Photophysical Properties of Fluorescent Core Dendrimers Controlled

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Photophysical Properties of Fluorescent Core Dendrimers Controlled by Size Valentina Paolucci, Søren Leth Mejlsøe, Mario Ficker, Tom Vosch, and Jørn Bolstad Christensen J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b05354 • Publication Date (Web): 23 Aug 2016 Downloaded from http://pubs.acs.org on August 28, 2016

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The Journal of Physical Chemistry

Photophysical Properties of Fluorescent Core Dendrimers Controlled by Size Valentina Paolucci, a Søren L. Mejlsøe, a Mario Ficker, a Tom Vosch, bJørn B. Christensen a* a

Department of Chemistry, University of Copenhagen, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark. b

Nano-science Center/ Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen, Denmark

ABSTRACT. A series of different generation PAMAM-dendrimers with Sulforhodamine B covalently attached to the dendrimer core was investigated regarding their optical properties. Steady-state and time-resolved spectroscopic techniques were used to determine the size influence of the dendrimers on the photophysical behavior of the luminescent core. New blue emissive species were formed as the generation increased from zero to four. The growth of the dendritic branches resulted in a rise of fluorescence quantum yield and fluorescence lifetime values. Rotational correlation times were used to determine the hydrodynamic diameters of the fluorescent-core dendrimers and good accordance was found with the values previously reported 1

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for unlabelled PAMAM-dendrimers, which makes them potentially suitable diagnostic tools for biomedical tracing.

INTRODUCTION. Luminescent labels are widely used in biomedical applications, such as in the tracing of drug delivery systems,1 detection of DNA for gene analysis2 and transfection, detection of cancer markers3 and the generation of singlet oxygen for photodynamic therapy4. Dendrimers are hyper branched polymers built up in layers, normally referred to as generations. High generation dendrimers present a large number of exterior surface moieties and interior pockets where guest molecules such as chromophores or drugs can be loaded. For this peculiar characteristic, dendrimers have been largely used in medicinal applications as drug delivery vehicles.5-6-7 When the dendritic structure is coupled with a chromophore the biological properties like cellular uptake can be modified. For example, it was observed by Wang et al. that conjugation of PAMAM dendrimers surface with aromatic fluorescent dyes induced aggregation.8 As a consequence of the interactions between the dendrimer and the chromophore, the optical properties of the chromophore may be altered as well. In a recent study by Dabrzalska et al., it was observed that the fluorescence of Rose Bengal was initially quenched by complexation with the cationic PAMAM dendrimers reducing the fluorophore brightness.9 Generally, two techniques are mainly exploited in order to conjugate a guest molecule in a carrier; complexation and covalent linkage. While complexation is easier, there is a risk of leakage of the fluorophore out of the dendrimer. Covalent linkage offers better control in regard to the number and placement in the dendrimer as shown by Sharma.10 2


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METHODS All optical spectra were acquired using quartz-cuvettes with 1 cm path length and milli-Q as solvent. A Jasco V 650 spectrophotometer with 1 nm slit width was used to record absorption spectra. Fluorescent measurements were performed using a Jasco Model FP 6200 spectrofluorometer with excitation and emission slit widths set to 5 nm. Fluorescence Quantum Yields. Fluorescence quantum yields were calculated according to the optically diluted method

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using Rhodamine 101 as fluorescence standard with a φst = 0.915.12

The emission spectra were subtracted by blank solvent and corrected for the detector sensitivity. The absorption value of the solutions was always kept below 0.1 over the long wavelength range in order to prevent inner-filter effect in study of luminescence. The integrated fluorescence intensity (IF) of four different concentrated solutions was plotted against the corresponding fraction of absorbed light at the excitation wavelength (f = 1-10^-A, A= absorbance) for each sample in order to obtain the gradient (Grad) from the linear fit. The fluorescence quantum yield value was obtained using the following equation: =

Eq.1

is the refractive index of the solvent and the subscripts sam and st indicate the sample and the standard, respectively. An example of this plot for Rhodamine 101 and SulfoB is depicted in Figure S1 (Supporting Information). The φsam values reported in Table 1 are the average of repeated experiments performed over a period of weeks. Time-resolved measurement. Time-resolved fluorescence anisotropy and intensity decay kinetics were investigated using Fluo Time 300 TCSPC instrument. The excitation source used was a picosecond laser diodes (LDH series) with a minimum pulse width of 104 ps and 3



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wavelength of 507 ± 3 nm. The detection wavelength was set at the emission maximum and a hybrid-PMT was used as detector. For fluorescence lifetimes, the luminescence decays were acquired at the magic angle until the maximum reached 104 counts. Time-resolved fluorescence anisotropy decays were acquired setting the polarizers to vertical and horizontal positions. FluoFit software (version 4.6 from PicoQuant) was used to fit all time-resolved data. The fitting 2

goodness was established evaluating two parameters: χ (reduced chi-squared value) and R 2

(residuals). χ estimates the agreement between the experimental data and the decay model used. All time-resolved measurements were performed using solutions with absorption value below 0.05. RESULTS and DISCUSSION In this work Sulforhodamine B (SulfoB) was chosen as organic fluorophore due to the well-established properties of rhodamine derivatives in fluorescence applications. 13 The dendrimers (Scheme 1) were synthesized from BIS-BOCprotected diethylenetriamine

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by reaction of the secondary amine with the acid chloride of

Sulforhodamine B, which gave the product 2 in a 80% yield. The PAMAM dendrimer core could be achieved by deprotecting the BOC group under acidic conditions, followed by a reaction with methyl acrylate. The fluorescent labeled dendrimer core could then be applied in the standard divergent PAMAM-synthesis. * This approach allowed us to keep intact the surface properties of the dendritic scaffold and simultaneously control the equimolar ratio of dye to dendrimer.

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Scheme 1. Brief outline of the synthesis of the Sulforhodamine B core dendrimers.

Figure 1. (A) Normalized absorption (left) and emission (right) spectra of free SulfoB (pink), G0-SulfoB (black), G1-SulfoB (orange) and G4-SulfoB (green) in MilliQ. (B). Pictures corresponding to absorption (white light, left) and emission (UV light, right) spectra of free SulfoB and SulfoB labelled dendrimers.

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The SulfoB labelled dendrimers developed in this study embrace all the characteristics and advantages of covalently linked systems and introduce dendrimer size control of photochemical properties of the organic dye. As shown in Figure 1, the absorption spectrum of free SulfoB presents the characteristic maximum centered around 565 nm and a shoulder at 530 nm. A small red shift to 569 nm of the main absorption band and more pronounced bands at shorter wavelengths arose from synthesis of the generation 0 PAMAM dendrimer. As the size of the dendrimer increased to G1-SulfoB dendrimer, the band at 525 nm became the dominant band and the band at 565 nm was almost completely gone. The position of the bands did not change significantly with further increase of generation. Indeed, the absorption spectra of G1-SulfoB and G4-SulfoB are fairly similar. Absorption measurement of G0-SulfoB showed that dilution from concentration 40 µM to 2 µM caused a decrease of the ratio between the higher and lower energy peaks that did not occur in the other dendrimer generations (Figure S2). This evidence permitted to exclude the hypothesis that the high energy peak arises from H-type aggregates formation due to dendritic synthesis. Furthermore, J-type aggregates formation could also be discarded since the peak is blue shifted compared to the band of free SulfoB. In Figure 1, the emission spectra of free SulfoB and generation 0, 1 and 4 of SulfoB labelled dendrimers are depicted. G0-SulfoB exhibited two peaks centered at 542 and 588 nm. While the lower energy transition was typical for free SulfoB, the emission at shorter wavelengths emerged in the synthesis of the dendrimer. This band became dominant as the size of the dendritic architecture increased from generation 0 to 1. It did not shift with following increase of dendrimer size (Figure 1, right). A similar behavior to the absorption spectra was observed and a 6


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40 nm blue shift was revealed. This resulted in a color change in emission from red to green upon UV excitation (Figure 1, (B)).

Table 1. Photophysical properties of free SulfoB and SulfoB labelled dendrimers. φsam a

λabs,max

λem,max

(nm)

(nm)

565

586

0.28

569

588

0.31

525

542

0.05

G1-SulfoB

524

546

0.13

G2-SulfoB

524

546

0.18

G3-SulfoB

525

546

0.22

G4-SulfoB

525

546

0.24

SulfoB b G0-SulfoBc

λabs,max and λem,max are the wavelengths of absorption and emission maxima, respectively; φsam is the fluorescence quantum yield of the sample. a

The error for repeat measurements of φsam was calculated to be always lower than ± 0.03.

b

All the photophysical parameters obtained in this work for the free Sulforhodamine B showed good accordance with the values reported in the previous study by Smith et al. 15 c

In the case of G0-SulfoB, it was assumed that it could be represented by the sum of two contributions. It exhibited spectroscopic properties of free SulfoB and the dendritic species. The fluorescence quantum yields were the result of a deconvolution process that considered the contribution of these two components. The procedure that allowed to obtain these values is described in the Supporting Information.

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Table 2. Average decay times (τavg), goodness of fitting (χ2), average radiative and non-radiative decay rate constants (κrad,avg,, κnonrad,avg; calculated from the average decay rate, considering it as one emissive species) are reported in the left part. Rotational correlation times (θi), goodness of fitting (χ2), experimental diameter (DH) and the diameter from literature (Dlit)16 are reported in the right part.

τavg χ2

κ rad,avg

κ nonrad,avg

θ1

θ2

(ns)

(108 s-1)

(108 s-1)

(ns)

(ns)

χ2

DH

Dlit16

(nm)

(nm)

1.5

1.3

1.9

4.8

0.2

-

1.6

-

-

1.4

1.2

2.2

4.9

0.3

-

1.2

-

-

0.7

1.3

0.7

13.5

0.3

-

1.7

-

G1-SulfoB

1.3

1.4

1.0

6.7

0.2

1.5

1.2

2.34

1.58

G2-SulfoB

1.6

1.2

1.1

5.1

0.2

2.2

1.2

2.66

2.20

G3-SulfoB

1.9

1.1

1.2

4.1

0.4

3.5

1.1

3.11

3.10

G4-SulfoB

2.1

1.2

1.1

3.6

0.5

6.7

1.3

3.86

4.00

SulfoB G0-SulfoB*

In the case of G0-SulfoB, the values reported are obtained setting the detection wavelengths at the two different emission maxima (542 nm, bottom line and 588 nm, top line). See Table S1 for details.

*

In order to evaluate the luminescence efficiency of these systems formed by dendrimer conjugation, fluorescence quantum yield values ( ) are calculated by a relative method11 (Rhodamine 101

as fluorescence standard with a φst = 0.915). The φsam values reported in

Table 1 are the average of experiments repeated over a period of weeks. 8


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The spectroscopic properties of G0-SulfoB were considered as the sum of two emissive species, one similar to the free SulfoB and one similar to G4-SulfoB. The contribution of each component was estimated as described in the Supporting Information (Figure S4).Comparing the luminescence efficiency of the new blue emissive species formed by dendrimer synthesis, as the generation grows and the dendritic scaffolds get bigger and more rigid, the fluorescence quantum yield increases from 0.05 (φsam of G0-SulfoB attributed to the dendritic species) to 0.24 (φsam of G4-SulfoB). (see Table 1). Time correlated single photon counting experiments were performed to record fluorescence decay curves. The detection wavelength was set at the emission maximum. Three exponentials decay functions were used to fit the fluorescence decays of all series of SulfoB labelled dendrimers. For solutions of free SulfoB and G0-SulfoB, a component with lifetime values around 1.5 ns is predominant (92%) (Table S1). Nevertheless, decay associated spectra (DAS) of G0-SulfoB show that a shorter component with lifetime value of 0.6 ns is responsible for the luminescence observed at shorter wavelength where higher generations SulfoB labelled dendrimers display their emission maxima (Figure S7). This points out that a new species with shorter fluorescence lifetime is present when the G0 dendritic structure is linked to the dye. This short component is then revealed in all solutions from G1 to G4 together with other two longer components (around 1.5 and 5 ns) as shown in Table S1. Generally, an increase of the average lifetime (intensity weighted)16 was observed with the increase of the dendrimer size. After an initial drop from free SulfoB (1.5 ns) to G0-SulfoB (0.7 ns for the new emissive species at 542 nm), a subsequent rise of average lifetime was observed until G4-SulfoB (2.1 ns). This pattern is in line with fluorescence quantum yield 9



values. When the dendritic scaffolds expand, the system becomes more compact and the interactions with the local environment are minimized. Isolation of the emitting fluorophore results in a reduction of the non-radiative processes that can deactivate the excited state (Figure 2).

Figure 2 Size effect of dendrimer on fluorescence quantum yield and average lifetime of the

0,25

2,4

0,20

2,0

0,15

1,6

0,10

1,2

0,05

0,8

G0-SulfoB

G1-SulfoB

G2-SulfoB

G3-SulfoB

Average Lifetime (ns)

emissive species around 546 nm. The values for this graph can be found in Table 1 and 2.

Fluorescence Quantum Yield

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G4-SulfoB

To get an estimation of this, we used the average lifetime to calculate “average” radiative and non-radiative decay rates (κrad,avg,, κnonrad,avg). Unlike the average radiative decay rate which remains fairly constant from G1-SulfoB to G4-SulfoB (for the new emissive species at 546 nm), the non-radiative average decay rate becomes smaller as the dendrimer size was increased (see Table 2).

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Rotational correlation times were obtained using the same instrumental set-up as for the lifetime measurements. In the case of G0-SulfoB, the rotational correlation time showed a similar value (0.3 ns) compared to the free SulfoB (0.2 ns). As a result, we cannot confirm or exclude the possibility that the 588 nm emissive species in G0-SulfoB is free SulfoB (although it seems a plausible explanation). Increasing the generation, a single rotational correlation time was no longer sufficient to describe the rotation of the system and an additional longer component was needed. This longer component increased from 1.5 ns to 6.7 ns as the size increased from G1-SulfoB to G4-SulfoB while the shortest component is slightly affected by the dendritic structure. It may be assumed that the short component is related to the free motion of SulfoB around its attachment point; while the longest value is associated with the global rotation of the whole dendritic structure. This assumption was supported by the good agreement between the values of the free PAMAM dendrimer diameters reported by Tomalia et al. (Dlit in Table 2)17 and the experimental diameters (DH) obtained rearranging the Perrin equation16 that relates the rotational correlation time with the hydrodynamic volume. In this calculation, the long rotational correlation time component (θ2) was introduced in the equation, using the viscosity of milliQ at 25 ̊ C,18 the value of temperature expressed in Kelvin (298 K), the Boltzmann constant and the hydrodynamic diameters were obtained. The discrepancy shown for the first smaller generation dendrimer might be attributed to a deviation from the spherical molecule which is used in the model to derive the Perrin equation. CONCLUSION The synthesis of PAMAM dendrimers using Sulforhodamine B as core resulted in the formation of blue shifted emission. These new dendrimers displayed optical properties depending on the dendritic size. The growth of the dendrimer branches increased the 11



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average fluorescence lifetime and quantum yield. The fluorescence quantum yield of the final G4-SulfoB is only slightly lower than the free SulfoB. The use of the organic dye as a core in the synthetic strategy permitted to preserve the surface properties of the dendrimer and add traceability to these systems. All these characteristics make the Sulforhodamine B labelled dendrimers potential biomarkers in future imaging studies.

ASSOCIATED CONTENT Supporting Information. Example of fluorescence quantum yield calculation plot (Figure S1), Excitation Spectra of G0-SulfoB (Figure S2, S3), Deconvolution Process of G0-SulfoB (Figure S4), Concentration Dependence of G0-SulfoB Absorption Spectra (Figure S5), Fluorescence Lifetimes (Table S1), Decay Associated Spectra of SulfoB and G0-SulfoB (Figure S6 and S7). UNPUBLISHED RESULTS *Wu, L.-p.; Mejlsøe, S. L.; Ficker, M.; Hall, A.; Paolucci, V.; Christensen, J. B.; Trohopoulos, P. N.; Moghimi, S. M. Intracellular Uptake and Trafficking of Sulforhodamine B Core Poly(amidoamine) (PAMAM) Dendrimers in Human Endothelial Cells

AUTHOR INFORMATION [email protected]

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ACKNOWLEDGMENT This work was supported by grant DFF – 4002-00344 from DFF|Natural Sciences. We would like to thank Bo Wegge Laursen and Thomas Just Sørensen for use of equipment. ABBREVIATIONS SulfoB: free organic dye Sulforhodamine B, Gn-SulfoB: Generation n of Sulforhodamine B core PAMAM-dendrimers. REFERENCES (1) Watson, P.; Jones, A. T.; Stephens, D. J. Intracellular trafficking pathways and drug delivery: fluorescence imaging of living and fixed cells. Adv. Drug Delivery Rev. 2005, 57, 4361. (2) Zhao, X.; Tapec-Dytioco, R.; Tan, W. Ultrasensitive DNA detection using highly fluorescent bioconjugated nanoparticles. J. Am. Chem. Soc. 2003, 125, 11474-11475. (3) Wu, X.; Liu, H.; Liu, J.; Haley, K. N.; Treadway, J. A.; Larson, J. P.; Ge, N.; Peale, F.; Bruchez, M. P. Immunofluorescent labeling of cancer marker Her2 and other cellular targets with semiconductor quantum dots. Nat. Biotechnol. 2003, 21, 41-46. (4) Kim, S.; Ohulchanskyy, T. Y.; Pudavar, H. E.; Pandey, R. K.; Prasad, P. N. Organically modified silica nanoparticles co-encapsulating photosensitizing drug and aggregation-enhanced two-photon absorbing fluorescent dye aggregates for two-photon photodynamic therapy. J. Am. Chem. Soc. 2007, 129, 2669-2675. (5) Wu, L.-p.; Ficker, M.; Christensen, J. B.; Trohopoulos, P. N.; Moghimi, S. M. Dendrimers in medicine: therapeutic concepts and pharmaceutical challenges. Bioconjugate Chem. 2015, 26, 1198-1211. (6) Tomalia, D. A.; Fréchet, J. M. Discovery of dendrimers and dendritic polymers: A brief historical perspective*. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 2719-2728. (7) Gillies, E. R.; Frechet, J. M. Dendrimers and dendritic polymers in drug delivery. Drug discovery today 2005, 10, 35-43. (8) Wang, B.-B.; Zhang, X.; Jia, X.-R.; Li, Z.-C.; Ji, Y.; Yang, L.; Wei, Y. Fluorescence and aggregation behavior of poly (amidoamine) dendrimers peripherally modified with aromatic chromophores: the effect of dendritic architectures. J. Am. Chem. Soc. 2004, 126, 15180-15194. (9) Dabrzalska, M.; Zablocka, M.; Mignani, S.; Majoral, J. P.; Klajnert-Maculewicz, B. Phosphorus dendrimers and photodynamic therapy. Spectroscopic studies on two dendrimerphotosensitizer complexes: Cationic phosphorus dendrimer with rose bengal and anionic phosphorus dendrimer with methylene blue. Int. J. Pharm. 2015, 492, 266-274. 13



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(10) Sharma, A.; Mejía, D.; Maysinger, D.; Kakkar, A. Design and synthesis of multifunctional traceable dendrimers for visualizing drug delivery. RSC Adv. 2014, 4, 1924219245. (11) Crosby, G. A.; Demas, J. N. Measurement of photoluminescence quantum yields. Review. J. Phys. Chem. 1971, 75, 991-1024. (12) Würth, C.; Grabolle, M.; Pauli, J.; Spieles, M.; Resch-Genger, U. Relative and absolute determination of fluorescence quantum yields of transparent samples. Nat. Protoc. 2013, 8, 1535-1550. (13) Beija, M.; Afonso, C. A.; Martinho, J. M. Synthesis and applications of Rhodamine derivatives as fluorescent probes. Chem. Soc. Rev. 2009, 38, 2410-2433. (14) Pittelkow, M.; Lewinsky, R.; Christensen, J. B. Selective synthesis of carbamate protected polyamines using alkyl phenyl carbonates. Synthesis 2002, 2195-2202. (15) Smith, S. N.; Steer, R. P. The photophysics of Lissamine rhodamine-B sulphonyl chloride in aqueous solution: implications for fluorescent protein–dye conjugates. J. Photochem. Photobiol., A 2001, 139, 151-156. (16) Lakowicz, J. R. Principles of fluorescence spectroscopy. Springer Science & Business Media: 2013. (17) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A. Starburst dendrimers: molecular‐level control of size, shape, surface chemistry, topology, and flexibility from atoms to macroscopic matter. Angew. Chem., Int. Ed. Engl. 1990, 29, 138-175. (18) Crittenden, J. C.; Trussell, R. R.; Hand, D. W.; Howe, K. J.; Tchobanoglous, G. MWH's water treatment: principles and design. John Wiley & Sons: 2012.

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Photophysical Properties of Fluorescent Core Dendrimers Controlled

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