Nanoparticles and nanocomposites for fluorescence sensing and imaging

Interlaboratory Comparison on Absolute Photoluminescence Quantum Yield Measurements of Solid Light Converting Phosphors with Three Commercial Integrating Sphere Setups

Scattering luminescent materials dispersed in liquid and solid matrices and luminescent powders are increasingly relevant for fundamental research and industry. Examples are luminescent nano- and microparticles and phosphors of different compositions in various matrices or incorporated into ceramics with applications in energy conversion, solid-state lighting, medical diagnostics, and security barcoding. The key parameter to characterize the performance of these materials is the photoluminescence/fluorescence quantum yield (Φf), i.e., the number of emitted photons per number of absorbed photons. To identify and quantify the sources of uncertainty of absolute measurements of Φf of scattering samples, the first interlaboratory comparison (ILC) of three laboratories from academia and industry was performed by following identical measurement protocols. Thereby, two types of commercial stand-alone integrating sphere setups with different illumination and detection geometries were utilized for measuring the Φf of transparent and scattering dye solutions and solid phosphors, namely, YAG:Ce optoceramics of varying surface roughness, used as converter materials for blue light emitting diodes. Special emphasis was dedicated to the influence of the measurement geometry, the optical properties of the blank utilized to determine the number of photons of the incident excitation light absorbed by the sample, and the sample-specific surface roughness. While the Φf values of the liquid samples matched between instruments, Φf measurements of the optoceramics with different blanks revealed substantial differences. The ILC results underline the importance of the measurement geometry, sample position, and blank for reliable Φf data of scattering the YAG:Ce optoceramics, with the blank's optical properties accounting for uncertainties exceeding 20%.

Recent progress in the development of small-molecule fluorescent probes for detection and imaging of selenocysteine and application in thyroid disease diagnosis

Selenocysteine (SeCys) is the 21st genetically encoded amino acid present in proteins and is involved in various biological functions. Inappropriate levels of SeCys can be considered as a sign of various diseases. Therefore, small molecular fluorescent probes for the detection and imaging of SeCys in vivo in biological systems are considered to be of significant interest for understanding the physiological role of SeCys. Thus, this article mainly provides a critical evaluation of recent advances made in SeCys detection along with the biomedical applications based on small molecular fluorescent probes published in the literature during the past half a dozen years. Therefore, the article primarily deals with the rational design of fluorescent probes, wherein these were selective towards SeCys over other biologically abundant molecules, in particular the thiol-based ones. The detection has been monitored by different spectral techniques, such as fluorescence and absorption spectroscopy and in some cases even visual color changes. Further, the detection mechanism and the utility of fluorescent probes for in vitro and in vivo cell imaging applications are addressed. For clarity, the main features have been conveniently divided into four categories based on the chemical reactions of the probe, viz., in terms of the cleavage of the responsive group by the SeCys nucleophile: (i) 2,4-dinitrobene sulphonamide group, (ii) 2,4-dinitrobenesulfonate ester group, (iii) 2,4-dinitrobenzeneoxy group and (iv) miscellaneous types. Overall this article deals with the analysis of more than two dozen fluorescent probes demonstrated for selective detection of SeCys along with their applications towards disease diagnosis.

Fluorescence Quantum Yield Standards for the UV/Visible/NIR: Development, Traceable Characterization, and Certification

The rational design of next generation molecular and nanoscale reporters and the comparison of different emitter classes require the determination of the fluorometric key performance parameter fluorescence quantum yield (Φ_f), i.e., the number of emitted photons per number of absorbed photons. Main prerequisites for reliable Φ_f measurements, which are for transparent luminophore solutions commonly done relative to a reference, i.e., a fluorescence quantum yield standard of known Φ_f, are reliable and validated instrument calibration procedures to consider wavelength-, polarization-, and time-dependent instrument specific signal contributions, and sufficiently well characterized fluorescence quantum yield standards. As the standard's Φ_f value directly contributes to the calculation of the sample's Φ_f, its accuracy presents one of the main sources of uncertainty of relative Φ_f measurements. To close this gap, we developed a first set of 12 fluorescence quantum yield standards, which absorb and emit in the wavelength region of 330-1000 nm and absolutely determined their Φ_f values with two independently calibrated integrating sphere setups. Criteria for standard selection and the configuration of these novel fluorescence reference materials are given, and the certification procedure is presented including homogeneity and stability studies and the calculation of complete uncertainty budgets for the certified Φ_f values. The ultimate goal is to provide the community of fluorescence users with available reference materials as a basis for an improved comparability and reliability of quantum yield data since the measurement of this spectroscopic key property is an essential part of the characterization of any new emitter.

Luminescent Polymer Composites for Optical Fiber Sensors

Optical fiber sensors incorporating luminescent materials are useful for detecting physical parameters and biochemical species. Fluorescent materials integrated on the tips of optical fibers, for example, provide a means to perform fluorescence thermometry while monitoring the intensity or the spectral variations of the fluorescence signal. Similarly, certain molecules can be tracked by monitoring their characteristic emission in the UV wavelength range. A key element for these sensing approaches is the luminescent composite, which may be obtained upon allocating luminescent nanomaterials in glass or polymer hosts. In this work, we explore the fluorescence features of two composites incorporating lanthanide-doped fluorescent powders using polydimethylsiloxane (PDMS) as a host. The composites are obtained by a simple mixing procedure and can be subsequently deposited onto the end faces of optical fibers via dip coating or molding. Whereas one of the composites has shown to be useful for the fabrication of fiber optic temperature sensors, the other shows promising result for detection of UV radiation. The performance of both composites is first evaluated for the fabrication of membranes by examining features such as fluorescent stability. We further explore the influence of parameters such as particle concentration and density on the fluorescence features of the polymer blends. Finally, we demonstrate the incorporation of these PDMS fluorescent composites onto optical fibers and evaluate their sensing capabilities.

Construction of an ultra-small hydrazone-linked covalent organic polymer for selective fluorescent detection of ferric ion in aqueous solution

A novel ultra-small hydrazone-linked covalent organic polymer (UHCOP) was synthesized based on the Schiff-base reaction between 2,4,6-trihydroxy-1,3,5-benzenetricarbaldehyde and 1,4-benzenedicarbohydrazide at room temperature and utilized as a sensitive fluorescent sensor for rapid (<2 min) and selective detection of Fe³⁺ in aqueous solution. The prepared UHCOP displayed ultra-small size with the diameter of 7.98 ± 0.97 nm and gave a stable fluorescent emission at 510 nm. UHCOP exhibited good sensitivity and highly selectivity towards Fe³⁺. The coordination interaction between UHCOP and Fe³⁺ resulted in the obviously aggregation-caused quenching response of UHCOP. The linear range was from 5.0 μM to 1.4 mM (R² = 0.999) with the detection limit of 2.5 μM. Finally, UHCOP has been successfully applied in the detection of Fe³⁺ in real water samples, proving the fabricated UHCOP is promising as a sensitive fluorescent sensor for selective detection of Fe³⁺ in aqueous solution.

Highly Sensitive Fingerprint Detection under UV Light on Non-Porous Surface Using Starch-Powder Based Luminol-Doped Carbon Dots (N-CDs) from Tender Coconut Water as a Green Carbon Source

This study aims to synthesize carbon dots from a natural resource and will be used to detect a latent fingerprint on a non-porous surface. The carbon dots (CDs) were prepared by adding luminol to coconut water and ethanol via a hydrothermal method. Luminol enhances the chemiluminescence of the CDs, which show more distinct blue light under a UV lamp compared with bare CDs. To detect the latent fingerprint, luminol carbon dots (N-CDs) were combined with commercial starch and stirred at room temperature for 24 h. Their characteristics and optical properties were measured using EDX-SEM, HR-TEM, FTIR, XPS, UV-visible absorption, and fluorescence. In this research, it was found that the N-CDs had a d-spacing of 0.5 nm and a size of 12.9 nm. The N-CDs had a fluorescence intensity 551% higher than the standard normally used. N-CDs can be used to detect latent fingerprints on a non-porous surface and are easy to detect under a UV lamp at 395 nm. Therefore, luminol has a high potential to increase sensitive and stable traces of chemiluminescence from the green CDs for forensic latent fingerprint detection.

Gold nanoparticle formation as an indicator of enzymatic methods: colorimetric l-phenylalanine determination

An enzymatic-colorimetric method has been developed based on the reaction between l-phenylalanine (l-Phe) and the l-amino acid oxidase (LAAO) in the presence of Au(III), which has led to the formation of gold nanoparticles. The intensity of the localized surface plasmon resonance (LSPR) band of the generated nanoparticles (550 nm) can be related to the concentration of l-Phe in the sample. The mechanism of the LAAO-l-Phe enzyme reaction in the presence of Au(III) has been studied through the evaluation and optimization of experimental conditions. These studies have reinforced the hypothesis that the catalytic center of the enzyme helps the Au(III) reduction and, thanks to the protein, the Au⁰ form is stabilized as gold nanoparticles (AuNPs). In the calibration study, a sigmoidal relationship between the concentration of the substrate and the LSPR of the nanoparticles was observed. The linearization of the signal has allowed the determination of l-Phe in the range from 17 to 500 µM with an RSD% (150 μM) of 4.8% (n = 3). The method is free of other amino acid interference normally found in blood plasma. These highly competitive results open the possibility of further development of a rapid method for l-Phe determination based on colorimetry.

Eggshell Derived Europium Doped Hydroxyapatite Nanoparticles for Cell Imaging Application

Hen's eggshell, a biological waste product, was turned into a cell imaging probe: europium doped hydroxyapatite (HAp: Eu) nanoparticle using hydrothermal method. Luminescence of the synthesized nanoparticle was studied for various doping concentrations of the lanthanide ion europium (Eu³⁺). Eu doped HAp showed a hexagonal crystal structure and rod-shaped morphology. Well-defined emission peaks of europium, corresponding to the substitution of Eu³⁺ at the Ca²⁺(I) site of HAp, were confirmed from the samples' photoluminescence (PL) spectra. Good biocompatibility up to 500 μg/mL of the samples indicates their potential applications in bioimaging. Synthesized nanoparticles were internalized and used for in vitro imaging of the PC12 cells without any surface modification. The materials' use as a potential in vivo imaging agent is proposed from the haemolysis study.

One-step synthesis of M13 phage-based nanoparticles and their fluorescence properties

Fluorescent carbon nanoparticles have been gaining more attention in recent years for their excellent fluorescence properties and simple synthesis routes. Different carbon sources have been reported for fluorescent carbon nanoparticle synthesis but the use of virus particles as a carbon source is scarce. Herein, we report the utilization of M13 bacteriophage particles as the carbon source to synthesize phage-based nanoparticles through facile, one-step microwave heating. M13 bacteriophage is a nanosized filamentous virus particle with a single-stranded DNA genome encapsulated by a large number of coat proteins. These amino acid rich building blocks provide a substantial amount of carbon source for the synthesis of fluorescent nanoparticles. The resulting nanoparticles from M13 bacteriophage showed good water solubility and exhibited bright blue luminescence. The selectivity and sensitivity of the phage-based nanoparticles towards Fe(iii) ions showed a quenching effect with a linear correlation and a detection limit of 8.0 μM. This process highlights the potential application of virus particles as a source for the synthesis of fluorescent carbon nanoparticles and the sensing application.

Exploring the Conformation and Thermal Stability of Human Serum Albumin Corona of Ferrihydrite Nanoparticles

In the last few years, a great amount of attention has been given to nanoparticles research due to their physicochemical properties that allow their use in analytical instruments or in promising imaging applications on biological systems. The use of ferrihydrite nanoparticles (Fh-NPs) in practical applications implies a particular control of their magnetic properties, stability, biocompatibility, interaction with the surface of the target, and low toxicity. In this study, the formation and organization of human serum albumin (HSA) molecules around the simple Fh-NPs and Fh-NPs doped with Co and Cu were examined by Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) in terms of morphology and particle size. The topology of all Fh-NPs shows an organized area of HSA around each type of Fh-NP. Molecular docking studies were used in order to determine the probable location of the ferrihydrite in the HSA structure. The thermal stability of these nanohybrids was further investigated by fluorimetry, using 214-Trp residue from HSA as a spectral sensor. The denaturation temperature (T_m) was determined, and stabilization of the HSA structure in the presence of Fh-NPs was discussed. This study could be a starting point for the development of different applications targeting the structure and stability of Fh-NPs complexes with proteins.

2 + 2 Can Make Nearly a Thousand! Comparison of Di- and Tetra-Meso-Alkyl-Substituted Porphycenes

Two porphycenes, substituted at the meso positions with two and four methyl groups, respectively, reveal similar absorption spectra, but their photophysical properties are completely different. 9,20-dimethylporphycene emits fluorescence with about 20% quantum yield, independent of the solvent. In contrast, fluorescence of 9,10,19,20-tetramethylporphycene is extremely weak in nonviscous solvents, but it can be recovered by placing the chromophore in a rigid environment. We propose a model that explains these differences, based on calculations and structural analogies with other extremely weakly emitting derivatives, dibenzo[cde,mno]porphycenes. The efficient S₁ deactivation involves delocalization of two inner cavity protons coupled with proton translocation toward a high-energy cis tautomer. The latter process leads to distortion from planarity. The probability of deactivation increases with the strength of the intramolecular NH···N hydrogen bonds. The model also explains the observation of biexponential fluorescence decay in weakly emitting porphycenes. It can be extended to other derivatives, in particular, the asymmetrically substituted ones. We also point to the possibility of using specific porphycenes as viscosity sensors, in particular, when working in single molecule regime.

Probing of Nucleic Acid Structures, Dynamics, and Interactions With Environment-Sensitive Fluorescent Labels

Fluorescence labeling and probing are fundamental techniques for nucleic acid analysis and quantification. However, new fluorescent probes and approaches are urgently needed in order to accurately determine structural and conformational dynamics of DNA and RNA at the level of single nucleobases/base pairs, and to probe the interactions between nucleic acids with proteins. This review describes the means by which to achieve these goals using nucleobase replacement or modification with advanced fluorescent dyes that respond by the changing of their fluorescence parameters to their local environment (altered polarity, hydration, flipping dynamics, and formation/breaking of hydrogen bonds).

pH-Controlled fluorescence switching in water-dispersed polymer brushes grafted to modified boron nitride nanotubes for cellular imaging

pH-Switchable, fluorescent, hybrid, water-dispersible nanomaterials based on boron nitride nanotubes (BNNTs) and grafted copolymer brushes (poly(acrylic acid-co-fluorescein acrylate) - P(AA-co-FA)) were successfully fabricated in a two-step process. The functionalization of BNNTs was confirmed by spectroscopic, gravimetric and imaging techniques. In contrast to "pure" BNNTs, P(AA-co-FA)-functionalized BNNTs demonstrate intense green fluorescence emission at 520 nm. Under neutral or alkaline pH values, P(AA-co-FA)-functionalized BNNTs are highly emissive in contrast to acidic pH conditions where the fluorescent intensity is absent or low. No increase in the absorption was observed when the suspension pH was increased from 7 to 10. The functionalized BNNTs are easily taken up by human normal prostate epithelium (PNT1A) and human prostate cancer cell lines (DU145) and are suitable for further evaluation in cellular imaging applications.

Colloidal semiconductor nanocrystals in energy transfer reactions

Excitonic energy transfer is a versatile mechanism by which colloidal semiconductor nanocrystals can interact with a variety of nanoscale species. This feature article will discuss the latest research on the key scenarios under which semiconductor nanocrystals can engage in energy transfer with other nanoparticles, organic fluorophores, and plasmonic nanostructures, highlighting potential technological benefits to be gained from such processes.

Organic Fluorescent Dye-based Nanomaterials: Advances in the Rational Design for Imaging and Sensing Applications

Self-assembled fluorescent nanomaterials based on small-molecule organic dyes are gaining increasing popularity in imaging and sensing applications over the past decade. This is primarily due to their ability to combine spectral properties tunability and biocompatibility of small molecule organic fluorophores with brightness, chemical and colloidal stability of inorganic materials. Such a unique combination of features comes with rich versatility of dye-based nanomaterials: from aggregates of small molecules to sophisticated core-shell nanoarchitectures involving hyperbranched polymers. Along with the ongoing discovery of new materials and better ways of their synthesis, it is very important to continue systematic studies of fundamental factors that regulate the key properties of fluorescent nanomaterials: their size, polydispersity, colloidal stability, chemical stability, absorption and emission maxima, biocompatibility, and interactions with biological interfaces. In this review, we focus on the systematic description of various types of organic fluorescent nanomaterials, approaches to their synthesis, and ways to optimize and control their characteristics. The discussion is built on examples from reports on recent advances in the design and applications of such materials. Conclusions made from this analysis allow a perspective on future development of fluorescent nanomaterials design for biomedical and related applications.

Quick Microwave Assisted Synthesis and In Vitro Imaging Application of Oxygen Doped Fluorescent Carbon Dots

In this paper, a fast and simplest one-pot tactic was used to synthesis fluorescent oxygen doped carbon dots from Tween-20 (TTO-CDs) is reported. The TTO-CDs were microwavically synthesized by using Tween-20 as both the carbon precursor and the oxygen dopant as well. The surface morphology, crystalline and/or amorphous nature, composition and optical assets of synthesized TTO-CDs were studied by means of existing techniques. From the results, it was confirmed that the as-synthesized TTO-CDs are amorphous in nature, monodispersed, sphere-shaped and the typical particle size range is 5 ± 1.5 nm. The synthesized TTO-CDs emits strong blue fluorescence at 390 nm under excitation of 335 nm. Most interestingly, the excitation dependent emission property of synthesized TTO-CDs was exposed from fluorescence results. The synthesized TTO-CDs have quantum yield of about 14% against quinine sulfate as reference standard. The biotoxicity of synthesized TTO-CDs on HeLa cells was assessed through cytotoxicity assay. These results implied that the fluorescent TTO-CDs showed less biotoxicity, and further which was efficaciously applied as a multicolor staining and bioimaging probe for the confocal imaging of HeLa cells.

Fluorescent Nanobiosensors for Sensing Glucose

Glucose sensing in diabetes diagnosis and therapy is of great importance due to the prevalence of diabetes in the world. Furthermore, glucose sensing is also critical in the food and drug industries. Sensing glucose has been accomplished through various strategies, such as electrochemical or optical methods. Novel transducers made with nanomaterials that integrate fluorescent techniques have allowed for the development of advanced glucose sensors with superior sensitivity and convenience. In this review, glucose sensing by fluorescent nanobiosensor systems is discussed. Firstly, typical fluorescence emitting/interacting nanomaterials utilized in various glucose assays are discussed. Secondly, strategies for integrating fluorescent nanomaterials and biological sensing elements are reviewed and discussed. In summary, this review highlights the applicability of fluorescent nanomaterials, which makes them ideal for glucose sensing. Insight on the future direction of fluorescent nanobiosensor systems is also provided.

Development of colloidally stable carbazole-based fluorescent nanoaggregates

Fluorescent nanomaterials require high colloidal stability for effective use in imaging and sensing applications. We herein report the synthesis of carbazole-based organic fluorescent nanoaggregates, and demonstrate the superior colloidal stability of alkyl-substituted dye aggregates over their non-alkylated analogs. The role of alkyl chains in self-assembly and stability of such nanoaggregates is discussed based on both experimental and molecular dynamics simulation data, and spectral characteristics of the precursor dyes and their aggregates are described. The obtained results provide new insights on development of colloidally stable organic fluorescent nanomaterials with low polydispersity.

Turn-on Fluorene Push-Pull Probes with High Brightness and Photostability for Visualizing Lipid Order in Biomembranes

The rational design of environmentally sensitive dyes with superior properties is critical for elucidating the fundamental biological processes and understanding the biophysical behavior of cell membranes. In this study, a novel group of fluorene-based push-pull probes was developed for imaging membrane lipids. The design of these fluorogenic conjugates is based on a propioloyl linker to preserve the required spectroscopic features of the core dye. This versatile linker allowed the introduction of a polar deoxyribosyl head, a lipophilic chain, and an amphiphilic/anchoring group to tune the cell membrane binding and internalization. It was found that the deoxyribosyl head favored cell internalization and staining of intracellular membranes, whereas an amphiphilic anchor group ensured specific plasma membrane staining. The optimized fluorene probes presented a set of improvements as compared to commonly used environmentally sensitive membrane probe Laurdan such as red-shifted absorption matching the 405 nm diode laser excitation, a blue-green emission range complementary to the red fluorescent proteins, enhanced brightness and photostability, as well as preserved sensitivity to lipid order, as shown in model membranes and living cells.

Bloch Surface Wave-Coupled Emission at Ultra-Violet Wavelengths

The interaction of fluorophores with nearby metallic structures is now an active area of research. Dielectric photonic structures offer some advantages over plasmonic structures, namely small energy losses and less quenching. We describe a dielectric one-dimensional photonic crystal (1DPC), which supports Bloch surface waves (BSWs) from 280 to 440 nm. This BSW structure is a quartz slide coated with alternating layers of SiO2 and Si3N4. We show that this structure displays BSWs and that the near-UV fluorophore, 2-aminopurine (2-AP), on the top surface of the structure couples with the BSWs. Fluorophores do not have to be inside the structure for coupling and show a narrow angular distribution, with an angular separation of wavelengths. The Bloch wave-coupled emission (BWCE) radiates through the dielectric layer. These BSW structures, with useful wavelength range for detection of intrinsic protein and cofactor fluorescence, provide opportunities for novel optical configurations for bioassays with surface-localized biomolecules and for optical imaging using the coupled emission.

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.

Highly Selective and Ultrasensitive Turn-on Luminescence Chemosensor for Mercury (II) Determination Based on the Rhodamine 6G Derivative FC1 and Au Nanoparticles

A method for the detection and quantitation of Hg²⁺ in aqueous samples by fluorescence spectroscopy is presented. It consists of a turn-on sensor developed by coupling Gold nanoparticles (AuNPs) with the rhodamine 6G derivative FC1, in which the response is generated by a mercury-induced ring-opening reaction. The AuNPs were included in order to improve the sensitivity of the method towards the analyte, maintaining its high selectivity. The method was validated in terms of linearity, precision and accuracy, and applied to the quantitation of Hg²⁺ in Milli-Q and tap water with and without spiked analyte. The limit of detection and quantitation were 0.15 μg·L⁻¹ and 0.43 μg·L⁻¹, respectively, constituting a substantial improvement of sensitivity in comparison with the previously reported detection of Hg²⁺ with free FC1.

Functionalized dye encapsulated polymer nanoparticles attached with a BSA scaffold as efficient antenna materials for artificial light harvesting

A potential strategy for a new generation light harvesting system is multi-chromophoric donor-acceptor pairs where light energy is absorbed by an antenna complex and subsequently transfers its energy to the acceptor via energy transfer. Here, we design a system of a functionalized polymer nanoparticle-protein scaffold for efficient light harvesting and white light generation where a dye doped polymer nanoparticle acts as a donor and a dye encapsulated BSA protein acts as an acceptor. Analysis reveals that 91.3% energy transfer occurs from the dye doped polymer nanoparticle to the dye encapsulated BSA protein. The antenna effect of this light harvesting system is found to be 31 at a donor to acceptor ratio of 0.82 : 1 which is unprecedented. The enhanced effective molar extinction coefficient of the acceptor dye is potential for the light harvesting system. Bright white light emission with a quantum yield of 14% under single wavelength excitation is obtained by changing the ratio of donor to acceptor. Analysis reveals that the efficient energy transfer in this polymer-protein assembly may open up new possibilities in designing artificial light harvesting systems for future applications.

Plasmon resonance and the imaging of metal-impregnated neurons with the laser scanning confocal microscope

The staining of neurons with silver began in the 1800s, but until now the great resolving power of the laser scanning confocal microscope has not been utilized to capture the in-focus and three-dimensional cytoarchitecture of metal-impregnated cells. Here, we demonstrate how spectral confocal microscopy, typically reserved for fluorescent imaging, can be used to visualize metal-labeled tissues. This imaging does not involve the reflectance of metal particles, but rather the excitation of silver (or gold) nanoparticles and their putative surface plasmon resonance. To induce such resonance, silver or gold particles were excited with visible-wavelength laser lines (561 or 640 nm), and the maximal emission signal was collected at a shorter wavelength (i.e., higher energy state). Because the surface plasmon resonances of noble metal nanoparticles offer a superior optical signal and do not photobleach, our novel protocol holds enormous promise of a rebirth and further development of silver- and gold-based cell labeling protocols.

Chemical sensing with nanoparticles as optical reporters: from noble metal nanoparticles to quantum dots and upconverting nanoparticles

A wide variety of biological and medical analyses are based on the use of optical signals to report specific molecular events. Thanks to advances in nanotechnology, various nanostructures have been extensively used as optical reporters in bio- and chemical assays. This review describes recent progress in chemical sensing using noble metal nanoparticles (gold and silver), quantum dots and upconverting nanoparticles. It provides insights into various nanoparticle-based sensing strategies including fluorescence/luminescence resonance energy transfer nanoprobes as well as activatable probes sensitive to specific changes in the biological environment. Finally we list some research challenges to be overcome in order to accelerate the development of applications of nanoparticle bio- and chemical sensors.

New diamidequat-type surfactants in fabrication of long-sustained theranostic nanocapsules: Colloidal stability, drug delivery and bioimaging

We report a new theranostic nanoformulation to transport both chemotherapeutic and imaging agents for successfully exterminating cancer cells. This strategy is based on encapsulation of colchicine (cytostatic drug) and coumarin-6 (fluorescent biomarker) in oil-core nanocarriers stabilized by diamidequat-type surfactants - N,N-dimethyl-N,N-bis[2-(N-alkylcarbamoyl) ethyl]ammonium methylsulfates (2xCnA-MS, n=8,10,12), and fabricated by the nanoprecipitation technique. The surfactants were synthesized using a technologically viable methodology and characterized. The potential of the encapsulated theranostic cargoes was evaluated in cytotoxicity studies as well as in imaging of intracellular localization, accumulation and distribution of cargoes delivered to well characterized human cancer cell lines - doxorubicin-sensitive breast (MCF-7/WT), alveolar basal epithelial (A549) and skin melanoma (MEWO) - performed by confocal laser scanning microscopy (CLSM). Backscattered profiles obtained by the turbidimetric technique were applied to evaluate physical stability of the obtained nanosystems. DLS measurements confirmed the particle diameter to be below 200nm, while AFM - its morphology and shape. Doppler electrophoresis provided a highly positive ζ-potential. UV-vis was applied to determine the encapsulation efficiencies (ca. 90%), and release profiles. The study demonstrates that the soft cationic diamidequat-type surfactants are suitable for the stabilization of theranostic nanodispersions, and they can constitute a new functional class of stabilizers of nanoparticles and have a progressive impact onto development of formulations. Furthermore, our results demonstrate excellent biocompatibility of the studied long-sustained monodisperse oil-core nanocapsules, stabilized by 2xCnA-MS, which makes them promising for cell imaging.

Tamm State-Coupled Emission: Effect of Probe Location and Emission Wavelength

We report the effect of the probe location and wavelength on the emission spatial distribution and spectral properties of fluorophores located on structures which display Tamm states. Our structure consists of a one-dimensional photonic crystal (1DPC)-that is, a multilayer structure of alternate high and low refractive index dielectrics-and a thin top silver film. Simulations show the presence of Tamm and surface plasmon modes in the structure. The electric field intensities for the Tamm modes are located mostly in the dielectric layer below the metal film. The corresponding field intensities for the surface plamon modes are located above the metal film in the distal side. Tamm states can be in resonance with the incident light normal or near normal to the surface, within the light line, and can be accessed without the use of a coupling prism or gratings. We investigated the emission spectra and angular distribution of the emission for probes located above and below the metal film to explore the interaction of fluorophores with Tamm plasmons and surface plasmons modes. Three probes were chosen with different overlap of the emission spectra with the Tamm modes. The fluorophores below the metal film coupled predominantly with the Tamm state and displayed more intense and only Tamm state-coupled emission (TSCE). Probes above the metal film display both surface plasmon-coupled emission (SPCE) and Tamm state-coupled emission. In contrast to SPCE, which shows only KR, P-polarized emission, the Tamm states can display both S- and P-polarized emission and can be populated using both RK and KR illuminations. The TSCE angle is highly sensitive to wavelength, which suggests the use of Tamm structures to provide both directional emission and wavelength dispersion. The combination of plasmonic and photonic structures with directional emission close to surface normal offers the opportunities for new design formats for clinical testing, portable devices, and other fluorescence-based applications.

Exploiting fluorescence for multiplex immunoassays on protein microarrays

Protein microarray technology is becoming the method of choice for identifying protein interaction partners, detecting specific proteins, carbohydrates and lipids, or for characterizing protein interactions and serum antibodies in a massively parallel manner. Availability of the well-established instrumentation of DNA arrays and development of new fluorescent detection instruments promoted the spread of this technique. Fluorescent detection has the advantage of high sensitivity, specificity, simplicity and wide dynamic range required by most measurements. Fluorescence through specifically designed probes and an increasing variety of detection modes offers an excellent tool for such microarray platforms. Measuring for example the level of antibodies, their isotypes and/or antigen specificity simultaneously can offer more complex and comprehensive information about the investigated biological phenomenon, especially if we take into consideration that hundreds of samples can be measured in a single assay. Not only body fluids, but also cell lysates, extracted cellular components, and intact living cells can be analyzed on protein arrays for monitoring functional responses to printed samples on the surface. As a rapidly evolving area, protein microarray technology offers a great bulk of information and new depth of knowledge. These are the features that endow protein arrays with wide applicability and robust sample analyzing capability. On the whole, protein arrays are emerging new tools not just in proteomics, but glycomics, lipidomics, and are also important for immunological research. In this review we attempt to summarize the technical aspects of planar fluorescent microarray technology along with the description of its main immunological applications.

Lighting up cells with lanthanide self-assembled helicates

Lanthanide bioprobes and bioconjugates are ideal luminescent stains in view of their low propensity to photobleaching, sharp emission lines and long excited state lifetimes permitting time-resolved detection for enhanced sensitivity. We show here how the interplay between physical, chemical and biochemical properties allied to microfluidics engineering leads to self-assembled dinuclear lanthanide luminescent probes illuminating live cells and selectively detecting biomarkers expressed by cancerous human breast cells.

Surface plasmon resonance mediated photoluminescence properties of nanostructured multicomponent fluorophore systems

The interaction between light and matter is the fundamental aspect of many optoelectronic applications. The efficiency of such devices is mainly dictated by the light emitting properties of fluorophores. Unfortunately, the intensity of emission is adversely affected by surface defects, scattering and chemical instability. Therefore, enhancing the luminescence of fluorophores is necessary for better implementation of nanocomposites in biological and optical applications. There are many interesting phenomena which can be observed if the characteristics of the fluorophores and metal nanoparticles are integrated. Photoluminescence (PL) by fluorophores can be enhanced or quenched by the presence of neighboring plasmonic metal nanostructures. An unambiguous study of the mechanism behind the enhancement and the quenching of emission is necessary to obtain new insight into the interactions between light and metal-fluorophore nanocomposites. In this review the core aspect of combining plasmonic metal nanostructures with fluorophores is discussed by considering various functional roles of plasmonic metals in modifying the PL properties reported by various research groups. A few representative applications of SPR mediated luminescence are also discussed.

Revolutionizing the FRET-based light emission in core-shell nanostructures via comprehensive activity of surface plasmons

We demonstrate the surface-plasmon-induced enhancement of Förster resonance energy transfer (FRET)using a model multilayer core-shell nanostructure consisting of an Au core and surrounding FRET pairs, i.e., CdSe quantum dot donors and S101 dye acceptors. The multilayer configuration was demonstrated to exhibit synergistic effects of surface plasmon energy transfer from the metal to the CdSe and plasmon-enhanced FRET from the quantum dots to the dye. With precise control over the distance between the components in the nanostructure, significant improvement in the emission of CdSe was achieved by combined resonance energy transfer and near-field enhancement by the metal, as well as subsequent improvement in the emission of dye induced by the enhanced emission of CdSe. Consequently, the Förster radius was increased to 7.92 nm and the FRET efficiency was improved to 86.57% in the tailored plasmonic FRET nanostructure compared to the conventional FRET system (22.46%) without plasmonic metals.

Optical methods for sensing and imaging oxygen: materials, spectroscopies and applications

We review the current state of optical methods for sensing oxygen. These have become powerful alternatives to electrochemical detection and in the process of replacing the Clark electrode in many fields. The article (with 694 references) is divided into main sections on direct spectroscopic sensing of oxygen, on absorptiometric and luminescent probes, on polymeric matrices and supports, on additives and related materials, on spectroscopic schemes for read-out and imaging, and on sensing formats (such as waveguide sensing, sensor arrays, multiple sensors and nanosensors). We finally discuss future trends and applications and summarize the properties of the most often used indicator probes and polymers. The ESI† (with 385 references) gives a selection of specific applications of such sensors in medicine, biology, marine and geosciences, intracellular sensing, aerodynamics, industry and biotechnology, among others.

Radiative decay engineering 7: Tamm state-coupled emission using a hybrid plasmonic-photonic structure

There is a continuing need to increase the brightness and photostability of fluorophores for use in biotechnology, medical diagnostics, and cell imaging. One approach developed during the past decade is to use metallic surfaces and nanostructures. It is now known that excited state fluorophores display interactions with surface plasmons, which can increase the radiative decay rates, modify the spatial distribution of emission, and result in directional emission. One important example is surface plasmon-coupled emission (SPCE). In this phenomenon, the fluorophores at close distances from a thin metal film, typically silver, display emission over a small range of angles into the substrate. A disadvantage of SPCE is that the emission occurs at large angles relative to the surface normal and at angles that are larger than the critical angle for the glass substrate. The large angles make it difficult to collect all of the coupled emission and have prevented the use of SPCE with high-throughput and/or array applications. In the current article, we describe a simple multilayer metal-dielectric structure that allows excitation with light that is perpendicular (normal) to the plane and provides emission within a narrow angular distribution that is normal to the plane. This structure consists of a thin silver film on top of a multilayer dielectric Bragg grating, with no nanoscale features except for the metal or dielectric layer thicknesses. Our structure is designed to support optical Tamm states, which are trapped electromagnetic modes between the metal film and the underlying Bragg grating. We used simulations with the transfer matrix method to understand the optical properties of Tamm states and localization of the modes or electric fields in the structure. Tamm states can exist with zero in-plane wavevector components and can be created without the use of a coupling prism. We show that fluorophores on top of the metal film can interact with the Tamm state under the metal film and display Tamm state-coupled emission (TSCE). In contrast to SPCE, the Tamm states can display either S or P polarization. The TSCE angle is highly sensitive to wavelength, which suggests the use of Tamm structures to provide both directional emission and wavelength dispersion. Metallic structures can modify fluorophore decay rates but also have high losses. Photonic crystals have low losses but may lack the enhanced light-induced fields near metals. The combination of plasmonic and photonic structures offers the opportunity for radiative decay engineering to design new formats for clinical testing and other fluorescence-based applications.

Fluorescent carbon nanomaterials: “quantum dots” or nanoclusters?

Despite many efforts, the mechanisms of light absorption and emission of small fluorescent carbon nanoparticles (C-dots) are still unresolved and are a subject of active discussion.