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'.

This perspective proposes principles to identify nanohybrids with novel properties relevant to nano EHS research, and discusses specific challenges for EHS research on these materials.

Conjugation of multiple nanomaterials has become the focus of recent materials development. This new material class is commonly known as nanohybrids or "horizon nanomaterials". Conjugation of metal/metal oxides with carbonaceous nanomaterials and overcoating or doping of one metal with another have been pursued to enhance material performance and/or incorporate multifunctionality into nano-enabled devices and processes. Nanohybrids are already at use in commercialized energy, electronics and medical products, which warrant immediate attention for their safety evaluation. These conjugated ensembles likely present a new set of physicochemical properties that are unique to their individual component attributes, hence increasing uncertainty in their risk evaluation. Established toxicological testing strategies and enumerated underlying mechanisms will thus need to be re-evaluated for the assessment of these horizon materials. This review will present a critical discussion on the altered physicochemical properties of nanohybrids and analyze the validity of existing nanotoxicology data against these unique properties. The article will also propose strategies to evaluate the conjugate materials' safety to help undertake future toxicological research on the nanohybrid material class.

A facile synthesis technique to generate 100s of mg of nanohybrids with carbon nanotubes and metal oxide of choice.