Sana Sandhu

Ultratrace Detection of Nitroaromatics: Picric Acid Responsive Aggregation/Disaggregation of Self-Assembled p-Terphenylbenzimidazolium-Based Molecular Baskets

1-(p-Terphenyl)-benzimidazolium (TRIPOD-TP) molecules undergo self-assembly to form rodlike structures in aqueous medium, as shown by field-emission scanning electron microscopy, transmission electron microscopy, and dynamic light scattering studies. Upon gradual addition of picric acid (PA), these aggregates undergo an aggregation/disaggregation process to complex morphological structures (10(-12)-10(-10) M PA) and spherical aggregates (10(-9)-10(-8) M PA). These spherical aggregates undergo further dissolution to well-dispersed spheres between 10(-7)-10(-6) M PA. During fluorescence studies, these aggregates demonstrate superamplified fluorescence quenching (>97%) in the presence of 10(-5) to 0.2 equiv of the probe concentration, an unprecedented process with PA. The lowest detection limits by solution of TRIPOD-TP are 5 × 10(-13) PA, 50 × 10(-12) M 2,4-dinitrophenol, 200 × 10(-12) M 2,4,6-trinitrotoluene, and 1 nM 1-chloro-2,4-dinitrobenzene. Paper strips dipped in the solution of TRIPOD-TP demonstrate quantitative fluorescence quenching between 10(-17) and 10(-6) M PA using front-surface steady state studies and can measure as low as 2.29 × 10(-20) g/cm(2) PA.

Impact of aggregation on fluorescence sensitivity of molecular probes towards nitroaromatic compounds

We have designed and synthesized three tripods TIBP4, TIBP8 and TIBP12 possessing, respectively, OC4H9, OC8H17 and OC12H25 alkoxy chains on the biphenyl units and have investigated the effect of the chain length on their ease in aggregation and their efficiency in the detection of nitroaromatic compounds. Tripods TIBP8 and TIBP12 self-assemble to form densely populated nano-spheres (60–100 nm) in a water–DMSO (98 : 2) mixture, as shown by field-emission scanning electron microscopy, transmission electron microscopy and dynamic light scattering studies. TIBP4 which has shorter n-butyl alkyl chains does not undergo aggregation under these conditions. Tripods TIBP8 and TIBP12 which remain in a self-assembled state in water reveal amplified fluorescence quenching with PA, 2,4-DNP, TNT and Cl-DNB, and are associated with NAC induced disaggregation to well-dispersed particles. TIBP8 can detect as low as 10−14 M PA and 2,4-DNP in solution and 2.29 × 10−20 g cm−2 (22.9 zg cm−2) PA by contact mode and is nearly 6000–16 000 times more selective towards PA and 2,4-DNP over TNT and Cl-DNB at 20% fluorescence quenching. However, the tripod TIBP12 can detect as low as 10−14 M of each PA, 2,4-DNP, TNT and Cl-DNB and can find application as a general probe for these NACs. TIBP4 which remains in a molecularly dissolved state shows poor sensitivity (LOD 1 nM) towards NACs.

Water dispersed fluorescent organic aggregates for the picomolar detection of ClO4− in water, soil and blood serum and the attogram detection of ClO4− in the solid state by a contact mode method

Fluorescent organic aggregates (FOAs) of CS-1 have been used for the fluorescence based selective estimation of ClO4− ions. CS-1 undergoes self-aggregation to form FOAs (ϕ = 0.35) with a diameter of 140 ± 50 nm in aqueous medium. Dynamic light scattering and field emission scanning electron microscopy studies reveal that FOAs of CS-1 undergo further aggregation to form larger particles upon addition of ClO4− (10 pM–1 nM concentration) but at higher concentrations of ClO4− ions, these FOAs undergo dis-aggregation to give finally a molecularly dissolved complex of CS-1 and ClO4−. This ClO4− induced aggregation–dis-aggregation process of FOAs of CS-1 is associated with super-amplified fluorescence quenching following two domains of non-linear complexation with Ksv values of 2.42 × 108 M−1 and 3.59 × 105 M−1 and variation in the lifetime measurements of FOAs of CS-1 at different concentrations of ClO4−. The lowest limit of detection is 10 pM in solution and 6 × 10−18 g cm−2 in the solid state by a contact mode method with a selectivity of ∼10 000 over other inorganic anions and allows the quantitative measurement of ClO4− ions using front surface steady state fluorescence of paper strips coated with CS-1. FOAs of CS-1 find applications in the determination of ClO4− in tap water, soil and also blood serum. Probe CS-2, which differs from CS-1 in lacking three methyl groups on the m-phenylene spacer, shows poor sensitivity (LOD 1.6 μM) towards ClO4−. DFT studies of CS-1 and CS-2 and their complexes with ClO4− reveal the effect of methyl substituents on their geometries.

Imidazolium based probes for recognition of biologically and medically relevant anions

The imidazolium derivatives due to their positive charge possess one of the most polarized and positively charged proton at C2-H to form strong ionic hydrogen bond (also termed as double ionic hydrogen bond) with anions and also provide opportunities for anion - π interactions with electron-deficient imidazolium ring. In the present review article, imidazolium based molecular probes for their ability to recognize inorganic anions like halides, cyanide, perchlorate, carboxylic acids, phosphate, sulfate etc. and their derived molecules viz. nucleotides, DNA, RNA, surfactants, proteins, etc have been discussed. The review covers the literature published after year 2009 and has > 130 references. The previous literature has already been discussed by Yoon et al. in two review articles published in Chem. Soc. Rev. 2006 and 2010.