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Voltammetric methods and electrodes. Electroanalytical methods Interfacial methods Bulk methods Static methods I ~ 0 Dinamic methods I # 0 Conductometry.

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Apresentação em tema: "Voltammetric methods and electrodes. Electroanalytical methods Interfacial methods Bulk methods Static methods I ~ 0 Dinamic methods I # 0 Conductometry."— Transcrição da apresentação:

1 Voltammetric methods and electrodes

2 Electroanalytical methods Interfacial methods Bulk methods Static methods I ~ 0 Dinamic methods I # 0 Conductometry (G = 1/R) Conductometric Titrations ( volume ) Potentiometry (E) Potentiometric titrations ( volume ) Constant current Coulometric Titrations (Q = It) Electrogravimetry (wt) Controlled potential Electrogravimetry (wt) Amperometric titrations ( volume ) Voltammetry [ I = f (E) ] Const. electrode potential coulometry (Q = 0 1 idt Introduction

3 Five Important interrelated concepts to understand electrochemistry: (1) the electrodes potential determines the analytes form at the electrodes surface; (2) the concentration of analyte at the electrodes surface may not be the same as its concentration in bulk solution; (3) in addition to an oxidation–reduction reaction, the analyte may participate in other reactions; (4) current is a measure of the rate of the analytes oxidation or reduction; and (5) we cannot simultaneously control current and potential. L. Faulkner, Understanding electrochemistry: some distinctive concepts, J. Chem. Educ., 60, 262 (1983) ; P. T. Kissinger, A. W. Bott, Electrochemistry for the Non- Electrochemist, Current Separations, 20, 51 (2002)

4 1) The Electrodes Potential Determines the Analytes Form Fig. Redox ladder diagram for Fe 3+ /Fe 2+ and for Sn 4+ / Sn 2+ redox couples. The areas in blue show the potential range where the oxidized forms are the predominate species; the reduced forms are the predominate species in the areas shown in pink. Note that a more positive potential favors the oxidized forms. At a potential of V (green arrow) Fe 3+ reduces to Fe 2+, but Sn 4+ remains unchanged.

5 2) Interfacial Concentrations May Not Equal Bulk Concentrations Fig. Concentration of Fe 3+ as a function of distance from the electrodes surface at (a) E = V and (b) E = V. The electrode is shown in gray and the solution in blue.

6 Nernst Equation E = E o /n + log [ox]/[red] Fe 3+ + e Fe 2+ E = E o /1 + log [Fe 3+ ]/[Fe 2+ ] 3) The Analyte May Participate in Other Reactions Fe 3+ + OH - FeOH 2+

7 4) We Cannot Simultaneously Control Both Current and Potential 5) Controlling and Measuring Current and Potential

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13 Controlled Potential Methods (Voltammetry)

14 Fig. Flow patterns and regions of interest near the work electrode in hydrodynamic voltammetry

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16 Controlled Potential Methods (Voltammetry) O + neR (1) E = E o` /n + log C s O / C s R (2) E = potential applied to electrode (mV) E o` = formal reduction potential of the couple vs E ref n = number of electrons in reaction (1) C s O = surface concentration of species O C s R = surface concentration of species R where

17 Table. Relationship of E to surface concentrations # E, mVC s O / C s R 23610,000/ ,000/ / /1 0 1/ / / /1, /10,000 For a reversible system, n = 1, E o` = V

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19 The current at an electrode is related to the flux (rate of mass transfer) of material to the electrode Considering x = and C = C O – C s O Where = Nernst diffusion layer (3)

20 4 (4) (5) Where i lc is limiting cathodic current and C s O is zero

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23 Cyclic Voltammetry

24 Fe III (CN) e Fe II (CN) 6 4- (1) Fe II (CN) 6 4- Fe III (CN) 6 3- (2)

25 E = E o` /1 log [Fe III (CN) 6 3- ] / [Fe II (CN) 6 4- ] (3) E o` = E 1/2 = (E pa + E pa )/ 2 (4) Nernst Equation for a reversible system E p = E pa – E pc = 0.059/ n (5) The peak current for a reversible system is described by Randles- Sevcic Equation for the forward sweep for the first cicle: i p = n 2/3 A D 1/2 1/2 C (6) Where: i p = peak current (A); n = number of electrons; A = electrode area (cm 2 ); D = diffusion coefficient (cm 2 / s); = scan rate (V /s) and C = concentration (mol / cm 3 )

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27 Cronoamperometria

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30 Figura (A) Representação esquemática da aplicação de potencial em voltametria de pulso diferencial. A corrente é amostrada em S 1 e S 2 e a diferença entre elas é registrada; (B) Voltamograma de pulso diferencial. SCHOLZ, F., ed (2005). Electroanalytical methods. New York: Springer.

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32 Figura (1) Forma de aplicação de potencial na voltametria de onda quadrada; (2) Voltamogramas de onda quadrada esquemáticos para um sistema reversível (A) e para um sistema totalmente irreversível (B). SOUZA, D.; MACHADO, S. A. S.; AVACA, L. A. Química Nova, Vol. 26, 81-89, LOVRIC, M.; KOMORSKY- LOVRIC, S.; MIRCESKI, V. Square Wave Voltammetry. ed. (2007), Berlin: Springer.

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39 Eletrodo de diamante dopado com boro 8000 ppm; 0,72 cm 2 Glassy carbon electrode Eletrodos de carbono vítreo da Tokai Carbon Co

40 M H 2 SO 4 1M NaOH 1M H 2 SO 4 1M KCl 1M NaOH Pt Hg 1M HClO 4 0.1M KCl C 0.5 M H 2 SO 4 BDD Approximate potential ranges for platinum, mercury, carbon and boro-doped diamond electrodes

41 Glassy carbon electrode application

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46 Aplicação de EQM em sistema FIA Eletrodo base: Eletrodo de carbono vítreo Preparação do Eletrodo: Ciclagem de potencial entre -0,2 e 0,6 V (vs. Ag/Cl) em solução de 1,0 mmol L -1 FeCl 3.6H 2 O e 10 mmol L -1 de K 3 [Fe(CN) 6 ] Funcionamento [Fe(CN) 6 ] 4- [Fe(CN) 6 ] 3- + e - [Fe(CN) 6 ] 3- + AA [Fe(CN) 6 ] 4- Comportamento voltamétrico do sistema

47 Aplicação de EQM em sistema FIA

48 Anodic stripping voltammetric determination of copper(II) using a functionalized carbon nanotubes paste electrode modified with crosslinked chitosan Janegitz, B.C., Marcolino-Junior, L.H., Campana-Filho, S.P.,Faria, R.C., Fatibello-Filho,O. Sensors and Actuators B, 142, 260 (2009)

49 49 Comparison: with and without carbon nanotube functionalization Anodic stripping voltammetry 80 % CNTs (w/w) + 20 % nujol (w/w) -0.2V for 270 s 25 mV s -1 Figure XX - Linear voltammograms obtained with electrodes containing functionalized nanotubes not (A) and functionalized (B), in 0.1 mol L -1 NaNO 3 solution in the presence of Cu x mol L -1. Carbon Nanotubes Functionalized (A)(B)

50 50 Anodic stripping voltammetry -0.2V por 270 s 25 mV s -1 Figura XX -. Stripping voltammetry for EPN (A), EPNM-QTS (B), EPNM-QTS-GA (C) and EPNM-QTS-ECH (D) in 0.1 mol L -1 NaNO 3 solution in the presence of Cu x mol L -1., = 25mV s -1, a 25ºC. EPNM-QTS-ECH

51 Analytical Curve Figura XX - Voltammograms obtained for the construction EPNM-QTS-ECH with 15% (w/w) QTS-ECH in 0.05 mol L -1 NaNO 3 solution. Figura XX - Analytical curve: 7.93 x a 1.6 x mol L -1 L D =1.06 x10 -8 mol L -1, LQ= 7.93 x mol L -1 RSD= 3.12%

52 Concentration de Cu (II) ( mol L -1 ) Sample Method comparative* Method proposed Erro relativo % Urine samples 0.50 ± ± 0,09 4, ± ± Industrial Waste 3.5 ± ± ± ± Determination of Cu 2+

53 Voltammetric determination of ciprofibrate using a glassy carbon electrode modified with functionalized carbon nanotube within a poly (allylamine hydrochloride) film o The ciprofibrate is a fibrate and present Antilipemic effect (lipid lowering); o Fibrates are indicated for patients who, after tests confirmed that the increase in endogenous triglecirideos is due to poor nutrition; o A possible interest in determining the ciprofibrate addition to quality control of drugs; Figure XXX - Ciprofibrate molecular estructure.

54 Functionalization of MWCNTs in acid solution (H 2 SO 4 /HNO 3 3:1) Carbon nanotubes dispertion in PAH solution [dispertion]=1mg mL -1 Film formation on the elcetrode by casting technique (20 μL)

55 Figure XX - PAH SEM images (a) and (b); MWCNTs/PAH SEM image (c) and (d)

56 Linear equation: i= x 10 4 x C Concentration range: 13.3 to132 mol L -1 Detection Limit: 8.34 mol L -1 Figure XX - Analytical curve obteined for ciprofibrate determination in phosphate buffer solution 0,01 mol L -1 by VPD. = 12 mV s -1, A= 60 mV, t= 100 ms Analytical curve

57 SampleLabel value (mg) DPV method HPLC method RE c 1 A ± 398±5 2 B ± 4100±7 C ± 6100±4 D ± 6104±2-4 Table XX - Ciprofibrate determination in pharmaceuticals formulations using GCE- MWCNTs/PAH and standard method RE c 1 = 100 x (VPD value – Reference method value) / Reference method value

58 Inhame (Alocasia macrorhiza) Abacate (Persea americana) Batata inglesa (Solanum tuberosum) Frutas e vegetais empregados como fontes da PFO (POLIFENOL OXIDASE) e PER (PEROXIDADE) em bioreatores e biossensores. Abobrinha (Cucurbita pepo) Alcachofra (Cynara scolymus L.) Banana (Musa paradisiaca) Batata doce (Ipomoea batatas L. Lam.) Berinjela (Solanum melongena) Cara (Dioscorea bulbifera) Coco (Cocus nucifera L.) Jaca ( Artocarpus integrifolia L.) Mandioca (Manihot utilissima) Nabo (Brassica campestre ssp.) Pêssego (Prunus persica) Rabanete (Raphanus sativus)

59 Enzimas São proteínas que agem como catalisadores biológicos: enzima Composto A Composto B Centro ativo ou sítio catalítico Não há consumo ou modificação permanente da enzima

60 Emil Fisher, década 50 Daniel Kosland, 1970 Modelo chave-fechadura Modelo Encaixe induzido E e S se deformam quando em contato (alteração conformacional), para otimizar o encaixe

61 Biossensor para glicose - Radiometer®

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63 Fig. Esquema de um biossensor PFO

64 Low cost Portability Practicality 64 Screen-printed electrodes

65 screen-printed or silk-screen Technology the possibility of mass production Extremely low cost Simplicity Complete electrochemical system Work Count er Referen ce 65 Screen-printed electrodes Figura 5: Struture of screen printed electrodes

66 Substrates Plastic materials (Polyester) Ceramics Metals Work electrode Addition Deposition Metalic films Nanoparticles Carbon nanotubes Enzymes Polymers Complexation agents 66 Screen-printed electrodes

67 Carbon nanotubes Boron-doped diamond (BDD) Carbon glassy (CG) Metallic films etc Copper Gold Iridium Antimony Bismuth Etc. 67 New Materials

68 2002 Vytras et al. Pauliukaite et al. Carbon paste modified with Bi 2 O Wang et al. Bismuth film electrode (BiFE) electrodeposited in CG 68 Bismuth film

69 Good cathodic potential window Interference of dissolved oxygen is minimal Low toxicity Electrochemical behavior is similar to that of mercury 69 Bismuth film

70 70 MEV-FEG Figura 10: Micrographs of the BiFE A) 10000x B) 50000x A) B) Bismuth film electrode

71 Bismuth film electrode for anodic stripping SWV lead determination A B C (A): PalmSens and (B): DropSens potentiostats and (C) BiSPE preparation

72 72 Confecção do minissensor 120 °C durante 200 s FeCl 3 0,50 mol L -1 em meio de HCl 0,10 mol L -1 durante minutos.

73 electrode Bismuth film electrode tt-type connector for printers

74 Bismuth redox process Figura 7: Cyclic voltammogram for 0.02 mol L -1 Bi(NO 3 ) 3 in 0.10 mol L - 1 acetate buffer (pH 4,5) solution as electrolyte support; the work electrode is a platinum foil and scan rate of 10 mV s I Bi e - Bi V II Bi 3+ Bi 0 + 3e V Filme de bismuto V vs. Ag/AgCl (3.0 mol L -1 KCl) during 200 s 0.02 mol L -1 Bi(NO 3 ) 3, 1.0 mol L -1 HCl in 0.15 mol L -1 Sodium citrate.

75 Anodic stripping voltammograms of 9.9 x – 8.3 x lead (LD of 5.8 x M) in 0.1 M acetate buffer (pH 4.5), using square-wave mode. Deposition at V for 2 min; pulse amplitude of 28 mV; increment of potential of 3 mV and frequency of 15 Hz. Determination of lead

76 Bismuth film electrode (BiFE) for paraquat determination In 0.1 mol L -1 HAC pH 4,5, using differential pulse voltammetry. Figueiredo-Filho, L. C. et al, Electroanalysis, 22, 1260 (2010)

77 DPV para determinação de Paraquat Besides of paraquat can be determined simultaneously Cd 2+ e Pb 2+.

78 Determination of PQ in six natural water samples by BIFE and HMDE (reference). Samples * / µ mol L -1 HMDEBIFEER (%) A ± ± A ± ± A ± ± A ± ± A ± ± A ± ± * The SD (±) was calculated from three replicates.

79 Filme de bismuto -0,18 V vs. Ag/AgCl (KCl 3,0 mol L -1 ) durante 200 s Bi(NO 3 ) 3 0,02 mol L -1, HCl 1,00 mol L -1 e citrato de sódio 0,15 mol L -1 Cola de prata Bright Silver Epoxy (BSE) + Gray Silver Hardener (GSH). Após a aplicação na superfície de cobre esperou-se 24 horas para a cura da cola 79 Confecção do minissensor Figura- Etapas da confecção do minissensor

80 Atrazina (ATZ) (2-cloro-4-etilenodiamino-6- isopropilamino-s-triazina) Pertence a classe das triazinas Composto polar, fracamente básico de coloração branca Figura. Fórmula estrutural da Atrazina 80 Atrazina

81 Figura 11- Voltamograma obtido para uma solução de Atrazina 4,00 x mol L -1, utilizando tampão acetato 0,10 mol L -1, pH *4,5 em 15 % v/v de etanol como eletrólito suporte. * pH condicional 81 Comportamento eletroquímico da Atrazina (ATZ)


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