Modification of fluorescent photoinduced electron transfer (PET) sensors/switches to produce molecular photo-ionic triode action

Lighting up Micro-/Nanorobots with Fluorescence

Micro-/nanorobots (MNRs) can be autonomously propelled on demand in complex biological environments and thus may bring revolutionary changes to biomedicines. Fluorescence has been widely used in real-time imaging, chemo-/biosensing, and photo-(chemo-) therapy. The integration of MNRs with fluorescence generates fluorescent MNRs with unique advantages of optical trackability, on-the-fly environmental sensitivity, and targeting chemo-/photon-induced cytotoxicity. This review provides an up-to-date overview of fluorescent MNRs. After the highlighted elucidation about MNRs of various propulsion mechanisms and the introductory information on fluorescence with emphasis on the fluorescent mechanisms and materials, we systematically illustrate the design and preparation strategies to integrate MNRs with fluorescent substances and their biomedical applications in imaging-guided drug delivery, intelligent on-the-fly sensing and photo-(chemo-) therapy. In the end, we summarize the main challenges and provide an outlook on the future directions of fluorescent MNRs. This work is expected to attract and inspire researchers from different communities to advance the creation and practical application of fluorescent MNRs on a broad horizon.

Naphthalenediimides with High Fluorescence Quantum Yield: Bright-Red, Stable, and Responsive Fluorescent Dyes

The naphthalenediimide (NDI) scaffold in contrast to its higher congeners possess low-fluorescence. In spite of elegant synthetic developments, a highly emissive NDI is quite rare to find, as well as, a green-light-emitting NDI is yet to be explored. Herein, we report a novel class of symmetric and asymmetric NH₂ -substituted core-NDIs (1-5) with tunable fluorescence in the visible region and extending to the NIR frontier. Importantly, the bis-NH₂ -substituted NDI 2 revealed quantum yield, Φ f of ≈81 and ≈68 % in toluene and DMSO, respectively, suggesting versatility of the fluorophore in a wide range of solvent polarity. The dye 1 is shown to be the first NDI-based green-light emitter. The donor piperidine group in 5 diminish the Φ f by 40-fold providing a lever to modulate the excited-state intramolecular proton transfer (ESIPT) process. Our synthetic protocol applies a Pd catalyst and a benign hydride source simplifying the non-trivial -NH₂ group integration at the NDI-core. TD-DFT calculations predicted strong intramolecular hydrogen bonds in the excited state in the bulk nonpolar medium and responsiveness to solvent polarity. The maximization of the NDI emission outlined here would further boost the burgeoning repertoire of applications of the NDI scaffold.

An Aggregation-Induced Emission-Based Indirect Competitive Immunoassay for Fluorescence "Turn-On" Detection of Drug Residues in Foodstuffs

A new fluorescent "turn-on" probe-based immunosensor for detecting drug residues in foodstuffs was established by combining the mechanism of aggregation-induced emission (AIE) and an indirect competitive enzyme-linked immunosorbent assay (ELISA). In this study, a luminogen, with negligible fluorescence emission (TPE-HPro), aggregated in the presence of H2O2, and exhibited astrong yellow emission based on its AIE characteristics. This AIE process was further configured into an immunoassay for analyzing drug residues in foodstuffs. In this approach, glucose oxidase (GOx) was used as an enzyme label for the immunoassay and triggered GOx/glucose-mediated H2O2 generation, which caused oxidation of TPE-HPro and a "turn-on" fluorescence response at 540 nm. To quantitatively analyze the drug residues in foodstuffs, we used amantadine (AMD) as an assay model. By combining the AIE-active "turn-on" fluorescent signal generation mechanism with conventional ELISAs, quantifying AMD concentrations in chicken muscle samples was realized with an IC50 (50% inhibitory concentration) value of 0.38 ng/mL in buffer and a limited detection of 0.06 μg/kg in chicken samples. Overall, the conceptual integration of AIE with ELISA represents a potent and sensitive strategy that broadens the applicability of the AIE-based fluorometric assays.

Microcapsule-Containing Self-Reporting Polymers

Self-reporting polymers, which can indicate damage or exposure to excessive stress with a clearly perceptible optical signal, are potentially useful for several technological applications, including stress-sensitive sensors that enable in situ monitoring of mechanical events and structural health monitoring systems. A versatile and simple concept to realize this function is the exploitation of microcapsules that are filled with solutions of dyes that are released and chemically or physically activated when the protective shell is damaged. Such microcapsules can readily be incorporated into polymers and the composites thus made can be processed into films, coatings, or other objects. Mechanochromic effects can be realized with different types of dyes and activation schemes. In this concept article, a selection of recent key studies is presented to provide an overview of the state of the field. Different architectures and operating principles and their advantages and drawbacks are reviewed. The parameters that influence the design of microcapsule-based mechanochromic systems are considered and unexplored chromophore systems that might be useful to design future self-reporting polymers are discussed. Finally, specific aspects of capsule design, fabrication, and integration into polymers are presented. Throughout the article, challenges and opportunities of the concept are highlighted and possible future directions are discussed.

Tuning the Photoinduced Electron Transfer in a Zr-MOF: Toward Solid-State Fluorescent Molecular Switch and Turn-On Sensor

The immobilization of fluorescent photoinduced electron transfer (PET) switches/sensors into solid state, which usually cannot maintain their identical properties in solution, has remained a big challenge. Herein, a water-stable anthracene and maleimide appended zirconium-based-metal-organic framework (Zr-MOF; UiO-68-An/Ma) is reported. Unlike the regular intramolecular "fluorophore-spacer-receptor" format, the separated immobilization of fluorescent (anthracene) and acceptor (maleimide) groups into the framework of a multivariate MOF can also favor a pseudo-intramolecular fluorescent PET process, resulting in UiO-68-An/Ma with very weak fluorescence. Interestingly, after Diels-Alder reaction or thiol-ene reaction of maleimide groups, the pseudo-intramolecular fluorescent PET process in UiO-68-An/Ma fails and the solid-state fluorescence of the crystals is recovered. In addition, UiO-68-An/Ma shows an interesting application as solid-state fluorescent turn-on sensor for biothiols, with the naked eye response at a low concentration of 50 µmol L-1 within 5 min. This study represents a general strategy to enable the efficient tuning of fluorescent PET switches/sensors in solid state, and considering the fluorescence of the PET-based MOFs can be restored after addition of analyte/target species, this research will definitely inspire to construct stimuli-responsive fluorescent MOFs for interesting applications (e.g., logic gate) in future.

Electrostatic Modification for Promotion of Flavin-Mediated Oxidation of a Probe for Flavin Detection

Electrostatic effects on the redox photochemistry of synthetic probes (1, 2, and 1-Zn) are examined by adjusting the thermodynamic driving force of their oxidation reactions. The redox photochemistry was simply controlled by introducing a zinc binding site (2,2'-dipicolylamine (DPA)) on the coumarin moiety of probe 2. Zinc complexation produced a positively charged environment on the coumarin (1-Zn), which lowered the electron density of a nearby 9 H-xanthene ring, attenuating the auto-oxidation of 1-Zn by 45 % compared with that of probe 1 at 298 K. The positive net charge of 1-Zn also provided an attractive Coulombic force toward the phosphate of flavin mononucleotide and flavin adenine dinucleotide, which lowered the reduction potential of the electron acceptor (isoalloxazine) and improved intermolecular electron transfer from the 9 H-xanthene ring to isoalloxazine. The flavin-mediated oxidation rate of 1-Zn was increased to 1.5 times that of probe 2. Probe 1-Zn showed highly selective sensing behaviour toward flavins, producing an intense brightness (ϵΦ_F =2.80×10³  m^-1  cm^-1 ) in the long-wavelength regions (λ_max =588 nm) upon flavin-mediated oxidation. Furthermore, probes 1-Zn and 2 were successfully applied to eosinophil imaging and the differential diagnosis of eosinophilia; this demonstrates their use as diagnostic tools.

Synthetic water soluble di-/tritopic molecular receptors exhibiting Ca2+/Mg2+ exchange

Structural integration of two synthetic water soluble receptors for Ca²⁺ and Mg²⁺, namely 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) and o-aminophenol-N,N,O-triacetic acid (APTRA), respectively, gave novel di- and tritopic ionophores (1 and 2). As Mg²⁺ and Ca²⁺ cannot be simultaneously complexed by the receptors, allosteric control of complexation results. Potentiometric measurements established stepwise protonation constants and showed high affinity for Ca²⁺ (log K = 6.08 and 8.70 for 1 and 2, respectively) and an excellent selectivity over Mg²⁺ (log K = 3.70 and 5.60 for 1 and 2, respectively), which is compatible with magnesium-calcium ion exchange. While ion-exchange of a single Mg²⁺ for a single Ca²⁺ is possible in both 1 and 2, the simultaneous binding of two Mg²⁺ by 2 appears prohibitive for replacement of these two ions by a single Ca²⁺. Ion-binding and exchange was further rationalized by DFT calculations.

Engineering a switch-based biosensor for arginine using a Thermotoga maritima periplasmic binding protein

The Thermotoga maritima arginine-binding protein (TmArgBP) has been modified to create a reagentless fluorescent protein biosensor. Two design methods for biosensor construction are compared: 1) solvent accessibility of environmentally-sensitive probes and 2) fluorescence deactivation due to photo-induced electron transfer (PET). Nine single cysteine TmArgBP mutants were created and labeled with three different environmentally sensitive fluorescent probes. These mutants demonstrated limited changes in fluorescence emission upon the addition of arginine. In contrast, the PET-based biosensor provides significant enhancements over the traditional approach and provides a fluorescence quenching mechanism that was capable of providing quantitative detection of arginine. Site-directed mutagenesis of TmArgBP was used to create attachment points for the fluorescent probe (K145C) and for an internal aromatic residue (D18X) to serve as the PET quencher. Both tyrosine and tryptophan, but not phenylalanine, were able to quench the emission of the fluorescent probe by more than 80% upon the addition of arginine. The dissociation constant for arginine ranged from 0.87 to 1.5 μM across the different sensors. This PET-based strategy provides a simple and broadly applicable approach for the analytical detection of small molecules that may be applied to any protein that exhibits conformational switching in a ligand dependent manner.

Proton-Gated Photoisomerization of Amino-Substituted Dibenzofulvene Rotors

Derivatives of 9-(2,2,2-triphenylethylidene)-fluorene (TEF) undergo E/Z photoisomerization and are candidates for light-powered molecular actuators and switches. The 2-substituted derivatives bearing an amino group (ATEF) or a dimethylamino group (DTEF) are weakly photoactive in the absence of acids, but protonation increases photoisomerization quantum yields by factors of 30-60. Such compounds may be useful for incorporation into pH-switchable photoactive devices.

Proof of principle of a three-input AND–INHIBIT–OR combinatorial logic gate array

A designed molecule provides a fluorescent signal according to a combinatorial logic array comprised of three-input AND, three-input INHIBIT and two-input OR logic gates.

Activatable Fluorescent Nanoprobe with Aggregation-Induced Emission Characteristics for Selective In Vivo Imaging of Elevated Peroxynitrite Generation

An activatable fluorescent nanoprobe with aggregation-induced emission signature is developed. The nanoprobe is nonfluorescent, but can be induced to emit intensely after reaction with peroxynitrite forming an intramolecular hydrogen bond. Excellent performance for selective in vivo imaging of inflammation with elevated peroxynitrite generation and efficient visualization of in vivo treatment efficacy of anti-inflammatory agents is demonstrated.

A multi-stimuli responsive switch as a fluorescent molecular analogue of transistors

Although the quantum nature of molecules makes them specially suitable for mimicking the operation of digital electronic elements, molecular compounds can also be envisioned to emulate the behavior of analog devices. In this work we report a novel fluorescent three-state switch capable of reproducing the analog response of transistors, an ubiquitous device in modern electronics. Exploiting the redox and thermal sensitivity of this compound, the amplitude of its fluorescence emission can be continuously modulated, in a similar way as the output current in a transistor is amplified by the gate-to-source voltage.

An acido- and photochromic molecular device that mimics triode action

A molecular triode that is operated by pH and light can be implemented with spiropyrans.

Construction of a novel INHIBIT logic gate through a fine-tuned assembly of anthryl fluorophores via selective anion recognition and host–guest interactions

A novel ligand containing of anthryl fluorophore was achieved. The assembly and disassembly of anthryl fluorophore by Pi and β-CD as chemical inputs and emission around 500 nm as output resulted in the construction of novel INHIBIT gate.

Water-soluble naphthalimide-based ‘Pourbaix sensors’: pH and redox-activated fluorescent AND logic gates based on photoinduced electron transfer

Naphthalimide-based ‘Pourbaix sensors’ for redox potential and pH fluoresce with a lifetime of 8.5 ns while photoinduced electron transfer occurs on a time scale of 20 ps.

Molecules with a sense of logic: a progress report

Ones and zeros can be handled by molecules through the input-control of their signaling features. The progress in this exciting field during the last five years is covered in this tutorial review.

Current developments in fluorescent PET (photoinduced electron transfer) sensors and switches

A fluorophore can be combined with a receptor according to a molecular engineering design in order to yield fluorescent sensing and switching devices.

A naphthalimide-based ‘Pourbaix sensor’: a redox and pH driven AND logic gate with photoinduced electron transfer and internal charge transfer mechanisms

A molecular logic gate with a ‘receptor–spacer–fluorophore–spacer–redox-unit’ format emits a fluorescent signal on simultaneous oxidation and protonation in aqueous methanol solution.

Taking baby steps in molecular logic-based computation

Constructs of fluorophores, receptors, spacers, ¹O₂ sensitizers, enzymes and oligonucleotides play their part in advancing the field of molecular logic-based computation.

Sugar-based molecular computing by material implication

A method to integrate an (in principle) unlimited number of molecular logic gates to construct complex circuits is presented. Logic circuits, such as half- or full-adders, can be reinterpreted by using the functional completeness of the implication function (IMP) and the trivial FALSE operation. The molecular gate IMP is represented by a fluorescent boronic acid sugar probe. An external wiring algorithm translates the fluorescent output from one gate into a chemical input for the next gate on microtiter plates. This process is demonstrated on a four-bit full adder.