An "Off-On" Rhodamine 6G Hydrazide-Based Output Platform for Fluorescence and Visual Dual-Mode Detection of Lead(II)

Paper-based sensors: affordable, versatile, and emerging analyte detection platforms

Paper-based sensors, often referred to as paper-based analytical devices (PADs), stand as a transformative technology in the field of analytical chemistry. They offer an affordable, versatile, and accessible solution for diverse analyte detection. These sensors harness the unique properties of paper substrates to provide a cost-effective and adaptable platform for rapid analyte detection, spanning chemical species, biomolecules, and pathogens. This review highlights the key attributes that make paper-based sensors an attractive choice for analyte detection. PADs demonstrate their versatility by accommodating a wide range of analytes, from ions and gases to proteins, nucleic acids, and more, with customizable designs for specific applications. Their user-friendly operation and minimal infrastructure requirements suit point-of-care diagnostics, environmental monitoring, food safety, and more. This review also explores various fabrication methods such as inkjet printing, wax printing, screen printing, dip coating, and photolithography. Incorporating nanomaterials and biorecognition elements promises even more sophisticated and sensitive applications.

Integrated microfluidic platforms for heavy metal sensing: a comprehensive review

Heavy metals are found naturally; however, anthropogenic activities such as mining, inappropriate disposal of industrial waste, and the use of pesticides and fertilizers containing heavy metals can cause their unwanted release into the environment. Conventionally, detection of heavy metals is performed using atomic absorption spectrometry, electrochemical methods and inductively coupled plasma-mass spectrometry; however, they involve expensive and sophisticated instruments and multistep sample preparation that require expertise for accurate results. In contrast, microfluidic devices involve rapid, cost-efficient, simple, and reliable approaches for in-laboratory and real-time monitoring of heavy metals. The use of inexpensive and environment friendly materials for fabrication of microfluidic devices has increased the manufacturing efficiency of the devices. Different types of techniques used in heavy metal detection include colorimetry, absorbance-based, and electrochemical detection. This review provides insight into the detection of toxic heavy metals such as mercury (Hg), cadmium (Cd), lead (Pb), and arsenic (As). Importance is given to colorimetry, optical, and electrochemical techniques applied for the detection of heavy metals using microfluidics and their modifications to improve the limit of detection (LOD).

A colorimetric and SERS dual-mode aptasensor for the detection of Shiga toxin type II based on Mn/Fe-MIL(53)@AuNSs

Shiga toxin type II (Stx2), the major virulence component of enterohemorrhagic Escherichia coli, is strongly associated with the life-threatening hemolytic uremic syndrome thus posing a substantial risk to food safety and human health. In this work, a dual-mode aptasensor with colorimetric and surface-enhanced Raman scattering was developed for Stx2 specific detection based on noble metal nanoparticles and Raman reporter loaded metal-organic framework (Mn/Fe-MIL(53)@AuNSs-MBA). The Mn/Fe-MIL(53)@AuNSs could catalyze the H2O2-mediated oxidation of 3,3',5,5'-tetramethylbenzidine (TMB), thereby enabling visual detection. Meanwhile, the SERS signal from MBA can be enhanced by the decorated AuNSs. Under optimal conditions, a linear range of 0.05-500 ng/mL with limit of detection (LOD) of 26 pg/mL was achieved in colorimetric mode and a linear range of 5-1000 ng/mL with LOD of 0.82 ng/mL in SERS mode, in which the dual-mode results complement each other, widening the linear range, increasing the accuracy and reliability of the detection. The method was further applied to the detection of Stx2 in milk with average recovery of 101.1 %, demonstrating its superior potential for bacterial toxin monitoring.

Portable sensors equipped with smartphones for organophosphorus pesticides detection

Organophosphorus pesticides (OPs) play an important role in agricultural production and the accurate detection of OP residues is essential to ensure food safety. Portable sensors are expected to be a potential device due to their high detection efficiency, easy-to-use processes and low cost. Due to the widespread popularity and powerful capabilities of smartphones, smartphone-based sensing systems have rapidly developed into ideal tools for portable detection, however, a systematic review on the detection of OPs is still lacking. Therefore, a comprehensive overview of sensors equipped with smartphones for OP detection in recent year is provided; this overview includes their sensing signals (colorimetric, fluorescent, chemiluminescent and electrochemical signals), detection mechanism, analysis applications, advantages/disadvantages and perspectives. Moreover, the progress of sensors equipped with smartphones for the detection of OPs in food is thoroughly summarized. This review contributes to food safety and the development of efficient and reliable methods for smartphone-based OPs detection.

Magnetic-fluorescent immunosensing platform applying AuNPs heterogeneous MIL-53(Al) composite for efficient detection of zearalenone

Rapid, sensitive, specific and stable detection of mycotoxin in food remains an extremely crucial issue. Herein, a magnetic-fluorescent immunosensing platform for the detection of zearalenone (ZEN) was proposed. The platform utilized Au nanoparticles (AuNPs) heterogeneous fluorescent metal-organic framework (MIL-53(Al)@AuNPs) labeled with ZEN-bovine serum albumin (ZEN-BSA) as signal probe and ZEN mono-antibodies coupled with magnetic NPs (MNPs-mAbs) as capture probe. Specifically, the heterogenization of AuNPs on the MIL-53(Al) surface improved its biocompatibility, and facilitated the loading of ZEN-BSA conjugates. The MNPs-mAbs could rapidly capture the target ZEN, simplify the immunoassay process and further improve the detection efficiency. The established competitive magnetic-fluorescent immunosensing platform had a wider linear response to ZEN in the range of 0.001-100 ng/mL with a lower limit of detection (LOD) at 0.0035 ng/mL, and could finish the whole detection process within 20 min, showing great potential for rapid and sensitive detection of food contaminants.

Dual-mode colorimetric and fluorescence biosensors for the detection of foodborne bacteria

Due to the growing demand for detection technologies, there has been significant interest in the development of integrated dual-modal sensing technologies, which involve combining two signal transduction channels into a single technique, particularly in the context of food safety. The integration of two detection signals not only improves diagnostic performance by reducing assumptions, but also enhances diagnostic functions with increased application flexibility, improved accuracy, and a wider detection linear range. The top two output signals for emerging dual-modal probes are fluorescent and colorimetric, due to their exceptional advantages for real-time sensitive sensing and point-of-care applications. With the rapid progress of nanotechnology and material chemistry, the integrated colorimetric/fluorimetric dual-mode systems show immense potential in sensing foodborne pathogenic bacteria. In this comprehensive review, we present a detailed summary of various colorimetric and fluorimetric dual-modal sensing methods, with a focus on their application in detecting foodborne bacteria. We thoroughly examine the sensing methodologies and the underlying principles of the signal transduction systems, and also discuss the challenges and future prospects for advancing research in this field.

Biomass-based indole derived fluorescence sensor composited with cellulose paper: Detection of picric acid in food and environment samples

Picric acid (PA) is highly water-soluble, the fact makes it stand out as the most hazardous environment pollutant. Therefore, accurate determination of PA is of great significance for human health and environmental protection. Herein, a novel indole-based fluorescent sensor (H1) with good water solubility and fluorescence stability was reported. H1 exhibited 'turn-off' fluorescence response for PA with fast reaction rate (<30 s), unique specificity and excellent selectivity and high sensitivity (limit of detection = 34 nM). Further, H1 was successfully applied to detect PA in real samples (tap water, Yangtze River, Xuanwu Lake, soil, food, fish and shrimp) with satisfactory recoveries at three spiking levels ranging from 98.0 to 112.0 %. In addition, H1 displayed high biocompatibility in mung beans and fresh blood. Moreover, aiming to attain portable analysis, H1 was composited with biomass cellulose paper (H1-FP) and integrated with smartphone for construction as a solid-state fluorescence platform to achieve fast and visual detection of PA in suit with significant stability, high sensitively and selectivity. The establishment of this sensing approach is expected to offer new insight into rapid, selective, and sensitive detection of major pollutants for food and environmental safety.

Noninvasive Imaging of Tumor Glycolysis and Chemotherapeutic Resistance via De Novo Design of Molecular Afterglow Scaffold

Chemotherapeutic resistance poses a significant challenge in cancer treatment, resulting in the reduced efficacy of standard chemotherapeutic agents. Abnormal metabolism, particularly increased anaerobic glycolysis, has been identified as a major contributing factor to chemotherapeutic resistance. To address this issue, noninvasive imaging techniques capable of visualizing tumor glycolysis are crucial. However, the currently available methods (such as PET, MRI, and fluorescence) possess limitations in terms of sensitivity, safety, dynamic imaging capability, and autofluorescence. Here, we present the de novo design of a unique afterglow molecular scaffold based on hemicyanine and rhodamine dyes, which holds promise for low-background optical imaging. In contrast to previous designs, this scaffold exhibits responsive "OFF-ON" afterglow signals through spirocyclization, thus enabling simultaneous control of photodynamic effects and luminescence efficacy. This leads to a larger dynamic range, broader detection range, higher signal enhancement ratio, and higher sensitivity. Furthermore, the integration of multiple functionalities simplifies probe design, eliminates the need for spectral overlap, and enhances reliability. Moreover, we have expanded the applications of this afterglow molecular scaffold by developing various probes for different molecular targets. Notably, we developed a water-soluble pH-responsive afterglow nanoprobe for visualizing glycolysis in living mice. This nanoprobe monitors the effects of glycolytic inhibitors or oxidative phosphorylation inhibitors on tumor glycolysis, providing a valuable tool for evaluating the tumor cell sensitivity to these inhibitors. Therefore, the new afterglow molecular scaffold presents a promising approach for understanding tumor metabolism, monitoring chemotherapeutic resistance, and guiding precision medicine in the future.

A colorimetric and fluorometric dual-mode probe for Cu2+detection based on functionalized silver nanoparticles

A novel colorimetric/fluorescent probe (AgNPs-GSH-Rh6G2) was prepared by linking silver nanoparticles (AgNPs) with rhodamine 6G derivative (Rh6G2) using glutathione (GSH) as a linker molecule. The prepared probe showed obvious fluorescence change and colorimetric response after adding copper ions. Based on this phenomenon, a colorimetric/fluorescence dual-mode detection method was constructed to recognize copper ions. The linear ranges of fluorescence detection and colorimetric detection were 0.10 to 0.45 mM and 0.15 to 0.65 mM, respectively, and the limit of detection were 0.18 μM and 24.90 μΜ. In addition, the dual-mode probe has achieved satisfactory results in the detection of copper ions in sediment samples. The successful construction of AgNPs-GSH-Rh6G2 not only provide a reliable tool for the detection of copper ions, but also shed light on a new idea for the multi-mode development of the detection platform.

Recent Progress on Fluorescent Probes in Heavy Metal Determinations for Food Safety: A Review

One of the main challenges faced in food safety is the accumulation of toxic heavy metals from environmental sources, which can sequentially endanger human health when they are consumed. It is invaluable to establish a practical assay for the determination of heavy metals for food safety. Among the current detection methods, technology based on fluorescent probes, with the advantages of sensitivity, convenience, accuracy, cost, and reliability, has recently shown pluralistic applications in the food industry, which is significant to ensure food safety. Hence, this review systematically presents the recent progress on novel fluorescent probes in determining heavy metals for food safety over the past five years, according to fluorophores and newly emerging sensing cores, which could contribute to broadening the prospects of fluorescent materials and establishing more practical assays for heavy metal determinations.

A convenient fluorescent/electrochemical dual-mode biosensor for accurate detection of Pb2+ based on DNAzyme cycle

The presence of heavy metals in the ecological environment is a serious threat to human health. Therefore, it is very important to establish a simple and sensitive method for the detection of heavy metals. Currently, most of the methods are single-channel sensing, and these methods are prone to false-positive signals, which reduces the accuracy. In this work, Pb²⁺-DNAzyme was immobilized on magnetic beads (MBs) using a linkage of biotin and streptavidin and successfully applied to the construction of a fluorescent/electrochemical dual-mode (DM) biosensor. The supernatant after magnetic separation formed a double strand on the electrode, which was combined with methylene blue (MB) for electrochemical detection (EC). At the same time, FAM-d was added to the precipitate, and after magnetic separation, the supernatant was subjected to fluorescent detection (FL). Under optimal conditions, the signal response of the constructed dual-mode biosensor showed a good linear relationship with the concentration of Pb²⁺. The DNAzyme-based dual-mode biosensor achieved sensitive and selective detection of Pb²⁺ with good accuracy and reliability, opening a new way for the development of biosensing strategies for the detection of Pb²⁺. More importantly, the sensor has high sensitivity and accuracy for the detection of Pb²⁺ in actual sample analysis.

Fluorescent and Colorimetric Dual-Mode Strategy Based on Rhodamine 6G Hydrazide for Qualitative and Quantitative Detection of Hg2+ in Seafoods

In this study, a rapid fluorescent and colorimetric dual-mode detection strategy for Hg²⁺ in seafoods was developed based on the cyclic binding of the organic fluorescent dye rhodamine 6G hydrazide (R6GH) to Hg²⁺. The luminescence properties of the fluorescent R6GH probe in different systems were investigated in detail. Based on the UV and fluorescence spectra, it was determined that the R6GH has good fluorescence intensity in acetonitrile and good selective recognition of Hg²⁺. Under optimal conditions, the R6GH fluorescent probe showed a good linear response to Hg²⁺ (R² = 0.9888) in the range of 0-5 μM with a low detection limit of 2.5 × 10^-2 μM (S/N = 3). A paper-based sensing strategy based on fluorescence and colorimetric analysis was developed for the visualization and semiquantitative analysis of Hg²⁺ in seafoods. The LAB values of the paper-based sensor impregnated with the R6GH probe solution showed good linearity (R² = 0.9875) with Hg²⁺ concentration in the range of 0-50 μM, which means that the sensing paper can be combined with smart devices to provide reliable and efficient Hg²⁺ detection.

Dual-Mode Optical Sensor Array for Detecting and Identifying Perillaldehyde in Solution Phase and Plant Leaf with Smartphone

Promising techniques for detecting and quantifying active components in the plants and foods have received global concern in smart agriculture. Dual-mode optical assays are becoming more attractive and popular thanks to robust and reliable analysis parameters. We herein unveil a novel turn-on and dual-mode sensor array comprising three kinds of reactive indicators including ring-closed rhodamine-hydrazine, squaraine-hydrazine, and 2,4-dinitrophenylhydrazine for evaluating perillaldehyde. Significant colorimetric and fluorescent changes were triggered through reacting primary amine/hydrazine with the active aldehyde group in perillaldehyde, thus turning on the chromogenic responses of all the indicators. Optimal colorimetric sensing showed good responses to perillaldehyde ranged up to 100 mM in ethanol. Dramatic fluorescence enhancement was also exhibited, illustrating good selectivity as well as high sensitivity (detection limit ∼20.0 μM). Inspired by rapid chemical reactions and distinct optical changes, distinct sensor array strips loaded with the optimal solid-state reactive indicators were developed for evaluating the perillaldehyde content in the perilla frutescence leaves. Smartphone-enabled readout system and digital data processing were further performed for chemometric analysis. A good correlation was obtained and the semiquantitative evaluation of the perillaldehyde content could be achieved within 15 min, possessing the significant features of naked-eye recognition, easy operation, and disposability. To the best of our knowledge, present work demonstrated the use of chromogenic sensing strips to evaluate the active perillaldehyde content in solution and vapor phases for the first time. Taken together, these characteristics also indicate that the present turn-on sensor array has great potential applications in the precise detection and evaluation of perillaldehyde in the forthcoming smart agriculture.

Detection of Heavy Metal Ions by Ratiometric Photoelectric Sensor

In recent years, heavy metal pollution has become increasingly serious. Heavy metals exist in an environment mainly in the form of ions (heavy metal ions, HMs). They can contaminate food, water, soil, and the atmosphere, leading to serious harm to plants and animals. With high bioavailability and nonbiodegradability, HMs can accumulate through biomagnification. Consequently, heavy metal pollution has become the cause of many fatal diseases threatening human health and ecological environment. Therefore, the accurate detection of HMs is vital and necessary. In this paper, the harm and limit standards of heavy metals were systematically summarized and the common analysis methods were overviewed and compared. Specifically, the latest research progress of ratiometric photoelectric sensor, including optical and electrical sensor, were mainly described. The research status and advantages and disadvantages of a photoelectric sensor were summarized. Furthermore, the future directions were proposed, which provided the reference for the further research and application of the ratiometric photoelectric sensor.

A novel fluorescent "OFF-ON" sensing strategy for Hg (II) in water based on functionalized gold nanoparticles

Mercury ion (Hg²⁺) is a heavy metal pollutant that can affect the safety of water environment and endanger human health. A novel detection strategy (GNPs-_L-Cys-Rh6G2) for Hg²⁺ based on a fluorescence "OFF-ON" was proposed. Gold nanoparticles (GNPs) were assembled with _l-cysteine (_L-Cys), which was used as a "bridge" to link with rhodamine 6G derivatives (Rh6G2). The fluorescence state transition of GNPs-_L-Cys-Rh6G2 switching from "OFF"-"ON" was observed because Hg²⁺ opened the spirolactam ring through a catalytic hydrolysis mechanism. The fluorescence signal of the GNPs-_L-Cys-Rh6G2 system mixed with Hg²⁺ in the concentration range of 10-100 μM was analyzed and determined with a limit of detection (LOD) of 2 μM (S/N = 3). Moreover, the spiked Hg²⁺ concentration in real water samples were successfully quantified by GNPs-_L-Cys-Rh6G2, which was in line with the ideal average recovery rate and relative standard deviation. The proposed strategy exhibited high specificity, sensitivity and stability, providing a novel sensing platform for heavy metal ions detection in water environment.

Synthesis of PVDF membrane loaded with wrinkled Au NPs for sensitive detection of R6G

A novel SERS membrane is synthesized by combining metal lattice and surface enhanced Raman scattering (SERS) technology. Since R6G is a carcinogenic and harmful pollutant, and traditional detection methods have many drawbacks and have research value, this paper selects R6G as the detection target. The SERS substrates are synthesized by loading Au nanoparticles (Au NPs) on the surface of polyvinylidene fluoride (PVDF) membrane. The Au NPs are synthesized through a controllable hydrothermal method. The synthesized AuNPs are covered by some gold particles, forming a fold pattern. Finally, the synthesized structure is immobilized on the surface of the PVDF membrane by the phase inversion method. It is suggested that the prepared Au NPs@PVDF membrane exhibits adjustable cavity structure, strong plasmon coupling, tunable magnetic plasmon resonance, prominent SERS performances. The prepared Au NPs@PVDF membrane showed sensitive SERS activity, good mechanical strength and reusability, expanding the application field of SERS detection. Overall, this study establishes a novel technique for the synthesis of SERS membrane with excellent SERS property and expands the application field of SERS detection.

Monitoring of glutathione using ratiometric fluorescent sensor based on MnO2 nanosheets simultaneously tuning the fluorescence of Rhodamine 6G and thiamine hydrochloride

L-glutathione (GSH) which has reducibility and integrated detoxification plays an important role in maintaining normal immune system function. Its abnormal levels are relevant to some clinical diseases. In this work, a facile ratiometric fluorescence sensor for GSH was designed based on MnO₂ nanosheets, Thiamine hydrochloride (VB₁) and Rhodamine 6G (R6G). VB₁ could be oxidized into fluorescent ox-VB₁ due to the strong oxidizing property of MnO₂, and MnO₂ nanosheets simultaneously could quench the fluorescence of R6G based on the inner filter effect (IFE). MnO₂ could react with GSH to form Mn²⁺, which caused its losing oxidizing property and quenching capacity. According to this principle, the concentration of ox-VB₁ diminished, resulting in its fluorescence intensity decreasing at 455 nm and the fluorescence of R6G recovering at 560 nm. Under optimal conditions, the VB₁-MnO₂-R6G detection system showed a wide linear range towards GSH in the range of 1.0-300.0 µmolL^-1 with a low detection limit reaching 0.52 µmolL^-1. Furthermore, the method was also applied in the determination of GSH in human serum.