Déborah Rupert May 22, ’18 ~ 6 min

Nanoparticle characterization: an overview of key physicochemical parameters

Nanoparticle suspensions are complex systems. So complex that understanding their interaction with their environment requires covering a broad range of physicochemical properties to get a full picture of what is happening at the nanoscale. The larger the umbrella of parameters available the better, but measuring them all can be a daunting task. So which properties are important and why? Here we present a brief overview of the wide range of physicochemical parameters that can be used to profile nanoparticle suspensions.

What is a nanoparticle?

Nano-objects come in various shape and sizes. They are classified according to their geometry and by how many of their key coordinate axes are confined within the 1-100 nanometer size range (ISO 2008). A nanoparticle is confined within the 1-100 nm nanometer in all directions, i.e. it fits in a sphere of 100 nm in diameter. A nanotube has two directions in the 100 nm range (diameter of the tube) but one dimension extends beyond 100 nm (length of the tube). A nanosheet only has one dimension confined within the 100 nm (thickness of the sheet) while the two other dimensions extend beyond 100 nm (the area of the sheet).  Nano-objects and nanoparticles can be found in nature in various forms such as volcano ash or viruses. They can also be fabricated synthetically with the aim to improve quality of life and performance of products. A thorough characterization of their physicochemical properties is required to understand how they interact with their environment and to assess the potential hazard risks they may pose to health and the environment.

The matter of size

Due to their intrinsic nanometer size range, nanoparticles have distinctive physicochemical properties compared to the bulk material they are composed of. This originates from the high surface area to volume aspect ratio which gives them a strong surface reactivity. Variations in nanoparticle size and geometry parameters generate new interesting functionalities but also raises potential hazard risks. This can be seen at the cellular level where the size of nanoparticles impacts cell toxicity from how they interact with the cell membrane to how they enter the cell and interact with the inner machinery. Geometry aspects also play a key role in how light interacts with nanoparticle suspensions through the so-called plasmonic effect which can be used for sensing changes in the nanoparticle environment.

Geometrical aspects to characterize nanoparticles include:

  • particle and hydrodynamic size
  • surface area
  • morphology
  • surface roughness
  • structure

The material also matters

The material composition of engineered nanoparticles is key to engineer nanomaterials with specific performance properties. Chemical composition can range from single raw material to complex multi-layered and composite objects. The outer surface composition plays a role in the interaction between nanoparticles and their environment. In nanomedicine, nanoparticles can be designed to affect specific cells in the body by functionalization of their surface with key molecules that act as target agent. Nanoparticles can also be decorated with coatings to protect their core material from harsh conditions. Coatings may naturally occur through the interaction between nanoparticles and their environment. For example, nanoparticles dispersed in biological fluids become coated with a layer of proteins called corona which becomes the new outer surface of the nanoparticles, bringing new properties to nanoparticles that need to be assessed for potential biohazard risks.

Parameters to be explored in the context of material composition and its interaction with the environment include:

  • particle density
  • molecular composition
  • surface chemistry
  • surface reactivity
  • surface charge

The difference between one and many – concentration and polydispersity

Beyond characterization of nanoparticles as single objects, nanoparticle suspensions can also be seen as particle ensembles. This gives access to new parameters such as nanoparticle concentration which is a key parameter during fabrication and functionalization of nanoparticles. Concentration and dose of exposure are also strategic parameters required to understand the effect on cells and perform biohazard risk assessment.  Since not all nanoparticles in a suspension are the same, the complexity of nanoparticle suspensions can be reflected by its degree of polydispersity in size.

Profiling of nanoparticle suspensions include:

  • number concentration
  • mass concentration
  • polydispersity index

What happens over time – stability and functionality with aging

Nanoparticle suspensions are susceptible to aging depending on their environment. It is therefore determinant to monitor their ability to remain stable and retain functionality over time. Aging may cause nanoparticle suspensions to aggregate or dissolve, generating by-products causing potential unforeseen biohazards risks.

Parameters to monitor in this context include:

  • dispersibility
  • solubility
  • agglomeration
  • aggregation behavior

The complexity of characterization

Overall, the complexity of nanoparticle suspensions is reflected by the diversity of physicochemical properties available to characterize their interaction with their environment. Which key parameters to consider depend on the study at hand. Deeper understanding and insights can be reached by broadening the profile of characterization.

Read more about the characterization of nanoparticle interaction with biological systems in our application note:

Download  Screening nanoparticle - protein interactions  Application note

 

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