Recent Advances in the Field of Amino Acid-Conjugated Aminoferrocenes-A Personal Perspective.

The development of turn-based inhibitors of protein-protein interactions has attracted considerable attention in medicinal chemistry. Our group has synthesized a series of peptides derived from an amino-functionalized ferrocene to investigate their potential to mimic protein turn structures. Detailed DFT and spectroscopic studies (IR, NMR, CD) have shown that, for peptides, the backbone chirality and bulkiness of the amino acid side chains determine the hydrogen-bond pattern, allowing tuning of the size of the preferred hydrogen-bonded ring in turn-folded structures. However, their biological potential is more dependent on their lipophilicity. In addition, our pioneering work on the chiroptical properties of aminoferrocene-containing peptides enables the correlation of their geometry with the sign of the CD signal in the absorption region of the ferrocene chromophore. These studies have opened up the possibility of using aminoferrocene and its derivatives as chirooptical probes for the determination of various chirality elements, such as the central chirality of amino acids and the helicity of peptide sequences.

Development of Novel Immobilized Copper-Ligand Complex for Click Chemistry of Biomolecules.

Copper-catalyzed azide-alkyne cycloaddition click (CuAAC) reaction is widely used to synthesize drug candidates and other biomolecule classes. Homogeneous catalysts, which consist of copper coordinated to a ligand framework, have been optimized for high yield and specificity of the CuAAC reaction, but CuAAC reaction with these catalysts requires the addition of a reducing agent and basic conditions, which can complicate some of the desired syntheses. Additionally, removing copper from the synthesized CuAAC-containing biomolecule is necessary for biological applications but inconvenient and requires additional purification steps. We describe here the design and synthesis of a PNN-type pincer ligand complex with copper (I) that stabilizes the copper (I) and, therefore, can act as a CuAAC catalyst without a reducing agent and base under physiologically relevant conditions. This complex was immobilized on two types of resin, and one of the immobilized catalyst forms worked well under aqueous physiological conditions. Minimal copper leaching was observed from the immobilized catalyst, which allowed its use in multiple reaction cycles without the addition of any reducing agent or base and without recharging with copper ion. The mechanism of the catalytic cycle was rationalized by density functional theory (DFT). This catalyst's utility was demonstrated by synthesizing coumarin derivatives of small molecules such as ferrocene and sugar.

Enhancing Glucose Biosensing with Graphene Oxide and Ferrocene-Modified Linear Poly(ethylenimine).

We designed and optimized a glucose biosensor system based on a screen-printed electrode modified with the NAD-GDH enzyme. To enhance the electroactive surface area and improve the electron transfer efficiency, we introduced graphene oxide (GO) and ferrocene-modified linear poly(ethylenimine) (LPEI-Fc) onto the biosensor surface. This strategic modification exploits the electrostatic interaction between graphene oxide, which possesses a negative charge, and LPEI-Fc, which is positively charged. This interaction results in increased catalytic current during glucose oxidation and helps improve the overall glucose detection sensitivity by amperometry. We integrated the developed glucose sensor into a flow injection (FI) system. This integration facilitates a swift and reproducible detection of glucose, and it also mitigates the risk of contamination during the analyses. The incorporation of an FI system improves the efficiency of the biosensor, ensuring precise and reliable results in a short time. The proposed sensor was operated at a constant applied potential of 0.35 V. After optimizing the system, a linear calibration curve was obtained for the concentration range of 1.0-40 mM (R2 = 0.986). The FI system was successfully applied to determine the glucose content of a commercial sports drink.

A small-molecule Fenton reagent for self-augmented chemodynamic therapy by intelligently regulating intracellular acidosis.

A small-molecule Fenton reagent, integrating ferrocene with a carbonic anhydrase inhibitor, was designed to intelligently regulate intracellular acidosis for self-augmented chemodynamic therapy. Acidosis coupled with up-regulated ROS levels demonstrated potent cytotoxicity and effective tumor suppression.

Exploring the modulatory influence on the antimalarial activity of amodiaquine using scaffold hybridisation with ferrocene integration.

Amodiaquine (AQ) is a potent antimalarial drug used in combination with artesunate as part of artemisinin-based combination therapies (ACTs) for malarial treatment. Due to the rising emergence of resistant malaria parasites, some of which have been reported for ACT, the usefulness of AQ as an efficacious therapeutic drug is threatened. Employing the organometallic hybridisation approach, which has been shown to restore the antimalarial activity of chloroquine in the form of an organometallic hybrid clinical candidate ferroquine (FQ), the present study utilises this strategy to modulate the biological performance of AQ by incorporating ferrocene. Presently, we have conceptualised ferrocenyl AQ derivatives and have developed facile, practical routes for their synthesis. A tailored library of AQ derivatives was assembled and their antimalarial activity evaluated against chemosensitive (NF54) and multidrug-resistant (K1) strains of the malaria parasite, Plasmodium falciparum. The compounds generally showed enhanced or comparable activities to those of the reference clinical drugs chloroquine and AQ, against both strains, with higher selectivity for the sensitive phenotype, mostly in the double-digit nanomolar IC50 range. Moreover, representative compounds from this series show the potential to block malaria transmission by inhibiting the growth of stage II/III and V gametocytes in vitro. Preliminary mechanistic insights also revealed hemozoin inhibition as a potential mode of action.

Quantification of Glycated Hemoglobin in Total Hemoglobin by a Simultaneous Dual-Signal Acquisition Approach.

The glycated hemoglobin (HbA1c) level, which is defined as the ratio of HbA1c to total hemoglobin (tHb, including glycated and unglycated hemoglobin), is considered one of the preferred indicators for diabetes monitoring. Generally, assessment of the HbA1c level requires separate determination of tHb and HbA1c concentrations after a complex separation step. This undoubtedly increases the cost of the assay, and the loss or degradation of HbA1c during the separation process results in a decrease in the accuracy of the assay. Therefore, this study explored a dual-signal acquisition method for the one-step simultaneous evaluation of tHb and HbA1c. Quantification of tHb: graphene adsorbed carbon quantum dots and methylene blue were utilized as the substrate material and linked to the antibody. tHb was captured on the substrate by the antibody. The unique heme group on tHb catalyzed the production of •OH from H2O2 to degrade methylene blue on the substrate, and a quantitative relationship between the tHb concentration and the methylene blue oxidation current signal was constructed. Quantification of HbA1c: complex labels with HbA1c recognition were made of ZIF-8-ferrocene-gold nanoparticles-mercaptophenylboronic acid. The specific recognition of the boronic acid bond with the unique cis-diol structure of HbA1c establishes a quantitative relationship between the oxidation current of the label-loaded ferrocene and the concentration of HbA1c. Thus, the HbA1c level can be assessed with only one signal readout. The sensor exhibited extensive detection ranges (0.200-600 ng/mL for tHb and 0.100-300 ng/mL for HbA1c) and low detection limits (4.00 × 10-3 ng/mL for tHb and 1.03 × 10-2 ng/mL for HbA1c).

Dual-potential ratiometric electrochemiluminescence based on single emitter and single coreactant for the sensitive detection of carcinoembryonic antigen.

Dual-potential ratiometric electrochemiluminescence (ECL) is in favor of resistance to environmental interference. However, two kinds of emitters or coreactants, and a wide scan potential range (>2 V) are mandatory. This work developed a new dual-potential ratiometric ECL sensor for detection of carcinoembryonic antigen (CEA) using single emitter (luminol) and single coreactant (H2O2) with a mild potential range from -0.1 to 0.6 V. Luminol could produce a strong cathodic ECL (Ec) induced by hydroxyl radicals (HO‧) from the reduction of H2O2, and a relatively weak anodic ECL (Ea). After the ferrocene modified CEA aptamer (Apt-Fc) was attached, Fc could promote Ea by catalyzing the oxidation of H2O2, and reduce Ec by consuming HO‧. With the cycling amplification of the exonuclease I, CEA could substantially reduce the amount of Apt-Fc, resulting in the decrease of Ea and the rise of Ec. So, the ratio of Ec to Ea (Ec/Ea) was used as the detection signal, realizing the sensitive determination of CEA from 0.1 pg mL-1 to 10 ng mL-1 with a LOD of 41.85 fg mL-1 (S/N = 3). The developed sensor demonstrated excellent specificity, stability and reproducibility, with satisfactory results in practical detection.

Homogeneous electrochemical ratiometric biosensor for MircoRNA detection based on UiO-66-NH2 signal probe and waste-free entropy-driven DNA machine.

The construction of efficient methods for highly sensitive and rapid detection of disease markers is essential for the early diagnosis of serious diseases. In this paper, taking advantage of the UiO-66-NH2 signal molecule in combination with a waste-free entropy-driven DNA machine, a novel homogeneous electrochemical ratiometric platform is developed to detect MircoRNA (miRNA). Metal-organic framework materials (UiO-66-NH2 MOF) and ferrocene were utilized as electrochemical signal tags and reference probes, respectively. The target-initiated waste-free three-dimensional (3D) entropy-driven DNA nanomachine is activated in the presence of miRNA, resulting in DNA-labeled-UiO-66-NH2 falling off from the electrode, leading to a decrease in the signal of UiO-66-NH2 at 0.83V. Our strategy can mitigate false positive responses induced by the DNA probes immobilized on electrodes in traditional distance-dependent signal adjustment ratiometric strategies. The proposed ratiometric platform demonstrates superior sensitivity (a detection limit of 9.8 fM), simplified operation, high selectivity, and high repeatability. The ratiometric biosensor is also applied to detect miRNA content in spiked serum samples.

Study on the anti-cancer activity of α-phenethylamine ferrocenecarboxylic acid co-crystals.

Ferrocene derivatives show a wide range of pharmacological activities in the medical field, especially in the anti-tumor field, and can be used as candidate drugs or lead compounds for the treatment of tumors and other diseases. And α-phenethylamine is an important intermediate for the preparation of fine chemical products. (R)-(+)-1-Phenethylamine ferrocenecarboxylic acid/(S)-(-)-1-phenethylamine ferrocenecarboxylic acid were prepared, named compounds 1 and 2, respectively. Single crystal X-ray diffraction showed that compounds 1 and 2 crystallized in the orthorhombic system space group P21 21 21 , and the crystal structures of compounds 1 and 2 exhibited mirror symmetry. The inhibitory effect of two compounds on SW480, MDA-MB-231, and H1299 cells was tested by MTT colorimetry. The IC50 values of the compounds against cancer cells were also calculated. The anti-cancer effect was more pronounced for compounds in the S-configuration. Compound 2 made the wild-type cancer cells undergo apoptosis, thus preventing cancer; it also had the function of helping the cell gene repair defects.

A signal-on photoelectrochemical aptasensor based on ferrocene labeled triple helix DNA molecular switch for detection of antibiotic amoxicillin.

A sensitive signal-on photoelectrochemical aptasensor for antibiotic determination was constructed based on the energy level matching between ferrocene and CuInS2. P-type CuInS2 microflower was complexed with reduced graphene oxide (CuInS2/rGO) to get photocathode current with good photoelectric conversion efficiency and stability. Then, hairpin DNA (HP) was covalently bonded to the electrode surface. A triple helix DNA (THMS) was used as a molecular switch. After the specific recognition between target and THMS in homogeneous solution, ferrocene labeled probe (Fc-T2) was released. Finally, Fc-T2 was captured by the HP, which leaded the obvious increase of photocurrent for the energy level matching between ferrocene and CuInS2. The increase of the photocurrent signal was proportional to the concentration of target amoxicillin (AMOX), the linear range was 100 fM-100 nM with detection limit of 19.57 fM. Meanwhile, the method has been successfully applied for milk and lake water samples analysis with satisfactory results.

Construction of a Dual-Mode Biosensor with Ferrocene as Both a Signal Enhancer and a Signal Tracer for Electrochemiluminescent and Electrochemical Enantioselective Recognition.

We demonstrate for the first time the construction of a dual-mode biosensor for electrochemiluminescent (ECL) and electrochemical chiral recognition of l- and d-isomers of amino acids, with ferrocene (Fc) as both a signal enhancer and a signal tracer. With the dissolved oxygen as a coreactant, ZnIn2S4 acts as the ECL emitter to generate a weak cathodic ECL signal. Fc can enter into the ß-cyclodextrin (ß-CD) cavity on ZnIn2S4-modified electrode as a result of host-guest interaction. Since Fc can promote H2O and O2 to produce abundant reactive oxygen species (ROS) (e.g., O2·- and ·OH), the ECL signal of ZnIn2S4 can be further amplified with Fc as a coreaction accelerator. Meanwhile, Fc molecules on the ß-CD/ZnIn2S4-modified electrode can be electrochemically oxidized to Fc+ to produce a remarkable oxidation peak current. When l-histidine (l-His) is present, the matching of the l-His configuration with the ß-CD cavity leads to the entrance of more l-His into the cavity of ß-CD than d-histidine (d-His), and the subsequent competence of l-His with Fc on the Fc/ß-CD/ZnIn2S4-modified electrode induces the decrease in both Fc peak current and ZnIn2S4-induced ECL intensity. This dual-mode biosensor can efficiently discriminate l-His from d-His, and it can sensitively monitor l-His with a detection limit of 7.60 pM for ECL mode and 3.70 pM for electrochemical mode. Moreover, this dual-mode biosensor can selectively discriminate l-His from other l- and d-isomers (e.g., threonine, phenylalanine, and glutamic acid), with potential applications in the chiral recognition of nonelectroactive chiral compounds, bioanalysis, and disease diagnosis.

Cascade signal amplification electrochemical biosensor based on AgNPs and ring opening polymerization for determination of Ochratoxin A.

An ochratoxin A (OTA) electrochemical biosensor based on a cascade signal amplification strategy with Ag nanoparticles (AgNPs) and ring opening polymerization (ROP) was constructed. The large specific surface area of AgNPs was used to increase the loading of OTA aptamer on the electrode surface, enhancing the ability to capture OTA as a way to achieve the first signal amplification. The OTA antibody modified with polyethylenimine specifically recognizes the OTA, forming an aptamer-OTA-antibody sandwich structure. The amino group on polyethylenimine initiates the ROP reaction with α-amino acid-n-carboxylic anhydride-ferrocene (NCA-Fc) as the monomer. A large number of electrochemical signal units of ferrocene are introduced into the sensing system for a second signal amplification. By amplifying the signal twice, the sensitivity of the sensor is improved. Under the optimal conditions, the detection range of the sensor is 1 pg·mL-1 ~ 1 µg·mL-1, while the detection limit is as low as 117 fg·mL-1. Moreover, the sensor has the advantages of high sensitivity, good stability and selectivity. Standard addition recovery experiment proved that the sensing system can be successfully used for the detection of OTA in four actual samples with recoveries in the range 90.0 to 113% with RSDs of 0.6 to 5.2%, providing a new idea for the pollution assessment of mycotoxins.

A GSH-activated AIE-based polymer photosensitizer for killing cancer cells.

Developing efficient photosensitizers which are sensitive to therapeutic tumor signals, but non-toxic to normal cells has always been a tremendous challenge in photodynamic therapy (PDT) process. Herein, a novel copolymer P1 was developed by ring-opening metathesis polymerization (ROMP) with disulfide bond linked ferrocene-norbornene dyad NB-SS-PyFc and the aggregation-induced emission (AIE) fluorephore anchored norbornene NB-TPE, and its nanoparticles (NPs) were obtained by using the amphiphilic Pluronic F-127 as the surfactant via a nanoprecipitation method. The P1 NPs show a weak emission and a low 1O2 generation for the quenching effect from the ferrocene moiety to the AIE group. However, the addition of GSH can recover the AIE fluorephore emission and 1O2 generation for cleavage the disulfide bond. Importantly, P1 NPs have been used for image-guided cancer cells apoptosis for the GSH activated 1O2 generation.

In silico selection of an aptamer for the design of aptamer-modified magnetic beads bearing ferrocene co-immobilized label for capacitive detection of acetamiprid.

In silico evaluation of aptamer/target interactions can facilitate the development of efficient biosensor with high specificity and affinity. In this work, we present in silico, i.e. structural similarity, molecular docking and molecular dynamics selection of the aptamer with sufficient binding properties for acetamiprid (ACE), a nicotine-like pesticide, and its use to design aptamer-modified magnetic beads bearing ferrocene co-immobilized label for capacitive detection of ACE. Taking advantages of the aptamer higher stability and binding affinity, the specific properties of magnetic beads and the redox properties of ferrocene moiety, the developed aptasensor showed promising analytical performances for ACE detection, using electrochemical capacitance spectroscopy, with a linear response ranging from 1 fM to 100 pM and a limit of detection of 0.94 fM (S/N = 3). Furthermore, it was successfully applied to detect ACE in fortified tomatoes samples, proving a promising approach for routine detection of pesticide in real agricultural samples.

Ferrocene Derivatives as New Generation of Antimalarial Agents: Opportunity or Illusion?

Despite significant scientific progress over the last two decades, malaria remains a global burden that causes thousands of deaths every year. In the absence of effective and practical preventive measures, the only current option for reducing the mortality and morbidity of malaria is chemotherapy. However, due to the minimal stock of active antiparasitic analogs, issues of toxicity, and the repeated appearance of drug resistance, scientists must broaden the arsenal of existing therapies beyond conventional medicinal chemistry. To curb this menace, a series of potential metal-based hybrids have been synthesized and screened. Ferrocene is one of the potent organometallic candidates and the hybridization of ferrocene with other pharmacophores results in compounds with enhanced biological activities. Many researchers have reported the ferrocene compounds as potent pharmacophores and useful as anticancer and antimalarial agents when hybridized with other pharmaceutical hybrids. Drug, such as Ferroquine (FQ, SSR97193), is currently the most advanced organometallic compound developed from the hybridization of ferrocene and chloroquine and has demonstrated great potency in clinical trials against both drug-sensitive and drug-resistant malaria. Not only ferroquine but its derivatives have shown significant activity as antimalarial agents. The present review focuses on the discovery of FQ, the hypothesis of its mode of action, and recent clinical trials of ferrocene compounds as a new class of antimalarial agents. The structure-activity relationship (SAR) of ferrocene derivatives is also discussed to provide insight into the rational design of more effective antimalarial candidates. Finally, efforts have been made to discuss the future expectations for ferrocene-based antimalarial drugs.

Synthesis, Electrochemical and Fluorescence Properties of the First Fluorescent Member of the Ferrocifen Family and of Its Oxidized Derivatives.

The first fluorescent ferrociphenol derivative (P797) has been synthesized via McMurry cross-coupling followed by copper-catalyzed [3 + 2] azide-alkyne cycloaddition of the fluorescent group coumarin. Cyclic voltammograms of P797 exhibit either a monoelectronic oxidation wave ascribed to the ferrocene Fe(II) → Fe(III) conversion or a three-electron oxidation process in the presence of a base, leading to a Fe(III) quinone methide adduct. This general sequence is consistent with those previously described for non-fluorescent ferrociphenols. Furthermore, the fluorescence properties of P797 and its oxidized intermediates appear to strongly depend on the redox state of the ferrocene group. Indeed, electrochemical generation of Fe(III) (ferrocenium) states markedly increases the fluorescence emission intensity. In contrast, the emission of the Fe(II) (ferrocene) states is partially quenched by photoinduced electron transfer (PET) from the Fe(II) donor to the coumarin acceptor and by concentration-dependent self-quenching. Owing to its switchable fluorescence properties, complex P797 could represent an innovative and useful tool to study the biodistribution and the redox state of ferrocifens in cancer cells.

Hydrogen Bonding Drives Helical Chirality via 10-Membered Rings in Dipeptide Conjugates of Ferrocene-1,1'-Diamine.

Considering the enormous importance of protein turns as participants in various biological events, such as protein-protein interactions, great efforts have been made to develop their conformationally and proteolytically stable mimetics. Ferrocene-1,1'-diamine was previously shown to nucleate the stable turn structures in peptides prepared by conjugation with Ala (III) and Ala-Pro (VI). Here, we prepared the homochiral conjugates of ferrocene-1,1'-diamine with l-/d-Phe (32/35), l-/d-Val (33/36), and l-/d-Leu (34/37) to investigate (1) whether the organometallic template induces the turn structure upon conjugation with amino acids, and (2) whether the bulky or branched side chains of Phe, Val, and Leu affect hydrogen bonding. Detailed spectroscopic (IR, NMR, CD), X-ray, and DFT studies revealed the presence of two simultaneous 10-membered interstrand hydrogen bonds, i.e., two simultaneous ß-turns in goal compounds. A preliminary biological evaluation of d-Leu conjugate 37 showed its modest potential to induce cell cycle arrest in the G0/G1 phase in the HeLa cell line but these results need further investigation.

High-sensitive ferrocene labeled aptasensor for the detection of Mucin 1 by tuning the sequence constitution of complementary probe.

A strand displacement-based "signal-off" electrochemical aptasensor is reported for the detection of Mucin 1 (MUC 1) based on a high original signal. Different from the conventional "signal-off" electrochemical biosensors where electrochemical substances are dispersed in electrolyte solution, here the current signal was generated by the complementary probe (CP) associated with ferrocene (Fc) labeled aptamer (Apt.-Fc). Because Apt.-Fc and MUC 1 have a higher affinity, Apt.-Fc dissociates from CP in the presence of MUC 1, resulting in a reduction of detection current signal generated by oxidation of labeled Fc. In this system, high detection signal is necessary to improve the sensor's performance. For this aim, a strategy is proposed for changing the modalities of electron transport and the quantity of Apt.-Fc introduced by simply tuning the sequence constitution of CP. As expected, a high detection current signal was obtained after selecting CP(Apt.-Fc)-TTT as the optimal CP. The aptasensor was then employed to detect MUC 1, and satisfactory detection results with a low detection limit (LOD) of 0.087 pM (S/N = 3), good specificity, good stability, and feasibility of detection of MUC 1 in artificial serum (recovery of 92-101%, RSD of 1.36-5.23%) were obtained.

Forever young: the first seventy years of ferrocene.

The discovery of ferrocene, [Fe(η5-C5H5)2], seventy years ago has significantly influenced chemical research and provided a key impetus for establishing and rapidly expanding organometallic chemistry, which has continued at a rapid pace until now. Over the years of intensive research, the ferrocene unit has been recognised as an extremely versatile platform for ligand design, materials research, and medicinal and analytical chemistry as well as many other research fields. Such wide applications of ferrocene and its derivatives are obviously rooted in the unique combination of the properties of the ferrocene moiety, which exhibits high chemical stability but is amenable to diverse synthetic modifications, has well-defined and highly specific steric properties, and displays defined and tuneable redox behaviour. The unrelenting research activity focused on ferrocene compounds and their widespread applications can be expected to continue even in the future to yield more attractive results in terms of both novelty and function.

Electrochemical Detection of Waterborne Bacteria Using Bi-Functional Magnetic Nanoparticle Conjugates.

Detection of microbial contamination in water is imperative to ensure water quality. We have developed an electrochemical method for the detection of E. coli using bi-functional magnetic nanoparticle (MNP) conjugates. The bi-functional MNP conjugates were prepared by terminal-specific conjugation of anti-E. coli IgG antibody and the electroactive marker ferrocene. The bi-functional MNP conjugate possesses both E. coli-specific binding and electroactive properties, which were studied in detail. The conjugation efficiency of ferrocene and IgG antibodies with amine-functionalized MNPs was investigated. Square-wave voltammetry enabled the detection of E. coli concentrations ranging from 101-107 cells/mL in a dose-dependent manner, as ferrocene-specific current signals were inversely dependent on E. coli concentrations, completely suppressed at concentrations higher than 107 cells/mL. The developed electrochemical method is highly sensitive (10 cells/mL) and, coupled to magnetic separation, provides specific signals within 1h. Overall, the bi-functional conjugates serve as ideal candidates for electrochemical detection of waterborne bacteria. This approach can be applied for the detection of other bacteria and viruses.