Creating hot nanoparticle pairs for surface-enhanced Raman spectroscopy through optical manipulation

We use optical tweezers to move single silver nanoparticles into near-field contact with immobilized particles, forming isolated surface-enhanced Raman spectroscopy (SERS) active Ag particle dimers. The surface-averaged SERS intensity increases by a factor approximately 20 upon dimerization. Electrodynamics calculations indicate that the final approach between the particles is due to "optical binding". The described methodology may facilitate controlled single molecule SERS analysis.     

Physicochemical and drug delivery aspects of lipid-based liquid crystalline nanoparticles: a case study of intravenously administered propofol

Liquid crystalline nanoparticles (LCNP) formed through lipid self-assembly have a range of attractive properties as in vivo drug delivery carriers. In particular they offer: a wide solubilization spectrum, and consequently high drug payloads; effective encapsulation; stabilization and protection of sensitive drug substances. Here we present basic physicochemical features of non-lamellar LCNP systems with a focus on intravenous drug applications. This is exemplified by the formulation properties and in vivo behavior using the drug substance propofol; a well-known anesthetic agent currently used in clinical practice in the form of a stable emulsion. In order to appraise the drug delivery features of the LCNP system the current study was carried out with a marketed propofol emulsion product as reference. In this comparison the propofol-LCNP formulation shows several useful features including: higher drug-loading capacity, lower fat-load, excellent stability, modified pharmacokinetics, and an indication of increased effect duration.     

Proton transfer reaction mass spectrometry and the unambiguous real-time detection of 2,4,6 trinitrotoluene

Fears of terrorist attacks have led to the development of various technologies for the real-time detection of explosives, but all suffer from potential ambiguities in the assignment of threat agents. Using proton transfer reaction mass spectrometry (PTR-MS), an unusual bias dependence in the detection sensitivity of 2,4,6 trinitrotoluene (TNT) on the reduced electric field (E/N) has been observed. For protonated TNT, rather than decreasing signal intensity with increasing E/N, which is the more usual sensitivity pattern observed in PTR-MS studies, an anomalous behavior is first observed, whereby the signal intensity initially rises with increasing E/N. We relate this to unexpected ion-molecule chemistry based upon comparisons of measurements taken with related nitroaromatic compounds (1,3,5 trinitrobenzene, 1,3 dinitrobenzene, and 2,4 dinitrotoluene) and electronic structure calculations. This dependence provides an easily measurable signature that can be used to provide a rapid highly selective analytical procedure to minimize false positives for the detection of TNT. This has major implications for Homeland Security and, in addition, has the potential of making instrumentation cost-effective for use in security areas. This study shows that an understanding of fundamental ion-molecule chemistry occurring in low-pressure drift tubes is needed to exploit selectivity and sensitivity for analytical purposes.     

Suppression of the zero-order diffracted beam from a pixelated spatial light modulator by phase compression

Phase compression is used to suppress the on-axis zero-order diffracted (ZOD) beam from a pixelated phase-only spatial light modulator (SLM) by a simple modification to the computer generated hologram (CGH) loaded onto the SLM. After CGH design, the phase of each SLM element is identically compressed by multiplying by a constant scale factor and rotated on the complex unit-circle to produce a cancellation beam that destructively interferes with the ZOD beam. Experiments achieved a factor of 3 reduction of the ZOD beam using two different liquid-crystal SLMs. Numerical simulation analyzed the reconstructed image quality and diffraction efficiency versus degree of phase compression and showed that phase compression resulted in little image degradation or power loss.     

Real-time scan assistant for echocardiography

A real-time scan assistant (SA) for use with echocardiography is presented. The motivation is to aid nonexpert users in capturing apical 4-chamber views (A4CH) during echocardiography. The algorithm is based on a parametric multi-chamber model of the A4CH view, updated in an extended Kalman filter framework. The regional model goodness-of-fit is used to calculate a score, which is provided to the user during acquisition, together with an icon (emoticon) indicating whether the current view is acceptable or not. The SA was implemented on a commercially available scanner. A feasibility test was performed using two healthy volunteers as models and 10 medical students acting as nonexpert users. The students examined the models on two occasions, separated more than four days in time. Half of the students used the SA during the first exam and no SA at the second exam. The other half used the opposite order. The recordings were later rated by a cardiologist. A Wilcoxon signed pair rank test revealed a statistically significant improvement when using SA. Nine cases were rated as poor without using the SA. In eight (89%) of these cases, view quality improved to acceptable when the SA was used.     

Lipid-based passivation in nanofluidics

Stretching DNA in nanochannels is a useful tool for direct, visual studies of genomic DNA at the single molecule level. To facilitate the study of the interaction of linear DNA with proteins in nanochannels, we have implemented a highly effective passivation scheme based on lipid bilayers. We demonstrate virtually complete long-term passivation of nanochannel surfaces to a range of relevant reagents, including streptavidin-coated quantum dots, RecA proteins, and RecA-DNA complexes. We show that the performance of the lipid bilayer is significantly better than that of standard bovine serum albumin-based passivation. Finally, we show how the passivated devices allow us to monitor single DNA cleavage events during enzymatic degradation by DNase I. We expect that our approach will open up for detailed, systematic studies of a wide range of protein-DNA interactions with high spatial and temporal resolution.     

Material-selective surface chemistry for nanoplasmonic sensors: optimizing sensitivity and controlling binding to local hot spots

Optical sensors utilizing the principle of localized surface plasmon resonance (LSPR) offer the advantage of a simple label-free mode of operation, but the sensitivity is typically limited to a very thin region close to the surface. In bioanalytical sensing applications, this can be a significant drawback, in particular since the surface needs to be coated with a recognition layer in order to ensure specific detection of target molecules. We show that the signal upon protein binding decreases dramatically with increasing thickness of the recognition layer, highlighting the need for thin high quality recognition layers compatible with LSPR sensors. The effect is particularly strong for structures that provide local hot spots with highly confined fields, such as in the gap between pairs of gold disks. While our results show a significant improvement in sensor response for pairs over single gold disks upon binding directly to the gold surface, disk pairs did not provide larger signal upon binding of proteins to a recognition layer (already for around 3 nm thin layers) located on the gold. Local plasmonic hot spots are however shown advantageous in combination with directed binding to the hot spots. This was demonstrated using a structure consisting of three surface materials (gold, titanium dioxide, and silicon dioxide) and a new protocol for material-selective surface chemistry of these three materials, which allows for controlled binding only in the gap between pairs of disks. Such a design increased the signal obtained per bound molecule by a factor of around four compared to binding to single disks.     

Fluorographene: a two-dimensional counterpart of Teflon

A stoichiometric derivative of graphene with a fluorine atom attached to each carbon is reported. Raman, optical, structural, micromechanical, and transport studies show that the material is qualitatively different from the known graphene-based nonstoichiometric derivatives. Fluorographene is a high-quality insulator (resistivity >10(12) Ω) with an optical gap of 3 eV. It inherits the mechanical strength of graphene, exhibiting a Young's modulus of 100 N m(-1) and sustaining strains of 15%. Fluorographene is inert and stable up to 400 °C even in air, similar to Teflon.     

Fluorinated graphene: Fluorographene: A Two‐Dimensional Counterpart of Teflon (Small 24/2010)

A stoichiometric derivative of graphene with a fluorine atom attached to each carbon is reported. Raman, optical, structural, micromechanical, and transport studies show that the material is qualitatively different from the known graphene‐based nonstoichiometric derivatives. Fluorographene is a high‐quality insulator (resistivity >10¹² Ω) with an optical gap of 3 eV. It inherits the mechanical strength of graphene, exhibiting a Young’s modulus of 100 N m^?1 and sustaining strains of 15%. Fluorographene is inert and stable up to 400 °C even in air, similar to Teflon.     

Variationally consistent computational homogenization of transient heat flow

A framework for variationally consistent homogenization, combined with a generalized macro‐homogeneity condition, is exploited for the analysis of non‐linear transient heat conduction. Within this framework the classical approach of (model‐based) first‐order homogenization for stationary problems is extended to transient problems. Homogenization is then carried out in the spatial domain on representative volume elements (RVE), which are (in practice) introduced in quadrature points in standard fashion. Along with the classical averages, a higher order conservation quantity is obtained. An iterative FE²‐algorithm is devised for the case of non‐linear diffusion and storage coefficients, and it is applied to transient heat conduction in a strongly heterogeneous particle composite. Parametric studies are carried out, in particular with respect to the influence of the ‘internal length’ associated with the second‐order conservation quantity. Copyright © 2009 John Wiley & Sons, Ltd.