An investigation of the Excitation Wavelength-Dependent Dynamic Changes in the Mechanism of Detection of Picric Acid using Pyrene-Based Donor-Acceptor Systems.

Picric acid (PA) is an important industrial feedstock and hence the release of industrial effluents without proper remediation results in its buildup in soil and water bodies. The adverse effects of PA accumulation in living beings necessitate the development of efficient methods for its detection and quantification. Herein, we describe pyrene-based fluorescent sensors for PA, where pyrene is appended with electron-withdrawing groups, malononitrile, and 2-(3-cyano-4,5,5-trimethylfuran-2(5H)-ylidene) malononitrile (DCDHF). These molecules displayed the typical emission of pyrene monomers, as well as a broad red-shifted emission resulting from an intramolecular charge transfer (ICT) in the excited state. Both the emissions displayed a turn-off response to PA with high selectivity and sensitivity and the lowest limit of detection was estimated as 27 nM. To prove the feasibility of on-site detection, test paper strips were prepared, which could detect PA up to 4.58 picograms. Using a combination of experimental and theoretical studies the mechanism of the detection was identified as primary/secondary inner filter effect, oxidative photoinduced electron transfer, or a combination of both depending on the excitation wavelength. Interestingly, the contribution of each of these mechanisms to the total quenching process varied with a change in the excitation wavelength.

Substituent-Controlled Photophysical Responses in Dihydropyridine Derivatives and Their Application in the Detection of Volatile Organic Contaminants.

In the ever-expanding realm of organic fluorophores, structurally simple and synthetically straightforward molecules with unique photophysical properties have received special attention. Among these, 1,4-dihydropyridine (DHP) is an important scaffold that permits fine-tuning of their photophysical properties through substituents on the periphery. Herein, we describe a series of solid-emissive N-substituted 2,6-dimethyl-4-methylene-1,4-dihydropyridine derivatives appended with electron-withdrawing substituents (dicyanomethylene or 2-dicyanomethylene-3-cyano-2,5-dihydrofuran) at the C-4 position and alkyl or alkylaryl groups on the DHP nitrogen. Electronic and steric tuning exerted by these substituents resulted in interesting photophysical properties such as negative solvatochromism, solidstate, and aggregation-induced emission (AIE). Theoretical calculations were carried out to explain the solvatochromic properties. Insight into the AIE properties was obtained through variable-temperature nuclear magnetic resonance and viscosity- and temperature-dependent emission studies. The variations in molecular packing in the crystal lattice with changes in the N-substituents contributed to the tuning of solid state emission properties. Detection of aromatic volatile organic compounds (VOCs) was achieved using the aggregates of the DHP derivatives. Among the VOCs, p-xylene elicited a significant enhancement in emission, allowing its detection at submicromolar levels.

Chiral 8-aminoBODIPY-based fluorescent probes with site selectivity for the quantitative detection of HSA in biological samples.

Human serum albumin (HSA) is one of the vital proteins in blood serum, and its optimum level is a reflection of the physiological well-being of an individual. Any abnormalities in serum HSA levels could often be a sign of disguised physiological disorders. The importance of fast and accurate determination of serum HSA levels has led to the development of various quantification methods. Among these, fluorescence-based methods employ molecular probes capable of producing selective responses on interaction with HSA. Herein, we report chiral 8-aminoBODIPY-based probes having blue emission for the quantitative detection of HSA in buffer and human blood serum. A pair of 8-aminoBODIPY enantiomers, namely R-PEB and S-PEB, were synthesized. They exhibited a fast 'turn-on' fluorescence response towards HSA, allowing its detection and quantification. In PBS buffer, R-PEB and S-PEB showed very good sensitivity with a limit of detection (LoD) of 25 nM (KD = 9.84 ± 0.14 µM) and 39 nM (KD = 18.67 ± 0.21 µM), respectively. The linear relationship observed between the fluorescence intensity of R-PEB/S-PEB and the HSA concentration in serum samples allowed us to generate a reference curve for HSA estimation for practical applications. Examination of unknown serum samples showed a good correlation with the results obtained by the benchmark BCG method. Interestingly, the difference in these probes' dissociation constants and LoD indicated their differential binding to HSA. Considering the availability of multiple ligand binding sites in HSA, their binding preferences were investigated in detail by displacement assays using site-specific drugs. These studies showed the preferential affinity of R-PEB towards site II, which was further substantiated using molecular docking studies. However, these displacement assays could not identify the preferred binding site of S-PEB. Blind docking studies indicated that S-PEB occupied a site closer to FA5. Selective binding of R-PEB to site II and its characteristic photophysical response can be utilized to quickly screen potential site II binding drugs.

Delocalization Effects and Tunable Emission in a Class of Charged Cyclazines with Nitrogen on the Periphery.

A new class of cyclazine analogues with periphery reminiscent of an aza[10]annulene framework, tethered internally by an sp3 carbon, is presented. In depth structure analysis based on NMR and X-ray diffraction data gave a deeper insight into the effect of electron delocalization on their structure and properties. A characteristic change in chemical shift positions suggested an aromatic ring current in these systems. Attractive emission properties in solid and solution states involving charge transfer is another highlight.

Electronically-tuned triarylmethine scaffolds for fast and continuous monitoring of H2S levels in biological samples.

A sensor for the detection and quantification of H2S in biological samples should ideally meet a set of criteria such as fast detection, high sensitivity in the desired concentration range, high selectivity, non-interference from biomolecules like proteins, ease of synthesis, long-term stability and water solubility. Although a number of H2S probes are known, none of them possess all the above attributes that are relevant for practical applications. As part of a program to develop reliable chemical probes for continuous monitoring of this gasotransmitter in the blood plasma of sepsis-prone individuals in post-operative wards, we have looked at the possibility of improving the reactivity and selectivity profile of triarylmethine dyes towards different nucleophiles. After achieving high sensitivity through electronic control, the interference from sulfite, thiosulfate and metabisulfite was addressed by introducing a metal salt-mediated desulfuration step that results in dye regeneration selectively from its H2S adduct. Typically, if the analyte contains only H2S, the loss of absorbance in the first step gets completely reinstated after the second step; absorbance changes in both steps vary linearly with sulfide concentration and either of these two steps can be used for the quantification of H2S with the help of standard plots. In the presence of interfering ions, the first step will show decolourization due to the presence of all of them whereas only the H2S-adduct will undergo desulfuration in the second step which can be used for quantification. The decolourization step is instantaneous while the desulfuration requires only about 50 s, making the entire protocol complete in less than a minute. The methodology optimized here also meets the requirements mentioned above for real-life applications.