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Excited State Electron, Energy Transfer Dynamics between 2D MoS and GO, RGO for Turn ON BSA, HSA Sensing 2

Hariharan Swaminathan, Venkadeshkumar Ramar, and Balasubramanian Karthikeyan J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 19 May 2017 Downloaded from http://pubs.acs.org on May 19, 2017

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The Journal of Physical Chemistry

Excited State Electron, Energy Transfer Dynamics between 2D MoS2 and GO, RGO for Turn ON BSA, HSA Sensing Hariharan Swaminathan, Venkadeshkumar Ramar, Karthikeyan Balasubramanian Nanophotonics Laboratory, Department of Physics, National Institute of Technology, Tiruchirappalli, India, 620015. Abstract We report excited state electron and energy transfer process from two dimensional MoS2 to graphene oxide (GO) and reduced graphene oxide (RGO), also extend its application towards turn-ON sensing of bovine serum albumin (BSA) and human serum albumin (HSA). Steady state fluorescence results reveal the quenching of MoS2 QDs by GO and RGO. The fluorescence quenching efficiency is found to be more in the presence of GO than RGO. The obtained SternVolmer plot suggests the occurrence of both static and dynamic quenching processes. The electron transfers from photoexcited MoS2 QDs to GO and RGO cause reduction and electron storage reactions respectively. Time resolved fluorescence results show significant reduction in the lifetime of a fast decay component of MoS2 upon addition of GO and RGO, which provide evidence for the excited state energy transfer interaction in addition to electron transfer between MoS2 and GO/RGO. The calculated non-radiative decay rate indicates the energy transfer process dominated in the case of interaction between MoS2 and RGO. Interestingly, the quenched fluorescence of MoS2 by both GO and RGO is recovering gradually upon addition of BSA and HSA in the range of 5 to 50 nM, which shows the highly sensitive and trun-ON sensing nature of MoS2 – GO and MoS2 – RGO couples. 

Corresponding Author.: Email: [email protected] (B.Karthikeyan). Tel. No: 0431-2503612, Fax: 0431-2500133

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Introduction Recent progress in spectroscopic research has been aiming towards advancement in medical diagnosis and detection. Spectroscopy is considered as a novel tool in health care and medical research for disease detection and bio sensing. Biosensors based on fluorescence and energy transfer techniques is attracting much due to very high sensitivity, environment friendly, cost effective and rapid diagnostic ability.1-3 Non-radiative energy transfers between a fluorescent donor and acceptor exists within close proximity. The fluorescence quenching phenomenon happens when the emission of donor fluorophore is absorbed by nearby acceptor where the distance between them is in the order of few nanometers. Fӧrster resonance energy transfer (FRET) and nanosurface energy transfer (NSET) are considered to be an efficient tool for study and detecting bioreactions because many bioreactions in human body, such as DNA hybridization, antibody based immunological recognition and enzyme catalyzed hydrolysis or transformation occur or cause in similar distance.4

Selective of suitable materials having

exceptional optical properties for energy donors and acceptors can significantly enhance the energy transfer efficiency and lead to high sensitive energy transfer based sensors. In traditional energy transfer based sensors, usually organic dye molecules (ODM) are used as a donor and acceptor. 5, 6 But it is well known that ODM are susceptible to photobleaching and low chemical stability. There is a high demand for developing new fluorescent materials as a substitute for ODM to overcome these obstacles. To replace the ODM in the energy transfer process, inorganic quantum dots (IQDs) come into existence. IQDs have narrow emission band, size tunable emission due to quantum confinement, excellent photostability, high quantum yield, long fluorescence lifetime and exhibit good resistance to photobleaching.

7-9

Therefore, IQDs have

received much attention and considered as an alternative to ODM for energy donor. Recently, 2 ACS Paragon Plus Environment

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two dimensional (2D) inorganic materials have been attained significant attention due to their remarkable electronic and optical properties. Molybdenum disulfide (MoS2) is one of the emerging 2D materials, popularly called as the inorganic cousin of graphene. MoS2 has composed of S-Mo-S triple layers weakly bounded by van der Waals forces.

10-13

The

exceptional properties of MoS2 give rise to applications in various molecule detection, hydrogen evolution and biosensing applications. 14-19 Metal nanoparticles (MNPs) have been used as a substitute for ODM as an energy acceptor in energy transfer based sensors due to its quenching character even at longer distance. 16-18

Recently, graphene oxide (GO) and reduced graphene oxide (RGO) have been attained

considerable attention and used as an energy acceptors in energy transfer based sensors.

20

It is

found that mechanism of energy transfer from ODM to graphene is similar to energy transfer from ODM to gold nanoparticles.

21

GO and RGO are found to be an effective fluorescence

quenchers and their estimated quenching distance is up to 30 nm. 20 This gives rise to develop a novel energy transfer based sensor to design a molecular or spectroscopic ruler. It is also reported that the GO has the ability to capture and store electrons from semiconductor nanoparticles.

22

Ian et al extensively studied the ability of GO and RGO as an electron and

energy acceptor. 23 Determination of the level of serum proteins in plasma fluids has become important. Bovine serum albumin (BSA) has the structure and composition similar to Human serum albumin (HSA) which is abundant in the circulatory system and constitutes the majority of plasma fluid in a variety of organisms.

24, 25

The interaction between nanomaterial and

biomolecules in complex biological fluids leads to potential biomedical applications such as drug delivery, biomolecules sensing, disease diagnosis and treatment. 3 ACS Paragon Plus Environment


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In the present work, for the first time, we report the excited state electron and energy transfer interactions between MoS2 and GO/RGO through steady state and time resolved spectroscopy. We extend these significant interactions towards the potential application of turnON based highly sensitive BSA sensing. Experimental Materials and methods All the reagents used are of high purity, purchased from Sigma Aldrich, U.S.A and are used without any further purification. Synthesis of MoS2 QDs Initially 0.25g of sodium molybdate is dissolved in 25 ml of Millipore water and sonicated for 5 mins. Then the mixture of 0.5 g of L-cysteine and 50ml of Millipore water are added to the solution and again sonicated for 20 mins. The final solution is transferred to 100 ml Teflon lined stainless steel autoclave and reacted at 200 ◦C for 36 hrs. After cooled naturally, the supernatant containing MoS2 quantum dots are collected by centrifugation at the speed of 12000 rpm for 30 mins. The collected MoS2 quantum dots are used for further studies. X- ray diffraction pattern of the prepared MoS2 is given in Figure S1 which confirms the formation of 2H- (Hexagonal symmetry) type of MoS2. Synthesis of GO and RGO: GO was synthesized by a modified Hummer’s method. Initially 0.5 g of graphite powder and 0.25 g of NaNO3 is added to 12 ml of Sulfuric acid. The mixture is kept in an ice bath until the temperature reached below 50C. Then 1.5 g of KMnO4 is slowly added to the above mixture and raised the temperature to 350C. The mixture is continuously stirred for 2 hrs. After the 4 ACS Paragon Plus Environment

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solution changed to a paste, 23 ml of distilled water is added. The resultant solution is heated to 900C for 15 minutes and then 80 ml of water is added. Then 3 ml of H2O2 is dropped into the mixture and the final solution appeared yellow in color. Precipitate of the final solution is washed with aqueous HCl and then with doubly distilled water for five times. Then it is dried in hot air oven at 60 0C for 48 hrs. The obtained GO is reduced by using hydrazine. The X-ray diffraction of the prepared GO and RGO is shown in Figure S1 which confirms the formation of GO and RGO. GO and RGO solutions for studies are prepared by dispersing 1mg of GO and RGO in each 5ml of doubly distilled water separately. Instrumentation Transmission electron microscope (TEM) image of prepared MoS2 QDs is taken using a JEOL-TEM-2010 electron microscope which is shown in Figure S2. Steady state fluorescence measurements and sensing studies are done in a Fluoromax-P-Horiba JobinYvon luminescence spectrometer. Time resolved fluorescence measurements are carried out using time correlated single photon counting (TCSPC) method. The decay dynamics is measured by exciting at wavelength of 330 nm using nanosecond diode laser with the repetition rates of 1 MHz and the emission decay dynamics are obtained using Deltaflex - Horiba JobinYvon TCSPC system. The obtained emission decay dynamics are analyzed using DAS 6 software. Results: Steady state fluorescence Figure 1 depicts the steady state fluorescence spectra of MoS2 QDs and GO added MoS2 QDs under the excitation of 330 nm. The MoS2 QDs shows strong fluorescence in the region of 350 – 550 nm, peak centered at 400 nm. The measured intense fluorescence is attributed to the excitonic transitions at the K point of the Brillouin zone and this characteristic emission of MoS2 is well agreed with the earlier reports. 26-29 When 100 L of GO solution is added into MoS2, the 5 ACS Paragon Plus Environment


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fluorescence intensity of MoS2 is decreased. An increase in the concentration of GO in MoS2, decreases the fluorescence intensity of MoS2. The fluorescence of MoS2 is observed to be completely quenched when 600 L of GO presents in it. The arrow line facing downward indicates the quenching of the fluorescence intensity of MoS2 QDs while increasing the concentration of the GO solution from 100 to 600 L. Figure 2 depicts the steady state fluorescence spectra of MoS2 QDs alone and RGO added MoS2 QDs under excitation at 330 nm. The fluorescence intensity of MoS2 QDs is found to be decreased, when 100 L of RGO solution is added into MoS2. An increase in the concentration of RGO in MoS2, decreases the fluorescence intensity of MoS2. The 600 L of RGO added MoS2 fluorescence spectrum shows that fluorescence intensity of MoS2 is decreased by 5 times when compared to fluorescence intensity of MoS2 QDs alone. The arrow line facing downward indicates quenching of the fluorescence intensity of MoS2 QDs while increasing the concentration of RGO solution from 100 to 600 L. Time resolved fluorescence: The decay dynamics of MoS2 QDs and GO, RGO added MoS2 QDs is studied by time resolved fluorescence spectroscopy. Time resolved fluorescence measurements are carried out for every increasing concentration of GO, RGO (from 100 to 600 L in steps of 100 L) in MoS2 QDs where the excitation is done at 330 nm and the emission wavelength is fixed at 400 nm. Figure 3(a) depicts the time resolved fluorescence spectra of MoS2 QDs with increasing concentration of GO from 100 to 600 L. The inset of Figure 3(a) shows time resolved fluorescence spectra of MoS2 QDs alone and 600 L of GO added MoS2 QDs. Figure 3(b) depicts time resolved fluorescence spectra of MoS2 QDs with increasing concentration of RGO 6 ACS Paragon Plus Environment

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from 100 to 600 L. The inset of Figure 3(b) shows time resolved fluorescence spectra of MoS2 QDs alone and 600 L of RGO added MoS2 QDs. The variation in decay dynamics of MoS2 QDs is observed in the presence of GO or RGO. The time resolved spectra obtained from measurements are fitted by the exponential decay equation given by

∑

y(x) =

( )

where, n is the number of discrete emissive species, A is a baseline correction usually called as “dc” offset. Bi is the pre exponential factor and τi is the excited-state fluorescence lifetime associated with the ith component. The fitted decay parameters for the time resolved spectra shown in Figure 3(a) and 3(b) are given in Table S1 and Table S2 respectively. The emission decay dynamics of MoS2 QDs follow three exponential decay, having decay times τ1 = 3.51 ns (41.19%), τ2 = 11.46 ns (41.70%) and τ3 = 0.50 ns (17.11%). The decay times are found to be decreased when GO or RGO is added into MoS2 QDs. When 600 L of GO is present in MoS2 QDs, the decay time τ1, τ2 and τ3 are decreased from 3.51 ns to 2.95 ns, 11.46 ns to 10.77 ns and 0.50 ns to 0.25 ns respectively. The arrow line slanting downward in Figure 3(a) indicates the emission decay dynamics become faster (decreasing lifetime) upon increasing concentration of GO in MoS2 QDs. From the Figure 3(a), it can be noted that the baseline of the spectrum is increased (denoted by encircled) when the concentration of GO is increased. The value of A is increased from 1.76 (MoS2 alone) to 13.22 (600 L GO added MoS2). This increased baseline might be due to the scattering effect of the measurement of emission decay dynamics at very low emission intensity.



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When increasing the concentration of RGO in MoS2 QDs, emission decay dynamics of MoS2 becomes fast, which is indicated by the arrow line slanting downward in Figure 3(b). When 600 L of RGO is added in MoS2 QDs, the decay time τ1, τ2 and τ3 are decreased from 3.51 ns to 2.77 ns, 11. 46 ns to 10.59 ns and 0.50 ns to 0.19 ns respectively. Discussions Steady state fluorescence results show that, when there is an increase in GO and RGO concentration, the fluorescence of MoS2 is gradually decreased. This is depicted in Figure 1 and Figure 2 which reveals that both GO and RGO assist as effective quenchers of excited MoS2. Decrease in fluorescence yield of MoS2 suggests that an additional pathway for the disappearance of charge carriers dominates due to the interaction between the excited MoS2 and GO or RGO. The observed quenching can be either due to excited state reaction, dynamic quenching (collisional interaction) or static quenching. To understand quenching mechanism, the relative change in fluorescence intensity has been plotted as the function of quencher concentration [Q]. Figure 4(a) depicts the plots of F0/F as a function of amount of GO/RGO added in MoS2 [Q] where F0 and F are the fluorescence intensity of MoS2 in the absence and presence of GO/RGO respectively. Similarly, the relative change in lifetime τ0/τ has been plotted as a function of quencher concentration which is depicted in Figure 4(b &c) where τ0 and τ are lifetime components of MoS2 in the absence and presence of GO/RGO respectively. Usually, for the dynamic (collisional) quenching, the relative change in fluorescence intensity and lifetime is linearly related to the concentration of quencher which is given by the Stern-Volmer equation F0/F = 1 + KSV[Q] and τ0/τ = 1 + KD[Q] 8 ACS Paragon Plus Environment

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Where KSV is the Stern-Volmer quenching constant and KD is the dynamic quenching constant. In the case of static quenching, F0/F increases, but τ0/τ = 1 i.e, τ remains unchanged. For pure dynamic (collisional) quenching, F0/F = τ0/τ where the slope of linear plot F0/F vs [Q] and τ0/τ vs [Q] should be equal. But in the case of modified Stern-Volmer equation when both dynamic and static quenching occur, F0/F is given by F0/F = (1+KD[Q]) (1+KS[Q]) where KD and KS represent the dynamic and static quenching constants respectively.30 Due to [Q]2 term in the above equation, the plot of F0/F vs [Q] will have an upward curvature instead of linear. From the Figure 4(a), it is observed that the F0/F vary linearly up to [Q] = 200 above which an upward curvature and then again linear up to [Q] = 400 and again an upward curvature and then become linear. The obtained results as shown in Figure 4(a) indicate the nonlinear upward curvature Stern-Volmer plot, which suggests that both the dynamic and static quenching mechanisms are occurring. Therefore, the observed quenching might be due to the charge or an electron transfer process (dynamic) and formation of complex in the ground state before excitation occurs or aggregation (static). The fluorescence of MoS2 QDs could be quenched by GO or RGO through two possible mechanisms: (a) Charge or electron transfer and (b) Energy transfer (FRET/NSET).

31, 32

In the

case of charge or electron transfer, the charge carriers or electrons are exchanged between the donor and the acceptor, where the overlap of the wave functions between the donor and acceptor is required. The quenching efficiency or the rate of charge/electron transfer decreases exponentially when the distance between fluorophore and quencher increases following the 9 ACS Paragon Plus Environment


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relation of exp[-(2r/L)], where r is the edge to edge distance between donor and acceptor and L is the sum of van-der Waals radii of donor and acceptor. Therefore, when the edge to edge distances increase more than one or two molecular diameters (0.5 – 1 nm), the rate of charge/electron transfer drops to very small values. Energy transfer process can occur through two possible ways which are FRET and NSET. FRET originates from the dipole-dipole interaction, where the overlap of an emission spectrum of the donor and absorption spectrum of the acceptor is required. In FRET process, the rate of energy transfer or energy transfer efficiency ( FRET) depends on 1/d6, where d is the separation distance between the donor and the acceptor. FRET occurs in the separation distance up to 10 nm. However, farFRET beyond 10 nm also reported. 33 In the present case, as shown in Figure S3, there is no spectral overlap between the emission spectrum of MoS2 and the absorption spectrum of GO or RGO. But, it is reported that when GO/RGO is an acceptor or quencher, FRET can be independent of the spectral overlap.

34, 35

Graphene has a unique electronic structure and has been reported to absorb a

significant fraction of incident white light. energy transfer efficiency (

NSET)

36

In NSET process, the rate of energy transfer or

is related to the separation distance by 1/d4. The NSET can

occur even in the longer separation distance than the FRET. As described by Perrson’s theory, 37 NSET requires inter-band electronic transitions instead of resonant interactions between electrons. In general, the energy transfer efficiency ( EnT) can be written as

( EnT) =

( )

In the case of FRET (dipole-dipole energy transfer), n = 6 and surface energy transfer), n = 4 and

= d0. 38


= R0, while for NSET (dipole-

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The results obtained from the steady state and time resolved fluorescence show that the excited state electron transfer and/or energy transfer are responsible for the quenching of MoS2 emission. By comparing the fluorescence quenching of MoS2 with GO and RGO, it is observed that the quenching efficiency is higher in the presence of GO than in the presence of the RGO. Even though both GO and RGO are good electron acceptors, GO has a higher capacity than RGO to consume more electrons due to its oxidation state. The photoexcited MoS2 transfers the electrons to GO and reduces to rGO. Here, rGO represents the GO reduced by electron transfer from photoexcited MoS2. MoS2 + h

MoS2 (e+h)

(1)

MoS2(e) + GO

MoS2 + rGO

MoS2(e) +RGO

MoS2 + RGO (e)

(2) (3)

As mentioned in reaction 3, further transfer of electrons to reduced GO leads to electron storage. The fluorescence quenching of MoS2 by GO is governed by both reduction and electron storage as mentioned in the reactions (2) and (3). The quenching by RGO might be as mentioned in the reaction (3). This supports the obtained results that the GO quenches more efficiently than the RGO at all concentrations as mentioned in Figure 4(a) and proving its higher electron accepting ability. If the energy transfer process is considered as predominant for the observed quenching, then the increased optical density of RGO over GO should enable improved overlap with MoS2 QDs fluorescence and resulting in higher quenching efficiency than GO. But in the present case, GO is found to be an effective quencher than the RGO. This evidences that the



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electron transfer severs to control the excited state deactivation, in addition to the energy transfer pathways, from MoS2 to GO and RGO. The obtained time resolved fluorescence results show that the fast time decay component (τ3) decreases from 0.50 ns to 0.25 ns upon the range of GO added from 0 to 600 L and its contribution to the overall fluorescence lifetime increases from 17.11% to 34.75 %. Similarly, the fast time decay component (τ3) decreases from 0.50 ns to 0.19 ns upon the range of RGO added from 0 to 600 L and its contribution to the overall fluorescence lifetime increases from 17.11% to 40.68 %.This enhanced contribution from the fast decay component is attributed to the fluorescence quenching of MoS2 by GO and RGO through electron and energy transfer, and also shows evidence for significant interaction between them. The rate of non-radiative excited state decay (KNRD) is the sum of the rate of electron transfer (Kelectron) and the rate of energy transfer (Kenergy). KNRD = Kelectron + Kenergy. The KNRD can be calculated using the following relation. KNRD =

-

The calculated KNRD is found to be 2 x 109 S-1 and 3.2631 x 109 S-1 in the presence of acceptors GO and RGO respectively. The higher Kenergy might be reason for the high KNRD obtained in case of RGO as an acceptor. In the case of energy transfer between MoS 2 and RGO, Kenergy >> Kelectron. Therefore, KNRD is almost equal to Kenergy. It is believed that, in the present case, the energy transfer process might occur through NSET mechanism similar to the energy transfer from organic dye molecule to graphene as reported earlier. 12 ACS Paragon Plus Environment

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The lifetime of the slow decay components τ1 and τ2 show variations from 3.51ns to 2.85 ns and 11.46 ns to 10.66 ns respectively and also their contributions observed to be decreasing. As the amount of GO or RGO increases, the contributions of the slow decay components decreases. These slow decay components, mainly arises from the free MoS2 QDs in the suspension. The obtained decreasing pre-exponential contribution is attributed to the decrease in the number of free MoS2 QDs, upon increasing GO/RGO concentration. These results also confirm the strong interactive nature of GO and RGO with MoS2 QDs. Turn ‘ON’ BSA sensing: To extend the significant interaction between the MoS2 and GO/RGO towards the potential BSA sensing applications, we studied the fluorescence response of fixed MoS 2 – GO/RGO pair (where the amount of GO/RGO in MoS2 is 300 L) in the presence of different concentrations of BSA. Figure 5(a) depicts the fluorescence spectra of fixed MoS2 – GO pair in the presence of BSA where the concentration of BSA is varied from 5 to 50 nM. When BSA is added to the MoS2-GO, a gradual recovery of the quenched fluorescence is observed with increasing concentration of BSA from 5 to 50 nM. The arrow line facing upwards in Figure 5(a) indicates the enhancement of fluorescence intensity upon increasing concentration of BSA. The change in fluorescence intensity (F-F0) of MoS2 – GO is plotted against the concentration of BSA which is depicted in Figure 5(b). F0 and F are the fluorescence intensity of MoS2 – GO in the absence and presence of BSA respectively. It is observed that the linear variation in fluorescence intensity upon increasing BSA concentration. Figure 5(c) depicts the fluorescence spectra of fixed MoS2 – RGO pair in the presence of BSA where the concentration of BSA is varied from 5 to 50 nM. When BSA is added to the MoS2-RGO, a gradual recovery of the quenched fluorescence is observed with increasing concentration of BSA from 5 to 50 nM. The arrow line facing upwards 13 ACS Paragon Plus Environment


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in Figure 5(c) indicates the enhancement of fluorescence intensity upon increasing concentration of BSA. The change in fluorescence intensity (F-F0) of MoS2 – GO is plotted against the concentration of BSA which is depicted in Figure 5(d). F0 and F are the fluorescence intensity of MoS2 – RGO in the absence and presence of BSA respectively. It is observed that the linear variation in fluorescence intensity upon increasing BSA concentration. The calibrated curves (concentration of BSA ( ) vs change in the fluorescence intensity of the MoS2 –GO/RGO ( ) is fitted using regression line equation by (1)

where

(∑

∑

)

and b

∑

̅̅

∑ ̅

.

The correlation coefficient ( ) is calculated from the relation ∑ √∑

∑

∑

∑

( ∑

(2) (∑

) )

Where n is the total number of data points, xi are x values and yi are y values. The calculated regression coefficient is found to be 0.9966 and 0.9918 for increasing BSA concentration in MoS2-GO and MoS2-RGO respectively. Figure 5(e) and 5(f) depict the turn ON sensing behavior of MoS2 - GO and MoS2 – RGO respectively. The quenched fluorescence of MoS2- GO and MoS2 – RGO is recovered in the presence of BSA and a slight shift towards the higher wavelength side is observed. The BSA added in MoS2- GO and MoS2 – RGO turns ON the quenched fluorescence. This might be due to the reason that the added BSA stacked with non-covalent π-π stacking with the rGO ( GO 14 ACS Paragon Plus Environment

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reduced by the transfer of electrons from MoS2) and reduces the further storage of excess electrons from MoS2. Similarly, in the case of BSA added in MoS2-RGO, the non-covalent covalent π-π stacking between the RGO and BSA decrease the electron/energy accepting nature. The interaction between Sulfur rich MoS2 and BSA/HSA also cannot be negligible and also quietly contribute for the observed fluorescence recovery. Thus, the quenching of fluorescence is controlled. Selectivity studies: To investigate the selectivity of the proposed approach towards the sensing of BSA, the fluorescence response of MoS2-GO/RGO is tested in the presence of molecules such as HSA, glutathione (GSH), glucose, maltose, fructose, lactose, ethanol, methanol, ethanediol, H2O2 and chloroform. Figure 6 depicts the fluorescence response with the molecules added where the concentration of each molecule is 50 nM. As shown in the Figure 6, enhancement of fluorescence signal or recovery (turn ON) response is observed only in the presence of BSA and HSA. Since HSA is similar to BSA in terms of structure and composition, it is responding similar to BSA and showing its interaction with MoS2 – GO/RGO. These results demonstrate that our approach has potential application towards BSA and HSA sensing. Conclusion: In summary, we extensively explored the excited state interactions between the MoS 2 QDs and GO/RGO. The gradual quenching of MoS2 fluorescence upon addition of GO and RGO was observed from the steady state fluorescence measurements. The occurrence of both static and dynamic quenching mechanisms was observed from the Stern-Volmer plots. GO was found to have a more effective quenching character than RGO and showed the evidence for its high 15 ACS Paragon Plus Environment


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electron capture ability. The quenching of MoS2 QDs fluorescence in the presence of GO and RGO was attributed to the reduction and electron storage reactions respectively. The gradual reduction in the lifetime of a fast decay component was observed from the time resolved fluorescence measurements which showed the evidence for energy transfer from MoS2 to GO/RGO in addition to the electron transfer process. From the calculated non-radiative decay rates for MoS2 – GO and MoS2 – RGO, it was observed that the energy transfer process was dominating for the later one. In the presence of BSA and HSA, the quenched fluorescence of MoS2 by GO/RGO was recovered back. The gradual enhancement in fluorescence intensity was found upon increasing concentration of BSA and HSA in both MoS2-GO and MoS2- RGO couples, which showed the turn-ON sensing application of them. Supporting information: X-ray diffraction pattern of the prepared MoS2, GO and RGO. TEM image of prepared MoS2 QDs. Spectral overlap between MoS2 emission and GO, RGO absorption. Table contains fitted parameters for the measured time resolved fluorescence spectra. Acknowledgement: Authors thank the National Institute of Technology, Tiruchirappalli for funding and accessing the facilities in the institute. References: 1. Li, J.; Wu, N. Biosensors Based on Nanomaterials and Nanodevices. 2013, CRC Press (ISBN: 978-1-4665-5152-7, 978-1-4665-5151-0). 2. Stanisavljevic, M.; Krizkova, S.; Vaculovicova, M.; Kizek, R., Adam, V. Quantum DotsFluorescence Resonance Energy Transfer-Based Nanosensors and Their Application. Biosens. Bioelectron. 2015, 74, 562–574. 16 ACS Paragon Plus Environment

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Figures: Figure 1:

Adding GO in MoS2

6

MoS2

6.0x10

MoS2 + GO 100 MoS2 + GO 200 MoS2 + GO 300

Intensity(CPS)

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


MoS2 + GO 400 MoS2 + GO 500

6

4.0x10

MoS2 + GO 600

6

2.0x10

0.0 350

400

450

500

550

600

Wavelength (nm)

Figure 1: Steady state fluorescence spectra of MoS2 QDs alone and MoS2 QDs upon increasing amount of GO from 100 to 600 L. Excitation is done at 330 nm and emission is recorded from 345 to 600 nm. The arrow line indicates the quenching of MoS2 fluorescence upon addition of GO.



Figure 2:

Adding RGO in MoS2 6

MoS2

6.0x10

MoS2 + RGO 100 MoS2 + RGO 200

Intensity(CPS)

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

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6

4.0x10


6

2.0x10

0.0 350

400

450

500

550

600

Wavelength (nm) Figure 2: Steady state fluorescence spectra of MoS2 QDs alone and MoS2 QDs upon increasing amount of RGO from 100 to 600 L. Excitation is done at 330 nm and emission is recorded from 345 to 600 nm. The arrow line indicates the quenching of MoS2 fluorescence upon addition of RGO.


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Figure 3:

Log Photon counts

(a)

Log Photon counts

1000

MoS2

1000

100

MoS2 + GO 10

25 30 35 40 45 50 55 60 65 70 75 Time (ns)

100

MoS2 + GO 10

Increasing concentration of GO in MoS2 QDs 25

30

35

40

45

50

55

60

65

70

75

Time (ns)

Log Photon counts

(b)

1000

Log Photon counts

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


1000

MoS2

100

MoS2 + RGO 10

25 30 35 40 45 50 55 60 65 70 75 Time (ns)

100

MoS2 + RGO 10

Increasing concentration of RGO in MoS2 QDs 25

30

35

40

45

50

55

60

Time (ns)


65

70

75


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

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Figure 3: (a) Time resolved fluorescence spectra of MoS2 QDs alone and MoS2 QDs upon increasing amount of GO from 100 to 600 L. Excitation is done at 330 nm using ns diode laser and emission wavelength is fixed at 400 nm. The arrow line indicates reduction in the life time of MoS2 fluorescence upon addition of RGO. Inset of (a) depicts the time resolved fluorescence spectra of MoS2 QDs alone and MoS2 QDs added with 600 L of GO. (b) Time resolved fluorescence spectra of MoS2 QDs alone and MoS2 QDs upon increasing amount of RGO from 100 to 600 L. Excitation is done at 330 nm using ns diode laser and emission wavelength is fixed at 400 nm. The arrow line indicates reduction in the life time of MoS2 fluorescence upon addition of RGO. Inset of (b) depicts the time resolved fluorescence spectra of MoS2 QDs alone and MoS2 QDs added with 600 L of GO.


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Figure 4:

(a)

(b)

(c)

9

7

MoS2+RGO

2

1.8

0/

5 4

1.6

1

2.4

2

2.2 2.0

3

0/

MoS2+GO

2.6

1

2.0

8

6

F0/F

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


1.4

3

1.8 1.6

3

1.4

1.2

2

1.2 1

1.0

1.0

0 0

100

200

300

400

500

Amount of GO/RGO added (L)

600

0

100

200

300

400

500

Amount of GO added (L)

600

0

100

200

300

400

Figure 4: (a) Variation of F0/F upon addition of GO/RGO from 100 to 600 L, showing the higher quenching ability of GO than RGO at all concentrations (b) Variation of τ0/τ upon addition of GO from 100 to 600 L. (c) Variation of τ0/τ upon addition of RGO from 100 to 600

L.


500

Amount of RGO added (L)

600


Figure 5: (a)

(b)

6

4x10

6

1.2x10

MoS2 - GO pair 6

Pair + BSA 5nM Pair + BSA 10nM Pair + BSA 15nM Pair + BSA 20nM Pair + BSA 25nM Pair + BSA 20nM Pair + BSA 35nM Pair + BSA 40nM Pair + BSA 45nM Pair + BSA 50nM

5

8.0x10

F-F0

6

2x10

1.0x10

F0-F

6

3x10

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6.0x10

5

4.0x10

5

6

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1x10

0.0

5

0 360

400

440

480

520

560

10

15

600

20

25

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45

50

Concentration of BSA (nM)

Wavelength (nm)

(C)

(d)

6

5x10

6

2.2x10

6

2.0x10

MoS2-RGO Pair

6

4x10

Pair + BSA 5nM Pair + BSA 10nM Pair + BSA 15nM Pair + BSA 20nM Pair + BSA 25nM Pair + BSA 30nM Pair + BSA 35nM Pair + BSA 40nM Pair + BSA 45nM Pair + BSA 50nM

6

2x10

6

1.6x10

6

1.4x10

F-F0 F-F0

3x10

6

1.8x10

F0-F

6

Intensity

Intensity

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

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6

1.2x10

6

1.0x10

5

8.0x10

5

6

6.0x10

1x10

5

4.0x10

5

2.0x10

0 360

420

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600

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5

10

15

20

25

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Concentration of BSA


40 (nM)

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(e)

(f) MoS2

6

6

6x10

Turn on sensing

6x10

MoS2

6

5x10

Turn on sensing

6

5x10

Intensity(CPS)

MoS2 + RGO + BSA

Intensity(CPS)

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


6

4x10

MoS2 + GO + BSA

6

3x10

MoS2 + GO 6

2x10

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4x10

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3x10

MoS2 + RGO

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1x10

0

0

350

400

450

500

550

350

600

400

450

500

550

600

Wavelength (nm)

Wavelength (nm)

Figure 5: (a) Fluorescence spectra of MoS2-GO couple upon on addition of BSA from 5 to 50 nM. (b) Plot of change in fluorescence intensity (F-F0) vs concentration of BSA increasing from 5 to 50 nM. (c) Fluorescence spectra of MoS2-RGO couple upon on addition of BSA from 5 to 50 nM. (d) Plot of change in fluorescence intensity (F-F0) vs concentration of BSA increasing from 5 to 50 nM. (e) Turn ON BSA sensing behavior of MoS2-GO couple (f) Turn ON BSA sensing behavior of MoS2-RGO couple. The arrow line facing upwards indicates the enhancement of fluorescence intensity and the dotted lines represent the shift in wavelength.



Figure 6:

2.0

MoS2 - GO MoS2 - RGO 1.5

F-F0

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

1.0

0.5

0.0

A BS

A H se tose tose anol anol diol 2O2 orm se HS GS luco alto c H c Eth ne th rof M La G Fru Me Etha olo Ch

Molecules added in MoS2 - GO/RGO

Figure 6: Selectivity of the proposed approach. Concentration of each molecule added is 50 nM.


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Table of Contents Graphic:

Electron, Energy Transfer Dynamics between MoS2 and GO, RGO and Turn ON BSA, HSA Sensing


Excited-State Electron and Energy Transfer ... - ACS Publications

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