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Resonance Energy Transfer Between Dye Molecules in Colloids of a Layered Silicate. the Effect of Dye Surface Concentration Silvia Belušáková, Virginia Martinez-Martinez, Inigo Lopez Arbeloa, and Juraj Bujdak J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00947 • Publication Date (Web): 27 Mar 2017 Downloaded from http://pubs.acs.org on April 5, 2017
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Resonance Energy Transfer between Dye Molecules in Colloids of a Layered Silicate. The Effect of Dye Surface Concentration.
S. Belušáková,1 V. Martinez-Martinez,2 I. Lopez Arbeloa,2 J. Bujdák*,1,3 1
Comenius University in Bratislava, Department of Physical and Theoretical Chemistry,
Faculty of Natural Sciences, 842 15 Bratislava, Slovakia 2
Molecular Spectroscopy Laboratory, Departamento Química Física, University of the
Basque Country UPV/EHU, P.O. Box 644, 48080, Bilbao, Spain 3
Institute of Inorganic Chemistry, Slovak Academy of Sciences, 845 36 Bratislava, Slovakia
*Corresponding author: Juraj Bujdák Comenius University in Bratislava, Department of Physical and Theoretical Chemistry, Faculty of Natural Sciences, 842 15 Bratislava, Slovakia Tel.: +421 2 60296602 E-mail:
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ABSTRACT: Colloidal dispersions were prepared from a synthetic layered silicate of saponite type (Sap), and two cationic laser dyes, rhodamine 6G (R6G) and oxazine 4 (Ox4). The adsorption of dye molecules on Sap particles led to neither molecular aggregation, nor segregation of dye molecules. Förster resonance energy transfer (FRET), investigated using steady-state and time-resolved fluorescence (TRF) spectroscopies, occurs from R6G to Ox4 and the efficiency increased with the surface concentration of adsorbed dye molecules, as determined by the dyes/Sap ratio. Theoretical modeling of FRET based on defining probability density functions of intermolecular distances is presented. The theoretical model and experimental results were in very good agreement. Diffusion of the molecules might have contributed to the increase of FRET efficiency, especially at lower dye concentrations, whereas the influence of anisotropy factors was most likely negligible.
INTRODUCTION Förster resonance energy transfer (FRET) is a non-radiative process between two molecules taking place at distances of a few nanometers. After light is absorbed by the first molecule (the energy donor, ED), the excitation energy is transferred to the second molecule (the energy acceptor, EA), in a process involving resonance between transition dipole moments of the interacting molecules. Spectral overlap and small distances between interacting molecules are the basic requirements for FRET to take place. FRET has been used as a tool in various research fields in chemistry, biochemistry and molecular biology. Its applications include measuring intermolecular distances and detecting molecular interactions, examining protein dynamics, and characterization of molecular conformation and biopolymer and cellular structures1-2. Various types of hybrid systems based on dyes and layered particles have been considered as potential photoactive materials. The main role of the layered substrates was to
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enhance the local concentration of dye molecules in order to promote their interaction3-5. Hybrid materials displaying FRET can have various applications5-6: i. detection of analytes via occurrence of FRET7-9; ii. photoprotection of photolabile chemicals, such as pesticides or insecticides, in order to slowing down their photochemical decomposition10-12; iii. as a sensitive method to detect competitive adsorption of dyes3. In other words, FRET has become well established as an analytical tool in biochemical and biophysical fields. However, FRET processes in colloids and nanomaterials have not been fully exploited. Since FRET is primarily influenced by the distances between interacting ED and EA molecules, it can be used for the characterization of adsorption. Besides the concentrations of adsorbed dye molecules, FRET can also be influenced by the segregation of either ED or EA molecules, molecular orientation, molecular aggregation, heterogeneity distribution of dye molecules, and stability of the colloids, among other factors. The objective of this work is the detailed characterization of FRET processes between a pair of laser dyes adsorbed in welldefined and stable colloidal dispersions of a synthetic layered silicate, saponite. The basic parameters of FRET obtained from spectroscopy data were compared with a theoretical model describing the distribution of dye molecules on the surface of the layered particles. These are important results for the design of photoactive materials of this type, as well as applicable for the use of FRET as a tool for characterising the surface properties of layered nanomaterials.
EXPERIMENTAL SECTION Materials. Synthetic saponite Sumecton SA (Sap) was purchased from Kunimine Industries Co., Ltd., Japan and used as received. The cation exchange capacity (CEC) of Sap is 0.72±0.05 mmol g−1. Its structural formula is:13 [(Si7.20Al0.80)(Mg5.97Al0.03)O20(OH)4]-0.77(Na0.49Mg0.14)+0.77
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The laser dyes rhodamine 6G (R6G) and oxazine 4 (Ox4) (Figure 1) were purchased from Acros Organics, and used as received. Deionized and purified Milli-Q water was prepared using a Merck Millipore Milli-Q water purification system and used for the preparation of all colloid samples and aqueous stock solutions.
Figure 1. Structural formulas of the cations of used dyes.
Sample preparation and basic characterization. Concentrations of aqueous stock dyes solutions were determined spectroscopically using the Lambert-Beer law after appropriate dilution in ethanol. Molar absorption coefficients are available from the literature14-15. Aqueous colloidal dispersions of Sap were prepared by immersing the silicate in water and stirring overnight, appropriately diluted to get the required concentration and left under stirring for an additional hour. The final concentration of each organic dye (cdye) in solutions and colloids was always 6×10-7 mol L-1, if not stated otherwise. In the systems 4 ACS Paragon Plus Environment
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containing both dyes, the total dye molar concentration (cdyes) was 2 × cdye = 1.2×10-6 mol L-1. For comparison, additional colloidal dispersions were prepared, in which each contained only one dye component (either R6G or Ox4) with cdye identical to that in the system of two dye components (6×10-7 mol L-1). The loadings (adyes, adye), representing the ratios ndyes/mSap and ndye/mSap, respectively, were mostly expressed as a percentage of the Sap's CEC (%CEC). The loadings were altered via changes of Sap concentration, and cdye and cdyes were always kept constant. The average concentrations of dye molecules on the surface (Γmol or Γ) were calculated from the value of dye loading (adye), and from the mass and theoretical dimensions of the structural unit of saponite (O20(OH)4, unit dimensions a=0.5333 nm, b=0.9233 nm, c=1.542 nm)16. The method for the calculation is shown in Supporting information (S1). In the first set of experiments, the procedure for preparing homogeneously distributed ED and EA molecules on Sap particles was verified. In these experiments, adyes was always 0.071 mmol g-1, which corresponds to 10 %CEC. The influence of two preparation procedures was examined (Table 1, Scheme S2). The tested methods (M.I, M.II) differed in the order in which the components were mixed, or in the particular steps leading to the preparation of the mixed R6G+Ox4+Sap systems. The colloids were allowed to equilibrate for 10 min after each mixing step. For example, M.II was designed to form homogeneous systems, since this procedure started from a mixed dyes solution (Table 1, Scheme S2) and was used as a standard method for the preparation of the colloids in the rest of the experiments. In contrast, method M.I, based on mixing two colloidal precursors R6G+Sap and Ox4+Sap, was designed to enhance the probability of the segregation of R6G cations from Ox4 ones and was investigated in more detail so as to characterize dye segregation directly by FRET. Fluorescence changes with time were measured for the colloid with adyes = 10 %CEC. The measurement started a few seconds before Ox410.0Sap colloid was added to a spectroscopy cell containing R6G10.0Sap. ED molecules were selectively excited at 475 nm. The time 5 ACS Paragon Plus Environment
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evolutions of the fluorescence signals using a front-face setup were measured at 550 and 630 nm, respectively. In addition, spectral evolution with time was tested for adyes = 2; 5; 10 and 20 %CEC to characterize the spectral changes over longer timescales (up to 24 h).
Table 1. Scheme Describing Procedures for the Preparation of Mixed Colloids. method
procedure
M.I
mixing Sap(coll)+R6G(aq); mixing Sap(coll)+Ox4(aq) mixing both the colloids together
M.II
preparation of mixed solution of R6G(aq)+Ox4(aq) mixing solution with Sap(coll)
(aq) – aqueous solution, (coll) - colloid
As mentioned above, the variable composition of the colloids with different loadings of the two dyes together (adyes) was achieved by changing the Sap concentration while keeping cdyes constant. The range for adyes was from 0.0036 to 1.071 mmol/g, giving loading values of 0.5, 1.0, 2.0, 5.0, 10, 20, 50 and 150 %CEC (Table 2). The loadings for each dye individually (adye) were half the values of adyes (0.25-75 %CEC). Additionally, colloids with single dye components and mixed solutions of the two dyes without Sap were also prepared. The cdye values of the solutions were the same as of the colloids. The abbreviations for the samples are as follows: Mix5.0Sap denotes a colloid based on the mixture of the dyes R6G, Ox4 and Sap. The number expresses the loading, adyes, in %CEC. Hence for Mix5.0Sap, adyes=5.0 %CEC. R6GxSap and Ox4xSap denote hybrid colloids containing only one dye (either R6G or Ox4) with a loading of x %CEC.
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Table 2. The loadings (aED, aEA), calculated values of dye concentrations at the smectite particle surface (Γmol, Γ), and average intermolecular distances (d).
Γmol
aED or aEA -1
Γ -2
d -2
%CEC
mol g
mol m
nm
0.25
1.78 10-6
5.19 10-9
3.13 10-3
17.9
0.50
3.55 10
-6
1.04 10
-8
6.25 10
-3
12.6
7.10 10
-6
2.08 10
-8
1.25 10
-2
8.94
2.5
1.78 10
-5
5.19 10
-8
3.13 10
-2
5.65
5.0
3.55 10-5
1.04 10-7
6.25 10-2
4.00
10
7.10 10-5
2.08 10-7
1.25 10-1
2.83
25
1.78 10-4
5.19 10-7
3.13 10-1
1.78
75
5.33 10-4
1.56 10-6
9.38 10-1
1.03
1.0
nm
METHODS The model for the calculation of FRET efficiency. The probability that a molecule is separated from the nearest molecule by a certain distance, r, can be characterized by a probability density function f(r, Γ). r is a continuous random variable defined as the distance between molecules in two-dimensional space (0
