Fluorescent carbon dots and their composites for multifunctional applications

Abstract

During the past few years, a novel nano form of carbon, carbon dots (C-dots) have emerged as a new class of functional material. Their particle sizes are generally below 10 nm and they are rich in oxygen functionality (up to 40%) owing to the presence of carbonyl, carboxyl and epoxy groups on the surface. Unlike most of the other carbon nanomaterials, C-dots are highly water dispersible and show splendid fluorescence properties originated both from their core and surface characteristics. Moreover, they possess good biocompatibility, photostability and energy conversion abilities which have led to their application in sensing, electronic, catalysis, bioimaging and nanomedicine. In this thesis, various physicochemical properties of C-dots have been utilized towards sensing, white light emission and development of functional nanocomposites. The thesis is divided into the following chapters:1. Introduction In this chapter, a general discussion and literature survey on the various forms of nanocarbons have been inscribed. Special focus has been on the synthetic routes and applications of C-dots as well as their composites which is the core of research included in this thesis. Also the prominent properties of this tiny luminescent form of carbon have been discussed. The literature reports on the various methods of fabricating C-dots with tunable properties for multifunctional applications have been covered. Further, a brief summary of the research reported in this thesis and their relevance in the prospects of recent developments are discussed. 2. Instrumentation and Methods In this chapter, specifications of the instruments used during the research work and description of certain experimental methods used have been mentioned. 3. Natural occurrence of carbon dots in honey: Harnessing honey as a functional material for sensing and white light emission In this chapter, the exploration of natural occurrence of fluorescent carbon dots in honey and its application as a functional material has been described. Honeyshows excitation dependent emission property which has been widely reported for engineered carbon dots. This prompted us to look for presence of nanoscale carbon particles in honey. The C-dots isolated from honey obtained from a bee hive were characterized by various spectroscopic and microscopic techniques which suggested that these particles are fluorescent with sizes in the range 3-5 nm and are enriched with hydroxyl, carbonyl and carboxylic acid groups at the surface. Although, the quantum yield of the C-dots isolated from honey was low (1.6%), a diluted honey solution without any processing could be used for sensing of a range of analytes like nitroaromatics, bilirubin and Fe3+. The fluorescence intensity of honey decreased with increasing the concentration of these analytes which could be detected to nanomolar concentrations. Moreover, a white light emitting composite could be generated by combination of honey with an orange emitting dye, resorufin. 4. White light emission by controlled mixing of carbon dot and rhodamine B for applications in optical thermometry and selective Fe3+ detection In this chapter, it is demonstrated that a simple mixing of rhodamine B (RhB) dye with fluorescent C-dots synthesized from β-carotene led to white light emission. Considering the bluish green emission of C-dots when excited at 365 nm, RhB with their reddish orange emissions was an obvious choice as a dual cooperating emitter as they are complementary colors of white light. RhB molecules aggregated on the C-dot surface which resulted in decrease in emission owing to free rhodamine. Controlling the emission of free rhodamine B dye with that of the resultant carbon dot-aggregated RhB composite resulted in efficient white light emission with the CIE coordinate (0.33, 0.32). The white light emitting system was found to be photostable and could be incorporated in a gel or polymer matrix for solid-state processibility. The emission intensity of peaks owing to both C-dots and RhB decreased with increasing temperature and this thermoresponse of the white light emitting composite was found to be reversible in nature. The system further provided selective and sensitive sensing for Fe3+ ions among a host of biologically relevant metal ions tested.Carbon Nanodots as Ligand Exchange Probes in Au@C-dot Nanobeacons for Fluorescent Turn-on detection of biothiols In this chapter, the exploitation of reducing and stabilizing properties of C-dots synthesized by microwave pyrolysis of β-carotene for the generation of Au NPs at room temperature has been described. The C-dots stabilized the Au NPs by forming a thin continuous layer around the nanoparticle surface. The participation of C-dots as reducing agents led to dramatic quenching of their fluorescence. A detailed investigation into the course of fluorescence turn off of C-dots revealed a combination of both static and dynamic nature of quenching.The Au@C-dot core shell composites could be used as a dual colorimetric and fluorometric detector for biothiols with high sensitivity and selectivity. The method relied on the fact that the thiol moiety could effectively ligand displace the C-dots from the Au NP surface owing to higher affinity of thiols for gold as comapred to the hydroxyl and carboxyl groups present on the C-dot surface. This led to the recovery of fluorescence of C-dots as they released from the Au NP surface. In addition, the Au NPs aggregated with the increasing concentration of thiol leading to rapid broadening and red shift of the SPR band of Au NPs. Thus, Au@C-dot composite served as a bidimensional optical sensing platform for biothiols. 6. Carbon dots as Nanodispersants for Multi-walled Carbon Nanotubes: Reduced Cytotoxicity and Metal Nanoparticle Functionalization In this chapter, it is demonstrated that C-dots (synthesized from PEG) with a hydrophilic surface and graphitic π-conjugated core could effectively stabilize a dispersion of multi walled carbon nanotubes (MWCNTs) in water. The noncovalent functionalization of CNTs with C-dots was achieved by probe sonication of a mixture of CNTs and C-dots which did not alter the intrinsic properties of CNTs. CNT dispersions with concentrations upto 0.5 mg/mL could be obtained using a 2: 1 C-dot to CNT mass ratio. C-dots efficiently wrapped the CNT surface and the resulting C-dot-CNT composites were found to be stable in a range of solvents like ethanol, acetonitrile, acetone and dimethyl sulfoxide. The composite did not precipitate even in the presence of high concentration of bovine serum albumin. The cytotoxicity assay along with cell proliferation studies revealed that the C-dots immobilized on the CNT surface couldeffectively alter their cellular interaction properties resulting in decreased cytotoxicity. The reducing capability of C-dots could be exploited further towards the generation of Au nanoparticles on CNT surface without further need of external reducing and stabilizing agents which were found to be catalytically active towards reduction of p-nitrophenol. 7. Aqueous exfoliation of transition metal dichalcogenide sheets using carbon dots: electrochemical sensing of dopamine and Au nanoparticle functionalization In this chapter, the exploitation of the amphiphilic nature of C-dots towards stabilization of hydrophobic two dimensional transition metal dichlcogenide flakes in water is reported along with their subsequent functionalization with gold nanoparticles without requirement of any additional chemicals. Sonication assisted liquid exfoliation of WS2 and MoS2 using C-dots resulted in long-term stable aqueous dispersions of these 2D materials where the flakes were found to be decorated with carbon nanoparticles. The thickness of the resultant metal dichalcogenide-C-dot composites as determined by AFM studies was found to be in the range 1-1.5 nm which suggested successful exfoliation of these lamellar bulk materials. These composites could be dispersed into other solvents like ethanol, acetone, acetonitrile and dimethyl sulfoxide. The C-dot-MoS2 composites were negatively charged which could be used as electrochemical sensors for selective sensing of positively charged dopamine. The carbon dot decorated 2D sheets were utilized as supports for anchoring Au nanoparticles which could be easily generated on their surface due to the reducing and stabilizing properties of C-dots, without addition of any external reducing agent. The resulting nanocomposites could function as efficient catalysts towards reduction of p-nitrophenol in aqueous environment.Conclusions and Scope for Future Works This chapter summarizes the works described in the thesis. Further, the relevance and future prospects of the works have been discussed.