Carbon Nanomaterials for Bioimaging, Bioanalysis, and Therapy

By Xueji Zhang

A comprehensive reference on biochemistry, bioimaging, bioanalysis, and therapeutic applications of carbon nanomaterials

Carbon nanomaterials have been widely applied for biomedical applications in the past few decades, because of their unique physical properties, versatile functionalization chemistry, and biological compatibility. This book provides background knowledge at the entry level into the biomedical applications of carbon nanomaterials, focusing on three applications: bioimaging, bioanalysis, and therapy.

Carbon Nanomaterials for Bioimaging, Bioanalysis and Therapy begins with a general introduction to carbon nanomaterials for biomedical applications, including a discussion about the pros and cons of various carbon nanomaterials for the respective therapeutic applications. It then goes on to cover fluorescence imaging; deep tissue imaging; photoacoustic imaging; pre-clinical/clinical bioimaging applications; carbon nanomaterial sensors for cancer and disease diagnosis; targeted cancer therapy; and photothermal/photodynamic therapy. Each chapter briefly introduces the biomedical application and emphasizes the most appropriate carbon nanomaterial(s) for the application.

• Provides an introduction to the biomedical applications of carbon nanomaterials for early-career scientists, as well as background and context for mid-career scientists and researchers
• Contains four sections covering biochemistry, bioimaging, bioanalysis, and therapeutic applications of carbon nanomaterials
• Presented by experts who have strong background in the field of nanotechnology for biomedical applications
• Covers a hot area of research which has very unique physical properties, versatile functionalization chemistry, and biological compatibility

Carbon Nanomaterials for Bioimaging, Bioanalysis and Therapy is an excellent resource for academic researchers and industrial scientists working on preparation and bio-application of carbon nanomaterials, biomedical engineering, and nanotechnology.

• Language: English
• Publisher: Wiley
• Release date: Nov 21, 2018
• ISBN: 9781119373469

Book preview

List of Contributors

Parinaz Abdollahiyan

Research Center for Pharmaceutical Nanotechnology

Biomedicine Institute

Tabriz University of Medical Sciences

Iran

Ayuob Aghanejad

Research Center for Pharmaceutical Nanotechnology

Biomedicine Institute

Tabriz University of Medical Sciences

Iran

Jaleh Barar

Research Center for Pharmaceutical Nanotechnology

Biomedicine Institute

Tabriz University of Medical Sciences

Iran

and

Department of Pharmaceutics

Faculty of Pharmacy

Tabriz University of Medical Sciences

Iran

Yu Cao

Beijing Key Laboratory for Bioengineering and Sensing Technology

Research Center for Bioengineering and Sensing Technology

School of Chemistry & Biological Engineering

University of Science & Technology Beijing

PR China

Yi‐Chun Chen

Department of Biotechnology

National Formosa University

YunlinTaiwan

Hung‐Hsiang Chen

Department of Biotechnology

National Formosa University

YunlinTaiwan

Petr Cigler

Institute of Organic Chemistry and Biochemistry of the CAS

Czech Republic

Laurent Cognet

Laboratoire Photonique Numérique et Nanosciences

Université de Bordeaux

Talence

France

and

Institut d'Optique d'Aquitaine & CNRS

France

John Czerski

Single Molecule Biophysics Laboratory

Department of Physics

Colorado School of Mines

GoldenUSA

Haifeng Dong

Beijing Key Laboratory for Bioengineering and Sensing Technology

Research Center for Bioengineering and Sensing Technology

School of Chemistry & Biological Engineering

University of Science & Technology

BeijingPR China

Jessica Fung Yee Fong

Swinburne Sarawak Research Centre for Sustainable Technologies

Swinburne University of Technology

Malaysia

Silvia Giordani

Department of Chemistry

University of Turin

Italy

and

School of Chemical Sciences

Dublin City University

Ireland

Simon Haziza

James H. Clark Center for Biomedical Engineering & Sciences

Stanford University

USA

and

CNC Program

Stanford University

USA

Yuen Yung Hui

Institute of Atomic and Molecular Sciences, Academia Sinica

TaipeiTaiwan

Mona Jani

Institute of Organic Chemistry and Biochemistry of the CAS

Czech Republic

Chulhong Kim

Department of Creative IT Engineering

Pohang University of Science and Technology

PohangRepublic of Korea

TaeYoung Kim

Department of Bionanotechnology

Gachon University

South Korea

Marek Kindermann

Institute of Organic Chemistry and Biochemistry of the CAS

Czech Republic

and

Institute of Microbiology of the CAS

Czech Republic

and

University of Chemistry and Technology

Czech Republic

Naoki Komatsu

Graduate School of Human and Environmental Studies

Kyoto University

Japan

Melinda Krebsz

School of Chemical Engineering

University of Adelaide

South Australia

and

Department of Chemistry

College of Engineering, Science and Technology

Fiji National University

Fiji

Seunghyun Lee

Department of Creative IT Engineering

Pohang University of Science and Technology

Republic of Korea

Donghyun Lee

Department of Creative IT Engineering

Pohang University of Science and Technology

Republic of Korea

Stefania Lettieri

Nano Carbon Materials

Istituto Italiano di Tecnologia (IIT)

TurinItaly

Chia‐Hua Lin

Department of Biotechnology

National Formosa University

YunlinTaiwan

Dusan Losic

School of Chemical Engineering

University of Adelaide

South Australia

and

ARC Research Hub for Graphene Enabled Industry Transformation

University of Adelaide

South Australia

Sing Muk Ng

Swinburne Sarawak Research Centre for Sustainable Technologies

Swinburne University of Technology

Malaysia

and

Faculty of Engineering, Computing and Science

Swinburne University of Technology

Malaysia

Yann Huey Ng

Swinburne Sarawak Research Centre for Sustainable Technologies

Swinburne University of Technology

Malaysia

Jitka Neburkova

Institute of Organic Chemistry and Biochemistry of the CAS

Czech Republic

and

First Faculty of Medicine, Charles University

PragueCzech Republic

Yadollah Omidi

Research Center for Pharmaceutical Nanotechnology

Biomedicine Institute

Tabriz University of Medical Sciences

Iran

and

Department of Pharmaceutics

Faculty of Pharmacy

Tabriz University of Medical Sciences

Iran

Tibor Pasinszki

Department of Chemistry

College of Engineering, Science and Technology

Fiji National University

Fiji

Goutam Pramanik

Institute of Organic Chemistry and Biochemistry of the CAS

Czech Republic

Susanta K. Sarkar

Single Molecule Biophysics Laboratory

Department of Physics

Colorado School of Mines

GoldenUSA

Meng‐Chih Su

Department of Chemistry, Sonoma State University

California

USA

François Treussart

Laboratoire Aimé Cotton, CNRS

Université Paris‐Sud, ENS Paris‐Saclay and Université Paris‐Saclay

OrsayFrance

Kumud M. Tripathi

Department of Bionanotechnology

Gachon University

South Korea

Tran Thanh Tung

School of Chemical Engineering

University of Adelaide

South Australia

and

ARC Research Hub for Graphene Enabled Industry Transformation

University of Adelaide

South Australia

Vaclav Vanek

Institute of Organic Chemistry and Biochemistry of the CAS

Czech Republic

Wen‐Cheng Wang

Research Center for Environmental Changes

Academia Sinica

TaipeiTaiwan

Junyan Yan

Jiangsu Provincial Key Laboratory of Parasite Molecular Biology

Jiangsu Institute of Parasitic Diseases

WuxiChina

Lingyan Yang

CAS Key Laboratory of Nano‐Bio Interface

Suzhou Institute of Nano‐Tech and Nano‐Bionics

Chinese Academy of Sciences

SuzhouChina

Jiantao Yu

School of Advanced Materials

Peking University Shenzen Graduate School

Peking UniversityShenzenChina

Series Preface

Carbon, the sixth element in the periodic table, is extraordinary. It forms a variety of materials because of its ability to covalently bond with different orbital hybridizations. For millennia, there were only two known substances of pure carbon atoms: graphite and diamond. In the mid‐1980s, a soccer‐ball‐shaped buckminsterfullerene, namely a new carbon allotrope C60, was discovered. Together with later found fullerene‐structures (C70, C84), the nanocarbon researcher was spawned. In the early 1990s, carbon nanotubes were discovered. They are direct descendants of fullerenes and capped structures composed of 5‐ and 6‐membered rings. This was the next major advance in nanocarbon research. Due to their groundbreaking work on these fullerene materials, Curl, Kroto, and Smalley were awarded the 1996 Nobel Prize in chemistry. In the beginning of the 2000s, graphene was prepared, using Scotch tape. It is a single sheet of carbon atoms packed into a hexagonal lattice with a bond distance of 0.142 nm. For their seminal work with this new nanocarbon material, Geim and Novoselov were awarded the 2010 Nobel Prize in physics. As new members, carbon nanoparticles such as diamond nanoparticles, carbon dots, and graphene (quantum) dots have emerged in the family of nanocarbon materials. Although all these materials consist only of the same carbon atoms, their physical, chemical, and engineering features are different, which are fully dependent on their structures.

The purpose of this series is to bring together up‐to‐date accounts of recent developments and new findings in the field of nanocarbon chemistry and interfaces, one of the most important aspects of nanocarbon research. The carbon materials covered in this series include diamond, diamond nanoparticles, graphene, graphene‐oxide, graphene (quantum) dots, carbon nanotubes, carbon fibers, fullerenes, carbon dots, carbon composites, and their hybrids. The formation, structure, properties, and applications of these carbon materials are summarized. Their relevant applications in the fields of electroanalysis, biosensing, catalysis, electrosynthesis, energy storage and conversion, environment sensing and protection, biology, and medicine are highlighted in different books.

I certainly want to express my sincere thanks to Sarah Higginbotham, Jenny Cossham, Emma Strickland, and Lesley Jebaraj from Wiley. Without Sarah’s efficient help and valuable suggestions during this project, the publication of this book series would not have been possible. Last, but not least, I want to thank my family, especially my wife, Dr. Xiaoxia Wang and my children Zimo and Chuqian Luisa, for their constant and strong support as well as for their patience in letting me finalize such a book series.

Nianjun Yang

Series Editor

Siegen, Germany

Preface

Carbon nanomaterials have different shapes and structures, including carbon nanotubes, graphene, nanodiamonds, fullerenes, carbon nanodots, and carbon nanohorns, which have been widely applied for biomedical applications in the past few decades. However, not every carbon nanomaterial is promising for the respective applications. During the production of this book, we interviewed our chapter contributors to propose the best three carbon nanomaterials in their field of interest and share their valuable opinions here. It is interesting to find that carbon nanotubes, graphene, and nanodiamonds are the popular choices among our contributors. Their discussions about various biomedical applications of carbon nanomaterials can be found in the forthcoming chapters.

This book focuses on the application of carbon nanomaterial for bioimaging and bioanalysis. They form an important basis for modern diagnostics. Bioimaging can help us visualize biological structure and function at both microscopic and macroscopic levels. Imaging provides detailed information about biological structures. For instance, magnetic resonance imaging, X‐rays, radio imaging, ultrasound imaging, and optical imaging are now routinely applied for disease diagnosis. We are interested in fluorescence imaging, the most widely used optical method, because it offers resolution down to the submicrometer scale near the surface. (Carbon nanomaterials, such as nanodiamonds, have already been applied for super‐resolution imaging.) Furthermore, the combination of optical bioimaging with therapeutical approaches has inspired extensive studies to explore the biomedical applications of carbon nanomaterials.

On the other hand, carbon nanomaterials can be applicable for bioanalysis. Bioanalysis is a term derived in 1970 [1] that has traditionally described the chemical analysis of drugs in biological fluids. However, these analytical techniques and technologies cross many disciplines, such as forensic science and have already been applied to measure a wide range of biological molecules in biological fluids [ 1 ] . In this book, we discuss the application of carbon nanomaterial to analyze biological molecules, which will promote interdisciplinary research.

Structure of This Book

The purpose of this book is to provide background knowledge for entry into the biomedical applications of carbon nanomaterials. This book should be accessible to both advanced undergraduate students and graduate students. The intended audience is broad, including academic researchers, industry professionals, as well as instructors in biology, chemistry, physics, materials science, and other related areas.

The first part of this book is focused on the basic properties of carbon nanomaterials. Careful reading of Chapter 1 is important for newcomers who are not familiar with the structure of carbon nanomaterials. Chapter 2 describes the chemical functionalization of carbon nanomaterials. Chapter 3 gives an overview of the current technology in optical bioimaging and phototherapy. In Chapters 4–7 we apply carbon nanomaterials for bioimaging in different areas, high‐resolution imaging, infra‐red imaging, long‐term tracking, and photoacoustic imaging, respectively. Chapters 8 and 9 elaborate the applications of carbon nanomaterials for bioanalysis. Chapters 10–12 discuss the applications of carbon nanomaterials for therapy, especially phototherapy. Each chapter will propose the best three carbon nanomaterials in the field of interest. In this book, we highlight the latest developments and potential clinical translation of carbon‐based technologies for imaging and bioanalysis with the promise of improved therapeutic outcomes. Finally, we hope this book can guide our readers in exploring the world of carbon nanomaterials.

Reference

1 Hill, H. (2009). Development of bioanalysis. Bioanalysis 1 (1): 3–7.

Part 1

Basics of Carbon Nanomaterials

1

Introduction to Carbon Structures

Meng‐Chih Su¹ and Yuen Yung Hui²

¹ Department of Chemistry, Sonoma State University, California, USA

² Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan

1.1 Carbon Age

Welcome to the Carbon World!

It seems that we already know the chemistry of carbon quite well, mostly in the form of graphite, through its long association with humans in history. Carbon is a simple element, predominantly (99% natural abundance of C‐12) containing the same number (6) of protons and neutrons, and therefore is radioactively stable. Of its other isotopes, C‐13 (1%) and C‐14 (<100 ppm), only C‐14 is radioactive with half‐life time lasting for a long 5730 year. It has been used routinely for the chronological tracking in archeology. With its relatively low mass, carbon is in the sixth place of all the hundred plus elements known today with an atomic number of Z = 6.

In nature, carbon ranks as the fourth element of cosmic abundance, after hydrogen, helium, and oxygen, and counts for about 20% by weight of all livestock and humans on earth, next only to oxygen (60%). Grouped together with silicon and germanium (4A group) in the Periodic Table of Elements, the chemical reactivity of elemental carbon may be considered inert compared to the neighboring nitrogen (5A) and oxygen (6A) groups because of its thermodynamic stability. Hence, graphite solid is used as a reference state for just about all thermodynamic measurements and data. Of course, such inertness is changed completely when carbon is combined with hydrogen, oxygen, and/or nitrogen − a whole world of interesting reactions will occur, and as we have learned in our college organic chemistry courses, the strong carbon chain becomes backbone of all organic molecules.

So, why carbon now? In science, carbon appears as one of the most common elements supporting of and interacting with a vast variety of materials since probably dawn of human civilization and, like a dear old friend, we thought we had known all there was to know about carbon. What is it so special about carbon, among all elements on our planet Earth, that scientists and researchers are now calling it the carbon age [1–3]? Specifically, why the carbon nanomaterials? Perhaps there is something about this old friend that we never knew, which can only be revealed as we look carefully into the nanometer regime of carbon. In this section, we trace the development of carbon nanomaterials in order to explore their molecular structures and intrinsic properties, which have shown great potential to many innovate practices and applications both in research and as commercial products.

1.2 Classification

Carbon nanomaterials have four major allotropes for biomedical applications: carbon nanotube (CNT), graphene (GR), nanodiamond (ND), and fullerene; all consisting of only carbon atoms, of which each has its own unique three‐dimensional nanostructure. These forms of carbon are often referred to as one‐dimensional CNT, two‐dimensional GR, three‐dimensional ND, and zero‐dimensional fullerene, based mainly on its structure and electronic properties.

Graphite is normally viewed as graphene layers stacking up in three dimensions and hence not an allotrope of its own. Multiple forms of carbon, including fullerene, CNT, GR, and ND, present a rich class of solid‐state materials that are both environmentally friendly and sustainable among other unique features each of their own.

1.3 Fullerene

A brief look in the history and development of carbon nanomaterials here seems in order to reacquaint us with some milestones in the development of new forms of carbon, with an emphasis on their unique structures and general features. A good place to start is when carbon nanoparticles first entered the center stage of science, as Richard Smalley, with help from Robert Curl, boldly introduced the world in 1985 with his soccer ball structure for the C60 molecule, now recognized as buckminsterfullerene (or fullerene, in short). An immense (and somewhat unexpected) passion caught up almost immediately in the science community to search for members of the fullerene family, to learn their chemistry, as well as to explore the possible applications of these new buckyballs. Indeed, C70, C76, C84, and other fullerene siblings were found in no time, all sharing a set of unique features as outlined here.

Fullerene is formed in a hollow cage made out of only carbon atoms with each carbon connected to three immediate neighbors in a small ring configuration, which consists of either five or six members, pentagonal, and hexagonal, respectively. As D'Arcy Thompson pointed out at the turn of the 1900s, no system of hexagons alone can enclose a space; it would need exactly 12 pentagonal rings in combination with a varying number of hexagonal rings to piece together a fullerene cage. This is known as Euler's theorem in geometry and topology, which states that the number of vertexes (V), edges (E), and faces (F) of a simple polyhedron is related by the formula V − E + F = 2.

For C60, as shown in Figure 1.1 [4], the 60 carbon atoms give V = 60; three bonds per atom and a total of 3 × 60 = 180 bonds, or 180/2 = 90 edges (each edge shared by two atoms). Following Euler's formula, 60 − 90 + F = 2, C60 has 32 faces (32 rings), i.e. 20 hexagonals and 12 pentagonals. Therefore, fullerenes are defined as "polyhedral closed cages made up entirely of n three‐coordinate carbon atoms and having 12 pentagonal and (n/2 − 10) hexagonal faces, where n is greater than or equal to 20." So, C70 has 25 hexagonals, C84 has 32, and so forth. Other polyhedral closed cages made up entirely of n three‐coordinate carbon atoms are known as quasi‐fullerenes. Also shown in Figure 1.1 is that no two pentagonals can share the same edge, consequently, every carbon atom must belong simultaneously to one pentagonal and two hexagonal rings. Therefore, while C60 is shaped as a true buckyball (spheroidal), other fullerenes such as C70 appear as an oblong figure more resembling a rugby ball rather than a soccer ball.

Image described by caption and surrounding text.

Figure 1.1 C60 fullerene (or soccer ball) [4].

Fullerenes are stable molecules. C60, for example, weighs only 720 amu in a tiny size of about 1 nm in diameter, likely to survive even in galaxy and interstellar journeys. This had motivated Smalley (based on Harold Kroto's suggestions) to do experiments in searching for new and stable carbon species that may be present in outer space. In an early study of the chemistry of fullerenes [5], both C60 and C70 were discovered become superconducting when doped with alkali metals yielding a record high temperature at the time: 18° in Kelvin. On the other hand, when placed under favorable conditions (mostly photolytic catalyzed reactions), fullerene may become reactive with incoming nucleophiles, converting carbon's sp² ‐hybridization to sp³ through nucleophilic additions. During the reaction, the hexagonal benzene rings remain largely inert as nucleophiles interacting with the electrons on the pentagonals. The change in hybridization, accompanied by a conformational change from the curved planar to a three‐dimensional structure, is believed to relax the overall surface strain, providing necessary stability for the final product.

In 1996, the Nobel Prize in chemistry was awarded to Robert F. Curl Jr., Harold W. Kroto, and Richard E. Smalley for their discovery of fullerenes. It is clear now as we look back that the impact brought by the discovery of C60 is not merely the discovery of a new form of carbon in material; rather, it has opened our eyes to see the infinitive possibilities of new structures that can be constructed by familiar elements, carbon and beyond, and the applications never imaginable that they can bring to humans. After all, we may not know carbon, a dear old friend of ours, as well as we have thought we did.

1.4 Carbon Nanotubes

Around 1990, while exploring new forms of fullerenes, Smalley suggested the possibility of tubular fullerene: a straight segment of carbon tube capped, perhaps, by two hemispheres of C60 on both ends of the tube. While Smalley's idea may or may not link directly to the grand entry of CNTs to the scientific world, certainly when Sumio Iijima of NEC (Japan) reported the first observation of multiwall nanotubes (MWNT) [6] in the following year (1991), the world was brought to witness yet another wave of awes and wonders of another new form of carbon. Iijima was able to demonstrate clearly the presence of concentric nanotubes produced by arc discharge evaporation of graphite, each comprised of 2–50 layers of carbon sheets. At the center, the smallest hollow tube has a diameter of 2.2 nm with additional layers each separated from the adjacent others by 0.34 nm, matching the distance between two adjacent layers in bulk graphite. Two years later, in 1993, single‐walled carbon nanotube (SWNT) was identified for the first time in a lab by Iijima and Ichihashi [7] and, independently, by Bethune et al. [8]. SWNT is recognized as rolling a single layer one atom thick of sp² ‐hybridized carbon sheet into a cylindrical configuration with the diameter ranging from few angstroms to nanometers, all depending on the particular method and conditions of synthesis. The length typically varies from hundreds of nanometers to micrometers [9].

The strong carbon–carbon bonds and the network they have formed make CNTs 375 times stronger than steel and only 1/16 as dense. As a result, CNTs have found their commercial use easily in lightweight applications, such as bicycle frames and golf clubs. With its enormous specific surface area, >1600 m² g−1 , SWNT shows an extraordinary surface adsorption capability comparable or, in some cases, better than activated carbon as an adsorptive material, all because of its highly unusual structure.

1.4.1 Structure

Almost all of CNT's intrinsic properties are derived from its 3‐D structure, which at first glance looks like chicken wire, all composed of six‐member rings. We use SWNT here as an example to describe parameters necessary to define the CNT structure. As shown in Figure 1.2 [10], a single‐layer nanotube can be pictured as cutting off a section from carbon sheet, represented by dashed lines on the figure, and rolling it over to seal on both edges (dashed lines) seamlessly so that at any point along one edge (for example, point A) will match an equivalent point A' on the other edge perfectly, and a cylindrical tube is thus formed. Depending on how the section is cut, the relative orientation of the cuts with respective to the sheet will determine the structure of CNT and, more than anything else, many of its physical properties. Furthermore, the orientation of these cuts and the size of the tube (in diameter) are all defined by a pair of numbers, called the chiral index (n, m), which is described below.

Image described by caption and surrounding text.

Figure 1.2 SWNT formed from carbon sheet to show unit vectors, chiral vector, chiral index, and angle [10].

The first and foremost structural parameter to describe nanotubes is the chiral vector, C h (Figure 1.2), which follows along the circumference of the tube and is perpendicular to the wall. C h connects two equivalent sites (A and A′) on a graphene sheet and is defined as C h = n a1 + m a2 , where a1 and a2 are unit vectors of the hexagonal honeycomb lattice and n and m are integers. Chiral vector C h also defines a chiral angle, θ, which is the angle between C h and the zigzag direction of the graphene sheet (Figure 1.2). In a sense, the zigzag vector originated at point A, which intersects only joining points of rings, defines the orientation of the carbon sheet, whereas C h results from CNT. The angle θ is related to chiral index as θ = tan −1 [(3)¹/² m/(2n + m)]. In addition, the diameter (d) of the tube is based on chiral index, d = [((3)¹/² /π)(m ² + mn + n ² )¹/² ] d cc, where d cc is the C─C bond length (0.142 nm). Now it is clear that every nanotube topology can be characterized by the pair of (n, m), which in turn defines the unique symmetry of CNT as chiral or achiral (armchair (n, n) or zigzag (n, 0)), see Figure 1.3 [11]. We will have a lot to say about chirality later because it is tied closely to the electronic properties of CNT. For now, the example in Figure 1.2 shows a chiral index of (5,3), which is a chiral SWNT, with a chiral angle of θ = 22° and diameter d = 0.55 nm.

Image described by caption.

Figure 1.3 Schematic diagram of three representative SWNT structures in similar diameter: (a) armchair; (b) zigzag; and (c) chiral [11].

1.4.2 Electronics

A promising direction for the future of electronics is aligned with molecular electronics [12], in which the active part of the device is composed of a single or a few molecules, and the most actively studied forms of molecular electronics are based on CNT. Depending on their chirality and diameter, CNTs can be metals or semiconductors. The huge aspect ratio (tube length/diameter) of CNT makes it an ideal one‐dimensional (1D) electronic system, drastically reducing charge carrier scattering. It was found that 1D carbon crystalline lattice could conduct electricity at room temperature with virtually no resistance, a phenomenon known as ballistic transport, where electrons can move freely through the structure without any scattering from atoms. Furthermore, the lack of interface states as in the silicon/silicon dioxide interface provides greater flexibility to the fabrication process. All these fascinating properties (and more) came from CNT's unique electronic structure, as briefly described below.

CNT can be considered as rolling a section from a carbon sheet, and as such, the electronic structure of CNT is also derived from graphene, whose electronic properties will be discussed latter and should be cross‐referenced if necessary. Instead of going over rigors of band theory, which is a fascinating subject of its own, we use examples here through developing CNT's energy band structure. In Figure 1.4a, a SWNT is folded in the (5, 2) chirality, i.e. C h = 5a 1 + 2a 2, which has a 0.49 nm tube diameter [13]. Let's now look into the band energies starting with graphene. Quantum mechanical computation has calculated the band energy of graphene, and a three‐dimensional representation of the valence and conduction bands is shown as π‐ and π*‐bands, respectively, in Figure 1.4b. Graphene is a semimetal with two energy bands degenerate only at six corners of the hexagonal first Brillouin zone in K‐space. This can be seen in the top diagram of Figure 1.4b, where the two energy surfaces of π‐ and π*‐bands touch each other at six quantum states (or, 6 K‐points of the first Brillouin zone), which leaves no energy gap between the two bands; electrons are free to conduct and, hence, graphene is metallic. The energy of these 6 K‐points are at the Fermi level, so they are referred to as the Fermi points located on the Fermi surface.

Image described by caption and surrounding text.

Figure 1.4 Construction of CNT (5, 2) and band structure of CNT (3, 3) [13].

Now we apply boundary conditions unique to SWNT and consider the transitions allowed between the valence and conduction bands. It is clear that the wavefunction describing electron behaviors on the nanotube must be forming a standing wave along the circumference of the tube, i.e. kC h = 2πq, where k is the wave vector aligned with the tube circumference (and C h) and q is an integer, so that k is quantized. Consequently, only a subset of graphene's states is allowed when it is rolled into a SWNT. For example, in the bottom diagram of Figure 1.4b, the dark lines represent the allowed states of a (3, 3) nanotube that transitions are allowed between bands. If the states pass through a K‐point (as in this case), the tube is metal and shows electrical conductivity, but if not, the tube is a semiconductor with a band energy gap. Whether a SWNT is a metal or semiconductor depends on its chiral index: If (n − m)/3 = 0 or integer, it is metallic; if not, semiconducting, which means, statistically, only one‐third of CNTs would show a metallic nature. Following this simple rule, both of our examples in Figure 1.4, chiral indices of (5, 2) and (3, 3) are metallic CNTs, whereas in the previous example shown in Figure 1.2, (5, 3), the SWNT is a semiconducting CNT.

With a basic understanding of how band energy is constructed for CNTs, we can zoom in those states responsible for electron transitions, which in turn will determine electronic properties unique only to CNTs. The density of states (DOS) is defined as the number of allowed electron states at a particular energy. Figure 1.5 includes a generalized representation of DOS in various dimensions for carbon nanomaterials [15]. The zero‐dimensional (0D) fullerene has DOS that is discrete in well‐defined energy levels. One‐dimensional (1D) CNTs split each band in sharp peaks DOS, van Hove singularities, which resemble molecular energy levels. In a simple term, for example, the allowed electronic transitions for SWNTs take place between matching peaks shown in the 1‐D DOS. These transitions are labeled as Ejj (E 11, E 22, etc.). The transitional energy varies inversely with nanotube diameter (d) according to

equation

where a C‐C is the C─C bond length (0.142 nm) and γ 0 is the adjacent C─C interaction energy [14]. Therefore, a large size CNT is expected to have relatively small transitional energies.

Graphs displaying descending and ascending curves for 3D DOS, descending and ascending step curves for 2D DOS, shaded and unshaded curves with 3 steep peaks for 1D DOS, and 2 sets of horizontal lines for 0D DOS.

Figure 1.5 Schematic diagram of energy vs. density of states (DOS) for carbon nanomaterials of various dimensions. Occupied electron energy levels (valence band) are shaded and unoccupied levels (conduction band) are white. The first two allowed absorption transitions (E11 and E22) are labeled for the 0D and 1D [14].

In summary, the unique electrical properties of CNTs arise from the confinement of the electrons in a tiny tubular structural configuration, which allows motion in only two directions along the tube axis: forward and backward. With the requirements that energy and momentum must be conserved, these constraints lead to a reduced phase space for the scattering processes that are responsible for the electrical resistance of all CNTs. Because of the reduced scattering, CNTs can carry enormous current densities, as reported, up to 10⁹ A cm−2 without being destroyed [15]. This density is about two to three orders of magnitude higher than Al or Cu. Indeed, CNT is a promising star rising to future electronics. In the following 10 years since the first discovery of CNT, there were nearly 18 000 published CNT papers, setting a phenomenal record for a single material in the public interests.

1.5 Graphene

For all we know, graphene may have been around us for ages. Nevertheless, it was only in 2004 that Konstantin Novoselov and Andre Geim elegantly (using Scotch tape) isolated single‐ and few‐layer of suspended graphene from highly oriented pyrolytic graphite [16]. Graphene is characterized as individual or few‐layer stacked sheets of sp² ‐ hybridized carbon where the number of sheets does not exceed 10 [17]. The structure of graphene is referred to as infinite polycyclic aromatic hydrocarbons (PAH) containing an infinite number of benzene rings fused together. Graphene is the first two‐dimensional nanomaterial known to exist in a suspended form, defying previous conventional knowledge that two‐dimensional material would have been too thermodynamically unstable to exist. In fact, with intrinsic strength of 130 GPa, Young's modulus per layer of 350 N m−1 , and breaking strength of 42 N m−1 , graphene is one of the world's strongest material ever discovered and thus warrants a title as a super carbon [18].

As a 2D crystalline membrane, graphene possesses a set of unique properties, collectively, surpassing any other material known today. These properties include an exceedingly large specific surface area (2630 m² g−1 ), low density (<1 g cm−3 ), ultrahigh charge mobility (>2 × 10⁵ cm² V−1 s−1 ), excellent electrical conductivity (106 S cm−1 ) and thermal conductivity (>5000 W m−1 K−1 ), a uniform broadband optical absorption ranging from ultraviolet UV to far infrared (IR), and superb mechanical strength and unusual flexibility (as mentioned above). To provide a general understanding and appreciation for these extraordinary physical properties, we take a structure–property approach in explaining the cause‐and‐consequence relation of graphene, starting with simple valence bond theory.

1.5.1 Structure

The honeycomb structure of graphene is constructed by PAH of benzene rings (see above) where each carbon atom uses three of its four valence electrons to form sp² ‐hybridized covalent bonds with three co‐planar neighboring carbons. The fourth valence electron occupies the carbon's pz orbital that forms π‐bonds shared equally in three directions leading to a C─C bond order of 1 and 1/3. Delocalized π‐electrons now spread over on a continuous layer of honeycomb consisting of short and rigid covalent bonds; together, the pool of π‐electrons provides graphene with an extremely strong stability capable of withstanding a great deal of mechanical strain and stress, indicated by its unusually high Young's modulus value (the ratio of normal stress over normal strain), making it one of the strongest material known. On a smooth flat surface like this, any injected charge carriers can run freely at an incredibly high speed, while the graphene sheet experiencing rapid lattice vibrations, i.e. phonons. Both effects contribute to excellent electrical and thermal conductivity. Furthermore, because all carbon atoms are identical and symmetrically arranged on the honeycomb plane, graphene is nonpolar and hydrophobic except at the edges; therefore, it has very poor solubility in water and/or common polar solvents used in lab, which actually imposes challenges on processing graphene functionalization.

1.5.2 Electronics

We now turn to electronic properties of graphene, the long‐continuing interest in physics since its debut in 2004, and the promising potential of graphene to be the building blocks for the next generation of electronics. It all starts with graphene's perfect conjugated structure. Following the tight‐binding model, in general, the valence electrons are assumed to occupy molecular orbitals delocalized throughout the entire solid molecule. Therefore, each carbon contributes one electron to the overall pool of π‐electrons on a graphene sheet. Meanwhile, the atomic orbital originally occupied by the valence electron of the carbon atom will now overlap with nearest neighbors to form a total of 2 N molecular orbitals for N carbon atoms of a graphene sheet. The energy levels of these molecular orbitals are close to each other and all together grouped in an energy band. Each band consists of (in order of increasing energy) bonding, nonbonding, and anti‐bonding states. At absolute zero in temperature, only the lower half of the graphene energy levels (or N molecular orbitals) are occupied and the energy level of the highest occupied molecular orbital (HOMO) is the Fermi level. Graphene has N π‐electrons that can fill all the way up to the Fermi level, and because there are empty orbitals very close to this level, it would take hardly any energy to excite the uppermost electrons. At temperatures above absolute zero, thermal motions of graphene can readily promote electrons to the unoccupied levels. Therefore, electrons can move extremely fast (and, hence, high charge carrier mobility), showing superb electrical and thermal conductivities.

Much interest has been focused on the behaviors of electrons around the Fermi level for the purposes of designing new electronic devices. The electronic structure of graphene was found to have a tiny overlap of its conduction and valence of π‐bands at Dirac point, according to Giem and Novoselov [19], which on the energy scheme is at the Fermi level. Therefore, graphene has zero band gap and behaves as a semiconductor. Some of its intriguing electronic properties derived from zero band gap, including features like massless Dirac fermions and anomalous quantum Hall effect, all require insight and understanding of the electronic structure of graphene that is beyond the scope of this book. Fortunately, because of intense interest and development in this area, the subject has been reviewed periodically and some are written at a level suited for beginners [20].

Now graphene is in the early stages of macroscale manufacturing for commercial use. As of January 2017, Project of Nanotechnology had surveyed 40 commercial products for CNT and graphene. There are examples of prototypes available as commercial products: conductive reinforcement coating on Kevlar fibers, fabrication of large‐area transparent electrodes, and high permittivity composites, only to name a few. Used in energy storage devices, the high surface area of graphene supports energy capacity that is cost‐effective far beyond any other material, i.e. in terms of energy per weight and cost. With all its intrinsic electronic properties, graphene is the top candidate to replace silicon‐based electronics, which is approaching its own material limit. Carbon‐based electronics when eventually commercially realized, promises to perform at lightning speed with superb capacity and, now more pressingly than ever, generate a green energy source that can benefit both socially and economically. Finally, as an endnote, Andre Geim and Konstantin Novoselov received the Nobel Prize in physics in 2010 for groundbreaking experiments regarding the two‐dimensional material graphene.

1.6 Nanodiamonds and Carbon Dots

Diamond is probably the best‐known allotrope of carbon, rooted in the history of the jewelry industry. In diamond crystal, every carbon atom is bonded to four other carbon atoms by sp³ hybridized orbitals. Diamond is a wide bandgap material with the highest hardness and thermal conductivity among natural bulk materials. Nanodiamond has similar physical properties, and has emerged as a promising candidate for biomedical applications owing to its high biocompatibility and low cytotoxicity [21]. In addition, its surface can be facilely functionalized for drug and gene delivery. Recently, the negatively charged nitrogen‐vacancy center in nanodiamond is of particular interest because its emission is in the optical region convenient for bioimaging as shown in Figure 1.6 [22]. The center absorbs strongly at 560 nm and emits fluorescence efficiently at ∼700 nm, well separated from cell autofluorescence. Furthermore, it is perfectly photostable without any sign of photobleaching even under high‐power excitation at the single molecule level. We will discuss more fully the bioimaging application of nanodiamonds in Chapter 4.

Image described by caption.

Figure 1.6 (a) Graphene honeycomb bipartite lattice structure in real space, where the black and white circles denote A‐ and B‐sublattice sites, respectively, a0 is the lattice constant and a = (a, 0) and b = (a/2, 3a/2) are the primitive vectors. The unit cell is indicated by a dotted hexagon. (b) First BZ of graphene. Reciprocal lattice vectors are denoted by a* and b*. K = 2π/a(1/3, 1/√3), K0 = 2π/a(2/3, 0), Γ = (0, 0). (c) π‐band structure of graphene along Γ‐M‐K‐Γ within the irreducible BZ. γ0 is the transfer integral between nearest‐neighbor carbon sites (γ0 = 3.16 eV). (d) Density of states (DOS) of the π band. (e) Three dimensional π‐band structure of graphene near the K point. Pseudospin is indicated by an arrow [20].

Finally, carbon dots are also known as carbon nanodots and carbon quantum dots. A carbon dot is a quasi‐spherical carbon nanoparticle with high oxygen content. Carbon dots have already inspired extensive studies for a wide range of applications [23], which will be the subject of Chapter 9.

Hope you enjoy the journey to the world of carbon nanomaterials!

Acknowledgment

This chapter, which sets the scene for the book, is an adaptation of the author’s own chapter Introduction to Nanotechnology from the forthcoming publication Fluorescent Nanodiamonds, First Edition, Huan‐Cheng Chang, Wesley W.‐W. Hsiao and Meng‐Chih Su. © 2018 John Wiley & Sons Ltd. Published 2018 by John Wiley & Sons Ltd.

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2

Using Polymers to Enhance the Carbon Nanomaterial Biointerface

Goutam Pramanik¹ Jitka Neburkova¹,² Vaclav Vanek¹ Mona Jani¹ Marek Kindermann¹,³,⁴ and Petr Cigler¹

¹ Institute of Organic Chemistry and Biochemistry of the CAS, Czech Republic

² Charles University, First Faculty of Medicine, Prague, Czech Republic

³ Institute of Microbiology of the CAS, Czech Republic

⁴ University of Chemistry and Technology, Czech Republic

2.1 Introduction

The unique ability of carbon atoms to participate in robust covalent bonds with each other and also with other elements in diverse hybridization states (sp, sp² , sp³ ) allows them to form a wide range of structures, from small molecules to nanomaterials [1]. Carbon‐based nanomaterials have garnered a great deal of interest over the past three decades, starting with the debut of fullerene in 1985 [2], followed by carbon nanotubes [3] (CNTs) and graphene [4] in 1991 and 2004, respectively. Fullerene, CNTs, and graphene are primarily composed of sp² carbon atoms with a network of conjugated π electrons. In recent years, carbon dots (CDs) with mixed sp² and sp³ carbon atoms plus defects and heteroatoms, as well as nanodiamonds consisting of sp³ carbon atoms, have also received a great deal of attention [5, 6]. Due to the quantum confinement effect, these carbon nanomaterials (CNMs) possess many interesting physicochemical properties that are not attainable in bulk carbon materials like diamond and graphite [7, 8]. Over the past decade, there has been a flurry of development of biomedical research applications for graphene, CNTs, nanodiamonds, and CDs, owing to their small size, unique optical properties, and large surface area. In this chapter, we will focus on functionalization of CNMs (CNTs, graphene, nanodiamonds, and CDs) with polymers for biomedical applications. For these applications, a stable dispersion of CNMs in biological medium is a fundamental prerequisite that can be achieved by proper surface functionalization of the nanomaterial [9, 10]. Nonfunctionalized CNMs tend to form stable aggregates due to strong intermolecular interactions, such as van der Waals (vdW) forces and dipole–dipole interactions. Aggregation of nanomaterials results in unwanted changes to their physicochemical properties and makes homogeneous cell labeling very difficult. Polymer coating is an effective, convenient, and widely used approach for transferring CNMs into aqueous solution. Polymer coatings provide a number of distinct advantages over small‐molecule coatings, including ease of synthesis and processing, structural diversity, tunable surface functionality, and flexibility. Multiple contact points between the CNM surface and polymers provide a more robust surface coating than coating with small molecules, which typically have only one binding group. The strong binding also prevents desorption of ligands from the surface, which has been one of the sources of toxicity in practical applications. Most importantly, polymers can be designed to include reactive functional groups that can be further functionalized with biomolecules. The design and structure of polymers for functionalization of CNMs depend on the chemistry of the CNM and its potential applications in a biological environment. Polymer coating of CNMs not only increases the colloidal stability of the particles but also selectively changes or influences the CNM properties, such as reducing nonspecific adsorption of biomolecules or tuning the charge (for gene therapy applications). The polymer shell on the surface of CNMs, rather than the core, determines the major final properties of the coated CNMs. These properties are crucial for potential interactions of particles with a biological environment.

2.2 Colloidal Stability of CNMs

To successfully develop nanomaterials for biomedical applications, a detailed understanding of the factors driving the colloidal stability of nanoparticles (NPs) and their aggregation in biological medium is crucial. A classical theoretical approach based on Derjaguin–Landau–Verwey–Overbeek (DLVO) theory is commonly used to predict the stability of a colloidal system. This approach balances attractive vdW forces and repulsive forces caused by the electrostatic double layer (EDL) (Figure 2.1a,c) [12]. The attractive vdW forces are competed by the EDL which stabilizes the colloidal dispersion. In addition to electrostatic stabilization, other repulsive forces (e.g. hydration forces) and specific attractive forces (e.g. depletion and hydrophobic or magnetic forces) also contribute to the stabilization or destabilization of the colloidal system [13]. The DLVO theory has been extended (X‐DLVO) (Figure 2.1d) by adding these individual forces, assuming that each interaction is completely independent.

Image described by caption and surrounding text.

Figure 2.1 Colloidal interactions of NPs. (a) At the most basic level, NP aggregation is governed by van der Waals interactions. Permanent or induced dipoles within the NP can result in net attractive forces between NPs and subsequent aggregation. (b) Macromolecules can physically stabilize NPs. Hydrophilic macromolecules can shield the NPs from interactions, and steric stabilization can be combined with charge stabilization for electro‐steric stabilization. (c) The inherent surface charge of NPs, caused by surface ions or functional groups, results in formation of the stern layer, oppositely charged ions adsorbed to the NP surface. Ions with an opposite charge from the stern layer form the diffuse layer. Together, this is referred to as the electric double layer (EDL). The EDL forms a net charge, and when two like particles are in proximity, the EDL repels the two. (d) Illustration of potential energy (ψ) distance (d) curves. They can be used to