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Multi-Sensing Capability of MoSe Quantum Dots by Tuning Surface Functional Groups Namasivayam Dhenadhayalan, Ta-Wei Lin, Hsin-Lung Lee, and King-Chuen Lin ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00634 • Publication Date (Web): 05 Jun 2018 Downloaded from http://pubs.acs.org on June 5, 2018

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ACS Applied Nano Materials

Multi-Sensing Capability of MoSe2 Quantum Dots by Tuning Surface Functional Groups

Namasivayam Dhenadhayalan,a,b,* Ta-Wei Lin,b Hsin-Lung Leeb and King-Chuen Lina,b,*

a

Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan.

b

Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan.

*Correspondence should be addressed. E-mail: [email protected] (King-Chuen Lin); Fax: +886-2-23621483; Tel.: +886-233661162. E-mail: [email protected] (Namasivayam Dhenadhayalan)

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Abstract Several distinct surface functionalized molybdenum diselenide (MoSe2) quantum dots (QDs) were developed as chemosensors based on the fluorescent probe. The carboxylic-, amine- and thiol-functionalized MoSe2 QDs (MoSe2/COOH, MoSe2/NH2, and MoSe2/SH) were synthesized by tuning their surface with thiol-containing capping agents. These MoSe2/COOH and MoSe2/NH2 QDs sensors were implemented towards the highly selective and sensitive detection of copper ion (Cu2+) and 2,4,6-trinitrophenol (TNP) with a lower detection limit of 4.6 and 45.3 nM, respectively. Similarly, the MoSe2/SH QDs while coupled with gold nanoparticles showed excellent selectivity towards melamine (MA) with a lower detection limit of 27.7 nM. It is surprising to find each functionalized QDs exhibits distinct sensing mechanism in the detection of Cu2+, TNP, and MA, based on metal ion-induced fluorescence turn-on, electron transfer, and energy transfer suppression, respectively. Moreover, these MoSe2 QDs based chemosensors were successfully utilized in real samples, confirming their propitious application.

Keywords: transition metal dichalcogenides, MoSe2 quantum dots, chemosensors, fluorescence, melamine, 2,4,6-trinitrophenol

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1. Introduction As new promising nanomaterials, transition metal dichalcogenides (TMD) such as MoS2, MoSe2, WS2, TiS2, ReS2, etc. exhibit different characteristic properties depending on their dimensions.1-5 For instance, the quantum confinement effect inherent in TMD nanosheets (NSs) brings about new optical and electronic properties compared to bulk material.1,2,6-8 In contrast, TMD quantum dots (QDs) exhibit distinct features such as strong fluorescence, stability, and low toxicity, thus leading to utilization in diverse applications including chemo- and bio-sensors, catalysis, and energy fields.9-14 However, TMD QDs have much less applied to chemosensors based on the fluorescent probe with respect to other QDs materials including carbon dots, graphene QDs, silicon QDs, and Cd-based QDs; therefore, by taking advantage of their intense emission merits, exploration of TMD QDs based-chemosensors from the aspects of size, shape and surface modification should be worthwhile. Recently, MoS2 and MoSe2 QDs have received much attention in the design and development of chemosensors, bioimaging, and photodynamic therapy.15-22 Haldar et al. have reported the 1,4-diaminobutane functionalized MoS2 QDs used as a fluorescent sensor for nitro-explosives.15 Similarly, Dong et al. have applied fluorescent MoS2 QDs to examine the behavior of bioimaging and photodynamic therapy.16 Yuwen et al. have developed ultrasmall MoSe2 nanodots for efficient photothermal therapy of cancer cells.19 Karfa et al. have prepared MoSe2:CdS and WSe2:CdS hybrid nanodots for their sensing of lead.22 Different from above-mentioned designs and applications, this work aims to fabricate the MoSe2 QDs based chemosensors by tuning their surface functional groups to 3



achieve detection of three different types of targets. Generally, exfoliation of MoS2/MoSe2 bulk material may form the defect sites due to the sulfur/selenium vacancies, which make feasible functionalization of the respective NSs and/or QDs with suitable capping agents.23-30 As reported, Cho et al. have investigated the functionalization of MoS2 NSs with thiol molecules and confirmed that the thiol groups could bind with Mo atoms at the sulfur vacancy sites.29 Similarly, Voiry et al. have investigated covalent functionalization of monolayered MoS2, WS2 and MoSe2 nanosheets with organohalide compounds and reported that the attachment of functional groups leads to dramatic changes in the optoelectronic properties of the material.30 Accordingly, in this work, we have modified MoSe2 QDs surface individually with thiol-capping agents including thioglycolic acid (TGA), cysteamine hydrochloride (CSH), and 1,3-propanedithiol (PDT), thus forming carboxylic-, amine- and thiolfunctionalized MoSe2 QDs (MoSe2/COOH, MoSe2/NH2 and MoSe2/SH), respectively. It is expected that each QDs can exhibit different sensing behavior due to the distinct nature of functional groups. The fluorescence titration method was used to investigate the sensing behavior of each QDs sensor system. As a result, the MoSe2/COOH, MoSe2/NH2 and MoSe2/SH QDs sensors are fabricated to detect copper ion (Cu2+), 2,4,6trinitrophenol (TNP) and melamine (MA), respectively, with higher sensitivity as well as good selectivity. Copper is a vital trace element which plays an important role in the biological processes especially in the nervous system. Abnormal level of copper present in the living system could cause several diseases including Alzheimer's, Menkes, Parkinson's, Wilson's, and Prion diseases.31,32,33 Similarly, melamine is a toxic material to human beings. Excess limit of MA may bring about serious kidney-related diseases and 4


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even death.34-36 Therefore, it is urgent to develop a higher sensitive sensor to effectively monitor Cu2+ and MA. TNP is one of the powerful aromatic nitro-explosives which is widely used in the military explosives and also in the industries such as leather, chemical, and pharmaceutical requirement.37-39 The aqueous solubility nature of TNP imposes on environmental pollution thereby producing health problems to living things. The development of a chemosensor sensitive to TNP is of great importance to prevent us from environmental pollution and also explosive threats.

2. Experimental Section 2.1. Materials MoSe2 powder was purchased from Acros organics. Thioglycolic acid, cysteamine hydrochloride, and 1,3-propanedithiol were purchased from Alfa Aesar and Acros. The isopropanol and N-methyl-2-pyrrolidone (NMP) solvents were purchased from Acros organics. Gold (III) chloride trihydrate (HAuCl4) was purchased from SigmaAldrich. HEPES buffer solution (1 M) was purchased from Gibco by Life technologies. All other commercially available inorganic and organic compounds were purchased from Acros and Sigma-Aldrich and used without further purification.

2.2. Methods Field emission transmission electron microscopy (FETEM) was conducted using JEOL JEM-2100F microscope. Fourier-transform infrared (FTIR) spectra were measured with a Thermo Scientific Nicolet iS5 Fourier-transform infrared spectrometer. Raman

spectra of QDs were recorded using Nicolet Almega XR Raman system (Thermo). X-ray 5



photoelectron spectroscopy (XPS) measurement was conducted with VG Scientific ESCALAB 250 spectrometer. The absorption and fluorescence spectra were recorded with a Thermo Scientific evolution 220 UV-visible spectrophotometer and Perkin-Elmer LS45 spectrometer, respectively.

2.3. Synthesis and Surface Functionalization of MoSe2 QDs The synthesis and subsequent surface functionalization of MoSe2 QDs were carried out by the following reported procedure with modifications.10,17,23,40,41 The MoSe2 QD was synthesized from bulk material by the facile hydrothermal method as illustrated in Scheme 1.10,40 Briefly, MoSe2 powder (0.3 g) was dissolved in NMP solvent (100 mL) followed by sonication for 12 h. The solution was centrifuged at 6000 rpm for 10 min to collect supernatant which contained MoSe2 NSs. Then, the supernatant solution was refluxed for 8 h at 180 °C to obtain MoSe2 QDs. Bulk particles and impurities were removed by the centrifugation at 10000 rpm for 30 min and followed by vacuum distillation to remove the solvent from the supernatant. Then, the QDs was washed thoroughly with Millipore water to obtain pure MoSe2 QDs. To synthesize the MoSe2/COOH QDs, the reaction mixture containing MoSe2 QDs (1 µg/mL, 25 mL) and TGA (100 mM) was refluxed for 12 h.23,41 After that, the solution was cooled down to room temperature followed by centrifugation at 10000 rpm for 15 min. The unreacted TGA was separated from collected supernatant through gel column, and each fluorescence fraction was collected after irradiation with UV light (365 nm). The molecular weight cut-off (MWCO) membrane-based centrifugal filters (3 and 10K) were used to further purification to afford pure MoSe2/COOH QDs. Similarly, CSH 6


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(100 mM) and PDT (100 mM) were added individually into MoSe2 QDs solution to synthesize amine- and thiol-functionalized MoSe2 QDs, respectively. Herein, PDT was dissolved in isopropanol due to the insoluble nature in water. Finally, each as-prepared functionalized QDs was concentrated and dissolved in pH 7.4 HEPES buffer.

Scheme 1. (a and b) Schematic representation of the synthesis of surface functionalized MoSe2 QDs and (c) photo image of QDs under UV light irradiation.

2.4. Preparation of Citrate-Stabilized AuNPs Gold nanoparticles (AuNPs) were prepared by a trisodium citrate reduction method according to previous literature.42,43 Briefly, 10 mL of HAuCl4 (1 mM) solution was heated with continuous stirring, and then 1 mL of citrate solution (38.8 mM) was rapidly added into the HAuCl4 solution. The mixture was heated for 20 min with vigorous stirring appearing a wine-red colored. Then, the solution was cooled to room temperature followed by the filtration through 0.22 µm filter membrane (Millipore) to remove impurities.

2.5. Fluorescence Quantum Yield Calculations

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The fluorescence quantum yield of as-prepared QDs was determined using coumarin 151 (C151) as a standard. For that, each QDs solution was prepared to have five different concentrations with which absorbance of the solution was fixed below ~0.1 at 380 nm. Similarly, five different concentrations of the C151 solution were prepared in ethanol. Then, the fluorescence spectra were recorded at the excitation of 380 nm. The quantum yield was determined by the following equation, Φ = Φ ⁄ ⁄

(1)

where Φ is the quantum yield, S is the slope value, and η is the refractive index of the solvent. The subscript st refers to the standard with known quantum yield (ΦC151 = 0.49)44 and x refers to the sample.

2.6. Fluorescence Sensing Experiments The HEPES buffer (10 mM) solution was prepared at pH 7.4. The sensing experiments were carried out under the condition of pH 7.4 HEPES buffered solution. Initially, the as-prepared MoSe2/COOH, MoSe2/NH2 and MoSe2/SH QDs exhibited the pH value of 2.4, 6.8 and 8.1, respectively. Then, each MoSe2 QDs was adjusted to pH 7.4 prior to dissolving in pH 7.4 HEPES buffer. The stock solution of each MoSe2 QDs was prepared in pH 7.4 HEPES, and the absorbance of each QDs at 400 nm was fixed around 0.2. The analyte (metal ions, nitro compounds, toxic organic molecules, and biomolecules) solutions were prepared in HEPES buffer. To investigate the selectivity, each metal ion (0.1 µM) solution was added individually to a set of MoSe2/COOH QDs solution. Then, the fluorescence spectrum of each sample solutions was monitored at the excitation wavelength of 380 nm to find out 8


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the metal ions selectivity. Similarly, the nitro-compounds (10 µM) and organic-/biomolecules (10 µM) were individually added into a set of MoSe2/NH2 and MoSe2/SH QDs solution, respectively. The fluorescence titration method was conducted to determine the sensitivity. In a typical assay, the Cu2+ ion solution (up to 30 µL; the maximum concentration of Cu2+ in solution is 20 µM) was gradually added into the MoSe2/COOH solution in a quartz cuvette. The solution was mixed well prior to recording the fluorescence spectrum. Likewise, the same procedure was followed for MoSe2/NH2 with TNP and MoSe2/SH with MA sensor systems. The sensitivity of each analyte was evaluated based on the changes in the fluorescence intensity of QDs after the addition of analyte.

2.7. Real Samples Analysis Tap water and pond water samples were collected from our university campus. The impurities of water samples were removed by filtration using a 0.22 µm filter membrane (Millipore) and followed by centrifugation at 6000 rpm for 30 min. The stock solution of MoSe2/COOH was prepared in each water samples, and the absorbance was fixed ~0.2 at 400 nm. Following standard addition method, different concentration of Cu2+ ion solution was added to each set of MoSe2/COOH solution. The fluorescence spectra of each solution were monitored. Similarly, the detection of TNP was examined in water samples as well as MA probed in milk samples. Raw milk and powder were purchased from local mart.

3. Results and Discussion 9



3.1. Characterization of QDs The MoSe2 QDs was synthesized initially from their NSs solution by using a simple hydrothermal method (Scheme 1). Then, the QDs surface was functionalized with individual carboxylic-, amine- and thiol-functional groups by using capping agents (TGA, CSH, and PDT). The morphology and surface functional groups of as-prepared QDs were characterized by FETEM, FTIR, Raman, and XPS measurements (Figure 1-3 and S1). The FETEM images of QDs were examined to evaluate the shape and size of prepared QDs, as shown in Figure 1. Note that all FETEM images display well dispersive QDs with a spherical shape, revealing the successful formation of QDs initialized from nanosheets. The average size of all QDs was determined to be ~4.5 nm. The existing functional groups on the surface of QDs were verified by the FTIR measurement. The FTIR spectrum of bare MoSe2 QDs shows no characteristic peaks whereas functionalized MoSe2 QDs display several prominent peaks as shown in Figure 2a-c. The capping agents (TGA, CSH, and PDT) show a significant strong peak at ~2500 cm-1 which was assigned to the –S-H vibration.26,45,46 After functionalization, this –S-H peak was found to disappear for all functionalized MoSe2 QDs (Figure 2a-c), thereby confirming that the thiol group in capping agents is bound on the surface of MoSe2 QDs through the sulfurmetal bond. As shown in Figure 2a, the strong peaks at ~1710 and 3530 cm-1 for MoSe2/COOH are ascribed to the stretching vibration of C=O and O−H band, respectively (Figure 2a),47 confirming the presence of carboxylic group on the surface of MoSe2/COOH QDs. The MoSe2/NH2 QDs exhibits peaks at ~3450, 1605, and 1125 cm-1 corresponding to the stretching vibration of amine, N-H bending, and amine C-N stretching, respectively,47 thus confirming the presence of the amine groups on the 10


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surface of MoSe2/NH2 QDs (Figure 2b). The appearance of characteristic peaks for functionalized QDs indicate that the thiol group in capping agents might interact with QDs via sulfur–metal bond as well as carboxylic-, amine- and thiol-functional groups are projected outward on the surface.

Figure 1. FETEM images and particle size distributions of (a, d) MoSe2/COOH, (b, e) MoSe2/NH2, and (c, f) MoSe2/SH QDs.

Figure 2. The FTIR spectra of (a) MoSe2/COOH, (b) MoSe2/NH2 and (c) MoSe2/SH QDs with respective capping agents and bare MoSe2 QDs. (d) The Raman spectra of MoSe2 NSs and functionalized QDs. The Raman spectra of functionalized QDs were recorded to further gain insight 11



into the structure of QDs with reference of MoSe2 NSs and the obtained spectra are compiled in Figure 2. The Raman spectrum of MoSe2 NSs exhibits two peaks at ~239 and 282 cm-1 (Figure 2d) corresponding to out-of-plane A1g and in-plane E12g vibration modes of 2H-MoSe2.30,48,49 Such similar peaks for all QDs were observed with less intensity, suggesting that the prepared QDs have a highly ordered structure as in NSs.18-20 To further analyze the structures of functionalized QDs, XPS measurements were investigated and the resultant spectra are shown in Figure 3. In all QDs, two characteristic peaks of Mo were observed at ~227 and 232 eV which are ascribed to Mo 3d5/2 and 3d3/2, respectively (Figure 3a,d,g), whereas two characteristic peaks of Se at ~54 and 56 eV are ascribed to Se 3d5/2 and 3d3/2, respectively (Figure 3b,e,h). The assigned peaks are in good agreement with those previously reported,11,16,18-20,50 thus indicating that Mo and Se are present in the Mo4+ and Se2- oxidation state, revealing a 2H phase in the MoSe2 crystal structure.18,19 The presence of –COOH group in MoSe2/COOH was confirmed by the observation of two peaks at 533.6 and 531.9 eV corresponding to C‒O and C=O groups (Figure 3c). Likewise, the peak at 400.3 eV corresponding to C-NH2 demonstrated the presence of –NH2 group in MoSe2/NH2 QDs (Figure 3f). Note that in Figure 3i, apart from the peaks of S in MoSe2, an additional peak at 167.1 eV appearing in the XPS spectrum of MoSe2/SH QDs is assigned to the -SH group. Based on FETEM, FTIR, Raman and XPS results, it can be concluded that dispersive spherical MoSe2 QDs were formed well with a size of ~4.5 nm; moreover, the –COOH, NH2, and SH groups were successfully functionalized on the surface of MoSe2/COOH, MoSe2/NH2 and MoSe2/SH QDs, respectively.

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Figure 3. XPS spectra of (a, b, c) MoSe2/COOH, (d, e, f) MoSe2/NH2, and (g, h, i) MoSe2/SH QDs.

3.2. Optical Spectral Properties of QDs For understanding the optical properties of MoSe2 QDs, the absorption and fluorescence spectra were recorded as shown in Figure S2. The absorption spectra of each functionalized QDs display strong absorbance in the region of 200 to 470 nm (Figure S2a). As shown in Figure S2b-d, the excitation wavelength dependence of the fluorescence spectra reveal the similar behavior of multi-emissions for each MoSe2 QDs. The emission of MoSe2 QDs is due to both intrinsic and trap emissions. The fluorescence maximum was found to redshift with increasing excitation wavelength from 340 to 460 nm. The higher fluorescence intensity was observed as excited at 380 nm. Thus, this wavelength was selected for the fluorescence probe in the chemosensor experiments. The fluorescence quantum yield of MoSe2/COOH, MoSe2/NH2 and MoSe2/SH QDs was determined to be 9.23, 10.14 and 8.58%, respectively (Figure S3). Moreover, the photostability of all QDs was examined under the continuous irradiation of xenon lamp 13



for 2 h. The fluorescence spectra of each QDs were recorded at the regular irradiation time. The resultant spectra of each QDs exhibit not much change in the fluorescence intensity even at the longer time (Figure S4), demonstrating their strong photostability. In addition, all QDs have an excellent storage stability, exhibiting the fluorescence as intense as the freshly prepared QDs, while stored in the refrigerator for nearly 4 months.

3.3. Sensing Performance of MoSe2 QDs Due to strong fluorescence and different nature of surface functional groups, the QDs are anticipated to achieve a distinct fluorogenic sensing of chemical compounds. Accordingly, the MoSe2/COOH and MoSe2/NH2 QDs were managed for the detection of metal ions and explosive nitro-compounds, respectively. Likewise, MoSe2/SH QDs was used for sensing of toxic organic- and essential bio-molecules. The selectivity and sensitivity of sensors were determined by monitoring the fluorescence intensity change upon addition of respective analytes. The fluorescence titration of each QDs was conducted with varying concentration of the selective analyte to determine the sensitivity of the sensor.

3.3.1 MoSe2/COOH QDs Based Cu2+ Ion Sensor Initially, the selectivity experiment was evaluated to find the specificity of MoSe2/COOH and thus the fluorescence spectra of QDs were recorded with the addition of several metal ions including Cr3+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+ and Pb2+. As a result, the fluorescence intensity increased substantially in the presence of a Cu2+ ion but turned out to be very weak when replaced by other metal ions. The 14


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histogram plot for the selectivity of MoSe2/COOH is shown in Figure 4a, explicitly confirming that the MoSe2/COOH exhibits higher selectivity towards the detection of the Cu2+ ion. To determine the sensitivity of Cu2+ ion, the fluorescence spectra of MoSe2/COOH were monitored with varying concentration of Cu2+ ion as shown in Figure 4b. The fluorescence intensity of QDs was found to increase with increasing concentration of the Cu2+ ion progressively. The fluorescence intensity of QDs becomes saturated at the Cu2+ ion concentration of ~10 µM. As shown in Figure 4c, the plot of fluorescence intensity change against the concentration of Cu2+ ion exhibits linearity in the initial concentration range. From the slope of the linear plot, the lower detection limit (LOD) was calculated to be 4.6 nM based on the 3(σ/slope) relation where σ is the standard deviation of the response,47,51 indicating that the MoSe2/COOH QDs is an excellent sensor towards the detection of Cu2+ ion. Then, the association constant (Ka) and stoichiometry for the interaction between MoSe2/COOH and Cu2+ ion were calculated using the Benesi-Hildebrand (B-H) method.48,50 The fluorescence intensity change 1/(F−F0) was plotted as a function of 1/[Cu2+] as shown in Figure 4d; the resultant plot shows linearity with satisfied R2 value. The observed linearity reveals the 1:1 stoichiometry between MoSe2/COOH and Cu2+ ion.47,51 Then the association constant was calculated as a result of a ratio of intercept and slope. The resulting Ka value of 6.0×106 M-1 was determined to reveal the stronger binding of MoSe2/COOH with Cu2+.

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Figure 4. (a) Selectivity plot of (a) MoSe2/COOH QDs in the presence of various metal ions. (b) Fluorescence spectra and (c) intensity response of MoSe2/COOH with varying concentration of Cu2+ ion. (d) Benesi-Hildebrand plot for MoSe2/COOH QDs.

3.3.2 MoSe2/NH2 QDs Based TNP Sensor To make known the selectivity, the change in the fluorescence intensity of MoSe2/NH2 QDs was monitored with various nitro-compounds including nitromethane (NM), nitroethane (NE), nitrobenzene (NB), 1,4-dinitrobenzene (DNB), 4-nitrophenol (NP), 2,4-dinitrophenol (DNP), 2,4,6-trinitrophenol (TNP), 4-nitrotoluene (NT), 2,4dinitrotoluene (DNT), 4-nitroaniline (NA), and 2,4-dinitroaniline (DNA). The MoSe2/NH2 exhibits strong fluorescence in the absence of nitro-compounds. Then, the fluorescence was strongly quenched in the presence of TNP (10 µM), but not much affected when other nitro-compounds were substituted. As shown in Figure 5a, the MoSe2/NH2 is found to be highly selective towards TNP. To explore the quantitative determination of TNP, the fluorescence titration of MoSe2/NH2 QDs was examined by varying the concentration of TNP. As shown in 16


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Figure 5b, the fluorescence intensity of QDs gradually decreased with increasing concentration of TNP. The efficiency of fluorescence quenching was achieved to above 90% at a higher concentration of TNP (50 µM). The lower detection limit of TNP was determined to be 45.3 nM by observing linearity as a function of initial concentration of TNP (10 to 600 nM). Fluorescence quenching of QDs was analyzed using the SternVolmer (S-V) relation as follows,52

= 1

(2)

where, I0 and I are the fluorescence intensity of QDs in the absence and presence of TNP, respectively, [TNP] is the concentration of TNP, and KSV is the S-V constant. The S-V plot of I0/I as a function of [TNP] yields linearity even at a higher concentration of TNP (Figure 5d), indicating that the quenching process is due to the dynamic quenching nature. Then, given the slope, the quenching constant KSV was calculated to be 7.5×104 M-1. This higher S-V quenching constant suggests a stronger interaction between the MoSe2/NH2 and TNP.

Figure 5. (a) Selectivity plot of (a) MoSe2/NH2 QDs in the presence of various nitrocompounds. (b) Fluorescence spectra and (c) intensity response of MoSe2/NH2 with 17



varying concentration of TNP. (d) The Stern-Volmer plot for MoSe2/NH2 QDs.

3.3.3 MoSe2/SH QDs Based Melamine Sensor The MoSe2/SH+AuNPs nanohybrid was initially prepared by a one-step process in which the as-prepared AuNPs solution (average size of NPs is ~13 nm) was mixed with QDs and stirred for overnight. The FETEM of nanohybrid confirms that the AuNPs were uniformly attached with QDs (Figure S5a). The absorption spectrum of nanohybrid gives rise to a peak at ~525 nm (Figure 6d) corresponding to the surface plasmon absorption band of AuNPs.42,43 The fluorescence spectrum of nanohybrid shows weak intensity compared to that of MoSe2/SH QDs alone. Then, the fluorescence recovery of QDs was examined with presence of diverse analytes including toxic organic molecules and essential biomolecules such as L-arginine (Arg), ascorbic acid (Asc), aspartic acid (Asp), bovine serum albumin (BSA), cisplatin (CPT), dopamine (DA), glutathione (GSH), L-histidine (His), α-lipoic acid (LA), lysine (Lys), melamine (MA), ractopamine (RA), and uric acid (UA). As a result, the higher fluorescence recovery was observed in the presence of MA whereas other analytes show weak fluorescence recovery (Figure 6a). This result indicates that the nanohybrid possesses stronger interaction with MA leading to fluorescence restoration and subsequently excellent selectivity towards MA.

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Figure 6. (a) Selectivity plot of (a) MoSe2/SH+AuNPs in the presence of diverse organicand

bio-molecules.

(b) Fluorescence

spectra and

(c) intensity response

of

MoSe2/SH+AuNPs with varying concentration of MA. (d) Absorption spectra of MoSe2/SH+AuNPs as a function of MA; inset shows zoomed view of spectra. Based on the aforementioned results, the sensitivity of MA was determined by monitoring fluorescence responses of nanohybrid as a function of MA. The quenched fluorescence intensity was found to increase with increasing concentration of MA (Figure 6b). The fluorescence restoration was obviously observed at the lower concentration of MA and became saturated in the range of 20 to 180 µM (Figure 6c). The lower detection limit of MA was thus calculated to be 27.7 nM from the linear plot of fluorescence responses in the concentration range of 0 to 1000 nM.

3.3.4 Sensing Mechanism According to abovementioned sensing results, it can be concluded that three distinctive sensor designs with functionalized MoSe2 QDs were applied individually to detect Cu2+, TNP, and MA exhibiting excellent selectivity as well as lower detection limit. 19



Each QDs follows different sensing mechanism depending on the interaction nature with respective analytes. The proposed sensing mechanism for Cu2+, TNP, and MA detection is illustrated in Scheme 2. The fluorescence turn-on mechanism is demonstrated for the Cu2+ ion sensing with the MoSe2/COOH QDs sensor. In fact, the fluorescence of bare MoSe2 QDs mainly originates from the excitonic recombination process between the electrons and holes. The excess free electrons in the unsaturated Mo atom due to the Se vacancy sites render the radiative recombination of electron-hole pairs more feasible leading to the strong fluorescence of bare MoSe2 QDs. These free electrons might be largely reduced by the bonding formation of capping agents with Se vacancy sites upon surface functionalization which restricts the efficiency of recombination process such that the fluorescence may decrease. In the meanwhile, the excited electrons to the conduction band may be delocalized and trapped in the surface states. The retarded electron-hole recombination concomitantly suppresses the fluorescence intensity. When appropriate metal ions are added, the tight association between metal ions and surface functional groups could repel these trapped conduction-band electrons, due to the d-orbital electrons of the metal ions, to facilitate the electron-hole recombination and restore the fluorescence intensity. For this reason, the fluorescence of MoSe2/COOH QDs is essentially enhanced upon interaction with appropriate metal ions. The association with some metal ions on the QDs surface leads to the fluorescence turn-on. The obtained higher association constant (6.0×106 M-1) reveals the stronger binding of MoSe2/COOH with Cu2+. Thus, the fluorescence intensity of MoSe2/COOH QDs is enhanced upon interaction with Cu2+ thereby showed high sensitivity of 4.6 nM.

20


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Scheme 2. Schematic representation of sensing mechanism for Cu2+, TNP, and MA detection. Regarding MoSe2/NH2 QDs sensor, the fluorescence quenching was observed in the presence of TNP. It is well known that the TNP is an electron deficient molecule which can easily bind to π-donor sites and serve as an electron acceptor. Moreover, it can quench the fluorescence via electron transfer while interaction with an electron-rich fluorophore. In this regard, the fluorescence quenching of MoSe2/NH2 is ascribed to the electron transfer from QDs to TNP. The electron-rich QDs acts as an electron donor due to the presence of amine functional groups. The electron donating nature of QDs makes stronger interaction with TNP to facilitate electron transfer resulting in the fluorescence quenching. The obtained higher quenching constant lends support to an efficient electron transfer occurring between MoSe2/NH2 and TNP. This mechanism of electron transfer is consistent with those reported.37-39,53-55 In addition to electron transfer, the inner filter effect could affect to some extent the fluorescence intensity of QDs at higher concentrations of TNP, because a significant UV absorption of TNP may overlap with the 21



excitation wavelength of QDs. Accordingly, the observed fluorescence quenching of QDs at higher concentrations of TNP might probably involve electron transfer in combination with partially the inner filter effect.37,55,56 The fluorescence quenching efficiency among the derivatives of nitrophenol was found to follow the order TNP>DNP>NP. This trend is well correlated with their acidity of phenolic protons such that TNP has a higher acidity nature compared with that of DNP and NP (TNP>DNP>NP), and the order is in good agreement with other reports.57,58 Consequently, the TNP could strongly interact with MoSe2/NH2 QDs via electrostatic interaction between the highly acidic nature of TNP and basic nature of MoSe2/NH2 QDs. Such interaction becomes weaker for DNP and NP due to a decrease of their acidity which results in the lower selectivity. The strong electrostatic interaction between TNP and MoSe2/NH2 can enhance the electron transfer between them. Moreover, the electron-withdrawing ability of nitro-compounds depends on the presence of nitro groups in the molecule; the electron-withdrawing ability increases with increasing number of nitro groups present in the molecule.59 Accordingly, the DNP and NP have less number of nitro groups, resulting in a weaker fluorescence quenching efficiency caused by the nature of lower electron-withdrawing ability. Thus, due to these combined effects, the MoSe2/NH2 QDs exhibit higher quenching efficiency with excellent selectivity towards TNP. As for the sensing of MA, initially the fluorescence spectrum of MoSe2/SH was checked with MA; the resultant fluorescence intensity does not change in the presence of MA. Similarly, the rest of analytes used in this sensing system have not influenced the fluorescence intensity of MoSe2/SH. These results exclude the possible interaction between MoSe2/SH and MA, even with other analytes. Thus, the MoSe2/SH QDs are 22


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modified with AuNPs for sensing the MA. The MA was detected based on fluorescence turn-on mechanism via suppression of fluorescence resonance energy transfer (FRET). The MoSe2/SH+AuNPs nanohybrid exhibits weak fluorescence with respect to the bare MoSe2/SH QDs. Such weak fluorescence is due to the FRET process from MoSe2/SH QDs to AuNPs. Then, the quenched fluorescence of QDs was found to restore with the addition of MA. The finding is interpreted in the following; MA can strongly interact with AuNPs to induce aggregation of AuNPs which prevents the FRET effect and subsequently leads to fluorescence turn-on of QDs. While examining, the absorption spectral changes of the nanohybrid as a function of MA, it is surprising to find the absorption maximum of AuNPs was gradually shifted from 525 to 536 nm accompanied by a decrease in the absorbance with increasing concentration of MA (Figure 6d). The redshift of absorption maximum is distinctly characteristic of aggregation of AuNPs induced by the interaction between the AuNPs and MA. This fact was further confirmed in FETEM as shown in Figure S5b obviously showing aggregated AuNPs in the presence of MA. When MA is added, its amine groups may anchor on the AuNPs surface through electrostatic/covalent interaction to cause the aggregation which subsequently reduces the FRET process between the MoSe2/SH and AuNPs and results in the fluorescence enhancement of MoSe2/SH.60-62 Control experiments were conducted to further support the interaction mechanism between surface functional groups of QDs with respective Cu2+, TNP, and MA analytes. To achieve that, the fluorescence spectra of bare MoSe2 QDs were recorded in the presence of each Cu2+, TNP, and MA. The resultant fluorescence intensity of MoSe2 QDs did not vary significantly with the presence of Cu2+ and MA even at their higher 23



concentration (Figure S6). These results demonstrate the interaction of Cu2+ and MA with surface functional groups instead of QDs core. Although the fluorescence intensity of MoSe2 QDs was found to slightly decrease at the higher concentration of TNP. This slight fluorescence quenching might be due to the involvement of inner filter effect of TNP as aforementioned. Accordingly, it can be concluded that each functionalized MoSe2 QDs follows diverse sensing mechanism; MoSe2/COOH, MoSe2/NH2, and MoSe2/SH QDs in the detection of Cu2+, TNP, and MA rely on metal ion-induced fluorescence turn-on, electron transfer, and FRET process, respectively. The resulting detection limit determined for each Cu2+, TNP, and MA was either better than or comparable with those obtained by nanomaterial-based fluorescence method (Table 1).17,33,35-37,56,60,63-77 The fluorescent sensing materials should possess unique features including ease of preparation and functionalization, higher quantum yield with tunable optical properties, reliable stability, non-toxicity and capability to detect analyte at lower detection limit. In this manner, the MoSe2 QDs display the following unique merits and deserve a wide attention for application of a potential sensor. First, the MoSe2 QDs can be readily prepared to obtain pure product from their bulk materials using the facile method and also easy for surface modification with diverse capping agents. Second, the MoSe2 QDs exhibit strong fluorescence with multi-emission behavior such that the fluorescence of QDs can be tuned for the desired applications. Third, the MoSe2 QDs can be used as multiple sensor systems by functionalizing their surface as MoSe2/COOH, MoSe2/NH2 and MoSe2/SH QDs to the sensing of diverse analytes with very low detection limit. Moreover, these functionalized MoSe2 QDs have higher fluorescence quantum yield 24


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along with excellent photostability, as compared to other transition metal dichalcogenides QDs such as MoS2 and WS2 QDs.14,17,78,79 Fourth, the sensing mechanism is feasible to be understood.

Table 1. Comparison of various nanomaterial-based fluorescent sensors for Cu2+, TNP, and MA detection. Sensors

Analyte

LOD

Ref.

(nM) CQDs

Cu2+

6.0

63

CQDs

Cu2+

6.3

64

GQDs

Cu2+

6.9

65

Carbon dots

Cu2+

8.8

66

77.0

67

2+

F-hPEI Au NPs

Cu

PEI-CuNCs

Cu2+

120.0

68

GQDs

Cu2+

226.0

69

BN-CNPs

TNP

10.0

70

P-doped Carbon dots

TNP

16.9

71

Polymer nanoparticle

TNP

26.0

37

Carbon dots

TNP

51.0

56

GQDs

TNP

91.0

72

MoS2 QDs

TNP

95.0

17

GQDs

TNP

300.0

73

Carbon dots

TNP

1800

74

Ag NCs

MA

30.0

75

Carbon dots

MA

36.0

76

Cu NCs

MA

95.0

60

GQD

MA

120.0

77

Carbon dots

MA

300.0

35

25



Page 26 of 41

AIE-based OFNs@Au NCs

MA

680.0

36

MoSe2/COOH

Cu2+

4.5

This work

MoSe2/NH2

TNP

45.3

This work

MoSe2/SH

MA

27.7

This work

3.4. Real Samples Analysis The species of Cu2+, TNP, and MA were detected in tap water and milk samples to verify the practical application of functionalized MoSe2 QDs. The fluorescence intensities of respective QDs in real samples were recorded in the presence of respective sensing molecule. Both tap and pond water samples were purified by filtration and centrifugation processes prior to mixing with QDs. Then, each MoSe2/COOH and MoSe2/NH2 QDs was dissolved in individual purified tap and pond water solutions. The absorbance of each QDs at 400 nm was fixed to ~0.2 and used as the bulk QDs solution. The fluorescence spectra of each QDs were found to be identical to those in HEPES, suggesting that these real samples do not have any specific external molecules. Upon addition of Cu2+, the fluorescence intensity of MoSe2/COOH QDs was increased with increase in the concentration of Cu2+. Similarly, the fluorescence intensity of MoSe2/NH2 QDs was quenched as a function of TNP in both tap and pond water samples. For the sensing of MA, the MoSe2/SH+AuNPs nanohybrid was dissolved in diluted raw milk and powder solutions. The fluorescence intensity of nanohybrid increases with increasing concentration of MA. Then, the concentration value and percentage of recovery for sensing molecules were determined. Table 2 shows the average recovery of analyte and relative standard deviations (RSD) for three replicates of experiments. The recovery of sensing molecule was determined to be above 90% for all QDs in real samples. This 26


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confirms that these functionalized MoSe2 QDs are capable of sensing of Cu2+, TNP, and MA in real samples.

Table 2. Detection of Cu2+, TNP, and MA in real samples. Tap water

Pond water

Added Found Recovery RSD

Cu2+

TNP

Found Recovery RSD

(µ µM)

(µ µM)

(%)

(%)

(µ µM)

(%)

(%)

0.04

0.039

97.5

1.52

0.042

105.0

2.49

0.2

0.192

96.0

4.19

0.195

97.5

2.04

1.0

1.02

102.0

3.45

1.10

110.0

1.03

0.1

0.104

102.0

2.87

0.097

97.0

1.68

1.0

0.93

93.0

1.96

1.05

105.0

4.68

10.0

9.90

99.0

1.74

9.64

96.4

1.91

Raw milk MA

Milk powder

0.1

0.096

96.0

0.98

0.101

101.0

2.09

1.0

1.10

110.5

3.81

1.05

105.0

3.66

10.0

9.28

92.8

2.44

9.94

99.4

2.85

4. Conclusions The distinct carboxylic-, amine- and thiol-surface functionalized MoSe2 QDs were successfully synthesized by hydrothermal method and well characterized by FETEM, FTIR, Raman and XPS measurements. Each QDs yielded significant fluorescence relying on excitation wavelength. The as-prepared QDs were demonstrated as chemosensors applied to different sensing molecules. The MoSe2/COOH and 27



MoSe2/NH2 QDs were applied to sensing of metal ions and explosive nitro-compounds, respectively, whereas MoSe2/SH+AuNPs nanohybrid was sensitive to essential organicand bio-molecules. The MoSe2/COOH, MoSe2/NH2 and MoSe2/SH QDs were demonstrated to show excellent selectivity towards the detection of Cu2+, TNP and MA, with higher sensitivity of 4.6, 45.3 and 27.7 nM, respectively, following a distinct mechanism of metal ion induced fluorescence turn-on, electron transfer, and FRET process. Further, the sensing capability was successfully extended to real water and milk samples in the detection of Cu2+, TNP and MA, demonstrating that these functionalized MoSe2 QDs can be performed well as chemosensors.

SUPPORTING INFORMATION Characterizations of bare MoSe2 QDs; the absorption and fluorescence spectra of functionalized MoSe2 QDs; the fluorescence integrated intensity change of C151 and functionalized MoSe2 QDs as a function of absorbance; the fluorescence intensity change of functionalized MoSe2 QDs under irradiation of xenon lamp; the FETEM images of MoSe2/SH+AuNPs in the absence and presence of melamine; the fluorescence spectra of bare MoSe2 QDs with Cu2+, TNP and MA.

ACKNOWLEDGMENT This work was supported by Ministry of Science and Technology of Taiwan, Republic of China, under the contract number NSC 102-2113-M-002-009-MY3. Thanks to Ms. C.-Y. Chien of Ministry of Science and Technology (National Taiwan University)

28


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for the assistance in FETEM experiments. N.D. wishes to thank Academia Sinica for a post-doctoral fellowship.

AUTHOR INFORMATION Corresponding Authors E-mail: [email protected] (King-Chuen Lin); Fax: +886-2-2362-1483; Tel.: +886-23366-1162. E-mail: [email protected] (Namasivayam Dhenadhayalan) Notes The authors declare no competing financial interest.

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