Optical Cavity Design and Functionality for Molecular Strong Coupling.

Optical cavity/molecule strong coupling offers attractive opportunities to modulate photochemical or photophysical processes. When atoms or molecules are placed in an optical cavity, they can coherently exchange photonic energy with optical cavity vacuum fields, entering the strong coupling interaction regime. Recent work suggests that the thermodynamic and kinetic properties of molecules can be significantly changed by strong coupling, resulting in the emergence of intriguing photochemical and photophysical phenomena. As more and more physico-chemical systems are studied under strong coupling conditions, optical cavities have also advanced in their sophistication, responsiveness, and (multi)functionality. In this review, we highlight some of these recent developments, particularly focusing on Fabry-Perot microcavities.

Visualizing Ribbon-to-Ribbon Heterogeneity of Chemically Unzipped Wide Graphene Nanoribbons by Silver Nanowire-Based Tip-Enhanced Raman Scattering Microscopy.

Graphene nanoribbons (GNRs), a quasi-one-dimensional form of graphene, have gained tremendous attention due to their potential for next-generation nanoelectronic devices. The chemical unzipping of carbon nanotubes is one of the attractive fabrication methods to obtain single-layered GNRs (sGNRs) with simple and large-scale production.  The authors recently found that unzipping from double-walled carbon nanotubes (DWNTs), rather than single- or multi-walled, results in high-yield production of crystalline sGNRs. However, details of the resultant GNR structure, as well as the reaction mechanism, are not fully understood due to the necessity of nanoscale spectroscopy. In this regard, silver nanowire-based tip-enhanced Raman spectroscopy (TERS) is applied for single GNR analysis and investigated ribbon-to-ribbon heterogeneity in terms of defect density and edge structure generated through the unzipping process.  The authors found that sGNRs originated from the inner walls of DWNTs showed lower defect densities than those from the outer walls. Furthermore, TERS spectra of sGNRs exhibit a large variety in graphitic Raman parameters, indicating a large variation in edge structures. This work at the single GNR level reveals, for the first time, ribbon-to-ribbon heterogeneity that can never be observed by diffraction-limited techniques and provides deeper insights into unzipped GNR structure as well as the DWNT unzipping reaction mechanism.

Nanoscale Chemical Diversity of Coke Deposits on Nanoprinted Metal Catalysts Visualized by Tip-Enhanced Raman Spectroscopy.

Coke formation is the prime cause of catalyst deactivation, where undesired carbon wastes block the catalyst surface and hinder further reaction in a broad gamut of industrial chemical processes. Yet, the origins of coke formation and their distribution across the catalyst remain elusive, obstructing the design of coke-resistant catalysts. Here, the first-time application of tip-enhanced Raman spectroscopy (TERS) is demonstrated as a nanoscale chemical probe to localize and identify coke deposits on a post-mortem metal nanocatalyst. Monitoring coke at the nanoscale circumvents bulk averaging and reveals the local nature of coke with unmatched detail. The nature of coke is chemically diverse and ranges from nanocrystalline graphite to disordered and polymeric coke, even on a single nanoscale location of a top-down nanoprinted SiO2 -supported Pt catalyst. Surprisingly, not all Pt is an equal producer of coke, where clear isolated coke "hotspots" are present non-homogeneously on Pt which generate large amounts of disordered coke. After their formation, coke shifts to the support and undergoes long-range transport on the surrounding SiO2 surface, where it becomes more graphitic. The presented results provide novel guidelines to selectively free-up the coked metal surface at more mild rejuvenation conditions, thus securing the long-term catalyst stability.

Remote Excitation of Tip-Enhanced Photoluminescence with a Parallel AgNW Coupler.

Tip-enhanced photoluminescence (TEPL) microscopy allows for the correlation of scanning probe microscopic images and photoluminescent spectra at the nanoscale level in a similar way to tip-enhanced Raman scattering (TERS) microscopy. However, due to the higher cross-section of fluorescence compared to Raman scattering, the diffraction-limited background signal generated by far-field excitation is a limiting factor in the achievable spatial resolution of TEPL. Here, we demonstrate a way to overcome this drawback by using remote excitation TEPL (RE-TEPL). With this approach, the excitation and detection positions are spatially separated, minimizing the far-field contribution. Two probe designs are evaluated, both experimentally and via simulations. The first system consists of gold nanoparticles (AuNPs) through photoinduced deposition on a silver nanowire (AgNW), and the second system consists of two offset parallel AgNWs. This latter coupler system shows a higher coupling efficiency and is used to successfully demonstrate RE-TEPL spectral mapping on a MoSe2/WSe2 lateral heterostructure to reveal spatial heterogeneity at the heterojunction.

A light-mediated covalently patterned graphene substrate for graphene-enhanced Raman scattering (GERS).

We report covalently patterned graphene with acetic acid as a new potential candidate for graphene-enhanced Raman scattering (GERS). Rhodamine 6G molecules in direct contact with the covalently modified region show an enormous enhancement (∼25 times) compared to the pristine region at 532 nm excitation. The GERS enhancement with respect to the layer thickness of the probed molecule, excitation wavelength, and covalently attached groups is discussed.

Differential toxicity and localization of arginine-rich C9ORF72 dipeptide repeat proteins depend on de-clustering of positive charges.

Arginine-rich dipeptide repeat proteins (R-DPRs), poly(PR) and poly(GR), translated from the hexanucleotide repeat expansion in the amyotrophic lateral sclerosis (ALS)-causative C9ORF72 gene, contribute significantly to pathogenesis of ALS. Although both R-DPRs share many similarities, there are critical differences in their subcellular localization, phase separation, and toxicity mechanisms. We analyzed localization, protein-protein interactions, and phase separation of R-DPR variants and found that sufficient segregation of arginine charges is necessary for nucleolar distribution. Proline not only efficiently separated the charges, but also allowed for weak, but highly multivalent binding. In contrast, because of its high flexibility, glycine cannot fully separate the charges, and poly(GR) behaves similarly to the contiguous arginines, being trapped in the cytoplasm. We conclude that the amino acid that spaces the arginine charges determines the strength and multivalency of the binding, leading to differences in localization and toxicity mechanisms.

Molecular Chemistry in Cavity Strong Coupling.

The coherent exchange of energy between materials and optical fields leads to strong light-matter interactions and so-called polaritonic states with intriguing properties, halfway between light and matter. Two decades ago, research on these strong light-matter interactions, using optical cavity (vacuum) fields, remained for the most part the province of the physicist, with a focus on inorganic materials requiring cryogenic temperatures and carefully fabricated, high-quality optical cavities for their study. This review explores the history and recent acceleration of interest in the application of polaritonic states to molecular properties and processes. The enormous collective oscillator strength of dense films of organic molecules, aggregates, and materials allows cavity vacuum field strong coupling to be achieved at room temperature, even in rapidly fabricated, highly lossy metallic optical cavities. This has put polaritonic states and their associated coherent phenomena at the fingertips of laboratory chemists, materials scientists, and even biochemists as a potentially new tool to control molecular chemistry. The exciting phenomena that have emerged suggest that polaritonic states are of genuine relevance within the molecular and material energy landscape.

SERS Endoscopy for Monitoring Intracellular Drug Dynamics.

Understanding the dynamics and distribution of medicinal drugs in living cells is essential for the design and discovery of treatments. The tools available for revealing this information are, however, extremely limited. Here, we report the application of surface-enhanced Raman scattering (SERS) endoscopy, using plasmonic nanowires as SERS probes, to monitor the intracellular fate and dynamics of a common chemo-drug, doxorubicin, in A549 cancer cells. The unique spatio-temporal resolution of this technique reveals unprecedented information on the mode of action of doxorubicin: its localization in the nucleus, its complexation with medium components, and its intercalation with DNA as a function of time. Notably, we were able to discriminate these factors for the direct administration of doxorubicin or the use of a doxorubicin delivery system. The results reported here show that SERS endoscopy may have an important future role in medicinal chemistry for studying the dynamics and mechanism of action of drugs in cells.

Liquid-phase photo-induced covalent modification (PICM) of single-layer graphene by short-chain fatty acids.

We report an efficient photo-induced covalent modification (PICM) of graphene by short-chain fatty acids (SCFAs) with an alkyl chain at the liquid-solid interface for spatially resolved chemical functionalization of graphene. Light irradiation on monolayer graphene under an aqueous solution of the SCFAs with an alkyl chain efficiently introduces sp3-hybridized defects, where the reaction rates of PICM are significantly higher than those in pure water. Raman and IR spectroscopy revealed that a high density of methyl, methoxy, and acetate groups is covalently attached to the graphene surface while it was partially oxidized by other oxygen-containing functional groups, such as OH and COOH. A greater downshift of the G-band in Raman spectra was observed upon the PICM with longer alkyl chains, suggesting that the charge doping effect can be controlled by the alkyl chain length of the SCFAs. The systematic research and exploration of covalent modification in SCFAs provide new insight and a potentially facile method for bandgap engineering of graphene.

Length-Controllable Gold-Coated Silver Nanowire Probes for High AFM-TERS Scattering Activity.

Tip-enhanced Raman scattering (TERS) microscopy is an advanced technique for investigation at the nanoscale that provides topographic and chemical information simultaneously. The TERS probe plays a crucial role in the microscopic performance. In the recent past, the development of silver nanowire (AgNW) based TERS probes solved the main tip fabrication issues, such as low mechanical strength and reproducibility. However, this fabrication method still suffers from low control of the protruded length of the AgNW. In this work, a simple water-air interface electrocutting method is proposed to achieve wide controllability of the length. This water cutting method was combined with a succedent Au coating on the AgNW surface, and the probe achieved an up to 100× higher enhancement factor (EF) and a 2× smaller spatial resolution compared to pristine AgNW. Thanks to this excellent EF, the water-cut Au-coated AgNW probes were found to possess high TERS activity even in the nongap mode, enabling broad applications.

The Patterning and Proportion of Charged Residues in the Arginine-Rich Mixed-Charge Domain Determine the Membrane-Less Organelle Targeted by the Protein.

Membrane-less organelles (MLOs) are formed by biomolecular liquid-liquid phase separation (LLPS). Proteins with charged low-complexity domains (LCDs) are prone to phase separation and localize to MLOs, but the mechanism underlying the distributions of such proteins to specific MLOs remains poorly understood. Recently, proteins with Arg-enriched mixed-charge domains (R-MCDs), primarily composed of R and Asp (D), were found to accumulate in nuclear speckles via LLPS. However, the process by which R-MCDs selectively incorporate into nuclear speckles is unknown. Here, we demonstrate that the patterning of charged amino acids and net charge determines the targeting of specific MLOs, including nuclear speckles and the nucleolus, by proteins. The redistribution of R and D residues from an alternately sequenced pattern to uneven blocky sequences caused a shift in R-MCD distribution from nuclear speckles to the nucleolus. In addition, the incorporation of basic residues in the R-MCDs promoted their localization to the MLOs and their apparent accumulation in the nucleolus. The R-MCD peptide with alternating amino acids did not undergo LLPS, whereas the blocky R-MCD peptide underwent LLPS with affinity to RNA, acidic poly-Glu, and the acidic nucleolar protein nucleophosmin, suggesting that the clustering of R residues helps avoid their neutralization by D residues and eventually induces R-MCD migration to the nucleolus. Therefore, the distribution of proteins to nuclear speckles requires the proximal positioning of D and R for the mutual neutralization of their charges.

Autotuning of Vibrational Strong Coupling for Site-Selective Reactions.

Site-selective chemistry opens new paths for the synthesis of technologically important molecules. When a reactant is placed inside a Fabry-Perot (FP) cavity, energy exchange between molecular vibrations and resonant cavity photons results in vibrational strong coupling (VSC). VSC has recently been implicated in modified chemical reactivity at specific reactive sites. However, as a reaction proceeds inside an FP cavity, the refractive index of the reaction solution changes, detuning the cavity mode away from the vibrational mode and weakening the VSC effect. Here we overcome this issue, developing actuatable FP cavities to allow automated tuning of cavity mode energy to maintain maximized VSC during a reaction. As an example, the site-selective reaction of the aldehyde over the ketone in 4-acetylbenzaldehyde is achieved by automated cavity tuning to maintain optimal VSC of the ketone carbonyl stretch during the reaction. A nearly 50 % improvement in site-selective reactivity is observed compared to an FP cavity with static mirrors, demonstrating the utility of actuatable FP cavities as microreactors for organic chemistry.

All-Optical and One-Color Rewritable Chemical Patterning on Pristine Graphene under Water.

We report a facile all-optical method for spatially resolved and reversible chemical modification of a graphene monolayer. A tightly focused laser on graphene under water introduces an sp3-type chemical defect by photo-oxidation. The sp3-type defects can be reversibly restored to sp2 carbon centers by the same laser with higher intensity. The photoreduction occurs due to laser-induced local heating on the graphene. These optical methods combined with a laser direct writing technique allow photowriting and erasing of a well-defined chemical pattern on a graphene canvas with a spatial resolution of about 300 nm. The pattern is visualized by Raman mapping with the same excitation laser, enabling an optical read-out of the chemical information on the graphene. Here, we successfully demonstrate all-optical Write/Read-out/Erase of chemical functionalization patterns on graphene by simply adjusting the one-color laser intensity. The all-optical method enables flexible and efficient tailoring of physicochemical properties in nanoscale for future applications.

Gold-coated silver nanowires for long lifetime AFM-TERS probes.

Tip-enhanced Raman scattering (TERS) microscopy is an advanced technique for investigation at the nanoscale because of its excellent properties, such as its label-free functionality, non-invasiveness, and ability to simultaneously provide topographic and chemical information. The probe plays a crucial role in TERS technique performance. Widely used AFM-TERS probes fabricated with metal deposition suffer from relatively low reproductivity as well as limited mapping and storage lifetime. To solve the reproducibility issue, silver nanowire (AgNW)-based TERS probes were developed, which, thanks to the high homogeneity of the liquid-phase synthesis of AgNW, can achieve high TERS performance with excellent probe reproductivity, but still present short lifetime due to probe oxidation. In this work, a simple Au coating method is proposed to overcome the limited lifetime and improve the performance of the AgNW-based TERS probe. For the Au-coating, different [Au]/[Ag] molar ratios were investigated. The TERS performance was evaluated in terms of changes in the enhancement factor (EF) and signal-to-noise ratio through multiple mappings and the storage lifetime in air. The Au-coated AgNWs exhibited higher EF than pristine AgNWs and galvanically replaced AgNWs with no remarkable difference between the two molar ratios tested. However, for longer scanning time and multiple mappings, the probes obtained with low Au concentration showed much longer-term stability and maintained a high EF. Furthermore, the Au-coated AgNW probes were found to possess a longer storage lifetime in air, allowing for long and multiple TERS mappings with one single probe.

Versatile and Robust Method for Antibody Conjugation to Nanoparticles with High Targeting Efficiency.

The application of antibodies in nanomedicine is now standard practice in research since it represents an innovative approach to deliver chemotherapy agents selectively to tumors. The variety of targets or markers that are overexpressed in different types of cancers results in a high demand for antibody conjugated-nanoparticles, which are versatile and easily customizable. Considering up-scaling, the synthesis of antibody-conjugated nanoparticles should be simple and highly reproducible. Here, we developed a facile coating strategy to produce antibody-conjugated nanoparticles using 'click chemistry' and further evaluated their selectivity towards cancer cells expressing different markers. Our approach was consistently repeated for the conjugation of antibodies against CD44 and EGFR, which are prominent cancer cell markers. The functionalized particles presented excellent cell specificity towards CD44 and EGFR overexpressing cells, respectively. Our results indicated that the developed coating method is reproducible, versatile, and non-toxic, and can be used for particle functionalization with different antibodies. This grafting strategy can be applied to a wide range of nanoparticles and will contribute to the development of future targeted drug delivery systems.

Failure-Experiment-Supported Optimization of Poorly Reproducible Synthetic Conditions for Novel Lanthanide Metal-Organic Frameworks with Two-Dimensional Secondary Building Units.

Invited for the cover of this issue are Daisuke Tanaka at Kwansei Gakuin University and co-workers at Kwansei Gakuin University, Hokkaido University, Kyoto University, Japan and KU Leuven, Belgium. The image is a depiction of exploring the desired crystal by decision tree analysis. Read the full text of the article at 10.1002/chem.202102404.

Selective crystallization via vibrational strong coupling.

The coupling of (photo)chemical processes to optical cavity vacuum fields is an emerging method for modulating molecular and material properties. Recent reports have shown that strong coupling of the vibrational modes of solvents to cavity vacuum fields can influence the chemical reaction kinetics of dissolved solutes. This suggests that vibrational strong coupling might also effect other important solution-based processes, such as crystallization from solution. Here we test this hitherto unexplored notion, investigating pseudopolymorphism in the crystallization from water of ZIF metal-organic frameworks inside optical microcavities. We find that ZIF-8 crystals are selectively obtained from solution inside optical microcavities, where the OH stretching vibration of water is strongly coupled to cavity vacuum fields, whereas mixtures of ZIF-8 and ZIF-L are obtained otherwise. Moreover, ZIF crystallization is accelerated by solvent vibrational strong coupling. This work suggests that cavity vacuum fields might become a tool for materials synthesis, biasing molecular self-assembly and driving macroscopic material outcomes.

Failure-Experiment-Supported Optimization of Poorly Reproducible Synthetic Conditions for Novel Lanthanide Metal-Organic Frameworks with Two-Dimensional Secondary Building Units*.

Novel metal-organic frameworks containing lanthanide double-layer-based secondary building units (KGF-3) were synthesized by using machine learning (ML). Isolating pure KGF-3 was challenging, and the synthesis was not reproducible because impurity phases were frequently obtained under the same synthetic conditions. Thus, dominant factors for the synthesis of KGF-3 were identified, and its synthetic conditions were optimized by using two ML techniques. Cluster analysis was used to classify the obtained powder X-ray diffractometry patterns of the products and thus automatically determine whether the experiments were successful. Decision-tree analysis was used to visualize the experimental results, after extracting factors that mainly affected the synthetic reproducibility. Water-adsorption isotherms revealed that KGF-3 possesses unique hydrophilic pores. Impedance measurements demonstrated good proton conductivities (σ=5.2×10-4  S cm-1 for KGF-3(Y)) at a high temperature (363 K) and relative humidity of 95 % RH.

Adaptive Optical Two-Photon Microscopy for Surface-Profiled Living Biological Specimens.

We developed adaptive optical (AO) two-photon excitation microscopy by introducing a spatial light modulator (SLM) in a commercially available microscopy system. For correcting optical aberrations caused by refractive index (RI) interfaces at a specimen's surface, spatial phase distributions of the incident excitation laser light were calculated using 3D coordination of the RI interface with a 3D ray-tracing method. Based on the calculation, we applied a 2D phase-shift distribution to a SLM and achieved the proper point spread function. AO two-photon microscopy improved the fluorescence image contrast in optical phantom mimicking biological specimens. Furthermore, it enhanced the fluorescence intensity from tubulin-labeling dyes in living multicellular tumor spheroids and allowed successful visualization of dendritic spines in the cortical layer V of living mouse brains in the secondary motor region with a curved surface. The AO approach is useful for observing dynamic physiological activities in deep regions of various living biological specimens with curved surfaces.

Gold-Etched Silver Nanowire Endoscopy: Toward a Widely Accessible Platform for Surface-Enhanced Raman Scattering-Based Analysis in Living Cells.

Recently, our group introduced the use of silver nanowires (AgNWs) as novel non-invasive endoscopic probes for detecting intracellular Raman signals. This method, although innovative and promising, relies exclusively on the plasmonic waveguiding effect for signal enhancement. It, therefore, requires sophisticated operational tools and protocols, drastically limiting its applicability. Herein, an advanced strategy is offered to significantly enhance the performance of these endoscopic probes, making this approach widely accessible and versatile for cellular studies. By uniformly forming gold structures on the smooth AgNW surface via a galvanic replacement reaction, the density of the light coupling points along the whole probe surface is drastically increased, enabling high surface-enhanced Raman scattering (SERS) efficiency upon solely focusing the excitation light on the gold-etched AgNW. The applicability of these gold-etched AgNW probes for molecular sensing in cells is demonstrated by detecting site-specific and high-resolved SERS spectra of cell compartment-labeling dyes, namely, 4',6-diamidino-2-phenylindole in the nucleus and 3,3'-dioctadecyloxacarbocyanine on the membrane. The remarkable spectral sensitivity achieved provides essential structural information of the analytes, indicating the overall potential of the proposed approach for cellular studies of drug interactions with biomolecular items.