Geon Joon Lee

The purpose of this paper is to provide an in-depth review of plasmonic metal nanoparticles made from rhodium, platinum, gold, or silver. We describe fundamental concepts, synthesis methods, and optical sensing applications of these nanoparticles. Plasmonic metal nanoparticles have received a lot of interest due to various applications, such as optical sensors, single-molecule detection, single-cell detection, pathogen detection, environmental contaminant monitoring, cancer diagnostics, biomedicine, and food and health safety monitoring. They provide a promising platform for highly sensitive detection of various analytes. Due to strongly localized optical fields in the hot-spot region near metal nanoparticles, they have the potential for plasmon-enhanced optical sensing applications, including metal-enhanced fluorescence (MEF), surface-enhanced Raman scattering (SERS), and biomedical imaging. We explain the plasmonic enhancement through electromagnetic theory and confirm it with finite-difference time-domain numerical simulations. Moreover, we examine how the localized surface plasmon resonance effects of gold and silver nanoparticles have been utilized for the detection and biosensing of various analytes. Specifically, we discuss the syntheses and applications of rhodium and platinum nanoparticles for the UV plasmonics such as UV-MEF and UV-SERS. Finally, we provide an overview of chemical, physical, and green methods for synthesizing these nanoparticles. We hope that this paper will promote further interest in the optical sensing applications of plasmonic metal nanoparticles in the UV and visible ranges.

This research investigated the capture of nitrate by magnesium ions in plasma-activated water (PAW) and its antifungal effect on the cell viability of the newly emerged mushroom pathogen Cryptococcus pseudolongus. Optical emission spectra of the plasma jet exhibited several emission bands attributable to plasma-generated reactive oxygen and nitrogen species. The plasma was injected directly into deionized water (DW) with and without an immersed magnesium block. Plasma treatment of DW produced acidic PAW. However, plasma-activated magnesium water (PA-Mg-W) tended to be neutralized due to the reduction in plasma-generated hydrogen ions by electrons released from the zero-valent magnesium. Optical absorption and Raman spectra confirmed that nitrate ions were the dominant reactive species in the PAW and PA-Mg-W. Nitrate had a concentration-dependent antifungal effect on the tested fungal cells. We observed that the free nitrate content could be controlled to be lower in the PA-Mg-W than in the PAW due to the formation of nitrate salts by the magnesium ions. Although both the PAW and PA-Mg-W had antifungal effects on C. pseudolongus, their effectiveness differed, with cell viability higher in the PA-Mg-W than in the PAW. This study demonstrates that the antifungal effect of PAW could be manipulated using nitrate capture. The wide use of plasma therapy for problematic fungus control is challenging because fungi have rigid cell wall structures in different fungal groups.

Polyethylene glycol-functionalized gold nanoparticles (Au@PEG NPs) were prepared by a simple plasma-assisted method without additional reducing chemicals. After irradiating tetrachloroauric acid (HAuCl₄) and polyethylene glycol (PEG) in aqueous medium with an argon plasma jet, the gold precursor transformed into an Au@PEG NP colloid that exhibited surface plasma resonance at 530 nm. When the plasma jet entered the water, additional reactive species were induced through interactions between plasma-generated reactive species and aqueous media. Interaction of the gold precursor with the plasma-activated medium allowed the synthesis of gold nanoparticles (AuNPs) without reductants. The plasma-synthesized Au@PEG NPs had a quasi-spherical shape with an average particle diameter of 32.5 nm. The addition of PEG not only helped to stabilize the AuNPs but also increased the number of AuNPs. Au@PEG NP-loaded paper (AuNP-paper) was able to detect the degradation of rhodamine B, therefore, indicating that AuNP-paper can act as a surface-enhanced Raman scattering platform. Dye degradation by plasma treatment was investigated by optical absorption and Raman spectroscopy. The method proposed for the fabrication of Au@PEG NPs is rapid, low-cost, and environment-friendly and will facilitate the application of plasma-synthesized nanomaterials in sensors.

The chiroptical properties of polyfluorene–phenylene (ℎ) thin films were studied using circular dichroism (CD), surface-enhanced Raman scattering (SERS), attenuated total reflection (ATR), and spectroscopic ellipsometry. The pristine ℎ films exhibited significant CD at room temperature, and the chirality increased by thermal annealing at 120 ℃. SERS and ATR spectra indicated that the chirality enhancement was due to the self-organization and/or rearrangement of the polymer backbones by thermal annealing. Optical sensing of the chiral polymers was demonstrated using SERS spectra of the chiral polymer films on gold. Optical sensing properties of the chiral ℎ films were compared with those of the achiral polyfluorene films.

In this research, we investigated the chiroptical properties of chiral–achiral polymer blends consisting of poly[(9,9-bis((S)-3,7-dimethyloctyl)-9H-fluoren-2,7-diyl)- alt -(4-phenylene)] and poly[(9,9-di-n-octylfluorenyl-2,7-diyl)- alt -(benzo[2,1,3]thiadiazol-4,8-diyl)] (PFPh-F8BT). The chirality of the blend film decreased on increasing the composition of the achiral polymer. However, the measured circular dichroism values of the blends with high content of F8BT were lower than expected considering the composition of the chiral PFPh polymer, suggesting that the chirality of the blend film was governed by the steric hindrance of the achiral polymeric dopant. Next, we examined optical sensing of the chiral–achiral polymer blends using gold nanoparticles (AuNPs) prepared using the citrate reduction and Ostwald ripening methods. Transmission electron microscopy images showed that the as-made AuNPs had a monodispersed spherical shape with an average diameter of 89 nm. The optical absorption spectrum of the calcined AuNP film exhibited a surface plasmon resonance wavelength at 550 nm. Raman scattering intensities of the PFPh-F8BT blends on the AuNPs were higher than those on fused silica glass; the observed Raman enhancement could be attributed to the optical confinement effects of the AuNPs. Surface-enhanced Raman scattering spectra of the PFPh-F8BT blends revealed that the 1546 cm ⁻¹ Raman peak could be used to estimate the composition of F8BT within the PFPh-F8BT blend.

To better understand the function of the red pigment in Cordyceps pruinosa, its effects on the cellular response of fungal spores to plasma-generated reactive oxygen species were studied. It is known that the degree of pigmentation in fungi is affected by environmental factors such as light exposure. C. pruinosa produced a red pigment when it grew in the light condition, but produced no pigment in the dark condition. An atmospheric-pressure plasma jet (APPJ) was used to generate reactive oxygen and nitrogen species in aqueous solution. Exposure of C. pruinosa spores to the APPJ decreased their viability in a time-dependent manner. For the same plasma treatment times, viability of spores was higher for light condition-grown C. pruinosa (LCP) than for dark-grown C. pruinosa (DCP). Moreover, analysis of oxygen radical scavenging activity in the ethanol extracts of LCP and DCP revealed that the higher spore viability of LCP could be attributed to the red pigment in the fungus. Induced circular dichroism revealed that the red pigment was embedded in the cell wall matrix. Liquid-chromatography and mass-spectroscopy results showed that the red pigment in C. pruinosa contains a compound with a molecular weight of 570 and its oxidation products. Overall, this study shows that the red pigment in C. pruinosa protects its spores from reactive species generated from the plasma itself and those derived from reaction of plasma radicals with the aqueous solution.