On-site Visual Detection Research Articles

Aggregation induced emission based “turn-off” fluorescent chemosensor for selective and swift sensing of mercury (II) ions in water

An Anthracene based fluorescent probe, 1-(Anthracen-2-yliminomethyl)-napthalen-2-ol (AIN), was synthesized via simple one-pot reaction from inexpensive reagents. Intriguingly, it exhibited novel AIE characteristics in H2O-THF mixtures at high water content. Size and growth process of particles in different volume percentage of water and THF mixtures were investigated using TRPL, DLS, SEM and optical microscope. The fluorescent aggregates exhibited distinct Hg2+-selective “on-off” type fluorescence switching upon binding. Additionally, the fluorescence quenching showed a linear correlation in the Hg2+ concentration range from 0.3 to 3.6 μM and the limit of detection was estimated as low as ∼3 ppb. The drastic decrease in emission intensity of AIN aggregates was observed in the presence of mercury ions via a complex interplay through the ground state complexation between AIN and Hg2+ ions and external heavy atom induced perturbation by Hg2+ ions to the excited states of the AIN. A large Stern–Volmer quenching constant was estimated to be 2.54 × 105 M−1. The proposed analytical system with clear AIE characteristics demonstrates a potential outlook for the on-site visual detection of Hg2+ in paper strips. Finally, the sensory probe was also applied to determine the concentration of Hg2+ in real water samples to demonstrate its practical applications.

“ON-OFF” novel fluorescent chemosensors based on nanoaggregates of triaryl imidazoles for superselective detection of nitro-explosive trinitrophenol in multiple solvent systems

Novel fluorescent chemosensors with a triaryl-substituted imidazole (TAI) moiety were synthesized, which exhibited strong fluorescence. These fluorescent chemosensors allowed highly sensitive and selective detection of nitroaromatic explosive, trinitrophenol (TNP). The TAI were successfully transformed to fluorescent organic nanoparticles (FONPs) in aqueous medium using re-precipitation method thus enabling fluorescent based detection of TNP in aqueous medium also. We hereby present simple and selective chemosensors that are highly sensitive towards TNP and not-sensitive towards interfering analytes. In light of Stern-Volmer plots, Benesi–Hildebrand plots and 1H NMR titrations; it was confirmed that there exist competent interactions between the chemosensors and TNP. A detailed study revealed that synergistic effect of photo-induced electron transfer, electrostatic interaction and inner filter effect resulted in fluorescence quenching of chemosensors on the addition of TNP. The compounds are practically applicable for sensing TNP in real environmental samples as they gave good sensing response towards TNP in pH range of 5–9. Fluorescent test strips were fabricated which allowed effective on-site visual detection of explosive compound (TNP). To the best of our knowledge, this chemosensor represents the first example of TNP detection in aqueous medium using FONPs of TAI.

Colorimetric immunoassay for rapid detection of Vibrio parahaemolyticus

Vibrio parahaemolyticus (V. parahaemolyticus) is one of the most common food-borne pathogens. The authors describe a rapid colorimetric assay for V. parahaemolyticus that is based on a combination of a magnetic bead-based sandwich immunoassay and signal amplification via an enzmye mimic. MnO2 nanoparticles are used as an artificial oxidase that oxidizes 3,3′,5,5′-tetramethylbenzidine in the presence of oxygen to form a blue (and readily visible) product with an absorption maximum at 652 nm. By combining the superior capture efficiency of magnetic beads with the high catalytic activity of the enzmye mimic, this method can detect V. parahaemolyticus concentration in the range between 10 to 105 cfu·mL−1 without pre-enrichment, and the limit of detection is as low as 10 cfu·mL−1. Recoveries ranging from 87.5% to 106.0% are found when analyzing spiked oyster samples. The assay is rapid, sensitive, and specific and specific. In our perception, it shows promise in rapid instrumental and on-site visual detection of V. parahaemolyticus.

A signal-enhanced lateral flow strip biosensor for ultrasensitive and on-site detection of bisphenol A

ABSTRACT A signal-enhanced lateral flow strip biosensor has been constructed for ultrasensitive and on-site visual detection of bisphenol A (BPA). The signal amplification principle is based on the capacity of binding a large number of antibody-assembled gold nanoparticle (AuNP–antibody) conjugates of the poly amidoamine (PAMAM), which bears a multitude of amino groups. This signal-enhanced method could achieve BPA detection at 10 ppb based on naked eye visual observation, which represents an at least 50-fold improvement of the sensitivity of a traditional method without PAMAM. Moreover, good linearity was observed in the range from 2 to 100 ppb after quantitative analysis using the ImageJ software. This signal amplification method could be adopted as a potential generous technique in different fields.

Griess reaction-based paper strip for colorimetric/fluorescent/SERS triple sensing of nitrite

This study demonstrates a novel strategy for colorimetric/fluorescent/surface enhanced Raman scattering (SERS) triple-mode sensing of nitrite based on Griess reaction modulated gold nanorods (GNRs)-Azo-gold nanoparticles (GNPs) (GNRs-Azo-GNPs) assembly. The p-aminothiophenol (PATP)-capped GNRs (GNRs@PATP) and 1,8-diaminonaphthalene (DAP)-modified GNPs (GNPs@DAP) are first synthesized through Au–S and Au–N bonds, respectively. Upon being excited at 365nm, the dispersion of GNRs-GNPs (GNRs@PATP and GNPs@DAP) emits cyan fluorescence. Next, the addition of nitrite into the GNRs-GNPs induces the formation of GNRs-Azo-GNPs assembly, resulting in the enhancement of color and decrease of fluorescence. Therefore, the GNRs-Azo-GNPs assembly can not only be used as a naked-eye indicator of nitrite changed from orange-yellow to purple, but also as a highly selective ‘‘fluorescence quenching’’ probe due to the fluorescence resonance energy transfer (FRET) between azo-moiety and DAP. The limit of detection (LOD) for nitrite is 0.05μM by colorimetry and 0.01μM by fluorescence. Meanwhile, the GNRs-Azo-GNPs assembly possesses controllable core–satellites nanostructures and enables on-field SERS detection of nitrite with the LOD of 0.8nM. More importantly, the GNRs-GNPs sensing system can not only be used as a paper-based test strip for on-site fast screening of nitrite with a high sensitivity and selectivity, but also as a SERS substrate for reliable quantitative analysis of nitrite. This study offers a new method for on-site visual detection of nitrite in human urine and meat products, as well as provides a strategy for designing multi-mode sensing platform for various applications.

A colorimetric sensor of cysteine based on self-assembly nanostructures of Fe3+-H2O2/Tetramethylbenzidine system with “On-Off” switching function

Many strategies have been explored for selectively and sensitively detecting cysteine in different samples. Here, a novel colorimetric sensor based on self-assembly nanostructures of Fe3+-H2O2/Tetramethylbenzidine system with dual-level logic gate function and colorimetric determination of cysteine were firstly explored. The proposed Fe3+-H2O2-TMB system provides a sensitive optical signal due to the selectively reductive ability of cysteine to the oxidized TMB and thus could be successfully applied to the construction of instant on-site visual detection method with a paper based test strip for cysteine determination in a sample solution as well as for a dual-level logic gate fabrication.

Pendant chain engineering to fine-tune the nanomorphologies and solid state luminescence of naphthalimide AIEEgens: application to phenolic nitro-explosive detection in water.

Strategically, a series of five angular "V" shaped naphthalimide AIEEgens with varying pendant chains (butyl, hexyl, octyl, cyclohexyl and methylcyclohexyl) have been synthesized to fine-tune their nanomorphological and photophysical properties. With similar aromatic cores and electronic states, unexpected tuning of the condensed state emission colors and nanomorphologies (reproducible on any kind of surface) of naphthalimides has been achieved for the first time simply by varying their side chains. Conclusive analysis by various spectroscopic techniques (SC-XRD, powder-XRD, DLS, FESEM) and DFT computational studies confirmed the full control of the pendant chain (in terms of bulkiness around the naphthalimide core, which restricts the ease of intermolecular π-π interactions) over the nanoaggregate morphology and solid state emissive properties of the AIEEgens; this can be rationalized to all aggregation-prone systems. These comprehensive studies establish a conceptually unique yet simple and effective method to precisely tune the nanomorphologies and the emission colors of aggregation-prone small organic molecules by judicious choice of the non-conjugated pendant chain. Thus, considering the prime role of the active layer nanomorphology in all organic optoelectronic devices, this methodology may emerge as a promising tool to improve device performance. Among all the congeners, the hexyl chain-containing congener (HNQ) forms well-defined nanoribbons with smaller diameters (as confirmed from DLS: 166 nm and FESEM: 150 nm) and provides a larger surface area. Consequently, the HNQ-nanoribbons were employed as a fluorescent sensor for the discriminative detection of trinitrophenol (TNP) in pure aqueous media. FE-SEM images revealed that, upon gradual addition of TNP (10 nM to 100 μM), these nanoribbons undergo an aggregation/disaggregation process, forming non-fluorescent co-aggregates with TNP, and provide highly enhanced sensitivity compared to existing state-of-the-art on aggregation-prone systems. Fluorescence titration studies confirmed that HNQ can detect the presence of TNP as low as 16.8 ppb and can serve as a cost-effective portable device incorporated with UV-light for on-site visual detection of TNP, even in the presence of potentially competing nitroaromatic compounds.

Highly selective and sensitive determination of copper ion based on a visual fluorescence method

A novel ratiometric fluorescence sensor for rapid and on-site visual detection of Cu2+ was designed and synthesized by integrating yellow-emission rhodamine fluorophore (RL) and red-emission CdTe@SiO2 QDs. The as-prepared nanohybrid fluorescence sensor shows dual-emissions at 537nm and 654nm under a single excitation at 500nm in the presence of Cu2+. Owing to the strong chelating ability of RL toward Cu2+, the yellow fluorescence of RL could be selectively enhanced while the red fluorescence of CdTe QDs is almost unchanged, leading to an obvious fluorescence color change from red to yellow, which could be used for visual and ratiometric detection of Cu2+. This nanohybrid sensor exhibits excellent selectivity, sensitivity and anti-interference to Cu2+ detection and the detection limit is as low as 8.4nM. Additionally, a simple device test strip for rapid and on-site detection of Cu2+ has been designed by immobilizing the RL-CdTe@SiO2 QDs on a common filter paper. This simple and effective paper-based sensor has a visual detection limit of 0.5μM, showing its promising application for on-site and rapid sensing of Cu2+ in pollution water.

A FRET-based ratiometric fluorescent aptasensor for rapid and onsite visual detection of ochratoxin A.

A color change observable by the naked eye to indicate the content of an analyte is considered to be the most conceivable way of various sensing protocols. By taking advantage of the Förster resonance energy transfer (FRET) principles, we herein designed a dual-emission ratiometric fluorescent aptasensor for ochratoxin A (OTA) detection via a dual mode of fluorescent sensing and onsite visual screening. Amino group-modified OTA's aptamer was firstly labeled with the green-emitting CdTe quantum dots (gQDs) donor. The red-emitting CdTe QDs (rQDs) which were wrapped in the silica sphere could serve as the reference signal, while the gold nanoparticle (AuNP) acceptors were attached on the silica surface to bind with the thiolated complementary DNA (cDNA). The hybridization reaction between the aptamer and the cDNA brought gQD-AuNP pair close enough, thereby making the FRET occur in the aptasensor fabrication, while the subsequent fluorescence recovery induced by OTA was obtained in the detection procedure. Based on the red background of the wrapped rQDs, the aptasensor in response to increasing OTA displayed a distinguishable color change from red to yellow-green, which could be conveniently readout in solution even by the naked eye. Since the bioconjugations used as the aptasensor can be produced at large scale, this method can be used for in situ, rapid, or high-throughput OTA detection after only an incubation step in a homogeneous mode. We believe that this novel aptasensing strategy provides not only a promising method for OTA detection but also a universal model for detecting diverse targets by changing the corresponding aptamer.

A ratiometric fluorescence sensor for highly selective and sensitive detection of mercuric ion

Mercury is widely used in industry and easily leads to environmental pollution if discharged without proper waste control. On-site visual detection of mercuric ion is thus in demand and remains a great challenge. We herein demonstrate a detection method for rapid and on-site visualization of mercuric ion on the basis of integrating green-emissive imidazol fluorophore (PIPT) and red-emissive quantum dots (QDs) embedded in silica nanospheres. Such a nanohybrid fluorescent probe exhibits dual emissions at 500nm and 657nm under a single excitation wavelength. Due to the high chelating ability of PIPT toward mercuric ion, the fluorescence of PIPT could be selectively quenched while the fluorescence of the QDs is almost constant, leading to a distinct fluorescence color change from green to red, which can be applied for the rapid visualization of mercuric ion. This ratiometric fluorescent approach shows high sensitivity, high selectivity and anti-interference toward mercury against other metal ions. The detection limit of this method is determined to be 6.5nM. The probe has also been successfully demonstrated for the determination of mercuric ion in real water samples.

On-site visual detection of hydrogen sulfide in air based on enhancing the stability of gold nanoparticles.

We have described a simple and low-cost visual method for on-site detection of hydrogen sulfide (H2S) in air based on the antiaggregation of gold nanoparticles (AuNPs). The bubbling of H2S into a weak alkaline buffer solution leads to the formation of HS-, which can stabilize the AuNPs and ensure the AuNPs maintain their red color even in a Tris buffer solution containing 80 mM NaCl with the presence of Tween 80. The stabilization of the AuNPs is attributed to the adsorption of negatively charged S2- on the AuNPs surface. In contrast, without the bubbling of H2S, AuNPs aggregate and change color from red to blue. Under optimal conditions, the proposed method exhibits excellent visual sensitivity with a naked-eye detectable limit of 0.5 ppm (v/v), making the on-site detection of H2S possible. This method also possesses good selectivity toward H2S over other gases by using a simple SO2 removal device. The successful determination of the concentrations of H2S in local air indicates the potential application of this cost-effective method.

One-step signal amplified lateral flow strip biosensor for ultrasensitive and on-site detection of bisphenol A (BPA) in aqueous samples

A one-step signal amplified lateral flow strip (LFS) biosensor has been developed for ultrasensitive and on-site visual detection of bisphenol A (BPA). This signal amplified LFS was based on the dual labeling using different-sized gold nanoparticles (Duo-LFS). This Duo-LFS could achieve BPA detection with 0.5ng/mL as the visual sensitivity by naked eye observation and with 0.076ng/mL as the limit of detection (LOD) for semi-quantitative detection by software analysis, which is at least 10-fold improvement of the sensitivity of traditional LFS based methods. This one-step signal amplified lateral flow strip biosensor and related signal enhancement method could be adopted as a potential generous technique for all LFS-based detection methods.

Gold nanoparticles based colorimetric aptasensor for theophylline

A simple colorimetric aptasensor for the detection of theophylline was investigated using stable RNA aptamer and gold nanoparticles (AuNPs). Theophylline, a purine derivative, structurally similar to caffeine is broadly used in the treatment of respiratory diseases and bronchial asthma. A highly stable RNA aptamer that selectively binds to the theophylline molecule was prepared by structural modification of the 2′-hydroxyl group of ribose to 2′-fluoro (5′GGC GAU ACC AGC CGA AAA GGC CCU UGG CAG CGU C3′) in all the pyrimidines in a 34-mer length aptamer. The RNA aptamer binds to AuNPs by simple electrostatic interactions thereby inhibiting salt induced aggregation. AuNPs are aggregated due to detachment of the theophylline specific RNA aptamer from the surface of the AuNPs as a result of interaction with the target leading to a color change from red to purple. By successfully optimizing the concentration of aptamer, AuNPs, salts and stability of the aptamer, a detection limit of 50 ng mL−1 theophylline was achieved. Analyses of theophylline in samples were carried out, and the observed recovery was 97–98.7% with a relative standard deviation in the range of 2.69–19.4%. The results obtained were compared with those of an HPLC method. This colorimetric aptasensor is advantageous for onsite visual detection.