Nanostructured MoS2-Based Advanced Biosensors: A Review

Nanostructured MoS2‑Based Advanced Biosensors: A ReviewShaswat Barua,†,‡ Hemant Sankar Dutta,† Satyabrat Gogoi,† Rashmita Devi,† and Raju Khan*,†

†Analytical Chemistry Group, Chemical Sciences & Technology Division, Academy of Scientific and Innovative Research,CSIR-North East Institute of Science & Technology, Jorhat 785006, Assam, India‡Department of Chemistry, School of Basic Sciences, Assam Kaziranga University, Koraikhowa, NH-7, Jorhat 785006, Assam, India

ABSTRACT: The introduction of nanotechnology in biosensorapplications has significantly contributed to human lifestyle byrendering advanced personalized diagnostics and health care andmonitoring equipment and techniques. Nanomaterials andnanostructures have recently gained impetus in the domain ofbiosensors because of their manifold applications. Transition-metaldichalcogenides (TMDs) newly attracted interest because of theirmultidimensional structures and structure-dependent uniqueelectronic, electrocatalytic, and optical properties, which can beexplored to design novel biosensing platforms. The content of thepresent article aspires to advocate a critical evaluation on the recentadvances in the domain of dimensionally different MoS2, the mostwidely explored TMD, and their relevance in biosensingapplication. This encompasses the major structural attributes and synthetic methodologies of zero-, one-, two-, and three-dimensional MoS2 nanostructures, pertaining to their biosensing potential. Herein, we described the prevailing and potentialapplications of MoS2 nanostructures in optical, electrochemical, and electronic biosensors.

KEYWORDS: MoS2, advanced biosensor, nanostructures, optical, electrochemical

1. INTRODUCTION

Human well-being is greatly dependent on the ease of lifestyle,which often loses its pace due to serious health issues. Thisraises the importance of modern tools and techniques to detectand diagnose various diseases or allied factors regularly asprecautionary measures. The development of biosensors hascontributed a significant share in this regard. Biosensing impliesthe use of some basic tools and techniques to detect diseasefactors easily and selectively.1 This selectivity ascertains thepossibility of using such biosensors in clinical real-time samplemonitoring.2 Another important parameter, sensitivity dictatesthe quality of a biosensor.3 A great deal of research has beeninvolved in attaining the desired selectivity and sensitivity bytailoring the sensor matrices.4−6

The development of nanotechnology has diverted theattention of the scientific community from conventionalsensing techniques and resulted in the fabrication of highlyselective biosensors with nanomolar-level capacity of sensingbioanalytes.5 Molybdenum disulfide (MoS2)-based nanomateri-als have attained the utmost attention in recent times becauseof their manifold advantageous attributes.7 MoS2 comprises S−Mo−S triple layers with well-known semiconducting propertiesof metal dichalcogenide compounds.8 Excellent electrochemicalattributes and luminescence properties have endorsed MoS2-based nanomaterials as novel biosensing probes for the carefuldetection of a range of analytes.9 Their multidimensionalstructures are the prime cause of attraction with theirmultifaceted application potentials.

In broader aspects, nanomaterials can be categorized as zero-(0D), one- (1D), two- (2D), and three-dimensional (3D)structures. The synthesis and applications of MoS2 withdifferent dimensions have been well documented in theliterature. Variation of the precursors, synthetic materials, andmethodologies mainly dictates the shape and size of the MoS2nanostructures.10−12 Each dimension has a unique attribute thatrenders tremendous potential for biosensing applications.0D MoS2 quantum dots, also referred to as “inorganic

fullerenes”, are nanooctahedral structures with size <10nm.13−15 Because of similar structural attributes with fullerene,MoS2 dots also exhibit excellent electronic and catalytic activity.Further, 1D MoS2 quantum dots show strong photo-luminescence at specific excitation wavelengths due to aquantum confinement effect that enables optical biosensing ofa range of analytes with a simple fluorimetric technique.16

Presently, MoS2 nanoprobes have been studied extensively forefficient fluorimetric detection of a range of bioanalytes.17,18

2D MoS2 nanostructures have been explored for electro-chemical biosensing because of their tunable electronic energystates.8 The state of literature cites promising aspects of MoS2sheets as electrode materials in amperometric and impedance-based biosensors.19,20 The graphene-like structure of MoS2 hasopened newer avenues for research in the domain of biosensorsand devices. MoS2 nanosheets of this dimension exhibit a direct

Received: November 7, 2017Accepted: December 22, 2017Published: December 22, 2017

Review

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© 2017 American Chemical Society 2 DOI: 10.1021/acsanm.7b00157ACS Appl. Nano Mater. 2018, 1, 2−25

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band gap, which shows strong fluorescence emission in thevisible range.21 This endorses such matrices for fabricating cost-efficient optical biosensors.Similar responses were observed for 1D MoS2, which are

analogous to carbon nanotubes (CNTs) with significantelectrical properties.21 Interestingly, the band gap of MoS2nanotubes can be tailored by tailoring the tube diameter andnumber of walls. CNT-based electrochemical biosensors werereported for sensing hydrogen peroxide (H2O2), glucose, DNA,proteins, etc.22,23 Thus, there is a huge potential for fabricating1D MoS2-based biosensors with analogous attributes. Figure 1

schematically represents the biosensing potential of dimension-ally different MoS2. However, it cannot be concluded that aparticular dimension can be specifically recommended for aparticular biosensing technique.Considering the immense potential of MoS2 in advanced

biosensing, this review presents a critical description of thestructure, synthesis, and application of 0D, 1D, 2D, and 3DMoS2 nanostructures. The basic idea of the paper is to bringcollective knowledge on morphologically different MoS2 underone umbrella and to promote new ideas toward developingnext-generation diagnostic tools in health care monitoring. Theperformance of a biosensor greatly depends on the structureand morphology as well as the method of preparation of thesensing probe.24 Thus, we are depicting a clear picture of thestructure and synthetic methodologies of dimensionally differ-ent MoS2 before elucidating their applicability in biosensors.

2. STRUCTURE AND SYNTHESIS OF MoS2NANOSTRUCTURES2.1. 0D MoS2. 0D nanomaterials are always of great interest

because of their easy preparative protocols.25 The representa-tive members of this family include metal nanoparticles,quantum dots, etc. Extensive research has been carried outon the use of silver, gold, copper, zinc, iron, and titanium oxidenanoparticles in biosensing.26−31 This class also includesquantum-sized nanodots of semiconductor materials withinteresting properties. Recent literature showcased the synthesisof 0D MoS2 nanoparticles and nanodots with excellentquantum-chemical properties.2.1.1. Structure of 0D MoS2. MoS2 quantum dots are

composed of monolayered hollow closed nanostructures, withthe smallest allowed “nanooctahedra” within a size window of3−8 nm.32 The larger MoS2 particles are analogous to fullerenewith polyhedral quasi-spherical structures. Their diameters

range from 20 to 150 nm, while 2H-MoS2 platelets can beobtained with diameters above 100 nm.33 The applicability ofMoS2 structures greatly depends on the structural integritybecause larger particles are used as solid lubricants, while ananooctahedral geometry imparts excellent catalytic attributes(Figure 2).34 Nanooctahedral quantum-sized MoS2 structures

were reported in 1993 with six rhombi in the corners.32,35

Pulsed-laser ablation (PLA) and subsequent quantum-chemicalcalculations revealed 2−4 walled MoS2 nanooctahedra withabout 104 atoms.36

Transformation of the morphology of MoS2 with the size andnumber of layers was also validated by theoretical calculations.Previously, Alexandrou et al. studied the structure of MoS2core−shell nanoparticles, synthesized by an electric arcdischarge method. An electric arc was passed between acathode, comprised of carbon, and a hollow molybdenumanode, containing MoS2 powder in the interior. A high-resolution transmission electron microscopy (HRTEM) studyindicated the agglomerated polyhedral structure of MoS2(Figure 3).37

2.1.2. Synthesis of 0D MoS2. In 1995, the Wilcoxon groupsynthesized quantum-sized (2−10 nm) MoS2 clusters usinginverse micellar cages.38 They demonstrated a crossover fromthe band-like spectra of the clusters to molecule-like spectrabecause of the quantum confinement effect. Again, Chhowallaet al. in 2000 reported that hollow MoS2 nanoparticles,synthesized via the arc discharge method, show ultralow frictioncompared to fullerene-like structures.39 Further, generation ofthe MoS2 nanoparticles on the reduced graphene oxide (RGO)sheets was reported by Li and co-workers in 2011.40 This workconcluded that RGO sheets acted as the support to influencethe size of the nanoparticles, which otherwise grew asaggregated MoS2 (Figure 4).Again, Dong et al. synthesized MoS2 quantum dots by an

ultrasonication-mediated technique using an “up-bottom”approach. These dots exhibited profound down-conversionphotoluminescence (PL) behavior, proving their strongpotential in photodynamic therapy (PDT).41 In a similarcontext, another research group reported a one-pot, facilesynthesis of hybrid MoS2/WS2 quantum dots by a solvothermalapproach (Figure 5), assisted by 3−6 h of ultrasonication.42

This work very clearly demonstrated the idea of preparing dotsfrom bulk MoS2, using ultrasonication, which delaminated thesheets, followed by exfoliation. Finally, the soluble part can be

Figure 1. Dimensionwise biosensing potential of MoS2.

Figure 2. (a) TEM micrograph of MoS2 nanooctahedra generatedfrom MoS2 powder by PLA. (b) Ball-and-stick models of thenanooctahedra. Reprinted with permission from ref 34. Copyright2011 John Wiley and Sons.

ACS Applied Nano Materials Review

DOI: 10.1021/acsanm.7b00157ACS Appl. Nano Mater. 2018, 1, 2−25

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isolated as MoS2 quantum dots, while the insoluble partincluded the MoS2 nanosheets. This report gave a spark to thesynthesis of MoS2 quantum dots in the following time primarilyby employing hydrothermal or solvothermal techniques.43,44

Gradually researchers tried to investigate MoS2 quantumdots for their analogous properties with graphene quantumdots. Wang and co-workers used such dots as a PL sensingplatform for 2,4,6-trinitrophenol, with high sensitivity down to95 nM.45 Another study reported the synthesis of heterodimen-sional MoS2 nanodots dispersed onto exfoliated MoS2 nano-

sheets by employing a liquid exfoliation technique withalteration of the sonication time and mode.46 However, thestudy targeted the fabrication of high-performance electrodeswith energy-based applications.Besides their wide diagnostic potential, MoS2 nanodots have

also been explored for a number of therapeutic potentials. Liuand co-workers investigated the glutathione (GSH)-modifieddots for photothermal cancer therapy.47 They witnessed thegrowth inhibition of Murine breast cancer 4T1 cells withefficient excretion via urine.

Figure 3. Typical 0D MoS2 particle transformation of the geometry from nanooctahedral (a and b) to quasi-spherical (c and d) outer shells. (e and f)HRTEM images of MoS2 layers. (g) Pictorial depiction of layers as in part f. Reprinted with permission from ref 34. Copyright 2011 John Wiley andSons.

Figure 4. Synthesis of MoS2 in solution with and without graphene sheets. (A) Schematic solvothermal synthesis with GO sheets to yield the MoS2/RGO hybrid. (B) SEM and (inset) TEM images of the MoS2/RGO hybrid. (C) Schematic solvothermal synthesis without GO sheets, resulting inlarge, free MoS2 particles. (D) SEM and (inset) TEM images of the free particles. Reprinted with permission from ref 40. Copyright 2011 AmericanChemical Society.

Figure 5. One-pot synthesis of MoS2/Ws2 quantum dots. Reproduced with permission from ref 42. Copyright 2015 John Wiley and Sons.



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The synthesis of monolayer MoS2 quantum dots has beendelved into to a large extent in recent time. The multilayeredbulk MoS2 structures are held together by weak van der Waalsforces, which can be overcome by exfoliation via a top-downapproach. Although a few research studies reported thesuccessful synthesis of monolayer dots, still an effective andtunable synthesis is in demand for various advancedapplications. A stepwise synthesis of monolayered MoS2quantum dots is presented in Figure 6.43

Some popular methods for synthesizing MoS2 quantum dotsinclude mechanical exfoliation, chemical vapor deposition,solvothermal or hydrothermal synthesis, liquid exfoliation,etc.48−54 Among all of these, the hydrothermal approach iswidely accepted because of its operational ease. Gu and co-

workers in 2016 synthesized water-soluble MoS2 dots by asimple hydrothermal process.55 They obtained quantum dotswith an average diameter of ∼2.8 nm (Figure 7) with excellentperformance as a fluorescent probe for hyaluronidase detectionwith high sensitivity. Again a facile colloidal chemical route wasemployed to generate MoS2 dots with diameters of ∼3 nm.56

They observed excitation-dependent fluorescence (λ max = 575nm with a quantum yield of 4.4%).MoS2 quantum dots have also been prepared by refluxing

few-layered MoS2 nanosheets in ethylene glycol.57 Thesynthesis of monodisperse MoS2 dots was reported through amechanical grinding method, followed by ultrasonication.58

Disruption of the bulky layered structure and cleavage of thecovalent chemical network was brought about by the synergistic

Figure 6. Stepwise synthesis of monolayered MoS2 quantum dots. Reprinted with permission from ref 43. Copyright 2015 John Wiley and Sons.

Figure 7. (A) TEM and (B) HRTEM micrographs of MoS2 prepared by a hydrothermal method using ammonium tetrathiomolybdate[(NH4)2MoS4] as the precursor. (C) (a) PL spectrum of MoS2 along with photographs of the samples (b) under daylight and (c) under UV light at365 nm. Reprinted with permission from ref 55. Copyright 2016 American Chemical Society. Reprinted with permission from ref 56. Copyright2015 Royal Society of Chemistry.

Figure 8. Preparation of TMD quantum dots from a layered bulk structure. Reprinted with permission from ref 22. Copyright 2015 John Wiley andSons.



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effect of grinding and ultrasonication.22 This group suggested acommon synthetic route for transition-metal dichalcogenide(TMD) by breaking the planar covalently bound X−M−Xsandwich structure (Figure 8).However, the top-down approaches need volatile organic

solvents, prolonged reaction time, and toxic reagents.55 Thus,the “bottom-up” route has also been followed for the synthesisof tunable MoS2 quantum dots. In this quest, a DNA matrix hasbeen used as a template to obtain crystalline MoS2 dots byusing MoCl5 as the precursor.58 These approaches, however,may lead to the simultaneous generation of carbon quantumdots.55 A recent work reported the synthesis of MoS2 quantumdots, which are hydrophobic in nature. This further necessitatesthe use of a surfactant such as cetyltrimethylammoniumbromide to make the dots water-soluble.56 However, use of asurfactant has some demerits like low quantum yield, inductionof cytotoxicity, etc., which restrict some important applicationsin the biomedical domain as well as in sensing. Although MoS2quantum dots have immense potential as excellent fluorescenceprobes for biosensing, still a considerable amount of work hasnot been reported.2.2. 1D MoS2. 1D nanomaterials include “nanosized

members” with two of the three dimensions in the nanometerrange, while the third is in the micrometer range. This classincludes nanotubes, nanofibers, nanowires, etc. Analogous tocarbon, TMD also forms tubular structures. MoS2- and WS2-based nanotubes have been investigated recently for theirexceptional electronic properties.59−61 Theoretical calculationsdemonstrated that the electronic properties of MoS2 nanotubescan be tailored by varying the diameter and chirality of thetubes.62−64

2.2.1. Structure of 1D MoS2. Armchair and zigzag structuresof MoS2 nanotubes were proposed by Seifert and co-workers in2000 (Figure 9).32 They witnessed that zigzag nanotubes show

a narrow direct band gap, while the armchair MoS2 nanotubesexhibit a nonzero moderate direct gap. Thus, depending uponthe structure, the electronic properties can be altered, whichdirectly influences the efficiency of an electrochemicalbiosensor.Ghorbani-Asl et al. forwarded a theoretical model depicting

the structural attributes (Figure 10) of single-walled andmultiwalled TMD nanotubes and estimated their electro-mechanical properties by density functional theory (DFT).65

They reported that large-diameter single-walled and multi-walled TMDs exhibit electrochemical properties similar tothose of monolayer and bulk TMDs. Electronic, vibrational,and electromechanical attributes of large-diameter planar MoS2and WS2 nanotubes were also predicted by structural modeling.Again, Rao et al. studied the morphology of MoS2 nanotubes

synthesized by the thermal decomposition of (NH4)2MoS4 byusing TEM (Figure 11a,b).66 They revealed that the nanotubesobtained from MoS3 exhibited onion-like morphology, whichthey assumed to be an intermediate between MoS3 and thecrystalline phase of MoS2, formed according to the followingsynthetic route:

+ → +MoS H MoS H S3 2 2 2 (1)

This work demonstrated the synthesis of MoS2 nanotubeswith inner and outer diameters within the ranges of 30−45 and7−8 nm, respectively. The morphology of these tubes isanalogous to that of CNTs. A tubular 1D structure not onlyfacilitates the electrical properties of MoS2 but also plays a vitalrole in mechanical reinforcement when used as a compositematerial.

2.2.2. Synthesis of 1D MoS2. The synthesis of 1D MoS2nanotubes has attained a high pace in last 2 decades. In 1996,Remikar and co-workers reported the synthesis of hollow MoS2microtubes by an iodine-transport method.67 Again, in 2001 thesame group reported the synthesis and structural aspects of“subnanometer-diameter monomolecular” single-walled MoS2by a similar transport reaction. Thus, synthesized MoS2nanotubes grew in twisted chiral bundles with hexagonalclose packing.68 A regular stacking arrangement was evidencedfrom TEM micrographs with varying lengths and a constantdiameter.Further, hybrid nanostructures have also been reported by

using MoS2 as one of the components. In 2006, Song and co-workers reported the synthesis of CNT@MoS2 nanotubes by ahydrothermal method by dispersing CNTs in an aqueoussolvent containing a mixture of Na2MoO4 and KSCN.69 1DMoS2 composite nanorods and nanotubes were prepared byZhang et al. via a sulfidation reaction using a MoOx/polyaniline(PANI) precursor. Heptamolybdate tetrahydrate and anilinewere reacted for 8 h at 50 °C in the presence of dilute 1 M HClto obtain Mo3O10(C6H8N)2·2H2O.70 The thus-obtainedproduct was again reacted with ammonium peroxydisulfateultrasonically in an acid medium, followed by annealing at 400°C. The 1D nanostructures boosted the electrochemicalattributes of the material as a promising anode for lithium-ion batteries.22

Again Yang et al. proposed a novel way to fabricate 1DMoS2/PANI nanowires from an anilinium trimolybdateprecursor by a hydrothermal process using thiourea (Figure12).71 They obtained a tunable hierarchical MoS2/PANInanoarchitecture by varying the amounts of the precursors.This work contributed to the potential applications of 1Dhybrid nanostructures based on MoS2 in the field of ionbatteries and storage cells. Hierarchical MoS2 nanotubes weresynthesized in 2014 by a solvothermal method using Na2MoO4·2H2O, MnCl2·4H2O, and (NH4)2CS. Here, an intermediateMnMoO4 acted as a self-sacrificed template. The synthesizedMoS2 nanotubes enhanced the lithium-ion storage capacitybecause of its hierarchical tubular structure.72 Again, multi-walled MoS2 nanotube and nanoribbon were synthesized by achemical transport method, which showed the presence of 10−11 monolayers stacked together to constitute the outer walls.73

Figure 9. Armchair and zigzag structures of MoS2 nanotubes.Reprinted with permission from ref 32. Copyright 2000 AmericanPhysical Society.



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Because of the excellent analogous attributes with CNTs, 1DMoS2 nanostructures have also attained interest in recent times.However, reports on MoS2 are much fewer compared to thoseon CNTs, which indicates the emergence of newer materialsand synthetic methods for obtaining nanostructured MoS2 inthe coming years.2.3. 2D MoS2. The materials having only one of the three

dimensions within the nanometer range fall into this category.The most common members of this class are the nanosheets,nanoflakes, nanoclays, etc. 2D graphene, graphene oxide (GO),and RGO have been explored extensively for their unique

electrical, mechanical, and antimicrobial properties.74,75 Recentresearch showcased the synthesis and potential applications of2D nanosheets made up of TMDs.76 Because of its inherentthickness-dependent band gap and natural abundance, 2DMoS2 is regarded as a prominent semiconductor material,among the TMDs. Compared to 0D or 1D MoS2, 2Dnanosheets have been explored widely because of their easyreaction conditions and broader scope of applicability.

2.3.1. Structure of 2D MoS2. Stratified crystals of hexagonalmolybdenum disulfide, comprised of the crystal structure ofMoS2, have unit-cell thickness. Exfoliated 2D MoS2 nanosheetsexhibit excellent optoelectronic attributes, which make thempromising candidates for biosensing applications.19,77−79 Thesenanosheets can be dispersed in liquid or gaseous media becauseof large lateral dimensions. The edges of the nanosheets can beengineered by termination with either Mo or S atoms.19 Thisprovides a tunable metallic property to the overall sheetelectrical performance, which is an essential prerequisite forelectrochemical biosensors. The layered structure of 2D MoS2sheets was visualized by TEM, which showed a hexagonalatomic arrangement. Yu et al. identified individual Mo and Satoms by scanning transmission electron microscopy−high-angle annular dark-field fluorescence (STEM−HAADF)imaging (Figure 13a−c). This also revealed the stackingpatterns of MoS2 layers.

80 The structure of 2D MoS2 can alsobe illustrated from field-emission scanning electron microscopy(FESEM) and TEM imaging, as demonstrated by Mishra etal.81 (Figure 14), and shows a flake-like morphology with lateraldimension distributions ranging from 80 to 120 nm.The electronic properties of MoS2 are primarily dictated by

the d orbital of molybdenum.82 The bonding states are filled bythe four electrons of molybdenum, while the lone pairsterminate the layer surfaces. The electronic structure of MoS2

Figure 10. Structural models of TMD nanotubes: (a) single-walled; (b) multiwalled. Color code: yellow, S; blue, metal (T) atoms. Reprinted withpermission from ref 65. Copyright 2013 Nature Research.

Figure 11. TEM micrographs of MoS2 nanotubes: (a) a bunch; (b) an individual tube. Reprinted with permission from ref 66. Copyright 2001 JohnWiley and Sons.

Figure 12. Fabrication of 1D hierarchical MoS2/PANI nanowiresthrough the hydrothermal treatment of anilinium trimolybdate.Reprinted with permission from ref 71. Copyright 2001 John Wileyand Sons.



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results in interesting excitonic and fluorescent attributes thathave immense utility in biosensing.2.3.2. Synthesis of 2D MoS2. The most common method for

synthesizing 2D MoS2 nanosheets includes the exfoliation oflayers from bulk MoS2 by ultrasonication.83 Mechanical orultrasonic exfoliation generally results in uniform monolayers,which, however, suffer some demerits like low yield, flakedeposition, etc. Solution-based exfoliation techniques are usedto obtain mixtures of single-layer and multilayered MoS2 sheets.Organic solvents, metal-ion intercalation, and the use ofsurfactants facilitate such exfoliation processes.84−89 Surfac-tant-free liquid-exfoliation methods were also reported, whichled to the production of 2D-layered MoS2.

85 Different solventsystems were investigated for the efficient dispersion of MoS2sheets, although low-boiling ones are preferred for thesubsequent drying process.90

Again, chemical gas or vapor deposition techniques havebeen employed to synthesize 2D MoS2 nanosheets. Liu et al.recently synthesized 2D MoS2 thin layers by a two-stepthermolysis process, taking (NH4)2MoS4 as the precursor(Figure 15).89 The high-temperature-annealed ammoniumthiomolybdate produced MoS2 layers with high surface area,which resulted in augmentation of the electrical properties.However, large-area uniformity with controllable synthesis is

still a challenge. Chemical vapor deposition techniques areuseful in synthesizing a wafer-scale 2D MoS2 with a widepotential for electronic applications. Lin et al. reported a wafer-scale synthesis of MoS2 layers by a two-step thermal process,using MoO3 as the precursor in the presence of sulfur (Figure16).8 The chemical equations involved in this process are

+ → +MoO (s) H (g) MoO (s) H O(g)3 2 2 2 (2)

+ → +MoO (s) 2S(g) MoS (s) O (g)2 2 2 (3)

Such methods diminish the possibility of interfacialcontamination during the layer-by-layer deposition of MoS2.These approaches help to obtain a continuous film with thedesired thickness. Another effective method for synthesizing 2DMoS2 is an ion-intercalation technique. One of such high-yielding synthesis was reported for MoS2, WS2, TiS2, and ZrS2by a complex lithiation method.88,87 Structural deformation isthe main demerit of the intercalation method, besidesprolonged time and environmental issues.

2.4. 3D MoS2. The bulk nanomaterials that are not confinedwithin the nanoregime are visualized by three arbitrarydimensions of >100 nm. 3D nanomaterials include nano-clusters, nanodispersions, etc. Recent literature showcased thesynthesis of 3D hierarchical architectures based on the self-assembly of MoS2. Such assembled nanomaterials have shownimmense potential as hydrogen evolution reactors, visible-lightphotocatalysts, high-performance flexible supercapacitors, lith-ium-ion batteries, etc.91−93 However, very few reports haveendorsed the biosensing capabilities of 3D MoS2.

2.4.1. Structure of 3D MoS2. Efficient 3D MoS2 nanostruc-tures, like nanoporous sheets, core−shell, double-gyroid,vertical nanoflakes, etc., have been studied by a few researchgroups.95−99 Kong et al. described the 3D structure of MoS2 incomparison with MoSe2.

97 One neutral layer was described toconsist of three covalently bonded sheets of atomic thickness,with an interlayer distance of 6 Å (Figure 17). Such crystalshave two kinds of sites, viz., terrace and edge. Anisotropicbonding and surface energy prefer platelet-like morphology forsuch layered nanomaterials.85,97,100,101 Figure 17d shows theTEM and HRTEM images (showing atomic planes) of thedensely packed grain-like morphology of MoS2 with anindividual grain size of about 10 nm. Raman spectra for 3DMoS2 nanostructures are shown in Figure 17e. Again, Figure17f represents the excited Ag

1 and E2g1 modes for the edge- and

terrace-terminated MoS2 respectively, while Figure 17g showsthe Raman spectrum for the terrace-terminated MoS2.

85

Again, Lu et al. observed a nanoflower-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) plane of MoS2with an interlayer spacing of ∼0.65 nm.95

2.4.2. Synthesis of 3D MoS2. 3D MoS2 and MoSe2 thin filmswere synthesized by Kong and co-workers in 2013, withvertically aligned layers.85 They demonstrated that thesulfurization reaction undergoes diffusion of sulfur into theMoS2 layers and converts it into sulfide. Again, 3D MoS2 sheetswere synthesized by using tetrakis(diethylaminodithiocarbo-

Figure 13. (a) STEM−HAADF image of 2D MoS2. (b and c) Filteredimages of MoS2 layers, distinguishing molybdenum and sulfur.Reprinted with permission from ref 80. Copyright 2013 NaturePublishing Group.

Figure 14. (a) FESEM image of MoS2 nanosheets. (b) TEM image showing lateral dimensions and (c) electron diffraction pattern of 2D MoS2.Reprinted with permission from ref 81. Copyright 2015 Nature Publishing Group.



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mato)molybdate(IV) as the precursor of both molybdenumand sulfur. The synthesis was comprised of a chemical vapordeposition method, where graphene was first synthesized ontoa 3D nickel foam. Subsequently, it was reacted withtetrakis(diethylaminodithiocarbomato)molybdate(IV) to ob-tain 3D MoS2/graphene/nickel (3D MoS2/G/Ni;Figure

19a).91 Parts b−d of Figure 19 show 4 and 15 layers ofgraphene and the selected-area electron diffraction (SAED)pattern, respectively. A schematic diagram of the experimentalsetup and the optical images of the samples are shown in Figure19e,f. Further, in 2014, the self-assembly of MoS2 nanostruc-tures with 3D hierarchical frameworks was performed by a

Figure 15. Two-step thermolysis process for synthesizing 2D MoS2. Reprinted with permission from ref 89. Copyright 2012 American ChemicalSociety.

Figure 16. (a) Schematic illustration for the synthesis of MoS2 layers by MoO3 sulfurization. (b) MoS2 layer grown on a wafer. Reprinted withpermission from ref 30. Copyright 2012 Royal Society of Chemistry.

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) 3Dnanostructures of MoS2. (d) TEM image of 3D MoS2 (the HRTEM image shows three atomic planes of S−Mo−S). (e) Raman spectrum fromMoS2. (f) Excited Ag

1 and E2g1 modes, respectively, for edge- and terrace-terminated MoS2. (g) Raman spectrum for terrace-terminated MoS2.

Reprinted with permission from ref 97. Copyright 2013 American Chemical Society.



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simple hydrothermal method.102 Such methods have also beenemployed for synthesizing MoS2 nanoflowers on a carbon fibercloth. These nanosheet-assembled MoS2 nanoflowers weresynthesized by using Na2MoO4 and CSN2H4 as precursors(Figure 20). Then GO was added to recover the rapid capacityfading of the MoS2 nanoflower-based anode.94

Further, MoS2-coated carbon foam/nitrogen-doped gra-phene were successfully synthesized by using 3D melaminefoams. Very recently, Zhang et al. reported the successfulsynthesis of a visible-light-responsive self-assembled MoS2/RGO by a hydrothermal method.92 Subsequent freeze-dryingyielded a 3D aerogel, which showed potential as a visible-lightphotocatalyst.

It has been observed that a hydrothermal approach is themost widely used method for synthesizing nanostructuredMoS2, regardless of the dimensions. However, literature reportson 3D MoS2 are very few to date.

3. PROPERTIES

3.1. Electronic Properties. Band structure and density ofstates dependent electronic properties of a MoS2 monolayer areinevitable in determining its application in electronic devices.The band structures of MoS2 have been calculated by Kuc et al.using the first-principle calculations with Perdew−Burke−Ernzerhof (PBE) exchange-correlation functionals. The incor-poration of the PBE scheme in DFT to obtain the band

Figure 18. (a) SEM and (b) TEM images showing the nanoflower-like morphology of 3D MoS2. (c) HRTEM image showing the interplanardistance with the (002) plane. Reprinted with permission from ref 95. Copyright 2017 Nature Publishing Group.

Figure 19. (a−c) Schematic depiction of 3D MoS2/G/Ni nanostructures. (b and c) HRTEM image of graphene. (d) SAED pattern. (e)Experimental setup for the synthesis. (f) Optical images of the samples. Reprinted with permission from ref 91. Copyright 2014 Wiley-VCH VerlagGmbH & Co. KGaA.



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structures increased the fraction of accuracy with theexperimental results.103 MoS2 is an indirect semiconductor in

bulk with a fundamental band gap of ∼1.2 eV, which initiatesbecause of the transition from the Γ high-symmetry point ofthe valence-band maximum to the conduction-band minimumpresent between the Γ−K high-symmetry points, as shown inFigure 21. However, the position of the conduction-bandminimum at the Γ point is dependent on the interlayerinteractions.104,105 Therefore, by a reduction of the number oflayers from the bulk form, the conduction-band minimum shiftsto a higher energy at the Γ point of the Brilluoin zone.Nevertheless, the band-energy values at the K point stay almostunchanged with a change in the slab thickness. This opens upthe direct-band-gap behavior at the K point in the monolayerstructure with an energy difference of ∼1.9 eV.103,106 However,it has been proposed that reducing the number of layersincreases the exciton binding energies from 0.1 eV (in the bulksystem) to about 1.1 eV (in the monolayer) because of thestrongly reduced dielectric constants.107 The excitonic effectsshould be incorporated in order to calculate the fundamentalband gap. The GW approach, where the self energy is given by

Figure 20. Synthesis of RGO-decorated MoS2 nanoflowers. Reprintedwith permission from ref 94. Copyright 2015 American ChemicalSociety.

Figure 21. (a) MoS2 band structure calculated at the DFT/PBE level with a reduction in the number of layers from the bulk to the monolayer. Thered dashed line shows the Fermi level, and the colored lines (blue and green) indicate the energy bands (valence and conduction) of the structure.The arrows indicate the fundamental band gap for the given system. Reprinted with permission from ref 103. Copyright 2011 American PhysicalSociety. (b) Brillouin zone and high-symmetry points of the MoS2 reciprocal lattice.



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the product of the Green’s (G) function and the screenedCoulomb interaction (W), has been used to calculate thefundamental band gap of the MoS2 monolayer and is found tobe ∼2.8 eV.108

The band-gap property of the MoS2 monolayer makes it apromising candidate for nanoelectronic biosensing applications.It has higher potential than its analogue graphene in suchapplications, which lacks the band-gap property in its originalform. Demonstration of the MoS2 monolayer with roomtemperature on/off ratios of 1 × 108, mobility of >200 cm2 V−1

s−1, and ultralow standby power dissipation has enabled therealization of practical electronic devices.109−111 Moreover, theMoS2 monolayer has a carrier lifetime of ∼100 ps and adiffusion coefficient of ∼20 cm2 s−1.112 Conclusively, it is worthmentioning that the above properties are highly suitable toestablish field-effect and electrochemical-based biosensors.The precision in fabricating MoS2 down to a monolayer

configuration with uniform thickness control enables accuratecontrol of the electrostatic characteristics of the transistor. Thelower in-plane dielectric constant is also attributed to precisecontrol of the electrostatic characteristics, thereby pushing itslimit to sub-5-nm-scale transistor technology.113,114 Notably,electron transport in the MoS2 monolayer is much slowercompared to that in bulk semiconductors. Yu et al. listedapproaches for improving carrier transport in the MoS2monolayer and accounted for intrinsic electron−phononscattering, surface optical phonon scattering, Coulombicimpurity scattering, atomic defect scattering, charge trapping,and metal-to-insulator transitioning as the reasons for low

mobility.115 Nevertheless, at sufficiently scaled lengths, suchissues may not be significant because the performance wouldmainly depend on the contacts rather than transport throughthe channel.116

3.2. Catalytic Properties. Recently significant works havebeen reported showing the catalytic activity of MoS2nanostructures and MoS2-based heterostructures.117−128 Edgesand metallic 1T polymorphic MoS2 nanostructures providedthe catalytic activity. The catalytic activity is further dictated bynanostructures with atomic-level precision.129 Recently, Yin etal. reported a poly(ethylene glycol)-functionalized MoS2nanoflower (PEG-MoS2 NF) system that showed profoundcatalytic effects during the decomposition of H2O2, generating ahydroxyl radical.130 This system induced a near-IR (NIR)photothermal effect, which imparted an excellent antibacterialeffect against both Gram-positive and Gram-negative bacteria.GSH oxidation was observed to be accelerated by NIRirradiation with effective bactericidal activity by inducinghyperthermia (Figure 22).Further, the edges of MoS2 mediate proton adsorption and,

subsequently, dihydrogen formation. The electrical conductivityand redox electrochemistry of MoS2 endorses them for energy-conversion- and storage-device-based applications.131 Yu et al.observed the electrocatalytic activity for hydrogen evolution byforming Ni−Co−MoS2 nanoboxes.131 Interestingly, it wasreported that the catalytic activity of MoS2 for such a reactiondecreases with increasing layers.130 Moreover, MoS2 nanostruc-tures have also been investigated for their electrocatalyticactivity in solar cells.132

Figure 22. (a) (i) PEG-MoS2 captured by bacteria. (ii) Catalytic decomposition of H2O2. (iii) Laser irradiation (808 nm) inducing hyperthermia andaccelerating GSH oxidation. (b) Bacterial colonies of E. coli incubated in (I) phosphate-buffered saline (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. Copyright2016 American Physical Society.



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3.3. Optical Properties. The layer-dependent band-structure variation in MoS2 is of particular interest amongresearchers for its optoelectronic properties. The generation of

a direct band gap in monolayer MoS2 provides strongluminescence properties, which are absent in the bulk material.In addition to the changes in the band structure, there is a

Figure 23. (a) Differential-reflectance spectrum showing A and B exciton absorption. The red, yellow, and green arrows represent the differentexciting photon energies and (b) PL spectrum for 2.33 eV (532 nm) excitation showing A and B exciton luminescence. Reprinted with permissionfrom ref 137. Copyright 2012 Nature Publishing Group. (c and d) Layer-dependent PL and Raman spectra showing the dramatic increase of theluminescence quantum efficiency in the MoS2 monolayer. Reprinted with permission from ref 138. Copyright 2010 American Chemical Society. (e)Peak position of the Raman modes and their dependency on the layer thickness. Reprinted with permission from ref 139. Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA. (f) Refractive index and extinction coefficient variation of the monolayer with the wavelength. Reprinted withpermission from ref 140. Copyright 2014 AIP Publishing LLC.

Figure 24. (a) Fabrication of a gold/MoS2 electrode: (1) molybdenum deposited on a gold electrode using an e-beam evaporator, (2) H2S + Arplasma reacting with a molybdenum film in a PECVD chamber, (3) H2S penetrating into molybdenum, and (4) a MoS2 biosensor device. Reprintedwith permission from ref 153. Copyright 2015 Royal Society of Chemistry. (b) Construction of a TMD biosensor electrode by drop-casting on a GCelectrode, followed by immobilization of GOx, cross-linked with glutaraldehyde (GTA) for the electrochemical sensing of gluocose. Reprinted withpermission from ref 147. Copyright 2017 American Chemical Society. (c) Biofunctionalization layers on a MoS2 device surface. Reprinted withpermission from ref 151. Copyright 2014 John Wiley and Sons. (d) MoS2-based modified electrodes for glucose sensing (S, source; D, drain).Reprinted with permission from ref 152. Copyright 2015 John Wiley and Sons.



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structural change in the thin-film MoS2. The inversionsymmetry existing in the bulk and thin films with an evennumber of layers is explicitly broken in films with odd numbersof layers. This leads to the generation of a valley-contrastingoptical selection rule, thus allowing spin-valley-coupled bandstructures.133 The broken inversion symmetry breaks downKramers’ degeneracy and splits the valence bands by ∼160meV.134 This leads to the possession of strongly bound excitonswhose binding energies depend on the number of layers. Theseexcitons decide the optical properties of the material, as shownin Figure 23. The optical absorption spectrum is obtained bymeasuring the differential reflectance of MoS2 samples on asubstrate and the bare substrate of hexagonal boron nitride.The pronounced absorption minima correspond to the A and Bexcitons (Figure 23a).135,136 Figure 23b shows the PL spectrafor the total unpolarized emission under 2.33 eV (532 nm)excitation. The strongest emission near 1.9 eV (652 nm) arisesfrom A exciton complexes, the weaker emission at 2.1 eV (590nm) is due to the B excitons, and the feature near 1.8 eV (688nm) corresponds to emissions from defect-trapped excitons.137

Again, Splendiani et al. demonstrated the layer dependenceof the PL and Raman signals in MoS2.

138 With a increase in thenumber of layers from the monolayer, the PL signal decreased;however, the Raman signal slightly improved because of theincreased amount of material interaction. Nevertheless, theintrinsic quantum efficiency increased dramatically on goingtoward the monolayer structure because there is a huge jump inthe PL emission.139,140 The weak interlayer coupling betweenthe restacked MoS2 sheets gradually decreased the emission

intensity. A slight red shift in the absorption resonance and PLenergy was observed with increasing film thickness.141 Theminor shift can be ascertained by the fact that the direct bandgap is only slightly sensitive at the K point to the layer thicknessbecause of the quantum confinement effect.142 The possessionof PL characteristics provides the possibility of using suchstructures in fluorescence-based applications, which can be usedto trace, image, and sense biological components.143,144 TheRaman characteristics of the MoS2 monolayer is also a functionof the dimension and permittivity of the environment.Attaching biological components may alter such characteristicsand thus can be used as a biosensing principle.145,146

4. APPLICATION OF MOS2 IN BIOSENSING

4.1. Electrochemical Biosensors. Recent development inelectrochemical biosensors mostly focuses on the use ofnanomaterials and nanostructures. TMDs have attainedsignificant attention of the scientific community because oftheir analogous attributes to that of graphene. In a recentreport, Rohaizad et al. demonstrated the fabrication of anelectrochemical biosensor by exfoliation of TMD for efficientsensing of glucose up to 2.8 μM.147 The construction of suchbiosensors includes the selection of the matrix material,fabrication of the electrode, use of a mediator (or mediatorfree) for immobilization of the labeling biomolecules (or labelfree), and detection of analytes using electrochemicaltechniques.144 Glassy carbon (GC), platinum, indium−tinoxide, and screen-printed electrodes are generally used foranchoring the biosensor matrix by different techniques like

Figure 25. (a) Voltametric detection of H2O2 by a MoS2/GOx-modified electrode and (b) amperometric responses of a MoS2-modified electrode.Reprinted with permission from ref 154. Copyright 2013 American Chemical Society. (C) SWV responses of the modified electrode in 0.1 M PBSwith injections of dsDNA. Reprinted with permission from ref 155. Copyright 2014 American Chemical Society. (d) EIS response (Nyquist plots) ofthrombin and adenosine triphosphate (ATP) detection by usinga gold/MoS2-based biosensor. Reprinted with permission from ref 156. Copyright2016 American Chemical Society.



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Table 1. List of Analytes, LODs, and Techniques Used in the Fabrication of MoS2-Based Electrochemical Biosensors

serial no. matrix analyte technique LOD ref

1 MoS2 glucose CV 2.8 μM 1472 MoS2 H2O2 CV 20 ng mL−1 1533 MoS2 nanoparticles H2O2 amperometry 2.5 μM 1544 MoS2/thionine dsDNA SWV 0.09 ng mL−1 1555 MoS2/poly(xanthurenic acid) adenine and guanine DPV 3 × 10−8 for adenine, 1.7 × 10−8 for guanine 1656 MoS2/PANI chloramphenicol DPV 6.9 × 10−8 mol L−1 1667 gold/MoS2 miR-21 DPV 0.26 pM 1678 MoS2 single layer dopamie CV 1719 gold/MoS2 nanoparticles thrombin and ATP CV 0.74 nm for ATP, 0.0012 nM for thrombin 17210 gold/MoS2 ribaflavin DPV 20 nM 17311 MoS2/PANI adenine and guanine DPV 3 × 10−9 for adenine, 5 × 10−9 for guanine 17412 poly(m-aminobenzenesulfonic acid)-reduced MoS2 dopamine DPV 0.22 μM 17513 nickel-doped MoS2/RGO glucose CV 17614 MoS2 glucose CV 0.042 μM 177

Figure 26. MoS2-based FET biosensor. (a) Schematic diagram of the sensing scheme showing the MoS2 channel functionalized with receptors forspecifically 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/silicon substrate (scale bar = 10 μm), (c) optical image of the FET device, (d) image and schematic (inset)showing the macrofluidic integrated FET chip, (e) transfer characteristics of a biotin-functionalized FET device measured in a pure buffer (0.01×PBS), with the addition of a streptavidin solution (10 μM in 0.01× PBS) and again with a pure buffer, and (f) comparison of the sensitivities in thesubthreshold, saturation, and linear regions of the transistor device. Reprinted with permission from ref 184. Copyright 2014 American ChemicalSociety. (g) Image of the FET biosensor integrated with a PDMS (polydimethylsiloxane) reservoir and (h) experimental setup showing an inlet/outlet tubing kit, driven by a motorized syringe pump. Reprinted with permission from ref 183. Copyright 2015 Nature Publishing Group.



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drop-casting, electrochemical deposition, etc.148−150 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 board, using a plasma-enhancedchemical vapor deposition (PECVD) technique for efficientsensing of H2O2.

153 Figure 24 shows different types of MoS2-based electrochemical biosensors for the sensitive detection ofbioanalytes.Most of the electrochemical biosensors based on MoS2

nanostructures employ potentiometric and amperometrictechniques for the detection of bioanalytes. Besides, square-wave voltammetry (SWV), differential-pulse voltammetry(DPV), and electron impedance spectroscopy (EIS) are thecommonly used techniques for detecting bioanalytes with ahigh level of sensitivity.154−156 Figure 25 shows the widely usedtechniques for MoS2-based electrochemical biosensors.A range of analytes have been successfully detected by MoS2-

based electrochemical biosensors. MoS2-based hybrid materialshave also been explored for the fabrication of sensor probes forthe highly selective and sensitive detection of bioana-lytes.157−164 Poly(xanthurenic acid) based on MoS2 supportwas electrochemically synthesized by Yang and co-workers.They prepared a highly electroactive biosensing matrix by anultrasonic method for the efficient detection of guanine andadenine.165 With a similar approach, a MoS2 and PANIcomposite was prepared through the in situ electrochemicaldetection of chloramphenicol.166 Again, gold nanoparticledecorated 2D MoS2 nanosheets were used for the electro-chemical sensing of glucose in the presence of commonlyinterfering species like ascorbic acid, dopamine, uric acid, andacetaminophen.21,167 A myoglobin-immobilized 0D MoS2nanoparticles−GO hybrid was fabricated by Yoon and co-workers for the detection of H2O2. They observed theenhancement of the electrochemical signal because of GO,which may be attributed to the high surface area available forthe immobilization of myoglobin.168 With a similar approach,an extremely sensitive (2.5 nM) H2O2 biosensor based onMoS2 quantum dots (∼2 nm) was fabricated by Wang et al. in2013.152 Another thionin-functionalized MoS2-based electro-chemical biosenser was fabricated for the direct detection ofDNA with sensitivity up to the ppb level.153 Electrochemicallyreduced single-layer 2D MoS2 nanostructures exhibited a fastelectron-transfer rate, which helped in the detection of glucosewith high sensitivity.169 Micromolar-level electrochemicalbiosensing of glucose was achieved by using a glucose oxidase(GOx)-immobilized gold/MoS2 nanohybrid.170 Literaturereports primarily showcased the utility of 2D MoS2nanostructures in the development of electrochemical bio-sensors.163 However, MoS2 nanostructures of other dimensionshave also been explored for the biosensing of different analytes,which will be discussed in the following sections. Table 1encompasses a list of analytes, lowest detection limits (LODs),and techniques used in the fabrication of MoS2-basedelectrochemical biosensors.4.2. Field-Effect Transistor (FET)-Based Biosensors.

FET-based biosensors are of great interest for researchers for itshighly desirable characteristics of label-free rapid electricaldetection capabilities, low power consumption, mass produc-tion, compactness, and the possibility of on-chip integration ofthe chip with measurement systems. FET conventionallyconsists of two electrodes (drain and source) that areconnected electrically via a semiconductor material (channel).The current flow between the drain and source through the

channel is modulated by a third electrode, namely, a gate, whichis capacitively coupled through a dielectric layer covering thechannel. Capturing biomolecules using a functionalized channelproduces an electrostatic effect that is transduced into areadable signal in the form of a change in the electricalcharacteristics of the FET device.178 Nevertheless, the perform-ance characteristics are dependent on the biasing strategy of thedevice.179 Figure 26 illustrates the sensing principle, detectionstrategy, and chip design of MoS2-based FET biosensors.The possession of a direct band gap in MoS2 allows better

control between the conductive and insulated states and alsoprevents leakage currents.180 This permits the designed FET toobtain higher sensitivity and accurate sensing with a very lowconcentration of the analytes. However, in comparison to itscounterpart, a graphene-based FET device possesses a zeroband gap and thus cannot be switched off, thereby inducingleakages and a higher potential for inaccuracies.181 Further-more, the absence of out-of-plane dangling bonds in MoS2reduces the surface roughness scattering and interface traps.This results in better electrostatic control due to the lowerdensity of the interface states on the semiconductor−dielectricinterface, thereby reducing the low frequency noise, whichhinders the performance of FET-based biosensors.182

MoS2-nanosheet-based FET biosensors have been demon-strated to detect proteins, pH, cancer biomarkers, etc., withhigh sensitivity and selectivity.183−186 It has been articulatedthat a few-layer MoS2-film-based FET device shows a morestable and sensitive response than the monolayer-baseddevice.139 Chip integration with microfluidics opens up hugeprospects in realizing compact sensor systems with clinicallymeaningful detection limits, allowing their usage in point-of-care diagnosis applications.187,188 Additionally, recent develop-ments in the synthesis and growth of MoS2 thin films as well asa demonstration of the integrated logic circuits on MoS2illustrate the viability of realizing MoS2-based FET biosensorswith low processing cost and multiplexed detection capabil-ities.21,189−197

Table 2 presents recent reports on the MoS2-based FETbiosensors, sensor material used, and detection limits.

4.3. Optical Biosensor. 4.3.1. MoS2 as a FluorescenceProbe. MoS2 has recently found widespread applications as afluorescence probe in the detection of various biological andenvironmental analytes (Table 3). As discussed in the previoussection, MoS2 possesses good optical properties likefluorescence and have been utilized to design various sensing

Table 2. Recent Reports on MoS2-Based FET Biosensors

target sensor materialdetectionrange

detectionlimit ref

streptavidin Si/SiO2/MoS2/biotin

100 fM 184

DNAhybridization

Si/SiO2/MoS2/DNA conjugates

10−100 fM 10 fM 196

PSA Si/SiO2/MoS2/anti-PSA

1 pg mL−1 to1 ng mL−1

1 pgmL−1

21

tumor necrosisfactor-α (TNF-α)

Si/SiO2/MoS2/HfO2/anti-TNF-α

60 fM to 6pM

60 fM 155

opioid peptideDAMGO

Si/SiO2/MoS2/wsMOR

3 nM to 1 μM 3 nM 190

PSA Si/SiO2/MoS2/HfO2/anti-PSA

375 fM to3.75 nM

375 fM 170



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platforms. The art of literature reveals multiple aspects of nano-MoS2 in biosensing applications.On the basis of the morphological characteristics and optical

properties, it can be used as either a fluorescence probe or aquencher. Generally, 2D nanosheets have been reportedly usedas quenchers in different sensing platforms. The efficiency ofMoS2 nanosheets to absorb emissive radiation over a widerange of wavelengths as well as their ability to interactspecifically with certain bioentities makes them suitable to actas quenchers. Forster-resonance-energy-transfer-based nanop-robes are known to use MoS2 nanosheets frequently as thefluorescence acceptor in an acceptor−donor couple. On theother hand, quantum-sized 0D MoS2 can be directly used as asensing probe because of its good fluorescence characteristics.In this section of the paper, we are trying to provide a conciseaccount of the working mechanism of different MoS2-basedsensors.198

DNA is one of the most widely reported biomolecules thatcan be detected by a MoS2 sensing platform with highselectivity and sensitivity. Generally, 2D MoS2 nanosheets havebeen used as quenchers that work in conjugation with astrongly fluorescent probe. Single-layer MoS2 can be consideredto be S−Mo−S sandwich structure in which each molybdenumis coordinated in a trigonal-prismatic geometry to six sulfuratoms. With such unique morphological characteristics, MoS2nanosheets can specifically absorb single-stranded DNA(ssDNA) via the van der Waals force between nucleobasesand the basal plane of MoS2. In most of the cases, the targetDNA molecule is labeled with a fluorescent dye or function-alized with a fluorescent nanostructure. As a result of specificMoS2 nanosheets/ssDNA interaction, the fluorescence effi-ciency of the probe gets hindered, and the diminished intensitycan be correlated to the quantitative amount of ssDNA presentin the system. Zhu et al. reported the detection of DNA andsmall biomolecules by using such a technique.199 Huang et al.demonstrated a slightly modified strategy for the detection ofssDNA but with a turn-on type of response (Figure 27).200 Adye-labeled DNA (P1:5′-TAMRA-TGCGAACCAGGAATT-

3′) was used as the probe for the detection of itscomplementary DNA (T1:5′-AATTCCTGGTTCGCA-3′).The probe worked at an excitation wavelength of 565 nmwith consequent emission at a wavelength of 580 nm. Becauseof the interaction of P1 and MoS2, the probe fluorescence isquenched. However, in the presence of target DNA, i.e., T1, P1forms a double-stranded form (dsDNA). Contrary to ssDNA,dsDNA has very weak binding affinity toward MoS2, and,hence, P1/MoS2 interaction is interrogated. As a result,fluorescence is restored, and measurement of the fluorescenceintensity provides the quantitative indication of T1.200 Singh etal. used the same technique for detection of pathogenSalmonella typhimurium. A fluorescein-labeled aptamer (Apt-FAM) was used as the probe for the specific recognition of thepathogen. In the presence of S. typhimurium, the Apt-FAMprobe prefers to bind with the target pathogen instead of MoS2nanosheets. As a result, a turn-on kind of response is achieved.The method can be selectively used for the detection of S.typhimurium over Escherichia coli and Pemphigus vulgaris.201

Kong et al. followed the detection of a prostate-specific antigen(PSA), a biomarker for the early diagnosis of prostate cancer.The quenched fluorescence intensity of a dye-labeled aptamer/MoS2 sensing platform was restored successfully in thepresence of the target PSA.202

Table 3. MoS2 as a Fluorescence Probe for Biosensing Applications

dimension matrix mode analyte LOD ref

2D single-layer MoS2 turn on DNA and small biomolecules 500 pM 199MoS2 nanosheets turn on ssDNA 500 pM 200MoS2 nanosheets turn on pathogens, S. typhimurium 10

CFU mL−1201

aptamer-functionalized MoS2 turn on PSA 0.2 ng mL−1 202MoS2nanosheets turn on DNA detection 0.67 ng mL−1 203MoS2 nanosheets turn on DNA detection 15 pM 204MoS2/RGO hybrid turn off GSH 25.0 μM 205MoS2 nanosheets turn on lead(II) 0.22 μM 206

0D MoS2 quantum dot turn on hyaluronic acid 0.7 U mL 552D MoS2 nanosheets turn on Ag+ in an aqueous medium and bacteria ∼10 nM 207

MoS2 nanosheets turn off S2− 0.42 μM 206boron- and nitride-doped MoS2nanosheets

turn off Hg2+ 1 nM 208

gold-modified MoS2 hybrid induced blueshift

DNA detection 209

single-layer MoS2/FA nanosheet NIR imaging tissue cancer phototherapy 210MoS2 bio-nanohybrid NIR imaging cancer cells 41

0D MoS2 quantum dot imaging SOSG cells 47MoS2 quantum dot@PANI imaging cancer cells 211

2D MoS2@Fe3O4−ICG/PtIV nanoflowers imaging MR/IR/PA and combined PTT/PDT/chemotherapy triggered by808 nm laser

212

Figure 27. MoS2/dye-labeled ssDNA (P1). Reprinted with permissionfrom ref 200. Copyright 2014 Royal Society of Chemistry.



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In addition to DNA molecules, other bioentities and differentenvironmental analytes have also been detected by using MoS2-based sensing platforms. Gu et al. used a MoS2 quantum dot asthe fluorescence probe for the detection of hyaluronic acid.Unlike 2D MoS2, this 0D quantum-sized nanoform is soluble inwater and, hence, offers further advantages in biosensingapplications. The formation of a MoS2 quantum dot/hyaluronicacid/gold nanoparticle nanoassembly leads to fluorescencequenching because of electron transfer from donors (MoS2quantum dots) to acceptors (hyaluronic acid/gold nano-particle). Thus, the quantification of a diminished fluorescenceintensity (turn-off response) facilitates the detection ofhyaluronic acid.55 Similarly, Wang et al. used fluorescentMoS2 nanosheets for the quantitative detection of PbII and S2−

ions. They observed that the doping of MoS2 with PbII cansignificantly enhance the fluorescence properties, while theaddition of S2− results in drastic quenching. These propertieswere explored to develop a MoS2-based sensing platform forthe detection of PbII (turn-on response) and S2− ions (turn-offresponse).206

Yang et al. reported a method for the detection of Ag+ insolution and bacteria. They used rhodamine B isothiocyanate(RhoBS) adsorbed MoS2, which suffers quenching of thefluorescence. On the surface of MoS2, Ag

+ undergoes reductionby the action of RhoBS. This resulted in the detachment ofsilver from the MoS2 surface, restoring the fluorescenceintensity (turn-on response).207 Liu et al. modified MoS2 bydoping with boron and nitride and used it in the detection ofenvironmental pollutant Hg2+. The whole mechanism relies onband-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 by up to 1.61 eV after dopingwith boron and nitride. With the introduction of Hg2+, the bandgap decreased significantly with a consequent decrease in thefluorescence intensity. Thus, it offers a turn-off response-dependent detection of Hg2+.208

Literature also shows some exciting detection techniques,where nanostructured MoS2 has been found to play a uniquerole (other than quencher or probe). For example, Zhang et al.described a nanohybrid comprised of RGO/MoS2 for thedetection of GSH that was used as the photocatalyst to produce•OH radicals at the photocatalyst−solution interface in thepresence of visible light.Terephthalic acid was used as a working probe, which accepts

the free •OH radicals and gets converted into 2-hydroxyter-ephthalic acid (HTA). HTA is a photoactive compound and

gives strong fluorescence at an excitation wavelength of 425nm. However, in the presence of GSH, free-radical reaction getshindered. GSH scavenges the free radicals, thereby lowering theHTA formation. Consequently, fluorescence is quenched with aturn-off type of response. Thus, measurement of the diminishedfluorescence intensity provides a quantitative estimation ofGSH.205

On the other hand, MoS2 has been used for bioimagingapplications as well. As described in the optical propertiessegment, MoS2 with variant morphology is suitable forbioimaging applications. Han et al. reported an upconversionmultifunctional nanostructure-based bioimaging platform.210

They covalently grafted upconversion nanoparticles withchitosan-functionalized MoS2 (MoS2/CS) and folic acid (FA).The MoS2 surface was loaded with zinc phthalocyanine. Thewhole nanoassembly was integrated into PDT with photo-thermal therapy (PTT) and used in upconversion luminescenceimaging.209 Similarly, Dong et al. used a MoS2 quantum dot asboth conventional and upconversion nanomaterials forbioimaging of SOSG cells.41 Wang and co-workers reportedMoS2 quantum dot@PANI (MoS2@PANI) inorganic−organicnanohybrids. Such a nanoassembly exhibits substantialpotentiality to enhance the photoaccoustic (PA) imaging/X-ray-computed tomography signal as well as perform efficientradiotherapy/PTT of cancer cells.211 Liu et al. designed amultifunctional composite system comprised of MoS2@Fe3O4−ICG/PtIV (MoS2@Fe-ICG/Pt). Such a nanoassemblywas obtained by covalent grafting of Fe3O4 nanoparticles withpolyethylenimine-functionalized MoS2 and then loadingindocyanine green molecules (ICG, photosensitizers) andplatinum(IV) prodrugs (PtIV prodrugs) on the surface ofMoS2@Fe3O4. The resultant nanohybrid system of Mo@Fe-ICG/Pt demonstrated good magnetic resonance/IR thermal/photoacoustic trimodal bioimaging utility.212

4.3.2. Colorimetric Detection. In addition to fluorescence-based techniques, a dimensionally different MoS2 has also beenutilized in the colorimetric assay by exploring its useful opticalproperties. MoS2 possesses a strong characteristic opticalabsorbance over a wide range of wavelengths. Such a uniqueabsorption of MoS2 can be attributed to the confinement ofelectronic movements, and the absence of interlayer interfer-ence confers monolayer band gaps. Both 0D and 2D MoS2possess the potential to be used in colorimetric-basedbiosensing applications. In this context, we highlight a few ofthe recent reports on MoS2-based colorimetric sensors. It hasbeen reported that 2D TMDs including MoS2 exhibit

Figure 28. Band-gap-dependent detection of Hg2+ based on a MoS2-doped sensing probe. Reprinted with permission from ref 208. Copyright 2015Royal Society of Chemistry.



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peroxidase-like catalytic activities, which can be utilized todesign various sensing platforms. Recently, Wang et al. reportedthe detection of FeII by using luminescent MoS2-nanosheet-based peroxidase mimetics. The developed nanosheets possessthe ability to catalyze oxidation of the peroxidase substrate o-phenylenediamine (OPD) in the presence of H2O2, which givesa yellow product. Fe2+ can enhance the catalytic activity ofMoS2 nanosheets greatly and thereby influence the OPDreaction, which can be followed by colorimetric measurements.This constitutes the basis of FeII estimation in a sensitivemanner.213 A similar strategy was adopted by Guo et al.showing that MoS2 nanosheets catalyze the reaction of3,3′,5,5′-tetramethylbenzidine (TMB) with H2O2 to give ablue product, 3,3′,5,5′-tetramethylbenzidinediimine. The for-mation of this blue product can be followed calorimetrically,which provides a method for the sensitive and selectivedetection of H2O2.

214 Lin et al. followed the same method forthe determination of glucose. They combined MoS2nanosheetswith GOx, which catalyzes the oxidation of glucose to gluconicacid, and oxygen of the solution is converted into H2O2. Theformation of H2O2 depends on the oxidation of glucose presentin the system. Thus, the estimation of H2O2 by using TMBprovides a quantitative amount of glucose (Figure 29a).215 In

addition to peroxidase-like catalytic activities, MoS2 alsopossesses size-dependent optical absorption, which has beenutilized in the detection of DNA molecules. In a salt solution,2D MoS2 exhibits a tendency to agglomerate and often formslarger aggregates than those in an aqueous medium. As a result,the optical absorption capacity decreases considerably.However, upon functionalization with ssDNA, such a tendencyof MoS2 gets hindered, and the original light absorption abilityis restored. However, in the presence of target DNA, 2D MoS2again forms aggregates, leading to a decrease in the opticalabsorbance. Thus, diminished optical absorption of the MoS2/

ssDNA hybrid in the presence of the target DNA can beutilized for the detection of DNA molecules (Figure 29b).216

Thus, the above motion gives us the impression that, byutilizing the optical properties of MoS2, it is possible to designsensing platforms for various analytes. In this context,understanding of the material properties of dimensionallydifferent MoS2 and their interaction with various bioentities isimperative to obtaining highly sensitive sensing probes.

5. CONCLUSION AND FUTURE SCOPESThe review addressed the structure, synthesis, and promisingbiosensing capabilities of MoS2 nanostructures of zero, one,two, and three dimensions. We demonstrated that differentdimensions of MoS2 implicate different optical and electro-chemical attributes. Electronic and optical properties of MoS2have been discussed thoroughly with special emphasis on theirbiosensing potential. This review also highlights the work onelectrochemical and fluorescence biosensors based on MoS2nanostructures. Further, transistor-based biosensors has alsobeen discussed pertaining to the fabrication of biosensingdevices.The preparation of composite materials based on dimen-

sionally different MoS2 may address various advancedapplications related to biosensing. All of the dimensionsdiscussed here have been explored to a considerable extent.In the coming years, novel synthetic methods are anticipated toregulate the desired shape and size of MoS2, which will dictateits overall performance as efficient biosensing probes.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] (R.K.).

ORCIDShaswat Barua: 0000-0003-0050-3698Satyabrat Gogoi: 0000-0002-8860-5615Raju Khan: 0000-0002-3007-0232NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe Director, CSIR-North East Institute of Science andTechnology, Jorhat, is deeply acknowledged for his kindsupport. S.B. thankfully acknowledges CSIR, India, for a CSIRNehru Post Doctoral Research Fellowship (HRDG/CSIR-Nehru PDF/CS/EMR-1/01/2017). S.G. acknowledges SERB,DST, India, for financial support through Grant PDF/2016/003142. R.K. acknowledges DBT, India. for financial supportthrough Grant BT/PR16223/NER/95/494/2016.

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Figure 29. (a) Colorimetric estimation of H2O2 and glucose.Reprinted with permission from ref 215, Copyright 2014 RoyalSociety of Chemistry. (b) Utilization of size-dependent opticalabsorption for the estimation of DNA. Reprinted with permissionfrom ref 216. Copyright 2015 John Wiley and Sons.



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mailto:[email protected]

http://orcid.org/0000-0003-0050-3698

http://orcid.org/0000-0002-8860-5615

http://orcid.org/0000-0002-3007-0232


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Nanostructured MoS2-Based Advanced Biosensors: A Review