Applications of Self-Assembled Monolayers
Self-assembled monolayers (SAMs) have numerous applications across various fields, including materials science, electronics, medicine, and nanotechnology, offering significant advantages in controlling surface properties, enhancing device performance, and developing new technologies. SAMs are highly organized single layers of molecules that form on a surface. This is driven by the chemisorption of specific functional groups on the SAM molecules such as MeO-2PACz that have a strong affinity for a particular surface. It provides one of the most elegant and convenient approaches to control the thickness and architectures of organic films.
SAMs represent a versatile and powerful tool for modifying and functionalizing surfaces at the molecular level:
- Terminal functionalities of the SAM organic molecules allow the precise control of the hydrophobicity or hydrophilicity of the surface
- The spacer can be used to tune the distant-dependent electron transfer behaviour
Adaptable organo-inorganic surfaces are extremely important in nanotechnology to construct nanoelectronics, sensor arrays, supercapacitors, catalysts, and rechargeable power sources etc.
Surface Modification and Functionalization using SAMs
One of the primary uses of SAMs is to modify surfaces to tailor their properties for specific applications. By altering the terminal groups of the molecules forming the SAM, you can control the chemical reactivity, hydrophobicity, and other surface characteristics.
For example, self-assembled monolayer surface modification is used for developing bioactive coatings on titanium. With antibacterial properties, good integration with bone and long-term stability, SAMs have had success as medical implants. Also, the treatment of SiO2 surfaces with n -octadecyltrichlorosilane (OTS) or hexamethyldisilazane (HMDS) SAM molecules created surfaces with low surface energy. This is known to improve the crystallinity of various organic semiconductors for electronic applications.
Wettability Control
Self-assembled monolayers can be used to control the wettability of surfaces, which is crucial in various industrial and biomedical applications.
- Hydrophobic SAMs can be applied to create water-repellent surfaces, which are valuable in coatings for self-cleaning materials.
- Hydrophilic SAMs can enhance water spreading, which is beneficial in microfluidic devices.
By mixing octadecylphosphonic acid (C18-PA) and an anthracene-terminated SAM material (ANT-PA), uniform mixed SAMs films can be formed. Surface wettability can be finely tuneable by varying the amounts of ANT-PA and C18-PA.
Adhesion Control
Self-assembled monolayers can be utilized to enhance or reduce adhesion between surfaces. This is useful for industries where adhesion properties are critical, such as in microelectronics and sensor technologies. Control is vital for ensuring the reliability and performance of devices. Complex chemical patterns can be fabricated with the help of electron beam lithography (EBL) and binary SAMs of controlled compositions to serve as templates for non-specific and specific proteins adsorption, growth of 3D DNA nanostructures and the attachment of nanoparticles.
Self-Assembled Monolayers in Biomedicine
Self-assembled monolayers provide an easy method to modify and functionalize sensor surfaces. SAM molecules bear both functional terminal groups and free anchor groups such as:
- thiols
- disulphides
- amines
- silanes
- acids
SAMs play a significant role in the development of biosensors and diagnostic devices due to their ability to immobilize biomolecules in a controlled and stable manner. The self-assembled monolayer produced is appropriate for several applications depending upon their terminal functionality or by varying the chain length.
Biosensors
Self-assembled monolayers are used to immobilize enzymes, antibodies, or other recognition elements on sensor surfaces. The self-assembled monolayer hosts the biomarkers and can serve as an interlayer on the surface of an electrode. This immobilization is essential for creating highly sensitive and specific biosensors, which are used for detecting various analytes, including glucose, toxins, and pathogens.
The biosensing systems are powerful tools for disease diagnosis at an early stage. The high selectivity of biological molecules integrated with either electrochemical, optical, or piezoelectric transduction mode of analyte recognition offers great promise to exploit them as efficient and accurate biosensors.
For example, SAMs on gold surfaces are often employed in electrochemical and optical biosensors. Bioreceptor-conjugated self-assembled monolayers on gold nanoparticles (AuNPs) can be used in a microfluidic chip. The chip is used to detect A549 human lung circulating tumour cells. The thiol-terminated DNA aptamers (bioreceptor) formed a SAM on the AuNP surface via the famously strong S–Au bonds.
Medical Diagnostics
In medical diagnostic applications, self-assembled monolayers have been used to develop microarrays and lab-on-a-chip devices. These devices require precise and stable immobilization of DNA, proteins, or other bioreceptor molecules. This ensures accurate and reproducible results in detecting diseases or genetic mutations.
A cost-effective, sensitive, and label-free electrochemical immunosensor based on a cysteamine (CYS) self-assembled monolayer decorated fluorine-doped tin oxide (FTO) electrode showed a linear dynamic range from 10 to 1000 ng/mL with a low detection limit of 1.13 ng/mL of α-synuclein (α-Syn) for the early diagnose of Parkinson’s disease in electrochemical impedance spectra (EIS) measurement.
Drug Delivery
Self-assembled monolayers are used to create nanoscale carriers for targeted drug delivery. By functionalizing the SAMs with specific ligands, these carriers can selectively bind to target cells or tissues, improving the efficacy and reducing the side effects of therapeutics. SAMs act as interface between the nanoparticles and drugs, control the final complex structure of the nanoparticle and its associated properties. By using self-assembled nanoparticles in drug delivery there are benefits such as:
- improved delivery system
- bioavailability
- biocompatibility
- enhanced circulation time
- controlled release of drugs
The anticancer drug Nintedanib (NTD) was conjugated with MNS-APTES to form self-assembled magnetic nanospheres (MNS). These are superparamagnetic iron oxide nanoparticles grafted with a monolayer of (3-aminopropyl)triethoxysilane (APTES). A controlled release of 85% of the drug NTD in 48 h was observed at pH 5.5. This mimics a cancerous environment, and prolonged release of NTD was found at physiological conditions at pH 7.4.
Biomedical Implants
As mentioned, self-assembled monolayers can be used to modify the surface of biomedical implants to improve their biocompatibility and reduce the risk of infection. By tailoring the surface properties, SAMs can enhance the integration of implants with the surrounding tissue, in particular, multifunctional titanium implants with good anti-infective ability, biocompatibility and even osteogenic ability are highly desirable.
With promising bone growing and antimicrobial properties, bioactive metallic complexes SrPhy and ZnPhy self-assembled monolayers on titanium surface demonstrate great capacity to promote the adhesion and proliferation of bone cell cultures over the titanium surface. Synergic effects between phytic acid and the corresponding cation in the self-assembled monolayer are observed in in vitro bone growth processes.
Self-Assembled Monolayers in Catalysis
Self-assembled monolayers (SAMs) have been used to modify traditional catalytic materials. For example, gold has been modified to create a more favourable surface environment for specific product formation. SAMs on metal or metal oxide nanoparticles surfaces are used in catalysis to create highly active and selective surfaces for various chemical reactions.
Supported Catalysts
SAMs can be used to immobilize catalytic species on supports, creating heterogeneous catalysts with improved activity and selectivity. This approach is valuable in:
- chemical synthesis
- environmental remediation
- energy conversion processes
Electrocatalysis
SAMs can enhance the performance of electrodes by improving their selectivity and reducing overpotentials. This is particularly important in applications such as fuel cells and electrochemical sensors. SAMs offer the possibility of a wide range of terminal groups that enable productive enzyme adsorption and fine-tuning in favourable orientations on the electrode. The chain length, and the contacting terminal functional group of the SAM material, are normally to be considered as the most significant controlling factors in direct electron transfer bio-electrocatalysis.
Electronics
Self-assembled monolayers (SAMs) are valuable in electronics for modifying surface energy and morphology. SAMs provide cost-effective and versatile surface modification and molecular-scale device creation. By choosing suitable head, spacer, and tail groups, chemical engineers can make surfaces more hydrophobic or hydrophilic, adjust surface energy, or add specific chemical functions. This is crucial for device fabrication, where surface properties affect performance.
SAMs can form ultra-thin insulating layers essential for nanoscale electronic components, reducing leakage currents and enhancing transistor performance. In molecular electronics, SAMs help build molecular-scale devices by arranging functional molecules precisely, allowing the creation of molecular diodes, transistors, and other components. This precise control enables the exploration of quantum mechanical effects in electronic devices.
Self-Assembled Monolayers (SAMs)
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Self-Assembled Monolayers in Electronic Devices
Self-assembled monolayers (SAMs) provide a versatile and cost-effective method for surface modification and the creation of molecular-scale electronic devices. By selecting the appropriate head, spacer and tail group for the SAM molecules the following properties can be adjusted:
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Deposition of Self-Assembled Monolayers
Self-assembled monolayers (SAMs) are well-organised, one molecule thick layers that form on a solid surface. SAM molecules adsorb on a substrate via chemical or physical bonds. They spontaneously organize themselves into ordered structures on the substrate. SAM materials have gained interest due to their potential applications in various fields such as solar cell research, nanotechnology, materials science, chemistry, and biology.
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J. Sánchez-Bodón et al. (2022); Bioactive Coatings on Titanium: A Review on Hydroxylation, Self-Assembled Monolayers (SAMs) and Surface Modification Strategies, Polymers, 14, DOI: 10.3390/polym14010165
T. Higashino (2020); Direct Preparation of Mixed Self-assembled Monolayers Based on Common-substructure-tailored Phosphonic Acids for Fine Control of Surface Wettability; Chem. Lett., 49 (1), DOI: 10.1246/cl.200460
J. Drzazgowska et al. (2020); Self-Assembled Monolayer Epitope Bridges for Molecular Imprinting and Cancer Biomarker Sensing, Anal. Chem., 92 (7), DOI: 10.1021/acs.analchem.9b03813