The influence of hemolysis upon admission on the performance of neuron-specific enolase in predicting death after resuscitated out-of-hospital cardiac arrest – a substudy of the IMICA trial

Background

The predominant cause of mortality among out-of-hospital cardiac arrest (OHCA) patients admitted to the intensive care unit in a comatose state is hypoxic-ischemic brain injury and consequent withdrawal of life sustaining therapies. Current guideline for post-resuscitation care suggests a multimodal approach for neuroprognostication including measuring a blood-born biomarker of neuronal injury termed neuron-specific enolase (NSE). However, NSE is also present in erythrocytes and therefore hemolysis – which frequently occurs immediately after resuscitation – must be assessed by measuring free-hemoglobin (free-hgb), and samples discarded if the threshold index is surpassed to minimize false positive results. In addition, the plasma half-life of NSE is much longer than free-hgb, and hemolysis occurring prior to the sampling point of NSE measurement, may cause elevated NSE levels due to destruction of erythrocytes rather than neurons, although free-hgb is no longer detectable in plasma.

Purpose

To investigate the relationship between hemolysis at hospital admission and NSE measured at 48 hours, and to determine the influence of hemolysis on the clinical performance of NSE in predicting all-cause mortality in comatose OHCA patients.

Methods

Free-hgb was measured at admission (0h) to quantify hemolysis. NSE was measured at 48 hours after admission. In each NSE sample hemolysis index was measured and mathematically corrected according to standard laboratory procedure. The relationship between free-hgb at 0h and NSE at 48h was investigated by Pearson's correlation analysis. The area under the receiver operating characteristic curves (AUROC) for prediction of all-cause mortality at 30 days were determined for both NSE and free-hgb, and the difference in AUROC between the two was calculated. Further, the positive and negative predictive values (PPV and NPV respectively) of NSE with a cut-off value at >60 μg/l (as recommended by guidelines) in predicting all-cause mortality were calculated.

Results

Admission free-hgb and NSE at 48 hours were available for 64 consecutive patients surviving beyond 48 hours. The 30-day mortality in this population was 33%. There was no correlation between free-hgb at 0h and NSE at 48h (r=0.13, p=0.30; Figure 1). The AUROC for NSE was 0.92 (Figure 2), and 0.59 for free-hgb at 0h. The PPV for NSE>60 μg/l was 1.0 (1.0–1.0) and NPV 0.84 (0.74–0.94).

Conclusions

The performance of NSE at 48 hours for predicting all-cause mortality among out-of-hospital cardiac arrest patients was not influenced by the degree of hemolysis at the time of admission, and NSE at 48 hours had both high positive and negative predictive values irrespectively of early hemolysis.

Funding Acknowledgement

Type of funding sources: None.

Early quantitative pupillary response parameters have high predictive value for 30-day mortality in patients resuscitated from out-of-hospital cardiac arrest

Background

Despite adequate post-resuscitation treatment following out-of-hospital cardiac arrest (OHCA), early risk of death remains high due to irreversible anoxic brain injury.

Objective

Neuroprognostication is pivotal, and automated quantitative pupillometry has become an essential part of early multimodal approach. Several quantitative pupillary response parameters are now available, but little evidence exist on their value and timing in neuroprognostication.

Purpose

To evaluate prognostic value of standard parameters of quantitative pupillometry in OHCA patients.

Methods

In this observational study we included 138 comatose patients resuscitated from OHCA admitted to a tertiary cardiac intensive care unit. With quantitative pupillometry, change in pupil size (%CH, %), Neurological Pupil index (NPi, scalar value 0–5), constriction velocity (CV, mm/s), maximum CV (MCV, mm/s), dilation velocity (DV, mm/s) and latency of constriction (Lat, s) were measured at admission, at the 24 hours (24h), and 48-hours (48h) time points. Receiver operating characteristics (ROC) curves with calculation of area under the curve (AUC) were applied for assessment of association with 30-day mortality.

Results

The population were predominantly males, had a mean age of 61±12 years, and 120 (87%) had shockable primary rhythm, 122 (88%) had witnessed cardiac arrest and 51 (37%) died in the first 30 days.

Median %CH, CV, MCV, DV and NPi values were significantly lower for diseased patients compared to survivors.

ROC analyses (Figure 1) present all parameters, except NPi, at admission with numerically highest values of AUC at 0.82, 0.76, 0.73, and 0.81 for %CH, CV, MCV, and DV respectively, and for NPi at 48h with AUC at 0.81. The admission time point still presented the highest AUC values, >0.84, for CV, MCV, and DV in an adjusted model. Of all the parameters, %CH and NPi maintained the highest prognostic values through all time points, and %CH performed best at admission with AUC at 0.87 and NPi at 24h with AUC at 0.91.

At admission the threshold value with 100% specificity for predicting 30-day mortality were 3.75, 0.09, 0.33, and 0.05, with negative predictive value (NPV [95% confidence interval]) at 71% [67–74], 66% [64–69], 67% [65–70], and 69% [66–72] for %CH, CV, MCV, and DV respectively. NPi had best performance at 24h with a cutoff value of 2.90 and NPV at 64% [63–64].

Conclusion

%CH and NPi were the strongest prognostic tools for 30-day mortality. Values below 3.75% for %CH at admission and 2.90 for NPi at 24h after archived 100% specificity for 30-day mortality, with NPV at 71% [67–74] and 64% [63–64] respectively. Although performance was persistently high, %CH, CV, MCV, and DV presented numerically highest AUC at admission.

Additional parameters of CV, MCV, and DV were also significantly associated with outcome and future research should focus on validation and possible combining these measures into useful indices.

Funding Acknowledgement

Type of funding sources: Private company. Main funding source(s): Novo Nordisk Foundation

Tocilizumab mitigates hypercoagulability after out-of-hospital cardiac arrest – results from a randomized controlled trial (IMICA)

Background

Systemic inflammation constitutes a key element of the post cardiac arrest syndrome (PCAS) which affects initially comatose resuscitated cardiac arrest patients. We have recently shown that treatment with tocilizumab, an IL-6 receptor antagonist, reduces systemic inflammation and myocardial injury after out-of-hospital cardiac arrest (OHCA). Inflammation and coagulation are interconnected, and changes in coagulation often accompany inflammatory states.

Purpose

To investigate if conventional coagulation parameters or a viscoelastic hemostatic assay differ in patients treated with tocilizumab compared to placebo.

Methods

Eighty comatose OHCA patients were randomized 1:1 in a double-blinded placebo-controlled trial to a single infusion of tocilizumab or placebo in addition to standard of care including targeted temperature management. Trial registration: Clinicaltrials.gov NCT03863015. Endpoints were plasma fibrinogen, platelet count, and Thrombelastography (TEG) variables. TEG analysis was performed utilizing whole blood in a citrated kaolin assay with heparinase (to neutralize any unfractionated or low molecular weight heparin that had been administered), and the following variables were analyzed: reaction time (R), angle, maximum amplitude (MA), and lysis at 30 minutes (Ly30) which represents clot initiation, propagation, strength and dissolution respectively. Data reported as median (Q1; Q3), statistical analysis performed by constrained linear mixed models, and a p<0.05 was considered statistically significant.

Results

Admission median levels of the investigated coagulation parameters were within normal levels for both the tocilizumab and placebo group. At 48 hours, for the placebo group, fibrinogen levels (figure 1) had risen to supra-normal levels, TEG angle had increased from 64 (10; 67) till 72 (69; 74), and TEG MA (figure 2) had increased to the hypercoagulable range (all p<0.05), while for the tocilizumab group fibrinogen levels and TEG MA remained within normal values. Both groups had a significant fall in platelet count from admission till 48h [placebo: from 252 109/L (208; 282) till 151 (126; 189); tocilizumab from 217 (183; 260) till 152 (127; 183)], with no group differences. Also for both groups there was significant shortening of the TEG R time [placebo: from 8 min (6; 10) till 6 (6; 8); tocilizumab from 8 (6; 9) till 7 (6; 8)], again with no difference between groups. Ly30 did not change over time or differ between groups.

Conclusion

Treatment with tocilizumab following resuscitated OHCA aids in maintaining a normo-coagulable state, whereas placebo patients developed supra-normal fibrinogen levels and a hypercoagulable TEG clot strength.

Funding Acknowledgement

Type of funding sources: Private grant(s) and/or Sponsorship. Main funding source(s): The Danish Heart Foundation (Reference no. 19-R135-A9302-22125); Region Hovedstadens Forskningsfond til sundhedsforskning (CapitalRegion Research Foundation, Denmark; reference no. A6030) Figure 1. FibrinogenFigure 2. Thrombelastography MA

Extinction-to-Absorption Ratio for Sensitive Determination of the Size and Dielectric Function of Gold Nanoparticles

Gold nanoparticles (AuNPs) have become an essential tool for a variety of fields across the biological, physical, and chemical sciences. The characterization of AuNPs by UV-vis spectroscopy is simple and commonly used but remains prone to error because of size and shape polydispersity and uncertainties in the dielectric function. We here propose and demonstrate a method to significantly improve this routine characterization technique by measuring not only the extinction but also the absorption spectrum. Specifically, we show that by considering the ratio of the extinction to absorption spectra, denoted η, we are able to determine the volume of AuNPs with a significant increase in accuracy compared to the UV-vis extinction method. We also prove an important property of η: it is independent of particle shape within the quasi-static/dipolar approximation, typically for particle sizes up to 100 nm. This shape independence results in very strong constraints for the theoretical predictions to agree with the experiments. We show that the spectral shape of η can therefore be used to discriminate between different proposed data sets for the dielectric function of gold, a long-standing challenge in plasmonics research.

Whispering-Gallery Mode Lasing in Perovskite Nanocrystals Chemically Bound to Silicon Dioxide Microspheres

Cesium lead halide perovskite nanocrystals exhibit high photoluminescence quantum efficiencies and tunability across the visible spectrum. This makes these crystals ideal candidates for solar panels, light-emitting diodes, lasers, and especially nanolasers. Due to the versatility of cation substitution in perovskite nanocrystals, they can be grown on amine-functionalized silicon dioxide nanoparticles, where the amine linker replaces the standard cation structure. Selectively growing luminescent nanocrystals on spherical silicon dioxide microspheres results in the opportunity to populate whispering-gallery modes in these spherical silica microspheres. In this case, the nanocrystal halide composition can be used to selectively tune the emission wavelength mode, and microsphere radius to tune the mode spacing. This silicon dioxide attachment also adds to the overall stability of the system. Through photoluminescence microscopy measurements, we show whispering gallery modes in individual perovskite-coated microspheres for CsPbBr₃ and CsPbI₃ nanocrystals on 9.2 μm diameter silica spheres and compare these to theoretically predicted optical modes. In CsPbBr₃, we provide evidence that these modes will lase under optical excitation, with a threshold of 750 μJ/cm². This study presents a novel system that, through optimization, could be a promising pathway to achieve facile and stable perovskite nanolasers.

Whispering-Gallery Mode Lasing in Perovskite Nanocrystals Chemically Bound to Silicon Dioxide Microspheres

Cesium lead halide perovskite nanocrystals exhibit high photoluminescence quantum efficiencies and tunability across the visible spectrum. This makes these crystals ideal candidates for solar panels, light-emitting diodes, lasers, and especially nanolasers. Due to the versatility of cation substitution in perovskite nanocrystals, they can be grown on amine-functionalized silicon dioxide nanoparticles, where the amine linker replaces the standard cation structure. Selectively growing luminescent nanocrystals on spherical silicon dioxide microspheres results in the opportunity to populate whispering-gallery modes in these spherical silica microspheres. In this case, the nanocrystal halide composition can be used to selectively tune the emission wavelength mode, and microsphere radius to tune the mode spacing. This silicon dioxide attachment also adds to the overall stability of the system. Through photoluminescence microscopy measurements, we show whispering gallery modes in individual perovskite-coated microspheres for CsPbBr₃ and CsPbI₃ nanocrystals on 9.2 μm diameter silica spheres and compare these to theoretically predicted optical modes. In CsPbBr₃, we provide evidence that these modes will lase under optical excitation, with a threshold of 750 μJ/cm². This study presents a novel system that, through optimization, could be a promising pathway to achieve facile and stable perovskite nanolasers.

Mind the gap: testing the Rayleigh hypothesis in T-matrix calculations with adjacent spheroids

The T-matrix framework offers accurate and efficient modelling of electromagnetic scattering by nonspherical particles in a wide variety of applications ranging from nano-optics to atmospheric science. Its analytical setting, in contrast to purely numerical methods, also provides a fertile ground for further theoretical developments. Perhaps the main purported limitation of the method, when extended to systems of multiple particles, is the often-stated requirement that the smallest circumscribed spheres of neighbouring scatterers not overlap. We consider here such a scenario with two adjacent spheroids whose aspect ratio we vary to control the overlap of the smallest circumscribed spheres, and compute far-field cross-sections and near-field intensities using the superposition T-matrix method. The results correctly converge far beyond the no-overlap condition, and although numerical instabilities appear for the most extreme cases of overlap, requiring high multipole orders, convergence can still be obtained by switching to quadruple precision. Local fields converge wherever the Rayleigh hypothesis is valid for each single scatterer and, remarkably, even in parts of the overlap region. Our results are validated against finite-element calculations, and the agreement demonstrates that the superposition T-matrix method is more robust and broadly applicable than generally assumed.

P6383Cardiac output during targeted temperature management and renal function after out-of-hospital cardiac arrest

Background

After resuscitation from out-of-hospital cardiac arrest (OHCA), renal injury and hemodynamic instability are common. Low blood pressure during targeted temperature management (TTM) is associated with acute renal injury (AKI). The aim of this study is to test the hypothesis, that low cardiac output during TTM is associated with acute kidney injury after OHCA.

Methods

Single-center substudy of 171 patients included in the prospective, randomized TTM-trial. Hemodynamic evaluation was performed with serial measurements by pulmonary artery catheter. Mean arterial pressure ≥65 mmHg and central venous pressure of 10 to 15 mmHg were hemodynamic treatment goals. Acute kidney injury (AKI) was the primary endpoint and was defined according to the KDIGO-criteria. Differences between groups were tested by repeated measurements mixed models.

Measurements and main results

Of 152 patients with available hemodynamic data, 49 (32%) had AKI and 21 (14%) had AKI with need for renal replacement therapy (RRT) in the first three days. At admission, cardiac index was higher in the AKI-group (mean (confidence interval): 2.6 (2.2–3.0) L/min/m2 versus 2.2 (2.0–2.3) L/min/m2, p=0.003). During 24 hours of targeted temperature management, patients with AKI had increased heart rate (11 beats/min, pgroup<0.0001) and increased lactate (1 mmol/L, pgroup<0.0001) compared to patients without AKI. However, there was no overall difference in cardiac index (pgroup = 0.25) (Figure). In multivariate models, adjusting for potential confounders including targeted temperature, mean arterial pressure (odds ratio: 0.69 (0.50–0.96) per 5 mmHg increase, p=0.03), heart rate (1.04 (1.01–1.08) per beat/min increase, p=0.01) and lactate (1.59 (1.14–2.2) per mmol/L increase, p=0.006) were independently associated with AKI, but cardiac index remained unrelated with AKI.

Figure 1

Conclusions

Blood pressure, heart rate and lactate, but not cardiac output, during 24 hours of targeted temperature management were associated with renal injury in comatose OHCA-patients.

Acknowledgement/Funding

The research fund Gangstedfonden and the Research fund of Rigshospitalet has supported this study with unrestricted salary in Dr. Grand's PhD project.

5230Association of cardiac output during targeted temperature management with mortality after out-of-hospital cardiac arrest

Purpose

Myocardial dysfunction and low cardiac output are common after out-of-hospital cardiac arrest (OHCA) as part of the post-cardiac arrest syndrome. This study investigates the association of cardiac output during targeted temperature management (TTM) with mortality. We hypothesized that low cardiac output during TTM is associated with mortality.

Methods

In the TTM-trial, which randomly allocated patients to TTM of 33°C or 36°C for 24 hours, we prospectively and consecutively monitored 171 patients with protocolized measurements from pulmonary artery catheters (PAC). Clinical and hemodynamic variables were registered at pre-specified time points in addition to 30-day survival status. Lactate, heart rate and cardiac index were measured at 3 time-points during TTM and averaged. We defined low cardiac output as a cardiac index during TTM <2.4 l/min/m2 in the TTM36-group and <1.8 l/min/m2 in the TTM33-group, since hypothermia affects cardiac output. We further stratified patients according to serum lactate (above/below 2 mmol/L) and heart rate (above/below median, which was 65 beats/min.).

Results

Of 152 patients with available hemodynamic measurements, 71 (47%) had low cardiac output during TTM (TTM33: 38 (49%), TTM36: 33 (44%)). Low cardiac output was not associated with mortality in univariate analysis (hazard ratio (HR): 1.47 [0.83–2.59], p=0.19) or multivariate analysis adjusted for potential confounders (HRadjusted: 0.74 [0.38–1.44], p=0.37). Low cardiac output combined with HR>65 was associated with increased mortality (HR: 2.69 [1.51–4.79], p=0.0007) in univariate, but not in multivariate analysis (p=0.22) (Figure). Low cardiac output and HR<65 was associated with decreased mortality in multivariate analysis (HRadjusted: 0.36 [0.14–0.93], p=0.03). Low cardiac output and lactate>2mmol/L was associated with increased mortality (HR: 2.73 [1.49–4.99], p=0.001) in univariate, but not in multivariate analysis (p=0.53), whereas patients with low cardiac output and lactate<2mmol/L had low mortality (HRadjusted: 0.58 [0.27–1.24], p=0.16) compared to the rest of the population (Figure).

Figure 1

Conclusion

This study found, that a frequent symptom during TTM is low cardiac output, which was not associated with mortality. However, patients with low cardiac output combined with either increased lactate or heart rate seems to be a population at risk. Whether low cardiac output should be corrected by inotropes or mechanical support to reduce mortality remains to be studied in prospective trials, but the efficacy of goal-directed therapy to increase cardiac output during TTM may be modest, especially if lactate and heart rate are normal.

Acknowledgement/Funding

The research fund Gangstedfonden and the Research fund of Rigshospitalet has supported this study with unrestricted salary in Dr. Grand's PhD project.

Plasmon-mediated chemical surface functionalization at the nanoscale

Controlling the surface grafting of species at the nanoscale remains a major challenge, likely to generate many opportunities in materials science. In this work, we propose an original strategy for chemical surface functionalization at the nanoscale, taking advantage of localized surface plasmon (LSP) excitation. The surface functionalization is demonstrated through aryl film grafting (derived from a diazonium salt), covalently bonded at the surface of gold lithographic nanostripes. The aryl film is specifically grafted in areas of maximum near field enhancement, as confirmed by numerical calculation based on the discrete dipole approximation method. The energy of the incident light and the LSP wavelength are shown to be crucial parameters to monitor the aryl film thickness of up to ∼30 nm. This robust and versatile strategy opens up exciting prospects for the nanoscale confinement of functional layers on surfaces, which should be particularly interesting for molecular sensing or nanooptics.

Highly stable silica-coated gold nanorods dimers for solution-based SERS

A bottom-up approach enabled us to have novel core–shell Au@SiO₂ dimers highly stable in water (at least 5 months) with substantially improved SERS performances as compared to isolated nanorods.

Tailoring Anisotropic Interactions between Soft Nanospheres Using Dense Arrays of Smectic Liquid Crystal Edge Dislocations

We investigated composite films of gold nanoparticles (NPs)/liquid crystal (LC) defects as a model system to understand the key parameters, which allow for an accurate control of NP anisotropic self-assemblies using soft templates. We combined spectrophotometry, Raman spectroscopy, and grazing incidence small-angle X-ray scattering with calculations of dipole coupling models and soft sphere interactions. We demonstrate that dense arrays of elementary edge dislocations can strongly localize small NPs along the defect cores, resulting in formation of parallel chains of NPs. Furthermore, we show that within the dislocation cores the inter-NP distances can be tuned. This phenomenon appears to be driven by the competition between "soft (nano)sphere" attraction and LC-induced repulsion. We evidence two extreme regimes controlled by the solvent evaporation: (i) when the solvent evaporates abruptly, the spacing between neighboring NPs in the chains is dominated by van der Waals interactions between interdigitated capping ligands, leading to chains of close-packed NPs; (ii) when the solvent evaporates slowly, strong interdigitation between the is avoided, leading to a dominating LC-induced repulsion between NPs associated with the replacement of disordered cores by NPs. The templating of NPs by topological defects, beyond the technological inquiries, may enable creation, investigation, and manipulation of unique collective features for a wide range of nanomaterials.

Photochromic–fluorescent–plasmonic nanomaterials: towards integrated three-component photoactive hybrid nanosystems

Silica-coated gold nanorods functionalized with grafted fluorescent and photochromic derivatives were synthesized and cross-coupled interactions between plasmonic, photochromic, and fluorescence properties have been demonstrated.

Revisiting Surface-Enhanced Raman Scattering on Realistic Lithographic Gold Nanostripes

In this article, we investigate the Surface-Enhanced Raman Scattering (SERS) efficiency of methylene blue (MB) molecules deposited on gold nanostripes which, due to their fabrication by electron beam lithography and thermal evaporation, present various degrees of crystallinity and nanoscale surface roughness (NSR). By comparing gold nanostructures with different degrees of roughness and crystallinity, we show that the NSR has a strong effect on the SERS intensity of MB probe molecules. In particular, the NSR features of the lithographic structures significantly enhance the Raman signal of MB molecules, even when the excitation wavelength lies far from the localized surface plasmon resonance (LSPR) of the stripes. These results are in very good agreement with numerical calculations of the SERS gain obtained using the discrete dipole approximation (DDA). The influence of NSR on the optical near-field response of lithographic structures thus appears crucial since they are widely used in the context of nano-optics or/and molecular sensing.

Silica-coated gold nanorod arrays for nanoplasmonics devices

A facile method for growing silica layer on lithographically designed gold nanorod arrays (GNRAs) using a convenient sol-gel method is presented herein. The silica layer thickness was controlled on GNRAs with the reaction time. The localized surface plasmon resonance (LSPR) spectra of these hybrid metal/dielectric nanoparticles were recorded before and after the coating and the effect of different solvents on the LSPR were also assessed. The change in the fluorescence and SERS intensities of a probe molecule (Rh6G) deposited on GNRAs and silica-coated GNRAs revealed that the as-fabricated silica layer does inhibit the quenching of molecular excited states and enhances photophysical/photochemical processes. This kind of hybrid metal/dielectric nanoparticle arrays hence turn out to be real good candidates to design new "plasmonic-active" devices.

Template-assisted deposition of CTAB-functionalized gold nanoparticles with nanoscale resolution

In this paper, we demonstrate the template-assisted deposition of cetyltrimethylammonium bromide (CTAB) stabilized gold nanorods at lithographically defined positions on a substrate. Overcoating of the nanoparticles with polystyrenesulfonate allows to switch the original nanoparticles positive surface charge to negative and to apply the template-assisted deposition technique developed for citrate-capped gold nanoparticles also to CTAB stabilized nanoparticles. The successful, selective deposition of gold nanorods in trenches with widths down to 50 nm is demonstrated. Our results indicate the potential of this method for the fabrication of well controlled, reproducible plasmonic biosensing substrates, applicable to the vast palette of anisotropic nanoparticle shapes synthesized with CTAB as the templating agent.

Linear self-assembly of nanoparticles within liquid crystal defect arrays

In the presence of oriented smectic liquid crystal defects, hybrid systems of nanoparticles/liquid crystals form straight chains of nanoparticles of length longer than tens of micrometers and width equal to one single nanoparticle. The interparticle distance in a chain can be varied between a few micrometers and 1.5 nm, highlighting the control of optical absorption by light polarization monitored by gold nanoparticle concentration.

A scheme for detecting every single target molecule with surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is now a well-established technique for the detection, under appropriate conditions, of single molecules (SM) adsorbed on metallic nanostructures. However, because of the large variations of the SERS enhancement factor on the surface, only molecules located at the positions of highest enhancement, so-called hot-spots, can be detected at the single-molecule level. As a result, in all SM-SERS studies so far only a small fraction, typically less than 1%, of molecules are actually observed. This complicates the analysis of such experiments and means that trace detection via SERS can in principle still be vastly improved. Here we propose a simple scheme, based on selective adsorption of the target analyte at the SERS hot-spots only, that allows in principle detection of every single target molecule in solution. We moreover provide a general experimental methodology, based on the comparison between average and maximum (single molecule) SERS enhancement factors, to verify the efficiency of our approach. The concepts and tools introduced in this work can readily be applied to other SERS systems aiming for detection of every single target molecule.

Influence of the number of nanoparticles on the enhancement properties of surface-enhanced Raman scattering active area: sensitivity versus repeatability

In the present work, the combination of chemical immobilization with electron beam lithography enables the production of sensitive and reproducible SERS-active areas composed of stochastic arrangements of gold nanoparticles. The number of nanoparticles was varied from 2 to 500. Thereby a systematic analysis of these SERS-active areas allows us to study SERS efficiency as a function of the number of nanoparticles. We found that the experimental parameters are critical, in particular the size of the SERS-active area must be comparable to the effective area of excitation to obtained reproducible SERS measurements. The sensitivity has also been studied by deducing the number of NPs that generate the enhancement. With this approach we demonstrates that the maximum enhancement, the best sensitivity, is obtained with the smallest number of nanoparticles that is resonant at a given excitation wavelength.

Active plasmonic devices with anisotropic optical response: a step toward active polarizer

Control of the optical properties of metallic nanoparticles (NP) is realized using an electrochemical switch consisting of a thin layer of conducting polymer (CP). It is shown that the quenching of localized surface plasmon (LSP) sustained by oblate particles depends of the frequency of the LSP resonance. This effect is attributed to the variation of the CP dielectric function with wavelength. As a consequence, prolate arrays show total quenching of the LSP resonance along the major axis of the particles whereas modulation and moderate damping are observed along the minor axis. Combining electroactive conducting polymer and prolate NP makes it possible to design active plasmonic devices with anisotropic optical response upon CP switching. In the present case, such devices can be used as active filters or polarizers.

Probing surface plasmon fields by far-field Raman imaging

This article reports on the imaging of the surface plasmon fields of lithographically designed micrometre-sized gold structures. We investigate rings made of disk-shaped particles and individual crescent-shaped particles. These structures are imaged with two techniques, dark field imaging of elastically scattered light and imaging the surface-enhanced Raman scattering signal of methylene blue dye adsorbed onto the structures. Although elastically scattered light images result from the coherently summed contributions from all elementary scattering volumes, surface-enhanced Raman scattering images reflect the optical near-field intensity incoherently averaged over a surface area corresponding to the spatial resolution of the microscope objective. The combination of both imaging methods enables us to emphasize the role of plasmon coupling and antenna effect in the surface-enhanced Raman scattering enhancement.

Characterization of strong electromagnetic field confinement on gold nanostructures by apertureless scanning near-field optical microscopy

We report on the detection of the optical near field of a 1D gold particle array by using an apertureless scanning near-field optical microscope. The strong near-field confinement measured above the grating proves unambiguously the near-field origin of the detected optical signal. Comparing the experiment with theory leads us to assign the optical near field to the first diffracted order of the grating, which is evanescent.

Near-field photochemical imaging of noble metal nanostructures

The sub-diffraction imaging of the optical near-field in nanostructures, based on a photochemical technique, is reported. A photosensitive azobenzene-dye polymer is spin coated onto lithographic structures and is subsequently irradiated with laser light. Photoinduced mass transport creates topographic modifications at the polymer film surface that are then measured with atomic force microscopy (AFM). The AFM images correlate with rigorous theoretical calculations of the near-field intensities for a range of different nanostructures and illumination polarizations. This approach is a first step toward additional methods for resolving confined optical near fields, which can augment scanning probe methodologies for high spatial resolution of optical near fields.