Characterization of electrodeposited gold and palladium nanowire gratings with optical diffraction measurements

Indirect Microcontact Printing to Create Functional Patterns of Physisorbed Antibodies

Microcontact printing (µCP) is a practical and versatile approach to create nanostructured patterns of biomolecular probes, but it involves conformational changes on the patterned bioreceptors that often lead to a loss on the biological activity of the resulting structures. Herein we introduce indirect µCP to create functional patterns of bioreceptors on solid substrates. This is a simple strategy that relies on physisorbing biomolecular probes of interest in the nanostructured gaps that result after patterning backfilling agents by standard µCP. This study presents the approach, assesses bovine serum albumin as backfilling agent for indirect µCP on different materials, reports the limitations of standard µCP on the functionality of patterned antibodies, and demonstrates the capabilities of indirect µCP to solve this issue. Bioreceptors were herein structured as diffractive gratings and used to measure biorecognition events in label-free conditions. Besides, as a preliminary approach towards sensing biomarkers, this work also reports the implementation of indirect µCP in an immunoassay to detect human immunoglobulin E.

Monitoring Volumetric Changes in Silicon Thin-Film Anodes through In Situ Optical Diffraction Microscopy

A high-resolution in situ spectroelectrochemical optical diffraction experiment has been developed to understand the volume expansion/contraction process of amorphous silicon (a-Si) thin-film anodes. Electrodes consisting of 1D transmissive gratings of silicon have been produced through photolithographic methods. After glovebox assembly in a home-built Teflon cell, monitoring of the diffraction efficiency of these gratings during the lithiation/delithiation process is performed using an optical microscope equipped with a Bertrand lens. When the diffraction efficiency along with optical constants obtained from in situ spectroscopic ellipsometry is utilized, volume changes of the active materials can be deduced. Unlike transmission electron microscopy and atomic force microscopy characterization methods of observing silicon's volume expansion, this experiment allows for real-time monitoring of the volume change at charge/discharge cycles greater than just the first few along with an experimental environment that directly mimics that of a real battery. This technique shows promising results that provide needed insight into understanding the lithium alloying reaction and subsequent induced capacity fade during the cycling of alloying anodes in lithium-ion batteries.

Double layer lift-off nanofabrication controlled gaps of nanoelectrodes with sub-100 nm by nanoimprint lithography

Basic research on nanoelectronics is often limited by the high cost and large-scale methods to fabricate electrodes with controlled gap size in nanometer scales. Here nanoelectrodes with a controlled gap size of sub-100 nm were fabricated by modified nanoimprint lithography (NIL) via a double-layer lift-off process utilizing polymethylmethacrylate (PMMA) and polydimethylglutarimide (PMGI) as the lift-off resist. Firstly the patterns of the electrode mold were transferred onto the upper PMMA layer by NIL techniques and then through controlling the developing time and concentration of developer of the PMGI under layer, regulating the exact gap size of the transferred metal nanoelectrode. The result indicated that the 'undercut' phenomenon was observed of the PMGI transfer layer during the developing process; through controlling the feature size of the undercut length, the gap size of the transferred metal nanoelectrode was precisely controlled, which showed shrinkage behavior. The nanoelectrodes with gap sizes of 800, 400, 200, and 100 nm can be reduced to about 440, 120, 80, and 70 nm. Our result provides a low-cost and large-scale route to prepare nanoelectrodes with controlled gap size, which can be valuable for current efforts in nanoelectronics.

Silica nanowire arrays for diffraction-based bioaffinity sensing

Arrays of electrodeposited silica nanowires (SiO2 NWs) have been fabricated over large areas (cm(2)) on fluoropolymer thin films attached to glass substrates by a combination of photolithography and electrochemically triggered sol-gel nanoscale deposition. Optical and scanning electron microscopy (SEM) measurements revealed that the SiO2 NW arrays had an average spacing of ten micrometers and an average width of 700 nm with a significant grain structure that was a result of the sol-gel deposition process. The optical diffraction properties at 633 nm of the SiO2 NW arrays were characterized when placed in contact with solutions by using a prism-coupled total internal reflection geometry; quantification of changes in these diffraction properties was applied in various sensing applications. Bulk refractive index sensing by using the SiO2 NW grating was demonstrated with a sensitivity of 1.30×10(-5) RIU. Toposelectively chemically modified SiO2 NW arrays were used for diffraction biosensing measurements of surface binding events, such as the electrostatic adsorption of gold nanoparticles and the bioaffinity adsorption of streptavidin onto a biotin monolayer. Finally, the application of the SiO2 NW arrays for practical medical-diagnostic applications was demonstrated by monitoring the diffraction of SiO2 NW arrays functionalized with a single-stranded (ss)DNA aptamer to detect human α-thrombin from solutions at sub-pathologic nanomolar concentrations.

DNAzyme footprinting: detecting protein-aptamer complexation on surfaces by blocking DNAzyme cleavage activity

A novel method to quantitatively measure the binding of proteins to single-stranded DNA (ssDNA) aptamers that employs the inhibition of the DNAzyme hydrolysis of aptamer monolayers is described. A 28-base DNAzyme was designed to specifically bind to and cleave a 29-base ssDNA sequence that can fold into a G-quartet aptamer and bind the protein thrombin. The binding strength of the DNAzyme to the aptamer sequence was designed to be less than the binding strength of the thrombin to the aptamer (ΔG° = -43.1 and -51.8 kJ/mol, respectively). Formation of the thrombin-aptamer complex was found to block DNAzyme cleavage activity both in solution and in an ssDNA aptamer monolayer. We denote this method for detecting protein-aptamer complexation as "DNAzyme footprinting" in analogy to the process of DNase footprinting for the detection of protein-DNA interactions. By attaching a 40-base reporter sequence to the ssDNA aptamer monolayer, the detection of any protein-aptamer complexes remaining on the surface after DNAzyme activity can be greatly enhanced (down to one thrombin-aptamer complex per 10,000 ssDNA molecules corresponding to 100 fM thrombin in solution) by a subsequent surface RNA transcription amplification reaction followed by RNA detection with nanoparticle-enhanced SPR imaging. In addition to RNA transcription, DNAzyme footprinting can be coupled to a wide variety of other nucleic acid surface amplification schemes and thus is a powerful new route for the enzymatically amplified detection of proteins via protein-aptamer complex formation.

Fabrication of periodic metal nanowires with microscale mold by nanoimprint lithography

In this paper, a simple method is demonstrated for fabricating periodic metal nanowires based on the unconventional nanoimprint lithography (NIL) technique. Using this method, sub-100 nm metal nanowires with the rectangular cross-section are fabricated with microscale stamp. Furthermore, the metal nanowires with different widths and heights can be generated by adjusting the imprinting parameters with the same stamp. The metal nanowires prepared with this method can be used for chemical sensing, such as ammonia sensing, and it may have applications in optical signal processing.

Characterization and Application of Surface Plasmon-Enhanced Optical Diffraction from Electrodeposited Gold Nanowire Arrays

Arrays of gold nanowires formed by the process of lithographically patterned nanowire electrodeposition (LPNE) were characterized by a combination of SEM, polarized UV-visible absorption spectroscopy and optical diffraction measurements. A transverse localized surface plasmon resonance (LSPR) was observed for gold nanowire arrays with an absorption maximum (λ(max)) that varied with nanowire width. Transmission optical diffraction measurements were measured with the even and odd diffraction orders creating an alternating, out of phase sinusoidal intensity pattern characteristic of the LPNE nanowire arrays. The intensities of the even diffraction order maxima were the strongest for nanowires with a width of 115 ± 10 nm; nanowires of this width exhibit a λ(max) of 635 ± 10 nm, verifying that the transverse LSPR has enhanced the optical diffraction signal. Real time total internal reflection diffraction intensity measurements were used to monitor in situ the electrodeposition of silver monolayers onto the gold nanowire arrays.

Fabricating nanoscale DNA patterns with gold nanowires

Surface patterns of single-stranded DNA (ssDNA) consisting of nanoscale lines as thin as 40 nm were fabricated on polymer substrates for nanotechnology and bioaffinity sensing applications. Large scale arrays (with areas up to 4 cm(2)) of ssDNA "nanolines" were created on streptavidin-coated polymer (PDMS) surfaces by transferring biotinylated ssDNA from a master pattern of gold nanowires attached to a glass substrate. The gold nano-wires were first formed on the glass substrate by the process of lithographically patterned nanowire electrodeposition (LPNE), and then "inked" with biotinylated ssDNA by hybridization adsorption to a thiol-modified ssDNA monolayer attached to the gold nanowires. The transferred ssDNA nanolines were capable of hybridizing with ssDNA from solution to form double-stranded DNA (dsDNA) patterns; a combination of fluorescence and atomic force microscopy (AFM) measurements were used to characterize the dsDNA nanoline arrays. To demonstrate the utility of these surfaces for biosensing, optical diffraction measurements of the hybridization adsorption of DNA-coated gold nanoparticles onto the ssDNA nanoline arrays were used to detect a specific target sequence of unlabeled ssDNA in solution.