Mass producible, robust SERS substrates based on metal film on nanosphere (MFON) on an adhesive substrate for detection of surface-adsorbed molecules and their evaluation by helium ion microscopy

We have developed a SERS stamp that can be pressed directly onto a solid surface for characterization of surface-adsorbed target molecules. The stamp was fabricated by transfer of a dense monolayer of SiO₂ nanospheres from a glass surface onto a piece of adhesive tape and subsequent evaporation of silver. The performance of the resulting SERS stamps was evaluated by their exposure to methyl mercaptan vapor, and immersion in rhodamine 6G and ferbam solutions. It was found that beside the nanosphere diameter and metal deposition thickness, the extent of burial of the nanospheres into the adhesive tape, dictated by the pressure during the nanosphere transfer process, had a significant effect. We carried out FDTD calculations of the near field. Models are based on morphological information obtained from helium ion microscopy, which can provide high-resolution images of poor electrical conductors such as our SERS stamp. While one of our main eventual goals is detection of pesticides on agricultural produce, we have begun to take a careful step by testing our SERS stamp on better characterized surfaces such as a porous gel surface, having been immersed in fungicides such as ferbam. We also present our preliminary results with ferbam on oranges. It is expected that our well-characterized SERS stamp will play a role in shedding light on the poorly studied transfer process of target molecules onto a SERS surface as well as serving as a new SERS platform.

Vapor Phase Exchange of Self-Assembled Monolayers for Engineering of Biofunctional Surfaces

We show that 4'-nitro-1,1'-biphenyl-4-thiol self-assembled monolayers (NBPT SAMs) on gold can be exchanged with 11-(mercaptoundecyl)triethylene glycol (C₁₁EG₃OH) SAMs via vapor deposition (VD). The pristine and the exchanged SAMs obtained by VD as well as solution method (SM) were characterized by X-ray photoelectron spectroscopy (XPS) and polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS). Using surface plasmon resonance (SPR), it is shown that C₁₁EG₃OH SAMs on gold obtained by vapor deposition exchange (VD_Ex) have the same protein resistivity as SAMs obtained by the direct self-assembly process. As expected, the cross-linked NBPT SAM are found to be resistive to both exchange processes, VD_Ex and solution method exchange (SM_Ex). In this way, VD_Ex opens up an elegant and new approach of patterning SAM surfaces in situ at vacuum conditions without using any solvents. By combining electron irradiation-induced chemical lithography of NBPT SAMs with VD_Ex, biofunctional patterned substrates were engineered and used for immobilization of protein arrays.

Formation and structure of copper(II) oxalate layers on carboxy-terminated self-assembled monolayers

Copper(II) oxalate was grown on carboxy-terminated self-assembled monolayers using a step-by-step approach by dipping the surfaces alternately in ethanolic solutions of copper(II) acetate and oxalic acid with intermediate thorough rinsing steps. The deposition was monitored by reflection absorption infrared spectroscopy (RAIRS), a quartz microbalance with dissipation measurement (QCM-D), scanning electron microscopy (SEM), and helium ion microscopy (HIM). Amounts of material corresponding to a coverage of 75% of a monolayer are deposited in each dipping step in copper(II) acetate solution while deposition of oxalic acid produces a viscoelastic layer that is partially removed by rinsing. This points toward initial aggregation but acid not bound to Cu(2+) ions as oxalate ions is removed by the rinsing steps. RAIRS further indicates that the material grows as copper(II) oxalate ribbons similar to the crystal structure but with ribbons oriented roughly parallel to the surface. SEM and HIM give evidence of the formation of needle-shaped structures which are a possible explanation for the viscoelastic behavior of the layer.

Focused electron beam induced processing and the effect of substrate thickness revisited

The current understanding in the study of focused electron beam induced processing (FEBIP) is that the growth of a deposit is mainly the result of secondary electrons (SEs). This suggests that the growth rate for FEBIP is affected by the SE emission from the support. Our experiments, with membranes thinner than the SE escape depth, confirm this hypothesis. We used membranes of 1.4 and 4.3 nm amorphous carbon as supports. At the very early stage, the growth is support-dominated and the growth rate on a 4.3 nm thick membrane is three times higher than on a 1.4 nm thick membrane. This is consistent with Monte Carlo simulations for SE emission. The results suggest that SEs are dominant in the dissociation of W(CO)6 on thin membranes. The best agreement between simulations and experiment is obtained for SEs with energies between 3 and 6 eV.With this work we revisit earlier experiments, working at a precursor pressure 20 times lower than previously. Then, despite using membranes thinner than the SE escape depth, we did not see an effect on the experimental growth rate. We explain our current results by the fact that very early in the process, the growth becomes dominated by the growing deposit itself.

Structural investigation of 1,1'-biphenyl-4-thiol self-assembled monolayers on Au(111) by scanning tunneling microscopy and low-energy electron diffraction

Self-assembled monolayers (SAMs) of 1,1'-biphenyl-4-thiol (H-(C(6)H(4))(2)-SH) on Au(111) were prepared from solution or via vapor deposition in ultrahigh vacuum and characterized by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS). In contrast to the typically observed for densely packed alkane-thiol SAMs on Au(111) (√3 × √3)R30° structure, the densely packed aromatic biphenylthiol SAMs prepared by both methods exhibit an unusual hexagonal (2 × 2) structure. Upon annealing at 100 °C, this structure evolves into the (2 × 7√3) structure resulting in the formation of highly ordered pinstripes oriented along the [1 -1 0] directions. Lower density SAMs, prepared by vapor deposition in vacuum, show mixed structures comprising the hexagonal (2 × 2) structure and two rectangular arrangements with the unit cells of (3√3 × 9) and (2√3 × 8). An extinction of the (3√3 × 9) structure in the favor of the (2√3 × 8) structure is observed upon annealing at temperatures of ~100 °C.

Functionalization of a self-assembled monolayer driven by low-energy electron exposure

Self-assembled monolayers (SAMs) of 10-undecene-1-thiol on Au were functionalized with nitrogen-containing groups using an approach in which multilayer ammonia (NH(3)) films were deposited at low temperature onto the SAMs and subsequently exposed to 15 eV electrons. The result of this process was investigated after removal of the remaining NH(3) by annealing to room temperature using high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). HREELS shows that the CC double bonds disappear during electron exposure, while XPS gives evidence that about 25% of the terminal double bonds of the SAM were functionalized. Also, XPS shows that a sufficiently thick NH(3) layer protects the underlying SAM from electron-induced damage. The process suggested here thus represents a particularly gentle approach to the functionalization of ultrathin molecular layers. Thermal desorption spectrometry (TDS) and electron-stimulated desorption (ESD) experiments on condensed layers of NH(3) reveal production of N(2) but show that significant amounts of the initial NH(3) as well as N(2) produced during electron exposure desorb. Hydrogen released upon formation of N(2) is held responsible for the reduction of double bonds and protection of the SAMs from damage.

Efficient animal-serum free 3D cultivation method for adult human neural crest-derived stem cell therapeutics

Due to their broad differentiation potential and their persistence into adulthood, human neural crest-derived stem cells (NCSCs) harbour great potential for autologous cellular therapies, which include the treatment of neurodegenerative diseases and replacement of complex tissues containing various cell types, as in the case of musculoskeletal injuries. The use of serum-free approaches often results in insufficient proliferation of stem cells and foetal calf serum implicates the use of xenogenic medium components. Thus, there is much need for alternative cultivation strategies. In this study we describe for the first time a novel, human blood plasma based semi-solid medium for cultivation of human NCSCs. We cultivated human neural crest-derived inferior turbinate stem cells (ITSCs) within a blood plasma matrix, where they revealed higher proliferation rates compared to a standard serum-free approach. Three-dimensionality of the matrix was investigated using helium ion microscopy. ITSCs grew within the matrix as revealed by laser scanning microscopy. Genetic stability and maintenance of stemness characteristics were assured in 3D cultivated ITSCs, as demonstrated by unchanged expression profile and the capability for self-renewal. ITSCs pre-cultivated in the 3D matrix differentiated efficiently into ectodermal and mesodermal cell types, particularly including osteogenic cell types. Furthermore, ITSCs cultivated as described here could be easily infected with lentiviruses directly in substrate for potential tracing or gene therapeutic approaches. Taken together, the use of human blood plasma as an additive for a completely defined medium points towards a personalisable and autologous cultivation of human neural crest-derived stem cells under clinical grade conditions.

X-ray holographic microscopy with zone plates applied to biological samples in the water window using 3rd harmonic radiation from the free-electron laser FLASH

The imaging of hydrated biological samples - especially in the energy window of 284-540 eV, where water does not obscure the signal of soft organic matter and biologically relevant elements - is of tremendous interest for life sciences. Free-electron lasers can provide highly intense and coherent pulses, which allow single pulse imaging to overcome resolution limits set by radiation damage. One current challenge is to match both the desired energy and the intensity of the light source. We present the first images of dehydrated biological material acquired with 3rd harmonic radiation from FLASH by digital in-line zone plate holography as one step towards the vision of imaging hydrated biological material with photons in the water window. We also demonstrate the first application of ultrathin molecular sheets as suitable substrates for future free-electron laser experiments with biological samples in the form of a rat fibroblast cell and marine biofouling bacteria Cobetia marina.

Ultrahigh resolution focused electron beam induced processing: the effect of substrate thickness

It is often suggested that the growth in focused electron beam induced processing (FEBIP) is caused not only by primary electrons, but also (and even predominantly) by secondary electrons (SEs). If that is true, the growth rate for FEBIP can be changed by modifying the SE yield. Results from our Monte Carlo simulations show that the SE yield changes strongly with substrate thickness for thicknesses below the SE escape depth. However, our experimental results show that the growth rate is independent of the substrate thickness. Deposits with an average size of about 3 nm were written on 1 and 9 nm thick carbon substrates. The apparent contradiction between simulation and experiment is explained by simulating the SE emission from a carbon substrate with platinum deposits on the surface. It appears that the SE emission is dominated by the deposits rather than the carbon substrate, even for deposits as small as 0.32 nm(3).

On the difficulties in characterizing ZnO nanowires

The electrical properties of single ZnO nanowires grown by vapor phase transport were investigated. While some samples were contacted by Ti/Au electrodes, another set of samples was investigated using a manipulator tip in a low energy electron point-source microscope. The deduced resistivities range from 1 to 10(3) Ωcm. Additionally, the resistivities of nanowires from multiple publications were brought together and compared to the values obtained from our measurements. The overview of all data shows enormous differences (10(-3)-10(5) Ωcm) in the measured resistivities. In order to reveal the origin of the discrepancies, the influence of growth parameters, measuring methods, contact resistances, crystal structures and ambient conditions are investigated and discussed in detail.

X-ray Absorption Spectromicroscopy Studies for the Development of Lithography with a Monomolecular Resist

Soft X-ray absorption microscopy was applied to image and characterize molecular patterns produced by electron irradiation of aliphatic and aromatic thiol-derived self-assembled monolayers (SAMs) on Au substrates. The measurements were performed at all relevant absorption edges. The fabricated patterns could be clearly imaged with a lateral resolution better than 150 nm, which, for example, allowed us to distinguish a fine structure of 1 microm features. The X-ray absorption microspot spectra derived from different areas of the SAM patterns provided specific chemical information on pristine and irradiated areas and unexpected features in these patterns. The quality of the microspot spectra is comparable with that of the analogous X-ray absorption spectra acquired with standard equipment from homogeneous SAMs. In particular, a chemical transformation of the functional tail groups within the irradiated areas of the patterned aromatic SAMs could be directly monitored.

Controlled three-dimensional immobilization of biomolecules on chemically patterned surfaces

We used electron-beam lithography to fabricate chemical nanostructures, i.e. amino groups in aromatic self-assembled monolayers (SAMs) on gold surfaces. The amino groups are utilized as reactive species for mild covalent attachment of fluorescently labeled proteins. Since non-radiative energy transfer results in strong quenching of fluorescent dyes in the vicinity of the metal surfaces, different labeling strategies were investigated. Spacers of varying length were introduced between the gold surface and the fluorescently labeled proteins. First, streptavidin was directly coupled to the amino groups of the SAMs via a glutaraldehyde linker and fluorescently labeled biotin (X-Biotin) was added, resulting in a distance of approximately 2 nm between the dyes and the surface. Scanning confocal fluorescence images show that efficient energy transfer from the dye to the surface occurs, which is reflected in poor signal-to-background (S/B) ratios of approximately 1. Coupling of a second streptavidin layer increases the S/B-ratio only slightly to approximately 2. The S/B-ratio of the fluorescence signals could be further increased to approximately 4 by coupling of an additional fluorescently labeled antibody layer. Finally, we introduced tetraethylenepentamine as functional spacer molecule to diminish fluorescence quenching by the surface. We demonstrate that the use of this spacer in combination with multiple antibody layers enables the controlled fabrication of highly fluorescent three-dimensional nanostructures with S/B-ratios of >20. The presented technique might be used advantageously for the controlled three-dimensional immobilization of single protein or DNA molecules and the well-defined assembly of protein complexes.

Optimization of the low energy electron point source microscope: imaging of macromolecules

Optimal conditions for low energy electron point source microscopy are investigated by the simulation and numerical reconstruction of holograms of phthalocyaninato polysiloxane, PcPS, a rod-like macromolecule. The effects of the electron energy, width of the electron beam and the detector size on the spatial resolution in the reconstructed images are modeled. We find that for electron energies around 200eV, with the specimen 0.1 microm from the source, a screen recording the image in a cone of at least 15 degrees half angle (7 cm lateral dimension at 10 cm from the source) with at least 512 x 512 pixel and 8-bit resolution will result in near atomic resolution.