Probing the protein orientation on charged self-assembled monolayers on gold nanohole arrays by SERS

SERS detection of streptavidin/biotin monolayer assemblies

A two-dimensional array of gold nanotriangles inscribed onto glass coverslips were optimized for the surface-enhanced Raman detection of streptavidin/biotin monolayer assemblies. The nanostructures were fabricated by electron beam lithography, and its optical parameters were optimized to be probed under a Raman microscope with a linearly polarized He-Ne laser with an excitation wavelength of λ = 632.8 nm. The platforms were first tested against a monolayer of biotinylated alkanethiols (BAT) functionalized over the gold nanostructure, showing that good-quality spectra could be acquired with a short acquisition time. The supramolecular interaction of streptavidin (strep) with BAT showed subsequent modification of the Raman spectrum that implies a change in the secondary structure of the host biomolecule (streptavidin). Compared to gold surfaces without nanoscale structures, the local enhancement that results from our nanostructured surfaces allows one to detect the vibrational signal of monolayers within a time on the order of seconds and under modest laser intensity, further demonstrating the utility of using plasmonic metallic nanostructures for molecular recognition.

Tailoring plasmonic nanostructures for optimal SERS sensing of small molecules and large microorganisms

Local electric fields can be tuned dramatically by varying the diameter of quasi-3D gold plasmonic nanostructure arrays, as indicated by 3D finite-difference time-domain calculations. Utilizing quasi-3D arrays that exhibit a maximum electric field intensity (i.e., a "hot" spot) either at the bottom (gold nanodisks) or on the top (gold film patterned with nanoholes), the optimal surface-enhanced Raman scattering (SERS) sensitivity for the detection of small molecules or large microorganisms can be achieved. The precisely fabricated and optimized SERS-active quasi-3D nanostructure arrays make it possible to quantitatively and reproducibly detect chemical and biological species using SERS, leading to a new sensing platform with molecular specificity based on SERS for many important applications.

Surface-enhanced Raman scattering on gold quasi-3D nanostructure and 2D nanohole arrays

A new method was developed to fabricate unique gold quasi-3D plasmonic nanostructures on poly(dimethylsiloxane) PDMS and 2D nanohole arrays on silicon as surface-enhanced Raman scattering (SERS) substrates using electron beam lithography (EBL) with negative tone resist Ma-N 2403 and soft lithography. The size and shape of nanopillars fabricated by EBL were well controlled via different beam conditions. An enhancement factor (EF) as high as 6.4 x 10(5) was obtained for 4-mercaptopyridine molecules adsorbed on the gold quasi-3D nanostructure array on PDMS with 400 nm diameter, 100 nm spacing and 300 nm depth, while no enhancement was observed for the gold 2D nanohole array on silicon with the same diameter and spacing. The experimental results were confirmed by finite-difference time-domain (FDTD) calculations. Furthermore, the calculated total electric fields showed that the strong SERS exhibited by the gold quasi-3D nanostructure arrays on PDMS is due to the strong localized electric fields at the gold-air interface of the bottom gold nanodisc. The strong and reproducible SERS spectroscopy for molecules adsorbed on precisely controlled gold quasi-3D nanostructure arrays on PDMS makes it possible for the integration of SERS-active nanopatterns into microfluidic devices as chemical and biological sensors with molecular specificity.

Highly sensitive protein concentration assay over a wide range via surface-enhanced Raman scattering of Coomassie brilliant blue

In the Bradford protein assay, protein concentrations are determined by the absorbance at 595 nm due to the binding of Coomassie brilliant blue G-250 (CBBG) to proteins. In a protein-CBBG liquid mixture, surface-enhanced Raman scattering (SERS) is sensitive to the amount of unbound CBBG molecules adsorbed on silver surfaces, and the bound CBBG amount is directly related to the target protein concentration. Accordingly, a novel method for detecting total protein concentration in a solution has been developed based on SERS of unbound CBBG with an internal standard of silicon. Two obvious advantages of the proposed protein assay over conventional Bradford protein assay are its much wider linear concentration range (10(-5)-10(-9) g/mL) and 200 times lower limit of detection (1 ng/mL), which demonstrates its great potential in rapid, highly sensitive concentration determination of high and low-abundance proteins.

Molecular-level evidence of the surface-induced transformation of peptide structures revealed by scanning tunneling microscopy

Molecular structures of polypeptide molecules (DELERRIRELEARIK) adsorbed at liquid-solid interfaces of graphite have been studied using scanning tunneling microscopy (STM). The polypeptide is originally stable with an alpha-helical conformation in solution and in its crystal states. STM observations reveal that the adsorbed polypeptides form homogeneous beta-sheet-like assemblies on the graphite surface. The separation (4.7+/-0.1 A) between two neighboring polypeptides and the full lengths of the polypeptides determined from STM images suggest distinctively different molecular conformations from the alpha-helical structure. The N 1s peak in the X-ray photoelectron spectroscopy (XPS) spectrum confirmed the presence of polypeptides on the graphite surface. In addition, the circular dichroism (CD) results provide supporting evidence that the polypeptides would undergo a structural transformation to beta-sheet secondary structure upon the addition of graphite particles to the peptide solution. Such conformational rearrangements upon adsorption on a hydrophobic surface could benefit the studies on protein-surface interactions.

Characterization of the electron transfer of a ferrocene redox probe and a histidine-tagged hemoprotein specifically bound to a nitrilotriacetic-terminated self-assembled monolayer

We report the selective, controlled binding of a model redox probe, 1,1'-bis(N-imidazolylmethyl)ferrocene (Fc-Im2), and a small redox hemoprotein, histidine-tagged recombinant human neuroglobin (hNb), at the surface of metal electrodes (gold and SER-active silver) modified by a self-assembled monolayer (SAM) of a nitrilotriacetic (NTA)-terminated thiol. The resulting SAMs were characterized by cyclic voltammetry and surface-enhanced resonance Raman (SERR) spectroscopy coupled to electrochemistry. Once specifically bounded to the Ni(II)-NTA-modified gold electrode, nearly ideal cyclic voltammetric behavior with relatively fast electron-transfer (ET) communication through the SAM was determined for the Fc-Im2 redox probe. However, no direct electron transfer could be evidenced for the hNb redox protein under the same conditions. This outcome was different from the result obtained during SERR experiments coupled to electrochemistry in which a direct electrochemical conversion of hNb immobilized on a Ni(II)-NTA-modified SER-active Ag electrode was observed. The SERR spectra of the immobilized hNb was the same as the resonance Raman spectra of the protein in homogeneous solution, allowing us to conclude that the native structure of hNb was retained upon immobilization and that the direct ET was not the result of some partial or complete protein denaturation. The long-range ET rate constant (kET) through the SAM was determined by time-resolved SERR spectroscopy. A value of kET=0.12 s(-1) was obtained, which is within the predicted range of a fully nonadiabatic ET through a SAM thickness of approximately 26 A and close to the values previously determined for analogous small redox proteins at similar long-range ET distances. A SERR spectroelectrochemical titration of the immobilized hNb was also carried out, showing both an apparent standard potential (E0') negatively shifted by 100 mV compared with hNb in solution and a gentle slope in the titration curve. These results suggest a range of chemical environments in the surroundings of the redox protein and a variety of interactions with the NTA-terminated SAM. The influence of protein immobilization on E0' is discussed together with the long-range ET rate constant and molecular orientation of the surface-immobilized hNb.

Conversion of just-continuous metallic films to large particulate substrates for metal-enhanced fluorescence

We report the effects of thermally annealing, non-, just-, and thick continuous silver films for their potential applications in metal-enhanced fluorescence, a near-field concept which can alter the free-space absorption and emissive properties of close-proximity fluorophores (excited states). We have chosen to anneal a noncontinuous particulate film 5 nm thick and two thicker continuous films, 15 and 25 nm thick, respectively. Our results show that the annealing of the 25 nm film has little effect on close-proximity fluorescence when coated with a monolayer of fluorophore-labeled protein. However, the 15 nm continuous film cracks upon annealing, producing large nanoparticles which are ideal for enhancing the fluorescence of close-proximity fluorophores that are indeed difficult to prepare by other wet-chemical deposition processes. The annealing of 5 nm noncontinuous particulate films (a control sample) has little influence on metal-enhanced fluorescence, as expected.

Surface-enhanced Raman spectroscopy substrates created via electron beam lithography and nanotransfer printing

The development of quantitative, highly sensitive surface-enhanced Raman spectroscopy (SERS) substrates requires control over size, shape, and position of metal nanoparticles. Despite the fact that SERS has gained the reputation as an information-rich spectroscopy for detection of many classes of analytes, in some isolated instances down to the single molecule detection limit, its future development depends critically on techniques for nanofabrication. Herein, an unconventional nanofabrication approach is used to produce efficient SERS substrates. Metallic nanopatterns of silver disks are transferred from a stamp onto poly(dimethysiloxane) (PDMS) to create nanocomposite substrates with regular periodic morphologies. The stamp with periodic arrays of square, triangular, and elliptical pillars is created via electron beam lithography (EBL) of ma-N 2403 resist. A modified cyclodextrin is thermally evaporated onto the stamp to overcome the adhesive nature of the EBL resist and to function as a releasing layer. Subsequently, Ag is physically vapor deposited onto the stamp at a controlled rate and thickness and used directly for nanotransfer printing (nTP). Stamps, substrates, and the efficiency of the nTP process were explored by scanning electron microscopy. Transferred Ag nanodisk-PDMS substrates are studied by SERS using Rhodamine 6G as the probe analyte. There are observed optimal conditions involving both Ag and cyclodextrin thickness. The SERS response of metallic nanodisks of various shapes and sizes on the original stamp is compared to the corresponding nTP created substrates with similar trends observed. Limits of detection for crystal violet and Mitoxantrone are approximately 10(-8) and 10(-9) M, respectively. As an innovative feature of this approach, we demonstrate that physical manipulation of the PDMS post-nTP can be used to alter morphology, e.g., to change internanodisk spacing. Additionally, stamps are shown to be reusable after the nTP process, adding the potential to scale-up regular morphology substrates by a stamp-and-repeat methodology.