We report a new strategy for the preparation of well-defined and mechanically stable porous nanostructures with tunable porosity. Silica inverse opals, which are known as a model system for a porous periodic nanostructure, were grafted with brushes of the thermoresponsive poly(N-isopropylacrylamide) grown via atom transfer radical polymerization. By tuning the temperature, the swelling state of the brush layer is reversibly altered, and with this we were able to control the overall porosity of the system and, thus, the mobility of small penetrants. Fluorescence correlation spectroscopy, a method combining single molecule sensitivity with small probing volume (<1 μm3), was used to directly monitor and quantify in situ the changes in the penetrants’ mobility.

•No adsorption of fluorescently labelled serum albumin on starch-based nanocapsules.•Adsorption suppression was independent of type and number of labels.•Albumin adsorption was not affected for hydrophobic polystyrene nanoparticles.Fluorescently labelled proteins are often used to study processes in vitro, e.g. the binding of proteins to cell surfaces or the adsorption of plasma proteins on drug nanocarriers. However, the fact that the fluorescent labelling may affect the protein properties is frequently neglected. On the example of a simple model system, we reiterate the importance of this issue by showing that even a single label may perturb interactions between hydrophilic starch-based nanocapsules and serum albumin and thus prevent binding.Fluorescence labels on serum albumin suppress adsorption onto hydroxyethyl starch nanocapsules while adsorption onto polystyrene nanoparticles is not affected.

Fluorescence correlation spectroscopy (FCS) has become an important tool in polymer science. Among various other applications the method is often applied to measure the hydrodynamic radius and the degree of fluorescent labeling of polymers in dilute solutions. Here we show that such measurements can be strongly affected by the molar mass dispersity of the studied polymers and the way of labeling. As model systems we used polystyrene and poly(methyl methacrylate) synthesized by atom transfer radical polymerization or free-radical polymerization. Thus, the polymers were either end-labeled bearing one fluorophore per chain or side-labeled with a number of fluorophores per chain proportional to the degree of polymerization.The experimentally measured autocorrelation curves were fitted with a newly derived theoretical model that uses the Schulz–Zimm distribution function to describe the dispersity in the degree of polymerization. For end-labeled polymers having a molecular weight distribution close to Schulz–Zimm, the fits yield values of the number-average degree of polymerization and the polydispersity index similar to those obtained by reference gel permeation chromatography. However, for the side-labeled polymers such fitting becomes unstable, especially for highly polydisperse systems. Brownian dynamic simulations showed that the effect is due to a mutual dependence between the fit parameters, namely, the polydispersity index and the number-average molecular weight. As a consequence, an increase of the polydispersity index can be easily misinterpreted as an increase of the molecular weight when the FCS autocorrelation curves are fitted with a standard single component model, as commonly done in the community.

Fluorescence correlation spectroscopy (FCS) was applied to directly monitor the hydrophobic collapse of pH-responsive hairy nanoparticles at the individual particle level. To this end, fluorescent nanoparticles (hydrodynamic radius 20 nm) with polystyrene core and poly(N,N-diethylaminoethyl methacrylate) (PDEA) shell were prepared and used as a model system. Dynamic light scattering and turbidity measurements showed that the hydrophobic collapse of the hairs at high pH values is associated with strong interparticle aggregation that hinders determination of individual particles size. However, at the ultralow concentrations assessable by FCS (less than one particle per femtoliter) the aggregation was prevented. Thus, the pH-induced change in the particles size caused by the swelling or the collapse of the PDEA hairs was systematically measured and compared with that of individual freely diffusing PDEA chains under similar conditions.

We investigated the equilibrium chain-exchange kinetics of amphiphilic diblock copolymer micelles, using a new method based on fluorescence correlation spectroscopy. The micelles were formed from polystyrene-block-poly[oligo(ethylene glycol) methyl ether methacrylate] (PS–POEGMA) in different solvents and studied at various temperatures. This linear-brush copolymer was chosen as a model system, forming micelles with short and bulky corona. Depending on the applied solvent, fast exchange could be observed even at temperatures well below the nominal glass transition of the core-forming PS block. The effect is caused by swelling of the core and allows extensive tuning of the chain-exchange rate by adding to the system minor amounts of good or bad solvent for the core block.

Fluorescence correlation spectroscopy was applied to study the diffusion of isolated surface-active molecules at air/water interfaces. Rhodamine 6G was used as a surface-active fluorescent tracer. Results show that the diffusion coefficient of the Rhodamine 6G at the interface is about 2.5 times higher than in the bulk. Effects of Rhodamine 6G concentration and added SDS or CTAB surfactants have been studied. Diffusion of Rhodamine 6G at the interface is slowed down at surfactant concentration corresponding to a mean distance between molecules of 10 and 40 nm, indicating a long-range interaction.

Conjugated polymers offer unique combination of easily tailored mechanical, electrical and optical properties that makes them perfect materials for the preparation of various devices such as light-emitting diodes, photovoltaic cells or field-effect transistors. However, the design and fabrication of such devices in a controlled and reproducible way are possible only if the behavior and the properties of individual polymer chains are well understood. One major problem in this respect is that aggregation often occurs even in dilute solutions and prevents the single polymer chain studies. To address this issue, in this work we employed fluorescence correlation spectroscopy (FCS) to study the behavior of a model conjugated polymer, poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) in several commonly used solvents. The very high sensitivity of FCS allowed measurements in ultradilute solutions and thus unambiguous determination of the hydrodynamic radius of single polymer chains. The solvent quality for MEH-PPV was then quantitatively evaluated from the measured logarithmic scaling of the single chain hydrodynamic radius versus the polymer molecular weight. Scaling exponents of 0.40, 0.41, and 0.43 were found in toluene, chloroform and 1,2-dichlorobenzene, respectively. These values are well below the θ-condition, emphasizing poor solvent quality for MEH-PPV, despite the fact that all studied solvents are commonly regarded as “good” solvents. In addition, by investigating the aggregation behavior of MEH-PPV at higher polymer concentrations, we found a clear relation between aggregates size and solvatochromism that indicates more extended chain conformation in larger aggregates..

Dual color fluorescence cross-correlation spectroscopy (DC FCCS) experiments were conducted to study the coalescence and aggregation during the formation of nanoparticles. To assess the generality of the method, three completely different processes were selected to prepare the nanoparticles. Polymeric nanoparticles were formed either by solvent evaporation from emulsion nanodroplets of polymer solutions or by miniemulsion polymerization. Inorganic nanocapsules were formed by polycondensation of alkoxysilanes at the interface of nanodroplets. In all cases, DC FCCS provided fast and unambiguous information about the occurrence of coalescence and thus a deeper insight into the mechanism of nanoparticle formation. In particular, it was found that coalescence played a minor role for the emulsion-solvent evaporation process and the miniemulsion polymerization, whereas substantial coalescence was detected during the formation of the inorganic nanocapsules. These findings demonstrate that DC FCCS is a powerful tool for monitoring nanoparticles genesis.

The successful non-invasive treatment of diseases associated with the central nervous system (CNS) is generally limited by poor brain permeability of various developed drugs. The blood–brain barrier (BBB) prevents the passage of therapeutics to their site of action. Polymeric drug delivery systems are promising solutions to effectively transport drugs into the brain. We recently showed that amphiphilic random copolymers based on the hydrophilic p(N-(2-hydroxypropyl)-methacrylamide), pHPMA, possessing randomly distributed hydrophobic p(laurylmethacrylate), pLMA, are able to mediate delivery of domperidone into the brain of mice in vivo. To gain further insight into structure–property relations, a library of carefully designed polymers based on p(HPMA) and p(LMA) was synthesized and tested applying an in vitro BBB model which consisted of human brain microvascular endothelial cells (HBMEC). Our model drug Rhodamine 123 (Rh123) exhibits, like domperidone, a low brain permeability since both substances are recognized by efflux transporters at the BBB. Transport studies investigating the impact of the polymer architecture in relation to the content of hydrophobic LMA revealed that random p(HPMA)-co-p(LMA) having 10 mol% LMA is the most auspicious system. The copolymer significantly increased the permeability of Rh123 across the HBMEC monolayer whereas transcytosis of the polymer was very low. Further investigations on the mechanism of transport showed that integrity and barrier function of the BBB model were not harmed by the polymer. According to our results, p(HPMA)-co-p(LMA) copolymers are a promising delivery system for neurological therapeutics and their application might open alternative treatment strategies.

In the last two decades fluorescence correlation spectroscopy (FCS) has been increasingly applied to analyze systems and processes relevant to colloid and interface science. The method has become a routine tool to measure the hydrodynamic radii of small fluorescent molecules, macromolecules and nanoparticles, characterize their interactions and follow a possible aggregation. It was also used to study the diffusion of such species in inhomogeneous media like polymer melts, solutions, gels or porous structures. The formation kinetics and size of micelles of surfactants or block copolymers has been quantified. FCS has also been applied to characterize diffusion of tracers at fluid–liquid and solid–liquid interfaces and study the hydrodynamic boundary condition. The review is intended to summarize these applications and highlight perspectives but also limits of FCS in colloid and interface science.Highlights► Fluorescence correlation spectroscopy has become a routine tool in soft matter science. ► Hydrodynamic radii of fluorescent molecules and nanoparticles can be measured. ► Dynamics in polymer melts, solutions, gels or porous structures can be studied. ► Diffusion coefficients at fluid–liquid and solid–liquid interfaces can be measured.

We employed fluorescence correlation spectroscopy (FCS) to study the diffusion of molecular and macromolecular tracers in polystyrene solutions over a broad range of concentrations (c) and molecular weights (Mw,m) of the matrix polymer. Molecular tracer diffusion scales only with the matrix concentration and superimposes on a single, nonpolymer specific, curve. On the contrary, the diffusion of macromolecular tracers in solutions of matrix polymers with Mw,m sufficiently larger than the tracer molecular weight scales with c/cp*, where cp* is the tracer overlap concentration. We further demonstrate that FCS can address local and global dynamics simultaneously.

Fluorescence correlation spectroscopy (FCS) was employed to study the diffusion of molecular tracers in different polymer melts (polydimethysiloxane (PDMS), 1,4-cis-polyisoprene (PI), poly(vinylethylene) (PVE), and a symmetric PI/PVE blend) as a function of molecular weight (Mw) and temperature (T). The single molecule sensitivity of the FCS technique precludes any modification of the matrix polymer properties. In all studied systems, the small tracer diffusion coefficient D(Mw,T) senses local segmental dynamics depending on the glass transition temperature Tg(Mw) of the polymer matrix and not its macroscopic viscosity. From the good representation of the D(T) data by the common non-Arrhenius (VFT) function, we found that the activation energy (BD) increases with tracer size (R) and for a given tracer the value of BD in PI is almost 2 times bigger than in PDMS. The possibility to establish a direct relation between D(T) and the segmental relaxation time τ(T) of the polymer matrix was critically addressed based on experimental data in dynamically homogeneous (homopolymers) and heterogeneous (miscible blend) systems and discussed in view of recent computer simulations of polymer/penetrant mixtures.

Fluorescence correlation spectroscopy (FCS) was employed to study the effect of chain topology on the small tracer diffusion in linear and star-shaped 1,4-polyisoprenes (PIs) at temperatures well above the glass transition. In the linear polyisoprene, a Fickian diffusion behavior was observed that is characterized by a single diffusion coefficient. In contrast, the diffusion of the same tracers in three-arm star PI was non-Fickian and could best be described assuming two diffusion modes. The fast mode diffusion has the same magnitude and temperature dependence as the tracer diffusion in linear PI. The slow mode present only in the star polymer is apparently related to topological restrictions that cause retention of the tracer. The temperature dependence of the retention time is very similar to that of the polymer arm relaxation, indicating a direct correlation between the two dynamic processes.