Fluorescence saturation spectroscopy in probing electronically excited states of silver nanoclusters

Journal of Luminescence 172 (2016) 175–179

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Fluorescence saturation spectroscopy in probing electronically excited states of silver nanoclusters Ivan Volkov a, Tomash Sych a, Pavel Serdobintsev a,b, Zakhar Reveguk a, Alexei Kononov a,n a b

Department of Molecular Biophysics and Polymer Physics, Saint-Petersburg University, 7/9 Universitetskaya nab., St. Petersburg 199034, Russia Saint-Petersburg State Polytechnical University, Polytechnicheskaya 29, St. Petersburg 195251, Russia

art ic l e i nf o

a b s t r a c t

Article history: Received 3 August 2015 Received in revised form 8 November 2015 Accepted 10 December 2015 Available online 17 December 2015

DNA-based fluorescent Ag clusters attract much attention due to their high brightness and sensitivity to environment, which can be used in chemical sensing and biosensing applications. Low chemical yield of the fluorescent Ag clusters in solution hinders measuring of their absorption cross-section in the ground state and rate constants in the excited states. We applied fluorescence saturation spectroscopy for determining the photophysical constants of an oligonucleotide-stabilized red emitting Ag cluster. Power dependencies of the fluorescence response to the pulse excitation with different pulse duration allowed us to obtain the values of absorption cross-section, dark states formation and deactivation rates of the cluster. The Ag fluorescent cluster exhibited a relatively high (17%) efficiency of a long-lived dark state formation. This feature of the Ag cluster might be further interesting for possible photodynamic and microscopy applications. & 2015 Elsevier B.V. All rights reserved.

Keywords: Fluorescent silver nanocluster DNA Saturation spectroscopy Excited states Dark state

1. Introduction Unique structure of DNA offers large possibilities for developing artificial nanostructures for nanophotonics and nanoelectronics applications [1–4]. DNA-stabilized silver nanoclusters have received much attention in past years because of their potential applications in biosensing and bioimaging due to their unique fluorescence properties such as high brightness and photostability [5,6]. For example, Ag clusters are now employed in detection of metal ions [7,8], microRNAs [9–11], target DNA strands [12], and mutations causing human diseases [13,14]. Though a fair amount of DNAstabilized silver nanoclusters has already been synthesized, their excited state properties, in particular, dark state formation, have been little investigated. Meantime, fluorophores with the dark states have raised increasing interest in fluorescence microscopy in recent years [15]. Dark non-emitting states of Ag nanoclusters have already been successfully used in optically modulated fluorescence imaging [16]. In spite of easy chemical synthesis, which involves formation of a complex of silver ions with nucleobases followed by reduction with NaBH4 in solution, a problem still exist in that the chemical yield of the fluorescent clusters in the mixture does not exceed a few percent. Ag cluster solutions, as prepared, are strongly n

Corresponding author. Tel.: þ 7 812 428 9971; fax: þ 7 812 428 7240. E-mail address: [email protected] (A. Kononov).

http://dx.doi.org/10.1016/j.jlumin.2015.12.019 0022-2313/& 2015 Elsevier B.V. All rights reserved.

heterogeneous systems with multiple presence of non-fluorescence species and need further purification to isolate fluorescent clusters. Most studies dealt with unpurified solutions, which hindered probing the photophysical properties of the fluorescent clusters. As a consequence, most of the data reported in the literature on the quantum yields and absorption cross-sections of the clusters are questionable, since the weight of the fluorescent species is unclear. Here we present a method for both the direct measuring the absorption cross-section of the fluorescent clusters in a mixture of synthesized species and also for determining the rate constants for the dark state of the clusters. In doing so, we modified a method of the fluorescence saturation spectroscopy [17] using three different pulse excitation regimes, namely picosecond and nanosecond pulse duration, and also CW (continuous wave) excitation. We studied Ag clusters emitting at about 620 nm on 12-mer singlestranded DNA 50 -CCTCCTTCCTCC-30 (S1). The same DNA sequence as a scaffold for Ag clusters has been successfully used in a series of probes for plant microRNA detection [9,10]. Red-emitting cluster on that DNA matrix was synthesized for the first time by Richards et al. [18]. We have determined the absorption crosssection, rates of the dark state formation and deactivation for the synthesized cluster. We show that the quantum yield of the dark state is relatively high (17%) and comparable to the previously obtained one for the cluster on calf thymus DNA [19] with similar spectral, and hence, structural properties.

I. Volkov et al. / Journal of Luminescence 172 (2016) 175–179

2. Experimental methods 2.1. Clusters synthesis Synthesis of Ag clusters was performed in a similar way as described in Ref. [18]. Oligonucleotide (S1 strand) 50 -CCTCCTTCCTCC30 (BioBeagle ltd.) and AgNO3 aqueous solutions in 20 mM sodium citrate buffer (pH 5) were mixed and stored for 15 min at room temperature. After that, NaBH4 aqueous solution was added followed by vigorous stirring. The final concentrations were CDNA ¼20 μM, CAgNO3 ¼120 μM, and CNaBH4 ¼30 μM. The sample was kept in the dark about 1 month at 21 °C to reach maximum fluorescence. After that, no spectral change was observed. 2.2. Spectral measurements Fluorescence spectra were obtained on Fluorolog 3 fluorometer at room temperature. All spectra were collected using 0.4 cm quartz cuvette. To remove scattering light, appropriate long wave pass filters were used. Fluorescent emission spectra were corrected for instrument sensitivity as supplied by manufacturers. Blank signals from the solvent, DNA, Agþ-DNA, AgNO3, NaBH4, and reduced Ag þ in the absence of DNA did not exceed 5% relative to the fluorescent maxima of the samples of Ag-DNA over all spectral range. For the fluorescence excitation spectra, the correction was done for the inner filter effect due to high absorbance of the sample in the 260 nm region. To eliminate polarization effects, a vertically oriented polarizer was set into the excitation channel and another polarizer was placed at “magic” angle in the emission channel. Fluorescence polarization degree was calculated as (I0  G  I90)/(I0 þG  I90), where I0, and I90 – intensities of vertically and horizontally polarized emission beam at vertically polarized excitation light, G – instrument factor calculated as I0/I90 at horizontally polarized excitation light. Fluorescence lifetime was determined using the same fluorometer and a LED as a pulse source (570 nm) with typical FWHM of about 2 ns. Emission bandwidth was set at 15 nm. Circular dichroism (CD) spectra were measured with Jasco J-815 polarimeter. Melting curve were obtained with Specord 210 Plus double-beam spectrophotometer equipped with Peltier cooled cell holder with stirrer.

Saturation curves obtained in the picosecond and CW excitation regimes were fitted analytically with MagicPlot Pro 2.5 software. For simulation of the curves in the nanosecond excitation regime we solved the set of Eq. (1) and integrated n(t) numerically with the step of 2  10  14 s  1 using Runge–Kutta formalism implemented in MatLab. The parameters were varied with appropriate step for the best fit to the experimental points using RMSD criterium.

3. Results and discussion 3.1. Spectral properties of Ag cluster Ag clusters synthesized at pH 5 on S1 strand exhibit mostly one type of emitting clusters. Absorption, fluorescence emission, fluorescence excitation, and excitation polarization spectra in a wide spectral range for the clusters with the excitation/emission maxima at 529 nm/ 620 nm are shown in Fig. 1 and S1. As can be seen from comparison of the excitation and absorption spectra (Fig. S1), chemical yield of the fluorescent clusters appears to be low enough, and most species are dark Ag clusters and nanoparticles. Even at 529 nm, the weight of the red-emitting cluster in the total absorbance is uncertain (Fig. S1). Nethertheless, fluorescence emission spectra practically does not depend on the excitation wavelength, though a minor presence of a green emitting cluster is seen, when exciting at 260 nm (Fig. 1). The 260 nm UV band in the excitation spectrum, practically coinciding with DNA absorption spectrum (Fig. 1), is typically observed for DNAstabilized Ag clusters due to efficient energy transfer from DNA to the clusters [20]. The difference between the excitation spectrum and DNA absorption spectrum (dashed curve in Fig. 1) evidently reflects intrinsic electronic transitions of the cluster. A fair amount of dark clusters is a typical for unpurified cluster solutions. Unfortunately, the sample appeared to be unstable during HPLC purification procedure, typically used for DNA-cluster complexes [21]. Typical chromatograms and comparison of the excitation and absorption spectrum after purification are shown in Figs. S2 and S3. Since only one emitting cluster contributed to the fluorescence emission spectrum, when excited at 530 nm, further measurements using fluorescence saturation technique were conducted with unpurified solutions. Fluorescence lifetime for the clusters was

2.3. Nonlinear fluorescence saturation spectroscopy

750

550

1.4

450

350

300

250

0.5

polarization

0.4 0.3

1.2 Intensity and absorbance, a.u

For the picosecond pulse excitation regime, we used Ti:Sapphire 800 nm femtosecond laser (Synergy 20) combined with an amplifier chain (Pulsar, Amplitude Technologies) that gave 2.5 mJ pulse energy and 50 fs pulse duration at 10 Hz. The beam was directed into an optical parametric amplifier (TOPAS-C, Light Conversion). The output pulse energy at 530 nm was about 100 μJ. The pulse was stretched up to ca. 1 ps to avoid efficient stimulated emission from the sample. The output beam was confined to 2 mm in diameter by an iris. For nanosecond pulse excitation, a Q-switched frequency-doubled Nd: YAG laser (Quantel, 532 nm, 10 ns pulse, 10 Hz, 100 mJ) was used. The output beam diameter was confined to 5 mm by a diaphragm. Integral fluorescence intensity within the range of 580–680 nm was measured with a portable spectrometer (Ocean Optics). In the case of CW excitation, we used a pump laser (Finesse, Laser Quantum) with 2 W output at 532 nm. The beam was confined by an iris to 2 mm. A short  0.2 s single flash was used for excitation to avoid photobleaching. Integral fluorescence was measured using a photodiode. In all cases the luminescence of the sample in 0.4 cm quarts cuvette was collected at right angle. A long pass filter was used to remove scattering light. The excitation intensity was adjusted with neutral glass filters and was measured with a pyroelectric energy meter (GentecEO). Solutions were extensively stirred during experiments, and the photobleaching did not exceed 2%.

, nm 1200

DNA absorption

1

0.2 0.1

0.8

excitation

emission ( ex = 520 nm)

0 cluster absorption

0.6

0.4

−0.1 −0.2

emission ( ex = 260 nm)

−0.3

0.2

0

Polarization degree

176

−0.4 1

2

3

4

5

−0.5

h , eV Fig. 1. Fluorescence excitation, polarization, and emission spectra of the red-emitting Ag nanoclusters synthesized on S1 DNA. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

I. Volkov et al. / Journal of Luminescence 172 (2016) 175–179

177

n1

60

k2

40

DNA F·σ1

k1

n2

mDeg

20 k3

Ag+-DNA Ag cluster-DNA n0

0

Fig. 3. Scheme of photoprocesses considered in the model of fluorescence response of Ag clusters.

c ¼ n0 þ n1 þ n2 :

−20

−40 220

240

260

280

300

320

λ, nm Fig. 2. CD spectra of S1 DNA, Agþ -DNA, and Ag cluster-DNA complexes at pH 5.

estimated to be 2 ns. The corresponding count trace is shown in Fig. S4. The same value was reported by Richards et al. [18]. 3.2. Structure of Ag cluster Previous experimental studies suggested that Ag fluorescent clusters were most likely stabilized by two neighboring DNA segments in the sites containing mismatch [22], hairpins [23], or self-dimers [21]. Circular dichroism (CD) spectrum of S1 strand, shown in Fig. 2, exhibits a positive band at 290 nm, which is typical for non-canonical C–C þ pairing [24]. Thermal denaturation profile, shown in Fig. S5, supports the conclusion about a supramolecular structure of S1 due to formation of C–C þ pairs, exhibiting a conformational transition at about 50°. C–C þ pairing might result in an intramolecular hairpin structure or a dimer formation, which stabilizes the Ag cluster. The experimentally obtained excitation spectrum of the cluster (dashed curve in Fig. 1) appears to be very close to the calculated earlier excitation spectrum of zig-zag threadlike cluster consisting of four silver atoms with the bond angle of about 130° [25]. Intrinsic electronic states of the cluster are seen in all spectral range in both the excitation and polarization spectra (Fig. 1), which might be further used to determine structure of the cluster. 3.3. Determining photophysical characteristics of Ag cluster Scheme of the photophysical processes involving three electronic states of Ag cluster is presented in Fig. 3. Our previous study showed that fluorescence response on pulse excitation of Ag clusters can be satisfactory described by the three-level scheme [19]. The three-level model of photoprocesses in Ag clusters involves excitation and deactivation of the first singlet state and also formation and deactivation of a dark state. It can be described by a set of the following kinetic equations for dilute solution: dn0 ðt Þ ¼  F ðt Þσ 1 n0 ðt Þ þ k1 n1 ðt Þ þ k3 n2 ðt Þ dt dn1 ðt Þ ¼ F ðt Þσ 1 n0 ðt Þ  k1 n1 ðt Þ  k2 n1 ðt Þ dt dn2 ðt Þ ¼ k2 n1 ðt Þ  k3 n2 ðt Þ dt

ð1Þ

Here n0, n1, and n2 are the concentrations of the clusters in the ground, first singlet, and dark states; c is the total concentration of the clusters; F is the excitation photon flux; σ1 is the absorption cross-section; k1 is rate of de-excitation of the singlet; k1 þk2 ¼ τ  1, where τ is the fluorescence life-time and k2 is rate of the transition from the singlet state to the dark state; k3 is rate of deactivation of the dark state. For nanosecond and picosecond excitation, repopulation of the ground state from the dark state may be neglected since the life-time of the dark states of Ag clusters is expected to be ca. 10 μs [5,19]. In the case of picosecond pulse excitation, when the pulse width tp {τ, Eq. (1) are simplified to: dn0 ðt Þ ¼  F ðt Þσ 1 n0 ðt Þ dt c ¼ n0 þ n1 :

ð2Þ

As shown in Supplement, this system may be solved analytically:
σ1 E  n1 ¼ c 1  e  shν ; ð3Þ Rt where E ¼ 0p F ðt Þshν dt is the pulse energy, s – cross section of the beam, h – Planck constant and ν – light frequency. The fluorescence intensity is proportional to n1 and may be obtained as:
σ1 E  ð4Þ I f l ¼ A 1 e  shν ; where A is a product of instrument factor, and fluorescence quantum yield. Reference measurements of absorption cross-section were made for rhodamine (Rhod 6G) solution. Pulse energy dependences of the fluorescence response obtained for Rhod 6G and Ag clusters are shown in Fig. 4. Fit of Eq. (4) to the data gives the value (4.170.4)  10  16 cm2 for the cross-section of Rhod 6G, which coincides with the reference value [26]. In doing so, we also took into account spectral width of 0.1 eV of the pulse that was comparable with Rhod 6G absorption line (see Supplement). For Ag cluster, saturation curve nearly superimposes the one obtained for Rhod 6G, thus implying close value of absorption cross section. The least square fit for the cluster gives the value of (3.570.3)  10  16 cm2, or (0.9070.08)  104 M  1 cm  1. For comparison, the value obtained for the same cluster with the use of fluorescence correlation spectroscopy (FCS) is (1.270.3)  104 M  1 cm  1 [18] that is 30% higher than the value obtained by the saturation spectroscopy. It is worth noting that FCS gives the absolute concentration of the fluorescent clusters. In order to calculate the absorption extinction coefficient, one also needs to know the relative absorbance of the emitting species in the complex mixture of emitting and non-emitting clusters and particles. If the relative weight of the fluorescent clusters in the total absorbance is low enough, this can lead to errors in calculation of the extinctions. Another uncertainty is

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25

Rhod 6G

50

40

Ag cluster-DNA

6

20

4 20

Intensity, counts (×103)

Intensity, counts (×103)

40

30

15

30

10

20

2 10

Rhod 6G

5

10

Ag cluster-DNA

0

20

40 E, μJ

60

0

5

10

15

20

0 25

E, mJ

Fig. 4. Pulse energy dependence of integral fluorescence intensities for Rhod 6G (λem ¼ 540–600 nm) and Ag clusters (λem ¼ 580–620 nm) at picosecond pulse excitation.

that it is not clear how many emitting clusters can be on one DNA matrix. In this respect, fluorescence saturation spectroscopy is the only direct method for the estimation of absorption extinction of the DNA-based fluorescent clusters. In the case of nanosecond pulse excitation, when the pulse width (Fig. S6) is comparable with the lifetime of the singlet excited state, numerical solution of Eq. (1) allows one to get the rate constant of formation of the dark non-fluorescent state. The saturation curves of the fluorescence response for Rhod 6G and Ag cluster are shown in Fig. 5. As can be seen, the curves for the dye and cluster are different in this case in contrast to the case of the picosecond pulse excitation (Fig. 4). Such the difference points at relatively high efficiency of formation of the dark state of Ag cluster in comparison with the dye, for which the rate of the dark triplet state formation is low enough ( 106 s  1 [27,28]). Numerical solution of Eq. (1) allowed us to obtain the value of k2 ¼ (0.970.1)  108 s  1. The value of the efficiency of the dark state formation of 17 73% appears to be close to the value obtained earlier for Ag clusters on CT DNA [19]. The similarity in the efficiencies of inter-crossing for the clusters on S1 oligonucleotide and CT DNA is interesting but not surprising, since the spectral, and hence, structural properties of those clusters are nearly identical in spite of the different DNA templates. The dark states in DNA-based Ag clusters were proposed to be the result of photoinduced electron transfer from the cluster to DNA [29]. For near-IR emitting Ag clusters, however, the yield of the dark state was reported to be a few percent [29]. Dark state yield is probably determined by the structure of Ag clusters, likely by the number of silver atoms and by the charge of cluster, which may change the redox properties of the clusters significantly [27]. Further studies of detailed structure of Ag-DNA complexes might shed light on the problem. Using the CW excitation regime, we estimated the rate of the dark state deactivation. In this case, the equilibrium concentrations are determined by the following system of linear equations: F σ 1 n0 ¼ n1 k1 þ n2 k3 n1 k2 ¼ n2 k3 c ¼ n0 þn1 þ n2 :

0

0 80

ð5Þ

Fig. 5. Fluorescence response for Rhod 6G and Ag clusters at nanosecond pulse excitation. The data for Ag clusters were fitted numerically using three-level model (1). The Rhodamine data are connected by straight lines for comparison with the cluster.

100

35

30 80 25 Intensity, a.u.

0

60

20

15

40

10 Rhod 6G Ag cluster-DNA

20

0

0

10

20

30

40

5

50

0

Iex, W/cm2 Fig. 6. Dependence of the fluorescence response of Rhod 6G and Ag clusters on the intensity of CW excitation. The initial slopes of the curves coincide.

Solving this system, we get n1 as: n1 ¼ c U

F σ1




F σ1  ; 1 þ kk23 þ k1 þ k2

ð6Þ

and finally the fluorescence intensity (Ifl) can be expressed as: I fl ¼ a U

σ Iex
hν  ; σ Iex 1 þ k2 þ 1 τ hν k3

ð7Þ

where a is a constant, proportional to concentration and radiative rate, Iex is the excitation intensity. The saturation curves obtained for both Rhod 6G and Ag nanoclusters at CW excitation are shown in Fig. 6. Linear plot for Rhod 6G agrees with a relatively low yield of the triplet state

I. Volkov et al. / Journal of Luminescence 172 (2016) 175–179

Table 1 Photophysical constants of the red-emitting Ag nanoclusters on S1 oligonucleotide. σ1, 10  16 cm2 3.5 70.3a

k1, 108 s  1 4.2 7 0.1b

k2, 106 s  1 857 10c

k3, 106 s  1 0.17 0.05d

179

Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2015.12.019.

a

Absorption cross-section at 530 nm. Rate of the excited singlet deactivation. Rate of the dark state formation. d Rate of the dark state deactivation. b c

[27,28]. A slight deviation of the curve for the Ag cluster from the Rhod 6G plot is explained by a relatively high rate of the dark state formation. Unfortunately, low signal allowed us to estimate the rate k3 only approximately with the error of 50%. A fit of expression (7) to the data gives the value of k3 ¼10 ms  1, close to the value obtained earlier for CT DNA-based clusters [19]. Table 1 summarizes the rate constants obtained for the redemitting Ag clusters on S1 DNA strand.

4. Conclusion We successfully applied the modified method of saturation fluorescence spectroscopy for a mixture of dark and fluorescent Ag nanoclusters synthesized on the 12-mer DNA strand. Using picosecond pulse excitation regime, we determined the absorption cross-section of the fluorescent clusters by the direct method avoiding uncertainties connected with extremely low and unknown concentration of emitting species. Nanosecond pulse excitation allowed us to obtain the rate and the quantum yield of the dark non-fluorescent state formation. Extremely high yield of the dark state of about 17% observed for Ag cluster deserves special attention. This feature of the clusters might be further interesting for possible photodynamic and microscopy applications. Thus, the nature and photochemical properties of the dark states of Ag clusters need further detailed investigation.

Acknowledgments The authors acknowledge Saint-Petersburg State University for a research grant 11.38.221.2014. Experimental data presented in this work were obtained using the equipment of Center for Optical and Laser Materials Research and Chemical Analysis and Materials Research Centre of St. Petersburg State University, Russia.

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