Editor?s Preface: Nanodiamonds - A Rising Star in Nanotechnology

Nanotechnology: From Big to Small

Delivered to the American Physical Society, Richard Feynman's famous 1959 Caltech lecture, "There's Plenty of Room at the Bottom," is today recognized as one of the first and most influential meditations on the topic known today as nanotechnology (6). Nanotechnology is exactly what Feynman spoke of when he "theorized" the compression of massive amounts of information into tiny spaces. Today, nanotechnology is on the brink of becoming precisely what Feynman conceived: A world in which machines are too tiny to be seen.

Though many overemphasize the "nano" of nanotechnology, size is not the most important aspect of this field. Rather, emphasis is on the concept of building machines from a bottom-up perspective. As nanotechnology practitioners, we operate at the atomic level to create improved machines, materials, medicines, energy sources, and commodities. This bottom-up approach has already led to many advances in the world of science and promises to lead to many more over the next century.

Nanotechnology relies on the use of nanomaterials, which are defined as anything on a scale of 1 to 100 nm. Just as Feynman found quantum mechanics to operate at a different level than classic Newtonian physics, scientists are finding that materials often operate on the nanoscale differently than they do on the macroscale. Using these unique nano-properties, scientists develop new technologies that would have been impossible to create in the past.

Many hold the misconception that nanotechnology is a niche field, which it is not. On the contrary, nanotechnology is a multipurpose field, as its impact reaches virtually all industries and research areas.

The Nanodiamond - A Scientist's Best Friend

A true diamond never overshadows.

It's the brilliance of the subtle shine.

That's most attractive.

- Dena Tyson, Xceptance, 2008

Humans have loved diamonds throughout history. On the surface, the reason is simple - they are sparkly. But the more philosophical of our ancestors gave more tangible reasons: diamonds are both rare and unbreakable. Today, researchers can synthesize diamonds, recreating the physical construction of what was once thought to be a singularity of nature.

To synthesize a diamond, we simply copy nature. In the laboratory, researchers convert graphite to physically and chemically identical diamonds found in nature through applying a high transient pressure under a high adiabatic temperature. The first proven synthetic diamond was created in 1954 by General Electric (GE) using the high-pressure, high-temperature (HPHT) method. However, as the size of this synthetic diamond was too small, with the largest size of 0.15 mm, it was applicable for the industrial field instead of gem use. Later, the first nanosized diamonds were developed by USSR in 1963. Today, these substances are known as nanodiamonds (NDs), as the commercial market (which began in the 1990s) calls it.

Modern analytical tools, such as scanning electron microscopy, infrared spectroscopy, optical microscopy, the X-ray diffraction technique, and Raman spectroscopy, help us understand the construction of NDs. The ND is unique in that it is both tiny and tough; traits that common sense often tells us are opposites. Not only are NDs used in the production of purposefully hard materials and polishing media, but they are also used in biotechnology and have the potential to play a major part in saving human lives.

As an allotrope (sp3 hybridization) of carbon, diamond has several important superlative physical properties:

• It is the hardest material known to date.
• It has the greatest thermal conductivity of any bulk material.
• It possesses by far the highest refractive index of all dielectric materials.

Nanoscale diamonds preserve some of these properties. However, the technicalities of ND production have traditionally been expensive, time-consuming, and crude. Since 1963, NDs have been prepared by detonation to generate "detonation nanodiamonds" (DNDs). Today, manufacturers crush or ball mill micrometer or millimeter diamonds obtained by HPHT and chemical vapor deposition (CVD) to reduce production costs. Despite some differences in surface structure, the core of HPHT NDs and CVD NDs are generally similar to DNDs. DNDs are particulates (with an average diameter of 5 nm). Their primary particles have a highly uniform size distribution. Due to the large number of disordered (sp2) carbon atoms on DNDs' surface during the synthesis period, they tend to bind together and form agglomerates. DND clusters can serve as a drug delivery system. Owing to the presence of less sp2 carbon on the surface, HPHT NDs exhibit reduced agglomeration. Therefore, even though their size distribution is broader than DNDs, HPHT NDs are amenable to many bioimaging applications. CVD NDs are thin films formed on substrates. Several industries have used these NDs as surface coating for a variety of cutting and dressing instruments.

DNDs are now taking on new roles. Biologists currently use them as diagnostic and therapeutic tools in biology and medicine, especially for nanometer-sized diamonds (1-4). Thanks to the excellent properties of biocompatibility, high loading capacity, and unique surface function, ND particles are well suited for many drug delivery applications. Another distinguishing attribute of ND is their bright, stable fluorescence, which comes from their crystals' defects. This discovery was made in 2005 (5), and from that, a wide array of studies have been done, marking the beginning of a new era in analytical and biological applications. For instance, fluorescent nanodiamond (FND) enables the use of cell labeling, imaging, and tracking. These applications directly result from FND's exceptionally high biocompatibility and unique optical properties (5). Recent research in the field has focused on applying surface-functionalized FNDs in bioimaging, quantum sensing, and drug or gene delivery. Results have shown that FND, with their surfaces' function group, can conveniently be immobilized with protein or nucleic acid (5). Additionally, when exposed to green-yellow light, FND containing nitrogen-vacancy (NV) color centers can emit bright, nonphotobleaching, nonphotoblinking, tissue-penetrating red photons.

Book Structure

The design and working principles of NDs are the focus of this book. A practical understanding of this field necessitates a multidisciplinary background: Physics, chemistry, materials science, electrical and optoelectronics engineering, as well as bioengineering are all viewpoints that may be added to the ND.

This book contains 13 chapters:

• Chapter 1: Nanodiamonds
• Chapter 2: Fluorescent Nanodiamonds
• Chapter 3: Fabrication of Fluorescent Nanodiamonds
• Chapter 4: Surface Modification of Nanodiamonds
• Chapter 5: Toxicity Assessments of Nanodiamonds
• Chapter 6: In Vitro Bioimaging of Fluorescent Nanodiamonds
• Chapter 7: In Vivo Bioimaging of Fluorescent Nanodiamonds
• Chapter 8: Quantum Sensing of Fluorescent Nanodiamonds
• Chapter 9: Nanoscale Thermometry with Fluorescent Nanodiamonds
• Chapter 10: Nanodiamond-Enabled Drug Delivery
• Chapter 11: Nanodiamond for Mass-Spectrometry-Based Analysis of Peptides, Proteins, and Proteomes
• Chapter 12: Emerging Roles of Artificial Intelligence in Nanodiamond Sensing
• Chapter 13: Perspective and Outlook on Nanodiamond Research

Research on the unique properties of ND/FND will likely lead scientists to find new uses for these materials, especially as particles in a multimodal imaging/therapy platform. This book touches on four major areas of ND/FND: surface modification, bioimaging, drug delivery, and quantum sensing. We hope this book can help readers choose an appropriate approach in successive ND research.

Surface Modification

As a carbon-based nanomaterial family member, NDs appear to be the most biocompatible compared with other nanoscale carbon-based materials. For this reason, biologists have begun researching their potential applications in nanomedicine (7). However, NDs or any materials practically used in biology face the following challenges: difficulties in directly modifying the NDs surface, low colloidal stability due to its aggregation in physiological media, and nonspecific protein adsorption.

In addition, colloidal stability is one of the most important features affecting the application of NDs. Like other nanoparticles (NPs), NDs' colloidal stability is not static and can decline in solution with high ionic strength. This poses a challenge for the usage of NDs in biological systems. All physiological buffers and culture mediums have high salt content. Therefore, prior to developing analytical and biological applications, researchers must first understand the surface features and aspects of different environments. This is because these environmental interactions influence the colloidal stability of NDs.

The book examines three strategies for modifying the surface of ND: functionalization,...