Gold Nanocluster-Assisted Fluorescent Detection for Hydrogen Peroxide and Cholesterol Based on the Inner Filter Effect of Gold Nanoparticles

Fabrication of Liquid-Crystal-Based Optical Sensing Platform for Detection of Hydrogen Peroxide and Blood Glucose

Rapid and accurate determination of H₂O₂ is of great importance in practical applications. In this study, we demonstrate construction of liquid-crystal (LC)-based sensing platforms for sensitive and real-time detection of H₂O₂ with high accuracy for the first time. Single-stranded DNA (ssDNA) adsorbed onto the surface of nanoceria is released to the aqueous solution in the presence of H₂O₂, which disrupts arrangement of the self-assembled cationic surfactant monolayer decorated at the aqueous/LC interface. Thus, the orientation of LCs changes from a homeotropic to planar state, leading to change in the optical response from dark-to-bright appearance. As H₂O₂ can be produced during oxidation of glucose by glucose oxidase (GOx), detection of glucose is also fulfilled by employing the H₂O₂ sensing platform. Our system can detect H₂O₂ and glucose with concentrations as low as 28.9 nM and 0.52 μM, respectively. It shows high promise of using LC-based sensors for the detection of H₂O₂ and its relevant biomarkers in practical applications.

N-(Aminobutyl)-N-(ethylisoluminol)-functionalized gold nanoparticles on cobalt disulfide nanowire hybrids for the non-enzymatic chemiluminescence detection of H2O2

N-(Aminobutyl)-N-(ethylisoluminol) (ABEI)-functionalized gold nanoparticles (AuNPs) on cobalt disulfide nanowires (ABEI/AuNPs/CoS2 NWs) are rapidly synthesized through a microwave-assisted reduction of chloroauric acid (HAuCl4) on CoS2 NWs with ABEI. The obtained nanohybrids with enhanced chemiluminescence are exploited for the non-enzymatic detection of H2O2.

Fluorescence chemodosimeter for dopamine based on the inner filter effect of the in situ generation of silver nanoparticles and fluorescent dye

A new strategy for the sensitive and selective detection of dopamine (DA) was proposed. The chemodosimeter design was based on the measurement of the fluorescent quenching of fluorescein dye caused by the in situ generation of silver nanoparticles (AgNPs). The AgNPs can be simply generated by a reaction between DA and Ag⁺ in the presence of polymethacrylic acid (PMAA). In addition, the generated AgNPs possess the maximum surface plasmon resonance (SPR) at 440 nm and an increase in the SPR intensity with an increasing DA concentration. Basically, fluorescein dye can emit the fluorescent intensity maximum at 513 nm with excitation at 487 nm. Thus, fluorescent quenching was achieved due to an inner filter effect from the overlap between the excitation spectrum of the fluorescein dye and the SPR spectrum of the generated AgNPs. The degree of fluorescent quenching linearly depends on the number of generated AgNPs that can be directly related to the concentration of DA. The proposed chemodosimeter can be used to detect DA in a working linear concentration range of 1.0-5.0 μM at a detection limit of 10.6 nM. This chemodosimeter was successfully applied to determine DA in a real urine sample and a dopamine injection formulation with satisfactory results.

Carbon dots derived from tobacco for visually distinguishing and detecting three kinds of tetracyclines

Although a number of methods to perform assays of tetracyclines using fluorescent probes have been reported, approaches for discriminating and detecting tetracyclines are few. Herein, bright-blue fluorescent carbon dots (CDs) with a quantum yield (QY) of up to ∼27.9% were hydrothermally synthesized using tobacco as the carbon source. Importantly, the as-prepared carbon dots were employed as a fluorescent probe, enabling selective differentiation of three tetracyclines using a test strip and the related quantitative detection. Towards the mechanism, three kinds of tetracyclines showed different interactions with the CDs, leading to variations in their fluorescence emissions. To be specific, the fluorescence of CDs was quenched by tetracycline (TC) without a fluorescence shift (Em = 440 nm), which was caused by an inner filter effect rather than a change in the energy band gap. Moreover, the introduction of chlorotetracycline (CTC) resulted in a blue shift (Em = 415 nm) of the fluorescence of the CD; this phenomenon was induced by the enlarged energy band gap. The CDs also responded to oxytetracycline (OTC), and their corresponding fluorescence experienced a red shift (Em = 500 nm) due to the narrowed band gap. Consequently, a visual detection strategy for three tetracyclines has been proposed based on the quantitative evaluation of TC, OTC, and CTC concentrations in broad range from 6 × 10-6 to 4 × 10-9 M, 2 × 10-6 to 2 × 10-8 M, and 2 × 10-7 to 2 × 10-8 M, respectively. Moreover, we have successfully applied the current CDs for visually distinguishing the three tetracyclines on a test strip on the basis of CDs exhibiting three types of fluorescence (weak-blue, navy-blue, and chartreuse).

Encapsulation of ionic nanoparticles produces reactive oxygen species (ROS)-responsive microgel useful for molecular detection

Encapsulation of ionic nanoparticles in a hydrogel microparticle, i.e. microgel, produces a target-stimulated probe for molecular detection. Selective reactive oxygen species (ROS) release the enclosed cations from the microgel which subsequently turn on the fluorogenic dyes to emit intense fluorescence, permitting rapid detection of ROS or ROS-producing molecules. The ROS-responsive microgel provides the advantages of simple fabrication, bright and stable signals, easy handling, and rapid response, carrying great promise in biomedical applications.

Horseradish Peroxidase-Encapsulated Hollow Silica Nanospheres for Intracellular Sensing of Reactive Oxygen Species

Reactive oxygen species (ROS) have crucial roles in cell signaling and homeostasis. Overproduction of ROS can induce oxidative damage to various biomolecules and cellular structures. Therefore, developing an approach capable of monitoring and quantifying ROS in living cells is significant for physiology and clinical diagnoses. Some cell-permeable fluorogenic probes developed are useful for the detection of ROS while in conjunction with horseradish peroxidase (HRP). Their intracellular scenario is however hindered by the membrane-impermeable property of enzymes. Herein, a new approach for intracellular sensing of ROS by using horseradish peroxidase-encapsulated hollow silica nanospheres (designated HRP@HSNs), with satisfactory catalytic activity, cell membrane permeability, and biocompatibility, was prepared via a microemulsion method.These HRP@HSNs, combined with selective probes or targeting ligands, could be foreseen as ROS-detecting tools in specific organelles or cell types. As such, dihydrorhodamine 123-coupled HRP@HSNs were used for the qualitative and semi-quantitative analysis of physiological H₂O₂ levels in activated RAW 264.7 macrophages. We envision that this HSNs encapsulating active enzymes can be conjugated with selective probes and targeting ligands to detect ROS in specific organelles or cell types of interest.

A single-electrode electrochemical system for multiplex electrochemiluminescence analysis based on a resistance induced potential difference

A single-electrode electrochemical system uses only one electrode for multiplex experiments, and is a highly cheap platform for high throughput analysis.

Developing low-cost and simple electrochemical systems is becoming increasingly important but still challenged for multiplex experiments. Here we report a single-electrode electrochemical system (SEES) using only one electrode not only for a single experiment but also for multiplex experiments based on a resistance induced potential difference. SEESs for a single experiment and multiplex experiments are fabricated by attaching a self-adhesive label with a hole and multiple holes onto an ITO electrode, respectively. This enables multiplex electrochemiluminescence analysis with high sensitivity at a very low safe voltage using a smartphone as a detector. For the multiplex analysis, the SEES using a single electrode is much simpler, cheaper and more user-friendly than conventional electrochemical systems and bipolar electrochemical systems using electrode arrays. Moreover, SEESs are free from the electrochemiluminescent background problem from driving electrodes in bipolar electrochemical systems. Since numerous electrodes and cover materials can be used to fabricate SEESs readily and electrochemistry is being extensively used, SEESs are very promising for broad applications, such as drug screening and high throughput analysis.

Fluorescent carbon dots nanosensor for label-free determination of vitamin B12 based on inner filter effect

A simple and effective fluorescent assay for the determination of vitamin B₁₂ was developed. In this study, carbon dots (CDs) were prepared by one-pot hydrothermal method and directly used as a fluorophore in the inner filter effect (IFE). Both of the maximum absorption peak of vitamin B₁₂ and excitation maxima of CDs are located at 360nm, hence, the excited light of CDs can be absorbed by vitamin B₁₂, resulting in the fluorescence reduction of CDs. And the fluorescence intensity of CDs decreases with the increasing concentration of vitamin B₁₂. This IFE-based sensing strategy shows a good linear relationship between the normalized fluorescence intensity and the concentration of vitamin B₁₂ ranging from 0 to 60μM, with a limit of detection (LOD) of 0.1μM at a signal-to-noise ratio of 3. Furthermore, this proposed approach was successfully applied to vitamin B₁₂ sensing in injections. This IFE sensing platform based on various fluorescent nanomaterials has a high promise for the detection of other biomolecules due to its inherent convenience.

Yellow-Emissive Carbon Dot-Based Optical Sensing Platforms: Cell Imaging and Analytical Applications for Biocatalytic Reactions

Carbon dots (CDs) have attracted increasing interest in bioimaging and sensing recently. Herein, we present a simple synthetic strategy to prepare yellow-emissive CDs (λ_em = 535 nm) by one-pot hydrothermal treatment of p-phenylenediamine and aspartic acid. The as-prepared CDs possess outstanding optical features, excellent biocompatibility, and low cytotoxicity, especially for fluorescence (FL) cellular imaging. Interestingly, by combining the quenching and recognition ability of silver nanoparticles (AgNPs) with the optical capacity of CDs, a label-free strategy for specifically monitoring H₂O₂-generated biocatalytic processes was proposed, such as glucose oxidase-induced conversion of glucose, cholesterol oxidase-catalyzed oxidization of cholesterol, and bienzyme of acetylcholinesterase and choline oxidase-mediated reaction of acetylcholine. In this process, AgNPs act as a "nanoquencher" to decrease the FL intensity of CDs via surface plasmon-enhanced energy-transfer mechanism. The enzymatic oxidation product (H₂O₂) subsequently etches the AgNPs to silver ions, thus recovering the FL of CDs, which enabled this proposed nanosensor to sensitively detect H₂O₂-generated biocatalytic processes. The above results pave the way to implement CDs as FL labels for biosensors and biological imaging.

Highly selective detection of p-nitrophenol using fluorescence assay based on boron, nitrogen co-doped carbon dots

p-Nitrophenol (p-NP) contaminants seriously endanger environmental and living beings health, hence to establish a sensitive and selective method is of great importance for the determination of p-NP. In this work, boron and nitrogen co-doped carbon dots (B,N-CDs) were synthesized by one-step hydrothermal method using 3-aminophenylboronic acid as the sole precursor. The product was characterized through high-resolution transmission electron microscopy, fluorescence spectroscopy, UV-visible absorption spectroscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. Without any functionalized modification, B,N-CDs can be directly applied as a 'turn-off' fluorescent probe for rapid, highly selective, and sensitive detection of p-NP. The fluorescent sensor based on the B,N-CDs exhibited a broad linear response to the concentration of p-NP in the range of 0.5 - 60 μM and 60 - 200 μM, respectively, and provided a detection limit of 0.2 μM. It was found that only the absorption spectrum of p-NP has a wide overlap with the fluorescence excitation and emission spectra of B,N-CDs compared to those of other representative analogues. The response mechanism was due to the inner filter effect and the formation of dynamic covalent B-O bonds between B,N-CDs and p-NP, which endowed the sensing platform with the rapid response and high selectivity to p-NP. Finally, the sensor showed the practicability of p-NP determination in environmental water samples.

Colorimetric detection of cholesterol based on enzyme modified gold nanoparticles

We develop a simple colorimetric method for determination of free cholesterol in aqueous solution based on functionalized gold nanoparticles with cholesterol oxidase. Functionalized gold nanoparticles interact with free cholesterol to produce H₂O₂ in proportion to the level of cholesterol visually is being detected. The quenching in optical properties and agglomeration of functionalized gold nanoparticles play a key role in cholesterol sensing due to the electron accepting property of H₂O₂. While the lower ranges of cholesterol (lower detection limit i.e. 0.2mg/dL) can be effectively detected using fluorescence study, the absorption study attests evident visual color change which becomes effective for detection of higher ranges of cholesterol (lower detection limit i.e. 19mg/dL). The shades of red gradually change to blue/purple as the level of cholesterol detected (as evident at 100mg/dL) using unaided eye without the use of expensive instruments. The potential of the proposed method to be applied in the field is shown by the proposed cholesterol measuring color wheel.

Phosphorescent inner filter effect-based sensing of xanthine oxidase and its inhibitors with Mn-doped ZnS quantum dots

Large bandgap semiconductor ZnS QDs (Mn-doped) were explored for inner filter effect-based sensing of xanthine oxidase and its inhibitors, due to the maximum spectral overlap between the absorption of uric acid (the enzymatic product of xanthine oxidase) and the excitation of Mn-doped ZnS QDs.

N-doped carbon dots as a fluorescent probe for the sensitive and facile detection of carbamazepine based on the inner filter effect

N-doped carbon dots are used as a fluorescent probe for the sensitive and facile determination of carbamazepine based on the inner filter effect.

A robust Zn(ii)/Na(i)-MOF decorated with [(OAc)2(H2O)2]n2n− anions for the luminescence sensing of copper ions based on the inner filter effect

A robust Zn(ii)/Na(i)-based MOF decorated with [(OAc)₂(H₂O)₂]_n²ⁿ⁻ anions was synthesized and employed as the probe for Cu²⁺ detection.

Secondary inner filter effect allows extremely efficient pure white light emission by spatially separated organic fluorophores

White light emission with 70 ± 3% efficiency under dilute conditions was obtained via the secondary inner filter effect between 2-(1-pyrenyl)benzimidazole (PyBIM) and pyrelene monoimide (PMI).

Highly Sensitive Ratiometric Fluorescent Sensor for Trinitrotoluene Based on the Inner Filter Effect between Gold Nanoparticles and Fluorescent Nanoparticles

In this work, we developed a simple and sensitive ratiometric fluorescent assay for sensing trinitrotoluene (TNT) based on the inner filter effect (IFE) between gold nanoparticles (AuNPs) and ratiometric fluorescent nanoparticles (RFNs), which was designed by hybridizing green emissive carbon dots (CDs) and red emissive quantum dots (QDs) into a silica sphere as a fluorophore pair. AuNPs in their dispersion state can be a powerful absorber to quench CDs, while the aggregated AuNPs can quench QDs in the IFE-based fluorescent assays as a result of complementary overlap between the absorption spectrum of AuNPs and emission spectrum of RFNs. As a result of the fact that TNT can induce the aggregation of AuNPs, with the addition of TNT, the fluorescent of QDs can be quenched, while the fluorescent of CDs would be recovered. Then, ratiometric fluorescent detection of TNT is feasible. The present IFE-based ratiometric fluorescent sensor can detect TNT ranging from 0.1 to 270 nM, with a detection limit of 0.029 nM. In addition, the developed method was successfully applied to investigate TNT in water and soil samples with satisfactory recoveries ranging from 95 to 103%, with precision below 4.5%. The simple sensing approach proposed here could improve the sensitivity of colorimetric analysis by changing the ultraviolet analysis to ratiometric fluorescent analysis and promote the development of a dual-mode detection system.

Inner filter effect-based fluorescent sensing systems: A review

Inner filter effect (IFE) was previously considered as an error in fluorescence measurement. In recent years, it has been developed as an important non-irradiation energy conversion model of spectroscopic technique and found wide applications in the fields of chemical sensing and biosensing. In comparison with traditional techniques based on forster resonance energy transfer (FRET), the IFE-based fluorescent approach is more flexible and straightforward without the link of absorber with fluorescer. The present review for the first time introduces the state of the art in the progress of the IFE-based fluorescent sensing systems, including sensing strategy, essential conditions, materials option, and their applications for the detection of various target analytes, e.g., ionic species, small molecules, and macromolecules. In addition, the benefits and limitations of the IFE-based fluorescent sensing systems are also critically discussed and highlighted.

Construction of a Nano Biosensor for Cyanide Anion Detection and Its Application in Environmental and Biological Systems

We have developed a Ag@Au core-shell nanoparticle (NP)/iridium(III) complex-based sensing platform for the sensitive luminescence "turn-on" sensing of cyanide ions, an acutely toxic pollutant. The assay is based on the quenching effect of Ag@Au NPs on the emission of complex 1, but luminescence is restored after the addition of cyanide anions due to their ability to dissolve the Au shell. Our sensing platform exhibited a high sensitivity toward cyanide anions with a detection limit of 0.036 μM, and also showed high selectivity for cyanide over 10-fold excess amounts of other anions. The sensing platform was also successfully applied to monitor cyanide anions in drinking water and in living cells.

Exploring the tunable excitation of QDs to maximize the overlap with the absorber for inner filter effect-based phosphorescence sensing of alkaline phosphatase

The inner filter effect (IFE) is an effective way for fluorescence modulation and thus has been extensively explored for the development of fluorescence assays. Theoretically, the key to maximize the sensitivity of IFE-based fluorescence assays is to enlarge the overlap between the absorption of the absorber and the excitation/emission of the fluorophore. Therefore, in this work, the tunable excitation of quantum dots (QDs) was explored for screening of the IFE pair having the best IFE-based assay sensitivity. A series of QDs, including CdTe QDs with different sizes, carbon dots, Cu-doped CdS QDs, and Mn-doped ZnS QDs, were investigated. PNPP (p-nitrophenylphosphate) was chosen as the absorber since its absorption overlapped with the above QDs. Besides, it can be catalytically converted to p-nitrophenol (PNP) by alkaline phosphatase (ALP) together with an absorption spectrum change (red-shift). Interestingly, it was found that the IFE efficiency of different PNPP-QD pairs increased almost linearly with the corresponding spectral overlap, and Mn-doped ZnS QDs were eventually chosen for the IFE assay of ALP because of the maximum spectral overlap and thus the best sensitivity. A simple and sensitive turn-on phosphorescence ALP assay was developed, with a detection limit of 4 × 10^-4 U L^-1. Because of the high sensitivity, we also found that ALP of different origins possessed different enzymatic activities. The developed ALP phosphorescence assay was successfully employed for the analysis of ALP in serum samples.

Multicolor Functional Carbon Dots via One-Step Refluxing Synthesis

Carbon dots are admirable fluorescent nanomaterials due to their low cost, high photostability, excellent biocompatibility, and environmental friendliness. Most conventional carbon dot fabrication approaches produce single-colored fluorescent material in the preparation process; different methods are therefore required to synthesize distinct carbon dots for specific optical applications. In this study, carbon dots carrying different emission colors are prepared through a one-step refluxing process. The emission of these materials can be well-tuned by sodium hydroxide content in the precursor solution. The carbon dots produced are used as sensing probes based on the spectrofluorometric inner filter effect for target molecule detection. Three sensing categories that combine carbon dots and inner filter effect are demonstrated, including direct, metal nanoparticle-assisted, and enzymatic reaction-supported detection. Caffeine, melamine, and fenitrothion are selected as targets to demonstrate the strategies, respectively. These multifunctional carbon dot-based sensors achieve comparable sensitivity toward analytes with a much more convenient preparation route.

Sensitive arginine sensing based on inner filter effect of Au nanoparticles on the fluorescence of CdTe quantum dots

Arginine plays an important role in many biological functions, whose detection is very significant. Herein, a sensitive, simple and cost-effective fluorescent method for the detection of arginine has been developed based on the inner filter effect (IFE) of citrate-stabilized gold nanoparticles (AuNPs) on the fluorescence of thioglycolic acid-capped CdTe quantum dots (QDs). When citrate-stabilized AuNPs were mixed with thioglycolic acid-capped CdTe QDs, the fluorescence of CdTe QDs was significantly quenched by AuNPs via the IFE. With the presence of arginine, arginine could induce the aggregation and corresponding absorption spectra change of AuNPs, which then IFE-decreased fluorescence could gradually recover with increasing amounts of arginine, achieving fluorescence "turn on" sensing for arginine. The detection mechanism is clearly illustrated and various experimental conditions were also optimized. Under the optimum conditions, a decent linear relationship was obtained in the range from 16 to 121μgL^-1 and the limit of detection was 5.6μgL^-1. And satisfactory results were achieved in arginine analysis using arginine injection, compound amino acid injection, even blood plasma as samples. Therefore, the present assay showed various merits, such as simplicity, low cost, high sensitivity and selectivity, making it promising for sensing arginine in biological samples.

A Turn-On Fluorescent Sensor for Selective and Sensitive Detection of Alkaline Phosphatase Activity with Gold Nanoclusters Based on Inner Filter Effect

In this work, a novel approach for simple and sensitive determination of alkaline phosphatase (ALP) is developed on the basis of an inner filter effect of p-nitrophenylphosphate (PNPP) on the fluorescence of gold nanoclusters (AuNCs). AuNCs with a high quantum yield of 12% were synthesized by one-pot strategy and were directly applied as fluorescent substance. When AuNCs were mixed with PNPP, the fluorescence of the AuNCs was remarkably quenched or was decreased via the inner filter effect since the absorption spectrum of PNPP overlaps well with the excitation spectrum of the AuNCs. While in the presence of ALP, PNPP was catalytically hydrolyzed into p-nitrophenol, which has different absorption characteristics from those of PNPP, resulting in the recovery of the AuNCs fluorescence. Thus, a novel "turn-on" fluorescent sensor for detecting ALP was established with a detection limit as low as 0.002 U/L (signal-to-noise ratio of 3). The turn-on fluorescent sensor exhibits many merits such as high sensitivity, excellent selectivity, and high signal output because of the low background signals. In addition, the developed sensing method was successfully applied to investigate ALP inhibitors and ALP determination in serum samples. A good linear relationship was obtained in the range from 0.02 to 50 U/L, and satisfactory recoveries at four spiking levels of ALP ranged from 95% to 106% with precision below 5%. The very simple sensing approach proposed here should promote the development of fluorescence turn-on chemosensors for chemo/biodetection.

Electrochemical Detection of Hydrogen Peroxide by Inhibiting the p-Benzenediboronic Acid-Triggered Assembly of Citrate-Capped Au/Ag Nanoparticles on Electrode Surface

Metal nanoparticles (NPs) possess unique physicochemical attributes for creating effective recognition and transduction processes in chem/bio-sensing. In this work, we suggested that citrate-capped Au/Ag NPs could be used as the reporters for the design of hydrogen peroxide (H₂O₂) sensors with a simple manipulation principle and an easy detection procedure. Specifically, p-benzenediboronic acid (BDBA) induced the aggregation of citrate-capped Au NPs through the cross-linking reaction between citrate and boronic acid of BDBA in solution. By modifying the electrode with a boronic acid derivative, the BDBA-induced assembly of Au NPs was achieved on the electrode surface. This led to a significant decrease in the electron transfer resistance due to the unique conductive ability of Au NPs. However, when the boronate group on the electrode surface was oxidized into its phenol format, the assembly of Au NPs on the electrode surface was not achieved. As a result, a higher electron transfer resistance was observed. The process could be monitored by electrochemical impedance technique. Furthermore, when Ag NPs were used instead of Au NPs in this design, the H₂O₂ concentration could be determined by measuring the linear-sweep voltammetry (LSV) current through the solid-state Ag/AgCl reaction of Ag NPs. The results indicated that NP-based colorimetric assays could be developed into more sensitive electrochemical analysis.

Sensitive fluorescent detection of fibrin based on the inner filter effect of gold nanoparticles

A simple, rapid and sensitive fluorescent assay for determination of fibrin has been developed based on the inner filter effect (IFE) of gold nanoparticles (AuNPs).

Fluorescent metal quantum clusters: an updated overview of the synthesis, properties, and biological applications

A brief history of metal quantum clusters, their synthesis methods, physical properties, and an updated overview of their applications is provided.

Thumb Imprint Based Detection of Hyperbilirubinemia Using Luminescent Gold Nanoclusters

Early and easy detection of diseases, using point-of-care and inexpensive devices, not only provides option for early treatment but also reduces the risk of propagation. Herein we report the fabrication of a robust film based luminescence indicator of bilirubin, which can indicate hyperbilirubinemia through the thumb imprint of the patient. The UV-light induced luminescence intensity of the film, made out of chitosan stabilised gold (Au) nanoclusters, which was effectively quenched in the presence of Cu²⁺ ions, recovered in the presence of bilirubin from skin or blood serum. Moreover, the sensitivity of detection of bilirubin was tuneable with the amount of Cu²⁺ added, thereby facilitating the detection of the desired concentration range of bilirubin.