12 research outputs found
Cloud Point Extraction for Electroanalysis: Anodic Stripping Voltammetry of Cadmium
Cloud point extraction (CPE) is a
well-established technique for
the preconcentration of hydrophobic species from water without the
use of organic solvents. Subsequent analysis is then typically performed
via atomic absorption spectroscopy (AAS), UV–vis spectroscopy,
or high performance liquid chromatography (HPLC). However, the suitability
of CPE for electroanalytical methods such as stripping voltammetry
has not been reported. We demonstrate the use of CPE for electroanalysis
using the determination of cadmium (Cd<sup>2+</sup>) by anodic stripping
voltammetry (ASV). Rather than using the chelating agents which are
commonly used in CPE to form a hydrophobic, extractable metal complex,
we used iodide and sulfuric acid to neutralize the charge on Cd<sup>2+</sup> to form an extractable ion pair. This offers good selectivity
for Cd<sup>2+</sup> as no interferences were observed from other heavy
metal ions. Triton X-114 was chosen as the surfactant for the extraction
because its cloud point temperature is near room temperature (22–25
°C). Bare glassy carbon (GC), bismuth-coated glassy carbon (Bi-GC),
and mercury-coated glassy carbon (Hg-GC) electrodes were compared
for the CPE-ASV. A detection limit for Cd<sup>2+</sup> of 1.7 nM (0.2
ppb) was obtained with the Hg-GC electrode. ASV with CPE gave a 20x
decrease (4.0 ppb) in the detection limit compared to ASV without
CPE. The suitability of this procedure for the analysis of tap and
river water samples was demonstrated. This simple, versatile, environmentally
friendly, and cost-effective extraction method is potentially applicable
to a wide variety of transition metals and organic compounds that
are amenable to detection by electroanalytical methods
Optically Transparent Carbon Nanotube Film Electrode for Thin Layer Spectroelectrochemistry
Carbon
nanotube (CNT) film was evaluated as an optically transparent
electrode (OTE) for thin layer spectroelectrochemistry. Chemically
inert CNT arrays were synthesized by chemical vapor deposition (CVD)
using thin films of Fe and Co as catalysts. Vertically aligned CNT
arrays were drawn onto a quartz slide to form CNT films that constituted
the OTE. Adequate conductivity and transparency make this material
a good OTE for spectroelectrochemistry. These properties could be
varied by the number of layers of CNTs used to form the OTE. Detection
in the UV/near UV region down to 200 nm can be achieved using these
transparent CNT films on quartz. The OTE was characterized by transmission
electron microscopy, scanning electron microscopy, Raman spectroscopy,
UV–visible spectroscopy, cyclic voltammetry, electrochemical
impedance spectroscopy, and thin layer spectroelectrochemistry. Ferricyanide,
trisÂ(2,2′-bipyridine) rutheniumÂ(II) chloride, and cytochrome
c were used as representative redox probes for thin layer spectroelectrochemistry
using the CNT film OTE, and the results correlated well with their
known properties. Direct electron transfer of cytochrome c was achieved
on the CNT film electrode
Carbon Nanotube Thread Electrochemical Cell: Detection of Heavy Metals
In this work, all
three electrodes in an electrochemical cell were
fabricated based on carbon nanotube (CNT) thread. CNT thread partially
insulated with a thin polystyrene coating to define the microelectrode
area was used as the working electrode; bare CNT thread was used as
the auxiliary electrode; and a micro quasi-reference electrode was
fabricated by electroplating CNT thread with Ag and then anodizing
it in chloride solution to form a layer of AgCl. The Ag|AgCl coated
CNT thread electrode provided a stable potential comparable to the
conventional liquid-junction type Ag|AgCl reference electrode. The
CNT thread auxiliary electrode provided a stable current, which is
comparable to a Pt wire auxiliary electrode. This all-CNT thread three
electrode cell has been evaluated as a microsensor for the simultaneous
determination of trace levels of heavy metal ions by anodic stripping
voltammetry (ASV). Hg<sup>2+</sup>, Cu<sup>2+</sup>, and Pb<sup>2+</sup> were used as a representative system for this study. The calculated
detection limits (based on the 3σ method) with a 120 s deposition
time are 1.05, 0.53, and 0.57 nM for Hg<sup>2+</sup>, Cu<sup>2+</sup>, and Pb<sup>2+</sup>, respectively. These electrodes significantly
reduce the dimensions of the conventional three electrode electrochemical
cell to the microscale
Simplified Nitrate-Reductase-Based Nitrate Detection by a Hybrid Thin-Layer Controlled Potential Coulometry/Spectroscopy Technique
A novel method for the detection
of nitrate was developed using
simplified nitrate reductase (SNaR) that was produced by genetic recombination
techniques. The SNaR consists of the fragments of the Mo–molybdopterin
(MO–MPT) binding site and nitrate reduction active site and
has high activity for nitrate reduction. The method is based on a
unique combination of the enzyme-catalyzed reduction of nitrate to
nitrite by thin-layer coulometry followed by spectroscopic measurement
of the colored product generated from the reaction of nitrite with
Griess reagents. Coulometric reduction of nitrate to nitrite used
methyl viologen (MV<sup>2+</sup>) as the electron transfer mediator
for SNaR and controlled potential coulometry in an indium tin oxide
(ITO) thin-layer electrochemical cell. Absorbance at 540 nm was proportional
to the concentration of nitrate in the sample with a linear range
of 1–160 μM and a sensitivity of 8000 AU M<sup>–1</sup>. The method required less than 60 μL of sample. Detection
of nitrate could also be performed by measuring the charge associated
with coulometry. However, the spectroscopic procedure gave superior
performance because of interference from the large background charge
associated with coulometry. Results for the determination of nitrate
concentration in several natural water samples using this device with
spectroscopic detection are in good agreement with analysis done with
a standard method
Electrospun Carbon Nanofiber Modified Electrodes for Stripping Voltammetry
Electrospun
polyacrylonitrile (PAN) based carbon nanofibers (CNFs)
have attracted intense attention due to their easy processing, high
carbon yield, and robust mechanical properties. In this work, a CNF
modified glassy carbon (GC) electrode that was coated with Nafion
polymer was evaluated as a new electrode material for the simultaneous
determination of trace levels of heavy metal ions by anodic stripping
voltammetry (ASV). Pb<sup>2+</sup> and Cd<sup>2+</sup> were used as
a representative system for this initial study. Well-defined stripping
voltammograms were obtained when Pb<sup>2+</sup> and Cd<sup>2+</sup> were determined individually and then simultaneously in a mixture.
Compared to a bare GC electrode, the CNF/Nafion modified GC (CNF/Nafion/GC)
electrode improved the sensitivity for lead detection by 8-fold. The
interface properties of the CNF/Nafion/GC were characterized by electrochemical
impedance spectroscopy (EIS), which showed the importance of the ratio
of CNF/Nafion on electrode performance. Under optimized conditions,
the detection limits are 0.9 and 1.5 nM for Pb<sup>2+</sup> and Cd<sup>2+</sup>, respectively
Carbon Nanotube-Loaded Nafion Film Electrochemical Sensor for Metal Ions: Europium
A Nafion film loaded with novel catalyst-free
multiwalled carbon
nanotubes (MWCNTs) was used to modify a glassy carbon (GC) electrode
to detect trace concentrations of metal ions, with europium ion (Eu<sup>3+</sup>) as a model. The interaction between the sidewalls of MWCNTs
and the hydrophobic backbone of Nafion allows the MWCNTs to be dispersed
in Nafion, which was then coated as a thin film on the GC electrode
surface. The electrochemical response to Eu<sup>3+</sup> was found
to be ∼10 times improved by MWCNT concentrations between 0.5
and 2 mg/mL, which effectively expanded the electrode surface into
the Nafion film and thereby reduced the diffusion distance of Eu<sup>3+</sup> to the electrode surface. At low MWCNT concentrations of
0.25 and 0.5 mg/mL, no significant improvement in signal was obtained
compared with Nafion alone. Scanning electron microscopy and electrochemical
impedance spectroscopy were used to characterize the structure of
the MWCNT–Nafion film, followed by electrochemical characterization
with Eu<sup>3+</sup> via cyclic voltammetry and preconcentration voltammetry.
Under the optimized conditions, a linear range of 1–100 nM
with a calculated detection limit of 0.37 nM (signal/noise = 3) was
obtained for determination of Eu<sup>3+</sup> by Osteryoung square-wave
voltammetry after a preconcentration time of 480 s
Carbohydrate-Based Label-Free Detection of <i>Escherichia coli</i> ORN 178 Using Electrochemical Impedance Spectroscopy
A label-free biosensor for <i>Escherichia coli</i> (<i>E. coli</i>) ORN 178 based on faradaic electrochemical impedance spectroscopy (EIS) was developed. α-Mannoside or β-galactoside was immobilized on a gold disk electrode using a self-assembled monolayer (SAM) via a spacer terminated in a thiol functionality. Impedance measurements (Nyquist plot) showed shifts due to the binding of <i>E. coli</i> ORN 178, which is specific for α-mannoside. No significant change in impedance was observed for <i>E. coli</i> ORN 208, which does not bind to α-mannoside. With increasing concentrations of <i>E. coli</i> ORN 178, electron-transfer resistance (<i>R</i><sub>et</sub>) increases before the sensor is saturated. After the Nyquist plot of <i>E. coli</i>/mixed SAM/gold electrode was modeled, a linear relationship between normalized <i>R</i><sub>et</sub> and the logarithmic value of <i>E. coli</i> concentrations was found in a range of bacterial concentration from 10<sup>2</sup> to 10<sup>3</sup> CFU/mL. The combination of robust carbohydrate ligands with EIS provides a label-free, sensitive, specific, user-friendly, robust, and portable biosensing system that could potentially be used in a point-of-care or continuous environmental monitoring setting
Optically Transparent Thin-Film Electrode Chip for Spectroelectrochemical Sensing
A novel microfabricated optically
transparent thin-film electrode
chip for fluorescence and absorption spectroelectrochemistry has been
developed. The working electrode was composed of indium tin oxide
(ITO); the quasi-reference and auxiliary electrodes were composed
of platinum. The stability of the platinum quasi-reference electrode
was improved by coating it with a planar, solid state Ag/AgCl layer.
The Ag/AgCl reference was characterized with scanning electron microscopy
and energy-dispersive X-ray spectroscopy. Cyclic voltammetry measurements
showed that the electrode chip was comparable to a standard electrochemical
cell. Randles-Sevcik analysis of 10 mM K<sub>3</sub>[FeÂ(CN)<sub>6</sub>] in 0.1 M KCl using the electrode chip gave a diffusion coefficient
of 1.59 × 10<sup>–6</sup> cm<sup>2</sup>/s, in comparison
to the value of 2.38 × 10<sup>–6</sup> cm<sup>2</sup>/s
using a standard electrochemical cell. By using the electrode chip
in an optically transparent thin-layer electrode (OTTLE), the absorption
based spectroelectrochemical modulation of [FeÂ(CN)<sub>6</sub>]<sup>3–/4–</sup> was demonstrated, as well as the fluorescence
based modulation of [RuÂ(bpy)<sub>3</sub>]<sup>2+/3+</sup>. For the
fluorescence spectroelectrochemical determination of [RuÂ(bpy)<sub>3</sub>]<sup>2+</sup>, a detection limit of 36 nM was observed
Bare and Polymer-Coated Indium Tin Oxide as Working Electrodes for Manganese Cathodic Stripping Voltammetry
Though
an essential metal in the body, manganese (Mn) has a number
of health implications when found in excess that are magnified by
chronic exposure. These health complications include neurotoxicity,
memory loss, infertility in males, and development of a neurologic
psychiatric disorder, manganism. Thus, trace detection in environmental
samples is increasingly important. Few electrode materials are able
to reach the negative reductive potential of Mn required for anodic
stripping voltammetry (ASV), so cathodic stripping voltammetry (CSV)
has been shown to be a viable alternative. We demonstrate Mn CSV using
an indium tin oxide (ITO) working electrode both bare and coated with
a sulfonated charge selective polymer film, polystyrene-<i>block</i>-polyÂ(ethylene-<i>ran</i>-butylene)-<i>block</i>-polystyrene-sulfonate (SSEBS). ITO itself proved to be an excellent
electrode material for Mn CSV, achieving a calculated detection limit
of 5 nM (0.3 ppb) with a deposition time of 3 min. Coating the ITO
with the SSEBS polymer was found to increase the sensitivity and lower
the detection limit to 1 nM (0.06 ppb). This polymer modified electrode
offers excellent selectivity for Mn as no interferences were observed
from other metal ions tested (Zn<sup>2+</sup>, Cd<sup>2+</sup>, Pb<sup>2+</sup>, In<sup>3+</sup>, Sb<sup>3+</sup>, Al<sup>3+</sup>, Ba<sup>2+</sup>, Co<sup>2+</sup>, Cu<sup>2+</sup>, Ni<sup>3+</sup>, Bi<sup>3+</sup>, and Sn<sup>2+</sup>) except Fe<sup>2+</sup>, which was
found to interfere with the analytical signal for Mn<sup>2+</sup> at
a ratio 20:1 (Fe<sup>2+</sup>/Mn<sup>2+</sup>). The applicability
of this procedure to the analysis of tap, river, and pond water samples
was demonstrated. This simple, sensitive analytical method using ITO
and SSEBS-ITO could be applied to a number of electroactive transition
metals detectable by CSV
<i>In Situ</i> Quantification of [Re(CO)<sub>3</sub>]<sup>+</sup> by Fluorescence Spectroscopy in Simulated Hanford Tank Waste
A pretreatment protocol
is presented that allows for the quantitative
conversion and subsequent <i>in situ</i> spectroscopic analysis
of [ReÂ(CO)<sub>3</sub>]<sup>+</sup> species in simulated Hanford tank
waste. In this test case, the nonradioactive metal rhenium is substituted
for technetium (Tc-99), a weak beta emitter, to demonstrate proof
of concept for a method to measure a nonpertechnetate form of technetium
in Hanford tank waste. The protocol encompasses adding a simulated
waste sample containing the nonemissive [ReÂ(CO)<sub>3</sub>]<sup>+</sup> species to a developer solution that enables the rapid, quantitative
conversion of the nonemissive species to a luminescent species which
can then be detected spectroscopically. The [ReÂ(CO)<sub>3</sub>]<sup>+</sup> species concentration in an alkaline, simulated Hanford tank
waste supernatant can be quantified by the standard addition method.
In a test case, the [ReÂ(CO)<sub>3</sub>]<sup>+</sup> species was measured
to be at a concentration of 38.9 μM, which was a difference
of 2.01% from the actual concentration of 39.7 μM