Summer Research 2007:
Caesar Tin-U, & An Vo
Abstract:
We propose a new, electroless, facile and controllable preparation of the transitional metal hydroxide mixture and its thin film. The film, Co/Ni blended hydroxide, is prepared by dipping coating techniques. The resultant, Co/Ni hydroxide film, is very stable in alkaline solution as evidenced by its well-defined, repeated redox (reduction/oxidation) peaks that may associate with their II/III surface transition. This newly formed thin film demonstrated electrocatalysis towards various analytes. Most noteworthy, the film can sense monosaccharides in the presence of disaccharides or multi-saccharides. Our results show that the oxidation of glucose, fructose, ribose, sorbose, galactose, and xylose (all monosaccharides) can be facilitated or catalyzed by the thin film, while the catalytic current relies on the respective concentration to certain extent.
Equal concentration of disaccharides, including lactose, sucrose, and maltose, and polysaccharides, such as starch, have revealed insignificant oxidation responses on the prepared thin film. Electrocatalysis occurs at the site where the surface is reaching its fastest electrochemical redox transition (II/III). To expand our method into a broad application, we found a conventional pencil can serve as an electrode to bear all sensing features as above, which provides a more cost-effective and public-accessible sugar sensor. Consequently, our newly fabricated mixture film has the potential to be developed into a simple, non-enzymatic alternative for the current sugar assay.
Introduction:
Monosaccharides are an important carbohydrate in metabolism. The living cell uses it as a source of energy and metabolic immediately. The basic carbohydrate units are glucose, galactose, and fructose which are found in blood. In addition, glucose is more important because there is only glucose levels are regulated via insulin and glucagon. The detection of sugar, either total determination or selective determination, are an active subject in life science and fermentation industry. Furthermore, the hydroxides and oxyhydroxides of transition metals have been focused not only on health care but also on coating, corrosion, and battery industry. There are considerable interests of these compounds in sensor technology recently. Traditionally, the bulky and thin film material of these hydroxides can be prepared by diverse techniques including physical vapor deposition (i.e. sputtering), pyrolysis of chemical sprays, immersion processes, casting, and electrochemical deposition. Depending the preparation conditions, the morphology and microstructure of the thin film can be manipulated resulting in different properties achieved. In this experiment, dipping (or immersing) coating technique has been used.
Cobalt/nickel hydroxides are highly insoluble material in water. In this presentation, we developed a new, facile fabrication of thin Co/Ni hydroxide films that can be deposited onto all conventional substrates. Our approach is electroless, fast in making stable thin films that is easily manipulated for its attachment and thickness. In this work, we found our newly made cobalt hydroxide thin films show electrocatalytic capacity towards monosacccharides while other carbohydrates are present. Our approach therefore constitutes a new, selective, and non-enzymatic protocol for monosaccharides. In addition, the effects of different experimental parameter, such as enrichment time, enrichment potetial and different concentration ions of Co/Ni were controled and investigated.
Materials:
Pencils were obtained from Michael’s Store. Polishing power (alpha alumina 1 Micron), 1.0M CoCl2, 1.0M NiCl2, 1.0M NaOH, 50% (wt%) NaOH, glucose, fructose, ribose, sorbose, xylose, galactose, sucrose, lactose, maltose, refined can sugar, starch, and conventional chemicals were used.
Glassy carbon (GC) electrodes, Ag/AgCl reference electrode, platinum wire electrode, the electrochemical analyzer (model CHI 400A), the stirring rode, the stirring plate, the picoamp booster, the Faraday cage was from CH Instruments (Austin, TX).
Procedures:
• Preparation of the thin Co/Ni hydroxide films by using dipping coating technique
To prepare the Co/Ni hydroxide film coated electrode, a glassy carbon (GC) electrode was firstly put into 1.0 mL 1:1 mixed 1M Co/Ni chloride solution for 30 seconds, then was taken off and dipped into 1.0 mL 50% (wt%) NaOH for another 30 seconds. The GC was subsequently withdrew from its solutions and rinsed gently with slow distilled water (or deionized water). Finally, the film coated GC electrode was placed into 1M NaOH for characterization and electrocatalysis.
• Voltammetry and electrocatalysis
In an electrochemical cell containing 10.0 mL of 1.0 M NaOH, aliquot of sugar stock solution or other analytes were added and mixed by using magnetic stirring. A three-electrode setup (including a bare or modified GC working electrode, a platinum counter electrode, and a Ag/AgCl reference electrode) was introduced into the cell for electrochemical study. The main techniques used in this study were cyclic voltammetry (CV) and chronoamperometry (CA). For the control purpose, a blank electrolyte that only contained plain 1.0 M NaOH was used for comparison. When using CA for hydrodynamic voltammetric experiment, a Faraday cage with magnetic stirring was utilized to promote mass transfer and to minimize the environmental noise. The CV was done from -1.0 ~ +0.6 V (vs. Ag/AgCl) range and the CA was carried out at fixed potential (+0.35 ~ + 0.6 V).
Results and Discussion:
Cobalt and nickel are transitional metals with variety of applications. Cobalt hydroxide and nickel hydroxide are well-known insoluble compounds with Ks.p. ~ 10-15. In this work we explore this insolubility for surface attachment and modification. Our new approach involves the in situ precipitation of Co/Ni hydroxide by a simple “dipping” protocol (see scheme 1). Fig.1 illustrates the resultant Co/Ni(OH)2 film (the red line) and the glassy carbon (GC) electrode (the blue line) in 1.0 M NaOH solution. As can be seen, the film modified electrode exhibits a well-defined, reversible redox peaks at ~ +0.3 V. Since the bare GC does not generate appreciable voltammetric signal in the NaOH media (the blue line), this pair of reversible peaks correspond to the reduction/oxidation of the deposited Co/Ni(OH)2, etc. The Co (II/III) and Ni (II/III) moieties within the insoluble hydroxide film network (see scheme 2). The Co/Ni(OH)2 film appears grey, and is very stable in alkaline media (500 voltammetric cycles result in only 5% activity loss). The voltammetry of the film under different voltammetric scan rates were investigated and its results were listed in Fig. 2. The voltammetric currents of both anodic peak and cathodic peak were found directly proportional to the scan rate (ip= kυ, plot not show) in the 0.05 – 0.30 V/s range while the peak potentials (Ep.a and Ep.a) have negligible shift, which are the typical features of a surface-confined process. Fig. 2 therefore further proves the strong adsorption of cobalt hydroxide and nickel hydroxide, probably due to their very low K s.p. values, or even through some synergic/co-crystalline precipitation.
The Co/Ni(OH)2 film has shown electrocatalysis towards sugar molecules, especially to monosaccharides, as evidenced in Fig. 3. As an example, 5 mM fructose does not show any response on the bare GC electrode (the black line) in the potential range examined. When using the film modified electrode, the anodic current is significantly enhanced at potentials ranging +0.3 ~ +0.6 V. Other carbohydrates of low molecular masses, such as glucose, xylose, and ribose, all exhibit similar behavior as fructose, that is, their enhanced oxidation current superimpose the anodic peak of the film itself. Considering the Co (II/III)/Ni (II/III) redox active center and this superimposition of the anodic current, we can conclude that these sugar molecules are electrocatalytically oxidized at the thin film modified electrode. Further, these electrocatalysis show features of the fast surface kinetics, leaving the bulky concentration gradient as the limiting step for the solution mass transport. Accordingly, the oxidation current of sugar molecules displays a concentration dependence, as witnessed by the linear increase of anodic current at +0.4 V upon successive addition of sugars, Fig. 3.
Our mechanistic study showed both cobalt and nickel metal wires are electroactive in alkaline media (data not shown). However these metal electrodes are not stable and are subjected to corrosion in alkaline media. Our results further revealed both cobalt hydroxide and nickel hydroxide exhibit redox (reduction/oxidation) activity when deposited onto the GC electrode separately (data not shown). Nevertheless, their individual sensing towards sugars is unstable and is more subjected to electrode fouling by the oxidized product. As a comparison, our current mixed Co/Ni(OH)2 film demonstrates a much more reversible, stable, and significant enhancement in the sugar electrocatalytic oxidation. This may be due to the formation of more surface-attachable film from a co-deposited Co/Ni hydroxide with lower K s.p than individual components. The mixed, defective Co (II/III)/Ni (II/III) network may have accounted for the much enhanced electrocatalysis through a synergic promotion within their co-crystalline structure. A possible mechanism for this electrocatalysis is proposed in Scheme 3.
The electrocatalysis of sugars on the mixed Co/Ni hydroxide film modified electrode has potential for the sensor application. Fig. 4 substantiates this sensing prospect by showing a near-linear responding pattern when using chronoamperometry. By using this method, the linear dynamic range for a prospective sugar sensor based on the Co/Ni hydroxide film is 0.0 – 10.0 mM in alkaline media. The film also demonstrates selectivity toward monosaccharides against other carbohydrate compounds with higher molecular masses. For this monosaccharide preference, Fig. 5 displays the response toward fructose (a monosaccharide), fructose mixed with maltose (a disaccharide), and fructose mixed with starch (a polysaccharide), respectively. As can be seen from Fig. 5, incorporation of disaccharide or polysaccharide do not interfere with the fructose sensing. To expand our sensing study into a broader application basis, we found conventional pencils also exhibit all of the preparation and electrocatalysis as above. As can be seen from Fig. 6, the film attached pencil gives response to ribose. Therefore, the monosaccharide sensing by the Co/Ni hydroxide film sheds a new, selective, direct, and non-enzymatic approach for the sugar assay. This new assay will be more cost-effective and public-accessible when combining with conventional material such as pencil.
Conclusion:
A new, facile preparation of thin cobalt/ nickel hydroxide films was introduced. The resultant film is stable and electroactive in alkaline solution. The film exhibits electrocatalytic capacities towards sugars, specifically, towards monosaccharide carbohydrates. The electrocatalysis originates from the fast surface Co (II/III)/Ni (II/III) process and its catalysis in the sugar oxidation. Pencils can be used in these study and application. Practically, these responses constitute a new base for the selective, direct, and non-enzymatic sensing protocol for monosaccharide sugars.
References:
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Acknowledgments:
The authors are grateful to Department of Natural Sciences and Scholars Academy at UHD through the scholarship (A.V.) sponsored by U.S. Army Research Office (Award No. W911NF-04-1-0024). This research was also supported by the UHD ORC grant, the Welch Foundation (No. BJ-0027), and the Undergraduate Analytical Research Program from SACP ( Pittsburgh, PA ). The assistance by SACNAS to present this work is greatly appreciated.
