A smartphone-integrated ratiometric fluorescence sensor for visual detection of cadmium ions

A ratiometric fluorescence and colorimetry dual-signal sensing strategy based on o-phenylenediamine and AuNCs for determination of Cu2+ and glyphosate

A ratiometric fluorescence sensing strategy has been developed for the determination of Cu2+ and glyphosate with high sensitivity and specificity based on OPD (o-phenylenediamine) and glutathione-stabilized gold nanoclusters (GSH-AuNCs). Water-soluble 1.75-nm size GSH-AuNCs with strong red fluorescence and maximum emission wavelength at 682 nm were synthesized using GSH as the template. OPD was oxidized by Cu2+, which produced the bright yellow fluorescence oxidation product 2,3-diaminophenazine (DAP) with a maximum fluorescence emission peak at 570 nm. When glyphosate existed in the system, the chelation between glyphosate and Cu2+ hindered the formation of DAP and reduced the fluorescence intensity of the system at the wavelength of 570 nm. Meanwhile, the fluorescence intensity at the wavelength of 682 nm remained basically stable. It exhibited a good linear relationship towards Cu2+ and glyphosate in water in the range 1.0-10 µM and 0.050-3.0 µg/mL with a detection limit of 0.547 µM and 0.0028 µg/mL, respectively. The method was also used for the semi-quantitative determination of Cu2+ and glyphosate in water by fluorescence color changes visually detected by the naked eyes in the range 1.0-10 µM and 0.30-3.0 µg/mL, respectively. The sensing strategy showed higher sensitivity, more obvious color changes, and better disturbance performance, satisfying with the detection demands of Cu2+ and glyphosate in environmental water samples. The study provides a reliable detection strategy in the environment safety fields.

A portable Eu-MOF-loaded paper-based probe integrated with smartphone for the visual and on-site detection of Cr2O72- in aqueous media

The high toxicity of dichromate anion (Cr2O72-) its accurate necessitates its sensitive and effective detection to safeguard human health. The study introduces a Eu-MOF fluorescent probe, named as Eu-TDCA, synthesized using 2,5-thiophenedicarboxylic acid (H2TDCA) as a bridging ligand for the detection of Cr2O72- in aqueous media. The probe suspension can detect Cr2O72- through fluorescence quenching, and the detection process exhibits a wide linear range (0-85 and 85-230 mg/L), low limit of detection (LOD, 5.1 μg/L) and rapid response speed (2 min). Furthermore, a portable Eu-TDCA-loaded paper-based probe, integrated with a smartphone color recognition app, was developed for the visual, sensitive and quantitative detection of Cr2O72- in real lake and river water samples, achieving satisfactory recoveries of 99.72%-103.75 %. Additionally, an advanced logic gate device was designed to simplify the detection process, providing a new direction for intelligent on-line detection of analytes.

Smartphone-integrated paper-based sensing platform for the visualization and quantitative detection of pymetrozine

Pymetrozine (PYM) is an effective pyridine insecticide for controlling aphids, while its residues pose a serious threat to human health. Herein, a europium complex (Eu-DBPA, DBPA represents deprotonated 2,5-dibromoterephthalic acid ligand) probe was prepared for the detection of PYM via fluorescence quenching. The detection process has the advantages of short response time (2 min), wide linear range (0-4 and 4-45 mg/kg) and low detection limit (2.2 μg/kg). Furthermore, a portable detection platform was designed by integrating Eu-DBPA-based paper strip with smartphone and applied for the visual detection of PYM in real cucumber, tomato, cabbage and apple samples, obtaining satisfactory recovery (99.00 %-107.00 %) and low standard deviation (RSD < 3.4 %). In addition, a logic gate device was designed to simplify the detection process. The smartphone-integrated paper-based probe detection platform provides a new strategy for intelligent and online identification of hazards in environmental and biological samples.

Development of a FRET aptasensor based on MoS2-doped Zn-MOF as luminophore for selective detection of cadmium in aqueous solutions

A robust "on-off" fluorescent aptasensor was developed using nanohybrids of molybdenum sulfide (MoS2) quantum dot (QD)-doped zinc metal-organic frameworks (Zn-MOF) for selective and sensitive detection of cadmium ions (Cd2+) in water. This nanohybrid (MoS2@Zn-MOF), synthesized via "bottle around the ship" methodology, exhibited a high-intensity fluorescence emission centered at 430 nm (λEm) (blue) on excitation at 320 nm (λEx). Further, the conjugation of this fluorophore to phosphate-modified cadmium aptamer (Cd-2-2) was achieved through carbodiimide reaction. The hybridization of prepared sensing probe (MoS2@Zn-MOF/Cd-2-2 aptamer) was done with dabcyl-conjugated complementary DNA (cDNA), acting as energy donor-acceptor pair in the fluorescence resonance energy transfer (FRET) system. This hybridization causes the fluorescence quenching of the nanohybrid. In the presence of Cd2+, the aptamer from the fabricated nano-biosensing probe binds to these ions, resulting in release of dabcyl-cDNA oligomer. This release of dabcyl-cDNA oligomer from the sensing probes restores the fluorescence of the nanohybrid. Under optimized conditions (sensing probe/dabcyl-cDNA ratio 1/7, pH 7.4, and temp 28 °C), the sensing probe showed a fast response time of 1 min. The fluorescence intensity of the nanohybrid can be utilized to determine the concentration of Cd2+. The proposed aptasensor achieved highly sensitive detection of Cd2+ with a limit of detection (LOD) of 0.24 ppb over the range of 1 × 10-9 to 1 × 10-4 M along with minimal effects of interferences (e.g., Hg2+, Pb2+, and Zn2+) and good reproducibility. The designed aptasensor based on MoS2@Zn-MOF nanofluorophore offers a highly sensitive and selective approach for rapid screening of metal ions in aqueous environments.

An ultrasensitive cellulose-based fluorescent sensor for Al3+ detection and its applications in plant tissue and food samples

Fluorescent sensors available for metal ions detection have been extensively developed in recent years. However, developing an ultrasensitive fluorescent sensor for highly selectively detecting Al3+ based on cellulose remains a challenge. In this study, an ethylcellulose-based flavonol fluorescent sensor named EC-BHA was synthesized by the esterification of ethylcellulose (EC) with a new flavonol derivative 4-(2-(2,3-bis(ethoxymeothy)phenyl)-3-hydroxy-4-oxo-4-H-chromen-7-yl) benzoic acid (BHA). The fluorescence intensity of EC-BHA exhibited a 180-fold increase at 490 nm after binding with Al3+ and provided an ultralow detection limit of 13.0 nM. The sensor showed some exceptional sensing properties including a broad pH range (4-10), large Stokes shifts (190 nm), and a short response time (3 min). This sensor was successfully applied for determining trace Al3+ in food samples as well as in plant tissue. Moreover, the electrostatic spun film EBP was fabricated by blending EC-BHA with PS (polystyrene) via electrostatic spinning technique and utilized for selective detection of Al3+ as soon as possible.

Physicochemically modulated fluorescence-scattering ratiometric sensor for selective and visual detection of levodopa

In this study, a facile fluorescence-scattering ratiometric sensor was designed for visual and selective detection of levodopa (LD) via a clever physicochemical modulation scheme. The alkalized products of LD can rapidly react with polyethyleneimine (PEI) to exhibit an intense blue fluorescence and decrease the second-order scattering (SOS) signal of PEI. As the concentration of LD increased, the fluorescence intensity at 420 nm increased and the SOS intensity at 675 nm decreased synchronously. Thus the fluorescence-scattering ratiometric sensor was constructed by virtue of the two simultaneously changed signals. Furthermore, red light-emitting Au nanoclusters (AuNCs) were added into the above mixture solution to enlarge the SOS signal and provide a stable red background fluorescence. The intensity ratio of fluorescence to SOS (F/(S/Sblank)) is linear dependent on CLD in the wide range of 50.0---30000.0 nM, and LD as low as 50.0 nM can be identified with the naked eye via change of fluorescence color. The developed ratiometric sensor is smart, simple and efficient, and has been applied to the convenient assay of LD in real samples. The proposed physicochemical modulation strategy provides a new and facile path for selectively and visually identifying the target from its analogues.

A novel [1, 2, 4]triazolo[5,1-b]quinazoline derivative as a fluorescent probe for highly selective detection of Fe3+ ions

A novel [1, 2, 4]triazolo[5,1-b]quinazoline fluorescent probe (VIi) for Fe3+ was developed, featuring with rapid response (< 5 s) and specific selectivity to Fe3+, low detection limit (1.3 × 10-5 M), as well as the ability to resist interference of chelating agent (e.g. EDTA). VIi-based fluorescent test paper can quickly recognize Fe3+ under irradiation at the wavelength of 365 nm. The fluorescence probe VIi has potential application prospects for the detection of Fe3+ in real circumstance.

Smartphone-Assisted EY@MOF-5-Based Dual-Emission Fluorescent Sensor for Rapid On-Site Detection of Daclatasvir and Nitenpyram

Fluorescent metal-organic frameworks (MOFs) are promising sensing materials with tunable and robust structural properties and remarkable luminescent capabilities. In this study, a novel dual-emission fluorescent metal-organic framework (EY@MOF-5) composite is synthesized by a one-pot bottle-around-ship approach. Eosin Y (EY) is encapsulated in MOF-5 to enhance its fluorescence properties and selectivity, effectively addressing typical MOF-5 limitations. EY@MOF-5 serves as a versatile dual-functional fluorescent sensor for two different analytes, daclatasvir (DCT) and nitenpyram (NTP), showing an impressive linear range of 10-200 nM and 0.1-300 μM, with detection limits of 233 pM and 65 nM, respectively. The established method is ultrafast, highly sensitive, and extremely selective for DCT and NTP detection in complex biological and food samples. Fluorescence results are compared and validated with the recommended UPLC method. Then, a smartphone-integrated sensing system is introduced for on-site, real-time, and quantitative analysis of DCT and NTP. The smartphone-assisted intelligent sensing method manifests promising results for DCT and NTP monitoring in biological and food samples, demonstrating its promising potential for the on-site detection of biologically and environmentally significant analytes.

A portable multi-channel fluorescent paper-based microfluidic chip based on smartphone imaging for simultaneous detection of four heavy metals

Due to the excessive contamination of heavy metals pollution, it is very urgent and necessary to develop a real-time detection method for the heavy metals in food. As a target sensing device, a paper-based microfluidic device (μPAD) has the advantages of simplicity, low-cost, and portability. In this study, a self-driven microfluidic paper-based chip was first developed for the simultaneous detection of four targets. The channels on the microfluidic chip were prepared by using wax printing and automatic screen printing on the filter paper, where liquid flowed by capillary force without pump assistance. Based on the specific binding ability of aptamers to heavy metals, a "turn-on" fluorescence aptasensor for the simultaneous detection of four heavy metal ions was developed on the proposed multi-channel device via smartphone imaging. The obtained fluorescence images were digitized into RGB color values by Image J software, and an M-mode was established to realize the quantitative detection of heavy metal ions. Under optimal conditions, the limits of detection for lead(II), mercury(II), cadmium(II), and arsenic(III) were 4.20 nM, 1.70 nM, 2.04 nM, and 1.65 nM, respectively. Furthermore, the aptasensor was successfully applied to the quantitative detection of four heavy metal ions in apple and lettuce samples with recovery rates of 84.0%-104.1%.

Machine learning integrated high quantum yield blue light carbon dots for real-time and on-site detection of Cr(VI) in groundwater and drinking water

The safety of groundwater and drinking water is directly related to the well-being of human beings and ecosystems. On-site monitoring and timely response to heavy metals in these water sources are crucial for water supply security. Fluorescent probes combined with machine learning technology have been applied to on-site detection of heavy metals. However, they were primarily focused on industrial-level detection and lacked the sensitivity required for detecting Cr(VI) in groundwater and drinking water. In this study, we developed an machine learning-integrated approach using high-quantum-yield (QY) N-doped blue-light carbon dots (N-BCDs) for instant detection of Cr(VI) in groundwater and drinking water. N-BCDs were synthesized within 3 min using a household microwave oven with citric acid and 1,2-diaminobenzene, resulting in a QY of approximately 90 %. The fluorescence of N-BCDs was quenched via the internal filter effect (IFE), enabling the detection of Cr(VI) within 1 min, with a detection limit of 0.1574 μg L-1 for Cr(VI) concentrations ranging from 0 to 60 μg L-1. We employed machine learning methods to determine Cr(VI) concentrations from simple shots, based on the red-green-blue (RGB) feature and Kmeans feature extraction. These features were input into four models (Ridge, XGB, SVR, and Linear), achieving a fitness of 95.2 %. Furthermore, the accuracies for Cr(VI) concentration identification in actual groundwater and drinking water were as high as 95.71 % and 96.81 %, respectively. Our work successfully extended the detection range of Cr(VI) to the μg level, significantly improving the practical applicability of the method and providing a new approach for on-site detection of Cr(VI) in groundwater and drinking water.

A Nanograss Boron and Nitrogen Co-Doped Diamond Sensor Produced via High-Temperature Annealing for the Detection of Cadmium Ions

The high-performance determination of heavy metal ions (Cd2+) in water sources is significant for the protection of public health and safety. We have developed a novel sensor of nanograss boron and nitrogen co-doped diamond (NGBND) to detect Cd2+ using a simple method without any masks or reactive ion etching. The NGBND electrode is constructed based on the co-doped diamond growth mode and the removal of the non-diamond carbon (NDC) from the NGBND/NDC composite. Both the enlarged surface area and enhanced electrochemical performance of the NGBND film are achievable. Scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, cyclic voltammetry, electrochemical impedance spectroscopy, and differential pulse anodic stripping voltammetry (DPASV) were used to characterize the NGBND electrodes. Furthermore, we used a finite element numerical method to research the current density near the tip of NGBND. The NGBND sensor exhibits significant advantages for detecting trace Cd2+ via DPASV. A broad linear range of 1 to 100 μg L-1 with a low detection limit of 0.28 μg L-1 was achieved. The successful application of this Cd2+ sensor indicates considerable promise for the sensitive detection of heavy metal ions.

Leveraging a smartphone to perform time-gated luminescence measurements

Empowered by advanced on-board sensors, high-performance optics packages and ever-increasing computational power, smartphones have democratized data generation, collection, and analysis. Building on this capacity, many platforms have been developed to enable its use as an optical sensing platform for colorimetric and fluorescence measurements. In this paper, we report the ability to enable a smartphone to perform laboratory quality time-resolved analysis of luminescent samples via the exploitation of the rolling shutter mechanism of the native CMOS imager. We achieve this by leveraging the smartphone's standard image capture applications, commercially available image analysis software, and housing the device within a UV-LED containing case. These low-cost modifications enable us to demonstrate the smartphone's analytical potential by performing tasks ranging from authentication and encryption to the interrogation of packaging, compounds, and physical phenomena. This approach underscores the power of repurposing existing technologies to extend the reach and inclusivity of scientific exploration, opening new avenues for data collection and analysis.

Ratio fluorescence test strip visualized by amino-functionalized metal-organic framework for rapid sensing of glyphosate

As glyphosate is a broad-spectrum herbicide extensively used in agriculture worldwide, rapid glyphosate detection is essential for food safety and human health. Herein, a ratio fluorescence test strip was prepared and coupled with an amino-functionalized bismuth-based metal-organic framework (NH₂-Bi-MOF) that bonded with copper ion for rapid visualization and determination of glyphosate. NH₂-Bi-MOF had excellent fluorescence performance, and the copper ion, a Lewis acid, was selected as the quencher. The strong chelation of glyphosate with copper ion and its quick interaction with NH₂-Bi-MOF would turn on the fluorescence signal, thus enabling the quantitative sensing of glyphosate, with a linear range of 0.10-200 µmol L^-1, and recoveries between 94.8% and 113.5%. The system was then expanded to a ratio fluorescence test strip, in which the fluorescent ring sticker was set as a binding-in self-calibration to reduce errors from the angle and light dependency. The method realized the visual semi-quantitation referring to a standard card, as well as the ratio quantitation using the gray value output with LOD of 0.82 µmol L^-1. And the as-developed test strip was accessible, portable, and reliable, thus offering a platform for the rapid on-site detection of glyphosate and other residual pesticides.

Cadmium ions mediated turn-on electrogenerated chemiluminescence of ZnS nanoparticles for highly selective cadmium detection in seafood

Cd²⁺ is one of the most toxic heavy metal ions that can be easily accumulated in human body via food chain. Thus, the onsite detection of Cd²⁺ in food is very important. However, present methods for Cd²⁺ detection either require the use of large equipment, or suffer from the severe interference from other analogical metal ions. This work establishes a facile Cd²⁺ mediated turn-on ECL method for highly selective detection of Cd²⁺ via cation exchanging with the nontoxic ZnS nanoparticles, owing to the unique surface-state ECL properties of CdS nanomaterials. The linear range of the calibration curve is from 7.0 × 10^-8 to 1.0 × 10^-6 M, while other analogical metal ions do not interfere, facilitating the selective detection of Cd²⁺ in oyster samples. The result agrees well with that obtained using atomic emission spectroscopy, indicating the potential for wider application of this approach.

A sensitive fluorescence sensor based on a glutathione modified quantum dot for visual detection of copper ions in real samples

Copper (Cu²⁺), as a heavy metal, accumulates in the human body to a certain extent, which can induce various diseases and endanger human health. Rapid and sensitive detection of Cu²⁺ is highly desired. In present work, a glutathione modified quantum dot (GSH-CdTe QDs) was synthesized and applied in a "turn-off" fluorescence probe to detect Cu²⁺. The fluorescence of GSH-CdTe QDs could be rapidly quenched in the presence of Cu²⁺ through aggregation-caused quenching (ACQ), resulting from the interaction between the surface functional groups of GSH-CdTe QDs and Cu²⁺ and the electrostatic attraction. In the range of 20-1100 nM, the Cu²⁺ concentration showed a good linear relationship with the fluorescence decline of the sensor, and the LOD is 10.12 nM, which was lower than the U.S. Environmental Protection Agency (EPA) defined limit (20 μM). Moreover, aiming to attain visual analysis, colorimetric method was also used for rapidly detecting Cu²⁺ by capturing the change in fluorescence color. Interestingly, the proposed approach has successfully been applied for the detection of Cu²⁺ in real samples (i.e., environment water, food and traditional Chinese medicine) with satisfactory results, which provides a promising strategy for the detection of Cu²⁺ in practical application with the merits of being rapid, simple and sensitive.

Smartphone-based on-site detection of hydrogen peroxide in milk by using a portable ratiometric fluorescent probe

The on-site detection of hydrogen peroxide (H₂O₂) is important for maintaining food safety as the ingestion of H₂O₂ can lead to serious pathological conditions. However, most reported fluorescent probes require a fluorometer to ensure readable signal output and reliable detection result. Consequently, the fluorescent detection of H₂O₂ can be realized only within a standard laboratory setting. Herein, we report a novel supramolecular strategy that can successfully convert the typical off-on response to H₂O₂ into a ratiometric response, which allows the on-site detection of H₂O₂ when used in conjunction with a smartphone-based 3D-printed miniaturized testing system. This method has acceptable sensitivity, good anti-interference ability, and desirable accuracy compared to a standard detection method. More importantly, this portable ratiometric method can be used to detect H₂O₂ residue in commercial milk samples with the simple testing apparatuses.

A smartphone-based ratiometric fluoroprobe based on blue-red dual-emission signals of thiochrome and copper nanoclusters for sensitive assay of metam-sodium in cucumbers

It is of great importance to develop the highly efficient fluorescence strategy for rapid/sensitive detection of metam-sodium (MES) in evaluating its residual safety, especially in fresh vegetables. Herein, we prepared an organic fluorophore (thiochrome, TC) and glutathione-capped copper nanoclusters (GSH-CuNCs), and their combination (TC/GSH-CuNCs) was sucessfully employed as a ratiometric fluoroprobe by means of the blue-red dual emission. The fluorescence intensities (FIs) of TC decreased upon the addition of GSH-CuNCs via the fluorescence resonance energy transfer (FRET) process. When fortified at the constant levels of GSH-CuNCs and TC, MES substantially reduced the FIs of GSH-CuNCs, while this was not the case in the FIs of TC except for the prominent red-shift of ∼30 nm. Compared to the previous fluoroprobes, the TC/GSH-CuNCs based fluoroprobe supplied wider linear range of 0.2-500 μM, lower detection limit (60 nM), and satisfactory fortification recoveries (80-107%) for MES in the cucumber samples. Based on the fluorescence quenching phenomenon, a smartphone application was used to output RGB values of the captured images for the colored solution. The smartphone-based ratiometric sensor could be utilized for the visual fluorescent quantitation of MES by virtue of the R/B values in cucumbers, which gave linear range (1-200 μM) and LOD (0.3 μM). By means of blue-red dual-emission fluorescence, the smartphone-based fluoroprobe provides a cost-effective, portable and reliable avenue for the on-site, rapid and sensitive assay of MES's residues in complex vegetable samples.

Visual and Real-Time Monitoring of Cd2+ in Water, Rice, and Rice Soil with Test Paper Based on [2 + 2] Lanthanide Clusters

Cadmium ions (Cd²⁺) are highly toxic to animal and human health, especially through the drinking of Cd²⁺-contaminated water and eating Cd²⁺-contaminated rice. Therefore, accurate detection of Cd²⁺ in water, rice, and rice soil is urgent. In this work, two [2 + 2] lanthanide clusters of Tb₂Tb₂ and Eu₂Eu₂ were synthesized and characterized in detail. Interestingly, Tb2Tb2 is a rapid sensor for Cd²⁺ through luminescence "turn-off". Further studies show that Tb2Tb2 is a highly sensitive and selective sensor toward Cd²⁺ in water, rice supernatants, and rice soil supernatants, with a very short response time of 20 s. The limit of detection (LOD) in the above three real samples is as low as 0.0112, 1.1240, and 0.1124 ppb, respectively, which is lower than the national standards for food safety in China (GB 2762-2022). More interestingly, a portable sensing device of test paper based on Tb2Tb2 is developed with a facile method, which shows visible, highly sensitive, and selective sensing toward Cd²⁺ in real samples of water, rice supernatants, and rice soil supernatants. Tb2Tb2 and its sensing device of test paper are an on-site analysis sensor for potentially non-expert users, especially for people in remote rural areas.

A smartphone-assisted optosensing platform based on chromium-based metal-organic framework signal amplification for ultrasensitive and real-time determination of oxytetracycline

Ultrasensitive and onsite detection of oxytetracycline (OTC) is of vital significance for ensuring public health. Herein, a novel and versatile fluorescence biomimetic nanosensor, Mg,N-CDs@MIL-101@MIP, was elaborately tailored for the assay of OTC. MIL-101 with extraordinarily high surface area and porosity, as a favorable supporter, suppressed self-quenching of Mg,N-CDs and boosted mass transfer rate, realizing signal amplification. As an ultrasensitive signal transducer, high luminescent Mg,N-CDs yielded conspicuous fluorescence responses for OTC, enhancing the sensitivity of Mg,N-CDs@MIL-101@MIP. High-affinity imprinting sites further endowed Mg,N-CDs@MIL-101@MIP with superior anti-interference ability and reusability. Given prominent merits, Mg,N-CDs@MIL-101@MIP demonstrated a good linear range (0.05-40 μg mL^-1) with a lower limit of detection (16.8 ng mL^-1), supplying high accessibility to realize ultrasensitive and highly selective measurement of OTC in samples. Additionally, to attain precise onsite profiling of OTC, an intelligent sensing platform was developed by integrating Mg,N-CDs@MIL-101@MIP with a portable smartphone-assisted optical device. As both signal reader and analyzer, smartphone can instantly capture concentration-dependent fluorescent images and accurately digitize them, accomplishing quantitative analysis of OTC. More delightfully, the portable platform was utilized for visual determination of OTC in milk samples with satisfactory results, offering a promising tool for the high-performance onsite evaluation of food safety and environmental health.

A ratiometric fluorescent biosensing platform for ultrasensitive detection of Salmonella typhimurium via CRISPR/Cas12a and silver nanoclusters

Rapid, sensitive and specific detection of bacteria is of great importance. Herein, we developed a versatile biosensing platform for ultrasensitive detection of pathogenic bacteria, termed as SCENT-Cas (Silver nanoCluster Empowered Nucleic acids Test using CRISPR/Cas12a). Simply, the species-specific invA gene of Salmonella typhimurium (S. typhi) was isothermally amplified using LAMP, which subsequently triggered the trans-cleavage of CRISPR/Cas12a. The trans-cleavage degraded any single-stranded DNA (ssDNA) non-specifically. A DNA-templated AgNCs probe was then employed, in which green fluorescence emissive AgNCs effectively converted to red fluorescence emissive AgNCs when placed in close vicinity to a pre-designed converter ssDNA. As such, the trans-cleavage was utilized for shredding converter ssDNA, enabling the green-to-red fluorescent change to form a ratiometric biosensing platform. With this strategy, target nucleic acid was dexterously converted into ratiometric fluorescence that was recorded to detect as low as 1 CFU/mL S. typhi with a dynamic range from 1 to 10⁸ CFU/mL. To our knowledge, this is the first report regarding the use of ratiometric fluorescence in CRISPR/Cas-based detection, which minimizes interference and improves reliability. Lastly, this proposed strategy was challenged by detecting S. typhi contamination in real food samples. Our work enriches CRISPR/Cas toolbox in biosensing by providing a desirable method for bacterial detection.

Engineering an Enzymatic Cascade Catalytic Smartphone-Based Sensor for Onsite Visual Ratiometric Fluorescence-Colorimetric Dual-Mode Detection of Methyl Mercaptan

Precise and reliable onsite detection of methyl mercaptan (CH₃SH) is of great significance for environmental surveillance. Here, we synthesized a novel blue fluorescence nanozyme CeO₂@TPE with high peroxidase-like activity by employing aggregation-induced emission (AIE) tetraphenylethene (TPE) to embed into hollow CeO₂ nanospheres. In the presence of ethanol oxidase (AOX) and o-phenylenediamine (OPD), we engineered an enzymatic cascade activation ratiometric fluorescence-colorimetric dual-mode system AOX/CeO₂@TPE + OPD toward CH₃SH. In this design, CH₃SH initiated AOX catalytic activity to convert it into H₂O₂ for activating the peroxidase-like activity of CeO₂@TPE, producing ^•OH for oxidizing the naked-eye colorless OPD into deep yellow 2,3-diaminophenazine (DAP) with an absorption enhancement at ∼425 nm, companied by a new emission peak at ∼550 nm to match with the intrinsic emission at ∼441 nm for observing ratiometric fluorescence response, enabling a ratiometric fluorescence-colorimetric dual-mode analysis. Interestingly, both the ratiometric fluorescence and colorimetric signals could be gathered for being converted into the hue parameter on a smartphone-based sensor, achieving the onsite visual fluorescence-colorimetric dual-mode detection of CH₃SH in real environmental media with acceptable results. This study gave a novel insight into designing target-responsive enzymatic cascade activation system-based efficient and reliable dual-mode point-of-care sensors for safeguarding environmental health.

A ratiometric fluorescent probe based on a dual-ligand lanthanide metal–organic framework (MOF) for sensitive detection of aluminium and fluoride ions in river and tap water

A dual-emission fluorescent probe towards Al3+ and F− using a Ln-MOF material Eu-BDC-NH2/TDA is employed with exceptional sensitivity, high selectivity, low LOD, excellent anti-interference characteristics and direct visual observation.

Novel Hydrazone Chromophore Sensor for Metallochromic Determination of Cadmium Ions

For the detection of Cd(II) in aquatic media, a novel dicyanomethylene dihydrofuran hydrazone(DCDHFH)-based colorimetric chemosensor was developed. DCDHFH was prepared by an azo-coupling process involving the diazonium chloride of 2, 4-dichloroaniline and a dicyanomethylene dihydrofuran heterocyclic moiety bearing an active methyl group. The DCDHFH chromophore showed strong solvatochromism depending on solvent polarity due to electronic delocalization. The pH sensory effects of the DCDHFH chromophore were also explored. DCDHFH could be used to identify Cd(II) in the presence of other competitive metals, as indicated by variations in color and absorbance spectra. In the presence of cadmium ions, the synthesized DCDHFH probe with hydrazone recognition moiety exhibited a significant sensitivity and selectivity to cadmium ions at the ppm concentration level (10–250 ppm). A DCDHFH-immobilized paper test strip was also prepared and effectively used for the detection of cadmium in aqueous media at various concentrations. According to CIE Lab’s criteria, colorimetric strength (K/S), and the UV–Vis absorbance spectra, the cadmium detection abilities of the DCDHFH-immobilized paper strips were evaluated. The optimal pH range for the determination of Cd(II) was monitored in the area of 5.5–6.3, with a fast chromogenic change from yellow to red relying on the Cd(II) concentration. The deposition of dicyanomethylene dihydrofuran hydrazone onto the paper strip’s surface was studied by scanning electron microscopy (SEM).

Cerium-terephthalic acid metal-organic frameworks for ratiometric fluorescence detecting and scavenging·OH from fuel combustion gas

Hydroxyl radical (•OH) in fuel combustion gas seriously damages human health. The techniques for simultaneously detecting and scavenging •OH in these gases are limited by poor thermal resistance. To meet this challenge, herein, metal organic frameworks (MOFs) with high thermal stability (80-400 °C) and dual function (•OH detection and elimination) are developed by coordinating Ce ions with terephthalic acid (TA) (Ce-BDC). Due to the reversible conversion between Ce³⁺ and Ce⁴⁺, and the high concentration of Ce³⁺ on the surface of Ce-BDC MOFs (89.6%), an •OH scavenging efficiency over 90% is realized. Ratiometric fluorescence (I_440 nm/I_355 nm) detection of •OH with a low detection limit of ∼4 μM is established by adopting Ce ions as an internal standard and TA as an •OH-responsive fluorophore. For real applications, the Ce-BDC MOFs demonstrate excellent •OH detection sensitivity and high •OH scavenging efficiency in gas produced from cigarettes, wood fiber and machine oil. Mouse model results show that the damage caused by •OH in cigarette smoke can be greatly reduced by Ce-BDC MOFs. This work provides a promising strategy for sensitively detecting and efficiently eliminating •OH in fuel combustion gas.

Paper-based LRET sensor for the detection of total heavy rare-earth ions

Based on the mechanism of luminescence resonance energy transfer (LRET) and using a special single strand DNA as the recognition element, a portable paper-based sensor for the accurate detection of total heavy rare-earth ions (mainly Gd3+, Tb3+ and Dy3+) concentration was proposed. The RNA cleaving-DNAzyme should recognize rare-earth ions to cleave RNA on DNA duplexes linking UCNPs and AuNPs, causing UCNPs and AuNPs to approach each other, inducing LRET, which attenuated the green upconversion luminescence (UCL) triggered by the 980 nm laser. UCL was captured by a charge-coupled device (CCD) image sensor and processed with the red-green-blue (RGB) image to quantitatively analyze heavy rare-earth ions in the samples. In the range of 5–50 μmol·L-1, the sensor has good sensitivity, with the limit of detection of 1.26 μmol L−1.

Dual-Color Fluorescent Hydrogel Microspheres Combined with Smartphones for Visual Detection of Lactate

Since it is difficult for human eyes to distinguish between two identical colors with only <15% variation in brightness, mono-color fluorescent hydrogel microspheres have some limitations in the detection of lactate. Herein, we prepared novel dual-color fluorescent hydrogel microspheres, which can achieve hue transformation. Microspheres were prepared by introducing a fluorescent nanoparticle as the reference signal while CdTe QDs were used as the response signal. We used smartphones with image processing software to collect and analyze data. In this way, the signal of lactate was converted to RGB (red, green, and blue) values, which can be quantitatively read. Within 10 to 1500 μM, the R/G values of the microspheres had a linear relationship with the logarithm of the lactate concentration. Moreover, color cards for lactate detection were prepared, from which the color change and concentration of lactate could be easily read by the naked eye. It is worth mentioning that this method was successfully applied to screen patients with hyperlactatemia.

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.

Reversible fluorescent test strip with red fluorescent carbon dots for monitoring water in organic solvents: Visual detection via a smartphone

Herein, a novel type of red-emitting carbon dots called TN-CDs was created via a one-step hydrothermal approach using neutral red and tartaric acid as raw materials. The fluorescence of TN-CDs was gradually quenched as the amount of water increased, and the color of the solution changed from yellow to pink mauve (or purple to pink). The reaction could be completed within only 5 s in various organic solvents such as N,N-Dimethylformamide (DMF), methanol (MeOH), acetonitrile (ACN), and ethanol (EtOH) with linear detection ranges of 1.2%-35.0%, 0.5%-20.0%, 0.25%-5.0% and 0%-16.0%, respectively. In addition, we prepared a reusable test strip and then combined it with TN-CDs to detect water content in DMF, as well as integrated it with smartphone software, a UV lamp, and a dark chamber for real-time, on-site, visual quantitative detection of the water content.

Ultra-sensitive detection of hydrogen peroxide and levofloxacin using a dual-functional fluorescent probe

Herein, a flower-shaped fluorescent probe was proposed for hydrogen peroxide (H₂O₂) and levofloxacin (LVF) sensing based on MoOx QDs@Co/Zn-MOFs with porous structure. Both MoOx QDs and Co/Zn-MOFs exhibited peroxidase-like properties, and the combination of them greatly aroused the synergistic catalytic capabilities between them. In o-Phenylenediamine (OPD)-H₂O₂ system, MoOx QDs@Co/Zn-MOFs efficiently catalyzed H₂O₂ to produce •OH and then oxidized OPD to its oxidation product (OxOPD). The OxOPD could not only emit blue fluorescence, but also inhibit the fluorescent intensity of MoOx QDs through fluorescence resonance energy transfer (FRET). Moreover, when introducing LVF into the system, the fluorescent intensities of MoOx QDs increased along with the aggregation of themselves while that of OxOPD remained unchanged, which was explained by the joint behavior of FRET and photo-induced electron transfer (PET) instead of the conventional aggregation-induced emission enhancement (AIEE). With these observation, the proposed probe was employed for H₂O₂ and LVF determination in biological samples with the limit of detection (LOD) of 32.60 pmol/L and 0.85 μmol/L, respectively, suggesting the method holds great promises for trace H₂O₂ and LVF monitoring in eco-environment.

Smartphone Case-Based Gas Sensing Platform for On-site Acetone Tracking

Gas sensor-embedded smartphones would offer the opportunity of on-site tracking of gas molecules for various applications, for example, harmful air pollutant alarms or noninvasive assessment of health status. Nevertheless, high power consumption and difficulty in replacing malfunctioned sensors as well as limited space in the smartphone to host the sensor restrain the relevant advancements. In this article, we create a smartphone case-based sensing platform by integrating the functional units into a smartphone case, which performs a low detection limit of 117 ppb to acetone and high specificity. Particularly, dimming glass-regulated light fidelity (Li-Fi) communication is successfully developed, allowing the sensing platform to operate with relatively low power consumption (around 217 mW). Experimental proof on harmful gas sensing and potential clinic application is implemented with the sensing platform, demonstrating satisfactory sensing performance and acceptable health risk pre-warning accuracy (87%). Thus, the developed smartphone case-based sensing platform would be a good candidate for realizing harmful gas alarms and noninvasive assessment of health status.

Temperature-Responsive Covalent Organic Framework-Encapsulated Carbon Dot-Based Sensing Platform for Pyrethroid Detection via Fluorescence Response and Smartphone Readout

In this paper, carbon dot (CD)-encapsulated 1,3,5-tris(4-formylphenyl)benzene (TFB)/2,5-dihydroxyterephthalohydrazide (DHTH) covalent organic frameworks (TDCOFs) grafted with thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) (CDs@TDCOFs@PNIPAM) were fabricated for the detection of pyrethroids. CDs@TDCOFs@PNIPAM achieved a temperature-responsive "on/off" detection of pyrethroids based on the target-triggered electron-transfer mechanism. The detection limit of pyrethroids was as low as 0.69 μg/L in the wide linear range of 5-400 μg/L (R² > 0.9523). Simultaneously, CDs@TDCOFs@PNIPAM with red, green, and blue (RGB) fluorescence emissions were integrated with a smartphone-assisted device, enabling the visual smart quantitative detection of pyrethroids with a detection limit of 4.875 μg/L. Ultimately, agricultural products were chosen as actual samples to verify the applicability of both recognition modes, and the calculated recovery rate was 105.48-113.40%. Accordingly, CDs@TDCOFs@PNIPAM featuring temperature-responsive switching behavior and RGB fluorescence emission provided a promising analytical strategy for ensuring agricultural and food safety.

An Ultrasensitive and Selective Determination of Cadmium Ions at ppt Level Using an Enzymic Membrane with Colorimetric and Electrochemical Detection

Cadmium ions (Cd²⁺) are extremely toxic heavy metal pollutants found in the environment, and which endanger human health. Therefore, it is critical to develop a sensitive and simple method for rapidly detecting Cd²⁺ in water samples. Herein, an enzymic membrane was developed based on an easy and rapid immobilization method of horseradish peroxidase (HRP), for determination of Cd²⁺ in drinking water. Hence, for the first time, an enzymic membrane was applied for the detection of Cd²⁺ without being pretreated. In the first format, the inhibition of horseradish peroxidase was performed using a colorimetric microplate reader. Under optimal conditions, the achieved limit of detection was 20 ppt. In addition, an electrochemical biosensor was developed, by combining the enzymic membrane with screen printed electrodes, which showed a linear calibration range between 0.02–100 ppb (R² = 0.990) and a detection limit of 50 ppt. The use of this enzymic membrane proved to be advantageous when reversible inhibitors such as the copper ion (Cu²⁺) were present in water samples, as Cu²⁺ can interfere with Cd²⁺ and cause erroneous results. In order to alleviate this problem, a medium exchange procedure was used to eliminate Cu²⁺, by washing and leaving only cadmium ions as an irreversible inhibitor for identification. The use of this membrane proved to be a simple and rapid method of immobilizing HRP with a covalent bond.

A novel signal amplification biosensor for detection of Cd2+ based on asymmetric PCR

In this work, a novel signal amplification biosensor was utilized to detect Cd²⁺ based on asymmetric PCR. In the presence of Cd²⁺, it can bind with Cd²⁺-aptamer C1 which caused the complementary strand C2 to be released from double-stranded DNA C1-C2. Because the single-stranded C1 cannot be hydrolyzed by Exo III, it can be used as a template to take part in asymmetric PCR reaction. In the absence of Cd²⁺, the C1-C2 was digested by Exo III and no PCR template was left. During the experiment, an interesting phenomenon was found that the asymmetric PCR can obtain higher level of fluorescent signal than that of symmetric PCR. To the best of our knowledge, this is the first report of using asymmetric PCR to detect Cd²⁺. Through the asymmetric PCR amplification strategy, this biosensor had a low detection limit (19.93 nM) and a wide linear range (0-500 nM). Meanwhile, this biosensor showed a satisfactory selectivity and recovery rate.

A ratiometric fluorescence platform based on carbon dots for visual and rapid detection of copper(II) and fluoroquinolones

A simple smartphone-integrated ratiometric fluorescent sensing system for visual determination of copper ions (Cu²⁺) and fluoroquinolones (FQs) was developed based on carbon dots (CDs) which were synthesized through the high-temperature pyrolysis of citric acid. In this system, with the fluorescence resonance energy transfer effect between CDs and 2,3-diaminophenazine (oxOPD), the detection of Cu²⁺ and ciprofloxacin (CIP, an example for FQs) was realized. Cu²⁺ catalyzes the oxidation of OPD to form oxOPD with yellow fluorescence, resulting in the quenching of CDs. In addition, CIP can inhibit the catalytic activity of Cu²⁺ and induce the recovery of CDs fluorescence. Under the excitation of 400 nm, the changes of CDs fluorescence at 472 nm and oxOPD fluorescence at 556 nm were monitored. The detection results showed that the sensing system exhibited good selectivity and sensitivity to Cu²⁺ and CIP with the limit of detection of 2.32 × 10^-8 mol L^-1 and 0.2 ng mL^-1, respectively. In addition, a smartphone was developed as a portable analyzer to capture the change of fluorescence color and quickly analyze the concentration of Cu²⁺ and CIP. The proposed smartphone-based sensing platform has satisfactory sensitivity, and it has application prospects for detecting Cu²⁺ and FQs in food safety monitoring.

A quinoline-benzothiazole-based chemosensor coupled with a smartphone for the rapid detection of In3+ ions

A newly designed quinoline-benzothiazole probe 2-((Z)-((E)-benzo[d]thiazol-2(3H)-ylidenehydrazono)methyl)quinolin-8-ol (L) was synthesized by reacting 8-hydroxyquinoline-2-carbaldehyde with 2-hydrazinobenzothiazole and structurally characterized by various spectroscopic techniques. The sensing ability of probe L was studied with various cations using colorimetry, test paper strips, a red-green-blue (RGB) model and UV-visible spectrophotometry in DMSO : H₂O (3 : 7, v/v). The pale yellow colour of L turns into orange on contact with In³⁺ ions, whereas other tested metal ions did not show any change in colour. The probe L exhibits an absorbance band at 360 nm due to ligand-to-ligand charge transfer (LLCT); upon interaction with In³⁺ ions, it exhibits a band at 445 nm due to ligand-to-metal charge transfer (LMCT). The probe L binds In³⁺ in a 2 : 1 ratio with an association constant of 8.1 × 10⁵ M^-1 and this is established using the Job's and Benesi-Hildebrand (B-H) methods. The probe L can work in the pH range of 4-8 without interfering with other competing ions. It can be used to detect quantities as low as 2.3 ppb and 85 ppb by spectrophotometry and RGB, respectively. The binding mechanism was studied by ¹H NMR titration, ESI mass and FT-IR spectral analysis and well supported by theoretical studies. Overall, probe L demonstrates promising potential for the detection of In³⁺ ions in the semi-aqueous phase and this is its first report as a colorimetric chromogenic probe.

Simple Design Concept for Dual-Channel Detection of Ochratoxin A Based on Bifunctional Metal-Organic Framework

A simple fluorescence and electrochemical dual-channel biosensor based on bifunctional Zr(IV)-based metal-organic framework (Zr-MOF) was proposed to detect Ochratoxin A (OTA). The bifunctional Zr-MOF, with photoluminescence properties and enormous electroactive ligands, was exploited to load OTA-specific aptamers for designing signal probes, greatly simplifying the probe-fabrication process and improving sensing reliability. Upon specific recognition of aptamer toward OTA, the anchored probe was released from the sensing interface into the reaction solution. In this circumstance, the increased amount of the signal probe in reaction solution led to an enhanced fluorescence response, while the decreased amount of the signal probe on the sensing interface resulted in a diminished electrochemical response. According to the dual-channel signal change with increasing OTA concentration, the visual fluorescence strategy was established for intuitive OTA detection, and meanwhile, sensitive electrochemical assay with a detection limit of 0.024 pg/mL was also achieved with the help of one-step electrodeposition as a sensing platform. Moreover, the proposed dual-channel assay has been successfully applied to determine OTA levels in corn samples with rapid response, superior accuracy, and high anti-interference capability, providing a promising method for food safety monitoring.

Rapid fluorescent color analysis of copper ions on a smart phone via ratiometric fluorescence sensor

A smartphone-assisted fluorescence color sensing system for rapid, convenient, and on-site detection of copper ions was developed. The ratiometric fluorescence sensor was fabricated by using silica-coated blue-light-emitting carbon dots and surface-grafted red-light-emitting cadmium-telluride quantum dots. After exposure to Cu²⁺ in 20 s, the red fluorescence was quenched obviously, while the blue fluorescence remained unchanged, and the sensor color changes continuously from red to blue under the ultraviolet lamp. The concentration (50-1200 nM) of copper ions could be measured by the fluorescence spectrum (excitation at 360 nm, dual-emission at 441 and 640 nm) with a detection limit of 7.7 nM. The fluorescence colors were converted to digital RGB values to calculate the concentration of copper ions by a smartphone with a detection limit of 9.6 nM. The method was applied to detecting copper ions spiked in real samples with recovery from 97.9 to 108.0% and RSD from 3.8 to 8.9%. Thus, this convenient and practical fluorescence color sensing system presents a new strategy for rapid, sensitive, and on-site determination of copper ions in environmental or biological samples.

A dual-signal fluorescent sensor based on MoS2 and CdTe quantum dots for tetracycline detection in milk

A dual-signal fluorescent sensor was developed for tetracycline (TET) detection in milk with excellent reproducibility and stability. In this protocol, molybdenum disulfide quantum dots (MoS₂ QDs) with blue fluorescence and cadmium telluride quantum dots (CdTe QDs) with yellow fluorescence were synthesized to establish the MoS₂/CdTe-based sensor with two fluorescence emission peaks at 433 nm and 573 nm. With the addition of TET, the fluorescence of MoS₂/CdTe were quenched by photoinduced electron transfer (PET), and the fluorescence of CdTe QDs were quenched more obvious than MoS₂ QDs. With the strategy, a calibration curve was established between the TET concentration in the range of 0.1-1 μM and the ratio of fluorescence intensity at 573 nm and 433 nm (F₅₇₃/F₄₃₃). Furthermore, the dual-signal sensor was applied for TET detection in milk samples with the recovery of 95.53-104.22% and the relative standard deviation (RSD) less than 5%, indicating the strong application potential.

Research progress in synthesis and biological application of quantum dots

Quantum dots are an excellent choice for biomedical applications due to their special optical properties and quantum confinement effects. This paper reviews the research and application progress of several quantum...

A highly selective fluorescence and absorption sensor for rapid recognition and detection of Cu2+ ion in aqueous solution and film

A fluorescence and absorption chemosensor (SAAT) based on 5-(hydroxymethyl)-salicylaldehyde (SA) and o-aminothiophenol (AT) was designed and synthesized. SAAT in DMSO-HEPES (20.0 mM, v/v, 1:99, pH=7.0) solution shows a highly selective and sensitive absorption and "on-off" fluorescence response to Cu²⁺ ions in aqueous solutions over all other competitive metal ions including Na⁺ , Ag⁺ , Ba²⁺ , Ca²⁺ , Cd²⁺ , Mg²⁺ , Zn²⁺ , Cr³⁺ , Al³⁺ , Hg²⁺ , K⁺ , Mn²⁺ , Ni²⁺ , Sr²⁺ , Tb³⁺ and Co²⁺ . SAAT exhibits ratiometric absorption sensing ability for Cu²⁺ ions. Importantly, SAAT also can sense Cu²⁺ ions by fluorescence quenching, the fluorescence intensity of SAAT showed a good linear relationship with Cu²⁺ concentration, and the detection limit of Cu²⁺ was 0.34 μM. The results of Job's plot, Benesi-Hildebrand plot, mass spectra, and DFT calculations confirmed that the selective absorption and fluorescence response were attributed to the formation of 1:1 complex between SAAT and Cu²⁺ . SAAT in test film can identify Cu²⁺ in water samples by the intuitive fluorescence color change under UV lamp. SAAT has great application value as a selective and sensitive chemosensor to discrimination and detection of Cu²⁺ ions.