Foreword

In the last decade, bioimaging and therapy based on near infrared (NIR) nanomaterials have played an important role in biotechnology due to their intrinsic advantages over traditional imaging probes and medicines, such as greater penetration depth, low detection threshold concentration, and better targeted performance. Nanomaterials based on organic dyes, lanthanides, carbon, quantum dots (QDs), and noble metals are major components in this big family of NIR bionanomaterials. Exciting developments have been made at a very fast pace by many research groups. The vast literature published about NIR nanomaterials over the past two decades is a clear witness to this; the number of papers has increased exponentially, with most of the activity and development happening in the last 10 years.

NIR materials have absorption/excitation/emission maxima falling in the region of minimal tissue absorbance/autofluorescence between 650 and 1700 nm, an “imaging window.” In tissues, light absorbance and scattering is minimal in this wavelength range, allowing light to penetrate more deeply. This enables animal bioimaging and therapy with high sensitivity in real time without the need for dissection or invasive procedures. In the past two decades, related theories, methods, and techniques have been explored. As a consequence, novel NIR materials are increasingly emerging, and their applications extend from traditional fields such as optical communication amplifiers and solid-state lasers to high-tech fields including biosensors, bioimaging, and disease therapy. Researchers in this field can therefore give a deep insight into synthesis strategies and behavior of NIR nanomaterials, and in particular, establish structure–function–synthesis relationships.

This book contains 11 chapters. Chapter 1 describes some distinctive characteristics of lanthanide-based NIR nanomaterials (upconversion and downconversion) which are directly related to their bioimaging applications, such as color tunability, energy transfer principles, and some strategies for enhancing luminescent efficiency. Bioimaging based on NIR QDs has advantages including lower absorption and relatively low autofluorescence, resulting in deeper penetrating depth and lower background. Chapter 2 summarizes developments in non-toxic QDs, especially for synthesis and in vivo bioimaging. Chapter 3 introduces the fabrication and fundamental properties of carbon dots (CDs) and nanodiamonds (NDs) and then focuses on their recent bioapplications in bioimaging. Challenges and perspectives for future developments are also briefly discussed. We hope this chapter will provide critical insights to inspire further exciting research on CDs/NDs for biological imaging applications, to better realize the potential of these intriguing materials in the near future. Chapter 4 summarizes recent progress in the synthesis and in vivo behavior of NIR-emitting gold nanoparticles, and discusses future challenges and opportunities for them.

Besides the inorganic NIR nanomaterials already mentioned, nanomaterials based on organic molecules are a novel type of NIR imaging agents. Chapter 5 focuses on recent progress in this area, including major NIR organic chromophores, luminescent principles, and construction methods, as well as biomedical applications and challenges. Photodynamic therapy (PDT) is a treatment for cancer that uses the reactive oxygen species generated by a photosensitizer drug following irradiation at a specific wavelength to destroy cancerous tissue. Chapter 6 provides an overview of the main principles and mechanisms for biosensing based on NIR QDs and the use of QDs for simultaneous diagnostics and therapy of disease. Chapter 7 focuses on state-of-the art use of NIR nanomaterials for PDT, including both in vitro and in vivo applications. Chapter 8 introduces NIR light-triggered drug and gene delivery platforms, including photoresponsive nanocarriers, photocaging of bioactive cargos, and photothermal transduction for NIR-triggered nanocarriers. It has been suggested that, due to the differences in cell-killing mechanisms, synergetic tumor responses may be achieved if two modalities are combined in an appropriate sequence. Therefore, multifunctional nanocarriers that enable combination cancer therapy with different therapeutic mechanisms in one system may play increasingly important roles in the fight against cancer due to their unique advantages such as minimal side effects and high efficacies. In Chapter 9, we discuss the most significant progress made in the field of NIR-responsive nanotheranostics for synergistic cancer therapy. NIR-induced photothermal ablation therapy (PAT) has attracted increasing interest as a minimally invasive and potentially effective treatment technology for cancer. A prerequisite for the development of NIR-induced PAT is to obtain low-cost and biocompatible photothermal agents with high photothermal conversion efficiency. In Chapter 10, we first introduce the measurement method for photothermal conversion efficiency, and then summarize the research progress of these photothermal agents as well as the combination of PAT with other nanobiotechnology techniques.

Numerous and extensive studies have been devoted to the design, synthesis, and development of various NIR nanomaterials for biomedical imaging and imaging-guided therapy of cancers; however, so far there is little information on the toxicological properties of NIR nanomaterials and their long-term toxicity or health effects. Moreover, most of the present data are either conflicting or not in the public domain, preventing the scientific community from properly evaluating the effect of nanomaterials on human health and environment. Therefore, Chapter 11 focuses primarily on recent progress in toxicity studies of NIR nanomaterials (including carbon-based materials, QDs, noble metal-based nanoparticles, upconversion nanoparticles, and narrow-bandgap semiconductors), discussing in detail how the biophysicochemical properties of NIR nanomaterials influence their toxicity, and finally presenting a broad overview of the available in vitro and in vivo toxicity assessments.

Research on NIR nanomaterials has developed rapidly in the past decade. A comprehensive review is thus necessary, and it is the main purpose of this book. Chapters are organized along the following lines: (1) following the forefront of current research, and striving to reflect the latest progress and developments; (2) comprehensive review focusing on basic fundamental research; and (3) practical research experience in methodology, experimental skills, and data analysis. Each chapter also includes understanding, induction, and summaries from the authors. We have taken care to include fundamental information about NIR nanomaterials, making the book especially suitable for beginners and graduate students who have just entered this field. We hope that, through reading this book, they can fully understand the chemistry of photon upconversion nanomaterials, grasp the skills required for their synthesis, obtain high-quality materials, and therefore learn to deeply appreciate the chemical and physical properties of these materials and their applications.

This book is a distillation of the authors' knowledge and hard work. We want it to help and inspire researchers who are working in the fields of chemistry and materials science, especially in nanobiology. We also hope it can provide a reference source or serve as a textbook for undergraduate and graduate students majoring in chemistry, chemical engineering, physics, materials science, and biology, as well as readers who are already interested in NIR nanomaterials.

Galen Stucky

University of California, Santa Barbara