CL-160124
Received: February 5, 2016 | Accepted: February 19, 2016 | Web Released: February 27, 2016
Highlight Review
Anisotropic Gold-based Nanoparticles: Preparation, Properties, and Applications Yasuro Niidome,*1 Aung Thu Haine,2,3 and Takuro Niidome*2 1
Department of Chemistry and Bioscience, Graduate School of Science and Engineering, Kagoshima University, 1-21-35 Korimoto, Kagoshima 890-8580 2 Department of Applied Chemistry and Biochemistry, Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555 3 Department of Chemical Engineering, Yangon Technological University, Gyogone, Insein Road, Insein P.O., Yangon, 11-011, Myanmar (E-mail:
[email protected],
[email protected],
[email protected]) Prof. Yasuro Niidome received his Bachelor degree of Science from Kagoshima University in 1989, and Master and Ph.D. degrees of Engineering from Tokyo Institute of Technology in 1991 and 1994, respectively. He was appointed as an Assistant Professor at Kyushu University in 1994. In 2007, he was appointed as an Associate Professor at Kyushu University. In 2014, he was appointed as an Associate Professor of the Department of Chemistry and Bioscience at Kagoshima University. His research interests are in the field of colloid chemistry and nanostructured materials for bioapplication. Mr. Aung Thu Haine received his Bachelor degree of Science from Yangon Technological University (Myanmar) in 2004. In May 2007, he received an International Program of Master of Science scholarship awarded by Thailand International Development Cooperation Agency (TICA) and graduated from Mahidol University (Thailand) in April 2010. After graduated from Mahidol University, he was appointed as Lecturer at Mandalay Technological University. In 2014, he received the JICA fellowship for his doctoral degree. He is now conducting his doctoral degree research in Kumamoto University (Japan). His research interests are in bio-nanotechnology and biomedical engineering fields. Prof. Takuro Niidome received his Bachelor degree of Science from Kagoshima University in 1989, and Master and Ph.D. degrees of Science from Kyushu University in 1991 and 1994, respectively. He was appointed as an Assistant Professor at Nagasaki University in 1994. From 2001 to 2002, he was a visiting researcher of University of Pittsburgh (Supervisor: Prof. L. Huang) in the USA. In 2004, he was appointed as an Associate Professor at Kyushu University. In 2007, he was assigned an additional post as a researcher for Program of Precursory Research of Embryonic Science and Technology, Japan Science and Technology Agency (JST) to 2011. In 2012, he was appointed as a full Professor of the Department of Applied Chemistry and Biochemistry at Kumamoto University. His research interests are in the field of nanomaterials and their medical application.
488 | Chem. Lett. 2016, 45, 488–498 | doi:10.1246/cl.160124
© 2016 The Chemical Society of Japan
Abstract Among the several types of nanoparticles in existence, anisotropic gold-based nanoparticles demonstrate special advantages because of their unique properties. Gold nanoparticles can be prepared and modified with a wide range of functional materials including polymers, surfactants, ligands, dendrimers, drugs, DNA, RNA, proteins, peptides, and oligonucleotides. Hence, they are a promising vehicle for bioimaging, drug delivery, and other therapies. This review mainly addresses the properties, preparation, and applications of anisotropic gold nanoparticles in biomedical applications and targeted drug delivery.
Table 1. Different shapes that have been achieved for various metal nanocrystals (Reprinted with permission from reference13)
Keywords: Gold nanoparticles | Optical properties | Drug delivery
Introduction Gold is the most popular material used in functional nanoparticles. Advantages of gold and gold-based nanoparticles are their chemical stabilities, shape and size controllability, and reproducibility in preparation.1 Nanoparticles of gold and silver show remarkable extinction bands in visible and near-infrared (near-IR) regions. Surface plasmon (SP) bands originating from the collective motions of free electrons encapsulated in nanoparticles have very different physics from that of organic molecules, whose spectroscopic properties originate from electron transitions. The SP bands of uniform gold nanoparticles are sensitive to size and shape changes, and show dramatic spectral changes when nanoparticles form aggregates. This property makes it very convenient for exploring surface modifications, because the spectroscopic properties of SP bands provide information about uniformity and colloidal dispersion before and after a surface modification. Microscopic observation is therefore not always required to evaluate the colloidal dispersion of gold nanoparticles. The SP bands of gold nanoparticles have been studied for almost 200 years.2 Theoretical modeling of spherical metal nanoparticles was carried out by Mie in 1908,3 through a rigorous analytical solution of Maxwell’s equations that describe absorption and scattering of spherical nanoparticles in homogeneous surroundings. “Mie’s Theory” provides a useful explanation for the mysterious red-color and green-scattering of colloidal gold nanoparticles. However, Mie’s theory cannot be applied to anisotropic nanoparticles and numerical methods are needed to describe anisotropic optical properties. The discrete dipole approximation (DDA),4,5 the finite difference time domain method (FDTD),6 the finite element method (FEM),7 and the multiple multipole method (MMP)8 are typical numerical methods used to describe the optical properties of nanoparticles with arbitrary shapes. Remarkable progress in computer hardware and software now provides versatile methods to simulate the optical properties of anisotropic metal nanoparticles. In contrast to anisotropic particles, ideal isotropic particles should be spherical and have minimum specific surface area, and their surface properties should be uniform. Real nanoparticles, however, are rarely spherical but show some facets originating from crystal structures.911 At the mesoscopic scale, rounded surfaces have large excess energies, whereas the facets minimize surface energies and regulate the shapes of crystallites. Thus, the surface properties of a crystallite are not isotropic but depend Chem. Lett. 2016, 45, 488–498 | doi:10.1246/cl.160124
[a] Platonic solid.
upon the facets of the nanoparticles.12 Crystal twinning, which occurs when two crystals share some of the same crystal lattice points, generates various crystallite shapes (Table 1).13 Collision of several crystallites produces further variations in the facets on a nanoparticle surface. If we average the various shapes of the particles, the averaged shape should be “isotropic”. In this context, Mie’s theory provides an approximation for real “spherical” nanoparticles. To prepare “spherical” nanoparticles, selective growth of specific facets should be suppressed. In other words, when we prepare anisotropic nanoparticles in a colloidal solution, facet-selective crystal growth is a key technique. Single crystals can be anisotropic through facet-selective growth, but a combination of crystallite twinning and facet-selective growth creates various anisotropic nanoparticles. Nanowires, nanoplates, nanostars, and nanorods are anisotropic nanoparticles obtained by the facet-selective growth of twinned crystallites. To obtain facet-selective crystal growth, we require chemical additives that regulate the activities of facets, in addition to surface-passivating agents that stabilize colloidal dispersions. Poly(vinylpyrrolidone) (PVP) and silver ions are typical shaperegulating chemicals. At present, there is no standard theory describing the selection of shape-regulating chemicals and they have, instead, been selected empirically. Variation in anisotropic nanoparticles implies variation in shape-regulating chemicals; however, in practice, the availability of reproducible methods for obtaining uniform anisotropic nanoparticles is limited. In this review, we introduce several practical methods for obtaining anisotropic gold-based nanoparticles. After proper surface modifications, these anisotropic nanoparticles are applicable as sensors, markers, and biofunctional materials.
© 2016 The Chemical Society of Japan | 489
The importance of gold-based anisotropic nanoparticles in bio-related applications lies in the tunability of their optical properties, which depend on their sizes and shapes. Spherical gold and silver nanoparticles with diameters of ca. 20 nm show SP bands at around 520 and 420 nm, respectively.11,14,15 The SP peaks are tunable in the visible region, depending on size, and have applications in biochemistry and other life sciences.16 Meanwhile, tunability in the near-IR region is the main reason for using anisotropic nanoparticles. Near-IR is invisible light, in the range 7502500 nm. For noninvasive optical imaging and sensing, light ranging from 650 to 900 nm is useful, because absorbance of water and hemoglobin are at a minimum in this region.17 This region is therefore named the “near-IR window” for living bodies. Near-IR light allows us to obtain information about deeper tissues (0.51 cm) without causing serious damage to living tissues. Near-IR fluorescent dyes, such as indocyanine green, are applicable in medical diagnostics, for determining cardiac output and liver blood flow, and for ophthalmic angiography.18 Metal nanoparticles, modified with various biofunctional molecules, are another type of near-IR functional materials used for medical diagnostics. Metal nanoparticles are mostly non-fluorescent when their diameters are larger than 5 nm; however, their extinction coefficients are much larger than those of organic dyes. The extinction coefficients of gold nanorods reach as high as 109/mol (particle)¢cm in the near-IR region. This large extinction enables sensitive probing and staining of bio-related molecules in a living body. For photothermal therapy, a large extinction tuned to the near-IR region is also advantageous for achieving efficient photothermal conversion. In the second half of this review, we provide an overview of bioimaging, therapeutic applications, and controlled release of biofunctional molecules, using anisotropic gold-based nanoparticles that show remarkable anisotropic surfaces and SP bands in the visible and near-IR regions.
Figure 1. TEM images of gold nanoshells. (a) As-deposited gold nanoparticles (
