Heavy metal ion sensing strategies using fluorophores for environmental remediation

The main aim of this review is to provide a holistic summary of the latest advances within the research area focusing on the detection of heavy metal ion pollution, particularly the sensing strategies. The review explores various heavy metal ion detection approaches, encompassing spectrometry, electrochemical methods, and optical techniques. Numerous initiatives have been undertaken in recent times in response to the increasing demand for fast, sensitive, and selective sensors. Notably, fluorescent sensors have acquired prominence owing to the numerous advantages such as good specificity, reversibility, and sensitivity. Further, this review also explores the advantages of various nanomaterials employed in sensing heavy metal ions. In this regard, exclusive emphasis is placed on fluorescent nanomaterials based on organic dyes, quantum dots, and fluorescent aptasensors for metal ion removal from aqueous systems, and to identify the fate of heavy metal ions in the natural environment.

Carbon and boron nitride quantum dots as optical sensor probes for selective detection of toxic metals in drinking water: a quantum chemical prediction through structure- and morphology-dependent electronic and optical properties

Toxic metals present in drinking water pose a serious threat to the environment and human beings when present in abundance. In this work, we investigated the sensing ability of quantum dots (pristine CQDs, boron/nitrogen/sulphur (B/N/S)-doped CQDs, and BNQDs) of various sizes and morphologies (rectangular, circular, and triangular) towards toxic metals such as arsenic (As), cobalt (Co), nickel (Ni), copper (Cu), and lead (Pb) using quantum chemical density functional theory calculations in both gas and water phases. We probed the structural, electronic, and optical properties of the QDs. All the modelled QDs are energetically stable. Frontier molecular orbital analysis predicted that BNQDs are more chemically stable than all other CQDs. UV-vis absorption and Raman spectra analyses helped to understand the optical properties of all the QDs. Further, adsorption studies revealed that triangular pristine CQDs and sulphur-doped CQDs show higher adsorption affinity towards the toxic metals. The magnitude of adsorption energies follows the trend Ni > Pb > As > Cu > Co in most of the QDs. Several pristine and doped CQDs exhibited chemisorption towards the toxic metals, and hence, they can be used as adsorbents. However, a majority of BNQDs showed physisorption towards the metals, and therefore, they can be used as efficient optical sensors compared to CQDs. Further, the sensing ability of the QDs was explored through optical phenomena such as changes in UV-vis absorption spectra and fluorescence after metal adsorption. When compared to pristine CQDs and B/N/S-doped CQDs, metal complexation caused significant changes in the UV-vis absorbance peak intensities in BNQDs along with peak shifts. Moreover, metal interaction with the QDs increased their fluorescence lifetime with the highest values observed in Co-adsorbed triangular H18C46 (152.30 ns), Pb-adsorbed rectangular H15C30S (21.29 ns), and As-adsorbed circular B27N27H18 (2.99 μs) among pristine CQDs, B/N/S-doped CQDs, and BNQDs, respectively. Overall, we believe that our first-of-its-kind computational prediction of the optical sensing ability of tailor-made zero-dimensional systems such as QDs will be a great aid for experimentalists in designing novel and rapid optical probes to detect toxic metals in drinking water.

Highly Sensitive and Selective Fluorescence and Smartphone-Based Sensor for Detection of Rutin Using Boron Nitrogen Co-doped Graphene Quantum Dots

This research explores the fluorescence properties and photostability of boron nitrogen co-doped graphene quantum dots (BN-GQDs), evaluating their effectiveness as sensors for rutin (RU). BN-GQDs are biocompatible and exhibit notable absorbance and fluorescence characteristics, making them suitable for sensing applications. The study utilized various analytical techniques to investigate the chemical composition, structure, morphology, optical attributes, elemental composition, and particle size of BN-GQDs. Techniques included X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The average particle size of the BN-GQDs was determined to be approximately 3.5 ± 0.3 nm. A clear correlation between the emission intensity ratio and RU concentration was identified across the range of 0.42 to 4.1 μM, featuring an impressively low detection limit (LOD) of 1.23 nM. The application of BN-GQDs as fluorescent probes has facilitated the development of a highly sensitive and selective RU detection method based on Förster resonance energy transfer (FRET) principles. This technique leverages emission at 465 nm. Density Functional Theory (DFT) analyses confirm that FRET is the primary mechanism behind fluorescence quenching, as indicated by the energy levels of the lowest unoccupied molecular orbitals (LUMOs) of BN-GQDs and RU. The method's effectiveness has been validated by measuring RU concentrations in human serum samples, showing a recovery range between 97.8% and 103.31%. Additionally, a smartphone-based detection method utilizing BN-GQDs has been successfully implemented, achieving a detection limit (LOD) of 49 nM.

Heterogenization of Ionic Liquid on Multiwalled Carbon Nanotubes for Lead(II) Ion Detection

The presence of lead(II) ion poses a significant threat to water systems due to their toxicity and potential health hazards. The detection of Pb2+ ions in contaminated water is very crucial. The ionic liquid functionalized multiwalled carbon nanotubes (IL@MWCNT) nanocomposite was fabricated using ionic liquid (IL) 1-methyl-3-(4-sulfobutyl)-imidazolium chloride and multiwalled carbon nanotubes (MWCNTs) for detection of lead(II) ions. It is a novel method to heterogenize the layer of IL on the surface of MWCNTs. The XPS and FTIR analyses confirm that the ionic liquid is not decomposed during annealing process. Moreover, the XRD analysis shows the presence of MWCNTs and carbon quantum dots (CQDs). The HRTEM results exhibit the aggregation of MWCNTs with IL, and formation of small distorted round shaped flakes of CQDs. Further, the successful heterogenization of IL on the surface of MWCNTs is also confirmed by TGA-DSC analysis. The quenching phenomenon of nanocomposite was observed by UV-Visible spectroscopy. The nanocomposite exhibits high performance for the selective detection of lead(II) ions in comparison to other metal ions. The presence of lead(II) ions eventually reduced the intensity of absorption. A limit of detection (LOD) of 9.16 nM was attained for Pb2+ ions in a concentration range of 0-20 nM.

Review on Hyaluronic Acid Functionalized Sulfur and Nitrogen Co-Doped Graphene Quantum Dots Nano Conjugates for Targeting of Specific Type of Cancer

Many people lose their lives to cancer each year. The prevalence of illnesses, metabolic disorders, high-risk infections, and other conditions has been greatly slowed down by expanding scientific research. Chemotherapy and radiation are still the initial lines of treatment for cancer patients, along with surgical removal of tumors. Modifications have been made in chemotherapy since medicines frequently have substantial systemic toxicity and poor pharmacokinetics and still do not reach the tumor site at effective concentrations. Chemotherapy may now be administered more safely and effectively thanks to nanotechnology. Nanotechnology-based graphene quantum dots (GQDs) are very applicable in breast cancer detection, as a drug delivery system, and in the treatment of breast cancer because of their physical and chemical properties, lower toxicity, small size, fluorescence, and effective drug delivery. This paper analyzes the GQDs as cutting-edge platforms for biotechnology and nanomedicine also its application in drug delivery in cancer. It shows that GQDs can be effectively conjugated with hyaluronic acid (HA) to achieve efficient and target-specific delivery.

Assessment of biomass-derived carbon dots as highly sensitive and selective templates for the sensing of hazardous ions

Access to safe drinking water and a hygienic living environment are the basic necessities that encourage healthy living. However, the presence of various pollutants (especially toxic heavy metal ions) at high concentrations in water renders water unfit for drinking and domestic use. The presence of high concentrations of heavy-metal ions (e.g., Pb2+, Hg2+, Cr6+, Cd2+, or Cu2+) greater than their permissible limits adversely affects human health, and increases the risk of cancer of the kidneys, liver, skin, and central nervous system. Therefore, their detection in water is crucial. Due to the various benefits of "green"-synthesized carbon-dots (C-dots) over other materials, these materials are potential candidates for sensing of toxic heavy-metal ions in water sources. C-dots are very small carbon-based nanomaterials that show chemical stability, magnificent biocompatibility, excitation wavelength-dependent photoluminescence (PL), water solubility, simple preparation strategies, photoinduced electron transfer, and the opportunity for functionalization. A new family of C-dots called "carbon quantum dots" (CQDs) are fluorescent zero-dimensional carbon nanoparticles of size < 10 nm. The green synthesis of C-dots has numerous advantages over conventional chemical routes, such as utilization of inexpensive and non-poisonous materials, straightforward operations, rapid reactions, and renewable precursors. Natural sources, such as biomass and biomass wastes, are broadly accepted as green precursors for fabricating C-dots because these sources are economical, ecological, and readily/extensively accessible. Two main methods are available for C-dots production: top-down and bottom-up. Herein, this review article discusses the recent advancements in the green fabrication of C-dots: photostability; surface structure and functionalization; potential applications for the sensing of hazardous anions and toxic heavy-metal ions; binding of toxic ions with C-dots; probable mechanistic routes of PL-based sensing of toxic heavy-metal ions. The green production of C-dots and their promising applications in the sensing of hazardous ions discussed herein provides deep insights into the safety of human health and the environment. Nonetheless, this review article provides a resource for the conversion of low-value biomass and biomass waste into valuable materials (i.e., C-dots) for promising sensing applications.

Dual metal ion (Fe3+ and As3+) sensing and cell bioimaging using fluorescent carbon quantum dots synthesised from Cynodon dactylon

In this study, water dispersible fluorescent carbon quantum dot (CQD) has been synthesised, having an average size of 8.6 ± 0.4 nm using Cynodon dactylon (CD) following microwave assisted green synthetic one-step method. As-prepared CQD fluoresces strongly at 444 nm having a quantum yield of 1% in water when excited at 350 nm. This fluorescence of CQD is sensitive toward As3+ and Fe3+ metal ions. These CQD are utilized for dual metal ion fluorescence sensing; turn-on fluorescence sensing for As3+ and turn-off fluorescence sensing for Fe3+ ions. Limit of detection for As3+ and Fe3+ ions has been found to be 19 nM and 0.10 μM respectively, which is the lowest value reported for As3+ without any functionalization. The adsorption kinetics of As3+ and Fe3+ ions on CQD have been examined using pseudo-first-order-kinetic model revealing that physical adsorption is dominant over chemical processes in this work. For 0.41 g/L and 1.90 g/L dose of CQD, the equilibrium adsorption capacity was found to be 1.57 × 10-6 mg/g, 2.91 × 10-7 mg/g, and 1.01 × 10-5 mg/g, 1.69 × 10-6 mg/g respectively for As3+ and Fe3+ ions. Despite having low quantum yield in water, as-prepared CQD showed low cytotoxicity and good tolerance against photodegradation of biological cells at concentrations lower than 62.5 μg/mL and when the cells are illuminated up to 12 h. Owing to this, the synthesised CQD have been utilized as fluorescent probes for in itro cell imaging.

Hydrothermal Synthesis of Functionalized Carbon Nanodots and Their Clusters as Ionic Probe for High Sensitivity and Selectivity for Sulfate Anions with Excellent Detection Level

Nitrogen-doped carbon nanodots (CNDs) were synthesized and utilized as sensing probes to detect different anions and metallic ions within aqueous solutions. The pristine CNDs were developed through a one-pot hydrothermal synthesis. o-Phenylenediamine was used as the precursor. A similar hydrothermal synthesis technique in the presence of polyethylene glycol (PEG) was adopted to form the PEG-coated CND clusters (CND-100k). Through photoluminescence (PL) quenching, both CND and PEG-coated CND suspensions display ultra-high sensitivity and selectivity towards HSO₄^- anions (Stern-Volmer quenching constant (K_SV) value: 0.021 ppm^-1 for CND and 0.062 ppm^-1 for CND-100k) with an ultra-low detection limit (LOD value: 0.57 ppm for the CND and 0.19 ppm for CND-100k) in the liquid phase. The quenching mechanism of N-doped CNDs towards HSO₄^- ions involves forming the bidentate as well as the monodentate hydrogen bonding with the sulfate anionic moieties. The detection mechanism of metallic ions analyzed through the Stern-Volmer formulation reveals that the CND suspension is well suited for the detection of Fe³⁺ (K_SV value: 0.043 ppm^-1) and Fe²⁺ (K_SV value: 0.0191 ppm^-1) ions, whereas Hg²⁺ (K_SV value: 0.078 ppm^-1) sensing can be precisely performed by the PEG-coated CND clusters. Accordingly, the CND suspensions developed in this work can be employed as high-performance PL probes for detecting various anions and metallic ions in the liquid phase.

Microwave synthesis of boron- and nitrogen-codoped graphene quantum dots and their detection to pesticides and metal ions

Through developing a highly efficient solid-phase microwave-assisted (SPMA) synthesis technique, we were able to synthesize graphene quantum dots (GQDs) that were doped with nitrogen and boron atoms. The as-synthesized GQDs were employed as sensing probes for detecting pesticides and iron ions within aqueous solutions. The SPMA approach is very versatile for in-situ doping of multiple atoms within the graphitic structure of GQDs. The maximal B/C and N/C atomic ratios within the GQD structures were reached as high as 28.6 and 86.4 at.%, respectively. For the B-/N-codoped GQDs, the N dopants comprises of pyrrolic/pyridinic N and graphitic N, whereas the B doping mainly involves two bonding types (i.e., B₄C and BCO₂) inserted into or decorated on the GQD skeleton structure. Based on the analysis of the Stern-Volmer plots, the B-/N-codoped GQDs can be employed as probing nanomaterials toward Fe²⁺ and paraquat detection thanks to their incredible sensitivity throughout the photoluminescent quenching. The PL quenching mechanism of GQDs is usually governed by the GQD‒(paraquat)_x intermediates formation and the resulting π-π stacking that can easily quench and aggregate. The findings of this work pave the pathway to engineering the chemical compositions as well as the crystalline structures of GQDs, used for energy and other sensing devices.

Graphene Quantum Dots: Novel Properties and Their Applications for Energy Storage Devices

Batteries and supercapacitors are the next-generation alternative energy resources that can fulfil the requirement of energy demand worldwide. In regard to the development of efficient energy storage devices, various materials have been tested as electrode materials. Graphene quantum dots (GQDs), a new class of carbon-based nanomaterial, have driven a great research interest due to their unique fundamental properties. High conductivity, abundant specific surface area, and sufficient solubility, in combination with quantum confinement and edge effect, have made them appropriate for a broad range of applications such as optical, catalysis, energy storage and conversion. This review article will present the latest research on the utilization of GQDs and their composites to modify the electrodes used in energy storage devices. Several major challenges have been discussed and, finally, future perspectives have been provided for the better implementation of GQDs in the energy storage research.

Nanoarchitectonics of neomycin-derived fluorescent carbon dots for selective detection of Fe3+ ions

The first-ever neomycin antibiotic-based carbon dots (Neo-CDs) were synthesized via a low-cost, eco-friendly, and single-step hydrothermal method using neomycin as a single precursor. The as-prepared Neo-CDs exhibited strong and stable blue fluorescence and were well characterized by TEM, UV-vis absorption, fluorescence emission, IR, XRD, Raman and XPS spectroscopy methods. The Neo-CDs showed a well-distributed size within the range of 4.5 to 7.8 nm, comprising various functional groups on the surface of the carbon core. The Neo-CDs exhibited exceptional emission behaviour, and fluorescence quantum yield was calculated to be 55% in double distilled water. Neo-CDs have been used as a fluorescent sensor for selective and sensitive detection of Fe³⁺ ions in aqueous solution in the fluorescence turn-off mode. From the set of metal ions, only the Fe³⁺ ion showed quenching of fluorescence due to photoinduced (PET) electron transfer from Neo-CDs to the half-filled 3d orbital of Fe³⁺ ions. The limit of detection for Fe³⁺ ions was calculated to be 0.854 μM. Further, the quenching efficiency and Stern-Volmer quenching constant have been calculated which are about 94% and 5.6 × 10⁶ M^-1, respectively.

Highly sensitive and selective detection of dopamine with boron and sulfur co-doped graphene quantum dots

In this work, we report, the synthesis of Boron and Sulfur co-doped graphene quantum dots (BS-GQDs) and its applicability as a label-free fluorescence sensing probe for the highly sensitive and selective detection of dopamine (DA). Upon addition of DA, the fluorescence intensity of BS-GQDs were effectively quenched over a wide concentration range of DA (0–340 μM) with an ultra-low detection limit of 3.6 μM. The quenching mechanism involved photoinduced electron transfer process from BS-GQDs to dopamine-quinone, produced by the oxidization of DA under alkaline conditions. The proposed sensing mechanism was probed using a detailed study of UV–Vis absorbance, steady state and time resolved fluorescence spectroscopy. The high selectivity of the fluorescent sensor towards DA is established. Our study opens up the possibility of designing a low-cost biosensor which will be suitable for detecting DA in real samples.

Graphene quantum dots: A review on the effect of synthesis parameters and theranostic applications

The rising demand for early-stage diagnosis of diseases such as cancer, diabetes, neurodegenerative can be met with the development of materials offering high sensitivity and specificity. Graphene quantum dots (GQDs) have been investigated extensively for theranostic applications owing to their superior photostability and high aqueous dispersibility. These are attractive for a range of biomedical applications as their physicochemical and optoelectronic properties can be tuned precisely. However, many aspects of these properties remain to be explored. In the present review, we have discussed the effect of synthetic parameters upon their physicochemical characteristics relevant to bioimaging. We have highlighted the effect of particle properties upon sensing of biological molecules through 'turn-on' and 'turn-off' fluorescence and generation of electrochemical signals. After describing the effect of surface chemistry and solution pH on optical properties, an inclusive view on application of GQDs in drug delivery and radiation therapy has been given. Finally, a brief overview on their application in gene therapy has also been included.

New bidental sulfur-doped graphene quantum dots modified with gold as a catalyst for hydrogen generation

Disadvantages of fossil fuels encourage researchers to develop clean combustion sources, in particular, H₂ due to the high potential energy and safe by-products. Herein, Au was deposited on S-doped graphene quantum dots to obtain a heterogeneous photocatalyst for the degradation of formic acid toward H₂ generation. Insertion of thiol groups on graphene quantum dots was carried out by self-condensation reaction of citric acid in the presence of Dimaval, as the thiol groups carrier. After the complexation of Au with the prepared S-doped graphene quantum dots, the catalytic activity of composite was evaluated in formic acid degradation to generate H₂ under visible light. Au@S-doped graphene quantum dots demonstrated superior catalytic activity with the turnover frequency up to 112 h^-1. The reaction enjoys significant benefits such as stability and recyclability of the catalyst, excellent reaction rate, and mild reaction conditions.

Quantum dot synthesis from waste biomass and its applications in energy and bioremediation

Quantum dots (QDs) are getting special attention due to their commendable optical properties and applications. Conventional metal-based QDs have toxicity and non-biodegradability issues, thus it becomes necessary to search for renewable precursor molecules for QDs synthesis. In recent years, biomass-based carbon rich QDs (CQDs) have been introduced which are mainly synthesised via carbonization (pyrolysis and hydrothermal treatment). These CQDs offered higher photostability, biocompatibility, low-toxicity, and easy tunability for physicochemical properties. Exceptional optical properties become a point of attraction for its multifaceted applications in various sectors like fabrication of electrodes and solar cells, conversion of solar energy to electricity, detection of pollutants, designing biosensors, etc. In recent years, a lot of work has been done in this field. This article will summarize these advancements along in a special context to biomass-based QDs and their applications in energy and the environment.

Structural and spectroscopic characterization of pyrene derived carbon nano dots: a single-particle level analysis

The bottom-up approach has been widely used for large-scale synthesis of carbon nanodots (CNDs). However, the structure and origin of photoluminescence in CNDs synthesized by the bottom-up approach is still a subject of debate. Here, using a series of separation techniques like solvent extraction, column chromatography, gel electrophoresis and dialysis, we present three distinct fluorescent components in CNDs synthesized from pyrene, a well-known precursor molecule. The separated components have qualitative and quantitatively different absorption and emission spectral features including quantum yield (QY). Optical and vibrational spectroscopy techniques combined with electron microscopy indicate that a subtle balance between the extent of graphitization and the presence of molecular fluorophores determines the nature of fluorescence emission. A substantial difference in photons/cycle, single-particle fluorescence blinking, ON-OFF photoswitching strongly supports the distinct nature of the components.

Hit Multiple Targets with One Arrow: Pb2+ and ClO- Detection by Edge Functionalized Graphene Quantum Dots and Their Applications in Living Cells

Recently, multimodal detection of analytes through a single nanoprobe has become an eminent approach for researchers. Herein a fluorescent nanoprobe, functionalized-GQD (F-GQD), has been designed through edge functionalization of graphene quantum dots (GQDs) by 2,6-diaminopyridine molecules. The fluorescence of F-GQD is quite sensitive to medium pH, making it a suitable pH sensor within the pH range 2-6. Interestingly, F-GQD shows dual sensing of Pb²⁺ and ClO^- by entirely different pathways; Pb²⁺ exhibits fluorescence turn-on performance while ClO^- triggers turn-off fluorescence quenching. The fluorescence enhancement may originate from the Pb²⁺-induced aggregation of the nanodots. The limit of detection (LOD) was also impressive, 1.2 μM and 12.6 nM for Pb²⁺ and ClO^-, respectively. The detailed mechanistic investigations reveal that both dynamic and static quenching effects operate together in the F-GQD-ClO^- system. The dynamic quenching was attributed to the energy migration from F-GQD to ClO^- through hydrogen bonding interaction (static quenching) between the amine group at the F-GQD surface and ClO^-. The F-GQD nanodot reveals excellent sensitivity toward the detection of ClO^- in real samples. Moreover, the F-GQDs also serve as multicolor fluorescent probes for cell imaging; the probe can easily penetrate the cell membrane and successfully detect intracellular ClO^-.

Multifunctional Biomedical Applications of Nitrogen and Sulfur Co-Doped Carbon Dots

Multifunctional carbon dots have drawn considerable attention due to their potential biomedical application value. We report the preparation of blue-green fluorescence-emitting, multifunctional, nitrogen-and-sulfur co-doped carbon dots (N, S-CDs) synthesized via a one-step process using 1-thioglycerol as a sulfur source, glucose and citric acid as carbon sources, and polyethyleneimine as a nitrogen source. Because of abundant amino and sulfur content, the CDs exhibited high sensibility and selectivity for detecting Cu₂₊ (detection limit: 0.01 μM, linear range: 0.025 to 50 μM). Fast and sensitive detection of tiopronin was also achieved on the basis of the fluorescence "off-on" mode considering the strong affinity between tiopronin and Cu₂₊. The N, S-CDs exhibited good biocompatibility as determined by fluorescence imaging using onion epidermal cells and gram-positive bacteria. The CDs also exhibited excellent antimicrobial ability against the gram-positive bacteria. Our results indicate that these novel N, S-CDs could be ideal candidates for several biochemical applications such as antibacterial treatment and detection of small biomolecules.

Recent advances in heteroatom-doped graphene quantum dots for sensing applications

Graphene quantum dots (GQDs) are carbon-based fluorescent nanomaterials having various applications due to attractive properties. But the low photoluminescence (PL) yield and monochromatic PL behavior of GQDs put limitations on their real-time applications. Therefore, heteroatom doping of GQDs is recognized as the best approach to modify the optical as well as electronic properties of GQDs by modifying their chemical composition and electronic structure. In this review, the new strategies for preparing the heteroatom (N, B, S, P) doped GQDs by using different precursors and methods are discussed in detail. The particle size, emission wavelength, PL emissive color, and quantum yield of recently developed heteroatom doped GQDs are reported in this article. The investigation of structure, crystalline nature, and composition of heteroatom doped GQDs by various characterization techniques such as high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) are also described. The recent progress on the impact of mono or co-doping of heteroatoms on PL behavior, and optical, electrochemiluminescence (ECL), and electrochemical properties of GQDs is also surveyed. Further, heteroatom doped GQDs with attractive properties used in sensing of various metal ions, biomolecules, small organic molecules, etc. by using various techniques with different limits of detection are also summarized. This review provides progressive trends in the development of heteroatom doped GQDs and their various applications.

A nanohybrid system based on covalently functionalized graphene quantum dots with dithienopyrrole derivative for the sensitive and selective fluorometric detection of Pb2+ ions

Recently graphene quantum dot (GQD) based nanohybrid materials have received much attention. Herein, a highly fluorescent biocompatible GQD and N-functionalized dithienopyrrole (DTP-PPD) modified nanohybrid system was fabricated (DTP-PPD-fn-GQD) for the first time. Modification resulted in stable fluorescence with a quantum yield of approximately 22% and 36 nm redshift compared to unmodified GQD. The apparent bandgap tuning along with fluorescence property changes was investigated by cyclic voltammetry measurements and density functional theory studies. This water-soluble system was then applied for the sensitive and selective detection of Pb²⁺ ions. As a result of a specific interaction towards Pb²⁺ ions, the fluorescence intensity was quenched. The detection limit is found to be 1.02 nM within the linear range of 3 to 30 nM. Finally, the feasibility of the developed probe was tested with real samples of water. Chemically modified GQDs are already widely exploited for Pb²⁺ sensing, whereas GQD/thiophene-based nanohybrid systems are less utilized. This newly developed nanohybrid of GQD (DTP-PPD-fn-GQD) has excellent fluorescence properties and bandgap tunability. Moreover, effective fluorometric sensing of Pb²⁺ ions in an aqueous medium is well investigated. This gives more insight into developing GQD-based highly promising nanohybrid colorimetric as well as fluorescent sensors.

Fluorescent nanoparticles as tools in ecology and physiology

Fluorescent nanoparticles (FNPs) have been widely used in chemistry and medicine for decades, but their employment in biology is relatively recent. Past reviews on FNPs have focused on chemical, physical or medical uses, making the extrapolation to biological applications difficult. In biology, FNPs have largely been used for biosensing and molecular tracking. However, concerns over toxicity in early types of FNPs, such as cadmium-containing quantum dots (QDs), may have prevented wide adoption. Recent developments, especially in non-Cd-containing FNPs, have alleviated toxicity problems, facilitating the use of FNPs for addressing ecological, physiological and molecule-level processes in biological research. Standardised protocols from synthesis to application and interdisciplinary approaches are critical for establishing FNPs in the biologists' tool kit. Here, we present an introduction to FNPs, summarise their use in biological applications, and discuss technical issues such as data reliability and biocompatibility. We assess whether biological research can benefit from FNPs and suggest ways in which FNPs can be applied to answer questions in biology. We conclude that FNPs have a great potential for studying various biological processes, especially tracking, sensing and imaging in physiology and ecology.

Nanomaterial-based fluorescent sensors for the detection of lead ions

Lead (Pb) poisoning has been a scourge to the human to pose sighnificant health risks (e.g., organ disorders, carcinogenicity, and genotoxicity) as observed from many different parts of the world, especially in developing countries. The demand for accurate sensors for its detection, especially in environmental media (soil, water, food, etc.) has hence been growing steadily over the years. The potential utility of fluorescent nanosensors as an important analytical tool is recognized due to their astonishing characteristics (e.g., high sensitivity/selectivity, enhanced detection performance, low cost, portability, and rapid on-site detection ability). This review is organized to offer insight into the recent developments in fluorescent nanosensing technology for the detection of lead ions (Pb²⁺). To this end, different types of nanomaterials explored for such applications have been classified and evaluated with respect to performance, especially in terms of sensitivity. This review will help researchers gain a better knowledge on the status and importance of optical nanosensors so as to remediate the contamination of lead and associated problems. The technical challenges and prospects in the development of nanosensing systems for Pb²⁺ are also discussed.

Carbon dots derived from Fusobacterium nucleatum for intracellular determination of Fe3+ and bioimaging both in vitro and in vivo

Intracellular Fe3+ amount is one of the critical determinants of human health. The development of simple and effective probes for the quantitative detection of Fe3+in vivo is of great significance for the early diagnosis of disease or disorder associated with iron deficiency or overload. In this study, remarkable carbon dots, which can serve as a biosensor for efficient intracellular Fe3+ detection, were synthesized by hydrothermal carbonization of Fusobacterium nucleatum, an anaerobic bacterium. The achieved F. nucleatum-carbon dots (Fn-CDs) possessed the features of strong fluorescence, high stability and excellent biocompatibility. The obtained Fn-CDs could easily internalize into both plant cells and human cells with excellent ability for cell tracking and biomedical labeling. The fluorescence of Fn-CDs could still remain for another 24 hours after penetrating into cells. Furthermore, the fluorescent Fn-CDs were very sensitive to the presence of Fe3+ ions even in cells, exhibiting great promising applications in in vivo detection of Fe3+ ions. In addition, the Fn-CDs posed no harm to the mice, being circulated and excreted within a short time, making the Fn-CDs an excellent candidate for bioimaging and biosensing in vivo.

Multienzymes activity of metals and metal oxide nanomaterials: applications from biotechnology to medicine and environmental engineering

With the rapid advancement and progress of nanotechnology, nanomaterials with enzyme-like catalytic activity have fascinated the remarkable attention of researchers, due to their low cost, high operational stability, adjustable catalytic activity, and ease of recycling and reuse. Nanozymes can catalyze the same reactions as performed by enzymes in nature. In contrast the intrinsic shortcomings of natural enzymes such as high manufacturing cost, low operational stability, production complexity, harsh catalytic conditions and difficulties of recycling, did not limit their wide applications. The broad interest in enzymatic nanomaterial relies on their outstanding properties such as stability, high activity, and rigidity to harsh environments, long-term storage and easy preparation, which make them a convenient substitute instead of the native enzyme. These abilities make the nanozymes suitable for multiple applications in sensing and imaging, tissue engineering, environmental protection, satisfactory tumor diagnostic and therapeutic, because of distinguished properties compared with other artificial enzymes such as high biocompatibility, low toxicity, size dependent catalytic activities, large surface area for further bioconjugation or modification and also smart response to external stimuli. This review summarizes and highlights latest progress in applications of metal and metal oxide nanomaterials with enzyme/multienzyme mimicking activities. We cover the applications of sensing, cancer therapy, water treatment and anti-bacterial efficacy. We also put forward the current challenges and prospects in this research area, hoping to extension of this emerging field. In addition to therapeutic potential of nanozymes for disease prevention, their practical effects in diagnostics, to monitor the presence of SARS-CoV-2 and related biomarkers for future pandemics will be predicted.

N-doped graphene quantum dots modified with CuO (0D)/ZnO (1D) heterojunctions as a new nanocatalyst for the environmentally friendly one-pot synthesis of monospiro derivatives

A new approach for the sustainable fabrication of a CuO/ZnO@N-GQDs nanocomposite as a powerful nanocatalyst for multicomponent reactions is demonstrated.

Graphene Quantum Dots as Flourishing Nanomaterials for Bio-Imaging, Therapy Development, and Micro-Supercapacitors

Graphene quantum dots (GQDs) are considerably a new member of the carbon family and shine amongst other members, thanks to their superior electrochemical, optical, and structural properties as well as biocompatibility features that enable us to engage them in various bioengineering purposes. Especially, the quantum confinement and edge effects are giving GQDs their tremendous character, while their heteroatom doping attributes enable us to specifically and meritoriously tune their prospective characteristics for innumerable operations. Considering the substantial role offered by GQDs in the area of biomedicine and nanoscience, through this review paper, we primarily focus on their applications in bio-imaging, micro-supercapacitors, as well as in therapy development. The size-dependent aspects, functionalization, and particular utilization of the GQDs are discussed in detail with respect to their distinct nano-bio-technological applications.

One-step synthesis of sulfur-incorporated graphene quantum dots using pulsed laser ablation for enhancing optical properties

To tune the electronic and optoelectronic properties of graphene quantum dots (GQDs), heteroatom doping (e.g., nitrogen (N), boron (B), and sulfur (S)) is an effective method. However, it is difficult to incorporate S into the carbon framework of GQDs because the atomic size of S is much larger than that of C atoms, compared to N and B. In this study, we report a simple and one-step method for the synthesis of sulfur-doped GQDs (S-GQDs) via the pulsed laser ablation in liquid (PLAL) process. The as-prepared S-GQDs exhibited enhanced fluorescence quantum yields (0.8% → 3.89%) with a huge improved absorption band in ultraviolet (UV) region (200 ∼ 400 nm) and excellent photo stability under the UV radiation at 360 nm. In addition, XPS results revealed that the PLAL process can effectively facilitate the incorporation of S into the carbon framework compared to those produced by the chemical exfoliation method (e.g., hydrothermal method). And also, the mechanisms related with the optical properties of S-GQDs was investigated by time-resolved photoluminescence (TRPL) spectroscopy. We believe that the PLAL process proposed in this study will serve as a simple and one-step route for designing S-GQDs and opens up to opportunities for their potential applications.

Embedding dual fluoroprobe in metal-organic frameworks for continuous visual recognition of Pb2+ and PO43- via fluorescence 'turn-off-on' response: Agar test paper and fingerprint

A novel dual-emissive ratiometric fluorescence (RF) probe CDs/QDs@ZIF-8 has been successfully constructed by employing a simple and effective strategy for in situ encapsulating carbon dots (CDs) and thioglycolic acid-modified CdTe quantum dots (QDs) into porous metal-organic frameworks (MOFs) "cage". The dual responsive colorimetric fluorescence probe was developed for the ultra-high selectivity and sensitivity continuous detection of Pb²⁺ (turn OFF) and PO₄^3- (turn ON) in biological samples. Blue CDs acts as a stable internal standard emission, the emission color of CDs/QDs@ZIF-8 changes from red to blue with introducing Pb²⁺, fluorescence of probe is quenched because of the binding of Pb²⁺ ions to thioglycolic acid on the surface of probe and e- transfer from the photoexcited QDs to Pb²⁺ ions, color can be recovered after the adding PO₄^3- to CDs/QDs@ZIF-8-Pb²⁺ system, which could take away Pb²⁺ ions from the surface of CDs/QDs@ZIF-8. More importantly, fabricated agar test papers was also successfully applied in visual detection of Pb²⁺ and PO₄³- in real samples, which can implement without instrument-specific calibration.

Effect of sulfur doping on fluorescence and quantum yield of graphene quantum dots: an experimental and theoretical investigation

Graphene quantum dots (GQDs) are one of the most promising luminescent carbon derived nanomaterials decorated with multiple useful functional groups and remarkable optoelectronic properties. Heteroatom doping of hexagonal carbon sheet of GQDs is an effective strategy to tailor their properties to meet desired application. In this work, sulfur doped GQDs (S-GQDs) were synthesized by simply pyrolyzing citric acid (CA) as a source of carbon and 3-Mercaptopropionic acid as a source of sulfur dopant. The optimal reaction conditions (ratio of the carbon to dopant source, temperature and time of reaction) were obtained while investigating their effect on the quantum yield and fluorescence properties of GQDs and, are hereby, reported for the first time. The as-synthesized S-GQDs were extensively characterized by different analytical techniques such as transmission electron microscopy (TEM), UV-vis Spectroscopy (UV), Fourier transform infrared spectroscopy, photoluminescence (PL) and x-ray Photoelectron Spectroscopy. S-GQDs were found uniform in size (∼4 nm) and spherical in shape with strong blue fluorescence. Further, for in-depth analysis of experimental results and underlying phenomena, theoretical studies based on density functional theory were performed for chemical structure optimization, possible sites of doping and density of states calculation. The synthesized S-GQDs exhibited excellent solubility in water, a stronger fluorescence and desirably higher quantum yield (57.44%) as compared to that of previously reported undoped GQDs. These successfully demonstrated unique and improved properties of S-GQDs present them as a potential candidate for biomedical, optical, electrical and chemical applications.

Development of Graphene Quantum Dots-Based Optical Sensor for Toxic Metal Ion Detection

About 71% of the Earth's surface is covered with water. Human beings, animals, and plants need water in order to survive. Therefore, it is one of the most important substances that exist on Earth. However, most of the water resources nowadays are insufficiently clean, since they are contaminated with toxic metal ions due to the improper disposal of pollutants into water through industrial and agricultural activities. These toxic metal ions need to be detected as fast as possible so that the situation will not become more critical and cause more harm in the future. Since then, numerous sensing methods have been proposed, including chemical and optical sensors that aim to detect these toxic metal ions. All of the researchers compete with each other to build sensors with the lowest limit of detection and high sensitivity and selectivity. Graphene quantum dots (GQDs) have emerged as a highly potential sensing material to incorporate with the developed sensors due to the advantages of GQDs. Several recent studies showed that GQDs, functionalized GQDs, and their composites were able to enhance the optical detection of metal ions. The aim of this paper is to review the existing, latest, and updated studies on optical sensing applications of GQDs-based materials toward toxic metal ions and future developments of an excellent GQDs-based SPR sensor as an alternative toxic metal ion sensor.

Fluorescent carbon dots functionalization

Carbon dots (CDs), as a new type of luminescent zero-dimensional carbon nanomaterial, have been applied in a variety of fields. Currently, functionalization of CDs is an extremely useful method for effectively tuning their intrinsic structure and surface state. Heteroatom doping and surface modification are two functionalization strategies for improving the photophysical performance and broadening the range of applications for fluorescent CDs. Heteroatom doping in CDs can be used to tune their intrinsic properties, which has received significant research interests because of its simplicity. Surface modification can be applied for varying active sites and the functional groups on the CDs surface, which can endow fluorescent CDs with the unique properties resulting from functional ligand. In this review, we summarize the structural and physicochemical properties of functional CDs. We focused our review on the latest developments in functionalization strategies for CDs and discuss the detailed characteristics of different functionalization methods. Ultimately, we hope to inform researchers on the latest progress in functionalization of CDs and provide perspectives on future developments for functionalization of CDs and their potential applications.

Fluorescent Sensors for the Detection of Heavy Metal Ions in Aqueous Media

Due to the risks that water contamination implies for human health and environmental protection, monitoring the quality of water is a major concern of the present era. Therefore, in recent years several efforts have been dedicated to the development of fast, sensitive, and selective sensors for the detection of heavy metal ions. In particular, fluorescent sensors have gained in popularity due to their interesting features, such as high specificity, sensitivity, and reversibility. Thus, this review is devoted to the recent advances in fluorescent sensors for the monitoring of these contaminants, and special focus is placed on those devices based on fluorescent aptasensors, quantum dots, and organic dyes.

Resonance light scattering sensor of the metal complex nanoparticles using diethyl dithiocarbamate doped graphene quantum dots for highly Pb(II)-sensitive detection in water sample

This study was aimed to detect Pb²⁺ using diethyl dithiocarbamate-doped graphene quantum dots (DDTC-GQDs) based pyrolysis of citric acid. The excitation maximum wavelength (λ_max, ex = 337 nm) of the DDTC-GQDs solution was blue shift from bare GQDs (λ_max, ex = 365 nm), with the same emission maximum wavelength (λ_max, em = 459 nm) indicating differences in the desired N, S matrices decorating in the nanoparticles. Their resonance light scattering intensities were peaked at the same λ_max, ex/em = 551/553 nm without any background effect of both ionic strength and masking agent. Under optimal conditions, the linear range was 1.0-10.0 μg L^-1 (R² = 0.9899), limit of detection was 0.8 μg L^-1 and limit of quantification was 1.5 μg L^-1. The precision, expressed as the relative standard deviations, for intra-day and inter-day analyses was 0.87% and 4.47%, respectively. The recovery study of Pb²⁺ for real water samples was ranged between 80.8% and 109.5%. The proposed method was also proved with certified water sample containing 60 μg L^-1 Pb²⁺ giving an excellent accuracy and was then implied satisfactorily for ultra-trace determination of Pb²⁺ in drinking water and tap water samples.

Nitrogen and sulfur co-doped graphene quantum dots for the highly sensitive and selective detection of mercury ion in living cells

In this work, a novel nitrogen and sulfur co-doped graphene quantum dots (N,S/GQDs) were synthesized by a simple and green pyrolysis method with citric acid as carbon source, d‑penicillamine as doped molecule. The obtained N,S/GQDs shows good water solubility, low cytotoxicity and high quantum yield of 56.4%. The doping of N and S can efficient promoted the coordination between the residual group of N,S/GQDs and Hg²⁺, thus, the fluorescence of N,S/GQDs can be dramatically and selectively quenched by Hg²⁺. Under the optimum conditions, the quenching effect of Hg²⁺ to N,S/GQDs was lined with its concentrations in the range of 0.9 to 30 nM with a detection limit of 0.69 nM. 100 fold of common metals had no influence for the determination of Hg²⁺. When Hela cells were incubated into the N,S/GQDs solutions for 2 h, more than 85% Cells still kept living even at a high concentration of 400 μg/mL suggesting the good biocompatibility and low toxicity of the obtained N,S/GQDs. Besides this, the N,S/GQDs also showed good cell permeability and could enter into the cell and showed bright blue light. Thus, they were successfully applied as a powerful fluorescent nano-sensor for imaging detection of mercury ions in the living cells.

Sulfur-doped graphene quantum dot-based paper sensor for highly sensitive and selective detection of 4-nitrophenol in contaminated water and wastewater

4-Nitrophenol (4-NP) is a promulgated priority pollutant, which can cause a negative impact on human health. The development of a direct and effective technique for the rapid detection and screening of 4-NP is, therefore, of urgent need. In this study, the blue luminescent sulfur-doped graphene quantum dots (S-GQDs) with a size of 1–5 nm are fabricated using a one-step pyrolysis procedure in the presence of citric acid and 3-mercaptosuccinic acid. The S-GQDs exhibit a strong emission band at 450 nm under the excitation of 330 nm UV light. 4-NP can serve as the fluorescence quencher by the π–π interaction with S-GQD, resulting in the linear decrease in fluorescence intensity after the addition of various 4-NP concentrations ranging from 10 nM to 200 μM. The S-GQDs serve as the sensing probe to enhance the analytical performance on 4-NP detection with the limit of detection values of 0.7 and 3.5 nM in deionized water and wastewater, respectively. The S-GQD based sensing platform can be used to detect 4-NP in different matrices of water and wastewater. In addition, the detected percentages of spiked 4-NP concentrations in the presence of different matrices and interferences are in the range of (98 ± 5)–(108 ± 2)%. Moreover, the S-GQD based paper sensor can rapidly screen 4-NP in wastewater within 1 min. Results obtained in this study clearly demonstrate the superiority of S-GQDs as a promising fluorescence probe for highly sensitive and selective detection of a wide concentration range of 4-NP in deionized water and wastewater.

Sulfur-doped graphene quantum dots have been prepared for effective and rapid detection of 4-nitrophenol.

Gram-scale synthesis of nitrogen doped graphene quantum dots for sensitive detection of mercury ions and l-cysteine

Sensitive and reliable detection of mercury ions (Hg²⁺) and l-cysteine (l-Cys) is of great significance for toxicology assessment, environmental protection, food analysis and human health. Herein, we present gram-scale synthesis of nitrogen doped graphene quantum dots (N-GQDs) for sensitive detection of Hg²⁺ and l-Cys. The N-GQDs are one-step synthesized using bottom-up molecular fusion in a hydrothermal process with gram-scale yield at a single run. N-GQDs exhibit good structural characteristics including uniform size (∼2.1 nm), high crystallinity, and single-layered graphene thickness. Successful doping of N atom enables bright blue fluorescence (absolute photoluminescence quantum yield of 24.8%) and provides unique selectivity towards Hg²⁺. Based on the fluorescence quenching by Hg²⁺ (turn-off mode), N-GQDs are able to serve as an effective fluorescent probe for sensitive detection of Hg²⁺ with low limit of detection (19 nM). As l-Cys could recover the fluorescence of N-GQDs quenched by Hg²⁺, fluorescent detection of l-Cys is also demonstrated using turn-off-on mode.

One-step and gram-scale synthesis of nitrogen doped graphene quantum dots is realized for their sensitive detection of Hg²⁺ and l-Cys.

Interband Absorption in Few-Layer Graphene Quantum Dots: Effect of Heavy Metals

Monolayer, bilayer, and trilayer graphene quantum dots (GQDs) with different binding abilities to elemental heavy metals (HMs: Cd, Hg, and Pb) were designed, and their electronic and optical properties were investigated theoretically to understand deeply the optical response under heavy metal exposure. To gain insight into the nature of interband absorption, we performed density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations for thickness-varying GQDs. We found that the interband absorption in GQDs can be efficiently tuned by controlling the thickness of GQDs to attain the desirable coloration of the interacting complex. We also show that the strength of the interaction between GQDs and Cd, Hg, and Pb is strongly dependent on the number of sp²-bonded layers. The results suggest that the thickness of GQDs plays an important role in governing the hybridization between locally-excited (LE) and charge-transfer (CT) states of the GQDs. Based on the partial density-of-states (DOS) analysis and in-depth knowledge of excited states, the mechanisms underlying the interband absorption are discussed. This study suggests that GQDs would show an improved sensing performance in the selective colorimetric detection of lead by the thickness control.

Facile and Highly Effective Synthesis of Controllable Lattice Sulfur-Doped Graphene Quantum Dots via Hydrothermal Treatment of Durian

Recently, the biomass "bottom-up" approach for the synthesis of graphene quantum dots (GQDs) has attracted broad interest because of the outstanding features, including low-cost, rapid, and environmentally friendly nature. However, the low crystalline quality of products, substitutional doping with heteroatoms in lattice, and ambiguous reaction mechanism strongly challenge the further development of this technique. Herein, we proposed a facile and effective strategy to prepare controllable sulfur (S) doping in GQDs, occurring in a lattice substitution manner, by hydrothermal treatment of durian with platinum catalyst. S atoms in GQDs are demonstrated to exist in the thiophene structure, resulting in good optical and chemical stabilities, as well as ultrahigh quantum yield. Detailed mechanism of the hydrothermal reaction progress was investigated. High-efficiency reforming cyclization provided by platinum was evidenced by the coexistence of diversified sp²-fused heterocyclic compounds and thiophene derivatives. Moreover, we also demonstrated that saccharides in durian with small molecular weight (<1000 Da) is the main carbon source for the forming GQDs. Because of the desulfurizing process, controllable photoluminescence properties could be achieved in the as-prepared GQDs via tuning doping concentrations.

Interband transitions in closed-shell vacancy containing graphene quantum dots complexed with heavy metals

High-performance optical detection of toxic heavy metals by using graphene quantum dots (GQDs) requires a strong interaction between the metals and GQDs, which can be reached through artificial creation of vacancy-type defects in GQDs.