Fluorescent sensor arrays with pattern recognition ability have been widely used to detect and identify multiple chemically similar analytes. In the present work, two particular bispyrene fluorophores containing hydrophilic oligo(oxyethylene) spacer, 6 and 4, were synthesized, but one is with and the other is without cholesterol unit. Their ensembles with cationic surfactant (CTAB) assemblies realize multiple fluorescence responses to different metalloproteins, including hemoglobin, myoglobin, ferritin, cytochrome c, and alcohol dehydrogenase. The combination of fluorescence variation at monomer and excimer emission of the two binary sensor ensembles enables the mini sensor array to provide a specific fingerprint pattern to each metalloprotein. Linear discriminant analysis shows that the two-ensemble-sensor-based array could well discriminate the five tested metalloproteins. The present work realizes using a mini sensor array to accomplish discrimination of complex analytes like proteins. They also display a very high sensitivity to the tested metalloproteins with detection limits in the range of picomolar concentration.Keywords: aggregate; bispyrene; metalloprotein; supramolecular assemblies; surfactant;

•A fluorescent ensemble based on dansyl derivative and SDS assemblies was prepared for sensing amino acids.•The supramolecular sensor exhibits selective and fast turn-off responses to aspartic acid and glutamic acid.•The binding of H+ released from acidic amino acids is accounting for the turn-off responses.•SDS assemblies play an important role in attracting acidic amino acids to complex with sensor system.Aspartic acid (Asp) and glutamic acid (Glu) both play critical roles in maintaining many life functions. Detection of both Asp and Glu is therefore a significant problem. A supramolecular binary ensemble based on an imidazolium-modified cationic dansyl derivative (1) and an anionic surfactant (SDS) assembly was prepared and used for detection of Asp and Glu in an aqueous medium. Fluorescence titration studies found that the binary ensemble displays selective turn-off responses to the acidic amino acids Asp and Glu. The detection limits for Asp and Glu were determined to be 0.6 and 2.1 μM, respectively. Time-resolved decays revealed that the quenching by Asp and Glu is static in nature. The results from fluorescence titration with H+ and control experiments in buffer solution and fluorescence recovery by NaOH demonstrated that it is the proton from Asp and Glu that is responsible for the fluorescence quenching. The UV–vis absorption changes upon titration of acidic amino acids and H+ approved the binding occurs between proton and the fluorophore. 1H NMR studies and control experiments with a similar fluorophore without alkylamine group further demonstrated the protonation of the dansyl alkylamine functionality. Control studies found that SDS assemblies also play an important role in the sensing process. Its electrostatic interaction with both the cationic dansyl probe and H+ helps to realize the binding of H+ with the sensor system and the resultant protonation leads to the fluorescence signal change.

•Several ferrocene-modified pyrene derivatives are synthesized.•They exhibit very low fluorescent quantum yield due to the presence of ferrocenyl unit.•These ferrocenyl fluorophores all show effective fluorescent turn-on responses to Cu2 +.•The turn-on responses of ferrocenyl fluorophores to Cu2 + could be influenced by the counter ion of Cu2 +.•The influence on the turn-on responses to Cu2 + could be further used for anion detection and fluorescent logic gate.Detection and identification of metal ions by fluorescent turn-on sensors are challenging due to the quenching effect of most of the tested metal ions. In the present work, three ferrocene-modified pyrene-based probes 2–4 were synthesized to act as turn-on fluorescent sensors for Cu2 +. The measurements of fluorescence quantum yield and fluorescence lifetime reveal that ferrocenyl unit can efficiently reduce the fluorescence emission of pyrene moiety. Steady-state fluorescence measurements find that the three ferrocene-modified fluorophores exhibit selective turn-on responses to Cu2 +. Moreover, this turn-on effect to Cu2 + is highly influenced by the type of the counter ion. It is found that the presence of Cl− or NO3− could realize the turn-on response to Cu2 +, whereas, the presence of SO42 − or Ac− could not induce any fluorescence enhancement to Cu2 +. Control experiments with ferrocene-free pyrene-based probe 1 reveal that the ferrocenyl unit plays the key role in the turn-on response to Cu2 +. The possible mechanism for the turn-on responses is attributed to the oxidation behavior of Cu2 + to the ferrocene unit, which is confirmed by the control experiments with sodium ascorbate. Cyclic voltammetry measurements show that Cu2 + can influence the redox behaviors of ferrocenyl derivatives, which is also highly dependent on the anion of the copper salts. The influence of anion on the turn-on responses to Cu2 + was further used for anion detection and fluorescent logic gate.Download high-res image (118KB)Download full-size image

Discrimination and identification of multiple metalloproteins by a single sensor system are challenging. Based on the strategy of surfactant assembly modulation effect, a strong discriminative sensor based on a pyrene-derived fluorophore and an anionic surfactant assembly was developed. A cholic acid-modified pyrene derivative, 1, was synthesized and it can form supramolecular aggregates in aqueous solution. Its fluorescence emission could be well modulated from monomer-dominant to monomer–excimer co-emission by the anionic surfactant, sodium dodecylbenzene sulfonate (SDBS). Fluorescence measurements found that the binary ensemble based on 1/SDBS exhibited cross-reactive sensing behavior towards different metalloproteins, and principal component analysis revealed that the present ensemble sensor could discriminate 7 different metalloproteins in aqueous solution. Moreover, the discrimination of metalloproteins could be further applied in biological fluids such as serum or urine.

•SDS assemblies can modulate fluorescence emission of mono-pyrene derivative.•SDS assemblies realizes monomer-excimer co-emission of mono-pyrene fluorophore in aqueous solution.•Fluorophore/SDS ensemble exhibits cross-reactive responses to metal ions.•Single supramolecular sensor system can discriminate 6 quenching metal ions at same or different concentration.A simple imidazolium-modified mono-pyrene derivative (Py-IM) is synthesized. SDS assemblies modulate its fluorescence emission from monomer emission in neat aqueous solution to monomer-excimer co-emission in pre-micelles and to monomer emission again in micellar solutions. The ensemble based on Py-IM and SDS assemblies exhibiting both monomer and excimer emission displays multiple-wavelength cross-reactive responses to cationic metal ions. This single sensor system could well discriminate 6 quenching metal ions (Fe3+, Cu2+, Pb2+, Ni2+, Co2+, and Mn2+) through principal component analysis method. Control experiments with Py-IM in the absence of SDS assemblies and in SDS micellar solutions reveal that the modulation effect of surfactant assemblies plays an important role in the cross-reactive responses of the cationic probe, which provides a general way of fabricating discriminative single sensor systems.

•A pyrene derivative-SDS assemblies-Cu2+ based ternary sensor system was built.•The ternary system is highly sensitive to arginine and lysine in aqueous solution.•The sensor shows the lowest detection limits of Arg (5.2 nM) and Lys (14.6 nM).•The sensor displays dual responsive signals of “excimer-off” and “monomer-on”.•Two different mechanisms occur to sensing different concentrated amino acids.A new cationic pyrene derivative-based fluorescent probe (IPy) was designed and synthesized. This cationic fluorophore with two imidazolium groups can be well-dissolved in aqueous solution and exhibits only monomer emission. The anionic surfactant SDS assemblies can modulate its fluorescence emission and enable both monomer and excimer emission by controlling SDS concentration. The selected IPy/SDS system shows turn-off responses to Cu2+, however, the quenching effect of Cu2+ can be increased by amino acids with high pI values such as arginine (Arg) and lysine (Lys). Therefore, the optimized ternary system based on IPy/SDS/Cu2+ can function as a highly sensitive and selective fluorescent sensor for these two amino acids, displaying detection limits of 5.2 nM and 14.6 nM for Arg and Lys, respectively. Moreover, this sensor system displays dual responsive signals of “excimer emission off” to low concentrated amino acids (smaller than 10 μM), and “monomer emission on” to high concentrated amino acids (larger than 10 μM). It turns out that low concentrated basic amino acids binds with negative SDS assemblies and facilitate fluorescence quenching of the cationic fluorophore by Cu2+, however, the high concentrated basic amino acids induce disassembly of SDS aggregates and lead to leak of the fluorophore back into aqueous environment, which gives rise to monomer emission.

Novel strategies of developing fluorescent sensors for proteins are highly demanded. In this work, we particularly synthesized a cholesterol-derivatized pyrene probe. Its fluorescence emission is effectively tuned by the aggregation state of a cationic surfactant dodecyltrimethylammonium bromide (DTAB). The used probe/DTAB assemblies exhibit highly sensitive ratiometric responses to pepsin and ovalbumin egg (o-egg) with detection limits of 4.8 and 18.9 nM, respectively. The fluorescence changes indicate the protein–surfactant interaction leads to further aggregation of DTAB assemblies. The results from Tyndall effect and dynamic light scattering verify this assumption. The responses to pepsin and o-egg are due to their strong electrostatic or hydrophobic interaction with DTAB assemblies at pH 7.4. The present noncovalent supramolecular sensor represents a novel and simple strategy for sensing proteins, which is based on the encapsulated fluorophore probing the aggregation variation of the surfactant assemblies.Keywords: DTAB; pepsin; pyrene; ratiometric sensor; supramolecular assembly

A particular bispyrene fluorophore (1) with two pyrene moieties covalently linked via a hydrophilic spacer was synthesized. Fluorescence measurements reveal that the fluorescence emission of 1 could be well modulated by a cationic surfactant, dodecyltrimethylammonium bromide (DTAB). Protein sensing studies illustrate that the selected ensemble based on 1/DTAB assemblies exhibits ratiometric responses to nonmetalloproteins and turn-off responses to metalloproteins, which can be used to differentiate the two types of proteins. Moreover, negatively charged nonmetalloproteins can be discriminated from the positively charged ones according to the difference in ratiometric responses. Fluorescence sensing studies with control bispyrenes indicate that the polarity of the spacer connecting two pyrene moieties plays an important role in locating bispyrene fluorophore in DTAB assemblies, which further influences its sensing behaviors to noncovalent interacting proteins. This study sheds light on the influence of the probe structure on the sensing performance of a fluorescent ensemble based on probe and surfactant assemblies.Keywords: DTAB; fluorescent sensor; metalloprotein; nonmetalloprotein; supramolecular assemblies

Pattern recognition has been widely used to detect and identify multiple analytes. The strategies for realizing pattern recognition of a single sensor are in significant demand. In the present work, a particular bispyrene-based fluorophore containing a hydrophilic spacer, Py-TOA-Py, was synthesized, and it was observed that its mixture with anionic surfactant assemblies realizes multiple fluorescence responses to different metal ions. The combination of fluorescence variation at four emission wavelengths enables the fluorophore/surfactant sensor system to provide specific recognition patterns to different metal ions. Principle component analysis shows that the present single sensor system could discriminate 6 metal ions, namely Fe3+, Co2+, Ni2+, Cu2+, Pb2+, and Hg2+, 5 of which are heavy metal ions. Results from UV-vis measurements rule out the possibility of the bispyrene fluorophore binding with metal ions. Fluorescence titration of metal ions with two other bispyrene fluorophores reveals that the ones with similar hydrophilic spacer, Py-EOA-Py, exhibit similar multiple fluorescence responses, whereas, the ones with hydrophobic spacer, Py-DDA-Py, display no cross-reactive responses. Time-resolved emission spectra measurements reveal that the spacer polarity plays an important role in determining the location of bispyrene in surfactant assemblies, which further influences its cross-reactive responses to metal ions. The present work provides a new strategy for developing fluorescent sensors with pattern recognition abilities.

A dansyl-functionalized fluorescent film sensor was specially designed and prepared by assembling dansyl on a glass plate surface via a long flexible spacer containing oligo(oxyethylene) and amine units. The chemical attachment of dansyl moieties on the surface was verified by contact angle, XPS, and fluorescence measurements. Solvent effect examination revealed that the polarity-sensitivity was retained for the surface-confined dansyl moieties. Fluorescence quenching studies in water declared that the dansyl-functionalized SAM possesses a higher sensitivity towards Hg2+ and Cu2+ than the other tested divalent metal ions including Zn2+, Cd2+, Co2+, and Pb2+. Further measurements of the fluorescence responses of the film towards Cu2+ and Hg2+ in three solvents including water, acetonitrile, and THF evidenced that the present film exhibits cross-reactive responses to these two metal ions. The combined signals from the three solvents provide a recognition pattern for both metal ions at a certain concentration and realize the identification between Hg2+ and Cu2+. Moreover, using principle component analysis, this method can be extended to identify metal ions that are hard to detect by the film sensor in water such as Co2+ and Ni2+.Keywords: interface; metal ions; pattern recognition; self-assembled monolayer; sensor array;

Lanthanides are valuable nonrenewable resources and widely used in a variety of industries. Detection and identification of lanthanide ions are in high demand but challenging because of the similarity among lanthanide ions. In the present work, a fluorescent sensor array of three cationic bispyrene derivatives mixed with anionic surfactant assemblies was developed. The sensor array exhibits cross-reactive responses to lanthanide ions when tested in aqueous solution. The combination of fluorescence variations at both monomer and excimer emission of each of the bispyrene sensor elements provides a six-signal recognition pattern for lanthanide ions. Principle component analysis illustrates that the sensor array could at least identify 6 of the 14 similar lanthanide ions including La3+, Pr3+, Nd3+, Eu3+, Ho3+, and Er3+. UV–vis absorption measurements rule out the possibility of binding lanthanides with fluorophores. Fluorescence titration experiments in both cationic and neutral surfactant aqueous solutions reveal that the three fluorophores show slight fluorescence responses to the lanthanide ions, indicating that electrostatic attraction between lanthanide ions and anionic surfactant plays an important role in the sensing behavior of the sensor array. Control experiments with divalent metal ions find no cross-reactive responses, suggesting that the stronger electrostatic interaction with trivalent lanthanide ions is responsible for the multiple fluorescence responses.Keywords: pattern recognition; pyrene; SDS; sensing; supramolecular assembly

The effect of surfactant micelles on the photophysical properties of a cationic bispyrene fluorophore, Py-diIM-Py, was systemically examined. The results from series of measurements including UV–vis absorption, steady-state fluorescence emission, quantum yield, fluorescence lifetime, and time-resolved emission spectra reveal that the cationic fluorophore is only encapsulated by the anionic sodium dodecyl sulfate (SDS) surfactant micelles and not incorporated in the cationic dodecyltrimethylammonium bromide (DTAB) and neutral Triton X-100 (TX100) surfactant micelles. This different fluorophore location in the micellar solutions significantly influences its sensing behavior to various explosives. Fluorescence quenching studies reveal that the simple variation of micellar systems leads to significant changes in the sensitivity and selectivity of the fluorescent sensor to explosives. The sensor exhibits an on–off response to multiple explosives with the highest sensitivity to picric acid (PA) in the anionic SDS micelles. In the cationic DTAB micelles, it displays the highest on–off responses to PYX. Both the sensitivity and selectivity to PYX in the cationic micelles are enhanced compared with that to PA in the anionic micelles. However, the poor encapsulation in the neutral surfactant TX100 micelles leads to fluorescence instability of the fluorophore and fails to function as a sensor system. Time-resolved fluorescence decays in the presence of explosives reveal that the quenching mechanism of two micellar sensor systems to explosives is static in nature. The present work demonstrates that the electrostatic interaction between the cationic fluorophore and differently charged micelles plays a determinative role in adjusting its distribution in micellar solutions, which further influences the sensing behavior of the obtained micellar sensor systems.

A new cationic dansyl derivative-based (DIlSD) fluorescence probe was designed and synthesized. Its combination with anionic surfactant SDS assemblies shows enhanced fluorescence intensity and blue-shifted maximum wavelength. Its fluorescence can be slightly quenched by Cu2+; however, the fluorescence quenching efficiency by Cu2+ is highly increased upon titration of arginine (Arg). As a result, the ternary system containing the cationic fluorophore, anionic surfactant, and Cu2+ functions as a highly sensitive and selective sensor to Arg. The optimized sensor system displays a detection limit of 170 nM, representing the highest sensitivity to Arg in total aqueous solution by a fluorescent sensor. Control experiments reveal that the imidazolium groups in the fluorophore, the anionic surfactant, and Cu2+ all play important roles in the process of sensing Arg. The electrostatic interaction between the cationic fluorophore and anionic surfactants facilitates the binding of imidazolium rings with Cu2+, the surfactant surface-anchored Cu2+ is responsible for further binding of Arg, and the electrostatic interaction between anionic surfactants and positively charged amino acids accounts for the selective responses to Arg.

A cationic bis-pyrene derivative was prepared and its assemblies with anionic surfactant to function as fluorescent sensor platforms for heavy metal ions in aqueous solution were evaluated. Optical spectroscopy measurements illustrated that both UV-vis absorption and fluorescence emission spectra of bis-pyrene could be well enhanced in the assemblies with the anionic surfactant, SDS. Moreover, fluorescence quenching studies revealed that the sensitivity of bis-pyrene/SDS assemblies is highly dependent on the concentration of SDS. The optimized sensor platform exhibited not only a high sensitivity towards both Cu2+ and Co2+ in aqueous solution with detection limits lower than 100 nM but also a high selectivity towards these two metal ions over a series of divalent metal ions including Ba2+, Ca2+, Hg2+, Mg2+, Ni2+, Pb2+, Zn2+, Cd2+, and Fe3+. The high sensitivity was demonstrated to be due to the electrostatic interaction between the metal cations and the anionic surfactants, which dramatically increases the local concentration of metal ions in the near vicinity of pyrene moieties. Moreover, the metal ion target could be identified by addition of glycine to the quenched system, where a dramatic turn-on fluorescence was observed for the Cu2+-quenched system whereas a further turn-off fluorescence was found for the Co2+-quenched system. Furthermore, the recovery of Cu2+-quenched fluorescence could be utilized to provide a turn-on fluorescence sensor for neutral amino acids.

The detection of nitroaromatics in aqueous solutions by a novel pyrene-functionalized film has been investigated in the present study. The pyrene moieties were attached on the glass surface via a long flexible spacer based on self-assembled monolayer technique. Steady-state fluorescence measurements revealed that these surface-attached pyrene moieties exhibited both monomer and excimer emission. Nitroaromatics such as 2,4,6-trinitrotoluene, 2,4-dinitrotoluene, and 2,4,6-trinitrophenol (picric acid) were found to efficiently quench the fluorescence emission of this film. The quenching results demonstrated that the excimer emission of these surface-confined pyrene moieties is more sensitive to the presence of nitroaromatics than the monomer emission. The quenching mechanism was examined through fluorescence lifetime measurement and it revealed that the quenching is static in nature and may be caused by electron transfer from the polycyclic aromatics to the nitroaromatics. Furthermore, the response of the film to nitroaromatics is fast and reversible, and the obtained film shows promising potentials in detecting explosives in aqueous environment.Graphical abstractHighlights► A pyrene-functionalized film sensor with high stability was prepared via self-assembled monolayer (SAM) technique. ► Nitroaromatics such as picric acid, TNT, and DNT can dramatically quench the fluorescence of pyrene-functionalized SAM film. ► Pyrene-functionalized SAM film can be used as sensitive and reversible sensors to nitroaromatics in aqueous solution.

To improve the photochemical stability of α-terthiophene (3T) in air, we purposely introduced a naphthalene unit into its conjugated backbone, resulting in a fluorescent compound, 5-(1-naphthyl)-2,2′:5′,2′′-terthiophene (NA-3T). The compound was further employed as a sensing element for the fabrication of a monolayer-chemistry based fluorescent sensing film. It was demonstrated that the fabricated film is highly sensitive and selective to the presence of picric acid (PA). The detection limit was found to be 3.2 × 10−7 mol/L. The high sensitivity of the film to PA has been attributed to the specific binding of the film to the analyte because of proton transfer from PA to the amino group in the spacer, which is in accordance with the static nature of the quenching as revealed by fluorescence lifetime measurements. Further experiments demonstrated that the sensing process is fully reversible and free of interference from common organic solvents, acids and bases, etc. In addition, the film is stable, at least, within half a year provided it is properly preserved. More importantly, the present work makes it possible to use oligothiophenes as a new class of sensing elements, which may combine the advantages of conjugated polymers or oligomers and those of fluorescent compounds of low-molecular masses. This effort enlarges, definitely, the applications of oligothiophenes and the space for creating monolayer-chemistry based fluorescent sensing films.Keywords: film sensor; fluorescence quenching; monolayer-chemistry; naphthyl-capped terthiophene; picric acid

A cationic bis-pyrene derivative was prepared and its assemblies with anionic surfactant to function as fluorescent sensor platforms for heavy metal ions in aqueous solution were evaluated. Optical spectroscopy measurements illustrated that both UV-vis absorption and fluorescence emission spectra of bis-pyrene could be well enhanced in the assemblies with the anionic surfactant, SDS. Moreover, fluorescence quenching studies revealed that the sensitivity of bis-pyrene/SDS assemblies is highly dependent on the concentration of SDS. The optimized sensor platform exhibited not only a high sensitivity towards both Cu2+ and Co2+ in aqueous solution with detection limits lower than 100 nM but also a high selectivity towards these two metal ions over a series of divalent metal ions including Ba2+, Ca2+, Hg2+, Mg2+, Ni2+, Pb2+, Zn2+, Cd2+, and Fe3+. The high sensitivity was demonstrated to be due to the electrostatic interaction between the metal cations and the anionic surfactants, which dramatically increases the local concentration of metal ions in the near vicinity of pyrene moieties. Moreover, the metal ion target could be identified by addition of glycine to the quenched system, where a dramatic turn-on fluorescence was observed for the Cu2+-quenched system whereas a further turn-off fluorescence was found for the Co2+-quenched system. Furthermore, the recovery of Cu2+-quenched fluorescence could be utilized to provide a turn-on fluorescence sensor for neutral amino acids.

Pattern recognition has been widely used to detect and identify multiple analytes. The strategies for realizing pattern recognition of a single sensor are in significant demand. In the present work, a particular bispyrene-based fluorophore containing a hydrophilic spacer, Py-TOA-Py, was synthesized, and it was observed that its mixture with anionic surfactant assemblies realizes multiple fluorescence responses to different metal ions. The combination of fluorescence variation at four emission wavelengths enables the fluorophore/surfactant sensor system to provide specific recognition patterns to different metal ions. Principle component analysis shows that the present single sensor system could discriminate 6 metal ions, namely Fe3+, Co2+, Ni2+, Cu2+, Pb2+, and Hg2+, 5 of which are heavy metal ions. Results from UV-vis measurements rule out the possibility of the bispyrene fluorophore binding with metal ions. Fluorescence titration of metal ions with two other bispyrene fluorophores reveals that the ones with similar hydrophilic spacer, Py-EOA-Py, exhibit similar multiple fluorescence responses, whereas, the ones with hydrophobic spacer, Py-DDA-Py, display no cross-reactive responses. Time-resolved emission spectra measurements reveal that the spacer polarity plays an important role in determining the location of bispyrene in surfactant assemblies, which further influences its cross-reactive responses to metal ions. The present work provides a new strategy for developing fluorescent sensors with pattern recognition abilities.