Self-Assembled Fluorescent Chemosensors

Selective and sensitive detection of metal ions by plasmonic resonance energy transfer-based nanospectroscopy

Highly selective and sensitive optical methods for the detection of metal ions have had a substantial impact on molecular biology, environmental monitoring and other areas of research. Here we demonstrate a new method for detecting metal ions that is based on selective plasmonic resonance energy transfer (PRET) between conjugated metal-ligand complexes and a single gold nanoplasmonic probe. In addition to offering high spatial resolution due to the small size of the probe, our method is 100 to 1,000 times more sensitive than organic reporter-based methods. Moreover, it can achieve high selectivity owing to the selective formation of Cu(2+) complexes and selective resonant quenching of the gold nanoplasmonic probe by the conjugated complexes. We expect that PRET-based metal ion sensing could have applications in cellular imaging, systems biology and environmental monitoring.

Metal-chelating and dansyl-labeled poly(N-isopropylacrylamide) microgels as fluorescent Cu2+ sensors with thermo-enhanced detection sensitivity

We report on the fabrication of Cu2+-sensing thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) microgels labeled with metal-chelating acceptor and fluorescent reporter moieties. Cu2+ detection sensitivity can be considerably enhanced via thermo-induced collapse of the sensing matrix, which can easily optimize the relative spatial distribution of Cu2+-binding sites and fluorescence readout functionalities. A novel picolinamine-containing monomer with Cu2+-binding capability, N-(2-(2-oxo-2-(pyridine 2-yl-methylamino)ethylamino)ethyl)acrylamide (PyAM, 3), was synthesized at first. Nearly monodisperse Cu2+-sensing microgels were prepared via emulsion polymerization of N-isopropylacrylamide (NIPAM) in the presence of a nonionic surfactant, N,N'-Methylene-bis(acrylamide) (BIS), PyAM (3), and fluorescent dansylaminoethyl- acrylamide (DAEAM, 5) monomers at around neutral pH and 70 degrees C. At 20 degrees C, as-synthesized microgels in their swollen state can selectively bind Cu2+ over other metal ions (Hg2+, Mg2+, Zn2+, Pb2+, Ag+, and Al3+), leading to prominent quenching of fluorescence emission intensity. Above the volume phase transition temperature, P(NIPAM-co-PyAM-co-DAEAM) microgels exhibit increased fluorescence intensity. It was observed that Cu2+ detection sensitivity can be dramatically enhanced via thermo-induced microgel collapse at elevated temperatures. At a microgel concentration of 3.0x10(-6) g/mL, the detection limit drastically improved from approximately 46 nM at 20 degrees C to approximately 8 nM at 45 degrees C. The underlying mechanism for this novel type of sensor with thermotunable detection sensitivity was tentatively proposed.

A facile approach for cupric ion detection in aqueous media using polyethyleneimine/PMMA core-shell fluorescent nanoparticles

A facile approach was developed to produce a dye-doped core-shell nanoparticle chemosensor for detecting Cu(2+) in aqueous media. The core-shell nanoparticle sensor was prepared by a one-step emulsifier-free polymerization, followed by the doping of the fluorescent dye Nile red (9-diethylamino- 5H-benzo[alpha] phenoxazine-5-one, NR) into the particles. For the nanoparticles, the hydrophilic polyethyleneimine (PEI) chain segments serve as the shell and the hydrophobic polymethyl methacrylate (PMMA) constitutes the core of the nanoparticles. The non-toxic and biocompatible PEI chain segments on the nanoparticle surface exhibit a high affinity for Cu(2+) ions in aqueous media, and the quenching of the NR fluorescence is observed upon binding of Cu(2+) ions. This makes the core-shell nanoparticle system a water-dispersible chemosensor for Cu(2+) ion detection. The quenching of fluorescence arises through intraparticle energy transfer (FRET) from the dye in the hydrophobic PMMA core to the Cu(2+)/PEI complexes on the nanoparticle surface. The energy transfer efficiency for PEI/PMMA particles with different diameters was determined, and it is found that the smaller nanoparticle sample exhibits higher quenching efficiency, and the limit for Cu(2+) detection is 1 microM for a nanoparticle sample with a diameter of approximately 30 nm. The response of the fluorescent nanoparticle towards different metal ions was investigated and the nanoparticle chemosensor displays high selectivity and antidisturbance for the Cu(2+) ion among the metal ions examined (Na(+), K(+), Mg(2+), Ca(2+), Zn(2+), Hg(2+), Mn(2+), Fe(2+), Ni(2+), Co(2+) and Pb(2+)). This emulsifier-free, biocompatible and sensitive fluorescent nanoparticle sensor may find applications in cupric ion detection in the biological and environmental areas.

Photoreversible fluorescent modulation of nanoparticles via one-step miniemulsion polymerization

A nitrobenzoxadiazolyl(NBD)-based fluorescent dye and a photochromic spiropyran derivative are incorporated into polymeric nanoparticles via a one-step miniemulsion polymerization. The diameter of the nanoparticles can be varied from approximately 40 nm to 80 nm by adjusting the polymerization conditions. The prepared nanoparticles exhibit the spectral properties of both NBD dye and spiropyran, indicating that the two chromophores are incorporated into the nanoparticles. The determined amount of NBD and spiropyran in the nanoparticles are about approximately 85-90% of the feed amount, while the determined weight ratios of spiropyran to NBD in nanoparticles are very close to that of feed ratios, suggesting the miniemulsion polymerization is a suitable approach for incorporating multiple chromophores into individual nanoparticles with controlled amounts (content) and ratio. UV and visible light can be applied to modulate the fluorescence emission of NBD dye in nanoparticles. Upon UV irradiation, the spiropyran moieties in nanoparticles are converted to the open-ring (McH form) structure and upon visible-light irradiation they return to the closed-ring (SP form) structure; as a result, the fluorescence of NBD can be reversibly "switched off" and "switched on". Fluorescence resonance energy transfer from the excited NBD dye molecules to the McH form of the spiropyran moieties is the drives the fluorescence modulation. The nanoparticles display fairly good photoreversibility, photostability, and relatively fast photoresponsivity upon alternate UV/Vis irradiation. This class of photoresponsive nanoparticles may find applications in biological fields, such as labeling and imaging, as well as in optical fields, for example, individually light-addressable nanoscale devices.

A molecular paddlewheel with a sliding organometallic latch: syntheses, X-ray crystal structures and dynamic behaviour of [Cr(CO)3{eta(6)-2-(9-triptycyl)indene}], and of [M(CO)3{eta(5)-2-(9-triptycyl)indenyl}] (M = Mn, Re)

In [eta(6)-2-(9-triptycyl)-indene]tricarbonylchromium (2a), the indenyl-chromium moiety is linked directly to the axis of the three-bladed triptycene paddlewheel. However, the molecular structure of 2a reveals that there is no steric interaction between these components, and the paddlewheel is free to rotate. Accordingly, its NMR spectrum indicates the full equivalence of the blades of the triptycene. Deprotonation of the indene induces a haptotropic shift of the organometallic fragment from the six-membered to the five-membered ring of the indene and, in the sodium [eta(5)-2-(9-triptycyl)-indenyl]tricarbonylchromium salt (3a), so formed, rotation of the three-bladed molecular paddlewheel is now blocked by the bulky tripod. NMR data for 3a, and also for the isostructural eta(5)-Mn(CO)(3) and eta(5)-Re(CO)(3) complexes, 3b and 3c, respectively, reveal a 2:1 splitting of the blades of the triptycyl moiety, thus breaking its original threefold symmetry. The X-ray crystal structures of the chromium complex, 2a, and of the manganese and rhenium complexes, 3b and 3c, provide pictures of the system in both its "ON" and "OFF" states, whereby the M(CO)(3) tripod has moved about 2 A towards the triptycene, thus blocking its rotation. Comparison of the rotational barriers in 2-(9-triptycyl)indene (1) and its complexes 2 and 3, suggests that rotation of the paddlewheel can be slowed by a factor of approximately 10(8).

A pyrenyl-appended triazole-based calix[4]arene as a fluorescent sensor for Cd2+ and Zn2+

The synthesis and evaluation of a novel calix[4]arene-based fluorescent chemosensor 8 for the detection of Cd(2+) and Zn(2+) is described. The fluorescent spectra changes observed upon addition of various metal ions show that 8 is highly selective for Cd(2+) and Zn(2+) over other metal ions. Addition of Cd(2+) and Zn(2+) to the solution of 8 results in ratiometric measurement.

Amplified fluorescence response of chemosensors grafted onto silica nanoparticles

In conventional fluorescent chemosensors, the recognition of the target by the receptor unit affects the fluorescence properties of a single covalently coupled fluorescent moiety. Here we show for the first time that when a suitable TSQ derivative is densely grafted onto the surface of preformed silica nanoparticles electronic interactions between the individual chemosensor units enable the free units to recognize the state of the surrounding complexed ones. As a result, the fluorescence transduction is not limited to the local site where binding occurs, but it involves a wider region of the fluorophore network that is able to transfer its excitation energy to the complexed units. Such behavior leads to an amplification of the fluorescence signal. What we report here is the first example of amplification in the case an off-on chemosensor due to its organization onto the surface of silica nanoparticles. We also describe a simple general model to approach amplification in multifluorophoric systems based on the localization of the excited states, which is valid for assemblies such as the supramolecular ones where molecular interactions are weak and do not significantly perturb the individual electronic states. The introduction of an amplification factor f in particular allows for a simple quantitative estimation of the amplification effects.

Convenient synthesis and properties of polypropyleneimine dendrimer-functionalized polymer nanoparticles

A simple synthesis of polymer core-dendrimer shell nanoparticles (NPs) in the 15-20-nm-diameter range is presented. Amine-terminated polypropyleneimine (PPI) dendrimers DAB-dendri-(NH(2))(4) and DAB-dendri-(NH(2))(16) (DAB4 and DAB16) are covalently attached to the surface of primary polystyrene-based NPs bearing reactive chlorobenzyl groups produced by microemulsion polymerization in the presence of a cationic surfactant. The grafting readily proceeds under mild conditions and leads to translucent aqueous suspensions of core-shell-type NPs with a high density of peripheral amine groups that have been characterized relative to their size and chemical composition. The dendritic shell acts as a protective ionizable outer layer and provides an improvement of the colloidal stability in neutral and acidic media. The metal-binding capacity of the PPI dendrimers is retained, and spectrophotometric titrations show that the dendrimer-grafted NPs can trap a large number of Cu(2+) ions (more than 900 Cu per NP-DAB16). These properties make them potentially valuable templates for the elaboration of hybrid nanomaterials. The reactivity of the external amine groups is used to link covalently azobenzene chromophores (disperse Red 1 residues) through aza-Michael addition in aqueous suspension. This simple method gives access to colored NPs with high dye contents in the outer layer (up to 1000-1500 dye molecules per NP), which indicates that dendrimer-functionalized NPs are valuable building blocks for the construction of multifunctional nanomaterials.

Fluorescent sensor array in a microfluidic chip

Miniaturization and automation are highly important issues for the development of high-throughput processes. The area of micro total analysis systems (muTAS) is growing rapidly and the design of new schemes which are suitable for miniaturized analytical devices is of great importance. In this paper we report the immobilization of self-assembled monolayers (SAMs) with metal ion sensing properties, on the walls of glass microchannels. The parallel combinatorial synthesis of sensing SAMs in individually addressable microchannels towards the generation of optical sensor arrays and sensing chips has been developed. [figure: see text] The advantages of microfluidic devices, surface chemistry, parallel synthesis, and combinatorial approaches have been merged to integrate a fluorescent chemical sensor array in a microfluidic chip. Specifically, five different fluorescent self-assembled monolayers have been created on the internal walls of glass microchannels confined in a microfluidic chip.

Self-organized fluorescent nanosensors for ratiometric Pb2+ detection

Silica nanoparticles (60 nm diameter) doped with fluorescent dyes and functionalized on the surface with thiol groups have been proved to be efficient fluorescent chemosensors for Pb2+ ions. The particles can detect a 1 microM metal ion concentration with a good selectivity, suffering only interference from Cu2+ ions. Analyte binding sites are provided by the simple grafting of the thiol groups on the nanoparticles. Once bound to the particles surface, the Pb2+ ions quench the emission of the reporting dyes embedded. Sensor performances can be improved by taking advantage of the ease of production of multishell silica particles. On one hand, signaling units can be concentrated in the external shells, allowing a closer interaction with the surface-bound analyte. On the other, a second dye can be buried in the particle core, far enough from the surface to be unaffected by the Pb2+ ions, thus producing a reference signal. In this way, a ratiometric system is easily prepared by simple self-organization of the particle components.

Design of a Hybrid Biosensor for Enhanced Phosphopeptide Recognition Based on a Phosphoprotein Binding Domain Coupled with a Fluorescent Chemosensor

Protein-based fluorescent biosensors with sufficient sensing specificity are useful analytical tools for detection of biologically important substances in complicated biological systems. Here, we present the design of a hybrid biosensor, specific for a bis-phosphorylated peptide, based on a natural phosphoprotein binding domain coupled with an artificial fluorescent chemosensor. The hybrid biosensor consists of a phosphoprotein binding domain, the WW domain, into which has been introduced a fluorescent stilbazole having Zn(II)-dipicolylamine (Dpa) as a phosphate binding motif. It showed strong binding affinity and high sensing selectivity toward a specific bis-phosphorylated peptide in the presence of various phosphate species such as the monophosphorylated peptide, ATP, and others. Detailed fluorescence titration experiments clearly indicate that the binding-induced fluorescence enhancement and the sensing selectivity were achieved by the cooperative action of both binding sites of the hybrid biosensor, i.e., the WW domain and the Zn(II)-Dpa chemosensor unit. Thus, it is clear that the tethered Zn(II)-Dpa-stilbazole unit operated not only as a fluorescence signal transducer, but also as a sub-binding site in the hybrid biosensor. Taking advantage of its selective sensing property, the hybrid biosensor was successfully applied to real-time and label-free fluorescence monitoring of a protein kinase-catalyzed phosphorylation.

Silica Nanoparticles for Fluorescence Sensing of ZnII: Exploring the Covalent Strategy

Silica nanoparticles (about 15 nm diameters), which contain a derivative of 6-methoxy-8-(p-toluensulfonamido)-quinoline (TSQ) as a Zn(II) fluorescent probe covalently linked to the silica network, were prepared and studied as Zn(II) fluorescent chemosensors. The systems selectively detect Zn(II) ions in water rich solutions with a submicromolar sensitivity: 0.13 microM concentrations of Zn(II) can be measured with the only interference of Cu(II) and Cd(II) ions. Compared with free TSQ, the nanoparticles based systems have the advantage that they can be employed in aqueous solutions without aggregation problems while at the same time, they maintain a similar Zn(II) affinity and sensing ability. Addition of a second, substrate insensitive, fluorophore to the particles leads to the realization of a ratiometric sensor.

Design of fluorescent materials for chemical sensing

There is an enormous demand for chemical sensors for many areas and disciplines. High sensitivity and ease of operation are two main issues for sensor development. Fluorescence techniques can easily fulfill these requirements and therefore fluorescent-based sensors appear as one of the most promising candidates for chemical sensing. However, the development of sensors is not trivial; material science, molecular recognition and device implementation are some of the aspects that play a role in the design of sensors. The development of fluorescent sensing materials is increasingly captivating the attention of the scientists because its implementation as a truly sensory system is straightforward. This critical review shows the use of polymers, sol-gels, mesoporous materials, surfactant aggregates, quantum dots, and glass or gold surfaces, combined with different chemical approaches for the development of fluorescent sensing materials. Representative examples have been selected and they are commented here.

Cp*Rh-Based Indicator-Displacement Assays for the Identification of Amino Sugars and Aminoglycosides

Indicator-displacement assays based on the organometallic complex [{Cp*RhCl2}2] (Cp*=pentamethylcyclopentadienyl) and the dye gallocyanine were used to sense amino sugars and aminoglycosides in buffered aqueous solution by conducting UV-visible spectroscopy. The data of three assays at pH 7.0, 8.0, and 9.0 were sufficient to distinguish between the amino sugars galactosamine, glucosamine, mannosamine and the aminoglycosides kanamycin A, kanamycin B, amikacin, apramycin, paromomycin, and streptomycin. Furthermore, the assays were used to characterize mixtures of aminoglycosides and obtain quantitative information about the respective analytes.

Quinoline-based molecular clips for selective fluorescent detection of Zn2+

New selective Zn2+ fluorescent sensors, di(2-quinoline-carbaldehyde)-2,2'-bibenzoyl-hydrazone (QB1) and di(2-quinolinecarbaldehyde)-6,6'-dicarboxylic acid hydrazone-2,2'-bipyridine (QB2), have been designed and prepared. Both QB sensors exhibit an emission band centered at 405 nm (excitation at 350 nm) with low quantum yield. Zinc binding not only red-shifts the emission band to 500 nm, but also enhances the fluorescence intensity by an order of magnitude based on the deprotonization strategy via self-assembly. These probes are highly selective for Zn2+ over biologically relevant alkali metals, alkaline earth metals and the first row transition metals such as Mn2+, Fe2+, Co2+ and Ni2+ in buffered aqueous DMSO solution.