Gold nanorods with localized surface plasmon resonance (LSPR) can be chemically synthesized. We systematically investigated the effects of reaction parameters and centrifugation on the fine tuning of the rod dimension in scale-up production (80–100 mL). Nanorods of absorption bands from 600–1050 nm were fabricated with precise control of the aspect ratio (AR) from 1.5 to 8.9. Although all chemicals are important in directing the nanostructure, silver ion concentration and seed/Au3+ ratio were the most effective variations to adjust the absorption wavelength. With a single surfactant under the influence of silver nitrate, short nanorods up to AR of 5 were synthesized with corresponding maximum absorption wavelength at 902 nm. To achieve higher aspect ratio with absorption band beyond 1,000 nm, two-surfactant growth solution was sought to further elongate the rod length. Centrifugation speed and times were found to exert significant influences on the final rod dimension, which is important during the purification process. In a relatively large quantity nanorod synthesis, even distribution and sufficient mixing of chemical ingredients play an essential role in determining the yield, uniformity, and stability of the final nanorod formation.
Nanoparticles provide unique physical and optical properties to serve as building blocks for a wide-ranging applications in nanoelectronics, nanophotonics, nanodevices, and nanomedicine [
Gold nanoparticles of varying morphologies and sizes can be fabricated by photolithography techniques and biosynthetic organisms. Nevertheless, chemical synthesis may be one of the favored and cost-effective routes. Nowadays, nanomaterials of various shapes can be precisely synthesized with seemingly limitless chemical functional groups. A variety of gold nanoparticles including nanospheres, nanorods, nanocages, nanoshells, nanoprisms, nanocubes, and nanorings have been chemically fabricated with high yield [
Numerous characteristics of nanoparticles depend on aspect ratios (length to width ratio), including optical and physical properties. As such, the longitudinal absorption wavelength can be selectively tuned from the visible to the near-infrared (NIR) region as aspect ratio (length to width ratio) increases. This provides a unique opportunity to develop a multiplexed nanodiagnostic device based on combination of different size of nanorods with distinctive plasmonic wavelengths (ongoing project). Therefore, control over the shape and size of fabricated nanorods precisely has been one of the important tasks to provide the desired nanomaterials for targeted biomedical applications. Most of the past studies discussed the influence of the various reaction parameters in the seed-mediated rod growth in a relatively small quantity (5–10 mL). In a scale-up production, precise tunability of the plasmon bands from visible to NIR region has not been thoroughly studied. This is important as the mass transport, therdynamics, and mixing effect can be dramatically different between small quantify and large volume. Here, we performed a systematic study to investigate the effects of various parameters in an 80–100 mL production volume. Additionally, centrifugation is an essential step to purify rod-shaped nanoparticles from excess chemicals and spheres. We found that centrifuge can significantly affect the final dimension and optical absorption of prepared GNR samples. Therefore, we systematically investigated, for the first time, the influence of centrifuge speed and times on the preparation, properties, and stability of nanorods.
Hydrogen tetrachloroaurate trihydrate (HAuCl4; 99%), sodium borohydride (NaBH4; 99%), cetyltrimethylammonium bromide (CTAB), L-ascorbic acid (AA), silver nitrate (AgNO3; 99%), and benzyldimethylhexadecylammonium chloride (BDAC) were obtained from Sigma-Aldrich (St. Louis, MO). All glasswares were cleaned with aqua regia (HCl/HNO3, 3 : 1 v/v), followed by thorough rinse with double distilled MilliQ water prior to use.
Gold nanorods with various aspect ratios up to 9 demonstrating tunable longitudinal plasmonic wavelengths (600–1050 nm) were synthesized as described previously with modifications [
Typically, HAuCl4 (5 mL, 0.5 mM) was mixed with CTAB (5 mL, 0.2 M) solution, followed by addition of freshly made, ice cold NaBH4 (600
A binary surfactant mixture of BDAC/CTAB is essential in the seed-mediated growth solution to fabricate longer nanorods with higher aspect ratio > 5 and plasmonic band over 1,000 nm. Briefly, the two-surfactant mixture was prepared by adding 0.8 g of CTAB to 0.15 M BDAC solution. After dissolving the mixture by sonication (20 min at 40°C), the solution was added to 200
The synthesized Au nanorods were purified to remove excess reagents such as CTAB surfactant by centrifugation. Typically, the solution was centrifuged at a speed of 8,500 rpm for 30 min to precipitate the Au nanorod solid. The colorless supernatant was carefully discarded without disturbing the bottom. The solid pellet was then redispersed in a suitable volume of DI water depending on the quantity of the residue. The centrifugation was repeated 2-3 times, and finally the solid residue was dispersed in DI water or biological buffers for biofunctionalization process.
Absorption spectra of the prepared Au nanorod solutions were obtained on a Beckman-Coulter UV-NIR (200–1100 nm) scan spectrophotometer. The SEM grid was dropcast with a total of 10
Nanorods are formed based on the structure-directing elongation of seeds. The seeds are basically 5–10 nm spheres which demonstrate a characteristic absorption band of gold at 520 nm. In contrast, rod-shaped nanoparticles have two bands in the absorption spectrum (Figure
(a) Representative absorption spectrum of gold nanorod with stronger band in the longitudinal direction. Gold seed (nanosphere) only shows a characteristic band (dashed line). (b) Scanning electron microscopy of rod-shaped gold nanoparticles. (c) Picture demonstrating a vivid color of the gold nanorod colloidals with tunable aspect ratio (AR) to result in distinguishable absorption wavelength from 650 to 1050 nm. The solutions are stable over 6 months. (d) Superimposed absorption spectrum of Au nanorods with various longitudinal peak wavelengths.
Gold nanosphere (5–10 nm) is required in the growth solution to serve as a material basis (seed) to allow attachment of gold atoms in a controlled direction to grow to a rod shape under influence of surfactant and silver nitrate. While the concentrations of other ingredients were kept constant, the amount of seeds added to the growth media was varied with increasing volume up to 6 times of normal amount (12
Effect of seed amount on the nanorod growth. (a) One to five times (a to e) of the normal seed amount (12
During the rod growth, gold (III) ions will be reduced to atomic elements for material supply to Au seeds to grow favorably in length under the influence of silver nitrate. When faster supply of monomer growth unit (Au0) is available to the seeds, there is a great tendency to induce the growth of seed particles in all directions and forming more spherical particles. AA is a mild reducing agent and a suitable amount of AA will facilitate the synthesis of nanorods with high yield. We found that in our experimental condition, 0.1 M AA was an optimal concentration to produce rods in high yield. Quantity less than that stoichiometric amount required for the reduction of Au3+ ions to Au0 failed to grow rods. For example, colloidal solution with 0.08 M AA did not produce a characteristic absorption peak of rod shape (Figure
Optimization of ascorbic acid (AA) amount for nanorod growth. Addition of excess AA causes “dogbone” shaped rod.
Silver ion is known to play an important role in assisting the growth of nanoparticles to rod shape. It is especially essential for the synthesis of short nanorods (AR < 5) with peak wavelength in visible region of the spectrum in very high yield (>95%) [
Additional amount of silver nitrate can effectively increase the aspect ratio and fabricate nanorods with a maximum absorption wavelength at 902 nm using a single CTAB surfactant. A: LSPR wavelength:
Although the mechanism to assist in the rod shape growth is not clearly elucidated, several studies [
Au nanorods with longer absorption wavelength in NIR region are beneficial for many biological applications such as thermal therapy and imaging [
Nanoparticles of various shapes and sizes can be synthesized with different chemical functional groups. Therefore, to synthesize rod-shaped nanoparticles with absorption > 900 nm, a binary surfactant system in growth solution was sought to enable production of higher aspect ratio during an extended aging period. Typically, a second surfactant BDAC is used in addition to CTAB at an appropriate molar ratio. Higher ratios of BDAC/CTAB tend to result in the formation of large amounts of spherical nanoparticles (Figure
(a) Electron microscopy of long nanorods (AR:
Nanospheres usually start to grow upon addition of gold seeds to the bisurfactant growth solution. As shown in Figure
Figure
After the growth of seed to rod-shaped nanoparticles, centrifuge was an essential step for purification by separating excess chemical reactants and small sphere particles from rod-shaped nanoparticles. This process may affect the concentration, dispersion, and dimension of the rods. Therefore, the effect of the centrifuge speed and times on the longitudinal peak wavelength of the nanorods was investigated. A batch of 180 mL nanorods solution was divided to multiple samples, and, at baseline (before centrifuge), the GNRs showed a plasmon band at 772 nm. The samples were then centrifuged at a speed of 5,500, 7,000, and 8,500 rpm, respectively, from one to three times. Readings of the absorption spectrum were taken at 4, 8, and 12 days after centrifuge to exam the stability of the purified nanorods. For all experimental conditions, the GNR samples showed a blue shift (reduction in the wavelength) of the peak after centrifugation. For example, for the samples centrifuged at 7,000 rpm, one time centrifuge resulted in a peak wavelength of 767 nm, and two times caused a shift to 757 nm, while centrifugation of three times resulted in a further shift down to 748 nm. The magnitude of the peak wavelength shortening between one and two centrifuge times depends on the centrifuge speed (Figure
(a) Effects of centrifugal times and speed on the final purified nanorod optical properties. (b) Stability of the rods over 14 days depending on the centrifuge speed and times.
The centrifuge also played an important role in the stability of the GNRs as shown in their corresponding longitudinal peak wavelength (Figure
We systematically studied the reaction parameters and centrifuge effect on the formation of rod-shaped gold nanoparticles in a relatively large quantity of 80–100 mL. Although all chemicals are important in directing the nanostructure, silver ion concentration and seed/Au3+ ratio were the most effective variations to adjust the absorption wavelength. This is consistent with the small quantity study reported by Murphy and El-Sayed groups. Since the molar concentrations of each ingredient for a large volume production multiply, even distribution by well mixing and precise control techniques play an essential role in success and determination of the final yield, uniformity, and stability. Temporal evolution of the gold nanorod dimensions fabricated by bisurfactant system was elucidated to follow the red shifts of longitudinal SPR peak wavelengths > 900 nm which is the limit for CTAB only method. In summary, nanorods of absorption bands from 600–1050 nm were fabricated with precise control of the AR from 1.5 to 8.9 in a relatively large quantity. The mass production of these rod-shaped gold nanoparticles with controllable aspect ratio and tunable optical properties will provide a solid material foundation for biological and therapeutic applications.
This work was partly supported by San Antonio Area Foundation and UTSA CRSGP funds.