Controlling Gold Morphology Using Electrodeposition for the Preparation of Electrochemical Aptamer-Based Sensors

Nanostructuring of gold surfaces to enhance electroactive surface area has proven to significantly enhance the performance of electrochemical aptamer-based (E-AB) sensors, particularly for electrodes on the microscale. Unlike for sensors fabricated on polished gold surfaces, predicting the behavior of E-AB sensors on surfaces with varied gold morphologies becomes more intricate due to the effects of surface roughness and the shapes and sizes of surface features on supporting a self-assembled monolayer. In this study, we explored the impact of gold morphology characteristics on sensor performance, evaluating parameters such as signal change in response to the addition of the target analyte, aptamer probe packing density, and continuous sensing ability. Our findings reveal that surface area enhancement can either enhance or diminish sensor performance for gold nanostructured E-AB sensors, contingent upon the surface morphology. In particular, our results indicate that the aptamer packing density and target analyte signal change results are heavily dependent on gold nanostructure size and features. Sensing surfaces with larger nanoparticle diameters, which were prepared using electrodeposition at a constant potential, had a reduced aptamer packing density and exhibited diminished sensor performance. However, the equivalent packing density of polished electrodes did not yield the equivalent signal change. Other surfaces that were prepared using pulsed waveform electrodeposition achieved optimal signal change with a deposition time, tdep, of 120 s, and increased deposition time with enhanced electroactive surface area resulted in minimized signal changes and more rapid sensor degradation. By investigating sensing surfaces with varied morphologies, we have demonstrated that enhancing the electroactive surface does not always enhance the signal change of the sensor, and aptamer packing density alone does not dictate observed signal change trends. We anticipate that understanding how electrodeposition techniques enhance or diminish sensor performance will pave the way for further exploration of nanostructure-aptamer relationships, contributing to the future development of optimized, miniaturized electrochemical aptamer-based sensors for continuous, in vivo sensing.

Magnetic Removal of Micro- and Nanoplastics from Water-from 100 nm to 100 µm Debris Size

Clean water is one of the most important resources of the planet but human-made contamination with diverse pollutants increases continuously. Microplastics (<5 mm diameter) which can have severe impacts on the environment, are present worldwide. Degradation processes lead to nanoplastics (<1 µm), which are potentially even more dangerous due to their increased bioavailability. State-of-the-art wastewater treatment plants show a deficit in effectively eliminating micro- and nanoplastics (MNP) from water, particularly in the case of nanoplastics. In this work, the magnetic removal of three different MNP types across three orders of magnitude in size (100 nm-100 µm) is investigated systematically. Superparamagnetic iron oxide nanoparticles (SPIONs) tend to attract oppositely charged MNPs and form aggregates that can be easily collected by a magnet. It shows that especially the smallest fractions (100-300 nm) can be separated in ordinary high numbers (1013  mg-1 SPION) while the highest mass is removed for MNP between 2.5 and 5 µm. The universal trend for all three types of MNP can be fitted with a derived model, which can make predictions for optimizing SPIONs for specific size ranges in the future.

Negative Charge-Carrying Glycans Attached to Exosomes as Novel Liquid Biopsy Marker

Prostate cancer (PCa) is the second most common cancer. In this paper, the isolation and properties of exosomes as potential novel liquid biopsy markers for early PCa liquid biopsy diagnosis are investigated using two prostate human cell lines, i.e., benign (control) cell line RWPE1 and carcinoma cell line 22Rv1. Exosomes produced by both cell lines are characterised by various methods including nanoparticle-tracking analysis, dynamic light scattering, scanning electron microscopy and atomic force microscopy. In addition, surface plasmon resonance (SPR) is used to study three different receptors on the exosomal surface (CD63, CD81 and prostate-specific membrane antigen-PMSA), implementing monoclonal antibodies and identifying the type of glycans present on the surface of exosomes using lectins (glycan-recognising proteins). Electrochemical analysis is used to understand the interfacial properties of exosomes. The results indicate that cancerous exosomes are smaller, are produced at higher concentrations, and exhibit more nega tive zeta potential than the control exosomes. The SPR experiments confirm that negatively charged α-2,3- and α-2,6-sialic acid-containing glycans are found in greater abundance on carcinoma exosomes, whereas bisecting and branched glycans are more abundant in the control exosomes. The SPR results also show that a sandwich antibody/exosomes/lectins configuration could be constructed for effective glycoprofiling of exosomes as a novel liquid biopsy marker.

Self-Assembled Monolayer and Nanoparticles Coenhanced Fragmented Silver Nanowire Network Memristor

Silver nanowire (AgNW) networks with self-assembled structures and synaptic connectivity have been recently reported for constructing neuromorphic memristors. However, resistive switching at the cross-point junctions of the network is unstable due to locally enhanced Joule heating and the Gibbs-Thomson effect, which poses an obstacle to the integration of threshold switching and memory function in the same AgNW memristor. Here, fragmented AgNW networks combined with Ag nanoparticles (AgNPs) and mercapto self-assembled monolayers (SAMs) are devised to construct memristors with stable threshold switching and memory behavior. In the above design, the planar gaps between NW segments are for resistive switching, the AgNPs act as metal islands in the gaps to reduce threshold voltage (Vth) and holding voltage (Vhold), and the SAMs suppress surface atom diffusion to avoid Oswald ripening of the AgNPs, which improves switching stability. The fragmented NW-NP/SAM memristors not only circumvent the side effects of conventional NW-stacked junctions to provide durable threshold switching at >Vth but also exhibit synaptic characteristics such as long-term potentiation at ultralow voltage (≪Vth). The combination of NW segments, nanoparticles, and SAMs blazes a new trail for integrating artificial neurons and synapses in AgNW network memristors.

Highly Reversible Molecular Photoswitches with Transition Metal Dichalcogenides Electrodes

The molecule-electrode coupling plays an essential role in photoresponsive devices with photochromic molecules, and the strong coupling between the molecule and the conventional electrodes leads to/ the quenching effect and limits the reversibility of molecular photoswitches. In this work, we developed a strategy of using transition metal dichalcogenides (TMDCs) electrodes to fabricate the thiol azobenzene (TAB) self-assembled monolayers (SAMs) junctions with the eutectic gallium-indium (EGaIn) technique. The current-voltage characteristics of the EGaIn/GaOx //TAB/TMDCs photoswitches showed an almost 100% reversible photoswitching behavior, which increased by ∼28% compared to EGaIn/GaOx //TAB/AuTS photoswitches. Density functional theory (DFT) calculations showed the coupling strength of the TAB-TMDCs electrode decreased by 42% compared to that of the TAB-AuTS electrode, giving rise to improved reversibility. our work demonstrated the feasibility of 2D TMDCs for fabricating SAMs-based photoswitches with unprecedentedly high reversibility.

Multiphysics Modeling of Electrochemical Impedance Spectroscopy Responses of SAM-Modified Screen-Printed Electrodes

In this work, we present a multiphysics modeling approach capable of simulating electrochemical impedance spectroscopy (EIS) responses of screen-printed electrodes (SPEs) modified with self-assembled monolayers of 11-Mercaptoundecanoic acid (MUA). Commercially available gold SPEs are electrochemically characterized through experimental cyclic voltammetry and EIS measurements with 10 mM [Fe(CN)6]3-/4- redox couple in phosphate buffered saline before and after the surface immobilization of MUA at different concentrations. We design the multiphysics model through COMSOL Multiphysics® based on the 3D geometry of the devices under test. The model includes four different physics considering the metal/solution interface electrochemical phenomena, the ion and electron potentials and currents, and the measurement set-up. The model is calibrated through a set of experimental measurements, allowing the tuning of the parameters used by the model. We use the calibrated model to simulate the EIS response of MUA-modified SPEs, comparing the results with experimental data. The simulations fit the experimental curves well, following the variation of MUA concentration on the surface from 1 µM to 100 µM. The EIS parameters, retrieved through a CPE-modified Randles' circuit, confirm the consistency with the experimental data. Notably, the simulated surface coverage estimates and the variation of charge transfer resistance due to MUA-immobilization are well matched with their experimental counterparts, reporting only a 2% difference and being consistent with the experimental electrochemical behavior of the SPEs.

The Role of Self-Assembled Monolayers in the Surface Modification and Interfacial Contact of Copper Fillers in Electrically Conductive Adhesives

Printing of electrical circuits and interconnects using isotropic conductive adhesives (ICAs) is of great interest due to their low-temperature processing and compatibility with substrates for applications in sensors, healthcare, and flexible devices. As a lower cost alternative to silver (Ag), copper (Cu)-filled ICAs are desirable but limited by the formation of high-resistivity Cu surface oxides. To overcome this limitation, self-assembled monolayers (SAMs) of octadecanethiol (ODT) have been demonstrated to reduce the oxidation of micrometer-scale Cu powder particles for use in ICAs. However, the deposition and function of the SAM require further investigation, as described in this paper. As part of this work, the stages of the SAM deposition process, which included etching with hydrochloric acid to remove pre-existing oxides, were studied using X-ray photoelectron spectroscopy (XPS), which showed low levels of subsequent Cu oxidation when ODT coated. The treated Cu powders were combined with one- or two-part epoxy resins to make Cu-ICAs, and the effect of the Cu surface condition and weight loading on electrical conductivity was examined. When thermally cured in an inert argon atmosphere, ICAs filled with Cu protected by ODT achieved electrical conductivity up to 20 × 105 S·m-1, comparable to Ag-ICAs, and were used to make a functional circuit. To understand the function of the SAM in these Cu-ICAs, scanning and transmission electron microscopy were used to examine the internal micro- and nano-structures along with the elemental distribution at the interfaces within sections taken from cured samples. Sulfur (S), indicative of the ODT, was still detected at the internal polymer-metal interface after curing, and particle-to-particle contacts were also examined. XPS also identified S on the surface of cured Cu-ICAs even after thermal treatment. Based on the observations, electrical contact and conduction mechanisms for these Cu-filled ICAs are proposed and discussed.

Dual-Use Self-Assembled Monolayer Controlling Charge Carrier Extraction in Organic Solar Cells

The development and use of interface materials are essential to the continued advancement of organic solar cells (OSCs) performance. Self-assembled monolayer (SAM) materials have drawn attention because of their simple structure and affordable price. Due to their unique properties, they may be used in inverted devices as a modification layer for modifying ZnO or as a hole transport layer (HTL) in place of typical poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) in conventional devices. In this work, zinc oxide (ZnO) is modified using five structurally similar SAM materials. This resulted in a smoother surface, a decrease in work function, a suppression of charge recombination, and an increase in device efficiency and photostability. In addition, they can introduced asfor hole extraction layer between the active layer and MoO3 , enabling the use of the same material at several functional layers in the same device. Through systematic orthogonal evaluation, it is shown that some SAM/active layer/SAM combinations still offered device efficiencies comparable to ZnO/SAM, but with improved device' photostability. This study may provide recommendations for future SAM material's design and development as well as a strategy for boosting device performance by using the same material across both sides of the photoactive layer in OSCs.

SpyDirect: A Novel Biofunctionalization Method for High Stability and Longevity of Electronic Biosensors

Electronic immunosensors are indispensable tools for diagnostics, particularly in scenarios demanding immediate results. Conventionally, these sensors rely on the chemical immobilization of antibodies onto electrodes. However, globular proteins tend to adsorb and unfold on these surfaces. Therefore, self-assembled monolayers (SAMs) of thiolated alkyl molecules are commonly used for indirect gold-antibody coupling. Here, a limitation associated with SAMs is revealed, wherein they curtail the longevity of protein sensors, particularly when integrated into the state-of-the-art transducer of organic bioelectronics-the organic electrochemical transistor. The SpyDirect method is introduced, generating an ultrahigh-density array of oriented nanobody receptors stably linked to the gold electrode without any SAMs. It is accomplished by directly coupling cysteine-terminated and orientation-optimized spyTag peptides, onto which nanobody-spyCatcher fusion proteins are autocatalytically attached, yielding a dense and uniform biorecognition layer. The structure-guided design optimizes the conformation and packing of flexibly tethered nanobodies. This biolayer enhances shelf-life and reduces background noise in various complex media. SpyDirect functionalization is faster and easier than SAM-based methods and does not necessitate organic solvents, rendering the sensors eco-friendly, accessible, and amenable to scalability. SpyDirect represents a broadly applicable biofunctionalization method for enhancing the cost-effectiveness, sustainability, and longevity of electronic biosensors, all without compromising sensitivity.

The Geometry of Nanoparticle-on-Mirror Plasmonic Nanocavities Impacts Surface-Enhanced Raman Scattering Backgrounds

Surface-enhanced Raman scattering (SERS) has garnered substantial attention due to its ability to achieve single-molecule sensitivity by utilizing metallic nanostructures to amplify the exceedingly weak Raman scattering process. However, the introduction of metal nanostructures can induce a background continuum which can reduce the ultimate sensitivity of SERS in ways that are not yet well understood. Here, we investigate the impact of laser irradiation on both Raman scattering and backgrounds from self-assembled monolayers within nanoparticle-on-mirror plasmonic nanocavities with variable geometry. We find that laser irradiation can reduce the height of the monolayer by inducing an irreversible change in molecular conformation. The resulting increased plasmon confinement in the nanocavities not only enhances the SERS signal, but also provides momentum conservation in the inelastic light scattering of electrons, contributing to the enhancement of the background continuum. The plasmon confinement can be modified by changing the size and the geometry of nanoparticles, resulting in a nanoparticle geometry-dependent background continuum in SERS. Our work provides new routes for further modifying the geometry of plasmonic nanostructures to improve SERS sensitivity.

Cholesterol-Enriched Hybrid Lipid Bilayer Formation on Inverse Phosphocholine Lipid-Functionalized Titanium Oxide Surfaces

Hybrid lipid bilayers (HLBs) are rugged biomimetic cell membrane interfaces that can form on inorganic surfaces and be designed to contain biologically important components like cholesterol. In general, HLBs are formed by depositing phospholipids on top of a hydrophobic self-assembled monolayer (SAM) composed of one-tail amphiphiles, while recent findings have shown that two-tail amphiphiles such as inverse phosphocholine (CP) lipids can have advantageous properties to promote zwitterionic HLB formation. Herein, we explored the feasibility of fabricating cholesterol-enriched HLBs on CP SAM-functionalized TiO2 surfaces with the solvent exchange and vesicle fusion methods. All stages of the HLB fabrication process were tracked by quartz crystal microbalance-dissipation (QCM-D) measurements and revealed important differences in fabrication outcome depending on the chosen method. With the solvent exchange method, it was possible to fabricate HLBs with well-controlled cholesterol fractions up to ~65 mol% in the upper leaflet as confirmed by a methyl-β-cyclodextrin (MβCD) extraction assay. In marked contrast, the vesicle fusion method was only effective at forming HLBs from precursor vesicles containing up to ~35 mol% cholesterol, but this performance was still superior to past results on hydrophilic SiO2. We discuss the contributing factors to the different efficiencies of the two methods as well as the general utility of two-tail CP SAMs as favorable interfaces to incorporate cholesterol into HLBs. Accordingly, our findings support that the solvent exchange method is a versatile tool to fabricate cholesterol-enriched HLBs on CP SAM-functionalized TiO2 surfaces.

Sensitive Detection of Trace Explosives by a Self-Assembled Monolayer Sensor

Fluorescence probe technology holds great promise in the application of trace explosive detection due to its high sensitivity, fast response speed, good selectivity, and low cost. In this work, a designed approach has been employed to prepare the TPE-PA-8 molecule, utilizing the classic aggregation-induced emission (AIE) property of 1,1,2,2-tetraphenylethene (TPE), for the development of self-assembled monolayers (SAMs) targeting the detection of trace nitroaromatic compound (NAC) explosives. The phosphoric acid acts as an anchoring unit, connecting to TPE through an alkyl chain of eight molecules, which has been found to play a crucial role in promoting the aggregation of TPE luminogens, leading to the enhanced light-emission property and sensing performance of SAMs. The SAMs assembled on Al2O3-deposited fiber film exhibit remarkable detection performances, with detection limits of 0.68 ppm, 1.68 ppm, and 2.5 ppm for trinitrotoluene, dinitrotoluene, and nitrobenzene, respectively. This work provides a candidate for the design and fabrication of flexible sensors possessing the high-performance and user-friendly detection of trace NACs.

Decoding the Self-Assembly Plasmonic Interface Structure in a PbS Colloidal Quantum Dot Solid for a Photodetector

Hybrid plasmonic nanostructures have gained enormous attention in a variety of optoelectronic devices due to their surface plasmon resonance properties. Self-assembled hybrid metal/quantum dot (QD) architectures offer a means of coupling the properties of plasmonics and QDs to photodetectors, thereby modifying their functionality. The arrangement and localization of hybrid nanostructures have an impact on exciton trapping and light harvesting. Here, we present a hybrid structure consisting of self-assembled gold nanospheres (Au NSs) embedded in a solid matrix of PbS QDs for mapping the interface structures and the motion of charge carriers. Grazing-incidence small-angle X-ray scattering is utilized to analyze the localization and spacing of the Au NSs within the hybrid structure. Furthermore, by correlating the morphology of the Au NSs in the hybrid structure with the corresponding differences observed in the performance of photodetectors, we are able to determine the impact of interface charge carrier dynamics in the coupling structure. From the perspective of architecture, our study provides insights into the performance improvement of optoelectronic devices.

Compact Hole-Selective Self-Assembled Monolayers Enabled by Disassembling Micelles in Solution for Efficient Perovskite Solar Cells

Self-assembled monolayers (SAMs) are widely employed as effective hole-selective layers (HSLs) in inverted perovskite solar cells (PSCs). However, most SAM molecules are amphiphilic in nature and tend to form micelles in the commonly used alcoholic processing solvents. This introduces an extra energetic barrier to disassemble the micelles during the binding of SAM molecules on the substrate surface, limiting the formation of a compact SAM. To alleviate this problem for achieving optimal SAM growth, a co-solvent strategy to disassemble the micelles of carbazole-based SAM molecules in the processing solution is developed. This effectively increases the critical micelle concentration to be above the processing concentration and enhances the reactivity of the phosphonic acid anchoring group to allow densely packed SAMs to be formed on indium tin oxide. Consequently, the PSCs derived from using MeO-2PACz, 2PACz, and CbzNaph SAM HSLs show universally improved performance, with the CbzNaph SAM-derived device achieving a champion efficiency of 24.98% and improved stability.

Self-Assembled Monolayer Doping for MoTe2 Field-Effect Transistors: Overcoming PN Doping Challenges in Transition Metal Dichalcogenides

Transition metal dichalcogenides (TMDs) have gained significant attention as next-generation semiconductor materials that could potentially overcome the integration limits of silicon-based electronic devices. However, a challenge in utilizing TMDs as semiconductors is the lack of an established PN doping method to effectively control their electrical properties, unlike those of silicon-based semiconductors. Conventional PN doping methods, such as ion implantation, can induce lattice damage in TMDs. Thus, chemical doping methods that can control the Schottky barrier while minimizing lattice damage are desirable. Here, we focus on the molybdenum ditelluride (2H-MoTe2), which has a hexagonal phase and exhibits ambipolar field-effect transistor (FET) properties due to its direct band gap of 1.1 eV, enabling concurrent transport of electrons and holes. We demonstrate the fabrication of p- or n-type unipolar FETs in ambipolar MoTe2 FETs using self-assembled monolayers (SAMs) as chemical dopants. Specifically, we employ 1H,1H,2H,2H perfluorooctyltriethoxysilane and (3-aminopropyl)triethoxysilane as SAMs for chemical doping. The selective SAMs effectively increase the hole and electron charge transport capabilities in MoTe2 FETs by 18.4- and 4.6-fold, respectively, due to the dipole effect of the SAMs. Furthermore, the Raman shift of MoTe2 by SAM coating confirms the successful p- and n-type doping. Finally, we demonstrate the fabrication of complementary inverters using SAMs-doped MoTe2 FETs, which exhibit clear full-swing capability compared to undoped complementary inverters.

Machine Learning Attacks-Resistant Security by Mixed-Assembled Layers-Inserted Graphene Physically Unclonable Function

Mixed layers of octadecyltrichlorosilane (ODTS) and 1H,1H,2H,2H-perfluorooctyltriethoxysilane (FOTS) on an active layer of graphene are used to induce a disordered doping state and form a robust defense system against machine-learning attacks (ML attacks). The resulting security key is formed from a 12 × 12 array of currents produced at a low voltage of 100 mV. The uniformity and inter-Hamming distance (HD) of the security key are 50.0 ± 12.3% and 45.5 ± 16.7%, respectively, indicating higher security performance than other graphene-based security keys. Raman spectroscopy confirmed the uniqueness of the 10,000 points, with the degree of shift of the G peak distinguishing the number of carriers. The resulting defense system has a 10.33% ML attack accuracy, while a FOTS-inserted graphene device is easily predictable with a 44.81% ML attack accuracy.

Detection of Aromatic Hydrocarbons in Aqueous Solutions Using Quartz Tuning Fork Sensors Modified with Calix[4]arene Methoxy Ester Self-Assembled Monolayers: Experimental and Density Functional Theory Study

Quartz tuning forks (QTFs), which were coated with gold and with self-assembled monolayers (SAM) of a lower-rim functionalized calix[4]arene methoxy ester (CME), were used for the detection of benzene, toluene, and ethylbenzene in water samples. The QTF device was tested by measuring the respective frequency shifts obtained using small (100 µL) samples of aqueous benzene, toluene, and ethylbenzene at four different concentrations (10-12, 10-10, 10-8, and 10-6 M). The QTFs had lower limits of detection for all three aromatic hydrocarbons in the 10-14 M range, with the highest resonance frequency shifts (±5%) being shown for the corresponding 10-6 M solutions in the following order: benzene (199 Hz) > toluene (191 Hz) > ethylbenzene (149 Hz). The frequency shifts measured with the QTFs relative to that in deionized water were inversely proportional to the concentration/mass of the analytes. Insights into the effects of the alkyl groups of the aromatic hydrocarbons on the electronic interaction energies for their hypothetical 1:1 supramolecular host-guest binding with the CME sensing layer were obtained through density functional theory (DFT) calculations of the electronic interaction energies (ΔIEs) using B3LYP-D3/GenECP with a mixed basis set: LANL2DZ and 6-311++g(d,p), CAM-B3LYP/LANL2DZ, and PBE/LANL2DZ. The magnitudes of the ΔIEs were in the following order: [Au4-CME⊃[benzene] > [Au4-CME]⊃[toluene] > [Au4-CME]⊃[ethylbenzene]. The gas-phase BSSE-uncorrected ΔIE values for these complexes were higher, with values of -96.86, -87.80, and -79.33 kJ mol-1, respectively, and -86.39, -77.23, and -67.63 kJ mol-1, respectively, for the corresponding BSSE-corrected values using B3LYP-D3/GenECP with LANL2dZ and 6-311++g(d,p). The computational findings strongly support the experimental results, revealing the same trend in the ΔIEs for the proposed hypothetical binding modes between the tested analytes with the CME SAMs on the Au-QTF sensing surfaces.

Molecular Mechanism of Selective Al2O3 Atomic Layer Deposition on Self-Assembled Monolayers

Area-selective atomic layer deposition (AS-ALD) of insulating metallic oxide layers could be a useful nanopatterning technique for making increasingly complex semiconductor circuits. Although the alkanethiol self-assembled monolayer (SAM) has been considered promising as an ALD inhibitor, the low inhibition efficiency of the SAM during ALD processes makes its wide application difficult. We investigated the deposition mechanism of Al2O3 on alkanethiol-SAMs using temperature-dependent vibrational sum-frequency-generation spectroscopy. We found that the thermally induced formation of gauche defects in the SAMs is the main causative factor deteriorating the inhibition efficiency. Here, we demonstrate that a discontinuously temperature-controlled ALD technique involving self-healing and dissipation of thermally induced stress on the structure of SAM substantially enhances the SAM's inhibition efficiency and enables us to achieve 60 ALD cycles (6.6 nm). We anticipate that the present experimental results on the ALD mechanism on the SAM surface and the proposed ALD method will provide clues to improve the efficiency of AS-ALD, a promising nanoscale patterning and manufacturing technique.

SAM-Support-Based Electrochemical Sensor for Aβ Biomarker Detection of Alzheimer's Disease

Alzheimer's disease has taken the spotlight as a neurodegenerative disease which has caused crucial issues to both society and the economy. Specifically, aging populations in developed countries face an increasingly serious problem due to the increasing budget for patient care and an inadequate labor force, and therefore a solution is urgently needed. Recently, diverse techniques for the detection of Alzheimer's biomarkers have been researched and developed to support early diagnosis and treatment. Among them, electrochemical biosensors and electrode modification proved their effectiveness in the detection of the Aβ biomarker at appropriately low concentrations for practice and point-of-care application. This review discusses the production and detection ability of amyloid beta, an Alzheimer's biomarker, by electrochemical biosensors with SAM support for antibody conjugation. In addition, future perspectives on SAM for the improvement of electrochemical biosensors are also proposed and discussed.

Fabrication of Highly Luminescent Quasi Two-Dimensional CsPbBr3 Perovskite Films in High Humidity Air for Light-Emitting Diodes

Perovskite light-emitting diodes (LEDs) have attracted extensive attention in recent years due to their outstanding performance and promise in lighting and display applications. However, the fabrication of perovskite LEDs usually requires a low-humidity atmosphere, which is unfavorable for industrial production. Herein, we report an effective strategy to fabricate highly luminescent quasi two-dimensional CsPbBr₃ perovskite films in an ambient atmosphere with a humidity up to 60%. We reveal that the hole transport layer (HTL) plays a significant role in the morphology and optical properties of the perovskite films. Using hydrophobic self-assembled monolayer materials as HTLs can remarkably improve the quality of the perovskite films processed in high humidity air. The resultant perovskite LEDs show reduced leakage current and significantly enhanced performance. Furthermore, surface treatment is conducted to prevent water invasion and promote radiative recombination in perovskite films and LEDs. Eventually, the perovskite LEDs exhibit bright green emission with an external quantum efficiency of 4.87%. The present work provides a feasible pathway to overcome the humidity limitation for obtaining bright perovskite films and LEDs, which would contribute to further reducing the fabrication cost of perovskite LEDs and promoting their applications.

Perovskite Solar Cell Using Isonicotinic Acid as a Gap-Filling Self-Assembled Monolayer with High Photovoltaic Performance and Light Stability

High photovoltaic performance and light stability are required for the practical outdoor use of lead-halide perovskite solar cells. To improve the light stability of perovskite solar cells, it is effective to introduce a self-assembled monolayer (SAM) between the carrier transport layer and the perovskite layer. Several alternative approaches in their molecular design and combination with multiple SAMs support high photovoltaic conversion efficiency (PCE). Herein, we report a new structure for improving both PCE and light stability, in which the surface of an electron transport layer (ETL) was modified by combining a fullerene-functionalized self-assembled monolayer (C₆₀SAM) and a suitable gap-filling self-assembled monolayer (GFSAM). Small-sized GFSAMs can enter the gap space of the C₆₀SAM and terminate the unterminated sites on the ETL surface. The best GFSAM in this study was formed using an isonicotinic acid solution. After a light stability test for 68 h at 50 °C under 1 sun illumination, the best cell with C₆₀SAM and GFSAM showed a PCE of 18.68% with a retention rate of over 99%. Moreover, following outdoor exposure for six months, the cells with C₆₀SAM and GFSAM exhibited almost unchanged PCE. From the valence band spectra of the ETLs obtained using hard X-ray photoelectron spectroscopy, we confirmed a decrease in the offset at the ETL/perovskite interface owing to the additional GFSAM treatment on the C₆₀SAM-modified ETL surface. Time-resolved microwave conductivity measurements demonstrated that the additional GFSAM improved electron extraction at the C₆₀SAM-modified ETL/perovskite interface.

Interfacial Molecular Compatibility for Programming Organic-Metal Oxide Superlattices

Artificially programming a sequence of organic-metal oxide multilayers (superlattices) by using atomic layer deposition (ALD) is a fascinating and challenging issue in material chemistry. However, the complex chemical reactions between ALD precursors and organic layer surfaces have limited their applications for various material combinations. Here, we demonstrate the impact of interfacial molecular compatibility on the formation of organic-metal oxide superlattices using ALD. The effects of both organic and inorganic compositions on the metal oxide layer formation processes onto self-assembled monolayers (SAM) were examined by using scanning transmission electron microscopy, in situ quartz crystal microbalance measurements, and Fourier-transformed infrared spectroscopy. These series of experiments reveal that the terminal group of organic SAM molecules must satisfy two conflicting requirements, the first of which is to promptly react with ALD precursors and the second is not to bind strongly to the bottom metal oxide layers to avoid undesired SAM conformations. OH-terminated phosphate aliphatic molecules, which we have synthesized, were identified as one of the best candidates for such a purpose. Molecular compatibility between metal oxide precursors and the -OHs must be properly considered to form superlattices. In addition, it is also important to form densely packed and all-trans-like SAMs to maximize the surface density of reactive -OHs on the SAMs. Based on these design strategies for organic-metal oxide superlattices, we have successfully fabricated various superlattices composed of metal oxides (Al-, Hf-, Mg-, Sn-, Ti-, and Zr oxides) and their multilayered structures.

Gold-Supported Lipid Membranes Formed by Redox-Triggered Vesicle Fusion on Binary Self-Assembled Monolayers: Ion-Pairing Association and Surface Hydrophilicity

The assembly of biomimetic, planar supported lipid bilayers (SLBs) by the popular vesicle fusion method, which relies on the spontaneous adsorption and rupture of small unilamellar vesicles from aqueous solution on a solid surface, typically works with a limited range of support materials and lipid systems. We previously reported a conceptual advance in the formation of SLBs from vesicles in the gel or fluid phase using the interfacial ion-pairing association of charged phospholipid headgroups with electrochemically generated cationic ferroceniums bound to a self-assembled monolayer (SAM) chemisorbed to gold. This redox-driven approach lays down a single bilayer membrane on the SAM-modified gold surface at room temperature within minutes and is compatible with both anionic and zwitterionic phospholipids. The present work explores the effects of the surface ferrocene concentration and hydrophobicity/hydrophilicity on the formation of continuous SLBs of dialkyl phosphatidylserine, dialkyl phosphatidylglycerol, and dialkyl phosphatidylcholine using binary SAMs of ferrocenylundecanethiolate (FcC₁₁S) and dodecanethiolate (CH₃C₁₁S) or hydroxylundecanethiolate (HOC₁₁S) comprising different surface mole fractions of ferrocene (χ_Fc^surf). An increase in the surface hydrophilicity and surface free energy of the FcC₁₁S/HOC₁₁S SAM mitigates the decrease in the attractive ion-pairing interactions resulting from a reduced χ_Fc^surf. SLBs of ≳80% area coverage form on the FcC₁₁S/HOC₁₁S SAM for all the phospholipid types down to χ_Fc^surf of at least 0.2, composition yielding a water contact angle (θ_W) of 44 ± 4°. By contrast, a greater number of ion-pairing interactions is required on the hydrophobic FcC₁₁S/CH₃C₁₁S surface to drive the vesicle fusion process; bilayers or bilayer patches form at χ_Fc^surf ≳ 0.6 (θ_W = 97 ± 3°). These findings will aid in tailoring the surface chemistry of redox-active modified surfaces to widen the conditions that yield supported lipid membranes.

Self-Assembled Monolayer for Low-Power-Consumption, Long-Term-Stability, and High-Efficiency Quantum Dot Light-Emitting Diodes

Quantum dot light-emitting diodes (QLEDs) are an emerging class of optoelectronic devices with a wide range of applications. However, there still exist several drawbacks preventing their applications, including long-term stability, electron leakage, and large power consumption. To circumvent the difficulties, QLEDs based on a self-assembled hole transport layer (HTL) with reduced device complexity are proposed and demonstrated. The self-assembled HTL is prepared from poly[3-(6-carboxyhexyl)thiophene-2,5-diyl] (P3HT-COOH) solution in N,N-dimethylformamide (DMF) forming a well-ordered monolayer on an indium-tin-oxide (ITO) anode. The P3HT-COOH monolayer has a smaller HOMO band offset and a sufficiently large electron barrier with respect to the CdSe/ZnS quantum dot (QD) emission layer, and thus it is beneficial for hole injection into and electron leakage blocking from the QD layer. Interestingly, the QLEDs exhibit an excellent conversion efficiency (97%) in turning the injected electron-hole pairs into light emission. The performance of the resulting QLEDs possesses a low turn-on voltage of +1.2 V and a maximum external quantum efficiency of 25.19%, enabling low power consumption with high efficiency. Additionally, those QLEDs also exhibit excellent long-term stability without encapsulation with over 90% luminous intensity after 200 days and superior durability with over 70% luminous intensity after 2 h operation under the luminance of 1000 cd m^-2. The outstanding device features of our proposed QLEDs, including low turn-on voltage, high efficiency, and long-term stability, can advance the development of QLEDs toward facile large-area mass production and cost-effectiveness.

Key research development by Prof Mark Reed in molecular electronic devices

In memory of Professor Mark Reed, who passed away on May 5, 2021, this article summarizes a series of his past groundbreaking developments in molecular electronic devices. Specifically, three key reports are summarized; measurement of the electrical conductance of molecular junctions using the mechanically controlled break junction technique and demonstration of negative differential resistance and orbital gating effect observed in molecular junctions. Also, a brief outlook on molecular electronics research field is addressed.

Guiding Metal Organic Framework Morphology via Monolayer Artificial Defect-Induced Preferential Facet Selection

Guiding metal organic framework (MOF) morphology, especially without the need for chemical additives, still remains a challenge. For the first time, we report a unique surface guiding approach in controlling the crystal morphology formation of zeolitic imidazole framework-8 (ZIF-8) and HKUST-1 MOFs on disrupted alkanethiol self-assembled monolayer (SAM)-covered Au substrates. Selective molecule removal is applied to generate diverse SAM matrices rich in artificial molecular defects in a monolayer to direct the dynamic crystal growth process. When a 11-mercaptoundecanol alkanethiol monolayer is ruptured, the hydroxyl tail groups of surface residue molecules act as nucleating sites by coordination with precursor metal ions. Meanwhile, the exposed alkane chain backbones stabilize a particular facet of MOF nuclei in the dynamic growth by slowing down their crystal growth rates along a specific direction. The competitive formation between the [110] and [100] planes of ZIF-8 ultimately regulates the crystal shapes from rhombic dodecahedron, truncated rhombic dodecahedron, and truncated cube to cube. Similarly, changeable morphologies of HKUST-1 crystals are also achieved from cube and tetrakaidekahedron to octahedron, originating from the competitive selection between the [100] and [111] planes. In addition to the artificial matrix preferred orientation of initial nucleation, parameters such as temperature also play a crucial role in the resulting crystal morphology. Standing on the additive-free MOF crystal morphology growth control, porous architectures prepared in this approach can act as templates for ligand-free metal (Au, Ag, and Cu) nanocluster synthesis. The nanocluster-embedded MOF structures represent distinct crystal morphology-dependent optical properties, and interestingly, their fluorescence emission can be highly enhanced by facet-induced nanocluster packing alignments. These findings not only provide a unique thought on MOF crystal morphology guidance but also pave a new route for the accompanied property investigation and further application.

Disordered Phase-Assisted Growth of Organic Semiconductor Crystals on Self-Assembled Monolayer Templates

Achieving high mobility and bias stability is a challenging obstacle in the advancement of organic thin-film transistors (OTFTs). To this end, the fabrication of high-quality organic semiconductor (OSC) thin films is critical for OTFTs. Self-assembled monolayers (SAMs) have been used as growth templates for high-crystalline OSC thin films. Despite significant research progress in the growth of OSC on SAMs, a detailed understanding of the growth mechanism of the OSC thin films on a SAM template is lacking, which has limited its use. In this study, the effects of the structure (thickness and molecular packing) of SAM on the nucleation and growth behavior of the OSC thin films were investigated. We found that disordered SAM molecules assisted in the surface diffusion of the OSC molecules and resulted in a small nucleation density and large grain size of the OSC thin films. Moreover, a thick SAM with disordered SAM molecules on the top was found to be beneficial for the high mobility and bias stability of the OTFTs.

Electrochemical Stability of Thiolate Self-Assembled Monolayers on Au, Pt, and Cu

Self-assembled monolayers (SAMs) of thiolates have increasingly been used for modification of metal surfaces in electrochemical applications including selective catalysis (e.g., CO₂ reduction, nitrogen reduction) and chemical sensing. Here, the stable electrochemical potential window of thiolate SAMs on Au, Pt, and Cu electrodes is systematically studied for a variety of thiols in aqueous electrolyte systems. For fixed tail-group functionality, the reductive stability of thiolate SAMs is found to follow the trend Au < Pt < Cu; this can be understood by considering the combined influences of the binding strength of sulfur and competitive adsorption of hydrogen. The oxidative stability of thiolate SAMs is found to follow the order: Cu < Pt < Au, consistent with each surface's propensity toward surface oxide formation. The stable reductive and oxidative potential limits are both found to vary linearly with pH, except for reduction above pH ∼10, which is independent of pH for most thiol compositions. The electrochemical stability across different functionalized thiols is then revealed to depend on many different factors including SAM defects (accessible surface metal atom sites decrease stability), intermolecular interactions (hydrophilic groups reduce the stability), and SAM thickness (stability increases with alkanethiol carbon chain length) as well as factors such as SAM-induced surface reconstruction and the ability to directly oxidize or reduce the non-sulfur part of the SAM molecule.

Impedimetric nanoimmunosensor platform for aflatoxin B1 detection in peanuts

This article developed a novel electrochemical immunosensor for the specific detection of aflatoxin B1 (AFB1). Amino-functionalized iron oxide nanoparticles (Fe₃ O₄ -NH₂ ) were synthesized. Fe₃ O₄ -NH₂ were chemically bound on self-assembly monolayers (SAMs) of mercaptobenzoic acid (MBA). Finally, polyclonal antibodies (pAb) were immobilized on Fe₃ O₄ -NH₂ -MBA. The sensor system was evaluated through atomic force microscopy (AFM), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). A reduction in the anodic and cathodic peak currents was observed after the assembly of the sensor platform. The charge transfer resistance (R_ct ) was increased due to the electrically insulating bioconjugates. Then, the specific interaction between the sensor platform and AFB1 blocks the electron transfer of the [Fe(CN)₆ ]^3-/4- redox pair. The nanoimmunosensor showed a linear response range estimated from 0.5 to 30 μg/mL with a limit of detection (LOD) of 9.47 μg/mL and a limit of quantification (LOQ) of 28.72 μg/mL for AFB1 identification in a purified sample. In addition, a LOD of 3.79 μg/mL, a LOQ of 11.48 μg/mL, and a regression coefficient of 0.9891 were estimated for biodetection tests on peanut samples. The proposed immunosensor represents a simple alternative, successfully applied in detecting AFB1 in peanuts, and therefore, represents a valuable tool for ensuring food safety.

Guiding Epilepsy Surgery with an LRP1-Targeted SPECT/SERRS Dual-Mode Imaging Probe

Accurate identification of the resectable epileptic lesion is a precondition of operative intervention to drug-resistant epilepsy (DRE) patients. However, even when multiple diagnostic modalities are combined, epileptic foci cannot be accurately identified in ∼30% of DRE patients. Inflammation-associated low-density lipoprotein receptor-related protein-1 (LRP1) has been validated to be a surrogate target for imaging epileptic foci. Here, we reported an LRP1-targeted dual-mode probe that is capable of providing comprehensive epilepsy information preoperatively with SPECT imaging while intraoperatively delineating epileptic margins in a sensitive high-contrast manner with surface-enhanced resonance Raman scattering (SERRS) imaging. Notably, a novel and universal strategy for constructing self-assembled monolayer (SAM)-based Raman reporters was proposed for boosting the sensitivity, stability, reproducibility, and quantifiability of the SERRS signal. The probe showed high efficacy to penetrate the blood-brain barrier. SPECT imaging showed the probe could delineate the epileptic foci clearly with a high target-to-background ratio (4.11 ± 0.71, 2 h). Further, with the assistance of the probe, attenuated seizure frequency in the epileptic mouse models was achieved by using SPECT together with Raman images before and during operation, respectively. Overall, this work highlights a new strategy to develop a SPECT/SERRS dual-mode probe for comprehensive epilepsy surgery that can overcome the brain shift by the co-registration of preoperative SPECT and SERRS intraoperative images.

Gap-directed chemical lift-off lithographic nanoarchitectonics for arbitrary sub-micrometer patterning

We introduce a unique soft lithographic operation that exploits stamp roof collapse-induced gaps to selectively remove an alkanethiol self-assembled monolayer (SAM) on Au to generate surface patterns that are orders of magnitude smaller than structures on the original elastomer stamp. The smallest achieved feature dimension is 5 nm using a micrometer-scale structured stamp in a chemical lift-off lithography (CLL) process. Molecular patterns retained in the gaps between stamp features and their circumscribed or inscribed circles follow mathematical predictions, and their sizes can be tuned by altering the stamp structure dimensions, including height, pitch, and shape. These generated surface molecular patterns can function as biorecognition arrays or be transferred to the underneath Au layer for metallic structure creation. By combining CLL process with this gap phenomenon, soft material properties that are previously thought as demerits can be used to achieve sub-10 nm features in a straightforward sketch.

Tailored Self-Assembled Monolayer using Chemical Coupling for Indium-Gallium-Zinc Oxide Thin-Film Transistors: Multifunctional Copper Diffusion Barrier

Controlling the contact properties of a copper (Cu) electrode is an important process for improving the performance of an amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistor (TFT) for high-speed applications, owing to the low resistance-capacitance product constant of Cu. One of the many challenges in Cu application to a-IGZO is inhibiting high diffusivity, which causes degradation in the performance of a-IGZO TFT by forming electron trap states. A self-assembled monolayer (SAM) can perfectly act as a Cu diffusion barrier (DB) and passivation layer that prevents moisture and oxygen, which can deteriorate the TFT on-off performance. However, traditional SAM materials have high contact resistance and low mechanical-adhesion properties. In this study, we demonstrate that tailoring the SAM using the chemical coupling method can enhance the electrical and mechanical properties of a-IGZO TFTs. The doping effects from the dipole moment of the tailored SAMs enhance the electrical properties of a-IGZO TFTs, resulting in a field-effect mobility of 13.87 cm²/V·s, an on-off ratio above 10⁷, and a low contact resistance of 612 Ω. Because of the high electrical performance of tailored SAMs, they function as a Cu DB and a passivation layer. Moreover, a selectively tailored functional group can improve the adhesion properties between Cu and a-IGZO. These multifunctionally tailored SAMs can be a promising candidate for a very thin Cu DB in future electronic technology.

Self-Assembled Monolayers of Molecular Conductors with Terpyridine-Metal Redox Switching Elements: A Combined AFM, STM and Electrochemical Study

Self-assembled monolayers (SAMs) of terpyridine-based transition metal (ruthenium and osmium) complexes, anchored to gold substrate via tripodal anchoring groups, have been investigated as possible redox switching elements for molecular electronics. An electrochemical study was complemented by atomic force microscopy (AFM) and scanning tunneling microscopy (STM) methods. STM was used for determination of the SAM conductance values, and computation of the attenuation factor β from tunneling current-distance curves. We have shown that SAMs of Os-tripod molecules contain larger adlayer structures compared with SAMs of Ru-tripod molecules, which are characterized by a large number of almost evenly distributed small islands. Furthermore, upon cyclic voltammetric experimentation, Os-tripod films rearrange to form a smaller number of even larger islands, reminiscent of the Ostwald ripening process. Os-tripod SAMs displayed a higher surface concentration of molecules and lower conductance compared with Ru-tripod SAMs. The attenuation factor of Os-tripod films changed dramatically, upon electrochemical cycling, to a higher value. These observations are in accordance with previously reported electron transfer kinetics studies.

Learning the relationship between nanoscale chemical patterning and hydrophobicity

The hydrophobicity of proteins and similar surfaces, which display chemical heterogeneity at the nanoscale, drives countless aqueous interactions and assemblies. However, predicting how surface chemical patterning influences hydrophobicity remains a challenge. Here, we address this challenge by using molecular simulations and machine learning to characterize and model the hydrophobicity of a diverse library of patterned surfaces, spanning a wide range of sizes, shapes, and chemical compositions. We find that simple models, based only on polar content, are inaccurate, whereas complex neural network models are accurate but challenging to interpret. However, by systematically incorporating chemical correlations between surface groups into our models, we are able to construct a series of minimal models of hydrophobicity, which are both accurate and interpretable. Our models highlight that the number of proximal polar groups is a key determinant of hydrophobicity and that polar neighbors enhance hydrophobicity. Although our minimal models are trained on particular patch size and shape, their interpretability enables us to generalize them to rectangular patches of all shapes and sizes. We also demonstrate how our models can be used to predict hot-spot locations with the largest marginal contributions to hydrophobicity and to design chemical patterns that have a fixed polar content but vary widely in their hydrophobicity. Our data-driven models and the principles they furnish for modulating hydrophobicity could facilitate the design of novel materials and engineered proteins with stronger interactions or enhanced solubilities.

AC Kelvin Probe Force Microscopy Enables Charge Mapping in Water

Mapping charged chemical groups at the solid-liquid interface is important in many areas, ranging from colloidal systems to biomolecular interactions. However, classical methods to measure surface charges either lack spatial resolution or─like Kelvin-probe force microscopy (KPFM)─cannot be applied in aqueous solutions because a DC bias voltage is used. Here, we show that using AC Kelvin probe force microscopy (AC-KPFM), in which the DC bias is replaced with an AC voltage of sufficiently high frequency, the surface potential of spatially fixated, charged surface groups can be mapped in aqueous solution. We demonstrate this with micropatterned, functionalized alkanethiol layers which expose ionized amino- and carboxy-groups. These groups are representative of the charged groups of most biomolecules such as proteins. By adjusting the pH of the solution, the charge of the groups was reversibly altered, demonstrating the electrostatic nature of the measured signal. The influence of the electric double layer (EDL) on the measurement is discussed, and we, furthermore, show how charged, micropatterned layers can be used to spatially direct the deposition of nanoparticles of opposite charge.

Threonine Phosphorylation of an Electrochemical Peptide-Based Sensor to Achieve Improved Uranyl Ion Binding Affinity

We have successfully designed a uranyl ion (U(VI)-specific peptide and used it in the fabrication of an electrochemical sensor. The 12-amino acid peptide sequence, (n) DKDGDGYIpTAAE (c), originates from calmodulin, a Ca(II)-binding protein, and contains a phosphothreonine that enhances the sequence's affinity for U(VI) over Ca(II). The sensing mechanism of this U(VI) sensor is similar to other electrochemical peptide-based sensors, which relies on the change in the flexibility of the peptide probe upon interacting with the target. The sensor was systematically characterized using alternating current voltammetry (ACV) and cyclic voltammetry. Its limit of detection was 50 nM, which is lower than the United States Environmental Protection Agency maximum contaminant level for uranium. The signal saturation time was ~40 min. In addition, it showed minimal cross-reactivity when tested against nine different metal ions, including Ca(II), Mg(II), Pb(II), Hg(II), Cu(II), Fe(II), Zn(II), Cd(II), and Cr(VI). Its reusability and ability to function in diluted aquifer and drinking water samples were further confirmed and validated. The response of the sensor fabricated with the same peptide sequence but with a nonphosphorylated threonine was also analyzed, substantiating the positive effects of threonine phosphorylation on U(VI) binding. This study places emphasis on strategic utilization of non-standard amino acids in the design of metal ion-chelating peptides, which will further diversify the types of peptide recognition elements available for metal ion sensing applications.

Optimizing Plasmonic Gold Nanorod Deposition on Glass Surfaces for High-Sensitivity Refractometric Biosensing

Owing to high surface sensitivity, gold nanorods (AuNRs) are widely used to construct surface-based nanoplasmonic biosensing platforms for label-free molecular diagnostic applications. A key fabrication step involves controlling AuNR deposition onto the target surface, which requires maximizing surface density while minimizing inter-particle aggregation, and is often achieved by surface functionalization with a self-assembled monolayer (SAM) prior to AuNR deposition. To date, existing studies have typically used a fixed concentration of SAM-forming organic molecules (0.2-10% v/v) while understanding how SAM density affects AuNR deposition and resulting sensing performance would be advantageous. Herein, we systematically investigated how controlling the (3-aminopropyl)triethoxysilane (APTES) concentration (1-30% v/v) during SAM preparation affects the fabrication of AuNR-coated glass surfaces for nanoplasmonic biosensing applications. Using scanning electron microscopy (SEM) and UV-visible spectroscopy, we identified an intermediate APTES concentration range that yielded the highest density of individually deposited AuNRs with minimal aggregation and also the highest peak wavelength in aqueous solution. Bulk refractive index sensitivity measurements indicated that the AuNR configuration had a strong effect on the sensing performance, and the corresponding wavelength-shift responses ranged from 125 to 290 nm per refractive index unit (RIU) depending on the APTES concentration used. Biosensing experiments involving protein detection and antigen-antibody interactions further demonstrated the high surface sensitivity of the optimized AuNR platform, especially in the low protein concentration range where the measurement shift was ~8-fold higher than that obtained with previously used sensing platforms.

Modification of Electrode Interface with Fullerene-Based Self-Assembled Monolayer for High-Performance Organic Optoelectronic Devices

Efficient charge transfer between organic semiconductors and electrode materials at electrode interfaces is essential for achieving high-performance organic optoelectronic devices. For efficient charge injection and extraction at the electrode interface, an interlayer is usually introduced between the organic active layer and electrode. Here, a simple and effective approach for further improving charge transfer at the organic active layer-interlayer interface was presented. Treatment of the zinc oxide (ZnO) interlayer, a commonly used n-type interlayer, with a fullerene-based self-assembled monolayer (SAM) effectively improved electron transfer at the organic-ZnO interface, without affecting the morphology and crystalline structure of the organic active layer on the cathode interlayer. Furthermore, this treatment reduced charge recombination in the device, attributed to the improved charge extraction and reduction of undesirable ZnO-donor polymer contacts. The photocurrent density and power conversion efficiency of organic solar cells employing the fullerene-SAM-treated interlayer were ~10% higher than those of the device employing the nontreated interlayer. This improvement arises from the enhanced electron extraction and reduced charge recombination.

Density Control over MBD2 Receptor-Coated Surfaces Provides Superselective Binding of Hypermethylated DNA

Using the biomarker hypermethylated DNA (hmDNA) for cancer detection requires a pretreatment to isolate or concentrate hmDNA from nonmethylated DNA. Affinity chromatography using a methyl binding domain-2 (MBD2) protein can be used, but the relatively low enrichment selectivity of MBD2 limits its clinical applicability. Here, we developed a superselective, multivalent, MBD2-coated platform to improve the selectivity of hmDNA enrichment. The multivalent platform employs control over the MBD2 surface receptor density, which is shown to strongly affect the binding of DNA with varying degrees of methylation, improving both the selectivity and the affinity of DNAs with higher numbers of methylation sites. Histidine-10-tagged MBD2 was immobilized on gold surfaces with receptor density control by tuning the amount of nickel nitrilotriacetic acid (NiNTA)-functionalized thiols in a thiol-based self-assembled monolayer. The required MBD2 surface receptor densities for DNA surface binding decreases for DNA with higher degrees of methylation. Both higher degrees of superselectivity and surface coverages were observed upon DNA binding at increasing methylation levels. Adopting the findings of this study into hmDNA enrichment of clinical samples has the potential to become more selective and sensitive than current MBD2-based methods and, therefore, to improve cancer diagnostics.

Self-Assembled 4-Aminopyridine Monolayer as a Nucleation-Inducing Layer for Transparent Silver Electrodes

The role of a self-assembled monolayer obtained by vacuum deposition of 4-aminopyridine (4-AP), a small organic molecule having amine and pyridine groups, as a metal nucleation inducer and adhesion promoter was verified, and the applicability was evaluated. 4-AP deposited to an extremely thin thickness effectively changed the substrate surface properties, increasing the nucleation density of silver (Ag) more than 3 times and eventually forming a more transparent, low-resistance Ag thin film. The optical transmittance of the Ag thin film, which was less than 60% when 4-AP was not applied, could be increased to about 77% by simply applying 4-AP, and the electrical resistance could be lowered from 37 to 14 Ω/square at the same time. Transmittance could be further improved to higher than 90% by depositing an antireflection layer for use as a transparent Ag electrode. It was also verified that 4-AP not only serves as a nucleation inducer but also contributes to improving interfacial adhesion. The Ag transparent electrode using 4-AP provided the improved performance of the organic light-emitting device due to higher transmittance, lower resistance, and surface roughness. Small organic molecules including functional groups that can be vacuum deposited, such as 4-AP, are expected to be used as surface pretreatment materials for various depositions because they can be easily patterned and can efficiently modify the surface even with extremely thin thickness.

Conductance Switching in Liquid Crystal-Inspired Self-Assembled Monolayer Junctions

We present the prototype of a ferroelectric tunnel junction (FTJ), which is based on a self-assembled monolayer (SAM) of small, functional molecules. These molecules have a structure similar to those of liquid crystals, and they are embedded between two solid-state electrodes. The SAM, which is deposited through a short sequence of simple fabrication steps, is extremely thin (3.4 ± 0.5 nm) and highly uniform. The functionality of the FTJ is ingrained in the chemical structure of the SAM components: a conformationally flexible dipole that can be reversibly reoriented in an electrical field. Thus, the SAM acts as an electrically switchable tunnel barrier. Fabricated stacks of Al/Al₂O₃/SAM/Pb/Ag with such a polar SAM show pronounced hysteretic, reversible conductance switching at voltages in the range of ±2-3 V, with a conductance ratio of the low and the high resistive states of up to 100. The switching mechanism is analyzed using a combination of quantum chemical, molecular dynamics, and tunneling resistance calculation methods. In contrast to more common, inorganic material-based FTJs, our approach using SAMs of small organic molecules allows for a high degree of functional complexity and diversity to be integrated by synthetic standard methods, while keeping the actual device fabrication process robust and simple. We expect that this technology can be further developed toward a level that would then allow its application in the field of information storage and processing, in particular for in-memory and neuromorphic computing architectures.

Biosensors Based on Bio-Functionalized Semiconducting Metal Oxides

Immobilization of biomaterials is a very important task in the development of biofuel cells and biosensors. Some semiconducting metal-oxide-based supporting materials can be used in these bioelectronics-based devices. In this article, we are reviewing some functionalization methods that are applied for the immobilization of biomaterials. The most significant attention is paid to the immobilization of biomolecules on the surface of semiconducting metal oxides. The improvement of biomaterials immobilization on metal oxides and analytical performance of biosensors by coatings based on conducting polymers, self-assembled monolayers and lipid membranes is discussed.

Temperature Dependence of the Rheology of Soft Matter on a MHz-oscillating Solid-liquid Interface

The temperature dependence of the resonant length, molecular weight, and rheology (shear viscosity and shear modulus) of chemisorbed soft matter on a solid-liquid interface oscillating at a megahertz frequency was studied using a quartz crystal microbalance. As a form of chemisorbed soft matter, self-assembled monolayers (SAMs) formed from six types of mercapto oligo(ethylene oxide) methyl ethers were used. A systematic analysis using the Voigt model showed that the variation in effective hydrated thickness (sensed mass), which is related to the resonant length, was classified into three types based on the molecular weight. As a result, a 2.2-nm change in the resonant length occurred in the studied temperature range from 10 to 35℃. Moreover, the variation in the effective hydrated thickness was dependent on the shear viscosity and shear modulus of the SAMs. A further investigation revealed that the relationships η₁∝M_n^0.13 and μ₁∝M_n^0.30 could be estimated regardless of the temperature, where η₁ and μ₁ are the shear viscosity and shear modulus of the SAM, and M_n is the molecular weight of mercapto oligo(ethylene oxide) methyl ether. As a result, we revealed that the experimental results followed the polymer formula irrespective of temperature.

Advanced Nonvolatile Organic Optical Memory Using Self-Assembled Monolayers of Porphyrin-Fullerene Dyads

Photo-switchable organic field-effect transistors (OFETs) represent an important platform for designing memory devices for a diverse array of products including security (brand-protection, copy-protection, keyless entry, etc.), credit cards, tickets, and multiple wearable organic electronics applications. Herein, we present a new concept by introducing self-assembled monolayers of donor-acceptor porphyrin-fullerene dyads as light-responsive triggers modulating the electrical characteristics of OFETs and thus pave the way to the development of advanced nonvolatile optical memory. The devices demonstrated wide memory windows, high programming speeds, and long retention times. Furthermore, we show a remarkable effect of the orientation of the fullerene-polymer dyads at the dielectric/semiconductor interface on the device behavior. In particular, the dyads anchored to the dielectric by the porphyrin part induced a reversible photoelectrical switching of OFETs, which is characteristic of flash memory elements. On the contrary, the devices utilizing the dyad anchored by the fullerene moiety demonstrated irreversible switching, thus operating as read-only memory (ROM). A mechanism explaining this behavior is proposed using theoretical DFT calculations. The results suggest the possibility of revisiting hundreds of known donor-acceptor dyads designed previously for artificial photosynthesis or other purposes as versatile optical triggers in advanced OFET-based multibit memory devices for emerging electronic applications.

Genetically Modified Soybean Detection Using a Biosensor Electrode with a Self-Assembled Monolayer of Gold Nanoparticles

In this study, we proposed a genosensor that can qualitatively and quantitatively detect genetically modified soybeans using a simple electrode with evenly distributed single layer gold nanoparticles. The DNA sensing electrode is made by sputtering a gold film on the substrate, and then sequentially depositing 1,6-hexanedithiol and gold nanoparticles with sulfur groups on the substrate. Then, the complementary to the CaMV 35S promoter (P35S) was used as the capture probe. The target DNA directly extracted from the genetically modified soybeans rather than the synthesized DNA segments was used to construct the detection standard curve. The experimental results showed that our genosensor could directly detect genetically modified genes extracted from soybeans. We obtained two percentage calibration curves. The calibration curve corresponding to the lower percentage range (1-6%) exhibits a sensitivity of 2.36 Ω/% with R2 = 0.9983, while the calibration curve corresponding to the higher percentage range (6-40%) possesses a sensitivity of 0.1 Ω/% with R2 = 0.9928. The limit of detection would be 1%. The recovery rates for the 4% and 5.7% GMS DNA were measured to be 104.1% and 102.49% with RSD at 6.24% and 2.54%. The gold nanoparticle sensing electrode developed in this research is suitable for qualitative and quantitative detection of genetically modified soybeans and can be further applied to the detection of other genetically modified crops in the future.

Streamlined Fabrication of Hybrid Lipid Bilayer Membranes on Titanium Oxide Surfaces: A Comparison of One- and Two-Tail SAM Molecules

There is broad interest in fabricating cell-membrane-mimicking, hybrid lipid bilayer (HLB) coatings on titanium oxide surfaces for medical implant and drug delivery applications. However, existing fabrication strategies are complex, and there is an outstanding need to develop a streamlined method that can be performed quickly at room temperature. Towards this goal, herein, we characterized the room-temperature deposition kinetics and adlayer properties of one- and two-tail phosphonic acid-functionalized molecules on titanium oxide surfaces in various solvent systems and identified optimal conditions to prepare self-assembled monolayers (SAMs), upon which HLBs could be formed in select cases. Among the molecular candidates, we identified a two-tail molecule that formed a rigidly attached SAM to enable HLB fabrication via vesicle fusion for membrane-based biosensing applications. By contrast, vesicles adsorbed but did not rupture on SAMs composed of one-tail molecules. Our findings support that two-tail phosphonic acid SAMs offer superior capabilities for rapid HLB coating fabrication at room temperature, and these streamlined capabilities could be useful to prepare durable lipid bilayer coatings on titanium-based materials.

Removal of Thiol-SAM on a Gold Surface for Re-Use of an Interdigitated Chain-Shaped Electrode

The self-assembled monolayer (SAM) is the most common organic assembly utilized for the formation of the monolayers of alkane-thiolates on gold electrode, resulting in a wide range of applications for the modified SAM on gold in various research areas. This study examined the desorption of a SAM that was developed on the gold surface of an interdigitated chain-shaped electrode (the ICE, a unique electrode design, was fabricated by our group) with the goal of determining the most efficient strategy of SAM removal for the ICE to be re-used. A simple and proficient solution-based cleaning procedure was applied for the removal of a SAM on the gold surface of the ICE by using a sodium borohydride solution within short-term treatment, resulting in efficiency for the recovery of the originally electrochemical characteristic of ICE of 90.3%. The re-use of ICE after the removal process was confirmed by the successful re-deposition of a SAM onto the electrode surface, resulting in the high efficiency percentage of 90.1% for the reusability of ICE with the SAM modification. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were used as tools to investigate the changes in the electrode interface at each stage of the SAM removal and the electrode recycling. X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy were employed, being powerful spectrum techniques, for the characterization of the bonding structure and chemical state of the bare ICE and the modified ICE at each treatment step. Based on the comprehensive discussion of analytical chemistry from the obtained EIS and CV data in this study, we confirmed and proved the effectiveness of this promising method for the removal of a SAM from the ICE and the re-use of ICE in the field of material deposition, with the aims of saving money, improving experimental handling, and protecting the environment.

Asymmetric Chemical Functionalization of Top-Contact Electrodes: Tuning the Charge Injection for High-Performance MoS2 Field-Effect Transistors and Schottky Diodes

The fabrication of high-performance (opto-)electronic devices based on 2D channel materials requires the optimization of the charge injection at electrode-semiconductor interfaces. While chemical functionalization with chemisorbed self-assembled monolayers has been extensively exploited to adjust the work function of metallic electrodes in bottom-contact devices, such a strategy has not been demonstrated for the top-contact configuration, despite the latter being known to offer enhanced charge-injection characteristics. Here, a novel contact engineering method is developed to functionalize gold electrodes in top-contact field-effect transistors (FETs) via the transfer of chemically pre-modified electrodes. The source and drain Au electrodes of the molybdenum disulfide (MoS₂ ) FETs are functionalized with thiolated molecules possessing different dipole moments. While the modification of the electrodes with electron-donating molecules yields a marked improvement of device performance, the asymmetric functionalization of the source and drain electrodes with different molecules with opposed dipole moment enables the fabrication of a high-performance Schottky diode with a rectification ratio of ≈10³ . This unprecedented strategy to tune the charge injection in top-contact MoS₂ FETs is of general applicability for the fabrication of high-performance (opto-)electronic devices, in which asymmetric charge injection is required, enabling tailoring of the device characteristics on demand.

Photochromic Dithienylethene Monolayer-Modified Gold Nanoparticles as a Tunable Floating Gate in the Fabrication of Nonvolatile Organic Memory

Nonvolatile memory (NVM) devices were fabricated by implanting a self-assembled monolayer (SAM) of functional dithienylethene (DTE) derivative on the gold nanoparticle (Au-NP) surface in a pentacene-based organic transistor. The Au-NPs and DTE served as a charge-trapping medium and tunneling barrier layer, respectively. The transfer characteristic of the NVM device showed a narrow hysteresis window and wide memory window, indicating that the DTE-SAM served as a variable barrier layer to regulate the trapping and detrapping of external free charges at the Au-NPs. The energy gap introduced by the DTE-SAM is modulated through photoisomerization between a ring-open form and a ring-closed form by absorbing UV or visible light. For a memory device, the ring-closed DTE allows more free charge injection into the trapping sites, and the ring-open one better retains the trapped charges. A longer anchoring alkanethiol chain at the DTE moiety can further extend the device's retention time. For the NVM operation, programming with the ring-closed DTE and then switching the DTE structure to the ring-open form for erasing can facilitate the charge trapping and charge retention with the same molecule compared to operating all in the ring-open form or all in the ring-closed form of DTE. The structural characterization and electronic characteristics of these devices are discussed in detail.

Surface NMR using quantum sensors in diamond

NMR is a noninvasive, molecular-level spectroscopic technique widely used for chemical characterization. However, it lacks the sensitivity to probe the small number of spins at surfaces and interfaces. Here, we use nitrogen vacancy (NV) centers in diamond as quantum sensors to optically detect NMR signals from chemically modified thin films. To demonstrate the method's capabilities, aluminum oxide layers, common supports in catalysis and materials science, are prepared by atomic layer deposition and are subsequently functionalized by phosphonate chemistry to form self-assembled monolayers. The surface NV-NMR technique detects spatially resolved NMR signals from the monolayer, indicates chemical binding, and quantifies molecular coverage. In addition, it can monitor in real time the formation kinetics at the solid-liquid interface. With our approach, we show that NV quantum sensors are a surface-sensitive NMR tool with femtomole sensitivity for in situ analysis in catalysis, materials, and biological research.