Photon Reabsorption and Nonradiative Energy-Transfer-Induced Quenching of Blue Photoluminescence from Aggregated Graphene Quantum Dots

A deep understanding of the photoluminescence (PL) from aggregated graphene quantum dots (GQDs) is very important for their practical applications. Here the PL spectra from GQDs solutions at different concentrations are studied. We find that the intensity of the green emission (ca. 530–560 nm) linearly relies on the concentration of GQDs, whereas the blue PL (ca. 425 nm) intensity is below the linear relationship, indicating a concentration-induced partial quenching of blue PL. Confocal fluorescence images explicitly demonstrate the aggregation of GQDs at high concentration. The concentration-induced PL quenching is successfully interpreted by a model of photon reabsorption and nonradiative energy transfer, indicating that, at the aggregated states, the excited electrons of GQDs may nonradiatively relax to ground states through couplings with neighboring ones. Simulated fluorescence decay results show that the energy transfer between neighboring GQDs results in a prolonged dwell time of electron on high-energy state and thus increases the decay time of 425 nm emission, while 550 nm emission remains unaffected, which is consistent with the experimental results. This work will contribute to a deep understanding on PL of GQDs and is also of huge importance to extend GQDs’ applications

Photothermal Contribution to Enhanced Photocatalytic Performance of Graphene-Based Nanocomposites

Photocatalysts possessing high efficiency in degrading aquatic organic pollutants are highly desirable. Although graphene-based nanocomposites exhibit excellent photocatalytic properties, the role of graphene has been largely underestimated. Herein, the photothermal effect of graphene-based nanocomposites is demonstrated to play an important role in the enhanced photocatalytic performance, which has not been considered previously. In our study on degradation of organic pollutants (methylene blue), the contribution of the photothermal effect caused by a nanocomposite consisting of P25 and reduced graphene oxide can be as high as ∼38% in addition to trapping and shuttling photogenerated electrons and increasing both light absorption and pollutant adsorptivity. The result reveals that the photothermal characteristic of graphene-based nanocomposite is vital to photocatalysis. It implies that designing graphene-based nanocomposites with the improved photothermal performance is a promising strategy to acquire highly efficient photocatalytic activity

Emission from Trions in Carbon Quantum Dots

The photoluminescence (PL) spectra acquired from 1 to 6 nm large carbon quantum dots (CQDs) prepared by refluxing activated carbon in HNO₃ show blue emission independent of the excitation wavelength as well as long-wavelength emission depending on the excitation wavelength. The dependence of the two emissions on pH is investigated, and the experimental results show that the peak position of the long-wavelength emission does not change with pH; however, the blue emission becomes more asymmetrical, and obvious shoulder peaks emerge as the pH increases. A model based on defect-bound trions in the CQDs is proposed to explain the shoulder peaks in the blue emission at high pH, and the calculated results agree well with experimental data concerning the integral intensity ratio of the trion to exciton emissions versus pH. Our experimental and theoretical results demonstrate for the first time emission from trions in CQDs

Ultrasensitive Detection of Matrix Metalloproteinase 2 Activity Using a Ratiometric Surface-Enhanced Raman Scattering Nanosensor with a Core–Satellite Structure

Matrix metalloproteinase 2 (MMP-2) has been considered a promising molecular biomarker for cancer diagnosis due to its related dysregulation. In this work, a core–satellite structure-powered ratiometric surface-enhanced Raman scattering (SERS) nanosensor with high sensitivity and specificity to MMP-2 was developed. The SERS nanosensor was composed of a magnetic bead encapsulated within a 5,5′-dithiobis(2-nitrobenzoic acid) (DTNB)-labeled gold shell as the capture core and a 4-mercaptobenzonitrile (MBN)-encoded silver nanoparticle as the signal satellite, which were connected through a peptide substrate of MMP-2. MMP-2-triggered cleavage of peptides from the core surface resulted in a decrease of the SERS intensity of MBN. Since the SERS intensity of DTNB was used as an internal standard, the reliable and sensitive quantification of MMP-2 activity would be realized by the ratiometric SERS signal, with a limit of detection as low as 2.067 ng/mL and a dynamic range from 5 to 100 ng/mL. Importantly, the nanosensor enabled a precise determination of MMP-2 activity in tumor cell secretions, which may provide an avenue for early diagnosis and classification of malignant tumors

Cubic In₂O₃ Microparticles for Efficient Photoelectrochemical Oxygen Evolution

Cubic In₂O₃ microparticles with exposed {001} facets as well as single morphology and size are produced on a large scale on silicon with a high yield. The morphological evolution during chemical vapor deposition is investigated and the new knowledge enables precise facet cutting. The synthesized Cubic In₂O₃ microparticles possess superior photoelectrocatalytic activity and excellent chemical and structural stability in oxygen evolution reaction on account of the unique surface structure and electronic band structure of the {001} facets. Our results reveal that it is feasible to promote the photolectrochemical water splitting efficiency of photoanode materials by controlling the growth on specific crystal facets. The technique and concept can be extended to other facet-specific materials in applications such as sensors, solar cells, and lithium batteries