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Sustainable Optical Nanomaterials | Synthesis | Application
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Optical Nanomaterials with Sustainability
Nanomaterials that have network covalent structures such as carbon nanodots, graphene quantum dots, and silica nanoparticles have suffered from their irregularity in size and shape. Such irregularity may be the consequence of the "irreversibility" of reactions that yield covalent structures (e.g. carbonization). In order to control the physical appearance of the covalent nanomaterials, we have developed the "passivation method" that uses surfactants to protect the surface of nanoparticles (see the left scheme). Surfactants have a "head" to anchor onto the surface of nanoparticles and a "tail" to suppress undesirable growth (or agglomeration) of nanomaterials. Thus, in the presence of surfactants, nanoparticle growth is terminated when surfactants are anchored onto the surface, and the rate of this surface reaction is a function of the surfactant concentration. Since the reaction rate is (directly) proportional to the concentration, the size of nanoparticles shall be inversely proportional to the surfactant concentration. Our previous papers including Chem. Commun. 48, 5256 (2012), Chem. Mater. 25, 1893 (2013), Small 10, 506 (2014), and Nano Lett. 14, 1306 (2014) have proved the concept in reality. The microscopy images below compare carbon nanoparticles prepared by the passivation method (right) with traditional carbon nanoparticles (left), which shows our carbon nanoparticles have a very uniform size without any agglomerates.
Using the passivation method, we have synthesized many kinds of nanomaterials including carbon nanodots, graphene quantum dots, silica nanoparticles and carbon nitride quantum dots, and recently, we are working to expand our reach to metal oxide nanoparticles and melanoidins. These nanomaterials have excellent biocompatibility and photostability, so they have been the subject of extensive research over the past decade. The possible applications of our nanomaterials cover a broad range of topics that include bioimaging, biomedical therapy, hydrogen generation, light-emitting devices, photovoltaics, and photochemical sensors.
Biomedical Applications
Recently, theranostic nanomedicines have been increasingly designed and developed for decreasing time and improving efficacy of medical treatment. The term "theranostics" is the portmanteau of therapeutics and diagnostics. Theranostic systems simultaneously deliver therapeutic drugs and diagnostic agents in the same dose. In detail, therapeutic techniques such as nucleic acid delivery, chemotherapy, photothermal therapy, photodynamic therapy, and radiation therapy are deliberately combined with imaging techniques such as confocal fluorescence microscopy (CFM), magnetic resonance imaging (MRI), positron emission tomography (PET), and photoacoustic imaging (PAI). The major advantage of theranostics is that key therapeutic factors including trafficking pathway, delivery kinetics, and therapeutic efficacy can be actually "seen" and analyzed to personalize therapies for every different patient.
Our nanomaterials have excellent biocompatibility and proper size for excretion, and also, they emit visible, near-infrared, and infrared (thermal) radiation, suitable for various types of imaging techniques. A variety of surface functional groups of our nanomaterials enable chemical conjugation with any therapeutic markers and drugs such as DNA, peptides, proteins, and bio-polymers.
We are working on the design of multimodal theranostic agents based on our nanomaterials for diagnosis and treatment of genetic diseases such as Alzheimer's disease and cancers. We have recently reported that our nanomaterials have a strong potential as a contrast agent in photoacoustic imaging. Photoacoustic imaging is a state-of-the-art imaging technique based on the photo-acoustic effect that describes the formation of sound or ultrasonic waves in materials by absorbing pulsed light.
This technique is promising because signal loss due to blood and tissues can be minimized. Some of our carbon and silica nanomaterials that absorb 600-800 nm light can enhance photoacoustic signals significantly by generating intense heat that leads to localized thermoelastic expansion. We expect that photoacoustic imaging may be combined with a variety of therapeutic techniques such as photothermal therapy to provide a complete theranostic system for cancer diagnosis and treatment. Recently, we successfully performed in vivo non-invasive photoacoustic imaging of sentinel lymph nodes and liver via targeted delivery of functionalized nanomaterials. Furthermore, their biocompatibility and biodegradability were verified by cell viability test, optical spectrum anlysis of urine, and inductively coupled plasma-mass spectroscopy analysis.
Optoelectronic and Environmental Applications
Resource depletion and environmental pollution are two of the biggest risks that the world is facing today. Particularly, rapid advance in nanotechnology has expedited the consumption of rare-earth and heavy metals, incurring the increase in not only their price but also the pollution. In this circumstance, our nanoparticles are expected to be potential candidates that can resolve the aforementioned risks because they are made from abundant and environmental-friendly sources. The related applications include photodiodes (solar cells and photoelectrochemical cells), phototransistors, photoresistors, laser diodes, and light-emitting diodes. We are working on the application of our fluorescent nanomaterials in solar cells, photoelectrochemical cells, phototransistors, and light-emitting diodes.
Solar cells are the devices that directly convert sunlight into electricity by the photovoltaic effect. Our nanomaterials including carbon nanodots and graphene quantum dots can be used as either electron donors (absorbing sunlight to generate electron-hole pairs) or electron acceptors (splitting electron-hole pairs into electrons and holes) in organic solar cells (solar cells made of organic materials). Photoelectrochemical cells are a kind of solar cells that produce hydrogen by splitting water (the electrolysis of water). Our nanomaterials that have excellent photostability can be used as photocatalysts that absorb sunlight to generate electron-hole pairs.
Light-emitting diodes are devices that convert electricity into light by recombining electrons and holes. Our nanomaterials such as graphene quantum dots can be used as phosphors that determine the wavelength of light or color. We have chemically functionalized graphene quantum dots with aniline derivatives to generate extrinsic energy levels and then used these graphene quantum dots to fabricate light-emitting diodes that exhibit green, orange, and red electroluminescence with high color purity. Transient absorption and time-resolved photoluminescence spectroscopies were used to study the electronic structures and related electronic transitions in our graphene quantum dots, which reveals that the underlying carrier dynamics is strongly related to their surface states. The external quantum efficiency of 1.28% was recorded with our light-emitting diodes, which is the highest value ever reported for light-emitting diodes based on carbonaceous phosphors. Considering resource depletion and environmental pollution related to use of rare-earth and heavy metals, our functionalized graphene quantum dots may have strong potential as clean light sources in future displays. We are now working on the improvement of the current efficiency that still remains below one-hundredth of the state-of-the-art light-emitting didoes.
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