Development of membrane based electrodes for electroanalytical applications 5

We exploited the electrochemical deposition technique to fabricate Prussian blue nanotubes inside the membrane nanochannels.. The platinum-coated layers on both sides of the membrane ser

5 CHAPTER 5

Applications of modified-nanoporous alumina membrane on potassium sensing

5.1 INTRODUCTION

5.1.1 Potassium ion sensors

Potassium monitoring in serum, urine and saliva is very important in biomedical field Potassium monitoring is also important in environmental water and soil samples [1, 2] In some cases, potassium concentration gives the information related to the physical condition of patients; for example the heart often stops when potassium in human serum increases to the concentration higher than 9 mmol L-1 Due to the importance of potassium analysis in clinical laboratory, highly selective and highly sensitive potassium sensors are required Sodium and other ions with the size and properties similar to those of potassium ions often challenges measurement of potassium ions in physiological matrix Many techniques have been developed to overwhelm

this problem There are different techniques for analyzing potassium ion such

as spectrophotometry, chromatography [3], thermogravimetry, inductively coupled plasma-atomic emission spectrometry, flow injection analysis [4] and electroanalytical methods [5-9] Common electrochemical techniques are potentiometry, amperometry and cyclic voltammetry The potentiometric method which uses a metal hexacyanoferrate modified electrode as the ion-selective electrode possesses many attractive advantages Potassium sensors with silver hexacyanoferrate modified electrode measure potassium ions in concentration ranging from 8.0×10-5 to 1 mol L-1 [10] Other Prussian blue analogues such as nickel [11], cobalt [12], and iron hexacyanoferrate [5] were also employed These sensors have short response time and good sensitivity

Several reports on use of Prussian blue (PB) for detection of potassium ion include thin film of PB on transparent tin oxide substrate in which K+ ionsconcentration was determined by in-situ measurement of the absorbance changes of PB film under variable reductive potentials [6] Other PB based sensors include potentiometric methods which typically require electrochemical or chemical pre-conditioning steps prior to K+ sensing in order

to maintain a constant [PB]/[ES] ratio during measurements and have poor reproducibility and long analysis time [6] In contrast, cyclic voltammetry method offers significant advantages of minimal pre-conditioning steps and fast analysis because constant [PB]/[ES] ratio is achieved at the cyclic voltammetric peak potentials under appropriate conditions However, PB films subjected to potential cycling conditions exhibit poor stability [10, 13] It is expected that the highly stable PB nanotubes derived in this work will significantly improve the analytical performance of PB sensors using the cyclic voltammetry detection In addition, higher loading of PB nanotubes within the membrane electrode compared to conventional PB films can be achieved without compromising film thickness This will enhance the cyclic voltammetric peak currents which enable accurate determination of the PB redox potentials

5.1.2 Prussian blue

PB (Fe4(Fe(CN) )6 3·nH2O), a dark blue pigment, was accidentally

discovered in 1704 by the Berlin artist Diesbach PB has received much attention due to their chemical stability, electrochromic reactions, electro- and photocatalytic activity, easy preparation, and low cost The PB films have

been investigated intensively for use in electrochromic displays, fuel cells, solid-state rechargeable batteries [7, 8], and as signal-enhancing devices due to photovoltaic and photoelectrochemical effects Keggin and Miles was the first researchers who analyzed the crystalline structure of PB based on power diffraction pattern [14] The PB structure was then determined more precisely

by Ludi and co-workers [15] PB is a mixed valence coordination compound with two redox centers [6, 16] and exhibits an open, zeolitic structure with the iron centers arranged in a cubic lattice with cyanide bridging the iron centers [9] The zeolitic nature of PB is the cubic cell of 10.2 Å Alternating iron (II) and iron (III) ions locate on the face of the center cubic lattice The iron (II) ions are surrounded by carbon atoms while the iron (III) ions are surrounded

by nitrogen atoms (Fig 5.1)

Fig 5.1 Cubic structure of PB Adapted from reference[17]

5.2 Template-based nano material synthesis

In the template-based nano material synthesizes, the templates play role

as the framework Either around or within the template, the typical materials can be grown in situ and shaped into the nanostructure with the morphology dictated by the size and shape of the template scaffold The template syntheses are relatively simple, reasonably high-throughput, cost-effective procedure and therefore applicable and environmentally acceptable Many templates have been used in researches by generally they can be divided into “hard” templates and “soft” templates [18] The soft templates can be associated with gels, micelles, chitin scaffolds DNA strands, polymer matrices, and reverse micelles are also listed in the soft template Materials such as nanoporous alumina membranes, track-etched polycarbonate and other nanoporous membranes are considered as the hard templates Nanoporous alumina membranes are ideal hard templates owing to their high porous density, cylindrical pore structure, and thermal and chemical ability Nanoporous alumina membranes are used to transfer the nanopore arrangement to other materials An enormous variety of nanomaterials (listed below) have been successfully fabricated on the basis of nanoporous alumina membranes The

nano materials synthesized using nanoporous alumina membranes can be categorized into the following groups[19]:

• Metal nanodots, nanowire, nanorods and nanotubes

• Metal oxide nanodots, nanowire and nanotubes

• Semiconductor nanodots, nanowires, nanopillars and nanopore arrays

• Polymer, organis and inorganic nanowires and nanotubes

We exploited the electrochemical deposition technique to fabricate Prussian blue nanotubes inside the membrane nanochannels The platinum-coated layers on both sides of the membrane served as the working and counter/reference electrodes for both electrodeposition of PB nanotubes and sensing of K+ ions

5.2.1 Depositions of Prussian Blue nano structure

The PB can be synthesized by chemical and electrochemical methods

By the chemical methods, PB is formed by mixing ferric salt with hexacyanoferrous complex (Fe3++[Fe (CN) ]II 6 4-) or ferrous salt with hexacyanoferric complex (Fe2+ + [Fe (CN) ]III 6 3-); while the nano structure PB was developed layer-by-layer using the diffusion-based fabrication method by which nanochannel structure templates were repeatedly dipped into a sequent solutions of Fe3+,[Fe (CN) ]II 6 4-, and H2O to wash electrode [20] The electrokinetic method was also applied in growing the PB nantotube inside the nanoporous alumina membrane [21] In the electrochemical methods, deposition of PB is carried from the solutions contained a mixture of ferric

Fe3+ and ferricyanide [FeIII(CN) ]6 3- The electrochemical methods are either spontaneous applying an open-circuit regime or by a reductive electrochemical driving force The open-circuit deposition is highly depending on the electrode support The (FeIII [Fe (CN)III 6]) is oxidized by the conductive material which form the PB after one-electron reduction [22, 23] In the published reports, electrochemical growth of PB started from the metal base which coves the entire pore openings on one side of the nano-scale porous membrane template

In this work, we use membrane-based electrode which is porous platinum-coated membrane template with partially covered pore openings described previously in Chapters 2 and 3 In this way, electrochemical growth

of PB within the nanochannels starts from the narrow ring of metal coating at the pore edges, producing only PB tubes even at long electrodeposition time

5.3 EXPERIMENTAL

5.3.1 Reagents and materials

Potassium ferricyanide, ferric chloride, potassium chloride, sodium chloride and hydrochloric acid (37%) were purchased from (Sigma-Aldrich, Singapore) Tris buffer (1 M, pH 7) was obtained from 1st Base, Singapore All chemicals and solvents used were of analytical grade and used as received All solutions were prepared in Milli-Q ultrapure water (Millipore) 60 μm thick nanoporous alumina membranes with 200 nm nominal pore size were obtained from Whatman (Maidstone, Kent, UK) All membranes were washed with hydrogen peroxide (Scharlau) before use Subsequently, one side of the membrane was sputter-coated with ca 50 nm thick porous platinum layer using platinum target (99.99% purity) in JEOL Auto Fine Coater (JFC-1600)

5.3.2 Instrumentation

Electrochemical measurements were performed with a CHI400 electrochemical workstation (CH instruments) A three-electrode system was employed for the electrodeposition, characterization and sensing application, using the metal layer of the membrane electrode or PB nanotube-modified membrane electrode as working electrode, Ag/AgCl electrode as reference electrode and a platinum wire as the auxiliary electrode was employed for the electrodeposition, characterization and sensing application The preparation of working electrode is reported in the next section Scanning electron micrographs (SEM) and energy dispersive X-ray spectra (EDX) of the membrane electrodes were obtained using the FE-SEM JSM-6701F with an EDX analyzer Samples in SEM analyses must be electrically conductive In our experiment, the samples such as Prussian blue or bare alumina membrane are not conductive enough, therefore before SEM analyses they were coated with a thin layer of platinum deposited by sputtering method

5.3.3 Development of PB nanotube membrane electrode

Scheme 5.1 illustrates the preparation of PB nanotube membrane electrode Nanoporous alumina membranes were washed and pre-treated in 35 %

hydrogen peroxide; subsequently, one side of the membrane was sputter-coated with ca 50 nm thick porous platinum layer using platinum target (99.99% purity) in JEOL Auto Fine Coater The coated membrane then was modified by electrodeposition of PB nanotubes along the nanochannels of the membrane electrode The electrodeposition was carried out by repeated potential scan from -500 to 600 mV forth and back at the scan rate of 50 mV

s-1 in a solution containing ferric chloride, potassium ferricyanide, potassium chloride and hydrochloric acid

Scheme 5.1 Schematics of the construction of PB-nanotube modified

membrane electrode from nanoporous alumina membrane template (a) Sputter coating of alumina membrane template with ca 50 nm thick conductive porous platinum layer (b) Electrodeposition of Prussian blue nanotubes along the nanochannels starting from the porous metal layer

5.3.4 Voltammetry of PB nanotubes

The PB nanotube-modified membrane electrodes were investigated using

cyclic voltammety at scan potential in the range from -200 to 600 mV Supporting electrolyte was 1 M Tris pH 7 buffer containing potassium ions in varying concentrations In order to characterize only the PB nanotubes, PB film formed on the surface of the membrane electrode was removed by gentle polishing To demonstrate the application of PB nanotubes in electrochemical sensing of K+ ions, the mid-peak potential Emp was recorded during each

potential cycle Average Emp values from 3 repeated potential cycles were used

to derive the potential-K+ concentration relationship

5.4 RESULT AND DISCUSSION

5.4.1 Electrodeposition of PB nanotubes in membrane electrode

Fig 5.2 shows cyclic voltammograms of Pt-coated alumina membrane electrode in a solution containing 2.5 mM FeCl , 2.5 mM K Fe(CN)3 3 6, 0.1 M KCl, 0.1 M HCl at the scan rate of 50 mV s-1 After electrodeposition of PB, the dark grey electromembrane acquired the typical intense blue color of PB The electrodeposition reaction of PB is given in Eqn 5.1 [22]

In general, the electrodeposition of PB nanotubes within the porous template is a faster method in comparison to the layer-by-layer method [20] In particular, electrodeposition using a metal coated porous membrane template can form either nanowires/nanorods [22, 24] or nanotubes [25] depending on the amount of charge passed during electrodeposition Growth of PB occurs at the alumina wall first before extending towards the pore center, likely due to electrostatic attraction of negatively charged Fe(CN)63- for the positively charged alumina surface with oxygen vacancies [25]

Fig 5.2 Cyclic voltammograms of Pt-coated alumina membrane electrode in

a solution containing 2.5 mM FeCl , 2.5 mM K Fe(CN)3 3 6, 0.1 M KCl, 0.1 M HCl Scan rate 50 mV s-1

Fig 5.3 A and B show the scanning electron micrographs of the tilted surface view of the nanoporous membrane and the metal-coated membrane

Under the optimal condition, the metal layer partially covers the pore and reduces the nominal pore size from 200 nm to 150 nm Thus, a ring-shaped metal base of ca 25 nm ring thickness is available for the electrodeposition of

PB nanotubes Cross-section profile of the membrane electrode after 15 electrodeposition potential cycles reveals growth of PB nanotubes starting from the metal layer (Fig 5.3 D) SEM/EDX spectrum of the deposited PB given in Fig 5.3 D shows the presence of K and Fe In the control sample of the alumina template without PB, only Al and O were observed Removal of the metal layer and partial removal of the alumina template using aqua regia and phosphoric acid (49%) respectively reveals polycrystalline structure of PB nanotubes (Fig 5.3C) It is clear that several nanotubes coalesce into larger tubes, likely because of dissolution or mechanical defects during acid dissolution of metal and alumina layers

The difference between our PB-nanotube membrane electrode and those

in the published reports on PB nanotubes [25] is that our metal layer does not cover the entire pore opening, but provide a ring-shaped metal electrode of ca

25 nm ring thickness at the pore opening The electrochemical growth of PB started at the edge of the pore opening from this ring-shaped metal base and

continued along the nanochannel walls due to electrostatic attraction between the oppositely charged Fe(CN)63- and alumina wall surface [25] No PB nanorods/nanowires were formed; the PB nanotube-modified membrane electrode remains permeable to water

Fig 5.3 Scanning electron micrograph of nanoporous alumina membrane

template (A) before and (B) after sputtering of ~50 nm thick metal (Pt) layer (450 tilted surface views); (C) 450 tilted surface view of PB nanotubes after removal of the metal layer and partial dissolution of alumina template; (D) cross-section of a PB-nanotube modified membrane electrode showing the penetration of PB into the nanochannels; Inset: SEM/EDX spectrum of PB in

the nanochannels showing the presence of Fe and K; (E) surface view of PB nanotubes after dissolution of alumina template

5.4.2 Cyclic voltammetry of PB nanotubes