Naked-eye Detection Research Articles

Fundamentals of Image-Based Assay (IBA) System for Affordable Point of Care Diagnostics

Cost-effective, high-performance diagnostic methods are a need for accessible, affordable, and high-quality healthcare in both developed and developing countries. Latest technological developments have steered the growth in the development of novel and rapid diagnostic tests having the potential for the better management and control of different diseases. However, most of these tests/assays rely on visual or naked-eye detection which is limited to qualitative or semi-quantitative analysis. Further, such detection approaches often lack consistency. The increase in adaptation of image-based assays (IBA) and point-of-care diagnostics is motivated by the above-mentioned need for quality quantitative assays with consistency apart from no requirement for trained technicians. In this review, we have focused on the basic requirements for an image-based assay system for point-of-care diagnostics which include: (i) matrices/substrates as a reaction zone, and (ii) the analytical tools for the quantification of such assays. Thousands of literature reports about image-based assays have been published and various substrates used in this context are available. Suitable applications of each substrate and the technique by which they are digitized are discussed. Also, the progress in affordable diagnostic technologies, the specific quantification methodology used, and the fusion of new and improved devices for diagnostics and clinical applications, associated challenges and ways of improvement have been discussed in the present work. Through this review, it is expected that researchers will get a clear perception of the image-based assay system, its benefits, challenges, and future improvements necessitated in achieving the ultimate purpose of managing disease and diagnostics cost-effectively.

CRISPR-Cas12-based field-deployable system for rapid detection of synthetic DNA sequence of the monkeypox virus genome.

The global outbreak of the monkeypox virus (MPXV) highlights the need for rapid and cost-effective MPXV detection tools to effectively monitor and control the monkeypox disease. Herein, we demonstrated a portable CRISPR-Cas-based system for naked-eye detection of MPXV. The system harnesses the high selectivity of CRISPR-Cas12 and the isothermal nucleic acid amplification potential of recombinase polymerase amplification. It can detect both the current circulating MPXV clade and the original clades. We reached alimit of detection (LoD) of 22.4 aM (13.5 copies/µl) using a microtiter plate reader, while the visual LoD of the system is 75 aM (45 copies/µl) in a two-step assay, which is further reduced to 25 aM (15 copies/µl) in a one-pot system. We compared our results with quantitative polymerase chain reaction andobtained satisfactory consistency. For clinical application, we demonstrated a sensitive and precise visual detection method with attomolar sensitivity and a sample-to-answer time of 35 min.

A novel TMD-based peroxidase-mimicking nanozyme: From naked eye detection of leukocytosis-related diseases to sensing different bioanalytes

Being emerged as alternatives to natural enzymes, nanozymes have recently drawn much attention in sensing. Herein, the first multicomponent transition metal dicalchogenide (TMD)-based nanozyme (MCFS/rGO) was synthesized by a facile hydrothermal method and characterized. This peroxidase-mimic nanozyme follows the typical Michaelis-Menten kinetics, showing a higher affinity for H2O2 substrate (Km = 9 μM) compared to that of natural peroxidase (Km = 3700 μM). The remarkable potential of the MCFS/rGO nanozyme to detect H2O2 provided us with a great opportunity to design some simple and fast colorimetric sensing systems. Coupling the efficient peroxidase-mimicking activity of the nanozyme with the H2O2 production capacity of white blood cells (WBCs) leads to the development of a novel, simple, rapid, and efficient colorimetric method to distinguish leukocytosis-related patients from healthy people by the naked eye. This pioneering diagnostic technique can also be utilized to quantitatively measure the WBC count. Moreover, we coupled the mentioned nanozyme-based system with the activity of glucose oxidase enzyme available in different types of honey samples, an innovative mechanism proved to be an effective quality indicator of the samples. Last but not least, the MCFS/rGO nanozyme is also able to determine the quantity of some biologically significant analytes, including glutathione (GSH), ascorbic acid (AA), and mercury ions (Hg2+), of which the limit of detection (LOD) was 9.3 nM, 22.5 nM, and 0.32 μM, respectively. Our results, however, demonstrated the superior performance of the MCFS/rGO nanozyme to determine the first two mentioned bioanalytes compared with other TMDs. Overall, this novel nanozyme-based sensor system can be considered a suitable candidate for developing multipurpose biosensors for medical and biochemical applications.

A novel polymeric nanocomposite based on TiO2 NPs containing β-cyclodextrin membrane for ultratrace naked-eye detection of Pb and Cd ions

A unique, selective, and sensitive colorimetric nanosensor for the naked-eye detection and adsorption of Pb(II) and Cd(II) in aqueous media has been synthesized based on the organic–inorganic hybrid. Acrylamide (AM) and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) as hydrophilic monomers, functionalized β-cyclodextrin (f-β-CD) as host-guest complex formation agent, and vinyl-dithizone (f-DTZ) as an efficient ligand were used. The β-CD with a hydrophobic interior and hydrophilic exterior can form a host-guest inclusion complex with a ligand that enhances nanosensors’ dispersion in aqueous media. The immediate color change of the sensor has been observed from gray to pink and blue for Pb(II) and Cd(II) with a wide linear range of [(1 ppb- 40 ppb)] and [(1 ppb- 60 ppb)], respectively. This study also considered the adsorption qualities of these heavy metals under various factors, including pH, time length, and initial ion concentration. The maximum adsorption capacity of Cd(II) via polymeric nanocomposite was 166 mg·g−1 at optimum condition. The experimental info has been investigated using the Langmuir and Freundlich isotherms and the pseudo-first-order and pseudo-second-order kinetic models.

The Eschenmoser's Salt as a Formylation Agent for the Synthesis of Indolizinecarbaldehydes and Their Use for Colorimetric Nitrite Detection.

C-H bond formylation is the most immediate way to incorporate the versatile formyl group into (hetero)aromatics. However, the type of reagents and severe conditions involved in the classical formylation methods often curtail their application, especially in the presence of other functional groups. Herein, we present the Eschenmoser's salt, a commercially available (dimethylamino)methylating chemical, as a useful reagent for the C-H formylation of indolizines and other compounds. The method is straightforward and mild, furnishing indolizinecarbaldehydes in modest-to-good yields with exclusive and remote regioselectivity. Furthermore, these compounds can be easily transformed into push-pull dyes and are highly selective in the colorimetric detection of nitrite, a substance extensively employed as preservative in the food industry, the concentration of which is crucial to control to prevent harmful effects in living organisms. The assay is simple, allowing the naked-eye detection of nitrite in solution or on a cotton swab for a wide range of concentrations.

Understanding the aggregation of excitation wavelength independent emission of amphiphilic carbon dots for bioimaging and organic acid sensing

Herein, excitation wavelength-independent, tunable emissive and amphiphilic CDs with high quantum yield were synthesized by a low-temperature oxidation method employing banana peel waste as a carbon source. These CDs showed longer wavelength emissions (green to yellow) independent of the excitation wavelength when dispersed in different polar to non-polar solvents. The quantum yields of the same CDs were 9–32% in different solvent polarities for different emissions. On the other hand, a large stokes-shifted emission (∼9606 cm−1) was observed for CDs in the non-polar and weak polar solvents. The particle size of CDs increases from a hydrophobic to a hydrophilic environment with the change in emission colour from yellow to green. A polar and a non-polar host matrix were used to overcome the limitation of aggregation-caused quenching of CDs in the solid state to obtain bright emissions. These CDs were potentially used for naked-eye detection of trifluoroacetic acid (TFA) by changing the emission colour from yellow to orange under UV 365 nm. Sensing of TFA was also shown reversibly switch emission colour and average lifetime for multiple cycles. Additionally, the highly emissive CDs show negligible cytotoxicity in 3T3 fibroblast cells, indicating possible bioimaging applications in 3T3 cells.

A “turn on” colorimetric system based on bimetallic UiO-66(Zr/Ce) for rapid sensing of tetracyclines through visible light triggering

In this work, a novel “turn on” colorimetric system based on UiO-66(Zr/Ce) for rapid sensing of tetracyclines through visible light triggering was facilely constructed. The construction of the system was significantly simplified without H2O2 addition and probe modification. It is also more economical and milder without high temperature requirement due to visible light triggering. Benefiting from the synergistic effect of its great tetracyclines adsorption performance and highly efficient photogenerated carrier separation, massive reactive oxygen species (ROS) can be generated rapidly and 3,3′,5,5′-tetramethylbenzidine (TMB) can be oxidized quickly, leading to a much faster visual naked-eye detection of tetracyclines within 2 min, nearly 5–45 times shorter than that of the reported methods. Moreover, the system can be used for the detection of tetracyclines in real samples accurately and reliably, exhibiting its bright potential for field or online monitoring of tetracyclines. We believe our work will provide a facile, mild, and effective strategy for the rapid naked-eye visualization detection of antibiotics in the environment, as well as present a novel methodology for rapid sensing of a certain target based on TMB chromogenic reaction.

A sensitive detection approach for Cu(II) ions in water samples using a solid-state chemical sensor

ABSTRACT We describe the creation of a solid-state optical sensor that is quick to respond, sensitive, and reliable for detecting and determining Cu(II) ions in both natural and wastewater sources. The solid-state template was prepared with mesoporous silica nanospheres of capacious porosity, high surface area, and uniform surface morphology. N,N’-bis(3-carboxysalcylidene)-4-chloro-1,2-phenylenediamine (CSCPD), a chromoionophoric probe with exclusive ion-selectivity and sensitivity for Cu(II), is created for use in naked-eye colorimetric detection. The mesoporous silica nanospheres (MSNs) template displayed several diffusible sites for the uniform anchoring of the CSCPD probe molecules, which serve as a colorimetric ion sensor for the naked eye. The surface profile and structure of the MSNs and the CSCPD sensor materials are studied using BET/BJH, XPS, FESEM, p-XRD, and HR-TEM analysis. With various concentrations of Cu(II), the CSCPD probe-anchored MSNs material reciprocates a vivid visual colour shift from an initial pale yellow colour to green with a reaction period of around one minute. In addition, the MSNs are simply covered with probe molecules as part of the sensor production process. The certified limits of detection for Cu(II) ions for the proposed CSCPD sensor is 1.57 ppb (2.48 × 10−8 M). The suggested CSCPD sensor is dual-functional in that it may be used as a concentrator to recover ultra-trace quantities of Cu(II) ions as well as an effective ion-sensing device.

Differential sensitization toward lanthanide metal–organic framework for detection of an anthrax biomarker

A novel Tb-doped Eu-based metal-organic framework (Eu-MOF@Tb) has been developed by incorporating hexanuclear europium cluster and 2,2'-bipyridine-5,5'-dicarboxylic acid as well as coordination with Tb(III). Owing to the diverse coordination status of Tb(III) and Eu(III) in MOF, antenna effect emission from Tb(III) can be invoked by dipicolinic acid (DPA), but the luminescence originating from Eu(III) remains unchanged. Taking advantage of this phenomenon, a ratiometric luminescent method for detection of DPA, a biomarker for Bacillus subtilis spores, was developed through differential sensitization toward lanthanide ions. This analysis method allowed for the detection of DPA in the 0.2-10 μM concentration range, with a detection limit of 60nM. It was further validated by spiked recoveries (89.3-110%) of real-world samples with RSD values in the range 3.9-11%. Alongside this, a paper indicator test was prepared for naked-eye detection of DPA via a dose-sensitive color evolution from red to green under UV light. The effectiveness of the proposed approach was explored in the detection of bacterial spores in real biological and environmental samples and indicated great potential for applications as a real-time monitoring system against the anthrax threat.

Azo-hydrazone tautomerism in a simple coumarin azo dye and its contribution to the naked-eye detection of Cu2+ and other potential applications

A new tailor-made azo dye of coumarin connected to phenolic derivative is presented herein. Azo-hydrazone tautomerism in aqueous solution of the dye was observed and studied using spectroscopic assays such as 1H NMR, absorption and emission assays, and theoretical studies. Tautomerism was attributed to the presence of a labile phenolic hydrogen in the ortho position to the azo functionality and the hydrazone was found to be the more dominant tautomer. Influence of metal ions on the azo-hydrazone chemical equilibrium and how the accompanying colour and spectroscopic changes can be exploited for various functions, especially the detection and quantification of Cu2+ in aqueous environments was explored. The presence of Cu2+ affects the azo-hydrazone equilibrium resulting in visual appearance and spectroscopic changes and the likely binding sites for Cu2+ were evaluated. Cu2+ pushes the azo-hydrazone equilibrium towards the more conjugated form and the presence of other metal ions does not have any perceivable impact on this mechanism. The dye showed potential applications as a sensor in colorimetric and spectroscopic detection and quantification of Cu2+ in domestic and environmental water samples, photo-imprinting and as a logic gate. The limits of detection (LOD) and quantification (LOQ) for Cu2+ were found to be 0.0779 mg/L and 0.236 mg/L, respectively, much lower than the World Health Organization (WHO) guideline limit for Cu2+ levels in drinking water.

A novel and simple naphthol azo dye chemosensor as a naked eye detection tool for highly selective, sensitive and accurate determination of thiourea in tap water, juices and fruit skins

In the present study, a highly accurate and sensitive azo-dye-based colorimetric sensor based on Eriochrome Black T (EBT) was proposed to detect and determine thiourea (TU). TU is truly an important toxic and carcinogenic hazardous pollutant as approved by EPA and IARC. This chemosensor shows a distinct color change from blue to pink during interaction with TU in aqueous medium. So EBT is capable as an applied tool for naked eye detection of TU as its color change is easily observed without any means. The sensing mechanism was also investigated using UV–vis absorption and FT-IR spectra. The linear range and the detection limit of TU sensing were respectively 0.15–18.5 μmol/L and 0.02 μmol/L. In addition, the relative standard deviation (RSD) based on ten repetitions calculated for two different TU concentrations 4.4 and 9.0 μmol/L were 2.3 % and 1.8 %, respectively. Besides its useful application as a naked eye detection tool, the advantages of the developed method include simplicity, elimination of tedious separation and pre-concentration steps, executable in neutral aqueous media, low costs, high accuracy, linear response for wide range of concentrations, low detection limit, high sensitivity, compatibility, and excellent selectivity. The concentration of TU in tap water, fruit juices or fruit skin samples can be visually detected and determined easily using this method. The results showed that EBT is an ideal colorimetric chemosensor for TU, which has been reported for the first time.

Solid-state naked-eye sensing of Cu(II) from industrial effluents and environmental water samples using probe integrated polymeric sensor materials

We report on the fabrication of two eco-friendly and fast-responsive solid-state optical sensors for the reliable sensing of Cu(II) ions from natural and industrial wastewater. The solid-state template constitutes indigenously prepared bimodal macro-/meso-porous channels of a monolithic polymer framework of uniform surface morphology, high surface area, and capacious porosity. For the naked-eye colorimetric detection, two chromoionophoric probes i.e., 4-butyl-N-((1,3-dioxo-1H-benzo[de]isoquinolin-2(3H)-yl)carbamothioyl)benzamide (BDICB) and 4-butyl-N-(2-(thiophene-2-carbonyl)hydrazine-1-carbonothioyl)benzamide (BTHCB) that offer exclusive ion-selectivity and sensitivity for Cu(II) are synthesized. The monolithic template exhibits multi-diffusible display sites for the uniform anchoring of the probe molecules that act as naked-eye colorimetric ion sensors. The structural and surface profile of the monolithic sensor materials are studied using HR-TEM, FE-SEM, EDAX, SAED, p-XRD, XPS, FT-IR, and BET/BJH analysis. The probe-anchored monolithic materials reciprocate a brilliant visual color shift from an initial colorless state to apple green and greyish black, respectively, for BDICB and BTHCB anchored sensor materials, with varying concentrations of Cu(II), with a response time of ≤1 min. Besides, the sensor fabrication pursues a simple direct encapsulation of probe molecules onto the polymer monolith. The proposed solid-state sensors are certified using simulated and real samples that reflect a reliable detection limit of 0.11 & 0.14 ppb and a quantification limit of 0.38 & 0.45 ppb for Cu(II) ions, respectively, using BDICB & BTHCB anchored polymer monolith sensors. The proposed solid-state colorimetric ion-sensors exhibit dual functionality in serving as a viable ion-sensing tool and concentrator for recovering ultra-trace concentrations of Cu(II) ions.

Supramolecular nanoarchitectonics with TPPS porphyrin as a fluorescent probe for detection of aminoglycoside antibiotics and their photocatalytic applications for the degradation of rhodamine B dye

The meso-tetra(4-sulfonatophenyl)porphyrin (TPPS) porphyrin was successfully synthesized and applied as a selective turn-off sensor for detecting neomycin and gentamicin antibiotics in aqueous media. The sensing ability of TPPS porphyrin was studied by naked-eye detection as well as UV–vis absorption and fluorescence spectroscopy. The emission intensity of TPPS porphyrin was observed to be decreased dramatically upon addition of neomycin and gentamicin antibiotics. The quantum yield of TPPS porphyrin (56%) which decreases to 2% and 9% upon addition of neomycin and gentamycin, respectively. The quenching efficiency of neomycin and gentamicin towards the TPPS porphyrin was 99.13% and 96% respectively. The detection limit was calculated to be 7.5 × 108 M and 1.1 × 107 M for gentamicin and neomycin respectively, indicating both the antibiotics exhibited efficient quenching abilities. Moreover, the self-assembly of TPPS porphyrin with gentamicin antibiotic have been investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD) and FTIR spectroscopy. Interestingly, TPPS porphyrin with 0–10 equiv. of gentamicin, produces nanorod-like structures. The key driving force for the formation of self-assembled nanostructure of TPPS porphyrin with gentamicin is H-bonding between sulfonate group of TPPS and –NH2 and –OH group of gentamicin. The-self-assembled nanostructure exhibited band gap energy in the range of 2.7 to 3 eV. Therefore, these nanostructures were used as a photocatalyst for degradation of RhB dye. Interestingly, nanostructure obtained from 1:2 equiv. of TPPS: gentamicin showed excellent photocatalytic performance, degraded almost 100% of RhB dye in aqueous media within 240 min. with simulated visible light irradiation with rate constant of 2 × 10−2 min−1. A possible mechanism for the photodegradation of RhB using nanorod like structure is also discussed.

"Enzyme-Like" Spatially Fixed Polyhydroxyl Microenvironment-Activated Hydrochromic Molecular Switching for Naked Eye Detection of ppm Level Humidity.

The detection and monitoring of ultralow humidity (<100ppm) are critical in many important industries, such as high-tech manufacturing, scientific research, and aerospace. However, the development of ppm level humidity sensors with portability, low cost, and ease of regeneration remains a significant challenge. Herein, an innovative "enzyme-like" construction strategy is proposed to address this problem by employing suitable molecular-level humidity-sensitive units and chemically constructing a multilevel spatial synergistic sensitization microenvironment around it. The as-prepared ultralow humidity-sensitive paper (UHSP) achieved a naked eye recognition humidity of 0.01-100ppm. UHSP not only is simple to prepare, handy and low-cost, but can also be simply and efficiently regenerated as well as recycled many times by skillfully utilizing the "unconventional sublimation" and "lime slaked" of calcium oxide. The molecular reaction mechanisms involved in the humidity response and regeneration of UHSP have been demonstrated in detail. UHSP can provide a promising new method for ultralow humidity detection in the form of portable kits or sirens. The demonstrated "enzyme-like" construction strategy can bring unlimited ideas and implications to the design and development of sensors with tunable response thresholds, particularly high sensitivity.

Naked-Eye Optical Recognition of Ammonia Vapor and Melamine in Water Using a Fluorophore Appended Polymer Matrix.

The generation of solid-state emitters is a challenge due to the intrinsic aggregation-caused quenching feature of the fluorophores. A conformationally twisted pyridyl π-conjugate as a solid-state emitter is appended with well-known and inexpensive poly(methylmethacrylate) [PMMA] to afford a handy, portable, and reusable solid-state emitting polymer matrix. Entrapment of the probe is noticed through non-covalent interactions, resulting in a green-emitting platform. It quickly accepts a proton upon acid vapor exposure and switches emission from green to red with a significant 107nm redshift. This shift is reversible with red to green emissions while exposed to base vapor. Thus, polymer-blended, homogeneous red-emitting pyridyl salt is employed as potential material to detect various basic vapors optically. Among different bases, naked-eye detection of essential analytes such as ammonia vapor and melamine shows potential demands. Hence, we have established an easy detection of ammonia vapor and aqueous melamine as low as 2.5 and 0.126ppm, respectively, using this solid-state emitter that displays an emission color change with an enhancement of emission intensity even in an aqueous solution.

A Coumarin-azo Derived Colorimetric Chemosensor for Hg2+ Detection in Organic and Aqueous Media and its Extended Real-world Applications.

Pollution caused by the release of toxic heavy metals into the environment by industrial and farming processes has been regarded as a major problem worldwide. This has attracted a great deal of attention into restoration and remediation. Mercury is classified as a toxic heavy metal which has posed significant challenges to public and environmental health. To date, conventional methods for mercury detection rely on expensive, destructive, complex, and highly specialized methods. Evidently, there is a need to develop systems capable of easily identifying and quantifying mercury within the environment. In this way, organic-based colorimetric chemosensors are gaining increasing popularity due to their high sensitivity, selectivity, cost-effectiveness, ease of design, naked-eye, and on-site detection ability. The formation of coumarin-azo derivative AD1 was carried out by a conventional diazotization reaction with coumarin-amine 1c and N,N-dimethylaniline. Sensor AD1 displayed remarkable visual colour change upon mercury addition with appreciable selectivity and sensitivity. The detection limit was calculated as 0.24µM. Additionally, the reversible nature of AD1 allowed for the construction of an IMPLICATION type logic gate and Molecular Keypad Lock. Chemosensor AD1 displayed further sensing applications in real-world water samples and towards on-site assay methods. Herein, we describe a coumarin-derived chemosensor bearing an azo (N = N) functionality for the colorimetric and quantitative determination of Hg2+ in organic and aqueous media.

8-Hydroxyquinoline-BODIPY based dual mode pH probe: Intuits acidic and basic environments through two different mechanisms

A new BODIPY based optical pH probe that effectively distinguishes acidic and basic solutions through opposite fluorescence responses was designed and subsequently synthesized. The fluorescence response in basic medium was governed by photoinduced electron transfer (PeT). Effective PeT quenched the emission of the probe in basic pH while it was inhibited in acidic pH. The visible colour change on addition of acidic and basic solutions enabled naked eye detection of pH. Comparing the optical property of this probe with other analogous compounds it was realized that substituent at BODIPY has significant impact on possibility of PeT from excited BODIPY moiety. Electron donating substituent facilitates the d-PeT (donor) of excited BODIPY moiety and absence of which inhibits such transition. These experimental observations were validated with theoretical study. The mechanism of pH sensing was also investigated by 1H NMR titration experiment. Interestingly, probe was found to be responsive to pH change in the entire range of pH. Moreover, the probe can reversely detect the pH change through absorption which allows extension of pH sensing in real life samples.

Thickness-Sensing Sandwiched Plasmonic Biosensors Enable Label-Free Naked-Eye Antibody Quantification.

Clinical serology assays for detecting the antibodies of the virus are time-consuming, are less sensitive/selective, or rely on sophisticated detection instruments. Here, we develop a sandwiched plasmonic biosensor (SPB) for supersensitive thickness-sensing via utilizing the distance-dependent electromagnetic coupling in sandwiched plasmonic nanostructures. SPBs quantitatively amplify the thickness changes on the nanoscale range (sensitivity: ∼2% nm-1) into macroscopically visible signals, thereby enabling the rapid, label-free, and naked-eye detection of targeted biomolecular species (via the thickness change caused by immunobinding events). As a proof of concept, this assay affords a broad dynamic range (7 orders of magnitude) and a low LOD (∼0.3 pM), allowing for the extremely accurate SARS-CoV-2 antibody quantification (sensitivity/specificity: 100%/∼99%, with a portable optical fiber device). This strategy is suitable for high-throughput multiplexed detection and smartphone-based sensing at the point-of-care, which can be expanded for various sensing applications beyond the fields of viral infections and vaccination.

Extraction-free techniques for sensitive detection of a petroleum marker on a paper-based platform

Fuel adulteration and cross-contamination lead to low-quality fuel products and loss of taxes. It is essential to develop methods of marking, identifying and monitoring tax classification of petroleum products in the market. In this work, a method for sensitive and fast detection of gasoline marker using paper-based platform and a smartphone camera detection are developed. A common dye, phenolphthalein (PhP), is used as a colorless dye marker in commercial gasoline. The color was developed by the interaction of PhP with NaOH on filter paper strips and evaluated by CMYK color systems. The newly developed counter-flowing technique, in which a gasoline sample containing PhP and NaOH solution are allowed to travel on a filter paper strip from the opposite end, gives the naked eye detection of PhP as low as 1 ppm with the limit of detection for a smartphone camera of 0.31 ppm. The high sensitivity is attributed to the preconcentration derived from accumulation of PhP at the immiscible liquid interface. The counter-flowing method can thus be a new facile analytical technique for enhancing chemical detection sensitivity.

2-Hydroxy-naphthalene hydrazone based dual-functional chemosensor for ultrasensitive colorimetric detection of Cu2+ and highly selective fluorescence sensing and bioimaging of Al3+

2-Hydroxy-naphthalene hydrazone based probe HNP demonstrated high selectivity towards sensing of Cu2+ and Al3+ in colorimetric and fluorimetric methods respectively. HNP can be used as “naked-eye” sensor based on the change of its color from very light yellow to bright yellow (for Cu2+) and colorless (for Al3+). The colorimetric detection mechanism for Cu2+ involved enhancement in internal charge transfer (ICT). In the detection of Al3+ through fluorescence, combined effect of ICT and excited-state intramolecular proton transfer (ESIPT) played major role. Job’s plots based on UV–vis (Cu2+) and fluorescence (Al3+) data along with mass spectral study indicated 1:1 metal to HNP stoichiometry. The binding of the probe HNP with Al3+ and Cu2+ was found chemically reversible on addition of EDTA. Binding mode of the probe with the metal ions was proposed based on spectroscopic data and density functional theory (DFT). Moreover, effect of pH and response time was also determined. The probe was found useful for in vivo detection and imaging of Al3+ in MDA-MB-231 cells. Paper strips of the probe were made and used for naked eye detection of Cu2+ and Al3+. The probe was also used in the construction of logic gate for detection of Al3+.