Nanosponges E-Book
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- Herausgeber: Wiley-VCH Verlag GmbH & Co. KGaA
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
- Veröffentlichungsjahr: 2019
An excellent overview of the field, covering in detail a wide range of different types of constituent materials, such as polymers, metals and metal oxides. It discusses their production and synthetic routes, as well as applications in several areas, including catalysis, drug delivery and environmental science. A must-have for scientists in academia and industry, as well as a valuable resource for both newcomers and more established researchers working in the field.
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Table of Contents
Cover
Preface
1 Nanomaterials: Classification and Properties
1.1 Nanomaterial Classifications
1.2 Nanomaterial Peculiarities
1.3 Manufacturing of Nanomaterials
1.4 Nanomaterials and Health Concerns
1.5 Legislation on Nanomaterials
1.6 ISO Classification and Standards
1.7 EPA Regulatory Approach for Nanomaterials and Manufacturing
References
2 Cyclodextrin Nanosponges1
2.1 Introduction
2.2 Nanosponge Evolution, Synthesis, and Characterization
2.3 Synthetic Routes for Different Types of Nanosponges
2.4 Synthetic Processes for Nanosponges
2.5 Characterization of Nanosponges
2.6 Applications of Nanosponges
2.7 Future Perspectives and Conclusions
References
3 Metal‐organic Framework Sponges
3.1 Introduction
3.2 Definition of Metal‐organic Framework
3.3 Applications
3.4 UiO‐66
Acknowledgments
References
4 Spongelike Functional Materials from TEMPO‐Oxidized Cellulose Nanofibers
4.1 Introduction
4.2 Synthesis and Characterization of bPEI–TOCNF Sponges
4.3 Applications of bPEI–TOCNF Sponges
4.4 Nanostructured TOCNF Templates
4.5 TEMPO‐Mediated Oxidation of Galactomannans: A New Class of Aerogels
4.6 Conclusions
Acknowledgments
References
5 Metal and Metal Oxide Nanosponges
5.1 Introduction
5.2 Types of Metal Oxide Nanosponge
5.3 Methods for the Synthesis of Metal Oxide Nanosponge
5.4 Applications
List of Abbreviations
References
6 Hybrid Nanosponges
6.1 Introduction
6.2 Hybrid Polymers
6.3 Toxicity
6.4 Characterization of Hybrid Nanosponges
References
7 Nanostructured Polymeric Hydrogels
7.1 Introduction
7.2 Hydrogel Design Features
7.3 Swelling Behavior
7.4 Gelation Theory
7.5 Cross‐linking
7.6 Chemical Cross‐links
7.7 Hydrogel Degradation
7.8 Network Structure and Characteristic Parameters
7.9 Drug Delivery Mechanisms and Experimental Evaluation
References
8 Vibrational Spectroscopic Methods for Nanosponges
8.1 Introduction
8.2 Molecular Vibrations and Principles of Raman Effect
8.3 Advantages/Utility of Raman Spectroscopy
8.4 Resonant Raman Scattering, Theory, and Applications to Investigations of Biosystems
8.5 Raman Measurements by Controlling Polarizations
8.6 Vibrational Dynamics of Cyclodextrin Nanosponges
8.7 Final Remarks
References
9 Nanosponges in Catalysis and Sensing
9.1 Introduction
9.2 Nanosponges in Catalysis
9.3 Nanosponges in Sensing
9.4 Conclusions
List of Abbreviations
References
10 Nanosponges for Gas Storage
10.1 Introduction
10.2 Hydrogen Storage
10.3 Methane Storage
10.4 Carbon Dioxide Adsorption
10.5 Conclusions
References
Index
End User License Agreement
List of Tables
Chapter 1
Table 1.1 Current definitions of terms with the “nano” suffix.
Table 1.2 List of the recommended upper limits suggested by different organizati...
Table 1.3 Techniques to measure particle sizes in the nanometer dimension range.
Table 1.4 Potential sources of exposure to nanoparticles.
Table 1.5 International harmonized stage codes.
Chapter 2
Table 2.2 Nanosponge evolution over the years.
Table 2.1 Compilation of pharmaceutical and biopharmaceutical applications of na...
Chapter 5
Table 5.1 List of soft templates.
Table 5.2 List of organic linkers for MOF synthesis.
Chapter 7
Table 7.1 Hydrogels in a view point of porosity‐related swelling kinetics.
Chapter 8
Table 8.1 Typical vibrational normal mode frequencies on a peptide system.
List of Illustrations
Chapter 1
Figure 1.1 Classification of nanomaterials (a) 0D spheres and clusters;...
Figure 1.2 Possible structure of cyclodextrin nanosponge.
Figure 1.3 The ISO definition of nano‐objects. Included as nano‐objects...
Figure 1.4 From macro‐ to nanomaterials: the effect on the surface.
Figure 1.5 Size dependence of the melting temperature of gold nanoparti...
Figure 1.6 Melting response for three representative narrow‐size distri...
Figure 1.7 Comparison of the relative size‐dependent melting point redu...
Figure 1.8 Experimental values of Δ
T
m
plotted as a function of the re...
Figure 1.9 Schematic representation of (c) 1D, (b) 2D, and (a) 3D confi...
Figure 1.10 (A) Molecular structure of meloxicam. (B) DSC thermograms o...
Figure 1.11 King Lycurgus Cup at British Museum – London: (a) reflected...
Figure 1.12 Pale yellow Au(III) salt (1 mM) (a) and red Au NP (b) solut...
Figure 1.13 Influence of gold shape on color change.
Figure 1.14 Color change of CdSe.
Figure 1.15 Methods of nanoparticle production: top‐down and bottom‐up....
Figure 1.16 Relationships between primary particles, agglomerates, and ...
Figure 1.17 Distribution of scientific papers for types of nanomaterial...
Figure 1.18 Diseases associated with nanoparticle exposure.
Figure 1.19 Risk assessment and risk management regarding possible adve...
Chapter 2
Figure 2.1 (a) Schematic representation of cyclodextrin nanosponge synt...
Figure 2.2 The many facets of cyclodextrin nanosponge pharmaceutical ap...
Chapter 3
Figure 3.1 van der Waals surface maps of four MOF structures, highligh...
Figure 3.2 Illustration of how one of the early MOFs is constructed. Co...
Figure 3.3 Construction of MOF‐5. Carbon, oxygen, and zinc atoms are sh...
Figure 3.4 Five of the sixteen IRMOFs published by Yaghi and coworkers ...
Figure 3.5 A selection of the most ubiquitous inorganic clusters based ...
Figure 3.6 A small selection of carboxylate‐based linkers with (a) diff...
Figure 3.7 The network topology of MOFs depends on the connectivity of ...
Figure 3.8 Schematic representation of the assembly of SBUs and linkers...
Figure 3.9 An overview of synthesis methods, possible reaction temperat...
Figure 3.10 Morphology of cubic MOF crystals (a) MOF‐5, simple cubic st...
Figure 3.11 Electron microscopy images of MOFs synthesized at high temp...
Figure 3.12 Structures of MOF‐76‐Ce. (a) MOF‐76‐Ce fully solvated; (b) ...
Figure 3.13 Classification of different flexibility modes of MOFs. One ...
Figure 3.14 Schematic representation of the pressure‐induced phase tran...
Figure 3.15 (A) Structural formulas of the asymmetric (C1) α‐1,4‐linked...
Figure 3.16 Schematic illustration of the synthesis of cubic gel partic...
Figure 3.17 Deliverable capacity for an idea adsorbent exhibiting a cla...
Figure 3.18 Contraction and expansion of the MOF Co(bdp). (a) The linke...
Figure 3.19 (a) Channel of [K
2
Co
3
(cit)
2
cit = C
6
H
4
O
7
] UTSA‐16 microporo...
Figure 3.20 (a) Reaction scheme illustrating the reversible CO
2
chemiso...
Figure 3.21 (a) Graphical model of CO
2
−mmen−M
2
(dobpdc). Green, gray, re...
Figure 3.22 Three metalated organic linkers. Coordination to metal cent...
Figure 3.23 Construction of UiO‐66. Carbon, oxygen, and zirconium atoms...
Figure 3.24 The (a) octahedral and (b) tetrahedral cages in the UiO‐66 ...
Figure 3.25 Dehydroxylation (dehydration) of the Zr
6
O
4
(OH)
4
(CO
2
)
12
clus...
Figure 3.26 Structure and composition of (a) UiO‐66, (b) UiO‐67, and (c...
Figure 3.27 Proposed scheme for the synthesis of UiO‐66 in the presence...
Figure 3.28 Reaction scheme for the hydrolysis of DMF.
Figure 3.29 UiO‐66 with (a) one missing linker defect and (b) three mis...
Figure 3.30 Illustration of missing linker defects terminated by differ...
Figure 3.31 Illustrations of (a) the structural and chemical difference...
Figure 3.32 Schematic illustrating the structural and compositional dif...
Chapter 4
Figure 4.1 Preparation of TOCNF.
Figure 4.2 Pictorial description of the process for the preparation of ...
Figure 4.3 (a) TOCNF aqueous suspension (≈3.5% w/v) at pH 11.0; (b) TOC...
Figure 4.4 Application of bPEI–TOCNF for water remediation. (a, b) Remo...
Figure 4.5 Fluoride sensing by bPEI–TOCNF sponges functionalized with
p
Figure 4.6 Ammonia‐induced solid‐state polymerization (AISSP) of DHI on...
Figure 4.7 Synthetic scheme for the one‐pot preparation and use of cera...
Figure 4.8 Comparison of fenugreek galactomannan viscosity before (left...
Figure 4.9 (a) TEMPO/laccase‐oxidized fenugreek galactomannan aerogels ...
Chapter 5
Figure 5.1 A schematic of hard‐template‐based approach.
Figure 5.2 A schematic of soft‐template‐based approach.
Figure 5.3 MOF synthesis and postsynthetic modification.
Figure 5.4 Dealloying process of a binary alloy.
Figure 5.5 The electrochemical deposition of a porous metallic film on ...
Figure 5.6 A schematic of sol–gel approach.
Chapter 6
Figure 6.1 Schematic representation of materials at different scale le...
Figure 6.2 Structural presentation of class I hybrids. (a) Blends. (b) ...
Figure 6.3 Structural presentation of class II hybrids. (a) Building bl...
Figure 6.4 The relationship between structurally hybridized materials, ...
Figure 6.5 Nanosponge‐loaded hydrogel (NS‐gel).
Chapter 7
Figure 7.1 Swelling of a hydrogel network.
Figure 7.2 Different types of water in hydrogels.
Figure 7.3 Picture shows the reversibility of the sol/gel process and q...
Figure 7.4 Scheme pointing out the different kinds of cross‐linking str...
Figure 7.5 Different routes for the creation of chemical and physical h...
Figure 7.6 Classification of different types of physical cross‐linking....
Figure 7.7 Carrageenan‐based gels physically cross‐linked [23].
Figure 7.8 Ionotropic gelation by interaction between anionic groups on...
Figure 7.9 Complex coacervation between polyanion and polycation.
Figure 7.10 Comparison between ionic interaction and complex coacervati...
Figure 7.11 Classification of different types of chemical cross‐linking...
Figure 7.12 Picture shows how the radical polymerization works.
Figure 7.13 Schematic methods for radical polymerization.
Figure 7.14 Graphical representation of a polycondensation reactive sys...
Figure 7.15 Schiff base reaction between PVA‐AL and PVA‐HY and gelation...
Figure 7.16 Potential applications of click reactions.
Figure 7.17 Degradation strategies for chemically cross‐linked hydrogel...
Figure 7.18 Selection of labile groups to control degradation rates. ...
Figure 7.19 Effects of hydrolytic degradation.
Figure 7.20 Repeating unit of the HA.
Figure 7.21 Features of a complex hydrogel network.
Figure 7.22 SANS experimental setup.
Figure 7.23 Typical example of best fitting results for cyclodextrin‐ba...
Figure 7.24 Comparison between conventional and controlled drug release...
Figure 7.25 Drug release from the polymeric matrix through solute diffu...
Figure 7.26 Drug release from the DDS through polymeric matrix swelling...
Figure 7.27 Drug release through two different mechanisms of polymer de...
Figure 7.28 Drug release, thanks to two different types of osmotic pump...
Figure 7.29 Schematic representation of drug delivery experiments.
Chapter 8
Figure 8.1 Schematic representation of a diatomic linear oscillator.
Figure 8.2 Energy level distributions given by Morse potential.
Figure 8.3 Schematic representation of the most common vibrational norm...
Figure 8.4 Schematic representation of the most relevant in‐plane vibra...
Figure 8.5 Schematic representation of a scattering Raman spectrum.
Figure 8.6 Schematic representation of the resonant Raman scattering ef...
Figure 8.7 Typical Raman spectra of cyclodextrin‐based nanosponges coll...
Figure 8.8 Schematic representation of a Raman experiment carried out i...
Figure 8.9 The analysis of vibrational dynamics of CDNS in a wide waven...
Figure 8.10 (a) Example of data handling of the low‐frequency Raman spe...
Figure 8.11 Overview of the changes of stiffness in cyclodextrin nanosp...
Figure 8.12 (a) Typical C
O stretching band profile observed in...
Figure 8.13 Total intensity of the C
O stretching band as a fun...
Figure 8.14 (a) Isotropic Raman profiles acquired on β‐CDPMA14 hydrogel...
Figure 8.15 Some representative results of the fitting procedure applie...
Figure 8.16 Hydration dependence of the I(C
C)
1
/I(C
C)
2
Figure 8.17 (a) Panel at the top: temperature evolution of isotropic Ra...
Figure 8.18 Temperature behavior of isotropic Raman spectra collected o...
Figure 8.19 (a) Representative example of Caf‐free isotropic profile ob...
Figure 8.20 (a) Temperature dependence of the dephasing time
τ
de
...
Chapter 9
Figure 9.1 Preparation of MNS by: (a) reduction of a metal salt; (b) r...
Figure 9.2 SEM (a) and TEM (b) images of PdPt bimetallic alloy nanospon...
Figure 9.3 TEM image of Pt/Ag nanosponges supported in graphite nanofib...
Figure 9.4 (a) Zeolites are formed by the self‐assembly of aluminosilic...
Figure 9.5 (a) SEM image of a MFI‐type zeolite nanosponge showing the l...
Figure 9.6 Structure of cyclodextrins.
Figure 9.7 Efficient CDNS catalyst for Sonogashira coupling based on en...
Figure 9.8 Construction of CDNS‐modified surfaces for biosensor applica...
Figure 9.9 Principle for signal amplification based on peroxidase‐loade...
Chapter 10
Figure 10.1 Physical and material‐based hydrogen storage volumetric ca...
Figure 10.2 Maximum hydrogen uptake (at 77 K) for carbon materials (▴),...
Figure 10.3 Influence of average pore size normalized to specific surfa...
Figure 10.4 Volumetric vs. gravimetric density of nearly 4000 MOFs (at ...
Figure 10.5 Hydrogen gravimetric uptake (at 77 K and ∼1 bar) vs. surfac...
Figure 10.6 Gravimetric (a) and volumetric (b) capacity of carbon mater...
Figure 10.7 Gravimetric storage capacity of zeolites, silica gel, and M...
Figure 10.8 Gravimetric isotherms relative to methane adsorption on NU‐...
Figure 10.9 Gravimetric isotherms relative to total and excess methane ...
Figure 10.10 Different technologies used to capture CO
2
gas.
Figure 10.11 Activated carbons do not have a well‐defined pore structur...
Figure 10.12 Four substrates of NPCs passivated with different function...
Figure 10.13 Schematic representation of eight cells of Cu‐BTC with one...
Figure 10.14 CO
2
adsorption isotherms on synthesized Cu‐BTC, measured u...
Figure 10.15 CO
2
isotherms of (a) ZIF‐8(MeOH), (b) ZIF‐8(DMF), and (c) ...
Figure 10.16 CO
2
adsorption isotherms of COP‐122 (blue) and COP‐122‐G1 ...
Figure 10.17 Pressure and temperature effect of CO
2
adsorption on zeoli...
Figure 10.18 CO
2
uptake capacity using porous carbon materials template...
Figure 10.19 CO
2
adsorption isotherms on PEI‐functionalized TUD‐1 mesop...
Guide
Cover
Table of Contents
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Nanosponges
Synthesis and Applications
Edited by
Francesco TrottaAndrea Mele
Copyright
Editors
Prof. Francesco Trotta
University of Torino
Department of Chemistry
Via Pietro Giuria 7
10125 Torino
Italy
Prof. Andrea Mele
Politecnico di Milano
Department of Chemistry, Materials and Chemical Engineering “G. Natta”
Piazza L. da Vinci, 32
20133 Milano
Italy
Cover Images:
(background) © majcot/Shutterstock;
(sponges) © ffolas/Shutterstock
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Print ISBN: 978‐3‐527‐34099‐6
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Preface
In the past few decades, the prefix “nano” was included in a great deal of words not only in the scientific language but also as part of everyday life vocabulary. Although the word “nanotechnology” was first introduced for the first time by Norio Toniguchi in 1974 during a talk at an international conference, the roots of this science is often linked to the Richard Feynman's lecture at Caltech in 1959. The famous sentence “there is a plenty of room at the bottom” can be considered the first milestone of nanotechnology and a visionary horizon. To date, the word nano has been included 86 635 times in the title of scientific articles, according to Scopus, or 342 190 times if abstract (194 056) and keywords (194 281) are included too. These simple numbers give the taste of the tremendous importance of nanomaterials and nanotechnology. Nanoscience and nanotechnology have shown – and still continuously show – an incredible growth of application leading to impressive advancement of fundamental knowledge and performances in medicine, electronics, mechanics, optics, computer science, drug delivery, paint, sensors, photonics, robotics, artificial intelligence, cosmetics, textiles, food packaging, just to mention the main fields. Actually, there is no field of modern technology without the presence of nanomaterials. The growing of the number of applications of nanomaterials is impressive: It is estimated that about three to four new products reach the market every week, and the list of such materials are now quite astonishing. In the recent years, several Nobel laureates in chemistry or physics took the prize for the studies in the nanoworld.
As it is explained in the first chapter of this book, nano‐world spans the range between 1 and 100 nm, i.e. below the submicron range (100–1000 nm) but above the quantum range (less than 1 nm), now referred to as the picorange.
A relevant limit on the applications and use of nanomaterials is derived from health concerns. This seems to be particularly true for nanofiber and, although to a lesser extent, for nanoparticles. On this basis, it is also understandable how nanocarriers for drug delivery still have limited applications.
Nanosponges do form a different class of nano‐objects. In fact, from a morphological point of view, they can assume dimensions in the microrange or even larger. Nevertheless, they generally present a nanoporous structure providing these materials with unique properties and ruling out the health concern related to the nanometric dimensions of the particles. From this standpoint, nanospanges can be considered a relatively safe material.
Although the papers directly linked to the term nanosponges are not so numerous, nevertheless, the world of porous, nanostructured materials or highly cross‐linked materials is quite large. In many cases, the synthetic processes leading to nanosponges are simple, especially if compared to classical nanomaterials. Nanosponges show tunable properties, high versatility, low‐cost synthetic routes, high stability, and excellent absorption properties, making them ideal materials for the removal of unwanted molecules from the environment or, on the other hand, for the selective release of added value compounds, as in drug delivery or in targeted and controlled release.
The aim of this book is to provide the reader with state‐of‐the‐art reviews of the fundamental and applicative aspects of nanosponges, thus filling the gap between nanomaterials and this new class of nanoporous, functional materials, underlining both basic and advanced information for those researchers willing to approach these fascinating, novel, and high‐performance materials.
Torino‐Milano
November 2018
Francesco Trotta
Andrea Mele
1Nanomaterials: Classification and Properties
Francesco Trotta1 and Andrea Mele2
1University of Torino, Department of Chemistry, Via Pietro Giuria 7, 10125 Torino, Italy
2Politecnico di Milano, Department of Chemistry, Materials, and Chemical Engineering “Giulio Natta”, Piazza Leonardo da Vinci 32, 20133 Milano, Italy
1.1 Nanomaterial Classifications
The word “nano” derives from the latin word “nanus” and Greek word “ννς,” both indicating a person of very low height, i.e. a dwarf. The International System (SI) of units considers nano as a prefix to indicate 10−9 part of a unit; thus, for instance, a billion of a meter, a billion of a liter, a billion of a kilogram, etc. Not always the term nano is referred to a very small object. For instance, in astronomy, a nanostar is a star having a mass comparable to our Sun or even less. A first, easy, and practical criterion to define nanomaterials is based on the dimensions “tout court”: Nanomaterials are conventionally defined as materials having at least a dimension between 1 and 100 nm. As a consequence, nanoparticles have all the three dimensions in the nanometer range, nanoplates present only one dimension below 100 nm, whereas nanofibers have two dimensions in the range of nano being the remaining remarkably longer. Some common terminologies of the nanorange world are listed in Table 1.1.
Table 1.1 Current definitions of terms with the “nano” suffix.
Source: Adapted from ISO/TS 27687.
Type
Description
Nanoscale
Size range from approximately 1–100 nm
Nano‐object
Material with one, two, or three external dimensions in the nanoscale
Nanoparticle
Nano‐object with all three external dimensions in the nanoscale
Nanotemplate
Nano‐object with one external dimension in the nanoscale and the other two external dimensions significantly larger (at least three times)
Nanofiber
Nano‐object (flexible or rigid) with two external dimensions in the nanoscale and the third dimension significantly larger
Nanotube
Hollow nanofiber
Nanorod
Solid nanofiber
Nanowire
Electrically conducting or semiconducting nanofiber
Nevertheless, 100 nm as an upper limit for a nanomaterial is not always accepted. Many organizations in the world fixed different thresholds for the nanoscale, although 100 nm still remains the most common shared limit. Table 1.2 presents some recommendations suggested by different organizations.
Table 1.2 List of the recommended upper limits suggested by different organizations.
Source: Adapted from Klaessig et al. [1].
Upper limit (nm)
Source
100
ISO
100
ASTM
100
Royal Society – SCENIHR
100
ETC group
100
Swiss Re
200
Soil Association
200
Defra
300
Chatham House
300
Friends of Earth
500
Swiss federal Office of Public Health
1000
House of Lords Science Committee
It is immediately clear that adequate techniques to determine the dimensions of the nano‐objects are required. Table 1.3 reports the methods till now available to measure the size of the objects in the nanometric range. To avoid incorrect results and classification, particular care and attention should be devoted to (i) prepare a representative sample for analysis, (ii) follow a correct sample preparation, (iii) use the most appropriate mathematical analysis to get size distribution, and (iv) consider the comparability among different laboratories. Detailed guidelines for sample preparation in GMO analysis were reported in the Joint Research Centre (JRC) technical report in 2014 [3].
Table 1.3 Techniques to measure particle sizes in the nanometer dimension range.
Source: Adapted from Lisinger et al. [2].
Method name (abbreviation)
Measurement range and medium (limiting factors)
Types of size distribution of raw data
Electron microscopy (EM)
1 nm or higher; dry (dynamic range)
Number based
Dynamic light scattering (DLS)
5–500 nm; suspension (sedimentation and scattering intensity)
(No distribution, or scattering intensity based)
Centrifugal liquid sedimentation (CLS)
20 nm and higher; suspension (particle density)
Extinction intensity based
Small‐angle X‐ray scattering (SAXS)
5 nm and higher; suspension (dynamic range)
Scattering intensity based
Field flow fractionation (FFF)
1–200 nm; suspension (dynamic range)
(Depends on detector)
Particle tracking analysis (PTA)
25 nm and higher; suspension (dynamic range)
Number based
Atomic force microscopy (AFM)
1 nm and higher; dry (dynamic range)
Number based
X‐ray diffraction
1 nm and higher; dry (only for crystalline materials)
(No distribution measured)
A sketch of nano‐objects is reported in Figure 1.1.
Figure 1.1 Classification of nanomaterials (a) 0D spheres and clusters; (b) 1D nanofibers, wires, and rods; (c) 2D films, plates, and networks; and (d) 3D nanomaterials.
Source: Adapted from Alagarasi [4].
Nanosponges, the subject of this book, can be considered as porous materials having all of the three external dimensions in the micro‐ or macrorange and the internal cavities, pores, or voids in the nanometer range. Actually, nanosponges can be either of organic or inorganic origin, natural or synthetic [5]. A simple sketch of a type of nanosponges based on cyclodextrins is reported in Figure 1.2.
Figure 1.2 Possible structure of cyclodextrin nanosponge.
Source: Reproduced under CC license from Singh et al. [6]. Published by The Royal Society of Chemistry.
In other words, nanosponges can be counterintuitively classified as nanomaterials because of the presence of a network of nanometer‐sized cavities in the bulk, despite the fact that the dimensions of a given specimen along the x‐, y‐, and z‐axes can be larger than 100 nm. From this viewpoint, nanosponges are characterized by nanometric structural features, but they are generally not nanoparticles. A hierarchical classification of nanomaterials has been proposed based on the particular feature falling in the nanometer size domain. A graphical summary, along with the relevant normation (vide ultra), is shown in Figure 1.3. The identification of the characteristic of a given material belonging to the nanometric range is particularly relevant for safety and health. IUPAC Glossary of Terms used in Toxicology indeed gave the following definition of “nanoparticle” [8]: “Microscopic particle whose size is measured in nanometers, often restricted to the so‐called nanosized particles (NSPs; <100 nm in aerodynamic diameter), also called ultrafine particles,” whereas no listed definition of nanomaterial has been proposed yet.
Figure 1.3 The ISO definition of nano‐objects. Included as nano‐objects are nanoparticles (nanoscale in all the three dimensions), nanofibers (nanoscale in two dimensions), and nanoplates or nanolayers (nanoscale only in one dimension).
Source: Krug and Wick 2011 [7]. Reprinted with permission from John Wiley & Sons.
Several times, nanomaterials can also be included in other bulk materials to form nanocomposites with external dimensions larger than 100 nm, but entrapping nanoparticles in the bulk. Although under discussion, even these materials should be seen as nanomaterials. This is the case of materials with antibacterial properties and made of nanoparticles dispersed in several environments such as fabrics, plastics, chitosans, etc. In a similar way, nanoparticles of reducing agents dispersed in a hydrogel provided superior performances in the reduction of organic dyes in comparison with molecular systems [9]. Polymers are particularly a suitable matrix to disperse nanoparticles. PVDF can host TiO2 nanoparticles showing an improved surface hydrophilicity and permeability of PVDF membranes and the decreasing adsorption capacity of protein, indicating an antifouling ability of such modified PVDF membranes [10].
On the other hand, it is important to note that even more ambiguity arises in the definition of the lower limit of the dimension of nanomaterials, generally set at 1 nm. Accordingly, also molecules, clusters, supramolecular assemblies – sometimes referred to as supermolecules – and not only nanoparticles can fall in this limit. In fact, a supramolecular assembly can be considered like the formation of a well‐defined complex of several molecules held together exclusively by noncovalent bonds. The main characteristic of such aggregates is that they possess an intrinsic mobility that leads to ordered nanostructures upon equilibration between aggregated and nonaggregated states, thus providing a number of interesting properties in biomedical applications, in therapy and delivery, information technologies, environmental sciences, chemical storage, separation and/or purification and transport, and catalysis among the other [11]. Nevertheless, the accepted 100 nm cutoff value is not based on a clear‐cut scientific basis, but it is much more related to political decisions. It should be stressed that changes in the physicochemical properties of nanomaterials related to the particle size take place as a continuum and that no switch of physical and chemical properties can be realistically expected at the exact value of 100 nm. In 2010, a complementary definition of nanomaterial was proposed by Kreyling et al. [12] on the basis of the volume‐specific surface area (VSSA). VSSA is easily determined by dividing the external surface area of particle S by its solid volume V. Alternatively, VSSA is obtainable by multiplying the specific surface area by the material density. This ratio is conventionally expressed as m2/cm3 units. Thus, a particulate material having a VSSA ≥ 60 m2/cm3 should be considered as a nanomaterial. This limit corresponds to monodispersed spherical particle with a diameter of 100 nm. As a consequence, a value higher than 60 m2/cm3 inevitably indicates the existence of spherical particles smaller than 100 nm, in accordance with the former classification. The main strength of such a definition is in the possibility to use parameters easily achievable for virtually any material. For example, surface area and density can either be retrieved from handbooks and databases or measured by BET and density measurements, respectively. The VSSA‐based definition overcomes the limitations of the definition of nanomaterials according to a single‐size parameter, as in the case of nanomaterials where a broad size distribution is present. In such a case, a fraction of the particle population may fulfill the 1–100 nm criterion, whereas a significant part may fall outside the nanorange, thus making difficult any decision on the classification of the material, especially for the legislator or for regulatory agencies. Similar cases are those where aggregation or clustering of nanoparticles are likely to occur, thus making a single‐size parameter insufficient for a clear‐cut discrimination of nanomaterials. From a practical point of view, the VSSA approach has the main advantage of providing legislators with a simple and clear cut‐off in order to approve or not a nanomaterial. Additionally, the VSSA criterion may also be useful in the regulatory field and helps correct political decisions. Nevertheless, it should be pointed out that the VSSA approach often leads to the false‐positive classification of nanomaterials, and to this regards, it appears excessively protective in consideration of health and safety concerns. Moreover, despite this criterion sounds simple and of practical uses without the necessity of sophisticated analyses, VSSA values highly depend on the shape of the nanoparticle: for example, the threshold value drops down to 40 m2/cm3 for fiber or rodlike nanoparticles and to 20 m2/cm3 for sheetlike nanoparticles.
1.2 Nanomaterial Peculiarities
Despite the recent explosion of the term nanomaterials in the common language, it is important to point out that a lot of nanomaterials are of natural origin and largely diffused in the environment. For instance, dust volatized, ocean spray, soot from forest fires, and volcanic eruptions are typical examples. Nevertheless, most of them are intentionally engineered and manufactured to proper applications. A constantly growing number of applications of engineered nanomaterials are reported in cosmetics, sporting wear and goods, tires, electronics, and medicine. In this latter case, nanomaterials can be used as an aid to make diagnosis easier and/or to improve the drug pharmacokinetics and bioavailability. Although the lexicon containing the nano‐ suffix is quite recent, some nanomaterials are on the market since longtime. This is the case of “carbon black” used from 1915 to reinforce car tires. It is estimated that today the annual production of carbon black exceeds 10 million tons, being about 85% used in rubbery industries. Other well‐known nanomaterials commercially available are fumed silica, titanium dioxide, zinc oxide, and, more recently, silver nanoparticles. In addition, nanoparticles are a component of smoke. However, the great interest toward nanomaterials arises from the observation that the basic properties of the matter change at the nanoscale level. Reduction of size, in fact, has many relevant effects on the properties of the nanoparticles. A nonexhaustive list of them includes the enhancement of hardness, strength, and fracture ductility; the decrease of the melting point; the increase of heat capacity; the decrease of Debye temperature; the increase of conductivity for nanometals; the decrease of Curie temperature; the onset of nonlinear optical properties; the enhancement of the catalytic activity; the swap of solubility; and even the color variation as a function of the particle size.
A key concept when dealing with nanomaterials is the dramatic enhancement of the surface area with decreasing object dimensions. This fact is clearly shown in Figure 1.4.
Figure 1.4 From macro‐ to nanomaterials: the effect on the surface.
Source: Image from www.nano.gov/nanotech‐101/special.
Actually, a relevant part of the unique properties of nanomaterials can be explained by the incredible increase in the surface area available.
The paradigmatic example depicted in Figure 1.4 illustrates this concept. In a virtual experiment, a cube of 1 cm on a side is progressively reduced in a collection of smaller cubic particles, keeping the total mass constant. The starting cube has a total surface area of 6 cm2. In the second step, the initial cube is decomposed into cubes of 1 mm on a side. The overall number of such cubes is 103. The overall surface can be calculated as (6 mm2/object × 103 objects) = 60 cm2. In a third step of the virtual experiment, the cube of 1 cm on a side is decomposed into cubes of 1 μm on a side. As 1 cm = 104 μm, the total number of little cubes is now (104)3 = 1012 cubes. The overall surface area is now 6 × 1012 μm2, corresponding to 6 m2. The final step is getting to the nanodimension. As 1 nm = 10−9 m = 10−7 cm, the calculation shown above leads to the astonishing numbers of 1021 little cubes, each accounting for a surface of 6 nm2. The sum of surface areas is 6 × 1021 nm2, i.e. 6 km2!
One of the most dramatic effects of dimension on the physical properties is their influence on the melting point of metals. Changes in melting point are due to the much larger surface‐to‐volume ratio than bulk materials, at nanoscale level, thus drastically altering their thermodynamic and thermal properties. This phenomenon was first theoretically predicted by Pawlow in 1909 on a thermodynamic model [13], experimentally proved by himself in 1910 [14].
For example, a decrease of almost 600 °C in the melting point of gold is observed on passing from 11 nm particles to 1 nm particles size, as shown in Figure 1.5. This effect may be estimated from the dimensionality of related physical values using the simple Gibbs–Thompson relationship [16]:
Figure 1.5 Size dependence of the melting temperature of gold nanoparticles. The subplot shows Tm from radius below 2 nm.
Source: Reprinted with permission from Font and Myers [15], Copyright © 2013, Springer.
where TmB is the bulk melting temperature, σsl is the solid–liquid interface energy (per unit area), Hf is the bulk enthalpy of fusion (per gram of material), and ρs is the density of solid considered of spherical shape having a diameter.
Very similar behavior can be observed with other elements such as aluminum as depicted in Figure 1.6.
Figure 1.6 Melting response for three representative narrow‐size distributed aluminum nanoparticles at 5 K/min under argon atmosphere; heat flows are normalized by mass of aluminum in the core of the nanoparticle, as determined from TGA. The radius indicated is the weight‐average radius.
Source: Reprinted with permission from Sun and Simon [17], Copyright © 2007, Elsevier.
Beside a great number of evidence for metallic nanoparticles, the linear dependence of Tm on 1/x was validated for other compounds such as oxide nanoparticles with covalent/ionic bonds, namely, Bi2O3 nanoparticles as reported in Figure 1.7. This effect appears to be even more accentuated than metallic melting point depression. This indicates that the effect is independent of bond nature and is purely caused by the geometry‐dependent surface/volume ratio. In addition, the melting point decrease is much more higher for 10 nm Bi2O3 nanoparticles than 10 nm gold nanoparticles.
Figure 1.7 Comparison of the relative size‐dependent melting point reduction between gold and bismuth oxide.
Source: Reprinted with permission from Guenther et al. [18], Copyright © 2014, American Chemical Society.
Another peculiar effect of nanomaterials much more related to the topics of this book is the melting behavior of organic materials once confined or entrapped in porous solid. This situation is quite common in nature and in biological systems like cell membrane, ion channels, etc. In these cases, the confined material has a great interaction with the surface wall. As one of the consequence, the melting point of the pore solid T(d) decreased with a decreasing pore diameter. According to the theories of solidification in a capillary, the decrease of the melting point was found to correlate linearly with the reciprocal of the diameter. Jackson and McKenna [16] studied a set of organic molecules inside pore glasses. The results shown in Figure 1.8 indicate the influence of small pores in reducing the melting points of the molecules.
Figure 1.8 Experimental values of ΔTm plotted as a function of the reciprocal pore diameter. (O) cis‐decalin, (•) trans‐decalin, (∇) cyclohexane, (Δ) benzene, (▴) chlorobenzene, (♦) naphthalene, and (□) heptanes.
Source: Jackson and McKenna 1990 [16]. Reprinted with permission from AIP Publishing.
As shown in Figure 1.9, the confinement is depending on the kind of nanomaterial, but it leads in any case to melting point depression.
Figure 1.9 Schematic representation of (c) 1D, (b) 2D, and (a) 3D confinement.
Source: Reprinted with permission from Singh et al. [19], Copyright © 2014, Elsevier.
Nanosponges can even suppress the melting point of a molecule, as reported in Figure 1.10 for meloxicam drug included in a cyclodextrin nanosponge. Meloxicam has a sharp melting point at 250 °C. This peak is absent when meloxicam is entrapped in nanosponges, thus indicating the formation of a complex between meloxicam and the nanosponges, and that meloxicam was protected by encapsulation in the nanosponges [20].
Figure 1.10 (A) Molecular structure of meloxicam. (B) DSC thermograms of (a) meloxicam and (b) meloxicam inclusion compound with CD‐NS. Source: Reprinted with permission from Shende et al. [20], Copyright © 2015, Elsevier.
Further spectacular effects associated with the nanosize dimensions of the particles in a material are the change in color. It is well known that gold has the characteristic yellow, brilliant color, but when gold is produced in the nanosized dimension range, its color invariably changes to red. In other words, gold shows a color depending on the size of the gold particles. This phenomenon has an impressive example in the so‐called Lycurgus cup now preserved at the British Museum in London (Figure 1.11) [21].
Figure 1.11 King Lycurgus Cup at British Museum – London: (a) reflected light and (b) transmitted light.
Source: Adapted under CC license from Freestone et al. 21. Published by SpringerNature.
This cup was probably created in Rome during the fourth century CE and shows a different color changing the light from opaque green to bright red. This is the most famous example of the so‐called dichroic glass. Actually, it presents some tiny amount of nanosized gold and silver that gives these unusual optical properties. This happens because at the nanoscale range, the electron cloud located on the surface of a gold nanoparticle has the possibility to resonate with different wavelengths of light depending on their frequency. This phenomenon is now well understood and reproduced even at laboratory demonstrative level as reported in Figure 1.12.
Figure 1.12 Pale yellow Au(III) salt (1 mM) (a) and red Au NP (b) solutions.
Source: Adapted under CC license from Pluchery et al. [22]. Published by SpringerNature.
As mentioned above, this behavior is influenced by the size of the nanoparticle. The result is that a nanoparticle of about 90 nm in size will absorb color on the red and yellow end of the color spectrum, making the nanoparticle appear blue‐green. A small‐sized particle, about 30 nm in size, absorbs blues and greens, resulting in a red appearance [23]. To date, it is proved that shape has also some influence in the color change. Figure 1.13, for example, reports the absorption spectra of several kinds of gold nanoparticles [24].
Figure 1.13 Influence of gold shape on color change.
Source: Adapted with permission from Eustis and El‐Sayed [24], Copyright © 2005, RSC.
Although it appears unlikely that this knowledge was in the background of those ancient masters, however, the effect obtained is still impressive! Nevertheless, Lycurgus cup was not the only one heritage that is connected with the presence of nanoparticles or nanomaterials in the ancient time. This is also the case of the red‐colored glasses of the Late Bronze Age (1200–1000 BCE) that were found to produce a plasmon surface resonance for the presence of copper nanoparticles. These latter were also used by Romans to produce mosaics. Finally, this expertise was not confined in the western part of the world but observed even in eastern countries. This is the case of the famous Japanese Satsuma glasses that appear ruby colored because of the presence of copper nanocrystals [24].
However, it should be pointed out that not only metallic nanoparticles show the above‐mentioned effect but also some salts such as CdS or CdSe crystals, which are also known to change their color when reduced to a few nanometers [25]. A visual sketch is reproduced in Figure 1.14.
Figure 1.14 Color change of CdSe.
Source: Adapted with permission from Eustis and El‐Sayed [24], Copyright © 2005, RSC.
Nanosized gold has another very particular behavior. It is well known that gold is considered chemically inactive; however, it was recently reported that gold nanoparticles, i.e. less than 2 nm, show an unexpected antimicrobial effect toward Gram‐positive and Gram‐negative bacteria. This broad antimicrobial activity was attributed to the ability of ultrasmall gold nanoparticles to interact with bacteria leading to a metabolic imbalance and a high production of reactive oxygen species (ROS) that kills the bacteria [26]. It should be quite evident from the above reported examples that the behavior and the properties of nanomaterials are much more because of the size and surface area rather than to their chemical composition.
1.3 Manufacturing of Nanomaterials
Despite the arbitrary 100 nm limit for numerous classifications of nanomaterials, it is sometimes reported, and generally accepted, that more precisely 30 nm is the upper limit for the experimental detection of size‐dependent property change. In fact, it is quite evident that the high increase in the surface area and new quantum effects are the two main factors driving the behavior of nanomaterials. A nanomaterial may be formed by just a single element or composed by a significant number of them. In addition, all nanomaterials can be further modified or functionalized. The choice of the method results to be different for 0D, 1D, 2D, and 3D nanomaterials. Generally, it is accepted that nanomaterials can be produced by physical, chemical, or bio‐based methods [25]. Two basic manufacturing processes are known [27], graphically displayed in Figure 1.15.
Figure 1.15 Methods of nanoparticle production: top‐down and bottom‐up.
Source: Reproduced under CC license from Gleiter [ 27]. Published by Elsevier.
The first approach is a defined “top‐down,” and the most common example is the milling procedure of the starting large‐sized material; this method requires relevant amounts of starting materials with a great loss of material and, as a consequence, to a significant waste production. This technique is well applicable to metallic and ceramic nanomaterials. It has to take into account that prolonged milling procedure involves severe thermal stress to the material. Often, a cryogenic liquid is used to facilitate the milling process and to increase the brittleness. In addition, contamination by abrading the container often occurs. Nanoparticles obtained invariably show relevant polydispersity and poor uniformity in particle shape. Photolithography, anodization, and plasma etching are other examples of top‐down methods used. Alternatively, nanomaterials can also be produced by a “bottom‐up” route based on the assembling of atoms or molecules to get the desired nanomaterial. Reaction in the gas phase and liquid phase as well is involved. Often, this technique is time consuming, complex, and expensive. In any case, this method allows the production of a much more regular particles size and shape. Some examples are sol–gel process, gas‐phase synthesis, flame‐assisted ultrasonic spray pyrolysis, gas condensation processing, chemical vapor condensation, sputtered plasma processing, microwave plasma processing, and laser ablation. Among these, gas‐phase process appears to be more prone to be used in industrial scale to produce nanomaterials both as powder and film. Fullerene and carbon nanotubes are the most known nanomaterials obtained in this way. On the other hand, the liquid‐phase process has the advantage to be performed at lower temperature than the gas‐phase process. Although it is possible to get almost completely monodisperse nanomaterials, nevertheless, often relevant polydispersity is observed. It has to be pointed out that the size distribution is a pivotal element to consider to classify the nanomaterials. In particular, the classification changes if the size distribution is considered on the number of particles or on the mass concentration. For safety concern, it is much more useful to consider the number distribution being a more cautious choice. The primary particle of the selected material can form larger unit formation, by adhesion phenomena, i.e. agglomerates or even more stable aggregates (Figure 1.16). The most important difference is that in agglomerates, the total surface area does not change to a great extent in comparison with the area of the single particles. On the contrary, aggregates always have a lower total surface area if compared with the parent particles. Agglomerates are, for instance, responsive to ultrasound being easily divided into smaller agglomerates.
Figure 1.16 Relationships between primary particles, agglomerates, and aggregates.
Source: Walter 2013 [28]. Reprinted with permission from John Wiley & Sons.
1.4 Nanomaterials and Health Concerns
The great interest toward nanomaterials in all aspects is well documented by the number of papers on this matter. For example, Figure 1.17 reports the number of articles published on some selected keywords dealing with nanomaterials in the article title (www.scopus.com).
Figure 1.17 Distribution of scientific papers for types of nanomaterials.
Source: Data obtained from www.scopus.com.
Applications of nanomaterials are growing continuously. Today, about one‐third of the nanomaterials produced in the world find applications in cosmetic products especially in sunscreen formulations. The broad range of applications of nanomaterials can be summarized in this nonexhaustive list of products containing nanomaterials of different origin:
(1) Cosmetic and personal care products
(2) Paints and coatings
(3) Household products
(4) Catalysts and lubricants
(5) Sport products
(6) Textiles
(7) Medical and health care products
(8) Food and nutritional ingredients
(9) Food packaging
(10) Agrochemicals
(11) Veterinary medicines
(12) Construction materials
(13) Weapons and explosives
(14) Consumer electronics
The wide and increased uses of nanomaterials rise the health and safety concerns for some major reasons: (i) nanomaterials are much more reactive than the molecular analogs in solvents or in condensed phase; (ii) their very small size allows them to migrate easier in biological systems; and (iii) they may pass the biological membranes in the lung, gut, and even in the brain causing damage to intracellular structures and cellular functions. Actually, lung is the preferred route to enter in the body by nanoparticles [7]. The 100 millions of alveoli present and their very high surface area in the lung highly facilitate the contact with the nanosized material. Fortunately, most of the introduced nano‐objects are removed efficiently by mucociliary transport or by macrophages for the smaller particles. It seems likely that only prolonged overexposure or stress created by smoking for instance can provoke serious danger to health. In any case, less than 0.05% of the quantity administered by inhalation reaches the bloodstream. The second possibility for a nano‐object to enter the body is to follow the olfactory nerve route. This latter route is much more effective, and it is directly connected to the brain; thus, in this case, the nanoparticle can reach the central nervous system despite the very low overall amount capable to reach the brain.
On the other hand, skin is an effective barrier to the penetration of nanoparticles, especially the nonlipophilic ones. Only very small particles, i.e. <10 nm, are reported to pass the skin, and the penetration can be influenced and promoted by particle surface coating, especially those used to prevent the agglomeration of the nanoparticles. Finally, gastrointestinal uptake of nanoparticles is minimal. In fact, the gastrointestinal tract is not efficient for nanoparticle adsorption, despite its large surface area. It was reported that 98% of the orally administered nanoparticles in animal tests was excreted [ 28–30].
Nanomaterials are recognized to produce the formation of ROS leading to severe inflammation processes and related consequences [31].
It should also point out that the factors of risk other than the particle size, e.g. the shape, contribute to the picture: this is the case, for example, of nanofibers that cannot be removed from the lung via mucociliary clearance, as happens with spherical particles, and they can generate fibrosis and even cancer. This is well known for asbestos fibers and probably also to carbon nanotubes [7].
A graphic summary of the possible pathologies associated with nanoparticle exposure is proposed in Figure 1.18. Risk assessment is defined as the product of hazard (toxicity) and exposure and determines the chronic and acute health concerns. This is also valid for adverse effects of nanomaterials toward CNS (central nervous system) [33,34].
Figure 1.18 Diseases associated with nanoparticle exposure.
Source: Buzea et al. 2007 [32]. Reprinted with permission from AVS.
Figure 1.18 reports the known routes and the possible effects of inhaled or ingested nanoparticles. Although nanoparticles can reach CNS, it is reported that by inhalation, only a minor part of them reaches the bloodstream and the target organs or brain. Moreover, a very slow translocation rate was always observed. On the other hand, nanoparticles can reach CNS more efficiently through the olfactory nerve. Long‐term exposure toward nonbiodegradable or nonexcreted nanoparticles is likely to accumulate in the brain with possible chronic toxic effects. Potential sources of exposure to nanoparticles are listed in Table 1.4.
Table 1.4 Potential sources of exposure to nanoparticles.
Source: Adapted from Safety & Wellbeing Team 2013 [35].
Process synthesis
Particle formation
Exposure source
Exposure route
Gas phase
In air
Leakage from reactor, especially if operated at positive pressure
Inhalation
Product recovery from bag filters in reactors
Inhalation/dermal
Processing and packaging of dry powder
Inhalation/dermal
Equipment cleaning/maintenance (including reactor evacuation and spent filters)
Dermal (and inhalation during reactor evacuation)
Vapor deposition
On substrate
Product recovery from reactor/dry contamination of workplace
Inhalation
Processing and packaging of dry powder
Inhalation/dermal
Equipment cleaning/maintenance (including reactor evacuation)
Dermal (and inhalation during reactor evacuation)
Colloidal
Liquid suspension
If suspension is processed into a powder, potential exposure during spray drying to create a powder, and processing and packaging of dry powder
Inhalation/dermal
Equipment cleaning/maintenance
Dermal
Attrition
Liquid suspension
If suspension is processed into a powder, potential exposure during spray drying to create a powder, and processing and packaging of dry powder
Dermal
Equipment cleaning/maintenance
Dermal
Note: Ingestion would be a secondary route of exposure from all sources/activities from the deposition of nanomaterials on food or mucous that is subsequently swallowed (primary exposure route inhalation) and from hand to mouth contact (primary exposure route dermal).
In addition, several nanomaterials may act as chemical catalysts and produce unanticipated reactions increasing the level of risk for explosions and fires. Because it is well known that dust explosion occurs below 500 μm size of the powder and the risk is increasing reducing the particle dimension, nanoparticles may be combustible and require less energy to ignite, enhancing the risk of accidents in industrial plants. In addition, metallic nanoparticles such as aluminum, titanium, and magnesium are even pyrophoric. Thus, it is necessary to properly characterize the nanomaterial from physical and chemical point of view in order to prepare a safety profile. A possible flowchart for the risk analysis is proposed in Figure 1.19.
Figure 1.19 Risk assessment and risk management regarding possible adverse substances or materials.
Source: Krug and Wick 2011 [ 7]. Reprinted with permission from John Wiley & Sons.
The most relevant physical properties to be monitored are size, shape, specific surface area, physical state, capability for aggregates via self‐assembly, size distribution, smoothness of the surface, crystal structure, and facility to dissolve in solvents. From a chemical point of view, other parameters should be considered: molecular structure, presence of impurities or additives, surface chemistry, and interaction with water or other solvents.
1.5 Legislation on Nanomaterials
The EU adopted a definition of a nanomaterial in 2011 (2011/696/EU) [36]. Its provisions include a requirement for review “in the light of experience and of scientific and technological developments. The review should particularly focus on whether the number size distribution threshold of 50% should be increased or decreased.” The commission was expected to conclude the review in 2016 or shortly after. According to the Recommendation, “Nanomaterial” means:
A natural, incidental or manufactured material containing particles, in an unbound state or as an aggregate or as an agglomerate and where, for 50% or more of the particles in the number size distribution, one or more external dimensions is in the size range 1 nm – 100 nm. In specific cases and where warranted by concerns for the environment, health, safety or competitiveness the number size distribution threshold of 50% may be replaced by a threshold between 1 and 50%. By derogation from the above, fullerenes, graphene flakes and single wall carbon nanotubes with one or more external dimensions below 1 nm should be considered as nanomaterials.
The definition will be primarily used to identify materials for which special provisions might apply (e.g. for risk assessment or ingredient labeling). Those special provisions are not part of the definition but of specific legislation in which the definition will be used. Nanomaterials are not intrinsically hazardous per se, but there may be a need to take into account specific considerations in their risk assessment. Therefore, one purpose of the definition is to provide clear and unambiguous criteria to identify materials for which such considerations apply. It is only the results of the risk assessment that will determine whether the nanomaterial is hazardous and whether or not further action is justified. Today, there are several pieces of EU legislation, and technical guidance supporting implementation of legislation, with specific references to nanomaterials. To ensure conformity across legislative areas, where often the same materials are used in different contexts, the purpose of the Recommendation is to enable a coherent cross‐cutting reference. Therefore, another basic purpose is to ensure that a material that is a nanomaterial in one sector will also be treated as such when it is used in another sector.
1.6 ISO Classification and Standards
ISO has shown interest in nanotechnologies and related fields: in 2005, ISO created the Technical Committee (TC) 299 – Nanotechnology – under the secretariat of BSI with the specific tasks of developing and spreading standards concerning materials, processes, and measurements on the matter in the size range 1–100 nm. The standards include nomenclature, terminology, reference materials, metrological aspects and characterization methods, and a large set of standards and recommendations related to safety, health, environment, and risk assessment. TC 229 is structured in Joint Working Groups (JWG), Working Groups (WG), and Task Groups (TG) engaged in the following sectors:
Terminology and nomenclature
Products and applications
Nanotechnology and sustainability
Measurements and characterization
Material specifications
Health, safety, and environmental
(
HSE
) aspects of nanotechnologies
Consumer and societal dimensions of nanotechnologies
Since its establishment and until 2017, ISO TC 229 published 55 standards, listed on the ISO website at the address: www.iso.org/committee/381983/x/catalogue/p/1/u/0/w/0/d/0 and released at different stages. It is worth mentioning here the code for the stage classification of the international standards. The code is a four‐digit number made of two couples of digits separated by a dot (e.g. 40.92). The first two digits identify the stage – ranging from “preliminary” to “withdrawal” – whereas the second two are associated with the substage – ranging from “registration” to “proceed.” The full matrix is reported in Table 1.5.
Table 1.5 International harmonized stage codes.
Source: Printable version downloadable from www.iso.org/stage‐codes.html.
Substage
90 Decision
Stage
00 Registration
20 Start of main action
60 Completion of main action
92 Repeat an earlier phase
93 Repeat current phase
98 Abandon
99 Proceed
00
Preliminary
00.00
Proposal for new project received
00.20
Proposal for new project under review
00.60
Close of review
00.98
Proposal for new project abandoned
00.99
Approval to ballot proposal for new project
10
Proposal
10.00
Proposal for new project registered
10.20
New project ballot initiated
10.60
Close of voting
10.92
Proposal returned to submitter for further definition
10.98
New project rejected
10.99
New project approved
20
Preparatory
20.00
New project registered in TC/SC work programme
20.20
Working draft (WD) study initiated
20.60
Close of comment period
20.98
Project deleted
20.99
WD approved for registration as CD
30
Committee
30.00
Committee draft (CD) registered
30.20
CD study/ballot initiated
30.60
Close of voting/comment period
30.92
CD referred back to Working Group
30.98
Project deleted
30.99
CD approved for registration as DIS
40
Enquiry
40.00
DIS registered
40.20
DIS ballot initiated: 12 weeks
40.60
Close of voting
40.92
Full report circulated: DIS referred back to TC or SC
40.93
Full report circulated: decision for new DIS ballot
40.98
Project deleted
40.99
Full report circulated: DIS approved for registration as FDIS
50
Approval
50.00
Final text received or FDIS registered for formal approval
50.20
Proof sent to secretariat or FDIS ballot initiated: eight weeks
50.60
Close of voting. Proof returned by secretariat
50.92
FDIS or proof referred back to TC or SC
50.98
Project deleted
50.99
FDIS or proof approved for publication
60
Publication
60.00
International standard under publication
60.60
International standard published
90
Review
90.20
International standard under periodical review
90.60
