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Silanization of silicon and mica is the coating of these materials with a thin layer of self assembling units.
Nanoscale analysis of proteins using atomic force microscopy (AFM) requires surfaces with well-defined topologies and chemistries for many experimental techniques. Biomolecules, particularly proteins, can be immobilized simply on an unmodified substrate surface through hydrophobic or electrostatic interactions. [1] However, several problems are associated with physical adsorption of proteins on surfaces. With metal surfaces, protein denaturation, unstable and reversible binding, nonspecific and random immobilization of protein have been reported. [2]
One alternative involves the interaction of chemically modified surfaces with proteins under non-denaturing circumstances. [2] Chemical modification of surfaces provides the potential to precisely control the chemistry of the surface, and with the correct chemical modifications, there are several advantages to this approach. First, the proteins adsorbed on the surface are more stable over a wide range of conditions. The proteins also adopt a more uniform orientation on the surface. Additionally, the higher density of protein deposition with greater reproducibility is possible.
Chemical modification of surfaces has been successfully applied in several instances to immobilize biomolecules in order to obtain valuable information. For instance, atomic force microscopy imaging of DNA has been performed using mica coated with 3-aminopropyltriethoxysilane (APTES). The negatively charged DNA backbone bound strongly to the positive charges on the amine functionality, leading to stable structures that could be imaged both in air and in buffer. [3] In a recent study by Behrens et al., amine-terminated silicon surfaces were successfully used to immobilize bone morphogenetic protein 2 (BMP2) for medical purposes (cf. hydrogen-terminated silicon surface) [4] . Molecules with amine groups (especially APTES) are important for biological applications, because they allow for simple electrostatic interactions with biomolecules. [5]
Self-assembled monolayers (SAM) are an extremely versatile approach that allows for precise control of surface characteristics. It was introduced in 1946 by Bigelow et al., [6] but it was not until 1983 that it attracted widespread interest, when the formation of SAMs of alkanethiolates on gold was reported by Allara et al. [7] Self-assembly of monolayers can be achieved using several systems. The basis for self-assembly is the formation of a covalent bond between the surface and the molecule forming the layer; and this requirement can be fulfilled using a variety of chemical groups such as organosilanes at hydroxylated materials (glass, silicon, aluminium oxide, mica) and organosulfur-based compounds species at noble metals . [7] [8] [9] While the latter system has been well characterized, much less is known about the behavior of organosilane layers on surfaces and the underlying mechanisms that control monolayer organization and structure.
Although silanization of silicate surfaces was introduced more than 40 years ago, the process of formation of smooth layers on surfaces is still poorly understood. Probably the most important reason for this situation is that a number of studies that have involved silanization as part of the procedure have not been concerned with thoroughly characterizing the silane layer formed. The one result that unifies recent studies on the characterization of silane layers is centered on the extreme sensitivity of the reactions that lead to the formation of silane layers. [9] Indeed, self-assembled layers of silanes on silicate surfaces have been reported to be dependent on various parameters such as humidity, temperature, impurities in the silane reagent and the type of silicate surface. In order to consistently and reproducibly make diverse functionalized surfaces with layers that are molecularly smooth, it is critical to understand the chemistry of the silicate surfaces and the ways in which various parameters affect the nature of the self-assembled layers.
Oxidized silicon has been extensively studied as a substrate for the deposition of biomolecules. Piranha solution can be used to increase the surface density of reactive hydroxyl groups on the surface of silicon. The –OH groups can hydrolyze and subsequently form siloxane linkages (Si-O-Si) with organic silane molecules. Preparation of silicon surfaces for silanization involves the removal of surface contaminants. This can be achieved by using UV-ozone and piranha solution. Piranha solution in particular constitutes quite a harsh treatment that can potentially damage the integrity of the silicon surface. Finlayson-Pitts et al. investigated the effect of certain treatments on silicon and concluded that both the roughness (3-5 Å) and the presence of scattered large particles were preserved after 1 cycle of plasma-treatment. [10] However, the silicon surface was significantly damaged after 30 cycles of treatment with piranha solution or plasma. In both cases, treatment introduced irregularities and large aggregates on the surface (aggregate size > 80 nm), with the effect being more pronounced when piranha was used. In either case, multiple treatments rendered the surface inadequate for deposition of small biomolecules.
Mica is another silicate that is widely used as substrate for the deposition of biomolecules. Mica bears a noticeable advantage over silicon because it is molecularly smooth and hence better suited for studies of small, flat molecules. [11] It has a crystalline structure with generic formula K[Si3Al]O10Al2(OH)2 and contains sheets of octahedral hydroxyl-aluminum sandwiched between two silicon tetrahedral layers. [12] In the silicon layer, one in four silicon atoms is replaced by an aluminum atom, generating a difference in charge that is offset by unbound K+ present in the region between neighboring silicon layers. [12] Muscovite mica is most susceptible to cleavage along the plane located in the potassium layer. When a freshly cleaved mica surface is placed in contact with water, hydrated potassium ions can desorb from the mica surface, leading to a negative charge at the surface.
Similar to silicon, the surface of mica does not contain an appreciable density of silanol groups for covalent attachment by silanes. [11] A recent study reported that freshly cleaved mica carries 11% silanol groups (i.e., approximately 1 in 10 silicon atoms bears a hydroxyl group). [12] Although it is possible that silanization may be carried out using untreated mica, the increased density of surface silanol groups on activated mica can significantly improve covalent attachment of silane molecules to the surface. Mica can be activated by treatment with argon/water plasma, leading to a silanol surface density of 30%. [12] [13] Working with activated surfaces introduces another consideration about the stability of the silanol groups on the activated surfaces. Giasson et al. reported that the silanol groups on freshly cleaved mica that was not subjected to any treatment were found to be more stable under high vacuum compared to the plasma-activated mica: after 64 hrs, surface coverage of the silanol groups for freshly cleaved mica plasma was roughly the same, while surface coverage for activated mica decreased 3-fold to 10%. [12]
Adsorption describes the process by which molecules or particles bind to surfaces and is distinguishable from absorption, whereby the particles spread in the bulk of the absorbing material. The adsorbed material is called the adsorbate, while the surface is called the adsorbent. It is common to distinguish between two types of adsorption, namely physical adsorption (which consists of intermolecular forces holding the adsorbed material to the surface) and chemical adsorption (which consists of covalent bonds tethering the adsorbed material to the surface). The nature of the layer of adsorbate formed depends on the interactions between the adsorbed material and the adsorbent. [14] More specifically, the mechanisms involved in adsorption include ion exchange (replacement of counter ions adsorbed from the solution by similarly charged ions), ion pairing (adsorption of ions from solution phase onto sites on the substrates that carry the opposite charge), hydrophobic bonding (non-polar attraction between groups on the substrate surface and molecules in solution), polarization of p-electrons polar interactions between partially charged sites on the substrate surface and molecules carrying opposite partial charges in solution, and covalent bonds. [9] [15] The variety of ways for adsorption to occur provides an indication of the complexities associated with controlling the type of layer that is adsorbed.
The type of silane used can further compound the problem, as in the case of APTES. APTES is the classical molecule used for the immobilization of biomolecules and has historically been the most widely studied molecule in the field by far. Since APTES contains three ethoxy groups per molecule, it can polymerize in the presence of water, leading to lateral polymerization between APTES molecules in horizontal and vertical directions and the formation of oligomers and polymers which can attach to the surface.
Self-assembly can be approached using solution-phase reactions or vapor-phase reactions. In solution-phase experiments, the silane is dissolved in an anhydrous solvent and placed in contact with the surface; in vapor-phase experiments, only the vapor of the silane reaches the substrate surface. [9]
Solution-phase reaction has historically been the method that has been most studied, and a general consensus that has evolved with regards to the conditions required for the formation of smooth aminosilane films includes the following: (1) an anhydrous solvent such as toluene is required, with a rigidly controlled trace amount of water to regulate the degree of polymerization of aminosilanes at the surface and in solution; (2) formation of oligomers and polymers is favored at higher silane concentrations (>10%); (3) moderate temperatures (60–90 °C) can disrupt non-covalent interactions such as hydrogen bonds, leading to fewer silane molecules that are weakly tethered to the surface. Additionally, condition (3) favors desorption of water from the substrate into the toluene phase20; (4) Rinsing with solvents such as toluene, ethanol and water following the silanization reaction favors the removal of weakly bonded silane molecules and the hydrolysis of residual alkoxy linkages in the layer; (5) drying and curing at high temperature (110 °C) favors the formation of siloxane linkages and also converts ammonium ions to the neutral amine, which is more reactive. [9]
Vapor-phase silanization has been approached as a way to circumvent the complexities of trace water in solution and silane purity. [9] Since oligomers and polymers of silanes have negligible vapor pressure at the reaction temperatures commonly used, they do not reach the surface of the silicate during deposition. Since there is no solvent in the system, it is easier to control the amount of water in the reaction. Smooth monolayers have been reported for vapor-phase silanizations of several types of silanes, including aminosilanes, octadecyltrimethoxysilane and fluoalkyl silanes. However, the nature of the attachment of the silane molecules to the substrate is uncertain, although siloxane bond formation can be favored by soaking the substrate in water following deposition.
In a recent study by Chen et al., APTES monolayers were obtained consistently at different temperatures and deposition times. The thicknesses of the layers obtained were 5 Å and 6 Å at 70 °C and 90 °C respectively, which corresponds to the approximate length of an APTES molecule and indicates that monolayers formed on the substrates in each case. [9]
Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. This process differs from absorption, in which a fluid is dissolved by or permeates a liquid or solid. While adsorption does often precede absorption, which involves the transfer of the absorbate into the volume of the absorbent material, alternatively, adsorption is distinctly a surface phenomenon, wherein the adsorbate does not penetrate through the material surface and into the bulk of the adsorbent. The term sorption encompasses both adsorption and absorption, and desorption is the reverse of sorption.
A thin film is a layer of materials ranging from fractions of a nanometer (monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films is a fundamental step in many applications. A familiar example is the household mirror, which typically has a thin metal coating on the back of a sheet of glass to form a reflective interface. The process of silvering was once commonly used to produce mirrors, while more recently the metal layer is deposited using techniques such as sputtering. Advances in thin film deposition techniques during the 20th century have enabled a wide range of technological breakthroughs in areas such as magnetic recording media, electronic semiconductor devices, integrated passive devices, light-emitting diodes, optical coatings, hard coatings on cutting tools, and for both energy generation and storage. It is also being applied to pharmaceuticals, via thin-film drug delivery. A stack of thin films is called a multilayer.
In chemistry, a dangling bond is an unsatisfied valence on an immobilized atom. An atom with a dangling bond is also referred to as an immobilized free radical or an immobilized radical, a reference to its structural and chemical similarity to a free radical.
Self-assembled monolayers (SAM) of organic molecules are molecular assemblies formed spontaneously on surfaces by adsorption and are organized into more or less large ordered domains. In some cases molecules that form the monolayer do not interact strongly with the substrate. This is the case for instance of the two-dimensional supramolecular networks of e.g. perylenetetracarboxylic dianhydride (PTCDA) on gold or of e.g. porphyrins on highly oriented pyrolitic graphite (HOPG). In other cases the molecules possess a head group that has a strong affinity to the substrate and anchors the molecule to it. Such a SAM consisting of a head group, tail and functional end group is depicted in Figure 1. Common head groups include thiols, silanes, phosphonates, etc.
A Langmuir–Blodgett trough is an item of laboratory apparatus that is used to compress monolayers of molecules on the surface of a given subphase and to measure surface phenomena due to this compression. It can also be used to deposit single or multiple monolayers on a solid substrate.
Brunauer–Emmett–Teller (BET) theory aims to explain the physical adsorption of gas molecules on a solid surface and serves as the basis for an important analysis technique for the measurement of the specific surface area of materials. The observations are very often referred to as physical adsorption or physisorption. In 1938, Stephen Brunauer, Paul Hugh Emmett, and Edward Teller presented their theory in the Journal of the American Chemical Society. BET theory applies to systems of multilayer adsorption that usually utilizes a probing gas (called the adsorbate) that does not react chemically with the adsorptive (the material upon which the gas attaches to) to quantify specific surface area. Nitrogen is the most commonly employed gaseous adsorbate for probing surface(s). For this reason, standard BET analysis is most often conducted at the boiling temperature of N2 (77 K). Other probing adsorbates are also utilized, albeit less often, allowing the measurement of surface area at different temperatures and measurement scales. These include argon, carbon dioxide, and water. Specific surface area is a scale-dependent property, with no single true value of specific surface area definable, and thus quantities of specific surface area determined through BET theory may depend on the adsorbate molecule utilized and its adsorption cross section.
Microcontact printing is a form of soft lithography that uses the relief patterns on a master polydimethylsiloxane (PDMS) stamp or Urethane rubber micro stamp to form patterns of self-assembled monolayers (SAMs) of ink on the surface of a substrate through conformal contact as in the case of nanotransfer printing (nTP). Its applications are wide-ranging including microelectronics, surface chemistry and cell biology.
Hydrosilylation, also called catalytic hydrosilation, describes the addition of Si-H bonds across unsaturated bonds. Ordinarily the reaction is conducted catalytically and usually the substrates are unsaturated organic compounds. Alkenes and alkynes give alkyl and vinyl silanes; aldehydes and ketones give silyl ethers. Hydrosilylation has been called the "most important application of platinum in homogeneous catalysis."
Plasma-enhanced chemical vapor deposition (PECVD) is a chemical vapor deposition process used to deposit thin films from a gas state (vapor) to a solid state on a substrate. Chemical reactions are involved in the process, which occur after creation of a plasma of the reacting gases. The plasma is generally created by radio frequency (RF) alternating current (AC) frequency or direct current (DC) discharge between two electrodes, the space between which is filled with the reacting gases.
The Vroman effect, named after Leo Vroman, describes the process of competitive protein adsorption to a surface by blood serum proteins. The highest mobility proteins generally arrive first and are later replaced by less mobile proteins that have a higher affinity for the surface. The order of protein adsorption also depends on the molecular weight of the species adsorbing. Typically, low molecular weight proteins are displaced by high molecular weight protein while the opposite, high molecular weight being displaced by low molecular weight, does not occur. A typical example of this occurs when fibrinogen displaces earlier adsorbed proteins on a biopolymer surface and is later replaced by high molecular weight kininogen. The process is delayed in narrow spaces and on hydrophobic surfaces, fibrinogen is usually not displaced. Under stagnant conditions initial protein deposition takes place in the sequence: albumin; globulin; fibrinogen; fibronectin; factor XII, and HMWK.
Silanization is the attachment of an organosilyl group to some chemical species. Almost always, silanization is the conversion of a silanol-terminated surface to a alkylsiloxy-terminated surface. This conversion confers hydrophobicity to a previously hydrophilic surface. This process is often used to modify the surface properties of glass, silicon, alumina, quartz, and metal oxide substrates, which all have an abundance of hydroxyl groups. Silanization differs from silylation, which usually refers to attachment of organosilicon groups to molecular substrates.
A model lipid bilayer is any bilayer assembled in vitro, as opposed to the bilayer of natural cell membranes or covering various sub-cellular structures like the nucleus. They are used to study the fundamental properties of biological membranes in a simplified and well-controlled environment, and increasingly in bottom-up synthetic biology for the construction of artificial cells. A model bilayer can be made with either synthetic or natural lipids. The simplest model systems contain only a single pure synthetic lipid. More physiologically relevant model bilayers can be made with mixtures of several synthetic or natural lipids.
Surface-assisted laser desorption/ionization (SALDI) is a soft laser desorption technique used for mass spectrometry analysis of biomolecules, polymers, and small organic molecules. In its first embodiment Koichi Tanaka used a cobalt/glycerol liquid matrix and subsequent applications included a graphite/glycerol liquid matrix as well as a solid surface of porous silicon. The porous silicon represents the first matrix-free SALDI surface analysis allowing for facile detection of intact molecular ions, these porous silicon surfaces also facilitated the analysis of small molecules at the yoctomole level. At present laser desorption/ionization methods using other inorganic matrices such as nanomaterials are often regarded as SALDI variants. As an example, silicon nanowires as well as Titania nanotube arrays (NTA) have been used as substrates to detect small molecules. SALDI is used to detect proteins and protein-protein complexes. A related method named "ambient SALDI" - which is a combination of conventional SALDI with ambient mass spectrometry incorporating the direct analysis real time (DART) ion source has also been demonstrated. SALDI is considered one of the most important techniques in MS and has many applications.
Adsorption is the accumulation and adhesion of molecules, atoms, ions, or larger particles to a surface, but without surface penetration occurring. The adsorption of larger biomolecules such as proteins is of high physiological relevance, and as such they adsorb with different mechanisms than their molecular or atomic analogs. Some of the major driving forces behind protein adsorption include: surface energy, intermolecular forces, hydrophobicity, and ionic or electrostatic interaction. By knowing how these factors affect protein adsorption, they can then be manipulated by machining, alloying, and other engineering techniques to select for the most optimal performance in biomedical or physiological applications.
Adsorption is the adhesion of ions or molecules onto the surface of another phase. Adsorption may occur via physisorption and chemisorption. Ions and molecules can adsorb to many types of surfaces including polymer surfaces. A polymer is a large molecule composed of repeating subunits bound together by covalent bonds. In dilute solution, polymers form globule structures. When a polymer adsorbs to a surface that it interacts favorably with, the globule is essentially squashed, and the polymer has a pancake structure.
Biomaterials are materials that are used in contact with biological systems. Biocompatibility and applicability of surface modification with current uses of metallic, polymeric and ceramic biomaterials allow alteration of properties to enhance performance in a biological environment while retaining bulk properties of the desired device.
Protein adsorption refers to the adhesion of proteins to solid surfaces. This phenomenon is an important issue in the food processing industry, particularly in milk processing and wine and beer making. Excessive adsorption, or protein fouling, can lead to health and sanitation issues, as the adsorbed protein is very difficult to clean and can harbor bacteria, as is the case in biofilms. Product quality can be adversely affected if the adsorbed material interferes with processing steps, like pasteurization. However, in some cases protein adsorption is used to improve food quality, as is the case in fining of wines.
Biomaterials exhibit various degrees of compatibility with the harsh environment within a living organism. They need to be nonreactive chemically and physically with the body, as well as integrate when deposited into tissue. The extent of compatibility varies based on the application and material required. Often modifications to the surface of a biomaterial system are required to maximize performance. The surface can be modified in many ways, including plasma modification and applying coatings to the substrate. Surface modifications can be used to affect surface energy, adhesion, biocompatibility, chemical inertness, lubricity, sterility, asepsis, thrombogenicity, susceptibility to corrosion, degradation, and hydrophilicity.
An electro-switchable biosurface is a biosensor that is based on an electrode to which a layer of biomolecules has been tethered. An alternating or fixed electrical potential is applied to the electrode which causes changes in the structure and position (movement) of the charged biomolecules. The biosensor is used in science, e.g. biomedical and biophysical research or drug discovery, to assess interactions between biomolecules and binding kinetics as well as changes in size or conformation of biomolecules.
Surface plasmon resonance microscopy (SPRM), also called surface plasmon resonance imaging (SPRI), is a label free analytical tool that combines the surface plasmon resonance of metallic surfaces with imaging of the metallic surface. The heterogeneity of the refractive index of the metallic surface imparts high contrast images, caused by the shift in the resonance angle. SPRM can achieve a sub-nanometer thickness sensitivity and lateral resolution achieves values of micrometer scale. SPRM is used to characterize surfaces such as self-assembled monolayers, multilayer films, metal nanoparticles, oligonucleotide arrays, and binding and reduction reactions. Surface plasmon polaritons are surface electromagnetic waves coupled to oscillating free electrons of a metallic surface that propagate along a metal/dielectric interface. Since polaritons are highly sensitive to small changes in the refractive index of the metallic material, it can be used as a biosensing tool that does not require labeling. SPRM measurements can be made in real-time, such as measuring binding kinetics of membrane proteins in single cells, or DNA hybridization.
