Characterization of hydrophobic ionic liquid-carbon nanotubes-gold nanoparticles composite film coated electrode and the simultaneous voltammetric determination of guanine and adenine

A novel composite film, comprising of hydrophobic ionic liquid (IL), multi-walled carbon nanotubes (MWNTs) and gold nanoparticles (GNP), was fabricated and characterized. The GNP was introduced through electrochemical deposition on IL-MWNT gel film coated glassy carbon electrodes (GCE). Experiments showed that both IL and MWNTs could facilitate the GNP deposition. With GNP the composite film exhibited smaller electron transfer resistance and higher sensitivity in sensing guanine (G) and adenine (A). Under the optimized experimental conditions, the anodic peak currents were linear to the analyte concentration in the ranges of 0.008-2.0 μM. The detection limits were down to nanomole level after an accumulation of 150 s on open-circuit. In addition, on the composite film coated GCE, the anodic peaks of G and A were well separated, and their response sensitivities kept almost unchanged no matter whether they coexisted or not. This proposed procedure was successfully applied to the detection of G and A in milk, plasma and urine samples.     

Hematoxylin multi-wall carbon nanotubes modified glassy carbon electrode for electrocatalytic oxidation of hydrazine

A new hydrazine sensor has been fabricated by immobilizing hematoxylin at the surface of a glassy carbon electrode (GCE) modified with multi-wall carbon nanotube (MWCNT). The adsorbed thin films of hematoxylin on the MWCNT modified GCE show one pair of peaks with surface confined characteristics. The hematoxylin MWCNT (HMWCNT) modified GCE shows highly catalytic activity toward hydrazine electro-oxidation. The results show that the peak potential of hydrazine at HMWCNT modified GCE surface shifted by about 167 and 255 mV toward negative values compared with that at an MWCNT and activated modified GCE surface, respectively. In addition, at HMWCNT modified electrode surface remarkably improvement the sensitivity of determination of hydrazine. The kinetic parameters, such as the electron transfer coefficient, α, and the standard heterogeneous rate constant, k⁰, for oxidation of hydrazine at the HMWCNT modified GCE were determined and also is shown that the heterogeneous rate constant, k′, is strongly potential dependent. The overall number of electron involved in the catalytic oxidation of hydrazine and the number of electrons involved in the rate-determining steps are 2 and 1, respectively. The amperometric detection of hydrazine is carried out at 220 mV in 0.1 M phosphate buffer solution (pH 7) with linear response range 2.0-122.8 μM hydrazine, detection limit of 0.68 μM and sensitivity of 0.0208 μA μM⁻¹. Finally the amperometric response for hydrazine determination is reproducible, fast and extremely stable, with no loss in sensitivity over a continual 400 s operation.     

Electrochemical determination of bisphenol A at Mg-Al-CO3 layered double hydroxide modified glassy carbon electrode

The electrochemical behavior of bisphenol A (BPA) was investigated on Mg-Al layered double hydroxide (LDH) modified glassy carbon electrode (GCE) by cyclic voltammetry (CV), differential pulse voltammetry (DPV), linear sweep voltammetry (LSV) and chronocoulometry (CC). The cyclic voltammogram of BPA on the modified electrode exhibited a well defined anodic peak at 0.454 V in 0.1 M pH 8.0 phosphate buffer solution (PBS). The experimental parameters were optimized and the kinetic parameters were investigated. The probable oxidation mechanism was proposed. Under the optimized conditions, the oxidation peak current was proportional to BPA concentration in the range from 1 × 10⁻⁸ to 1.05 × 10⁻⁶ M with the correlation coefficient of 0.9959. The detection limit was 5.0 × 10⁻⁹ M (S/N = 3). The fabricated electrode showed good reproducibility, stability and anti-interference. The proposed method was successfully applied to determine BPA in plastic products and the results were satisfactory.     

Gold nanoparticle/carbon nanotube hybrids as an enhanced material for sensitive amperometric determination of tryptophan

In this work, a highly sensitive electrochemical sensor for the determination of tryptophan (Trp) was fabricate by electrodeposition of gold nanoparticles (AuNPs) onto carbon nanotube (CNT) films pre-cast on a glassy carbon electrode (GCE), forming an AuNP-CNT composite-modified GCE (AuNP-CNT/GCE). Scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were used for the surface analysis of the electrode. The results indicate that the hybrid nanomaterials induced a substantial decrease in the overpotential of the Trp oxidation reaction and exhibited a remarkable synergistic effect on the electrocatalytic activity toward the oxidation of Trp. In phosphate buffer solution (pH 7.4), the modified electrode showed excellent analytical performance for the amperometric determination of Trp. The peak currents possess a linear relationship with the concentration of Trp in the range of 30 nM to 2.5 μM, and the detection limit is 10 nM (S/N = 3). In addition, the modified electrode was used to determine Trp concentration in pharmaceutical samples with satisfactory results.     

Towards the conception of an amperometric sensor of l-tyrosine based on Hemin/PAMAM/MWCNT modified glassy carbon electrode

A novel amperometric sensor was fabricated based on the immobilization of hemin onto the poly (amidoamine)/multi-walled carbon nanotube (PAMAM/MWCNT) nanocomposite film modified glassy carbon electrode (GCE). Electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and ultraviolet visible (UV-vis) adsorption spectroscopy were used to investigate the possible state and electrochemical activity of the immobilized hemin. In the Hemin/PAMAM/MWCNT nanocomposite film, MWCNT layer possessed excellent inherent conductivity to enhance the electron transfer rate, while the layer of PAMAM greatly enlarged the surface average concentration of hemin (Γ) on the modified electrode. Therefore, the nanocomposite film showed enhanced electrocatalytical activity towards the oxidation of l-tyrosine. The kinetic parameters of the modified electrode were investigated. In pH 7.0 phosphate buffer solution (PBS), the sensor exhibits a wide linear range from 0.1 μM to 28.8 μM l-tyrosine with a detection limit of 0.01 μM and a high sensitivity of 0.31 μA μM⁻¹ cm⁻². In addition, the response time of the l-tyrosine sensor is less than 5 s. The excellent performance of the sensor is largely attributed to the electro-generated high reactive oxoiron (IV) porphyrin (O = Fe^IV-P) which effectively catalyzed the oxidation of l-tyrosine. A mechanism was herein proposed for the catalytic oxidation of l-tyrosine by oxoiron (IV) porphyrin complexes.     

Voltammetric determination of bisoprolol fumarate in pharmaceutical formulations and urine using single-wall carbon nanotubes modified glassy carbon electrode

The electrochemistry of bisoprolol fumarate (BF) has been investigated by differential pulse voltammetry at a single-wall carbon nanotubes (SWNTs) modified glassy carbon electrode (GCE). The prepared electrode showed an excellent electrocatalytic activity towards the oxidation of BF leading to a marked improvement in sensitivity as compared to bare glassy carbon electrode where electrochemical activity for the analyte cannot be observed. The SWNTs-modified GCE exhibited a sharp anodic peak at a potential of ∼950 mV for the oxidation of BF. Under optimum conditions linear calibration curve was obtained over the BF concentration range 0.01-0.1 mM in 0.5 M phosphate buffer solution (pH 7.2) with a correlation coefficient of 0.9789 and detection limit of 8.27 × 10⁻⁷ M. The modified electrode has been applied for the drug determination in human urine with no prior extraction and in commercial tablets. The proposed method has also been validated.     

Electrochemical characterization of poly(eriochrome black T) modified glassy carbon electrode and its application to simultaneous determination of dopamine, ascorbic acid and uric acid

A polymerized film of eriochrome black T (EBT) was prepared on the surface of a glassy carbon (GC) electrode in alkaline solution by cyclic voltammetry (CV). The redox response of the poly(EBT) film at the GC electrode appeared in a couple of redox peak in 0.1 M hydrochloride and the pH dependent peak potential was −55.1 mV/pH which was close to the Nernst behavior. The poly(EBT) film-coated GC electrode exhibited excellent electrocatalytic activity towards the oxidations of dopamine (DA), ascorbic acid (AA) and uric acid (UA) in 0.05 mM phosphate buffer solution (pH 4.0) and lowered the overpotential for oxidation of DA. The polymer film modified GC electrode conspicuously enhanced the redox currents of DA, AA and UA, and could sensitively and separately determine DA at its low concentration (0.1 μM) in the presence of 4000 and 700 times higher concentrations of AA and UA, respectively. The separations of anodic peak potentials of DA-AA and UA-DA reached 210 mV and 170 mV, respectively, by cyclic voltammetry. Using differential pulse voltammetry, the calibration curves for DA, AA and UA were obtained over the range of 0.1-200 μM, 0.15-1 mM and 10-130 μM, respectively. With good selectivity and sensitivity, the present method provides a simple method for selective detection of DA, AA and UA in biological samples.     

Detection of hydrazine based on Nano-Au deposited on Porous-TiO2 film

The fabrication of Nano-Au/Porous-TiO₂ composite modified glassy carbon electrode (GCE) and its application in the determination of hydrazine were proposed. The morphological characterization was examined by transmission electron microscope and scanning electron microscopy. The Nano-Au/Porous-TiO₂/GCE exhibited a wide linear range of hydrazine from 2.5 to 500 μM, with a detection limit of 0.5 μM at a signal-to-noise ratio of 3 and with a fast response time (within 3 s). Furthermore, the reaction mechanism of the hydrazine on the Nano-Au/Porous-TiO₂/GCE was explored. The ease of fabrication, high stability, and low cost of the modified electrode are the promising features of the proposed sensor.     

Simultaneous voltammetric determination of tramadol and acetaminophen using carbon nanoparticles modified glassy carbon electrode

A sensitive and selective electrochemical sensor was fabricated via the drop-casting of carbon nanoparticles (CNPs) suspension onto a glassy carbon electrode (GCE). The application of this sensor was investigated in simultaneous determination of acetaminophen (ACE) and tramadol (TRA) drugs in pharmaceutical dosage form and ACE determination in human plasma. In order to study the electrochemical behaviors of the drugs, cyclic and differential pulse voltammetric studies of ACE and TRA were carried out at the surfaces of the modified GCE (MGCE) and the bare GCE. The dependence of peak currents and potentials on pH, concentration and the potential scan rate were investigated for these compounds at the surface of MGCE. Atomic force microscopy (AFM) was used for the characterization of the film modifier and its morphology on the surface of GCE. The results of the electrochemical investigations showed that CNPs, via a thin layer model based on the diffusion within a porous layer, enhanced the electroactive surface area and caused a remarkable increase in the peak currents. The thin layer of the modifier showed a catalytic effect and accelerated the rate of the electron transfer process. Application of the MGCE resulted in a sensitivity enhancement and a considerable decrease in the anodic overpotential, leading to negative shifts in peak potentials. An optimum electrochemical response was obtained for the sensor in the buffered solution of pH 7.0 and using 2 μL CNPs suspension cast on the surface of GCE. Using differential pulse voltammetry, the prepared sensor showed good sensitivity and selectivity for the determination of ACE and TRA in wide linear ranges of 0.1-100 and 10-1000 μM, respectively. The resulted detection limits for ACE and TRA was 0.05 and 1 μM, respectively. The CNPs modified GCE was successfully applied for ACE and TRA determinations in pharmaceutical dosage forms and also for the determination of ACE in human plasma.     

Nonenzymatic glucose sensor based on flower-shaped Au@Pd core-shell nanoparticles-ionic liquids composite film modified glassy carbon electrodes

A novel nonenzymatic glucose sensor based on flower-shaped (FS) Au@Pd core-shell nanoparticles-ionic liquids (ILs i.e., trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl) imide, [P(C₆)₃C₁₄][Tf₂N]) composite film modified glassy carbon electrodes (GCE) was reported. The Au@Pd nanocatalysts were prepared by seed-mediated growth method, forming the three-dimensional FS nanoparticles, where tens of small Pd nanoparticles (∼3 nm) aggregated on gold seeds (∼20 nm). The FS Au@Pd nanoparticle was a good candidate for the catalytic efficiency of nanometallic surfaces because of its flower-shaped nature, which has greater adsorption capacity. XPS analysis and zeta potential indicated that the surface of Pd atoms is positively charged, profiting the oxidation process of glucose. And ILs acted as bridge connecting Au@Pd one another and bucky gel as platform within the whole nanocomposite. So the modified electrode has higher sensitivity and selectivity owing to intrinsic synergistic effects of this nanocomposite. Amperometric measurements allow observation of the electrochemical oxidation of glucose at 0.0 V (vs. Ag/AgCl), the glucose oxidation current is linear to its concentration in the range of 5 nM-0.5 μM, and the detection limit was found to be 1.0 nM (S/N = 3). The as-prepared nonenzyme glucose sensor exhibited excellent stability, repeatability, and selectivity.     

Electrocatalytic oxidation of l-cysteine with a stable copper-cobalt hexacyanoferrate electrochemically modified carbon paste electrode

A stable composition of hybrid copper-cobalt hexacyanoferrate (Cu-CoHCF) film was electrodeposited on a carbon paste electrode (CPE). There are a few reports for using this hybrid as a mediator, but all of them require almost 12 h conditioning time before usage. Contrary to previous reports this electrode does not require any conditioning and can be used immediately after film formation. The electrocatalytic activity of this film was investigated and showed a good electrocatalytic effect for oxidation of l-cysteine (Cys) in phosphate buffer solution (PBS) in pH range of 1-7. A linear range of 6 μM to 1 mM of Cys and an experimental detection limit of 5 μM of Cys were obtained using cyclic voltammetry method. The diffusion coefficient of Cys and catalytic rate constant for electrocatalytic reaction were also calculated. The major problem reported in electro oxidation of Cys is poisoning of electrode surface with reaction product, but in this study oxidation of Cys had no significant fouling effect on the modified electrode surface for the concentrations below 0.5 mM of Cys.     

Glassy carbon electrodes modified with a film of nanodiamond-graphite/chitosan: Application to the highly sensitive electrochemical determination of Azathioprine

A novel modified glassy carbon electrode with a film of nanodiamond-graphite/chitosan is constructed and used for the sensitive voltammetric determination of azathioprine (Aza). The surface morphology and thickness of the film modifier are characterized using atomic force microscopy. The electrochemical response characteristics of the electrode toward Aza are investigated by means of cyclic voltammetry. The modified electrode showed an efficient catalytic role for the electrochemical reduction of Aza, leading to a remarkable decrease in reduction overpotential and enhancement of the kinetics of the electrode reaction with a significant increase of peak current. The effects of experimental variables, such as the deposited amount of modifier suspension, the pH of the supporting electrolyte, the accumulation potential and time were investigated. Under optimal conditions, the modified electrode showed a wide linear response to the concentration of Aza in the range of 0.2-100 μM with a detection limit of 65 nM. The prepared modified electrode showed several advantages: simple preparation method, high stability and uniformity in the composite film, high sensitivity, excellent catalytic activity in physiological conditions and good reproducibility. The modified electrode can be successfully applied to the accurate determination of trace amounts of Aza in pharmaceutical and clinical preparations.     

Selective oxidation of serotonin and norepinephrine over eriochrome cyanine R film modified glassy carbon electrode

A novel ECR-modified electrode is fabricated by electrodeposition of Eriochrome Cyanine R (ECR) at a glassy carbon (GC) electrode by cyclic voltammetry (CV) in double-distilled water. The characterization of the ECR film modified electrode is carried out by atomic force microscopy (AFM), infrared spectra (IR), spectroelectrochemistry and cyclic voltammetry. The results show that a slightly heterogeneous film formed on the surface of the modified electrode, and the calculated surface concentration of ECR is 2 × 10⁻¹⁰ mol/cm⁻². The ECR film modified GC electrode shows excellent electrocatalytic activities toward the oxidation of serotonin (5-HT) and norepinephrine (NE). Furthermore, the modified electrode can separately detect 5-HT and NE, even in the presence of 200-fold concentration of ascorbic acid (AA) and 25-fold concentration of uric acid (UA). Using differential pulse voltammetry (DPV), the peak currents of 5-HT and NE recorded in pH 7 solution are linearly dependent on their concentrations in the range of 0.05-5 μM and 2-50 μM, respectively. The limits of detection are 0.05 and 1.5 μM for 5-HT and NE, respectively. The ECR film modified electrode can be stored stable for at least 1 week in 0.05 M PBS (pH 7) at 4 °C in a refrigerator. Owing to its excellent selectivity and sensitivity, the modified electrode could provide a promising tool for the simultaneous determination of 5-HT and NE in complex biosamples.     

Study of nimesulide and its determination using multiwalled carbon nanotubes modified glassy carbon electrodes

A new method for the determination of nimesulide was established based on the multiwalled carbon nanotubes (MWCNTs) modified glassy carbon electrode (MWCNTs/GCE). In 0.2 M PBS (pH 6.6) buffer solution, the MWCNTs/GCE showed a remarkable catalytic and enhancement effect on reduction of the nimesulide. The reduction peak potential of nimesulide shifted positively from −0.665 V at bare GCE to −0.553 V at MWCNTs/GCE, and the sensitivity increased ca. 7 times. A linear dynamic range of 3.2 × 10⁻⁷-6.5 × 10⁻⁵ M (R = 0.9992) with a detection limit of 1.6 × 10⁻⁷ M was obtained. The electrochemical behaviors of nimesulide were studied and electron-transfer coefficient (α = 0.45), proton number (X = 1) and electron-transfer number (n = 2) have been determined. This method has been used to determine the content of nimesulide in medical tablets. The recovery was determined to be 93.2-106.2% by means of standard addition method. Compared with UV-vis spectrometry, the method was not remarkable difference.     

Separation and concentration effect of f-MWCNTs on electrocatalytic responses of ascorbic acid, dopamine and uric acid at f-MWCNTs incorporated with poly (neutral red) composite films

A novel conductive composite film containing functionalized multi-walled carbon nanotubes (f-MWCNTs) with poly (neutral red) (PNR) was synthesized on glassy carbon electrodes (GC) by potentiostatic method. The composite film exhibited promising electrocatalytic oxidation of mixture of biochemical compounds such as ascorbic acid (AA), dopamine (DA) and uric acid (UA) in pH 4.0 aqueous solutions. It was also produced on gold electrodes by using electrochemical quartz crystal microbalance technique, which revealed that the functional properties of composite film were enhanced because of the presence of both f-MWCNTs and PNR. The surface morphology of the polymer and composite film deposited on transparent semiconductor tin oxide electrodes were studied using scanning electron microscopy and atomic force microscopy. These two techniques showed that the PNR was fibrous and incorporated on f-MWCNTs. The electrocatalytic responses of neurotransmitters at composite films were measured using both cyclic voltammetry (CV) and differential pulse voltammetry (DPV). These experiments revealed that the difference in f-MWCNTs loading present in the composite film affected the electrocatalysis in such a way, that higher the loading showed an enhanced electrocatalytic activity. From further electrocatalysis studies, well separated voltammetric peaks were obtained at the composite film modified GC for AA, DA and UA with the peak separation of 0.17 V between AA-DA and 0.15 V between DA-UA. The sensitivity of the composite film towards AA, DA and UA in DPV technique was found to be 0.028, 0.146 and 0.084 μA μM⁻¹, respectively.     

Monolayer covalent modification of 5-hydroxytryptophan on glassy carbon electrodes for simultaneous determination of uric acid and ascorbic acid

5-Hydroxytryptophan (5-HTP) was covalently grafted on the surface of glassy carbon electrodes (GCEs) using cyclic voltammetric method in a phosphate buffer solution. The prepared electrode, denoded as 5-HTP/GCE, was characterized by X-ray photoelectron spectroscopy, cyclic voltammetry and differential pulse voltammetry (DPV). Tryptophan grafted GCE (TRP/GCE) and 5-hydroxytryptamine grafted GCE (5-HTP/GCE) were also prepared by the same method for comparison. It was found that the electrocatalytic activities toward the oxidation of uric acid (UA) and ascorbic acid (AA) was in the order of 5-HT/GCE > 5-HTP/GCE > TRP/GCE for UA oxidation and 5-HT/GCE = 5-HTP/GCE > TRP/GCE for AA oxidation. However, the CV current sensitivity was estimated as 4:2:1 for 5-HTP/GCE:5-HT/GCE:TRP/GCE. The DPV peaks for UA and AA oxidation appeared at 0.07 V and 0.34 V versus SCE, respectively, allowing simultaneous determination in mixtures. A linearly response in the range of: 5.0 × 10⁻⁷ to 1.1 × 10⁻⁵ M with the detection limit (s/n = 3) of 2.8 × 10⁻⁷ M for UA determination, and a linear response in the range of: 5.0 × 10⁻⁶ to 1.0 × 10⁻⁴ M with the detection limit of 4.2 × 10⁻⁶ M for AA determination were obtained. This electrode was used for UA and AA determinations in human urine samples satisfactorily.     

Electrodeposited MnO2/Au composite film with improved electrocatalytic activity for oxidation of glucose and hydrogen peroxide

The major drawback of currently used MnO₂ film sensor is the loss of electrical conductivity due to the formation of a poorly conductive MnO₂ layer. To overcome this problem, a coating in which the Au is alloyed with MnO₂ has been developed. The fabrication of the codeposited film electrode of Au and MnO₂ by using a cyclic voltammetric (CV) method was described, and systematic physical and electrochemical characterization was performed. This MnO₂/Au film electrode enhanced MnO₂ electrocatalytic activity. The oxidation process of glucose at the codeposited MnO₂/Au shows a well-defined peak at 0.27 V in alkaline aqueous solution. In contrast, the glucose oxidation at Au modified glassy carbon electrode (GCE) just shows a shoulder wave at 0.42 V. The experimental results indicate that the modification of MnO₂ on the surface of GCE significantly improved the electrocatalytic activity towards the oxidation of glucose. Further study shows that the MnO₂/Au could also effectively catalyze the oxidation of hydrogen peroxide in pH 7.0 phosphate buffer solution (PBS).     

Amperometric detection of acetaminophen by an electrochemical sensor based on cobalt oxide nanoparticles in a flow injection system

This paper reports the use of a carbon ceramic electrode as a highly-porous substrate for the electrochemical formation of cobalt oxide nanoparticles. The electrocatalyst was characterized by energy dispersive X-ray (EDX) spectroscopy, scanning electron microscopy (SEM) and cyclic voltammetry techniques, and it was used in a homemade flow injection analysis (FIA) system for acetaminophen determination using 0.1 M KOH as the carrier solution. The rate constant (k_s) and charge transfer coefficient (α) were calculated for the electron exchange reaction of the modified film. The kinetic parameters and the mechanism of acetaminophen electrooxidation at the electrode surface were studied by cyclic voltammetry and chronoamperometry. The effects of working potential and flow rate on the performance of the FIA system were studied. Under optimized conditions, the electrode response due to the electrocatalytic oxidation of acetaminophen at 450 mV (vs. SCE) is proportional to the concentration of acetaminophen over a 5-35 μM range with an associated detection limit (S/N = 3) of 0.37 μM and a sensitivity of 0.0296 μA/μM. The relative standard deviation (RSD) was 1.6% for eight replicate measurements. The modified electrode was used to determine the acetaminophen content in tablet samples.     

银-亮氨酸复合膜修饰电极测定去甲肾上腺素

利用循环伏安法(CV)制备了银和亮氨酸聚合物膜修饰电极(Ag-Leu/GCE)。研究表明,电极对NE的电化学氧化具有明显的催化作用。氧化峰电流与去甲肾上腺素浓度在2.4×10-7~4.8×10-5mol/L范围内呈线性关系,检出限(3S/N)为1.2×10-8mol/L。修饰电极用于药物中去甲肾上腺素的测定,加标回收率在97.5%~101.3%之间。     

A hydroxylamine electrochemical sensor based on electrodeposition of porous ZnO nanofilms onto carbon nanotubes films modified electrode

A novel route (electrodeposition) for the fabrication of porous ZnO nanofilms attached multi-walled carbon nanotubes (MWCNTs) modified glassy carbon electrodes (GCEs) was proposed. The morphological characterization of ZnO/MWCNT films was examined by scanning electron microscopy (SEM) and X-ray powder diffraction (XRD). The performances of the ZnO/MWCNTs/GCE were characterized with cyclic voltammetry (CV), Nyquist plot (EIS) and typical amperometric response (i-t). The potential utility of electrodes constructed was demonstrated by applying them to the analytical determination of hydroxylamine concentration. An optimized limit of detection of 0.12 μM was obtained at a signal-to-noise ratio of 3 and with a fast response time (within 3 s). Additionally, the ZnO/MWCNTs/GCE exhibited a wide linear range from 0.4 to 1.9 × 10⁴ μM and higher sensitivity. The ease of fabrication, high stability, and low cost of the modified electrode are the promising features of the proposed sensor.