Mixed Self-assembled Monolayers Research Articles

Mediating Mesenchymal Stem Cells Responses and Osteopontin Adsorption via Oligo(ethylene glycol)-amino Mixed Self-assembled Monolayers

Oligo(ethylene glycol) (-OEG) and amino (-NH2) mixed self-assembled monolayers (SAMs) were employed as model substrates to investigate the effect of charge density on the fate of mesenchymal stem cells (MSCs) and osteopontin (OPN) adsorption. We found that all surfaces presenting -NH2 groups favored cell responses regardless of the surface charge. Meanwhile, OPN adsorption could remain stable on the mixed SAMs over a certain range of charge densities. Our work provides some insights into cell responses and protein adsorption to surface charge.

Sensitive and rapid detection of aflatoxin M1 in milk utilizing enhanced SPR and p(HEMA) brushes

The rapid and sensitive detection of aflatoxin M1 (AFM1) in milk by using surface plasmon resonance (SPR) biosensor is reported. This low molecular weight mycotoxin is analyzed using an indirect competitive immunoassay that is amplified by secondary antibodies conjugated with Au nanoparticles. In order to prevent fouling on the sensor surface by the constituents present in analyzed milk samples, an interface with poly(2-hydroxyethyl methacrylate) p(HEMA) brush was employed. The study presents a comparison of performance characteristics of p(HEMA)-based sensor with a regularly used polyethylene glycol-based architecture relying on mixed thiol self-assembled monolayer. Both sensors are characterized in terms of surface mass density of immobilized AFM1 conjugate as well as affinity bound primary and secondary antibodies. The efficiency of the amplification strategy based on Au nanoparticle is discussed. The biosensor allowed for highly sensitive detection of AFM1 in milk with a limit of detection (LOD) as low as 18pgmL−1 with the analysis time of 55min.

Electrostatic Titrations Reveal Surface Compositions of Mixed, On-Nanoparticle Monolayers Comprising Positively and Negatively Charged Ligands

A general-purpose procedure based on electrostatic titrations was developed to determine the compositions of mixed self-assembled monolayers (m-SAMs) of oppositely charged ligands on metal nanoparticles (NPs). In this procedure, nanoparticles of unknown m-SAM compositions and charges are titrated with the positively charged NP “standards”. These compositions and the differences in the free energies of adsorption of the [+] and [−] are then determined from the points of precipitation at which the overall charge on all NPs is neutralized. The results of the titrations agree well with traditional core-etching/NMR studies. However, the titration-based approach is nondestructive and requires significantly less material than NMR.

A pH-Sensitive Supramolecular Switch Based on Mixed Carboxylic Acid Terminated Self-Assembled Monolayers on Au(111).

We show that homogeneously mixed self-assembled monolayers (SAMs) of mercaptoalkanoic acids of different chain lengths can be used to build up a pH-sensitive supramolecular switch. The acids with short and long alkyl chains interact via the strong hydrogen bond between carboxylic acid groups. The pH acts as a trigger by breaking or restoring the hydrogen bond interaction in basic or acidic solutions, respectively. The corresponding changes in the monolayer structure were determined by ellipsometry, surface-enhanced Raman spectroscopy, and contact angle measurements. Density functional theory (DFT) calculations were performed to elucidate the structures of interacting molecules compatible with the surface coverage obtained from electrochemical reductive desorption experiments. The simplicity of the preparation procedure assures a high reproducibility whereas the stability of the homogeneous mixed SAM guarantees the reversibility of the switching process.

Raman spectroscopy and surface wetting of self-assembled monolayer (SAM) of 1-octanethiol and 1,10-decanedithiol

We report the preparation of mixed self-assembled monolayer of 1-octanethiol and 1,10-decanedithiol on Au thin film with preferential Au(111) surface and their characterization using Raman spectroscopy of cysteine adsorbed on mixed self-assembled monolayer mediated isolated Ag nanoparticles. The self-assembled monolayer characterization has been also performed through water contact angle measurement. A significant enhancement in water contact angle from 24° to 103° has been observed on Au surface after self-assembled monolayer formation, primarily due to the hydrophobic nature of the methyl group at the terminal end of the 1-octanethiol, which confirms regular self-assembled monolayer formation. Availability of –SH group from 1,10-decanedithiol on the self-assembled monolayer surface and so, the formation of mixed self-assembled monolayer has been ascertained by immobilization of Ag nanoparticles probed via scanning electron microscopy and Raman spectroscopy of cysteine adsorbed on Ag nanoparticles. Raman spectrum of cysteine on self-assembled monolayer mediated Ag nanoparticles in the fingerprint region of 500–1800 cm−1 shows appreciable increase in the band intensity due to surface enhanced Raman scattering as compared to the band intensity on bare Au surface. These results clearly indicate that the mixed self-assembled monolayer with adequate proportion of component molecules can be utilized as a suitable and inexpensive host for surface enhanced Raman scattering substrates.

Protein-G-based human immunoglobulin G biosensing by electrochemical impedance spectroscopy

A highly sensitive biosensor based on electrochemical impedance spectroscopy (EIS) was developed for the determination of human immunoglobulin G (IgG). Protein G, which specifically binds to IgG, was employed as the molecular receptor. Protein G was covalently immobilized on interdigitated electrodes through a mixed self-assembled monolayer (SAM) composed of 11-mercaptoundecanoic acid (MUA) and 6-mercaptohexanol. It was found that the mixing ratio of the SAM markedly affected the sensor performance. The sample prepared on 25% MUA SAM exhibited a linear behavior in the concentration range of 0.01–10 ng/mL, which is a record low detection for EIS-based IgG sensors. On the other hand, the sample on 100% MUA SAM showed no IgG-sensing action. A possible mechanism of the mixing ratio that affects the sensing performance was proposed.

Structure and Phase Behavior of Mixed Self-Assembled Alkanethiolate Monolayers on Gold Nanoparticles: A Monte Carlo Study.

Configurational-bias Monte Carlo (CBMC) simulations are carried out to investigate the structure and phase behavior of self-assembled monolayers consisting of equimolar alkanethiolate mixtures chemisorbed on the surface of gold nanoparticles. The simulations probe the effects of variations in the chain length, nanoparticle curvature, and exchange of alkanethiolates between nanoparticles. The TraPPE-UA force field is used for the alkanethiolates, whereas the nanoparticle is represented by gold atoms placed on the surface of a sphere. CBMC identity exchange moves are used to enhance sampling of the spatial distribution of the different ligands and to ensure that thermodynamic equilibrium is reached. At a temperature of 298 K, mixtures differing in length by four methylene units exhibit some degree of local segregation. In contrast, the hexanethiolate/tetradecanethiolate mixture yields Janus-like arrangement when the ligands are confined to a single nanoparticle but global demixing when the ligands are allowed to distribute between two nanoparticles.

Rhombic Coulomb diamonds in a single-electron transistor based on an Au nanoparticle chemically anchored at both ends.

Rhombic Coulomb diamonds are clearly observed in a chemically anchored Au nanoparticle single-electron transistor. The stability diagrams show stable Coulomb blockade phenomena and agree with the theoretical curve calculated using the orthodox model. The resistances and capacitances of the double-barrier tunneling junctions between the source electrode and the Au core (R1 and C1, respectively), and those between the Au core and the drain electrode (R2 and C2, respectively), are evaluated as 4.5 MΩ, 1.4 aF, 4.8 MΩ, and 1.3 aF, respectively. This is determined by fitting the theoretical curve against the experimental Coulomb staircases. Two-methylene-group short octanedithiols (C8S2) in a C8S2/hexanethiol (C6S) mixed self-assembled monolayer is concluded to chemically anchor the core of the Au nanoparticle at both ends between the electroless-Au-plated nanogap electrodes even when the Au nanoparticle is protected by decanethiol (C10S). This is because the R1 value is identical to that of R2 and corresponds to the tunneling resistances of the octanedithiol chemically bonded with the Au core and the Au electrodes. The dependence of the Coulomb diamond shapes on the tunneling resistance ratio (R1/R2) is also discussed, especially in the case of the rhombic Coulomb diamonds. Rhombic Coulomb diamonds result from chemical anchoring of the core of the Au nanoparticle at both ends between the electroless-Au-plated nanogap electrodes.

SPR Biosensing MUA/Poly-L-lysine Platform for the Detection of 2,4-Dinitrophenol as Small Molecule Model System

Surface Plasmon Resonance assays are being developed as alternative biodetection methods for a great number of pesticides and toxins. These substances typically have low molecular weight, making it necessary to perform competitive inhibition immunoassays. In most of the cases, the strategy is to immobilize a protein derivative of the analyte, which usually involves the appearance of nonspecific protein binding which limits the detection range of the assay. In this work we present results of a poly-L-lysine (Au-MUA-PLL) based sensor platform for quantitative determination of 2,4-dinitrophenol as model system for small molecular weight substances detection. The prepared sensor chip was characterized by means of Atomic Force Microscopy, Surface Plasmon Resonance, and Surface Enhanced Raman Spectroscopy. Experiments verified the absence of nonspecific protein adsorption to Au-MUA-PLL surfaces and the improvement of the competitive inhibition assays performance in comparison with single and mixed thiol self-assembled monolayers. The possibility of directly immobilizing 2,4-dinitrophenol to the poly-L-lysine containing platforms leads to an improvement in the detection of the soluble analyte by the competitive inhibition assay avoiding undesirable nonspecific protein adsorption. Therefore, Au-MUA-PLL surfaces constitute a suitable alternative for quantitative detection of small molecules when nonspecific adsorption cannot be avoided.

Assembly of Mid-Nanometer-Sized Gold Particles Capped with Mixed Alkanethiolate SAMs into High-Coverage Colloidal Films.

We investigated the influence of the mixed n-alkanethiolate self-assembled monolayer (SAM) formed on gold nanoparticles (AuNPs: 50.0 ± 3.2 nm in diameter) on their assembly into colloidal films. Dodecanethiol and octadecanethiol were selected as the short- and long-chain alkanethiols, respectively. The mixed SAMs were formed by immersing AuNPs in a mixed alkanethiol solution at different molar ratios. Au colloidal films were fabricated on indium tin oxide substrates by our previously reported hybrid method. The composition of the two alkanethiolates in the SAM was deduced from the intensity ratio of two Raman bands at 1080 and 1105 cm(-1). The surface coverage of the colloidal films increased by forming equimolar or dodecanethiolate-dominant mixed SAMs on AuNPs instead of a pure dodecanethiolate or octadecanethiolate SAM. The highest coverage exceeded 80%. This improvement is attributed to the high dispersion stability of AuNPs covered with equimolar or dodecanethiolate-dominant mixed SAMs.

Signal Enhancement of Surface Plasmon Resonance Imaging for Detection of <i>Acidovorax avenae</i> subsp. <i>citrulli</i>

Signal enhancement based surface plasmon resonance imaging (SPR imaging) was developed using gold nanoparticle (AuNP) for detection of Acidovorax avenae subsp. citrulli (Aac). The antibodies against Aac, monoclonal antibody 11E5 (MAb 11E5) and polyclonal antibody MPC (PAb MPC), were covalently immobilized on the 1:40 of mixed self-assembled monolayer (mixed SAMs) surface for detection of Aac. The 107 cfu/ml of Aac was injected over the surface and was captured by immobilized antibodies on the sensing surface. PAb MPC conjugated to 10 nm of gold nanoparticle (PAb-AuNP) was flowed over the surface to enhance the SPR signal for detection of Aac. The MAb/Aac/PAb-AuNP assay provides a higher in signal enhancement than that of PAb/Aac/PAb-AuNP assay. Moreover, SPR signal using PAb-AuNP enhancement increases 23 – fold in signal enhancement when comparing to PAb enhancement. Thus, the detection of Aac based SPR imaging can be used PAb-AuNP in signal enhancement for further improvement in limit of detection (LOD).

Sensitivity improvement of a sandwich-type ELISA immunosensor for the detection of different prostate-specific antigen isoforms in human serum using electrochemical impedance spectroscopy and an ordered and hierarchically organized interfacial supramolecular architecture

A gold millielectrode (GME) functionalized with a mixed (16-MHA + EG3SH) self-assembled monolayer (SAM) was used to fabricate an indirect enzyme-linked immunosorbent assay (ELISA) immunosensor for the sensitive detection of prostate-specific antigen (PSA), a prostate cancer (PCa) biomarker, in human serum samples. To address and minimize the issue of non-specific protein adsorption, an organic matrix (amine-PEG3-biotin/avidin) was assembled on the previously functionalized electrode surface to build up an ordered and hierarchically organized interfacial supramolecular architecture: Au/16-MHA/EG3SH/amine-PEG3-biotin/avidin. The electrode was then exposed to serum samples at different concentrations of a sandwich-type immunocomplex molecule (BtnAb-AgPSA-HRPAb), and its interfacial properties were characterized using electrochemical impedance spectroscopy (EIS). Calibration curves for polarization resistance (RP) and capacitance (1/C) vs. total and free PSA concentrations were obtained and their analytical quality parameters were determined. This approach was compared with results obtained from a commercially available ELISA immunosensor. The results obtained in this work showed that the proposed immunosensor can be successfully applied to analyze serum samples of patients representative of the Mexican population.

Phase Segregation in the Mixed Alkyl Thiol Self‐assembled Monolayers on a Gold Surface at a High Incubation Temperature in a Sealed Container

Self‐assembled monolayers (SAMs) of mixed 1‐octanethiol (OT) and 1‐decanethiol (DT) that were formed on gold substrates at a high incubation temperature in a sealed container were characterized using scanning tunneling microscopy (STM) and X‐ray photoelectron spectroscopy (XPS). In mixed‐composition monolayer films, self‐assembly of the mixture could induce phase separation into discrete molecular domains at higher incubation temperatures. Mixtures of the alkanethiol molecules affected the domain size, defect density, packing density, and binding features of the resultant SAMs prepared on gold substrates. Single‐molecule‐resolved STM images of mixed SAMs revealed both c(4 × 2) − ns (non‐shifted) for DT and c(4 × 2) − s (shifted) for OT domains. For the mixed SAMs, the peak S2 (XPS) increased in intensity relative to the peak S1, reflecting significant modifications in the binding characteristics of the thiols and surface roughness upon mixing.

Hybridization performance of DNA/mercaptohexanol mixed monolayers on electrodeposited nanoAu and rough Au surfaces

Abstract In this article we investigated the hybridization performance of thiol-modified probe DNA/mercaptohexanol (MCH) mixed self-assembled monolayers (SAMs) on nanoAu and rough Au surfaces with target DNA in 1 M electrolytical solution. The nanoAu surfaces were prepared by electrodeposition in 2 mM HAuCl4 and 0.1 M Na2SO4 aqueous solution based on different deposition time (0.33 min, 2 min, 5 min, 10 min and 20 min), deposition potential (− 0.4 V, − 0.8 V, − 1.2 V) and deposition methods (direct electrodeposition or firstly self-assembly of 1-dodecanethiol, then electrodeposition). Scanning electron microscopy (SEM) observed that size and density of gold nanoparticles formed were related to electrodeposition conditions. The rough Au surfaces were prepared by roughly hand-polishing and the gold roughness factors (Rf) were from 1 to 3. Chronocoulometry (CC) experiments showed that DNA hybridization density (HD) arrived at the biggest for 16.3 × 10− 12 mol cm− 2 when electrodeposition was performed at − 1.2 V for 10 min and the concentration ratio of probe DNA and MCH (CDNA/CMCH) for mixed assembly was 1:1. DNA hybridization performance of probe DNA/MCH mixed SAMs on some nanoAu surfaces was much better than that on rough Au or planar Au surfaces and the biggest HD for DNA optimal hybridization increased by 85% and 51% respectively. For rough Au, DNA hybridization performance was almost independent of Rf. Based on our previous reported simple DNA hybridization model, the size fitting coefficient (dc/dt) for DNA optimal hybridization on nanoAu was calculated to be 0.71, smaller than that on rough Au (0.99) or planar Au (0.93). It meant that surface coverage of probe DNA in probe DNA/MCH mixed SAMs should be much bigger on nanoAu than that on rough Au or planar Au surface for DNA optimal hybridization. These results indicated that Au surface configuration played the important role on DNA hybridization. The intermolecular repulsion and steric hindrance for DNA hybridization might be reduced on nanoAu than those on rough Au or planar Au, which possibly led to the improvement of DNA hybridization performance. The conclusions provided the important reference for constructing electrochemical DNA sensor with the optimal performance.

Mixed SAMs of backbone-functionalized tribenzotriquinacenes and alkanethiols: Synthesis, preparation and STM-investigations

We present the synthesis of a backbone-functionalized tribenzotriquinacene (TBTQ) 3, serving as a prototype for TBTQ-based stators comprising thioether anchor groups for self-assembled monolayers on Au-(111) surfaces. Self-assembled monolayers (SAM) consisting of isolated molecules of the minor component 3 embedded in a close-packed matrix of decanethiol (C10-SH) were prepared, leading to stable monolayers which were characterized by scanning tunneling microscopy (STM) under ambient conditions. The structural features of the highly ordered mixed monolayers point to the fact that isolated TBTQ-headgroups are exposed to the outer monolayer surface, making them available for the interaction with shape-complementary guest molecules.

Detection of Escherichia coli with a label-free impedimetric biosensor based on lectin functionalized mixed self-assembled monolayer

A highly sensitive impedimetric lectin-based biosensor for the determination of Escherichia coli (E. coli) was developed based on the affinity of lectin with glycosyl complex on cellular surface. The biosensor was fabricated by co-assembling 11-mercaptoundecanoic acid (MUA) and dithiothreitol (DTT) on the surface of gold electrode, and then the lectin was covalently immobilized using carbodiimide chemistry. A well-known lectin, Concanavalin A (Con A) was used to demonstrate proof-of-concept of the recognition interface. In the measurement of E. coli DH5 (as a model target), the change in interfacial electron transfer resistance of lectin-based biosensor was monitored using a redox couple of Fe(CN)63−/4−. The increase in the electron transfer resistance was in logarithmically direct proportion to the concentration of E. coli over a range from 1.0×102 to 1.0×105cells/mL. The detection limit of this sensor was 75cells/mL. Additionally, the EIS biosensor also showed satisfactory selectivity in discriminating gram-negative E. coli from gram-positive bacteria. The strategy developed in this study may be a promising approach and could be extended to the design of EIS biosensors for highly sensitive and rapid detection of other desired bacteria.

The effect of supporting electrolyte on the electron transfer at mixed self-assembled monolayers containing ferrocene moieties

The effect of supporting electrolyte on the apparent redox potential (Eapp∘) of ferrocene (Fc) moieties in mixed self-assembled monolayers (SAMs) has been studied by means of cyclic and ac voltammetry. When the Fc moieties are exposed on the surface of the SAM (i.e., for the mixed SAM of Fc(CH2)6SH and 1-hexylmercaptan), the Eapp∘ is little dependent on the supporting electrolytes used, i.e., sodium salts of 4-hydroxybenzenesulfonate (HBS), p-toluenesulfonate (TS), and 2-naphtalenesulfonate (NS). On the other hand, when the Fc moieties are buried in the SAM (i.e., for the mixed SAM of Fc(CH2)6SH and 1-octadecylmercaptan), the Eapp∘ is significantly shifted to more positive potentials and is further affected by the nature of the electrolyte anions. The more hydrophobic the anion becomes in the order HBS<TS<NS, the less positive the Eapp∘ value. This effect has been successfully explained by the theory proposed by Smith and White (Anal. Chem., 64 (1992) 2398–2405), with an additional consideration on the anions' ability to neutralize the positive charge of oxidized Fc moieties.

Fast Electron Transfer Exchange at Self-Assembled Monolayers of Organometallic Ruthenium(II) σ-Arylacetylide Complexes.

A new series of ruthenium organometallic carbon-rich complexes, exhibiting fast electron transfer kinetics combined to a low oxidation potential, was synthesized for self-assembled monolayer (SAM) formation on gold surfaces. The molecules consist of highly conjugated ruthenium(II) mono(σ-arylacetylide) or bis(σ-arylacetylide) complexes functionalized with different bridge units with specific (protected) anchoring groups that possess high affinity for gold, such as thiol, carbodithioate, and isocyanide. Single component and mixed SAMs were prepared and fully characterized by wettability studies, infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS), and electrochemical analyses. By applying the Laviron's formalism, fast electron transfer kinetics (≈10(4) s(-1)) were found at the derived self-assemblies while no significant effect could have been evidenced with variation of the bridging unit and of the anchoring moiety. Interestingly, a hexyl aliphatic spacer in the bridging unit with a thiol group and dilution with suitable nonelectroactive thiols lead to better SAM organization and packing, in comparison with undiluted complexes with shorter spacers. Such features make these compounds suitable alternatives to the widely used ferrocene center as redox-active building blocks for reversible charge storage devices.

Structural Investigations of Self-Assembled Monolayers for Organic Electronics: Results from X-ray Reflectivity

Self-assembled monolayers (SAMs) have been established as crucial interlayers and electronically active layers in organic electronic devices, such as organic light emitting diodes (OLEDs), organic photovoltaics (OPVs), organic thin film transistors (OTFTs), and nonvolatile memories (NVMs). The use of self-assembling functionalized organic molecules is beneficial due to mainly three advantages compared with common thin film deposition approaches. (1) Molecular self-assembly occurs with surface selectivity, determined by the interaction between the functional anchor group of the organic molecules and the target surface. (2) The film thickness of the resulting layers is perfectly controllable on the angstrom scale, due to the self-terminating film formation to only a single molecular layer. And finally, (3) the wide variability in the chemical structure of such molecules enables different SAM functionalities for devices, ranging from electrical insulation to charge storage to charge transport. The SAM approach can be further expanded by employing several functionalized molecules to create mixed SAMs with consequently mixed properties. The function of SAMs in devices depends not only on the chemical structure of the molecules but also on their final arrangement and orientation on the surface. A reliable and nondestructive in-depth characterization of SAMs on nonconductive oxide surfaces is still challenging because of the very small thickness and the impracticality of methods such as scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). In this Account, we illustrate how X-ray reflectivity (XRR) provides analytical access to major questions of SAM composition, morphology, and even formation by means of investigations of pure and mixed SAMs based on phosphonic acids (PAs) of various chain structures on flat alumina (AlOx) surfaces. XRR is an analytical method that provides access to spatially averaged structural depth profiles over a relatively large area of several square micrometers. The key outcome of XRR, the surface-normal electron density profile of the SAMs, leads to precise information on the SAM thickness with subangstrom resolution and allows for the determination of molecular tilt angles and packing densities. We have systematically increased the chemical complexity of PA molecules and the resulting SAMs, utilizing XRR to provide insight into the SAM structures. In SAMs composed of functionalized molecules or complex chain structures, the distribution of electron rich and electron poor signatures is detected and thus the molecular order within the SAM is determined. In mixed SAMs of two different molecular species, electron density profiles reveal the morphology and how the surface-normal structure changes if one component of the mixed SAM is altered. Furthermore, XRR was applied to investigate in situ the self-assembly of functionalized PA molecules from solution by tracking the monolayer growth over time. Even though the results provided by XRR on in-plane molecular arrangement are limited, it presents excellent information on the molecular scale along the surface normal and in addition allows for drawing conclusions on the intermolecular interactions within the SAM.

Poly(L-lysine) Interfaces via Dual Click Reactions on Surface-Bound Custom-Designed Dithiol Adsorbates.

Surfaces modified with poly(L-lysine) can be used to immobilize selected biomolecules electrostatically. This report describes the preparation of a set of self-assembled monolayers (SAMs) from three different azide-terminated adsorbates as platforms for performing controlled surface attachments and as a means of determining the parameters that afford stable poly(L-lysine)-modified SAM surfaces having controlled packing densities. A maleimide-terminated alkyne linker was "clicked" to the azide-terminated surfaces via a copper-catalyzed cycloaddition reaction to produce the attachment sites for the polypeptides. A thiol-Michael addition was then used to immobilize cysteine-terminated poly(L-lysine) moieties on the gold surface, avoiding adsorbate self-reactions with this two-step procedure. Each step in this process was analyzed by ellipsometry, X-ray photoelectron spectroscopy, polarization modulation infrared reflection-absorption spectroscopy, and contact angle goniometry to determine which adsorbate structure most effectively produced the targeted polypeptide interface. Additionally, a series of mixed SAMs using an azidoalkanethiol in combination with a normal alkanethiol having an equivalent alkyl chain were prepared to provide data to determine how dilution of the azide reactive site on the SAM surface influences the initial click reaction. Overall, the collected data demonstrate the advantages of an appropriately designed bidentate absorbate and its potential to form effective platforms for biomolecule surface attachment via click reactions.