Nonspecific interactions of a carboxylate-substituted PPE with proteins. A cautionary tale for biosensor applications

Water-Soluble Fluorescent Polymer Dyes with Tunable Emission Spectra for Flow Cytometry Applications

The application of spectrally unique, bright, and water-soluble fluorescent dyes is indispensable for the analysis of biological systems. Multiparameter flow cytometry is a powerful tool for characterization of mixed cell populations. To discriminate the different cell populations, they are typically stained by a set of fluorescent reagents, e.g., antibody-fluorophore conjugates. The number of parameters which can be studied simultaneously strongly depends on the availability of reagents which can be differentiated by their spectral properties. In this study a series of fluorescent polymer dyes was developed, that can be excited with a single violet laser (405 nm) but distinguished by their unique emission spectra. The polyfluorene-based polymers can be used on their own, or in combination with covalently bound small-molecule dyes to generate energy transfer constructs to red-shift the emission wavelength based on Förster resonance energy transfer (FRET). The polymer dyes were utilized in a biological flow cytometry assay by conjugating several of them to antibodies, demonstrating their effectiveness as reagents. This report represents the first systematic investigation of structure-property relationships for this type of fluorescent dyes.

Aggregation and Binding-Directed FRET Modulation of Conjugated Polymer Materials for Selective and Point-of-Care Monitoring of Serum Albumins

Nonspecific interactions of conjugated polymers (CPs) with various proteins prove to be a major impediment for researchers when designing a suitable CP-based probe for the amplified and selective recognition of particular proteins in complex body fluids. Herein, a new strategy is presented for the precise and specific monitoring of clinically important serum albumin (SA) proteins at the nanomolar level using fluorescence resonance energy transfer (FRET)-modulated CP-surfactant ensembles as superior sensing materials. In brief, the newly designed color-tunable CP PF-DBT-Im undergoes intense aggregation with the surfactant sodium dodecyl sulfate (SDS), enabling drastic change in the emission color from violet to deep red due to intermolecular FRET. The emission of PF-DBT-Im/SDS ensembles then changed from deep red to magenta specifically on addition of SAs owing to the exclusive reverse FRET facilitated by synergistic effects of electrostatic interactions, hydrophobic forces, and the comparatively high intrinsic quantum yield of SAs. Interestingly, PF-DBT-Im itself could not differentiate SAs from other proteins, demonstrating the superiority of the PF-DBT-Im/SDS self-assembly over PF-DBT-Im. Finally, an affordable smartphone-integrated point-of-care (PoC) device is also fabricated as a proof-of-concept for the on-site and rapid monitoring of SAs, validating the potential of the system in long-term clinical applications.

A Portrait of the OPE as a Biological Agent

Oligophenylene ethynylenes, known as OPEs, are a sequence of aromatic rings linked by triple bonds, the properties of which can be modulated by varying the length of the rigid main chain or/and the nature and position of the substituents on the aromatic units. They are luminescent molecules with high quantum yields and can be designed to enter a cell and act as antimicrobial and antiviral compounds, as biocompatible fluorescent probes directed towards target organelles in living cells, as labelling agents, as selective sensors for the detection of fibrillar and prefibrillar amyloid in the proteic field and in a fluorescence turn-on system for the detection of saccharides, as photosensitizers in photodynamic therapy (due to their capacity to highly induce toxicity after light activation), and as drug delivery systems. The antibacterial properties of OPEs have been the most studied against very popular and resistant pathogens, and in this paper the achievements of these studies are reviewed, together with almost all the other roles held by such oligomers. In the recent decade, their antifungal and antiviral effects have attracted the attention of researchers who believe OPEs to be possible biocides of the future. The review describes, for instance, the preliminary results obtained with OPEs against severe acute respiratory syndrome coronavirus 2, the virus responsible for the COVID-19 pandemic.

Finding Value in Wastewaters from the Cork Industry: Carbon Dots Synthesis and Fluorescence for Hemeprotein Detection

Valorisation of industrial low-value waste residues was preconized. Hence, carbon dots (C-dots) were synthesized from wastewaters of the cork industry-an abundant and affordable, but environmentally-problematic industrial effluent. The carbon nanomaterials were structurally and morphologically characterised, and their photophysical properties were analysed by an ensemble of spectroscopy techniques. Afterwards, they were successfully applied as highly-sensitive fluorescence probes for the direct detection of haemproteins. Haemoglobin, cytochrome c and myoglobin were selected as specific targets owing to their relevant roles in living organisms, wherein their deficiencies or surpluses are associated with several medical conditions. For all of them, remarkable responses were achieved, allowing their detection at nanomolar levels. Steady-state and time-resolved fluorescence, ground-state UV-Vis absorption and electronic circular dichroism techniques were used to investigate the probable mechanisms behind the fluorescence turn-off of C-dots. Extensive experimental evidence points to a static quenching mechanism. Likewise, resonance energy transfer and collisional quenching have been discarded as excited-state deactivating mechanisms. It was additionally found that an oxidative, photoinduced electron transfer occurs for cytochrome c, the most electron-deficient protein. Besides, C-dots prepared from citric acid/ethylenediamine were comparatively assayed for protein detection and the differences between the two types of nanomaterials highlighted.

Synthesis and photophysical properties of water-soluble fluorinated poly(aryleneethynylene)s

Reported are the synthesis of water-soluble fluorinated conjugated polymers, and photophysical properties, and fluorescence quenching response to arylamines.

Intercalation of Alkynylplatinum(II) Terpyridine Complexes into a Helical Poly(phenylene ethynylene) Sulfonate: Application to Protein Sensing

The interactions of two anionic poly(phenylene ethynylene) sulfonate-conjugated polyelectrolytes (mPPESO₃^- and pPPESO₃^-) with two alkynylplatinum(II) terpyridine complexes (Pt²⁺ and Pt³⁺) were studied. The Pt(II) complexes interact with helical mPPESO₃^- by intercalation within the polymer helix to form a "guest-host" ensemble. Titration of Pt(II) complexes into an aqueous solution of mPPESO₃^- gives rise to efficient quenching of the polymer's fluorescence; meanwhile, triplet metal-metal-to-ligand charge transfer (³MMLCT) state emission from the intercalated Pt(II) complexes appears when the ensembles are excited into the polymer's absorption band. The ³MMLCT state emission implies that the Pt(II) complexes aggregate or dimerize on the mPPESO₃^- scaffold. The responses of the mPPESO₃^- and Pt(II) complex ensembles to various proteins were examined by monitoring the mPPESO₃^- fluorescence change. Negatively charged proteins recover the mPPESO₃^- fluorescence more than the positively charged proteins under physiological pH, indicating that electrostatics play an important role in the protein-ensemble interaction.

Chemical Tongues and Noses Based upon Conjugated Polymers

We report the uses of conjugated polymers in multisensory applications and in chemical and optoelectronic tongues. We look at the potential of single polymers to discriminate multiple analytes and into small libraries of conjugated polymers that represent sensors. These small libraries combine several barely selective, promiscuous sensor elements and react with the analytes in a fairly non-selective fashion by change of color, emission wavelength, or emission intensity. In such optoelectronic noses and tongues, response of a single element is not specific or particularly useful at all, but the response pattern after the combination of several sensor elements is often specific for an analyte and allows discrimination and identification without any problem. These types of tongues and noses are well suited for quality control of foodstuff, beverages, and biological species such as proteins or cells. The discriminative process is often not well understood but it is powerful, particularly if the obtained data are analyzed by sophisticated statistical methods, i.e., linear discriminant analysis and/or principal component analysis. This added layer of analysis extracts the hidden information/patterns out of the data and allows visualization of the results.

End Functionalized Nonionic Water-Dispersible Conjugated Polymers

2,7-Dibromofluorene monomers carrying two or four oligo(ethylene glycol) (OEG) side chains are synthesized. Heck coupling between the monomers and 1,4-divinylbenzene followed by end capping with [4-(4-bromophenoxy)butyl]carbamic acid tert-butyl ester leads to two nonionic water-dispersible poly(fluorene-alt-1,4-divinylenephenylene)s end-functionalized with amine groups after hydrolysis. In water, the polymer with a lower OEG density (P1) has poor water dispersibility with a quantum yield of 0.24, while the polymer with a higher OEG density (P2) possesses excellent water-dispersibility with a high quantum yield of 0.45. Both polymers show fluorescence enhancement and blue-shifted absorption and emission maxima in the presence of surfactant sodium dodecyl sulfate and dodecyltrimethylammonium bromide. The polymers are also resistant to ionic strength with minimal nonspecific interactions to bovine serum albumin. When biotin is incorporated into the end of the polymer backbones through N-hydroxysuccinimide/amine coupling reaction, the biotinylated polymers interact specifically with streptavidin on solid surface.

Optical probes and sensors as perspective tools in epigenetics

Modifications of DNA cytosine bases and histone posttranslational modifications play key roles in the control of gene expression and specification of cell states. Such modifications affect many important biological processes and changes to these important regulation mechanisms can initiate or significantly contribute to the development of many serious pathological states. Therefore, recognition and determination of chromatin modifications is an important goal in basic and clinical research. Two of the most promising tools for this purpose are optical probes and sensors, especially colourimetric and fluorescence devices. The use of optical probes and sensors is simple, without highly expensive instrumentation, and with excellent sensitivity and specificity for target structural motifs. Accordingly, the application of various probes and sensors in the recognition and determination of cytosine modifications and structure of histones and histone posttranslational modifications, are discussed in detail in this review.

Fostering protein–calixarene interactions: from molecular recognition to sensing

An highly selective direct sensing of cytochrome c by a bis-calix[4]arene-carbazole conjugate (CCC-1) in aqueous-based medium at nanomolar levels is disclosed. An electron transfer (ET) between complexed partners mediates the sensory event.

Fluorescent Ensemble Based on Bispyrene Fluorophore and Surfactant Assemblies: Sensing and Discriminating Proteins in Aqueous Solution

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.

Oligo(p-phenylene ethynylene) Electrolytes: A Novel Molecular Scaffold for Optical Tracking of Amyloids

Finding new optical probes to detect and track amyloid protein aggregates is key to understanding and defeating the myriad of neurodegenerative and other diseases associated with these misfolded proteins. Herein we report that a series of fluorescent, soluble oligo(p-phenylene ethynylene)s (OPEs) are able to detect amyloids in vitro by massive binding-activated superluminescence, with low micromolar affinity and high selectivity for the amyloid conformer. The OPEs track the kinetics of amyloid fibril formation from native hen egg white lysozyme (HEWL) similarly to thioflavin T (ThT), and the dependence of binding affinity on OPE length supports the theory of a linear binding groove. We hypothesize, based on spectral properties, induced circular dichroism, and previous work in analogous systems, that the fluorescence turn-on mechanism is a combination of the reduction of static solvent-mediated quenching at the ethyl ester end groups of the phenylene ethynylene fluorophore and the formation of chiral J-type aggregates templated on the amyloid fibril surface.

Polymer - Nanoparticle Assemblies for Array Based Sensing

Sensing clinically relevant biomolecules is crucial for the detection and prevention of disease. Currently used detection methods tend to be expensive, time intensive, and specific for only one particular biomolecule of interest. Nanoparticle-based arrays using conjugated polymers have emerged as an analytical and potential clinical tool, allowing detection of a wide range of biomolecules using selective, not specific, sensor components. In this report, we highlight recent progress in nanoparticle - polymer sensor arrays in both the fundamental understanding of how the sensor arrays function as well as the detection of clinically relevant bacteria and cells.

Pyridine-based poly(aryleneethynylene)s: a study on anionic side chain density and their influence on optical properties and metallochromicity

We report the Pd-catalyzed synthesis of six new water soluble, alternating poly(p-phenylene-ethynylene-p-pyridinylene-ethynylene) (abcb-alternating) copolymers and one poly(p-pyridinyleneethynylene).

Monodispersed nanoparticles of conjugated polyelectrolyte brush with high charge density for rapid, specific and label-free detection of tumor marker

Rapid and label-free detection of human α-fetoprotein (AFP) based on selective superquenching of monodispersed nanoparticles of conjugated polyelectrolyte.

Nanoparticles made of π-conjugated compounds targeted for chemical and biological applications

This feature article summarizes the recent applications of nanoparticles made of π-conjugated compounds in bio/chemo-sensing, disease therapy, and photoacoustic imaging.

Synthesis and chemosensory application of water-soluble polyfluorenes containing carboxylated groups

Detection of metal ions in aqueous solutions is a major issue for environmental protection. Conjugated polyelectrolytes showing high sensitivity and selectivity towards the detection of metal ions are highly desirable. We report a water-soluble polyfluorene containing carboxylated groups (P1), poly[9,9'-bis(3''-propanoate)fluoren-2,7-yl] sodium salt, which shows high recognition capability toward Cu(+) and Cu(2+). P1 was prepared via the hydrolysis of poly[9,9'-bis(tert-butyl-3''-propanoate)fluoren-2,7-yl] (P0) which was synthesized by Suzuki coupling polymerization. The photoluminescence (PL) spectra of P1 in aqueous solution are significantly quenched in the presence of Cu(+) and Cu(2+). P1 shows high selectivity and sensitivity toward Cu(+) and Cu(2+), with the Stern-Volmer constants (Ksv) being 3.5 × 10(6) and 5.78 × 10(6) M(-1), respectively. Moreover, the stoichiometric ratio of the P1 repeat unit to Cu(+) or Cu(2+) is 2 : 1 obtained from Job's plot. P1 maintains high selectivity towards Cu(+) or Cu(2+) in the presence of various metal cations. Our results demonstrate that P1 shows very high sensitivity and selectivity in recognizing Cu(+) and Cu(2+), indicating that it is a promising functional material for chemical sensors.

Protein induced aggregation of conjugated polyelectrolytes probed with fluorescence correlation spectroscopy: application to protein identification

The interaction of a series of water-soluble conjugated polyelectrolytes with varying backbone structure, charge type (cationic and anionic), and charge density with a set of seven different proteins is explored by using fluorescence correlation spectroscopy (FCS). The FCS method affords the diffusion time for a particular CPE/protein pair, and this diffusion time is a reflection of the aggregation state of the polymer/protein in the solution. The diffusion time is larger for oppositely charged CPE/protein combinations, reflecting the tendency toward the formation of CPE/protein aggregates in these systems. However, by careful analysis of the data, other factors emerge, including possible effects of hydrophobic interaction in specific CPE/protein systems. The final diffusion time for each CPE/protein mixture varies and the diffusion time response pattern created by the six-CPE array for a typical protein is unique, and this effect was leveraged to develop a sensor array for protein identification by using linear-discriminant analysis (LDA) methods. By application of multimode linear discrimination analysis, the unknown protein samples have been successfully identified with a total accuracy of 93%.

Fluorescent organic nanoparticles based on branched small molecule: preparation and ion detection in lithium-ion battery

Fluorescent organic nanoparticles (FONs) as a new class of nanomaterials can provide more advantages than molecule based probes. However, their applications in specific metal ion detection have rarely been exploited. We design and synthesize a branched small-molecule compound with triazole as a core and benzothiadiazole derivative as branches. By a facile reprecipitation method, nanoparticles (NPs) of this compound can be prepared in aqueous solutions, which can show high selectivity and sensitivity to Fe(III) ions based on fluorescence quenching. In addition, the fluorescence intensity of these NPs is resistant to pH changes in solutions. Such characters of this kind of NPs can be utilized in Fe(3+) impurity detection in a promising cathode material (LiFePO4) for lithium ion batteries. When exposed to Fe(3+), both the triazole and benzothiadiazole groups contribute to the fluorescence quenching of NPs, but the former one plays a more important role in Fe(3+) impurity detection. The sensing mechanism has also been investigated which indicates that a Fe-organic complex formation may be responsible for such sensing behavior. Our findings demonstrate that specific metal ion detection can be realized by FONs and have extended the application field of FONs for chemical sensing in aqueous solutions.

Multivalent glycoconjugate syntheses and applications using aromatic scaffolds

Glycan-protein interactions are of utmost importance in several biological phenomena. Although the variety of carbohydrate residues in mammalian cells is limited to less than a dozen different sugars, their spatial topographical presentation in what is now associated as the "glycocodes" provides the fundamental keys for specific and high affinity "lock-in" recognition events associated with a wide range of pathologies. Toward deciphering our understanding of these glycocodes, chemists have developed new creative tools that included dendrimer chemistry in order to provide monodisperse multivalent glycoconjugates. This review provides a survey of the numerous aromatic architectures generated for the multivalent presentation of relevant carbohydrates using covalent attachment or supramolecular self-assemblies. The basic concepts toward their controlled syntheses will be described using modern synthetic procedures with a particular emphasis on powerful organometallic methodologies. The large variety of dendritic aromatic scaffolds, together with a brief survey of their unique biophysical and biological properties will be critically reviewed. The distinctiveness of the resulting multivalent glycoarchitectures, encompassing glycoclusters, glycodendrimers and molecularly defined self-assemblies, in forming well organized cross-linked lattices with multivalent carbohydrate binding proteins (lectins) together with their photophysical, medical, and imaging properties will also be briefly highlighted. The topic will be presented in increasing order of aromatic backbone complexities and will end with fullerenes together with self-assembled nanostructures, thus complementing the various scaffolds described in this special thematic issue dedicated to multivalent glycoscience.

Nonionic, water self-dispersible "hairy-rod" poly(p-phenylene)-g-poly(ethylene glycol) copolymer/carbon nanotube conjugates for targeted cell imaging

The generation and fabrication of nanoscopic structures are of critical technological importance for future implementations in areas such as nanodevices and nanotechnology, biosensing, bioimaging, cancer targeting, and drug delivery. Applications of carbon nanotubes (CNTs) in biological fields have been impeded by the incapability of their visualization using conventional methods. Therefore, fluorescence labeling of CNTs with various probes under physiological conditions has become a significant issue for their utilization in biological processes. Herein, we demonstrate a facile and additional fluorophore-free approach for cancer cell-imaging and diagnosis by combining multiwalled CNTs with a well-known conjugated polymer, namely, poly(p-phenylene) (PP). In this approach, PP decorated with poly(ethylene glycol) (PEG) was noncovalently (π-π stacking) linked to acid-treated CNTs. The obtained water self-dispersible, stable, and biocompatible f-CNT/PP-g-PEG conjugates were then bioconjugated to estrogen-specific antibody (anti-ER) via -COOH functionalities present on the side-walls of CNTs. The resulting conjugates were used as an efficient fluorescent probe for targeted imaging of estrogen receptor overexpressed cancer cells, such as MCF-7. In vitro studies and fluorescence microscopy data show that these conjugates can specifically bind to MCF-7 cells with high efficiency. The represented results imply that CNT-based materials could easily be fabricated by the described approach and used as an efficient "fluorescent probe" for targeting and imaging, thereby providing many new possibilities for various applications in biomedical sensing and diagnosis.

Design and synthesis of monofunctionalized, water-soluble conjugated polymers for biosensing and imaging applications

Water-soluble conjugated polymers with controlled molecular weight characteristics, absence of ionic groups, high emission quantum yields, and end groups capable of selective reactions of wide scope are desirable for improving their performance in various applications and, in particular, fluorescent biosensor schemes. The synthesis of such a structure is described herein. 2-Bromo-7-iodofluorene with octakis(ethylene glycol) monomethyl ether chains at the 9,9'-positions, i.e., compound 4, was prepared as the reactive premonomer. A high-yielding synthesis of the organometallic initiator (dppe)Ni(Ph)Br (dppe = 1,2-bis(diphenylphosphino)ethane) was designed and implemented, and the resulting product was characterized by single-crystal X-ray diffraction techniques. Polymerization of 4 by (dppe)Ni(Ph)Br can be carried out in less than 30 s, affording excellent control over the average molecular weight and polydispersity of the product. Quenching of the polymerization with [2-(trimethylsilyl)ethynyl]magnesium bromide yields silylacetylene-terminated water-soluble poly(fluorene) with a photoluminescence quantum efficiency of 80%. Desilylation, followed by copper-catalyzed azide-alkyne cycloaddition reaction, yields a straightforward route to introduce a wide range of specific end group functionalities. Biotin was used as an example. The resulting biotinylated conjugated polymer binds to streptavidin and acts as a light-harvesting chromophore to optically amplify the emission of Alexa Fluor-488 chromophores bound onto the streptavidin. Furthermore, the biotin end group makes it possible to bind the polymer onto streptavidin-functionalized cross-linked agarose beads and thereby incorporate a large number of optically active segments.

FRET detection of proteins using fluorescently doped electrospun nanofibers and pattern recognition

This paper reports the fabrication of solid-state nanofiber sensor arrays and their use for detection of multiple proteins using principal component analysis (PCA). Four cationic and anionic fluorescently embedded nanofibers are generated by an electrospinning method, yielding unique patterns of fluorescence change upon interaction with protein samples. Five metal and nonmetal containing proteins, i.e., hemoglobin, myoglobin, cytochrome c, BSA, and avidin, have been investigated; and the results show that distinct fluorescent patterns can be formed upon the addition of protein samples to the array of solid nanofiber substrates, allowing their unambiguous identification. The nanofiber films are highly repeatable with a batch-to-batch variation of approximately 5% and demonstrated outstanding reusability with less than a 15% loss of fluorescence intensity signal after 5 regenerations of test cycles. For a more practical visualization, a cluster map was generated using PCA of the change-in-fluorescence (ΔI) composite patterns, demonstrating the potential of the method for diagnostic applications.