Use Of Self-assembled Monolayer Research Articles

Impact of Buried Interface Texture on Compositional Stratification and Ion Migration in Perovskite Solar Cells

AbstractDespite the striking increase in the power conversion efficiency (PCE) of lead‐based perovskite solar cells (PSCs), their poor operational stability impedes their commercialization. Among the various factors that influence device stability, ion migration has been identified as a key driver of degradation. In this work, the focus is on studying ion migration‐induced degradation in inverted architecture PSCs, which employ either a thin polymer layer or a self‐assembled monolayer (SAM) for hole extraction. It is demonstrated that the difference in texture imposed by the use of these hole transport layers (HTL) is an important and thus far inconspicuous factor that impacts ion migration, and consequently device stability. By investigating the buried interface in detail, it is revealed that its texture has a strong impact on the vertical compositional stratification in the perovskite active layer. By monitoring bias‐induced ion migration in devices with different hole extraction layers, it is demonstrated that the smooth polymer‐based HTL results in a higher degree of ion migration than the rough SAM HTL, corresponding to a stronger degradation in the former. These results further indicate that the use of SAMs for hole extraction is a promising strategy to suppress ion migration and improve device efficiency.

Functionalized Self-Assembled Monolayers: Versatile Strategies to Combat Bacterial Biofilm Formation.

Bacterial infections due to biofilms account for up to 80% of bacterial infections in humans. With the increased use of antibiotic treatments, indwelling medical devices, disinfectants, and longer hospital stays, antibiotic resistant infections are sharply increasing. Annual deaths are predicted to outpace cancer and diabetes combined by 2050. In the past two decades, both chemical and physical strategies have arisen to combat biofilm formation on surfaces. One such promising chemical strategy is the formation of a self-assembled monolayer (SAM), due to its small layer thickness, strong covalent bonds, typically facile synthesis, and versatility. With the goal of combating biofilm formation, the SAM could be used to tether an antibacterial agent such as a small-molecule antibiotic, nanoparticle, peptide, or polymer to the surface, and limit the agent’s release into its environment. This review focuses on the use of SAMs to inhibit biofilm formation, both on their own and by covalent grafting of a biocidal agent, with the potential to be used in indwelling medical devices. We conclude with our perspectives on ongoing challenges and future directions for this field.

Molecular Features for Probing Small Amphiphilic Molecules with Self-Assembled Monolayer-Protected Nanoparticles.

The sensing of small molecules poses the challenge of developing devices able to discriminate between compounds that may be structurally very similar. Here, attention has been paid to the use of self-assembled monolayer (SAM)-protected gold nanoparticles since they enable a modular approach to tune single-molecule affinity and selectivity simply by changing functional moieties (i.e., covering ligands), along with multivalent molecular recognition. To date, the discovery of monolayers suitable for a specific molecular target has relied on trial-and-error approaches, with ligand chemistry being the main criterion used to modulate selectivity and sensitivity. By using molecular dynamics, we showcase that either individual molecular characteristics and/or collective features such as ligand flexibility, monolayer organization, ligand local ordering, and interfacial solvent properties can also be exploited conveniently. The knowledge of the molecular mechanisms that drive the recognition of small molecules on SAM-covered nanoparticles will critically expand our ability to manipulate and control such supramolecular systems.

Electrochemical Detection of Fertility Hormones.

Fertility hormone levels are constantly changing, but it is crucial for a woman to be able to monitor her fertility levels if she is interested in conceiving. Women and physicians often have a difficult time determining ovulation windows due to fluctuating menstrual cycles and inaccurate interpretations of hormone levels. Current methods of fertility monitoring include physical or vaginal exams, laparoscopy, ultrasound scans, as well as evaluation of hormone levels. A rapid, at-home fertility monitoring tool can help alleviate the apprehensiveness associated with routine screenings and give women the privacy desired when trying to conceive. Herein, we discuss the development of an electrochemical biosensor for quantification of three fertility hormones: beta-estradiol, progesterone, and FSH. Each biomarker's MRE was immobilized onto a gold disk electrode through the use of self-assembled monolayer linking chemistry. Using electrochemical impedance spectroscopy (EIS), the biomarker concentration was correlated to impedance magnitude. An optimal binding frequency was identified for each biomarker, permitting simplistic hardware requirements and investigation into multimarker detection. Analytes were tested in both purified solutions and 1%-90% whole blood. Each biomarker exhibited a unique imaginary impedance peak and optimal binding frequency. The determination was made by assessing the response parameters including the linear fit correlation across the physiological hormone ranges. The existence of unique optimal frequencies permits for simultaneous detection of multiple hormones in a single test. Additionally, the identified frequency was robust across purified and complex solutions. Response characteristics were negatively impacted by the introduction of blood-based contaminants. However, the introduction of Nafion membranes, similar to ones used in commercial glucose sensors, is both feasible and beneficial.

Regulating the Intermolecular Hydrogen Bonding: The Reversible Assembly and Disassembly in the Diffusion Layer

The intermolecular hydrogen bonding between 3-mercaptopropionic acid (MPA) and ferrocenylmethanol (FcCH2OH) on the tip electrode was studied by scanning electrochemical microscopy (SECM). The reversible assembly and disassembly of the intermolecular hydrogen bonding were designed with the aid of the dissociation equilibrium of the terminal carboxyl groups (-COOH) of the MPA self-assembled monolayer (SAM). Based on this design, the pH-modulated assembly and disassembly of intermolecular hydrogen bonding have been implemented. The use of well-defined SAM as model membrane provides a simple yet practicable approach to explore the molecular recognition at the cell membrane surfaces.

Tunable doping in PbS nanocrystal field-effect transistors using surface molecular dipoles

We study the effect of self-assembled monolayer (SAM) treatment of the SiO2 dielectric on the electrical characteristics of PbS transistors. Using SAMs, we observe threshold voltage shifts in the electron transport, allowing us to tune the electrical properties of the devices depending on the SAM molecule used. Moreover, the use of a specific SAM improves the charge carrier mobility in the devices by a factor of three, which is attributed to the reduced interface traps due to passivated silanol on the SiO2 surface. These reduced traps confirm that the voltage shifts are not caused by the trap states induced by the SAMs.

Sequence Detection of Unlabeled DNA Using the Sandwich Assay: Strand-Displacement, Hybridization Efficiency, and Probe- Conformation Considerations for the Tethered Surface

We report the development of a “sandwich-hybridization” electrochemical DNA-detection assay on gold electrodes using osmium tetroxide-labeled reporter strands, while avoiding chemical modification of analyte strands. Successful design of the oligonucleotides used in the assay required a significant adjustment to solution-based estimates of melting temperature for the immobilized-probe/target duplex, to achieve optimal displacement kinetics. This allowed for reduced hybridization times and temperatures, and a higher sensitivity at low concentrations of DNA-analyte than oligos designed using solution-based melting temperatures as a guide. In addition we found that for the highest ranges of probe surface-density, signal response was sluggish until the DNA self-assembled monolayer (SAM) was exposed to several hybridization procedures, suggesting a modification of the SAM in response to a hybridization experiment. Use of a less-dense SAM eliminated this effect, suggesting increased influence of molecular crowding on the sandwich assay versus a simple hybridization. The high sensitivity, quick response time, relative simplicity and low cost demonstrated here helps pave the way for a field-level electrochemical genetic sensor using our method, as the unmodified target from samples in question can be tested directly without a chemical labeling step.

Evaluating the use of self-assembled monolayer (SAM) release coatings for replicated optics

Carbon fiber-reinforced composite replicated mirrors offer weight savings, higher stiffness, tailorable CTEs, higher thermal conductivities, and excellent damage tolerant mechanical properties in comparison to traditionally processed glass mirrors. These mirrors can also be rapidly manufactured by replicating the surface of a high-precision glass mandrel multiple times. The mold release coating used is critical and must be on the molecular scale, uniform, and exhibit low adhesion in order to produce high-fidelity replication. This study investigates the use of a self-assembled monolayer (SAM) as the mold release agent for the manufacture of replicated mirrors. We have synthesized and tested perfluoropolyether (PFPE) coatings formed on glass substrate via self-assembly. Ellipsometry, X-ray photoelectron spectroscopy, and atomic force microscopy were used to characterize the morphology, thickness uniformity and the chemical profile of the coatings. The release force necessary to remove the replicated surfaces from the mandrel is critical and was directly related to the surface distortion of the replica. The release force increased with each successive replication due to pinholes and defects. Incorporating a secondary, lower molecular weight SAM reagent significantly reduced both surface agglomeration and transfer of SAM material to the replicated resin surface. A 60% reduction in the replica release force and a 50% improvement in the replica surface figure were achieved due to reductions in the defect density of the SAM coating.

Switching “on and off” faradaic electrochemistry at an otherwise passivated electrode using gold-coated magnetic nanoparticles

Herein we combined the use of self-assembled monolayer (SAM) modified electrodes and gold coated magnetic nanoparticles (Au@MNPs) to modulate faradaic electrochemistry when nanoparticles are absorbed on and removed from, a passivated electrode using an external magnetic field. Substantial faradaic electrochemistry of potassium ferricyanide in solution is first prevented by modifying gold electrodes with 11-mercaptoundecanoic acid to form a passivating SAM. Restoration of the faradaic electrochemistry is then achieved by introducing Au@MNPs which are brought to the surface using an external magnetic field. The faradaic electrochemistry can again be suppressed by removing Au@MNPs from the SAM using another external magnetic field.

Discovery of SIRT3 Inhibitors Using SAMDI Mass Spectrometry

Lysine acetylation plays a critical role in cellular regulation and is implicated in human disease. Sirtuin deacetylases remove acetyl groups from modified lysine residues, and sirtuin 3 (SIRT3) has been identified as a target for cancer therapeutics. Robust and high-throughput screening methods for these targets will be important to the development of therapeutics. This article describes the use of self-assembled monolayer desorption/ionization mass spectrometry, or SAMDI-MS—a label-free drug discovery tool—to characterize SIRT3 activity and discover inhibitors. SAMDI-MS was used to analyze a peptide array having 361 distinct acetylated peptides to identify an active SIRT3 substrate (GYKAcRGC). This peptide was used in a screen of 100,000 small molecules to identify inhibitors of SIRT3. A total of 306 SIRT3 inhibitors were identified, with one compound, SDX-437, having an IC50 of 700 nM with >100-fold selectivity for SIRT3 over SIRT1.

Self-assembled monolayer modified ITO in P3HT:PC61BM organic solar cells with improved efficiency

In this paper we demonstrate the use of self-assembled monolayer modified indium tin oxide (ITO) electrode in bulk heterojunction solar cell structure based on a photoactive layer of poly(3-hexylthiophene) and [6,6]-phenyl C61 butyric acid methyl ester (P3HT:PC61BM). The device studies show that there is a significant enhancement in the short-circuit current as well as in the shunt resistance on use of the hexamethyldisilazane (HMDS) layer. The enhancement in short-circuit current density is attributed to the good contact formed between the organic materials and the methyl groups of HMDS. A significant improvement in the external quantum efficiency is also observed in case of the HMDS modified ITO based solar cells.

Control of field-effect transistor threshold voltages by insertion of self-assembled monolayers

We report the control of the threshold voltage of field-effect-transistors (FET) by inserting a self-assembled monolayer (SAM) between the insulator and the gate electrode. The surface potentials of the SAMs, measured by Kelvin force microscopy, depended on the dipole moments of their monomer units. The SAMs were formed by chemical vapor deposition at low temperatures from various monomer units with different dipole moments. The dipole moments were determined by ab initio calculations. The FET properties, such as the threshold voltage, could be controlled by SAMs inserted between the insulator and the gate electrode. The threshold voltage shift was linearly correlated with the surface potential of the SAM. The transconductance also depended on the dipole moment of the SAM monomer unit. Therefore, altering the transconductance could also increase the range of the threshold voltage. The use of SAMs provides a simple method for controlling the properties of transistors, and can be used in thin film transistors, such as organic and oxide devices.

Multidentate Adsorbates for Self-Assembled Monolayer Films

The spontaneous adsorption of organic molecules on a variety of planar and nonplanar substrates, that is, self assembly, can generate films just one molecule thick. These nanoscale, self-assembled monolayer (SAM) films have been extensively used to engineer surfaces with well-defined properties. Their utility has been demonstrated in a wide range of applications, including wetting, adhesion, lubrication, patterning, and molecular recognition. Many SAM systems have been investigated, but alkanethiols adsorbed on gold are the most successful combination. This pairing offers a variety of advantages, including the ability to tune precisely the interfacial properties of a surface through the well-established organic synthetic methodologies that have been developed for preparing custom ω-terminated alkanethiols. Alkanethiolate monolayers are moderately stable at room temperature; however, these films degrade over time and readily desorb upon moderate heating. This shortcoming limits the use of SAMs in applications involving elevated temperatures or harsh environments. Accordingly, new adsorbates with multiple bonding moieties have been created to enhance the stability and versatility of SAMs. In this Account, we examine a variety of multidentate adsorbate structures that have been used to generate SAMs on planar substrates and on nanoparticles. Each of these chelating adsorbates (bidentates and tridentates) has been designed to generate well-defined organic monolayer films with multiple attachment points to the underlying substrate. This bonding arrangement allows the formation of SAMs with enhanced stability through the entropy-driven "chelate effect". The research examined here demonstrates that multidentate adsorbates provide robust films: they enable the use of SAMs under conditions that are incompatible with SAMs derived from normal alkanethiols. Another advantage offered by multidentate adsorbates is the capacity for new paradigms in thin-film composition. In particular, appropriately designed chelating adsorbates can be engineered to have two or more chemically distinct terminal groups that are covalently linked to the same underlying headgroup, without adding steric bulk that might prove detrimental to the resultant assembly. This strategy allows the generation of homogeneously mixed multicomponent surfaces, overcoming the problem of phase separation or "islanding" that is pervasive when two or more chemically distinct adsorbates are used to form mixed SAMs. Such homogeneously mixed films offer the opportunity to fine-tune the interfacial properties of a substrate and to create unique heterogeneous interfaces that are well defined by the chemical composition of the tailgroups exposed at the surface. The insight derived from these studies opens the door to new uses for SAMs, both in surface engineering applications (such as corrosion resistance and soft lithographic patterning) and in the stabilization and manipulation of nanoparticles.

Gold electrode modified by self-assembled monolayers of thiols to determine DNA sequences hybridization

The process of immobilization of biological molecules is one of the most important steps in the construction of a biosensor. In the case of DNA, the way it exposes its bases can result in electrochemical signals to acceptable levels. The use of self-assembled monolayer that allows a connection to the gold thiol group and DNA binding to an aldehydic ligand resulted in the possibility of determining DNA hybridization. Immobilized single strand of DNA (ssDNA) from calf thymus pre-formed from alkanethiol film was formed by incubating a solution of 2-aminoethanothiol (Cys) followed by glutaraldehyde (Glu). Cyclic voltammetry (CV) was used to characterize the self-assembled monolayer on the gold electrode and, also, to study the immobilization of ssDNA probe and hybridization with the complementary sequence (target ssDNA). The ssDNA probe presents a well-defined oxidation peak at +0.158 V. When the hybridization occurs, this peak disappears which confirms the efficacy of the annealing and the DNA double helix performing without the presence of electroactive indicators. The use of SAM resulted in a stable immobilization of the ssDNA probe, enabling the hybridization detection without labels. This study represents a promising approach for molecular biosensor with sensible and reproducible results.

Electrochemical Study and Preparation of Gold Substrates Functionalized with Single-Walled Carbon Nanotubes for DNA Biosensor Application

We report here the use of self-assembled monolayer (SAM) on gold modified with single-walled carbon nanotubes (SWNTs) as a suitable substrate for deoxyribonucleic acid (DNA) oligonucleotides attachment. For our purpose, the molecule 4- aminothiophenol (4-ATP) was self-assembled on gold by forming a thiolate (Au-S) bond. The amine terminal group of the SAM was used to covalently link SWNTs, which serve as template for the covalent attachment of DNA. Each step were characterized by cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR).

Controlled Amine Functionality in Self-Assembled Monolayers via the Hidden Amine Route: Chemical and Electronic Tunability

A synthetic strategy for fabricating a dense amine functionalized self-assembled monolayer (SAM) on hydroxylated surfaces is presented. The assembly steps are monitored by X-ray photoelectron spectroscopy, Fourier transform infrared- attenuated total reflection, atomic force microscopy, variable angle spectroscopic ellipsometry, UV-vis surface spectroscopy, contact angle wettability, and contact potential difference measurements. The method applies alkylbromide-trichlorosilane for the fabrication of the SAM followed by surface transformation of the bromine moiety to amine by a two-step procedure: S(N)2 reaction that introduces the hidden amine, phthalimide, followed by the removal of the protecting group and exposing the free amine. The use of phthalimide moiety in the process enabled monitoring the substitution reaction rate on the surface (by absorption spectroscopy) and showed first-order kinetics. The simplicity of the process, nonharsh reagents, and short reaction time allow the use of such SAMs in molecular nanoelectronics applications, where complete control of the used SAM is needed. The different molecular dipole of each step of the process, which is verified by DFT calculations, supports the use of these SAMs as means to tune the electronic properties of semiconductors and for better synergism between SAMs and standard microelectronics processes and devices.

Quartz crystal microbalance bioaffinity sensor for biotin based on mixed self-assembled monolayers and metastable molecular complex receptor

A quartz crystal microbalance (QCM) sensor was proposed for the detection of small molecule biotin based on the mixed self-assembled monolayer (SAM) of thiols on gold substrate and the bioaffinity difference between an analyte (biotin) and an analogue compound (HABA) in binding avidin. Avidin formed a metastable complex with 2-[(4-hydroxyphenyl)azo]benzoic acid (HABA) immobilized on the crystal surface. When the sensor contacts a sample solution containing biotin, the avidin was released from the sensor surface to form a more stable complex with biotin in solution. The frequency change recorded is proportional to the desorbed mass of avidin, and there is a clear mathematic relationship between the frequency change and the biotin concentration. The use of mixed SAMs allows the stable attachment of bioreceptor molecules on the QCM, and enhances the amount of the immobilized molecules on the QCM, as a longer "space arm" in the mixed SAMs makes this monolayer membrane more accessible to capture the immobilized molecules. The proposed bioaffinity sensor has nice response to biotin in the range of 0.017-1.67 microg/mL. The sensor could be regenerated under very mild conditions simply by reimmersion of the sensor into a biotin solution to desorb the surplus avidin.

Surface modification and functionalization through the self-assembled monolayer and graft polymerization

The modification of a surface at the molecular level with precise control of the building blocks generates an integrated molecular system. This field has progressed rapidly in recent years through the use of self-assembled monolayer (SAM) interfaces. Recent developments on surface-initiated chemical reactions, functionalization, and graft polymerization on SAM interfaces are emphasized in the present review. A number of surface modifications by grafting are reviewed. The grafting of polyaniline on a glass surface, previously modified with a silane self-assembled monolayer (SAM), is examined in detail for both planar and 3-D systems, such as fibers, nanoparticles, and even polymer patterned surfaces. We also discuss the graft polymerization of water-soluble polymers on the surface of silicon nanoparticles, which generate stable aqueous colloidal solutions and have numerous applications. Finally, we compare and review some surface-modification techniques on the surfaces of polymers, such as two-solvent entrapment, polymer blending, and chemical grafting, which improve their biocompatibility.

Self-Assembled Monolayer as a Template to Deposit Silicon Nanoparticles Fabricated by Laser Ablation

We demonstrate a use of self-assembled monolayer (SAM) as a template to deposit in-situ Si nanoparticles fabricated by laser ablation. Scanning tunneling microscopy, atomic force microscopy observations, and photoluminescence measurements show that the Si nanoparticles deposited in situ on SAMs are round shaped, firmly attached to the surface, and remain stable for at least a couple of months. Control over the average size of the Si nanoparticles could be achieved, in the region where quantum confinement effect is important, by changing the Ar ambient pressure. Our results show that SAM endures the fierce heat, ions, and plasma generated during the laser ablation process, and the use of SAM could be extended as a substrate to deposit in-situ materials fabricated by laser ablation. In-situ deposition is important because it would facilitate fabrication of high functional nanoarchitectures based on this easily oxidized material. We believe that the wide range of available SAMs, different in chemical and ele...

Application of Self Assembled Monolayer Approach to Probe Fiber Matrix-Adhesion

ABSTRACTAdhesion at the fiber-matrix interface of composites is often related to a combination of factors such as mechanical interlocking, physico-chemical interactions, and chemical bonding of the fiber-matrix interphase region. We demonstrate the use of self-assembled monolayer (SAMs) approach for depositing silane coupling agent on glass fiber and studying the impact of the individual interactions on the adhesion process. Through some unique chemistry, functionalized and non-functionalized C11 chlorosilanes were deposited on to E-glass fiber and modified. The adhesion of diglycidyl ether of bisphenol-A (DGEBA) cured with meta-phenylene diamine (m-PDA) to SAM layer on E-glass fibers was measured by performing single fiber fragmentation tests (SFFT). The extent of adhesion between the fiber and matrix was found to be dependent on carbon chain length of coupling agents, and the functional group at the end of the SAMs layer. Furthermore, the contributions to adhesion by physico-chemical interaction and covalent bonding has been individually assessed.