Synthesis, Morphology & Properties of GNPs
GNPs come in different shapes and sizes. Control over shape, size and chemical composition of GNPs itself is a challenging area of research that eventually determines the surface functionality and activity. The most common shapes are spherical particles, rods, shells, cages, prisms and stars. Figure 3A presents the transmission electron microscopy images of spheres, rods and stars, and their optical absorbance (Figure 3B). Spectral comparison in Figure 3C shows their utility in surface-enhanced Raman spectroscopy (SERS). The following paragraphs review the synthesis and physical properties of GNPs.
Figure 3.
Physical properties of some select nanoprobes. (A) Transmission electron microscopy images of gold nanoprobes: (a) spheres; (b) rods; and (c) stars, (B) optical absorbance versus wavelength for the three different morphologies of gold nanoprobes and (C) SERS spectra comparison of 2 MPy absorbed on: (a) nanostars (140 nm), (b) nanorods (65 × 30 nm) and (c) nanospheres (150 nm).
Reproduced with permissions from [33,227].
Stable and spherical gold nanoparticles ranging between 10 and 100 nm can be formed in a number of ways. The classic method is gold reduction from HAuCl4 via citrate at 100°C, and was developed in the early 1950s by Turkevich et al.[4–6] By varying the concentration of the gold versus citrate, one can tune the size of the gold nanoparticles synthesized. Recently, Turkevich's method has been revisited and compared with its variants, namely UV-initiated reduction and ascorbate reduction.[7,8] Results showed that control of the simple Turkevich process by the reduction conditions is sufficient to define particles' shape and size in a wide interval. The UV-initiated particle growth in contrast resulted in more spherical-like particles even at larger sizes. Ascorbate reduction ensured the best spherical definition of the particles. Extensive networks of gold nanowires were also formed as a transient intermediate in the citrate reduction method.[7] These gold nanowires were shown to exhibit nonlinear optical properties and also contributed to the dark appearance of the reaction solution before it turned ruby-red. Another popular fabrication method is called the 'Brust synthesis' developed in 1994 by Brust and Schiffrin et al.[9,10] In this method, organic liquids such as toluene, reducing agents such as NaBH4 and tetraoctylammonium bromide (TOAB) are used to synthesize gold colloid particles from HAuCl4. TOAB, a phase transfer agent, acts as a stabilizer; however, it does not form a strong bond with gold, which may cause particle aggregation after a few weeks. Another more recently developed method by Perrault et al. uses hydroquinone in an aqueous environment to grow nanoparticles of various sizes by the reduction of HAuCl4. This method can produce monodispersed gold nanospheres in sizes ranging from 50 to 200 nm.[11]
A simple but versatile variation of the spherical gold nanoparticle is the nanorod.[12] GNRs can be synthesized with different aspect ratios and are particularly useful in imaging, because they produce strong and tunable plasmonic resonance properties in the red to near-infrared (NIR) region of the electromagnetic spectrum.[13–16] GNRs and silver nanorods can be prepared using electrochemical and seed-mediated growth methods.[12,17–20] In the electrochemical method, a platinum cathode and gold anode are immersed in an electrolytic solution in an ultrasonic bath at 36°C containing hexadecyltrimethylammonium bromide and a small amount of hydrophobic cosurfactant tetradodecylammonium bromide. The surfactant serves to stabilize and prevent the aggregation of nanoparticles. Controlled electrolysis at 3 mA for 30 min leads to the formation of nanorods. In the seed-mediated approach, citrate-capped gold nanoparticle seeds with a diameter of 3.5 nm are used for the nucleation and reduction of gold salt in the presence of cetyl trimethyl ammonium bromide and ascorbic acid. This produced nanorods with a 4.6 ± 1 aspect ratio. It was found that addition of silver nitrate enhanced the formation and aspect ratio of nanorods. While the initial yield was poor using this method (˜4%), nanorods of high yield (90%) using seed-mediated growth have been reported by slightly increasing the pH of the solution.[21]
Gold-coated dielectric nanoparticles are called gold nanoshells.[22–24] This con?guration of a dielectric core coated with a metal nanoshell occur naturally in the growth of Au-Au2S nanoparticles. Silica-coated gold nanoshells are produced by growing silica nanoparticles and coating then with a layer of gold to form a core–shell structure. Reduction of tetraethylorthosilicate in ammonium hydroxide and ethanol produces silica nanoparticles of 40–120 nm in diameter. Next, colloidal gold nanoparticles are used as seeds to stick to the silica nanoparticles, which form a discontinuous gold layer. Further gold is added by reducing HAuCl4 in the presence of potassium carbonate and formaldehyde. By tuning the diameter of the core and thickness of the shell, one can tune the optical properties of the nanoshells to a wide range of wavelengths, including the 'water window', which is the IR range of the electromagnetic spectrum.[25] Gold nanoshells possess remarkable optical properties that differ dramatically from solid spheres. Another type of nanoparticle, gold nanoclusters (2–6 nm), has been a topic of recent interest because of its unique size, which may be advantageous in cellular uptake as well as in nuclear targeting, in addition to the unique optical and electrical properties exhibited by gold nanoclusters.[26–29] Despite their small size, these nanoclusters were found to be stable because the thiolate groups form a protective layer around gold clusters, improving their stability and functionality.[30,31]
Star polyhedral gold nanocrystals were synthesized recently by colloidal reduction of gold with ascorbic acid in water under ambient conditions.[32] Two distinct classes of star nanocrystals were identified: multiple-twinned crystals with fivefold symmetry and monocrystals. These respective classes correspond to icosahedra and cuboctahedra, two Archimedian solids, with preferential growth of their {111} planes. Due to this preferential growth, the {111} faces of the original Archimedean solids grow to become tetrahedral pyramids, the base of each pyramid being the original polyhedral face. By assuming a star morphology, gold nanocrystals increase in proportion to the exposed {111} planes and have low surface energy that could be highly useful for creating stable and biologically active surfaces. Gold nanostars exhibit high plasmonic resonance due to juxtapositioning of the tips that radially emerge from the core making these structures useful for imaging by dark field microscopy, SERS and plasmonics.[33–37]
Gold nanocages also represent a unique design in the field of nanomaterials for biology and medicine.[38–42] Gold nanocages with porous walls can be formed using simple galvanic replacement reaction in an electrochemical bath between solutions containing metal precursor salts and silver nanostructures prepared through polyol reduction. The electrochemical potential difference drives the reaction with gold reduction depositing on the surface of the silver nanocubes. Typically, HAuCl4 is used as a metal precursor, the resultant gold is deposited epitaxially on the surface of the silver nanocubes, adopting their underlying cubic form. Concurrent with this deposition, the interior silver is oxidized and removed, together with alloying and de-alloying to produce hollow and eventually, porous structures that are commonly referred to as gold nanocages. By controlling the molar ratio of silver to HAuCl4, the extinction peak of the resultant nanostructures can be continuously tuned from the blue (400 nm) to the NIR (1200 nm) region of the electromagnetic spectrum (Figure 3). Calculations suggest that the magnitudes of both scattering and absorption cross-sections of nanocages can be tailored by controlling their dimensions, as well as the thickness and porosity of their walls. This novel class of hollow nanostructures is expected to find use as both a contrast agent for optical imaging in early-stage tumor detection and as a therapeutic agent for photothermal cancer treatment.[43–45]
Newer shapes and morphologies of nanoprobes and multifunctional composite nanostructures are being researched. The future of nanoprobe synthesis lies in creative techniques for combining the geometries of two or more probes to enable sharp tuning of optical properties and effective targeted therapy. For example, one could imagine nanorods with needle- shaped structures along the longitudinal axis with surface plasmon bands at lower energies, for use as SERS probes, and in SPR imaging, drug delivery and photothermal/RF therapy. The use of gold with other materials, namely magnetic and polymeric materials, to create new morphologies for imaging and therapy needs to be further investigated to make it cost effective for simultaneous imaging and therapy. However, the creation of new morphologies are only justified if the new materials produce surface plasmon bands at lower energies, enable sensitive colorimetric contrast for imaging, can be utilized as improved SERS probes or have useful photothermal effects for therapy. Finally, the mass production of different types of nanoprobes and their impact on the environment needs to be thoroughly investigated.
Nanomedicine. 2011;6(10):1787-1811. © 2011 Future Medicine Ltd.
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