Gold Nanocage Assemblies for Selective Second Harmonic Generation Imaging of Cancer Cell

Second harmonic generation (SHG) imaging using near infrared laser light is the key to improving penetration depths, leading to biological understanding. Unfortunately, currently SHG imaging techniques have limited capability due to the poor signal‐to‐noise ratio, resulting from the low SHG efficiency of available dyes. Targeted tumor imaging over nontargeted tissues is also a challenge that needs to be overcome. Driven by this need, in this study, the development of two‐photon SHG imaging of live cancer cell lines selectively by enhancement of the nonlinear optical response of gold nanocage assemblies is reported. Experimental results show that two‐photon scattering intensity can be increased by few orders of magnitude by just developing nanoparticle self‐assembly. Theoretical modeling indicates that the field enhancement values for the nanocage assemblies can explain, in part, the enhanced nonlinear optical properties. Our experimental data also show that A9 RNA aptamer conjugated gold nanocage assemblies can be used for targeted SHG imaging of the LNCaP prostate cancer cell line. Experimental results with the HaCaT normal skin cell lines show that bioconjugated nanocage‐based assemblies demonstrate SHG imaging that is highly selective and will be able to distinguish targeted cancer cell lines from other nontargeted cell types. After optimization, this reported SHG imaging assay could have considerable application for biology.     

Coumarin 6 and 1,6-diphenyl-1,3,5-hexatriene (DPH) as fluorescent probes to monitor protein aggregation

We report the use of Coumarin 6 and 1,6-diphenyl-1,3,5-hexatriene (DPH) for the identification of protein aggregates for the first time. The two dyes can be used at very low (nanomolar) concentrations and do not interfere with the aggregation process, as is reported for other commonly used fluorescent protein probes. In the presence of protein aggregates, their quantum yields are significantly high. DPH is able to recognize both amorphous and fibrillar aggregates but cannot distinguish between them. Coumarin 6 can distinguish between both types of aggregates. It also exhibits the characteristic sigmoidal curve of amyloid formation, with higher sensitivity for detection of fibrillation than the conventionally used Thioflavin T.     

Unique contrast patterns from resonance-enhanced chiral SHG of cell membranes

Chirality can produce novel nonlinear optical effects that may form the basis for new imaging contrast agents. In this paper, we developed a new chiral chromophore 2, which is the dimer of a known voltage sensitive dye, monomer 1, with the chirality originating from the twisted orientation between two subunits. Racemic dimer and monomer 1 were used as the references to study the effect of chirality in SHG microscopy of live cells. All these dyes selectively stain the outer leaflets of cell membranes, producing strong resonance-enhanced SHG images. At the symmetric junction between two adherent cells, monomer or racemic dimer SHG is forbidden due to centrosymmetry, and indeed little SHG was observed (10 +/- 1% relative to nonjunction). When stained with the chiral dimer, the junction is no longer centrosymmetric and much stronger SHG was observed (39 +/- 4% relative to nonjunction). Plane polarized light produces highly polarized images of spherical cells stained with racemic dye, but for the chiral dye, the polarized pattern is largely eliminated by the chiral SHG emanating from the subresolution membrane convolutions.     

Aggregation‐Induced Emission Active Probe for Light‐Up Detection of Anionic Surfactants and Wash‐Free Bacterial Imaging

Anionic surfactants are widely used in daily life and industries, but their residues can cause serious damage to the environment. The current detection methods for anionic surfactants suffer from various limitations and a new detection strategy is highly desirable. Based on 2‐(2‐hydroxyphenyl)benzothiazole fluorogen with aggregation‐induced emission characteristics, we have developed a fluorescent probe HBT‐C₁₈ for selective and sensitive detection of anionic surfactants. By in situ formation of catanionic aggregates or micelles with anionic surfactants, the emission intensity of the HBT‐C₁₈ probe can increase with increasing keto/enol emission ratio through restriction of intramolecular motion and excited‐state intramolecular proton‐transfer mechanisms. The probe can also be used for wash‐free imaging of bacteria enveloped by a negatively charged outer membrane. The results of this study provide a new strategy for sensitive detection of anionic surfactants and wash‐free bacterial imaging.     

Electrodeposition of flattened Cu nanoclusters on a p-GaAs(001) electrode monitored by in situ optical second harmonic generation

In situ optical second harmonic generation (SHG) technique was employed to investigate the shape and density of Cu nanoclusters, which were electrochemically formed on p-GaAs(001) electrode surfaces. Since GaAs is not a centrosymmetric medium, a significant portion of SHG signal arises from the bulk dipole susceptibility, but it was possible to separate a surface-induced signal from a bulk-induced signal by choosing an appropriate experimental geometry and appropriate data processing. The rotational anisotropy (RA) pattern of the SHG signal from a p-GaAs(001) electrode changed in both shape and magnitude during potential cycling in an electrolyte solution containing Cu2+. The surface plasmon-induced SHG signal from Cu nanoclusters deposited on GaAs was attributed to the modulation source for the RA-SHG pattern. More detailed study was carried out with both in situ SHG and ex situ AFM measurements for Cu nanoclusters deposited by potential step. The results showed that the SHG signal at the present optical geometry was sensitive to the number of oblate or flattened Cu nanoclusters with lateral diameter larger than 30 nm and that the SHG enhancement occurred because of resonant coupling between the surface plasmon induced in the flattened Cu nanoclusters and the near-infrared fundamental light.     

Visualizing tributyltin (TBT) in bacterial aggregates by specific rhodamine-based fluorescent probes

Here we present the first examples of fluorescent and colorimetric probes for microscopic TBT imaging. The fluorescent probes are highly selective and sensitive to TBT and have successfully been applied for imaging of TBT in bacterial Rhodobacter ferrooxidans sp. strain SW2 cell-EPS-mineral aggregates and in cell suspensions of the marine cyanobacterium Synechococcus PCC 7002 by using confocal laser scanning microscopy.     

Effect of pH on thermal- and chemical-induced denaturation of GFP

Green fluorescent protein (GFP) is an unusually stable autofluorescent protein that is increasingly being exploited for many applications. In this report, we have used fluorescence spectroscopy to study the effect of pH on the denaturation of GFP with sodium dodecyl sulfate (SDS), urea, and heat. Surprisingly, SDS (up to 0.5%) did not have any significant effect on the fluorescence of GFP at pH 7.5 or 8.5 buffers; however, at pH 6.5, the protein lost all fluorescence within 1 min of incubation. Similarly, incubation of GFP with 8 M urea at 50°C resulted in time dependent denaturation of GFP, but only in pH 6.5 buffer. At higher pH values (pH 7.5 and pH 8.5), the GFP was quite stable in 8 M urea at 50°C, showing only a slight decrease in fluorescence. Heat denaturation of GFP was found to be pH dependent as well, with the denaturation being fastest at pH 6.5 as compared to pH 7.5 or pH 8.5. Like the denaturation studies, renaturation of heat-denatured GFP was most efficient at pH 8.5, followed by pH 7.5, and then pH 6.5. These results suggests that GFP undergoes a structural/stability shift between pH 6.5 and pH 7.5, with the GFP structure at pH 6.5 being very sensitive to denaturation by SDS, urea, and heat.     

Surfactant-induced amorphous aggregation of tobacco mosaic virus coat protein: a physical methods approach

The interactions of non-ionic surfactant Triton X-100 and the coat protein of tobacco mosaic virus, which is an established model for both ordered and non-ordered protein aggregation, were studied using turbidimetry, differential scanning calorimetry, isothermal titration calorimetry, and dynamic light scattering. It was found that at the critical aggregation concentration (equal to critical micelle concentration) of 138 x 10(-6) M, Triton X-100 induces partial denaturation of tobacco mosaic virus coat protein molecules followed by protein amorphous aggregation. Protein aggregation has profound ionic strength dependence and proceeds due to hydrophobic sticking of surfactant-protein complexes (start aggregates) with initial radii of 46 nm. It has been suggested that the anionic surfactant sodium dodecyl sulfate forms mixed micelles with Triton X-100 and therefore reverses protein amorphous aggregation with release of protein molecules from the amorphous aggregates. A stoichiometric ratio of 5 was found for Triton X-100-sodium dodecyl sulfate interactions.     

Nanobody‐Conjugated Nanotubes for Targeted Near‐Infrared In Vivo Imaging and Sensing

Fluorescent nanomaterials such as single‐walled carbon nanotubes (SWCNTs) have many advantages in terms of their photophysics, but it is difficult to target them to specific locations in living systems. In contrast, the green fluorescent protein (GFP) has been genetically fused to proteins in many cells and organisms. Therefore, GFP can be seen not only as a fluorophore but as a universal target/handle. Here, we report the conjugation of GFP‐binding nanobodies to DNA‐wrapped SWCNTs. This approach combines the targeting capabilities of GFP‐binding nanobodies and the nonbleaching near‐infrared fluorescence (850–1700 nm) of SWCNTs. These conjugates allow us to track single Kinesin‐5‐GFP motor proteins in developing embryos of Drosophila melanogaster. Additionally, they are sensitive to the neurotransmitter dopamine and can be used for targeted sensing of dopamine in the nm regime.     

A second-harmonic-active unnatural amino acid as a structural probe of biomolecules on surfaces

Second-harmonic generation (SHG) is highly sensitive to the net, average orientation of SH-active molecules on surfaces and has recently emerged as a technique for detecting biomolecules and their conformational changes. As most biomolecules are not intrinsically SH-active, they must be labeled with probes to render them detectable. To date, exogenous probes have been used to do this, but second-harmonic-active unnatural amino acids offer important advantages for the long-range goal of precisely and directly determining structural changes in real time and may be used for both buried and surface sites. Results of the first known SH-active unnatural amino acid, Aladan, are presented here. Aladan is found to be SH-active by detecting it at an interface, both alone and incorporated into the B1 domain of protein G (GB1), a globular immunoglobulin-binding protein, at both buried and exposed sites. The tilt angle of Aladan alone adsorbed on a mica surface is determined by polarization experiments, and its nonlinear polarizability alpha((2)) is found to be ca. 10(-30) esu. Aladan GB1 mutants are detectable by SHG, either when coupled covalently to a derivatized glass surface or bound to IgG immobilized via protein A. Addition of an Fc domain to this GB1 complex causes a small but defined change in the SH signal when Aladan is incorporated at site Ala(24), but not at Leu(7), consistent with a local conformational change of GB1. This structural change is not apparent in either X-ray crystallography or fluorescence studies, demonstrating that SHG can detect subtle orientational changes, including protein-protein interactions in which no significant rearrangements occur.     

Light-activated reassembly of split green fluorescent protein

Truncated green fluorescent protein (GFP) with the 11th β-strand removed is potentially interesting for bioconjugation, imaging, and the preparation of semisynthetic proteins with novel spectroscopic or functional properties. Surprisingly, the truncated GFP generated by removing the 11th strand, once refolded, does not reassemble with a synthetic peptide corresponding to strand 11 but does reassemble following light activation. The mechanism of this process has been studied in detail by absorption, fluorescence, and Raman spectroscopy. The chromophore in this refolded truncated GFP is found to be in the trans configuration. Upon exposure to light a photostationary state is formed between the trans and cis conformations of the chromophore, and only truncated GFP with the cis configuration of the chromophore binds the peptide. A kinetic model describing the light-activated reassembly of this split GFP is discussed. This unique light-driven reassembly is potentially useful for controlling protein-protein interactions.     

Enhancement of second harmonic generation in helical Sc liquid crystals

In a ferroelectric liquid crystal, a special type of phase-matching for optical second harmonic generation (SHG) is possible, where two counter-propagating fundamental waves create second harmonic waves at the edge of the selective reflection band. We compute the SHG intensity in such a situation and show that, at slight detuning from exact phase-matching, useful resonance enhancement can be obtained. A considerable amount of SHG also appears when the second harmonic frequency is in the reflection band, where the SHG wave is non-propagating.     

Mechanism of Crystalline Self‐Assembly in Aqueous Medium: A Combined Cryo‐TEM/Kinetic Study

Understanding the crystallization of organic molecules is a long‐standing challenge. Herein, a mechanistic study on the self‐assembly of crystalline arrays in aqueous solution is presented. The crystalline arrays are assembled from perylene diimide (PDI) amphiphiles bearing a chiral N‐acetyltyrosine side group connected to the PDI aromatic core. A kinetic study of the crystallization process was performed using circular dichroism spectroscopy combined with time‐resolved cryogenic transmission electron microscopy (cryo‐TEM) imaging of key points along the reaction coordinate, and molecular dynamics simulation of the initial stages of the assembly. The study reveals a complex self‐assembly process starting from the formation of amorphous aggregates that are transformed into crystalline material through a nucleation–growth process. Activation parameters indicate the key role of desolvation along the assembly pathway. The insights from the kinetic study correlate well with the structural data from cryo‐TEM imaging. Overall, the study reveals four stages of crystalline self‐assembly: 1) collapse into amorphous aggregates; 2) nucleation as partial ordering; 3) crystal growth; and 4) fusion of smaller crystalline aggregates into large crystals. These studies indicate that the assembly process proceeds according to a two‐step crystallization model, whereby initially formed amorphous material is reorganized into an ordered system. This process follows Ostwald’s rule of stages, evolving through a series of intermediate phases prior to forming the final structure, thus providing an insight into the crystalline self‐assembly process in aqueous medium.     

Confocal/TEM overlay microscopy: a simple method for correlating confocal and electron microscopy of cells expressing GFP/YFP fusion proteins.

Genetic manipulation allows simultaneous expression of green fluorescent protein (GFP) and its derivatives with a wide variety of cellular proteins in a variety of living systems. Epifluorescent and confocal laser scanning microscopy (confocal) localization of GFP constructs within living tissue and cell cultures has become routine, but correlation of light microscopy and high resolution transmission electron microscopy (TEM) on components within identical cells has been problematic. In this study, we describe an approach that specifically localizes the position of GFP/yellow fluorescent protein (YFP) constructs within the same cultured cell imaged in the confocal and transmission electron microscopes. We present a simplified method for delivering cell cultures expressing fluorescent fusion proteins into LR White embedding media, which allows excellent GFP/YFP detection and also high-resolution imaging in the TEM. Confocal images from 0.5-microm-thick sections are overlaid atop TEM images of the same cells collected from the next serial ultrathin section. The overlay is achieved in Adobe Photoshop by making the confocal image somewhat transparent, then carefully aligning features within the confocal image over the same features visible in the TEM image. The method requires no specialized specimen preparation equipment; specimens are taken from live cultures to embedding within 8 h, and confocal transmission overlay microscopy can be completed within a few hours.     

用激光第二谐振光研究阴离子在多晶铜电极表面上的吸附

首次用激光产生的第二谐振光(SHG)检测到金属/水溶液界面上阴离子在多晶铜电极表面上的吸附,阴离子吸附特性对SHG强度影响明显,由多晶铜电极在(0.5-x)mol/L NaClO_4+xmol/L NaBr溶液中的SHG强度-电位曲线表明铜电极表面对ClO_4~-的吸附非常弱,对Br~-有特定的吸附,SHG强度随Br~-浓度增加而增强,结果表明SHG是定量研究电化学界面区吸附特性的灵敏有效的探针,可揭示金属与吸附质间相互作用的本质。     

A DNA G-quadruplex converts SOD1 into fibrillar aggregates

Nucleic acids with G4 elements play a role in the formation of aggregates involved in intracellular phase transitions. Our previous studies suggest that different forms of DNA could act as an accelerating template in Cu/Zn superoxide dismutase (SOD1) aggregation. Here, we examined the regulation of formation and cytotoxicity of the SOD1 aggregates by single-stranded 12-mer deoxynucleotide oligomers (dN)₁₂ (N = A, T, G, C; ssDNAs) under acidic conditions. The ssDNAs can be divided into two groups based on their roles in SOD1 binding, exposure of hydrophobic clusters in SOD1, accelerated formation, morphology and cytotoxicity of SOD1 aggregates. G-quadruplexes convert SOD1 into fibrillar aggregates as a template, a fact which was observed for the first time in the nucleic acid regulation of protein aggregation. Moreover, the fibrillar or fibril-like SOD1 species with a G-quadruplex provided by (dG)₁₂ were less toxic than the amorphous species with (dN)₁₂ (N = A, T). This study not only indicates that both morphology and cytotoxicity of protein aggregates can be regulated by the protein-bound DNAs, but also help us understand roles of nucleic aid G-quadruplexes in the formation of aggregates and membraneless organelles involved in intracellular phase transitions.     

Computational prediction of absorbance maxima for a structurally diverse series of engineered green fluorescent protein chromophores

By virtue of its self-sufficiency to form a visible wavelength chromophore within the confines of its tertiary structure, the Aequorea victoria green fluorescent protein (GFP) is single-handedly responsible for the ever-growing popularity of fluorescence imaging of recombinant fusion proteins in biological research. Engineered variants of GFP with altered excitation or emission wavelength maxima have helped to expand the range of applications of GFP. The engineering of the GFP variants is usually done empirically by genetic modifications of the chromophore structure and/or its environment in order to find variants with new photophysical properties. The process of identifying improved variants could be greatly facilitated if augmented or guided by computational studies of the chromophore ground and excited-state properties and dynamics. In pursuit of this goal, we now report a thorough investigation of computational methods for prediction of the absorbance maxima for an experimentally validated series of engineered GFP chromophore analogues. The experimental dataset is composed of absorption maxima for 10 chemically distinct GFP chromophore analogues, including a previously unreported Y66D variant, measured under identical denaturing conditions. For each chromophore analogue, excitation energies and oscillator strengths were calculated using configuration interaction with single excitations (CIS), CIS with perturbative correction for double substitutions [CIS(D)], and time-dependent density functional theory (TD DFT) using several density functionals with solvent effects included using a polarizable continuum model. Comparison of the experimental and computational results show generally poor quantitative agreement with all methods attempted. However, good linear correlations between the calculated and experimental excitation energies (R2>0.9) could be obtained. Oscillator strengths obtained with TD DFT using pure density functionals also correlate well with the experimental values. Interestingly, most of the computational methods used in this work fail in the case of nonaromatic Y66S and Y66L protein chromophores, which may be related to a significant contribution of double excitations to their excited-state wavefunctions. These results provide an important benchmark of the reliability of the computational methods as applied to GFP chromophore analogues and lays a foundation for the computational design of GFP variants with improved properties for use in biological imaging.     

Combined Second Harmonic Generation and Fluorescence Analyses of the Structures and Dynamics of Molecules on Lipids Using Dual-Probes: A Review

Revealing the structures and dynamic behaviors of molecules on lipids is crucial for understanding the mechanism behind the biophysical processes, such as the preparation and application of drug delivery vesicles. Second harmonic generation (SHG) has been developed as a powerful tool to investigate the molecules on various lipid membranes, benefiting from its natural property of interface selectivity, which comes from the principle of even order nonlinear optics. Fluorescence emission, which is in principle not interface selective but varies with the chemical environment where the chromophores locate, can reveal the dynamics of molecules on lipids. In this contribution, we review some examples, which are mainly from our recent works focusing on the application of combined spectroscopic methods, i.e., SHG and two-photon fluorescence (TPF), in studying the dynamic behaviors of several dyes or drugs on lipids and surfactants. This review demonstrates that molecules with both SHG and TPF efficiencies may be used as intrinsic dual-probes in plotting a clear physical picture of their own behaviors, as well as the dynamics of other molecules, on lipid membranes.