Electrochemical evaluation of alternating duplex-triplex conversion effect on the anthraquinone-photoinjected hole transport through DNA duplex immobilized on a gold electrode

Amperometry was employed to characterize the anthraquinone (AQ)-photoinjected hole transport through a 20-mer oligodeoxynucleotide (ODN) duplex, as immobilized on the surface of a gold electrode, and its triplex forms converted by association with several third oligopyrimidine (OPD) short strands. While the cathodic photocurrent was observed upon irradiation at 365 nm of the AQ photosensitizer linked to the end of DNA duplex, a marked lowering of the current density was identified to occur by the triplex formation of a duplex with a given third OPD short strand. The photocurrent through the DNA duplex showed a reversible fall-rise response concomitant with alternating association-dissociation cycle of the OPD short-strand, as regulated by temperature change around the corresponding melting temperature of the DNA triplex. Both the switched photoirradiation and the thermally alternating duplex-triplex conversion could provide tools of regulating the DNA hole transport.     

Molecular chirality and charge transfer through self-assembled scaffold monolayers

The effect of molecular chirality on electron transmission is explored by photoelectrochemistry. Thiol-terminated chiral scaffold molecules containing a porphyrin chromophore were self-assembled on gold surfaces to form a monolayer. Incorporation of the SAM-coated gold into an electrochemical cell and illumination with visible light generated a cathodic photocurrent. When using circularly polarized light, the photocurrent displayed an asymmetry (different magnitude of photocurrent for right versus left polarization) that changed with the molecular chirality (left- or right-handedness of the scaffold). A symmetry constraint on the electronic coupling between the porphyrin and the organic scaffold is proposed as a possible mechanism for the photocurrent asymmetry.     

pH-controlled switching of photocurrent direction by self-assembled monolayer of helical peptides

A novel molecular system, where the photocurrent direction can be reversibly switched by changing the pH of the solution, was prepared on gold from helical peptides carrying a photosensitizer and a carboxyl group at the terminal. Upon photoexcitation of the photosensitizer in an aqueous solution containing an electron donor and acceptor at pH 10, the monolayer generated an anodic photocurrent due to enhancement of the dipole moment by a carboxylate anion, while it generated an opposite cathodic photocurrent at pH 3.4     

DNA hole transport on an electrode: application to effective photoelectrochemical SNP typing

A useful feature of DNA is that long-range hole transport through DNA is readily achieved. Photostimulated long-range hole transport through DNA has prospective use in the development of a conceptually new electrochemical single-nucleotide polymorphism (SNP) typing method for use as a versatile platform for gene diagnostics and pharmacogenetics. We have applied artificial DNAs designed for photostimulated long-range hole transport through DNA to SNP typing. By hybridizing photosensitizer-equipped DNA probes, immobilized on gold working electrodes, with a target DNA strand containing an SNP site, we observed a cathodic photocurrent, which markedly changed depending on the nature of the base at the specific site. The use of a combination of hole-transporting bases constitutes a very powerful method for a single-step electrochemical assay applicable to SNP typing of all types of sequences.     

Sequence‐Dependent Photocurrent Generation through Long‐Distance Excess‐Electron Transfer in DNA

Given its well‐ordered continuous π stacking of nucleobases, DNA has been considered as a biomaterial for charge transfer in biosensors. For cathodic photocurrent generation resulting from hole transfer in DNA, sensitivity to DNA structure and base‐pair stacking has been confirmed. However, such information has not been provided for anodic photocurrent generation resulting from excess‐electron transfer in DNA. In the present study, we measured the anodic photocurrent of a DNA‐modified Au electrode. Our results demonstrate long‐distance excess‐electron transfer in DNA, which is dominated by a hopping mechanism, and the photocurrent generation is sequence dependent.     

Photocurrent response after enzymatic treatment of DNA duplexes immobilized on gold electrodes: electrochemical discrimination of 5-methylcytosine modification in DNA

We demonstrate a photoelectrochemical approach to the detection of the methylation status of cytosine bases in DNA. We prepared anthraquinone (AQ) photosensitizer-tethered oligodeoxynucleotide (ODN) duplexes bearing 5-methylcytosine (mC) or the corresponding cytosine (C) at a restriction site of the ODN strand immobilized on gold electrodes, and measured their photocurrent responses arising from hole transport after enzymatic digestion. Treatment with HapII or HhaI of the duplexes bearing normal C led to strand cleavage, and the photosensitizer unit was eliminated from the ODN strand immobilized on the gold electrode, exclusively reducing the photocurrent density. With a similar treatment, the duplexes bearing mC showed higher photocurrent responses arising from hole transport through the duplex. This significant difference in the photocurrent response between mC and normal C residues in DNA on the gold electrodes is potentially applicable to the detection of mC modification in DNA.     

Surface-grafted multiporphyrin arrays as light-harvesting antennae to amplify photocurrent generation

Organized multiporphyrin arrays were developed on the conductive surface by a novel coordination-directed molecular architecture aiming at efficient photoelectric conversion. The basic strategy employs the mutual coordination of two imidazolylporphyrinatozinc(II) units to form a cofacial dimer. Thus, meso,meso-linked bis(imidazolylporphyrinatozinc) (Zn2(ImP)2) was organized onto imidazolylporphyrinatozinc on the gold substrate as a self-assembled monolayer. The organized Zn2(ImP)2 bearing allyl side chains was covalently linked by ring-closing olefin metathesis catalyzed with Grubbs catalyst. Alternating coordination/metathesis reactions allow the stepwise accumulation of multiporphyrin arrays on the gold electrode. A successive increase in absorption over a wide wavelength range occurred after each accumulation step of Zn2(ImP)2 on the gold electrode, and cathodic photocurrent generation was enhanced in the aqueous electrolyte system, containing viologen as an electron carrier. The significant increase of the photocurrent indicates that the multiporphyrin array works as a "light-harvesting antenna" on the gold electrode.     

Photoelectrochemical evaluation of pH effect on hole transport through triplex-forming DNA immobilized on a gold electrode

We characterized pH effect on hole transport through DNA duplexes possessing a partial triplex-forming region. Direct electrochemical measurement of the current response of photosensitizer-tethered DNA immobilized on a gold electrode revealed that the partial triplex formation under acidic conditions suppressed photocurrent due to hole transport, while dissociation of the triplex into the duplex as occurred upon increasing pH values recovered the photocurrent efficiency. Reversible conversion between duplex and triplex induced upon cyclic alternation of pH values resulted in a rise and fall of photocurrent responses, indicating that pH change may feature in the switching function of hole transport in DNA. These electrochemical behaviors could be correlated to the results obtained in long-range photo-oxidative DNA cleavage experiments, in which DNA cleavage at the hole trapping site beyond the triplex region was significantly suppressed under triplex-forming acidic conditions.     

Facile synthesis of chlorophyll analog possessing a disulfide bond and formation of self-assembled monolayer on gold surface

A chlorophyll analog forming self-assembled monolayers (SAMs) on a gold surface was synthesized for the first time. 13(2)-(Demethoxycarbonyl)pheophorbide-a, which was converted from naturally occurring chlorophyll-a, was condensed with 2-hydroxyethyl disulfide to give a chlorin dyad linked by a disulfide bond. The chlorin analog was spontaneously immobilized on a gold substrate by soaking in an acetone solution of the dyad for 24 h. The resulting gold plate exhibited a visible absorption spectrum with about 420- and 675-nm maxima as the Soret and Qy peaks, respectively, indicating that chlorin pi-conjugates were modified on the gold substrate through Au-S bonding. Both visible absorption and fluorescence emission bands of the chlorin chromophores on the gold substrate were red-shifted compared with those of the synthesized chlorin dyad in a homogeneous acetone solution. The measured absorbance at the Soret maximum suggests that the chlorin chromophores on the gold plate were densely packed on a gold surface to form a SAM. Cathodic photocurrents were generated from SAMs of the chlorins on a gold substrate with irradiation of visible-lights above 400 nm. Photoinduced electron transfer from chlorins on the gold substrate to oxygen molecules in an electrolyte solution were attributed to the cathodic photocurrent generation.     

Light-driven OR and XOR programmable chemical logic gates

Photoelectrodes made of nanocrystalline titanium dioxide modified with various pentacyanoferrates exhibit unique photoelectrochemical properties; photocurrent direction can be switched from anodic to cathodic and vice versa upon changes in photoelectrode potential and incident light wavelength (PhotoElectrochemical Photocurrent Switching, PEPS effect). At certain potentials, anodic photocurrent generated upon UV irradiation has the same intensity as the cathodic photocurrent generated upon visible irradiation. Under these conditions, simultaneous irradiation with UV and visible light results in compensation of anodic and cathodic photocurrents, and zero net photocurrent is observed. This process can be used for construction of unique light-driven chemical logic gates.     

SPECTRAL SENSITIZATION OF SnO2 THIN FILM ELECTRODE BY THE J-AGGREGATE OF CYANINE DYES IN MONOLAYERS

Arachidic acid monolayers containing 5,6,5',6'-dibenzo-I,I-diethyl-2,2'-cyanine chloride were prepared by the spreading method and deposited on the SnO₂ surface using the Langmuir-Blodgett technique. The SnO₂ : dye assembly prepared in this manner was used as a photoelectrode of the conventional electrochemical cell. A distinct J-band was observed in the action spectra of sensitized anodic and cathodic photocurrents. Effects of calcium arachidate barriers inserted between dye layer and either SnO₂ electrode (inside barrier) or electrolyte solution (outside barrier) on the photocurrent were examined. Although the inside barrier was effective in decreasing both anodic and cathodic photocurrents, the outside barrier did not suppress cathodic photocurrent. The following mechanism involving a molecular exciton of the J-aggregate is proposed for the sensitized photocurrent. The anodic photocurrent is caused by hole trapping by some reducing agent and concomitant injection of the electron from molecular exciton to the conduction band of SnO₂. Extraction of conduction-band electron of SnO₂ by molecular exciton and supplying to some oxidizing agent such as dissolved oxygen are responsible for the cathodic photocurrent.     

Abnormal Cathodic Photocurrent Generated on an n‐Type FeOOH Nanorod‐Array Photoelectrode

A simple, wet‐chemical method for the synthesis of an FeOOH nanorod‐array photoelectrode on fluorine‐doped tin oxide (FTO) glass is reported. Nanorods of diameter about 35 nm and length about 300 nm have been vertically grown on an FTO substrate. Upon calcination, the FeOOH phase could be easily converted to a hematite structure while maintaining the shape of the nanorod array. An interesting abnormal cathodic photocurrent is generated on the FeOOH nanorod‐array photoelectrode under illumination, which is totally different from that obtained on a calcined hematite photoelectrode under the same experimental conditions. The cathodic photocurrent density generated on the FeOOH photoelectrode can also be tuned by applying an electrochemical anodic or cathodic treatment. Detailed analysis has revealed that higher valence state Fe^IV species in the FeOOH photoelectrode play an important role in sacrificing the photoexcited electrons for generation of the cathodic photocurrent. Comparison between the FeOOH and hematite photoelectrodes allows for a better understanding of the interplay between crystal structure, surface reactions, and photocurrent. The findings on this new abnormal phenomenon could also provide guidance for the design of new types of semiconducting photoelectrochemical devices.     

氯离子对铜或黄铜表面膜半导体性质的影响

Di Quarto等对Cu在弱酸性溶液中氧化膜层的光电化学进行了较为广泛的研究.关于Cl~-离子的影响,他们认为,在低浓度下([Cl~-]≤5×10~(-2)mol·L~(-1),即质量比0.0029),仅增加Cu 电极的腐蚀速度,而不影响Cu 电极表而氧化层Cu_2O 的半导体特性.但尚未见细致的工作报导:Cl~-离子浓度究竟达到多大时能对Cu 电极氧化膜层的半导体特性产生影响?我们在弱碱性溶液中逐步添加Cl~-,通过在周期性光照(14Hz)下,用锁定放大器测得的光电流i_(ph)对E 的关系,来研究Cl~-对铜或黄铜表面膜层半导体性质的影响.     

石墨烯氧化物薄膜电极的光电化学特性

采用浸渍-提拉法制备了一系列石墨烯氧化物(GO)薄膜,并通过X射线衍射(XRD),扫描电镜(SEM),傅里叶变换红外光谱,紫外-可见吸收光谱和光电化学测量等技术对样品进行了表征.在GO电极上观察到阴极光电流,且光电流密度受薄膜的厚度影响.GO薄膜电极厚度为27nm时,光电流密度为0.25μA·cm-2.此外,GO电极的光电响应还受紫外光照影响,随着紫外光照时间的延长,阴极光电流逐渐减小.该工作提供了简便的通过控制薄膜厚度或紫外光照时间来控制GO薄膜半导体光电化学性能的方法.     

Hydrogen bonding effect on photocurrent generation in porphyrin-fullerene photoelectrochemical devices

A hydrogen bonding effect on photocurrent generation has been evaluated successfully in a mixed film of porphyrin and fullerene with hydrogen bonding on an ITO electrode, which exhibits efficient cathodic photocurrent generation as compared to the reference system without hydrogen bonding.     

Effect of Chemical Treatment on TiO2 Particles by IMPS

The photodegradation reaction rate of CHCl3 in TiO2 particulate suspension was imperoved significantly by HCl-treatment.The effect of HCl-treatment on the photocatalytic activity of TiO2 was strdied in a PEC cell by using Intensity-Modulated Photocurrent Spectroscopy(IMPS).The magntude of photocurrent response and the characteristic frequencies of the upper and lower semicircles in the complex plane of IMPS response were analyzed,and the ccathodic and anodic reaction processes of photogenerated holes and electrons were discussed.The increases in the cathodic and anodic photocurrent response and in the time constants of both cathodic and anodic reaction processes of photogenerated holes and electrons were discussed.The increases in the cathodic and anodic photocurrent response and in the time constants of both cathodic and anodic reaction processes indicate that HCl-treatment leads to the improvement of the photocatalytic activity of TiO2 and a change of the photocatalytic kinetic mechanism.     

Photoelectrochemical water splitting using a Cu(In,Ga)Se2 thin film

The effects of surface modification and reaction conditions on the photoelectrochemical properties of polycrystalline Cu(In,Ga)Se₂ (CIGS) thin films for water splitting were studied. CIGS modified with platinum particles (Pt/CIGS) generated a cathodic photocurrent at potentials up to + 0.4 V vs. RHE at pH = 9.5. The photocurrent was stable for 16 h, which resulted in a turnover number of over 500. A CdS-inserted film (Pt/CdS/CIGS) had significantly improved properties compared to Pt/CIGS: a 0.3 V higher onset potential of cathodic photocurrent and a three-fold increase in the quantum efficiency. Our results suggest the feasibility of CIGS as a photocathode for biphotoelectrochemical water splitting.     

A photoelectronic switching device using a mixed self-assembled monolayer

A cathodic-anodic biway photoelectronic device has been successfully constructed using a self-assembled monolayer (SAM). The SAM consists of two kinds of photofunctional thiol derivatives, a ruthenium complex-viologen linked compound (RuVS) and a phthalocyanine derivative (PcS), on a gold electrode. Structural characterization of the SAM has been carried out by absorption spectroscopy, cyclic voltammetry, and differential pulse voltammetry. Photocurrent responses were measured in the presence of methyl viologen (MV2+) and oxygen as electron acceptors and triethanolamine (TEOA) as a sacrificial reagent. For the SAM of RuVS alone, intramolecular electron transfer (ET) was superior to intermolecular ET, resulting in anodic photocurrents even in the presence of MV2+ and oxygen at 0 V vs Ag/AgCl. On the contrary, only cathodic photocurrents were observed at 0 V for the SAM of PcS alone. Photocurrents from the mixed SAM of RuVS and PcS were roughly the sum of individual photocurrents from RuVS and PcS. In fact, photocurrents from the mixed SAM of RuVS and PcS were observed in the anodic direction below approximately 550 nm, and in the cathodic direction above approximately 550 nm at 0 V vs Ag/AgCl. In the case of the mixed SAM of RuS (ruthenium complex disulfide) and PcS, only cathodic photocurrents were observed at 0 V vs Ag/AgCl, due to the lack of an intramolecular ET pathway. The results indicate that in the mixed SAM of RuVS and PcS both dyes can individually function for opposite photocurrent generation. We have also applied the mixed SAM as a photoelectronic logic device by using two LEDs (470 and 640 nm). The system clearly operated as an XOR logic device.     

Photoelectrochemical Behavior of Poly(3,4-ethylenedioxythiophene) in Acetonitrile Solutions during Cathodic Reduction of Oxygen

The photoelectrochemical behavior of a polymer electrode based on poly(3,4-ethylenedioxythiophene) (PEDT) is studied in acetonitrile solutions during the cathodic reduction of oxygen. The measurements are taken on PEDT films 490 nm thick deposited onto a platinum support in oxygen-saturated 0.1 M Bu₄NBF₄ solutions. The electrode was illuminated with a light of a wavelength 487 nm from a helium–cadmium laser in conditions of modulated illumination, and in doing so, the real and imaginary components of an alternating photocurrent were measured at different modulation frequencies. The peculiarity of the photoelectrochemical behavior of PEDT manifests itself in the emergence of an anodic photocurrent, which increases with a change in potential in the cathodic direction. At low modulation frequencies, the real and imaginary components are of the same sign, whereas at high modulation frequencies the components have opposite signs (photocurrent lags behind a change in the light intensity). The emergence of an anodic photocurrent is connected with the cathodic reduction of oxygen and the formation in a near-electrode layer of radical anion O^– ₂. Such a behavior of PEDT, which makes it different from other polymers of the thiophene series, is explained by different potentials of their anodic oxidation, to which a conductive state of the polymer is related.     

纳米尺度TiO2微粒多孔膜电极光电化学

用光电流作用谱、光电流-电势图和瞬态光电流谱等光电化学方法研究了TiO2多孔膜电极在含不同氧化还原体系的电解质溶液中的光电转换过程.结果说明TiO2多孔股为n-型半导体，其禁带宽度为3.26eV.当在电解质溶液中加入醌二苯酸（BQ／HQ），TiO2多孔膜电极的光电流作用谱形基本与没加氧化还原对时类似。在可见光区的光电流拖尾是由于醌被光激发，然后给出电子到TiO2多孔膜导带而产生阳极光电流.而在电解质溶液中加入Fe（CN）3-6-／4-时，TiO2多孔膜电极的光电流作用谱有明显的改变.除了在小于380nm短波区有光电流峰外，还在400-600nm的可见光区观察到宽的光电流峰，大大增加了光电流转换效率.同时在小于-0.2V下为阳极光电流，在-0.2V～0.3V电势区间为明显阴极光电流，在大于0.3V下可观察到较弱的阳极光电流.当电极电势大于-0.2V时，光电流瞬态谱在开始光照时有一阴极瞬态光电流尖峰，然后转变为阳极稳态光电流.这是因为当电极电势较负时，Fe（CN）4-6与TiO2的电子传递络合物可以吸收光子，光生电子迅速注入TiO2导带，然后还原溶液中的而产生阴极光电流.