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2H-WS Quantum Dots Produced by Modulating the Dimension and Phase of 1T-Nanosheets for Antibody-free Optical Sensing of Neurotransmitters Man-Jin Kim, Su-Ji Jeon, Tae Woog Kang, Jong-Min Ju, DaBin Yim, Hye-In Kim, Jung Hyun Park, and Jong-Ho Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01644 • Publication Date (Web): 20 Mar 2017 Downloaded from http://pubs.acs.org on March 21, 2017

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ACS Applied Materials & Interfaces

2H-WS2 Quantum Dots Produced by Modulating the Dimension and Phase of 1T-Nanosheets for Antibody-free Optical Sensing of Neurotransmitters Man-Jin Kim,+ Su-Ji Jeon,+ Tae Woog Kang, Jong-Min Ju, DaBin Yim, Hye-In Kim, Jung Hyun Park, and Jong-Ho Kim* Department of Chemical Engineering, Hanyang University, Ansan 426-791, Republic of Korea +

These authors contributed equally to this work.

* To whom correspondence should be addressed: [email protected] Keywords: dopamine detection, optical modulation, optical biosensors, phase engineering, WS2 quantum dots

1 Environment ACS Paragon Plus


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Abstract Modulating the dimensions and phases of transition metal dichalcogenides is of great interest to enhance their intrinsic properties or to create new physicochemical properties. Herein, we report an effective approach to synthesize 2H-WS2 quantum dots (QDs) via the dimension and phase engineering of 1T-WS2 nanosheets. The solvothermal reaction of chemically-exfoliated 1T-WS2 nanosheets in N-methyl-2-pyrrolidone (NMP) under an N2 atmosphere induced their chopping and phase transition at lower temperature to produce 2H-WS2 QDs with a high quantum yield (5.5 ± 0.3%). Interestingly, this chopping and phase transition process showed strong dependency on solvent; WS2 QDs were not produced in other solvents such as 1,4-dioxane and dimethyl sulfoxide. Mechanistic investigations suggested that NMP radicals played a crucial role in the effective production of 2H-WS2 QDs from 1T-WS2 nanosheets. WS2 QDs were successfully applied for the selective, sensitive and rapid detection of dopamine in human serum (4 min, as low as 23.8 nM). The intense fluorescence of WS2 QDs was selectively quenched upon the addition of dopamine and Au3+ ions due to fluorescence resonance energy transfer between WS2 QDs and the quickly-formed Au nanoparticles. This new sensing principle enabled us to discriminate dopamine from dopamine-derivative neurotransmitters including epinephrine and norepinephrine, and other interference compounds.


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Introduction Transition metal dichalcogenides (TMDs) have gained tremendous attention in a variety of research fields due to their unique physical and chemical properties.1-4 Among them, W and Mo dichalcogenides, such as WS2 and MoS2, exhibit thickness-dependent properties. Bulk WS2 and MoS2 with stacked layers are indirect-bandgap semiconductors; however, they change to directbandgap semiconductors as their thickness is reduced to a monolayer.5 This novel property has enabled TMD monolayers to be used in interesting and diverse applications. For instance, WS2 monolayers have a direct bandgap and emit fluorescence in the visible range, which allows them to be used in promising applications for bioimaging,6 photocatalysis,7 H2 evolution,8 and optoelectronic devices.4,

9

Therefore, many efforts have been made to obtain WS2 and MoS2

monolayers, and a variety of methods, such as liquid exfoliation,8, 10 CVD,11-12 and mechanical exfoliation,13-14 have been well established. However, WS2 and MoS2 monolayers generally show very low fluorescence quantum yields (QYs); the QYs of WS2 and MoS2 monolayers have been found to be less than 0.1 and 0.4%, respectively,15-17 which restricts their wide use in bioimaging, optical sensing, and photocatalysis. Tailoring the dimensions of 2D nanomaterials is an effective way to adjust their intrinsic properties or induce new optical/electrical characteristics. Very recently, TMD quantum dots (QDs) with lateral sizes smaller than 10 nm have been prepared by several approaches.18-27 The obtained TMD QDs showed blue-shifted and excitation wavelength-dependent fluorescence with higher QYs than TMD monolayers. In addition, TMD QDs possess larger surface-to-volume ratios and more active edge sites than TMD monolayers,28 which may be beneficial for catalysis, sensing, and energy storage. However, only a few methods for the synthesis of TMD QDs have been reported to date, and some of these approaches still suffer from long preparation times and low



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production yields: In particular, there are few reports for the effective production of WS 2 QDs. Additionally, the QYs of these synthesized TMD QDs are still not satisfactory, despite the fact that they have higher values than TMD monolayers. Therefore, for widespread application of TMD QDs, it is essential to develop an efficient approach for high-yield production of TMD QDs with improved QYs under relatively mild conditions. In addition, more investigations into the applications of TMD QDs, to demonstrate their potential and value in a range of research fields, should be undertaken. Dopamine (Dopa) is an important neurotransmitter and a chemical messenger in the central and peripheral nervous systems.29-30 Dopa is involved in various neural signaling pathways and brain functions. The level of Dopa in biological fluids is an indicator that can be used to determine neural or brain disorders that cause severe neural diseases such as Parkinson’s disease and schizophrenia.31-32 Therefore, it is important to detect Dopa sensitively and selectively in blood. Many different types of sensing platforms for Dopa detection have been reported,33-39 but developing biosensors that are able to discriminate Dopa from Dopa derivatives such as epinephrine and norepinephrine with high sensitivity and selectivity is still a challenge. In addition, there has yet to be a report investigating TMD QD-based biosensors for Dopa detection. WS2 QDs, in particular, would be a good fluorophore for the optical detection of Dopa in human serum since they exhibit higher QYs than MoS2 and WS2 nanosheets. Herein, we demonstrate a simple and effective approach for the production of WS2 QDs with the highest reported QY, and also investigate their application for the sensitive, selective, and rapid detection of Dopa in serum. Monodisperse 2H-WS2 QDs were prepared via chopping and phase transition of chemically-exfoliated 1T-WS2 nanosheets40-42 during a solvothermal reaction in Nmethyl-2-pyrrolidone (NMP) at a relatively low temperature (100 oC) under an N2 atmosphere


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(Scheme 1). As-prepared WS2 QDs were 2.7 nm in size and possessed a semiconducting phase (2H), exhibiting strong fluorescence with a QY of 5.5 ± 0.3% in the visible range. Finally, a WS2 QD fluorescence-based biosensor for Dopa detection with high sensitivity and selectivity was demonstrated; the sensing mechanism of this sensor platform was fully investigated.

Results and discussion To prepare WS2 QDs, bulk WS2 was chemically exfoliated with butyl lithium,41 which produced WS2 monolayers with a 1T-phase (Figure S1). As reported in the literature, the production yield of WS2 nanosheets was fairly high (200 g/mL), and they were made up mainly of the 1T-phase rather than the 2H-phase (Figure S1b). After exchanging the aqueous medium of the 1T-WS2 nanosheets with NMP, the resulting solution was heated at 100 oC for 90 min under an N2 atmosphere. As shown in Figure S2a, the color of the NMP solutions of 1T-WS2 nanosheets turned from a dark brown to a light yellow as heating time increased. Along with this color change, the resulting solution became fluorescent (Figure S2b). In order to confirm the structural change of WS2 nanosheets, WS2 nanosheets at each time interval of heating were analyzed by transmission electron microscopy (TEM). As shown in Figure 1, 1T-WS2 nanosheets had an average lateral size of 379 nm and became smaller as heating time increased. Eventually, the large 1T-WS2 nanosheets were chopped into 2.7 nm WS2 QDs. According to the literature,43-45 NMP can be oxidized at elevated temperatures to produce several radical species. Recently, we have also reported that NMP radical species, which were generated at mild condition, produced carbon dots via their polymerization and carbonization.46 Therefore, we speculate that NMP radicals might break the W-S bonds of 1T-WS2 nanosheets through a radical reaction as proposed in Figure S3. To verify this hypothesis, benzoquinone (BQ) used as a radical scavenger was added into the NMP



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solution of 1T-WS2 nanosheets and heated at 100 oC for 90 min under an N2 atmosphere. As shown in Figure S4, the size of the 1T-WS2 nanosheets remained almost the same after 90 min of heating in the presence of BQ, indicating that the radical species derived from NMP were involved in the formation of WS2 QDs. In order to further verify that the NMP radicals played an important role in the formation of WS2 QDs, other solvents such as 1,4-dioxane and dimethyl sulfoxide (DMSO) were employed in the solvothermal reaction of 1T-WS2 nanosheets under an N2 atmosphere. As shown in Figure S5a, the color of the 1,4-dioxane solutions of 1T-WS2 nanosheets remained almost the same after heating at 100 oC for 90 min. No color change was observed in the DMSO solutions of 1T-WS2 nanosheets after heating (Figure S5b), which was dissimilar from the result found in the NMP solutions of 1T-WS2. The sizes of the 1T-WS2 nanosheets in the 1,4-dioxane and DMSO solutions were also analyzed by TEM. As shown in Figure S6 and S7, the size distributions remained almost the same during the solvothermal reactions in both 1,4-dioxane and DMSO. According to this control experiment, no WS2 QDs were produced in both 1,4-dioxane and DMSO because no radicals were generated in these two solvents, which was also confirmed by electron spin resonance (ESR) spectroscopy (Figure S8). No characteristic peaks for radical species were found in the ESR spectra. These results clearly suggest that NMP has a crucial role, and its radical species are involved in the formation of WS2 QDs from large 1T-WS2 nanosheets during the reaction. Next, we analyzed the lattice structure of WS2 QDs to confirm their phase using high-resolution TEM (HR-TEM). Figure 2a and 2b showed the HR-TEM images of the 1T-WS2 nanosheets prior to the solvothermal reactions and the as-prepared WS2 QDs, respectively. The lattice structures were then magnified to examine their atomic arrangements. As shown in Figure 2c, 1T-WS2 nanosheets had a trigonal atomic arrangement with a strong contrast between the W and S sites;


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this is characteristic of 1T-phase WS2, indicating that they were metallic. The WS2 QDs, however, exhibited a honeycomb-like atomic arrangement with a minor contrast between the W and S sites, corresponding to the atomic structure of 2H-phase WS2. This atomic structure indicates that the WS2 QDs were semiconducting.47-48 Atomic mapping along the lines (blue and green lines in Figure 2c and 2d) were conducted for both 1T-WS2 nanosheets and WS2 QDs. The atomic linemapping revealed that the intensity ratio between W and S sites for 1T-WS2 nanosheets (Figure 2e) was much greater than that for semiconducting WS2 QDs (Figure 2f). This atomic intensity mapping was in good agreement with the previously reported lattice structures of 1T- and 2H-WS2 as previously reported.49 All of the aforementioned results reveal that the phase transition from metallic (1T) to semiconducting (2H) phases occurred after chopping of the large 1T-WS2 nanosheets during the solvothermal process. According to a previous study,50 the activation barrier for the phase transition of TMDs from the 1T-phase to the 2H-phase becomes lower as the lateral size is decreased. The atomic force microscopy (AFM) image of WS2 QDs shows that their average height was 2.4 nm (Fig. S9), indicating that as-prepared WS2 QDs are spherical. To further investigate the phase of as-prepared WS2 QDs, they were analyzed with X-ray photoelectron spectroscopy (XPS). As shown in Figure 3a, chemically-exfoliated WS2 nanosheets showed a dominant 1T-phase, as denoted by the red peaks at 32 and 34 eV; this was in good agreement with previous studies.8 However, the proportion of the 2H-phase, which is denoted by blue peaks at 33 and 35 eV, increased with increasing heating time in the solvothermal reaction of 1T-WS2 nanosheets (Figure 3b and 3c). After heating for 90 min under an N2 atmosphere where WS2 QDs were produced, 2H-phase WS2 became dominant (Figure 3d and 3e), which originated from the formation of semiconducting WS2 QDs. These XPS data corresponded well to the atomic structure of WS2 QDs that was characterized by HR-TEM. The XPS analysis clearly suggests that



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the phase transition occurred during chopping of the large 1T-WS2 nanosheets in the solvothermal reaction, leading to the production of semiconducting WS2 QDs. Next, we investigated the optical properties of the as-prepared WS2 QDs. Since the fluorescence emission is one of the important features of semiconducting QDs, the fluorescence of WS2 QDs was measured as a function of heating time during the solvothermal reaction of 1T-WS2 nanosheets. As shown in Figure 4a, 1T-WS2 nanosheets did not emit any measurable fluorescence (red line, 0 min), which is expected due to their metallic property. However, as the reaction time increased, strong fluorescence appeared in the solutions, indicating the production of semiconducting WS2 QDs via chopping and phase-transition of the 1T-WS2 nanosheets. WS2 QDs had a fluorescence emission maximum at 480 nm under excitation at 390 nm (blue line, Figure 4a). The fluorescence intensity of WS2 QDs did not increase further when the reaction time increased to 100, 120, and 150 min. This indicates that all 1T-WS2 nanosheets were chopped into WS2 QDs within 90 min (Figure S10). In addition, the fluorescence wavelength of WS2 QDs was dependent on the excitation wavelength. Their emission maximum shifted to longer wavelengths as the excitation wavelength increased (Figure 4b). This excitation wavelength-dependent fluorescence might be attributed to the various defect states of WS2 QDs as well as their different sizes, as reported in other QDs.28 The defect states in WS2 QDs can be created by oxidation of WS2, which was observed in the XPS data (Figure 3). We then measured the fluorescence QY of WS2 QDs using quinine sulfate as a standard (Figure S11) and compared our results with the QYs of previous TMD QDs. As shown in Table S1, WS2 QDs had a QY of 5.5 ± 0.3%; this represents the highest value among TMD QDs that have been reported to date. The bright WS2 QDs with a high QY might be beneficial for the sensitive and selective detection of dopamine in serum. In addition to the QY, the lifetime of the singlet excited states of WS2 QDs was also measured via


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time-resolved fluorescence spectroscopy (Figure S12). It was found that the singlet excited states could decay via several different pathways in the WS2 QDs (Table S2). This also indicates that WS2 QDs had several defect states that could trap the excited electrons, resulting in the excitation wavelength-dependent fluorescence of WS2 QDs. The average lifetime of WS2 QDs was calculated to be 4.35 ns. The absorption features of WS2 QDs were also investigated. As shown in Figure 4c, WS2 QDs exhibited strong absorption in the UV region (blue line) with negligible absorption of visible light; alternatively, the original metallic 1T-WS2 nanosheets showed a broad range of absorption from UV to visible light without featured excitonic absorption peaks (red line).8, 51-52 Figure 4d shows the absorbance ratios between the values at 270 and 550 nm. WS2 QDs had a much larger absorbance ratio than the 1T-WS2 nanosheets, demonstrating their capability for strong UV absorption. This UV absorption feature of WS2 QDs can arise from the larger quantum confinement effect in the much smaller nanoparticles compared to the relatively large 1T-WS2 nanosheets. One other interesting feature was noted in the Raman scattering results of WS2 QDs. As shown in Figure S13, the in-plane (E2g, 353 cm-1) and out-of-plane (A1g, 420 cm-1) vibrational modes of WS2 were clearly observed in 1T-WS2 nanosheets (black line). However, these two peaks were not present for WS2 QDs (red line). This interesting phenomenon was also observed in the literature for MoS2 QDs.20 We speculate that these collective vibrational modes might be diminished by the much smaller size of WS2 QDs (ca. 2.7 nm) compared to the larger nanosheets (ca. 379 nm). This abnormal Raman scattering of WS2 QDs will be studied in greater detail in the future. Next, we utilized these WS2 QDs, which possess intense fluorescence emission, for the selective and sensitive detection of Dopa in serum. The fluorescence intensity of WS2 QDs was much more



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intense than the background signal of proteins in human serum (Figure S14), which allowed the effective detection of Dopa in a mixture of various fluorescent biomolecules. First, we investigated the fluorescence response of WS2 QDs to Dopa or Au3+ ions. After WS2 QDs were dispersed in HEPES buffer (50 mM, pH 7.4), Dopa or Au3+ ions were added to the solution. As shown in Figure S15a, the fluorescence of WS2 QDs did not change in the presence of Dopa (12.5 M). The fluorescence slightly decreased when Au3+ ions (250 M) were added to the WS2 QD solution (Figure S15a, green). However, the solution color remained the same (Figure 5a), and Au nanoparticles (NPs) were not formed. In addition, no plasmonic absorption of Au NPs was observed in the WS2 QDs solution containing only Au3+ ions (Figure S15b, green). When Dopa was added to the solution of Au3+ ions that did not contain WS2 QDs, no color change or plasmonic absorption was observed (Figure 5a and Figure S15b), indicating that no Au NPs were formed. However, when both Dopa and Au3+ ions were simultaneously added to the solution of WS2 QDs, the resulting solution turned red very rapidly (Figure 5a), indicating the formation of Au NPs. In addition, the characteristic plasmonic absorption of Au NPs clearly appeared in the spectrum of the red-colored solution (Figure 5b). The formation of Au NPs were also confirmed by TEM and energy-dispersive X-ray spectroscopy (Figure S16 and Table S3). Along with the formation of Au NPs in the presence of Dopa, the fluorescence of the WS2 QDs started to become quenched. As shown in Figure 5c, fluorescence quenching significantly increased as the incubation time of the solution (WS2 QDs + Au3+ + Dopa) increased. However, the fluorescence of WS2 QDs remained constant in the absence of Dopa (Figure 5d, black) because Au NPs did not form under these conditions. When Dopa was present with Au3+ ions, the fluorescence of WS2 QDs was quenched due to formation of Au NPs (Figure 5d, red). The fluorescence quenching of WS2 QDs is attributed to fluorescence resonance energy transfer (FRET) between the QDs and the quickly-formed Au


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NPs. The fluorescence energy of WS2 QDs can be effectively transferred to the Au NPs because the emission wavelengths significantly overlapped with the absorption wavelengths of Au NPs. The FRET-based quenching of the WS2 QD fluorescence was also confirmed by time-resolved fluorescence spectroscopy (Figure S17). The lifetime of WS2 QD fluorescence decreased to 1.9 ns from 2.4 ns when the Au NPs were quickly formed in the presence of Dopa. These experimental results clearly suggest that Dopa can be detected effectively by measuring the fluorescence of WS2 QDs. According to our experimental results, we propose the sensing mechanism for the detection of Dopa. As shown in Figure 6, the oxidation of Dopa into o-dopaminoquinone occurred in the presence of WS2 QDs, which provided many electrons to Au3+ ions in a short period of time to create Au NPs. The oxidation of Dopa might be accelerated by WS2 QDs after the catechol group bound to the sulfur vacancy sites of WS2 QDs. It was found that the vacancy defects of W nanomaterials effectively promote oxidation reactions.53 Then, the formed Au NPs effectively quenched the fluorescence of WS2 QDs via FRET. This novel sensing principle based on WS2 QDs fluorescence allowed for the rapid detection of Dopa in less than 5 min. In order to emphasize the necessity of WS2 QDs in this sensing principle, other brighter QDs such as graphene quantum dots (GQDs) with a QY of 18% were employed as fluorophores.54 When Dopa (12.5 M) and Au3+ ions (250 M) were added into the GQD solution (Figure S18a), no color changes were observed, indicating no Au NPs formed. No plasmonic absorption of Au NPs was observed in the GQD solution as well (Figure S18b), and the fluorescence of GQDs was not quenched at any reaction time (Figure S18c). These results clearly imply that WS2 QDs can selectively accelerate the reduction and nucleation of Au ions in the presence of Dopa to produce Au NPs.



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In order to verify whether this sensing principle is selective only to Dopa, various types of interference compounds and neurotransmitters, such as ascorbic acid (AA), uric acid (UA), glucose (Glu), adenosine triphosphate (ATP), epinephrine (Epi), and norepinephrine (Nor), were examined. As shown in Figure 7a, the WS2 QD solution containing Au3+ ions (250 M) turned red only in the presence of Dopa (12.5 M); this color change was due to the formation of Au NPs. This Au NP formation was also clearly confirmed in the absorption spectrum of the solution (Figure 7b, red), showing the plasmonic absorption peak at 520 nm. However, the presence of interference materials, including AA and UA at much higher concentrations (200 M), did not induce any color change in the solution of WS2 QDs. No plasmonic absorption peak appeared in the solutions of WS2 QDs in the presence of AA or UA (Figure 7b). Especially, the dopamine derivatives, Epi and Nor led to no color changes in the WS2 QD solutions because no Au NPs were formed. Therefore, plasmonic absorption was not observed. The selective formation of Au NPs in the presence of Dopa might be attributed to the fact that the amino group of Dopa has a higher pKa value than that of Epi and Nor.55 The more basic amino group of Dopa could facilitate the reduction and nucleation of Au ions in the presence of WS2 QDs to form Au NPs. We have also measured the oxidation potential of Dopa, Epi, and Nor. As shown in Figure S19, Dopa has a less positive value (0.21 V) than Epi (0.24 V) and Nor (0.25 V). This result indicates that the oxidation of Dopa more readily occurs than Epi and Nor, which leads to the facile formation of Au NPs in the presence of Dopa. As shown in Figure 7c, along with formation of Au NPs in the presence of Dopa, the fluorescence of WS2 QDs was significantly quenched via FRET. However, no fluorescence quenching was observed in the presence of any of the other interference materials and neurotransmitters. These results clearly verify that the WS2 QD fluorescence sensor was able to selectively detect Dopa. In particular, it is worth noting that the WS2 QD fluorescence sensor was able to distinguish Dopa


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from other Dopa derivatives such as Epi and Nor which are also important neurotransmitters in the human body. Since the molecular structure of Dopa is very similar to those of Epi and Nor, it is generally very difficult to discriminate Dopa from these other compounds using biosensors.39 The WS2 QDs fluorescence sensor, however, was able to selectively detect Dopa against Epi and Nor. Next, we investigate the sensitivity of WS2 QDs fluorescence sensors for Dopa detection. As shown in Figure 8a, the fluorescence quenching of WS2 QDs gradually increased as the concentration of Dopa was increased from 24 to 781 nM. In addition, the fluorescence response of WS2 QDs exhibited a linear correlation with the Dopa concentration (Figure 8b), indicating that this WS2 QD sensor can detect Dopa in a quantitative manner. The limit of detection (LOD) of the WS2 QD sensor for Dopa was calculated based on the standard deviation of the response (SD) and the slope of the calibration curve (S) at levels approximating the LOD according to the formula: LOD = 3.3 (SD/S).56 The LOD of the sensor was found to be 23.8 nM. We compared the performance of the WS2 QD sensor for Dopa detection to that of previous biosensors in terms of sensitivity, assay time, and selectivity, as shown in Table S4. It was found that the developed WS2 QD sensor was more sensitive than other biosensors with the exception of an enzyme-linked immunosorbent assay (ELISA). ELISA was more sensitive than the WS2 QD sensor; however, it required a significantly longer assay time (8 h). The WS2 QD sensor, however, requires only 4 min for detection of Dopa. This was the most rapid detection method. In addition, the WS2 QD sensor was able to selectively recognize Dopa against Epi and Nor. The other biosensors could not discriminate Dopa from Epi and Nor. Finally, we detected Dopa in human serum using the WS2 QD sensor. First, we added a certain amount of Dopa into human blood serum, in which various proteins and small molecules existed.



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Then, we detected Dopa with the WS2 QD sensor and calculated its concentration using the calibration curve in Figure 8b. As shown in Figure S20, Table 1, the WS2 QD sensor was able to detect Dopa in human serum with high precision. The amount of Dopa added to the human serum was consistent with the concentration detected by the WS2 QD sensor. The p-values clearly indicate that detection of Dopa with the WS2 QD sensor was very reliable, even in human blood serum. It will be very interesting to detect biomolecules including Dopa in vivo using WS 2 QDs in the future. WS2 QDs are applicable for in-vivo sensing since it was found that they are biocompatible.57-58

Conclusions In conclusion, we demonstrated an effective approach for the synthesis of WS2 QDs from metallic 1T-WS2 nanosheets. WS2 QDs were produced through chopping and phase transition of 1T-WS2 nanosheets in NMP under relatively mild conditions. The as-prepared WS2 QDs were found to have a semiconducting phase (2H) and to exhibit very intense fluorescence with the highest QY in the visible region. These WS2 QDs were then successfully applied for the sensitive and selective detection of Dopa with an LOD of 23.8 nM. The fluorescence of WS2 QDs was selectively quenched in the presence of Dopa and Au3+ ions via FRET. On the basis of this fluorescence quenching mechanism, WS2 QD sensor wase able to discriminate Dopa from Epi and Nor, as well as from other interference materials, with high precision. This insight into the mechanism responsible for the formation of WS2 QDs will lead to many opportunities for designing other unique nanomaterials. In addition, this new sensing principle based on WS2 QDs fluorescence can be employed to decipher the biological roles of Dopa in the human body.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI. Experimental details, the TEM and XPS data of 1T-WS2 nanosheets, the data for the control experiments in the presence of a radical scavenger and in other solvents, Raman spectra, additional fluorescence spectra for dopamine detection, the TEM image of Au NPs formed, and comparison for sensing performance.

AUTHOR INFORMATION Corresponding Author Prof. Jong-Ho Kim ([email protected]) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This

work

was

supported

by

the

Basic

Science

Research

Program

(NRF-

2014R1A2A1A11051877) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning.



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32. Zhang, M. N.; Yu, P.; Mao, L. Q., Rational Design of Surface/Interface Chemistry for Quantitative in Vivo Monitoring of Brain Chemistry. Accounts Chem Res 2012, 45 (4), 533-543. 33. Dan Wen, W. L., Anne-Kristin Herrmann , Danny Haubold , Matthias Holzschuh, Frank Simon, Alexander Eychmüller, Simple and Sensitive Colorimetric Detection of Dopamine Based on Assembly of Cyclodextrin-Modified Au Nanoparticles. Small 2016, 12 (18), 2439-2442. 34. Yildirim, A.; Bayindir, M., Turn-on Fluorescent Dopamine Sensing Based on in Situ Formation of Visible Light Emitting Polydopamine Nanoparticles. Anal Chem 2014, 86 (11), 5508-5512. 35. Sun, H. F.; Chao, J.; Zuo, X. L.; Su, S.; Liu, X. F.; Yuwen, L. H.; Fan, C. H.; Wang, L. H., Gold nanoparticle-decorated MoS2 nanosheets for simultaneous detection of ascorbic acid, dopamine and uric acid. Rsc Adv 2014, 4 (52), 27625-27629. 36. Wu, S. X.; Zeng, Z. Y.; He, Q. Y.; Wang, Z. J.; Wang, S. J.; Du, Y. P.; Yin, Z. Y.; Sun, X. P.; Chen, W.; Zhang, H., Electrochemically Reduced Single-Layer MoS2 Nanosheets: Characterization, Properties, and Sensing Applications. Small 2012, 8 (14), 2264-2270. 37. Ma, W. G.; Wang, L. N.; Zhang, N.; Han, D. X.; Dong, X. D.; Niu, L., Biomolecule-Free, Selective Detection of o-Diphenol and Its Derivatives with WS2/TiO2-Based Photoelectrochemical Platform. Anal Chem 2015, 87 (9), 4844-4850. 38. Mu, Q.; Xu, H.; Li, Y.; Ma, S. J.; Zhong, X. H., Adenosine capped QDs based fluorescent sensor for detection of dopamine with high selectivity and sensitivity. Analyst 2014, 139 (1), 9398. 39. Jeon, S. J.; Kwak, S. Y.; Yim, D.; Ju, J. M.; Kim, J. H., Chemically-Modulated Photoluminescence of Graphene Oxide for Selective Detection of Neurotransmitter by "Turn-On" Response. J Am Chem Soc 2014, 136 (31), 10842-10845. 40. Heising, J.; Kanatzidis, M. G., Structure of restacked MoS2 and WS2 elucidated by electron crystallography. J Am Chem Soc 1999, 121 (4), 638-643. 41. Eda, G.; Yamaguchi, H.; Voiry, D.; Fujita, T.; Chen, M. W.; Chhowalla, M., Photoluminescence from Chemically Exfoliated MoS2. Nano Lett 2011, 11 (12), 5111-5116. 42. Eng, A. Y. S.; Ambrosi, A.; Sofer, Z.; Simek, P.; Pumera, M., Electrochemistry of Transition Metal Dichalcogenides: Strong Dependence on the Metal-to-Chalcogen Composition and Exfoliation Method. Acs Nano 2014, 8 (12), 12185-12198. 43. Poulain, L.; Monod, A.; Wortham, H., Development of a new on-line mass spectrometer to study the reactivity of soluble organic compounds in the aqueous phase under tropospheric conditions: Application to OH-oxidation of N-methylpyrrolidone. J Photoch Photobio A 2007, 187 (1), 10-23. 44. Berrueco, C.; Alvarez, P.; Venditti, S.; Morgan, T. J.; Herod, A. A.; Millan, M.; Kandiyoti, R., Sample Contamination with NMP-oxidation Products and Byproduct-free NMP Removal from Sample Solutions. Energ Fuel 2009, 23, 3008-3015. 45. Viet, H. P.; Tran, V. C.; Hur, S. H.; Oh, E.; Kim, E. J.; Shin, E. W.; Chung, J. S., Chemical functionalization of graphene sheets by solvothermal reduction of a graphene oxide suspension in N-methyl-2-pyrrolidone. J Mater Chem 2011, 21 (10), 3371-3377. 46. Park, J. H.; Raza, F.; Jeon, S. J.; Yim, D.; Kim, H. I.; Kang, T. W.; Kim, J. H., Oxygenmediated formation of MoSx-doped hollow carbon dots for visible light-driven photocatalysis. J Mater Chem A 2016, 4 (38), 14796-14803. 47. Qi, Y. H.; Xu, Q.; Wang, Y.; Yan, B.; Ren, Y. M.; Chen, Z. M., CO2-Induced Phase Engineering: Protocol for Enhanced Photoelectrocatalytic Performance of 2D MoS2 Nanosheets. Acs Nano 2016, 10 (2), 2903-2909.


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48. Wang, K. P.; Wang, J.; Fan, J. T.; Lotya, M.; O'Neill, A.; Fox, D.; Feng, Y. Y.; Zhang, X. Y.; Jiang, B. X.; Zhao, Q. Z.; Zhang, H. Z.; Coleman, J. N.; Zhang, L.; Blau, W. J., Ultrafast Saturable Absorption of Two-Dimensional MoS2 Nanosheets. Acs Nano 2013, 7 (10), 9260-9267. 49. Eda, G.; Fujita, T.; Yamaguchi, H.; Voiry, D.; Chen, M. W.; Chhowalla, M., Coherent Atomic and Electronic Heterostructures of Single-Layer MoS2. Acs Nano 2012, 6 (8), 7311-7317. 50. Hu, Z. Y.; Zhang, S. L.; Zhang, Y. N.; Wang, D.; Zeng, H. B.; Liu, L. M., Modulating the phase transition between metallic and semiconducting single-layer MoS2 and WS2 through size effects. Phys Chem Chem Phys 2015, 17 (2), 1099-1105. 51. Liu, Q.; Li, X. L.; He, Q.; Khalil, A.; Liu, D. B.; Xiang, T.; Wu, X. J.; Song, L., GramScale Aqueous Synthesis of Stable Few-Layered 1T-MoS2: Applications for Visible-Light-Driven Photocatalytic Hydrogen Evolution. Small 2015, 11 (41), 5556-5564. 52. Pagona, G.; Bittencourt, C.; Arenal, R.; Tagmatarchis, N., Exfoliated semiconducting pure 2H-MoS2 and 2H-WS2 assisted by chlorosulfonic acid. Chem Commun 2015, 51 (65), 1295012953. 53. Zhang, N.; Li, X. Y.; Ye, H. C.; Chen, S. M.; Ju, H. X.; Liu, D. B.; Lin, Y.; Ye, W.; Wang, C. M.; Xu, Q.; Zhu, J. F.; Song, L.; Jiang, J.; Xiong, Y. J., Oxide Defect Engineering Enables to Couple Solar Energy into Oxygen Activation. J Am Chem Soc 2016, 138 (28), 8928-8935. 54. Jeon, S. J.; Kang, T. W.; Ju, J. M.; Kim, M. J.; Park, J. H.; Raza, F.; Han, J.; Lee, H. R.; Kim, J. H., Modulating the Photocatalytic Activity of Graphene Quantum Dots via Atomic Tailoring for Highly Enhanced Photocatalysis under Visible Light. Adv Funct Mater 2016, 26 (45), 8211-8219. 55. Armstrong, J.; Barlow, R. B., Ionization of Phenolic Amines, Including Apomorphine, Dopamine and Catecholamines and an Assessment of Zwitterion Constants. Brit J Pharmacol 1976, 57 (4), 501-516. 56. Kang, T. W.; Jeon, S. J.; Kim, H. I.; Park, J. H.; Yim, D.; Lee, H. R.; Ju, J. M.; Kim, M. J.; Kim, J. H., Optical Detection of Enzymatic Activity and Inhibitors on Non-Covalently Functionalized Fluorescent Graphene Oxide. Acs Nano 2016, 10 (5), 5346-5353. 57. Teo, W. Z.; Chng, E. L. K.; Sofer, Z.; Pumera, M., Cytotoxicity of Exfoliated TransitionMetal Dichalcogenides (MoS2, WS2, and WSe2) is Lower Than That of Graphene and its Analogues. Chem-Eur J 2014, 20 (31), 9627-9632. 58. Appel, J. H.; Li, D. O.; Podlevsky, J. D.; Debnath, A.; Green, A. A.; Wang, Q. H.; Chae, J., Low Cytotoxicity and Genotoxicity of Two-Dimensional MoS2 and WS2. Acs Biomater Sci Eng 2016, 2 (3), 361-367.



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1T-WS2 Nanosheet

Page 20 of 40

2H-WS2 Quantum Dots

Chopping & Phase Transition Scheme 1. Schematic illustration for the preparation of WS2 QDs through chopping and phasetransition of 1T-WS2 nanosheets during a solvothermal reaction.


(a)

0 min (f) ca. 379 nm

100 nm

Frequency (%)

20 min (g) ca. 148 nm

100 nm

40 min (h) Frequency (%)

(c)

ca. 79 nm

100 nm

(d)

60 min (i) ca. 8.9 nm

Frequency (%)

(b)

50 nm

(e)

90 min (j)

ca. 2.7 nm

20 nm 1

10

Frequency (%)

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


Frequency (%)

Page 21 of 40

100

Size (nm)

Figure 1. Size changes of 1T-WS2 nanosheets during solvothermal reactions. TEM images of (a) original 1T-WS2 nanosheets, (b) 20 min, (c) 40 min, (d) 60 min and (e) 90 min after heating



at 100oC under N2 atmosphere. (f)-(j) Size distributions for the samples at each time interval of heating.

(a) 1T-WS2

(b) WS2 QDs

10 nm

10 nm

(c)

(d)

1 nm

1 nm

Intensity (a.u)

.

W

0.8

(e)

0.4

S

0.0 0.0

.

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

Page 22 of 40

W S

(f) S S W

0.2

SS W

0.4

0.6

0.8

Distance (nm)

WS 1.0

0.2

W

S 0.4

0.6

W 0.8

S W 1.0

Distance (nm)

Figure 2. Atomic structures of WS2 QDs and 1T-WS2 nanosheets. Magnified HR-TEM images of (a) original 1T-WS2 nanosheet, (b) as-prepared WS2 QDs. (c) Trigonal atomic structure of 1TWS2 nanosheet. (d) Honeycomb-like atomic structure of WS2 QDs. Intensity mappings between W and S sites along (e) the blue line in (c), and (f) the green line in (d).


Page 23 of 40

(a)

0 min

1T-WS2 2H-WS2 WO3

(b)

40 min

(c)

60 min

(d)

90 min

40

38

36

34

32

30

80

100

Binding Energy (eV) Area Ratio (2H/1T)

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


12

(e)

9 6 3 0

0

20

40 60 Reaction Time (min)

Figure 3. XPS analysis of WS2 QDs. XPS spectra of W4f for (a) origi-nal 1T-WS2 nanosheets, and the samples (b) 40 min, (c) 60 min, and (d) 90 min after heating at 100oC under N2 atmosphere. (e) Area ratios between 2H-WS2 and 1T-WS2 as a function of heating time.


0.5

0.0 450

500

Fluorescence Intensity

1.0

90 min 60 40 20 1T-WS2

(a)

550

1.0

0.6 0.4 0.2 400

0.6 0.3 0.0

300

400 500

15

560

640

(d)

12

600 700

Wavelength (nm)

480

Wavelength (nm)

A270 / A550

0.9

Ex = 350 370 390 410 430 450

0.8

WS2 QDs 1T-WS2

(c)

Page 24 of 40

(b)

Wavelength (nm) Absorbance (a.u)

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



9 6 3 0

0

20 40 60 80 100

Reaction Time (min)

Figure 4. Optical properties of WS2 QDs. (a) Fluorescence spectra of WS2 QDs and 1T-WS2 nanosheets at excitation of 390 nm. (b) Excitation wavelength-dependent fluorescence spectra of WS2. (c) UV-Vis absorption spectra of WS2 QDs and 1T-WS2 nanosheets. (d) Absorbance ratios between the values at 270 and 550 nm as a function of reaction time in a solvothermal process.


0

(a)

1

2

WS2 QDs +

3

4 5 min

Au3+ +

Dopa

Au3+ + Dopa WS2 QDs + Au3+

0.16

(b)

0 min 2 4

0.12 0.08 0.04 0.00 400

500

600

700

Wavelength (nm) 1.0

0 min 2 4

(c)

0.8 0.6

(d) 1.0

I / I0


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


Absorbance (a.u)

Page 25 of 40

0.4

w/o Dopa w/ Dopa

0.6

0.2 0.0 450

0.8

500

550

600

Wavelength (nm)

0

1

2

3

4

5

Reaction Time (min)

Figure 5. Detection of dopamine via fluorescence quenching of WS2 QDs. (a) Optical photos for the solutions; WS2 QDs + Au3+ ions + Dopa, Au3+ ions + Dopa, and WS2 QDs + Au3+ ions. The concentrations of Au3+ and Dopa were 250 and 12.5 M, respectively. (b) Absorption spectra of the solution consisting of WS2 QDs + Au3+ ions + Dopa, showing the plasmonic absorption of Au NPs. (c) Fluorescence response of WS2 QDs in the solution of WS2 QDs + Au3+ ions + Dopa, showing a quenching response. (d) Quenching response kinetics of WS2 QD fluorescence in the absence and the presence of Dopa. I0 and I were the fluorescence intensities of WS2 QDs including Au3+ ions before and after addition of Dopa, respectively.



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

Au3+ Au3+ e-

ee-

Oxidation

WS2 QD

Page 26 of 40

FRET

Au3+ Reduction & Nucleation

Au NP

Dopamine

Au NP

Fluorescence Quenching

Figure 6. Proposed mechanism responsible for the selective detection of dopamine on the basis of fluorescence quenching of WS2 QDs by Au NPs selectively formed in the presence of both WS2 QDs and dopamine.


0

(a)

1

2

3 min Dopa

AA

UA

0.15

Glu

(b)

0.10

Dopa UA ATP

0.05

400

AA Glu Epi Nor

500

600

700

Wavelength (nm)

0.5

ATP

(c)

Nor

Epi

ATP

0.1

Glu

0.2

UA

Nor

0.3

AA

Epi

Dopa

0.4

(I0 - I) / I0

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


Absorbance (a.u)

Page 27 of 40

0.0

Figure 7. Sensitivity of WS2 QD fluorescence sensors to dopamine. (a) Optical photos of the WS2 QD solutions with Au3+ ions (250 M) upon addition of various substances; Dopa, AA (ascorbic acid), UA (uric acid), Glu (glucose), ATP (adenosine tri-phosphate), Epi (Epinephrine), and Nor (Norepinephrine). The concentration of all substances was 12.5 M except AA and UA (200 M). (b) Absorption spectra of the WS2 QD solutions 3 min after addition of each substance. (c) Fluorescence response of WS2 QDs to various substances. I0 and I were the fluorescence intensities of WS2 QDs including Au3+ ions before and after addition of each substance, respectively.


1.0

0 nM 24.4 48.8 97.7 195.3 390.6 781.3

(a)

0.8 0.6 0.4

0.6

(I0 - I) / I0

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

Normalized Intensity (a.u)


Page 28 of 40

(b)

R2 = 0.997

0.4

0.2

0.2

LOD = 23.8 nM

0.0

0.0 450

500

550

Wavelength (nm)

600

0

200

400

600

800

Dopa Concentration (nM)

Figure 8. Sensitivity of WS2 QD fluorescence sensors for Dopa detection. (a) Fluorescence spectra of WS2 QDs containing Au3+ ions (375 M) for various concentration of Dopa. (b) Plot of fluorescence responses of WS2 QDs against various Dopa concentrations from 24 to 781 nM. I0 and I were the fluorescence intensities of WS2 QDs including Au3+ ions before and after addition of Dopa, respectively.


Page 29 of 40

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


Table 1. Detection of Dopa in human serum with WS2 QD fluorescence sensors

Sample

Human blood-serum

Added conc. (nM)

Detected conc. (nM)

Consistencya (%)

p-Valueb

98

95.6

97.6

3.75 x 10-3

195

193.3

99.0

6.35 x 10-3

391

392.3

100.5

3.06 x 10-3

781

786.9

99.0

3.91 x 10-6

a

(Detected conc./Added conc.) x100 p-value indicate the accuracy of each assay; if p < 0.05, >95% confidence;, if p < 0.01, >99% confidence; if p < 0.001, >99.9% confidence. b



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

TOC

1T-WS2 Nanosheet

Chopping & Phase Transition

Dopamine

Highly Fluorescent

Fluorescence Quenching

2H-WS2 Quantum Dots


Page 30 of 40

Page 31 of 40

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


Scheme 1. Schematic illustration for the preparation of WS2 QDs through chopping and phase-transition of 1T-WS2 nanosheets during a solvothermal reaction. 257x100mm (150 x 150 DPI)

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Figure 1. Size changes of 1T-WS2 nanosheets during solvothermal reactions. TEM images of (a) original 1TWS2 nanosheets, (b) 20 min, (c) 40 min, (d) 60 min and (e) 90 min after heating at 100oC under N2 atmosphere. (f)-(j) Size distributions for the samples at each time interval of heating. 97x225mm (150 x 150 DPI)


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Figure 2. Atomic structures of WS2 QDs and 1T-WS2 nanosheets. Magnified HR-TEM images of (a) original 1T-WS2 nanosheet, (b) as-prepared WS2 QDs. (c) Trigonal atomic structure of 1T-WS2 nanosheet. (d) Honeycomb-like atomic structure of WS2 QDs. Intensity mappings between W and S sites along (e) the blue line in (c), and (f) the green line in (d). 153x190mm (150 x 150 DPI)



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Figure 3. XPS analysis of WS2 QDs. XPS spectra of W4f for (a) original 1T-WS2 nanosheets, and the samples (b) 40 min, (c) 60 min, and (d) 90 min after heating at 100oC under N2 atmos-phere. (e) Area ratios between 2H-WS2 and 1T-WS2 as a function of heating time. 109x182mm (150 x 150 DPI)


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Figure 4. Optical properties of WS2 QDs. (a) Fluorescence spectra of WS2 QDs and 1T-WS2 nanosheets at excitation of 390 nm. (b) Excitation wavelength-dependent fluorescence spectra of WS2. (c) UV-Vis absorption spectra of WS2 QDs and 1T-WS2 nanosheets. (d) Absorbance ratios between the values at 270 and 550 nm as a function of reaction time in a solvothermal process. 171x138mm (150 x 150 DPI)



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Figure 5. Detection of dopamine via fluorescence quenching of WS2 QDs. (a) Optical photos for the solutions; WS2 QDs + Au3+ ions + Dopa, Au3+ ions + Dopa, and WS2 QDs + Au3+ ions. The concentrations of Au3+ and Dopa were 250 and 12.5 µM, respectively. (b) Absorption spectra of the solution consisting of WS2 QDs + Au3+ ions + Dopa, showing the plasmonic absorption of Au NPs. (c) Fluorescence response of WS2 QDs in the solution of WS2 QDs + Au3+ ions + Dopa, showing a quenching response. (d) Quenching response kinetics of WS2 QD fluorescence in the absence and the presence of Dopa. I0 and I were the fluorescence intensities of WS2 QDs including Au3+ ions before and after addition of Dopa, respectively. 169x134mm (150 x 150 DPI)


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Figure 6. Proposed mechanism responsible for the selective detection of dopamine on the basis of fluorescence quenching of WS2 QDs by Au NPs selectively formed in the presence of both WS2 QDs and dopamine. 169x62mm (150 x 150 DPI)



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Figure 7. Sensitivity of WS2 QD fluorescence sensors to dopamine. (a) Optical photos of the WS2 QD solutions with Au3+ ions (250 µM) upon addition of various substances; Dopa, AA (ascorbic acid), UA (uric acid), Glu (glucose), ATP (adenosine tri-phosphate), Epi (Epinephrine), and Nor (Norepinephrine). The concentration of all substances was 12.5 µM except AA and UA (200 µM). (b) Absorption spectra of the WS2 QD solutions 3 min after addition of each substance. (c) Fluo-rescence response of WS2 QDs to various substances. I0 and I were the fluorescence intensities of WS2 QDs including Au3+ ions before and after addition of each substance, respectively. 133x131mm (150 x 150 DPI)


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Figure 8. Sensitivity of WS2 QD fluorescence sensors for Dopa detection. (a) Fluorescence spectra of WS2 QDs containing Au3+ ions (375 µM) for various concentration of Dopa. (b) Plot of fluorescence responses of WS2 QDs against various Dopa concentrations from 24 to 781 nM. I0 and I were the fluorescence intensities of WS2 QDs including Au3+ ions before and after addition of Dopa, respectively. 265x108mm (150 x 150 DPI)



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Table 1. Detection of Dopa in human serum with WS2 QD fluorescence sensors 159x79mm (150 x 150 DPI)


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2H-WS2 Quantum Dots Produced by Modulating ... - ACS Publications

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