Solid-Phase Pyrolysis Synthesis of Highly Fluorescent Nitrogen/Sulfur Codoped Graphene Quantum Dots for Selective and Sensitive Diversity Detection of Cr(VI)

A holistic review on red fluorescent graphene quantum dots, its synthesis, unique properties with emphasis on biomedical applications

Graphene quantum dots (GQDs) are an evolving class of carbon-based nanomaterial, seizing tremendous attention owing to their intense optical property, engineered shapes and structures, and good photostability. Being a zero-dimensional form of carbon structure, GQDs have superior photoluminescent behavior, tunable emission and absorption, excellent biocompatibility, low cytotoxicity, hydrophilic nature, modifying surface states. Their water dispersibility and functionalized surface structure, involving heteroatoms and various functional groups onto the surface of GQDs, make them particularly suitable for biological applications. Based on their absolute luminescence properties, GQDs emit blue, green, yellow, and red light under ultraviolet irradiation. Amongst the three colors, red luminescence can achieve deeper penetration of light into tissues, good cellular distribution, bio-sensing property, cell imaging, drug delivery, and serves as a better candidate for photodynamic therapy. The overall objective of this review is to provide a comprehensive overview of the synthesis methods for red fluorescence graphene quantum dots (RF-GQDs), critical comparative analyses of spectral techniques used for their characterization, the tunable photoluminescence mechanisms underpinning red emission, and the significance of chemically functionalizing GQDs' surface edges in achieving red fluorescence are discussed in depth. This review also discusses the effective biological applications and critical challenges associated with RF-GQDs are examined, providing insights into their future potential in clinical and industrial applications.

Coupled Poly(ethylenimine) Coreactant to Enhance Electrochemiluminescence of Polymer Dots for Array Imaging of Protein Biomarkers

Traditional electrochemiluminescent (ECL) bioanalysis suffers from the demand for excessive external coreactants and the damage of reaction intermediates. In this work, a poly(ethylenimine) (PEI)-coupled ECL emitter was proposed by covalently coupling tertiary amine-rich PEI to polymer dots (Pdots). The coupled PEI might act as a highly efficient coreactant to enhance the ECL emission of Pdots through intramolecular electron transfer, reducing the electron transfer distance between emitter and coreactant intermediates and avoiding the disadvantages of traditional ECL systems. Through modification of the PEI-Pdots with tDNA, a sequence partially complementary to cDNA that was complementary to the aptamer of target protein biomarker (aDNA), tDNA-PEI-Pdots were obtained. The biosensors were produced using Au/indium tin oxide (ITO) with an aDNA/cDNA hybrid, and an ECL imaging biosensor array was constructed for ultrasensitive detection of protein biomarkers. Using vascular endothelial growth factor 165 (VEGF165) as a protein model, the proposed ECL imaging method containing two simple incubations with target samples and then tDNA-PEI-Pdots showed a detectable range of 1 pg mL-1 to 100 ng mL-1 and a detection limit of 0.71 pg mL-1, as well as excellent performance such as low toxicity, high sensitivity, excellent selectivity, good accuracy, and acceptable fabrication reproducibility. The PEI-coupled Pdots provide a new avenue for the design of ECL emitters and the application of ECL imaging in disease biomarker detection.

A Review of Dual-Emission Carbon Dots and Their Applications

Carbon dots (CDs), as a rising star among fluorescent nanomaterials with excellent optical properties and fascinating dual-emission characteristics, have attracted increasing attention in sensing, bio-imaging, drug delivery, and so on. The synthesis of dual-emission CDs (DE-CDs) and the establishment of ratiometric fluorescence sensors can effectively diminish background interference and provide more accurate results than single-emission CDs. Although DE-CDs have generated increased attention in many fields, the review articles about DE-CDs are still insufficient. Therefore, we summarized the latest results and prepared this review. This review first provides an overview of the primary synthesis route and commonly used precursors in DE-CDs synthesis. Then, the photoluminescence mechanism behind the dual-emission phenomenon was discussed. Thirdly, the application of DE-CDs in metal cation detection, food safety analysis, biosensing, cell imaging, and optoelectronic devices has been extensively discussed. Finally, the main challenges and prospects for further development are presented. This review presents the latest research progress of DE-CDs synthesis and its application in ratiometric sensing; hopefully, it can help and encourage researchers to overcome existing challenges and broaden the area of DE-CDs research.

Ultrasound-Assisted Hydrothermal Synthesis of Highly Fluorescent Sulfur Quantum Dots for Fe3+ Ion and Ascorbic Acid Detection in Real Samples

In this work, the ultrasound-assisted hydrothermal synthesis method offers a facile method to synthesize highly efficient photoluminescence sulfur quantum dots (SQDs). Impressively, a switchable fluorescent "on-off-on" sensor was developed using the acquired SQDs, which are capable of sequentially detecting iron ions (Fe3+) and ascorbic acid (AA) with exceptional sensitivity and selectivity. Meanwhile, SQDs and Fe3+ formed complexes through coordination, causing the fluorescence quenching of SQDs because of the static quenching effect. Upon the addition of AA into the SQDs/Fe3+ system, a redox-reaction-mediated mechanism leads to the recovery of fluorescence. The fluorescence intensity of SQDs exhibits a linear relationship with the concentrations of Fe3+ and AA in the ranges 5-30 and 20-100 μM, respectively. Notably, the detection limits achieved are 14.31 nM for Fe3+ and 0.64 μM for AA. Moreover, the chemosensor was successfully employed for monitoring Fe3+ in real water samples and AA in fruits. These results demonstrate the excellent analysis and detection capabilities of SQDs in the complex environment.