의생명공학적 응용을 위한 탄소나노입자의 에너지 구조 조절 연구

Abstract

In the past decade, carbon nanodots (CNDs) have received great attention due to their unique optical properties and for potential applications. Their strong photoluminescence (PL), high photostability, chemical inertness, low toxicity, and biocompatibility make CNDs valuable in variety of fields ranging from biomedical to optoelectronic and photocatalytic applications. The unique PL property of CNDs arises from their surface functional groups and carbon core. However, poor understanding of the chemical origin of the visible and near-infrared PL from CDs hinders attempts at energy modification and limits their practical use. This thesis reports investigations of energy structure of CNDs and rational structure-engineering methods to tailor their energy structure. The chemical origin of the PL of CNDs is investigated by manipulating their chemical structures via reduction and deprotonation. By manipulating CNDs by two methods, the roles of oxygen-containing groups and the carbon core in the visible and NIR PL are identified. The excitation-dependent visible PL of CNDs originates from recombination at localized energy states (surface states) formed by oxygen-related surface functional groups, whereas the excitation-independent visible PL seems to originate from recombination within the oxidized sp3 carbon matrix. It is also found that the NIR PL from CNDs shows completely different behavior from the visible PL. On the basis of steady-state and transient optical characterization, it is suggested that the NIR PL of CNDs originates from energy states formed by small (1–2 nm) sp2 carbon clusters. To tailor the energy structure of CNDs, rational structure-engineering methods are developed. First, blue PL of CNDs is controlled by mild-condition carbonization of two carbon sources with different molar ratio. By regulating molar ratio of citric acid and ethanolamine, the surface chemistry of CNDs is controlled without changing their sizes and atomic ratio, lead to formulate surface-localized energy states (surface states, molecule-like states, and defect states). The relations between structural changes and changes of luminescence pathways (surface state emission, molecule-like state emission, and core-state emission) are investigated by steady state spectroscopy and time-correlated single photon counting spectroscopy. Next, it is also found that carbon-based nanostructures (CNs) with red PL can be synthesized via air oxidation of para-substituted anilines (F-, C-, S-, and N-anilines). The chemical origin of the red PL is investigated and revealed. The obtained F-, C-, S-, and N-CNs show highly crystalline 1D nanorod structures. The CNs are composed of π-π stacked phenazine-like structures, which are concluded to be responsible for the red PL on the basis of optical and ab initio analyses. The CNs show bright red PL with high QYs (up to 46%), small FWHM (≈ 25 nm), and high photostability. Finally, based on the understanding about chemical origins of PL, dual-color-emitting CNDs with two distinct excitation and emission centers have been successfully developed via the facile surface modification with para-substituted aniline (4-octyloxyaniline). CND and para-substituted aniline provide the blue-luminescent surface state (intrinsic state) and red-luminescent molecule-like state (extrinsic state), respectively. The two emission centers show no spatial overlap with high quantum yield (>30 %). The CNDs show the minimal cytotoxicity and demonstrate the great potential as a bioimaging agent for multicolor bioimaging, two-photon microscopy and in vivo optical imaging. Moreover, CNDs can be successfully exploited for optogenetic applications via the wavelength-dependent activation of ion channels. Taken together, the feasibility of CNDs can be confirmed with the unique characteristics of multicolor-emitting, facile and robust preparation, and biocompatibility for various biomedical applications. It is believed that these results would pave the way for understanding nature of CNDs and their widespread application in biomedical area.