Ofer Kedem

Sensitivity and optimization of localized surface plasmon resonance transducers.

Gold nanoisland films displaying localized surface plasmon resonance optical response were constructed by evaporation on glass and annealing. The surface plasmon distance sensitivity and refractive index sensitivity (RIS) for island films of different nominal thicknesses and morphologies were investigated using layer-by-layer polyelectrolyte multilayer assembly. Since the polymer forms a conformal coating on the Au islands and the glass substrate between islands, the relative sensitivity of the optical response to adsorption on and between islands was evaluated. The RIS was also determined independently using a series of solvents. An apparent discrepancy between the behavior of the RIS for wavelength shift and intensity change is resolved by considering the different physical nature of the two quantities, leading to the use of a new variable, that is, RIS (for intensity change) normalized to the surface density of islands. In the present system the surface plasmon decay length and RIS are shown to be directly correlated; both parameters increase with increasing average island size. This result implies that a higher RIS is not always beneficial for sensing; maximizing the transducer optical response requires the interrelated RIS and decay length to be optimized with respect to the dimensions of the studied analyte-receptor system. It is shown that, as a rule, transducers comprising larger islands furnish better overall sensitivity for thicker adlayers, whereas thinner adlayers produce a larger response when sensed using transducers comprising smaller islands, despite the lower RIS of the latter.

Improved Sensitivity of Localized Surface Plasmon Resonance Transducers Using Reflection Measurements

The refractive index sensitivity (RIS) of a localized surface plasmon resonance (LSPR) transducer is one of the key parameters determining its effectiveness in sensing applications. LSPR spectra of nanoparticulate gold films, including Au island films prepared by evaporation on glass and annealing as well as immobilized Au nanoparticle (NP) films, were measured in the transmission and reflection modes. It is shown that the RIS, measured as the wavelength shift in solvents with varying refractive index (RI), is significantly higher in reflection measurements.

Stabilization of metal nanoparticle films on glass surfaces using ultrathin silica coating.

Metal nanoparticle (NP) films, prepared by adsorption of NPs from a colloidal solution onto a preconditioned solid substrate, usually form well-dispersed random NP monolayers on the surface. For certain metals (e.g., Au, Ag, Cu), the NP films display a characteristic localized surface plasmon resonance (LSPR) extinction band, conveniently measured using transmission or reflection ultraviolet-visible light (UV-vis) spectroscopy. The surface plasmon band wavelength, intensity, and shape are affected by (among other parameters) the NP spatial distribution on the surface and the effective refractive index (RI) of the surrounding medium. A major concern in the formation of such NP assemblies on surfaces is a commonly observed instability, i.e., a strong tendency of the NPs to undergo aggregation upon removal from the solution and drying, expressed as a drastic change in the LSPR band. Since various imaging modes and applications require dried NP films, preservation of the film initial (wet) morphology and optical properties upon drying are highly desirable. The latter is achieved in the present work by introducing a convenient and generally applicable method for preventing NP aggregation upon drying while preserving the original film morphology and optical response. Stabilization of Au and Ag NP monolayers toward drying is accomplished by coating the immobilized NPs with an ultrathin (3.0-3.5 nm) silica layer, deposited using a sol-gel reaction performed on an intermediate self-assembled aminosilane layer. The thin silica coating prevents NP aggregation and maintains the initial NP film morphology and LSPR response during several cycles of drying and immersion in water. It is shown that the silica-coated NP films retain their properties as effective LSPR transducers.

Identification of two mechanisms for current production in a biharmonic flashing electron ratchet

Ratchets rectify the motion of randomly moving particles, which are driven by isotropic sources of energy such as thermal and chemical energy, without applying a net, time-averaged force between source and drain. This paper describes the behavior of a damped electron, modeled by a quantum Lindblad master equation, within a flashing ratchet (a one-dimensional potential that oscillates between a flat surface and a periodic asymmetric surface). By examining the complete space of all biharmonic potential shapes and a large range of oscillation frequencies, two modes of ratchet operation, differentiated by their oscillation frequencies (relative to the rate of electron relaxation), are identified. Slow-oscillating, strong friction ratchets operate by a classical, overdamped mechanism. In fast-oscillating, weak friction ratchets, current is primarily produced when the frequency of the oscillating potential is resonant with the beating of the electron wave function in the potential well. The shape of the ratchet potential determines the direction of the current (and, in some cases, straightforwardly accounts for current reversals), but the maximum achievable current at any shape is controlled by the degree of friction applied to the electron.

An introduction to ratchets in chemistry and biology

This article is an accessible introduction to ratchets and their potential uses. A ratchet can dramatically improve directional transport of classical or quantum particles in systems that are dominated by random diffusion. The key idea is that ratchets do not overcome poor conductivity with strong gradients, but rather use non-directional sources of energy like heat or chemical energy to power unidirectional transport, making the ratchet a Maxwells demon. We introduce the ratchet concept and its inspiration from biology, discuss the terminology used in the field, and examine current progress and ideas in ratcheting electrons and classical particles.

Light-responsive organic flashing electron ratchet

Significance Transport of objects in man-made systems typically relies on energy gradients that span the distance over which the particles must travel. Biological systems do not have these large-scale gradients available and instead transport particles with repeated local interactions between asymmetric structures, powered by nondirectional sources of energy like heat or chemical reactions. This process is called “ratcheting.” Here, we exploit the ratcheting mechanism to transport charge-carrying particles, electrons, through an amorphous organic film in which local structural inhomogeneities disrupt the gradients needed for typical electrical conduction to occur efficiently. This work explores basic mechanisms by which energy that is normally wasted in an electronic or photoelectrical device could be harnessed to do electrical work. Ratchets are nonequilibrium devices that produce directional motion of particles from nondirectional forces without using a bias, and are responsible for many types of biological transport, which occur with high yield despite strongly damped and noisy environments. Ratchets operate by breaking time-reversal and spatial symmetries in the direction of transport through application of a time-dependent potential with repeating, asymmetric features. This work demonstrates the ratcheting of electrons within a highly scattering organic bulk-heterojunction layer, and within a device architecture that enables the application of arbitrarily shaped oscillating electric potentials. Light is used to modulate the carrier density, which modifies the current with a nonmonotonic response predicted by theory. This system is driven with a single unbiased sine wave source, enabling the future use of natural oscillation sources such as electromagnetic radiation.

Mechanisms of Symmetry Breaking in a Multidimensional Flashing Particle Ratchet

Ratcheting is a mechanism that produces directional transport of particles by rectifying nondirectional energy using local asymmetries rather than a net bias in the direction of transport. In a flashing ratchet, an oscillating force (here, an AC field) is applied perpendicular to the direction of transport. In an effort to explore the properties of current experimentally realizable ratchet systems, and to design new ones, this paper describes classical simulations of a damped flashing ratchet that transports charged nanoparticles within a transport layer of finite, non-zero thickness. The thickness of the layer, and the decay of the applied field in the z-direction throughout that thickness, provide a mechanism of symmetry breaking in the system that allows the ratchet to produce directional transport using a temporally unbiased oscillation of the AC driving field, a sine wave. Sine waves are conveniently produced experimentally or harvested from natural sources but cannot produce transport in a 1D or pseudo-1D system. The sine wave drive produces transport velocities an order of magnitude higher than those produced by the common on/off drive, but lower than those produced by a temporally biased square wave drive (unequal durations of the positive and negative states). The dependence of the particle velocity on the thickness of the transport layer, and on the homogeneity of the oscillating field within the layer, is presented for all three driving schemes.