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

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

Endogenous Enzyme-Activatable Spherical Nucleic Acids for Spatiotemporally Controlled Signal Amplification Molecular Imaging and Combinational Tumor Therapy

Due to the adjustable hybridization activity, antinuclease digestion stability, and superior endocytosis, spherical nucleic acids (SNAs) have been actively developed as probes for molecular imaging and the development of noninvasive diagnosis and image-guided surgery. However, since highly expressed biomarkers in tumors are not negligible in normal tissues, an inevitable background signal and the inability to precisely release probes at the chosen region remain a challenge for SNAs. Herein, we proposed a rationally designed, endogenous enzyme-activatable functional SNA (Ep-SNA) for spatiotemporally controlled signal amplification molecular imaging and combinational tumor therapy. The self-assembled amphiphilic polymer micelles (SM-ASO), which were obtained by a simple and rapid copper-free strain-promoted azide–alkyne cycloaddition click reaction between dibenzocyclooctyne-modified antisense oligonucleotide and azide-containing aliphatic polymer polylactic acid, were introduced as the core elements of Ep-SNA. This Ep-SNA was then constructed by connecting two apurinic/apyrimidinic (AP) site-containing trailing DNA hairpins, which could occur via a hybridization chain reaction in the presence of low-abundance survivin mRNA to SM-ASO through complementary base pairing. Notably, the AP site-containing trailing DNA hairpins also empowered the SNA with the feasibility of drug delivery. Once this constructed intelligent Ep-SNA nanoprobe was specifically cleaved by the highly expressed cytoplasmic human apurinic/apyrimidinic endonuclease 1 in tumor cells, three key elements (trailing DNA hairpins, antisense oligonucleotide, and doxorubicin) could be released to enable subsequent high-sensitivity survivin mRNA imaging and combinational cancer therapy (gene silencing and chemotherapy). This strategy shows great application prospects of SNAs as a precise platform for the integration of disease diagnosis and treatment and can contribute to basic biomedical research

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

Visual Biopsy by Hydrogen Peroxide-Induced Signal Amplification

Visual biopsy has attracted special interest by surgeons due to its simplicity and practicality; however, the limited sensitivity of the technology makes it difficult to achieve an early diagnosis. To circumvent this problem, herein, we report a visual signal amplification strategy for establishing a marker-recognizable biopsy that enables early cancer diagnosis. In our proposed approach, hydrogen peroxide (H₂O₂) was selected as a potential underlying marker for its compact relationship in cancer progression. For selective recognition of H₂O₂ in the process of visual biopsy, a benzylbenzeneboronic acid pinacol ester-decorated copolymer, namely, PMPC–Bpe, was synthesized, affording the final formation of the H₂O₂-responsive micelles in which amylose was trapped. The presence of H₂O₂ activates the boronate ester recognition site and induces it releasing abundant indicator amylose, leading to signal amplification. The indicator came across the solution of KI/I₂ added to the sample, and the formative amylose–KI/I₂ complex has a distinct blue color at 574 nm for visual amplification detection. The feasibility of the proposed method is demonstrated by visualizing the H₂O₂ content of cancer at different stages and three kinds of actual cancerous samples. As far as we know, this is the first paradigm to rationally design a signaling amplification-based molecular recognizable biopsy for visual and sensitive disease identification, which will extend new possibilities for marker-recognition and signal amplification-based biopsy in disease progressing

Poly β‑Cyclodextrin/TPdye Nanomicelle-based Two-Photon Nanoprobe for Caspase‑3 Activation Imaging in Live Cells and Tissues

Two-photon excitation (TPE) with near-infrared (NIR) photons as the excitation source has important advantages over conventional one-photon excitation (OPE) in the field of biomedical imaging. β-cyclodextrin polymer (βCDP)-based two-photon absorption (TPA) fluorescent nanomicelle exhibits desirable two-photon-sensitized fluorescence properties, high photostability, high cell-permeability and excellent biocompatibility. By combination of the nanostructured two-photon dye (TPdye)/βCDP nanomicelle with the TPE technique, herein we have designed a TPdye/βCDP nanomicelle-based TPA fluorescent nanoconjugate for enzymatic activity assay in biological fluids, live cells and tissues. This sensing system is composed of a <i>trans</i>-4-[<i>p</i>-(<i>N</i>,<i>N</i>-diethylamino)­styryl]-<i>N</i>-methylpyridinium iodide (DEASPI)/βCDP nanomicelle as TPA fluorophore and carrier vehicle for delivery of a specific peptide sequence to live cell through fast endocytosis, and an adamantine (Ad)-GRRRDEVDK-BHQ2 (black hole quencher 2) peptide (denoted as Ad-DEVD-BHQ2) anchored on the DEASPI/βCDP nanomicelle’s surface to form TPA DEASPI/βCDP@Ad-DEVD-BHQ2 nanoconjugate by the βCD/Ad host–guest inclusion strategy. Successful in vitro and in vivo enzymatic activities assay of caspase-3 was demonstrated with this sensing strategy. Our results reveal that this DEASPI/βCDP@Ad-DEVD-BHQ2 nanoconjugate not only is a robust, sensitive and selective sensor for quantitative assay of caspase-3 in the complex biological environment but also can be efficiently delivered into live cells as well as tissues and act as a “signal-on” fluorescent biosensor for specific, high-contrast imaging of enzymatic activities. This DEASPI/βCDP@Ad-DEVD-BHQ2 nanoconjugate provides a new opportunity to screen enzyme inhibitors and evaluate the apoptosis-associated disease progression. Moreover, our design also provides a methodology model scheme for development of future TPdye/βCDP nanomicelle-based two-photon fluorescent probes for in vitro or in vivo determination of biological or biologically relevant species

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

Noninvasive and Highly Selective Monitoring of Intracellular Glucose via a Two-Step Recognition-Based Nanokit

Accurate determination of intracellular glucose is very important for exploring its chemical and biological functions in metabolism events of living cells. In this paper, we developed a new noninvasive and highly selective nanokit for intracellular glucose monitoring via two-step recognition. The liposome-based nanokit coencapsulated the aptamer-functionalized gold nanoparticles (AuNPs) and the Shinkai’s receptor together. When the proposed nanokit was transfected into living cells, the Shinkai’s receptor could recognize glucose first and then changed its conformation to endow aptamers with binding and sensing properties which were not readily accessible otherwise. Then, the binary complexes formed by the intracellular glucose and the Shinkai’s receptor can <i>in situ</i> displace the complementary oligonucleotide of the aptamer on the surface of AuNPs. The fluorophore-labeled aptamer was away from the AuNPs, and the fluorescent state switched from “off” to “on”. Through the secondary identification of aptamer, the selectivity of the Shinkai’s receptor could be greatly improved while the intracellular glucose level was assessed by fluorescence signal recovery of aptamer. In the follow-up application, the approach exhibits excellent selectivity and is noninvasive for intracellular glucose monitoring under normoxia and hypoxia. To the best of our knowledge, this is the first time that the advantages of organic receptors and nucleic acids have been combined and highly selective monitoring of intracellular glucose has been realized via two-step recognition. We expect it to open up new possibilities to integrate devices for diagnosis of various metabolic diseases and insulin delivery

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