A latest research revealed in Superior Optical Supplies presents a novel methodology for depositing and patterning plasmonic gold nanorods (AuNRs) onto stable surfaces, particularly glass, utilizing a mixture of optical manipulation and tailor-made chemical environments.
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Background
Exact management over nanoparticle positioning is central to advancing nanophotonic and sensing applied sciences. Varied strategies—chemical self-assembly, lithography, and field-driven methods—have been developed to govern particles on the nano- and microscale. Amongst these, optical strategies similar to opto-thermophoresis have proven promise through the use of localized thermal gradients to information particles into well-defined patterns with excessive spatial decision.
Nevertheless, making use of opto-thermophoresis to anisotropic particles like AuNRs has confirmed tough. In aqueous options, these nanorods are sometimes stabilized by surfactants like cetyltrimethylammonium chloride (CTAC), which hinder their means to mixture and type ordered assemblies. Whereas earlier work has demonstrated profitable patterning of spherical gold nanoparticles, extending this to nanorods has remained a problem on account of variations in floor chemistry and geometry.
Examine Overview
The experimental setup started with the synthesis of AuNRs utilizing well-established colloidal chemistry methods, permitting exact management over particle measurement and side ratio. The nanorods used within the research measured roughly 16 nm in diameter and 60 nm in size, yielding a facet ratio of about 3.8.
On the core of the meeting course of was a centered 532 nm laser beam, expanded and directed by a high-quality microscope. When centered onto the pattern, the laser created a localized warmth supply that generated a thermal gradient throughout the colloidal resolution. The AuNRs, suspended in aqueous mixtures containing various concentrations of ethanol and polyvinylpyrrolidone (PVP) of various molecular weights, have been deposited on completely cleaned glass substrates.
The experiment was performed beneath tightly managed situations. Laser energy ranged from roughly 8.6 to 14 mW, ethanol concentrations diverse as much as 20% (v/v), and polymer concentrations have been exactly tuned. Glass coverslips with molded wells offered confinement, guaranteeing the particles remained throughout the focal space throughout meeting and minimizing drift.
Ethanol performed a key position by altering the colloidal stability of the dispersion. Particularly, it influenced micelle formation by the first surfactant, CTAC, and modified the interfacial interactions between nanorods. The presence of ethanol lowered surfactant protection on the AuNR surfaces, growing their mobility beneath the affect of the thermal gradient and enabling more practical particle meeting.
To investigate the outcomes, the researchers employed scanning electron microscopy (SEM) to visualise the ensuing nanostructures and surface-enhanced Raman spectroscopy (SERS) to evaluate the electromagnetic enhancement created by densely packed plasmonic areas. Extra optical characterization utilizing UV-Vis spectroscopy offered insights into how ethanol content material and polymer measurement affected each the structural integrity and optical properties of the assembled nanorods.
Outcomes and Dialogue
The research revealed that in purely aqueous environments, AuNRs didn’t assemble beneath laser-induced thermal gradients, largely on account of dense surfactant layers that prevented interparticle attraction. The introduction of ethanol, nonetheless, dramatically modified the system’s conduct.
Ethanol raised the crucial micelle focus (CMC) of CTAC—from roughly 1.1 mM in water to about 125 mM in water-ethanol mixtures—weakening the surfactant’s maintain on the nanorods and permitting them to reply extra readily to thermophoretic forces.
As ethanol content material elevated, the meeting high quality improved noticeably. Patterns grew to become extra outlined, with larger development charges and better anisotropy. Polymer traits additional influenced the meeting. Greater molecular weight PVP, particularly PVP360K, promoted ordered, densely packed preparations, notably within the sample’s central area. This was seemingly on account of depletion interactions described by the Asakura–Oosawa (A–O) mannequin, which favors side-by-side and tip-to-tip aggregation of nanorods.
Laser energy additionally performed a job. Whereas larger intensities accelerated meeting, extreme energy typically lowered sample uniformity, highlighting the necessity for cautious tuning of optical enter.
SERS measurements confirmed important electromagnetic subject enhancements, with enhancement components reaching as much as 108. These enhancements correlated with the diploma of structural order within the AuNR assemblies. SEM photos confirmed the formation of organized patterns aligned with the laser’s focal zone, with meeting high quality enhancing alongside ethanol focus and polymer molecular weight.
Conclusion
This analysis efficiently demonstrates a quick, scalable, and surface-functionalization-free methodology for assembling gold nanorods into ordered constructions utilizing opto-thermophoresis enhanced by ethanol as a co-solvent. The approach permits fast (<10 seconds) formation of dense plasmonic patterns with tunable morphology and robust SERS responses.
Key parameters—ethanol content material, polymer measurement, and laser energy—have been proven to affect surfactant conduct, colloidal stability, and meeting dynamics. By harnessing the interaction between chemical formulation and optical manipulation, the research opens new potentialities for growing nanostructured supplies for chemical sensing, biosensing, and nanophotonic system purposes.
Journal Reference
Jiménez Amaya, A., et al. (2025). Co‐Solvent‐Aided Opto‐Thermophoretic Printing of Gold Nanorod Assemblies. Superior Optical Supplies. DOI: 10.1002/adom.202500375, https://superior.onlinelibrary.wiley.com/doi/10.1002/adom.202500375
