Anastasia Meristoudi

Thermoresponsive behavior of poly(N-isopropylacrylamide)s with dodecyl and carboxyl terminal groups in aqueous solution: pH-dependent cloud point temperature

It was recently reported that poly(N-isopropyl acrylamide) (PNIPAm) polymers synthesized by RAFT polymerization using S-1-dodecyl-S′-(α,α′-dimethyl-α′′-acetic acid)trithiocarbonate as a chain transfer agent form micelles in aqueous solutions with the core of hydrophobic terminal dodecyl groups and the corona of PNIPAm chains with carboxylic groups at the periphery, the ionization of which prevents the micelles from phase separation above the lower critical solution temperature of PNIPAm in water (Langmuir 30:7986–7992). In this paper, we study the pH- and ionic strength-dependence of the aggregation behavior of two HOOC-PNIPAm-C12 polymers, differing in the degree of polymerization, in aqueous solutions. We show that the cloud point temperature (CPT) of HOOC-PNIPAm-C12 can be shifted up to several tens of K by changing pH of the solution. The aggregation of the PNIPAms above the CPT can be efficiently accelerated by screening electrostatic repulsion between PNIPAm micelles by changing ionic strength of the solution.

Lysozyme complexes with thermo- and pH-responsive PNIPAM-b-PAA block copolymer

Lysozyme is an enzyme responsible for the damage of bacterial cell walls and is abundant in a number of secretions such as tears and human milk. In the present study, we investigated the structure, the physicochemical characteristics, and the temperature-responsiveness of lysozyme complexes with poly(N-isopropylacrylamide)-b-poly(acrylic acid) block polyelectrolyte in aqueous media. A gamut of light-scattering techniques and fluorescence spectroscopy were used in order to examine the complexation process, as well as the structure, solution behavior, and temperature response of the nanosized complexes. The concentration of copolymer polyelectrolyte was kept constant. The values of the scattering intensity, I90, which is proportional to the mass of the species in solution, increased gradually as a function of CLYS, providing proof of the occurring complexation, while the size of the nanostructures decreased. The structure of the complexes became more open as the CLYS increased. The increase of the salinity did not affect the structural characteristics of the supramolecular nanoparticulate aggregates. On the other hand, the physicochemical and structural characteristics of the complexes changed upon increasing temperature, and the changes depended on the initial ratio block polyelectrolyte/lysozyme. The knowledge on developing block polyelectrolyte/protein complexes through electrostatic interactions, obtained from this investigation, may be applied to the design of nutraceuticals.

Characterization of Industrial Coolant Fluids and Continuous Ageing Monitoring by Wireless Node-Enabled Fiber Optic Sensors

Environmentally robust chemical sensors for monitoring industrial processes or infrastructures are lately becoming important devices in industry. Low complexity and wireless enabled characteristics can offer the required flexibility for sensor deployment in adaptable sensing networks for continuous monitoring and management of industrial assets. Here are presented the design, development and operation of a class of low cost photonic sensors for monitoring the ageing process and the operational characteristics of coolant fluids used in an industrial heavy machinery infrastructure. The chemical, physical and spectroscopic characteristics of specific industrial-grade coolant fluids were analyzed along their entire life cycle range, and proper parameters for their efficient monitoring were identified. Based on multimode polymer or silica optical fibers, wide range (3–11) pH sensors were developed by employing sol-gel derived pH sensitive coatings. The performances of the developed sensors were characterized and compared, towards their coolants’ ageing monitoring capability, proving their efficiency in such a demanding application scenario and harsh industrial environment. The operating characteristics of this type of sensors allowed their integration in an autonomous wireless sensing node, thus enabling the future use of the demonstrated platform in wireless sensor networks for a variety of industrial and environmental monitoring applications.

Design and development of pH-sensitive liposomes by evaluating the thermotropic behavior of their chimeric bilayers

This study is focused on mixed/chimeric advanced drug delivery nanosystems and specifically on pH-sensitive liposomes, combining lipids and pH-responsive amphiphilic block copolymers. Chimeric liposomes are composed of hydrogenated soy phosphatidylcholine (HSPC) and two different poly(n-butylacrylate)-b-poly(acrylic acid) (PnBA-b-PAA) block copolymers with 85 and 70% content of PAA, at six different molar ratios. PAA block exhibits pH responsiveness, because of the regulative group of –COOH. Chimeric bilayers are composed of HSPC and PnBA-b-PAA. Experiments are carried out by using differential scanning calorimetry (DSC) in order to investigate their thermotropic properties. DSC indicated disappearance of the pretransition effect in all chimeric lipid bilayers, at both buffers [phosphate buffer saline (PBS) and citrate buffer], and slight changes of the main transition temperature (Tm). Contrariwise, the cooperativity (T1/2) presented alterations between the two different buffers. Chimeric liposomes have been prepared and their physicochemical characteristics have been explored in PBS and citrate buffer by measuring the size, size distribution and ζ-potential. Liposomes are found to retain the mean value of their size during the stability studies. The physicochemical characteristics and the stability assessment of chimeric liposomes are correlated with DSC measurements of mixed bilayers. The incorporation of the appropriate amount of these novel pH-responsive block copolymers affects the cooperativity and the liposomal stabilization and imparts pH responsiveness (functionality), which was confirmed by performing experiments in acidic environment (citrate buffer). In conclusion, the results from DSC measurements provide useful information regarding the quality by design process for rationally preparing mixed/chimeric liposomal platforms to incorporate bioactive molecules.

A dual-stimuli-responsive polymer into phospholipid membranes A thermotropic approach

In this study, we investigate the thermotropic effects of diblock copolymer poly(N-isopropylacrylamide)-block-poly(acrylic acid) (PNIPAM-b-PAA) on fully hydrated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipid bilayers and its ability to alter the membranes’ organization, fluidity and phase behavior. The composition of the diblock copolymer and the nature of dispersion medium (pH and ionic strength) were also examined. For these purposes, pure DPPC lipid and polymer–lipid mixed systems, hydrated in three different dispersion media (i.e., HPLC-grade water, phosphate buffer saline and hydrochloric acid solution of pH 4.5), were investigated by differential scanning calorimetry. Two compositions of PNIPAM-b-PAA with different molar ratio of the polymeric blocks were used. PNIPAM-b-PAA presents great scientific interest due to the combination of the special characteristics of its homopolymer components; it is dual responsive both in temperature and in pH changes. The incorporation of the PNIPAM-b-PAA into the DPPC bilayers causes particularly significant perturbations in their thermotropic behavior, slightly different in each dispersion medium. The results indicated the ordering of the polymer guest near the polar head group surface probably by its PAA block and, on the other hand, the penetration of the PNIPAM block into the hydrophobic bilayer core, causing membrane disruption in a temperature-depended manner. We can conclude that the lipid–polymer interactions seem to be affected by the pH and the ionic strength of the hydration medium, as well as the polymer content incorporated in the DPPC bilayer. These studies could be a roadmap in order to rationally design and develop chimeric liposomes.

Assessment of fiber optic sensors for aging monitoring of industrial liquid coolants

Lately the demand for in situ and real time monitoring of industrial assets and processes has been dramatically increased. Although numerous sensing techniques have been proposed, only a small fraction can operate efficiently under harsh industrial environments. In this work the operational properties of a proposed photonic based chemical sensing scheme, capable to monitor the ageing process and the quality characteristics of coolants and lubricants in industrial heavy machinery for metal finishing processes is presented. The full spectroscopic characterization of different coolant liquids revealed that the ageing process is connected closely to the acidity/ pH value of coolants, despite the fact that the ageing process is quite complicated, affected by a number of environmental parameters such as the temperature, humidity and development of hazardous biological content as for example fungi. Efficient and low cost optical fiber sensors based on pH sensitive thin overlayers, are proposed and employed for the ageing monitoring. Active sol-gel based materials produced with various pH indicators like cresol red, bromophenol blue and chorophenol red in tetraethylorthosilicate (TEOS), were used for the production of those thin film sensitive layers deposited on polymers and silicas large core and highly multimoded optical fibers. The optical characteristics, sensing performance and environmental robustness of those optical sensors are presented, extracting useful conclusions towards their use in industrial applications.

Nonlinear Optical Properties of Au and Ag Nanoparticles Embedded into Hybrid‐block Copolymer Micelles

The nonlinear optical (NLO) properties of two classes block‐copolymer micelles containing metallic nanoparticles of gold are silver are reported. The NLO properties of the incorporated organic‐inorganic materials were determined by means of Optical Kerr Effect (OKE) and Z‐scan technique, under 35 ps and 4 ns at 532 nm excitation wavelength. The results showed that the nonlinear response depends on the metallic load of the micelles and from the size of the forming micelles.

Calorimetric study on pH-responsive block copolymer grafted lipid bilayers: rational design and development of liposomes

Abstract This study is focused on chimeric advanced drug delivery nanosystems and specifically on pH-sensitive liposomes, combining lipids and pH-responsive amphiphilic block copolymers. Chimeric liposomes composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and two different forms of block copolymers, i.e. poly(n-butylacrylate)-b-poly(acrylic acid) (PnBA-b-PAA) at 70 and 85% content of PAA at six different molar ratios, each form respectively. PAA block exhibits pH-responsiveness, because of the regulative group of –COOH. –COOH is protonated under acidic pH (pKa ca. 4.2), while remains ionized under basic or neutral pH, leading to liposomes repulse and eventually stability. Lipid bilayers were prepared composed of DPPC and PnBA-b-PAA. Experiments were carried out using differential scanning calorimetry (DSC) in order to investigate their thermotropic properties. DSC indicated disappearance of pre-transition at all chimeric lipid bilayers and slight thermotropic changes of the main transition temperature. Chimeric liposomes have been prepared and their physicochemical characteristics have been explored by measuring the size, size distribution and ζ-potential, owned to the presence of pH-responsive polymer. At percentages containing medium to high amounts of the polymer, chimeric liposomes were found to retain their size during the stability studies. These results were well correlated with those indicated in the DSC measurements of lipid bilayers incorporating polymers in order to explain their physicochemical behavior. The incorporation of the appropriate amount of these novel pH-responsive block copolymers affects thus the cooperativity, the liposomal stabilization and imparts pH-responsiveness.

Development and Nonlinear Optical Properties of Block Copolymers Micelles Encapsulating Metal Nanoparticles

Hybrid materials, consisting of an amphiphilic block copolymer matrix incorporating metal nanoparticles, have attracted great attention due to their unique properties, such as the catalytic behavior or the optical properties, making them potential candidates for photonic devices. High purity block copolymers, such as poly(isoprene‐b‐acrylic acid) (PI‐b‐PAA) and poly[(sodium sulfamate‐carboxylate‐isoprene)‐b‐tert‐butylstyrene] (BS‐SCI), have been synthesized by anionic polymerization and dissolved in selective solvents for the formation of well defined micelles. These block copolymers have been used as matrices for the in situ synthesis of gold nanoparticles either inside the dense core or at the periphery of the corona of their micelles. The systems were investigated by dynamic light scattering (DLS), UV‐visible and ATR‐IR spectroscopy, atomic force microscopy (AFM) and transmission electron microscopy (TEM). Additionally, the third non linear optical response of the hybrid materials synthesized has been ...

The modulation of physicochemical characterization of innovative liposomal platforms: the role of the grafted thermoresponsive polymers.

Abstract This study is focused on chimeric advanced drug delivery systems and specifically on thermosensitive liposomes, combining lipids and thermoresponsive polymers. In this investigation, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) chimeric liposomal systems were prepared, incorporating the homopolymer C12H25-poly(N-isopropylacrylamide)-COOH (C12H25-PNIPAM-COOH) and the block copolymer poly(n-butylacrylate-b-N-isoropylacrylamide) (PnBA-PNIPAM), at six different molar ratios. Both of these polymers contain the thermoresponsive PNIPAM block, which exhibits lower critical solution temperature (LCST) at 32 °C in aqueous solutions, changing its nature from hydrophilic to hydrophobic above LCST. During the preparation of liposomes, the dispersions were observed visually, while after the preparation we studied the alterations of the physicochemical characteristics, by measuring the size, size distribution and ζ-potential of prepared liposomes. The presence of polymer, either C12H25-PNIPAM-COOH or PnBA-PNIPAM, resulted in liposomes exhibiting different physicochemical characteristics in comparison to conventional DPPC liposomes. At the highest percentage of the polymeric guest, chimeric liposomes were found to retain their size during the stability studies. The incorporation of the appropriate amount of these novel thermoresponsive polymers yields liposomal stabilization and imparts thermoresponsiveness, due to the functional PNIPAM block.