Detection Limit Of Electrode Research Articles

Estimation of trace element of strontium ion using ion selective electrode based on a ceramic cordierite nanoparticle in some vegetarian foods

The development of an ion-selective electrode for the potentiometric measurement of strontium ions using ceramic nanoparticles (cordierite) as an ionophore was a concern in this study. The nano cordierite (Mg2Al4Si5O18) was synthesized by sol–gel method and subsequently characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) analysis, transmission electron microscope (TEM), scanning electron microscope (SEM) with EDAX unit and contact angle. The XRD results confirm the purity and crystalline structure of nanoparticles. The average crystallite size and particle size were studied by Debye-Scherrer equation, TEM, and SEM and it is found to be 16.9 and 29.29 nm which is a mesoporous structure. Cordierite nanoparticles were used to accelerate the electron transfer which refers to the extensive surface area, electrical characteristics, porous design, and nanoscale dimension. The carbon paste electrode optimization was done to study the potentiometric performance of Sr(II) ion detection. The electrode composition was 5.0 % synthetic ionophere, 28.9 % tricresylphosphate (TCP), and 65.8 % graphite powder (w/w). Stable potentiometric behavior was shown in the concentration range 1.0 × 10−6 −1.0 × 10−2 mol/L (R2 = 0.998). The electrode detection limit and response time were 5.0 × 10−7 mol/L and 6 s, respectively. As well as the electrode showed selectivity towards the Sr(II) ion in the vicinity of certain cation species. Also, it was performed without any variations in response within the pH range of 3–8. Additionally, Sr(II) ion determination was detected in some real samples (plant origin) such as legumes (kidney bean), cereals (wheat), leafy vegetables (cabbage), stems (onion), roots (carrot), fruits (tomatoes) and nuts (almond) with high recoveries and reproducibility through around 72 days. Subsequently, the outcome was assessed against the ICP technique's superior performance.

Fabrication of a simple and sensitive mitoxantrone electrochemical sensor based on topdown recycling of iron wool and tissue paper

The development of rapid, sensitive detection and quantification techniques for mitoxantrone is crucial for monitoring treatment, cancer research and pharmaceutical analysis. In this context, The aim was to research an electrochemical sensor with a cheaper source of electrode material through recycling. The current study successfully recycled domestically used iron wool and tissue paper wastes into functional electrode materials: iron oxide nanoparticles (Fe3O4 NP) and black carbon (BC). The synthesis of BC was achieved by completely charring used tissue paper in sulphuric acid followed by ultrasonic disintegration. Also, Fe3O4 NP was synthesised through a simple, rapid rusting of iron chippings with hydrogen peroxide. The current research is the first to report this synthetic approach to obtain iron oxide and black carbon composite. The materials were then mixed to produce nanocomposite Fe3O4 NP:BC. Cyclic voltammetry and differential pulse voltammetry were used to study the behaviour of the modified electrodes towards the drug. The result showed the combination of the two materials in a Fe3O4 NP:BC = 1:2 ratio improved the active surface area of the glassy carbon electrode and conferred superior electrocatalytic properties compared to the individual materials. The Fe3O4 NP:BC modified electrode was highly sensitive towards mitoxantrone with a linear range of 5 × 10−6 to 1.0 × 10−4 M and a detection limit of 2.55 × 10−10 M. The current electrode's detection limit is lower than all but one of the electrodes reported for the drug whiles being the simplest and cheapest.

A highly efficient electrochemical sensor containing polyaniline/cerium oxide nanocomposites for hydrogen peroxide detection.

An efficient electrochemical sensor containing polyaniline/cerium oxide (PANI/CeO2) nanocomposites for the detection of hydrogen peroxide has been fabricated using the traditional in situ oxidative polymerization process. PANI/CeO2 nanocomposite-based modified glassy carbon electrodes were utilized as an electrochemical sensor for the detection of hydrogen peroxide. Before the fabrication, CeO2 was prepared by a hydrothermal method, and common techniques confirmed its structure. PANI/CeO2 nanocomposites were prepared by adding variable loadings of the pre-prepared CeO2 nanoparticles (weight%) inside the polymer host matrix. All the nanocomposites were characterized to determine their chemical structures and suitability for electrode materials. The electrode detection limit, sensitivity, and effect of pH on the sensor performance were investigated using different electrochemical methods, including cyclic voltammetry, electrochemical impedance spectroscopy, and linear sweep voltammetry. The results indicated that the sensing abilities of the synthesized PANI/CeO2(10) nanocomposite-modified GCE presented good electrocatalytic oxidation properties towards H2O2 with an enhanced low limit of detection and good repeatability. The fabricated electrode sensor was successfully used to detect H2O2 in real samples.

Sensitive electrochemical detection of toxic nitro-phenol in real environmental samples using enzymeless oxidized-carboxymethyl cellulose-sulfate/sulfated polyaniline composite based electrode

In current study, sulfated-polyaniline and oxidized-sulfated-carboxymethyl cellulose (S-Pani/Ox/CMCS) deposited on a glassy carbon electrode (GCE) was investigated in a mediator-free, non-enzymatic electrochemical sensing for ortho-nitrophenol (2-NP). The resulting S-Pani/Ox/CMCS bio-composites were analyzed by scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, and X-ray photoelectron. The electrode detection limit, sensitivity, and pH effect on the performance of the fabricated sensor were evaluated by using different electrochemical methods including differential pulse voltammetry, cyclic voltammetry, and electrochemical impedance spectroscopy. The –OH, -SO3, and –COO- groups present in Ox/CMCS verified it could act as an brilliant substrate for polymerization and adsorption of aniline. Under optimal conditions of Ox/CMCS concentration in the fabricated S-Pani/Ox/CMCS bio-composite, the signal and the logarithm of 2-NP concentration showed a linear correlation, and a very low detection limit of 0.05 μM was observed. The S-Pani/Ox/CMCS modified GCE confirmed high reproducibility, good stability, and durability was found applicable to the detection of 2-NP in milk, water, and orange juice samples. The obtained result for the fabricated sensor revealed that it was highly selective for the detection of 2-NP, which might play a significant role in human health protection and environmental monitoring applications.

Analysis on Electrochemical Property of Gold Electrode Modification for Water Quality Examination

Water plays an indispensable role in manufacturing industry. However, discharge of polluted water containing heavy metal ions often appears in streams and causes serious damage to creatures and environment. Therefore, water-quality monitoring is essential to ensure both developments and safeties. To address this need, different approaches of metal ion detection include spectrophotometry, conductivity test, membrane electrode method and atomic emission spectroscopy. Nevertheless, the accuracy and efficiency of these approaches could still be improved. For these reasons, this study aims to develop an innovative water quality examination technique by using electrochemical impedance spectroscopy (EIS) measurements. By improving the sensitivity and reproducibility of the developed approach, this technique could contribute to metal ion detection. In this work, the main approach to detect the target ion, Cu2+, is surface modification for electrochemical impedance analysis method. To accomplish the development, first, the relationship between ion concentration and the impedance of solution was analyzed. CuCl2 solutions with a concentration between 10-2 to 10-6 M were used as samples for measurement. The experiment, in the Faradaic Mode, was carried out at a starting voltage of 0.34 V. The amplitude was 200 mV, with frequencies varied from 1 MHz to 1 Hz. The effect of temperature variations between 40 to 140 degree Celsius was also discussed. Then, the electrodes were respectively immersed in solutions which are 11-MUA (24 hours), 3-MPA (6 hours) and 2-thiobarbituric acid molecules (24 hours) to construct different exterior structures. Finally, histidine is connected to the electrode for capturing and recognizing copper ions [1].With the modified electrode, the measured ion concentration was changed to 10-3 to 10-9 M. The same experimental parameters as mentioned above were used to measure Ca2+, Mg2+, Na+ and Cu2+, respectively. Also, the copper ion selectivity of each type of monolayers was evaluated. The diameter of the semicircle extrapolated in the Nyquist diagram represents the charge transfer resistance (Rct ) of the solution. In Faradaic mode, Rct should have a negative relationship with ion concentration [2]. By adopting the method proposed by Edward & Craig (2013), the copper ions can be identified by the Rct value obtained in this study.According to experimental results illustrated in Fig. 1, the diameter of the impedance diagram becomes smaller as the concentration increases, meaning a decrease of the Rct. Besides, as the temperature increases, Rct decreases gradually (Fig. 2). On the other hand, there is no significant difference between the trend of Rct when using electrodes with different modification materials (Fig. 3). The detection limit of the bare electrode or modified with planar structured thiolates falls on 10-6 M. However, the detection limit of the electrode with 11-MUA or 3-MPA is approximately reduced to 10-8 M (Fig. 4). Compared to other ions, Rct of Ca2+ and Mg2+ in specific concentration performs similarly when using 11-MUA modified electrode. The correlation coefficient of the calibration relationship between Na+ concentration and the resistance are lower than those in Cu2+ detection (Fig. 5). Based on the above results, it is illustrated that MUA modified electrode performs the best ion selectivity in Cu2+ detection. Finally, for the MPA electrode, the quadratic calibration function between ion concentration and Rct of Cu2+ can also be established as shown in Fig. 6. Based on this experimental result, MPA modified electrode also has high selectivity to Cu2+.In the study, it has been confirmed that the lower ion concentration results a higher impedance of the solution. Besides, the negative logarithm of the concentration (-log[Cu2+], pCu2+) is positively related to Rct in a quadratic form. Also, as the temperature increases, the ion migration rate becomes faster, and the Rct value gets lower; hence, the environmental temperature was controlled to be 25 degree Celsius during the experiments. By modifying 11-MUA and 3-MPA on the electrode, the measurement limit has been significantly reduced, verified by a smoother fitting trend. In conclusion, the proposed 11-MUA and 3-MPA modified approaches have a higher copper ion selectivity and concentration measurement accuracy. In other words, an efficient and sensitive ion detection approach becomes available for water quality examination by utilizing the method proposed in the study.[1] Perrin D. D. (1959). Histidine-Copper (II) Complexes. [2] Edward P, and Craig E (2013). Electrochemical impedance spectroscopy: an overview of bioanalytical applications, Analytical Methods, 2013(5), 1098–1115. Figure 1

Influence of plasticizer nature on the analytical characteristics of ion-selective electrodes based on higher quaternary ammonium salts reversible to doubly charged inorganic anions

It has been established that membranes of carbonate and hydrogen phosphate selective electrodes based on higher quaternary ammonium salts are preferable to be plasticized with o-nitrophenyldecyl ether, membranes of sulfate, selenate, selenite-selective electrodes with 1-bromonaphthalene, sulfite, molybdate, tungstate-selective electrodes with dibutylphthalate. The use of an optimum plasticizer allows, in general, reducing both the lower detection limit of electrodes and the values of selectivity coefficients by about 1 order (depending on the interfering ion). A qualitative explanation is given to the results obtained from the Fouoss theory and the Born model.

A voltammetric sensor based on glassy carbon electrode modified with 1H-1,2,4-triazole-3-thiol coating for rapid determination of trace lead ions in acetate buffer solution

In this paper 1H-1,2,4-triazole-3-thiol-GC electrode was prepared successfully, and its properties were characterized by cyclic voltammetry, electrochemical AC impedance and X-ray photoelectron spectroscopy. The surface hydrophilicity was determined by contact angle measurement. Moreover, the electrochemical behavior of lead ions on the prepared electrode was studied by differential pulse voltammetry. Experimental parameters, such as number of coating cycles, the pH value of buffer solution, were optimized. Under the optimized conditions, the modified electrode responds to lead was linear between 0.03 and 8.59μM (with sensitivity of 1.3558μA/μM). The detection limit of electrode was 6.64nM (S/N=3). The reproducibility and repeatability of the analysis were also examined and calculated as the relative standard deviations were 2.02% and 5.16%, respectively. Finally, the proposed analytical procedure was applied to determine the trace lead ions in synthetic and real samples with satisfactory results.

Highly Selective and Sensitive Coated-Wire Yttrium (III) Cation Selective Electrode Based on Kryptofix-22DD

An yttrium (III) cation selective electrode based on kryptofix-22DD (1,10-didecyl-1,10-diaza-18-crown-6) incorporated into a polyvinylchloride (PVC) membrane has been developed. The optimized membrane incorporating dibutylphtalate (DBP) as plasticizer, kryptofix-22DD and palmitic acid as a new component in a highly selective and sensitive coated wire electrode (CWE) for yttrium (III) cation was directly coated on the surface of a platinum wire electrode. The selective electrode showed more effective performance than other yttrium (III) cation PVC based sensors. The electrode exhibits a Nernstian slope (18.89 ± 0.43 mv.decade−1) in a wide linear concentration range of 1.0 × 10−8–1.0 × 10−1 M. The detection limit of electrode is 2.15 × 10−9 M. The effects of the pH and possible interfering ions were investigated and the optimized conditions for electrode were evaluated. The electrode was applied as an indicator electrode for potentiometric titration of yttrium (III) cation in solutions. Determination of fluoride ion in tap water, mouthwash and toothpaste gave results that compare favorably with those obtained by the fluoride ion selective electrode.

Used gold nano-particles as an on/off switch for response of a potentiometric sensor to Al(III) or Cu(II) metal ions

The potentiometric response of a carbon paste electrode modified with silica sol–gel and mercaptosuccinic acid (MSA) in the presence and absence of gold nano-particles was studied. The results showed that the electrode with gold nano-particles was responded to Al 3+ ions as a hard metal ion. On the other hand, the electrode without gold nano-particles was responded to copper ions as a soft metal ion. The electrodes without and with gold nano-particles exhibits a Nernstian slope of 29.1 and 19.2 mV decade −1 for copper and aluminum ions over a wide concentration range of 4.3 × 10 −7–1.0 × 10 −2 and 4.5 × 10 −7–1.6 × 10 −3 mol L −1, respectively. The detection limits of electrodes were 4.0 × 10 −7 and 1.6 × 10 −7 mol L −1 for copper and aluminum ions, respectively.

Comparative studies of mercapto thiadiazoles self-assembled on gold nanoparticle as ionophores for Cu(II) carbon paste sensors

Comparative studies of the potentiometric behavior of three mercapto compounds [2-((5-mercapto-1,3,4-thiadiazol-2-ylimino)methyl)phenol] (MTMP), [5-(2-methoxy benzylidene amino)-1,3,4-thiadiazole-2-thiol] (MBYT) and [5-(pyridin-2-ylmethyleneamino)-1,3,4-thiadiazole-2-thiol] (PYTT) self-assembled on gold nanoparticles (GNPs) as ionophores in carbon paste electrodes (CPEs) have been made. These mercapto thiadiazole compounds were self-assembled onto gold nanoparticles and then incorporated within carbon paste electrode. The self-assembled ionophores exhibit a high selectivity for copper ion (Cu 2+), in which the sulfur and nitrogen atoms in their structure play a role as the effective coordination donor site for the copper ion. These carbon paste electrodes were applied as indicator electrodes for potentiometric determination of copper ions. The sensor based on PYTT exhibits the working concentration range of 4.0 × 10 −9 to 7.0 × 10 −2 M and a Nernstian slope of 28.7 ± 0.3 mV decade −1 of copper activity. The detection limit of electrode was 1.0 × 10 −9 M and potential response was pH independent across the range of 3.0–6.5. It exhibited a quick response time of <5 s and could be used for a period of 45 days. The ion selectivity of this electrode for Cu 2+ was over 10 4 times that for other metal cations. The application of prepared sensors has been demonstrated for the determination of copper ions in spiked water and natural water samples.

Nile Blue-hexacyanoferrate carbon paste modified electrode as an amperometric sensor for determination of hydrazine

The main problem associated with metal hexacyanoferrates (MHCFs) is their instability at high pH values. We synthesized a new Prussian Blue analogue, Nile-Blue hexacyanoferrate (NBHCF), that remains stable in 0.2 M KOH. A carbon paste (CPE) chemically modified electrode (ME) containing NBHCF was prepared as a stable electrochemical sensor for measuring hydrazine. A detailed characterization of the electrochemical and electrocatalytic behavior of NBHCF was performed using cyclic voltammetric, chronoamperometric, differential pulse voltammetric (DPV) and hydrodynamic amperometric methods. The NBHCF-modified electrode produced reproducible redox peaks and resulted in a linear increase in the oxidation signal of hydrazine with increasing concentration of hydrazine in the range of 0.1-6.0 mM (in hydrodynamic amperometry method (HDA)). The electrode detection limit was 40 \mu M and possessed a surface coverage of \Gamma = 2.0 \times 10^{-8} mol cm^{-2}.

Self‐Assembled Mercapto‐Compound‐Gold‐Nanoparticle‐Modified Carbon Paste Electrode for Potentiometric Determination of Cadmium(II)

AbstractA new modified carbon paste electrode (CPE) based on a recently synthesized ligand [2‐mercapto‐5‐(3‐nitrophenyl)‐1,3,4‐thiadiazole] (MNT), self‐assembled to gold nanoparticles (GNP) as suitable carrier for Cd(II) ion with potentiometric method are described. The proposed electrode exhibits a Nernstian slope of 29.4±1.0 mV per decade for Cd(II) ion over a wide concentration range from 3.1×10−8 to 3.1×10−4 mol L−1. The detection limit of electrode was 2.0×10−8 mol L−1 of cadmium ion. The potentiometric responses of electrode based on MNT is independent of the pH of test solution in the pH range 2.0–4.0. It has quick response with response time of about 6 s. The proposed electrode show fairly good selectivity over some alkali, alkaline earth, transition and heavy metal ions. Finally, the proposed electrode was successfully employed to detect Cd(II) ion in hair and water samples.

Copper(II) modified carbon paste electrodes based on self-assembled mercapto compounds-gold-nanoparticle

Three mercapto compounds [2-mercapto-5-(1-methyl-5-nitroimidazole-2-yl)-1,3,4-thiadiazole] (MMNIT), [2-mercapto-5-(5-nitrofuran-2-yl)-1,3,4-thiadiazole] (MNFT) and [2-mercapto-5-(5-nitrothiophen-2-yl)-1,3,4-thidiazole] (MNTT) were used for self-assembled-gold nanoparticle (SAGNP) modified carbon paste electrodes. The electrodes were applied as indicator electrodes for potentiometric determination of Cu(II) ion. The prepared electrodes exhibit a Nernstian slope of 31.0 ± 0.5 mV per decade for Cu(II) ion over a wide concentration range of 7.9 × 10 −9–3.2 × 10 −2, 7.9 × 10 −9–7.9 × 10 −4, and 2.8 × 10 −8–7.9 × 10 −3 mol L −1 for MMNIT, MNFT and, MNTT, respectively. The detection limits of electrodes were 3.5 (±0.2) × 10 −9, 4.1 × 10 −9, and 4.1 × 10 −8 mol L −1 of copper ion, respectively. The potentiometric responses of electrodes based on MMNIT, MNFT, and MNTT are independent of the pH of test solution in the pH range 2.0–5.5, 2.5–7.0, and 2.0–6.5, respectively. They have quick response with response time of about 5 s. The proposed electrodes show fairly good selectivity over some alkali, alkaline earth, transition and heavy metal ions. Finally, the proposed electrodes were successfully employed to detect Cu(II) ion in hair and water samples.

Thiocyanate Ion-selective PVC Membrane Electrode Based on N,N′-Ethylene-bis(4-methylsalicylidineiminato)nickel(II)

A highly selective poly(vinyl chloride) (PVC) membrane electrode based on an N,N'-ethylene-bis(4-methyl-salicylidineiminato) nickel(II) [Ni(EBMSI)] complex as a carrier for a thiocyanate-selective electrode is reported. The influences of the membrane composition, pH and possible interfering anions were investigated based on the response properties of the electrode. The electrode exhibited a good Nernstian slope of -58.9 +/- 0.7 mV decade(-1), over a wide pH range of 3.5 - 8.5 and a linear range of 1.0 x 10(-6) - 1.0 x 10(-1) M for thiocyanate. The detection limit of electrode was 3.1 x 10(-7) M SCN(-). The selectivity coefficients determined by a fixed interference method (FIM) indicate that a good discriminating ability towards the SCN- ion compared to other anions. The proposed sensor had a fast response time of about 5 - 15 s and could be used for at least 3 months without any considerable divergence in the potential. It was applied as an indicator electrode in the titration of thiocyanate with Ag+ and in the potentiometric determination of thiocyanate in saliva and urine samples.

Sensor for Silver(I) Ion Based on Schiff-base-p-tertbutylcalix[4]arene

The preparation of polymer membrane selective to silver ion and its application to ion-selective electrode is reported here. PVC membrane contain silver-selective calix[4]arene derivative as ionophore. The membrane electrode is highly selective to silver ion and exhibit good linear response over a wide concentration range of 1.0×10-1 to 1.0×10-5M AgNO3 with Nernstian slope of 58.9 mV per decade. The detection limit of electrode is 6.31×10-6 M .The calix[4]arene based membrane electrode exhibit enhanced selectivities for silver ion over other cations; log KPotAg,Na = -2.35, log KPotAg,K= -2.65, log KPotAg,Mg = -4.57, log KPotAg,Ba = -4.10, log KPotAg,Cd = -3.42, log KPotAg,Pb = -3.45.Even the strongest interferent Hg2+ do not interfere to great extent, log KPotAg,Hg = -2.0.The electrode response is stable over wide pH range of 1.0-6.0. The response time of the sensor is 30 seconds and the membrane can be used for more than 6 months without observing any divergence. The sensor can also be applied as an indicator electrode for potentiometric titration of Ag+ ions with Cl-. It is possible to determine Ag(I) in the natural water spiked with silver with this electrode assembly.

Cesium ion selective electrode based on calix[4]crown ether–ester

The potentiometric response characteristics of cesium ion selective PVC membrane electrode employing calix[4]crown ether–ester as an ionophore were investigated. The electrode exhibit a good response for cesium ion over wide concentration range of 5.0×10 −6–1.0×10 −1 M with a Nernstian slope of 59 mV per decade. The detection limit of electrode is 5.0×10 −6 M. The electrode was found to have selectivity for cesium ion over alkali, alkaline and transition metals. The response time of the electrode is less than 20 s and can be used for more than 4 months without observing any divergence in potentiometric response. The electrode response was stable over wide pH range.

Prussian Blue- and lactate oxidase-based amperometric biosensor for lactic acid

The properties of amperometric lactate biosensors based on Prussian Blue (PB) and lactate oxidases (LOX) of different origin (from Pediococcus sp. and Enterococcus sp. RL1) immobilised in Nafion were investigated by flow injection analysis. The operating potential was −50 mV (versus Ag/AgCl, 0.1 M KCl). A linear range in the dependence of current response upon the concentration of lactic acid was up to 0.8 mM. The detection limit of electrodes containing a high amount (6 U) of enzyme was below 1 μM. Operational stability could be maintained by exposing the electrodes to low (20 μM) concentrations of lactic acid.

Evaluation of stearate-graphite paste electrodes for chronic measurement of extracellular dopamine concentrations in the mammalian brain

Chronoamperometric procedures, in combination with pharmacological treatments, were used to verify whether stearate-modified graphite paste recording electrodes (SGEs) could measure basal extracellular dopamine (DA) concentrations in the striatum of awake rats over a 3-week period of implantation. Baseline Chronoamperometric signals were unaffected by systemic injections of ascorbate (AA) or the monoamine oxidase inhibitor pargyline, or by intraventricular infusions of the AA degrading enzyme AA-oxidase. In contrast, systemic injections of d-amphetamine or nomifensine increased, and γ-butyrolactone decreased, the signal in a reproducible fashion over a similar test period. In addition, 6-hydroxydopamine lesions of the nigrostriatal DA pathway attenuated the ability of d-amphetamine to increase, and γ-butyrolactone to decrease, the Chronoamperometric signal. In separate studies, reverse microdialysis, performed with dialysis probes implanted directly adjacent to SGEs in the striatum, permitted the assessment of electrode selectivity, sensitivity, response linearity, and detection limits to DA. Perfusion of the probe with normal Ringer solution (5 μl/min) decreased the baseline Chronoamperometric signal by 10 nA. Comparable decreases in the baseline signal were observed after systemic injections of γ-butyrolactone or medial forebrain bundle infusions of tetrodotoxin, suggesting these decreases reflected depletion of extracellular DA to levels below the electrode's detection limit. Reverse dialysis with high concentrations of AA, DOPAC, 5-HT, or 5-HIAA, failed to reverse the decrease in the Chronoamperometric signal induced by dialysis. In contrast, reverse dialysis with a physiologically relevant range of DA concentrations, in rats pretreated with the DA uptake blocker nomifensine, increased the Chronoamperometric signal in a linear fashion with a detection threshold of < 20 nM. Combined, these results indicate that the baseline Chronoamperometric signals recorded at +0.20 V in the striatum with SGEs do not reflect changes in extracellular concentrations of AA, DA metabolites, or indoles, but rather represent neuronally mediated nanomolar changes in extracellular DA concentrations, even after extended periods of implantation in brain tissue.