Chemical and Biosensors Based on Graphene Analogue 2D Materials and Their Hybrids Padmanathan Karthick Kannan1,*, Rogerio V. Gelamo2, Chan-Hwa Chung1, Chandra Sekhar Rout3,* 1
School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea 2
Instituto de Ciências Tecnológicas e Exatas, UFTM, Uberaba, Minas Gerais 38064-200, Brazil 3
School of Basic Sciences, Indian Institute of Technology Bhubaneswar, Bhubaneswar, Odisha 751013, India * Email:
[email protected],
[email protected],
[email protected],
TABLE OF CONTENTS 1 Introduction 2 Synthesis of Graphene Analogue 2D Materials 2.1 Physical Approaches 2.2 Chemical Approaches 3 Chemical Sensing Applications 3.1 Principle 3.1.1 Electrochemical Sensors 3.1.2 Electrochemical Biosensors 3.1.3 Chemiresistive Sensors 3.1.4 Field effect transistor based Sensors 3.2 Current status 3.2.1 Electrochemical Sensors and Biosensors 3.2.2 Chemiresistive Sensors 3.2.3 FET based sensors 4 Conclusion Glossary References
1
1 Introduction The development of chemical sensors based on nanostructured materials has always been an important area during the past few years and it is still being growing up because of its essential importance in daily life. According to IUPAC, chemical sensor is a device that transduces chemical information into a useful measurable signal [1]. Chemical sensors have been widely used in many sectors especially in environmental monitoring and industries. In industrial areas, many toxic and pollutant gases are released as a result of chemical processes which may cause serious health problems to the mankind. Likewise, from the automobile and other combustion sources, many toxic chemicals are expelled into the atmosphere and it creates an adverse effect to the ecosystem. Therefore, the use of chemical sensors is of great importance in both indoor and outdoor atmospheric system, so that the information about the chemical species and their concentration level can be easily studied. In the past few decades, a variety of materials including nanostructured metal oxides, nano conducting polymers, carbon nanotubes, and their hybrid materials have been widely explored as an active material in the fabrication of chemical sensors. Voluminous literature are available on the use of above-mentioned materials in chemical sensors and many articles and papers have been already reviewed and their potentiality in chemical sensing applications have been reported [2–12]. However, there has always been a surge for the discovery of novel nanomaterials for the development of chemical sensors with superior sensing performance such as high sensitivity and selectivity with fast response time. The application of two-dimensional (2D) nanomaterials in chemical sensing finds tremendous attention in very recent years because of their excellent semiconducting nature with layer dependent physical and chemical properties and high surface to volume ratio [13]. Graphene, one of the first real 2D nanomaterials created great explosion among the researchers in the scientific and industrial community since its invention in 2004. From then, umpteenth numbers of research articles, review papers and books 2
have been published on the use of graphene in plethora of applications [13–20]. The unique physical and fascinating electrical properties of graphene have attracted considerable attention across the researchers globally and inspired to find new class of 2D nanomaterials similar to graphene, i.e graphene analogue 2D nanomaterials (GA2DNs) with exceptional physicochemical properties. To date, a variety of GA2DNs have been prepared and demonstrated for numerous applications including chemical sensors, electro-catalysts, solar cells, biosensors, energy storage and optoelectronics etc [21–31]. Some of the GA2DNs such as molybdenum disulphide (MoS2), tungsten disulphide (WS2), gallium arsenide (GaAs), gallium telluride (GaTe), bismuth telluride (Bi2Te3) have been employed in aforementioned applications. This book chapter is aimed to present the recent developments and current status on the application of GA2DNMs nanomaterials in chemical sensor applications. Several book chapters have been published on the development of chemical and biosensors based on conventional nanomaterials [32–34]. However, to the best of our knowledge, there is no book chapter focusing on the chemical sensing properties of graphene like two dimensional nanomaterials (GA2DNMs). Based on this view, the present chapter is categorized into three parts. The first part describes about the overview of the preparative methods so far developed for the synthesis of GA2DNMs. Both physical and chemical methods have been briefly reviewed. In the second part of this chapter, principles of sensing techniques have been discussed with suitable illustrations. Chemical sensors can be classified based on their detection techniques including electrochemical [35, 36], conductometric [37, 38], mass-sensitive [39, 40] and optical [41, 42]. Although, different techniques are explored in chemical sensing, this chapter is devoted mainly to electrochemical, chemiresistor, field effect transistor based sensors because of their low cost, compact and simple fabrication. In addition to that, they also have some credentials which are given below: Electrochemical sensors offer many advantages such as high sensitivity, high selectivity, wide linear range with very low detection limit and 3
fast response time [43]. With the same detection principle, electrochemical biosensors also find potential attention in chemical sensing in which biomolecules get immobilized on the electrode surfaces and act as a bio-recognition element [44]. The detection principle of biosensors has also been discussed in the second part of this chapter. The principle of conductometric sensors is based on the change in electrical parameters such as resistance[45], current [46], voltage [47] and impedance [48, 49] of active material with respect to change in the concentration of chemical species. Moreover, conductometric sensors are very compact and rugged and it can be operated in a very simple manner. Though, various electrical parameters are measured as a transducing element in conductometric sensors, resistance is the widely reported parameters in majority of literatures based on chemical sensors. In recent years, chemiresistive field effect transistors (FETs) based sensors received considerable interest in chemical sensing because of its simplicity, miniature size, rapid response and its high detectivity even for small amount of analyte molecules [50–52].. The third part of the present chapter deals with the recent developments and current trends on chemical sensors based on GA2DNs. Finally, the conclusion of the chapter has been addressed
2 Synthesis of Graphene Analogue 2D Materials In recent years, several synthesis methods have been used to prepare graphene and its analogue materials either in thin film or powder form. The synthesis methodologies basically include two categories such as top-down and bottom up methods and both physical and chemical approaches have been employed for this purpose. Figure 1 shows the range of preparative methods used for the preparation of GA2DNs.
2.1 Physical Approaches In order to prepare GA2DNs, various physical synthetic strategies have been employed. Some of the reported preparative methods are micromechanical cleavage, atomic layer deposition and
4
pulsed laser deposition. These methods play crucial role in determining their physical and chemical properties required for a suitable chemical and biosensor application. Micro-mechanical exfoliation commonly known as “Scotch tape” method is the foremost technique used for the preparation of graphene nanosheets. It is generally a familiar method for preparing scalable graphene nanosheets. However, researchers are currently using this technique for the preparation of MoS2 and other 2D layered nanosheets. Recently, Kis and his group have developed an ultrasensitive photodetector using MoS2 layer prepared on SiO2/Si substrate by micro-mechanical cleavage method [21]. Similarly, Late et al., reported the use of micro-mechanical exfoliation for generating nanosheets from various nanomaterials [53, 54]. Though, micro-mechanical exfoliation yields 2D nanosheets with high quality, the main limitation is that its low productivity. Atomic layer deposition (ALD), one of the physical vapour deposition methods has been used for the preparation of atomically thin coatings of 2D materials on a wide range of substrates [55, 56]. Using molybdenum (V) chloride (MoCl5) and hydrogen sulphide (H2S) as a precursor material, thin films of MoS2 were deposited on SiO2/Si substrate using ALD [55].
5
Various methods used for the preparation of GA2DNMs
Micromechanical cleavage
Physical methods Chemical unzipping
Hydrothermal and solvothermal
Chemical exfoliation
Chemical Chemical methods methods
Pulsed laser deposition
Wet Chemical Synthesis
Chemical vapour deposition
Figure 1. Various synthetic approaches to prepare GA2DNs
6
Using the same precursor materials, another work on ALD has been reported on the preparation of thin layer of MoS2 on SiO2 substrates for the fabrication of top gated FET. It is demonstrated that ALD technique is capable of producing wafer scale MoS2 film with 95% layer uniformity over large areas [57]. Pulsed laser deposition has also been used to prepare thin films of 2D materials in which laser pulse are exposed on a bulk target material. Serna et al. and Samani et al. have applied this technique for the preparation of layered MoS2 and GaSe thin films [58, 59]. From the reports on preparation of GA2DNs by physical methods it is evident that the obtained materials are of high quality with sufficiently large size to allow the fabrication of proof-of-concept devices with a variety of new experiments. But the demerit of the methods is that the yield with few-layer nanosheets by these methods is low and not scalable to mass production.
2.2 Chemical Approaches Chemical approaches have been commonly adapted because of the mass production of GA2DNs. Liquid based Chemical exfoliation is the most widely applied technique for the preparation of GA2DNs [60]. Under the chemical exfoliation category, several methods have been employed viz. direct ultrasonic exfoliation, electrochemical exfoliation, shear exfoliation, ion intercalation exfoliation, anion exchange exfoliation. In direct ultrasonic exfoliation, bulk materials are mixed with suitable organic solvents and under sonication, it get exfoliated. Some of the solvents such as N-methyl pyrrolidone [61], 1-methyl 2-pyrrolidone [62], dimethyl sulfoxide [63], N-N’ dimethylformamide [64] and aqueous solution of tetrabutylammonium hydroxide [65] have been used. Compared to direct ultrasonic exfoliation, electrochemical based is a fast and large scale method in which exfoliation takes place under the application of electrical bias [66, 67]. Generally, in electrochemical exfoliation, ionic species (i.e. both anionic and cationic) get intercalated into the bulk material by an electrochemical reaction that actually weakens the vander waals force between the layers, thus ease the exfoliation process. 7
Recently, there is another exfoliation process called shear exfoliation which has been used for the scalable synthesis of GA2DNs. In this method, a shear mixer with a rotor and stator is used in which layered powders are added. Under the generation of strong shear force, exfoliation takes place and the nanosheets get detached from the bulk crystal. Many reports are available on the preparation of graphene nanosheets by shear exfoliation [68–70]. In addition to that, recently, some of the layered materials like MoS2 and WSe2 have been prepared by employing shear exfoliation technique [71]. Apart from these exfoliation processes, intercalation assisted exfoliation has also been used to prepare GA2DNs. Normally in this method, Li+ [72–74] and acids such as H2SO4 [75], H3PO4 [76] are used as an intercalant which intercalate the bulk material. This intercalation process increases the spacing between the layers in the bulk material and also weakens the van der waals force. Due to this phenomenon, layered nanosheets can be easily separated from the bulk material upon applying ultrasonication. Chemical vapour deposition (CVD) is a typical fabrication technique in which ultrathin films of GA2DNs have been prepared. CVD offers films with several advantages such as high crystal quality, variable thickness with tunable electronic properties. By CVD graphene [77], MoS2 and WS2 [78, 79], Ga2Te3 and Ga2Se3 [80], GaSe [81] have been prepared. Wet chemical synthesis has also been used to prepare a variety of GA2DNs and their composites. Some of the methods such as hydrothermal and solvothermal synthesis, template synthesis, selfassembly and soft colloid synthesis have been extensively employed for the preparation of GA2DNs and their composites [82].
3 Chemical Sensing Applications 8
3.1 Principle In general, the detection method for chemical species occurs through chemical or physical reaction with the material surface [83] which can be semiconductors, conductors, conducting polymers and other composite materials [84–87]. Since the materials at nanometric scale present a high surface area/volume ratio, [88] favoring the enhancement of reaction/interaction with molecules in their neighbor [89]. Thin film deposition techniques have been used extensively for chemical and biological sensors construction [83, 90–92]. In most cases, these sensors are constructed using functionalization (or decoration) of some transducer material [93]. The transductor collects information, i.e. electric current [92], occurring during the interaction process as it can be seen in chemiresistors [89, 94, 95] In order to construct a chemical or biological sensor, nanostructures such as carbon nanotubes[88, 93], graphene [96, 97], nanowires [98], nanosheets [99, 100], nanorods [101, 102] and nanobelts [103, 104] are being deposited on solid substrates or directly over electrochemical electrodes which can be insulators, semiconductors,. Nanotubes, graphene, and other nanostructures without or with functionalization can be deposited by dip-coating [105, 106], dielectrophoresis [88, 107] or by drop casting [105, 108] over electrodes that can be fabricated by lithography techniques in order to establish electric contacts [84, 109]. Figure 2 shows the construction of a typical chemiresistor sensor. The functionalization above cited can be made using chemical or physical vapour deposition techniques such as plasma enhanced chemical vapour deposition, sputtering, thermal evaporation [83, 88], ion implantation [110], sol-gel [83], dip-coating [106] or several chemical process reported elsewhere.
9
Figure 2. Illustrative representation of a chemiresistor sensor construction and operation. The better sensor operation is obtained with a nanoparticle decoration that can react with specific molecules in order to alter the electric conductivity as shown above. Reprinted with permission from ref. [111], Z. Zhang, X. Zou, L. Xu, L. Liao, W. Liu, J. Ho, X. Xiao, C. Jiang, Nanoscale 4, 1166 (2010). Copyright@ Royal Society of Chemistry
10
Reactive plasma is another method used for nanostructures surface functionalization [112]. In this case, nanostructures are exposed to reactive species existing in the plasma and that induces a modification in the chemical structure of the material or formation of defect sites [113]. Sensors constructed with this plasma-modified structures have activated sites propitious for interacting with other substances, promoting a change in some physical or chemical material property, such as electrical conductivity[114], optical absorption or other[92]. Figure 3 shows the photograph of a multilayer graphene pellet treated by reactive plasma method to create defects in the graphene. Thus different kinds of sensors can be designed and developed through the techniques and materials herein described. The main sensors currently being studied are based on electrochemical reaction, chemiresistor, field effect transistor (FET), quartz crystal microbalance, and others. We discuss some results and principles involving chemical sensors in the next sections.
Figure 3. Multilayer graphene pellet treatment using reactive plasma. 11
3.1.1 Electrochemical Electrochemical sensors have been widely used for the detection of various chemical species due to their simplicity, low cost, high sensitivity and selectivity. Nanostructured materials such as metal nanoparticles, nanostructured metal oxides, conducting polymers, carbon nanotubes, graphene and nanocomposites consisting of either metal oxides, CNTs and graphene with metals or metal oxides or conducting polymers have been used to prepare electrodes of electrochemical devices. On the other hand GA2DNs materials such as, MoS2, WS2 and SnS2 and Ni3S2 with graphene and CNTs have been used to improve the sensitivity (for example) in electrochemical sensors. The operation principle of an electrochemical sensor is based on charge transfer between the electrode surface (working electrode) and an analyte target present in an appropriate electrolyte solution. The setup of an electrochemical apparatus consists of a working, a counter and a reference electrode. In order to detect some electroactive species, three techniques are most used, cyclic voltammetry, amperometry and potentiometry, but in general the first one is the most used due to its versatility in obtaining the information about the electrochemical reactions occurring in the electrodes [115]. The working principle of an electrochemical sensor is depicted in Figure 4. High sensitivity and selectivity performance can be achieved for different chemical and biological species using the electrochemical configuration.
12
Figure 4. Illustrative representation of the working principle of electrochemical sensor
13
3.1.2 Electrochemical Biosensors Electrochemical biosensors are recently emerging as a potential analytical technique for the detection of various chemical species. In this sensor type, bio-receptor molecule get immobilized on the surface of the working electrode using a specific binder material, which acts as a transducer and the subsequent electrochemical analysis is carried out. A schematic of the design of electrochemical biosensor is shown in Figure. 56. Some of the biological materials such as enzymes, tissues, anti-bodies, nucleic acids and hormones have been used as a bio-receptor molecule [116]. During the measurement, sensing layer composed of bio-active material and react with the analyte species and due to the reaction, electrical signal is generated. Electrochemical biosensor offers several advantages such as low cost, high sensitivity, high selectivity and rapid detection of chemical species (fast response time). Like conventional electrochemical sensors, analytical techniques such as potentiometric, amperometric and electrochemical impedance spectroscopy have also been used in electrochemical biosensors for detection analysis.
Bio-recognition element
Product
Electrode
Signal Processing Output
Current / A
Current / A
Electrochemical Analyzer
Potential / V
Analyte
Time / s
Figure 5. Schematic diagram showing the design of an electrochemical biosensor
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3.1.3 Chemiresistive Sensors Sensing of chemical species using chemical resistors (chemiresistor devices) have been widely used mostly for gas and vapour detection [89]. The operation principle of this sensor is relatively simple. The electrical conductivity of a resistive element (CNTs, graphene oxide, functionalized graphene, thin films, nanowires, nanosheets and others nanostructured materials) is monitored between electrodes [88, 92]. The electrodes can be constructed directly over the resistor element by lithography for example, or using tips connecting the resistor to the external equipment (electrometer for example). The materials used as chemical resistor must be carefully chosen to promote the chemical or physical interaction with the molecule to be detected [28]. A significant change in resistor conductivity is observed when the interaction occurs. It is essential that this measurement be accurate at a determined pressure and thus a good device calibration is obtained. Chemo resistors are a simple method of detecting and measuring substances but it is not selective for simultaneous measurements. However, very low detection limit can be achieved through these device [92]. In order to estimate the sensor sensitivity the equation below is used, where R and Ro are the electrode in a presence of some specie and reference resistances respectively. 𝑅 − 𝑅𝑜 ∆𝑅% = ( ) 𝑥100 𝑅𝑜
On the other hand, the possibility of simultaneous detection of different species as well as pressure measurements have been investigated worldwide by electrode functionalization with GA2DNs [88]. Lab-on-Chip devices have been constructed with different chemo resistors and making it possible for the simultaneous detection of several substances at the same time [117, 118]. Figure 6 shows the schematic diagram of a GNRFET gas sensor array on a SiO2/Si
15
substrate in which 10 by 10 numbers of sensing electrodes have been fabricated using photolithography technique.
3.1.4 Field effect transistor based Sensors The sensors based on Field Effect Transistor (FET) have been widely used for the detection of chemical substances, either liquid or gaseous. In general, their operation involve monitoring of the voltage necessary to sustain the current between the source-drain electrodes [119]. A modification in the band structure of the gate due to its interaction with the chemical substances of interest induces changes in the current between source-drain electrodes yielding a sensing mechanism[120]. Frequently, the material used as a gate is a thin film of metal oxide or chalcogenide. Advantages of the FET based sensors are that they operate at room temperature and the performance of the sensor can be tuned by applying a desired gate voltage. With the advanced studies in low dimensionality materials, carbon nanotubes, graphene and graphene oxide, nanowires, nanosheets and other two-dimensional materials have been widely used [83, 84, 109, 121] in FET sensors. Figure 6 and 7 show the construction of a typical FET based chemical sensor and its sensing characteristics. A large number of work related to construction and use of FET devices for chemical and biological sensing are available in literature. Silicon platform as substrate is more common for its lithography process used to construct FET electrodes, however, work on flexible commercial polymers as a substrate in sensors for liquid and gas sensing [109, 122] have been reported. FET devices offers advantages such as selectivity to many different substances, low operation temperature, low power consumption and good stability during operation [119, 120]. However, the need for electrodes with micro and nanometric spacing
16
(a)
(b)
(c)
Figure 6. (a) Schematic diagrams showing a GNRFET gas sensor array on a Si substrate. (b) Schematic of GNRFET sensor device (c) Image of a patterned device consists of 144 GNRFET sensor devices on a 4 inch Si wafer. Reprinted with permission from ref. [123], W. Xu, H.-K. Seo, S.-Y. Min, H. Cho, T.-S. Lim, C. Oh, Y. Lee, and T.-W. Lee, Adv. Mater. 26, 3459 (2014). Copyright@ John Wiley and Sons
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(a)
(b)
(c)
Figure 7. (a) Illustrative representation of a WS2 FET-based device used in ammonia and ethanol sensing; (b) Time-dependent photocurrent response of WS2 FET device under various gas atmospheres and (c) Measured dark current and photocurrent of WS2 FET device under different gas atmospheres. Reprinted with permission from ref.[124], N. Huo, S. Yang, Z. Wei, S.-S. Li, J.-B. Xia, and J. Li, Sci. Rep. 4, 5209 (2014). Copyright @ Nature Publishing.
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among source, drain and gate requires a great deal of manufacturing processes which are only accessible in major research centers (for example lithography technique).
3.2 Current Status 3.2.1 Electrochemical sensors and Biosensors Great quantity of work related to the GA2DNs structures and their hybrid used as electrochemical sensors have been reported in literature. Table 1 shows the list of some of the reported electrochemical sensors baesd on GA2DNs. The construction or modification of electrodes for electrochemical measurements using GA2DNs and the ability to detect large range of chemical substances through this kind of sensor makes this system one of most used worldwide. In general, nanocomposites constructed from graphene (or nanotubes) conjugated with nanoparticles of some active material (such as conductive polymers or other GA2DNs) are readily available and they are most effective in sensing purposes. [115, 125]. Using a set of nanocomposites consisting of graphene and conducting polymers [86], it is possible to detect H2O2, glucose, other organic molecules, and oligomers. Very recently, few reports have appeared on the use of graphene-polymer based nanocomposites in electrochemical sensing towards bisphenol [126], vitamin K3 [127], dihydronicotinamide adenine dinucleotide [128] and iodate species [129]. An electrochemical sensor for acetaminophen (AP) has been constructed with reduced graphene oxide (RGO) and poly(3,4ethylenedioxythiophene) (PEDOT) nanotube modified GCE. The developed sensor exhibited very high selectivity towards AP in the presence of common interferants with the high sensitivity value of 16.85 A M-1 cm-2. A linear range of 1-35 M has been achieved with the LOD value of
0.4 M [130]. The SEM images of RGO-PEDOT composite used in
electrochemical detection of AP and their corresponding sensor data is shown in Figure 8. A work reporting the characterization and development of an amperometric sensor based on
19
graphene nanosheets doped with bismuth for hydrazine detection has been reported by Devasenathipathy et al. [131]. The sensor exhibited a wide linear range (20 nM to 0.28mM) for the detection of hydrazine and a very low detection limit (LOD) of 5 nM, which is an extremely low concentration to determine hydrazine in neutral pH. This work shows the possibility to detect several other chemical species such as Cl-, Br-, I-, CO32-, NO-3, NO2-, glucose, fructose, ascorbic acid and others.
(b)
(a)
(d)
(c)
(e)
Figure 8. SEM images of (a) PEDOT nanotubes, (b) RGO and (c) RGO-PEDOT nanocomposites used in AP detection. (e)
DPV curves obtained for RGO-PEDOT
nanocomposite modified GCE at different concentrations of AP and (f) selectivity data of RGO-PEDOT/GCE towards AP with common interferants such as glucose, nitrite, methanol and ethanol. Reprinted with permission from ref.[130], T.-Y. Huang, C.-W. Kung, H.-Y. Wei,
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K. M. Boopathi, C.-W. Chu, and K.-C. Ho, J. Mater. Chem. A 2, 7229 (2014). Copy Right @ Royal Society of Chemistry Wu et al. [132] developed a cadmium sensor based on nafion functionalized-graphene nanosheets electrodes and compared this sensor with another one based on multiwall carbon nanotubes. The results confirmed that the graphene-based sensor present some advantages in terms of repeatability, sensitivity and detection limit over the MWCNT-based sensor. The graphene-based sensor show superior analytical performance. Another advantage observed is that the sensor could successfully differentiate cadmium ions from interferents. The concentration range studied for Cd2+ was 0.25 µg L-1 to 5 µg L-1, with a detection limit of 3.5 mg L-1. The superior sensing performance of graphene-nafion composite compared to MWCNT can be attributed to its large specific surface area, high electrical conductivity and unique nanosheet structure. These factors greatly enhances following three phenomena: (i) the adsorption of cadmium ion over the graphene surface (ii)electron transfer rate between the surface of the GC electrode and cadmium ions in the bulk solution (iii) easy transfer of cadmium ions due to the broad space of nanographene. Pokpas et al. [105] used pencil graphite rods modified with a nafion-graphene nanocomposite in conjunction with an in situ plated bismuth-film to construct an electrochemical sensor for the determination of trace amounts of Zn2+, Cd2+ and Pb2+ in tap water samples. The electroanalytical sensor was successfully applied for the determination of metal ions in concentrations of 5 mg L-1, 5 μg L-1 and 15 μg L-1 for Zn2+, Cd2+ and Pb2+, respectively, with a good reproducibility. Roushani et al.[133] constructed a GA2DNs electrochemical sensor composed of graphene/quantum dots/riboflavin modified glassy carbon (GC/GQDs/RF) electrode to determine persulfate (S2O82-). The results indicated a catalytic reduction of the composites allowing an amperometric detection of S 2O82− at a potential of −0.1 V at detection limit of 0.2 µM, in concentration range of 1.0 µM to 1 mM and sensitivity of 4.7 nA µM−1. Very recently, electrochemical simultaneous detection of 21
heavy metals such as Cd2+, Pb2+,Cu2+ and Hg2+ has been reported by employing GCE modified with Au nanoparticles decorated RGO in which RGO was prepared by using vegetable extract [134]. The developed electrode have shown high sensitivity and selectivity towards heavy metal ions and the achieved sensor data was found to be compared with the standard ICP-MS data. Figure 9 show the schematic representation showing the preparation of Au decorated RGO using vegetable extract with their corresponding calibration data.
(A)
(B)
Figure 9 (A) Schematic showing the preparation of Au decorated RGO using vegetable extract. (B) Square wave anodic stripping voltammetry data and the calibration data of Au decorated RGO modified GCE measured in different concentrations of heavy metal ions. Reprinted with permission from ref. [134], P. Gnanaprakasam, S. E. Jeena, D. Premnath, and T. Selvaraju, Electroanalysis 28, 1885 (2016). Copy Right @ John Wiley and Sons 22
The possibility to detect hydrogen peroxide using an electrochemical sensor based on MoS 2 nanoflowers decorated with platinum nanoparticles was proposed by Lin D. et al. [135] . MoS2 nanoflowers were prepared by a simple one-step hydrothermal synthesis and the MoS2 Ptdecorated was transferred to glass carbon electrodes using nafion to fix this nanosctructures over the electrode surface. Figure 10 shows the illustrative scheme of Pt decorated MoS2 nanoflower preparation demonstrated by Lin D. et al. The sensor shows good stability for more than 2 weeks and a wide linear range of 0.02 to 4.72 mM and a low detection limit 0.345 µM for hydrogen peroxide detection. Electrochemical characterization and sensing data of MoS2Pt NP towards H2O2 is shown in Figure 11. An advantage of the developed sensor mentioned by Lin et al. is the simpler, efficient, and economic MoS2–Pt nanohybrids obtaining compared with other works.
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Figure 10. (a) Illustration of one-step hydrothermal synthesis and the MoS2 nanoflower Ptdecorated for a hydrogen peroxide detection. TEM images of (b) MoS2 nanoflowers and (c) Pt decorated MoS2 nanoflowers (Reprinted with permission from ref. [135], D. Lin, Y. Li, P. Zhang, W. Zhang, J. Ding, J. Li, G. Wei, and Z. Su, RSC Adv. 6, 52739 (2016). Copy Right @ Royal Society of Chemistry
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Figure 11. Electrochemical characterization of hydrogen peroxide sensor using MoS2 nanoflowers decorated with platinum nanoparticles. Low concentrations of hydrogen peroxide were detected using this electrochemical sensor. Reprinted with permission from ref. [135], D. Lin, Y. Li, P. Zhang, W. Zhang, J. Ding, J. Li, G. Wei, and Z. Su, RSC Adv. 6, 52739 (2016). Copy Right @ Royal Society of Chemistry A chloramphenicol sensor operating by differential pulse voltammetry was demonstrated by Yang T et. al. [136]. The detection of chloramphenicol is important for health purposes because it act strongly against infections caused by bacteria. The sensor is fabricated by drop casting the nanocomposite composed of molybdenum disulphide (MoS2) and polyaniline (PANI) on a carbon paste electrode (CPE) which was prepared through in-situ polymerization of aniline on the surface and interlayer of thin-layered MoS2. The The linear range and the detection limit value of the developed sensor were found to be from 1x10-7 mol L-1 to 1x10-4 mol L-1and 6.9x10-8 mol L-1 with high stability. Like PANI, nanocomposite of MoS2 with PEDOT has been used as an active material for the electrochemical simultaneous detection of dopamine, uric acid and ascorbic acid [137]. The electrode material showed good catalytic activity towards the oxidation of DA, UA and AA which has been attributed to the
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synergetic effect of both MoS2 and PEDOT. Furthermore, the fabricated electrode material has very high anti-interference effect in the presence of glucose. Additionally, the obtained nanocomposite material have shown high sensitivity towards each of the analyte molecules during mutual interference experiment in the presence of DA, UA and AA confirming MoS2/PEDOT, a potential material for the fabrication of low cost electrochemical biosensors. Au decorated MoS2 nanosheets have been used for the simultaneous detection of DA, UA and AA [138]. Compared to pure Au and MoS2, the composite material showed enhanced electrocatalytic activity towards DA, UA and AA with well resolved oxidation peaks. In another report by Wang and his group, electrochemical dopamine sensing properties of Au decorated MoS2 nanosheets have been demonstrated [139]. The developed nanocomposite material exhibited reproducible results with very high performance towards DA detection in the presence of excess of AA. By employing MoS2-graphene nanocomposite as an electrode material, electrochemical detection has been investigated towards honkiol, one of the chemical compounds used in Chinese herbal medicines [140]. Figure 12 shows the SEM images of MoS2 and MoS2/graphene nanocomposite used in honkiol sensing. Their sensor data have shown high performance such as wide linear range, low detection limit, high selectivity with superior recovery from real samples. Differential pulse voltammogram data recorded for MoS2graphene modified GCE at different concentrations of honkiol with its corresponding calibration data is shown in Figure 13.
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Figure 12 SEM images of (A) MoS2 and (B) MoS2-Graphene nanocomposite. Reprinted with permission from ref. [140], X. Zhao, X. Xia, S. Yu, and C. Wang, Anal. Methods 6, 9375 (2014). Copy Right @ Royal Society of Chemistry
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Figure 13. (A) Differential pulse voltammograms recorded at the MoS2/graphene modified GCE at different concentrations of honkiol in 0.2 M PBS (pH 5.5) (B) Corresponding calibration curve plot Reprinted with permission from ref. [140], X. Zhao, X. Xia, S. Yu, and C. Wang, Anal. Methods 6, 9375 (2014). Copy Right @ Royal Society of Chemistry
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Table 1 List of reported electrochemical sensors based on various GA2DNs S. No
Material
Analyte detected
Sensitivity
Linear range
LOD
(M)
(M)
Ref.
(1)
Fe3O4-MoS2
Nitrite
-
1- 2630
0.5
[141]
(2)
MoS2-Au-PEI-hemin
Clenbuterol
-
0-
0.0106
[142]
(3)
MoS2
DA,AA, UA
-
(4)
MoS2-PEDOT
DA,AA, UA
DA : 1 - 900
DA
: 0.15
AA : 5 - 1200
AA
: 0.82
UA : 1-
60
UA
: 0.06
DA: 36.40 A mM−1 m−2
DA : 1 - 80
DA
: 0.52
1.20 A mM−1 m−2
AA : 20 - 140
AA
: 5.83
UA: 105.17 A mM−1 m−2
UA : 2 - 25
UA
: 0.95
dGTP: 100
dGTP : 1.6
AA:
(5)
MoS2-poly(xanthurenic acid)
dGTP, BPA, TNT
7.22
-
BPA : TNT : (6)
CuNFs-MoS2
H2O2
-
- 1000
0.1 - 10 11
- 132
0.04 - 1.88
BPA
: 0.037
TNT
: 3.35
[143]
[137]
[144]
0.021
[145]
0.32
[145]
0.31
[146]
1.88 - 35.6 (7)
CuNFs-MoS2
Glucose
-
1-20 20-70
(8)
MoS2-Ni
Glucose
1824 A mM-1
0- 4000
(9)
MoS2-Cu
Glucose
1055 A mM-1
0- 4000
(10) Au-decorated MoS2
AA, DA and UA
-
AA: 0.001 - 0.07
-
[147]
AA- 100
[139] 29
DA: 0.05 - 0.004
DA-
UA:10
- 0.001
UA - 10
20000
- 4720000
(11) Pt decorated MoS2
H2O2
-
(12) Au-decorated MoS2
Dopamine
-
0.1 - 200
(13) MoS2-PANI-Au
Dopamine
-
1 - 500
(14) MoS2–graphene
Acetaminophen
-
0.1 - 100
(15) WS2–graphene
CT, RS and HQ
-
(16) WS2-silver
Theophylline
-
(17) MoS2-Chitosan-Ag
Tryptophan
-
(18) FeS2
H2O2
(19) MoS2-Chitosan-Au
Bisphenol-A
-
(20) Poly(diallyldimethylammoniu
Eugenol
-
604.8 A mM-1
0.05
0.345
[135]
0.08
[138]
0.1
[148]
0.02
[149]
CT : 1 - 100
CT- 0.2
[150]
RS : 1 - 100
RS- 0.1
HQ :1 - 100
HQ- 0.1
0.05 - 150
0.003
[151]
- 120000
0.05
[152]
- 1900
4
[153]
0.05 - 100
0.005
[154]
0.1
0.036
[155]
500 10
- 440
m chloride) - graphene-MoS2
LOD – Lower Detection Limit, PEI -Polyethylenimine; PEDOT - Polyethylenedioxythiophene; CuNFs – Copper nanoflowers; dGTP - 2′deoxyguanosine-5′-triphosphate trisodium salt, BPA- Bisphenol; TNT - trinitrotoluene; AA – Ascorbic acid; DA- Dopamine; UA Uric acid; CT Catechol, RS – Resorcinol, HQ - Hydroquinone
30
During the last few years, voluminous amount of literatures have already been reported on the development of electrochemical biosensors based on graphene nanosheets [156–159]. Recently, GA2DNs such as MoS2, WS2 nanosheets and their graphene and metal nanoparticle based composites have been used in electrochemical biosensing. The following section briefly shows the current trends in the development of electrochemical biosensors based on GA2DNs. Most of the recent works on electrochemical biosensors are based on molybdenum disulphide (MoS2) and their composites with metal and carbon nanomaterials. A label free ultrasensitive electrochemical DNA biosensor has been developed with high electrochemical activity using thin layer of MoS2 nanosheets modified carbon paste electrode as a working electrode [160]. The hybridization of the working electrode to sensor DNA was achieved by immobilized probe DNA on the electrode surface and after that, single strand DNA was dropped onto it. Thus, the hybridization reaction happened which helps in the effective sensing of DNA. The developed sensor shows label free DNA detection performance in the concentration range varying from 1 ×10-16 to 1 ×10-10 M with a detection limit of 1.9 × 10-17 M. In another work by Wang et al. direct detection of DNA was demonstrated by using thionin functionalized layered MoS2 nanosheets modified glassy carbon electrode [161]. The thionin functionalization process was assisted by an ionic liquid namely 1-butyl-3-methylimidazolium hexafluorophosphate. The sensor results shown that the modified GCE exhibits linear range in the detection of double strand DNA from 0.09 ng mL-1 to 1.9 ng mL-1. Also, the fabricated sensor shows satisfactory performance in the detection of circulation DNA from healthy human serum sample. Gold (Au) nanoparticles based MoS2 composites have been widely used in electrochemical biosensors because it significantly increase the effective surface area of the working electrode. Moreover, the electrocatalytic performances of composite materials have shown synergetic behaviour of both bare Au and MoS2 modified electrodes. Very recently, Parlak et al and Lin et al have developed an electrochemical biosensor for the detection of 31
glucose [162] and cholesterol [163] using MoS2-Au nanocomposite modified gold and glassy carbon electrode. In these reports, glucose oxidase and cholesterol oxidase enzymes act as a bio-recognition element which was immobilized on the working electrode surfaces. The immobilized enzyme increases the bioelectrocatalytic reactions between the electrode and the electrolyte interfaces, helping to increases the effective sensing of glucose and chlorestrol. Immobilization of heme proteins such as haemoglobin [164], myoglobin [165], cytochrome C [166] on the surface of the working electrode find wide interest on increasing the performance of the electrochemical biosensors. In a recent work by Wang and his group [167], an electrochemical biosensor has been developed in which they explored the direct electrochemistry of haemoglobin for the detection of H2O2 and NO.
Au nanoparticles
decorated MoS2 nanosheets modified GCE was used as a working electrode in which haemoglobin was immobilized. It has been shown that the presence of haemoglobin enhance the electron transfer reaction between the electrode and the electroactive center of haemoglobin due to its excellent conductivity and biocompatibility. The fabricated sensor showed linear detection range of 10 to 300 μM and 10 to 1100 μM with a detection limit of 4 and 5 μM, for H2O2 and NO respectively. Similar to MoS2, aptamer immobilized Au decorated WS2 modified GCE has been used in electrochemical sensing of 17-estradiol, which is a natural steroid estrogen secreted by the ovary [168]. Figure 14 shows the schematic diagram of the fabrication of electrochemical biosensor for the detection of 17-estradiol. The fabricated sensor showed linear range of 0.01 nM to 5 nM with the LOD value of 2 pM.
32
Figure 14. Schematic diagram showing the fabrication of aptamer immobilized AuNPs/MoS2 modified GCE for the detection of 17-estradiol. Reprinted with permission from ref. [168], K.-J. Huang, Y.-J. Liu, J.-Z. Zhang, and Y.-M. Liu, Anal. Methods 6, 8011 (2014). Copy Right @ Royal Society of Chemistry
Nanocomposites of GA2DNs with carbon nanomaterials find immense interest in electrochemical biosensors due to its fascinating physical and chemical properties, fast electron transfer capability and good biocompatibility. Fang et al. recently prepared a novel nanocomposite of MoS2 with carbon aerogel by simple hydrothermal synthesis for the fabrication of electrochemical aptamer biosensor [169]. Carbon aerogel is nothing but a three dimensional network structure of carbon which has good electrode characteristics such as high surface area and electrical conductivity. In addition that, they have studied the electrochemical biosensing by modified the electrode surface with gold nanoparticles and obtained superior sensing performance towards aptamer. In a similar way, MoS2-graphene nanocomposites were used as an active material in electrochemical biosensing. Myoglobin immobilized MoS 2graphene nanocomposite modified glassy carbon electrode has been used in electrochemical 33
biosensing towards H2O2 and NO2 [170]. On the surface of the myoglobin electrode, nafion was also decorated in order to obtain a very tight electrode with high electron transfer rate. In another work by recent times, MoS2-graphene quantum dots
nanocomposite has been
demonstrated as a support material for the fabrication of bio-electrode in which trametes versicolour laccase enzyme was successfully immobilized [171]. The fabricated electrode has shown high electrochemical sensing property towards caffeic acid. Also, it has been successfully tested for the determination of polyphenols such as Chlorogenic acid and (-) epicatechin in red wine samples. Layered tungsten disulphide (WS2) sample has also been studied for electrochemical biosensor application. Nanocomposites of WS2 with multiwalled carbon nanotube and acetylene black have been used as an electrode material for electrochemical DNA biosensor [172, 173]. In both the reports, nanocomposite materials were prepared by simple hydrothermal synthesis and after that, gold nanoparticles were decorated on the electrode surface by thiol immobilization method. The presence of gold nanoparticles on the electrode increases the electron transfer rate of the reaction. In addition to that, it also retained the immobilized biomolecules on the surface; thereby it shows enhanced stability towards DNA sensing. A list of electrochemical biosensors based on various GA2DNs is shown in Table 2.
34
Table 2 List of electrochemical biosensors based on various 2D layered inorganic materials S. No
Material
Analyte detected
Sensitivity
(1)
Chox/MoS2-AuNPs
Cholesterol
4460 μA mM−1 cm−2
(2)
MoS2-GQDs-Laccase
CA, CGA, EC
CA
: 17.92±0.21 nA µM−1 : 7.32±0.14 nA µM−1
(µM)
(µM)
48
0.26 ± 0.015
[163]
0.38 -
10
0.32
[174]
100
0.19
10
-
: 5.63±0.12 nA µM−1
8.26 -
100
:16.42±0.09 nA µM−1
2.86 -
100
MoS2/Au NPs/GOx
Glucose
13.8 µAµM-1cm-2
(4)
Hb-AuNPs/MoS2/GCE
H2O2, NO
-
Glucose
-
Glucose
-
Ref.
0.50 -
0.38 -
(3)
APTES/chitosan/rMoS2/GO
LOD
CGA: 11.25±0.36 nA µM−1
EC
(5)
Linear range
250
8.26 2.04
- 13200
0.042
[175]
10
-
300
4
[167]
10
- 1100
5
0
- 20000
-
[176]
-
[177]
D (6)
GOx/MoS2
-
35
(7)
PB/Bi2Se3/GOx
Glucose
24.55 μA mM-1 cm-2
(8)
HRP/MoS2
H2O2
-
1
(9)
HRP/MoS2/Gr
H2O2
679.7 μA mM-1 cm-2
0.2 - 1103
(10) GOx/Au/SnS2/chitosan
Glucose
21.78 mA M-1 cm-2
(11) GOx/MWCNTs–SnS2
Glucose
-
(12) SWCNT/CS–SnS2
AA, DA and UA
-
(13) Nafion-GOx/Au/MoS2
Glucose
-
(14) GOD/Au/MoS2/MWCNTs
DNA
-
(15) Nafion/Hb/MoS2
H2O2
21.65 mA M-1 cm-2
10
20
- 11000
3.8
[178]
-
0.26
[179]
0.049
[180]
1
[181]
950
- 1320
0.00002 - 0.00195 0.000004 10
-300
0.00001 - 0.1 20
- 180
[182]
-
[183]
2.8
[184]
0.00000011
[185]
6.7
[186]
Chox- Cholestrol oxidase; Au NPs- Gold nanoparticles; GQDs- Graphene quantum dots; GOx and GOD- Glucose oxidase; Hb – Hemoglobin; CA- Caffeic acid; CGA - Chlorogenic acid; EC - (−) Epicatechin; APTES - 3-aminopropyltriethoxysilane; Pb- Prussian blue; HRP – Horseradish peroxidase; CS- Chitosan
36
3.2.2 Chemiresistive Sensors The use of chemical resistors (chemiresistor) as sensors is widespread reported in literature [86, 88, 92, 115, 187] due to the versatility of its construction and operation [89] to detect environmental contamination [188], gases [189, 190], biological substances [191] and others [90]. This technique is still widely used despite the progress made in the development of other kinds of sensors. The detection of toxic gases such as ammonia (NH3) is important for environmental control in industrial activities. Late et al. [188] demonstrated the possibility of using nanosheets of molybdenum diselenide (MoSe2) to detect NH3 (ammonia) in the concentration range of 50 to 500 ppm. Electron beam lithography is used for the fabrication of gas sensor device. Figure 15a and b shows the optical image of bulk and single layer MoSe2 along with the fabricated sensor device. They confirmed the interaction between NH3 and the MoSe2 by a substantial change in the carrier concentration which interferes in electrical measurements. Moreover, Raman measurement was employed as a tool to demonstrate the charger transfer mechanism of MoSe2 upon exposure towards ammonia. The sensing performance of MoSe2 towards ammonia is shown in Figure 15 c and d. Wu et al. [192] constructed a chemiresistor sensor using a graphene/polyaniline composite film to detect a wide range of NH3 concentrations (from 1 to 6400 ppm) with high stability. The detection limit for this sensor is 1ppm. The use of graphene homogeneously dispersed in PANI, was essential to increase the surface-tovolume ratios and therefore increases the sensitivity of graphene/PANI thin films in the presence of NH3. Tran et al. [193] used a sensor based on graphene and silver nanowire composite to detect NH3. The presence of NH3 in low concentrations (SCCM) was detected successfully in their work.
37
(a)
(b)
(c)
(d)
(e)
(f)
(g)
Figure 15. (a, b) Optical images of bulk and single layer MoSe2. (c, d) SEM and AFM images of single layer MoSe2 nanosheets (e) Optical image showing the fabricated MoSe2 sensor device to detect ammonia by electron beam lithography (f) Sensitivity plot against ammonia concentration (f) Raman spectral data of MoSe2 sensor measured at ambient and in the presence of argon and 1000 ppm of NH3. Reprinted with permission from ref. [188], D. J. Late, T. Doneux, and M. Bougouma, Appl. Phys. Lett. 105, 233103 (2014) Copy Right @ AIP Publishing. 38
Another important gas to be detected is nitrogen dioxide (NO2), a toxic gas whose origin is from internal combustion engines burning fossil fuels. A flexible and transparent large-scale graphene-based sensor was proposed by Hongkyw Choi et al. [121] for NO2 detection. A complete study related to the sensor efficiency as a function of temperature operation and NO2 concentration in 0.5 to 40 ppm range is shown. Applications such as smart window and wearable nose-sensors application with transparent flexible 2D materials are demonstrated. Flexible graphene-based sensors using soft lithographic patterning method has been proposed by Jung et al. [91] for the detection in a range of 2.5 to 100 ppm of NO2 and a better response at 45oC was mentioned (Figure 16). An ultrasensitive and selective NO2 sensor based on selfassembled graphene/polymer (PVA/PEI) composite nanofiber has been proposed by Yuan et al.[189]. Using this sensor the limit of detection was experimentally measured to be as low as 150 ppb, a value much lower than the threshold exposure limit proposed by American Conference of Governmental Industrial Hygienists (200 ppb). The maximum NO2 concentration detectable with their sensor was 5 ppm with a good stability at room temperature. A high surface area 3D MoS2/graphene hybrid aergol material was employed for the fabrication of ultra-high sensitive NO2 gas sensor [194]. For the fabrication of gas sensor, the obtained MoS2-graphene aerogel material was integrated as a low power heater platform. Figure 17 a and b shows the schematic and optical image of micro-heater sensor. The developed sensor exhibited very low detection limit of 50 ppb at both room temperature and 200 C. However, at 200 C, sensor showed very fast response and recovery time of
