An evidence-based environmental perspective of manufactured silver nanoparticle in

Prévia do material em texto

Review
An evidence-based environ
syntheses and applications
peer-reviewed scientific p
Thabet M. Tolaymat a,⁎, Amro M.
Todd P. Luxton a, Makram Suidan
a USEPA Office of Research and Development, National Ri
b Dept. of Civil & Environmental Engineering, University o
c WorldTek Inc, Cincinnati, OH, United States
a r t i c l e i n f o
Article history:
Received 3 April 2009
Received in revised form 20 October 2009
Accepted 2 November 2009
Available online 27 November 2009
Science of the Total Environment 408 (2010) 999–1006
Contents lists available at ScienceDirect
Science of the Total Environment
j ourna
objectives. Specific inclusion criteria were applied to gather the most pertinent research articles. Data and
information extraction reliedon the type of synthesismethods, that is, synthesized silver nanoparticles in general
and specific applications, nanocomposites, and bimetallic techniques. The following items were gathered for:
type of silver salt, solvent, reducing agent, stabilizing agent, size, and type of application/nanocomposite/
bimetallic, and template (for nanocomposites). The description of evidencewas presented in tabular format. The
critical appraisal was analyzed in graphical format and discussed.
Results: An analysis of the scientific literature suggests that most synthesis processes produce spherical silver
nanoparticles with less than 20 nm diameter. Silver nanoparticles are often synthesized via reduction of AgNO3,
dissolution in water, and utilization of reductants also acting as capping or stabilizing agents for the control of
particle size to ensure a relatively stable suspension. Two of the most commonly used reductants and stabilizing
agents are NaBH4 and citrate which yield particles with a negative surface charge over the environmental pH
range (3–10). The environmental perspectives of these parameters are discussed.
Concluding remarks: It is expected that theantibacterial propertyof bulk silver is carriedover andperhapsenhanced,
to silver nanoparticles. Therefore, when one examines the environmental issues associated with the manufacture
and use of silver nanoparticle-based products, the antibacterial effects should always be taken into account
particularly at the different stages of the product lifecycle. Currently, there are two arguments in the scientific
⁎ Corresponding author. Tel.: +1 513 487 2860; fax:
E-mail address: tolaymat.thabet@epa.gov (T.M. Tola
0048-9697/$ – see front matter. Published by Elsevier
doi:10.1016/j.scitotenv.2009.11.003
nanoparticles to form bimetallic nanoparticles; Aim 3 — to provide a summary of the morphology of silver
nanoparticles; and (4) Aim4— to provide an environmental perspective of the evidencepresented inAims1 to 3.
Methods: A comprehensive electronic search of scientific databases was conducted in support of the study
Keywords:
Silver
Nanoparticle
Syntheses
Evidence–based approach
in the regulatory and research communities about potential environmental impacts associated with the use
of silver nanoparticles. In particular, the unlimited combinations of properties emerging from the syntheses
and applications of silver nanoparticles are presenting an urgent need to document the predominant salt
precursors, reducing agents and stabilizing agents utilized in the synthesis processes of silver nanoparticles
to guide the massive efforts required for environmental risk assessment and management.
Objectives: The primary objective of this study is to present an evidence-based environmental perspective of
silver nanoparticle properties in syntheses and applications. The following specific aims are designed to achieve
the study objective: Aim 1 — to document the salt precursors and agents utilized in synthesizing silver
nanoparticles; Aim 2 — to determine the characteristics of silver nanoparticles currently in use in the scientific
literature when integrated in polymer matrices to form nanocomposites and combined with other metal
mental perspective of manufactured silver nanoparticle in
: A systematic review and critical appraisal of
apers
El Badawy b, Ash Genaidy c, Kirk G. Scheckel a,
b
sk Management Laboratory, 26 West Martin Luther King Drive, Cincinnati, OH 45224, United States
f Cincinnati, Cincinnati, OH, United States
a b s t r a c t
Background: Most recently, renewed interest has arisen in manufactured silver nanomaterials because of
their unusually enhanced physicochemical properties and biological activities compared to the bulk parent
materials. A wide range of applications has emerged in consumer products ranging from disinfecting medical
devices and home appliances to water treatment. Because the hypothesized mechanisms that govern the fate
and transport of bulk materials may not directly apply to materials at the nanoscale, there are great concerns
l homepage: www.e lsev ie r.com/ locate /sc i totenv
literature about the mechanisms of antimicrobial properties of silver nanoparticles as they relate to colloidal silver
particles and inonic silver. Methodologies of risk assessment and control have to account for both arguments.
Published by Elsevier B.V.
+1 513 569 7879.
ymat).
B.V.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
1000 T.M. Tolaymat et al. / Science of the Total Environment 408 (2010) 999–1006
involves the dissolution of silver salt into a solvent and the subsequent
to 15% to the total silver released into water in the European Union.
addition of a reducing agent, with the supplemental use of stabilizing
assist, among other things, in developing relative risk analyses. Such
approach was recently adopted to evaluate the relative risk of
manufactured nanomaterials. A representative synthesis method
was selected by Robichaud et al. (2005) for a given manufactured
nanomaterial based on its potential for scale-up. A list of input and
output materials and waste streams for each step of fabrication was
developed and entered into a database that included key process
characteristics. The physical–chemical properties and quantities of
inventoriedmaterials were used to qualitatively assess the risks based
on factors such as volatility, carcinogenicity, flammability, toxicity,
and persistence.
In general, silver nanoparticles are synthesized using various
techniques resulting in different shapes and sizes for use in numerous
applications. The synthesis techniques are categorized into top-down
and bottom-up approaches (del Rocío Balaguera-Gelves, 2006). The
top-down techniques use silver metal in its bulk form, then,
mechanically reduce its size to the nanoscale via specialized
methodologies such as lithography and laser ablation (Amendola
et al., 2007). The bottom-up (also known as self-assembly) technique
syntheses and applications. The following specific aims are designed
to achieve the study objective: Aim 1 — to document the salt
precursors and agents utilized in synthesizing silver nanoparticles;
Aim 2 — to determine the characteristics of silver nanoparticles
currently in use in the scientific literature when integrated in polymer
matrices to formnanocomposites and combinedwith othermetal nano-
particles to form bimetallic nanoparticles; Aim 3 — to provide a
summary of themorphology of silver nanoparticles; and (4) Aim 4— to
provide an environmental perspective of the evidence presented in
Aims 1 to 3.
The work presented herein supplements but do not duplicate
earlier reviews on the subject (Boxall et al., 2007; Luoma, 2008; Blaser
et al., 2008; Mueller and Nowack, 2008). Boxall et al. (2007)
developed a range of models aiming at predicting the concentrations
of a wide variety of engineered nanoparticles in water and soils.
Luoma (2008) presented an overview of silver and the environment
including the expected quantities to be released, sources of release,
expected pathways, and toxicity. Blaser et al. (2008) modeled
environmentalconcentrations of silver and demonstrated that, by
2010, nanosilver in plastics and textiles are expected to contribute up
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .
2. Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1. Search strategy and inclusion criteria . . . . . . . . . . . .
2.2. Data/information extraction . . . . . . . . . . . . . . . .
2.3. Data collection, management and analysis . . . . . . . . . .
3. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1. Identification of studies . . . . . . . . . . . . . . . . . .
3.2. Description of evidence and critical appraisal . . . . . . . .
3.2.1. Synthesis . . . . . . . . . . . . . . . . . . . . .
3.2.2. Nanocomposites and bimetallic nanoparticles . . . .
3.2.3. Particle morphology . . . . . . . . . . . . . . . .
3.3. Synopsis of silver nanoparticle applications . . . . . . . . .
4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5. Concluding remarks. . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . .
Appendix A. Supplementary data . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction
Historically, silver compounds and ions have been extensively
used for both hygienic and healing purposes (Chen and Schluesener,
2008). However, over time, the use of silver compounds and ions has
faded as an anti-infection agent due to the advent of antibiotics and
other disinfectants and the poorly understood mechanisms of their
toxic effects. Most recently, renewed interest has arisen in manufac-
tured silver nanomaterials because of their unusually enhanced
physicochemical and biological properties activities compared to the
bulk parent materials. A wide range of applications has emerged in
consumer products ranging from disinfecting medical devices and
home appliances to water treatment (Li et al., 2008a,b).
Because the hypothesized mechanisms that govern the fate and
transport of bulk materials may not directly apply to materials at the
nanoscale, there are great concerns in the regulatory and research
communities about the potential environmental impacts associated
with the manufacture and use of silver nanoparticles. This research
provides an evidence-based environmental perspective of the sub-
stances used in synthesizing silver nanoparticles as well as their
properties, thus, leading to focused attention to the most appropriate
steps to be taken in environmental risk assessment and control
throughout the product lifecycle. Until more detailed findings emerge
from toxicological studies, the evidence provided in this research can
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1000
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1001
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1001
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1001
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1001
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1001
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1001
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1001
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1001
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1003
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1003
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1003
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1004
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1005
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1005
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1005
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1005
agents, if warranted, to prevent agglomeration of nanoparticles.
Indeed, the solvents and reducing agents used in these processes
affect the physical and morphological characteristics of manufactured
silver nanoparticles. In turn, these specific characteristics will
influence the fate, transport and toxicity of nanoparticles in the
environment. For example, the use of sodium citrate as a reducing
agent generates a negatively charged silver nanoparticle which may
behave differently than a positively charged silver nanoparticle
generated via branched polyethyleneimine (BPEI) (Tan et al., 2007).
Intermediates or by-products are often generated in these techniques,
thereby playing a critical role in nanoparticle synthesis. Furthermore,
the incorporation of silver nanoparticles in nanocomposites and
bimetallic nanoparticlesmay augment environmental concerns due to
the uncertainty surrounding the inclusion of another metal or poly-
mer, possibly increasing nanoparticle toxicity.
In light of the above, one can realize the unlimited combinations of
properties and substances involved in the syntheses and applications
of silver nanoparticles. Consequently, there is an urgent need to
document the predominant salt precursors, reducing agents and
stabilizing agents utilized in the synthesis processes of silver
nanoparticles to guide the massive efforts required for environmental
risk assessment and management.
The primary objective of this study is to present an evidence-based
environmental perspective of silver nanoparticle properties in
3.2.1. Synthesis
Analysis of the reviewed articles suggests that the bottom-up
technique was predominantly used in the synthesis of silver
nanoparticles relative to the top-down technique (b4% of reviewed
articles). This is mainly attributed to the surface imperfection of
formed particles used in the top-down approach, thereby, impacting
their applicability (Kildeby et al., 2005). Consequently, the description
of evidence presented in Tables S1 and S2 is based on the bottom-up
synthesis techniques. An analysis of the evidence is presented
separately by metal salt precursor, solvents, and reducing and
stabilizing agents.
3.2.1.1. Metal salt precursor. Silver salt precursors are used in bottom-
up techniques to produce ionic silver, which can be reduced and
precipitated to form nanoparticles. Fig. 1 suggests that silver nitrate
(AgNO3) is the most widely used salt precursor accounting for almost
83% of those reported in studies of general and specific synthesis
methods. The dominant use of AgNO3 is attributed to its low cost and
chemical stability when compared to other types of silver salts (Lee
et al., 2007). Thus, although it is likely that nitrate will be the
dominant anion associated with the production of silver nanoparti-
cles, nitrate is a reaction/synthesis byproduct that is classified by the
1001T.M. Tolaymat et al. / Science of the Total Environment 408 (2010) 999–1006
Similarly, Mueller and Nowack (2008) presented the results of
modeling environmental concentrations of nanoparticles including
nanosilver.
2. Methods
2.1. Search strategy and inclusion criteria
The electronic search was initially conducted on the ‘Scopus’
database with the following keywords: silver, nanoparticles, synthe-
sis. To further refine the search, the following inclusion criteria were
adopted: general applications, specific applications, nanocomposites,
and bimetallic. In addition to Scopus, general search engines such as
Google and Yahoo were used. The bibliographies of retrieved articles
were also searched for pertinent references not included in the
electronic search. Only reports and papers published in English were
used, with the electronic search conducted in August of 2008.
2.2. Data/information extraction
Data and information extraction relied on the type of synthesis
methods. The following items were gathered for synthesizedsilver
nanoparticles in general applications: type of silver salt, solvent,
reducing agent, stabilizing agent, and size. Specific applications
featured the following items for synthesized silver nanoparticles:
type of application, silver salt, solvent, size as well as reducing
and stabilizing agents. Similar data and information were assem-
bled for silver nanocomposites and bimetallic with the addi-
tional items of type of nanocomposite or bimetal and template for
nanocomposites.
2.3. Data collection, management and analysis
The description of evidence was presented separately in tabular
format for general synthesis applications, specialized synthesis
methods, nanocomposite and bimetallic forms. In addition, a critical
appraisal of those general classifications was graphically displayed
and presented in terms of the type of salt, solvents, reducing agents,
stabilizing agents and particle size used in the synthesis methods. An
environmental perspective of each of those items was presented.
3. Results
3.1. Identification of studies
The general electronic search yielded 18,793 articles from the
Scopus database. A subsequent focused search was followed with the
previously stated inclusion criteria. About 400 articles were deemed
relevant to the reported research. All abstracts were reviewed for
relevance and only needed articles were obtained. The final electronic
and manual search resulted in nearly 200 research articles for use in
this study.
3.2. Description of evidence and critical appraisal
From an environmental perspective, the description of evidence
was presented separately in tabular format for synthesized silver
nanoparticles for general applications (see Supplementary informa-
tion: Table S1), synthesized silver nanoparticles for specific applica-
tions (Supplementary information: Table S2), silver nanocomposites
(Supplementary information: Table S3), and bimetallic nanoparticles
(Supplementary information: Table S4). Table S5 includes the details
of acronyms listed in Tables S1 to S4, respectively of Supplementary
information. The details of description of evidence and critical
appraisal for the reviewed articles are presented in the following
sections.
United States Environmental Protection Agency (US EPA) as a primary
drinking water contaminant (MCL=10 mg L−1). In light of the above,
it is logical to assume that an environmental release from a manu-
facturing facility will be concerned with not only silver nanoparticles
but also nitrate.
3.2.1.2. Solvents. Solvents are used to solubilize silver salts and other
chemicals included in the synthesis processes. Although organic and
inorganic solvents are used in synthesizing silver nanomaterials,
approximately 80% of synthesis processes usewater as a solvent in the
reported studies (Fig. 2). A good share of organic solvents is
particularly used in the production of relatively high particle
concentrations coupled with predefined shapes and sizes (e.g.:
Dorjnamjin et al., 2008; Yang et al., 2005). Thus, the emergence of
organic solvents in scalable synthesis processes raises concerns for
their environmental impacts in comparison to the low environmental
impact of water. One can deduce from the use of water as a solvent
that most processes yield potentially stable and mobile silver
nanoparticles in an aqueous environment. Although research is
lacking in the area of silver nanoparticle transport, the potential for
transport into and via surface and ground water may be greater than
Fig. 1. Analysis of silver salt precursors reported in studies of silver nanoparticle
synthesis.
with borides (Bönnemann et al., 1994, Sardar et al., 2007). For
example, Bönnemann et al. (1994) reported that the reduction of
nickel choride with sodium borohydride results in a nickel powder
with 5% boron.
3.2.1.4. Stabilizing agents. A stabilizing agent (also known as capping
agent) is used in the synthesis process to prevent nanoparticles from
aggregation and to control the size of final product, with agglomera-
tionmainly caused by excess surface energy and high thermodynamic
instability of the nanoparticle surface (Olenin et al., 2008). A
stabilizing agent relies on electrostatic repulsion force caused by
surface charge, steric stabilization, or both (Guo et al., 2008; Sun and
Luo, 2005; Hasell et al., 2007). Consequently, there is a wide selection
of stabilizing agents to choose from as shown in Fig. 4 including
surfactants such as sodium dodecyl sulfate (SDS) or ligands and poly-
mers that contain functional groups such as thiol (–SH), cyano (–CN),
carboxyl (–COOH) and amino (–NH2) acting as stabilizers (Olenin
et al., 2008; Si and Mandal, 2007).
The selection of stabilizing agent depends on the application. For
example, polynisopropylacrylamide (PNIPAM) is commonly used as a
temperature-sensitive polymer possessing a lower critical solutionFig. 2. Analysis of solvents reported in studies of silver nanoparticle synthesis.
1002 T.M. Tolaymat et al. / Science of the Total Environment 408 (2010) 999–1006
previously thought in the event of these particles forming stable sus-
pensions in water.
3.2.1.3. Reducing agents. In the context of the bottom-upmethodology,
a reducing agent is a chemical agent, plant extract, biological agent, or
irradiation method that provides the free electrons needed to reduce
silver ions and to form silver nanoparticles. Although a strong
reducing agent such as sodium borohydride (NaBH4) tends to produce
a narrow range of small monodispersed particles, a weaker reducing
agent such as ascorbic acid produces larger particle sizes (Chen et al.,
2007a). A critical appraisal of the scientific literature embodied in the
peer-reviewed articles suggests that NaBH4 (∼23%) and sodium
citrate (∼10%) accounted for 33% of all the used reducing agents
(Fig. 3). Smaller percentages of other reducing agents were used in
special synthesis applications. It should be noted that the “other”
category refers to reducing agents mentioned only one time in the
reviewed literature and did not fall under any of the specified
categories. From an environmental perspective, although sodium
citratemay not pose an immediate environmental threat as a reducing
agent, NaBH4 may lead to contamination due to the hydrogen atoms
in the BH4− anion. In essence, the four hydrogen atoms participate in
the reduction process leaving a surface particle that is contaminated
Fig. 3. Analysis of reducing agents reported in studies of silver nanoparticle synthesis.
temperature (LCST). Below the LCST, it is hydrophilic and soluble in
aqueous solution; however, the polymer becomes hydrophobic,
insoluble and aggregates in solution when the temperature is raised
above the LCST. Thus, silver nanoparticles capped by PNIPAM allow
for combined surface plasmon and thermal switching applications
(Guo et al., 2008).
In addition to the above, the stability, reactivity, solubility, particle
shape and size are determined by the concentration of a given type of
a stabilizing agent (Balan et al., 2007). For instance, smaller particles
are obtained by increasing the carboxylate stabilizer concentration
(Wang et al., 1999). The catalytic properties of silver nanomaterials
can also depend on the type of stabilizers used (e.g., cetyltrimethy-
lammonium bromide (CTAB), SDS), hence, decreasing the adsorption
of reactants to the silver surface (Jiang et al., 2005). The commonly
used synthesis methods usually produce negatively charged silver
nanoparticles; however, the use of branched polyethyleneimine
(BPEI) results in positively charged silver nanoparticles that show
greater surface enhanced Raman scattering (SERS) activity over
negatively charged silver nanoparticles (reduced via citrate) for the
detection of thiocyanate and perchlorate ions.
As demonstrated in Fig. 4, an appraisal of the scientific literature
revealed that sodium citrate (27%) was themost commonly used
stabilizing agent, followed by polyvinyl pyrrolidone (PVP) (18%), then
Fig. 4. Analysis of stabilizing agents reported in studies of silver nanoparticle synthesis.
amines (8%). Cetyltrimethylammonium bromide (CTAB), polyvinyl
alcohol (PVA), sugars and amides accounted each for 5% of the
stabilizing agents used in the reviewed literature. The “other”
category in Fig. 4 represents 23% of all stabilizing agents with
individual chemicals being used in only one article. It should be noted
that sodium citrate was used in 50% of the stabilizing agents used in
specific applications, followed by PVP (18%), then CTAB/amines/
amides/fatty acids with each representing less than 10% of used
agents. Indeed, one cannot overlook the crucial role of surface
charging properties of silver nanomaterials in affecting their potential
mobility and behavior in the environment. Therefore, an assessment
of the chemical properties of commonly used stabilizing agents is
warranted.
3.2.2. Nanocomposites and bimetallic nanoparticles
A combination of useful properties such as optical/catalytic/
conductive is achieved when silver nanoparticles are integrated into
polymer matrices to form nanocomposites or when they are
combined with other metal nanoparticles acting as a shell or a core
1003T.M. Tolaymat et al. / Science of the Total Environment 408 (2010) 999–1006
to form bimetallic nanoparticles (Koh et al., 2008; Dong et al., 2008).
For example, the antibacterial activity of silver nanoparticles was
enhanced when it was incorporated into an Ag/TiO2 nanocomposite
(Nino-Martınez et al., 2008). The successful catalytic reduction of
specific dyes may be achieved via use of silver nanoparticles
supported on silica spheres (Jiang et al., 2005). Furthermore, silver
is often used as a substrate formagnifying the SERS providing stronger
spectrum than data obtained from using each individual nanoparticle
(Pande et al., 2007).
An account of the substances used in synthesizing silver
nanocomposites and bimetallic nanoparticles as well as particle
sizes is provided in Tables S3 and S4, respectively. Other materials
incorporated with silver include multi-walled carbon nanotubes and
metallic nanoparticles (Sn, Ru and Pd). As can be observed, the
integration of silver into nanocomposites and bimetallic nanoparticles
is limitless. With each combination, silver is being used to generate
nanoparticles with new characteristics. Consequently, the environ-
mental impact of nanocomposites and bimetallic nanoparticles has to
be examined on a case-by-case basis. For example, the enhanced
antibacterial effect of Ag/TiO2 may be evidence of higher toxicity
levels in the ecosystem in the event of release to the environment,
thus requiring further investigation.
3.2.3. Particle morphology
As shown in Fig. 5, the dominant size of silver nanoparticles ranges
from 1 to 10 nm in general applications (approximately 45.5%), with
the same trend demonstrated for specific applications (51% of the
Fig. 5. Particle size (nm) reported in studies of synthesized silver nanoparticles.
particles). In particular, the optimum combination of these properties
is obtained for particles in the range of 3 to 10 nm (Olenin et al., 2008).
This is attributed to the high ‘surface area to volume’ ratio and the
quantum confinement effect caused by extremely reduced size (i.e.,
electron confinement in a small area) (Morones and Frey, 2007).
It has been shown that silver nanoparticles with 15 nm diameters
had the highest toxic effect on rats' alveolar macrophages in
comparison to larger nanoparticles (i.e., 30 to 55 nm) (Cataleya,
2006). This trend is most likely the result of increased surface reactive
area of silver nanoparticles and the comparable size of particle to
protein in biological cells. Silver nanoparticles with 1–10 nm size
demonstrated interaction with HIV by inhibiting the virus from
binding to the host cells (Navaladian et al., 2008). Smaller particle size
does not always translate to an enhancement in the particle
properties. For example, an SERS application shows enhanced signals
with larger particle sizes (Dong et al., 2007).
The size can have an effect on the thermodynamic properties of
silver nanoparticles, most notably due to the low ‘surface area to
volume’ ratio which seems to control these properties (Luo et al.,
2008). In general, the thermodynamic properties of silver nanopar-
ticles (e.g., melting point and molar heat of fusion) are directly
proportional to particle diameter (Luo et al., 2008). For example, silver
nanoparticles of less than 10 nm in diameter are utilized in the
semiconductor industry and printed electronic products due to their
lower melting point (Kashiwagi et al., 2006).
Silver nanoparticles can be synthesized as 1-D objects (e.g., thin
films), 2-D objects (e.g., nanowires, nanorods) and 3-D objects (e.g.,
spheres) (Muraviev, 2005). Although nanosilver is synthesized in
various shapes, silver spheres are the dominant shape accounting for
approximately 90% of those used in the bottom-upmethods. With this
in mind, one cannot overlook other shapes. Triangular silver
nanoparticles, for example, had stronger biocidal action against
gram-negative bacterium Escherichia coli than spherical and rod-
shaped nanoparticles. This was mainly attributed to the arrangement
of atoms in the crystal structure (Pal et al., 2007).
3.3. Synopsis of silver nanoparticle applications
The scientific literature points to the wide use of silver in nume-
rous applications. It is well established that silver nanoparticles are
known for their strong antibacterial effects for a wide array of
organisms (e.g., viruses, bacteria, fungi) (Wen et al., 2007). Therefore,
silver nanoparticles are widely used in medical devices and supplies
such as wound dressings, scaffold, skin donation, recipient sites,
sterilized materials in hospitals, medical catheters, contraceptive
devices, surgical instruments, bone prostheses, artificial teeth, and
bone coating. One can also observe their wide use in consumer
products such as cosmetics, lotions, creams, toothpastes, laundry
detergents, soaps, surface cleaners, room sprays, toys, antimicrobial
paints, home appliances (e.g., washing machines, air and water
filters), automotive upholstery, shoe insoles, brooms, food storage
containers, and textiles (Amendola et al., 2007; Navaladian et al.,
2008; Fernandez et al., 2008; Thomas et al., 2007).
Other unique properties of silver nanoparticles include their
utilization in fluorescence and surface plasmon resonance (SPR). For
example, SPR occurs upon irradiating silver nanoparticles with visible
light, which then causes the oscillation of free electrons in the
conduction band of nanoparticles. The width and position of SPR peak
depend mainly on the particle size and shape (Basavaraja et al., 2008;
Jiang et al., 2007). These properties allow silver nanoparticles to be
used in sensing applications such as detection of DNA sequences
(Jacob et al., 2008), laser desorption/ionization mass spectrometry of
peptides (Hua et al., 2007), colorimetric sensor for histidine (Xiong
et al., 2008), determination of fibrinogens in human plasma (ZhiLiang
et al., 2007), real-time probing of membrane transport in living
microbial cells (Xu et al., 2004), enhanced IR absorption spectroscopy
(Huo et al., 2006), colorimetric sensors for measuring ammonia
concentration (Dubas and Pimpan, 2008a), biolabeling, optical
imaging of cancer (Wiley et al., 2007), biosensors for detection of
herbicides (Dubas and Pimpan, 2008b), and glucose sensor for
medical diagnostics (Mishra et al., 2007). Furthermore, SERS analysis
techniques can be used to examine trace analysis of pesticides,
anthrax, prostate-specific antigen glucose, nuclear waste, identifica-
tion of bacteria, genetic diagnostics, detection of nitro-explosives
(Marcia, 2006), and immunoassay labeling (Moronesand Frey, 2007).
Electronic applications span the preparation of active waveguides
(provided by NanoSys GmbH, Switzerland) to assess the toxicity of
silver nanoparticles to chlamydomonas reinhardtii. The information
was not presented in this systematic review because the synthesis
reactants and methods were not disclosed in the Navarro paper.
A possible explanation for the lack of such information can be that
the innovative methods developed by the nano-manufacturing
enterprises are considered as proprietary information. Based on our
own experience, we asked a supplier of silver nanomaterial utilized in
various experiments in our laboratory about certain details of the
synthesis methods, however, the manufacturer was not willing to
tab
r
r
r
r
odi
r
1004 T.M. Tolaymat et al. / Science of the Total Environment 408 (2010) 999–1006
in optical devices, inks for printed circuit boards, optoelectronics,
nanoelectronics (e.g., single-electron transistors, electrical connects),
sub-wavelength optics, data storage devices, nonlinear optics, high
density recording devices, battery-based intercalation materials,
provision of micro-interconnects in integrated circuits (IC) and
integral capacitors (Navaladian et al., 2008; Kim et al., 2007;
Deshmukh and Composto, 2007). In addition, the large surface area
of silver nanoparticles provides high surface energy, which promotes
surface reactivity (e.g., adsorption, catalysis). For example, silver
nanoparticles and silver nanocomposites have been used to catalyze
many reactions such as CO oxidation (Liu et al., 2005), benzene
oxidation for phenol (Ameen et al., 2007), photodegradation of
gaseous acetaldehyde (Hamal and Klabunde, 2007), and reduction of
p-nitrophenol to p-aminophenol (Fernandez et al., 2008).
4. Discussion
An examination of silver nanomaterial synthesis is warranted for
two primary reasons. First, it allows the identification of dominant
silver nanomaterials currently in use to allow a focused approach in
evaluating their environmental impacts as well as the substances used
in their synthesis. Second, the great majority of synthesis processes
are not environmentally friendly, with only 24% of the reported
methods relying on green or environmentally-friendly techniques.
This is expected since non-green methods provide more control over
the reaction process and produce monodisperse silver nanoparticles
with peculiar characteristics. According to Sharma et al. (2009), green
synthetic methods include mixed-valence polyoxometallates, poly-
saccharides, tollens, irradiation, and biological; on the other hand,
conventional or non-green methods involve chemical agents associa-
ted with environmental toxicity. As can be seen from Tables S1 and S2,
the green approaches generate large particle sizes. It can also be
inferred that particle sizes less than 20 nm are considered the most
reactive and are seldom generated using green synthesis methods.
A specific aim of the evidence-based methodology employed in
this research was to extract the information from the scientific
literature embodied in peer-reviewed papers (as obtained from
electronic scientific databases). We did not take into account other
evidence which may be present in industrial enterprises. Although
this issue can be regarded as a serious limitation in the present study,
we have attempted to consult general purpose databases such as
Google to gather additional evidence for potential inclusion in the
supplementary information. We found a lack of information which
cannot be presented in the reported supplementary information. For
example, Navarro et al. (2008) used carbonate-coated nanoparticles
Table 1
Samples of work reported in patent electronic database.
Ag salt Reducing agent Solvent S
nr Hydrazine Organic solvent n
nr Organic solvent Water n
nr Polycyclic hydrocarbon/carboxyl acids nr n
nr Probiotic bacteria nr n
AgNO3 Ferrous sulfate Water S
AgNO3 Formalin Water n
Note: ‘nr’ — not reported.
share the information with us. We made another attempt to search
electronic databases documenting patents. Our earlier observation
about the lack of some details in the reporting of synthesis methods
can be demonstrated from examination of Table 1. It should be,
therefore, be kept in mind that the systematic review reported herein
only includes synthesis methods that are available to the general
public for review and may not reflect the universe of all synthesis
processes currently in use.
On the basis of evidence extracted from the scientific literature, it
is clear that silver nanoparticles have unique properties allowing
them to be one of themost commonly used nanoparticles in consumer
products. Thus, they may eventually reach the environment at
different stages of the product lifecycle, that is, manufacturing, con-
sumer use, and/or end-of-life management. The body of knowledge
suggests that most synthesis processes produce silver nanoparticles
that are spherical with a diameter of less than 20 nm. Silver
nanoparticles are often synthesized using AgNO3, dissolved in water
with NaBH4 or citrate as a capping agent, which gives these particles a
negative surface charge. Thus, there is a need to evaluate the effect of
particle-specific factors (e.g., shape and size) and environmental
factors (e.g., pH, ORP) on the fate, mobility and toxicity of silver
nanoparticles, fueling the need for new methodologies as silver
nanoparticles may behave differently from bulk silver. One point to
keep in mind is that the conversion of silver ions into silver
nanoparticles is not always 100%. Therefore, one should be cautioned
about the environmental release of those ions that do not make it into
the final silver nanoparticle-based product. According to synthesis
methods in our lab, the point of release may reach up to 30%.
In general, the fate and transport of silver nanoparticles are
governed by many variables such as capping agent, particle size, and
surrounding environmental conditions (Wiesner et al., 2006).
Furthermore, silver nanoparticles have a surface charge. Since citrate
and PVP are the dominant capping agents, the surface charge of
nanoparticles will bemainly negative under typical environmental pH
range (4–9). Particle charge will have a profound impact on its
mobility in the environment. For example, transport through a
negatively charged sandy soil will be dominated by repulsion forces,
thus, causing silver nanoparticles to be more mobile. Conversely,
transport through positively charged soils will be dominated by
attraction forces, thus, hindering silver nanoparticles from reaching
groundwater. In this latter case, silver nanomaterial may pose a
hazard to soil organisms that play a vital role in maintaining natural
processes.
Silver nanoparticles have a reactive surface that is evident by their
usage as substrates to adsorb a variety of analytes for study by SERS. In
ilizing agent Size (nm) Source Notes
nr Li et al. (2008a,b) –
nr Liu (2007) –
nr Nakamoto et al. (2007) –
nr De et al. (2008) –
um citrate 20 Uno and Sato (2006) Spherical
1–20 Fujieda and Nishihara (2006) Spherical
1005T.M. Tolaymat et al. / Science of the Total Environment 408 (2010) 999–1006
fact, a number of SERS studies reported the adsorption of 1,2,3-triazole
(Pergolese et al., 2004), organo-sulfur compounds and benzyl phenyl
sulfide (BPS) (Kim et al., 2004), organo-sulfur compounds and benzyl
phenyl sulfide (BPS) (Kim et al., 2004), carbonyl species Cr(CO)4dpp
(Tan et al., 2005,), and cationic dyes (e.g., acridine, 9-aminoacridine and
Nile Blue) (Xingcan et al., 2003) in silver nanoparticles. Additionally,
SO2 (i.e., a pollutant gas) was adsorbed in silver nanoparticles sur-
face causing their agglomeration and settlement from the solution
(Patakfalvi et al., 2008). It should be noted that silver nanoparticles can
potentially absorb some contaminants from wastewater andconvey
them to surface water via the final treated effluent. One can be equally
concerned about thepresence of silver inwastewater streamsbecause it
can impact the microorganisms treating wastewaters.
It is expected that the antibacterial property of bulk silver is carried
over to and perhaps enhanced in silver nanoparticles. Therefore, when
one examines the environmental issues associated with the manu-
facture and use of product-based silver nanoparticles, the antibac-
terial effects should always be taken into account, particularly at the
different stages of product lifecycle depending on the environmental
release in the form of colloidal silver particles or silver ions. Different
mechanisms have been proposed for the antimicrobial properties of
silver nanoparticles: (a) adhesion of nanoparticles to the surface
altering the membrane properties, with possible degradation of
lipopolysaccharide molecules, accumulation inside the membrane
via forming pits and causing large increases in membrane permeabi-
lity; (b) penetration of silver nanoparticles inside the bacterial cell
resulting in DNA damage; and (c) dissolution of silver nanoparticles
with releases of antimicrobial ionic silver (Li et al., 2008a,b).
The above mechanisms suggest that the impact of environmental
releases throughout the product lifecycle seems to take place in the
form of colloidal silver particles or ionic silver. Contrary to the
bactericideal effects of ionic silver, the antimicrobial activity of
colloidal silver particles is influenced by the dimensions of the
particles; the smaller the particles, the greater the antimicrobial effect
(Guzman et al., 2008). Silver nanoparticles may attach to the surface
of the cell membrane and disturb its permeability properties, possibly
affecting respiration in aerobes. The binding of the particles to
bacteria may depend on the surface area available for interaction.
Smaller particles, possessing larger surface area to volume ratio, will
be more bactericidal than larger particles. Ionic silver, however, as
pointed out by Damm et al. (2007) forms complexes with the sulfur-,
nitrogen- or oxygen-containing functional groups of bacterial
enzymes. This leads to a destabilization of the bacterial cell wall and
disturbs metabolic function. Moreover, silver ions interact with the
bacterial DNA preventing the reproduction of the cells (bacteriostatic
action). In this respect, one may argue that silver nanoparticles are a
delivery system that may degrade over time resulting in the release of
ionic silver. Elemental silver nanoparticles can provide a large
reservoir of silver ions. When silver nanoparticles come in contact
with the dissolved oxygen (O2(aq.)) in water, they release silver ions
according to the following equation:
O2ðaq:Þ þ 4H3Oþ þ 4AgðsÞ→ 4Agþðaq:Þ þ 6H2O
A study by Benn and Westerhoff (2008) suggests that both
colloidal and ionic silver leach from commercially available sock
fabrics incorporating nanosilver as an antimicrobial agent. Variable
leaching rates indicate that the sock manufacturing process may
control the release of silver. Thus, it appears that one form of silver or
both may be the outcome. In this respect, the impact of colloidal silver
can be short- or long-term, while the environmental impact of ionic
silver is long-term. This issue requires further investigation to better
understand the fate and transport of colloidal and ionic silver. In
particular, because of their long-term effects, if the same trend
continues in the use of silver nanoparticles, there will be a need to
evaluate their effects on the liquid and solid waste management
systems that rely heavily on microbial decomposition for treatment
(landfills and wastewater treatment plants). Indeed, nano-silver
toxicity to aerobic and anaerobic microorganisms should be an
integral part of this evaluation. It is equally warranted that the
accumulation of silver nanoparticle in biosolids and their effects on
management practices of these materials should be investigated.
5. Concluding remarks
It is expected that the antibacterial property of bulk silver is carried
over to and perhaps enhanced in silver nanoparticles. Therefore, when
one examines the environmental issues associated with the manu-
facture and use of silver nanoparticle-based products, the antibac-
terial effects should always be taken into account particularly at the
different stages of the product lifecycle. Currently, there are two
arguments in the scientific literature about the mechanisms of
antimicrobial properties of silver nanoparticles as they relate to
colloidal silver particles and inonic silver. Methodologies of risk
assessment and control have to account for both arguments.
Acknowledgments
This research was funded by the National Risk Management
Research Laboratory of the U.S. Environmental Protection Agency
Office of Research and Development. The paper has not been
subjected to the Agency's internal review; therefore, the opinions
expressed are those of the authors and do not necessarily reflect the
official positions and policies of the USEPA.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.scitotenv.2009.11.003.
References
Ameen KB, Rajasekar K, Rajasekharan T. Silver nanoparticles in mesoporous aerogel
exhibiting selective catalytic oxidation of benzene in CO2 free air. Catal Lett 2007;119:
289–95.
AmendolaV, Polizzi S,MeneghettiM. Free silver nanoparticles synthesizedby laser ablation
in organic solvents and their easy functionalization. Langmuir 2007;23:6766–70.
Balan L, Malval J, Schneider R, Burget D. Silver nanoparticles: new synthesis, charac-
terization and photophysical properties. Mater Chem Phys 2007;104:417–21.
Basavaraja S, Balaji SD, Lagashetty A, Rajasab AH, Venkataraman A. Extracellular
biosynthesis of silver nanoparticles using the fungus fusarium semitectum. Mater
Res Bull 2008;43:1164–70.
Benn TM, Westerhoff P. Nanoparticle silver released into water from commercially
available cock fabrics. Environ Sci Technol 2008;42:4133–9.
Blaser SA, Scheringer M, MacLeod M, Hungerbuhler K. Estimation of cumulative aquatic
exposure and risk due to silver: contribution of nano-functionalized plastics and
textiles. Sci. Total Environ. 2008;390:396–409.
Bönnemann H, Brijoux W, Brinkmann R, Fretzen R, Joussen T, Köppler R, et al.
Preparation, characterization, and application of fine metal particles and metal
colloids using hydrotriorganoborates. J Mol Catal 1994;86:129–77.
Boxall ABA, Chaudhry Q, Sinclair C, Jones A, Aitken R, Jefferson B, et al. Current and
future predicted environmental exposure to engineered nanoparticles. Sand
Hutton, UK: Central Science Laboratory; 2007.
Cataleya Carlson. In vitro toxicity assessment of silver nanoparticles in rat alveolar
macrophages. 2006, Master thesis, Wright State University.
Chen X, Schluesener HJ. Nanosilver: a nanoproduct in medical applications. Toxicol Lett
2008a;176:1-12.
ChenM, Feng Y,Wang X, Li T, Zhang J, Qian D. Silver nanoparticles capped by oleylamine:
formation, growth, and self-organization. Langmuir 2007;23:5296–304.
Damm C, Munstedt H, Rosch A. Long-term antimicrobial polyamide 6/silver-
nanocomposites. J Mater Sci 2007;42:6067–73.
De WW, Vrecauteren T, Verstraete W. Method for production metal nanoparticle.
http://www.rexresearch.com/alchemy5/nanogold.htm. 2008 WO2008003522.
del Rocío Balaguera-Gelves, M. Detection of nitroexplosives by surface enhanced Raman
spectroscopy on colloidal metal nanoparticles. 2006, Master thesis, University of
Puerto Rico, Mayaguez Campus.
Deshmukh RD, Composto RJ. Surface segregation and formation of silver nanoparticles
created in situ in poly(methyl methacrylate) films. Chem Mater 2007;19:745–54.
Dong Y, Ma Y, Zhai T, Shen F, Zeng Y, Fu H, et al. Silver nanoparticles stabilized by
thermoresponsivemicrogel particles: synthesis and evidence of an electron donor-
acceptor effect. Macromol Rapid Commun 2007;28:2339–45.
Dong Q, Su H, Cao W, Han J, Zhang D, Guo Q. Biogenic synthesis of hierarchical hybrid
nanocomposites and patterning of silver nanoparticles. Mater Chem Phys 2008;110:
160–5.
Dorjnamjin D, Ariunaa M, Shim YK. Synthesis of silver nanoparticles using hydroxyl
functionalized ionic liquids and their antimicrobial activity. Int J Mol Sci 2008;9:
807–20.
Dubas ST, Pimpan V. Green synthesis of silver nanoparticles for ammonia sensing. Talanta
2008a;76:29–33.
Dubas ST, Pimpan V. Humic acid assisted synthesis of silver nanoparticles and its
application to herbicide detection. Mater Lett 2008b;62:2661–3.
Fernandez EJ, Garcıa-Barrasa J, Laguna A, Lopez-de-Luzuriaga J, Monge M, Torres C. The
Mueller NC, Nowack B. Exposure modeling of engineered nanoparticles in the environ-
ment. Environ Sci Technol 2008;42:4447–53.
Muraviev DN. Inter-matrix synthesis of polymer stabilised metal nanoparticles for
sensor applications. Contrib Sci 2005;3:19–32.
Nakamoto M, Yamamoto M, Kashiwagi Y, Yoshida Y, Kakiuchi H, HaradaM. Silver nano-
particle and production method thereof. http://www.rexresearch.com/alchemy5/
nanogold.htm. 2007 JP2007063580.
Navaladian S, Viswanathan B, Varadarajan TK, Viswanath RP. Microwave-assisted rapid
synthesis of anisotropic Ag nanoparticles by solid state transformation. Nanotech-
nology 2008;19:1–7.
Navarro E, Piccapietra F, Wagner B, Marconi F, Kaegi R, Odzak N, et al. Toxicity of silver
nanoparticles to chlamydomonas reinhardtii. Environ Sci Technol 2008;42:8959–64.
Nino-Martınez N, Martınez-Castanon GA, Aragon-Pina A, Martınez-Gutierrez F,
Martınez-Mendoza JR, Ruiz F. Characterization of silver nanoparticles synthesized
1006 T.M. Tolaymat et al. / Science of the Total Environment 408 (2010) 999–1006
approach. Nanotechnology 2008;19:1–6.
Fujieda N, Nishihara K. Production method of silver nanoparticle, silver nanoparticle
and application thereof. http://www.rexresearch.com/alchemy5/nanogold.htm.
2006 JP2006328472.
Guo L, Nie J, Du B, Peng Z, Tesche B, Kleinermanns K. Thermoresponsive polymer-
stabilized silver nanoparticles. J Colloid Interface Sci 2008;319:175–81.
Guzman MG, Dille J, Godet S. Synthesis of silver nanoparticles by chemical reduction
method and their antimicrobrial activity. Proceedings of World Academy of
Science, Engineering and Technology 2008;33:367–74.
Hamal DB, Klabunde KJ. Synthesis, characterization, and visible light activity of new
nanoparticle photocatalysts based on silver, carbon, and sulfur-doped TiO2.
J Colloid Interface Sci 2007;311:514–22.
Hasell T, Yang J, Wang W, Brown PD, Howdle SM. A facile synthetic route to aqueous
dispersions of silver nanoparticles. Mater Lett 2007;61:4906–10.
Hua L, Chen J, Ge L, Tan SN. Silver nanoparticles as matrix for laser desorption/
ionization mass spectrometry of peptides. J Nanopart Res 2007;9:1133–8.
Huo S, XueX, Li Q, XuS, CaiW. Seeded-growth approach to fabrication of silver nanoparticle
films on silicon for electrochemical atr surface-enhanced ir absorption spectroscopy.
J Phys Chem B 2006;110:25721–8.
Jacob JA, Mahal HS, Biswas N, Mukherjee T, Kapoor S. Role of phenol derivatives in the
formation of silver nanoparticles. Langmuir 2008;24:528–33.
Jiang Z, Liu C, Sun L. Catalytic properties of silver nanoparticles supported on silica
spheres. J Phys Chem B 2005;109:1730–5.
JiangX, ZengQ, YuA. Thiol-frozen shape evolution of triangular silver nanoplates. Langmuir
2007;23:2218–23.
Kashiwagi Y, Yamamoto M, Nakamoto M. Facile size-regulated synthesis of silver nano-
particles by controlled thermolysis of silver alkylcarboxylates in the presence of
alkylamines with different chain lengths. J Colloid Interface Sci 2006;300:169–75.
Kildeby NL, Roge RE, Larsen T, Petersen R, Riis JF, Bozhevolnyi SI. Silver nanoparticles.
ProjectGroupN344. Faculty of Physics andNanotechnology,AalborgUniversity; 2005.
Kim SJ, Kim TG, Ah CS, Kim K, Jang D. Photolysis dynamics of benzyl phenyl sulfide
adsorbed on silver nanoparticles. J Phys Chem B 2004;108:880–2.
KimSH,Choi BS, KangK, ChoiY, Yang SI. Low temperature synthesis andgrowthmechanism
of Ag nanowires. J Alloys Compd 2007;433:261–4.
Koh JH, Kim YW, Park JT, Kim JH. Templated synthesis of silver nanoparticles in
amphiphilic poly(vinylidene fluoride-cochlorotrifluoroethylene) comb copolymer.
J Polym Sci B 2008;46:702–9.
Lee KJ, Lee Y, Shim I, Jun BH, Cho HJ, Joung J. Large-scale synthesis of polymer-stabilized
silver nanoparticles. Sol St Phen 2007;124–126:1189–92.
Li Q, Mahendra S, Lyon DY, Brunet L, LigaMV, Li D, et al. Antimicrobial nanomaterials for
water disinfection and microbial control: potential applications and implications.
Water Res 2008a;42:4591–602.
Li Y, Wu Y, Ong B. Stabilized silver nanoparticle composition. http://www.rexresearch.
com/alchemy5/nanogold.htm. 2008 US2008000382.
Liu CZ. Method for preparing redispersible Ag nanoparticle. http://www.rexresearch.
com/alchemy5/nanogold.htm. 2007 CN1994631.
Liu J, Wang A, Chi Y, Lin H, Mou C. Synergistic effect in an au-ag alloy nanocatalyst: CO
oxidation. J Phys Chem B 2005;109:40–3.
LuoW, HuW, Xiao S. Size effect on the thermodynamic properties of silver nanoparticles.
J Phys Chem C 2008;112:2359–69.
Luoma SN. Silver nanotechnologies and the environment: old problems or new challenges?
Project on Emerging Nanotechnologies, The Pew Charitable Trusts; 2008.
Marcia del Rocío B. Detection of nitroexplosives by surface enhanced Raman
spectroscopy on colloidal metal nanoparticles. 2006, Master thesis, University of
Puerto Rico, Mayaguez Campus.
Mishra YK, Mohapatra S, Kabiraj D, Mohanta B, Lalla NP, Pivin JC, et al. Synthesis and
characterization of Ag nanoparticles in silica matrix by atom beam sputtering. Scr
Mater 2007;56:629–32.
Morones JR, Frey W. Environmentally sensitive silver nanoparticles of controlled size
synthesized with PNIPAM as a nucleating and capping agent. Langmuir 2007;23:
8180–6.
on titanium dioxide fine particles. Nanotechnology 2008;19:1–8.
Olenin AY, Krutyakov YA, Kudrinskii AA, Lisichkin GV. Formation of surface layers on
silver nanoparticles in aqueous and water-organic media. Colloid J 2008;70:71–6.
Pal S, Tak YK, Song JM. Does the antibacterial activity of silver nanoparticles depend on
the shape of the nanoparticle? A study of the gram-negative bacterium Escherichia
coli. Appl Environ Microbiol 2007;73:1712–20.
Pande S, Ghosh SK, Praharaj S, Panigrahi S, Basu S, Jana S, et al. Synthesis of normal and
inverted gold-silver core-shell architectures in β-Cyclodextrin and their applications
in SERS. J Phys Chem C 2007;111:10806–13.
Patakfalvi R, Diaz D, Velasco-Arias D, Rodriguez-Gattorno G, Santiago-Jacinto P. Synthesis
and direct interactions of silver colloidal nanoparticles with pollutant gases. Colloid
Polym Sci 2008;286:67–77.
PergoleseB,Muniz-MirandaM, BigottoA.Studyof theadsorptionof 1, 2, 3-Triazoleon silver
and gold colloidal nanoparticles bymeans of surface enhancedRaman scattering. J Phys
Chem B 2004;108:5698–702.
Robichaud CO, Tanzil D, Weilenmann U, Weisner M. Relative risk analysis of several manu-
factured nanomaterials: an insurance industry context. Environ Sci Technol 2005;39:
8985–94.
SardarR, Park J-W, Shumaker-Parry JS. Polymer-induced synthesis of stable gold and silver
nanoparticles and subsequent ligand exchange inwater. Langmuir 2007;23:11883–9.
Sharma VK, Yngard RA, Lin Y. Silver nanoparticles: green synthesis and antimicrobial
activities. Adv Colloid Interface Sci 2009;145:83–96.
Si S, Mandal TK. Tryptophan-based peptides to synthesize gold and silver nanoparticles: a
mechanistic and kinetic study. Chem Eur J 2007;13:3160–8.
Sun X, Luo Y. Preparation and size control of silver nanoparticles by a thermal method.
Mater Lett 2005;59:3847–50.
Tan H, Wong L, LaiMY, Kiruba GSM, Leong WK, Wong MW, et al. Preparation and
characterization of Cr(CO)4d pp (Chromium Tetracarbonyl 2, 3-Bis(2′-pyridyl)
pyrazine) adsorbed on silver nanoparticles. J Phys Chem B 2005;109:19657–63.
Tan S, Erol M, Attygalle A, Du H, Sukhishvili S. Synthesis of positively charged silver
nanoparticles via photoreduction of agno3 in branched polyethyleneimine/HEPES
solutions. Langmuir 2007;23:9836–43.
Thomas V, Yallabu MM, Sreedhar B, Bajpai SK. A versatile strategy to fabricate hydrogel–
silvernanocomposites and investigationof their antimicrobial activity. J Colloid Interface
Sci 2007;315:389–95.
UnoT, SatoK. Silver nanoparticle andproductionmethod thereof. http://www.rexresearch.
com/alchemy5/nanogold.htm. 2006 JP2006045655.
Wang W, Chen X, Efrima S. Silver nanoparticles capped by long-chain unsaturated
carboxylates. J Phys Chem B 1999;103:7238–46.
Wen H, Lin Y, Jian S, Tseng S, Weng M, Liu Y, et al. Observation of growth of human
fibroblasts on silver nanoparticles. J Phys 2007;61:445–9.
Wiesner MR, Lowry GV, Alvarez P, Dionysiou D, Biswas P. Assessing the risks of manu-
factured nanomaterials. Environ Sci Technol 2006;40:4336–45.
WileyBJ, ChenY,McLellan JM, XiongY, Li Z, GingerD, et al. Synthesis andoptical properties
of silver nanobars and nanorice. Nano Lett 2007;7:1032–6.
Xingcan S, Qi Y, Hong L, Haigang Y, Xiwen H. Hysteresis effects of the interaction
between serum albumins and silver nanoparticles. Sci China Ser B 2003;46:387–98.
Xiong D, Chen M, Li H. Synthesis of para-sulfonatocalix[4]arene-modified silver
nanoparticles as colorimetric histidine probes. Chem Commun 2008:880–2.
Xu X, Brownlow WJ, Kyriacou SV, Wan Q, Viola JJ. Real-time probing of membrane
transport in living microbial cells using single nanoparticle optics and living cell
imaging. Biochemistry 2004;43:10400–13.
Yang J, Lee JY, Too H. Core-shell Ag–Au nanoparticles from replacement reaction in
organic medium. J Phys Chem B 2005;109:19208–12.
ZhiLiang J, Yuan CY, AiHui L, HuiLin T, NingLi T, FuXin Z. Silver nanoparticle labeled
immunoresonance scatter-ing spectral assay for trace fibrinogen. Sci China Ser B
2007;50:345–50.
preparation of highly active antimicrobial silver nanoparticles by an organometallic
	An evidence-based environmental perspective of manufactured silver nanoparticle in syntheses an.....
	Introduction
	Methods
	Search strategy and inclusion criteria
	Data/information extraction
	Data collection, management and analysis
	Results
	Identification of studies
	Description of evidence and critical appraisal
	Synthesis
	Metal salt precursor
	Solvents
	Reducing agents
	Stabilizing agents
	Nanocomposites and bimetallic nanoparticles
	Particle morphology
	Synopsis of silver nanoparticle applications
	Discussion
	Concluding remarks
	Acknowledgments
	Supplementary data
	References