Hong Qun Luo

Determination of the critical premicelle concentration, first critical micelle concentration and second critical micelle concentration of surfactants by resonance Rayleigh scattering method without any probe.

The purpose of this work is to determine the values of critical premicelle concentration (CPMC), first critical micelle concentration (FCMC) and second critical micelle concentration (SCMC) of surfactants using a common spectrofluorophotometer by recording resonance Rayleigh scattering (RRS) signal without any probe. The plot of the RRS intensities at the maximum scattering wavelength (I(RRS)(max)) versus surfactant concentrations (c) was constructed to obtain the I(RRS)(max)-c curve. From the inflexions in I(RRS)(max)-c curve, the CPMC, FCMC and SCMC values of a surfactant can be obtained sensitively. The FCMC of some anionic, cationic and nonionic surfactants such as sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS), cetyltrimethylammonium bromide (CTAB), cetylpyridinium chloride (CPC), Tween-20, and Tween-80 were determined by RRS method and the values are in good agreement with those obtained from conductivity and surface tension measurements and literature values. The CPMC and SCMC of SDS and CTAB were also determined by RRS method respectively and the values conform to literature values too. Furthermore, RRS method can also be used to determine the FCMC of an amphiphilic macromolecule-hemoglobin, whose structure resembles a surfactant. From the experimental results, it is concluded that RRS method can be applied to the simultaneous determination of the CPMC, FCMC and SCMC values in a sensitive, accurate and no probe way.

A simple and facile strategy based on Fenton-induced DNA cleavage for fluorescent turn-on detection of hydroxyl radicals and Fe2+

We provide a novel, simple, and general strategy based on a DNA–graphene oxide (GO)–Fenton hybrid system for fluorescence turn-on detection of hydroxyl radicals (HO˙) and Fe2+, in which GO as a nanoquencher is utilized for reducing the background signal and simplifying procedures. Fluorescence resonance energy transfer (FRET) between GO and dye units incorporated into DNA is switched off as a result of HO˙-induced strand breakage in the single-stranded DNAs, restoring the fluorescence of the quenched fluorophores. In our assay, HO˙ is generated by Fe2+ catalyzing decomposition of hydrogen peroxide through the Fenton reaction. Therefore, switching off FRET depends on the amounts of Fe2+ and HO˙. In vitro assays with Fe2+ and HO˙ demonstrated increases in fluorescence intensity with a linear range from 10 nM to1 μM and a detection limit as low as 2.4 nM.

Electrochemical detection of copper(II) at a gold electrode modified with a self-assembled monolayer of penicillamine

A penicillamine (PCA) self-assembled monolayer (SAM) was prepared on a gold electrode. It has been found that the modified electrode exhibited a selective response to copper ions. As demonstrated by cyclic voltammetric experiments, the SAM-based electrode showed an attractive ability to preconcentrate efficiently traces of copper(II) from solutions. Under optimum conditions, the anodic peak current was proportional to the concentration of Cu(II) in the range from 8.0 × 10−7 to 1.0 × 10−4 M with a detection limit of 4.0 × 10−7 M. Moreover, this modified gold electrode is also characterized by excellent repeatability, showing a relative standard deviation of 3.2% for nine successive measurements of 1.0 × 10−5 M Cu(II). The PCA/Au SAM gold electrode was used for the determination of Cu(II) in a tap water sample and the results showed a good agreement with the data obtained by atomic emission spectrometry.

Electrochemical Behavior of Uric Acid and Epinephrine at a Meso-2,3-Dimercaptosuccinic Acid Self-Assembled Gold Electrode

Abstract.A self-assembled electrode with a meso-2,3-dimercaptosuccinic acid (DMSA) monolayer has been characterized by electrochemical quartz crystal microbalance and complex impedance analysis, surface enhanced Raman spectroscopy and cyclic voltammetry. The self-assembled electrode was used for the simultaneous electrochemical detection of epinephrine (EP) and uric acid (UA) in phosphate buffer of pH 7.7. The simultaneous oxidation of EP and UA was performed by cyclic voltammetry (CV) and differential pulse voltammetry (DPV), and the signals for each method were well separated with a potential difference of over 330 mV and without interference by each other. The detection limit of EP is 5.4 × 10−8 mol L−1 by CV and 5.3 × 10−8 mol L−1 by DPV and that of UA is 8.4 × 10−8 mol L−1 by CV and 4.2 × 10−8 mol L−1 by DPV. The DMSA self-assembled electrode can be applied to the simultaneous determination of EP and UA.

Fast and sensitive dye-sensor based on fluorescein/reduced graphene oxide complex

We report on a fast, sensitive, label-free, and general dye-sensor platform for synthetic organic dyes detection by competitive adsorption on reduced graphene oxide (rGO) against a fluorescent dye (FD). Fluorescein (Fl) as fluorescence indicator and a cationic dye methylene blue (MB) as model analyte were employed to investigate the analytical feature of this assay platform. An anionic dye sunset yellow FCF (SY) was chosen as a comparison analyte to test the generality of this strategy. Results show that rGO can bind Fl and quench the fluorescence by fluorescence resonance energy transfer (FRET), while MB can displace Fl quickly from the Fl/rGO complex by competitive adsorption, inducing the fluorescence recovery which provides a quantitative readout for MB. Besides, this design was simply based on the competitive adsorption of rGO between dye and FD, and can be generally applied to other dyes for label-free detection. The fluorescence enhancement efficiency (FEE) is proportional to the dye concentration over the range of 7.60-420.00 ng mL(-1) MB and 7.28-400.25 ng mL(-1) SY, respectively. The linear regression equations were calculated as FEE(MB) = 0.0192c(MB)- 0.3103 for MB and FEE(SY) = 0.0142 c(SY)- 0.0427 for SY, with the detection limits of 1.03 and 1.15 ng mL(-1), respectively. The MB in waste water and SY in an orange-flavored sports drink sample were assayed with satisfactory results.

An ultrasensitive and selective fluorescence assay for Sudan I and III against the influence of Sudan II andIV

We report on an ultrasensitive and selective fluorescence assay for Sudan I and III against the influence of Sudan II and IV based on ligand exchange mechanism. Calcein as a fluorescence indicator and Sudan I-IV as model analytes were employed to investigate the analytical feature of this assay platform. Results show that the fluorescence of calcein can be efficiently quenched by Cu(II). When the ligand exchange reaction proceeds, calcein is deprived of Cu(II) by Sudan I and III, resulting in the fluorescence recovery of calcein. However, the ligand exchange reaction does not happen in the presence of Sudan II or IV due to the 2-methyl steric effects, which is favorable for selective determination of Sudan I and III against the influence of Sudan II and IV. It was found that the fluorescence enhancement efficiency (FEE) against the concentration of Sudan (c(Sudan), nmol L⁻¹) shows a linear relationship. The calibration equations are FEE(Sudan I)=0.0032 c(Sudan I)-0.02613, and FEE(Sudan III)=0.0033 c(Sudan III)-0.02467 over the corresponding linear range of 11.25-2078.29 and 9.44-1035.78 nmol L⁻¹ with the correlation coefficients (R(2)) of 0.9984 and 0.9955, respectively. And the detection limits (3σ/slope) are calculated to be 211.3 and 208.5 pmol L⁻¹ for Sudan I and III, respectively, showing ultralow detection limit. The Sudan dye in a commercial chilli powder sample was assayed with satisfactory results.

Resonance Rayleigh scattering, second-order scattering and frequency doubling scattering spectra for studying the interaction of erythrosine with Fe(phen)32+ and its analytical application

In a weak alkaline Britton-Robinson buffer medium, erythrosine (Ery) can react with Fe(phen)(3)(2+) to form 1:1 ion-association complex, which will cause not only the changes of the absorption spectra, but also the remarkable enhancement of resonance Rayleigh scattering (RRS), second-order scattering (SOS) and frequency doubling scattering (FDS) spectra, and the appearance of new spectra of RRS, SOS and FDS. The maximum RRS, SOS and FDS wavelengths (λ(ex)/λ(em)) of the ion-association complex are located at 358/358 nm, 290/580 nm and 780/390 nm, respectively. The increments of scattering intensities (ΔI) are directly proportional to the concentration of Ery in a certain range. The detection limits for Ery are 0.028 μg mL(-1) for RRS method, 0.068 μg mL(-1) for SOS method and 0.11 μg mL(-1) for FDS method, respectively. Among them, the RRS method has the highest sensitivity. Based on the above researches, a new highly sensitive and simple method for the determination of Ery has been developed. In this work, the spectral characteristics of absorption, RRS, SOS and FDS spectra, the optimum conditions of the reaction and influencing factors for the RRS, SOS and FDS intensities were investigated. In addition, the reaction mechanism was discussed.

Electrochemical behavior of epinephrine at a meso-2,3-dimercaptosuccinic acid self-assembled gold electrode and its analytical application

Abstract Meso‐2,3‐dimercaptosuccinic acid (DMSA) can bind onto the surface of a gold electrode through the strong gold‐sulfur interaction. The formation mechanism and electrochemical properties of the DMSA monolayer were studied by electrochemical quartz crystal microbalance, complex impedance analysis, surface enhanced Raman spectroscopy, and cyclic voltammetry. The DMSA‐modified gold electrode shows an obvious electrocatalytic activity for the oxidation of epinephrine (EP). There are good linear relationships between peak current and the concentration of EP in the range of 8.0×10−6∼4.0×10−3 mol L−1 and 5.0×10−7∼6.0×10−3 mol L−1 for the cyclic voltammetry (CV) and differential pulse voltammetry (DPV) methods with the detection limits of 4.8×10−7 and 4.7×10−8 mol L−1, respectively. The reaction mechanism was primarily discussed. The electrode reaction of EP is an irreversible process with two electrons and two protons at the DMSA‐modified gold electrode. The modified electrode was applied to the determination of EP in epinephrine hydrochloride injection samples with satisfactory results. This project is supported by the Municipal Science Foundation of Chongqing City (No. CSTC‐2004BA4024) and the Doctor Foundation of Southwest China Normal University (2003‐10, 2003‐11), and all authors here express their deep thanks.

Novel and sensitive methods for the determination of dopamine based on the resonance Rayleigh scattering, second-order scattering and frequency doubling scattering quenching effects.

Three novel methods were designed for the determination of dopamine (DA) by means of the quenching effects on resonance Rayleigh scattering (RRS) and non-linear scattering such as second-order scattering (SOS) and frequency doubling scattering (FDS) intensities. In Britton-Robinson buffer medium (pH 6.02), I(3)(-) could react with DA to produce I(-), which resulted in the decreases of the RRS, SOS and FDS intensities of the ethyl violet (EV)-I(3)(-) system, and the decreases of scattering intensity (DeltaI) were directly proportional to the concentrations of DA in a certain range. The detection limit (3sigma) for DA was 0.0195 micromol/L for the RRS method, 0.286 micromol/L for the SOS method and 0.0985 micromol/L for the FDS method, respectively. In this work, the characteristics of absorption, RRS, SOS and FDS spectra of this reaction have been studied; the optimum reaction condition and influencing factors have been investigated. The methods were applied to the determination of DA in pharmaceutical samples with satisfactory results.

Resonance Rayleigh Scattering of K2Zn3[Fe(CN)6]2 Nanoparticles and its Application for the Determination of Vitamin C

Abstract In Britton-Robinson buffer medium, (pH 4.43), potassium ferricyanide (K3[Fe(CN)6]) could react with vitamin C (VC) to produce potassium ferrocyanide (K4[Fe(CN)6]), which further reacted with Zn2+ to form potassium zinc hexacyanoferate K2Zn3[Fe(CN)6]2 nanoparticles. The shapes and diameters of the K2Zn3[Fe(CN)6]2 nanoparticles have been observed with transmission electron microscopy, which showed the shapes of these nanoparticles was cubic and their average sizes were about 50 nm in the presence of 2.0 × 10−5 mol L−1 VC. The characteristics of resonance Rayleigh scattering (RRS) spectra of this reaction have been studied. The optimum reaction condition for the determination of VC has been investigated. It was found that the RRS intensity of the system at the RRS peak of 363.4 nm was proportional to the VC concentration in the range of 4.0−80.0 µmol L−1, and the detection limit (3σ) for VC was 0.075 µmol L−1. A novel and simple RRS method for the determination of VC based on the formation of K2Zn3[Fe(CN)6]2 nanoparticles has been established.