An optical fiber sensor for berberine based on immobilized 1,4-bis(naphth[2,1-d]oxazole-2-yl)benzene in a new copolymer

A new copolymer made of 8-quinolyl methacrylate, methyl methacrylate and n-butyl acrylate(PQMB) has been used as a solid substrate for the preparation of an optical fiber berberine sensor. The sensing is based on the fluorescence of 1,4-bis(naphth[2,1-d]oxazole-2-yl)benzene(BNOB) immobilized in the copolymer quenched by berberine with the formation of 1:1 complex between them. The membrane composition was optimized using the orthogonal experimental design. Using the sensor described above, berberine in a sample solution from 4.02x10(-7) mol l(-1) to 2.82x10(-4) mol l(-1) can be detected. The sensor has satisfactory reversibility and a short response time of less than 30 s. The relative standard deviations for repeated measurements (n=15) of 8.05x10(-6) mol l(-1) and 4.02x10(-5) mol l(-1) beberine are 0.43% and 0.33%, respectively. The sensor shows good selectivity over alkali and alkali-earth metal salts and some common pharmaceutical species and can be used for the determination of berberine in pharmaceutical preparations.     

Internal solid contact sensor for the determination of doxycycline hydrochloride in pharmaceutical formulation

The internal solid contact sensor for the determination of doxycycline hydrochloride (DC) was developed based on a conducting polypyrrole (PPy) film immobilized on a glassy carbon electrode surface casted by a plasticized polyvinyl chloride (PVC) membrane containing an ion-pair compound of DC with tetraphenylborate (TPB) and dibutylphthalate (DBP) as plasticizer. Effects of various factors for the electropolymerization of pyrrole or aniline, including monomer concentration, acidity or inorganic salt and thickness of polymer film were investigated experimentally. It was found that the slope and the linear range of SCSs changed with both the different concentration of monomer and of KCl in electrolyte solution and with the different substrate material and a marked influence of the change of solution pH on the potential response of sensor occurred when sample solution pH>3.5. Under the condition of pH 2.8, the sensor showed a near-Nernstian response over the range of DC concentration of 1.0x10(-2)-1.0x10(-5) mol l(-1) with the slope (at 25 degrees C) of 54.4 mV per decade. The detection limit obtained was 4.0x10(-6) mol l(-1).The sensor was successfully applied to determination of DC in pharmaceutical formulation.     

N-allyl-4-(N-2'-hydroxyethyl)amino-1,8-naphthalimide as a fluorophore for optical chemosensing of nitrofurantoin

N-Allyl-4-(N-2'-hydroxyethyl)amino-1,8-naphthalimide (AHEAN), a naphthalimide derivative, was synthesized as a new fluorophore for optical chemical sensor preparation. To prevent leakage of the fluorophore, AHEAN was photo-copolymerized with 2-hydroxypropyl methacrylate on a glass surface treated with a silanizing agent. An optical chemical sensor based on AHEAN can be utilized for nitrofurantoin assay based on fluorescence quenching. The sensor shows sufficient repeatability, selectivity and a fast response of less than 30 s. Nitrofurantoin can be determined in the range between 1.00 x 10(-6) and 1.00 x 10(-3) mol l(-1) with a detection limit of 4.8 x 10(-7) mol l(-1). Most commonly co-existing drug substances and ions do not interfere with the nitrofurantoin assay. The sensor was applied to the analysis of pharmaceutical and urine samples.     

A selective sensing membrane for the determination of tetracycline with heptakis (2,6-di-O-isobutyl)-β-cyclodextrin as the substrate

When heptakis (2,6-di-O-isobutyl)-β-cyclodextrin(DOB-β-CD) is immobilized in a plasticized poly vinyl chloride (PVC) membrane, it extracts tetracycline (TC) from the sample solution into the organic membrane phase to form a complex of DOB-β-CD and TC. Since the complex formation results in an enhancement of fluorescence intensity of TC at 506 nm, the chemical recognition process can be directly translated into an optical signal. The maximum response of the sensitive membrane for TC was obtained in 0.2 mol/l KH₂PO₄–KOH buffer solution (pH 8.01). In the optimum conditions described, the proposed sensor responds linearly in the measuring range of 2.00×10⁻⁶ mol/l to 4.00×10⁻⁴ mol/l, and has a detection limit of 8.00×10⁻⁷ mol/l. The response time of the sensor is within 2.0 min. In addition to high reproducibility and reversibility, the sensor also exhibits good selectivity over some common pharmaceutical species and some common organic and inorganic compounds.     

Diffusion coefficient measurements in microfluidic devices

A glassy carbon electrode (GCE) modified with Pd/IrO(2) provides excellent electrocatalytic oxidation of hydrogen peroxide. Glucose oxidase (GOD) and xanthine oxidase (XOD) were co-immobilized on the modified electrode with a thin film Nafion coated on the enzyme layer to form a glucose (Glu)/hypoxanthine (Hx) sensor, without interference from electroactive species such as ascorbic acid (AA) and uric acid (UA). Its response was evaluated with respect to the enzyme amount on the electrode, pH and temperature of the electrolyte. The prepared bienzymic biosensor, used as the detector of HPLC gave a detection limit of 1.0x10(-6) mol l(-1) Glu and 2.0x10(-7) mol l(-1) Hx (Hx) with a linear concentration range of 5.0x10(-6)-2.5x10(-3) mol l(-1) and 1.0x10(-6)-5.0x10(-4) mol l(-1), respectively. Coupled with microdialysis, it was used to monitor the concentrations of Glu and Hx in rat brain.     

Covalently immobilized aminonaphthalimide as fluorescent carrier for the preparation of optical sensors

By replacing the hydrogen of the 4-amino group of a 4-amino-1,8-naphthalimide derivative with an N-acryloxyethyl group, the fluorophore has been covalently immobilized on an optical sensor surface by UV photopolymerization. The optical sensor obtained can be used for the determination of picric acid. The linear range and detection limit of the sensor are 9.80x10(-7)-1.96x10(-4) mol L(-1) and 7.1x10(-7) mol L(-1), respectively. Leaching of the fluorophore from the membrane is effectively prevented by covalent immobilization, resulting in a sensor with a relatively long lifetime. The response time of the sensor is short, and the reproducibility and reversibility are good. The sensor has been used for the indirect determination of the chloroquine content of pharmaceutical tablets.     

Novel bilirubin-based PVC calcium sensors.

Poly(vinyl chloride) membrane sensors for Ca(II) ions were constructed based on bilirubin (1,3,6,7-tetramethyl-4,5-dicarboxyethy-2,8-divinyl-[b-13]-dihydrobilenone) as a neutral carrier in the presence of various plasticizers (o-nitrophenyloctyl ether, dioctyl phethalate and dibutyl sebacate), which were used as solvent mediators to incorporate the carrier into the membranes. The role of plasticizers as a medium of complexation was demonstrated. The influences of an anionic excluder, the pH and foreign ions were investigated. The selectivity coefficients were measured by a separate solution method. Among the plasticizers used, o-nitrophenyloctyl ether gave the best selectivity in the presence of large concentrations of alkali and alkaline earth's metal ions. Using sensor number I, a linear calibration graph with a good Nernstian slope of 29.5 +/- 0.8 mV was obtained for Ca(II) concentrations of 5 x 10(-7)-10(-2) mol l(-1) with a detection limit of 3 x 10(-7) mol l(-1) (12 ppb). The sensor can be used for more than 3 months with good reproducibility and a fast response time of 10 s. The developed sensor has been successfully applied to the analysis of some pharmaceutical formulations, baby-food products and human plasma. Moreover, it can be used as an indicator sensor in the potentiometric titration of Ca(II) with EDTA.     

A selective PVC membrane for di- or trinitrophenol based on N,N-dibenzyl-3,3',5,5'-tetramethylbenzidine

A new fluorophore, N,N-dibenzyl-3,3',5,5'-tetramethylbenzidine (NBTMB), was prepared and shown to exhibit significant and analytical usefulness for optical sensing toward 2,4-dinitrophenol or 2,4,6-trinitrophenol (picric acid) when it was immobilized in a plasticized poly(vinyl chloride) (PVC) membrane. When the membrane was applied to aqueous nitrophenol solution, NBTMB was able to extract selectively nitrophenol into the membrane phase. Since the extraction equilibrium was accompanied by fluorescence quenching of NBTMB, the chemical recognition process could be directly translated into an optical signal. The sensor showed reversible response in the concentration range from 2.0 x 10(-7) to 6.0 x 10(-5) mol L(-1) for the detection of 2,4-dinitrophenol in NaOAc-HOAc buffer at pH 4.0. It also showed a fast response time (t95% < 1.5 min) when the sensor was applied to 2,4-dinitrophenol solution at concentration levels of 5.26 x 10(-6) and 2.10 x 10(-5) mol L(-1) alternatively. A working principle is proposed and the responses of this sensor to various kinds of nitrophenol were studied. The sensor was applied to the direct determination of 2,4-dinitrophenol in prepared water samples and the indirect assay of the drug cinchonine and the results obtained were satisfactory.     

Determination of trace chromium(VI) and molybdenum(VI) in natural and bottled mineral waters using long pathlength absorbance spectroscopy (LPAS)

A flow-through solid phase spectrophotometric (SPS) sensing device is proposed for the determination of minoxidil. The analyte is concentrated on Sephadex SP-C25 ion-exchanger packed in a flow cell and it is monitored by UV-Vis spectrophotometry at 282 nm, without derivatization reaction. When a HCl (10(-2) mol l(-1))/NaCl (5x10 (-2) mol l (-1)) solution is used as carrier/desorbing solution, the sensor responds linearly in the measuring range of 0.2-7, 0.1-4 and 0.05-2 mug ml(-1) with detection limits of 60, 33 and 6 ng ml(-1) for 600, 1000 and 2000 mul of sample, respectively. The relative standard deviations (%) for these volumes are 0.38, 1.06 and 2.63, respectively. The method was satisfactorily applied to the determination of minoxidil in pharmaceutical preparations and the results were compared with those obtained by high performance liquid chromatography (HPLC).     

Constructing a new optical sensor for monitoring ammonia in water samples using bis(acetylacetoneethylendiamine)-tributylphosphin cobalt(III) tetraphenylborate complex-coated triacetylcellulose.

A new ammonia optical sensor was designed using bis(acetylacetoneethylendiamine)tributylphosphin cobalt(III) tetraphenylborate complex, coated on transparent triacetylcellulose film as membrane. The change in the absorbance of the optode at the maximum wavelength of 408 nm was related to ammonia concentration in aqueous samples. A buffer solution with a pH of 9 (sodium borate-HCl) was used. The optode was fully regenerated in pH 2. The linear dynamic range for determination of ammonia was 3.3 x 10(-4) to 6.9 x 10(-3) mol l(-1) with a detection limit of 5.0 x 10(-5) mol l(-1) and a response time range of 4 - 6 min. This membrane was successfully applied for determination of ammonia in drinking water.     

Fiber-optic sensor for iodine based on a covalently immobilized aminobenzanthrone Schiff base.

An aminobenzanthrone Schiff base has been synthesized as a new fluorescence carrier for the preparation of an optical chemical sensor for iodine. The response of the sensor is based on fluorescence quenching of the aminobenzanthrone Schiff base by iodine. The sensor shows a linear response toward iodine in the range of 1.0 x 10(-5) to 1.0 x 10(-3) mol l(-1), with a detection limit of 6.0 x 10(-6) mol l(-1) at pH 8.0. Leaching of the fluorophore from the membrane is effectively hindered by covalent immobilization, resulting in an enhanced sensor lifetime. In addition to satisfactory reproducibility and reversibility, the prepared sensor exhibits sufficient selectivity toward iodine with respect to other coexisting ions. The sensor has been applied to the determination of iodine in common salt samples.     

A multi-wall carbon nanotubes-dicetyl phosphate electrode for the determination of hypoxanthine in fish.

An enzymeless sensor based on a multi-walled carbon nanotubes-dicetyl phosphate (MWCNT-DCP) film modified vitreous carbon electrode was developed for the determination of hypoxanthine. The MWCNT-DCP film modified electrode showed a remarkable enhancement effect on the oxidation peak current of hypoxanthine. Under the optimized conditions, the oxidation peak current is proportional to the concentration of hypoxanthine over the range from 5.0 x 10(-7) to 2.0 x 10(-4) mol L(-1) with a detection limit (S/N = 3) of 2.0 x 10(-7) mol L(-1). The MWCNT-DCP film modified electrode has been successfully used to detect hypoxanthine in fish samples.     

Flow injection analysis using carbon film resistor electrodes for amperometric determination of ambroxol

Flow injection analysis (FIA) using a carbon film sensor for amperometric detection was explored for ambroxol analysis in pharmaceutical formulations. The specially designed flow cell designed in the lab generated sharp and reproducible current peaks, with a wide linear dynamic range from 5x10(-7) to 3.5x10(-4) mol L(-1), in 0.1 mol L(-1) sulfuric acid electrolyte, as well as high sensitivity, 0.110 Amol(-1) L cm(-2) at the optimized flow rate. A detection limit of 7.6x10(-8) mol L(-1) and a sampling frequency of 50 determinations per hour were achieved, employing injected volumes of 100 microL and a flow rate of 2.0 mL min(-1). The repeatability, expressed as R.S.D. for successive and alternated injections of 6.0x10(-6) and 6.0x10(-5) mol L(-1) ambroxol solutions, was 3.0 and 1.5%, respectively, without any noticeable memory effect between injections. The proposed method was applied to the analysis of ambroxol in pharmaceutical samples and the results obtained were compared with UV spectrophotometric and acid-base titrimetric methods. Good agreement between the results utilizing the three methods and the labeled values was achieved, corroborating the good performance of the proposed electrochemical methodology for ambroxol analysis.     

Flow-through sensor with Fourier transform Raman detection for determination of sulfonamides

A flow-through sensor system with Fourier transform (FT) Raman spectroscopy as detection technique is described. The molecular and structural information contained in Raman spectra together with the selective retention of the species of interest on the sorbent make the proposed methodology highly selective. The flow-through sensor allowed the direct quantitative determination of sulfathiazole and sulfamethoxazole in the presence of other species that are normally encountered with these analytes. The system used Sephadex QAE A-25 resin as packing material of a flow-through cell on which sulfonamides were temporarily retained. Samples were transported by a carrier solution of NaOH 10(-2) mol l(-1) (pH = 12), and 2 ml of a [NaCl (0.10 mol l(-1))/NaOH (10(-2) mol l(-1))] solution was employed as eluent. Using a sample volume of 1 ml, the analytical signal was linear in the range 0.5-7 g l(-1) and 0.5-10 g l(-1), for sulfathiazole and sulfamethoxazole, respectively. RSDs (%) lower than 4% were obtained for both analytes. The sensor was satisfactorily applied to several commercial pharmaceutical preparations for human and animals in different physical presentations, including capsules, syrup, tablets, powders, injectables and suspensions.     

Flow injection spectrophotometric determination of tetracycline in a pharmaceutical preparation by complexation with aluminium(III).

A rapid flow injection (FI) spectrophotometric procedure for tetracycline determination is described. It is based on the injection of a 100 microl sample solution containing tetracycline into merged streams of aluminium(III) chloride (0.01 mol 1(-1)) and Tris-buffer in the presence of KCl (0.06 mol l(-1)), pH 7.0, with the same optimum flow rate of 3.2 ml min(-1). A yellow Al(III)-tetracycline complex was monitored at 376 nm. The flow injection system and the experimental conditions were optimized by means of the univariate method. The procedure was applied to the determination of tetracycline in pharmaceutical preparations with a high sampling rate of at least 165 h(-1). A high precision with a relative standard deviation was obtained less than 0.72 and 0.30% of 5.0 and 10 microg ml(-1) (n=11), respectively. The detection limit (3sigma) and the quantification limit (10sigma) were 0.07 and 0.72 mg l(-1), respectively. There were no interference effects from traditional excipients in the dosage forms when the method was applied to pharmaceutical preparations. The matrix effect could be reduced by the standard addition method.     

PVC matrix membrane sensor for potentiometric determination of cetylpyridinium chloride.

A novel cetylpyridinium chloride-selective membrane sensor consisting of cetylpyridinium-ferric thiocyanate ion pairs dispersed in a PVC matrix placticized with dioctylphthalate is described. The electrode shows a stable, near-Nernstian response for 1 x 10(-3)-1 x 10(-6) mol l-1 cetylpyridinium chloride (CPC) at 25 degrees C over the pH range 1-6 with a cationic slope of 57.5 +/- 0.4. The lower detection limit is 8 x 10(-7) mol l-1 and the response time is 30-60 s. Selectivity coefficients for CPC relative to a number of interfering substances were investigated. There is negligible interference from many cations, anions and pharmaceutical excipients; however, cetyltrimethylammonim bromide (CTMAB) interfered significantly. The determination of 0.5-350 micrograms/ml of CPC in aqueous solutions shows an average recovery of 98.5% and a mean relative standard deviation of 1.6% at 56.0 micrograms/ml. The direct determination of CPC in Ezafluor mouthwash gave results that compare favorably with those obtained by the British Pharmacopoeia method. Precipitation titrations involving CPC as titrant are monitored with a CP sensor. The CP electrode has been utilized as an end point indicator electrode for the determination of anionic surfactants in some commercial detergents.     

Chronoamperometric determination of paracetamol using an avocado tissue (Persea americana) biosensor

A biosensor based on vaseline/graphite modified with avocado tissue (Persea americana) as the source of polyphenol oxidase was developed and used for the chronoamperometric determination of paracetamol in pharmaceutical formulations. This enzyme catalyses the oxidation of paracetamol to N-acetyl-p-benzoquinoneimine whose electrochemical reduction back to paracetamol was obtained at a potential of -0.12 V. After addition of paracetamol reference solutions in glass cell and stirring for 60 s for the accumulation of N-acetyl-p-benzoquinoneimine at the electrode surface under open-circuit conditions, the current response was monitored by 120 s without stirring. The currents obtained at 70 s were proportional to the paracetamol concentration from 1.2x10(-4) to 5.8x10(-3) mol l(-1) (r=0.9927) with a detection limit of 8.8x10(-5) mol l(-1). The recovery of paracetamol from two samples ranged from 97.9 to 100.7% and a relative standard deviation lower than 0.5% for a solution containing 5.0x10(-3) mol l(-1) paracetamol in 0.10 mol l(-1) phosphate buffer solution (pH 7.0; n=10) was obtained. The results obtained for paracetamol in pharmaceutical formulations using the proposed biosensor and those obtained using a pharmacopoeial procedure are in agreement at the 95% confidence level.     

Flow injection turbidimetric determination of thiamine in pharmaceutical formulations using silicotungstic acid as precipitant reagent

A simple flow injection system is proposed for the determination of thiamine in pharmaceutical formulations. The determination is based on the precipitation reaction of thiamine with silicotungstic acid in acidic medium to form a thiamine silicotungstate suspension that is measured at 420 nm. Adding 0.05% (w/v) poly(ethyleneglycol) in the carrier solution (0.5 mol l(-1) hydrochloric acid), an improvement in the sensitivity, repeatability and baseline stability of the flow injection system was obtained. The calibration graph was linear in the thiamine concentration range from 5.0x10(-5) to 3.0x10(-4) mol l(-1) with a detection limit of 1.0x10(-5) mol l(-1). The relative standard deviations for ten successive measurements of 1.0x10(-4) mol l(-1) and 2.5x10(-4) mol l(-1) thiamine were less than 1% and an analytical frequency of 90 h(-1) was obtained.     

A potentiometric rhodamine-B based membrane sensor for the selective determination of chromium ions in wastewater.

The construction and performance characteristics of a novel chromate ion-selective membrane sensor are described and used for determining chromium(III) and chromium(VI) ions. The sensor is based on the use of a rhodamine-B chromate ion-associate complex as an electroactive material in a poly(vinyl chloride) membrane plasticized with o-nitrophenyloctyl ether as a solvent mediator. In a phosphate buffer solution of pH 6 - 7, the sensor displays a stable, reproducible and linear potential response over the concentration range of 1 x 10(-1) - 5 x 10(-6) mol l(-1) with an anionic Nernstian slope of 30.8 +/- 0.5 mV decade(-1) and a detection limit of 1 x 10(-6) mol l(-1) Cr(VI). High selectivity for Cr(VI) is offered over many common anions (e.g., I-, Br-, Cl-, IO4-, CN-, acetate, oxalate, citrate, sulfate, phosphate, thiosulfate, selenite, nitrate) and cations (e.g., Ag+, Ca2+, Sr2+, Co2+, Ni2+, Cu2+, Mn2+, Fe2+, Zn2+, Cd2+, Al3+, Cr3+). The sensor is used for determining Cr(VI) and/or Cr(III) ions in separate or mixed solutions after the oxidation of Cr(III) into Cr(VI) with H2O2. As low as 0.2 microg ml(-1) of chromium is determined with a precision of +/-1.2%. The chromium contents of some wastewater samples were accurately assessed, and the results agreed fairly well with data obtained by atomic absorption spectrometry.     

Spectrophotometric determination of fluoxetine by batch and flow injection methods

A rapid, simple, and accurate spectrophotometric method is presented for the determination of fluoxetine by batch and flow injection analysis methods. The method is based on fluoxetine competitive complexation reaction with phenolphthalein-beta-cyclodextrin (PHP-beta-CD) inclusion complex. The increase in the absorbance of the solution at 554 nm by the addition of fluoxetine was measured. The formation constant for fluoxetin-beta-CD was calculated by non-linear least squares fitting. Fluoxetine can be determined in the range 7.0 x 10(-6)-2.4 x 10(-4) mol l(-1) and 5.0 x 10(-5)-1.0 x 10(-2) mol l(-1) by batch and flow methods, respectively. The limit of detection and limit of quantification were respectively 4.13 x 10(-6) mol l(-1) and 1.38 x 10(-5) mol l(-1) for batch and 2.46 x 10(-5) mol l(-1) and 8.22 x 10(-5) mol l(-1) for flow method. The sampling rate in flow injection analysis method was 80+/-5 samples h(-1). The method was applied to the determination of fluoxetine in pharmaceutical formulations and after addition to human urine samples.