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Graphene quantum dots (GQDs) are a newly developed graphene family with good electrical conductivity and high theoretical capacitance, while halloysite nanotubes (HNTs) are naturally occurring layered mineral materials containing high active sites for energy storage support. The combination of HNTs and GQDs can offer a new strategy on the fabrication of eco-friendly electrode materials for high performance supercapacitor applications. Herein, an environmentally friendly GQD-HNT nanocomposite is fabricated in the presence of (3-aminopropyl)-triethoxysilane to provide increased charge storage sites as well as to allow for the fast charge transport for supercapacitor application. Morphological and surface analytical results show that 5–10 nm GQDs are homogeneously distributed on the surface of APTES-coated HNTs via amide linkage. This new and novel layered nanocomposite can provide accessible electroactive sites and low resistance to accelerate the electrons and electrolyte ion transport, resulting in excellent specific capacitance and high energy density. The specific capacitances of 363–216 F/g at current densities of 0.5–20 A/g are obtained. In addition, the GQD-HNTs exhibit excellent energy density of 30–50 Wh/kg. Results obtained in this study clearly demonstrate the feasibility of using GQD-HNTs as alternative energy storage materials with increased charge storage sites and fast charge transport for high energy density supercapacitor applications.

Herein, a fluorescence turn off–on nanosensor has been successfully developed using functionalized N-doped carbon dots (N-CDs) as the label-free sensing probe for the ultrasensitive detection of Cu2+ ions first and then ciprofloxacin (CIP), one of the most commonly used antibiotics for disease control, in the presence of bipyridine. The homogeneous and narrowly distributed N-CDs with a mean size of 5.7 nm and a high quantum yield of 84% are fabricated via the hydrothermal method in the presence of citric acid and ethylenediamine as the carbon and nitrogen sources, respectively. The Cu2+ ions serve as both analyte and fluorescence quenchers in the sensing platform of N-CDs, and a good linear response to Cu2+ in the range of 0.01–0.35 μM with a limit of detection (LOD) of 0.076 nM is observed. Then, 0.35 μM Cu2+ is used as the fluorescence quencher of N-CDs to build up the fluorescence turn off–on sensing probe for the detection of CIP using bipyridine (bipy) as the linker for CIP and Cu2+ ions. The addition of CIP to the bipy-Cu@N-CD composites triggers the formation of CIP-bipy-Cu conjugate as well as the release of N-CDs, resulting in the recovery of fluorescence intensity after 6 min of incubation. The sensing probe exhibits a two-phase linear response to CIP in the concentration range of 0.05–1 and 1–50 μM with a LOD of 0.4 nM. In addition, the bipy-Cu@N-CD probe shows high sensitivity toward CIP over the 19 other interferences. Good recovery of 96–110% is also observed when 0.1–0.9 μM CIP is spiked into the real samples. Results obtained in this study clearly demonstrate a newly developed sensing platform with rapid detection of metal ions and antibiotics, which can open an avenue to develop highly efficient and robust sensing probes for the detection of metal ions, organic metabolites, and biomarkers in biological applications.

The development of a fast-response sensing technique for detection of cysteine can provide an analytical platform for prescreening of disease. Herein, we have developed a fluorescence turn off-on fluorescence sensing platform by combining nitrogen-doped graphene quantum dots (N-GQDs) with VO nanosheets for the sensitive and selective detection of cysteine in human serum samples. VO nanosheets with 2-4 layers are successfully synthesized via a simple and scalable liquid exfoliation method and then deposited with 2-8 nm of N-GQDs as the fluorescence turn off-on nanoprobe for effective detection of cysteine in human serum samples. The VO nanosheets serve as both fluorescence quencher and cysteine recognizer in the sensing platform. The fluorescence intensity of N-GQDs with quantum yield of 0.34 can be quenched after attachment onto VO nanosheets. The addition of cysteine triggers the reduction of VO to V as well as the release of N-GQDs within 4 min, resulting in the recovery of fluorescence intensity for the turn off-on detection of cysteine. The sensing platform exhibits a two-stage linear response to cysteine in the concentration range of 0.1-15 and 15-125 μM at pH 6.5, and the limit of detection is 50 nM. The fluorescence response of N-GQD@VO exhibits high selectivity toward cysteine over other 22 electrolytes and biomolecules. Moreover, this promising platform is successfully applied in detection of cysteine in human serum samples with excellent recovery of (95 ± 3.8) - (108 ± 2.4)%. These results clearly demonstrate a newly developed redox reaction-based nanosensing platform using N-GQD@VO nanocomposites as the sensing probe for cysteine-associated disease monitoring and diagnosis in biomedical applications, which can open an avenue for the development of high performance and robust sensing probes to detect organic metabolites.

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