Toward SERS based localized thermometry of Polymer-Supported silver and gold nanostructures.

In order to accurately account for the contribution of different plasmon mediated phenomena when developing materials for applications in photothermal therapy, photovoltaics, or photocatalysis reliable, precise, and localized temperature measurements are required. In this work we applied two surface-enhanced Raman scattering (SERS) spectroscopy based methods to measure the local temperature increase due to the thermoplasmonic effect in gold and silver nanoparticles on thin polystyrene films. The first method relies on the temperature dependence of the anti-Stokes to Stokes Raman bands intensity ratio for a label Rhodamine 6G deposited on the nanostructures. We found that the method enables good measurements in the 20-60 °C temperature range but becomes less reliable at higher temperatures when the system undergoes transformations and the plasmonic response of the nanoparticles changes with heating. The second method makes use of the temperature dependent adsorption geometry of phenyl isocyanide (PIC) and a corresponding shift of ν(C≡N) vibration. The method demonstrates greater temperature sensitivity of gold nanoparticles than silver. The difference in sensing capability is related to the difference in molecular adsorption geometry of PIC on Au compared to Ag. We conclude that for universal thermometry of the nanoparticle/ thin film composite a combination of the two methods provides more precise localized temperature measurements.

Discrimination of Bulk and Surface Refractive Index Change in Plasmonic Sensors with Narrow Bandwidth Resonance Combs.

A method to enable surface plasmon resonance (SPR) sensors to discriminate between bulk and surface-localized refractive index changes is demonstrated with modified gold-coated tilted fiber Bragg grating SPR sensors (TFBG-SPR). Without this capability, all high-resolution SPR sensors should be using reference channels and strict temperature control to prevent the contamination of the desired detection of surface-localized chemical or binding events by drift of the refractive index of the medium, in which the experiment is carried out. The very fine comb of high-quality-factor resonances of a TFBG-SPR device coupled to the large differential sensitivity of some of the resonances to various perturbations is used to measure unambiguously the refractive index changes within a surface layer thinner than 25 nm from those of the bulk surrounding. The enabling modification of the conventional TFBG-SPR is a reduction of the gold coating from its optimum value near 50-30 nm: at this lower thickness, a surface plasmon wave can still be excited by a limited number of cladding mode resonances, but at the same time, the metal is thin enough to allow modes away from the SPR to tunnel across the metal and probe the bulk RI value. Measurements and simulations of the deposition of a self-assembled monolayer of 1-dodecanethiol in ethanol show that the bulk refractive index changes as small as 0.0004 can be distinguished from the formation of a 1 nm thick coating on the surface of the fiber.

Silver Nanocubes Coated in Ceria: Core/Shell Size Effects on Light-Induced Charge Transfer.

Plasmonic sensitization of semiconductors is an attractive approach to increase light-induced photocatalytic performance; one method is to use plasmonic nanostructures in core@shell geometry. The occurrence and mechanism of synergetic effects in photocatalysis of such geometries are under intense debate and proposed to occur either through light-induced charge transfer (CT) or through thermal effects. This study focuses on the relation between the dimensions of Ag@CeO2 nanocubes, the wavelength-dependent efficiency, and the mechanism of light-induced direct CT. A 4-mercaptobenzoic acid (4-MBA) linker between core and shell acts as a Raman probe for CT. For all Ag@CeO2 nanocubes, CT increases with decreasing excitation wavelength, with notable increase at and below 514 nm. This is fully explainable by CT from silver to the 4-MBA LUMO, with the increase for excitation wavelengths that exceed the Ag/4-MBA LUMO gap of 2.28 eV (543 nm). A second general trend observed is an increase in CT yield with ceria shell thickness, which is assigned to relaxation of the excited electron further into the ceria conduction band, potentially producing defects.

Role of Lipid Composition in the Interaction and Activity of the Antimicrobial Compound Fengycin with Complex Membrane Models.

Fengycins are compounds produced by bacteria of the Bacillus genus with strong antifungal activity. In this work, lipids extracted from fungal and oomycetal molds were used to assess the ability of fengycin to bind and insert into complex membrane models prepared as Langmuir lipid monolayers. In addition, fengycin-induced leakage in liposomes prepared from these complex lipid extracts was also evaluated. Fengycin's ability to bind and incorporate into these membranes seemed to be mainly related to ergosterol content. Other membrane characteristics such as phospholipid fatty acyl chain length played a more peripheral role. A high ergosterol concentration appeared to allow other membrane characteristics generally associated with fengycin binding and/or insertion, such as higher proportion of phosphatidylcholine head groups or increased fatty acyl unsaturation, to be present without adversely affecting membrane integrity. Increased membrane leakage was also generally associated with the presence of low or no ergosterol. Leakage was also correlated with the previously reported biological activity of fengycin on these molds.

Self-heating tilted fiber Bragg grating device for melt curve analysis of solid-phase DNA hybridization and thermal cycling.

We demonstrate a DNA-based optical fiber device that uses an in-fiber grating, a light absorbing coating with surface anchored DNA, and a built-in optical thermometer. This device is used for precisely thermal cycling surface DNA spots bound by a simple UV cross-linking technique. Near-infrared light of wavelengths near 1550 nm and guided power near 300 mW is coupled out of the fiber core by a tilted fiber Bragg grating inscribed in the fiber and absorbed by the coating to increase its temperature to more than 95 °C. A co-propagating broadband light signal (also in the near-infrared region) is used to measure the reflection spectrum of the grating and thus the temperature from the wavelength shifts of the reflection peaks. The device is capable of sensitive DNA melt analysis and can be used for DNA amplification. Graphical abstract.

Selective embedment of silver nanocrystals into spatially segregated domains in thin polymer films for controlled fabrication of functional nanocomposites.

Fabrication of polymer-nanoparticle nanocomposites typically relies on mixing nanoparticle and polymer solutions, which renders little control over nanoparticle incorporation, and homogeneity of the resulting composite material. This work focuses on the thermally induced embedment of monocrystalline silver nanocubes (AgNCs) into polymer surfaces. The AgNCs are initially deposited through a Langmuir approach onto films of immiscible blended polymer films, which allows fine control over nanoparticle density and aggregation state. This nanoparticle/polymer composite is then heated above the glass transition temperature (T g) of a polymer, which initiates the irreversible embedding of the AgNCs. The immiscible ternary polymer films featured discrete domains (with different T gs), which were altered by changing the amount of polystyrene, poly(2-vinylpyridine) and poly(methyl methacrylate) within the polymer solution. The T g dependence of the embedding process allowed the selective embedment of AgNCs into discrete polymer domains. The process was monitored in real time by using spatially separated hybrid plasmon modes, through peak shifts observed in a UV-vis spectrum. Enhanced surface confinement was observed for certain tripolymer films when compared to polystyrene-AgNC nanocomposites, due to changes in the surface energy within the blend. This work brings interesting insight on nanoparticle-blended polymer interactions and provides a fairly universal approach for the fabrication of these polymer-metal nanoparticle nanocomposites, which is of particular interest in fields that require fine control over nanoparticle incorporation within segregated polymer domains.

Unusually Sharp Localized Surface Plasmon Resonance in Supported Silver Nanocrystals with a Thin Dielectric Coating.

An unusually sharp localized surface plasmon resonance (sLSPR) is observed for a monolayer of glass-supported silver nanocubes coated with a thin, 5-20 nm, Al2O3 film. The resonance becomes significantly narrower and stronger while losing optical anisotropy and sensitivity to the surroundings with increasing overlayer thickness. Surface-enhanced Raman scattering excitation profiles indicate an additional enhancement to the electric field brought in by the sLSPR. The resonance is thought to originate from a Fano-like constructive interference between the quadrupolar and dipolar LSPR modes in supported silver nanocubes leading to enhanced light extinction. This phenomenon is of significance for plasmon-induced charge-transfer processes in photovoltaics and catalysis.

Experimental Parameters Leading to Optimal Bilayers for Total Internal Reflection Fluorescence Microscopy Visualization.

Supported lipid bilayer systems were evaluated following various experimental procedures in an effort to determine their appropriateness for visualization using total internal reflection fluorescence (TIRF) microscopy. The incorporation and distribution of Texas Red® 1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (TR-DHPE) was studied when incorporated into bilayers of variable lipid composition using different forms of mechanical shearing. Results showed that 0.8 mol% TR-DHPE provides the most optimum TIRF images. At this concentration, a sufficient level of photostability can be achieved without an undesirable increase in TR-DHPE aggregates caused by excess probe molecules. Solutions composed of a 3:1 molar ratio of DOPC:DPPC with 0.8 mol% TR-DHPE produce bilayers that consistently display clear, distinct, rounded domains, whereas other lipid compositions did not. This optimum phase separation appears to be influenced by an increase in mechanical shearing during the vesicle formation process, when the lipid solutions were exposed to sonication and extrusion processes. The combination of a sonication and extrusion process also helped with eliminating the presence of TR-DHPE aggregates within the model membranes. It was also shown that bilayers formed on conditioned glass, placed on a slide, produced more highly detailed bilayers in which distinct lipid phase separation could be optimally visualized using TIRF microscopy.

Biofunctionalization of Plasmonic Nanoparticles with Short Peptides Monitored by SERS.

In order for plasmonic nanoparticles to be usable in biomedical applications their surface requires functionalization with biocompatible material. For this purpose short peptides, CFY, CFFY, CLY, were designed and replacement of the capping agent poly(vinylpyrrolidone) (PVP) on the surface of silver nanocubes by the peptides was investigated. The primary sequences of the peptides were designed such that they enable the covalent attachment to silver via the cysteine thiols, contain amino acids that can interact via hydrophobic interactions, and therefore are likely to form tightly packed films. Finally, the peptides contained UV-vis and SERS markers, allowing the dynamics of the biomolecule attachment to the nanoparticles to be monitored spectroscopically. The ligand exchange was observed for nanocubes suspended in solution and supported on a dielectric substrate. Formation of the peptide film around the nanocubes was confirmed by electron microscopy and SERS measurements. The film thickness was found to be 4-6 nm and independent of peptide solution concentration, suggesting multilayer formation. The surface density of these cysteine-containing peptides was found to be between 0.59 and 4.92 molecules per nm2.

Dynamics of nanocubes embedding into polymer films investigated via spatially resolved plasmon modes.

Integration of nanoparticles into thin films is essential for the development of functional materials, studies of fundamental interfacial processes, and exploitation of inherent properties from the particles themselves. In this work, we systematically investigate the process of incorporation of silver nanocubes into thin polystyrene films at temperatures just above the polymer glass transition. The process of nanocrystal incorporation can be precisely monitored via far-field spectroscopy to observe the response of spatially resolved hybrid plasmon modes. Each plasmon resonance has a distinct dynamic range and maximum sensitivity forming a complementary set of nanorulers that operates over a distance comparable to the edge length of the cubes. The approach explored in this work is a general robust method for the development of long-range polychromatic nanorulers.

Molecular imprinted polymer-coated optical fiber sensor for the identification of low molecular weight molecules.

A biomimetic optical probe for detecting low molecular weight molecules (maltol, 3-hydroxy-2-methyl-4H-pyran-4-one, molecular weight of 126.11 g/mol), was designed, fabricated, and characterized. The sensor couples a molecular imprinted polymer (MIP) and the Bragg grating refractometry technology into an optical fiber. The probe is fabricated first by inscribing tilted grating planes in the core of the fiber, and then by photopolymerization to immobilize a maltol imprinted MIP on the fiber cladding surface over the Bragg grating. The sensor response to the presence of maltol in different media is obtained by spectral interrogation of the fiber transmission signal. The results showed that the limit of detection of the sensor reached 1 ng/mL in pure water with a sensitivity of 6.3 × 10(8)pm/M. The selectivity of the sensor against other compounds and its reusability were also studied experimentally. Finally, the unambiguous detection of concentrations as little as 10nM of maltol in complex media (real food samples) by the MIP-coated tilted fiber Bragg grating sensor was demonstrated.

Improved detection limits of protein optical fiber biosensors coated with gold nanoparticles.

The study presented herein investigates a novel arrangement of fiber-optic biosensors based on a tilted fiber Bragg grating (TFBG) coated with noble metal nanoparticles, either gold nanocages (AuNC) or gold nanospheres (AuNS). The biosensors constructed for this study demonstrated increased specificity and lowered detection limits for the target protein than a reference sensor without gold nanoparticles. The sensing film was fabricated by a series of thin-film and monolayer depositions to attach the gold nanoparticles to the surface of the TFBG using only covalent bonds. Though the gold nanoparticle integration had not yet been optimized for the most efficient coverage with minimum number of nanoparticles, binding AuNS and AuNC to the TFBG biosensor decreased the minimum detected target concentrations from 90 nM for the reference sensor, to 11 pM and 8 pM respectively. This improvement of minimum detection is the result of a reduced non-specific absorption onto the gold nanoparticles (by functionalization of the external surface of the gold nanoparticles), and of an optical field enhancement due to coupling between the photonic modes of the optical fiber and the localized surface plasmon resonances (LSPR) of the gold nanoparticles. This coupling also increased the sensitivity of the TFBG biosensor to changes in its local environment. The dissociation constant (Kd) of the target protein was also characterized with our sensing platform and found to be in good agreement with that of previous studies.

Fine tuning of plasmonic properties of monolayers of weakly interacting silver nanocubes on thin silicon films.

Plasmonic properties, such as refractive index sensitivity (RIS), surface enhancement of the Raman signal (SERS), fluorescence quenching, and photocatalytic activity, of monolayers of weakly interacting monodisperse silver nanocubes were qualitatively modified in a very well controlled manner by supporting them on thin silicon films with varying thickness. Such fine tunability is made possible by the strong dependence of the nanocube dipolar (D) and quadrupolar (Q) plasmon mode hybridization on the refractive index of the supporting substrate. By increasing the Si film thickness from zero to ~25 nm we were able to "shift" the D resonance mode by up to 200 nm for ~80 nm cubes without significantly affecting the Q mode. The silicon supported nanocubes showed a significant improvement in RIS via the Q mode with a figure of merit greater than 6.5 and about an order of magnitude enhancement of the SERS signal due to the stronger electric field created by the D mode. Such substrates also showed a ~10 times decrease in rhodamine 6G fluorescence as well as the rates of amorphous carbon formation. The study proposes a new way to design and engineer plasmonic nanostructures.

Imaging the membrane lytic activity of bioactive peptide latarcin 2a.

Latarcin 2a (ltc2a, GLFGKLIKKFGRKAISYAVKKARGKH-COOH) is a short linear antimicrobial and cytolytic peptide extracted from the venom of the Central Asian spider, Lachesana tarabaevi, with lytic activity against Gram-positive and Gram-negative bacteria, erythrocytes, and yeast at micromolar concentrations. Ltc2a adopts a helix-hinge-helix structure in membrane mimicking environment, whereas its derivative latarcin 2aG11A (ltc2aG11A, GLFGKLIKKFARKAISYAVKKARGKH-COOH), likely adopts a more rigid structure, demonstrates stronger nonspecific interaction with the zwitterionic membrane, and is potentially more toxic against eukaryotic cells. In this work, interactions of these two ltc2a derivatives with supported "raft" lipid bilayer (1,2-dioleoyl-sn-glycero-3-phosphocholin/egg sphingomyelin/cholesterol 40/40/20mol%) were studied by in situ atomic force microscopy in order to investigate the potential anticancer activity of the peptides since some breast and prostate cancer cell lines contain higher levels of cholesterol-rich lipid rafts than non-cancer cells. Both peptides induced reorganization of the raft model membrane by reducing line tension of the liquid ordered phase. Ltc2aG11A induced membrane thinning likely due to membrane interdigitation. Formation of large pores by the peptides in the bilayer was observed. Cholesterol was found to attenuate membrane disruption by the peptides. Finally, leakage assay showed that both peptides have similar membrane permeability toward various model membrane vesicles.

Investigating the effect of a single glycine to alanine substitution on interactions of antimicrobial peptide latarcin 2a with a lipid membrane.

Latarcins are linear, α-helical antimicrobial peptides purified from the venom of the Central Asian spider Lachesana tarabaevi, with lytic activity against Gram-positive and Gram-negative bacteria, erythrocytes, and yeast at micromolar concentrations. In this work, we investigated the role of the hinge in latarcin 2a (ltc2a, GLFGKLIKKFGRKAISYAVKKARGKH-COOH), which adopts a helix-hinge-helix conformation in membrane-mimicking environments, on peptide-membrane interactions and its potential effect on the selective toxicity of the peptide. A modified latarcin 2a, ltc2aG11A, obtained by replacing the glycine at position 11 with alanine (ltc2aG11A, GLFGKLIKKFARKAISYAVKKARGKH-COOH), adopts a more rigid structure due to the reduced conformational flexibility. Langmuir monolayer measurements combined with atomic force microscopy and X-ray photoemission electron microscopy (X-PEEM) indicate that both peptides bind and insert preferentially into anionic compared with zwitterionic phospholipid monolayers. Modified ltc2aG11A was found to be more disruptive of supported phospholipid bilayer modeling mammalian cell membrane. However, no considerable difference in lytic activity of the two peptides toward bacterial membrane was found. Overall the data indicate that decrease in the flexibility of ltc2a induced by the modification in the hinge region is likely to increase the peptide's nonspecific interactions with zwitterionic cell membranes and potentially increase its toxicity against eukaryotic cells.

Imaging interactions of cationic antimicrobial peptides with model lipid monolayers using X-ray spectromicroscopy.

The interaction of antimicrobial peptide anoplin with 1,2-dipalmitoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] lipid monolayers was imaged with atomic force microscopy, scanning transmission X-ray microscopy, and X-ray photoemission electron microscopy. X-ray absorption spectromicroscopy of the surface revealed the domains of the phase-segregated surface to be composed of 98(±5)% lipid while the matrix consisted of a ~50:50 lipid-peptide mixture. We show X-ray spectromicroscopy to be a valuable quantitative tool for label-free imaging of lipid monolayers with antimicrobial peptides at a lateral spatial resolution below 80 nm.

Effect of point mutations on the secondary structure and membrane interaction of antimicrobial peptide anoplin.

Anoplin (GLLKRIKTLL-NH(2)) is the smallest linear α-helical antimicrobial peptide found naturally to date. Antibacterial and hemolytic properties of anoplin depend strongly on physicochemical properties. Two anoplin derivatives, anoplin-8K (Ano8K, GLLKTIKKLL-NH(2)) and anoplin-1K5 V8K (Ano1K5 V8K, KLLKVIKLL-NH(2)), were found to have increased bacterial and low hemolytic activity. In the present work physicochemical properties of these three peptides were studied by UV resonance Raman (UVRR) spectroscopy, Langmuir-Blodgett monolayer technique, and carboxyfluorescein (CF) leakage assay. UVRR data indicated that all three peptides adopt predominantly unordered conformation in aqueous buffer solution. In membrane-mimicking trifluoroethanol, the α-helical content increases for all three peptides with Ano1K5 V8K having the highest α-helix percentage, followed by Ano8K and anoplin. Critical micelle concentrations were found to be similar for all three peptides, and the saturation pressure decreases in the sequence Ano1K5 V8K, anoplin, Ano8K. Critical pressure of insertion was found to be greater for anionic lipid monolayer DPPG than for zwitterionic lipid DPPC indicating preferential adsorption of all three peptides to DPPG. Finally, membrane lytic activities of all three peptides toward various model lipid vesicles were compared through CF leakage assay. Overall the data indicate that antimicrobial activity of anoplin increases with charge, whereas membrane lytic activity correlates with peptides helicity and amphipathicity.

Investigating the effects of L- to D-amino acid substitution and deamidation on the activity and membrane interactions of antimicrobial peptide anoplin.

Isolated from the venom sac of solitary spider wasp, Anoplius samariensis, anoplin is the smallest linear α-helical antimicrobial peptide found naturally with broad spectrum activity against both Gram-positive and Gram-negative bacteria, and little hemolytic activity toward human erythrocytes. Deamidation was found to decrease the peptide's antibacterial properties. In the present work, interactions of amidated (Ano-NH2) and deamidated (Ano-OH) forms of anoplin as well as Ano-NH2 composed of all D-amino acids (D-Ano-NH2) with model cell membranes were investigated by means of Langmuir Blodgett (LB) technique, atomic force microscopy (AFM), X-ray photoemission electron microscopy (X-PEEM) and carboxyfluorescein leakage assay in order to gain a better understanding of the effect of these peptide modifications on membrane binding and lytic properties. According to LB, all three peptides form stable monolayers at the air/water interface with Ano-NH2 occupying a slightly greater area per molecule than Ano-OH. All three forms of the peptide interact preferentially with anionic 1,2-dipalmitoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (DPPG), rather than zwitterionic 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipid monolayer. Peptides form nanoscale clusters in zwitterionic but not in anionic monolayers. Finally, membrane lytic activity of all derivatives was found to depend strongly on membrane composition and lipid/peptide ratio. The results suggest that amidated forms of peptides are likely to possess higher membrane binding affinity due to the increased charge.

In situ monitoring of the formation of nanoscale polyelectrolyte coatings on optical fibers using Surface Plasmon Resonances.

Deposition of a conformal nanoscale polymer coating was characterized using a fiber SPR sensor. The sensor platform consisted of an unmodified gold-coated single mode fiber where SPR was excited through the coupling of the core mode into the cladding modes using a Tilted Fiber Bragg Grating. The results from this study show how the sensor can monitor in real time the formation of polyelectrolyte coatings during a process consisting of several stages of immersion. The experimental data was further calibrated by simulations and Atomic Force Microscope imaging allowing us to determine the thickness and refractive index of the adsorbed polyelectrolyte.