Ordered mesoporous carbon for electrochemical sensing: A review

Ordered mesoporous carbon for electrochemical sensing: A review

Analytica Chimica Acta 747 (2012) 19–28 Contents lists available at SciVerse ScienceDirect Analytica Chimica Acta journal homepage: www.elsevier.com...

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Analytica Chimica Acta 747 (2012) 19–28

Contents lists available at SciVerse ScienceDirect

Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca

Review

Ordered mesoporous carbon for electrochemical sensing: A review Jean Chrysostome Ndamanisha a,b , Li-ping Guo a,∗ a b

Faculty of Chemistry, Northeast Normal University, Changchun 130024, People’s Republic of China Université du Burundi, Institut de pédagogie appliquée, B.P. 5223, Bujumbura, Burundi

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

 The preparation and functionalization of ordered mesoporous carbon.  Their applications as electrochemical sensors with high electrocatalytic activity.  A promising electrode material based on its interesting properties.

a r t i c l e

i n f o

Article history: Received 31 May 2012 Received in revised form 29 July 2012 Accepted 16 August 2012 Available online 28 August 2012 Keywords: Ordered mesoporous carbon Electrochemical properties Sensors Biosensors Functionalization Edge plane-like defective sites

a b s t r a c t With its well-ordered pore structure, high specific surface area and tunable pore diameters in the mesopore range, ordered mesoporous carbon (OMC) is suitable for applications in catalysis and sensing. We report recent applications of OMC in electrochemical sensors and biosensors. After a brief description of the electrochemical properties, the functionalization of the OMC for improvement of the electrocatalytic properties is then presented. We show how the ordered mesostructure of OMC is very important in those applications. The high density of edge plane-like defective sites (EDSs), oxygen-containing groups and a large surface area on OMC may provide many favorable sites for electron transfer to compounds, which makes OMC a potential novel material for an investigation of the electrochemical behavior of substances. Moreover, the structural capabilities of OMC at the scale of a few nanometers agree with immobilization of other electrocataytic substances. Interesting properties of this material may open up a new approach to study the electrochemical determination of other biomolecules. © 2012 Elsevier B.V. All rights reserved.

Contents 1. 2. 3. 4.

5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis and characterization of OMC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrochemical properties of OMC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications as electrochemical sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Influence of the oxygen-containing functional groups and the edge plane defect sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Influence of the large surface area and the widely open and ordered mesostructure of OMC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrochemical biosensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Immobilization of biomolecules on OMC with usual pore size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Incorporation active molecules in OMC with large pore . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Corresponding author. Tel.: +86 431 85099762; fax: +86 413 85099762. E-mail address: [email protected] (L.-p. Guo). 0003-2670/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.aca.2012.08.032

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6. 7.

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Functionalization of OMC for electrochemical detection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Jean Chrysostome Ndamanisha received his Ph.D. in Analytical Chemistry from Faculty of Chemistry, Northeast Normal University, P.R. China in 2009. He worked for University of Burundi from 1998. Now he has published more than 8 papers and he is working at University of Burundi as a Senior Lecturer. His current research interest field is Analytical Chemistry.

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Liping Guo received her Ph.D. degree in Electroanalytical chemistry from Changchun Institute of Applied Chemistry, Chinese Academy of Sciences in 1997. She became a professor in 2000. Now she works at Faculty of Chemistry, Northeast Normal University as Professor and Ph.D. Candidate Supervisor of Analytical Chemistry. Her current research interests focuses on the development of electrochemical sensors and biosensors based on ordered porous carbon and its derivatives.

1. Introduction

2. Synthesis and characterization of OMC

Carbon materials are widely used in electroanalytical investigations because of their chemical inertness, relatively wide potential window, low background current, and suitability for different types of analysis [1–3]. By combining the advantages of carbon materials with those of nanostructured materials, carbon-based nanoscaled materials have been widely used in preparation of modified electrodes including carbon nanotubes (CNT) [4,5]. The abilities of carbon nanotubes (CNT) and graphene modified electrodes to promote electron-transfer reactions and to offer resistance to surface fouling have been documented in connection with biomolecules [3,6–9]. Besides the carbon-based materials mentioned above, there has been significant interest in the development of one such novel nanostructured carbon material, i.e. highly ordered mesoporous carbon (OMC). Since its discovery in 1999 [10], OMC has been receiving much attention owing to the extremely well-ordered pore structure, high specific pore volume, high specific surface area, and tunable pore diameters in the mesopore range, which make it suitable for applications in catalysis [11] and sensing [12,13]. The high density of edge plane defect sites (EDSs) on OMC may provide many favorable sites for electron transfer to biomolecules [12,13], which makes OMC potential novel materials for investigating the electrochemical behavior of the substances. This family of materials is characterized by a long-range structural order made of mesopore channels, specially arranged in hexagonal, cubic, lamellar or wormlike structures displaying very high specific surface area [10]. Carbon can bond with itself principally via sp3 and sp2 covalence linkages and this unique ability leads to diversified molecular configurations including fullerene, carbon nanotubes and ordered mesoporous carbons. The applications of carbon nanotubes [14] and graphene [3] as electrochemical sensors have been recently reviewed. Here, we review the use of OMC in electrochemical (bio)sensors. We will first describe the general methods for the preparation of ordered mesoporous carbons, then the applications of their electrochemical properties as electrochemical (bio)sensors and finally highlight their functionalization for the improvement of their electrocatalytic properties. This area is interesting because a number of articles on applications of OMC in the electrochemical detections are growing exponentially.

Ordered mesoporous carbon and polymers are generally fabricated by a hard-templating approach [15–17]. The first ordered mesoporous carbon was synthesized by Ryoo’s group in 1999 [10]. Mesoporous silicates derived from the surfactant self-assembly approach were used as the hard templates. Briefly, the mesopores in the silica molecular sieve were impregnated with sucrose and sulphuric acid using their aqueous solution. The silica material after the impregnation was heated to a desired temperature in the range of 1073–1373 K under vacuum or in an inert atmosphere. The sucrose was converted to carbon by such a process using sulphuric acid as the catalyst. Removal of the silica scaffold resulted in ordered mesoporous carbon replicating the silica mesochannels. Variable carbon precursors, e.g., sucrose, furfuryl alcohol, naphthalene, mesophase pitch, C2 H2 , polyacrylonitrile, and phenolic resin, can be also utilized [18]. The synthesis of the template silicas has been reviewed by Tagushi and Schuth [18]. A general drawback of hard-templating methods is the need for an inorganic template, and a process that involves several time-consuming and costly steps for the impregnation of the template, and selective etching of the silica with hydrofluoric acid or sodium hydroxide solution. To overcome the problem, a soft-templating approach can be used. An amphiphilic surfactant templating method via organic–organic assembly based on the evaporation-induced self-assembly (EISA) process has been developed using amphiphilic block copolymers as structural directing agents (templates) [19]. From the view of practical applications, morphology control of mesoporous carbon materials is highly important. Various shapes and morphologies of mesoporous carbons, such as single crystals [20], monoliths [21], fibers [21], nanospheres [22], vesicles [23]. and films [24] have been synthesized. Another effective approach [25] to produce OMC with extremely high porosity is based on triconstituent self-assembly of resol as the carbon precursor, tetraethylorthosilicate (TEOS) as the silica precursor and the block copolymer Pluronic F127 as the structure directing agent. Carbonization is followed by etching of the silica in the carbon–silica nanocomposite with hydrofluoric acid. Through the silica in the walls, shrinkage during carbonization is reduced and additional porosity is produced by etching it, therefore mesoporous carbon with extremely high porosity is obtained.

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Fig. 1. (A)Small-angle XRD patterns of OMC. Inset. Some symmetries of OMC. Copyright 2009 Elsevier. Reproduced with permission from Ref. [26]. (B) TEM images of OMC viewed from [1 0 0] directions. (C) TEM images of OMC viewed from [0 0 1] directions. Copyright 2009 Elsevier. Reproduced with permission from Ref. [64].

It is interesting to note that it is possible to obtain different types of ordered structures, owing to the diversity in the achievable 3-dimensional (3D) structures of the ordered mesoporous silica (OMS) templates [17]. In particular, CMK-1 carbon with I41/a (or lower) symmetry and CMK-4 carbon with cubic Ia3d symmetry are synthesized using MCM-48 and FDU-5 silica templates respectively because these templates are of Ia3d symmetry. CMK-2 carbon with cubic Pm3n symmetry is obtained using an SBA-1 template of Pm3n space group CMK-3 and CMK-5 carbons with 2D hexagonal (p6mm) symmetry are obtained using an SBA-15 template (p6mm symmetry). Moreover, it is possible to synthesize OMC with cubic Im3m symmetry using for instance the SBA-16 silica template [17]. Some symmetries are shown in the inset of Fig. 1A. It is important to remember that the structure of the obtained mesoporous carbon material is not simply a negative replica of the used silica template, but the synthesis mechanism involves the unique transformation into a new ordered array that was triggered by the removal of the silica framework [10]. The OMC is often characterized with Fourier Transform IR (FTIR) spectroscopy, X-ray diffraction (XRD), transmission electron microscopy and N2 adsorption–desorption isotherms. From the FT-IR, it has been shown that the OMC spectrum shows the oxygencontaining functional groups [26]. Fig. 1 shows the structure of the OMC synthesized in our group and it is consistent with other prepared ordered mesoporous carbons [10,16–18]. The XRD patterns (Fig. 1A) and transmission electron microscopy (TEM) (Fig. 1B and C) of OMC display that the ordered structure corresponding to the silica template is preserved. The structure is not simply a negative replica of the used silica template to OMC. The synthesis mechanism involves the unique transformation into a new ordered array that was triggered by the removal of the silica frameworks. It is clear that the transmission electron microscopy (TEM) for the carbon materials gives the regular pore image shown in Fig. 1B and C. The TEM image from thin edges of the carbon particles shows that the carbon molecular sieve has a uniform pore distribution, without carbon deposition on the external surface, which is in agreement with the XRD results (Fig. 1A) [10]. N2 adsorption–desorption isotherms and the The Barrett–Joyner–Halanda (BJH) calculations for pore volume and pore size distribution curves of OMC show that the nitrogen sorption is essentially type IV with a clear hysteresis loop, indicating the mesoporous structure. The specific surface area is calculated with the Brunauer–Emmett–Teller (BET) equation. Moreover, the pore size distributions are narrow [10,26,27].

3. Electrochemical properties of OMC The modification of a substrate electrode with OMC is very simple. A modified electrode is prepared by casting 4 ␮L of OMC suspension on the electrode surface, and dried under an infrared lamp. The suspension can be obtained with 1 mL of Nafion solution, 9 mL of distilled water and 5 mg of OMC [26] or with 10 mL N,N-dimethylformamide (DMF) and 5 mg of OMC [28]. In order to get a very stable modified electrode Chitosan and Nafion have been used after a dispersion of OMC in water [29]. Reversible redox system, such as Fe(CN)6 3−/4 − , due to its well studied electrochemical behavior, is usually employed as the probe to characterize the electrochemical reactivity of OMC using cyclic voltammetry (CV). The OMC modified electrode exhibits well defined a quasi-reversible redox waves and the improved electron transfer kinetics are evidenced by a very low peak separations. The CV shows that the electrochemical response currents at OMC modified electrodes are higher than that of the glassy carbon electrode (GCE) and CNT modified electrode [30]. It has been shown that the electron transfer resistance of OMC is much lower than that on CNT [24] suggesting that OMC can form good electron pathway between the electrode and electrolyte. Recent works showed that the apparent electrode area is about 0.063 cm2 [31] higher than that of CNT. This large value of the electrode area is attributed to the higher surface to volume ratio and roughness factors of OMC [31]. The apparent capacitance of OMC was calculated being a value of 197 ␮F cm−2 by Zhu et al. [32], which is much lower than that of CNT. This is very interesting because a lower capacitance results in a lower charging current. The low electron-transfer resistance is clear in Fig. 2A where the capability of electron transfer of OMC electrode is compared to that of CNT and GC electrode surface. The Randles circuit (inset) is chosen to fit the obtained impedance data. The resistance to charge transfer (Rct ) and the diffusion impedance (W) are both in parallel to the interfacial capacitance (Cdl ). By fitting the data, Rct can be estimated to be 225.8 , 100.3  and 18.5  at GCE, CNT/GCE and OMC/GCE, respectively [28]. This shows that the much lower electron-transfer resistance on OMC/GCE. 4. Applications as electrochemical sensors OMC electrochemical sensors are based on the structure of such carbon-based materials. Many reports tried to investigate the influence of (1) oxygen-containing functional groups and the edge plane defect sites available on OMC and (2) its large surface area and

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Fig. 2. (A) Nyquist plots at GE (a), CNT/GE (b) and OMC/GE (c) in 5 mM Fe(CN)3−/4−6 containing 0.1 M KCl. Inset is the equivalent circuit. The frequency range is from 1 Hz to10 kHz. Inset is the equivalent circuit. Copyright 2008 Elsevier. Reproduced with permission from Ref. [47]. (B) Raman spectrum of OMC. Copyright 2009 Elsevier. Reproduced with permission from Ref. [26].

the widely open and ordered mesostructure. A discussion will be provided on their implication in selected sensors and biosensors. 4.1. Influence of the oxygen-containing functional groups and the edge plane defect sites The first use of OMC for electrochemical sensing is the work of Jia et al. [12]. The prepared OMC modified electrode demonstrated a very good performance for the simultaneous determination of dopamine (DA) and ascorbic acid (AA) with cyclic voltammetry. The authors thought that the good performance of the OMC modified electrode was attributed to the high density of edge plane defect sites on the ordered mesoporous carbon materials, which may provide many favorable sites for electron transfer to biomolecules. However, the sensitivity and the stability of the modified electrode were not studied because the technique used was not very sensitive. At the same time, the electrochemical behavior of l-cysteine (CySH) was investigated thoroughly at an ordered mesoporous carbon-modified glassy carbon (OMC/GC) electrode [27] and a sensitive CySH sensor was developed based on an OMC/GC electrode, which showed a large determination range (18–2500 ␮M), a high sensitivity (23.6 ␮A mM), and a remarkably low detection limit (2.0 nM). With an amperommetric technique, this detection limit was the lowest value ever reported for direct CySH determination on the electrodes at pH 2.0. At pH 7.0, the good linear ranges were from 18 to 1000 ␮M (correlation coefficient of 0.995) for the detection of CySH with a sensitivity of 8.0 ␮A mM and a detection limit of 50 nM. In this paper, the stability of OMC/GC electrode was studied and it has been found very high. The amperometric current of CySH remained nearly unchanged after the OMC/GC electrode was stored at 4 ◦ C for 1 month and only 8.0% current loss after 2 months was observed. The influence of edge plane defect sites on electrocatalytic properties of OMC was again suggested. The Raman spectrum of OMC (as shown in Fig. 2B) exhibits the presence of D and G bands located at 1355 cm−1 and 1602 cm−1 which indicate the presence of edge plane defect sites. Moreover, it was claimed that the oxygen-containing functional groups on OMC played an important role and their presence on OMC has been proved [26]. As those functional groups are available on OMC, it became important to study their influence on the electrocatalytic properties. For this goal, two kinds of experiments were carried out [26]. First, OMC was oxidized with HNO3 to generate a high concentration of oxygen-containing groups and the bands attributed to O H groups became stronger and shifted to higher wavenumbers. The material obtained (OMC-Ox) was used to modify a GC electrode to observe the influence of oxygen-containing groups. In the

second experiment, OMC was treated electrochemically to see the influence of edge plane defect sites. The results demonstrated that the ordered structure of OMC was very important for its electrochemical and electrocatalytic properties. The destruction of this structure resulted in the decrease of its electrochemical and electrocatalytic properties. The investigation on the electrochemical activity of OMC showed that the biomolecules could be detected at this electrode due to the presence of the oxygen-containing functional groups and edge plane-like defective sites available on the OMC electrode. It is then possible to affirm that those groups are very important for the electrocatalytic activity of OMC. An acceptable sensor for glutathione (GSH) and CySH has been developed. In order to detect CySH in the presence of GSH, it has been proposed to use the amperometric method at 0.4 V, where the interference by GSH is well reduced. Song and coworkers [33] found the same conclusions with dopamine. An OMC with carboxylic groups was prepared with concentrated nitric acid treatment at 80 ◦ C for 24 h. The obtained OMC was used to detect DA. A detection limit of 4.4 × 10−2 ␮M was achieved and the sensor is suitable for measurement of DA at a trace level under an environment similar to organism liquid. The detection of dopamine in the presence of ascorbic acid was then obtained with a low detection limit. It has also found that the oxygen functional groups play an important role in the catalytic activity for the dehydrogenation of propane to propylene [34]. The discussion shown above may come to a conclusion that the intrinsic electrocatalytic properties of OMC attributed to oxygencontaining functional groups and edge plane defect sites take advantage of the large surface area and open porous structure. The direct detection of the analytes above results in well-defined voltammetric signals occurring at lower overvoltage than other carbon materials. 4.2. Influence of the large surface area and the widely open and ordered mesostructure of OMC The large surface area and the widely open and ordered mesostructure of OMC are expected to offer high preconcentration efficiency and fast diffusion processes which should lead to a very sensitive voltammetric or amperometric detections. In this case, the analytes are first accumulated prior their amperometric or voltammetric detection. They are adsorbed onto the large surface area of OMC and they are electrochemically detected as adsorbed species on such conductive surface. The phenomenon has been analytically applied in the fabrication of a sensitive electrochemical sensor to detect ultratrace nitroaromatic explosives using OMC [35] as it is shown in Fig. 3A.

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Fig. 3. (A) Adsorptive stripping voltammetry in 0.5 M NaCl solution for 50 ppb TNT on OMC modified GCE (scan rate of 100 mV s−1 ). (B) The schematic illustration of the electrochemical reduction of 2,4,6-TNT on OMC electrode. Copyright 2011 Elsevier. Reproduced with permission from Ref. [35].

In the presence of 2, 4, 6 trinitrotoluene (TNT) the curve reveals three well-defined peaks at −0.37, −0.51 and −0.62 V (with adsorptive stripping voltammetry), respectively, which can be assigned to sequential reduction of the three nitro groups of TNT. The schematic illustration of the electrochemical reduction of TNT on OMC electrode is shown in Fig. 3B. By comparison with other materials such as carbon nanotubes and ordered mesoporous silica, it has been found that the high performance of OMC toward sensing trinitrotoluene is attributed to its large specific surface area and fast electron transfer capability. In fact, the sensitivity of 62.7 ␮A cm−2 ppb−1 was much higher than that of ordered mesoporous silica (8.5 ␮A cm−2 ppb−1 ) and that of multi-walled carbon nanotubes (1.57 ␮A cm−2 ppb−1 ). As low as 0.2 ppb TNT, 1 ppb 2,4dinitrotoluene and 1 ppb 1,3-dinitrobenzene could be detected at OMC electrode. Other substances have been detected at OMC modified electrode with the same procedure. Yu et al. [36] demonstrated that, in comparison with multi-walled carbon nanotubes (MWCNTs) and Vulcan XC-72 carbon, the CMK-3 modified electrode shows larger peak currents and higher adsorbed amounts for the two dihydroxybenzene isomers. They developed a sensor with sensitivity for catechol (CC) and hydroquinone (HQ) which is close to that of one compound alone. The sensitivity for CC and HQ was respectively 41 ␮A cm−2 ␮M−1 and 52 ␮A cm−2 ␮M−1 . The detection limits for CC and HQ were 1 × 10−7 M after a time of accumulation of 4 min. Morphine was also detected with high sensitivity (1.74 ␮A ␮M−1 ) and the time of accumulation was 5 min [37]. The OMC/GC modified electrode was also used to detect riboflavin in aqueous solutions [31]. With good stability and reproducibility, this OMC/GC electrode was applied in the determination of vitamin B2 content in vitamin tablets, and satisfactory results were obtained with a time accumulation time of 4 min. The sensitivity was 769 ␮A mM−1 with a detection limit of 2 × 10−8 M. In these cases OMC contributes to bring enhanced electrical conductivity and/or ensure lower resistance to mass transport, which results in rather short preconcentration times and/or fast response

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time. Moreover, the accumulation can be made by applying a potential and the example is the detection of Sudan I where differential pulse voltammetry was used [38]. In the same way, the anodic stripping voltammetry was used to deposit Pb2+ and differential pulse voltammetry was used to determine the metal ion [13]. Sometimes, the intrinsic electrocatalytic properties of OMC are combined to the accumulation ability of the material. An example is the detection of morphine [37] where electrocatalytic activity towards morphine is remarkably enhanced. The influence of the BET surface area on the electrocatalytic activity of OMC [39] has been studied. The results showed that OMC with high BET surface area exhibits improved electrocatalytic activity. Additional active sites are available on OMC-modified electrode with a higher BET surface area for electrocatalysis of morphine. The OMC (with high BET surface area) modified electrode showed high sensitivity, low detection limit, and fast response toward the electrochemical determination of morphine [39]. Moreover, the effect of the pore size has been investigated [40,41]. In general, in order to get a higher electrocatalytic activity, the porous modifier should have higher specific surface area theoretically. In this way the surface area is electrochemically accessible when the pore size is larger [41]. Thus, along with the decrease of the sucrose to silica weight ratio in the synthesis, the increase in the pore volume, pore size and surface heterogeneity of the different OMC samples might make more surface area electrochemically accessible and more exposure of edge-plane-like defective sites, which is consistent with the observed structure [40]. Those properties can also be combined to the accumulation ability of the binder. This is illustrated in the detection of epinephrine (EP) [42]. By combining the advantages of OMC with those of Nafion, the anodic peak of EP and that of AA were separated successfully (by ca. 144–270 mV) in the pH range of 2.0–10.0, which may make Nafion–OMC/GC electrode potential for selective determination of EP in the presence of AA at a broad pH range [42]. The authors saw that the EP amperometric response at Nafion–OMC/GC electrode in pH 7.0 PBS is extremely stable, with 99% of the initial activity remaining (compared to 32% at GC surface) after 120 min stirring of 0.20 mM EP. The advantage of Nafion was also used in the determination of DA, AA acid and UA at OMC/Nafion electrode [43]. 5. Electrochemical biosensors Here we will use the IUPAC definition of a biosensor. “Electrochemical biosensor is a self-contained integrated device, which is capable of providing specific quantitative or semi-quantitative analytical information using a biological recognition element (biochemical receptor) which is retained in direct spatial contact with an electrochemical transduction element” [44]. Therefore, we call electrochemical biosensor, a sensor that integrates a biorecognition element into the sensor [3]. The unique characteristics of OMC, including large pore and large electrocative area enables a high loading of active biomolecules and catalysts, which is a major requirement of a biosensor. 5.1. Immobilization of biomolecules on OMC with usual pore size With its large pore, OMC is able to load simple active molecules and several examples are available. Using chitosan as an effective linker between CMK-3 and GCE surface, Chen and coworkers [45] constructed a novel biosensor for the determination of H2 O2. The biosensor was based on a {Hb/CMK-3}6 film where hemoglobin (Hb) is adsorbed on OMC through layer-by-layer assembly. The detection limit was 0.6 ␮M. When OMC is used as a carbon paste, immobilization of the active molecules becomes easier. This

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Table 1 Performances of different sensors and biosensors based on OMC. Substance detected

Modifier

Glutathione l-Cysteine NADH Dopamine Epinephrine Riboflavin Catechol Hydroquinone Morphine H2 O2 Glucose Trinitrotoluene Ascorbic acid Uric acid H2 O2 (with enzyme)

Nafion–OMC/GC OMC/GC DMF–OMC/GC DMF–OMC/GC Nafion–OMC/GC DMF–OMC/GC Nafion–OMC/GC Nafion–OMC/GC DMF–OMC/GC {Hb/CMK-3}6 /GC OMC–GOD/GC Nafion–OMC/GC Nafion–OMC/GC Nafion–OMC/GC FDU-15-HB/GC

procedure has been used by Zhu et al. [46]. For the construction of this biosensor using an enzyme, glucose oxidase (GOD) was incorporated in OMC by mechanical mixing OMC, glucose oxidase and mineral oil to fabricate a sensitive and selective glucose biosensor. The obtained electrode (OMC–GOD) was compared to that constructed with CNT and GOD (CNT–GOD). The OMC–GOD electrode provided an evidently larger current signals with a sensitivity augmentation of 6.3-fold compared to the CNT–GOD electrode (18.42 ␮A M−1 and 2.92 ␮A M−1 for OMC–GOD and CNT–GOD electrodes, respectively) so as to decrease the detection limit from 0.13 mM (CNT–GOD electrode) to 0.072 mM (OMC–GOD electrode). This was due to the substantial decrease in the over voltage of the hydrogen peroxide oxidation along with the facile incorporation of glucose oxidase (GOD) into the composite matrix. The incorporation of GOD can also be achieved with Nafion as a linker in the construction of an enzyme-based device. For example, Zhou et al. [47] described a simple method for the construction of electrochemical alcohol dehydrogenase (ADH) and GOD-based biosensors for the determination of nicotinamide adenine dinucleotide (NADH) and hydrogen peroxide (H2 O2 ). The high density of edge-plane-like defective sites and high specific surface area of OMC could be responsible for the electrocatalytic behavior at OMC modified glassy carbon electrode (OMC/GCE), which induced a substantial decrease in the overpotential of NADH and H2 O2 oxidation reaction compared to carbon nanotubes modified glassy carbon electrode (CNT/GCE). The oxidation potential appeared at about 0.6 V, 0.4 V and 0.2 V for GCE, CNT/GCE and OMC/GCE, respectively. Especially, as an amperometric glucose biosensor, Nafion/GOD–OMC/GCE showed large determination range (500–15,000 ␮M), high sensitivity (0.053 nA ␮M−1 ), fast (9 s) and stable response.

5.2. Incorporation active molecules in OMC with large pore One can figure out that OMC with large pore and/or large surface area can load easily other active molecules. In this way, a three-dimensional (3D) ordered mesoporous carbon, a bicontinuous gyroidal mesoporous carbon (BGMC) has been used to immobilize myoglobin [48]. In contrast with carbon nanotubes and general carbon mesoporous materials, BGMC is of a relatively isotropic graphited structure and thus can more effectively enhance the heterogeneous electron transfer. The electrocatalytic activity of immobilized protein on BGMC was used to detect H2 O2 . Similarly, You et al. [49] demonstrated a strategy of a protein entrapment within mesoporous carbon matrices to probe the electrochemistry of glucose oxidase. The large surface area and remarkable electro-catalytic properties of mesoporous carbon materials make them suitable candidates for high loading of protein

Linear range 3–130 ␮M 18–2500 ␮M 5–900 ␮M 0.05–0.99 ␮M 0.1–1200 ␮M 0.4–1 ␮M 0.5–35 ␮M 1–30 ␮M 0.2–197.6 ␮M 1–57 ␮M 0.5–15 mM 0–0.22 ␮M 40–800 ␮M 5–80 ␮M 2–300 ␮M

Detection limit

Reference

90 nM 2 nM 1.61 ␮M 4.5 nM 35 nM 20 nM 0.1 ␮M 0.1 ␮M 30 nM 0.6 ␮M 156.52 ␮M 0.9 nM 20 ␮M 4 ␮M 0.8 ␮M

[26] [27] [30] [33] [42] [31] [36] [36] [39] [45] [47] [35] [43] [43] [51]

molecules and the promotion of heterogeneous electron transfer. Moreover, it has been noted that highly ordered 3D-mesoporous carbon material exhibited larger adsorption capacity for glucose oxidase and the immobilized enzymes retained a higher bioactivity compared with 2D-mesoporous carbons. The assembled glucose biosensor showed a broader linear response scale and higher detection sensitivity. The pore volume can be enlarged again to incorporate enzymes easily. In this way, an ordered mesoporous carbon (called FDU-15) was used by Wang et al. [50] to immobilize GOD. FDU-15 is a mesoporous material with uniform tubular channels varying from 2.8 to 7.4 nm. Furthermore, it was found that, in the presence of O2 , GOD immobilized on Nafion and FDU-15 matrices could produce a linear response to glucose. Pei et al. [51] claimed that Hb immobilized on FDU-15 film had performed direct electrochemistry and retained high electrocatalytic efficiency toward H2 O2 . The obtained Hb/FDU-15 electrode exhibited good analytical performance features, such as a wide determination range and low detection limit for H2 O2 determination. One can note then that a large number of substances can be detected at OMC modified electrode. This is clear in Table 1 where the performances of the sensors are shown. It is observed that the linear ranges are very large with very low detection limits, which indicates that OMC is a promising material. 6. Functionalization of OMC for electrochemical detection Although ordered mesoporous materials can act as catalysts, additional catalytic functions are introduced by incorporation of active sites in the walls or by deposition of active sites on the inner surface of the material. In this case, we have relatively large pores which facilitate mass transfer and very high surface area which allows a high concentration of active sites per mass of material. This is important, not only for enhancing further the sensitivity of the method, but also for improving the selectivity. There are many substances incorporated to functionalize mesoporous materials and some of them are briefly described: (1) Noble metal nanoparticles can be incorporated in the mesopore network. For example, the Pt nanoparticles can be incorporated inside the pores of the OMC. Fig. 4A and B shows the XRD patterns before (A) and after (B) loading 20 wt% Pt. One can observe the presence of Pt nanoparticles in OMC, consistent with Pt peaks in Fig. 4B. Fig. 4C shows the tomogram of the Pt/OMC. It is very clear from this snapshot that Pt has been deposited throughout the porous OMC network [52]. The obtained material has then improved electrocatalytic

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Fig. 4. XRD patterns of OMC before (A) and after (B) 20 wt% Pt loading. (C) Tomography slices through samples of 20 wt% Pt-loaded OMC. Copyright 2011 Elsevier. Reproduced with permission from Ref. [52].

properties. As an example of the scheme of immobilization of other substances on OMC, Liu and Chen [53] showed, in Fig. 5, the preparation of Pt/OMC nanocomposite. Compared with the original OMC modified electrode, the Pt/OMC-modified electrode displayed improved current response towards hydrogen peroxide. An electrodeposition method can also be used to attach gold nanoparticles on OMC. A nonenzymatic hydrogen peroxide (H2 O2 ) sensor based on GNPs–OMC/GCE has been fabricated using the method [54]. It exhibited good reproducibility and long-term stability. (2) Inorganic mediators have been immobilized. Zhou et al. [55] developed a convenient and efficient method for the functionalization of OMC using polyoxometalate H6 P2 Mo18 O62 ·xH2 O (P2 Mo18 ). By the method, GCE modified with P2 Mo18 which was immobilized on the channel surface of OMC was prepared. The electrochemical behavior of the modified electrode was studied in detail, including pH-dependence, stability and so on. The CV and amperometry studies demonstrated that P2 Mo18 /OMC/GC electrode has high stability, fast response and good electrocatalytic activity for the reduction of nitrite, bromate, iodonate, and hydrogen peroxide. Cerium(III) 12-tungstophosphoric acid (CePW) can also be immobilized on OMC and the CePW/OMC/GC modified electrode showed an enhanced electrocatalytic activity [56]. This property was applied in the determination of some biomolecules. Especially, the detection and determination of the guanine (G) in the presence of adenine (A) was achieved with good stability, low detection limit and reproducibility. (3) Organometallic catalysts can also play this role. OMC was functionalized with ferrocenecarboxylic acid (Fc) and the OMC–Fc modified electrode improved electrocatalytic properties compared to OMC alone [57]. Ascorbic acid was detected in the presence of dopamine with a separation peak-to-peak of 170 mV. The same modified electrode has been used in the electrochemical determination of uric acid (UA) [57]. From Fig. 6, one can note that the electrocatalytic properties of OMC are improved after Fc incorporation. We can figure out the increase of the anodic peak current at OMC–Fc electrode compared to that of GC and OMC electrodes. With amperometric method, at a constant potential of 375 mV, the catalytic current of UA versus its concentration shows a good linearity in the range 60–390 ␮M (R = 0.998) with a detection limit of 1.8 ␮M (S/N = 3). These results are not influenced by the presence of AA in the sample solution. With good stability and reproducibility, the present OMC–Fc-modified electrode was applied in the determination of UA content in urine sample and satisfactory results were obtained. Binuclear cobalt phthalocyaninehexasulfonate sodium salt (bi-CoPc) can be adsorbed onto didodecyldimethylammonium

bromide (DDAB/OMC) film by ion exchange [58]. Oxygen, hemoglobin and 2-mercaptoethanol were detected with this bi-CoPc/DDAB/OMC modified electrode with higher sensitivity and low detection limits. (4) Organic redox mediators, molecular and polymeric ones have been immobilized by impregnation or electropolymerization. It has been demonstrated that the integration of OMC and fullerene (C60 ) can provide a remarkable synergistic augmentation of the current [28]. To illuminate the concept, eight kinds of inorganic and organic electroactive compounds were employed to study the electrochemical response at an OMC–C60 modified glassy carbon (OMC–C60 /GC) electrode for the first time. The modified electrode showed more favorable electrontransfer kinetics than OMC/GC, carbon nanotube modified GC, C60 /GC, and GC electrodes. Such electrocatalytic behavior at OMC–C60 /GC electrode could be attributed to the unique physicochemical properties of OMC and C60 , especially the unusual host–guest synergy of OMC–C60 . It has also demonstrated that an excellent electron donor, tetrathiafulvalene (TTF) can be incorporated in OMC and the electrocatalytic behavior of OMC–TTF is attributed to the unique physicochemical properties of OMC and TTF [29]. An amperometric oxygen biosensor has been constructed based on OMC–TTF and it exhibited good response to dissolved oxygen with a large linear range and a very low detection limit. The interferences of AA and AA were suppressed. Zhu et al. [59] prepared a novel amperometric NADH sensor based on a Nile blue A (NB)/ordered mesoporous carbon (OMC) composite (NB/OMC) electrode. Under a lower operation potential of −0.1 V, NADH could be linearly detected with an extremely lower detection limit of 1.2 ␮M (S/N = 3). Conducting polymers such as polyaniline (PANI) has been used to functionalize OMC and the OMC–PANI electrode was used to detect Cu2+ and Pb2+ [60]. The detection limit was 6.00 × 10−9 M for Cu2+ and 4.00 × 10−9 M for Pb2+ . OMC can also be functionalized to overcome the problem of poor coverage of OMC on the substrate electrode that leads to large interfacial capacitance. For instance, OMC film was constructed by adsorbing OMC onto a self-assembled monolayer of C18 H37 SH chemisorbed on Au electrode [61]. The advantages of OMC and those of nanoparticles can be combined to construct an immunosensor. In this way, a class of novel redox-active hybrid nanostructures comprising of metal platinum and ordered mesoporous carbon (Pt–OMC) was designed as molecular tags for sensitive electrochemical immunoassay (alpha-fetoprotein, AFP, as a model analyte) on the cyclodextrin-functionalized graphene sensing platform [62]. Under optimal conditions, the developed immunoassay displayed a wide linear range from 0.2 pg mL−1 to 10.0 ng mL−1

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Fig. 5. Schematic illustrations of synthesis procedure for Pt–OMC samples. Copyright 2011 Elsevier. Reproduced with permission from Ref. [53].

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Fig. 6. Cyclic voltammograms of GC (a), OMC (b) and OMC–Fc (c) electrodes in 0.1 M LiClO4 + 0.1 M PBS (pH 7.3) containing 0.4 mM of UA. Scan rate: 20 mV s−1 . Copyright 2008 Elsevier. Reproduced with permission from Ref. [57].

toward AFP standards with a low detection limit (LOD) of 1.5 pg mL−1 . (5) Metal oxides or sulfides have been incorporated. It has been noted that electrocatalytic properties of OMC could be improved when the cobalt oxide nanoparticles (Co) were incorporated into the material [63]. A sensitive GSH sensor was developed based on the OMC–Co/GC electrode and it showed a high sensitivity and a remarkably low detection limit. Moreover, OMC–Co/GC modified electrode could be used for selective amperometric determination of GSH in the presence of glucose, DA and UA. Other oxide nanoparticles have been incorporated in OMC [64]. The performance of ordered mesoporous carbon containing iron oxide (OMC–Fe) has been compared to OMC and the properties of the new material have been found to be improved. It has been demonstrated that OMC–Fe combines electrochemical properties of OMC and iron oxide species. Electrocatalytic properties towards H2 O2 are enhanced because of synergetic effect of ordered mesoporous carbon and iron oxide species. The results are based on the combination of OMC and Fe3 O4 because it has been shown that the reduction of H2 O2 occurs at OMC but its detection is less sensitive than at OMC–Fe. This high sensitive determination can be explained by the OMC–Fe structure. The presence of iron oxide species results in the increase of the surface area. Having a catalytically active surface, OMC (in OMC–Fe) can promote the H2 O2 reduction and provide a base for the mediation for H2 O2 at OMC–Fe modified electrode with Fe3 O4 as a mediator. In that case, the oxidative valencestate (Fe3+ ) and the reductive valence-state (Fe2+ ) on Fe3 O4 are reduced and oxidized respectively. The electrochemical determination of H2 O2 at the OMC–Fe modified electrode was more sensitive than that at OMC electrode because the sensitivity for OMC–Fe was 8.4 ␮A mM−1 and not only OMC substantially offered smaller signals, but also the response is affected by regenerating the surface of the electrode. A sensitive and stable H2 O2 sensor was developed based on OMC–Fe electrode with a large determination range (7–4000 ␮M) and a good reproducibility. A simple and facile synthetic method to incorporate copper sulfide (Cu2 S) nanoparticles inside the mesopores of OMC has also been reported [65]. A nonenzymatic amperometric sensor of hydrogen peroxide based on the Cu2 S/OMC nanocomposite modified glassy carbon (GC) electrode was then developed. The combination of the unique properties of Cu2 S nanoparticles and the ordered mesostructure of OMC matrix resulted in the excellent electrocatalysis for hydrogen peroxide. The peak potential

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for reduction of hydrogen peroxide at Cu2 S/OMC/Nafion/GCE was more positive than that at OMC/Nafion/GCE, shifting positively by about 70 mV with higher cathodic response. (6) Ionic liquids with their high chemical stabilities, relatively high ionic conductivity and wide electrochemical windows can be used to functionalize OMC. They form gel with OMC by grinding. In this way, a quick and sensitive biosensor based on functionalized ordered mesoporous carbon and an ionic liquid has been developed for the first time for the detection of DA and UA in the presence of a large amount of AA [66]. Similarly, Zhu et al. [67] prepared OMC modified carbon ionic liquid electrode (CILE). CILE was prepared by mixing graphite powder with 1-ethyl3-ethylimidazolium ethylsulphate ([EMIM]EtOSO3 ) and liquid paraffin. Using Nafion as a binder, a double-stranded (dsDNA) biosensor based on Nafion–OMC/CILE was constructed. Under the optimal conditions the oxidation peak current increased with dsDNA concentration in the range of 10.0–600.0 ␮g mL−1 by differential pulse voltammetry (DPV) with the detection limit of 1.2 ␮g mL−1 .

7. Conclusions and outlook As shown in the previous sections, we have described several applications of OMC as electrochemical sensors and biosensors. OMC with its ordered mesostructure can be used alone in the construction of an electrochemical sensor. Especially, the high density of edge plane-like defective sites (EDSs) and oxygen-containing groups are the main functional groups that play an important role in the electrocatalytic properties for the design of those sensors. However, the large surface area and the widely open and ordered mesostructure of OMC can be applied in the construction of many sensors and biosensors. It has been also shown that the combination of such characteristics can be used. Other sensors and biosensors are based on the unique properties of OMC and those of other active sites immobilized on OMC because OMC has a high surface area and large pore volume. This may open up a new challenge and approach to construct more sensitive electrochemical sensors and biosensors because a kilogram-scale synthesis of OMC is now possible [68]. This field, however, has many points to be addressed for future developments. First, we need to understand exactly the electrochemical behavior of this material. In fact we have described the influence of pore size, surface area, functional groups on OMC. However, other works are still needed in this way. For example, there are many factors that affect the performance of the modified electrode and each contribution must be known. In particular, rationalization of the observed electrocatalytic effects, apparently different from one material to another one, would help at selecting to most appropriate OMC for target applications. Second, more than 75% of the applications used the CMK-3 material and more than 95% of the studies were made on the basis of a single material type. Only few works investigated, in the same time, several materials exhibiting distinct structural and/or porosity parameters. The future will certainly require a study of the other ordered mesoporous carbons for the analytical performance of the resulting devices. Moreover, OMC materials do often exist actually as powders and their durable immobilization onto solid electrode surfaces may require the use of an additional binder to improve the long-term mechanical stability of the device, especially if used in stirred solutions. A possible alternative would be the generation of continuous and uniform thin films of ordered mesoporous carbon directly onto the electrode surface. As most electroanalytical methods are diffusion-controlled, attention should be paid to get such continuous films with features of OMC.

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