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A novel ferrocene encapsulated palladium-linked ormosil-based electrocatalytic dopamine biosensor

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A novel ferrocene encapsulated palladium-linked ormosil-based electrocatalytic dopamine biosensor
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  A Novel Ferrocene-Encapsulated Palladium-LinkedOrmosil-Based Electrocatalytic Biosensor. The Roleof the Reactive Functional Group  P. C. Pandey,* S. Upadhyay, Ida Tiwari,  and   Soma Sharma Department of Chemistry, Banaras Hindu University, Varanasi-221005, India; e-mail: p_c_pandey@satyam.net.inReceived: February 16, 2001Final version: April 10, 2001 Abstract A novel palladium-linked ormosil material with encapsulated ferrocene is reported along with its application in bioelectrocatalysis. The Pd-glycidoxypropyltrimethoxysilane is made by mixing an aqueous solution of palladium chloride and glycidoxypropyltrimethoxysilane. Thelinkage of palladium with glycidoxypropyltrimethoxysilane is confirmed by UV-vis, mass, and   13 C spectroscopy. It is suggested that Pd issandwiched between two molecules of glycidoxypropyltrimethoxysilane replacing oxygen. The new ormosil is made using Pd-linked silane precursor containing ferrocene monocarboxylic acid, trimethoxysilane and HCl. The formation of ormosil at two different temperatures(10 and 30  C) is also studied, with the result that the ormosil formed at 10  C does not show electrocatalysis of glucose oxidase whereas theormosil made at 30  C is found to be an efficient bioelectrocatalyst. The cyclic voltammetry results show peak separation of 57–59mVof encapsulated ferrocene made at 30  C and relatively large peak separation of the one made at 10  C. The performance, stability, and reproducibility of the new ormosil based glucose biosensor are discussed. Another important investigation in support of the above outcomeis reported showing the self-assembly of palladium on the reactive solid state ormosil surface. The reactive ormosil is developed using amixture of trimethoxysilane and 2-(3,4-epoxycyclohexyl) ethyltrimethoxysilane in acidic medium. Keywords:  Ferrocene, Palladium, Ormosil, Bioelectrocatalysis, Cyclic voltammetry 1. Introduction Several examples of the synthesis of sol-gel glasses [1–21]have become available during the last few years. One of the possible applications of such materials in the development of sensors is the attachment of the sensing material to the surface of  physicochemical transducers. The use of sol-gel glass for thedevelopment of electrochemical biosensors has received great attention associated with the coupling of biological componentsto electrochemical transducers. Apparently the synthesis of suitable biocompatible sol-gel glass of desired thickness and  porosity is of considerable interest.The encapsulation of a redox material within a sol-gel glasshas gained significant attention in sensor designing. Pankratovand Lev [14] also reported tetrathiafulvalene mediated carbonceramic electrode (CCEs) with limited storage-in-use stability.Several other articles on ferrocene encapsulated sol-gel glassesincluding those of Lev et al. [11–13] are available. Audebert et al.[15] reported modified electrodes from organic-inorganic hybrid gels containing ferrocene unit covalently bonded inside a silicanetwork and modified electrode from organic-inorganic hybrid gels formed by hydrolysis-polycondensation of some trimethox-ysilylferrocenes. Collinson et al. [17, 21] reported eletroactivityof redox probes encapsulated within sol-gel derived silicate film based on anionic, i.e.,  ½ Fe ð CN Þ 3  = 4  6   ;  ½ IrCl 2  = 3  6    and cationic,i.e., ferrocenemethanol [FeCH 2 OH 0 ] gel doped probes. Other articles on ferrocene based sol-gel sensors are also available[18]. There is great potential to study ferrocene encapsula-ted  = linked sol-gel glasses for mediated biosensor applications.We recently reported on glucose biosensors [9, 10, 20] based ona sol-gel matrix of controlled porosity followed by an enzymelayer and subsequently above another layer of sol-gel glass of controlled porosity using 3-aminopropyltriethoxysilane and 2-(3,4-epoxycyclohexyl)-ethyltrimethoxysilane as sol-gel glass precursors [9]. We studied both mediated and nonmediated response of glucose biosensor. The mediated response was based on the soluble ferrocene for the regeneration of the glucoseoxidase. However, co-immobilization of the mediator together with glucose oxidase does not permit the occurrence of medi-ated electrochemical reaction associated to restricted degree of translational motion which is essentially required in the devel-opment of mediated enzyme biosensors. Accordingly, immobi-lization of ferrocene derivatives within sol-gel glass with good electrochemistry is an attractive requirement. The present investigation is aimed to develop a novel biocompatible ormosilmaterial with encapsulated ferrocene with redox electrochemistrysimilar to that observed using ferrocene in solution.The use of palladium in metal-ceramic composites has gained attention. Burkhart et al. [22] have studied the reaction of glycidoxypropyltriethoxysilane with palladium chloride and suggest the opening of the epoxide ring of the glycidoxy moiety.In view of the reactivity of the glycidoxy moiety with palladiumchloride we intended to study the interaction of palladiumchloride with glycidoxypropyltrimethoxysilane and subsequentlyto use the reaction product for the encapsulation of ferrocenewithin ormosil matrix. The present investigation has beenundertaken on these lines. The new material was developed usingPd-linked silane precursor, trimethoxysilane, ferrocene mono-carboxylic acid and HCl. The linkage of palladium to one of the ormosil precursors (glycidoxypropyltrimethoxy silane) isconfirmed by NMR, Mass and UV-vis spectroscopy and theresults are reported.Although a chemical reaction in the solid state is sparse withinnormal phasic boundaries and of course even not known on thedesired solid surface within required technological systems infitting sites of commercial = technological = scientific interest. Thissrcinates the need of a material which is reactive in solid-stateunder the above-mentioned conditions for the scientific = technologist world. Further if the untreated material has better electromagnetic radiation transmission, the suitability of such 1519 Electroanalysis  2001 , 13, No. 18  # WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2001 1040-0397/01/1812–1519 $17.50 þ .50 = 0  material would be a boon for human welfare since optics havecontributed in depth for growing knowledge of mankind about the in fi nite universe. The present research output lights on suchworld class requirement. Here we describe a novel reactiveormosil which ful fi lls several of such requirements using amixture of trimethoxysilane and 2-(3,4-epoxycyclohexyl) ethyl-trimethoxysilane in acidic medium. The result on self-assemblyof Pd on reactive ormosil is reported based on SEM and image photography. 2. Experimental 2.1. Reagents Trimethoxysilane, ferrocene monocarboxylic acid and palla-dium chloride were obtained from Aldrich; 3-glycidoxypropyl-trimethoxysilane, 2-(3,4-epoxycyclohexyl) ethyltrimethoxysilanewere obtained from United Chemical Technologies, Inc., Petrachsilanes and silicones, Bristol, PA, USA. Glucose oxidase and dialysis membrane (cut off level 5000) was obtained from Sigma.The aqueous solutions were prepared in double distilled de-ionized water. All other chemicals employed were of analyticalgrade. Palladium chloride with palladium of molecular weight 106 was used during the present investigation.The electrode body used for the construction of ormosilmodi fi ed electrode and subsequently the electrocatalytic glucose biosensor was the same as described in earlier publications[19, 20]. 2.2. Construction of Ferrocene Encapsulated Pd-LinkedOrmosil The new material was prepared as follows: ferrocene mono-carboxylic acid (4mg) was dissolved in 3-glycidoxypropyl-trimethoxysilane, palladium chloride (1mg) was dissolved in500 m L distilled water. This solution was added into 3-glycid-oxypropyltrimethoxysilane solution containing ferrocene (70 m L)which resulted in a black solution. The resulting reaction product was mixed with trimethoxysilane (30 m L), and 0.1N HCl (5 m L).The resulting reaction product was stirred thoroughly up to 5minat 30  C and the desired amount of the homogeneous solutionranging between 40 and 70 m L was added to the well of thespecially designed electrode body. The gelation was allowed tooccur at 30  C for 30h. In order to study the effect of temperatureon the ormosil structure and subsequently the electrochemistry of encapsulated ferrocene, the gelation was allowed to form ormosilat 10  C and also at 30  C in a thermostatic cage keeping other conditions constant. A smooth very thin ferrocene encapsulated  palladium-linked ormosil on the Pt surface was evidenced byelectrochemical and SEM measurements. 2.3. Construction of Ferrocene Encapsulated Pd-LinkedOrmosil Based Electrocatalytic Glucose Biosensor Glucose oxidase 300units (35 m L) was poured on the surfaceof new ormosil and allowed to be adsorbed overnight at 4  C.A dialysis membrane was mounted over the adsorbed glucoseoxidase using an O-ring. The performance of the resultingglucose biosensor based on bioelectrocatalysis was studied. Insome cases the enzyme was also immobilized over ferroceneencapsulated ormosil within a polyvinyl alcohol matrix. 2.4. Construction of Reactive Ormosil The reactiveormosilwas developed from a mixture of ormosil ’ s precursors; trimethoxysilane (30 m L), 2-(3,4-epoxycyclohex-ylethyltrimethoxysilane) (70 m L), distilled water (300 m L) and HCl (5 m L). Desired amount of this ormosil ’ s precursors solutionwas added on a suitable solid surface, e.g., on theglass slide, or onindium tin oxide (ITO) electrode. The precursors ’  solution wasallowed to form ormosil for 12  –  20h at 25  C. A transparent smooth layer of ormosil exactly matching the glass surfaceappeared on the solid surface. This ormosil was studied for itsreactivity in the solid-state. 2.5. Electrochemical Measurement The electrochemical measurements were performed with aSolartron Electrochemical Interface (Solartron 1287 Electroche-mical Interface). A one-compartment cell with a working volumeof 4mL and a new ormosil based glucose biosensor as workingelectrode, Ag = AgCl reference electrode and a platinum foilauxillary electrode were used for the measurements. The cyclicvoltammetry using GOD modi fi ed ormosil electrode was studied  between   0.2 and 0.6V (vs. Ag = AgCl). The amperometricmeasurements using GOD immobilized sol-gel modi fi ed elec-trode was operated at 0.35V (vs. Ag = AgCl). The experimentswere performed in phosphate buffer (0.1M, pH 7) employing thenew ormosil based electrocatalytic glucose biosensor. All themeasurements were performed at 25  C. Before each set of measurement the working solution was degassed by nitrogen for 15min. 2.6. Spectroscopic Characterization The new Pd-linked ormosil precursor (Pd-glycidoxypropyl-trimethoxysilane) was characterized using  13 C, mass and UV-visspectroscopy. The scanning electron micrograph (SEM)measurment were made from using JEOL-JSM 840A scanningelectron microscope. The self-assembly of palladium on reactiveormosil was investigated by SEM and image photograph. 3. Results and Discussion 3.1. Chemistry of Palladium Chlorideand Glycidoxypropyltrimethoxysilane Linkage Glycidyl group of one of the present ormosil precursor ishighly reactive. When aqueous solution of palladium chloride,which acts as Lewis acid, was added to glycidoxypropyl-trimethoxysilane, we observed two following remarkable  fi nd-ings: the color of the solution became black and evolution of gas bubble was visibly seen. Because of the Lewis acid character-istics of palladium salt, it opens the epoxide of the glycidylmoieties and in turn palladium is reduced. The reduction of  palladium by glycidyl moieties has already been reported byBurkhart et al. [22]. The reduced palladium may undergo coor-dination with carbon atoms which were initially bonded toepoxide linkage. Further experimental observation to characterizethe structural components of resulting Pd-glymo precursor solution was observed below. 1520  P. C. Pandey et al. Electroanalysis  2001 , 13, No. 18  3.1.1. UV-vis Spectroscopy The UV-vis spectroscopy was used to characterize the solu-tions: (1) glycidoxypropyltrimethoxy silane containing the sameamount of water as used to prepare the desired palladiumchloride solution; (2) the subsequent black solution obtained after adding palladium chloride; (3) palladium chloride solution alone.The UV-vis spectroscopy was recorded in two ranges (I) between200 and 340 and (II) 250  –  700nm. These results are shown inFigure 1A and B. Curve a (Fig. 1A) shows the spectra of solution(1) where as curve b (Fig. 1A) shows the spectra of solution (2) between 250 and 700nm. There is a peak at 320  –  340nm in curve b which is absent in curve a. The UV-vis spectroscopy of thesame solution between 200 and 250nm is shown in inset (b) toFigure 1A. There is a peak near 235nm in the spectroscopy of solution 1 (inset a) which is shifted to 320  –  340nm in the spec-troscopy recorded for solution 2 (inset b to Fig. 1A). Thesespectroscopic observations justify the linkage of palladium toglycidyl moieties. This linkage of palladium to glycidyl moietieswas further conformed from the UV-vis spectroscopy recorded inFigure 1B. Curve c of Figure 1B shows the spectra of palladiumchloride solution, which shows sharp peaks at 304 and 421nm.The presence of a peak at ca. 320  –  340nm in curve b (Fig. 1A)clearly justi fi es the palladium linkage to glycidyl moieties. Sincethe UV-vis spectroscopy of the sample was recorded containingan aqueous solution, it is obvious to consider the hydrolysis of the methoxy-group attached to silicon. It has been observed earlier that in absence of any catalyst the rate of such hydrolysisis very slow. Further slow kinetics of the methoxy group of thissilane is due to inhomogeneity of water and glycidoxypropyl-trimethoxysilane. Accordingly we recorded the UV-vis spectro-scopy just after adding the palladium chloride solution and alsoafter the incubation of the same solution overnight at 10  C.Curve d (Fig. 1B) shows the spectroscopy of resulting solution(after incubation overnight at 10  C). The occurrence of two peaks the  fi rst one being at 320nm and the second one being at 391nm suggest a two site attachment of palladium to glycidox-ypropyltrimethoxysilane. The second linkage is possiblysuggested due to hydrolysis of the methoxy group followed byattachment of palladium chloride at the hydroxyl group. Further evidence of such attachment is discussed below. 3.1.2. NMR Spectroscopy In order to have further insight of palladium chloride and glycidoxypropyltrimethoxysilane, we have taken  13 C NMR spectra of: 1) glycidoxypropyltrimethoxysilane containing sameamount of water and 2) the  13 C spectra of black solution after adding palladium chloride. These spectroscopy results arerecorded in Figure 2a and b, respectively. The presence of aminimum seven carbon peaks (Fig. 2a and b) in both casesstrongly suggest that the number of carbon atoms after additionof palladium chloride is not lost. The black color of the solutionthen itself indicates the coordination of palladium to either oxygen or carbon atom of the glymo-precursor. However, when Fig. 1. A) UV-vis spectroscopy of a) glycidoxypropyltrimethoxysilane, b) palladium chloride treated glycidoxypropyltrimethoxysilane between 250and 700nm. Insets  ‘ a ’  and   ‘  b ’ show the UV spectroscopy of (a) and (b), respectively, between 200 and 340nm. B) UV-vis spectroscopy of c) aqueoussolution of palladium chloride, d) palladium chloride treated glycidoxypropyltrimethoxysilane and after incubation of this solution at 10  C overnight  between 250 and 700nm. Palladium-Linked Ormosil Based Biosensor 1521 Electroanalysis  2001 , 13, No. 18  epoxide of glycidyl moieties is broken the possibility of palla-dium coordination to the same carbon atom is suggested. 3.1.3. Mass Spectroscopy In order to have a further clear picture of glycidoxypropyl-trimethoxysilane and palladium chloride linkage, we have takenmass spectroscopy of the solution obtained just after adding palladium chloride solution for overnight at 10  C. The structureof glycidoxypropyltrimethoxysilane is given below.When an aqueous solution of palladium chloride is added to theabove mentioned liquid the solution turned black and evolution of a gas bubble is seen. The mass spectroscopy of the resultingsolution is recorded and shown in Figure 3a and b. Figure 3bshows the mass spectra between molecular weight 220 and 450 at an ampli fi ed scale. The presence of a cluster of molecular weight 326 (Fig. 3a) clearly suggest the following type of binding between palladium and glycidoxypropyltrimethoxysilane.The presence of the peak corresponding to the cluster of molecular weight 382amu suggest the following structure:Additionally the presence of another cluster corresponding tothe molecular weight of 352amu suggests the presence of thefollowing structure: Fig. 2.  13 C NMR spectroscopy of a) glycidoxypropyltrimethoxysilaneand b) palladium chloride treated glycidoxypropyltrimethoxysilane.Fig. 3. Mass spectroscopy of palladium chloride treated glycidox-ypropyltrimethoxysilane. Curve (a) shows the results of molecular weight between 220 and 450 at 30  C whereas curve (b) shows the resultsof the same solution after incubation at 10  C overnight. 1522  P. C. Pandey et al. Electroanalysis  2001 , 13, No. 18  There is a cluster recorded at 390amu in mass spectroscopy.The possibility of such a cluster under the present experimentalconditions can be as follows:The presence of such a cluster in mass spectroscopy is possibly due to hydrolysis of the  –  OCH 3  group in aqueoussolution although in absence of any catalyst the kinetics of suchhydrolysis is very slow. If the Pd-glymo-precursor is allowed toincubate overnight at 10  C the mass spectroscopy of this solu-tion shows increments in this peak. This observation suggest theexistence of such a cluster in the black solution. All thesespectroscopic observation suggest the following structure of thePd-glymo-precursor:The gas evolution was also identi fi ed. The pyrogallol test clearly indicates the evolution of oxygen. All these results clearlysuggest the above structure of the Pd-glymo-precursor. 3.2. Electrochemistry of Ferrocene EncapsulatedOrmosil The electrochemistry of ferrocene in these two types of ormosils was studied. The ormosil prepared using unmodi fi ed glycidoxypropyltrimethoxysilane shows quasi-reversible electro-chemistry of ferrocene [19] and does not mediate electrontransport from the active site of redox proteins = cofactorsencapsulated together. The electrochemistry of ferrocene encap-sulated within Pd-linked ormosil prepared at 30  C is shown inFigure 4. The cyclic voltammogram of ormosil encapsulated ferrocene is reversible (Fig. 4). A plot of the peak current vs. thesquare root of the scan rate (inset to Fig. 4) shows a linear relation passing through the origin. This observation showsremarkable behavior of ormosil encapsulated ferrocene witha peak separation of 57  –  59mV as compared to the electro-chemistry of ferrocene reported earlier in absence of palladiumlinkage [19].In order to have deeper insight in the encapsulation of ferro-cene and the behavior of the subsequent ormosil we let the Pd-glymo-precursor incubated overnight at 10  C. The electro-chemistry of ferrocene encapsulated within ormosil prepared at 10 and 30  C is shown in Figure 5. Curve B shows the cyclicvoltammogram of ormosil encapsulated ferrocene at 30  C and curve A shows the voltammogram of the one prepared using thePd-glymo-precursor incubated overnight at 10  C.When the two palladium linked ormosil precursors are treated with trimethoxysilane and other common ingredients of gelation,there were three sites available in case I (single site palladiumlinkage) for hydrolysis and polycondensation of these twoormosil precursors. Trimethoxysilane undergoes rapid hydrolysisand subsequent polycondensation alone in the presence of common gelation ingredients and forms a composite sol-gelnetwork. Accordingly, we optimized the concentration of trime-thoxysilane for cross hydrolysis and polycondensation with Pd-glymo precursor. Under these conditions when the two-site palladium attached glymo-precursor is treated with trimethox-ysilane, the polycondensation chain is restricted to only two siteswhereas the three-sites chain of polycondensation based ormosilis desirable = suggested with single-site linkage of palladium-glymo. Accordingly, the nanoporous geometry of ormosil whichsubsequently determines the presence of palladium in nanoporesof ormosil surrounding encapsulated ferrocene becomes verycrucial. The number of palladium ions around encapsulated ferrocene is expected to be greater when single-site linkage of  palladium-glymo is used for ormosil formation whereas a smaller number of palladium ions is expected when two-sites linkage of  Fig. 4. Cyclic voltammetry of ferrocene encapsulated Pd-linked ormosilin 0.1M phosphate buffer pH 7.0 at the scan rate of 2, 5, 10, 20, 50 and 100mV = s. The inset shows the plot of the peak current vs. the squareroot of the scan rate.Fig. 5. Cyclic voltammetry of ferrocene encapsulated Pd-linked ormosilin 0.1M phosphate buffer pH 7.0 at a scan rate of 10mV = s, at twodifferent temperatures: A) 10  C and B) 30  C. Palladium-Linked Ormosil Based Biosensor 1523 Electroanalysis  2001 , 13, No. 18
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