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Fluorescence Correlation Spectroscopy in Polymer Science Cite This: RSC Adv.,2014,4, 2447 Ab Dominik Woll¨ *

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Fluorescence Correlation Spectroscopy in Polymer Science Cite This: RSC Adv.,2014,4, 2447 Ab Dominik Woll¨ * Erschienen in: RSC Advances ; 4 (2014), 5. - S. 2447-2465 https://dx.doi.org/10.1039/C3RA44909B RSC Advances REVIEW View Article Online View Journal | View Issue Fluorescence correlation spectroscopy in polymer science Cite this: RSC Adv.,2014,4, 2447 ab Dominik Woll¨ * Fluorescence correlation spectroscopy (FCS) is a well-established technique for studying dynamic processes and interactions with minimal invasion into the corresponding system. Even though FCS has been mainly applied to biological systems, within the last 15 years an increasing number of studies in material sciences have appeared, demonstrating its enormous potential also for this field. Apart from investigations on colloidal systems, polymer science has benefited significantly from this technique. This review will summarize FCS studies on polymer systems and, in particular, focus on the diffusion of differently sized molecular and macromolecular probes in polymer solutions, classical and responsive polymer gels, polymer melts and glasses. It will be discussed how FCS can be used to determine Received 5th September 2013 translational and rotational diffusion in polymer solutions and at interfaces, scaling laws, micellization Accepted 5th November 2013 and aggregation and, to some extent, polymer structure including heterogeneity. Thus, FCS should be DOI: 10.1039/c3ra44909b considered a powerful complement to other methods for the investigation of polymer structure and www.rsc.org/advances dynamics. Polymers have emerged as the most important materials of the and only combining their strengths will allow us to gain a modern world. The reason for their success lies in the variety of consistent picture of polymers from the nanoscopic to the functions they can cover due to their tunable properties. A macroscopic scale. faceted knowledge of how these properties are determined by Within the last 15 years, uorescence correlation spectros- the structure and dynamics on different length scales is still copy (FCS) has signicantly contributed to the insights into amongst the scientists' dreams. The complexity of this rela- polymer systems, in particular to diffusion measurements. Even tionship challenges all experimental and theoretical methods, though the rst paper on FCS by Magde, Webb and Elson was already published in 1972,1 several developments were neces- sary to exploit the power it has reached today.2–7 One major step Published on 05 November 2013. a Zukun skolleg, University of Konstanz, Universitatsstr.¨ 10, 78464 Konstanz, Germany in the evolution of FCS was its combination with confocal b Department of Chemistry, University of Konstanz, Universitatsstr.¨ 10, 78464 microscopy which allows for enhanced spatial resolution and Konstanz, Germany. E-mail: [email protected]; Tel: +49 7531 88 2024 sensitivity.8 Further important improvements concern the quality of optical components, the sensitivity and time-resolu- tion of detectors, and better labels and labelling strategies. The Dominik Woll¨ received his advantages of FCS are (i) small sample consumption, (ii) diploma in chemistry from the negligible dye concentration and thus perturbance of sample University of Konstanz (Germany) properties, (iii) the possibility of in situ measurements, (iv) good in 2002 and nished his PhD at spatial resolution at the diffraction limit and (v) the possibility the same university in 2006, to observe diffusion of different species depending on their working with Prof. Ulrich E. uorescence labelling. The majority of FCS studies have been Steiner on intramolecular sensi- conducted in biological systems. Transferring the concepts and tization of photolabile protecting technical knowledge gained in these studies to polymer science, groups for DNA chip synthesis. however, is straight-forward. Aer a postdoctoral stay with In this review, I will report on FCS studies in polymer Prof. Johan Hoens at the KU systems and how this modern technique contributed to new Leuven (Belgium) where he insights in polymer structure and dynamics. The review starts became acquainted with single with an introduction to uorescence correlation spectroscopy molecule spectroscopy techniques, he started an independent junior covering theoretical and technical aspects of diffusion research group as fellow of the Zukunskolleg of the university of measurements, their possibilities, limitations and pitfalls. In Konstanz in 2008. His current research focus is single molecule the next section, I briey report on diffusion of polymer chains uorescence spectroscopy and microscopy in so matter. This journal is © The Royal Society of Chemistry 2014 Konstanzer Online-Publikations-System (KOPS) RSC Adv.,2014,4, 2447–2465 | 2447 URL: http://nbn-resolving.de/urn:nbn:de:bsz:352-0-277334 View Article Online RSC Advances Review in pure solvents, followed by an extended section on diffusion of diffraction limited confocal volume within the sample placed molecular and macromolecular probes in polymer solutions, on a glass coverslip. Part of the light emitted from this confocal gels, melts and glasses. In the end of that section, I will also volume is collected by the same objective and separated from briey review how the diffusion of large nanoparticles can probe excitation light using a dichroic mirror and an emission lter. microrheology in polymer systems and, furthermore, report on The emission light is focused onto a pinhole blocking most of FCS studies in responsive hydrogels and at solid–liquid inter- the light not originating from the confocal volume, thereby faces, and demonstrate the power of this method to study improving the axial z-resolution as shown in Fig. 2. The confocal micellization and aggregation. volume typically amounts to ca. 0.1 femtolitre. The photons I will concentrate on synthetic polymers neglecting the passing through the pinhole are detected with an avalanche impressive results on biopolymers, such as dynamics9–13 and photo diode (APD). Such APDs possess good quantum effi- hybridization14–16 of DNA, polysaccharides,17–21 protein binding ciencies and, with appropriate electronics, allow for the deter- and dynamics,22–28 motor proteins,29,30 brin polymerization,31 mination of the arrival times of single photons with an accuracy enzyme kinetics,32 diffusion through meshes formed by semi- in the picosecond range. rigid biopolymers,33 and polymer–membrane interaction.34 It Typically, the arrival times are binned and an autocorrela- will be also not further discussed how FCS can assist in ana- tion function G(s) calculated over the resulting time traces of lysing the photokinetics in conjugated polymers,35 or detect the intensity I(t) or its deviations dI(t) ¼ I(t) hI(t)i from the molecular mobility at interfaces,36,37 nor will this review cover mean intensity: 38–44 FCS studies performed with nanoparticles. With respect to hdIðtÞ$dIðt þ sÞi hIðtÞ$Iðt þ sÞi GðsÞ¼ ¼ À 1 (1) the latter topic, I refer to the review on FCS studies in the hIðtÞi2 hIðtÞi2 journal “Current Opinion in Colloid & Interface Science” by 45 Koynov et al. The autocorrelation function contains information on all processes which cause intensity uctuations within the Fluorescence correlation spectroscopy confocal volume. Such processes are translational and rota- tional diffusion or photophysical or photochemical trans- Basics of uorescence correlation spectroscopy formations of the uorescent probes such as transitions to the In the following, the basic principles of FCS will be summa- triplet state. Due to the large span in time scales of these rized. In a typical FCS experiment, as shown in Fig. 1, a colli- different processes, the time axis of the autocorrelation func- mated laser beam is focused by an objective lens into a tion is oen represented in a logarithmic scale. In the following paragraph, I will concentrate on trans- lational diffusion. As the uorescent probe enters or leaves the confocal volume, it causes uctuations which can be typically observed as a drop in the autocorrelation in the millisecond to second range. The autocorrelation is described by the following function2 !À1 À1 2 2 Published on 05 November 2013. 1 s wxy s GðsÞ¼ 1 þ 1 þ (2) N sD wz sD where N is the number of independently diffusing dye mole- s ff cules in the confocal volume, D is the average di usion time of the probes through the confocal volume with the radial and the axial diameter wxy and wz, respectively. Typical values are ca. 300 nm for wxy, and ca. 1.5 mm for wz. The square root term in Fig. 2 Effect of the pinhole on the axial resolution of a confocal microscope. Only light originating from the focal plane (red solid line, orange area) can pass the pinhole without loss. Light from other planes Fig. 1 Typical FCS setup as described in the text. (green dotted line and blue dotted line) is mainly blocked. 2448 | RSC Adv.,2014,4, 2447–2465 This journal is © The Royal Society of Chemistry 2014 View Article Online Review RSC Advances eqn (2) accounts for the ellipsoid shape of the confocal with each additional free parameter improving the t, oen volume. however without obtaining additional physical information. s ff ff The highest sensitivity for the determination of D is realized Instead, if one molecular species di uses in di erent environ- when in average approx. one independently diffusing probe ments bearing different diffusion coefficients but a constant molecule is present in the confocal volume, i.e. at sub-nano- uorescence yield, it oen makes more sense to obtain diffu- molar concentrations. For much higher concentrations, the sion time distributions. This requires solving the ill-posed changes in intensity caused by probes entering and leaving the problem of calculating the distribution P(sD) from the auto- confocal volume have to be detected against an intensity correlation function2 s ð !À1 background limiting the sensitivity of determining D.As À1 2 2 s wxy s obvious from eqn (2), a high value of N results in a low auto- GðsÞ¼ PðsDÞ 1 þ 1 þ dsD (6) s wz s correlation which oen cannot be tted appropriately.
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