Large Area Film of Highly Crystalline, Cleavable, and Transferable Semi-Conducting 2D-Imine Covalent Organic Framework on Dielectric Glass Substrate.

Two dimensional (2D) imine-based covalent organic framework (COF), 2D-COF, is a newly emerging molecular 2D polymer with potential applications in thin film electronics, sensing, and catalysis. It is considered an ideal candidate due to its robust 2D nature and precise tunability of the electronic and functional properties. Herein, we report a scalable facile synthesis of 2D imine-COF with control over film thickness (ranging from 100 nm to a few monolayers) and film dimension reaching up to 2 cm on a dielectric (glass) substrate. Highly crystalline 2D imine polymer films are formed by maintaining a quasi-equilibrium (very slow, ∼15 h) in Schiff base condensation reaction between p-phenylenediamine (PDA) and benzene-1,3,5-tricarboxaldehyde (TCA) molecules. Free-standing thin and ultrathin films of imine-COF are obtained using sonication exfoliation of 2D-COF polymer. Insights into the microstructure of thin/ultrathin imine-COF are obtained using scanning and transmission electron microscopy (SEM and TEM) and atomic force microscopy (AFM), which shows high crystallinity and 2D layered structure in both thin and ultrathin films. The chemical nature of the 2D polymer was established using X-ray photoelectron spectroscopy (XPS). Optical band gap measurements also reveal a semiconducting gap. This is further established by electronic structure calculation using density functional theory (DFT), which reveals a semiconductor-like band structure with strong dispersion in bands near conduction and valence band edges. The structural characteristics (layered morphology and microscopic structure) of 2D imine-COF show significant potential for its application in thin film device fabrication. In addition, the electronic structure shows strong dispersion in the frontier bands, making it a potential semiconducting material for charge carrier transportation in electronic devices.

Membrane Permeability Dominates over Electrostatic Interactions in Dictating Drug Transport in Osmotically Shocked Escherichia coli.

To combat surging multidrug-resistant Gram-negative bacterial infections, better strategies to improve the efficacy of existing drugs are critical. Because the dual membrane cell envelope is the first line of defense for these bacteria, it is crucial to understand the permeation properties of the drugs through it. Our recent study shows that isosmotic conditions prevent drug permeation inside Gram-negative bacteria, Escherichia coli, while hypoosmotic stress enhances the process. Here, we unravel the reason behind such differential drug penetration. Specifically, we dissect the roles of electrostatic screening and low membrane permeability in the penetration failure of drugs under osmotically balanced conditions. We compare the transport of a quaternary ammonium compound malachite green in the presence of an electrolyte (NaCl) and a wide variety of commonly used organic osmolytes, e.g., sucrose, proline, glycerol, sorbitol, and urea. These osmolytes of different membrane permeability (i.e., nonpermeable sucrose and NaCl, freely permeable urea and glycerol, and partially permeable proline and sorbitol) clarify the role of osmotic stress in cell envelope permeability. The results showcase that under balanced osmotic conditions, drug molecules fail to penetrate inside E. coli cells because of low membrane permeabilities and not because of electrostatic screening imposed by the osmolytes. Contribution of the electrostatic interactions, however, cannot be completely overruled as at osmotically imbalanced conditions, drug transport across the bacterial subcellular compartments is found to be dependent on the osmolytes used.

Hypo-osmotic Stress Increases Permeability of Individual Barriers in Escherichia coli Cell Envelope, Enabling Rapid Drug Transport.

Survival of foodborne Gram-negative bacteria during osmotic stress often leads to multidrug resistance development. However, despite the concern, how osmoadaptation alters drug penetration across the Gram-negative bacterial cell envelope has remained inconclusive for years. Here, we have investigated drug permeation and accumulation inside hypo-osmotically shocked Escherichia coli. Three different quaternary ammonium compounds (QACs) are used as cationic amine-containing drug representatives; they also serve as envelope permeability indicators in different assays. Propidium iodide fluorescence reveals cytoplasmic accumulation and overall envelope permeability, while crystal violet sorption and second harmonic generation (SHG) spectroscopy reveal periplasmic accumulation and outer membrane permeability. Malachite green sorption and SHG results reveal transport across both the outer and inner membranes and accumulation in the periplasm as well as cytoplasm. The findings are found to be complementary to one another, collectively revealing enhanced permeabilities of both membranes and the periplasmic space in response to hypo-osmotic stress in E. coli. Enhanced permeability leads to faster QACs transport and higher accumulation in subcellular compartments, whereas transport and accumulation both are negligible under isosmotic conditions. The QACs' transport rates are found to be highly influenced by the osmolytes used, where phosphate ion emerges as a key facilitator of transport across the periplasm into the cytoplasm. E. coli is found viable, with morphology unchanged under extreme hypo-osmotic stress; i.e., it adapts to the situation. The outcome shows that the hypo-osmotic shock to E. coli, specifically using phosphate as an osmolyte, can be beneficial for drug delivery.

Tuning nanoscale plasmon-exciton coupling via chemical interface damping.

Understanding the exact role of each plasmon decay channel in the plasmon-exciton interaction is essential for realizing the translational potential of nanoscale plexciton hybrids. Here, using single-particle spectroscopy, we demonstrate how a particular decay channel, chemical interface damping (CID), influences the nanoscale plasmon-exciton coupling. We investigate the interaction between cyanine dye J-aggregates and gold nanorods in the presence and absence of CID. The CID effect is introduced via surface modification of the nanorods with 4-nitrothiophenol. The relative contribution of CID is systematically tuned by varying the diameter of the nanorods, while maintaining the aspect ratio constant. We show that the incorporation of the CID channel, in addition to other plasmon decay channels, reduces the plasmon-exciton coupling strength. Nanorods' diameter-dependency measurements reveal that in the absence of CID contribution, the plasmon mode-volume factor gradually dominates over the plasmon decoherence effects as the diameter of the nanorods decreases, resulting in an increase in the plasmon-exciton coupling strength. However, the situation is entirely different when the CID channel is active: plasmon dephasing determines the plasmon-exciton coupling strength by outweighing the influence of even a very small plasmon mode-volume. Most importantly, our findings indicate that CID can be used to controllably tune the plasmon-exciton coupling strength for a given plexciton system by modifying the nanoparticle's surface with suitable adsorbates without the need for altering either the plasmonic or excitonic systems. Thus, judicious exploitation of CID can be tremendously beneficial in tailoring the optical characteristics of plexciton hybrid systems to suit any targeted application.

Simultaneous Harvesting of Multiple Hot Holes via Visible-Light Excitation of Plasmonic Gold Nanospheres for Selective Oxidative Bond Scission of Olefins to Carbonyls.

Using visible photoexcitation of gold nanospheres we successfully demonstrate the simultaneous harvesting of plasmon-induced multiple hot holes in the complete oxidative scission of the C=C bond in styrene at room temperature to selectively form benzaldehyde and formaldehyde, which is a reaction that requires activation of multiple substrates. Our results reveal that, while extraction of hot holes becomes efficient for interband excitation, harvesting of multiple hot holes from the excited Au nanospheres becomes prevalent only beyond a threshold light intensity. We show that the alkene oxidation proceeded via a sequence of two consecutive elementary steps; namely, a binding step and a cyclic oxometallate transition state as the rate-determining step. This demonstration of plasmon-excitation-mediated harvesting of multiple hot holes without the use of an extra hole transport media opens exciting possibilities, notably for difficult catalytic transformations involving multielectron oxidation processes.

Second harmonic generation-based nonlinear plasmonic RI-sensing in solution: the pivotal role of the particle size.

Here, we demonstrate the utility of the second harmonic generation (SHG) for refractometric sensing in the solution phase. We employ an aqueous colloid of gold nanorods as our sensors, and modulation in their SHG with the surrounding refractive index (RI) is mirrored using second-harmonic light scattering (SHLS). A limit of detection (LOD) as low as 9 × 10-4 RIU is achieved. The RI sensitivity of our SHLS-based approach is two orders of magnitude higher than that obtained using linear Rayleigh scattering. Most importantly, we show that the particle size plays a crucial role in controlling the nonlinear plasmonic sensing performance of gold nanorods.

S. boulardii Fails to Hold Its Cell Wall Integrity against Nonpathogenic E. coli: Are Probiotic Yeasts Losing the Battle?

Probiotic yeast Saccharomyces boulardii exerts direct probiotic action on pathogenic E. coli by trapping them on surfaces and inactivating toxic lipopolysaccharides. Using optical dark-field microscopy, we show that nonpathogenic E. coli cells also readily bind probiotic S. boulardii. More importantly, the adhered nonpathogenic E. coli progressively damage S. boulardii cell walls and lyse them. Co-cultured methylene blue-supplemented agar-plate assay indicates that rough lipopolysaccharides might be playing a key role in S. boulardii cell wall damage. When experiments are repeated with lipopolysaccharide-depleted E. coli and also lipopolysaccharide-deficient E. coli, adhesion decreases substantially. The co-cultured assay further reveals that free lipopolysaccharides, released from E. coli, are also causing damage to S. boulardii walls like adhered E. coli. These new findings contradict the known S. boulardii-E. coli interaction mechanisms. We confirm that E. coli cells do not bind or damage human erythrocyte cell walls; therefore, they have not developed pathogenicity. The combined results demonstrate the first example of nonpathogenic E. coli being harmful to probiotic yeast S. boulardii. This finding is important because gut microbial flora contain large numbers of nonpathogenic E. coli. If they bind or damage probiotic S. boulardii cell walls, then the probiotic efficiency toward pathogenic E. coli will be compromised.

The Pivotal Role of Hot Carriers in Plasmonic Catalysis of C-N Bond Forming Reaction of Amines.

Here, we demonstrate the simultaneous utilization of both the hot carriers (electrons and holes) in the photocatalytic transformation of benzylamine to N-benzylidenebenzylamine and the scope of reaction has also been successfully demonstrated with catalytic oxidation of 4-methoxybenzylamine. The wavelength-dependent excitation of AuNP allows us to tune the potential energy of charge carriers relative to the redox potential of the reactants which leads to energetically favorable product formation on the nanoparticle surface. We capture the formation of reaction intermediates and products by using in situ Raman spectroscopy, complemented by NMR spectroscopy and GC-MS. Based on the experimental substantiations, a plausible reaction mechanism has been proposed.

Probing the role of oscillator strength and charge of exciton forming molecular J-aggregates in controlling nanoscale plasmon-exciton interactions.

In this study, we probe into the roles of exciton oscillator strength and charge of J-aggregates as well as nanoparticle's surface capping ligands in dictating the plasmon-exciton interaction. We systematically compare the plasmon-exciton coupling strengths of two hybrid plexcitonic systems involving CTAB-capped hollow gold nanoprisms (HGNs) and two different cyanine dyes, TDBC and PIC, having very similar J-band spectral positions and linewidths, but different oscillator strengths and opposite charges. Both HGN-PIC and HGN-TDBC systems display large Rabi splitting energies which are found to be extremely dependent on dye-concentrations. Interestingly, for our plexciton systems we find that there is interplay between the exciton oscillator strength and the electrostatic interaction amid dyes and HGN-surfaces in dictating the coupling strength. The oscillator strength dominates at low dye-concentrations resulting in larger Rabi splitting in the HGN-PIC system while at high concentrations, a favorable electrostatic interaction between TDBC and CTAB-capped HGN results in larger exciton population of the HGN-surface and in turn larger Rabi splitting for the HGN-TDBC system than the HGN-PIC system even though TDBC has a lower oscillator strength than PIC. The trend in Rabi splitting is just reversed when the HGN surface is modified with a negatively charged polymer, confirming the role of electrostatic interactions in influencing the plasmon-exciton coupling strength.

Nanoscale plasmon-exciton interaction: the role of radiation damping and mode-volume in determining coupling strength.

Unravelling the exact role of each individual plasmon decay channel in plasmon-exciton coupling is pivotal for successful realization of the exciting potential applications of plexcitonic nanostructures. Here, we successfully demonstrate how exactly one specific plasmon dephasing channel, radiation damping, influences plasmon-exciton coupling in Au nanorod-J-aggregate hybrids. We systematically and selectively varied the contribution of radiation damping, keeping the contributions of other damping channels negligible or invariant, by controllably varying nanorod diameter (above 20 nm) while maintaining the aspect ratio constant and studied the optical response of the corresponding plexcitons using single-particle spectroscopy. Our results show that decreasing radiation damping inversely drives the plasmon-exciton interaction toward a strong coupling regime. However, we find that plasmon mode-volume is a more fundamental parameter in dictating coupling strength than radiation damping. Overall, this comprehensive study provides a significant step toward developing a predictive understanding of how exactly excitation decay channels influence plasmon-exciton coupling.

Nonlinear Plasmonic Sensing for Label-Free and Selective Detection of Mercury at Picomolar Level.

We present the concept of a nonlinear plasmonic sensing approach for rapid, sensitive, and label-free detection of mercury. Nonlinear plasmonic sensing of mercury relies on a systematic combination of nonlinear optics (NLO) with well-known concepts of amalgamation chemistry and plasmonic properties of gold nanorods. Exploiting the extreme sensitivity of the NLO process toward Hg-induced change in the local electric field of plasmonic nanorods, we succeed in improving the limit of detection (LOD) of mercury by 2-3 orders of magnitude as compared to the commonly used linear localized surface plasmon resonance (LSPR) based sensing. Using our method, an LOD of as low as 58 pM (11 ppt) has been achieved with high selectivity. Nonlinear plasmonic sensing aproach is found to work excellently for detecting mercury in real samples like blood plasma.

Modulation of LSPR spectra and enhanced RI-sensitivity through symmetry breaking in hollow gold nanoprism.

Optical responses of plasmonic nanostructures can be tailor-made by judiciously controlling their structural parameters. Here in this article, we describe how symmetry-breaking influences the optical properties of an anisotropic hollow nanostructure, a hollow gold nanoprism (HGN). We find that the introduction of structural asymmetry by shifting the cavity position alters the plasmon hybridization conditions, which, in turn, lifts the degeneracy of bonding plasmon modes and thereby causes mode splitting. The splitting between the nondegenerate bonding modes is directly correlated with the extent of the cavity offset. Interestingly, it is found that a reduced symmetry HGN having a cavity of any arbitrary size does not necessarily show such spectral modulation as a function of the cavity offset. Rather, there is a threshold value of (cavity diameter/edge length) ratio for observing this kind of optical behavior. Symmetry breaking not only leads to spectral modulation but also improves the refractive index (RI) sensitivity as well as the associated figure of merit of the HGN nanosensors tremendously. This comprehensive study develops a predictive understanding of the structure-specificity of the optical properties of HGNs and also suggest that sensible tailoring of the structural parameters can make HGNs as one of the most suitable candidates for RI sensing based applications.

Unveiling the Modulating Role of Extracellular pH in Permeation and Accumulation of Small Molecules in Subcellular Compartments of Gram-negative Escherichia coli using Nonlinear Spectroscopy.

Quantitative evaluation of small molecule permeation and accumulation in Gram-negative bacteria is important for drug development against these bacteria. While these measurements are commonly performed at physiological pH, Escherichia coli and many other Enterobacteriaceae infect human gastrointestinal and urinary tracts, where they encounter different pH conditions. To understand how external pH affects permeation and accumulation of small molecules in E. coli cells, we apply second harmonic generation (SHG) spectroscopy using SHG-active antimicrobial compound malachite green as the probe molecule. Using SHG, we quantify periplasmic and cytoplasmic accumulations separately in live E. coli cells, which was never done before. Compartment-wise measurements reveal accumulation of the probe molecule in cytoplasm at physiological and alkaline pH, while entrapment in periplasm at weakly acidic pH and retention in external solution at highly acidic pH. Behind such disparity in localizations, up to 2 orders of magnitude reduction in permeability across the inner membrane at weakly acidic pH and outer membrane at highly acidic pH are found to play key roles. Our results unequivocally demonstrate the control of external pH over entry and compartment-wise distribution of small molecules in E. coli cells, which is a vital information and should be taken into account in antibiotic screening against E. coli and other Enterobacteriaceae members. In addition, our results demonstrate the ability of malachite green as an excellent SHG-indicator of changes of individual cell membrane and periplasm properties of live E. coli cells in response to external pH change from acidic to alkaline. This finding, too, has great importance, as there is barely any other molecular probe that can provide similar information.

Structure-specific chiroptical responses of hollow gold nanoprisms.

Chiroptical responses of plasmonic chiral nanostructures can be controllably tuned by judicious tailoring of their structural parameters. In this article, the chiroptical properties of a newly designed plasmon-supporting nanostructure, chiral hollow gold nanoprisms (HGNs), has been numerically investigated in detail. The most compelling observation is that the CD response and the dissymmetry factor (g, which is a measure of the strength of chiroptical responses) of the chiral HGNs are large and at the same time, highly structure-specific. Also, we observed finite CD activity not only in absorption and scattering but also in the extinction spectra, which is a signature of a typical 3D chiral structure. We show that the chiroptical responses of HGNs can be exponentially enhanced simply by controlling the cavity-position or cavity size. Our results reveal that the structure-specific chiroptical response is a result of structure-dependent interplay between the non-radiative (Ohmic) and radiative losses. We also show that the CD intensity of a suitably designed chiral HGN is higher than other nanoscale metasurfaces of comparable volume. The insights obtained from this comprehensive study assert that this unique chiral nanostructure has great potential for being used in numerous applications.

Exploring the coherent interaction in a hybrid system of hollow gold nanoprisms and cyanine dye J-aggregates: role of plasmon-hybridization mediated local electric-field enhancement.

In this work, we probed the possibility of observing strong plasmon-exciton interactions in hollow gold nanoprism-J-aggregate nanocomposites. Several different hollow gold nanoprisms (HGNs) with different aspect ratios were synthesized. This allowed us to systematically tune the LSPR energies through the exciton energy of the PIC-J-aggregate, which in turn allowed us to have direct determination of the coupling strength of HGN-J-aggregate composites. Hybrid nanosystems were prepared by adsorbing and assembling 1,1'-diethyl-2,2'-cyanine (pseudoisocyanine or PIC) iodide onto the surface of hollow gold nanoprisms. Plasmon-exciton interactions were studied using extinction spectroscopy. The experimental results were analysed, and complemented by the results obtained from numerical simulations. Our results reveal that the HGN-PIC-J-aggregate hybrid nanosystem shows coherent coupling between the localized surface plasmons of the HGN and excitons of the PIC-J-aggregate, as obvious from the observation of a clear transparency dip and the formation of two new hybrid plexcitonic modes in the plexcitonic spectra. Anti-crossing behaviour of the plexcitonic modes, together with large Rabi splitting and coupling constant, asserts strong coupling between the plasmon and the exciton, overwhelming the decoherence effects, in our hybrid nanosystem. Analysis of the calculated near-field distribution establishes that the plasmon-hybridization mediated large electric-field enhancement holds the key to the strong coupling.

Large second harmonic generation from hollow gold nanoprisms: role of plasmon hybridization and structural effects.

The structure and morphology of nanomateials strongly influence their nonlinear optical properties. In this work, we report a systematic investigation of second order nonlinear optical responses and their structural dependencies in the case of a plasmonically hybrid nanostructure, hollow gold nanoprisms (HGNs). The first hyperpolarizabilities (ß) of the HGNs have been measured using the two-photon Rayleigh scattering (TPRS) technique. The measured hyperpolarizability values are extremely large for the HGNs, larger than those for gold nanospheres or gold nanorods with similar size and surface area. The larger ß values of the HGNs are due to a strong local electromagnetic field enhancement owing to efficient plasmon hybridization. We find that the ß values for the HGNs studied here have a purely local dipolar origin, as confirmed by their surface area dependence. Moreover, the SH responses of the HGNs are found to be a linear function of their aspect ratios. Our results suggest that the nonlinear optical (NLO) properties of HGNs can be tailor made and utilized to suit various practical applications.

Optimization of nonlinear optical localization using electromagnetic surface fields (NOLES) imaging.

The use of plasmon amplification of nonlinear optical wave-mixing signals to generate optical images in which the position of the scattering point source can be determined with nanometer accuracy is described. Solid gold nanosphere dimers were used as a model system for the nonlinear medium, which converted the Ti:sapphire fundamental to its second harmonic frequency. Matching the fundamental wave energy to the localized surface plasmon resonance of the electromagnetically coupled nanospheres was critical for achieving the high localization accuracy. Our technique, named Nonlinear Optical Localization using Electromagnetic Surface fields (NOLES) imaging, routinely yielded nonlinear optical images with 1-nm localization accuracy at rates ≥2 fps and can also be used as a photo-switching localization contrast method. This high level of accuracy in pinpointing the signal point source position exceeded that made possible using conventional diffraction-limited far-field methods by 160×. The NOLES technique, with its high temporal resolution and spatial accuracy that far surpass the performance typical of fluorescence-based imaging, will be relevant for imaging dynamic chemical, biological, and material environments.

Nanoparticle surface electromagnetic fields studied by single-particle nonlinear optical spectroscopy.

Polarization-resolved second harmonic generation (SHG) measurements were performed on solid gold nanosphere (SGN) dimers at the single-particle level. The results indicated that single-particle SHG measurements could be used to quantify the localized electromagnetic surface fields that result from excitation of inter-particle localized surface plasmon modes. For several dimers, the polarization-resolved measurements revealed that the surface fields localized between SGNs in a dimer were chiral. Quantitative analysis of SHG line shapes obtained from single-particle continuous polarization variation (CPV-SHG) experiments confirmed that the chirality originated from non-zero magnetic-dipolar contributions to the SGN dimer nonlinear optical response. Correlation of SEM images and single-particle SHG measurements obtained for several SGN dimers, as well as lithographically generated nanostructures, were used to demonstrate the structure sensitivity of the CPV-SHG method.

Probing the Structure-Property Interplay of Plasmonic Nanoparticle Transducers Using Femtosecond Laser Spectroscopy.

The characteristic feature of noble metal nanoparticles is the localized surface plasmon resonance (LSPR). Plasmon-supporting nanoparticles can function as transducers because of the LSPR's ability to amplify electromagnetic fields and its sensitivity to changes in the surrounding dielectric. The performance of these materials in transducer applications is inherently related to nanoparticle structure. This Perspective describes the use of femtosecond laser-based spectroscopies to elucidate the nanoscale structure-property interplay. First, femtosecond time-resolved transient extinction measurements that probe the LSPR following nanoparticle photoexcitation are described. These measurements illustrate how nanostructure dimensions influence sensitivity to changes in the interfacial dielectric. The combination of single-particle nonlinear optical (NLO) measurements and electron microscopy is also used to describe the symmetry of plasmon surface fields in nanoparticle assemblies. In particular, the use of continuous polarization variation-detected second-harmonic generation to describe electric and magnetic dipolar contributions to NLO properties is discussed.

Magnetic dipolar interactions in solid gold nanosphere dimers.

We report the first observation of a magnetic dipolar contribution to the nonlinear optical (NLO) response of colloidal metal nanostructures. Second-order NLO responses from several individual solid gold nanosphere (SGN) dimers, which we prepared by a bottom-up approach, were examined using polarization-resolved second harmonic generation (SHG) spectroscopy at the single-particle level. Unambiguous circular dichroism in the SH signal was observed for most of the dimeric colloids, indicating that the plasmon field located within the interparticle gap was chiral. Detailed analysis of the polarization line shapes of the SH intensities obtained by continuous polarization variation suggested that the effect resulted from strong magnetic-dipole contributions to the nanostructure's optical properties.