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A Critical Review of Nanohybrids: Synthesis, Applications and Environmental Implications

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A Critical Review of Nanohybrids: Synthesis, Applications and Environmental Implications CSIRO PUBLISHING Environ. Chem. 2014, 11, 609–623 Review http://dx.doi.org/10.1071/EN14127 A critical review of nanohybrids: synthesis, applications and environmental implications Nirupam Aich,A Jaime Plazas-Tuttle,A Jamie R. LeadB and Navid B. SalehA,C ADepartment of Civil, Architectural and Environmental Engineering, University of Texas, Austin, TX 78712, USA. BCenter for Environmental Nanoscience and Risk, Department of Environmental Health Sciences, Arnold School of Public Health, University of South Carolina, Columbia, SC 29208, USA. CCorresponding author. Email: [email protected] 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 environ- mental 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 multi- dimensional 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 multi- Materials development at the nanoscale has progressed from ple NMs, each possessing unique and novel advantages. For single particle synthesis to multi-component assemblies or example, nanoscale iron oxide, nanogold and graphene hierarchical structures, where two or more pre-synthesised nanosheets individually possess paramagnetism, plasmon reso- nanomaterials (NMs) are conjugated to extract multi- nance and superior charge carrying capability respectively. functionality.[1] These ensembles are termed as nanohybrids However, careful combination of two or more of these materials (NHs).[2,3] The underlying focus of NH synthesis is property enhanced their functional performance as observed in the modulation, which results in alteration to inherent physico- development of the first sets of bimetallic NHs. Iron oxide when chemical properties, i.e. size, shape, composition and surface conjugated with gold to form core–shell particles, provided chemistry. Such changes also give rise to novel emerging inherent magnetism of the iron oxide shell, while preserving properties[4] that are not observed during classical NM health the surface plasmon resonance of the gold core.[7] Such multi- and safety (EHS) evaluation. This new direction in NH synthesis functional bimetallics were used as magnetic resonance imaging and use thus presents unique challenges and necessitates (MRI) agents with added nanoheating capabilities, useful for systematic evaluation of nano EHS. laser irradiated drug delivery systems.[8] Similarly, gold, when Demand for multifunctionality has resulted in physical and intercalated within layered clay, was used for protein or organic chemical modification to NMs, in general. Size and shape molecule immobilisation, applicable for biocatalysis and modulation alongside physical or chemical functionalisation sensors.[9,10] Paramagnetic iron oxides, in contrast, when com- are used to achieve hierarchical[5] and heterostructures.[6] Such bined with novel graphene oxides, resulted in unique drug functionalisation has altered inherent surface attributes and delivery systems with superior drug release and targetability.[11] extracted novel electronic configuration, intrinsic hydropho- Again, graphene nanosheets have also been combined with bicity, dissolution properties, etc., from nanoscale materials. porphyrins, titanium dioxide (TiO2), carbon nanotubes, quan- The successes of such manipulations have further encouraged tum dots, etc., and have generated NHs for enhanced optical Journal compilation Ó CSIRO 2014609 www.publish.csiro.au/journals/env N. Aich et al. emitting[12] and limiting[13] devices, supercapacitors,[14] lithium- from the knowledge-gap of ‘conjugated materials’ in an envi- ion batteries[15,16] or transparent conductors.[17] It is evident that ronmental setting – because an ensemble of multiple materials benefits of conjugation and ensembles of multiple materials are will most likely behave differently compared with their parent well realised and thus will likely widen the NH material domain, components. For example, carbonaceous NMs (CNMs), such as affecting a much larger application space and in large amounts. fullerenes[24] and carbon nanotubes (CNTs),[25] show a high For example, it is projected that by the year 2050, at least aggregation propensity due to their inherent hydrophobicity and 1.0 Â 107 kg of platinum carrying titania-modified multiwalled strong van der Waals interaction forces; whereas, metallic carbon nanotube (MWNT) NHs will be deployed in fuel nanomaterials (MNMs) (such as silver or zinc oxide), possess cells for vehicles alone, assuming 20 % platinum in the NH unique dissolution and complexation properties.[26,27] When by mass.[18,19] combined, behavioural manifestation of metal–carbonaceous The development of novel materials comes with an intrinsic conjugates can either present dominant hydrophobicity or dis- uncertainty regarding their potential environmental and biologic solution–complexation reactions; which will be influenced by consequences. Material release can occur from nano-laden the nature of conjugation. Thus risk evaluation of these hierar- products and devices as well as during their manufacture and chical NHs will require systematic environmental studies. use.[20] Upon release, NMs undergo transport and transforma- This account presents an EHS-relevant definition of hybrid tion in either occupational or environmental settings.[21] Such NMs, classifies the NHs, reviews the NH literature, and dis- processes are highly influenced by the material attributes and the cusses the need for environmental studies. Probable environ- form of release; e.g. NM release from personal care products and mental exposures of NHs and relevant altered fate, transport and medicinal applications will possess distinctive physicochemical toxicity as a result of transformed physicochemical and emer- properties compared with their release from solid-state gent properties are discussed. Challenges regarding the predic- optoelectronic systems. As the material complexity increases tion of environmental behaviour of NHs from their individual with conjugation and assemblages of materials with uniquely component characteristics are also delineated. Overall, this different properties, their environmental processes will also be account will serve as an environmentally relevant summary of altered and likely present higher uncertainty when predicted the ever-expanding class of NHs, and hopefully will accentuate using their parent material classes. To date, environmental fate, the importance of evaluating these nano-ensembles for transport and transformation literature of NMs have systemati- enhanced risk assessment. cally generated a critical information mass – by measuring physicochemical properties and their influence on environmen- Defining nanohybrids tal behaviour manifestation – that has begun to effectively Definitional ambiguities are evident in NH literature[28] similar to determine material safety and risk.[22,23] However, the uncer- the debate that exists for singular nanoscale materials (National tainty of environmental behaviour for hierarchical and conju- Nanotechnology Initiative, see http://www.nano.gov/nanotech- gated materials continues to prevail. The uncertainty emanates 101/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
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