The development of reconfigurable metamaterials capable of real-time optical tuning represents a major milestone in nanophotonics. In this work, we demonstrate a highly dynamic system based on silica-gold core-shell microparticles that can be reversibly assembled into chain-like structures using external alternating current (AC) electric fields. The resulting plasmonic metamaterial exhibits strong, tunable mid-infrared resonances, enabling precise control over light-matter interactions without permanent structural changes.
Core-shell particles were synthesized through a chemical route involving surface modification with 3-aminopropyltriethoxysilane (APTES), followed by sequential deposition and reduction of gold ions to form a uniform metallic shell. Scanning electron microscopy confirmed spherical morphology with a consistent gold layer thickness of 10–13 nm. These particles were dispersed in ethanol and introduced into a microfluidic cell consisting of a glass substrate, a 6 μm polymer spacer, and a calcium fluoride (CaF₂) window. The CaF₂ layer provides high transparency across the mid-infrared spectrum (0.2–7 μm) while minimizing evaporation, ensuring stable conditions for extended spectroscopic measurements.
An AC electric field was applied parallel to the substrate via coplanar electrodes. At low frequencies (10 Hz), positive dielectrophoresis (p-DEP) dominates, driving particles toward regions of high electric field gradient and initiating aggregation.Saccharocin Epigenetics As frequency increases to 300 Hz, induced-charge electro-osmosis (ICEO) becomes significant, generating localized fluid flows that assist in alignment and promote ordered chain formation.2-(2-Bromoethyl)-1,3-dioxane web At 100 kHz, negative dielectrophoresis (n-DEP) prevails due to the permittivity mismatch between the particles and the surrounding medium, leading to repulsion from high-field zones and stabilization of extended chains.
A critical innovation of this study is the in situ Fourier transform infrared (FTIR) spectroscopy performed directly on the suspended particle system. Unlike traditional methods requiring sample drying or substrate immobilization, our setup allows continuous monitoring of optical changes during dynamic assembly and disassembly. This eliminates artifacts caused by environmental shifts and enables real-time observation of functional transitions.
Spectra recorded under s-polarized and p-polarized illumination revealed pronounced differences between the dispersed and assembled states. Dispersed particles exhibited nearly identical spectra regardless of polarization, confirming isotropic behavior. In contrast, chain structures showed significantly enhanced reflectivity under p-polarized light, indicating strong coupling with longitudinal plasmonic modes aligned along the chain axis. Four distinct resonant features appeared at 2.6 μm, 3.4 μm, 6.1 μm, and 9.1 μm—none of which were present in the disordered state.
Numerical simulations using COMSOL Multiphysics accurately reproduced these spectral characteristics. The 2.6 μm peak corresponds to an electric quadrupole mode, arising from asymmetric charge distribution around the particle perimeter.PMID:35102602 The 3.4 μm resonance originates from dipole oscillation of the SiO₂ core, acting as a cavity resonator. The 6.1 μm feature results from plasmonic confinement at the Au-water interface. The 9.1 μm peak is attributed to a gap plasmon mode localized in the narrow region between gold and silica, particularly strong near wavelengths where silica exhibits negative permittivity.
Crucially, the entire process is fully reversible. Applying a brief 1 Hz AC pulse for 1–3 seconds disrupts the chain structure, returning the system to a dispersed state. This demonstrates the feasibility of creating programmable, switchable optical components with no permanent modifications.
These findings establish a scalable, low-cost platform for dynamically tunable infrared devices. By adjusting the AC field parameters—frequency, amplitude, and waveform—the optical response can be precisely controlled in real time. Applications include adaptive filters, tunable waveguides, real-time biosensors, and integrated photonic circuits. The soft, liquid-phase nature of the material also enables compatibility with microfluidic systems, wearable sensors, and lab-on-a-chip platforms. This work opens new pathways toward intelligent, responsive optical materials that combine self-assembly with external field control—ushering in a new era of smart photonics.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com