CSIRO PUBLISHING
Environ. Chem. 2014, 11, 609–623
http://dx.doi.org/10.1071/EN14127
Review
A critical review of nanohybrids: synthesis, applications
and environmental implications
Nirupam Aich,A Jaime Plazas-Tuttle,A Jamie R. Lead B and Navid B. SalehA,C
A
Department of Civil, Architectural and Environmental Engineering, University of Texas,
Austin, TX 78712, USA.
B
Center for Environmental Nanoscience and Risk, Department of Environmental Health Sciences,
Arnold School of Public Health, University of South Carolina, Columbia, SC 29208, USA.
C
Corresponding author. Email: navid.saleh@utexas.edu
Environmental context. Recent developments in nanotechnology have focussed towards innovation and
usage of multifunctional and superior hybrid nanomaterials. Possible exposure of these novel nanohybrids can
lead to unpredicted environmental fate, transport, transformation and toxicity scenarios. Environmentally
relevant emerging properties and potential environmental implications of these newer materials need to be
systematically studied to prevent harmful effects towards the aquatic environment and ecology.
Abstract. Nanomaterial synthesis and modification for applications have progressed to a great extent in the last decades.
Manipulation of the physicochemical properties of a material at the nanoscale has been extensively performed to produce
materials for novel applications. Controlling the size, shape, surface functionality, etc. has been key to successful
implementation of nanomaterials in multidimensional usage for electronics, optics, biomedicine, drug delivery and green
fuel technology. Recently, a focus has been on the conjugation of two or more nanomaterials to achieve increased
multifunctionality as well as creating opportunities for next generation materials with enhanced performance. With
incremental production and potential usage of such nanohybrids come the concerns about their ecological and
environmental effects, which will be dictated by their not-yet-understood physicochemical properties. While environmental implication studies concerning the single materials are yet to give an integrated mechanistic understanding and
predictability of their environmental fate and transport, the importance of studying the novel nanohybrids with their multidimensional and complex behaviour in environmental and biological exposure systems are immense. This article critically
reviews the literature of nanohybrids and identifies potential environmental uncertainties of these emerging ‘horizon
materials’.
Received 6 July 2014, accepted 22 August 2014, published online 16 December 2014
Introduction
achieving a higher degree of functionality by combining multiple NMs, each possessing unique and novel advantages. For
example, nanoscale iron oxide, nanogold and graphene
nanosheets individually possess paramagnetism, plasmon resonance and superior charge carrying capability respectively.
However, careful combination of two or more of these materials
enhanced their functional performance as observed in the
development of the first sets of bimetallic NHs. Iron oxide when
conjugated with gold to form core–shell particles, provided
inherent magnetism of the iron oxide shell, while preserving
the surface plasmon resonance of the gold core.[7] Such multifunctional bimetallics were used as magnetic resonance imaging
(MRI) agents with added nanoheating capabilities, useful for
laser irradiated drug delivery systems.[8] Similarly, gold, when
intercalated within layered clay, was used for protein or organic
molecule immobilisation, applicable for biocatalysis and
sensors.[9,10] Paramagnetic iron oxides, in contrast, when combined with novel graphene oxides, resulted in unique drug
delivery systems with superior drug release and targetability.[11]
Again, graphene nanosheets have also been combined with
porphyrins, titanium dioxide (TiO2), carbon nanotubes, quantum dots, etc., and have generated NHs for enhanced optical
Materials development at the nanoscale has progressed from
single particle synthesis to multi-component assemblies or
hierarchical structures, where two or more pre-synthesised
nanomaterials (NMs) are conjugated to extract multifunctionality.[1] These ensembles are termed as nanohybrids
(NHs).[2,3] The underlying focus of NH synthesis is property
modulation, which results in alteration to inherent physicochemical properties, i.e. size, shape, composition and surface
chemistry. Such changes also give rise to novel emerging
properties[4] that are not observed during classical NM health
and safety (EHS) evaluation. This new direction in NH synthesis
and use thus presents unique challenges and necessitates
systematic evaluation of nano EHS.
Demand for multifunctionality has resulted in physical and
chemical modification to NMs, in general. Size and shape
modulation alongside physical or chemical functionalisation
are used to achieve hierarchical[5] and heterostructures.[6] Such
functionalisation has altered inherent surface attributes and
extracted novel electronic configuration, intrinsic hydrophobicity, dissolution properties, etc., from nanoscale materials.
The successes of such manipulations have further encouraged
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N. Aich et al.
emitting[12] and limiting[13] devices, supercapacitors,[14] lithiumion batteries[15,16] or transparent conductors.[17] It is evident that
benefits of conjugation and ensembles of multiple materials are
well realised and thus will likely widen the NH material domain,
affecting a much larger application space and in large amounts.
For example, it is projected that by the year 2050, at least
1.0 107 kg of platinum carrying titania-modified multiwalled
carbon nanotube (MWNT) NHs will be deployed in fuel
cells for vehicles alone, assuming 20 % platinum in the NH
by mass.[18,19]
The development of novel materials comes with an intrinsic
uncertainty regarding their potential environmental and biologic
consequences. Material release can occur from nano-laden
products and devices as well as during their manufacture and
use.[20] Upon release, NMs undergo transport and transformation in either occupational or environmental settings.[21] Such
processes are highly influenced by the material attributes and the
form of release; e.g. NM release from personal care products and
medicinal applications will possess distinctive physicochemical
properties compared with their release from solid-state
optoelectronic systems. As the material complexity increases
with conjugation and assemblages of materials with uniquely
different properties, their environmental processes will also be
altered and likely present higher uncertainty when predicted
using their parent material classes. To date, environmental fate,
transport and transformation literature of NMs have systematically generated a critical information mass – by measuring
physicochemical properties and their influence on environmental behaviour manifestation – that has begun to effectively
determine material safety and risk.[22,23] However, the uncertainty of environmental behaviour for hierarchical and conjugated materials continues to prevail. The uncertainty emanates
from the knowledge-gap of ‘conjugated materials’ in an environmental setting – because an ensemble of multiple materials
will most likely behave differently compared with their parent
components. For example, carbonaceous NMs (CNMs), such as
fullerenes[24] and carbon nanotubes (CNTs),[25] show a high
aggregation propensity due to their inherent hydrophobicity and
strong van der Waals interaction forces; whereas, metallic
nanomaterials (MNMs) (such as silver or zinc oxide), possess
unique dissolution and complexation properties.[26,27] When
combined, behavioural manifestation of metal–carbonaceous
conjugates can either present dominant hydrophobicity or dissolution–complexation reactions; which will be influenced by
the nature of conjugation. Thus risk evaluation of these hierarchical NHs will require systematic environmental studies.
This account presents an EHS-relevant definition of hybrid
NMs, classifies the NHs, reviews the NH literature, and discusses the need for environmental studies. Probable environmental exposures of NHs and relevant altered fate, transport and
toxicity as a result of transformed physicochemical and emergent properties are discussed. Challenges regarding the prediction of environmental behaviour of NHs from their individual
component characteristics are also delineated. Overall, this
account will serve as an environmentally relevant summary of
the ever-expanding class of NHs, and hopefully will accentuate
the importance of evaluating these nano-ensembles for
enhanced risk assessment.
Defining nanohybrids
Definitional ambiguities are evident in NH literature[28] similar to
the debate that exists for singular nanoscale materials (National
Nanotechnology Initiative, see http://www.nano.gov/nanotech101/what/definition, accessed 30 November 2014).[20,29]
Nirupam Aich is a Ph.D. student at the Department of Civil, Architectural and Environmental Engineering in the University of
Texas at Austin. Prior to joining UT in 2014, he completed his M.Sc. in Environmental Engineering from University of South
Carolina, Columbia, SC and B.Sc. in Chemical Engineering from Bangladesh University of Engineering and Technology,
Dhaka, Bangladesh. His research interests include systematic evaluation of environmental implications of nanohybrid
materials and application of nanomaterials for environmental remediation and sustainable infrastructure.
Jaime Plazas-Tuttle is a Ph.D. student at the Department of Civil, Architectural and Environmental Engineering in the
University of Texas at Austin. He earned his M.Sc. in Environmental Engineering from the University of Illinois at UrbanaChampaign in 2012, and a M.Sc. in Desert Studies, in Water Resources and Management, from Ben Gurion University of the
Negev, Sde Boker, Israel in 2004. His B.Sc. degree is in Civil Engineering, earned at Pontificia Universidad Javeriana,
Bogotá, Colombia in 2000. He was the recipient of a Fulbright Scholarship in 2009. His research interests focus on the
development and application of nanomaterials in drinking water treatment.
Professor Jamie Lead is an endowed Professor and Director of the SmartState Center for Environmental Nanoscience and Risk
in the Department of Environmental Health Sciences, University of South Carolina, USA, and an adjunct Professor and coDirector of the Facility for Environmental Nanoscience Analysis and Characterisation, in the School of Geography, Earth and
Environmental Sciences, University of Birmingham, UK. His research aims at (i) understanding nanoscale phenomena in the
environment including natural nanomaterials, manufactured nanomaterials and their interactions and impacts on pollutant
behaviour and (ii) the development of manufactured nanomaterials for environmentally beneficial processes such as
remediation of organic contaminants.
Navid Saleh is an Assistant Professor of Civil, Architectural and Environmental Engineering at the University of Texas at
Austin. He holds a Ph.D. in Civil and Environmental Engineering from Carnegie Mellon University and has been trained as a
postdoctoral scholar at the Department of Chemical Engineering, Yale University. His research focuses on the fundamental
understanding of nanomaterial fate, transport and transformation and on physicochemical characterisation of nanomaterials
to provide mechanistic insights on nanotoxicity. Use of nanomaterials for water treatment and environmental remediation has
also been a focus of his research.
610
143
181
Number of publications
NH of Env. importance
62
57
58
53
74
29
26
37
31
50
77
80
100
93
95
110
150
2
2
4
4
8
8
12
12
15
13
13
11
Number of publications
200
71
We attempt to clarify the nuances in the NH literature and also to
make way for defining NHs from a EHS perspective. A strong
tendency of claiming simple surface modification – with inorganic, organic and soft molecules – as hybridisation has been
observed in the material science literature. For example,
attaching a monomer or polymeric molecule onto a metallic
nanoscale material has been claimed to form a NH[30,31]; similarly, large polymeric structures with conjugated inorganic–
organic atoms–molecules are claimed to be NHs as well.[32]
Although such minor surface modifications can enhance the
material performance, it is likely that the parent physicochemical properties will be preserved and therefore they should
not be considered as novel NHs for environmental evaluation
purposes. Our rendition of an environmentally unique NH definition can be formulated as follows: when more than one NM
of unique chemical origin or differing dimensionality are
conjugated by molecular or macromolecular links or physicochemical forces or when one nanomaterial overcoats another
possessing a unique chemical identity or when complex soft
molecules are engineered to chemically bind to NM surfaces, all
to enhance the existing functionality or achieve multifunctional
usage, can be defined as NHs. This definition concurs with the
literature definition of NHs[2–4]; however, it confines the
material class to those NHs that will likely result in unpredicted
and unique environmental fate, transport and toxicity.
199
A critical review of nanohybrids
2012
2011
2010
2009
2008
2007
2006
2005
2004
2003
2002
2001
2000
1999
1998
0
Year
Fig. 1. Total number of publications per year from 1998 to 2012 using the
Web of Science search engine searching for ‘nanohybrid’ or ‘nano-hybrid’,
and total number of nanohybrids of environmental importance. Literature
was selected when it originated from scientific articles and referred specifically to the following combination of keywords, special character (*), and
search field (Title): ‘Title ¼ (nano-hybrid*) OR Title ¼ (nanohybrid*)’.
Title was selected as the search criteria to try and limit the results to those
articles dealing particularly with nanohybrid research. Meeting abstracts,
reviews and proceeding papers, were not included. More search combinations ‘Title ¼ (nano-horn* OR nanohorn*) AND Title ¼ (hybrid*)’, ‘Title
¼ (peapod* OR pea-pod*) AND Title ¼ (hybrid*)’, ‘Title ¼ (nanobud* OR
nano-bud*) AND Title ¼ (hybrid*)’, and ‘Title ¼ (nanoonion* OR nanoonion*) AND Title ¼ (hybrid*)’ were used to identify some popular carbonaceous nanohybrids having speciality names because of their interesting
morphologies.
Classification, synthesis and applications of nanohybrids
The growth of NH literature in the recent decade has been
noticeable. To assess the importance of this emerging material
class, a comprehensive literature search using the Web of
Science database was performed (Fig. 1). A list of 758 peerreviewed journal articles and 123 additional publications on
speciality carbonaceous NHs (peapods, nano-onions, nanobuds, nano-horns, etc.) during the years 1998–2012 were identified. After careful screening on the basis of the NH definition,
752 articles dealing with NHs of environmental importance
were selected and classified (Table S1, Supplementary material). The remaining 129 articles were not considered as they were
deemed beyond the definitional scope. Overall, the literature
search shows an exponential increase in publication number
over the last decade (Fig. 1). This substantial published body of
literature thus makes a strong case to carefully evaluate their
physicochemical properties, relevant to environmental safety.
The environmentally relevant classification of NHs is established based on the primary constituents. Four major classes of
NHs are identified, namely: carbon–carbon, carbon–metal,
metal–metal, and organic molecule-coated NHs (Fig. 2a).
The simple classification above should not deceive the
readers of the inherent complexity of each of these NH classes;
e.g. carbon–carbon NHs include rather simple CNMs such as
single-walled and multiwalled carbon nanotubes (SWNTs and
MWNTs), fullerenes and graphene sheets as the primary components, which are then conjugated with other carbonaceous
entities[33,34] to form hierarchical structures. Similarly, carbon–
metal NHs are formed by a conjugation of carbonaceous
materials with metallic NMs.[35,36] Metal–metal NHs, however,
are assemblies of individual metallic NMs[37] or are formed as
core–shell structures of different metals[38] and metal oxides.[39]
When metallic NMs combine with long chain polymers,[40]
drug molecules,[41] cell-synthesised proteins,[42] DNA,[43] long
chain organic molecules,[44] etc. they form organic moleculecoated NHs.
NH of Enviromental importance
(a)
(b)
CCNH
14%
OMCNH
58%
CMNH
11%
MMNH
17%
Others
11%
NH Synthesis and
Characterisation
24%
Imaging &
Luminiscence
12%
Electronics
24%
Enviromental
11%
Medical &
Health
18%
Fig. 2. Distribution of research article publications based on (a) environmental classification of nanohybrids and (b) relevant application premise.
The retrieved literature also provided information in relation
to the application potential of the NHs (Fig. 2b and Table S2,
Supplementary material). NH applications are categorised as
follows: (1) electronic: solar and fuel cells, Li–ion batteries,
semiconductors–superconductors–conductive materials, imaging and sensing applications; (2) environmental: contaminant
sorption, membrane technology, catalytic–photocatalytic–
electrocatalytic applications, and antimicrobial–antibacterial
processes and devices and (3) medical: cancer treatment and
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(a)
(i)
(b)
TiO2
(ii)
Gd
(i)
Pd
Pd
Pd
(iii)
(iii)
(ii)
(c)
(d)
SiO2
N
CH3
Fe3O4
Au
N
Ag
TiO2
N
N
CdS
(i)
Zn
N
N
(ii)
Fig. 3. Schematic representations of nanohybrids (NHs). (a) Carbon–carbon: (i) nanobud (fullerenes covalently
bound to the outer sidewalls of single-wall carbon nanotube), (ii) peapod (fullerenes encapsulated inside a singlewall carbon nanotube), and (iii) nano-onion (multi-shelled fullerenes); (b) Carbon–metallic: (i) titanium dioxide
nanoparticle conjugated with single-wall carbon nanotube, (ii) gadolinium encapsulated within a fullerene, and
(iii) graphene decorated with palladium; (c) Metal–metal: (i) multimetallic core–shell structure of TiO2–CdS–
Fe3O4@SiO2 and (ii) bimetallic Au–Ag core–shell; (d) Organic molecule-coated: zinc tetraphenylporphyrin
coordinated with pyridyl fulleropyrrolidine (C60Py-ZnTPP) dyad.
attachment of chemically active molecules[54] or polymeric
assemblies.[55] For example, fullerenes functionalised with
porphyrin-derivatives are refluxed with acid-treated CNT–
COOH suspensions to generate fullerene–CNT NHs by reaction
between the carboxyl functionality on the CNT and aminegroups on the porphyrin molecules.[56] Producing seamless
exohedral bonding between CNT and graphene[52] or CNT and
fullerene[33] (nanobuds) through covalent modification is typically achieved by catalytic reaction processes involving vapour
phase reactant molecules. Moreover, drop-cast,[57] spin-cast[58]
and dipping[55] methods of these graphitic NMs can produce
layered assemblies of NH-based thin films by electrostatic and
non-covalent interactions.
The usefulness of hybridisation among CNMs has been
obtained from multifunctional and improved properties emanating from individual species. Whereas graphene has a high
reactive surface area, mechanical and thermal stability and high
electrical conductivity, CNTs present unique electrical,
mechanical, optical and charge carrying properties. Fullerenes,
in contrast, provide high electron density and photoactivity.
Thus, fullerenes when conjugated with graphene or CNTs can
lead to improved organic photovoltaics[59] and optoelectronic
devices, optical limiting and switching,[60] field effect transistors[61] by enhancement of the photoinduced electricity production, charge transfer and electron–hole shuttling,[61] singlet
excited state quenching,[54] non-linear optical properties,[60]
bandgap tenability,[45] etc. Hybridised graphene can act as a
major candidate for transparent conducting films for optoelectronic and photovoltaic devices, which possess high surface
detection, biomaterial–biohybrids, delivery carriers and drug
compound controlled release, UV protection, etc. Detailed and
more specific usage of NHs along with their synthesis processes
(Tables S3–5, Supplementary material) will be discussed in the
following section in context of their environmental release and
interaction.
Carbon–carbon nanohybrids (CCNHs)
Carbon-based NHs include combinations of three major carbon
nanostructures – zero dimensional fullerenes (Fig. 3a), 1-D
CNTs (SWNTs and MWNTs) and 2-D graphene and carbon
nanohorns (CNHs). Open-ended hollow structures of CNTs or
CNHs and cage-like fullerenes offer unique advantages to produce endohedral NHs as well as allow for generation of their
exohedral forms.[45] Fullerenes or graphene (pristine or functionalised) when encapsulated within the CNTs or CNHs by
thermal annealing,[34] by in situ growth from vapour-based
deposition reactions[46] or by dispersion-assisted cavity filling
processes, are called ‘nano-peapods’.[47] Similar synthesis
processes as well as the water-assisted electric arc process can
create an exotic multi-layered hybrid fullerene structure named
a ‘carbon nano-onion’.[48–50] In contrast, the exohedral conjugation of CNTs, graphene and fullerenes employ long-range
electrostatic or short-ranged specific interactions[51]; where
conjugating molecules or polymers and covalent functionalities[52] drive the ensemble process. Such functionalisations
include: oxidation of CNTs and graphene to attach polar
carboxyl or hydroxyl surface groups (–COOH or –OH)[51,53] and
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A critical review of nanohybrids
decomposition of metal salts are the most used synthesis techniques.[96] Wide variations of wet chemical processes include:
polyol methods,[97] photochemical deposition,[98] electroless
plating,[99] solvothermal,[100] hydrothermal,[101] sol–gel,[39]
ion-implantation,[102] epitaxial growth,[103] etc. Vapour–gas
phase processes, such as flame aerosol[104] and plasma-assisted
deposition[105] are also commonly used. Core–shell based
nanostructures can be formed by co-reduction[106] or sequential
reduction,[97] where a metal NM previously formed can act as a
‘seed’ for subsequent growth of another NM with different
chemical origin. Optical lithography is also combined with
common methods to obtain patterned growth.[107] Templatebased growth processes can be used to obtain hollow spherical,[108] porous[109] or tubular[110] structures. Matrix bound
methods, however, utilise inorganic silica, the oil–water interface, and polymer or block-co-polymer matrices, where
co-precipitation,[111] ion implantation,[102] emulsification[112]
and reverse micellisation[113] processes grow NHs. Core–shell
metallic layers sometimes include inorganic[114] or organic[115]
linkers or spacers between them. Biogenic or green synthesis
approaches for MMNHs have also been developed using natural
extracts as solvents or reducing agents.[116] This is a synopsis of
MMNH synthesis processes. Careful review of the existing
literature will further elaborate on such techniques.
Property synergies in MMNHs allow their application in the
diverse fields of photovoltaics and solar cells,[100] biomedical
engineering and nanotherapeutics,[117] catalysis,[118] chemical
sensing[119] and degradation[118] and bactericidal applications.[120] For example: co-axial Ag–TiO2 core–shell nanowire
arrays with high specific surface area and rapid electron transport can improve the electron collection efficiency for application in dye-sensitised solar cells.[100] Bioapplications, such as
enhancement of contrast in MRI for disease[121] and pathogen
detection,[122] photo-thermal destruction of these cells by nearIR irradiation[121] and separation of cancer cells from cell
mixtures[8] have begun to employ plasmonic, semiconducting
and magnetic metal NM-based MMNHs.[117] Plasmonic properties of Au and Ag are combined to produce high efficiency
localised surface plasmon resonance (SPR) and surface
enhanced Raman scattering (SERS) to detect disease-specific
biomolecules.[96] Photoluminescent properties of semiconducting quantum dots have been shown to be enhanced when
combined with magnetic (e.g. Fe3O4 CdS[123]) or plasmonic
particles (e.g. Au–CdSe–ZnS[124]) and can be used for bioimaging or fluorescence microscopy. Conjugating TiO2, Ag or ZnO
with other metal NMs has also been shown to enhance photocatalytic activities and bandgap modulation combined with
excellent charged separation and charge transfer processes have
made them excellent candidates for organic contaminants degradation[109] and bacteria inactivation under UV to visible light
irradiation.[120] Such diverse applications, particularly in
biomedicine, increase the MMNHs’ environmental relevance.
area, conductivity, transmittance and low physical thickness as
they conjugate with CNTs or fullerenes.[62,63] Such modifications also render their applications in various avenues; such as in
electrochemical and biomolecular sensing,[64] structural health
monitoring,[65] etc.
Carbon–metal nanohybrids (CMNHs)
Carbon-metal nanohybrid (CMNH) synthesis processes involve
a combination of CNMs (CNTs, graphenes and fullerenes) with
different metallic or metal oxide NMs[66] (Fig. 3b). CMNHs
include assemblies with a variety of metallic NMs (MNMs)
ranging from noble metals like Ag, Au, Pt, Pd, Ru, Rh, etc. to
lanthanide series metals (La, Sc, Gd, etc.), metal oxide NMs
(ZnO, TiO2, SiO2, Fe3O4, CuO, etc.), semiconducting quantum
dots (CdSe, CdTe, etc.) and ligand-based metallic compounds
(ferrocene). CMNHs can be synthesised following four key
pathways – (i) filling the inner cavities of CNTs and fullerenes
with MNMs using vapour deposition,[67] arc discharge,[68]
thermal annealing[69] and wet chemical approach[70];
(ii) attaching MNMs onto CNT surfaces functionalised with
pyrene, porphyrin derivatives[71] and similar linking molecules[72]; (iii) decorating CNM surfaces with MNMs by sol–
gel,[73] hydrothermal[74] and aerosol-based processes[75] and
(iv) in-situ growth of MNMs on CNM surfaces by electrochemical,[76] eletroless deposition[77] and redox reactions.[78]
Combinations of graphitic and metallic nanostructures result
in the emergence of unique and synergistic electrical, optical,
mechanical, catalytic, sensing ability and magnetic properties,
which can be utilised for applications in various fields;
e.g. chemical reactivity and catalysis,[79,80] organic photovoltaics and solar cells,[81] optoelectronics,[82] supercapacitors[83]
and batteries,[84] proton exchange fuel cells,[85] gas and chemical sensing,[86] biomedical imaging,[87] environmental pollution
monitoring and mitigation,[88,89] etc. The thermal and mechanical stability of CNTs and graphene with high active surface
area are particularly promising in the development of Li–ion
storage units with high efficiency, capacity and durability.[74]
Similarly, antibacterial activities of TiO2, ZnO or Ag are
enhanced when conjugated with CNMs and thus facilitate their
use in water treatment and other purification or detoxification
applications.[89] Better sensors for gas, protein or chemicals
(H2O2,[86] trinitrotoluene,[86] etc.) are being prepared using
CMNHs utilising their enhanced sorption and electrical sensitivity. In contrast, endohedral metallofullerenes by themselves[90] or when encapsulated inside CNTs or CNHs[91] have
great potential to be used as MRI contrast agents with extremely
high water relaxivities – a holy grail in MRI contrast agent
research. Such a wide range of applications of CMNHs has
encouraged major research efforts in material development
and their application necessitating extensive environmental
implications studies.
Metal–metal nanohybrids (MMNHs)
Metal NMs, i.e. metals and metal oxides, when conjugated to
form multi-metallic ensembles are classified as metal–metal
nanohybrids (MMNHs, Fig. 3c). Metals can be grouped based
on their functionalities; e.g. plasmonic (Au, Ag, Pt),[92] magnetic (Fe3O4, Fe2O3)[93] and semiconducting oxides (TiO2),[94]
quantum dots (CdSe, ZnS, CdTe, ZnO, PbS),[95] etc. Synthesis
processes to prepare conjugated metallic NMs depend on the
desired hybrid properties, structures and applications.
Wet chemical processes involving reduction or thermal
Organic molecule-coated nanohybrids (OMCNHs)
A wide body of literature identifies metallic, carbonaceous or
polymeric NMs coated with organic molecules, biomolecules or
polymers as NHs (Fig. 3d). Layer-by-layer hierarchical thin
films have also been called NHs. Although such identification is
debatable, environmental evaluation of NHs in this category
should be pursued with reflection on already existing classical
coated-NM studies. The literature on OMCNH involves a wide
range of synthesis processes that include: physisorption of
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organic molecules,[125] electrochemical immobilisation of
protein, enzyme or DNA molecules,[44] polymer grafting from
or grafting to NM surfaces,[126] emulsification[40] and ionexchange.[127] Such coated NMs are researched in the
application areas of nanoelectronics,[125] photovoltaics,[125]
chemical and bio-sensing,[128] bio-imaging,[129] controlled drug
delivery[130] and cancer therapy.[131] CNMs are surface functionalised with porphyrin,[125] phthalocyanine[125] and other
molecules to attain higher efficiency in charge transfer for
photovoltaics and dye sensitised solar cells. Similarly, magnetic
or plasmonic particles are grafted or coated with organic polymers, such as polyethylene glycol (PEG)[132] and poly(vinyl
pyrollidone) (PVP)[133] to enhance their solubility for enhanced
bio-imaging, drug delivery or sensing. Metallic NMs are also
attached to organic fluorophores for enhanced tagging and
contrasting.[129]
Most of these materials appear to be merely coated-NMs for
environmental purposes, thus might not require systematic and
independent environmental evaluation for accurate risk estimation. Already established environmental fate and toxicological
literature have focussed on physisorbed coatings. For example,
citrate, PVP, PEG, gum arabic, copolymers, etc. are typically
adsorbed onto the NMs to enhance dispersion in a desired
solvent and have been studied for environmental implications.[134–137] However, the recent surface modification of
NMs are performed with rather complex supramolecules or
heterocyclic structures (e.g. porphyrins), which are covalently
bound to the NM surfaces.[138–140] As per the NH definition,
chemically bound coatings of this nature will lead to altered
nano-EHS behaviour. For example, heterocyclic porphyrins not
only provide stabilisation to NH dispersions but will also
provide excellent electronic charge transfer properties[138] and
antimicrobial capabilities.[141] Moreover, conformational differences of organic molecules or polymers present on the NM
surface are known to present unique fate, transformation and
toxicity behaviour.[142] Systematically evaluating nano-EHS
behaviour of these complex chemically coated NHs is thus
imperative. Existing environmental literature on NMs with
physisorbed coatings will enhance the understanding of
OMCNH environmental behaviour.
aqueous environment. Moreover, transformation of NMs[153]
can occur by sorption of geo- and bio-macromolecules, reaction
with chemical species (presence of reactive ions, ozone or
oxygen) and by solar irradiation in case of photoactive NMs –
contributing towards NM fate and toxicological effects. The
fate, transport and transformation of NMs in the environment are
also highly dependent on the intrinsic NM properties. As these
NMs conjugate to form hierarchical ensembles, their physicochemical properties alongside their environmental behaviour
and toxicity response will likely be altered. How such alterations
will occur depends on the mode of conjugation as well as the
application type, influencing their release and exposure. Here
the altered fate, transport, transformation and toxicity of some
common NHs will be discussed to lay out the uncertainties in
nano-EHS.
Fate and transport
Singular NMs, either carbonaceous or metallic, have been
studied extensively to evaluate their aggregation, deposition and
transport behaviour. Such behaviour has been characterised in
relation to their physicochemical properties and major
mechanisms are elucidated in terms of electrostatic interactions,[24,151] van der Waal’s attraction forces, steric hindrances
contributed by physical morphology and unique materialspecific forces, such as magnetism (in case of iron-based
NMs[154]) or chirality.[155] However, conjugation of two or more
NMs will likely alter contributions from these forces, resulting
in uncertain stability and mobility of the NHs.
Carbon nano-peapods, that are highly attractive for solid
state electronics[156] or MRI contrast agents,[91] are prepared by
encapsulation of fullerenes (C60, C70 or higher order fullerenes)
inside CNTs or CNHs. Such conjugation exhibits bandgap
tuning[157] and electron density differences.[158] Such alterations
occur as a result of SWNT diameter changes upon conjugation
as well as of the entrapment of fullerenes that causes overlap of
electron clouds.[157,159] Peapod formation often involves SWNT
oxidation in the presence of acid mixtures that forms surface
defects and also causes shortening of the SWNT length.[159]
Such surface property changes will likely influence van der
Waals and electrokinetic interactions of nano-peapods (Fig. 4a).
For example C60@SWNT peapod bundles can have stronger van
der Waals forces compared with C70@SWNT bundles as demonstrated by spectral characterisation.[158] Moreover, other
higher order fullerenes also induced size-dependent electronic
structure variation in peapods followed by van der Waals’
disparity.[160] Furthermore, fullerene encapsulation may also
result in increased mechanical strength of SWNTs,[161] resulting
in stiffer tubules.[162] Altered van der Waals forces and shorter,
stiffer tubes, will likely demonstrate unique environmental
behaviour compared with the component fullerenes and
SWNTs.
Similarly, emergent properties, such as dimensional modifications, occur as a result of hybridisation. For example, nanopeapods mask the presence of zero-dimensional fullerenes[158]
and two-dimensional graphene[163] inside one-dimensional
CNTs; whereas their exohedral conjugation results in unique
three-dimensional configurations. Covalently bonded fullerenes
on the surface of the graphene[60] or CNTs[33] (in case of nanobuds) can have debundling or intercalating effects and can result
in enhanced stability. However, such dispersion enhancement
can also be compromised by a superimposed or combined
inherent hydrophobicity of the CNMs.[45] Exohedrally attached
Environmental interaction of nanohybrids
The novelty in NH ensembles lies in multifunctionality,
resulting from a non-linear combination of advantageous
properties of each of the component nanostructures.[45,75] Such
assemblies not only contribute to enhanced functionality but
also present unknown and unique physicochemical properties,
which will likely cause unpredictable environmental behaviour
from their release and exposure. However, while researchers
focus on the merits of such NHs, their potential toxic and
environmental implication studies have gained attention only
recently and require a significant systematic approach.
Eco-toxicity of singular NMs and their microbial and organismal uptake are known to be influenced by material-specific
physicochemical properties such as size,[143] shape,[144] aggregation state,[145] surface functionality and coating,[146] reactive
oxygen species (ROS) generation capability,[143,147] photoactivity,[148] crystallinity[149] and dissolution[26,145] of metal
NMs and bandgap[150] of metal oxide NMs. When NMs are
exposed to the environment they experience aggregation in
aqueous media[25] and deposition onto solid surface[151] and
porous media,[152] which contribute to their mobility in the
614
A critical review of nanohybrids
(a)
Increased
van der Waals
(b)
Sand
Grain
Enhanced aggregation
(c)
Increased physical straining
High surface area
Altered dissolution & activity
(d)
Ag⫹
Ag⫹
Ag⫹
Toxic Ag⫹
Ag0
Ag2O
shell
Ag⫹
Ag
Au
shell
Ag⫹
Ag⫹
⫹
Ag
⫹
Ag
Ag
Enhanced sorption
of macromolecules
Au
(f)
(e)
Detachment &
release
UV to visible
Altered
band-gap
Increased
ROS
(g)
Mechanical stiffness & cell
disruption
Unknown toxic response
Fig. 4. Plausible environmental interactions of nanohybrids (NHs). (a) Increased van der Waals attraction
forces in fullerene–carbon nanotube (CNT) peapods may lead to enhanced aggregation. (b) Exohedrally
conjugated fullerenes with CNTs may enhance physical straining during transport through porous media.
(c) Ag–Au core–shell NHs may show decreased dissolution and enhanced chemical stability. (d) Exohedral
conjugation of CNTs and fullerenes may provide more surface for sorption of geo- and bio-macromolecules.
(e) Fullerenes may be released from nano-peapods during transformation and result in different surface
chemistry compared with component NMs. (f) Bandgap alteration of TiO2 by conjugation with graphene can
increase reactive oxygen species (ROS) production under visible light, leading to enhanced nanotoxicity.
(g) Increased stiffness as a result of hybridisation may induce greater cellular interaction, uptake and
membrane disruption.
be the roles of the linking molecules? How will overcoating
influence the aggregation and deposition behaviour of metallic
NHs? Such questions require immediate attention to address
uncertainties from the emerging properties of NHs.
fullerenes may increase physical straining during their transport
through porous media (Fig. 4b). Altered stability and porous
media transport will likely lead to uncertain NH fate and
transport in the natural environment.
Understanding of NH aggregation and transport necessitates
resolving the following key questions. Will altered electrostatic
or van der Waals forces dictate aggregation or deposition of
exohedrally hybridised nanotube–fullerene conjugates? How
will metal NMs change the NH surface interaction? What will
Transformation
Upon environmental release, NM characteristics can be altered
by various transformation processes. For example, fullerenes
615
N. Aich et al.
Toxicity
Substantial literature exists regarding the toxicity of singular
NMs, delineating mechanisms and correlating the effects with
physicochemical properties. Several carbonaceous (fullerene,[180,181] CNT[182] or graphene oxide[183]) and metallic NMs
(Ag,[26] TiO2,[149,184] ZnO,[184] CuO[184]) are known to illicit
toxicological effects on biological species. The key mechanisms associated with such toxic responses include: ROS
mediated oxidative stress,[143] direct interaction of metal
NMs with cell membranes,[144] lipid peroxidation,[185] ROS
independent protein oxidation,[186] dissolution and relevant
reactive membrane or enzymatic damage,[26] asbestos-like
inflammation by CNTs[187] and physical rupture of cell
membranes.[183] Material characteristics such as size and surface area,[188] shape,[144] crystalline structure,[144] surface
coatings,[189] aggregation state[145,190] and electronic properties,[150] have been known to influence NM bioaccumulation
and toxicity. However, the likelihood of altering the toxicity
following NM hybridisation has not been well studied. Among
few recent efforts, most are directed towards beneficial antimicrobial applications but only a handful of studies report
concerns regarding NHs’ harmful implications.[4,191] For
example, bimetallic conjugation of non-toxic parent materials
Au and Pt with variable compositions has generated antimicrobial responses against E. coli, Salmonella choleraesius,
and Pseudomonas aeruginosa by cell membrane damage and
incremental increses in the intracellular level of adenosine
triphosphate (ATP).[192] Recent studies involving graphene–
ZnO[193] and graphene–Cu[194] NHs showed increased toxicity
in comparison to their parental components towards a model
organism transgenic Drosophila melanogaster as demonstrated
by enhanced lipid peroxidation and apoptosis. On the contrary,
the presence of a silica-based shell structure reduced ZnO
toxicity towards E. coli.[195] A comprehensive toxicity evaluation of E. coli on exposure to iron-based bimetallic NHs has
shown differences in toxicity based on the presence and type of
a second metal.[191] Component dependent toxicity was
observed as bare Fe, Fe–Cu and Fe–Ni showed comparable
toxicity whereas Fe–Pd and Fe–Pt presented with significantly
lower toxicity. These differences were attributed to diverse
interactions of these NHs with the cellular membrane as a
result of differences in surface charge, particle size and
reactivity, caused by conjugation. This evidence of altered
bio-compatibility hints towards the necessity of a systematic
and mechanistic exploration of NH toxicity.
A recent study[196] involving colloidally stable graphene–
TiO2 NHs showed enhancement in photocatalytic ROS generation under visible light irradiation, whereas pristine TiO2
showed photoactivity, only in the UV spectrum (Fig. 4f). This
has been possible because of the excellent charge separation
abilities of graphene; which could reduce TiO2’s bandgap in the
hybridised form. However, the NHs didn’t exhibit enhanced
toxicity compared with singular TiO2 to model aquatic organisms, Daphnia magna and Oryzias latipes (Japanese Medaka
fish). The lack of toxicity may be explained by ROS quenching,
which resulted from rapid aggregation of the NHs in high ionic
strength culture media. Hybridisation of NMs thus has been
shown to alter nanotoxicity.
Emergent properties, such as changes in surface roughness
and mechanical stiffness, have shown to be responsible
for differential cell–NH interactions; as was observed in
the case of multicomponent hierarchical NHs prepared by
and CNTs can undergo various transformation processes that
include: reaction with atmospheric oxygen or ozone,[164] ultraviolet (UV) or solar light mediated photochemical change,[165]
adsorption of macromolecules[166] and natural organic matter
(NOM).[165,167,168] Similarly, TiO2 and ZnO transformation can
also occur under UV-exposure and during interaction with geoand bio-macromolecules.[169] These transformations take place
because of the NMs’ inherent photoactivity, chemical reactivity
and sorption ability; which are functions of their size, shape,
surface charge and chemistry.
NOM sorption on carbonaceous and metallic NMs showed
enhanced stability in aqueous media.[166,169] After NOM sorption, TiO2 has exhibited reduced photoactivity and suppressed
ROS production.[169] However, unknown alterations of transformation results may be experienced by hybridised NMs. For
example, the photoactivity of TiO2 (under visible light) has been
shown to enhance upon conjugation with CNTs or graphene,
because of lowering of the bandgap energy.[73,170] Such
enhancement is attributed to the synergy in electronic properties
between titania and carbon nanostructures; e.g. small-sized
TiO2 particles on CNT surfaces reduce the electron–hole pair
recombination rate and thus enhance the photoactivity.[73]
Moreover, the high electron transport ability through hollow
CNT structures and conductive graphene – e.g. photoactivity
transfer from UV region to visible range – is also known to
improve photodynamic activity.[171,172] Similarly, a substantial
increase in the available surface area during hybridisation can
also invoke excellent sorption properties[75]; as demonstrated in
the case of flowerlike hierarchical structures of TiO2 on
CNTs.[173] Sorption of geo- and bio-macromolecules on CNTs
can also be enhanced by exohedral attachment of fullerenes,
which will likely add to available sorption sites (Fig. 4c).[33]
Increased adsorption can enable higher coverage of the NH
surfaces with geo- and bio-macromolecules and thus can alter
the subsequent fate, transport and toxicity.
Dissolution and reaction with inorganic species such as
sulfide (S2 ) or chloride (Cl ) ions in the aquatic environment
are two important transformation processes for metallic NMs,
such as Ag[26,145] (or ZnO[27] and CuO[174]). These transformations are governed by the inherent solubility, reactivity and
sorption ability of AgNMs, influenced by physicochemical
characteristics such as: size,[175] shape,[26] surface structures,[26]
surface chemistry[176] or coatings.[134] However, hybridisation
of chemically active AgNMs with a relatively inert gold overcoating can reduce Agþ dissolution (Fig. 4d).[177] Electron
transfer properties of a Ag-core through to a Au-shell were
shown to increase the oxidative and chemical stability of these
NHs.[178] On the contrary, an 18 times higher catalytic activity
was observed for Ag–Au core–shell structures when compared
with monometallic Au particles.[179] Thus, overlapping of
chemical or electronic characteristics can have an unprecedented effect on the transformation behaviour of Ag–Au NHs.
In addition to the above discussed probable uncertain alterations of transformation behaviour, some key questions arise that
necessitate systematic transformation studies of NHs. What will
the relative roles of parent materials be in such transformations?
Will there be new transformation processes resulting from the
instability of NHs in the environmental matrices – e.g. detachment of TiO2 from CNT surfaces or release of fullerenes from
nano-peapods (Fig. 4e)? How does the release of NMs from NHs
alter their previously predicted environmental interactions?
Such questions need to be researched to better understand NH
environmental transformation.
616
A critical review of nanohybrids
Supplementary material
sequential coating of functionalised CNTs with Ag, DNA, and
poly(vinyl alcohol) (PVA).[197] Similarly a stiffness increase
attributable to fullerene encapsulation inside CNTs may also
have physical interaction mediated toxicological consequences
(Fig. 4g).
Thus questions may arise when combining graphitic nanostructures with metallic ones. Will emergent mechanical
properties dominate the NH toxicity? How will metal dissolution be altered and mediate nanotoxicity? Will alteration of
dimensionality, e.g. from 2-D (graphene) to 3-D (fullerene–
graphene), influence shape-dependent toxicity? Thus the potential environmental interaction of emerging nanoscale hybrid
materials are ostensibly unique, complex and may not be
predictable from simple one or two parametric combinations
of physicochemical characteristics; addressing the aforementioned questions can be a starting point for NH toxicity
evaluation.
Table S1 includes the annual number of publications regarding
NHs from 1998 to 2012. Table S2 includes the total number of
publications categorised according to the potential applications.
Table S3 includes all the listing of the retrieved articles, classifications according to the material types, their usage, and
research areas. Table S4 includes specific NH class examples and
their synthesis processes. Table S5 includes specific NH class
examples and their corresponding potential application premises.
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Conclusion and perspectives
This article has reviewed the NH literature and has highlighted
the emergent properties of this new ensemble material class. The
novel properties already emerging from conjugation and overcoating are altering fundamental physicochemical properties.
Such differences will alter EHS behaviour of NHs, thereby
warranting careful consideration and strategising for systematic
evaluation. Their environmental exposure appears to be more
eminent when NH-laden real-world applications are marketed in
electronics (e.g. silicon oxycarbide–CNTs[198] and CNF–
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related products,[200,201] antimicrobial coatings[202] and
protective devices[203] (e.g. Ag–TiO2) and in bio-imaging or
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supercapacitors (graphene–Mn3O4) and solar cells (CNT–fullerenes), are enabling active technology transfer. The key issue is
not about the NHs’ unique EHS behaviour, but about how and by
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aggravated risk potential realised from the component
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