Positive microcontact printing with mercaptoalkyloligo(ethylene glycol)s

The soft lithographic replication of patterns with a low filling ratio by microcontact printing (microCP) is problematic due to the poor mechanical stability of common elastomeric stamps. A recently described strategy to avoid this problem employs a modified patterning method, positive microcontact printing ((+)microCP), in which a stamp with a mechanically more stable inverted relief pattern is used. In contrast to conventional negative microCP ((-)microCP), in the contact areas a self-assembled monolayer (SAM) is printed of a "positive ink", which provides only minor etch protection, whereas the noncontacted areas are subsequently covered with a different, etch-resistant SAM, prior to development by chemical etching. With the aim to identify novel, highly versatile positive inks, the patterning of gold by (+)microCP with mercaptoalkyloligo(ethylene glycol)s (MAOEGs), the subsequent adsorption of octadecanethiol (ODT), and the final development by wet chemical etching have now been studied. A polydisperse mixture of mercaptoundecylocta(ethylene glycol) derivatives was found to provide the best patterning results. The surface spreading of the positive ink during stamping, the exchange of printed MAOEGs with ODT, and the choice of the right etching bath were identified as key parameters that influence the achievable pattern resolution and contrast. Due to the modular composition of functionalized alkyloligo(ethylene glycol) derivatives, (+)microCP with these positive inks has the potential for easy adaptation to a variety of materials and development conditions.     

Covalent attachment of TAT peptides and thiolated alkyl molecules on GaAs surfaces

Four TAT peptide fragments were used to functionalize GaAs surfaces by adsorption from solution. In addition, two well-studied alkylthiols, mercaptohexadecanoic acid (MHA) and 1-octadecanethiol (ODT) were utilized as references to understand the structure of the TAT peptide monolayer on GaAs. The different sequences of TAT peptides were employed in recognition experiments where a synthetic RNA sequence was tested to verify the specific interaction with the TAT peptide. The modified GaAs surfaces were characterized by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS). AFM studies were used to compare the surface roughness before and after functionalization. XPS allowed us to characterize the chemical composition of the GaAs surface and conclude that the monolayers composed of different sequences of peptides have similar surface chemistries. Finally, FT-IRRAS experiments enabled us to deduce that the TAT peptide monolayers have a fairly ordered and densely packed alkyl chain structure. The recognition experiments showed preferred interaction of the RNA sequence toward peptides with high arginine content.     

Dip-pen刻蚀技术直接制造蛋白质纳米阵列

美国西北大学 Mirkin等 [1～ 3] 发明的 Dip- pen( DPN,译为蘸水笔技术 )纳米刻蚀技术是以 SPM的针尖为笔 ,通过超分子相互作用使被书写的分子或纳米材料粘在针尖上 ,以某种材料为基底 ,通过合理的超分子相互作用的设计将针尖上的分子或纳米材料书写到基底上 ,从而实现纳米刻蚀和纳米制造的目的 .很显然 ,这种技术对纳米器件、纳米传感器、高密度存储以及生物芯片的制造具有重要意义[4～ 7] .近年来 ,Mirkin研究组和其他几个研究集体利用这种技术成功地制造了有机分子纳米图形与阵列 [8] ;无机氧化物 [9]、金属纳米粒子 [10 ,11]、高分…     

Surface modification of elastomeric stamps for microcontact printing of polar inks

Chemical modification of the surface of a stamp used for microcontact printing (microCP) is interesting for controling the surface properties, such as the hydrophilicity. To print polar inks, plasma polymerization of allylamine (PPAA) was employed to render the surface of poly(dimethylsiloxane) (PDMS), polyolefin plastomers (POP), and Kraton elatomeric stamps hydrophilic for long periods of time. A thin PPAA film of about 5 nm was deposited on the stamps, which increased the hydrophilicity, and which remained stable for at least several months. These surface-modified stamps were used to transfer polar inks by microCP. The employed microCP schemes are as follows: (a) a second generation of dendritic ink having eight dialkyl sulfide end groups to fabricate patterns on gold substrates by positive microCP, (b) fluorescent guest molecules on beta-cyclodextrin (beta-CD) printboards on glass employing host-guest recognition, and (c) Lucifer Yellow ethylenediamine resulting in covalent patterning on an aldehyde-terminated glass surface. All experiments resulted in an excellent performance of all three PPAA-coated stamp materials to transfer the polar inks from the stamp surface to gold and glass substrates by microCP, even from aqueous solutions.     

Supramolecular microcontact printing and dip-pen nanolithography on molecular printboards

The transfer of functional molecules onto self-assembled monolayers (SAMs) by means of soft and scanning-probe lithographic techniques-microcontact printing (muCP) and dip-pen nanolithography (DPN), respectively-and the stability of the molecular patterns during competitive rinsing conditions were examined. A series of guests with different valencies were transferred onto beta-cyclodextrin- (beta-CD-) terminated SAMs and onto reference hydroxy-terminated SAMs. Although physical contact was sufficient to generate patterns on both types of SAMs, only molecular patterns of multivalent guests transferred onto the beta-CD SAMs were stable under the rinsing conditions that caused the removal of the same guests from the reference SAMs. The formation of kinetically stable molecular patterns by supramolecular DPN with a lateral resolution of 60 nm exemplifies the use of beta-CD-terminated SAMs as molecular printboards for the selective immobilization of printboard-compatible guests on the nanometer scale through the use of specific, multivalent supramolecular interactions. Electroless deposition of copper on the printboard was shown to occur selectively on the areas patterned with dendrimer-stabilized gold nanoparticles.     

Reactive microCP on ultrathin block copolymer films: investigation of the microCP mechanism and application to sub-microm (bio)molecular patterning

In this paper, the mechanism of the recently introduced soft lithographic patterning approach of reactive microcontact printing on thin substrate-supported polystyrene-block-poly(tert-butyl acrylate) (PS690-b-PtBA1210) films using trifluoroacetic acid (TFA)-inked elastomeric poly(dimethylsiloxane) (PDMS) stamps is investigated in detail. In this approach, solventless deprotection reactions are carried out with very high spatial definition using TFA as a volatile reagent that partitions into the PtBA skin layer. On the basis of a systematic investigation of the process, ink loading was identified as a crucial parameter for obtaining faithful pattern transfer. Using optimized conditions, submicrometer-sized patterns were successfully fabricated. In combination with subsequent wet chemical covalent coupling of various (bio)molecules, reactive microCP is established as an approach to afford positive, as well as negative, images of the features of the stamps used. In addition, the size of the patterned areas was manipulated by exploiting the controlled spreading of the ink; thus, stamps with identical features yield patterns with different sizes, yet identical periodicity, as shown for bovine serum albumin (BSA)-poly(ethylene glycol) patterns. The reactive microCP methodology affords new pathways for submicrometer-scale patterning of bioreactive surfaces.     

Elastomeric nanoparticle composites covalently bound to Al2O3/GaAs surfaces

This article reports the modification of Al2O3/GaAs surfaces with multifunctional soft materials. Siloxane elastomers were covalently bound to dopamine-modified Al2O3/GaAs semiconductor surfaces using MPt (M = Fe, Ni) nanoparticles. The sizes of the monodisperse FePt and NiPt nanoparticles were less than 5 nm. The surfaces of the nanoparticles as well as the Al2O3/GaAs substrates were modified with allyl-functionalized dopamine that utilized a dihydroxy group as a strong ligand. The immobilization of the elastomers was performed via a hydrosilation reaction of the allyl-functionalized dopamines with the siloxane backbones. X-ray photoelectron spectroscopy (XPS) experiments confirmed the covalent bonding of the siloxane elastomers to the oxide layer on the semiconductor surface. Fourier transform-infrared reflection absorption spectroscopy (FT-IRRAS) measurements revealed that the allyl functional groups are bonded to the siloxane backbones. The FT-IRRAS data also showed that the density of the allyl groups on the surface was lower than that of the siloxane backbones. The mechanical properties of the surface-bound nanocomposites were tested using nanoindentation experiments. The nanoindentation data showed that the soft matrix composed of the elastomeric coating on the surfaces behaves differently from the inner, hard Al2O3/GaAs substrate.     

Comparative height measurements of dip-pen nanolithography-produced lipid membrane stacks with atomic force, fluorescence, and surface-enhanced ellipsometric contrast microscopy

Dip-pen nanolithography (DPN) with phospholipids has been shown to be a powerful tool for the generation of biologically active surface patterns, but screening of the obtained lithographic structures is still a bottleneck in the quality control of the prepared samples. Here we performed a comparative study with atomic force microscopy (AFM), fluorescence microscopy (FM), and surface-enhanced ellipsometric contrast (SEEC) microscopy of phospholipid membrane stacks consisting of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) with high admixing of 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-[6-[(2,4-dinitrophenyl)amino]hexanoyl] (DNP Cap PE) produced by DPN. We present a structural model of membrane stacking based on the combined information gained from the three microscopic techniques. Domains of phase-separated DNP Cap PE can be detected at high DNP Cap PE admixing that are not present at medium or low admixings. While the optical methods allow for a high-throughput screening of lithographic structures (compared to AFM), it was found that, when relying on FM alone, artifacts due to phase-separation phenomena can be introduced in the case of thin membrane stacks.     

Mapping the interaction forces between TAR RNA and TAT peptides on GaAs surfaces using chemical force microscopy

The complexation of the HIV transactivation response element (TAR) RNA with the viral regulatory protein TAT is of enormous interest for the design of new sensing and therapeutic strategies. In this work, we anchored TAT peptides on GaAs surfaces using microcontact printing. Atomic force microscopy was used to quantify the interaction between TAR RNA and model TAT peptide sequences. Different pH conditions were utilized in order to assess specific vs nonspecific interactions. AFM tips functionalized with TAR RNA molecules were used to collect adhesion maps that displayed stronger interaction with peptide sequences that contained a greater number of arginine residues. All of the studies consistently showed a pH dependence of the interaction between the surface bound peptides and the TAR RNA on the AFM tips. This work quantifies the TAR RNA/TAT peptide interaction after one of the molecules is anchored on a surface. The conclusions in this paper are consistent with previous work and demonstrate that cationic residues are responsible for the polyelectrolyte-like affinity of TAT peptides for TAR RNA.     

Dip-pen刻蚀技术直接构建聚-L-赖氨酸纳米结构

Dip-pen纳米刻蚀技术(简称DPN技术)为在目标基底上沉积一个有序或连续的图案提供了一条简单而有效的途径,DPN技术是一种直接书写的扫描探针刻蚀技术,它使用原子力显微镜探针针尖,在一定的驱动力下,直接将化学试剂“墨水”转移到目标基底上．近年来,利用DPN技术已经成功地实现了多种“墨水一基底”组合。     

Creation of cadmium sulfide nanostructures using AFM dip-pen nanolithography

A dip-pen nanolithography (DPN) process capable of depositing nanoscaled structures of semiconducting CdS materials was developed by careful control of the reaction speed between the precursors. The new development expanded the scope of the powerful DPN process and provided more insight in the deposition mechanism. Features ranging from several hundreds of nanometers to sub-50 nanometers were generated and characterized. The effects of the surface property of the substrate, the relative humidity, the translating rate, and the temperature were systematically investigated. X-ray photoelectron spectroscopy (XPS) was used to verify the chemical composition of the patterns. In principle, this simple and convenient method should be applicable to deposit various metal sulfides on suitable substrates.     

PTFE超细颗粒的表面活化与化学接枝

采用钠萘络合物化学腐蚀液对聚四氟乙烯(PTFE)超细颗粒表面进行活化, 对活化后的表面用氨基十一酸碳链进行化学接枝, 并用IR和XPS技术对活化及接枝前后颗粒的表面结构和价键状态进行了表征. 结果表明：活化后的PTFE超细颗粒表面上存在羟基、羰基、羧基等活性官能团, 并出现炭化现象；氨基十一酸的氨基能与表面羟基发生缩合反应, 并接枝于PTFE超细颗粒表面.     

Dip-pen nanolithography of high-melting-temperature molecules

Direct nanopatterning of a number of high-melting-temperature molecules has been systematically investigated by dip-pen nanolithography (DPN). By tuning DPN experimental conditions, all of the high-melting-temperature molecules transported smoothly from the atomic force microscope (AFM) tip to the surface at room temperature without tip preheating. Water meniscus formation between the tip and substrate is found to play a critical role in patterning high-melting-temperature molecules. These results show that heating an AFM probe to a temperature above the ink's melting temperature is not a prerequisite for ink delivery, which extends the current "ink-substrate" combinations available to DPN users.     

Effect of chain length of self-assembled monolayers in dip-pen nanolithography using molecular dynamics simulations

The pattern transfer mechanism of an alkanethiol self-assembled monolayer (SAM) with different chain lengths during the dip-pen nanolithography (DPN) process and pattern characterizations are studied using molecular dynamics (MD) simulations. The mechanisms of molecular transference, alkanethiol meniscus characteristics, surface adsorbed energy, transfer number, and pattern formation are evaluated during the DPN process at room temperature. The simulation results clearly show that the molecular transfer ability in DPN is strongly dependent on the chain length. Shorter molecules have significantly better transport and diffusion abilities between the meniscus and substrate surface, and the transport period can be maintained longer. The magnitude of adsorbed energy increases with chain length, so many more molecules can be transferred to the surface when shorter molecules are used. After deposition, the magnitude of the adsorbed area and pattern height decrease with increasing chain length.     

Printing biomacromolecules on a bovine serum albumin precursor layer

Various biomacromolecules including proteins and polysaccharides are printed on a substrate capped with a bovine serum albumin (BSA) precursor layer to create clear co-patterns of these molecules. Characterizations by confocal laser scanning microscopy (CLSM) and atomic force microscopy (AFM) demonstrate the successful production and clear boundaries of the co-patterns. Rinsing the BSA-adsorbed substrate and the biomacromolecules-inked stamp before microcontact printing (microCP) is crucial for the creation of clear and stable co-patterns. The patterns are mainly stabilized by electrostatic interactions and van der Waals forces. Characterizations by ellipsometry, UV-Vis and fluorescence spectroscopy reveal that printing by a flat PDMS stamp yields a denser layered structure of proteins with a higher amount than that of adsorbed proteins. By printing, however, a lower enzymatic catalytic activity for horseradish peroxidase (HRP) or binding capability for avidin (both normalized to amount) is determined. A conformational transition from alpha-helix to beta-sheet of HRP is observed by ATR-IR. By contrast, a BSA precursor layer can effectively improve the functionality of the printed HRP or avidin and preserve the original conformation of the proteins, although the absolute transferred amount of these proteins is decreased.     

Experimental and theoretical study of the adsorption of fumaramide [2]rotaxane on Au(111) and Ag(111) surfaces

Thin films of fumaramide [2]rotaxane, a mechanically interlocked molecule composed of a macrocycle and a thread in a "bead and thread" configuration, were prepared by vapor deposition on both Ag(111) and Au(111) substrates. X-ray photoelectron spectroscopy (XPS) and high-resolution electron-energy-loss spectroscopy were used to characterize monolayer and bulklike multilayer films. XPS determination of the relative amounts of carbon, nitrogen, and oxygen indicates that the molecule adsorbs intact. On both metal surfaces, molecules in the first adsorbed layer show an additional component in the C 1s XPS line attributed to chemisorption via amide groups. Molecular-dynamics simulation indicates that the molecule orients two of its eight phenyl rings, one from the macrocycle and one from the thread, in a parallel bonding geometry with respect to the metal surfaces, leaving three amide groups very close to the substrate. In the case of fumaramide [2]rotaxane adsorption on Au(111), the presence of certain out-of-plane phenyl ring and Au-O vibrational modes points to such bonding and a preferential molecular orientation. The theoretical and experimental results imply that the three-dimensional intermolecular configuration permits chemisorption at low coverage to be driven by interactions between the three amide functions of fumaramide [2]rotaxane and the Ag(111) or Au(111) surface.     

Interaction of ester functional groups with aluminum oxide surfaces studied using infrared reflection absorption spectroscopy

The bonding of two types of ester group-containing molecules with a set of different oxide layers on aluminum has been investigated using infrared reflection absorption spectroscopy. The different oxide layers were made by giving typical surface treatments to the aluminum substrate. The purpose of the investigation was to find out what type of ester-oxide bond is formed and whether this is influenced by changes in the composition and chemistry of the oxide. The extent by which these bonded ester molecules resisted disbondment in water or substitution by molecules capable of chemisorption was also investigated. The ester groups were found to show hydrogen bonding with hydroxyls on the oxide surfaces through their carbonyl oxygens. For all oxides, the ester groups showed the same nu(C = O) carbonyl stretching vibration after adsorption, indicating very similar bonding occurs. However, the oxides showed differences in the amount of molecules bonded to the oxide surface, and a clear relation was observed with the hydroxyl concentration present on the oxide surface, which was determined from XPS measurements. The two compounds showed differences in the free to bonded nu(C = O) infrared peak shift, indicating differences in bonding strength with the oxide surface between the two types of molecules. The bonding of the ester groups with the oxide surfaces was found to be not stable in the presence of water and also not in the presence of a compound capable of chemisorption with the aluminum oxide surface.     

Direct and indirect causes of Fermi level pinning at the SiO/GaAs interface

The correlation between atomic bonding sites and the electronic structure of SiO on GaAs(001)-c(2x8)/(2x4) was investigated using scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and density functional theory (DFT). At low coverage, STM images reveal that SiO molecules bond Si end down; this is consistent with Si being undercoordinated and O being fully coordinated in molecular SiO. At approximately 5% ML (monolayer) coverage, multiple bonding geometries were observed. To confirm the site assignments from STM images, DFT calculations were used to estimate the total adsorption energies of the different bonding geometries as a function of SiO coverage. STS measurements indicated that SiO pins the Fermi level midgap at approximately 5% ML coverage. DFT calculations reveal that the direct causes of Fermi level pinning at the SiO GaAs(001)-(2x4) interface are a result of either local charge buildups or the generation of partially filled dangling bonds on Si atoms.     

Surface-enhanced Raman scattering interaction of p-aminobenzoic acid on a silver-coated alumina substrate

The adsorption behavior of p-aminobenzoic acid (PABA) molecules on a silver-coated alumina surface-enhanced Raman scattering (SERS) substrate was investigated. For spotted PABA and PABA in non-polar solvents, the PABA molecule is adsorbed flat on the surface of the SERS substrate. In this orientation, the benzene ring is π-bonded to the substrate, and the molecule is further anchored to the substrate by the binding of the lone pairs of NH₂ and COO⁻ groups onto the metal surface. On the other hand, the adsorption behavior of PABA in a polar solvent is greatly influenced by the hydrogen bonding of the amine group with the polar solvent. In this orientation, the molecule is preferentially adsorbed through the COO^± and assumes a non-flat orientation on the metal surface.     

Effect of stamp deformation on the quality of microcontact printing: theory and experiment

Microcontact printing (microCP) is an effective way to generate micrometer- or submicrometer-sized patterns on a variety of substrates. However, the fidelity of the final pattern depends critically on the coupled phenomena of stamp deformation, fluid transfer between surfaces, and the ability of the ink to self-assemble on the substrate. In particular, stamp deformation can produce undesirable effects that limit the practice and precision of microCP. Experimental observations and comparison with theoretical predictions are presented here for three of the most undesirable consequences of stamp deformation: (1) roof collapse of low aspect ratio recesses, (2) buckling of high aspect ratio plates, and (3) lateral sticking of high aspect ratio plates. Stamp behavior was observed visually with an inverted optical microscope while load-displacement data were collected during compression and retraction of stamps. Additionally, a "robotic stamper" was used to deliver ink patterns in precise locations on substrates. These monomolecular ink patterns were then observed in high contrast using the surface potential scanning mode of an atomic force microscope. Theoretical models based on continuum mechanics were used to accurately predict both physical deformation of the stamp and the resultant inking patterns. The close agreement between these models and the experimental data presented clearly demonstrates the essential considerations one must weigh when designing stamp geometry, material, and loading conditions for optimal pattern fidelity.