ASTM E2859-11

Standard Guide for Size Measurement of Nanoparticles Using Atomic Force Microscopy

SIGNIFICANCE AND USE
As AFM measurement technology has matured and proliferated, the technique has been widely adopted by the nanotechnology research and development community to the extent that it is now considered an indispensible tool for visualizing and quantifying structures on the nanoscale. Whether used as a stand-alone method or to complement other dimensional measurement methods, AFM is now a firmly established component of the nanoparticle measurement tool box. International standards for AFM-based determination of nanoparticle size are nonexistent as of the drafting of this guide. Therefore, this standard aims to provide practical and metrological guidance for the application of AFM to measure the size of substrate-supported nanoparticles based on maximum displacement as the probe is rastered across the particle surface to create a line profile.
SCOPE
1.1 The purpose of this document is to provide guidance on the quantitative application of atomic force microscopy (AFM) to determine the size of nanoparticles deposited in dry form on flat substrates using height (z-displacement) measurement. Unlike electron microscopy, which provides a two-dimensional projection or a two-dimensional image of a sample, AFM provides a three-dimensional surface profile. While the lateral dimensions are influenced by the shape of the probe, displacement measurements can provide the height of nanoparticles with a high degree of accuracy and precision. If the particles are assumed to be spherical, the height measurement corresponds to the diameter of the particle. In this guide, procedures are described for dispersing gold nanoparticles on various surfaces such that they are suitable for imaging and height measurement via intermittent contact mode AFM. Generic procedures for AFM calibration and operation to make such measurements are then discussed. Finally, procedures for data analysis and reporting are addressed. The nanoparticles used to exemplify these procedures are National Institute of Standards and Technology (NIST) reference materials containing citrate-stabilized negatively charged gold nanoparticles in an aqueous solution.
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

General Information

• Status: Historical
• Publication Date: 30-Nov-2011
• ICS: 07.120 - Nanotechnologies
• Technical Committee: E56 - Nanotechnology
• Drafting Committee: E56.02 - Physical and Chemical Characterization

Standards Content (Sample)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: E2859 − 11
Standard Guide for
Size Measurement of Nanoparticles Using Atomic Force
Microscopy
This standard is issued under the fixed designation E2859; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom-
1.1 The purpose of this document is to provide guidance on
mendations issued by the World Trade Organization Technical
thequantitativeapplicationofatomicforcemicroscopy(AFM)
2 Barriers to Trade (TBT) Committee.
todeterminethesizeofnanoparticles depositedindryformon
flat substrates using height (z-displacement) measurement.
2. Referenced Documents
Unlikeelectronmicroscopy,whichprovidesatwo-dimensional
projection or a two-dimensional image of a sample, AFM 2.1 ASTM Standards:
provides a three-dimensional surface profile. While the lateral E1617Practice for Reporting Particle Size Characterization
dimensions are influenced by the shape of the probe, displace- Data
ment measurements can provide the height of nanoparticles E2382Guide to Scanner and Tip Related Artifacts in Scan-
with a high degree of accuracy and precision. If the particles ning Tunneling Microscopy and Atomic Force Micros-
are assumed to be spherical, the height measurement corre- copy
spondstothediameteroftheparticle.Inthisguide,procedures E2456Terminology Relating to Nanotechnology
are described for dispersing gold nanoparticles on various E2530Practice for Calibrating the Z-Magnification of an
surfaces such that they are suitable for imaging and height AtomicForceMicroscopeatSubnanometerDisplacement
measurement via intermittent contact mode AFM. Generic Levels Using Si(111) Monatomic Steps
procedures for AFM calibration and operation to make such E2587Practice for Use of Control Charts in Statistical
measurements are then discussed. Finally, procedures for data Process Control
analysisandreportingareaddressed.Thenanoparticlesusedto
2.2 ISO Standards:
exemplify these procedures are National Institute of Standards
ISO 18115-2Surface ChemicalAnalysis - Vocabulary - Part
and Technology (NIST) reference materials containing citrate-
2: Terms Used in Scanning-Probe Microscopy
stabilized negatively charged gold nanoparticles in an aqueous
ISO/IEC Guide 98-3:2008Uncertainty of measurement—
solution.
Part 3: Guide to the Expression of Uncertainty in Mea-
1.2 The values stated in SI units are to be regarded as surement (GUM:1995)
standard. No other units of measurement are included in this
standard. 3. Terminology
1.3 This standard does not purport to address all of the
3.1 For definitions pertaining to nanotechnology terms,
safety concerns, if any, associated with its use. It is the
refer to Terminology E2456.
responsibility of the user of this standard to establish appro-
3.2 For definitions pertaining to terms associated with
priate safety and health practices and determine the applica-
scanning-probe microscopy, including AFM, refer to ISO
bility of regulatory limitations prior to use.
18115-2.
1.4 This international standard was developed in accor-
3.3 Definitions of Terms Specific to This Standard:
dance with internationally recognized principles on standard-
3.3.1 agglomerate, n—in nanotechnology, an assembly of
particles held together by relatively weak forces (for example,
This guide is under the jurisdiction of ASTM Committee E56 on Nanotech-
nology and is the direct responsibility of Subcommittee E56.02 on Physical and
Chemical Characterization.
Current edition approved Dec. 1, 2011. Published January 2012. DOI: 10.1520/ For referenced ASTM standards, visit the ASTM website, www.astm.org, or
E2859-11. contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Having two or three dimensions in the size scale from approximately 1nm to Standards volume information, refer to the standard’s Document Summary page on
100nmasinaccordancewithTerminologyE2456;thisdefinitiondoesnotconsider the ASTM website.
functionality, which may impact regulatory aspects of nanotechnology, but which Available from International Organization for Standardization (ISO), 1, ch. de
are beyond the scope of this guide. la Voie-Creuse, CP 56, CH-1211 Geneva 20, Switzerland, http://www.iso.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2859 − 11
Van der Waals or capillary), that may break apart into smaller surface on the back side of the cantilever onto a split
particles upon processing, for example. E2456 photodiode. A schematic diagram of the system is shown in
3.3.1.1 Discussion—Using imaging based techniques, such Fig.1.Thecantileverdeflectionismeasuredbythedifferential
as AFM, it is generally difficult to differentiate between output(differenceinresponsesoftheupperandlowersections)
agglomerates formed during the deposition process (that is, of the split photodiode. The deflections are very small relative
artifacts) and agglomerates or aggregates that pre-exist in the tothecantileverthicknessandlength.Thus,theprobedisplace-
test sample. ment is linearly related to the deflection. The cantilever is
typicallysiliconorsiliconnitridewithatipradiusofcurvature
3.3.2 aggregate, n—in nanotechnology, a discrete assem-
on the order of nanometers. More detailed and comprehensive
blage of particles in which the various individual components
information on theAFM technique and its applications can be
are not easily broken apart, such as in the case of primary
found in the published literature (1, 2).
particlesthatarestronglybondedtogether(forexample,fused,
sintered, or metallically bonded particles). E2456
4.2 Based on the nature of the probe-surface interaction
3.3.2.1 Discussion—Using imaging based techniques, such
(attractive or repulsive), anAFM can be selected to operate in
as AFM, it is generally difficult to differentiate between
various modes, namely contact mode, intermittent contact
aggregates and agglomerates.
mode, or non-contact mode. In contact mode, the interaction
between the tip and surface is repulsive, and the tip literally
3.4 Acronyms:
contacts the surface. At the opposite extreme, the tip interacts
3.4.1 AFM—atomic force microscopy
with the surface via long-range surface force interactions; this
3.4.2 APDMES—3-aminopropyldimethylethoxysilane
is called non-contact mode. In intermittent contact mode (also
3.4.3 DI—deionized
referred to as tapping mode), the cantilever is oscillated close
3.4.4 HEPA—high efficiency particulate air to its resonance frequency perpendicular to the specimen
surface, at separations closer to the sample than in non-contact
3.4.5 NIST—NationalInstituteofStandardsandTechnology
mode. As the oscillating probe is brought into proximity with
3.4.6 PLL—poly-L-lysine
the surface, the probe-surface interactions vary from long
3.4.7 RM—reference material
range attraction to weak repulsion and, as a consequence, the
amplitude (and phase) of the cantilever oscillation varies.
4. Summary of Practice
During a typical imposed 100 nm amplitude oscillation, for a
4.1 This standard guide outlines the procedures for sample
short duration of time, the tip extends into the repulsive region
preparation and the determination of nanoparticle size using
close to the surface, intermittently touching the surface and
atomic force microscopy (AFM).AnAFM utilizes a cantilever
thereby reducing the amplitude. Intermittent contact mode has
with a sharp probe to scan a specimen surface. The cantilever
the advantage of being able to image soft surfaces or particles
beam is attached at one end to a piezoelectric displacement
weakly adhered to a surface and is hence preferred for
actuator controlled by the AFM. At the other end of the
nanoparticle size measurements.
cantilever is the probe tip that interacts with the surface. At
4.3 Amicroscope feedback mechanism can be employed to
close proximity to the surface, the probe experiences a force
maintainauserdefinedAFMsetpointamplitude,inthecaseof
(attractive or repulsive) due to surface interactions, which
imposes a bending moment on the cantilever. In response to
this moment, the cantilever deflects, and this deflection is 5
The boldface numbers in parentheses refer to a list of references at the end of
measured using a laser beam that is reflected from a mirrored this standard.
FIG. 1 Schematic Illustration of AFM Measurement Principle
E2859 − 11
intermittent contact mode.When such feedback is operational, 6.4 Colloidal gold, citrate-stabilized in aqueous solution,if
constant vibration amplitude can be maintained by displacing neededtotestorvalidatesamplepreparationandmeasurement
the built-in end of the cantilever up and down by means of the procedures.
piezo-actuator.
6.5 Deionizedwater,filteredto0.1 µm,asneededforsample
preparation or to rinse substrates.
NOTE 1—Operation of an AFM with feedback off enables the interac-
tions to be measured and this is known as force spectroscopy.
6.6 Ethanol, reagent or chromatographic grade, as needed
This displacement directly corresponds to the height of the
to rinse substrates.
sample. A topographic image of the surface can be generated
6.7 HCl, concentrated (37 %),ifneededtocleansilicon(Si)
byrasteringtheprobeoverthespecimensurfaceandrecording
substrates.
thedisplacementofthepiezo-actuatorasafunctionofposition.
Although the lateral dimensions are influenced by the shape of 6.8 H O , 30 % solution, if needed to clean Si substrates.
2 2
the probe (see Guide E2382 for guidance on tip related
6.9 Inert compressed gas source (for example, nitrogen,
artifacts), the height measurements can provide the height of
argon, or air), filtered to remove particles.
nanoparticles deposited onto a substrate with a high degree of
6.10 Mica disc, if needed as a substrate material.
accuracy and precision. If the particles are assumed to be
spherical, the height measurement corresponds to the diameter 6.11 Poly-l-lysine, solution (0.1 %), if needed for prepara-
or “size” of the particle. tion of functionalized substrates.
4.4 Procedures for dispersing nanoparticles on various sur- 6.12 Single crystal Si wafers, diced to appropriate size,if
faces such that they are suitable for imaging and height needed as a substrate material.
measurement via intermittent contact mode AFM are first
described. The nanoparticles used to exemplify these proce- 7. Apparatus
dures were National Institute of Standards and Technology
7.1 Atomic Force Microscope, capable of making
(NIST)goldnanoparticlereferencematerials,RM8011(nomi-
z-displacement measurements at sub-nanoscale dimensions.
nally 10 nm), RM 8012 (nominally 30 nm), and RM 8013
7.2 Bath Ultrasonicator, as needed to clean substrates.
(nominally 60 nm), all of which contained citrate-stabilized
negatively charged gold nanoparticles in an aqueous solution.
7.3 Microcentrifuge (“Microfuge”), as needed for sample
preparation.
4.5 Generic procedures for AFM calibration and operation
to perform size measurements in intermittent contact mode are
7.4 RF Plasma Cleaner with O , as needed to clean Si
discussed, and procedures for data analysis and reporting are substrates.
outlined.
8. Procedure
5. Significance and Use
8.1 Nanoparticle Deposition—For AFM measurements,
5.1 As AFM measurement technology has matured and
nanoparticle samples must be deposited on flat surfaces. The
proliferated, the technique has been widely adopted by the
roughness of the surface should be much less than the nominal
nanotechnology research and development community to the
size of the nanoparticles (preferably less than 5%) in order to
extent that it is now considered an indispensible tool for
provide a consistent baseline for height measurements. High-
visualizing and quantifying structures on the nanoscale.
qualitymica,atomicallyflatgold(111)(depositedonmica),or
Whether used as a stand-alone method or to complement other
single crystal silicon can all be used as substrates to minimize
dimensional measurement methods, AFM is now a firmly
the effect of surface roughness on nanoparticle measurements.
established component of the nanoparticle measurement tool
Example procedures are provided for depositing nanoparticles
box. International standards for AFM-based determination of
on these three substrates. The sample deposition procedures
nanoparticle size are nonexistent as of the drafting of this
outlinedbelowweredevelopedforusewithnegativelycharged
guide. Therefore, this standard aims to provide practical and
citrate-stabilized gold nanoparticles suspended in an aqueous
metrological guidance for the application of AFM to measure
solution at a mass concentration nominally 50 µg/g (as exem-
the size of substrate-supported nanoparticles based on maxi-
plified by NIST RMs 8011, 8012, and 8013). The procedures
mum displacement as the probe is rastered across the particle
should work with other nanoparticles that carry a negative
surface to create a line profile.
surface charge or zeta potential, including, but not limited to,
commercially available citrate-stabilized colloidal gold. As
6. Reagents
suggested below, these procedures can also be applied to
6.1 Certain chemicals and materials may be necessary in
positively charged or neutral nanoparticles with some modifi-
order to perform one or more of the steps discussed in this
cation.Eachproceduremayrequireoptimizationbytheuserin
guide,butthespecificreagentsusedareatthediscretionofthe
order to obtain satisfactory deposition density and to minimize
testerandmaydependonwhichspecificalternativeprocedures
artifacts such as agglomerate formation on the substrate or
are chosen or relevant for a particular application.
build-upoforganicfilmsresultingfromadditivesthatmightbe
6.2 Adhesive tape, if needed to cleave mica substrates.
present in the solution phase.
6.3 Atomically flat gold (111) on mica, if needed as a
NOTE 2—Substrate preparation and sample deposition should be con-
substrate material. ducted in a manner that minimizes the potential for contamination and
E2859 − 11
artifacts. For instance, to the extent possible, these operations should be
plasma cleaner, treat for 10 min in a clean glass beaker with
conducted in a HEPA filtered clean bench or work area. Similarly,
acetone placed in a low intensity ultrasonic bath followed by
prepared samples should be stored in a manner that maintains their
10 min sonication in a clean glass beaker with ethanol. Blow
integ
...