Amperometric biosensors based on carbon paste electrodes modified with nanostructured mixed-valence manganese oxides and glucose oxidase

Nanomedicine: Nanotechnology, Biology, and Medicine 1 (2005) 130 – 135 www.nanomedjournal.com

Basic Research

Amperometric biosensors based on carbon paste electrodes modified with nanostructured mixed-valence manganese oxides and glucose oxidase Xiaoli Cui, PhD, Guodong Liu, PhD, Yuehe Lin, PhDT Pacific Northwest National Laboratory, Richland, Washington Received 23 February 2005; accepted 29 March 2005

Abstract

Nanostructured, multivalent, manganese-oxide octahedral molecular sieves (OMS), including cryptomelane-type manganese oxides and todorokite-type manganese oxides, were synthesized and evaluated for chemical sensing and biosensing at low operating potential. Both cryptomelane-type manganese oxides and todorokite-type manganese oxides are nanofibrous crystals with subnanometer open tunnels that provide a unique property for sensing applications. The electrochemical and electrocatalytic performance of OMS for the oxidation of H2O2 have been compared. Both cryptomelane-type manganese oxides and todorokite-type manganese oxides can be used to fabricate sensitive H2O2 sensors. With glucose oxidase (GOx) as an enzyme model, amperometric glucose biosensors are constructed by bulk modification of carbon paste electrodes with GOx as a biocomponent and nanostructured OMS as a mediator. A Nafion thin film was applied as an immobilization/encapsulation and protective layer. The biosensors were evaluated as an amperometric glucose detector at phosphate buffer solution with a pH 7.4 at an operating potential of 0.3 V (vs Ag/AgCl). The biosensor is characterized by a well-reproducible amperometric response, linear signal-to-glucose concentration range up to 3.5 mmol/L and 1.75 mmol/L, and detection limits (S/N = 3) of 0.1 mmol/L and 0.05 mmol/L for todorokite-type manganese oxide and cryptomelane-type manganese oxide– modified electrodes, respectively. The biosensors based on OMS exhibit considerable good reproducibility and stability, and the construction and renewal are simple and inexpensive. D 2005 Elsevier Inc. All rights reserved.

Key words:

Biosensors; Glucose; Mixed-valence manganese oxides; Hydrogen peroxide; Carbon paste electrode

A reliable, rapid, and economic method to monitor glucose is of great importance in numerous areas, such as in clinical diagnostics and biotechnology, and in food, pharmaceutical, and environmental analyses [1- 10]. In the field of biotechnology and bioanalytical chemistry, manganese dioxide is an important functional material and has been used as a mediator to fabricate chemical sensors and biosensors [11 -17]. Hocevar et al [11] reported a glucose microbiosensor using electrochemical codeposition of glucose oxidase (GOx) along with MnO2 as mediator with an operating potential of +0.58 V (vs Ag/AgCl) on a single carbon fiber microelectrode. Schachl et al [15] reported a carbon paste electrode (CPE ) modified with commercially available manganese No financial conflict of interest was reported by the authors of this paper. T Corresponding author. E-mail address: [email protected]. 1549-9634/$ – see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.nano.2005.03.005

dioxide and used for H2O2 detection in connection to flow-injection analysis. The operating potential is +0.46 V in a solution of pH 9.5. Recently, the development of sensors and biosensors based on heterogeneous carbon electrodes modified with manganese dioxide has been reviewed by Beyene et al [13]. On the basis of the special reaction capability of MnO2 nanoparticles with H2O2, a glucose biosensor has been fabricated by coimmobilizing GOx and MnO2 nanoparticles on the gate of an ion-sensitive fieldeffect transistor [16]. MnO2 nanoparticles have also been used to detect L-ascorbic acid with an ion-sensitive fieldeffect transistor [18]. Both todorokite and hollandite are microporous and mixed-valence manganese oxides with tunnel structures, and their basic unit structure is made of sheets of MnO6 edgesharing octahedral, so-called octahedral molecular sieve (OMS) materials [19]. Synthetic todorokite has been designated OMS-1, which has a 3  3 tunnel structure having a

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Fig. 1. Tunnel structure of OMS manganese oxides. OMS-1 is synthetic todorokite with a 3  3 structure and a pore size of 6.9 2, and OMS-2 is synthetic hollandite with a 2  2 structure and a pore size of 4.6 2.

pore size of about 6.9 2. Natural todorokite may contain different tunnel counter cations, such as Mg2+, Ba2+, and K+, while synthetic torodokite was prepared with Mg2+ as the counter cation in the tunnel. OMS-1 is composed of Mg2+0.98-1.35Mn2+1.89-1.94Mn4+4.38-4.54O12d 4.47-4.55 H2O, and it is believed to be a mixed-valence species having primarily Mn4+. However, it also has some Mn3+ and Mn2+, based on an average oxidation state of about 3.6. A related structure is hollandite, and the synthetic one has been designated OMS-2 with a 2  2 tunnel structure, having a pore size of 4.6 2. The K+ form of hollandite is known as cryptomelane, and it also is a mixed valence and has an average oxidation state of 3.9. Octahedral molecular sieve materials are of considerable interest and have been widely used as catalyst, ion exchanger, solid ionic conductor, and battery materials. Suib and colleagues [19 -27] did excellent and extensive research on these materials. The studies of the synthesis and application of microporous manganese oxides are of both theoretical and practical interest [21,22,28]. Recent studies show microporous manganese oxides can be used as catalyst for the low-temperature carbon monoxide oxidation [23], 2-propanol decomposition [24], aerobic oxidation of benzyl alcohol [25,26], and cyclohexanol to cyclohexanone [27]. Figure 1 presents the tunnel structure of OMS-1 and OMS-2 [29]. We are particularly interested in the application of OMS materials in electrocatalysis because they are multivalent manganese oxides with nanostructure, which results in outstanding catalytic properties for H2O2 oxidation if employed as a mediator. We have demonstrated that cryptomelane-type manganese oxides (OMS-2) can be used for amperometric detection of H2O2 at a low operating potential of +0.3 V (vs Ag/AgCl) with potential applications to fabricate a glucose sensor [30]. Carbon-paste enzyme electrodes have attracted significant interest because of their simplicity, chemical stability, and high sensitivity in constructing sensors and biosensors [14,15,30,31]. A glucose biosensor has been developed by bulk modification of a CPE with GOx as a biocomponent and commercially available manganese dioxide as a mediator [14]. An enzyme electrode for amperometric measurement of D-amino acid has also

Fig. 2. Cyclic voltammograms of CPE modified by OMS-1 (a), and OMS-2 (b) in PBS (pH 7.4).

been developed using a fatty acid modified with flavin adenine dinucleotide as an enzyme stabilizing agent [31]. In this article, the electrochemical and electrocatalytic performance of OMS-1 and OMS-2 are compared. Amperometric glucose sensors have been constructed by bulk modification of CPEs with GOx as a biocomponent and nanostructured OMS as a mediator. Methods Chemicals Glucose oxidase (EC 1.1.3.4, type X-S) from Aspergillus niger with an activity of 157500 Ug-1 and glucose were purchased from Sigma. H2O2 (35 wt%) was purchased from Aldrich (Milwaukee, WI). Water used in all the experiments was purified through an ion exchange system with a resistivity of about 18.0 MVcm (Barnstead Nanopure). Buffer solution was prepared from phosphate buffered saline (PBS) (Sigma). Mineral oil from Aldrich was used as a pasting liquid for the CPE. Graphite powder (particle sizes 1 to 2 Am) from Aldrich was used as the working electrode substrate. An aqueous stock standard solution containing 0.05 mol/L H2O2 was freshly prepared every day. Glucose stock solutions were allowed to store overnight at room temperature before use. Synthesis of OMS-1 and OMS-2 OMS-2 (cryptomelane-type manganese oxides) and OMS-1 (todorokite-type manganese oxides) were prepared according to the method reported in the literature [19,32]. Briefly, birnessite was first synthesized and used as a precursor for the synthesis of todorokite. Birnessite (a layered manganese oxide, Na 0.55Mn2O4 d 1.5 H2O) was prepared using a method described by Golden et al [32]. A typical synthesis was carried out as follows: 250 mL of 6.4 mol of NaOH solution was mixed with 200 mL of 0.5 mol MnSO4 at

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Fig. 3. Cyclic voltammogram of modified CPE with OMS-1 in PBS (a) and in the presence of H2O2 (b). Scan rate, 50 mV/s; concentration of H2O2, 1.27  10-3 mol/L; supporting electrolyte, PBS (pH 7.4). The percentage of mixed-valence manganese oxides in modified CPEs was 5.5 wt%.

Fig. 4. Cyclic voltammogram of modified CPE with OMS-2 in PBS (a) and in the presence of H2O2 (b). Scan rate, 50 mV/s; concentration of H2O2, 1.27  10-3 mol/L; supporting electrolyte, PBS (pH 7.4). The percentage of mixed-valence manganese oxides in modified CPEs was 5.5 wt%.

room temperature. Oxygen was immediately bubbled through a glass frit at a rate of 4 mL/min. After 4.5 hours the oxygenation was stopped and the precipitate was filtered, washed with deionized water, and dried in air at 1008C. About 13 g of birnessite product was obtained. Todorokite was prepared according to literature [33]. Approximately 3 g of birnessite was added to 100 mL of 1 mol of MgCl2 solution, and the slurry was stirred overnight at room temperature to ion exchange Mg2+ for Na+. The product was washed with deionized water, then added to 25 mL of autoclaved H2O and heated at 1508C for 48 hours. The product was washed and dried in air at 1008C. About 2 g of the todorokite product was obtained.

5.5% OMS and 5% GOx (wt). The mixture was homogenized completely and allowed to store at 08C. The modified paste was packed into the hole of the electrode and smoothed with a weighting paper. Finally, the electrode was covered with 5 AL of 1% Nafion solution (diluted from the 5% Nafion solution with pure water) and dried in air at room temperature for half an hour. The prepared glucose sensor was stored in a dry state at 48C. The electrode was rinsed thoroughly with pure water before the electrochemical experiments.

Preparation of CPEs A Teflon rod (8 cm in length) with a hole at one end (3 mm in diameter, 3 mm in depth, BAS) for filling carbon paste was used as the electrode body. Electrical contact was provided by a copper piston within the center of the rod. Unmodified carbon paste was prepared by adding 0.05 g mineral oil to 0.15 g graphite powder. Modified carbon pastes were prepared by replacing corresponding graphite powder with OMS-1 and OMS-2 powder and then adding mineral oil. The mixture was homogenized carefully and packed into the hole of the electrode holder. The electrode surface was smoothed with a weighing paper. When necessary, a new electrode surface was obtained by removing 2 mm of the outer paste layer, adding freshly made mixed-valence manganese oxides/carbon paste mixture, and polishing it. Fabrication of glucose sensors OMS/GOx-modified carbon paste was prepared by adding the desired amount of GOx to 5.5% (wt) OMS carbon paste. The final OMS/GOx modified carbon paste contained

Instruments Amperometric and cyclic voltammetric measurements were carried out with CH Instruments (CHI 660 A, CH Instruments, Austin, Tex) in a phosphate buffer (pH 7.4) supporting an electrolyte medium. A carbon-paste working electrode, Ag/AgCl reference electrode (Model CH 111, CH Instruments), and a platinum-wire counter electrode were inserted into the 20-mL cell through holes in its Teflon cover. Procedures Amperometric detection proceeded under a batch condition in an unpurged stirred measuring solution at low applied potential, such as 0.2 V or 0.30 V (vs Ag/AgCl). A magnetic stirrer and a Teflon-coated stirring bar provided convective transport during the amperometric measurement. All measurements were performed at room temperature after applying the desired working potential and allowing the transient baseline current to decay to a steady-state value. Results and discussion Voltammetric behavior of the CPE modified with OMS-1 and OMS-2 Figure 2 shows the cyclic voltammetric behavior of the CPEs modified with OMS-1 (a) and OMS-2 (b) with similar

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1.80E-06 1.60E-06

1.20E-06 1.00E-06

0.30 0.25

8.00E-07

Current / µA

Current / A

1.40E-06

6.00E-07

0.20 0.15 0.10

4.00E-07

0.05

2.00E-07

0.00 0.0

4.0x10-4 8.0x10-4 1.2x10-3 1.6x10-3 Concentration / M

0.00E+00 0

0.2

0.4

0.6

0.8

1

1.2

Potential / V vs Ag/AgCl Fig. 5. Hydrodynamic voltammograms for 1  10-4 H2O2 at plain (x) and OMS-1– (E) and OMS-2– (n) modified CPEs. Other conditions are the same as in Figures 3 and 4.

loading in the carbon-paste composite in PBS (pH 7.4). A pair of redox peaks was observed in the potential range of 0 to 1.0 V for both electrodes. This can be attributed to the oxidation and reduction between Mn (+3) and Mn (+4), which are located in octahedral sites of mixed-valence manganese oxides [19]. Figures 3 and 4 show the cyclic voltammetric behavior of the CPE modified with OMS-1 (Figure 3) and OMS-2 (Figure 4) in the absence (a) and presence (b) of H2O2. Both OMS-1 and OMS-2 exhibited electrocatalytic properties for the oxidation of H2O2. The oxidative current increased from 0.2 V in the presence of H2O2, indicating high electrocatalytic activity of multivalence manganese oxides toward the oxidation of H2O2. To compare the difference in their electrocatalytic activity, hydrodynamic voltammograms were generated with CPEs, as shown in Figure 5. There was no response observed at the plain electrode at a potential lower than 0.7 V. The modified CPE, in contrast, responded from 0.2 V, and its peak was at 0.61 V. OMS-2– modified CPE shows higher electrocatalytic activity for the oxidation of H2O2 than OMS-1– modified CPE. The higher activity of OMS-2 may result from its higher average oxidation state of manganese (3.9) than that of OMS-1 (3.6). Response to oxidation of H2O2 We have demonstrated the electrocatalytic activity of OMS-2 for amperometric detection of H2O2 at a low operating potential of +0.3 V in a previous communication [30]. When compared with OMS-2, OMS-1 also has considerable electrocatalytic activity for the oxidation of H2O2. Figure 6 shows the amperometric response (at +0.2 V) of OMS-1– modified CPE to successive 1  10-4 mol/L additions of H2O2. The plain electrode shows no response to these concentration changes under this low-detection poten-

Fig. 6. Current-time recording obtained on increasing the H2O2 concentration in 1  10-4 mol/L steps at OMS-1– modified CPE at an operating potential of 0.2 V and the corresponding calibration plot (inset). The supporting electrolyte is PBS (pH 7.4). The percentage of OMS-1 in carbon paste is 5.53%, and the stirring rate is ~350 rpm.

tial. In contrast, the OMS-1 modified electrode responds very rapidly to the changes of H2O2 concentration, producing steady-state signals within 10 seconds. The favorable signals are accompanied by a low noise level. The inset shows the corresponding calibration curve at 0.2 V. This fact implies that OMS-1 would also be good material to fabricate sensitive H2O2 sensors. Figure 7 illustrates the calibration plots for H2O2 responses at CPEs modified by OMS-2 and OMS-1 (both with 5.5% percentage in carbon paste composite) at the working potential of 0.3 V. As shown, OMS-2 exhibits a higher activity than that of OMS-1, which is in agreement with the results from hydrodynamic voltammograms. It should be noted that OMS-2 has a higher electrocatalytic activity, but the measurement is often related with a larger background current than the OMS-1– modified electrode. Performance of glucose sensors based on glucose oxidase and OMS The nanostructure and multivalence of the OMS materials provide more active sites and higher electrocatalytic activity than the commercially available manganese dioxide powder for the determination of H2O2. In view of its sensitivity, stability, low working potential, simplicity, and low cost of construction, the CPEs modified with the nanostructured, mixed-valence manganese oxides exhibit great prospects for biosensor application. Glucose biosensors were constructed by using OMS as a mediator and GOx as a biocomponent, and their performance was investigated. Figure 8 shows the response to successive addition of glucose with CPE modified with OMS-1(A) and OMS-2(B) and GOx at a working potential of 0.3 V. Table 1 summarizes the related data for glucose sensors based on OMS and GOx. The percentage of OMS is 5.5% in the carbon

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Current / µA

134

5

Table 1 Performance of glucose sensors based on glucose oxidase and OMS at operating potential of 0.3 V

4

Mediator

OMS-1

OMS-2

Linear range/mmol/L Sensitivity/AA/mmol/L Correlation coefficient Detection limit/mmol/L RSD (number of samples = 8) %

0.1 to 3.5 0.44 0.989 0.1 (3 S/N) 1.5 (to 0.5 mmol/L glucose)

0.05 to 1.75 1.73 0.987 0.05 (3 S/N) 5.2 (to 0.25 mmol/L glucose)

3

2

OMS = octahedral molecular sieve.

1

0 0.0

3.0x10-4

6.0x10-4

9.0x10-4

1.2x10-3

Concentration / M Fig. 7. Calibration plots for H2O2 response at CPEs modified by OMS-2 (.) and OMS-1 (n) with a concentration of 5.5% at an operating potential of 0.3 V. The supporting electrolyte is PBS (pH 7.4). The stirring rate is ~350 rpm.

paste. As shown in Table 1, the modified CPEs composed of mixed-valence manganese oxides show good reproducibility for the detection of glucose (relative SD 1.5% to 5.2%; n = 8). The glucose sensor based on OMS-1 has a larger linear range than that of OMS-2, while the OMS-2 modified electrode has a higher sensitivity, which reflects its higher electrocatalytic activity. This is in agreement with the response of H2O2. Conclusions The nanostructured todorokite-type (magnesium) and cryptomelane-type (potassium) manganese oxides were prepared, and their analytical applications were evaluated by electrochemical sensing H2O2 and glucose. We demonstrated that the nanostructured OMS-modified CPE offered low-potential amperometric detection for H2O2 and glucose. The sensors were easily prepared with an inexpensive, chemically stable, and practically harmless mediator. The low-potential detection and its applicability in neutral solution show great promise for bioanalytical applications with electrochemical biosensors. Compared with the CPE modified with commercially available MnO 2 powder (working potential +0.48 V) [12,14,17], and electrodeposited MnO2 on carbon fiber microelectrode (working potential +0.58 V) [11], the operating potential of the new CPEs modified with OMS materials is much lower. The glucose biosensors based on OMS and GOx show good reproducibility and high sensitivity for glucose detection. The OMS-2 based electrode has a lower detection limit, while the OMS-1 based electrode shows a larger linear range. The biosensor fabrication technology demonstrated in this work should be transferable to the fabrication of other biosensors based on oxidases, such as biosensors for

Fig. 8. Amperogram for successive additions of 0.5 mmol/L (A) and 0.25 mmol/L (B) glucose obtained with OMS-1(A), OMS-2(B)/GOx/ Nafion-modified CPE at operating potential 0.3 V. The supporting electrolyte is PBS (pH 7.4). The stirring rate is ~350 rpm.

cholesterol, alcohol, lactate, acetylcholine, choline, hypoxanthine, and xanthine. Acknowledgments This work is supported by a laboratory-directed research and development program at Pacific Northwest National Laboratory (PNNL). The research described in this article

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was performed at the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the U.S. Department of Energy (DOE) Office of Biological and Environmental Research and located at PNNL. PNNL is operated by Battelle for the DOE (contract DE-AC05-76RL01830). We are grateful to Dr. Liyu Li for help in preparing OMS-1 and OMS-2 materials and for fruitful discussions, to Dr. Guanguang Xia for help in drawing the schematics, and to Wayne Cosby for editing the manuscript.

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