MoS2 Nanoparticles as Electrocatalytic Labels in Magneto

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

MoS2 Nanoparticles as Electrocatalytic Labels in MagnetoImmunoassays Daniel Bouša,†,‡ Carmen C. Mayorga-Martinez,†,‡ Vlastimil Mazánek,†,‡ Zdeněk Sofer,† Kristýna Boušová,§ and Martin Pumera*,†,‡ †

Department of Inorganic Chemistry, University of Chemistry and Technology Prague, Technická 5, 166 28 Prague 6, Czech Republic ‡ Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371, Singapore § Institute of Organic Chemistry and Biochemistry ASCR, Vvi, Flemingovo Náměstí 2, 16610 Prague 6, Czech Republic S Supporting Information *

ABSTRACT: Ability to detect biomolecules with a simple and cost-effective approach has been very demanding in today’s medicine. The nanoparticles and two-dimensional materials have been extensively used within this field in devices with high selectivity and sensitivity. Here, we report the use of MoS2 nanoparticles (MoS2 NPs) as a signal-enhancing label in a standard immunoassay test. MoS2 NPs were prepared by a bipolar electrochemistry method. The current response during the hydrogen evolution reaction catalyzed by MoS2 was measured. This current was directly proportional to the amount of the MoS2 NPs and thus also to the concentration of desired protein. The immunoassay containing the MoS2 NPs displays extraordinary low limit of detection (1.94 pg mL−1), good selectivity, and reproducibility. This MoS2 NP detection system could have profound implication for analytical applications. KEYWORDS: bipolar electrochemistry, nanoparticles, molybdenum disulphide, immunoassay, hydrogen evolution reaction

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INTRODUCTION Layered materials attract great attention of scientific community because of their huge application potential in advanced technologies. Weak van der Waals forces between individual sheets and strong in-plane covalent bonds enable exfoliation of layered materials to single layers.1,2 Electrical, optical, and mechanical properties of single layers dramatically change compare with their bulk counterparts.3−5 Graphene represents a typical material with exciting properties related to its thickness;6,7 however, there were more single-layer materials like graphene oxide,8,9 phosphorene,10,11 boron nitride,12,13 layered double hydroxides,14 and transition metal dichalcogenides (TMDs).15−18 Molybdenum disulphide (MoS2) is a usual member of the TMD family. It has been used for a long time as a lubricant in industrial applications.19,20 Today, research of the MoS2 material is related to the exfoliation down to the single-layer, intercalation21 and mainly to study its interesting electrochemical properties like electrocatalytic activity toward the hydrogen evolution reaction (HER) and oxygen reduction reaction.22−29 Application of MoS2 as an electrode catalyst in fuel cell is of great importance because of its significantly lower cost compared with the platinum currently used.24,30−33 Detection of diseases plays a crucial role for the successful therapy. Therefore, the invention of sensitive and fast detection © XXXX American Chemical Society

methods of biomolecular markers is currently very important in biomedical research.34 The ability to evaluate physiological states, detect the onset and progression of morbidity, and track therapeutic outcomes after treatment with noninvasive approaches is one of the most sought-after research and healthcare delivery goals. The second important sector for the detection of biomolecules is the biotechnological industry which will also benefit from the advanced proteins quantitative detections.35 Nowadays, the magnetic immunoassay represents technology for the separation, preconcentration, and detection of biomolecules. Recently, catalytic materials based on TMDs and black phosphorus have been reported as signal-enhancing labels for immunoassays though the HER.36−39 These systems utilize novel label materials. Previously, the immunoassay tests used exclusively precious metal nanoparticles40−43 or quantum dots.44,45 Here, we explore the possibility of using MoS2 nanoparticles (MoS2 NPs) prepared by bipolar electrochemistry (electroexfoliation method) as signal-enhancing labels in the magnetoimmunoassay through the HER. Received: February 1, 2018 Accepted: April 24, 2018

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DOI: 10.1021/acsami.8b01607 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (A) Electro-exfoliation via bipolar electrochemistry of t-BuLi MoS2 to form MoS2 NPs. (B) Magneto-immunoassay steps: (1) adsorption of anti-rabbit IgG onto the MB (anti-IgG-MB); (2) labeling of rabbit IgG protein with MoS2 NPs (IgG-MoS2 NPs); (3) final conjugation of antiIgG-MBs with IgG-MoS2 NPs yield IgG-MoS2 NPs/anti-IgG-MB complex; and (4) rabbit IgG-MoS2 NP detection through the HER mediated by the MoS2 label. The magneto-immunoassay was held onto the WE of the SP electrode by the help of a magnet placed under the WE.

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the other hand, to connect the MoS2 NPs to the rabbit IgG protein, 150 μL of MoS2 suspension obtained by bipolar electrochemistry, 25 μL of PBS, and 10 μL of rabbit IgG were mixed and incubated at 25 °C for 2 h with 650 rpm agitation. In total volume of 200 μL, the concentration of rabbit IgG was 5 μg/mL. Subsequently, the blocking step was achieved by adding 15 μL of PBS solution containing BSA (5 wt %) to IgG-MoS2 NPs solution, and the incubation was performed for 20 min more under the same conditions. Then, the sample was centrifuged at 4 °C for 2 h and 14 000 rpm. After centrifugation, 180 μL of supernatant solution was removed and 55 μL of PBS solution was added to obtain a final volume of 75 μL. The final incubation was performed by preconcentrating of 75 μL of anti-IgG-MB using an external magnet followed by the addition of 75 μL of the MoS2 NPs labeled rabbit IgG and subsequent agitation of the resulting mixture at 650 rpm and 25 °C for 30 min. Next, PBS Tween 20 (0.5 vol %) and PBS were used during washing steps. Prior to the electrochemical testing, the assembly was resuspended in Milli-Q water. Instrumentation. A PHOIBOS 100 spectrometer together with a monochromatic Mg X-ray radiation source (SPECS, Germany) were used for X-ray photoelectron spectroscopy (XPS) measurements. High-resolution and wide-scan (survey) spectra were also recorded for sample coated as a uniform layer onto a silicon wafer from suspension. Zetasizer Nano ZS (Malvern, England) was used for the dynamic light scattering (DLS) measurement to determine the particle size. The temperature during measurement was 20 °C and the glass cuvette was used. A energy-filtered transmission electron microscopy JEOL 2200 FS microscope (JEOL, Japan) was used to obtaining high-resolution transmission electron microscopy (HR-TEM) images. Measurement was performed under a 200 kV acceleration voltage. Sample suspension (1 mg mL−1 in H2O) was drop-casted on a TEM grid (Cu; 200 mesh; Formvar/carbon) and dried at 60 °C for 12 h. Electrochemical Measurements. Performance of the samples toward the HER was judged by recording linear sweep voltammetry (LSV) measurements using a three-electrode system. The cell was composed of a glassy carbon (GC) as the working electrode (WE) with a diameter of 3 mm, Ag/AgCl reference electrode and Pt counter electrode. MoS2 NPs suspension (3 μL) was drop-casted onto surface of the WE and dried at room temperature. The study of the experimental conditions and magneto-immunoassays experiments

EXPERIMENTAL SECTION

Reagents. Solution of tert-butyllithium (t-BuLi, 1.7 M) in pentane and molybdenum disulfide MoS2 (99.9%) were purchased from SigmaAldrich (Czech Republic) and Alfa Aesar (Germany), respectively. Tosyl-activated magnetic beads (MBs, Dynabeads M-280) were acquired from Invitrogen (Singapore). Phosphate-buffered saline (PBS) tablet, albumin from bovine serum (BSA), sodium hydroxide, boric acid, antirabbit IgG produced in goat, rabbit IgG from serum, Tween 20, and lyophilized powder of hemoglobin were purchased from Sigma-Aldrich (Singapore). MoS2 NPs Preparation. MoS2 NPs were prepared by a combination of chemical intercalation and bipolar electrochemistry exfoliation method. First, bulk MoS2 was exfoliated and after that the electro-exfoliation method was used for completion of exfoliation and for reduction of particle lateral dimensions. t-BuLi was used as a source of Li during intercalation of the layered MoS2.46 After that, the MoS2 sheets were further exfoliated electrochemically in aqueous electrolyte similarly to the procedure reported in the literature.37,38 Repeated centrifugation was used for purification. The obtained t-BuLi MoS2 powder was mixed with ultrapure water (5 mg mL−1) and ultrasonicated for 3 h at 25 °C. Afterward, electrolyte solution was prepared by using of 400 μL of ultrasonicated t-BuLi MoS2 suspension to produce 4 mL of solution containing 0.5 M Na2SO4. The resultant electrolyte with t-BuLi MoS2 particles was inserted into cell between two Pt electrodes separated by the distance of 2 cm and magnetically stirred. Direct current potential of +10 V was applied between the two Pt electrodes for the duration of 30 min. The resulting suspension was allowed to sediment for 2 days, and finally, the suspension above the sediment was collected. Magneto-Immunoassay. The magneto-immunoassay was prepared similar to the procedure described elsewhere.32−34 First, the MB suspension (3 mg mL−1) was washed twice with borate buffer solution at pH = 9.2 and incubated overnight with anti-rabbit IgG (40 μg mL−1) at 37 °C and 400 rpm agitation, followed by two washing steps of the anti-IgG-MB complex with PBS/Tween 20 (0.5 vol %). Further, the obtained complex was resuspended in a PBS solution containing 5 wt % BSA followed by 650 rpm agitation for 1 h at 25 °C to perform the blocking step. The samples were washed two times with PBS containing BSA (0.1 wt %) after the blocking step was completed. On B


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ACS Applied Materials & Interfaces were performed using commercial screen-printed (SP) electrodes purchased from eDAQ Instruments. These electrodes are designed to contain a small WE with 3 mm of diameter and counter electrodes from graphitic carbon as well as Ag/AgCl reference electrode. In all cases, chronoamperometry response were evaluated in 0.5 M solution of H2SO4; for this reason, 15 μL of 1 M H2SO4 solution and 15 μL of IgG-MoS2 NPs/anti-IgG-MB complex solution was gently mixed onto the surface of SP electrode. In addition, SP electrodes were treated before measurement using cyclic voltammetry in solution of H2SO4 (0.5 M). Such treatment led to the smoothening of current response as well as the increase of reproducibility. Scan rate during all LSV experiments was 2 mV s−1. The working potential for chronoamperometric measurements was −1.1 V versus reversible hydrogen electrode (RHE) applied for 200 s.

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RESULTS AND DISCUSSION MoS2 NPs were synthesized by bipolar electrochemical exfoliation which led to the exfoliation and reduction of MoS2 sheet’s lateral dimension (see Figure 1A). First, we characterized the obtained MoS2 NPs by DLS, TEM/HR-TEM, and XPS. After that, rabbit IgG was incubated with the obtained MoS 2 NPs to produce the IgG-MoS 2 NPs complex. Simultaneously MBs were incubated with an anti-rabbit IgG to obtain anti-IgG-MB complex. Finally, these two complexes were conjugated to yield the IgG-MoS2 NPs/anti-IgG-MB magneto-immunoassay. To detect and quantify the protein, electrocatalytic properties of the MoS2 NPs toward the HER were utilized and chronoamperometry was selected as the electrochemical method for measurement of the current resulting from HER catalysis at a fixed potential. The absolute current value is proportional to the rabbit IgG protein concentration. This process is schematically depicted in Figure 1B. At the beginning, we would like to confirm the previous statements that bipolar electrochemistry can be used as an efficient exfoliation and downsizing method to obtained MoS2 NPs. In the first step, we exfoliated micrometer-sized sheets by chemical exfoliation of bulk MoS2 crystal using t-BuLi as an intercalating agent. Micrometer-sized sheets of t-BuLi MoS2 are not suitable for use as labels in the immunoassay because of their large dimensions (tens of micrometers). Thus, the t-BuLi MoS2 sheets were subjected to bipolar electrochemical treatment to obtain labels with comparable dimensions of proteins for the preparation of the magneto-immunoassay. The main advantage of this exfoliation method lies in the downsizing of the t-BuLi MoS2 particle in an aqueous solution that is compatible with the immunoassays. DLS measurement allows determining the particle size of MoS2 NPs (Figure S1). Average hydrodynamic radius of the resulting MoS2 NPs after bipolar electrochemistry was 186 nm. To observe dimensions and shape of MoS2 NPs, TEM was used. Figure 2A shows aggregates of platelet particles with size around 200 nm which is in good agreement with DLS measurement (see Figure S1). With greater magnification, the small MoS2 NPs appears with an approximate size of 4−5 nm (lower right inset of Figure 2B). Particles with this size were not observed in DLS measurement. However, it was reported that relatively small volume of bigger NPs could completely screen the presence of smaller particles during DLS measurement.10 Elemental composition of MoS2 NPs was confirmed by energy-dispersive spectrometry elemental mapping (Figure S2). Selected-area electron diffraction (SAED) pattern in upper left inset of Figure 2B exhibits the circular character which suggests

Figure 2. (A) TEM image of MoS2 NPs aggregates. (B) HR-TEM image and SAED pattern (upper left inset) of individual MoS2 NPs. Scales bars are 1 μm for TEM image, 20 and 1.5 nm for HR-TEM image and 200 μm for the SAED picture.

nanometric dimensions of the randomly oriented nanoparticles within individual aggregates. Surface chemical composition and bonding states of atoms in the MoS2 NPs was determined using high-resolution XPS (HRXPS). The HR-XPS spectra of MoS2 NPs are shown in Figure S3. Analysis and deconvolution of Mo bonding states47 showed the presence of 1T-MoS2 and also significant amount of the Mo oxidized states. This suggested the oxidation of MoS2 NPs caused by positive potential applied during bipolar electroexfoliation. To get information about how the bipolar electrochemistry treatment affects the crystal structure of MoS2−t-BuLi precursor material, X-ray diffraction measurement was performed (see Figure S4). The MoS2−t-BuLi precursor material exhibited a P63/mm structure with noticeably diffusive peaks related to the intercalation of the t-BuLi exfoliation agent. After bipolar electrochemical treatment, the material preserved its crystal structure. The reduction of the particle size also led to the widening of the most intense [002] peak. After the characterization, the catalytic activity of MoS2 NPs and starting t-BuLi MoS2 toward the HER was performed using LSV measurement in 0.5 M H2SO4 (see Figure 3A). The lowest overpotential was observed for t-BuLi MoS2 (−0.90 V vs RHE), followed by MoS2 NPs (−0.97 V vs RHE) and the GC electrode (−1.12 V vs RHE). The overpotential in all these cases was measured at a current density of 10 mA/cm2. The slight difference in overpotential value between t-BuLi MoS2 and MoS2 NPs can be attributed to the: (i) partial oxidation of the MoS2 material during bipolar electrochemical exfoliation C


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surface of the SP electrode and catalyze the HER. The magnitude of the measured current during the HER is directly proportional to the rabbit IgG protein concentration (see Figure 1B(4)). Interaction between IgG and MoS2 NPs was studied using the ζ-potential measurement (see Figure S5). ζPotential of MoS2 NPs was determined to be −27.7 mV, and its value shift to the more positive after conjugation with IgG (−16.6 mV). The efficiency of the conjugation is caused by electrostatic interactions between MoS2 NPs (negatively charged) and the IgG (positively charged).48 Moreover, for probing biomolecular detection, the ideal label must have sufficiently small size to be comparable with immune molecules sizes. For this reason the preparation of the immunoassay with t-BuLi MoS2 or bulk MoS2 would not be efficient because the lateral size of t-BuLi MoS2 sheets are between 2 and 5 μm and bulk MoS2 particles had even bigger size between 5 and 20 μm (see Figure S6). First, selectivity of the system was evaluated, and its results are shown in Figure 4A. For this aim, two control experiments

Figure 3. (A) Responses of bare GC (black line), t-BuLi MoS2 (blue line), and MoS2 NPs (red line) recorded by LSV during the HER in 0.5 M H2SO4. The desired material was drop-casted onto a surface of the GC electrode and a scan rate of 2 mV s−1 was used for the measurements. (B) Current responses in 0.5 M H2SO4 of MoS2 NPs on the surface of the SP electrode obtained at three different potentials for the duration of 200 s (Inset: Recorded chronoamperograms of bare SP electrode compare to the MoS2 NPs modified SP electrode). The working potential applied for the chronoamperograms was −1.1 V vs Ag/AgCl.

(see Figure S3) and (ii) amount of MoS2 NPs used for HER measurement which was much smaller compared with the tBuLi MoS2. Similar phenomena have been observed on other materials under bipolar electrochemical exfoliation.5 However, the ability of MoS2 NPs to catalyze the HER is significantly higher than that of the control GC electrode. In addition, we have carried out chronoamperometry measurements to examine the usefulness of MoS2 NPs as useful labels. HER catalysis was carried out on a SP electrode at three different potentials (−0.9, −1.0, and −1.1 V vs RHE). The current responses were recorded for a duration of 200 s (Figure 3B). The current response was highest for −1.1 V and decrease with the decreasing applied potential. The current response at −1.1 V for MoS2 NPs was in order of milliamperes, while the response for bare SP electrode was in order of microamperes (see the inset of the Figure 3B). For the further testing of MoS2 NPs as electrochemical labels, working potential of −1.1 V and 200 s measurement time were chosen. After evaluation of the MoS2 NP efficiency toward the HER, the immunoassay was prepared using anti-IgG-MB complex for capture, preconcentration, and detection of the rabbit IgG decorated by MoS2 NPs (see Figure 1B). For this purpose, the anti-IgG-MB complex was conjugated with different concentrations of IgG decorated with MoS2 NPs. The resulting magneto-immunoassay was placed on the WE of the SP electrode with the help of a permanent magnet. The HER catalysis was evaluated in an acid environment. Once the acid dissolved, the biological materials (anti IgG and rabbit IgG), MoS2 NPs adsorbed onto the protein were able to reach the

Figure 4. (A) Comparison of chronoamperometric responses of the immunoassay and control experiments: current signals of IgG/antiIgG MB (red line), of IgG-MoS2/anti-IgG MB (lacking the MoS2 NP label) (black line), and in presence of hemoglobin noncomplementary protein (green line). All experiments were carried out in 0.5 M H2SO4 and the working potential of −1.1 V vs RHE was used. (B) Calibration curve of rabbit IgG at different concentrations as a function of absolute current values (error bars of n = 3 and 95% of confidence interval).

were performed, one without MoS2 NP labels and the other in the presence of human hemoglobin (nonspecific protein). High selectivity of the immunoassay was observed because IgG-MoS2 NPs/anti-IgG-MB complex showed superior current response (red curve) in comparison to the immunoassay without a label (black curve) and the nonspecific protein immunoassay (green curve). In addition, immunoassay sensitivity was tested. Figure 4B shows linear dependence of the rabbit IgG concentration on the current value measured at the time of 200 s. The present system showed very small limit of detection (LOD) of 1.94 pg D



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mL−1, reasonable detection range with R = 0.968, and high reproducibility with relative standard deviation (RSD) = 7.5%. The RSD was obtained from the mean of five different protein concentrations and triplicate runs for each concentration. The suitability of the MoS2 NPs as labels for the magnetoimmunoassay is shown by comparing performance parameters (LODs and R) with similar reported systems (see Table 1). To

ACKNOWLEDGMENTS The project was supported by Czech Science Foundation (GACR no. 17-11456S), Specific University Research (MSMT no. 20-SVV/2017) and by the project Advanced Functional Nanorobots (reg. no. CZ.02.1.01/0.0/0.0/15_003/0000444 financed by the EFRR). This work was created with the financial support of the Neuron Foundation for science support. M.P. thanks to A*Star grant (No. SERC A1783c0005), Singapore.

Table 1. Comparison of LOD and Linear Correlation Coefficient for Immunoassays Utilizing Different Nanomaterials as Labels label

LOD [ng mL−1]

linearity correlation coefficient

references

WS2 MoSe2 PbS R-phycoerythrin Au nanoparticles Ag nanoparticles MoS2

2 1.23 0.8 0.48 0.52 11 1.94 × 10−3

0.960 0.950 0.995 0.894 0.992 0.994 0.968

36 38 49 50 51 52 this work

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CONCLUSION Bipolar electrochemistry has been successfully employed as a solution-based electro-exfoliation method for the preparation of MoS2 NPs with small lateral dimensions. The reduction of the particles size was confirmed by HR-TEM. A small particle size facilitates anchoring of MoS2 NP labels to protein. The resulting MoS2 NPs were utilized as labels for the detection of rabbit IgG by HER using chronoamperometry and a fixed potential of −1.1 V which corresponds to previously measured overpotential. The presented immunoassay exhibits approximately thousand times smaller LOD (1.94 pg mL−1) compared with similar reported immunoassay systems.36,38 Moreover detection systems based on hydrogen evolution catalyzed by nanosized TMD show great potential for sensitive biosensing. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b01607. Material characterization: particle size distribution, electron-dispersive X-ray spectroscopy (EDS), and HRXPS spectra of MoS2 NPs obtained by bipolar electrochemical exfoliation (PDF)

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the best of our knowledge, the immunoassay reported in this work exhibit thousand times smaller LOD (1.94 pg mL−1) compared with the systems reported in the literature. High sensitivity of the system containing MoS2 NPs, low cost of MoS2, simple and quick electrochemical modification method are main advantages of the proposed magneto-immunoassay system.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zdeněk Sofer: 0000-0002-1391-4448 Martin Pumera: 0000-0001-5846-2951 Notes

The authors declare no competing financial interest. E


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MoS2 Nanoparticles as Electrocatalytic Labels in Magneto

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