Forming mechanism of nitrogen doped graphene prepared by thermal solid-state reaction of graphite oxide and urea

Nitrogen doped graphene was synthesized from graphite oxide and urea by thermal solid-state reaction. The samples were characterized by transmission electron microscopy, atomic force microscopy, scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectra, element analysis, and electrical conductivity measurement. The results reveal that there is a gradual thermal transformation of nitrogen bonding configurations from amide form nitrogen to pyrrolic, then to pyridinic, and finally to “graphitic” nitrogen in graphene sheets with increasing annealing temperature from 200 to 700 °C. The products prepared at 600 °C and 700 °C show that the quantity of nitrogen incorporated into graphene lattice is ∼10 at.% with simultaneous reduction of graphite oxide. Oxygen-containing functional groups in graphite oxide are responsible for the doping reaction to produce nitrogen doped graphene.     

Efficiency of energy transfer by resonance in quenching of protein fluorescence by dyes

Considering the quenching of tryptophan fluorescence of bovine serum albumin by various dyes as an example, it is shown that overlap of radiation and absorption spectra does not necessarily lead to energy transfer by resonance. No correlation is revealed between the limiting quenching and the Forster overlap integral. Quenching can occur even in the absence of overlap. The magnitude of energy transfer is markedly lower than that of quenching owing to competing processes, namely, excitation deactivation by the dye and, probably, by the protein itself which undergoes conformation upon sorption of the dye. Negatively charged and neutral dyes posses, on the average, a higher quenching activity relative to albumin than do positively charged dyes. Institute of the Biophysics of Cells of the Russian Academy of Sciences, Pushchino, 142292, Moscow Region, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 2, pp. 290–293, March–April, 1998.     

Investigation of the fluorescence quenching of 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) by certain substituted uracils

The fluorescence quenching of 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) by a series of uracils has been studied in water and acetonitrile solvents using steady-state and time-resolved fluorescence techniques. The steady-state fluorescence quenching technique has been performed in three different pHs (i.e. 4, 8 and 12). The bimolecular quenching rate constant (k_q) increases with increase in pH of uracils. In acidic pH, a pure hydrogen atom abstraction is proposed as the quenching mechanism. This is supported by a pronounced solvent deuterium isotope effect. Electron transfer from the anionic form of uracil to the excited state of DBO is proposed as a mechanism for quenching in basic pH on the basis of highly exergonic thermodynamics obtained from the Rehm-Weller equation. The variation of k_q is explained on the basis of the electronic effect of substitution in uracils as well.     

An investigation on fluorescence quenching of certain porphyrins by colloidal CdS

Certain porphyrin derivatives namely meso-tetraphenylporphyrin (TPP), meso-tetrakis(4-carboxyphenyl)porphyrin (TCPP), meso-tetrakis(4-sulfonatophenyl)porphyrin (TSPP) were examined as sensitizers for colloidal CdS. The interaction of these porphyrins and colloidal CdS were studied by absorption, infrared, steady state and time resolved fluorescence spectroscopy and transient absorption techniques. The apparent association constants (K_app) resulting from adsorption of porphyrins on CdS surface were calculated from both absorption and fluorescence studies and they agree well. Using all the spectroscopic measurements we confirmed that the interaction between porphyrins and colloidal CdS occurs through ground state complex formation and the quenching follows static mechanism.     

Fe~(3+)对石墨烯氧化物荧光淬灭机理的研究

采用改进的Humer法合成了石墨烯氧化物,利用搭建的时间分辨光谱探测系统详细探究了Fe~(3+)(浓度为0.5、1、2 mmol/L)对石墨烯氧化物荧光淬灭物理机制。稳态荧光发射光谱中,随着Fe~(3+)浓度的增加,石墨烯氧化物的荧光强度急剧减弱。时间分辨荧光光谱和飞秒瞬态吸收光谱研究证实,加入不同浓度Fe~(3+)的GO其动力学衰减曲线基本没有任何变化。结果表明,Fe~(3+)对石墨烯氧化物的荧光淬灭主要是静态的荧光淬灭过程。     

Distance-dependent quenching of Nile Blue fluorescence byN,N-diethylaniline observed by frequency-domain fluorometry

Fluorescence quenching of Nile Blue by amines is thought to be due to electron transfer to the excited dye molecule from the amine electron donor. We used electron transfer quenching of Nile blue byN,N-diethylaniline in propylene glycol as a model system for an interaction which depends exponentially on distance. We investigated the time dependence of the presumed distance-dependent process using gigahertz harmonic-content frequency-domain fluorometry. The frequency-domain data and the steady-state quantum yield were analyzed globally based on either the Smoluchowski-Collins-Kimball radiation boundary condition (RBC) model or the distancedependent quenching (DDQ) model, in which the rate of quenching depends exponentially on the flourophore-quencher distance. We performed a global analysis which included both the frequencydomain time-resolved decays and the steady-state intensities. The latter were found to be particularly sensitive to the model and parameter values. The data cannot be satisfactorily analyzed using the RBC model for quenching. The analysis shows the excellent agreement of the DDQ model with the experimental data, supporting the applicability of the DDQ model to describe the quenching by the electron transfer process, which depends exponentially on the donor-acceptor distance.     

Surface adhesion properties of graphene and graphene oxide studied by colloid-probe atomic force microscopy

Surface adhesion properties are important to various applications of graphene-based materials. Atomic force microscopy is powerful to study the adhesion properties of samples by measuring the forces on the colloidal sphere tip as it approaches and retracts from the surface. In this paper we have measured the adhesion force between the colloid probe and the surface of graphene (graphene oxide) nanosheet. The results revealed that the adhesion force on graphene and graphene oxide surface were 66.3 and 170.6 nN, respectively. It was found the adhesion force was mainly determined by the water meniscus, which was related to the surface contact angle of samples.     

Photoinduced interaction between MPA capped CdTe QDs and certain anthraquinone dyes

Photoinduced interaction of mercapto propionic acid (MPA) capped CdTe quantum dots (QDs) with certain anthraquinone dyes namely alizarin, alizarin red S, acid blue 129 and uniblue has been studied by steady state and time resolved fluorescence measurements. Addition of anthraquinone dyes to CdTe QDs results in the reduction of electron hole recombination has been observed (i.e., fluorescence quenching). The Stern-Volmer constant (K_SV), quenching rate constant (k_q) and association constants (K) were obtained from fluorescence quenching data. The interaction of anthraquinone dyes with QDs occurs through static quenching was confirmed by unaltered fluorescence lifetime. The occurrence of electron transfer quenching mechanism has been proved by the negative free energy change (ΔG_et) obtained as per the Rehm-Weller equation.     

One-pot synthesis of fluorescent nitrogen-doped graphene quantum dots for portable detection of iron ion

Nitrogen-doped graphene quantum dots (N-GQD) were synthesized by direct thermal decomposition of ammonium citrate tribasic. With the increment of torrefaction temperature, the average size of N-GQD was increased from 2.56 to 3.73 nm. Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy analysis (XPS) proved the successful doping of nitrogen atoms. Besides, the N-GQDs showed blue fluorescence which was quenched by Fe³⁺ ions, and the fluorescence intensity of N-GQDs decayed exponentially. Accordingly, the same quenching effect was observed on a test paper prepared by soaking paper in N-GQDs dispersion. The quenching mechanism was due to electron transfer between Fe³⁺ and functional groups on the surface of N-GQDs which could be confirmed by XPS and diameter growth. Therefore, through this simple method, N-GQDs with high blue fluorescence and high production yield (64%) can be prepared, which provided a new strategy for monitoring and collecting Fe³⁺ in environmental water.     

NO_2分子激光诱导荧光辐射寿命和碰撞猝灭速率的实验研究

实验测定了激发波长在 591～ 594nm范围 ,压力在 0 .1～ 15Pa时 ,NO2 分子激光诱导荧光衰减曲线 ,作几何修正后应用Stern Volmer模型 ,得到了零压辐射寿命τ0 ( =1/k0 )和碰撞猝灭速率常数kq,结果与文献报道基本一致。对NO2 分子激发态能级的反常长寿命和一定压力下出现的双指数衰减等进行了较详细的讨论。     

A study on the fluorescence quenching of 9-Aminoacridine by certain antioxidants

The fluorescence quenching of 9-Aminoacridine by certain estrogens and flavonoids in water was studied using absorption, steady state and time-resolved measurements. The bimolecular quenching rate constants for the chosen estrogens and flavonoids were found to be in the range of 3.2-9.2×10¹¹ and 0.36-14.46×10¹¹ M⁻¹s⁻¹, respectively. From lifetime measurement we observed that the quenching was mainly due to static mechanism through ground state complex formation. The binding constant (K) and the number of binding sites (n) were calculated based on the fluorescence quenching data. The free energy change (ΔG_et) for electron transfer process was calculated by Rehm-Weller equation.     

Synthesis and characterization of graphene oxide,reduced graphene oxide and their nanocomposites with polyethylene oxide

This work describes the synthesis of GO, rGO and their nanocomposites with PEO. GO and rGO were prepared by the modified Hummers method and in-situ reduction of GO utilizing green reductant L (+) Ascorbic acid. The nanocomposites were characterized by Fourier-transform infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), Field emission scanning electron microscopy (FE-SEM), Thermogravimetric Analysis (TGA), and Universal Testing Machine (UTM). FT-IR and XRD confirmed the synthesis of GO and rGO. FE-SEM confirmed the uniformly exfoliated GO and rGO nanosheets in the polymer matrix. Hydrogen bonding was the main interaction mechanism for GO with PEO while no interaction was detected by FT-IR for rGO. Enhanced thermal stability was observed for both GO/PEO and rGO/PEO nanocomposites. The mechanical analysis showed an increase in Young's modulus, tensile strength, and elongation at break for GO/PEO nanocomposites, which is attributed to the homogeneous dispersion and hydrophilic hydrogen bonding interaction of GO with PEO.     

Fabrication and characterization of graphene oxide/zinc oxide nanorods hybrid

This work presented a hybrid architecture of graphene oxide (GO)/ZnO nanorods (ZNs) with ZNs attached parallel onto GO sheets. ZNs were synthesized by refluxing zinc acetate dehydrate in methanol solution under basic conditions followed by surface modification of 3-aminopropyl triethoxysilane (ATS), and then the preformed ZNs were attached onto GO sheets by reaction of the amino groups on the outer wall of ZNs with the carboxyl groups on the GO surface. Transmission electron microscopy (TEM) image of the as-prepared hybrid reveals the morphology of the architecture of GO/ZNs hybrid. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA) ultraviolet-visible (UV-vis) and fluorescence spectroscopy were also performed to characterize the structure and properties of the GO/ZNs hybrid. It was shown that ZNs maintained their initial morphology and crystallinity in the hybrid and the luminescence quenching of yellow-green emission of ZNs confirmed the electron transfer from excited ZnO to GO sheets.     

Diffusion-influenced fluorescence quenching dynamics in one to three dimensions

The fluorescence decay curves obtained from diffusion-influenced quenching in various spatial dimensions are discussed. The two-dimensional quenching has, because of intractable fitting functions, previously been dealt with only in the completely diffusion-controlled case (corresponding to the Smoluchowski boundary condition). In this paper, an approximation for the two-dimensional (2D)-quenching behavior with the Collins-Kimball boundary condition is presented. The nonlinear least-squares method has been used to analyze simulated decay data. The consequences the choice of an incorrect model has on the final results as well as the possibility to discriminate between different dimensionalities are investigated. Also, some inherent properties of the fitting functions are studied.     

荧光光谱法研究核黄素与核黄素结合蛋白的相互作用

运用荧光光谱研究了核黄素与核黄素结合蛋白的相互作用,并探讨了两者间的结合类型、结合常数、结合过程中热力学参数和能量转移。结果表明:核黄素结合蛋白内源荧光的猝灭是由于核黄素与蛋白质之间形成复合物,并符合静态猝灭机理。298,308,318K下核黄素与核黄素结合蛋白的结合常数分别为:5.35×108,1.54×108,0.56×108 L.mol-1。热力学数据表明核黄素与核黄素结合蛋白之间主要作用力为氢键和范德华力。Frster能量转移理论确定了核黄素与核黄素结合蛋白的作用距离与能量转移效率分别为0.70nm与0.39。利用同步荧光光谱研究了核黄素结合蛋白与核黄素结合过程中构象的变化。     

Effect of copper oxide on the resistive switching responses of graphene oxide film

The role of CuO films in meliorating resistive switching behavior of graphene oxide (GO) in CuO/GO/CuO memory structure was investigated. An increase in the set voltage from 1.3 to 3.0 V and a step-like switching current was clearly observed when the GO film was sandwiched between two CuO layers. It is attributed to the fact that the set voltage of GO is lower than that of CuO and accumulated charge carriers located at the interface of GO and CuO can pass through CuO abruptly at set voltage of 3.0 V. Our results suggested that designed sandwich structure of materials with different set voltage enables to amend resistive switching response characteristics.     

Oxygen sensing by fluorescence quenching of [Cu(btmgp)I]

A fluorescence study of acetonitrile solutions of bis(tetramethylguanidine)propane, copper(I)-iodide and [Cu(btmgp)I] was performed and the chemical reaction of the latter species with O₂ was investigated at room temperature. The actual quenching process via O₂ gassing was studied and an exponential dependence of the fluorescence intensity with respect to the complex concentration was observed.Furthermore the survey was deepened on time resolved fluorescence properties of solved [Cu(btmgp)I] in a wider concentration range. The applicability of this complex for O₂ sensing inside a microreactor system was proven by confocal fluorescence measurements. It was shown that the investigated system can be used for oxygen sensing in the copper concentration range from 10⁻² to 10⁻⁹ mol/l.     

壳聚糖包覆的CdTe量子点荧光猝灭法测定吉米沙星

以壳聚糖包覆的CdTe量子点为荧光探针,基于荧光猝灭法建立了吉米沙星定量测定方法。结果表明,体系的荧光强度与吉米沙星浓度在3.46×10-9～3.46×10-7 g.L-1范围内呈良好的线性关系(r=0.999 2),线性回归方程为F0/F=1.063 7+0.016 7c(g.L-1)。对2.77×10-7 g.L-1吉米沙星溶液进行7次平行测定的相对标准偏差为2.7%,基于荧光猝灭法相关理论证明了该相互作用过程为静态猝灭。本方法灵敏度高,检测线性范围宽,为吉米沙星定量测定提供了简便可靠的方法。     

Ultrasound assisted reduction of graphene oxide to graphene in l-ascorbic acid aqueous solutions: Kinetics and effects of various factors on the rate of graphene formation

The reduction of graphene oxide (GO) to graphene (rGO) was achieved by using 20 kHz ultrasound in l-ascorbic acid (l-AA, reducing agent) aqueous solutions under various experimental conditions. The effects of ultrasound power, ultrasound pulse mode, reaction temperature, pH value and l-AA amount on the rates of rGO formation from GO reduction were investigated. The rates of rGO formation were found to be enhanced under the following conditions: high ultrasound power, long pulse mode, high temperature, high pH value and large amount of l-AA. It was also found that the rGO formation under ultrasound treatment was accelerated in comparison with a conventional mechanical mixing treatment. The pseudo rate and pseudo activation energy (E_a) of rGO formation were determined to discuss the reaction kinetics under both treatment. The E_a value of rGO formation under ultrasound treatment was clearly lower than that obtained under mechanical mixing treatment at the same condition. We proposed that physical effects such as shear forces, microjets and shock waves during acoustic cavitation enhanced the mass transfer and reaction of l-AA with GO to form rGO as well as the change in the surface morphology of GO. In addition, the rates of rGO formation were suggested to be affected by local high temperatures of cavitation bubbles.     

Rotamer-specific fluorescence quenching in tyrosinamide: Dynamic and static interactions

We have examined the environments of the three phenol rotamers about the C–C bond in tyrosinamide by fluorescence quenching. Steady-state acrylamide quenching yields a nonlinear stern-Volmer plot. With three distinct emitting species and no other information about the system, it is impossible to analyze the data due to the number of variables which have to be determined. We therefore reduced the number of variables by independently determining the fractional intensity and dynamic quenching constant for each rotamer through time-resolved fluorescence quenching studies. These parameters were then used to analyze the steady-state data for any contribution of static quenching. We conclude that the nonlinear Stern-Volmer plot for the quenching of tyrosinamide by acrylamide is a consequence of each rotamer having a distinct dynamic quenching constant and the presence of static quenching. The static quenching can be represented by either the sphere-of-action model involving two of the three rotamers or the ground-state complex model involving all three rotamers.