Classical vs. quantum plasmon-induced molecular transformations at metallic nanojunctions.

Chemical transformations near plasmonic metals have attracted increasing attention in the past few years. Specifically, reactions occurring within plasmonic nanojunctions that can be detected via surface and tip-enhanced Raman (SER and TER) scattering were the focus of numerous reports. In this context, even though the transition between localized and nonlocal (quantum) plasmons at nanojunctions is documented, its implications on plasmonic chemistry remain poorly understood. We explore the latter through AFM-TER-current measurements. We use two molecules: i) 4-mercaptobenzonitrile (MBN) that reports on the (non)local fields and ii) 4-nitrothiophenol (NTP) that features defined signatures of its neutral/anionic forms and dimer product, 4,4'-dimercaptoazobenzene (DMAB). The transition from classical to quantum plasmons is established through our optical measurements: It is marked by molecular charging and optical rectification. Simultaneously recorded force and current measurements support our assignments. In the case of NTP, we observe the parent and DMAB product beneath the probe in the classical regime. Further reducing the gap leads to the collapse of DMAB to form NTP anions. The process is reversible: Anions subsequently recombine into DMAB. Our results have significant implications for AFM-based TER measurements and their analysis, beyond the scope of this work. In effect, when precise control over the junction is not possible (e.g., in SER and ambient TER), both classical and quantum plasmons need to be considered in the analysis of plasmonic reactions.

Subnanometer Visualization of Spatially Varying Local Field Resonances that Drive Tip-Enhanced Optical Spectroscopy.

Our knowledge of the electromagnetic fields that power modern nanoscale optical measurements, including (non)linear tip-enhanced Raman and photoluminescence, chiefly stems from numerical simulations. Aside from idealized in silico vs heterogeneous (nano)structures in the laboratory, challenges in quantitative descriptions of nanoscale light-matter interactions more generally stem from the very nature of the problem, which lies at the interface of classical and quantum theories. This is particularly the case in ultrahigh spatial resolution measurements that are sensitive to local optical field variations that take place on subnanometer length scales. This work approaches this challenge through extinction-based spectral nanoimaging experiments. We demonstrate <1 nm spatial resolution in hyperspectral extinction measurements that track spatially varying plasmon resonances. We describe the principles behind our experiments and highlight more general implications of our observations.

Antiblood Cell Adhesion of Mussel-Inspired Chondroitin Sulfate- and Caffeic Acid-Modified Polycarbonate Membranes.

We fabricated a mussel-inspired hemocompatible polycarbonate membrane (PC) modified by the cross-linking of chondroitin sulfate and caffeic acid polymer using CA-CS via a Schiff base and Michael addition reaction and named it CA-CS-PC. The as-fabricated CA-CS-PC membrane shows excellent hydrophilicity with a water contact angle of 0° and a negative surface charge with a zeta potential of -32 mV. The antiadhesion property of the CA-CS-modified PC membrane was investigated by enzyme-linked immunosorbent assay (ELISA), using human plasma protein fibrinogen adsorption studies, and proved to have excellent antiadhesion properties, because of the lower fibrinogen adsorption. In addition, the CA-CS-PC membrane also shows enhanced hemocompatibility. Finally, blood cell attachment tests of the CA-CS-PC membrane were observed by CLSM and SEM, and the obtained results proved that CA-CS-PC effectively resisted cell adhesion, such as platelets and leucocytes. Therefore, this work disclosed a new way to design a simple and versatile modification of the membrane surface by caffeic acid and chondroitin sulfate and apply it for cell adhesion.

Ambient Tip-Enhanced Two Photon Photoluminescence from CdSe/ZnS Quantum Dots.

Nonlinear nano-optical measurements that combine ultrafast spectroscopy with tools of scanning probe microscopy are scarce. This is particularly the case when high spatial resolution on the order of a few nanometers is sought after in experiments performed under ambient laboratory conditions. In this work, we demonstrate the latter through measurements that track two-photon photoluminescence from aggregates of CdSe/ZnS quantum dots with sub-5 nm spatial resolution. Our proof-of-principle measurements that only take advantage of a plasmonic probe (as opposed to a gap mode) pave the way for nonlinear photoluminescence-based spectral nanoimaging of realistic/heterogeneous (bio) molecular and (bio) material systems.

Multipolar Raman Scattering vs Interfacial Nanochemistry: Case of 4-Mercaptopyridine on Gold.

Caution needs to be exercised in associating changes in plasmon-enhanced Raman spectra with chemical transformations. This is demonstrated through a detailed analysis of tip-enhanced Raman (TER) scattering from 4-mercaptopyridine (MPY) on gold. The substrate used consists of gold nanoplates atop a gold surface featuring heterogeneous grooves, all coated with a monolayer of MPY. The brightest spectra across the substrate exhibit features that can only be recovered by considering the generalized polarizability of oriented MPY molecules. The complex TER spectra we observe do not mark interfacial chemistry but rather multipolar TER scattering driven by local field gradients.

LSPR Sensing of Epithelial Cell Adhesion Molecules through Sphere and Cavity Plasmons of a Composite Ring Array of Poly[2-(dimethylamino)ethyl methacrylate]/Gold Nanoparticles.

Self-organization facilitates the formation of specific structures as a result of constituent interactions. In this study, the bottom of a 600 nm hole array photoresist template, which was deposited with a hydrophobic atom transfer radical polymerization (ATRP) initiator, was wetted by treatment with oxygen plasma. After the removal of the photoresist template, ring patterns of the ATRP initiator were formed at the interface between the hydrophobic and wetting regions. Poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) was grafted from the ring array of the initiator to immobilize gold nanoparticles (AuNPs) as a uniform ring array on a silicon substrate via repeated swelling/shrinking cycles. The localized surface plasmon resonance (LSPR) peak of the PDMAEMA-AuNP hybrid ring (PAHR) red-shifted after 12 swelling/shrinking cycles. In comparison to gold nanoparticles, scalable gold nanorings can effectively develop a variety of nanostructures to design LSPR-based sensors and optimize the sensing accuracy and stability. To detect epithelial cell adhesion molecules (EpCAM) during the structural change from a ring to a disk, antiEpCAM was anchored onto the PAHR as a biosensor during swelling/shrinking. The coupling of antiEpCAM and EpCAM led to asymptotical convergence from rings to disks as well as blue shifts of the LSPR peaks. Linear correlation between the blue shift and EpCAM concentration showed a limit of detection of ∼27 pg mL-1 and a linear range of 25-200 pg mL-1 for the detection of EpCAM within 30 min. The simple method of combining lithography and plasma technology provides a versatile platform for developing the scalable ring structure of AuNPs for highly sensitive and selective biosensing.

Fluorine-Free Superhydrophobic Covalent-Organic-Polymer Nanosheet Coating for Selective Dye and Emulsion Separation.

Covalent organic polymer nanosheets (COPNs) endowed with porous networks and large surface areas in their structures offer great advantages over other materials in addressing environmental problems. In this study, fluorine-free superhydrophobic COPNs were designed and applied to selective dye absorption. Notably, COPNs selectively adsorb dyes with a high hydrophobic index (HI) and reject low HI dyes with maximum adsorption capacities of 361 and 263 mg/g for crystal violet and methylene blue, respectively. The adsorption isotherm model showed that the COPNs follow the Langmuir adsorption isotherm model and pseudo-second-order kinetics. Next, we explored the superhydrophobicity of the COPNs by in situ fabrication with melamine sponge (COPNs-MS), which incorporates the superhydrophobicity of COPNs [water contact angle (WCA) of >150°] with the structure and flexibility of the MS skeleton. The COPNs-MS shows various oil-adsorbing properties with good adsorption capacity (from 60 to 120 g/g) and also effectively separates various surfactant-stabilized emulsions with a separation efficiency of over 99%. The as-fabricated COPNs-MS retains its superhydrophobicity in various solvents and hazardous conditions (WCA ≥ 150°) and exhibits good flame retardancy and excellent compression properties with excellent antifouling property due to the superhydrophobic COPN coating. Furthermore, COPNs-MS also demonstrates excellent recyclability because the strong COPN coating in the MS skeleton retains its hydrophobicity. Therefore, our fluorine-free superhydrophobic COPNs are not only capable of selective dye adsorption but also exhibit very good oil adsorption and surfactant-stabilized emulsion separation performance.

Resonant Coherent Raman Scattering from WSe2.

Low-dimensional transition-metal dichalcogenides (TMDs) continue to comprise a subject of intense research because of their unique optical and electronic properties that may be harnessed in modern devices. Intense photoluminescence (PL) from few-/monolayer TMDs rendered PL-based micro- and nanospectroscopic characterization ideal in the quest to understand the correlation between structure and function in these materials. Nonlinear optical methods are by comparison far less utilized for this purpose. In this work, we describe an approach based on electronically resonant four-wave-mixing that allows spatio-spectral characterization of excitons in monolayer WSe2. Due to the coherent nature of the response that we exploit to trace exciton resonances, and recent demonstrations of electronic four-wave-mixing-based nanoimaging and nanospectroscopy, our present work is an important step toward characterizing TMDs on the nano-femto scale using light.

Nanoindentation-enhanced tip-enhanced Raman spectroscopy.

We combine nanoindentation, herein achieved using atomic force microscopy-based pulsed-force lithography, with tip-enhanced Raman spectroscopy (TERS) and imaging. Our approach entails indentation and multimodal characterization of otherwise flat Au substrates, followed by chemical functionalization and TERS spectral imaging of the indented nanostructures. We find that the resulting structures, which vary in shape and size depending on the tip used to produce them, may sustain nano-confined and significantly enhanced local fields. We take advantage of the latter and illustrate TERS-based ultrasensitive detection/chemical fingerprinting as well as chemical reaction imaging-all using a single platform for nano-lithography, topographic imaging, hyperspectral dark field optical microscopy, and TERS.

Observation of Intersubband Polaritons in a Single Nanoantenna Using Nano-FTIR Spectroscopy.

Strong coupling of an intersubband (ISB) electron transition in quantum wells to a subwavelength plasmonic nanoantenna can give rise to intriguing quantum phenomena, such as ISB polariton condensation, and enable practical devices including low threshold lasers. However, experimental observation of ISB polaritons in an isolated subwavelength system has not yet been reported. Here, we use scanning probe near-field microscopy and Fourier-transform infrared (FTIR) spectroscopy to detect formation of ISB polariton states in a single nanoantenna. We excite the nanoantenna by a broadband IR pulse and spectrally analyze evanescent fields on the nanoantenna surface. We observe the distinctive splitting of the nanoantenna resonance peak into two polariton modes and two π-phase steps corresponding to each of the modes. We map ISB polariton dispersion using a set of nanoantennae of different sizes. This nano-FTIR spectroscopy approach opens doors for investigations of ISB polariton physics in the single subwavelength nanoantenna regime.

Biocompatible Meshes with Appropriate Wettabilities for Underwater Oil Transportation/Collection and Highly Effective Oil/Water Separation.

In this study, we prepared biocompatible superhydrophilic and underwater superoleophobic tannic acid (TA)/polyvinylpyrrolidone (PVP)-coated stainless-steel meshes that mediated extremely efficient separations of mixtures of oil and water. These TA/PVP-coated stainless-steel meshes displayed excellent antifouling properties and could be used to separate oil/water mixtures continuously for up to 24 h. Moreover, a funnel-like TA/PVP-coated stainless-steel mesh device could be used for underwater oil transportation and collection. In conjunction with our continuous oil removal system, this device allowed for the continuous collection and removal of oil pollutants from underwater environments. The high performance of these TA/PVP-coated stainless-steel meshes and their green, low-energy, cost-effective preparation suggests great potential for practical applications.

Preparation of Superwetting Porous Materials for Ultrafast Separation of Water-in-Oil Emulsions.

Functional materials with a superwetting surface property have been extensively explored to achieve emulsion separation. In this paper, we report a simple and inexpensive method for fabricating superhydrophobic/superoleophilic porous materials from polymeric sponges. These microstructured porous materials, which do not contain any fluorinated compounds, maintain their superhydrophobicity and superoleophilicity after long-term organic solvent immersion and display environmental stability. These superhydrophobic porous materials can effectively separate a wide range of water-in-oil emulsions including surfactant-free and surfactant-stabilized water-in-oil emulsions with high efficiency (>99.98%) and high flux (up to 155 000 L m-2 h-1 bar-1). Meanwhile, these materials exhibited excellent pH resistance and antifouling properties. The high performance of our superhydrophobic porous materials and their efficient, low-energy, cost-effective preparation suggest that they have a great potential for practical applications.

Visualizing nanoscale heterogeneity in perylene thin films via tip-enhanced photoluminescence with unsupervised machine learning.

We investigate the properties of ultrathin 3,4,9,10-perylenetetracarboxylic diimide (PTCDI) films using a combination of tip-enhanced photoluminescence and unsupervised machine learning. We expose nanoscale spectral heterogeneities that can be understood on the basis of the interplay between vibronic effects, intermolecular excitons, and intramolecular excitons in PTDCI films.

An Eco-Friendly Manner to Prepare Superwetting Melamine Sponges with Switchable Wettability for the Separation of Oil/Water Mixtures and Emulsions.

Oil/water separation processes have garnered significant global attention due to the quick growth in industrial development, recurring chemical leakages, and oil spills. Hence, there is a significant demand for the development of inexpensive superwetting materials in an eco-friendly manner to separate oil/water mixtures and emulsions. In this study, a superwetting melamine sponge (SMS) with switchable wettabilities was prepared by modifying melamine sponge (MS) with sodium dodecanoate. The as-prepared SMS exhibited superhydrophobicity, superoleophilicity, underwater superoleophobicity, and underoil superhydrophobicity. The SMS can be utilized in treating both light and heavy oil/water mixtures through the prewetting process. It demonstrated fast permeation fluxes (reaching 108,600 L m-2 h-1 for a light oil/water mixture and 147,700 L m-2 h-1 for a heavy oil/water mixture) and exhibited good separation efficiency (exceeding 99.56%). The compressed SMS was employed in separating surfactant-stabilized water-in-oil emulsions (SWOEs), as well as surfactant-stabilized oil-in-water emulsions (SOWEs), giving high permeation fluxes (reaching 7210 and 5054 L m-2 h-1, respectively). The oil purity for SWOEs' filtrates surpassed 99.98 wt% and the separation efficiencies of SOWEs exceeded 98.84%. Owing to their remarkable capability for separating oil/water mixtures and emulsions, eco-friendly fabrication method, and feasibility for large-scale production, our SMS has a promising potential for practical applications.

Changes in the Bio-Compounds and Biological Activities of Eight Whole Grains Fermentation Starter with Different Oxidized Chin-Shin Oolong Teas.

Chin-shin oolong tea is the most widely planted variety in Taiwan. This study fermented eight whole grains fermentation starter (EGS) with light (LOT), medium (MOT), and fully (FOT) oxidized Chin-shin oolong teas for ten weeks. Comparing the three fermentation beverages, it was found that LOT fermentation can obtain the highest catechins (1644.56 ± 60.15 ppm) among the functional and antioxidant components. MOT can obtain the highest glucuronic acid (19,040.29 ± 2903.91 ppm), tannins, total phenols, flavonoids, and angiotensin-converting enzyme (ACE) inhibitory activity. FOT can obtain the highest GABA (1360.92 ± 123.24 ppm). In addition, both the LOT and MOT showed a significant increase in their ability to scavenge DPPH radicals after fermentation. EGS fermented with lightly or moderately oxidized Chin-shin oolong tea may be considered a novel Kombucha.

Probing Local Optical Fields via Ultralow Frequency Raman Scattering from a Corrugated Probe.

We revisit nanoscale local optical field imaging via tip-enhanced Raman scattering (TERS). Rather than taking advantage of molecular reporters to probe different aspects of the local fields, we show how ultralow frequency Raman (ULF) scattering from the (nanocorrugated) metallic probe itself can be used for the same purpose. The bright ULF-TERS response we record allows non-invasive (tapping mode feedback) local field imaging, enables visualization of the local fields of small (≥20 nm) isolated plasmonic particles, and can also be exploited to distinguish between Si and SiO2 domains with 5 nm spatial resolution. We describe our approach and its limitations, particularly when it comes to using all-metallic versus molecular reporters.

Ultraviolet-durable superhydrophobic zinc oxide-coated mesh films for surface and underwater-oil capture and transportation.

In this paper, we report a simple and an inexpensive method for fabricating superhydrophobic/superoleophilic mesh films from microstructured ZnO coatings. The microstructured ZnO coatings, which do not contain any fluorinated compounds, maintain their superhydrophobicity and superoleophilicity after ultraviolet irradiation and display environmental stability. Furthermore, those microstructured ZnO-coated mesh films exhibit good selectivity (even underwater) and excellent recyclability, making them promising candidates for many potential applications, including liquid-liquid separation, water treatment, and liquid transportation.

Multimodal (Non)Linear Optical Nanoimaging and Nanospectroscopy.

This Perspective highlights recent advances in linear and nonlinear spectral nanoimaging. The described developments are motivated by the need to characterize molecular and material systems noninvasively with nanometer spatial and femtosecond temporal resolution. Indeed, the ability to image and chemically characterize heterogeneous interfaces with joint nano-femto resolution is a prerequisite to advancing our fundamental understanding of processes as diverse as heterogeneous catalysis, microbial communication, and energy flow in pristine/defect-containing low-dimensional quantum materials, to name a few. We describe pioneering work and recent demonstrations of (non)linear optical nanoimaging and nanospectroscopy, with an emphasis on high spatial resolution measurements conducted under ambient laboratory conditions.

Sustainable, biocompatible, and mass-producible superwetting water caltrop shell biochars for emulsion separations.

The separation of oily wastewater, specifically emulsions, is a crucial global issue. Possible strategies for the efficient separation of emulsified oil/water mixtures through sustainable and environmentally friendly materials have recently drawn considerable attention. In our study, we prepared superwetting water caltrop shell biochar (WCSB) via a top-lit-updraft carbonization procedure. The as-prepared WCSB was characterized by superhydrophilicity, underwater superoleophobicity, underoil superhydrophilicity, and underoil water adsorption ability. Because of its superwetting properties, WCSB was used for the separation of both surfactant-stabilized oil-in-water emulsions (SOIWEs) and surfactant-stabilized water-in-oil emulsions (SWIOEs) with very high fluxes (up to 74,700 and 241,000 L m-2 h-1 bar-1 for SOIWE and SWIOE, respectively). The separation performances were excellent, with oil contents in all SOIWE filtrates lower than 10 ppm and oil purities in all SWIOE filtrates higher than 99.99 wt%. Moreover, WCSB was applied to separate dye-spiked emulsions. Due to their high emulsion separation ability, sustainability, good biocompatibility, and ease of mass production, the as-prepared WCSBs have notable potential for utilitarian applications.

Multimodal Tip-Enhanced Nonlinear Optical Nanoimaging of Plasmonic Silver Nanocubes.

Optical field localization at plasmonic tip-sample nanojunctions has enabled high-spatial-resolution chemical analysis through tip-enhanced linear optical spectroscopies, including Raman scattering and photoluminescence. Here, we illustrate that nonlinear optical processes, including parametric four-wave mixing (4WM), second-harmonic/sum-frequency generation (SHG and SFG), and two-photon photoluminescence (TPPL), can be enhanced at plasmonic junctions and spatiospectrally resolved simultaneously with few-nm spatial resolution under ambient conditions. Through a detailed analysis of our spectral nanoimages, we find that the efficiencies of the local nonlinear signals are determined by sharp tip-sample junction resonances that vary over the few-nanometer length scale. Namely, plasmon resonances centered at or around the different nonlinear signals are tracked through TPPL, and they are found to selectively enhance nonlinear signals with closely matched optical resonances.