Electron transport through a self-assembled monolayer of thiol-end-functionalized tetraphenylporphines and metal tetraphenylporphines

The monolayers of several thiol-end-functionalized tetraphenylphophines (SH-TPP) and metal tetraphenylporphines (SH-MTPP) were self-assembled on gold surfaces and identified by cyclic voltammetry (CV), electrochemical impedance spectroscopy, scanning electrochemical microscopy, and the contact angle. The CV peaks of the [Fe(CN)6]3-/ [Fe(CN)6]4- couple were used to identify the efficiency of electrons transferring through the self-assembled monolayer (SAM). The results suggested that SH-TPP and SH-MTPP could form high-quality SAMs on gold surfaces. The SAMs blocked electron transport from the gold electrode to solution. When the length of the thiol-end-link spacer (alkyl group) increased, the electron transport ability of the SAM decreased because of the increased insulator properties of the alkyl chain. With the insertion of metallic ions, the electron transport ability of the SAM of SH-MTPP increased compared to that of the SAM of SH-TPP, which was probably due to the fact that (i) the insertion of metallic ions changed the molecular structure and the molecular structure of SH-MTPP played an important role in electron transport through the SAM and (ii) the insertion of metallic ions increased the electron tunneling probability through the monolayer.     

The [Ru(CN)5(pyS)](4-) complex, an efficient self-assembled monolayer for the cytochrome c heterogeneous electron transfer studies

The organothiol 4-mercaptopyridine (pyS) has been used extensively as facilitator for the assessment of heterogeneous electron transfer reaction of cytochrome c (cyt c). Its efficiency, however, is strongly affected by the instability of the adlayer due to the C-S bond cleavage. The K(4)[Ru(CN)(5)(pyS)].3H(2)O complex was synthesized and characterized aiming its utilization as an inorganic self-assembled monolayer (SAM) that would enhance the gold adlayer stability. The SAM formed by this complex onto gold (RupySAu) was characterized by spectroscopic (FTIRRAS and SERS) and electrochemical (LSV) techniques. The ex situ vibrational SERS and FTIRRAS spectra data of this SAM formed onto gold suggest a sigma interaction between the gold and sulfur atoms of the complex, inducing a perpendicular arrangement in relation to the surface normal. Additionally, SERS and FTIRRAS spectra performed for freshly prepared RupySAu adlayer and for large immersion times in the precursor solution have not shown any significant change that would reflect the degradation of the adlayer. The LSV desorption curves of this SAM indicate an enhancement in the C-S bond strength of the pyS ligand when coordinated to the [Ru(CN)(5)](3-) moiety. Comparatively to the data obtained for the desorption process of the pyS monolayer, the reductive desorption potential, E(rd), of the RupySAu presents a shift of -17 mV. This bond strength intensification leads to an increase in the stability of the monolayer. The voltammetric curves of cyt c carried out with the RupySAu electrode showed electrochemical parameters consistent with those reported for the native protein, as well as the maintenance of the electrochemical kinetic data after repetitive cycles. The results all together suggest that the pi back-bonding effect from the [Ru(CN)(5)](3-) metal center plays an important role in the stability of the RupySAu adlayer, improving the assessment of the cyt c heterogeneous electron transfer reaction.     

喹啉衍生物自组装单分子膜的制备与结构

由于自组装单分子膜（SeifAssewhledMonolnyer,SAM）技术提供了在分子水平上构造理想界面的方便手段,而且所得到的膜具有优于传统m膜的均一性与稳定性,所以近年来SAM成为研究的热点．大量的研究工作表明SAM在润滑、防腐、催化老u蚀、电子转移反应研究、分子器件、非线性光学等众多领域都有广泛的应用前景［‘］．在以前的工作中我们曾报导了含酸胺键［’］、偶氮苯基团”,‘］等链内取代基的SAM的电化学与红外研究的结果,并采用化学力显微镜（CF）技术对末端为竣基的SAM进行了力滴定研究l’1．由于受合成等因素的制约,目前国…     

Studies on Gold Electrode Modified by Alkanethiol SAM

Self-assembled monolayer (SAM) on gold electrode has been extensively studied in electrochemistry. It is a good model for study the electron transfer through the SAM from metal to redox couple in the solution or tethered on the surface of monolayer. For a pinehole-free monolayer, electron tunneling is considered as the mechanism of electron transfer through the film. The detail of the process in electrochemistry is not clear though there are a lot of publications on SAM research. In this paper,the electrochemical behavior of pinehole-free alkanethiol modified SAM electrode was investigated at different potential in the solution containing various concentration Fe(CN)₆^3-/4- ions. It was found that the apparent resistance could be attributed mainly to the resistance of SAM film.     

膜电阻对自组装膜修饰电极电化学行为的影响

应用循环伏安和交流阻抗技术研究了 16烷基硫醇自组装膜修饰的金电极在Fe(CN) 63 - /Fe(CN) 64 - 溶液中的电化学行为 .无“针孔”缺陷的自组装膜对溶液与基底间的界面电子转移具有强烈的阻碍作用 ,当过电位较大时 ,In(I/ η)对 η1/2 之间具有良好的线性关系 .通过对Au/SAM /Hg模拟体系的电流———电压曲线进行测定 ,得到了自组装膜膜电阻的特征 .指出由于膜电阻的存在 ,自组装膜修饰电极在Fe(CN) 63 - /Fe(CN) 64 - 溶液中的行为实质上反映了膜自身的电阻特征     

维生素B_1自组装膜修饰金电极对多巴胺、尿酸的电催化作用

用维生素B1(VB1)在金电极上进行自组装,制备了VB1自组装膜修饰金电极(VB1-Au/SAMs/CME).利用循环伏安法初步研究了此自组装单分子膜修饰电极的电化学行为.结果表明: VB1在金电极表面具有特性吸附.以\3-/ 4-氧化还原电对为探针,考察了VB1自组装膜修饰金电极的电化学性质, VB1自组装膜的存在对\3-/4-的电子转移具有明显的阻碍作用.研究了多巴胺(DA)和尿酸(UA)在此电极上的电化学行为.实验结果表明, DA和UA在此电极上均可被电催化氧化.差分脉冲伏安(DPV)氧化峰电流与DA浓度在2.0×10-5～4.0×10-4 mol/L范围内呈线性关系;测定UA的线性范围为6.0×10-5～2.2×10-4 mol/L,而且可实现这两种物质的同时测定.     

Self-assembled monolayer of organic iodine on a Au surface for attachment of redox-active metal clusters

The attachment of a bifunctional iodo-organo-phosphinate compound to gold (Au) surfaces via chemisorption of the iodine atom is described and used to chelate a redox-active metal cluster via the phosphinate group. XPS, AFM, and electrochemical measurements show that (4-iodo-phenyl)phenyl phosphinic acid (IPPA) forms a tightly bound self-assembled monolayer (SAM) on Au surfaces. The surface coverage of an IPPA monolayer on Au was quantified by an electrochemical method and found to be 0.40 +/- 0.03 nmol/cm2, roughly corresponding to 0.4 monolayers. We show that the Au/IPPA SAM, but not the underivatized Au, adsorbs Mn4O4(Ph2PO2)6 from solution by a phosphinate exchange reaction to yield Au/IPPA/Mn4O4(Ph2PO2)5 SAM. The resulting SAM is firmly bound and not removed by sonication, as confirmed by manganese XPS (Mn 2p1/2) and by AFM. Electrochemistry confirms that Mn4O4(Ph2PO2)6 is anchored on the Au/IPPA surface and that redox chemistry can be mediated between the electrode and the surface-attached complex. Mn4O4(Ph2PO2)6 contains the reactive Mn4O46+ cubane core, a redox-active bioinspired catalyst.     

Voltammetric Study of Methylene Blue at Thiol SAMs‐Modified Gold Electrodes

The voltammetric behavior of methylene blue (MB) at thiol self‐assembled monolayers modified gold electrodes (SAMs/Au) has been investigated. MB exhibited a redox peak at about ?0.35 V (vs.SCE) in alkaline solution at bare gold electrodes. When the gold electrodes were modified with thiol SAMs, the peak grew due to the accumulation of MB at SAMs. With the solution pH rising, more MB was accumulated, hence the peak height increased, which differed from that at bare gold electrodes. The electrode process at SAMs/Au featured the characteristics of adsorption and/or electrode reaction controlled. The enhancing action of glutathione monolayer (GSH SAM), 3‐mercaptopropionic acid monolayer (3MPA SAM) and other thiol SAMs was compared. Among these, GSH SAM made the MB peak increase more. At GSH SAM/Au, the peak height varied linearly with MB concentration over the range of 2 μM to 400 μM. So this can be developed for the determination of MB and studies concerned. The accumulation behavior caused by GSH SAM and native fish sperm dsDNA was compared. The interaction between DNA and MB was also discussed under this condition.     

Electrochemical metal deposition on top of an organic monolayer

Electrochemical deposition of metals (platinum or gold) only on top of an organothiolate, 1,4-benzenedimethanethiol (BDMT) or hexanedithiol (HDT), self-assembled monolayer (SAM) on a Au(111) substrate was achieved by electrochemical reduction of PtCl(4)(2-) or AuCl(4)(-) ion, which was preadsorbed on one free thiol end group of the dithiol SAM formed on a Au surface, in a metal-ion-free sulfuric acid solution at potentials more negative than the reduction potential of the metal ion. Angle-resolved X-ray photoelectron spectroscopy (AR-XPS) measurement after the reduction of preadsorbed PtCl(4)(2-) ion on BDMT/Au(111) electrode showed the presence of Pt not underneath but on top of the BDMT SAM. After a negative potential scan of the Pt/BDMT/Au(111) electrode to -1.30 V in 0.1 M KOH solution, a typical cyclic voltammogram of a clean Au(111) electrode was obtained, showing that the BDMT SAM with a Pt layer was reductively desorbed. These results proved that a Pt-BDMT SAM-Au substrate sandwich structure without a short circuit between the two metals was successfully constructed by this technique. Furthermore, a decanethiol (DT) monolayer was constructed on a Au layer, which was formed by the reduction of preadsorbed AuCl(4)(-) ion on HDT/Au(111) electrode. The formation of DT/Au/HDT/Au(111) structure was confirmed as two cathodic peaks corresponding to reductive desorption of DT from Au on top of the HDT/Au(111) at -0.97 V and that of Au/ HDT from Au(111) at -1.12 V were observed when potential was scanned negatively to -1.35 V.     

Synthesis of Crown Ether-tethered b-Cyclodextrin and Fabrication of Its Self-assembled Monolayer on Gold Surface

A number of cyclodextrin (CD) derivatives and crown ethers have been designed and synthesized in order to examine their molecular/ions recognition mechanics and control molecular assembly and self-assembly. However, CD derivatives linked by crown ether have rarely been synthesized and their molecular assembly has not been extensively investigated1. In the present communication, we report the synthesis of 4', 5'-dimercap -tomethylene-benzo-15-crown-5 tethered b-cyclodextrin and its self-asse…     

Surface stress, kinetics, and structure of alkanethiol self-assembled monolayers

The surface stress induced during the formation of alkanethiol self-assembled monolayers (SAMs) on gold from the vapor phase was measured using a micromechanical cantilever-based chemical sensor. Simultaneous in situ thickness measurements were carried out using ellipsometry. Ex situ scanning tunneling microscopy was performed in air to ascertain the final monolayer structure. The evolution of the surface stress induced during coverage-dependent structural phase transitions reveals features not apparent in average ellipsometric thickness measurements. These results show that both the kinetics of SAM formation and the resulting SAM structure are strongly influenced both by the surface structure of the underlying gold substrate and by the impingement rate of the alkanethiol onto the gold surface. In particular, the adsorption onto gold surfaces having large, flat grains produces high-quality self-assembled monolayers. An induced compressive surface stress of 15.9 +/- 0.6 N/m results when a c(4x2) dodecanethiol SAM forms on gold. However, the SAMs formed on small-grained gold are incomplete and an induced surface stress of only 0.51 +/- 0.02 N/m results. The progression to a fully formed SAM whose alkyl chains adopt a vertical (standing-up) orientation is clearly inhibited in the case of a small-grained gold substrate and is promoted in the case of a large-grained gold substrate.     

Soft-landing isolation of vanadium-benzene sandwich clusters on a room-temperature substrate using n-alkanethiolate self-assembled monolayer matrixes

Gas-phase synthesized vanadium-benzene 1:2 (VBz(2)) sandwich clusters were size-selectively deposited onto bare gold and long-chain n-alkanethiolate [-S-(CH(2))(n-1)-CH(3); n = 16, 18, and 22] self-assembled monolayer (SAM)-coated gold substrates under ultrahigh vacuum (UHV) conditions. Investigation of the resulting deposited clusters was performed by infrared reflection absorption spectroscopy (IRAS) and thermal desorption spectroscopy (TDS). The IR frequencies of the soft-landed VBz(2) clusters show excellent agreement with the fundamentals reported in IR data of VBz(2) in an argon matrix. The analysis of IRAS spectra reveals that while there was no orientational preference of the VBz(2) clusters on a bare gold substrate, the VBz(2) clusters deposited onto the SAM substrates were highly oriented with the molecular axis 70-80 degrees tilted off the surface normal. In addition, analysis of TDS spectra revealed unusually large adsorption heats of the physisorbed VBz(2) clusters. The present results are explained by cluster penetration into the long-chain alkanethiolate SAM and for the first time demonstrate the matrix isolation of gas-phase organometallic clusters around room temperature.     

3,4-二羟基苯甲酸在自组装结构中的电化学行为

电极与有确定取向的电活性基团之间的电子传递是电化学领域的研究热点．利用二维有序薄膜固定电活性官能团是一个成熟的方法［１］,主要包括ＬＢ技术和自组装技术,这两者都存在着样品合成困难的问题．近年来,通过表面逐层反应来固定电活性官能团已有研究,但是反应过程...     

Immobilization of the nanoparticle monolayer onto self-assembled monolayers by combined sterically enhanced hydrophobic and electrophoretic forces

The immobilization of surface-derivatized gold nanoparticles onto methyl-terminated self-assembled monolayers (SAMs) on gold surface was achieved by the cooperation of hydrophobic and electrophoretic forces. Electrochemical and scanning probe microscopy techniques were utilized to explore the influence of the SAM's structure and properties of the nanoparticle/SAM/gold system. SAMs prepared from 1-decanethiol (DT) and 2-mercapto-3-n-octylthiophene (MOT) were used as hydrophobic substrates. The DT SAM is a closely packed and organized monolayer, which can effectively block the underlying gold and inhibit a variety of solution species including organic and inorganic molecules from penetrating, whereas the MOT monolayer is poorly packed or disorganized (because of a large difference in dimension between the thiophene head and the alkylchain tail) and permeable to many organic probes in aqueous solution but not to inorganic probes. Thus, the MOT monolayer provides a more energetically favorable hydrophobic surface for the penetration and adsorption of organic species than the DT monolayer. This hypothesis is supported by experiments in which the density of hydrophobically immobilized nanoparticles on the MOT SAM is much larger than that on the DT SAM. The results also suggest new approaches for modification of macroscopic surfaces with nanoscopic particles.     

Electrochemical response of dopamine at a penicillamine self-assembled gold electrode.

The penicillamine (Pen) self-assembled monolayer (SAM) modified gold electrode (Pen/Au) is demonstrated to catalyze the electrochemical response of dopamine (DA) by cyclic voltammetry. A pair of well-defined redox waves was obtained and the calculated standard rate constant (k(s)) is 3.88 x 10(-3) cm/s at the self-assembled electrode. The electrode reaction is a quasi-reversible process. The oxidation peak of DA can be used to determine the concentration of DA. The peak current and the concentration of DA are a linear relationship in the range of 2.0 x 10(-5) M to 8.0 x 10(-4) M. The detection limit is 4.0 x 10(-6) M. By ac impedance spectroscopy the apparent electron transfer rate constant (k(app)) of Fe(CN)(3-)/Fe(CN)(4-) at the Pen/Au electrode was obtained as 2.08 x 10(-5) cm/s. The Pen SAM was characterized with X-ray photoelectron spectroscopy (XPS), grazing angle FT-IR spectroscopy and contact angle goniometer.     

Studies on biotin–avidin indirect conjugated technology for a piezoelectric DNA sensor

Because of their high sensitivity, piezoelectric sensor techniques are extremely useful for environmental or clinical analysis. We developed a piezoelectric crystal DNA biosensor for the detection of the hybridization reaction based on the self-assembled monolayer technology and biotin–avidin system. 3,3′-Dithiopropionic acid was applied to form a self-assembled monolayer (SAM) on the gold surface of the quartz crystal. Avidin was coated on the gold electrode conjugated with 1-ethyl-3-[3-(dimethylamino)propyl]-carbodiimide (EDC) and N-hydroxysuccinimide (NHS), and then biotinylated nucleotide acids were immobilized on the gold electrode surface through the specific interaction of biotin and avidin. Our results indicated that, using this immobilization method, the piezoelectric DNA sensor shows a higher sensitivity and specificity in detecting the hybridization reaction. The sensor can be used repeatedly by electrode regeneration.     

Mediator free cholesterol biosensor based on self-assembled monolayer platform

Self-assembled monolayer (SAM) of 4-aminothiophenol (4-ATP) has been investigated for immobilization of bi-enzymes (ChOx and ChEt) towards development of enzyme biosensors for detection of free and total cholesterol. This enzyme immobilized SAM surface has been characterized by scanning electron microscopy and electrochemical measurements. The results of electrochemical response studies reveal fast enzymatic reaction in phosphate buffer saline solution without using any artificial mediator. This may be attributed to the molecular wire type behavior of short 4-ATP molecule that promotes electron transfer between enzyme and the electrode surface due to its conjugated structure. Interference free estimation of free and total cholesterol has been realized at low operating potential of 0.33 V with range of detection as 25 to 400 mg dl(-1), sensitivity of 542.3 nA mM(-1) (for ChOx/4-ATP/Au) and 886.6 nA mM(-1) (for ChEt-ChOx/4-ATP/Au) with a response time of 20 s at pH 7.4.     

Surface coordination of nitric oxide to a self-assembled monolayer of a triruthenium cluster: an in situ infrared spectroscopic study

Coordination of nitric oxide (NO) to a self-assembled monolayer (SAM) of a triruthenium (Ru(3)) cluster, [Ru(3)(micro(3)-O)(micro-CH(3)COO)(6)(CO)(L(1))(L(2))] (0) (L(1) = [(NC(5)H(4))CH(2)NHC(O)(CH(2))(10)S-](2), L(2) = 4-methylpyridine), on a gold electrode surface has been studied by electrochemical and in situ infrared (IR) spectroscopic measurements. Ligand substitution reaction of NO for carbon monoxide (CO) ligands in the SAM strongly depends on the oxidation state of the terminal Ru(3) cluster. NO can be introduced into the Ru(3) cluster in the SAM with a high yield after one-electron oxidation of the Ru(3) core to a (III,III,III) oxidation state, while no coordination reaction occurs at the initial oxidation state (II,III,III) of the Ru(3) cluster. The kinetics of the NO coordination and desorption processes is also evaluated by time-resolved in situ IR spectroscopy. Finally, we demonstrate that the SAM with NO/CO randomly mixed ligands at a desired ratio can be constructed on the gold surface by tuning a suitable oxidation state of the Ru 3 cluster under electrochemical control.     

Enantioselectivity of redox reaction of DOPA at the gold electrode modified with a self-assembled monolayer of homocysteine

The enantioselectivity of the self-assembled monolayer (SAM) of homocysteine formed on the (111)-oriented gold surface was investigated. We analyzed the redox behavior of 3,4-dihydroxyphenylalanine (DOPA), which is an electrochemically active chiral molecule, by means of cyclic voltammetry at a gold electrode modified with one enantiomeric form of homocysteine. It was demonstrated that the homocysteine SAM of one enantiomeric form blocked the redox reaction of only one enantiomer of DOPA, with cross inversion for the other enantiomer, in acidic solution.     

Reduced Pd density of states in Pd/SAM/Au junctions: the role of adsorbed hydrogen atoms

Experiments have shown that a Pd monolayer deposited electrochemically on a Au-supported self-assembled monolayer (SAM) of 4-mercaptopyridine (Mpy) exhibits a strongly reduced Pd local density of states (LDOS) at the Fermi energy (E(f)). Understanding the origin of this modified electronic structure is crucial for the use of the sandwich design as a platform for future nanoelectronics. Here we suggest that hydrogen adsorption might be the origin of the modified electronic properties. We performed periodic density functional theory calculation to explore the influence of hydrogen adsorption on the geometric and electronic structure of a Pd/Mpy/Au(111) complex. Dissociative adsorption of H(2) on a Pd monolayer on top of a Mpy SAM is a strongly exothermic process leading to atomic hydrogen atoms preferentially located at the hollow sites. Due to the formation of a strong Pd-H bond the Pd-SAM interaction realized via one-fold N-Pd bonds is substantially weakened. Upon hydrogen adsorption, the Pd LDOS becomes significantly modified exhibiting a drastic reduction of the density of states at E(f). The calculated spectra are in a good agreement with the experiment for a hydrogen coverage corresponding to two monolayers which is still thermodynamically allowed.