Surface Plasmon Field-Enhanced Raman Scattering Co-Excited by P-Polarized and S-Polarized Light Based on Waveguide-Coupled Surface Plasmon Resonance Configuration.

We constructed a waveguide-coupled surface plasmon resonance (WCSPR) structure to enhance Raman scattering. In this structure, P-polarized and S-polarized incident lasers can simultaneously coexcite the evanescent field, thereby further enhancing Raman scattering. This configuration is a five-phase Kretschmann resonance setup that consists of a SF10 prism/inner Ag film/SiO2 film/outer Ag film/water structure. The WCSPR configuration effectively concentrates and confines the evanescent field excited by the incident light. Ag nanoparticles assembled on the outer Ag film surface enhance the evanescent field further by means of surface plasmon resonance. By finely tuning the thickness of the Ag and SiO2 films, it is possible to achieve a coincidence between the SPR angle of P-polarized light and that of S-polarized light. At this angle, both P- and S-polarized light can jointly elevate the evanescent field intensity, leading to the simultaneous enhancement of the electric fields at the upper, lower, left, and right parts of the silver nanoparticles and generating maximum evanescent field enhancement. We achieved an electric field enhancement of up to 103 around the nanoparticles, leading to more SERS hotspots and comparable SERS enhancement capability to gap-type hotspots. Our WCSPR structure combined with the nanoparticles offers a feasible strategy for the SERS detection of large molecules that cannot be placed in traditional gap-type hotspots. It is highly convenient for SERS detection of large molecules.

Preparation and characterisation of linalool oil-in-water starch-based Pickering emulsions and the effects of the addition of cellulose nanocrystals on their stability.

Creaming could be generated during storage of the starch-based Pickering emulsions. And cellulose nanocrystals in the solution are usually dispersed by relatively strong mechanical force, otherwise they may appear in the form of aggregates. In this work, we investigated the effects of cellulose nanocrystals on the stability of the starch-based Pickering emulsions. Results showed that the stability of Pickering emulsions was significantly improved by adding cellulose nanocrystals. Cellulose nanocrystals increased the viscosity, electrostatic repulsion and steric hindrance of the emulsions, which delayed the movement of droplets and obstructed the contact between droplets. This study provides new insights into the preparation and stabilisation of starch-based Pickering emulsions.

Tuning Organic Microcrystal Morphologies through Crystal Engineering Strategies toward Anisotropic Optical Waveguide.

The construction of organic optoelectronic materials with desirable size and morphology remains a challenge now. Crystal engineering strategies (polymorphs and cocrystals) provide convenience for tailoring molecular packing and further controlling the growth morphology and photofunctionality of materials. Herein, we prepare polymorphic 2D plate crystals and 3D microhelixes by assembly of a cyanostilbene derivative (2-(3',5'-bis(trifluoromethyl)-biphenyl-4-yl)-3-(4-(pyridin-4-yl)phenyl)acrylonitrile, CF3-CN-Py). The former emits blue emission, while the latter emits green emission. Different crystallization environments contribute to the adjustable morphologies. Then, novel cocrystals are fabricated with the introduction of 1,4-diiodotetrafluorobenzene (FDIB) to CF3-CN-Py. Both molecular conformation and packing are totally changed in the cocrystal system. Such cocrystal displays a 1D sky-blue emissive rod shape on account of a long-range ordered π-stacking of molecules. In addition, the 2D plate crystal and 1D rod cocrystal are further applied to optical waveguides. In the plate crystal, a packing of transition dipole moment (µ) inclined to the upper surface leads to an anisotropic optical waveguide. In the cocrystal, owing to the nearly horizontal µ orientation, the cocrystal exhibits light propagation along the primary growth direction and a low optical loss coefficient. The present study supplies an effective way to construct materials with controlled morphology and optical waveguide.

Single-Cell Oxidative Stress Events Revealed by a Renewable SERS Nanotip.

A nanotip sensitive to reactive oxygen species (ROS) and NAD+/NADH (oxidized/reduced forms of nicotinamide adenine dinucleotide) was designed and prepared to identify the redox events in a single living cell by surface-enhanced Raman scattering (SERS) spectroscopy. The nanotips were prepared by the one-step laser-induced Ag growth and deposition. A redox-reversible Raman reporter, 4-mercaptophenol (4-MP), was employed for the nanotip decoration along with the Ag deposition. 4-MP can be converted to SERS-inactive 4-mercaptocyclohexa-2,5-dienone (4-MC) by Fe3+ ions to complete signal rezeroing for multiple oxidative stress event loops. The SERS signal conversion from 4-MC to 4-MP provides a cue for the reduction process that is NADH-dependent. In contrast, by the conversion from 4-MP to 4-MC, the oxidative stress events and the signal transduction mechanism of cells stimulated by drugs (phorbol 12-myristate 13-acetate and H2O2) can be explored by SERS. This sensor is easy to fabricate and can be recycled. This tip-typed SERS nanosensor can be extendedly available for tracing other key markers in other NAD+/NADH-mediated respiratory chain and glycolysis, e.g., lactic acid, pyruvic acid, adenosine triphosphate, and antioxidants. It will be useful for investigating the diseases of abnormal oxidative stress and mitochondrial metabolism at the single-cell level.

Plasmon-Enhanced Four-Wave Mixing Imaging for Microdroplet-Based Single-Cell Analysis.

A high-throughput single-cell analytical technique based on the microdroplet array integrated with the plasmon-enhanced-four-wave mixing (PE-FWM) imaging was developed, which is applicable for the highly sensitive and automatic assessment of the surface receptors of cells. The metal nanoprobes were prepared by simply decorating metal nanoparticles with capturing molecules (antibody or molecules with surface identification function). Owing to the multifrequency selection of lasers via resonating their plasmonic bands, these metal nanoprobes are highly recognizable under the FWM imaging and display high photostability above fluorescent dyes. This PE-FWM imaging technique shows superior to dark-field imaging due to almost no interference from off-resonant species and exhibits the antifade feature that is suitable for long-period cell monitoring. The automated processing of images is available for the analysis of cell heterogeneity according to the cell surface receptors. Emerging applications such as single-cell analysis, bioimaging, metabolite, and drug tracing offer many biological and medical possibilities with broad application prospects.

A two-photon fluorescence, carbonized polymer dot (CPD)-based, wide range pH nanosensor: a view from the surface state.

A green-emitting, low-toxicity carbonized polymer dot (CPD) with a high fluorescence quantum yield was synthesised by a simple hydrothermal method, and has been applied as a three-mode pH indicator and the pH readouts involve the intensity ratio of the absorption bands, the single-photon fluorescence, and the two-photon fluorescence (TPF) signals. The pH sensing mechanism of this CPD is dependent on the hydrogen ion regulation on its surface states, which is evidenced for the first time by transient spectroscopy. The rich surface states of this CPD allow a wider pH-responsive range relative to other carbon nanodot-based pH nanosensors. Its ultra-small size, low cell toxicity, high brightness and stability are conducive to intracellular pH sensing under the TPF imaging. Our study is helpful for the development of novel carbon-based sensing materials based on the design of the surface states. It also provides a new candidate for up-conversion photoluminescence-responsive imaging agents and it has potential applications in the diagnosis and dynamic monitoring of cells relying on the pH evolution.

Deep Red Emissive Carbonized Polymer Dots with Unprecedented Narrow Full Width at Half Maximum.

Development of high-performance carbon dots (CDs) with emission wavelength longer than 660 nm (deep red emission) is critical in deep-tissue bioimaging, yet it is still a major challenge to obtain CDs with both narrow full width at half maximum (FWHM) and high deep red/near-infrared emission yield. Here, deep red emissive carbonized polymer dots (CPDs) with unprecedented FWHM of 20 nm are synthesized. The purified CPDs in dimethyl sulfoxide (DMSO) solution possess quantum yield (QY) as high as 59% under 413 nm excitation, as well as recorded QY of 31% under 660 nm excitation in the deep red fluorescent window. Detailed characterizations identify that CPDs have unique polymer characteristics, consisting of carbon cores and the shells of polymer chains, and π conjugated system formed with N heterocycles and aromatic rings governs the single photoluminescence (PL) center, which is responsible for high QY in deep red emissive CPDs with narrow FWHM. The CPDs exhibit strong absorption and emission in the deep red light region, low toxicity, and good biocompatibility, making them an efficient probe for both one-photon and two-photon bioimaging. CPDs are rapidly excreted via the kidney system and hepatobiliary system.

Long-Range Surface Plasmon Resonance Configuration for Enhancing SERS with an Adjustable Refractive Index Sample Buffer to Maintain the Symmetry Condition.

We propose a method to maintain the symmetry condition of the refractive index with respect to a dielectric buffer layer for a long-range surface plasmon resonance (LRSPR) configuration. The symmetry condition was maintained by changing the concentration of the ethylene glycol aqueous solution (sample buffer layer) to match the refractive index of the MgF2 film. Maintenance of the symmetry condition is necessary for exciting the LRSPR mode and increasing the electric field intensity near the film. We used a four-phase Kretschmann resonance setup composed of a K9 prism, MgF2 film, Ag film, and sample buffer layer. The incident angle-dependent surface-enhanced Raman scattering (SERS) spectra were measured in the evanescent field. At the SPR angle, the SERS signal of the symmetric configuration was 60 times higher than that of the conventional SPR configuration. Moreover, the electric field penetration depth of the symmetric long-range surface plasmon configuration (>1000 nm) was longer than that of their asymmetric counterparts. The enhancement factor of the symmetric configuration was 8.6 × 107, which corresponded to the lowest detectable concentration for 4-mercaptopyridine, reaching 1.0 × 10-10 M at the resonance angle. Thus, the symmetric LRSPR configuration has great potential for label-free sensing and detection of macromolecules and biomolecules.

Solvation-Enhanced Intermolecular Charge Transfer Interaction in Organic Cocrystals: Enlarged C-C Surface Close Contact in Mixed Packing between PTZ and TCNB.

The mixed π-π packing of the donor (D) and acceptor (A) molecules is the highlighting feature of the intermolecular interactions following charge transfer (CT) issues in organic cocrystal systems. There is an inverse relationship between the D-A interplanar distance and the intermolecular CT interaction. However, the D-A C-C surface close contact (relative areas) on the intermolecular CT interactions in organic cocrystal systems is rarely investigated. Herein, we designed and constructed a novel cocrystal and its solvate cocrystal. The structural and electrostatic potential analyses suggest that the solvation destroys the N-H···N hydrogen bond interaction between phenothiazine (PTZ) and 1,2,4,5-tetracyanobenzene (TCNB), which causes the TCNB molecules to have a 90° rotation along the normal axis of the PTZ plane. Thus, the D-A C-C surface close contact is enlarged, strengthening the intermolecular π-π stacking interactions and intermolecular CT interaction between PTZ and TCNB, which are further evidenced by the absorption and Raman spectroscopic analyses. This study provides rare evidence of the enlarged C-C surface close contact in the mixed packing between D and A that greatly contributes to the intermolecular CT interaction in a D-A cocrystal system. It also provides a deeper understanding of the role of solvation in the structure-property relationship of organic cocrystal materials.

Remarkable pressure-induced emission enhancement based on intermolecular charge transfer in halogen bond-driven dual-component co-crystals.

A series of two-component co-crystals driven by IN interactions based on the bipyridine (BIPY) chromophore with one among three different co-former building blocks, iodopentafluorobenzene (IPFB), 1,4-diiodotetrafluorobenzene (DITFB) and 1,3,5-trifluoro-2,4,6-triiodobenzene (IFB), were prepared and analysed via infared spectroscopy and single-crystal X-ray diffraction. By comparing the IN distances in the co-crystal structures, we found that the higher the -F ratio in the building blocks the closer the contact of the IN bond, enhancing the intermolecular interactions in these co-crystals as well. That is, the positive electrostatic potential on the iodine atom(s) in the co-formers was enhanced by the presence of strong electron-withdrawing groups. The distinct spectroscopic behaviours (fluorescence and Raman spectra) among the two-component BIPY co-crystal systems in response to hydrostatic pressure were also investigated. Interestingly, the fluorescence of BIPY-DITFB presented intriguing abnormal evolution from dark to bright, suggesting a new charge transfer state due to the decreased intermolecular distance and the enhanced IN interactions. Theoretical simulations by Materials Studio also showed the shortened IN distance and the increased angle of C-IN, evidencing the enhanced IN interactions. In contrast, BIPY-IFB showed only slightly enhanced fluorescence intensity at 550 nm consistent with BIPY-DITFB. Once the pressure was relieved, both the Raman and fluorescence spectra for BIPY co-crystal systems entirely self-recovered. Remarkable emission enhancement in a solid-state co-crystal has been rarely reported in previous publications and in fact, this study paves a unique way for designing and developing novel stimuli-responsive photo-functional materials.

Investigation of supramolecular interaction in 4, 4'-bipyridine crystal by hydrostatic pressure spectroscopies.

The luminescence and structural changes of 4, 4'-bipyridine in the crystal and powder forms under the effect of high pressure applied by a diamond anvil cell has been investigated through the fluorescence and Raman spectroscopies. In its single crystal structure, the 4, 4'-bipyridine molecules are paralleled arranged with the identifiable CH⋯N and π⋯π interactions among molecules. However, in the powder form, these intermolecular interactions nearly diminish. The 4, 4'-bipyridine crystal shows the obvious bathochromic-shifting of the emission band, which is different from the powder sample that displays a fixed luminescent band during compression. Additionally, the Raman bands of them both show shifts to higher wavenumbers as different degrees. The detailed peak assignments are performed based on the theoretical calculation through B3LYP method. Comparisons in spectral behaviors between the crystal and powder under compression show the crystal form exhibits a superior mechanochromic performance relative to the powder one, because the intermolecular interactions in the crystal form play dominating roles and they can be easily tuned along with pressure in such a highly ordered structure compared to the powder form. The relation investigation between property and supramolecular interactions not only makes deeper understanding in the mechanochromic mechanisms of 4, 4'-bipyridine, but also gives a helpful reference for the molecular designs of coordination polymers and co-crystals with 4, 4'-bipyridine involved.

Design of Metal-Free Polymer Carbon Dots: A New Class of Room-Temperature Phosphorescent Materials.

Polymer carbon dots (PCDs) are proposed as a new class of room-temperature phosphorescence (RTP) materials. The abundant energy levels in PCDs increase the probability of intersystem crossing (ISC) and their covalently crosslinked framework structures greatly suppress the nonradiative transitions. The efficient methods allow the manufacture of PCDs with unique RTP properties in air without additional metal complexation or complicated matrix composition. They thus provide a route towards the rational design of metal-free RTP materials that may be synthesized easily. Furthermore, we find that RTP is associated with a crosslink-enhanced emission (CEE) effect, which provides further routes to design improved PCDs with diverse RTP performance. Our results show the potential of PCDs as a universal route to achieve effective metal-free RTP.

Note: A portable Raman analyzer for microfluidic chips based on a dichroic beam splitter for integration of imaging and signal collection light paths.

An integrated and portable Raman analyzer featuring an inverted probe fixed on a motor-driving adjustable optical module was designed for the combination of a microfluidic system. It possesses a micro-imaging function. The inverted configuration is advantageous to locate and focus microfluidic channels. Different from commercial micro-imaging Raman spectrometers using manual switchable light path, this analyzer adopts a dichroic beam splitter for both imaging and signal collection light paths, which avoids movable parts and improves the integration and stability of optics. Combined with surface-enhanced Raman scattering technique, this portable Raman micro-analyzer is promising as a powerful tool for microfluidic analytics.

Note: Mobile micro-Raman analyzer integrated with a lab-on-a-chip.

A mobile micro-Raman microfluidic analyzer was designed and built for label-free, nondestructive, fingerprint detection of samples on microfluidic chip systems. It mainly includes an optical module (including the Raman detection system and the microscopic imaging system), 3-axis stages with step motors and other auxiliary circuits. The setup of the analyzer was designed with fully considering the characters of the microfluidics system. The experimental results prove that this microfluidics analyzer is practical and very convenient to use. This micro-Raman microfluidics analyzer with high performance to cost ratio has wide application potential in lab-on-a-chip fields as a powerful analytical tool.