Substrate effects on photophysical properties of fluorescent self‐assembled monolayers (SAMs)

Substrate materials play an important role to modulate the photophysical properties of fluorescent self‐assembled monolayers (SAMs) constructed on them. The substrate–fluorophore interactions in fluorescent SAMs can affect their fluorescence intensity. Hence, it is important to take into account such alteration of fluorescence properties and study the underlying mechanisms. In this brief overview, substrate effects on fluorescence properties of chemisorbed or physisorbed fluorescent SAMs on two‐dimensional (chips) and three‐dimensional (nanoparticles) surfaces are highlighted. Examples of fluorescence quenching and enhancement on various substrate materials by different factors are discussed and analyzed. Additionally, some strategies to limit metallic substrate–fluorophore interactions are discussed briefly. Copyright © 2016 John Wiley & Sons, Ltd.     

Subdiffraction resolution in total internal reflection fluorescence microscopy with a grating substrate

We propose a fluorescence surface imaging system that presents a power of resolution beyond that of the diffraction limit without resorting to saturation effects or probe scanning. This is achieved by depositing the sample on an optimized periodically nanostructured substrate in a standard total internal reflection fluorescence microscope. The grating generates a high-spatial-frequency light grid that can be moved throughout the sample by changing the incident angle. An appropriate reconstruction procedure permits one to recover the fluorescence amplitude from the images obtained for various incidences. Simulations of this imaging system show that the resolution is not limited by diffraction but by the period of the grating.     

Influence of groove depth and surface profile on fluorescence enhancement by grating-coupled surface plasmon resonance

The fluorescence intensity of a sample placed on a metal grating pattern is enhanced due to excitation by the electric field of the grating-coupled surface plasmon resonance (GC-SPR). The dependence of the enhancement on groove depth and surface profile was studied with the aim of improving the sensitivity of fluorescence detection. The enhancement was found to depend on the groove depth, with intensity most enhanced on grating substrate of about 20 nm depth, which produced an intensity about 30 times greater than that on a flat borosilicate glass substrate. Rigorous coupled wave analysis calculation showed that the shape of the groove influenced GC-SPR, suggesting that controlling not only the depth but also the shape of the grating surface profile can be an important factor in improving the sensitivity of detection by fluorescence microscopy.     

Increasing efficiency of laser fluorescence diagnostics of microbial diseases

The laser-induced fluorescence spectra of five types of mycobacteria are studied in various associations. The results obtained are used to demonstrate the possibility of a diagnostic system for the differential diagnostics of microbial diseases. The method employed for the specific analysis of microbial species in the substrate that contain various associations is based on a regression of the spectral characteristics of the fluorescence intensity to the principal components. The diagnostic efficiency is increased when the fluorescence spectra are reconstructed with regard to the optical characteristics of the medium. The detergent effect on the parameters of microbial fluorescence diagnostics is studied.     

In situ observation of cell-detachment process initiated by femtosecond laser-induced stress wave

When a stress wave generated by focusing a femtosecond laser is loaded on an animal cell adhered on a substrate, the cell is detached from the substrate. There are two possible mechanisms for the cell detachment: (a) The cell is detached from a scaffold coated on a glass plate, and (b) the cell is detached from the glass plate with the scaffold. In this work, we have studied the cell-detachment mechanism by visualizing the scaffold with a fluorescence probe of quantum dots. When the cell was detached from the substrate, fluorescence from the scaffold simultaneously disappeared from the glass plate, although the scaffold was not irradiated by the laser. This indicates that detachment due to the stress wave is attributed to mechanism (a). On the other hand, when the cell was detached from the substrate by a trypsin treatment, the fluorescence from the scaffold remained, suggesting mechanism (b). By comparing both results, it is considered that physiological damage of the cell membrane during the detachment process by femtosecond laser-induced stress wave is less than that due to the trypsin treatment.     

有机分子隔离层对机械抛光金属银表面的罗丹明6G荧光增强效应的影响

采用物理吸附法,在机械抛光纯银表面引入对巯基苯胺(p-Amiothiophenol,PATP)分子充当隔离层,运用激光光谱学方法研究隔离层对位于银表面附近的罗丹明6G(Rh6G)分子的荧光增强效应影响。实验结果表明,未经PATP分子修饰的机械抛光金属衬底对Rh6G分子表现为猝灭效应,而经过PATP分子修饰后的银表面对Rh6G分子的荧光发射具有增强效应。根据局域表面等离子共振及辐射能量转移模型对实验观测所得结果进行了分析研究,结果表明,PATP有机分子隔离层的引入有效地减小了荧光分子与金属衬底之间的无辐射能量速率,提高了荧光辐射强度。     

Self-assembled synthesis of SEF-active silver dendrites by galvanic displacement on copper substrate

In this paper, an effective substrate for surface-enhanced fluorescence, which consists of dendritic silver nanostructure on copper surface, was fabricated by modified galvanic displacement reaction at room temperature. It was found that the fluorescence efficiency of Rh6G probe molecules on the substrate depended on the period of the nanostructure growth. And it increased with the enrichment of fine-branches of the dendritic silver nanostructure. The experimental results indicated that the fine nanostructure with dendritic distribution can produce better fluorescence enhancement with the help of proper control over the reaction condition. It is important for studying the mechanism and the potential applications of enhanced fluorescence effect.     

Vibrationally resolved fluorescence from organic molecules near metal surfaces in a scanning tunneling microscope

Intrinsic molecular fluorescence from porphyrin molecules on Au(100) has been realized by using a nanoscale multimonolayer decoupling approach with nanoprobe excitation in the tunneling regime. The molecular origin of luminescence is established by the observed well-defined vibrationally resolved fluorescence spectra. The molecules fluoresce at low "turn-on" voltages for both bias polarities, suggesting an excitation mechanism via hot electron injection from either tip or substrate. The excited molecules decay radiatively through Franck-Condon pi(*)-pi transitions.     

Optimization of Gold Nanoparticle-Based DNA Detection for Microarrays

DNA microarrays are promising tools for fast and highly parallel DNA detection by means of fluorescence or gold nanoparticle labeling. However, substrate modification with silanes (as a prerequisite for capture DNA binding) often leads to inhomogeneous surfaces and/or nonspecific binding of the labeled DNA. We examined both different substrate cleaning and activating protocols and also different blocking strategies for optimizing the procedures, especially those for nanoparticle labeling. Contact angle measurements as well as fluorescence microscopy, atomic force microscopy (AFM), and a flatbed scanner were used to analyze the multiple-step process. Although the examined different cleaning and activating protocols resulted in considerably different contact angles, meaning different substrate wettability, silanization led to similar hydrophobic surfaces which could be revealed as smooth surfaces of about 2–4 nm roughness. The two examined silanes (3-glycidoxypropyltrimethoxysilane (GOPS) and 3-aminopropyltriethoxysilane (APTES)) differed in their DNA binding homogeneity, maximum signal intensities, and sensitivity. Nonspecific gold binding on APTES/PDC surfaces could be blocked by treatment in 3% bovine serum albumin (BSA).     

Excimer emission from glassy films of some aromatic hydrocarbons

The fluorescence emission spectra and fluorescence decay times of amorphous films prepared by evaporation of a number of aromatic hydrocarbons on to a substrate cooled with liquid nitrogen were measured. The fluorescence of such films appears to originate from sites where the interaction between neighbouring molecules is of the same kind as that present in the well-known excimer state. The density of such sites is found to be so high as effectively to prevent energy migration. Some interesting exceptions to this behaviour are noted and comparisons drawn with experimental data reported for excimer formation in solution and in the crystalline phase.     

Fluorescence spectroscopy of semiconductor CdTe nanocrystals: preparation effect on photostability

We report on continuous-wave and time-resolved fluorescence spectroscopy studies of CdTe water-soluble nanocrystals at room temperature. For nanocrystals spread directly on the substrate we observe large variation both in fluorescence maximum energy and fluorescence lifetime. We attribute this to the influence of the surface of the nanocrystals on the stability of excitations in the nanocrystals. As the fluorescence lifetime of the nanocrystals is monitored, we find it increases with time from 6 to 18 ns and then saturates. Placing the nanocrystals in a polymer matrix remarkably improves the photostability and all the above-mentioned effects are diminished. Upon mixing the nanocrystals with gold spherical nanoparticles we observe a decrease of the fluorescence intensity due to efficient energy transfer to the nanoparticles.     

Optical Properties of Planar Nanostructures Based on Semiconductor Quantum Dots and Plasmonic Metal Nanoparticles

The optical properties of a composite material consisting of a thin polymer film, which is activated by semiconductor CdSe/ZnS quantum dots (QDs) and silver nanoparticles, on a transparent dielectric substrate have been investigated. It is revealed that the presence of silver nanoparticles leads to an increase in the QD absorption (by a factor of 4) and in the fluorescence intensity (by a factor of 10), whereas the fluorescence time drops by a factor of about 10. Excitation of the composite medium by a pulsed laser is found to result in narrowing of the fluorescence band and a sublinear dependence of its intensity on the pulse energy. In the absence of silver nanoparticles, the fluorescence spectrum of QDs is independent of the excitation-pulse energy density, and the fluorescence intensity depends linearly on the pulse energy in the entire range of energy densities, up to 75 mJ/cm².     

Segment distributions of end-tethered polymers in a good solvent

We use confocal fluorescence microscopy to study the conformation of single DNA molecules end-tethered to a solid substrate. The segment distribution rho(z) measured for chains with contour lengths 15.4 microm 相似文献   

Photo-physical and lasing characterisation of a polyparaphenylenevinylene (PPV) neat film

The wave-guided travelling-wave laser action (amplified spontaneous emission) of a neat film of poly(p-phenylenevinylene) (PPV) on a quartz glass substrate prepared by a sulfinyl precursor technique is studied. The samples are transversally pumped with picosecond excitation pulses (wavelength 347.15 nm, duration 35 ps). Lasing occurs at 550 nm. The optical constants of the neat films are determined by transmittance measurements exploiting the multiple beam interference in the transparency region. A fluorescence spectroscopic characterisation is carried out determining the fluorescence quantum distribution, fluorescence quantum yield, degree of fluorescence polarisation, and fluorescence lifetime. The emitting chromophore size (emitting singlet exciton extension) is determined by the ratio of exciton radiative lifetime to repeat-unit based radiative lifetime. The obtained size of about two repeat units is discussed in a disordered solid-state polymer model.     

Supercritical angle fluorescence biosensor for the detection of molecular interactions on cellulose-modified glass surfaces

Cellulose films have been proposed as a convenient substrate for producing flat and homogeneous surface coatings. Additionally, amino-labelled cellulose species, like aminopropyltrimethylsilylethercellulose (ATMSC), are excellent support matrices for covalent binding of biomolecules, with low probe density and prevention of non-specific adsorption of unbound analyte molecules. Due to ATMSC films fulfil important requirements as substrate for analyse techniques of surface-tethered proteins and nucleic acids, we consequently report a new preparation for DNA-functionalised surfaces. Single-stranded DNA molecules are covalently coupled to cellulose-coated glass cover slips to interact with complementary free Cy5-labelled oligonucleotides in solution. Hybridisation efficiencies at the new substrate and at standard surface coatings are determined by detection of the surface-generated fluorescence. In order to discriminate against the fluorescence from unbound oligonucleotides the detection volume was restricted to the surface by collecting supercritical angle fluorescence (SAF). Thus, it is demonstrated that cellulose films are utilised to investigate DNA-hybridisation reactions highly sensitive.     

Tryptophan Fluorescence Yields and Lifetimes as a Probe of Conformational Changes in Human Glucokinase

Five variants of glucokinase (ATP-D-hexose-6-phosphotransferase, EC 2.7.1.1) including wild type and single Trp mutants with the Trp residue at positions 65, 99, 167 and 257 were prepared. The fluorescence of Trp in all locations studied showed intensity changes when glucose bound, indicating that conformational change occurs globally over the entire protein. While the fluorescence quantum yield changes upon glucose binding, the enzyme’s absorption spectra, emission spectra and fluorescence lifetimes change very little. These results are consistent with the existence of a dark complex for excited state Trp. Addition of glycerol, L-glucose, sucrose, or trehalose increases the binding affinity of glucose to the enzyme and increases fluorescence intensity. The effect of these osmolytes is thought to shift the protein conformation to a condensed, high affinity form. Based upon these results, we consider the nature of quenching of the Trp excited state. Amide groups are known to quench indole fluorescence and amides of the polypeptide chain make interact with excited state Trp in the relatively unstructured, glucose-free enzyme. Also, removal of water around the aromatic ring by addition of glucose substrate or osmolyte may reduce the quenching.     

A novel natural surface-enhanced fluorescence system based on reed leaf as substrate for crystal violet trace detection

The preparation of surface-enhanced fluorescence (SEF) substrates is often influenced by experimental strategies and factors such as the morphology and size of the nanostructures. In this study, using the natural reed leaves (RLs) without any special pretreatment as the substrate, metal silver is modified by magnetron sputtering technology to prepare a stable and efficient SEF system. The abundant “hedgehog-like”protrusions on the RL substrate surface can generate high-density “hot spots”, thus enhancement factor (EF) is enhanced up to 3345 times. The stability and reproducibility are verified in many measurements. The contribution of the intervention of silver nanostructure to the radiation attenuation process of fluorescent molecules is analyzed with the aid of Jablonski diagrams. Three-dimensional (3D) finite difference time domain (FDTD) simulates the spatial electric field and “hot spots”distribution of the substrate. The “hedgehog-like”protrusion structure generates multiple “hot spots”, which produce an excellent local surface plasmon resonance (LSPR) effect and provide higher fluorescence signal. Finally, RL/Ag-35 substrate is used to detect crystal violet (CV), and the detection limit is as low as 10^-13 M. This “hedgehog-like”SEF substrate provides a new strategy for the trace detection of CV, which has a good practical application value.     

The formation of chlorine-induced alterations in daguerreotype image particles: a high resolution SEM-EDS study

The daguerreotype image, composed of nanosized silver–mercury or silver–mercury–gold amalgam particles formed on a polished silver substrate, is particularly sensitive to deterioration by chlorine-containing compounds resulting in the formation of AgCl that generates redeposited silver upon exposure to UV and visible lights. In the present study, alterations caused by chlorides on daguerreotype test samples prepared following 19th century recipes were studied. The dependence of variations in the production steps of daguerreotypes, such as multiple sensitization and gilding, on the impact of the exposure to chlorine were analyzed by scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDS) and atomic force microscopy (AFM), complemented by X-ray fluorescence (XRF) and Raman spectroscopy. It was observed that AgCl nucleates on the image particles and in the substrate defects, regardless of the particle density or the sensitization process. In gilded samples, Au was observed over the image particles and the polished silver substrate as a tightly packed grainy layer, which conformably follows the polishing irregularities. For the first time it is shown that Au preferentially accumulates on top of the image particles. This gold layer does not protect the image from chlorine-induced deterioration.     

Surface-enhanced fluorescence and application study based on Ag-wheat leaves

Wheat leaves with natural microstructures as substrates were covered by the silver nanoislands by magnetron to prepare a low-cost, environment-friendly and mass production surface-enhanced fluorescence (SEF) substrate (Ag-WL substrate). The best SEF substrate was selected by repeatly certifying the fluorescence intensity of 10^-5 M Rhodamine B (RB) and 10^-5 M Rhodamine 6G (R6G) aqueous solutions. The abundant semi-spherical protrusions and flake-like structures on the surface of the Ag-WL substrate produce high-density hot spots, which provides a new and simple idea for the preparation of biomimetic materials. The results of 3D finite-different time-domain (FDTD) simulation show that the nanoisland gap of semi-spherical protrusions and flake-like structures has produced rich hotspots. By adjusting the time of magnetron sputtering, the enhancement factor (EF) was as high as 839 times, relative standard deviation (RSD) reached as low as 10.7%, and the substrate was very stable and repeatable, which shows that Ag-WL substrate is trustworthy. Moreover, semi-spherical protrusions provide stronger surface-enhanced Raman scattering (SERS) effects compared to flake-like structure. What is more surprising is that the detection limit of the substrate for toxic substance crystal violet (CV) is as low as 10^-10 M.     

Kinetics of long-lived luminescence of eosin in Langmuir-Blodgett films

The spectral and kinetic properties of photoluminescence of mixed Langmuir-Blodgett (LB) films of eosin decyl ether and palmitic acid on a solid substrate are studied. The electronic absorption and fluorescence spectra of the films are identical to the spectra of the dye in ethanol. An increase in the dye concentration in a monolayer results in the appearance of a dimer absorption band, quenching of fluorescence of monomers, and the red shift of the spectral bands. At 90 K, the distinct phosphorescence and delayed fluorescence bands of LB films were observed. The decay kinetics of phosphorescence and delayed fluorescence is nonexponential. It is shown that the decay curve of delayed fluorescence is determined by triplet-triplet annihilation (TTA) and T ₁→S₁ triplet-singlet intersystem crossing (IS). The initial nonexponential phosphorescence decay is caused the dominant contribution of TTA to the decay of triplet molecules. The experimental data are interpreted based on the mechanisms of exchange-resonance and inductive-resonance annihilation.