Chemical, electrochemical, and structural stability of low-density self-assembled monolayers

The stability of low-density self-assembled monolayers of mercaptohexadecanoic acid on gold is studied under a variety of storage conditions--air at room temperature, argon at room temperature and 4 degrees C, and ethanol at room temperature. The structural monotony of the low-density monolayers was assessed by monitoring the alkyl chains of LDSAMs by grazing-angle Fourier transform infrared spectroscopy as a function of time. Independently of the storage conditions, both symmetric and asymmetric methylene stretches at 2923 and 2852 cm-1 decreased after 4 weeks to 2919 and 2849 cm-1, respectively. These data suggest an increased ordering of the alkyl chains that is distinctly different from that of conventional high-density monolayers of mercaptohexadecanoic acid included as a reference in this study. As a further extension of this observation, the electrochemical barrier properties of the low-density monolayers were assessed by electrochemical impedance spectroscopy and did not change significantly for any of the storage conditions over a period of 4 weeks. Moreover, X-ray photoelectron spectroscopy was used to assess the chemical changes in the low-density monolayers over time. The chemical composition was essentially unaltered for all storage conditions. Specifically, oxidation of the sulfur headgroup, a common cause of monolayer degradation, was excluded for all test conditions on the basis of XPS analysis. This study confirms excellent storage stability for low-density monolayers under commonly used storage conditions and bridges an important technological gap between these systems and conventional high-density systems.     

Multipotential electrochemical detection of primer extension reactions on DNA self-assembled monolayers

Electroactive nucleoside triphosphates ("electrotides") have been incorporated into primers by DNA polymerase and detected on oligonucleotide surface-assembled monolayers. Four electrotides bearing three different electroactive moieties-ferrocene, vinylferrocene, and anthraquinone-are detected in four alternative formats.     

Switching the electrochemical impedance of low-density self-assembled monolayers

Because the active remodeling of biointerfaces is a paramount feature of nature, it is very likely that future, advanced biomaterials will be required to mimic at least certain aspects of the dynamic properties of natural interfaces. This need has fueled a quest for model surfaces that can undergo reversible switching upon application of external stimuli. Herein, we report the synthesis and characterization of a model system for studying reversibly switching surfaces based on low-density monolayers of mercaptohexadecanoic acid and mercaptoundecanoic acid. These monolayers were assembled on both gold and silver electrodes. When conducting electrochemical impedance spectroscopy under physiological conditions, these monolayers exhibit significant changes in their electrochemical barrier properties upon application of electrical DC potentials below +400 mV with respect to a standard calomel electrode. We further found the impedance switching to be reversible under physiological conditions. Moreover, the impedance can be fine-tuned by changing the magnitude of the applied electrical potential. Before and during impedance switching at pH 7.4 in aqueous buffer solutions, the low-density monolayers showed good stability according to grazing angle infrared spectroscopy data. We anticipate low-density monolayers to be potentially useful model surfaces when designing active biointerfaces for cell-based studies or rechargeable biosensors.     

Dynamic and collective electrochemical responses of tetrathiafulvalene derivative self-assembled monolayers

Electroactive tetrathiafulvalene (TTF)-containing alkanethiol self-assembled monolayers (SAMs) were designed and synthesized to elucidate the relationship between electrochemical responses and film structures. Two TTF derivative molecules having one alkanethiol chain (1) and two alkanethiol chains (2) were utilized to modulate the molecular packing arrangements in the SAMs, and the formation and structure of the SAMs were characterized by surface plasmon resonance spectroscopy (SPR). SPR measurements in various contacting media demonstrated loose packing of SAM 1 and close packing of SAM 2 due to the different space fillings of the molecules. Two successive one-electron redox waves were observed for both SAMs by cyclic voltammetry. The peak widths of the redox waves were strongly dependent on the oxidation states of the TTF moieties, the packing arrangement of the SAMs, and the contacting medium. We found that TTF-based SAMs exhibited collective electrochemical responses induced by dynamic structural changes, depending on the degree of freedom for the component molecules in the SAMs. These results imply that the molecular design, taking into account the electrochemical responses, extends the available range of molecular-based functionalities in TTF-based SAMs.     

Directed assembly and separation of self-assembled monolayers via electrochemical processing

Separated domains of 1-dodecanethiolate were fabricated via solution displacement of preformed 1-adamantanethiolate self-assembled monolayers on Au{111}. Subsequently, the 1-adamantanethiolate domains were desorbed selectively, and the substrate was exposed to a 1-octanethiol solution, creating artificially separated self-assembled monolayers of 1-dodecanethiolate and 1-octanethiolate. The molecular order of each lattice type and the apparent height differences imaged with scanning tunneling microscopy and the two distinct cathodic peaks observed with cyclic voltammetry indicated distinct separated domains of each lattice type in the separated self-assembled monolayers. By manipulating the intermolecular interaction strengths of the patterned molecules, we are able to control the structure and properties of the separated self-assembled monolayers via the exploitation of competitive adsorption and the utilization of electrochemical processing, which can be extended to other self-assembly patterning techniques such as microdisplacement printing.     

Room-temperature ionic liquids as media to enhance the electrochemical stability of self-assembled monolayers of alkanethiols on gold electrodes

The electrochemical stability of self-assembled monolayers was greatly enhanced by using room-temperature ionic liquids as media.     

Characterization of carboxylic acid-terminated self-assembled monolayers by electrochemical impedance spectroscopy and scanning electrochemical microscopy

Electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) are used to monitor changes in the ionization of monolayers of 11-mercaptoundecanoic acid. When using an anionic redox probe, Fe(CN)6(-4), the charge-transfer resistance of the 11-mercaptoundecanoic acid monolayer-modified interface increases in a sigmoidal fashion as the solution is made basic. The opposite effect is observed when using a cationic redox probe. The inflection points of these two titration curves, however, differ when using the different redox probes. This result is taken as being characteristic of the influence that applied potential has on the ionization of the monolayer. The role of substrate potential on the ionization of the monolayer is further investigated by SECM. The SECM measurement monitors the concentration of Ru(NH3)6(+3) as the potential of the substrate is varied about the potential of zero charge. For monolayers of 11-mercaptoundecanoic acid in solutions buffered near the pKa of the terminal carboxylic acid, potential excursions positive of the PZC cause an increase in the concentration of Ru(NH3)6(+3) local to the interface, and potential excursions negative of the PZC cause a decrease in the local concentration of Ru(NH3)6(+3). Similar experiments conducted with an interface modified with 11-undecanethiol had no impact on the local concentration of Ru(NH3)6(+3). These results are interpreted in terms of the influence that applied potential has on the pH of the solution local to the interface and the impact that this has on the ionization of the monolayer.     

Selective electrochemical recognition of ions in solution and at self-assembled monolayers

Quinone-functionalized calix[4]arenes having carboxylic acid groups or thiol groups were prepared and their spontaneous adsorption on silver and gold surfaces, respectively, was studied. Since the cavity-like structure of calixarenes was immobilized on the noble metal electrodes, they exhibited a selective affinity towards specific hard metal ions in aqueous media. Voltammetric and spectroscopic studies showed the well-ordered deposition of organic receptors and entrapment of metal ions. It also was found that the repeated capture and removal of metal ions reversibly with chelating agents such as ethylenediaminetetraacetic acid (EDTA) was possible. This is the first example, to our knowledge, of voltammetric detection of hard metal ions in aqueous media using a chemically modified electrode with redox-active macrocyclic receptors.     

Scanning electrochemical microscopy. 51. Studies of self-assembled monolayers of DNA in the absence and presence of metal ions

Scanning electrochemical microscopy was used to examine electron transfer across a self-assembled monolayer of thiol-modified DNA duplexes on a gold electrode. The apparent rate constant for heterogeneous ET from a solution redox probe, Fe(CN)6(3-/4-), to the gold surface through ds-DNA was 4.6 (+/-0.2) x 10(-7) cm/s. With the addition of Zn2+, which resulted in the formation of a metalated DNA (M-DNA) monolayer, the rate constant increased to 5.0 (+/-0.3) x 10(-6) cm/s. Upon treating M-DNA with EDTA, the zinc ions were released from the monolayer and the original rate constant for the DNA duplexes was restored. The enhanced ET rate was also observed at a DNA monolayer treated with Ca2+ or Mg2+, which does not complex by the DNA bases to form M-DNA. The binding of these cations facilitated the monolayer penetration by the probe mediator Fe(CN)6(3-/4-) and accordingly caused an increased redox signal of the mediator at the ds-DNA-modified electrode. Cationic or neutral mediators were not blocked by the ds-DNA monolayer. These results suggest that although the increased electron transport through M-DNA could partially be ascribed to the intrinsic enhancement of electric conductivity of M-DNA, which has been confirmed by photochemical studies, the change in the surface charge of DNA monolayers on the electrode caused by the binding of metal ions to DNA molecules may play a more important role in the enhancement of current with M-DNA.     

Attachment of tyrosinase on mixed self-assembled monolayers for the construction of electrochemical biosensor

<正>A mixed self-assembled monolayers(SAMs) of thioctic acid(T-COOH) and thioctic acid amide(T-NH_2) were used to immobilize tyrosinase for fabricating biosensor.The results showed that the mixed SAMs prepared from solution at the ratio of 1:4 provided an excellent microenvironment for enzymatic reaction between tyrosinase and substrate.The biosensor exhibited a fast response and high sensitivity for sensing substrate.     

Nanomechanics of the formation of DNA self-assembled monolayers and hybridization on microcantilevers

Biomolecular interactions over the surface of a microcantilever can produce its bending motion via changes of the surface stress, which is referred to nanomechanical response. Here, we have studied the interaction forces responsible for the bending motion during the formation of a self-assembled monolayer of thiolated 27-mer single-stranded DNA on the gold-coated side of a microcantilever and during the subsequent hybridization with the complementary nucleic acid. The immobilization of the single-stranded DNA probe gives a mean surface stress of 25 mN/m and a mean bending of 23 nm for microcantilevers with a length and thickness of about 200 microm and 0.8 microm, respectively. The hybridization with the complementary sequence could not be inferred from the nanomechanical response. The nanomechanical response was compared with data from well-established techniques such as surface plasmon resonance and radiolabeling, to determine the surface coverage and study the intermolecular forces between neighboring DNA molecules anchored to the microcantilever surface. From both techniques, an immobilization surface density of 3 x 10(12) molecules/cm(2) and a hybridization efficiency of 40% were determined. More importantly, label-free hybridization was clearly detected in the same conditions with a conventional sensor based on surface plasmon resonance. The results imply that the nanomechanical signal during the immobilization process arises mainly from the covalent attachment to the gold surface, and the interchain interactions between neighboring DNA molecules are weak, producing an undetectable surface stress. We conclude that detection of nucleic acid hybridization with nanomechanical sensors requires reference cantilevers to remove nonspecific signals, more sensitive microcantilever geometries, and immobilization chemistries specially addressed to enhance the surface stress variations.     

Syntheses of anthraquinone capped hairpin DNAs and electrochemical redox responses from their self-assembled monolayers on gold electrode

An anthraquinone (AQ) based DNA linker and hairpin-forming DNAs linked by the AQ linker with variable A-T base pairs were synthesized for the investigation of electron transfer through double helical DNA (DNA-ET) in self-assembled monolayers (SAMs). The spectroscopic analysis of absorption spectra indicated that the AQ of the hairpin DNA stacked with adjacent A-T base pair. Electrochemical redox response due to the AQ was observed from the hairpin DNA immobilized on gold electrode, thus the hairpin DNA is suitable for the investigation of DNA-ET in SAMs.     

Comparison of the structure and stability of monolayers prepared with 12-phenyldodecyl mercaptan and 11-phenoxyundecyl mercaptan

Monolayers are prepared by self-assembly of 12-phenyldodecyl mercaptan or 11-phenoxyundecyl mercaptan on gold substrates. Reflection-absorption infrared spectra of these monolayers show that the 12-phenyldodecyl mercaptan monolayer forms a well-organized interface while 11-phenoxyundecyl mercaptan monolayers do not. Infrared spectra also suggest that the C1-C4 axis of the phenyl ring is largely parallel to the Au-substrate for 12-phenyldodecyl mercaptan layers, but has a more perpendicular orientation relative to the substrate for 11-phenoxyundecyl mercaptan layers. Despite these structural differences, Zisman analysis of contact angle data shows that these monolayers have similar surface energies, 27.1 (+/-6.7) mN/m for 12-phenyldodecyl mercaptan and 26.1 (+/-5.3) mN/m for 11-phenoxyundecyl mercaptan. Fowkes analysis of the contact angle data suggest that dispersive interactions account for all of the measured surface energy for both modified interfaces. The structural differences are, however, reflected in the lateral stabilization energy measurements where monolayers of 12-phenyldodecyl mercaptan are found to be 24 (+/-5) kJ/mol more stable than those of 11-phenoxyundecyl mercaptan.     

Thermal stability and electrochemical performance of Polyaniline/Pr6O11 composites

In order to describe the influence of Pr₆O₁₁ on thermal and electrochemical performance of polyaniline (PANI), PANI/Pr₆O₁₁ composites were synthesized by in situ polymerization in the presence of sulfosalicylic acid (as dopant). The composites obtained was characterized by scanning electron microscope, Fourier transform infrared spectra, and X‐ray diffraction. The thermal stability of the composites was investigated by thermogravimetry and derivative thermogravimetry. Finally, the comparative analysis on the electrochemical properties of PANI/Pr₆O₁₁ composites electrodes, such as cyclic voltammetry, electrochemical impedance spectroscopy, and constant current charge–discharge test, was conducted under similar conditions. Scanning electron microscope, infrared, and X‐ray diffraction results show that interaction exists between PANI and Pr₆O₁₁, and the composites are more crystalline than pure PANI. Thermogravimetry and derivative thermogravimetry analyses indicate that PANI/Pr₆O₁₁ composites and pure PANI have similar thermal stability. Moreover, the studies regarded to capacitive properties show that the electrochemical performance of the composites is improved greatly when Pr₆O₁₁ is added in small amount. However, with increasing the amount of Pr₆O₁₁, the electrochemical performance of the composites become worse, which indicates a large amount of Pr₆O₁₁ has a negative effect on the capacitance. These results suggest that PANI is still in the conductive form after inserting the Pr₆O₁₁, and the capacitance performance of PANI can be improved only by a low percentage of Pr₆O₁₁. Copyright © 2013 John Wiley & Sons, Ltd.     

Electrochemical investigations on stability and protonation behavior of pyridine-terminated aromatic self-assembled monolayers

Based on electrochemical methods such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), Au(111) electrodes modified by self-assembled monolayers (SAMs) of a homologous series of pyridine-terminated thiols with aromatic backbones have been investigated. An important correlation between the chain structure and film integrity in electrolytic media was found. Monolayers with odd numbers of methylene spacers in the molecular chain showed superior barrier properties compared to even numbered counterparts. A positive influence of an increase in the number of attached phenyl rings on the integrity of SAMs was observed. Furthermore, cathodic desorption of the investigated SAMs is characterized by multiwave desorption peaks and extraordinarily large cathodic charges indicating an unusual desorption process. Moreover, protonation behavior of the SAMs has been investigated by X-ray photoelectron spectroscopy (XPS) and electrochemical methods. Protonation has been found to be reversible and surface pK(a) values have been determined to be around 5 for all investigated monolayers.     

Modification and stability of aromatic self-assembled monolayers upon irradiation with energetic particles

We have studied ion and electron irradiation of self-assembled monolayers (SAMs) of 2-(4'-methyl-biphenyl-4yl)-ethanethiol (BP2, CH3-C6H4C6H4CH2CH2-SH), phenyl mercaptan (PEM, C6H5CH2CH2-SH), and 4'-methyl-biphenyl-4-thiol (BP0, CH3-C6H4C6H4-SH) deposited on Au(111) substrates. Desorption of neutral particles from PEM/Au and BP2/Au was investigated using laser ionization in combination with mass spectrometry. The ion-induced damage of both BP2 and PEM SAMs is very efficient and interaction with a single ion leads to the modification of tens of molecules. This feature is the result of a desorption process caused by a chemical reaction initiated by an ion impact. Both for ions and electrons, experiments indicate that the possibility for scission of the Au-S bond strongly depends on the chemical nature of the SAM system. We attribute the possible origin of this effect to the orientation of the Au-S-C angle or adsorption sites of molecules. The analysis of electron-irradiated PEM/Au and BP2/Au, using ion-initiated laser probing, enabled measurements of the cross section for the electron-induced damage of the intact molecule or specific fragment. Analysis of electron-irradiated BP0/Au by using time-of-flight secondary ion mass spectrometry (TOF-SIMS) provides direct evidence for the quasi-polymerization process induced by electron irradiation.     

硒杂环化合物及其自组装膜的电化学性质研究

硒与芳香邻二胺类化合物反应定量、快速,且产物苤硒脑易于用光谱和电化学检测,在分析化学上具有重要地位[1 4],本文报道该类型化合物及其在金电极上的自组装膜的电化学特性。苤硒脑结构式如下:1　实验部分1 1试剂与仪器2,3 二氨基萘(2,3 diaminonaphthalene,DAN,Sigma公司),1,2 二氨基蒽醌(1,2 diamino qthraquinone,DAAQ,Aldrich公司),Na2SeO3及其余试剂均为A.R.级,丙酮经重蒸提纯,石英亚沸蒸馏器制高纯水,BAS-100B型电化学分析仪(美国)。1 2　实验方法苤硒脑按文献[4]法制备。DAN和DAAQ分别与Na2SeO3在暗室中反应制取BP和…     

Comparison of the stem-loop and linear probe-based electrochemical DNA sensors by alternating current voltammetry and cyclic voltammetry

Here we systematically characterized the sensor performance of the stem-loop probe (SLP) and linear probe (LP) electrochemical DNA sensors using alternating current voltammetry (ACV) and cyclic voltammetry (CV), with the goal of generating the set of operational criteria that best suits each sensor architecture, in addition to elucidating the signaling mechanism behind these sensors. Although the LP sensor shows slightly better % signal suppression (SS) upon hybridization with the perfect match target at 10 Hz, our frequency-dependent study suggests that it shows optimal % SS only in a very limited AC frequency range. Similar results are observed in CV studies in which the LP sensor, when compared to the SLP sensor, displays a narrower range of voltammetric scan rates where the optimal % SS can be achieved. More importantly, the difference between the two sensors' performance is particularly pronounced if the change in integrated charge (Q) upon target hybridization, rather than the peak current (I), is measured in CV. The temperature-dependent study further highlights the differences between the two sensors, where the LP sensor, owing to the flexible linear probe architecture, is more readily perturbed by temperature changes. Both SLP and LP sensors, however, show a loss of % SS when operated at elevated temperatures, despite the significant improvement in the hybridization kinetics. In conjunction with the ACV, CV, and temperature-dependent studies, the electron-transfer kinetics study provides further evidence in support of the proposed signaling mechanism of these two sensors, in which the SLP sensor's signaling efficiency and sensor performance is directly linked to the hybridization-induced conformational change in the redox-labeled probe, whereas the performance of the LP sensor relies on the hybridization-induced change in probe dynamics.     

Comparison of the anchoring of nematic liquid crystals on self-assembled monolayers formed from semifluorinated thiols and alkanethiols

The anchoring of nematic liquid crystals on self-assembled monolayers (SAMs) formed by the chemisorption of semifluorinated thiols or alkanethiols on gold is compared and contrasted. The planar anchoring of 4-n-pentyl-4-cyanobiphenyl (5CB) observed in the past on SAMs formed from alkanethiols is also observed on SAMs formed from semifluorinated thiols. The azimuthal anchoring of 5CB, however, differs on these two types of surfaces: nematic 5CB anchored on SAMs formed from alkanethiols has a grainy appearance due to the formation of domains with sizes 10 mum whereas 5CB forms large domains ( 100 mum) with diffuse branches emerging from defects of strength 1/2 when anchored on SAMs formed from semifluorinated thiols. Mixed (two-component) SAMs formed from either short and long semifluorinated thiols or short and long alkanethiols cause homeotropic anchoring of 5CB. We discuss these results in light of the known differences in the structure of SAMs formed from alkanethiols and semifluorinated thiols, i.e. the tilt of the chains and conformational freedom (flexibility) of the chains within these SAMs.     

Monitoring the hydration of DNA self-assembled monolayers using an extensional nanomechanical resonator

We have fabricated an ultrasensitive nanomechanical resonator based on the extensional vibration mode to weigh the adsorbed water on self-assembled monolayers of DNA as a function of the relative humidity. The water adsorption isotherms provide the number of adsorbed water molecules per nucleotide for monolayers of single stranded (ss) DNA and after hybridization with the complementary DNA strand. Our results differ from previous data obtained with bulk samples, showing the genuine behavior of these self-assembled monolayers. The hybridization cannot be inferred from the water adsorption isotherms due to the low hybridization efficiency of these highly packed monolayers. Strikingly, we efficiently detect the hybridization by measuring the thermal desorption of water at constant relativity humidity. This finding adds a new nanomechanical tool for developing a label-free nucleic acid sensor based on the interaction between water and self-assembled monolayers of nucleic acids.