Exonuclease I-Assisted General Strategy to Convert Aptamer-Based Electrochemical Biosensors from "Signal-Off" to "Signal-On"

Rapid and specific detection of thiabendazole: enzymatic digestion-enabled fluorescent aptasensor

Thiabendazole, a widely used broad-spectrum fungicide in agriculture, poses risks to human health. To monitor its presence in water, we propose a fluorescent aptasensor utilizing Escherichia coli exonuclease I (Exo I). The findings demonstrate a linear correlation between thiabendazole concentrations and digestion percentage, with a detection limit (LOD) exceeding 1 µM and a determination coefficient (R2) of 0.959. This aptamer-based fluorescence spectroscopy detection system holds promise for a rapid, specific, and sensitive analysis of thiabendazole in environmental waters and food matrices.

A signal-on photoelectrochemical aptasensor based on ferrocene labeled triple helix DNA molecular switch for detection of antibiotic amoxicillin

A sensitive signal-on photoelectrochemical aptasensor for antibiotic determination was constructed based on the energy level matching between ferrocene and CuInS2. P-type CuInS2 microflower was complexed with reduced graphene oxide (CuInS2/rGO) to get photocathode current with good photoelectric conversion efficiency and stability. Then, hairpin DNA (HP) was covalently bonded to the electrode surface. A triple helix DNA (THMS) was used as a molecular switch. After the specific recognition between target and THMS in homogeneous solution, ferrocene labeled probe (Fc-T2) was released. Finally, Fc-T2 was captured by the HP, which leaded the obvious increase of photocurrent for the energy level matching between ferrocene and CuInS2. The increase of the photocurrent signal was proportional to the concentration of target amoxicillin (AMOX), the linear range was 100 fM-100 nM with detection limit of 19.57 fM. Meanwhile, the method has been successfully applied for milk and lake water samples analysis with satisfactory results.

Aptamer-Protein Interactions: From Regulation to Biomolecular Detection

Serving as the basis of cell life, interactions between nucleic acids and proteins play essential roles in fundamental cellular processes. Aptamers are unique single-stranded oligonucleotides generated by in vitro evolution methods, possessing the ability to interact with proteins specifically. Altering the structure of aptamers will largely modulate their interactions with proteins and further affect related cellular behaviors. Recently, with the in-depth research of aptamer-protein interactions, the analytical assays based on their interactions have been widely developed and become a powerful tool for biomolecular detection. There are some insightful reviews on aptamers applied in protein detection, while few systematic discussions are from the perspective of regulating aptamer-protein interactions. Herein, we comprehensively introduce the methods for regulating aptamer-protein interactions and elaborate on the detection techniques for analyzing aptamer-protein interactions. Additionally, this review provides a broad summary of analytical assays based on the regulation of aptamer-protein interactions for detecting biomolecules. Finally, we present our perspectives regarding the opportunities and challenges of analytical assays for biological analysis, aiming to provide guidance for disease mechanism research and drug discovery.

Advances in the application of metal-organic framework nanozymes in colorimetric sensing of heavy metal ions

Nanozymes, which can be defined as nanomaterials with excellent catalytic function, are well known to the scientific community due to their distinct merits, such as low cost and high stability, which render them preferable to natural enzymes. As porous organic-inorganic coordination materials, metal-organic frameworks (MOFs) possess a large number of active sites and thus can effectively mimic the properties of natural enzymes. Recently, MOF-based nanozymes have also exhibited good application potential for the analysis of heavy metal ions. In comparison to the traditional detection methods for heavy metal ions, nanozyme-based colorimetric sensing permits intuitive visual analysis by using relatively simple instruments, facilitating rapid and simple on-site screening. In this minireview, the preparation of MOF-based nanozymes and the different nanozyme activity types are briefly described, such as peroxidase-like and oxidase-like, and the relevant catalytic mechanisms are elaborated. Based on this, different response mechanisms of MOF-based colorimetric methods to heavy metal ions, such as turn-off, turn-on, and turn-off-on, are discussed. In addition, the colorimetric sensing applications of MOF-based nanozymes for the detection of heavy metal ions are summarized. Finally, the current research status of MOF-based nanozymes and the future development direction are discussed.

Glyphosate-Exonuclease Interactions: Reduced Enzymatic Activity as a Route to Glyphosate Biosensing

N-phosphonomethyle-glycine (glyphosate) is the most widely used pesticide worldwide due to its effectiveness in killing weeds at a moderate cost, bringing significant economic benefits. However, owing to its massive use, glyphosate and its residues contaminate surface waters. On site, fast monitoring of contamination is therefore urgently needed to alert local authorities and raise population awareness. Here the hindrance of the activity of two enzymes, the exonuclease I (Exo I) and the T5 exonuclease (T5 Exo) by glyphosate, is reported. These two enzymes digest oligonucleotides into shorter sequences, down to single nucleotides. The presence of glyphosate in the reaction medium hampers the activity of both enzymes, slowing down enzymatic digestion. It is shown by fluorescence spectroscopy that the inhibition of ExoI enzymatic activity is specific to glyphosate, paving the way for the development of a biosensor to detect this pollutant in drinking water at suitable detection limits, i.e., 0.6 nm.

Exonuclease-based aptasensors: Promising for food safety and diagnostic aims

As of today's requirement, developing cost-effective smart sensing tools with ultrahigh sensitivity for food safety insurance is of special importance. For this purpose, aptamer-based biosensors (aptasensors) powered by the superiorities of the recycling signal amplification strategies have been expanded especially. Target recycling supported by enzymes is an appealing approach for implementing signal amplification. As the supreme biocatalyst enzymes, exonucleases can inaugurate signal improvement by involving a single target in a process would result in appreciable repeating cycles of the cleavage of the phosphodiester bonds between the building blocks of the nucleic acid strands, and also, their terminals. Although there are diverse substances for catalyzing amplification strategies, including nanoparticles, carbon-based nanocomposites, and quantum dots (QDs), exonucleases are of superiority over them by simplifying the amplification process with no need for the complicated pre-treatment processes. The outstanding selectivity and great sensitivity of the aptasensors tuned by amplification potency of exonucleases nominate them as the promising sensing tools for label-free, ease-of-use, cost-effective, and real-time diagnosis of diverse targets. Here, we summarize the achievements and perspectives in the scientific branch of aptasensor design for the qualitative monitoring of diverse targets by cooperation of exonucleases with the conspicuous potential for the signal amplification. Finally, some results are expressed to provide a comprehensive viewpoint for developing novel nuclease-based aptasensors in the future.

Highly Sensitive and Selective Detection of Inorganic Phosphates in the Water Environment by Biosensors Based on Bioluminescence Resonance Energy Transfer

The accurate detection of phosphate in water is very important to prevent water eutrophication and ensure the health of water quality. However, traditional phosphomolybdenum blue spectrophotometry is not sensitive, is time-consuming, and demands large amounts of chemical reagents. Therefore, highly sensitive, rapid, and environmentally friendly Pi detection methods are urgently needed. Here, we developed a bioluminescence resonance energy transfer (BRET)-based biosensor, which can detect Pi in water quickly, highly sensitively, and highly selectively. The NanoLuc and the Venus fluorescent protein were selected as the bioluminescence donor and energy acceptor, respectively. The best-performing BRET sensor variant, VenusΔC10-PΔC12-ΔN4Nluc, was identified by Pi-specific binding protein (PiBP) screening and systematic truncation. Single-factor experiments optimized the key parameters affecting the detection performance of the sensor. Under the optimal detection conditions, the detection limit of this method was 1.3 μg·L^-1, the detection range was 3.3-434 μg·L^-1, and it had excellent selectivity, repeatability, and stability. This low-cost and environment-friendly BRET sensor showed a good application prospect in real water quality detection.

A versatile CRISPR/Cas12a-based biosensing platform coupled with a target-protected transcription strategy

Besides the critical role in gene editing, CRISPR/Cas system also brings a new signal amplification mechanism to the development of next generation biosensing technologies. Herein, we have developed a versatile CRISPR/Cas12a sensing platform by combining a target protection-based transcription amplification strategy with the Cas12a-based signal amplification mechanism, which allows for the sensitive detection of both nucleic acid and non-nucleic acid targets. In this design, a rationally designed transcription template sequence is able to avoid Exonuclease I (Exo I) degradation only in the existence of the target-mediated binding events including either nucleic acid hybridization or protein-based affinity interactions. This target binding-induced protection effect can facilitate the subsequent transcription amplification to generate crRNA and activate the subsequent Cas12a trans-cleavage signal amplification mechanism to yield target dosage-responsive fluorescence signal. In contrast, if the target is absent, the protection-free transcription template will be completely digested by Exo I, thus no fluorescence response is produced. This new strategy well eliminates the T7 polymerase-associated non-specific transcription background and realizes the sensitive detection of various kinds of biomolecules including microRNA, protein, as well as exosome, broadening the application scenarios of CRISPR/Cas system in the field of bioanalysis and biosensing.

In vitro electrochemical detection of the degradation of amyloid-β oligomers

The clearance of overloaded amyloid β (Aβ) oligomers is thought to be an attractive and potential strategy for the therapy of Alzheimer's disease (AD). A variety of strategies have already been utilized to study Aβ degradation in vitro. Here, the electrochemical detection based on direct electrooxidation of specific Tyr residues within Aβ peptide has been developed as a simple and robust approach for monitoring the oligomers' degradation. C60 was employed for photodegrading Aβ oligomers due to the generated ROS under light irradiation. The oxidation current of Tyr residues by square wave voltammetry (SWV) increased upon the Aβ degradation, confirming that the structure variation of Aβ peptide indeed influenced the exposure of those redox species to the electrode surface and final signal output. Chronoamperometric assay also found the electrooxidation of Tyr undergone an irreversible process. Additionally, the direct electrochemistry was capable of detecting the aggregation with rapid test and better sensitivity in compared with dynamic light scattering (DLS), atomic force microscopy (AFM) and thioflavin T (ThT) based fluorescence assay. Thus, this work indicated the potential application of direct electrochemistry in the in vitro measurement of Aβ degradation and clearance, providing new insights and a complementary means into the AD theranostics.

Review on the Selection of Aptamers and Application in Paper-Based Sensors

An aptamer is a synthetic oligonucleotide, referring to a single-stranded deoxyribonucleic acid or ribonucleic acid ligand produced by synthesis from outside the body using systematic evolution of ligands by exponential enrichment (SELEX) technology. Owing to their special screening process and adjustable tertiary structures, aptamers can bind to multiple targets (small molecules, proteins, and even whole cells) with high specificity and affinity. Moreover, due to their simple preparation and stable modification, they have been widely used to construct biosensors for target detection. The paper-based sensor is a product with a low price, short detection time, simple operation, and other superior characteristics, and is widely used as a rapid detection method. This review mainly focuses on the screening methods of aptamers, paper-based devices, and applicable sensing strategies. Furthermore, the design of the aptamer-based lateral flow assay (LFA), which underlies the most promising devices for commercialization, is emphasized. In addition, the development prospects and potential applications of paper-based biosensors using aptamers as recognition molecules are also discussed.

An electrochemical biosensor for the detection of aflatoxin B1 based on the specific aptamer and HCR biological magnification

Aflatoxin B1 (AFB1) is a highly toxic mycotoxin, which causes severe acute or cumulative poisoning. Therefore, it is important to develop sensitive and selective detection methods for AFB1 for the safety of food and medicinal herbs. Herein, we have developed a "signal-on" electrochemical aptasensor based on the high specificity of the aptamer and hybridization chain reaction (HCR) biological amplification for AFB1 detection. In this work, thiol-modified complementary DNA (cDNA) immobilized on the surface of a gold electrode (GE) served as an initiator DNA. When AFB1 was present, it competed with the cDNA for binding to the aptamers, which resulted in the detaching of aptamers from the cDNA-aptamer duplexes. Then, the single-stranded cDNA acted as an initiator to trigger the HCR signal amplification. Therefore, long double-stranded DNA (dsDNA) products were produced, which could load large amounts of methylene blue (MB) molecules to generate a distinct electrochemical signal. Under the optimized conditions, the proposed electrochemical aptasensor achieved the ultrasensitive detection of AFB1 with a linear detection range of 0.01-100 pg mL^-1, and a limit of detection (LOD) down to 2.84 fg mL^-1. Furthermore, the electrochemical aptasensor was successfully applied for detecting AFB1 in corn and two kinds of traditional Chinese medicine samples, indicating the potential value for AFB1 detection in practical samples.

Engineering the Signal Transduction between CdTe and CdSe Quantum Dots for in Situ Ratiometric Photoelectrochemical Immunoassay of Cry1Ab Protein

Controllable modulation of a response mode is extremely attracting to fabricate biosensor with programmable analytical performances. Here, we reported a proof-of-concept ratiometric photoelectrochemical (PEC) immunoassay of Cry1Ab protein based on the signal transduction regulation at the sensing interface. A sandwich-type PEC structure was designed so that gold nanorods sensitized quantum dots to fix primary antibody (Au NRs/QDs-Ab₁) and methylene blue sensitized QDs to combine a second antibody (MB/QDs-Ab₂), which served as photoelectric substrate and signal amplifier, respectively. Unlike common recognition element, such a sandwich-type PEC structure allowed for the in situ generation of two specific response signals. For analysis, Cry1Ab captured by Au NRs/QDs-Ab₁ led to a decreased photocurrent (I_Cry1Ab), while the subsequently anchored MB/QDs-Ab₂ produced another photocurrent (I_MB). Noteworthy, by taking advantage of the different energy band gaps of QDs, varying locations of CdTe and CdSe QDs could realize different signal transduction strategies (i.e., Mode 1 and Mode 2). Investigations on data analysis of I_Cry1Ab and I_MB via different routes demonstrated the superior analytical performances of ratiometry (Mode 1). Consequently, the ratiometric PEC immunosensor offered a linear range of 0.01-100 ng mL^-1 with a detection limit of 1.4 pg mL^-1. This work provides an efficient strategy for in situ collection of multiple photocurrents to design ratiometric PEC sensors.

Hydrogel Nanoarchitectonics: An Evolving Paradigm for Ultrasensitive Biosensing

The integration of nanoarchitectonics and hydrogel into conventional biosensing platforms offers the opportunities to design physically and chemically controlled and optimized soft structures with superior biocompatibility, better immobilization of biomolecules, and specific and sensitive biosensor design. The physical and chemical properties of 3D hydrogel structures can be modified by integrating with nanostructures. Such modifications can enhance their responsiveness to mechanical, optical, thermal, magnetic, and electric stimuli, which in turn can enhance the practicality of biosensors in clinical settings. This review describes the synthesis and kinetics of gel networks and exploitation of nanostructure-integrated hydrogels in biosensing. With an emphasis on different integration strategies of hydrogel with nanostructures, this review highlights the importance of hydrogel nanostructures as one of the most favorable candidates for developing ultrasensitive biosensors. Moreover, hydrogel nanoarchitectonics are also portrayed as a promising candidate for fabricating next-generation robust biosensors.

Recent Progress on Highly Selective and Sensitive Electrochemical Aptamer-based Sensors

Highly selective, sensitive, and stable biosensors are essential for the molecular level understanding of many physiological activities and diseases. Electrochemical aptamer-based (E-AB) sensor is an appealing platform for measurement in biological system, attributing to the combined advantages of high selectivity of the aptamer and high sensitivity of electrochemical analysis. This review summarizes the latest development of E-AB sensors, focuses on the modification strategies used in the fabrication of sensors and the sensing strategies for analytes of different sizes in biological system, and then looks forward to the challenges and prospects of the future development of electrochemical aptamer-based sensors.

Target-modulated competitive binding and exonuclease I-powered strategy for the simultaneous and rapid detection of biological targets

Simultaneous multiple-target detection is essential for the prevention, identification, and treatment of numerous diseases. In this study, a novel strategy based on target-modulated competitive binding and exonuclease I (Exo I)-powered signal molecule release was established with the advantages of rapid response and high selectivity and sensitivity. The strategy holds substantial potential for the development of versatile platforms for the simultaneous detection of biological targets. To mitigate the low load capacity and time-consuming responsive process of the Zr-MOF system, UiO-67 was chosen to replace UiO-66 (a typical Zr-MOF) as the nanocarrier for encapsulating more signal molecules, whereby the assembled double-stranded DNA (dsDNA) structures of UiO-67 acted as gatekeepers to form dsDNA-functionalized MOFs. Additionally, Exo I was introduced into the system to accelerate the release of the signal molecules. In the presence of biological targets, the competitive binding between the targets and aptamers caused the hydrolysis of the free DNA sequence by Exo I, promoting the release of signal molecules and leading to a rapid and significant increase in the fluorescence intensity. For adenosine triphosphate (ATP) and cytochrome c (cyt c), which were chosen as model biological targets, this sensor displayed detection limits as low as 5.03 and 6.11 fM, respectively. Moreover, the developed biosensor was successfully applied to the simultaneous detection of ATP and cyt c in spiked serum samples. Therefore, this strategy provides guidance for further research of biosensors for simultaneous multiple-target detection and propels the application of MOF carriers in biomedicine.

A novel electrochemical insulin aptasensor: From glassy carbon electrodes to disposable, single-use laser-scribed graphene electrodes

Insulin, a peptide hormone secreted by pancreatic β cells, affects the development of diabetes and associated complications. Herein, we propose an electrochemical aptasensor for sensitive and selective detection of insulin using laser-scribed graphene electrodes (LSGEs). Before using disposable LSGEs, the development and proof-of-concept sensing experiments were firstly carried out on research-grade glassy carbon electrode (GCE). The aptasensor is based on using Exonuclease I (Exo I) that catalyses the hydrolysis of single-stranded aptamers attached to the electrode surface; however, the hydrolysis does not occur if the insulin is bound to the aptamer. Therefore, the unbound aptamers are cleaved by Exo I while insulin-bound aptamers remain on the electrode surface. In the next step, the gold nanoparticle - aptamer (AuNPs-Apt) probes are introduced to the electrode surface to form a 'sandwich' structure with the insulin on the surface-attached aptamer. The redox probe, methylene blue (MB), intercalates into the aptamers' guanine bases and the sandwich structure of AuNPs-Apt/insulin/surface-bound aptamer amplifies electrochemical signal from MBs. The signal can be well-correlated to the concentrations of insulin. A limit of detection of 22.7 fM was found for the LSGE-based sensors and 9.8 fM for GCE-based sensors used for comparison and initial sensor development. The results demonstrate successful fabrication of the single-use and sensitive LSGEs-based sensors for insulin detection.

A highly specific aptamer probe targeting PD-L1 in tumor tissue sections: Mutation favors specificity

Although DNA aptamers can show comparable affinity to antibodies and have the advantage of having high batch-to-batch consistency, they often suffer from unsatisfied specificity for complex samples. The limited library size used for aptamer in vitro isolation (SELEX) has been recognized as one of the major reasons. Programmed cell death-ligand 1 (PD-L1) is both a key protein in cancer diagnostics and also immunotherapy. We report here a DNA aptamer that highly specifically binds PD-L1 expressed on the surface of various cancer cells and multiple types of tissue sections. The aptamers were selected from a DNA library containing a type II restriction endonuclease Alu I recognition site in the middle of the 40-nt random sequences, against recombinant PD-L1 rather than the whole cell or tissue section. The library enrichment was achieved by Alu I mediated-SELEX, named as REase-SELEX, in which Alu I cut off the non-binders at the recognition site and, more importantly, induced library mutations to substantially increase the library diversity. 8-60, a representative aptamer with high affinity (K_D = 1.4 nM determined by SPR) successfully detected four types of cancer cells with PD-L1 expression levels from low to high by flow cytometry, normal human tonsil (gold standard for PD-L1 antibody evaluation), clinical non-small cell lung cancer (high PD-L1 expression level), and malignant melanoma (low PD-L1 expression level) tissue sections by fluorescence microscopy imaging, showing unprecedented high specificity. The results demonstrate that 8-60 is an advanced probe for PD-L1 cancer diagnostics and mutations in SELEX greatly favor aptamer specificity.

Using dual exonucleases to finely distinguish structural adjustment of aptamers for small-molecule detection

The binding of small molecules to their DNA aptamers can modulate their susceptibility to digestion by exonucleases, however, absolute differentiation between digestion and inhibition has never been reported. Here, we show that the digestion of aptamers by T7 exonuclease can be completely inhibited upon binding of small-molecule targets and exploit this finding for the first time to achieve sensitive, label-free small-molecule detection. We use a quinine-binding aptamer to show that target binding entirely halts T7 exonuclease digestion, leaving behind an intact double-stranded product that retains strong target affinity. On the contrary, digestion of nontarget-bound aptamer produces a single-stranded product incapable of target binding. Exonuclease I efficiently eliminates these single-stranded products but is unable to digest the target-bound double-stranded product. The remaining products can be fluorescently quantified with SYBR Gold to determine target concentrations, giving a limit of detection of 100 nM with the linear range from 0 to 8 μM. We demonstrate the first example of a dual-exonuclease-mediated approach capable of producing a concentration-dependent response in terms of aptamer digestion modules, therefore improving performance of the current aptamer-based assay for small-molecule detection.

Dual exonucleases to finely distinguish structural adjustment of aptamers to produce absolute differentiation between digestion and inhibition.

Triple-signaling amplification strategy based electrochemical sensor design: boosting synergistic catalysis in metal-metalloporphyrin-covalent organic frameworks for sensitive bisphenol A detection

A covalent organic framework (COF) is a promising type of porous material with customizable surface characteristics. Confining multiple catalytic units within a mesoporous COF can generate abundant active sites and improve the catalytic performance. In this work, a COF with both metalloporphyrin and a metal nanoparticle complex denoted as hemin/TAPB-DMTP-COF/AuNPs (TAPB: 1,3,5-tris(4-amino-phenyl)benzene, DMTP: 2,5-dimethoxyterephaldehyde, AuNPs: Au nanoparticles) has been successfully fabricated through a hierarchical encapsulation method. The as-synthesized composite was then employed to construct an electrochemical sensing platform for the efficient detection of bisphenol A (BPA). Under the optimal conditions, the hemin/TAPB-DMTP-COF/AuNP sensor presented a linear range of 0.01-3 μmol L^-1 and a low detection limit of 3.5 nmol L^-1. The satisfactory signal amplification is based on a triple-signaling amplification strategy due to the abundant Fe³⁺ sites of Fe-porphyrin, high conductivity of AuNPs and a large specific surface area of the TAPB-DMTP-COF. The proposed method was used to measure the content of BPA in different water samples with a satisfactory recovery from 95.5 to 104.0%, suggesting the great potential of the sensor in practical applications.

An electrochemical sensor for high sensitive determination of lysozyme based on the aptamer competition approach

Protein is a kind of basic substance that constitutes a life body. The determination of protein is very important for the research of biology, medicine, and other fields. Lysozyme is relatively small and simple in structure among all kinds of proteins, so it is often used as a standard target detector in the study of aptamer sensor for protein detection. In this paper, a lysozyme electrochemical sensor based on aptamer competition mechanism is proposed. We have successfully prepared a signal weakening electrochemical sensor based on the lysozyme aptamer competition mechanism. The carboxylated multi-walled carbon nanotubes (MWCNTs) were modified on the glassy carbon electrode, and the complementary aptamer DNA with amino group was connected to MWCNTs. Because of the complementary DNA of daunomycin into the electrode, the electrochemical signal is generated. When there is a target, the aptamer binds to lysozyme with higher binding power, and the original complementary chain breaks down, resulting in the loss of daunomycin inserted into the double chain and the weakening of electrochemical signal. Differential pulse voltammetry was used to determine lysozyme, the response range was 1–500 nM, the correlation coefficient was 0.9995, and the detection limit was 0.5 nM. In addition, the proposed sensor has good selectivity and anti-interference.

High yield synthesis of graphene quantum dots from biomass waste as a highly selective probe for Fe3+ sensing

Graphene quantum dots (GQDs), a novel type of zero-dimensional fluorescent materials, have gained considerable attention owing to their unique optical properties, size and quantum confinement. However, their high cost and low yield remain open challenges for practical applications. In this work, a low cost, green and renewable biomass resource is utilised for the high yield synthesis of GQDs via microwave treatment. The synthesis approach involves oxidative cutting of short range ordered carbon derived from pyrolysis of biomass waste. The GQDs are successfully synthesised with a high yield of over 84%, the highest value reported to date for biomass derived GQDs. As prepared GQDs are highly hydrophilic and exhibit unique excitation independent photoluminescence emission, attributed to their single-emission fluorescence centre. As prepared GQDs are further modified by simple hydrothermal treatment and exhibit pronounced optical properties with a high quantum yield of 0.23. These modified GQDs are used for the highly selective and sensitive sensing of ferric ions (Fe3+). A sensitive sensor is prepared for the selective detection of Fe3+ ions with a detection limit of as low as 2.5 × 10-6 M. The utilisation of renewable resource along with facile microwave treatment paves the way to sustainable, high yield and cost-effective synthesis of GQDs for practical applications.