Synthesis and characterisation of spiropyran-polymer brushes in micro-capillaries: towards an integrated optical sensor for continuous flow analysis

Fused silica-capillaries were successfully functionalised with spiropyran-polymer brushes using spiropyran functionalised norbornyl derivative as monomer. The polymerisation was achieved by surface-initiated ring- opening metathesis polymerisation. A three-dimensional arrangement, covalently attached to the inner wall of the fused-silica capillary, was obtained. The spiropyran moiety has the freedom to open and close in response to light (ultraviolet, white light) within the polymer brushes. The coating was fully characterised by Scanning Electron Microscopy, absorbance measurements and kinetic studies. The photo-response of the coatings showed very good reproducibility comparable with spiropyran monomers in solution demonstrating that this platform can be used for the develop of capillary integrated sensors based on the inherited sensing proprieties of spiropyran moieties

Spiropyran polymeric microcapillary coatings for photodetection of solvent polarity

Fused silica micro-capillaries were functionalised with spiropyran-polymer brushes using surface-initiated ring-opening metathesis polymerisation. Based on the inherited spiropyran properties, the functionalised capillaries were successfully used to photo-identify solvents of different polarity when passing through the micro-capillary in continuous flow. In the present study, six different solvents (toluene, tetrahydrofuran, acetone, acetonitrile, ethanol and methanol) can be easily detected while passing through the modified micro-capillary by simply irradiating a portion of it with UV light (365 nm). This converts the closed spiropyran moiety to the open merocyanine form and as a consequence, the micro-capillary gains a distinct colour and spectral response depending on the polarity of the solvent. The rate of ring opening of the spiropyran-polymer brushes coatings has been determined in-situ in the presence of different solvents, showing that the coloration rate is also influenced by the solvent polarity and therefore can be used as an additional parameter for solvent sensing

Stimuli-responsive materials: developing integrated opto-molecular systems as sensors and actuators in micro-fluidic devices

Micro-fluidic platforms have been conferred with inherent optical sensing capabilities by coating the walls of micro-fluidic channels or micro-capillaries with stimuli-responsive materials. These adaptive materials respond optically to environmental stimuli, such as changes in pH, solvent polarity, the presence of certain metal ions and light. This approach confers sensing capabilities along the entire length of the coated micro-channel or micro-capillary. Adaptive coatings based on two types of materials are presented: 1. Conductive polymer polyaniline - The optical properties of these coatings respond to changes in the pH of the solution that is passing through the micro- channel or micro-capillary, and therefore can be used for dynamic pH monitoring (pH 2-8) or for aqueous ammonia sensing. 2. Photochromic spiropyrans - Photoswitchable coatings based on spiropyran are used to photo-detect solvents of different polarity when passing through the micro-capillary in continuous flow. This sensing behaviour can be switched on/off remotely using light. Finally, it is reported, for the first time, the potential of using spiropyran as a pH pump in fluidic channels for photo-activated chemopropulsion of organic droplets and the solvato-morphological control of self-assembled micro-structures based on spiropyran

Photo-responsive polymeric structures based on spiropyran

Spiropyrans are one of the most popular classes of photochromic compounds that change their optical and structural properties in response to external inputs such as light, protons and metal ions, making them ideal molecules for the fabrication of multifunctional stimuli-responsive materials. Nowadays, the emphasis in polymeric materials incorporating spiropyran units, focuses on the effectiveness of their reversible response to external photonic stimuli. Photo-control of a range of key characteristics for flow systems, such as wettability, permeability, photo-modulation of flow by photo-actuation of valves, photonic control of uptake and release of guests using films and coatings, and colorimetric sensing of various species, are highlighted and discussed

Multi-purpose capillary-integrated optical sensors based on spiropyran

Optical chemical sensors for liquid phase monitoring (overwhelmingly focused on water-based samples) often employ a dye or indicator that is immobilised onto a solid support material. However, this strategy presents two main disadvantages: firstly, the immobilisation process may lead to losses in dye sensitivity and secondly, the stability of the sensor is affected by dye leaching into the sample solution over time, making long-term use impractical. Therefore, effective optical-chemical sensors require new materials that are able to overcome all these problems. In this context, inclusion of photocromic molecules in solid matrices is of particular interest for the development of new approaches for such opto-sensing. Spiropyrans offer new routes for the fabrication of multifunctional materials since it is possible to take advantage of their photo-reversible interconversion between the two thermodynamically stable states of the molecule: a spiropyran (SP) form, and a merocyanine (MC) form, which have dramatically different charge, polarity and molecular conformations. It is well known that the open-chain merocyanine isomers of spiropyrans derivatives present negative solvatochromism, meaning that their absorption bands undergo a hypsochromic (blue) shift in solvents of increasing polarity, property that made them used as solvatochromic dyes. Another interesting property of spiropyran is their sensitive towards pH. By adding acid, the opened MC is protonated to form the protonated merocyanine (MCH+) form. A metal ion-binding center can also be formed by a spatial rearrangement of opened MC form, thus spiropyrans present a high interest for photo-reversible metal ion- complexation. Spiropyran derivatives are therefore sensitive towards many external agents, thus making them compounds of choice in the next generation sensors. In this context, here we present the functionalisation of the inner walls of micro-capillaries with polymeric coatings based on a spiropyran derivative and their successful use as capillary integrated optical sensors for a variety of target analytical species (divalent metal ions, solvents of different polarities, H+). The polymeric brushes approach offers a nanostructured to microstructured responsive coating insuring small diffusion paths and fast response times towards the target species. Moreover, this sensing behavior can be switched ON-OFF using light of appropriate wavelengths

Stimuli-responsive materials as sensors and actuators in microfluidic devices

The integration of stimuli-responsive materials into microfluidic systems can provide a means for external control over fluid flow and can reduce the overall complexity of microfluidic devices. Herein we present several approaches for introducing fluid movement and sensing using stimuli-responsive materials. The first approach comprises the use of adaptive nanostructured coatings for direct sensing of flow in continuous flow mode. For this, the inner walls of micro-capillaries and micro-channels were coated with polymeric materials that can be used to detect a variety of target species. Two types of adaptive coatings will be discussed. The first one is based on the conductive polymer polyaniline (PAni) [1,2] while the second consists of polymeric brushes based on spiropyran [3,4]. Using the “grafting” approach homogeneous coatings were obtained on the micro-channel/micro-capillary surface that retained their inherent nano-morphology. The optical proprieties of these coatings change in response to a variety of target analytical species (divalent metal ions, solvents of different polarities, ammonia, H+) passing through the microfluidic device in continuous flow mode. The grafting approach can provide nanostructured to microstructured coatings that combine small diffusion paths with relatively thick optical pathlengths, thereby providing sensitive and fast optical responses to the target analytes. The second approach comprises the use of porous photo-actuated hydrogels as photo-controlled micro-valves in microfluidic systems for repeatable ON/OFF flow modulation in flowing streams over a wide range of pH values (acidic to ca. pH 7.0). Incorporation of such stimuli-controlled structures in microfluidic devices offers unprecedented versatility and external flow control. We envision using these systems to create a new generation of sustainable, low-cost, photonically-controlled and self-reporting fluidic systems

The toolbox of porous anodic aluminum oxide–based nanocomposites: from preparation to application

Anodic aluminum oxide (AAO) templates have been intensively investigated during the past decades and have meanwhile been widely applied through both sacrificial and non-sacrificial pathways. In numerous non-sacrificial applications, the AAO membrane is maintained as part of the obtained composite materials; hence, the template structure and topography determine to a great extent the potential applications. Through-hole isotropic AAO features nanochannels that promote transfer of matter, while anisotropic AAO with barrier layer exhibits nanocavities suitable as independent and homogenous containers. By combining the two kinds of AAO membranes with diverse organic and inorganic materials through physical interactions or chemical bonds, AAO composites are designed and applied in versatile fields such as catalysis, drug release platform, separation membrane, optical appliances, sensors, cell culture, energy, and electronic devices. Therefore, within this review, a perspective on exhilarating prospect for complementary advancement on AAO composites both in preparation and application is provided

Application and Development of Mechanoresponsive Polymer Structures

Mechanoresponsive Systeme antworten auf mechanische Reize mit einer Eigenschaftsänderung. Diese Dissertation umfasst die Arbeiten mit zwei mechanoresponsiven Systemen, die optisch auf mechanische Reize antworten. Sie basieren auf polymeren Strukturen, einer Polymerbürste und einem Hydrogelnetzwerk. Ihr optischer Antwortmechanismus ermöglicht die Beobachtung wirkender Kräfte als ein Ansatz zur in situ-Kraftmessung. Im ersten Teil wird ein existierendes, mechanoresponsives System zur Anwendung gebracht, das auf einer mit Fluoreszenzfarbstoff markierten Polyelektrolytbürste basiert. Die Ladungen des Polyelektrolyts können die Fluoreszenz des Farbstoffs unterdrücken, sodass lokale Kompression und Zugspannung über die Fluoreszenzintensität unterschieden werden können. Die mechanoresponsive Polymerbürste wurde als mechanosensitive Oberflächenbeschichtung angewandt, um Unterschiede in der Kontaktspannungsverteilung von Gecko-inspirierten adhäsiven Mikrostempelstrukturen aufzuklären. Die erarbeiteten Ergebnisse und daraus abgeleiteten Ablösemechanismen der Mikrostempeltypen deckten sich qualitativ mit Vorhersagen aus theoretischen Ansätzen. Aufgrund geometrischer Einschränkungen einer planaren Oberflächenbeschichtung zielt der zweite Teil darauf ab, dieses mechanoresponsive Prinzip in ein dreidimensionales Netzwerk zu überführen und ein mechanoresponsives Hydrogelnetzwerk als Plattform zur Kraftmessung zu entwickeln. Konzeptionell besitzt ein homogenes Netzwerk vorhersagbare mechanische Eigenschaften, sodass lokale optische Antworten auf mechanische Kräfte ermöglichen könnten, die wirkenden Kräfte zu lokalisieren und quantifizieren. Basierend auf einer Gestaltung nach der Flory-Rehner-Theorie wurden Präkursoren mit vordefinierter Größe und Architektur für die Hydrogelherstellung eingesetzt, um auf ein homogenes Netzwerk abzuzielen. Zu diesem Zweck wurde das Mischungsvolumen durch Tropfenmikrofluidik reduziert. Für den optischen Antwortmechanismus wurden die Hydrogelnetzwerk-Präkursoren mit zwei verschiedenen Fluorophoren markiert, die sich durch abstandsabhängige Emission über Förster-Resonanzenergietransfer auszeichnen. Die Funktionalität des optischen Antwortmechanismus wurde auf globaler Ebene durch Kollabieren und kontrolliertes Quellen des Netzwerks, dann auf lokalisierter Ebene durch definierte mechanische Belastung mit Rasterkraftmikroskopie gezeigt. Durch ihre Anpassbarkeit könnte die Hydrogelplattform zukünftig verschiedenste Anwendungen im Bereich intrisischer Kraftmessung weicher Materie bedienen.Mechanoresponsive systems respond to mechanical triggers by changes in a certain property. This thesis covers the work conducted with two mechanoresponsive systems that respond optically to mechanical triggers. These two systems are based on polymer structures, a polymer brush and a hydrogel network. Thus, the optical response mechanism allows observing acting forces as an approach to force sensing in situ. In the first part, an existing mechanoresponsive system based on a polyelectrolyte brush labeled with a fluorescent dye is engaged in application. The charges of the polyelectrolyte are able to quench the fluorescence of the dye so that local compression or tension can be distinguished from the local fluorescence intensity. The mechanoresponsive polymer brush was applied as mechanosensitive surface coating to elucidate differences in the contact stress distributions of gecko-inspired adhesive micropillar structures. The determined results and the derived detachment mechanisms of the micropillar types were in qualitative accordance with predictions from theoretical approaches. Overcoming the geometrical limitations of a planar surface coating, the second part aims at translating the mechanoresponse principle to a three-dimensional network and developing a mechanoresponsive hydrogel as a platform for force sensing. Conceptually, a homogeneous network allows to predict mechanical properties so that localized optical mechanoresponses could enable locating and quantifying acting forces. Based on network design principles from the Flory-Rehner theory, precursors with predefined size and architecture were utilized in hydrogel preparation, aiming for a homogeneous network. Further in this regard, the mixing volume was reduced by employing droplet microfluidics. As optical response mechanism, the hydrogel network precursors were labeled with two kinds of fluorophore, featuring distance-dependent emission from Förster Resonance Energy Transfer. The functionality of the optical response mechanism was demonstrated on global level by collapsing and controlled swelling of the network, and on a localized level by defined mechanical stress, applied with Atomic Force Microscopy. Owing to its adjustability, the hydrogel platform might be employed in various applications that require intrinsic force sensing of soft matter in future