# Nanotechnology/Health effects of nanoparticles#Nanoecotoxicology 2

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# Nanotoxicology: Health effects of nanotechnology

The environmental impacts of nanotechnology have become an increasingly
active area of research.

Until recently the potential negative impacts of nanomaterials on human
health and the environment have been rather speculative and
unsubstantiated[^1].

However, within the past number of years several studies have indicated
that exposure to specific nanomaterials, e.g. nanoparticles, can lead to
a gamut of adverse effects in humans and animals [^2], [^3], [^4].

This has made some people very concerned drawing specific parallels to
past negative experiences with small particles [^5], [^6].

Some types of nanoparticles are expected to be benign and are FDA
approved and used for making paints and sunscreen lotion etc. However,
there are also dangerous nanosized particles and chemicals that are
known to accumulate in the food chain and have been known for many
years:

-    Asbestos
-    Diesel particulate matter
-   ultra fine particles
-   DDT
-   lead

The problem is that it is difficult to extrapolate experience with bulk
materials to nanoparticles because their chemical properties can be
quite different. For instance, anti-bacterial silver nanoparticles
dissolve in acids that would not dissolve bulk silver, which indicates
their increased reactivity[^7].

An overview of some exposure cases for humans and the environment shown
in the table. For an overview of nanoproducts see the section  Nanotech
Products in this
book.

  ------------------------- -------------------------------------------------------------------------------------------------------------------------------------- -----------------------------------------------------------------------------------------------------------------------------------------
  Product                   Examples                                                                                Potential release and exposure
  Cosmetics                 IV absorbing TiO~2~ or ZnO~2~ in sunscreen                                                                                Directly applied to skin and late washed off. Disposal of containers
  Fuel additives            Cerium oxide additives in the EU                                                                                Exhaust emission
  Paints and coatings       antibacterial silver nanoparticles coatings and hydrophobic nanocoatings                                                               wear and washing releases the particles or components such as Ag^+^.
  Clothing                  antibacterial silver nanoparticles coatings and hydrophobic nanocoatings                                                               skin absorption; wear and washing releases the particles or components such as Ag^+^.
  Electronics               Carbon nanotubes are proposed for future use in commercial electronics                                                                 disposal can lead to emission
  Toys and utensils         Sports gear such as golf clubs are beginning to be made from eg. carbon nanotubes                                                      Disposal can lead to emission
  Combustion processes      Ultrafine particles are the result of diesel combustion and many other processes can create nanoscale particles in large quantities.   Emission with the exhaust
  Soil regeneration         Nanoparticles are being considered for soil regeneration (see later in this chapter)                                                   high local emission and exposure where it is used.
  Nanoparticle production   Production often produces by products that cannot be used (e.g. not all nanotubes are singlewall)                                      If the production is not suitably planned, large quantities of nanoparticles could be emitted locally in wastewater and exhaust gasses.
  ------------------------- -------------------------------------------------------------------------------------------------------------------------------------- -----------------------------------------------------------------------------------------------------------------------------------------

  : Ways nanoparticles can escape into the environment (adapted from
  [^8])

The peer-reviewed journal
Nanotoxicology is
dedicated to Research relating to the potential for human and
environmental exposure, hazard and risk associated with the use and
development of nano-structured materials. Other journals also report on
the research, for see the full Nano-journal
list in
this book.

## Nanoecotoxicology

In response to the above concerns a new field of research has emergence
termed "nano(eco-)toxiciolgoy" defined
as the "science of engineered nanodevices and nanostructures that deals
with their effects in living organisms" [^9].

In the following we will first try to explain why some people are
concerned about nanomaterials and especially nanoparticles. This will
lead to a general presentation of what is known about the hazardous
properties of nanoparticles in the field of the environment and
nanoecotoxicology. This includes a discussion of the main areas of
uncertainty and gaps of knowledge.

## Human or ecotoxicology

The focus of this chapter is on the
ecotoxicological - and environmental
effects of nanomaterials, however references will be made to studies on
human toxicology where it is assumed that such analogies are warranted
or if studies have provided new insights that are relevant to the field
of nanoecotoxicology as well. As further investigations are made, more
knowledge will be gained about human toxicology of nanoparticles.

# Production and applications of nanotechnology

At present the global production of nanomaterials is hard to estimate
for three main reasons:

-   Firstly, the definition of when something is "nanotechnology" is not
    clear-cut.
-   Secondly, nanomaterials are used in a great diversity of products
    and industries and
-   Thirdly, there is a general lack of information about what and how
    much is being produced and by whom.

In 2001 the future global annual production of carbon-based
nanomaterials was estimated to be several hundred tons, but already in
2003 the global production of nanotubes alone was estimated to be around
900 tons distributed between 16 manufacturers [^10].

The Japanese company, Frontier Carbon Corp, plan to start an annual
production of 40 tons of C~60~ [^11].

It is estimated that the global annual production of nanotubes and fiber
was 65 tons equal to €144 million worth and it is expected to surpass €3
billion by 2010 representing an annual growth rate of well over 60%
[^12].

Even though the information about the production of carbon-based
nanomaterials is scarce, the annual production volumes of for instance
quantum dots, nano-metals, and materials with nanostructured surfaces
are completely unknown.

The development of nanotechnology is still in its infancy, and the
current production and use of nanomaterials is most likely not
representative for the future use and production. Some estimates for the
future manufacturing of nanomaterials have been made. For instance the
Royal Society and the Royal Academy of Engineering [^13] estimated that
nanomaterials used in relation to environmental technology alone will
increase from 10 tons per year in 2004 to between 1000-10.000 tons per
year before 2020. However, the basis of many of these estimations is
often highly unclear and the future production will depend on a number
of things such as for instance:

1.  Whether the use of nanomaterials indeed entails the promised
    benefits in the long run;
2.  which and how many different applications and products will
    eventually be developed and implemented;
3.  And on how nanotechnology is perceived and embraced by the public?

With that said, the expectations are enormous. It is estimated that the
global market value of nano-related products will be U.S. \$1 trillion
in 2015 and that potentially 7 million jobs will be created globally
[^14] , [^15]

# Exposure of environment and humans

Exposure of nanomaterials to workers, consumers, and the environment
seems inevitable with the increasing production volumes and the
increasing number of commercially available products containing
nanomaterials or based on nanotechnology [^16].

Exposure is a key element in risk assessment of nanomaterials since it
is a precondition for the potential toxicological and ecotoxicological
effects to take place. If there is no exposure -- there is no risk.
Nanoparticles are already being used in various products and the
exposure can happen through multiple routes.

Human routes of exposure are:

-   dermal (for instance through the use of cosmetics containing
    nanoparticles);
-   inhalation (of nanoparticles for instance in the workplace);
-   ingestion (of for instance food products containing nanoparticles);
-   and injection (of for instance medicine based on nanotechnology).

Although there are many different kinds of nanomaterials, concerns have
mainly been raised about free nanoparticles [^17], [^18].

Free nanoparticles could either get into the environment through direct
outlet to the environment or through the degradation of nanomaterials
(such as surface bound nanoparticles or nanosized coatings).

Environmental routes of exposure are multiple.

One route is via the wastewater system. At the moment research
laboratories and manufacturing companies must be assumed to be the main
contributor of carbon-based nanoparticles to the wastewater outlet.

For other kinds of nanoparticles for instance titanium dioxide and
silver, consumer products such as cosmetics, crèmes and detergents, is a
key source already and discharges must be assumed to increase with the
development of nanotechnology.

However, as development and applications of these materials increases
this exposure pattern must be assumed to change dramatically. Traces of
drugs and medicine based on nanoparticles can also be disposed of
through the wastewater system into the environment.

Drugs are often coated, and studies have show that these coatings can be
degraded through either metabolism inside the human body or
transformation in environment due to UV-light [^19]. Which only
emphasises the need to studying the many possible process that will
alter the properties of nanoparticles once they are released in nature.

Another route of exposure to the into the environment is from wastewater
overflow or if there is an outlet from the wastewater treatment plant
where nanoparticles are not effectively held back or degraded.

Additional routes of environmental exposure are spills from production,
transport, and disposal of nanomaterials or products [^20].

While many of the potential routes of exposure are uncertain scenarios,
which need confirmation, the direct application of nanoparticles, such
as for instance nano zero valent iron for remediation of polluted areas
or groundwater, is one route of exposure that will certainly lead to
environmental exposure. Although, remediation with the help of free
nanoparticles is one of the most promising environmental
nanotechnologies, it might also be one the one raising the most
concerns. The Royal Society and The Royal Academy of Engineering [^21]
actually recommend that the use of free nanoparticles in environmental
applications such as remediation should be prohibited until it has been
shown that the benefits outweigh the risks.

The presence of manufactured nanomaterials in the environment is not
widespread yet, it is important to remember that the concentration of
xenobiotic organic chemicals in the environment in the past has
increased proportionally with the application of these [^22] -- meaning
that it is only a question of time before we will find nanomaterials
such as nanoparticles in the environment -- if we have the means to
detect them.

The size of nanoparticles and our current lack of metrological methods
to detect them is a huge potential problem in relation to identification
and remediation both in relation to their fate in the human body and in
the environment [^23].

Once there is a widespread environmental exposure human exposure through
the environment seems almost inevitable since water- and sediment living
organisms can take up nanoparticles from water or by ingestion of
nanoparticles sorbed to the vegetation or sediment and thereby making
transport of nanoparticles up through the food chain possible [^24].

# Nanoecotoxicology

Despite the widespread development of nanotechnology and nanomaterials
through the last 10-20 years, it is only recently that focus has been
turned onto the potential toxicological effects on humans, animals, and
the environment through the exposure of manufactured nanomaterials
[^25].

With that being said, it is a new development that potential negative
health and environmental impacts of a technology or a material is given
attention at the developing stage and not after years of application
[^26].

The term "nano(eco-)toxicology" has been developed on the request of a
number of scientists and is now seen as a separate scientific discipline
with the purpose of generating data and knowledge about nanomaterials
effects on humans and the environment [^27], [^28].

Toxicological information and data on nanomaterials is limited and
ecotoxicological data is even more limited. Some toxicological studies
have been done on biological systems with nanoparticles in the form of
metals, metal oxides, selenium and carbon [^29], however the majority of
toxicological studies have been done with carbon fullerenes [^30].

Only a very limited number of ecotoxicological studies have been
performed on the effects of nanoparticles on environmentally relevant
species, and, as for the toxicological studies, most of the studies have
been done on fullerenes. However, according to the European Scientific
Committee on Emerging and Newly Identified Health Risks [^31] results
from human toxicological studies on the cellular level can be assumed to
be applicable for organisms in the environment, even though this of
cause needs further verification. In the following a summary of the
early findings from studies done on bacteria, crustaceans, fish, and
plants will be given and discussed.

## Bacteria

The effect of nanoparticles on bacteria is very important since bacteria
constitute the lowest level and hence the entrance to the food chain in
many ecosystems [^32].

The effects of C~60~ aggregates on two common
soil bacteria E. coli (gram negative) and B. subtilis (gram positive)
was investigated by Fortner et al. [^33] on rich and minimal media,
respectively, under aerobe and anaerobe conditions. At concentrations
above 0.4 mg/L growth was completely inhibited in both cultures exposed
with and without oxygen and light. No inhibition was observed on rich
media in concentration up to 2.5 mg/L, which could be due to that
C~60~ precipitates or gets coated by proteins
in the media. The importance of surface chemistry is highlighted by the
observation that hydroxylated C~60~ did not
give any response, which is in agreement with the results obtained by
Sayes et al. [^34] who investigated the toxicity on human dermal- and
liver cells. The antibacterial effects of
C~60~ has furthermore been observed by
Oberdorster [^35] , who observed remarkably clearer water during
experiments with fish in the aquarium with 0.5 mg/L compared to control.

Lyon et al. [^36] explored the influence of four different preparation
methods of C~60~ (stirred
C~60~, THF-C~60~,
toluene-C~60~, and
PVP-C~60~) on Bacillus subtilis and found that
all four suspensions exhibited relatively strong antibacterial activity
ranging from 0.09 ± 0.01 mg/L- 0.7 ± 0.3 mg/L, and although fractions
containing smaller aggregates had greater antibacterial activity, the
increase in toxicity was disproportionately higher than the associated
increase in surface area.

Silver nanoparticles are increasingly used as antibacterial agent [^37]

## Crustacean

A number of studies have been performed with the freshwater crustacean
Daphnia magna, which is an important ecological important species that
furthermore is the most commonly used organisms in regulatory testing of
chemicals.

The organism can filter up to 16 ml an hour, which entails contact with
large amounts of water in its surroundings. Nanoparticles can be taken
up via the filtration and hence could lead to potential toxic effects
[^38].

Lovern and Klaper [^39], [^40] observed some mortality after 48 hours of
exposure to 35 mg/L C~60~ (produced by
stirring and also known as "nanoC~60~" or
"nC~60~"), however 50% mortality was not
achieved, and hence an LC50 could not be determined [^41].

A considerable higher toxicity of LC50 = 0.8 mg/L is obtained when using
nC~60~ put into solution via the solvent
tetrahydrofuran (THF) -- which might indicate that residues of THF is
bound to or within the C~60~-aggreggates,
however whether this is the case in unclear at the moment. The
solubility of C~60~ using sonication has also
been found to increase toxicity [^42], whereas unfiltered
C~60~ dissolved by sonication has been found
to cause less toxicity (LC50 = 8 mg/L). This is attributed to the
formation of aggregates, which causes a variation of the bioavailability
to the different concentrations. Besides mortality, deviating behavior
was observed in the exposed Daphnia magna in the form of repeated
collisions with the glass beakers and swimming in circles at the surface
of the water [^43]. Changes in the number of hops, heart rate, and
appendage movement after subtoxic levels of exposure to
C~60~ and other
C~60~-derivatives [^44]. However, Titanium
dioxide (TiO2) dissolved via THF has been observed to cause increased
mortality in Daphania magna within 48 hours (LC50= 5.5 mg/L), but to a
lesser extent than fullerenes, while unfiltered TiO2 dissolved by
sonication did not results in a increasing dose-response relationship,
but rather a variation response [^45]. Lovern and Klaper [^46] have
furthermore investigated whether THF contributed to the toxicity by
comparing TiO2 manufactured with and without THF and found no difference
in toxicity and hence concluded that THF did not contribute to neither
the toxicity of TiO2 or fullerenes.

Experiments with the marine species Acartia tonsa exposed to 22.5 mg/L
stirred nC~60~ have been found to cause up to
23% mortality after 96 hours, however mortality was not significantly
different from control25. And exposure of Hyella azteca by 7 mg/L
stirred nC~60~ in 96 hours did not lead to any
visible toxic effects -- not even by administration of
C~60~ through the feed [^47].

Only a limited number of studies have investigated long-term exposure of
nanoparticles to crustaceans. Chronic exposure of Daphnia magna with 2.5
mg/L stirred nC~60~ was observed to cause 40%
mortality besides causing sub-lethal effects in the form of reduced
reproducibility (fewer offspring) and delayed shift of shield [^48].

Templeton et al. [^49] observed an average cumulative life-cycle
mortality of 13 ± 4% in an Estuarine Meiobenthic Copepod Amphiascus
tenuiremis after being exposed to SWCNT, while mean life-cycle
mortalities of 12 ± 3, 19 ± 2, 21 ± 3, and 36 ± 11 % were observed for
0.58, 0.97, 1.6, and 10 mg/L.

Exposure to 10 mg/L showed:

1.  significantly increased mortalities for the naupliar stage and
    cumulative life-cycle;
2.  a dramatically reduced development success to 51% for the nauplius
    to copepodite window, 89% for the copepodite to adult window, and
    34% overall for the nauplius to adult period;
3.  a significantly depressed fertilization rate averaging only 64 ±
    13%.

Templeton also observed that exposure to 1.6 mg/L caused a significantly
increase in development rate of 1 day faster, whereas a 6 day
significant delay was seen for 10 mg/L.

## Fish

A limited number of studies have been done with fish as test species. In
a highly cited study Oberdorster [^50] found that 0.5 mg/L
C~60~ dissolved in THF caused increased lipid
peroxidation in the brain of largemouth bass (Mikropterus salmoides).
Lipid peroxidation was found to be decreased in the gills and the liver,
which was attributed to reparation enzymes. No protein oxidation was
observed in any of the mentioned tissue, however a discharge of the
antioxidant glutathione occurred in the liver possibility due to large
amount of reactive oxygen molecules stemming from oxidative stress
caused by C~60~ [^51].

For Pimephales promelas exposured to 1 mg/L THF-dissolved
C~60~, 100 % mortality was obtained within 18
hours, whereas 1 mg/L C~60~ stirred in water
did not lead to any mortality within 96 hours. However, at this
concentration inhibition of a gene which regulates fat metabolism was
observed. No effect was observed in the species Oryzia latipes at 1 mg/L
stirred C~60~, which indicates different
inter-species sensitivity toward C~60~ [^52],
[^53].

Smith et al. [^54] observed a dose-dependent rise in ventilation rate,
gill pathologies (oedema, altered mucocytes, hyperplasia), and mucus
secretion with SWCNT precipitation on the gill mucus in juvenile rainbow
trout.

Smith et al. also observed:

-   dose-dependent changes in brain and gill Zn or Cu, partly attributed
    to the solvent;
-   a significant increases in Na+K+ATPase activity in the gills and
    intestine;
-   a significant dose-dependent decreases in TBARS especially in the
    gill, brain and liver;
-   and a significant increases in the total glutathione levels in the
    gills (28 %) and livers (18 %), compared to the solvent control (15
    mg/l SDS).
-   Finally, they observed increasing aggressive behavior; possible
    aneurisms or swellings on the ventral surface of the cerebellum in
    the brain and apoptotic bodies and cells in abnormal nuclear
    division in liver cells.

Recently Kashiwada [^55]29 reported observing 35.6% lethal effect in
embryos of the medaka Oryzias latipes (ST II strain) exposed to 39.4 nm
polystyrene nanoparticles at 30 mg/L, but no mortality was observed
during the exposure and postexposure to hatch periods at exposure to 1
mg/L. The lethal effect was observed to increase proportionally with the
salinity, and 100% complete lethality occurred at 5 time higher
concentrated embryo rearing medium. Kashwada also found that 474 nm
particles showed the highest bioavailability to eggs, and 39.4 nm
particles were confirmed to shift into the yolk and gallbladder along
with embryonic development. High levels of particles were found in the
gills and intestine for adult medaka exposed to 39.4 nm nanoparticles at
10 mg/L, and it is hypothesized that particles pass through the
membranes of the gills and/or intestine and enter the circulation.

## Plants

To our knowledge only one study has been performed on phytotoxicity, and
it indicates that aluminum nanoparticles become less toxic when coated
with phenatrene, which again underlines the importance to surface
treatments in relation to the toxicity of nanoparticles [^56].

# Identification of key hazard properties

Size is the general reason why nanoparticles have become a matter of
discussion and concern.

The very small dimensions of nanoparticles increases the specific
surface area in relation to mass, which again means that even small
amounts of nanoparticles have a great surface area on which reactions
could happen.

If a reaction with chemical or biological components of an organism
leads to a toxic response, this response would be enhanced for
nanoparticles. This enhancement of the inherent toxicity is seen as the
main reason why smaller particles are generally more biologically active
and toxic that larger particles of the same material [^57].

Size can cause specific toxic response if for instance nanoparticles
will bind to proteins and thereby change their form and activity,
leading to inhibition or change in one or more specific reactions in the
body [^58].

Besides the increased reactivity, the small size of the nanoparticles
also means that they can easier be taken up by cells and that they are
taken up and distributed faster in organism compared to their larger
counterparts [^59], [^60].

Due to physical and chemical surface properties all nanoparticles are
expected to absorb to larger molecules after uptake in an organism via a
given route of uptake [^61].

Some nanoparticles such as fullerene derivates are developed
specifically with the intention of pharmacological applications because
of their ability of being taken up and distributed fast in the human
body, even in areas which are normally hard to reach -- such as the
brain tissue [^62]. Fast uptake and distribution can also be interpreted
as a warning about possible toxicity, however this need not always be
the case [^63]. Some nanoparticles are developed with the intension of
being toxic for instance with the purpose of killing bacteria or cancer
cells [^64], and in such cases toxicity can unintentionally lead to
adverse effects on humans or the environment.

Due to the lack of knowledge and lack of studies, the toxicity of
nanoparticles is often discussed on the basis of ultra fine particles
(UFPs), asbestos, and quartz, which due to their size could in theory
fall under the definition of nanotechnology [^65], [^66].

An estimation of the toxicity of nanoparticles could also be made on the
basis of the chemical composition, which is done for instance in the
USA, where safety data sheets for the most nanomaterials report the
properties and precautions related to the bulk material [^67].

Within such an approach lies the assumption that it is either the
chemical composition or the size that is determining for the toxicity.
However, many scientific experts agree that that the toxicity of
nanoparticles cannot and should not be predicted on the basis of the
toxicity of the bulk material alone [^68], [^69].

The increased surface area-to-mass ratio means that nanoparticles could
potentially be more toxic per mass than larger particles (assuming that
we are talking about bulk material and not suspensions), which means
that the dose-response relationship will be different for nanoparticles
compared to their larger counterparts for the same material. This aspect
is especially problematic in connection with toxicological and
ecotoxicological experiments, since conventional toxicology correlates
effects with the given mass of a substance [^70], [^71].

Inhalation studies on rodents have found that ultrafine particles of
titanium dioxide causes larger lung damage in rodents compared to larger
fine particles for the same amount of the substance. However, it turned
out that ultra fine- and fine particles cause the same response, if the
dose was estimated as surface area instead of as mass [^72].

This indicates that surface area might be a better parameter for
estimating toxicity than concentration, when comparing different sizes
of nanoparticles with the same chemical composition5. Besides surface
area, the number of particles has been pointed out as a key parameter
that should be used instead of concentration [^73].

Although comparison of ultrafine particles, fine particles, and even
nanoparticles of the same substance in a laboratory setting might be
relevant, it is questionable whether or not general analogies can be
made between the toxicity of ultrafine particles from anthropogenic
sources (such as cooking, combustion, wood-burning stoves, etc.) and
nanoparticles, since the chemical composition and structure of ultrafine
particles is very heterogeneous when compared to nanoparticles which
will often consists of specific homogeneous particles [^74].

From a chemical viewpoint nanoparticles can consist of transition
metals, metal oxides, carbon structures and in principle any other
material, and hence the toxicity is bound to vary as a results of that,
which again makes in impossible to classify nanoparticles according to
their toxicity based on size alone [^75].

Finally, the structure of nanoparticles has been shown to have a
profound influence on the toxicity of nanoparticles. In a study
comparing the cytotoxicity of different kinds of carbon-based
nanomaterials concluded that single walled carbon nanotubes was more
toxic that multi walled carbon nanotubes which again was more toxic than
C~60~ [^76].

# Hazard identification

In order to complete a hazard identification of nanomaterials, the
following is ideally required

-   ecotoxicological studies
-   data about toxic effects
-   information on physical-chemical properties
    -   Solubility
    -   Sorption
-   biodegradability
-   accumulation
-   and all likely depending on the specific size and detailed
    composition of the nanoparticles

In addition to the physical-chemical properties normally considered in
relation to chemical substances, the physical-chemical properties of
nanomaterials is dependent on a number of additional factors such as
size, structure, shape, and surface area. Opinions on, which of these
factors are important differ among scientists, and the identification of
key properties is a key gap of our current knowledge [^77], [^78],
[^79].

There is little doubt that the physical-chemical properties normally
required when doing a hazard identification of chemical substances are
not representative for nanomaterials, however there is at current no
alternative methods. In the following key issues in regards to
determining the destiny and distribution of nanoparticles in the
environment will be discussed, however the focus will primarily be on
fullerenes such as C~60~.

## Solubility

Solubility in water is a key factor in the estimation of the
environmental effects of a given substance since it is often via contact
with water that effects-, or transformation, and distribution processes
occur such as for instance bioaccumulation.

Solubility of a given substance can be estimated from its structure and
reactive groups. For instance, Fullerenes consist of carbon atoms, which
results in a very hydrophobic molecule, which cannot easily be dissolved
in water.

Fortner et al. [^80] have estimated the solubility of individual
C~60~ in polar solvents such as water to be
10-9 mg/L. When C~60~ gets in contact with
water, aggregates are formed in the size range between 5-500 nm with a
greater solubility of up to 100 mg/L, which is 11 orders of magnitude
greater than the estimated molecular solubility. This can, however, only
be obtained by fast and long-term stirring in up to two months.
Aggregates of C~60~ can be formed at pH
between 3.75 and 10.25 and hence also by pH-values relevant to the
environment [^81].

As mentioned the solubility is affected by the formation of
C~60~ aggregates, which can lead to changes in
toxicity [^82].

Aggregates form reactive free radicals, which can cause harm to cell
membranes, while free C~60~ kept from
aggregation by coatings do not form free radical [^83]. Gharbi et al.
[^84] point to the accessibility of double bonds in the
C~60~ molecule as an important precondition
for its interactions with other biological molecules.

The solubility of C~60~ is less in salt water,
and according to Zhu et al. [^85] only 22.5 mg/L can be dissolved in 35
‰ sea water. Fortner et al. [^86] have found that aggregate precipitates
from the solution in both salt water and groundwater with an ionic
strength above 0.1 I, but aggregates would be stable in surface- and
groundwater, which typically have an ionic strength below 0.5 I.

The solubility of C~60~ can be increased to
about 13,000-100,000 mg/L by chemically modifying the
C~60~--molecule with polar functional groups
such as hydroxyl [^87]. The solubility can furthermore be increased by
the use of sonication or the use of none-polar solvents.

C~60~ will neither behave as molecules nor as
colloids in aqueous systems, but rather as a mixture of the two [^88],
[^89].

The chemical properties of individual C~60~
such as the log octanol-water partitioning coefficient (log Kow) and
solubility are not appropriate in regard to estimating the behavior of
aggregates of C~60~. Instead properties such
as size and surface chemistry should be applied a key parameters
[^90]18.

Just as the number of nanomaterials and the number of nanoparticles
differ greatly so does the solubility of the nanoparticles. For instance
carbon nanotubes have been reported to completely insoluble in water
[^91]. It should be underlined that which method is used to solute
nanoparticles is vital when performing and interpreting environmental
and toxicological tests.

## Evaporation

Information about evaporation of C~60~ from
aqueous suspensions has so far not been reported in the literature and
since the same goes for vapor pressure and Henry's constant, evaporation
cannot be estimated for the time being. Fullerenes are not considered to
evaporate -- neither from aqueous suspension or solvents -- since the
suspension of C~60~ using solvents still
entails C~60~ after evaporation of the solvent
[^92], [^93].

## Sorption

According to Oberdorster et al. [^94] nanoparticles will have a tendency
to sorb to sediments and soil particles and hence be immobile because of
the great surface area when compared to mass. Size alone will
furthermore have the effect that the transport of nanoparticles will be
dominated by diffusion rather than van der Waal forces and London
forces, which increases transport to surfaces, but it is not always that
collision with surfaces will lead to sorption [^95].

For C~60~ and carbon nanotubes the chemical
structure will furthermore result in great sorption to organic matter
and hence little mobility since these substances consists of carbon.
However, a study by Lecoanet et al. [^96]45 found that both
C~60~ and carbon nanotubes are able to migrate
through porous medium analogous to a sandy groundwater aquifer and that
C~60~ in general is transported with lower
velocity when compared to single walled carbon nanotubes, fullerol, and
surface modified C~60~. The study further
illustrates that modification of C~60~ on the
way to - or after - the outlet into the environment can profoundly
influence mobility. Reactions with naturally occurring enzymes [^97],
electrolytes or humid acid can for instance bind to the surface and make
thereby increase mobility [^98], just as degradation by UV-light or
microorganisms could potentially results in modified
C~60~ with increased mobility [^99]45.

## Degradability

Most nanomaterials are likely to be inert [^100], which could be due to
the applications of nanomaterials and products, which is often
manufactured with the purpose of being durable and hard-wearing.
Investigations made so far have however showed that fullerene might be
biological degradable whereas carbon nanotubes are consider biologically
non-degradable [^101], [^102]. According to the structure of fullerenes,
which consists of carbon only, it is possible that microorganisms can
use carbon as an energy source, such as it happens for instance with
other carbonaceous substances.

Fullerenes have been found to inhibit the growth of commonly occurring
soil- and water bacteria [^103], [^104] , which indicates that toxicity
can hinder degradability. It is, however, possible that biodegradation
can be performed by microorganisms other than the tested microorganisms,
or that the microorganism adapt after long-term exposure. Besides that
C~60~ can be degraded by UV-light and O
[^105]. UV-radiation of C~60~ dissolved in
hexane, lead to a partly or complete split-up of the fullerene structure
depending on concentration [^106].

## Bioaccumulation

Carbon based nanoparticles are lipophilic which means that they can
react with and penetrate different kinds of cell membranes [^107].
Nanomaterials with low solubility (such as
C~60~) could potentially accumulate in
biological organisms [^108] , however to the best of our knowledge no
studies have been performed investigating this in the environment.
Biokinetic studies with C~60~ in rats result
in very little excretion which indicates an accumulation in the
organisms [^109]. Fortner et al. [^110] estimates that it is likely that
nanoparticles can move up through the food chain via sediment consuming
organisms, which is confirmed by unpublished studies performed at Rice
University, U.S. [^111] Uptake in bacteria, which form the basis for
many ecosystems, is also seen as a potential entrance to whole food
chains [^112].

# Surface chemistry and coatings

In addition to the physical and chemical composition of the
nanoparticles, it is important to consider any coatings or modifications
of a given nanoparticles [^113].

A study by Sayes et al. [^114] found that the cytotoxicity of different
kinds of C~60~-derivatives varied by seven
orders of magnitude, and that the toxicity decreased with increasing
number of hydroxyl- and carbonyl groups attached to the surface.
According to Gharbi et al. [^115], it is in contradiction to previous
studies, which is supported by Bottini et al. [^116] who found an
increased toxicity of oxidized carbon nanotubes in immune cells when
compared to pristine carbon nanotubes.

The chemical composition of the surface of a given nanoparticle
influences both the bioavailability and the surface charge of the
particle, both of which are important factors for toxicology and
ecotoxicology. The negative charge on the surface of
C~60~ is suspected to be able to explain these
particles ability to induce oxidative stress in cells [^117].

The chemical composition also influences properties such as
lipophilicity, which is important in relation to uptake through cells
membranes in addition to distribution and transport to tissue and organs
in the organisms5. Coatings can furthermore be designed so that they are
transported to specific organs or cells, which has great importance for
toxicity [^118].

It is unknown, however, for how long nanoparticles stay coated
especially inside the human body and/or in the environment, since the
surface can be affected by for instance light if they get into the
environment. Experiments with non-toxic coated nanoparticles, turned out
to be very cell toxic after 30 min. exposure to UV-light or oxygen in
air [^119].

# Interactions in the Environment

Nanoparticles can be used to enhance the bioavailability of other
chemical substances so that they are easily degradable or harmful
substances can be transported to vulnerable ecosystems [^120].

Besides the toxicity of the nanoparticles itself, it is furthermore
unclear whether nanoparticles increases the bioavailability or toxicity
of other xenobiotics in the environment or other substances in the human
body. Nanoparticles such as C~60~ have many
potential uses in for instance in medicine because of their ability to
transport drugs to parts of the body which are normally hard to reach.
However, this property is exactly what also may be the source to adverse
toxic effects [^121]. Furthermore research is being done into the
application of nanoparticles for spreading of contaminants already in
the environment. This is being pursued in order to increase the
bioavailability for degradation of microorganisms [^122]54, however it
may also lead to increase uptake and increased toxicity of contaminants
in plants and animals, but to the best of our knowledge, no scientific
information is available that supports this [^123], [^124].

# Conclusion

It is still too early to determine whether nanomaterials or
nanoparticles are harmful or not, however the effects observed lately
have made many public and governmental institutions aware of

1.  the lack of knowledge concerning the properties of nanoparticles
2.  the urgent need for a systematic evaluation of the potential adverse
    effect of Nanotechnology

[^125], [^126].

Furthermore, some guidance is needed as to which precautionary measures
are warranted in order to encourage the development of "green
nanotechnologies" and other future innovative technologies, while at the
same time minimizing the potential for negative surprises in the form of
adverse effects on human health and/or the environment.

It is important to understand that there are many different
nanomaterials and that the risk they pose will differ substantially
depending on their properties. At the moment it is not possible to
identify which properties or combination of properties make some
nanomaterials harmful and which make them harmless, and properly it will
depend on the nanomaterial is question. This makes it is extremely
difficult to do risk assessments and life-cycle assessment of
nanomaterials because, in theory, you would have to do a risk assessment
for each of the specific variation of nanomaterial -- a daunting task!

# Contributors to this page

This material is based on notes by

-   Steffen Foss Hansen, Rikke Friis Rasmussen, Sara Nørgaard Sørensen,
    Anders Baun. Institute of Environment & Resources, Building 113,
    NanoDTU Environment, Technical University of Denmark
-   Stig Irving Olsen, Institute of Manufacturing Engineering and
    Management, Building 424, NanoDTU Environment, Technical University
    of Denmark
-   Kristian Mølhave, Dept. of Micro and Nanotechnology - DTU -
    www.mic.dtu.dk

# References

See also notes on editing this book about how to add references
Nanotechnology/About#How_to_contribute.

```{=html}
<references />
```

------------------------------------------------------------------------

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