Carbon dots as an "on-off-on" fluorescent probe for detection of Cu(II) ion, ascorbic acid, and acid phosphatase

Composite Eu@Cd-CP as a fluorescent probe for the detection of some food additives

A transition metal coordination polymer (CP), [Cd(Hdpcp)]n (Cd-CP) was prepared based on 3-(2,4-dicarboxyphenyl)-6-carboxypyridine ligand (H3dpcp), and then its composite Eu@Cd-CP was synthesized by the post-modification through loading Eu3+ ions on Cd-CP. Eu@Cd-CP has outstanding fluorescence stability in aqueous solution with a wide range of pH. Furthermore, Eu@Cd-CP can distinguish sodium salicylate (SS) and sodium dehydroacetate (SA) in some food additives by quenching the characteristic fluorescence of Eu3+ ion. Eu@Cd-CP is the first known CP-based fluorescent probe for selective detection of SS and SA. In addition, the fluorescence mechanisms of discerning above analytes by Eu@Cd-CP have been thoroughly evaluated. It has found that synergistic effect of the dynamic process, photoinduced electron transfer (PET) process, energy absorption competition, and formation of Eu-O bonding interactions in sensing SA lead to the fluorescence quenching of Eu@Cd-CP. The fluorescence response mechanism of Eu@Cd-CP with SA is ascribed to the combination of the dynamic process, PET process, and energy absorption competition. A series of portable devices based on Eu@Cd-CP including fluorescence test strips, lamp beads, and composite films were developed to discern SS and SA via visual changes in luminescence color. This composite material can be potentially used as a multifunctional fluorescent probe for practical applications.

Construction of nitrogen-doped carbon dots-based fluorescence probe for rapid, efficient and sensitive detection of chlortetracycline

Antibiotics are widely used in clinical medicine due to their excellent antibacterial abilities. As typical emerging pollutants, their misuse can lead to excess antibiotics entering the environment, causing antimicrobial resistance and leading to serious health problems via food chain. Herein, a nano-fluorescent probe based on nitrogen-doped carbon dots (N-CDs) was constructed for the sensitive detection of chlortetracycline (CTC). N-CDs with stable fluorescence were synthesized by hydrothermal method using alizarin red and melamine as raw materials. The N-CDs exhibited significant independence to excitation wavelength. The fluorescence of N-CDs was significantly quenched by CTC ascribing to the fluorescence resonance energy transfer mechanism. The concentration of N-CDs, solution pH and incubation time were optimized to obtain the optimal detection parameters. Under optimal conditions, CTC exhibited excellent linearity over the range of 20-1200 μg/L, and the detection limit was 8.74 μg/L. The method was validated with actual water samples and achieved satisfied spiked recoveries of 97.6-102.6%. Therefore, the proposed method has significant application value in the detection of CTC in waters.

Fe3+-induced luminescence quenching in carbon dots - mechanism unveiled

Carbon dot (CD)-based fluorimetric sensors have attracted immense attention for the detection of metal ions. Among the available works in this direction, more than 70% of the studies reported the detection of Fe3+ through luminescence quenching. Ferric ions are significant species from environmental and biological point of view. Excited-state electron transfer from carbon dots to ferric ions is suggested as the reason for the luminescence quenching. However, to date, no solid proof was provided to demonstrate this electron transfer process. Herein, N-doped blue luminescent carbon dots prepared via hydrothermal carbonization are used to demonstrate the exact mechanism operating in the above-mentioned detection strategy. The carbon dots possessed an average size of 4.9 nm, and exhibited good aqueous solubility as well as an excitation wavelength-dependent emission. Fe3+-mediated luminescence quenching was quantitatively achieved at the micromolar level, with a detection limit of 1.426 μM. The CD-mediated reduction of ferric ions is confirmed by spectral analysis. Fe3+-induced luminescence quenching was partially restored in the presence of ascorbic acid, enabling the sub-micromolar level monitoring of this analyte, with the lowest detection amount of 276 nM. Turnbull's blue method is adopted for confirming the reducing role of ascorbic acid, which eventually increased the luminescence of the system, evoking a turn-on response.

A fluorescence "turn-on" sensor for ascorbic acid in fruit juice and beverage based on ascorbate oxidase-like activity of citric acid-derived carbon dots

Citric acid-derived carbon dots (CA-CDs) without any modifications were found to have the ascorbate oxidase (AAO)-like activity. The CA-CDs have high affinity for ascorbic acid (AA), which is similar to natural AAO. The robustness of CA-CDs is greater than that of AAO. Based on the AAO mimetic activity of CA-CDs, a sensitive turn-on mode and natural enzyme-free fluorescence detection method has been developed for AA in some fruit juice and beverage samples with satisfied recoveries. This study provides CDs-based AAO mimetic nanozymes to replace the expensive natural enzymes or heavy metal-based nanozymes, which will show great potential in biological and food assays.

Green-emitting carbon dots as a "turn on" fluorescence bio-probe for highly sensitive and selective detection of lipase in human serum

Enzyme activity assays play a crucial role in numerous fields, including biotechnology, the food industry, and clinical diagnostics. Lipases are particularly important enzymes due to their widespread use in lipid metabolism and esterification reactions. Here, we present a pioneering method for the sensitive and selective determination of lipase activity using green carbon dots (G-CDs) for first time. G-CDs are a fascinating class of carbon nanomaterials with unique optical properties and biocompatibility, making them ideal candidates for enzyme activity assays. This approach eliminates the need for traditional fluorophores or chromogenic substrates, reducing costs, fast response time (1 min), and environmental impact with a quantum yield (QY) of 7.42%. As designed, the G-CDs fluorescent probe turn-on demonstrated a reliable linear detection range from 0 to 9 mg/mL under ideal conditions, with detection limit of 0.01 mg/mL and limit of quantification (LOQ) of 0.045 mg/mL, respectively. Furthermore, the G-CDs system was thoroughly evaluated in human serum samples, showing recoveries ranging from 100.0 to 105.0%. These findings highlight the promising applicability of the G-CDs probe for lipase detection, yielding highly favorable results.

Using Cu2+ to regulate the emission feature of near-infrared fluorescent sensor with AIE: To detect ascorbic acid in food samples and its application in bioimaging

Conventional ascorbic acid (AA) detection methods such as chromatography, capillary electrophoresis, colorimetry, electrochemical detection, and enzymatic analysis require expensive equipment and complicated operation. Simple, rapid, and accurate AA detection is essential to inspect food quality, diagnose diseases, and assess immunity in humans. In this study, the first near-infrared fluorescence sensor DBHM with aggregation-induced emission was developed to detect AA under the involvement of Cu2+. The DBHM + Cu2+ sensor showed high sensitivity to AA with a limit of detection of 2.37 μM. The AA detection mechanism was investigated by optical studies, 1H NMR titration, high-resolution mass spectrometry, and infrared spectroscopy. AA was detected qualitatively and quantitatively by the DBHM + Cu2+ sensor in beverages, fruits, and Vitamin C tablets using a dual-mode (fluorescence and smartphone app) sensing platform. The new sensing system also showed low toxicity and excellent bioimaging in HeLa cells, C. elegans, and mice. This sensor could advance AA detection technology in the food industry and has potential bioimaging applications.

Histamine Recognition by Carbon Dots from Plastic Waste and Development of Cellular Imaging: Experimental and Theoretical Studies

The present work highlights the sustainable approach for the transformation of plastic waste into fluorescent carbon dots (CDs) through carbonization and then they were functionalized with L-cysteine and o-phenylenediamine. CDs which were characterized by different analytical techniques such as X-ray diffraction (XRD), thermogravimetric analysis (TGA), Scanning electron microscopy (SEM), and transmission electron microscopy (TEM) are employed to recognize Cu2+, Fe2+, and Hg2+ ions. The results show that the fluorescence emission was considerably quenched, and it is consistent with the interference and Jobs plots. The detection limit was found to be 0.35µM for Cu(II), 1.38 µM for Hg(II), and 0.51µM Fe(III). The interaction of CDs with metal ions enhances the fluorescence intensity detecting histamine successfully. It shows that plastic waste-based CDs can be applied clinically to detect toxic metals and biomolecules. Moreover, the system was employed to develop the cellular images using Saccharomyces cerevisiae cells with the support of a confocal microscope. Furthermore, theoretical studies were performed for the naphthalene layer (AR) as a model for C-dots, then optimized its structure and analyzed by using the molecular orbital. The obtained TD-DFT spectra coincided with experimental spectra for CDs/M2+/histamine systems.

An "on-off-on" electrochemiluminescence aptasensor based on a self-enhanced luminophore for ochratoxin A detection

A highly selective and sensitive "on-off-on" electrochemiluminescence (ECL) aptasensor based on a self-enhanced luminophore was developed for the detection of ochratoxin A (OTA). Specifically, polyethyleneimine functionalized multi-walled carbon nanotubes decorated with gold nanoparticles (AuNPs-PEI-MWCNTs) were used as the electrode matrix to accelerate electron transfer and provide a favorable microenvironment for self-enhanced luminophore loading and ECL signal enhancement. In addition, black phosphorus quantum dots (BPQDs) were used as co-reactants of the ECL reagent tris (2,2'-bipyridyl) ruthenium(II) (Ru(bpy)32+) in ECL experiments, and the reaction mechanism was investigated. The self-enhanced luminophore Ru@SiO2-BPQDs was obtained by encapsulating Ru(bpy)32+ in silica (SiO2) nanoparticles and then combining it with BPQDs through electrostatic interaction. In conventional ECL systems, the emitter and its co-reactants reacted via the inter-nanoparticle pathway, leading to long distance electron transfer. However, the electron transfer distance in the self-enhanced luminophore was significantly shortened due to the intra-nanoparticle electron transfer pathway because BPQDs and oxidized Ru(bpy)32+ were bound within one nanoparticle, thereby improving ECL efficiency to achieve the first "switch-on" state. Then, the luminophore was quenched using ferrocenes (Fc) modified on an aptamer to achieve the "switch-off" state. Finally, OTA was specifically identified by the adapter, causing Fc to be released from the sensor interface, restoring the ECL intensity to achieve the second "switch-on" state. Under optimal conditions, the aptasensor exhibited good sensitivity, stability, and reproducibility, with a linear detection range from 0.1 to 320 ng/mL and a detection limit of 0.03 ng/mL. The novel ECL aptasensor provided a common analytical tool for the detection of mycotoxins and other small molecules.

Synthesis, properties and potential applications of photoluminescent carbon nanoparticles: A review

Photoluminescent-carbon nanoparticles (PL-CNPs) are a new class of materials that received immense interest among researchers due to their distinct characteristics, including photoluminescence, high surface-to-volume ratio, low cost, ease of synthesis, high quantum yield, and biocompatibility. By exploiting these outstanding properties, many studies have been reported on its utility as sensors, photocatalysts, probes for bio-imaging, and optoelectronics applications. From clinical applications to point-of-care test devices, drug loading to tracking of drug delivery, and other research innovations demonstrated PL-CNPs as an emerging material that could substitute conventional approaches. However, some of the PL-CNPs have poor PL properties and selectivity due to the presence of impurities (e.g., molecular fluorophores) and unfavourable surface charges by the passivation molecules, which impede their applications in many fields. To address these issues, many researchers have been paying great attention to developing new PL-CNPs with different composite combinations to achieve high PL properties and selectivity. Herein, we thoroughly discussed the recent development of various synthetic strategies employed to prepare PL-CNPs, doping effects, photostability, biocompatibility, and applications in sensing, bioimaging, and drug delivery fields. Moreover, the review discussed the limitations, future direction, and perspectives of PL-CNPs in possible potential applications.

Ganoderma lucidum bran-derived blue-emissive and green-emissive carbon dots for detection of copper ions

Ganoderma lucidum bran (GB) has a broad application prospect in the preparation of activated carbon, livestock feed, and biogas, but the preparation of carbon dots (CDs) from GB has never been reported. In this work, GB was applied as a carbon source and nitrogen source to prepare both blue fluorescent CDs (BCDs) and green fluorescent CDs (GCDs). The former were prepared at 160 °C for 4 h by a hydrothermal approach, while the latter were acquired at 25 °C for 24 h by chemical oxidation. Two kinds of as-synthesized CDs exhibited unique excitation-dependent fluorescence behavior and high fluorescent chemical stability. Based on the fantastic optical behavior of the CDs, they were utilized as probes for fluorescent determination of copper ions (Cu²⁺). In the range of 1-10 μmol L^-1, the fluorescent intensity of BCDs and GCDs decreased linearly with the increase of Cu²⁺ concentration; the linear correlation coefficient reached 0.9951 and 0.9982, and the limit of detection (LOD) was 0.74 and 1.08 μmol L^-1, respectively. In addition, these CDs remained stable in 0.001-0.1 mmol L^-1 salt solutions; BCDs were more stable in the neutral pH range, but GCDs were more stable in neutral to alkaline conditions. The CDs prepared from GB are not only simple and low-cost, but also can realize the comprehensive utilization of biomass.

The fabrication of N-doped carbon dots by methionine and their utility in sensing Cu2+ in real water

Copper ions (Cu²⁺) are ubiquitous in the ecosystem and cause serious environmental pollution, posing a threat to human health. Therefore, sensitive detection of Cu²⁺ is urgently needed. Herein, we employed a solvothermal method to prepare blue-emitting carbon dots (Met-CDs) using formamide (FA) and methionine (Met) as precursors, with a high quantum yield (QY) of 38%. Based on the good optical stability of Met-CDs and selective quenching by Cu²⁺, a sensitive probe using Met-CDs for the detection of Cu²⁺ in water was successfully designed. Within the linear range of 0.15-2 μM, the limit of detection (LOD) was determined to be as low as 47.7 nM, enabling the quantitative detection of Cu²⁺. Moreover, the recovery data of the spiked analysis of lake/river water samples were also satisfactory and verified the feasibility of the probe by the analysis of Cu²⁺ in natural conditions.

Carbon dots fabricated by solid-phase carbonization using p-toluidine and l-cysteine for sensitive detection of copper

In this study, a label-free "turn off" fluorescent sensor has been resoundingly fabricated using carbon dots (CDs) for ultrasensitive detection of copper ions (Cu²⁺). CDs are prepared by solid phase carbonization method using p-toluidine and l-cysteine as the precursors. The synthesized CDs exhibited the highest fluorescence intensity with excitation and emission wavelengths set at 300 nm and 400 nm, respectively. The CDs were selective and sensitive to Cu²⁺ due to the static quenching mechanism. The concentration of CDs, and solution pH and incubation time were important parameters for the developed sensor. The experimental results showed that 20 mgL^-1 was enough for the analysis. As the solution pH was concerned, it was apparent that the sensor was endowed with an excellent response signal to Cu²⁺ and provided high sensitivity at pH 12. The interaction occurred very quickly, and the incubation time could be set at 1 min. The sensor provided a two-stage calibration curve to Cu²⁺ in the range of 0.05-0.7 and 0.7-4 μM with a limit of detection of 47 nM. The obtained results clearly demonstrated that this facile method was fast, reliable and selective for detecting Cu²⁺, which would explore a prospective strategy for developing effective and low-cost sensors for monitoring metal ions in aqueous environments.

Carbon dots with polarity-tunable characteristics for the selective detection of sodium copper chlorophyllin and copper ions

Compared to water-soluble carbon dots (CDs) the properties and applications of hydrophobic CDs are rarely addressed. In this study, a one-pot, simple chemical oxidation approach has been applied to synthesize hydrophobic carbon dots (TO-CDs) at room temperature from triolein (TO) in concentrated sulfuric acid solution. Sodium copper chlorophyllin (SCC) quenches the fluorescence of TO-CDs by a photoinduced electron transfer process. Upon excitation at 400 nm, the fluorescence intensity of TO-CDs probe at 500 nm shows a linear response against the SCC concentration ranging from 1.0 to 10 μM, with a limit of detection (LOD) of 0.61 μM. Quantitation of SCC in flavored drinks shows percentage recovery (%R) vaues of 98-103% and relative standard deviation (RSD) values less than 6.5%. The hydrophobic TO-CDs can be converted into hydrophilic TO-CDs through hydrolysis in NaOH solution. The presence of sulfonyl groups on the hydrophilic TO-CDs enhances the coordination ability of the CDs toward Cu²⁺ ions, leading to fluorescence quenching which allows for the detection of Cu²⁺ ions with LOD of 0.21 μM and a linear range of 0.5-10 μM. The hydrophilic TO-CD probe possesses high selectivity toward Cu²⁺ ions (tolerance at least ten-fold comparative to other metal ions). The assay has been validated with the analysis of spiked soil samples, with %R values of Cu concentration of 97.8-99.0% and RSDs below 2.0%. The surface tunable CD probes demonstrate their potential for the rapid screening of Cu²⁺ ions in environmental samples and SCC in foods.

Green- and Red-Emitting Fluorescent Silicon Nanoparticles: Synthesis, Mechanism, and Acid Phosphatase Sensing

Until now, the green and facile synthesis of multicolor fluorescent silicon nanoparticles (SiNPs) with favorable biocompatibility for cellular imaging and biosensors is still a challenge. Herein, a facile one-step room temperature method for preparing fluorescent SiNPs displayed different emission wavelengths was reported. Green and red fluorescent SiNPs (G-SiNPs and R-SiNPs) were synthesized by adjusting the concentration of the reducing agent 2,4-diaminophenol hydrochloride when the amount of N-[3-(trimethoxysilyl)-propyl]-ethylenediamine was consistent. Characterized by Fourier transform infrared spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy, the results revealed that the G-SiNPs and R-SiNPs were assembled by polymerization of different building blocks, and the emission characteristics of these SiNPs were attributed to the difference in their structural composition and particle size. Interestingly, these fluorescent SiNPs exhibited excellent water solubility, salt tolerance, pH stability, photobleaching resistance, and low cytotoxicity, which facilitated multicolor cell imaging, and further led to these SiNPs were highly attractive in a variety of applications, such as multi-channel sensing and biological imaging. Furthermore, the R-SiNPs have shown the potential to detect acid phosphatase, which is a biomarker of prostate cancer.

Metal-organic gel as a fluorescence sensing platform to trace copper(II)

Metal-organic gel (MOG), as a novel type of metallic organic hybrid material, exhibits diverse properties. However, its application in fluorescence detection for specific metal ions has rarely been exploited. In this work, we have designed and synthesized a MOG based on Al-carboxylate coordination assemblies (denoted as MOG-Al). The resultant MOG-Al shows good specific fluorescence signal response to trace Cu²⁺. Under optimal conditions, the fluorescence quenching degrees (F₀ - F) of the MOG-Al have a linear correlation with Cu²⁺ concentration ranging from 0.05 to 100 μM, and the limit of detection (LOD) is 45.00 nM. The proposed sensing platform was also applied for the detection of Cu²⁺ in real samples. Satisfactory recoveries (92-116%) for Cu²⁺ in rice, soybean milk powder and pork liver were obtained. These results indicate that MOG-Al is a promising material for the specific and sensitive sensing of Cu²⁺.

Dual colorimetric sensing of ascorbic acid and thyroxine using Ag-EGCG-CTAB via a DFT approach

In this work, a colorimetric approach for the detection of ascorbic acid (AA) and thyroxine (TH) was developed by synthesizing cost-effective silver nanoparticles (AgNPs) decorated with epigallocatechin gallate (EGCG) and CTAB. EGCG is the major bioactive chemical constituent that played a significant role in this study. The environment around the nanoparticle (NP) was controlled by adding CTAB surfactants. The synthesized NPs were characterized by different advanced techniques including XRD, XPS, SEM, and TEM. UV-visible spectra were thoroughly analyzed for sensing of AA and TH and the colour change of the solution can be visually monitored. The change in the localized surface plasmon resonance (LSPR) properties was used as an asset for the detection of AA and TH. A good linear relationship was obtained in both the sensing schemes with a limit of detection (LoD) of 0.67 μM and 0.33 μM for AA and TH respectively. Furthermore, the nanoparticles (NP) were implemented for real-sample analysis (pharmaceutical tablets). A cost-effective filter paper strip-based method coupled with smartphone scanning sensing was developed for the detection of AA. The interaction of AA and TH with the probe was depicted by a density functional theory (DFT) analysis. The synthesized NPs show tremendous selectivity towards AA and TH and excellent potential for practical applications.

A feasible self-assembled near-infrared fluorescence sensor for acid phosphatase detection and cell imaging

The single signal amplification strategy is significant for detecting various disease biomarkers but is restricted by its limited accuracy. The multi-signal and multi-mode methods have overcome this deficiency. Acid phosphatase (ACP) is an important intracellular enzyme but one-step cell imaging material-based probes are scarce for ACP. Herein, we designed a one-step self-assembled polymer probe using neutral red (NR), modified-(pyridoxal-5'-phosphate (PLP)) and Eu³⁺. The polymer exhibited non-emission and excellent stability. Upon the catalytic hydrolysis reaction of ACP, the polymer exhibited two strong fluorescence signals at 373 nm and 613 nm and an appreciable decline of absorbance at 395 nm. The probe has excellent selectivity and higher sensitivity with a limit of detection as low as 0.02 mU mL^-1. It possesses favorable biocompatibility and has been successfully used to detect and image intracellular ACP in several living cells.

Simple and Sensitive Multi-components Detection Using Synthetic Nitrogen-doped Carbon Dots Based on Soluble Starch

Although carbon dots (CDs) as fluorescent sensors have been widely exploited, multi-component detection using CDs without tedious surface modification is always a challenging task. Here, two kinds of nitrogen-doped CDs (NCD-m and NCD-o) based on soluble starch (SS) as carbon source were prepared through one-pot hydrothermal process using m-phenylenediamine and o-phenylenediamine as nitrogenous dopant respectively. Through fluorescence "on-off" mechanism of CDs, NCD-m and NCD-o could be used as a fluorescence sensor for detection of Fe 3+ and Ag + with LOD of 0.25 and 0.51 μM, respectively. Additionally, NCD-m could be used for indirect detection of ascorbic acid (AA) with LOD of 5.02 μM. Moreover, fluorescence intensity of NCD-m also exhibited the sensitivity to pH change from 2 to 13. More importantly, Both NCD-m and NCD-o had potential application for analysis of complicated real samples such as tap water, Vitamin C tablets and orange juice. Ultimately, the small size of NCD-m could contribute to reinforcing intracellular endocytosis, which allowed them to be used for bacteria imaging. Obviously, these easily obtainable nitrogen-doped CDs were able to be used for multi-components detection. Strategy for synthesis of nitrogen-doped carbon dots (NCDs) and a schematic for fabrication of as-prepared NCDs for detection of Fe 3+, Ag + and ascorbic acid (AA).

Cancer cell identification by facile imaging of intracellular reductive substances with fluorescent nanosensor

Ascorbic acid (AA) and glutathione (GSH), the most abundant intracellular reductive substances, have been widely used as biomarkers for cancer cells identification. The current methods relying on imaging of AA or GSH alone to identify cancer cells may cause systematic errors, since a mutual conversion relationship exists between AA and GSH. In this work, we propose a fluorescent nanosensor for the simultaneous imaging of intracellular reductive substances including AA and GSH. Biocompatible fluorescent silicon nanoparticles (SiNPs) with rich surface amine and carboxyl groups were synthesized. The fluorescence of the SiNP was initially quenched by chelation of Fe³⁺ ions, forming SiNP/Fe³⁺ complex as the fluorescent nanosensor. Upon the redox reaction with reductive substances, the nanosensor showed sensitively fluorescent recovery. Moreover, benefited from the efficient cellular uptake of the SiNP/Fe³⁺ and the overexpressed intracellular reductive substances in cancer cells, the fluorescent nanosensor was used to accurately identify the human breast carcinoma (MCF-7) cells from normal mammary epithelial (MCF-10A) cells by imaging of intracellular AA and GSH simultaneously. This strategy would be promising in imaging-guided precision cancer diagnosis.

Two-dimensional nano-layered materials as multi-responsive chemosensors constructed by carbazole- and fluorene-based polyaniline-like derivatives

The development of multi-responsive chemosensors has a bright application prospect in environmental monitoring and biological diagnosis. In this paper, we report two kinds of fluorescent polyaniline-like derivatives containing of carbazole or fluorene moieties with two-dimensional (2D) nano-layered structure and their applications in the detection of Al³⁺, Fe³⁺, Cu²⁺ and HCl in different environments. Through the analysis of the structure and properties of these two 2D materials, we find that the prepared (Poly(9,9'-(9,9-dihexyl-9H-fluorene-2,7-diyl)bis(9H-carbazol-3-amine))) PDFCA material performs excellent sensing properties for above analytes. Relevant density functional theory (DFT) calculation further confirms the potential application of 2D nano-layered PDFCA material in sensing field. This study presents that 2D nano-layered PDFCA material is considerably competitive in the development of multi-responsive chemosensors, and it will greatly accelerate the research of 2D polymer materials.

A colorimetric indicator-displacement assay based on stable Cu2+ selective carbon dots for fluorescence turn-on detection of pyrophosphate anions in urine

Determination of PPi levels in urine represents a measurable factor for diagnostic, treatment, and monitoring of urolithiasis. Owing to the quenching ability of Cu²⁺ on fluorescent carbon dots (CDs) and strong binding affinity between Cu²⁺ and PPi, we develop a new off-on assay for PPi detection using newly BPHA CDs (BPHA: N,N-bis(pyridin-2-ylmethyl)hexan-1-amine). The fluorescence intensity of BPHA CDs was significantly quenched by Cu²⁺ ("off") through forming BPHA CDs/Cu²⁺ complexes and the fluorescence intensity of BPHA CDs /Cu²⁺ system was completely resumed by PPi ("on") owing to the release of free Cu²⁺. The fluorescence turn-off/on approach showed a highly selective response to PPi over the large family of other anions. The detection limits were 0.094 μM for Cu²⁺ and 0.025 μM for PPi, respectively. A wide linear range for PPi was up to 4400 μM. The indicator displacement assay (IDAs) using pyrocatechol violet (PV) as a colorimetric indicator was carried out to detect PPi with the naked eyes. The "off-on" fluorescent sensor based on BPHA CDs shows many merits, including convenient operation, cost-saving, high sensitivity, selectivity, stability and wide detecting range, which is applied to PPi detection in human urine sample.

Portable smartphone device-based multi-signal sensing system for on-site and visual determination of alkaline phosphatase in human serum

To provide the basis for clinical diagnosis in an emergency case, a portable smartphone device-based multi-signal sensing system for on-site determination of alkaline phosphatase (ALP) is introduced. In this system, cobalt hydroxide (CoOOH) nanoflakes can oxidize O-phenylenediamine (OPD) to produce 2,3-diaminophenazine (OxOPD), resulting in a strong fluorescence at 565 nm and an absorbance at 420 nm, respectively. The ascorbic acid 2-phosphate (AAP) can be hydrolyzed by alkaline phosphatase (ALP) to yield ascorbic acid (AA). Then, AA reduces the CoOOH nanoflakes to produce Co²⁺, and AA is oxidized to form dehydroascorbic acid (DHAA), thereby inhibiting the formation of OxOPD. The reaction product DHAA further combines with OPD to yield 3-(1,2-dihydroxyethyl)furo[3,4-b]quinoxalin-1(3H)-one (DFQ) accompanied by a strong fluorescence at 430 nm. Based on this, the fluorometric assay for ALP has a wide linear range from 0.8 to 190 U/L with a low detection limit of 0.16 U/L, and the colorimetric assay from 3 to 130 U/L with a detection limit of 1.94 U/L. Moreover, a portable smartphone sensing platform integrated with fluorescent and colorimetric signals was established for rapid determination of ALP without spectrometers. Recoveries of 97-104% for spiked samples and relative standard deviations (RSD) of less than 2% (n = 3) confirmed the feasibility of the developed platform in complicated samples, opening up new horizons for on-site evaluation in the biomedical field.

Paper microfluidic device using carbon dots to detect glucose and lactate in saliva samples

Bioanalyses are commonly performed with blood or serum samples. However, these analyses often require invasive and painful blood collection using a needle or finger pricking. Saliva is an alternative and very attractive biological medium for performing clinical analyses, since it contains many types of clinically relevant biomarkers and compounds. Its collection is straightforward and can be achieved in a non-invasive and stress-free way. However, the analytes are frequently present at low concentrations, while the viscosity of whole saliva hinders its analysis using paper devices, especially those with multiple layers (3D-μPADs). This work explores the use of a simple, fast, and low-cost saliva sample pretreatment using a cotton-paper-syringe filtration system, allowing the analysis of saliva samples using multilayer paper devices. The proposed methodology employs the oxidation of glucose and lactate, catalyzed by specific oxidase enzymes, producing hydrogen peroxide. The detection is based on the fluorescence quenching of carbon dots in the presence of hydrogen peroxidase. The concentrations of the analytes showed good linear correlations with the fluorescence quenching, with LODs of 2.60 × 10^-6 and 8.14 × 10^-7 mol L^-1 for glucose and lactate, respectively. The proposed method presented satisfactory intra-day and inter-day repeatabilities, with %RSD values in the range 3.82-6.61%. The enzymatic systems proved to be specific for the analytes and the matrix had no significant influence on the glucose and lactate determinations. The proposed methodology was successfully applied to saliva and serum samples and was validated using certified material.

Synthesis of Fluorescent Carbon Dots and Their Application in Ascorbic Acid Detection

Water-soluble fluorescent carbon dots (CDs) were synthesized by a hydrothermal method using citric acid as the carbon source and ethylenediamine as the nitrogen source. The repeated and scale-up synthetic experiments were carried out to explore the feasibility of macroscopic preparation of CDs. The CDs/Fe3+ composite was prepared by the interaction of the CDs solution and Fe3+ solution. The optical properties, pH dependence and stability behavior of CDs or the CDs/Fe3+ composite were studied by ultraviolet spectroscopy and fluorescence spectroscopy. Following the principles of fluorescence quenching after the addition of Fe3+ and then the fluorescence recovery after the addition of asorbic acid, the fluorescence intensity of the carbon dots was measured at λex = 360 nm, λem = 460 nm. The content of ascorbic acid was calculated by quantitative analysis of the changing fluorescence intensity. The CDs/Fe3+ composite was applied to the determination of different active molecules, and it was found that the composite had specific recognition of ascorbic acid and showed an excellent linear relationship in 5.0-350.0 μmol·L-1. Moreover, the detection limit was 3.11 μmol·L-1. Satisfactory results were achieved when the method was applied to the ascorbic acid determination in jujube fruit. The fluorescent carbon dots composites prepared in this study may have broad application prospects in a rapid, sensitive and trace determination of ascorbic acid content during food processing.

Smartphone colorimetric assay of acid phosphatase based on a controlled iodine-mediated etching of gold nanorods

A simple but efficient colorimetric assay was developed for the detection and quantification of acid phosphatase (ACP) using a smartphone. This strategy is based on target-controlled iodine-mediated etching of gold nanorods (AuNRs). Due to effective hydrolysis of the substrate pyrophosphate (PPi) by ACP, chelated Cu²⁺ with PPi was released, which promoted the redox reaction with an iodide ion (I^-), leading to the formation of I₃^-. As the etching agent of AuNRs, I₃^- caused a blueshift of the localized surface plasmon resonance peak and, more importantly, an observable color change. The vivid colors were recorded with a smartphone camera and directly analyzed using an image-processing app. On the basis of the direct correlation between ACP concentration and the etching degree of AuNRs as well as color change, this smartphone nanocolorimetry technique showed a good linear response toward ACP over the range of 0-15.0 U/L, with a detection limit of 0.97 U/L. Using the standard addition method, the practical applicability of the proposed smartphone-based assay was successfully demonstrated by determining ACP in human serum samples, with results consistent with those obtained by UV-Vis spectrophotometry.

Hyperbranched Poly(amido amine) Entrapped Tetraphenylethene as a Fluorescence Probe for Sequential Quadruple-Target Detection and Its Potential as a Chemical Logic Gate

Fluorescence sensors exhibit great potential as molecular logic gates to perform computation on a nanometer scale. For achieving the more complex artificial intelligence activities, developing complex logic gates using multitarget sensing systems with multi-input characteristics is highly desirable. Herein, a water-soluble quadruple-target fluorescence sensor that embeds a small amount (4.1 wt %) of tetraphenylethene (TPE) units into hyperbranched poly(amido amine) (TPE-HPA) has been designed. The nonfluorescent TPE-HPA could experience the fluorescence "off-on-off-on-off" by sequential addition of sodium hexametaphosphate (SHMP), Fe³⁺, ascorbic acid (AA), and H₂O₂. The as-prepared quadruple-target sensor showed good sensitivity and selectivity to SHMP, Fe³⁺, AA, and H₂O₂, and the limit of detection values were 29 nM, 20 nM, 0.66 μM, and 0.78 μM, respectively. On the basis of the multitarget sensing nature of TPE-HPA, chemical or electrochemical-induced logic gates were constructed, including YES, NOT, OR, NOR, NAND, INHIBIT, IMP, and higher logic systems.

Silk Fibroin-Confined Star-Shaped Decahedral Silver Nanoparticles as Fluorescent Probe for Detection of Cu2+ and Pyrophosphate

A green, one-step method for the fabrication of silk fibroin (SF) protected silver decahedral nanoparticles (SF@AgNPs) has been developed. High-resolution transmission electron microscopy characterization demonstrated that the silver decahedral nanoparticles can provide more binding sites with (111) facets. Moreover, a facile strategy based on Cu²⁺ mediated SF@AgNPs was reported as an on-off-on fluorescent system for the detection of Cu²⁺, and the SF@AgNPs can be also used for the determination of pyrophosphate ion (P₂O₇^4-, PPi). The fluorescence of SF@AgNPs was quenched by Cu²⁺ as a complex formed between SF and Cu²⁺ and was restored when PPi was introduced into the system due to the higher binding affinity between PPi and Cu²⁺. Herein, a novel SF@AgNPs-Cu²⁺ fluorescent probe for Cu²⁺ and PPi detection is presented. The proposed assay shows a linear relationship at a Cu²⁺ concentration range from 1 to 6 μM with a detection limit of 33.3 nM. This simple, reliable, selective, and environmentally friendly fluorescent probe also has a wide concentration range from 100 to 700 μM with a detection limit of 6.7 μM for PPi. The 16 types of anions lead to negligible changes in the SF@AgNPs-Cu²⁺ complexes, while restoring the fluorescence intensity of the complexes when added with PPi. Additionally, it is demonstrated that the SF@AgNPs-based fluorescent assay works in real biological samples. It suggests that this proposed method has the potential for application in the clinical detection of Cu²⁺ and PPi.

MnO2 nanoparticle mediated colorimetric turn-off determination of ascorbic acid

The scheme of the turn-off colorimetric response of AA to the TMB–MnO₂ NP system.