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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.^[1]^[2] 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.

Mechanism

Silanization mechanisms vary with substrate and with silanization reagent. In the usual circumstance, surface MOH groups react as nucleophiles with silyl chlorides or silyl alkoxides. The stoichiometry for these reactions are shown:

M−OH + R₃SiCl → M−OSiR₃ + HCl

M−OH + R₃SiOCH₃ → M−OSiR₃ + CH₃OH

M is typically Si, but could be many other elements.

The process is assumed to follow the pathways that apply to silylation of molecular substrates, such as alcohols.^[1]

Properties of organofunctional alkoxysilanes

Structural formula of (3-aminopropyl)triethoxysilane (APTES)

Silanizing reagents often have the formula (RO)_3-n(CH₃)_nSiR' or R'SiCl₃, where R = methyl or ethyl and R' is a long alkyl chain or a functionalized alkyl group.^[1]

Selected Silanizing Agents
Silane coupling agent	Structural formula	Comment
octadecyltrichlorosilane (ODTS)	CH₃(CH₂)₁₇SiCl₃	long-chain alkyl^[3]
vinyltrimethoxysilane	CH₂=CHSi(OCH₃)₃	vinyl-terminated
3-methacryloxypropyltrimethoxysilane	CH₂=C(CH₃)CO₂(CH₂)₃Si(OCH₃)₃	acrylate terminated
3-aminopropyltriethoxysilane	H₂NCH₂CH₂CH₂Si(OC₂H₅)₃	amino terminated;^[4] see also 3-aminopropyl)diethoxymethylsilane (APDMES), 3-aminopropyl)dimethylethoxysilane (APTMS), 3-aminopropyl)trimethoxysilane (CAS# 13822-56-5)
N-(2-amino ethyl)-3-aminopropyltrimethoxysilane	H₂NCH₂CH₂NH(CH₂)₃Si(OCH₃)₃	diamino-terminated
bis[3-triethoxysilyl-propyl]tetrasulfide	[(CH₃CH₂O)₃SiCH₂CH₂CH₂SS]₂	crosslinking group for silica-rubber composites
3-mercaptopropyltrimethoxysilane	HSCH₂CH₂CH₂Si(OCH₃)₃	thiol-terrminated (see also MPDMS (3-mercaptopropyl)-methyl-dimethoxysilane
3-chloropropyltrimethoxysilane	CICH₂CH₂CH₂Si(OCH₃)₃	chloroalkyl-terminated
3-glycidoxypropyl)-dimethylethoxysilane (GPMES)	CH₂OCHCH₂Si(CH₃)₂(OC₂H₅)	epoxide-containing

The nature of the organic group on the silanization agent strongly influences the properties of the surface. Simple alkyl-containing silanizing groups confer hydrophobicity.

When side chains of the silane compound are amines or thiols, the surfaces assume the properties of those functional groups.^[5] These surfaces are susceptible to further reactions characteristic of the appended functional groups. Grafting can be performed, for example.^[6]

Applications

Reversed-phase chromatography

A popular stationary phase is an octadecyl carbon chain (C18)-bonded silica (USP classification L1).^[7] Such materials are produced by reaction of silica gel with trimethoxyoctadecylsilane]]. The individual links involve silanol groups displacing the methoxy groups, forming an Si-O-Si bonds. This reaction changes the hydrophilic silanol groups into hydrophobic coatings. Other silanizing groups install C8-bonded silica (L7), pure silica (L3), cyano-bonded silica (L10) and phenyl-bonded silica (L11). Note that C18, C8 and phenyl are dedicated reversed-phase resins, while cyano columns can be used in a reversed-phase mode depending on analyte and mobile phase conditions.

Glassware

Silanization (or siliconization) of glassware is a common application that increases the hydrophobicity of a glass container. Thus treated, the glassware produces a flat meniscus and allowing for more complete transfer of aqueous solutions.^[2]^[8]

Silanization of glassware is used in cell culturing to minimize adherence of cells to flask walls.^[9] Additionally, the silanization process is also used in biomedical fields for a wide variety of purposes, including anchoring DNA to substrates.^[10]

Silanization of glassware can be achieved by dipping into a solution of 5-10% dimethyldiethoxysilane followed by heating.^[2]

Silanization is also used for DNA chips. Nucleic acids do not bond to untreated glass surfaces. Silanization can be providing a better bonding site for the nucleic acids onto the chip.^[11] A common silane used to treat glass surfaces for this application is (3-mercaptopropyl) trimethoxysilane, which increases the number of reactive thiol groups on the surface^[11] The nucleic acids can bond to these available thiol groups on the surface of the glass DNA chip after silanization occurs.^[11]

Dental

Silanization is often used to treat ceramics used for dental restorations and repairs. Applying silane coupling agents after grit blasting the ceramic material has been shown to produce durable resin bonding.^[12] Additionally, for titanium and other metal implant features in wires and crowns, application of silane coupling agents followed by resin composite cement has produced durable bonding in a clinical application.^[12] While silane coupling agents are widely used in dental practices, they are subject to bond degradation due to the environment of the oral cavity, weakening the adhesion between the surfaces that they are used to connect.^[12]^[13]

Additional reading

Zelzer, M. (2015), "Peptide-based switchable and responsive surfaces", Switchable and Responsive Surfaces and Materials for Biomedical Applications, Elsevier, pp. 65–92, doi:10.1016/b978-0-85709-713-2.00003-1, ISBN 978-0-85709-713-2, retrieved 2023-04-30
Schwartz, Jeffrey; Avaltroni, Michael J; Danahy, Michael P; Silverman, Brett M; Hanson, Eric L; Schwarzbauer, Jean E; Midwood, Kim S; Gawalt, Ellen S (2003). "Cell Attachment and Spreading on Metal Implant Materials". Materials Science and Engineering: C. 23 (3): 395–400. doi:10.1016/S0928-4931(02)00310-7.

References

^ ^a ^b ^c Pape, Peter G. (2017). "Silylating Agents". Kirk-Othmer Encyclopedia of Chemical Technology. pp. 1–15. doi:10.1002/0471238961.1909122516011605.a01.pub3. ISBN 9780471238966.
^ ^a ^b ^c Arkles, Barry (1997). "Tailoring Surfaces with Silanes". Chemtech. 7: 766–777 – via ResearchGate.
^ Žuvela, Petar; Skoczylas, Magdalena; Jay Liu, J.; Ba̧Czek, Tomasz; Kaliszan, Roman; Wong, Ming Wah; Buszewski, Bogusław; Héberger, K. (2019). "Column Characterization and Selection Systems in Reversed-Phase High-Performance Liquid Chromatography". Chemical Reviews. 119 (6): 3674–3729. doi:10.1021/acs.chemrev.8b00246. PMID 30604951. S2CID 58631771.
^ Vashist, Sandeep Kumar; Lam, Edmond; Hrapovic, Sabahudin; Male, Keith B.; Luong, John H. T. (2014). "Immobilization of Antibodies and Enzymes on 3-Aminopropyltriethoxysilane-Functionalized Bioanalytical Platforms for Biosensors and Diagnostics". Chemical Reviews. 114 (21): 11083–11130. doi:10.1021/cr5000943. PMID 25299435. S2CID 206900258.
^ Brehm, Marius; Scheiger, Johannes M.; Welle, Alexander; Levkin, Pavel A. (2020). "Reversible Surface Wettability by Silanization". Advanced Materials Interfaces. 7 (12): 1902134. doi:10.1002/admi.201902134. ISSN 2196-7350.
^ Zhao, Jie; Milanova, Maria; Warmoeskerken, Marijn M. C. G.; Dutschk, Victoria (2012-11-05). "Surface modification of TiO2 nanoparticles with silane coupling agents". Colloids and Surfaces A: Physicochemical and Engineering Aspects. 25th Meeting of the European Colloid and Interface Society. 413: 273–279. doi:10.1016/j.colsurfa.2011.11.033. ISSN 0927-7757.
^ USP Chromatographic Reagents 2007-2008: Used in USP-NF and Pharmacopeial Forum. United States Pharmacopeia. 2007.
^ Seed, B. (May 2001). "Silanizing glassware". Current Protocols in Cell Biology. Appendix 3: Appendix 3E. doi:10.1002/0471143030.cba03es08. ISSN 1934-2616. PMID 18228287. S2CID 20511249.
^ Seed, Brian (May 2001). "APPENDIX 3E Silanizing Glassware". Current Protocols in Cell Biology. Appendix 3: A.3E.1. doi:10.1002/0471143030.cba03es08. PMID 18228287. S2CID 20511249.
^ Labit, Hélène; Goldar, Arach; Guilbaud, Guillaume; Douarche, Carine; Hyrien, Olivier; Marheineke, Kathrin (2008-12-01). "A simple and optimized method of producing silanized surfaces for FISH and replication mapping on combed DNA fibers". BioTechniques. 45 (6): 649–658. doi:10.2144/000113002. ISSN 0736-6205. PMID 19238795.
^ ^a ^b ^c Halliwell, Catherine M.; Cass, Anthony E. G. (2001-06-01). "A Factorial Analysis of Silanization Conditions for the Immobilization of Oligonucleotides on Glass Surfaces". Analytical Chemistry. 73 (11): 2476–2483. doi:10.1021/ac0010633. ISSN 0003-2700.
^ ^a ^b ^c Matinlinna, Jukka Pekka; Lung, Christie Ying Kei; Tsoi, James Kit Hon (2018). "Silane adhesion mechanism in dental applications and surface treatments: A review". Dental Materials. 34 (1): 13–28. doi:10.1016/j.dental.2017.09.002.
^ Wagner, William; Zhang, Guigen; Sakiyama-Elbert, Shelly; Yaszemski, Michael (2020). Biomaterials Science - An Introduction to Materials in Medicine (4th ed.). Elsevier. pp. 495–496. ISBN 978-0128161371.