A colorimetric and fluorescent chemosensor for detection of Hg2+ using counterion exchange of cationic polydiacetylene

Structural, optimization of and mechanistic insights into a new thiosemicarbazone derivative as a highly sensitive and selective chemosensor for Hg2+ recognition using DFT, COSMO-RS, RSM, and molecular dynamics simulation approaches

A new thiosemicarbazone derivative, N-(2-hydroxyphenyl)-2-[1-(pyridin-4-yl)ethylidene]hydrazinecarbothioamide (HPEH), has been synthesized, characterized, and further developed as a highly selective and sensitive colorimetric chemosensor for Hg2+ recognition in environmental water samples. Structural conformers of HPEH were successfully identified using a combination of the potential energy surface (PES) and time-dependent density functional theory (TD-DFT) methods. The synthesized HPEH was successfully characterized further and analyzed based on its harmonic vibrational frequencies, NMR spectra, and electronic transitions using the DFT approach. Sigma profiles were generated using the COSMO-RS approach to identify a compatible medium for HPEH to act as a chemosensor. The conditions for the highly sensitive and selective detection of Hg2+ by HPEH were successfully optimized using the statistical response surface methodology approach. The optimum sensing of HPEH occurred in an 8:2 v/v DMSO/pH 7.8 solution at a 20:60 μM HPEH/Hg2+ concentration and after a reaction time of 18 min, with statistically significant independent variables (p < 0.05) for all parameters. Under optimal conditions, the lowest Hg2+ concentration detected by HPEH was 3.56 µM, indicating that HPEH can serve as an alternative and comparable probe for the detection of Hg2+ in aqueous systems. Using the optimized results, the interaction between HPEH and Hg2+ in the chemosensor system was successfully modeled, and the model was subsequently used with the TD-DFT, non-covalent interaction-reduced density gradient (NCI-RDG), and molecular dynamics approaches to gain mechanistic insights into the interaction. The results showed that the newly synthesized HPEH, in addition to being cost-effective, could serve as a suitable alternative and comparable chemosensor for Hg2+ recognition in water samples, with the advantages of being efficient, portable, and eco-friendly, and offering rapid analysis without the need of specialized training.

Thermochromic Behavior of Polydiacetylene Nanomaterials Driven by Charged Peptide Amphiphiles

The tunability of chromatic phases adapted by chromogenic polymers such as polydiacetylene (PDA) is key to their utility for robust sensing applications. Here, we investigated the influence of charged peptide interactions on the structure-dependent thermochromicity of amphiphilic PDAs. Solid-state NMR and circular dichroism analyses show that our oppositely charged peptide-PDA samples have distinct degrees of structural order, with the coassembled sample being in between the β-sheet-like positive peptide-PDA and the relatively disordered negative peptide-PDA. All solutions exhibit thermochromicity between 20 and 80 °C, whereby the hysteresis of the blue, planar phase is much larger than that of the red, twisted phase. Resonance Raman spectroscopy of films demonstrates that only coassemblies with electrostatic complementarity stabilize coexisting blue and red PDA phases. This work reveals the nature of the structural changes responsible for the thermally responsive chromatic transitions of biomolecule-functionalized polymeric materials and how this process can be directed by sequence-dictated electrostatic interactions.

Self-Associated 1,8-Naphthalimide as a Selective Fluorescent Chemosensor for Detection of High pH in Aqueous Solutions and Their Hg2+ Contamination

A novel diamino triazine based 1,8-naphthalimide (NI-DAT) has been designed and synthesized. Its photophysical properties have been investigated in different solvents and its sensory capability evaluated. The fluorescence emission of NI-DAT is significantly impacted by the solvent polarity due to its inherent intramolecular charge transfer character. Moreover, the fluorescence emission quenched at higher pH as a result of photo-induced electron transfer (PET) from triazine moiety to 1,8-naphthalimide after cleaving hydrogen bonds in the self-associated dimers. Furthermore, the new chemosensor exhibited a good selectivity and sensitivity towards Hg²⁺ among all the used various cations and anions in the aqueous solution of ethanol (5:1, v/v, pH = 7.2, Tampon buffer). NI-DAT emission at 540 nm was quenched remarkably only by Hg²⁺, even in the presence of other cations or anions as interfering analytes. Job's plot revealed a 2:1 stoichiometric ratio for NI-DAT/Hg²⁺ complex, respectively.

Synthesis and Properties of Aromatic-Terminated Diacetylene Organogelators and Their Application to Photopatterning of Polydiacetylenes

A series of simply structured diacetylene-diamide-based gelators (DAGs) with aromatic terminals were synthesized, and their gelation and subsequent photopolymerization abilities were analyzed. DAGs with an adequate spacer length (n) and tolyl terminals (DA-Tn) interacted with aromatic solvents, such as benzene and xylenes, at elevated temperatures. During the subsequent cooling process, the DAGs interacted with each other through CH-π interactions at their terminal positions. They also formed one-dimensional hydrogen bonding arrays through secondary amides, leading to stable organogels. These gels polymerized into π-conjugated polydiacetylenes (PDAs) under ultraviolet irradiation. In the p-xylene gels of DA-Tn, the spacer length exerted characteristic odd-even effects on the photopolymerization rates over a certain range (n = 3-6), which can be explained by periodic changes in the uniformity of the molecular packing modes. When the gelling solvent was changed to cyclohexane, the gelation and photopolymerization abilities were greatly improved because the DA-Tn gel networks became highly crystallized and transparent to ultraviolet light (254 nm). The ultimate conversion to PDA from DA-T8/cyclohexane gels was 45.2 wt %. Applying photolithographic techniques to the DAG with excellent photopolymerizability in the film state, we successfully fabricated microscale photopatterns of PDA. We also established a convenient removal process (development process) of DA monomers in unexposed areas. The resulting PDA patterns were quite stable to ambient light stimuli.

An optical sensor for the detection and quantification of lidocaine in cocaine samples

An optical sensor (OS) was synthesized by mixing 10,12-pentacosadiinoic acid (PDA) with a triblock copolymer for use in the detection/quantification of lidocaine (LD) in seized cocaine hydrochloride (seized CH) samples. In the presence of LD, the OS presented a chromatic transition from blue to red, while no chromatic transition was observed for other typical cocaine adulterants or cocaine hydrochloride. Isothermal titration calorimetry analysis revealed specific interactions between the PDA molecules of the OS and the LD molecules, with these interactions being enthalpically favorable (-1.20 to -36.7 kJ mol^-1). Therefore, the OS color change only occurred when LD was present in the sample, making the OS selective for LD. Consequently, LD was successfully detected in seized CH samples, irrespective of the type of adulteration. The OS was used for the quantification of LD in seized CH samples containing different adulterants, providing a linear range of 0.0959 to 0.225% (w/w), a precision of 7.2%, an accuracy ranging from -10 to 10%, and limits of detection and quantification of 0.0110% (w/w) and 0.0334% (w/w), respectively.

Highly Sensitive and Selective Colorimetric Sensor of Mercury (II) based on Layer-by-Layer Deposition of Gold/Silver Bimetallic Nanoparticles

A new colorimetric sensor based on gold/silver bimetallic nanoparticles (Au-Ag BNPs) for the sensitive and selective detection of mercury (II) was developed. Gold nanoparticles (AuNPs) were synthesized by Turkevich method. The surface modification of AuNPs was modified by the layer-by-layer technique using poly(diallyl dimethylammonium chloride) which provided positively charged of AuNPs. Negatively charged silver nanoparticles (AgNPs) were synthesized by chemical reduction using poly(4-styrenesulfonic acid-co-maleic acid) as the stabilizing agent. The layer-by-layer assembly deposition technique was used to prepare Au-Ag BNPs of positively and negatively charged of AuNPs and AgNPs, respectively. The synthesized Au-Ag BNPs were characterized by a UV-visible spectrophotometer, zeta potential analyzer, FT-IR, TEM, XRD, and EDX. The Au-Ag BNPs sensor was able to detect mercury (II) in aqueous solution, visibly changing from brownish-orange to purple. The linear relationships of the UV-visible spectrometry demonstrate that the Au-Ag BNPs-based colorimetric sensor can be used for the quantitative analysis of mercury (II) in the range of 0.5-80 mg L^-1, with the correlation coefficient, r² = 0.9818. The limit of detection (LOD) of mercury (II) was found to be 0.526 + 0.001 mg L^-1. The BNPs is also verified to have a good practical applicability for mercury (II) detection in the real samples.

Ultrasensitive impedimetric mercury(II) sensor based on thymine-Hg(II)-thymine interaction and subsequent disintegration of multiple sandwich-structured DNA chains

An impedimetric method is described for ultrasensitive analysis of mercury(II). It is based on thymine-Hg(II)-thymine interaction which causes the disintegration of multiple-sandwich structured DNA chains. DNA strands were selected that are partially complementary to the T-rich Hg(II)-specific oligonucleotides (MSO). They were immobilized on a gold electrode via Au-S interaction. Next, the MSO and the bridging strands (BS) that can connect adjacent MSOs were alternately attached through layer-by-layer hybridization. Thus, a multiple-sandwich structured interface in created that carries numerous MSOs. This leads to a change-transfer resistance (R_ct) values of the electrode-electrolyte interface at faradic electrochemical impedance spectroscopy measurements in the presence of the hexacyanoferrate(II)/(III) redox probe at 0.2 V (vs. Ag/AgCl). If Hg(II) is added to the solution, the MSOs selectively interact with Hg(II) to produce T-Hg(II)-T structures. Hence, the multiple-sandwich hybridization chains become disintegrated, and this causes a decrease in resistivity. The effect can be used to quantify Hg(II) over an analytical range that extends over four orders of magnitude (1 fM to 10 pM), and it has a 0.16 fM limit of detection under optimal conditions. Graphical abstract An electrochemical sensor for femtomolar level detection of Hg²⁺ is realized on the basis of thymine-Hg²⁺-thymine interaction which causes disintegration of multiple sandwich DNA hybridization strands.