Enhanced optical trapping and arrangement of nano-objects in a plasmonic nanocavity.

Gentle manipulation of micrometer-sized dielectric objects with optical forces has found many applications in both life and physical sciences. To further extend optical trapping toward the true nanometer scale, we present an original approach combining self-induced back action (SIBA) trapping with the latest advances in nanoscale plasmon engineering. The designed resonant trap, formed by a rectangular plasmonic nanopore, is successfully tested on 22 nm polystyrene beads, showing both single- and double-bead trapping events. The mechanism responsible for the higher stability of the double-bead trapping is discussed, in light of the statistical analysis of the experimental data and numerical calculations. Furthermore, we propose a figure of merit that we use to quantify the achieved trapping efficiency and compare it to prior optical nanotweezers. Our approach may open new routes toward ultra-accurate immobilization and arrangement of nanoscale objects, such as biomolecules.

Comparison of the conformational behavior of amino acids and N-acetylated amino acids: a theoretical and matrix-isolation FT-IR study of N-acetylglycine.

Within the structure determination task for peptides, which is of large interest due to the relation between structure and functionality, infrared spectra can provide detailed information on the conformational behavior. The conformational landscape ofN-acetylgycine has been studied by a combined theoretical and matrix-isolation FT-IR study. The acetylation simulates an amino acid a peptide bond. Four stable conformations were found at the MP2/6-31++G** level of theory. Among these, only one contains an intramolecular H-bond that has a small abundance at the considered temperature. Apart from this one, three other different conformations could be detected in an Ar matrix. The experimental rotamerization constants NAG2 ⇌ NAG1 and NAG3 ⇌ NAG1 could be estimated. The values of the rotamerization constants as well as the mean frequency deviation of N-acetylglycine were combined with previously obtained data of other N-acetylated amino acids and they appeared to be similar to the data for nonsubstituted amino acids. This suggests that the used methodology can be in the future applied to investigate small peptides. Analysis of H-bond frequency shifts and distance demonstrates that the intramolecular H-bonds in N-acetylated amino acids are stronger compared to those in nonsubstituted amino acids.

Potential energy surface and matrix isolation FT-IR study of isoleucine.

The conformational landscape of isoleucine was investigated by a theoretical DFT and MP2 study. This investigation has revealed new important conformations in comparison to a previous study. Five conformations have been predicted with an abundance larger than 5% at both levels of theory. Among these, three different types of amino acid backbone were present, each characterized by a typical intramolecular H-bond. Due to the H-bonding, the frequencies of the involved groups are strongly shifted, and these modes were probably observed with FT-IR spectroscopy in an Ar matrix. The experimental abundances could be estimated and were in good accordance with the MP2 stabilities, whereas the accordance with the DFT abundances was poor. In contrast to this, the correspondence of the DFT and the experimental frequencies was excellent, as demonstrated by the mean frequency deviation of only 4.7 cm(-1).

Theoretical study of the regioselectivity of the interaction of 3-methyl-4-pyrimidone and 1-methyl-2-pyrimidone with Lewis acids.

A density functional theory (DFT) study is performed to determine the stability of the complexes formed between either the N or O site of 3-methyl-4-pyrimidone and 1-methyl-2-pyrimidone molecules and different ligands. The studied ligands are boron and alkali Lewis acids, namely, B(CH(3))(3), HB(CH(3))(2), H(2)B(CH(3)), BH(3), H(2)BF, HBF(2), BF(3), Li(+), Na(+), and K(+). The acids are divided into two groups according to their hardness. The reactivity predictions, according to the molecular electrostatic potential (MEP) map and the natural bond orbital (NBO) analysis, are in agreement with the calculated relative stabilities. Our findings reveal a strong regioselectivity with borane and its derivatives preferring the nitrogen site in both pyrimidone isomers, while a preference for oxygen is observed for the alkali acids in the 3-methyl-4-pyrimidone molecule. The complexation of 1-methyl-2-pyrimidone with these hard alkali acids does not show any discrimination between the two sites due to the presence of a continuous delocalized density region between the nitrogen and the oxygen atoms. The preference of boron Lewis acids toward the N site is due to the stronger B-N bond as compared to the B-O bond. The influence of fluorine or methyl substitution on the boron atom is discussed through natural orbital analysis (NBO) concentrating on the overlap of the boron empty p-orbital with the F lone pairs and methyl hyperconjugation, respectively. The electrophilicity of the boron acids gives a good overall picture of the interaction capabilities with the Lewis base.

Temperature determination of resonantly excited plasmonic branched gold nanoparticles by X-ray absorption spectroscopy.

The fields of bioscience and nanomedicine demand precise thermometry for nanoparticle heat characterization down to the nanoscale regime. Since current methods often use indirect and less accurate techniques to determine the nanoparticle temperature, there is a pressing need for a direct and reliable element-specific method. In-situ extended X-ray absorption fine structure (EXAFS) spectroscopy is used to determine the thermo-optical properties of plasmonic branched gold nanoparticles upon resonant laser illumination. With EXAFS, the direct determination of the nanoparticle temperature increase upon laser illumination is possible via the thermal influence on the gold lattice parameters. More specifically, using the change of the Debye-Waller term representing the lattice disorder, the temperature increase is selectively measured within the plasmonic branched nanoparticles upon resonant laser illumination. In addition, the signal intensity shows that the nanoparticle concentration in the beam more than doubles during laser illumination, thereby demonstrating that photothermal heating is a dynamic process. A comparable temperature increase is measured in the nanoparticle suspension using a thermocouple. This good correspondence between the temperature at the level of the nanoparticle and at the level of the suspension points to an efficient heat transfer between the nanoparticle and the surrounding medium, thus confirming the potential of branched gold nanoparticles for hyperthermia applications. This work demonstrates that X-ray absorption spectroscopy-based nanothermometry could be a valuable tool in the fast-growing number of applications of plasmonic nanoparticles, particularly in life sciences and medicine.

Poly(acrylic acid)-stabilized colloidal gold nanoparticles: synthesis and properties.

Combining the intriguing optical properties of gold nanoparticles with the inherent physical and dynamic properties of polymers can give rise to interesting hybrid nanomaterials. In this study, we report the synthesis of poly(acrylic acid) (PAA)-capped gold nanoparticles. The polyelectrolyte-wrapped gold nanoparticles were fully characterized and studied via a combination of techniques, i.e. UV-vis and infrared spectroscopy, dark field optical microscopy, SEM imaging, dynamic light scattering and zeta potential measurements. Although PAA-capped nanoparticles have been previously reported, this study revealed some interesting aspects of the colloidal stability and morphological change of the polymer coating on the nanoparticle surface in an electrolytic environment, at various pH values and at different temperatures.

Strong location dependent surface enhanced Raman scattering on individual gold semishell and nanobowl particles.

We demonstrate strong spatial localization of SERS on single symmetry-reduced gold semishell and nanobowl particles. A ∼30 nm carbon nanoparticle acts as a Raman reporter and is placed on different locations on a single semishell or nanobowl by e-beam induced deposition method, and remarkably different SERS intensities are observed.

Dynamic light scattering as a powerful tool for gold nanoparticle bioconjugation and biomolecular binding studies.

Dynamic light scattering (DLS) is an analytical tool used routinely for measuring the hydrodynamic size of nanoparticles and colloids in a liquid environment. Gold nanoparticles (GNPs) are extraordinary light scatterers at or near their surface plasmon resonance wavelength. In this study, we demonstrate that DLS can be used as a very convenient and powerful tool for gold nanoparticle bioconjugation and biomolecular binding studies. The conjugation process between protein A and gold nanoparticles under different experimental conditions and the quality as well as the stability of the prepared conjugates were monitored and characterized systematically by DLS. Furthermore, the specific interactions between protein A-conjugated gold nanoparticles and a target protein, human IgG, can be detected and monitored in situ by measuring the average particle size change of the assay solution. For the first time, we demonstrate that DLS is able to directly and quantitatively measure the binding stoichiometry between a protein-conjugated GNP probe and a target analyte protein in solution.

Focusing plasmons in nanoslits for surface-enhanced Raman scattering.

The focusing of plasmons to obtain a strong and localized electromagnetic-field enhancement for surface-enhanced Raman scattering (SERS) is increasing the interest in using plasmonic devices as molecular sensors. In this Full Paper, we report the successful fabrication and demonstration of a solid-state plasmonic nanoslit-cavity device equipped with nanoantennas on a freestanding thin silicon membrane as a substrate for SERS. Numerical calculations predict a strong and spatially localized enhancement of the optical field in the nanoslit (6 nm in width) upon irradiation. The predicted enhancement factor of SERS was 5.3 x 10(5), localized in an area of just 6 x 1.5 nm(2). Raman spectroscopy and imaging confirm an enhancement factor of approximately 10(6) for SERS from molecules chemisorbed at the nanoslit, and demonstrate the electromagnetic-field-enhancing function of the plasmonic nanoantennas. The freestanding membrane is open on both sides of the nanoslit, offering the potential for through-slit molecular translocation studies, and opening bright new perspectives for SERS applications in real-time (bio)chemical analysis.

Symmetry breaking induced optical properties of gold open shell nanostructures.

We use the finite difference time domain method to predict how optical plasmon properties are modified if the symmetrical geometry of gold shell nanostructures is broken. The simulations include three kinds of gold open shell nanostructures of nanobowls, open nanocages, and open eggshells. For all structures, the optical extinction spectra commonly display a distinct red shift when the full shell geometry is broken and a hyperbola-like dipolar plasmonic shift when the fractional height continuously decreases. The optical transitions of gold open shell nanostructures are explained by the plasmon hybridization theory combined with numerical calculations. Furthermore, the calculations exhibit that the local electric fields are strongly enhanced at the edges of the open nanoapertures on those symmetry-broken structures, which suggests a potential application in surface-enhanced Raman spectroscopy.

Shrinking solid-state nanopores using electron-beam-induced deposition.

Solid-state nanopores of only a few nanometres in size show a great potential for applications such as molecule detection and DNA sequencing. In most cases, the fabrication of such a nanopore requires the high energy beam of a transmission electron microscope (TEM) or focused ion beam (FIB) tool to drill or reshape a small hole in a freestanding membrane. Here, we present a novel method to reduce the size of existing nanopores using electron-beam-induced deposition (EBID) of carbon in a conventional scanning electron microscope (SEM). The existing nanopores are etched in a silicon membrane using anisotropic wet etching and can be shrunk down to a few nanometres using EBID. This paper discusses the parameters that influence the rate of shrinking and provides an insight into the underlying mechanism.

Observation of plasmonic dipolar anti-bonding mode in silver nanoring structures.

We report on a clear experimental observation of the plasmonic dipolar anti-bonding resonance in silver nanorings. The data can be explained effectively by the plasmon hybridization model, which is confirmed by the numerical calculations of the electromagnetic field and surface charge distribution profiles. The experimental demonstration of the plasmon hybridization model indicates its usefulness as a valuable tool to understand, design and predict optical properties of metallic nanostructures.

Surface modification of gamma-Fe2O3@SiO2 magnetic nanoparticles for the controlled interaction with biomolecules.

Modifying the surface of magnetic nanoparticles (MNPs) to allow for controlled interaction with biomolecules enables their implementation in biomedical applications such as contrast agents for magnetic resonance imaging, labels in magnetic biosensing or media for magnetically assisted bioseparation. In this paper, self-assembly of trialkoxysilanes is used to chemically functionalize the surface of gamma-Fe2O3@SiO2 core-shell particles. First, the silane deposition procedure was optimized using infrared analysis in order to obtain maximum packing density of the silanes on the particles. The surface coverage was determined to be approximately 8 x 10(14) molecules/cm2. It was shown that the magnetic, crystalline, and morphological properties of the MNPs were not altered by deposition of a thin silane coating. The optimized procedure was transferred for the deposition of aldehyde and poly(ethylene glycol) (PEG) presenting silanes. The presence of both silanes on the particle surface was confirmed using XPS and FTIR. The interaction of proteins with silane-modified MNPs was monitored using a Bradford protein assay. Our results demonstrate that, by introducing aldehyde functions, the MNPs are capable of covalently binding human IgG while retaining their specific binding capacity. Maximum surface coverage occurs at 46 microg antibodies per mg particle, which corresponds to 35 antibodies bound to an average sized MNP (54 nm in diameter). The human IgG functionalized MNPs exhibit a high degree of specificity (approximately 90%) and retained a binding capacity of 32%. Using the same approach, streptavidin was coupled onto the MNPs and the biotin binding capacity was determined using biotinylated fluorescein. At maximum surface coverage, a biotin binding capacity of 1500 pmol/mg was obtained, corresponding to a streptavidin activity of 76%. On the other hand, by introducing PEG functions the non-specific adsorption of serum proteins could be significantly suppressed down to approximately 3 microg/mg. We conclude that self-assembly of silane films creates a generic platform for the controlled interactions of MNPs with biomolecules.

Matrix-isolation FT-IR spectroscopic study and theoretical DFT(B3LYP)/6-31++G** calculations of the vibrational and conformational properties of tyrosine.

The complicated conformational isomerism of tyrosine is studied by experimental matrix-isolation FT-IR spectroscopy combined with theoretical DFT(B3LYP)/6-31++G** calculations. Not less than 18 possible conformations of tyrosine have been considered theoretically. The results revealed that the most and the less stable forms of neutral tyrosine have the same conformation of the main part of amino acid (conformation II) but they differ in orientation of the phenyl ring. The calculated values of the relative energies suggest that all conformations would be detectable in the experimental spectrum. However, it appeared that it is not possible to distinguish in the experimental spectrum between the bands due to the forms with the same conformation of the main part of amino acid but a different orientation of the phenol ring.

Full wetting of plasmonic nanopores through two-component droplets.

Benefiting from the prospect of extreme light localization, plasmonic metallic nanostructures are bringing advantages in many applications. However, for use in liquids, the hydrophobic nature of the metallic surface inhibits full wetting, which is related to contact line pinning in the nanostructures. In this work, we use a two-component droplet to overcome this problem. Due to a strong internal flow generated from the solutal Marangoni effect, these droplets can easily prime metallic nanostructures including sub-10 nm nanopores. We subsequently evaluate the local wetting performance of the plasmonic structures using surface enhanced Raman spectroscopy (SERS). Compared with other commonly used surface cleaning based wetting methods such as the oxygen plasma treatment, our two-component drop method is an efficient method in resolving the pinning of contact lines and is also non-destructive to samples. Thus the method described here primes plasmonic devices with guaranteed performances in liquid applications.

Reduced nonspecific adsorption on covalently immobilized protein surfaces using poly(ethylene oxide) containing blocking agents.

In a number of applications, e.g. DNA/protein micro-array technology, enzyme-linked immunosorbent assay (ELISA) technology or surface plasmon resonance (SPR) technology, the covalent coupling of proteins to surfaces is required. Following the covalent coupling of proteins, the remaining reactive groups should be blocked in order to avoid covalent binding of the analyte to the reactive surface. To this end, preferably blocking agents containing groups that avoid nonspecific adsorption should be used. These blocking agents are typically ethanolamine and cysteine for protein coupling via amino groups and thiol groups, respectively. This report presents novel blocking agents containing poly(ethylene oxide) (PEO) groups. These blocking agents show enhanced qualities to avoid nonspecific adsorption and can therefore have advantages in versatile protein-surface technologies.

On the contribution of intramolecular H-bonding entropy to the conformational stability of alanine conformations.

The experimental and theoretical rotamerization constants for the rotameric equilibrium between the two most stable conformations of the alpha-amino acid alanine are compared. The experimental technique of matrix-isolation Fourier transform infrared spectroscopy in combination with the density functional theory (DFT) (B3LYP) and the 6-31++G** basis set is used for this study. A large disagreement between the experimental and theoretical value of the equilibrium constant is found. A relatively strong intramolecular H-bond in conformation II is at the origin of this discrepancy. From the difference between the experimental and theoretical rotamerization constant, a DeltaS degrees value of -6.6 J K(-1) mol(-1) is found for the intramolecular H-bond formation.

Matrix isolation FT-IR and theoretical DFT/B3LYP spectrum of 1-naphthol.

The FT-IR spectrum of 1-Naphthol isolated in an argon matrix is performed and compared to the infrared spectra calculated at the DFT (B3LYP)/6-31+G(d) level for cis-1-Naphthol and trans-1-Naphthol rotamers in order to clarify the existence of both rotamers in the standard temperature. Comparison of the computed and the experimental matrix spectra reveals the presence in 1-Naphthol argon matrices in the standard temperature of both cis and trans rotameric forms of 1-Naphthol, the last predominating. The relative stability of the trans-1-Naphthol rotamer has also been supported by a fit comparison between the difference of predicted total energy (ETC) of both rotamers of 0.00195 a.u. corresponding to 5.12 kJ mol(-1) and the variation of the standard free Gibbs energy of rotamerization (ΔGr°) of 5.06 kJ mol(-1). Almost all 51 active vibrational modes of 1-Naphthol have been assigned. The stretching vibration of the OH group (νOH) appears to be the unique vibrational mode distinguishing the cis-1-NpOH rotamer from the trans-1-NpOH rotamer in FT-IR spectrum.

Experimental and theoretical observation of different intramolecular H-bonds in lysine conformations.

Due to the importance of the structure of amino acids for the folding and functionality of proteins, the conformational behavior of lysine has been investigated. Experimental matrix-isolation FT-IR spectra have been recorded. These spectra were interpreted using an extended theoretical DFT and MP2 study. Theoretically, 28 (DFT) and 18 (MP2) conformations were found with ΔE < 10 kJ mol(-1). Incorporation of the entropy term changed the relative order of the stability because of the large unfavorable effect of this term for the conformations with one or two intramolecular H-bonds. As a matter of fact, the predicted abundances are strongly temperature dependent. The abundant conformations of lysine at sublimation temperature can be characterized by the type of amino acid backbone and the eventual additional H-bond in four groups. These groups are predicted to be detectable in the matrix, as their abundances are all larger than 5%. The theoretical spectral data of the most abundant conformation of a particular group are used to represent the group. In the matrix-isolation FT-IR spectrum all the important, H-bonded involved modes (ν(OH), ν(NH(2)), ν(C═O), γ(OH), δ(OH) and γ(NH(2))) of the four conformational groups were observed. A linear correlation between the stretching frequency shift ν(XH) and the elongation of the XH distance Δr(XH) in different conformations of lysine and other amino acids has been observed. The experimental frequencies are in good relationship with the theoretically obtained data, which is proven by a mean frequency deviation for the most abundant conformation is 12.6 cm(-1).