Digital Theses Archive


Tesi etd-02272017-105239

Type of thesis
Stimuli-Responsive Ordered Micro- Nanostructures based on Polyelectrolyte Hydrogels for Sensing Applications
Scientific disciplinary sector
INGEGNERIA - Biorobotics
relatore DARIO, PAOLO
  • CNT forests
  • metal sensors
  • opals
  • photonic crystals
  • vertically aligned carbon nanotubes
Exam session start date
The research activity performed over the past three years was devoted to the fabrication, optimization and characterization of polyelectrolyte hydrogels containing micro- and nanostructures, able to act as sensors for metal ions in water. These stimuli responsive systems could transduce chemical stimulations into detectable responses, depending on how the polyelectrolytes were embedded into the structures. In particular, two different categories of ordered structures were considered for these investigations, such as (a) photonic crystals and (b) carbon nanotubes forests. The present thesis is divided into two main sections, according to these different areas of research.<br><br><br>a) Photonic Crystals<br>Stimuli-responsive photonic crystals represent an intriguing class of smart materials very promising for sensing applications. These ordered structures can alter their reflected wavelength or intensities upon exposure to physical or chemical stimuli. In this project, new selective ionic strength responsive polymeric PCs were developed through the sulfonation of polystyrene (PS) opals, without using toxic or expensive monomers and etching steps. Different aspects of the fabrication process were widely investigated, optimized and characterized, such as the synthesis of PS particles through the emulsion polymerization method, the fabrication of ordered opal structures and the sulfonation process.<br>The color of resulting sulfonated PS opals showed a peculiar dependence on the ionic strength of the surrounding solution, since it could be continuously shifted over the entire visible range (405−760 nm) by changing the content of ions over an extremely wide range of concentration (from about 70 μM to 4 M). Moreover, the optical response was totally pH and temperature independent and the initial color could be fully recovered by washing the opals with pure water. Interestingly, based on preliminary results, sulfonated PS opal showed also a peculiar optical response with bivalent cations, allowing for a discrimination of charged metal ions. In particular, the higher sensitivity for Pb2+ ions could be promising for sensing this metal in water.<br>These achievements represented an extraordinary improvement in the field of ionic strength responsive photonic crystals, enabling the use of sulfonated PS opals as inexpensive, reusable and not photo-bleachable chromogenic ionic strength sensors.<br>This project was carried out at The BioRobotics Institute of Scuola Superiore Sant’Anna and Center for Micro-BioRobotics @SSSA, Istituto Italiano di Tecnologia, under the supervision of Dr. Virgilio Mattoli and Dr. Francesco Greco.<br><br><br>b) Carbon Nanotube Forests<br>Carbon nanotube (CNT) forests constitute an important class of highly ordered nanomaterials that can find applications in many technological fields, including high-sensitive sensors. In this project, the fabrication of sensor devices for the detection of metal ions based on new CNT/polyelectrolyte composite structures was investigated. In particular, thanks to the sensor design, CNT arrays could act as microtransducers to transform a chemical stimulus, coming from the polyelectrolyte network, into a variation of the electrical resistance. The sensor device was constituted by a polystyrene sulfonate (PSS) functionalized CNT forest grown between two molybdenum electrodes. Different aspects of the device fabrication were investigated and optimized, such as the thermal stability of the molybdenum electrodes, the production of well-densified CNT forests by chemical vapor deposition (CVD) technique and the covalent functionalization of CNT forests with PSS through a thermal activation of the nanotubes.<br>These devices are based on an elegant and novel approach for fabricating metal sensors, which allows an easy integration in complex circuit systems and the detection of different analytes, by changing the nature of the polymer.<br>This study was performed over a period of six months at the NanoManufacturing Group led by Dr. Michael De Volder – Institute for Manufacturing, University of Cambridge (UK).