Protein sensors based on reversible π–π stacking on basal plane HOPG electrodes

Investigating the Stacking Interactions Responsible for Driving 3D Self-Association of Tricarb Macrocycles

We have investigated the noncovalent forces that play a crucial role in the three-dimensional (3D) self-association of the tricarb macrocycle (composed of alternating triazoles and carbazoles) to understand the multilayer stacks observed through electron microscopy. To explore this idea quantitatively, we have investigated a stacked dimer model of tricarb, where we consider homochiral as well as heterochiral forms of the dimer. We have computed the rotational potential energy surface of the dimer by conducting an angle-dependent scan between the two macrocycles using different levels of theory including the RI-MP2 ab initio method. We observe that dimers oriented at an angle of 60° exhibit the highest stability, while a secondary minimum is observed at an angle of 30°. While density functional theory (DFT) describes the behavior of both minima very close to that obtained with RI-MP2, semiempirical and MM models appear to obtain only a shoulder instead of the second minimum. To further understand the underlying interactions responsible for stabilizing the self-assembly of the macrocycles, we employed energy decomposition analysis (EDA) using SAPT0. This quantitative assessment allowed us to identify the major contributing noncovalent interactions, including electrostatic, exchange-repulsion, dispersion, and induction. Finally, we expanded our study to evaluate the accuracy of the MIM (molecules-in-molecules) fragmentation methodology in capturing the crucial π-stacking interactions. Our benchmarking results using the MIM method accurately replicated the angle-dependent PES results. This shows the efficacy of MIM in predicting the noncovalent interactions responsible for the construction of 3D and other higher-order nanoarchitectures for tricarb and related compounds.

Modified Graphite Pencil Electrode Based on Graphene Oxide-Modified Fe3O4 for Ferrocene-Mediated Electrochemical Detection of Hemoglobin

This study describes the synthesis of graphene oxide-modified magnetite (rGO/Fe₃O₄) and its use as an electrochemical sensor for the quantitative detection of hemoglobin (Hb). rGO is characterized by a 2θ peak at 10.03° in its X-ray diffraction, 1353 and 1586 cm^-1 vibrations in Raman spectroscopy, while scanning electron microscopy coupled with energy-dispersive spectroscopy of rGO and rGO/Fe₃O₄ revealed the presence of microplate structures in both materials and high presence of iron in rGO/Fe₃O₄ with 50 wt %. The modified graphite pencil electrode, GPE/rGO/Fe₃O₄, is characterized using cyclic voltammetry. Higher electrochemical surface area is obtained when the GPE is modified with rGO/Fe₃O₄. Linear scan voltammetry is used to quantify Hb at the surface of the sensor using ferrocene (FC) as an electrochemical amplifier. Linear response for Hb is obtained in the 0.1-1.8 μM range with a regression coefficient of 0.995, a lower limit of detection of 0.090 μM, and a limit of quantitation of 0.28 μM. The sensor was free from interferents and successfully used to sense Hb in human urine. Due to the above-stated qualities, the GPE/rGO/Fe₃O₄ electrode could be a potential competitive sensor for trace quantities of Hb in physiological media.

Electrochemically produced local pH changes stimulating (bio)molecule release from pH-switchable electrode-immobilized avidin-biotin systems

Immobilized avidin-biotin complexes were used to release biotinylated (bio)molecules upon producing local pH changes near an electrode surface by electrochemical reactions. The nitro-avidin complex with biotin was dissociated by increasing local pH with electrochemical O₂ reduction. The avidin complex with iminobiotin was split by decreasing local pH with electrochemical oxidation of ascorbate. Both studied systems were releasing molecule cargo species in response to small electrical potentials (-0.4 V or 0.2 V for the O₂ reduction or ascorbate oxidation, respectively) applied on the modified electrodes.

Graphene Biodevices for Early Disease Diagnosis Based on Biomarker Detection

The early diagnosis of diseases plays a vital role in healthcare and the extension of human life. Graphene-based biosensors have boosted the early diagnosis of diseases by detecting and monitoring related biomarkers, providing a better understanding of various physiological and pathological processes. They have generated tremendous interest, made significant advances, and offered promising application prospects. In this paper, we discuss the background of graphene and biosensors, including the properties and functionalization of graphene and biosensors. Second, the significant technologies adopted by biosensors are discussed, such as field-effect transistors and electrochemical and optical methods. Subsequently, we highlight biosensors for detecting various biomarkers, including ions, small molecules, macromolecules, viruses, bacteria, and living human cells. Finally, the opportunities and challenges of graphene-based biosensors and related broad research interests are discussed.

A Nanoporous Supramolecular Metal-Organic Framework Based on a Nucleotide: Interplay of the π···π Interactions Directing Assembly and Geometric Matching of Aromatic Tails

Self-assembly is the most powerful force for creating ordered supramolecular architectures from simple components under mild conditions. π···π stacking interactions have been widely explored in modern supramolecular chemistry as an attractive reversible noncovalent tool for the nondestructive fabrication of materials for different applications. Here, we report on the self-assembly of cytidine 5’-monophosphate (CMP) nucleotide and copper metal ions for the preparation of a rare nanoporous supramolecular metal-organic framework in water. π···π stacking interactions involving the aromatic groups of the ancillary 2,2’-bipyridine (bipy) ligands drive the self-assemblies of hexameric pseudo-amphiphilic [Cu6(bipy)6(CMP)2(µ-O)Br4]²⁺ units. Owing to the supramolecular geometric matching between the aromatic tails, a nanoporous crystalline phase with hydrophobic and hydrophilic chiral pores of 1.2 and 0.8 nanometers, respectively, was successfully synthesized. The encoded chiral information, contained on the enantiopure building blocks, is transferred to the final supramolecular structure, assembled in the very unusual topology 8T6. These kinds of materials, owing to chiral channels with chiral active sites from ribose moieties, where the enantioselective recognition can occur, are, in principle, good candidates to carry out efficient separation of enantiomers, better than traditional inorganic and organic porous materials.

Aggregation Enhanced Excimer Emission Supported, Monomeric Fluorescence Quenching of Dendritic Hyperbranched Polyglycerol Coupled 1-Pyrene Butyric Acid Lumophore as a Sensing Probe for Fe2O3 Nanoparticles

Pyrene butyric acid (PBA) is a well studied lumophore for its exciting fluorescent properties. The current study focussed on a dendritic modification of PBA with hyperbranched polyglycerols (HPG) by Steglich esterification and further doping with iron oxide nanoparticles (IONP) of α-Fe₂O₃ phase. The covalent coupling between HPG and PBA was confirmed by FTIR and ¹H-NMR spectra. The main objective of the study was to monitor the fluorescent properties of the modified and doped products. Steady state PL emission studies showed a considerable decrease in fluorescence intensity on HPG modification which almost completely disappeared on doping with IONP. This suggests that this fluorosensing property can be explored in identification and estimation of iron oxide nanoparticles which has a great significance in biomedical field both in diagnostics and therapeutics. Lifetime measurements with TCSPC suggested an aggregation enhanced quenching of HPG-PBA conjugates and mixed static and dynamic mechanisms in IONP doped HPG-PBA conjugates. Graphical Abstract ᅟ.

Electrochemically stimulated molecule release associated with interfacial pH changes

Molecular release was activated with an electrochemical signal, resulting in the hydrolysis of a linker.

Multifunctional solid-state electrochemiluminescent chemosensors and aptasensor with free-standing active sites based on task-specific pyrene-terminated polymers via RAFT polymerization

Based on the flexible molecular engineering technique of reversible addition-fragmentation chain transfer (RAFT) polymerization, various polymers carrying positive or negative charges and different terminal groups such as pyrene or tertiary amine were synthesized for fabricating multifunctional solid-state electrochemiluminescent (ECL) sensors. Accordingly, the chemosensors immobilizing the ECL probe or co-immobilizing the ECL probe and the coreactant were realized for the quantification of small molecules (e.g., tripropylamine, tetracycline), and an aptasensor was developed for the specific and sensitive lysozyme assay (limit of detection: 0.1 ng/mL). All of the sensors were realized via a simple design exploiting the π-π stacking and electrostatic interactions. It was confirmed that the proposed strategy is simple but universal for the fabrication of versatile ECL sensors that showed simplicity, cost-effectiveness, high sensitivity, long-term stability, and excellent reproducibility.

On Monolayer Formation of Pyrenebutyric Acid on Graphene

As a two-dimensional material with high charge carrier mobility, graphene may offer ultrahigh sensitivity in biosensing. To realize this, the first step is to functionalize the graphene. This is commonly done by using 1-pyrenebutyric acid (PBA) as a linker for biomolecules. However, the adsorption of PBA on graphene remains poorly understood despite reports of successful biosensors functionalized via this route. Here, the PBA adsorption on graphene is characterized through a combination of Raman spectroscopy, ab initio calculations, and spectroscopic ellipsometry. The PBA molecules are found to form a self-assembled monolayer on graphene, the formation of which is self-limiting and Langmuirian. Intriguingly, in concentrated solutions, the PBA molecules are found to stand up and stack horizontally with their edges contacting the graphene surface. This morphology could facilitate a surface densely populated with carboxylic functional groups. Spectroscopic analyses show that the monolayer saturates at 5.3 PBA molecules per nm² and measures ∼0.7 nm in thickness. The morphology study of this PBA monolayer sheds light on the π-π stacking of small-molecule systems on graphene and provides an excellent base for optimizing functionalization procedures.

Chemical modification of gold electrodes via non-covalent interactions

Chemically modifying electrode surfaces with redox active molecular complexes is an effective route to fabricating tailored functional materials.