Correction: Effect of DNA density immobilized on gold nanoparticles on nucleic acid detection.

[This corrects the article DOI: 10.1039/D3RA06528F.].

Effect of DNA density immobilized on gold nanoparticles on nucleic acid detection.

Gold nanoparticles (AuNPs) have been utilized as colorimetric biosensors, where target molecule-induced AuNP aggregation can be recognized by a colour change from red to blue. Particularly, single-stranded DNA (ssDNA)-immobilized AuNPs (ssDNA-AuNPs) have been applied to genetic diagnosis due to their rapid and sequence-specific aggregation properties. However, the effect of the density of immobilized ssDNA have not been investigated yet. In this study, we developed a method to control the amount of immobilized ssDNA by use of ethylene glycol, which is expected to control the ice crystal spacing in a freezing-thawing ssDNA-AuNP synthesis method. We also investigated the effect of the DNA density on the sensitivity of the target ssDNA detection, and found that the detection sensitivity was improved at lower DNA densities. To discuss the reason for the improved detection sensitivity, we modified the ssDNA-AuNPs with alkane thiol for better dispersion stability against salt. The results suggest that the DNA density, rather than the dispersion stability, has a significant impact on detection sensitivity.

Charge-Segregated Stacking Structure with Anisotropic Electric Conductivity in NIR-Absorbing and Emitting Positively Charged π-Electronic Systems.

Squarylium-based π-electronic cation with an augmented dipole was synthesized by methylation of zwitterionic squarylium. The cation formed various ion pairs in combination with anions, and the ion pairs exhibited distinct photophysical properties in the dispersed state, ascribed to the formation of J- and H-aggregates. The ion pairs provided solid-state assemblies based on cation stacking. It is noteworthy that complete segregation of cations and anions was observed in a pseudo-polymorph of the ion pair with pentacyanocyclopentadienide as a π-electronic anion. In the crystalline state, the ion pairs exhibited photophysical properties and electric conductivity derived from cation stacking. In particular, the charge-segregated ion-pairing assembly induces an electric conductive pathway along the stacking axis. The charge-segregated mode and fascinating properties were derived from the reduced electrostatic repulsion between adjacent π-electronic cations via dipole-dipole interactions.

Electron-donating curved π-electronic systems that complex with buckyballs.

Modifications in curved π-electronic systems, dipyrrolylbenzodiazepines, resulted in various derivatives with modulated electronic properties and assembly behaviour. The electron-rich pyrrole-based curved π-system exhibited C60 complexation in the form of a hydrogen-bonding cyclic hexamer, giving rise to solid-state photo-induced electron transfer as elucidated by transient absorption spectroscopy.

Ion-pairing π-electronic systems: ordered arrangement and noncovalent interactions of negatively charged porphyrins.

In this study, charged π-electronic species are observed to develop stacking structures based on electrostatic and dispersion forces. i π- i π Interaction, defined herein, functions for the stacking structures consisting of charged π-electronic species and is in contrast to conventional π-π interaction, which mainly exhibits dispersion force, for electronically neutral π-electronic species. Establishing the concept of i π- i π interaction requires the evaluation of interionic interactions for π-electronic ion pairs. Free base (metal-free) and diamagnetic metal complexes of 5-hydroxy-10,15,20-tris(pentafluorophenyl)porphyrin were synthesized, producing π-electronic anions upon the deprotonation of the hydroxy unit. Coexisting cations in the ion pairs with porphyrin anions were introduced as the counter species of the hydroxy anion as a base for commercially available cations and as ion-exchanged species, via Na+ in the intermediate ion pairs, for synthesized π-electronic cations. Solid-state ion-pairing assemblies were constructed for the porphyrin anions in combination with aliphatic tetrabutylammonium (TBA+) and π-electronic 4,8,12-tripropyl-4,8,12-triazatriangulenium (TATA+) cations. The ordered arrangements of charged species, with the contributions of the charge-by-charge and charge-segregated modes, were observed according to the constituent charged building units. The energy decomposition analysis (EDA) of single-crystal packing structures revealed that electrostatic and dispersion forces are important factors in stabilizing the stacking of π-electronic ions. Furthermore, crystal-state absorption spectra of the ion pairs were correlated with the assembling modes. Transient absorption spectroscopy of the single crystals revealed the occurrence of photoinduced electron transfer from the π-electronic anion in the charge-segregated mode.

Molecular detection using aptamer-modified gold nanoparticles with an immobilized DNA brush for the prevention of non-specific aggregation.

Gold nanoparticles (AuNPs) are often used for biosensing. In particular, aptamer-modified AuNPs are often used for colorimetric molecular detection, where target molecule-induced AuNP aggregates can be recognized by a color change from red to blue. However, non-specific aggregation could be induced by various compounds, leading to false-positive results. In this work we employed high-density ssDNA modification on the AuNP surface to prevent non-specific aggregation. The covalently immobilized DNA brush was used as an anchor for an aptamer specific for the target molecule. Herein, as a proof-of-concept study, we demonstrated detection of estradiol (E2), one of the endocrine-disrupting estrogen molecules as a model target, in the presence of antibiotic kanamycin (KN) as a model of co-contaminating compounds that induce non-specific aggregation of AuNPs. We also developed a smartphone dark field microscope (DFM) to visualize AuNP aggregation. Our previous study demonstrated that the observation of light scattering by AuNP aggregates with DFM can be applied for versatile molecular detection. In this work, we could successfully detect E2 with the smartphone DFM, and the results were verified by the results from a conventional benchtop DFM. This study would contribute to the future field applicability of AuNP-based sensors.

Nanosecond laser photothermal effect-triggered amplification of photochromic reactions in diarylethene nanoparticles.

Intense ns pulse laser excitation to nanoparticle colloids of a photochromic diarylethene induced an amplified cycloreversion reaction. The mechanism was explained as a 'photosynergetic response' coupled with nanoscale laser heating and the photochemical reaction in nanoparticles.

Detection of Gold Nanoparticles Aggregation Using Light Scattering for Molecular Sensing.

Gold nanoparticles (AuNPs) have been commonly used in molecular sensing, in the form of observation of the color change from red to blue of the AuNP solution, caused by target-molecule-induced AuNP aggregation. In this work, the changes in absorbance and scattering spectra caused by AuNP aggregation were studied using thrombin-induced AuNP aggregation as a model. We demonstrated for the first time that scattering spectra is more sensitive to the changes owing to AuNP aggregation than absorbance spectra. Moreover, a digital color analysis of darkfield images using dark field microscopy (DFM) facilitated a simple method for detection of AuNPs aggregation without the use of spectroscopic analysis. Furthermore, we demonstrated that DFM is useful for detecting AuNPs aggregation in a colored solution, in which the color change by AuNPs aggregation is not visible.

Synthesis of Bare Iron Nanoparticles from Ferrocene Hexane Solution by Femtosecond Laser Pulses.

Iron-based nanoparticles (FeNPs) have unique and attractive properties such as superparamagnetism, biocompatibility, and catalytic activity. Although the synthesis of precious metal NPs from a metal in liquid and/or metal salt solution by a pulsed laser has been investigated, comparably little effort has been devoted to examine the production of FeNPs. Here we report the synthesis of carbon-shell free spherical NPs of iron oxide (magnetite) from ferrocene hexane solution by femtosecond near infrared laser pulses. Nanosecond UV laser pulses are used to compare the evolution of the particle size distribution as a function of laser irradiation time. The size of NPs remains constant even for extended exposure to femtosecond laser pulses, whereas it grows with exposure to nanosecond laser pulses. The primary particles are generated by photochemical reactions regardless of pulse duration; however, the fragmentation of NPs by successive femtosecond laser pulses regulates the particle size.

Femtosecond excited-state dynamics of fullerene-C60 nanoparticles in water.

Femtosecond excited-state dynamics of fullerene-C60 nanoparticles (nC60) having a mean size of 50 nm dispersed in pure water was studied by means of femtosecond transient absorption spectroscopy. The intermolecular charge-transfer (CT) excited state in solid C60 was directly and firstly observed by femtosecond 350 nm and 420 nm excitations, and its intrinsic lifetime of 0.35 ps was found. The CT excited state relaxed to the locally excited S1 state and excimers or directly to the ground state through geminate charge recombination. We also examined the laser fluence dependence of the CT excited-state dynamics. At a high laser fluence, the mutual interactions between neighboring CT excited states were observed immediately after the excitation. The interaction disappeared through the charge recombination in the geminate CT pair or between the neighboring CT excited states with a lifetime of 0.45 ps. After that, the locally excited S1 state decayed with a few ps lifetime independent of the fluence. In this paper, the mechanism and dynamics of the intermolecular CT excited state generated by UV light excitation is discussed in detail.

Preparation and Fluorescence Properties of Perylenediimide Nanodispersions Having a One-Dimensional π-Stacked Structure.

We prepared stable nanodispersions of a fluorescent perylenediimide (PDI) derivative having long alkyl chains by nanosecond laser fragmentation of its microcrystalline powder in acetonitrile (ACN). The nanoparticles had cube-like or rod shapes with a mean size of 100 nm, and they dispersed stably for longer than 1 month. The prepared nanobricks exhibited absorption and fluorescence spectra characteristic of one-dimensional aggregates with cofacial stacking of PDI planes. Single-particle fluorescence measurements demonstrated that nanobricks had a well-aligned structure of one-dimensional columns of PDI. The aqueous dispersions were also fabricated by redispersing the prepared nanobricks, utilizing lipophilic interactions of surfactants having long alkyl chains. We examined the fluorescence properties of nanoparticles dispersed in ACN and in water, and observed amplified fluorescence quenching by the surface-adsorbed dye.

Femtosecond Pump-Probe Microspectroscopy of Single Perylene Nanoparticles.

We have developed a femtosecond pump-probe light scattering microspectroscopic system in which the output of a femtosecond Ti:sapphire oscillator (1 W, 82 MHz) was used as a light source; the pump light is the second harmonics (395 nm) of the laser output, and the probe light is a femtosecond white-light continuum (490-900 nm) generated with a photonic crystal fiber. Detection of the backscattered light from single nanoparticle on a glass substrate allowed us to obtain higher gain of the transient signals by ∼20 times in comparison with the conventional transmittance-mode experiment. This high-sensitivity of the backscattering detection makes it possible to examine ultrafast relaxation dynamics of excited states in organic nanoparticles, which, in general, are lower photodurability than the inorganic one. We applied the system to single nanocrystals of α-form perylene and then succeeded in direct observation of the excimer formation dynamics on a picosecond time scale. Single nanoparticle measurements for the perylene nanocrystals having a size range of 100 to 500 nm suggested that the excimer formation time became short from 2 ps to <0.3 ps for decreasing of the size.

Photoswitchable interactions between photochromic organic diarylethene and surface plasmon resonance of gold nanoparticles in hybrid thin films.

Hybrid materials combining gold nanoparticles (GNP) of variable diameter and an organic thin layer of photochromic diarylethenes were achieved. Solid-state photoswitching based on ring-closure/ring-opening reaction was carried out under alternate UV and visible irradiations. In addition to the spectral changes due to the photochromism itself, the surface plasmon resonance related to the GNP is significantly modified, influenced by a photoinduced change in the refractive index of its environment. These two contributions were sorted out, showing the possibility of probing a photochromic switch by following the plasmon band. The shape change of the plasmon band was consistently compared to calculations based on the Mie theory. Additionally, with one given diarylethene compound, both UV-visible spectroscopy and surface enhanced Raman scattering (SERS) spectroscopy showed an acceleration of the ring-opening photochromic reaction in the presence of GNP.

Plasmonic enhancement of a photocycloreversion reaction of a diarylethene derivative using individually dispersed silver nanoparticles.

The fabrication of silver nanoparticles covered with polymers with a well-defined core-shell structure and the quantitative evaluation of the plasmonic enhancement effect on a photochemical reaction in the vicinity of these silver nanoparticles individually dispersed in a medium are described. The photocycloreversion reaction of a diarylethene polymer in the vicinity of silver nanoparticles was enhanced by 2-6 times relative to the reaction without the nanoparticles. The promotion of the photocycloreversion reaction is due to enhancement of the electromagnetic field near the surface of the silver core.

Plasmonic enhancement of gold nanoparticles on photocycloreversion reaction of diarylethene derivatives depending on particle size, distance from the particle surface, and irradiation wavelength.

We newly synthesized various sized gold nanoparticles covered with photochromic polymers consisting of diarylethenes with various structures to investigate an effect of the gold nanoparticles on the photocycloreversion reaction of the diarylethene chromophores upon irradiation with visible light. The gold nanoparticles covered with the photochromic polymers exhibited reversible changes in localized surface plasmon resonance (LSPR) absorption along with the photochromic reaction depending on the diameter of the particle, the distance between the gold surface and the chromophore, and the structure of the diarylethene chromophore. The rate of the photocycloreversion reaction of the chromophores around the particle was enhanced by the gold nanoparticles and the degree of the enhancement was affected by the diameter of the particle and the distance from the gold surface, while a structural difference in the diarylethene chromophore had no effect on the degree of the enhancement. The larger enhancement of the photocycloreversion reaction was observed by irradiation at longer wavelength side than visible light corresponding to the LSPR frequency.

Fabrication of the smallest organic nanocolloids by a top-down method based on laser ablation.

Stable aqueous colloids of 10-nm sized organic nanoparticles were tailored by laser ablation of microcrystalline quinacridone in water. The nanocolloids were flaky in shape and had the dimension of a width of 13 (±5) nm and a height of 1.4 (±0.5) nm. The formation mechanism is discussed in terms of laser-induced fragmentation of organic solids and the potential application of aqueous organic nanocolloids free from any additives and chemicals is considered.

Photochemical reaction of p-hydroxycinnamic-thiophenyl ester in the microcrystalline state.

We have studied the photochromic reaction of p-hydroxycinnamic-thiophenyl ester in the microcrystalline state. We attributed the fluorescence spectral evolution of the microcrystal, under UV irradiation, to the photoinduced trans-to-cis isomerization. The photocyclic behavior of the chromophore was demonstrated by cis-to-trans back reaction under a subsequent visible light irradiation. In addition, the [2 + 2] topochemical photocyclodimer was observed as another photoproduct. It is considered that the cooperative photoisomerization is initiated at the local lattice distortion and free spaces around the [2 + 2] cyclodimer near the crystal surface, and the photoisomerization induces larger lattice deformation and further photoisomerization in the interior of the crystal.

trans-cis Photoisomerization of a photoactive yellow protein model chromophore in crystalline phase.

We have studied the photoinduced trans/cis isomerization of the protonated form of p-hydroxycinnamic thiophenyl ester, a model chromophore of the photoactive yellow protein (PYP), in crystalline phase, by both fluorescence and infrared spectroscopies. The conversion from trans to cis configuration is revealed by a shift of the fluorescence peak and by inspection of the infrared maker bands. The crystal packing apparently stabilizes the cis photoproduct, suggesting different environmental effects from the solvent molecules for this model chromophore in liquid solutions or from the amino acid residues for the PYP chromophore.

Development of near-infrared 35 fs laser microscope and its application to the detection of three- and four-photon fluorescence of organic microcrystals.

Femtosecond near-infrared laser microscope was developed with a home-built cavity-dumped chromium:forsterite laser as a light source centered at 1.26 microm. Optimization of the pulse duration achieved 35 fs fwhm at the sample position of the microscope after passing through a 100x objective. This system was applied to the detection of multiphoton fluorescence of some organic microcrystals. Excitation intensity dependence and the interferometric autocorrelation detection of the fluorescence clearly demonstrated that simultaneous three- and four-photon absorption processes are responsible for the production of the excited state for perylene and anthracene microcrystals, respectively. The spatial resolution along the optical axis and its dependence on the order of the multiphoton process were also discussed.

Cooperative photochemical reaction mechanism of femtosecond laser-induced photocoloration in spirooxazine microcrystals.

Microcrystalline powders of spirooxazine and spiropyran compounds do not show photocoloration under steady-state illumination, whereas they undergo photochromism on intense femtosecond laser-pulse excitation. We investigated the characteristic mechanism of the crystalline photochromism by studying the photocoloration of spironaphthooxazine (SNO) and its chloro-substituted derivative (Cl-SNO) with our femtosecond diffuse-reflectance spectroscopic system. In particular, femtosecond double-pulse excitation using 390+780-nm pulses and 390+390-nm pulses, with a variable time interval between the two pulses, was applied to reveal an intermediate species involved in the photocoloration. Although 780-nm excitation of an intermediate produced by 390-nm excitation did not lead to isomerization, the 390+390-nm excitation resulted in photocoloration. The yield for SNO decreased on increasing the interval from 40 ps to 5 ns, while that for Cl-SNO was constant. The photocoloration mechanism in the crystalline phase is considered from the viewpoint of the time-dependent density of short-lived transient species, and it is concluded that cooperative interactions of excited states and nonplanar open forms play an important role in femtosecond laser-induced photochromism in these crystals.