Dynamic Self-Stiffening and Structural Evolutions of Polyacrylonitrile/Carbon Nanotube Nanocomposites

The self-stiffening under external dynamic strain has been observed for some artificial materials, especially for nanocomposites. However, few systematic studies have been carried out on their structural evolutions, and the effect of the types of nanofillers was unclear. In this study, we used a semicrystalline polymer, polyacrylonitrile (PAN), and various types of carbon nanomaterials including C₆₀, carbon nanotube (CNT), and graphene oxide (GO). An external uniaxial dynamic strain at small amplitude of 0.2% was applied on the prepared nanocomposite films. It was observed that PAN/CNT exhibited significant self-stiffening behavior, whereas PAN/GO showed no response. Systematic characterizations were performed to determine the structural evolutions of PAN/CNT film during dynamic strain testing, and it was found that the external dynamic strain not only induced the crystallization of PAN chains but also aligned CNT along the strain direction

Simultaneous Nucleophilic-Substituted and Electrostatic Interactions for Thermal Switching of Spiropyran: A New Approach for Rapid and Selective Colorimetric Detection of Thiol-Containing Amino Acids

Complementary electrostatic interaction between the zwitterionic merocyanine and dipolar molecules has emerged as a common strategy for reversibly structural conversion of spiropyrans. Herein, we report a concept-new approach for thermal switching of a spiropyran that is based on simultaneous nucleophilic-substitution reaction and electrostatic interaction. The nucleophilic-substitution at spiro-carbon atom of a spiropyran is promoted due to electron-deficient interaction induced by 6- and 8-nitro groups, which is responsible for the isomerization of the spiropyran by interacting with thiol-containing amino acids. Further, the electrostatic interaction between the zwitterionic merocyanine and the amino acids would accelerate the structural conversion. As proof-of-principle, we outline the route to glutathione (GSH)-induced ring-opening of 6,8-dinitro-1′,3′,3′-trimethylspiro [2H-1-benzopyran-2,2′-indoline] (<b>1</b>) and its application for rapid and sensitive colorimetric detection of GSH. In ethanol–water (1:99, v/v) solution at pH 8.0, the free <b>1</b> exhibited slight-yellow color, but the color changed clearly from slight-yellow to orange-yellow when GSH was introduced into the solution. Ring-opening rate of <b>1</b> upon accession of GSH in the dark is 0.45 s^–1, which is 4 orders of magnitude faster in comparison with the rate of the spontaneous thermal isomerization. The absorbance enhancement of <b>1</b> at 480 nm was in proportion to the GSH concentration of 2.5 × 10^–8–5.0 × 10^–6 M with a detection limit of 1.0 × 10^–8 M. Furthermore, due to the specific chemical reaction between the probe and target, color change of <b>1</b> is highly selective for thiol-containing amino acids; interferences from other biologically active amino acids or anions are minimal

Highly Selective Two-Photon Fluorescent Probe for Ratiometric Sensing and Imaging Cysteine in Mitochondria

A novel ratiometric mitochondrial cysteine (Cys)-selective two-photon fluorescence probe has been developed on the basis of a merocyanine as the fluorophore and an acrylate moiety as the biothiol reaction site. The biocompatible and photostable acrylate-functionalized merocyanine probe shows not only a mitochondria-targeting property but also highly selective detection and monitoring of Cys over other biothiols such as homocysteine (Hcy) and glutathione (GSH) and hydrogen sulfide (H₂S) in live cells. In addition, this probe exhibits ratiometric fluorescence emission characteristics (<i>F</i>₅₁₈/<i>F</i>₄₅₂), which are linearly proportional to Cys concentrations in the range of 0.5–40 μM. More importantly, the probe and its released fluorophore, merocyanine, exhibit strong two-photon excited fluorescence (TPEF) with two-photon action cross-section (Φσ_max) of 65.2 GM at 740 nm and 72.6 GM at 760 nm in aqueous medium, respectively, which is highly desirable for high contrast and brightness ratiometric two-photon fluorescence imaging of the living samples. The probe has been successfully applied to ratiometrically image and detect mitochondrial Cys in live cells and intact tissues down to a depth of 150 μm by two-photon fluorescence microscopy. Thus, this ratiometric two-photon fluorescent probe is practically useful for an investigation of Cys in living biological systems

Self-Assembly of Graphene Oxide with a Silyl-Appended Spiropyran Dye for Rapid and Sensitive Colorimetric Detection of Fluoride Ions

Fluoride ion (F^–), the smallest anion, exhibits considerable significance in a wide range of environmental and biochemical processes. To address the two fundamental and unsolved issues of current F^– sensors based on the specific chemical reaction (i.e., the long response time and low sensitivity) and as a part of our ongoing interest in the spiropyran sensor design, we reported here a new F^– sensing approach that, via assembly of a F^–-specific silyl-appended spiropyran dye with graphene oxide (GO), allows rapid and sensitive detection of F^– in aqueous solution. 6-(<i>tert</i>-Butyldimethylsilyloxy)-1′,3′,3′-trimethylspiro [chromene- 2,2′-indoline] (SPS), a spiropyran-based silylated dye with a unique reaction activity for F^–, was designed and synthesized. The nucleophilic substitution reaction between SPS and F^– triggers cleavage of the Si–O bond to promote the closed spiropyran to convert to its opened merocyanine form, leading to the color changing from colorless to orange-yellow with good selectivity over other anions. With the aid of GO, the response time of SPS for F^– was shortened from 180 to 30 min, and the detection limit was lowered more than 1 order of magnitude compared to the free SPS. Furthermore, due to the protective effect of nanomaterials, the SPS/GO nanocomposite can function in a complex biological environment. The SPS/GO nanocomposite was characterized by XPS and AFM, etc., and the mechanism for sensing F^– was studied by ¹H NMR and ESI-MS. Finally, this SPS/GO nanocomposite was successfully applied to monitoring F^– in the serum

Hemicyanine-based High Resolution Ratiometric near-Infrared Fluorescent Probe for Monitoring pH Changes in Vivo

Intracellular pH is an important parameter associated with cellular behaviors and pathological conditions. Quantitative sensing pH and monitoring its changes by near-infrared (NIR) fluorescence imaging with high resolution in living systems are essential but challenging due to the lack of effective probes. To achieve good adaptability, in this study, a class of resolution-tunable ratiometric NIR fluorescent probes, which possess a stable NIR hemicyanine skeleton bearing different substituents, are rationally designed and synthesized, enabling detection through noninvasive optical imaging of organisms. Based on the protonation/deprotonation of the hydroxy group, a marked NIR emission shift provides a ratio signal in response to pH. Meanwhile, two states exhibit good photostability, sensitivity and reversibility, conducive to longtime monitoring of persistent pH changes without disturbance of other biological active species. Among the series, NIR-Ratio-BTZ modified with an electron-withdrawing substituent of benzothiazole exhibited the largest emission shift of about 76 nm from 672 to 748 nm with the pH environment changing from acidic to basic, which could be considered as a good candidate for high resolution pH imaging in live cells, tissues and organisms. Moreover, NIR-Ratio-BTZ has an ideal p<i>K</i>_a value (p<i>K</i>_a ≈ 7.2) for monitoring the minor fluctuations of physiological pH near neutrality. The ratiometric fluorescence measurement is beneficial to ensure the accuracy of quantitative measuring pH changes, as well as the real-time monitoring pH-related physiological effects both in living cells and living mice. The results demonstrate that NIR-Ratio-BTZ is a highly sensitive ratiometric pH probe in vivo, giving it potential for biological applications

A Zero Cross-Talk Ratiometric Two-Photon Probe for Imaging of Acid pH in Living Cells and Tissues and Early Detection of Tumor in Mouse Model

Acid–base disorders disrupt proper cellular functions, which are associated with diverse diseases. Development of highly sensitive pH probes being capable of detecting and monitoring the minor changes of pH environment in living systems is of considerable interest to diagnose disease as well as investigate biochemical processes in vivo. We report herein two novel high-resolution ratiometric two-photon (TP) fluorescent probes, namely, PSIOH and PSIBOH derived from carbazole–oxazolidine π-conjugated system for effective sensing and monitoring acid pH in a biological system. Remarkably, PSIOH exhibited the largest emission shift of ∼169 nm from 435 to 604 nm upon pH changing from basic to acidic with an ideal p<i>K</i>_a value of 6.6 within a linear pH variation range of 6.2–7.0, which is highly desirable for high-resolution tracking and imaging the minor fluctuation of pH in live cells and tissues. PSIOH also exhibits high pH sensitivity, excellent photostability, and reversibility as well as low cytotoxicity. More importantly, this probe was successfully applied to (i) sense and visualize the pH alteration in HeLa cells caused by various types of exogenous stimulation and (ii) detect and differentiate cancer and tumors in liver tissues and a mouse model, realizing its practical <i>in vitro</i> and <i>in vivo</i> applications

Fluorescence Modulation by Absorbent on Solid Surface: An Improved Approach for Designing Fluorescent Sensor

Inner filter effect (IFE), a well-known phenomenon of fluorescence quenching resulting from absorption of the excitation or emission light of luminescent species by absorbent, has been used as a smart approach to design fluorescent sensors, which are characterized by the simplicity and flexibility with high sensitivity. However, further application of IFE-based sensors in complex environment is hampered by the insufficient IFE efficiency and low sensitivity resulting from interference of the external environment. In this paper, we report that IFE occurring on a solid substrate surface would solve this problem. As a proof of concept, a fluorescent sensor for intracellular biothiols has been developed on the basis of the absorption of a newly designed thiols-specific chromogenic probe (<b>CP</b>) coupled with the use of a thiols-independent fluorophore, rhodamine 6G (R6G), operative on the IFE on graphene oxide (GO). To construct an efficient IFE system, R6G was covalently attached to GO, and the <b>CP</b> molecules were adsorbed on the surface of <b>R6G-GO</b> via π–π stacking interaction. The reaction of thiols with <b>CP</b> on <b>R6G-GO</b> decreases the absorption of <b>CP</b>, resulting in the increase of the intensity of R6G fluorescence. The results showed that the IFE efficiency, sensitivity, and dynamic response time of <b>R6G-GO/CP</b> for biothiols could be significantly improved compared with <b>R6G/CP</b>, and furthermore, <b>R6G-GO/CP</b> functioned under complex system and could be used for assaying biothiols in living cells and in human serum samples. This new strategy would be general to explore the development of more effective IFE-based sensors for other analytes of interest

Competitive Assembly To Increase the Performance of the DNA/Carbon-Nanomaterial-Based Sensing Platform

Increasing the rate of target binding on the surface and enhancing the fluorescence signal restoration efficiency are critical to the desirable biomedical application of carbon nanomaterials, for example, single-walled carbon nanotubes (SWNTs). We describe here a strategy to increase the target binding rate and enhance the fluorescence signal restoration efficiency on the DNA-functionalized SWNT surface using a short complementary DNA (scDNA) strand. The scDNA causes up to a 2.5-fold increase in association rate and 4-fold increase in fluorescence signal restoration by its competitive assembly on the nanostructure’s surface and inducing a conformational change that extends the DNA away from the surface, making it more available to bind target nucleic acids. The scDNA-induced enhancement of binding kinetics and fluorescence signal restoration efficiency is a general phenomenon that occurred with all sequences and surfaces investigated. Through this competitive assembly strategy of scDNA, performance improvement of the carbon-nanomaterial-based biosensing platform for both in vitro detection and live cell imaging can be reached

Graphene Oxide Assisted Fluorescent Chemodosimeter for High-Performance Sensing and Bioimaging of Fluoride Ions

Fluorescent chemodosimeters for a fluoride ion (F^–) based on a specifically F^–-triggered chemical reaction are characterized by high selectivity. However, they are also subjected to intrinsic limits, such as long response time, poor stability under aqueous solution, and unpredictable cell-member penetration. To address these issues, we reported here that the self-assembly of fluorescent chemodosimeter molecules on a graphene oxide (GO) surface can solve these problems by taking advantage of the excellent chemical catalysis and nanocarrier functions of GO. As a proof of concept, a new F^–-specific fluorescent chemodosimeter molecule, <b>FC-A</b>, and the GO self-assembly structure of <b>GO/FC-A</b> were synthesized and characterized. Fluorescent sensing and imaging of F^– with <b>FC-A</b> and <b>GO/FC-A</b> were performed. The results showed that the reaction rate constant of <b>GO/FC-A</b> for F^– is about 5-fold larger than that of <b>FC-A</b>, so that the response time was shortened from 4 h to about 30 min, while for F^–, the response sensitivity of <b>GO/FC-A</b> was >2-fold higher than that of <b>FC-A</b>. Furthermore, <b>GO/FC-A</b> showed a better bioimaging performance for F^– than <b>FC-A</b> because of the nanocarrier function of GO for cells. It is demonstrated that this GO-based strategy is feasible and general, which could help in the exploration of the development of more effective fluorescent nanodosimeters for other analytes of interest

Ultrasensitive Detection of Single Nucleotide Polymorphism in Human Mitochondrial DNA Utilizing Ion-Mediated Cascade Surface-Enhanced Raman Spectroscopy Amplification

Although surface-enhanced Raman spectroscopy (SERS) has been featured by high sensitivity, additional signal enhancement is still necessary for trace amount of biomolecules detection. In this paper, a SERS amplified approach, featuring “ions-mediated cascade amplification (IMCA)”, was proposed by utilizing the dissolved silver ions (Ag⁺) from silver nanoparticles (AgNPs). We found that using Ag⁺ as linkage agent can effectively control the gaps between neighboring 4-aminobenzenethiol (4-ABT) encoded gold nanoparticles (AuNPs@4-ABT) to form “hot spots” and thus produce SERS signal output, in which the SERS intensity was proportional to the concentration of Ag⁺. Inspired by this finding, the IMCA was utilized for ultrasensitive detection of single nucleotide polymorphism in human mitochondrial DNA (16189T → C). Combining with the DNA ligase reaction, each target DNA binding event could successfully cause one AgNP introduction. By detecting the dissolved Ag⁺ from AgNPs using IMCA, low to 3.0 × 10^–5 fm/μL targeted DNA can be detected, which corresponds to extractions from 200 nL cell suspension containing carcinoma pancreatic β-cell lines from diabetes patients. This IMCA approach is expected to be a universal strategy for ultrasensitive detection of analytes and supply valuable information for biomedical research and clinical early diagnosis