Nanotechnology and the Environment - Chapter 5 potx

In addition, the target nanoparticles mayonly be a minor component of an environmental sample and fall below the detec-tion limits of standard EPA chemical analysis methods.. Instrumenta

Nanoparticles in

the Environment

Marilyn Hoyt

AMEC Earth & Environmental

CONTENTS

5.1 Analytical Methods 101

5.1.1 Nanoparticle Imaging: Size, Shape, and Chemical Composition 101

5.1.1.1 Electron Microscopy 101

5.1.1.2 Scanning Probe Microscopy (SPM) 106

5.1.2 Compositional Analysis 108

5.1.2.1 Single Particle Mass Spectrometer 108

5.1.2.2 Particle-Induced X-Ray Emission (PIXE) 109

5.1.3 Surface Area: Product Characterization and Air Monitoring 109

5.1.3.1 The Brunauer Emmett Teller (BET) Method 109

5.1.3.2 Epiphaniometer 109

5.1.3.3 Aerosol Diffusion Charger 110

5.1.4 Size Distribution 110

5.1.4.1 Electrostatic Classifiers 110

5.1.4.2 Real-Time Inertial Impactor: Cascade Impactors 110

5.1.4.3 Electrical Low Pressure Impactor (ELPI) 111

5.1.4.4 Dynamic Light Scattering (DLS) 111

5.2 Workplace Air Monitoring 112

5.2.1 Condensation Particle Counter (CPC) 113

5.2.2 Surface Area: Total Exposure 113

5.3 Sampling and Analysis of Waters and Soils for Nanoparticles 114

5.4 Nanotechnology Measurement Research and Future Directions 115

5.4.1 United States 115

5.4.1.1 NIOSH 115

5.4.1.2 U.S Government-Sponsored Research 117

5.4.1.3 National Institute of Standards and Technology (NIST) 117

5.4.2 European Union 118

5.4.3 Asia-Pacific 118

5.5 Summary 119

References 119

The rapid explosion of production and use of engineered nanoparticles has outpacedthe scientific community’s ability to monitor their presence in the environment.Without measurement data, it is not possible to fully evaluate whether the promises

of nanoparticles are accompanied by significant ecological or human health risks.Numerous national and international agencies and research groups have recognizedthis gap and put in place research programs to address it However, the technicalrequirements for the detection and characterization of nanoparticles in complexenvironmental systems push the limits of current sampling techniques and instru-mentation In most cases, multiple complementary measurements are likely neces-sary to detect and understand the importance of nanoparticles in air, water, or soilbecause physical properties as well as chemical composition determine activity andenvironmental impact or risk Environmental analyses of nanoparticles are not com-mon offerings at commercial environmental laboratories at this time, and they arenot likely to become so in the near future

In the manufacturing industry, the development and production of cle materials for commercial applications are supported by an array of analyticalmethods While numerous methods can successfully characterize the chemistry andphysical properties of nanoparticles in relatively pure states and under defined condi-tions, the applicability of these methods to nanoparticles in environmental settingsmay be more limited Once nanoparticles enter the environment, they may cluster toform larger particles, interact with particles from natural sources, or change chemi-cally Conventional environmental analysis methods as developed and standardized

nanoparti-by the U.S Environmental Protection Agency (EPA) are bulk analyses; they candetect the primary chemical constituents of nanoparticle materials but little else

of use for characterizing risk from them In addition, the target nanoparticles mayonly be a minor component of an environmental sample and fall below the detec-tion limits of standard EPA chemical analysis methods Collection and separation ofnanoparticles from larger environmental particles, when even possible, are difficult,and their analysis is in most cases time-consuming and costly No standard methodswith prescribed quality control requirements for environmental nanoparticle analy-ses exist, and only limited traceable standards have been developed

Aside from the technical challenges to nanoparticle measurement in mental media, the lack of specific regulations limits the incentive for commercialenvironmental laboratories to put in place the costly instrumentation and the highdegree of expertise that will be required to offer nanoparticle analyses to government,private industry, or public groups While there is some concern for possible envi-ronmental risks from nanoparticles, manufacturers, users, and site owners currentlyare not required to address these concerns with actual environmental measurementdata As a result, most technical advances and data that do exist for environmen-tal analyses have come from academic laboratories and governmental or privatelyfunded research laboratories The applicability of regulatory statutes as discussed in

environ-Chapter 4of this book continues to be debated The Toxic Substances Control Act(TSCA), the Clean Water and Clean Air Acts (CWA, CAA), the Resource Conserva-tion and Recovery Act (RCRA), and the Federal Insecticide, Fungicide, and Roden-ticide Act (FIFRA) drove method development for numerous industrial chemicals

in the environment Regulatory requirements applicable to nanomaterials likewise

would be expected to drive the development and standardization of environmentalnanoparticle analytical methods for wider application, as well as to foster competi-tion in an emerging market for laboratory services Instrumentation and staffingcosts will, however, remain a barrier to entry into the field for most commerciallaboratories currently offering environmental services.

5.1 ANALYTICAL METHODS

The production of nanoparticle materials typically requires control of the chemicalcomposition, size, shape, and surface characteristics of the material Many of theanalytical techniques applied for the analysis of nanoparticles during developmentand production also are critical to laboratory studies of fate and transport and expo-sure effects to ensure that the material being tested is fully understood These meth-ods also may be components of analyses to detect nanoparticles after their releaseinto the environment, dispersion in air or water, or uptake into organisms [1].This chapter discusses highlights of the most widely used techniques, provid-ing the basic science of the analyses and describing the type of information thatcan be expected and reported for possible environmental applications These tech-

researchers to address environmental issues; it is likely that over time, other currenttechniques or newly developed instrumentation will also prove useful Representa-tive citations are provided where methods have proven successful for analyses ofnanoparticles present in air, water, or soils However, it should be noted that mostenvironmental analyses reported to date for nanoparticles have focused on naturalspecies such as colloids in water or on combustion-related emissions Engineerednanoparticles have been characterized in laboratory studies and in indoor air moni-toring programs, but only limited studies designed to detect their releases into or fate

in ambient air, surface or ground waters, or soils or waste have been reported [2].More in-depth discussions of the theoretical basis for each measurement tech-nique, specifics for instrument design, detection options, and data examples can befound in a review article [3] that discusses more than 30 measurement techniques

in detail, presenting the theory and advantages and limitations to each tory analyses, real-time methods, and portable instrumentation for particulatecharacterization from mobile source emissions are reviewed in a literature surveyfor the California Air Research Board (ARB) [4] Many of the methods discussedand equipment illustrated are also potentially applicable to measurement of nanopar-ticles from other sources in the environment A recent U.S EPA symposium onnanoparticles in the environment discussed the challenges involved, and also pre-sented highlights of applicable measurement methods [5]

Labora-5.1.1 NANOPARTICLE IMAGING: SIZE, SHAPE,ANDCHEMICALCOMPOSITION

5.1.1.1 Electron Microscopy

Electron microscopy is comparable to light microscopy, except that a beam of trons rather than light is used to form images Electron beams have a much shorterwavelength than light and, as a result, they can provide the resolution required to

Methods for Environmental Analyses of Nanoparticles

Technique Parameters Measured Resolution/Sensitivity Limitations/Advantages Environmental Applications

1 nm SEM, <0.1 nm TEM Particle-by-particle analysis,

time-consuming Sample preparation, high vacuum for SEM, TEM may alter particles ESEM allows imaging in water or other liquid media

Ambient air studies [11], nanoparticle characterization for laboratory studies of fate, toxicity [7–10]

Scanning probe microscopy

(STM, AFM)

Analysis at ambient pressure, particles may be in solution

Ambient air studies, natural colloids [15–17, 20, 21]

Characterization for laboratory studies of fate, toxicity [29]

m 2 /cm 3

Requires radioactive lead source Ambient air studies [30]

will not necessarily be same as from imaging technique

Releases during nanopowder use [33]

aerodynamic diameter

<30 nm diameter <10 nm (MOUDI)

Time-integrated average distributions; particles collected may be analyzed subsequently by microscopy

Ambient air studies, vehicle emissions [35]

Electrical impactor (ELPI) Particle distribution based on

Particle Concentration/Surface Area in Air

Condensation particle counter Particle concentration in air

stream

shape composition Hand-held units available, real-time data.

Indoor air monitoring, worker exposure studies [43]

Electrical aerosol detector Aerosol diameter concentration,

calculated from a number concentration multiplied by average diameter

field-portable instrumentation

Ambient air studies [45]

Particles in Aqueous Samples

ng/L for elemental composition

Must be combined with subsequent analysis to assess size, (e.g., DLS).

Can combine with ICPMS, ESEM.

Natural colloids, iron oxide/

hydroxide colloids [49, 50]

form clear images of nanomaterials There are two major types of electron copy: (1) transmission electron microscopy (TEM) and (2) scanning electron micros-copy (SEM) As a beam of electrons hits the surface of a particle or film, electronscan be deflected off the surface or, in collisions with atoms of the material, releaselight, knock off secondary electrons from atoms in the material, or cause the emis-sion of x-rays Some electrons also pass through the material, either directly or withsome scattering due to collisions with the particle atoms (Figure 5.1).

micros-With SEM, emissions from the top of a surface impacted by the electron beamare detected and measured A variety of instruments can be used to detect the back-scattered electrons, secondary electrons, x-rays, or light generated above the surface.Each detector adds its own acronym to the analysis technique (e.g., EDS [energydispersive x-ray spectroscopy], EDX [energy dispersive x-ray], and XEDS [x-rayenergy dispersive spectroscopy] all refer to x-ray detection techniques that providestructural or chemical composition information when paired with SEM) Auger elec-tron microscopy or spectroscopy (AEM or AES), which measures the energy of

FIGURE 5.1 Electron microscopy (From J Mansfield, University of Michigan Withpermission.)

ejected electrons, also is useful for elemental composition information Paired withthese different detectors, SEM can provide information on the size and shape of aparticle, three-dimensional topographic information on surface features and texture,crystalline or amorphous structure, and elemental composition The technique ismost useful for measurements of particles in the range of 50 nanometers (nm) orhigher, although stronger electron sources can achieve spatial resolution of 1 nm.More advanced detectors are available now that can charactize the difference inchemistry between the top 2 nm of a particle and its interior.

With TEM, the measurements are taken underneath the material The portion ofthe electron beam that passes through the particle can be projected onto a fluorescentscreen to form a two-dimensional image of the particle Resolution of less than 0.1

nm can be achieved, making it a primary tool for characterization of the smallestnanoparticles As with SEM, a variety of detectors can be used to detect scatteredelectrons and x-rays released by the interactions of the electron beam with the atoms

of the particles TEM analyses can be designed to determine the elemental tion of the particle and the chemical bonding environment, particle shape and size,and its crystalline or amorphous structure TEM also can be conducted in a scanningmode (STEM), where the narrowly focused electron beam scans over the particle formaximum sensitivity and resolution A more detailed introduction to TEM is avail-able on the Internet [6]

composi-Researchers frequently use SEM and TEM to characterize nanoparticles beforetheir use in laboratory experiments and to monitor progress or results TEM has been

nanoparticles absorbed into red blood cells; and Sipzner et al [10] monitored the

Reported environmental applications include the use of SEM and TEM to terize fine and ultrafine particulates present in ambient air In an urban air study [11],Utsunomiya et al conducted analyses using several TEM techniques to characterizethe particulate size associated with heavy metals and to speciate the metals detected.Metals of particular interest for engineered nanomaterials — titanium, iron, andsilver — were all detected in nanoparticles Titanium and iron were present at com-paratively high concentrations and were attributable to fractal rock and numerousnatural and anthropogenic sources, highlighting the difficulty of determining poten-tial air sources from the manufacture or use of zero-valent iron or titanium dioxidenanoparticles against naturally high backgrounds Silver was present at low levels,primarily associated with soot particles, and tentatively attributed to backgroundcombustion sources

charac-SEM and TEM provide invaluable information for many purposes They do,however, have several limitations for environmental applications Although SEMhas a larger field of view than TEM, both SEM and TEM can analyze only a rela-tively small number of particles at a time Representativeness for a nonhomogeneoussample is difficult to achieve The instrumentation is costly and requires a high level

of technical expertise to operate properly The sample preparation and analysis aretime-consuming The particles must be deposited on a support film, and the differ-ent ways of achieving this deposition may allow some nanoparticles to aggregate

or to fragment, losing some of the characteristics responsible for their activity ForTEM, nonconductive materials must be coated with a conducting material such asgraphite, potentially obscuring critical features On most available instruments, thesample must be at high vacuum during analysis, and results for nanoparticles withvolatile components, such as hydrated salts or oxides, may not be representative forthe material as it exists outside the vacuum.

Environmental SEM (ESEM) instruments have been developed recently thatutilize differential pressure zones These do allow analyses with the sample at pres-sures closer to atmospheric, and ESEM instrumentation also can be modified toallow imaging of nanoparticles while in suspension in water or other liquid media.Condensation, evaporation, and transport of water inside carbon nanotubes have

and silica nanoparticles and carbon nanotubes dispersed in water using this nique, which they have named “wet scanning transmission electron microscopy,”(wet STEM)

tech-5.1.1.2 Scanning Probe Microscopy (SPM)

Scanning probe microscopy (SPM), a relatively newer tool, provides a true dimensional surface image SPM includes a variety of different techniques, includ-ing atomic force microscopy (AFM) and scanning tunneling microscopy (STM),which have proven useful for imaging and measuring materials at the nanoscale.SPM techniques are based on a mechanical survey of the surface of an object or par-ticle A very fine tip mounted on a cantilever scans over the surface of interest, fol-lowing the surface profile Interactions between the tip and the surface deflect the tip

three-as it follows the surface profile The movement of the tip in response to the tion can be monitored with a laser reflected from the cantilever to a photodiode array(Figure 5.2) STM monitors the weak electrical current induced as the tip is held aset distance from the surface STM, under some conditions, can provide chemicalcomposition information for the surface With AFM, the tip responds to mechanicalcontact forces as well as atom-level interactions between the tip and surface (such aschemical bonding forces, van der Waals forces, or electrostatic forces)

interac-Since their development in the late 1980s, both techniques have found wideapplication for nanotechnology materials development, as illustrated by the

environmental applications AFM can be operated at ambient pressure and can acterize a wide range of particle sizes in the same scan, from 1 nm to 8 μm (microm-eter) It can analyze particles on a solid substrate at atmospheric pressure or in aliquid medium such as water It has been used to characterize the morphology andsize distribution of nanometer-sized environmental aerosol particles collected from

distribu-tion and morphology of natural aquatic colloids, which play important roles in taminant binding, transport, and bioavailability, also have been characterized withAFM after their absorption onto a mica substrate [15–17] A detailed discussion ofAFM is provided in the review article by Burleson et al [3]; further information on

con-FIGURE 5.2 Atomic force microscopy (From A Nadarajah With permission.)

FIGURE 5.3 STM images of buckyballs (From Nanoscience Instruments With permission.)

applications of and images from AFM for nanotechnology are available on ment manufacturers’ websites [18, 19].

instru-5.1.2 COMPOSITIONAL ANALYSIS

5.1.2.1 Single Particle Mass Spectrometer

Mass spectrometry forms the basis of several U.S EPA methods for environmentalsample analysis on a bulk basis, providing chemical composition data on an ele-mental level for metals, and on a molecular level for organics Mass spectrometryalso applies to the analysis of single particles on a real-time basis, although theinstrumentation has major differences from mass spectrometers used in U.S EPAmethod analyses The single particle mass spectrometer, first developed in the 1970sfor atmospheric aerosol research, analyzes particles from a continuous air streamdrawn directly into the ion source Both organic and inorganic constituents can bedetected and identified The instrument has been widely used for air monitoringstudies of particles with aerodynamic diameters in the low micron range [20, 21], butthe technology has been extended now to the nanoparticle range

Most current single particle mass spectrometers are time-of-flight instruments,with some that can detect and analyze particles down to 3 nm in diameter [22] As

a solid particulate or droplet suspended in the air stream enters the source region ofthe mass spectrometer, a pulsed laser beam desorbs and ionizes the particle compo-nents; immediately afterward, a pulsed electric field accelerates all ions of the samecharge to the same energy, after which, depending on their mass and charge, they

“fly” at different velocities to a charged detector Both positive and negative ionscan be detected in some time-of-flight instruments These instruments can be field-deployed and have been used in upper atmospheric studies [23] and for on-site ambi-ent air monitoring [24] Of the nanomaterials specifically discussed in this book,fullerene is the only one for which detection by single particle mass spectrometryhas been reported [25]

A recent modification to the technology adds particle size measurement prior

to the introduction of the particle into the mass spectrometer source These ments, called aerosol time-of-flight mass spectrometers (ATOFMS) [26], employtwo distinct time-of-flight technologies One determines particle size; the otherdetermines particle chemical composition As a particle enters the instrument, asupersonic expansion of the carrier gas accelerates the particle to terminal veloc-ity Because smaller particles reach a higher velocity than the larger particles, theaerodynamic diameter can be calculated from the time it takes the particle to travelbetween two lasers As the particle passes the second laser and enters the massspectrometer source, the high-intensity laser of the source is triggered to hit theparticle and desorb and ionize particle constituents These instruments have beenused for nanoparticle emission studies from vehicle emissions [27] as well as foratmospheric studies [23]

instru-5.1.2.2 Particle-Induced X-Ray Emission (PIXE)

PIXE measurements can provide major, minor, and trace constituent analyses ofnanoparticles The instrument directs a beam of protons from a high-energy particleaccelerator that will knock out core electrons from the atoms of the sample X-raysare then emitted when outer shell electrons drop into the orbital from which the pro-ton-ejected electron came The resulting x-ray spectrum of the sample can be usedfor elemental identifications The requirement for a particle accelerator to generatethe proton beam makes PIXE techniques very costly and available in only a lim-ited number of research laboratories The technique has been used for trace elementanalysis of background aerosol particles in the heavily polluted air of Mexico City[28], but it is likely to remain a research tool with limited use

5.1.3 SURFACE AREA: PRODUCTCHARACTERIZATION AND AIR MONITORING

Surface area is a critical parameter influencing the properties and activity of ticles In large part, this is believed due to the comparatively high number of atoms onthe surface of the particle as opposed to larger particles where most atoms are interior.Surface areas for individual particles can be estimated from the imaging techniquesdiscussed above, but techniques for determining the average surface area for a bulksample of nanoparticles are more commonly used to monitor production of nano-materials for specific uses Some of these methods also are applicable for materialscharacterization before laboratory exposure studies, and for environmental samples

nanopar-5.1.3.1 The Brunauer Emmett Teller (BET) Method

The BET method is named for the three scientists who recognized that particulatesurface area can be determined based on the volume of gas that will adsorb to thesurface of a given mass of sample The BET equation relates the volume of gasadsorbed to form a monolayer, the size of the gas molecules, and the mass of thematerial to derive surface area per unit mass Commercial analyzers are availablethat perform this measurement, which may be used during development and produc-tion In a representative research application, BET measurements were relied uponfor size characterization of nitrogen-doped titanium dioxide prepared as a photocat-

alyst for Escherichia coli disinfection [29] Because BET requires a relatively pure

bulk sample of a chemically homogeneous material, it has not found application forenvironmental analyses

5.1.3.2 Epiphaniometer

The epiphaniometer is a relatively simple device that measures the active surface area

of aerosol particles Particles entering the instrument are charged with radioactivelead ions and then collected on a collection filter The measured total radioactivity

is a measure of the attachment rate, which then allows calculation of the total activesurface area of particles in the sample The requirement for a radioactive source lim-its the wide use of this instrument, but it has been used in research programs such asmobile laboratory studies of on-road air quality as related to traffic emissions [30]

5.1.3.3 Aerosol Diffusion Charger

The same measurement principle as used for the epiphaniometer is applied in sol diffusion chargers but without the requirement for a radioactive source Ions areproduced in a carrier gas by electrical discharge The ions attach to the surface ofthe particles, which are then collected in an electrically insulated particle filter Theelectric charge is converted to a direct current (DC) voltage signal in an electrometeramplifier Studies have shown that these devices provide a good estimate of aero-sol surface area in ambient air when airborne particles are smaller than 100 nm indiameter [31]

5.1.4.1 Electrostatic Classifiers

Electrostatic classifiers operate on the basic principle that the velocity of a chargedspherical particle in an electrical field relates directly to its diameter Particles aresuspended in air to form an aerosol, charged, and then introduced into a cylindri-cal apparatus The classifier has an outer cylinder that is a ground electrode and aninner rod that can have precisely controlled negative voltage applied The chargedparticles are introduced near the wall of the outer cylinder, with a sheath of clean airmoving through the cylinder at a constant flow rate The positively charged particleswill move toward the negatively charged center electrode at a rate determined bytheir operative diameter and the applied voltage Only those particles within a nar-row velocity range will pass through a thin sampling slit near the bottom of the cen-ter electrode Particles exit through this slit into a particle-counting instrument Byscanning the voltage on the central rod, analysts can obtain a full particle-size dis-tribution for the aerosol It should be noted, however, that the particle size measured

is based on the assumption of a spherical shape, and the dimensions of nonsphericalparticles determined by this technique will correlate with but not necessarily equalthose determined by an imaging technique

Various types of (and names for) electrostatic classifiers are in common use.These include the differential mobility analyzer (DMA), nanodifferential mobilityanalyzer (NDMA), the differential mobility particle sizer (DMPS), and the scanningmobility particle sizer (SMPS) Electrostatic classifiers can be used in a variety ofways, including real-time monitoring of the length of carbon nanotubes during syn-

5.1.4.2 Real-Time Inertial Impactor: Cascade Impactors

Cascade impactors have a long history with ambient air monitoring programs, viding size selectivity to the collection of suspended particles These units take