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Understanding engineered nanomaterials


In recent years there has been ever-increasing activity and excitement within the scientific and engineering communities, driven heavily by government investment, about engineered nanotechnology applications. The US National Science Foundation has estimated that the global nanotechnology market could be worth US $1.2 trillion by 20201. In parallel, much has been written and presented about the excitement and possible dangers of these materials. The tone of these media articles range from how these wonder materials are going to revolutionize all aspects of our lives to how they might harm us!

What follows is a basic introduction to engineered nanomaterials. It provides insight and appreciation of some of the potential new applications of these materials along with a discussion of the wide range and types of measurements needed to characterize them.

What is nanotechnology?

Nanotechnology is the science and technology of precisely manipulating the structure of matter at the molecular level. The term nanotechnology embraces many different fields and specialties, including engineering, chemistry, electronics, and medicine, among others, but all are concerned with bringing existing technologies down to a very small scale, measured in nanometers.2 Processes and functionality take place at the nanoscale, exhibiting properties not available in the bulk material. But what is a nanometer? Figure 1 compares the nano-region to things we know, such as a pin, insect and cells and provides a visual perspective.

A nanometer is a thousandth of a micron and a micron is a thousandth of a millimeter, so a nanometer is a millionth of a millimeter or 10-9 meters. To be classified as a nanomaterial (NM), the material must be less than 100 nm in size in at least one direction. According to the International Standards Organization® (ISO) a nano-object is a material with at least one, two or three external dimensions in the nanoscale range of 1 to 100 nm and a nanoparticle is a nano-object with all three external dimensions in the 1 to 100 nm range and showing a property not evident in the bulk material. Hence, a nanofiber, 400 nm long and 12 nm in diameter, and a 20nm diameter nanoparticle, are both classified as NMs.3

NMs are materials that range in size from approximately 1 nm to 100 nm. There are more rigorous definitions that are specific to certain applications such as cosmetics. In Europe’s efforts to label cosmetics that contain nanoparticles, this definition evolved: “nanomaterial means an insoluble or biopersistent and intentionally manufactured material with one or more external dimensions, or an internal structure, on the scale from 1 to 100 nm.” 3

Different size characteristics of the various NMs are summarized below:

Type of NM and number of dimensions and size:

Nanoparticle:

Three dimensions in the 1 to 100 nanometers (nm) range

Nanotubes/nanowires:

Two dimensions in the 1 to 100 nm range

Nanofibres:

Length ranges between 50nm and 300nm with diameter <50nm

Nanofilms:

One dimension in the 1 to 100 nm range

Nanoplates:

Two dimensions in the 1 to 100 nm range

Engineered nanoparticles are of great scientific interest. They effectively bridge a gap between bulk materials and atomic and molecular structures. Nanoparticle mechanical properties are different than bulk material. Surface area is disproportionate to weight, for instance, an 8nm gold material has a surface area of 32 sq m per gram.

Engineered NMs

Natural occurring NMs

Gold, silver, copper, selenium, iron, titanium, zinc, and aluminium

Ubiquitous in ground water, fresh water, and sea water

Zinc oxide, titanium oxide

Iron oxyhydroxides

Carbon

Manganese oxides

Clay

Aluminium hydroxides and alumina silicates

Organic materials

Humic substances (humic and fulvic acids) and acid polysaccharides

Material parameters

To completely characterize NM, it is necessary to know a multitude of chemical and physical pararmeters including: the size of the particle, shape, surface characteristics, the presence of surface coatings, and the presence of impurities.

Consequently, at the nanoscale, analytical measurement challenges are considerable. The choice of just one analytical technique for manufacturing quality control may be insufficient. An example is choosing to measure the elemental concentration of gold in a suspension by inductively coupled plasma-mass spectrometry (ICP-MS) as the only metric. This may be incomplete to control a manufacturing process. Complementary characterization of size, size distribution, or shape may be required to best control the process.

Measuring engineered NMs

Seven of the nine NM characteristics – particle size distribution, surface charge, surface area, shape, agglomeration, and structure – are characterized by one of the following analytical techniques: scanning electron microscopy (SEM); transmission electron microscopy (TEM); atomic force microscopy (AFM); confocal microscopy (CFM); dynamic light scattering (DLS); field flow fractionation (FFF); molecular gas adsorption (BET); and electrophoresis particle size. Ultraviolet/visible spectroscopy and fluorescence spectroscopy are used for particle size identification as long as the material is known and is reflective. Fluorescence spectroscopy is also used for agglomeration studies.

Nanoparticle concentration and composition are not covered by the analytical techniques described above but other techniques do cover them. The correct analytical technique is determined by the material, coatings, and nano application and could include inductively coupled plasma and mass spectroscopy (ICP-MS); liquid chromatography and mass spectroscopy (LC-MS); ultraviolet/visible spectroscopy (UV/Vis); or fluorescence spectroscopy (FL).

For nanoparticle composition, there is the choice of these analytical techniques: ICP-MS; LC-MS; UV/Vis; FL; thermogravimetry (TGA); differential scanning calorimetry (DSC); dynamic mechanical analysis (DMA); Fourier transform infrared spectroscopy (FTIR); Raman spectroscopy; thermogravimetry, gas chromatography, and mass spectroscopy (TGA-GC/MS); and thermogravimetry and mass spectroscopy (TGA-MS).

For composition, you may be concerned with purity or the coatings on NMs besides the substrate material of the nanoparticle. The multitude of nano-characterization instrumentation reminds us that there is not one analytical technique that can characterize a NM.

Why is characterization important?

Engineers know that, “If you can’t measure it, you can’t build it.” To understand this, an overview of the NM manufacturing process and value chain is necessary. This includes:

  • Source and quality of raw materials (QA/QC);
  • Control the NM manufacturing process (QA/QC);
  • End product formulation (QA/QC);
  • Incorporation into another product; and
  • End use.

First and foremost is to know why characterizing materials is important for a stable process. Without material characterization (QC and QA), the end product man
ufacturing process will be difficult to control. This results in products that do not meet specification and manufacturing inefficiencies.

Environmental issues

Process waste has always been a manufacturing issue. It is slightly different today when NMs are considered. Nano-waste is different than bulk material waste. Laboratory experiments have shown that NMs can enter the human body by dermal exposure, inhalation, and ingestion. While there are no NM regulations yet, there is increasing review and concern both within industry and in the environmental field as to the fate and behaviour of these materials.4

In the United States, the federal government has established the National Nanomaterial Initiative (NNI) where government agencies and private industry meet to discuss and understand NM implications of the environment and human health.5

The waste interaction with the environment could occur from material taken to a land fill, incinerated, or washed down the drain. Environmental Health, and Safety (EHS) applies to NM-workers as human exposure could occur during the manufacturing process.4, 5, 6, 7

It will take time for manufacturers to identify what NM characteristics are important for their manufacturing process and how to apply this knowledge to specific nano-business processes. Skilled people will be needed to operate the existing analytical instruments and new instruments on the horizon. Lastly is environmental stewardship, to allow NM applications to realize their full potential, there must be safeguards for the environment where needed.5, 6, 7

Additional readings and websites

References

  • 1. Roco, M Nat’l Science Foundation and the National Nanotechnology Initiative, Nanotechnology Research Directions for Societal Needs in 2020, presented at the Woodrow Wilson Center for Scholarship, Dec 1, 2010.
  • 2. American Heritage Dictionary, March 2010. http://dictionary.reference.com/browse/Nanotechnology
  • 3. International Standards Organization (ISO), Tech Spec ISO/TS80004-1 Nanotechnologies –Vocabulary Part 1: Core Terms.
  • 4. Hasselhov, M, Kaegl, R, “Analysis and Characterization of Manufactured Nanomaterials in Aquatic Environment,” Chapter 6 of Environmental and Human Health Impacts of Nanomaterials, Eds Lead, J and Smith, E, Blackwell Publishing Ltd.
  • 5. Klaine, S., Alvareez, J., Bately, G., et al., (2008), Critical Review, Nanomaterials in the Environment:
  • 6. Behavior, Fate, Bioavailability, and Effects, Environ. Toxicol.Chemsitry 27, 1825-1851.
  • 7. National Nanomaterial Initiative (2010): www.nano.gov/
  • 8. PerkinElmer, Nanomaterials Reference Library, (2010): www.perkinelmer.com/nano
  • 9. US EPA, Office of Pollution Prevention and Toxics (2010).

This article appeared in the April 2011 edition of Lab Product News.