A Reversible Dual‐Response Fluorescence Switch for the Detection of Multiple Analytes

This paper reports a reversible dual fluorescence switch for the detection of a proton target and 2,4,6‐trinitrotoluene (TNT) with opposite‐response results, based on fluorophore derivatization of silica nanoparticles. Fluorescent silica nanoparticles were synthesized through modification of the surface with a nitrobenzoxadiazole (NBD) fluorophore and an organic amine to form a hybrid monolayer of fluorophores and amino ligands; the resultant nanoparticles showed different fluorescence responses to the proton target and TNT. Protonation of the amino ligands leads to fluorescence enhancement due to inhibition of photoinduced electron transfer (PET) between the amine and fluorophore. By contrast, addition of TNT results in fluorescence quenching because a fluorescence resonance energy transfer (FRET) happens between the NBD fluorophore and the formed TNT–amine complex. The fluorescence signal is reversible through washing with the proper solvents and the nanoparticles can be reused after centrifugal separation. Furthermore, these nanoparticles were assembled into chips on an etched silicon wafer for the detection of TNT and the proton target. The assembled chip can be used as a convenient indicator of herbicide (2,4‐dichlorophenoxyacetic acid) and TNT residues with the use of only 10 μL of sample. The simple NBD‐grafted silica nanoparticles reported here show a reversible signal and good assembly flexibility; thus, they can be applied in multianalyte detection.     

Pore size and channel structure study of mesoporous SiO2 films on sensing trace explosive

In this work, a new type of naphthalene dye was synthesized for detecting nitro explosives in solution and mesoporous film system. The fluorescence quenching mechanism between dye and explosive was determined. For detecting nitro explosives in gas system, mesoporous silica films doped with this fluorescent dye were successfully fabricated using the sol–gel technology with different templates. Two-dimensional hexagonal structure mesoporous silica film was found sensitive to traces of nitroaromatic trinitrotoluene. When enlarging the pore size, to improve the sensitivity, a lower sensitivity was actually obtained. This can be explained by the small molecule diameter of the explosive and the relatively large surface area of the mesoporous films. The Brunauer–Emmett–Teller surface area can be enlarged by a suitable 3D pore channel structure so that the fluorescence quenching sensitivity is improved for the nitro aromatic compounds vapors. Optimizing the pore size and channel structure can therefore well improve the sensing efficiency in this system.     

Instant visual detection of trinitrotoluene particulates on various surfaces by ratiometric fluorescence of dual-emission quantum dots hybrid

To detect trace trinitrotoluene (TNT) explosives deposited on various surfaces instantly and on-site still remains a challenge for homeland security needs against terrorism. This work demonstrates a new concept and its utility for visual detection of TNT particulates on various package materials. The concept takes advantages of the superior fluorescent properties of quantum dots (QDs) for visual signal output via ratiometric fluorescence, the feasibility of surface grafting of QDs for chemical recognition of TNT, and the ease of operation of the fingerprint lifting technique. Two differently sized CdTe QDs emitting red and green fluorescences, respectively, have been hybridized by embedding the red-emitting one in silica nanoparticles and covalently linking the green-emitting one to the silica surface, respectively, to form a dual-emissive fluorescent hybrid nanoparticle. The fluorescence of red QDs in the silica nanoparticles stays constant, whereas the green QDs functionalized with polyamine can selectively bind TNT by the formation of Meisenheimer complex, leading to the green fluorescence quenching due to resonance energy transfer. The variations of the two fluorescence intensity ratios display continuous color changes from yellow-green to red upon exposure to different amounts of TNT. By immobilization of the probes on a piece of filter paper, a fingerprint lifting technique has been innovated to visualize trace TNT particulates on various surfaces by the appearance of a different color against a yellow-green background under a UV lamp. This method shows high selectivity and sensitivity with a detection limit as low as 5 ng/mm(2) on a manila envelope and the attribute of being seen with the naked eye.     

Influences of Acid on Molecular Forms of Fluorescein and Photoinduced Electron Transfer in Fluorescein‐Dispersing Sol–Gel Titania Films

Fluorescein‐dispersing titania gel films were prepared by the acid‐catalyzed sol–gel reaction using a titanium alkoxide solution containing fluorescein. The molecular forms of fluorescein in the films, depending on its acid–base equilibria, and the complex formation and photoinduced electron transfer process between the dye and titania surface were investigated by fluorescence and photoelectric measurements. The titanium species were coordinated to the carboxylate and phenolate‐like groups of the fluorescein species. The quantum efficiencies of the fluorescence quenching and photoelectric conversion were higher upon excitation of the dianion species interacting with the titania, i.e. the dye–titania complex. This result indicated that the dianion form was the most favorable for formation of the dye–titania complex exhibiting the highest electron transfer efficiency. Using nitric acid as the catalyst, the titania surface bonded to the fluorescein instead of the adsorbed nitrate ion during the steam treatment. The dye–titania complex formation played an important role in the electron injection from the dye to the titania conduction band.     

Complex Formation of d‐Metal Ions at the Interface of TbIII‐Doped Silica Nanoparticles as a Basis of Substrate‐Responsive TbIII‐Centered Luminescence

The complex formation of d‐metal ions at the interface of Tb^III‐doped silica nanoparticles modified by amino groups is introduced as a route to sensing d‐metal ions and some organic molecules. Diverse modes of surface modification (covalent and noncovalent) are used to fix amino groups onto the silica surface. The interfacial binding of d‐metal ions and complexes is the reason for the Tb^III‐centered luminescence quenching. The regularities and mechanisms of quenching are estimated for the series of d‐metal ions and their complexes with chelating ligands. The obtained results reveal the interfacial binding of Cu^II ions as the basis of their quantitative determination in the concentration range 0.1–2.5 μM by means of steady‐state and time‐resolved fluorescence measurements. The variation of chelating ligands results in a significant effect on the quenching regularities due to diverse binding modes (inner or outer sphere) between amino groups at the interface of nanoparticles and Fe^III ions. The applicability of the steady‐state and time‐resolved fluorescence measurements to sense both Fe^III ions and catechols in aqueous solution by means of Tb^III‐doped silica nanoparticles is also introduced.     

Porphyrin-doped mesoporous silica films for rapid TNT detection

Two kinds of porphyrin-doped silica films with mesoporous structures were fabricated using evaporation-introduced self-assembly approach and examined for chemosensor applications to detect explosive compounds such as 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT), and nitrobenzene (NB). All synthesized silica films showed high fluorescence quenching sensitivity toward the vapors of TNT, DNT, and NB but is strongly dependent on pore structure. The silica film with three dimensional pore structure exhibits the highest quenching efficiency close to the quenching efficiency reported for emissive conjugated polymers, indicating these kinds of mesostructured composites are potentially useful chemosensory materials for rapidly detecting trace explosives. The preparation conditions, the structures of the resulting films, their sensing performances, and the fluorescence quenching mechanism were discussed in this paper.     

Multicomponent Assembly of Fluorescent‐Tag Functionalized Ligands in Metal–Organic Frameworks for Sensing Explosives

Detection of trace amounts of explosive materials is significantly important for security concerns and pollution control. Four multicomponent metal–organic frameworks ( MOFs‐12 , 13 , 23 , and 123 ) have been synthesized by employing ligands embedded with fluorescent tags. The multicomponent assembly of the ligands was utilized to acquire a diverse electronic behavior of the MOFs and the fluorescent tags were strategically chosen to enhance the electron density in the MOFs. The phase purity of the MOFs was established by PXRD, NMR spectroscopy, and finally by single‐crystal XRD. Single‐crystal structures of the MOFs‐12 and 13 showed the formation of three‐dimensional porous networks with the aromatic tags projecting inwardly into the pores. These electron‐rich MOFs were utilized for detection of explosive nitroaromatic compounds (NACs) through fluorescence quenching with high selectivity and sensitivity. The rate of fluorescence quenching for all the MOFs follows the order of electron deficiency of the NACs. We also showed the detection of picric acid (PA) by luminescent MOFs is not always reliable and can be misleading. This attracts our attention to explore these MOFs for sensing picryl chloride (PC), which is as explosive as picric acid and used widely to prepare more stable explosives like 2,4,6‐trinitroaniline from PA. Moreover, the recyclability and sensitivity studies indicated that these MOFs can be reused several times with parts per billion (ppb) levels of sensitivity towards PC and 2,4,6‐trinitrotoluene (TNT).     

Ring Size Effects on Multi‐Stimuli Responsive Luminescent Properties of Cyclic Amine Substituted β‐Diketones and Difluoroboron Complexes

Emissive β‐diketones (bdks) and difluoroboron complexes (BF₂bdks) show multi‐stimuli responsive luminescence in both solution and the solid state. A series of bdk ligands and boron coordinated dyes were synthesized with different cyclic amine substituents in the 4‐position to explore ring size effects on various luminescent properties, including solvatochromism, viscochromism, aggregation‐induced emission (AIE), mechanochromic luminescence (ML) and halochromism. Red‐shifted absorption and emission were observed in CH₂Cl₂ for both bdk ligands and boron dyes with increasing substituent ring size. The compounds displayed bathochromic emission in more polar solvents, and higher fluorescence intensity in more viscous media. The AIE compounds exhibited enhanced emission when aggregated. For solid‐state properties, a large emission wavelength shift was shown for the piperidine substituted bdk after melt quenching on weighing paper. Large blue‐shifted emissions were observed in all the boron dye spin cast films after trifluoroacetic acid vapor annealing, and the original emissions were partially recovered after triethylamine vapor treatment.     

Rhodamine B nanocrystals: elaborations,characterizations and functionalizations for biosensing applications

A biological sensor based on fluorescent organic nanocrystals (NCs) of Rhodamine B grown in sol–gel thin films was developed. The original signalization function is based on fluorescence contrasts of NCs, which exhibit a simple fluorescence signature, good photostability and higher fluorescence intensities compared to dispersed dye molecules. Thanks to a well-controlled dissolution process of the sol–gel surface, accurately followed by atomic force microscopy, the NCs were made emerging just a few nanometers above the silicate thin films to be directly accessible to biological macromolecules. Thus, hairpin-shaped DNA, functionalized by a probe-molecule (DNA probe), has been grafted onto nanocrystal surfaces leading to a fluorescence quenching by Forster resonance energy transfer. After hybridization of these hairpin-shaped DNA probes with their complementary DNA-target, the molecular probes and NCs are pulled apart, stopping thus the quenching. This “turn-on” of nanocrystal fluorescence allows thus a label-free DNA detection. The preparation methodology of the signalization function, its functionalization by hairpin-shaped DNA probes and first DNA-sensor experiments are presented.     

Femtogram Detection of Explosive Nitroaromatics: Fluoranthene‐Based Fluorescent Chemosensors

Herein we report a novel fluoranthene‐based fluorescent fluorophore 7,10‐bis(4‐bromophenyl)‐8,9‐bis[4‐(hexyloxy)phenyl]fluoranthene ( S₃ ) and its remarkable properties in applications of explosive detection. The sensitivity towards the detection of nitroaromatics (NACs) was evaluated through fluorescence quenching in solution, vapor, and contact mode approaches. The contact mode approach using thin‐layer silica chromatograp‐ hic plates exhibited a femtogram (1.15 fg cm^?2) detection limit for trinitrotoluene (TNT) and picric acid (PA), whereas the solution‐phase quenching showed PA detection at the 2–20 ppb level. Fluorescence lifetime measurements revealed that the quenching is static in nature and the quenching process is fully reversible. Binding energies between model binding sites of the S₃ and analyte compounds reveal that analyte molecules enter into the cavity created by substituted phenyl rings of fluoranthene and are stabilized by strong intermolecular interactions with alkyl chains. It is anticipated that the sensor S₃ could be a promising material for the construction of portable optical devices for the detection of onsite explosive nitroaromatics.     

Fluorescent Detection of 2,4‐DNT and 2,4,6‐TNT in Aqueous Media by Using Simple Water‐Soluble Pyrene Derivatives

Pyrene‐containing water‐soluble probes for the fluorescent detection of nitroaromatic compounds (NACs), such as explosive components (2,4‐DNT and 2,4,6‐TNT) and herbicides (2,4‐dinitrocresol, 2,4‐DNOC), in aqueous media are reported. In the probes, the introduction of surface‐active hydrophilic “heads” at the periphery of lipophilic (i.e., hydrophobic) pyrene “tails” resulted in the formation of highly fluorescent micelle‐like aggregates/pre‐associates in aqueous solutions at concentrations of ≤10^?5 m . The enhanced fluorescence quenching of the herein reported architectures is achieved in the presence of ultra‐trace amounts of TNT or 2,4‐DNT with values of Stern–Volmer quenching constant close to 1×10⁵ m ^?1 and a detection limit as low as 182 ppb. The most hydrophilic probes demonstrated higher response to 2,4‐DNT over TNT. Filter paper test strips impregnated with 1×10^?5 m solutions of the probes were able to detect TNT, 2,4‐DNT, and other NACs at levels as low as 50 ppb in water.     

基于四联苯臂星型小分子凝胶剂的TNT薄膜荧光传感器

基于具有三苯胺中心核、咔唑外围和四联苯连接臂的星型荧光小分子三(4″″-(3, 6-二叔丁基-咔唑-9氢-9-基)-(1, 1': 4', 1″':4″', 1?:4?, 1″″-四苯基)-4-基)胺(N5), 采用溶液旋涂的方法制备了用于检测三硝基甲苯(TNT)饱和蒸汽的高效荧光传感薄膜。 不同于具有单苯连接臂的模型化合物三(4'-(3, 6-二叔丁基-咔唑-9氢-9-基)-(1, 1'-二苯基)-4-基)胺(N2), N5由于长共轭臂的存在, 容易在苯类溶剂中通过π-π相互作用组装形成凝胶。 同时, 其旋涂薄膜具有细小纳米级纤维状的组装形貌, 表面出现一定的多孔结构, 有利于气体分子的渗透。 因此, 应用于TNT蒸汽的检测, 化合物N5薄膜表现出比N2薄膜更快的荧光淬灭响应速度和更高的淬灭效率。 特别是TNT传感特性几乎不受化合物N5膜厚的影响, 当膜厚为94 nm时, 在1和30 min的淬灭程度仍然可以达到44%和90%。     

A fluorescent and chemiluminescent difunctional mesoporous silica nanoparticle as a label for the ultrasensitive detection of cancer cells

A new kind of ultrabright fluorescent and chemiluminescent difunctional mesoporous silica nanoparticle (FCMSN) is reported. A luminescent dye, Rhodamine 6G or tris(2,2′-bipyridyl)dichlororuthenium(II) hexahydrate (Rubpy), is doped inside nanochannels of a silica matrix. The hydrophobic groups in the silica matrix avoid the leakage of dye from open channels. The amines groups on the surface of the FCMSN improve the modification performance of the nanoparticle. Because the nanochannels are isolated by a network skeleton of silica, fluorescence quenching based on the inner filter effect of the fluorescent dyes immobilized in nanochannels is weakened effectively. The Quantum Yield of obtained 90 nm silica particles was about 61%. Compared with the fluorescent core–shell nanoparticle, the chemiluminescence reagents can freely enter the nanoparticles to react with fluorescent dyes to create chemiluminescence. The results show that the FCMSN are both fluorescent labels and chemiluminescent labels. In biological applications, the NaIO₄ oxidation method was proven to be superior to the glutaraldehyde method. The amount of amino could affect the specificity of the FCMSN. The fluorescence microscopy imaging demonstrated that the FCMSN is viable for biological applications.     

Two-step resonance energy transfer between dyes in layered silicate films

Single- and two-step fluorescence resonance energy transfer (FRET) was investigated between laser dyes rhodamine 123 (R123), rhodamine 610 (R610), and oxazine 4 (Ox4). The dye molecules played the role of molecular antennas and energy donors (ED, R123), energy acceptors (EA, Ox4), or both (R610). The dye cations were embedded in the films based on layered silicate laponite (Lap) with the thickness of several μm. Optically homogeneous films were prepared directly from dye/Lap colloids. Dye concentration in the films was high enough for FRET to occur but sufficiently low to prevent the formation of large amounts of molecular aggregates. The films were characterized by absorption and fluorescence spectroscopies, and their optical properties were compared with colloid precursors and dye aqueous solutions. The phenomenon of FRET was confirmed by means of steady-state and time-resolved fluorescence spectroscopies. Significant quenching of ED emission in favor of the luminescence from EA molecules was observed. FRET led to the decrease in the lifetimes of excited states of ED molecules. Molecular orientation of dye molecules was determined by polarized absorption and fluorescence spectroscopies. Almost parallel orientation with respect to silicate surface (～30°) was determined for all fluorescent species of the dyes. Theoretical model on relationship between anisotropy and molecular orientation of the fluorophores fits well with measured data. The analysis of anisotropy measurements confirmed the significant role of FRET in the phenomenon of light depolarization.     

Charge‐Recombination Fluorescence from Push–Pull Electronic Systems Constructed around Amino‐Substituted Styryl–BODIPY Dyes

A small series of donor–acceptor molecular dyads has been synthesized and fully characterized. In each case, the acceptor is a dicyanovinyl unit and the donor is a boron dipyrromethene (BODIPY) dye equipped with a single styryl arm bearing a terminal amino group. In the absence of the acceptor, the BODIPY‐based dyes are strongly fluorescent in the far‐red region and the relaxed excited‐singlet states possess significant charge‐transfer character. As such, the emission maxima depend on both the solvent polarity and temperature. With the corresponding push–pull molecules, there is a low‐energy charge‐transfer state that can be observed by both absorption and emission spectroscopy. Here, charge‐recombination fluorescence is weak and decays over a few hundred picoseconds or so to recover the ground state. Overall, these results permit evaluation of the factors affecting the probability of charge‐recombination fluorescence in push–pull dyes. The photophysical studies are supported by cyclic voltammetry and DFT calculations.     

功能化荧光素掺杂二氧化硅纳米粒子用于免疫层析试验

A simple but effective approach was developed to synthesize amino functionalized fluorescein isothiocyanate-doped silica nanoparticles based upon polycondensation of tetraethoxysilane. Organic dye molecule (fluorescein isothiocyanate) coupled with a silane coupling agent, 3-aminopropyltriethoxysilane, was incorporated into silica sphere through controlled hydrolysis and polymerization of tetraethoxysilane. The dye was connected with silica sphere through 3-aminopropyltriethoxysilane, which avoided the leakage of the dye. The cohydrolysis and polymerization of tetraethoxysilane and 3-aminopropyltriethoxysilane outside the surface of the silica sphere formed another thin silica shell with the functionalized amino groups on the surface. With amino groups on the surface, the nanoparticle surface was affluent in positive charges. The amino-functionalized nanoparticles were linked with mouse monoclonal antibody against hepatitis B virus surface antigen through electrostatic interaction to form fluorescence probes, which were tested by immunochromatographic assay using immunochromatography test strip. It was indicated that the fluorescence probe was suitable for immunoassay.     

Iron Oxide Nanoparticles Labeled with an Excited‐State Intramolecular Proton Transfer Dye

Excited‐state intramolecular proton transfer (ESIPT) is a particularly well known reaction that has been very little studied in magnetic environments. In this work, we report on the photophysical behavior of a known ESIPT dye of the benzothiazole class, when in solution with uncoated superparamagnetic iron oxide nanoparticles, and when grafted to silica‐coated iron oxide nanoparticles. Uncoated iron oxide nanoparticles promoted the fluorescence quenching of the ESIPT dye, resulting from collisions during the lifetime of the excited state. The assembly of iron oxide nanoparticles with a shell of silica provided recovery of the ESIPT emission, due to the isolation promoted by the silica shell. The silica network gives protection against the fluorescence quenching of the dye, allowing the nanoparticles to act as a bimodal (optical and magnetic) imaging contrast agent with a large Stokes shift.     

Selective fluorescence response and magnetic separation probe for 2,4,6-trinitrotoluene based on iron oxide magnetic nanoparticles

Despite the rapid development of nanomaterials and nanotechnology, it is still desirable to develop novel nanoparticle-based techniques which are cost-effective, timesaving, and environment-friendly, and with ease of operation and procedural simplicity, for assay of target analytes. In the work discussed in this paper, the dye fluorescein isothiocyanate (FITC) was conjugated to 1,6-hexanediamine (HDA)-capped iron oxide magnetic nanoparticles (FITC–HDA Fe₃O₄ MNPs), and the product was characterized. HDA ligands on the surface of Fe₃O₄ MNPs can bind 2,4,6-trinitrotoluene (TNT) to form TNT anions by acid–base pairing interaction. Formation of TNT anions, and captured TNT substantially affect the emission of FITC on the surface of the Fe₃O₄ MNPs, resulting in quenching of the fluorescence at 519 nm. A novel FITC–HDA Fe₃O₄ MNPs-based probe featuring chemosensing and magnetic separation has therefore been constructed. i.e. FITC–HDA Fe₃O₄ MNPs had a highly selective fluorescence response and enabled magnetic separation of TNT from other nitroaromatic compounds by quenching of the emission of FITC and capture of TNT in aqueous solution. Very good linearity was observed for TNT concentrations in the range 0.05–1.5 μmol?L^?1, with a detection limit of 37.2 nmol?L^?1 and RSD of 4.7 % (n?=?7). Approximately 12 % of the total amount of TNT was captured. The proposed methods are well-suited to trace detection and capture of TNT in aqueous solution.

Ultrasensitive determination of 1,4-dihydroxybenzene based on fluorescence resonance energy quenching of luminescent quantum dots modified on surface of silica nanoparticles

An ultrasensitive solid-phase fluorescence resonance energy quenching (FREQ) method for determination of 1,4-dihydroxybenzene (DHB) using mercaptosuccinic acid (MSA)-capped CdTe quantum dots (QDs) immobilized on silica nanoparticles (NPs) as donors was developed. In the method, silica NPs were first modified with 3-aminopropyltriethoxysilane (APTS). Then, MSA-capped CdTe QDs were immobilized on the surface of the APTS-modified silica NPs. Finally, DHB in the solution was attached to the empty sites on the surface of silica NPs with QDs through electrostatic interaction. The fluorescence emission of the QDs was quenched by the proximal DHB molecules on the silica NPs. The quenching efficiency of the solid-phase FREQ method was 200-times higher than that of the solution-phase FREQ method. Using the ultrasensitive solid-phase FREQ method, DHB as low as 2.4 × 10⁻¹² mol/L could be detected. The method was applied to quantify trace DHB in water samples.     

A novel nanoaggregation detection technique of TNT using selective and ultrasensitive nanocurcumin as a probe

Here, for the first time, we designed an ultrasensitive nanocurcumin based nanomaterials surface energy transfer (NSET) probe for detection of trace amount of TNT with excellent sensitivity (1 nM) and selectivity over other nitro explosives via nanoaggregation and we found the largest fluorescent enhancement to date for sensing TNT (upto 800 fold).