Nanostructured MoS2-Based Advanced Biosensors: A Review - ACS

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Nanostructured MoS2 Based Advanced Biosensors: A Review Shaswat Barua, Hemant S Dutta, Satyabrat Gogoi, Rashmita Devi, and Raju Khan ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00157 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 25, 2017

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Nanostructured

MoS2

Based

Advanced

Biosensors: A Review Shaswat Barua1,2, Hemant Sankar Dutta1, Satyabrat Gogoi1, Rashmita Devi1, Raju Khan1* 1

Analytical Chemistry Group, Chemical Sciences & Technology Division, Academy of

Scientific and Innovative Research, CSIR-North East Institute of Science & Technology, Jorhat, 785006, Assam, India 2

Department of Chemistry, School of Basic Sciences, Assam Kaziranga University,

Koraikhowa, NH-7, Jorhat-785006, Assam, India

ABSTRACT: Introduction of nanotechnology in biosensor applications has significantly contributed to human lifestyle by rendering advanced personalized diagnostics, health care and monitoring equipment and techniques. Nanomaterials and nanostructures have recently gained impetus in the domain of biosensors due to their manifold applications. Transition metal dichalcogenides (TMD) newly attracted interests because of their multi-dimensional structures and structure dependent unique electronic, electrocatalytic and optical properties, which can be explored to design novel biosensing platforms. The content of the present article aspires to advocate a critical evaluation on the recent advances in the domain of dimensionally different MoS2, the most widely explored TMD and their relevance in biosensing application. This encompasses the major structural attributes and synthetic methodologies of zero, one, two and three-dimensional MoS2 nanostructures, pertaining to

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their biosensing potential. Herein, we described the prevailing and potential applications of MoS2 nanostructures in optical, electrochemical and electronic biosensors.

KEYWORDS: MoS2, Advanced biosensor, Nanostructures, Optical, Electrochemical

1. INTRODUCTION Human wellbeing is greatly dependent on the ease of life style, which often loses its pace due to serious health issues. This raises the importance of modern tools and techniques to detect and diagnose various diseases or allied factors regularly as precautionary measures. Development of biosensors has contributed a significant share in this regard. Biosensing implies the use of some basic tools and techniques to detect disease-factors easily and selectively.1 This selectivity ascertains the possibility of using such biosensors in clinical real time sample monitoring.2 Another important parameter, sensitivity dictates the quality of a biosensor.3 A great deal of research has been involved to attain desired selectivity and sensitivity by tailoring the sensor matrices.4-6 Development of nanotechnology has diverted the attention of the scientific community from the conventional sensing techniques and resulted in the fabrication of highly selective biosensors with nano-molar level capacity of sensing bio-analytes.5 Molybdenum disulphide (MoS2) based nanomaterials have attained utmost attention in recent times due to their manifold advantageous attributes.7 MoS2 comprises of S-Mo-S triple layers with well-known semiconducting properties of metal dichalcogenide compounds.8 Excellent electrochemical attributes and luminescence properties have endorsed MoS2 based nanomaterials as novel biosensing probes for the careful detection of a range of analytes.9 Their multi-dimensional structures are the prime cause of attraction with their multifaceted application potentials. In broader aspects, nanomaterials can be categorized as zero, one, two and threedimensional structures. Synthesis and applications of MoS2 with different dimensions have 2 ACS Paragon Plus Environment

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been well documented in literature. Variation of precursors, synthetic materials and methodologies mainly dictate the shape and size of MoS2 nanostructures.10-12 Each dimension has unique attribute that renders tremendous potential for biosensing applications. Zero dimensional (0D) MoS2 quantum dots, also referred to as “inorganic fullerenes” are nano-octahedral structures with size 100 nm. Three dimensional (3D) nanomaterials includes nanoclusters, nanodispersions etc. Recent literature showcased the synthesis of 3D hierarchical architectures based on the self-assembly MoS2. Such assembled nanomaterials have shown immense potential in hydrogen evolution reaction, visible‐light photocatalyst, high-performance flexible supercapacitors, lithium-ion batteries etc.91-93 However, very few reports have endorsed the biosensing capabilities of 3D MoS2. 2.4.1. Structure of three dimensional MoS2 Efficient 3D MoS2 nanostructures, like nano-porous sheets, core-shell, double-gyroid, vertical nanoflakes etc. are studied by a few research groups.95-99 Kong et al. described the 3D structure of MoS2 in comparison with MoSe2.97 One neutral layer was described to be consisting of three covalently bonded sheets of atomic thickness, with an interlayer distance of 6 Å (Figure 17). Such crystals have two kinds of sites, viz. terrace sites and edge sites. Anisotropic bonding and surface energy prefers platelet-like morphology for such layered nanomaterials.85,97,100,101 Figure 17-d shows the TEM and HRTEM image (showing atomic planes) of densely packed grain-like morphology of MoS2 with individual grain size of about 10 nm. Raman spectra for 3D MoS2 nnostructure is shown in Figure 17-e. Again, Figure 17-f

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represents the excited A1g and E12g modes, for the edge- and terrace-terminated MoS2 respectively, while Figure 17-g shows the Raman spectra for the terrace-terminated MoS2.85

Figure 17: (a) Layered crystal structure of S-Mo-S (or Se-Mo-Se), (b) platelet-like nanostructures and nanotubes of MoS2 and MoSe2, (c) 3D nanostructures of MoS2, (d) TEM image of a 3D MoS2 (HRTEM image shows, three atomic planes of S-Mo-S, (e) Raman spectra from MoS2, (f) excited A1g and E12g modes, respectively for edge-terminated and terrace-terminated MoS2 and (g) Raman spectrum for terrace-terminated MoS2 (Reprinted with permission from ref. 97, Copyright © 2013, American Chemical Society)

Figure 18: (a) SEM and (b) TEM images showing the nano-flower like morphology of 3D MoS2 and (c) HRTEM image showing the inter-planer distance with (002) plane (Reprinted with permission from ref. 95, Copyright © 2017, Nature Publishing Group) 20 ACS Paragon Plus Environment

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Again, Lu et al. observed nano-flower like morphology (Figure 18) of 3D MoS2 nanostructures, with larger stretched “thin folding leaves”, when observed under SEM and TEM. Further, HRTEM images revealed the (002) planes of MoS2 with interlayer spacing of ~ 0.65 nm.95 2.4.2. Synthesis of three dimensional MoS2 Three dimensional MoS2 and MoSe2 thin films were synthesized by Kong and co-workers in 2013, with vertically aligned layers.85 They demonstrated that sulfurization reaction undergoes via diffusion of sulphur into the MoS2 layers and converts it into sulfide. Again, 3D MoS2 sheets were synthesized by using tetrakis(diethylaminodithiocarbomato) molybdate(IV) as the precursor of both molybdenum and sulphur. The synthesis comprised of a chemical vapour deposition method, where graphene was first synthesized onto 3D Ni foam. Subsequently, it was reacted with tetrakis(diethylaminodithiocarbomato)molybdate(IV) to obtain 3D MoS2-graphene-Ni (3D MoS2-G-Ni (Figure 19 a).91 Figure 19 b,c and d shows 4 and 15 layers of graphene and the SAED pattern, respectively. Schematic diagram of the experimental set and the optical images of the samples are shown in Figure 19 e,f. Further, in 2014, self-assembly MoS2 nanostructures with 3D hierarchical frameworks were synthesized by simple hydrothermal method.102 Such methods have also been employed for synthesizing MoS2 nanoflowers on carbon fiber cloth. These nanosheet-assembled MoS2 nanoflowers were synthesized by using Na2MoO4 and CSN2H4 as the precursors (Figure 20). Then graphene oxide was added to recover rapid capacity fading of the MoS2 nanoflowers based anode.94 Further, MoS2-coated carbon foam/N-doped graphene were successfully synthesized by using 3D melamine foams. Very recently, Zhang et al. reported a successful synthesis of visible‐light responsive self-assembly MoS2/reduced graphene oxide, by hydrothermal

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method.92 Subsequent freeze drying yielded a 3D aerogel, which showed potential as visible light photocatalyst.

Figure 19: (a-c) Schematic depiction of 3D MoS2-graphene-Ni nanostructures, (b,c) HRTEM image of graphene, (d) SAED pattern, (e) experimental set-up for the synthesis and (f) optical images of the samples (Reprinted with permission from ref. 91, © 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) It has been observed that hydrothermal approach is the most widely used method for synthesizing nanostructured MoS2, regardless of the dimension. However, literature reports on 3D MoS2 are very few till date.

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Figure 20: synthesis of RGO decorated MoS2 nanoflowers (Reprinted with permission from ref. 94, Copyright © 2015, American Chemical Society) 3. PROPERTIES 3.1. Electronic properties Band structure and density of states dependent electronic properties of MoS2 monolayer is inevitable in determining its application in electronic devices. Band structures of MoS2have been calculated by Kuc et al. using the first-principle calculations with PBE (Perdew-BurkeErnzerhof) exchange-correlation functionals. Incorporation of PBE scheme in DFT to obtain the band structures increased the fraction of accuracy with the experimental results.103 MoS2 is an indirect semiconductor in bulk with a fundamental band gap of ~1.2 eV, which initiates because of the transition from the Γ high symmetry point of the valence band maximum to the conduction band minimum present in between the Γ-K high symmetry points as shown in Figure 21. However, the position of conduction band minimum at the Γ point is dependent on the interlayer interactions.104,105 Therefore, in reducing the number of layers from the bulk form, the conduction band minimum shifts to a higher energy at the Γ point of the Brilluoin zone. Nevertheless, the band energy values at the K point stay almost unchanged with the change in the slab thickness. This opens up the direct bandgap behaviour at the K point in the monolayer structure with an energy difference of ~1.9 eV.103,106 However, it has been proposed that reducing the number of layers increases the exciton binding energies from 0.1 23 ACS Paragon Plus Environment

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eV (in bulk system) to about 1.1 eV (in monolayer) due to the strongly reduced dielectric constants.107 The excitonic effects should be incorporated in order to calculate the fundamental band gap. The GW approach has been used to calculate the fundamental band gap of MoS2 monolayer and is found to be ~2.8 eV.108

Figure 21: (a) MoS2 band structure calculated at DFT/PBE level with reducing the number of layers from bulk to monolayer. Red dashed line showed the fermi level and colored lines (blue and green) indicated the energy band (valence band and conduction band) of the structure. The arrows indicated the fundamental band gap for the given system. (Reprinted with permission from ref. 103, Copyright © 2011, American Physical Society) and (b) Brillouin zone and high symmetry points of MoS2 reciprocal lattice 24 ACS Paragon Plus Environment

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The band gap property of MoS2 monolayer makes it a promising candidate for nanoelectronic biosensing applications. It has higher potential than its analogue graphene in such applications, which lacks the band gap property in its original form. Demonstration of MoS2 monolayer with room temperature on/off ratios of 1 x 108, mobility of >200 cm2V-1s-1 and ultralow standby power dissipation have enabled the realization of practical electronic devices.109-111Moreover, MoS2 monolayer has a carrier lifetime of ~100 ps and diffusion coefficient of ~20 cm2/s.112 Conclusively, it is noteworthy to mention that the above properties are highly suitable numbers to establish field effect and electrochemical based biosensors. The precision in fabricating MoS2 down to monolayer configuration with uniform thickness control enables the controlling of electrostatic characteristics of the transistor accurately. The lower in-plane dielectric constant also attributes in precise controlling of the electrostatic characteristics, thereby pushing its limit to sub-5-nm scale transistor technology.113-114Notably, electron transport in MoS2 monolayer is much slower as compared to bulk semiconductors. Yu et.al listed the approaches to improve the carrier transport in MoS2 monolayer and accounted intrinsic electron-phonon scattering, surface optical phonon scattering, Columb impurity scattering, atomic defect scattering, charge trap and metal-toinsulator transition as the reasons for low mobility.115 Nevertheless, at sufficiently scaled lengths, such issues may not be significant as the performance would mainly depend on the contacts rather than transport through channel.116 3.2. Catalytic properties Recently significant works have been reported showing the catalytic activity of MoS2 nanostructures and MoS2 based heterostructures.117-128 Edges and metallic 1T polymorph MoS2 nanostructures provided the catalytic activity. Catalytic activity is further dictated by nanostructures with atomic level precision.129 Recently, Yin et al. reported a polyethylene 25 ACS Paragon Plus Environment

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glycol functionalized MoS2 nanoflowers (PEG-MoS2 NFs) system which showed profound catalytic effect during the decomposition of H2O2, generating hydroxyl radical.130 This system induced NIR photothermal effect, which imparted excellent antibacterial effect against both Gram-positive and Gram-negative bacteria. Glutathione (GSH) oxidation was observed to be accelerated by NIR irradiation with effective bactericidal activity, by inducing hyperthermia (Figure 22).

Figure 22: (a) (i) PEG-MoS2 captured by bacteria, (ii) Catalytic decomposition of H2O2 (iii) laser irradiation (808 nm) induces hyperthermia, accelerating accelerates GSH oxidation and (b) bacterial colonies of E. coli incubated in (I) PBS, (II) MoS2, (III) H2O2, (IV) MoS2+H2O2, (V) PBS+NIR, (VI) MoS2+NIR, (VII) H2O2+NIR and (VIII) MoS2+H2O2+NIR (Reprinted with permission from ref. 130, Copyright © 2016, American Physical Society)

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Further, the edges of MoS2 mediate proton adsorption and subsequently, dihydrogen formation. Electrical conductivity and redox electrochemistry of MoS2 endorses them for energy conversion and storage devices based applications.131 Yu et al. observed electrocatalytic activity for hydrogen evolution by forming Ni-Co-MoS2 Nanoboxes.131 Interestingly, it was reported that the catalytic activity of MoS2 for such reaction reduces with increasing layers.130 Moreover, MoS2 nanostructures have also been investigated for their electrocatalytic activity in solar cells.132

2.3. Optical properties The layer dependent band structure variation in MoS2 is of particular interest amongst researchers for its optoelectronic properties. The generation of direct band gap in monolayer MoS2 provides strong luminescence properties, which is absent in the bulk material. In addition to the changes in the band structure, there is a structural change in thin film MoS2. The inversion symmetry existing in the bulk and in thin films with an even number of layers is explicitly broken in films with odd number of layers. This leads to generation of valleycontrasting optical selection rule, thus allowing spin-valley coupled band structures.133The broken inversion symmetry breaks down the Kramer’s degeneracy and splits the valence bands by ~160 meV.134 This leads to the possession of strongly bound excitons whose binding energies depend on the number of layers. These excitons decide the optical properties of the material as shown in Figure 23. The optical absorption spectrum is obtained by measuring the differential reflectance of MoS2 samples on a substrate and the bare substrate of hexagonal boron nitride (h-BN). The pronounced absorption minima correspond to the A and B excitons (Figure 23-a).135,136 Figure 23-b shows the photoluminescence (PL) spectra for total unpolarized emission under 2.33 eV (532 nm) excitation. The strongest emission near 1.9 eV (652 nm) arises from A excitons complexes, weaker emission at 2.1 eV (590 nm) is

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due to the B excitons and the feature near 1.8 eV (688 nm) corresponds to emissions from defect-trapped excitons.137

Figure 23: (a) Differential reflectance spectrum showing A and B exciton absorption spectrum. Red, yellow and green arrows represent the different exciting photon energies, (b) PL spectrum for 2.33 eV (532 nm) excitation showing the A and B exciton luminescence (a and b are reprinted with permission from ref. 137, Copyright © 2012, Nature Publishing Group), (c, d) Layer dependent PL and Raman spectra showing the dramatic increase of luminescence quantum efficiency in MoS2 monolayer (Reprinted with permission from ref. 138, Copyright © 2010, American Chemical Society). (e) Peak position of Raman modes and their dependency on the layer thickness. (Reprinted with permission from ref. 139, Copyright © 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim). (f) Refractive index and extinction coefficient variation of the monolayer with wavelength (Reprinted with permission from ref. 140, 2014, AIP Publishing LLC). Again, Splendiani et al. demonstrated the layer dependence of PL and Raman signal in MoS2.138 With increasing the number of layers from monolayer, the PL signal reduced; however, the Raman signal slightly improved because of the increased amount of material interaction. Nevertheless, the intrinsic quantum efficiency increased dramatically on going 28 ACS Paragon Plus Environment

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towards monolayer structure, as there is a huge jump in the PL emission.139,140 The weak interlayer coupling between the restacked MoS2 sheets gradually decreases the emission intensity. A slight red shift in the absorption resonance and PL energy has been observed with increasing film thickness.141The minor shift can be ascertained to the fact that the direct band gap is only slightly sensitive at the K point to the layer thickness due to the quantum confinement effect.142The possession of PL characteristics provide the possibility of using such structures in fluorescence based applications, which can be used to trace, image and sense biological components.143,144 The Raman characteristics of MoS2 monolayer is also a function of dimension and permittivity of the environment. Attaching biological components may alter such characteristics and thus can be used as a biosensing principle.145-146

4. APPLICATION OF MoS2 IN BIOSENSING 4.1. Electrochemical biosensors Recent development in electrochemical biosensors mostly focuses on the use of nanomaterials and nanostructures. TMDs have attained significant attention of the scientific community because of their analogous attributes to that of graphene. In a recent report, Rohaizad et al. demonstrated the fabrication of an electrochemical biosensor by exfoliation of TMD for efficient sensing of glucose upto 2.8 µM.147 Construction of such biosensors includes the selection of the matrix material, fabrication of the electrode, use of a mediator (or, mediator free) for immobilization of the labelling biomolecules (or, label free) and detection of analytes using electrochemical techniques.144 Glassy carbon (GC), platinum, Indium tin oxide (ITO) and screen printed electrodes are generally used for anchoring the biosensor matrix by different techniques like drop-casting, electrochemical deposition etc.148150

Further, HfO2, silicon wafer etc. are used for device fabrication.151,152 Kim et al. Recently

demonstrated an in situ synthesis of MoS2, on a polymeric printed circuit boards (PCB), using 29 ACS Paragon Plus Environment

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plasma enhanced chemical vapour deposition (PECVD) technique for efficient sensing of H2O2.153 Figure 24 shows different types of MoS2 based electrochemical biosensors for sensitive detection of bio-analytes.

Figure 24: (a) fabrication of a gold-MoS2 electrode, (1) Mo deposited on Au electrode using an e-beam evaporator, (2) H2S + Ar plasma reacting with Mo film in PECVD chamber, (3) H2S penetrating into Mo, (4) MoS2 biosensor device (a 1-4 are reprinted with permission from ref. 153,Copyright © 2015, Royal Society of Chemistry), (b) Construction of a TMD biosensor electrode by drop casting on GC electrode, followed by immobilization of glucose oxidase (GOx), crosslinked with glutaraldehyde (GTA) for electrochemical sensing of gluocose (Reprinted with permission from ref. 147, Copyright © 2017, American Chemical Society), (c) Biofunctionalization layers on MoS2 device surface (Reprinted with permission from ref. 151, Copyright © 2014, John Wiley and Sons) and (d) MoS2-based modified electrodes for glucose sensing (S: source, D: drain, Reprinted with permission from ref. 152, Copyright © 2015, John Wiley and Sons) 30 ACS Paragon Plus Environment

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Most of the electrochemical biosensors based on MoS2 nanostructures employ potentiometric and amperometric techniques for detection of bio-analytes. Besides, square wave voltammetric (SWV), differential pulse voltammetry (DPV), electron impedance spectroscopy (EIS) are the commonly used technique for detecting bio-analytes with high level of sensitivity.154-156 Figure 25 shows the widely used techniques for MoS2 based electrochemical biosensors.

Figure 25: (a) Voltametric detection of H2O2 by MoS2/GOx modified electrode, (b) Amperometric responses of MoS2 modified electrode (a and b are reprinted with permission from ref. 154 Copyright © 2013, American Chemical Society), (C) square wave voltammetric (SWV) responses of the modified electrode in 0.1 M PBS with injections of dsDNA (Reprinted with permission from ref. 155, Copyright © 2014, American Chemical Society) and (d) EIS response (Nyquist plots) of thrombin and ATP detection by using goldMoS2 based biosensor (Reprinted with permission from ref. 156, Copyright © 2016, American Chemical Society)

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A range of analytes have been successfully detected by MoS2 based electrochemical biosensors. MoS2 based hybrid materials have also been explored for fabricating sensor probes for highly selective and sensitive detection of bio-analytes.157-164 Poly(xanthurenic acid, Xa) based on MoS2 support was electrochemically synthesized by Yang and coworkers. They prepared a highly electroactive biosensing matrix by ultrasonic method for efficient detection of guanine and adenine.165 With similar approach, MoS2 and polyaniline (PANI) composite was prepared through in situ for electrochemical detection of chloramphenicol.166 Again, gold nanoparticles decorated 2D MoS2 nanosheets were used for electrochemical sensing of glucose in the presence of commonly interfering species like ascorbic acid (AA), dopamine (DA), uric acid (UA) and acetaminophen.21,167 Myoglobin immobilized zero dimensional MoS2 nanoparticles-graphene oxide (GO@ MoS2) hybrid was fabricated by Yoon and co-workers for detecting hydrogen peroxide. They observed enhancement of electrochemical signal due to graphene oxide which may be attributed to the high surface area available for immobilization of myoglobin.168 With a similar approach, an extremely sensitive (2.5 nM) H2O2 biosensor based on MoS2 quantum dots (~2 nm) was fabricated by Wang et al. in 2013.152 Another thionin functionalized MoS2 based electrochemical was fabricated for the direct detection of DNA with sensitivity up to ppb level.153 Electrochemically reduced single-layer 2D MoS2 nanostructures exhibited fast electron transfer rate which help in the detection of glucose with high sensitivity.169 Micromolar level electrochemical biosensing of glucose was achieved by using a glucose oxidase immobilized gold-MoS2 nanohybrid.170 Literature reports primly showcased the utility of 2D MoS2 nanostructures in the development of electrochemical biosensors.163 However, MoS2 nanostructures of other dimensions have also been explored for biosensing of different analytes which would be discussed in the following sections.Table 1 encompasses a list of

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analytes, lowest detection limits (LOD), and technique used in fabricating MoS2 based electrochemical biosensors. Table 1: List of analytes, lowest detection limits (LOD), and technique used in fabricating MoS2 based electrochemical biosensors

Sl No. 1 2 3

Matrix MoS2 MoS2 MoS2 nanoparticles MoS2-thionine

Analyte Glucose H2 O2 H2 O2

Technique CV CV Amperometry

LOD 2.8 µM 20 ng/mL 2.5 µM

References 147 153 154

Ds-DNA

0.09 ng/mL

155

5

MoS2poly(xanthurenic acid)

Adenine & Guanine

Square Wave Voltamerty DPV

165

6

MoS2/PANI

Chloramphenicol

DPV

7 8

MoS2-Gold MoS2 SingleLayer MoS2-gold nanoparticles

miR-21 Dopamie

DPV CV

3 × 10-8 for adenine 1.7 × 10-8 for guanine 6.9 × 10-8 mol/L 0.26 pM -

Thrombin & Adenosine triphosphate Ribaflavin Adenine & Guanine

CV

4

9

10 11

MoS2-Gold MoS2/PANI

12

Poly(maminobenzene sulfonic acid) reduced MoS2 Ni-doped MoS2RGO MoS2

13 14

166 167 171

Dopamine

DPV

0.74 nm for ATP 0.0012 nM for thrombin 20 nM 3 × 10-9 for adenine 5 × 10-9 for guanine 0.22 µM

Glucose

CV

-

176

Glucose

CV

0.042 µM

177

DPV DPV

172

173 174

175

4.2. Field effect transistor based biosensors Field effect transistors (FETs) based biosensors are of great interests for the researchers for its highly desirable characteristics of label-free rapid electrical detection capabilities, low 33 ACS Paragon Plus Environment

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power consumption, mass production, compactness and possibility of on-chip integration of the chip with the measurement systems. FET conventionally consists of two electrodes (drain and source) that are connected electrically via a semiconductor material (channel). The current flowing between the drain and source through the channel is modulated by a third electrode namely gate, which is capacitively coupled through a dielectric layer covering the channel. Capturing biomolecules using functionalized channel produce an electrostatic effect that is transduced into a readable signal in the form of change in electrical characteristics of the FET device.178 Nevertheless, the performance characteristics is dependent on the biasing strategy of the device.179 Figure 26 illustrates the sensing principle, detection strategy and chip design of MoS2 based FET biosensors. The possession of direct band gap in MoS2 allows better controllability between conductive and insulated states and also prevents leakage currents.180 This permits the designed FET to obtain higher sensitivity and accurate sensing with very low concentration of analytes. However, in comparison to its counterpart, graphene based FET device possesses a zero band gap and thus cannot be switched off, thereby inducing leakages and higher potential for inaccuracies.181 Furthermore, absence of out-of-plane dangling bonds in MoS2 reduces the surface roughness scattering and interface traps. This results in better electrostatic control due to the lower density of interface states on the semiconductor-dielectric interface, thereby reducing the low frequency noise, which hinders the performance of FET based biosensors.182 MoS2 nanosheet based FET biosensors have been demonstrated to detect proteins, pH, cancer biomarkers etc. with high sensitivity and selectivity.183-186 It has been articulated that a few layer MoS2 film based FET device shows more stable and sensitive response than the monolayer based.139 Chip integration with microfluidics opens up huge prospects in realizing compact sensor systems with clinically meaningful detection limits, allowing their usage in

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point-of-care diagnosis applications.187,188 Additionally, recent developments in the synthesis and growth of MoS2 thin films as well as demonstration of integrated logic circuits on MoS2 illustrates the viability of realizing MoS2 based FET biosensors with low processing cost and multiplexed detection capabilities.21,189-197

Figure 26: MoS2 based FET biosensor (a) Schematic diagram of the sensing scheme showing the MoS2 channel functionalized with receptors for specifically capturing the target biomolecules and the drain and source contacts and the Ag/AgCl reference electrode for biasing the device, (b) Optical image of a MoS2 flake in a SiO2/Si substrate. Scale bar, 10 µm, (c) Optical image of the FET device, (d) Image and schematic (inset) showing the 35 ACS Paragon Plus Environment

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macro-fluidic integrated FET chip, (e) Transfer characteristics of a biotin functionalized FET device measured in pure buffer (0.01X PBS), with addition of streptavidin solution (10 µM in 0.01X PBS) and again with pure buffer, (f) Comparison of sensitivities in the subthreshold, saturation and linear regions of the transistor device (a-f are reprinted with permission from ref. 184 Copyright © 2014, American Chemical Society), (g) Image of FET biosensor integrated with PDMS reservoir and (h) Experimental setup showing an inlet/outlet tubing kit, driven by motorized syringe pump. (g and h are reprinted with permission from ref. 183, Copyright © 2015, Nature Publishing Group) Table 2 presents the recent reports on MoS2 based FET biosensors, the sensor material used and the detection limits. Table 2: Recent reports on MoS2 based FET Biosensors Target

Sensor Material

Streptavidin DNA hybridization Prostate specific antigen (PSA) Tumor necrosis factor-alpha (TNF-α) Opioid peptide DAMGO Prostate specific antigen (PSA)

Si/SiO2/MoS2/biotin Si/SiO2/MoS2/DNA conjugates Si/SiO2/MoS2/anti-PSA Si/SiO2/MoS2/HfO2/antiTNF-α Si/SiO2/MoS2/wsMOR Si/SiO2/MoS2/HfO2/antiPSA

Detection Range 10 to 100 fM

Detection Limit 100 fM 10 fM

References

1 pg/mL to 1 ng/mL 60 fM to 6 pM

1 pg/mL

21

60 fM

155

3 nM to 1 µM 375 fM to 3.75 nM

3 nM

190

375 fM

170

184 196

4.3. Optical Biosensor 4.3.1. MoS2 as a fluorescence probe MoS2 has recently found widespread application as fluorescence probe in the detection of various biological and environmental analytes (Table 3). As discussed in the previous section, MoS2 possesses good optical properties like fluorescence which have been utilized to design

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various sensing platforms. Art of literature reveals multiple aspects of nano MoS2 in biosensing application. Based on the morphological characteristics and optical properties, it can either be used as the fluorescence probe or as quencher. Generally, two dimensional nanosheets have been reportedly used as quencher in different sensing platforms. The efficiency of MoS2 nanosheet to absorb emissive radiation over a wide range of wavelength as well as their ability to interact specifically with certain bio-entities make them suitable to act as quencher. Förster Resonance Energy Transfer (FRET) based nanoprobes are known to use MoS2 nanosheets frequently as florescence acceptor in an acceptor-donor couple. On the other hand, quantum sized, zero dimensional MoS2 can be directly used as sensing probes because of their good fluorescence characteristics. In this section of the article, we are trying to provide a concise account on the working mechanism of different MoS2 based sensors.198 DNA is one of the most widely reported bio-molecule which can be detected by MoS2 sensing platform with high selectivity and sensitivity. Generally, 2D MoS2 nanosheets have been used as quencher which works in conjugation with a strongly fluorescent probe. Singlelayer MoS2 can be considered as S-Mo-S sandwish structure in which each Mo is coordinated in a trigonal prismatic geometry to six S atoms. With such unique morphological characteristics, MoS2 nanosheets can specifically absorb single-stranded DNA (ssDNA) via the van der Waals force between nucleobases and the basal plane of MoS2. In most of the cases, target DNA molecule is labelled with a fluorescent dye or functionalized with a fluorescent nanostructure. As a result of specific MoS2 nanosheets/ssDNA interaction, the fluorescence efficiency of the probe gets hindered and diminished intensity can be co-related to the quantitative amount of ssDNA present in the system. Zhu et al. reported detection of DNA

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and small biomolecules by using such technique.199 Huang et al. demonstrated a slightly modified strategy for detection of ssDNA, but with a turn on type of response (Figure 27).200 Table 3: MoS2 as fluorescence probe for biosensing application Dimen sion

Probe

Mode

Analyte

LOD

Single-layer MoS2

Turn off

DNA and small biomolecules Single-stranded DNA Pathogens, S. typhimurium

Refer ence 199

1 nM 10 CFU·mL−

200 201

MoS2 nanosheets MoS2 nanosheetTurn on

2D

1

Aptamer functionalized MoS2 MoS2nanosheetMoS2 nanosheets MoS2/rGO hybrid MoS2 nanosheets MoS2 quantum dot

Prostate specific antigen (PSA) DNA Detection Both Turn on and off Turn off

0D 2D 0D

0.2 ng/mL

202

0.67 ng mL-1

203

DNA Detection

204

Glutathione Pb(II) Hyaluronic acid

0.7 U/mL

205 206 55

∼10 nM

206

1 nM

Turn on

MoS2 nanosheets

Turn on

Ag+ in aqueous medium and bacteria S2-

Boron- and nitridedoped MoS2 nanosheets Au modified MoS2 hybrid Single-layer MoS2/Folic Acid nanosheet MoS2-GSH bionanohybrid MoS2 quantum dot

Turn off

Hg2+

Induced blue shift

DNA Detection

208

Tissue cancer phototherapy

209

Cancer cells

41

MoS2 nanosheets

2D

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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MoS2 Quantum Dot@Polyaniline MoS2@Fe3O4ICG/Pt(IV) Nanoflowers

206 207

NIR imaging

SOSG cells

-

210

Cancer cell

211

MR/IR/PA and Combined PTT/PDT/Chemotherapy Triggered by 808 nm Laser

47

Imaging

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Figure 27: MoS2/dye-labeled single-stranded probe DNA (P1). (Reprinted with permission from ref. 200, Copyright © 2014, Royal Society of Chemistry) A dye-labeled DNA (P1: 5’-TAMRA-TGCGAACCAGGAATT-3’) was used as the probe for the detection of its complementary DNA (T1: 5’-AATTCCTGGTTCGCA-3’). The probe worked at an excitation wavelength of 565 nm with consequence emission at the wavelength of 580 nm. Due to the interaction of P1 and MoS2, the probe fluorescence is quenched. However, in presence of target DNA, i.e. T1, P1 forms a double stranded form (dsDNA). Contrary to ssDNA, dsDNA has very weak binding affinity towards MoS2 and hence P1/MoS2 interaction is interrogated. As a result, the fluorescence is restored and measurement of fluorescence intensity provides the quantitative indication of T1.200 Singh et al. used the same technique for detection of pathogen S. typhimurium. A fluorescein-labelled aptamer (Apt-FAM) was used as the probe for the specific recognition of the pathogen. In presence of S. typhimurium, Apt-FAM probe prefers to bind with the target pathogen instead of MoS2 nanosheets. As a result, turn on kind of response is achieved. The method can be selectively used for the detection of S. typhimurium over E. coli and P. vulgaris.201 Kong et al. followed detection of Prostate Specific Antigen (PSA)-a biomarker for the early diagnosis of prostate cancer. The quenched fluorescence intensity of dye labelled aptamer/MoS2 sensing platform was restored successfully in presence of target PSA.202

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In addition to DNA molecules, other bio-entities and different environmental analytes have also been detected by using MoS2 based sensing platforms. Guet al. used MoS2 quantum dot as fluorescence probe for the detection of hyaluronic acid. Unlike 2D MoS2, this zero dimensional quantum sized nano-form is soluble in water, hence offers further advantage in biosensing application. The formation of a MoS2 quantum dot/hyaluronic acidgold nanoparticle nano-assembly leads to fluorescence quenching due to electron transfer from donors (MoS2 quantum dots) to acceptors (hyaluronic acid-gold nanoparticle). Thus, quantification of diminished fluorescence intensity (turn off response) facilitates detection of hyaluronic acid.55Similarly, Wang et al. used fluorescent MoS2nanosheets for quantitative detection of Pb(II) and S2- ions. They observed that doping of MoS2 with Pb(II) can significantly enhance the fluorescence property, while addition of S2- results in drastic quenching. These properties were explored to develop MoS2 based sensing platform for detection of Pb(II) (turn on response) and S2- ions (turn on response).206 Yang et al. reported method for detection of Ag+ in solution and bacteria. They used rhodamine B isothiocyanate (RhoBS) adsorbed MoS2 which suffer quenching of fluorescence. On the surface of MoS2, Ag+ undergoes reduction by the action of Rhodamine B isothiocyanate (RhoBS). This resulted in the detachment of silver from MoS2 surface, restoring of fluorescence intensity (turn on response).206 Liu et al. modified MoS2 by doping with boron and nitride and used it in the detection of environmental pollutant Hg2+. The whole mechanism relies on band gap dependent fluorescence of doped MoS2 (Figure 28). In the pristine state MoS2 possesses a band gap of 1.20 eV. However, this value increased up to 1.61 eV after doping with boron and nitride. With the introduction of Hg2+ the band gape decreases significantly with consequent decrease in fluorescence intensity. Thus, it offers a turn off response dependent detection of Hg2+.207 Literature also shows some exciting detection techniques, where nanostructured MoS2 has been found to play unique role (other

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than quencher or probe). For example, Zhang et al. described a nanohybrid comprises of rGO/MoS2 for detection of glutathione (GSH) which was used as photo-catalyst to produce ·OH radicals at the photocatalyst/solution interface in presence of visible light.

Figure 28: Band gap dependent detection of Hg2+ based on MoS2 doped sensing probe. (Reprinted with permission from ref. 208, Copyright © 2015, Royal Society of Chemistry) Terephthalic acid (TA) was used as the working probe, which accepts the free ·OH radicals and gets converted into 2-hydroxyterephthalic acid (HTA). HTA is a photo-active compound and gives strong fluorescence at excitation wavelength of 425 nm. However, in presence of GSH, free radical reaction gets hindered. GSH scavenges the free radicals thereby lowering the HTA formation. Consequently, fluorescence is quenched with turn off type of response. Thus, measurement of diminished fluorescence intensity provides quantitative estimation of GSH.205 On the other hand, MoS2 has been used for bio-imaging applications as well. As described in the optical properties segment, MoS2 with variant morphology is suitable for bio-imaging application. Han et al. reported an upconversion multifunctional nanostructure based bioimaging platform.210 They covalently grafted upconversion nanoparticles (UCNPs)

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with chitosan functionalized MoS2 (MoS2-CS) and Folic acid (FA). MoS2 surface was loaded with phthalocyanine (ZnPc). The whole nano-assembly was integrated into photodynamic therapy (PDT) with photothermal therapy (PTT) and used in upconversion luminescence imaging.209 Similarly, Dong et al. used MoS2 quantum dot both as conventional and upconversion nanomaterial for bioimaging of SOSG cells.41 Wang and group reported MoS2 quantum dot@polyaniline (MoS2@PANI) inorganic-organic nanohybrids. Such nanoassembly exhibits substantial potentiality to enhance photoaccoustic (PA) imaging/X-ray computed

tomography

(CT)

signal

as

well

as

perform

efficient

radiotherapy

(RT)/photothermal therapy (PTT) of cancer cells.211 Liu et al. designed a multifunctional composite system comprises of MoS2@Fe3O4-ICG/Pt(IV) (MoS2@Fe-ICG/Pt). Such a nanoassembly was obtained by covalently grafting Fe3O4 nanoparticles with polyethylenimine (PEI) functionalized MoS2, and then loading indocyanine green molecules (ICG, photosensitizers) and platinum (IV) prodrugs (Pt(IV) prodrugs) on the surface of MoS2@Fe3O4. The resultant nanohybrid system of Mo@Fe-ICG/Pt demonstrated good magnetic resonance/infrared thermal/photoacoustic trimodalbioimaging utility.212 4.3.2. Colorimetric detection In addition to fluorescence based techniques, dimensionally different MoS2 have also been utilized in the colorimetric assays by exploring its useful optical properties. MoS2 possesses a strong characteristic optical absorbance over a wide range of wavelength. Such unique absorption of MoS2 can be attributed to the confinement of electronic movements and the absence of interlayers interference confers monolayers band gaps. Both 0D and 2D MoS2 possess the potentiality to be used in colorimetric based biosensing applications. In this context, we would like to highlight few of the recent reportson MoS2 based colorimetric sensors. It has been reported that 2D TMDs including MoS2 exhibits peroxidase-like catalytic activities which can be utilized to design various sensing platforms. Recently, Wang et al. 42 ACS Paragon Plus Environment

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reported detection of Fe(II) by using luminescent MoS2nanosheet-based peroxidase mimetics. The developed nanosheets possess the ability to catalyse oxidation of peroxidase substrate ophenylenediamine (OPD) in the presence of hydrogen peroxide (H2O2) which gives yellow colour product. Fe2+ can enhance the catalytic activity of MoS2 nanosheets greatly and thereby influence the OPD reaction which can be followed by colorimetric measurements. This constitutes the basic of Fe(II) estimation in a sensitive manner.213 Similar strategy was adopted by Guo et al. showing that MoS2 nanosheets catalyse the reaction of 3,3′,5,5′tetramethylbenzidine (TMB) with H2O2 to give a blue colouredproduct 3,3′,5,5′tetramethylbenzidine diimine. Formation of this blue coloured product can be followed by calorimetrically, which providesthe method for sensitive and selective detection of H2O2.214 Lin et al. followed the same method for determination of glucose. They combined MoS2nanosheet with glucose oxidase which catalyse oxidation of glucose to gluconic acid and oxygen of the solution is converted into H2O2.Formation of H2O2 depends on the oxidation of glucose present in the system. Thus estimation of H2O2 by using TMB provides the quantitate amount of glucose (Figure 29 a).215 In addition to peroxidase-like catalytic activities, MoS2 also possesses size dependent optical absorption which have been utilized in the detection ofDNA molecules. In salt solution, 2D MoS2 exhibits tendency to agglomerate and often forms larger aggregates than in aqueous medium. As a result, optical absorption capacity decreases considerably. However, on functionalization with ssDNA, such tendency of MoS2 gets hindered, and the original light absorption ability restored. But, in presence of target DNA, 2D MoS2 again forms aggregates leading to decrease in optical absorbance. Thus, diminished optical absorption of MoS2/ssDNA hybrid in presence of target DNA can be utilized for the detection of DNA molecules (Figure 29 b).216 Thus, above motion gives us an impression that by utilizing optical properties of MoS2, it is possible to design sensing platforms for various analytes. In this context,

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understanding of material properties of dimensionally different MoS2 and their interaction with various bio-entities is imperative to get highly sensitive sensing probes.

Figure 29. (a) Colorimetric estimation of H2O2 and glucose (Reprinted with permission from ref. 215, Copyright © 2014, Royal Society of Chemistry) and (b) Utilization of size dependent optical absorption for the estimation of DNA (Reprinted with permission from ref. 216, Copyright © 2015, John Wiley and Sons) 5. CONCLUSION AND FUTURE SCOPES The review addressed the structure, synthesis and promising biosensing capabilities of MoS2 nanostructures of zero, one, two and three dimensions. We demonstrated that different dimensions of MoS2 implicate different optical and electrochemical attributes. Electronic and optical properties of MoS2 have been discussed thoroughly with special emphasis to their biosensing potential. This review also highlights the works on electrochemical and fluorescence biosensors based on MoS2 nanostructures. Further, transistor based biosensors has also been discussed in pertinence with the fabrication of biosensing devices.

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Preparation of composite materials based on dimensionally different MoS2 may address various advanced applications related to biosensing. All the dimensions discussed here, have been explored to a considerable extent. In coming times, novel synthetic methods are anticipated to regulate the desired shape and size accord of MoS2, which dictate the overall performance as efficient biosensing probes.

Corresponding Author E-Mail: [email protected] (R Khan) ORCID Raju Khan: 0000-0002- 3007-0232 Shaswat Barua: 0000-0003-0050-3698 Satyabrat Gogoi: 0000-0002-8860-5615 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT Director, CSIR-North East Institute of Science and Technology, Jorhat is deeply acknowledged for his kind support. SB thankfully acknowledges CSIR, India for CSIR Nehru Post Doctoral Research Fellowship (HRDG/CSIR-Nehru PDF/CS/EMR-1/01/2017). SG acknowledges SERB, DST, India for financial support through grant no. PDF/2016/003142. RD acknowledges DBT, India for financial support through grant no. GPP 0318.

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Dimensionally different MoS2 nanostructures for advanced biosensor applications 230x155mm (96 x 96 DPI)

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