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Enhanced Radiative Decay Rate of Confined Green Fluorescent Protein in Polyvinylpyrrolidone-based Nanofiber Sabriye Acikgoz, Yakup Ulusu, Hasan Yungevis, Faruk Ozel, Abdurrahman Ozen, Isa Gokce, Koray Kara, and Mahmut Kus J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04074 • Publication Date (Web): 14 Jul 2016 Downloaded from http://pubs.acs.org on July 16, 2016

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The Journal of Physical Chemistry

Enhanced Radiative Decay Rate of Confined Green Fluorescent Protein in Polyvinylpyrrolidone-based Nanofiber

Sabriye Acikgoz,1,* Yakup Ulusu,2 Hasan Yungevis,1 Faruk Ozel,1 Abdurrahman Özen,1 Isa Gokce,3 Koray Kara,4 Mahmut Kuş4 1

Department of Metallurgical and Materials Engineering, Faculty of Engineering, Karamanoğlu Mehmetbey University, 70100 Karaman, Turkey

2

Department of Bioengineering, Faculty of Engineering, Karamanoğlu Mehmetbey University, 70100 Karaman, Turkey

3

Department of Bioengineering, Faculty of Engineering, Gaziosmanpaşa University, 60240 Tokat, Turkey 4

Advanced Technology Research and Application Center & Department of Chemical Engineering, Faculty of Engineering, Selcuk University, 42100 Konya, Turkey

Corresponding Author * E-mail: [email protected] Phone: (+90) 338 2262000 Address: Department of Material Science and Engineering, Faculty of Engineering, Karamanoğlu Mehmetbey University, 70100 Karaman, Turkey.

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ABSTRACT Green fluorescent protein (GFP) molecules are encapsulated by Polyvinylpyrrolidone (PVP) material in the form of nanofibers to study their diameter dependence of the fluorescence decay rate. It is investigated that the fluorescence dynamics of the confined GFP is governed by the Purcell effect. It is demonstrated that the electrospun nanofibers are quite controllable geometries and are suitable local photonic environments for exploring such effects. The chromophore of GFP, responsible for the intense green fluorescence, is attached to the alpha-helix and perfectly surrounded by an eleven-stranded beta-barrel cylinder. It is clearly observed that the molecular structures of the confined GFP protein molecules are well protected and are able to maintain their fluorescence properties.


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I. INTRODUCTION Radiative decay rate of a fluorescent molecule depends strongly on the density of propagating photon modes in the surrounding photonic environment. The first experimental work on the modification of decay rate of a molecule inside a confined geometry is performed by Drexhage.1 He has explained the distance dependence of the radiative decay rate of emitters near a metal surfaces. It is reported that the fluorescence lifetime of the emission oscillates as function of the distance away from the planar metal surface. Purcell proposed in 1946 that the mode structure of the vacuum field and local photonic density of states can be dramatically altered in a cavity.2 The physical properties of the cavity determine the enhancement rate of the spontaneous emission, which is known as the Purcell effect. This effect is investigated in many different forms of confined geometries such as metal nanocavities, hyperbolic metamaterials, microdisks, micropillars and photonic crystals.3-7 Recently, the enhancement of fluorescence characteristics of protein molecules due to confinement has gained considerable attention. GFP is the most widely used fluorescent protein molecule in biological applications due to its sustaining structure, high stability and the unique spectral properties. GFP exhibits a bright green fluorescence with a peak wavelength at 509 nm when excited with ultraviolet or blue light. The emission of the GFP of the jellyfish Aequora victoria originates from the spontaneous formation of an emitting chromophore inside a rigid β-barrel structure.8 The dynamic behavior and stability of GFP encapsulated in organic nanotubes has been investigated by Kameta et al.9 It is demonstrated that fluorescence emission of GFP chromophore can be greatly enhanced when it is confined in classic rod-like plant virus and octaacid capsule.10,11 Three dimensional cylindrical geometries of polymer nanofibers can be a perfect photonic environment to confine fluorescent dye and protein molecules due to their subwavelength dimensions. Polymer based fibers with diameters in the range of nano to 3 ACS Paragon Plus Environment


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micrometer can be produced by electrospinning method.12 In this method, the resulting fiber morphology can be controlled by changing the polymer solution properties and operating conditions.13 Electrospun nanofibers have been gaining increasing attention with potential applications such as medicine, tissue engineering, drug delivery control, filtration, sensors, energy and environmental protection.14-19 In our previous work, the emission wavelength dependent photonic interaction between a perylene dye molecule and Polymethyl methacrylate-based polymer nanofiber is investigated and excimer formation map of perylene molecules is obtained via fluorescence lifetime imaging microscopy (FLIM) method.20 Although the immobilization of surface-display enhanced green fluorescent protein (eGFP) within nanofibers is investigated, the radiative decay rate of GFP molecules embedded into a nanofiber has not yet been clarified.21 In this work, the modification of the radiative decay rate of GFP molecules encapsulated in PVP-based three-dimensional cylindrical nanofiber (NF) is experimentally demonstrated and the Purcell factor is improved by controllably changing the nanofiber diameter. When a fluorescent molecule is encapsulated in a confined geometry, whose dimensions are much smaller than the radiation wavelength, surface curvature, effective mode volume and quadruple transitions as well as the plasmon resonances become effective on the spontaneous decaying rates. For GFP-NFs hybrid system, effective mode volume can be controlled by the diameter of nanofiber. Therefore, nanofiber diameter plays a crucial role in determining the enhancement rate. Due to the high surface area, nanofibers behave as a novel platform for the immobilization of the biological objects and a nanocontainer for drug delivery systems.


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II. MATERIALS AND METHODS Expression and purification of recombinant GFP:

Expression and purification of hexa-histidine tagged GFP molecule is performed as described elsewhere.22 The purity of GFP protein is evaluated by a 12% sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) and obtained results are given in Figure 1. The absorbance of the protein solution at 280 nm is measured using UV/VIS spectrophotometer (Varian Cary 50 Scan) and protein concentration is calculated using BeerLambert law. The molecular weight of the histidine tagged GFP is 27,883 Dalton.

Figure 1. 12% SDS-PAGE of expressed His-tagged GFP protein. Samples are boiled in the presence of SDS-PAGE buffer before loading onto gel. Lane1. BL21 DE3 (pLysE) cells expressing His-tagged GFP, Lane 2 loading fraction Lanes 3 and 4 are washings Lane 5 molecular weight marker (Thermo scientific PageRuler Plus Prestained protein ladder), Lanes 6, 7, 8 and 9, elution of His-tagged GFP with 300 mM imidazole. Synthesis of fluorescence nanofibers: Fabrication of GFP-PVP polymer composite nanofibers are carried out under atmospheric conditions (T = 296 ºK, RH = 35%) by using GFP solution (10 mg/mL) and Polyvinylpyrrolidone, (PVP, Mw=1300000). A schematic presentation of fabrication 5 ACS Paragon Plus Environment


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process is given in Figure 2. For tuning the fiber diameter, a solution comprising of 0.40, 0.60 and 0.80 g PVP (depending upon the desired fiber diameter) and 100 mg GFP in 10 mL water:ethanol (3:2) mixtures are used. After the solution is transferred into a 2 mL syringe with metallic needle tip which have 21 gauges in diameter, the range between the square plate and needle tip was the fixed to 15 cm. Then the solutions are spun on collecting plate via electrospinning at 10 kV with a feed-rate of 0.25 mL/h using electrospinning system. Spellman SL30 brand DC power supply and New Era pump firm Ne-300 model are used for electrospinning.

Figure 2. Schematic illustration of production procedure for GFP-PVP composite nanofibers. Time-domain fluorescence lifetime measurements: The fluorescence lifetime of a GFP molecule is measured using a Picoharp 300 (Picoquant, GmbH) time correlated single photon counting instrument. Time-domain fluorescence lifetime spectroscopy setup is given in Figure 3. GFP protein molecules are excited by a picosecond pulsed diode laser head with a wavelength of 470 nm (LDH-C-D-470 Picoquant, GmbH). In order to increase laser beam quality and obtain a Gaussian beam illumination, the output of laser is coupled into a single mode optical fiber (Thorlabs, S4056 ACS Paragon Plus Environment

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HP). A dichroic mirror is used to separate the excitation light from the fluorescence emission. The laser beam is focused onto the sample using an extra long working distance microscope objective (Nikon, ELWD 100X). A pinhole is placed in the focal plane; thus the possibility of getting illuminations apart from the focal center is eliminated. Fluorescence emission is collected by a single-photon avalanche diode photodetector (MPD, SPAD).

Figure 3. Time-domain fluorescence lifetime spectroscopy setup. The fluorescence decay curves are analyzed using the FluoFit software (Picoquant, GmbH) based on multi-exponential tailfit model. The fluorescence intensity decays is recovered from the frequency-domain data in terms of a multiexponential model, ∑ ⁄

(1)

where Ai and τi are the amplitude and fluorescence lifetime of each component, respectively. The intensity weighted average lifetime is given by the following formula, 〈〉

∑ ∑

.

(2)

III.RESULTS AND DISCUSSION The surface structure and morphology of the prepared nanofibers are photographed by the scanning electron microscopy (Zeiss Evo, SEM). Figure 4a, 4b and 4c show SEM images 7 ACS Paragon Plus Environment


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of GFP-PVP composite nanofibers with polymer concentrations of 4%, 6% and 8%. It is clearly seen in the SEM pictures that the obtained nanofibers demonstrate a semi-polydisperse distribution with the average size of nanofibers of 40 ± 10 nm, 150 ± 20 nm, and 325 ± 25 nm, respectively. All the nanofibers show a smooth surface structure due to the amorphous nature of the polymers and proteins. Applied voltage and right polymer concentration parameters have crucial effects on the quality of the electrospun nanofibers. For example, if the applied electrical potential of the electrospinning system is below the desired threshold and the required amount of the polymer concentration is insufficient, some beads and entangling structures are observed to occur together with the nanofibers. It is observed that the nanofibers show a uniform appearance and bead-free structure when the polymer concentration is increased from 4% to 8%. Incorporation of GFP fluorescent molecules into PVP-based nanofiber network can be confirmed by energy dispersive X-ray spectroscopy (EDS) technique. In order to decide the most proper element in EDS analysis, various physico-chemical properties of GFP protein molecules are determined by ProtParam bioinformatics computer program. The results obtained from ProtParam bioinformatics program are given in the Supporting Information. According to the atomic composition analysis, GFP amino acid chains contain carbon (C), hydrogen (H), oxygen (O), nitrogen (N) and sulfur (S) atoms. In elemental analysis of the GFP-PVP composite system, the presence of sulfur, which is not present in a PVP polymer, can indicate the presence of GFP within nanofiber. EDS map of sulfur elements in a single GFP-PVP nanofiber is shown in Figure 4d. It is obvious that the S atoms are homogeneously distributed throughout the nanofiber.


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Figure 4. SEM images of GFP-PVP nanofibers with containing of a) 4%, b) 6%, c) 8% polymer and d)EDS map of sulfur in GFP-PVP nanofibers. All scale bars are 200 nm. A Nikon A1R confocal laser scanning microscope system attached to an upright ECLIPSE FN1 machine is used to capture the confocal images of GFP doped nanofibers. Results show that the protein molecules are uniformly distributed within three dimensional nanofiber photonic environments. Figure 5 displays individual nanofibers, which are clearly visible in the confocal microscope images. Additionally, the confocal images evidence that the green fluorescent light of GFP protein molecules is uniformly emitted from the nanofibers membranes.

Figure 5. Confocal pictures of GFP-PVP nanofibers with containing of a) 4%, b) 6% and c) 8% polymer. All scale bars are 3 µm. 9 ACS Paragon Plus Environment


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Localization of GFP molecules within nanofibers is also monitored using our homemade Fluorescence Lifetime Imaging Microscope (FLIM) system. FLIM is a special fluorescence microscopy technique to map the spatial distribution of the nanosecond excited state lifetimes within macroscopic images.23 Moreover, FLIM provides information about the local environments of the dye molecules due to its sensitivity to changes in environmental conditions like pH, refractive index, ion and oxygen concentration.24 The resolution of our home-made FLIM microscope is restricted by diffraction limit of light. Confocal FLIM configuration allows us to visualize fluorescent nanosurfaces with improved spatial resolution and our FLIM microscope generates high resolution images when the minimum distance between two objects is greater than 200 nm. We have performed the FLIM experiments for nanofibers with the diameter of 325 nm only due to the diffraction limited resolution. Intensity and lifetime based FLIM images are given in Figure 6. The lifetime based FLIM image is colored according to the mean fluorescence lifetime in each pixel using a continuous pseudocolor scale ranging from 1 to 4 ns.

Figure 6 (a) Intensity and (b) lifetime based FLIM images of GFP molecules within NF. All scale bars are 1 µm. In this work, GFP molecules are encapsulated within polymer based nanofibers having different average fiber diameters and the fluorescence lifetimes of these molecules in a three dimensional confined photonic geometry are investigated. Nanofibers whose diameters are smaller than the radiation wavelength of the GFP molecules are specially fabricated by an electrospinning method to investigate the diameter dependent photonic interactions between the 10 ACS Paragon Plus Environment

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fluorescent protein molecules and the nanofiber cylindrical photonic geometry. The experimental decay kinetics for different fiber diameters (40, 150 and 325 nm) are given in Figure 7. The radiative decay curves are analyzed using double exponential tailfit model and the best fits are obtained by minimizing χ2 values.

Calculated decay parameters are

summarized in Table 1. The average fluorescence lifetime of GFP molecules embedded in nanofibers are compared to the free GFP molecules. We had studied the decay parameters of free GFP protein molecules and its concentration dependency in elsewhere.22 The fluorescence lifetime of the GFP molecules on glass substrates are independent of fluorescent protein concentration, and the intensity weighted fluorescence lifetime is measured as 2.411 ns. It is observed that all GFP molecules exhibit a shorter average fluorescence lifetime than the free GFP molecules under the same experimental conditions. Moreover, the reduction in the fluorescence lifetime of the GFP molecules increases with decreasing the average nanofiber diameter and the largest reduction from 2.411 to 1.671 ns is observed when the average nanofiber diameter equals to 40 nm.



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Figure 7. (a) Fluorescence lifetime decay curves of GFP on 8% NF (blue), 6% NF (red), 4% NF (yellow). (─) is double-exponential fitting curve. (b) Residuals for fittings.

Table 1. Calculated decay parameters for GFP molecules Sample A1 (au) τ1 (ns) A2 (au) τ2 (ns)

〈〉 (ns)

GFP@Glass GFP@ NF(d=325 nm)

13.88 9.80

3.006 2.126

18.24 0.26

0.903 0.021

2.411 2.127

1.81 1.07

GFP@ NF(d=150 nm)

8.56

2.097

1.2

0.756

2.032

1.04

GFP@ NF(d=40 nm)

7.33

1.807

2.58

0.895

1.671

1.10


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The strong manipulation of the local photonic density of states alters the photophysical properties of the emitters for the dimensions below the wavelength of the radiation. Therefore, a great decrease in the radiative decay rate of the fluorescence molecules is observed when it is embedded into sub-wavelength confined geometry. This effect is firstly proposed by E. M. Purcell in 1946.2 The radiative decay rate of an emitter in an optical three dimensional cavity increases relative to that in a homogeneous medium by a certain factor, which is known as the Purcell factor (FP).

!

"

#

(3)

$%&&

Where λ is the radiation wavelength, Q is the quality factor, n refractive index of the medium and Veff is the effective mode volume. For a large Purcell enhancement effect, the effective mode volume should be smaller than

!

" , or approximately

!

" in each dimension. For a

confined cylindrical geometry, the effective mode volume can be defined as '()) ()) *()) +,-()) .*())

(4)

where ()) is the effective mode area, *()) and -()) are the length and radius of the cylinder, respectively.25 It is obvious that the Purcell factor is inversely proportional to the radius of the cylinder. The enhancement of the radiative decay rates of the GFP fluorescent protein molecules within nanofiber can be attributed to the Purcell effect. In GFP-PVP composite system, the effective mode area is altered by the nanofiber diameter. Time resolved experiments show that the decrease in effective mode volume causes an increase in spontaneous emission rate of the GFP molecules. The Purcell factor is also given by the ratio of the decay rate / in the vicinity of the cavity to the decay rate of the same emitter in free space / .



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0

(5)

01

Using this relation, the Purcell factor for the GFP molecules, which are restricted in cylindrical geometry, can be obtained from the fluorescence lifetime of the GFP. Calculated effective mode area and the Purcell factor values for different nanofiber diameters are given in Table 2. When the radiation wavelength of the GFP and refractive index of Polyvinylpyrrolidone are taken as 509 nm and 1.53, two dimensional Purcell limit can be calculated as

!

" 110675 7 1089: . It is observed that the effective mode area values for

nanofibers are smaller than the Purcell limit. The relation between the Purcell factor and the nanofiber diameter is given in Figure 8. It is obvious that the Purcell factor is highly enhanced with the decrease of the nanofiber diameter. Table 2. Calculated Purcell Factors NF Diameter d (nm)

40 150 325

Effective mode area

()) ,; ⁄2

1256 × 10

2

(m )

-18

17671 × 10 82958× 10

-18

-18

Decay rate

Purcell Factor

/ 1⁄ (s ) -1

/ ⁄/

0.598 × 10

9

1.44

0.492 × 10

9

1.19

0.470 × 10

9

1.13

Figure 8. Purcell factor (FP) vs nanofiber diameter (d).


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IV. CONCLUSION Fluorescence dynamics of the GFP molecules confined in polymer nanofiber structures is investigated by a time correlated single photon counting technique. Time-resolved experiments show that the fluorescence decay rate of the GFP molecules is enhanced when the protein molecules are encapsulated in three dimensional photonic environments. It is demonstrated that the Purcell enhancement factor is improved by decreasing the nanofiber diameter. This work is the first experimental verification of the size dependent photonic interactions of nanofibers and fluorescent protein molecules. Controlling the spontaneous emission rate of the protein molecules in nanostructured geometries can be quite useful for various biological and photonic applications, such as imaging in the living cell, tissues engineering, sensing, drug delivery, bio-catalysis, bio-lasing and energy harvesting. Acknowledgments : This work was supported by TUBITAK (Contract numbers: 114F451).

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