Synthesis And Fluorescence Properties Of Carbon-based Hydrogel Nano Inks

In recent years, with the one-dimensional carbon nanotubes, two-dimensional graphene and other carbon nanomaterials breakthrough, zero-dimensional carbon quantum dots become another hotspot in the large family of carbon materials Photo luminescent (PL) carbon nanodots (CNDs) are attracting considerable attention as nascent quantum dots (QDs) owing to their favorable optical properties along with chemical inertness, low toxicity and excellent biocompatibility, which give rise to their exciting application in labeling, biomedical imaging, sensing and photovoltaic devices. Currently, CNDs stand to have a huge impact in biotechnological and environmental applications due to their potential as nontoxic alternatives to traditional heavy-metal-based QDs.For such applications, embedding QDs into an appropriate matrix is an effective method of preventing their agglomeration because strong fluorescence quenching occurs in dry and aggregate states. Among the possible matrices, poly(N-isopropyl-acrylamide)(PNIPAM), with a coil-to-globule transition in water at a lower critical solution temperature (LCST) around32℃, is a well-known temperature-responsive polymer as an active matrix transducing environmental temperature changes to embedded QDs. Fluorescent PNIPAM hydrogels, which endow the’smart’hydrogel matrix with luminescent properties of QDs, can be applied to a new generation of fluorescence markers with temperature-sensitive characteristics.In this thesis, we have successfully synthesis blue fluorescent carbon dots. Designed and synthesized carbon-based hydrogel photoluminescent nano inks with semi-interpenetrating networks and reversible photoluminescent thermoresponses. The PL intensity and the optical transmiuance of the hydrogels are reversibly sensitive to external temperature stimuli in the temperature range of20-55℃. which is attributed to thermoresponsive behavior of the PNIPAM matrix. Furthermore, the prepared hydrogels can be coated on various substrates and the gravure printed pattern exhibits blue emission upon exciting with a UV lamp, showing promise as a smart fluorescent ink. The main points and conclusions of this thesis are summarized as follows:1. We prepared blue fluorescent carbon dot by pyrolysis and hydrothermal methods and characterized through XRD. Raman, XPS. TEM, PL and so on. The results show that the carbon dots prepared by pyrolysis are well-dispersed and average particle diameter of4.5nm. The Raman spectrum exhibits ID/IG is around0.97, indicating that they have a similar structure to graphite. The carbon dots exhibit excitation-wavelength-dependent PL behavior; the PL maximum shifts from～410to～500nm when the excitation wavelength is varied from340to480nm. The strongest fluorescence emission band located at420nm is observed under360nm excitation. The synthesized carbon nanoparticles have various functional groups like C-OH, C-O-C and C=O on the surface which may result in the PL emission. Compared to the carbon dots synthesised by pyrolysis. the carbon dots prepared by hydrothermal average particle diameter is smaller (3.6nm) and have optimal excitation and emission wavelengths at360nm and443nm. The fluorescence quantum yield increased to60%. Through the analysis of the elements and surface functional groups, the higher fluorescence yield could be due to the introduction of the amino, which change the surface state of carbon dots. Comparison the microstructure and optical properties of the carbon dots synthesis by different methods, found that the quantum confinement effect and surface defects may be the main factors of fluorescence emission, and the surface defects emission plays a leading role in the high luminescence of the carbon dots.2. PNIPAM hydrogel spheres were prepared by surfactant-free radical polymerization and the PAAm/PNIPAM-CND hydrogels inks that combine the features of semi-interpenetrating network structure and strong photoluminescence through the crosslink of PAAm in the presence of PNIPAM and CNDs at room temperature. Since PNIPAM changes aggregation state and chemical microenvironment at different ambient temperatures, can create a unique controllable composite state for carbon quantum dots. As the result shown, carbon-based hydrogel nano inks were sensitive to external temperature stimuli (20-55℃) and reversible. Furthermore, the PL properties of PAAm/PNIPAM-CND hydro gels inks sample have no obvious change before and after repeated heating and cooling cycles. It can be concluded the CNDs in the PAAm/PNIPAM gels had little effect on the polymer characteristics of the temperature-induced phase transition.3. The prepared carbon-based hydrogel nano inks can be coated on various substrates, like paper, quartz plate, polyimide (PI) film and Polyethylene terephthalate (PET) film. The gravure printed pattern exhibits blue emission upon exciting with a UV lamp.