Gold Nanorod Microneedles Supercharge Infrared Detection of Microplastics

Microplastics have become one of the most pervasive pollutants on the planet, turning up everywhere from the deepest ocean trenches to the tea in our cups. Yet detecting them, especially at low concentrations and at the nanoscale, remains a stubborn analytical challenge. A research team in South Korea now reports a strikingly simple solution: microneedle arrays tipped with densely packed clusters of gold nanorods that dramatically amplify the infrared fingerprints of polystyrene microplastics and nanoplastics. The work, published in the journal Advances in Industrial and Engineering Chemistry, demonstrates that surface-enhanced infrared absorption (SEIRA) spectroscopy, run in attenuated total reflection (ATR) mode, can identify irregularly shaped plastic fragments at concentrations as low as 0.1 milligrams per milliliter.

The problem the team set out to solve is well known among environmental analysts. Conventional Fourier transform infrared (FTIR) spectroscopy, the workhorse technique for identifying plastics, struggles badly with particles smaller than about 50 micrometers, because the infrared absorption cross-section of such tiny targets is simply too weak. Irregularly shaped fragments, which are exactly what environmental weathering produces, scatter incident light and generate artifacts, background signals, and ambiguous interpretations. Raman spectroscopy has partly filled the gap and is now more intensively used for microplastic identification than FTIR, but it too suffers from low sensitivity and diffraction-limited spatial resolution, and particles smaller than roughly 20 micrometers are often considered non-detectable in the standard Raman microscopy format.

Surface-enhanced infrared absorption offers a way around these limits. The technique exploits resonant electron oscillations, or plasmons, within metallic nanoantennas to dramatically boost the magnitude of infrared signals from molecules close to the metal surface. The catch has been that gold nanorods, the most commonly used plasmonic nanoantennas, naturally resonate in the visible to near-infrared range, not the mid-infrared where the characteristic vibrational fingerprints of polymers live. Achieving genuine plasmonic enhancement in the mid-IR has been rare, hampered by intrinsic noise, the limitations of mid-infrared light emitters, and the need for sophisticated waveguide-based light control. Previous successes required painstaking lift-off lithography or meta-films with stringent lattice fabrication to create short-range-ordered gold nanostructures under sub-wavelength gaps.

The Korean team, led by Hyun Ho Lee of Myongji University together with Jonghoon Choi of Chung-Ang University, took a deliberately looser approach. Instead of precisely patterning nanostructures, they allowed gold nanorods to randomly stack and cluster at the tips of polymeric microneedles during a simple imprint fabrication process. When packed closely together at sub-wavelength spacings, these short-range-ordered clusters act as nanoresonators capable of exciting both plasmon and phonon resonances in the mid-infrared. The random light confinement created by the tightly packed rods, combined with the reflection prism of the ATR setup, proved sufficient to evoke strong signal enhancement without any elaborate nanofabrication.

The microneedles themselves were made by a soft-lithography stamping method. For conical-shaped arrays, the researchers engraved molds directly into polydimethylsiloxane (PDMS) using a carbon dioxide laser, varying laser intensity and speed to control the tip geometry. For pyramidal arrays, they used a commercially available polycarbonate microneedle patch as a master to create PDMS molds. A drop of Norland Optical Adhesive 63, a UV-curable polymer, was cast onto the mold, drawn into every recess under vacuum, and stamped onto glass before being cured with ultraviolet light for 25 minutes and baked at 70 degrees Celsius. Critically, the gold nanorod solution was applied to the mold first, so that the rods became concentrated and embedded precisely at the tip end of each needle as the adhesive cured around them.

Selectivity came from chemistry rather than geometry. The researchers functionalized the exposed nanorods with a short oligopeptide called polystyrene binding peptide, or PSBP, a seven-amino-acid sequence (HWGMWSY) with a terminal thiol group that anchors it efficiently to gold surfaces. This peptide binds strongly and specifically to polystyrene, so any polystyrene microplastic drifting near a needle tip gets captured right where the plasmonic hotspots are concentrated. Before conjugation, the team used a plasma treatment to oxidize the needle tips and expose the gold nanorods that had been passivated by the adhesive during stamping, confirming the exposure with energy-dispersive X-ray spectroscopy. The microneedles were then incubated in the peptide solution, rinsed thoroughly, and dipped into samples containing polystyrene fragments at concentrations ranging from 0.1 to 100 milligrams per milliliter.

The evidence that the system works came from multiple directions. Scanning electron microscopy showed polystyrene particles attached specifically to the needle tips and nowhere else, with captured particles spanning sizes from about 10 to 100 micrometers despite the sieving step used to prepare the samples, which targeted fragments between 75 and 106 micrometers. The samples themselves were made to mimic real environmental debris: polystyrene litter was cryo-ground into irregularly shaped fragments rather than using the smooth spherical beads common in laboratory studies. The fact that nanoscale particles were also identified at the tips suggests the platform reaches down into the nanoplastic regime, where toxicity concerns are most acute.

The spectroscopic results were the centerpiece. When the peptide-functionalized, particle-capturing microneedles were pressed face-down against the flat surface of an ATR-FTIR spectrometer, characteristic polystyrene peaks appeared at 1450, 1700, and 1750 wavenumbers, amplified more than a hundredfold compared with controls. Those controls told a convincing story: needles carrying only the peptide showed no plastic peaks, and needles incubated in plastic solution without the peptide, allowing only weak physical adsorption, showed nothing detectable either. Because the adhesive itself is a benzene-containing polymer similar to polystyrene, the team checked whether the enhanced peaks might simply be background from the needle material, but the bare adhesive produced no strong signals at the characteristic positions. The enhancement, they concluded, arises from the plasmon polariton resonance of the clustered nanorods.

Raman measurements added a complementary dimension, though with an important caveat about geometry. Direct top illumination of the needles with a 532-nanometer laser failed to produce enhanced peaks, apparently because the needle apexes shadowed the signal. When the team tilted the samples and optimized the angle on a position-gauged stage, side illumination revealed surface-enhanced Raman scattering peaks, including a highlighted band near 2900 wavenumbers that distinguished tip-captured particles from bulk polystyrene powder, which instead shows its strongest aromatic carbon-hydrogen stretch at 3058 wavenumbers. That shift between the powder spectrum and the tip-captured spectrum is itself evidence of the plasmonic effect at work, and the finite number of nanorods exposed at each tip confers a moderate spatial resolution that ordinary Raman microscopy cannot match.

Beyond the raw sensitivity, the microneedle format brings practical advantages that could matter for field deployment. The needle array acts as a physical sieve during sampling, capturing particles while letting much of the background matrix wash away, which reduces interfering signals and substitutes for the filtration steps that conventional Raman workflows require. The high surface energy of the needles also lets aqueous samples dry quickly, and the arrays are cheap to replicate, with a single PDMS mold surviving three to five stamping cycles without mechanical failure. The researchers suggest that with further development of portable infrared measurement tools, the platform could enable rapid point-of-care style testing for environmental microplastic contamination, turning a laboratory-bound spectroscopic technique into something closer to a field instrument for tracking one of the defining pollutants of the plastic age.

Subject of Research: Surface-enhanced infrared spectroscopy using gold nanorod clusters on microneedle tips for detecting microplastics and nanoplastics

Article Title: Enhanced mid-IR detection characteristics of microplastics and nanoplastics using gold nanorods cluster at microneedle tips

Article References: Hur, H., Kim, C., Jo, A., Kim, G., Choi, J., & Lee, H. H. (2025). Enhanced mid-IR detection characteristics of microplastics and nanoplastics using gold nanorods cluster at microneedle tips. Advances in Industrial and Engineering Chemistry, 1(1), Article 9. https://doi.org/10.1007/s44405-025-00008-x

Image Credits: AI Generated

DOI: 10.1007/s44405-025-00008-x

Keywords: microplastics, nanoplastics, gold nanorods, SEIRA, microneedles, infrared spectroscopy, ATR-FTIR, SERS, polystyrene binding peptide, plasmonics, environmental monitoring, nanotechnology