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Energy transfer kinetics in Basic Fuchsin dye sensitized CdS quantum dots
Materials Chemistry and Physics 242 (2020) 122560
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Energy transfer kinetics in Basic Fuchsin dye sensitized CdS quantum dots Alina C. Kuriakose *, V.P.N. Nampoori, Sheenu Thomas International School of Photonics, Cochin University of Science and Technology, Cochin, 682022, Kerala, India
H I G H L I G H T S
� We prepared CdS QDs by chemical method. � F€ orster type energy transfer is observed in QD-dye pair. � FRET parameters are determined. � This study enhances the scope in nanosensors and in light harvesting systems. � This QD-dye pair can be used in subcellular imaging and in photodynamic therapy. A R T I C L E I N F O
A B S T R A C T
Keywords: Basic fuchsin CdS QDs Fluorescence quenching Energy transfer
Sensitization of QDs (Quantum Dots) with a dye of good spectral overlap maximizes and extends the absorption range of incident photons. Interactions of Cadmium Sulfide Quantum Dots (CdS QDs) with Basic Fuchsin (BF) dye has been investigated in the present work using steady state and time resolved fluorescence emission techniques. The analysis has led to the inference that fluorescence resonance (F€ orster type) energy transfer (FRET) is the mechanism behind the fluorescence quenching of CdS QDs in the presence of dye molecules. An efficient energy transfer (~90%) from CdS donor to BF dye acceptor was observed and the FRET parameters such as F€ orster distance and spectral overlap integral were determined. The non-radiative energy transfer mechanism in QDsdye binary mixtures holds great promise in analytical chemistry, biodetection system and in light harvesting systems.
1. Introduction FRET is a non-radiative process in which energy is transferred from an excited state donor to a ground state acceptor through long range dipole interactions [1–4]. The rate of energy transfer depends on factors like spectral overlap, distance between donor and acceptor molecules and the relative orientation of the transition dipoles [5,6]. The elec tronic coupling by FRET is enabled by the spectral overlap between the photoluminescence (PL) of the donor and the absorption of the acceptor [7]. FRET is very appealing for the fabrication of nano sensors and has been widely used for photosensitization in various biological systems [8]. FRET studies has been conducted mostly on several organic dyes, fluorescent proteins, polymers and inorganic nanocrystals for various potential applications. There are reports that FRET systems with QDs as donors can overcome the limitations associated with organic molecules when used in pairs. Only few studies exist on FRET mechanism between
QDs and dyes. The use of semiconductor QDs in FRET has recently been proposed to improve its efficiency [9]. The unique photophysical properties of semiconductor QDs like high photo stability, broad tunable absorption spectra, negligible photo bleaching and longer excited state life time make them an ideal photonic material for applications in op tical sensing and photonics [8,10]. FRET based on QD is a significant phenomenon in fluorescence spectroscopy and it can be used as a spectroscopic ruler for the measurement of distances between donors and acceptors. It is a powerful photophysical tool to investigate dis tances in the range of 10- 100 Å [11,12]. The extended optical absorption spectra of the QDs allows efficient excitation at different wavelengths. Dye molecules that serve as FRET acceptor possess a narrow absorption band which allow selective optical excitation of the QD behind the optical absorption of the dye molecules [13]. For dye sensitized solar cell (DSSC) applications, extension of the optical absorption spectra of dyes is important to enhance the efficiency [14]. FRET based DSSCs employing QD donor and dye acceptor allows
* Corresponding author. E-mail address: [email protected] (A.C. Kuriakose). https://doi.org/10.1016/j.matchemphys.2019.122560 Received 25 June 2018; Received in revised form 17 May 2019; Accepted 13 December 2019 Available online 14 December 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
A.C. Kuriakose et al.
Materials Chemistry and Physics 242 (2020) 122560
Scheme 1. Structure of Basic Fuchsin.
simultaneous utilization of wide solar spectrum and higher conversion efficiency. One of the prerequisite for FRET to occur is that donor and acceptor molecules must be in close proximity, apart from the spectral overlap between the fluorescence emission spectrum of the donor and the absorption spectrum of the acceptor [15–20]. FRET studies between QDs and dye molecule has recently gained interest with a view to broaden its potential applications in the area of material science [21,22]. Mutluganet al. has developed a FRET system with CdTe QDs and RhB dye for light harvesting in solutions where the QDs act as optical antennas for the dye acceptor molecules [23]. The use of CdSe/CdS QDs donor and dye acceptor in FRET based dye sensitized solar cells has been demonstrated by Lee et al. [24]. There are reports on the use of QD-dye as potential pairs to enhance the luminescent solar concentration efficiency using FRET by Tummeltshammer et al. [25]. Dey et al. have reported FRET between CdS and Eosin dye for the sensing of a toxic pesticide chlorpyrifos in water [26]. QDs used as donors in FRET can be further developed as nano biochemical and fluorescent sensors. Energy transfer studies in QD-dye pair enhance the scope of FRET analysis in multiplexed biodetection systems. Recently, the potential applications of the semiconductor QDs have been extended in the areas of medical diagnostics, luminescence tagging and imaging. Nikki et al. have reported the use of Basic Fuchsin (BF) for imaging that reveals subcellular patterning and ecotype varia tion of lignification in Brachypodium distachyon [27]. In many of the biological imaging, dye incorporated subcellular patterning is important especially when analyzing pathological cases. It has been reported that the incorporation of nanoparticles in dye modifies the properties of subcellular imaging [28]. One of the important parameters for such imaging application of dyes is their fluorescence quantum yield. Hence dye incorporated quantum dot find tremendous applications in imaging and related fields. Such energy transfer may sometimes reduce radiative relaxation process of the dye thereby enhancing its thermal energy contribution to the medium. In the present work, we have studied the FRET mechanism between the CdS QDs and Basic Fuchsin dye. Basic fuchsin is a triaminotriphenylmethane dye with molecular formula C20H20CIN3(Scheme 1). This dye is been widely used as a colouring agent in textile industry and as a stain in cell biology. We have chosen this QD-dye pair for our FRET study since a highly efficient energy transfer was expected on observing strong overlap be tween the emission spectrum of QDs and the absorption spectrum of dye. With a view to extend the absorption range in solar applications and also to initiate research on QD based FRET sensors and in subcellular inter action studies using QD-dye conjugates, we have taken up the study on CdS QDs as donors and BF as acceptor.
Fig. 1a. TEM image of CdS QDs.
Fig. 1b. Statistical particle size distribution.
above reaction mixture was then kept at 80 � C for 45 min with rapid stirring. The CdS nanoparticles thus formed was washed, filtered and dried. A series of CdS: BF binary mixtures were prepared in distilled water by mixing different concentrations of BF with a fixed concentra tion of CdS. Absorption and fluorescence spectra of the prepared sam ples were recorded using JascoV570 UV/Vis/IR spectrophotometer and a Varian Cary Eclipse spectrofluorimeter respectively. The size of CdS QDs were determined with Transmission Electron Microscopy (TEM) instrument Philips Tecnai G2 operating at 120 kV. For the fluorescence lifetime measurements, time-correlated single photon counting (TCSPC) spectrometer was used (HoribaJobinYvon). All the samples were excited at 367 nm and the fluorescence decays were deconvoluted using the DAS6 software. By the inspection of residuals and χ2 (a statistical parameter), the goodness of fit has been determined. The fluorescence quantum yield (QY) of CdS QDs were obtained experimentally by using sodium fluorescein as the reference dye (QY ¼ 0.9) [30].
1.1. Experimental Cadmium acetate dihydrate (Merck), thiourea (SRL) and trietha nolamine (TEA), Basic Fuchsin(SRL) were purchased and used without further purification. The synthesis of semiconductor CdS QDs is described elsewhere [29]. Briefly, cadmium acetate and thiourea were mixed together and stirred using a magnetic stirrer. TEA has been used as the capping agent and pH was adjusted to 10.5 by adding NH4OH. The 2
A.C. Kuriakose et al.
Materials Chemistry and Physics 242 (2020) 122560
12
10
I0/I
8
6
4
KSV= 15.88 + 0.05 x 106M-1 R2 = 0.99
2 0.1 Fig. 2. Spectral overlap between the fluorescence spectrum of CdS and exci tation spectrum of BF.
0.2
0.3
0.4
0.5
0.6
0.7
Concentration of BF (µM) Fig. 4. Stern-Volmer plot of I0/I against BF concentration.
with increase in the concentration of BF. Several mechanisms like collisional processes, complex formation and energy transfer processes have been reported for the quenching effects [32,33]. Due to the good spectral overlap between emission spectrum of the donor and the ab sorption spectrum of acceptor, the observed quenching of the donor can be attributed to the non-radiative resonance energy transfer from CdS to the dye. The dye does not show fluorescence emission at 400 nm exci tation due to the absence of absorption in this spectral region. Existence of effective energy transfer between the donor and the acceptor and the absence of fluorescence emission corresponding to the dye implies that the excited dye molecules get de-excited to the ground state through non-radiative relaxation process. The FRET process manifests as a quenching in the fluorescence in tensity of the donor and the quenching efficiency can be measured experimentally either by monitoring the fluorescence intensity changes in the donor or from the changes in the fluorescence lifetimes of the donor in the absence and presence of the acceptor. The fluorescence quenching of the CdS QDs is described by Stern-Volmer equation [31]. I0 ¼ 1 þ KSV ½Q� I
Fig. 3. Fluorescence quenching of CdS with increasing concentration of BF at an excitation wavelength of 400 nm.
2. Results and discussions The TEM image reveal CdS QDs as spherical in shape (Fig. 1a) with an average diameter of 4 � 0.03 nm from the histogram distribution as shown in Fig. 1b. The fluorescence spectra of CdS and the absorption spectra of BF is shown in Fig. 2. The fluorescence peak of CdS is observed at 536 nm on excitation with 400 nm while an absorption peak at 545 nm is observed for BF. The emission of CdS at 536 nm is due to band edge transitions. The spectral overlap between the donor (CdS QDs) emission spec trum and the acceptor (BF dye) absorption spectrum is evident in Fig. 2 proving them to be a good donor – acceptor pair for the FRET study. The spectral overlap integral is given by [31]. Z ∞ JðλÞ ¼ FD ðλÞεA ðλÞλ4 dλ M 1 cm 1 (1) 0
where ðλÞ is the fluorescence intensity of the donor at wavelength λ and
εA ðλÞ is the molar absorption coefficient of the acceptor at λ (M 1cm 1).
The calculated spectral overlap is 7.191 � 1014 nm4 M 1cm 1. The fluorescence quenching of CdS QDs (1.76 mM) in the presence of BF is shown in Fig. 3. It is seen that the PL intensity of CdS QDs decreases
Fig. 5. Energy transfer efficiency Vs. BF concentration. 3
(2)
A.C. Kuriakose et al.
Materials Chemistry and Physics 242 (2020) 122560
Table 1 Decay parameters of CdS in the presence and absence of BF. Sample
τ1 (ns)
(a1)
τ2 (ns)
(a2)
τ3(ns)
(a3)
<τ> ns
CdS
3.08 � 0.07 0.11 � 0.01
(81%)
0.47 � 0.01 1.03 � 0.06
(17%)
12.9 � 0.17 3.76 � 0.05
(2%)
1.14
(1%)
0.17
CdS þ BF
(97%)
(2%)
2.1. Time - resolved measurements Time resolved measurements were performed to confirm the energy transfer from QDs to dye molecules as decay measurements provide significant information about the molecular interactions. Fig. 6 shows the fluorescence decay curves of CdS QDs in the presence and absence of BF dye. The effect of dye on the lifetime properties of CdS QDs has been studied. Nonradiative energy transfer is expected to alter the lifetime of the donor. The fluorescence decay data I (t) was analysed using X t αi e =τi (7) IðtÞ ¼
Fig. 6. Fluorescence decay curve of CdS in the presence and absence of BF.
where I0 is the fluorescence intensity of the donor alone, I is the fluo rescence intensity of the donor in the presence of acceptor and [Q] is the quencher concentration. The Stern - Volmer plot of I0/I vs concentration of BF is shown in Fig. 4. Stern-Volmer constant, Ksv from the slope of the plot was found to be 15.88 � 0.05 � 106 M 1. €rster’s theory, the energy transfer efficiency E is According to Fo €rster critical energy transfer distance R0 and the center related to the Fo to center distance’r’ by [31]. E¼
R0 6 R0 þ r 6
i
where αi corresponds to the amplitude of the ith exponential component and τi is the corresponding fluorescence lifetime. CdS QDs exhibited triple exponential decay curve with an average decay time of 1.14 ns. In the presence of dye acceptor (0.7 μM BF) decay time is shortened to 0.17ns. The observed decrease in lifetime of the donor CdS QDs upon addition of BF dye further confirmed the energy transfer from CdS to BF. The three decay time components with pre exponential values obtained are tabulated below (See. Table 1). It has to be noted that the fast decay component shows reduction in the lifetime from 3.08 ns to 0.11ns indicating an effective molecular interaction between donor and acceptor. The FRET efficiency can also be determined from time resolved studies by measuring the reduced life time of the donor in the presence of acceptor using [31].
(3)
6
FRET efficiency can also be determined as follows [31]. E¼1
I I0
(4)
The energy transfer efficiency plot shown in Fig. 5 reveals that the transfer efficiency is increased with increasing concentration of dye. The fluorescence quenching efficiency of the acceptor calculated from equation (4) is found to be 91%. The magnitude of R0 depends on the spectral properties of the donor and acceptor given by [31]. R0 6 ¼ 8:79*10
5
� 2 4 � 6 k n φD JðλÞ � A
E¼1
(8)
where τDA is the fluorescence lifetime of the donor in the presence of acceptor and τD is the fluorescence lifetime of the donor in the absence of acceptor. The energy transfer efficiency calculated according to equation (8) is 85%.
(5)
here, k2 is the orientation factor, n is the refractive index of the medium, φD is the quantum yield of the donor and J(λ) is the spectrum overlap integral. The QY can be determined from the equation [31]. I s AR n S 2 QS ¼ QR IR AS nR 2
τDA τD
3. Conclusion FRET studies have been carried out in the CdS QDs and BF dye pair. The influence of dye molecules in tailoring the PL characteristics of CdS donors has been experimentally investigated through FRET mechanism and it was found that the prepared CdS QDs are efficient FRET donors for the BF dye. The energy transfer mediated dynamic quenching from the steady state measurements was confirmed by the reduction in the excited state lifetime of the QDs from time-resolved spectroscopy. The €rster associated energy transfer parameters were determined and the Fo distance calculated falls within the FRET limits. Highly efficient energy transfer observed reveal the potential use of the QD-dye pair for the design of biochemical sensors and in light harvesting systems. The enhancement of fluorescence quenching further helps in considering CdS QDs in the presence of dye for subcellular interaction applications.
(6)
where Q is the quantum yield, I is the integrated intensity, A is the absorbance and n is the refractive index. The subscript R and S corre sponds to the reference and sample respectively. It has to be noted that the absorption factors must be accurately determined (absorbance, A< 0.1) in order to eliminate the inner filter effects. With the values, k2 ¼ 2/ 3, n ¼ 1.33, φD ¼ 0.11 and J(λ) ¼ 7.191 � 1014nm4 M 1cm 1, the €rster radius R0 is estimated to be 34.05 Å and the value of ‘r’obtained Fo is 23 Å. The present studies reveal enhancement in the fluorescence quenching efficiency of CdS by BF dye. New fluorescence emission with the presence of dye in CdS QDs in the visible or near IR region were not observed proving that the excitation of CdS yields enhanced non radi ative relaxation resulting in increased thermal energy in the medium. Such enhancement of thermal energy in the medium has practical ap plications. For example, in sub cellular structures, deposition of thermal energy in the medium is helpful for medical imaging and photodynamic therapy applications.
Acknowledgement A C. K is grateful for the Maulana Azad National Fellowship, Uni versity Grants Commission, New Delhi. Author ST thank KSCSTE, Govt of Kerala for research funding.
4
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Materials Chemistry and Physics 242 (2020) 122560
References
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[1] G.A. Kumar, N.V. Unnikrishnan, Energy transfer and optical gain studies of FDS: Rh B dye mixture investigated under cw laser excitation, J. Photochem. Photobiol. A Chem. 144 (2–3) (2001) 107–117. [2] B. Wieb Van Der Meer, George Coker, S.-Y. Simon Chen, Resonance Energy Transfer: Theory and Data, Wiley-VCH, 1994. [3] David L. Andrews, A unified theory of radiative and radiationless molecular energy transfer, Chem. Phys. 135 (2) (1989) 195–201. [4] Wan Zakiah Wan Ismail, Ewa M. Goldys, Judith M. Dawes, Extended emission wavelength of random dye lasers by exploiting radiative and non-radiative energy transfer, Appl. Phys. B 122 (2) (2016) 40. [5] Elizabeth A. Jares-Erijman, Thomas M. Jovin, FRET imaging, Nat. Biotechnol. 21 (11) (2003) 1387. [6] Aaron R. Clapp, et al., Can luminescent quantum dots be efficient energy acceptors with organic dye donors? J. Am. Chem. Soc. 127 (4) (2005) 1242–1250. [7] Paul C. Stanish, Pavle V. Radovanovic, Energy transfer between conjugated colloidal Ga2O3 and CdSe/CdS core/shell nanocrystals for white light emitting applications, Nanomaterials 6 (2) (2016) 32. [8] Gengwen Chen, et al., Fluorescent nanosensors based on fluorescence resonance energy transfer (FRET), Ind. Eng. Chem. Res. 52 (33) (2013) 11228–11245. [9] E. Alphandery, et al., Highly efficient F€ orster resonance energy transfer between CdTenanocrystals and Rhodamine B in mixed solid films, Chem. Phys. Lett. 388 (1–3) (2004) 100–104. [10] Manuela Frasco, Nikos Chaniotakis, Semiconductor quantum dots in chemical sensors and biosensors, Sensors 9 (9) (2009) 7266–7286. [11] M.A. Sepulveda-Becerra, et al., Refolding of triosephosphateisomerase in lowwater media investigated by fluorescence resonance energy transfer, Biochemistry 35 (49) (1996) 15915–15922. [12] Ye Chen, AmmasiPeriasamy, Intensity range based quantitative FRET data analysis to localize protein molecules in live cell nuclei, J. Fluoresc. 16 (1) (2006) 95–104. [13] Mikhail V. Artemyev, Resonance energy transfer in conjugates of semiconductor nanocrystals and organic dye molecules, J. Nanophotonics 6 (1) (2012), 061705. [14] Brian O’regan, Michael Gr€ atzel, A low-cost, high-efficiency solar cell based on dyesensitized colloidal TiO2 films, Nature 353 (6346) (1991) 737. [15] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, second ed., Klawer Academic, New York, 1999. [16] Suparna Sadhu, Masanori Tachiya, AmitavaPatra, A stochastic model for energy transfer from CdS quantum dots/rods (donors) to Nile Red dye (acceptors), J. Phys. Chem. C 113 (45) (2009) 19488–19492. [17] Paresh Chandra Ray, et al., Gold nanoparticle based FRET for DNA detection, Plasmonics 2 (4) (2007) 173–183.
5
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Energy transfer kinetics in Basic Fuchsin dye sensitized CdS quantum dots Alina C. Kuriakose *, V.P.N. Nampoori, Sheenu Thomas International School of Photonics, Cochin University of Science and Technology, Cochin, 682022, Kerala, India
H I G H L I G H T S
� We prepared CdS QDs by chemical method. � F€ orster type energy transfer is observed in QD-dye pair. � FRET parameters are determined. � This study enhances the scope in nanosensors and in light harvesting systems. � This QD-dye pair can be used in subcellular imaging and in photodynamic therapy. A R T I C L E I N F O
A B S T R A C T
Keywords: Basic fuchsin CdS QDs Fluorescence quenching Energy transfer
Sensitization of QDs (Quantum Dots) with a dye of good spectral overlap maximizes and extends the absorption range of incident photons. Interactions of Cadmium Sulfide Quantum Dots (CdS QDs) with Basic Fuchsin (BF) dye has been investigated in the present work using steady state and time resolved fluorescence emission techniques. The analysis has led to the inference that fluorescence resonance (F€ orster type) energy transfer (FRET) is the mechanism behind the fluorescence quenching of CdS QDs in the presence of dye molecules. An efficient energy transfer (~90%) from CdS donor to BF dye acceptor was observed and the FRET parameters such as F€ orster distance and spectral overlap integral were determined. The non-radiative energy transfer mechanism in QDsdye binary mixtures holds great promise in analytical chemistry, biodetection system and in light harvesting systems.
1. Introduction FRET is a non-radiative process in which energy is transferred from an excited state donor to a ground state acceptor through long range dipole interactions [1–4]. The rate of energy transfer depends on factors like spectral overlap, distance between donor and acceptor molecules and the relative orientation of the transition dipoles [5,6]. The elec tronic coupling by FRET is enabled by the spectral overlap between the photoluminescence (PL) of the donor and the absorption of the acceptor [7]. FRET is very appealing for the fabrication of nano sensors and has been widely used for photosensitization in various biological systems [8]. FRET studies has been conducted mostly on several organic dyes, fluorescent proteins, polymers and inorganic nanocrystals for various potential applications. There are reports that FRET systems with QDs as donors can overcome the limitations associated with organic molecules when used in pairs. Only few studies exist on FRET mechanism between
QDs and dyes. The use of semiconductor QDs in FRET has recently been proposed to improve its efficiency [9]. The unique photophysical properties of semiconductor QDs like high photo stability, broad tunable absorption spectra, negligible photo bleaching and longer excited state life time make them an ideal photonic material for applications in op tical sensing and photonics [8,10]. FRET based on QD is a significant phenomenon in fluorescence spectroscopy and it can be used as a spectroscopic ruler for the measurement of distances between donors and acceptors. It is a powerful photophysical tool to investigate dis tances in the range of 10- 100 Å [11,12]. The extended optical absorption spectra of the QDs allows efficient excitation at different wavelengths. Dye molecules that serve as FRET acceptor possess a narrow absorption band which allow selective optical excitation of the QD behind the optical absorption of the dye molecules [13]. For dye sensitized solar cell (DSSC) applications, extension of the optical absorption spectra of dyes is important to enhance the efficiency [14]. FRET based DSSCs employing QD donor and dye acceptor allows
* Corresponding author. E-mail address: [email protected] (A.C. Kuriakose). https://doi.org/10.1016/j.matchemphys.2019.122560 Received 25 June 2018; Received in revised form 17 May 2019; Accepted 13 December 2019 Available online 14 December 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
A.C. Kuriakose et al.
Materials Chemistry and Physics 242 (2020) 122560
Scheme 1. Structure of Basic Fuchsin.
simultaneous utilization of wide solar spectrum and higher conversion efficiency. One of the prerequisite for FRET to occur is that donor and acceptor molecules must be in close proximity, apart from the spectral overlap between the fluorescence emission spectrum of the donor and the absorption spectrum of the acceptor [15–20]. FRET studies between QDs and dye molecule has recently gained interest with a view to broaden its potential applications in the area of material science [21,22]. Mutluganet al. has developed a FRET system with CdTe QDs and RhB dye for light harvesting in solutions where the QDs act as optical antennas for the dye acceptor molecules [23]. The use of CdSe/CdS QDs donor and dye acceptor in FRET based dye sensitized solar cells has been demonstrated by Lee et al. [24]. There are reports on the use of QD-dye as potential pairs to enhance the luminescent solar concentration efficiency using FRET by Tummeltshammer et al. [25]. Dey et al. have reported FRET between CdS and Eosin dye for the sensing of a toxic pesticide chlorpyrifos in water [26]. QDs used as donors in FRET can be further developed as nano biochemical and fluorescent sensors. Energy transfer studies in QD-dye pair enhance the scope of FRET analysis in multiplexed biodetection systems. Recently, the potential applications of the semiconductor QDs have been extended in the areas of medical diagnostics, luminescence tagging and imaging. Nikki et al. have reported the use of Basic Fuchsin (BF) for imaging that reveals subcellular patterning and ecotype varia tion of lignification in Brachypodium distachyon [27]. In many of the biological imaging, dye incorporated subcellular patterning is important especially when analyzing pathological cases. It has been reported that the incorporation of nanoparticles in dye modifies the properties of subcellular imaging [28]. One of the important parameters for such imaging application of dyes is their fluorescence quantum yield. Hence dye incorporated quantum dot find tremendous applications in imaging and related fields. Such energy transfer may sometimes reduce radiative relaxation process of the dye thereby enhancing its thermal energy contribution to the medium. In the present work, we have studied the FRET mechanism between the CdS QDs and Basic Fuchsin dye. Basic fuchsin is a triaminotriphenylmethane dye with molecular formula C20H20CIN3(Scheme 1). This dye is been widely used as a colouring agent in textile industry and as a stain in cell biology. We have chosen this QD-dye pair for our FRET study since a highly efficient energy transfer was expected on observing strong overlap be tween the emission spectrum of QDs and the absorption spectrum of dye. With a view to extend the absorption range in solar applications and also to initiate research on QD based FRET sensors and in subcellular inter action studies using QD-dye conjugates, we have taken up the study on CdS QDs as donors and BF as acceptor.
Fig. 1a. TEM image of CdS QDs.
Fig. 1b. Statistical particle size distribution.
above reaction mixture was then kept at 80 � C for 45 min with rapid stirring. The CdS nanoparticles thus formed was washed, filtered and dried. A series of CdS: BF binary mixtures were prepared in distilled water by mixing different concentrations of BF with a fixed concentra tion of CdS. Absorption and fluorescence spectra of the prepared sam ples were recorded using JascoV570 UV/Vis/IR spectrophotometer and a Varian Cary Eclipse spectrofluorimeter respectively. The size of CdS QDs were determined with Transmission Electron Microscopy (TEM) instrument Philips Tecnai G2 operating at 120 kV. For the fluorescence lifetime measurements, time-correlated single photon counting (TCSPC) spectrometer was used (HoribaJobinYvon). All the samples were excited at 367 nm and the fluorescence decays were deconvoluted using the DAS6 software. By the inspection of residuals and χ2 (a statistical parameter), the goodness of fit has been determined. The fluorescence quantum yield (QY) of CdS QDs were obtained experimentally by using sodium fluorescein as the reference dye (QY ¼ 0.9) [30].
1.1. Experimental Cadmium acetate dihydrate (Merck), thiourea (SRL) and trietha nolamine (TEA), Basic Fuchsin(SRL) were purchased and used without further purification. The synthesis of semiconductor CdS QDs is described elsewhere [29]. Briefly, cadmium acetate and thiourea were mixed together and stirred using a magnetic stirrer. TEA has been used as the capping agent and pH was adjusted to 10.5 by adding NH4OH. The 2
A.C. Kuriakose et al.
Materials Chemistry and Physics 242 (2020) 122560
12
10
I0/I
8
6
4
KSV= 15.88 + 0.05 x 106M-1 R2 = 0.99
2 0.1 Fig. 2. Spectral overlap between the fluorescence spectrum of CdS and exci tation spectrum of BF.
0.2
0.3
0.4
0.5
0.6
0.7
Concentration of BF (µM) Fig. 4. Stern-Volmer plot of I0/I against BF concentration.
with increase in the concentration of BF. Several mechanisms like collisional processes, complex formation and energy transfer processes have been reported for the quenching effects [32,33]. Due to the good spectral overlap between emission spectrum of the donor and the ab sorption spectrum of acceptor, the observed quenching of the donor can be attributed to the non-radiative resonance energy transfer from CdS to the dye. The dye does not show fluorescence emission at 400 nm exci tation due to the absence of absorption in this spectral region. Existence of effective energy transfer between the donor and the acceptor and the absence of fluorescence emission corresponding to the dye implies that the excited dye molecules get de-excited to the ground state through non-radiative relaxation process. The FRET process manifests as a quenching in the fluorescence in tensity of the donor and the quenching efficiency can be measured experimentally either by monitoring the fluorescence intensity changes in the donor or from the changes in the fluorescence lifetimes of the donor in the absence and presence of the acceptor. The fluorescence quenching of the CdS QDs is described by Stern-Volmer equation [31]. I0 ¼ 1 þ KSV ½Q� I
Fig. 3. Fluorescence quenching of CdS with increasing concentration of BF at an excitation wavelength of 400 nm.
2. Results and discussions The TEM image reveal CdS QDs as spherical in shape (Fig. 1a) with an average diameter of 4 � 0.03 nm from the histogram distribution as shown in Fig. 1b. The fluorescence spectra of CdS and the absorption spectra of BF is shown in Fig. 2. The fluorescence peak of CdS is observed at 536 nm on excitation with 400 nm while an absorption peak at 545 nm is observed for BF. The emission of CdS at 536 nm is due to band edge transitions. The spectral overlap between the donor (CdS QDs) emission spec trum and the acceptor (BF dye) absorption spectrum is evident in Fig. 2 proving them to be a good donor – acceptor pair for the FRET study. The spectral overlap integral is given by [31]. Z ∞ JðλÞ ¼ FD ðλÞεA ðλÞλ4 dλ M 1 cm 1 (1) 0
where ðλÞ is the fluorescence intensity of the donor at wavelength λ and
εA ðλÞ is the molar absorption coefficient of the acceptor at λ (M 1cm 1).
The calculated spectral overlap is 7.191 � 1014 nm4 M 1cm 1. The fluorescence quenching of CdS QDs (1.76 mM) in the presence of BF is shown in Fig. 3. It is seen that the PL intensity of CdS QDs decreases
Fig. 5. Energy transfer efficiency Vs. BF concentration. 3
(2)
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Materials Chemistry and Physics 242 (2020) 122560
Table 1 Decay parameters of CdS in the presence and absence of BF. Sample
τ1 (ns)
(a1)
τ2 (ns)
(a2)
τ3(ns)
(a3)
<τ> ns
CdS
3.08 � 0.07 0.11 � 0.01
(81%)
0.47 � 0.01 1.03 � 0.06
(17%)
12.9 � 0.17 3.76 � 0.05
(2%)
1.14
(1%)
0.17
CdS þ BF
(97%)
(2%)
2.1. Time - resolved measurements Time resolved measurements were performed to confirm the energy transfer from QDs to dye molecules as decay measurements provide significant information about the molecular interactions. Fig. 6 shows the fluorescence decay curves of CdS QDs in the presence and absence of BF dye. The effect of dye on the lifetime properties of CdS QDs has been studied. Nonradiative energy transfer is expected to alter the lifetime of the donor. The fluorescence decay data I (t) was analysed using X t αi e =τi (7) IðtÞ ¼
Fig. 6. Fluorescence decay curve of CdS in the presence and absence of BF.
where I0 is the fluorescence intensity of the donor alone, I is the fluo rescence intensity of the donor in the presence of acceptor and [Q] is the quencher concentration. The Stern - Volmer plot of I0/I vs concentration of BF is shown in Fig. 4. Stern-Volmer constant, Ksv from the slope of the plot was found to be 15.88 � 0.05 � 106 M 1. €rster’s theory, the energy transfer efficiency E is According to Fo €rster critical energy transfer distance R0 and the center related to the Fo to center distance’r’ by [31]. E¼
R0 6 R0 þ r 6
i
where αi corresponds to the amplitude of the ith exponential component and τi is the corresponding fluorescence lifetime. CdS QDs exhibited triple exponential decay curve with an average decay time of 1.14 ns. In the presence of dye acceptor (0.7 μM BF) decay time is shortened to 0.17ns. The observed decrease in lifetime of the donor CdS QDs upon addition of BF dye further confirmed the energy transfer from CdS to BF. The three decay time components with pre exponential values obtained are tabulated below (See. Table 1). It has to be noted that the fast decay component shows reduction in the lifetime from 3.08 ns to 0.11ns indicating an effective molecular interaction between donor and acceptor. The FRET efficiency can also be determined from time resolved studies by measuring the reduced life time of the donor in the presence of acceptor using [31].
(3)
6
FRET efficiency can also be determined as follows [31]. E¼1
I I0
(4)
The energy transfer efficiency plot shown in Fig. 5 reveals that the transfer efficiency is increased with increasing concentration of dye. The fluorescence quenching efficiency of the acceptor calculated from equation (4) is found to be 91%. The magnitude of R0 depends on the spectral properties of the donor and acceptor given by [31]. R0 6 ¼ 8:79*10
5
� 2 4 � 6 k n φD JðλÞ � A
E¼1
(8)
where τDA is the fluorescence lifetime of the donor in the presence of acceptor and τD is the fluorescence lifetime of the donor in the absence of acceptor. The energy transfer efficiency calculated according to equation (8) is 85%.
(5)
here, k2 is the orientation factor, n is the refractive index of the medium, φD is the quantum yield of the donor and J(λ) is the spectrum overlap integral. The QY can be determined from the equation [31]. I s AR n S 2 QS ¼ QR IR AS nR 2
τDA τD
3. Conclusion FRET studies have been carried out in the CdS QDs and BF dye pair. The influence of dye molecules in tailoring the PL characteristics of CdS donors has been experimentally investigated through FRET mechanism and it was found that the prepared CdS QDs are efficient FRET donors for the BF dye. The energy transfer mediated dynamic quenching from the steady state measurements was confirmed by the reduction in the excited state lifetime of the QDs from time-resolved spectroscopy. The €rster associated energy transfer parameters were determined and the Fo distance calculated falls within the FRET limits. Highly efficient energy transfer observed reveal the potential use of the QD-dye pair for the design of biochemical sensors and in light harvesting systems. The enhancement of fluorescence quenching further helps in considering CdS QDs in the presence of dye for subcellular interaction applications.
(6)
where Q is the quantum yield, I is the integrated intensity, A is the absorbance and n is the refractive index. The subscript R and S corre sponds to the reference and sample respectively. It has to be noted that the absorption factors must be accurately determined (absorbance, A< 0.1) in order to eliminate the inner filter effects. With the values, k2 ¼ 2/ 3, n ¼ 1.33, φD ¼ 0.11 and J(λ) ¼ 7.191 � 1014nm4 M 1cm 1, the €rster radius R0 is estimated to be 34.05 Å and the value of ‘r’obtained Fo is 23 Å. The present studies reveal enhancement in the fluorescence quenching efficiency of CdS by BF dye. New fluorescence emission with the presence of dye in CdS QDs in the visible or near IR region were not observed proving that the excitation of CdS yields enhanced non radi ative relaxation resulting in increased thermal energy in the medium. Such enhancement of thermal energy in the medium has practical ap plications. For example, in sub cellular structures, deposition of thermal energy in the medium is helpful for medical imaging and photodynamic therapy applications.
Acknowledgement A C. K is grateful for the Maulana Azad National Fellowship, Uni versity Grants Commission, New Delhi. Author ST thank KSCSTE, Govt of Kerala for research funding.
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References
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