Molybdenum disulfide–graphene van der Waals heterostructures as stable and sensitive electrochemical sensing platforms

Abstract

Atomic layers are sought after for molecular sensing due to their available high surface interaction area, and different types of monolayers are attempted for sensing in the recent past. However, their chemical stability towards these molecules is questioned in recent times and alternate methods need to be developed to circumvent such issues, while maintaining high sensitivity. Here, the van der Waals (vdWs) stacks of molybdenum disulfide (MoS2) and graphene are shown for their stable electrochemical sensing towards ascorbic acid (AA) and dopamine (DA)—two important biomolecules. AA is known to chemically react with MoS2 leading to unstable sensing platform, while here the graphene coverage is shown to protect the MoS2 even from low-energy plasma exposure while keeping the same high sensitivity. Upon proving the graphene-based protection of the sensor, such a sensing platform is shown for its applicability in DA sensing, where it is found to give a linear response in a wide range of concentrations (2.5 to 600 µmol·L−1) and even selective sensing in the presence of AA. Such a stack is found to be not merely giving protection to the beneath MoS2 layer but also the inter-layer charge transfer due to work function differences being beneficial in bringing fast and high sensitivity to the next-generation sensors and point-of-care devices.

Introduction

Being a highly explored transition metal dichalcogenide, molybdenum disulfide (MoS2) has been extensively studied for its potential in biomedical applications, and these include the sensing of molecules such as dopamine (DA), ascorbic acid (AA) and glucose, and biomarker and deoxyribonucleic acid (DNA) detections [1,2,3].The atomically thin surface of MoS2 with trigonal prismatic geometry allows the molecules to sit on its surface [4, 5], and the negative charge on its surface makes it feasible for selective detection, too [1]. The electrochemical stability of its layers in a large voltage window opens its possibility as an electrochemical sensing platform, too [6]. The cytotoxicity studies on MoS2 nanosheets also show its advantage over the other two-dimensional (2D) counterparts [4]. The high specific surface area of MoS2 (321 m2·g−1) also allows to immobilize large amounts of biomolecules per unit area [7, 8], which is beneficial in developing highly efficient biosensors. Furthermore, the surface negative charges present over MoS2 make it capable of selective detection toward DA in presence of high concentrations of AA, where they co-exist in the physiological conditions and have similar electrochemical oxidation potential (~ 0.5 V vs Ag/AgCl for AA and DA) [1, 9]. However, the sensitivity of MoS2 platform in its long use is of concern [8], where the poor chemical stability of MoS2 can alter its properties, affecting adversely its long-term stable performance. For example, Wang et al. [10] have shown that even a few minutes exposure of low concentrated AA can reduce and etch MoS2. It has been shown that AA can reduce the exposed Mo (IV) atoms at the edges to lower the oxidation state and hence decompose the MoS2 structure.

Graphene, another atomic layer of carbon having better chemical stability [11,12,13,14], has good charge transfer properties than semiconducting MoS2, and it has been used widely for selective detection of biomolecules in its modified state [15, 16]. Poor response of a sensor signal due to thermal fluctuations and defects in the sensing platform can be evaded by interfacing it with a low electronic noise system such as graphene [17]. The main advantage of graphene in sensor applications is its perfect two dimensionality with low Johnson–Nyquist noise due to its high conductivity [18, 19]. This facilitates enhanced amplification of the signal from the interactions with the analyte as all the atoms are in direct contact [20, 21]. However, graphene in its pure form cannot act as an efficient electrochemical sensing platform (basal plane), though it is known for its ultra-fast, low-noise resistive, or electronic sensing properties [22], and hence, surface modification or engineering defects is needed to make it active towards electrochemical molecular sensing [23, 24].

Vertical interfacing of dissimilar atomic layers to form van der Waals (vdWs) structures has received enormous potential from the recent past, and a variety of synergetic effects due to charge redistribution and structural changes have been observed from such structures [25,26,27]. Such structures stabilized in sub-nanometre separation have sharp electronic boundaries [28] and it leads to inter-layer charge transfer within the layers of different work functions [29]. Hence, such interfacing is found to be resulting in new solids of interesting electronic, optical, and electrochemical properties [30,31,32]. It is shown that even pristine graphene surfaces become electrochemically active while forming vdWs structures with MoS2 and hBN [33, 34]. Furthermore, vdWs structures of graphene with MoS2 monolayers (MS) were found to provide enhanced hydrogen evolution catalytic activity compared with bare MS along with the enhanced stability even in harsh acid conditions and the interfacial charge transfer from graphene to MS was also established [35]. Such a charge transfer makes active adsorption centers on graphene basal plane and, hence, facilitates electrochemical sensing efficiency of graphene surface. Furthermore, being a relatively stable material, graphene can also protect the beneath MS layer from erosion and such possibilities are not explored much in the literature. In the present study, the stability and performance of graphene protected MS in sensing AA and DA are tested along with the possibilities of selective sensing using such structures.

Graphene-MS vertical stacks with graphene on top of the transferred MS were developed using a layer by layer transfer method (Fig. 1a), and such structures were studied for electrochemical sensing with them as working electrodes. Chemical vapor deposition (CVD) technique was used to develop both the atomic layers, and the layers were studied with micro-Raman analyses, X-ray photoelectron spectroscopy (XPS), and field emission scanning electron microscopy (FESEM) along with the electrochemical techniques to establish the inter-layer charge transfer, stability, and highly efficient sensing properties of these vdWs structure-based sensors.

Fig. 1: a
figure1

Scheme showing transfer process of graphene and MoS2 over GCE using wet transfer process. The GMS electrode sensing DA is represented in the scheme. b FESEM image of MoS2 (MS) over SiO2/Si wafer indicating ~ 20 µmol·L−1 wide (side) triangular crystal. c Optical microscope image (× 100 objective) of freestanding (over holey carbon substrate) GMS; blue dots show PL from MS (532 nm excitation and emission at 670 nm) confirming the presence of freestanding MoS2 (red dots show MoS2 triangular crystal which is uniformly covered by graphene). d Raman spectra of freestanding GMS, MS, and G, confirming the formation of monolayers of graphene and MoS2. e PL spectra of MS and GMS showing a broad emission due to A exciton and A trion

Experimental

MS and graphene growth

CVD technique was used for the growth of monolayer MoS2 triangular crystals over 300 nm SiO2/Si wafer, as discussed in our previous report [5]. Large-area monolayer graphene was grown in a horizontal muffle attached with a quartz tube (1 m long and 5.08 cm diameter). A copper foil (25 µm) was used as a substrate for growth of graphene, and this was pre-treated in acetic acid for 5 min and was washed off with acetone and isopropyl alcohol. The copper foil was moved to the center of the quartz tube, which is exactly positioned below the thermocouple. Initially, a continuous flow of 200 standard cubic centimeters per minute (sccm) of Ar/H2 (10% H2) gas was maintained. The quartz tube was heated to 1000 °C in 50 min, and the temperature was maintained at 1000 °C for 55 min to generate grain boundaries in copper. During the growth time (next 4 min), 20 sccm CH4 was introduced to the tube, and the Ar/H2 flow was increased to 400 sccm. Furthermore, the CH4 flow was stopped and Ar/H2 flow was reduced to 200 sccm, and the quartz tube was then allowed for an immediate cool-down by opening the furnace. The time-dependent temperature profile of graphene growth and schematic of CVD is shown in Fig. S1a and b.

Electrode development: MS and graphe-MS (GMS)

Graphene and MoS2 were transferred over glassy carbon electrode (GCE), SiO2/Si wafer, and holey carbon substrate for electrochemical and spectroscopic characterization using wet transfer technique. The monolayer of graphene grown over copper foil or MoS2 grown over SiO2/Si wafer was initially spin-coated with polymethyl methacrylate (PMMA) at 3000 rotations per minute (r·min−1) and is dried at 70 °C, which was then allowed to slowly cool to room temperature. PMMA-coated graphene was then dipped for 12 h in aqueous 0.2 mol·L−1 Fe(NO3)3 solution whereas MoS2 over Si/SiO2 is dipped in 2 mol·L−1 KOH solution to etch copper and SiO2 layer, respectively. Consequently, the PMMA to float over the solution along with the material and then was washed in Milli Q water to remove the residual solution. PMMA/2D material was scooped over the substrates and dried for a few hours. It was then immersed in acetone for 3 h, wherein every 1 h acetone is replaced with fresh solvent. The removal of PMMA is confirmed using optical microscopy and micro-Raman analyses. Finally, the transferred materials (graphene or MoS2) on the respective substrates were washed with isopropanol. To form graphene-MoS2 heterostructures, the copper-etched floating PMMA/graphene is scooped over MoS2 transferred substrates and the washing steps were repeated.

Characterization techniques

The FESEM images were captured using JEOL JSM 7200F FESEM (Japan). Raman spectroscopy and photoluminescence (PL) measurements were taken using Renishaw inVia micro-Raman spectrometer with 532 nm laser excitations. All Raman characterizations were carried out using 5 µW laser power to avoid local heating and 10 s acquisition time was used to decrease the noise-to-signal ratio. XPS characterization was performed by PHI Versa Probe II (USA) using a monochromated Al Kα excitation source (1486.6 eV). Harrick Basic Plasma Cleaner PDC-32G (USA) is used to plasma treat electrodes. The electrodes were treated with a high power plasma (18 W) for 5 s.

Electrochemical characterizations were carried out at room temperature using Bio Logic SP-300 (France) in a three-electrode setup. A 3 mm-diameter GCE with material transferred over it served as working electrodes; platinum is used as a counter electrode and Ag/AgCl as the reference electrode. The sensing studies using cyclic voltammetry (CV), chronoamperometry (CA), and differential pulse voltammetry (DPV) were carried out in 0.1 mol·L−1 phosphate-buffered saline (PBS) and with pH = 7.4. Ultra-high pure N2 was purged to PBS for 30 min to degas the electrolyte prior to all electrochemical experiments. CV was carried out from potential of − 0.3 to 1 V vs Ag/AgCl with a scan rate of 100 mV·s−1. While DPV was carried out by sweeping the potential from − 0.1 to 0.6 V with 8 mV step height and 50 mV pulse size. Initially, the electrodes were activated by ten consecutive potential sweeps at a scan rate of 100 mV·s−1 to attain a stable electrochemical response in blank PBS. Furthermore, all the electrochemical experimental measurements were converted to current densities.

Results and discussion

Graphene and MoS2 were grown via CVD technique to get uniform high-quality and defect-free monolayers, and the details can be found in the experimental section. The vdWs’ heterostructures from the individual layers were formed by (PMMA-assisted wet transfer method (Fig. 1a), as discussed in the experimental section [33]. The transferred materials are abbreviated as G (graphene), MS (MoS2), and GMS (graphene–MoS2) throughout the article. Optical images of the CVD-grown graphene over the copper foil and MoS2 over SiO2/Si are shown in Fig. S2a and b, respectively. Large-area growth of graphene (100 µm × 100 µm) and MS (100 µm × 100 µm) can be ensured from these images. The FESEM image of MoS2 grown over SiO2/Si via CVD is given in Fig. 1b, and the same of GMS is shown in Fig. S2c. Freestanding MS (over holey carbon substrate) and GMS, GMS over SiO2/Si wafer, and GMS over GCE are optically imaged (Fig. 1c and Fig. S2d-f), where the triangular MS crystals (size ~ 18–23 µm) can be seen covered by the transparent monolayer of graphene. The coverage of graphene over MS is evident from the scanning electron microscope-energy dispersive spectroscopy (SEM–EDS) elemental mapping shown in Fig. S2g. Furthermore, Raman mapping of the freestanding (over holey carbon substrate) GMS showing the graphene coverage is given in the supporting information (Fig. S3).

The micro-Raman analysis was carried on G, MS, and GMS to study the graphene and MoS2 interactions and verify the quality of the transferred films. Figure 1d shows the Raman spectra of freestanding G, MS, and GMS. The in-plane (E2g) and out-of-plane (A1g) vibration modes of MS show a peak separation of ~ 19 cm−1 confirming the formation of MoS2 monolayer [36]. The Raman spectrum of graphene shows the presence of 2D vibration mode at 2693 cm−1 and G peak at 1585 cm−1 with the intensity ratio of 2D and G bands (I2D/IG) ratio ~ 2.2, indicating the presence of monolayers of graphene. The ID/IG ratio of 0.2 shows minimal structural defects on graphene [33]. Furthermore, the Raman spectrum of GMS shows no shift in the graphene Raman modes’ positions, confirming the formation of high-quality heterostructures [37].

The PL spectrum of MoS2 arises due to the direct optical transition between the highest valence band and lowest conduction band [38]. MoS2 has two prominent peaks denoted as A and B excitons arise as a result of the spin–orbit splitting of the valance band. Figure 1e shows the PL of MS and GMS electrodes, where the high intense peak corresponds to A exciton [39]. The peak corresponds to B exciton is not visible as this optical transition is not the lowest energy transition [40]. Furthermore, the broad A exciton emission can be deconvoluted into two Lorentzian peaks—A exciton and A trion in both MS and GMS [41]. For GMS, the PL peak intensity shifts to high wavelengths by ~ 2 nm and this can be due to the charge transfer from graphene to MoS2 as its work function is higher than graphene making MoS2 more n-type, as reported by others [35, 42].

The preliminary electrochemical response of G, MS, and GMS electrodes on GCE towards AA sensing was studied by CV (scan rate 100 mV·s−1), and CA. The details of the electrochemical sensing setup are given in the experimental section. Figure S4a-d shows the CVs for bare GCE, G, MS, and GMS electrodes in the presence of 100 µmol·L−1 AA in 0.1 mol·L−1 PBS at pH = 7.4. In case of bare GCE, the oxidation of AA to dehydroascorbic acid (DAA) is observed at an anodic peak potential of 0.30 V vs Ag/AgCl leading to a Faradaic peak with an insignificant current [43]. On the other hand, the oxidative peaks for AA are seen at 0.18 V for G and at 0.28 V for both MS and GMS electrodes with an enhanced anodic peak current. The CA studies were conducted through the successive addition of 100 µmol·L−1 AA at definite time intervals. The electrode stability of G, MS, and GMS towards AA sensing stability is studied by repeating the CA measurements on these electrodes after washing them thoroughly before each measurement [10].

Figure 2a displays the CA plots of G, MS, and GMS electrodes, where MS and GMS electrodes showed ~ 13 µA·cm−2 increment in current density, while the response of G was ~ 3 µA·cm−2 less than those of MS and GMS electrodes with each addition of 100 µmol·L−1 AA and also a linear sensing response (Fig. S5a-c) over a wide range of concentrations (100 µmol·L−1 to 1 mmol·L−1). However, the MS electrode showed sluggishness in raising the signal upon each addition, while an instant response is demonstrated by GMS (Fig. 2a). This can be due to the vdW interaction of MoS2 with the zero band-gap graphene, and the delocalized π bonds formed from pz orbital of sp2 carbon help in faster electron transfer, which leads to instant detection of AA [43]. The second round CA study with the same electrode for the detection of AA was conducted to evaluate the stability and repeatability of these electrodes (Fig. 2b). The MS electrode is found to be losing its initial sensitivity, while the GMS and G electrodes still showed linear sensing responses (Fig. S5d and Fig. S6a), indicating that graphene is stable against the reducing effect of AA and also acts as a protecting layer over MS in GMS heterostructure. To confirm the sensor stability, further G, MS, and GMS electrodes were treated with mild oxygen plasma exposure (details of plasma generation and exposure are given in the experimental section) followed by the CA study (Fig. 2c). It is interesting to note that the GMS and G electrodes showed similar sensing response as the initial cycle towards each addition of 100 µmol·L−1 AA even after exposure to the oxygen plasma. In contrast, the MS electrode has lost its initial linear response and electrochemical oxidation of AA become more sluggish due to the formation of MoO3 [44], while the GMS and G electrodes still gave a linear response with less than 5% error in amperometric signal (Fig. S6b-d, Fig. 2d). However, multiple plasma treatment can affect the performance of graphene electrode (or GMS) too as the prolonged oxygen plasma can oxidize the surface of graphene and this oxidation can eventually affect the electronic properties of graphene. These results confirm the protective effect of graphene over MoS2 and formation of a stable and robust sensor with graphene even in harsh conditions.

Fig. 2
figure2

Electrochemical response studied using CA for GMS, MS, and G electrodes at constant potential 0.28 V vs Ag/AgCl towards 100 µmol·L−1 AA for every 100-s time of interval (supporting electrolyte: 0.1 mol·L−1 PBS at pH = 7.4). a Initial CA cycle the GMS electrode is showing instant response compared to MS. b Second CA cycle: the MS electrode lost its sensitivity; GMS shows instant response towards AA. c CA after mild plasma treatment; MS lost its sensitivity completely, while GMS still shows linear response. d Linear fit of GMS CA with error bar showing its sensitivity with less than 5% standard deviation

Figure S7a and b shows the optical images of MS over the GCE electrode before and after the initial CA measurement, where the change in MoS2 surface texture is visible. Moreover, MS and GMS over SiO2/Si were treated with 1 mmol·L−1 AA solution, and the corresponding FESEM images are presented in Fig. 3a and b. The images show clear changes in the texture of MS after treating with AA. Lifting off of the MoS2 flakes from SiO2/Si can be due to the interaction and accumulation of water molecules between MoS2 and SiO2/Si wafer and this is in tune with that reported by Wang et al. [10] Apart from the lifting off, here, it is shown that the AA can chemically attack the MS and the same has been shown with the freestanding MS and GMS. The high-resolution FESEM images and Raman spectra of freestanding MS before and after AA treatment are shown in Fig. 3c and d, where the changes in the MoS2 texture (erosion) after AA treatment are clearly evident. In accordance with these results, the Raman spectra show clear blue shifts in E2g (3 cm−1) and A1g (~ 1–2 cm−1) peaks, which is discussed in the following section.

Fig. 3
figure3

FESEM images of GMS and MS treated in 1 mmol·L−1 AA for 30 min: a, b MS and GMS over SiO2/Si wafer after AA treatment. MoS2 etching is visible in a MS and graphene seems protecting MoS2 in b GMS. c, d Freestanding (on holey carbon substrate) MS and its Raman spectra (insets) before and after AA treatment; the FESEM shows a clear change in MoS2 texture (erosion) after AA treatment. The Raman spectra (insets) show blue shifts in E2g and A1g peaks in AA-treated MS

CVD-grown MoS2 is reported to have a few sulfur vacancies [45], and these vacancies are highly active sites that can assist oxidative etching of MoS2 resulting in the formation of MoO3 and SO2 [46]. The recent research shows that AA molecules can also interact with these active sites of MoS2. This interaction decouples more sulfur from MoS2 resulting in the formation of unsaturated sulfur, and furthermore, Mo gets oxidized to MoO3 leading to lattice distortions of MoS2 [10, 47]. The FESEM images of freestanding GMS half-covered by graphene before and after treatment with AA are given in Fig. S7c and d. The MoS2 exposed to AA shows similar texture changes, while MoS2 covered by graphene is protected from the reducing effect of AA.

To study the protecting effect of graphene over MoS2 against the reducing effect of AA, micro-Raman spectroscopy and PL were carried out on MS and GMS electrodes. The Raman spectra of MS and GMS electrodes after the first cycle CA studies in AA detection (i.e., 1 mmol·L−1 AA) and after mild plasma treatment are shown in Fig. 4a and b. The Raman spectrum of MS shows clear blue shifts in E2g and A1g peaks (3 cm−1 and up to ~ 2 cm−1, respectively). This can be due to the p-type doping of MoS2 lead by the lattice distortion due to the generation of sulfur vacancies and further the oxidation of Mo [10, 47, 48]. With the mild oxygen plasma treatment, the lattice distortion is found to be increased, resulting in a further blue shift of both E2g and A1g peaks by 2 cm−1 [49, 50]. No change in MoS2 Raman peak position is observed for GMS treated with AA or after exposed to the mild plasma. These observations confirm that graphene is protecting MoS2 atomic layers in GMS. The Raman spectrum of freestanding G treated in 1 mmol·L−1 AA for 30 min is given in Fig. S7e, where no change in Raman peaks indicates the stability of graphene against AA.

Fig. 4
figure4

Micro-Raman and PL spectra of MS and GMS electrode after the first cycle CA studies in AA detection (i.e., 1 mmol·L−1 AA). a, b Raman spectra of MS and GMS electrodes after AA and mild plasma treatment, showing blue shift in the modes of MS, while no shifts are seen in those of GMS. c PL spectra of MS and GMS electrodes after AA treatment indicating broadening of A exciton and A trion in MS, but no such effect is visible in GMS

The PL spectra of MS and GMS electrodes after the first round CA studies of AA detection are shown in Fig. 4c. The PL spectra of A exciton and A trion got broadened with the effect of AA on MS. This can be due to the mid-gap states in the band structure of MoS2 arises as an effect of defects generated by AA [29, 51]. While in GMS, where the MoS2 is protected by a stable layer of graphene shows no such shift and broadening in PL peak, confirming the graphene preventing propagation of defects in MoS2.

To further confirm the effect of the protective layer of graphene over MoS2, XPS measurements were performed on the MS and GMS transferred over SiO2/Si wafer after dipping in 1 mmol·L−1 AA solution for 30 min. The S 2p spectrum of GMS and MS are shown in Fig. 5a where the peaks at 162.4 eV and 163.7 eV are corresponding to that of sulfides of MoS2 (S2− 2p3/2 and S2− 2p1/2) [52]. The MS peaks at binding energies 163.5 eV and 164.8 eV indicate the presence of unsaturated sulfur as an after-effect of AA treatment [53, 54].

Fig. 5
figure5

XPS analyses on MS and GMS over SiO2/Si wafer treated in 1 mmol·L−1 AA for 30 min: a S 2p spectrum shows sulfur terminal peaks for MS which is not visible in GMS. b Mo 3d spectrum showing the conversion of Mo4+ to Mo6+ in MS

Figure 5b shows the Mo 3d spectrum of GMS and MS, where the S 2s peak can also be seen in the Mo region at a binding energy of 226.8 eV, in both GMS and MS. At lower binding energies, the Mo 3d spectrum of GMS shows two Mo4+ peaks at 229.5 eV and 232.6 eV corresponding to Mo4+ 3d5/2 and Mo4+ 3d3/2, respectively, due to the spin–orbit splitting [55]. Small oxidation peaks can be seen at binding energies 233.5 eV and 235.9 eV corresponding to Mo6+ 3d5/2 and Mo6+ 3d3/2 for GMS and which can be due to the exposure to air during the transfer process [56, 57]. The lower energy peaks of MS at binding energies 230.3 eV and 233.1 eV correspond to Mo4+ 3d5/2 and Mo4+ 3d3/2, while the peaks at 233.8 eV and 236.1 eV correspond to Mo6+ 3d5/2 and Mo6+ 3d3/2. The results show an increment in the area of Mo6+ peaks for MS in comparison to that of GMS. The conversion of Mo4+ to Mo6+ is due to its coordination with unsaturated sulfur (MoS42−), where Mo is getting reduced to a lower oxidation state leading to the formation of MoO3 [10, 47, 58]. These results confirm distortion of MoS2 structure and the graphene is shielding MoS2 from the reducing effect of AA. The core level C 1s spectrum of GMS (Fig. S8) can be deconvoluted into three different components. The peak at 284.8 eV corresponds to sp2 hybridized carbon (C=C), at 286.4 eV is of epoxide (C=O) and the one at 288.7 eV is due to carboxyl functional groups (O–C=O) [59]. The higher intensity sp2 peak and absence of sp3 peak show the presence of defect-free graphene even after AA exposure.

In contrast to negatively charged AA electrochemical oxidation, we also studied the electrochemical sensing of DA using the G, MS, and GMS electrodes. Along with the CV and CA measurements, DPV is also used for the selective sensing studies of DA in the presence of AA molecules. The anodic peak potential of DA to form dopamine-o-quinone for the various modified (G, MS, and GMS) and bare GCEs are confirmed from the respective CVs recorded in the presence of 20 µmol·L−1 DA (Fig. S9a-d). Amperometric detection of DA was conducted for G, MS, and GMS electrodes through the sequential addition of 10 µmol·L−1 DA in every 100-s time intervals (Fig. 6a). Here, all electrodes showed linear responses (Fig. S10a-c). The relative response of the GMS sensor towards DA showed an enhancement factor of two in comparison to that of the MS electrode for each addition. As reported by others, DA is more sensitive to the density of states at the binding sites than the functional groups at the surface [60]. It results in charge transfer between the 2D layers of heterostructure, which is further revealed from the PL studies (Fig. 1e). The charge transfer from graphene to MoS2 makes graphene p-doped and hence effectively increases the density of states at the Fermi level. This could be the reason for the enhanced sensitivity of GMS in comparison to MS electrode.

Fig. 6
figure6

DA sensing and its selectivity: a CA of GMS and MS electrodes with 10 µmol·L−1 DA for every 100-s time at a constant potential 0.31 V vs Ag/AgCl shows increment in current with each addition (supporting electrolyte: 0.1 mol·L−1 PBS at pH = 7.4). b CA studies on GMS electrode at a constant potential 0.31 V vs Ag/AgCl with increasing DA from 2 to 600 µmol·L−1 addition (Inset: magnified image from 2 to 90 µmol·L−1). c CA of GMS electrode at constant potential 0.28 V vs Ag/AgCl with increasing AA from 10 to 5 mmol·L−1 addition (Inset: magnified image from 10 to 550 µmol·L−1). d DPV of GMS electrode in 10 mmol·L−1 AA with DA addition; no peaks are seen that corresponds to AA

The linearity and sensitivity of GMS electrodes are studied using CA by gradually increasing the concentrations of AA and DA (Fig. 6b and c). The corresponding response of GMS sensor showed linearity over a wide range of concentrations from 2.5 to 600 µmol·L−1 for DA and from 10 µmol·L−1 to 5 mmol·L−1 for AA (Fig. S11a and b). From these analytical curves, it is evident that the GMS-based sensor has high sensitivity towards DA while wide linearity to a higher concentration for AA. To study the selectivity of GMS towards DA, DPV was carried out individually for AA and DA as well as DA in the presence of AA. Figure S11c shows the oxidation of AA at a negative potential (− 0.19 V vs Ag/AgCl) [61]. For DA (Fig. S11d), the GMS electrode is able to detect even 0.5 µmol·L−1concentration at an anodic peak potential 0.20 V vs Ag/AgCl. Further selectivity is studied by adding varying concentrations of DA in 10 mmol·L−1 AA. Figure 6d shows DPVs recorded with the sequential addition of DA at a constant concentration of AA, and there is no corresponding oxidation peak of AA observed. The sensor can detect DA at 0.20 V vs Ag/AgCl, with the lowest detection range of 0.5 µmol·L−1. It is observed that even 10 µmol·L−1 of DA in the solution can suppress the oxidation peak of AA completely, and thus, DA can be easily detected selectively. The results demonstrate that the GMS sensor exhibited wide linear ranges for both DA and AA using CA and highly selective detection towards DA even in the presence of 10 mmol·L−1 AA using the DPV technique. The selectivity of graphene-based electrochemical sensors towards DA in the presence of AA was shown by other researchers too [62], where the enhanced selectivity is attributed to the atomically thin highly conductive nature of graphene and the possibility of π–π interactions among graphene and DA molecules, while it is not possible in AA. The comparison of the present method with other material composite sensors is given in Table 1. While almost no studies exist on the electrochemical sensing of DA using 2D hybrids, it is shown here that MoS2 can be protected from the etching effect of AA where it coexists with DA in human blood serum, enabling 2D hybrid-based sensors having improved stability.

Table 1 Comparison of different electrodes for sensing AA and DA

Conclusion

One of the major drawbacks of atomic layers in electrochemical sensing, their destabilization via chemical/physical interactions with molecules and exposure, has been addressed here with the development of vdW heterostructures as sensing platforms. In this study, the chemical interaction of MoS2 with AA leading to its erosion or lifting off from the substrate is addressed by developing a graphene protective layer over MoS2 (GMS), while keeping the high sensitivity. The GMS electrodes showed a wide range of linearity (10 µmol·L−1 to 5 mmol·L−1) in sensing for AA where the sensing platform is protected from the AA’s chemical interactions with MoS2 as well as the oxygen plasma exposure-induced MoS2 distortion.

AA exists with DA, one of the important neurotransmitters, in physiological conditions, and it is much lower in concentration (10 ± 6 pg·mL−1) in comparison to AA. Both molecules have similar oxidative potentials and, hence, selective electrochemical sensing of DA is highly important. Though MoS2 is widely accepted as a selective platform for DA, its instability in the electrochemical condition is a bottleneck and, here, we have shown that GMS vertical heterostructures can be selective towards DA in the presence of AA and it has a sensitivity as low as 0.5 µmol·L−1 even in the presence of large amount (10 mmol·L−1) AA. Hence, a detailed electrochemical study using various electrochemical techniques including differential pulse voltammetry along with microscopic and spectroscopic analyses establishes the potential of vdW structures as possible futuristic high-sensitivity small molecule sensing platforms, where the issues such as stability, sensitivity, and selectivity have been resolved. Furthermore, the present work also opens up the wide possibilities of vdWs’ stacks of different atomic layers in bio-molecular sensing where the synergistic activities of the hybrids can able to address the present drawbacks in sensors such as reusability of the sensor, ultra-fast detection, high sensitivity, and long stability, and such stacks can find applicability in both non-enzymatic and enzymatic sensing platforms.

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Acknowledgements

This work was financially supported by Department of Science and Technology, India extramural research grant (Grant No. EMR/2017/000513) and intramural grants at TIFR Hyderabad from the Department of Atomic Energy (DAE). T. N. N. also thank Mr. Venu Uppalapati for the partial financial support towards this research in the form of TIFR Endowment grant.

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Supporting Information:

A few experimental results are available in the supporting information: optical images of G, MS, and GMS, CVs’ comparison of various modified electrodes in the presence of AA, freestanding (on holey carbon substrate) GMS Raman Mapping, linear fitted calibration curves of G, MS, and GMS electrodes for AA detection, optical images and micro-Raman spectrum of MS before and after treated with AA for 30 min, CV of electrodes with 20 µmol·L−1 DA and liner fitting for G, MS, and GMS electrodes in the presence of 10 µmol·L−1 DA, linear fitting for CA of GMS electrode with increasing DA and AA concentration, and determination of DA or AA using DPV. The schematic of CVD for graphene growth and time–temperature profile for graphene growth is also given. (DOCX 5993 kb)

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Figerez, S.P., Tadi, K.K., Sahoo, K.R. et al. Molybdenum disulfide–graphene van der Waals heterostructures as stable and sensitive electrochemical sensing platforms. Tungsten (2020). https://doi.org/10.1007/s42864-020-00061-7

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Keywords

  • Graphene
  • Molecular sensing
  • MoS2 etching
  • MoS2 protection
  • van der Waals structures
  • Electrochemical sensing
  • Ascorbic acid
  • Dopamine