Secondary Ag/AgCl Pseudo-Reference Electrode on Silicon Substrate F. L. Almeida a, M. B. A. Fontes b, C. Jimenez c and I. Burdallo c a Laboratório de Sistema Integráveis LSI, University of São Paulo, São Paulo, Brazil. b Faculdade de Tecnologia de São Paulo FATEC-SP/CEETEPS, São Paulo, Brazil. c Centro Nacional de Microeletronica CNM/IMB, Barcelona, Spain. Amperometric analysis systems with two or three require a potential of reference electrode stable in function of time. In this work we developed an Ag/AgCl pseudo-reference onto gold film on silicon substrate. The amperometric electrodeposition method utilized (constant potential) provided a uniform deposited film and a controlled process characterization. It allows us to obtain electrodes with excellent performance during fourteen consecutives day. The characterization in function of the chloride concentration and time were also presented. Keywords: pseudo-reference, secondary electrode, electrochemical analysis Introduction In electrochemical analysis systems with two or three electrodes, it is necessary to assert a stable potential of reference electrode in function of the time. Reference electrodes are well-known for provide a trustful measuring of working electrode potential, enabling to apply a potential difference on working electrode, with accuracy because it presents a fixed, known, solution-independent potential (1). The role of the reference electrode is, therefore, to provide a precise potential which will assure an exact measuring of specimens to be oxidized or reduced in the analysis solution (2). Mercury-mercurous chloride sat KCl (SCE), mercury-mercurous chloride 1 mol L -1 KCl (Calomel), mercury-mercurous sulphate, mercury-mercuric oxide and silver-silver chloride are secondary reference electrodes more communes found in the literature hydrogen reference electrode is referred as primary reference electrode (3). Silver-silver chloride are widely used due to great advantages: robust and stable, easy construction and low contamination, suitable for medical applications. Pseudo-reference Ag/AgCl electrodes consist in the direct contact of chloride silver layer within analysis solution. In this article we presented the fundaments, fabrication, characterization and application of the secondary pseudo-reference electrodes.
General specifications about reference electrodes Primary and Secondary Reference Electrodes It is very important to observe that thermodynamic equilibrium studies were based on primary reference electrode developed for hydrogen redox reaction (reaction a), where hydrogen gas was bubbled on a platinum electrode at temperature of 25 ºC, unitary ionic activity and atmospheric pressure. 2H + + 2e - H 2 E 0 = 0 V [reaction a] This reaction was attributed as zero electrode potential and the potentials of subsequent reactions are considered as secondary reference electrodes, which potentials are related to the primary reference electrode. Reference Electrodes: pseudo and non-pseudo Pseudo-reference Ag/AgCl electrodes consist in the direct contact of chloride silver layer within analysis solution. This fabrication method presents the facility of silver chloride electrochemical deposition directly on silver deposited onto gold layer. On the other hand non-pseudo reference electrodes have a porous membrane that makes ionic contact with the electrolyte of the inner solution or polymer doped. Liquid-junction and Solid-state Reference Electrode The liquid-junction reference electrodes consist, generally, in a silver wire where silver chloride is electrodeposited to obtain Ag/AgCl junction. This wire is inserted in a pipe filled with electrolyte solution inside and with a porous membrane in extremity (it permits ionic contact and keeps stable the silver chloride concentration in the inner solution). The solid-state reference electrodes type Ag/AgCl are based on the same principle liquid-junction, changing only the inner solution for a material with solid characteristics and redox capacity, normally, a polymer doped with ionic salts as NaCl, KCl or KNO 3 (little stable). Surface cleaning of the electrode Fabrication of the electrodes The first fabrication step is the silver layer electrodeposition on top of the underneath gold. Before initiating the electrodeposition process, it is necessary to clean the electrode surfaces to eliminate all adhered particles (organics, ionics, oxides, hydroxides and metals). It is well-known that the best functioning of reference electrode is achieved when the electrochemical deposition characteristics of silver/silver chloride layer are quite controlled (4). The cleaning process efficiency is crucial to obtain the desired electrodeposited silver characteristics (with adhesion, hardness, homogeneity and stability) and consequently a better control of silver chloride electrochemical deposition which results in a reference electrode with excellent performance (4).
The following sequence was used for surface cleaning: I) physical cleaning, II) chemical cleaning and III) electrochemical cleaning. I) Physical cleaning: mechanic polishing with brush impregnated in 0.05 µm gel alumina removed crude material. It was realized thirty 8-shape movements. a) b) Figure 01: Influence of the brush clean electrodes: in dark field; a) more abrasive brush and b) more soft brush (make-up case). There were ridges formations during physical cleaning process with brush impregnated in gel alumina. With soft brush it was observed less ridges and minor particles. II) Chemical cleaning: we realized bath in alcohol and acetone to remove organic composts (grazes, oleos and others) stirred at 400 rpm. The alcohol eliminated particles and also permitted to remove alumina residues. In this process we utilized 1.0 mol L -1 HCl to clean the metal residues. III) Electrochemical cleaning: it is very important to eliminate materials adsorbed in the gold electrode surface and increase roughness providing more adherences at layer deposited. This occurs probably for oxidation (corrosion) of gold thin layer at potentials up 0.8 V: Using an electrochemical workstation type III µ-autolab / Ecochemie, we performed a cyclic voltammetry technique (CV), sweeping ninety consecutives cycles of the potential applied to the electrode from 0 V to +0.9 V at 0.3 V s -1 scanning rate in standard solution of 0.1 mol L -1 H 2 SO 4. We observed after each cycle that the redox current increases, but it is necessary to control this process to prevent the electrode damage. Silver and silver chloride deposition The silver chloride and silver electrodeposition were realized using an TB amperiometric technique (Single Potential Time Base) and a system with three electrodes: working where is applied the constant potential for redox reaction in time function; reference stable potential provider; and counter to close electric circuit in the solution and drain the current. In the fabrication of reference electrodes, the silver deposition process is important because define the good characteristics of the silver chloride layer which is directly
related to the cleaning process. The thickness of deposited layer can be obtained through equation 02. Q. M th = [02] q. N. d. A 0 This equation is the relation between multiplication of the electric charge (Q), that is necessary to deposit a thickness determined (th), for the molecular mass (M) in function of some constants values. The electric charge (q = 1.602x10-19 C), the Avogadro number (6.02x10 23 mol -1 ), structural factors (the reference electrode area utilized of 4x10-2 cm 2 ) and relative factors of the specimen to deposit, as molecular mass (M Ag = 107.87 g; M AgCl = 143.32 g mol -1 ) and density (d Ag = 10.5 g cm -3 ; d AgCl = 5.56 g cm -3 ). We calculated the amount of charge to obtain 20 µm of silver and 5 µm of silver chloride. The relation of 1/4 between silver and chloride thickness indicates the best performance of the reference electrode, considering to deposits silver chloride is a precise consumer 1/3 of silver thickness (5). Silver electrodeposition on gold in silicon substrate: in this work we developed a procedure for electrochemical deposition of silver by using a silver commercial solution (Silvrex S Enthone). Following the chemical reaction (reaction b), we can observed that one electron is necessary to deposit each silver atom. 1Ag ( aq) + 1e 1Ag 0 ( S ) + E 0 = +0.8 V [reaction b] By LSV (Linear Sweep Voltammetry) technique we realized one cycle with potential from 0 V to +1.3 V at 0.3 V s -1 in silver commercial solution. The target potential is the one which provides a current density between 0.4 to 0.5 ma/cm 2. Our best results pointed to 0.5 ma/cm 2 density. In following graphic it is shown the electrodeposition potential and correspondent current (figure 02). 0.1 2x10-5 A 0.0-0.1-0.2 Current (ma) -0.3-0.4-0.5-0.6-0.7-0.8-1.2-1.0-0.8-0.6-0.4-0.2 0.0-936 mv Potential (V) Figure 02: Calculation of the electrodeposition potential of silver for LSV technique.
The electrodeposition by TB (Single Potential Time Base) of silver using the silver commercial solution (Silvrex S Enthone) on gold layer, stirring at 400 rpm and potential in order of -0.94 V by around of 60 min, permits to obtain 20 µm of silver. Silver chloride electrochemical deposition: similarly silver electrodeposition, the silver chloride electrodeposition (reaction c) needs one silver atom for each chloride atom deposited. Ag 0 ( S ) + Cl ( aq.) AgCl( S + e ) [reaction c] Also by LSV (Linear Sweep Voltammetry) technique, we did one cycle with potential from 0 V to +0.3 V at 0.3 V s -1 in 0.1 mol L -1 HCl. We observed the best results for the same 0.5 ma/cm 2 density. The graphic that follows indicates potential applied and current measured for electrochemical deposition of chloride (figure 03). 2.5 2.0 Current (ma) 1.5 1.0 0.5 2x10-5 A 0.0 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.3 52 mv Potential (V) Figure 03: Calculation of the electrodeposition potential of chloride for LSV technique. Similarly the silver electrodeposition by TB, silver chloride over the silver surface was made in 0.1 mol L -1 HCl solution (6), stirring at 400 rpm, and potential in order of 50 mv by around of 27 min. This procedure permits to obtain 5 µm of silver chloride. Reversibility test Reference Electrode Characterization A reference electrode is considered with good characteristics when the applied redox potential is the same for oxidation and reduction current. Any variation of current indicates alteration in the reference potential. The curve recorded (figure 04) shows linear behavior of current versus potential and any discrepancy occur due to hysterisis effect which is an indicative of irreversibility.
After the silver chloride electrodeposition, it is necessary the stabilization of the reference electrode. We swept fifty consecutives cycles for the potential from 0.3 V to +0.3 V at 0.3 V s -1 in a commercial solution containing AgCl. It was followed by six consecutives cycles for the reversibility characterization from 0.5 V to +0.5 V at 0.1 V s -1 in the same solution. It is expected a redox response at the same potential, therefore producing a linear relationship. 2 1 Current (µa) 0-1 -2-0.6-0.4-0.2 0.0 0.2 0.4 0.6 Potential (V) Figure 04: Example of the standard reversibility test. It is necessary to characterize the reference electrode reversibility to guarantee a stable reference potential (03). This stability is related to the adhesion of the electrodeposited silver chloride film. Through of the relation between fabricating electrode and (double junction Orion) Ag/AgCl commercial reference electrode curt circuit with counter electrode, we expected a linear response (03) of current oxidation and reduction regarded to the potential applied. This result indicates the best uniformity deposition and a stable potential in time. Stability in function of time We determined electrodes stability in function of time (figure 05), utilizing the same configuration of the reversibility test. It was measured the electrode potential in relation to commercial reference electrode in commercial inner solution of Orion Reference Electrode. The test was initialized with potential in order of 29 mv and before of fourteen days was verified a potential in order 35 mv. The relation between reference electrode potential and test days permitted to calculate the degradation slope.
-20-22 -24-26 Potential (mv) -28-30 -32-34 -36-38 -40 0 2 5 7 9 12 14 16 Time (days) Figure 05: Function of time test in commercial solution. The graphic indicates the potential performance of fabricated reference electrode during fourteen days, with a degradation of 18 µv h -1. We observed that the stability is directly influenced for the temperature, for 5 to 7 and 12 to 14 days the signal is more stable due the constant temperature in this period. Stability in function of the silver chloride concentration Thermodynamic equilibrium can be demonstrated using the Nernst equation [03]. In practice, the reference electrode of silver chloride is selective at chloride concentration (07). The electrodeposition reaction following indicates the formation of silver chloride. AgCl (s) + e - Ag(s) + Cl - E 0 = +0.222 V [reaction d] The equilibrium potential associated with this chemical reaction is related to ions activity in solution, Nernst equation: where, E = equilibrium potential E 0 = potential in standard conditions R = universal gas constant, 8.314 J K -1 mol -1 T = absolute temperature n = electrons number of reaction F = Faraday constant, 9.648E4 C mol -1 a cl - = chloride ions activity in solution a = γ.[cl - ] E = E 0 - RT ln a [03] cl nf For temperature equal to 25ºC, electrode potential is:
E = +0.222-0.0592 log a - Cl = +0.222 + 0.0592pCl - [04] n Keeping constant the ion concentration in solution, the temperature and ph provided a constant potential electrode. This is the basic fundament in reference electrodes, but in pseudo-reference electrode, it is necessary to control the solution concentration and chloride surface characteristic to obtain a reference stable with low drift. Below is shown the study of chloride concentration influence, with addition of chloride (figure 06). 0.25 0.18 Potential (V) 0.20 0.15 0.10 0.05 Vout (V) 0.16 0.14 0.12 0.10 0.08 0.06 0.04 f08 f09 f10 f11 f13 0.00 0 500 1000 1500 2000 Time (s) Figure 06: response at ions of chloride concentration in solution. To each addition there is an increase in electrode response; lack of linearity (59 mv/pacl - ) indicates degradation of reference electrode, consequently, low quality of Ag/AgCl interface. Table 01: show the reply for chloride concentration for various electrodes. Unity Origen Ordinate Slope Linearity [V] [V/paCl - ] F08-0.043 0.058 0.999 F09-0.044 0.059 0.999 F10-0.042 0.059 0.999 F11-0.049 0.049 0.998 F13-0.044 0.059 0.999 The results indicate in table 01 show a good quality of silver chloride layer deposited with slope equal to 59 mv/pacl - and linearity next at one has the best performance of reference electrode. We obtained excellent results obeying the deposition relation of 1/4 in thickness. The preservation influence of electrodes in solution with AgCl 1.0 1.5 2.0 2.5 3.0 3.5 4.0 pa - cl In our study we observe a compacting in surface structure of silver chloride after keeping one day in (Orion) commercial inner solution containing AgCl. This procedure provided a reduction of electrodeposition imperfections, with the chloride film becoming more homogeneous and adhered. 0.02
a) b) c) Figure 07: show the preservation effect in solution with AgCl picture microscopy increase 5x. a) clear field, b) and c) dark field without and with preservation, respectively. The figure 07c indicates the best result for silver chloride layer after preservation that to guarantee a reference electrode more stable and with minor drift. Application of fabricated reference electrode Experimental analysis We used an amperometric system with three electrodes (fabricated reference, platinum working and counter electrodes) to test the functionality of fabricated reference electrode. We realized five additions of 0.25 µmol L -1 potassium ferricyanide in 0.1 mol L -1 KNO 3 to obtain the linearity curve at 0.75 V potential. Linear coefficient next to one demonstrate good performance of reference electrode. 1.0 80 Current (µa) 0.8 0.6 0.4 0.25 µm addition of ferricyanide Current (µa) 70 60 50 40 R SD --------------------- 0.9997 0.5908 --------------------- 0.2 30 20 Experimental Fit Linear 0.0 0 1000 2000 3000 4000 5000 6000 Time (s) 10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Concentration (µm) Figure 08: a) experimental curve with five additions of 0.25 µmol L -1 potassium ferricyanide in 0.1 mol L -1 KNO 3 and b) current by concentration curve with respective standard deviation (SD) and linear coefficient (R). Conclusion The specification, fabrication, characterization and application were performed for secondary pseudo-reference electrode using a gold film deposited on a silicon substrate. The fabrication parameters are important for silver electrodeposition and to control process for electrochemical deposition of silver chloride. The cleaning process occurred in three sequences: I) physical cleaning, II) chemical cleaning and III) electrochemical cleaning). The verification of current density by LSV
(Linear Sweep Voltammetry) technique and the electrodeposition by TB amperiometric technique (Single Potential Time Base) guaranteed layers of silver and silver chloride with better adhesion, hardness, homogeneity and stability and consequently good performance of fabricated electrode. We used three characterization methods to test the pseudo-references: (i) Ag/AgCl reversibility, (ii) chloride concentration in solution and (iii) time function. We verified stability acceptable of fabricated electrodes which was confirmed for the response at measuring of ferricyanide potassium using an amperometric system. The presented results with secondary pseudo-reference electrode on silicon substrate is very interesting for application in clinic analysis. Measuring small interval of time showed efficient, low cost, minor decline (18 µv h -1 ) and no there is no necessity of place electrolyte layer and porous membrane. On the other hand it also provides the solid-state reference electrode that becomes possible to create an entire reference electrode on silicon substrate. Acknowledgments We acknowledge to CNM-IMB Microelectronic National Centre - Microelectronic Institute of Barcelona for the use of its infrastructure and for the group work. The funding of F.A. from the CYTED project 506PI0292 is also acknowledged. References 1. Ives & Janz, Reference Electrodes. Theory and Practice, New York Academic Press, 1961. 2. Cardoso, J. L., Desenvolvimento de um instrumento virtual aplicado a um potenciostato para detecção eletroquímica, Trabalho de Graduação, FATEC-SP, 2003. 3. Horwood, E., Instrumental methods in Electrochemistry, Southampton Electrochemistry Group, University of Southampton, series in physical chemistry, pp. 360. 4. Jahir, Informe caracterización de microelectrodos de 4 barras de platino por CV, CNM. 5. Janz, G. J.; Silver-Silver Halide Electrodes, Reference Electrodes : Theory and Practice, chap 4, Edited by David J.G. Ives and George J. Janz, Academic Press, 1961. 6. Almeida, F. L.; Montagem e Caracterização de sensores eletroquímicos em cateteres, Trabalho de Graduação, FATEC-SP, 2006. 7. Eric Bakker, Hydrophobic membranes as liquid Junction-Free Reference Electrodes, Electroanalysis 1999, 11, No. 10-11