Fluorescence excitation enhancement by waveguiding nanowires

The optical properties of vertical semiconductor nanowires can allow an enhancement of fluorescence from surface-bound fluorophores, a feature proven useful in biosensing. One of the contributing factors to the fluorescence enhancement is thought to be the local increase of the incident excitation light intensity in the vicinity of the nanowire surface, where fluorophores are located. However, this effect has not been experimentally studied in detail to date. Here, we quantify the excitation enhancement of fluorophores bound to a semiconductor nanowire surface by combining modelling with measurements of fluorescence photobleaching rate, indicative of the excitation light intensity, using epitaxially grown GaP nanowires. We study the excitation enhancement for nanowires with a diameter of 50-250 nm and show that excitation enhancement reaches a maximum for certain diameters, depending on the excitation wavelength. Furthermore, we find that the excitation enhancement decreases rapidly within tens of nanometers from the nanowire sidewall. The results can be used to design nanowire-based optical systems with exceptional sensitivities for bioanalytical applications

Fluorescence Signal Enhancement in Antibody Microarrays Using Lightguiding Nanowires

Fluorescence-based detection assays play an essential role in the life sciences and medicine. To offer better detection sensitivity and lower limits of detection (LOD), there is a growing need for novel platforms with an improved readout capacity. In this context, substrates containing semiconductor nanowires may offer significant advantages, due to their proven light-emission enhancing, waveguiding properties, and increased surface area. To demonstrate and evaluate the potential of such nanowires in the context of diagnostic assays, we have in this work adopted a well-established single-chain fragment antibody-based assay, based on a protocol previously designed for biomarker detection using planar microarrays, to freestanding, SiO2-coated gallium phosphide nanowires. The assay was used for the detection of protein biomarkers in highly complex human serum at high dilution. The signal quality was quantified and compared with results obtained on conventional flat silicon and plastic substrates used in the established microarray applications. Our results show that using the nanowire-sensor platform in combination with conventional readout methods, improves the signal intensity, contrast, and signal-to-noise by more than one order of magnitude compared to flat surfaces. The results confirm the potential of lightguiding nanowires for signal enhancement and their capacity to improve the LOD of standard diagnostic assays

Optical properties and self assembly of biotemplated gold nanoparticle chains

Gold nanochains are assembled on amyloid fibers and their optical properties are investigate

Lightguiding of Fluorescence in Nanowires : Principles, Optimization and Implementation for Biosensing

Due to their unique optical properties, III-V nanowires are promising candidates for the development of efficient solar cells, LEDs and nanoscaled lasers. The interaction of nanowires with fluorescent molecules is also of interest, as these nanostructures can act as nanoscaled optical fibers. The light emitted by a fluorophore close to a nanowire surface can in fact couple into the supported waveguide modes and get re-emitted at the nanowire tip. This thesis explores the possibility of using III-V nanowires to enhance the detection of fluorescent molecules, focusing on the development of a platform to characterize an artificial molecular motor, the Lawnmower, and the possibilities these nanostructures present for optical biosensing. Confocal imaging was used to investigate the dependence of the lightguiding effect over nanowire diameter and fluorophore emission range, and the role of multiple waveguide modes was identified by comparing experimental data with finite-difference simulations and analytical calculations. These results show how the normalized frequency parameter can be used to predict the lightguiding ability of a nanowire at a certain wavelength, serving as a useful design guide for optimizing the effect for a specific fluorophore.The potential of nanowires for enhanced fluorescence detection was investigated by coating the nanowires with a supported lipid bilayer containing fluorescently labelled proteins. Diffusion of single proteins from and to the nanowires surface caused the nanowire tips to blink over time, and the parallel observation of the blinking pattern of hundreds of nanowires allowed to estimate simultaneously both protein coverage and diffusion constant in the bilayer.This thesis also discusses how nanowires can be used as a track and characterization device for the Lawnmower, and the development of the surface chemistry necessary for this scope. These results open the possibility of developing nanowire-based optical biosensors, as due their lightguiding properties nanowires could be used as signal integrators to improve the sensitivity of fluorescence-based assays

Single-Molecule Detection with Lightguiding Nanowires : Determination of Protein Concentration and Diffusivity in Supported Lipid Bilayers

Determining the surface concentration and diffusivity of cell-membrane-bound molecules is central to the understanding of numerous important biochemical processes taking place at cell membranes. Here we use the high aspect ratio and lightguiding properties of semiconductor nanowires (NWs) to detect the presence of single freely diffusing proteins bound to a lipid bilayer covering the NW surface. Simultaneous observation of light-emission dynamics of hundreds of individual NWs occurring on the time scale of only a few seconds is interpreted using analytical models and employed to determine both surface concentration and diffusivity of cholera toxin subunit B (CTxB) bound to GM1 gangliosides in supported lipid bilayer (SLB) at surface concentrations down to below one CTxB per μm2. In particular, a decrease in diffusivity was observed with increasing GM1 content in the SLB, suggesting increasing multivalent binding of CTxB to GM1. The lightguiding capability of the NWs makes the method compatible with conventional epifluorescence microscopy, and it is shown to work well for both photostable and photosensitive dyes. These features make the concept an interesting complement to existing techniques for studying the diffusivity of low-abundance cell-membrane-bound molecules, expanding the rapidly growing use of semiconductor NWs in various bioanalytical sensor applications and live cell studies

Through the Eyes of Creators: Observing Artificial Molecular Motors : ACS Nanoscience Au

Inspired by molecular motors in biology, there hasbeen significant progress in building artificial molecular motors, usinga number of quite distinct approaches. As the constructs become moresophisticated, there is also an increasing need to directly observe themotion of artificial motors at the nanoscale and to characterize theirperformance. Here, we review the most used methods that tacklethose tasks. We aim to help experimentalists with an overview of theavailable tools used for different types of synthetic motors and tochoose the method most suited for the size of a motor and the desiredmeasurements, such as the generated force or distances in the movingsystem. Furthermore, for many envisioned applications of syntheticmotors, it will be a requirement to guide and control directed motions.We therefore also provide a perspective on how motors can be observed on structures that allow for directional guidance, such asnanowires and microchannels. Thus, this Review facilitates the future research on synthetic molecular motors, where observations ata single-motor level and a detailed characterization of motion will promote applications

Nanowires for Biosensing: Lightguiding of Fluorescence as a Function of Diameter and Wavelength

Semiconductor nanowires can act as nanoscaled optical fibers, enabling them to guide and concentrate light emitted by surface-bound fluorophores, potentially enhancing the sensitivity of optical biosensing. While parameters such as the nanowire geometry and the fluorophore wavelength can be expected to strongly influence this lightguiding effect, no detailed description of their effect on in-coupling of fluorescent emission is available to date. Here, we use confocal imaging to quantify the lightguiding effect in GaP nanowires as a function of nanowire geometry and light wavelength. Using a combination of finite-difference time-domain simulations and analytical approaches, we identify the role of multiple waveguide modes for the observed lightguiding. The normalized frequency parameter, based on the step-index approximation, predicts the lightguiding ability of the nanowires as a function of diameter and fluorophore wavelength, providing a useful guide for the design of optical biosensors based on nanowires