Revealing the role of nitrogen dopants in tuning the electronic and optical properties of graphene quantum dots via a TD-DFT study

Impact of BSA and Au3+ concentration on the formation and fluorescence properties of Au nanoclusters

Bovine serum albumin-stabilized Au nanoclusters (BSA-Au NCs) have emerged as promising contenders for imaging agents and highly sensitive fluorescence sensors due to their biocompatibility and strong photoluminescence. Optimizing the synthesis conditions of BSA-Au NCs is crucial for enhancing fluorescence imaging and other nanocluster applications. In this study, for the first time, we systematically investigated the effects of BSA concentration and Au3+ on both particle size and optical characteristics of BSA-Au NCs. When the two components achieved a suitable concentration ratio, it was beneficial to form BSA-Au NCs with a high quantum yield (QY = 74.30%) and good fluorescence stability. In contrast, an inappropriate concentration ratio would lead to the formation of gold nanoparticles (Au NPs), and their internal filtration effect (IFE) would attenuate the fluorescence emission of BSA-Au NCs. The BSA-Au NCs were then employed as efficient fluorescence sensors for detecting Hg2+. Furthermore, the growth mechanism of BSA-Au NCs was elucidated by monitoring fluorescence changes during different incubation times. The BSA-Au NCs with a high quantum yield introduce a novel synthetic concept for sensitive fluorescent probes and expanding versatile applications of BSA-Au NCs in catalysis, chemical sensing and biomedicine.

Carbon Dot Synthesis in CYTOP Optical Fiber Using IR Femtosecond Laser Direct Writing and Its Luminescence Properties

Luminescent carbon dots (CDs) were locally synthesized in the core of CYTOP fibers using IR femtosecond laser direct writing (FLDW), a one-step simple method serving as a post-treatment of the pristine fiber. This approach enables the creation of several types of modifications such as ellipsoid voids. The CDs and photoluminescence (PL) distribute at the periphery of the voids. The PL spectral properties were studied through the excitation/emission matrix in the visible range and excitation/emission spectra in the UV/visible range. Our findings reveal the presence of at least three distinct luminescent species, facilitating a broad excitation range extending from UV to green, and light emission spanning from blue to red. The average laser power and dose influence the quantity and ratio of these luminescent CD species. Additionally, we measured the spatially resolved lifetime of the luminescence during and after the irradiation. We found longer lifetimes at the periphery of the laser-induced modified regions and shorter ones closer to the center, with a dominant lifetime ~2 ns. Notably, unlike many other luminophores, these laser-induced CDs are insensitive to oxygen, enhancing their potential for display or data storage applications.

Competitive fluorescent immunoassay for the ultrasensitive determination of amyloid beta peptide1-42 based on Ag@SiO2@N, S-GQD nanocomposites

A competitive fluorescent immunoassay is described for the ultrasensitive determination of amyloid beta peptide1-42 (Aβ1-42), a biomarker for early diagnosis of Alzheimer's disease. N, S-doped graphene quantum dots (N, S-GQDs) were freely assembled on the surface of Ag@SiO₂ nanoparticles to obtain a composite (Ag@SiO₂@N, S-GQD nanocomposite), which was successfully prepared and characterized. By theoretical study, the optical properties of nanocomposites are improved compared with GQDs, due to the advantages of combining N, S co-doping and metal-enhanced fluorescence (MEF) effect of Ag NPs. In addition, Aβ1-42 was modified by Ag@SiO₂@N, S-GQDs to prepare a probe with high photoluminescence properties (Ag@SiO₂@N, S-GQDs-Aβ1-42). In the presence of Aβ1-42, a competitive reaction towards anti-Aβ1-42 fixed on the ELISA plate was proceeded between Aβ1-42 and Ag@SiO₂@N, S-GQDs-Aβ1-42 by specific capture of antigen-antibody. The emission peak of Ag@SiO₂@N, S-GQDs-Aβ1-42 (400 nm emission) was used for the quantitative determination of Aβ1-42. Under the optimal conditions, the fluorescent immunoassay exhibited a linear range of 0.32 pg·mL^-1-5 ng·mL^-1 with a detection limit of 0.098 pg·mL^-1. The results show that the immunoassay has good analytical ability and can provide a new method for the clinical determination of Aβ1-42.

Tuning the properties of graphene quantum dots by passivation

The electronic and optical properties of graphene quantum dots (GQDs) of size less than 10 nm are different from those of graphene sheets due to quantum confinement effects. For analyzing their use in optoelectronic applications, it is very crucial to optimally tune the bandgap and engineer the controlling parameters. In the present work, a systematic investigation of the band gap of hexagonal GQDs has been carried out by examining their HOMO and LUMO energies. Passivation of dangling bonds of these GQDs has been carried out with the help of electron-withdrawing substituents in order to tune the band gap and engineer their optical properties such as absorption and emission spectra by carrying out the simulation with Density Functional Theory formalism using Gaussian 09 software. Carrying out passivation with electronegative element fluorine (F) effectively decreases the band gap of these QDs resulting in a redshift in the absorption spectra. The HOMO and LUMO topographical surfaces have been used to understand the absorption spectra. These surfaces show some σ-bond characteristics along with π-bond properties on passivating GQDs with the F-atom which further results in alteration of their energies and a corresponding decrease in their band gap. Here, the absorption is found to be dependent on the size of GQDs and the type of passivating atoms (H or F). The results thus obtained are found to be in good consonance with those reported in the literature engaging different methods. The present analysis may prove to be useful in improving the working of solar cells and other optoelectronic devices.

Alkaline n-gqds fluorescent probe for the ultrasensitive detection of creatinine

Creatinine (Crn) is an important excretory product of the human body. Medical laboratory technology has improved over years and brought many advancements in clinical diagnostics equipment, and testing techniques and made the tests more efficient. Yet, the quantitative analysis of Crn is still carried out by the classical Jaffe's reaction (using Picric acid (PA) with NaOH) method. Since PA is hazardous to human health, alternative solutions such as; nanoparticles and surface-modified nanoparticles can be used. Exploring the optoelectronic properties of carbon-based quantum dots for biomolecule sensing is of current interest among researchers. Nitrogen functionalized graphene quantum dots (Alk-NGQDs) measured featured Crn easier and reduced the time taken for the test carried out in laboratories. The synthesized Alk-NGQDs optical, structural, morphological properties, surface and compositions are studied through XPS, HRTEM, XRD, FTIR, and spectroscopic techniques. Alk-NGQDs at alkaline conditions (pH 9.5) form a stable complex with Crn through intermolecular charge transfer (ICT). The fluorescence titration method is used to sense Crn in commercial Crn samples and human blood serum. To understand the efficacy of sensing creatinine using Alk-NGQDs, working concentration, fluorescence quantum yield, the limit of detection, and quenching constant are calculated using the Stern-Volmer plot. The emission property of Alk-NGQDs is aimed to bring an alternative to the traditional colorimetric Jaffe's reaction.

Shedding Light on Graphene Quantum Dots: Key Synthetic Strategies, Characterization Tools, and Cutting-Edge Applications

During the last 20 years, the scientific community has shown growing interest towards carbonaceous nanomaterials due to their appealing mechanical, thermal, and optical features, depending on the specific nanoforms. Among these, graphene quantum dots (GQDs) recently emerged as one of the most promising nanomaterials due to their outstanding electrical properties, chemical stability, and intense and tunable photoluminescence, as it is witnessed by a booming number of reported applications, ranging from the biological field to the photovoltaic market. To date, a plethora of synthetic protocols have been investigated to modulate the portfolio of features that GQDs possess and to facilitate the use of these materials for target applications. Considering the number of publications and the rapid evolution of this flourishing field of research, this review aims at providing a broad overview of the most widely established synthetic protocols and offering a detailed review of some specific applications that are attracting researchers’ interest.