The challenge of measuring nanotoxicity
With any material comes the potential for a harmful or damaging interaction with a biological system, which we refer to as toxicity. There are three-types of toxicity, including biological, chemical and physical and all depend on the mode of action from a material with a biological system. Not surprisingly, the effect of toxicity can be varied with minor and short-term effects being experienced all the way through to cumulative long-term effects and potential death of an organism.
To try to understand nanomaterials toxicology we must consider the type of nanomaterials that might interact with a biological system. Nanomaterials can be separated into three categories, including zero-dimensions (quantum dots), one-dimension (carbon nanotubes and nanoparticles) and two-dimensions (thin-films). This list is not exhaustive but is demonstrative of materials at this scale. Nanomaterial production can also be split into biological (microorganism production of nanoparticles), chemical (synthesis of nanoparticles and chemical vapour deposition (CVD) of thin-films and carbon nanotubes) and physical (physical vapour deposition (PVD) production of thin-films and nanoparticles) methods. Nanoscale materials produced by these methods can vary in size for all three dimensions, have different; elemental compositions, size distributions depending on production method, crystal structures, surface roughness, surface architectures, stabilities and a plethora of interactions with complex solutions such as blood and bacterial solutions. With this level of variation within some nanomaterials samples, we can start to see that measuring and understanding nanomaterials toxicology is not without challenge.
Arguably the first step towards understanding material toxicology is to characterise the material. In many ways this sounds a routine task easily within the capability of many laboratories, and in many ways this is true. However with the end goal being to correlate material characteristics with toxicity, this task is not as simple as it would seem. The first problem often encountered is the lack of cross training and communication between the sciences. The challenge is to convey material characterisation data to toxicologists and vice versa. What we can end up with (and often associated with protection of intellectual property) is material characterisation being carried out by scientists trained in material characterisation, with limited characterisation data being disseminated to the toxicologists assessing the toxicology. This can result in virtually no understanding of what nanomaterial property results in toxicity. It also raises the question, what biological system was tested for toxicology? And at what stage of the product life cycle was it tested for – considering that product life cycles have a production, usage and disposal stage. Testing at stages other than at the end user is rare and leaves unanswered questions about the toxicity of a product at the other stages.
Conversely, when research to elucidate nanomaterials toxicology is carried out by scientists with a lack of knowledge of materials characterisation, the methods of characterisation are often limited, which may also be linked with a lack of access to specialist equipment. An example of this is found in microorganism production of nanoparticles from metallic salts, where microorganism growth parameters are varied to examine the production of nanoparticles. Interestingly, the characterisation of nanoparticles is commonly limited to the use of UV-visible spectrophotometry, which is often present in biological laboratories.
This again limits the understanding of the mechanisms of toxicity, where there is the realisation that a nanomaterial sample is toxic but with little knowledge of what is causing the toxicity. Unfortunately we are not yet at a stage scientifically where we can say that a certain nanomaterial characteristic results in an understood mechanism of toxicity. To increase the scientific understanding, it is important that business and academia more rigorously examine these effects.
Measuring nanotoxicity
To try to understand the challenges of understanding nanomaterials toxicology, we shall examine a hypothetical nanoparticle solution. Although a nanoparticle example is given, there are potential similarities with other nanoscale systems such as nanostructured thin-films. Nanoparticle technology is based upon the production of particles within a 1 – 100 nm range.
The variation in nanoparticle composition can be vast as we can have metallic, organometallic, organic and even what some might say are biological nanoparticles (viruses). The measurement of nanoparticle toxicity is also related to the nanoparticle product life cycle and its application. One of the first challenges for measuring nanoparticle toxicity is to decide what aspect of toxicology is being tested for. Is this simply a case of exposing bacteria, mammalian cells, DNA or an animal model to nanoparticles and looking for a damaging effect? For a simple answer, we could say yes, but this is a very limiting and quite misleading view. This leads to the crux of nanomaterials toxicity testing, is understanding required of what causes toxicity important? Or just that a material is or is not toxic?
Ideally, we should be working towards understanding the cause of toxicity and its mechanisms, as with advances in materials engineering and science, there is the possibility of producing the nanomaterial without its toxic functionality. For this to be achieved however, the question must again be asked, what is causing the toxicity? From a materials point this is difficult to answer, as there are many factors, particularly with different nanoparticle solutions varying in distributions of size, geometries, crystal structures, material desorption and functionalisation (if present). It is rare that these factors will all be considered with it being far more likely to consider nanoparticle toxicity as being related to an increase in surface area to volume ratio or to do with the chemical composition.
To reduce the concept of toxicity to surface area neglects all previous potential factors and steps away from the concept of ‘structure being related to function’. For any nanomaterials product, the ideal situation is to deduce how the nanomaterial interacts with a biological system (not necessarily every part of that system though) at a bulk and molecular scale.
At Spartan Nano, we are particularly interested in the mechanisms that result in toxicity and are currently working with a range of microbial and eukaryotic systems to elucidate and remove potential product toxicity. We believe that measuring the underlying chemical and physical characteristics of a material is paramount to understanding toxicity. This typically involves a battery of analytical techniques such as atomic force microscopy to measure surface roughness and surface structuring, ellipsometry and Rutherford backscattering for material thickness, quartz rate monitor to measure material oxidation and fouling and inductively coupled plasma mass spectrometry to measure material desorption. Once these tests and others have been carried out, we are in a position to be able to examine the material from a toxicological point of view. Using the product life cycle of the material, we are able to expose different aspects of a material to different biological systems. An example of this is the loss of a material from a thin-film via desorption, would lead us to test this material loss against an appropriate biological model to assess its relevance as a toxic agent.
Future challenges
Approximately 60% of all synthesized drug candiExamining nanomaterials toxicology is not about creating hypothetical scenarios that act as barriers to technology commercialisation or scaremongering, but asking fundamental questions that answer questions about the interaction of nanomaterials with biological systems.
The issue of nanomaterials toxicity is not just a scientific challenge as it goes well beyond the science of whether a nanomaterial is toxic or not. The issues are wide and complex, including for many materials, bulk and nanoscale products being legislatively regarded as the same. Not only can this result in a lack of testing for nanomaterial toxicology but also should there be a problem with toxicity, it has a great potential to damage consumer confidence.
With many non-scientists and non-engineers not knowing what nanotechnology is, and whether ‘nano’ is smaller than an atom or the other way around, there is a real possibility for mistrust of nanoscale products. It is interesting to consider other easily misunderstood technologies such as genetically modified organisms (GMOs) and the damage that negative public perception did to the GMO markets. If a move towards a greater level of toxicity testing is carried out, consumer confidence can be maintained, and toxic functionalities removed from products.
About the author
Dr Andrew Dean is Chief Executive Officer at Spartan Nano (www.spartannano.com), based in Durham, UK. Spartan Nano was formed in 2008 to understand and commercialise fundamental research in the biophysical sciences. Over this time Spartan Nano has worked towards understanding biological interactions with nanomaterials. These core competencies have led to the development and use of an array of analytical procedures for the determination of nanomaterial behaviour with biological systems.
