Electrochemical signal modulation in homogeneous solutions using the formation of an inclusion complex between ferrocene and β-cyclodextrin on a DNA scaffold

Recent Advances in Transition-Metal-Catalyzed C-H Functionalization of Ferrocene Amides

During the past decades, in synthetic organic chemistry, directing-group-assisted C-H functionalization is found to be a key tool for the expedient and site-selective construction of C-C and hybrid bonds. Among C-H functionalization of ferrocene derivatives, the directed group strategy is undoubtedly the most commonly used method. Compared to the other directing groups, ferrocene amides can be synthesized easily and are now recognized as one of the most efficient devices for the selective functionalization of certain positions because its metal centre permits fine, tuneable and reversible coordination. The family of amide directing groups mainly comprises monodentate and bidentate directing groups, which are categorized on the basis of coordination sites. In this review, various C-H bond functionalization reactions of ferrocene using amide directing groups are broadly discussed.

Metal Ion-Directed Specific DNA Structures and Their Functions

Various DNA structures, including specific metal ion complexes, have been designed based on the knowledge of canonical base pairing as well as general coordination chemistry. The role of metal ions in these studies is quite broad and diverse. Metal ions can be targets themselves in analytical applications, essential building blocks of certain DNA structures that one wishes to construct, or they can be responsible for signal generation, such as luminescence or redox. Using DNA conjugates with metal chelators, one can more freely design DNA complexes with diverse structures and functions by following the simple HSAB rule. In this short review, the authors summarize a part of their DNA chemistries involving specific metal ion coordination. It consists of three topics: (1) significant stabilization of DNA triple helix by silver ion; (2) metal ion-directed dynamic sequence edition through global conformational change by intramolecular complexation; and (3) reconstruction of luminescent lanthanide complexes on DNA and their analytical applications.

Competitive Self-Assembly Interaction between Ferrocenyl Units and Amino Acids for Entry into the Cavity of β-Cyclodextrin for Chiral Electroanalysis

At present, chiral electroanalysis of nonelectroactive chiral compounds still remains a challenge because they cannot provide an electrochemical signal by themselves. Here, a strategy based on a competitive self-assembly interaction of a ferrocene (Fc) unit and the testing isomers entering into the cavity of β-cyclodextrin (β-CD) was carried out for chiral electroanalysis. First of all, the Fc derivative was directly bridged to silica microspheres, followed by inclusion into the cavity of β-CD. As expected, once it was modified onto the surface of a carbon working electrode as an electrochemical sensor, SiO₂@Fc-CD-WE, its differential pulse voltammetry signal would markedly decrease compared with the uncovered Fc. Next, when l- and d-isomers of amino acids that included histidine, threonine, phenylalanine, and glutamic acid were examined using SiO₂@Fc-CD-WE, it showed an enantioselective entry of amino acids into the cavity of β-cyclodextrin instead of Fc, resulting in the release of Fc with signal enhancement. For histidine, glutamic acid, and threonine, l-isomers showed a higher peak current response compared with d-isomers. The peak current ratios between l- and d-isomers were 2.88, 1.21, and 1.40, respectively. At the same time, the opposite phenomenon occurred for phenylalanine with a peak current ratio of 3.19 between d- and l-isomers. In summary, we are assured that the recognition strategy based on the supramolecular interaction can enlarge the detection range of chiral compounds by electrochemical analysis.

Two-Dimensional Cobalt-Doped Ti3C2 MXene Nanozyme-Mediated Homogeneous Electrochemical Strategy for Pesticides Assay Based on In Situ Generation of Electroactive Substances

Common homogeneous electrochemical (HEC) sensors usually suffer from the drawbacks of high background signal, low signal-to-noise ratio, and even false positive results due to the preaddition of electroactive substances. Thus, it is necessary to develop novel HEC sensors based on in situ generation of electroactive substances to overcome these shortcomings, which, however, is underexplored. In this work, two-dimensional (2D) nanozymes, i.e., cobalt-doped 2D Ti₃C₂ MXene nanosheets (CMNSs), with excellent peroxidase-like properties were utilized to develop HEC sensors based on the in situ generation of electroactive substances for organophosphate pesticides (OPs) detection. The 2D CMNSs were synthesized via a template-directed wet chemical approach and displayed outstanding features of hydrophilia and water dispersibility, which could catalyze the oxidation of o-phenylenediamine (OPD) to generate significantly increased reduction current. Interestingly, the 2D CMNSs with peroxidase-like properties exhibited a unique response to thiol compounds and were thus employed as highly efficient catalysts to develop HEC sensors for OPs based on the hydrolysis of acetylthiocholine (ATCh) to form thiocholine catalyzed by acetylcholinesterase (AChE) and the inhibition of AChE activity by OPs. The recovery for OPs analysis of pakchoi extract solutions ranged from 97.4% to 103.3%. The as-proposed HEC sensor based on in situ generation of electroactive substances will provide a new way for the development of high-performance electrochemical sensors and demonstrate potential applicability for the determination of pesticide residues in real samples.

Electrochemical Molecular Beacon for Nucleic Acid Sensing in a Homogeneous Solution

Ferrocene (Fc) and β-cyclodextrin (βCyD) were modified at each end of stem-loop structured DNA as an electrochemical signal generator and its quencher, respectively, to give an electrochemical molecular beacon (eMB). A relatively high efficiency of signal quenching was achieved by an inclusion complex (βCyD ⊃ Fc) formation that was induced on the stem structure of the closed form (= stem-loop structure) of eMB. With the addition of target DNA, the structure of eMB opened to form a linear duplex, where the Fc dissociated from the βCyD to restore its intrinsic electrochemical signal. The signal contrast of the electric current for this off/on-type sensor was high, ca. 95. This technique did not require any modification of the electrode surface, and it realized the detection of the target nucleic acids in a homogeneous solution with a high sensitivity using high-performance liquid chromatography (HPLC) equipped with electrochemical detector.

Detection of prostate-specific antigen in semen using DNA aptamers: an application of nucleic acid aptamers in forensic body fluid identification

In forensics, body fluid identification plays an important role because it aids in reconstructing a crime scene. Therefore, it is essential to develop simple and reliable techniques for body fluid identification. Nucleic acid aptamers are useful tools in analytical chemistry that can be used to improve conventional forensic analytical techniques. They have numerous advantages over antibodies including their low cost, long shelf life, and applicability for chemical modification and PCR amplification. A DNA aptamer against a human prostate-specific antigen (PSA), which is a well-known protein marker for semen identification in forensics, has been reported previously. In this study, as a proof-of-concept for nucleic acid aptamer-based identification of body fluids, we developed a technique of aptamer-based PSA assays for semen identification that employed enzyme-linked oligonucleotide assay (ELONA) and real-time PCR. We evaluated their sensitivity and specificity for semen compared with those for blood, saliva, urine, sweat, and vaginal secretion. The assays have equivalent procedures compared to enzyme-linked immunosorbent assay; their results were consistent with those produced by the conventional immunochromatographic assay. The minimum volume of semen required for detection was 62.5 nL in ELONA and 5 nL in real-time PCR, making this assay applicable for semen detection in actual criminal investigation. Aptamers can be a cost-effective and versatile tool for forensic body fluid identification.

Dual-Stimuli-Responsive Nanoassemblies as Tunable Releasing Carriers

Two end-decorated homopolymers, methoxy polyethylene glycol-ferrocene (mPEG-Fc) and poly(N-isopropylacrylamide)-β-cyclodextrin (PNIPAM-β-CD), were further orthogonally self-assembled into stable micelles in aqueous solution by controlling the temperature of the solution via terminal host-guest interactions. Because of the H₂O₂ cleavable CD/Fc connection and thermoresponsive PNIPAM, an H₂O₂ and thermo dual-controlled drug release based on this system was also achieved. Interestingly, the cytotoxicity evaluation of mPEG-Fc/PNIPAM-β-CD indicated good biocompatibility. Compared with free doxorubicin, the doxorubicin-loaded supramolecular micelles exhibited equal cellular proliferation inhibition toward A549 cells. This supramolecular complex is thus anticipated to serve as a promising new type of alternative drug-delivery system.

Switchable mechanical DNA "arms" operating on nucleic acid scaffolds associated with electrodes or semiconductor quantum dots

Functional footholds linked to DNA scaffolds associated with surfaces provide nano-engineered assemblies acting as switching devices. By the assembly of a β-cyclodextrin receptor on one foothold, and a ferrocene-modified nucleic acid on a second foothold, the switchable and reversible, fuel-driven activation of "molecular arms" proceeds, transduced by electrochemical or optical signals.

Rational design for cooperative recognition of specific nucleobases using β-cyclodextrin-modified DNAs and fluorescent ligands on DNA and RNA scaffolds

We propose a binary fluorimetric method for DNA and RNA analysis by the combined use of two probes rationally designed to work cooperatively. One probe is an oligonucleotide (ODN) conjugate bearing a β-cyclodextrin (β-CyD). The other probe is a small reporter ligand, which comprises linked molecules of a nucleobase-specific heterocycle and an environment-sensitive fluorophore. The heterocycle of the reporter ligand recognizes a single nucleobase displayed in a gap on the target labeled with the conjugate and, at the same time, the fluorophore moiety forms a luminous inclusion complex with nearby β-CyD. Three reporter ligands, MNDS (naphthyridine-dansyl linked ligand), MNDB (naphthyridine-DBD), and DPDB (pyridine-DBD), were used for DNA and RNA probing with 3'-end or 5'-end modified β-CyD-ODN conjugates. For the DNA target, the β-CyD tethered to the 3'-end of the ODN facing into the gap interacted with the fluorophore sticking out into the major groove of the gap site (MNDS and DPDB). Meanwhile the β-CyD on the 5'-end of the ODN interacted with the fluorophore in the minor groove (MNDB and DPDB). The results obtained by this study could be a guideline for the design of binary DNA/RNA probe systems based on controlling the proximity of functional molecules.