Stable Magnetic Hot Spots for Simultaneous Concentration and Ultrasensitive Surface-Enhanced Raman Scattering Detection of Solution Analytes

A simple and robust strategy is reported in this Article for the synthesis of stable magnetic surface-enhanced Raman scattering (SERS) hot spots in superparamagnetic, raspberry-shaped, mesoscopic gold particles that are composed of superparamagnetic Fe₃O₄ cores, amorphous SiO₂ mediation shells, and outer individual Au nanoparticles. The average interparticle gaps between the Au nanoparticles can be finely tuned by controlling the synthesis conditions, resulting in the formation of adequate SERS hot spots. The magnetic cores provide the capability to concentrate solution analytes adsorbed on the surfaces of the composite particles with the assistance of an external magnetic field, leading to ultrasensitive SERS detection of target species with concentration as low as femtomolar

Determination of Solvation Layer Thickness by a Magnetophotonic Approach

Derjaguin–Landau–Verwey–Overbeek (DLVO) theory fails in explaining the superior stability of colloid particles in aqueous suspensions under conditions of high ionic strengths where electrostatic forces are effectively screened. Accumulating evidence shows that the formation of a thin rigid layer of solvent molecules in the vicinity of a colloidal particle surface provides an additional repulsive interaction when the interparticle distance is reduced to several nanometers. The effective determination of the thickness of the solvation layer however remains a challenge. Here, we demonstrate a simple yet powerful magnetophotonic technique that can be used to study the thickness of the solvation layers formed on the colloidal silica surface in various polar solvents. A relationship between the hydrogen-bonding ability of the solvents and the thickness of solvation layer on colloidal silica surfaces has been identified; this observation is found to be consistent with the previously proposed hydrogen-bonding origin of the solvation force

Fully Alloyed Ag/Au Nanospheres: Combining the Plasmonic Property of Ag with the Stability of Au

We report that fully alloyed Ag/Au nanospheres with high compositional homogeneity ensured by annealing at elevated temperatures show large extinction cross sections, extremely narrow bandwidths, and remarkable stability in harsh chemical environments. Nanostructures of Ag are known to have much stronger surface plasmon resonance than Au, but their applications in many areas have been very limited by their poor chemical stability against nonideal chemical environments. Here we address this issue by producing fully alloyed Ag/Au nanospheres through a surface-protected annealing process. A critical temperature has been found to be around 930 °C, below which the resulting alloy nanospheres, although significantly more stable than pure silver nanoparticles, can still gradually decay upon extended exposure to a harsh etchant. Nanospheres annealed above the critical temperature show a homogeneous distribution of Ag and Au, minimal crystallographic defects, and the absence of structural and compositional interfaces, which account for the extremely narrow bandwidths of the surface plasmon resonance and may enable many plasmonic applications with high performance and long lifetime, especially for those involving corrosive species

Assembly and Photonic Properties of Superparamagnetic Colloids in Complex Magnetic Fields

Interparticle magnetic dipole force has been found to drive the formation of dynamic superparamagnetic colloidal particle chains that can lead to the creation of photonic nanostructures with rapidly and reversibly tunable structural colors in the visible and near-infrared spectrum. Although most studies on magnetic assembly utilize simple permanent magnets or electromagnets, magnetic fields, in principle, can be more complex, allowing the localized modulation of assembly and subsequent creation of complex superstructures. To explore the potential applications of a magnetically tunable photonic system, we study the assembly of magnetic colloidal particles in the complex magnetic field produced by a nonideal linear Halbach array. We demonstrate that a horizontal magnetic field sandwiched between two vertical fields would allow one to change the orientation of the particle chains, producing a high contrast in color patterns. A phase transition of Fe₃O₄@SiO₂ particles from linear particle chains to three-dimensional crystals is found to be determined by the interplay of the magnetic dipole force and packing force, as well as the strong electrostatic force. While a color pattern with tunable structures and diffractions can be instantly created when the particles are assembled in the form of linear chains in the regions with vertical fields, the large field gradient in the horizontal orientation may destabilize the chain structures and produces a pattern of 3D crystals that compliments that of initial chain assemblies. Our study not only demonstrates the great potential of magnetically responsive photonic structures in the visual graphic applications such as signage and security documents but also points out the potential challenge in pattern stability when the particle assemblies are subjected to complex magnetic fields that often involve large field gradients

Photonic Labyrinths: Two-Dimensional Dynamic Magnetic Assembly and <i>in Situ</i> Solidification

Creating novel structures by self-assembly processes and fixing the resultant assemblies are both critical to the design and fabrication of functional materials through bottom-up approaches. We demonstrate magnetically induced self-assembly of 2D photonic labyrinth structures and their solidification through a sol–gel method. The photonic labyrinth structures can be patterned into more regular arrangements using nonmagnetic substrates. This work may provide a platform for fabricating novel materials and devices with complex morphologies and spatial configurations