In Situ Growth of Surfactant-Free Gold Nanoparticles on Nitrogen-Doped Graphene Quantum Dots for Electrochemical Detection of Hydrogen Peroxide in Biological Environments

Nitrogen doped carbon dots: mechanism investigation and their application for label free CA125 analysis

Nitrogen-doped CDs (N-CDs) were firstly prepared by using pear juice as the carbon source and ethanediamine as a nitrogen doping precursor with a microwave assisted pyrolysis technique. Based on the fluorescence recovery induced by competitive adsorption and desorption, a label-free “turn on” fluorescence assay with high sensitivity and selectivity was proposed for the analysis of CA125.

Carbon-based quantum particles: an electroanalytical and biomedical perspective

Carbon-based quantum particles, especially spherical carbon quantum dots (CQDs) and nanosheets like graphene quantum dots (GQDs), are an emerging class of quantum dots with unique properties owing to their quantum confinement effect.

A novel hydrogen peroxide sensor based on electrodeposited copper/cuprous oxide nanocomposites

Copper/cuprous oxide nanocomposites were electrodeposited on a fluorine doped tin oxide (FTO) glass substrate for sensitive determination of hydrogen peroxide.

Nanochannel-Confined Graphene Quantum Dots for Ultrasensitive Electrochemical Analysis of Complex Samples

Herein, we present an electrochemical sensing platform based on nanochannel-confined graphene quantum dots (GQDs) that is able to detect a spectrum of small analytes in complex samples with high sensitivity. Vertically ordered mesoporous silica-nanochannel film (VMSF) is decorated on the supporting electrode, conferring the electrode with excellent antifouling and anti-interference properties through steric exclusion and electrostatic repulsion. The synthesized GQDs with different functionalities are confined in the nanochannels of VMSF through electrophoresis, serving as the recognition element and signal amplifier. Without the usual need of tedious pretreatment, ultrasensitive and fast detection of Hg²⁺, Cu²⁺, and Cd²⁺ (with limits of detection (LOD) of 9.8 pM, 8.3 pM, and 4.3 nM, respectively) and dopamine (LOD of 120 nM) in complex food (Hg²⁺-contaminated seafood), environmental (soil-leaching solution), and biological (serum) samples are realized as proof-of-concept demonstrations.

MoS2/Pt nanocomposite-functionalized microneedle for real-time monitoring of hydrogen peroxide release from living cells

This work describes the adaptive use of a conventional stainless steel acupuncture needle as the electrode substrate for construction of a molybdenum disulfide (MoS₂) and platinum nanoparticles (PtNPs) layer-modified microneedle sensor for real-time monitoring of hydrogen peroxide (H₂O₂) release from living cells. To construct the nanocomposite-functionalized microneedle, the needle surface was first coated with a gold film by ion sputtering to enhance the conductivity. Subsequently, an electrochemical deposition method was successfully employed to deposit MoS₂ nanosheet and Pt nanoparticles on the needle tip as the sensing interface. Electrochemical study demonstrated that the MoS₂/PtNPs nanocomposite-modified needle exhibited excellent catalytic performance and low over-potential toward the reduction of H₂O₂. Not only did the microneedle achieve a wide linear range from 1 to 100 μmol L^-1 with a limit of detection down to 0.686 μmol L^-1, but it also realized the highly specific detection of H₂O₂. Owing to these remarkable analytical advantages, the prepared microneedle was applied to determine H₂O₂ release from living cells with satisfactory results. The MoS₂/PtNPs nanocomposite-functionalized microneedle sensor is simple and affordable, and can serve as a promising electrochemical nonenzymatic sensing platform. Moreover, this superfine needle sensor shows great potential for real-time monitoring of reactive oxygen species in vivo with minimal damage.

Gold foil-based biosensor for the determination of hydrogen peroxide

Hydrogen peroxide ($\text{H}_{\mathbf {2}} \mathbf {O} _{\mathbf {2}}$) plays a critical role in the regulation of multifarious physiological processes. We developed a sensor containing a mercaptopropionic acid (MPA) monolayer covalently immobilized with Horseradish peroxidase (HRP) enzyme for the electrochemical detection of hydrogen peroxide ($\textbf{H}_{\mathbf {2}} \mathbf {O} _{\mathbf {2}}$). A gold foil substrate was chemically treated with nitric acid and were used as working electrode. Platinum wire and Ag-Ag/Cl were used as counter and reference electrodes, respectively. The acid treated gold electrode with the immobilized enzyme shown to have improved catalytic activity in the reduction of $\textbf{H}_{\mathbf {2}} \mathbf {O} _{\mathbf {2}}$. The steady-state current response increases linearly with $\textbf{H}_{\mathbf {2}} \mathbf {O} _{\mathbf {2}}$ concentration from 10 $\mu \textbf{M}$ to 9 mM with a low detection limit of 60 $\mu \textbf{M}$ and showed a sensitivity of 0.4 mA/ mM cm$^{\mathbf {2}}$. This electrochemical sensor is demonstrated to be highly selective and sensitive in the presence of interfering analytes. The improved activity and simple preparation method of the electrode makes the MPAHRP modified gold electrode promising for being developed as an attractive robust material for electrochemical $\textbf{H}_{\mathbf {2}} \mathbf {O} _{\mathbf {2}}$ sensing.

Vertical Gold Nanowires Stretchable Electrochemical Electrodes

Conventional electrodes produced from gold or glassy carbon are outstanding electrochemical platforms for biosensing applications due to their chemical inertness and wide electrochemical window, but are intrinsically rigid and planar in nature. Hence, it is challenging to seamlessly integrate them with soft and curvilinear biological tissues for real-time wearable or implantable electronics. In this work, we demonstrate that vertically gold nanowires (v-AuNWs) possess an enokitake-like structure, with the nanoparticle (head) on one side and nanowires (tail) on the opposite side of the structure, and can serve as intrinsically stretchable, electrochemical electrodes due to the stronger nanowire-elastomer bonding forces preventing from interfacial delamination under strains. The exposed head side of the electrode comprising v-AuNWs can achieve a detection limit for H₂O₂ of 80 μM, with a linear range of 0.2-10.4 mM at 20% strain, with a reasonably high sensitivity using chronoamperometry. This excellent electrochemical performance in the elongated state, in conjunction with low-cost wet-chemistry fabrication, demonstrates that v-AuNWs electrodes may become a next-generation sensing platform for conformally integrated, in vivo biodiagnostics.

Chromogenic, Fluorescent, and Redox Sensors for Multichannel Imaging and Detection of Hydrogen Peroxide in Living Cell Systems

Hydrogen peroxide (H₂O₂) is an important reactive oxygen species (ROS). Maintaining the H₂O₂ concentration at a normal level is critical to achieve the normal physiological activities of cells, which otherwise might trigger various diseases. Therefore, it is necessary to develop new and practical multisignaling sensors for both visualization of intracellular H₂O₂ and accurate detection of extracellular H₂O₂. In this paper, a novel multichannel signaling fluorescence-electrochemistry combined probe 1 (FE-H₂O₂) is presented for imaging and detection of H₂O₂ in living cell systems. In our design, the probe FE-H₂O₂ consists of a H₂O₂ reaction site and 4-ferrocenyl(vinyl)pyridine unit which affords chromogenic, fluorescent, and electrochemical signals. These structural motifs yield a combined chromogenic, fluorescent, and redox sensor in a single molecule. Probe FE-H₂O₂ showed a "Turn-On" fluorescence response to H₂O₂, which can be used for monitoring intracellular H₂O₂ in vivo. Furthermore, the electrochemical response of probe FE-H₂O₂ was decreased after the addition of H₂O₂, which can be applied for accurate detection of H₂O₂ released from living cells. When the fluorescence imaging method is combined with electrochemical analysis technology, it is hopeful that the well-designed multimodule probe can serve as a practical tool for understanding the metabolism and homeostasis of H₂O₂ in a complex biological system.

Highly passivated phosphorous and nitrogen co-doped carbon quantum dots and fluorometric assay for detection of copper ions

Carbon quantum dots are becoming powerful fluorophore materials for metal ion analysis. Here, highly passivated green phosphorous and nitrogen co-doped carbon quantum dots (C-dots) were prepared using low-temperature carbonization route. Strong green fluorescence emission around 490 nm and excitation wavelength independent C-dots were obtained. Morphological, surface, and optical properties of the C-dots were characterized. Fluorescence emission of C-dots was quenched selectively by copper ions and restored by adding copper chelators, such as EDTA and sulfide ions. Thus, C-dots were successfully used for direct determination of copper ions. Detection limit as low as 1.5 nM (s/n = 3) was achieved for copper ions. Such a low detection limit is very significant for metal analysis using our proposed facile method and low-cost substrates. Experimental results showed that the prepared C-dots demonstrated high sensitivity and selectivity for Cu²⁺ ion detection and the method is robust and rugged. Graphical abstract ᅟ.

TEMPO-Functionalized Nanoporous Au Nanocomposite for the Electrochemical Detection of H2O2

A novel nanocomposite of nanoporous gold nanoparticles (np-AuNPs) functionalized with 2,2,6,6-tetramethyl-1-piperidinyloxy radical (TEMPO) was prepared; assembled carboxyl groups on gold nanoporous nanoparticles surface were combined with TEMPO by the “bridge” of carboxylate-zirconium-carboxylate chemistry. SEM images and UV-Vis spectroscopies of np-AuNPs indicated that a safe, sustainable, and simplified one-step dealloying synthesis approach is successful. The TEMPO-np-AuNPs exhibited a good performance for the electrochemical detection of H₂O₂ due to its higher number of electrochemical activity sites and surface area of 7.49 m²g⁻¹ for load bigger amount of TEMPO radicals. The TEMPO-functionalized np-AuNPs have a broad pH range and shorter response time for H₂O₂ catalysis verified by the response of amperometric signal under different pH and time interval. A wide linear range with a detection limit of 7.8 × 10⁻⁷ M and a higher sensitivity of 110.403 μA mM⁻¹cm⁻² were obtained for detecting H₂O₂ at optimal conditions.

Hydrothermal conversion of Magnolia liliiflora into nitrogen-doped carbon dots as an effective turn-off fluorescence sensing, multi-colour cell imaging and fluorescent ink

The present work illustrates the potential uses of nitrogen-doped multi-fluorescent carbon dots (N-CDs) for Fe³⁺ sensing, cellular multi-colour imaging, and fluorescent ink. N-CDs were synthesized using Magnolia liliiflora flower by the simple hydrothermal method. The resulted N-CDs was found to be nearly spherical in shape with the size of about 4 ± 1 nm and showed competitive quantum yield around 11%. The synthesized N-CDs with uniform size distribution and high content of nitrogen and oxygen-bearing functional groups exhibit excellent dispersibility in aqueous media. The N-CDs were able to detect a high concentration of Fe³⁺ ions (1-1000 μM) with a limit of detection is about 1.2 μM by forming N-CDs-Fe³⁺ complex due to the functional groups such as nitrogen, carbonyl and carboxyl on the surface of N-CDs. Thus they could be used to remove pollutants from industrial wastewater. The electronic charge on the surface of the N-CDs and N-CDs-Fe³⁺ complex (zeta potential) is around -36 and 18 mV, respectively. In addition, these N-CDs show excitation-dependent fluorescence that was utilized for multi-colour in vitro cellular imaging in rat liver cells (Clone 9 hepatocytes). The N-CDs are rapidly uptake in the cell cytoplasm and showed high cytocompatibility on cellular morphology. Moreover, as the N-CDs possess strong fluorescence and anti-coagulation they could be utilized in fluorescent ink pens.

Cost-effective three dimensional Ag/polymer dyes/graphene-carbon spheres hybrids for high performance nonenzymatic sensor and its application in living cell H2O2 detection

We describe a facile method to synthesize a new type of catalyst by electrodepositing Ag nanocrystals (AgNCs) on the different polymer dyes, Poly (methylene blue) (PMB) or Poly (4-(2-Pyridylazo)-Resorcinol) (PAR) modified graphene‑carbon spheres (GS) hybrids. The self-assembled GS take dual advantages of carbon spheres and graphene. Carbon spheres acts as nano-spacers prevent the aggregation of graphene and guarantee the fast electron transfer of GS. Secondly, polymerized dyes used here are beneficial for AgNCs growing as a linker. The effects of dyes on the growth habits, morphologies and catalytic properties for AgNCs were investigated. A novel electrochemical nonenzymatic sensor for hydrogen peroxide (H₂O₂) detection is fabricated based on the Ag/Polymer dyes/GS ternary composites modified glass carbon electrode (GCE) for the first time. It was found that the proposed electrodes, especially for Ag/PMB/GS/GCE, displayed a peculiar electrocatalytic activity towards H₂O₂ reduction synergistically as compared to Ag/PAR/GS/GCE or Ag/GS/GCE alone. Ag/PMB/GS/GCE showed a linear response over the H₂O₂ concentration range of 0.5 to 1112 μM. The detection limit and sensitivity is 0.15 μM and 400 μA mM^-1 cm^-2, respectively. These outstanding results enable the practical application of Ag/PMB/GS/GCE for the H₂O₂ tracking released from MCF-7 (human breast cancer cells) with satisfactory results.

Hydrogen peroxide and glucose concentration measurement using optical fiber grating sensors with corrodible plasmonic nanocoatings

We propose and demonstrate hydrogen peroxide (H₂O₂) and glucose concentration measurements using a plasmonic optical fiber sensor. The sensor utilizes a tilted fiber Bragg grating (TFBG) written in standard single mode communication fiber. The fiber is over coated with an nm-scale film of silver that supports surface plasmon resonances (SPRs). Such a tilted grating SPR structure provides a high density of narrow spectral resonances (Q-factor about 10⁵) that overlap with the broader absorption band of the surface plasmon waves in the silver film, thereby providing an accurate tool to measure small shifts of the plasmon resonance frequencies. The H₂O₂ to be detected acts as an oxidant to etch the silver film, which has the effect of gradually decreasing the SPR attenuation. The etching rate of the silver film shows a clear relationship with the H₂O₂ concentration so that monitoring the progressively increasing attenuation of a selected surface plasmon resonance over a few minutes enables us to measure the H₂O₂ concentration with a limit of detection of 0.2 μM. Furthermore, the proposed method can be applied to the determination of glucose in human serum for a concentration range from 0 to 12 mM (within the physiological range of 3-8 mM) by monitoring the H₂O₂ produced by an enzymatic oxidation process. The sensor does not require accurate temperature control because of the inherent temperature insensitivity of TFBG devices referenced to the core mode resonance. A gold mirror coated on the fiber allows the sensor to work in reflection, which will facilitate the integration of the sensor with a hypodermic needle for in vitro measurements. The present study shows that Ag-coated TFBG-SPR can be applied as a promising type of sensing probe for optical detection of H₂O₂ and glucose detection in human serum.

Ultraviolet Photoluminescence of Carbon Nanospheres and its Surface Plasmon-Induced Enhancement

Ultraviolet (UV) light can be used in versatile applications ranging from photoelectronic devices to biomedical imaging. In the development of new UV light sources, in this study, stable UV emission at ≈350 nm is unprecedentedly obtained from carbon nanospheres (CNSs). The origin of the UV fluorescence is comprehensively investigated via various characterization methods, including Raman and Fourier transform infrared analyses, with comparison to the visible emission of carbon nanodots. Based on the density functional calculations, the UV fluorescence is assigned to the carbon nanostructures bonded to bridging O atoms and dangling -OH groups. Moreover, a twofold enhancement in the UV emission is acquired for Au-carbon core-shell nanospheres (Au-CNSs). This remarkable modification of the UV emission is primarily ascribed to charge transfer between the CNSs and the Au surface.

Recent Progress in the Development of Conducting Polymer-Based Nanocomposites for Electrochemical Biosensors Applications: A Mini-Review

Among various immobilizing materials, conductive polymer-based nanocomposites have been widely applied to fabricate the biosensors, because of their outstanding properties such as excellent electrocatalytic activity, high conductivity, and strong adsorptive ability compared to conventional conductive polymers. Electrochemical biosensors have played a significant role in delivering the diagnostic information and therapy monitoring in a rapid, simple, and low cost portable device. This paper reviews the recent developments in conductive polymer-based nanocomposites and their applications in electrochemical biosensors. The article starts with a general and concise comparison between the properties of conducting polymers and conducting polymer nanocomposites. Next, the current applications of conductive polymer-based nanocomposites of some important conducting polymers such as PANI, PPy, and PEDOT in enzymatic and nonenzymatic electrochemical biosensors are overviewed. This review article covers an 8-year period beginning in 2010.

Fluorescent TPA@GQDs Probe for Sensitive Assay and Quantitative Imaging of Hydroxyl Radicals in Living Cells

A fluorescent probe TPA@GQDs is fabricated by the conjugation of terephthalic acid (TPA) on the surface of graphene quantum dots (GQDs). The TPA@GQDs probe not only has favorable dispersibility but also exhibits excellent fluorescence stability over a wide pH range and high ionic strength and favorable antiphotobleaching ability. The great fluorescence enhancement of TPA@GQDs induced by the reaction between TPA and hydroxyl radicals makes the TPA@GQDs a powerful probe for the sensitive assay of hydroxyl radicals, giving rise to a low detection limit down to 12 nmol L^-1. Meanwhile, the obtained fluorescent TPA@GQDs probe shows low cytotoxicity and favorable biocompatibility. Its potential in bioimaging is demonstrated by the quantitative fluorescent imaging of hydroxyl radicals in living HeLa cells under different circumstances, which enables the opportunities to study hydroxyl radicals dynamics in living cells.

Preparation of a nanocomposite material consisting of cuprous oxide, polyaniline and reduced graphene oxide, and its application to the electrochemical determination of hydrogen peroxide

A method is described for the preparation of a nanocomposite material consisting of cuprous oxide/polyaniline/reduced graphene oxide (Cu₂O/PANI/rGO). Aniline was employed as both the precursor for PANI and the reducing agent for Cu²⁺ and graphene oxide. A glassy carbon electrode was modified with the nanocomposite material. Chronoamperometric studies with the modified electrode showed it to enable an efficient electroreduction of hydrogen peroxide at -0.2 V vs. saturated calomel electrode. All measurements were performed in the absence of oxygen. Figures of merit include a wide linear response range (0.8 μM to 12.78 mM) and a low limit of detection of 0.5 μM (S/N = 3). Graphical abstract Cuprous oxide/polyaniline/reduced graphene oxide nanocomposites were synthesized through one-step process for fabricating an nonenzymatic electrochemical sensor for hydrogen peroxide.

Biomimetic Mineralization Guided One-Pot Preparation of Gold Clusters Anchored Two-Dimensional MnO2 Nanosheets for Fluorometric/Magnetic Bimodal Sensing

A novel fluorometric/magnetic bimodal sensor is reported based on gold nanoclusters (Au NCs)-anchored two-dimensional (2D) MnO₂ nanosheets (Au NCs-MnO₂) that are synthesized through a one-pot biomimetic mineralization process. Bovine serum albumin (BSA) was used as the template to guide the formation and assembly of the Au NCs-MnO₂ under physiological conditions and without use of any strong oxidizing agent and toxic surfactants as well as organic solvent. The fluorescence of Au NCs was first quenched by MnO₂ nanosheets, while upon H₂O₂ introduction, the MnO₂ nanosheets can be sensitively and selectively reduced to Mn²⁺ with enhanced magnetic resonance (MR) signal and rapid recovery of Au NCs fluorescence simultaneously. This dual-modal strategy can overcome the weakness of a single-fluorescence detection mode. A linear range of 0.06-2 μM toward H₂O₂ was obtained for the fluorescence mode, whereas the MR mode also allowed detection of H₂O₂ at a concentration that ranged from 0.01 to 0.2 mM. Benefiting from the BSA molecule residual on the product surface, the as-prepared Au NCs-MnO₂ displays low cytotoxicity and good biocompatibility. Importantly, the successful application of Au NCs-MnO₂ for analysis of H₂O₂ in biological samples and cells indicates that the integration of Au NCs fluorescence with Mn²⁺ MR response provides a promising bimodal sensing platform for H₂O₂ in vivo monitoring.

3D Network and 2D Paper of Reduced Graphene Oxide/Cu2O Composite for Electrochemical Sensing of Hydrogen Peroxide

In this study, two-dimensional (2D) and three-dimensional (3D) freestanding reduced graphene oxide-supported Cu₂O composites (Cu₂O-rGO) were synthesized via simple and cost-efficient hydrothermal and filtration strategies. The structural characterizations clearly showed that highly porous 3D graphene aerogel-supported Cu₂O microcrystals (3D Cu₂O-GA) have been successfully synthesized, and the Cu₂O microcrystals are uniformly assembled in the 3D GA. Meanwhile, paper-like 2D reduced graphene oxide-supported Cu₂O nanocrystals (2D Cu₂O-rGO-P) have also been prepared by a filtration process. It was found that the products prepared from different precursors and methods exhibited different sensing performances for H₂O₂ detection. The electrochemical measurements demonstrated that the 3D Cu₂O-GA has high electrocatalytic activity for the H₂O₂ reduction and excellent sensing performance for the electrochemical detection of H₂O₂ with a detection limit of 0.37 μM and a linear detection range from 1.0 μM to 1.47 mM. Meanwhile, the 2D Cu₂O-rGO-P structure also showed good electrochemical sensing performance toward H₂O₂ detection with a much wider linear response over the concentration range from 5.0 μM to 10.56 mM. Compared to the previously reported sensing materials, the as-obtained 2D and 3D Cu₂O-rGO materials exhibited higher electrochemical sensing properties toward the detection of H₂O₂ with high sensitivity and selectivity. The 2D and 3D Cu₂O-rGO composites also exhibited high sensing performance for the real-time detection of H₂O₂ in human serum. The present study indicates that 2D and 3D graphene-Cu₂O composites have promising applications in the fabrication of nonenzymatic electrochemical sensing devices.

Recent progress in two-dimensional inorganic quantum dots

This review critically summarizes recent progress in the categories, synthetic routes, properties, functionalization and applications of 2D materials-based quantum dots (QDs).

Facile preparation of highly sensitive and selective fluorescent paper sensor for the visual and cyclic detection of Cu2+ and Hg2+

The fluorescent paper sensor based on LAA-CQDs was prepared and applied to detect heavy metal ions Cu²⁺ and Hg²⁺. Notably, the paper sensor can be recycled for detecting at least four times, which greatly reduced resource consumption.

Novel carbon quantum dots for fluorescent detection of phenol and insights into the mechanism

Phenol is considered as one of the most important pollutants in the water environment, and thus its detection plays a cardinal role in environmental assessment and treatment.

Facile synthesis of nitrogen and sulfur co-doped carbon dots for multiple sensing capacities: alkaline fluorescence enhancement effect, temperature sensing, and selective detection of Fe3+ ions

In this work, we prepared nitrogen and sulfur co-doped carbon dots (C-dots) via a one-pot facile hydrothermal method using methionine and ethylenediamine as the precursors.

Green synthesis of surface-passivated carbon dots from the prickly pear cactus as a fluorescent probe for the dual detection of arsenic(iii) and hypochlorite ions from drinking water

Efforts were made to develop a simple new approach for the green synthesis of surface-passivated carbon dots from edible prickly pear cactus fruit as the carbon source by a one-pot hydrothermal route. Glutathione (GSH) was passivated on the surface of the CDs to form a sensor probe, which exhibited excellent optical properties and water solubility. The prepared sensor was successfully characterized by UV-visible spectrophotometry, fluorescence spectrophotometry, Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The simple sensing platform developed by the GSH-CDs was highly sensitive and selective with a “turn-off” fluorescence response for the dual detection of As³⁺ and ClO⁻ ions in drinking water. This sensing system exhibited effective quenching in the presence of As³⁺ and ClO⁻ ions to display the formation of metal complexes and surface interaction with an oxygen functional group. The oxygen-rich GSH-CDs afforded a better selectivity for As³⁺/ClO⁻ ions over other competitive ions. The fluorescence quenching measurement quantified the concentration range as 2–12 nM and 10–90 μM with the lower detection limit of 2.3 nM and 0.016 μM for the detection of As³⁺ and ClO⁻ ions, respectively. Further, we explored the potential applications of this simple, reliable, and cost-effective sensor for the detection of As³⁺/ClO⁻ ions in environmental samples for practical analysis.

Efforts were made to develop a simple new approach for the green synthesis of surface-passivated carbon dots from edible prickly pear cactus fruit as the carbon source by a one-pot hydrothermal route.