Table of contents
Optical Properties
NanoComposix’s gold nanorods are surface plasmon resonant (SPR) rod-shaped nanoparticles with narrow size distributions and uniform shape. The gold nanorods have absorption peaks that are tunable throughout the visible and short wave near-infrared (SWNIR) regions of the electromagnetic spectrum. Our purification methods remove ammonium surfactants used during synthesis and replace it with citrate, producing a stable, easily displaceable surface chemistry to use as a base for functionalization with a wide variety of molecules (silica, DNA, antibodies, etc.). We also offer PEG-functionalized nanorods, as well as carboxyl-terminated surfaces for bioconjugation.
Gold nanorods with a high aspect ratio (length/width) have a longer peak resonant wavelength than gold nanorods with a lower aspect ratio. The three resonances that are available as standard products are listed below.
| Resonance | Typical Aspect Ratio |
|---|---|
| 660 nm | 2.7 |
| 800 nm | 4.1 |
| 980 nm | 5.6 |
By tuning the synthesis conditions to control the nanorod length and diameter, nanorods with peak resonance from 600–1100 nm can be produced. To receive nanorods with a custom resonance wavelength peak please contact us.
Surface Functionalization
The surface of gold nanorods is very important for use in a variety of applications. Most gold nanorods available commercially are coated with cetyl trimethylammonium bromide (CTAB), a common surfactant used during nanorod synthesis. CTAB coated nanorods are toxic in biological systems and are readily destabilized when transferred to different solvents. NanoComposix’s gold nanorods are CTAB-free and, like all our materials, highly purified to remove all residual reactants following synthesis.
The gold nanorod surface options available include a covalently-bound 5 kDa methoxy-terminated PEG, which has been shown to have a high degree of salt stability, and a discrete carboxylic acid-terminated PEG, (PEG)12-Carboxyl, in which the free acid functional group is available for subsequent coupling chemistry and can provide anchor points for covalent protein attachment. Also available is the citrate surface option which can be readily functionalized with other ligands, biomolecules and DNA. Other custom surfaces are available, and include PVP, lipoic acid, and BPEI.
For more information, check out our guide for conjugating PEG-carboxyl gold nanorods to free amines.
Applications
Gold nanorods are utilized in a range of applications including drug delivery, photothermal therapy, bioassays, biomedical imaging, and biosensing. The sections that follow explain why gold nanorods are useful for these applications and provide guidance for applying nanoComposix products to develop new technologies and improve existing ones.Drug Delivery
The optical properties, tunable surface chemistry, and relative biocompatibility of gold nanorods lend them unique capabilities as vehicles for targeted drug delivery. This can occur via biofunctionalization of the nanorod surfaces for targeted drug delivery, and/or by engineering a photoinduced release of drug molecules for targeted and site-specific drug delivery. Additionally, gold nanorods can be hybridized with other materials, such as mesoporous silica, for targeted and controlled drug release.
For additional information about targeted drug delivery, visit our page about Nanobiotechnology.
Photothermal Therapy
Gold nanorods are widely used for photothermal therapy technologies, where near-infrared (NIR) lasers selectively irradiate nanorods at their resonant wavelengths to induce local heating without causing damage to surrounding (IR transparent) tissues. PEG-capped gold nanorods are aqueous compatible and ideal for facilitating nanorod pore penetration for cosmetic therapies such as acne treatment and laser hair removal. Citrate-capped and PEG-carboxyl ligands can also be further functionalized with diverse terminal groups or biomolecules for cell targeting, enabling nanorod utility in cancer therapies and tumor ablation.
At nanoComposix, we offer gold nanorods with optical properties tuned to match standard NIR laser wavelengths, including 660 nm, 808 nm, and 980 nm. Click here to read more about our standard product offerings. We also have extensive experience working with individual clients to develop photothermal therapy technologies, designing custom nanoparticles and testing materials in-house for clinical application. For more information on our capabilities, check out this case study about a photothermal acne treatment technology that was brought to market, co-developed and enabled by nanoComposix products and testing.
Photoacoustic Imaging
Gold nanorods are commonly used for photoacoustic imaging, where they serve as molecular-targeted contrast agents for in vitro bioimaging. This technique, also known as optoacoustic imaging, relies on the photoacoustic effect, where absorbed light induces a thermoelastic expansion in biological tissue, which in turn generates ultrasound waves. The ultrasound waves are detected by a transducer and converted into images with optical contrast.
The surfaces of gold nanorods are readily functionalized for selective tissue targeting. Nanorods act as chromophores to absorb incoming light efficiently and with site specificity for improved contrast and spatial resolution in the images. Other naturally occurring chromophores, such as hemoglobin and red blood cells, can also absorb incident irradiation and cause thermal expansion, but incorporating strong absorbers like gold nanorods amplifies the effect, enabling non-invasive imaging at deeper penetration depths than would otherwise be possible.
Biosensors
Gold nanorods are garnering considerable attention for their use as biosensors due to their tunable surface plasmon resonances and sensitivity to local refractive index, serving as probes of their chemical environment. Their surface chemistry is easily modified for attaching biomolecules, from antibodies and proteins, to enzymes and aptamers. These bioreceptors can be used to target molecules of interest with high specificity, and their interaction with gold nanorod surfaces can induce a visual and quantifiable indicator of molecular detection. Gold nanorods have unique capabilities as optical indicators due to the exceptional tunability of their surface plasmon resonances, covering wavelengths from the visible to the near-infrared (NIR) region of the electromagnetic spectrum. In addition, their anisotropic shape offers two distinct plasmon modes which can tuned by changing the aspect ratios (length to width) of the rods during fabrication. These qualities enable sensing technologies with extraordinary control.
Their anisotropic shapes enable arrangement of gold nanorods into superstructures, offering enhanced stability from long-range ordering and additional modes of optical tunability. Construction of oriented arrangements can even be guided by surface interactions sensitive to biotin-streptavidin binding, DNA hybridization, and antibody-antigen recognition facilitated by surface functionalization of nanorod ensembles.
After nanorod synthesis, the surfaces of CTAB-capped gold nanorods are nearly always modified for subsequent conjugation or superstructure formation due to the cytotoxicity of CTAB. At nanoComposix, our citrate-functionalized gold nanorods provide bare, CTAB-free surfaces for easy displacement with thiols and other organic capping agents to facilitate creation of custom biosensors and aptasensors.
Another approach to making nanorod-based biosensors utilizes polyethyleneglycol (PEG) surface modification, which renders the nanorods highly stable in aqueous conditions to facilitate in vivo bioimaging. nanoComposix offers gold nanorods surface-functionalized with PEG for enhanced stability in aqueous buffers and protic solvents, and PEG carboxyl for ready EDC/NHS coupling or conjugation to proteins, antibodies, and peptides. For more information, check out our guide for conjugating PEG-carboxyl gold nanorods to free amines.
As an alternative to ligand exchange, coating CTAB-capped nanorods with silver or silica shells is common for subsequent application in biosensing technologies. At nanoComposix, we fabricate materials of this nature on a custom basis. For more information about our custom products and capabilities, please contact us.
Additional Resources
- Cao, J.; Sun, T.; Grattan, K. T. V. Gold Nanorod-based Localized Surface Plasmon Resonance Biosensors: A Review Sensor Actuator 2014, 195, 332-351.
- Chen, H.; Shao, L.; Ming, T.; Sun, Z.; Zhao, C.; Yang, B.; Wang, J. Understanding the Photothermal Conversion Efficiency of Gold Nanocrystals Small 2010, 6 (20), 2272-2280.
- Huang, X.; El-Sayed, I. H.; Qian, W.; El-Sayed, M. A. Cancer Cell Imaging and Photothermal Therapy in the Near-Infrared Region by Using Gold Nanorods J. Am. Chem. Soc. 2006, 128 (6), 2115-2120.
- Knights, O. B.; Ye, S. Ingram, N.; Freear, S.; McLaughlan, J. R. Optimising Gold Nanorods for Photoacoustic Imaging In Vitro Nanoscale Adv. 2019, 1, 1472-1481.
- Mayer, K. M.; Lee, S.; Liao, H.; Rostro, B. C.; Fuentes, A.; Scully, P. T.; Nehl, C. L.; Hafner, J. H. A Label-free Immunoassay Based Upon Localized Surface Plasmon Resonance of Gold Nanorods ACS Nano 2008, 2 (4), 687-692.
- Qin, Z.; Wang, Y.; Rnadrianalisoa, J.; Raeesi, V.; Chan, W. C. W.; Lipiński, W.; Bischof, J. C. Photothermal Heat Generation between Gold Nanospheres and Nanorods Sci. Rep. 2016, 6, 29836.
- Xu, X.; Xu, C.; Ying, Y. Aptasensor for Simple Detection of Ochratoxin A Based on Side-by-Side Assembly of Gold Nanorods RSC Adv. 2016, 6, 50437-50443.
- Zhang, Z.; Wang, L.; Wang, J.; Jiang, X.; Li, X.; Hu, Z.; Ji, Y.; Wu, X.; Chen, C.Mesoporous Silica Coated Gold Nanorods as a Light Mediated Multifunctional Theranostic Platform Adv. Mater. 2012, 24 (11), 1418-1423.
- Zhong, J.; Wen, L.; Yang, S.; Xiang, L.; Chen, Q.; Xing, D. Imaging-guided High-efficient Photoacoustic Tumor Therapy with Targeting Gold Nanorods Nanomed. Nanotechol. 2015, 11 (6), 1499-1509.
