Fabrication and Photoelectrochemical Properties of [Ru(bpy)2dppz]2+ on ITO Electrode Associated with the Oxidation of Guanine

<正>Electrochemical assembly of[Ru(bpy)_2dppz]~(2+){bpy=2,2'-bipyridine,dppz=dipyrido[3,2-a:2',3'-c]phenazine} on an ITO electrode in the presence of guanine and photoelectrochemical properties of the assembled layer were investigated.It has been found that[Ru(bpy)_2dppz]~(3+/2+) can be assembled onto the ITO electrode by the method of repetitive voltammetric sweeping,and the assembly is enhanced by guanine.The peak currents of prewaves increase linearly up to a guanine concentration of 0.25 mmol/L.More importantly,upon illumination with 470 nm light source and at an applied potential of 0.2 V,cathodic current for the fabricated layer on the ITO electrode indicate a linear enhancement with the rise of guanine concentration.Meanwhile,[Ru(bpy)_2dppz]~(2+) can be served as an excellent mediator to prompt the oxidation of guanine,and the mediated peak current increases linearly with added guanine concentration from 0.01 to 0.25 mmol/L.In addition,the assembly mechanism of[Ru(bpy)_2dppz]~(2+) on the ITO electrode associated with the oxidation of guanine and the assistance of light irradiation were discussed.     

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.     

A novel ferroceneylazobenzene self-assembled monolayer on an ITO electrode: photochemical and electrochemical behaviors

A novel ferroceneylazobenzene self-assembled monolayer (SAM) has been constructed on an indium-tin oxide (ITO) electrode via the covalent attachment of 4-(4'-11-ferrocenyl-undecanoxyphenylazo)benzoic acid ( FcAzCOOH) onto a silanized ITO substrate surface and verified by reflectance infrared spectroscopy and water contact angle. Atomic force microscopy (AFM) and cyclic voltammogram (CV) indicated that the FcAzCOOH formed a uniform and reproducible SAM on the ITO electrode with a surface coverage of ca. 1.9 x 10 (-10) mol/cm (2) (87 A (2)/molecule). The reversible photoisomerization behavior of the SAM was characterized by UV-vis spectra. The azo pi-pi* transition band intensity of the SAM gradually decreased with UV (365 nm) irradiation and was almost recovered again when subsequent exposure to ambient room light (400-800 nm). The increased tilt angle of the molecules on the ITO substrate after UV irradiation further confirmed the trans-to- cis isomerization of azobenzene moieties. The CV of the trans- FcAzCOOH modified ITO electrode showed a pair of waves due to redox of the ferrocene groups in the potential range of 0 to +800 mV (vs SCE), and the peak separation of the redox wave became larger after UV irradiation and almost returned to its original value after subsequent exposure to the visible light. Rate-dependent CV curves indicated that the charge transfer rate between the ferrocene species in the SAM and the ITO electrode was slowed down after UV irradiation due to the smaller porosity of the monolayer film and the more compact barrier layer between the redox species and the ITO electrode. It is the first time to directly observe the influence of photoisomerization of the azobenzene moiety on the redox behavior of redox species in the ferroceneylazobenzene-functionalized SAM. The present results provide profound insight into the role of redox microenvironment on electron transfer kinetics and also provide a simple and facile approach to the preparation of photocontrollable electrodes.     

Efficient charge injection from the S2 photoexcited state of special-pair mimic porphyrin assemblies anchored on a titanium-modified ITO anode

A novel surface fabrication methodology has been accomplished, aimed at efficient anodic photocurrent generation by a photoexcited porphyrin on an ITO (indium-tin oxide) electrode. The ITO electrode was submitted to a surface sol-gel process with titanium n-butoxide in order to deposit a titanium monolayer. Subsequently, porphyrins were assembled as monolayers on the titanium-treated ITO surface via phosphonate, isophthalate, and thiolate groups. Slipped-cofacial porphyrin dimers, the so-called artificial special pair at the photoreaction center, were organized through imidazolyl-to-zinc complementary coordination of imidazolylporphyrinatozinc(II) units, which were covalently immobilized by ring-closing olefin metathesis of allyl side chains. The modified surfaces were analyzed by means of X-ray photoelectron spectroscopy. Photoirradiation of the porphyrin dimer generated a large anodic photocurrent in aqueous electrolyte solution containing hydroquinone as an electron sacrificer, due to the small reorganization energy of the dimer. The use of different linker groups led to significant differences in the efficiencies of anodic photocurrent generation. The apparent flat-band potentials evaluated from the photocurrent properties at various pH values and under biased conditions imply that the band structure of the ITO electrode is modified by the anchoring species. The quantum yield for the anodic photocurrent generation by photoexcitation at the Soret band is increased to 15 %, a surprisingly high value without a redox cascade structure on the ITO electrode surface, while excitation at the Q band is not so significant. Extensive exploration of the photocurrent properties has revealed that hot injection of the photoexcited electron from the S2 level into the conduction band of the ITO electrode takes place before internal conversion to the S1* state, through the strong electronic communication of the phosphonyl anchor with the sol-gel-modified ITO surface.     

Reconstitution of apo-glucose dehydrogenase on pyrroloquinoline quinone-functionalized au nanoparticles yields an electrically contacted biocatalyst

An electrically contacted glucose dehydrogenase (GDH) enzyme electrode is fabricated by the reconstitution of the apo-GDH on pyrroloquinoline quinone (PQQ)-functionalized Au nanoparticles (Au-NPs), 1.4 nm, associated with a Au electrode. The Au-NPs functionalized with a single amine group were attached to the Au surface by 1,4-benzenedithiol bridges, and PQQ was covalently linked to the Au-NPs. The apo-GDH was then reconstituted on the PQQ cofactor sites. The surface coverage of GDH corresponded to 1.4 x 10(-12) mol cm(-2). The reconstituted enzyme revealed direct electrical contact with the electrode surface, and the bioelectrocatalytic oxidation of glucose occurred with a turnover number of 11,800 s(-1). In contrast, a system that included the covalent attachment of GDH to the PQQ-Au-NPs monolayer in a random, nonaligned, configuration revealed lack of electrical communication between the enzyme and the electrode, albeit the enzyme existed in a bioactive structure. The bioelectrocatalytic function of the later system was, however, activated by the diffusional electron mediator 2,6-dichlorophenol-indophenol. The results imply that the alignment of GDH on a Au-NP through the reconstitution process leads to an electrically contacted enzyme-electrode, where the Au-NP acts as a charge-transfer mediator.     

An amperometric hydrogen peroxide biosensor based on immobilizing horseradish peroxidase to a nano-Au monolayer supported by sol-gel derived carbon ceramic electrode

A novel strategy for fabricating horseradish peroxidase (HRP)-based H(2)O(2) sensor has been developed by combining the merits of carbon sol-gel supporting matrix and nano-scaled particulate gold (nano-Au) mediator. The thiol functional group-derived carbon ceramic electrode (CCE) was first constructed using (3-mercaptopropyl) trimethoxy silane as sol-gel monomer. Then, the stable nano-Au monolayer was obtained through covalent linkage between nano-Au and thiol group on the surface of CCE. The experimental results showed that nano-Au monolayer formed not only could steadily immobilize HRP but also efficiently retain its bioactivity. Hydrogen peroxide was detected with the aid of hydroquinone mediator to transfer electrons between the electrode and HRP. The process parameters for the fabrication of the enzyme electrode and various experimental variables such as the operating potential, mediator concentration and pH of background electrolyte were explored for optimum analytical performance of the enzyme electrode. The biosensor had a fast response of less than 8 s with linear range of 1.22 x 10(-5) to 1.10 x 10(-3)mol l(-1) and a detection limit of 6.1 x 10(-6)mol l(-1). The sensitivity of the sensor for H(2)O(2) was 0.29 A l mol(-1) cm(-2). The activation energy for enzyme reaction was calculated to be 10.1 kJ mol(-1). The enzyme electrode retained 75% of its initial activity after 5 weeks storage in phosphate buffer at pH 7.     

Advanced surface modification of indium tin oxide for improved charge injection in organic devices

A new method is described for surface modification of ITO with an electroactive organic monolayer. This procedure was done to enhance hole injection in an electronic device and involves sequential formation of a monolayer of a pi-conjugated organic semiconductor on the indium tin oxide (ITO) surface followed by doping with a strong electron acceptor. The semiconductor monolayer is covalently bound to the ITO, which ensures strong adhesion and interface stability; reduction of the hole injection barrier in these devices is accomplished by formation of a charge-transfer complex by doping within the monolayer. This gives rise to very high current densities in simple single layer devices and double layer light emitting devices compared to those with untreated ITO anodes.     

苝四羧酸自组装单分子膜的制备及其光电转换性质研究

通过共价键连接的方式在亲水性基底上制备了刚性功能分子3,4,9,10- NFDA1 四羧酸的自组装单分子膜,利用接解角、紫外-可见光谱、电化学循环伏安等方法对所制备的NFDA1 四羧酸自组装膜进行了表征,并初步研究了该自组装膜在ITO电极表面光电转换性质.     

Molecular lego for the assembly of biosensing layers

We propose a procedure to assemble monolayers of redox mediator, coenzyme, enzyme and stabilizing polyelectrolyte on an electrode surface using essentially electrostatic and complexing interactions. In a first step a monolayer of redox mediator, substituted nitrofluorenones, is adsorbed. In a second step, a layer of calcium cations is immobilized at the interface. It establishes a bridge between the redox mediator and the subsequently adsorbed coenzyme NAD⁺. In the next step we use the intrinsic affinity of the NAD⁺ monolayer for dehydrogenases to build up a multilayer composed of mediator/Ca²⁺/NAD⁺/dehydrogenase. The so obtained modified electrode can be used as a biosensor. Quartz crystal microbalance measurements allowed us to better understand the different parameters responsible for the adsorption. A more detailed investigation of the system made it possible to finally stabilize the assembly sufficiently by the adsorption of a polyelectrolyte layer in order to perform rotating disk electrode measurements with the whole supramolecular architecture on the electrode surface.     

Covalent attachment of glucose oxidase to an Au electrode modified with gold nanoparticles for use as glucose biosensor

A feasible method to fabricate glucose biosensor was developed by covalent attachment of glucose oxidase (GOx) to a gold nanoparticle monolayer modified Au electrode. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) of ferrocyanide followed and confirmed the assemble process of biosensor, and indicated that the gold nanoparticles in the biosensing interface efficiently improved the electron transfer between analyte and electrode surface. CV performed in the presence of excess glucose and artificial redox mediator, ferrocenemethanol, allowed to quantify the surface concentration of electrically wired enzyme (Gamma(E)(0)) on the basis of kinetic models reported in literature. The Gamma(E)(0) on proposed electrode was high to 4.1 x 10(-12) mol.cm(-2), which was more than four times of that on electrode direct immobilization of enzyme by cystamine without intermediate layer of gold nanoparticles and 2.4 times of a saturated monolayer of GOx on electrode surface. The analytical performance of this biosensor was investigated by amperometry. The sensor provided a linear response to glucose over the concentration range of 2.0 x 10(-5)-5.7 x 10(-3) M with a sensitivity of 8.8 microA.mM(-1).cm(-2) and a detection limit of 8.2 microM. The apparent Michaelis-Menten constant (K(m)(app)) for the sensor was found to be 4.3 mM. In addition, the sensor has good reproducibility, and can remain stable over 30 days.     

Electrochemically‐controlled DNA Release under Physiological Conditions from a Monolayer‐modified Electrode

An indium tin oxide (ITO) electrode prepared on a flexible polymeric support was modified with an amino‐silane and then functionalized with trigonelline and 4‐carboxyphenylboronic acid covalently bound to the amino groups. The trigonelline species containing quarterized ammonium group produced positive charge on the electrode surface regardless of the pH value, while the phenylboronic acid species were neutral below pH 8 and negatively charged above pH 9 (note that their pK_a=8.4). The total charge on the monolayer‐modified electrode was positive at the neutral pH and negative at pH>9 (note that 4‐carboxyphenylboronic acid was attached to the electrode surface in excess to trigonelline, thus allowing the negative charge to dominate on the electrode surface at basic pH). Single‐stranded DNA molecules were loaded on the modified electrode at pH 7.0 due to their electrostatic attraction to the positively charged surface. By applying electrolysis at −1.0 V (vs. Ag/AgCl reference) electrochemical oxygen reduction resulted in the consumption of hydrogen ions and local pH increase in the vicinity of the electrode surface. The process resulted in the transition to the total negative charge due to the negative charges formed on the phenylboronic acid species. This resulted in the electrostatic repulsion and release of the loaded DNA. The developed approach allowed the electrochemically‐triggered DNA release not only in the aqueous solutions, but also in human serum solution, thus giving promise for future biomedical applications.     

Fabrication of an electrode-viologen-hydrogenase heterogeneous system and the electrochemical hydrogen evolution

An indium tinoxide (ITO) electrode was chemically modified by one layer of viologen (VIO) deri vative, which possessed a persistent and reproducible electrochemical response. A monolayer of a thermal stable hydrogenase from Thiocapsa roseopersic ina was stabilized on a synthesized poly-l-lysine subphase surface and transferred on to the electrode for fabrication of an ITO-VIO-hydrogenase heterogeneous system. Electrochemical properties of both the ITO-VIO monolayer and the heterogeneous ITO-VIO-hydrogenase system have been investigated. Hydrogen evolution could be measured by potentiostating the VIO-hydrogenase-covered ITO electrode to “electroplate” [(VIO⁺)_n]_surf and a large increase in hydrogen evolution was observed when using an electrolytesolution containing sodium dithionite. We discuss the possible electron transfer process.     

Use of zirconium-phosphate-carbonate chemistry to immobilize polycyclic aromatic hydrocarbons on boron-doped diamond

We report on the formation of monomolecular layers of perylene- and pyrene-alkanoic acids on boron-doped diamond (BDD) substrates. The carboxylic acid layers are bound by coordination to zirconium phosphate (ZP) functionalities on the BDD substrate surface. The resulting Zr-phosphate-carbonate (ZPC) linkages between the substrate and the adlayer are asymmetric, of the form -(OPO3(2-) Zr4+-O2C-R)+ X-. Pyrene and perylene are well-established optical probes of polarity and viscosity at interfaces. We have used electrochemical and steady-state fluorescence techniques to study the loading density and behavior of these monomolecular films, allowing comparison of BDD and indium-doped tin oxide (ITO) substrates. Electrochemical data suggest that the pyrene chromophores are positioned roughly at the same distance from the surface, regardless of the length of the anchoring alkanoic acid chain, a finding that can be explained by the pyrene lying on the substrate surface. Such a conformation is plausible given the surface coverage (5 x 10(-11) mol/cm2, ca. 0.1 monolayer) we measure for these systems.     

Pd纳米粒子敏化的纳米单层TiO_2-Ru(Ⅱ)螯合物修饰电极对单磷酸鸟苷的电催化氧化行为

利用LB膜技术可控制备了纳米单层的二氧化钛-有机钌螯合物杂化膜,并研究了上述无机-有机杂化膜修饰电极在Pd纳米粒子敏化后对单磷酸鸟苷(GMP)的电催化氧化行为.实验结果表明:(1)纳米单层TiO2/[Ru(phen)2(dC18bpy)]2+(简称为TiO2-Ru)杂化膜的平均厚度为(3.2±0.5)nm;(2)在光照条件下TiO2-Ru杂化膜能有效催化还原[Pd(NH3)4]2+形成粒径位于20～200nm之间的Pd纳米粒子;(3)纳米单层TiO2-Ru/Pd杂化膜能高效催化氧化具有供电子能力的单磷酸鸟苷(GMP),与纳米单层TiO2-Ru杂化膜修饰的ITO电极(ITO/TiO2-Ru)相比,当工作电压为1200mV时,ITO/TiO2-Ru/Pd电极在含有1×10-3molL-1GMP的磷酸盐缓冲液中,单位面积的催化氧化电流提高了约36倍;(4)Pd纳米粒子的引入消除了金属钌螯合物中配体对电子传递的阻碍作用,改变了电子传递途径,从而有效减少了电子空穴对的复合,提高了杂化膜修饰电极(ITO/TiO2-Ru/Pd)的电子传递效率.     

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.     

铅于旋转银盘电极上欠电位沉积的计时库仑法研究

用计时库仑法研究了Ｐｂ２＋于旋转Ａｇ盘电极（Ａｇ－ＲＤＥ）上的欠电位沉积（ＵＰＤ）性质。实验证明,支持电解质（０．０１ｍｏｌ／Ｌ　ＨＮＯ３＋０．０１　ｍｏｌ／Ｌ ＮａＣＩ）中Ｃｌ－离子的存在有利于取得稳态条件下ＵＰＤ Ｐｂ的电量值。其氧化过程电量（Ｑａ）,代表Ｐｂ２＋吸附单层被定量转化为ＵＰＤ Ｐｂ单层的量,Ｑａ的平均值为３９３．３μＣ／ｃｍ２,相应于２×１０－９ｍｏｌ／ｃｍ２Ｐｂ２＋表面复盖量。     

Catalytic Au Electrodes Based on SAMs of per(6‐deoxy‐6‐thio‐2,3‐di‐O‐methyl)‐β‐cyclodextrin

A three‐step sequential self‐assembly procedure was applied in preparing gold electrodes modified in a stable and controlled way by a monolayer of thiolated β‐cyclodextrin (β‐CD), with methylene blue (MB) included in its cavity as the active component of the monolayer, and octanethiol as the nonelectroactive spacer blocking the electrode surface not occupied by β‐CD. MB acted as a mediator of electrons with respect to a solution soluble analyte, H₂O₂, and provided electrical contact between the electrode and solution resident enzyme, laccase, catalyzing reduction of oxygen to water.     

Determination of porphyrin dimer in a mixed monolayer of porphyrin/phospholipid transferred on ITO electrodes

The Langmuir–Blodgett method has been used to transfer mixed monolayers of a porphyrin (TMPyP) and a phospholipid (DMPA) from the air|water interface onto optically transparent indium–tin oxide (ITO) electrodes. The surface concentration of porphyrin, Γ, transferred on the ITO surface, has been obtained by integration of the reduction current from the cyclic voltammograms. The experimental Γ values ranged from Γ_m=5.19×10⁻¹¹ mol cm⁻², and corresponding to a compact monolayer of porphyrin monomers in a plane orientation with respect to the surface, to Γ_d=8.65×10⁻¹¹ mol cm⁻², and equivalent to the total amount of the porphyrin molecules at the air|water interface under a compression of 35 mN m⁻¹. Prior to the electrochemical experiments, the transmission spectrum was recorded. The surface concentration obtained of the porphyrin is not directly proportional to the transmission of the film, ΔT. This phenomenon is assigned to the dimer formation and, depending on the surface activity of the ITO electrodes, toward porphyrin adsorption. The dimer structure of TMPyP on an intact ITO electrode is altered with respect to that found at the air|water interface. A simple model has been developed to evaluate the contribution of monomer and dimer phases of the porphyrin in the mixed monolayer. Furthermore, spectroscopic measurements with linearly polarized light under oblique incidence have been performed in order to infer the plane orientation of the TMPyP molecules with respect to the ITO surfaces.     

An Electrochemical Sensor for the Detection of Cu2+ Based on Gold Nanoflowers‐modifed Electrode and DNAzyme Functionalized Au@MIL‐101 (Fe)

In this study, we developed an electrochemical sensor for sensitive detection of Cu²⁺ based on gold nanoflowers (AuNFs)‐modified electrode and DNAzyme functionalized Au@MIL‐101(Fe) (MIL: Materials of Institute Lavoisier). The AuNFs‐modified indium tin oxide modified conductive glass electrode(AuNFs/ITO) prepared via electrodeposition showed improved electronic transport properties and provided more active sites to adsorb large amounts of oligonucleotide substrate (DNA1) via thiol‐gold bonds. The stable Au@MIL‐101(Fe) could guarantee the sensitivity because of its intrinsic peroxidase mimic property, while the Cu²⁺‐dependent DNA‐cleaving DNAzyme linked to Au@MIL‐101(Fe) achieved the selectivity toward Cu²⁺. After the DNAzyme substrate strand (DNA2) was cleaved into two parts due to the presence of Cu²⁺, the oligonucleotide fragment linked to MIL‐101(Fe) was able to hybridize with DNA1 adsorbed onto the surface of AuNFs/ITO. Due to the peroxidase‐like catalytic activity of MIL‐101(Fe) and the affinity recognition property of DNAzyme toward Cu²⁺, the electrochemical biosensor showed a sensitive detection range from 0.001 to 100 μM, a detection limit of 0.457 nM and a high selectivity, demonstrating its potential for Cu²⁺ detection in real environmental samples.     

Electrocatalytic oxidation of nucleobases by TiO2 nanobelts

Two kinds of TiO(2) nanobelts were prepared from commercial P-25 powders via an alkaline hydrothermal method with and without an acid etching process. The uncauterized nanobelts (TNs) exhibited a smooth surface, and mixed phases of anatase and TiO(2) (B), whereas the cauterized ones (CTNs) displayed a rough surface and a pure anatase structure. TNs and CTNs were then deposited onto a glassy carbon electrode (GCE) surface with a conductive adhesive (CA), and the resulting chemically modified electrodes exhibited electrocatalytic activities in the oxidation of nucleobases in a 0.1 M phosphate buffer solution (PBS) at pH 7.4. For guanine and adenine, well-defined oxidation peaks were observed in voltammetric measurements at about +0.62 and +0.89 V, respectively, at a potential sweep rate of 100 mV s(-1), whereas for cytosine, uracil and thymine, the voltammetric features were not obvious. The average surface coverages (Γ) of guanine and adenine on the CTNs/CA/GCE electrode were estimated to be 4.75 × 10(-10) and 7.44 × 10(-10) mol cm(-2), respectively, which were about twice those at the TNs/CA/GCE electrode. The enhanced activity of the CTN-based electrode towards purine nucleobase oxidation was ascribed to the large specific surface area and anatase structures with enhanced (001) facets of the CTN that facilitated adsorption of the analytes onto the electrode surface and charge transport through the electrode surface layer.