The last decade has seen dramatic progress in the principle, design, and fabrication of photonic nanomaterials with various optical properties and functionalities. Light-emitting and light-responsive nanomaterials, such as semiconductor quantum dots, plasmonic metal nanoparticles, organic carbon, and polymeric nanomaterials, offer promising approaches to low-cost and effective diagnostic, therapeutic, and theranostic applications. Reasonable endeavors have begun to translate some of the promising photonic nanomaterials to the clinic. Here, current research on the state-of-the-art and emerging photonic nanomaterials for diverse biomedical applications is reviewed, and the remaining challenges and future perspectives are discussed.

Chemically derived graphene quantum dots (GQDs) to date have showed very broad emission linewidth due to many kinds of chemical bondings with different energy levels, which significantly degrades the color purity and color tunability. Here, we show that use of aniline derivatives to chemically functionalize GQDs generates new extrinsic energy levels that lead to photoluminescence of very narrow linewidths. We use transient absorption and time-resolved photoluminescence spectroscopies to study the electronic structures and related electronic transitions of our GQDs, which reveals that their underlying carrier dynamics is strongly related to the chemical properties of aniline derivatives. Using these functionalized GQDs as lumophores, we fabricate light-emitting didoes (LEDs) that exhibit green, orange, and red electroluminescence that has high color purity. The maximum current efficiency of 3.47 cd A−1 and external quantum efficiency of 1.28% are recorded with our LEDs; these are the highest values ever reported for LEDs based on carbon-nanoparticle phosphors. This functionalization of GQDs with aniline derivatives represents a new method to fabricate LEDs that produce natural color.

Generally, the optical properties of CNDs are directly related to their electronic structure. It has been reported that the electronic structure of CNDs originates from their polyaromatic carbon domains, defective states, and surface states, presumably due to auxochromic effect. [ 7,[15][16][17] Currently, doping heteroatoms, especially nitrogen and/or sulfur, in a carbon framework has been found to be an attractive strategy to control photoluminescence (PL) of CNDs. Such dopants can be considered to generate defective states, where their different electronegativity, lone pairs of electrons, etc., change the electronic structure of CNDs. Many research groups have attempted to synthesize a variety of heteroatom-doped CNDs with distinguished optical properties. For example, Dong's group produced highly luminescent nitrogen and sulfur co-doped carbon-based dots through a one-step hydrothermal treatment using L -cysteine and citric acid as a dopant and carbon precursor, respectively. [ 37 ] In another study, Li et al. reported a facile method to prepare sulfur-doped graphene quantum dots for tuning their electronic structure by using sulfuric acid and fructose as a sulfur and carbon precursor, respectively. [ 38 ] Although these attempts have proven themselves promising; however, there have been limitations on the control of a degree of doping because heating a physical mixture of two separated precursors that have different structures and reactivity cannot guarantee the formation of desired chemical bondings between them. In this case, the bonding state of dopant atoms cannot be clear to make their roles in CNDs ambiguous. Thus, it is necessary to use a new precursor carrying both dopant and carbon atoms to not only guarantee effi cient doping but also examine the effect of doping on the optical properties of CNDs in a molecular level.Herein, we now present the synthesis of nitrogen and sulfur-doped CNDs (denoted as N-CNDs and S-CNDs, respectively) with single molecular precursors: ethylenediamine-N,N ′-diacetic acid (EDDA) and 2,2′-(ethylenedithio)diacetic acid (ETDA), respectively. [ 7,12 ] Our doping would lead to signifi cant changes in the electronic structure of CNDs and give rise to broad light absorption and strong PL in a long-wavelength visible light region. We fi nally demonstrated light-emitting diodes (LEDs) with our CNDs to show the effects of doping on their electronic structure and related electroluminescence (EL). OurIn this work, nitrogen and sulfur-doped carbon nanodots (CNDs) have been synthesized from ethylenediamine-N,N′ -diacetic acid and 2,2 ′ -(ethylenedithio) diacetic acid, respectively. The method used in this work features the use of "single" molecular precursors that contain both carbon and dopant atoms, which allows examining the effects of doping in a molecular level. The effects of doping on the electronic structure of CNDs could be examined by a series of spectroscopic measurements including UV-vis absorption and photoluminescence. It is found that doping gives rise to new light abs...

Carbon nanodots (CNDs) are organic-based particulate fluorophores prepared using various carbon-containing sources such as bulk carbon materials (e.g., graphite and coal) and organic molecules (e.g., carbohydrates, organic acids, and amino acids). Despite the wide variety of sources, the formation of CNDs almost always requires a specific type of oxidation reaction, and CNDs are generally regarded as highly oxidized carbon materials, similar to graphitic oxide. The oxide structures of CNDs are known to not only play a crucial role in the realization of photoluminescence but also induce oxygen-related defects that may degrade the optoelectronic properties. Therefore, we report an oxygen-free synthesis of CNDs with extremely low oxygen contents based on the pyrolysis-induced decarboxylation of aromatic carboxylic acids. The CNDs exhibit very strong excitation-independent photoluminescence in the yellow to orange range of the visible spectrum, with an absolute quantum yield that ranges up to 80%. Finally, we successfully fabricated freestanding color filters using these CNDs to demonstrate their potential in future display applications.

In this article, we demonstrate that TiO 2 @carbon core/shell (TiO 2 @C) nanocomposite photocatalysts prepared by carbonizing a single molecular layer of aromatic compounds adsorbed on the surface of TiO 2 nanoparticles selectively enhance the generation of hydrogen peroxide (H 2 O 2 ). Atomically thin carbon shells have been formed directly on the surface of TiO 2 nanoparticles through pyrolytic decarboxylation of the adsorbed aromatic compounds, benzoic acid (BA), and 1-naphthoic acid (NA), which yields two types of TiO 2 @C nanocomposites, TiO 2 @C(BA) and TiO 2 @C(NA). Raman spectroscopy shows that the as-obtained nanocomposites have similar degrees of graphitization (D/G band ratio), regardless of the type of aromatic precursors, but TiO 2 @C(NA) contains more oxygenic species than TiO 2 @C(BA) (D*/G band ratio). Such oxygenic species predominantly exist in the form of epoxide groups, as determined by attenuated total reflection infrared spectroscopy. The sp 2 carbon atoms near the epoxide groups in the carbon shell can act as active sites for the two-electron reduction of O 2 . Therefore, TiO 2 @C(NA) can generate H 2 O 2 more efficiently than TiO 2 @C(BA). Furthermore, the carbon shells retard the reconsumption of the generated H 2 O 2 by inhibiting the adsorption of H 2 O 2 on the surface of TiO 2 nanoparticles, thereby improving the photocatalytic efficiency of H 2 O 2 generation. Finally, we have shown the durability and reproducibility of our TiO 2 @C-based photocatalytic systems. We believe that our research may offer a potentially improved strategy for H 2 O 2 generation and other photocatalytic applications.