Marcin Fiałkowski

Highly reproducible, stable and multiply regenerated surface-enhanced Raman scattering substrate for biomedical applications

We fabricated a Surface Enhanced Raman Scattering (SERS)-active surface based on photo-etched and Au-coated GaN. The highest enhancement factor (EF) in SERS and high reproducibility of spectra were obtained from surfaces covered with bunched nanopillars which were produced by relatively long defect-selective photo-etching. The surfaces exhibited SERS enhancements of the order of 2.8 × 106 for malachite green isothiocyanate (MGITC) and 2 × 106 for p-mercaptobenzoic acid (PMBA). These SERS enhancement factors were comparable to those of conventional SERS substrates, while the EF for MGITC was two orders of magnitude larger than the corresponding one reported for the SERS platform made on porous GaN. The standard deviation of the relative intensity of the 1180 cm−1 mode of MGITC was less than 5% for 100 randomly distributed locations across a single platform and less than 10% between different platforms. The SERS signal of MGITC at our GaN/Au surface (kept under ambient conditions) was extremely stable. We could not detect any peak shift or appreciable change of intensity even after three months. We used these surfaces to detect biological molecules such as amino acids and bovine serum albumin (BSA) at low concentration and with short detection time. We developed simple and effective cleaning procedures for our substrates. After cleaning, the same substrate could be used multiple times retaining the SERS activity. We are not aware of any other multiply regenerated SERS substrate which provides simultaneously such high stability with high enhancement, good uniformity, and high reproducibility.

Close-packed monolayers of charged Janus-type nanoparticles at the air–water interface

We present a new method to obtain close-packed monolayers composed of noble metal nanoparticles (NP) possessing well-defined permanent charge of either sign. The method is based on the fact that ligands forming the protecting layer exhibit ability to rearrange at the NPs surface. We demonstrate that if the protecting layer is composed of a mixture of hydrophobic and hydrophilic charged ligands in appropriate proportion, the NP exhibits properties of a Janus-type particle with one of the hemispheres hydrophilic and the other hydrophobic. Such amphiphilic NPs self-assemble into a monolayer of well defined surface charge at the air-water interface. Due to strong stabilizing effect of the lateral electrostatic repulsions, such monolayer can be compressed to form close-packed hexagonal structure, and then easily transferred onto a solid substrate with the Langmuir-Blodgett technique.

Gold Micro-Flowers: One-Step Fabrication of Efficient, Highly Reproducible Surface-Enhanced Raman Spectroscopy Platform

We present a new method enabling simultaneous synthesis and deposition of gold micro-flowers (AuMFs) on solid substrates in a one-pot process that uses two reagents, auric acid and hydroxylamine hydrochloride, in aqueous reaction mixture. The AuMFs deposited onto the substrate form mechanically stable gold layer of expanded nanostructured surface. The morphology of the AuMFs depends on and can be controlled by the composition of the reaction solution as well as by the reaction time. The nanostructured metallic layers obtained with our method are employed as efficient platforms for chemical and biological sensing based on surface-enhanced Raman spectroscopy (SERS). SERS spectra recorded by such platforms for p-mercaptobenzoic acid and phage lambda exhibit enhancement factors above 106 and excellent reproducibility.

Autonomous Self‐Assembly of Ionic Nanoparticles into Hexagonally Close‐Packed Lattices at a Planar Oil–Water Interface

Lets get charged! Positively charged nanoparticles (NPs) spontaneously self-assemble into hexagonally close-packed lattices at a planar CH(2)Cl(2)-water interface. The self-assembly process is fully autonomous and occurs without any external manipulation.

Morphological changes of gold nanoparticles due to adsorption onto silicon substrate and oxygen plasma treatment

Deposition of gold nanoparticles (AuNPs) on a solid substrate followed by oxygen plasma treatment is a commonly used protocol for surface functionalization. Surprisingly, the effect of the deposition process and oxygen plasma on the morphology of the AuNPs is usually overlooked in research. Here, we investigated morphological changes caused by (i) adsorption of small ligand-capped AuNPs (∼5 nm in diameter) onto a silicon substrate and (ii) subsequent oxygen plasma treatment. AuNPs coated with positively and negatively charged as well as uncharged ligands have been investigated. It is found that upon the adsorption the AuNPs undergo plastic deformations and their shapes can be approximated by spherical caps. The degree of the deformation depends strongly on the AuNP coating. During the plasma treatment the AuNPs behave like droplets of a non-wetting liquid, exhibiting the ability to move and merge. We argue that the AuNP coarsening is dominated by the diffusion coalescence mechanism and show that time evolution of the surface AuNP density follows the Smoluchowski coagulation equation. The diffusivity of the AuNP scales with its mass as D(m) ∼ m−α with α = 2.6.

Fabrication of nanocomposites by covalent bonding between noble metal nanoparticles and polymer matrix

We present a novel approach to the synthesis of polymeric nanocomposites in which noble metal nanoparticles (NP) are covalently bonded to a polymer matrix. In this approach, the NPs are functionalized with novel aminothioalkil ligands that are capable to bind chemically to a wide range of polymers possessing carbonyl groups or hydroxyl groups that can be oxidized into carbonyl groups. This method is illustrated by the ex situ incorporation of gold, platinum, and silver NPs decorated with the aminothioalkil ligands into matrices of natural polymers such as potato starch, cellulose, as well as synthetic polymers – poly(vinyl)alcohol, carboxymethylcellulose, and copolymer of maleic anhydride and styrene. It is demonstrated that in the obtained nanocomposites the functionalized NPs are covalently linked to the polymer matrix via amide, imide, imine, or aminal bonds. We show that the binding reaction of the aminothioalkil ligands with the polymer chains occurs spontaneously at room temperature only when the aminothioalkil ligands are attached to the surface of the NPs.

Orientational phenomena in a plastic flow of a two-dimensional square crystal

Within the framework of irreversible thermodynamics, a nonlinear relaxation equation for the 4-rank alignment tensor is formulated for a two-dimensional square crystal subjected to the shear flow. An evolution equation governing the dynamics of the orientation of the crystal, analogous to the Ericksen–Leslie balance equation used in the theory of liquid crystals, is derived from the relaxation equation and analyzed for a Couette flow geometry. It is found that two solutions of the evolution equation are possible: (i) the crystal aligns at a specific angle or (ii) rotates in the shear plane (tumbling). The flow alignment angle is predicted to depend on the value of the shear rate. Furthermore, a generalized Fokker–Planck equation for the probability distribution function is formulated within the mean-field approximation. It is used to derive a scalar counterpart, expressed in terms of microscopic quantities, of the phenomenological evolution equation. The stationary solution is also investigated, based on the kinetic equation.

Dynamic charge separation in a liquid crystalline meniscus

Oscillating electric fields can sustain a macroscopic and steady separation of electrostatic charges. The control over the dynamic charge separation (dyCHASE) is presented for the example of circular menisci of thin, free standing smectic films. These films are subject to an in-plane, alternating radial electric field. The boundaries of the menisci become charged and unstable in the electric field and deform into pulsating, flower-like shapes. This instability ensues only at frequencies of the electric field that are lower than a critical one. The critical frequency is a linear function of the strength of the electric field. Since the speed of electrophoretic drift of ions is also linearly related to the strength of the field, the linear relation between critical frequency and the amplitude of the field sets a characteristic length scale in the system. We postulate that dyCHASE is due to (i) electrophoretic motion of ions in the liquid crystalline (LC) film, (ii) microscopic separation of charges over distances similar in magnitude to the Debye screening length, and (iii) further, macroscopic separation of charges through an electro-hydrodynamic instability. Interestingly, the electrophoretic motion of ions couples with the macroscopic motion of the LC material that can be observed with the use of simple optical microscopy.

Phenomenological approach to viscosity of a two-dimensional square crystal

Constitutive equations for the viscous stress tensor and tensorial molecular field for an incompressible two-dimensional square crystal are proposed, based on general symmetry considerations. The viscous stress derived involves 15 viscosity coefficients and contains terms which are linear as well as quadratic in the velocity gradient. The entropy production is expressed in terms of thermodynamic fluxes and forces. An equilibrium condition for the antisymmetric part of the viscous stress is used to derive a balance of angular momentum. Furthermore, it is found that viscous stresses in a plane Couette geometry are characterized by four effective viscosity coefficients. In the limit of slow flow, the first normal stress difference is predicted to be independent of the shear rate.