iron oxide nanoparticles (such
as, magnetite, Fe3O4) can be used, for example, for cell
sorting, for magnetic guidance in the body, and for tumor
thermotherapy.[116–119] If the particles are subjected to a
rapidly changing magnetic field, they can destroy the tissue
of a tumor by hyperthermia.[117,120] Another approach is the
magnetic guidance to a selected
part of the body, for example, into
a tumor.[121–124]
Gould et al. reported iron
oxide particles with diameters
ranging from less than 10 nm to
300 nm that can serve as a carrier
for DNA.[125] Cheng et al. prepared
magnetite nanoparticles with a
diameter of 9 nm from Fe2+,Fe
3+,
and tetramethylammonium hy-
droxide. The nanoparticles were
tested on Cos-7 monkey kidney
cells, and they showed no cytotoxic
effect at various doses of magnet-
ite.[126] We note that magnetic iron
oxides are often applied together
with a suitable coating to improve
their biocompatibility and functio-
nalizability. Silica-coated magnet-
ite nanoparticles were prepared by
Bruce et al. and functionalized
with amine groups to which oligo-
nucleotides were covalently bound
(Figure 3).[127,128]
A new approach was presented
by Farle et al. where magnetite was
incorporated into silica and then
coated with gold. These magnetic
particles can then be surface-func-
tionalized and subsequently
directed within the body, for exam-
ple, to tumor cells.[129] Landfester
and Ramirez showed how magnet-
ite nanoparticles can be encapsu-
lated in polymers by microemul-
sions.[130] Plank et al. presented the
concept of magnetofection and
showed a strongly enhanced
uptake of DNA by cells after treat-
ment with transfection agents,
superparamagnetic particles (mag-
netite or neodymium-iron-boron),
and application of an external
magnetic field (Figure 4).[131–133]
Table 2: Some key properties of inorganic nanoparticles which are used for transfection in cell biology.[a]
Kind of
nanoparticle
Chemical composition[b] Typical size
range
Solubility in
mgL1[c]
Comments
Cadmium
sulfide
CdS 2–5 nm 0.69 ngL1toxic, fluorescent, semicon-
ducting
Calcium
phosphate
Ca5(PO4)3OH (hydroxy-
apatite)
10–100 nm 6.1 mgL1[d] biodegradable, biocompat-
ible; may be made fluorescent
by incorporation of lantha-
nides; cations and anions
may be substituted
Carbon
nanotubes
Cndiameter of a
few nm and
length of a few
mm
0 Not biodegradable, hollow;
may be covalently functional-
ized to improve solubility and
may be loaded with mole-
cules
Cobalt-plati-
num
CoPt33–10 nm 0 ferromagnetic or superpara-
magnetic; toxic in uncoated
form
Gold Au 1–50 nm 0 easily covalently functional-
ized, for example, with thiols
Iron oxide
(magnetite)
Fe3O45–20 nm 0 ferromagnetic or superpara-
magnetic; harmful for cells in
uncoated form; solubility
increases with falling pH
Layered
double
hydroxides
Mg6Al2(CO3)(OH)16·4H2O
(hydrotalcite)
50–200 nm moderate,
increases
below
pH 5–6
high selective anion exchange
capacity; biodegradable in
slightly acidic environment;
cations can be substituted
Nickel Ni 5–100 nm 0 immunogenic, toxic
Silica SiO2·nH2O 3–100 nm ca. 120 mg
SiO2L1(for
silica parti-
cles)
Biodegradable; available also
in micro- or mesoporous
form (e.g., zeolites) ; easily
functionalizable, for example,
by chlorosilanes
Silver Ag 5–100 nm 0 Bactericidal; dissolution
product (Ag+) potentially
harmful for cells
Zinc oxide ZnO 3–60 nm 1.6 to
5mgL
1
fluorescent, semiconducting
Zinc sulfide ZnS 3–50 nm 67 ngL1fluorescing, semiconducting
[a] Note that in general it must be distinguished between the solubility in ionic form (which is given
here) and the solubility in the form of a nanoparticulate dispersion (i.e. as intact nanoparticles).
[b] Sometimes idealized. [c] The solubility was computed for standard solids in pure water (pH 7) at
258C, using the solubility products of CdS (1.40 1029 m2), hydroxyapatite (Ca10(PO4)6(OH)2;
10116.8 m18), and ZnS (2.91  1025m2). The other solubilities are taken from the literature. The solubility
of the other compounds cannot be computed because it depends on the chemical species present on
their surface. In any case, nanoscopic systems have a higher solubility than macroscopic phases owing
to their higher specific surface area, and an appropriate surface functionalization can strongly enhance
the solubility. For metals and alloys, the solubility also depends on the composition of the surrounding
solution (e.g. its oxidative potential). [d] Computed for stoichiometric hydroxyapatite.
Figure 3. TEM micrograph of silica-coated magnetite nanoparticles
used for transfection. The silica layer can in turn be covalently
functionalized by organic molecules, using the silanol groups in the
surface. Reprinted from Ref. [128], Copyright 2005, with permission
from Elsevier.
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Morishita et al. also showed that it is possible to increase
the transfection efficiency of viral vectors with superpara-
magnetic iron oxide nanoparticles (SPION).[134] The interac-
tion of surface-modified superparamagnetic iron oxides
nanoparticles with cells was investigated by Gupta
et al.[135–137] The unfunctionalized iron oxide nanoparticles
alone were cytotoxic (disruption of the cytoskeleton organ-
ization) whereas the same nanoparticles functionalized with
pullulan (a polysaccharide obtained from yeast) did not show
such adverse effects. This study emphasizes the importance of
the particle surface for the biological performance. Zhang
et al. showed that polyethylene glycol-functionalized magnet-
ite nanoparticles were taken up by macrophages
(RAW 264.7) to a much lower extent than unfunctionalized
magnetite nanoparticles, whereas for breast cancer cells
(BT20), the opposite effect was observed. Clearly, different
cell lines show a different selectivity towards the hydro-
philicity of the particles surface when it comes to the uptake
of nanoparticles.[138] Berry et al. investigated the effect of pure
and functionalized (either with dextran or albumin) iron
oxide nanoparticles (diameters of 8–10 nm) on fibroblasts.
They found that all three kinds of nanoparticles were well
taken up by the cells, and that both unfunctionalized and
dextran-functionalized nanoparticles induced cell death
whereas albumin-coated nanoparticles did not hinder cell
proliferation. Again, the nanoparticle surface appears to be
more important than the composition of its core.[139]
4.2.3. Carbon Nanotubes
Following the discovery of carbon nanotubes (CNT) by
Iijima in 1991,[140] they were the subject of many investigations
because of their special structural, mechanical, electrical, and
chemical properties. Two different types are known: single-
walled carbon nanotubes (SWCNTs) and multi-walled carbon
nanotubes (MWCNTs),[141] with diameters of a few nano-
meters and lengths up to 1 mm.[142, 143] Their main character-
istic property is their high ratio of length to diameter. Carbon
nanotubes can be prepared on the gram-scale. They have
found application as efficient biosensors,[144] as substrates for
directed cell growth,[145] as supports for the adhesion of
liposaccharides to mimic the cell membrane,[146] for trans-
fection,[147] and for controlled drug release.[148] Carbon nano-
tubes are practically insoluble in biological (aqueous) envi-
ronment and only a surface functionalization, for example,
with polymers, can increase their solubility. Their chemical
inertness, together with the option to functionalize them or to
load the inside of the tube with biomolecules,[149] makes them
attractive as carriers.[141,148, 150] However, as carbon nanotubes
are not biodegradable, their fate inside a cell is unclear. They
must be excreted by suitable mechanisms without degrada-
tion. Carbon nanotubes were found to be cytotoxic in vitro to
various mammalian cell lines.[148] Interestingly, the cytotox-
icity of carbon nanotubes towards macrophages strongly
depends on their structure. Jia et al. found a decrease in
cytotoxicity in the row of SWNTs >MWNT (with diameters
ranging from 10 to 20 nm) >quartz >C60.[151] Major efforts
were therefore directed to increase the solubility and to
reduce the toxicity of the carbon nanotubes to obtain a better
delivery system.
Harrison and Atala have reviewed the use of carbon
nanotubes for tissue engineering, and conclude with the
following sentences that bring the present state and the
possible problems to the point:[150] “While new uses of carbon
nanotubes for biomedical applications are being developed,
concerns about cytotoxicity may be mitigated by chemical
functionalization. However, there will be some limitations to
this nanomaterial since it is not biodegradable. Yet, it has been
shown to be excreted in vivo and so could be cleared from the
body once it is no longer needed.”
Recently, Liu et al. have demonstrated that carbon nano-
tubes functionalized with covalently bound siRNA can lead to
an efficient delivery of these nucleic acids into human T-cells
and primary cells (Figure 5).[147]
4.2.4. Double Hydroxides/clays
Layered double hydroxides (LDHs; also known as anionic
clays or hydrotalcites) constitute a class of clays which contain
positively charged layers. They have the general formula
MII1xMIIIx(OH)2(A)x·nH2O with the archetype hydrotalcite,
Mg6Al2(OH)16CO3·4H2O.[152] Interlayer anions and water
molecules are present in the interlayer space and can be
exchanged by other molecules.[153–155] LDHs with high anion-
exchange capacity have attracted particular attention in the
field of bio-hybrid nanomaterials owing to their high bio-
compatibility, high chemical stability, and controlled release
rate. LDHs have a high potential to exchange intercalated
anions by a variety of negatively charged biomolecules such
as DNA, vitamins, drugs, or sugars.[153] Organic molecules can
be released from LDHs at a rate that depends on the
pH value and the ionic strength of the surrounding
medium.[92,156] Choy et al. reported a biomolecular–inorganic
hybrid, a class of anionic exchanging clays, incorporating
DNA.[157] Because of its negative charge, DNA can be
Figure 4. Efficiency of antisense-oligodesoxyribonucleotide (ODN)
uptake by magnetofection (addition of superparamagnetic particles
and application of an external magnetic field) using different trans-
fection reagents. Comparison of the uptake of fluorescence-labeled
(Cy3) antisense-ODN 4 h after 15 min of standard transfection (black
bars) or magnetofection (white bars) using different transfection
reagents (PEI/DOTAP-cholesterol, FuGENE, Effectene), followed by
intense washing and addition of new medium. The numbers above the
bars show the n-fold increase achieved by magnetofection. Reprinted
by permission from Macmillan Publishers Ltd., Ref. [133], Copyright
2003.
M. Epple and V. Sokolova
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strongly incorporated into such a layered double hydroxide. If
the material is prepared in nanoparticulate form with
incorporated DNA, the particles can be used for transfection
with high efficiency.[157–159] The solubility of such particulate
LDHs strongly depends on the composition and properties of
the solvent and strongly increases at lower pH values (see
Refs. [160,161] for solubility data). It is likely that these
compounds can be dissolved (for example, by lysosomes
which have an acidic internal environment) and removed
from the cells in ionic form.
4.2.5. Silica
The preparation of silica nanoparticles by suitable sol–gel
processing routes is well established.[162] The presence of
silanol groups on the surface allows an easy functionalization,
for example, by attaching functionalized chlorosilanes. This
property together with the high biocompatibility of silica has
inspired many researchers to use them as carriers for drug
release or transfection. A successful transfer of DNA into
living cells was reported by Chen et al. Sodium chloride-
modified silica nanoparticles had diameters of 10–100 nm and
showed a transfection efficiency of about 70 % without
cytotoxicity. The administration of such silica nanoparticles
to mice showed no pathological cell changes.[163] Radu et al.
reported a novel gene-delivery system, where polyamido-
amine dendrimers were covalently attached to the surface of
mesoporous silica nanoparticles. These nanoparticles, with a
size of 250 nm, formed a complex with plasmid DNA. A
successful introduction of these nanoparticles into neural glia
cells, human cervical cancer cells, and Chinese hamster
ovarian (CHO) cells was observed with a higher transfection
efficiency than that obtained with commercial transfection
agents.[164] This concept is promising because the mesoporous
particles can be used as carriers for nucleic acids, and in
addition, dye molecules can be brought into the mesopores to
allow the tracing of the nanoparticles in the cell by, for
example, fluorescence microscopy. However, the particles
were found in the cytoplasm but not in the nucleus, a fact
which underscores the barrier action of the nuclear mem-
brane (Figure 6).[164]
Luo et al.[165] noticed that unfunctionalized silica nano-
particles can serve as mediators for the uptake of DNA into
cells by adsorbing on the cell surface.[166] This observation was
developed into a modular system where silica nanoparticles
(diameter about 225 nm) increased the concentration of DNA
in the presence of a transfection reagent on the cell surface
(simply by sedimentation of the nanoparticles on the cells),
thereby increasing the transfection efficiency by a factor of
ten. The silica nanoparticles alone were not active for cell
transfection.[167] The co-precipitation of other inorganic or
polymeric particles together with DNA on cell surfaces also
led to a good transfection efficiency, comparable with
commercial transfection agents. The increase in transfection
efficiency could be directly related to the rate of sedimenta-
tion, for example, very small or low-density nanoparticles did
not show an effect. The chemical composition of the nano-
particles was not of any influence, that is, this enhanced
uptake of DNA is a kind of “mechanical” effect where the
nanoparticles appear to exert some pressure upon the cell
surface.[168]
4.2.6. Calcium Phosphate
Calcium phosphates are the inorganic component of
biological hard tissues, for example, bone, teeth, and tendons,
where they occur as carbonated hydroxyapatite. With the
exception of enamel, they are always found as nanoparti-
cles.[169–171] Because of their biocompatibility there are no
concerns about an inherent cell toxicity. However, they may
increase the (usually very low) intracellular level of calcium
after biodegradation which could be harmful to cells. There-
fore, these particles have to be excreted from the cell before
they dissolve in the cytoplasm and cause a harmful increase in
the intracellular concentration of calcium.
The standard calcium phosphate transfection method,
originally discovered by Graham and van der Eb in 1973, is
very easy and straightforward.[172] The preparation of the
calcium phosphate carrier for transfection consists of only a
few steps: Mixing of calcium chloride solution with DNA and
a subsequent addition of phosphate-buffered saline solution
results in the formation of fine precipitates (nano- and
microparticles) of calcium phosphate with DNA. This dis-
persion is added to a cell suspension, and the nanoparticles
are taken up by the cells. The affinity of calcium phosphate to
the phosphate groups in nucleic acids is probably the reason
for the good adherence of the DNA to calcium phosphate
(Figure 7).
Figure 5. Carbon nanotubes for siRNA delivery into human T-cells. The
nanotubes are functionalized with PL-PEG2000-NH2(PL=phospholi-
pid, noncovalent bond; PEG =poly(ethylene glycol, Mw2000) followed
by the covalent attachment of thiol-siRNA through disulfide linkages.
Reprinted with permission from Ref. [147] .
Figure 6. Dye-loaded mesoporous silicate particles (black) which were
functionalized with DNA and endocytosed by a Chinese hamster
ovarian cell (TEM picture). Reprinted with permission from Ref. [164] .
Copyright 2004 American Chemical Society.
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The precipitation conditions of the standard calcium
phosphate method are decisive for the cell transfection
efficiency. The main parameters are pH value, concentrations
of CaCl2and DNA, temperature, and the time between
precipitation and transfection.[172] The transfection efficiency
also strongly depends on the kind of cells.[172] Quite often, the
reproducibility is poor. Orrantia and Chang followed the
pathway of 32P-marked DNA inside the cell and concluded
that the morphology of the colloids (mainly the particle size)
and the protection from enzymes that degrade nucleic acid
played a major role for the transfection efficiency.[173] Loyter
et al. also stressed the importance of the nanoparticle size
after studies with 3H-marked DNA.[34] From a chemical point
of view, it is understandable that this process depends on
many variables that all influence the nucleation and subse-
quent crystal growth of calcium phosphate. With time,
insufficiently protected nanocrystals will grow to microcrys-
tals by Ostwald-ripening, and their ability for transfection will
be lost.
Following these pathways, custom-made calcium phos-
phate nanoparticles were prepared for transfection by differ-
ent groups. This activity was also inspired by the observation
that calcium phosphate nanoparticles in general have a high
biocompatibility and a good biodegradability compared to
other types of nanoparticles. Maitra, even denoted them as
“second-generation nonviral vectors in gene therapy”.[174] A
successful transfection was reported with DNA-loaded cal-
cium phosphate nanoparticles functionalized with bovine
serum albumin (BSA; particle diameter 23.5–34.5 nm).[175]
Block-copolymer/calcium phosphate nanoparticle assemblies
were prepared by Kakizawa et al. and used for cell trans-
fection. The high biocompatibility of this system was empha-
sized.[176–178] Olton et al. prepared monodisperse calcium
phosphate nanoparticles (with an unusually high Ca:P ratios
of 110 :1 to 300:1 and a typical diameter of 25–50 nm) by
precipitation in the presence of DNA and found a most
efficient transfection.[179] The Ca:P ratio in crystalline calcium
phosphates is typically around 1.5:1,[170] thus, it is not clear
from which chemical compound these particles were formed,
although X-ray diffraction indicated hydroxyapatite. Other
earth-alkaline phosphate nanoparticles showed a similar
behavior. Bhakta et al. prepared magnesium phosphate and
manganese phosphate nanoparticles with a particle size of
100–130 nm functionalized with DNA.[180] Brash et al.
reported the preparation and characterization of strontium
phosphate nanoparticles and their application for both
transient and stable transfection.[181]
Concerning the biocompatibility of calcium phosphate
nanoparticles, Liu et al. reported an apoptotic action of
unfunctionalized calcium phosphate nanoparticles of about
50 nm diameter on a hepatoma cell line in the concentration
range of 50–200 mg l1.[182] However, questions remain about
the actual size of the nanoparticles investigated because the
crystal growth was not inhibited (no surface functionaliza-
tion). The adverse effect on the cells may be due to a harmful
increase in the intracellular calcium concentration. Euro-
pium-doped calcium phosphate nanoparticles showed fluo-
rescence, and the pathway of the nanoparticles could be
followed inside pancreatic cells.[183–185] It was also possible to
prepare terbium-doped (green fluorescence) and europium-
doped (red fluorescence) calcium phosphate nanoparticles,
colloidally stabilized by DNA, which were easily taken up by
cells, and showed a sufficiently high internal crystallinity to
give a reasonable fluorescence signal.[186] The accumulation of
DNA-loaded calcium phosphate nanoparticles which also
contained red-fluorescing tetramenthylrhodamin isothiocya-
nate (TRITC) BSA inside a cell and its nucleus was observed
by fluorescence microscopy (Figure 8).[187]
Calcium phosphate nanoparticles can be prepared by
rapid precipitation, followed by an immediate surface func-
tionalization with DNA[188] or oligonucleotides.[189] These
particles typically have a size of 80 nm and form stable
colloidal solutions. As discussed in Section 2, a major problem
Figure 7. Model of the interaction between the surface of a calcium
phosphate nanoparticle and a nucleic acid.
Figure 8. Transmission light microscopy (top row), fluorescence
microscopy (center row), and overlay of both pictures (bottom row) of
transfection experiments with human T-HUVEC cells. In light micros-
copy (top), the cells and their nuclei are visible. In the central row, the
calcium phosphate/DNA/TRITC-BSA nanoparticles appear as bright
red dots. In the bottom row, arrows indicate binding of nanoparticles
to the cell surface after 2 h (a), penetration into the cytoplasm after
8 h (b), and accumulation on the nuclear membrane after 48 h (c).
After 48 h, the transfected cells appear green as a result of the
expression of enhanced green-fluorescent protein (EGFP). The incor-
porated red-fluorescing nanoparticles are also clearly visible (d).
M. Epple and V. Sokolova
Reviews
1390 www.angewandte.org  2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2008,47, 1382 – 1395

is the intracellular degradation of the DNA-loaded nano-
particles on their pathway towards the nucleus. Some studies
were carried out to elucidate the pathway of the calcium
phosphate/DNA conjugates into the cell. Strain and Wyllie
found less than 7% of the added DNA inside the cytoplasm
and less than 4% in the nucleus. Only 0.5% of the DNA was
still undegraded and active.[190] We have prepared multishell
nanoparticles (Figure 9) in which DNA is incorporated both
inside the particle, where it is protected from degradations,
and outside, where it serves as a protecting layer against
aggregation and precipitation.[191] The transfection efficiency
was considerably increased by this process.[192] The same
concept worked well for gene silencing/antisense experiments
with HeLa-EGFP cells where the green fluorescence was
effectively inhibited by siRNA-functionalized calcium phos-
phate nanoparticles.[189]
4.2.7. Quantum Dots
Quantum dots are small nanoparticles with typical
diameters of a few nanometers (typically <10 nm) which
consist of II–VI or III–V semiconductors (e.g. CdS, CdSe,
ZnS, ZnSe, ZnO, GaAs, InAs; sometimes in a core–shell
structure).[193] They are protected against aggregation by
suitable capping agents which can also be functionalized.
They show favorable optical properties (highly efficient
fluorescence owing to quantum confinement effects and a
good resistance towards photobleaching) which are exploited,
for example, for biomedical imaging.[194,195] Although their
major application lies in the field of imaging, they were also
employed for transfection. Tan et al. showed the preparation
of self-tracking chitosan nanoparticles (diameter about
40 nm) with encapsulated CdSe/ZnS quantum dots and their
application for siRNA interference. A high efficiency in gene
silencing occurred after functionalization of the particle
surface with suitable antibodies (HER2) that target specific
receptors on the cell surface (Figure 10).[196]
Akerman et al. showed how ZnS/CdSe quantum dots
coated with specific peptides can be used to target different
cells and organs both in vitro and in vivo.[197] Srinivasan et al.
encapsulated CdSe/ZnS quantum dots in a functionalized
block-copolymer and attached DNA to the particle surface.
The quantum dots served as fluorescence marker to visualize
the transport of DNA into living cells during transfection.[198]
Nikolic et al. showed how different nanoparticles (CdSe/CdS,
Fe3O4, and CoPt3) could be coated with amine-functionalized
polyethylene oxide. In this way, their solubility in water was
greatly increased.[199]
The inherent toxicity of II–VI and III–V semiconductor
quantum dots (such as, CdSe, CdTe) is a serious issue for
biological applications. As shown by Aryal et al., there are
two possible reasons for the toxicity of quantum dots: The
presence of surface cations (such as Cd2+) and the formation
of photoinitiated radicals.[200] Metallothioneins, that is, cys-
teine-rich proteins which are present in a cell, are able to
mobilize cadmium from the nanoparticle surface by complex-
ation, which leads to an enhanced rate of dissolution and
higher toxicity. It was proposed that capping of the surface,
either by silica or by compounds which form stronger
complexes with cadmium than metallothioneins, might dimin-
ish this effect.[200] However, it may be argued that the long-
term fate of such toxic quantum dots inside a cell is not clear,
even if the surface is kinetically stabilized. Nevertheless, as
the quantity of material in such nanoparticles is very small,
the toxic effect may not be very serious.
A method to increase the biocompatability was demon-
strated by Zhang et al. who showed, in a very comprehensive
analysis, how the gene expression of fibroblasts changed when
they were exposed to silica-coated quantum dots. The surface
of CdSe/ZnS core–shell quantum dots was first silanized and
then coated with polyethylene glycol (PEG). These surface-
modified nanoparticles were not harmful to the cells, genes
which are upregulated by heavy-metal exposure were not
effected by the presence of these nanoparticles.[201]
5. Summary
Many different kinds of nanoparticles can be loaded with
nucleic acids (DNA or RNA) and cells appear to be quite
indifferent to the chemical nature of these nanoparticles when
it comes to an uptake by endocytosis. Regarding their size, the
Figure 9. Scanning electron micrograph of calcium phosphate/DNA/
BSA nanoparticles (left). Transmission electron micrograph of calcium
phosphate/oligonucleotide nanoparticles (right).
Figure 10. Green-fluorescing chitosan/CdSe/ZnS nanoparticles which
were functionalized on their surface with antibodies that recognize the
cell wall (HER2 antibody surface labeling). The nuclei of the SKBR3
cells were stained blue with 4’,6-diamino-2-phenylindol (DAPI). Mag-
nification=40 . Reprinted from Ref. [196] , Copyright 2007, with
permission from Elsevier.
Transfection
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upper limit for an efficient uptake through the cell membrane
appears to be around 100 nm. The surface functionalization of
the nanoparticle is important for uptake and short-term
cellular interaction, whereas the chemical composition of the
interior (the “bulk”) is important for long-term biodegrada-
bility and biocompatibility. For transfection, the nucleic acids
must be protected from premature degradation, for example,
by nucleases, inside the cell, so that they can transfer their
genetic information. Both magnetic and mechanical factors
can be beneficial for the cellular uptake of nanoparticles. The
transfer of nanoparticles into the cell nucleus is necessary for
transfection with DNA, whereas for antisense strategies with
siRNA (gene silencing), it is sufficient to deliver siRNA into
the cytoplasm. Therefore, the optimal carriers may be differ-
ent for these two applications.
For a clinical application, such as in gene therapy, there is
of course the requirement for a high transfection efficiency,
but the aspects of biocompatibility, long-term biodegradation,
and site-selective application have to be addressed as well.
Inorganic nanoparticles offer many ways to prepare systems
with a defined particle size, surface functionalization, nucleic
acid protection, and biocompatibility. As it is possible to fine-
tune their nanostructure, for example, by coating them with
different layers or by loading internal nanopores, their use as
carriers can be extended. For example, such coatings allow the
shielding of internal, toxic ions (such as Cd2+), the protection
of internal nucleic acids from degradation, and the fine-tuning
of the hydrophobic/hydrophilic surface properties.
Finally, we believe that a better understanding of the fate
of the nanoparticles inside of the cell, and of the interactions
between the organic and inorganic parts of the particles will
lead to a delivery system suitable for clinical use.
We thank our collaborators in this field of research, especially
Prof. R. Heumann (Bochum), for many helpful discussions.
This project was supported by the Deutsche Forschungsge-
meinschaft.
Received: July 8, 2007
Published online: December 20, 2007
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