Turn-on fluorescent assay for antioxidants based on their inhibiting polymerization of dopamine on graphene quantum dots

A photoelectrochemical sensor based on In2O3/In2S3/ZnIn2S4 ternary Z-scheme heterojunction for ultrasensitive detection of dopamine in sweat

A novel ternary heterojunction material In2O3/In2S3/ZnIn2S4 was synthesized, and a photoelectrochemical sensor was fabricated for the non-invasive test of dopamine (DA) in sweat. In2O3 multihollow microtubules were synthesized and then In2S3 was formed on their surface to construct a type-I heterojunction between In2S3 and In2O3. ZnIn2S4 was further introduced to form a Z-scheme heterojunction between In2S3/ZnIn2S4. Under photoexcitation, the photogenerated holes of In2O3 transferred to the valence band of In2S3, superimposed with the holes produced by In2S3, leads to a significantly higher photocatalytic oxidation capacity of In2O3/In2S3/ZnIn2S4 ternary composites than that of In2O3/In2S3. The Z-scheme heterojunction accelerates the transfer of photogenerated electrons accumulated on the type-I heterojunction. In the presence of DA, it is rapidly oxidized into polydopamine (PDA) by In2O3/In2S3, and the benzoquinone groups of PDA compete for the photogenerated electrons to reduce the current in the external circuit, whereby DA determination is achieved. Owing to the combination of type-I and Z-scheme heterojunction, the sensor showed extremely high sensitivity, with a detection limit of 3.94 × 10-12 mol/L. It is one of the most sensitive methods for DA detection reported and has been applied to the determination of DA in human sweat.

N,Si co-doped GQDs: Facile green preparation and application in visual identifying dihydroxybenzene isomers and selective quantification of catechol, hydroquinone and antioxidants

A green economical procedure for preparing N,Si co-doped graphene quantum dots (N,Si-GQDs) using waste toners and ethylene diamine was reported, which not only minimizes waste and promotes recycling but also offers an alternative method for producing N,Si-GQDs. At a pH of 8.5, hydroquinone and catechol underwent oxidation in the presence of air, resulting in the formation of diquinones, specifically p-phenyldiquinone and o-phenyldiquinone. Resorcinol, on the other hand, was converted into monoquinone. The interaction between diquinones and N,Si-GQDs caused a linear fluorescence quenching effect when catechol and hydroquinone were present. However, this effect was minimal in the case of resorcinol. Furthermore, the antioxidants glutathione (GSH) and ascorbic acid (AA) were observed to disrupt the redox equilibrium of catechol and o-phenyldiquinone, leading to the activation of fluorescence. Conversely, hydroquinone and p-phenyldiquinone, due to the highly stable and symmetrical structure of p-phenyldiquinone, did not exhibit this fluorescence activation. Based on the described "Off-On" sensor system, it was possible to visually identify dihydroxybenzene isomers and selectively quantify catechol and hydroquinone in environmental samples, as well as GSH and AA in human serum. The method detection limits were 0.93, 1.35, 2.34, and 1.37 μM for catechol, hydroquinone, GSH, and AA, respectively. In conclusion, the presented procedure offers several advantages, including environmental friendliness, cost-effectiveness, and a means of recycling waste toners. It also demonstrates the successful synthesis of N,Si-GQDs, as well as the potential for their application in the "Off-On" sensor system for the detection and quantification of various analytes.

Multi-applications of carbon dots and polydopamine-coated carbon dots for Fe3+ detection, bioimaging, dopamine assay and photothermal therapy

Carbon dots (CDs) or CDs/polymer composites have been applied in numerous fields. Here, novel CDs were synthesized by carbonization of egg yolk, and characterized by TEM, FTIR, XPS and photoluminescence spectra. The CDs were found to be approximate sphere in shape with an average size of 4.46 ± 1.17 nm, and emitted bright blue photoluminescence under UV irradiation. The photoluminescence of CDs was found selectively quenched by Fe³⁺ in a linear manner in the range of 0.05-0.45 mM, meaning they could be applied for Fe³⁺ detection in solution. Moreover, the CDs could be uptaken by HepG2 cells to exhibit bright blue photoluminescence. The intensity could reflect the level of intracellular Fe³⁺, indicating they could be further used for cell imaging and intracellular Fe³⁺ monitoring. Next, dopamine was polymerized on the surface of CDs to obtain the polydopamine (PDA)-coated CDs (CDs@PDA). We found PDA coating could quench the photoluminescence of CDs via inner filter effect, and the degree of quenching was linearly related to the logarithm of DA concentration (Log C_DA). Also, the selectivity experiment indicated the method had a high selectivity for DA over a number of possible interfering species. This indicated the CDs in combination with Tris buffer could be potentially applied as the assay kit of dopamine. At last, the CDs@PDA exhibited excellent photothermal conversion capability, and they could efficiently kill HepG2 cells under NIR laser irradiation. Overall, the CDs and CDs@PDA in this work exhibited many excellent advantages, and could be potentially used for multi-applications, such as Fe³⁺ sensor in solution and cellular, cell imaging, dopamine assay kit, as well as photothermal agents for cancer therapy.

Role of Förster Resonance Energy Transfer in Graphene-Based Nanomaterials for Sensing

Förster resonance energy transfer (FRET)-based fluorescence sensing of various target analytes has been of growing interest in the environmental, bioimaging, and diagnosis fields. Graphene-based zero- (0D) to two-dimensional (2D) nanomaterials, such as graphene quantum dots (GQDs), graphene oxide (GO), reduced graphene oxide (rGO), and graphdiyne (GD), can potentially be employed as donors/acceptors in FRET-based sensing approaches because of their unique electronic and photoluminescent properties. In this review, we discuss the basics of FRET, as well as the role of graphene-based nanomaterials (GQDs, GO, rGO, and GD) for sensing various analytes, including cations, amino acids, explosives, pesticides, biomolecules, bacteria, and viruses. In addition, the graphene-based nanomaterial sensing strategy could be applied in environmental sample analyses, and the reason for the lower detection ranges (micro- to pico-molar concentration) could also be explained in detail. Challenges and future directions for designing nanomaterials with a new sensing approach and better sensing performance will also be highlighted.

Post-synthetic modification of graphene quantum dots bestows enhanced biosensing and antibiofilm ability: efficiency facet

Graphene quantum dots (GQDs) are a luminescent class of carbon nanomaterials with a graphene-like core structure, possessing quantum confinement and edge effects. They have gained importance in the biological world due to their inherent biocompatibility, good water dispersibility, excellent fluorescence and photostability. The improved properties of GQDs require the logical enactment of functional groups, which can be easily attained through post-synthetic non-covalent routes of modification. In this regard, the present work has for the first time employed a simple one-pot post-modification method utilizing the salt of amino caproic acid, an FDA approved reagent. The adsorption of the modifier on GQDs with varying weight ratios is characterized through DLS, zeta potential, Raman, absorption and fluorescence spectroscopy. A decrease of 20% in the fluorescence intensity with an increase in the modifier ratio from 1 to 1000 and an increased DLS size as well as zeta potential demonstrate the efficient modification as well as higher stability of the modified GQDs. The modified GQDs with a high weight ratio (1 : 100) of the modifier showed superior ability to sense dopamine, a neurotransmitter, as well as competent biofilm degradation ability. The modified GQDs could sense more efficiently than pristine GQDs, with a sensitivity as low as 0.06 μM (limit of detection) and 90% selectivity in the presence of other neurotransmitters. The linear relationship showed a decrease in the fluorescence intensity with increasing dopamine concentration from 0.0625 μM to 50 μM. Furthermore, the efficiency of the modified GQDs was also assessed in terms of their antibiofilm effect against Staphylococcus aureus. The unmodified GQDs showed only 10% disruption of the adhered bacterial colonies, while the modified GQDs (1 : 100) showed significantly more than 60% disruption of the biofilm, presenting the competency of the modified GQDs. The unique modifications of GQDs have thus proven to be an effective method for the proficient utilization of zero-dimensional carbon nanomaterials for biosensing, bioimaging, antibacterial and anti-biofilm applications.

Ascorbic Acid Sensor Based on CdS QDs@PDA Fluorescence Resonance Energy Transfer

An ascorbic acid (AA) sensor was constructed based on the fluorescence resonance energy transfer (FRET) between CdS quantum dots (CdS QDs) and polydopamine (PDA) to detect trace AA sensitively. FRET occurred due to the broad absorption spectrum of PDA completely overlapped with the narrow emission spectrum of CdS QDs. The fluorescence of CdS QDs was quenched and in the "off" state. When AA was present, the conversion of DA to PDA was hindered and the FRET disappeared, resulting in the fluorescence of CdS QDs in an "on" state. Importantly, the degree of fluorescence recovery of CdS QDs displayed a desirable linear correlation with the concentration of AA in the range of 5.0-100.0 μmol/L, the linear equation is y=0.0119cAA+0.3113, and the detection limit is 1.16 μmol/L (S/N = 3, n = 9). There was almost no interference with common amino acid, glucose and biological sulfhydryl small molecules to AA. Trace amount of AA in vitamin C tablets were determined and satisfactory results were obtained; the recoveries were observed to be 98.01-100.7%.

Dual recognition strategy for selective fluorescent detection of dopamine and antioxidants based on graphite carbon nitride in human blood serum

In this work, a strong blue-emitting fluorescent biosensor based on graphite carbon nitride nanoparticles (GCNNs) (E_x = 340 nm and E_m = 435 nm) was synthesized by a facile one-step hydrothermal method. With the aid of hydrogen peroxide and horseradish peroxidase, pyrocatechol structure of dopamine (DA) was oxidized to o-quinone structure of polydopamine (PDA) by hydroxyl radical. PDA was able to rapidly and significantly quench fluorescence of GCNNs. In the meanwhile, oxidative self-polymerization from DA to PDA would be blocked by antioxidants, such as glutathione (GSH) and ascorbic acid (AA). Thus, the fluorescence of GCNNs@DA sensor would be recovered owing to the decrease of o-quinone. Based on above-mentioned dual recognition strategy of "turn-off" and "turn off-on", a fast, simple and ultrasensitive method was developed to measure DA and antioxidants. Under the optimal experimental conditions, the detection limits of DA, GSH and AA were 0.064 μmol L^-1, 0.11 μmol L^-1 and 0.16 μmol L^-1 with relative standard deviations of 1.7%, 9.3% and 8.0%, respectively. As one of metal-free quantum dots, our GCNNs-based sensors were also successfully applied to the determination of DA as well as GSH and AA in human serum. The recoveries for the spiked samples were in the range of 93.8%-109% and 95.0%-110% of DA and antioxidants, which shows great promise to clinicalapplication.

Recent progress on graphene quantum dots-based fluorescence sensors for food safety and quality assessment applications

The versatile photophysicalproperties, high surface-to-volume ratio, superior photostability, higher biocompatibility, and availability of active sites make graphene quantum dots (GQDs) an ideal candidate for applications in sensing, bioimaging, photocatalysis, energy storage, and flexible electronics. GQDs-based sensors involve luminescence sensors, electrochemical sensors, optical biosensors, electrochemical biosensors, and photoelectrochemical biosensors. Although plenty of sensing strategies have been developed using GQDs for biosensing and environmental applications, the use of GQDs-based fluorescence techniques remains unexplored or underutilized in the field of food science and technology. To the best of our knowledge, comprehensive review of the GQDs-based fluorescence sensing applications concerning food quality analysis has not yet been done. This review article focuses on the recent progress on the synthesis strategies, electronic properties, and fluorescence mechanisms of GQDs. The various GQDs-based fluorescence detection strategies involving Förster resonance energy transfer- or inner filter effect-driven fluorescence turn-on and turn-off response mechanisms toward trace-level detection of toxic metal ions, toxic adulterants, and banned chemical substances in foodstuffs are summarized. The challenges associated with the pretreatment steps of complex food matrices and prospects and challenges associated with the GQDs-based fluorescent probes are discussed. This review could serve as a precedent for further advancement in interdisciplinary research involving the development of versatile GQDs-based fluorescent probes toward food science and technology applications.

Fluorescent DNA-templated silver nanoclusters for highly sensitive detection of D-penicillamine

Herein, fluorescent DNA-templated silver nanoclusters (DNA-AgNCs) with red emission were synthesized and utilized as novel probe to detect D-penicillamine (D-Pen) for the first time. D-Pen molecules contain a thiol which can combine with Ag to form a non-fluorescent ground state complex, inducing the aggregation of DNA-AgNCs followed by the fluorescence quenching. The quenching mechanism is well-studied and found to be a static quenching process. This method can detect D-Pen in the range of 0.025-0.7 μM with the detection limit as low as 8 nM, which is 1-3 orders of magnitude more sensitive than those based on other fluorescent nanoprobes. More importantly, the preparation procedure for DNA-AgNCs is fast and without the requirement of heavy metal ions. Thus, this detection strategy is time-saving and eco-friendly. Satisfactory recoveries have been acquired for monitoring D-Pen in human serum samples and pharmaceutical samples owing to the high sensitivity.

A ratiometric fluorescence assay for bleomycin based on dual-emissive chameleon DNA-templated silver nanoclusters

The authors design dual-emissive DNA-templated silver nanoclusters (DNA-AgNCs) for ratiometric fluorescence sensing bleomycin (BLM) for the first time. A hairpin probe containing two different C-rich DNA templates at two terminals is used to synthesize chameleon DNA-AgNCs, which possess two emission peaks when they are in close proximity. A strong emission is founded at 622 nm (λex = 570 nm) while a weak one is located at 572 nm (λex = 504 nm). Meanwhile, the loop of this probe contains the scission site (5'-GC-3') of BLM. The loop can be cleaved into two parts by BLM-Fe(II) complex, inducing the two DNA-AgNCs away from each other. The fluorescence intensity at 572 nm and 622 nm increases and decreases, respectively. Such chameleon DNA-AgNCs exhibit an obvious fluorescence discoloration from orange to yellow. Therefore, a sensitive ratiometric fluorescent strategy for BLM detection has been proposed with the detection limit of 67 pM. Finally, this ratiometric method is used to detect BLM in serum samples.