Graphene quantum dots from chemistry to applications

Heteroatom doped graphene engineering for energy storage and conversion

This is the first comprehensive review on methods and materials for use in optical sensing of pH values and on applications of such sensors. The Review starts with an introduction that contains subsections on the definition of the pH value, a brief look back on optical methods for sensing of pH, on the effects of ionic strength on pH values and pKa values, on the selectivity, sensitivity, precision, dynamic ranges, and temperature dependence of such sensors. Commonly used optical sensing schemes are covered in a next main chapter, with subsections on methods based on absorptiometry, reflectometry, luminescence, refractive index, surface plasmon resonance, photonic crystals, turbidity, mechanical displacement, interferometry, and solvatochromism. This is followed by sections on absorptiometric and luminescent molecular probes for use pH in sensors. Further large sections cover polymeric hosts and supports, and methods for immobilization of indicator dyes. Further and more specific sections summarize the state of the art in materials with dual functionality (indicator and host), nanomaterials, sensors based on upconversion and 2-photon absorption, multiparameter sensors, imaging, and sensors for extreme pH values. A chapter on the many sensing formats has subsections on planar, fiber optic, evanescent wave, refractive index, surface plasmon resonance and holography based sensor designs, and on distributed sensing. Another section summarizes selected applications in areas, such as medicine, biology, oceanography, bioprocess monitoring, corrosion studies, on the use of pH sensors as transducers in biosensors and chemical sensors, and their integration into flow-injection analyzers, microfluidic devices, and lab-on-a-chip systems. An extra section is devoted to current challenges, with subsections on challenges of general nature and those of specific nature. A concluding section gives an outlook on potential future trends and perspectives.

Optical Sensing and Imaging of pH Values: Spectroscopies, Materials, and Applications

Impurity doping is a promising method to impart new properties to various materials. Due to their unique optical, magnetic, and electrical properties, rare-earth ions have been extensively explored as active dopants in inorganic crystal lattices since the 18th century. Rare-earth doping can alter the crystallographic phase, morphology, and size, leading to tunable optical responses of doped nanomaterials. Moreover, rare-earth doping can control the ultimate electronic and catalytic performance of doped nanomaterials in a tunable and scalable manner, enabling significant improvements in energy harvesting and conversion. A better understanding of the critical role of rare-earth doping is a prerequisite for the development of an extensive repertoire of functional nanomaterials for practical applications. In this review, we highlight recent advances in rare-earth doping in inorganic nanomaterials and the associated applications in many fields. This review covers the key criteria for rare-earth doping, including basic electronic structures, lattice environments, and doping strategies, as well as fundamental design principles that enhance the electrical, optical, catalytic, and magnetic properties of the material. We also discuss future research directions and challenges in controlling rare-earth doping for new applications.

Rare-Earth Doping in Nanostructured Inorganic Materials.

In the past decades, the energy consumption of nonrenewable fossil fuels has been increasing, which severely threatens human life. Thus, it is very urgent to develop renewable and reliable energy storage devices with features of environmental harmlessness and low cost. High power density, excellent cycle stability, and a fast charge/discharge process make supercapacitors a promising energy device. However, the energy density of supercapacitors is still less than that of ordinary batteries. As is known to all, the electrochemical performance of supercapacitors is largely dependent on electrode materials. In this review, we firstly introduced six typical transition metal oxides (TMOs) for supercapacitor electrodes, including RuO2, Co3O4, MnO2, ZnO, XCo2O4 (X = Mn, Cu, Ni), and AMoO4 (A = Co, Mn, Ni, Zn). Secondly, the problems of these TMOs in practical application are presented and the corresponding feasible solutions are clarified. Then, we summarize the latest developments of the six TMOs for supercapacitor electrodes. Finally, we discuss the developing trend of supercapacitors and give some recommendations for the future of supercapacitors.

https://www.mdpi.com/2079-4991/11/5/1248/pdf

Transition Metal Oxide Electrode Materials for Supercapacitors: A Review of Recent Developments.

Although, great attention is paid to synthesize fluorescent carbon quantum dots (CQDs) for versatile applications, the field remains still attractive to achieve white light using these nano materials. In the present work, CQDs are synthesized from citric acid and lanthanide ions viz. Europium (Eu) and Terbium (Tb) are doped in CQD moiety to explore superior optical response for multifunctional applications. By proper tuning of excitation wavelength, perfect white light with Commission Internationale de l’Elcairage (CIE) index (0.345, 0.344) is obtained using these Europium Terbium co-doped CQDs (Eu-Tb-CQD). The observed photoluminescence of white light emitting lanthanide based CQD is pH dependent and will be used as a visual pH sensor. These luminescent Eu and Tb co-doped CQDs are also very useful to detect toxic Cr (VI) with excellent selectivity and sensitivity as compared to pure CQDs. It shows high quenching efficiency (∼95%) in presence of only 160 µM Cr(VI). The selectivity and lower detection limit are also obtained as ∼80% and 0.175 µM respectively.

White light emitting lanthanide based carbon quantum dots as toxic Cr (VI) and pH sensor.

Study of sensing properties of nitroexplosives using luminescent carbon based quantum dots is an interesting area of research. Enhancement in sensing ability exploiting edge states and doped states in quantum dots is an elegant approach. In the present work, graphene quantum dots (GQD) are co-doped by nitrogen and sulphur to produce large number of localized energy levels near conduction band. Remarkable enhancement in fluorescence quenching effect compared to their individual doped states either by nitrogen or sulphur is observed using only 90 μM solution of 2,4,6-trinitro phenol (TNP) via charge transfer among these doped states to detect trinitrophenol selectively. The detection limit has been calculated to about 19.05 ppb. The origin of this ultra high fluorescence quenching and low detection limit is the presence of electron rich edge states due to ‘N’ and ‘S’ dopants as verified by Zeta potential results.

Nitrogen, sulphur co-doped graphene quantum dot: An excellent sensor for nitroexplosives

We report very fast, green, and large-scale synthesis of amino-functionalized carbon quantum dots (CQDs) using a domestic microwave to investigate CQD-Tb-based dual emission for visual detection of toxic Hg2+. Citric acid and p-phenylenediamine are used as precursor materials to synthesize the CQD, which shows excitation-independent blue luminescence. To achieve the dual emission, Tb-containing CQD is synthesized in a very easy and cost-effective way. These dual-emissive fluorescent materials have been successfully used as a fluorescent indicator for visual detection of toxic Hg2+ metal ions. An instant color change from blue to green in the presence of a very low amount of Hg2+ under a UV lamp (λ365nm) is observed. The material is highly sensitive and selective toward detection of mercury ions in the presence of other metal ions. The photoluminescence quenching mechanism (photoinduced electron transfer process) has been explained using an electronic band diagram supported by zeta-potential and time-corre...

Dual-Emissive Carbon Quantum Dot-Tb Nanocomposite as a Fluorescent Indicator for a Highly Selective Visual Detection of Hg(II) in Water.

Although few works on highly luminescent carbon quantum dots (CQD) with high photostability for sensing applications have already been reported in the literature, the study of photoluminescence of ...

Facile Approach To Synthesize Nitrogen- and Oxygen-Rich Carbon Quantum Dots for pH Sensor, Fluorescent Indicator, and Invisible Ink Applications

Here we demonstrate a simple, low cost, and green synthetic approach to synthesizing water-soluble, nitrogen-doped, fluorescent carbon quantum dots (NCQDs) from lemon juice and ammonia by hydrothermal treatment. Chemical characterizations and low temperature photoluminescence and photoconductivity results show interesting structural features of the as-prepared NCQDs. These new NCQDs consist of a ring type moiety (porphyrin/chlorin) in the centre surrounded by the graphitic network and serve as an efficient fluorescent probe for label-free, sensitive, and selective detection of Fe3+ with a detection limit of 140 ppb (2.5 μM), which is remarkably lower than the earlier reports on CQDs-based sensing systems. DFT calculations are carried out to optimise the structural aspects for selective detection of Fe3+. This extremely low detection limit (140 ppb) arises due to static quenching in addition to dynamic quenching which generally occurs in most cases.

Highly luminescent N-doped carbon quantum dots from lemon juice with porphyrin-like structures surrounded by graphitic network for sensing applications

Tapas Kumar Mondal

Papers

White light emitting lanthanide based carbon quantum dots as toxic Cr (VI) and pH sensor.

Nitrogen, sulphur co-doped graphene quantum dot: An excellent sensor for nitroexplosives

Dual-Emissive Carbon Quantum Dot-Tb Nanocomposite as a Fluorescent Indicator for a Highly Selective Visual Detection of Hg(II) in Water.

Facile Approach To Synthesize Nitrogen- and Oxygen-Rich Carbon Quantum Dots for pH Sensor, Fluorescent Indicator, and Invisible Ink Applications

Highly luminescent N-doped carbon quantum dots from lemon juice with porphyrin-like structures surrounded by graphitic network for sensing applications