Conductive AFM patterning on oligo(ethylene glycol)-terminated alkyl monolayers on silicon substrates: proposed mechanism and fabrication of avidin patterns

Anisotropic Effects in Local Anodic Oxidation Nanolithography on Silicon Surfaces: Insights from ReaxFF Molecular Dynamics

Fully understanding the anisotropic effect of silicon surface orientations in local anodic oxidation (LAO) nanolithography processes is critical to the precise control of oxide quality and rate. This study used ReaxFF MD simulations to reveal the surface anisotropic effects in the LAO through the analysis of adsorbed species, atomic charge, and oxide growth. Our results show that the LAO behaves differently on silicon (100), (110), and (111) surfaces. Specifically, the application of an electric field significantly increases the quantity of surface-adsorbed -OH2 while reducing -OH on the (111) surface, and results in a higher charge on a greater number of Si atoms on the (100) surface. Moreover, the quantity of surface-adsorbed -OH plays a pivotal role in influencing the oxidation rate, as it directly correlates with an increased formation rate of Si-O-Si bonds. During bias-induced oxidation, the (111) surface appears with a high initial oxidation rate among three surfaces, while the (110) surface underwent increased oxidation at higher electric field strengths. This conclusion is based on the analysis of the evolution of Si-O-Si bond number, surface elevation, and oxide thickness. Our findings align well with prior theoretical and experimental studies, providing deeper insights and clear guidance for the fabrication of high-performance nanoinsulator gates using LAO nanolithography.

Lab on a tip: Applications of functional atomic force microscopy for the study of electrical properties in biology

Electrical properties, such as charge propagation, dielectrics, surface potentials, conductivity, and piezoelectricity, play crucial roles in biomolecules, biomembranes, cells, tissues, and other biological samples. However, characterizing these electrical properties in delicate biosamples is challenging. Atomic Force Microscopy (AFM), the so called "Lab on a Tip" is a powerful and multifunctional approach to quantitatively study the electrical properties of biological samples at the nanometer level. Herein, the principles, theories, and achievements of various modes of AFM in this area have been reviewed and summarized. STATEMENT OF SIGNIFICANCE: Electrical properties such as dielectric and piezoelectric forces, charge propagation behaviors play important structural and functional roles in biosystems from the single molecule level, to cells and tissues. Atomic force microscopy (AFM) has emerged as an ideal toolkit to study electrical property of biology. Herein, the basic principles of AFM are described. We then discuss the multiple modes of AFM to study the electrical properties of biological samples, including Electrostatic Force Microscopy (EFM), Kelvin Probe Force Microscopy (KPFM), Conductive Atomic Force Microscopy (CAFM), Piezoresponse Force Microscopy (PFM) and Scanning ElectroChemical Microscopy (SECM). Finally, the outlook, prospects, and challenges of the various AFM modes when studying the electrical behaviour of the samples are discussed.

Interfacial Electron Beam Lithography: Chemical Monolayer Nanopatterning via Electron-Beam-Induced Interfacial Solid-Phase Oxidation

Chemical nanopatterning-the deliberate nanoscale modification of the chemical nature of a solid surface-is conveniently realized using organic monolayer coatings to impart well-defined chemical functionalities to selected surface regions of the coated solid. Most monolayer patterning methods, however, exploit destructive processes that introduce topographic as well as other undesired structural and chemical transformations along with the desired surface chemical modification. In particular in electron beam lithography (EBL), organic monolayers have been used mainly as ultrathin resists capable of improving the resolution of patterning via local deposition or removal of material. On the basis of the recent discovery of a class of radiation-induced interfacial chemical transformations confined to the contact surface between two solids, we have advanced a direct, nondestructive EBL approach to chemical nanopatterning-interfacial electron beam lithography (IEBL)-demonstrated here by the e-beam-induced local oxidation of the -CH₃ surface moieties of a highly ordered self-assembled n-alkylsilane monolayer to -COOH while fully preserving the monolayer structural integrity and molecular organization. In this conceptually different EBL process, the traditional resist is replaced by a thin film coating that acts as a site-activated reagent/catalyst in the chemical modification of the coated surface, here the top surface of the to-be-patterned monolayer. Structural and chemical transformations induced in the thin film coating and the underlying monolayer upon exposure to the electron beam were elucidated using a semiquantitative surface characterization methodology that combines multimode AFM imaging with postpatterning surface chemical modifications and quantitative micro-FTIR measurements. IEBL offers attractive opportunities in chemical nanopatterning, for example, by enabling the application of the advanced EBL technology to the straightforward nanoscale functionalization of the simplest commonly used organosilane monolayers.

Orthogonal thiol-ene 'click' reactions: a powerful combination for fabrication and functionalization of patterned hydrogels

A combination of 'orthogonal' thiol-ene 'click' reactions is utilized for fabrication and functionalization of micro-patterned hydrogels. A furan-protected maleimide-containing parent copolymer is partially activated via the retro Diels-Alder reaction to obtain an 'orthogonally' functionalizable copolymer, where the different functional groups can be exploited for multi-functionalization or fabrication of functional hydrogels using combination of the nucleophilic and radical thiol-ene reactions.

Spatially selective binding of green fluorescent protein on designed organosilane nanopatterns prepared with particle lithography

A practical approach for preparing protein nanopatterns has been to design surface templates of nanopatterns of alkanethiols or organosilanes that will selectively bind and localize the placement of biomolecules. Particle lithography provides a way to prepare millions of protein nanopatterns with a few basic steps. For our nanopatterning strategy, organosilanes with methoxy and sulfhydryl groups were chosen as a surface template. Green fluorescent protein (GFP) was selected as a model for patterning. Areas of 2-[methoxy (polyethyleneoxy)6-9propyl]trichlorosilane (MPT-silane) are effective as a matrix for resisting the attachment of proteins, whereas nanopatterns with sulfur groups provide reactive sites for binding linker groups to connect proteins. A protocol with particle lithography was designed to make a surface template of nanopatterns of (3-mercaptopropyl)trimethoxysilane (MPTMS) surrounded by a methoxy terminated matrix. The sulfhydryl groups of the MPTMS nanopatterns were activated with a sulfosuccinimidyl-4-(N-maleimidomethyl)cyclohexane-1-carboxylate linker. The activated regions of MPTMS furnished sites for binding GFP. Samples were characterized with atomic force microscopy after successive steps of the patterning protocol to evaluate the selectivity of protein binding. Direct views of the protein bound selectively to designated sites of MPTMS are presented, as evidence of robust and reproducible patterning. Nanoscale patterns of proteins can be used for surfaces of biochips and biosensors, and also for immunochemistry test platforms.

Advanced oxidation scanning probe lithography

Force microscopy enables a variety of approaches to manipulate and/or modify surfaces. Few of those methods have evolved into advanced probe-based lithographies. Oxidation scanning probe lithography (o-SPL) is the only lithography that enables the direct and resist-less nanoscale patterning of a large variety of materials, from metals to semiconductors; from self-assembled monolayers to biomolecules. Oxidation SPL has also been applied to develop sophisticated electronic and nanomechanical devices such as quantum dots, quantum point contacts, nanowire transistors or mechanical resonators. Here, we review the principles, instrumentation aspects and some device applications of o-SPL. Our focus is to provide a balanced view of the method that introduces the key steps in its evolution, provides some detailed explanations on its fundamentals and presents current trends and applications. To illustrate the capabilities and potential of o-SPL as an alternative lithography we have favored the most recent and updated contributions in nanopatterning and device fabrication.

Preparation, characterization, and protein-resistance of films derived from a series of α-oligo(ethylene glycol)-ω-alkenes on H–Si(111) surfaces

Films on Si(111) were prepared by photo-activated grafting of CH₂&#xe001;CH(CH₂)m(OCH₂CH₂)nOCH₃ (m = 8, 9; n = 3–7) by using different vacuum conditions. High vacuum produced a higher thickness (40 Å) and <0.8% fibrinogen adsorption (C₁₀EG₇). Films were stable even after 28 days.

Recent development of biotin conjugation in biological imaging, sensing, and target delivery

Despite encouraging results from preliminary studies of anticancer therapies, the lack of tumor specificity remains an important issue in the modern pharmaceutical industry. New findings indicate that biotin or biotin-conjugates could be favorably assimilated by tumor cells that over-express biotin-selective transporters. Furthermore, biotin can form stable complexes with avidin and its bacterial counterpart streptavidin. The strong bridging between avidin and biotin moieties on other molecules is a proven adaptable tool with broad biological applications. Under these circumstances, a biotin moiety is certainly an attractive choice for live-cell imaging, biosensing, and target delivery.

Photothermal laser fabrication of micro- and nanostructured chemical templates for directed protein immobilization

Photothermal patterning of poly(ethylene glycol) terminated organic monolayers on surface-oxidized silicon substrates is carried out using a microfocused beam of a CW laser operated at a wavelength of 532 nm. Trichlorosilane and trimethoxysilane precursors are used for coating. Monolayers from trimethoxysilane precursors show negligible unspecific protein adsorption in the background, i.e., provide platforms of superior protein repellency. Laser patterning results in decomposition of the monolayers and yields chemical templates for directed immobilization of proteins at predefined positions. Characterization is carried out via complementary analytical methods including fluorescence microscopy, atomic force microscopy, and scanning electron microscopy. Appropriate labeling techniques (fluorescent markers and gold clusters) and substrates (native and thermally oxidized silicon substrates) are chosen in order to facilitate identification of protein adsorption and ensure high sensitivity and selectivity. Variation of the laser parameters at a 1/e(2) spot diameter of 2.8 μm allows for fabrication of protein binding domains with diameters on the micrometer and nanometer length scale. Minimum domain sizes are about 300 nm. In addition to unspecific protein adsorption on as-patterned monolayers, biotin-streptavidin coupling chemistry is exploited for specific protein binding. This approach represents a novel facile laser-based means for fabrication of protein micro- and nanopatterns. The routine is readily applicable to femtosecond laser processing of glass substrates for the fabrication of transparent templates.

Spatially selective surface platforms for binding fibrinogen prepared by particle lithography with organosilanes

We introduce an approach based on particle lithography to prepare spatially selective surface platforms of organosilanes that are suitable for nanoscale studies of protein binding. Particle lithography was applied for patterning fibrinogen, a plasma protein that has a major role in the clotting cascade for blood coagulation and wound healing. Surface nanopatterns of mercaptosilanes were designed as sites for the attachment of fibrinogen within a protein-resistant matrix of 2-[methoxy(polyethyleneoxy)propyl] trichlorosilane (PEG-silane). Preparing site-selective surfaces was problematic in our studies, because of the self-reactive properties of PEG-organosilanes. Certain organosilanes presenting hydroxyl head groups will cross react to form mixed surface multi-layers. We developed a clever strategy with particle lithography using masks of silica mesospheres to protect small, discrete regions of the surface from cross reactions. Images acquired with atomic force microscopy (AFM) disclose that fibrinogen attached primarily to the surface areas presenting thiol head groups, which were surrounded by PEG-silane. The activity for binding anti-fibrinogen was further evaluated using ex situ AFM studies, confirming that after immobilization the fibrinogen nanopatterns retained capacity for binding immunoglobulin G. Studies with AFM provide advantages of achieving nanoscale resolution for detecting surface changes during steps of biochemical surface reactions, without requiring chemical modification of proteins or fluorescent labels.

Immobilization of proteins on carboxylic acid functionalized nanopatterns

The immobilization of proteins on nanopatterned surfaces was investigated using in situ atomic force microscopy (AFM) and ex situ infrared reflectance-absorption spectroscopy (IRAS). The AFM-based lithography technique of nanografting provided control of the size, geometry, and spatial placement of nanopatterns within self-assembled monolayers (SAMs). Square nanopatterns of carboxylate-terminated SAMs were inscribed within methyl-terminated octadecanethiolate SAMs and activated using carbodiimide/succinimide coupling chemistry. Staphylococcal protein A was immobilized on the activated nanopatterns before exposure to rabbit immunoglobulin G. In situ AFM was used to monitor changes in the topography and friction of the nanopatterns in solution upon protein immobilization. Complementary studies with ex situ IRAS confirmed the surface chemistry that occurred during the steps of SAM activation and subsequent protein immobilization on unpatterned samples. Since carbodiimide/succinimide coupling chemistry can be used for surface attachment of different biomolecules, this protocol shows promise for development of other aqueous-based studies for nanopatterned protein immobilization.