Sensitive Pb(2+) probe based on the fluorescence quenching by graphene oxide and enhancement of the leaching of gold nanoparticles

Aptamer-modified paper-based analytical devices for the detection of food hazards: Emerging applications and future perspective

Food analysis plays a critical role in assessing human health risks and monitoring food quality and safety. Currently, there is a pressing need for a reliable, portable, and quick recognition element for point-of-care testing (POCT) to better serve the demands of on-site food analysis. Aptamer-modified paper-based analytical devices (Apt-PADs) have excellent characteristics of high portability, high sensitivity, high specificity, and on-site detection, which have been widely used and concerned in the field of food safety. The article reviews the basic components and working principles of Apt-PADs, and introduces their representative applications detecting food hazards. Finally, the advantages, challenges, and future directions of Apt-PADs-based sensing performance are discussed, to provide new directions and insights for researchers to select appropriate Apt-PADs according to specific applications.

An ultrasensitive "mix-and-detect" kind of fluorescent biosensor for malaoxon detection using the AChE-ATCh-Ag-GO system

Malaoxon, a highly toxic metabolite of malathion, can lead to severe harm or death if ingested. This study introduces a rapid and innovative fluorescent biosensor that relies on acetylcholinesterase (AChE) inhibition for detecting malaoxon using Ag-GO nanohybrid. The synthesized nanomaterials (GO, Ag-GO) were evaluated with multiple characterization methods to confirm their elemental composition, morphology, and crystalline structure. The fabricated biosensor works by utilizing AChE to catalyze the substrate acetylthiocholine (ATCh), which generates positively charged thiocholine (TCh) and triggers citrate-coated AgNP aggregation on the GO sheet, leading to an increase in fluorescence emission at 423 nm. However, the presence of malaoxon inhibits the AChE action and reduces the production of TCh, resulting in a decrease in fluorescence emission intensity. This mechanism allows the biosensor to detect a wide range of malaoxon concentrations with excellent linearity and low LOD and LOQ values of 0.001 pM to 1000 pM, 0.9 fM, and 3 fM, respectively. The biosensor also demonstrated superior inhibitory efficacy towards malaoxon compared to other OP pesticides, indicating its resistance to external influences. In practical sample testing, the biosensor displayed recoveries of over 98% with extremely low RSD% values. Based on the results obtained from the study, it can be concluded that the developed biosensor has the potential to be used in various real-world applications for detecting malaoxon in food, and water samples, with high sensitivity, accuracy, and reliability.

Graphene Nanobeacons with High-Affinity Pockets for Combined, Selective, and Effective Decontamination and Reagentless Detection of Heavy Metals

Access to clean water for drinking, sanitation, and irrigation is a major sustainable development goal of the United Nations. Thus, technologies for cleaning water and quality-monitoring must become widely accessible and of low-cost, while being effective, selective, sustainable, and eco-friendly. To meet this challenge, hetero-bifunctional nanographene fluorescent beacons with high-affinity pockets for heavy metals are developed, offering top-rated and selective adsorption for cadmium and lead, reaching 870 and 450 mg g^-1 , respectively. The heterobifunctional and multidentate pockets also operate as selective gates for fluorescence signal regulation with sub-nanomolar sensitivity (0.1 and 0.2 nm for Pb²⁺ and Cd²⁺ , respectively), due to binding affinities as low as those of antigen-antibody interactions. Importantly, the acid-proof nanographenes can be fully regenerated and reused. Their broad visible-light absorption offers an additional mode for water-quality monitoring based on ultra-low cost and user-friendly reagentless paper detection with the naked-eye at a limit of detection of 1 and 10 ppb for Pb²⁺ and Cd²⁺ ions, respectively. This work shows that photoactive nanomaterials, densely-functionalized with strong, yet selective ligands for targeted contaminants, can successfully combine features such as excellent adsorption, reusability, and sensing capabilities, in a way to extend the material's applicability, its life-cycle, and value-for-money.

Probing the Interaction of Heavy and Transition Metal Ions with Silver Nanoparticles Decorated on Graphene Quantum Dots by Spectroscopic and Microscopic Methods

We report a comprehensive study of the interaction of transition and heavy metal ions with graphene quantum dots-capped silver nanoparticles (AgGQDs) using different spectroscopic and microscopic techniques. High-resolution transmission electron microscopy studies show that the interaction of metal ions with AgGQDs leads to the formation of metal oxides, the formation of zerovalent metals, and the aggregation of Ag nanoparticles (AgNPs). The metal ions may interact with AgGQDs through selective coordination with -OH and -COOH functionalities, adsorption on the graphene moiety, and directly to AgNPs. For instance, the interaction of Cd²⁺ with AgGQDs altered the spherical shape of AgNPs into a chain-like structure. On the contrary, the formation of PbO is observed after the addition of Pb²⁺ to AgGQDs. Interestingly, the interaction of AgGQDs with Hg²⁺ results in the complete dissolution of Ag⁰ from the surface of GQDs and subsequent deposition of Hg⁰ on the graphene moiety of GQDs. Unlike transition metal ions, Cd²⁺, Pb²⁺, and Hg²⁺ can adsorb strongly on the graphene surface at the bridge, hollow, and top sites, respectively. This special interaction of heavy metal ions with the graphene surface would decide the mechanistic pathway in which the reaction proceeds. The transition metal ions Cu²⁺, Zn²⁺, Co³⁺, Mn²⁺, Ni²⁺, and Fe³⁺ induced the aggregation of AgNPs.

Recyclable Target Metal-Enhanced Fluorometric Naked Eye Aptasensor for the Detection of Pb2+ and Ag+ Ions Based on the Structural Change of CaSnO3@PDANS-Constrained GC-Rich ssDNA

Reliable, label-free, and ultraselective detection of Pb2+ and Ag+ ions is of paramount importance for toxicology assessment, human health, and environmental protection. Herein, we present a novel recyclable fluorometric aptasensor based on the Pb2+ and Ag+-induced structural change of the GC-rich ssDNA (guanine cytosine-rich single-strand DNA) and the differences in the fluorescence emission of acridine orange (AO) from random coil to highly stable G-quadruplex for the detection of Pb2+ and Ag+ ions. More interestingly, the construction and principle of the aptasensor explore that the GC-rich ssDNA and AO can be strongly adsorbed on the CaSnO3@PDANS surface through the π-π stacking, hydrogen-bonding, and metal coordination interactions, which exhibit high fluorescence quenching and robust holding of the GC-rich ssDNA. However, in the presence of Pb2+, the specific G-rich ssDNA segment could form a stable G-quadruplex via G4-Pb2+ coordination and capture of AO from the CaSnO3@PDANS surface resulting in fluorescence recovery (70% enhancement). The subsequent addition of Ag+ ion induces coupled cytosine base pairs in another segment of ssDNA to get folded into a duplex structure together with the G-quadruplex, which highly stabilizes the G-quadruplex resulting in the maximum recovery of AO emission (99% enhancement). When the Cys@Fe3O4Nps are added to the above solution, the sensing probe was restored by complexation between the Cys in the Cys@Fe3O4Nps and target metal ions, resulting in the fabrication of a highly sensitive recyclable Pb2+ and Ag+ assay with detection limits of 0.4 and 0.1 nM, respectively. Remarkably, the Cys@Fe3O4Nps can also be reused after washing with EDTA. The utility of the proposed approach has great potential for detecting the Pb2+ and Ag+ ions in environmental samples with interfering contaminants.

A Microfluidic Aptamer-Based Sensor for Detection of Mercury(II) and Lead(II) Ions in Water

Heavy metal contaminants have serious consequences for the environment and human health. Consequently, effective methods for detecting their presence, particularly in water and food, are urgently required. Accordingly, the present study proposes a sensor capable of detecting mercury Hg(II) and lead Pb(II) ions simultaneously, using graphene oxide (GO) as a quenching agent and an aptamer solution as a reagent. In the proposed device, the aptamer sequences are labeled by FAM and HEX fluorescent dyes, respectively, and are mixed well with 500 ppm GO solution before injection into one inlet of the microchannel, and the heavy metal sample solution is injected into another inlet. The presence of Hg(II) and Pb(II) ions is then detected by measuring the change in the fluorescence intensity of the GO/aptamer suspension as the aptamer molecules undergo fluorescence resonance energy transfer (FRET). The selectivity of these two ions is also shown to be clear among other mixed heavy metal ions. The experimental results show that the aptamer sensors have a linear range of 10~250 nM (i.e., 2.0~50 ppb) for Hg(II) ions and 10~100 nM (i.e., 2.1~20.7 ppb) for Pb(II) ions. Furthermore, the limit of detection is around 0.70 ppb and 0.53 ppb for Hg(II) and Pb(II), respectively, which is lower than the maximum limits of 6 ppb and 10 ppb prescribed by the World Health Organization (WHO) for Hg(II) and Pb(II) in drinking water, respectively.

Highly Sensitive and Selective Fluorescence "Turn-On" Detection of Pb (II) Based on Fe3O4@Au-FITC Nanocomposite

New nanocomposites, Fe₃O₄@Au-FITC, were prepared and explored to develop a fluorescent detection of Pb²⁺. The Fe₃O₄@AuNPs-FITC nanocomposites could be etched by Pb²⁺ in the presence of Na₂S₂O₃, leading to fluorescence recovery of FITC quenched by Fe₃O₄@Au nanocomposites. With the increase of Pb²⁺ concentration, the fluorescence recovery of Fe₃O₄@AuNPs-FITC increased gradually. Under optimized conditions, a detection limit of 5.2 nmol/L of Pb²⁺ with a linear range of 0.02-2.0 µmol/L were obtained. The assay demonstrated negligible response to common metal ions. Recoveries of 98.2-106.4% were obtained when this fluorescent method was applied in detecting Pb²⁺ spiked in a lake-water sample. The above results demonstrated the high potential of ion-induced nanomaterial etching in developing robust fluorescent assays.

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.

In-Situ Gold-Ceria Nanoparticles: Superior Optical Fluorescence Quenching Sensor for Dissolved Oxygen

Cerium oxide (ceria) nanoparticles (NPs) have been proved to be an efficient optical fluorescent material through generating visible emission (~530 nm) under violet excitation. This feature allowed ceria NPs to be used as an optical sensor via the fluorescence quenching Technique. In this paper, the impact of in-situ embedded gold nanoparticles (Au NPs) inside ceria nanoparticles was studied. Then, gold–ceria NPs were used for sensing dissolved oxygen (DO) in aqueous media. It was observed that both fluorescence intensity and lifetime were changed due to increased concentration of DO. Added gold was found to enhance the sensitivity of ceria to DO quencher detection. This enhancement was due to optical coupling between the fluorescence emission spectrum of ceria with the surface plasmonic resonance of gold nanoparticles. In addition, gold caused the decrease of ceria nanoparticles’ bandgap, which indicates the formation of more oxygen vacancies inside the non-stoichiometric crystalline structure of ceria. The Stern–Volmer constant, which indicates the sensitivity of optical sensing material, of ceria–gold NPs with added DO was found to be 893.7 M⁻¹, compared to 184.6 M⁻¹ to in case of ceria nanoparticles only, which indicates a superior optical sensitivity to DO compared to other optical sensing materials used in the literature to detect DO. Moreover, the fluorescence lifetime was found to be changed according to the variation of added DO concentration. The optically-sensitivity-enhanced ceria nanoparticles due to embedded gold nanoparticles can be a promising sensing host for dissolved oxygen in a wide variety of applications including biomedicine and water quality monitoring.

Nitrogen-doped graphene oxide as a catalyst for the oxidation of Rhodamine B by hydrogen peroxide: application to a sensitive fluorometric assay for hydrogen peroxide

The authors report that nitrogen-doped graphene oxide (NGO) catalyzes the oxidative decomposition of the fluorophore Rhodamine B (RhB) by hydrogen peroxide. The catalytic decomposition of hydrogen peroxide yields free hydroxyl radicals that destroy RhB so that the intensity of the yellow fluorescence is reduced. Nitrogen doping enhances the electronic and optical properties and surface chemical reactivities of GO such as widening of bandgap, increase in conductivity, enhanced quenching and adsorbing capabilities etc. The catalytic properties of NGO are attributed to its large specific surface and high electron affinity of nitrogen atoms. The chemical and structural properties of GO and NGO were characterized by XRD, FTIR, SEM, UV-visible and Raman spectroscopies. The method was optimized by varying the concentration of RhB, nitrogen dopant and hydrogen peroxide. The fluorescent probe, best operated at excitation/emission wavelengths of 554/577 nm, allows hydrogen peroxide to be determined in concentrations as low as 94 pM with a linear range spanning from 1 nM to 1 μM. Graphical abstract Schematic illustration of a fluorescence quenching method for the determination of H₂O₂. Upon addition of H₂O₂, nitrogen-doped graphene oxide (NGO) catalyzes the oxidation of Rhodamine B dye due to hydroxyl radical generation, which leads to a sensitive quenchometric methd for H₂ O₂.

Leaching of AuNPs from the surface of GO: Sensitive turn on fluorescence detection of toxic preservative

Development of a novel colorimetric and turn-on fluorescent sensor for potassium metabisulphite (KMS) using graphene oxide stabilized gold nanoparticles (GO-AuNPs) was described. The red color GO-AuNPs was changed to violet while adding 250 × 10^-5 M KMS whereas the absorbance band at 523 nm was decreased. The observed changes were ascribed to the leaching of AuNPs from GO. The emission maximum was observed at 448 nm for GO while exciting at 335 nm. However, the GO emission was "turn-off" after the formation of AuNPs on GO surface due to masking of oxygen functional groups responsible for emission. Interestingly, the emission of GO-AuNPs becomes "turn-on" after the addition of 75 × 10^-6 M KMS. Further addition of KMS from 150 to 1125 × 10^-6 M, the emission intensity of GO-AuNPs linearly increases with the correlation coefficient of 0.9980 and the limit of detection was found to be 9.4 µM L^-1/1.2 mg L^-1 (S/N = 3).

Electrochemical lead(II) biosensor by using an ion-dependent split DNAzyme and a template-free DNA extension reaction for signal amplification

A voltammetric biosensor for lead(II) (Pb²⁺) is described that is based on signal amplification by using an ion-dependent split DNAzyme and template-free DNA extension reaction. The Pb²⁺-dependent split DNAzyme was assembled on gold nanoparticles (Au@Fe₃O₄), and this nanoprobe then was exposed to Pb²⁺ which causes the split-off of DNAzymes to release primers containing 3'-OH groups (S₁ and S₂). The template-free DNA extension reaction triggers the generation of long ssDNA nanotails, which then can bind the free redox probe N,N'-bis(2-(trimethylammonium iodide)propylene)perylene-3,4,9,10-tetracarboxyldiimide (PDA⁺) via electrostatic adsorption. Hence, the concentration of PDA⁺ in solution is reduced. Therefore, less free PDA⁺ can be immobilized on a glassy carbon electrode modified with electrodeposited gold nanoparticles (depAu) to produce an electrochemical signal, typically measured at ∼0.38 V (vs. SCE) for quantitation of Pb²⁺. The use of a Pb²⁺-dependent split DNAzyme avoids the usage of a proteinic enzyme. It also increases the sensitivity of the sensor which has a lower detection limit of 30 pM of Pb²⁺. Graphical abstract Novel electrochemical biosensor based on the amplification of ion-dependent split DNAzyme and template-free DNA extension reaction for trace detection of Pb²⁺.

Amphiphilic Polymer-Mediated Aggregation-Induced Emission Nanoparticles for Highly Sensitive Organophosphorus Pesticide Biosensing

Biosensing applications require signal reporters to be sufficiently stable and biosafe as well as highly efficient. Aggregation-induced emission (AIE) nanoparticles have proven to be capable of cell-imaging and cancer therapy; however, realizing sensitive detection of biomolecules remains a great challenge because of their instability, biotoxicity, and lack of modifiable functional groups. Herein, we report a self-assembling strategy to fabricate AIE nanoparticles (PTDNPs) through the dispersion of amphiphilic polymers (PTDs) in phosphate-buffered saline. The PTDs were prepared through radical copolymerization of N-(1,2,2-triphenylvinyl)-4-acetylaniline and dimethyl diallyl ammonium chloride. We found that the particle size, morphology, functional groups, and fluorescence property of PTDNPs can be fine-tuned. Further, PTDNPs-0.10 were chosen as signal reporters to detect organophosphorus pesticides (OPs) with the aid of gold nanoparticles. Their sensing performance on OPs is superior to that using C-dot/quantum dot/rhodamine B as the signal reporter. This study not only provides new possibilities to fabricate novel AIE nanoparticles with exceptional properties, but also facilitates the AIE nanoparticle's application for target analyte biosensing.

Biomimetic Sensing System for Tracing Pb2+ Distribution in Living Cells Based on the Metal-Peptide Supramolecular Assembly

Metal-peptide interactions provide plentiful resource and design principles for developing functional biomaterials and smart sensors. Pb²⁺, as a borderline metal ion, has versatile coordination modes. The interference from competing metal ions and endogenous chelating species greatly challenges Pb²⁺ analysis, especially in complicated living biosystems. Herein, a biomimetic peptide-based fluorescent sensor GSSH-2TPE was developed, starting from the structure of a naturally occurring peptide glutathione. Lewis acid-base theory was employed to guide the molecular design and tune the affinity and selectivity of the targeting performance. The integration of peptide recognition and aggregation-induced emission effect provides desirable sensing features, including specific turn-on response to Pb²⁺ over 18 different metal ions, rapid binding, and signal output, as well as high sensitivity with a detection limit of 1.5 nM. Mechanism investigation demonstrated the balance between the chelating groups, and the molecular configuration of the sensor contributes to the high selectivity toward Pb²⁺ complexation. The ion-induced supramolecular assembly lights up the bright fluorescence. The ability to image Pb²⁺ in living cells was exhibited with minimal interference from endogenous biothiols, no background fluorescence, and good biocompatibility. With good cell permeability, GSSH-2TPE can monitor changes in Pb²⁺ levels and biodistribution and thus predict possible damage pathways. Such metal-peptide interaction-based sensing systems offer tailorable platforms for designing bioanalytical tools and show great potential for studying the cell biology of metal ions in living biosystems.

A review on graphene-based nanocomposites for electrochemical and fluorescent biosensors

Biosensors with high sensitivity, selectivity and a low limit of detection, reaching nano/picomolar concentrations of biomolecules, are important to the medical sciences and healthcare industry for evaluating physiological and metabolic parameters.

A label-free and ultrasensitive electrochemical aptasensor for lead(ii) using a N,P dual-doped carbon dot-chitosan composite as a signal-enhancing platform and thionine as a signaling molecule

Herein, a label-free and ultrasensitive electrochemical aptasensor for the determination of lead(ii) (Pb2+) was described. It was based on the application of a N,P dual-doped carbon dot-chitosan (N,P-CD-CS) composite as the signal molecule carrier and an aptamer (APT) as the specific binding probe for Pb2+ that were self-assembled on the surface of a gold electrode (GE). 6-Mercapto-1-hexanol (MCH) was used to block the nonspecific binding sites, and the electro-active molecule thionine (THi) was used as the signaling probe. The differential pulse voltammetry (DPV) response of THi at a rather low working potential of -0.17 V (vs. Ag/AgCl) was used to detect Pb2+. The electrochemical performances of the resulting modified electrode were investigated by cyclic voltammetry and electrochemical impedance spectroscopy. Under optimal experimental conditions, the modified electrode exhibited excellent DPV response depending on the concentration of Pb2+ in the 0.01 nM to 10 nM range. The limit of detection was 3.8 pM (at S/N = 3). The modified electrode displayed good reproducibility and excellent stability. It was successfully applied for the determination of Pb2+ in real water samples.

Graphene and its derivatives as biomedical materials: future prospects and challenges

Graphene and its derivatives possess some intriguing properties, which generates tremendous interests in various fields, including biomedicine. The biomedical applications of graphene-based nanomaterials have attracted great interests over the last decade, and several groups have started working on this field around the globe. Because of the excellent biocompatibility, solubility and selectivity, graphene and its derivatives have shown great potential as biosensing and bio-imaging materials. Also, due to some unique physico-chemical properties of graphene and its derivatives, such as large surface area, high purity, good bio-functionalizability, easy solubility, high drug loading capacity, capability of easy cell membrane penetration, etc., graphene-based nanomaterials become promising candidates for bio-delivery carriers. Besides, graphene and its derivatives have also shown interesting applications in the fields of cell-culture, cell-growth and tissue engineering. In this article, a comprehensive review on the applications of graphene and its derivatives as biomedical materials has been presented. The unique properties of graphene and its derivatives (such as graphene oxide, reduced graphene oxide, graphane, graphone, graphyne, graphdiyne, fluorographene and their doped versions) have been discussed, followed by discussions on the recent efforts on the applications of graphene and its derivatives in biosensing, bio-imaging, drug delivery and therapy, cell culture, tissue engineering and cell growth. Also, the challenges involved in the use of graphene and its derivatives as biomedical materials are discussed briefly, followed by the future perspectives of the use of graphene-based nanomaterials in bio-applications. The review will provide an outlook to the applications of graphene and its derivatives, and may open up new horizons to inspire broader interests across various disciplines.

Cerium Dioxide-Mediated Signal "On-Off" by Resonance Energy Transfer on a Lab-On-Paper Device for Ultrasensitive Detection of Lead Ions

In this report, a 3D microfluidic lab-on-paper device for ultrasensitive detection of lead cation was designed using phoenix tree fruit-shaped CeO₂ nanoparticles (PFCeO₂ NPs) as the catalyst and 50 nm silver NPs (Ag NPs) as the quencher. First, snowflake-like Ag NPs were grown on the paper working electrode through an in situ growth method and used as a matrix for DNAzymes that were specific for lead ions (Pb²⁺). After the addition of Ag NP-labeled substrate strands, the Ag NPs restrained the electrochemiluminescence (ECL) intensity of luminol greatly through the resonance energy transfer from luminol to Ag NPs. However, under the existence of Pb²⁺, the substrate strands were separated, and then PFCeO₂ NP-labeled signal strands were hybridized with the DNAzymes. The ECL signal was improved greatly under the fast catalytic reaction between PFCeO₂ NPs and H₂O₂, which converted the response from signal off to signal on state, resulting in sensitive detection of Pb²⁺. Under the optimal conditions, the ECL signal response exhibited a good linear relationship with the logarithm of lead cation in a wide linear range of 0.05-2000 nM and an ultralow detection limit of 0.016 nM. Meanwhile, a sensor featured with good specificity, acceptable stability, reproducibility, and low cost provides a promising portable, simple, and effective strategy for Pb²⁺ detection.

A novel silicon-oxygen aurone derivative assisted by graphene oxide as fluorescence chemosensor for fluoride anions

A novel silicon-oxygen aurone derivative TBDPSA was synthesized and used for the detection of fluoride anions in aqueous solution based on a specifically F^--triggered silicon-oxygen cleavage. Even though the compound has shown high selectivity, obvious absorption and fluorescence response for fluoride anions in aqueous solution, but it also is suffered from many limits, such as low detection sensitivity and long response time. Here the compound was successfully assembled on the graphene oxide (GO) surface by π-π stacking. GO improves recognition sensitivity and shortens response time of TBDPSA for fluoride anions by taking advantage of the nanocarrier GO. Compared with TBDPSA, the response time of GO/TBDPSA is shortened greatly from 1h to <5s and the detection limit is lowered about four times with fluorescence as detected signal. Generally speaking, GO is an excellent promoter for accelerate recognition.

Enhanced Electronic Communication and Electrochemical Sensitivity Benefiting from the Cooperation of Quadruple Hydrogen Bonding and π-π Interactions in Graphene/Multi-Walled Carbon Nanotube Hybrids

By designing a molecule labeled as UPPY with both ureidopyrimidinone (UP) and pyrene (PY) units, the supramolecular self-assembly of multiwalled carbon nanotube (MWNT) and reduced graphene oxide (rGO) was driven by the UP quadruple hydrogen-bonding and PY-based π-π interactions to form a novel hybrid of rGO-UPPY-MWNT in which the morphology of rGO-wrapped MWNT was found. Bridged by the two kinds of noncovalent bonding, enhanced electronic communication occurred in rGO-UPPY-MWNT. Also, under the cooperation of UP quadruple hydrogen-bonding and PY-based π-π interactions, higher electrical conductivity and better charge transfer were observed for rGO-UPPY-MWNT, compared with the rGO-MWNT composite without such noncovalent bonds, and that with just single PY-based π-π interaction (rGO-PY-MWNT) or UP quadruple hydrogen bond (rGO-UP-MWNT). Specifically, the electrical conductivity of rGO-PY-MWNT hybrids was increased approximately sevenfold, and the interfacial charge transfer resistance was nearly decreased by 1 order of magnitude compared with rGO-MWNT, rGO-UP-MWNT, and rGO-PY-MWNT. Resulting from its excellent electrical conductivity and charge transfer properties, the rGO-UPPY-MWNT modified electrode exhibited enhanced electrochemical activity toward dopamine with detection limit as low as 20 nM.

A new macrocyclic ligand as a turn-on fluorescent chemosensor for the recognition of Pb2+ions

The chemosensor L has a detection limit of 9.63 nM for Pb²⁺in an acetonitrile/H₂O (9 : 1, v/v) mixture.

One-pot synthesis of sulfur-doped graphene quantum dots as a novel fluorescent probe for highly selective and sensitive detection of lead(ii)

A facile strategy was developed for the one-step synthesis of S-GQDs with a monolayer-graphene crystal structure. The change of surface chemistry by S-doping resulted in selective and sensitive detection of Pb²⁺.

Merocyanine 540 adsorbed on polyethylenimine-functionalized graphene oxide nanocomposites as a turn-on fluorescent sensor for bovine serum albumin

We suggest a simple, fast, sensitive and selective BSA sensor designed by assembling MC540 molecules on PEI–GO nanocomposites.

Gold nanoparticles as sensitive optical probes

Recent advances in Au NP based optical sensing systems for various analytes based on absorption, fluorescence and SERS are summarized.

Highly sensitive chemiluminescent detection of lead ion based on its displacement of potassium in G-Quadruplex DNAzyme

A simple and highly sensitive chemiluminescence (CL) detection method for Pb²⁺ in biosamples, such as human hairs, based on its displacement of potassium in G-Quadruplex DNAzyme, was successfully developed.