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Quantitative identification of mixed-valence metal ions in solution by the distinctive fluorescence responses of graphene quantum dots. [Display omitted] • Quantitative mixed-valence state identification of Fe3+/Fe2+ solution by graphene quantum dots (GQDs). • The real-time monitoring of the dynamic changes in mixed-valence states triggered by the redox process was realized. • This method can be extended to other metal ions (Cr3+/Cr2+ or Cu2+/Cu+) and typically functionalised GQDs. • A universal and facile method for the quantitative mixed-valence state identification of metal ions in a solution. Quantitative identification of mixed-valence metal ions is essential for gaining deeper insights into critical chemical and biological processes in environmental science, chemical engineering, and biological systems. However, a simple approach of quantitative identification mixed-valence metal ions in solution has remained a challenge. In this study, we have experimentally observed a significant linear correlation (R2 = 0.99) between the concentration of high-valence metal ions (using iron ions as an example) and the fluorescence intensity of graphene quantum dots (GQDs). Utilizing the distinct fluorescence responses of GQDs to high-valence and low-valence metal ions, reliable quantitative detection of mixed-valence metal ions has been successfully achieved. Remarkably, we introduced real-time monitoring of mixed-valence metal ions, revealing a shift from a molar ratio of approximately 4.0 to 2.0. Density functional theory calculations have revealed significant differences in charge transfer between high-valence and low-valence states of metal ions adsorbed onto GQDs. Furthermore, the versatility of this method can extend to various types of GQDs and metal ions, highlighting its universal applicability. This work presents a simple, convenient, and cost-effective approach for quantitatively identifying mixed-valence metal ions in solution, offering a new avenue and opportunity for applications in biochemistry, environmental science, catalysis and materials science. [ABSTRACT FROM AUTHOR]

Graphene-based free-space optical modulators are usually enabled by shifting electrically the resonant frequency of a plasmonic or optical resonator. However, this resonance-shifting approach could lead to a potential loss of the on/off ratio, considering the fact that photodetectors always respond in a spectral range. Here, to modulate directly the amplitude of mid-infrared light, we propose a graphene-based modulator employing a graphene layer and a magnetic plasmonic array. With this novel design, the transmission intensity through the plasmonic array can be modulated in a direct manner without shifting the transmission peak. Magnetic plasmon resonance (MPR) accounts for the observed transmission, while Wood anomaly (WA) is utilized to fix the resonant position. The proposed modulator shows an excellent tolerance upon variation of structure parameters, with an insertion loss as low as 0.7 dB and a typical on/off ratio of 20, as well as a large modulation depth of 95%. This work offers a new route to design graphene-based optical modulators and may find applications in planar optics and the free-space optical (FSO) communication. [ABSTRACT FROM AUTHOR]

The effective regulation of the absorption properties of Au nanosphere arrays, as well as effective enhancement of its SERS performance, are of great significance for its applications in the field of optical sensing. In this paper, the polyol synthesis method was used to fabricate monolayer and bilayer periodic Au nanosphere arrays, onto which monolayer graphene was coated, and the optical properties of the obtained composite structures were investigated. The results showed that the coverage of graphene enhances the absorption peak intensities of the monolayer and bilayer Au nanoarrays by up to 32.92%, and the resonance peaks were red-shifted by up to 17 nm. The Raman signals of graphene-covered monolayer and bilayer Au nanosphere composite arrays are 180 and 280 times stronger than that of pristine graphene, due to the combination of chemical enhancement and electromagnetic enhancement. And the detection sensitivity of R6G molecules by graphene-bilayer Au nanosphere composite arrays is 10−6 M. In addition, the staggered stacking of bilayer Au nanosphere arrays can provide more plasmonic hotspots, resulting in higher Raman intensity of the substrate. These results will pave the way for the fabrication of highly efficient SERS substrates by providing various promising candidates. After being covered by monolayer graphene sheets, the intensity of the plasmon resonance absorption peak of Au nanosphere arrays is enhanced and the peak positions are red-shifted. At the same time, the Raman signal strength of the composite arrays is stronger than the original arrays. [Display omitted] • Graphene-monolayer/bilayer Au nanosphere composite arrays were prepared by polyol synthesis and wet transferred method. • The absorption peak intensities of the composite arrays were enhanced by up to 32.92% compared to Au nanosphere arrays. • The Raman signals of the graphene-Au nanosphere composite arrays are 180-280 times stronger than that of pristine graphene. • The detection sensitivity of the graphene-bilayer Au nanosphere composite array for R6G molecules can reach 10−6 mol/L. [ABSTRACT FROM AUTHOR]