Simultaneous determination of N-acetyl-p-aminophenol and p-aminophenol with poly(3,4-ethylenedioxythiophene) modified glassy carbon electrode

A sensitive and selective method was developed for the determination of N-acetyl-p-aminophenol (APAP) and p-aminophenol (PAP) using poly(3,4-ethylenedioxythiophene) (PEDOT)-modified glassy carbon electrode (GCE). Cyclic voltammetry and differential pulse voltammetry were used to investigate the electrochemical reaction of APAP and PAP at the modified electrode. Both APAP and PAP showed quasireversible redox reactions with formal potentials of 367 mV and 101 mV (vs. Ag/AgCl), respectively, in phosphate buffer solution of pH 7.0. The significant peak potential difference (266 mV) between APAP and PAP enabled the simultaneous determination both species based on differential pulse voltammetry. The voltammetric responses gave linear ranges of 1.0 × 10⁻⁶-1.0 × 10⁻⁴ mol L⁻¹ and 4.0 × 10⁻⁶-3.2 × 10⁻⁴ mol L⁻¹, with detection limits of 4.0 × 10⁻⁷ mol L⁻¹ and 1.2 × 10⁻⁶ mol L⁻¹ for APAP and PAP, respectively. The method was successfully applied for the determination of APAP and PAP in pharmaceutical formulations and biological samples.     

Capillary electrophoresis-electrochemiluminescent detection of N,N-dimethyl ethanolamine and its application in impurity profiling and stability investigation of meclophenoxate

Numerous drugs are carboxylic acid derivatives containing amino group, and hydrolysis reaction of these agents often generates toxic amines. Thus, the detection of amine impurity is of great importance in drug quality control of these amino group-containing ester and amide. A capillary electrophoresis method coupled with end-column electrochemiluminescent detection based on tris(2,2′-bipyridyl)ruthenium(II) system was proposed for the analysis of N,N-dimethyl ethanolamine (DMEA, the degradation product of meclophenoxate) in the presence of its precursor. Baseline separation of DMEA and meclophenoxate can be easily achieved under the selected conditions. DMEA can be assayed within 3 min over the concentration range of 5.0 × 10⁻⁸ to 3.0 × 10⁻⁶ mol L⁻¹ with a detection limit of 2.0 × 10⁻⁸ mol L⁻¹ at the signal-to-noise ratio of 3. The relative standard deviations of the signal intensity and the migration time were less than 5.3 and 2.5% for a standard sample containing 1.0 × 10⁻⁷ mol L⁻¹ DMEA (n = 5), respectively. The presented method has been successfully applied for the profiling of DMEA resulting from the hydrolysis of meclophenoxate in commercial formulations. A primary stability investigation of meclophenoxate in aqueous solution was also carried out at different temperatures, and the results showed that the degradation of meclophenoxate accelerated at the higher temperature.     

Langmuir-Blodgett film of p-tert-butylthiacalix[4]arene modified glassy carbon electrode as voltammetric sensor for the determination of Hg(II)

The π-A isotherms and UV-vis spectra of the transferred films suggested that the monolayer of p-tert-butylthiacalix[4]arene can coordinate with Hg²⁺ at the air-water surface. From these observations, a glassy carbon electrode coated with Langmuir-Blodgett film of p-tert-butylthiacalix[4] arene as a new voltammetric sensor is designed for the determination of trace amounts of Hg²⁺. Compared with bare glassy carbon electrode and modified glassy carbon electrode using direct coating method, the Langmuir-Blodgett film-modified electrode can greatly improve the measuring sensitivity of Hg²⁺. Under the selected conditions, the Langmuir-Blodgett film-modified electrode in 0.1 mol L⁻¹ H₂SO₄ + 0.01 mol L⁻¹ KCl solution shows a linear voltammetric response for Hg²⁺ in the range of 5.0 × 10⁻¹⁰ to 1.5 × 10⁻⁷ mol L⁻¹, with a detection limit of 2.0 × 10⁻¹⁰ mol L⁻¹. The proposed method was also applied to determine Hg²⁺ in water samples (tap, lake and river water). In addition, the fabricated electrode exhibited a distinct advantage of simple preparation, non-toxicity, good reproducibility and good stability.     

An ultrasensitive method for the determination of melamine using cadmium telluride quantum dots as fluorescence probes

An ultrasensitive and simple method for the determination of melamine was developed based on the fluorescence quenching of thioglycolic acid (TGA) capped CdTe quantum dots (QDs) at pH 11.0. In strong alkaline aqueous solution, the selectivity of the method has been greatly improved due to most heavy metal ions show no interference as they are in the precipitation form or in their anion form. Furthermore, CdTe quantum dots have higher quantum yields at higher pH. The method has a wider concentration range and lower detection limit. The influence factors on the determination of melamine were investigated and the optimum conditions were determined. Under optimum conditions, the fluorescence intensity change of TGA coated CdTe quantum dots was linearly proportional to melamine over a concentration range from 1.0 × 10⁻¹¹ to 1.0 × 10⁻⁵ mol L⁻¹ with a correlation coefficient of 0.9943 and a detection limit of 5 × 10⁻¹² mol L⁻¹. The mechanism of fluorescence quenching of the QDs has been proposed based on the infrared spectroscopy information and electrophoresis experiments in presence of melamine under alkaline condition. The proposed method was employed to detect trace melamine in milk powder and pet feeds with satisfactory results.     

Ultrasensitive cysteine sensing using citrate-capped CdS quantum dots

The importance of cysteine (Cys) in biological systems has stimulated a great deal of efforts in the development of analytical methods for the determination of this amino acid. In this work, a novel fluorescent probe for Cys based on citrate (Cit)-capped CdS quantum dots (QDs) is reported. The Cit-capped CdS QDs fluorescent probe offers good sensitivity and selectivity for detecting Cys. A good linear relationship was obtained from 1.0 × 10⁻⁸ mol L⁻¹ to 5.0 × 10⁻⁵ mol L⁻¹ for Cys. The detection limit was calculated as 5.4 × 10⁻⁹ mol L⁻¹. The proposed method was applied to detect Cys in human urine samples, which showed satisfactory results. This assay is based on both the lability of Cit and the strong affinity of thiols to the surface of CdS QDs. The addition of Cys improved the passivation of the surface traps of CdS QDs and enhanced the fluorescence intensity.     

Photovoltammetric behavior and photoelectrochemical determination of p-phenylenediamine on CdS quantum dots and graphene hybrid film

A photoelectroactive film composed of CdS quantum dots and graphene sheets (GS) was coated on F-doped SnO₂ (FTO) conducting glass for studying the electrochemical response of p-phenylenediamine (PPD) under photoirradiation. The result indicated that the cyclic voltammogram of PPD on CdS–GS hybrid film became sigmoidal in shape after exposed under visible light, due to the photoelectrocatalytic reaction. Such a photovoltammetric response was used to rapidly optimize the photoelectrocatalytic activity of hybrid films composed of different ratios of CdS to GS toward PPD. The influences of scan rate and pH on the photovoltammetric behavior of PPD on CdS–GS film revealed that although the controlled step for electrochemical process was not changed under photoirradiation, more electrons than protons might participate the photoelectrocatalytic process. Furthermore, the photoelectroactive CdS–GS hybrid film was explored for PPD determination based on the photocurrent response of film toward PPD. Under optimal conditions, the photocurrent signal on CdS–GS film was linearly proportional to the concentration of PPD ranging from 1.0 × 10⁻⁷ to 3.0 × 10⁻⁶ mol L⁻¹, with a detection limit (3S/N) of 4.3 × 10⁻⁸ mol L⁻¹. Our work based on CdS–GS hybrid film not only demonstrated a new facile photovoltammetric way to study the photoinduced electron transfer process of PPD, but also developed a sensitive photoelectrochemical strategy for PPD determination.     

Functionalized carbon dots as sensors for gold nanoparticles in spiked samples: Formation of nanohybrids

This paper reports the synthesis, passivation and functionalization of luminescent carbon dots (CDs) possessing surface thiol ending groups. A simple procedure involving amidation of passivated carbon dots (p-CDs) with cysteamine boosts their photoluminescent properties and enables their use as easily controlled fluorescent nanosensors for determining citrate–gold nanoparticles (AuNPs). The mechanism behind the quenching phenomenon was established from fluorescence measurements at high temperatures and lifetime tests, and found to involve static quenching leading to the formation of CD–AuNP nanohybrids. A method for determining AuNPs in complex matrices was developed and validated by application to spiked drinking water and mussel tissues. The limits of detection and quantitation for AuNPs thus obtained were 0.20 and 0.66 nmol L^–1, respectively.     

Facile synthesis of N-acetyl-L-cysteine capped ZnS quantum dots as an eco-friendly fluorescence sensor for Hg2+

This paper described an investigation of a novel eco-friendly fluorescence sensor for Hg²⁺ ions based on N-acetyl-l-cysteine (NAC)-capped ZnS quantum dots (QDs) in aqueous solution. By using safe and low-cost materials, ZnS QDs modified by NAC were easily synthesized in aqueous medium via a one-step method. The quantitative detection of Hg²⁺ ions was developed based on fluorescence quenching of ZnS QDs with high sensitivity and selectivity. Under optimal conditions, its response was linearly proportional to the concentration of Hg²⁺ ions in a range from 0 to 2.4 × 10⁻⁶ mol L⁻¹ with a detection limit of 5.0 × 10⁻⁹ mol L⁻¹. Most of common physiologically relevant cations and anions did not interfere with the detection of Hg²⁺. The proposed method was applied to the trace determination of Hg²⁺ ions in water samples. The possible quenching mechanism was also examined by fluorescence and UV-vis absorption spectra.     

Naked-eye colorimetric analysis of Hg2+ with bi-color CdTe quantum dots multilayer films

A novel direct quantificational method through naked-eye colorimetric analysis of Hg²⁺ was constructed based on different degree of the fluorescence quenching of bi-color quantum dots (QDs) multilayer films (2-QDMF). The functional multilayer films were assembled by layer-by-layer (LBL) deposition of oppositely charged CdTe QDs and poly(dimethyldiallylemmonium chloride) (PDDA). Then the outermost layer of 2-QDMF was cross-linked to bovine serum albumin (BSA), polyethylene glycol (PEG) or glutathione (GSH). It was found that when BSA modified quartz slides were immersed into solutions containing Hg²⁺ and Cu²⁺ respectively, the 2-QDMF can be quenched by Hg²⁺, but not by Cu²⁺. Under the optimization conditions, the quenched photoluminescence (PL) intensities of multilayer films were almost linearly proportional to the concentration of Hg²⁺ in the range of 1.0 × 10⁻⁸ to 1.0 × 10⁻⁶ mol L⁻¹ and the detection limit was 4.5 × 10⁻⁹ mol L⁻¹. The proposed method is intuitional and convenient, which can be applied to the determination of trace Hg²⁺ in the artificial water sample with satisfactory results.     

A potential visual fluorescence probe for ultratrace arsenic (III) detection by using glutathione-capped CdTe quantum dots

The interaction between mercaptoacetic acid (MA)-capped CdTe QDs, MA-capped CdTe/ZnS QDs or glutathione (GSH)-capped CdTe QDs with As(III) was studied using fluorescence spectrometry. As (III) has a high-affinity to reduced-GSH to form As(SG)₃, and the emission of the GSH-capped CdTe QDs (λ_em. = 612 nm) is quenched effectively. Thus, a novel fluorescence spectrometric method was developed for As (III) determination by using GSH-CdTe QDs. Under optimal conditions, the quenched fluorescence intensity (F₀/F) increased linearly with the concentration of As (III) ranging from 5.0 × 10⁻⁶ to 25 × 10⁻⁵ mol L⁻¹. The limit of detection (3σ) for As (III) was found to be 2 × 10⁻⁸ mol L⁻¹. This method is potentially useful in visual detection of As (III) under irradiation of the ultraviolet light.     

Size-dependent electrochemiluminescence behavior of water-soluble CdTe quantum dots and selective sensing of l-cysteine

Water-soluble CdTe quantum dots (QDs) with five sizes (2.25, 2.50, 2.77, 3.12, and 3.26 nm) were synthesized with the hydrothermal method. The electrochemiluminescence (ECL) of CdTe QDs was investigated in detail in air-saturated solution without adding foreign oxidant. It was found that the ECL of CdTe QDs displayed a size-dependent property. With the increasing in the particle size of the CdTe QDs, the ECL intensity was gradually increased, in addition, both ECL peak potentials and ECL onset potentials of CdTe QDs were shifted positively. Influences of some factors on the ECL intensity were investigated. Under the optimal conditions, the ECL intensity had a linear relationship with the concentration of l-cysteine (l-Cys) in the range from 1.3 × 10⁻⁶ to 3.5 × 10⁻⁵ mol L⁻¹ (R² 0.996) with a detection limit of 8.7 × 10⁻⁷ mol L⁻¹ (S/N = 3). The proposed method was applied to the determination of l-Cys in real samples with satisfactory results. Compared with previous reports, it has better selectivity for the determination of l-Cys.     

Selective electrochemical sensing of calcium dobesilate based on the nano-Pd/CNTs modified pyrolytic graphite electrode

A new palladium nanoparticle functionalized multi-wall carbon nanotubes (nano-Pd/CNTs) modified pyrolytic graphite electrode (PGE) has been fabricated for electrochemical sensing of calcium dobesilate (CD) in pharmaceutical capsules. The nano-Pd/CNTs were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The nano-Pd/CNTs composite showed a strong electrocatalytic property for CD. The anodic peak current is 6-fold than that obtained in bare PGE and the oxidation potential has an obvious shift to negative. The anodic peak current is proportional to the concentration of CD in the range of 1.0 × 10⁻⁷ to 7.0 × 10⁻⁴ mol L⁻¹, with a linear relative coefficient r = 0.999 and a detection limit 4.0 × 10⁻⁸ mol L⁻¹ (S/N = 3). This kind of electrode shows good stability, sensitivity, reproducibility, large linear range and low detection limit towards electrochemical determination of CD. The proposed method provides a selective and sensitive electrochemical sensor of calcium dobesilate.     

ZnS quantum dots derived a reagentless uric acid biosensor

A reagentless amperometric uric acid biosensor based on zinc sulfide (ZnS) quantum dots (QDs) was firstly developed. It could detect uric acid without the presence of an electron mediator. The carboxyl group functionalized ZnS QDs were synthesized, and they were soluble biocompatible and conductive. ZnS QDs conjugates could provide increased enzyme binding sites, which may result in higher enzyme loading. Thus, the proposed uricase/ZnS QDs/l-cys biosensor exhibited higher amperometric response compared to the one without QDs (uricase/l-cys biosensor). In addition, there was little AA interference. It showed a linear dependence on the uric acid concentration ranging from 5.0 × 10⁻⁶ to 2.0 × 10⁻³ mol L⁻¹ with a detection limit of 2.0 × 10⁻⁶ mol L⁻¹ at 3σ.     

A novel method for iodate determination using cadmium sulfide quantum dots as fluorescence probes

We have developed a novel method for the determination of iodate based on the carboxymethyl cellulose-capped CdS quantum dots (QDs). Factors affecting the iodate detection were investigated, and the optimum conditions were determined. Under the optimum conditions, the relative fluorescence intensity of CdS quantum dots was linearly proportional to IO₃⁻ over a concentration range from 1.0 × 10⁻⁸ to 1.0 × 10⁻⁵ mol L⁻¹ with a correlation coefficient of 0.9987 and a detection limit of 6.0 nmol L⁻¹. Iodide, being oxidized by bromine to form iodate, was detected indirectly. The method was successfully applied to the determination of iodate and total amount of iodine in table salt samples. The related mechanism was also discussed.     

A novel dual-function molecularly imprinted polymer on CdTe/ZnS quantum dots for highly selective and sensitive determination of ractopamine

A novel dual-function material was synthesized by anchoring a molecularly imprinted polymer (MIP) layer on CdTe/ZnS quantum dots (QDs) using a sol–gel with surface imprinting. The material exhibited highly selective and sensitive determination of ractopamine (RAC) through spectrofluorometry and solid-phase extraction (SPE) coupled with high performance liquid chromatography (HPLC). A series of adsorption experiments revealed that the material showed high selectivity, good adsorption capacity and a fast mass transfer rate. Fluorescence from the MIP-coated QDs was more strongly quenched by RAC than that of the non-imprinted polymer, which indicated that the MIP-coated QDs acted as a fluorescence sensing material could recognize RAC. In addition, the MIP-coated QDs as a sorbent was also shown to be promising for SPE coupled with HPLC for the determination of trace RAC in feeding stuffs and pork samples. Under optimal conditions, the spectrofluorometry and SPE-HPLC methods using the MIP-coated QDs had linear ranges of 5.00 × 10⁻¹⁰–3.55 × 10⁻⁷ and 1.50 × 10⁻¹⁰–8.90 × 10⁻⁸ mol L⁻¹, respectively, with limits of detection of 1.47 × 10⁻¹⁰ and 8.30 × 10⁻¹¹ mol L⁻¹, the relative standard deviations for six repeat experiments of RAC (2.90 × 10⁻⁹ mol L⁻¹) were below 2.83% and 7.11%.     

Ultrasensitive Cu sensing by near-infrared-emitting CdSeTe alloyed quantum dots

The near-infrared (NIR)-emitting CdSeTe alloyed quantum dots (AQdots) that capped with l-cysteine were applied for ultrasensitive Cu²⁺ sensing. The sensing approach was based on the fluorescence of the AQdots selectively quenched in the presence of Cu²⁺. Experimental results showed a low interference response towards other metal ions. The possible quenching mechanism was discussed on the basis of the binding between l-cysteine and the metal ions. In addition, biomolecules have low effect on the fluorescence due to the minimized interferences in NIR region. The response of the NIR optical sensor was linearly proportional to the concentration of Cu²⁺ ranging from 2 × 10⁻⁸ to 2 × 10⁻⁶ mol L⁻¹. Furthermore, it has been successfully applied to the detection of Cu²⁺ in vegetable samples.     

Construction of a novel molecularly imprinted sensor for the determination of O,O-dimethyl-(2,4-dichlorophenoxyacetoxyl)(3′-nitrophenyl)methinephosphonate

A novel voltammetric sensor for O,O-dimethyl-(2,4-dichlorophenoxyacetoxyl)(3′-nitrophenyl)methinephosphonate (Phi-NO₂) based on molecularly imprinted polymer (MIP) film electrode is constructed by using sol-gel technology. The sensor responds linearly to Phi-NO₂ over the concentration range of 2.0 × 10⁻⁵ to 1.0 × 10⁻⁸ mol L⁻¹ and the detection limit is 1.0 × 10⁻⁹ mol L⁻¹ (S/N = 3). This sensor provides an efficient way for eliminating interferences from coexisting substances in the solution. The high sensitivity, selectivity and stability of the sensor demonstrates its practical application for a simple and rapid determination of Phi-NO₂ in cabbage samples.     

Electrochemical determination of L-phenylalanine at polyaniline modified carbon electrode based on β-cyclodextrin incorporated carbon nanotube composite material and imprinted sol-gel film

A sensitive and selective electrochemical sensor based on a polyaniline modified carbon electrode for the determination of l-phenylalanine has been proposed by utilizing β-cyclodextrin (β-CD) incorporated multi-walled carbon nanotube (MWNT) and imprinted sol-gel film. The electrochemical behavior of the sensor towards l-phenylalanine was investigated by cyclic voltammetry (CV), differential pulse voltammetry (DPV), and amperometric i-t curve. The surface morphologies of layer-by-layer assembly electrodes were displayed by scanning electron microscope (SEM). The response mechanism of the imprinted sensor for l-phenylalanine was based on the inclusion interaction of β-CD and molecular recognition capacity of the imprinted film for l-phenylalanine. A linear calibration plot was obtained covering the concentration range from 5.0 × 10⁻⁷ to 1.0 × 10⁻⁴ mol L⁻¹ with a detection limit of 1.0 × 10⁻⁹ mol L⁻¹. With excellent sensitivity, selectivity, stability, reproducibility and recovery, the electrochemical imprinted sensor was used to detect l-phenylalanine in blood plasma samples successfully.     

Electrogenerated chemiluminescence from thiol-capped CdTe quantum dots and its sensing application in aqueous solution

In this paper, the electrogenerated chemiluminescence (ECL) from thiol-capped CdTe quantum dots (QDs) was reported. The ECL emission was occurred at −1.1 V and reached a maximum value at −2.4 V when the potential was cycled between 0.0 and −2.5 V. The reduced species of CdTe QDs could react with the coreactants to produce the ECL emission. The CdTe QD concentration (6.64 × 10⁻⁷ mol L⁻¹) of ECL is lower than that (1.0 × 10⁻³ mol L⁻¹) of chemiluminescence (CL). Based on the enhancement of light emission from thiol-capped CdTe QDs by H₂O₂ in the negative electrode potential, a novel method for the determination of H₂O₂ was developed. The light intensity was linearly proportional to the concentration of H₂O₂ between 2.0 × 10⁻⁷ and 1.0 × 10⁻⁵ mol L⁻¹ with a detection limit of 6.0 × 10⁻⁸ mol L⁻¹. Compared with most of previous reports, the proposed method has higher sensitivity for the determination of H₂O₂. In addition, the ECL spectrum of thiol-capped CdTe QDs exhibited a peak at around 620 nm, which was substantially red shifted from the photoluminescence (PL) spectrum, suggesting the surface states play an important role in this ECL process.     

High selective SiO2-Al2O3 mixed-oxide modified carbon paste electrode for anodic stripping voltammetric determination of Pb(II)

The main purpose of this study is to develop an inexpensive, simple, selective and especially highly selective modified mixed-oxide carbon paste electrode (CPE) for voltammetric determination of Pb(II). For the preliminary screening purpose, the catalyst was prepared by modification of SiO₂-Al₂O₃ mixed-oxide and characterized by TG, CHN elemental analysis and FTIR spectroscopy. Using cyclic voltammetry the electroanalytical characteristics of the catalyst have been determined, and consequently the modified mixed-oxide carbon paste electrode was constructed and applied for determination of Pb(II). The electroanalytical procedure for determination of the Pb(II) comprises two steps: the chemical accumulation of the analyte under open-circuit conditions followed by the electrochemical detection of the preconcentrated species using differential pulse anodic stripping voltammetry. During the preconcentration step, Pb(II) was accumulated on the surface of the modifier by the formation of a complex with the nitrogen atoms of the pyridyl groups in the modifier. The peak currents increases linearly with Pb(II) concentration over the range of 2.0 × 10⁻⁹ to 5.2 × 10⁻⁵ mol L⁻¹ (r² = 0.9995).The detection limit (three times signal-to-noise) was found to be 1.07 × 10⁻⁹ mol L⁻¹ Pb(II). The chemical and instrumental parameters have been optimized and the effect of the interferences has been determined. The Proposed method was used for determination of lead ion in the real samples.